TThhiiss ppuubblliiccaattiioonn iiss CCIIRRIIAA ccooppyyrriigghhtt aanndd iiss aavvaaiillaabbllee ffoorr tthhee uusseeooff CCIIRRIIAA CCoorree MMeemmbbeerrss oonnllyy.. IItt mmuusstt nnoott bbee bbee rreepprroodduucceedd oorrttrraannssmmiitttteedd iinn aannyy ffoorrmm oorr bbyy aannyy mmeeaannss,, iinncclluuddiinngg pphhoottooccooppyyiinngg aanndd rreeccoorrddiinngg,, wwiitthhoouutt tthhee wwrriitttteenn ppeerrmmiissssiioonn oofftthhee ccooppyyrriigghhtt--hhoollddeerr,, aapppplliiccaattiioonn ffoorr wwhhiicchh sshhoouulldd bbeeaaddddrreesssseedd ttoo tthhee ppuubblliisshheerr.. SSuucchh wwrriitttteenn ppeerrmmiissssiioonn mmuusstt aallssoobbee oobbttaaiinneedd bbeeffoorree aannyy ppaarrtt ooff tthhiiss ppuubblliiccaattiioonn iiss ssttoorreedd iinn aarreettrriieevvaall ssyysstteemm ooff aannyy nnaattuurree..

PPuubblliisshheedd bbyy CCIIRRIIAA,, CCllaassssiicc HHoouussee,, 117744––118800 OOlldd SSttrreeeett,,LLoonnddoonn EECC11VV 99BBPP,, UUKK..

TThhiiss ddooccuummeenntt mmaayy bbee pprriinntteedd –– oorr ppaassssaaggeess eexxttrraacctteedd –– ffoorr rreeffeerreennccee ppuurrppoosseess oonnllyy.. TToo pprriinntt,, pplleeaassee uussee tthhee rriigghhtt bbuuttttoonn oonn yyoouurr mmoouussee ttoo bbrriinngg uupp tthhee mmeennuu..

CClliicckkiinngg tthhee rriigghhtt bbuuttttoonn wwiillll aallssoo aallllooww yyoouu ttoo uussee tthhee sseeaarrcchh ffuunnccttiioonn..

CCIIRRIIAA ppuubblliiccaattiioonnss ffoorr CCoorree MMeemmbbeerrss

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CCIIRRIIAA CC665566 LLoonnddoonn,, 22000066

MMaassoonnrryy aarrcchh bbrriiddggeess::ccoonnddiittiioonn aapppprraaiissaall aannddrreemmeeddiiaall ttrreeaattmmeenntt

LLeeoo DD MMccKKiibbbbiinnss Mott MacDonald Ltd

CClliivvee MMeellbboouurrnnee University of Salford

NNiissaarr SSaawwaarr Birse Rail Ltd

CCaarrllooss SSiicciilliiaa GGaaiillllaarrdd KW Ltd

Classic House, 174–180 Old Street, London EC1V 9BPTEL: +44 (0)20 7549 3300 FAX: +44 (0)20 7253 0523EMAIL: enquiries@ciria.orgWEBSITE: www.ciria.org

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Masonry arch bridges: condition appraisal and remedial treatment

McKibbins, L; Melbourne, C; Sawar, N; Sicilia Gaillard, C

CIRIA

CIRIA C656 © CIRIA 2006 RP692

British Library Cataloguing in Publication Data

A catalogue record for this book is available from the British Library.

Published by CIRIA, Classic House, 174–180 Old Street, London EC1V 9BP, UK.

This publication is designed to provide accurate and authoritative information on the subjectmatter covered. It is sold and/or distributed with the understanding that neither the authorsnor the publisher is thereby engaged in rendering a specific legal or any other professionalservice. While every effort has been made to ensure the accuracy and completeness of thepublication, no warranty or fitness is provided or implied, and the authors and publishershall have neither liability nor responsibility to any person or entity with respect to any lossor damage arising from its use.

All rights reserved. No part of this publication may be reproduced or transmitted in anyform or by any means, including photocopying and recording, without the writtenpermission of the copyright-holder, application for which should be addressed to thepublisher. Such written permission must also be obtained before any part of this publicationis stored in a retrieval system of any nature.

If you would like to reproduce any of the figures, text or technical information from this orany other CIRIA publication for use in other documents or publications, please contact thePublishing Department for more details on copyright terms and charges at:publishing@ciria.org Tel: 020 7549 3300.

CIRIA C6562

Keywords

Transport infrastructure, ground engineering, health and safety, materialstechnology, site management, water infrastructure, whole-life costing

Reader interest

Owners, asset andmaintenance managers,geotechnical engineers,environmental engineersinvolved in masonry archbridge management

Classification

AVAILABILITY Unrestricted

CONTENT Enabling document

STATUS Committee-guided

USER Maintenance, geotechnical,environmental and civil engineers

ISBN-13: 978-0-86017-656-5

ISBN-10: 0-86017-656-8

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SSuummmmaarryy

Masonry arch bridges have proved to be reliable, enduring structures and remain a vitalpart of the road, rail and waterway infrastructure in the UK and other countries. Howeverthey are facing a number of challenges associated with their extended period in serviceand the changing requirements of modern transport systems. In order to ensure thecontinued efficient use of these assets in the future it is necessary to manage and maintainthem carefully, with due regard to, and an adequate understanding of, their specialcharacteristics and needs. In a number of important ways these are distinct from those ofmodern structures and the effective stewardship of masonry arch bridges requires somespecialist knowledge and a particular approach. The report provides information andguidance which will assist those responsible for this task in achieving their aims.

The guidance provides infrastructure owners, consulting engineers, contractors andmaintenance managers with guidance on the management, condition appraisal,maintenance and repair of masonry (stone and brick) arch bridges. It is based on adetailed review of published literature and infrastructure owner’s procedures,consultation with experts and practitioners within the field and includes case studiesdemonstrating good practice.

The purpose of the book is to:

� present good practice (2005)

� provide a guide for routine management

� recommend assessment, maintenance and repair strategies to give value for money

� facilitate knowledge sharing.

The guidance is divided into five chapters, each including information and guidance onparticular aspects of masonry and brick arch bridges, followed by appendices withdetailed information for practitioners.

Chapter 1: Introduction and general background information on the document,including advice on how and where to find information.

Chapter 2: Construction and behaviour of arch bridges and an overview of the basicprinciples of arch bridge history, construction and materials, behaviour andperformance which is intended to be particularly useful to readers with less experiencein this type of structure.

Chapter 3: Asset management and condition appraisal of masonry arch bridges, includinginformation and guidance on bridge inspection, investigation and structural assessment.

Chapter 4: Selection, planning and implementation of maintenance, repair andstrengthening works on masonry arch bridges, including health and safety,environmental and heritage considerations.

Chapter 5: Summary of recommendations for good practice, discussion of futureresearch and development needs, and a list of references used in the guidance.

Appendix 1: Case studies which illustrate particular aspects of the practicalimplementation of topics discussed in the main body of the guidance.

Appendices 2 to 6: Additional information on topics covered in the main body of theguidance.

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AAcckknnoowwlleeddggeemmeennttss

Steering group This book has been produced at the request of the Bridge Owners’ Forum by CIRIA asCIRIA Research Project 692. The detailed research for this project was carried out byMott MacDonald in partnership with the University of Salford, Birse Rail Ltd and KW Ltd.

Authors Leo McKibbins BSc(Hons) MSc CEng MIMMM FGS

After graduating from University College London and gaining an MSc in Geomaterialsfrom Queen Mary College, Leo worked for Sandberg Consulting Engineers beforejoining the Special Services unit of Mott MacDonald Ltd. in 1998 where he is aprincipal materials engineer. He is experienced in the performance, specification,investigation and repair of brick and stone masonry in bridges, tunnels and otherstructures.

Clive Melbourne BEng(Hons) PhD CEng FIStructE FICE

After graduating from Sheffield University, Clive worked as a bridge engineer for 12years before returning to academia. He has been professor of structural engineering atthe University of Salford since 1995 where he supervises a research team studyingseveral aspects of masonry arch bridge behaviour. Additionally, he leads the masonryarch bridges sub-group for the European funded Sustainable Bridges project.

Nisar Sawar CEng MICE

Nisar is a senior design manager with Birse Rail Ltd. and has over 20 years ofexperience in the design, inspection, maintenance and renewal of railway structures.He has been responsible for the design and implementation of a wide variety ofpreventative, remedial and strengthening measures on masonry arch bridges.

Carlos Sicilia Gaillard Dr. Ingeniero de Caminos, PhD

Carlos studied at the UPC in Barcelona and obtained his PhD at the University ofWales, where he performed centrifuge tests on masonry arch bridges and developed aFinite Element code to simulate masonry structures. After a period working forconsultants Mott MacDonald Ltd, he is now responsible for the numerical modelling ofa wide range of structures at KW Ltd.

Steering group Following CIRIA’s usual practice, the research project was guided by a steering groupwhich comprised:

Chair Brian Bell* Network Rail

Members Graham Bessant Metronet Rail SSL Limited

Edward Bunting* Department for Transport

Graham Cole* Surrey County Council

Matthew Collings Gifford and Partners Ltd

Prof Robert Falcolner University of Greenwich

Andrew Fewtrell Up and Under Group

Amrit Ghose Faber Maunsell

CIRIA C6564

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Dr Matthew Gilbert University of Sheffield

Rod Howe* British Waterways

Prof Tim Hughes Cardiff University

William Kerr* Northern Ireland Roads Service

Ron Ko* Highways Agency

David Steer Owen Williams

*Bridge Owners’ Forum representatives

CIRIA managers CIRIA’s research managers for the project were Dr Andrew Pitchford and Mrs NatalyaBrodie-Greer (neé Brodie-Hubbard).

Project funders The project was funded by:

The Department for Transport

Network Rail

CIRIA Core Programme sponsors

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CCoonntteennttss

SSuummmmaarryy .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. 33

AAcckknnoowwlleeddggeemmeennttss .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. 44

CCoonntteennttss .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. 66

LLiisstt ooff ffiigguurreess .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. 1100

LLiisstt ooff ttaabblleess .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. 1133

GGlloossssaarryy .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. 1155

LLiisstt ooff aaccrroonnyymmss aanndd aabbbbrreevviiaattiioonnss .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. 2233

11 IInnttrroodduuccttiioonn aanndd bbaacckkggrroouunndd .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. 2255

1.1 Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

1.2 Purpose and scope of work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

1.3 Application . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

1.4 Issues of special importance for masonry arch bridges . . . . . . . . . . . 26

1.5 How to use this guidance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

22 BBrriiddggee ccoonnssttrruuccttiioonn aanndd ppeerrffoorrmmaannccee .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. 2299

2.1 History of masonry arch bridges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

2.1.1 Bridge design and construction . . . . . . . . . . . . . . . . . . . . . . . . 29

2.1.2 Bridge materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

2.1.3 Construction methodology . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34

2.2 Structural elements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

2.2.1 Arch geometry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38

2.2.2 Arch construction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38

2.2.3 Spandrels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

2.2.4 Piers and abutments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

2.3 Construction materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

2.3.1 Mortars . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42

2.3.2 Stone . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48

2.3.3 Bricks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52

2.4 Structural behaviour . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53

2.4.1 Behaviour of masonry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53

2.4.2 Loads on arch bridges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55

2.4.3 Performance requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56

2.4.4 Behaviour under quasi-static loading . . . . . . . . . . . . . . . . . . . . 57

2.4.5 Behaviour under dynamic and cyclic loading . . . . . . . . . . . . . 62

2.4.6 Performance of highway bridge parapets . . . . . . . . . . . . . . . . 62

2.5 Loss of bridge performance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63

2.5.1 Boundary conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64

2.5.2 Structural condition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69

2.5.3 Material condition and deterioration . . . . . . . . . . . . . . . . . . . . 77

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33 BBrriiddggee mmaannaaggeemmeenntt aanndd ccoonnddiittiioonn aapppprraaiissaall .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. 8855

3.1 The need for masonry arch bridge management . . . . . . . . . . . . . . . . 85

3.2 Consequences of loss of performance . . . . . . . . . . . . . . . . . . . . . . . . . 86

3.2.1 Safety in operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86

3.2.2 Synergy with other assets and infrastructure . . . . . . . . . . . . . . 86

3.2.3 Disruption and customer dissatisfaction . . . . . . . . . . . . . . . . . . 86

3.2.4 Costs of failure and repair . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87

3.3 General principles of asset management . . . . . . . . . . . . . . . . . . . . . . . 87

3.3.1 Strategic, tactical and operational management . . . . . . . . . . . 89

3.4 Managing bridge maintenance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89

3.4.1 Appraisal of condition and maintenance needs . . . . . . . . . . . . 90

3.4.2 Bridge management systems and data . . . . . . . . . . . . . . . . . . . 93

3.4.3 Availability of resources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94

3.4.4 Statutory and regulatory obligations . . . . . . . . . . . . . . . . . . . . 95

3.4.5 Practical constraints . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95

3.4.6 Prioritisation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95

3.4.7 Risk assessment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96

3.4.8 Whole-life asset costs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98

3.4.9 Reconstruction and replacement . . . . . . . . . . . . . . . . . . . . . . . 99

3.4.10 Business case . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99

3.5 Environmental management . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100

3.5.1 Bridge conservation and heritage . . . . . . . . . . . . . . . . . . . . . 101

3.5.2 Environmental conservation and ecology . . . . . . . . . . . . . . . 103

3.6 Sources of bridge information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106

3.6.1 Data collection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106

3.6.2 Historical data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107

3.6.3 Inspection data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107

3.7 Bridge inspection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 108

3.7.1 Types and frequencies of inspection . . . . . . . . . . . . . . . . . . . . 108

3.7.2 Visual inspection procedures . . . . . . . . . . . . . . . . . . . . . . . . . 111

3.7.3 Optimising inspection procedures . . . . . . . . . . . . . . . . . . . . . 116

3.7.4 Programming and timing . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116

3.7.5 Health and safety and environmental considerations . . . . . . 116

3.7.6 Planning and preparation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117

3.7.7 Competence of inspection personnel . . . . . . . . . . . . . . . . . . . 117

3.7.8 Strengths and weaknesses of visual inspection . . . . . . . . . . . 118

3.8 Bridge investigation and monitoring . . . . . . . . . . . . . . . . . . . . . . . . . 119

3.8.1 Optimising investigation procedures . . . . . . . . . . . . . . . . . . . 120

3.8.2 Investigation techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121

3.8.3 Sampling and materials testing/analysis techniques . . . . . . . 124

3.8.4 Monitoring techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 134

3.9 Interpreting inspection and investigation results . . . . . . . . . . . . . . . 137

3.9.1 The importance of good interpretation . . . . . . . . . . . . . . . . . 137

3.9.2 Considerations for interpretation . . . . . . . . . . . . . . . . . . . . . . 138

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3.9.3 Condition assessment and assigning ratings . . . . . . . . . . . . . 139

3.10 Structural assessment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 139

3.10.1 Analysis methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 139

3.10.2 Analysis procedures and selection of method of analysis . . . 142

3.10.3 Considerations when carrying out analyses . . . . . . . . . . . . . . 146

3.10.4 Assessment results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .157

44 SSeelleeccttiinngg aanndd ccaarrrryyiinngg oouutt bbrriiddggee wwoorrkkss .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. 116633

4.1 Health and safety considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . 163

4.1.1 Hazards, risks and safe working . . . . . . . . . . . . . . . . . . . . . . . 163

4.1.2 Statutory legislation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 164

4.2 Environmental considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 165

4.2.1 Executing work on historic bridges . . . . . . . . . . . . . . . . . . . . 165

4.2.2 Dealing with protected species . . . . . . . . . . . . . . . . . . . . . . . . 168

4.2.3 Prevention and control of pollution . . . . . . . . . . . . . . . . . . . . 170

4.3 Preventative, remedial and strengthening measures . . . . . . . . . . . . 171

4.3.1 Guidance on selection of measures . . . . . . . . . . . . . . . . . . . . 171

4.3.2 Routine and preventative maintenance . . . . . . . . . . . . . . . . . 172

4.3.3 Repairing deteriorating masonry . . . . . . . . . . . . . . . . . . . . . . 175

4.3.4 Repair and strengthening techniques . . . . . . . . . . . . . . . . . . 182

4.3.5 Minimising risk in contracting and execution of works . . . . 190

4.4 New masonry arch bridges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 191

4.4.1 Existing design codes and guidance . . . . . . . . . . . . . . . . . . . . 191

4.4.2 Selection and specification of materials . . . . . . . . . . . . . . . . . 193

55 SSuummmmaarryy aanndd rreeccoommmmeennddaattiioonnss .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. 119977

5.1 Recommendations for good practice . . . . . . . . . . . . . . . . . . . . . . . . . 197

5.2 Areas requiring further research and future needs . . . . . . . . . . . . . 199

AA11 CCaassee ssttuuddiieess .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. 220033

A.1.1 Saddling and repair of Elsage Farm bridge . . . . . . . . . . . . . . . . . . . 205

A1.2 Cross-country route bridge strengthening works . . . . . . . . . . . . . . . 210

A1.3 Rockshaw road overbridge – pier repair of a three span masonryarch . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 214

A1.4 Ecological issues when working on a bridge in Wales . . . . . . . . . . . . 218

A1.5 Sympathetic repair of Brynich Aqueduct . . . . . . . . . . . . . . . . . . . . . 220

A1.6 Refurbishment of Berwyn Viaduct . . . . . . . . . . . . . . . . . . . . . . . . . . 224

A1.7 Strengthening and refurbishment of Hungerford Canal bridge . . . 229

A1.8 Assessment of Llanharan Bridge using discrete element analysis . . 234

A1.9 Strengthening of Gumley Road bridge using retrofittedreinforcement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 236

A1.10 Repairs to Caergwrle Packhorse bridge . . . . . . . . . . . . . . . . . . . . . . . 239

A1.11 Reconstruction of brick arch bridges on the Chesterfield Canal . . . 242

A1.12 Egglestone Abbey Bridge strengthening and repairs . . . . . . . . . . . . 247

AA22 BBrriiddggee ccoonnddiittiioonn aasssseessssmmeenntt gguuiiddeelliinneess .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. 224499

AA33 BBrriiddggee aasssseessssmmeenntt mmeetthhooddss .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. 225533

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A3.1 Assessment methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 253

A3.2 Verification of solid mechanics methods . . . . . . . . . . . . . . . . . . . . . . 259

A3.3 Simulation of masonry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 259

AA44 SSppeecciiaalliisstt iinnssppeeccttiioonn aanndd mmoonniittoorriinngg tteecchhnniiqquueess .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. 226622

AA55 HHeeaalltthh,, ssaaffeettyy aanndd eennvviirroonnmmeennttaall lleeggiissllaattiioonn .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. 227711

AA66 RReeppaaiirr aanndd ssttrreennggtthheenniinngg tteecchhnniiqquueess .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. 227733

A6.1 Arch distortion remedial works . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 274

A6.2 Arch grouting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 276

A6.3 Backfill replacement or reinforcement . . . . . . . . . . . . . . . . . . . . . . . 279

A6.4 Concrete saddle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 281

A6.5 Parapet upgrading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 285

A6.6 Patch repair of masonry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 288

A6.7 Prefabricated liners . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 292

A6.8 Relieving slabs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 294

A6.9 Retro-reinforcement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 298

A6.10 Spandrel tie bars/patress plates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 303

A6.11 Sprayed concrete lining . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 307

A6.12 Spandrel wall strengthening (“Stratford method”) . . . . . . . . . . . . . . 309

A6.13 Thickening surfacing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 313

A6.14 Through-ring stitching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 315

A6.15 Underpinning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 319

A6.16 Waterproofing and drainage improvements . . . . . . . . . . . . . . . . . . . 325

RReeffeerreenncceess .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. ..333311

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Glossary Elements of a typical masonry arch bridge . . . . . . . . . . . . . . . . . . . . . . 15

Figure 2.1 A Roman bridge - Ponte Saint-Martin (c 25 BC) near Torino (Italy).Span to pier thickness ratio of Roman bridges was typically twoor three . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

Figure 2.2 Bridge of Khaju (1667) at Isfahan (Iran) functioned as a bridge, a dam, and a resort for desert travellers . . . . . . . . . . . . . . . . . . . . . . . . . . 31

Figure 2.3 Pontypridd Bridge (1756) in South Wales had to be rebuilt several times before the correct rise-to-span ratio was achieved to ensure that the 43 m span arch did not collapse after removal of the centring . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

Figure 2.4 Construction of Gayton canal bridge on wooden centering . . . . . . . . . 35

Figure 2.5 Construction of a modern canal bridge arch barrel using archcentring . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

Figure 2.6 Old rural bridge with an arch constructed from irregular limestoneblocks, some held in place by friction and some roughly packed with lime mortar. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

Figure 2.7 Removal of the provisional supports (on left) of a stone arch of theNogent-sur-Seine Bridge (Perronet, 1782-83) . . . . . . . . . . . . . . . . . . . . 36

Figure 2.8 Main elements of a masonry arch rail overbridge (top) and underbridge (below) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

Figure 2.9 Skew bridge construction patterns . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38

Figure 2.10 Skew bridge with arch barrel constructed from a single ring of sandstone blocks in the “English pattern” . . . . . . . . . . . . . . . . . . . . . . . 39

Figure 2.11 Typical backing of semi-elliptical arches. . . . . . . . . . . . . . . . . . . . . . . . 40

Figure 2.12 Masonry construction patterns; (a) coursed rubble, (b) random rubble inmortar matrix, (b) dry random rubble, (c) cut-stone ashlar, (d) commonbrickwork bonds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

Figure 2.13 (a) Knocking up lime mortar to prepare it for use; (b) lime mortarready for application using a “pointing key” . . . . . . . . . . . . . . . . . . . . . 45

Figure 2.14 Limestone voussoirs in an arch barrel. . . . . . . . . . . . . . . . . . . . . . . . . . 50

Figure 2.15 Masonry under uniaxial compression . . . . . . . . . . . . . . . . . . . . . . . . . . 54

Figure 2.16 Four hinge failure mechanism of an arch bridge . . . . . . . . . . . . . . . . . 59

Figure 2.17 Transmission of pressure from loaded span to adjacent span in a multi-span bridge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60

Figure 2.18 Multi-span failure mechanisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61

Figure 2.19 Results of differential settlement on bridge superstructure . . . . . . . . . 68

Figure 2.20 Arch soffit showing cracking caused by differential settlement . . . . . . 68

Figure 2.21 Longitudinal cracking in bridge superstructure . . . . . . . . . . . . . . . . . . 71

Figure 2.22 Transverse cracking in the arch barrel . . . . . . . . . . . . . . . . . . . . . . . . . 71

Figure 2.23 Displaced crown voussoir in arch soffit . . . . . . . . . . . . . . . . . . . . . . . . . 72

Figure 2.24 Plan of typical crack pattern in a skewed barrel . . . . . . . . . . . . . . . . . . 73

Figure 2.25 Spandrel wall defects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74

Figure 2.26 Cracking in arch barrel intrados indicating spandrel wall detachment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74

Figure 2.27 Visible separation of the intrados ring of a four ring thick brickworkarch, showing a failed attempt at repointing of the crack . . . . . . . . . . . 77

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Figure 2.28 Freeze-thaw spalling affecting individual susceptible bricks in a bridge parapet . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80

Figure 2.29 Leachate deposits of calcium carbonate showing water ingress through masonry joints . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81

Figure 2.30 Vegetation growing through and damaging parapet and spandrel wall . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82

Figure 2.31 Repointing with a impermeable Portland-cement based mortar hasconcentrated deterioration in the softer brick units, leaving the cement standing proud . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83

Figure 2.32 Stone copings damaged by corrosion of an embedded iron clamp . . . 83

Figure 3.1 The asset management cycle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88

Figure 3.2 Process of routine maintenance management for a bridge . . . . . . . . . . 92

Figure 3.3 Deep open joints leading to loose masonry units and loss of loadtransfer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114

Figure 3.4 Loss of mortar between arch barrel rings at spandrel . . . . . . . . . . . . 114

Figure 3.5 Delamination of intrados arch ring despite presence of headers, crack concealed by repointing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114

Figure 3.6 Leachate deposits on arch soffit indicating water penetration and mortar deterioration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115

Figure 3.7 Fragmented 100mm diameter masonry cores taken through massbrickwork into fill . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125

Figure 3.8 Masonry thin-section viewed through a microscope; brick is on bottomleft, mortar on top right, divided by a lighter band of secondary calcitesuggesting that the masonry has been exposed to wet conditions inservice . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 128

Figure 3.9 Results of collapse load test on a full-scale model of a multi-spanbridge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132

Figure 3.10 Flat jack developed by Cardiff University . . . . . . . . . . . . . . . . . . . . . . 133

Figure 3.11 Scour protection works on a masonry arch bridge . . . . . . . . . . . . . . . 137

Figure 3.12 Basis of multi-stage assessment process . . . . . . . . . . . . . . . . . . . . . . . . 145

Figure 3.13 Orleans bridge (Perronet, 1782-83) transversal relieving arches over pier and haunches . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 148

Figure 3.14 Internal structure of springings and piers: a) Verde viaduct: masonry and poorly supported springings of the lower tier of arches; b) QueenMarguerita bridge, Turin: masonry backfill and stiff stone support of the springings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 149

Figure 3.15 Verde viaduct: arches of the lower tier; (a) external front view; (b)internal structure with stone springing and brick multi-ring arch . . . 150

Figure 3.16 Traversa Bridge at km 52.133 of the Savona-Carmagnola line, North-Western Italy. a) Hollow abutment-pier and rubble masonry also in the spandrels; b) river pebbles masonry with courses of regular brickmasonry and hollow pier . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151

Figure 3.17 Ring separation detected using endoscope down corehole through arch barrel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 152

Figure 4.1 Proprietary “bat brick” artificial roost and suggested locations forinstallation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 170

Figure 4.2 Removal of deteriorated mortar using a long-necked chisel to avoiddamage to brickwork . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 177

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Figure 4.3 Appropriate techniques should be selected and contractor’s personnelshould be suitably experienced else quality of finished repointing workmay be very poor! . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 178

Figure 4.4 Selection of methods for the repair of masonry arch barrels in single-span bridges (after Broomhead, 1991) . . . . . . . . . . . . . . . . . . . 184

Figure A1.1 General view on east elevation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 205

Figure A1.2 Water ingress through the arch barrel; previous large patch repair toarch intrados is visible on the right . . . . . . . . . . . . . . . . . . . . . . . . . . . 206

Figure A1.3 Arch extrados prepared for reinforcement and shuttering . . . . . . . . 209

Figure A1.4 Machine-mounted drill rig in operation . . . . . . . . . . . . . . . . . . . . . . . 211

Figure A1.5 Wall mounted drill rig in operation . . . . . . . . . . . . . . . . . . . . . . . . . . . 212

Figure A1.6 Erection of access scaffold for close inspection . . . . . . . . . . . . . . . . . . 214

Figure A1.7 Crushing brickwork in pier below springing showing longitudinalcracks, with patch repairs to spalled brickwork and steel corset added for temporary support . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 215

Figure A1.8 Dropped string course above pier suffering from settlement . . . . . . 216

Figure A1.9 General view of Brynich Aqueduct . . . . . . . . . . . . . . . . . . . . . . . . . . . 221

Figure A1.10 Placement of mass concrete fill over arch between internalspandrels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 222

Figure A1.11 General view of Berwyn Viaduct before refurbishment . . . . . . . . . . . 224

Figure A1.12 Severe displacement of masonry from arch spandrel caused by rootgrowth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 225

Figure A1.13 Repairs to displaced brickwork at arch spandrels . . . . . . . . . . . . . . . . 226

Figure A1.14 Fill removed to expose bitumen coated arch extrados and originaldrainage points above piers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 226

Figure A1.15 Reapplied sprayed waterproofing system draining through granularmaterial to re-cored and re-lined drainage points through piers . . . . 227

Figure A1.16 Construction of concrete apron at pier bases as support for scaffolding, eventually left in place to provide additional scour protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 227

Figure A1.17 General appearance after strengthening works . . . . . . . . . . . . . . . . . 229

Figure A1.18 Drilling circumferential holes for reinforcement in arch intrados . . . 232

Figure A1.19 Close-up of installed circumferential reinforcement . . . . . . . . . . . . . . 232

Figure A1.20 Elevation of Llanharan Bridge showing brick piers added at quarter-points of arch . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 234

Figure A1.21 Analysis results with additional piers for 40/44 tonne triple axle at the crown . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 235

Figure A1.22 General view of Gumley Road Bridge . . . . . . . . . . . . . . . . . . . . . . . . . 236

Figure A1.23 Anchor arrangement and installation . . . . . . . . . . . . . . . . . . . . . . . . . 237

Figure A1.24 Caergwrle Packhorse Bridge after repairs . . . . . . . . . . . . . . . . . . . . . . 239

Figure A1.25 Reconstruction of brick arch on centring, Osberton Bridge . . . . . . . . 245

Figure A1.26 Osberton Bridge after reconstruction, with conserved features andmaterials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 246

Figure A1.27 General view of Egglestone Abbey Bridge . . . . . . . . . . . . . . . . . . . . . . 247

Figure A6.1 Grout injection of arch barrel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 278

Figure A6.2 Layout of diagonal reinforcement in a parapet with retrofittedreinforcement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 287

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Figure A6.3 Brickwork patch repair works being undertaken on a “hit and miss”basis to arch ring with no live loading . . . . . . . . . . . . . . . . . . . . . . . . . 291

Figure A6.4 Typical arrangement of internally installed retro–reinforcement . . . . 299

Figure A6.5 Coring for installation of internal retro-reinforcement, from above (a)and below (b) the arch barrel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 301

Figure A6.6 Drilling guide-holes in arch intrados for installation of dowels . . . . . 316

Figure A6.7 Underpinning works in progress . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 321

Figure A6.8 Brush application of bitumen to upper surface of concrete arch saddle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 327

Figure A6.9 Loose laid system and protective geotextile membranes being provided for masonry arch structure . . . . . . . . . . . . . . . . . . . . . . . . . . 328

LLiisstt ooff ttaabblleess

Table 1.1 Where to find guidance in this book . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

Table 2.1 Time-line for construction of masonry arch bridges in the UK . . . . . . 29

Table 2.2 Mortar mixes and compressive strengths used in the UK, andcorresponding strengths of masonry using different bricks. . . . . . . . . 47

Table 2.3 Comparison of typical strength and density values of some common UK building stones with other construction materials . . . . . . . . . . . . . 49

Table 2.4 Physical properties of bricks sampled from old railway structures . . . . 53

Table 2.5 Potential consequences of instability of support on structuralperformance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65

Table 2.6 Actions (loads) and consequences for masonry arch barrels . . . . . . . . . 76

Table 2.7 Summary table of factors causing masonry deterioration . . . . . . . . . . . 79

Table 3.1 Safety hazards and risk mitigation measures for bridges . . . . . . . . . . . 97

Table 3.2 Principal statutory designations relating to the conservation of British bridges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102

Table 3.3 Inspection requirements of main UK bridge owners: Network Rail (NR), Highways Agency (HA), British Waterways (BW) and LondonUnderground (LU). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109

Table 3.4 Specialist investigation, testing and monitoring techniques for bridge investigation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 120

Table 3.5 Specialist inspection techniques for bridge investigation . . . . . . . . . . 123

Table 3.6 Common geotechnical testing techniques for masonry arch bridgeinvestigations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 131

Table 3.7 Possible levels of structural analysis for masonry arch bridges . . . . . . 144

Table 3.8 Significance of defects for assessment purposes . . . . . . . . . . . . . . . . . 154

Table 3.9 Comparison of main analysis methods for bridge assessment . . . . . . 160

Table 3.10 Sensitivity of analysis techniques to input parameters and techniquesfor obtaining them . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 161

Table 4.1 Application of remedial measures to treat common defects . . . . . . . . 183

Table 4.2 Common factors for consideration when carrying out bridge works . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 186

Table 4.3 Repair and strengthening techniques considered in detail in this book . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 187

Table 4.4 Summary table of considerations for common remedial andstrengthening measures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 186

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Table 4.5 Equivalent mortar mix designations of current British and EuropeanStandards and cement gauging information . . . . . . . . . . . . . . . . . . . . 194

Table 4.6 Compressive strengths of hydraulic lime and natural hydrauliclime . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 196

Table A1.1 Summary of case studies included in this appendix . . . . . . . . . . . . . . 203

Table A2.1 Guidance on how to assign condition ratings based on evidencecollected in the course of bridge inspection, investigation andmonitoring . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 250

Table A4.1 Application table of GPR Methods, from BA86/04 . . . . . . . . . . . . . . . 267

Table A5.1 Principal health and safety legislation relevant to infrastructure bridges valid in Great Britain at the year 2005 . . . . . . . . . . . . . . . . . . 271

Table A5.2 International, national and locally designated sites with environmental and wildlife protection . . . . . . . . . . . . . . . . . . . . . . . . . 272

Table A6.1 Common repair and strengthening techniques for masonry archbridges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 273

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GGlloossssaarryy

Note on the use of the term – Masonry

Although sometimes used specifically to refer exclusively to building stone, here theword “masonry” will be used in the broader sense ie to refer generally to both brickand stone construction. When referring to brick or stone in particular, specific termswill be used, eg “stonework”, “stone arch bridge”, “brickwork” and “brick arch bridge”.

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Abutment a body, usually of masonry, which provides the resistance tothe vertical forces and the thrust of the arch.

Adobe regularly shaped body made of dried clay, usuallyincorporating straw to give it cohesion.

Antifunicular for a given set of loads, this is the geometry that results ingeometry an equilibrium state free from bending stresses ie simply

under axial section forces

Appraisal includes the range of activities involved with the evaluation ofa bridge’s condition and performance ie the gathering ofexisting data, inspection, investigation and structuralassessment.

Archivolt a projecting moulding which follows the curve of an archabove the extrados, for example the arch ring on the façade,or the shape of the arch curve.

Arch a curved structural member capable of supporting verticalloads across an opening and transferring these loads to piersor abutments.

Arch barrel the load-bearing part of the arch. It contains a single(or barrel) thickness of voussoir tones or several rings of brickwork or

coursed random rubble.

Ashlar type of masonry consisting of regularly shaped blocks of stonesquare-dressed to given dimensions and laid in courses withthin joints.

Aspect ratio the ratio of the span (longitudinal axis) of a bridge to its width(its transverse axis).

Assessment here used specifically to imply the evaluation of a bridge’sstructural capacity and performance, typically by one of anumber of prescribed methods and possibly making use ofproprietary software applications.

Autogenous healing the “self healing” of fine cracks in mortars by the precipitationof dissolved calcium ions as calcium carbonate; a slow andgradual process which may occur in wet conditions wherethere is adequate free lime (and thus particularly in lime-richmortars).

Backfill (or backing/ material (usually low quality fill) used to give supportfill/infill) behind a structure. For a masonry arch bridge, backfill

material is placed in the spandrels between the arch barreland the road surface and retained laterally by the spandrelwalls and/or wingwalls. It normally consists of granularmaterial eg gravel or building debris, which may have beenexcavated for the foundations or is waste from theconstruction.

Backing see Backfill.

Barrel see Arch barrel.

Bastion a section of solid masonry projecting from a wall to provideadditional structural stability.

Bedding mortar the mortar between masonry units which forms a part of thestructural masonry, as distinct from the pointing mortar,which is that used for the outer finish of the joints; in originalconstruction, these are normally identical.

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Bedding plane a plane of stratification in natural sedimentary stone.

Bed joint a joint between masonry courses.

Bond an arrangement of masonry units so that the vertical joints ofone course do not coincide with those immediately above andbelow.

Bond types refers to the relative arrangement of masonry units,particularly the arrangement of header and stretcher units,the main types being: (1) Header bond: units laid so that theirends only (short dimension) appear on the face of theelement, (2) Stretcher bond: units laid so that their long sideonly appears on the face of the element, (3) English bond:with alternate courses composed of headers or stretchers only,(4) Flemish bond: with alternate headers or stretchersappearing in each course. The most common type used in UKbridge arch barrels is stretcher bond, in which there is noconnection between rings.

Brick a masonry unit comprising a shaped and kiln-fired block ofclay or shale which can be used as an element for the fabric ofa bridge.

Bridge engineer a person responsible for the technical and engineeringprocesses of bridge management eg carrying out or makingdecisions regarding condition appraisal, bridge capacity andserviceability, performance restrictions and requirements formaintenance, repair and strengthening.

Bridge strike an incident in which a road, rail or waterborne vehicle, or itsload, impacts on any part of a bridge structure.

Calcining The heating of calcite or limestone (CaCO3) to its temperatureof dissociation so that its carbon dioxide is driven off, leaving“quicklime” (CaO) which can be reacted with water (“slaking”)to produce lime putty.

Centring temporary structure on which an arch is supported duringconstruction, normally made from timbers.

Clamp a large stack of moulded dry clay bricks with crushed fuel,which is then fired.

Common brick a type of brick whose characteristics suit it to general use egwhere there are no special requirements for appearance,strength and durability.

Condition appraisal see Appraisal.

Conservation work carried out to with the aim of maintaining or restoringthe important features of a bridge, in particular the visibleparts of its structure.

Coping a cap or covering to the top of a wall, which may comprisesingle or multiple units, the primary function being tochannel water away from the building.

Course a continuous layer of brick or stone masonry units.

Corbel horizontal outward masonry projection (in brickwork usuallyconstructed of headers) to provide an outstand from thenormal line of masonry.

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Corbelling structural form preceding the construction of true arches, inwhich the masonry units of successive horizontal coursesproject progressively further inward from the bottom courseup, to create a “stepped” structure capable of spanning anopen space.

Elliptical arch indicating a “flattened” semicircular arch, used to keep heightreasonable, to reduce approach gradients, and to increase thewidth of gauge clearance below.

Engineering brick a dense, strong and durable brick, often used for constructionor just for facing of engineering structures.

Extrados in an arch or vault is the top surface of the arch barrel ie theouter (convex) curve of an arch.

Facing brick a brick with suitable colour and durability for use in theexposed face of a masonry element.

Fatigue the reduction of the failure load by the repeated applicationof loads.

Fill see Backfill.

Gauging the addition of cement to lime and sand mixes to impart anelement of hydraulicity (ability to set by chemical reaction withwater) to the set of a mortar (see also Hydraulic lime).

Haunch the lower section of the arch barrel towards the springing

Header a masonry unit laid with its longer dimension normal to theface of a wall or arch barrel, used to interconnects adjacentrings of brickwork. See also bond types.

Hinge a more or less local situation in which, due to the formation oftensile openings, the structure can rotate as if it were anarticulation.

Historic bridge one that has some recognised historical value, through rarityor in terms of social, cultural or engineering heritage.

Hydraulic/ a non-hydraulic lime is a more or less pure calcium/non-semi-hydraulic hydroxide substance, used as cement, which can only achievehydraulic lime a set through reaction with atmospheric carbon-dioxide.

Hydraulic or semi-hydraulic limes also contain calciumsilicates or calcium aluminates, and their set is to a greater orlesser degree assisted by chemical reaction with water (see alsoNatural hydraulic lime).

Impost the upper element of an abutment or pier which supports anarch barrel or other superstructure.

Infill see Backfill.

Inspection refers to a visually-based examination of the bridge andassociated structures, which may be supported by othersimple methods of evaluation or measurement

Intrados in an arch or vault is the inner surface of the arch barrel iethe inner (concave) curve of the barrel.

Investigation refers to an enquiry into one or more specific aspects of abridge’s structure, its environment, performance orbehaviour, typically using techniques of measurement, testingor sampling of relevant parameters which go beyond thenormal scope of visual inspection.

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Keystone the highest and last-placed stones in an arch. In the arch barrelof a bridge there are a series of keystones at the crown, acrossits width, which are often left projecting on side elevations.

Leaching a deteriorative process where moisture movement through orover the surface of a material causes the removal of solublecomponents from it; the “leachates” may crystallise out ofsolution elsewhere or be redeposited at surfaces whereevaporation occurs causing distinctive staining anddiscolouration, and gradual build-up of mineral deposits.

Lime mortar a lime mortar is produced by combining slaked lime, sand andwater and relies at least in part upon gradual reaction withatmospheric carbon dioxide (“carbonation”) to harden anddevelop strength. Pure limes (also known as “fat” or “non-hydraulic” limes) produce a mortar that is typically weaker andmore porous and permeable than limes with a degree ofhydraulic (water-dependent) set or those which have beengauged with Portland cement.

Maintenance all the operations necessary to maintain it in a serviceablecondition until the end of its life, comprising routinemaintenance (routine work carried out with the aim ofpreventing or controlling deterioration, including inspectionand monitoring activities) and essential maintenance(rehabilitation works required to address specific inadequaciesin function and performance eg strengthening).

Masonry the work of a mason, strictly referring to work in stone, butcommonly used to refer generally to work in either brick orbuilding stone, as it is here.

Masonry cement a blend of Portland-type cement (typically comprising around75 per cent) with the remainder being fillers, admixtures andsometimes other binders, often used for general purposeapplications.

Mortar mix of one or more inorganic binders, aggregates, water andsometimes additions and/or admixtures for bedding ,jointingand pointing of masonry.

Multi-ring arch an Arch with more than one ring. Rings can be separated fullyby mortar joints, or can be structurally connected by masonryunits laid as headers between rings.

Natural hydraulic a lime produced by burning of more or less impure limestoneslime with reduction to powder by slaking (the addition of water) with

or without grinding. They have the property of setting andhardening under water, although the presence of atmosphericcarbon dioxide can contribute to the hardening process.

Open-spandrel arch one that has apertures between the bridge deck/roadway andthe arch ring, which can have the benefits of minimising deadweight loading and reducing hydraulic pressure on bridgescrossing rivers whose levels rise in spate.

Parabolic arch a very strong arch shape defined by the intersection of a coneand a plane parallel to the plane tangent of the cone. Foruniform loads a parabola is theoretically an ideal arch shapebecause the line of thrust coincides with the centre-line of thearch ring.

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Parapet usually a vertical continuation of the spandrel wall; an upwardextension of a spandrel wall above road surface level toprotect those on and below the bridge.

Pattress plate Load-spreading plate fitted at ends of tie-bars to restrainspandrels.

Performance operation and/or functionality of a bridge or bridge element,in relation to the requirements of owners/operators/users.

Pier has two definitions: (a) an intermediate support betweenadjoining bridge spans, or (b) a thickened section located atintervals along a masonry wall to strengthen it.

Pointing the filling and finishing of mortar on the outer part of a jointwhere the bedding mortar has been raked back from themasonry face or left recessed from it in construction.

Polycentric arch an arch shape with more than one centre ie one that is notdefined as part of a single circle or curve.

Pozzolan a cement additive comprising silica in reactive form, whichcan impart hydraulic set; can be either naturally occurring (egvolcanic ash) or artificially produced (eg brick dust orpulverised fuel ash, PFA).

Puddled clay a thoroughly mixed combination of pure clay with a proportionof water, forming a plastic material which can be used inconstruction to prevent the passage of water – particularly forlining canals, aqueducts and as a waterproof backing to arches.

Pulverised fuel ash a waste product of coal fired power stations consisting (PFA)of tiny spherules of reactive silica, sometimes used as acomponent in mortars and grouts.

Rehabilitation work that involves bringing features of a deteriorated bridgeback into a satisfactorily functional state.

Relieving arch one built over a lintel, flat arch or smaller arch to divert loads,thus relieving the lower member from excessive loading.

Ring a layer of transverse single masonry elements that form slenderunits which make up an arch barrel. In brickwork, multipleadjacent rings are commonly used to produce a multi-ring arch.

Ring separation loss of bonding between adjacent rings (not necessarily a gap)in a multi-ring arch.

Rise vertical height of arch from springing level to the crown ofthe intrados.

Risk a summation of the likelihood and consequences of anundesirable incidence.

Roadway or road the upper surface of the bridge on which vehicular trafficsurface runs, used here also to include the equivalent surface of

bridges carrying rail traffic or waterways.

Roman cement a quick-setting naturally hydraulic cement produced bycalcination of limestone containing clay materials (principallysilica and alumina) in a coal or coke-fired kiln. Used fromabout 1800 onwards, it was so named because its red/browncolour and hardness resembled mortars of the Roman period,although rather misleading since this type of cement was notin use in Roman times.

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Rubble masonry the term describes many different types of masonry, the maintypes being random rubble (irregularly shaped stoneelements, typically as it comes from the quarry) either coursedor uncoursed, and squared rubble (more regularly shapedstone), either coursed or uncoursed.

Saddle a concrete slab cast over an arch to strengthen it or distributeloads upon it.

Scour the removal of material from around structural supports byflowing water.

Segmental arch arch whose intrados comprises a segment of a circle which issmaller than a semicircle.

Semicircular arch arch with an intrados the shape of a semicircle (ie a 180° arc)so that the rise is half the span.

Shallow arch arch in which the rise is smaller than a quarter of the span.

Skew arch arch where the longitudinal and transverse axes are not atright angles.

Skewback The inclined surface of the course of masonry located at theextremity of an arch which transmits the stresses of the archto an abutment or pier; surface of an inclined springing.

Slaking see calcining.

Snap-through mechanism in which sufficient rotations take place at a hingeso as to produce instability and local failure, prior to theformation of a global hinge failure mechanism. This type oflocal failure can occur in highly confined arches andprecipitates the global collapse of the structure.

Soffit the underside of an element – in masonry arch bridges,equivalent to the intrados.

Soldier masonry unit laid with its longer dimension upright andparallel with the face of the wall ie bedded on its smaller face.

Spalling loss of material from the face of a masonry unit, eitherthrough “flaking” or delamination.

Span the distance between the supports of an individual arch alongits longitudinal axis.

Spandrel the area overlying the arch barrel under the road surface (orequivalent), occupied by the spandrel walls, fill material orvoids, and occasionally hidden elements such as internalspandrel walls.

Spandrel wall masonry wall that sits on the edge of the arch barrel and thatlimits the extent of, and retains, the backfill. Sometimes“internal” spandrel walls may be present at other locations onthe arch.

Spandrel separation usually refers to lateral separation, in which the spandrel wallmoves horizontally due to the action of applied loads, sometimesover the extrados of the arch and sometimes by forming a crackthrough the arch barrel close to its outer face. However, it couldalso be tangential separation, in which a crack tangential to archforms at the contact between the arch and the spandrel walls.

Spreader beam load spreading strip over the length of the span and fitted atends of tie-bars to restrain the spandrels.

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Springing plane from which an arch springs ie the junction between thevertical face of the abutment and the arch barrel.

Square spanning non-skewed arches.

Stretcher a masonry unit laid with its longer dimension parallel to theface of the wall or arch barrel. See also bond types.

Stock brick originally meaning a soft mud brick that is hand made using astock mould, later coming to mean a large number (stock) ofbricks all manufactured in the one locality eg London stockbrick.

Thrust line the locus of the positions of the centroid of the compressiveforce within the arch. The point on a given section where ifyou transfer the stresses, there is no bending moment butonly axial force.

Tie-bar a structural tensile element used to provide restraint, typicallycomprising steel rods installed transversely through a bridge,and attached to pattress plates, to provide restraint to thespandrel walls.

Unit an individual stone or brick that is laid, normally in mortar, toform masonry.

Vault either (a) the arched ceiling over a void, or (b) any spacecovered by arches.

Voussoir a wedged-shaped masonry unit used to make an arch or vault(voussoirs can be flat or irregular in rubble construction).

Voussoir arch arch with one ring only ie not a multi-ring arch.

Width the transverse dimension of a bridge, perpendicular to itsspan.

Wing wall a wall at the abutment of a bridge, which extends beyond thebridge to retain the earth behind the abutment.

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AAccrroonnyymmss aanndd aabbbbrreevviiaattiioonnss

ALARP as low as reasonably practicable

BMS bridge management system

BR British Rail (now Network Rail)

BRR British Rail Research

BW British Waterways

CBA cost-benefit assessment

CEEQUAL the Civil Engineering Environmental Quality Awards Scheme(<www.ceequal.com>)

CL calcium lime

DE discrete element (method of structural analysis)

Defra the Department for Environment, Food and Rural Affairs

FE finite element (method of structural analysis)

HA Highways Agency

HL hydraulic lime

KEL knife edge loading

LU London Underground

MEXE Military Engineering Experimental Establishment

NHL natural hydraulic lime

NR Network Rail

PAL provisional axle loading

PFA pulverised fuel ash

QRA quantitative risk assessment

SAC special area for conservation (EC designation relating toenvironmental conservation)

SNCO statutory nature conservation organisation

SPA special protection area (EC designation relating toconservation of wild birds)

SSSI site of special scientific interest

UDL uniformly distributed loading

WLC whole-life costing

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11 IInnttrroodduuccttiioonn aanndd bbaacckkggrroouunndd

11..11 BBaacckkggrroouunndd

Safe and efficient transport is fundamental to the freedom, wellbeing and prosperity ofsociety. By their nature, bridges are essential elements in the road, rail and waterwaytransport networks of the UK and are vital to their operation. Restrictions to theoperation of bridges or their closure can have effects beyond the immediate localdisruption, including undesirable health and safety, economic, environmental andpolitical consequences.

In the UK the present transportation network is the result of development that hastaken place over hundreds of years, and bridges form a valuable part of our historicallegacy. Although the oldest bridges still in existence date from medieval times, asignificant proportion of the UK’s current bridge stock was constructed between 1760and 1900 as first the canal, then the railway, and finally the road networks were subjectto rapid development and expansion. The great majority of these bridges wereconstructed in the form of arches, either from stone or brickwork, depending on theavailability of local materials, skills and experience. Few such bridges were constructedafter 1925 when iron, steel and, later, reinforced concrete became the engineeringmaterials of choice and the UK trunk road and motorway systems were developed. Ahigh proportion of the bridges on the waterway, rail and local road network comprisemasonry arches that have now been in service for at least 100 years and frequentlymuch longer. Masonry arch bridges are not only a vital part of our transportinfrastructure but also make an important contribution to our cultural and engineeringheritage.

Today the transport network in the UK, as in many other countries, is under constantpressure to expand and increase capacity, with attendant economic and environmentalcosts. In this climate it is vitally important that the existing infrastructure is usedefficiently and to its full capacity. This can only be achieved by careful management ofexisting bridge assets. Changes in the requirements of the transport system and thegradual deterioration of existing structures in service mean that there is a growingneed to maintain, repair, widen and strengthen masonry arch bridges over the comingdecades. The success of this will be dependent on accurately determining the needs ofbridges and understanding how best to undertake and allocate resources for theirmaintenance, repair and renewal.

Masonry arch bridges can be viewed as among the most sustainable structures ever tohave been built. Many have already been in service for hundreds of years withoutsignificant repair or strengthening works – exceeding the design life requirements ofmodern structures. By contrast, many of the steel and concrete bridges built in the lastcentury have required considerable expenditure on maintenance and repair or evenreplacement within the first 30–40 years of service. A recent review of funding requiredfor bridge and retaining wall maintenance carried out by the Bridges Group of theCounty Surveyors Society (CSS, 2000) which involved several methods of assessment,suggested that the annual maintenance cost of masonry arch bridges appeared to bemuch lower than for other bridge types, and half that of steel bridges with reinforcedconcrete supports. Other studies have produced similar results (Bouabaz, 1990; CSS,1999).

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The aim of this document is to gather existing knowledge from the UK and abroadand to provide examples of good (and poor) practice in the management of masonryarch bridges. It is hoped that the advice and information included here will benefitthose involved with the preservation of such bridges, suggest improvements in theirinspection and assessment, and assist in the budgeting, selection, planning andexecution of maintenance and repair works. While intended primarily for the UKmarket, the methods and advice included are generally applicable in other countries.

11..22 PPuurrppoossee aanndd ssccooppee ooff wwoorrkk

This guidance provides guidelines on good practice for the appraisal, maintenance,repair and strengthening of masonry arch bridges, as well as advice on issues such asinspection, investigation and monitoring, bridge management, conservation, health andsafety and environmental issues.

The purpose of the guidance is to:

� present good practice (as of 2005)

� provide a guide for routine management

� recommend assessment, maintenance and repair strategies to give best value formoney

� facilitate knowledge sharing.

11..33 AApppplliiccaattiioonn

This guidance is intended for:

� clients who are infrastructure owners

� those responsible for the management and care of bridge assets

� engineers who are responsible for assessing, maintaining, repairing andstrengthening bridges.

There are around 40 000 masonry arch bridges in the UK, representing an estimated40–50 per cent of the total bridge stock. The main UK arch bridge owners are railwayauthorities, highway authorities and navigable waterway authorities.

11..44 IIssssuueess ooff ssppeecciiaall iimmppoorrttaannccee ffoorr mmaassoonnrryy aarrcchh bbrriiddggeess

Topics of particular importance in the management of older masonry arch bridgesinclude:

� the need to investigate and evaluate the existing structure, its performance andmaterials, taking into account issues such as complex structural behaviour, lack ofdesign to modern codes, the presence of defects and the original variability and in-service deterioration of materials

� the importance of a thorough knowledge of this particular bridge form in order tomake good assessments of their condition, and understanding the significance ofobserved features

� the necessity of regular maintenance to ensure continued performance andserviceability while minimising unnecessary repair expenditure, closures and trafficrestrictions

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� the impacts of changes in usage and in traffic loading regimes

� consideration of the effectiveness of repairs and alterations and their likelyinfluence on the long-term performance and maintenance of the structure

� the importance of careful selection and planning of works so as to minimisedisruption to the normal operation of the bridge

� the particular access, safety and environmental issues, their associatedrequirements and cost implications, when carrying out works

� consideration of the historic or aesthetic value of the bridge and the need torespect and preserve the existing structure by carrying out repairs and alterationssympathetically

� lack of recent experience in, and modern guidance for, building new masonry archbridges.

This guidance aims to provide guidance in each of these areas. For a quick referenceguide in dealing with these issues, see the table below in Section 1.5.

11..55 HHooww ttoo uussee tthhiiss gguuiiddaannccee

This guidance is divided into five chapters each comprising of advice and guidance onparticular aspects of masonry and brick arch bridges, followed by appendices whichinclude supporting information. It is intended that the main sections of the book can beread to provide further information on each topic, and that readers requiringadditional detail for application are referred to the appendices or to other availablesources of information where appropriate.

Section A1 includes a number of case studies intended to illustrate the practicalapplication of some of the concepts discussed in this publication.

The construction, materials and structural behaviour of masonry arches is not a topicwidely taught in modern engineering courses,and it is not greatly understood by manymodern engineers. Chapter 2 of the guidance is intended to provide the reader with abasic level of understanding. The guidance is written so that more experienced readerswith a greater depth of knowledge of masonry arch bridges can omit Chapter 2 andconcentrate on those chapters of the document most relevant or useful to them.Frequent cross-references are included where the reader may require furtherexplanation or clarification of points not fully covered in that chapter, but a certainamount of repetition has been included to enable readers to “dip into” sections of thedocument without excessive cross-referencing.

CIRIA C656 27

11.. IInnttrroodduuccttiioonnGeneral background information; principal bridge asset owners; how to use this guide.

22.. CCoonnssttrruuccttiioonn aanndd bbeehhaavviioouurrBasic principles of arch bridges; their history; construction and materials; behaviour andperformance.

33.. MMaannaaggeemmeenntt,, ccoonnddiittiioonn aapppprraaiissaall aanndd aasssseessssmmeennttAsset management; bridge condition appraisal; structural assessment.

44.. SSeelleeccttiinngg aanndd ccaarrrryyiinngg oouutt bbrriiddggee wwoorrkkssHealth and safety, environmental and heritage considerations; maintenance, repair andstrengthening techniques; selection and execution of works.

55.. SSuummmmaarryy ooff rreeccoommmmeennddaattiioonnss aanndd ffuuttuurree nneeeeddssOverall summary of recommendations; discussion of future research and development needs; listof references.

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Table 1.1 provides a quick guide to finding relevant information on a number of theprincipal topics included in the publication.

TTaabbllee 11..11 WWhheerree ttoo ffiinndd gguuiiddaannccee iinn tthhiiss bbooookk

CIRIA C65628

GGeenneerraall ttooppiiccWWhheerree ttoo ffiinndd gguuiiddaannccee iinn tthhiiss bbooookk

IIssssuuee SSeeccttiioonn

UUnnddeerrssttaannddiinngg tthhee hhiissttoorryy ooffbbrriiddggeess,, hhooww tthheeyy wweerree bbuuiilltt aannddtthhee mmaatteerriiaallss uusseedd

History and construction of bridges 2.1

Bridge structural elements 2.2

Bridge materials 2.3

SSttrruuccttuurraall bbeehhaavviioouurr aanndd ccaauusseessssiiggnnss ooff lloossss ooff ppeerrffoorrmmaannccee aannddddeetteerriioorraattiioonn

Structural behaviour 2.4

Structural problems 2.5

Materials deterioration 2.5.3

EEnnssuurriinngg sseerrvviicceeaabbiilliittyy tthhrroouugghh aammaaiinntteennaannccee aanndd rreeppaaiirrpprrooggrraammmmee

Maintenance management (general) 3.4

Maintenance requirements 3.4.1

Bridge management systems 3.4.2

IInnssppeeccttiioonn,, iinnvveessttiiggaattiioonn aannddmmoonniittoorriinngg

Sources of information 3.6.1

Bridge inspection (general) 3.7

Inspection frequencies 3.7.1

Investigation techniques 3.8.2

Materials sampling and testing 3.8.3

Monitoring techniques 3.8.4

Interpretation of results 3.9

SSttrruuccttuurraall aasssseessssmmeenntt

Methods of analysis 3.10.1

Influence of construction features and defects 3.10.4

Assessment results. 3.10.5

TThhee sseelleeccttiioonn,, ddeessiiggnn aannddeexxeeccuuttiioonn ooff mmaaiinntteennaannccee aannddrreeppaaiirr mmeetthhooddss

Considerations for selection 4.3.1

Health and safety issues 4.1.1

Environmental issues 4.2

Routine and preventative maintenance 4.3.2

Deteriorating masonry 4.3.3

Repair and strengthening measures 4.3.4

DDeeaalliinngg wwiitthh hhiissttoorriicc bbrriiddggeess oorrtthhoossee wwiitthh ssppeecciiaall hheerriittaaggee vvaalluuee

Management issues and legislation 3.5.1

Works on historic bridges 4.2.1

DDeeaalliinngg wwiitthh bbrriiddggeess wwhhiicchh hhaavveeeeccoollooggiiccaall aanndd ccoonnsseerrvvaattiioonn iissssuueess

General management issues 3.5

Wildlife conservation and ecology 3.5.2

Dealing with protected species 4.2.2

Prevention of pollution. 4.2.3

DDeessiiggnn aanndd ccoonnssttrruuccttiioonn ooff nneewwmmaassoonnrryy aarrcchh bbrriiddggeess

Existing design codes 4.4.1

Materials selection and specification 4.4.2

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22 BBrriiddggee ccoonnssttrruuccttiioonn aanndd ppeerrffoorrmmaannccee

22..11 HHiissttoorryy ooff mmaassoonnrryy aarrcchh bbrriiddggeess

22..11..11 BBrriiddggee ddeessiiggnn aanndd ccoonnssttrruuccttiioonn

Table 2.1 summarises the history of bridge development and construction in the UK,with more detail (including international examples) given in the following text.

TTaabbllee 22..11 TTiimmee--lliinnee ffoorr ccoonnssttrruuccttiioonn ooff mmaassoonnrryy aarrcchh bbrriiddggeess iinn tthhee UUKK

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BBrroonnzzee--aaggee ttoo 11880000“Clapper bridges” – simply supported structures with single or multiple spans.Early examples are very primitive, with stone slabs laid across vertical supportstones. Later examples have slabs suspended between mass brickwork piers.

4433––440000Roman bridges and aqueducts constructed in the UK and throughout the Romanempire. UK bridges had a wooden superstructure and have not survived, butoccasionally foundations remain and some are still in use.

440000––11110000Long period during which time few masonry arch bridges were built in thewestern world and vital skills were lost. Many existing bridges were left todeteriorate through lack of maintenance or destroyed in warfare.

11118800––11445500

Main period for construction of medieval monastic bridges, mainly built fromstone, often with distinctive ribbed arches, a number of which are still in use inthe UK. Occasional use of pointed gothic arches with sturdy piers and load-bearing spandrel walls.

11667755Robert Hooke realises the fundamental action of an arch and the concept ofhow load is transferred through the line of thrust.

11666600––11885500Development of “turnpike roads” system in UK leads to construction of manymasonry arch bridges in brick and stone.

11773344

Construction of Westminster Bridge over the Thames in London, whichintroduces several innovations including higher span/pier ratios,compartmentalised spandrels and inner relieving arches, which become popularthereafter.

11775566

After some “trial and error” to get the correct span-to-rise ratio, requiring severalphases of reconstruction, Pontypridd Bridge over the River Taff in Wales useslonger span (43 m) and a more slender arch design than any previous bridge inthe UK. It includes open cylindrical voids over the haunches of the arch barrel, tolimit weight.

11776600––11883300

Expansion of the canal system in the UK, involving the construction of many newsingle-span arch bridges, typically of brickwork with spans of 5–7 m. Earlierbridges were segmental arches. Elliptical bridges came later, Circa 1900, to giveincreased headroom over towpaths.

11884466Following a series of high-profile bridge failures, W H Barlow submits a paper tothe Institution of Civil Engineers outlining the mechanisms of arch failure, whichform the basis of modern theories of structural collapse.

11882255––11990000Expansion of the rail system in the UK, involving the construction of many newmulti-span bridges and viaducts, typically of brickwork, some tall and with verylong spans.

11990000++

Reinforced concrete and steel supersede brickwork as the engineering materialsof choice for new bridges and prompt the use of new structural forms. Archesbecomes rare. Short-span masonry “jack arches” – supported on iron beams –survive a little longer into the early 20th century.

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EEaarrllyy bbrriiddggeess

It is not clear where and when the construction of arch bridges originated, but it iswidely accepted that the use of the arch as a structural form developed independentlyin China and the Middle East more than 5000 years ago (Howe, 1897 and Van Beek,1987). In Europe, however, the establishment of masonry arches as part of the commoncultural heritage is a legacy of the Roman Empire. The development of transportinfrastructure for the movement of armies, trade and communication was vital to thespread and successful administration of the empire, and bridge building was a key partof that infrastructure. Roman engineers applied new concepts of design, standardisedforms and introduced new materials, construction techniques and technology to bridge-building (see Figure 2.1). Their bridges were made of cut stone voussoirs, usuallywithout mortar and with arches semicircular in shape. Multi-span bridges had thickpiers, with span to pier thickness ratio between two and three, which made each of thespans an independent structure. This clearly simplified the construction process andhad advantages in unsettled times. It is often said that longevity of Roman arch bridgeswas based on their expertise in constructing foundations and in the UK several Romanbridge foundations are still in use today, despite the demise of the typically woodensuperstructures.

FFiigguurree 22..11 AA RRoommaann bbrriiddggee -- PPoonnttee SSaaiinntt--MMaarrttiinn ((cc 2255 BBCC)) nneeaarr TToorriinnoo iinn IIttaallyy.. SSppaann ttoo ppiieerr tthhiicckknneessssrraattiioo ooff RRoommaann bbrriiddggeess wwaass ttyyppiiccaallllyy ttwwoo oorr tthhrreeee ((ccoouurrtteessyy SShhuunnssuukkee BBaabbaa,, OOkkaayyaammaaUUnniivveerrssiittyy))

TThhee MMiiddddllee AAggeess

With the fall of the Roman Empire, the art of bridge building in Europe plunged intodarkness, where it remained for centuries. During the early Middle Ages many bridgeswere destroyed or left to deteriorate, but the secrets of masonry building were keptalive by the church, and from the 12th century it started promoting the construction ofnew bridges to complement the deteriorated Roman network. New arch profiles wereintroduced, although often purely for aesthetic reasons, for example pointed arches,broken at the crown, or polycentric, to smooth the pointed shape. Segmental arches,which started to flatten the arch profiles, improved functionality and structuralperformance but increased lateral thrust on the abutments. Typical proportions of archbridges from this period were span to rise ratios of three and span to pier thicknessratios of four, and construction was predominantly stone masonry. Bastions at the endsof bridges were also common in medieval bridges; although adopted for practicalreasons they improved the capacity of the abutments to resist the lateral thrusts. Duringthis period, bridge builders had little structural understanding and bridges were

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conceived from secret rules and proportions kept by closed groups of masons whotravelled throughout Europe building bridges and cathedrals.

TThhee RReennaaiissssaannccee

The next period of development was in the Renaissance, which saw significantadvances in bridge design, particularly on the continent. The main innovations werethe progressive increases in the span to rise ratio, allowing long spans with adequaterises, the introduction of parabolic arches and the increase of the span to pier thicknessratio, exceptionally up to five (see Figure 2.2). At the end of the Renaissance, theelliptical arch was introduced. Although a result of the search for aesthetic perfection,the elliptical arch also provided an efficient structural form.

FFiigguurree 22..22 BBrriiddggee ooff KKhhaajjuu ((11666677)) aatt IIssffaahhaann ((IIrraann)) ffuunnccttiioonneedd aass aa bbrriiddggee,, aa ddaamm aanndd aa rreessoorrtt ffoorrddeesseerrtt ttrraavveelllleerrss ((ccoouurrtteessyy SShhuunnssuukkee BBaabbaa,, OOkkaayyaammaa UUnniivveerrssiittyy))

1188tthh cceennttuurryy ttoo pprreesseenntt ddaayy

The 18th century saw a number of important developments in British bridge designwhich were to become accepted common usage thereafter. The construction ofWestminster Bridge in 1734, for example, introduced to Britain several continentalinnovations and also some of its own, which were adopted in subsequent bridges (afterColla et al, 2002):

� high span/pier thickness ratios (about 4.5)

� attention to achieving better structural connection between bridge elements (archspringings/piers, arch/backing and spandrel wall/infill)

� the use of tapering secondary arches to back the main arches (thick at thespringings, thin at the crown) to allow better transfer of thrust while minimisingdead load on the arch

� arches stiffened with longitudinal walls

� compartmentalised spandrels with dry-stone dividers filled with gravel to restrainfill and reduce pressure on spandrels

� the use of thinner relieving arches spanning internal voids above piers to reduceload on them (originally implemented in 1748 as a remedial measure in responseto settlement of a pier, subsequently becoming a common design feature on largerbridges).

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Following the construction of Westminster Bridge, William Edwards’ 1756 PontypriddBridge over the River Taff in Wales (see Figure 2.3) introduced the concept of a long,thin arch supporting a light charcoal fill. The 43 m span arch, the longest in Britain,had to be rebuilt several times before the correct rise to span ratio was achieved toensure that it did not collapse after removal of the centring. Pontypridd Bridge alsointroduced the innovation of including open cylindrical voids in the rubble infill of thespandrel above the haunches of the arch barrel to limit dead weight, which becamewidely used. The bridge has a very steep angle of entry and exit that was acceptable forlivestock but caused problems for heavily loaded horse-drawn wagons. A heavy drag-chain was used to slow them on the downhill side which caused continual wear to theroad surface.

FFiigguurree 22..33 PPoonnttyypprriidddd BBrriiddggee ((11775566)) iinn SSoouutthh WWaalleess ((oolldd bbrriiddggee iinn ffoorreeggrroouunndd)) hhaadd ttoo bbee rreebbuuiilltt sseevveerraallttiimmeess bbeeffoorree tthhee ccoorrrreecctt rriissee ttoo ssppaann rraattiioo wwaass aacchhiieevveedd ttoo eennssuurree tthhaatt tthhee 4433 mm ssppaann aarrcchhddiidd nnoott ccoollllaappssee aafftteerr rreemmoovvaall ooff tthhee cceennttrriinngg ((ccoouurrtteessyy SShhuunnssuukkee BBaabbaa,, OOkkaayyaammaa UUnniivveerrssiittyy))

During the 18th century the great French bridge builders, in particular Perronet,began to “design” bridges in the modern sense, based on an early understanding oftheir structural behaviour. The main innovations were the use of polycentric arches, toreach span to rise ratios of 10, and the introduction of arches with thickness increasingtowards the abutments or piers. The latter led to increased stability and the dramaticreduction of pier thickness in multi-span bridges by breaking the independence ofadjacent spans (ie achieving “global equilibrium”) and leaving the intermediate piers tosimply transfer the vertical loads.

In the UK, the next significant period of bridge construction took place during thesecond half of the 18th century. The need for efficient transport to serve expandingindustry led to the development of the canal networks and later the railway networks.The main development of this era was the concept of mass production of arch bridges,which resulted in the standardisation of construction and materials. The vast majorityof bridges built in this period were modest examples with spans below 10 m, built inseries along canals or railway lines. However, some more impressive arch bridges andviaducts were built and advances made in the structural theory of arches, paving theway for modern concepts of design and failure analysis.

In 1779 Abraham Darby III began the construction of the 100-foot span cast iron archover the Severn at Coalbrookdale which marked the beginning of the end for masonryarch bridge building, and by the mid-1800s masonry arch bridge construction was indecline, although hundreds more were built in the UK before the end of the century. Inthe 20th century masonry gave way to iron and steel construction and, after the SecondWorld War, to concrete, and particularly pre-stressed and post-tensioned concrete.

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Today, masonry arch bridges are rarely built but those that remain from the past form avital part of our transport infrastructure, and increasingly it is recognised that theirmaintenance and repair are of enormous importance to our modern way of life.

22..11..22 BBrriiddggee mmaatteerriiaallss

Bridges built from stone were potentially very solid and durable structures, but greatlyrelied on the quality of locally available materials and the skill of the masons. Thequarrying, selection, weathering and shaping of stone was very labour intensive anddependent on the employment of skilled and knowledgeable craftsmen. Until the endof the Middle Ages, brickwork was seen as inferior to stone for construction, butincreasing scarcity of easily available building stone led to a revival and development ofbrick making in Britain in the 13th and early 14th centuries.

By Tudor times (16th century) brickwork had come to rival stone in popularity as astructural material and brick makers and layers were seen as skilled craftsmen. At thistime, bricks were irregular in size and shape and so were used with irregular andrelatively thick joints typically comprising non-hydraulic lime mortars. By Georgiantimes (late 18th and early 19th century) the technology of brick making had advancedand resulted in an improved and more consistent quality and shape of brick, forexample the familiar yellow London Stocks, allowing a high standard of construction tobe achieved. Early 19th century masonry tended to be thick, solid and jointed with limemortar or Roman cement. The compressive strength of the materials in this type ofmasonry construction easily accommodated the high dead loads and imposed loads,and bending stresses were avoided. However, gradual advances in materials and designtheory were reflected in the increasing sophistication of bridge forms.

As brick construction increased in popularity, stone masonry construction wanedfurther and by the Victorian period (1830 to 1901) brickwork was firmly established.The development of the Hoffman kiln, which allowed the production of a consistentquality of bricks on an industrial scale, at low cost and without the need for aspecialised workforce, was one of the factors that led to brickwork replacing stone asthe construction material of choice, resulting in multi-ring arch barrels. The benefits ofusing elements with standardised dimensions removed the need for selection andshaping of individual blocks by skilled stonemasons and made handling and placementof materials easier.

Earlier brick bridges tended to be of variable quality but the accelerating demand forconstruction materials was a driving force behind the mechanisation of materialsmanufacture and mass production techniques which led to greater standardisation ofmaterials and consistency of quality. Brick making technology improved further andwas becoming mechanised, allowing the production of strong and durable engineeringbricks for use in important structures. The dimensional consistency of machine-pressedbricks allowed more accurate construction and the use of very thin (<10 mm) andregular mortar joints between courses. The washing and grading of aggregates andincreasing use of “natural cements” (particularly Roman cement) produced stronger,more durable mortars with a more rapid hydraulic set. Portland cement, patented in1824 by Joseph Aspdin, became popular toward the end of the 19th century and wasused to produce mortars that were stronger and quicker-setting than before, and bettersuited to the requirements of engineers and contractors. Their ability to set underwaterand provide improved durability in wet conditions made them ideal for theconstruction of aqueducts and the piers and abutments of bridges over rivers.

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By the early 20th century masonry bridge construction was a rarity, and the massivestyle of construction gave way to newer and more economical ways of using masonry inwhich the materials were required to resist greater flexural and shear stresses. Thistype of construction placed greater demands on strength and consistency of materials,and was a significant factor in the widespread adoption of machine-pressed bricks andmasonry mortars based on Portland-type cements. Lime mortars, which were typicallyproduced locally on a smaller scale, tended to be more variable in character and qualityand were not well suited to these new methods of construction. Increasingly, asstandardised approaches to structural design were developed, the use of lime mortarswas not supported by the new design codes, contributing further to its neglect. It isonly in recent years that conservators and engineers have begun to reappraise thequalities of lime mortars, in particular their usefulness in repairing old structures.

22..11..33 CCoonnssttrruuccttiioonn mmeetthhooddoollooggyy

The basic principles and processes of constructing a modest masonry arch bridge fromRoman times through to the beginning of the 18th century remained fairly constant:

� initially, some basic surveying and setting-out would be required. For aqueducts,levels were particularly critical. Since Roman times it was understood that theperformance of this type of bridge is highly dependent on the adequacy of itsfoundations, and examination of ground conditions would be undertaken.Dependent on the local conditions, excavations were made to expose bedrock or toremove any weak and unstable materials at the surface to allow foundationconstruction

� rafts or piled foundations were used where the near-surface substrate was notsuitably solid, with piles (frequently timber) driven either from a barge or from atemporary scaffold. Foundations for piers could simply be constructed on piles ofstone dumped into the shallowest part of a river (often giving rise to bridges withunequal spans), although for most bridges cofferdams, formed by rows of adjacentpiles, were used to divert watercourses. In later times, pressurised caissons wereused to allow the construction of foundations in deep water

� some type of grillage would be laid down on the piles to serve as a platform, andconstruction of abutments and piers would begin. Bridges from medieval times arecharacterised by massive construction, normally with solid rubble masonry or witha masonry outer-wall and an internal fill of random rubble, which was occasionallycemented. Cut-waters and other anti-scour and washout devices might beconstructed to protect these pier-bases

� construction of the bridge superstructure and parts of the substructure (eg tallabutments and piers) would make use of scaffolding. For stone bridges, largeindividual elements were typically lifted into place using a system such as a block-and-tackle mounted on a wooden derrick

� the stone would have been quarried, aged, cut and dressed as necessary by masonsbefore being stockpiled to “mature” before use. Bridges varied considerably intheir masonry quality, dependent on the supply of local materials, the skill of themasons, the prestige of the bridge and the funds available

� with no transport system in place, bricks were often manufactured on site, usingmaterial dug from cuttings in the vicinity. This led to much variability in brickquality, even for bridges close together

� once the piers were completed, temporary formwork comprising a timber truss inthe shape of an arch, known as “centring”, was erected to define the intrados ofthe arch barrel. Voussoir stones or brick rings were positioned on this centring,which had to support the full weight of the arch until the keystones were placed or

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the brick rings completed (see Figures 2.4 and 2.5). Wooden centring was oftenre-used for a number of bridges on a route where they formed part of a singlecontract (this fact can provide a useful check on survey results for bridges).

FFiigguurree 22..44 CCoonnssttrruuccttiioonn ooff GGaayyttoonn ccaannaall bbrriiddggee oonn wwooooddeenn cceennttrriinngg ((ccoouurrtteessyy BBrriittiisshh WWaatteerrwwaayyss))

FFiigguurree 22..55 CCoonnssttrruuccttiioonn ooff aa mmooddeerrnn ccaannaall bbrriiddggee aarrcchh bbaarrrreell uussiinngg cceennttrriinngg ((ccoouurrtteessyy BBrriittiisshhWWaatteerrwwaayyss))

CIRIA C656 35

FFiigguurree 22..66

OOlldd bbrriiddggee iinn rruurraall ssoouutthheerrnn FFrraannccee wwiitthh aannaarrcchh ccoonnssttrruucctteedd ffrroomm iirrrreegguullaarr lliimmeessttoonneebblloocckkss,, ssoommee hheelldd iinn ppllaaccee bbyy ffrriiccttiioonn aannddssoommee rroouugghhllyy ppaacckkeedd wwiitthh lliimmee mmoorrttaarr

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� at its most basic, a stone arch could be built using flat or irregularly shaped stonesarranged in the shape of an arch, typically with mortar packed into the gaps toprovide support and improve cohesion (see Figure 2.6). However, most used cutand shaped stone at least for the arch ring, since this resulted in a more perfect arch.

� the spandrel walls of the bridge were less critical; their function was simply toretain the fill for the roadway. Although in stone bridges where appearance wasimportant the spandrel wall masonry was typically ashlar (regularly shaped dressedstone blocks), less prestigious bridges often had spandrel walls constructed fromrandom rubble. In order to achieve good friction and allow transfer of shearbetween the arch and the backing, the arch extrados surface was often constructedto be “rough” eg by using alternating thicknesses of stone in the arch ring

� fill material used in the spandrel often comprised whatever materials were at handlocally; as a result it could be anything from ash to concrete

� once the arch barrel was complete it was preferably left for some weeks or monthsbefore the centring was removed, but this was not always the case. Sometimes thesupports had to be removed soon after completion of the arch, which could causesagging. In order to limit this movement, timber wedges were sometimes driveninto the extrados of the central voussoirs. Long-term movements of the archresulted from creep over a period of months or years, the rate and final magnitudeof the displacement being influenced by the properties of the materials(particularly the mortar) used in the arch barrel. Older bridges built with limemortars tended to suffer from greater displacements

� many of the visible defects in old masonry arch bridges are longstanding, andvisible cracking and distortion often dates back to the removal of the centring, afterwhich the bridge structure found a new equilibrium (see Figure 2.7). In order tominimise cracking of the spandrel walls and parapets they were sometimesconstructed after removal of the arch supports and preferably after some of thecreep deformation of the arch barrel had occurred (Ruddock, 2000).

FFiigguurree 22..77 RReemmoovvaall ooff tthhee pprroovviissiioonnaall ssuuppppoorrttss ((oonn lleefftt)) ooff aa ssttoonnee aarrcchh ooff tthhee NNooggeenntt--ssuurr--SSeeiinnee BBrriiddggee((PPeerrrroonneett,, 11778822--8833)) ((ccoouurrtteessyy BBrreenncciicchh aanndd CCoollllaa,, 22000022)).. NNoottee:: TThhee ooppeenniinngg ooff tthhee jjooiinnttssbbeettwweeeenn tthhee aarrcchh eexxttrraaddooss aanndd bbrriicckkwwoorrkk ssppaannddrreellss aafftteerr ddee--cceennttrriinngg ((rriigghhtt))

Hidden bridge construction forms are illustrated, and their influence on bridgestructural behaviour discussed further, in Section 3.10.3 Considerations when carrying outanalyses.

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An excellent review of masonry arch bridge construction in the UK between 1720 and1840, illustrating developments in design and the hidden structural features of bridges,is included in Colla et al (2002).

A collection of papers on the historic aspects of masonry arch bridge constructiontechniques and features, including case studies of individual bridges from the UK andabroad, is included in Ruddock (2000).

22..22 SSttrruuccttuurraall eelleemmeennttss

As a result of the long history of arch bridges, many variations exist around the simplebut effective structural concept presented schematically in Figure 2.8. The arch spans aspace between two abutments, with backfill which provides a transition between thearch and the bridge surface and two external walls containing the backfill andextended by the wing walls on the abutments.

FFiigguurree 22..88 MMaaiinn eelleemmeennttss ooff aa mmaassoonnrryy aarrcchh rraaiill oovveerrbbrriiddggee ((ttoopp)) aanndd uunnddeerrbbrriiddggee ((bbeellooww))

The following sections include a description of the main structural elements of an archbridge; further consideration of the structural significance of construction features onbridge performance and behaviour is included in Section 3.10.3 Considerations whencarrying out analyses.

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22..22..11 AArrcchh ggeeoommeettrryy

The Roman idea of a “perfect” arch was that of a semicircle, a concept in which nodistinction was made between aesthetic quality and structural performance. Since then,however, the geometry and proportions of the arches in masonry bridges have been thesubject of intense debate and investigation among mathematicians, architects andengineers. The result was the development of a wide variety of arch geometries whichincluded semicircles, pointed arches, segmental arches, parabolas, ellipses andcombinations of a number of circular segments to achieve forms that intended tooptimise the structural performance of the arch and its function.

22..22..22 AArrcchh ccoonnssttrruuccttiioonn

In addition to the thickness of the arch barrel, another very significant characteristic isthe number of rings within it and most importantly, the connection between them. Inbrickwork constructions, the arches will almost invariably contain multiple rings. Theserings will either be simply connected by uninterrupted mortar joints or will be crossedby headers, which provide an interlocking effect that greatly increases the shearstrength of the connection between rings. In stone masonry, arches normally consist ofa single ring, either in the form of large blocks of regular depth, each the thickness ofthe arch barrel, or an irregular distribution of blocks which gives a varying archthickness, so that the extrados of the arch is not a simple curved plane.

Bridges with no skew or very small degrees of skew (square bridges) can be constructedusing masonry bedding planes parallel to each other and to the abutments, but thispresents significant constructional difficulties in bridges with larger skews. The twoalternatives used for skew bridge construction are the English (or helicoidal) pattern, inwhich the brick courses are parallel to each other, at right angles to the longitudinalaxis at the crown, and at a constant inclined angle to the springings, and the French (ororthogonal) pattern, in which the courses are not parallel to each other and at varyingangles to the springings (see Figures 2.9 and 2.10).

FFiigguurree 22..99 SSkkeeww bbrriiddggee ccoonnssttrruuccttiioonn ppaatttteerrnnss ((MMeellbboouurrnnee aanndd HHooddggeessoonn,, 11999955))

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FFiigguurree 22..1100 SSkkeeww bbrriiddggee wwiitthh aarrcchh bbaarrrreell ccoonnssttrruucctteedd ffrroomm aa ssiinnggllee rriinngg ooff ssaannddssttoonnee bblloocckkss iinn tthhee““EEnngglliisshh ppaatttteerrnn”” ((ccoouurrtteessyy BBrriittiisshh WWaatteerrwwaayyss))

22..22..33 SSppaannddrreellss

In the simplest and possibly the most common case (as in Figure 2.7) the spandrelspace is occupied by soil, retained transversely by the spandrel walls which sit on thetransverse edges of the arch. The material used to fill this space varies very significantlyand will depend on what was available locally. In general, the backfill will be thematerial that was excavated to build the foundations, although it could have been wellcompacted and treated in some way to improve its strength. In most cases, after manyyears of compaction carrying loads in service, the backfill will have reached a significantstrength and possibly some degree of cementation.

In addition to this, in many bridges, the space near the haunches and, most typically,the space above intermediate piers in multi-span bridges was backfilled with acemented material. Providing the cemented material and the arch act compositely thispractice is, in effect, equivalent to significantly increasing the thickness of the arch and,therefore, has a positive effect on the stability of the structure.

In some cases, the stability of masonry arch bridges was increased by optimising thedistribution of the backfill weights either by backfilling different areas with materials ofdifferent densities (from compact soils to very porous volcanic material or charcoal) orby introducing cylindrical openings. In some cases, these voids were capped by thespandrel walls and cannot be detected by a visual inspection. Where left open, theseopenings would also improve the hydraulic performance of the bridge in the event ofsevere floods.

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FFiigguurree 22..1111 TTyyppiiccaall bbaacckkiinngg ooff sseemmii--eelllliippttiiccaall aarrcchheess ((BBaaggggii,, 11992266))

To improve the durability of the masonry, waterproofing layers of tar or puddled claywere sometimes included between the backfill and the bridge surface. Thewaterproofing prevents water percolating through the backfill and the masonry, whichdamages the mortar joints and washes out fine particles in the backfill. In many caseshowever, these layers have become ineffective either by general wear (for instancethrough washout or desiccation), lack of maintenance or damaged by subsequent workson the bridge.

The primary functions of the spandrels/parapet walls are to act as retaining walls forthe backfill material, to provide safety for the bridge users and containment for strayvehicles. In many instances, the thickness of the parapets is different from the thicknessof the spandrel walls, which can increase towards their base and increase stability.Additionally, in some cases, the spandrel walls have an arched geometry in plan, whichboosts their capacity to resist the outwards pressure of the backfill by adding alongitudinal arching effect to the gravity wall action. The original masonry parapets ofroad bridges may not have sufficient capacity to meet modern requirements for vehiclecontainment, and in some cases have been replaced or reinforced.

In some large span structures, instead of a spandrel area contained by two externalspandrel walls, the arch is topped by a number of longitudinal internal spandrel wallsthat divide the spandrel area into compartments, which can either be backfilled orspanned by transverse arches or large stones. These arrangements provide lighter andstiffer structures. Another alternative used in large spans is the use of open spandrels,in which the transition between the main arch and the bridge surface is achieved by aseries of pillars and small longitudinal arches. In some cases, this type of constructioncan be hidden behind conventional external spandrel walls. Such hidden features areillustrated and discussed further in Section 3.10.3 Considerations when carrying outanalyses.

22..22..44 PPiieerrss aanndd aabbuuttmmeennttss

Multi-span masonry arch bridges can be constructed as a sequence of independentsingle spans or as a unit, in which the thrusts imposed by adjacent spans at the heads ofthe piers balance each other. The proportions of the piers are greatly affected by thephilosophy of design adopted. In cross-section, the pier can be solid, hollow or hollowbut filled with clay or rubble. In hollow, backfilled piers, it is important that the weightof the backfill is transferred to the pier masonry perimeter (not directly to itsfoundation) to achieve an adequate stability of the pier and the entire bridge. For thisreason, some “hollow” piers are compartmentalised, with layers of continuous masonryat certain heights.

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The foundations of masonry arch bridges often comprise relatively shallow spreadfootings. In cases where suitably solid ground was too deep below the land surface,timber piles were often used and connected to a surface grillage as a platform forconstruction. Over the years, these may have rotted resulting in loss of support andmovements of abutments and piers. It is not uncommon for the foundations ofmasonry arch bridges to be one of their weak points.

22..33 CCoonnssttrruuccttiioonn mmaatteerriiaallss

The main materials used in masonry construction include a variety of bricks and stoneunits, typically separated by beds and vertical joints comprising some type of mortar.

As discussed in Section 2.1.2, until the dawn of the industrial revolution (18th century)and the beginning of the “mass production” of masonry arch bridges, most bridgeswere built from stone. This ranged from random rubble, which consists of uncut,irregular stone, with or without large amounts of mortar filling the gaps between theirregularly shaped blocks (see Figures 2.12a and 2.12b respectively), to ashlar (seeFigure 2.12c), which consists of square dressed stone, laid in courses with thin mortarjoints (Page, 1993). More recent bridges are in most cases built in brick masonry with avariety of construction patterns (see Figure 2.12d).

FFiigguurree 22..1122 MMaassoonnrryy ccoonnssttrruuccttiioonn ppaatttteerrnnss;; ((aa)) ccoouurrsseedd rruubbbbllee ((bb)) rraannddoomm rruubbbbllee iinn mmoorrttaarr mmaattrriixx,,((cc)) ccuutt--ssttoonnee aasshhllaarr,, ((dd)) ccoommmmoonn bbrriicckkwwoorrkk bboonnddss

CIRIA C656 41

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The response of masonry to loads is influenced by the way in which these materialshave been used in construction, their original physical characteristics and anysubsequent changes, including deterioration. Masonry structural elements which areuniformly constructed throughout their section will behave differently to those whichhave a rubble core, and differences can also be expected between elements withdifferent proportions of mortar, or mortars of different strength and compressibility.

22..33..11 MMoorrttaarrss

Depending on the type of masonry the volume of mortar per unit volume variessignificantly, from 0 per cent in the case of some ancient arches which were constructedfrom perfectly fitted dressed stone to over 20 per cent in the case of some randomrubble bridges. Historically, lime mortars were used and these tend to be plastic andforgiving of small movements. The proportion of mortar, its type and characteristics arean important influence on the performance and behaviour of a masonry bridge’sstructural elements, particularly the arch barrel.

MMoorrttaarr ffuunnccttiioonn aanndd bbeehhaavviioouurr

The functions of mortar in masonry are:

� to provide an even contact surface between the masonry units (brick or stone) topromote even load transfer between them, avoiding excessive local stresses whichmight otherwise develop at points of contact

� to physically bind the masonry units together to form a cohesive structural fabricand allow it to function as a composite material, ie by influencing its importantphysical characteristics such as compressive strength and modulus of elasticity

� to provide a preferential pathway for the movement of moisture through amasonry structure, allowing it to “breathe”, and to act as a sacrificial componentwhere deterioration would be concentrated, rather than in the masonry unitsthemselves.

