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6 May 2003 – The Structural Engineer|15

technical update: thaumasite

Following the identification of the

thaumasite form of sulfate attack

(TSA) on the concrete foundations

to a number of bridges on the M5

motorway in 1998, the then Minister

for Construction, Nick Raynsford,

established the Thaumasite Expert

Group (TEG) under the chairmanship

of Les Clark, Professor of Structural

Engineering, University of 

Birmingham. It was asked to report on

the nature and threat of this phenome-

non and to provide interim guidance on

its avoidance.

The Report of the TEG1 was

published by the Department of 

Environment, Transport and the

Regions (DETR) in January 1999, and

a summary of the structural implica-

tions of TSA was published in The

 Structural Engineer the following month2. At the Minister’s request, the

Report was followed up in Spring 2000

by a One-Year Review3 of the Report’s

factual statements and guidance in the

light of new developments on TSA.

 A second review4 of the Thaumasite

Expert Group Report has now been

undertaken following 3 years experi-

ence up to March 2002. Following the

demise of DETR, this review was

accepted by the Office of the Deputy

Prime Minister (ODPM) in January

2003. The principal sections of the

Review are summarised in a paper in

the journal Concrete prepared by theGroup5. This article gives a brief 

summary of the main conclusions of 

the Review and highlights a structural

issue.

Current impacts and awarenessThe TEG Report continues to be well

received by the construction industry

and is broadly viewed as well balanced,

safe and robust. Its publication has not

had a significant adverse impact on the

 various stakeholder sectors.

In respect of existing buried concrete

construction, the greatest impact of 

TSA to date has been on the Highways Agency (HA). The HA is undertaking a

strategic assessment of structures in

its care to check for TSA. Its experience

has shown that where the risk factors

identified in the TEG Report are all

present, the prevalence of TSA is high,

although the severity varies.Where

necessary, remedial measures involv-

ing concrete repair and protection have

been undertaken, providing assurance

of serviceability and safety.

The TEG is concerned that no other

owners of existing structures with

buried concrete elements have under-

taken a similar assessment. The expe-

rience of the HA indicates that there

could be a large number of structures

with some degree of attack to buried

concrete. It is therefore important that

owners of key structures do assess the

risks and take necessary measures.

New guidance A substantial body of new guidance

has been published in 2000 – 2002.

This has evolved from the TEG Reportbut has taken into account industry

consultations and new research find-

ings. Publications include a new BRE

Special Digest 1: Concrete in aggressive

 ground6, Amendments to BS 5328:

Concrete7, Amendments to BS 882:

 Specification for aggregates from

natural sources for concrete8, new

BS 8500: Concrete – complementary

 British Standard to BS EN 206-19, and

a new BCA Guide: Concrete resistant to

chemical attack10.

There have been a few problems in

interpreting and/or implementing the

intentions of the guidance given in theTEG Report in some situations. These

problems have been addressed through

publication of the new guidance listed

above and through discussions with

the relevant industry bodies. In partic-

ular, problems were encountered by the

manufacturers of precast concrete

construction products, including those

for precast concrete pipes and tunnel

linings, in complying with the TEG

Recommendations. These have been

resolved by the inclusion in the new

BRE SD16 of specific design guidance

for these products.

Guidance in preparationKey items of guidance which have been

prepared for publication in 2003 are:

• A second edition of BRE SD1 to align

included cements or combination

groups with the new BS EN 197-

11111 classification.

• A revision of the NHBC Standards,

Chapter 2.1, Concrete and its rein-

forcement12.

New field cases of TSA About 50 new cases of TSA have been

identified since the publication of the

TEG Report and 30 since the publica-

tion of the first review in 2000. Most of 

these were in the foundations of 

bridges founded on Lower Lias Clay

(Charmouth Mudstone Formation),

found as a result of HA investigations

in Gloucestershire, Somerset and

Wiltshire. The remaining cases have

occurred in a wide range of structures,

buildings and environmental condi-

tions. Most of these new cases have

taken place in conditions that were

anticipated in the TEG Report.

Significant features which have

emerged from inspection of these new

cases of TSA are:

• Three further sources of sulfate in

the ground have been identified

leading to TSA: Kimmeridge Clay,

Rheatic Mudstone and railway ash.

• Two new field cases of TSA in buried

concrete containing siliceous aggre-

gates, reinforcing a concern

expressed in the TEG Report that

concretes containing little or no

carbonate in aggregates can be

affected by TSA if an external source

of carbonate ions is available, for

example from groundwater. This is of 

significance since currently the guid-ance for specification of concrete

presumes that restriction of the

carbonate content of aggregate is an

effective measure for combating 

TSA. Present understanding of the

problem is insufficient to underpin

amendment of the guidance, but

further research is underway.

• Investigations have concluded that

bituminous waterproofing applied to

some of the buried concrete exposed

to sulfates may have prevented the

onset of TSA.

Research on occurrence andmitigation of TSARecommendations for future research

given in the TEG Report are being 

largely met by projects initiated by the

BRE, the universities and industry.

Initial findings of particular signifi-

cance are:

• The mix designs recommended in

the TEG Report are robust.

• Significant deterioration in some

concretes not meeting the recom-

mendations in the TEG Report have

been observed in both laboratory and

field trials after as little as 3 years

exposure to sulfate Class AC-3conditions.

• Mixes containing ground granulated

blastfurnace slag (ggbs), in sufficient

proportions, continue to demonstrate

excellent resistance to TSA.

New guidance andprogress on

thaumasite control A summary the findings of the Thaumasite Expert GroupReport: ‘Review of structural aspects after 3 years experience’

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