Innovative Ductile Fiber Reinforced CementiousComposites for Structural Applications

(Materials, Analysis, Design & Industrial Scope)

Prof. Shamsher Bahadur Singh, Ph.D., PDF, PE (Mich., USA)

Civil Engineering Department

Birla Institute of Technology and Science, Pilani -333031

E-mail: sbsingh@bits-pilani.ac.in; sbsinghbits@gmail.com

Web: (http://discovery.bits-pilani.ac.in/Homepage/disciplines/civil/sbsingh/sbsingh.htm)

Brief Bio-data

S.B. Singh is an ICI member and Professor &Head of Civil Engineering Department at BirlaInstitute of Technology and Science (BITS), Pilani.His current areas of research are development ofdesign guidelines for Fiber Reinforced Polymer(FRP) reinforced prestressed concrete structures inparticular and composite structures in generalincluding nonlinear finite element modeling. He hasbeen Indian Team Leader for prestigious UKIERIcollaborative research project on sustainableconcrete Infrastructure. He is also member ofvarious committee and serving as editorial boardmember of various Journals. Currently, he isrepresenting BITS Pilani on committee of Ireland-India Concrete Research Initiatives (IICRI).

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Content

(1) Fiber Reinforced Polymers (FRP) – Analysis, Design and

Applications

~ Scope for composite manufacturing industries

~ Scope for structural engineering Consulting companies

~ Scope for engineering construction companies

~ Scope for structural repair and rehabilitation companies

(2)Engineered Cementitious Composites (ECC) (Ductile

Concrete) – Applications

~ Scope for concrete industry

Fiber Reinforced Polymers (FRP) (Analysis, Design and Applications)

Introduction - Time Line

World War II : The evolution of FRP materials started.1960s- The combination of high strength, high stiffness, less density and lowcost FRP materials such as boron, aramid and carbon were commercializedfor air travel and space exploration

1970s :The cost of FRP materials continues to decrease and the aggressive infrastructural renewal has started

Late 1980s and 1990s :The funding agencies encouraged research for FRP materials for infrastructure development

2000 onwards : Many projects have been successfully completed and structures are performing efficiently as demonstration projects

Ref: (ACI 440-96)

FRP Materials:Introduction

Fiber Reinforced Plastics(FRP) is a compositematerial comprising apolymer matrix reinforcedwith fibers

Fibers are usuallyfiberglass, carbon, oraramid, while the polymeris usually an epoxy,vinylester or polyesterthermosetting plastic

High tensile strength to weight ratio

Non-corrodible characteristics

Chemically inert properties

Non-sensitive to magnetic effects

But, linear elastic behavior till rupture

Ref: (ACI 440-07)

Scope for Composite Manufacturing Industries

FRP Products

Ref: (ACI 440-07)

Composite Fabrication Hand Lay-up

Ref: (ACI 440-96)

Fibers in the form of unidirectional mats, fabric or braid are cut and laid up to produce laminate

For small quantities of complex and/or high quality parts. Very labor-intensive and thus expensive

Until recently, even high volume parts for aerospace applications were produced by this process

This process has been partially automated. Woven or Non-woven fabric are used with this process

Produces laminates with relatively higher fiber volume fractions (50-60%) and low void contents (1-2%). Used to make fly rods and most golf club shafts

Filament Winding

Ref: (ACI 440-96)

Fibers are pulled from single or multiple continuous fiber - spools and passed through a resin bath

The primary advantage of the filament winding process is high processing speed (i.e., up to 700 lbs of material/hr) -resulting in a low cost. In spite of higher capital costs, cost of filament wound parts can be one third of that of hand lay-up

Examples:

Rocket motor casings

Pressure vessels

Light poles

Oil pipes

Aircraft fuselages

Pultrusion

Ref: (ACI 440-96)

Continuous fiber reinforcement is impregnated with resin by passing through a resin bath. The impregnated fibers are pulled through a forming die, consolidated, cured, cooled quickly, and cut to length; all as a continuous automated process

Production speeds are usually two to four feet per minute

Pull forming which allows changing cross sections and fabrication of curved parts.