In stone masonry of well-cut ashlar, blocks typically rest directly on one another or on avery thin bed of mortar which was probably used as much as a lubricant for moving theblocks into place during construction as for any other reason. In such construction, thebehaviour of the masonry is principally dependant on the properties of the stone.Where stone is less well-dressed, thicker mortar beds are required to provide a uniformbearing surface, and the mortar becomes an important influence on masonrybehaviour. Likewise, mortar characteristics also become more significant to thestructural behaviour of the masonry where the mortar is present in high proportionseg in mortared rubble. Similarly, in brickwork, structural behaviour will be influencedby the thickness of mortar beds.

In thin mortar beds between well-dressed stone, mortar may be in a state of triaxialcompression, where its strength is less significant. Conversely, where mortar is presentin irregular masonry with thicker and less even beds it is most likely to be subject tonon-uniform stresses and there is a greater potential for compression, which may resultin load transfer directly between masonry units.

MMoorrttaarr ttyyppeess aanndd cchhaarraacctteerriissttiiccss

Modern Portland-type cements have been the predominant type used in mortarsthroughout the 20th century but such cements did not come into common use until the

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second half of the 19th century, and the great majority of older masonry arch bridgeswere originally constructed using mortars based on lime cements.

“Traditional” lime-based mortars

Traditionally, mortars were produced using lime (calcium carbonate, CaCO3) which isderived primarily from natural limestones. The raw material was broken into lumpsand heated in a kiln, which drove off carbon dioxide and any water present to producecalcium oxide (CaO – also known as “quicklime” or “unslaked lime”). This was then“slaked” by reaction with water (a process of maturation traditionally carried out in pitsover a period of months or even years) giving out heat and resulting in the formationof hydroxides of calcium (Ca(OH)2) and, where present, magnesium (Mg(OH)2). This“slaked lime” could be mixed with sand in a ratio of 1:3, in which the lime slightlyoverfills the available void space in a dry and well-graded sand, to produce a stiff mixknown as “coarse stuff ” which, when exposed to air, dried out and set by slowlycombining with atmospheric carbon dioxide to produce a mortar cemented withcalcium carbonate.

The characteristics of lime mortars were dependent on the nature of the raw materialsused, the presence of any impurities or additions, and the process of production,particularly the firing conditions in the kiln. There are two principal types, withconsiderable variation between them:

� pure limes (also known as “fat limes”) are produced by heating pure limestone orsimilar materials; they are non-hydraulic cements which harden slowly by reactionwith atmospheric carbon dioxide only – know as “carbonation” or “air-setting”.They will not set in permanently wet conditions

� hydraulic limes rely partly on reaction with atmospheric carbon dioxide to set, butalso have a greater or lesser element of hydraulicity (ie set by chemical reactionwith water). The hydraulic properties result from the inclusion of clay impurities inthe limestone or from the direct addition of hydraulic material.

A very pure lime produces a mortar which strengthens and cures very gradually over along period, but remains relatively weak and plastic, typically with a crumbly texturesimilar to a digestive biscuit. During this prolonged curing period, they are susceptibleto damage from water and frost. As the degree of hydraulicity of lime increases, itscharacteristics can become similar to those associated with a Portland-type cement,exhibiting more rapid set, greater strength and brittleness, and lower permeability.

Many of the lime mortars used in the past have been produced from limestone withsome degree of clay impurity, which may have imparted a very slight degree ofhydraulicity to the mortar. However, certain limestone resources were suitable for theproduction of stronger hydraulic materials, although still perhaps the equivalent ofwhat are classified as feebly or even moderately hydraulic limes today (Ellis, 2002). Anumber of these “naturally hydraulic limes” were produced in the UK, the principalexamples being from the Lias limestones of Somerset, Devon, and Aberthaw in SouthWales, and the Arden lime in Scotland. “Roman cement” was a type of eminentlyhydraulic lime which could be used to produce a relatively strong, dense andimpermeable mortar and was commonly used for construction and repair in the 19thcentury. However, since the mid 20th century, although the raw material is stillplentiful, the only naturally hydraulic lime available in the UK at the present time isimported from the continent.

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MMooddeerrnn cceemmeennttiittiioouuss mmoorrttaarrss

Since the late 19th century, modern Portland-type cements have been included inmortar mixes in an attempt to overcome the potential disadvantages of traditional limemortars. Portland-type cements (now referred to simply as “cement”) are in some wayssimilar to very strongly hydraulic natural cements, but are artificially produced byburning a mixture of limestone and clay like minerals to form a clinker to whichgypsum is added during grinding to produce the typical fine powder. They relyentirely on chemical reaction with water to set, and are hydraulic cements. Since mortarsbased only on cement and sand at an optimal ratio of 1:3 are too harsh and difficult towork with, and produce a mortar that is too strong and inflexible, cement is insteadadded to lime and sand mixes to impart hydraulicity to the set by a process know asgauging. The most commonly used type of mortar in modern masonry iscement:lime:sand, with various additives such as plasticisers, retarders, air entrainersand pigments to modify fresh and set properties. The lime included in such mixes is anon-hydraulic, hydrated type which is available either dry-batched or, more commonly,delivered to site mixed with fine aggregate and water ready for gauging with a suitablequantity of cement. Mortars based on modern cements are quite different in characterto the traditional lime-based materials used in the construction of the majority ofmasonry arch bridges.

MMoorrttaarr sseelleeccttiioonn aanndd uussee

A relatively recent increase in awareness of the potential advantages of lime-basedmortars has led to something of a renaissance in their use for the maintenance andrepair of older structures, and even for some new-build projects.

Cement-rich mortars have frequently been used to carry out repairs to existing archbridges which were constructed with much weaker and more flexible lime mortars.Despite the potential benefits, this can cause problems, since patch repairs with overlystrong mortars can attract additional load and result in localised distress, and the use ofhard impermeable mortars for repointing can result in deterioration of the masonryunits themselves.

Lime-based mortars have properties which can be advantageous, particularly whendealing with older masonry (Figure 2.13):

� their relative weakness and flexibility when set allows them to deform plasticallyunder load, rather than cracking, imparting to the masonry a limited tolerance tomovement, which can be particularly useful in elements such as arch barrels whichdepend on a degree of articulation. In contrast, cement-rich mortars have a muchstronger crystalline structure but once fractured their strength is permanently lost

� their low strength makes them well suited to use with weak masonry units, commonin older arch bridges, where a stronger cement mortar might cause damage

� they are typically more porous and permeable than mortars based on cement,allowing moisture to pass out of the structure through the mortar rather thanthrough the masonry units. Mortar is intended to act sacrificially so thatdeterioration is concentrated at its surface, which can be easily repointed, ratherthan in the masonry units, which are more difficult to repair

� they are more resistant to deterioration from some forms of sulfate attack, andhave a limited “self healing” capability (autogenous healing) which allows very finecracks to seal themselves under certain conditions

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� their malleability means that they tend to retain good surface contact with themasonry units, which is an important factor in minimising the secondarypermeability of the masonry. In contrast, when small movements occur, hardcement-rich mortars have a greater tendency to separate from the units, generatingfine cracks at the mortar/brick interface which allow water to pass through

� lime has the advantage over cement because it is less environmentally damaging,principally due to the lower energy consumption and carbon dioxide emissionsassociated with its production, and its uptake of atmospheric carbon dioxide on curing.

However, there are potential drawbacks to using lime mortars:

� they can be very slow to cure, particularly where they are constantly in conditionswhich are too wet or too dry, or where they are used in thick sections

� their low curing rate means they are slow to gain strength, and ultimate strength isrelatively low

� while curing they are susceptible to deterioration through leaching and frost attack.

FFiigguurree 22..1133 ((aa)) ““KKnnoocckkiinngg uupp”” lliimmee mmoorrttaarr ttoo pprreeppaarree iitt ffoorr uussee;; ((bb)) lliimmee mmoorrttaarr rreeaaddyy ffoorr aapppplliiccaattiioonnuussiinngg aa ““ppooiinnttiinngg kkeeyy””

Today a variety of cements and mortars based on limes and hydraulic limes areavailable, but it should not be assumed that these modern versions of traditionalmaterials are entirely similar to their historic predecessors. Their properties andperformance can vary considerably and they may require special techniques for theirhandling on-site and application. Lime mortars require more mixing, more care inapplication, and more protection from the elements while curing than cement mortars,which undoubtedly have their place. Widespread use of “bag lime” – the type available

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from builders merchants – has given lime a poor name either because of the instanceswhere due to poor storage it simply has little reactive lime left by the time the end-userworks with it, or it is specified and/or used incorrectly — hence it fails, dusts etc.

To take advantage of the potential benefits of lime mortars requires specialistknowledge on the part of specifiers and skill and experience on the part of tradesmen.It is recommended that care is taken in the very important task of mortar selection andapplication for construction, repairs and repointing, particularly where structures withspecial historic value are concerned. After many years of disuse, and with sparseexperience and guidance currently available, it may be necessary to gradually re-learnhow best to make use of lime-based mortars and avoid potential problems. This is likelyto require a certain amount of experimentation. However, errors are not tolerablewhen carrying out work on infrastructure bridges. Measures may be taken to increasethe likelihood of success when dealing with these materials (Ashurst, 1997):

� taking time to read about, and understand the materials

� production a full and detailed specification covering materials and work with fullreference to manufacturers’ instructions

� selection of experienced contractor and contractor’s operatives

� submittal of material samples with manufacturers’ data sheets

� execution of trial work for approval including mixing and placing procedures

� allowance made for slow curing

� close supervision of works and monitoring of performance.

Although the use of naturally hydraulic limes is currently limited in the UK, it iscommon for lime to be gauged with Portland cement (or sometimes pozzolans, whichare natural or artificially produced hydraulically reactive additives, eg volcanic ash,brick dust or PFA) to confer an element of hydraulicity. This has potential advantagesover use of the pure lime mortar, including:

� more rapid strength gain

� reduced likelihood of shrinkage cracking

� less sensitive to inclement weather during application and curing.

However, there are potential disadvantages, including:

� reduction in workable time

� can introduce soluble salts to mortar, which may damage masonry

� segregation of cement and lime as the mortar hardens.

Gauging with cement is a common practice and can potentially produce a mortar withthe desired properties (see Section 4.4 on new bridges). However, care should beexercised, particularly when working with older structures where the masonry is weakor deteriorated, to match new mortar to old in terms of its strength and permeability.This may require the addition of only a small proportion of cement to a lime:sand mix,but this increases the risk of segregation, weakening the mortar and reducing itsdurability. Research carried out by English Heritage suggests that because of the risk ofsegregation an un-gauged lime mortar, based on a well matured lime putty with sharp,well graded aggregate, properly applied and carefully cured to ensure that it fullycarbonates, is likely to be more durable in the long run than a mortar gauged with onlya small amount of cement (Tutonico et al, 1994). If a hydraulic set is required, a safer

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alternative in most circumstances would be to use a good natural hydraulic lime mortar(O’Hare, 1995).

Table 2.2 gives mix proportions for mortars and indicative ranges for their compressivestrengths, as well as values for the compressive strength of the masonry produced usingdifferent brick strengths (assuming standard brick dimensions).

TTaabbllee 22..22 MMoorrttaarr mmiixxeess aanndd ccoommpprreessssiivvee ssttrreennggtthhss uusseedd iinn tthhee UUKK,, aanndd ccoorrrreessppoonnddiinngg ssttrreennggtthhss ooffmmaassoonnrryy uussiinngg ddiiffffeerreenntt bbrriicckkss ((SSoowwddeenn,, 11999900))

a from BS5628: Part 1 (BSI, 1992)

b class B engineering brick

c class A engineering brick

d hydraulic lime

e pure lime

Standards relating to mortars include:

� specifications for mortar mixes are given in Table 1 of BS 5628-1:1992, which atthe present time is current but partially replaced by BS EN 998-2:2003 Specificationfor mortar for masonry: masonry mortar

� BS EN 459-1:2001 Building lime part 1: Definitions, specifications and conformity criteriahas superseded BS 890:1995 Specification for building limes.

The specification and use of cements and mortar in modern masonry and theapplication of current mortar standards is discussed further in Section 4.4. Repointingof existing masonry and the selection of suitable repointing mortars is discussed inSection 4.3.3.

Both English Heritage (www.english-heritage.org.uk) and the Scottish Lime Centre(www.scotlime.org) can provide information and support on the use of lime cements,and sources of lime materials are listed in the English Heritage directory of building limes(Teutonico, 1997). Further information on lime cements, aggregates and mortars isavailable:

� Mortars, plasters and renders (Ashurst and Ashurst, 1988).

� Use of traditional lime mortars in modern brickwork (BDA, 2001a).

� The technology and use of hydraulic lime (Ashurst, 1997).

CIRIA C656 47

MMoorrttaarr

ddeessiiggnnaattiioonnaa

TTyyppee ooff mmoorrttaarr ((bbyy vvoolluummee)) MMoorrttaarr

ssttrreennggtthh

rraannggee

((NN//mmmm22))

BBrriicckk ssttrreennggtthh ((NN//mmmm22))

CCeemmeenntt::

lliimmee:: ssaanndd

MMaassoonnrryy

cceemmeenntt::

ssaanndd

CCeemmeenntt

ssaanndd++

ppllaassttiicciisseerr

7 20 35 50b 70c

CChhaarraacctteerriissttiicc ccoommpprreessssiivvee ssttrreennggtthh ooffbbrriicckkwwoorrkk ((NN//mmmm22))

(i) 1:0– 0.25:3 - 11-16 3.5 7.5 11 15 19

(ii) 1:0.5:4.5 1:2.5-3.5 1:3-4 4.5-6.5 3.5 6.5 9.5 12 15

(iii) 1:1:5-6 1:4-5 1:5-6 2.5-3.6 3.5 6 8.5 11 13

(iv) 1:2:8-9 1:5.5-6.5 1:7-8 1-1.5 3 5 7 9 11

(v) 1:3:10-12 1:6.5-7 1:8 0.5-1 2 4 6 7.5 8.5

(vi) 0:1:2-3d - - 0.5-1 2 4 6 7.5 8.5

(vii) 0:1:2-3e - - 0.5-1 2 3 3.5 4.5 5

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� Preparation and use of lime mortars: An introduction to the principles of using lime mortars(Scottish Lime Centre, 1995).

� Hydraulic lime mortar for stone, brick and block masonry (Allen et al, 2003).

� Directory of sands and aggregates (Chapman and Fidler, 2000).

� Gauging lime mortars (Ellis, 2002).

22..33..22 SSttoonnee

Stone is a term applied to construction material quarried from a natural rock. It is oneof the oldest building materials known to man, and since the earliest times ofcivilisation has been the preferred material for the construction of permanent andimportant buildings. Stone is important because of its potential for exceptionaldurability and unique aesthetic appeal, and the oldest bridges in existence are all ofstone construction. Despite the development and prevalence of man-made buildingmaterials in the 19th and 20th centuries, stone is still used today in prestigiousstructures, although it is more often used in thin sheets as a facing material rather thanas a structural component.

An understanding of the geological origin of building stones, their physical structure,chemical composition and engineering properties is important in selecting stone thatwill perform as required in either a new or an existing structure. Original sources ofstone are often unavailable, so finding stone that is a suitable match for the repair of anexisting bridge requires expert advice and frequently entails consideration of a varietyof alternative resources. When stone bridges were built, experienced masons wouldtypically have had a good knowledge of how local stone performed in differentcircumstances and how best to use it within a structure. However, some existing bridgesdo have problems with stone deterioration either because of original poor selection anduse of stone, subsequent implementation of inappropriate repairs, or becausedeterioration has been hastened by changes in the bridge’s environment.

Particular care should be taken when selecting stone for repairing existing bridges since“new” stone can have significantly different properties and perform quite differently tothe original stone in a structure, even where it appears to come from the originalsource. It is important to appreciate that stone is a natural and variable material.Unlike man-made materials, which are typically produced so as to be homogeneousand uniform in quality and character, stone can be highly variable even within thespace of a single small unit, although some types and resources are more consistentthan others in this respect. Innate characteristics such as inhomogeneity and isotropycan affect the physical properties of stone and influence its performance within astructure. For instance, strength can vary across a wider range than that of man-madeconstruction materials, and can change according to the orientation of imposedstresses. Most quarries, though, are located in areas where there is some degree ofconsistency of raw material, and selective quarrying can help to provide an adequatesupply of similar stone for a particular need.

A description of the range of rock and stone types available and their geological andengineering characteristics is beyond the scope of this document, which is limited to adiscussion of some of the main factors affecting their selection and performance inservice particularly pertinent to their use in masonry arch bridges.

Whether selecting stone for a new bridge or for repairing an existing one, theimportant physical characteristics of building stone are:

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� strength; most building stone has good compressive and shear strength

� hardness and workability; stone subject to wear should be hard, and in suchapplications softer stone (such as poorly cemented sandstone) is unlikely to provideadequate performance. However, harder stone tends to be less workable and canbe more difficult (and expensive) to cut and shape

� durability; this is the ability of stone to resist deterioration in service. It is dependenton characteristics such as the mineralogy, physical structure, texture and porosity ofthe stone, and also on the way it is used in the structure and its environment ofexposure. A stone that is perfectly durable in one part of a structure under one setof environmental conditions may perform differently in another. Stone durability isan important influence on the maintenance-free life of a bridge

� porosity; the degree of porosity will determine the amount of moisture that may beabsorbed into the stone, and is related to its durability. Porosity has a direct bearingon the ability of the stone to withstand frost action and resist other deteriorativemechanisms. It also influences how easily the stone becomes marked and stained

� aesthetics; since stone was often selected for use on prestigious bridges, andbecause existing stone bridges often have considerable heritage value, the visualcharacteristics of stone are an important factor. Over time, weathering canconsiderably change the appearance of stone in a structure and this should beconsidered when trying to match new stone to old

� availability; this is an important factor, since stone can be very expensive to processand transport over long distances. Potential stone resources should be assessed toensure that they can provide a consistent supply of suitable stone adequate tocomplete the project.

A wide range of rock-types has been used as building stone, but in the UK the mostcommonly used were the sedimentary rocks limestone and sandstone, and, in someareas (particularly the north and west of England, Scotland and Northern Ireland),igneous rocks such as granite. Historically, because of the difficulties and cost associatedwith the transport of large quantities of stone, the type used for most bridges was onethat was locally available. Occasionally, for more prestigious bridges, stone was selectedfor other reasons (eg strength, appearance, durability) and where necessary broughtfrom further afield.

TTaabbllee 22..33 CCoommppaarriissoonn ooff ttyyppiiccaall ssttrreennggtthh aanndd ddeennssiittyy vvaalluueess ooff ssoommee ccoommmmoonn UUKK bbuuiillddiinngg ssttoonneess wwiitthhootthheerr ccoonnssttrruuccttiioonn mmaatteerriiaallss ((aafftteerr GGeeoollooggiiccaall SSoocciieettyy,, 11999999))

CIRIA C656 49

MMaassoonnrryy uunniitt mmaatteerriiaallTTyyppiiccaall ccoommpprreessssiivvee ssttrreennggtthh

((NN//mmmm²²))TTyyppiiccaall ddeennssiittyy

((kkgg//mm³³))

Stokeground bath – base bed 22.5 2126

Stokeground bath – top bed 13.8 1988

Portland roach 52 2100

Portland whitbed 36 2200

Welsh blue pennant 158 2630–2850

Clipsham limestone 32 1826

Woodkirk Yorkstone 54 2400

Granites 100–350 2500–3200

Concrete (typical) 40 2240

Bricks (typical commons) 20 1800

Bricks (engineering class A) 70 up to 2800

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SSeeddiimmeennttaarryy rroocckkss

Sedimentary rocks, for example sandstones and limestones (Figure 2.14), are composed ofindividual grains of rock or minerals which have been compacted over time and are heldtogether by a cement which may wholly or partly fill the interstices between the grains.The hardness and durability of such rock can vary greatly and is dependent principally onthe physical and chemical nature of the grains and of the cement, and the proportion ofinter-grain spaces which are not filled with cement, which gives the rock much of itsporosity. Sedimentary rocks often have natural parallel lamina known as bedding planes,which were horizontal when the rock was formed. Such features contribute to theanisotropic nature of stone ie its characteristics may vary according to its physicalorientation. This means that the properties of stone can vary according to how it isquarried, processed and used within a structure. It is important that stone containing suchfeatures is correctly orientated within a structure in order to optimise its performance. Ifthese planes are parallel to the exposed face of a structure then weathering processes maycause delamination and rapid deterioration. A skilled and experienced mason will have anunderstanding of such characteristics and how best to use stone in a structure.

Sandstones and limestones are the most commonly used types of stone in masonry archbridges (and other stone structures in the UK) since they are relatively widespread inoccurrence and are relatively easy to quarry and cut to shape. However, in the longterm they can be less resistant to deterioration than some of the stronger and moredurable igneous rocks.

FFiigguurree 22..1144 LLiimmeessttoonnee vvoouussssooiirrss iinn aann aarrcchh bbaarrrreell((ccoouurrtteessyy ooff BBrriittiisshh WWaatteerrwwaayyss))

IIggnneeoouuss rroocckkss

Igneous rocks, such as granites, basalts and dolerites, are those that have crystallisedfrom molten intrusions and lava. They are present in great quantities in certain parts ofthe UK but absent from much of England, particularly the Midlands and the SouthEast. They tend to be harder and more durable than sedimentary rocks and morehomogeneous since they do not contain bedding planes, so their orientation within astructure is often less critical.

CIRIA C65650

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CIRIA C656 51

Certain igneous rocks exhibit exceptional durability when used in bridges, but they aretypically more difficult to quarry and shape into cut stone than the softer sedimentaryrocks, and so may be more expensive, particularly if they have to be transported fromsome distance away.

MMeettaammoorrpphhiicc rroocckkss

Over geological time, the nature of both igneous and sedimentary rocks may bechanged by a process of metamorphism, caused by exposure to extremes of pressureand/or temperature in the earth’s crust. They exhibit a very wide range of physical,mineralogical and textural characteristics. Some types of metamorphic rock arerelatively homogeneous and isotropic, but others have textures and fabrics which resultin greater variability and can act as planes of weakness in certain orientations, forinstance the development of pronounced cleavage that is a characteristic of slates.

As with igneous rocks, metamorphic rocks tend to occur in the hillier and moremountainous parts of the UK (Scotland, Wales, Northern Ireland and the north andsouth-west of England) and are used in various construction applications. Some, such ascertain gneisses or quartzites, may be very strong and durable and ideal for use as cutstone blocks in structures such as bridges.

CCaasstt ssttoonnee

Cast stone is not a true stone but one of a number of names given to various concrete-like mixtures that were (and still are) used to produce moulded shapes which simulatenatural stone. It is also known as “artificial stone” – or by the names of a number ofproprietary systems. BS 1217:1997 defines cast stone as any material manufacturedwith aggregate and cementitious binder and intended to resemble in appearance, andbe used in a similar way to, natural stone.

Although the history of cast stone dates back to the Middle Ages, the ability toconsistently mass-produce complicated shapes and replicate intricate architecturaldetails without recourse to the labour intensive and highly skilled methods of stonemasons saw a rise in the popularity and use of these materials in Europe (particularlyBritain and France) during the 19th century, and their widespread acceptance as aneconomical alternative to stone in the 20th century. Although most frequently used inbuilding elements such as lintels, cast stone was occasionally used to provide animpressive appearance to a bridge constructed using more modest materials, eg brickor random rubble. Such materials could be used consistently over the whole exterior ofa bridge, for the vertical elevations only, or most economically, for the verticalelevations of the arch barrel. Sometimes cast stone was used as an attractive “trim” overnatural stone with poor or unfinished appearance.

Although this type of cladding is not common on bridges, engineers should be awarethat it may be encountered, and that care should always be exercised so as not to bemisled as to the internal composition of such bridges by their outer appearance.

FFuurrtthheerr iinnffoorrmmaattiioonn

For further information on stone properties, use, conservation, repair and findingsuitable matches:

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� Stone: Building stone, rock fill and armourstone in construction (Geological Society, 1999).

� The building sandstones of the British Isles (Leary, 1986).

� The building limestones of the British Isles (Leary, 1989).

� The building magnesian limestones of the British Isles (Hart, 1988).

� Selecting natural building stone (BRE Digest 420, 1997).

� Building stone resources of the United Kingdom (BGS, 2001).

22..33..33 BBrriicckk

Clay bricks are produced by firing natural clay at high temperatures until the clayminerals melt and fuse to form a combination of vitreous and new mineral phases, thecomposition and characteristics of the fired brick being dependent on the originalcomposition of the clay and the temperature and duration of the firing process. Brickcolour is dependent upon the raw clay materials used in their manufacture, and can beinfluenced by the addition of other minerals and pigments.

Traditionally, clay known to be suitable for brickmaking was dug from the ground and“weathered” for some time to dry it, before being mixed and hand-thrown intoindividual moulds. The earliest firings were done by heaping the bricks and fueltogether and covering with turf, but simple kilns followed – a single “clamp” consistingof a brick arch covered with turf being one of the earliest, followed by round brickkilns. The enormous demand for bricks by the middle of the 19th century led to thedevelopment of the first brickmaking machines and improvements in kiln design andefficiency.

The quality of bricks within a single structure, and particularly in large structures, canvary because of the composition and quality of the raw materials used at theintroduction of the brickmaking process itself. For instance, bricks that were fired in thecentre of the clamp were subject to burning and baking at high temperatures and wereoften better quality. In contrast, bricks from the outer part of the clamp were poorlyfired, weaker and less durable. The fired bricks were graded according to their“quality” so that they could be used appropriately, the best being reserved for facingwork and in areas most exposed to the weather. Poor quality bricks were frequentlyused behind facings or as random rubble or fill, or set aside for re-firing.

Beyond their original variability, the process of ageing and deterioration of bricks inold arch bridges is another factor which has influenced the current condition andphysical characteristics of their masonry.

Modern technology has improved the efficiency of brick production and theconsistency of the product, and bricks can be specified to meet a wide variety ofrequirements in terms of strength, durability and appearance. The designations andclassifications of modern brick units is discussed in Section 4.4.2 on new bridges.

As with mortars, it is important to carry out brick replacement with materials that aresympathetic to the original masonry materials, and new bricks should not be too hardor dense as they may result in accelerated deterioration of adjacent masonry throughredistribution of stresses and moisture movement. There are environmental andtechnical reasons why re-use of bricks may in certain circumstances be desirable, butreclaimed bricks should be carefully selected to ensure that they will performadequately in their intended environment. If it is necessary to use new bricks to repairan old structure, it may be possible to find some that are similar in appearance or touse treatments such as soot washes to help blend them visually.

CIRIA C65652

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Research conducted by British Rail (Temple and Kennedy, 1989) involved an extensivetesting programme to determine the compressive strength and elastic properties ofbrickwork from old masonry structures of different ages (predominantly between 1840and 1910) from across the UK, and applied statistical techniques to draw generalconclusions that could be used for arch assessments. The results, which are summarisedin Table 2.4, illustrate the considerable difference in strength and modulus of old bluebricks (engineering bricks) and old red or yellow bricks (probably various non-engineering class bricks eg stock and gault bricks) used in these structures. It was alsonoted that individual bricks from the same sample often showed considerable variabilityin their physical characteristics. It should be noted that these are typical values only,and may not correspond to those found in other structures.

TTaabbllee 22..44 PPhhyyssiiccaall pprrooppeerrttiieess ooff bbrriicckkss ssaammpplleedd ffrroomm oolldd rraaiillwwaayy ssttrruuccttuurreess

a value exceeded by 90 per cent of bricks tested in a large sample (not mean strength, as used in modernBritish Standards)

b value is suggested, based on typical results from a small sample size

Further guidance and information:

� Bricks and brickmaking (Hammond, 1981) – on the history of brickmaking

� Observations on the use of reclaimed clay bricks (BDA, 2001) – on reclaimed bricks.

22..44 SSttrruuccttuurraall bbeehhaavviioouurr

22..44..11 BBeehhaavviioouurr ooff mmaassoonnrryy

Masonry is essentially unable to resist tensile or bending stresses and in masonrystructures loads are resisted only by compressive axial stresses. Masonry structures aregeometrical elements that resist actions when they can include, within their geometry, athrust line in equilibrium with the external loads. In general, from a structural point ofview, of the three conditions any structure has to verify – strength, stiffness and stability– stability (static equilibrium) is most important in masonry structures, although clearlyserviceability requirements should also be satisfied.

As a result of their inability to resist bending forces, masonry structures under loadingwill deform and crack unless they can resist those loads through a path of compressiveinternal forces. As a consequence of this, cracking is quite common in masonrystructures and should not be automatically associated with structural distress. Moreover,the durability of masonry is not as severely affected by cracking as, say, reinforcedconcrete, and in many cases the plasticity of most historical lime mortars will allowthose cracks to be gradually sealed by autogenous healing.

As a composite material, the stress state of masonry, even under simple loadingconditions is quite complex. Under uniaxial compression, the most common loadingcondition in masonry structures, the internal stress state can be simplified as illustratedin Figure 2.15. As a result of this, masonry fails by developing indirect tension cracks inthe units, parallel to the direction of load. A full discussion is given in Hendry (1998).

CIRIA C656 53

CChhaarraacctteerriissttiicc ssttrreennggtthh((NN//mmmm²²))a

MMoodduulluuss ooff eellaassttiicciittyy((kkNN//mm²²))

PPooiissssoonn’’ssrraattiioo

Red and yellow bricks 16.5 5.2 0.11

Blue bricks 70b 15.6 0.16

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FFiigguurree 22..1155 MMaassoonnrryy uunnddeerr uunniiaaxxiiaall ccoommpprreessssiioonn

CCrreeeepp aanndd sshhrriinnkkaaggee

Creep often has a forgiving effect in masonry by allowing a beneficial redistribution ofstresses. The simulation of creep in the calculation of initial stresses in masonry archbridges by finite element (FE) analysis is a topic of current research, and is notadequately developed for routine application.

Shrinkage effects in masonry should have no influence, except in newly built masonryarches and even then, its effects should have been minimised by following constructiongood practice advice (see Section 4.4 on new bridges).

BBeehhaavviioouurr uunnddeerr hhiigghh ccyycclliicc ffaattiigguuee llooaaddiinngg

The quasi-static compressive strength of brick masonry (ie that displayed under veryslow loading, such as is specified by British Standard tests for masonry structures)depends upon the compressive strengths of the bricks and mortar, the nature of theapplied loading, and environmental conditions such as the degree of saturation. Thequasi-static compressive strength under concentric loading is well documented andinformation relating the strength of the masonry to the strengths of the units andmortar appears in codes of practice. It is also well known that the quasi-staticcompressive strength of masonry under non-uniform loading may be significantlygreater than its corresponding strength under uniform loading (Page and Hendry,1988; Hendry, 1990). However, recent experimental studies have indicated that thelower bound increase in compressive strength due to non-uniform loading, typically upto 20 per cent can be accounted for by assuming plastic, parabolic or rectangular stressdistributions to determine the maximum induced stress (Roberts et al, 2004).

Little information is available concerning the high cycle fatigue strength of brickmasonry. The earliest tests, undertaken by British Rail Research (BRR) on brickmasonry piers, indicated the fatigue strength of dry brick masonry at 108 cycles, basedon the maximum induced stress, to be approximately 50 per cent of its quasi-staticcompressive strength under similar loading conditions (Clark, 1994).

Recent and more extensive tests conducted for Network Rail have confirmed this(Roberts et al, 2004). These tests also indicate that the high cycle fatigue strength of drybrick masonry, based on the induced stress range, depends upon the maximuminduced stress and the quasi-static compressive strength under similar loading

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conditions. A lower bound fatigue strength curve was proposed in the form:

where ∆S and Smax are the induced stress range and maximum induced stressrespectively, Su is the quasi-static compressive strength under similar loading conditionsand N is the number of constant amplitude load cycles.

EEffffeeccttss ooff ssaattuurraattiioonn

Relatively little information is available concerning the influence of moisture content onthe strength and stiffness of brick masonry. A recent experimental study has indicatedthat the presence of moisture can reduce the quasi-static compressive strength andelastic modulus by up to approximately 20 per cent and 10 per cent respectively (Amdeet al, 2004). This study confirmed a much older study by Baker (1909).

Grimm (1999) suggested that poor workmanship, coupled with a soaking rain, mayreduce the factor of safety in brick masonry structures.

Tests reported by Clark (1994) indicated that the high cycle fatigue strength ofsaturated brick masonry, based on the maximum induced stress, reduces to zero atbelow 108 load cycles ie there is no fraction of the ultimate load below which 108 cyclescan be resisted. This conclusion was not confirmed by a more recent andcomprehensive study reported by Roberts et al (2004) which indicated that the highcycle fatigue strengths of dry and saturated brick masonry, based on the induced stressrange, are similar and can be represented by the fatigue strength curve given in theprevious section, provided the appropriate value of Su (dry or saturated) is used.

The effect of water on the deterioration of masonry is discussed in Section 2.5.3.

22..44..22 LLooaaddss oonn aarrcchh bbrriiddggeess

DDeeaadd llooaaddss

Dead loads are essential for the stability of masonry arch bridges. Accordingly, it isimportant to consider accurately the weight and distribution of the bridge and itssuperimposed dead loads. It is particularly important to take this into account whenmaintenance works are undertaken and these loads are temporarily changed.

In the case of an arch bridge prone to flooding consideration should be given to use ofbuoyant weights, which act against dead loads, when calculating the load carryingcapacity.

When applying factors to these loads it should be taken into account that dead loadshave a beneficial effect, but they can also have a negative effect if the pattern of deadweight loading in relation to the shape of the arch is inappropriate.

TTrraaffffiicc llooaaddss –– ssttaattiicc,, ddyynnaammiicc aanndd ccyycclliicc

The following types of loads need to be considered:

� vertical traffic loads (trains, vehicles, etc)

� pedestrian loads

CIRIA C656 55

NSSS

u

log05.07.0)( 5.0

max −=×∆

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� equipment load during works

� lateral loads due to centrifugal effects

� lateral internal loads due to the transfer of loads through the backfill

� accidental loads on the parapets

� accidental bridge bashing

� longitudinal loads caused by traction and braking. These would only be significanton multi-span bridges with slender piers.

Specific guidance on the load magnitudes, positions, frequency etc to consider shouldbe provided by the standard appropriate for the specific bridge. When determining theloaded lengths masonry arch bridges do not behave as elastic structures and thereforeapproaches based on lines of influence are not valid.

EEnnvviirroonnmmeennttaall eeffffeeccttss

The effects of wind loading on masonry arch bridges can be ignored as a result of thehigh mass of this type of structure. However, other environmental effects that need tobe taken into account are floods and droughts, which can have significant effects onone of the weakest points of masonry arch bridges, their foundations. Similarly,although stone and brick masonry is generally a very durable material, the effects ofweathering and different deterioration mechanisms need to be taken into considerationwhen assessing their performance (see Section 2.5.3).

The thermal properties of masonry can vary quite significantly between the differenttypes of masonries. In general, however, although thermal changes will no doubt affectthe results of a monitoring programme, they are not considered to have significanteffects on the integrity of masonry arch bridges.

GGrroouunndd mmoovveemmeennttss

As previously mentioned, older masonry arch bridges often have weak and shallowfoundations. As a result of this, foundation movements when the centring was removedand during its service life are common in these structures. As these movements takeplace, the arch will adapt its geometry to the new conditions and in doing so it willcrack. These types of cracks are found in many masonry arch bridges and in most casestheir effects on the structure can be neglected. Details on the effects of groundmovements on the performance of masonry arch bridges are given in Section 2.5.1.

22..44..33 PPeerrffoorrmmaannccee rreeqquuiirreemmeennttss

The following structural performance requirements have been identified:

� stability

� strength

� stiffness.

These are in addition to the normal serviceability requirements of bridges, which varyaccording to the infrastructure type, owner’s requirements and type of use (adequateclearance, crack closure, levels, drainage, leakage, safety to users, appearance etc).

Loss of bridge performance may be attributed to a number of factors, discussed furtherin Section 2.5.

CIRIA C65656

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Many bridges suffer from sub-optimal serviceability, but are forced to continue inservice as they form an essential part of the infrastructure of the system for which theywere constructed. The loss of performance may be slow and insidious or inherent asbuilt-in defects, but may not be associated with excessive movement or overallinstability. Structural instability and collapse is less frequent but when it occurs it isusually dramatic and catastrophic, and may be without advance warnings unless signsof distress are apparent during inspections. Structural failure may develop ifserviceability problems are not addressed. It may result in loss of life if the change incondition is sudden and no advance warning given. It is important to be aware of thecauses and significance of changes and defects that become apparent during theinspection and maintenance programme.

SSttrruuccttuurraall iinnssttaabbiilliittyy aanndd ccoollllaappssee

Structural instability and potential collapse of bridges can have extreme andunacceptable consequences. Failure of a bridge structure can:

� lead to injury or loss of life

� severely disrupt transport routes and networks

� cause frustration for users and associated parties/neighbours

� damage service and infrastructure furniture that may be housed within thestructure or cause damage to adjacent services and neighbouring properties

� require the implementation of costly emergency remedial/replacement works.

In reality, structural failures of masonry arch bridges are rare but have happened,particularly where the failure has been caused by a process that is not easily identifiedby the routine inspection process, for instance, where foundation support has been lostdue to the effects of scour. The potential for structural failure may be assessed throughthe bridge condition and structural appraisal process described in Chapter 3.

LLoossss ooff sseerrvviicceeaabbiilliittyy aanndd iinnccrreeaassiinngg mmaaiinntteennaannccee rreeqquuiirreemmeennttss

Loss of serviceability results in sub-optimal performance of a bridge and may requirerestrictions (eg on weight or speed of vehicles, or number of lanes of traffic) thatcompromise its normal usage. It may also require increased maintenance and theimplementation of temporary risk management measures, for example throughincreased frequency of inspection or the installation of monitoring equipment. Bridgesin such condition are a problem for asset managers, since they not only haveinadequate performance but are also an increased drain on limited resources.

If left unattended, serviceability problems may lead to complete deterioration of thestructure and, in the worst case, structural failure. In order to control deterioration ofthe structure a programme of condition appraisal, maintenance and, where necessary,remedial measures, is essential. This may require taking the structure out of servicetemporarily to allow this work to be carried out, albeit with the consequence ofdisruption to users.

22..44..44 BBeehhaavviioouurr uunnddeerr qquuaassii--ssttaattiicc llooaaddiinngg

In order to understand the behaviour of masonry arch bridges, it is important tounderstand which resisting mechanisms are mobilised under loading and how failurestake place. Loading of a bridge structure is first considered under static loads and thenthis behaviour is adapted to consider its response under dynamic loading.

CIRIA C656 57

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SSqquuaarree bbrriiddggeess

The main resisting mechanisms in square masonry arch bridges are as follows:

� the main resisting mechanism is the geometry of the arch. From a strict structuraldefinition, an arch would be the antifunicular geometry of a certain set of loads.Under loads that take the arch away from being the antifunicular geometry, itresists as a result of having been pre-stressed by gravity. The geometrical nature ofthe way in which arches resist loads means that its capacity is dependent on thewhole arch shape, not just its span and rise. In the case of multi-ring arches, if ringseparation takes place, the arch will not behave as a unit, but as a stack of thinindependent arches, which has been shown by laboratory experiments to reduceload carrying capacity (Melbourne and Gilbert, 1995).

� as a relatively uniform load, the weight of the spandrel has a stabilising effect onthe structure. It further confines or pre-stresses the arch, moving it away fromtensile stresses it can seldom resist. In some cases, however, depending on the archgeometry and on the spandrel’s profile, the weight of the spandrels at thehaunches can be much bigger than at the crown, causing negative effects

� as a structural material in contact with the arch, when the arch moves against thebackfill under external actions, the strength of the backfill is mobilised. This effectcan be seen as a series of normal and shear stresses applied at the arch extrados, inaddition to those induced by the weight of the backfill

� in the same way as the backfill, the spandrel walls constrain the arch movements asa result of their stiffness. Additionally, backfill-spandrel walls interaction also takesplace, making the restriction of the arch movements by the backfill and thespandrel walls complex. Disagreement exists on whether the backfill-spandrel wallsinteraction is sufficient for the spandrel walls to still constrain the arch movements,after transverse spandrel wall-arch separation has taken place

� as an extension of the spandrel walls, the wing walls contribute to the strength ofmasonry arch bridges by restricting the rotation of the spandrel walls around theirbase, consequently increasing their in-plane stiffness. Similarly, the movements ofthe backfill are constrained by the surrounding soil. On deep arches, the at-restlateral pressures provided by the surrounding soil can be essential to ensure thestability of the structure under dead loads

� finally, another factor contributing to the strength of masonry arch bridges is thespread of the load through the backfill, but the extent of this is unknown.

Three main modes of failure have been identified for square masonry arch bridges(Hughes, 1995a; Page, 1995):

� failure by formation of a hinge mechanism. This may involve the formation ofhinges and/or “sliders”. A hinge characterised by the progressive opening of cracksat the positions of the arch where the line of thrust is sufficiently eccentric. Thenumber of hinges required to convert a single span structure into a mechanism isfour, except in the special case of central symmetric load, in which five hinges arerequired. Figure 2.16 shows an example of a four hinge mechanism reproduced ina full-scale laboratory model. When the abutments contract or spread sufficiently(the equivalent of the mechanism of a “slider”) a three-hinge mechanism can occur

� snap-through failure prior to the full formation of hinges. In highly restrictedarches, sufficient concentrated rotations can develop at the hinge under the load soas to produce an instability local failure, prior to the formation of the remainingfourth hinge. This results in a local failure that precipitates the global collapse ofthe structure

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� crushing failure. This occurs when compression failure of the masonry in a certainzone of the structure results in local damage that instigates the global failure. Highcompressions develop at hinges as cracking progresses and the compressed area ofthe section is reduced. This type of failure can be quite brittle and if signs ofcrushing are observed in the arch, immediate action should be taken.

These failure modes are to some extent assumed, but an understanding of the idealisedfailure modes is useful in gaining an understanding of the response of masonry archbridges to loadings and their effect on serviceability. Real failures observed in bridgesare often more complex, since they normally comprise combinations of these idealisedmodes in addition to general loss of performance. More complex modes of failure candevelop as combinations of the three main modes with local failures such as ringseparation, in the case of multi-ring arches, spandrel wall-arch separation or radialshear failure in between units.

FFiigguurree 22..1166 FFoouurr hhiinnggee ffaaiilluurree mmeecchhaanniissmm ooff aann aarrcchh bbrriiddggee;; ((aa)) iinn pprraaccttiiccee,, ((bb)) iinn tthheeoorryy ((ccoouurrtteessyyUUnniivveerrssiittyy ooff SSaallffoorrdd))

In shallow square masonry arch bridges, the critical load position is generally thequarter point, whereas in deep square masonry arch bridges the critical load position isprimarily near the 1/3 point.

CIRIA C656 59

((aa))

((bb))

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SSqquuaarree mmuullttii--ssppaann bbrriiddggeess

The behaviour of square multi-span arch bridges will be different to that of single-spanarch bridges when the piers in between arches are sufficiently slender to ensure thatthe application of load in one span will mobilise the adjacent spans. Additionally, evenwhen stocky piers are present it may be found that interaction between adjacent spansis important. For example, the critical collapse mechanism might involve a four hingemechanism in the loaded span with horizontal pressures transmitted onto an adjacentspan leading to failure, as in Figure 2.17. Although this might seem unlikely, it ispossible where concrete haunching or effectively compacted backfill is present.Considering the possibility of interaction and the effect that it might have on the modeof failure and long-term performance is important.

FFiigguurree 22..1177 TTrraannssmmiissssiioonn ooff pprreessssuurree ffrroomm llooaaddeedd ssppaann ttoo aaddjjaacceenntt ssppaann iinn aa mmuullttii--ssppaann bbrriiddggee

From a comprehensive parametric study using a simple mechanism approach, theequation below was proposed to determine when single rather than multiple spanbehaviour would take place (Hughes, 1995b).

r is the arch rise

s is the arch span

d is the arch thickness

f is the depth of the fill over the arch crown

w is the loaded length

t is the pier thickness

h is the pier height

Where the value of this expression is greater than or equal to one then the multi-spanfailure load is equal to the equivalent single-span failure load, and the single-spancarrying capacity may be used unmodified. Where the value of the expression is lessthan one, the multi-span failure load is less than that of the equivalent single-span, andthe multi-span carrying capacity should be assessed by multiplying the single-spancapacity by this factor.

The main difference between single and multi-span arch bridges is that sufficientrotation around the base of a pier can take place for the structure to fail with a sevenhinge mechanism. In this mechanism, illustrated in Figure 2.17, only three hinges arenecessary on the loaded span since the slenderness of the pier is such that sufficientoutwards movement of the intermediate support takes places for the three hingemechanism in the loaded span to induce failure. For this particular failure mode, thecritical load position is near the centre of the span (Melbourne et al, 1997).

CIRIA C65660

10417.0

375.0218.035.2436.028.111.0

≥⎟⎠⎞

⎜⎝⎛

⎟⎠⎞

⎜⎝⎛

⎟⎠⎞

⎜⎝⎛ +

⎟⎠⎞

⎜⎝⎛ +

⎟⎠⎞

⎜⎝⎛

⎟⎠⎞

⎜⎝⎛

−−−−

th

dt

ssw

ddf

sd

sr

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FFiigguurree 22..1188 MMuullttii--ssppaann ffaaiilluurree mmeecchhaanniissmmss ((MMeellbboouurrnnee,, 11999977))

SSkkeewweedd bbrriiddggeess

The behaviour of all masonry arch bridges has a 3D component, but in the case ofskewed bridges this is more pronounced.

Although the behaviour of skewed bridges is quite complex and not completelyunderstood, the following aspects have been observed (Melbourne and Hodgson,1995):

� the stiffness of the arch varies considerably across its width

� the loads are generally transferred across the shortest span available. As a result ofthis, stresses concentrate in the obtuse corners and torsional moments are appliedto the abutments and/or piers. This tends to make the abutments rotate as well aslean backwards and this may be a particular problem on a slender pier supportingtwo arches skewed in the same direction (Page, 1993)

� as a result of this, the failure mechanisms involve complex 3D hinge patterns, quitedifferent to those on square bridges. In particular, the orientation of the hingeswith relation to the load and/or abutments can change across the width of thestructure and diagonal cracks “isolating” the acute corners

� the hinges that develop are also more diffused than in square spans

� if ring separation takes place, the kinematics of the system are such that the ringswill slide transversely over each other

� multi-ring tests (Melbourne, 2001) demonstrated the significance of the interactionbetween the skew barrels and the piers which ultimately failed in torsion. Initiallythe barrel tried to form a complex five hinge similar to that of the previous singlespan tests but when the pier failed in torsion the barrel was released to initiallyform a four hinge mechanism and finally a three hinge mechanism. Each of thesemodes of failure should be considered in arriving at a “safe” load carrying capacity.

As in multiring arches, the brickwork pattern used to build skewed arches has asignificant effect on the stiffness and strength of the structure (Melbourne and

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Hodgson, 1995). The worst case scenario relates to a barrel where each ring is notconnected to the next ring except by the mortar bed jointing material. The best casescenario is one where headers are incorporated into the barrel every third course – thisshould minimise the effects of ring separation. Additionally, positive features like astone voussoir barrel elevation and/or stone skewbacks saw-toothed to accommodate thebrickwork bedding planes as the brickwork reaches the abutment or pier support,should be taken into account when considering the barrel edge effects and thepossibility of springing sliding effects respectively. Advice on the analysis of skewbridges is given in Section 3.10.3.

WWiiddeenneedd bbrriiddggeess

A comprehensive review of methods for widening arch bridges and their advantagesand disadvantages is provided by Tilly (2002). Essentially, the main problems associatedwith widening masonry arch bridges using different structural systems are thedifference in stiffness of both the structure and its foundations. As a result of this,unless there are good reasons for structural connection between the two structures, thebest option is to let the two structures work independently. It is important however, tointroduce a joint with sufficient movement capacity to prevent it from being damagedand allow water to penetrate.

22..44..55 BBeehhaavviioouurr uunnddeerr ddyynnaammiicc aanndd ccyycclliicc llooaaddiinngg

Fatigue has not been positively identified to date in masonry arch bridges. In fact,research undertaken in the 1940s by Pippard on the effect of cyclic loading on masonryarch bridges indicated that although the load to produce the first cracking wassignificantly reduced by repeated loading, the final collapse load was hardly altered(Pippard, 1948). These observations could be explained by the low stresses developingin masonry arch structures until very advanced stages of failure and the limitedinfluence of the strength of materials on capacity of masonry arch bridges. More recentresearch at Nottingham University on the effects of repeated loading (Peaston andChoo, 1997) found the scatter of the results too big to be conclusive. Both Roberts et al(2004) and Melbourne et al (2004) have independently undertaken laboratoryexperiments that indicate an endurance limit of about 50 per cent of the static loadstrength.

However, the experience of some maintenance engineers with regards to the veryheavy traffic load increases experienced in the last 30 to 40 years suggests that therepeated application of heavy loads could accelerate the deterioration of masonry archbridges. Moreover, the recent nature of these increases, in terms of the fatigueresponse, suggests that the full potential fatigue consequences of the loads currentlyimposed upon loading the masonry arch bridge stock have not manifested themselvesin any structure.

For highway structures, the current assessment code simply indicates that it “will beprudent” to limit the regularly applied loads to half the ultimate failure load.

In terms of the dynamic response of masonry arch bridges to constant frequencyloading, experience suggests that their high mass and damping is sufficient to preventsignificant accelerations or dynamic amplification of the displacements.

CIRIA C65662

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22..44..66 PPeerrffoorrmmaannccee ooff hhiigghhwwaayy bbrriiddggee ppaarraappeettss

Research conducted in the 1990s indicated that masonry parapets which were notreinforced could often contain errant cars; further advice on this is provided in BS 6779-4:1999. However, since the research work underpinning this standard wasprimarily focused on measuring the ability of a parapet to contain a car, otherimportant issues such as determination of the amount of loose masonry, if any, whichmight be ejected following an impact were largely overlooked, and in some cases thiscan pose a serious hazard.

More recent research has indicated that provision of retrofitted drilled-in diagonalreinforcement can significantly reduce the likelihood of loose masonry being ejected.Conversely, provision of bed-joint reinforcement was found to be either largelyineffective or even counterproductive. Alternatively rock-fall type netting could beprovided to catch any ejected masonry.

It should be appreciated that anchoring an existing parapet to an underlying spandrelwall can be problematic since the affected area following a major vehicle impact mightthen be greater. Conversely, if retro-fit reinforcement is provided only in the parapet,overturning failure of the whole parapet, acting as a single monolithic entity, should beguarded against (eg by fixing the parapet ends to underlying material beyond the archspringings, ideally using anchors incorporating an energy absorbing element).

Additionally, even if retro-fit reinforcement is installed, existing masonry parapets areunlikely to be able to contain heavy goods vehicles. Where this is required replacementof the original parapet should be considered, though an alternative method, oftenmore cost-effective and sympathetic to the original structure, may be to providesecondary elements to divert an errant vehicle before it hits the parapet.

Parapet strengthening measures are considered further in Section 4.3.4 (Table 4.4) andin Section A6, Section 6.5.

22..55 LLoossss ooff bbrriiddggee ppeerrffoorrmmaannccee

A survey of those responsible for the management and maintenance of masonry archbridges in the UK carried out for this guidance, indicates that the most commondefects in these bridges are:

� deteriorating masonry (particularly spalling and loss of mortar)

� spandrel wall movements (bulging and/or detachment from arch barrel)

� cracking and deformation of arch barrels

� movement of piers and abutments

� separation between brick rings in multi-ring arch barrels (delamination)

� parapet damage.

In the same survey, respondents identified the most common causes of bridgedeterioration and loss of performance:

� water percolation from inadequate waterproofing/drainage or leaking services

� loading/overloading

� instability of foundations

CIRIA C656 63

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� growth of vegetation

� vehicle impact

� thermal movements (in large structures).

The relationship between visible defects, their root causes and the mechanisms whichhave brought them about are in some cases relatively straightforward but are oftenmore complex, resulting from a combination of many different factors. When assessingdefects and loss of bridge performance it is important to consider every aspect of thebridge that might cause movement, change in load path or material deterioration. Toapproach these aspects in a systematic manner it is vital to ensure that a correctdiagnosis of the symptoms is made and that the evidence is collected in a systematicand open-minded way.

The performance of a bridge to loading will be influenced by:

� boundary conditions, which relate to the interface between the “hard” elements ofthe structure and its support

� structural condition of the main structural elements of the bridge and their abilityto transfer load to the foundations

� material condition of the bridge fabric.

Each of these conditions is considered in greater detail in the following sections. Themethods of identifying, investigating and monitoring them and determining theireffects are considered further in Chapter 3.

22..55..11 BBoouunnddaarryy ccoonnddiittiioonnss

Boundary conditions relate to the conditions at the interface between the structuralelements of the bridge with its backfill and supports. In the case of the boundarybetween bridge foundations and the ground, the location and nature of that interface isclear, but it is less clear when considering the backfill. The backfill plays a key role inrestraining the barrel and abutments and in spreading the load, and loss of backfillperformance is as crucial to the strength of the bridge as are its foundations andstructural elements.

The potential structural consequences of loss of support to an arch bridge are outlinedin Table 2.5.

CIRIA C65664

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TTaabbllee 22..55 PPootteennttiiaall ccoonnsseeqquueenncceess ooff iinnssttaabbiilliittyy ooff ssuuppppoorrtt oonn ssttrruuccttuurraall ppeerrffoorrmmaannccee

CIRIA C656 65

SSeettttlleemmeenntt CCoonnsseeqquueennccee

Vertical differential settlement betweenadjacent supports.

Springings stay parallel.

Barrel develops three hinges (it may however beable to accommodate movement using twohinges).

If three hinges form and vertical settlementcontinues then this represents a failuremechanism and should be treated immediately.

Horizontal spread of supportBarrel develops three hinges.

If three hinges form and the horizontal movementcontinues then this represents a failuremechanism and should be treated immediately.

Horizontal inward movementBarrel develops three hinges.

If three hinges form and the horizontal movementcontinues then this represents a failuremechanism and should be treated immediately.

Transverse settlement of an abutment or pier:

Rotation

Local differential settlement

Can cause diagonal cracking in the barrel andspandrel wall movement.

Stress redistribution causes stepped cracking inthe abutment; differential settlement along thespringing results in cracking in the barrel.

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The boundary condition effects which can affect the structural performance of a bridgeinclude scour, mining subsidence, differential settlement and backfill integrity.

SSccoouurr

Bridges that span over natural and man-made watercourses may be subject to scour.Scour has been linked to many instances of severe bridge damage and failure, and suchfailures are particularly dangerous since they tend to occur suddenly and without priorwarning or sign of distress to the structure.

There are two issues associated with such scour-induced damage to bridge pierfootings:

� loss of foundation material, which exposes the footing and lowers its factor ofsafety with regard to sliding or lateral deformation. The greatest loss of sedimentoccurs at high water velocities, such as during floods

� pier movement may occur because of material loss beside and beneath the base ofthe footing, which produces a change in stress distribution in the bridge structureand may ultimately result in structural collapse.

Potential consequences for arch bridges subject to scour include (May et al, 2002):

� pier settlement due to loss of support to foundation

� pier tilting

� abutment settlement and/or tilting

� damage due to hydraulic loading, perhaps aggravated by debris accumulation

CIRIA C65666

Combination of settlements

Examples

Aggravates the effects of the individualsettlements.

Bank seat translation and rotation interacts tocause translation of the pier foundation resultingin severe cracking of the barrels and spandrelwalls

Relative settlement and rotation causes diagonalcracking in the barrel.

Arch barrel distortion This can have a serious detrimental effect on theload carrying capacity of the bridge

Translates

Translates

and rotates

rotation and

settlement

diagonal crack

in barrel

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� damage due to sediment abrasion, boulder impact and debris abrasion

� scour hole or washout of embankment behind abutment

� twisting of arch due to differential abutment/pier movement

� loss of intrados/spandrel masonry due to suction/washout

� total or partial collapse.

There are three types of scour:

� natural

� local

� contraction.

Natural scour processes (degradation or aggregation of the channel, lateral channelmigration etc) are associated with long-term erosion and deposition of bed material.They usually result from a series of progressive steps that are dominated by floodevents. It is important that the vulnerability of the watercourse to natural scour bedetermined. Methods for undertaking this are presented in the literature (CIRIA,2002). The best way of reducing the susceptibility of the bridge to scour is to ensuregood flow conditions at the bridge. It is outside the scope of this manual to providedetailed methods for studying river stability. However, it is important to check thegeology of the site and the river morphology, looking for evidence of channel stability(both vertically and horizontally) using historic maps and aerial and satellitephotography. If the prevailing or predicted hydraulic conditions would indicate thatsignificant scour is possible within a return period of 200 years then remedial measuresshould be taken to reduce the risk. These may take the form of major rivermanagement scheme and/or local remediation in the form of streamlining thestructure, river training (including weirs and invert slabs) and deflectors.

Local scour occurs in the vicinity of individual structural elements like piers andabutments as a result of their effect on the flow, causing localised removal of supportingmaterial, which can happen with relative rapidity in the right conditions. It is one ofthe major causes of bridge failure and collapse. The most critical factors contributing tolocal scour are the velocity and depth of flow, both of which are significantly increasedduring heavy storms and floods. As the velocity and/or depth increase, the amount ofscour increases. Other factors affecting bridge scour include the dimensions andorientation of piers, bed configuration and material size/gradation, and accumulation ofice and debris along the piers.

Contraction scour is normally the result of confining the width of the watercourse inthe vicinity of the bridge, and although many have cut-waters – some do not. Theabsence or poor design of a cutwater results in turbulent flow as the water passes underthe bridge creating vortexes that culminates in local scour. In times of flood thesephenomena are aggravated by the reduction in width as the water rises under the arch.

It is important to determine the relative significance of the types of scour in order todesign the most appropriate form of defence.

MMiinniinngg ssuubbssiiddeennccee

The effects of mining subsidence can cause large ground movement. Settlements inexcess of 1 m and ground strains up to one per cent are common (Sowden, 1990). Inthese circumstances, damage is severe and total reconstruction is necessary. It is

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possible, though unlikely, for a small mine working to collapse adjacent to or under theabutment/pier, in which case it may be possible to stabilise the bridge and to return it toservice. Given the age of the masonry arch bridge stock, it is likely that any subsidenceof this nature will already have taken place and will be evident in the condition/geometry of the bridge.

DDiiffffeerreennttiiaall sseettttlleemmeenntt

Masonry arch bridges by their very nature as particulate structures are tolerant ofmovement. Structural integrity is not compromised by a few cracks. As discussed inSection 2.3.1, the historical use of lime mortar adds to the characteristic “plasticity” oftheir behaviour. Many masonry arch bridges were built with shallow foundations. Somecorbelling of the base reduced the bearing stress and in some cases, where it was feltthat the ground was particularly weak, wooden piles would be incorporated into thestructure. However, differential settlement, particularly when it is transverse, can leadto loss of load carrying capacity and local material failure (Figure 2.19).

FFiigguurree 22..1199 RReessuullttss ooff ddiiffffeerreennttiiaall sseettttlleemmeenntt oonn bbrriiddggee ssuuppeerrssttrruuccttuurree

FFiigguurree 22..2200 AArrcchh ssooffffiitt sshhoowwiinngg ccrraacckkiinngg ccaauusseedd bbyy ddiiffffeerreennttiiaall sseettttlleemmeenntt

Differential movement can be caused by the bridge being founded in variable stratawhich results in a different magnitude or rate of settlement between abutments and/orpiers. Additionally, changes in hydrostatic water pressure can cause fines to be washedout of the backfill and founding strata which can lead to changes in volume and hence

CIRIA C65668

Spandrel wall rotation

local pier

settlement

voussoir

slippage

Diagonal

cracking

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led to movement of the structural elements of the bridge. The signs of distress usuallydevelop over a long period of time and so may have been masked by repointing themasonry. In this case it may be evident from irregularities in the masonry beddingplanes, which will give a better assessment of the movement than measuring cracks.The presence of cracks may indicate more recent movement – this should be a causefor concern and should be investigated immediately (particularly if there has beenrecent flooding). In any case, movement of the foundations will affect the carryingcapacity and residual life of the bridge. It is important to determine the extent to whichthe arch barrel is receiving support from that part of the foundation that is suspectedto have settled. It may be that the settlement occurred during construction and was“adjusted for” as the structure was built. If the entire pier or abutment has settleduniformly transversely but there is a longitudinal differential settlement then this willbe detectable by irregularities in the string course and spandrel wall bedding planes. Itshould be noted that it was usual to backfill up to the stringcourse level before addingthe parapet. This should be a good indicator of whether or not the settlement occurredat the time of construction or some time later.

BBaacckkffiillll

The nature and condition of the backfill and carriageway construction or track andballast will have a significant effect on the assessed carrying capacity of the bridge.

As described in Section 2.1.1, the spandrel of the bridge may take several formsincluding the incorporation of longitudinal spandrel walls with cover slabs, internalrelieving arches, open spandrel arches etc. Over time most bridges have experiencedsome modification. For example, cover slabs may have been removed and the voidsbetween the longitudinal spandrel walls filled with graded stone or even concrete.

Any changes in backfill pressures change the equilibrium of the barrel and causemovement of the abutments which will subsequently affect those structural elementsthat depend upon them for their support.

Increases in backfill pressures may be caused by:

� increased surcharge loading resulting from vertical road/rail realignment

� overconsolidation due to an increased loading regime

� expansion of the structure

� changes in the water table (this includes flooding as well as seasonal changes).

Decreases in backfill pressures may be caused by:

� reduced surcharge loading resulting from vertical road/rail realignment

� contraction of the structure

� washout of the backfill

� changes in the water table (as above).

22..55..22 SSttrruuccttuurraall ccoonnddiittiioonn

AAbbuuttmmeennttss aanndd ppiieerrss

The abutments and piers transfer the actions from the arch barrel and walls down tothe foundations. They primarily behave as mass structures with little or no strength in

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tension and relying entirely on compressive strength, mass and geometry to maintainstability. The structural performance of a bridge can be influenced by anything thataffects the equilibrium of these elements. This includes the fill behind and over theabutments and the condition of the material under the foundation, which allowshorizontal resistance to the thrust from the arch (see Table 2.5).

Piers are particularly susceptible to any imbalance between the thrusts from each of theadjacent spans. Additionally, in the case of skew bridges the propensity for the thrust tospan at right angles to the abutments results in the pier of a multi-span bridgeexperiencing significant torsion. Tests have shown that even for piers with aheight/thickness ratio of 3.4, failure in torsion is possible (Melbourne at al, 1997). Oftenthe outer skin was constructed using better quality material than that used to fill thecore – especially in the case of stone piers when random rubble might have been used(see Section 3.10.3). Additionally, due to settlement of the core, the top of the piers arevulnerable, especially as this is subjected to the most stress variability.

In most cases the lateral earth pressures acting on the back of abutments, wing wallsand retaining walls may be assumed to be constant and stable, and at-rest soilconditions can be assumed. However, recent work on the soil-structure interaction ofintegral bridges subjected to diurnal/seasonal thermal movement has demonstrated thatsoil pressures have to be given careful and specific consideration. In the case of single-span arch bridges it is likely that seasonal changes will dominate although this,depending on the bridge dimensions, may not be true for multi-span bridges whichtend to have a smaller thermal mass and consequently are more thermally responsive.The Stockport railway viaduct suffered problems associated with thermal movementsthat came to a head in 1929 resulting in the replacement of one of its 26 semi-circulararches (Morris, 1949).

As discussed in Section 2.5.1, any changes in backfill pressures will cause movement ofthe abutments, to what extent these changes will cause distress is dependent on manyparameters including the relative stiffness of the structural elements, the geometry andthe soil stiffness. These could cause movement of the abutment and/or a significantchange in the stresses in the arch barrel.

AArrcchh bbaarrrreell

Table 2.6 shows actions (loads) on the arch barrel and their consequences.

Longitudinal cracks can occur anywhere within the barrel (Figure 2.21). They reducethe capacity of the arch to distribute the load evenly throughout the arch and onto theabutments or piers. Additionally, if the cracks occur immediately behind the spandrelwall it can isolate the wall and hence reduce its contribution to supporting the arch.

If the longitudinal cracks are limited to the crown region then they are possibly due totransverse bending. On the other hand if the cracks extend down to the skewbacksthen this is usually associated with live loading due to directional flow of the trafficcarried by the bridge. Each half of the arch tries to “sway” in the direction of the trafficflow. This is a result of the incremental permanent deformation due to the lack ofrecovery of the sway and is indicative of inadequate transverse distributional strength.It is usual that such cracks are “live” and consequently will need regular monitoring.Haunch support is restricted to the segments between the cracks. Lateral backfillpressures on the spandrel walls will increase their outward movement becausetransverse continuity has been lost. This will open the longitudinal cracks further withpotential loss of material from the arch.

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FFiigguurree 22..2211 LLoonnggiittuuddiinnaall ccrraacckkiinngg iinn bbrriiddggee ssuuppeerrssttrruuccttuurree

Transverse cracks are frequently reported; their significance in respect to the loss ofbridge performance depends upon a number of factors.

If the cracks are very longstanding and shown no sign of recent movement, then it islikely that they formed at the time of construction. They will have been caused by theredistribution of stress during construction as the centring was removed and thebackfill installed (historical cracking will often be lost as a consequence of anyrepointing operations). Sometimes bridges are subjected to major reconstruction,widening etc that result in further redistribution of stress and, potentially, cracking ofthe structure. If all movement has ceased, such cracking can be treated as benignprovided that the presence of it is taken into account when idealising the bridge forstructural assessment.

Recent transverse cracks (Figure 2.22) present a more immediate concern and shouldbe dealt with expeditiously after determining their cause however there may not be asingle cause. The position and extent of the cracking assists in a diagnosis. Crackslocated in the quarter-span region of single-span bridges are usually associated withbarrels that are forming hinges. Multi-span arch bridges form mechanisms bydeveloping hinges at the crown of the arches and so for this type of bridge transversecracking at the crown indicates potential hinge formation.

FFiigguurree 22..2222 TTrraannssvveerrssee ccrraacckkiinngg iinn tthhee aarrcchh bbaarrrreell

CIRIA C656 71

Longitudinal

cracking

Spandrel

buldging

Spandrel wall rotation

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Cracking in the crown region of a single span bridge may be associated with apunching type failure mechanism and may be accompanied by the formation of a“yield-line” type failure. This could be caused by the reduction in cover to the extradosat the crown, by increases in the loading regime, by spreading of the abutments etc.Each induces tensile stresses in the crown intrados that may result in cracking.

Diagonal cracking is invariably caused by non-uniform settlement/spread of theabutments which results in torsion being induced in the arch barrel.

In some cases, individual masonry units of the arch barrel can become displaced,indicating a local failure; this may be the result of point-loading or more frequentlybecause the mortar around it has severely deteriorated and been washed out (seeFigure 2.23).

FFiigguurree 22..2233 DDiissppllaacceedd ccrroowwnn vvoouussssooiirr iinn aarrcchh ssooffffiitt

Based upon current understanding, it may be expected that the pattern of cracks inskew bridges may be different to those observed in square arches. This will be afunction of the relative stiffness of the arch and its supports. The less stiff the supportsthe more likely that a four hinge mechanism will be critical. If the supports are morerigid then five or more hinges will be required to create sufficient releases in the archto form a mechanism. It is also possible that the release resulting from crack formationmay change/influence the arch response to further movement – so determining therelative sequence of crack formation, in chronological order, is important when tryingto interpret behaviour and predict future behaviour and repair strategy.

CIRIA C65672

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FFiigguurree 22..2244 PPllaann ooff ttyyppiiccaall ccrraacckk ppaatttteerrnn iinn aa sskkeewweedd bbaarrrreell

Arch barrel distortion due to long-term movements should be carefully monitored as,particularly, flat spots can significantly reduce carrying capacity.

Temperature effects have traditionally been given a low priority but they can besignificant in long span bridges and viaducts and are worthy of consideration in suchstructures.

SSppaannddrreell aanndd wwiinngg wwaallllss

The movement of spandrel walls may take the form of tilting, bulging or sliding overthe extrados or, in the case of longitudinal cracking immediately behind the spandrelwall, outward movement taking the cracked arch barrel with it (Figures 2.25 and 2.26).

CIRIA C656 73

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FFiigguurree 22..2255 SSppaannddrreell wwaallll ddeeffeeccttss

FFiigguurree 22..2266 CCrraacckkiinngg iinn aarrcchh bbaarrrreell iinnttrraaddooss iinnddiiccaattiinngg ssppaannddrreell wwaallll ddeettaacchhmmeenntt

CIRIA C65674

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There are several possible reasons for these movements. The walls may have beeninadequately designed in the first place, which should not be surprising given therudimentary understanding of soil mechanics which existed at the time of theirconstruction. In larger bridges the covering slabs which originally spanned betweeninternal spandrel walls, and protected spandrel walls from backfill pressures may havebeen removed and the voids filled, contributing to the movement of spandrel walls.However, in some cases, extensive haunching to the arch barrel over the pier oradjacent to the abutments reduces the effective height of the spandrel walls andsubsequently the soil pressure.

Vertical live loading will surcharge the fill resulting in lateral loading on the spandrelwall. Over the years, the volume and weight of the loading has increased. Additionally,carriageways have been widened and realigned or track realigned. This can result intraffic running immediately behind the wall – increasing the soil pressures on the walland aggravating distortion. Centrifugal forces transmitted through the fill have alsoincreased as traffic speeds have increased.

As most arch bridges have no waterproofing, if the backfill is not free-draining, then itis usually saturated. Freeze-thaw cycles may cause incremental permanent movementwhich, if left unchecked or uncorrected, will cause collapse. Each cycle will allowmigration of fines during thawing and these will “set” in the gaps created by theexpansion of the freezing water. Consequently, it is important to maintain any drainagesystem that exists in the bridge and to seal the carriageway to minimise anyaccumulation of water in the backfill.

Vehicular impact with the parapet will inevitably have an effect on the spandrel wall.Merely repairing the parapet after an accident may cause trouble in the spandrel walllater. The latter should always be checked for movement and rehabilitatedappropriately.

In skew arch bridges it is important to be aware that because the barrel tries to spansquare to the abutments/piers (ie between the obtuse corners) the spandrel walls nolonger offer their full longitudinal stiffness/support to the barrel. Additionally, shouldthe barrel become distorted or “sway” then the spandrel wall may rotate about thebedding planes thus de-stabilising the wall.

PPaarraappeettss

Parapets are expected to restrain vehicles. They are usually adjacent to carriagewaysand consequently in recent years have been subjected to de-icing salts that causedeterioration of the mortar and masonry units. However, the extent of thedeterioration is not tested until the parapet is struck, so it is vital to pay specificattention to the condition of the parapet fabric, including chemical contaminationassessment. It is usual that if the fabric is found to be wanting in any way that someform of reinforced grouted cavity wall construction will be installed with the incumbentproblem of providing an adequate base support.

The impact resistance of masonry parapets which are not reinforced depends on twoprincipal factors. These are the internal forces which are governed by the wall massand geometry and the bond conditions between the mortar and the units and the baseof the parapet.

CIRIA C656 75

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TTaabbllee 22..66 AAccttiioonnss ((llooaaddss)) aanndd ccoonnsseeqquueenncceess ffoorr mmaassoonnrryy aarrcchh bbaarrrreellss

CIRIA C65676

AAccttiioonn ((llooaadd)) CCoonnsseeqquueennccee

Point loading

Individual masonry units pushed through barrel – a local

failure (the unit moves because the mortar around it has

perished and/or been washed out), see Figure 2.23.

Yield-line pattern failure (this should be considered in

conjunction with diagonal cracking due to other causes

such as differential settlement).

Hinge formation and incremental loss of

statical indeterminacy.

In 2D, an arch has three redundancies that require four

releases to create a mechanism.

Cracks may only open when the bridge is loaded and

completely closes up when unloaded.

Additionally, settlement or movement during the

construction may already have introduced releases into

the structure. Although they may have long since been

covered up by routine pointing, they will still be latently

present within the fabric of the bridge. So the

development of one hinge in the quarter-span region

may actually herald the onset of a four hinge

mechanism.

The formation of a four hinge mechanism implies that

the barrel has reached its ultimate limit state.

Shear loading

Radial slippage. Stone voussoir barrels are more

susceptible to this than other forms of construction.

Longitudinal slippage/debonding (ring separation), see

Figure 2.27. Multi-ring brickwork barrels are most

susceptible. Skew barrels are more vulnerable than

square barrels because of the longitudinal shear induced

as a consequence of the kinematic complexities of the

structure.

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FFiigguurree 22..2277 VViissiibbllee sseeppaarraattiioonn ooff tthhee iinnttrraaddooss rriinngg ooff aa ffoouurr--rriinngg tthhiicckk bbrriicckkwwoorrkk aarrcchh,, sshhoowwiinngg aa ffaaiilleeddaatttteemmpptt aatt rreeppooiinnttiinngg ooff tthhee ccrraacckk

22..55..33 MMaatteerriiaall ccoonnddiittiioonn aanndd ddeetteerriioorraattiioonn

Masonry, both brick and stone, if properly selected and used, is capable of providingexceptional durability. However, deteriorative processes are relentless in their actionand begin to incrementally weaken and disintegrate construction materials from theday of their creation and incorporation into the fabric of a structure. Although thisprocess is typically very slow, it becomes significant over long periods of time. Themajority of older bridges, those built in the 19th century and before, show someevidence of materials deterioration. Although the rate of deterioration is stronglyinfluenced by the original quality of materials and construction, it can be greatlyaccelerated by neglect of routine maintenance or changes to the structure and its use,or where the in-service environment is particularly harsh.

CIRIA C656 77

Transverse bending

Transverse bending, lateral pressure on the

spandrel, longitudinal flexing of the barrel

relative to the longitudinally stiff spandrel

wall, or any other combination.

Directional flow of traffic

Longitudinal crack limited to the crown region

Spandrel wall separation with a longitudinal crack in the

barrel adjacent to the spandrel wall.

Longitudinal crack in the barrel that extends beyond the

crown region, possibly over 2/3 of the span.

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Problems occur where:

� the original masonry units are not very durable, and deteriorate along with themortar

� repointing is not carried out as required and the jointing mortar itself begins todeteriorate

� the in-service environmental conditions are particularly aggressive or tend to affectthe jointing mortar or masonry units themselves rather than the pointing mortar

� repairs or alterations are carried out unsympathetically, for example withincompatible materials

� there are adverse changes in structural behaviour, loading or environment.

In such circumstances, the serviceable life of the structural fabric may be shortened,leading to increased maintenance costs and reduced performance. This couldpotentially hasten structural problems and the point at which relatively drastic anddisruptive remedial intervention is required.

The deterioration of stone, brick and mortar is a very complex and wide-ranging topic,and can only be summarised here. It is worth considering that the majority ofdeterioration is related either directly or indirectly to the presence of water and thechemical contaminants it often contains; this underlines the importance of takingmeasures to keep masonry dry, and where this is not possible to allow it to dry anddrain freely. Recent surveys of bridge owners across Europe have shown that the effectof water on the condition of arch bridges is considered to be their major concern. Thiscan be justified in terms of the “external” effects, such as scour which can causecollapse; and the “internal” effects, including the saturation and washout of fines inbackfill and masonry deterioration. Contributory mechanisms for deterioration ofmasonry in all types of structures include:

� moisture saturation

� freeze-thaw cycling

� physical salt attack (“subflorescence”/“cryptoflorescence”)

� sulfate attack (thaumasite conversion, gypsum formation) in mortars

� leaching of mortar

� biological attack

� repair with unsympathetic materials (eg patch repairs with hard bricks or strongmortar)

� expansion and contraction (from thermal and wetting, and drying cycles)

� other physical movements (development of additional stress or change in stressdistribution)

� cyclic loading and fatigue effects.

Current knowledge regarding these mechanisms is reviewed in the following sections,with comments on their relevance to masonry in bridges. These are summarised inTable 2.7.

CIRIA C65678

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TTaabbllee 22..77 SSuummmmaarryy ttaabbllee ooff ffaaccttoorrss ccaauussiinngg mmaassoonnrryy ddeetteerriioorraattiioonn

CIRIA C656 79

DDeetteerriioorraattiioonnmmeecchhaanniissmm

CCoonnsseeqquueenncceess

Freeze-thaw cyclingWhere masonry is persistently wet and exposed to repeated freeze-thawcycles, can cause spalling of masonry units (Figure 2.28) and mortar lossfrom joints.

Physical salt weatheringTransport and precipitation of salts can cause softening, crumbling,flaking, blistering and laminar spalling of mortar and masonry units.

Sulfate attackPrincipally affects the mortar causing its deterioration into a flaky, crumblynon-structural material. It may also attack brick and some types of stonewith similar results.

Leaching

The mortar’s calcium hydroxide and calcium carbonate components areparticularly vulnerable and their loss creates secondary porosity that canweaken materials and in turn aggravates the effects of other agents likefreeze-thaw. Leaching may result in staining and whitish deposits onmasonry surfaces (Figure 2.29).

Vegetation growth

Grasses are not likely to cause physical damage, but tree roots can causeserious damage to the structural fabric of the bridge (Figure 2.30).Smaller organisms that may be found in damp areas of the bridge fabriccan cause deterioration by increasing porosity and facilitating leachingand other mechanisms.

Repair with unsympatheticmaterials

The use of overly-hard mortar can lead to masonry units losing their facesand edges. The use of overly hard masonry units in repairs can damageadjacent original fabric. Use of impermeable materials can increasesaturation and redirect moisture into other components or parts of thestructure, accelerating their deterioration (Figure 2.31). Corrosion offerrous elements can cause spalling of adjacent masonry (Figure 2.32).

Expansion and contraction(thermal, and wetting anddrying cycles)

This can result in internal fracture of the units and spalling, and loss ofmortar from the joints.

Fatigue (loading andenvironmental effects)

Reduction in load carrying capacity. Current experimental results suggestthat masonry has an endurance limit of about 40–50 per cent of itsultimate carrying capacity.

Moisture saturation

Increases the vulnerability of masonry to environmental agents that causedeterioration. The nature and extent of the saturation is a function of thetype and amount of porosity. Movement of moisture can result in washoutof fines from particulate materials eg fill, causing weakening andinstability.

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FFrreeeezzee--tthhaaww ccyycclliinngg

Where spalling and mortar loss occurs in masonry structures, it is frequently attributedto freeze-thaw action. Certainly freezing and thawing is considered to be one of theprincipal and most aggressive causes of masonry deterioration in climates with cold, wetwinters, and is probably the most common cause of spalling in above-ground structuresin such areas (Figure 2.28) although it is likely that in some instances spalling caused byother factors (for example, delamination from physical salt attack or “crushing” whererepointing has been carried out with overly strong mortar) is mistakenly attributed tofreeze-thaw action.

A considerable amount of literature has been published on this subject, and the basicmechanism of freeze-thaw damage is fairly well understood. A comprehensivediscussion of the issues relating to freeze-thaw damage of brickwork, and a review ofpublished literature is available (Stupart, 1989).

FFiigguurree 22..2288 FFrreeeezzee--tthhaaww ssppaalllliinngg aaffffeeccttiinngg iinnddiivviidduuaall ssuusscceeppttiibbllee bbrriicckkss iinn aa bbrriiddggee ppaarraappeett

CChheemmiiccaall aattttaacckk aanndd pphhyyssiiccaall ssaalltt wweeaatthheerriinngg

The physical salt-related deterioration of masonry has been investigated by a number ofresearchers (eg Larsen and Nielsen, 1990). Alternative theories have been suggested asto the precise mechanism involved, which can vary dependent upon the type of salt,the type of substrate and its environment, but there is consensus that the crystallisationof water-soluble salts near to the surface of stone or brick can result in disintegration.Various mechanisms have been proposed to explain this disintegration (eg Neilsen,1988, Shuh et al, 1986) which can be manifest variously as:

� crumbling

� flaking

� blistering

� laminar spalling.

Sources of potentially damaging salts include:

� groundwater

� polluted rainwater and contaminated runoff from the bridge surface

� airborne pollutants such as traffic fumes and sooty deposits from steam-trains

� the bricks, mortar, fill, ballast and other construction materials of the bridge itself.

CIRIA C65680

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Hard bricks and durable stone are less susceptible to chemical attack and physical saltweathering than mortar, but they can be affected where overly hard and impermeablemortar has been used in the original construction or for repointing since this concentratesmoisture movement through the masonry units themselves (see Figure 2.31). Certaintypes of stone in particular limestone, marble and calcareous sandstones, aresusceptible to attack by acids, especially sulphuric acid from acid rain or polluted air;calcium carbonate is attacked by sulphuric acid to form gypsum which forms a skin onthe surface and prevents evaporation, leading to blistering and spalling.

LLeeaacchhiinngg

In mortar, the principal components that are vulnerable to leaching are calciumhydroxide and calcium carbonate from the cement and possibly also from theaggregate. Leaching has a physical effect on the structure of the mortar, with loss ofsoluble components resulting in an increase in permeability. This increase inpermeability is progressive, because it results in greater water flow and furtherleaching. The leached material becomes more susceptible to other mechanisms ofphysical and chemical deterioration such as freeze-thaw cycling and salt attack. The lossof solid mass associated with leaching results in physical loss of strength and adhesion.Mortar that has undergone severe leaching can become weak and friable, and is easilylost from joints by washing-out or compressive extrusion in areas of high stress,resulting in local stress concentrations and loosening masonry units. Typically, as watercontaining leachates passes out of the body of the masonry and over its surfaces, thesedissolved salts are precipitated as unsightly surface staining and deposits which canbuild up into thick mineral deposits over time (Figure 2.29).

As with chemical attack, hard bricks and durable stone are typically less susceptible toleaching unless they are jointed with a hard and impermeable mortar. Damage is mostlikely to occur in weak and porous bricks and limestone, and naturally weaklycemented calcareous sandstones.

CIRIA C656 81

FFiigguurree 22..2299

LLeeaacchhaattee ddeeppoossiittss ooff ccaallcciiuummccaarrbboonnaattee sshhoowwiinngg wwaatteerr iinnggrreesssstthhrroouugghh mmaassoonnrryy jjooiinnttss

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Voussoirs of certain types of sedimentary stone can experience leaching of their naturalcementing material, leaving the cores of the individual voussoirs with little strength.The outer skin of the voussoir can appear, on external inspection, to be in goodcondition and it is only by coring that this defect can be detected.

VVeeggeettaattiioonn aanndd ootthheerr ffoorrmmss ooff bbiioollooggiiccaall aattttaacckk

Vegetation can cause significant physical disruption. Grasses, weeds and small flowersare unlikely to cause any loss of carrying capacity, but shrubs and tree roots can (Figure2.30). Creeping plants can disrupt the mortar, inhibit evaporation and hinder properinspection thus potentially concealing incipient defects.

Smaller living organisms such as bacteria, fungi, algae, mosses and liverworts cancolonise masonry surfaces, particularly where they are damp or wet, and may be anagent of deterioration of brick, stone and mortar. The effect of such organisms can bephysical (eg by osmotic pressure and leverage of roots) or chemical (eg by theproduction of organic acids which can dissolve carbonates). Certain bacteria are capableof utilising sulfates from groundwater or even concentrating them from theatmosphere, and forming gypsum and sulphuric acid which may accelerate the decayof masonry. Others have been shown to have the ability to dissolve silica and silicates.Biodeterioration involves a complex interaction of a number of chemical, physical andbiological processes.

FFiigguurree 22..3300 VVeeggeettaattiioonn ggrroowwiinngg tthhrroouugghh aanndd ddaammaaggiinngg ppaarraappeett aanndd ssppaannddrreell wwaallll

RReeppaaiirr wwiitthh uunnssyymmppaatthheettiicc mmaatteerriiaallss

Old masonry bridges were typically constructed using weakly or moderately hydrauliclime mortars for bedding and pointing. The use of strong, impermeable cementmortars for re-pointing and repair of old masonry is a cause of damage anddeterioration. The strong mortar produces hard points which reduce the innateflexibility of old masonry and, as the masonry moves (in response to thermal cycles,changes in moisture content or loading), transfers stresses into the masonry units thiscauses stress concentrations which can “pinch off ” the brick faces and damage theedges of stone blocks. Using impermeable cementitious mortar for repair and re-pointing also promotes moisture movement through the masonry units themselves,which focuses any deterioration on the brick or stone rather than the more easilyrepaired mortar (Figure 2.31).

CIRIA C65682

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Spalling damage may also be caused through the inclusion of ferrous elements such aspins, clamps, dowels and supports which rust and create expansive forces that maydamage adjacent masonry (Figure 2.32).

Deterioration may also be hastened by poor selection and improper juxtaposition ofstone types within a structure, particularly if they have very different physical andchemical characteristics. For instance, when positioned below a limestone string coursethe coarse pores of sandstone can collect leachates of calcite, gypsum and other saltswashed out from the limestone, which can damage and substantially reduce thesandstone’s durability.

FFiigguurree 22..3311 RReeppooiinnttiinngg wwiitthh aann iimmppeerrmmeeaabbllee PPoorrttllaanndd--cceemmeenntt bbaasseedd mmoorrttaarr hhaass ccoonncceennttrraatteeddddeetteerriioorraattiioonn iinn tthhee ssoofftteerr bbrriicckk uunniittss,, lleeaavviinngg tthhee cceemmeenntt ssttaannddiinngg pprroouudd

FFiigguurree 22..3322 SSttoonnee ccooppiinnggss ddaammaaggeedd bbyy ccoorrrroossiioonn ooff aann eemmbbeeddddeedd iirroonn ccllaammpp

CIRIA C656 83

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The importance of selecting and using compatible materials for repairs to old masonryis discussed further in Section 4.3.3 on the repair of deteriorating masonry.

EExxppaannssiioonn aanndd ccoonnttrraaccttiioonn

Bricks undergo a progressive and permanent dilation/expansion on removal from thekiln and exposure to water vapour, which is influenced by their degree of firing andporosity, and their calcitic content. The degree of expansion is greater in poorly firedbricks. Additionally there is a reversible element of dilation when the brick is wettedand dried. Although the amount of dilation is very small, recurrent cycles of expansionand contraction, resulting from wetting/drying and warming/cooling effects, can cause agradual softening of brick. Similar mechanisms can operate in stone, with constituentminerals taking in moisture and undergoing dimensional changes.

In thick sections of brick and stone masonry, moisture and temperature gradients mayoccur which could give rise to differential expansion and contraction across the section,with resultant differential stress distribution.

The outer skin of brick exposed at the element surface is likely to have differentproperties to the bulk masonry, through its original firing and subsequent weatheringin-service. It may respond differently to dimensional changes caused by moisturemovement and/or thermal variations. As discussed previously, the relative flexibility oflime rich mortars allows brickwork to absorb movements and this may have a beneficialinfluence on the capability of masonry to accommodate expansion and contractionwithout damage.

FFaattiigguuee

The effects of fatigue from high cyclic loading on masonry structures is not greatlyunderstood at the present time, but the results of available research suggest that undersuch loading masonry has a fatigue strength significantly less than its ultimate carryingcapacity, discussed further in Section 2.4.1.

MMooiissttuurree ssaattuurraattiioonn

Moisture is a necessary ingredient for most mechanisms of masonry deteriorationdiscussed previously in this section. It may also affect the strength and modulus ofmasonry and its resistance to fatigue loading (see Section 2.4.1). Pressures arising fromsaturated fill, particularly when it freezes, can cause damage to spandrel walls, andsaturated ground can lead to instability and movement of abutments and wingwalls.

These undesirable effects of saturation indicate the importance of keeping masonryarch bridges dry and their drainage systems in good condition, and this should be oneof the main priorities of any maintenance programme, as discussed in Section 4.3.2.