Examples:

I-beams, Rebar

Prestressing strand

Twisted cables

Automotive shafts

Development of Ductile Fiber Reinforced Polymer (DFRP)

Development of Ductile Fiber Reinforced Cementitious Composite (DFRCC)

Investigation on various strengthening patterns applied to masonry structures

Development of design guidelines

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INTRODUCTION AND CHARACTERISTICS OF FRP

FRP is a special type of two-component composite material consisting of high-strength fibers embedded in a polymer matrix.

High tensile strength ( can reach upto 3000 MPa)

Low density ( 1800 kg/m3)

High modulus (300-400 GPa)

Resistant to corrosion

Good dimensional stability (extremely low coefficient of thermal expansion)

Outstanding fatigue characteristics

Electromagnetic neutrality

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THEN

WHY GO

FOR

HYBRID FRP ??

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Lack of Ductility

Current FRP bars behave linearly

elastic till rupture

No early warning of

failure

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In Hybrid FRP Bars :

The proposed hybridrebar consists of differenttypes of fibers which failat different strains duringthe load history of therebar, thereby allowing agradual failure of therebar.

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REDUCTION OF COST

Carbon fibers have high ultimate tensile strength but are very expensive. In order to make the FRP reinforcements economically viable, it is fabricated by combining with other cheap fibers such as glass fibers and/ or metallic fibers. Glass fibers have UTS less than carbon but are much cheaper than carbon fibers.

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Deliverables Planned at BITSPilani

Development of Low cost FRP Manufacturing Techniques

Development of Viable Anchorage Systems for Tensile Tests of FRP Specimens

Development of Ductile FRP Rebars, fabrics and Plates

Development of Precast DFRCC Wall Panels and Plates/Strip

Development of Design Approach for FRP Strengthened Masonry Structures (Beams and Columns)

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Methodology

Stage 1: Develop Ductile Fiber Reinforced Polymer (FRP) rebars, fabrics, Ductile Fiber Reinforced CementitiousComposite (DFRCC) wall panels, plates and examine their mechanical properties for structural applications

The project has been planned to be executed in four stages as described herewith.

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Methodology (Contd..)

Stage 2: Investigate the behavior of masonry columns externally strengthened using newly developed ductile FRP rebars, fabrics and DFRCC plates in compression and flexure.

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Methodology (Contd..)

Stage 3: Examine thebehavior of masonry wallsexternally strengthenedusing newly developedductile FRP rebars,fabrics and DFRCCplates in compressionand flexure.

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Methodology (Contd….)

Stage 4: Preparation of technical papers, Submission of research findings and Completion of Report

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Fabrication of Hybrid FRP BarsFRP bars with a diameter of 10 mm were designed and made by hand lay-up process.

Six types of bars were manufactured.

Carbon FRP

Glass FRP

Hybrid- 70% glass & 30% carbon

Hybrid- 50% glass & 50% carbon

Hybrid- 60% glass & 40% carbon

Hybrid- 80% glass & 20% carbon

Bars having a length of 2.4 m and a nominal diameter of 10 mmwere manufactured with 76 longitudinal fiber ribbons and Epoxy wasused as resin. Hardener dose of 7% by weight was added to Epoxyfor acquiring sufficient strength, hardening and curing purpose inwinter season.

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Tensile Testing of Bars

Tensile testing was done on Universal Testing Machine

The length of specimen is taken as 600 mm.

Anchors were prepared for these specimen to ensure proper grip. Anchors were prepared with mild steel pipes with 20mm diameter & 200mm length and attached to the

specimen using epoxy.

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Uni-axial Tensile Strength

CharacteristicsS. No. FRP Type Diameter, mm

(Area, mm2 )

Failure Load, kN Tensile

Strength, MPa

1 CFRP 8 (50.27) 51.54 1025.4

2 GFRP 9 (63.62) 32.26 507.1

3 Hybrid FRP

(30% CFRP

plus 70%

GFRP)

9 (63.62) 43.23 679.4

Tensile strength of the hybrid FRP bar (30%CFRP and 70% GFRP) is 66.3% of carbon FRP bar and 170% of the strength of steel(400MPa).Thus, it could be concluded that the fabricated hybrid bar possess reasonable strength (>400 MPa) while maintaining the benefits of composite characteristics.