CIRIA C65684

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33 BBrriiddggee mmaannaaggeemmeenntt aanndd ccoonnddiittiioonnaapppprraaiissaall

33..11 TThhee nneeeedd ffoorr mmaassoonnrryy aarrcchh bbrriiddggee mmaannaaggeemmeenntt

Masonry arch bridges form the majority of the UK bridge stock. Historically, they haveperformed well in service since their construction, for at least 100 years and oftenconsiderably more, with relatively little in the way of maintenance, repair andalteration. However, the gradual deterioration of materials over time, coupled with theincrease in loading from modern road and rail vehicles, make maintenance and repairsinevitable in order to ensure that safety, performance and serviceability are sustained atan acceptable level. In the future, it seems reasonable to expect further demands willbe made on existing infrastructure.

There are several features and characteristics of masonry arch bridges which requirespecial consideration in bridge management:

� they are among the oldest elements of the transport infrastructure and have specialmaintenance and repair needs which may differ from those of “modern” structures

� they are often very individual in their character, behaviour and maintenance needs

� typically they lack information regarding their design, construction and internalstructure

� their performance and structural capacity is not as well understood as it is withstructures designed to modern design codes, and may be hard to assess

� the effectiveness of repairs and alterations and their likely influence on the long-term performance and maintenance of the structure are not thoroughlyunderstood

� it is not feasible to carry out widespread renewal or replacement, so maintainingfuture serviceability is vital to the operation of all major transport networks

� they may have historic importance which necessitates a specific approach to theirmanagement and conservation.

In addition to satisfying the fundamental requirements of preventing failure andmaintaining safety, it is necessary to ensure continued serviceability and prevent loss ofperformance of masonry arch bridges through good practice in bridge managementand maintenance, ie good asset management.

CIRIA C656 85

Even if it were desirable, it is not feasible to carry out wholesale replacement of masonryarch bridges that do not comply with current requirements. Neither the economic, materialnor construction resources are available to undertake such a task. With over 40 000masonry arch bridges in service in the UK at the present time, even at a rate of replacementof one per week this would take almost 1000 years. The development and implementationof effective and sustainable methods for bridge maintenance, repair, strengthening andrefurbishment are vitally important to the future wellbeing of the UK’s transportinfrastructure.

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Asset management is a specialist field in its own right, and this book does not attemptto cover it in detail other than to comment on some of the principal considerations ofasset management where they relate to bridges in general, and to masonry arch bridgesin particular, and to guide the reader to other references where a wider or moredetailed appreciation of the subject is required.

33..22 CCoonnsseeqquueenncceess ooff lloossss ooff ppeerrffoorrmmaannccee

33..22..11 SSaaffeettyy iinn ooppeerraattiioonn

The principal responsibility of owners and operators is to ensure that arch bridges aremaintained in a condition such that the safety of users and the public is notcompromised. Factors such as age, increased traffic loading, inadequate or poormaintenance and deferred repairs reduce the performance of an arch bridge and mayultimately compromise operational safety. When a bridge is experiencing serviceabilityproblems it is often necessary to impose temporary restrictions on speed or loading, orclose traffic lanes to maintain operational safety. When its structural capacity isinadequate and there is a risk of collapse, public safety is jeopardised and completeclosure of the bridge may be necessary.

33..22..22 SSyynneerrggyy wwiitthh ootthheerr aasssseettss aanndd iinnffrraassttrruuccttuurree

Bridges are integral to the operation of the road, rail and waterways transportinfrastructure and loss of performance of a single bridge can impact detrimentally onthe performance of one or a number of roads, rail routes or waterways. Utilitycompanies may use bridges for their services (eg water, electricity andtelecommunications), and loss of bridge condition may damage these assets. Placingincreased strain on other transport routes and infrastructure elements may hasten theirdeterioration and require increased expenditure on their maintenance, repair andrenewal. These hidden costs may add to the eventual total cost of remediation of thebridge, particularly where necessary repairs have been deferred or delayed.

33..22..33 DDiissrruuppttiioonn aanndd ccuussttoommeerr ddiissssaattiissffaaccttiioonn

Where loss of condition leads to traffic disruption, speed restriction, delay, closure ordeterioration of ride quality, customers will inevitably express dissatisfaction. This isparticularly so with unplanned closures. While it is impossible to eliminate unplannedclosures completely planned bridge maintenance, strengthening and renewal strategiessignificantly contribute to reducing them. Increasing demand to improve publictransportation by government regulators will place greater pressures (and possiblypenalties for failure to comply with operational performance targets) on infrastructureowners to ensure the smooth operation of services. As well as the negative user-perceptions associated with disruption and delays, where road traffic is subject torestrictions and diversions there is normally some increase in user-risk, as accident ratesare often higher in these situations.

CIRIA C65686

In the past a reactive approach to infrastructure management has frequently prevailed butthis is disruptive, inefficient and uneconomic, and inconsistent with long-term transportationobjectives. There is increasing pressure to adopt a more proactive approach, which favourscontinued and undisrupted asset performance in the long-term and the development of asustainable transport network. This requires the implementation of a reliable system ofinspection, assessment, maintenance and repair so that existing bridges can be kept in goodcondition and their capacity fully utilised, minimising unnecessary and expensive unplannedworks, reducing the environmental impact of bridgeworks, closures and diversions andavoiding increased repair costs.

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33..22..44 CCoossttss ooff ffaaiilluurree aanndd rreeppaaiirr

When repair becomes necessary significant costs may be incurred, particularly wherethe repairs are unplanned. These costs may extend beyond the direct costs ofemploying consultants and contractors for repair works to the provision of access,temporary speed restrictions, line and lane closures and reduction in revenue. Thehigh cost of unplanned repairs, loss of serviceability, additional disruption justify a pro-active, rather than a reactive, approach to the management of bridge assets oneconomic and budgetary terms alone. Many businesses are to some degree dependentupon the efficiency and reliability of the transport system, for the efficient movement ofpeople, goods and raw materials, so disruption and restrictions may have a hiddeneconomic cost, particularly on a local scale.

33..33 GGeenneerraall pprriinncciipplleess ooff aasssseett mmaannaaggeemmeenntt

The basic principles of bridge management are essentially common with that of otherelements of the infrastructure and apply at all levels, from the formulation of nationaland regional transport policy to the management of individual structures. However,their means of practical application and method of achievement may differ.

Typical high level performance requirements include:

� safety and reliability

� operational efficiency

� satisfying statutory and regulatory obligations

� value for money and business improvement

� minimising environmental impact

� preserving historical value

� satisfying customer and employee expectations and perceptions.

Additionally, benefits of an effective asset management system include:

� converting owner policy and objectives into appropriate actions

� assisting with the prioritisation of expenditure at regional and national level

� providing comparative analysis between regional and nationwide assets

� supporting submissions to financial sponsors for funding maintenance works

� allowing progress against strategic and financial targets to be monitored andreported

� providing information on the serviceability and improvement or deterioration ofthe asset

� helping to quantify and mitigate risks associated with loss of performance andfailure

� allowing efficient resource planning by identifying immediate and futureinvestment requirements.

Figure 3.1 illustrates the asset management cycle, which follows a continuous process ofinspection, assessment and improvement or repair, leading to a continuous awarenessof asset quality and achieving a steady state, or improvement. An integral part of thisprocess is the management of asset data and the provision of information links betweeninspections and planning.

CIRIA C656 87

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FFiigguurree 33..11 TThhee aasssseett mmaannaaggeemmeenntt ccyyccllee ((bbaasseedd oonn PPeerrrryy eett aall,, 22000033))

For further information on management concepts and principles for bridges and forphysical infrastructure in general the reader is referred to the following publications:

� International infrastructure management manual, UK Edition (Institute of AssetManagement, 2003).

� PA 55-1 Asset management: Part 1: Specification for the optimised management of physicalinfrastructure assets, and PA 55-2 Asset management: Part 2: Guidelines for the applicationof PA 55-1 (The Institute of Asset Management and BSI, 2004).

� The code of practice for the management of highway structures (DfT, 2005).

� Bridge management systems: Extended review of existing systems and outline framework for aEuropean system (Godart and Vassie, 1999).

� The output of the BRIME (BRIdge Management in Europe) project, undertakenby the European Commission, concerning effective bridge management (moreinformation at <http://www.trl.co.uk/brime/index.htm)>.

CIRIA C65688

Operation

Inspection

Strategic level risk assessment/business case

Assessment

Tactical level risk assessment/business case

Remedial treatment/preventative measures/monitoring or increased inspection

Renewal

RReess

eeaarrcc

hh aann

dd ddee

vveelloo

ppmmeenn

tt

Maintenance

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33..33..11 SSttrraatteeggiicc,, ttaaccttiiccaall aanndd ooppeerraattiioonnaall mmaannaaggeemmeenntt

Asset management comprises a hierarchy of three main levels of organisation. Theseare:

Tactical management objectives should align with higher-level strategic goals, andsuitable performance indicators be identified. Wherever possible, measurable (preferablyquantifiable) targets should be set and a suitable time-frame specified for achievement.For instance, a typical tactical objective might be to reduce a bridge maintenancebacklog from £2 m to £1 m over a five year period, or to reduce the number of bridgeson a particular route with weight or speed restrictions from ten to five over a three-year period. It is then the function of management at the operational level to developand implement plans for achieving these objectives. It is important that tacticalobjectives are achievable and supported by adequate physical and financial resources.

Organisations have their own frameworks and procedures for each level ofmanagement to support their policies and strategic aims. The infrastructure-wide levelof strategic planning is above the scope of this document, which is principallyconcerned with operational management and provides some additional generalguidance and recommendations relevant to tactical management.

Further details on strategic, tactical and operational management and planning forbridges and other structures are included in the Code of practice for the management ofhighway structures (DfT, 2005).

33..44 MMaannaaggiinngg bbrriiddggee mmaaiinntteennaannccee

The maintenance of a bridge can be defined as all the operations necessary to maintainit in a serviceable condition until the end of its life. These include:

CIRIA C656 89

Strategic management: The highest level of infrastructure management, by which the overalldirection of the transport infrastructure is steered. Organisational policies are agreed,strategic long-term objectives are identified, and suitable high-level performance-basedtargets are defined.

Tactical management: High-level objectives defined by strategic management are translatedinto specific management plans for individual asset types. The performance of theinfrastructure is measured, its requirements are assessed and resources allocated toimprove or maintain it in order to achieve the strategic level performance targets.

Operational management: Translation of tactical level strategies into specific plans formaintenance related activities such as inspection, assessment and works on structures byappropriate direction of available resources. Maintenance requirements are determined andprioritised, detailed work plans are devised and resources directed as necessary forimplementation.

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� inspections

� investigations (testing and monitoring)

� structural assessments

� routine maintenance (minor works to maintain efficient functioning and preservecondition)

� essential maintenance (structural repairs and rehabilitation)

� emergency actions (eg in response to unforeseen incidents).

A suitable bridge maintenance planning process includes a number of elements andstages:

� compile and maintain a bridge inventory and database

� carry out periodic condition appraisal of bridge stock

� identify maintenance needs

� assess and prioritise maintenance needs (value management)

� develop optimal solutions for prioritised maintenance needs (value engineering)

� consider resource availability and prepare forward work plans and schedules

� monitor and improve the management process through continual feedback.

This approach can be used to ensure that safety, performance and business objectivesare met, determine the resources required and make best use of available resourcesthrough sustainable maintenance work plans. If such a system is properly devised, fullyimplemented and adequately resourced it has the potential to provide enhanced assetperformance and return on maintenance investment.

33..44..11 AApppprraaiissaall ooff ccoonnddiittiioonn aanndd mmaaiinntteennaannccee nneeeeddss

Condition appraisal is the process used to ascertain maintenance requirements bygathering and periodically updating information relating to the performance andcondition of bridge stock. Each of the main infrastructure owners has its own internalprocedures and systems for determining the maintenance needs of its bridges, but theyare all based on a similar principle (DfT, 2005):

� confirming basic bridge data for the bridge inventory

� establishing and recording the condition of the structure

� detecting defects and evidence of deterioration

� detecting changes in bridge condition, loading or environment since the previousinspection

� providing information necessary for assessing serviceability and structural capacity

� identifying causes of deterioration, determining their extent and estimating theirrate

� diagnosing the causes of defects and deterioration

� evaluating the need, urgency and most appropriate course of action for routineand essential maintenance.

The information produced in the course of condition appraisal must be collated andrecorded in a suitable format, incorporated in the bridge inventory, used to highlightany changes in the bridge condition and determine its serviceability and level of

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performance against specified performance requirements. This information forms thebasis for assessing the bridge’s needs and determining appropriate managementactions, for instance:

� adequacy of existing routine maintenance regime

� additional routine maintenance requirements

� changes in frequency of inspections

� requirements for additional inspections and their objectives

� need for assessment of load capacity

� essential maintenance requirements

� requirements for safety measures (restrictions of traffic and usage, regularmonitoring).

The interval between inspections is related to the importance of the structure and theperceived degree of risk associated with it (and in accordance with minimumrequirements set out in national policy and the asset owner’s own standards).Additionally, where a special need is identified, or at some regular time interval,appraisal by inspection is supported by the structural assessment which uses the basicbridge data gathered during an inspection, and where necessary may be supplementedby more detailed information from investigations, involving a variety of testing andmonitoring techniques. These data can be used to assess the structural capacity of thebridge, the nature and cause of any defects, their extent and potentially the rate ofdeterioration. The results of such assessments are used to inform the managementstrategy for the bridge, allow comparison of its current performance againstrequirements, and to determine its need for maintenance, remedial or strengtheningworks.

In addition to routine/planned inspections, certain observations and incidents trigger aprescribed reaction, particularly those where there is may be a risk to bridge or publicsafety. For instance, the necessity to carry out an additional inspection in response to avehicle strike on an arch barrel, flooding of a watercourse which might affect bridgestability, the passage of an abnormally high load, or the requirement to carry out areassessment of load capacity in response to an increase in permissible loading. Themaintenance needs of the bridge are evaluated and assessed by the bridge engineer.

When the assessment (see Section 3.10) has demonstrated adequate structural capacityof a bridge to carry imposed traffic loads, then reassessment may be required only inresponse to observed changes in the structure and its materials (eg deterioration andevidence of structural distress) or incidents which may have reduced the bridge’scapacity (eg vehicle-strikes). A typical management process for an individual bridge isillustrated in Figure 3.2.

Where intervention is required, this involves implementation of routine works (minorand often cyclic maintenance tasks to repair defects and slow future deterioration egclearing vegetation, repointing masonry) and essential works (bridge rehabilitation egmajor structural repairs and strengthening). The range of maintenance, repair andstrengthening works, as well as their selection and implementation, is discussed inSection 4.3.

When maintenance resources are limited it is sometimes the case that routine works areneglected or given a lower priority than they deserve and this can becounterproductive in the longer term. What began as minor maintenance issues candevelop into serious problems if not dealt with at an early stage, often with significant

CIRIA C656 91

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repercussions for bridge serviceability in the interim period and the eventual cost anddisruption associated with rectifying problems that were avoidable in the first place.Asset managers should establish a proactive regime of preventative maintenance for allmasonry arch bridges and maintenance and repair programmes should deal with thecauses, and not just the effects, of deterioration. Advice on routine and preventativemaintenance is given in Section 4.3.2.

FFiigguurree 33..22 PPrroocceessss ooff rroouuttiinnee mmaaiinntteennaannccee mmaannaaggeemmeenntt ffoorr aa bbrriiddggee

CIRIA C65692

Monitor and record theeffectiveness of works

carried out over time; addto bridge database

Plan works, assembleresources, execute works

and record details inbridge database

Monitor bridge conditionand reasses situation atan appropriate frequency

Defer maintenance, imposeinterim safety measures

(eg weight or speedrestriction) if necessary

Obtain and collate allavailable current and

historic data on the bridge

Carry out regular routineinspections record detailedand objective observations

Can work bejustified and resourced

at presenttime?

Compare latest inspectionrecords with those ofprevious inspections

Evidenceof progressive

deterioration ordistress?

Investigate, identify likelycause, assess significance

and requirement forpreventative or corrective

action

Arepreventive or

corrective actionsrequired?

Consider available optionsin the light of policy and

available resources

YYEESS

YYEESS

YYEESS

NNOO

NNOO

Provide feedback torelevant personnel andidentify any areas forfuture improvement

NNOO

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33..44..22 BBrriiddggee mmaannaaggeemmeenntt ssyysstteemmss aanndd ddaattaa

A bridge management system (BMS) comprises a framework which allows efficientorganisation of bridge maintenance, including activities such as informationmanagement, condition appraisal and maintenance and repair planning, and which canbe used to inform, guide and support management decisions. A BMS storesinformation on individual bridges, such as inspection and assessment results, which itcan use to carry out a variety of engineering and economic appraisals.

A BMS can be a powerful tool for owners, providing assistance with implementingorganisational policy, adhering to statutory requirements, making, recording andjustifying management decisions, determining the best use of limited resources, andformulating and presenting business cases for obtaining funding.

The level of complexity and sophistication required of such a system will depend onthe size and character of the bridge stock. Historically bridge management has reliedupon written records, including card indexing systems and files containing paper-basedinformation on individual bridges. Although such systems may still be adequate for themanagement of very small numbers of bridges and limited works, the demands ofmanaging large numbers of structures and dealing with an ever increasing quantity ofinformation mean that computer-based management systems, relying on electronicinformation storage and retrieval, offer definite advantages.

The various types of data collected are collated in a database, where they are stored ina useable and easily accessible format. Typical data included are (after Godart andVassie, 1999):

� unique bridge identifiers (name, number)

� location data (map reference, road/route details, obstacle crossed)

� bridge owner and maintaining agent

� bridge type and main elements

� bridge dimensions and geometric data

� form of construction

� bridge materials

� year built

� traffic and loading data

� performance data (eg load or speed restrictions)

� history and results of inspections, investigations and structural assessments

� history of maintenance, repairs, strengthening and other works

� schedules for planned inspections, investigations and appraisals

� schedules for planned maintenance and repairs

� information on services carried by the bridge and relevant contacts

� any statutory designations or restrictions (eg listed status or environmentaldesignations)

� new Roads and Street Acts designations

� other information (eg incidents such as bridge-strikes, traffic accidents).

These data form the heart of the management system, and can be manipulated andinterrogated to provide specific information required for the analysis of individual

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bridges or for all or part of the total bridge stock. More advanced systems may offeranalytical functions beyond that of mere record-keeping, which may include:

� scheduling inspections and prioritising appraisals

� assisting with risk assessments

� generating prioritised lists of maintenance and repair works

� allowing comparative evaluation of management strategies

� assisting with programming and planning of works

� calculating costs, assisting with budget control and forecasting

� assessing the overall “health” of the bridge stock and identifying trends incondition and performance over time

� assisting with responses to incidents (eg accidents and emergencies).

Even the most sophisticated management systems are reliant upon the quality andreliability of the data with which they are fed. Inadequate or inaccurate data can lead topoor management decisions, whereas good quality data allows more effective andefficient management of the bridge stock.

The collection and validation of data is discussed in Section 3.6.

Additional guidance on bridge management systems can be found in Godart andVassie, 1999.

33..44..33 AAvvaaiillaabbiilliittyy ooff rreessoouurrcceess

The most efficient use of limited economic resources can be achieved by adhering tosensible periodic maintenance regimes, and in carrying out repairs as needed on time.Allowing a backlog of maintenance and repair activities to accumulate over time andcarrying out unplanned and reactive interventions represents poor value for moneyand increases disruption to the transport infrastructure. It may also increase essentialcapital expenditure on replacement in the long-term, which is incompatible with asustainable transport policy.

Financial, engineering and material resources are unavoidably constrained by varioussocial, economic and political factors, and a rational approach aimed at directingavailable resources to provide maximum benefit, aligned with existing managementpolicy and objectives, needs to be adopted. The role of asset managers is to understandthe implications of alternative management strategies and make the best and mostbeneficial use of finite resources in achieving their objectives. The optimal assetmanagement strategy may not aim to maintain all parts of the infrastructure inexcellent condition. A recent major international study of transport policy (OECD,2003) concluded that:

Roads and bridges should not be kept in good or excellent condition for their own sake, but for thesake of the users. Thus, there may be instances in which disinvestment, or a lack of rehabilitationor maintenance, may be a wise action, obviously within the legal constraints for safety andenvironment, allowing for redirection of these funds to new facilities where they better satisfy theneeds of users.

CIRIA C65694

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33..44..44 SSttaattuuttoorryy aanndd rreegguullaattoorryy oobblliiggaattiioonnss

Owners and operators of bridges have obligations to maintain assets in a safe conditionto protect employees, persons not in their employment and the environment fromunreasonable or unacceptable risks. These obligations arise from civil and criminal lawand licence conditions, and also from professional and moral responsibilities, and mustbe taken into account in asset management policy and operational procedures. SectionA5 includes a listing of the current key legislation applicable to health and safety (as of2005). It should be noted that legislation is liable to change.

Environmental legislation relating to bridge management is discussed in Section 3.4.9.

33..44..55 PPrraaccttiiccaall ccoonnssttrraaiinnttss

The same aspects that make bridges essential elements of transport routes also makethem potential “bottlenecks” which can have a strong negative influence on theefficiency of local transport systems. Since the purpose of a bridge is to cross obstacles,provision of an alternative route is seldom straightforward and typically involvesdiversion to the nearest accessible bridge or the provision of alternative forms oftransport. This may require increases in journey times and additional congestion andpollution. Because of this, long possessions of bridge structures for maintenance andrepair works are seldom feasible, and even off-peak lane or line closures for shortdurations can be disruptive and result in increased accidents. Traffic control and accesscosts often comprise the majority of the budget required to undertake investigation of abridge, and can add substantially to the cost of any works undertaken. The need toensure continuity of service and minimise disruption are often the overridinginfluences in the selection of maintenance and repair schemes, and planning andprogramming are key elements in the success of any works carried out. The selection ofinspection, investigation, maintenance and repair techniques, equipment, materials andaccess provisions which minimise disruption to normal bridge use are importantconsiderations in bridge management.

33..44..66 PPrriioorriittiissaattiioonn

Resources for bridge maintenance are often limited, and in order to achieve their high-level performance requirements it is important that infrastructure owners andoperators identify those elements of their networks, routes and structural assets that aremost critical to ensuring safety and efficient operation.

Criteria for prioritisation frequently include:

� risk assessment

� condition of bridges

� degree and consequences of substandard performance and failure

� importance of route

� minimisation of maintenance costs

� organisational policy

� environmental considerations (heritage and ecology)

� budgetary constraints.

“Top-level” prioritisation is often considered on a route-by-route basis, the most criticalroutes being given a high priority for management activities. Closures or restrictions

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that are tolerable for a bridge on a lightly trafficked local road, or a little used branchline of a railway or canal, can have much greater consequences on a bridge that formspart of a primary transport route. On a particular route, bridges are often highlycritical structures, and the importance to operations, safety considerations and bridge-related works are frequently allocated as high priority.

Operationally critical bridges can be identified by an assessment of their location withinprimary transport routes, volume of transportation and possible detour options so as toconsider the impact of loss of performance and bridge closure on the networkinfrastructure. For critical bridges, dependent on condition, the frequency of routineexaminations may be increased above that of non-critical bridges and other structures,and maintenance and repair works given a higher priority. This approach requirescareful consideration of the relative risks and benefits involved.

� a risk-based approach (see Section 3.4.7) can be used to assist with identifyinghighly critical structures and with prioritisation and planning to achieve optimumuse of resources; an example of this approach is included in Shetty et al (1996)

� strategies for prioritising bridge maintenance work and the principal factors to beconsidered are discussed in further detail in Bridge management systems: Extendedreview of existing systems and outline framework for a European system (Godart andVassie, 1999).

Where maintenance and repair is deferred, this may have negative impact on thewhole-life cost of maintenance and reduction in bridge performance and serviceability.Deferral of essential maintenance may require the implementation of appropriateprecautions to ensure the continuing safety of the bridge and its users, for instanceweight or speed restrictions, monitoring and special inspections (see Section 3.10.5).

33..44..77 RRiisskk aasssseessssmmeenntt

The purpose of the risk assessment process is to systematically identify significant risks,allowing prioritisation of actions to minimise and manage them. Risk assessmentprocedures can be applied by asset managers to ensure that both performance andsafety objectives are met within a business framework and that funds are justified andallocated in response to safety and business needs.

The need for risk assessment arises principally to satisfy statutory safety obligations(discussed in Section 3.2.1) under which employers have a duty to conduct regular riskassessments. Hazards should be identified and assessed to ensure that safety is notdegraded, and that risks are maintained as low as is reasonably practicable (ALARP). Allof the principal transport infrastructure owners and operators have asset managementprocesses that encompass such risk assessment procedures.

Examples of safety hazards that might be considered, along with possible risk reductionmeasures for existing bridges, are given in Table 3.1.

CIRIA C65696

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TTaabbllee 33..11 SSaaffeettyy hhaazzaarrddss aanndd rriisskk mmiittiiggaattiioonn mmeeaassuurreess ffoorr bbrriiddggeess

The risk assessment process permits a logical analysis of the safety risks; how these canbe mitigated or controlled; and assists with planning efficient use of financial andphysical resources to meet asset management objectives.

For bridges, prioritisation should take into account the stability of the structure, thelikelihood of failure or other hazards occurring, and the consequences to bridge usersand the public in general. Identification of the hazards, their likelihood of occurrenceand consequences requires a technical assessment based on an assessment of bridgecondition and capacity, its usage and environment. The risk is equal to the product oflikelihood and consequences for each hazard. Risk is therefore higher if a bridge is inpoor condition, of inadequate capacity, on a heavily trafficked route, or in anenvironment where it can cause significant problems. Carrying out risk assessments fora number of bridges on a route or transport network provides a relative assessment ofrisk and allows ranking and prioritisation of remedial actions, thereby allowing efficientuse of limited resources for inspection, assessment, maintenance and repair.

CIRIA C656 97

HHaazzaarrdd RRiisskk mmiittiiggaattiioonn mmeeaassuurreess

Accidental impacts(due to road, water or rail vehicles)

� likelihood of impact reduced by considering vehicleusage and adequate clearances, providing appropriatehigh-visibility signage and lane markings for roadvehicles

� consequences reduced by ensuring appropriate vehicle-strike response procedures (reporting and responsesystem, emergency bridge closures and engineeringassessments), by frequent inspection and possiblyreinforcement of vulnerable elements, and by ensuringadequacy of containment systems eg parapets.

Deliberate damage(eg due to vandalism or terrorism)

� risk reduced by protecting and discouraging access tothe structure eg use of high fencing

� regular inspection can identify any necessary repairs tobridge or its protection system.

Scour and flood effects (for bridgesover water).

� risk reduced by inspection and assessment, periodicand in response to flooding events

� mitigation measures include physical anti-scourmeasures.

Foundation movements � risk reduced by regular inspection to detect evidence ofmovement and identify potential influences on localground stability.

Overloading(due to increase in magnitudeand/or frequency of vehicle loading,natural events such as floods orhigh winds, or weakening anddeterioration of structural fabric)

� controlling traffic loading� checking adequacy of structural capacity and upgrading

where necessary� periodic inspection and maintenance� special inspections and engineering assessment in

response to rapid deterioration or other triggers.

Parapet failure(from vehicle impact, or possiblyloading from wind or floodwater)

� assessment of structural adequacy of parapet andupgrading if necessary

� periodic inspection and maintenance of parapetcondition.

Falling debris(eg deteriorated masonry, loosebricks etc or sections of parapet onvehicle impact)

� periodic inspection and maintenance of bridge fabriccondition

� consider potential for falling debris and usage of areabelow, and implement repairs or other protectivemeasures (eg crash-decks or netting) if necessary.

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Minimal extension of the scope of the risk analysis can lead to secondary benefits, suchas identifying the exposure to non-safety related business risks and helping toformulate business cases and overall spending forecasts and plans. For example,comparison of the relative risk (and benefit, through risk reduction) presented bydifferent asset maintenance and repair strategies may assist in determining theoptimum strategic maintenance policy. Socio-economic and environmental risks egeffects on local communities and environmental impact, may also be incorporated intosuch an assessment.

Risk assessment procedures are dealt with in further detail in:

� C592 Infrastructure embankments (CIRIA, 2003) includes further discussion of tacticaland strategic risk management for transport infrastructure.

� Application of QRA in operational safety issues (HSE, 2002) covers the use of regulatoryguidance, risk analysis, cost-benefit analysis and risk reduction measures.

� Quantified risk assessment: its input to decision making (HSE, 1989) includes case studiesfrom the nuclear industry where QRA was used to help judge risks.

33..44..88 WWhhoollee--lliiffee aasssseett ccoossttss

Investment in new development is often perceived as desirable and leading toadditional benefit, in contrast to maintenance expenditure which is perceived asunavoidable but, on the whole, unproductive and to be minimised (OECD, 2003) – thisis a fundamental conceptual error. One of the functions of maintenance is to preserveassets which are the products of prior investment, and so the benefits of maintenanceare identical to those which investment in new development aims to provide in thefuture. Whole-life costing is a method whereby the benefits of expenditure on activitiessuch as maintenance may be clearly demonstrated and the allocation of resourcesjustified.

The whole-life cost of an asset measures, over a number of years, the total cost ofdesigning, constructing, operating, maintaining, repairing and ultimately demolishingit. It provides a rational basis for the decision-making process, the main benefit beingthat it allows the owner of the asset to compare a variety of alternative constructionand/or maintenance schemes and to choose the one that is most economical orappropriate to the current or expected financial position.

Although whole-life costing is a potentially useful tool it does have its limitations,particularly for existing assets which are expected to have very long service lives such asmasonry arch bridges. These limitations should be understood and the process usedwith care to ensure sensible results. In practice it is difficult to set up a reliable modelfor the maintenance and repair of existing masonry arch bridges because the long-termrequirements, the likely frequency of expenditure and an appropriate discount rate aredifficult to estimate. There is a need to consider the particular network and routerequirements, which can dominate the maintenance costs. For example, if railpossessions or highway contraflow traffic management are required, they will distortthe relative merits/costs of strengthening/maintenance methods (or new build overexisting infrastructure). Determining and including such factors with adequateweighting can present problems. There is a risk that whole-life cost models can becomeimmensely complicated, however if they are too simplistic this defeats the whole objectof the exercise and their results may be misleading.

The most appropriate and realistic discount rate to apply is a contentious issue, sincethis has a significant influence on the results, particularly when considering long-term

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assets such as bridges. The current relatively high discount rate (as of 2005)recommended by HM Treasury and applied in the public sector emphasises “upfront”costs and can result in poorly calculated rates of return on maintenance expenditure,which may encourage avoidance of maintenance in favour of the deferred cost ofeventual bridge replacement. The danger is that this could lead to an unfeasibly largerequirement for future bridge replacements, which might be unsustainable in terms ofresources demands and the disruption to transport networks.

For more information:

� Ferry and Flanagan (1991) provide a review of the general applications of whole-life costing

� a more detailed review of whole-life costing for transport infrastructure is includedin the Code of practice for the management of highway structures (DfT, 2005).

33..44..99 RReeccoonnssttrruuccttiioonn aanndd rreeppllaacceemmeenntt

In certain circumstances it may be necessary to rebuild parts of individual masonryarch bridges, or replace them with modern structures. The decision as to whether torepair or replace an existing bridge should balance the cost, remaining serviceable lifeand performance of the existing bridge after maintenance works (which may bedifficult to assess) against the cost, design life and performance of a replacementstructure, and the disruption associated with each option. Where structures havesensibly reached the end of their useful life, it is wasteful and clearly a false economy tocontinually drain resources from maintenance and repair budgets in order to avoid thehigh initial capital outlay of replacement. Where the retention of an existing bridge isdeemed uneconomic or unfeasible it may, for instance, be desirable to preserve itsfunctional elements and where possible integrate these into renewal schemes. In certainsituations, there may be overriding historic, environmental or statutory reasons fordoing so. Guidance on the design and construction of new masonry arch bridges isincluded in Section 4.4.

33..44..1100 BBuussiinneessss ccaassee

Within an infrastructure network there will always be competition for funding of capitaland maintenance projects, within corporate financial constraints. Asset managersshould justify the funding of bridge repair, maintenance and strengtheningprogrammes over other network assets at a strategic level, and the funding ofindividual bridges at a tactical level. Subsequently, a business case is required to justifythe return on investment or funds provided.

The business case for bridge works should reflect the strategic objectives of theinfrastructure owner and the performance requirements of the bridge asset. Anexample of the former would be to set a target figure for the maximum number ofbridges with weight restrictions over the next ten years, or to limit the number ofunplanned line/lane closures on a route to less than one day per year within the nextfive years. The business case needs to demonstrate the anticipated risk relating to thelikelihood of disruption if no action was taken (the “do nothing” option) and use this asa baseline for assessing the relative cost/benefit of risk mitigation works across theroute.

When compiling a business case both safety and business risks need to be assessed.Analysis of these risks helps identify the scale of potential problems, the cost of solvingthem and how quickly they should be resolved. As discussed in Section 3.4.7, there are

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recognised techniques across industry for safety risk assessments, whereas business riskassessment models are undeveloped and can be specific to infrastructure types andstrategic objectives. Assessment tools such as whole-life cost analysis can also be useful indemonstrating the implications of alternative actions and schemes.

The fundamental considerations in developing a business case are:

� current condition and serviceability issues (“fitness for purpose”)

� consequences of failure (eg personal injury/fatality, effect on neighbours, damage toproperty)

� strategic value of route and effects on other infrastructure assets

� capital cost of works, maintenance requirements and residual life of structure afterrepair

� cost of disruption to service (including disruption while awaiting repair)

� environmental impact and improvement.

Further guidance on assessing and presenting information from a variety of sources aspart of a business plan is included in Assessment and decision making for sustainabletransport (OECD, 2004).

33..55 EEnnvviirroonnmmeennttaall mmaannaaggeemmeenntt

Issues such as climate change, groundwater pollution and damage to ecosystems nowdominate the natural environment agenda, while the man-made environment raisesconcerns over issues such as excessive development and loss of cultural and industrialheritage. Infrastructure owners have statutory obligations in respect of theenvironment and these should be reflected within their asset management policy. Inaddition to these statutory requirements there are various other reasons why it is in theinterest of bridge owners to consider the environmental aspects of their bridges:

� users benefit from the contribution of bridges to the surrounding landscape

� bridges offer habitats to species of plant and wildlife, and protected landscape,some of which the owner has a specific duty to protect

� closures, restrictions and works on bridges can have a number of direct andindirect impacts on the environment

� the general public is becoming increasingly aware of the environment and willjudge infrastructure owners on their environmental performance andenhancement.

Some asset owners are already taking steps to satisfy their obligations in these respectsby the formulation of environmental policies and action plans, with a requirement tocarry out environmental audits on infrastructure projects. A good example of theapplication of environmental impact assessment techniques to bridge management isgiven in Steele et al (2003).

Environmental issues associated with carrying out maintenance and repair works onbridges are considered further in Section 4.2.

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33..55..11 BBrriiddggee ccoonnsseerrvvaattiioonn aanndd hheerriittaaggee

Arch bridges are among the oldest existing structures built by man, having beenimportant landmarks and the focus for the movement of people and goods over manygenerations. They have lived through significant environmental, social and economicchanges. As a testament to and record of the technological achievement and skill ofdesigners and craftsmen of past times, they form an essential part of cultural, historicand engineering heritage and the fine forms and aesthetic appeal of many old bridgesmake them integral features of the landscapes that they grace. Bridges may haveconsiderable value beyond their immediate functional purpose, which meritsrecognition and special stewardship by those responsible for their upkeep.

To varying degrees, all old arch bridges have some innate historical significance asrecords of past engineering, technological and cultural achievements and trends.Although some have survived relatively unaltered, either because they still adequatelyserve their original function, they no longer serve any function, or because they havebeen preserved predominantly for historical reasons, many are living examples of ahistory of adaptation, repair, recycling of materials, abandonment, collapse, rebuildingand renovation. Often, these adaptations form an essential part of their character, butsometimes mistakes have been made which have resulted in loss of historical value ofbridges through the removal or damage of historical features or the use of unsympatheticmodern construction techniques and materials. It is the responsibility of bridge ownersand maintainers to recognise this value and minimise such losses in the future.

The management of bridges with historic value requires consideration of a number ofpotentially conflicting objectives. The principal duty of the owner is to ensure the safeand adequate performance of the bridge, and in performing this task it is oftennecessary to carry out works to maintain or improve the structure. Ideally, such worksshould be carried out in a manner that is sensitive to the important heritage features ofthe structure. Optimal rehabilitation strategies should be determined within legislative,financial, technical, operational constraints while considering the requirements forconservation of the bridge’s historic character. Where these conflict, some form ofcompromise is necessary. However, where a bridge’s historic value is recognised bysome form of statutory designation, such as listed building or scheduled monumentstatus, this may have a significant effect on the options available for such a compromise.

HHeerriittaaggee bbooddiieess,, lleeggiissllaattiioonn aanndd hheerriittaaggee ssttaattuuss

The heritage bodies listed below have a general duty to conserve heritage. Works onarch bridges with recognised historic value, and those within certain areas which havespecial environmental protection, require their consultation and co-operation:

� English Heritage

� Historic Scotland

CIRIA C656 101

Many of the bridges to be assessed by this standard are of considerable age and representimportant features of our cultural heritage. Their survival to this day owes a great deal to thecare of past generations. Where remedial or strengthening works are found to be necessary,the proposals should reflect the duty to retain these structures for the benefit of futuregenerations. Early remediation measures, which restore the carrying capacity and extend thelife of these structures, are preferable to urgent reconstruction, as the former prove generallyto be more cost-effective, but also retain the existing character of these structures.

BD21 – The assessment of highway bridges and structures (HA, 2001a)

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� Northern Ireland Environment and Heritage Services

� Cadw (the historic environment agency of the Welsh Assembly Government).

Some masonry arch bridges will be subject to statutory controls under one (or both) oftwo pieces of legislation; they can be scheduled under the Ancient Monuments andArchaeological Areas Act (1979) and/or listed under the Planning (Listed Buildings andConservation Areas) Act 1990 (see Table 3.2). Typically, bridges that remain in serviceare more likely to be listed than scheduled. Where a bridge is both scheduled andlisted, the requirements of scheduling are the controlling ones for statutory purposes.In addition to scheduling and listing, bridges may have protection under a variety ofdesignations of the land on which they are sited, for instance in a conservation area,Site of Special Scientific Interest (SSSI), Special Area for Conservation (SAC) ornational park (see Section 3.5.2).

TTaabbllee 33..22 PPrriinncciippaall ssttaattuuttoorryy ddeessiiggnnaattiioonnss rreellaattiinngg ttoo tthhee ccoonnsseerrvvaattiioonn ooff BBrriittiisshh bbrriiddggeess

Such designations highlight the need for a special approach to the management andconservation of bridge structures, and frequently indicate special statutory protectionand restrictions on any works which might affect them or the surrounding land. Worksthat only affect the settings of listed or scheduled structures do not require scheduledmonument consent or listed building consent, but setting is a material consideration inplanning applications.

Non-statutory planning policy guidance notes (PPGs) are prepared by the governmentto provide guidance to local authorities and others on planning policy and theoperation of the planning system. Those most relevant to the management of masonryarch bridges are:

� PPG15: Planning and the historic environment lays out government policies for theidentification and protection of historic buildings, conservation areas, and otherelements of the historic environment. It explains the role of the planning system intheir protection.

� PPG16: Archaeology and planning sets out the government’s policy on archaeologicalremains on land and how they should be preserved or recorded both in an urbansetting and in the countryside.

Further discussion of legislation concerning historic masonry arch bridges andguidance on issues associated with planning and permissions is included in thepublication Conservation of bridges (Tilly, 2002) and in BS 7913 (BSI, 1998).

CIRIA C656102

SScchheedduulleeddaanncciieennttmmoonnuummeennttss

Any works to or within a scheduled ancient monument and likely to damage thatmonument (including removing, repairing, altering or making additions to it) requireprior consent of the Secretary of State, who is required to consult with theappropriate heritage body before making a decision. Where consent is issued it isfrequently subject to conditions to prevent damage or limit it to agreed levels withappropriate archaeological recording. Unauthorised works are a criminal offence andcan result in stiff penalties.

LLiisstteeddbbuuiillddiinnggss

Listed bridges are subject to planning controls exercised through listed buildingconsent procedures, which are the responsibility of local planning authorities. Thedemolition, alteration or extension of listed structures cannot be undertaken withoutconsent. Where the structure is Grade I or Grade II* the local planning authoritiesare required to notify and consult with the appropriate Heritage Body before makinga decision. Listed building consent is not required for repairs, unless they wouldaffect the character of the structure, in which case they constitute an alteration.

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33..55..22 EEnnvviirroonnmmeennttaall ccoonnsseerrvvaattiioonn aanndd eeccoollooggyy

CCoonnsseerrvvaattiioonn bbooddiieess aanndd eennvviirroonnmmeennttaall lleeggiissllaattiioonn

Works associated with existing structures such as masonry arch bridges may impact onprotected sites or protected species. The nature conservation bodies (known as thestatutory nature conservation organisations or SNCOs) have responsibility forpromoting the conservation of wildlife and natural features:

� English Nature.

� Scottish Natural Heritage.

� Northern Ireland Environment and Heritage Service.

� Countryside Council for Wales.

There are various categories of sites with designations for environment andconservation both statutory and non-statutory at international, national, regional orlocal level which can affect areas surrounding or adjacent to bridges. Thesedesignations afford varying levels of protection and carry with them restrictions on thetypes of activities that can take place in these areas, which are likely to have a significantinfluence on any works undertaken within them. They stipulate procedures that mustbe followed for notifying relevant authorities and gaining permissions to undertake anywork on bridges, which must be considered from the very outset of a project, and mayhave significant effect on the selection of works and method of working, and hence onthe programming and cost of works, for example:

� under the Wildlife and Countryside Act 1981, operations on sites of specialscientific interest (SSSI) must be agreed with the appropriate SNCO, and specieslisted as protected must not be killed or have their habitat damaged without alicence. Special considerations must be given to any work on such sites to minimisedisruption to habitats and employ environmentally friendly methods of working

� under the Habitat Regulations,1994 SNCOs can permanently ban operations thatthey consider may damage SAC (special areas for conservation) or SPA (specialprotection area) designated sites. Although appeals can be made, these must be onthe basis that the works are for “imperative reasons of overriding public interest” andthat no alternative solutions exist. Where appeals are granted, compensatory worksare likely to be required – ie the creation of suitable replacement habitat whichshould ideally be “ecologically functional” before the original habitat is damaged.

CIRIA publication C587 Working with wildlife (Newton et al, 2004) provides a usefulsummary of wildlife legislation and planning guidance relevant to the UK constructionindustry, and how it affects those involved with construction. The principalconservation-related site designations affecting the UK, and associated legislation, arehighlighted in Section A5.

The ethos underpinning EU environmental legislation is “the precautionary principle” iethat prevention is better than cure and that it is important to prevent foreseeable acts ofenvironmental damage. The continual review of legislation to make sure that goodpractice is followed at all times should be come part of the design process. The actionsrequired to prevent pollution during construction are usually relatively easy and cheap toimplement compared with the cost of the clean-up if pollution occurs. It is particularlyimportant to take measures to prevent pollution where bridges cross watercourses.

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WWiillddlliiffee ccoonnsseerrvvaattiioonn

Bridges and associated earthworks, waterways etc. frequently provide habitats for avariety of flora and fauna, including bats, birds, amphibians, reptiles, insects and othersmall mammals. The Wildlife and Countryside Act 1981 as amended and theCountryside and Rights of Way Act 2000 afford protection to certain endangeredspecies of wildlife, and the presence of certain species of plants and animals can have aprofound effect on routine maintenance and renewals works on bridges.

The major infrastructure owners typically recognise the value of wildlife on their landand work with the SNCOs to manage protected habitats (eg Railtrack, 1999). Thepreservation and management of wildlife habitats should be incorporated into overallasset management plans for structures such as bridges, and reflect overallenvironmental management targets. To this end, the major infrastructure ownersemploy environmental management personnel as part of management teams to assistwith determining environmental policy and to liaise with other specialists in measuringand achieving environmental targets.

Seeking early ecological advice before starting work is important in determining thepotential impacts and concerns relating to protected species and habitats. Ecologicalscoping studies or an initial desk-based study will identify restrictions and implicationsfor development. Works may have to be restricted to certain times of the year – forinstance, to avoid bird-nesting periods in spring and summer, (bird nesting season is 1 March to 31 August) or the disturbance of hibernating animals in winter. If protectedspecies are of concern advice can be provided on optimal times of surveying, mitigationmeasures and licensing requirements.

For more detailed information in relation to protected flora and fauna and legislationuseful reference sources are:

� C587 Working with wildlife (CIRIA, 2004)

� C650 Environmental good practice on site (second edition) (CIRIA, 2005)

� Urban environments and wildlife law (Rees, 2002)

� Developing naturally (Oxford, 2000)

� Planning for biodiversity (RTPI, 1999)

� and also from <www.defra.gov.uk>

MMaannaaggiinngg eennvviirroonnmmeennttaall iimmppaacctt

Over the last few decades awareness of environmental issues has increased and it hasbecome an important consideration in many areas where society’s actions may havedirect or indirect environmental consequences. There are a wide range ofenvironmental aspects to be considered when assessing the sustainability of amanagement policy, the impact of executing a specific maintenance, repair of anexisting bridge or the building of a new one, including:

� consumption of limited resources (materials, energy)

� air pollution

� noise pollution

� water pollution

� soil and waste

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� safety

� visual impact

� land-use

� flora and fauna.

Where transport and construction/development are concerned the environment andsustainability are particularly sensitive issues. Environmental appraisals are nowmandatory elements of the planning and design of new transport routes, but are notalways considered for maintenance and repair of existing infrastructure. Environmentallegislation, coupled with a greater understanding of the potential impact ofconstruction activities, is leading to the incorporation of environmental concepts andaims as a core policy of national and local authorities. All aspects of transport planningand works, including the construction of new bridges and the maintenance and repairof existing ones, should be aligned with and assist in achieving these objectives. Thisrequires the definition and consideration of environmental issues and imperativesalongside the more traditional constraints (social, economic and technological) whichinfluence asset management policy. The integrated approach remains at an early stage,but efforts are being made to develop methodologies for assessing and comparing thereal environmental impact of alternative management policies. A good example of suchan approach is given in Steele et al (2003) where a life-cycle assessment (LCA) has beenapplied to the management of brick arch bridges.

A detailed consideration of the topics of managing environmental impact andsustainability is beyond the scope of this book, but the reader is referred to existinggood practice guidance included in the following documents:

� C650 Environmental good practice on site (second edition) (CIRIA, 2005) for practicaladvice on environmental responsibilities when planning and executing civilengineering works and how to fulfil them satisfactorily

� C571 Sustainable construction procurement (CIRIA, 2001) includes advice on successfultechniques and strategies for delivering construction projects that encourageenvironmental responsibility

� Building Research Establishment Information Paper IP14/04 (Steele, 2004) onEnvironmental sustainability in bridge management

Some points of particular relevance to arch bridges include:

Use of new materials

The choice of materials used for the maintenance, repair and construction of bridgescan affect the wider environment such as by influencing carbon dioxide (CO2) outputinto the atmosphere. With the exception of some renewable sources, all energy sourcesand processes requiring the use of energy release CO2 into the atmosphere. CO2 is agreenhouse gas and is implicated in climate change which affects the species, habitatsand built environment around us. The production and processing of new materialsinevitably requires energy and may have other environmental impacts eg noise,pollution and land-use. In a wider environmental context, there is a strong basis for therevival of the use of hydraulic lime not just for repairs but also for new build, as theenergy consumption and pollution caused by its production are considerably less thanthat of Portland cement: for equivalent mortar quantities, lime production usesbetween 47 and 70 per cent of the energy needed for cement production, withcorresponding reductions in emission of pollutants (Pritchett, 2003).

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Waste, reuse and recycling

Waste impacts on the environment in a number of ways: loss of valuable resources,need for landfill space, and the unnecessary production of additional materials.Wherever practicable, original materials should be reused unless they have alreadyproven unsuitable or are in a state such as they are unlikely to provide adequateperformance. Where original materials are unavailable or unsuitable, used and recycledmaterials that are not a part of the original structure may be considered and sourcedlocally wherever feasible to reduce the demand for production and transportation ofnew materials. Where waste is unavoidable, measures should be taken to avoidpollution and minimise its environmental impact.

Key guidance on minimising waste and re-using and recycling materials:

� C513 The reclaimed and recycled materials handbook (CIRIA. 1999) summarises theopportunities for re-using and recycling materials with information on theirproperties, performance, specification and use

� Observations on the use of reclaimed bricks (BDA, 2001b) discusses technical andperformance issues associated with the selection and reuse of bricks.

Pollution

Although the consequences of pollution immediately evoke contamination of the air,water and soil, the impact of noise pollution and other less tangible, transient andindirect consequences of carrying out works should also be considered. For instance,congestion caused by implementation of bridgeworks and diversions can lead toincreases in emissions of pollutants such as benzene and oxides of nitrogen and carbonfrom vehicle exhausts, resulting in additional greenhouse gases, affecting human healthand potentially harming heath land and other low nutrient habitats. For this reason,transport management should minimise the disruption caused to the transportinfrastructure for environmental reasons as well as for functional purposes. InGermany, restrictions have been placed on the use of heavy road vehicles at weekendsto control their impact (air and noise pollution) in urban and rural areas. Interestingly,this has led to a transfer of some unitised freight to waterways and rail.

33..66 SSoouurrcceess ooff bbrriiddggee iinnffoorrmmaattiioonn

33..66..11 DDaattaa ccoolllleeccttiioonn

A significant challenge facing those involved with the asset management of bridges isthe frequent lack of comprehensive and accurate data. Without adequate information itis impossible to develop coherent and cost-effective strategies for sustaining the bridgestock in an efficient manner. It is important for bridge owners to systematically collectand record data on their own bridges and to retain this information for future use.Asset information, such as the results of inspections, investigations and records frommaintenance, repair and strengthening works, is highly valuable and its loss may incuradditional expenses for re-investigation at a later time. Lost or inadequate records ofworks that have been carried out can lead to inaccurate structural assessments,strengthening schemes being effectively repeated and defects being wrongly identified.Gradually accumulating such information paves the way to developing a systematic andeffective long-term maintenance policy, and allows asset managers and engineers tomake better and more informed judgements.

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Types of data collected for bridge assets are considered in Section 3.4.2. The primarysources of bridge data include:

� historical records

� routine and emergency inspections/examinations and investigations (testing andmonitoring).

The information from these primary sources is used as the basis for carrying outassessments of bridge condition and structural capacity, which is important “secondary”information for use in the bridge management process.

33..66..22 HHiissttoorriiccaall ddaattaa

Historical data may include:

� original design information (drawings, structure, materials, construction records)

� reports of past inspections, investigations and structural assessments

� records of maintenance, repairs and alterations

� old photographs and drawings

� incidents involving the bridge (sometimes recorded in local papers and journals)

� anecdotal information from staff, local interest groups or members of the public.

Original design and construction records are seldom available and subsequent recordsmay be inaccurate or incomplete. To further aggravate the problem, valuableinformation is often lost, misplaced or destroyed, members of staff leave and retire,departments are reorganised and move between premises. It is important that this lossis minimised in future.

Where they are available, historical records such as drawings and inspection andinvestigation reports provide essential information on the structure and form of abridge as well as some history of its performance, deterioration and repair. Some caremay be necessary though, as such information can potentially be inaccurate ormisleading, and problems can occur when previous assumptions and incorrectconclusions are accepted later on. Also, details shown in original records may be invaliddue to changes made in the course of construction or through subsequent alteration.

33..66..33 IInnssppeeccttiioonn ddaattaa

Regular inspection and reporting regimes for masonry bridges typically compriserelatively frequent basic visual inspections, and additionally, at a lower frequency,inspections where a more detailed examination is carried out; these are described inSection 3.7.1.

Types of data that can be gathered in routine visual inspections include:

� administrative and background data (bridge ID, location, traffic flow, weather,inspection details etc)

� structural information (type of structure, materials, dimensions, description ofmain structural elements)

� bridge site data (presence of embankments, slopes, ditches, bridge furniture, access etc)

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� apparent condition, damage and deterioration (visible defects, changes sinceprevious inspection noted, including photographs and sketches)

� requirements for additional investigation (proposals, features of interest, accessrequirements).

Some degree of data verification is advisable, and should form an integral part of theinspection management process. A suitable proportion of inspection reports fromroutine inspections should be subject to checking as part of a routine validationexercise. This should involve an independent on-site check of the reported data from arandom selection of inspected bridges, carried out by a professional bridge engineer.The frequency of checking may vary but should be at least 5 per cent, and ideally 10 per cent, of the inspections carried out. As well as providing a basic checkingfunction, feedback from this validation exercise should be used to identify areas whereinspection data are typically less reliable or subjective, where errors are most likely tooccur, and where inspection staff might benefit from improved guidance or furthertraining.

33..77 BBrriiddggee iinnssppeeccttiioonn

Inspection (also referred to as examination) performs a vital role in bridgemanagement, and is integral to bridge condition appraisal. In addition to existinghistorical records, inspections are the principal source of information on bridgecondition and performance. Inspections are typically restricted to the visual observationof condition and monitoring of defects, often supported by some simple on-site testing,with observations and test results recorded in a standardised fashion on inspection pro-forma. The reporting systems used by different owners vary widely, particularly in theway that the condition of bridge elements is described.

Visually-based inspections can provide data on bridge geometry and externaldimensions, and evidence of visible deterioration and distress may be discerned. Thedesigns, materials and construction methods employed for masonry arch bridges wereconsidered adequate and economical at the time and have proven their worth in-service since; the bridges have proven to be robust and typically show visible evidenceof distress in advance of structural failure. The routine process of periodic visualinspection, supplemented by more detailed inspection on a less frequent basis,supported by additional investigations as required, is therefore a potentially adequatestrategy for ensuring fitness for continued service. However, deterioration and loss ofperformance which is not apparent to the naked eye typically remains undetected andit is important to consider hidden variations in quality and construction whenconsidering bridge characteristics for structural assessment.

There are assumptions implicit in taking a bridge “at face value” and where the risksassociated with such assumptions are not tolerable, it is necessary to carry out furtherinvestigative work to achieve the necessary confidence in visually based inspection data.Such investigations can provide additional information on specific aspects of thebridge’s structure and its behaviour, hidden details, materials type, and condition anddeterioration, using a variety of intrusive, non-destructive and analytical techniques.Investigation techniques are discussed further in Section 3.8.

33..77..11 TTyyppeess aanndd ffrreeqquueenncciieess ooff iinnssppeeccttiioonn

The regime of bridge inspection should ensure that any deterioration in the conditionof the bridge is detected in good time to allow remedial action. The intervals betweeninspections are typically specified by organisations to satisfy compliance with their

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statutory obligations and internal policies; for the main UK bridge infrastructureowners these are:

� for Network Rail (NR) bridges, examination types requirements and intervals areset out in Railway Group Standard GC/RT5100 Safe management of structures

� for London Underground (LU) masonry bridges, inspection types requirementsand intervals are set out in Engineering Standard E3701 Structural assets inspection

� the requirements of the Highways Agency (HA) for inspections and intervals areset out in BD63 Inspection of highway structures (HA, 1994)

� British Waterways (BW) carry out inspections generally in accordance with BD63(HA, 1994).

The terminology and frequencies of inspection vary between the main UKinfrastructure owners, although a similar basic principle of four types of inspection,varying in terms of typical objectives and methodology, is a common approach as setout in Table 3.3.

TTaabbllee 33..33 IInnssppeeccttiioonn rreeqquuiirreemmeennttss ooff mmaaiinn UUKK bbrriiddggee oowwnneerrss:: NNeettwwoorrkk RRaaiill ((NNRR)),, HHiigghhwwaayyss AAggeennccyy((HHAA)),, BBrriittiisshh WWaatteerrwwaayyss ((BBWW)) aanndd LLoonnddoonn UUnnddeerrggrroouunndd ((LLUU))

1 Where structural parts of bridges are under water in a water course, and where the depth of waterprevents a visual examination, the normal interval between detailed examinations is three years.

2 Intervals can exceptionally be up to 10 years

3 London Underground also require “special inspections” – which are regular visual inspections carriedout at short intervals – for masonry structures awaiting repairs.

CIRIA C656 109

TTyyppee KKnnoowwnn aass SSccooppee aanndd oobbjjeeccttiivvee IInntteerrvvaallssRoutinesurveillance

Superficial inspection (HA)

Length inspection(BW)

Cursory visual check for obvious deficiencieswhich might lead to accidents or increasedmaintenance, Part of the day-to-daysurveillance of the transport network carriedout by infrastructure owner’s staff (notnecessarily trained inspectors) in the course oftheir normal duties.

When staff visit thebridge site.

Routine visualinspection

General inspection (HA)

Visual examination (NR)

Superficial inspection (LU)

Intermediate inspection (BW)

Visual inspection of accessible representativeparts of the structure (including adjacentearthworks, waterways etc) from ground level orfrom other readily available walkways, platformsetc to identify hazards and changes in conditionand determine requirements for detailedinspection.

Maximum interval:(NR, LU) one year(HA) two years after last General or PrincipalInspection(BW) five years after lastIntermediate or principalinspection.

Routinedetailedinspection

Principal inspection (HA, LU, BW)

Detailed examination (NR)

Close, or tactile (ie touching distance),inspection of all accessible parts of thestructure, including adjacent earthworks,waterways etc with provision of special accessif necessary. Visually based but can besupported by measurement and simple testing(eg hammer-tapping) of structure to gatheradditional data.

Normal intervals:(LU) four years(NR) six years1

(HA) six years2

(BW) maximum interval10 years.

Non-routineinspection

Special inspection (HA)

Additional examination (NR)

Defect advice inspection (LU3)

Undertaken in response to a specific need (egwhere significant deterioration or evidence ofstructural distress is observed; before, duringand after the passage of abnormal loads, afterflooding and accidents such as bridge-strikes,fires or chemical spillage). Visual inspectioncan be augmented by specialist techniques forinvestigation of structure (in situ testing,sampling and laboratory analysis) as required.

As required, toinvestigate particularfeature or gather specificinformation.

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When referring to generic inspection types, this document adopts the terminology used incolumn one of Table 3.3.

The fundamental principle of the inspection process is that the infrastructure should bemaintained in a safe and serviceable condition, and inspection intervals should beappropriate to ensure that this is achieved. Aside from satisfying statutory obligations insuch respects, the period between inspections for an individual bridge should bedetermined dependent upon the findings of the previous inspection, its sensitivity todeterioration and the criticalness of the structure within the transport network.

Fixed-schedule inspection and assessment schemes have some negative consequences,since valuable resources are spent on bridges that are known to be in excellentcondition whereas bridges in poor condition may not be inspected as regularly asnecessary. A measure of flexibility is desirable, based on a good risk assessment, so thatresources can be directed where they will be most effective, while ensuring the primeobjectives of safety and functionality. Subject to the policy of the bridge owner, limitedvariations in inspection frequencies may be permissible dependent on the use, type,condition, deterioration and accessibility of the bridge, and the perceived effectivenessof the inspection itself. This requires justification, typically through a risk assessmentprocess to demonstrate the acceptability of the proposed inspection frequency. Thisapproach is considered advantageous, but the risks associated with reduced inspectionfrequency need to be adequately assessed on a structure-by-structure basis.

Risk assessments can be used to justify increases or decreases in inspection frequencies.Reductions in inspection frequencies may be considered if it has been demonstratedthat:

� the condition of the structure is good and there is no potential for rapiddeterioration

� the capacity of the structure exceeds the applied loading by a significant margin

� there is a good level of confidence in the results of inspections and assessments

� it is not envisaged that there will be any significant changes in use, loadings orenvironment which might detrimentally affect the bridge

� the potential modes of failure of the bridge are understood and there is adequateconfidence that the proposed inspection type and frequency can adequatelyidentify structural distress in advance of failure, or that the consequences of failureare low

� the likelihood of incidents which might affect the capacity of the bridge structure(eg bridge strikes, excessive traffic or environmental loading) is low.

Conversely, increases in inspection frequency may be necessary if:

� the condition of the structure is poor or there is the potential for rapiddeterioration

� the capacity of the structure barely exceeds the applied loading or if there arerestrictions on its use

� the level of confidence in the results of inspections and assessments is not good

� changes in the use, loading or environment of the bridge, which mightdetrimentally affect its performance, are envisaged

� the potential mode of failure of the bridge is poorly understood and there isinadequate confidence that the current inspection regime can identify structuraldistress in advance of failure

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� the consequences of failure are perceived to be particularly high

� the likelihood of incidents that might affect the capacity of the bridge structure ishigh.

Where risk assessment is used to justify reductions in inspection frequency it isparticularly important that they are updated with current data, reviewed and re-assessed at suitably regular intervals.

RRoouuttiinnee ssuurrvveeiillllaannccee

Routine surveillance is the responsibility of all persons associated with the managementand maintenance of the infrastructure. This can be useful for detecting obvious suddenchanges in bridge condition or circumstances which might lead to increased risk to thebridge structure or to its users, for instance damage caused by accidents or vandalism.Infrastructure owners should ensure that there are suitable procedures for reporting,recording and where necessary responding to such observations, particularly whereurgent action is required. Relevant employees should be made aware of theseprocedures and the need for vigilance.

33..77..22 VViissuuaall iinnssppeeccttiioonn pprroocceedduurreess

Traditionally, visual inspection has been the first (or most basic) level of inspection. Visualsigns of deterioration should inform the process whereby further more sophisticatedmodes of inspection are authorised which may lead to remedial work to the bridge.

IInnssppeeccttiioonn tteecchhnniiqquueess

Inspections normally consist of a visual examination of the external parts of the bridgewhich may be supplemented by photography. It is recommended that photography isused routinely when recording the condition of bridges, in particular their generalappearance and the appearance of any features of interest such as defects and evidenceof deterioration. As well as being invaluable for communicating information in arelatively objective manner to others they also improve the continuity of inspectionsand are useful for comparison of condition over time. It is recommended that allinspectors are equipped with cameras and any ancillary equipment (tripods, flash units)necessary for obtaining good photographic records, and trained to an adequatestandard in their use. Where photographs are likely to be used for later comparisons,efforts should be made to ensure consistency in the images captured to facilitate this (iein the area photographed, angle and position of view, lighting conditions, inclusion ofscale measures etc).

For detailed or “tactile” (touching distance) inspections, visual observation may beaugmented by simple physical testing, for example a hammer-tapping survey. Hammertapping is a simple technique routinely used to give a quick appreciation of thecondition of masonry, in particular, delamination between rings of brickwork in multi-ring brickwork arches. The surface of the structure is tapped and listened to, with anyvoiding or delamination causing a lower pitch than that from solid brickwork. Anexperienced operative may be able to achieve good though not always repeatableresults. The method detects not only delamination but also loose bricks and internalvoids. Delamination from deep within the brickwork cannot be detected. The methodshould not be used on frozen brickwork because water within the masonry may befrozen and hence give a “solid” response.

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During the inspection, inspectors carry drawings and photographs with the findings ofthe previous inspection to bring to their attention the defects previously identified andallow them to superimpose the two pictures of the bridge and compare observations.Comparison allows inspectors and bridge engineers to identify whether the appearanceof existing defects has changed and identify any additional defects which havedeveloped since the last inspection. These observations are the basis for any furtherappraisal or investigation.

OObbsseerrvvaattiioonn aanndd rreeccoorrddiinngg

Successful inspections rely upon the recording of accurate and relevant observations ina systematic and objective manner that facilitates comparison with the observations ofprevious inspections, and allows inspectors (on site) and bridge engineers (off site) todiscern current condition and identify any changes.

The type of material and construction as well as its condition should be recorded anddefects should be mapped. It is vital that in addition to clear extensive notes andlogging of information, a full and comprehensive set of detailed and generalphotographs are taken.

For cracks, the following details should be recorded:

� position

� orientation (in 3D and relative to the bedding planes)

� length

� displacement (check if the crack is wider at one end)

� whether or not the crack has been repointed in the past.

Cracks may only open when the bridge is loaded, and may completely close up whenthe bridge is unloaded. Inspectors need to be aware of this, since most inspections areundertaken when the bridge is not loaded. It is necessary to look carefully for evidenceof closed cracks and loss of bond. Where visible, cracks in road surfaces etc can beequally important in the assessment of the bridge.

Any repairs or previous works should be recorded. This could be of vital importance asit may hide ongoing defects. Cracks may have been covered up by routine pointing, inwhich case they may be very difficult or impossible to detect, but extra-thick mortarjoints may provide an indication, particularly where the mortar is clearly different tothat surrounding it. Resurfacing of a highway may mask movements in the backfill.

As described in Section 2.5 the reasons for masonry to crack are varied but the natureof masonry is such that it can often accommodate large differential movement withoutcracking. This movement may manifest itself in various guises, all of which should berecorded:

� distortion of regular shape

� misalignment

� tilting

� bulging

� hollowness (eg ring separation)

� excessive movement under load.

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These movements can usually be measured using simple techniques such as a line and/or a plumb bob or possibly a microlevel.

It is important that the form of construction and the dimensions of parapets areaccurately recorded together with any evidence of impacts.

The presence of water and the evidence of its effect on the fabric of the bridge shouldbe noted, (everything from surface deposits to damp patches to visible evidence ofscour). It is important to record:

� the weather condition at the time of the inspection and in the period prior to theinspection in as much as they may influence the “as inspected” condition of thebridge eg flooding

� the extent and degree of any masonry wetness, which may be influenced by thelevel of the water table in the backfill and the porosity of the masonry. It should beremembered that leakage from sewers and water pipes are a potential source ofwater in the bridge

� bridges over water should be checked for obvious signs of scour – especially thosewhich are prone to flooding and/or tidal flow (for further guidance refer to May,2002).

To provide some degree of objectivity, the degree of masonry wetness should bedescribed in a standardised way, for example:

� dry – no discolouration of the masonry surface, feels dry to the touch

� damp – discolouration of the masonry surface, feels moist to touch

� wet – film of water at the surface and/or dripping

� running – constant trickle or jet of water.

Mortar loss and the condition of the mortar are important parameters. The single mostdifficult problem associated with any subsequent sophisticated assessment of thecarrying capacity of the bridge is the evaluative description of the mortar and its bondwith the units. It is therefore important to record:

� the depth of the open joints (Figure 3.3) their location (Figure 3.4) and an estimateof the percentage of the structure affected

� where repointing has occurred since the last inspection, it is advisable that thedepth of the repointing should be determined and any voids behind the repointingshould be investigated (Figure 3.5), and that the condition of any mortar behindthe repointing is reported (including dampness)

� the presence of obvious signs of leaching (whitish mineral deposits on masonrysurfaces, Figure 3.6) and their extent, and any evidence of chemical attack anddeterioration (detectable in the course of tactile examinations by softening,crumbling and disintegration of masonry).

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FFiigguurree 33..33 DDeeeepp ooppeenn jjooiinnttss lleeaaddiinngg ttoo lloooossee mmaassoonnrryy uunniittss aanndd lloossss ooff llooaadd ttrraannssffeerr

FFiigguurree 33..44 LLoossss ooff mmoorrttaarr bbeettwweeeenn aarrcchh bbaarrrreell rriinnggss aatt ssppaannddrreell

FFiigguurree 33..55 DDeellaammiinnaattiioonn ooff iinnttrraaddooss aarrcchh rriinngg ddeessppiittee pprreesseennccee ooff hheeaaddeerrss –– ccrraacckk ccoonncceeaalleedd bbyyrreeppooiinnttiinngg

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FFiigguurree 33..66 LLeeaacchhaattee ddeeppoossiittss oonn aarrcchh ssooffffiitt iinnddiiccaattiinngg wwaatteerr ppeenneettrraattiioonn aanndd mmoorrttaarr ddeetteerriioorraattiioonn

The condition of the masonry fabric should be recorded, including:

� the extent and depth of any spalled or eroded surfaces

� the extent of any softening and deterioration of materials

� the extent of any vegetation (including lichen and other normally visible coloniesof micro-organisms which might be indicative of, or an agent of, internaldeterioration).

Most authorities require the severity of defects to be assessed and classified, with theintention of using this information to inform an assessment of load carrying capacity.This usually relates to the MEXE method of assessment – in particular the conditionfactor (see Section 3.10).

Additionally, this first level of inspection may be the precursor to an extensiveinvestigation which gathers information for a more sophisticated assessment. A non-routine inspection should be undertaken where there is evidence of severe progressivedeterioration or structural instability, for instance:

� new or ongoing cracking or deformation of the arch barrel

� deformation or bulging in masonry

� evidence of foundation movement or displacement of spandrel walls or parapets.

The inspection should be adequate to identify the cause of the problem, assess thesafety and serviceability of the structure and identify any immediate and longer-termmeasures required.

Non-routine inspections may also be required as a response to incidents which mightthreaten bridge integrity or affect its performance, such as flooding, vehicle impacts,accidents, chemical spillages etc. Such inspections often require the use of additionalinvestigation techniques such as in situ non-destructive testing, sampling and laboratorytesting and monitoring (see Section 3.8).

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33..77..33 OOppttiimmiissiinngg iinnssppeeccttiioonn pprroocceedduurreess

The success of bridge inspections can be optimised in several ways:

� inspection procedures and classification systems for observations should becarefully devised and recorded with clear, illustrated descriptions and examples,supplied to each inspector

� where possible, reporting should be standardised to reduce the risk of error and/orimportant data not being recorded and to facilitate comparison of observations

� inspection proforma should be devised to capture the required range and detail ofinformation, and prompt inspectors to observe and record information in aconsistent and systematic way

� suitable lighting and access should be provided where necessary, and items such ascameras (preferably digital), tape measures and binoculars carried as an aid toinspection

� inspectors should be encouraged to make liberal use of annotated diagrams,photographs and direct measurements of the structure to illustrate andcommunicate features of interest eg condition and deterioration.

33..77..44 PPrrooggrraammmmiinngg aanndd ttiimmiinngg

Where programming of inspections is concerned, consideration should be given tomaking advantageous use of existing access opportunities. The disruption to transportand cost associated with inspections can be minimised by coordinating them with otheractivities that might affect the normal use of the bridge, for example route closures andrestrictions, access provisions, programmed maintenance and repair works (whichmight also open up and allow inspection of hidden parts of the structure). However, atall times the priority is that the timing of inspections satisfies regulatory requirementsby meeting owner’s procedures, and that any delay or deferral is justified by anadequate assessment of possible increased risk to the safety of the structure and thepublic.

The timing of inspections may influence the state of the structure itself and the natureand quality of observations that can be made. Adverse weather conditions can influencethe quality of inspection, making the inspector’s task more uncomfortable and difficult,and it is preferable not to carry out inspection under conditions of heavy rain or snowor when the light is poor. The adequacy of lighting is an important factor in the qualityof observations, and inspections should wherever feasible be carried out in daylight.Where this cannot be arranged it is necessary to provide good artificial lighting. Theenvironmental conditions (temperature and weather) should be recorded as a routinepart of any inspection, and the current and recently prevailing conditions may beimportant: for example, the moisture state of the masonry may be higher after rain,cracks may open more in cold weather, and the adequacy and functioning of existingdrainage provisions may be more apparent in wet periods.

33..77..55 HHeeaalltthh aanndd ssaaffeettyy aanndd eennvviirroonnmmeennttaall ccoonnssiiddeerraattiioonnss

Inspection of bridges involves exposure of those involved (and in some cases thegeneral public) to a variety of health and safety hazards, including but not limited toexposure to live traffic, working over or near water, falls from height, contact withservices and equipment and with dirty and unhygienic conditions. There may also berisks to the environment, including pollution of the air or watercourses with harmfulfumes or substances.

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Those involved with planning and executing inspections should be aware of therelevant health and safety and environmental hazards and, at a minimum, ensure thatthey are dealt with in accordance with the relevant statutory requirements. Riskassessments should be carried out to ensure that hazards are identified, risks areassessed and where necessary measures are taken to minimise them to acceptable levels.Inspectors should remain alert at all times and be aware of the procedures to befollowed and people to be contacted in the case of emergencies. This can be assisted bythe preparation of a method statement for the works, which is a formal requirementfrom many of the larger infrastructure owners (see Section 3.7.6).

33..77..66 PPllaannnniinngg aanndd pprreeppaarraattiioonn

Inspection of bridges requires careful planning and execution. Before undertaking aninspection, particularly one involving special access or investigation and testing of thestructure, it may be useful to undertake a reconnaissance visit to gather informationabout the site and identify potential problems and hazards. It may be necessary toarrange the removal of vegetation or other obstructions prior to the inspection if theywill obscure parts of the bridge or hinder access. A method statement that summarisesall the relevant information should be prepared, and agreed by all parties. Thestatement should take into account the review of records and reconnaissance of thestructure, access requirements, health and safety and environmental considerations (seeSection 3.7.5). The level of detail given should be appropriate to the complexity,circumstances and type of inspection. Normally, the following information should beincluded in any method statement (after DfT, 2005):

� details and programme of the work to be undertaken

� equipment required

� methods of access to be used

� traffic management details

� the risk assessment including safe procedures for dealing with hazards

� the resources and competence of the staff to be employed

� planned working times

� temporary works to be employed

� protection from highway, rail, waterway and other traffic

� requirements for action by others

� any co-ordination or notification required

� any environmental impacts of the work.

33..77..77 CCoommppeetteennccee ooff iinnssppeeccttiioonn ppeerrssoonnnneell

It is vital that inspection personnel are equipped with the skills, knowledge andexperience to adequately perform their duties, commensurate with the complexity ofthe task, and are supported with the necessary resources. It is also necessary that theyhave an adequate level of understanding to be able to judge when emergency measuresare required for safety reasons.

Routine visual inspections are not carried out by professionally qualified bridgeengineers but by bridge inspectors or examiners, who may also be involved with theday-to-day maintenance of these bridges. Such staff perform the function of “a trainedpair of eyes” able to spot obvious signs of damage and distress, and often have a goodunderstanding of the requirements for routine maintenance and straightforward repairs.

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The basic qualities of a good inspector are (after DfT 2005):

� a knowledge of safe working practices and access requirements for inspection

� experience of the techniques and tools available, and an understanding of their useand limitations

� an adequate understanding of the construction, materials and behaviour ofmasonry arch bridge structures

� a knowledge of the causes of structural defects and deterioration of masonrymaterials

� an adequate understanding of the modes of failure of masonry arch bridgestructures and the ability to recognise and interpret features which might requireurgent action

� the ability to make and record objective observations accurately, clearly andconsistently.

For a novice inspector to attain these qualities and become fully effective, formaltraining in addition to experience gained by working alongside a well practiced andtrusted examiner to allow the transfer of knowledge and skills, is necessary. In certainsituations specialist training and skills may be required, for instance where inspectionsrequire roped access, working in confined spaces or underwater. Some of the majorinfrastructure owners specifically define the necessary standards of competence forbridge inspectors and provide training schemes which lead to formal qualifications inthis respect to ensure compliance.

33..77..88 SSttrreennggtthhss aanndd wweeaakknneesssseess ooff vviissuuaall iinnssppeeccttiioonn

The main advantage of visually based inspection techniques is that they are simple,rapid, inexpensive, do not require any specialist equipment or techniques and can beundertaken with minimal disruption. They are therefore ideal when gathering basicinformation on large bridge stocks. Moreover, if inspections are carried out by a well-trained and sufficiently knowledgeable team who regularly inspect the same structuresand are adequately supervised, this should provide the bridge owner with a goodknowledge of its bridge stock and of how it is evolving with time.

Unfortunately, this system does have weaknesses, the main ones being reliance onvisible features and subjectivity of observations. The early signs of structural distressand deterioration may manifest themselves in relatively subtle changes in the bridgestructure that are easily overlooked or may be perceived as inconsequential,particularly where more dramatic defects are present, although these may belongstanding. Whether such symptoms are observed and recorded depends upon thelevel of skill and knowledge of the inspection personnel. These factors reduceconfidence in inspection results and should be considered when comparingobservations from different inspections of the same bridge, and observations made ondifferent bridges. This variability may be exacerbated by a lack of consistency in theinspection methods used and the teams carrying out the inspections, leading to a lackof continuity in the available knowledge. This increases the level of subjectivity inassessing changes in the performance of bridges over time.

Attempts have been made to ensure greater objectivity in the inspection process such asbetter training of examiners, clearly prescribing a comprehensive range of observationsto be made during examinations, and wherever possible ensuring observations arequantitative or semi-quantitative, for example by requiring measurements to be madeor observations to be assigned an index value in accordance with a prescribed rating

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system. This “systematised” data, in standardised and often numerical form, is suitablefor recording and comparison as part of bridge management systems and can bemanipulated and analysed far more easily than non-standardised information such asan inspector’s general comments on condition. Each of the major UK infrastructureowners has their own systematised procedure for condition assessment and reporting,so that requirements for data collection are dictated by the needs of the owner. Forinstance, condition reporting of highway bridges has recently been subject to review,resulting in the trial of a standardised system for reporting known as the bridge conditionindex (BCI), and Network Rail’s structures condition marking index (SCMI) performs asimilar function for railway bridges.

It is important to note that it is impossible to determine the hidden internal structureof a bridge by visual inspection, and important features are often missed even whenintrusive investigations are carried out. For instance, the presence or absence of voidingor internal spandrels and significant changes in construction, material type and quality.Certain defects, such as separation between brick rings of an arch barrel, may bedifficult or impossible to discern from visual inspection alone. Often such unexpectedfeatures are only discovered at the point where works are already being carried out onthe bridge, at which point they can lead to considerable problems.

The application of new and emerging survey and monitoring techniques holds somepromise for the development of more objective inspection methods in future.

33..88 BBrriiddggee iinnvveessttiiggaattiioonn aanndd mmoonniittoorriinngg

Where it is necessary to gather information on parts or features of the bridge that arenot readily obtained by visually-based inspection techniques, this may require the use ofmore rigorous methods of investigation, for instance intrusive and/or non-destructive insitu testing, which can be supported by sampling and laboratory testing of materials ifrequired.

Bridge investigations are often required to determine the parameters needed forundertaking structural assessments analyses (see Section 3.10). Over the last twentyyears, the understanding of the behaviour of masonry arch bridges has improvedsignificantly and the level of sophistication of the available mathematical models hasadvanced in line with the power of computers. This has meant that detail, collectedabout a bridge, is no longer only used to inform an experienced (though subjective)estimate of a condition factor for a simple semi-empirical assessment, but also may beneeded in the development of a three-dimensional finite element model. It is thereforeimportant to recognise the developments that have taken place and reflect these in thequality and extent of the data that is collected.

The objectives of investigation are:

� to confirm or establish the details of the bridge

� to detect the presence of any defect or signs of deterioration not observed by visualinspection

� to record any significant change in the condition, loading or environment that mayhave occurred since the last observation

� to identify any hazards to the performance of the bridge and the safety of its users.

The investigation techniques available to achieve these objectives can be categorised asthose for:

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� specialist investigation (advanced techniques requiring expert application andinterpretation)

� sampling and testing techniques (in situ testing, obtaining samples, laboratorytesting of samples)

� monitoring techniques (for periodic measurement and comparison of results).

The range of investigative techniques employed for these purposes is summarised inTable 3.4, and each technique discussed in more detail in Sections 3.8.2, 3.8.3 and 3.8.4respectively.

TTaabbllee 33..44 SSppeecciiaalliisstt iinnvveessttiiggaattiioonn,, tteessttiinngg aanndd mmoonniittoorriinngg tteecchhnniiqquueess ffoorr bbrriiddggee iinnvveessttiiggaattiioonn

Bridge investigations require a full consideration of programming and timing, healthand safety and environmental, and planning and preparation issues similar to those forbridge inspections, as discussed in Sections 3.7.4, 3.7.5 and 3.7.6 respectively.

33..88..11 OOppttiimmiissiinngg iinnvveessttiiggaattiioonn pprroocceedduurreess

Where investigations (ie non-routine bridge inspections, invasive sampling, testing andmonitoring) of bridges are concerned, these are typically carried out in response to aspecific requirement and therefore have very specific objectives. For instance, a changein permissible bridge loading may provoke the need to obtain geometrical andstructure data required to carry out a detailed structural appraisal, or a sudden visibledeterioration of the bridge fabric may provoke the need to investigate its causes andconsequences. It is important that these objectives are clearly understood and stated,and that the investigation is designed to meet them efficiently. Such investigationstypically require disruption to the normal function of the bridge, need to be carried outin restricted (often very short) periods, require special traffic management and accessprovisions, and may use a variety of specialist techniques and sub-contractors.Investigations should be efficient and well-organised, and require careful planning andcoordination between the various parties involved:

� investigations need to be focused; obtaining superfluous information results inunnecessary cost, damage to the structure and disruption to the bridge’s normalfunction and should be avoided

� investigation, testing and monitoring techniques should be carefully selected with agood understanding of the benefits and limitations of each technique, the results itis expected to yield, how they will be used to achieve the investigation objectives,and the level of confidence that is required

CIRIA C656120

SSppeecciiaalliisstt iinnvveessttiiggaattiioonntteecchhnniiqquueess

SSaammpplliinngg aanndd mmaatteerriiaallsstteessttiinngg//aannaallyyssiiss tteecchhnniiqquueess

MMoonniittoorriinngg tteecchhnniiqquueess

SonicsConductivityUltrasonicsInfra-red thermographyImpact EchoTomographyGPR TechniquesPhotogrammetryLaser scanningStrain measurementAcoustic emissionElectrical conductivityEndoscopeScour detection

CoringWhole-brick samplingGeotechnical investigationLoad testingFlat-jack testingPetrological examinationAnalytical laboratory techniques

Crack monitoringStrain measurementsDisplacementmeasurementScour detectionFibre-optic sensors

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� techniques should wherever possible be used in a complimentary fashion, ie theirstrengths and weaknesses and the results yielded should combine to provide thenecessary range and quality of information to adequately fulfil the investigationobjectives

� those responsible for carrying out different elements of the investigation (forinstance, specialist sub-contractors and testing laboratories) should have anunderstanding of its overall objectives, how their activities fit into it, theirresponsibilities and what is required of them

� specialist sub-contractors should be carefully selected and are required todemonstrate suitable skills and past experience; often it is useful to involve them inthe process of specifying and planning the investigation so as to ensure thatadequate resources and support are available and potential problems and risks areidentified and dealt with at an early stage

� risks to achieving the investigation objectives should be identified and measurestaken to minimise them to acceptable levels wherever practicable – for instance, byhaving backup equipment and personnel available on stand-by

� a clear method statement should be produced, setting out the scope of theinvestigation and the parties involved and their responsibilities; listing the activitiesto be undertaken, where, when, who by and what equipment is to be used;identifying the hazards associated with the work, with details of how they are to bemitigated; procedures in the event of unforeseen circumstances and emergencies.

33..88..22 IInnvveessttiiggaattiioonn tteecchhnniiqquueess

SSuurrvveeyyiinngg tteecchhnniiqquueess

It is important to undertake an accurate dimensional survey of the structure forstructural assessment purposes. Making assumptions about the shape of the arch barrelcan have a significant effect on the calculated carrying capacity. Due regard to the 3Dnature of the structure is important; even if a 3D analysis is not planned, it may showup structural distortions which later prove to be critical.

Modern surveying techniques are accurate to within a few millimetres, so referencepoints should be installed to facilitate long-term monitoring.

More advanced “specialist” surveying techniques, such as photogrammetry and laser-surveying, have improved in recent years and now offer the possibility of determiningthe overall dimensions of bridge structures and the 3D shape of elements such as archbarrels. These techniques may provide particularly useful data for more advancedmethods of structural analysis or for monitoring changes in structural condition. Moreinformation on these techniques is included in Section A4.

SSppeecciiaalliisstt NNDDTT tteecchhnniiqquueess

A brief description is given below for each of the principal specialist non-destructivetesting (NDT) techniques and it should be appreciated that many require the use ofsophisticated equipment, and a high level of expertise is necessary to interpret theiroutput. Consequently, they can be expensive procedures and need to be used carefullyto optimise their chances of success and the usefulness of data collected. Thosespecifying the use of specialist methods of investigation should have a goodunderstanding of the potential benefits and the limitations of each technique, how theserelate to the objectives of the investigation and situation of the bridge in question. A

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comprehensive overview is presented in Highways Agency Advice Note BA86 (HA,2004a) and in McDowell et al (2002). Guidance on the need to understand “black box”outputs and interpretation of results is given in Turner (1997) in the context of pilinginvestigations.

There is often uncertainty surrounding the interpretation of the output of the specialisttechniques described here, so it is recommended that sole reliance upon one techniqueshould be avoided where a high level of confidence is required in the results. Suchtechniques should ideally be used in situations where more conventional and reliabletechniques cannot be used, are unsuitable, or their use is minimised. It is oftenadvantageous to combine the use of specialist and conventional techniques in acomplementary manner, making best use of their strengths and minimising theirweaknesses. Typically this means using NDT techniques to reduce the need forintrusive/destructive testing by “filling in the gaps” between test locations, or to useintrusive/destructive testing to check and/or calibrate the results of NDT techniquesand improve confidence in their results.

For example, if trying to determine the thickness of an arch barrel a radar survey maybe used to rapidly cover the whole area of the arch soffit and identify the boundarybetween arch extrados and fill material, and this can be supplemented by drilling alimited number of small diameter cores through the full thickness of the barrel anddirectly measuring its thickness using the cores and (more reliably) the holes. In thiscase, the benefits of the radar survey over using coring only are rapidity, good areacoverage and minimisation of damage to the arch and disruption to the normal use ofthe bridge; the potential weakness of the technique (lack of confidence in accuracy) isminimised by the accurate “spot” results from the coring, which allow calibration of theradar results and additional verification. In such an investigation there is anopportunity to gather additional information by examining the core holes using anendoscope (to assess arch barrel condition in situ and identify delamination betweenrings) and to use the masonry cores for laboratory-based examination and testing (todetermine materials characteristics and condition).

Initially it may be advisable to validate a high proportion of NDT results, which can bereduced if confidence improves. However, it is important to understand that the qualityof NDT results are often sensitive to environmental and other factors which may varyfrom bridge to bridge, and even within the same structure, so that a technique thatgives good results in one location may not perform similarly in another. NDTtechniques are a potentially valuable tool in bridge investigation, but their successfulapplication requires a cautious and rigorous approach.

A summary of available specialist NDT techniques is included in Table 3.5. More detaileddescription of each of the techniques and their capabilities are given in Section A4.

CIRIA C656122

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TTaabbllee 33..55 SSppeecciiaalliisstt iinnssppeeccttiioonn tteecchhnniiqquueess ffoorr bbrriiddggee iinnvveessttiiggaattiioonn

CIRIA C656 123

TTeecchhnniiqquuee CCoommmmeennttss

Sonics This technique is dependent on measuring changes in the velocity of sonicpulses travelling in a solid material, on the basis that velocity is dependent onthe density and elastic properties of the material. Internal discontinuities (egcracks, voids, boundaries between material types) can potentially be detectedusing this technique.

Conductivity Electrodes are inserted into the structure or ground in order to determine itselectromagnetic conductivity and hence estimate the moisture content of themasonry or the presence of voids etc.

Ultrasonics The velocity of ultrasonic pulses travelling in a solid material depends on thedensity and elastic properties of the material. Pulses are not transmitted acrossvoids, so by measuring apparent speeds of pulses it is possible to determinethe competence of the material and the location of delaminations etc.

Infra-redthermography

Thermography involves the measurement of small variations in surfacetemperature (0.1°C) which are used to predict internal conditions.

Impact echo This technique can be used to determine the depth of delaminations (eg ringseparation) and the thickness of structural elements. It is based on the use ofimpact-generated stress waves that propagate through the structure and arereflected by internal flaws and/or external surfaces.

Tomography Involves the measurement of stress waves (sonic) through the structure. Thisgives information in 3D that enables an assessment of the location of possibledefects to be made.

GPR techniques Ground penetrating radar (GPR) is an echo sounding technique whereelectromagnetic impulses are transmitted into the bridge and a receiver detectsreflections from material boundaries. It can be used to determine constructiondetails and conditions including delamination and voiding.

Photogrammetry Several digital images are recorded of the structure from different locations.Using the collected information a 3D image of the structure can be createdusing specialist computer software and used to study the condition of themasonry and crack patterns.

Strainmeasurement

Strain measurements may be taken as part of an ongoing monitoringprogramme or during loading of the bridge to determine the structuralresponse. Two things must be remembered when measuring strain in masonryor brick structures. Firstly, the structure already has a strain history and so anymeasurement only relate to a change in the strain-state. Secondly, masonryand brickwork are not isotropic, homogeneous materials and so interpreting thestrain gauge output is not straightforward.

Acousticemission

As micro-cracking develops in the structure under increasing load and/ormaterial deterioration, small amounts of strain energy are released in the formof elastic stress waves that travel through the material. These waves can be“listened” to by a transducer and the level of activity interpreted in terms ofstructural condition.

Electricalconductivity

This electromagnetic technique is used to map variations in the electricalconductivity of the sub-surface. The technique can be used to locate areas ofmoisture, voids and variation in fill material.

Remote visualinspection

Remote visual inspection can take the form of rigid endoscopes or flexible image-scopes, both provide still and video records. Access can be through existingholes/cracks or small holes (typically 10–20 mm) drilled into the structure.

Scour detectiontechniques

There are a number of techniques currently available to measure and monitorthe effects of scour. CIRIA publication C551 Manual on scour at bridges andother hydraulic structures (May et al, 2002) presents a detailed review of thecurrent techniques.

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33..88..33 SSaammpplliinngg aanndd mmaatteerriiaallss tteessttiinngg//aannaallyyssiiss tteecchhnniiqquueess

The following sections provide a summary of sampling and testing techniquesapplicable to masonry sampling and testing. For further detail, the reader is directed tothe comprehensive review of sampling and testing of brick masonry given in Edgell,2005 and the review of techniques for sampling and testing of building stone inGeological Society Special Publication No. 16 Stone: Building stone, rock fill andarmourstone in construction (Geological Society, 1999).

CCoorriinngg

Cores are normally taken to provide samples for laboratory testing to allow materialproperties and presence and extent of deterioration to be established. They are alsoused to confirm construction details such as ring thickness, spandrel wall thickness, ringseparation and material variation. If taken through the full thickness of the arch barrelthen the nature of the backfill material can be determined and further inspectionundertaken using an endoscope.

Cores are usually cut perpendicular to the masonry surface using a water-flusheddiamond-tipped core barrel to produce parallel-sided cores. Avoiding ridging of thecores is difficult and requires skill and a rigid support platform. Where it is difficult toprovide support from the ground (eg working above water) a drilling rig can be boltedto the structure itself.

If the cores are used to determine strength, they are usually not taken in the directionof principal stress and so results should be interpreted accordingly. Typically cores areapproximately 100 mm in diameter, but larger diameter cores (300 mm or more) maybe necessary for certain purposes, and smaller diameter cores (as small as 25 mm) mayin some cases be acceptable, dependent on the aims of the investigation.

Where masonry is satisfactorily cemented and in good condition long cores through thefull depth of thick masonry elements can be obtained by skilled and motivatedoperatives using good equipment. However, the disruption caused by the drillingprocess means that masonry cores often fragment and there is significant material loss(Figure 3.7). Dependent on the masonry characteristics and condition this can beminimised by good drilling practices, equipment and technique, and the operativesshould understand that the objective of the exercise is the production of a good qualityand substantially intact core sample, not the making of a hole in the structure. Thecore, including estimated materials loss, should be carefully logged, noting the typesand features of bricks, stone and mortar, their moisture condition, the presence of voidsand empty joints etc. Immediately after cutting, each core should be given a uniquereference, and its location and orientation within the structure recorded. Cores shouldbe wrapped in cling film to preserve their condition since exposure to the atmospheremay influence the results of tests to be performed on them. Careful handling andpacking for transportation from the site is very important to avoid damage.

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Reinstatement should be undertaken sympathetically using appropriate materials (seeSection 4.3.3).

WWhhoollee--bbrriicckk ssaammpplliinngg

It should be remembered that there can be considerable variation in the properties ofbricks within a bridge. Often well-fired bricks were used to face the structure, while lesswell-fired bricks may have been used internally. Consequently, the sampling shouldattempt to obtain a representative sample of each type. Earlier manufacturers’ marksare often useful in identifying bricks from different origins. The best advice is toassume nothing, investigate the nature of the structure as far as the project allowsbefore finalising the sampling scheme.

It is unlikely that a statistically significant sample of whole bricks can be taken from thestructure and so some engineering judgement will be required. The selection needs tobe mindful of brick types, their variability, the importance of the various elements inthe structure and the criticalness of the load bearing capacity.

Depending on the location, taking brick samples varies in difficulty and the rely uponthe strength and adhesion of the mortar. Occasionally a brick can be removed by simplyusing a hammer and chisel, but there is always the risk of percussive damage to thesample so using a wheel cutter is preferred. It is very important to individually bageach brick and securely attach identification labels with information on the location andorientation of the sample within the structure. As with core samples old bricks requirecareful packing and transportation to the laboratory to avoid fragmentation, whichmight render them useless.

CIRIA C656 125

FFiigguurree 33..77

FFrraaggmmeenntteedd 110000 mmmm ddiiaammeetteerrmmaassoonnrryy ccoorreess ttaakkeenn tthhrroouugghhmmaassss bbrriicckkwwoorrkk iinnttoo ffiillll

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BBrriicckkwwoorrkk ssttrreennggtthh tteessttiinngg

The strength of masonry units in a bridge can be estimated by testing whole bricksremoved from it, but there are difficulties with obtaining statistically adequate samplesand with the preparation of samples for testing. At the laboratory the adhering mortarof whole brick samples must be removed prior to testing. This can be a problem if thebrick is weak as dressing may cause it to break. Examination of the brick will confirmwhether or not the British Standard approach of using thin plywood platen sheets maybe successful. Old bricks are often misshapen and the plywood sheets would not be ableto accommodate the irregular shape. In such cases, surface grinding has been usedsuccessfully, but care should be taken not to alter the proportions of the brick. If thishappens, then the non-standard proportions should be taken into account whenestimating strength. Light grinding followed by mortar capping or plywood packingmay be preferred.

Reasonable estimates of masonry strength may be expected on the basis of unit andmortar strength for ashlar, squared rubble and brickwork. Random rubble masonry ismuch more problematic given the relatively large variation in the mortar joints. Thestrength of masonry is significantly affected by the thickness of the mortar joint, and inbrickwork a doubling of the joint thickness from 10 to 20 mm results in a 15 per centloss in strength. It is unlikely that sufficient material will be available to undertake arepresentative series of tests to establish a statistically acceptable value. Even whensufficient material is available for tests to be carried out, the masonry strength forassessment purposes may be assumed to be 0.75 of the strength so determined(Hendry, 1990). A 20 per cent overstress can be allowed on these values where it isjustified that the masonry is subjected to a concentrated loading.

Tests on masonry prisms, constructed from bricks removed from the bridge and usinga similar mortar, may be justified if there are any unusual features such as bonding inthe masonry. Otherwise, an estimate based upon the unit strength and knowledge ofthe type of the mortar may be appropriate.

An alternative to testing a prism made from sampled bricks is to drill a 300 mmdiameter core and test it on its end, applying a correction factor that will need to bederived from a limited number of validating tests on laboratory-built prisms (Edgell,2005). This can work well for fairly friable brickwork, and may be merited whereextensive sampling and testing is required on large structures.

SSttoonnee ssaammpplliinngg aanndd tteessttiinngg

There is no British or European standard for testing stone in a structure and thereforesome judgement should be exercised when dealing with this material. The effect ofspecimen size is significant, with smaller specimens overestimating strength. Althoughsmall diameter cores are usual, they do not give very reliable values. Larger cores arepreferred. If cuboid specimens not less than 90 mm side can be tested then the factorsin Eurocode 6 (EN 1996) could be used to adjust the stone size to the standard200 mm × 200 mm. Fortunately the strength of cylinders with a height/diameter ratioof 1.0 have almost the same strength as cubes. Tests on cylinders with this ratio used inconjunction with the Eurocode 6 shape factors could be used to give an estimate ofstrength.

A detailed review of the sampling and testing of building stone is included inGeological Society Special Publication No.16 Stone: Building stone, rock fill andarmourstone in construction (Geological Society, 1999).

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Stone may not be homogeneous or isotropic and may have bedding planes, but thenumber of cores available will usually be statistically small and so there are difficultiesin adequately characterising stone in existing structures. Coring is usuallyperpendicular to the surface (radial in the case of the barrel) and the sample willinevitably be tested in a direction that will not be the direction of the principal stress.Published data from stone quarry sources should be used in conjunction with testresults to give what is at best an estimate of the stone strength.

Where the condition of the masonry is suspect, for example in situations where thebarrel is subject to frequent wetting and drying cycles and the atmosphere and groundconditions are aggressive, coring can be used to obtain samples for laboratoryinspection and testing, for instance by Petrography. There have been cases where theinternal competence of sedimentary stone has been lost while the external face hasremained sound.

MMoorrttaarr ssaammpplliinngg aanndd tteessttiinngg

Mortar sampling and testing is particularly problematic. It is difficult to obtainspecimens of sufficient size on which to carry out a test, and also the structure mayhave been repointed several times within its life. Additionally, due to water percolationand atmospheric ingress, the internal mortar will have changed, been washed out orwill have deteriorated. Furthermore, the material is liable to be damaged in removing itfrom the structure. Taking all these difficulties into account, it is often the case thatsome nominal value of mortar strength will have to be assumed. Where lime mortarhas been used (which is likely to be the case in old bridges), a strength of 0.5 to 1.0N/mm² would be realistic. In Eurocode 6 (EN1996) the strength of masonry is afunction of the mortar strength raised to the power 0.25, so that an increase in mortarstrength from 0.5 to 1.0 N/mm² would increase the masonry strength by approximately15 per cent.

A chemical analysis of samples of the mortar coupled with petrographic examinationshould give an indication of the mortar type, mix proportions and hence the mortardesignation from which an estimate of masonry strength can be determined. Athorough analysis is particularly important when dealing with the conservation andrepair of bridges with historic importance, where it may be necessary to obtain anauthentic match of repair mortars with existing mortar.

PPeettrroollooggiiccaall eexxaammiinnaattiioonn

This technique is carried out on samples of masonry from a structure, commonly in theform of core samples but even small fragments of brick, stone and mortar can beusefully examined. Petrological (also known as “petrographic”) examination canprovide a variety of information on the type of materials present, their quality,composition and condition as well as the presence, causes and potential effects ofdeterioration. It is useful for a variety of purposes in masonry bridge investigation:

� to characterise brick, stone and mortar types

� to assess the original quality and durability of materials

� to investigate materials condition and physical characteristics

� to investigate materials deterioration

� to assess suitable materials for repair and matches for replacement brick, stone andmortar (particularly for the conservation of heritage structures).

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Petrographic examination involves a close visual examination of masonry samples,supplemented by the examination of very thinly ground slices of the samples (“thinsections”) using special microscopy techniques and equipment (Figure 3.8). Wherenecessary it can be supported by additional analytical techniques which further assistwith determining the physical and chemical characteristics of the masonry and itscomponents, for instance wet-chemistry, x-ray diffraction (XRD) and electronmicroscopy and analysis.

Petrological examination is carried out by a materials specialist, typically working for aspecialist materials consultancy or laboratory, and has the potential to yield very usefuland detailed information. The benefit obtained is typically related to the extent towhich the materials specialist is involved in the process of the investigation itself. Ideallythey should have an opportunity to see the structure, understand the objectives of theinvestigation and ensure that adequate samples are obtained from suitable locations(normally by coring). Although the investigation is laboratory-based, simply sendingmasonry samples to the laboratory with no explanation of the purpose of theinvestigation, details of the structure, the location of the samples, their environment etcmay result in less useful information.

GGeeootteecchhnniiccaall iinnvveessttiiggaattiioonn

Geotechnical investigations aim to establish the type and nature of the material presentover (fill) and adjacent to the bridge and of the founding strata. These are undertakenwhen there are doubts about the vertical or lateral stability of the abutments/foundations, or when the bridge has failed an assessment and the acquisition of moreaccurate soil information would improve the accuracy of the assessment.

Most commonly required backfill parameters are self weight and shear strength. In situpressures and soil stiffness may also be required depending on the nature of theanalysis to be undertaken. There may be significant variation in the nature of thebackfill both vertically and laterally. Recent works may have stripped off the surfacelayer of material and replaced it with a more competent fill. Additionally reinforced

CIRIA C656128

FFiigguurree 33..88

MMaassoonnrryy tthhiinn--sseeccttiioonn vviieewweedd tthhrroouugghh aammiiccrroossccooppee;; bbrriicckk iiss oonn bboottttoomm lleefftt,,mmoorrttaarr oonn ttoopp rriigghhtt,, ddiivviiddeedd bbyy aa lliigghhtteerrbbaanndd ooff sseeccoonnddaarryy ccaallcciittee ssuuggggeessttiinnggtthhaatt tthhee mmaassoonnrryy hhaass bbeeeenn eexxppoosseedd ttoowweett ccoonnddiittiioonnss iinn sseerrvviiccee

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concrete slabs, steel plates etc may have been incorporated into the fill or parts of thefill have been lost through washout.

The investigation will need to establish the variation in character and quality of thebackfill across the site. As discussed in Section 2.5.1, this should be established over thearch and for some distance behind the abutment (normally 1.5 times the depth fromrunning surface to foundation). Dynamic probing over a regular grid of points usinghand-operated equipment should be used to provide initial profiling of the backfill,followed by more detailed testing at specific locations, thereby permitting theinterpolation of data between these locations. Use of hand operated equipment overthe arch barrel itself also minimises risk of damage to the masonry beneath.

Where there is the risk of saturation of the fill, the permeability of the soil and overalldrainage capacity of the backfill should be established. Under conditions of highsaturation, rapid loading of a low permeability backfill can lead to destabilising highwater pressure conditions and an undrained response from the soil.

Full details of applicable geotechnical investigations are outside the scope of thepublication, for which the reader is referred to BS 5930:1999 Code of practice for siteinvestigations. However, it is appropriate to give a brief description of some of the morerelevant methods here.

A considerable variety of methods of ground investigation exist, consisting of those inwhich samples are retrieved from the ground for description and testing and those inwhich the properties of the soil are described or measured in situ. Sampling inevitablyintroduces sample disturbance. This is reduced in the case of in situ testing which cangive more representative results. However since the strength of certain fills is a functionof moisture content, in situ test results may be representative only of backfill moistureconditions on the day of testing, and would require additional laboratory tests toinvestigate alternative moisture conditions, for example, full saturation on flooding.

It is sometimes easier to ensure that the sample orientation is correctly related to theproposed loading conditions in the laboratory than with an in situ test. Long-term testsare better handled in the laboratory. However, it is possible to test a morerepresentative volume of soil in situ than in the laboratory (eg in a direct load platetest). Any factor that influences the soil properties should be carefully considered whendeciding the mode of testing.

Trial pit investigation

Shallow trial pits provide an economical method of examining in situ conditions.Exploration depths are typically between 1 m and 4 m (although smaller depths may beachievable over the crown of an arch) and therefore require temporary support ifpersonnel are to enter them. Investigation is normally limited to levels abovegroundwater in non-cohesive soils.

A close examination of the ground is carried out using a systematic scheme for thedescription of the soils. Colour photographs of side faces should be taken with aprominent scale marker and located on a plan. Trial pits can be used to locate statutoryundertakers installations and any other buried equipment, and to examine the archextrados. Sampling can be carried out from either disturbed samples obtained byexcavation, or open-tubes driven into the base or side face. High quality “block”samples may be cut from a bench formed in a trial pit. In all cases, the exact locationand orientation of the sample relative to the pit and walls needs to be recorded.

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Borehole investigation

Exploration by means of boring or drilling, and the recovery of samples, is a well-established technique, and may be used exclusively or to supplement trial pits. It isunlikely to be used unless there are real concerns about the competence of the bridgefoundations or refurbishment includes new-build and/or change of loading regime.

Many of the most frequently used sampling and in situ testing methods can be carriedout with a wide variety of boring rigs, so that the accessibility or labour costs can oftenbe controlling factors regarding rig selection. For rough terrain or inaccessiblelocations, light rigs are advantageous. Environmental issues need to be strictly observedin particular with respect to the use of water and the disposal of material.

In situ density tests by replacement

The principle of determining the density of soil by the removal of a representativesample is well-established. In cohesive soils, a core may be cut (core cutter method),and its volume determined directly. In other soils, a replacement method is used. Driedgraded sand is poured into the void from which the soil sample is taken to determineits volume. The volume of soil removed should be large enough to be representative. Apouring cylinder is used to run sand into the void and an additional cone of knownvolume. The density can then be determined. Alternatively a water replacementmethod can be employed. BS 1377-9:1990 describes these methods in detail.

Test results are often variable, and it is important to carry out at least three tests at anygiven location.

Other common techniques

Other common geotechnical testing techniques are presented in Table 3.6, which is inpart adapted from CIRIA SP25, 1983.

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TTaabbllee 33..66 CCoommmmoonn ggeeootteecchhnniiccaall tteessttiinngg tteecchhnniiqquueess ffoorr mmaassoonnrryy aarrcchh bbrriiddggee iinnvveessttiiggaattiioonnss

CIRIA C656 131

MMeetthhoodd AApppplliiccaattiioonn AAddvvaannttaaggeess DDiissaaddvvaannttaaggeess

100 mmdiameteropen-tubesampler(U100)

Firm to stiff clays, insensitive orstoney clays, clayey silts, someweathered rocks.

Simple robust equipment, usuallydynamically driven. Provides areasonably large sample.Inexpensive. Rapid. Widelyaccepted and used.

Produces disturbed samples.Accurate control of samplerpenetration is difficult. Qualitydependent on the care taken by thedriller.

StandardPenetrationTest (SPT)

Derivation of a standardised blowcount from dynamic penetration ingranular soils (silts, sands, gravels)and in certain cases, other materialssuch as weak rock or clayscontaining gravels which are notreadily sampled by other methods.Convenient both above and belowthe groundwater table.The blow count (N value) may beused directly in empirical formulaeto determine soil strengthparameters.

Simple, robust equipment.Procedure is straightforward andpermits frequent tests.Inexpensive.

Simplicity of the equipment beliesits sensitivity to operatortechniques, equipment malfunctionand poor boring practiceResults require interpretation.Test insensitive in loose sands, cangive misleading results in fissuredclays.

ConePenetrometerTest (CPT)

Continuous measurement ofresistance to penetration of a coneon the end of a series of rodspushed into the ground at aconstant rate into sands, silts andclay. Convenient both above andbelow the groundwater table.Resistance parameters may be useddirectly in empirical formulae todetermine soil strength parameters.

Provides continuous record ofground conditions. Reduceddisturbance of ground comparedwith boring and sampling.

Specialist equipment and driving rigrequired. Results requireinterpretation and should beaccompanied by additionaltesting/sampling.

Vane test Measurement of undrained shearstrength of clays and measurementof remoulded strength.The results should be used inconjunction with laboratory derivedvalues of cohesion andmeasurement of plastic index inorder that an assessment of thevalidity of the results may be made.

Permits in situ measurement ofthe undrained strength ofsensitive clays with cohesionsgenerally up to 100 kN/m². Theremoulded shear strength mayalso be measured in situ.Causes little disturbance to the soil.Can be used directly from thesurface, or from the base of aborehole.Results are direct and immediate.Tests can be rapid.Small hand-operated vane testinstruments are available for usein sides or base of excavations.

The results are affected by silty orsandy pockets, or significant organiccontent in the clay.There is some dependence on thePI of the clay.To be used in conjunction withcareful description and backed upwith high quality sampling andlaboratory testing.Results are in terms of total stressonly.Specialist technicians required.

Plate bearingtest

For determination of elasticmodulus and bearing capacity ofsoils and weak rock, with minimumdisturbance.Horizontal load tests are possible todetermine backfill capacity anddeformational characteristics fornumerical modelling.

There is close control of loadingintensity, rate and duration.More representative thanlaboratory testing.Can be carried out in pits orboreholes.Lateral loading is possible.

Expensive and time consuming test.Scale effects should be considered.Specialist technician required.Ground conditions changed be test.Porewater changes difficult to takeaccount of.

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LLooaadd tteessttiinngg

Load testing is undertaken usually to determine the actual response of a bridge. Itshould be noted that some asset owners (in particular Network Rail) do not allow loadtesting of their bridges in excess of normal loading, so the owner’s policy on this shouldbe checked in advance. There are four types of load test that can be used:

� supplementary load testing

� proving load testing

� collapse load testing

� dynamic load testing.

Supplementary load testing, most commonly adopted in general practice, is used tosupplement the analytical methods of assessment based on calculation and the use ofcodes of practice. Loading should not exceed the normal loading regime as this maycause permanent damage to the bridge.

Proving load tests involves applying loads greater than the normal loading regime. Thistype of test should not be undertaken in the case of masonry arch bridges as it is likelyto cause permanent damage. This will reduce the residual life and ultimate strength ofthe bridge.

Collapse load testing is carried out on obsolete bridges or those that are being replaced(Figure 3.9). Its purpose is usually to develop the understanding of bridge behaviour. Itis important when such tests are organised that the research community is involved toensure that the maximum value is gained. Full constructional details should berecorded (many of these will not be available until after the test) including backfill andsurfacing and foundations. As much instrumentation as can be afforded should beinstalled and monitored during the test and the results made publicly available.

FFiigguurree 33..99 RReessuullttss ooff aa ccoollllaappssee llooaadd tteesstt oonn aa ffuullll--ssccaallee mmooddeell ooff aa mmuullttii--ssppaann bbrriiddggee

Dynamic load testing uses either ambient or forced vibrations to evaluate structuralperformance. To date, this type of testing has only been used as a research tool andwith very limited success.

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FFllaatt jjaacckkss

Flat jacks have been used to measure in situ stresses in one direction with some success(Hughes and Pritchard 1994a, Abdunur 1995). Various types of jack have been triedfrom simple rectangular jack (Figure 3.10) through to series of segmental jacks that areincrementally introduced into successively deeper grooves. The principle is the sameand involves fixing a series of reference points on the surface of the masonry, cuttingan appropriate groove into the surface and installing the jack such that it is in contactwith the sides of the groove. The jack is then pressurised until the reference points aredeemed to have returned to their original position. This basic procedure assumesuniform stress for the depth of the jack, which is a simplified approach. More sophisticatedtechniques try to compensate for the variation in stress with depth by progressively cuttingdeeper grooves and inserting larger jacks which create a picture of the stress state.

FFiigguurree 33..1100 FFllaatt jjaacckk ddeevveellooppeedd bbyy CCaarrddiiffff UUnniivveerrssiittyy

There are several sources of error; one of which is the simplification of the procedurein ignoring potential stress variations mentioned above. Additionally, the materialstiffness may vary with depth making the surface measurements somewhat circumspect.Also, it can be difficult to achieve a close and even contact between the jack and thematerial, and the jack should be calibrated because of edge effects that can cause thepressure exerted to be less than the internal pressure.

International standard recommendations for the use of flat jacks to measure stress andelastic moduli in a compressive environment have been published by RILEM (1994).Standard test methods include those from the ASTM (1991a and 1991b) and a digeston flat jack testing has been published (BRE, 1995).

If synthetic or natural rubber sheet material is used to manufacture the jack, then thejack’s performance is such that the apparent and physical active areas are almostidentical. Unfortunately, the open edge of the jack is vulnerable to bursting and needsto be restrained; Hughes and Pritchard (1994b). Steel jacks do not have this problembut have to be carefully calibrated to take account of their inherent stiffness in order toestablish their apparent active area. As might be expected, the efficiency of the jackimproved with size because the significance of the edge effects reduces. However, thereis a compromise to be struck between the efficiency of the jack and the variation ofstress with depth: the pressure will not be uniform and so the jack will measure anaverage value. Using jacks of various dimensions in the same location can partiallyovercome this problem (Abdunur, 1995).

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It should be noted that the calibration factor for a jack will vary depending on itsapplication. For example, the efficiency of a jack in brickwork and stonework where thejack is over 50 per cent of the unit depth is approximately 80 per cent but it drops tohalf this value if the jack is much less than 50 per cent of the unit depth. The problemcan be reduced if the slots are made in the masonry unit (although reinstatement canbe difficult, particularly for heritage structures).

Where there is eccentricity in the stress field, it is important to take this into account. Asit is anticipated that this will usually be the case in an arch subjected to asymmetricalloading, it will be necessary to consider this when interpreting output. Using jacks atseveral depths of penetration can give useful insight. It should be remembered at eachstage and location that the method only gives reliable output when used in acompressive situation.

As noted above, flat jacks can be used to determine the elastic modulus of the masonry.This can be done by cutting two slots in the masonry at approximately the length of thejacks apart. The slots should not be more than 1.5 times the length of the jacks apart.Best results are achieved by cyclic loading at increasing stress levels. Constant load testscan be performed to check the effects of creep.

33..88..44 MMoonniittoorriinngg tteecchhnniiqquueess

Most monitoring will not proceed beyond the routine inspection stage. However,observations or results of routine inspection may indicate gradual or sudden changes incondition or other potential problems. At this stage an investigation to collect relevantinformation will have to be undertaken, which may involve one-off investigation orlong-term monitoring of some aspect of the bridge’s appearance, structuralcharacteristics or performance.

Monitoring may also be used to assess the success and effectiveness of maintenance,repair or strengthening works on the bridge. Such works are usually undertaken at oneof two levels; either a short-term repair to allow continued use, or a long-termrehabilitation/strengthening. In either case, it is expedient to install someinstrumentation to monitor performance and hence prove the efficacy of the solution.For certain applications, such instrumentation can be installed so that it can beinterrogated remotely.

In the past, monitoring bridges was fundamentally a time consuming and labourintensive activity, but this situation is changing with developments in monitoringtechnology. With the development of wireless technology there is no need to havecables trailing between instruments and logging devices. In fact, several instrumentscan be logged simultaneously using a single logging device. Additionally, many systemscan be connected directly, by means of telephony, to a computer which is programmedto download data at user defined periods. This means that not only may site visitsbecome less frequent but also less accessible parts of the bridge will be regularlymonitored. In the past, these parts would only have been inspected at principalinspections. Depending upon the sampling frequency, simple battery operated mini-loggers can run for many months logging the information from several instruments.Such systems may be used to alert (by text message, email etc) relevant personnel ifpre-defined action levels are reached, for example if the structure is deemed to be atrisk – due, perhaps, to a significant change in the loading regime or the proximity ofmajor construction works. Information can be updated in “real time” with results beingplotted onto CAD drawings or superimposed onto photographs of the actual site. Thesoftware enables all structural elements to be interrogated individually as well aslooking at the structure as a whole. Such systems may be relatively expensive but costs

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are constantly reducing over time, and their use may be justified for importantstructures or where gaining the required level of access is difficult or dangerous.

Despite the availability of complex and technologically advanced systems as describedabove, experience has shown the wisdom in keeping things simple wherever possible – thisapplies both to monitoring equipment and system design. Basic methods which do not relyon sophisticated electronic instrumentation are typically more robust and less prone tofailure or error. Only where such methods cannot meet the specific requirements of theproject may the use of more sophisticated techniques and equipment be justified.

CCrraacckk mmoonniittoorriinngg

Crack monitoring (including monitoring of joints or other localised movements) can becarried out by a number of methods with varying accuracy and reliability, usinginstrumentation ranging from simple crack width gauges (tell-tales and graduatedplastic rules) to sophisticated data logging systems capable of continuously measuringmovement along with changes in temperature.

Crack width gauges, although cheap, easy to install and read, are prone to beingbroken off either accidentally or, where they are too obvious, through vandalism.Additionally, they need to be read manually and therefore have to be accessible. Theyalso give only a snapshot of movement, which may be due to seasonal temperaturevariations rather than a deteriorating structural condition. However, where theselimitations are not too significant, the demec-type surface-installed studs, read withvernier callipers, are preferable since they are simple, cheap and robust and not asobvious as some other types.

In some situations it may be beneficial or necessary to use an automated system, usuallyincorporating an extensometer which can be read remotely via a data logger. Athermometer can be incorporated so that both displacement and temperature can belogged simultaneously which enables the effects of cyclic variations to be monitored.They are relatively expensive compared with a graduated plastic rule.

SSttrraaiinn mmoonniittoorriinngg

Two things should be remembered when measuring strain in masonry or brickstructures. Firstly, the structure already has a strain history and so any measurementonly relates to a change in the strain-state. Secondly, masonry and brickwork are notisotropic materials and so interpreting the strain gauge output is not straightforward.

There are two methods of measuring surface strain that are appropriate to use onmasonry: vibrating wire gauges and Demec gauges. Vibrating wire (VW) gaugescomprise a fine wire tensioned between mounting blocks and protected by a tube. Thewire is plucked by a coil which also serves to read the frequency at which the wire thenvibrates. Temperature can also be monitored. A resolution of 0.5 micro-strain over arange of 3000 micro-strain is achievable for a gauge with a gauge length of 140 mm.Installation can be difficult particularly when the surface is wet or cold. Masonry isoften irregular resulting in the necessity to provide make-up shims to ensure that thewire’s protective tube does not make contact with the surface. Additionally, the adhesiveshould be selected to ensure rigid attachment. If the surface layer of material is suspectthen careful consideration to installation is necessary and may result in the need for steelpins or bolts being installed to which the VW gauge can be secured. The maindisadvantage of VW gauges is that they need a finite period of time to be read and socannot be used in dynamic load situations.

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Demec (demountable mechanical) gauges comprise a spring loaded lever systemoperating a dial gauge. Pins protruding from the instrument are located in pre-drilledstuds that have been fixed to the structure. The dial gauge is read manually and anumber of gauge lengths are available (100 mm, 200 mm and 250 mm are the mostcommon). The main advantage of the method is its simplicity and robustness. Theremay be a health and safety issue, as the method requires the operative to be close to thestructure, probably underneath it, while taking the readings.

DDiissppllaacceemmeenntt mmoonniittoorriinngg

Displacements are usually measured using either potentiometric, linear variabledisplacement transducers (LVDTs) or occasionally manually read dial gauges. Thesetransducers need to be mounted directly or via invar wires to an independent frame.Invar wires are used to minimise temperature effects and are kept short to reduce windinduced oscillation.

There are other ways of measuring displacements such as electro-levels, lasertechniques and photogrammetry. Laser theodolite systems, when sited on a targetattached to the structure, can be used to determine deflections under rapidly movingloads. They can be affected by atmospheric distortion. Photogrammetric methods havebeen used to check rail tunnels for displacement and a scanning displacement system ofgreater accuracy could be useful for comparative tests on bridges.

Tiltmeters (electrolevels) are used to monitor arch deformation and differential movementof piers. They are small fluid based sensors with no internal moving parts. The system isenergised with an electric circuit with the resultant output depending on the amount anddirection of tilt of the instrument. The electrolevels can be mounted within beams that arefixed mechanically to the structure. To determine the actual movement of an arch there isa need to mount a string of several beams in series. Once the instruments are fixed inplace they can be zeroed and any subsequent movement logged at user defined intervals.They can be used in conjunction with digital tape extensometers to improve theirreliability. Although they have a proven track record of being very accurate, they aredelicate to install and a full series of beams can be expensive.

SSccoouurr mmoonniittoorriinngg

Despite its importance, the detection of scour, or the determination of the susceptibilityof occurrence, is problematic. Riverbed depths are sounded during principalinspections but the competence of the foundation can easily be misinterpreted. Groundpenetrating radar (GPR) techniques have been developed to determine the depth ofscouring following flooding. These techniques require specialist expertise andexpensive equipment.

Given the increased incidence of flooding in recent times, it is important that the scourrisk assessment of each bridge over water is regularly updated. If it can be justified interms of risk, installation of a remote sensing monitoring system is a potential solution.

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FFiigguurree 33..1111 SSccoouurr pprrootteeccttiioonn wwoorrkkss oonn aa mmaassoonnrryy aarrcchh bbrriiddggee

Inspection after flooding by experienced divers, using probes, can distinguish betweenthe undisturbed bed and a filled-in scour hole. These post-flood inspections may bedelayed for some time while the river levels subside and flows reduce to safe levels.This may close the bridge until the perceived danger is over.

There is now a range of real-time monitoring equipment available. The cost of suchequipment should be balanced against the cost of physical measures to protect thefoundations - although their reliability has not been entirely proven.

CIRIA publication C551 Manual on scour at bridges and other hydraulic structures (May et al,2002) presents a detailed review of the current techniques.

33..99 IInntteerrpprreettiinngg iinnssppeeccttiioonn aanndd iinnvveessttiiggaattiioonn rreessuullttss

33..99..11 TThhee iimmppoorrttaannccee ooff ggoooodd iinntteerrpprreettaattiioonn

The safety and serviceability of individual bridges and the bridge stock as a whole isreliant on the quality of the data obtained in the course of routine inspections andinvestigations, and on the quality of the interpretation by which bridge condition isassessed and maintenance and repair needs identified.

The importance of good interpretation cannot be overstated, and there is no substitutefor a thorough understanding of masonry arch bridges, the factors which influencetheir performance and behaviour, and the significance of observations and defects. Inthis way, the knowledge and experience of bridge inspectors, assessors and engineershas a direct influence on the quality of bridge management.

It is also important to remember that the interpretation can only be as good as the datafrom the investigation and great care should be taken when extrapolating data andmaking judgement about parameters which were either not directly measured or wherethe accuracy of the data is suspect.

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33..99..22 CCoonnssiiddeerraattiioonnss ffoorr iinntteerrpprreettaattiioonn

It is important to appreciate that old bridges and their environments are subject togradual change and that information from a site investigation represents only currentcondition. While this is adequate for some purposes, for instance in determining archbarrel thickness and geometry for structural analysis, used in isolation it cannot provideinformation on how parameters have changed over time, which is frequently desirable.Additionally, the geometry of the bridge will change with time and this should becarefully recorded and supplemented with photographic evidence (imperial unitswould have been used in the construction of the bridge and so metric dimensionsshould be checked in this context). A single site investigation can, for example, identifya crack in an arch barrel, and possibly even allow its likely cause to be discerned, buttaken in isolation it is difficult to determine whether this is an inactive defect that hasbeen stable for a long time or whether it is recent and rapidly developing – scenarioswhich might prompt very different reactions. Although there are sometimes clues as towhether phenomena are recent or longstanding, such as fresh surfaces on spalledbrickwork or the presence of thick leachate deposits, such indicators cannot always beconfidently relied upon to provide adequate or accurate information.

An understanding of the heterogeneous nature of masonry is vital for theinterpretation of crack patterns and their significance. Cracking usually follows themasonry unit/mortar interface. If this is not the case then there may be some concernover the integrity/competence of the masonry units and this should be reflected in theassessment model. Over time the mortar may have been washed out or “ground out”by the working of the barrel. Although the bridge may have been repointed regularlythis will not have prevented internal damage and the possibility of voussoir “drop” instone barrels or ring separation in the case of multi-ring brickwork barrels.

Care should be exercised in the interpretation of test results from localised samplingand testing. The fabric of the constituent elements of masonry arch bridges may varyconsiderably and so it is important that undue weight should not be given to individualresults but rather that the data are seen in the context of the holisticbehaviour/performance of the bridge. This is not to say that individual rogue resultsshould be ignored – they may hold the key to the problem. Reliance on one type of testto determine key parameters is discouraged in favour of a broader approach.

Samples taken from the external faces of the fabric of the bridge may not berepresentative of the internal material. However, if the bridge is one of a seriesconstructed under one contract, it is likely that the same centring will have been usedalong with similar materials, construction techniques and workmanship. Sampling andtesting should not be confined to the masonry but should include the backfill up to adistance of 1.5 times the depth of the foundations. This information is vital if a moresophisticated analysis is necessary or if the bridge is showing signs of deterioration.

Where rates of change are important, as they often are, comparison of the current statewith a previous one is necessary, and there is no option but to rely on whateverhistorical records that may exist. These are particularly useful where it is necessary toextrapolate observations into the future and make predictions, but great care should beexercised here since although a good understanding of previous behaviour is anextremely valuable starting point, the past is not always the key to the present andfuture. Many aspects of bridge behaviour and performance are the result of complexinteractions between parameters which undergo changes over time, and the rates ofthese changes can vary. It is often desirable to supplement historical information withongoing assessments to monitor current state and discern any changes. Monitoring can

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be achieved by carrying out discrete repeat observations and measurements ofphenomena at suitable time periods, or gathering such data using a continuousautomated approach eg by installing suitable dedicated monitoring instrumentationand logging devices (see Section 3.8.4).

33..99..33 CCoonnddiittiioonn aasssseessssmmeenntt aanndd aassssiiggnniinngg rraattiinnggss

Major bridge owners have their own systems of assigning condition ratings basedanalysis of results from inspection and investigation. In the absence of any existingmethodology, Table A2.1 in Section A2 presents guidance on how to assign a conditionrating of “good”, “moderate” or “poor” based upon the evidence collected in the courseof bridge inspection, investigation and monitoring. It is intended to provide assistanceto personnel who are experienced in the inspection of masonry arch structures, ratherthan providing a substitute for their experience. The table details the visualobservations and investigation results required for classification, describing whatconstitutes each condition. It also gives guidance on the parametric values that shouldbe achieved for each condition. Additionally, the table presents qualitative assessment ofthe application of the techniques, so that the level of confidence for their results can beassessed. It should be noted that it is possible to have a “good” data output with a poorapplication of the technique and vice versa – for example a poor application of hammertapping with “less than 75 per cent coverage” but a good output with “no loose orspalling bricks or stones and no dull areas”. In such situations it is suggested that acautious approach to assessment is adopted, supplementing engineering judgementwith further direct verification where additional confidence is required.

33..1100 SSttrruuccttuurraall aasssseessssmmeenntt

Structural assessments are carried out when it is necessary to ascertain a bridge’scapability for carrying required loadings and to estimate the magnitude of any sparecapacity to better understand the sensitivity of the bridge to defects and deterioration.It is an important technique in ensuring that bridges are kept in a safe and serviceablecondition. The assessment makes use of data from inspections and investigations.

Such assessments should be carried out in accordance with national standards and thelarge UK infrastructure owners have their own internal policies and procedures whichmeet their statutory imperatives and organisational objectives.

33..1100..11 AAnnaallyyssiiss mmeetthhooddss

The principal analytical methods for assessing the structural capacity of masonry archbridges are:

� semi-empirical methods

� limit analysis methods

� solid mechanics methods.

A brief review of these methods is included below, and the basic principles, application,strengths and limitations of each technique is summarised in Table 3.9.

More detailed information on the principles and application of each method is given inSection A3.

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SSeemmii--eemmppiirriiccaall mmeetthhooddss:: MMEEXXEE

The MEXE (Military Engineering Experimental Establishment) method evolved fromwork undertaken by Pippard in the 1930s. This included both field and laboratory teststhat were used to calibrate theoretical work. During World War II, this research wasused to develop a quick field method to classify bridges according to their capacity tocarry military vehicles and was subsequently adapted for civil use and adopted by theMinistry of Transport in 1967.

The method comprises the calculation of a provisional axle load (PAL) that relates tothe performance of a “standard” arch barrel. The provisional axle load is then modifiedby the application of factors that take into account the extent of the departures of the“real” arch from the “standard” arch. The result is then subject to the application of a“condition factor” which relates to the condition of the bridge and the presence ofdefects. The results of a MEXE analysis are often considered to be conservative, but incertain circumstances this may not always be the case, for example:

� small span arches

� arches where the cover over the crown is greater than the ring thickness

� multi-ring brickwork arches where ring separation is suspected

� misshapen arches.

The MEXE method is quick and easy to use, and there is a great deal of experience inits use. However, it is a semi-empirical method, and heavily dependent on theexperience of the assessor since engineering judgement is required to arrive at arealistic final carrying capacity. Great care should be taken not to infer a greateraccuracy than the method is capable of delivering. Nowadays, there are severalcomputer packages that can be used in parallel to a MEXE assessment to giveconfidence where uncertainty might exist.

LLiimmiitt aannaallyyssiiss mmeetthhooddss

An alternative approach recognises the particulate nature of masonry and the observedcollapse mechanisms. It was from the work of Pippard (1951) and Heyman (1966) thatthe modern mechanism methods evolved. In their basic form the methods consider a2D arch comprising a series of blocks such that the arch possesses the followingproperties:

� it cannot resist tensile stresses

� it has infinite compressive strength

� it has finite stiffness

� sliding between voussoirs cannot occur.

These hypotheses force the local failure to occur by the formation of hinges betweenadjacent voussoirs and global failure to occur by the formation of a sufficient number ofhinges in order to convert the structure into a mechanism.

This type of model has been used in a modern framework since the 1950s (Kooharian,1952; Pippard, 1951). It was Heyman (1966) who demonstrated that masonry could beanalysed using either the upper or lower bound theorems of plasticity. Since then, anumber of computerised versions of this approach have been developed and arecurrently available.

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As a cautionary note, when using computerised versions of this approach it is importantto check that the modelling criteria do not exclude other possible modes of failure,including:

� movement of the supports (which might be interpreted as a release “roller”)

� material failure

� sliding between voussoirs

� slippage between rings (ring separation)

� “snap-through” failure.

SSoolliidd mmeecchhaanniiccss mmeetthhooddss

In recent years, experience has been gained in modelling masonry arches usingincreasingly sophisticated techniques. In the early days, simple linear elastic modelswere constructed and used to determine allowable carrying capacity by limiting stresslevels. Nowadays, software is available to create models that can take into account notonly the non-linear material behaviour of masonry but also the 3D behaviour andholistic soil-structure interaction. These latter formulations require a great amount ofskill and computer time.

The principal solid mechanics methods used in the analysis of masonry arch bridgesare:

� castigliano’s non-linear analysis

� finite element analyses

� discrete element analyses.

The castigliano method, by the incremental application of loads, allows the area of thearch ring to be modified as tension develops and masonry yields. The application ofload is continued until, ultimately, sufficient hinges form to create a mechanism.

Finite element analysis allows 2D and 3D elements and complex constitutiverelationships for the materials to be represented.

2D plane strain models allow the analysis of problems such as ring separation to beinvestigated and may allow some advanced soil models to be adopted.

3D finite element analysis using curved shell or appropriate elements should only beused where specific structural problems warrant such complex analytical techniques.The discrete element method (DEM) relates to a group of several formulations thatmodel the behaviour of a structure as elements whose interface can accommodatesevere discontinuities without convergence problems. In some packages, FE anddiscrete elements can be used simultaneously, creating a very powerful tool.

It is important to remember that sophisticated computational methods of analysis areonly as good as their input data and the expertise of the assessor. The inputparameters, in particular, may be difficult and costly to adequately characterise, and areoften estimated, which is a potential source of error.

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LLooaadd tteessttss

The use of load testing as a method of appraising masonry arch bridges has thedisadvantage that no accepted relationship has been found between the load at whichthe first signs of damage develop and the load capacity of the bridge. As a result,although a load test could be used to prove that a certain load can be safely resisted by astructure, it would be difficult to predict the level of safety with which it can resist it.Nevertheless, it is considered that load testing should be a final option to consider beforea structure is replaced. Load tests are described in Section 3.8.3 and in ICE (1998).

Load testing beyond normal service loads is not permitted by some bridge owners,most notably Network Rail.

QQuuaalliittaattiivvee aannaallyyssiiss

In some circumstances, and for certain structures, a quantitative approach to analysismay be unnecessary or technically inappropriate – for example in masonry jack archesor very short-span bridges built from natural stone. In such cases a qualitative analysisof structural adequacy, based on engineering judgement and past performance, may besatisfactory if such an approach is permissible by the bridge owner or other responsibleauthorities. Where qualitative analysis is proposed, the reasons why quantitativemethods are inappropriate should be clearly stated and the use and adequacy ofqualitative assessment needs to be justified by satisfying a number of basicrequirements, for example (after Network Rail, 2001):

� no significant increased capacity is required

� the structure has demonstrated satisfactory performance over a long period of time(eg more than five years) since any significant repairs or alteration

� detailed close inspection does not reveal significant damage, distress ordeterioration

� review of the structure confirms its force transfer system

� predicted future deterioration does not jeopardise safety

� no significant changes in the loads and actions on the structure are anticipated.

SSttoocchhaassttiicc aapppprrooaacchheess

Due to the level of uncertainty invariably associated with the parameters used in theassessment of masonry arch bridges, there is scope for the development of stochasticapproaches based on parametric studies. In order to extract all the information fromthese studies, reliability techniques could be used to produce bridge capacitiesassociated with a certain probability of failure. Such approaches are likely to needconsiderable further development and compilation of data before they are confidentlyused; and may not be acceptable to some asset owners at the present time. Details onreliability analysis techniques can be found in Baker, 1973.

33..1100..22 AAnnaallyyssiiss pprroocceedduurreess aanndd sseelleeccttiioonn ooff mmeetthhoodd ooff aannaallyyssiiss

The appropriateness of the analysis methods discussed in Section 3.10.1 depends onthe particular structure being assessed, the available bridge data and the level ofaccuracy and confidence in results that is required and that can be afforded.

CIRIA C656142

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The procedure for an assessment typically involves three distinct sequential phases:

� desk study of available information (this should be current and verified forcorrectness)

� inspection and investigation (to gather the necessary data required for analysis andverify existing data)

� analysis (using an appropriate method, using the data gathered during theprevious two phases).

The scope of assessment and the selection of appropriate analysis procedures should becommensurate with the type and function of the bridge, the information available, itscurrent condition and the consequences of its failure. The typical informationrequirements for each method of assessment are indicated in Table 3.9, along withpotential techniques for obtaining that information through inspection andinvestigation of the bridge.

MMuullttii--lleevveell aasssseessssmmeenntt pprroocceedduurree

To minimise effort and use of resources, a multi-level assessment procedure may beadopted so that where necessary assessment can be carried out with increasing levels ofrefinement. In the absence of any existing system, a simple multi-level system as set outhere may be adopted. Initially, a simple and conservative method of analysis should beused. Given the large number of bridges that most owners are responsible for,application of such a simple method could act as a “sieving process” in which structuresare initially assessed and categorised or possibly ranked according to their assessmentresults. More sophisticated analyses are only introduced if the simple methods predictfailure and the use of more complex methods is deemed cost-effective. Frequently suchmethods require additional input information which might not be available oradequately reliable, necessitating site investigation, testing and laboratory analysis ofsamples.

Although the details of such a phased multi-level approach would need to be tailored tothe needs and characteristics of each bridge owner, possible “levels of analysis” aredefined in Table 3.7, and the process of analysis is outlined in Figure 3.12.

CIRIA C656 143

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TTaabbllee 33..77 PPoossssiibbllee lleevveellss ooff ssttrruuccttuurraall aannaallyyssiiss ffoorr mmaassoonnrryy aarrcchh bbrriiddggeess

In all cases the level of analysis applied should be appropriate to the specific situationof the bridge and its circumstances, and the level of confidence required in the resultsof the assessment. Some degree of engineering judgement should be exercised in everyanalysis, at least to perform a “plausibility check” on the assessment results and the dataused to obtain them. This will minimise the risk of spurious results that can arise in theapplication of routine procedures, caused by mistakes, incorrect assumptions orinappropriate use. In particular, discrepancies between assessment results and observedbridge condition, damage and deterioration should be adequately explained.

It is important to remember that carrying out assessment using more complex methodsof analysis will not always give more favourable results, as is sometimes assumed.

CIRIA C656144

LLeevveell ooff aannaallyyssiiss EElleemmeennttss ooff aannaallyyssiiss

LLeevveell 11Basic analysis

� semi-empirical methods should only be considered as part of theappraisal by inspection where the owner considers this procedure asappropriate

� basic 2D limit analysis methods should be used to assess all structuresexcept significantly skewed bridges, long spans, bridges with unusualgeometries and important structures.

LLeevveell 22Detailed analysis

� the bridges excluded from the previous group and those failing itsassessment should be analysed using solid mechanics methods

� these analyses should be adapted to the available bridge data andcombined with site investigations and monitoring as appropriate, if morerefined analyses are required to demonstrate the adequacy of thestructure to fulfil its purpose

� the use of characteristic or worst credible strengths of materials may beused, based on test results from samples taken from the structure

� the level of refinement achieved before it is decided that the structure isunfit for purpose will depend on owner needs and constraints.

LLeevveell 33Special analysis

� where the use of refined solid mechanics methods cannot demonstratestructural adequacy, it may be possible to demonstrate its adequacy andinherent safety by comparison of its safety characteristics with othersimilar structures using stochastic approaches and probability analysis

� actual live traffic loadings of the bridge might be determined statisticallyand used for analysis

� the safety criteria of the bridge in question might be assessed andspecific relaxations considered if this can be justified and theacceptability of risks clearly demonstrated with adequate confidence

� this level of assessment would require considerable specialist knowledgeand research, and the benefit is unlikely to be justifiable except in themost critical cases.

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CIRIA C656 145

FFiigguurree 33..1122 BBaassiiss ooff mmuullttii--ssttaaggee aasssseessssmmeenntt pprroocceessss

Review interim safety measures

Update bridge database;review optimum

inspection/assessmentschedule; continuewith management

programme

Keep safety measuresunder review; assess

remedial/strengtheningoptions; prioritise,

programme and carryout works

Review existing data(historical, inspection,

performance data)

Bridge consideredprovisionally sub-standard

Interim safetymeasures required?

Is LLeevveell 33assessmentappropriate?

Obtain necessary data,carry out LLeevveell 33

assessment

FFaaiill

YYeess

NNoo

YYeess

PPaassss

PPaassss

FFaaiill

PPaassss

FFaaiill

Implement interimsafety measures

NNoo

Obtain necessary data, carry out

LLeevveell 22 assessment

Carry out LLeevveell 11assessment

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PPrriioorriittiissaattiioonn ooff aasssseessssmmeennttss

It is suggested that in addition to its use as a tool for initially assessing load capacity, asimple semi-empirical method such as MEXE, based on existing or readily obtainabledata, could be suited to rapidly and economically prioritising the needs of sizeablebridge stocks to allow inspection and detailed assessment resources to be used moreeffectively, directing them where they are most needed. Such an approach wouldrequire trialling and development, and continual feedback, validation andimprovement before sufficient experience is gained to apply the refined prioritisationmethodology with adequate confidence.

The preliminary results of research carried out by Mott MacDonald Ltd for Network Railsuggest that in such a system MEXE assessment results could be used to sort individualbridges from a bridge population into categories with a low, medium or high priority forfurther inspection and more sophisticated structural assessment where necessary.

33..1100..33 CCoonnssiiddeerraattiioonnss wwhheenn ccaarrrryyiinngg oouutt aannaallyysseess

The structural behaviour of masonry arch bridges was considered in Section 2.4. Thissection contextualises the issues in relation to the different levels of assessment.

It is important that the assessing engineer never looses sight of the holistic behaviour ofthe bridge and the range of the contribution that each element can make to it. It is vitalthat the efficacy of the assumed (or derived) load paths are checked. This is particularlytrue when the bridge contains defects – for example, using “smeared” elements mayrely on load paths that cannot exist in the actual structure.

Following the load path through the structure begins with an uncertainty – the loaditself. There are legislative limits but it is well known that highway vehicles arefrequently overloaded. Additionally, there are unlikely to be reliable records of theactual loading history of the bridge. This is not the case for railway bridges where, witha knowledge of axle loading, load histories can be estimated with reasonable accuracyusing timetable and movement records.

All masonry arch bridges have a common uncertainty over the way the load dispersesthrough the backfill. This is a particular problem in the case of high-speed trains andheavy road vehicles where dispersal of the load through the backfill is not whollyunderstood and certainly has never been measured in the field. There is a tacitassumption in the assessment methods that the backfill is in good condition and it iswell compacted. The MEXE method does allocate type and condition factors that canbe applied. Alternative methods leave this to the judgment of the assessing engineer, socare should be exercised. The nature of the backfill, whether it is granular or cohesive,should be considered when modeling its contribution to the carrying capacity of thebridge. It is very important to substantiate the nature and condition of the backfill for adistance of at least 1.5 times the depth to the abutment foundations (this is equally truefor hollow construction where its full extent must be determined and its contributionassessed). Additionally, it is important to take note of seasonal and special events such asflooding, when determining the contribution that the backfill and/or internal supportstructure, will make to the load carrying capacity of the bridge.

The soil-structure interaction is not referred to in the MEXE method and is onlycrudely represented in all but the most sophisticated (and expensive) models. Manymodels assume fixed abutments which predetermines the pattern of soil strain (andfailure mechanism in the arch barrel). In reality the barrel thrust causes spread of theskewbacks. This may be manifest by sliding of the skewbacks, and/or sliding and

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rotation of the abutments, all of which has a significant influence on the soil strain andhence the stabilising effect of the backfill.

The mechanical response of an existing masonry arch bridge is significantly influencedby its constructional details, the presence and extent of any defects and its materialproperties, as described in the following sections.

CCoonnssttrruuccttiioonn ffeeaattuurreess

The construction of a bridge and the materials used are the result of reasoned decisionstaken by the designers and builders based on their experience, knowledge and skill,and influenced by a variety of specific environmental and economic considerations, forexample the local weather conditions, the “prestige” of the project and the availabilityof resources. However, because this information is not available to the modern engineerundertaking a structural assessment, they will have to make assumptions regarding thedesign and construction of a bridge which may be 100 or more years of age.

The MEXE method assumes a solid masonry structure with a “solid” backfill. Anydeparture from this renders the method inappropriate and an alternative method mustbe adopted.

In any case, the non-visible construction features of a bridge can have a considerableinfluence on structural behaviour, for example, the additional stiffening effect ofinternal spandrels can lead to significant error in calculating its structural capacity orinterpreting its performance. It may also result in erroneous interpretation of cracksand other defects. Additionally, serious technical problems and expensive delays canresult when remedial work gets underway only to find that the internal structure of thebridge is not as was assumed at the tender stage.

Since the internal structure of a bridge is not always apparent from a simple visualinspection, it may be necessary to carry out intrusive investigations, possibly supportedby non-destructive techniques, to gain confidence in the assumptions made in thestructural assessment (see Section 3.8). Such investigations can confirm parameters suchas arch ring thickness as well as providing information on the type, bonding andcondition of materials, and identify hidden features such as internal spandrels andhollow areas. For instance:

� spandrel walls, arch backings and infill may contain systems of relieving arches andsupports to produce hollow areas which reduce dead-load on the structure (seeFigure 3.13). Internal spandrels, which may be longitudinal or transverse, aretypically supported directly by the arch barrel extrados, and can have a significantstiffening effect. Relieving arches were typically designed to keep the line of thrustclose to the arch shape. The MEXE method of assessment cannot be applied to suchstructures. Any model should take into account the particular construction details

� piers may also use systems of hollowing or non-homogeneous construction toreduce their dead-weight (see Figures 3.14 and 3.16). Such features may notpresent externally visible clues. If there are concerns due to visual evidence thatcalculations would not have predicted, then further investigation is advised toestablish construction details

� Where backing material, such as rubble infill or massive masonry, is extended overthe pier and has a good frictional interaction with the arch extrados, this may alterthe effective springing location and cause the effective arch profile to differ fromthe apparent geometric shape of the barrel. Knowledge of the backfill dimensionsand properties may be useful for structural analyses

CIRIA C656 147

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� it should be remembered that statutory undertakers have apparatus in mostbridges and that their installation, maintenance and repair can significantlyinfluence the contribution that the backfill makes to the bridge capacity. There is atleast one recorded case of a statutory undertaker cutting a chase longitudinallythrough a masonry arch to accommodate his apparatus without the knowledge ofthe bridge owner. Additionally, the variation in ground water levels and leakagefrom water services can also affect the backfill performance, as can flooding

� the external materials used for the bridge may be quite different in type and qualityto those found internally, for example, the external leaf of masonry might be dressedstone or strong engineering brick, concealing an internal structure of weaker bricks,rubble infill or voids. If there are no signs of deterioration and adequate loadcarrying capacity has been demonstrated by analysis, then the stress levels are suchthat even with inferior infill materials the structure may still be adequate

� such variation in quality is also found in stone bridges, where well-jointed andregular shaped ashlar blocks present on external surfaces are frequently backed byan irregular fill of random rubble or cobbles loosely cemented by a weak mortarmatrix. The rear face of stone blocks used for ashlar facings is frequentlyundressed, giving rise to variations in depth between blocks; this should beconsidered when assessing the true thickness of masonry courses. Multi-ring archesoccasionally comprise one material on the visible sections and another internally,which can mask defects (for example ring separation) and lead to erroneousassumptions regarding vault thickness and arch strength (see Figure 3.15).

CIRIA C656148

FFiigguurree 33..1133

OOrrlleeaannss bbrriiddggee ((PPeerrrroonneett,, 11778822--8833))ttrraannssvveerrssaall rreelliieevviinngg aarrcchheess oovveerrppiieerr aanndd hhaauunncchheess ((BBrreenncciicchh aannddCCoollllaa,, 22000022))

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There is no universal agreement about the assessment of skewed masonry arch bridges;however, some major asset owners do give guidance on this issue in their standards.While Network Rail accept a 2D analysis for skewed bridges (Network Rail, 2006), forskews up to 30° the Highways Agency advises that “average increase in load carryingcapacity of a skew bridge over a square bridge with the same span and width can beestimated from (b/w)², where b = abutment width and w = bridge width” (HA, 1997).For greater skews a 3D analysis should be undertaken.

However, large scale laboratory tests have demonstrated that arches with modest skewsof 22.5° display behaviour which is different to that of a square span arch (Melbourneand Hodgson, 1995; Hodgson, 1996). Some care is needed when applying the aboverules. Certainly bridges with a span/width ratio of greater than 1 should be consideredfor a 3D analysis, even those with modest skews of 15°. On the other hand, bridges withlow span/width ratios (less than 0.5) may be assessed based on a two dimensionalanalysis of the square span having due regard to the arch/spandrel wall interaction.

As discussed in Section 2.5.2, piers are particularly susceptible to any imbalancebetween the thrusts from each of the adjacent spans. Additionally, in the case of skewbridges the propensity for the thrust to span at right angles to the abutments results inthe pier of a multi-span bridge experiencing significant torsion. Tests have shown thateven for piers with a height/thickness ratio of 3.4, failure in torsion is possible(Melbourne et al, 1997 and 1998). This should be checked using an appropriate shearstress limit of 0.15 N/mm² for weak mortar. Piers should also be checked for soundness.

CIRIA C656 149

FFiigguurree 33..1144

IInntteerrnnaall ssttrruuccttuurree ooff sspprriinnggiinnggss aanndd ppiieerrss::aa)) VVeerrddee vviiaadduucctt:: mmaassoonnrryy aanndd ppoooorrllyyssuuppppoorrtteedd sspprriinnggiinnggss ooff tthhee lloowweerr ttiieerr ooffaarrcchheess;; bb)) QQuueeeenn MMaarrgguueerriittaa bbrriiddggee,, TTuurriinn::mmaassoonnrryy bbaacckkffiillll aanndd ssttiiffff ssttoonnee ssuuppppoorrtt oofftthhee sspprriinnggiinnggss ((BBrreenncciicchh aanndd CCoollllaa,, 22000022))

((aa))

((bb))

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FFiigguurree 33..1155 VVeerrddee vviiaadduucctt:: aarrcchheess ooff tthhee lloowweerr ttiieerr;; ((aa)) eexxtteerrnnaall ffrroonntt vviieeww;; ((bb)) iinntteerrnnaall ssttrruuccttuurree wwiitthhssttoonnee sspprriinnggiinngg aanndd bbrriicckk mmuullttii--rriinngg aarrcchh ((BBrreenncciicchh && CCoollllaa,, 22000022))

CIRIA C656150

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FFiigguurree 33..1166 TTrraavveerrssaa BBrriiddggee aatt kkmm 5522..113333 ooff tthhee SSaavvoonnaa--CCaarrmmaaggnnoollaa lliinnee,, NNoorrtthh--WWeesstteerrnn IIttaallyy.. aa)) HHoolllloowwaabbuuttmmeenntt--ppiieerr aanndd rruubbbbllee mmaassoonnrryy aallssoo iinn tthhee ssppaannddrreellss;; bb)) rriivveerr ppeebbbblleess mmaassoonnrryy wwiitthhccoouurrsseess ooff rreegguullaarr bbrriicckkmmaassoonnrryy aanndd hhoollllooww ppiieerr ((BBrreenncciicchh && CCoollllaa,, 22000022))

Tests on brickwork barrels subjected to quarter span point loading have shown that, forbarrels with an aspect ratio of 1, the full width of the unloaded side of the barrel isactive in resisting the load (Melbourne and Wang, 2002). Most small span bridges, upto about 6 m, are wider than their span, so there is some logic in analysing thesebridges on the basis of an aspect ratio of 1. Spandrel walls contribute to barrel stiffness.If the barrel is wider than the span then this contribution reduces; also if spandrel wallseparation is present then the stiffening effect is usually ignored –although research hasshown that if the backfill is in good condition the spandrel wall continues to stiffen thebarrel (Melbourne and Gilbert, 1997). However, current advice is to ignore anycontribution from the spandrel walls.

For a more thorough review of the influence of construction features on the structuralperformance and capacity of masonry arch bridges, the reader is referred to:

� Using construction history as an aid to masonry bridge assessment (Colla et al, 2002)

� The influence of construction technology on the mechanics of masonry railway bridges(Brencich and Colla, 2002).

DDeeffeeccttss

The assessing engineer is often faced with having to follow up a MEXE assessmentwhich has raised concerns about the carrying capacity of the bridge. This may be as aconsequence of the bridge’s condition or a function of the MEXE method itself – forexample, a small span bridge with cover greater than the arch thickness often exceedsthe 70 tonne maximum set by the method even when the bridge may be showingvisible signs of distress.

Multi-ring brickwork arches present a particular mode of failure which involves thebreakdown in the bond between the individual rings. This is referred to as ringseparation (see Figure 3.17). Where headers (bricks laid radial that offer shearconnection between the rings) are provided, this may be sufficient to bond the two

CIRIA C656 151

((aa)) ((bb))

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rings together. Even so, there are cases where the headers have been cracked at theinterface, with a resultant loss of homogeneity (see Figure 3.5). Ring separation can alsobe caused by deterioration and washout of the mortar; this may have occurred at sometime in the past and so may have affected even bridges that now appear to be “dry”.Large scale laboratory experiments (Melbourne and Gilbert, 1995) have shown that thenet effect of ring separation is to significantly reduce the load carrying capacity of thebridge. Full ring separation may cause up to 50 per cent reduction in carrying capacity.Even partial ring separation can significantly reduce the residual life of the bridgebecause propagation of internal cracking is accelerated by its presence (Melbourne,2001). There are examples where changes in loading regimes have accelerated suchdeterioration. Inspections should recognise the importance of ring separation and theassessment method should fully recognise its significance. There is no specific referenceto ring separation in the MEXE method and so caution should be exercised by theassessing engineer when dealing with multi-ring brickwork arch bridges.

FFiigguurree 33..1177 RRiinngg sseeppaarraattiioonn ddeetteecctteedd uussiinngg eennddoossccooppee ddoowwnn ccoorreehhoollee tthhrroouugghh aarrcchh bbaarrrreell

It is important to note that for any analysis, however empirical (in the case of theMEXE method) or sophisticated (as in the case of FEM/DEM methods), the result canonly be a good as the input data. This not only includes the field data for the particularbridge being assessed but also the compatibility of the validating field/laboratory data.

In particular, caution should be exercised in the interpretation of computer output.

The main advantage of computerised modelling techniques is that stress levels anddeflections can be calculated; how meaningful they are is open to question but they doprovide an idea of the potential problems. However, it is universally accepted thatmasonry arch bridges crack, even before the centering is removed. This is a veryimportant observation that should not be obscured by the sophistication of some of thecurrently available software.

Where the analytical technique specifically excludes any of the potential modes ofbridge failure then the assessor should check whether or not such modes can occur andtake appropriate action; these might include, for example, the possibility of snap-through in a flat barrel or ring separation in a multi-ring brickwork barrel.

CIRIA C656152

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Additionally, care should be taken when modelling the soil-structure interaction not tooversimplify the soil contribution without detailed knowledge of the soil properties andits condition. This is particularly important for small span and deep arch bridges.

Longitudinal cracks can occur anywhere within the barrel and reduce the capacity ofthe arch to distribute the load evenly throughout the arch and onto the abutments orpiers. Additionally, if the cracks occur immediately behind the spandrel wall it canisolate the wall reducing its contribution to supporting the arch. This is a point fordebate as most computer models ignore the contribution from the spandrel walls and itcan be argued that spandrel wall separation has no effect on the predicted carryingcapacity using such software. However, it is likely that the software has been validatedusing test data from field tests where spandrel walls were present. It is important tocheck such assumptions – in particular, the material from which the bridge wasconstructed, the nature of the foundations and backfill, the aspect ratio and the span torise ratio and depth of fill. If these characteristics individually or collectively differ fromthe validating test by more than 25 per cent then the validating test data should not beused, or at least treated with some caution and subjected to engineering judgment.

As discussed in Section 2.5.2, longitudinal cracks which extend down to the springingsmay be as a result of stresses caused by live loading from bi-directional traffic. Theyindicate that the arch is performing beyond its serviceability limit state and should bemonitored.

Transverse cracking may not be significant if longstanding, and can be treated asbenign but its presence should be taken into account when idealising the bridge forstructural assessment. Recent or “live” transverse cracks are a more immediate concern,particularly where they form at the quarter-span positions of single span bridges, or atthe crown position of multi-span bridges, where they may signify the formation of amechanism. In the case of voussoir arches this may not mean that the bridge is about tocollapse but frequent monitoring is recommended. However, in the case of multi-ringbrickwork arches such cracking may be accompanied by the formation of ringseparation which, as discussed earlier, has been shown in laboratory tests to reduce loadcarrying capacity (Gilbert and Melbourne, 1994; Melbourne and Gilbert, 1995;Melbourne et al, 1997).

Cracking in the crown region of a single span bridge may be associated with a“punching” type failure mechanism and which may be accompanied by the formationof a “yield-line” type failure. The assessment methodology should consider this mode offailure.

Diagonal cracking is invariably caused by non-uniform settlement/spread of theabutments/piers which results in torsion being induced in the barrel. The presence ofsuch cracking should be incorporated into the assessment model.

Table 3.8 gives some guidance regarding defects and how the various methods dealwith them.

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TTaabbllee 33..88 SSiiggnniiffiiccaannccee ooff ddeeffeeccttss ffoorr aasssseessssmmeenntt ppuurrppoosseess ((ccoonnttiinnuueedd))

CIRIA C656154

DDeeffeecctt MMEEXXEE RRiiggiidd bblloocckk FFEEMM//DDEEMM

Vertical differentialsettlement betweenadjacent supports;springings stay parallel.

Not specifically mentioned but isrolled up in the “lateral cracks orpermanent deformation of the archwhich may be caused by partialfailure of the arch or movement atthe abutments”. This is given acondition factor of 0.6–0.8.

No specificconsideration.

Settlement can be incorporatedinto the model.

Horizontal spread ofsupport; springings stayparallel.

Not specifically mentioned but isrolled up in the “lateral cracks orpermanent deformation of the archwhich may be caused by partialfailure of the arch or movement atthe abutments”. This is given acondition factor of 0.6–0.8.

No specificconsideration.

Horizontal spread of the supportscan be incorporated into themodel.

Horizontal inwardmovement of support;springings stay parallel.

Not specifically mentioned but isrolled up in the “lateral cracks orpermanent deformation of the archwhich may be caused by partialfailure of the arch or movement atthe abutments”. This is given acondition factor of 0.6–0.8.

No specificconsideration.

Horizontal inward movement ofthe supports can be incorporatedinto the model

Transverse settlementof an abutment or pier� rotation� local differential

settlement.

In the absence of any barrel crackingno specific guidance is given.If diagonal cracking is present thenthis is considered to be dangerous ifextensive.A condition factor of between0.3–0.7 is given.

No specificconsideration.

With a 3D model it is possible tomodel the cracking but this wouldmake the analysis very expensiveand could only be justified inspecial cases (eg for a listedstructure).

Effects of point loadactions (slipped units,fan yield-line patterns)

Considered when determining thecondition factor.

No specificconsideration.

With a 3D model it is possible tomodel the cracking but this wouldmake the analysis very expensiveand could only be justified inspecial cases (eg listedstructure).

Hinge formation andincremental loss ofstatical indeterminacy.

Not specifically mentioned but isrolled up in the “lateral cracks orpermanent deformation of the archwhich may be caused by partialfailure of the arch or movement atthe abutments”. This is given acondition factor of 0.6–0.8.

The 2D ultimate limitstate load carryingcapacity iscalculated.

The successive hinge formationcan be followed through theanalysis.2D and 3D models can be built.

Shear loading No specific consideration but shouldbe incorporated into the conditionfactor.

RING programspecifically modelsthe interfacebetween the units toallow for unitslippage and ringseparation.

This can be modelled but thereare problems with validating theoutput given that the parametersthat govern this mode ofbehaviour are not wellunderstood or measurable for anexisting structure.

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CIRIA C656 155

Transverse bendingeffects:Longitudinal cracking

The position and extent of the crackinginfluences the condition factor.This varies depending upon the bridgeauthority.Longitudinal cracks due to differentialsettlement are considered dangerousby the trunk road authorities. If thecracks are greater than 3 mm and atless than 1 m centres then a conditionfactor of 0.4 should be adopted.Otherwise, a factor of up to 0.6 maybe used.Network Rail, on the other hand, offersa range of condition factors from 0.95for a situation where the longitudinalcracks are outside the centre third ofthe barrel, less than one tenth of thespan in length, to 0.85 for a crackwithin the centre third of the barrellonger than one tenth of the span inlength.

Currently, all rigid blockformulations analysethe barrel as a 2Dproblem and so do notconsider the transversedistribution.Consequently, whetherlongitudinal cracks arepresent or not does notaffect the calculatedcarrying capacity permetre width.

Like the rigid block programs,2D models will not beaffected by the longitudinalcracking. In the 3D models,but it is possible to introducethe discontinuities caused bythe cracking. This will makethe model very complex andthe validation of its resultswould be highly questionablegiven our current knowledgeand material property data.

Transverse bending.Spandrel wallseparation.

Suggests that spandrel wall separationdoes not per se affect the conditionfactor but that its presence should beconsidered within the remit of theoverall condition factor.

Currently, all rigid blockformulations analysethe barrel as a 2Dproblem and so do notconsider the transversedistribution.Consequently, whetherlongitudinal cracks arepresent or not does notaffect the calculatedcarrying capacity permetre width.

Like the rigid block programs,2D models will not beaffected by the longitudinalcracking. In the 3D models,but it is possible to introducethe discontinuities caused bythe cracking. This will makethe model very complex andthe validation of its resultswould be highly questionablegiven our current knowledgeand material property data.

Material deterioration. Material factors take into account thecondition of the material.

Engineering judgementis applied to representthe barrel by adjustingthe geometry andstrength of the barrel.

It is possible for a 3D modelto vary the properties of thebridge material throughoutits thickness and width andthus represent the actualcondition of the bridge. Thisis very difficult and expensiveto do. More usually a“smear” approach is taken toreplicate the actual structure.

Ring separation Not specifically mentioned in any ofthe MEXE methods of assessment.This should be considered verycarefully. Large-scale laboratory testshave shown that there is the potentialfor crack propagation and that thisresults in loss of carrying capacity.Ignoring the bottom ring and carryingout an assessment using the reducedeffective thickness is notrecommended unless it has beenshown that the remaining ring isintact.

RING programspecifically models theinterface between theunits to allow for unitslippage and ringseparation.

This can be modelled butthere are problems withvalidating the output giventhat the parameters thatgovern this mode ofbehaviour are not wellunderstood or measurablefor an existing structure.

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MMaatteerriiaallss

The simulation of masonry in analysis of masonry arch bridges can be very complex.A2.3 includes a review of simulation methods for basic masonry properties. Details onmore advanced modelling methods can be found in Sicilia, 2001.

The structural performance of the arch barrel has been the subject of much research,both experimental and theoretical, but there are still many uncertainties regardingpredicting performance. In particular, the prediction of serviceability limit stateperformance continues to be a challenge, and the prediction of brittle phenomena suchas ring separation in multi-ring brickwork barrels is problematic.

Recent studies at the Universities of Salford and Cardiff (Melbourne et al, 2004) haveindependently confirmed that fatigue is an issue for brickwork barrels and should beconsidered. At present, the advice is to limit the stress to below that created by applyingsomewhere between 40 to 50 per cent of the ultimate carrying capacity of the arch.

When considering the effect of fatigue the condition of the bridge is important. Thepresence of cracks indicates that sections of the barrel have experienced redistributionof stress leaving elements of the bridge in a continuous state of stress in excess of thelimits given above, creating a situation where fatigue, or incremental loss of strength, islikely. This possibility should be investigated. There is an important distinction to bemade between cracks that result from settlement and those that result fromoverloading. In the latter case the working stress levels in the barrel are high and thestress range during each load cycle may be high. Conversely, in the former case, thereis no stress cycling although the working loads will produce some low stress cyclingwhich may be below the endurance limit and not a long-term problem.

In recent years, data from many laboratory and field tests have confirmed thecomplexity of the behaviour of masonry arch bridges. The assessing engineer shouldalways be aware of the limitations of the model that is used to assess carrying capacityand the effects of working loads on the residual life of the bridge. The published testdata that are used to validate the models have limitations that should not be ignored,not least of which is that all field tests were undertaken under monotonic loading.Additionally, most of the test results suggested a ductile behaviour of the arch barrel.

The loading systems used in these tests were very stiff compared with the stiffness ofthe bridge; this meant that as the arch barrel deflected the load would drop off veryquickly. In some cases servo-systems were used to follow the load but even in thesecases a falling load-deflection curve was observed. This means that had the load beenapplied as Kentledge, the bridge would have collapsed when the maximum load wasreached. There would have been some warning but there would have been littlereserve of extra strength.

CIRIA C656156

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33..1100..44 AAsssseessssmmeenntt rreessuullttss

WWhheerree aasssseessssmmeenntt ddeemmoonnssttrraatteess aaddeeqquuaattee ccaappaacciittyy

Where assessment results indicate that bridge capacity is adequate to meet loadingrequirements:

� structural adequacy should be kept under review and re-assessment may benecessary where there are changes in vehicle loading, in the bridge’s condition (iethere is new evidence of structural distress or deterioration) or in its environment

� the structural sensitivity of each bridge should be assessed, and critical elementsidentified. Additional measures (eg monitoring or special maintenance) may berequired to reduce risk and ensure the continued adequacy of bridges and bridgeelements which are considered particularly sensitive, where deterioration is ongoing,or where incidents might damage the bridge and compromise its structural integrity

� reassessment may also be required in response to specific needs, such as arequirement for the bridge to carry abnormal loads, or where the bridge is to bemodified in some way

� some organisations require reassessment of old bridges to be undertaken on a regularbasis at some specified interval, irrespective of any specific need, in order to ensurethat assessments are kept reasonably current and that the bridge’s capacity has notbeen significantly reduced by the processes associated with gradual ageing in service.

WWhheerree aasssseessssmmeenntt ffaaiillss ttoo ddeemmoonnssttrraattee aaddeeqquuaattee ccaappaacciittyy

Where assessment results indicate that a bridge does not meet the required capacity itshould, at least provisionally, be considered inadequate to meet requirements. A“failed” assessment does not necessarily mean that the bridge is inadequate to carry therequired loading and consideration needs to be given to other factors, including thebridge’s history, current condition and the potential degree of conservatism in theassessment results.

� the bridge should be managed in a safe and responsible manner until such a timeas either (i) its capacity can be proven to be adequate, either by reassessment or byimplementing remedial/strengthening works, or (ii) until it can be replaced. Thismay require the execution of precautionary measures, restrictions on its use (eg onvehicle weight or speed) and close monitoring of its condition (eg throughincreased frequency of inspections)

� where the assessment was based on a simple assessment technique (eg MEXE,thrust-line analysis) but the bridge shows no evidence of structural distress andengineering judgement suggests that the assessment technique is likely to beconservative, re-assessment using a more refined (and complex) assessmenttechnique may be appropriate. This may require additional investigation of thebridge to determine the parameters needed for such an analysis, and “hiddenstrengths” may be identified and taken into account. This approach is that of amulti-level assessment procedure (see Section 3.10.2)

� where structural distress is evident, where engineering judgement suggests thatbridge capacity is inadequate, and where any additional (more refined) secondaryassessment of the bridge has failed to demonstrate its structural adequacy, remedial(strengthening) works or replacement/renewal of the bridge should be considered.

CIRIA C656 157

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Wherever assessment indicates that a bridge has inadequate capacity to sustain therequired loadings, it is necessary to implement measures to mitigate the safety risksand, as far as possible, preserve the integrity and function of the bridge. The bridgeowner has a statutory obligation to ensure the safety of their staff and the public insuch situations, and all of the large UK infrastructure owners have their own internalprocedures and policies for dealing with under-strength bridges which satisfy theirstatutory obligations and align with their own organisational objectives. For example,BA79 (HA, 2001c) sets out requirements for highway bridges on primary routes andnecessitates replacement or strengthening of bridges which fail assessments areundertaken “without undue delay” – but it does also acknowledge that carrying outworks immediately may not be feasible or represent good management of the transportinfrastructure or limited resources; other measures, such as permanent weightrestrictions or a reduction in the number of traffic-carrying lanes, may be moreappropriate for bridges which are not part of the primary road network.

Available remedial options should be identified, considered and assessed based ontechnical, operational and policy considerations, and recommendations made.Appropriate precautions may be required to protect the structure and/or the publicbefore long-term remedial actions can be implemented.

Aspects to consider when determining the optimal course of action include:

� bridge condition

� strategic importance of route

� volume of traffic

� cost/benefit

� availability of alternative routes

� legal requirements

� environmental impact.

The range of options available for dealing with bridges which fail to meet structuralperformance requirements include:

� monitoring

� further testing and investigation

� refined analysis

� load tests (not permitted by some asset owners, see Section 3.8.3)

� weight or width restrictions

� strengthening

� reconstruction to current standards

� closure.

The presence of services in bridge structures and the cost of diverting or supportingthem during reconstruction can make some repair or replacements unviable.

The optimal timing of works to replace or repair a weak structure is dependent upon anumber of factors, including:

� volume and weight of traffic normally carried by it

� variations in traffic (eg periods of peak usage)

CIRIA C656158

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� the effect of traffic restrictions and redirection on the local transport infrastructure

� the availability of alternative unrestricted crossings

� interaction with works scheduled on other elements of the local infrastructure

� nuisance and costs of delays and diversions to customers

� environmental impact of redirection, longer journeys etc

� the availability of resources

� economic factors, corporate policy and long-term performance objectives andtargets.

In certain instances, immediate replacement or repair may not be feasible, or mayrepresent poor value for money. In such situations it may be possible to justify thedecision to postpone such works until a more favourable time and continue with thenecessary restrictions in usage until that point. In taking this decision it is important togive due consideration of all of the above factors, and to understand that increases inthe volume of traffic in other areas of the transport network, for instance on nearbybridges, may hasten their deterioration. The cost/benefit assessment of maintenancedeferral is discussed in Section 3.4.8.

CIRIA C656 159

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TTaabbllee 33..99 CCoommppaarriissoonn ooff mmaaiinn aannaallyyssiiss mmeetthhooddss ffoorr bbrriiddggee aasssseessssmmeenntt

CIRIA C656160

MMeetthhoodd MMaaiinn ppaarraammeetteerrss AApppplliiccaabbiilliittyy AAddvvaannttaaggeess DDiissaaddvvaannttaaggeess

MEXE � span� rise at crown and quarter

point� arch thickness and crown

cover� arch material� backfill material� mortar joint depth and

condition� general condition factor.

� only applicable to spans shorterthan 18 m

� not applicable for flat orappreciably deformed arches

� not applicable for multi-spanbridges, although the BR versionof MEXE accounted for that usingan extra factor.

� relatively quick and easy,uses readily available data,and therefore aneconomical method forassessing large numbers ofbridges

� if applied by an experiencedengineer, the conditionfactors may be used toaccount for effects whichare difficult to modelaccurately

� longstanding and extensiveexperience with the methodgives confidence in its use.

� the only resisting mechanismsconsidered are the arch and the weight ofthe backfill

� the limiting load criterion is not realistic� unnecessary assumptions on geometry

and load locations� the subjectivity of some modifying factors

require the judgement of an experiencedengineer

� its results are assumed to beconservative, but can be over-conservative or, in some circumstances(eg very short spans) potentiallyunconservative

� cannot consider the effect ofstrengthening measures.

Heyman’slimitanalysismethods

� arch geometry� compressive strength of

masonry (in some models)� masonry and backfill

densities.

� can be difficult to apply to deeparches

� can be difficult to apply toshallow arches with large spans

� can be difficult to apply tobridges with complex geometries.

� for simple structures, it canproduce safe results fromvery limited input and at alimited cost

� these methods are veryeffective when the engineerhas a clear idea of themechanism by which thestructure will fail.

� for upper bound methods, if some failuremechanisms are ignored, the methodwould provide an unsafe prediction

� for lower bound methods, if somekinematically admissible equilibriumstates are ignored, the method wouldprovide a conservative prediction.Similarly, if an assumed equilibrium is notpossible (because some failure criterionhas been ignored) the method willproduce unsafe results

� cannot consider ring separation� cannot consider snap-through failures� cannot consider the contribution of the

spandrel walls.

Discreteandindiscreterigid blockmethods

� arch geometry� compressive strength of

masonry (in some models)� masonry and backfill

densities� dilatancy� angles of friction (radial

and tangential).

� can be applied to multi-ringarches

� can be applied to multi-spanarches

� *can produce unsafe results inshallow arches with large spans(bridges where snap-throughfailure is possible)

� some methods might be able toconsider skewed arches.

� quick and reliable for asignificant range of bridgeconfigurations

� it is a very versatile tool foran experienced engineer.

� cannot consider snap-through failures� cannot consider the contribution of the

spandrel walls� the separation between rings cannot be

reproduced. Instead, the used has toassume whether ring separation will orwill not take place

� consideration of masonry compressivefailure might increase the computationaltime.

Castigliano’s non-linearanalyses

� arch geometry� compressive strength of

masonry (in some models)� masonry and backfill

densities� the deformational and

strength parameters of thebackfill.

� cannot be used with skewedbridges.

� simple and easy to use � the prediction of the in-service behaviourcan be quite sensitive to the boundaryconditions and the initial stress state,which are very difficult to determine

� cannot consider ring separation.

Finiteelement

� arch geometry� initial stress state� compressive strength of

masonry (in some models)� masonry and backfill

densities� the deformational and

strength parameters of thebackfill

� masonry tensile strengthand post-yielding response(softening rule etc).

� these methods provide results onthe in-service behaviour of thestructure. As such, they can beused to analyse existing defects,their origin and relevance on thesafety of the structure

� similarly, they can be used toconsider strengthening and/orrepair options taking into accountnot only their effect on thecapacity of the structure, but alsoon the performance of thestructure and its components.This allows more consideration tobe given to the performance ofthe structure and hence the long-term effects of the methodsadopted.

� can be extremely versatileand allow almost anysophistication required

� the versatility of thesemethods means that theycan be used to explore thebenefits of variousstrengthening options.

� the prediction of the in-service behaviourcan be quite sensitive to the boundaryconditions and the initial stress state,which are very difficult to determine

� the lack of customised packages meansthat the preparation of the models canbe quite time consuming

� as the complexity of the model increases,so does the time required to obtainresults. Since parametric studies areessential, this option can become tooexpensive

� the results are sensitive to inputparameters difficult to determine such asthe backfill properties, the masonrystrength and the interface between thedifferent structural elements.

Discreteelement

� arch geometry� initial stress state� compressive strength of

masonry (in some models)� masonry and backfill

densities� the deformational and

strength parameters of thebackfill

� masonry tensile strengthand post-yielding response(softening rule, etc)

� contact properties.

� same as FE. � same as FE, with theadvantage of coping betterwith discontinuities (such asring separation andspandrel separation)

� uses explicit solver whichimproves convergence.

� the prediction of in-service behaviour canbe quite sensitive to the boundaryconditions and the initial stress state,which are very difficult to determine

� the lack of customised packages meansthat the preparation of the models canbe quite time consuming

� as the complexity of the model increases,so does the time required to obtainresults. Since parametric studies areessential, this option can become tooexpensive

� the explicit solver requires a rigorouscheck of the results predicted.

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Table 3.10 is intended to identify both the importance and the sensitivity of differentassessment methods to the different properties of masonry arch bridges. Given thewide range of methods that currently exists and that will be developed in the nearfuture within each category of assessment considered, the values presented can only beindicative. Similarly, the sensitivity of the assessment methods to the input parameterswill vary with the characteristics of the masonry arch bridge being considered.

TTaabbllee 33..1100 SSeennssiittiivviittyy ooff aannaallyyssiiss tteecchhnniiqquueess ttoo iinnppuutt ppaarraammeetteerrss aanndd tteecchhnniiqquueess ffoorr oobbttaaiinniinngg tthheemm

CIRIA C656 161

IInnppuutt ppaarraammeetteerrPPaarraammeetteerr sseennssiittiivviittyy

((���� == mmoosstt sseennssiittiivvee,, �� == sseennssiittiivvee,, �� == nnoott sseennssiittiivvee)) HHooww ttoo oobbttaaiinn11

BBaassiicc 22DD lliimmiitt

aannaallyyssiiss22BBaassiicc 22DD rriiggiidd

bblloocckk33 33DD FFEEMM44 33DD DDEEMM55

GGeeoommeettrryy

Number of spans �� �� �� �� Visual inspection

Arch profile �� �� �� �� Measurement by standard surveying techniques or“advanced” techniques eg laser profiling, photogrammetry.

Arch thickness profile �� �� �� ��

Coring (E), external measurement (it should be rememberedthat arch barrels are often not the same thickness throughoutthe width of the bridge; it cannot be assumed that anyprevious surveys took this into account. This is a fundamentalparameter that must be correct).

Number of rings � �� �� �� Coring (E), visual inspection (see “Arch thickness profile”above)

Backfill profile � � �� �� Trial pits (E), radar (E), Standard survey techniques

Presence of backing �� �� �� �� Trial pits (E), radar (E), electrical conductivity (E), acoustictomography (E)

Surfacing thickness � � � � Trial pit, coring, radar (E)

Spandrel wall thickness andheight � � � � Coring (E), radar (E) – both coupled with standard surveying

techniques

Width of bridge loaded � � � � Direct measurement

Pier height and thickness � � � � Standard surveying techniques, coring (E), radar (E)

MMaatteerriiaall pprrooppeerrttiieessMaterial properties are typically determined using in situ or laboratory tests which are often expensive and also relate only to the location of thetest/sample, so may not be fully representative of the element that is being modelled. Additionally some of the parameters required for certain analysistechniques/software cannot be measured, in which case a parametric sensitivity analysis should be undertaken and carefully validated before resultscan be relied upon..

Masonry elastic properties � � � �

Standard tests on core samples. Test specimens should beintact on arrival at the laboratory. Remember that the corewas probably taken perpendicular to the direction of theprincipal compressive stress. Flat jacks have been used toestimate elastic modulus of masonry in situ (E)

Masonry density � � � � Tests on core samples

Masonry crushing strength � � � � Tests on core samples

Masonry shear strength � � � � No test – estimate

Masonry tensile strength � � � � No test – estimate

Masonry post-yield behaviour � � � � Tests on core samples

Ring-ring friction properties � �� � ��No test – estimate (could use the core sample to assess thenature and properties of the mortar and hence estimate theinternal friction between the units and the mortar)

Spandrel-arch and spandrel-backfill friction properties � � � � No test – estimate from backfill properties

Backfill density �� �� �� �� Determine using standard tests (eg sand replacement)

Backfill deformational properties � � � � Determine using standard geotechnical tests

Backfill strength properties � � �� �� Determine using standard geotechnical tests

Surfacing density � � � � Tests on core samples

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1 Potential methods of measurement are indicated where applicable; these are listed in order of typical reliability. (E)indicates that the technique is likely to be relatively expensive to apply. For more information on these techniques, seeSection 23.8

2 Assuming a Heyman type analysis model, incorporating extensions such as the ability to model multi-span arches andfinite masonry strength (eg ArchieM).

3 Assuming the masonry is modelled explicitly using rigid blocks, with the backfill modelled in a simplified manner.Supports assumed fixed (eg RING 1.5).

4 Non-linear finite element model in which the masonry and surrounding fill are modelled using continuum elements.Smeared crack model for the masonry.