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Beam Specifications

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Grooving of Beams

Mounting of NSM FRP Bars

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Mix Proportion of DFRCC Beams made of ECC

Cement Silica

Sand

Fly-Ash Water Plasticizer PVA fiber

452 kg/m3 452 kg/m3 452 kg/m3 199 kg/m3 9.03 kg/m3 20 kg/m3

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Flexural Testing of NSM Hybrid FRP Reinforced ECC Beams

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60% GFRP- 40% CFRP

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Flexural Response of DFRCC Beams (ECC Beams)

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Achievements

Development of FRP fabrication Equipment for

processing with hand

Development of Tensile Anchor

Systems

Development of Ductile Hybrid FRP System and DFRCC

Beams

Demonstration of Effectiveness of Hybrid FRP System for low cost DFRCC

Beams

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Scope for Structural Engineering

Consultancies

Prestressing with FRPs

FRP materials exhibit several properties including high tensile strength, which make them suitable for the use as structural reinforcement and prestressingtendons

FRP tendons for prestressing is the ability to configure the reinforcement to meet specific performance and design objectives

FRP tendons may be configured as rods, bars, and strands and characteristics of an FRP tendon are dependent on fiber and resin properties, as well as manufacturing process

FRP tendons are produced from a wide variety of fibers, resins, shapes, and sizes. Especially carbon fibers, however, are recommended for prestressingapplications, since glass fibers have poor resistance to creep

Tensile stress-strain behavior of various reinforcing fibers and tendons

Ref: (ACI 440-4R-04)

General Design Considerations

Allowable stresses in FRP tendons are limited to 40 to 65% of their ultimate strength due to stress-rupture limitations

During the overall design a prestress level of 40 to 50% of the tendon strength is selected as initial stresses and service stresses are checked

If the section is sufficient, the flexural design is over. Otherwise, it is prescribed to increase the number of tendons rather than stress level in tendons

Nonprestressed FRP rods can be used to increase the strength

Typical cross-sections for DT-beam bridge with bonded and unbonded tendons

Ref: Grace and Singh (2003): ACI Structural Journal

Cross-sections for Box Beam bridgewith bonded and unbonded tendons

Ref: Grace, Singh, Mathew, Shenouda (2004), PCI Journal

Design and Analysis of FRP Prestressed Beams

Design approach

1. Balanced

Section

2. Reinforcement

Ratio

3. Cracking

Moment

4. Flexural

Capacity

a. Strains and

stresses in

tendons

b. Internal

forces in the

section

c. Nominal

moment

capacity

Ref: Grace and Singh (2003): ACI Structural Journal

Experimental Studies and Validation of Design Approach

Ref: Grace and Singh (2003): ACI Structural Journal

Full Scale test of Double Tee Beams

An experimental study was conducted in Lawrence Technological

University (LTU), Michigan, USA

A special purpose computer program was developed to compute the overall

response of the beams such as deflections, strains, cracking loads,

and post-tensioning forces.

The design equations and the accuracy of the nonlinear computer

program were validated by comparing the analytical results with

experimental results from a full-scale Double-Tee (DT) test beam

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Load Vs. Deflection Response

Ref: Grace and Singh (2003): ACI Structural Journal

Energy Ratio Based Ductility Index

Ref: Grace, Singh, Mathew, Shenouda (2004), PCI Journal

Scope for Engineering Construction Companies

Successful Deployment of CRRP PrestressingTechnology

Bridge Street Bridge, Southfield,MI,USA(First vehicular FRP

prestressed bridge in USA)‏

Bridge Street Bridge, City of Southfield, Michigan,USA( Construction Site)‏

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Scope for Structural Repair and Rehabilitation Companies

An experimental study:

Examined the efficiency of the CFRP plates in strengthening of the RC beams.

All flexural specimens loaded to a predetermined cracking load to simulate a deficient structure. Strengthened with CFRP plates using the proper epoxy adhesive. Specimens were then loaded to failure.

Significantly improved the load carrying capacity of the beams. Debonding of the plates and concrete shear failure were failure modes in all of the strengthened beams

Strengthening In Flexure

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Strengthening of Negative Moment Regions of RC Beams

An experimental study:

The first strengthening of beams was designed to fail in flexure, while the second dealt with strengthening of beams designed to fail in shear

Figure shows failures in some of tested beams by debonding of the plates

The maximum stress experienced in the CFRP plates was observed to be 52% of their ultimate load carrying capacity for the beams designed to fail in flexure and 28.5% for the beams designed to fail in shear

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Shear Strengthening

An experimental study:

The strengthening of beams in shear was dealt with using three different lay-ups (45°, 0°/90°, and 0°/90°/45°) of CFRP fabric sheets on the beams

A total of four beams were tested while the fourth beam was unstrengthened and served as the control beam.