5 Assuming the masonry and surrounding fill are both modelled using 3D discrete elements.

CIRIA C656162

CCoonnddiittiioonn aassppeeccttss

Spandrel arch connectionconditions � � � � Visual, hammer tapping, radar (E)

Connection between rings � �� �� �� Hammer tapping, sonics (E), radar (E)

Abutment conditions � � � � Core (E), radar (E), sonics (E), hammer tapping, visual

Masonry condition � � � � Core (E), radar (E), sonics (E), hammer tapping, visual

Initial conditions � � � �Flat jack investigation and crack survey together withdeformation analysis

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44 SSeelleeccttiinngg aanndd ccaarrrryyiinngg oouutt bbrriiddggee wwoorrkkss

44..11 HHeeaalltthh aanndd ssaaffeettyy ccoonnssiiddeerraattiioonnss

44..11..11 HHaazzaarrddss,, rriisskkss aanndd ssaaffee wwoorrkkiinngg

Working on railways, highways or waterways can be dangerous and regulations andpractical measures exist to ensure the safety of those working on them and themembers of the public who may use them or be affected by them.

Risks associated with loss of bridge performance or collapse are managed by theprocess of condition assessment, maintenance and repair, which is the principal subjectof this document. Carrying out maintenance, repair, strengthening and reconstructionworks on masonry arch bridges present a range of hazards in common with thoseassociated with working on other types of bridge:

� hazards inherent to the environment; the potential hazards inherent in working onmasonry bridges include working at height, working near live roads or rail traffic,above or adjacent to water courses, and slipping/tripping. When work is carriedout on bridges that are at least in part unstable or unsound the hazard of beingstruck by falling masonry is also likely to be present

� exposure to hazardous materials; these may be included in the fabric of the bridgeor be used in carrying out the works. The structural fabric of masonry arch bridgesseldom contains hazardous materials, but they may be present in bridge services,or in adjacent areas. Materials used in bridge repair, such as resins and grouts, maybe hazardous in their application, and work which generates dust and fumes maypresent a hazard to those carrying out the works and to members of the public

� use of plant and equipment; as with any other civil engineering work, there arerisks associated with the use of heavy plant, such as cranes and excavators, lightplant, such as generators, and hand-held tools, such as angle-grinders. Specialaccess equipment is often required, and this can also introduce hazards to the work

� risks to bridge users and members of the public; in many situations it is necessary tocarry out work in areas where a bridge, or the area beneath it, is not entirely closedto use or where members of the public are in close proximity and might be affected.Safe systems of work should consider the safety of the public as well as workers.

The hazards from any work on bridges should be identified and risks carefully managedto reduce them to an acceptable level and to comply with statutory requirements.Owners of infrastructure have their own health and safety management systems to allowthem to meet legal requirements. Consultants and contractors should adhere to theowner’s systems when carrying out any work on site, in addition to complying with theirown safety management systems. This might include the need for site observation visitsand visual inspections. When construction work is being carried out on an operationalsite that is under the control of the owner, co-ordination may be necessary to clarify whois in control of the work area. The Construction (Design and Management) regulationsinclude requirements relating to the control of construction work.

The CDM Regulations require that the hazards of construction and maintenance areavoided or reduced during the design phase. All those involved in design decisions

CIRIA C656 163

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have a part to play in this, including the client and contractor. Active risk managementcan be aided by involving all parties in a risk workshop at the early planning stages.When identifying hazards and designing out risk, records of designers’ actions shouldbe kept and where significant residual risks remain after design action has been taken,information on these should be communicated to allow those exposed to them to plan asafe system of work. If the risks relate to construction, contractors will receive thisinformation via the health and safety plan. If the risks relate to future maintenance thisinformation will be placed in the health and safety file which is held by the client forthe work.

44..11..22 SSttaattuuttoorryy lleeggiissllaattiioonn

The Health and Safety at Work etc Act 1974 places duties on many parties includingemployers, employees and those in control of premises. The principal requirement isthe requirement for employers to provide a safe system of work for their employees “asfar as is reasonably practicable”. It establishes the framework for safe working in theUK, allowing regulations bought in under the act to introduce more specificrequirements. Among the principal regulations under this Act that are pertinent to andhave an impact on the maintenance of railway, highway and waterway bridges are:

� the Management of Health and Safety at Work Regulations 1999. The mostimportant requirement of these regulations is for all employers to take steps toeliminate or reduce risk based on a sufficient assessment of the risks to employeesand others as a result of their undertakings. Requirements also include co-operation between employers on health and safety matters and the provision ofinformation

� the Construction (Design and Management) Regulations 1994, as amended andcurrently under revision, which require all those involved in designing and planningconstruction work to consider health and safety risk arising from construction,during the design process in an attempt to eliminate or reduce that risk

� the Construction (Health, Safety and Welfare) Regulations 1996 (under revision)set the minimum standard for physical conditions on a construction site includingprotection from physical hazards and provision of welfare facilities.

Additionally, there are a number of other items of health and safety legislation whichare pertinent to working on bridges, including (but not limited to):

� the Control of Substances Hazardous to Health (COSHH) Regulations 2002 is themain piece of legislation covering control of the risks to employees and otherpeople arising from exposure to harmful substances generated out of or inconnection with any work activity under the employer’s control. The regulationsset out a simple framework for controlling hazardous substances in the workplace.Employers must ensure that the exposure of employees to hazardous substances iseither prevented (ie no exposure) or adequately controlled

� the Working at Height Regulations (WAHR) 2005 apply to all work at heightwhere there is a risk of a fall liable to cause personal injury. They place duties onemployers, the self-employed, and any person who controls the work of others (egbridge managers or owners who may contract others to work at height) to theextent they control the work.

Note that the health and safety legislation discussed here is valid in Great Britain only, and hasequivalents in Northern Ireland.

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Section A5 includes a listing of the principal legislation applicable to health and safety(Table 5.1) current at the time of publication. Further information concerning currenthealth and safety legislation can be found in Tyler and Lamont (2005). However, itshould be noted that legislation is liable to change and it is the responsibility of thoseinvolved in the management of bridges and bridge works to ensure that currentlegislation is adhered to.

OOtthheerr rreessttrriiccttiioonnss aanndd rreegguullaattiioonnss

In addition to statutory regulations, infrastructure owners may have policies in thefollowing areas which workers and organisations should be aware of:

� alcohol and drugs policies

� safety training of individuals and safety certification and approval of contractors

� passes, permits and certification required to access bridges and trafficked areas(requiring suitable allowances for training costs and a lead in time to projects)

� working on or near a railway, road or canal, which is only possible with restrictionsto traffic or time of working

� railway possessions, working within engineering hours or road and canal bookingall require a lead-in time

� precautions should be taken against fire, for proper storage of equipment, foradequate welfare facilities and safe storage and disposal of waste.

44..22 EEnnvviirroonnmmeennttaall ccoonnssiiddeerraattiioonnss

44..22..11 EExxeeccuuttiinngg wwoorrkk oonn hhiissttoorriicc bbrriiddggeess

If the historic significance or statutory protection status of a bridge is unknown oruncertain, it is important to consult the county conservation officer and the countyarchaeologist, preferably at an early stage of any project being considered. Theprincipal source of information on historic assets is the planning authority’s sites andmonuments record, normally available through the county planning department orcounty museums service (Streeten, 1990). Information on listed structures is held bythe Department for Culture, Media and Sport.

Rigorous specification, careful control and adherence to statutory procedures arerequired for works on any bridge with recognised historical value. Permissions for workwhich may affect the bridge structure or appearance are unlikely to be granted unless itcan be demonstrated that the project has considered the full heritage implications ofthe works and takes an approach that conforms to good conservation practice. Planningconsent may have to be applied for and negotiated some time prior to commencing anyworks. It is worth remembering that carrying out work on a protected structurewithout consent is a criminal offence, and may result in a fine or even a prison sentencefor responsible individuals. Additionally there are a wide range of other statutory andnon-statutory designations which can affect the approach to carrying out bridge works.

Work on a structure with recognised historic value, particularly where it is affordedstatutory protection, typically requires planning, liaison and project managements skillsover and above that required for works on modern structures. Good investigative andpreparation work are particularly important elements of a successful project. It is likelythat specialist expertise will be required at all stages of the work from its planning andinception through to its completion.

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Successful rehabilitation of historic bridges requires a conservation-led approach.Success is achievable where there is a team effort between all parties involved to selectand agree on appropriate methods, materials and working practices to achieve a safeand functional structure that satisfies the technical and heritage objectives of theproject. Engineers and conservationists need to adopt a cooperative attitude whenconsidering the balance between functional and heritage requirements of bridges.

� engineers should seek to gain an appreciation of the historical significance of thebridge and be sympathetic to the conservationist’s concerns so as to betterunderstand and address the conservation needs of the structure

� for the conservationist, it is important to understand the technical and other issuesthat constrain the engineer, to appreciate that most bridges are primarilyfunctional elements of the transport infrastructure, and be prepared to makecompromises where necessary.

Further guidance on the conservation of historic stuctures, including legal andmanagement issues, is given in BS 7913 (BSI, 1998) and in Tilly (2002).

CCoonnsseerrvviinngg hheerriittaaggee vvaalluuee

The designation of scheduled and listed structures carries a presumption that every effortwill be made to preserve them. However, even where a bridge does not have the benefitof statutory protection, its historical and heritage significance should be taken intoaccount when considering carrying out any works, especially those involving alterations.Old structures often require adaptation to meet modern functional requirements,including those of health and safety. It is preferred that old masonry bridge structures berehabilitated sympathetically to allow them to fulfil their original function.

For example, existing bridge parapets may need altering to increase their height andimprove their impact resistance, or strengthening may be required to accommodateincreases in permitted vehicle loading. Wherever possible this should be carried outsympathetically with the aim of retaining the overall character of the bridge. Originalfabric should be retained and complete replacement by new elements should beavoided. Where replacement is unavoidable, new elements should follow the originalpattern and design. It is good practice to keep any alterations in the style of theoriginal yet make them identifiable as part of the historical development of thestructure. Clearly it is important that any works carried out do not hasten thedeterioration of remaining original fabric.

A number of questions should be considered:

1. How has the structure functioned to date and how well has it survived?

2. Are any changes of use proposed and how are they likely to affect the structure?

3. Are works required to preserve the structure or accommodate changes of use?

4. Will these works damage or require the removal of the historic fabric of the bridgeor change its appearance in any way?

5. Will the works change the function of the bridge and could they result in furtherdeterioration of its historic fabric?

6. Will the original character and essential historical and aesthetic value bemaintained?

The restoration of a bridge does not necessarily involve returning it to close to itsoriginal state. The great age of many bridges means that over time they have been

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altered or augmented in some way, and these changes may themselves have historicvalue which should be considered when undertaking restoration or repair works.Gratuitous alteration is always undesirable. However, in certain instances it may besuitable or even necessary to reverse previous misguided works by replacinginappropriate materials, repairs or additions to the structure in order to restore abridge’s architectural composition or to ensure its performance and longevity. For themajority of bridges, work carried out on the principle of “repair as found” will oftensatisfy the imperative to preserve its historic integrity and character. Further guidanceon sympathetic alteration is given in the Highways Agency publication The appearance ofbridges and other highway structures (HA, 1996).

Whenever changes are made to an historic structure suitable records of its originalform should be made before the works commence, and in certain circumstances duringthe works. These records should be compiled by a cultural heritage specialist.

It may not always be preferable to rehabilitate historic bridges; occasionally thenecessary works may damage the historic value of a bridge, so it would be better topreserve it untouched and provide an alternative crossing.

AArrcchhaaeeoollooggiiccaall vvaalluuee

Work on an historic bridge may be essential for its preservation, but could entail thedisturbance or destruction of valuable archaeological information. A bridge’sfoundations and backfill, as well as the areas of ground around it, can provideimportant evidence concerning its original construction and subsequent history, as wellas that of its use, since most river crossings have a long history of human travel andcongregation. Where work unavoidably causes damage or removal of historic featuresor fabric, whether visible or hidden, this may require archaeological appraisal andshould be carefully recorded by an appropriate authority. This is also the case whereworks, including temporary works, will involve excavation around the foundations andin the area adjacent to the bridge.

Unless dealing with scheduled structures, an appropriate heritage body should be thefirst point of call. Initially contact the local planning authority archaeologist forarchaeological matters and the local authority conservation officer for listed buildings.

Further guidance on the archaeological aspects of bridge works is provided in Tilly (2002).

PPrreesseerrvviinngg hhiissttoorriicc ffaabbrriicc aanndd mmaatteerriiaallss

Wherever practicable, original materials should be reused unless they have alreadyproven unsuitable or are unlikely to provide adequate performance. For works overwatercourses, dredging may recover original elements such as copings which maycleaned and considered for reuse (although this may also damage archaeologicaldeposits and should be considered with care; archaeological supervision may berequired). Where new materials are required to replace old, they should be carefullyselected to replicate the originals, not only in appearance – for aesthetic value – but alsoin their strength and durability for sound technical reasons. For instance:

� historic brick structures require hand-cast bricks of the same dimensions as thosethey replace, since chopping modern machine-made bricks to size results inunappealing sharp edges and makes for visually obtrusive repairs. The use ofstrong modern bricks may also create “hard spots” which attract load and result incracking and spalling

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� where stonework needs replacement, efforts should be made to find materials ofthe same type, quality and appearance as the original. Selection should be basedon careful comparison of samples, and include consideration of aspects beyond thesuperficial eg comparative performance and durability. Particularly where originalquarries have been exhausted, this is likely to require investigation of currentsources for suitable matches and a consideration of the physical and chemicalattributes of original and potential replacement materials

� use of unsuitable mortars is a frequent cause of accelerated decay, since modernmortars based on Portland cement are much harder and less permeable than thelime mortars they typically replace. Mortar should be durable but sacrificial anddecay in preference to the masonry units (see Section 4.3.3 for further guidance).

Knowledge of the performance, selection and specification of traditional and modernmaterials is vital to carrying out successful repairs to the fabric of historic buildings, andrequires the involvement of a suitably experienced specialist engineers andconservators. Likewise, it is necessary to use craftsmen and contractors with provenskills in the use of traditional materials and conservation techniques on sensitive tasks.

Further guidance on the recovery and use of reclaimed bricks is given in Observations onthe use of reclaimed clay bricks (BDA, 2001b).

44..22..22 DDeeaalliinngg wwiitthh pprrootteecctteedd ssppeecciieess

All bridge works should include consideration of the potential presence of bats andother protected species. The first stage of any investigation into the presence of suchspecies will normally comprise a desk study, to include the records of the bridge owneror overseeing authority, as well as the appropriate SNCO, other non-statutoryconsultees, such as local wildlife trusts, local records offices, county recorders and anylocal wildlife groups.

Where the presence of protected species such as bats, badgers and great crested newtsis suspected or established, ecological surveys should always be carried out by anecologist and the appropriate SNCO contacted and allowed a reasonable time toprovide advice or agree on a course of action suggested by an ecologist before anyworks are carried out. Where a bridge spans a watercourse protected species such asfreshwater crayfish, otters and water voles should also be considered along with thelegislation relating to invasive plants. The Wildlife and Countryside Act 1981 makes itan offence to plant or otherwise cause to grow in the wild any plant listed on Schedule9 Part (II), such as Japanese knotweed an invasive non-native plant growing rapidly tothe detriment of other plants and animals. The list is not exhaustive and each siteshould be subject to desk study and SNCO consultation to address particular areas ofinterest.

If protected species are discovered while work is being carried out on site, work shouldbe stopped immediately and advice from the appropriate SNCO or a suitablyexperienced and licensed ecologist should be sought on how best to proceed.Damaging or disturbing protected species can result in prosecution under a range oflegislation, and the penalties can be harsh. Fines for non-compliance vary according tothe species and type of damage caused but disturbing protected species such as bats canresult in fines of £5000 per animal – it is important to note that a single small bridgecan be home to a considerable number of individual bats or other protected species.Vehicles or other equipment used to commit the offence can be confiscated, and inEngland and Wales, the person committing the offence can be imprisoned for up to sixmonths.

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Further guidance on dealing with protected species is included in:

� CIRIA C587 Working with wildlife: a resource and training pack for the constructionindustry (Newton et al, 2004) – species briefing sheets

� CIRIA C650 Environmental good practice on site (second edition) (CIRIA, 2005).

A case study illustrating good practice in dealing with protected wildlife species isincluded in Section A1.4

DDeeaalliinngg wwiitthh bbaattss

The soffits of old arch bridges are a favourite roosting place of bats, which are strictlyprotected by UK and EU law due to their rapidly declining numbers. They inhabitboth rural and urban sites and are easily disturbed by maintenance and repair works.Both English Nature and the Countryside Council for Wales have highlighted damageand losses to bat populations associated with the routine maintenance and demolitionof arch bridges.

The Bat Conservation Trust summaries bat legislation within their Professional SupportSeries leaflets as follows:

� the Wildlife and Countryside Act 1981 provides protection for all bats and theirroosts and requires consultation with English Nature before carrying out activitiesthat might harm or disturb bats and/or roosts

� the Countryside and Rights of Way Act 2000 adds the word “reckless” (in Englandand Wales) to the offence of disturbing a bat or damaging/destroying a place a batuses for shelter of rest (ie a bat roost). This is important legislation because itprotects bats and roots from reckless and/or international disturbance/damage

� under the EC Habitats Directive it is considered an offence to damage or destroy abreeding site or resting place of any bat, or to deliberately capture, kill or disturb abat. Most development and maintenance works affecting bats and/or roosts egbridge/tree maintenance works require a Habitats Regulations Licence that mustbe applied for and obtained from Defra.

Activities such as repointing and repair of masonry may result in the disturbance ofbats and loss of the cracks and crevices necessary for roosting and hibernation, whichmay be illegal under the above legislation. In some circumstances licences may beobtained from Defra to permit actions affecting bats or their roosts that are normallyprohibited by law, but it will be necessary to demonstrate that the proposed works arenecessary for public health or safety, or for reasons of overriding public interest.Applicants should demonstrate that there is no satisfactory alternative and suitablemitigation measures are likely to be required; these may include restrictions on thetiming of works, protection of existing roosts or the provision of alternative roosts.There is likely to be a requirement to monitor the bats and the adequacy of themitigation measures, and this may take considerable time. It is advisable to seek theservices of a professional environmental consultant with appropriate experience at anearly stage of planning when considering works that might affect bats or their roosts.

Where provision of alternative roosts is required, a variety of proprietary bat boxes andother artificial roosts are available for such uses, including “bat bricks” which can beincluded at suitable locations within a masonry arch (Figure 4.1). When considering theuse of such artificial roosts it is important that expert advice from a bat specialist issought to assist with their selection and location.

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Bat mitigation guidelines (Mitchell-Jones, 2004) have been published by English Nature,and include detailed guidance on bats, their habitats, bat surveys and acceptablemitigation plans for development and construction, with case studies which may beuseful to those who have to deal with bat occupied structures.

Further guidance on dealing with bats is included in:

� Bats in buildings (SNH, 2004)

� Bats, development and planning in England (BCT, 2002)

� Bats and bridges (BCT, 2003)

� Nature conservation in relation to bats (HA, 1999).

44..22..33 PPrreevveennttiioonn aanndd ccoonnttrrooll ooff ppoolllluuttiioonn

When working on bridges it is important to prevent debris or materials from fallingonto the ground or water below, and suitable mitigation measures should beincorporated into the working methods where this might occur. This is especiallyimportant where potential pollutants and hazardous materials are being used, or forworks over public areas or near watercourses and sensitive ecological sites. Under theEnvironmental Protection Act 1990 it is an offence to deliberately or accidentallypollute controlled waters (all watercourses, lakes, lochs, coastal waters andgroundwater) and any discharges into them require consent from the relevantenvironmental agency. Other waste produced on construction sites is subject to theduty of care under the Environmental Protection Act 1990 and may be subject to

CIRIA C656170

FFiigguurree 44..11

PPrroopprriieettaarryy ““bbaatt bbrriicckk”” aarrttiiffiicciiaall rroooossttaanndd ssuuggggeesstteedd llooccaattiioonnss ffoorr iinnssttaallllaattiioonn((ccoouurrtteessyy tthhee NNoorrffoollkk BBaatt GGrroouupp//AAuurruumm EEccoollooggyy 22000055))

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control under the Waste Management Licensing Regulations 1994. Separate legislationapplies in Northern Ireland.

Detailed guidance on water pollution is given in CIRIA C532 Control of water pollutionfrom construction sites – guidance for consultants and contractors (Masters-Williams et al, 2001)which identifies potential sources of water pollution from within construction sites anddiscusses effective methods of preventing its occurrence. Further guidance is given inthe Pollution Prevention Guidelines (PPGs) published by the Environment Agency forEngland and Wales and equivalent agencies in Scotland and Northern Ireland, inparticular:

� PPG1: General guide to the prevention of pollution (EA, 2001a)

� PPG5: Works in, near or liable to affect watercourses (EA, 2000)

� PPG6: Working at construction and demolition sites (EA, 2001b)

� PPG23: Maintenance of structures over water (EA, 2002).

All the above may be downloaded free from <www.environment-agency.gov.uk>.

Waste minimisation and recycling of materials is an important factor in controllingenvironmental impact and minimising pollution associated with maintenance andrepair works. Useful guidance for those involved with site work is provided in CIRIASP133 Waste minimisation in construction – site guide (Guthrie et al, 1997).

44..33 PPrreevveennttaattiivvee,, rreemmeeddiiaall aanndd ssttrreennggtthheenniinngg mmeeaassuurreess

44..33..11 GGuuiiddaannccee oonn sseelleeccttiioonn ooff mmeeaassuurreess

This guidance encompasses a range of preventative, remedial and strengtheningtechniques that can be successfully implemented to minimise deterioration and addressvarious structural defects and inadequacies commonly encountered in masonry archbridges.

There are three levels at which work is undertaken on a bridge:

� routine maintenance, eg repointing, which is often preventative in nature

� repair, eg replacement of defective masonry, which is corrective in nature

� strengthening, eg where an enhanced carrying capacity is required, which isintended to provide improvement.

In all cases the bridge may have multiple defects of various degrees of severity, and itmay be advantageous to coordinate such works so that problems are tackled together,minimising disruption to the bridge’s normal service and potentially saving time andmoney. Selection of the most appropriate approach to bridge works should includeconsideration of a number of factors (after Broomhead and Clark, 1995):

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� type of fault or faults to be repaired

� ease of access

� health and safety, environmental and heritage considerations and constraints

� available clearances

� length of possession times/lane closure requirements

� cost of repair options

� expertise required to execute repairs and contractor availability

� performance, long-term durability and maintenance requirements of repairs

� purpose of repair and ability to satisfy requirements

� obstruction of future arch barrel inspections.

As well as aiming for an effective and economical solution to immediate problems, it isparticularly important to consider the potential influence of any works on the bridge’slong-term performance. Since most masonry arch bridges will be expected to remain inservice for considerable periods of time in the future, and it is important not to carryout works which might compromise their inherent durability. These “engineeringconsiderations” and other factors influencing the selection of remedial or strengtheningmethods are discussed further in Section 4.3.4.

� routine and preventative maintenance activities are considered in Section 4.3.2

� preservation of masonry fabric is considered in Section 4.3.3

� repair and strengthening techniques are considered in Section 4.3.4.

44..33..22 RRoouuttiinnee aanndd pprreevveennttaattiivvee mmaaiinntteennaannccee

There are a number of basic maintenance activities that should be carried out regularlyon any bridge in order to maintain its performance, prolong its serviceable life andreduce its requirement for more significant remedial works over time. Failure to carry outregular basic maintenance is a short-sighted approach and a false economy. Basic cyclicmaintenance should therefore be seen as a routine and very beneficial element of bridgemanagement, rather than an unnecessary and avoidable drain on valuable resources. Aproactive approach based on preventative maintenance may assist an asset manager inimproving the condition and performance of the whole bridge stock, minimisingrestrictions and reducing urgent, expensive and disruptive works to the structures.

Routine maintenance typically comprises minor and minimally disruptive activitiesaimed at preserving the bridge’s structural fabric in good condition, keeping it in astate in which it is performing as intended, and dealing with any apparent threats to itscontinued function.

Although the specific regular maintenance activities required for individual bridges willvary depending on their nature, condition and environment, typical activities thatshould be considered on a cyclic basis at an appropriate frequency include:

� ensuring bridge drainage is working efficiently by clearing drainage channels,weepholes etc

� removal of vegetation from all parts of the structure

� repointing of masonry following vegetation removal

� clearance of vegetation from areas immediately adjacent to the structure if thesepresent a hazard or obstruction, or obscure parts of the bridge

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� dealing with other hazards eg removing obstructions from culverts, flood channelsand arches that span watercourses.

Observations made during routine inspections should direct and inform maintenanceactivities by identifying changes in condition and external factors which mightdetrimentally affect the bridge or its function (eg unstable adjacent ground, blockeddrainage, fallen or encroaching trees etc).

It is very important to keep a detailed record of all maintenance work carried out on abridge, preferably including good before and after photographs and measurementswhere appropriate. This information is a valuable part of the bridge’s history and islikely to be useful when trying to budget and programme future maintenancerequirements, assessing bridge capacity or investigating the cause and significance ofnew bridge defects.

MMaaiinnttaaiinniinngg ddrraaiinnaaggee

This is one of the most important and worthwhile elements of a routine maintenanceprogramme, and frequently one of the most neglected. Water is a key factor in most ofthe processes that result in gradual deterioration of a bridge’s structural fabric.Effective management of water is fundamental to the long-term serviceability ofbridges. Most old masonry arches did not incorporate any kind of waterproofingsystem but relied on the structure’s permeability to allow water to drain out. Sometimesthis was enhanced by the use of weepholes which allowed drainage of the fill materialabove the arch and behind retaining walls. Drainage channels were sometimes includedin the parapet walls to allow water to drain away easily from the roadway rather thanpooling and finding its way down into the fill. Also the use of lime-based mortars madethe masonry itself “breathable” – it was able to dry out in good weather and did notremain permanently saturated.

Where such provisions have been made, whether as part of the original structure or addedsubsequently, they should be maintained by ensuring all drainage paths are kept clear andfunctional, and avoiding the use of impermeable mortars for repointing and repair.

Where original drainage channels have been neglected and cannot be properlycleaned, it may be necessary to carefully re-core them and insert new pipework. Mostbridges will benefit from drainage maintenance on an annual basis. The best time forthis is around late autumn, allowing clearance of fallen leaves which may be blockingdrainage channels and before the wettest and coldest part of the year.

MMaannaaggeemmeenntt aanndd rreemmoovvaall ooff vveeggeettaattiioonn

Although sometimes aesthetically pleasing, plants have the potential to disrupt anddisplace the fabric of a bridge, block drainage channels and retard the drying out ofwet masonry. Ideally they should be completely removed from the structure, andmonitored in the adjacent area. Vegetation should be cleared away from all parts of thestructure and roots removed. It may also be beneficial to treat any remaining roots witha suitable herbicide, although the potential environmental impact should beconsidered. Vigorously growing plants and shrubs immediately adjacent to thestructure should also be cleared away since their roots may penetrate the masonry andfoundation, and they obscure the structure of the fabric and hinder inspection.

Some bridges have more of a problem with vegetation than others, but most benefitfrom vegetation removal and management on an annual basis. The best time to do this

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is during the spring. Care should be exercised where flora on masonry may containrare and protected species (see Section 4.2.2).

RReeppooiinnttiinngg

Masonry is constructed so that deterioration should be concentrated in the mortar,which acts sacrificially to preserve the masonry units themselves, be they brick or stone.This is because the mortar is intended to be weaker, more porous and permeable thanthe masonry units. Water plays an important role in almost all deteriorative processes,and the movement of water through the mortar in the joints, in preference to the lesspermeable masonry units, is one of the casuses of its more rapid deteriotation. Thisdeterioration is typically most rapid and severe at the external surface of the joints,where the mortar is exposed to the atmosphere. If the masonry is behaving as itshould, deterioration should be concentrated in the pointing mortar of the masonry,which is easily dealt with by replacement (repointing). This optimises the durability ofthe masonry as a whole.

Selective repointing should be considered as routine maintenance and carried outwhen necessary. This is likely to be more frequent in bridges which are in exposedlocations or subject to severe weather conditions, particularly wetness and freezing.Waiting until the majority of the pointing on a bridge has completely deteriorated orfallen out before carrying out repointing is not advisable, since by that time otherdamage may have occurred to the structure. Ideally repointing should not be carriedout in cold and wet weather (particularly where lime-based mortars are used) and thebest time is during the spring, immediately after removal of vegetation. At this timejoints damaged by the vegetation and freezing winter weather are apparent, and therepointing mortar has a long period to cure and develop strength before again beingsubjected to very wet and freezing conditions.

The selection of pointing mortars and their application is discussed in Section 4.3.3.

Care should be exercised where gaps in mortar may contain protected species such asbats (see Section 4.2.2).

DDeeaalliinngg wwiitthh ootthheerr hhaazzaarrddss

A combination of periodic routine visual inspection and observations made incidentallyduring other attendances to the bridge site should identify any other potential immediateor long-term hazards to the performance and serviceability of the bridge. Some suchhazards should be dealt with as routine maintenance activities. For example, in bridgesover watercourses flooding and damage to the bridge structure may result if culverts,flood channels and arches are blocked by an accumulation of branches and twigs. Bridgesprone to this should be checked and obstructions removed periodically, preferably beforethe autumn’s increase in rainfall. Other apparent threats to the bridge should beidentified and dealt with as necessary, for example evidence of scour or silting-up.

CIRIA C656174

Where cracking and distortion of a structure has occurred, it is very important that this is notsimply covered up by repointing, since this can mask serious structural problems and makethe task of an assessing engineer difficult. Where cracking is known to be longstanding andnon-progressive, repointing may be considered but detailed records of such defects shouldbe made before and after remediation, including drawings with measurements andphotographs.

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44..33..33 RReeppaaiirriinngg ddeetteerriioorraattiinngg mmaassoonnrryy

Often the focus is placed on the structural causes of problems without giving dueconsideration to the contribution of the materials that make up the structural fabric.Keeping a bridge performing efficiently in the long-term and optimising its serviceablelife depends to a great extent on monitoring the deterioration of its materials.

IInnvveessttiiggaattiinngg tthhee ccaauusseess

The main causes of masonry deterioration are outlined in Section 2.5.3. When treatingdeteriorated masonry it is important that a good understanding of the cause ofdeterioration is achieved before remedial works are embarked upon. Failure to do thismay result in the problem recurring or possibly new problems becoming manifest. Forinstance, local softening and deterioration of mortar in the spandrel walls, associatedwith local leakage of water through the masonry, may be the result of saturated fill withno suitable drainage path. Simply carrying out local repairs to the masonry with newmaterials does not address the root cause of the problem, and risks its recurrence orpossibly even an increase in fill saturation which could result in increased pressure onthe spandrel walls. In order to investigate the cause of materials deterioration,appropriate assessment by a knowledgeable engineer or engineering materials specialistis required. This is likely to involve a close visual inspection of the deterioration,possibly supported by on-site testing, materials sampling and laboratory analysis usingtechniques such as petrographic examination (see Section 3.8.3). Damage with apotential structural cause, such as new or progressive cracking or distortion, shouldalways be investigated to determine its cause prior to carrying out any repairs.

RReemmeeddiiaall ttrreeaattmmeennttss

As discussed previously, the root cause of masonry deterioration should be addressedbefore undertaking any direct measures to preserve, improve or replace the masonryitself. However, frequently the problem is associated with inadequate maintenance andcan be solved by local masonry repairs and replacement, combined with improvedmaintenance in future (see Section 4.3.2).

RReeppooiinnttiinngg

Deterioration and loss of mortar loosens masonry units, which may present a hazard totraffic and members of the public using the area below the bridge. Loss of mortar from jointsalso reduces the ability of the masonry to transmit and evenly distribute forces, focusingstresses in localised areas and potentially leading to cracking and distortion. Repointingshould be considered a routine maintenance item (see Section 4.3.2) or, if extensive or a partof other works, a repair. Although the need for repointing can vary considerably dependingon the structure, its materials, design, location and exposure, masonry typically requiresextensive repointing at intervals of between 25 and 50 years in relatively harsh exposureconditions, and between 50 and 100 years or even longer for a durable mortar in lessdemanding conditions.

CIRIA C656 175

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The overriding principle of mortar selection is to choose a mortar that is slightlyweaker and more permeable than the masonry units. In particular mortar used forrepointing arch barrels should not be too strong or brittle to accommodate a smallamount of movement. Conversely mortar should not be too weak, since this will affectits durability.

Finding an ideal mortar mix may require some experimentation, but a good place tostart for ‘general purpose’ repointing of bridges with no special historic value is likelyto be a cement:lime:sand mix with a ratio between 1:1:6 and 1:3:12, using a well-graded sand and just enough water to provide a workable mixture that can be readilyshaped to the desired finish. The lime and sand are mixed to produce a “coarse stuff ”which is then gauged with the cement.

� a relatively weak mortar mix of 1:3:12 (cement:lime:sand) may be appropriate forrepointing sheltered areas with weak masonry units, and/or where significantmovement is anticipated, but using such a small amount of cement requires verycareful mixing to avoid problems

� a mix with proportions 1:2:9 (cement:lime:sand) will produce a moderately strongmix with a good all-round performance, potentially suitable for most bridgerepointing, including arch barrels, but may lack durability used in elementsexposed to harsh environments

� for a stronger more durable mortar for exposed areas which are not expected toaccommodate movement, such as spandrel and wing-walls, a 1:1:6cement:lime:sand mix may be suitable.

Mortar selection, specification and performance are discussed further in Section 4.4.2.

For bridges with special historic value it may be necessary to take more stringentmeasures to ensure an accurate match of repointing mortar with the existing mortar interms of its components, performance and appearance, to maintain its historicalintegrity. This is likely to require laboratory analysis of the existing mortar and is aspecialist task requiring the involvement of a suitably experienced conservator. Ratherthan gauging the mortar with cement, suitable mixes may be based on a hydraulic limeor a pure (non-hydraulic) lime as follows:

� lime is available as lime putty and as hydrated lime, and can be mixed with sandand water to produce lime mortar. Lime putty should be properly “matured” andis available from specialist suppliers (Teutonico, 1997). Although hydrated lime ismore freely available, its use is not recommended since it is a less consistentmaterial and its properties and quality may vary compared to lime putty. Mortarsbased on lime putties would normally be a mix of one part lime to three partsaggregate (1:3). In sheltered locations a weaker mix may be considered (1:4) andin exposed areas a richer mix (1:2) can be used to provide enhanced durability. Invery thin joints, a stronger and more workable mix of 1:1 might be most suitable,

CIRIA C656176

When undertaking repointing work it is important to ensure that materials used in themortar are obtained from a reliable source and that due regard is given to the appearanceand performance characteristics of the original mortar. It is good practice to match closelythe strength of the existing mortar, and new mortar should always be weaker than themasonry units themselves. New mortar should also have adequate permeability to allow thebrickwork to “breathe” and for moisture to evaporate through the joints rather than throughthe masonry units. The choice and application of suitable mortars for conservation andrestoration requires a careful approach and it is recommended that the advice of specialistconservators and material scientists is sought. Testing is normally carried out to determinethe existing mortar constituents and approximate mix proportions.

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and the use of more finely graded aggregate will be necessary. Where a slightlystronger mortar is required, gauging lime mortar with a pozzolan may beconsidered, or the use of a weakly naturally hydraulic lime

� hydraulic limes tend to give a more rapid set and produce stronger, more durablemortar than pure limes (see Section 2.3.1). They are supplied in bags of powder,similar to cement, and have a limited storage life before use. For conservationwork, on bridges with historic value, limes classified by BS EN 459 (BSI, 2001a) asnatural hydraulic limes (NHL) should be used. For mortars based on such limes anall-purpose mix suitable for jointing and pointing would be a 1:2 or 1:3 ofmoderately hydraulic lime (NHL3.5). A weaker mix suitable for use existingmaterials that are weak and friable could be a 1:2 mix of feebly hydraulic lime(NHL2) and aggregate. Where a stronger and more durable mortar is required,this might be achieved with a 1:2.5 mix of eminently hydraulic lime (NHL5) andaggregate (CADW, 2003). Care should be taken when using some of the morestrongly hydraulic limes, since these can produce mortars nearly as strong andimpermeable as some cement-based mixes.

Joints are cleaned out to a depth of at least twice the width of the joint, or to a minimumdepth of 15 mm from the finished face of the joint, using hand tools (quirks and longnecked jointing chisels with parallel faces) and taking care to avoid damaging the arisesof the brick/stone (Figure 4.2). Where mortar is weak and soft hand tools are adequatebut where joints are thin and dense mortar has been used it can be very difficult toremove, requiring considerable care to avoid damaging the masonry units. Cutting outusing angle grinders is not advised, and should only be considered when necessitated bythe scale of work, in which case appropriate equipment should be used by skilled andmotivated staff to prevent damage to brickwork. Particular caution should be exercisedwhere aesthetic or historic value is a concern (Figure 4.3). When removing hard mortar,a skilled mason may sometimes use a hand-grinder fitted with a thin diamond blade toscore the centre of a joint, then remove the rest with a hand chisel. However, thistechnique requires the utmost care and skill. The use of small pneumatic chisels, such asthose used to tool stone, can work well for mortar removal, but even this method cancause chipping to the edges of masonry units if it is not done carefully.

FFiigguurree 44..22 RReemmoovvaall ooff ddeetteerriioorraatteedd mmoorrttaarr uussiinngg aa lloonngg--nneecckkeedd cchhiisseell ttoo aavvooiidd ddaammaaggee ttoo bbrriicckkwwoorrkk((ccoouurrtteessyy BBrriittiisshh WWaatteerrwwaayyss))

CIRIA C656 177

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FFiigguurree 44..33 AApppprroopprriiaattee tteecchhnniiqquueess sshhoouulldd bbee sseelleecctteedd aanndd ccoonnttrraaccttoorr’’ss ppeerrssoonnnneell sshhoouulldd bbee ssuuiittaabbllyyeexxppeerriieenncceedd eellssee tthhee qquuaalliittyy ooff ffiinniisshheedd rreeppooiinnttiinngg wwoorrkk mmaayy bbee vveerryy ppoooorr

Once the deteriorated mortar has been removed, joints should be brushed and flushedout with water to remove dust and loose material, any joints which have dried out sincecleaning should be re-wetted. The new mortar should be plastic and workable but stiffas possible and it should be pushed into the back of the joints in layers and finishedflush with the surrounding brick/masonry with a bucket handle or weather-struck finish(recessed joints should be avoided).

� further description of mortar types and their characteristics is given in Section 2.3.1

� further information and guidance on good practice in the selection and applicationof mortars for repointing is included in Mack and Speweick (1998) and in Ashurst(1990).

DDeeeepp ppooiinnttiinngg aanndd ffiilllliinngg ooff jjooiinnttss

For narrow or deeply eroded joints, specially shaped pointing keys or tamping rodsmay have to be used, and lime mortars may need to be built up in layers up to 25 mmthick to assist with its curing, with each layer firmly tamped in place. Wheredeterioration of jointing mortar is extensive, resulting in voids and friable mortar wellback in the joint, greater than approximately 50 mm, normal repointing techniques areoften unsuitable and pressurised mechanical pointing using a “dry-mix process”becomes necessary. Loose and very soft mortar should be removed by hand tools or byhigh-pressure water jetting, back to more solid material, to a depth of up to 100 mmfor brickwork and potentially more for stonework. This may cause loosening of thefacing course of brickwork, and care should be taken not to damage or displace themasonry units which can be pinned in place using suitable “pinning stones”. A suitablemortar can be injected to fill the joints under pressure using spray pointing equipment,which uses compressed air to pressurise the mortar and force it through a hose to agun nozzle which is used by an operator to build up mortar in layers from the back ofthe joint to the front. Such techniques have been successfully used by Network Rail(and formerly British Rail) for deep-pointing of masonry tunnels and bridges foralmost 50 years (Sowden, 1990). The resulting mortar surface can be rough and rathermessy, with mortar projecting proud of the joints, but this can be tooled to anacceptable finish once the mortar begins to set. Care should be exercised where thistechnique is used on structures where the aesthetic appearance of the bridge is important.

CIRIA C656178

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PPrreessssuurree iinnjjeeccttiioonn ooff ggrroouutt wwiitthhiinn tthhee ssttrruuccttuurree

Where there are voids within the structure or excessively deep mortar loss from joints,pressure injection of grout into the structure may be necessary. As with shallowpointing, it is important to select a grout mix which will have suitable properties oncehardened and not adversely affect the performance of the bridge or its originalmaterials, for example allowing elements such as arches to accommodate a limiteddegree of movement. This technique is discussed further in Section 4.3.4 and inSections 6.2 and A6, and further detail is given in Sowden (1990).

SSuuppeerrffiicciiaall ccrraacckk rreeppaaiirrss

Cracking should never be repaired until its cause has been adequately established and,where necessary, dealt with. Crack repairs are only worthwhile if the forces that causedthe crack are unlikely to recur, or if provision is made for future movements.Superficial repairs to cracking involve sealing the surface of the crack to prevent theingress of moisture and deterioration of the adjacent materials, but do not restorestructural connection between the masonry either side of the crack.

Longstanding inactive cracks can be repaired using mortar materials which should notbe too hard or brittle, or else small movements are likely to result in a recurrence of thecracking and failure of the repair. Cracks which are expected to experience furthermovement, for example through cyclic moisture or thermal variations, can be treatedas joints and sealed with a flexible material that can accommodate the anticipated rangeof movement.

Where cracks will have confined themselves to the mortar joint lines they can berepaired using normal pointing methods. Cracks that pass through the masonry unitsthemselves are more difficult to treat, and patch repairs may be required.

Alternatively, cracks can be widened and undercut with a saw or chisel so that they areabout 10-15 mm wide at the surface, a few millimetres wider at the back, and 15-20mm deep. Loose dust should be brushed away, the crevice and surrounding blockdampened with water, and a suitably stiff repair mortar applied and properly cured. Itmay be permissible to use dry-pack mortar or carefully selected proprietary materials inthis respect. If cracks are fine, they should not be widened but a mortar containing finesand may be worked into them to seal them, or they may be injected under pressureusing a proprietary sealant material selected to meet suitable performance requirements.

PPaattcchh rreeppaaiirrss

Where deterioration has affected the masonry units themselves, particularly where theyare deeply spalled or unstable, they may have to be replaced with new units. This istypically a local repair. For brickwork it will normally involve the careful removal of asingle skin of brick over a limited area, including the damaged bricks, and replacementwith new, or sometimes recycled, brick or stone. This procedure is detailed in SectionA6 (Section 6.6). With stone units, damage of the stone surface is more likely to beaddressed by “piecing in” which involves cutting out the damaged section and using amortared-in fillet of stone, matched with the original stone to preserve its appearance,and possibly pinned in place using dowels. More extensive damage may, however,require the complete replacement of one or more blocks.

This type of repair may be superficial in nature, used to restore the appearance ofdeteriorated elevation and to protect underlying materials, or can be expected to act

CIRIA C656 179

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structurally and compositely with the surrounding structure, in which case the selectionof materials with appropriate physical properties is particularly important. The repairshould be able to take some load but avoid forming hard spots and risking damage ofthe surrounding structure. It is particularly important that such repairs are wellbonded to the adjacent masonry, normally by pinning and grouting.

AApppplliiccaattiioonn ooff ccoonnssoolliiddaannttss aanndd sseeaallaannttss

There is much debate about the use of consolidants and sealants to preserve masonry.Although it appears that they can in certain circumstances be beneficial to thestructure, incautious use can have the reverse effect and they should certainly not beconsidered a quick and easy solution to the problem of masonry deterioration. Manysuch materials are enthusiastically marketed by their producers who are quick to pointout their potential benefits, but their long-term effects are less well known. Great careshould be exercised in considering their use, particularly in old and historic structures,and expert advice should be sought before using any such treatments so as to fullyunderstand their potential benefits and disadvantages for the specific structure in question.

Masonry consolidants are either applied to the surface or can be pressure-injectedthrough a network of holes drilled in the masonry. They are low-viscosity liquids whichwork by penetrating the pore-spaces in permeable masonry and either depositing aninorganic substance, typically a silicate, or reacting with the mineral phases present.The intention is to reduce porosity and improve cohesiveness and strength. Whenapplied to soft and porous masonry which is crumbling away it can in certain situationsprovide some preservative benefit. However, such techniques and materials are nottried and tested and their effects are unpredictable, particularly in the long-term.When surface-applied, consolidants penetrate only a limited distance into the masonryto produce a hard skin at its surface; this layer has different properties to theunderlying masonry, both in terms of thermal response and moisture permeability andin some situations could accelerate deterioration by delaminating from it. Whenpressure-injected, weak masonry may be damaged by the drilling of injection holes andthe pressure of injection itself. Penetration is unlikely to be uniform, particularly ifconservative limits are placed on injection pressures, resulting in incomplete andpatchy impregnation of the masonry.

Although more widely used, masonry sealants and water-repellent applications can alsopresent problems, particularly for old masonry. These are surface-applied and areintended to inhibit the ingress of moisture into the masonry through the sealed surface.Although they may be beneficial in certain circumstances, for instance reducing thepenetration of moisture through the treated surface from driving rainfall,unfortunately they may also inhibit or reduce surface evaporation of moisture fromother sources and lead to an increase in masonry saturation. Also, moisture movementsand evaporation may be concentrated through certain parts of the structure, leading toaccelerated localised deterioration through leaching, salt crystallisation or freeze-thawaction. Another consideration is that the application of sealants can significantly changethe masonry’s appearance, sometimes leading to patchy discolouration.

CClleeaanniinngg

Where it has become dirty and discoloured, disfigured by waterborne or airbornedeposits, or colonised by unsightly biological growth, it may be desirable to restore theattractive original appearance of brick or stone masonry by cleaning. In certain cases,the soiling may be contributing to the deterioration of the masonry and cleaning may

CIRIA C656180

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be desirable for its preservation – for example where sulfate-bearing sooty depositshave been left by steam trains on railway bridges.

A variety of techniques and proprietary cleaning products are available and are oftenactively marketed by their producers/applicators. However, with old masonry inparticular, every structure and situation is unique and there is no single technique orproduct that can be relied upon to achieve the desired result while avoidingundesirable effects.

There are three main groups of masonry cleaning methods (Mack and Grimmer, 2000):

� water methods soften the dirt or soiling material and rinse the deposits from themasonry surface

� chemical cleaners react with dirt, soiling material or paint, allowing it to be rinsed offthe masonry surface with water

� abrasive methods mechanically remove the dirt, soiling material or paint (and,usually, some of the masonry surface) and may also be followed with a water rinse.

Often it is possible to adequately clean masonry surfaces by soaking using a low-pressure water spray followed by light brushing, but the most suitable technique andproduct will depend on a variety of factors including the type and condition of themasonry materials, the nature of the material needing to be removed, and theacceptability of the change in appearance (and possible damage) that might result.Great care should be taken to select an appropriate cleaning method, particularlywhere the masonry has historic or aesthetic value, or is subject to any kind ofpreservation order.

Applying the wrong cleaning materials or techniques to old masonry can havedisastrous results and leave the masonry surface in a weakened and disfigured state.For instance, some stones, such as limestone and sandstones with calcareous cement,can be damaged by acidic treatments. Others can contain impurities such as iron, whichmay result in streaky reddish discolouration on cleaning. Very harsh abrasive methods,such as grit blasting, are always damaging and their use is seldom justified.

The “golden rules” are:

� do not clean historic masonry unless there is a good reason and a definite benefit

� use the gentlest method possible, commensurate with achieving the desired result

� trials need to be carried out on parts of the structure that are not clearly visible

� cleaning should only be carried out by experienced contractors.

A thorough understanding of the physical and chemical properties of the masonry willhelp avoid the inadvertent selection of damaging cleaning agents, and wherehistorically or aesthetically valuable masonry is to be treated it is essential that expertadvice is sought.

Information on cleaning masonry is given by BS 8221 Code of Practtice for cleaning andsurface repair of buildings (BSI, 200b). Further guidance on masonry preservation andthe selection of suitable cleaning methods is given by Ashurst, J (1988) and Ashurst, N(1994).

CIRIA C656 181

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OOtthheerr ssppeecciiaalliisstt ttrreeaattmmeennttss

In addition to the above, there are a variety of specialist treatments for preserving andrepairing masonry, depending on its type and the cause of the damage. These are oftentechnologically advanced, experimental or labour-intensive techniques, more frequentlyused over limited areas in the course of preserving important stone buildings and theirarchitectural details, statuary etc and would not normally be routinely considered forapplication to bridges. For example, the concentration of soluble salts at stone surfacescan cause them to “blister” and flake off or delaminate in sheets, and this can be treatedby the use of poultices to draw out the salts and reduce their concentration at exposedsurfaces. Also laser-based techniques are available for cleaning masonry, but are tooslow and expensive to be used for treating large areas. Although not suitable forroutine use, specialist treatments might be worth considering in certain circumstances,for instance to treat limited areas of bridges with substantial historic value.

44..33..44 RReeppaaiirr aanndd ssttrreennggtthheenniinngg tteecchhnniiqquueess

It is essential that the cause of the deterioration is understood and the effects of anystrengthening or repair techniques are considered before commencing any work on thebridge. At all times it is important to consider that masonry arch bridges derive theirstrength and tolerance to movement from their ability to articulate – it is theirparticulate nature that gives them their unique structural characteristics. If this is lostthen they behave in a different way that should be taken into account when consideringtheir strength and residual life.

If remedial or strengthening works are selectedwithout due consideration and understanding of theirshort and long-term effects, they may result in moreharm than good and can address one failure modeonly to allow, or even cause, another. Particular care isrequired when work is necessary to one span of amultispan bridge.

Sometimes repair and strengthening options are limited or their execution complicatedby the presence of previous works on the bridge. The response of the bridge to pastworks and their success can provide useful information for assessing the potentialeffects of the proposed works and their chances of success.

Table 4.1 suggests techniques that might be considered to deal with common defects.

Figure 4.4 includes a flow-chart which suggests a selection process for determiningappropriate repairs to arch barrels of single-span bridges. The repair methodssuggested by this decision-making process are for consideration only, and should besubject to full engineering assessment based on the specific circumstances of the bridge,its construction, materials, defects, performance requirements and constraints.

CIRIA C656182

Wherever possible, repairsmust be sympathetic to thestructure, not alter itsworking mode and usematerials compatible withthose already existing.

In order to aid the process of selection of appropriate preventative, remedial andstrengthening techniques, this section includes guidance on the selection of potentiallysuitable techniques for addressing common structural problems and deficiencies ofmasonry arch bridges. This information is for guidance only; it is not a substitute forexperience and knowledge of masonry arch structures.

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TTaabbllee 44..11 AApppplliiccaattiioonn ooff rreemmeeddiiaall mmeeaassuurreess ttoo ttrreeaatt ccoommmmoonn ddeeffeeccttss ((aafftteerr PPaaggee,, 11999933))

CIRIA C656 183

DDeeffeecctt TTeecchhnniiqquueeDeteriorated pointing Repoint

Deterioration of arch ring material Masonry repairSaddleSprayed concrete to soffitPrefabricated liner to soffitGrout arch ring

Arch ring thickness assessed to beinadequate to carry required traffic loads

SaddleSprayed concrete to soffitPrefabricated liner to soffitReplaced fill with concreteSteel beam relieving archesRelieving slabRetro-reinforce

Internal deterioration of mortar eg ringseparation

Grout arch ringStitch

Foundation movement Mini-pilesGrout piers and abutmentsUnderpin

Scour of foundations UnderpinInvert slabStone pitchingRip rap

Outward movement of spandrel walls Tie barsSpreader beamsReplace fill with concreteTake down and rebuildGrout fill if it is suitableStrengthening by the “Stratford method” (see Table 4.3)

Spandrel wall separation StitchingTie-bars and patress plates

Weak fill Replace fill with concreteGrout fill if suitableReinforced fill

Water leakage through arch ring Make bridge surfacing water resistantHigh level waterproofing layerWaterproof extrados and improve drainage

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CIRIA C656184

Is it

poss

ible

toex

cava

te b

ack

toth

e ar

ch b

arre

lex

trad

os?

Is t

he c

rack

ing

of t

he a

rch

barr

elex

tens

ive?

Is t

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rch

heav

ily d

isto

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?

Is t

he s

pan

of t

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5 m

?

Doe

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Is t

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quire

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be

perm

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Is t

hest

ruct

ural

inte

grity

of t

he a

rch

barr

el v

ery

poor

?

Is t

he fa

ult

due

to a

ctiv

esu

bsid

ence

?

It is

ass

umed

tha

t th

esp

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all i

s in

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ny b

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c ha

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repa

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usin

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Und

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and/

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ater

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it re

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ent

Use

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trad

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Use

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Inst

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Sadd

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Inst

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Yes

Yes

Yes

Yes

Yes

Yes

Yes

Yes

No

No

No

No

No

No

No

No

FFiigguu

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..44

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ccttiioo

nn ooff

mmeett

hhooddss

ffoorr tt

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ppaaiirr

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Not

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entif

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of

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ent

tech

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Once a repair or strengthening option has been selected, there are many factors thatmust be considered prior to its implementation. Some of these are common to mostbridges and are presented in Table 4.2.

TTaabbllee 44..22 CCoommmmoonn ffaaccttoorrss ffoorr ccoonnssiiddeerraattiioonn wwhheenn ccaarrrryyiinngg oouutt bbrriiddggee wwoorrkkss

In order to carry out the works it is vital that all the appropriate preparatory work isundertaken before the contract is awarded:

� identify and establish causes of deterioration and defects and ensure that these areaddressed where appropriate

� an initial desktop study should be undertaken to identify important parameters

� a site investigation is required to confirm the structural dimensions and theproperties of the fabric of the bridge, including the backfill

� consideration of the effects of temporary loading conditions of plant etc on thebridge

� ensure that material specifications are compatible with existing fabric of the bridge

� it should be checked that the works do not change the mode of behaviour of thebridge – ideally this should be avoided but if they do, full account should be takennot only of their immediate consequences but also their effects on the long-termperformance

� consideration should be given to the effect of local repairs on the globalperformance of the bridge and vice versa

� assess the adequacy of the existing fabric of the bridge at each stage of the works;requirements for temporary support works should be assessed

� location and diversion of utility company equipment should be carried out asnecessary

� site access should be secured (eg railway possessions)

� clearance for temporary works should be checked

CIRIA C656 185

FFaaccttoorr IItteemmss ttoo bbee ccoonnssiiddeerreedd

Access TrafficTopographyServices (location and diversion)Nature of obstacle (eg river)Wayleaves.

Procurement Traditional tendering methods for one-off contractsDesign and build contracts with specialist subcontractorsCall off contractsMaintenance contracts.

Health and safety Rrisk assessmentsMethod statement (including manual handling, COSHH, dust and debris,Working at height and/or over water etc use of power tools and mobilePlant, buried services, structural collapse).

Environment Protection of watercoursesNoise, light and dust pollutionSpecies protection (especially protected species such as bats)Minimisation and safe disposal of wasteReuse of materials and minimisation of new materialsSSSI and other designations

Heritage Listed building, national monument and other designationsMaintenance of original appearance and featuresPreservation of original structural fabric.

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� personnel should be appropriately trained and have the necessary skills andexperience to undertake the work. (In particular, working with historical structuralmaterials and at height, over water etc)

� the requirements for post-repair monitoring and the most suitable techniquesshould be considered.

On completion, full details of “as-built” drawings, material performance records andmonitoring installations should be made available to the bridge owner for the bridgearchive.

Repair and strengthening techniques considered are in detail this guide and are listedin Table 4.4. A summary of each technique, its purpose, principal engineeringconsiderations, advantages and limitations is included. Additional detail on the designand implementation of each technique is included in Section A6.

TTaabbllee 44..33 RReeppaaiirr aanndd ssttrreennggtthheenniinngg tteecchhnniiqquueess ccoonnssiiddeerreedd iinn ddeettaaiill iinn tthhiiss gguuiiddaannccee

Specifically, for each of the techniques there are considerations that should be takeninto account at the design stage. These are presented in Table 4.4.

CIRIA C656186

TTeecchhnniiqquuee AAppppeennddiixx 66 rreeffeerreennccee

Arch distortion remedial works A6.1

Arch grouting A6.2

Backfill replacement or reinforcement A6.3

Concrete saddle strengthening A6.4

Parapet upgrading A6.5

Patch repairs A6.6

Pre-fabricated liners A6.7

Relieving slabs A6.8

Retro-reinforcement A6.9

Spandrel tie-bars/patress plates A6.10

Sprayed concrete lining A6.11

Spandrel strengthening “stratford method” A6.12

Thickening surfacing A6.13

Through ring stitching A6.14

Underpinning A6.15

Waterproofing and drainage improvements A6.16

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TTaabbllee 44..44 SSuummmmaarryy ttaabbllee ooff ccoonnssiiddeerraattiioonnss ffoorr ccoommmmoonn rreemmeeddiiaall aanndd ssttrreennggtthheenniinngg mmeeaassuurreess

CIRIA C656 187

TTeecchhnniiqquuee CCaatteeggoorryySSttrruuccttuurraall ddeeffeeccttaanndd llooccaattiioonn

EEnnggiinneeeerriinngg aassppeeccttssAAddvvaannttaaggeess//ddiissaaddvvaannttaaggeess

Arch distortionremedialworksSection A6.1

Remedial Inadequate overallload carryingcapacity of archbarrel.Distortion,misalignment ortilting of the regularshape of the archbarrel.

� establish causes of distortion� lining support should be designed

to be either composite with theexisting bridge or offer independentsupport to it. This must be clearlyrecorded in the bridge records

� the foundations should be checkedfor competence.

AAddvvaannttaaggeess� ease of application/

installation� possible enhanced live

load.DDiissaaddvvaannttaaggeess� potential bond problems� appearance and

clearance� measures for protected

species (if present).

Arch groutingSection A6.2

Remedial Arch ring separation.Fractures andextensive cracking.

� it is recommended that the groutingholes are stopped at least 100 mmshort of the extrados. If no radialpins are installed then the new loadcarrying capacity should bedetermined assuming full ringseparation of the top ring with dueregard to the shear capacity of theinter-ring mortar

� if radial pins are installed then theshear capacity should be checkedwith due regard to the otherpossible modes of failure

� the brickwork should be checked forthe new loading regime.

AAddvvaannttaaggeess� simple established

methods� structure life expectancy

extended.DDiissaaddvvaannttaaggeess� traffic disruption during

construction� further

inspection/testingrequired to confirmrepair effectiveness.

BackfillreplacementSection A6.3

Remedial Incompetent existingfill material orinadequate overallload carryingcapacity.

� check stability of the structure whenthe backfill has been removed (archmay require temporary support)

� method statement should take theremoval of the backfill into account

� determination of new carryingcapacity should take into accountthe effects of the new backfillparticularly in terms of the loaddispersal and lateral support to thestructure

� consider change in loading on thespandrel walls.

AAddvvaannttaaggeess� possible enhanced live

load capacity� relative cost.DDiissaaddvvaannttaaggeess� traffic disruption during

construction� structure life expectancy

unaffected by works� further maintenance

works may be required.

ConcretesaddleSection A6.4

Strengthening Inadequate overallload carryingcapacity of archbarrel in conjunctionwith spandrel walland waterproofingfailures.

� the adequacy of the existingstructure should be checked toensure that it is capable ofsustaining the enhanced loadingwith the saddle in place

� decide whether the saddle is to actcompositely with the existingstructure or not

� check the structural interaction,bearing in mind that the barrel isparticulate and heterogeneouswhile the reinforced concrete saddleis not

� particular attention should be givento the load paths through theabutments, piers and theirfoundations

� ensure adequate support ofstructural elements during saddling.

AAddvvaannttaaggeess� no change to

appearance as hidden� facilitates other

repairs/parapetupgrades/waterproofing

� enhanced live loadcapacity.

DDiissaaddvvaannttaaggeess� traffic disruption during

construction� relative cost� increase in crown depth

possible.

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CIRIA C656188

ParapetupgradingSection A6.5

Strengthening Inadequate impactresistance ofparapet(s).

� the consequence of impact shouldbe given detailed considerationparticularly in respect of vehicularcontainment and falling material

� where the parapet is a new build, itsinteraction with the existingstructure should be checked.

AAddvvaannttaaggeess� Enhanced vehicle

containment and/orreduction in likelihood offalling material.

DDiissaaddvvaannttaaggeess� traffic disruption during

construction � provision of access� relative cost.

RepointingSection 4.3.3

Preventative/remedial

Deterioration ofmortar joints.

� careful consideration should begiven to the selection of the repairmaterials. These should becompatible particularly with respectto stiffness and thermalcharacteristics.

AAddvvaannttaaggeess� simple established

methods� structure life expectancy

extended.DDiissaaddvvaannttaaggeess� labour intensive� possible traffic

disruption duringconstruction

� possible provision ofaccess.

Patch repairSection A6.6

Remedial Bulging or loosemasonry/brickwork.Heavily spalledbrickwork.

� careful consideration should begiven to the selection of the repairmaterials. These should becompatible particularly with respectto stiffness and thermalcharacteristics

� initially, the patched masonry willcarry very little load and so the loadcarrying capacity should be basedupon the capacity of the originalbarrel – over time, this mayredistribute as a result of creep andgeneral other movement. (It may bethat the removal of bulging materialetc could lead to overstressing andfatigue damage to the remainingmasonry).

AAddvvaannttaaggeess� simple established

methods� structure life expectancy

extended.DDiissaaddvvaannttaaggeess� labour intensive� works phasing required� possible traffic

disruption duringconstruction

� possible provision ofaccess.

PrefabricatedlinersSection A6.7

Strengthening Inadequate overalllive load carryingcapacity of archand/or abutmentswhere depth of fillover the arch barrelis excessive. Thiscan also addressspandrel wall andwaterproofingfailures.

� the nature and condition of thebackfill and any services should bedetermined

� determine the extent of the spreadof the loading and its effect uponthe load carrying capacity

� attention should be given to thechange in load paths which mayincrease the load effects on thespandrel walls, substructure andfoundations.

AAddvvaannttaaggeess� no change to

appearance as hidden� facilitates other repairs� enhanced live load

capacity.DDiissaaddvvaannttaaggeess� traffic disruption during

construction � relative cost� increase in crown depth.

Retro-reinforcementSection A6.9

Strengthening Inadequate overallload carryingcapacity of archbarrel.

� it should be recognised by thedesigner and bridge owner that theuse of a retro-fit solution changesthe nature and behaviour of thebarrel from that of a particulateheterogeneous material to that of areinforced masonry

� particular care should be exercisedto ensure that the bond betweenthe masonry, adhesive andreinforcement can be relied upon

� in the case of multi-ring brickworkbarrels, the possibility of ringseparation should be investigated

AAddvvaannttaaggeess� repairs hidden� much less disruption

than saddle/slab/reconstruction

� relative cost� speed of

implementation.DDiissaaddvvaannttaaggeess� independent

verification/validation ofanalysis, design,installation, fatigue anddurability of systems.

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CIRIA C656 189

Retro-reinforcementSection A6.9(cont’d)

due to the change in structuralbehaviour accompanying theintroduction of the reinforcement.(Radial pinning is recommended toensure through-thickness continuity)

� it should be noted that unreinforcedareas still possess little, if any, tensilestrength.

Spandrel tie-bars andpatress platesSection A6.10

Preventative/remedial

Bulging ordisplacedspandrels.Inadequate lateralload capacity.

� the long-term effect of theintroduction of a “stiff” local restrainton the spandrel wall stability shouldbe investigated

� it should be noted that tie-bars havea negligible strengthening effect onthe barrel serviceability performanceas they only affect long-term stabilityand soil-structure interaction as theultimate state is approached(Melbourne and Gilbert, 1995).

AAddvvaannttaaggeess� simple� minimal traffic

disruption. � relative cost.DDiissaaddvvaannttaaggeess� specialist

subcontractor� service avoidance� localised high stresses

to spandrel at plates� possible water paths

into bridge� change in structure

appearance.

Stratfordmethod(spandrelstrengthening)Section A6.12

S/R Bulging ordisplacedspandrels.Inadequate lateralload capacity.

� the change in structural behaviour ofthe strengthened spandrel wallshould be investigated

� the effect of any potential “hard-spot”loading onto the barrel should beinvestigated.

AAddvvaannttaaggeess� non-specialist

implementation � can accommodate

parapet strengtheningDDiissaaddvvaannttaaggeess� traffic disruption during

construction� relative cost� service avoidance/

diversion� not widely used� change in structure

appearance.

ThickeningsurfacingSection A6.13

Strengthening Inadequate overalllive load carryingcapacity of archbarrel.

� determination of new carryingcapacity should take into account theeffects of the increased surfacingdepth, particularly in terms of theload dispersal and lateral support tothe structure

� loading on the spandrel walls willalso change.

AAddvvaannttaaggeess� possible enhanced live

load capacity� relative cost.DDiissaaddvvaannttaaggeess� traffic disruption during

construction� structure life

expectancy unaffectedby works

� further maintenanceworks may be required.

Through-ringstitchingSection A6.1

Remedial Arch ringseparation at anylocation over planarea of archbarrel.Extensive crackingof arch barrel.

� cross-stitching is not recommendedas it changes the behaviour of themasonry to that of a brittle“continuum”

� radial pinning is preferred as it allowsparticulate behaviour whilereinstating the longitudinal continuitybetween the rings and has thethrough-thickness performance of thering

� the longitudinal shear capacity of theinter-ring connection should bedetermined and used in thedetermination of the bridge loadcarrying capacity.

AAddvvaannttaaggeess� simple established

methods� structure life

expectancy extended.DDiissaaddvvaannttaaggeess� labour intensive� possible traffic

disruption duringconstruction

� possible provision ofaccess.

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44..33..55 MMiinniimmiissiinngg rriisskk iinn ccoonnttrraaccttiinngg aanndd eexxeeccuuttiioonn ooff wwoorrkkss

Unforeseen circumstances, such as encountering unanticipated conditions orrestrictions, or the discovery of hidden internal details of the bridge structure, cancause considerable increase in disruption and cost when carrying out bridge works,resulting in significant variation to contracts. Although thorough research and acarefully designed and executed site investigation can help to minimise such risks, thefull extent of repairs cannot always be determined in advance. Where uncertaintyremains it is important to reduce the potential consequences of unforeseen problems byensuring that all parties involved adopt a flexible approach and that good channels ofcommunication are established at an early stage in the project. It is worthwhile givingconsideration to possible alternative construction details, work scopes or methods basedon the most likely scenarios and ensuring that contracts allow for such variationswherever it is practical.

Generic risks that should be assessed as a part of contracts for bridge works include:

� design risks

� construction risks

� health and safety and environmental risks

� programme risks

� economic risks

� incidental and indirect risks.

As an alternative to the normal tendering procedure, some asset owners and managershave adopted partnering agreements or contracts for carrying out bridgeworks. Thisavoids repeatedly going through the tendering process each time work needs to bedone, removes some degree of risk from the contractor, and typically results in reducedfees for consultancy, appraisals and site supervision. This approach is becomingincreasingly popular with larger asset owners with many structures and much work to

CIRIA C656190

UnderpinningSection A6.15

Remedial Inadequate bearingcapacity ofsubstructureformation.

� the cause of the foundationmovement must be establishedand the effectiveness ofunderpinning justified

� the effect of a stiffer foundationshould be considered in respect ofthe rest of the structure, (especiallyif the underpinning is restricted topart of the structure).

AAddvvaannttaaggeess� allows existing structure

to remain in service� commonly used

technique.DDiissaaddvvaannttaaggeess� phasing of works� relative cost� service avoidance/

diversion.

WaterproofingSection A6.16

Preventative/remedial

Water seepageresulting in masonrydegradation.

� sufficient falls must be provided bythe substrate to allow run-off

� selection of system to suitsubstrate.

AAddvvaannttaaggeess� removes many

problems associatedwith water penetration

� reduced maintenancecosts.

DDiissaaddvvaannttaaggeess� disruption/closures� need to provide positive

drainage at springinglevel (if barrelwaterproofed)

� suitable substrate forbackfill waterproofingrequired.

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be done, where the potential benefits are greatest. In any case, contractingarrangements which clearly assign responsibilities to each party and encouragecooperative working are frequently beneficial.

44..44 NNeeww mmaassoonnrryy aarrcchh bbrriiddggeess

Although stone and brick are still commonly used to face structural concrete and steelbridges, primarily for aesthetic purposes, the construction of true structural masonryarches is now rare and much of the skill-base of previous generations in both designand construction seems to have been lost.

At the present time the initial construction or renewal cost of a masonry arch bridge islikely to be notably higher than an equivalent structure in “modern” materials, andexisting procurement processes, focused on minimising construction cost, do notencourage their design. Also, at the present time, the construction of masonry arches ishampered by the lack of design guidance and the skills of major contractors. However,given their relatively low maintenance and good performance in the past, and in thelight of the relatively high costs and poor performance of other types of bridge built inthe last century, refurbishment of existing masonry arch bridges, and even theconstruction of new ones in certain circumstances, is worthy of consideration. Thecurrent trend towards whole-life cost (Section 3.4.8) and sustainability may contributetoward this re-evaluation.

A case study giving an example of the design and construction of new masonry archcanal bridges is included in Section A1 (Section A1.11).

44..44..11 EExxiissttiinngg ddeessiiggnn ccooddeess aanndd gguuiiddaannccee

Currently there is no definitive UK or European standard for the design of newunreinforced masonry arch bridges, but design guidance is available from a number ofsources. Mass brickwork may be designed on the gravity principle, where no tension ispermitted in the brickwork, or on the basis that the development of small tensilestresses is permissible. Until 1978 the structural design of brickwork was carried out inaccordance with CP111:1970 Structural recommendations for loadbearing walls. This wassuperseded by the limit state code for the design of structural masonry, BS 5628-1:1992Code of practice for use of masonry. The limit state design philosophy is based on theprinciple that there should be an acceptable probability that a structure, or any of itsparts, will not become unfit for purpose (ie will not reach its limit state).

BS 5628 Code of practice for the use of masonry includes three parts:

� BS 5628-1:1992 Structural use of unreinforced masonry includes updated wall tieprovisions, wind load design and loading factors for earth retaining structures

� BS 5628-2:2000 Structural use of reinforced and pre-stressed masonry

� BS 5628-3:2001 Materials and components, design and workmanship incorporates a newversion of BS 8000-3: Workmanship on building sites – code of practice for masonry (alsoavailable separately).

These documents provide guidance relating to masonry units, mortars and ancillarycomponents and materials.

At the time of writing, the only specific standard for construction of masonry arch bridgesis BD 91/04 Unreinforced masonry arch bridges issued by the Highways Agency (HA, 2004b).

CIRIA C656 191

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The objective of BD91 is to encourage a renaissance in arch building using unreinforcedmasonry materials, adopting a limit state approach. It states the design and constructionrequirements for arch bridges, and compliments the Highways Agency’s Specification forhighway works (MCHW1) in this respect. BD91 applies to bridges incorporating masonryarches either as single or multiple spans with a span/rise ratio of between two and 10 andspans not exceeding 40 m. It does not apply to open-spandrel arch bridges or bridgescarrying railway loadings. However, the present issue of BD91 (BD91/04) does notcomply with the recent Euronorm (EN) standards for masonry units and mortars, ie BSEN 771:2003, BS EN998-2:2003 and the draft revisions to BS 5628-1:1992.

In addition to BD91 the following standards and guidance are available, relating tomasonry arch design and construction:

� United States’ Brick Industry Association (BIA) has published Technical Notes 31AStructural design of brick masonry arches (BIA, 1967, rev.1986) giving design guidancefor masonry arches.

� BS5628: Structural use of masonry, part 1: Unreinforced masonry handbook (Haseltineand Moore, 1981)

� Eurocode 6 1996: Design of masonry structures, part 2: Design, selection of materials andexecution of masonry

� Brickwork arch bridges (Cox and Halsall, 1996) gives practical advice on brick archbridge design

Other aspects to consider in masonry arch bridge design and specification include:

� in most old masonry arch bridges there are no movement joints, and movementsfrom creep, moisture expansion, thermal effects and small foundation movementsare accommodated by the inherent plasticity and flexibility of the old lime mortarsused. The cement mortars typically used for modern structures are lessaccommodating of such movements and tend to crack where stresses exceed limitingvalues. It is normal practice to include movement joints in modern structures

� technical and safety advice for vehicle restraint systems on the trunk road systemhas been updated; HA Interim Advice Note 44/05 Interim requirements for roadrestraint systems (vehicle and pedestrian) (IRRRS) has been produced to advise on thesechanges to technical and safety advice and ensure compliance with therequirements of EN 1317. Where masonry parapets are permitted, they can bedesigned to BS 6779-4:1999 and formed as extensions to spandrel walls, butchecks should be performed and measures taken to ensure that parapet failurewould not result in failure of the spandrel wall

� bonding patterns for masonry arches should be selected so as to ensure compositeaction between arch rings and provide resistance to ring separation. Forms of bondingwhich rely on mortar alone to join rings should not be used, unless the separation ofrings has been assumed in the design analysis. Arch rings should be flush jointed

� where masonry is likely to be exposed to freeze-thaw cycles, it should be detailed sothat the risk of saturation is minimised, particularly in exposed areas. This requiresthe use of details which shed water and the use of cappings, and copings whichhelp to reduce the wetness of walls

� guidance on the architectural design and appearance of bridges and theirrelationship to their surroundings is given in The appearance of bridges and otherhighway structures (HA, 1996)

� workmanship is a crucial factor in achieving good quality masonry, and this is also dealtwith in BS 5628-3:2001 Use of masonry; materials, components, design and workmanship

CIRIA C656192

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� methods for the consideration of sustainability and the whole-life environmentalcost of bridge construction and maintenance are discussed in Steele et al (2003a,2003b), Steele (2004).

44..44..22 SSeelleeccttiioonn aanndd ssppeecciiffiiccaattiioonn ooff mmaatteerriiaallss

The durability of masonry constructionis achieved by a consideration of theelement type, exposure and in-serviceenvironment, an appropriate choice ofmortar and masonry units, selection ofa suitable mortar joint profile andachievement of an adequate standardof workmanship.

MMaassoonnrryy uunniittss

At the time of writing, the existing British Standard specification for clay bricks, BS 3921:1985, is in the process of being replaced by the new European Standard BSEN 771-1:2003 Standard specification for clay brick. The changeover is expected to occurin early 2006, but both standards will co-exist until then. Specifying clay masonry unitsto EN 771-1:2003 has been theoretically possible since 2003. The amended EN 771-1 isexpected to be published in April 2005 and the coexistent period with BS 3921 isextended to April 2006.

Bricks are available with compressive strengths of between about 7 N/mm² (soft brick,potentially suitable for use in restoring old bridges with equally weak original masonry)and 100 N/mm²; the National Annex to EN 771-1 provides a link between BS 3921 andEN 771-1. Bricks selected for new bridges would normally be Class A or B Engineeringbrick, which requires them to meet minimum specifications for compressive strengthand maximum limits for water absorption, and with durability designation FL, whichindicates an adequate level of frost resistance and a low soluble salt content.

Designation letters for brick durability and soluble salts content differ between BS 3921and EN 771-1.

In BS 3921:

In BS EN 771-1:

Bricks can be handmade (hand-thrown in moulds), stocks (produced in moulds butthrown mechanically) or wirecut (extruded and cut by wire) depending on theappearance required.

Natural stone masonry units, which were previously required to comply with BS3921may currently be specified to comply with either BS 5628-3:2001 or EN 771-6:2001.

CIRIA C656 193

Brick durability: F2 = severe exposure F1 = moderate exposure F0 = passive exposure.

Soluble salts content: S2 = lower content category S1 = higher content category S0 = Norequirement.

Brick durability: F = frost resistant M = moderately frost resistant O = not frost resistant.

Soluble salts content: L = low soluble salts content N = normal soluble salts content.

British Standards are currently being revisedin line with European requirements. The newstandards are at varying stages of preparationand/or publication, and the reader shouldrefer to the British Standards Institution (BSI)for guidance as to the current status ofstandards and latest requirements formaterials and construction before proceeding.

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They should be selected on the basis of proven durability and resistance to weatheringin a similar climate and exposure condition. Reconstructed stone should comply with BS6457:1984 Specification for reconstructed stone units and manufacturers should demonstratethat the units have adequate strength for design purposes and have demonstratedadequate durability in the environment in which they are proposed for use.

MMoorrttaarrss

The selection of mortars is important for the performance, maintenance and long-termdurability of masonry arch bridges. The most commonly used mortars in modernmasonry are mixes of cement:lime:sand, with a variety of additions such as plasticisers,retarders, air entrainers and pigments. Table 15 of BS 5628-3:2001 includesprescriptive mixes for four such mortar types, (i) to (iv), with (i) being the strongest,most cement-rich, mix, and (iv) being the weakest, most lime-rich. The type of mortarand designation selected for a particular element of masonry depends upon a numberof considerations. In particular, it should be selected to be durable in the environmentfor which it is intended, but not stronger than the masonry units themselves or thismay lead to their damage and rapid deterioration. Table 10 of BS5628-3 provides aclassification of severity of masonry exposure, ranging from “sheltered” to “very severe”to aid selection of mortar mixes with suitable durability.

In contrast to the traditional prescriptive approach of BS 5628-3, BS EN 998-2:2003Specification for mortar for masonry – part 2: Masonry mortar adopts a more performance-based approach to mortar specification, with mortar specified in terms of compressivestrength (“mortar class”). EN 998-2 includes equivalent mix designations to thosetraditionally used in the UK and included in BS 5628; these are shown in Table 4.5along with guidance for gauging factory produced lime:sand mixes with cement. EN998-2 also requires a consideration and declaration of mortar durability, but in theabsence of any agreed European test method this is currently based on a considerationof local experience. EN 998-2, Annex B provides a means of assessing the suitability ofmortar for different exposure conditions (“severe”, “moderate” and “passive”) based ontemperature, moisture conditions and the presence of any aggressive substances. Theseand other issues are discussed in the National Annex to BS EN 998-2:2003.

TTaabbllee 44..55 EEqquuiivvaalleenntt mmoorrttaarr mmiixx ddeessiiggnnaattiioonnss ooff ccuurrrreenntt BBrriittiisshh aanndd EEuurrooppeeaann SSttaannddaarrddss aanndd cceemmeennttggaauuggiinngg iinnffoorrmmaattiioonn ((MMIIAA,, 22000055))

1 Mortar class (compressive strength) as defined in the National Annexe to BS EN 998-2:2003, ClauseNA.1

Mortar mixes can be specified to BS EN 998-2:2003 when tested by the methods givenin BS EN 1015:1999 and BS 4551:2005. Selection of fine aggregate with a suitablegrading, particle shape and purity is important for the workability and set properties ofmortars, and fine aggregate can be specified to BS 1200:1976 Specification for sand for

CIRIA C656194

BBSS 55662288ddeessiiggnnaattiioonn

BBSS EENN 999988--22mmoorrttaarr ccllaassss11

MMoorrttaarr bbyy vvoolluummeeCCeemmeenntt::lliimmee::ssaanndd

FFaaccttoorryy--pprroodduucceedd bbyy

vvoolluummeelliimmee::ssaanndd

SSiittee--mmiixxiinngg cceemmeenntt::((ffaaccttoorryy pprroodduucceedd lliimmee::ssaanndd))

BByy vvoolluummeeLLiimmee::ssaanndd bbyy wweeiigghhtt

kkgg::ttoonnnneeAAiirr--

eennttrraaiinneeddNNoonn aaiirr--

eennttrraaiinneedd

i 12 1:¼:3 1:12 1:3 - 250

ii 6 1:½:4–4½ 1:9 1:4½ 190 170

iii 4 1:1:5–6 1:6 1:6 150 125

iv 2 1:2:8–9 1:4½ 1:9 100 90

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mortars, which at the time of writing is still current, or to BS EN 13139:2002 Aggregatesfor mortar for which additional guidance is given in PD 6682-3:2002.

The fact that BS 5628 does not deal specifically with the use of lime mortars andnatural hydraulic limes has not encouraged the use of these materials in the UKindustry. However, the more prescriptive approach adopted by the existing andincoming normalised European standards accommodates the use of such materials andprovides additional specific guidance on hydraulic limes.

BS EN 459-1:2001 Building lime – part 1: Definitions, specifications and conformity criteriahas now superseded BS 890:1995 Specification for building limes. It classifies non-hydraulic lime (calcium lime, or “CL”) according to its chemical composition. Allhydrated lime used in the construction industry in the United Kingdom conforms tothe requirements of CL90; this designation indicates the purity of the product, ie thatthe calcium oxide content plus any magnesium oxide present is in excess of 90 percent.

In addition to calcium limes, BS EN 459-1 includes two categories of hydraulic limes.Artificial hydraulic limes (designated “HL”) are unregulated in terms of their overallcomposition, and are likely to contain cement additions to achieve the desired level ofhydraulicity. Two types of natural hydraulic lime are distinguished – “pure” naturalhydraulic lime products which contain no additions (eg pozzolans, gypsum, ash, airentrainers or cement) and are designated as “NHL”, and products where up to 20 percent of additional materials (typically cement) has been added to achieve and maintainthe required degree of hydraulicity, which are designated as “NHL-Z”. Since certainadditions can be detrimental to masonry, when using HL or NHL-Z products it isadvisable to check with the manufacturer or supplier about the quantity and type ofany such additions and the proportion of potentially undesirable components such assulfates (SO3) and other salts which may result in damaging chemical reactions.Although products with additions may be acceptable for newbuild, for conservationwork mortar based on the purer NHL product is likely to be the most suitable and isavailable in a range of grades, NHL2 to NHL5, dependent on degree of hydraulicity:

� feebly hydraulic lime (NHL2) is closest to pure lime in its properties ie relativelylow-strength and slow setting, and suitable for use in mortars for weak masonryunits eg for repointing older masonry

� moderately hydraulic lime (NHL3.5) has a moderate strength and shorter settingtime than NHL1, and can be used for more general bedding and pointing mortarapplications

� eminently hydraulic lime (NHL5) is closest to Portland cement in its properties, iehas high strength and a fast set, although it may not be suitable for use on older,weaker masonry.

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TTaabbllee 44..66 CCoommpprreessssiivvee ssttrreennggtthhss ooff hhyyddrraauulliicc lliimmee aanndd nnaattuurraall hhyyddrraauulliicc lliimmee ((TTaabbllee 33 ffrroomm BBSS EENN445599--11::22000011))

The use of lime-based mortars may be advantageous for elements that arepredominantly subject to compressive forces and requiring some tolerance to minormovements, for instance arch barrels. It is recommended that, where the use of limemortars (including hydraulic limes) is to be considered, expert guidance is sought.Although their use offers potential benefits, care should be taken in the selection anduse of lime-based mortars for the construction of new masonry arch bridges, since theirperformance is less well known than that of Portland cement based mortars. Althoughthey have exhibited good performance in the massive masonry elements constructed inthe past, they may be less suitable for use in modern thin-section designs. At thepresent time, few contractors have the necessary skills and experience to be confidentof achieving an adequate level of workmanship, and the relative sensitivity of limemortars to damage while curing means avoiding construction during periods whenthere might be wet and freezing weather. Modern lime is likely to be much purer thanthe limes traditionally used in mortar production, which frequently had some element ofhydraulicity due to clay impurities or even added pozzolans. Compared to pure limes, useof limes with an element of hydraulicity can produce a stronger and more durable mortarwith many of the same characteristics and advantages of cement based mortars; the use ofsuch materials should be considered where durability in an aggressive environment isrequired. Naturally hydraulic limes are now commercially available, and there is also atrend towards the use of set additives, or pozzolans, which induce a hydraulic set in non-hydraulic lime, of which brick dust is perhaps the most common type.

Advice on the specification and handling of hydraulic lime mortars is included in thegood practice guide Hydraulic lime mortar for stone, brick and block masonry (Allen et al,2003). Guidance is also available from the manufacturers of lime products.

For all mortar joints, jointing and pointing mortars should have mix proportionssimilar to the bedding mortar. Jointing (where the joint finish is achieved while thebedding mortar is still unset) is preferable to pointing since it does not disturb thebedding mortar and is likely to be more durable. The finished joint profile affects boththe durability and appearance of masonry, and selection requires consideration of thesefactors. Tooled finishes are preferred where elements are exposed to rainwater.

MMaassoonnrryy ttrreeaattmmeennttss

The use of sealants and water repellent treatments on masonry can cause problems.They can reduce the rate of evaporation of moisture from the masonry and, dependingon exposure conditions, may increase saturation because the surface drying is inhibitedby the sealant on the face. In certain situations it could be enough to saturate themasonry units sufficiently for frost attack to occur causing damage through spalling.Also, they need periodic reapplication which increases maintenance. The use of suchmaterials should be very carefully considered; such treatments are discussed further inSection 4.3.3.

CIRIA C656196

TTyyppee ooff bbuuiillddiinngg lliimmeeCCoommpprreessssiivvee ssttrreennggtthh MMPPaa

7 days 28 days

HL2 and NHL2 – ≥ 2 to ≤ 7

HL3.5 and NHL3.5 – ≥ 3.5 to ≤ 10

HL5 and NHL5 ≤ 2 ≥ 5 to ≤ 15a

a HL5 and NHL5 with a bulk density lower than 0.90 kg/dm³ is allowed to have a strength up to 20MPa

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55 SSuummmmaarryy aanndd rreeccoommmmeennddaattiioonnss

55..11 RReeccoommmmeennddaattiioonnss ffoorr ggoooodd pprraaccttiiccee

At a strategic level, recommendations for the management of masonry arch bridgeinfrastructure are:

� in the past a reactive approach to infrastructure management has frequentlyprevailed, but this is now viewed as being a disruptive, inefficient and uneconomicapproach and not consistent with achieving a sustainable transport network. Thereis considerable benefit in adopting a more proactive approach, setting out policieswhich aim to meet the long-term objective of preserving the serviceability of ageingbridge infrastructure in the future

� the achievement of long-term objectives depends on the development andimplementation of effective management procedures that support them in the short-and medium-term. These should be geared toward identifying the maintenance needsof bridges and developing and justifying maintenance plans which make efficient useof resources. This requires application of current good practice in the core activities ofbridge inspection, assessment, maintenance, repair and enhancement, and continualassessment and feedback to ensure that procedures are refined

� sufficient resources should be allocated to enable long-term asset managementobjectives to be realised in an efficient way, ie to fund the clearance of any existingmaintenance and repair backlog and to achieve an overall steady state of fullyserviceable condition for masonry arch bridge stocks.

To provide the necessary support for achieving these strategic aims, at the operationallevel, recommendations are:

� those involved with the management and maintenance of masonry arch bridgesshould recognise and develop an understanding of their special characteristics andneeds, as distinct from other types of bridge and structures, to allow them to bemore effective in ensuring their continued serviceability (Section 1.4)

� efforts should be made to gather and organise existing data on all masonry archbridges to form a valuable resource for present and future asset management;steps should be taken to ensure that this data is regularly updated and to minimisethe loss of information in future (Section 3.6.1)

� efforts should be made to identify and prioritise the needs of bridges whoseperformance is particularly sensitive to deterioration or have special maintenanceand repair requirements, and to direct resources appropriately. This will requirethe development and continual improvement of techniques for conditionassessment and prioritisation (Section 3.4.6)

� efforts should be made to establish and justify cost-effective maintenance, repairand replacement schemes for masonry arch bridges which ensure that they areused efficiently, based on the analysis of accumulated bridge data and an improvedunderstanding of the influence of alternative management strategies (Section 3.4.3)

� efforts should be made to improve the quality and objectivity of visual inspections,since the resulting information provides the basis for all other managementactivities. This can be achieved through:

CIRIA C656 197

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� careful selection and thorough training of inspectors (Section 3.7.7)

� use of inspection methodologies which promote the accurate and objectiverecording of data and allow reliable assessment and comparison of bridgecondition (Section 3.7.3)

� optimisation of inspection programmes to direct resources to where they aremost needed without compromising the safety and serviceability of any partof the bridge stock (Section 3.7.1)

� in order in to ensure that the full capacity of bridges is utilised and avoidunnecessary expenditure of resources on assessment and remedial orstrengthening work, a staged approach to the assessment of bridge capacity isrecommended, with increasingly sophisticated techniques of analysis beingemployed where structural adequacy cannot be demonstrated using simple andconservative methods (Section 3.10.2)

� assessing engineers should be aware of the capabilities and limitations of availableassessment techniques and understand how the parameters required for analysisare influenced by the specific construction, materials and defects of bridges. Thesignificance of hidden construction features and materials deterioration should beappreciated and investigation of these factors undertaken where necessary toimprove confidence in assessment results (Section 3.10)

� although preventative maintenance is often overlooked or given a low priority, it islikely to have considerable benefit in the long-term. Asset managers shouldestablish a proactive regime of preventative maintenance for all masonry archbridges to reduce the rate of deterioration and focus on minor problems beforethey become significant. Maintenance and repair should examine the causes, andnot just the effects, of deterioration (Section 3.4.1)

� before implementation of maintenance, repair and in particular strengtheningworks to a bridge, the potential effects on its long-term performance should becarefully considered. The techniques and materials used need to be compatiblewith the existing structure and maintain its inherent flexibility (Section 4.3.4)

� due consideration should be given to the historic, aesthetic and environmentalvalue of masonry arch bridges and the need to respect and preserve them bycarrying out repairs and alterations sympathetically, making full use of existingfabric where reconstruction is necessary. The environmental and ecological impactof the works should be considered and measures taken to minimise undesirableeffects (Section 3.4.9)

� unforeseen circumstances and variations in the original scope of works often causeproblems in the execution of bridge works; these can be minimised by adoptingpractices which encourage co-operative working and being prepared for possiblechanges. It is important for all parties to maintain a flexible and cooperativeapproach during the works (Section 4.3.5)

� given their relatively low maintenance requirements and good performance in thelong-term, in certain circumstances it may be appropriate to consider theconstruction of new masonry arch bridges as a viable alternative to other bridgeforms and construction materials. The current trend toward consideration of“whole-life cost” and sustainability may contribute toward this re-evaluation(Section 4.4).

CIRIA C656198

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55..22 AArreeaass rreeqquuiirriinngg ffuurrtthheerr rreesseeaarrcchh aanndd ffuuttuurree nneeeeddss

Increasingly there is a realisation that economic and material resources are limited, andthat there is a real cost associated with the environmental impact of building, operatingand maintaining transport infrastructure. There is a distinct and pressing need toconserve valuable natural and economic resources by maintaining existing road, railand waterway routes in as efficient and sustainable a manner as possible. This need isillustrated by the considerable sums expended on maintaining the UK transportnetwork, despite which the amount of maintenance, repair and replacement worksgradually increases. However, for at least the last 50 years, the focus of government,clients, industry and academia seems to have been on improving technology for newconstruction. This focus needs to change to meet the objective of sustaining existingtransport infrastructure efficiently into the future. In spite of the accumulatedknowledge of centuries of experience, the behaviour of masonry arch bridges is still notsufficiently understood to be able to benefit from modern analytical, construction andrepair techniques.

Specific requirements for future research include the following:

� there is a requirement for the development of “smart” integrated assetmanagement systems which can consider a wider variety of factors than is possibleat present, for instance environmental issues and changes in loading regimes. Thenext generation of management systems should be mutually compatible withexisting systems and allow bridge owners to “buy into” an over-arching system thatcan deliver best value for the owners, society and the environment

� in order to avoid the subjectivity intrinsic to condition appraisal based ontraditional inspection methods, there is scope for the development of “intelligentinspection techniques” which provide a more objective view of the changes inbridge condition over time. This would involve the periodic collection of specificobjectively measurable bridge parameters. The obvious parameter to measure isthe arch intrados profile, since this is directly linked to structural performance andcan be determined rapidly, with considerable accuracy and at limited cost usingmodern surveying techniques, but other parameters may be suitable

� benefit would be gained by the development of more efficient investigationtechniques, in particular non-destructive investigation and monitoring techniques,to the point where they can be applied routinely and efficiently obtain adequatelyreliable data on bridge construction, condition and performance, and particularly“hidden” elements such as backfill

� although useful guidance is now available in this area (May et al, 2002), improvedunderstanding of the effects of scour on the behaviour of masonry arch bridges,and guidance on assessing the effect of scour, is desirable since scour has beenresponsible for the majority of unforeseen structural failures of bridges in service(Tilly et al, 2002)

� there has been little published research into spandrel wall failure, which is acommon problem and would benefit from improved understanding of its causesand the most effective long-term method of treatment

� existing methods of assessing the structural capacity of bridges are prone to beconservative. There can be a large discrepancy between actual and predictedfailure loads (Tilly et al, 2002, Cullington and Beales, 1994). This can create aconsiderable margin of safety in the interpretation of assessment results, likely tolead to unnecessary restrictions and diversions, and waste of resources onstrengthening or replacement works. A better understanding of the structuralperformance of masonry arches and improved methods of assessment are required

CIRIA C656 199

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to avoid carrying unnecessary works while maintaining adequate factors of safety.Two areas in particular merit further development:

� the assessment of masonry arch bridges has arrived at a significant point inits evolution. Existing experience is wedded to visual inspection andMEXE assessment but new technologies and more advanced analyticalmethods have been developed and are now starting to consolidate a bodyof knowledge and experience which is challenging the status quo. Thereare several initiatives that are attempting to develop acceptable waysforward. Clearly, it is important that new assessment methods aredeveloped. A more holistic approach is needed which considers not onlythe changes in loading and the condition of the arch barrel but alsoconsiders other construction details, the boundary conditions, soil-structure interaction, the type of materials used and their condition. Ascomputer power increases, the possibility of a probabilistic approach toassessment becomes more feasible

� advances in assessment should be supported by a better understanding ofthe behaviour of masonry arch bridges, based on data from further fieldand laboratory testing. Much existing data is limited in its usefulness sinceit was carried out when the importance of certain parameters was not fullyunderstood, and was not fully or adequately recorded. New and morecomprehensive data will be central to the development and validation ofnew assessment techniques and relating them to existing methods(including MEXE). Future development should address a number offundamental aspects of masonry arch bridge performance and behaviour,including:

� load transfer through the bridge – especially through the backfill. Thisrequires development of theory supported by field and laboratory tests

� soil-structure interaction, particularly development of a more holisticapproach including abutment and foundation movement. Differenttypes of backfill should be considered – particularly cohesive material.In addition to theoretical studies, field and laboratory tests should beundertaken

� the performance of masonry arch bridges subjected to cyclic (fatigue)loading.

� a better understanding of the performance of masonry parapets and their vehiclecontainment capabilities would be beneficial. The test data that does exist relates tonewly constructed masonry but methods of assessing existing masonry parapetsshould be capable of considering the effects of material deterioration. Developmentof simple and economical methods of enhancing masonry parapet performance isdesirable, particularly systems which are sympathetic to the appearance andheritage value of masonry arch bridges

� there is a need for improved understanding of the deterioration mechanismswhich affect masonry materials, the potential rates of deterioration, effects on theirphysical characteristics and subsequently their influence on bridge performance.This would allow more reliable prediction of maintenance and repair requirementsfor the bridge fabric, and identification of preventative maintenance opportunities

� there is a need for those involved with selection and specification of bridge worksto learn from growing experience with “traditional” materials such as lime mortars,to improve awareness of their potential advantages and to provide better guidanceon their use for the repair and rehabilitation of existing masonry bridges. Likewise,

CIRIA C656200

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contractors should develop their skills and experience with such materials andapplication techniques to allow their routine use in maintenance and repair

� as our understanding of the behaviour of masonry arch bridges improves and withthe benefit of hindsight, it has become apparent that inappropriate repairs havebeen carried out in the past, with the consequence that further work has beenrequired to bring affected bridges back into full service. There is a need forindependent assessment of the efficacy of established and novel repair andstrengthening techniques and in particular a better understanding of their long-term effects. Repaired bridges should be instrumented and monitored to providethe necessary data

� low-energy and sustainable maintenance and repair solutions should be sought

� to encourage the consideration of masonry arches as a viable form of constructionfor new bridges, there is a need for improved understanding of masonry archdesign, development and publication of suitable design codes and guidance, anddevelopment of techniques for improving the efficiency and economy of masonryconstruction, for instance pre-fabrication of masonry elements and modularconstruction.

CIRIA C656 201

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CIRIA C656202

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AA11 CCaassee ssttuuddiieess

This appendix includes a number of case studies involving works on masonry archbridges to illustrate some of the issues associated with their practical implementation.

Table A1.1 lists each project and provides a brief summary of the main problems andsolutions dealt with in each case study.

TTaabbllee AA11..11 SSuummmmaarryy ooff ccaassee ssttuuddiieess iinncclluuddeedd iinn tthhiiss aappppeennddiixx

CIRIA C656 203

SSeeccttiioonn BBrriiddggee//pprroojjeecctt PPrroobblleemmss SSoolluuttiioonnss

AA11..11

Saddling and repair ofElsage Farm Bridge.

Cracking and deterioration ofmasonry arch andabutments, severe leakagethrough deck.

Addition of reinforced concretesaddle, brickwork repairs anddeck waterproofing.

AA11..22

Railway cross-countryroute bridgestrengthening works.

Increased dynamic loadingfrom trains requiredstrengthening of spandrelwalls.

Addition of spandrel tie-barsand patress plates.

AA11..33

Rockshaw RoadOverbridge.

Settlement of an abutmentcaused overstress of a pierand damage to brickwork,urgently requiring a reductionin dead load on the pier.

Repairs to damaged masonry,grout injection of the damagedpier and infill of relieving archwith concrete to re-distributethe load and reduce the stressin the damaged leg of the pier.

AA11..44

Ecological issues incarrying out works ona bridge in Wales.

A number of protectedspecies, including bats, birdsand otters, were potentiallyaffected by maintenance andrepair works to the bridge.

Statutory bodies and naturegroups consulted, works weretimed to minimise ecologicalimpact and provisions made toprotect local fauna.

AA11..55

Sympathetic repair ofBrynich Aqueduct.

Movement and leakage,excessive pressure onspandrel walls, accessdifficulties, working on ascheduled monument withprotected species (bats).

Grouting and anchoring works,replacement of fill withconcrete saddle, consultationwith statutory bodies andnature groups and adoption ofconservation-focused approachto works.

AA11..66

Refurbishment ofBerwyn Viaduct.

Cracking to arch barrel andspandrels, brickworkdamaged by root growth,constraints on access andtiming in construction phase.

Stitching and grouting, patchrepair to masonry, improvementof drainage, careful design andplanning of construction phase.

AA11..77

Strengthening andrefurbishment ofHungerford CanalBridge.

Assessment indicatedinadequate structuralcapacity; strengtheningrequired; Grade II listedbridge with integral services;working over a navigablewaterway.

Options assessed on basis ofcost, safety and disruption;system of circumferential retro-reinforcement of arch ringimplemented from floatingplatform; bridge appearancepreserved and disruptionminimised.

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CIRIA C656204

AA11..88

Assessment ofLlanharan Bridgeusing discreteelement analysis.

MEXE assessment indicatedinadequate bridge capacity butwas unable to take into accountthe benefit of additional pierspreviously added in response tostructural distress of arch barrel.

Discrete element analysisundertaken to improveunderstanding of originaldeterioration and additionalcapacity provided by newpiers, resulting in increase inassessed bridge capacity.

AA11..99

Strengthening ofGumley Road Bridgeusing retrofittedreinforcement.

Assessment demonstratedinadequate capacity;strengthening required; Grade IIlisted bridge.

Installation of proprietaryretrofitted reinforcementsystem assisted by 3D solidmodelling techniques.

AA11..1100

Repairs to CaergwrlePackhorse Bridge.

Stone bridge suffering fromneglect and flood damage;silting of arches and inadequateflow capacity; partial rebuildingrequired; listed as ancientmonument.

Sympathetic rebuilding ofdamaged parapets andspandrel walls; works tomaximise the flow capacity;masonry repairs and re-pointing; archaeologicalassessment.

AA11..1111

Reconstruction ofbrick arch bridges onthe ChesterfieldCanal.

Two deteriorated and unstablebrick and stone masonrybridges; heritage value;presence of protected species

Sympathetic reconstruction;retention of heritage value(features and materials);species habitat maintained;safety features improved.

AA11..1122

Egglestone AbbeyBridge strengtheningand repairs.

Displacement of spandrel walls;voids in masonry; weightrestriction; Grade II listed bridge;presence of bat roosts.

Temporary structural repairs;masonry repairs and voidgrouting; installation ofproprietary anchoring system;provision of alternative roostsfor bats.

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AA11..11 SSaaddddlliinngg aanndd rreeppaaiirr ooff EEllssaaggee FFaarrmm BBrriiddggee

Railway Underbridge 121, known as Elsage Farm Bridge, is a single span brickworkarch structure carrying four tracks of the London Euston to Rugby Junction line over aprivate farm access road (Figure A1.1). The presence of a number of transversefractures, visible movement under traffic and extensive seepage indicated that thecondition of the bridge was deteriorating rapidly.

FFiigguurree AA11..11 GGeenneerraall vviieeww oonn eeaasstt eelleevvaattiioonn

DDeeffeeccttss

The arch barrel exhibited significant transverse joint fractures around 2 m abovespringings, with extensive water seepage and staining to 80 per cent of the soffit area(Figure A1.1). Slight movement was observed at these fractures under live load. Hollowareas were identified in the crown. Weep holes had previously been cored into theintrados to assist drainage. The abutments also contained hollow areas with multiplehairline fractures.

CIRIA C656 205

Client: Network Rail Ltd

Principal contractor: Birse Rail Ltd

Designer: Owen Williams Railways

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FFiigguurree AA11..22 WWaatteerr iinnggrreessss tthhrroouugghh tthhee aarrcchh bbaarrrreell;; pprreevviioouuss llaarrggee ppaattcchh rreeppaaiirr ttoo aarrcchh iinnttrraaddooss iiss vviissiibblleeoonn tthhee rriigghhtt

OOppttiioonnss ccoonnssiiddeerreedd

� do nothing

� reinforced concrete saddle over arch, brickwork repairs and waterproofing of the deck

� brickwork repairs and waterproofing of the deck

� brickwork repairs, installation of sprayed concrete lining and waterproofing of the deck

� deck reconstruction.

PPrreeffeerrrreedd ooppttiioonn

Doing nothing at this location was not an option as the condition of the bridge wasdeteriorating rapidly. Brickwork repairs would not give adequate strength to the bridgedeck as the fractures were so severe. A sprayed concrete lining would reduce headroomand exacerbate disruption to the access road. Reconstruction was beyond the scope ofwhat was required. The option selected was the addition of a reinforced concretesaddle with brickwork repairs and deck waterproofing.

Although strengthening was not required to increase the assessed capacity, thetransverse fractures implied that there might be a stiffness problem with the structure.The saddle is expected to improve the arch barrel stiffness while providing a soundsubstrate for the waterproofing membrane. The additional work required to install thesaddle was not unduly expensive compared with simply waterproofing the existingextrados since the main cost is removal of the tracks, which would apply in either case.A design life of 75 years was specified for concrete saddle (considered non-structural),assuming no damage and regular structure maintenance.

PPrrooppoosseedd rreemmeeddiiaall ssoolluuttiioonnss

� stitch cracks and grout

� pin hollow brickwork and grout

� remove spalled/loose brickwork and recase

� rake out loose mortar and repoint

CIRIA C656206

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� reinforced concrete saddle over arch

� application of membrane waterproofing to saddle

� rebuild fractured revetment.

IImmpplleemmeennttaattiioonn

PPoosssseessssiioonnss

The concrete saddle work was carried out over a number of possessions. The civilswork scope within these possessions included:

� possession one (10 hrs): support S&T cables

� possession two (10 hrs): install access ramp

� possession three (96 hrs): excavate existing fill, erect shuttering, pour concretesaddle, lay waterproofing and drainage, backfill, reinstate S&T cables

� possession four (10 hrs): remove access ramp.

PPllaanntt

The large plant used during the main implementation works included:

� 14 tonnes tracked excavators fitted with height restrictors + spare

� 5 tonnes rubber-tracked excavator

� 120 rollers + spare

� 6 tonnes dumpers + spare

� Wacker plates + spare

� TE75 drills and bits + spares

� Task lighting/generators + spares

� Certified 6 tonnes lifting chains + spare

� 31 m concrete pump + spare static pump

� Compressor unit + spare

� Vibrating pokers + spare

� 9 m tower lights + spare.

Spares were included for contingency against plant breakdown.

AAcccceessss//ccoonnssttrraaiinnttss

Possession access was gained via a section of a nearby field set aside for allotments.However, this only allowed for access from the south side of the bridge, which meantthat plant and materials had to be brought across the bridge and stationed on thenorth side prior to the excavation work commencing. This also restricted plantmovement to and from the bridge as there was insufficient room for a passing point tobe built into the access ramp.

The project possession works would have been easier to implement if access could havebeen gained to the north side of the structure.

CIRIA C656 207

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Access to the bridge arch, spandrels and wingwalls was from the private access roadwhich ran beneath the structure. A thoroughfare had to be maintained for residents atall times; proprietary scaffold towers were used for high level access as these could bequickly moved out of the way or demobilised if necessary.

Temporary closure of the access road would have been beneficial to the works, but itwas not an option in this case.

IInntteerrffaacceess

Interfaces had to be managed with the parish council for compound access through thevillage hall car park, taking into account functions booked at the village hall. Workswere undertaken on the car park to reinstate any damage, and Network Rail agreed topay for the full surfacing of the car park after completion.

Heras fencing panels were erected around the compound and access road to protectthe public and allotment owners, and to prevent unauthorised access.

Interfaces with other contractors had to be managed to ensure there was minimaldisruption of planned work scope or access during implementation.

Sensitive issues between Network Rail and the private access road and local landownerhad to be managed on site to ensure the relationship did not deteriorate during theworks.

SSeerrvviicceess

Only the trackside signalling and telecoms (S&T) services were affected by the works,and these were temporarily supported in situ. The access ramp crossing to the westincluded protection to the services to prevent damage.

EEnnggiinneeeerriinngg

The standard brickwork methodologies were followed. However, additional grout holeshad to be drilled into the abutments to allow full grouting as hollow-sounding areasremained after the main pinning and grouting work was completed. This wouldsuggest that the grid pattern used for the original grouting did not allow grout to bepumped into all voided or fractured areas within the substructure.

The excavation, concrete pour and backfill operations were strictly controlled tominimise imbalance of loading on the arch barrel. A maximum level difference of 0.5 mwas allowed for fill or concrete either side of the brick arch, and an exclusion zone wasorganised to ensure surcharge pressure from heavy plant was retained by theabutments. This reduced the potential for arch instability during construction works.

One of the main problems identified during the works was the presence of internalspandrel walls encountered within the excavations. As the concrete shutters to be usedfor the works were proprietary panel forms, the internal spandrels had to be brokenout to allow the shutters to be placed at the correct level. This used up additional timeduring the 96-hour possession, which had not been allocated to the shuttering activity.

CIRIA C656208

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FFiigguurree AA11..33 AArrcchh eexxttrraaddooss pprreeppaarreedd ffoorr rreeiinnffoorrcceemmeenntt aanndd sshhuutttteerriinngg

Three particular parameters were determined as requirements for concrete strengthgain during the 96 hour possession; these were to progressively strip formwork, to passheavy plant over the structure and to pass trains over the structure. The requiredconcrete strength-gain was plotted against time to decide what mix design would bestfit the project needs. From this, concrete suppliers were requested to forward historicalcube strength data to show consistency with the requirements. Cube tests were alsotaken during the concrete pour to confirm the actual strength gain was in line with therequirements.

Loose-laid membrane waterproofing was the preferred option to use in this case as it isrelatively quick to install and can be used in a wide range of weather conditions.Sprayed acrylic waterproofing would have taken a much longer time to place andwould have introduced more onerous requirements on the concrete finish. Further, acuring period would have been necessary prior to the application of the acrylic system.Finally a membrane system would have had to be employed in conjunction with thesprayed acrylic to ensure adequate waterproofing against the brickwork spandrels. Thiswould have introduced a discontinuity into the system, which could have proveddetrimental over time. Bonded membrane waterproofing system would again havetaken longer to place and was discounted.

CIRIA C656 209

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AA11..22 CCrroossss--ccoouunnttrryy RRoouuttee BBrriiddggee ssttrreennggtthheenniinngg wwoorrkkss

A qualitative risk assessment undertaken on the effects of trains travelling at higherspeeds on a railway cross-country route indicated that lateral load effects on thespandrel walls would increase. To mitigate the potential risk of spandrel failure,spandrel ties were proposed to be installed on nine bridge structures.

Minimal brickwork and structural defects were identified on the structures, the mainissue being the increase in lateral load.

OOppttiioonnss ccoonnssiiddeerreedd aanndd pprreeffeerrrreedd ooppttiioonn

Installing spandrel tie-bars and pattress plates was the only option considered as apreventative measure. This was so that disruption of traffic could be kept to a minimumruling out “the Stratford method” (see Section A6.12) or saddle strengthening.

PPrrooppoosseedd rreemmeeddiiaall wwoorrkkss

� minor repointing

� replace missing brickwork

� drill and install spandrel tie-bars and pattress plates

� design life of 50 years assuming no damage and regular structure maintenance.

IImmpplleemmeennttaattiioonn

PPoosssseessssiioonnss

The first tie-bar casing installation was carried out during a possession. The civils workscope within the possessions included:

� possession 1 (12 hrs): excavate trial pits at services; install 1no tie-bar casing

� follow-on possessions (12 hrs): contingencies in case of installation failure duringnormal running of trains

� a follow-on possession was booked to allow at least one tie-bar to be installed eachweekend ie six ties in total would require five follow-on possessions.

PPllaanntt

The large plant used during the main implementation works included:

� 13 tonnes excavator fitted with a drilling arm

� wall-mounted coring rig

� access scaffolding.

CIRIA C656210

Client: Network Rail Ltd

Principal contractor: Birse Rail Ltd

Designer: Mott Macdonald Ltd

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An on-call fitter and spares were available for contingency against plant breakdown.

AAcccceessss//ccoonnssttrraaiinnttss

Some of the structures were above road and footpaths and some were above waterways.Access was available at ground level on the roads and footpaths.

Access scaffolding had to be put in place on the structures above waterways and analternative method of drilling used as there was no way of stabling the machine-mounted drill rig over the water. Standard coring equipment was fixed to the spandrelwalls and the tie-bar cores drilled as per the standard method.

Track access was gained via authorised access points adjacent to structures, but in someinstances access steps were cut into the embankments to prevent slips, trips and falls.

No track access was available during the normal running of trains for safety reasons. Toovercome this problem, temporary survey stations were established in a position ofsafety at track level and remote targets fixed to each rail web above the proposed coreposition. This allowed the track to be monitored for movement or settlement whiledrilling/coring during the normal running of trains.

The drill rig used was fitted with a height restrictor during weekday drilling works toprevent the arm moving over the rails and coming into contact with the normalrunning of trains.

FFiigguurree AA11..44 MMaacchhiinnee--mmoouunntteedd ddrriillll rriigg iinn ooppeerraattiioonn

IInntteerrffaacceess

Interfaces had to be managed with the local authorities for footpath and road closures.Footpaths were either temporarily closed or pedestrians were diverted to prevent aconflict between operating plant and the public. A temporary closure was put in placeon the highway to eliminate traffic management at the structure.

Heras fencing panels were erected around the compound and access road to protectthe public and to prevent unauthorised access.

Interfaces with other contractors had to be managed to ensure there was minimaldisruption of planned work scope or access during implementation.

CIRIA C656 211

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The Environment Agency had to be contacted to give temporary works consent forscaffold access above a river and netting protection to a stream affected by the works.

SSeerrvviicceess

All the bridges strengthened had signalling and telecoms (S&T) services adjacent to atleast one parapet. During booked possessions, trial pits had to be excavated and levelstaken to confirm the extent and depth of the services, minimising the potential forservice damage during drilling/coring works.

IImmpplleemmeennttaattiioonn pphhaassee

The initial drill rig setup and subsequent monitoring regime is critical as there is asmall risk of the drill head deflecting if obstructions are encountered. A large scale setsquare and spirit level was used to position the drill rig at the proposed core entrypoint and shoring props were used at the front and back of the drilling arm forsupport (Figure A1.4). Track monitoring was put in place (see access/constraints) anddimensions taken to monitor when critical depths were achieved, ie between or beyondthe tracks. The drilling equipment was continuously visually monitored during theworks to note any changes in drilling progress or drill head rotation rate, which wouldgive an indication of change in material or change in drilling direction.

At one structure, concrete and reinforced steel were encountered within the bridge fill.Steel is impenetrable with the Symmetrix drill bit normally used (compressed airpulverises the material immediately ahead), so another method had to be employed. Inthis instance, a “down the hole” hammer drill bit was used for the concrete (hammer headpulverises the material immediately ahead) and a core rig fixed to the spandrel wall to corethrough the steel (Figure A1.5). See Section A6, Section 6.10 for standard methodologies.

FFiigguurree AA11..55 WWaallll mmoouunntteedd ddrriillll rriigg iinn ooppeerraattiioonn

After further investigation, archive drawings were found which showed this structurehad been strengthened using a concrete saddle within the previous 10 years. Had thisinformation been available at the design stage the special requirements for drilling atthis bridge could have been identified in advance and problems during implementationavoided.

Additional work methodologies were employed when required. For the structure over ariver, standard coring equipment was fixed to the spandrel walls and the tie-bar coresdrilled as per the standard method.

CIRIA C656212

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AA11..33 RRoocckksshhaaww RRooaadd OOvveerrbbrriiddggee –– ppiieerr rreeppaaiirr ooff aa tthhrreeee ssppaannmmaassoonnrryy aarrcchh

IInnttrroodduuccttiioonn

Rockshaw Bridge at Merstham, carrying a road over Network Rail’s London toBrighton line, exhibited serious defects in the downside pier over the London end half.

Visible increase of the existing vertical fractures, localised crushing of brickwork andbulging of the faces indicated a deteriorating situation.

Built in the 1870s the 36 m long bridge spanned a cutting and consisted of three equal semielliptical arches of rise 4.6 m and spans of 9.2 m. The central arch is supported on two piers6.5 m to springing point from ground level in height and 1.3 × 5.4 m in plan area.

FFiigguurree AA11..66 EErreeccttiioonn ooff aacccceessss ssccaaffffoolldd ffoorr cclloossee iinnssppeeccttiioonn

TTeemmppoorraarryy wwoorrkkss aanndd ssiittee iinnvveessttiiggaattiioonn

The first consideration was safety. Freshly broken bricks on the bulging face of therelieving arch on the downside pier and cracks that were wider at their centres indicatedthe damage was live, progressive and probably in its final stages. Immediate action wasrequired to arrest further movement. A steel corset was constructed and clampedaround the brick pier and the road over closed by the local authority (Figure A1.7).

While the steel corset was under construction a remote monitoring system usingvibrating wire strain gauges was installed over existing main cracks. Movementreadings were logged hourly.

Following these temporary measures a detailed survey of the bridge was executed usingtotal station surveying instruments. At the same time a careful inspection of thestructure recording all defects was completed.

CIRIA C656 213

Courtesy Crouch Waterfall & Partners, Bill Harvey Associates,CMS Pozzament and Network Rail

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With the completion of the steel corset movement of the pier was reduced dramatically.

The results of the survey and inspection also provided evidence of the causes. Thedownside London end abutment corner was 200 mm lower than the country endcorner and both corners of the upside abutment, all of which were virtually at the samelevel. This drop is visible in the string course between the spandrel and parapet wall(Figure A1.8).

� the opening of the vertical cracks at their centres and the bulging faces of the pierindicate excessive compression

� the longitudinal profile of the road over produces a sag curve immediately behindthe downside abutment

� a drainage gully is located on the London side of the road behind the abutmentand discharge through the wing wall directly onto the cutting face

� at times of heavy rain the gully proved inadequate and water ponded behind theabutment

� simultaneously water issued through the front face of the abutment

� in wet weather a series of diagonal cracks are clearly visible across the face of thedownside spandrel and parapet wall on the London end face.

Intrusive boroscope investigation revealed the pier was constructed with a 9 in (225mm) outer skin infilled with voided and unbonded brickwork.

FFiigguurree AA11..77 CCrruusshhiinngg bbrriicckkwwoorrkk iinn ppiieerr bbeellooww sspprriinnggiinngg sshhoowwiinngg lloonnggiittuuddiinnaall ccrraacckkss,, wwiitthh ppaattcchh rreeppaaiirrss ttoossppaalllleedd bbrriicckkwwoorrkk aanndd sstteeeell ccoorrsseett aaddddeedd ffoorr tteemmppoorraarryy ssuuppppoorrtt

CIRIA C656214

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FFiigguurree AA11..88 DDrrooppppeedd ssttrriinngg ccoouurrssee aabboovvee ppiieerr ssuuffffeerriinngg ffrroomm sseettttlleemmeenntt

CCoonncclluussiioonnss

It was concluded that settlement of the London end corner of the downside abutmenthad occurred due to constant saturation of the soil beneath it. In turn this had throwntoo great a load onto the pier causing overstress of the brickwork removal of dead loadon the pier was immediately required.

RReemmeeddiiaall aaccttiioonn rreeqquuiirreedd

� analysis of the bridge showed that removal of fill over the bridge, while relievingstress, could cause instability in the pier if not strictly controlled

� construction vehicles could not exceed 5 tonnes

� the fill had to be removed in narrow strips over the entire length of the bridges

� if approximately 1 metre depth of fill was removed the dead load would bereduced by 100 tonnes

� removal of the fill to 1m depth over the length of the bridge would providemaximum relief and the thrust lines show that the pier remains stable.

SSttrruuccttuurraall iinntteeggrriittyy aanndd ppeerrffoorrmmaannccee

It was concluded that the piers were built as filled shells. Their structural performancedepends on the degree of integrity of the shell. It is likely that the core also carries itsshare of load but it is likely to be marginally less stiff and, because of the open perpendjoints found, more liable to creep fracture.

The piers will be subjected to some bending effects and in that condition will functionas filled tubes while the shells are complete. In the present condition, one face of oneleg has buckled outwards breaking away from the brick bond at the corners and fromthe core. Possibly as a result of this but possibly a cause, some masonry is badly crushed.

In order to restore the integrity of the pier it was concluded that:

� those shell bricks which are crushed beyond value are replaced

� the integrity of the connections at the corner of the shell are restored

� as much as possible create an interaction between shell and core

CIRIA C656 215

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� the base area of material carrying the load is increased by filling the void in the pier

� the badly constructed and shattered core is grouted to restore its integrity.

It was also concluded that the downside centre span arch rings are stitched and thecracks in the intrados grouted.

The vertical alignment is improved to remove the sag curve at the downside abutment.

Several options for repair were explored and various materials considered for use. Itwas imperative that any void filling material used should help create a densehomogenous support, but not exceed the compressive strength of the original Victorianlime mortar. A medium strength proprietary lime-based grout specifically formulatedfor use with “heritage structures” was chosen. To prevent seepage of the grout into thetrack bed a plug of a proprietary specialist anchor grout was to be pumped into thebase of the pier.

To redistribute the load and reduce the stress in the damaged leg of the pier therelieving arch was infilled. A concrete “mushroom” was cast in the upper half of therelieving arch and jacked-up to 104 tonnes. The jacks were locked off and the lowerhalf of the relieving arch infilled with concrete.

Finally foam concrete was pumped over the bridge to replace the previously removedfill. This reduced the original dead load weight of the bridge by some 60 tonnes.

CCoonnssttrruuccttiioonn sseeqquueennccee

� repoint to seal all mortar joints in the pier

� drill and grout up pier (11 tonnes of grout were injected into the pier)

� drill transversely and longitudinally horizontally into the pier and install stainlesssteel ties

� cast in place upper half of concrete “mushroom”

� jack load upwards into pier via the concrete mushroom, lock off jacks

� infill lower half of relieving arch with concrete

� remove steel corset

� pump foam concrete as backfill to deck

� place road surfacing to new vertical alignment.

Monitoring continued for one year after the works were completed and no furthermovements were recorded.

CIRIA C656216

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AA11..44 EEccoollooggiiccaall iissssuueess wwhheenn wwoorrkkiinngg oonn aa bbrriiddggee iinn WWaalleess

This case study concerns a three-span masonry arch bridge in west Wales. One of thebridge abutments had been damaged by water scouring and it was also discoveredduring inspection that not only did the abutment require underpinning but some jointsneeded pointing and much of the structure required pressure grouting. The bridgewas not considered unsafe in the short term so ecological surveys to be undertaken atthe same time as further structural investigations.

It was found that Daubenton’s bats were using crevices in the arch of the central span.Sparrows were nesting in the weep holes of a connected revetment wall and there wasanecdotal evidence of sand martins having also used these structures. Otters wereutilising the car park access and crossing the road rather than swimming under thebridge during spate conditions.

Otters and bats are European protected species under the Conservation (NaturalHabitats &c.) Regulations 1994 and are also protected by the Wildlife and CountrysideAct 1981 (as substituted by the Countryside and Rights of Way Act 2000). Birds’ nests,eggs and nestlings are protected by the Wildlife and Countryside Act 1981.

Advice was sought from the statutory body the Countryside Council for Wales andfurther studies of the bat roosts was undertaken throughout the summer and whilework on the underpinning of the abutment was continuing. Before work started onpressure grouting the joints in the arch it had been established that it was unlikely thatthe bats were hibernating there and all the evidence from the local bat group indicatedthat it was a summer roost only. Work therefore started in October and decisions onwhich crevices to keep were negotiated between the bridge engineer and the ecologist.The remaining crevices were highlighted with paint and then filled with straw duringthe pressure grouting, this preserved the spaces effectively. As the bridge did notrequire major construction works there was no opportunity to build in artificial batboxes.

Work on re-pointing the revetment wall was undertaken after the bird nesting season.There was an opportunity to create artificial nest sites for sand martins by bedding 1 mlengths of 40 mm pipe into a proportion of the weep holes sloping gently (1/60)downwards towards the entrance. They have not been found to have an adverse effecton the function of weep holes.

An otter pass was installed under the bridge to prevent them crossing the road. Thesimple bolt on structure cost approximately £1000 and was made by a local fabricator.The decision to install the ledge was taken after the Welsh Assembly Government(WAG) commissioned a study of otter mortality on roads and recognised that many ofthe deaths were as a result of the animals being unable to swim under bridges in spateconditions. WAG and the Environment Agency flood control section have approved thedesign for an otter pass and this was installed above normal flood levels. Fencing andappropriate ramps that meet the river bank direct the otters to the ledge.

CIRIA C656 217

Supplied by Judi Shorrock, Ecologist, Morrison Construction

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Monitoring of bats and nesting birds was undertaken the following year. Bat numberswere almost identical to those before the bridge was grouted. Sand martins were flyingclose to the pipes and as they are known to seek nesting sites a year in advance it ishoped that the artificial nest holes will be in use shortly. There have been no furtherotter mortalities on the road.

CIRIA C656218

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AA11..55 SSyymmppaatthheettiicc rreeppaaiirr ooff BBrryynniicchh AAqquueedduucctt

SSuummmmaarryy

Problems with movement and leakage, loss of mortar and water seepage weakeningclay fill required the replacement of fill material to relieve pressures on spandrels andsidewalls. Flood risk meant that additional strengthening measures could not rely onsupport from ground-level, necessitating grouting and anchoring works carried out“from within the structure”. This is a scheduled monument with protected bat species,and the successful completion of the project was dependent on working closely with theconservation and heritage authorities and local environmental groups to developacceptable strategies for dealing with these issues and obtaining the necessarypermissions in advance of starting work on-site.

BBaacckkggrroouunndd

Brynich Aqueduct is a 50 m long multi-span masonry arch structure dating from 1804,spanning the Monmouthshire and Brecon Canal over the River Usk (Figure A1.9). Ithas been extensively repaired with rebuilt spandrel walls and arch barrels and theconstruction of supporting brick arches beneath two spans over the River Usk. Mostelements of the structure showed evidence of movement and deformation, and theaqueduct had a history of repairs including:

� reconstruction of areas of spandrel walls and arch voussoirs in stone

� replacement of stone voussoirs with brick

� construction of supporting brickwork arch beneath stone arch

� filling of scour holes to piers in riverbed

� lining of aqueduct trough with sprayed concrete.

Some of the above have been carried out recently.

CIRIA C656 219

Courtesy British Waterways

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FFiigguurree AA11..99 GGeenneerraall vviieeww ooff BBrryynniicchh AAqquueedduucctt

Investigations indicated that ongoing leakage and groundwater movements hadresulted in loss of mortar from joints and weakening of the clay fill, contributing to anoverall weakening of the aqueduct. In response to these problems, a £500 000strengthening and repair contract was carried out over a six month period between1996 and 1997.

The principal engineering issues were:

� the need to replace fill material to relieve internal pressure on the spandrels andside-walls

� the inadequacy of the masonry structure to withstand the works without additionalstructural support

� the timing of the works and flood risk meant that it was not possible to providestructural support from the ground.

In addition to the engineering issues, a number of environmental and heritage issueswere identified:

� the location of the structure in the Brecon Beacons National Park and its presencein the landscape

� the scheduled monument status of the aqueduct, requiring consent for the worksfrom CADW

� the presence of protected bat species in the soffits of the arches (Dabenton’s andPipistrelle)

� impacts on the operation of the navigation

� impacts on the water quality of the canal and the River Usk below

� impacts on fish and other wildlife, including dipping birds

� variable river levels

� approval of the Environment Agency.

CIRIA C656220

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EEnnggiinneeeerriinngg aassppeeccttss

Investigations had indicated deterioration and loss of mortar from the stone archbarrels and some separation of the brick barrels. After the initial investigations anddesign phases the works were undertaken in three main stages: grouting of masonrystructure, replacement of fill with mass concrete and construction of aqueduct trough.A system of grouting and anchoring was adopted to strengthen the arches and pierswhich then allowed concrete fill to be placed working from “within the structure”, andexcavation of fill material and replacement with concrete was undertaken in carefullycontrolled stages:

� to improve integrity and increase strength the whole structure was repointed usinga lime based mortar grouted with a low strength cement based grout. These workswere undertaken with the canal remaining in water and open to navigation

� on completion of grouting and draining the canal the trough and fill wereexcavated to the underside of trough level. From this point a detailed plan forsequence of excavation and placement of mass concrete fill was required to preventout of balance forces developing (Figure A1.10). This plan included limitations onmachine size and positioning. Excavations revealed internal rib walls built off thearches extrados and the extent of some of the previous repairs

� a reinforced concrete trough was constructed off the fill, connected into the canallining at each end.

FFiigguurree AA11..1100 PPllaacceemmeenntt ooff mmaassss ccoonnccrreettee ffiillll oovveerr aarrcchh bbeettwweeeenn iinntteerrnnaall ssppaannddrreellss

RReessttoorraattiioonn iissssuueess

Great care was taken with the choice of mortar for grouting and pointing; a hydrauliclime mortar was used to allow the structure to “breathe” and accommodate limitedmovement. Development of the repair options required close cooperation with CADWto gain their approval for the works and keep the repair contract running smoothly.The pointing detail was of particular concern, since it has a significant effect on theappearance of the structure. CADW required a full photographic record of thestructure pre-works. They also inspected the raking out of the old mortar from thejoints to ensure this was not causing damage to the brickwork, and required thecontractor to demonstrate the suitability of their proposed repointing techniques andmaterials by trialling using test-panels.

CIRIA C656 221

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The presence of protected bat species in the decaying arches required measures to betaken to provide suitable alternative roosts during and after the works. Artificial roostswere provided on the structure, designed by the Countryside Council for Wales andmodified by CADW to offer greater concealment within the arch barrels and lessentheir visual impact. Additional consultation was carried out with the local bat group.The operation proved a success, the bats adopting the new roosts the first season aftertheir completion.

In addition to the bat roosts, additional roosts were provided for dipping birds.

It was vital to the overall success of the project that the environmental and heritageissues were fully considered at the outset, and that all the necessary consultations werecarried out in advance of the need to obtain formal consents for the works and the on-site execution of the contract. Without this, it the project would have suffered costlydelays and setbacks and possibly compromised the ultimate achievement of theengineering works.

CIRIA C656222

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AA11..66 RReeffuurrbbiisshhmmeenntt ooff BBeerrwwyynn VViiaadduucctt

IInnttrroodduuccttiioonn

Berwyn Viaduct is located at Berwyn Station on the Llangollen Railway in Wales, whichwas opened in 1865. The line was heavily used, both by passengers and freight, untilthe 1960s when, the line was run down and closed, after which the viaduct fell intodisrepair and an original cantilever platform was removed for scrap. In July 1975, theFlint and Deeside Preservation Society (the forerunner of the Llangollen Railway Trust)leased Llangollen Station and the trackbed, and this led to the refurbishment ofBerwyn Viaduct and reinstatement of the platform.

Berwyn Viaduct consists of six equal arch spans (Figure A1.11). The first three spansout from the station cross open ground. The next two span a small fast-flowingtributary of the River Dee and the final span is across the minor road leading to theHorseshoe Pass, which is heavily used during the summer months. The location of thebridge was to prove one of the major challenges in the construction of the platform andthe remedial works.

FFiigguurree AA11..1111 GGeenneerraall vviieeww ooff BBeerrwwyynn VViiaadduucctt bbeeffoorree rreeffuurrbbiisshhmmeenntt

AAsssseessssmmeenntt

In July 1992 an inspection and assessment of all the structures on the re-opened linewas carried out. The assessment of Berwyn Viaduct showed that, in good condition, thestructure was able to take axle loads up to 22 tonnes. This would be sufficient for alllocomotives that the railway intended to run on the line. However, the assessmentinspection identified that there were a number of areas of distress to the structure,particularly on the spandrels where significant vegetation growth had resulted in

CIRIA C656 223

Courtesy Jonathan Symonds, David Symonds Associates

Winner in the 2003 Institution of Civil Engineers Historic Bridgeand Infrastructure Awards

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displacement of the masonry (Figure A1.12). There was also cracking and separation ofthe arch barrel. This was most noticeable on the arch over the minor road. There wasalso a problem of significant water penetration through the arches, due to the failure ofthe original drainage outlets.

FFiigguurree AA11..1122 SSeevveerree ddiissppllaacceemmeenntt ooff mmaassoonnrryy ffrroomm aarrcchh ssppaannddrreell ccaauusseedd bbyy rroooott ggrroowwtthh

The assessment report recommended that major remedial works should beundertaken, in order to preserve the structure and ensure it was able to safely carry theintended locomotives. Initial estimates put the cost of the works at around £100 000(2002 prices). The feasibility of reinstating the original cantilever platform andrestoring the station to its original operating capacity was also considered.

DDeessiiggnn

The work to the viaduct fell into two distinct areas; firstly there was the refurbishmentand reinstatement of the viaduct structure itself; secondly the reinstatement of theoriginal cantilever platform (which is not discussed here).

The design of the remedial works to the viaduct was further sub-divided into two areas.The first was the stitching of the cracking, which had developed in the arch barrel andthe spandrels using proprietary anchors with cementitious grout. This aspect of theworks also included repairs to the brickwork disrupted by root growth. The extent ofthe root growth was far more significant than originally envisaged from the preliminaryinspection, and necessitated significant cutting out and replacement of the brickwork.

CIRIA C656224

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FFiigguurree AA11..1133 RReeppaaiirrss ttoo ddiissppllaacceedd bbrriicckkwwoorrkk aatt aarrcchh ssppaannddrreellss

The original drainage to the structure was provided by a bitumen coating to thebrickwork of the arches, shaped to a drainage outlet at the springing (Figure A1.14).These had become blocked over the years. On previous refurbishment schemes ofviaducts on other preserved lines, the consultant had utilised concrete haunching to thearches, shaped to a drainage point through the spandrel to external drainage points.Such a system facilitates ease of future maintenance. However, due to the listed natureof the viaduct, this proposal was rejected by CADW which required that the originaldrainage points should be reinstated. This necessitated careful coring out and cleaningof the drainage outlets and insertion of new pipe work.

FFiigguurree AA11..1144 FFiillll rreemmoovveedd ttoo eexxppoossee bbiittuummeenn--ccooaatteedd aarrcchh eexxttrraaddooss aanndd oorriiggiinnaall ddrraaiinnaaggee ppooiinnttss aabboovveeppiieerrss

CIRIA C656 225

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FFiigguurree AA11..1155 RReeaapppplliieedd sspprraayyeedd wwaatteerrpprrooooffiinngg ssyysstteemm ddrraaiinniinngg tthhrroouugghh ggrraannuullaarr mmaatteerriiaall ttoo rree--ccoorreedd aannddrree--lliinneedd ddrraaiinnaaggee ppooiinnttss tthhrroouugghh ppiieerrss

Perhaps the most onerous aspect of the planning and design of the works, was theliaison between the various statutory bodies to obtain regulatory approval for theworks. Not only was the local authority involved in obtaining listed building consentand the Railway Inspectorate to approve the design of the new platform, consultationwas also required with the highway authority to obtain a temporary road closure of theminor road, to erect scaffolding and to allow access to carry out repairs to the archbarrel and install a section of platform over the carriageway.

The final body requiring consultation was the Environment Agency as the works wereimmediately adjacent to the River Dee and therefore a licence had to be obtained forthe scaffolding works, as they encroached within the flood plain of the river. In theend, in order to obtain a satisfactory foundation for the scaffolding, the contractorintroduced the construction of a concrete apron to the piers (Figure A1.16). This wasconstructed in such a way as to replicate the stonework of the viaduct, so that theEnvironment Agency agreed to the temporary works being left in place as additionalscour protection to the piers. This also saved the contractor considerable sums inbreaking out the concrete work and disposal of the surplus material.

FFiigguurree AA11..1166 EErreeccttiioonn ooff ssccaaffffoolldd ffoorr aacccceessss ttoo uuppppeerr ppaarrttss ooff tthhee bbrriiddggee

CIRIA C656226

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TThhee ccoonnssttrruuccttiioonn pphhaassee

The consultant and contractor faced a number of challenges in the construction phaseof the works:

� one of the biggest problems for working on any railway, whether the national railsystem or one of the many heritage lines, is the difficulty in obtaining anappropriate window of opportunity to complete the works, while causing theminimum disruption to the line. For the preserved line, the difficulties of notoperating can be more problematic than for the National Rail Network, as anyrestriction of the operation of the line can have a significant impact on the incomeof the railway. It is for this reason that work on preserved lines is often carried outin the most inhospitable winter months

� when producing contract documents, it is always difficult to accurately assess thelength of time required for works. On this contract to suit the operational needs ofthe railway, the contract was split into two distinct phases. In the first, the railwaywould be completely shut and the contractor would have free access to the viaduct.In the second stage, the tracks would be relayed over the structure, and furtherwork would need to be carried out under lookout protection, with a 10 mph speedlimit imposed across the structure

� the most difficult aspect from the contractors’ perspective was to programme theworks around the various constraints imposed by the different statutory bodies.Not only were there time constraints imposed by the railway due to theiroperational needs, but there were limits on when the scaffolding could affect thehighway, due to the possible effect on the local tourism and limits on when andhow scaffolding could be placed on the river bank, due to the risk of flooding.

� further significant difficulties, were experienced by the contractor in the delivery ofmaterials to the site. There appeared to be good road access from the adjacentmain road, but the poor visibility at the junction with the minor road meant that,other than when the minor road was subject to closure, the local highway authoritywould not permit any deliveries to take place. This meant many deliveries had tobe made to the railway sidings immediately to the west of the structure andtransported back to the site by dumper.

Overall the reinstatement of the platform and the refurbishment of the viaduct wascompleted on time and within the original budget, which is a testament to the co-operation of all parties to the works.

CIRIA C656 227

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AA11..77 SSttrreennggtthheenniinngg aanndd rreeffuurrbbiisshhmmeenntt ooff HHuunnggeerrffoorrdd CCaannaallBBrriiddggee

IInnttrroodduuccttiioonn

Hungerford Canal Bridge is a Grade II listed, brick arch structure built in the late 18thcentury (Figure A1.17). The bridge is situated in the centre of Hungerford and carriesthe A338 over the Kennet and Avon Canal. The structure is a 7 m single span brickarch supported on brickwork abutments with brick spandrels and wing walls. Little isknown about the abutment and wing wall foundations. The arch ring is 330 mm thickand is elliptical in shape. The average depth of fill at the crown is 460 mm. The rise ofthe arch at the crown is 3 m. Site investigations established that there is no backing tothe arch although the wing walls thicken at the ends of the arch ring.

FFiigguurree AA11..1177 GGeenneerraall aappppeeaarraannccee aafftteerr ssttrreennggtthheenniinngg wwoorrkkss

AAsssseessssmmeenntt

An assessment of the structure undertaken by the consultant determined the arch tohave a 3 tonne capacity. West Berkshire Council entered into an agreement with BritishWaterways, who owned the bridge, whereby it would be strengthened at the Council’sexpense and the ownership of the bridge would then be transferred to the Council withan agreed commuted sum. The consultant was then commissioned by West BerkshireCouncil to undertake a feasibility study to investigate and compare the variousstrengthening options available.

CIRIA C656228

Client: West Berkshire District Council

Bridge owner: British Waterways

Consulting engineers: Babtie Group

Main contractor: Bersche-Rolt

Winner in the 2004 Institution of Civil Engineers Historic Bridge and Infrastructure Awards

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OOppttiioonnss ccoonnssiiddeerreedd

The feasibility study considered the following three options:

� provision of concrete backing to the arch

� strengthening of the arch using anchors

� retro-reinforcement of the arch ring.

The options were evaluated on the basis of cost, safety and the minimisation ofinconvenience to highway users, canal users and nearby residents. Consideration wasalso given to the presence of existing electricity, gas, water and telecommunicationsservices in the bridge.

� Option 1: Provision of concrete backing to the arch. Under this option the fillmaterial at the back of the arch would have been removed down to the archspringing and replaced with lean mix concrete up to 2.2 m above springing level.The concrete would then require seven days to cure. The top of the arch would bebackfilled with class 6N material to 100 mm of the wearing course. The location ofservices in the footways and the carriageway would need to be established. Thework would have involved excavation and concreting while providing temporarysupport to the services. Consideration was given to using this option andundertaking the works in two phases, enabling the road to be kept open usingtemporary traffic signals. However this would have caused significant delays tohighway users over a long period. The bridge is very narrow and there were alsoconcerns about safety in keeping the bridge open to traffic and pedestrians duringthe works. With no convenient alternative route complete closure of the road wasconsidered to be an unsatisfactory solution.

� Option 2: Strengthening of the arch using anchors. This method involvesproprietary masonry stitching consisting of stainless steel ribbed bars, pressuregrouted in predrilled holes in the structure. Each bar is enclosed in a mesh fabricsock, which inflates under grout pressure to fill completely the masonry hole, andallow the grout to pass through the expanded sock to form a chemical andmechanical bond with the structural material. The stitching work is carried outfrom road level. Using this method the works could have been undertaken at nightunder road closures and temporary traffic signals, reducing inconvenience tohighway users. It was also considered a safer option for both site operatives andthe general public. It would have caused little if any inconvenience to canal users.However the existence of residential properties within 5 m of the bridge gavecause for concern regarding the noise levels imposed on residents as a result ofnight working. There were also concerns that the large amount of services in thebridge would hinder the works and the need for trial holes in advance of the workswould cause traffic delays.

� Option 3: Retro-reinforcement of the arch ring. This method requires thegrouting of stainless steel reinforcement into the arch ring. Small diameter bars(up to 16 mm) are placed in 50 mm diameter holes drilled laterally andcircumferentially 50 mm from the intrados. The reinforcement is placed at closecentres over the whole face of the arch and grouted in using a proprietary grout.The holes drilled laterally are straight and therefore relatively easy to drill. Thecircumferential holes follow the curve of the arch and are therefore much moredifficult to drill. The circumferential drilling is achieved by removing several bricksat the springings and crown of the arch to insert the drill. The drill is thencarefully guided on its radial path, just below the surface of the brickwork, aroundthe circumference of the arch. Reinforcing bars are then fed through the holes and

CIRIA C656 229

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grouted into place. The bricks at the springing and crown are replaced leaving novisible traces of the strengthening works.

AAddoopptteedd ooppttiioonn

The estimated costs for the three options were very similar with less than 10 per centvariance between the highest and lowest estimates. Cost was not considered whenevaluating the options and consideration was given to safety and the minimisation ofinconvenience to the public.

Option 3, arch ring retro-reinforcement, was selected as the preferred option.

TThhee ddeessiiggnn

Retro-reinforcement is based on the use of stainless steel reinforcing bars insertedbelow the surface of the intrados bonded in place with thixotropic cementitious grouts.

Masonry arch bridges mainly constructed during this period (1750–1920) can bestrengthened by inserting a comprehensive mesh of reinforcement behind the surfaceof the intrados, to strengthen and support the masonry while allowing the bridge tocontinue to flex and move as normal in response to imposed loads. Stainless steelreinforcement is used because of its excellent strength and corrosion resistance, bondedin place with thixotropic cementitious shrink resistant grouts. This technique hasrequired the development of special drills and reinforcement installation techniques.

Before the bridge engineer can decide on the extent of the reinforcement and otherremedial works, it is important to carry out a proper survey to establish the form ofconstruction, including the thickness of the arch ring, the height of the haunching andto identify any weakened and damaged areas.

Transverse reinforcement is designed to strengthen arches with circumferentialcracking and spandrel wall separation. Reinforcement 18 mm in diameter is installed in50 mm diameter holes which pass through the arch ring at a depth of about 75 mmfrom the surface.

The depth of the hole will vary as the brick surface is often uneven and it is also normalto find voids in the arch barrel due to leaching by surface water. The reinforcing barshave fixed spacers which locate the bar in the centre of the hole allowing the grout tocompletely surround the bar and pumping of the grout continue until all voids havebeen completely filled.

Circumferential holes are drilled below the surface of the intrados by removing bricksfrom the crown and springing. Diamond-tipped drills mounted on flexible drive shaftsare guided around the arch and the path of the drill head is continually adjusted toensure that the depth and path of the hole does not go beyond the permissible limits.

Reinforcement is inserted with laps at joins to form a continuous length ofreinforcement which passes around the arch and into the springings. In this way eachhinge is reinforced regardless of the eventual position and tests show that increases of50 per cent in the load bearing capacity are possible.

As radial reinforcement, stainless steel dowels are grouted into holes drilled throughthe arch rings, to allow the prevent ring separation.

CIRIA C656230

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EExxeeccuuttiioonn ooff tthhee wwoorrkkss

Using the retro-reinforcement method allowed the works to be undertaken frombeneath the bridge eliminating delays or inconvenience to highway users. It was agreedwith British Waterways that the work would be undertaken from a floating platformthat would be moved out of the way periodically to allow the passage of boats throughthe works ensuring that delays to canal users were kept to an acceptable level. Usingthe retro-reinforcement method also eliminated the need for night working and theresulting disturbance to nearby residents.

West Berkshire Council entered into an NEC short contract with the main contractor,who specialise in the design and construction of reinforced masonry systems and whodeveloped the unique system to achieve the circumferential drilling required for thismethod of reinforcement.

The strengthening works commenced on 24 February 2003. For the first four weeks ofthe contract British Waterways were able to offer a complete canal closure on the backof other works being undertaken on this reach of the canal. After four weeks BritishWaterways required that the canal be re-opened and the remaining work wasundertaken from the floating platform. The canal was opened at set and pre-advertisedtimes during the day. The contractor also agreed to open the canal immediately forcommercial craft and when special circumstances arose.

FFiigguurree AA11..1188 DDrriilllliinngg cciirrccuummffeerreennttiiaall hhoolleess ffoorr rreeiinnffoorrcceemmeenntt iinn aarrcchh iinnttrraaddooss

FFiigguurree AA11..1199 CClloossee--uupp ooff iinnssttaalllleedd cciirrccuummffeerreennttiiaall rreeiinnffoorrcceemmeenntt

CIRIA C656 231

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SSuummmmaarryy

The main objective of the council was to execute the works with minimum delays anddisruption to bridge users and local residents. The working methods adopted weresuccessful in achieving this and a good working relationship developed between thecontractor and local residents and businesses. With the goodwill of these parties WestBerkshire Council took the opportunity to extend the contract with the contractor toinclude much needed repair and reinforcement of the retaining walls on the bridgeapproaches. These walls are extremely close to shop frontages and residentialentrances. After full consultation with the occupiers and owners of these premises it wasagreed that the works be carried out during weekday early evenings, and the works tothe bridge and approach retaining walls were completed in August 2003.

CIRIA C656232

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AA11..88 AAsssseessssmmeenntt ooff LLllaannhhaarraann BBrriiddggee uussiinngg ddiissccrreettee eelleemmeennttaannaallyyssiiss

TThhee pprroobblleemm

Llanharan Bridge, constructed in the nineteenth century, is a single span sandstonemasonry arch which carries the A437 over the South Wales mainline railway. The archhas a span of 16 m and a rise of approximately 3.5 m with no skew. At some time afterconstruction, two concrete and brick piers were built just below the quarter points onboth sides of the bridge.

FFiigguurree AA11..2200 EElleevvaattiioonn ooff LLllaannhhaarraann BBrriiddggee sshhoowwiinngg bbrriicckk ppiieerrss aaddddeedd aatt qquuaarrtteerr--ppooiinnttss ooff aarrcchh

An inspection of the bridge carried out in February 1999 revealed several significantdefects, including transverse cracking at the crown. It was suspected that smallabutment movements since construction had caused the cracking and had promptedthe installation of the additional piers.

Glamorgan Engineering Consultancy carried out a strength assessment of the bridge,on behalf of Network Rail, using the MEXE Method and calculated a live loadassessment rating of only 17 tonnes. However, this work did not investigate theinfluence of abutment movements nor did it determine the effectiveness of theadditional piers.

TThhee ssoolluuttiioonn

It had been recognised in the first phase assessment that to obtain a betterunderstanding of the bridges structural behaviour and a realistic strength assessment amore advanced and comprehensive method of analysis would be necessary.Consequently, a strength assessment and investigation based on the Finite/DiscreteElement (DE) technique was commissioned.

CIRIA C656 233

Courtesy Gifford and Partners, Glamorgan EngineeringConsultancy, Rhondda Cynon Taff Borough Council and NetworkRail.

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The DE method is suited to the simulation of a non-homogenised continuum such asmasonry and in the case of Llanharan Bridge the ability to predict both displacementand strength could be used to model the structural interaction of the masonry barrel,soil and additional piers. By representing separate parts that can deform and interactwith each other the highly non-linear behaviour of the bridge as live load traversescould be modelled simply at a fundamental level. In addition, it would allow theinfluence of the construction sequence to be investigated including the introduction ofthe additional piers after the main construction and the bridges sensitivity to anysubsequent abutment settlements.

To understand the occurrence of the transverse cracks and represent their influence inthe strength assessment their cause had to be introduced. The most likely cause seemedto be lateral movement of the abutments and so a sensitivity analysis was undertaken onthe original bridge to help quantify the resulting damage. It was found that the damageresulted in a crack at the crown and with a movement of about 50 mm generated asimilar pattern to that observed during the visual inspection. Using this damage historyand eliminating the need for somewhat subjective condition factors, subsequentanalyses were then carried out to investigate the influence of the additional piers and todetermine a live load rating.

Not surprisingly, without the piers the arch was found to exhibit classic arch behaviour.After the introduction of the piers the behaviour was modified; the piers attractedvertical load and tended to brace the barrel with axle loads at the quarter points; therewas a corresponding reduction in passive soil pressure on the opposite half of the span.Interestingly, with axle loads at the crown, overall displacements and crack widths atthe crown were found to be greater than those without the piers.

FFiigguurree AA11..2211 AAnnaallyyssiiss rreessuullttss wwiitthh aaddddiittiioonnaall ppiieerrss ffoorr 4400//4444 ttoonnnnee ttrriippllee aaxxllee aatt tthhee ccrroowwnn

Using the DE technique it was possible to determine the influence of the piers and theearlier settlement induced damage and using a sensitivity approach undertake a morerealistic strength assessment. It was possible to increase the assessment live load ratingof this bridge to 40/44 tonnes. As the rating was influenced by the earlier settlementand subsequent damage it was also recommended that monitoring of the transversecrown crack is continued.

CIRIA C656234

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AA11..99 SSttrreennggtthheenniinngg ooff GGuummlleeyy RRooaadd BBrriiddggee uussiinngg rreettrrooffiitttteeddrreeiinnffoorrcceemmeenntt

TThhee pprroobblleemm

Gumley Road Bridge, is a Grade II listed masonry arch which carries the unclassifiedGumley Road over the Grand Union Canal in Leicestershire. The bridge constructionis typical for masonry canal arches, consisting of a single elliptical span constructedfrom three rings of red brick masonry.

The single span of 6.7 m is square, with a rise at the crown of approximately 2.5 m.The carriageway is 4.2 m wide with a total width between parapets of 4.9 m. Situated inthe middle of a sharp s-bend, the road profile over the bridge is humpbacked with littlefill and surfacing at the crown.

FFiigguurree AA11..2222 GGeenneerraall vviieeww ooff GGuummlleeyy RRooaadd BBrriiddggee

A strength assessment of the bridge carried out in May 1995 using the Modified MEXEMethod determined a live load rating of 17 tonnes. To increase the rating to a required40/44 tonnes the decision to strengthen was taken.

TThhee ssoolluuttiioonn

Traditional saddling, barrel lining and intrados reinforcement were ruled out becauseof the limited depth of fill, the listed status of the bridge and closure time limitations.However, a proprietary internal barrel strengthening system appeared to provide asolution.

Principally, the system involves the retrofit of reinforcement to increase the bendingcapacity of the barrel and numerical simulation based on the finite/discrete element(DE) technique to carry out the design. Using precise diamond drilling, stainless steelbars are installed tangentially to the intrados at predicted hinge locations. Generally

CIRIA C656 235

Courtesy Gifford and Partners, Leicestershire County Council andCintec International Ltd

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installed from the road surface, the bars are bonded to the masonry using grout and afabric sock delivery system (the whole assembly referred to as an anchor).

After modelling the bridge to confirm the strength shortfall and to gain anunderstanding of the mode of failure a design was developed and checked with furthersimulations including strengthening. The final design consisted of 26 no 65 mmdiameter anchors, each approximately 3 m in length and containing one 25 mmdiameter bar.

FFiigguurree AA11..2233 AAnncchhoorr aarrrraannggeemmeenntt aanndd iinnssttaallllaattiioonn

CCoonnssttrruuccttiioonn

Installation of the strengthening was carried out from the road surface. Accuratesetting out and drilling directions are critical aspects of the work, so 3D solid modellingwas used for all setting out calculations. With just three rings of brick in the barrel, theminimum cover to the anchors was designed to be 30 mm, any less would have riskedspalling of the masonry.

After the anchors were marked out on the road surface, the road was scanned forservices and trial trenches made where drilling locations were likely to interfere withservice locations. After minor adjustments, installation commenced, and each hole wasdrilled and anchors grouted in turn. The total duration of the site work was 10 days,averaging 2.6 anchors per day.

CIRIA C656236

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CCoosstt,, aacccceessss aanndd ssaaffeettyy

The cost of the strengthening work was £34 500 (at 2004) including all design,installation and material costs.

Although the technique is particularly suitable for flexible night time closures, on thisoccasion installation was carried out under a full day time road closure as traffic couldmake use of a short diversion route. Cyclist and pedestrian access was maintainedacross the bridge throughout the duration of the works. Canal boat and towpath trafficwere unaffected by the work and special measures were taken to prevent hazards tocanal users and pollution of the watercourse from falling debris.

CIRIA C656 237

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AA11..1100 RReeppaaiirrss ttoo CCaaeerrggwwrrllee PPaacckkhhoorrssee BBrriiddggee

SSuummmmaarryy

This case study describes the repair works to Caergwrle Packhorse Bridge, an historic17th century stone footbridge over the River Alyn, between Hope and Caergwrle,which was badly damaged during the floods of November 2000. The Packhorse Bridge,believed to be one of the oldest bridges over the River Alyn, is one of the best examplesof this type of bridge in North Wales. It is listed as an ancient monument so FlintshireCounty Council had to gain approval from CADW for all aspects of the works.

The repair works comprised rebuilding the damaged stone parapets and spandrelwalls, opening up buried flood relief arches and river profiling works to maximise theflow capacity through the bridge. The remainder of the bridge was repaired and re-pointed.

FFiigguurree AA11..2244 CCaaeerrggwwrrllee PPaacckkhhoorrssee BBrriiddggee aafftteerr rreeppaaiirrss

IInnttrroodduuccttiioonn

The date of construction of Packhorse Bridge is unknown but local historians estimatethat it was built in the late 1600s. The overall length of the bridge is 56 m andcomprises seven arches, three carrying the main river channel, one admitting anancillary branch of the river, two flood relief arches and one that admitted an artificialmillstream, now disused. There are three cut waters on the upstream side and twodownstream. The construction is random masonry.

The bridge provides pedestrian access over the River Alyn between Fellows Lane andDerby Road. The bridge is widely used by parents and children to walk to Hope School

CIRIA C656238

Courtesy Flintshire County Council

Winner in the 2002 Institution of Civil Engineers Historic Bridge and Infrastructure Awards

Client: Flintshire County Council

Designer: High-Point Rendel

Contractor: F G Whitley & Sons

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so works which affected the footpath over the bridge were carried out during theschool summer holidays and the bridge was closed to pedestrians for this period. Thebridge was reopened on the first day of the new school term in September.

Prior to the works, only three of the arches allowed passage of water, the rest beingsilted up, with two of the arches being completely buried. This had been the situationfor many years, even local residents in their eighties could not recall these archesfunctioning.

During the flooding of November 2000 flood waters demolished about 30 m of theparapets and sections of the spandrel walls and walkway.

Initially a temporary walkway was constructed to allow pedestrian access whilepreparations were made for the permanent repairs.

RReeppaaiirr ssttrraatteeggyy

It was necessary to rebuild the demolished sections of the bridge, to bring it back to itsoriginal form. This was done where possible using the original masonry washed fromthe bridge. However, much of the stone was weathered and unsuitable for inclusion inthe reconstruction so local stone was used which was sourced from a building whichhad been demolished. The coping stones to the parapets are a worked bull-nosed typesandstone. About 30 m of coping had been lost or damaged so it was necessary to havethis re-manufactured in sandstone.

Caergwrle Packhorse Bridge was probably built with unskilled or semiskilled labourand the masonry work is crude and random in nature. In the rebuilding of thedemolished sections reference was made to old photographs to try to replicate theoriginal form and style of construction, this included the use of lime mortar in therebuilding and repointing works. Also sections of the bridge which were still standingwere in poor condition and needed to be taken down and rebuilt. Prior to any workstaking place a photographic survey of the bridge was carried out using rectifiedphotography, these records were referenced in the rebuilding to ensure the areas wererebuilt in its original form.

Two arches which have been buried for many years were opened up and another onewhich was partially silted up was cleared. During the excavation works an archaeologistwas on hand to record any finds of historic significance. Nothing of value was foundbut fragments of pottery were unearthed which were dated at about the thirteenthcentury, which would suggest that this location has been used for crossing the riverprior to the construction of this bridge, wooden bridges probably predate the currentmasonry bridge.

Fortunately the arch rings escaped significant damage so only needed local repairs andre-pointing. The foundations of one of the flood relief arches are relatively shallow soto prevent future scour problems the invert was pitched with river cobbles.

The main rebuilding phase was limited to the six weeks of the summer school holidays,so careful planning was required to ensure all the necessary materials and labour wasavailable. There would appear to be a shortage of skilled stone masons that meet theapproval of CADW so it was necessary to arrange all works around the masons’availability. Due to limited vehicular access to the bridge materials were delivered atone end of the bridge by transit pick up and manually taken to where they wereneeded.

CIRIA C656 239

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To minimise the likelihood of future flood damage the scope of the scheme waswidened to improve the flow capacity of the bridge. A hydrological survey showed thatthe capacity of the bridge prior to the works was 41 m³/s which had to be raised to 65 m³/s. This was achieved by exposing three of the four buried and silted arches andriver profiling works.

The total cost of the works was £80 000 (at 2001 prices).

CIRIA C656240

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AA11..1111 RReeccoonnssttrruuccttiioonn ooff bbrriicckk aarrcchh bbrriiddggeess oonn tthhee CChheesstteerrffiieellddCCaannaall

British Waterways owns and maintains around 2400 accommodation bridges. Many ofthese are original masonry or brick arch bridges, which form an important part of theheritage of the canal network. It is British Waterways’ policy to preserve the historicinfrastructure of the canals through preventative maintenance. In the case of bridgeswhich are, or have become, under-capacity or have suffered structural or foundationfailure, strengthening or even reconstruction may over-rule this policy. However, majorworks of this nature will always strive to retain the original features, form and materialwhere practicable.

Osberton Bridge was a failing brick arch that had become unstable and dangerous andthe arch barrel of Manor Farm bridge had started cracking into segments. The boatingenvelope was restricted at both bridges and boat impacts had displaced some bricks.

This case study discusses:

� how features of the existing bridges have been retained and the brick arch bridgesrebuilt using traditional methods and materials to preserve the heritage of theChesterfield Canal

� the design philosophy for the brick arch structures

� how the habitat for a protected species was maintained

� how measures were taken to improve safety features without compromisingtraditional appearances.

HHiissttoorryy

The bridges which are over the Chesterfield Canal opened in 1777, and were originallybuilt to serve the Derbyshire and Nottinghamshire lead, coal and iron industries. Aftera century in operation, the railways developed and boat traffic began to decline; allcommercial traffic ceased in 1955. Under the 1968 Transport act the Chesterfield Canalis a remainder canal therefore British Waterways’ statutory obligation was only tomaintain a safe passage. This meant little money was invested in enhancement orplanned preventative maintenance. However, the increasing popularity of canals forleisure purposes means that money can been made available through European grantsand lottery funds as well as British Waterways money, and it has been possible torestore sections of this canal, which is now navigable.

CCoonnddiittiioonn ooff tthhee oorriiggiinnaall bbrriiddggeess

Osberton Bridge had been failing for a number of years, and was in a very poor state ofrepair with numerous defects and signs of deterioration:

� the arch had flattened and there were distinct bulges in the soffit

� the wing walls had moved and had been repaired several times in the past

CIRIA C656 241

Courtesy Fiona Smith, British Waterways

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� there were signs that the abutment on the towpath side had moved, and beenrepaired a number of times using different sizes and colours of bricks and differenttypes of mortar

� the west voussoir was dressed masonry. The east voussoir was brick headers andmost of the bricks had cracked in half, leaving gaps between the two halves ofbrickwork up to 10 mm wide, a symptom of delamination or ring separation

� there were very few of the original masonry copings to the parapets and pilasters onthe bridge. Repairs had been made in brick and poor quality mass concrete. Most ofthe copings were found on the canal bed following the demolition works, these weretaken to the Chesterfield Canal restoration site which was running concurrently

� there was no fill over the arch extrados for a length of approximately two metres.The bricks on the extrados were uneven, and this was mirrored on the intrados,where punching of the bricks was visible. A number of the bricks had punched outcompletely leaving the remaining bricks unstable.

� only three of the four corner stones on the abutments were remaining. One of theremaining stones had been damaged by boat impacts, and split when it wasremoved during the demolition works. The original bricks in the structure were inpoor condition, and many had spalled and crumbling faces. The original brickscould not be used in the new structure due to their poor condition.

Manor Farm Bridge is an accommodation bridge and is the primary means of access toManor Farm. Manor Farm is a smaller structure than Osberton, with a clear span ofjust over five metres. The towpath under the bridge had been widened in the past, andthis has caused problems for boaters who misjudged the line that should be taken andhave subsequently collided with the bridge elevations.

� there was damage to the brickwork on the elevations and along the arch intradoson the wet abutment side

� the arch barrel had split into three sections across its width. The cracks in thebrickwork on the soffit tied in with the cracks in the concrete surfacing

� the bridge had been strengthened with a reinforced concrete saddle, but therewere no details of the saddle available. During the demolition, two railway railswere found cast in to the concrete. It is thought that the strengthening work hadbeen undertaken by the farmer

� the parapets and pilasters had been damaged by impacts from farm vehicles, andthere were a few of the stone copings missing from the parapets

� although there were obviously structural problems with the bridge, it was theproblem with the poor boating envelope and British Waterways’ duty of care to itscustomers that prompted the replacement of the bridge.

PPrreesseerrvviinngg tthhee hheerriittaaggee

The very poor condition of both bridges meant that substantial reconstruction wasnecessary, but considerable effort was taken to preserve as much as possible of thestructural fabric of each bridge and to retain their heritage features.

The new bridges both have vertical wing walls, pilasters, spandrels and parapets. Thecopings on the parapets and pilasters were stone and there were grooves in the brickson the corners of the abutments on the towpath side, created by towing ropes rubbingagainst them. The parapets on both bridges were very low, and form a series of threestraights in elevation rather than a curved profile.

CIRIA C656242

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As many of the original features as possible were preserved. Heritage style bricks withClass B engineering properties were used with lime mortar in the joints on the archbarrel. Rope grooved bricks were retained and placed in the new abutments. Theoriginal copings were re-used on Manor Farm Bridge. Items, which were not reused inthe bridges, such as the masonry copings that were found on the canal bed at Osbertonand the masonry from the abutments at Manor Farm, were taken to the ChesterfieldCanal restoration site for use there. A holistic approach on the architectural style of theChesterfield Canal was adopted to assure that sympathetic materials and appearanceswere adopted.

The Chesterfield Canal Trust, were consulted and they approved the generalarrangement drawings for the new bridges. There were no problems encountered ingaining planning approval.

TThhee ddeessiiggnn pphhiilloossoopphhyy

The original bridges were over 200 years old and had been carrying heavier trafficthan they would have been designed for. The traffic of the day is invariably regarded tobe the 5 tonne horse drawn cart. Osberton is a towpath turnover bridge and carriespedestrians only, but had been used by farm vehicles in the past.

The design of the new bridges has ensured that excessive tension or compression willnot develop in either the extrados or the intrados for the serviceability limit state. Thelinear strip analysis was conservative, as it didn’t allow for the stiffening effect of thespandrel walls. The arch barrels were assumed to be pinned at the springing points,which is also a conservative view. The arch barrels are the main structural elements.Sheet waterproofing membranes over the extrados on both bridges, debonds thebrickwork from the mass concrete fill. Mass concrete was chosen, as it can be classed asa high-grade durable fill. However, in the analysis, a granular fill was assumed for thepurpose of load distribution in the arch ring. The parapets were designed to resist theimpact forces in current design codes.

Ground investigations and coring to abutments to establish existing foundation levelswas carried out at an early stage. The ground conditions at both sites are good withsandstone at Osberton and very stiff clay at Manor Farm, and hence mass concretespread foundations were used. At Manor Farm, the original timber piles were left intactand concrete was placed around them.

Osberton has a clear span of 6.645 m and Manor Farm has a clear span of 5.02 m. Bothbridges had an original barrel thickness of 1 brick and now have a barrel thickness of1.5 bricks. It was not possible to obtain the required strength with a 1 brick barrelthickness as the permissible stresses due to flexure were exceeded. Both bridges weredesigned to carry full HA loading. The HA Uniformly Distributed Load (UDL) andKnife Edge Load (KEL) was applied over different loaded lengths to ensure that theworse case load effects were considered. Braking loads and temperature effects werealso considered.

CIRIA C656 243

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FFiigguurree AA11..2255 RReeccoonnssttrruuccttiioonn ooff bbrriicckk aarrcchh oonn cceennttrriinngg,, OOssbbeerrttoonn BBrriiddggee

HHaabbiittaatt ffoorr pprrootteecctteedd ssppeecciieess

Before it was replaced Osberton Bridge was in a very poor condition. The arch barrelhad deformed and bricks had started to punch out. The joints had opened up to fulldepth at hinge positions. This provided an ideal habitat for bats. A bat survey wascarried out and Daubentons bats were found in the bridge. The ground investigationwork was delayed until an approved bat exclusion process was carried out. As part ofthe approval process to remove the bats, assurances had to be given to English Naturethat a suitable habitat would be provided in the new structure.

A bat brick was placed on the intrados of the new arch and a bat box was placed on theextrados with an access slot to the intrados. A bat brick and bat box were also placed onManor Farm Bridge despite there being no bats present in the original structure.

IImmpprroovveedd ssaaffeettyy ffeeaattuurreess

One of the main problems with Manor Farm Bridge was poor headroom. As with allarches, the headroom decreases towards the supports. At Manor Farm, the towpathhad been widened in the past and the crown of the bridge was relatively close to theedge of the towpath. Consequently there was a history of boat impacts on the wetabutment side of the bridge.

While the new bridge has the same span as the existing, the arch was reconstructed 400mm further out from the towpath. The canal wall was retained and a small ledge hasbeen formed above water level. This coupled with a modification of the arch profile,has resulted in a much improved boating envelope. Although this has resulted in areduction in the towpath width, the headroom for pedestrians is 1.8 m along thecentreline of the towpath.

One of the features of the brick arch bridges on the Chesterfield Canal are the very lowparapets. At Osberton the extrados was exposed and the parapets were less that 0.5 mhigh. Because the parapets were so low, local children used the parapets to stand on tojump in to the canal. To bring the parapets in line with current standards, the newparapets are over 1.0 m high throughout.

CIRIA C656244

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FFiigguurree AA11..2266 OOssbbeerrttoonn BBrriiddggee aafftteerr rreeccoonnssttrruuccttiioonn,, wwiitthh ccoonnsseerrvveedd ffeeaattuurreess aanndd mmaatteerriiaallss

CCoonncclluussiioonnss

It is possible to design and construct new bridges incorporating traditional methodsand, which are able to carry modern traffic loads. The new bridges look very similar tothe originals, but have been modified to improve strength, provide better boatingenvelopes and improve the containment of the parapets. Both bridges have beenwelcomed and praised by the waterway, the farmer, boaters and the Chesterfield CanalTrust.

Osberton Bridge was replaced in 2002 and won a British Waterways Built Heritageaward. It subsequently achieved a very good award in the CEEQUAL scheme. ManorFarm Bridge was completed in March 2003. The success of these projects has allowedthe heritage of the Chesterfield Canal to be retained.

CIRIA C656 245

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AA11..1122 EEgggglleessttoonnee AAbbbbeeyy BBrriiddggee ssttrreennggtthheenniinngg aanndd rreeppaaiirrss

IInnttrroodduuccttiioonn

The bridge was a former toll bridge, owned by Mortham Estates before it wastransferred to the former North Riding of Yorkshire CC and Durham County Councilin 1958 to be maintained jointly. After local government reorganisation in 1974 ittransferred to Durham County Council.

The bridge is a 24 m single span masonry arch which carries road C146 over the RiverTees Gorge. It has a single width carriageway controlled by traffic signals and thecastellated parapets extend to 75 metres. The bridge is Grade II listed.

In the early 1980s there were very low temperatures especially during the winter of1982, and freezing of waterlogged fill forced the parapet and spandrel walls outwards,causing a gap to appear between the carriageway kerb and walls.

Strategically the bridge carries the local HGV route through the area linking the trunkroad A66 with Barnard Castle which has a 7.5 tonne weight restriction on the countybridge crossing of the River Tees.

FFiigguurree AA11..2277 GGeenneerraall vviieeww ooff EEgggglleessttoonnee AAbbbbeeyy BBrriiddggee

TTeemmppoorraarryy rreeppaaiirrss

The form of temporary repair in 1985 comprised a 26 mm diameter “Dividag” tie-barbeing inserted through a 35 mm inside diameter plastic sleeve. The steel bars tiedtimber walings were placed horizontally at intervals over the height of the wing wallsspandrel wall and buttresses. This was considered to be a temporary repair untilfunding was identified for a permanent repair – 12 years later.

CIRIA C656246

Courtesy Brian Poole, Durham County Council

Winner of ICE Historic Bridge and Infrastructure Awards 1988

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In the early 1990s a concrete slab was cast over the arch and waterproofed. Theimportance of the local route for HGVs between the trunk road A66 and the BarnardCastle area became more focused in the late 1990s and funding was identified to affecta permanent repair to the bridge.

PPeerrmmaanneenntt rreeppaaiirrss

Voids within the arch fill material were suspected and this was confirmed usingspecialist radar equipment.

A scheme was identified to tie the wing walls and spandrel wall together with a stainlesssteel bar and also grout up the voids.

A proprietary anchor system was used, in which stainless steel bars are installed in acementitious grouted sock anchor. The bar was stressed prior to being locked-off andthe end of the cored hole made good. The area of surrounding stonework was alsostrengthened using helical rebar bedded in the mortar joints so that a composite zonewould act around the anchor.

The bridge was fully scaffolded on both faces which provided access when a close-upinspection could be undertaken. Areas were identified for repointing using cement,lime and sand mortar.

Closer inspection of the stone voussoirs revealed heavy weathering in places andrepairs were undertaken by a specialist stone repair company. The repair system usedstainless steel pins dowelled into the stone with mesh reinforcement.

Stone stitching of the arch barrel was also included to tie the voussoirs area to the mainbarrel where circumferential cracking had occurred.

The bridge, and also the temporary timber walings, included bat roosts and permissionwas needed to destroy the roost while also making provision for a new roost. Prior togrouting the arch barrel fill open joints were temporarily sealed at night to try andensure the bats had left the roost and could not return. A series of open perpends inthe stonework were created in the spandrel walls and wing walls to allow bats a clearroute to the river.

Grouting of the arch fill used a proprietary lime-based “heritage grout” which waspumped through the side walls.

The steel bar arrangement in the parapet castellations was modified by inserting twovertical bars to reduce the size of the openings in the intervals of safety. The contractoroffered to replace the whole unit rather than introduce only vertical bars.

The success of the project was in restoring the original elevations of the 19th centurybridge for use by modern day traffic. The works were also carried out without the needfor a temporary road closure.

CIRIA C656 247

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AA22 BBrriiddggee ccoonnddiittiioonn aasssseessssmmeenntt gguuiiddeelliinneess

This appendix includes Table A2.1 Guidance on how to assign condition ratings basedon evidence collected in the course of bridge inspection, investigation and monitoring(see Section 3.9 for further guidance).

CIRIA C656248

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CIRIA C656 249

TTaabbllee AA22..11 GGuuiiddaannccee oonn hhooww ttoo aassssiiggnn ccoonnddiittiioonn rraattiinnggss bbaasseedd oonn eevviiddeennccee ccoolllleecctteedd iinn tthhee ccoouurrssee ooffbbrriiddggee iinnssppeeccttiioonn,, iinnvveessttiiggaattiioonn aanndd mmoonniittoorriinngg

Method Comments Good Moderate Poor

Visual Traditionally, visualinspection has been thefirst level of inspection.Visual signs ofdeterioration have usuallyled to further inspectionand/or repair.Visual inspection has themajor disadvantage of onlyrecording that which canbe seen which may ofcourse be a consequenceof that which cannot beseen.

All the basic dimensions ofthe structure should berecorded.

The type of material fromwhich the structure isconstructed, its generalcondition and any defectsshould be mapped such ascracks, settlements,distortions etc (includingthe location of any slippedvoussoirs or bricks).

Any repairs or previousworks should also berecorded.The presence of watershould be recorded.

IInnssppeeccttiioonn::Full visual inspection of all aspects ofthe bridge and comprehensive reportincluding drawings and photographs.

AAsssseessssmmeenntt ddaattaa oouuttppuutt::Material:hard stone, engineering class bricks,sealed surface, well pointed

Shape:Arch barrel defined shapeWalls, abutments and piers plumb (oras built).

Fabric condition:Units and mortar in good condition.

Cracks:Longitudinal:None present

Transverse:None present

Diagonal:None present

Abutment, pier and wall cracks:None present (less than 1 mmmeasured over a 1m gauge length inany direction)

Settlement:Longitudinal and transverse relativesettlement less than 1 in 100.

Vehicular surface:No significant defects

Wall alignment:No evidence of wall sliding, bulging ortilting

Vegetation:None present

Water:None present and no evidence thatwater has been present and causeddeterioration and/or damage

IInnssppeeccttiioonn::80 per cent coverage visualinspection of all aspects of the bridgeand full report including drawings andphotographs.

AAsssseessssmmeenntt ddaattaa oouuttppuutt::Material:Medium stone, building brick, up to20 per cent not well pointed

Shape:Arch barrel some movement over upto 25 per cent of the arch surface butno areas flatter than a 0.1 m offseton a 2 m straight edge setlongitudinally or 0.05 m offset on a 2 m straight edge set transversely.

Fabric condition:Up to five per cent of the mortarjoints displaying signs of deteriorationie missing or crumblingUp to five per cent of the unitsdisplaying signs of deterioration iespalling, crumbling, fissures.

Cracks:Longitudinal:Outside the middle third of the arch,less than 1/10 of the span in length

Transverse:None present

Diagonal:None present

Abutment, pier and wall cracks:Some present over 20 per cent ormore of the surface such that theyare up to 6 mm measured over a 1 mgauge length in any direction.

Settlement:Longitudinal and transverse relativesettlement less than 1 in 25

Vehicular surface:Some minor differential longitudinalalignment

Wall Alignment:Up to 25 mm over a 2 m gauge lengthaffecting up to 10 per cent of thestructural element.It should be recognised that a localfailure of a wall can result in a globalfailure of the bridge.

Vegetation:Some present in embankmentsadjacent to walls etc and greenvegetation present in up to two percent of the wall elements.

Water:Evidence that water has been presentin sufficient quantities to causedeterioration and/or damage to up to25 per cent of the fabric of thestructure including the backfill andsurfacing.

IInnssppeeccttiioonn::Less than 80 per cent coverage visualinspection of a representative sample of someaspects of the bridge and a full reportincluding drawings and photographs.

AAsssseessssmmeenntt ddaattaa oouuttppuutt::Material:Soft stone, weak brick (fk less than 20N/mm²), up to 20 per cent not well pointed.

Shape:Arch barrel has general movement over morethan 25 per cent of the arch surface.Areas identified than are flatter than a 0.1 moffset on a 2 m straight edge set longitudinallyor 0.05 m offset on a 2 m straight edge settransversely.Seriously mis-shaped arch barrels anddistorted walls can be dangerous and requireimmediate investigation.

Fabric condition:More than five per cent of the mortar jointsdisplaying signs of deterioration ie missing orcrumbling.More than five per cent of the units displayingsigns of deterioration ie spalling, crumbling,fissures

Cracks:Longitudinal:Outside the middle third of the arch, more than1/10 of the span in length.Any longitudinal cracking within the middlethird of the arch.

Transverse:Within the middle third of the span –potentially very dangerous and may requireimmediate attention.

Diagonal:Normally start near the sides of the arch at thespringing and spread up towards the centre ofthe bridge. The bridge is probably in adangerous state and requires immediateattention.

Abutment, pier and wall cracks:Present over 20 per cent or more of thesurface and/or greater than 6 mm measuredover a 1 m gauge length in any direction.

Settlement:Longitudinal and transverse relative settlementgreater than 1 in 25

Vehicular surface:Evidence of significant defects like rutting,potholes, track alignment etc

Wall Alignment:More than 25 mm over a 2 m gauge lengthaffecting up to 10 per cent of the structuralelement and/or up to 25 mm over a 2m gaugelength affecting more than 10 per cent of thestructural element.

Vegetation:Some present in embankments adjacent towalls etc and green vegetation present in morethan two per cent of the wall elements and/orwoody vegetation is present in the bridgestructure or close enough for the root systemto affect the mortar joints and foundations.

Water:Evidence that water has been present insufficient quantities to cause deteriorationand/or damage to more than 25 per cent of thefabric of the structure and surfacing.

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CIRIA C656250

Load testing This is usually only undertaken asa last resort or as part of aresearch programme; it is notpermitted by all asset owners.

There are several types of loadingtests:supplementaryprovingproofcollapsedynamic.

Technique:Comprehensive loading system and fullinstrumentation of the bridge.Comprehensive report including details ofthe bridge construction, fabric, backfilland foundations, together with a fullanalysis of the results.

Assessment data output:Good correlation with model – predictedbehaviour. Load capacity proven.

Technique:Specific loading system andinstrumentation. Full reportincluding details of the bridgeconstruction, fabric, backfill andfoundations, together with a fullanalysis of the results.

Assessment data output:Marginal compliance

Technique:Limited loading system andinstrumentation. Full reportincluding details of the bridgeconstruction, fabric, backfill andfoundations, together with a fullanalysis of the results

Assessment data output:Load capacity not proven;Model non-compliant

Coring This is undertaken to determinematerial properties and condition.