It was also noted that there exits a critical value of shear force up to which there is no appreciable strain in the beam

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Shear Strengthening

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Scope for Concrete Industry (Engineered Mortar/Precast/Ready-mix)

Engineered Cementitious Composites

(Ductile Concrete)

~ Introduction~ Precast Products

~ Micromechanics Based Design of ECC~ Structural Response of FRP/Steel reinforced

ECC beams

How ECC differs from FRC ?How ECC differs from FRC ?

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Precast ECC Products

ECC Ductile plate ECC conoe

Extrusion of a 100mm ECC-pipe

Research Significance

+

=

Brittle FRP materials

Ductile ECC

High flexural strength & Ultra-high ductility

of

structures

Design Approach

Fiber Reinforced Plastics (FRP)Reinforced

Engineered CementitiousComposite (ECC) beam

Verification of Present Analytical Models

Flexural load vs. deflection response has been evaluated analytically

A special purpose computer program was developed

Computer program incorporates the tensile strain hardening behavior of PE-ECC and considers its tensile load carrying capacity

Strain controlled approach

Weighted‏average‏of‏Young’s‏modulus / Numerical integration of the curvature

Comparison of Load vs. Deflection Curves Beam-GRE16

The curtailment of the simulation is governed by rupture strain

of GFRP bars

Difference in peak

load = 13.1%

Difference in

deflection = 1.6%

Experimental Investigation on ECC

Demonstration of DFRCC as Strengthening Structural Material

Effect of DFRCC Plastic Hinge

Length

Effect of Thickness of

External DFRCC Layer

Response of RC Frames

strengthened with DFRCC

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Beam Design

Over Reinforced Beam

Under Reinforced Beam

Loading Pattern

Effect of DFRCC Plastic Hinge Length & Thickness of DFRCC Layer

Effect of Plastic Hinge Length

Effect of ECC Layer Thickness

Response of RC Beams Strengthened with DFRCC Plastic Hinges

In the present study, a number of parameters such as

Compressive strength of DFRCC

Compressive strength of concrete

Length of plastic hinge

Load span of control beams and load span of strengthened beams

were maintained different for each beam specimen so as to incorporate the

diverse effects of materials and geometry. The load versus deflection

response, the applied load was normalized with a parameter. Normalized

Load Parameter is given below

XParameter

loadAppliedY

,

spanOverall

spanLoad

spanOverall

lengthhingePlastic

f

fXParameter

ecc

ck 1,'

'

Response of RC Over-Reinforced Beams

Strengthened with DFRCC Plastic Hinges

ORPH 600 Load capacity= 6.34 times of the control beam

ORPH 400 Load capacity=4.24 times of the control beam

ORPH 200 Load Capacity= 2.85 times of the control beam

Failure of RC Over-Reinforced Beams Strengthened with DFRCC Plastic Hinges

Ultimate failure of all over reinforced

beams (ORPH 200, ORPH 400 and ORCB) was governed by flexural compression except ORPH 600.

Ultimate failure of ORPH 600 was governed by

one single flexural crack at tension side

Response of RC Under-Reinforced Beams Strengthened with DFRCC Plastic Hinges

URPH 600 Load Capacity= 4.83 times of control beam

URPH 400 Load capacity =1.72 times of control beam

URPH 200 Load Capacity= 1.55 times of control beam

Failure of RC Under-Reinforced Beams Strengthened with DFRCC Plastic Hinges

Ultimate failure of all UR beams was governedby flexural tension. Beams URPH 200 andURPH 400 exhibited multiple microcracks atECC plastic hinge zone.

But beam URPH 600 did not show multiple cracks and its failure was similar to beam ORPH 600 with one single crack at tension side.