It should be remembered that theprincipal axes of stress may notbe those of the direction oftesting. Additionally, cores onlygive local information in what is avery variable material. (Withfriable masonry it may be best toobtain a large 300 mm diametersample).

Endoscopic inspection of the holeprior to reinstatement isrecommended.

Technique:Good representative samples arerecovered; Consistent test results.

Assessment data output:Consistent test results.Unit strength > 60N/mm²90 per cent presence of mortar

Technique:Up to 25 per cent of the sample isunusable but remaining samplesin good condition and givereasonably consistent test results.

Assessment data output:Stone strength >40 N/mm² Brickstrength >25 N/mm²And/or more than 65 per centpresence of mortar

Technique:Up to 50 per cent of the sample isunusable. Results variable (+/- 35per cent).

Assessment data output:Stone strength <40 N/mm²Brick strength <25 N/mm²And/or less than 65 per centpresence of mortar

Hammertapping Surface is tapped –voids ordelamination cause lower pitch(dull) response compared withsolid masonry. Also loose stonesand bricks can be detected.

Delamination deep in thestructure cannot be detected.The method should not be usedwhen the masonry is frozen aswater within the structure may befrozen and hence given a “solid”response.

Technique:100 per cent coverage

Assessment data output:No loose or spalling bricks or stones.

No “dull” areas

Technique:75 per cent coverage

Assessment data output:Up to 10 per cent of the archsurface experiencing spallingand/or brick/stone loss.

Up to 10 per cent of the archsurface recording a “dull”response.

Technique:Less than 75 per cent coverage

Assessment data output:Over 10 per cent of the archsurface experiencing spallingand/or brick/stone loss.

Over 10 per cent of the archsurface recording a “dull”response.

Crackmonitoring

Can be carried out in a number ofways from simple observation,crack width gauges to datalogging systems. Crack widthgauges are prone to being brokenoff. Automated systems arepreferred which also monitortemperature as this affectsdeformation.

Note: that repointing destroyscrack pattern evidence.

Technique:Comprehensive inspection and monitoringinstallation

Assessment data output:No significant cracks are present.Summation of crack widths over a 1metre gauge length (or 1/10 span,whichever is the lesser) in any direction isless than 1 mm.

Technique:Full inspection. Monitoring only alimited number of sites.

Assessment data output:Summation of crack widths over a1 metre gauge length (or 1/10span, whichever is the lesser) inany direction is greater than 1mmbut less than 6 mm.

Technique:Reliance on visual inspection and“tell-tail” installations

Assessment data output:Summation of crack widths over a1 m gauge length (or 1/10 span,whichever is the lesser) in anydirection is greater than 6 mm.

Surveying Survey techniques range fromsimple tape surveys to digitalimaging.

Technique:Full 3D survey of structure including thearch barrel, abutments, piers, spandreland wing walls and running surfaceincluding approach and run-off surfaces

Assessment data output:Geometrical data allows MEXEassessment and all current numericalmethods.

Technique:A 3D survey of main structuralelements at key points only.

Assessment data output:Geometrical data allows MEXEassessment and some alternativenumerical methods.

Technique:Only data relating to a MEXEassessment collected.

Assessment data output:Geometrical data limited to MEXEassessment and crude alternativenumerical methods

Strainmeasurements

There are a number of ways ofmeasuring surface strainincluding:vibrating wire gaugesdemec gaugesLVDT’s

Good resolution is possible but itshould be remembered that thestructure has already experienceda strain history and so anymeasurement only relates to achange in the strain state. Also,masonry and brickwork areheterogeneous and anisotropic.

Technique:Instrumentation coincides with thevulnerable parts of the structure andsufficient gauges are installed to berepresentative of the overall structuralbehaviour.

Assessment data output:Significant help in validating numericalmodels and assessment methods

Technique:Instrumentation captures theresponse of the vulnerable partsof the structure.

Assessment data output:Moderate help in validatingnumerical models and assessmentmethods

Technique:Instrumentation only captures upto 75 per cent of the response ofthe vulnerable parts of thestructure.

Assessment data output:Limited help in validatingnumerical models and assessmentmethods.

Displacementmeasurements

Traditionally displacement hasbeen measured usedpotentiometric, linear variabledisplacement transformers(LVDT’s) and manually read dialgauges. Resolution is good.

Additionally, electro-level, laserand photogrammetric methodshave been used.

Technique:Sufficient gauges are independentlyinstalled to monitor the 3D structuraldisplacement of the bridge in order tocapture the holistic response of thebridge.

Assessment data output:Significant help in validating numericalmodels and assessment methods.

Technique:Sufficient gauges areindependently installed to monitorthe 3D structural displacement ofthe arch and its support.

Assessment data output:Moderate help in validatingnumerical models and assessmentmethods.

Technique:Sufficient gauges areindependently installed to monitorthe arch only.

Assessment data output:Limited help in validatingnumerical models and assessmentmethods.

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CIRIA C656 251

Tiltmeters Used to monitor arch deformation anddifferential support movement.

Delicate to install.

Expensive for a full series of beams.

Technique:Full series of beams installed.

Assessment data output:Significant contribution to thevalidation of numerical modelsand assessment methods.

Technique:Sufficient gauges installed toenable triangulation of results.

Assessment data output:Moderate contribution to thevalidation of numerical modelsand assessment methods.

Technique:Results dependent on isolatedgauges.

Assessment data output:Limited contribution to thevalidation of numerical modelsand assessment methods.

Endoscope/boroscope/fibrescope/video_imagescopes

Used to inspect regions where accesscan be gained through small holeswhich are not passable by aninspector.

In spite of the clarity of the imagesproduced by modern equipment itshould be remembered that they onlyrepresent that part of the structurethat can be viewed. Provides apermanent record.

Can detect delamination.

Technique:Representative sample of thewhole internal structure of thebridge recorded.

Assessment data output:Significant contribution to theassessment method and whereappropriate to the validation ofnumerical models.

Technique:Limited (67 per cent) sample ofthe whole internal structure ofthe bridge recorded.

Assessment data output:Moderate contribution to theassessment method and whereappropriate to the validation ofnumerical models.

Technique:Very limited (less than 33 percent) sample of the wholeinternal structure of the bridgerecorded – possibly only in thevicinity of defects.

Assessment data output:Limited contribution to theassessment method and whereappropriate to the validation ofnumerical models.

Scour detection There are several methods:

Sounding weights and probing –both ofthese methods are time consumingand potentially dangerous ifundertaken during flooding.

Sounding rods – sleeved vertical ornear-vertical metal rods connected tothe bridge structure and resting on theriverbed.

Sliding collars – the collar slides downa vertical support (it can be prone tojamming) and can be located after aflood.

Sonic fathometers use reflectedacoustic waves to detect the water-bedinterface. They give a continuous real-time record but require expertinterpretation.

The above methods are not universallytrusted to give reliable data duringflood conditions.

Technique:A continuous record of thestructure – bed interfaceautomatically monitored atvulnerable sites.

Assessment data output:Assessment can take account ofscour warning

Technique:Regular inspection at vulnerablesites with limited permanentlyinstalled instrumentation.

Assessment data output:Assessment should consider theeffects of scour and reliability ofwarning

Technique:Regular inspection at vulnerablesites with no permanentlyinstalled instrumentation.

Assessment data output:Assessment should take accountof the consequences of scouroccurring during operation.

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CIRIA C656252

AA33 BBrriiddggee aasssseessssmmeenntt mmeetthhooddss

This appendix includes a brief summary of the principal methods commonly used forbridge assessment.

AA33..11 AAsssseessssmmeenntt mmeetthhooddss

SSeemmii--eemmppiirriiccaall mmeetthhooddss:: MMEEXXEE

During the Second World War, the need for a method to rapidly assess the capacity ofthe most common type of bridge ie the masonry arch, resulted in the development ofthe Military Engineering Experimental Establishment method, known as MEXE. Themethod, based on Pippard’s work in the 1930s, was subsequently adapted for civilianuse and adopted by the Ministry of Transport in 1967. With some variations, thismethod remains the most commonly used for the assessment of masonry arch bridgesand the method recommended by most assessment codes. The main assumptions of themethod are summarised below (Pippard, 1948):

� the arch is assumed to be parabolic, with a span to rise ratio of 4 and a cross-section increasing from the crown to the abutment in proportion to the arch slope.From this assumption, the load carrying capacity is determined based on thegeometry of the span of the arch, the ring depth and the crown cover

� the arch is assumed to be pinned at the abutments. This assumption is based onPippard’s observation that a very slight spread of the abutments results in thiscondition

� only the arch is assumed to be structural

� the backfill and the masonry are assumed to have a unit weight of 2.24 t/m³

� a knife edge load is applied at the crown of the arch, as this was considered thecritical load location for the bridge, although not for the arch

� a strip of arch with a width equal to two times the depth of the backfill isconsidered (45° load spread angle)1

� the arch is assumed to behave as a linear elastic continuum

� the maximum permissible load is defined as the load at the crown that combinedwith the bridge self-weight produces a compressive stress at the crown extrados of13 t/ft² (1.4 N/mm²) and a tensile stress limit of 0.7 N/mm. This criterion waspreferred to the middle third rule ie no tensile stresses in the arch, asexperimental evidence suggested the middle third rule was too conservative.

From solving this structural problem, a load defined as the provisional axle loading(PAL) is obtained. Multiplying the PAL by a series of modifying factors, the allowableaxle loading for a double axled bogie with no lift-off is defined. These empirical factors

1 Pippard’s original paper refers to distribution a wedge with an apex angle of 90°, which was used todetermine the effective “rib” dimensions. Pippard was considering crown loading where there isminimum cover and so wheel load dispersal does not overlap at the extrados but will be concentratedon part of the transverse available width. Current guidance on MEXE assessment, for instance HABA16, refers to a distribution of 1H:2V since critical loading is now normally considered to be at thequarter span so that the dispersal of the axle loading is across the full width of the barrel.Consequently, loading per unit width is appropriate.

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are intended to account for the geometrical simplifications of the method andvariations on the condition of the structure.

It is clear that MEXE is a quick and easy-to-use method and a great deal of experienceexists in applying it. This tends to give comfort to bridge owners and managers and isprobably the reason why it is still so commonly used. Although undeniably useful, it isimportant to understand the limitations of this method in order to successfully use it:

� the results of a MEXE analysis are often considered to be conservative, but in certaincircumstances this may not always be the case, for example: small span arches, archeswhere the cover over the crown is greater than the ring thickness, multi-ringbrickwork arches where ring separation is suspected and misshapen arches

� the results of a MEXE assessment are heavily dependent on the experience of theassessor since engineering judgement is required to arrive at a realistic finalcarrying capacity. Great care should be taken not to infer a greater accuracy thanthe method is capable of delivering.

Nowadays, there are several computer packages that can be used in parallel to a MEXEassessment to give confidence where uncertainty might exist.

LLiimmiitt aannaallyyssiiss mmeetthhooddss

BBaassiicc mmeetthhooddss:: HHeeyymmaann’’ss mmooddeerrnn ffoorrmmuullaattiioonn

The hypotheses required for the application of the limit analysis theory to masonryarch bridges are as follows (Heyman, 1996):

� masonry cannot resist tensile stresses

� masonry has infinite compressive strength

� masonry has infinite stiffness

� sliding between voussoirs cannot occur.

These hypotheses force the local failure to occur by formation of a hinge betweenadjacent voussoirs and the global failure to occur by formation of a sufficient number ofthese articulations in order to convert the structure into a mechanism.

Under these conditions, the theorems of the limit analysis are valid and can beformulated for the specific case of masonry arches as:

� the lower bound theorem: if any statically admissible line of thrust can be drawnentirely within the arch, then the arch is safe

� the upper bound theorem: if any kinematically admissible collapse state is found,collapse will occur.

It is worth noting that neither the line of thrust of the lower bound theorem nor thecollapse state of the upper bound theorem need be the actual ones. Using thesetheorems, a lower bound method consists in maximising the load corresponding to allthe statically admissible lines of thrust and an upper bound method consists inminimising the load corresponding to all the kinematically admissible failuremechanisms.

When using this type of analysis, the level of safety could be considered through the

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concept of the “geometrical factor of safety”, defined, for a given load, as the ratio ofthe real arch thickness over the minimum arch thickness required for including a lineof thrust in equilibrium within it.

These types of models have been used in a modern framework since the 1950s(Kooharian, 1952 and Pippard, 1951), although it was not until the 1970s that the basesof their application to masonry arch bridges were established (Heyman, 1996). Sincethen, a number of computerised versions of this approach have been developed andare currently available (Hughes et al, 2002; Crisfield and Packham, 1987; Taylor andMallinder, 1993; Gilbert, 2001; Falconer, 1987). Without describing in detail any ofthese packages, these are some of the main features that have been incorporated toplastic methods for the assessment of masonry arch bridges:

� iterative solution algorithms to easily maximise/minimise lower/upper bound loadsfor a given load location by moving the location of the hinges and for any loadlocation by moving it along the span

� consideration of the lateral restraint of the arch movements by the backfill, byintroducing a horizontal force in the equations, representing a fraction of the fullpassive pressure on the side of the arch remote from the load. In some models,this has been modified to allow for the effect of different backfill layers

� consideration of the effective springing position taking into account the effect ofthe stiffness of the backing material

� consideration of the spread of the load through the backfill by a pre-establishedspread rule (constant angle, Boussinesq etc)

� simplified inclusion of a compressive strength by limiting how close from the edgeof the arch cross-section the line of thrust can get and assuming a certain shape ofthe cross-sectional stress distribution

� a number of models consider one of the effects of mortar loss by thinning the arch(generally at the intrados) by the depth of mortar loss measured on-site

� some of these methods are capable of considering multi-span bridges.

DDiissccrreettee rriiggiidd bblloocckk mmeetthhooddss

Discrete rigid block methods (Livesley, 1978) constitute a significant improvement frombasic limit analyses formulations. The main improvement of discrete rigid blockmethods is that instead of just considering the basic mechanisms, which for a singlespan, would comprise varying the positions of the four hinges, much more complexmechanisms are taken into account. To do so, the structure is divided into a largenumber of discrete rigid blocks connected by zero thickness and zero tensile strengthjoints. All the possible combinations of virtual displacements compatible with thekinematic laws of the system of rigid blocks can be considered. The inclusion of slidingbetween the blocks can be achieved by the introduction of dilatancy at the blockinterface to ensure that the virtual work equations remain compatible with the limitstate theory (Gilbert, 1993). This can have a significant effect upon the calculated loadcarrying capacity of the arch and its mode of failure. For example, ring separation maybe critical.

In addition to these extra capabilities, discrete rigid block methods can incorporate allthe features described for basic limit analysis methods.

It is worth noting that for any limit analysis method to take into account materialfailures such as compression failure or ring separation occurring during loading, an

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iterative process should be followed until convergence is achieved, as follows; the limitanalysis is performed; with the obtained critical virtual displacements, internal stressesare calculated and using those and strength parameters the different failuresconsidered are checked for. If any failure is exceeded, the constraining equations or thegeometry of the problem are accordingly modified and the limit analysis is repeated.However, these methods can easily consider pre-existing conditions such as ringseparation present before live load is applied. In this case the user needs to determineinitially, or after a few trial runs, whether ring separation will or will not take placeduring loading to failure.

Performing these verifications is very important otherwise an equilibrium configuration,allegedly verifying the lower bound theorem, might be impossible, in which case, theresult of the lower bound method would be unsafe. In this respect it should be takeninto account that failures involving geometric instability cannot be considered in limitanalysis introducing a certain level of uncertainty for some type of bridges, althoughadmittedly, the number of structures affected by this will be quite limited.

A method of this type was extended to 3D a few years ago to allow consideration of theeffect of the spandrel walls (Livesley, 1992). However, information on this method is notreadily available and its usage appears confined to the academic community at thepresent time.

IInnddiissccrreettee rriiggiidd bblloocckk mmeetthhooddss

An intermediate step between the basic limit analysis methods and the discrete rigidblock methods is the approach known as the indiscrete rigid block method (Hodgson,1999). This approach takes into account the fact that in a discrete rigid block analysismost rigid blocks do not undergo movements relative to each other and, therefore, theycould be fixed to allow for bigger and geometrically more complex models beinganalysed more efficiently in a limited timescale. In particular, it is claimed that thisapproach would allow capturing the complex failure mechanisms of skewed arches witha limited computational cost. This approach clearly requires a very advancedunderstanding of the behaviour of masonry arch bridges, which might be beyond usefor everyday assessments.

SSoolliidd mmeecchhaanniiccss mmeetthhooddss

The arch is a statically indeterminate problem and the equilibrium equations are notsufficient to solve the redundancy and find the actual line of thrust that transports theloads to the abutments. To do so, the equations of elasticity theory are needed and toconsider that masonry cannot resist tension, non-linear analyses are needed.

Solving the equilibrium equations and obtaining the actual line of thrust implies thatdisplacements and strains as well as forces and stresses are obtained from theseanalyses. This is invaluable when considering the in-service behaviour of bridges and toassess the design of strengthening and repair measures but should be treated withcaution as the initial stress state is not known nor are the effects of the environment.

CCaassttiigglliiaannoo’’ss nnoonn--lliinneeaarr aannaallyysseess

A range of computer programs to analyse masonry arch bridges using Castigliano’snon-linear analyses have long been available as commercial packages as well as researchtools (Roca et al, 1998). These models simulate masonry arch bridges by using variousconfigurations of beam elements.

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The key aspects differentiating these models is the way in which the restraining effectsof the backfill are simulated, which in these approaches can be defined as a function ofthe arch movements, and the way in which thinning of the arch section takes place astensions try to develop.

An additional advantage of this approach is that it can incorporate geometric non-linearity and reproduce snap-through failures.

FFiinniittee eelleemmeenntt

Finite element analyses allow 2D and 3D elements, and complex constitutiverelationships for the materials to be represented. This allows the incorporation of avery high degree of sophistication into the models. Validation of FE models isextremely important and should be built into all assessments. This aspect ofcomputerised assessment cannot be over-emphasised.

Whereas quite a number of limit analysis-based computer programs specificallyprepared for the analysis and assessment of masonry arch bridges are available, nofinite element packages customised for considering masonry arch bridges are known.As a result, although a number of ingenious ways of modelling complex aspects of thebehaviour of masonry arch bridges have been proposed, the lack of specific subroutinesmeans that the preparation of the models can be tedious and time consuming. In thissection, the key methodologies proposed are presented. It should be possible to adoptthese methodologies in most commercially available FE packages or even in-house FEcodes.

� all the modes of failure described in Section 2.4 could be considered

� in particular, using 3D models, the constraining effects of the spandrel walls couldbe considered. This approach requires attention to the interaction between thearch, the spandrel walls and the backfill and in particular to the possibility ofdifferent forms of spandrel wall separation (see Section 2.5.2)

� similarly, 3D models can consider any complex bridge geometry ie skewed multi-span, open spandrels, spandrels with openings, internal spandrels and even hybridstructures

� introducing interfaces with appropriate failure criteria, ring separation that formsduring loading as well as pre-existing ring separation may be considered

� similarly, slippage between the backfill and the arch extrados could be investigated

� deterioration of masonry could be incorporated in the model. However, this wouldrequire tools to translate site observation into model parameters, which arecurrently not available

� the effect of specific defects, such as longitudinal and diagonal cracks could beintroduced in the models

� the effect of non-load-related actions, such as differential settlements may beconsidered.

� the behaviour of masonry and in particular its anisotropy (both in terms ofdeformation and failure) can be incorporated into the model with different levelsof complexity

� the dynamic effects of loading can be simulated. In most cases the dynamic effectswill probably be negligible, but in some light structures, such as open spandrelbridges, this could be relevant

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� the effects of cyclic loading on the response of masonry and the structure could beincluded, although this aspect requires further research.

When analysing highly non-linear problems with large displacements, convergence to asolution with the more conventional implicit solvers can prove difficult. Alternativelyusing explicit solvers can always find a solution however, great care should be taken tocheck the results, as those from explicit analyses are not always valid.

It is particularly important when modelling brittle fracture that a constitutive modelwhich takes account of fracture energy is employed (using simple strain-softeningmodels will lead to results which are highly sensitive to the mesh size). It is unlikely thatreliable fracture energy data will be available for the bridge being assessed, so aparametric/sensitivity study is recommended.

The selection of the constitutive model for masonry can have a significant effect on theresults of the analyses. However, very few commercial programs incorporate bespokemasonry models. If possible, homogenisation-based models that account for theanisotropy of the material should be employed. Otherwise it would be adequate to usea typical “no-tension concrete model” and define the elastic modulus of masonryfollowing a homogenisation approach as described in Section A3.3 Simulation of masonry.If a no-tension concrete model is used, convergence difficulties will occur because zerotensile strength has been used. To avoid this, a low tensile strength should be used – areasonable starting point is to assume that the tensile strength is about 1 per cent of thecompressive strength.

As the material cracks, its shear stiffness diminishes. The reduced shear modulus foropen and closed cracks is usually considered by shear transfer parameters. Fortunately,the overall response is not strongly dependent on the amount of shear retention.

DDiissccrreettee eelleemmeenntt

The discrete element method (DEM) groups together several formulations on which,essentially, the behaviour of structures composed of multiple blocks or particles aremodelled using a discontinuous series of individual blocks.

These formulations can be considered to operate in a manner analogous to that of FE,but with the advantage of allowing severe discontinuities (in tensile and shear) to occurwithin the model, without convergence problems. Additionally, the use of automaticcontact detection greatly simplifies the definition of the contact surface behaviour of themany interfaces present in masonry arch bridges. In addition to these advantages, DEmethods should be able to incorporate all the features described in the FE methods. Ina number of DEM packages, finite elements and discrete elements can be usedsimultaneously, obtaining a very powerful combination. As in the case of FE models, theuse of DE models requires a user experienced with the tool and familiar with thespecific behaviour of masonry arch bridges.

Practical applications of DEM for arch simulation have generally been limited to 2Dsimulations, although 3D models have been considered in academic applications.

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AA33..22 VVeerriiffiiccaattiioonn ooff ssoolliidd mmeecchhaanniiccss mmeetthhooddss

The main difference between limit analysis methods and solid mechanics methods isthat while the former consider the line of thrust at collapse, or a stress distribution in amore general situation, the latter also consider the thrust line prior to collapse. As aresult of this, solid mechanics methods provide not only an estimation of the capacity ofthe structure, but they attempt to predict the in-service behaviour and the contributionof each structural component. This is essential when considering strengthening and/orrepair methods for assessing the origin of an observed deterioration that is consideredto have a structural element and when dealing with highly sensitive/importantstructures.

Consequently these methods rely on additional hypotheses on the deformationalbehaviour of the materials involved, the boundary conditions and other modelassumptions, which are not required in limit analysis methods. The results of theseanalyses may be sensitive to many of these assumptions and the relevant parametersshould be determined as accurately as is necessary.

The sensitivity of each analysis to particular parameters should be established byverification of the analytical approach against full scale tests, sensitivity/parametricstudies and experience of previous analysis of similar structures.

Those parameters to which the analysis is found to be sensitive need to be as accurateas possible and this may require in situ or laboratory testing or monitoring on site.Details of testing and monitoring techniques are presented in Section 3.8. Whereuncertainty as to the value of particular parameters remains, parametric studies should beundertaken to establish the range of the solution within which the actual behaviour lies.

The stress distribution under self-weight has been highlighted by some studies ashaving a potentially significant effect on the prediction by solid mechanics methods ofthe in-service behaviour of masonry arch bridges (Hughes and Pritchard, 1994a). Thestress distribution under self-weight on old structures is virtually impossible to predict,since it is affected by a complex and unknown loading history, foundation movementsand creep. Ideally, this should be obtained from in situ stress measurements. However,while methods of determining the initial stress condition in masonry arch bridges havebeen developed (Hughes and Pritchard, 1994a), they have not yet been widely used. Inmany cases, the low stress levels, the difficulties in obtaining representativemeasurements and the lack of sufficient practical experience may make this taskimpractical. It is recommended that care should be taken to address this issue,particularly if the in-service behaviour is being considered and, when in situ testing isnot possible, sensitivity studies are made to confirm the validity of the analysis results.

Bridge records should be used to investigate possible causes of distress and hence thehistorical pattern of stress within the structure.

Unless properly verified, used carefully and with adequate input data, these analysismethods can give inaccurate results that may be unsafe.

AA33..33 SSiimmuullaattiioonn ooff mmaassoonnrryy

The simulation of masonry in bridge analyses can be a complex subject. In this section,however, only the basic masonry properties will be considered. Details on moreadvanced modelling methods can be found in Sicilia, 2001.

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The following formulae have been proposed to determine the main parameters ofmasonry:

BBSS 55662288--11::11999922

The British Standard estimates the characteristic compressive strength in a tabularform from the kind of blocks and their compressive strength and from the mortardesignation. The modulus of elasticity of brick masonry is approximated by thestandard as a fraction of the compressive strength of masonry, as shown in thefollowing rquation. No alternative for stone masonry is proposed.

EECC 66 ((EENN 11999966--11--11))

The characteristic compressive strength is estimated by the eurocode as shown in thefollowing equation and the modulus of elasticity is approximated as a fraction of thestrength.

where are the uniaxial compressive strengths of unit and mortar and K, áand â are constants.

BBDD 2211//0011 ((HHAA,, 22000011)) aanndd SSBB33//8844 ((SSDDDD,, 11998844))

Curves for estimating the compressive strength of brick and stone masonry as functionsof the type and compressive strength of the masonry unit (Hendry, 1990) are given inthese standards for the assessment of highway bridges.

AAnnaallyyttiiccaall ffoorrmmuullaaee

Brooks and Baker (1998) proposed the following formula to determine the elasticmodulus of masonry.

where C is the number of units per row, H is the total height of the element, Aw, Aband Am are the transversal sections of the masonry, units and head joints, Eb and Emare the modulus of the units and mortar and my and by are the thickness of the bedjoints and of the units.

Hilsdorf (given in Anthoine, 1992) proposed the following formula for the compressivestrength of masonry:

where are the uniaxial tension and compressive strengths of unitsand mortar and are the thickness of units and mortar.

CIRIA C656 259

, . 0.9 masonry masonry

compression charactE f=

( ) ( )masonry u mcompression c cf K f f

α β=

, . 1.0 masonry masonry

secant compression charactE f=

and u mc cf f

( )( )11 y w y

b b m m m

b CA m CE H E A E A HE

+= +

+

;

u m mmasonry u t c m tcompression c u u m

t c u c

f Kf t ff f Kf Kf t f

+= =

+

, , and u u m mt c t cf f f f

and u mt t

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Tassios (1988) proposed:

More details on masonry properties for the assessment of masonry arch bridges can befound in Hendry (1990) and Anthoine (1992).

A the location of hinges, it is considered by Hendry (Hendry, 1992) that the uniaxialcompressive strength of masonry could be increased by 20 per cent to account for theconcentrated load enhancement.

Currently no detailed study has been presented on how the masonry materialproperties are degraded as deterioration takes place. As a result, to take these effectsinto account in the assessment, engineering judgement needs to be used in reducingthese parameters.

CIRIA C656260

( )

( ) ( )1

1 1

umasonry ccompression

mm u u mu

c uu

mtm u u m

u

fft E E

f ttf E Et

ν ν

ν ν

=⎛ ⎞−⎜ ⎟⎜ ⎟+⎜ ⎟− + −⎜ ⎟⎝ ⎠

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AA44 SSppeecciiaalliisstt iinnssppeeccttiioonn aanndd mmoonniittoorriinnggtteecchhnniiqquueess

This appendix includes a brief description of the principal specialist geophysical andother non-destructive techniques for the investigation and monitoring of masonrybridges:

� accelerometers

� impact echo analysis

� acoustic emission

� sonic and ultrasonic methods

� electrical conductivity measurements

� moisture monitoring

� endoscopy

� thermography

� ground penetrating radar

� in situ stress measurement

� over-coring

� optical fibre sensors

� scour detection techniques.

AAcccceelleerroommeetteerrss

Traditionally, accelerometers have not been used extensively in the monitoring ofmasonry arch bridges. There may be a role for such gauges to investigate whether ornot there is a relationship between the accelerations that the bridge experiences andthe rate of deterioration that is observed. This is particularly the case when there is achange in the loading regime, eg increased rail freight traffic.

An accelerometer produces a continuous electrical record of acceleration in a givendirection. It can be used to obtain displacements by double integration of theacceleration signal. This enables displacements to be determined at locations whichwould otherwise be quite difficult to instrument. The accuracy is transducer/structuredependent with problems being reported when the structure’s frequency of oscillationis below 20 Hz.

IImmppaacctt eecchhoo aannaallyyssiiss

The technique was originally developed to test the integrity of concrete piles. Thesurface of the structure is excited with an impulse, eg a hammer blow that sets up aresonant frequency in the structure which is related to the member thickness or depthof defect. The response is measured in terms of surface displacement by a displacementtransducer having a suitable frequency range.

CIRIA C656 261

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The method may be used for determining the thickness and shape of the backfill andidentifying cracks and voids within the masonry. It should give the best results whenthere is a void behind the wall or the wall has lost contact with the backfill. It isimportant to ensure that there is good contact between the material and thetransducers. Petroleum jelly is sometimes used but even this may not work when themasonry has a rough surface. The results of this method are complex and requireexpert interpretation.

AAccoouussttiicc eemmiissssiioonn

As micro-cracking develops in the structure under increased loading, small amounts ofstrain energy are released in the form of elastic stress waves that travel through thematerial at the speed of sound for that material. These waves can be detected bypiezoelectric crystal accelerometers attached to the structure.

There is much attenuation and high levels of background noise make interpretationdifficult. This makes the setting of threshold levels critical so that only acousticemissions above the level are recorded. There have been attempts to correlate the levelof acoustic emissions to material deterioration but to date this has not beensubstantiated.

SSoonniicc aanndd uullttrraassoonniicc mmeetthhooddss

The direct transmission sonic pulse velocity method involves the passing of amechanical stress wave through the material. The transmission wave is initiated byimpacting an instrumented hammer on one side of the structure and receiving it onthe other side using an accelerometer. The frequency of the resulting propagating waveis low, typically around 10Hz. The resulting wave velocity along the path of the brick orstone, mortar joints, soil fill and air voids or other defects is an average and so it is notpossible to establish the position and extent of any potential inhomogeneity. Directtransmission tests between spandrel walls have successfully detected and located voidsin the fill and other internal discontinuities.

The sonic tomography imaging method uses pulse velocity information taken througha section to develop a 3D reconstruction of the velocity distribution in that section. Thisis an improvement on the direct method described previously. The structure is crossedby a dense net of transit paths that allows a detailed map of wave velocities to beproduced. The map can be used to determine the extent and location of flaws. It isimportant that transit paths are uniformly distributed across the section and that themaximum possible area is crossed by the sonic paths. There is a marked reduction inmeasured velocity as the angle between the transmitter and the receiver is increased.This effect is due to the anisotropic properties of the masonry which prevent the wavefrom propagating with a spherical migrating wavefront.

The main benefit of the method is that access is only required from the outside edgesof the arch thus giving information of the condition of the brickwork without having tohave direct access to the barrel. However the complexity of this method means that thecollection, processing and interpretation of the data is a specialist task requiring expertoperatives.

Although this technique has been used on similar structures there is little informationavailable on its accuracy. The technique does appear to have the potential to provideinformation on the integrity of the arch barrel in situations where direct access to thebarrel is not possible.

CIRIA C656262

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EElleeccttrriiccaall ccoonndduuccttiivviittyy mmeeaassuurreemmeennttss

Electrical conductivity can be defined as a measure of the ease with which an electricalcurrent can be made to flow through a substance (this is the reciprocal of electricalresistivity).

The application of this electromagnetic technique for measuring conductivity involvesthe use of a transmitter coil energised with an alternating current and a receiver coillocated a short distance away. The time-varying magnetic field arising from the currentinduces very small currents in the structure. The currents generate a secondarymagnetic field which is sensed, together with primary field, by the receiver coil. Thereis no need for physical contact with the surface of the structure being investigated. Theconductivity equipment permits the measurement of near surface average conductivity.It should be noted that the results are averaged over the depth of penetration.

The electromagnetic conductivity of masonry is a function of the degree of the watersaturation and its electrical properties of the masonry.

Conductivity measurements can be used to assess:

� moisture content in the masonry

� salt content in the masonry associated with moisture content

� height of moisture capillary rise

� multi-wythe nature of the masonry wall

� composite construction of the masonry structure

� presence of voids or inhomogeneities in the wall

� presence of metal reinforcement, pipes, drains etc in the wall.

The method can be used to produce profiles of readings from different areas of thestructure, or to assess changes in the measured parameters over time. It isrecommended that the results are calibrated against a core sample from the structure.

The system comprises a conductivity meter emitting continuously, and receivingelectromagnetic fields through two coils. Typical commercial instruments would beoperating at approximately 15 Hz for a penetration depth range up to 0.75–1.5 m. Alower operating frequency (around 10 Hz) could be used for penetration depths up to6 m.

The procedure involves marking out a grid on the structure and taking readings whichcan be recorded either manually or using a digital data logger. An attempt should bemade to measure the variability in material quality and condition based upon arepresentative sample of the structure – typically an area 3 m square.

The results have to be interpreted in the context of the influence of variousparameters. These include the moisture content by weight (conductivity increasesapproximately as the square of the moisture content), the concentration of salts in thepore water, temperature (2.2 per cent change per degree) and material consolidation.

MMooiissttuurree mmoonniittoorriinngg

As with the above technique establishing the moisture content is difficult given theheterogeneity of the material, especially with brickwork and random rubble where

CIRIA C656 263

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there is a high percentage of mortar. The moisture content of the masonry units isparticularly significant since this is an important influence on their susceptibility to frostdamage. One possible method is to use dry timber plugs inserted into sealed holeswithin the masonry; since timber is a hygroscopic material it will, over the course of afew days, take in water to equilibrate with the moisture content of the surroundingmaterial. The ambient temperature within the hole should be measured and themoisture content of the timber plug can be determined by gravimetric methods usingoven drying or by monitoring the resistance between two probes inserted into thetimber plug. The method is not very accurate at extremes but accuracies of +/- 3 percent over the range 10 to 90 per cent relative humidity (RH) are claimed.

A detailed discussion of available methods of moisture measurement for masonry andother construction materials is included in CIRIA C583 A review of testing for moisture inbuilding elements (Dill, 2000).

EEnnddoossccooppyy

An endoscope is a piece of equipment in which the image is transmitted down a rigidor flexible tube by a series of lenses or optical fibres to a viewing eyepiece.

There are currently three forms of endoscopes: boroscopes; fibroscopes; and video-imagescopes.

� boroscopes have a rigid tube and use optical fibres to illuminate the subject andconventional lenses to transmit the images to the eyepiece. The images may beviewed by the naked eye and recorded using a camera, CCTV or video. Theyrange in length from 0.2 m to 1.5 m and in diameter from 1.2 mm to 16 mm. Theview can be forward-facing, perpendicular to the axis of the tube or in a variabledirection

� fibrescopes have a flexible tube and use optical fibres both to transmit the imageand to illuminate the object to be viewed. The image can be viewed by the nakedeye and recorded using a camera, CCTV or video. They range in length from 0.7m to 6 m and in diameter from 0.66 mm to 13 mm. They may have a two-way orfour-way angulation tip, either forward-facing or with a 90° view. Tips may beinterchangeable

� video-imagescopes are similar to fibrescopes in operation, the principal differencebeing that the image is transmitted using an electronic “chip” camera situated atthe end of the system. The image may be recorded directly on to a video system.They range in length from 1.5 m to 22 m and in diameter from 6 mm to 16 mm.The tip details are as for the fibrescope.

Fibrescopes and video-imagescopes are less robust than borescopes and need carefulhandling. The intensity of the light illuminating the object should be varied andphotographs or video should be taken under various light conditions to ensure clearimages are recorded.

A carefully located small diameter core through the arch in conjunction with remotevisual inspection could give information on delamination/mortar loss/moisture contentand detail of the condition and construction of the materials above the arch as well asgiving accurate arch thickness and samples of material testing.

Additionally, such surveys provide a permanent digital record with only a minimum ofintrusion albeit at a limited number of sites.

CIRIA C656264

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Although very clear images can be obtained, coverage of the structure is limited toimages of the few areas where access can be gained and this should be taken intoconsideration when interpreting results over larger areas of structures, particularly inas variable a material as old masonry. An experienced operator is needed in order togive the maximum benefit from the system.

TThheerrmmooggrraapphhyy

Infrared thermography records variations in the heat radiated from surfaces, whichcorresponds to variations in their surface temperature. It is claimed that this methodcan, in the right circumstances, detect variations in construction and moisturecondition, and identify defects such as voiding and delamination. In practice the resultsare highly dependent on the specific thermal conditions prevailing. However it isrelatively straightforward and inexpensive technique to apply and is in no wayintrusive, and in the right situation it can produce useful information, especially whenused selectively alongside other investigation techniques.

A typical thermographic imaging system comprises a hand-held infrared camera. Thesystem is self-powered and is operated in a similar way to a video camera. Data can berecorded as both stills and video images. Post processing is possible on still images.

GGrroouunndd ppeenneettrraattiinngg rraaddaarr

The principle behind this method is one of applying high frequency electromagneticimpulses to the structure through the use of antennae to probe the subsurfacestructure. Radar (radio detection and ranging) antennae for structural engineeringapplications operate at centre frequencies between 100 MHz and 1000 MHz or higher.Radio waves can travel through slightly conducting dielectric solids and liquids,including common building materials, but not metal. The energy of theelectromagnetic pulse will be partially transmitted and partially reflected at eachchange of interface represented by a change in the dielectric properties of the material.By recording the energy reflected from (or, alternatively, transmitted through) differentinterfaces, a representation of the subsurface may be built up. Since the energyradiated by the antenna is a divergent beam, reflections from any layers or anomaliesmay be recorded even if the antenna is not positioned directly above them. This resultsin a distortion of the radar plot which requires expert knowledge of the technique tointerpret.

There is a balance to be struck between high frequency antennae that give good spatialresolution but shallow penetration and lower frequency antennae that give deeperpenetration but have poorer resolution.

Features and their resolution can be masked by radar reflective materials such asmetals, which can severely limit the effectiveness of the survey. Masking can also affectresults in areas of engineering bricks (“blue bricks”) which have a high ferrous content,where there are highly conductive soot deposits, or electrical services and wiring.Features may not be detectable if their electromagnetic response is similar to that of thesurrounding material, and conductive (wet and particularly saline) materials can causeproblems by severely attenuating electromagnetic waves so that results may be poor insaturated fill materials. Notwithstanding the above difficulties, the technique has thepotential to “see” into the bridge and locate discontinuities like voids and confirminternal structure.

CIRIA C656 265

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Table A4.1 identifies several GPR methods and lists their advantages and disadvantagesalong with their relative costs.

TTaabbllee AA44..11 AApppplliiccaattiioonn ttaabbllee ooff GGPPRR mmeetthhooddss ffrroomm BBAA8866//0044 ((HHAA,, 22000044))

In summary, the benefits of the method include being able to cover large areas rapidly,continuously recording in real time permitting preliminary assessment of the findings.It can penetrate multiple layers and can accurately measure the depth to defects. Itdoes have some limitations due to compromises between resolution and depth ofpenetration, although this can be improved if the apparatus is calibrated. It cannotpenetrate conductive materials like metal, soot, clay-rich or highly saline ground. Thisis a sophisticated technique that produces complex data that requires expert analysisand interpretation.

IInn ssiittuu ssttrreessss mmeeaassuurreemmeenntt

It is important to determine the existing stress state of the structure when undertakingany form of more sophisticated assessment of the carrying capacity. This is not easy asmost techniques and experience relate to homogeneous materials and not to masonryand brickwork.

OOvveerr--ccoorriinngg

Over-coring involves the installation of a strain-measuring device into the structure andrecording the strains. A concentric annulus is then removed (over-coring) to create aquasi-stress-free zone to which the device had already been attached. The strains arethen re-recorded. The difference between the readings gives an indication of theoriginal stress condition.

CIRIA C656266

GGPPRRmmeetthhoodd

CCoonnttrraaccttoorr’’sseeqquuiippmmeenntt ssppeecc

AAddvvaannttaaggeeRReellaattiivvee

aaddvvaannttaaggeeDDiissaaddvvaannttaaggee CCoosstt

1

Analogue radarsystem.

Low cost. Visualinterpretation ofraw data only

Low

2

Digital: singlechannel; mono-static bow tieantenna.

Facility for bothvisualinterpretation andpost-processing.

More flexibleinterpretationthan analoguemethod 1.

Resolution andpenetration intothe structure islimited

Medium

3

Digital: singlechannelbi-static.

Facility for bothvisualinterpretation andpost-processing.

Better resolutionthan monostatic,method 2.

Medium

4

Digital:multi-channel.

Facility for bothvisualinterpretation andpost-processing.

Potential forhigher resolutionthan singlechannel bistatic,method 3

Higher

5

Post-processingsoftware.

Facility for“cleaning-up” data;more effective onbi-static; and evenmore effective onmulti-channel

May give betterinterpretationthan methods 2-4 withoutsignal processing

Requires extraprocessing,usually on adesk-topcomputer;requires moretechnicalexpertise

Extraovercosts

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The method may have some credibility if the original stressed state of a large block ofstone is needed. However, the method is not recommended for brickwork and randomrubble where over-coring will inevitably cut across mortar joints thus exacerbating analready complex heterogeneous situation.

OOppttiiccaall ffiibbrree sseennssoorrss

Optical fibre sensors are based on the principle of the measurement of the change inintensity of light passing down a glass fibre and is reflected back to a sensor such as afibre bragg grating which measures changes in light intensity. If there is microbendingwithin the fibres then there is loss of light intensity, and if this is compared with theoriginal light intensity, it can be translated into a measurement of crack formation,stress changes, crack width variation and temperature measurement. They arelightweight, corrosion resistant and immune from magnetic and electrical interference.However, they are fragile and so great care needs to be given to the planning andinstallation of such devices. The authors are unaware of any instances where opticalfibre sensors have been used on masonry arch bridges, although they have been usedsuccessfully on other types of bridges.

SSccoouurr ddeetteeccttiioonn tteecchhnniiqquueess

Scour is the single most common cause of bridge failure. Its detection, or thedetermination of the susceptibility of occurrence, is quite difficult. Riverbed depths aresounded during principal inspections but the competence of the foundation can easilybe misinterpreted. Ground penetrating radar (GPR) techniques have been developedto determine the depth of scouring following flooding. These techniques requirespecialist expertise and expensive equipment.

Given the increased incidence of flooding in recent times, it is important that the scourrisk assessment of each bridge over water be constantly updated. If it is felt necessary aremote sensing monitoring system should be installed.

Inspection after flooding by experienced divers, using probes, can distinguish betweenthe undisturbed bed and a filled-in scour hole. These post-flood inspections may bedelayed for some time while the river levels subside and flows reduce to safe levels.This may close the bridge until the perceived danger is over.

There is now a range of real-time monitoring equipment available. The cost of suchequipment should be balanced against the cost of physical measures to protect thefoundations.

The CIRIA publication C551 Manual on scour at bridges and other hydraulic structures (Mayet al, 2002) presents a detailed review of the current techniques. However, a briefdescription of the available methods is given below:

SSoouunnddiinngg wweeiigghhttss aanndd pprroobbiinngg

The bed profile may be determined using lead sounding weights on the end of a cable.Clearly, it is difficult and dangerous to undertake this type of survey during the peakflood but at later stages scour holes may have refilled with the consequence that falsebed levels may be recorded.

CIRIA C656 267

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Probing with a rod can be carried out immediately after flooding. This can beundertaken by experienced divers who may be able to detect the interface between thedisturbed and the undisturbed bed.

Both methods are time consuming and only provide a “snap-shot” of the bed profile.

SSoouunnddiinngg rrooddss

Sounding rods are sleeved vertical or near-vertical metal rods connected to a bridgepier or other structure. They rest on the riverbed and can slide down as the scour holedevelops during the flood. There have been some concerns over their operation butthey have been largely overcome by modifications based on laboratory and field studies.However, a healthy scepticism is recommended, as they can be susceptible to:settlement due to vibration (particularly in non-cohesive materials) caused by bridgeloading and/or hydrodynamic vortex shedding on the rod; jamming due to corrosion,damage during flooding and/or bending due to hydraulic loading.

Sounding rods should not be mounted at projections by drilling through theobstruction but should be either inclined to miss the obstruction or mounted onoutstands to miss the obstruction.

SSlliiddiinngg ccoollllaarrss

The sliding collar installation comprises a vertical support rod driven into the riverbedwith a horseshoe-shaped or circular collar around it. The collar rests on the riverbedand slides down the support rod as a scour hole develops. The collar can jam and thesupport rod can experience all the problems of the sounding rod installations. Thedepth of the collar can then be determined using a magnet or metal detector, or ageiger counter if a radioactive collar is used.

SSoonniicc ffaatthhoommeetteerrss

Sonic fathometers use reflected acoustic waves to detect the water-bed interface. Theyconsist of a robust downward-pointing transducer (which acts as an emitter andreceiver) mounted on the structure below the flood level. Both the housing and thetransducer need to be robust and streamlined to avoid damage and hydrodynamiceffects that might affect the readings. The instrumentation should be mounted inregions of stable flow as vortices can create bubbles that can interfere with the reflectedacoustic waves. The transducer heads require regular maintenance to clean offbiological organisms. Their main advantage is that they can give a continuous real-timemonitoring of scour.

BBuurriieedd iinnssttrruummeennttaattiioonn

Hydraulics Research Wallingford has developed a device that can be buried at selecteddepths mounted on the structure in the riverbed in regions that are suspected to bevulnerable to scour. The device comprises a robust but sensitive motion sensormounted on a flexible rubber tail. The lowest of the devices can be located at a depththat is thought to put the structure at risk. If scour occurs and the sensor is exposed,the tail oscillates and sends an electrical signal to a data-logger or warning device.

CIRIA C656268

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Should the lowest device be exposed then the owner can be alerted to the situation andcan take appropriate action. Additionally, as the flood abates the devices may becomereburied and the structure may then be considered safe to re-open.

Of course, as with any device malfunction and/or incorrect positioning can lead to afalse sense of security.

CIRIA C656 269

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AA55 HHeeaalltthh,, ssaaffeettyy aanndd eennvviirroonnmmeennttaalllleeggiissllaattiioonn

This appendix includes details of some of the principal health, safety andenvironmental legislation applicable in the UK that is liable to affect the managementand maintenance of masonry arch bridges. For a more detailed review of constructionhealth and safety legislation, see Tyler and Lamont (2005).

TTaabbllee AA55..11 PPrriinncciippaall hheeaalltthh aanndd ssaaffeettyy lleeggiissllaattiioonn rreelleevvaanntt ttoo iinnffrraassttrruuccttuurree bbrriiddggeess vvaalliidd iinn GGrreeaatt BBrriittaaiinnaatt tthhee yyeeaarr 22000055

TThhiiss lliisstt ooff hheeaalltthh aanndd ssaaffeettyy lleeggiissllaattiioonn iiss nnoott iinntteennddeedd ttoo bbee eexxhhaauussttiivvee aanndd,, wwiitthh tthhee ssppeeeeddaatt wwhhiicchh nneeww lleeggiissllaattiioonn iiss iinnttrroodduucceedd ssoommee ooff tthhee lleeggiissllaattiioonn mmaayy nnoo lloonnggeerr bbee uupp ttoo ddaattee

Note: Additional health and safety legislation will apply, depending on the work activities being carriedout. This may include, for example, Provision and Use of Work Equipment Regulations (1998),Control of Asbestos at Work legislation (including the Control of Asbestos at Work Regulations2002), Confined Spaces Regulations (1997), Control of Substances Hazardous to Health (2002)and the Working at Height Regulations (WAHR) (2005).

Similar legislation is enacted to fulfil these obligations elsewhere in the UK.

CIRIA C656270

LLeeggiissllaattiioonn PPrriinncciippaall rreeqquuiirreemmeennttssDDeessiiggnnaattiinngg//

rreegguullaattoorryy aauutthhoorriittyyEEffffeecctt oonn bbrriiddggee

mmaaiinntteennaannccee

Health and Safetyat Work etc Act1974.

Places a general duty onemployers to ensure “so faras is reasonably practicable”the health and safety of theiremployees throughdevelopment of a safesystem of work.

Health and SafetyExecutive (HSE)

Applies when anywork is takingplace, therefore toall remedial worksand groundinvestigationworks.

Management ofHealth and Safetyat WorkRegulations 1999.

Requires employers toassess risks and put in placepreventive and protectivemeasures.

HSE Applies to allsites, propertyand activities.

Construction(Design andManagement)Regulations 1994 ,amended 2000.

Places duties on all thoseinvolved in planning andcarrying out constructionwork to allow risks to beavoided or reduced. Dutiesare placed on clients,designers and contractors.Requires the appointment oftwo statutory duty holders:the planning supervisor andprincipal contractor.

HSE Applies toremedial worksand to somegroundinvestigationwork, dependingon the duration ofconstruction work.

Construction(Health, Safety andWelfare)Regulations 1996

Set a minimum standard forphysical conditions on aconstruction site, includingprotection from physicalhazards and provision ofwelfare facilities.

HSE Applies to all siteswhereconstruction workis being carriedout.

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TTaabbllee AA55..22 IInntteerrnnaattiioonnaall,, nnaattiioonnaall aanndd llooccaallllyy ddeessiiggnnaatteedd ssiitteess wwiitthh eennvviirroonnmmeennttaall aanndd wwiillddlliiffee pprrootteeccttiioonn((rreepprroodduucceedd ffrroomm CCIIRRIIAA CC558877 WWoorrkkiinngg wwiitthh WWiillddlliiffee NNeewwttoonn eett aall,, 22000044))

This list of site designations and legislation is not intended to be exhaustive and, withthe speed at which new legislation is introduced some of the legislation may no longerbe up to date.

Where legislation is particular to Great Britain, similar legislation is enacted to fulfilthese obligations elsewhere in the UK.

CIRIA C656 271

IInntteerrnnaattiioonnaall ddeessiiggnnaattiioonn SSuuppppoorrtteedd bbyy

RRaammssaarr ssiitteess (Ramsar sites are designated as SPAs and protected as SSSIsin Britain and as ASSIs in Northern Ireland)

Convention on Wetlands of International Importance Especially Waterfowl Habitat1971 (Ramsar Convention), 1972

BBiioosspphheerree rreesseerrvveess (All biosphere reserves receive statutory protection asnational nature reserves)

UNESCO Man & The Biosphere Programme 1970

BBiiooggeenneettiicc rreesseerrvveess (receive statutory protection as SSSIs or NNRs) Berne Convention 1979

WWoorrlldd hheerriittaaggee ssiitteess (include features such as Hadrian’s Wall, Stonehengeand the new Lanark Industrial Landscape)

UNESCO Convention for the Protection of World Cultural & Natural Heritage 1972

European sites

EC Habitats Directive 1992 and;� special areas of conservation (GB Regulations, 1994)� special protection areas (EC Wild Birds Directive, 179)� sites of community importance.

Candidate/potential European sitesEC Habitats Directive 1992 and;� candidate SACs (GB Habitats Regulations, 1994)� potential SPAs (EC Wild Birds Directive, 1979)

European diploma sites Council of Europe

Sites hosting habitats/species of (European) community interest Annexes 1 and 2, Habitats Directive 1992

Site hosting significant species populations under the Bonn Convention Convention on the Conservation of Migratory Sepcies of WIld Animals 1979

Sites hosting significant populations under the Berne Convention Convention on the Conservation of European WIldlife and Natural Habitats 1979

NNaattiioonnaall ddeessiiggnnaattiioonn SSuuppppoorrtteedd bbyy

Sites of special scientific interest (SSSIs)/areas of special scientific interest(ASSIs) (NI)

National Parks & Access to the Countryside Act (NP&AC) 1949, WCA 1981, CRoWAct 2000, Environment (NI) Order 2003

Natural conservation order WCA 1981

Special nature conservation order Habitats Regs 1994

Marine nature reserves WCA 1981, Nature Conservation and Amenity Lands (NCL) (NI) Order 1985

Areas of special protection for birds WCA 1981

Bird sanctuaries Protection of Birds Act 1954

National parks NP & AC Act 1949 (as amended), NCAL (NI) Order 1985

Areas of outstanding natural beauty/national scenic areas (in Scotland) NP & AC Act 1949 (as amended), NCAL (NI) Order 1985

Environmentally sensitive areasAgriculture Act 1986 (as amended)Agriculture (Environment Areas) (NI) Order 1987

Natural heritage areas Natural Heritage (Scotland) Act 1991

Limestone pavement orders Wildlife & Countryside Act 1981

Nature conservation review sites Listed in the Nature Conservation Review (NCR)

Geological conservation review (GCR) sites

Sites hosting Red data book species

Sites hosting species not covered by Berne Convention

Areas of special protection for birds (ASPs) WCA 1981

LLooccaall ddeessiiggnnaattiioonn SSuuppppoorrtteedd bbyy

Local nature reservesNational Parks and Access to the Countryside Act 1949 (as amended), NCAL (NI)Order 1985

Sites of importance for nature conservation (SINCs), sites of natureconservation importance (SSNCIs) county wildlife sites or similar

Usually confirmed by the LPA in conjunction with the local wildlife trust and listedwithin attendant policies in the respective local plan

Regionally important geological sites (RIGs)

Important “inventory” sites (eg ancient semi-natural woodland and grasslandinventories)

Usually kept by the SNCO

Other sites (not described above) with biodiversity action plan (BAP) priorityhabitats/species

Listed in the local BAP

Other natural/semi-natural sites of significant biodiversity importance, notreferred to above (eg sites relevant to local BAP/natural areas objectives)

Possibly listed in the local BAP

Sites not in the above categories, but with some biodoiversity or earthheritage interest

Could be any site (eg a notified hedgerow)

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AA66 RReeppaaiirr aanndd ssttrreennggtthheenniinngg tteecchhnniiqquueess

This appendix includes a discussion of the principal design and implementation issuesassociated with a number of commonly used repair and strengthening techniques formasonry arch bridges.

Table A6.1 lists techniques commonly used for the repair and strengthening of masonryarch bridges which are dealt with in this appendix.

TTaabbllee AA66..11 CCoommmmoonn rreeppaaiirr aanndd ssttrreennggtthheenniinngg tteecchhnniiqquueess ffoorr mmaassoonnrryy aarrcchh bbrriiddggeess

For each technique, a relative indication of typical cost (design plus implementation) is given.

Key to cost ratings:

��� low cost

��� moderate cost

��� high cost

These assume “typical” conditions (moderate size, single-span bridge, no unusual access difficulties,significant costs associated with full closure to traffic) and are likely to vary from structure to structuredependent on site-specific and infrastructure-specific issues.

CIRIA C656272

TTeecchhnniiqquuee AA66 rreeffeerreennccee

Arch distortion remedial works 6.1

Arch grouting 6.2

Backfill replacement or reinforcement 6.3

Concrete saddle strengthening 6.4

Parapet upgrading 6.5

Patch repairs 6.6

Pre-fabricated liners 6.7

Relieving slabs 6.8

Retro-reinforcement 6.9

Spandrel tie-bars/patress plates 6.10

Sprayed concrete lining 6.11

Spandrel strengthening “Stratford method” 6.12

Thickening surfacing 6.13

Through ring stitching 6.14

Underpinning 6.15

Waterproofing and drainage improvements 6.16

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AA66..11 AArrcchh ddiissttoorrttiioonn rreemmeeddiiaall wwoorrkkss

SSuummmmaarryy

DDeessccrriippttiioonn

Distortion, misalignment or tilting of the regular shape of the arch usually indicatesthat movement has occurred within the structure either through subsidence,settlement, overloading or the vertical or lateral movements within the substructure.Distortion can occur without necessarily giving rise to cracking because of the inherentflexibility imparted by lime mortars in joints.

It should be noted that distortion of the arch could have occurred during or immediatelyfollowing construction. In order to determine whether it is stable or progressivemeasurement of distortion should be taken periodically from a fixed datum point.

This repair is applied to arches suffering from distortion and it will improve theintegrity of the arch through provision of an arch lining in the form of a truss, steelribs, prefabricated liners or concrete.

PPuurrppoossee

The benefits in undertaking this work are primarily that any further movement of thearch is arrested and that the arch will be strengthened through designing the lining tocarry the live loads. However, it should be noted that further movement and distressmay not be arrested if the distortion is the result of ongoing settlement or subsidence.

CIRIA C656 273

TTeecchhnniiqquuee ssuummmmaarryyAArrcchh ddiissttoorrttiioonn rreemmeeddiiaall wwoorrkkAims to improve the integrity of the arch through provision of an arch lining inthe form of a truss, steel ribs, prefabricated liners or concrete lining.

CCoossttss ���

SSppeecciiaall ccoonnssiiddeerraattiioonnss

Engineering considerations – see Table 4.4 in Section 4.3.4Temporary works designEffect on gauging/clearance should be assessedHealth and safety issues – see Section 4.1.

HHeerriittaaggee aannddeennvviirroonnmmeenntt

Assess impact on bridge’s heritage value; permissions required for bridgesafforded statutory protection – see Section 3.5.1.If protected species (such as bats) are present they are likely to be disturbedor their habitat damaged – see Section 3.5.2.Certain treatments have the potential to cause environmental pollution,particularly working over or near a watercourse – see Section 3.5.2.

DDuurraabbiilliittyyThe design life will vary depending on the actual repair carried out and if therepair is undertaken as a temporary or permanent measure.

IInnssppeeccttiioonnExamination of the repair should be undertaken at an agreed regular periodwith the asset owner, to ensure its continuing effectiveness.

PPeerrffoorrmmaanncceeEffective implementation will allow the structure to continue to perform safelyfor the designed life of the repair.

RReeffeerreennccee//ffuurrtthheerrgguuiiddaannccee

BD21 (HA, 2001a)BS 5400 Guidance on design of steel, concrete and composite bridgesBS 5628-1:1992.

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DDeessiiggnn ccrriitteerriiaa

� primary causes of the distortion should be investigated and determined,particularly associated with foundation failure. Monitoring should be undertakento confirm if distortion is stable or progressive

� the liners for the arch will normally be designed to be sufficient to carry the liveloads

� design of liners will be in accordance with the asset owner’s requirements as well asBritish and industry standards

� a detailed survey of the structure should be undertaken to allow accurate profilingof the arch to enable the steel lining to be manufactured

� vertical and lateral clearances will normally be affected and should be agreed withthe affected parties at an early stage

� consideration should be given to whether the installation of the proposed lining isundertaken while the bridge is closed or open to traffic.

IImmpplleemmeennttaattiioonn

There are a number of repair methods available in undertaking remedial works todistorted arch structures. Provision of prefabricated and concrete linings to archstructures is discussed in Sections A6.7 and A6.11 (respectively) of this report.

Steel ribs are commonly used for supporting arches suffering from faults caused bysubsidence, overloading or lateral/vertical movement within the abutments/piers. Aspecialist contractor is normally employed to manufacture the ribs to the requiredprofile. The ribs are then installed on site with an appropriate offset from the archbarrel with timber wedges positioned between the ribs and arch barrel.

Concrete foundations are normally necessary at ground level for supporting the steelribs and these would need to be specifically designed for each location.

The installation of the steel ribs can be undertaken without closing the bridge to traffic,but consideration should be given to stopping traffic while providing support to thearch lining using timber wedges. Regular inspection of the steel ribs and supportshould be undertaken to ensure its continuing effectiveness.

Another method for providing support to a distorted arch is for the provision of asupportive truss. A number of these trusses are normally required along the length ofthe arch barrel with timber supports to the arch using wedges. The truss is normally oftimber construction but can be made up of other materials and is supported at aroundthe springing level of the arch using robust guides. The condition of the existingbrickwork should be assessed to ensure its adequacy in carrying the loading. Regularinspections should be undertaken to ensure the continuing effectiveness of the repair.

This type of repair will normally result in infringement of the arched area of thestructure and consequently is only suitable for arches with sufficient headroom wherevertical clearances are not critical. The agreement of other authorities and undertakerswho are affected by the works should be sought at an early stage.

CIRIA C656274

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AA66..22 AArrcchh ggrroouuttiinngg

SSuummmmaarryy

DDeessccrriippttiioonn

Grouting is used to fill voids within the arch barrel, which are often caused by waterpercolating through the structure due normally to failure of the waterproofing system,resulting in washing out of the mortar or erosion of the masonry. Grouting is also usedto repair barrel cracks such as ring separation, but if the cracking is extensive thenpinning of the cracks in conjunction with grouting would normally be required.

Cracks and fractures within the arch barrel face can also be repaired by cross stitchingof cracks and injection of a suitable grout.

PPuurrppoossee

The purpose of the arch grouting is to fill he voids present within the arch barrel &along with the provision of pinning bars, the re-establishment of the mechanicalconnection between the arch barrel rings is achieved.

CIRIA C656 275

TTeecchhnniiqquuee ssuummmmaarryy

AArrcchh ggrroouuttiinnggAims to fill voids present within the arch barrel, consolidate the masonry and,along with the provision of pinning bars, re-establish the mechanicalconnection between the arch barrel rings.

CCoossttss ���

SSppeecciiaall ccoonnssiiddeerraattiioonnss

Engineering considerations – see Table 4.4 in Section 4.3.4Selection of specialist grouting contractor.Selection of grout material compatible with the existing structureHealth and safety issues – see Section 4.1.

HHeerriittaaggee aannddeennvviirroonnmmeenntt

Repair should not be visible and therefore heritage issues are likely to beminimised; pining & stitching bars are terminated within the brickwork & thensealed with mortar to match the existing structure. However, permission willstill be required for work on protected structures – see Section 3.5.1.The filling of cracks and voids within the arch ring may kill any bats present ordestroy their roosts, so their presence (and that of any other protectedspecies) should be ascertained before planning any work – see Section3.5.2.Certain grouts have the potential to cause environmental pollution; careshould be exercised, particularly working over or near a watercourse – seeSection 3.5.2.

DDuurraabbiilliittyy Repairs normally designed to give a minimum design life of 25 years.

IInnssppeeccttiioonn

In accordance with asset stewards requirements and using appropriateinstrumentation undertake visual and tactile inspections including:Tactile examination using inspection hammer to check for hollow soundingbrickwork which may signify further ring separation.Tactile examination after completion of works to confirm adequacy of repair.Consideration can be given to undertaking coring.Where surface evident cracks have been repaired, install tell tales to checkfor post-remediation movement.

PPeerrffoorrmmaanncceeEffective implementation will allow the structure to continue to performsatisfactorily in carrying imposed loading and prolong its serviceable life.

RReeffeerreennccee//ffuurrtthheerrgguuiiddaannccee

Standard brickwork repair detailsSpecifications for brickwork and masonry repairs.

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DDeessiiggnn ccrriitteerriiaa

� a tactile examination of the arch barrel using an inspection hammer should beundertaken to determine the extent of ring separation, signified by a hollowsound. This should also be repeated at the end of the work to confirm that thehollow sound is no longer present and if warranted coring can be undertaken toconfirm extent of ring separation and to prove the effectiveness of the groutpenetration

� proposed grout strength characteristics to be similar to existing mortar; overlystrong and inflexible grouts should not be used in bridges constructed with softlime mortars

� false refusal can occur if grouts are not adequately mobile or set too quickly egwhen overly dry mixes are used, and on the other hand it may be difficult toachieve refusal on structures with extensive voids. Grout vents should be spacedand sited appropriately to allow effectiveness to be assessed as grouting operationproceeds

� grout pressure can cause instability of spandrels and wing-walls or local “blow-out”of brickwork if not properly controlled; this can be achieved by using groutingequipment which allows monitoring of the pressure and volume of injected grout,and setting reasonable limits on both

� grouting work should be undertaken while there is no live loading of the structure.Premature passage of vehicles across the structure should be avoided as this couldlead to cracking of the repair before it has had time to develop its requiredstrength.

IImmpplleemmeennttaattiioonn

There are various types of grouting, but by far the most common technique used ispressure grouting. In this method a matrix of holes is drilled into the structure, priorto injection pipes being inserted. The grid pattern for drilling will depend on thecondition and thickness of the underlying brickwork or masonry. Low vibrationpercussive drilling equipment is normally used and by employing experienced drillingoperators, the extent of any voids should be readily determined. It is important thatwhen grouting of voided areas within the arch due to arch ring separation that thegrouting holes are stopped a minimum of 100 mm from the back of the arch barrel.This is to prevent drilling through the extrados of the arch and damaging thewaterproofing membrane or causing grout contamination of the track ballast of railwaybridges. Site investigation should be undertaken to prevent this problem from arising.Examples of investigation techniques may include trial drilling to determine actual archbarrel depth & monitoring of grout intake.

Once the holes have been drilled and flushed clean the grout is then injected into thearch ring at the lowest point and gradually progressing upwards. At each point thegrouting should continue until no more grout is accepted or for larger voids where thegrout may emerge from a neighbouring hole, which should then be sealed usingtemporary timber bungs. The grout should be injected at a pressure which allows it toflow through and permeate the target area but minimises excessive leakage, and thispressure may need to be varied as grouting proceeds. This operation needs to be closelymonitored with due consideration being given to the condition of the brickwork andprevent the possibility of brickwork “blow out”.

After grouting operations have been completed and while the grout is still “green”holes should be re-drilled and anchor grout injected and then insert the pinning bars

CIRIA C656276

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into the holes. While inserting the bars it may be necessary to place timber bungs toprevent grout loss, which can be removed after initial set has taken place. The piningbars should be recessed into the brickwork and sealed with mortar to match theexisting structure.

Due consideration should be given to the diameter of the proposed pinning bars.These bars should be kept to a minimum size to prevent any local over stiffening of thearch thus possibly altering its structural behaviour. Bars in the order of 8 to 10mm indiameter should be considered with holes of around 16 mm diameter. See photographbelow showing grout hole grid pattern on arch.

FFiigguurree AA66..11 GGrroouutt iinnjjeeccttiioonn ooff aarrcchh bbaarrrreell

For repairing cracks within the intrados of the arch barrel it is common practice tocross-stitch the cracks using stainless steel bars prior to grouting. Before the groutoperation is commenced it is important that each crack is clean of all water, dust anddebris. The crack is then sealed with mortar or a sealing compound and left to gainsufficient strength to withstand grouting pressure. The injection sequence and pressureused must ensure that the crack is completely filled and that no damage occurs to theexisting brickwork of structure.

CIRIA C656 277

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AA66..33 BBaacckkffiillll rreeppllaacceemmeenntt oorr rreeiinnffoorrcceemmeenntt

SSuummmmaarryy

DDeessccrriippttiioonn

Backfill replacement comprises the removal of the existing fill over the arch structureand replacement with a more competent, possibly lighter, material.

Reinforcement of the existing backfill is undertaken using geotextile/geogrid materialsin order improve the properties and action of the fill material.

PPuurrppoossee

Brick and Masonry arch structures rely on the fill over the arch to help distribute liveloads more evenly through the structure. For a variety of reasons including the effect ofwater, or the action of vehicles passing over the bridge, the fill may become unable tofulfil this function.

The purpose of replacement or reinforcement of the fill is to provide a morecompetent fill material over the structure, capable of distributing live loads more evenlyover the arch.

Another possible reason for replacement of the fill is to reduce dead loading on thestructure by replacing the existing fill with lower-density material. This is of particularrelevance when considering shallow arches.

CIRIA C656278

TTeecchhnniiqquuee ssuummmmaarryy

BBaacckkffiillll rreeppllaacceemmeenntt oorr rreeiinnffoorrcceemmeennttAims to provide a more competent fill material over the structure, capable ofdistributing live loads more evenly over the arch, or to reduce dead loadingon the structure by replacing the existing fill with lower density material.

CCoossttss ���

SSppeecciiaall ccoonnssiiddeerraattiioonnss

Engineering considerations – see Table 4.4 in Section 4.3.4Reinforced earth materials supplyRemoval from site and disposal of original fill materialsHealth and safety – see Section 4.1

HHeerriittaaggee aannddeennvviirroonnmmeenntt

Repair should not be visible and therefore heritage issues are likely to beminimised, but permission will still be required for work on protectedstructures – see Section 3.5.1. The original fill material may havearchaeological significanceConsider potential for damage to/disturbance of protected species or theirhabitat and environmental pollution – see Section 3.5.2.

DDuurraabbiilliittyy Design life of original structure unaffected by works to the fill.

IInnssppeeccttiioonn Structure inspection to continue as part of ongoing inspection regime.

PPeerrffoorrmmaanncceeImproved performance and capacity of original structure by introduction ofmore competent fill material should prolong serviceable life.

RReeffeerreennccee//ffuurrtthheerrgguuiiddaannccee

BD37 Loads for highway bridges (HA, 2001b)BS 5400 Steel, concrete and composite bridgesBS 6031:1981 Code of practice for earthworksSpecialist manufacturer’s literature.

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DDeessiiggnn ccrriitteerraa

� site investigation to be carried out in order to confirm fill properties

� site investigation to be carried out in order to identify all buried services over thestructure

� provide geogrid reinforcement to existing fill where suitable

� ensure geogrid terminations along the edges and at the ends of the bridge deckare secure to ensure movement does not occur

� if existing fill is unsuitable then specify replacement with compacted granular fillor concrete

� consider whether use of lightweight fill eg foamed concrete, offers benefits in termsof dead load reduction

� temporary loading conditions of plant on exposed arch to be considered

� temporary loading conditions due to imbalanced fill removal to be considered andany required restrictions identified

� consider application of waterproof membrane over the fill to prevent furtherdeterioration due to water.

IImmpplleemmeennttaattiioonn

All buried services are to be identified, temporarily or permanently diverted, ortemporarily supported in place during excavation. Taking care to avoid damage tocables during excavation.

All excavation of fill is to be carried out in accordance with designer’s requirements inorder to avoid overloading of the structure due to plant loading and/or due toimbalanced fill removal.

Excavation is to be carried out by non-percussive methods as the arch is approached, inorder to avoid possible damage and fracture of the arch itself.

Geogrid reinforcement to be installed in accordance with manufacturer’srecommendations.

Works can be carried out utilising, as a minimum, half closures of the bridge with workbeing undertaken to one half of the bridge before switching over to the opposite side.

CIRIA C656 279

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AA66..44 CCoonnccrreettee ssaaddddllee

SSuummmmaarryy

DDeessccrriippttiioonn

Construction of a concrete saddle requires the excavation of structural fill between thespandrel wall and down to the arch barrel. The void is filled with reinforced concreteformed on the extrados of the arch barrel, to which the spandrel walls and extradosare sometimes stitched using structural ties formed from high tensile reinforcing orstainless steel bars.

The method relies upon the concept of creating a composite structure from theexisting brick arch and the new concrete saddle, thus enhancing stability. However,saddles can be installed simply to act as backing for waterproofing repairs.

This method is practically and financially viable where the existing bridge fill can beexcavated and the concrete saddle placed within a reasonable duration. Bridgereconstruction should be considered if this is not the case. Relieving arches should beconsidered where there is an excessively large depth of fill and headroom orappearance is not important.

CIRIA C656280

TTeecchhnniiqquuee ssuummmmaarryy

CCoonnccrreettee ssaaddddlleeReplacement of existing fill material with a reinforced concrete saddle, towhich the spandrel walls and extrados are sometimes stitched usingstructural ties, aiming to create a composite structure with enhanced stabilityand to facilitate waterproofing repairs.

CCoossttss ���

SSppeecciiaall ccoonnssiiddeerraattiioonnss

Engineering considerations – see Table 4.4 in Section 4.3.4Temporary works design for all stagesRemoval and disposal of original fill materialLightweight concrete supplyRequirements for waterproofingHealth and safety – see Section 4.1.

HHeerriittaaggee aannddeennvviirroonnmmeenntt

Repair should not be visible and therefore heritage issues are likely to beminimised, but permission will still be required for work on protectedstructures – see Section 3.5.1. The original fill material may havearchaeological significance.Consider potential for damage to/disturbance of protected species or theirhabitat and environmental pollution – see Section 3.5.2.

DDuurraabbiilliittyyStructural concrete saddle normally designed to give a minimum design lifeof 120 years. However, the overall life of the structure will be governed by thecurrent level of dilapidation.

IInnssppeeccttiioonnNot applicable as the saddle will be buriedRoutine visual and tactile inspection of the structure in accordance with theasset steward’s requirements.

PPeerrffoorrmmaanncceeEffective implementation and inspection/maintenance will enhance structuralperformance in line with the strengthening or repair design life.

RReeffeerreennccee//ffuurrtthheerrgguuiiddaannccee

BA 16 (HA, 1997)BS 5400-4:1990BS 5628 Code of practice for use of masonry

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PPuurrppoossee

The purpose of concrete saddle strengthening is to increase the load-carrying capacityof the structure.

The advantage of this method is that it not only strengthens the arch but also improvesload distribution and ties together any cracked sections.

This method will negate the need for wholesale demolition and rebuilding of theexisting structure, which would be highly disruptive to end users.

DDeessiiggnn ccrriitteerriiaa

Key considerations and actions at the design phase include;

� general condition of spandrel wall and arch barrel constituent materials anddetermination of as-constructed geometry. This can be established through tactileexamination and by intrusive investigation using a combination of coring and trialpits

� degree of geometric dilapidation of the spandrel wall to assess the overall viabilityof employing concrete saddle strengthening as an alternative to reconstruction

� quantify the available construction depth and plan area for reinforced concreteinfill. For bridges carrying railway lines the top of the concrete infill should notencroach within 300mm of the underside of sleeper to facilitate future trackmaintenance

� to reduce induced shrinkage stresses the saddle should be thoroughly cured andconsideration given to casting segmentally

� design life for structural concrete saddles to be a minimum of 120 years

� design of the reinforced concrete to be carried out in accordance with BS 5400

� lightweight concrete should be considered for non-structural concrete saddles (ieacting as waterproofing backing only) to reduce the additional load placed on theexisting structure

� exposure conditions to be assessed to allow the correct grade and composition ofconcrete mix to be specified

� exposure conditions to be defined to allow the required cover to reinforcement tobe determined

� the designer should assess at an early stage whether or not the existing services canbe elevated or temporarily supported during the works

� the existing structure fill should be assessed for temporary stability duringimplementation to achieve a suitable excavation profile

� excavation works should take into account existing structure revetments, spandrelwalls and wingwalls. The designer should assess the temporary stability of theseelements during implementation

� backfill material should be well graded free-draining fill and provision should beconsidered for membrane waterproofing and drainage measures to mitigate thebuild up of hydrostatic pressures, thus preserving constituent materials of theexisting structure

� the design should take account of the required reinstatement measures to preventexcessive loading of the arch barrel in its temporary state

CIRIA C656 281

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� to ensure safety in construction and following the relevant procedures the designteam should at an early stage identify adequate possessions of the highway orrailway infrastructure required to permit the construction of the works

� design of temporary works including required speed restrictions, barriers andtrench supports or excavation profile required to maintain operational safetythroughout the construction period.

IImmpplleemmeennttaattiioonn

The key components for the successful implementation of this method of strengtheningare as follows:

� before starting work the location of all buried services should be identified andwhere necessary temporary or permanent diversions effected in conjunction withthe relevant utility or asset owner/maintainer

� where services are to remain in situ measures for their temporary/permanentprotection and /or support should be implemented

� the working area for the construction of the spandrel strengthening works shouldbe clearly defined and adjacent traffic and public should be protected from theworks. Intrusive survey works will be carried out at an early stage in the works toconfirm the integrity and cross sectional dimensions of the spandrel wall. Thisinformation will be utilised in planning and undertaking excavation works as wellas providing the design team with the information necessary to confirm the designof the in situ concrete saddle. Core samples taken during the intrusive surveys willbe examined by the design team to confirm the existing condition and structuralintegrity of the spandrel wall and arch barrel

� all required brickwork repairs for the spandrels and/or arch barrel are to becompleted and allowed to set prior to concrete saddle placement.

A benched or battered profile should be formed in the existing fill to reduce local slopefailure during the works. Excavated spoil is likely to be contaminated and will requiredisposal at a local licensed waste establishment. It is likely that the structure backingcannot be completely and accurately surveyed without extensive excavation andtherefore the contractor should prepare suitable methods to break out unseenelements, eg buried wingwalls, if required to accommodate the saddle. The abutmentsmay not follow the assessed dimensions and again the contractor should preparesuitable methods to overcome this during the works

Shuttering to be used during the works should be trial-erected prior to the main worksshould it be time-dependant eg during possession/road closure. Dowels should be fixedinto the arch extrados and spandrel walls to ensure a composite structure is formed.Reinforcement should be placed ensuring that adequate cover will be afforded by theconcrete pour.

Saddling should be approached with care since otherwise it can result in structuralinstability and even partial collapse. Concrete pours should be strictly controlled toensure that hydrostatic pressure generated by the wet concrete does not adverselyaffect the stability of the spandrel wall. Vulnerable structural elements such as wing-walls and spandrels should be adequately supported. The concrete supplier shouldprovide evidence that the mix meets with specification and this should be validated byindependent site and laboratory testing regimes in accordance with the specification.The staging of the concrete pours should be determined by the designer and specifiedin the construction method statement.

CIRIA C656282

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Where only limited construction periods are available the use of cement-rich or rapidhardening mixes should be considered. This will allow productivity to be optimisedwhile having the benefit of concrete achieving a high early strength when the adjacentinfrastructure is brought into use. It will also allow the shutters to be removed at anearly stage.

Guidance on waterproofing and drainage requirements is included within SectionA6.16 of this report.

The trafficked surface should be reinstated as per the required standards andspecification. It should be immediately apparent if the waterproofing layer is effectiveas no water should issue from the arch barrel after strengthening/repair. No inspectionwill be required as the saddle is buried. However, the structure should be maintained atregular periods to ensure structural longevity.

CIRIA C656 283

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AA66..55 PPaarraappeett uuppggrraaddiinngg

SSuummmmaarryy

DDeessccrriippttiioonn

Masonry and brick arch bridge parapets are often found to be incapable of containingerrant vehicles. Additionally, even in the case of an existing masonry parapet whichdoes have adequate containment capacity, the risk of loose masonry being ejectedduring an impact event may be unacceptable. Various options exist for parapetupgrading, including parapet reconstruction and provision of retrofittedreinforcement.

Methods of parapet reconstruction involve pre-cast or in situ reinforced concreteparapet units. In some cases the decision may be taken to dowel into the existingstructure and/or tie parapets together across the bridge deck. However, connecting areinforced parapet to unreinforced masonry below can be problematic, with a severeimpact event then conceivably leading to disproportionate damage to the structure as awhole.

A variation on the plain reinforced concrete solution is provision of a brick/concretesandwich type parapet, comprising outer faces of brickwork with an in situ reinforcedconcrete core. It has been shown that this form of construction can provide themaximum level of containment capacity currently specified, and can also beaesthetically compatible with the existing structure.

CIRIA C656284

TTeecchhnniiqquuee ssuummmmaarryy

PPaarraappeett uuppggrraaddiinnggParapet upgrading methods either increase the containment capacity, reducethe likelihood of loose masonry from being ejected, or both, thereby reducingthe risk and severity of any incidents involving parapet collision.

CCoossttss ���

SSppeecciiaall ccoonnssiiddeerraattiioonnss

Engineering considerations – see Table 4.4 in Section 4.3.4Implementation issues: temporary edge-protection and containment;temporary access; demolition of existing parapetsHealth & safety issues – see Section 4.1.

HHeerriittaaggee aannddeennvviirroonnmmeenntt

Assess impact on bridge’s heritage value; retro-reinforcement likely to bemore acceptable than replacement; permissions required for bridgesafforded statutory protection – see Section 3.5.1.Consider potential for damage to/disturbance of protected species or theirhabitat and environmental pollution - see Section 3.5.2.

DDuurraabbiilliittyyOverall design life limited to that of existing structure.New parapet construction design life up to 120 years.Existing parapet strengthening design life normally 20–25 years.

IInnssppeeccttiioonnAll strengthened parapets inspected as part of on-going structure inspectionregime.

PPeerrffoorrmmaanncceeParapet strengthening introduced to reduce consequences of vehicle collisionwith parapets and comply with statutory requirements for vehiclecontainment where applicable.

RReeffeerreennccee//ffuurrtthheerrgguuiiddaannccee

BS 6779-4:1999CSS Guidance Note The assessment and design of unreinforced masonryvehicle parapets (CSS, 1995)BS EN 1317:1998.

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Existing masonry parapets can also be upgraded by introducing reinforcement into theexisting structure by drilling and grouting in bars. Independent verification of thesesystems is required in order to confirm suitability of these retro-reinforcing methods(see Section A6.9).

PPuurrppoossee

During structural assessment, masonry and brick arch bridge parapets may be found tohave insufficient capacity to prevent vehicle incursion in the event of accidental vehicleimpact. Parapet upgrading methods either increase the containment capacity, reducethe likelihood of loose masonry from being ejected, or both, thereby reducing the riskand severity of any incidents involving parapet collision.

DDeessiiggnn ccrriitteerriiaa

� traditional unreinforced masonry parapets are not mechanically fixed to theunderlying masonry and, as ostensibly sacrificial elements, may have to be rebuiltseveral times during the lifetime of a bridge. Upgrading schemes fall into twotypes: (i) those which maintain the lack of mechanical fixing between the wall andunderlying masonry, thereby not interfering with the main part of the structure,and (ii) those which attempt to make parapets fully integral elements of the bridge,with substantial mechanical fixings to try to mobilise the dead load of the structureto help resist overturning

� parapet upgrading may sometimes result in significant dead load increases on thestructure. Assessment of the structure should be carried out to confirm adequatecapacity is available

� upgrading measures should be designed to take account of both local and globaleffects

� bridge owners may permit design of masonry parapets to BS 6779-4:1999 and CSSGuidance note assessment and design of unreinforced masonry parapets (CSS, 1995) incircumstances where masonry scatter in not an issue and desired level ofcontainment can be achieved (the likelihood of scatter can be reduced by addingsmall amounts of retrofitted reinforcement)

� the design life of the parapet will be limited to that of the existing structure, butstrengthening measures are typically designed to give 20–25 years whilereconstructed parapets can be designed for up to 120 years.

IImmpplleemmeennttaattiioonn

Parapet strengthening and reconstruction works often requires access to both sides ofthe parapet, thereby introducing a temporary works access issue. Care should also betaken to ensure that brickwork is not allowed to fall and strike pedestrians or trafficbeneath the structure. Hence, consideration of a designed temporary crash deck maybe appropriate.

Alternative temporary measures should also be provided throughout the works inorder to in order to protect the workforce and retain vehicles travelling over thestructure.

Any demolition works to the existing parapets should be carried out in a controlled andphased manor such that the stability of the parapet is not affected. Temporary stability ofthe structure as a whole at all stages of the works also needs to be considered.

CIRIA C656 285

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New reinforced concrete parapets can be pre-cast and lifted into place usinglightweight lifting equipment. The pre-cast sections may then be placed on a simplemortar bed or mechanically fixed in place by drilling and grout/resin fixing throughthe parapet base into the existing structure.

The in situ concrete version requires substantial formwork to be fixed in place along theedge of the bridge before placing reinforcement and pouring concrete in a controlledmanner along the length of the structure. Composite brick/concrete parapets areconstructed in a similar way with the inner and outer faces of brickwork acting asformwork for the reinforced concrete core.

Existing parapet upgrading comprises installation of reinforcement into the existingstone or brickwork. One method involves chasing out vertical and horizontal grooves inboth faces of the parapet and grouting bars in place. The two layers of reinforcementmay be tied together with transverse bars, which are drilled and grouted in place fromone face of the parapet. Another method involves installation of diagonal reinforcementthrough pre-drilled holes in the top of the wall. The bars are then fixed in positionusing epoxy resin based grout. In this case bar spacing along the length of the wall ischosen to ensure adequate bar overlap (eg see Figure A1.28). An advantage of the useof diagonal reinforcement is that access is only required to the top of the wall.Additionally, the method has recently been shown (in a full-scale laboratory test) tosignificantly reduce the likelihood of loose masonry from being ejected from a weaklymortared wall.

FFiigguurree AA66..22 LLaayyoouutt ooff ddiiaaggoonnaall rreeiinnffoorrcceemmeenntt iinn aa ppaarraappeett wwiitthh rreettrrooffiitttteedd rreeiinnffoorrcceemmeenntt

All methods are likely to have some visual impact on the structure and henceconsideration should be given to aesthetics throughout the project, particularly forlisted and historical structures.

CIRIA C656286

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AA66..66 PPaattcchh RReeppaaiirr ooff mmaassoonnrryy

SSuummmmaarryy

DDeessccrriippttiioonn

Patch repairs involve the local replacement of defective stonework or brickwork when itis heavily damaged or deteriorated.

PPuurrppoossee

Patch repairs are carried out to reinstate structural integrity and/or to improveappearance. Repair is often required to prevent patches of masonry becoming detachedand creating a falling hazard, and to protect underlying masonry from deterioration andthus prolong its serviceable life. Often these repairs are in themselves sufficient, butsometimes they can serve to delay the need for more extensive repair or reconstruction.

DDeessiiggnn ccrriitteerriiaa

� examination of masonry structures is undertaken in accordance with the assetowner’s requirements will result in the identification of defects, allow assessment ofthe extent of repairs needed and their priority. A structural examination should becarried out before deciding on repair strategy (Yu and Dean, 1997), and if damageis caused by the action of live loads then further investigation will be required

CIRIA C656 287

TTeecchhnniiqquuee ssuummmmaarryy

PPaattcchh rreeppaaiirr ooff mmaassoonnrryyPatch repairs involve the local replacement of defective stonework orbrickwork when it is heavily damaged or deteriorated, aiming to reinstatestructural integrity and/or to improve appearance.

CCoossttss ���

SSppeecciiaall ccoonnssiiddeerraattiioonnss

Engineering considerations – see Table 4.4 in Section 4.3.4Temporary works design.Specialist access requirements.Effect on gauging/clearance.Compatibility of repair materials with existing masonry.Health and safety issues – see Section 4.1.

HHeerriittaaggee aannddeennvviirroonnmmeenntt

Assess impact on bridge’s heritage value; preserve original fabric andappearance where possible; permissions required for bridges affordedstatutory protection – see Section 3.5.1.If protected species (such as bats) are present they are likely to be disturbedor their habitat damaged – particularly where repairs to the arch barrel areinvolved – see Section 3.5.2.The potential for environmental pollution should be considered, particularlyworking over or near a watercourse – see Section 3.5.2.

DDuurraabbiilliittyy Brickwork repair design life normally 20–25 years.

IInnssppeeccttiioonn Repairs inspected as part of structure inspection programme.

PPeerrffoorrmmaanncceeRepairs carried out as maintenance measure to enable structure to continueto perform as originally designed.

RReeffeerreennccee//ffuurrtthheerrgguuiiddaannccee

Standard brickwork repairs detailsSpecification for brickwork and masonry repairsYu and Dean (1997) for compatibility of patch repairs with existing fabricTemple and Kennedy (1989) for engineering properties of old bricks.

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� where patch repairs are to be made to spalled or bulging masonry, the cause ofdeterioration should be properly investigated by coring and examination. In someinstances water may be responsible, in which case further deterioration may beavoided by relieving water pressure, either by simple maintenance of the existingdrainage system or by improving drainage

� if the repair is required to behave in a composite manner with adjacent brickwork,for instance in repairs to arch rings, the repair patch should be keyed into it by theuse of shear connectors (dowels or pins), by “toothing in” of additional headers orby the use of weld mesh or steel ties

� repair materials should be carefully selected to behave sympathetically with theexisting fabric of the bridge. In order to achieve this, it is necessary to removesamples of masonry for examination and laboratory testing

� the strength and elastic properties of the repair should be closely matched to thatof the surrounding masonry

� for brickwork repairs, engineering class B bricks are commonly used in repairsalong with a mortar mix which will match closely the strength and permeabilitycharacteristics of the existing mortar. Although engineering bricks are commonlyused, they are often much stronger than the bricks used in the original structure,which can lead to premature failure of repairs and damage to the structure

� similar problems occur when strong, dense and impermeable cement-basedmortars are used in structures which were built using softer and more flexiblelime-based mortars

� the strength of a masonry patch repair may be reduced by increasing the mortar:brickratio, ie by using greater joint thicknesses in the patch (Yu and Dean, 1997)

� particularly for structures with special heritage or aesthetic value, it is importantthat repair materials are selected to carefully match the appearance of the originalbridge fabric and that the approval of statutory conservation agencies is obtainedwhere necessary (see Section 3.5.1)

� when breaking out and repairing old masonry, care should be taken not to damageor disturb the habitats of protected species, for instance bat roosts in arch soffits(see Section 3.5.2)

� where temporary works are required to enable brickwork replacement to beundertaken then full consideration should be given to the reduction of existingroad, canal or railway bridge clearances.

IImmpplleemmeennttaattiioonn

BBrriicckkwwoorrkk rreeppaaiirrss

Where individual bricks are replaced then temporary formwork is not necessary astimber wedging of the brick/s is sufficient. Similarly relining of vertical faces ofabutments, wingwalls or piers can be undertaken without the need for temporaryworks provided the extent of repair is limited to 1 m square. In arches wherebrickwork is to be renewed (or “recased”) then a programme of work will be necessaryto allow determination of the number of possessions that will be required and otherauthorities or statutory undertakers that may be affected by the works contacted eglocal roads authorities and waterways authorities etc. The removal of the brickworkwithin the arch will require the aid of temporary formwork or centring. This formworkis installed to support the new bricks and is only removed once the mortar has attainedsufficient strength. Such formwork should be suitably designed and have a temporaryworks design certificate.

CIRIA C656288

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It is critically important that when it is intended to install temporary works overoperational railway lines, highway or canal traffic that sufficient clearance to thesetemporary works is maintained at all times. Permission should be sought from theappropriate Authority prior to installation works commencing. Where infringement ofexiting clearances is not permitted or limitations are placed then alternative method ofsupporting the new brickwork may have to be incorporated eg low construction depthtemporary works.

Small patch repairs (up to 1 m² in area) are most effective, larger repairs less so.Defective brickwork is therefore broken out to a maximum of 1 m square areas andonce removed the exposed surface of the next ring or leaf of brickwork should beinspected to determine if further repairs are required. Prior to carrying out any suchfurther, deeper, remedial works it is important that full consideration be given to thestability of the element (particularly in arches) and that appropriate experienced &qualified personnel are involved in the decision making.

New brickwork is normally pinned back into the arch barrel but the proposed pinningmethod should not result in local stiffening of the arch, which may change its structuralbehaviour. Use of stainless steel brick ties is one suitable method of tying back of newbrickwork.

Finally it should be noted that bricks produced today are metric sized, typically 215 × 102.5 × 65 mm. In the past a much wider variation in size of bricks wasmanufactured, particularly on the height. When renewing brickwork considerationshould be given to matching the existing brick size as otherwise the bed joints will haveto be deeper than those in the original brickwork, which may give the impression ofpoor workmanship.

SSttoonneewwoorrkk rreeppaaiirrss

Repairs to stonework require similar general considerations to those discussed abovefor brickwork, but employ slightly different techniques and labour skills, so are likely torequire the use of a specialist contractor with suitably skilled and experienced masons.Local repairs, which involve the replacement of a small number of masonry units ordamaged parts of units only, can be achieved either by replacement with new stone orby “piecing in” to repair the damaged areas only.

Stone is normally cut out with a hammer and chisel to a depth of at least 50 mm, or tothe depth of the whole stone (which is often easier). New stone elements should be cutsimilarly to those they replace. They should be carefully handled and be laid “on bed”(ie with its natural plane of stratification in the horizontal plane) with the outer faceflush with the original outer plane of the stonework ie not flush with adjacentweathered stone. Before placement the cavity should be dampened and the stone slidinto position on a mortar bed with lead or slate packing as necessary to ensure correctpositioning. Mortar joints can be sealed at the surface (often by pointing them) beforethe void behind is filled by low-pressure injection of grout, typically a mix of lime witheither low-sulphate fly-ash or HTI powder. When “piecing in” the damaged areas ofstone are removed using a hammer and chisel to create a regular cavity and a piece ofstone cut to size and inserted in place of the removed material; in this case, jointsshould be kept as thin as possible.

Stonework repairs are typically pinned back to the original fabric of the bridge in asimilar way to brickwork repairs. The use of “plastic repairs” where mortar is used toreplace original stonework, or the use of bricks to replace stone, should be avoided

CIRIA C656 289

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wherever possible, since the results are typically unsightly and repairs may failprematurely or damage adjacent masonry fabric.

FFiigguurree AA66..33 BBrriicckkwwoorrkk ppaattcchh rreeppaaiirr wwoorrkkss bbeeiinngg uunnddeerrttaakkeenn oonn aa ““hhiitt aanndd mmiissss”” bbaassiiss ttoo aarrcchh rriinngg wwiitthh nnoolliivvee llooaaddiinngg

CIRIA C656290

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AA66..77 PPrreeffaabbrriiccaatteedd lliinneerrss

SSuummmmaarryy

DDeessccrriippttiioonn

Prefabricated liners provide a secondary support mechanism within an existingdeformed or deteriorated arch. The system generally comprises corrugated steel linersor precast concrete liners, which are placed beneath the existing arch structure. Anyresultant gaps between the liner and the existing structure are then grouted up toprovide continuous support to the arch. Other types of liners available include GRPpanels and steel section ribs profiled to the intrados of the existing arch or fixed intochased grooves/slots in the existing arch brickwork in order to reduce the effect onclearances.

PPuurrppoossee

The new liners are installed where the existing structure has inadequate structuralcapacity or is exhibiting significant signs of distortion or deterioration. The idealsituation for use of prefabricated liners is where a small reduction in clearance belowthe structure is acceptable and where disruption of access over the bridge is expensiveor impractical. Although the contribution of the existing structure is commonly ignoredin the design of the liner, liners may also be installed as a pure strengthening measuretaking into account the residual strength of the existing.

CIRIA C656 291

TTeecchhnniiqquuee ssuummmmaarryy

PPrreeffaabbrriiccaatteedd lliinneerrssStructural lining (normally corrugated steel or precast concrete liners) areinstalled beneath the existing arch structure to provide a secondary supportmechanism within an existing deformed or deteriorated arch.

CCoossttss ���

SSppeecciiaall ccoonnssiiddeerraattiioonnss

Engineering considerations – see Table 4.4 in Section 4.3.4.Temporary works design.Effect on gauging/clearance.Provision of suitable foundation/support for lining.Health and safety issues – see Section 4.1.

HHeerriittaaggee aannddeennvviirroonnmmeenntt

Assess impact on bridge’s heritage value; permissions required for bridgesafforded statutory protection – see Section 3.5.1. Affects appearance of thearch intrados but careful detailing can minimise the effect on the appearanceof bridge elevations.If protected species are present they are likely to be killed, disturbed or theirhabitat damaged, particularly bats with roosts in the arch intrados – seeSection 3.5.2.The potential to cause environmental pollution should be considered,particularly working over or near a watercourse – see Section 3.5.2.

DDuurraabbiilliittyyOriginal structure made redundant and lining system design for up to 120years design life.

IInnssppeeccttiioonnNo inspection of original structure possible following installation of liningsystem. New lining to be included within structure inspection programme.

PPeerrffoorrmmaanncceeExisting structure assumed to be redundant with liner designed to take fulldead and live loading.

RReeffeerreennccee//ffuurrtthheerrgguuiiddaannccee

Specialist manufacturer dataBD 91 Unreinforced masonry arch bridges (HA, 2004b)BS 5400 Steel concrete and composite bridges.

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DDeessiiggnn ccrriitteerriiaa

� the new liner is designed to take all dead and live loads, based on the assumptionthat the existing arch will eventually deteriorate into fill material above the new liner

� detailed ground investigations are required in order to establish groundconditions, together with the existing structure foundation details

� foundations of the existing structure require checking for the new loadingarrangement, new strip or raft foundations may be required in order to supportthe new lining system

� proposed foundation arrangement may be affected by the presence of existingburied services beneath the structure

� stability of the existing structure in the temporary condition, allowing forexcavation for installation of new foundations, should be considered

� lining of the structure will lead to reduction of clearances. The new, reducedprofile should be checked for clearances to traffic as part of the design process

� provide alternative route for water that is currently penetrating the arch barrel, inthe form of weep pipes

� the durability of liners should be considered in the light of their serviceenvironment – for example whether they might be exposed to water containingaggressive salts

� the appearance of the structure from beneath will be significantly affected, but withcareful detailing the outer elevation of the bridge may remain apparently unaltered

� design should consider that no inspection of the existing structure will be possiblefollowing installation of the lining system. However, this problem is overcome byassuming that the existing structure becomes redundant

� the design life for the prefabricated liner is up to 120 years.

IImmpplleemmeennttaattiioonn

Specialist fabricators manufacture both steel and concrete liners and any lead in timerequired should be allowed for within the scheme programme.

Following excavation for, and casting of, new foundations the prefabricated liningsections can either be erected in situ underneath the existing structure, or may be fixedtogether alongside the existing structure and slid into place.

Where the lining is installed in situ, bolted anchors may be drilled and fixed into theintrados in order to provide temporary support.

For structures affected by water seepage, permanent drainage in the form of weeppipes is to be installed prior to grouting behind the liner.

Careful grouting of the annulus between the new and existing structure is required toensure continuous support. Follow- up grouting may be required in order tocompensate for any initial shrinkage or settlement that may take place.

The main advantage during the implementation process for this type of work is thataccess can be maintained over the structure while all works are carried out below.

Environmental impacts during grouting operations should be considered, particularlyin the vicinity of watercourses.

CIRIA C656292

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AA66..88 RReelliieevviinngg ssllaabbss

SSuummmmaarryy

DDeessccrriippttiioonn

This method of strengthening masonry or brickwork arch structures involves theinstallation of a horizontal reinforced concrete slab over the plan area of the arch afterthe fill has been partially excavated. The slab extends over the abutments and isgenerally thickened at the ends. Relieving slabs are also sometimes constructed withintegral side walls that have the effect of removing horizontal loading from the existingspandrel walls.

The implementation of this method also allows the improvement of the structuresdrainage by the installation of waterproofing systems, repairs to spandrel walls andcracks evident to the intrados of the arch.

As the relieving slab spans over the existing abutments the load transfer to the arch isradically altered. This is due to the reactions to loading being vertical reactions ratherthan a combination of vertical and horizontal reactions normally encountered at thearch springing.

CIRIA C656 293

TTeecchhnniiqquuee ssuummmmaarryy

RReelliieevviinngg ssllaabbssInstallation of a horizontal reinforced concrete slab over the plan area of thearch, extending over the abutments. Aims to improve live load carryingcapacity of the arch while eradicating the generation of additional horizontalthrust from the arch into the abutments at springing level.

IInnddiiccaattiivvee ccoossttss ���

SSppeecciiaall ccoonnssiiddeerraattiioonnss

Engineering considerations – see Table 4.4 in Section 4.3.4.Temporary Works design for all stages.Lightweight concrete supply.Removal and disposal of existing fill materials.Waterproofing requirements.Health and safety issues – see Section 4.1.

HHeerriittaaggee aannddeennvviirroonnmmeenntt

Repair should not be visible and therefore heritage issues are likely to beminimised, but permission will still be required for work on protectedstructures – see Section 3.5.1. The original fill material may havearchaeological significance.Consider potential for damage to/disturbance of protected species or theirhabitat and environmental pollution – see Section 3.5.2.

DDuurraabbiilliittyyConcrete slab to be designed to give a minimum design life of 120 years.Overall design life of structure limited to that of the existing structure.

IInnssppeeccttiioonn

To check the effectiveness of the repair the following should be undertakenat intervals and for periods agreed with the asset steward:Track or road surface monitoring using fixed targets and EDM to check forany settlement or excessive deflections.Observation of historic cracks visible on the intrados to ensure spreading ofthe cracks has been arrested. Tell tales can be used to monitor movement ofcracks.

PPeerrffoorrmmaanncceeEffective implementation will allow the structure to continue to perform asoriginally designed with increased capacity.

RReeffeerreennccee//ffuurrtthheerrgguuiiddaannccee

BD 24 (HA, 1992).BS 5400-4:1990.BA16 (HA, 1997). Railway bridge maintenance (Turton, 1972).

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Following implementation the existing masonry or brickwork arch continues to supportits own dead weight as well as the fill between the soffit of the relieving slab and thearch extrados. The relieving slab transmits its own dead weight and live load direct tothe abutments in the form of vertical reactions with no horizontal components.

PPuurrppoossee

The principle of the relieving slab method is to improve the live load carrying capacityof the arch while eradicating the generation of additional horizontal thrust from thearch into the abutments at springing level.

The method is particularly useful where the fill over the arch cannot be fully excavatedto expose the extrados of the arch ring. Viaduct spans are a prime example as in manycases the cover to the arch barrel is substantial

DDeessiiggnn ccrriitteerriiaa

Key considerations and actions at the design phase include;

� through topographic survey, trial pitting and review of available structural recordsthe geometric constraints of the relieving slab can be established

� quantify the available construction depth and plan area for reinforced concreterelieving slab. For structures carrying railways the upper surface of the slab shouldnot be less than 350 mm below the bottom of sleeper level to ensure that futuremechanised maintenance of the track can proceed unhindered

� assessment of condition and load carrying capacity of abutments to ensure thatrevised load paths are structurally acceptable

� identify live loading regime in accordance with BD37/01 to allow design of RC slab

� consideration to be given to introduction of compressible layer between existing filland relieving slab in order to aid load distribution to the abutments

� design life is normally a minimum of 120 years for the proposed slab. Overalldesign life for the structure is dependant on its condition and owner requirements

� design of the reinforced concrete to be carried out in accordance with BS 5400

� exposure conditions to be assessed to allow the correct grade and composition ofconcrete mix to be specified.

� exposure conditions to be defined to allow the required cover to reinforcement tobe determined

� provision for location and diversion of existing buried services should be made

� provision of membrane forms of waterproofing and drainage measures to improvestructure drainage to mitigate any pre-existing seepage or percolation through thestructure

� the design should take into account the required reinstatement measures for thearea of deck.

� development of a robust construction sequence at the design stage that inparticular addresses the structural stability of the spandrel wall and adjacent rail orroad infrastructure during the excavation phase of the works.

CIRIA C656294

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IImmpplleemmeennttaattiioonn

The key components for the successful implementation of this method of repair are asfollows. Acquisition of available desktop information including;

� buried services records

� structural examination and assessment reports

� record construction drawings of structure and drainage provisions

� structural monitoring results.

This information will allow the designer to develop specifications for site dataacquisition including topographical and intrusive survey works

Prior to the commencement of the works the location of all buried services should beidentified and where necessary temporary or permanent diversions effected inconjunction with the relevant utility or asset owner/maintainer. Where services are toremain in situ measures for their temporary/permanent protection and/or supportshould be implemented.

The topographical survey data should confirm the principle dimensions of the bridgeand the location of any structural defects. The span and abutment dimensions andwidth between parapets is particularly important to ensure the correct sizing of therelieving slab. The survey should also identify the position of existing deck enddrainage outfalls.

Intrusive survey works should be carried out at an early stage in the form of hand dugtrial pits from deck level, and core samples from the abutments. The trial pit resultswill be utilised as the basic design input information to confirm the following keyaspects:

� type and properties of proposed formation for relieving slab

� condition and thickness of spandrel walls

� construction depth of infrastructure to be supported on the relieving slab.

The core samples taken from the abutments will be utilised to;

� assess the structural integrity of the abutments

� confirm the thickness to allow the length of the relieving slab to be determined toprovide concentric load path to the abutment structure.

To ensure safety in construction and following the relevant procedures theimplementation team should, at an early stage, identify adequate possessions of thehighway or railway infrastructure required to permit the construction of the works.

Relieving slabs within the railway environment are generally constructed over aminimum 54 hour possession for a single span structure. This period will allowadequate time for removal and reinstatement of the track, excavation to formation, aswell as a minimum curing time of 12 hours for the in situ concrete.

The use of concrete with high early strength should be considered to permit theinfrastructure to be brought into as early as possible.

CIRIA C656 295

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For highway structures carrying more than one carriageway it may be feasible toconstruct the relieving slab in two halves. This will allow ongoing restricted use of thestructure for highway traffic. Temporary support to the interface between the existingcarriageway and the relieving slab pour site will need to take account of live loadingduring construction.

Spoil from the excavation to accommodate the relieving slab is likely to becontaminated and will require disposal at a local licensed waste establishment.

The concrete supplier should provide evidence that the mix meets with specificationand this should be validated by independent site and laboratory testing regimes inaccordance with the specification.

The staging of the concrete pours should be determined by agreement between thecontractor and designer and specified in the construction method statement.

The designer should take account of the staged nature of the proposed construction inthe design of the reinforcement to ensure that structural continuity of the structure ismaintained

Waterproofing is installed in the form of an approved flexible membrane to the insideface of the cast in situ concrete. The membrane should extend to and up the internalface of the spandrel wall. Once the membrane is installed backfilling can commenceand trench supports withdrawn.

Measures such as tell tales and survey targets should be installed to repaired fracturesor cracks on the intrados of the arch to allow the effectiveness of the repair to bemonitored following the completion of the works.

Prior to demobilising the works the area where the works have been constructedshould be reinstated to the design specification. For rail bridges where the works havebeen adjacent to the permanent way then consideration should be given to reinstatingand maintaining the track alignment prior to hand over to the permanent waymaintainer. This work is likely to involve monitoring and hand packing the track forapproximately two weeks following completion.

CIRIA C656296

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AA66..99 RReettrroo--rreeiinnffoorrcceemmeenntt

SSuummmmaarryy

Retro reinforcement techniques, which are intended primarily to increase the loadbearing capacity of arches, are still relatively novel, although their use is increasing.While design criteria have been verified at ultimate capacity by experimental workthere is little experimental evidence available to justify increases in load capacity underservice conditions. There is also very little experience (or experimental data) relating tothe long-term effects of their use on bridge durability and/or serviceability. Whatexperimental data exists relates to brick-built arches; no work appears to have beendone to check their applicability on stone arch bridges.

DDeessccrriippttiioonn

Retro reinforcement techniques for strengthening masonry arch bridges comprise theintroduction of supplementary steel reinforcement into the barrel of the arch. Theprincipal function of the reinforcement is to strengthen the structure by providingsome resistance to the development of hinges in the arch barrel. Retro-reinforcement islikely to alter the load paths within the arch and also to render it stiffer.

Two basic forms of retro-reinforcement exist.

Internally installed reinforcement, involves the grouting of reinforcement into holescored through the arch barrel from the crown to the springing, from either above, as

CIRIA C656 297

TTeecchhnniiqquuee ssuummmmaarryy

RReettrroo--rreeiinnffoorrcceemmeennttInstallation of additional structural reinforcement to the arch barrel aims toincrease its structural capacity while not reducing structure clearances orsignificantly affecting the bridge’s appearance.

IInnddiiccaattiivvee ccoossttss ���

SSppeecciiaall ccoonnssiiddeerraattiioonnss

Engineering considerations – see Table 4.4 in Section 4.3.4.Proprietary retro-reinforcement systems.Specialist access requirements.Health and safety issues – see Section 4.1.

HHeerriittaaggee aannddeennvviirroonnmmeenntt

Assess impact on bridge’s heritage value; essentially the repairs will not alterits appearance but care should be taken to ensure that the colour of thestructural adhesive is compatible to the existing mortar; permissions will stillbe required for protected structures – see Section 3.5.1.The impact of the works on any protected species present should beassessed, particularly where bats may have roosts in the arch intrados – seeSection 3.5.2.The potential to cause environmental pollution should be considered, eg fromgrout loss, particularly working over or near a watercourse – see Section3.5.2.

DDuurraabbiilliittyy Unknown, but probably 50 years minimum.

IInnssppeeccttiioonn Visual and tactile inspection in accordance with asset stewards requirements.

PPeerrffoorrmmaanncceeEffective implementation will allow the structure to support specific enhancedloadings

RReeffeerreennccee//ffuurrtthheerrgguuiiddaannccee

BS 5400BD37 and BD86System-specific design information.

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illustrated in Figure A6.4, or, in the case of multi span bridges, from below. Transversereinforcement can also be provided if necessary to improve transverse load distributionacross cracks or joints or to improve the integrity of poor quality masonry.

FFiigguurree AA66..44 TTyyppiiccaall aarrrraannggeemmeenntt ooff iinntteerrnnaallllyy iinnssttaalllleedd rreettrroo––rreeiinnffoorrcceemmeenntt ((ssiimmpplliiffiieedd ddiiaaggrraamm ooff aassiinnggllee ssppaann wwiitthh ffrroomm--aabboovvee iinnssttaallllaattiioonn.. NNoottee aanncchhoorr aarrrraannggeemmeennttss vvaarryy ttoo ssuuiitt eeaacchh bbrriiddggee))

Surface installed reinforcement involves the installation of longitudinal and transversereinforcing bars into a series of channels chased into the intrados of the arch barrel thatare in turn filled with structural adhesive. The reinforcing is usually also dowelled backinto the arch ring, which in brick arches can provide an element of inter-ring stitchingas well.

Retro reinforcement techniques can be applied to both single and multi-span arches,although in the case of the latter the failure mechanisms to be considered in design areoften more complex. This can lead to complications in positioning internally installedreinforcement. While the primary purpose of both techniques is to strengthen the archbarrel, the techniques can be adapted and extended to the other parts of the structure,eg abutments, wing walls and parapets, to provide more conventional strengtheningand tying as appropriate.

PPuurrppoossee

The principle purpose of retro-reinforcement systems is to increase the structuralcapacity of the arch barrel while not reducing structure clearances or significantlyaffecting the appearance. The reinforcement can generally be installed quite quicklyand with minimal disruption to bridge users compared to more conventionaltechniques such as saddling.

DDeessiiggnn ccrriitteerriiaa

Some bridge owners have established policies relating to the use of retro-reinforcementon their structures. Checks should be made to ensure that any proposal to use one ofthese systems complies with the owner’s policies and requirements.

A standardised approach to the design of retro-reinforcement does not yet exist anddifferent design approaches have been developed for different proprietary systems. Ineach case the approach adopted should have been verified against appropriate testing.The detailed design process, while verified against tests, should be capable of takinginto account the condition, geometry and loading requirements of each specific bridge.As noted in Section 3.10.1, solid mechanism methods are required to assess the effectsof such strengthening on masonry arch structures.

CIRIA C656298

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� before giving consideration to the use of a retro-reinforcement system, thedesigner and owner should agree on the degree of strengthening that ispermissible and whether the changed nature of the bridge after strengthening willbe detrimental to its future serviceability. For instance the increase in loadtransmitted to the foundations could cause additional settlement, which a stifferreinforced arch may not be as capable of resisting, or compressive stresses withinthe arch material could be increased to unsatisfactory levels

� design loading is principally governed by dead and vehicle loads. For highwaybridges these are defined by BD21 (reference to BD37 and BD86 may be requiredif spans exceed 20 m or if accidental loading is to be considered or if abnormalvehicle loading is required). For railway bridges design live loadings can beobtained from BS 5400 or BD37

� thorough topographic, tactile and intrusive surveys and a review of availablestructural inspection and graphical records, along with relevant assessment dataare essential to establish a sound basis for design. It is particularly important toestablish the thickness of the barrel and it will usually be necessary to confirm thisby coring. It is normally adequate to determine conservative masonry strengths fordesign from published data on the basis of simple characterisation of the masonry;compressive tests are expensive and not usually warranted. Particular attentionshould be paid to the condition of the material forming the arch to ensure thatadequate holes or chases can be formed without detriment to the overall integrityand stability of the structure and that the reinforcement can achieve an adequatebond to the masonry. Where there is heavy weathering or spalling of the intradossurface it may be necessary to undertake masonry repairs prior to coring holes orcutting chases for the introduction of retro-reinforcement. The designer shouldconsider whether any such masonry repairs could reduce or negate the need forretro-reinforcement

� while the cutting or drilling of chases or holes into the ring will remove relativelylittle material from an arch, the effect on temporary stability and load capacityduring implementation should be considered during the design

� to date the reinforcement used has been stainless steel to provide adequatedurability and avoid the risk of subsequent damage to the masonry by expansivecorrosion. Some experimental work has been done with FRP reinforcement butcommercial applications are not yet available. With regard to surface installedretro-reinforcement, the exposure conditions should be assessed and the requireddurability confirmed in order to allow the correct composition of structuraladhesive to be established.

In particular, it is recommended that the following questions should be addressed whenprocuring retro reinforcement strengthening techniques;

� compliance with the bridge owner’s policy, if one exists

� track record and relevant experience of contractor and designer

� basis of design and extent of verification undertaken, including consideration ofthe extent to which the design method takes account of the real condition of thespecific arch and is not based on simple extrapolation of test results that may notbe appropriate to the structure under consideration

� adequacy of investigations and inspections undertaken to confirm condition andstructural dimensions, including confirmation that the degree of masonryweathering and the condition of the arch does not make retro-reinforcementinappropriate

� long-term durability of the reinforced bridge

CIRIA C656 299

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� where several systems are being considered; a simple comparison of the magnitudeof strengthening proposed

� consideration of the temporary conditions during the works

� effect on the appearance of the bridge.

IImmpplleemmeennttaattiioonn

Retro reinforcement is available in the form of a number of proprietary systems.Installation is undertaken by specialist contractors, who generally provide a design andbuild service. The technique, in its basic form, is intended to address primary structuralinadequacy and consideration should be given to the need to address any otherdeficiencies in the bridge at the same time eg locally deteriorated masonry, poorpointing, local cracking and ring separation etc. In some cases, particularly in relationto local cracking and ring separation, retro reinforcement techniques can be extendedto address these.

Internally installed reinforcement is grouted into holes cored into the barrel of the archfrom either above or below, as illustrated in Figure A6.5. Cementitious grout inconjunction with a socked anchor (to prevent excessive grout loss into theenvironment) is generally used. Designs are generally bespoke for each bridge andanchor arrangements vary considerably, but typically comprise reinforcing bars in therange 20–25 mm diameter in cored holes from 50–65 mm diameter at a spacing ofbetween 200 mm and 1000 mm. Accurate survey and setting out is essential andanchors are regularly and successfully installed to within 65 mm of the intrados of thebarrel. Where anchors are installed from above, particular attention is required toavoid services below the road surface and verges. However, provided logical proceduresare followed this is seldom an insurmountable problem and the location of anchors cangenerally be adjusted to avoid existing services while at the same time providingadequate levels of strengthening.

FFiigguurree AA66..55 CCoorriinngg ffoorr iinnssttaallllaattiioonn ooff iinntteerrnnaall rreettrroo--rreeiinnffoorrcceemmeenntt,, ffrroomm aabboovvee ((aa)) aanndd bbeellooww ((bb)) tthhee aarrcchhbbaarrrreell

Coring is undertaken relatively quickly with lightweight plant and jigs that are easilymanoeuvrable. Typically a bridge can be strengthened in the course of 3–10 days on site.

When working on highway bridges and where coring from on top is necessary,temporary traffic management can usually be established to maintain single wayworking, as a minimum, for the majority of the time. On smaller bridges, where closureof the road for a period is inevitable, the speed of assembling and dismantling thecoring plant is such that it has been possible, on occasion, to make arrangements

CIRIA C656300

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whereby the bridge can be reopened to emergency vehicles and locals at very shortnotice, if required.

When coring from the upper surface on railway underline bridges it will usually benecessary to obtain a full line closure, which will normally mean that the work has tospread over a number of weekends. Care will also be needed to ensure that there is nocontamination of the ballast from either the drilling or grouting operations. If anchorsare to be installed in a bridge carrying an electrified railway isolation of the overheadcables will usually be necessary.

Surface installed reinforcement is achieved by the installation of longitudinal andtransverse reinforcing bars into formed chases over the intrados of the arch and oftenthe installation of radial pins into holes drilled into the arch barrel. The reinforcingmay also be extended to cover abutments, wing walls and parapets as well. Onlylightweight plant is required and the works can therefore be undertaken fromlightweight access scaffold erected under the bridge, which can, if necessary, berelatively easily removed and replaced to suit available possessions.

For bridges over water care will need to be taken to ensure that no pollution, fromeither the chasing/drilling or grouting operations, enters the watercourse. Permissionfor the provision of scaffolding in or over a waterway will have to be obtained andwhere scaffolding is erected in or over watercourses that may be subject to suddenfloods a suitable alarm system with the relevant authorities should be established.

Generally the reinforcement comprises small diameter stainless steel high tensile barswith the transverse bars being fixed in pairs to sandwich single longitudinal bars.Typically the grid comprises transverse bars at 450 mm centres with longitudinal bars at225 mm centres. The chases are disk cut to a cross section, which are generally notmore than 40 mm deep and 20 mm wide. Vertical and horizontal holes are drilled intothe spandrel walls and abutments respectively to anchor the transverse bars. Followingcompletion of reinforcement fixing structural adhesive is pumped into the rebates(Typical curing times are approximately 1–2 hours at 5°C). The adhesive is tooled at thesurface to provide a compatible and consistent appearance over the arch soffit andabutment faces to help minimise the visual impact of the works. The visual impact canbe further mitigated by tinting the adhesive or roughening and coating its finishedsurface with masonry dust.

CIRIA C656 301

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AA66..1100 SSppaannddrreell ttiiee--bbaarrss//ppaattrreessss ppllaatteess

SSuummmmaarryy

DDeessccrriippttiioonn

Spandrel tie-bars require the formation of bored holes between and perpendicular tothe spandrel walls that extend across the arch barrel. Structural reinforcing tie-bars areplaced into the holes, the patress placed and bolts fixed at each end of the bar and thebars torqued to the prescribed tension. Grout may then be pumped into the void tosurround the bar, depending on the agreed method.

The method relies upon creating a tensile force between the spandrels to counteractthe internal lateral forces of passing traffic on the spandrel, thus enhancing lateralstability.

This method is practically and financially viable where a potential risk in increasedlateral load is identified. It has the advantages over the “Stratford method” (see SectionA6.12) that there is less intrusive work, less disruption to traffic passing over the bridgeand is less expensive. The width of bridge is not limited for this method ofstrengthening, but wider bridges increase the potential for displaced exit wall corepositions due to deflection or incorrect drill rig setup.

CIRIA C656302

TTeecchhnniiqquuee ssuummmmaarryy

SSppaannddrreell ttiiee--bbaarrss//ppaattrreessss ppllaatteessThe aim is to prevent arch spandrels from experiencing excessive lateralforces or movements due to lateral pressure from the fill eg from passingtraffic. Reinforcing tie-bars provide structural connection between thespandrels and load is transferred to the new patress plates and tie bars viathe spandrel walls.

CCoosstt bbaanndd ���

SSppeecciiaall ccoonnssiiddeerraattiioonnss

Engineering considerations – see Table 4.4 in Section 4.3.4.Directional drilling.Specialist access/plant.Protective treatments.Health and safety issues – see Section 4.1.

HHeerriittaaggee aannddeennvviirroonnmmeenntt

Assess impact on bridge’s heritage value; patress plates will be visible onbridge elevations but careful design can minimise impact; permissionsrequired for bridges afforded statutory protection – see Section 3.5.1.Consider potential for damage to/disturbance of protected species or theirhabitat and environmental pollution – see Section 3.5.2.

DDuurraabbiilliittyySpandrel ties and patress plate materials should normally be designed togive a minimum design life of 50 years.

IInnssppeeccttiioonn

In accordance with asset stewards requirements and using appropriateinstrumentation undertake visual and tactile inspections of:Arch ring/spandrel interface to check for differential movement.Check verticality of spandrel wall for signs of rotation and evidence oflocalised bulging.Check torque bolts for tension and tighten with torque wrench if necessary.

PPeerrffoorrmmaanncceeEffective implementation and maintenance will increase structureserviceability in line with the strengthening design life, excluding otherreasons for structural deterioration.

RReeffeerreennccee//ffuurrtthheerrgguuiiddaannccee

BA 16 (HA, 1997).Proprietary system literature.

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PPuurrppoossee

The purpose of spandrel ties and patress plates is to prevent arch spandrels fromexperiencing excessive lateral forces or movements due to passing traffic. The method,if successfully implemented, will reduce the ongoing degradation of spandrel walls andnegate the need for wholesale demolition and rebuilding of existing walls that would behighly disruptive to users of the structure.

DDeessiiggnn ccrriitteerriiaa

Key considerations and actions at the design phase include:

� all existing survey records should be consulted as it is essential that the existingstructural fill and any previous remedial work undertaken is known; spandrel tieworks have been carried out on bridges with previously placed reinforced concretesaddles. Core drilling will also adversely affect previous remedial work eg stitchingand grouting, Stratford method repairs etc

� to ensure safety in construction and following the relevant procedures the designteam should, at and early stage, identify adequate possessions of the highway orrailway infrastructure required to allow the construction of the works

� condition of spandrel wall and general fill constituent material and determinationof as constructed geometry. This can be established through tactile examinationand by intrusive investigation using a combination of coring and trial pits

� degree of geometric dilapidation of the spandrel wall to assess the overall viabilityof employing spandrel ties and patress plates as an alternative to reconstruction

� core samples taken during an intrusive survey should be examined by the designteam to confirm the existing condition and structural integrity of the spandrelwalls to be stabilised

� tie bars in close proximity to the brick arch crown should have adequate clearanceto prevent damage to possible pitching acting as a waterproof membrane with asand protection layer near the crown

� quantify the available construction depth. For bridges carrying railway lines thecrown of the hole or casing should not encroach within 300 mm of the bottom ofsleeper to facilitate future track maintenance

� design life to be a minimum of 50 years

� lateral load for the spandrel ties to resist to be calculated using simple hand calcs.Nominal torque per tie-bar to be specified by the designer

� consideration should be given to obstructions and existing buried services.Adequate clearances should be detailed to reduce the potential for the core drill todamage the services if deflected

� a temporary road or footpath closure should be arranged if the work is to becarried out over the public highway or footpath. This can take up to 12 weeks toput in place and therefore is best applied for by the design team at an early stage

� development of a robust construction sequence at the design stage that inparticular addresses the structural stability of the spandrel wall and adjacent rail orroad infrastructure during the core drilling phase of the works.

CIRIA C656 303

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IImmpplleemmeennttaattiioonn

The key components for the successful implementation of this method of repair are asfollows.

Prior to the commencement of the works the location of all buried services should beidentified and, where necessary, temporary or permanent diversions put in place inconjunction with the relevant utility or asset owner/maintainer. Where services are toremain in situ measures for their temporary/permanent protection and/or supportshould be implemented. The working area for the construction of the spandrelstrengthening works should be clearly defined and the normal undertaking of adjacentwater flow, traffic and/or public should be protected from the works.

Intrusive survey works should be carried out at an early stage of implementation toconfirm the integrity and cross sectional dimensions of the entry and exit spandrelwalls. This survey should include pilot drilling through a proposed entry and exit pointto confirm the spandrel wall thicknesses. Core drilling will not work within cohesive fill;therefore this information is essential during implementation to allow the Contractor toknow when the drilling methodology should be changed. Standard brickwork repairsmay be required at the spandrel walls prior to core drilling, eg repointing, relining orcrack injection. These repairs should be carried out in good time to allow the repairmaterials to set. Before drilling begins a photographic and drawing survey shouldrecord any cracking in the walls. This is to confirm whether or not the core drilling isadversely affecting the spandrels as a whole.

The first cores should be drilled under track possession or road closure due to thepotential for the core drill to deflect upwards into traffic. Should the contractor be ableto demonstrate that they are capable of installing the ties within the acceptabletolerances then they should be allowed to install the remaining ties withoutpossessions/closures. However, the tracks or road should be continually monitoredduring non-possession/closure installation to confirm there is no adverse movement orsettlement. Core drilling should commence on the spandrel wall nearest to any knownservices to reduce the potential for core drill damage if deflected. Should there beservices on both sides, the drilling should commence on the side closest to the services.Aluminium tower scaffolds should be set below the entry and exit points to allowperson access to both points.

Setup of the drill rig is critical. It should be checked in a horizontal plane and forperpendicular orientation relative to the entry spandrel wall. Deviations from theseplanes will be visible by differences in actual exit positions from the proposed exitpositions. However, exit positions are also affected by deflections during drilling causedby changes in fill material eg rock, steel etc. Tie-bars in close proximity to the archcrown should have adequate clearance to reduce the potential for the borehole todamage the crown if deflected. Ducts for tie-bars should be installed within a casedbore. This reduces the potential for local settlement above the bore position.

Standard tolerances should be considered for the proposed tie-bar positions. Core drillscan exit the spandrel wall up to 300 mm from the proposed exit positions. Skewedbridges should also be considered in plan to determine the proposed exit positions.

Monitoring measures such as tell tales and survey targets should be installed to theouter face of the spandrel to allow the effectiveness of the repair to be monitoredfollowing the completion of the works.

CIRIA C656304

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Upon reaching the front of the exit wall, the drill bit should be changed back to a coredrill. Upon reaching the front of the exit wall, the drill bit should be changed overfrom an air hammer method back to a core drill. The exit core should be water flushedto prevent blow-out of the core at the exit wall. This should be given earlyconsideration when working above watercourses or surface water drainage systems.

The potential obstructions likely to delay or prevent drilling of tie-bar hole using amachine-mounted drill rig and suitable implementation measures are:

1. Encountering of hard concrete infill behind the entry wall

� pre-drill the concrete using an oversize diameter full face air flush “down the hole”hammer bit without permanent casing until through the concrete into the fill

� upon entering the fill the hammer action will slow dramatically. The drilling willcease and the “down the hole” hammer will be withdrawn

� the drilling equipment with permanent liner will be installed to the end of the holedrilled in the concrete and the procedure will recommence as previous.

2. Encountering of steel obstructions/access obstructions that prevent the drillingmachine from aligning onto the tie-bar entry location.

� a man access scaffold should be erected at both exit and entry points

� power packs for the coring equipment should be positioned at ground level.Coring drills should be fixed to the bridge walls

� the coring entry point should be inspected and hammer sounded for drummy ordamaged brickwork. Loose drummy or damaged brickwork in coring position tobe stitched using stainless threaded rod and quick set epoxy resin

� the coring rig should be fixed to the wall using mechanical or chemical means.Position and level to be checked by the engineer on-site

� a water line should be established at the coring position and a collection hopperand settlement tanks/piping installed

� the core should be drilled continuously with alignment and level being checkedduring and after each core

� a permanent sleeve should be drawn through the cored hole

� after completion of the liner installation the procedure will recommence asprevious.

Prior to demobilising the works the area where the works have been constructedshould be monitored. Where the works have been adjacent to the permanent way ofrail systems, consideration should be given to monitoring the track alignment prior tohand over to the permanent way maintainer. This work is likely to involve monitoringtrack line and level using sighting targets and remote surveying equipment, normallyfor approximately two weeks following completion.

CIRIA C656 305

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AA66..1111 SSpprraayyeedd ccoonnccrreettee lliinniinngg

SSuummmmaarryy

DDeessccrriippttiioonn

Sprayed concrete lining of arch structures involves the application of concrete to thearch intrados using a spray nozzle. The main advantage of this type of concreteplacement is that it removes the requirement for expensive curved formwork to theface of the arch.

The layer of sprayed concrete provides additional strength to the arch structure andcan be reinforced with dowel connections into the existing arch. The sprayed concretelining can extend from springing to springing of the arch or all the way to thefoundations of the structure.

PPuurrppoossee

The purpose of sprayed concrete is to repair and strengthen arches which are sufferingfrom major defects such as arch barrel distortion, deteriorated brickwork and severecracking.

CIRIA C656306

TTeecchhnniiqquuee ssuummmmaarryy

SSpprraayyeedd ccoonnccrreettee lliinniinnggApplication of structural sprayed concrete to the arch barrel intrados to repairand strengthen arches which are suffering from major defects such as archbarrel distortion, deteriorated masonry and severe cracking.

CCoosstt bbaanndd ���

SSppeecciiaall ccoonnssiiddeerraattiioonnss

Engineering considerations – see Table 4.4 in Section 4.3.4.Competence of spray operatives.Carry out test panels.Effect on gauging/clearances should be assessed.Secure fixing of reinforcement to sound masonry in arch.Spacing of reinforcement to avoid “shadowing”.Health and safety issues – see Section 4.1.

HHeerriittaaggee aannddeennvviirroonnmmeenntt

Assess impact on bridge’s heritage value; permissions required for bridgesafforded statutory protection – see Section 3.5.1. Affects appearance of thearch intrados but careful detailing can minimise the effect on the appearanceof bridge elevations.If protected species are present they are likely to be killed, disturbed or theirhabitat damaged, particularly bats with roosts in the arch intrados – seeSection 3.5.2.The potential to cause environmental pollution should be considered,particularly working over or near a watercourse – see Section 3.5.2.

DDuurraabbiilliittyyOverall design life limited to that of existing structure.Spray concrete lining design life normally up to 120 years.

IInnssppeeccttiioonnNo inspection of original structure possible following application of concrete;concrete lining should be included in the structure inspection regime.

PPeerrffoorrmmaanncceeSpayed concrete provides strengthening mechanism for weakeneddeteriorated structures.

RReeffeerreennccee//ffuurrtthheerrgguuiiddaannccee

Sprayed Concrete Association guidelines.

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DDeessiiggnn ccrriitteerriiaa

� the design loading of the sprayed concrete is generally for live loading, but athicker lining can be provided to resist dead and live loads, thereby making theexisting arch redundant

� concrete mix specification to be suitable for spray application, possibly withinclusion of “natural cements” which are typically more costly but can exhibitenhanced properties for this type of application

� spacing of reinforcement within the concrete should be maximised in order toprevent shadowing, where the sprayed concrete fails to fill voids behind thereinforcement

� for bridges over waterways where sprayed concrete is taken down to water levelthere should be a clear break above the water-line to minimise the effects of freeze-thaw action

� sprayed concrete to the arch intrados should not be continued up the sideelevations of the barrel since this encourages water penetration between themasonry and concrete, and accelerates deterioration

� lining of the structure will lead to reduction of clearances beneath. The new profileshould be checked for clearances to traffic as part of the design process

� where clearances are limited it may be possible to remove the inner course of archbrickwork and use this thickness to install the concrete

� fibre reinforced concrete is also available for spraying, making traditional meshreinforcement redundant

� design should consider that no inspection of the existing structure will be possiblefollowing application of the sprayed concrete

� the design life of the bridge will be limited to that of the remaining structure, butthe sprayed concrete lining can be designed for up to 120 years.

IImmpplleemmeennttaattiioonn

Concrete spraying should be carried out by trained, experienced professionals andtight quality control systems need to be adopted.

Prior to implementation, test panels should be sprayed and tested for effectiveness ofspraying. These panels should be orientated to simulate the profile of the structure tobe sprayed.

Large amounts of waste due to rebound of the concrete occurs during the sprayingoperations, this needs to be considered when ordering materials. Mitigatingenvironmental impact should also be considered, particularly over watercourses.

Sprayed concrete reinforcement is placed prior to spraying and is connected to dowelsfixed into the existing structure. Care should be taken due to the condition of theexisting arch that the dowels are securely fixed prior to fixing the reinforcement.

This method of strengthening is unlikely to be permitted on structures where aestheticsis an issue or where the structure is heritage listed.

CIRIA C656 307

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AA66..1122 SSppaannddrreell wwaallll ssttrreennggtthheenniinngg ((““SSttrraattffoorrdd mmeetthhoodd””))

SSuummmmaarryy

DDeessccrriippttiioonn

The Stratford repair method comprises the excavation of a trench directly behind andparallel to the spandrel wall that extends downwards to the arch barrel. The trench isfilled with reinforced concrete placed on the extrados of the arch barrel to which thespandrel wall is stitched using structural ties formed from reinforcing or stainless steelbars as deemed appropriate. Patress plates can be used in conjunction with thestructural ties.

The method relies upon the concept of creating a composite spandrel that has greatermass having its centre of gravity positioned well back from the face of the arch ringthus enhanced stability. The increased mass, contact area and mechanical bondinggenerated by the in situ concrete will in addition produce greater adhesion at thejunction between the spandrel wall and the arch barrel. This method is practically andfinancially viable where the depth of excavation does not exceed 1.2 m to the archbarrel and is therefore limited to the area between the arch crown and haunches. Itmay be employed between the haunches and springing only where the design does not

CIRIA C656308

TTeecchhnniiqquuee ssuummmmaarryy

SSppaannddrreell wwaallll ssttrreennggtthheenniinngg ((““SSttrraattffoorrdd mmeetthhoodd””))Aims to prevent spandrel walls from undergoing excessive horizontal orrotational movements relative to their arch barrel supports by adding andconnecting them to linear reinforced concrete elements, creating a compositespandrel with enhanced stability.

CCoosstt bbaanndd ���

SSppeecciiaall ccoonnssiiddeerraattiioonnss

Engineering considerations – see Table 4.4 in Section 4.3.4.Temporary works design for all stages.Lightweight concrete supply.Specialist access.Health and safety – see Section 4.1.

HHeerriittaaggee aannddeennvviirroonnmmeenntt

Assess impact on bridge’s heritage value; in most cases the repair will not bevisible but where patress plates are to be fitted to the outer face of thespandrel walls or additional impact protection is to be provided to the innerfaces of the parapet wall the impact is likely to be greater. Permissionsrequired for bridges afforded statutory protection – see Section 3.5.1.Consider potential for damage to/disturbance of protected species or theirhabitat and environmental pollution – see Section 3.5.2.

DDuurraabbiilliittyyConcrete repairs normally designed to give a minimum design life of 25years.

IInnssppeeccttiioonn

In accordance with asset stewards requirements and using appropriateinstrumentation undertake visual and tactile inspections of:Arch ring/spandrel interface to check for differential movement.Check verticality of spandrel wall for signs of rotation and evidence oflocalised bulging.Following implementation of the repair scheme it is recommended that tell-tales are installed to confirm that the works have mitigated any previousmovement and or rotation.

PPeerrffoorrmmaanncceeEffective implementation will allow the structure to continue to perform asoriginally designed.

RReeffeerreennccee//ffuurrtthheerrgguuiiddaannccee

BS 5400-4:1990.CIRIA RO97 (Irvine and Smith, 1983).BS 5628 Code of practice for use of masonry.

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rely on structural connection between the concrete and the arch barrel. Whereproblems exist between the haunches and springings consideration should be given toalternative methods of stabilisation such as through deck ties with patress plates.

PPuurrppoossee

The purpose of the Stratford method of repair is to prevent spandrel wall structuresfrom undergoing excessive horizontal or rotational movements relative to their archbarrel supports.

The method if successfully implemented will negate the need for wholesale demolitionand rebuilding of existing spandrel walls that would be highly disruptive to users of thestructure.

DDeessiiggnn ccrriitteerriiaa

Key considerations and actions at the design phase include:

� general condition of spandrel wall constituent material and determination of asconstructed geometry. This can be established through tactile examination and byintrusive investigation using a combination of coring and trial pits

� degree of geometric dilapidation of the spandrel wall to assess the overall viabilityof employing the Stratford method as an alternative to reconstruction

� quantify the available construction depth and plan area for reinforced concreteinfill. For bridges carrying railway lines the edge of the concrete infill should notencroach within 150 mm of the sleeper ends to facilitate future track maintenance

� identify loading regime in accordance with BD52 to allow the scheme to bedesigned using geometric constraints to deliver the appropriate factor of safety

� design life normally a minimum of 25 years

� design of the reinforced concrete to be carried out in accordance with BS 5400

� exposure conditions to be assessed to allow the correct grade and composition ofconcrete mix to be specified

� exposure conditions to be defined to allow the required cover to reinforcement tobe determined

� provision for existing buried services

� provision of membrane forms of waterproofing and drainage measures to mitigatethe build up of hydrostatic pressures and to preserve constituent materials of theexisting spandrel wall

� the design should take account of the required reinstatement measures for the areaof deck

� development of a robust construction sequence at the design stage that inparticular addresses the structural stability of the spandrel wall and adjacent rail orroad infrastructure during the excavation phase of the works

� design of temporary works including required speed restrictions barriers and trenchsupports required to maintain operational safety throughout the construction period

� for bridges carrying road vehicles consideration should be given to providingadditional impact protection to the parapet above highway or footway surface level

� when providing supplementary impact protection the proposed structure gaugeshould be checked for the intended vehicular use post remediation

CIRIA C656 309

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� where the spandrel wall requires only localised stabilisation and this method is tobe employed consideration should be given to providing suitable transitionbetween the proposed strengthened section of wall and the un-strengthenedsection of wall thus preventing localised stiffening of the structure.

IImmpplleemmeennttaattiioonn

The key components for the successful implementation of this method of repair are asfollows:

Prior to the commencement of the works the location of all buried services should beidentified and where necessary temporary, permanent diversions effected inconjunction with the relevant utility or asset owner/maintainer

Where services are to remain in situ measures for their temporary/permanentprotection and/or support should be implemented.

The working area for the construction of the spandrel strengthening works should beclearly defined and adjacent traffic and public should be protected from the works.

Intrusive survey works will be carried out at an early stage in the works to confirm theintegrity and cross sectional dimensions of the spandrel wall. This information will beutilised in planning and undertaking excavation works as well as providing the designteam with the information necessary to confirm the design of the in situ concretesupport and associated structural ties. Core samples taken during the intrusive surveyswill be examined by the design team to confirm the existing condition and structuralintegrity of the spandrel wall to be stabilised.

To ensure safety in construction and following the relevant procedures theimplementation team should at and early stage identify adequate possessions of thehighway or railway infrastructure required to permit the construction of the works.

Prior to excavation for the in situ concrete the length of the trench to be excavated atany one time should be agreed with the design team and asset steward to ensurestructural stability of the wall and the adjacent infrastructure during construction. Thisis particularly important when working adjacent to railways where permanent wayconstructed from stressed continuous welded rail is required to be afforded adequatelateral support even when the line is under possession during the construction period.

The design and method of support for the trench will be determined by the type ofmaterial and the depth to which this material is to be retained. The system should bedesigned to take due account of surcharge loading imposed by adjacent infrastructure.CIRIA R97 Trenching practice (Irvine and Smith, 1983) provides invaluable guidance onthe assessment, design, selection and installation methods of temporary support system.

Particular care should be taken to ensure that any temporary point loads are nottransferred to the spandrel wall being stabilised once the required trench supportsystem is installed excavation can progress following the agreed method. Spoil is likelyto be contaminated and will require disposal at a local licensed waste establishment.The reinforcement should be placed ensuring that adequate cover will be afforded bythe concrete pour.

CIRIA C656310

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Concrete pours should be strictly controlled to ensure that hydrostatic pressuregenerated by the wet concrete does not adversely affect the stability of the spandrelwall. The concrete supplier should provide evidence that the mix meets withspecification and this should be validated by independent site and laboratory testingregimes in accordance with the specification. The staging of the concrete pours shouldbe determined by the designer and specified in the construction method statement.The designer should take account of the staged nature of the proposed construction inthe design of the reinforcement to ensure that structural continuity of the structure ismaintained. Where only limited construction periods are available the use of concretewith rapid early strength development may offer advantages. This will allowproductivity to be optimised while having the benefit of concrete achieving a high earlystrength when the adjacent infrastructure is brought into use.

Waterproofing is installed in the form of a flexible approved membrane to the insideface of the cast in situ concrete. The membrane should extend to and up the internalface of the spandrel wall. Once the membrane is installed backfilling can commenceand trench supports withdrawn.

Structural ties can then be installed to a pre-determined grid by competent diamonddrilling teams. The holes shall be drilled at an angle of approximately 30° through thespandrel wall into the reinforced concrete and can take the form of galvanised orstainless steel resin anchors as appropriate. Pattress plates can be fitted to the outer faceof the spandrel in order to distribute the tie force over an area of brickwork

Monitoring measures such as tell-tales and surveying targets should be installed to theouter face of the spandrel to allow the effectiveness of the repair to be monitoredfollowing the completion of the works.

Prior to demobilising the works the area where the works have been constructedshould be reinstated. For bridges carrying railways, where the works have beenadjacent to the permanent way then consideration should be given to reinstating andmaintaining the track alignment prior to hand over to the permanent way maintainer.This work is likely to involve monitoring and hand packing the track for approximatelytwo weeks following completion.

CIRIA C656 311

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AA66..1133 TThhiicckkeenniinngg ssuurrffaacciinngg

SSuummmmaarryy

DDeessccrriippttiioonn

Surface thickening over an arch structure comprises the installation of a replacementsurfacing material of thicker depth or of additional surfacing on top of the existing.

PPuurrppoossee

The surfacing over an arch structure helps with distribution of live loads throughoutthe arch. Thin layers of surfacing do little to distribute the load and result in highconcentration of loads on the arch. Provision of an additional thickness of surfacingdistributes the live loads more evenly through the arch and can result in a higher liveload capacity for the structure.

DDeessiiggnn ccrriitteerriiaa

� site investigation to be carried out in order to confirm fill properties

� site investigation to be carried out in order to identify all buried services over thestructure

� leave existing surfacing in place if possible

� if existing surfacing is unsuitable then specify replacement with competentsurfacing material

� consider application of waterproof membrane while existing surfacing is removedto prevent deterioration of the arch due to water

� proposed surfacing to meet requirements of bridge users.

CIRIA C656312

TTeecchhnniiqquuee ssuummmmaarryy

TThhiicckkeenniinngg ssuurrffaacciinnggProvision of an additional thickness of surfacing distributes the live loadsmore evenly through the arch and can result in a higher live load capacity forthe structure.

CCoossttss ���

SSppeecciiaall ccoonnssiiddeerraattiioonnssEngineering considerations – see Table 4.4 in Section 4.3.4Health and safety – see Section 4.1.

HHeerriittaaggee aannddeennvviirroonnmmeenntt

Assess impact on bridge’s heritage value; original appearance of structurecan normally be maintained - use can be made of pigmented surfacing, butpermissions may be required for bridges afforded statutory protection – seeSection 3.5.1.Consider potential for damage to/disturbance of protected species or theirhabitat and environmental pollution – see Section 3.5.2.

DDuurraabbiilliittyy Design life of original structure unaffected by works to the surfacing.

IInnssppeeccttiioonnStructure inspection to continue as part of ongoing structure inspectionregime.

PPeerrffoorrmmaanncceeImproved performance and capacity of original structure by introduction ofincreased depth of surfacing material.

RReeffeerreennccee//ffuurrtthheerrgguuiiddaannccee

BD37 Loads for highway bridges (BSI, 2001b)

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IImmpplleemmeennttaattiioonn

All buried services are to be identified, taking care to avoid damage to cables duringexcavation of surfacing and excavating by hand if necessary.

All excavation of surfacing is to be carried out in accordance with designer’srequirements in order to avoid overloading of the structure due to plant loadingand/or due to imbalanced fill removal.

Works can be carried out utilising, as a minimum, half closures of the bridge with workbeing undertaken to one half of the bridge before switching over to the opposite side.

CIRIA C656 313

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AA66..1144 TThhrroouugghh--rriinngg ssttiittcchhiinngg

SSuummmmaarryy

DDeessccrriippttiioonn

Through ring stitching is a technique that involves the installation of stainless steel orgalvanised dowels into bored holes drilled to predetermined angles and staggered gridon the intrados of the arch structure. It can be employed to arrest ring separation ofbetween two and six successive constituent arch rings. The system is used inconjunction with cementitious or resin based grout that is injected at low pressure intothe holes prior to installation of the dowels and is allowed to penetrate the voids thatexist between the arch rings. Figure A6.6 illustrates the principle of this technique.

CIRIA C656314

TTeecchhnniiqquuee ssuummmmaarryy

TThhrroouugghh--rriinngg ssttiittcchhiinnggAims to re-establish the mechanical connection between the constituent ringsof an arch barrel where ring separation has occurred, restoring its structuralintegrity by installing overlapping reinforcement dowels into holes drilledthrough the intrados of the arch and grouted into place.

IInnddiiccaattiivvee ccoossttss ���

SSppeecciiaall ccoonnssiiddeerraattiioonnss

Engineering considerations – see Table 4.4 in Section 4.3.4.Specialist access.Selection of suitable grout.Health and safety issues – see Section 4.1.

HHeerriittaaggee aannddeennvviirroonnmmeenntt

Repair should not be visible and therefore heritage issues are likely to beminimised; stitching bars are terminated within the brickwork & then sealedwith mortar to match the existing structure. However, permission will still berequired for work on protected structures – see Section 3.5.1.The filling of cracks and voids within the arch ring may kill any bats present ordestroy their roosts, so their presence (and that of any other protectedspecies) should be ascertained before planning any work – see Section3.5.2.Certain grouts have the potential to cause environmental pollution; careshould be exercised, particularly working over or near a watercourse – seeSection 3.5.2.

DDuurraabbiilliittyy Repairs normally designed to give a minimum design life of 25 years.

IInnssppeeccttiioonn

In accordance with asset stewards requirements and using appropriateinstrumentation undertake visual and tactile inspections including:Tactile examination using inspection hammer to check for hollow soundingbrickwork which may signify further ring separation.Check intrados profile using laser profiling equipment where ring separationhas been remediated.Where surface evident cracks have been repaired, install tell tales to checkfor post remediation movement.

PPeerrffoorrmmaanncceeEffective implementation will allow the structure to continue to perform asoriginally designed.

RReeffeerreennccee//ffuurrtthheerrgguuiiddaannccee

Standard brickwork repair detailsBS 5628 Code of practice for use of masonry.

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FFiigguurree AA66..66 DDrriilllliinngg gguuiiddee--hhoolleess iinn aarrcchh iinnttrraaddooss ffoorr iinnssttaallllaattiioonn ooff ddoowweell

PPuurrppoossee

The purpose of through ring stitching systems is to remediate existing and preventfurther separation of brickwork or masonry arch rings thus restoring the structuralintegrity of the arch barrel by re-establishing the mechanical connection between theconstituent rings. This method also ensures the effective restoration of shear transferacross surface evident shear and rotational longitudinal cracks that may exist inconjunction with ring separation.

DDeessiiggnn ccrriitteerriiaa

Key considerations and actions at the design phase include:

� through tactile examination with an inspection hammer identify the extent of ringseparation, (signified by hollow brickwork) to enable the extent of the repair to bedesigned and quantified

� during the inspection identify any cracking as well as areas of loose or spalledbrickwork on the intrados

� where ring separation occurs close to the face of the arch barrel it may be possibleto observe the degree of separation by visually examining the bed joints at thispoint

� using the results of the tactile survey, carry out intrusive investigations by takingcore samples through the arch rings. Careful examination of the cores and theholes (ideally using an endoscope or similar) will allow the designer to identifybetween which rings separation is occurring. This is particularly important as thesystem relies on the steel dowels intersecting at the separation point

� using the separation point identified by the intrusive survey the information can beutilised by the designer to determine the pitch of the pinning and stitching gridalong with the diameter, length and angle of penetration of the steel dowelsrequired to achieve intersection and adequate anchorage

� in designing the grid the designer should ensure that the likelihood of inducingfurther cracking through installation is eliminated

� specify patch repairs to any loose or spalled brickwork in the vicinity of theproposed area to be stitched in advance of the grouting and stitching operations

� design life to be approximately 25 years

CIRIA C656 315

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� exposure conditions and geometric constraints to be assessed to allow the groutmix and dowel including cover to be specified

� consideration should be given to the use of resin and epoxy based grouts wherethe structure is to support live loading soon after injection.

IImmpplleemmeennttaattiioonn

The key components for the successful implementation of this method of repair are asfollows.

The working area for the works should be clearly defined and users should beprotected from the works. When working on a structure spanning the operationalhighway it may be feasible to undertake the works under a half road closure trafficmanagement system thus avoiding the need for disruptive diversions

Tactile and intrusive survey works will be carried out at an early stage. Thisinformation will be utilised by the design team with the information necessary toconfirm the design and location of the remedial patch repairs and the through ringstitching respectively.

� to ensure safety in construction and following the relevant procedures theimplementation team should at and early stage, identify adequate possessions ofthe highway or railway infrastructure required to allow the construction of theworks

� this method of repair can be carried out in either a piecemeal or wholesale fashion,hence a progressive work plan can be developed and timed to utilise periods thatcause minimum disruption to the users of the structure

� when planning the works the Contractor should carefully assess the temporarymeans of access to provide a working platform to the intrados for operatives andhand held plant within the geometric and time constraints

� for example when working on a structure that affords significant clearance (egviaducts) to the operational infrastructure it may be feasible and cost effective toconsider the installation of a crash deck/working platform constructed frommedium duty scaffolding under the intrados for the duration of the works

� for structures where adequate clearance between the working platform and theinfrastructure cannot be achieved the access solution should be quick to install anddismantle. In this scenario a range of access solutions are available including lorrymounted/rough terrain hydraulic access platforms and mobile lightweightaluminium tower scaffolding

� compressors and grout pumps can normally be site conveniently on or adjacent tothe bridge structure.

Prior to commencing the through ring stitching works the contractor should carry outany specified ancillary patch repairs to the intrados to ensure that the grouting andstitching process itself does not adversely affect the stability of the existing brickwork.Such patch repairs may comprise localised re-casing, re-pointing and removal of facedeposits and loose material.

The designed stitching grid should be set out and clearly marked on the intrados of thearch. When setting the grid out the contractor should ensure that the holes are centredon whole bricks thus avoiding the drilling of holes through mortar joints which woulddecrease the effectiveness of the system.

CIRIA C656316

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Prior to drilling any holes which are not perpendicular to the intrados it is advisable toconstruct a triangular plywood drilling template to assist the drilling operative inmaintaining a consistent angle when drilling the dowel holes. The template should beconstructed to reflect the angle of the stitching bars as specified by the designer. Thedrilling angle normally varies between 30° and 45° depending upon the number ofrings to be stitched and the location of the separation within the arch barrel.

The holes are formed using a conventional masonry bit that will produce an irregularfinish to the circumference, hence when grouted additional adhesion along the lengthof the anchor will be generated. The operative should ensure that the depth of eachhole is achieved by the use of appropriate drill markers or depth gauges. Once the gridhas been drilled the holes may be grouted using a cementitious or resin based grout asappropriate. The grout should be injected at a low but variable pressure (limited toapproximately 3.0 bar or less) to ensure that it will penetrate the voids and permeatesthe brickwork but avoids excessive leakage and potential blow-out of individual bricksor delamination between brick rings.

� proprietary lockable grouting nipples or timber bungs should be used to preventloss of grout from each hole as work progresses

� the grouting sequence should commence from the lowest hole on the intradoswhere grout should be pumped until it issues from the hole above. At this pointthe hole being grouted is locked off and the procedure repeated at the hole above

� while the grout is still “green” the holes should be re-drilled and the anchorsinstalled to the correct depth. The hole should then be re-grouted and the securedwith a temporary timber bung to prevent grout loss

� following the set period the bung should be removed and the resultant holepointed up using a mortar to match that of the existing brickwork

Following completion of the works the agreed monitoring measures should be installed.

The principles of the above stitching method can be utilised to repair rotational andshear cracks occurring in the longitudinal or transverse direction. In addition topointing the dowel holes the contractor should ensure that the crack itself is raked outand re-pointed following grouting to ensure that it is sealed and compatible inappearance to the surrounding brickwork.

CIRIA C656 317

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AA66..1155 UUnnddeerrppiinnnniinngg

SSuummmmaarryy

DDeessccrriippttiioonn

Underpinning involves the construction of new substructure supports under existingbrickwork or masonry abutment and wing wall foundations whose ability to transfer thedead and imposed loads from the structure to the formation has deteriorated or failed.Reasons for this failure may be attributed to structural defects, changes in loadingregime, subsidence and time related deterioration of the available bearing capacityafforded by the formation or a combination of any of these.

The method could also be employed if there is a future risk of settlement fromproposals to increase imposed loadings (eg increase in axle loads) or to reduce passivepressures by excavation (eg provision of enhanced headroom under structure)

Many brickwork and masonry substructures bear directly on the formation material byway of corbelled brickwork or masonry spread footings as well as mass concretefoundations.

The most common form of underpinning techniques employed to remedy failing orfailed substructures include mass concrete strip foundations and piles. These are brieflydescribed as follows.

CIRIA C656318

TTeecchhnniiqquuee ssuummmmaarryy UUnnddeerrppiinnnniinngg

IInnddiiccaattiivvee ccoossttss ���

SSppeecciiaall ccoonnssiiddeerraattiioonnss

Engineering considerations – see Table 4.4 in Section 4.3.4.Temporary Works design for all stages.Piling.Specialist access/plant.Ground investigation.Health and safety issues – see Section 4.1.

HHeerriittaaggee aannddeennvviirroonnmmeenntt

Repair should not be visible and therefore heritage issues are likely to beminimised but if the repair is visible above ground level or involves themaking good of resultant defects within the visible elements then visualimpact should be considered. Permission will be required for work onprotected structures – see Section 3.5.1. The original fill material may havearchaeological significance.Consider potential for damage to/disturbance of protected species or theirhabitat and environmental pollution – see Section 3.5.2.

DDuurraabbiilliittyy Repairs normally designed to give a minimum design life of at least 50 years.

IInnssppeeccttiioonn

In accordance with asset stewards requirements and using appropriateinstrumentation undertake visual inspection, including:Installation of tell-tales to historic settlement cracks.Precise levelling on remediated elements referenced to known fixed points orreference stations.

PPeerrffoorrmmaanncceeEffective implementation will allow the structure to continue to perform asoriginally designed.

RReeffeerreennccee//ffuurrtthheerrgguuiiddaannccee

Guidance from the Federation of Piling Contractors.Institution of Civil Engineers Piling specification (ICE, 1996).

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CCoonnttiinnuuoouuss ssttrriipp ffoouunnddaattiioonnss

This method involves the excavation of rectangular working pits at predeterminedintervals that will not compromise stability of the structure being underpinned. Thepits are filled with mass concrete to the underside of the existing foundation. Thisprocess is repeated at designed intervals until the desired extent of underpinning isachieved.

This method is considered suitable where:

� the depth to competent formation is relatively shallow

� material can be feasibly excavated and can be adequately supported via temporaryworks

� the water table is below the level of the proposed excavation

� the structure is able to tolerate temporary undermining to form strip foundation.

PPiilliinngg mmeetthhooddss

Various piling techniques can be employed with cased or uncased (depending onground conditions) bored piles being the most common. Displacement piles are notnormally favoured due to the potential for vibration and displacement of the ground toadversely affect the structure being stabilised. Bored piles are also favoured from thestandpoint of accessibility as the installation rigs are capable of operating in locationswhere low headroom exists (eg when underpinning abutments)

In many cases it is possible to only access the foundations of the abutments and wingwalls from the exposed face of these elements.

Two main structural principles can be applied for piled underpinning solutions tosuccessfully address this situation as follows.

The first of these principles involves the installation of piles adjacent to the foundationin plan on the inside face of the wing wall or abutment. The piles are capped with areinforced concrete pile cap that extends under the existing foundation. This methodinvolves excavation to the underside of the pile cap to allow installation and shouldfollow the same principles of “hit and miss” construction as described for continuousstrip footings. This will ensure structural stability is maintained throughoutconstruction.

Where the existing foundations are of significant width it may be more practical toinstall mini-or micro piles known as root piles that can be drilled using low percussiveequipment at a steep raking angle through the structure and foundations into theformation. Once formed galvanised or stainless steel bars are threaded into the holesand grouted with concrete. These piles are particularly useful where limited space isavailable for installation and where ground conditions prohibit hand excavation. Theyare also particularly appropriate if the area adjacent to the base of the wing walls orabutments are congested with services and utilities.

CIRIA C656 319

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FFiigguurree AA66..77 UUnnddeerrppiinnnniinngg wwoorrkkss iinn pprrooggrreessss

PPuurrppoossee

The purpose of underpinning is to arrest and/or prevent settlement of substructures bytransferring the dead and imposed loads to a competent bearing material.

DDeessiiggnn ccrriitteerriiaa

Key considerations and actions at the design phase include;

� initial dimensional survey to establish location, extent and magnitude of structuraldeterioration/failure. Particular attention to be given to visual signs of distress inbody of brickwork such as bulging, vertical stepped cracks and fractures

� desktop study of structural records to determine the existing form of substructureconstruction and available geological/geotechnical information, both to aid theplanning and specification of the site investigatory works

� identification of services and utilities likely to impact on the planning of theground investigations and/or the development of a permanent underpinningsolution

� trial pitting to confirm the dimensions, construction and condition of the existingsubstructures

� boreholes to establish formation material type, depth to interfaces between varyingmaterials and to provide samples for laboratory testing

� confirmation of precise location of buried services

� confirmation of type, magnitude and rate of structural failure by precise levellingand/or tell tales

� range of laboratory testing with required outputs for design including strength,angles of internal friction and settlement/consolidation (eg tri-axial or shear box,oedometer testing etc as appropriate for material type)

� groundwater monitoring in the form of piezometers should be considered toestablish the position of the natural water table to allow hydrostatic pressure to beconsidered in the design calculations as well as the planning of construction andassociated temporary works.

� chemical analysis of soil and ground water samples to allow where necessarysulfate/acid resisting mix as required.

CIRIA C656320

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Once acquired the above data should be presented in a Geotechnical interpretive report.This coupled with the desktop information will provide the basis for assessing the causeof existing settlement problems and/or the potential for settlement in the future.

Prior to commencing the design the designer should satisfy himself of the mode offailure of the formation material that is causing settlement and/or instability.

At an early stage following review of the geotechnical interpretive and survey reportsthe following criteria should be considered in selecting the underpinning method to beemployed

� depth at which the required bearing capacity is encountered (formation level)

� nature of material to be excavated

� total length of structure to be underpinned

� available programme for completion of the works

� physical site constraints (eg limited plan area, headroom etc).

There is no definitive rule that states the depth at which traditional strip footings andpiles should or should not be employed, as each site should be judged upon its ownmerits. However in addition to above the following factors will assist in reaching themost appropriate solution

� where consolidation settlement is occurring in a clay band strip footings alonewould be ineffective as the loads will merely be transferred to a lower levelpermitting the cycle of settlement to recommence

� underpinning measures should ideally be taken down to a relatively incompressiblestrata

� establish the true cause of settlement (eg clay shrinkage, de-vegetation, subsidence,overloading etc).

It should be noted that where the failure is deep seated due to mining subsidence orthe like then traditional methods of underpinning would be rendered as ineffective.

Fundamental criteria to be input into the design stage includes

� exposure conditions and cover to reinforcement, (contingency in cover to noncased reinforced piles in granular or loose strata)

� dead and imposed loading from structure

� chemical composition of formation stratum and ground water to allow specificationof durable mix

� design life 50 years or over

� connectivity between eccentric pile caps or strip foundations and foundationstructure

� where continuous strip foundations are to be employed to ensure structuralstability, the designer should specify the maximum length of foundation that canbe underpinned at any one time. The designer should indicate the resulting hitand miss sequence for the construction

� for continuous strip foundations the designer should develop a reinforcementschedule that will achieve structural continuity in conjunction with the intendedconstruction sequence

CIRIA C656 321

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� for continuous strip foundations the designer should give consideration to thespecification of mixes that are capable of achieving high early strength, thusallowing installation periods to be reduced

� for piled solutions the designer should specify the length and diameter of the pilethat will be governed by the structure loadings, allowable bearing pressure of theformation, end surface area and shaft surface area

� the designer should provide details for the repair of any cracked brickwork ormasonry consequential of historic settlement.

IImmpplleemmeennttaattiioonn

The generic key components for the successful implementation of this method of repairare as follows:

� the working area for the works should be clearly defined and users and publicshould be protected from the works. When working on a structure spanning theoperational highway it may be feasible to undertake the works under a half roadclosure traffic management system thus avoiding the need for disruptive diversion

� ground investigation and survey works to be carried out at an early stage toprovide base information for design

� confirmation of position and marking of services to ensure that design andconstruction phases take due account of their presence

� to ensure safety in construction and following the relevant procedures theimplementation team should at an early stage identify adequate possessions of thehighway or railway infrastructure required to permit the construction of the works.

The key aspects to be observed for the implementation of the continuous stripfoundation underpinning technique are as follows:

� development of approved design for the temporary support of the working pits(for guidance refer to CIRIA R97 Trenching Practice (Irvine and Smith, 1983)

� planning of “hit and miss” sequence for excavation and installation of concrete

� careful excavation of working pits and portion of failed formation underneathexisting foundations to proposed new formation level using compressed air handtools

� shuttering to be installed between working pit and proposed underpinning tocontain pour

� concrete mix to be checked for compliance with specification via laboratory andfield testing.

� concrete to be placed in excavated void underneath existing foundation as soon aspossible following excavation. Concrete to be placed by pump and hand tools andcompacted in the normal manner using appropriate vibrators and agitators

� concrete to be poured to within 50 mm to 100 mm of underside of existingfoundations

� once setting and initial shrinkage has taken place the void between the cast in situconcrete and the existing foundation can be filled with a suitable non shrink grout.Pressure grouting using a cementitious grout could also be employed to ensurethat all voids at the interface between old and new structures are removed

� once complete the shutter can be removed and the working pit backfilled withengineering fill using appropriate means of compaction

CIRIA C656322

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� the process can then be repeated for the remaining length of structure to bestabilised to completion in strict accordance with the specified intervals.

When implementing the piled underpinning solutions the major steps and points forconsideration are as follows:

� a competent specialist piling contractors should be appointed to at the earliestopportunity during the design phase. By involving this expertise early thedesigner will be able to specify the pile type with buildability input

� through previous experience and expertise the piling contractor will be able toreview the site constraints and ground conditions to develop the most appropriatecost effective means of installing the piles

� physical site constraints may limit the available space for installation thus the pilesize, numbers and arrangement will need to be designed around an installation rigthat it is feasible to operate within such constraints

� where early strength is required or where piling loose material it may be prudentto consider utilising permanent casings that will provide enhanced structuralperformance and will ensure that pile does not suffer localised necking duringinstallation. If necking occurs then little or no cover is afforded to reinforcementleading to potential corrosion and early failure of the pile

� in water bearing sandy soils boring should proceed carefully to prevent draw downof sand into the hole. Pumping of ground water from the hole is also notrecommended as this will lead to settlement caused by boiling at the base of thehole which in turn will be generated by surrounding pore water pressure

� when the cantilevered pile cap method is employed the piles are installed to thedesigned arrangement in advance of excavating for the pile cap itself. Onceinstalled the piles are exposed and reduced to underside of pile cap level. It isrecommended that the pile is dowelled with stainless steel kicker bars to providedurability in the structural connection between the pile cap and piles. Theconstruction of the pile cap itself is consistent with the construction of thecontinuous strip method of underpinning.

Following completion of the works the agreed monitoring measures should be installedand reviewed at intervals and over a period agreed with the asset steward to ensurethat the underpinning measures are effective.

CIRIA C656 323

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AA66..1166 WWaatteerrpprrooooffiinngg aanndd ddrraaiinnaaggee iimmpprroovveemmeennttss

SSuummmmaarryy

DDeessccrriippttiioonn

Waterproofing of masonry arch bridges is an important preventative repair measure, aswithout a satisfactory waterproofing layer and efficient drainage an arch can deterioratevery rapidly. Two types of waterproofing systems commonly used are bonded or looselaid systems. It is normally preferable to use a bonded system as any subsequentdamage or puncturing of the membrane will not result in total failure of the system butwill be contained locally and be more easily repaired.

However, bonded systems are not normally suitable for arch structures unless aconcrete saddle or concrete screed is provided. It is more common to provide a looselaid system for arch structures, particularly within the railway environment as the timeavailable to undertake such works is normally short.

PPuurrppoossee

The purpose of waterproofing of arch structures and providing an effective drainagesystem is to prolong the structures life through prevention of water ingress into thestructural elements. Water ingress invariably results in the break down of the structural

CIRIA C656324

TTeecchhnniiqquuee ssuummmmaarryy

WWaatteerrpprrooooffiinngg aanndd ddrraaiinnaaggee iimmpprroovveemmeennttssProvision of an effective drainage system, comprising either a bonded orloose-laid waterproofing membrane above the arch, to prevent water ingressinto the structural elements thus prolonging the bridge’s serviceable life andreducing its maintenance requirements.

CCoossttss ���

SSppeecciiaall ccoonnssiiddeerraattiioonnss

Engineering considerations – see Table 4.4 in Section 4.3.4.Temporary Works design for deep excavations.Alternative waterproofing systems.Health and safety – see Section 4.1.

HHeerriittaaggee aannddeennvviirroonnmmeenntt

Repair should not be visible and therefore heritage issues are likely to beminimised but may affect hoppers & down pipes are fitted to the elevations.Permission will still be required for work on protected structures – seeSection 3.5.1. Original fill material may have archaeological interest.Consider potential for damage to/disturbance of protected species or theirhabitat and environmental pollution – see Section 3.5.2.

DDuurraabbiilliittyyProvided the waterproofing works are undertaken in accordance withmanufacturer’s instructions the life expectancy of a bonded system would bein the region of 40 years and around 25 years for a loose laid system.

IInnssppeeccttiioonn

Examination of all structures in undertaken periodically in accordance withthe asset owners requirements. Damage or failure of the waterproofingsystem would be evident either as a damp/wet patch or water drips in wetweather, or by the development of staining/leachate deposits on masonrysurfaces.

PPeerrffoorrmmaanncceeEffective implementation will allow the structure to continue to perform asoriginally designed.

RReeffeerreennccee//ffuurrtthheerrgguuiiddaannccee

Network Rail Company Standards RT/CE/C/001, Waterproofing of undertrackbridge decks and RT/CE/S/041, Waterproofing system for underline bridgedecks.Relevant British and Highways Standards.

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building elements and together with chemical activity, perishes the bonding mortar.This occurs typically when waterproofing to an arch fails and water runs through thearch lining resulting in mortar washout.

Brickwork/masonry when saturated and subjected to freeze-thaw cycles will deterioratein the form of spalling and combined with mortar loss may result in loss of structuralperformance of bridge elements.

DDeessiiggnn ccrriitteerriiaa

BBoonnddeedd ssyysstteemmss::

� bonded waterproofing systems are normally used on concrete or steel surfaces &are not usually applied to masonry arch structures, as the backfill materials wouldnot normally be suitable for bonded systems. A concrete saddle, slab or screedwould be necessary before consideration could be given to the application of thissystem to masonry arch structures

� bonded systems should be applied in accordance with manufacturers instructionsand can be liquid applied or as a preformed sheet membrane bonded to thesubstrate

� sufficient falls in the substrate should be provided to allow water to freely drain offthe structure and into a deck end drainage system.

LLoooossee--llaaiidd ssyysstteemmss::

� fill materials should be covered with a layer of geotextile material as protectionprior to laying of the waterproofing membrane

� any large debris or sharp objects that are likely to be present a risk of damage tothe membrane should be removed. The substrate may be made smooth by blindingwith a layer of sand to give sufficient cover

� the waterproofing membrane is loose laid onto the substrate and terminated inaccordance with manufacturers’ standard details. The membrane should be laidwith no voids beneath and minimising the number of membrane welds required

� the waterproofing membrane is normally extended beyond the length of thebridge where it will terminate into a suitable drainage system. For multi-archbridges or viaducts drainage provisions may be provided at each pier location

� the upper surface of the waterproofing membrane should be protected using anadditional protective layer of geotextile material

� sufficient falls in the substrate should be provided to allow water to freely drain offthe structure and into a deck-end drainage system.

IImmpplleemmeennttaattiioonn

When undertaking waterproofing works on arch structures it may not be possible orpreferable to fully close an operational highway or railway line. In this case it may befeasible to undertake the works under a half road closure for highway bridges andsimilarly waterproofing railway structures over a number of consecutive track closures.The number of road, railway or canal closures that are required needs to be establishedby the project team at an early stage.

CIRIA C656 325

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Other aspects that need to be considered at planning stage include possibleenvironmental effects of works, planning consent issues, management of wastematerials, services that may be affected and logistics of undertaking the works.

For bonded waterproofing systems it is important that the preparation of concretesurface is in compliance with the manufacturers requirements. Normally a U4 finish isrequired and the surface should be free of any dirt or other contaminants. An adhesiontest is undertaken to confirm that minimum tensile adhesion strength is achieved forconcrete surfaces.

Prior to application of the waterproofing membrane a primer is applied by airlessspray, brush or roller at a coverage rate in accordance with manufacturersrequirements. Once the primer has cured the waterproofing membrane can then beapplied. No further preparation is normally required other than removing any dirt orcontaminants from the primer.

A tack coat is usually provided to the cured waterproofing membrane on bridgescarrying highways where an additional protection layer of sand asphalt or hot rolledasphalt is provided. For railway bridges geotextile sheets are provided to protect thewaterproofing membrane from possible damage caused by ballast tamping.

Integrity tests of the applied membrane can be undertaken through use of non-destructive electronic test method for identifying any defects such as pin-hole withinthe membrane. This test should be undertaken prior to the application of any tack coat.

FFiigguurree AA66..88 BBrruusshh aapppplliiccaattiioonn ooff bbiittuummeenn ttoo uuppppeerr ssuurrffaaccee ooff ccoonnccrreettee aarrcchh ssaaddddllee

Loose laid waterproofing systems are mainly used on masonry arch structures and inparticular over structures carrying railway lines. The primary reasons for using suchsystems include their suitability for usage under a wide range of weather conditionsand quickness of application when working under tight bridge closure constraints.

Under full or partial closure of the bridge the deck and backfill materials are removedto the appropriate level with care being taken to remove any large debris or sharpobjects that are likely to present a risk of damage to the waterproofing membrane.Alternatively the substrate may be made smooth by blinding with a layer of sand, whichshould be laid at a suitable depth to provide sufficient cover to the debris. The fillmaterial should be profiled to a suitable gradient with drainage provision at deck ends& possibly additional drainage provision at pier locations for viaducts. Care should be

CIRIA C656326

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taken to ensure that the depth of the fill materials is at a level which will provideadequate cover to the waterproofing membrane, when applied.

Prior to application of the waterproofing membrane it is sometimes good practice toprovide a layer of geotextile protection over the fill material, where there is large debrisor sharp objects present. The waterproofing membrane can be usually laid in allweather conditions and can be off site or site welded using hot rollers. It is terminatedin accordance with manufacturers standard details to bridge parapets and to drainagepipes at deck ends. The position of the deck end drainage pipe is important,particularly its depth in relation to the adjacent arch springing level. Carefulconsideration needs to be given to this at design stage as it is important to terminatethe waterproofing membrane as low a level as practical, otherwise there is risk thatwater may seep to the lower arch area.

Once the waterproofing membrane has been laid it is important to protect it fromdamage by covering it with a protective geotextile layer. For railway bridges where it isnot possible to provide a minimum ballast depth of 300 mm an additional layer ofgeotextile can be provided. Care should also be taken when replacing either fillmaterial or ballast using mechanical plant. The fill/ballast should be laid progressivelyover the protected membrane such that any plant drives over the fill/ballast and not thewaterproofing membrane.

FFiigguurree AA66..99 LLoooossee llaaiidd ssyysstteemm aanndd pprrootteeccttiivvee ggeeootteexxttiillee mmeemmbbrraanneess bbeeiinngg pprroovviiddeedd ffoorr mmaassoonnrryy aarrcchhssttrruuccttuurree

CIRIA C656 327

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CIRIA C656328

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RReeffeerreenncceess

ABDUNAR, C (1995)Direct assessment and monitoring of stresses and mechanical properties in masonry arch bridgesArch bridges, C Melbourne (ed), Thomas Telford Ltd, pp 327-335, ISBN 0727720481

ADDIS, B and TALBOT, R (2001)Sustainable construction procurement. A guide to delivering environmentally responsible projectsC571, CIRIA, London

ALLEN et al (2003)Hydraulic lime mortar for stone brick and block masonryDonhead Publishing, London, ISBN 1873394640

AMDE, A M; MARTIN, J V and COLVILLE, J (2004)The effects of moisture on compressive strength and modulus of brick masonry13th Int. Conf. on brick and block masonry, Amsterdam, July 2004, pp 65-72

ANTHOINE, A (1992)In-plane behaviour of masonry: A literature reviewReport EUR 13840 EN, Commission of the European Communities, JRC – Institute forsafety technology, Italy

ASHURST, J (1988)Practical building conservation: Stone masonryEnglish Heritage, UK

ASHURST, J and ASHURST, N (1988)Mortars, plasters and rendersPractical Building Conservation. Vol 3, Halsted Press, New York

ASHURST, J (1990)Mortars for stone buildingsConservation of building and decorative stone – Volume 2 (Ashurst, J and Dimes, F G),Reed Educational and Professional Publishing Ltd

ASHURST, J (1997)The technology and use of hydraulic limeThe Building Conservation Directory<http://www.buildingconservation.com/>

ASHURST, N (1994)Cleaning historic buildings. Volume one: Substrates, soiling and investigation. Volume two:Cleaning materials and processesDonhead Publishing, London

ASTM (1991a)C1196-04 Standard test method for in situ compressive stress within solid unit masonry estimatedusing flatjack measurementsStandard C 1196-91, ASTM

ASTM (1991b)C1197-92(1997) Standard test method for in situ measurement of masonry deformabilityproperties using the flatjack methodStandard C 1197-91, ASTM

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BAGGI, V (1926)Road and hydraulic constructions: Part one – road constructionsUTET, Italy

BAKER, M J (1973)Reliability considerations in structural design: A state of the art reportTN050M, CIRIA, London

BAKER, I O (1909)A treatise on masonry constructionJohn Wiley & Sons New York, New York

BCT (2002)Bats, development and planning in EnglandThe Bat Conservation Trust, professional support series information sheet<www.bats.org.uk>

BCT (2003)Bats and bridgesThe Bat Conservation Trust, professional support series information sheet<www.bats.org.uk>

BDA (2001)Observations on the use of reclaimed clay bricksProperties of brick and mortar generally, No 1.4, Brick Development Association (BDA)<http://www.brick.org.uk/publications/PDFs/reclaimed_clay_bricks.pdf>

BDA (2001a)Use of traditional lime mortars in modern brickworkProperties of bricks and mortar generally, No 1.3, Brick Development Association(BDA)

BGS (2001)Building stone resources of the United KingdomPublished by the British Geological Society (BGS)(This is a large fold-out map with references to types of stone, quarry sites etc useful for resourcelocation) <http://www.bgs.ac.uk/mineralsuk/minequar/stones/home.html>

BIA (1967, reissued 1986)Structural design of brick masonry archesTechnical notes 31A, The Brick Industry Association (BIA), Reston, Virginia, USA<http://www.brickinfo.org/bia/technotes/t31a.htm>

BOUABAZ, M and HORNER, M W (1990)Modelling and predicting bridge repair and maintenance costsBridge management, Taylor & Francis Ltd

BRE (1995)Masonry and concrete structures: measuring in situ stress and elasticity using flat jacksBRE Digest 409, Building Research Establishment (ISBN 1860810330)

BRE (1997)Selecting natural building stoneDigest 420, Building Research Establishment, Garston, Watford

CIRIA C656330

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BRENCICH, A and COLLA, C (2002)The influence of construction technology on the mechanics of masonry railway bridges5th Int. Conf. Railway engineering, M C Forde (ed), London, 3-4 July 2002,Engineerings Technics Press, proceedings on CD-Rom and book of abstracts<http://prinpontimuratura.diseg.unige.it/>

BROOKS, J J and ABU BAKER, B H (1998)The modulus of elasticity of masonryMasonry International, 12, No 2, 58-63

BROOMHEAD, S F and CLARK, G W (1995)Strengthening masonry archesBridge modification, B Pritchard (ed), Thomas Telford Ltd, London, pp 174-184,ISBN 0727720287

BROOMHEAD, S F (1991)Masonry structures project: Survey of arch repair techniquesBritish Rail Research (BRR) Technical Report RR CES 006

CADW (2003)The repair and preservation of historic masonryTechnical Conservation Note 1

CHAPMAN, S and FIDLER, J (2000)Directory of sands and aggregatesPublished by English Heritage in association with Donhead Publishing, London,ISBN 1873394225

IRVINE, D J and SMITH, R J H (1983)Trenching practice (second edition)R97, CIRIA, London

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BS 3921:1985 British standard specification for clay bricks

BS 1377-9:1990 Methods of test for soils for civil engineering purposes. General requirementsand sample preparation

BS 5400-4:1990 Steel, concrete and composite bridges. Code of practice for design of concretebridges

BS 1217:1997 Specification for cast stone

BS 7913:1998 Guide to the principles of the conservation of historic buildings

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BS 5930:1999 Code of practice for site investigation

BS 5628-2:2000 Code of practice for use of masonry. Structural use of reinforced and prestressedmasonry

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