Response of RC Beams Strengthened with DFRCC Layer

In the present study, a number of parameters such as

Compressive strength of concrete

Thickness of DFRCC Layer

Load span of control beams and load span of strengthened beams

were maintained different for each beam specimen so as to incorporate the

diverse effects of materials and geometry. The load versus deflection

response, the applied load was normalized with a parameter. Normalized

Load Parameter is given below

MParameter

loadAppliedY

,

spanOverall

spanLoad

depthOverall

LayerECCofThickness

f

fMParameter

ecc

ck 1,'

'

Response of RC Under-Reinforced Beams Strengthened with DFRCC Layer

URLY 70 (Load capacity= 8.31 times of control beam)

URLY 50 (Load capacity=5.37 times of control beam)

Failure of RC Under-Reinforced Beams Strengthened with DFRCC Layer

Ultimate failure of allUR beams occurreddue to flexuraltension accompaniedby yielding of internalsteel rebars.

Unlike under reinforced ECC strengthened plastic hinge beams, the under reinforced layered ECC beams have not shown microcracks.

The failure was governed by a single flexural crack at the tension side.

Response of RC Over-reinforced Beams Strengthened with DFRCC Layer

ORLY 70mm (Load capacity =8.45 times of control beam )

ORLY 50mm (Load capacity=5.71times of control beam)

Failure of RC Over-Reinforced Beams Strengthened with DFRCC Layer

Ultimate failure of allOver-reinforcedstrengthened beamsoccurred due tocrushing of concreteat Compression side

Applications of DFRCC in RC Frames as Strengthening Material

Steel Reinforced DFRCC FramesTotal of Steel reinforced DFRCC

frames (6 Nos.)

Category I – 3 Nos. (Detailed as per IS 13920-2002)

Category II – 3 Nos. (Detailed as per IS 456-2000)

Fully DFRCC

Fully Concrete

Both DFRCC & Concrete

Frame Design Following IS 13920-2002

Frame Design Following IS 456-2000

Loading Pattern

Load vs. Deflection Response for IS 13920-2002 Frames

33 % More load carrying capacity & 2 times more ductile

21.8 % More load carrying capacity & less ductile*

Load vs. Deflection Response for IS 456-2000 Frames

0

6

37.2 % More load capacity & 1.5 times more ductile

27.2 % More load capacity & 2.8 times more ductile

Category Frame Yield

deflection,

mm

Ultimate

deflection,

mm

Ductility

factor

Failure

mode

1 CON 13920 6.36 11.77 1.85 Flexural compression

and moderately brittle

DFRCON

13920

6.93 8.06 1.16 Flexural tension and

local failure

DFR 13920 4.52 17.08 3.78 Flexural tension with

multiple micro cracks

and highly ductile

2 CON 456 4.49 5.44 1.21 Flexural compression

and highly brittle

DFRCON

456

4.35 14.65 3.37 Flexural tension and

highly ductile

DFR 456 6.0 11.11 1.85 Flexural tension with

multiple micro cracks

and ductile

CONCLUSIONS

Fiber Reinforced Polymers are potential constructionmaterials and they give lot of scope for various industries likeComposite, Structural Consultancies, ConstructionCompanies, Repair & Rehabilitation Companies andConcrete industries.

These materials always need performance based designapproaches for better exploitation of the material strengthand economy.

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CONCLUSIONS (Contd..)

NSM hybrid FRP rods provide a promising reinforcing technique to enhance theflexural capacity of ductile fiber reinforced cementitious composites (DFRCC) beamsfabricated with engineered cementitious composites (ECC) materials with significantwarning and cost effectiveness based on replacement of carbon fibers with low costglass fibers without compromising the strength of DFRCC beams. Among the varioustested beams, the strengthened ones showed an increase in capacity ranging from42% to 53% over the control beam, which demonstrates the effectives of NSM FRPbars for rehabilitation of beams under flexural loading. Moreover, it also exhibits thecost effectiveness by incorporating the hybrid configuration of FRP systems.

However, it appears that the bond strength between FRP bars and the beamis of critical importance for the effectiveness of this technique. Maximumstrength of FRP rods could not be achieved due to bond failure. Therefore, allthe different compositions show somewhat similar results. Hence, it may bepoint of future research to find the solution for preventing the debondingmode of failure of NSM bars reinforcing ECC beams

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CONCLUSIONS (Contd..)

Experimental study conducted on framedstructures has provided additionalconfidence to use DFRCC as potentialstrengthening material in RC structures

Further research programme of this project will focus ondeveloping hybrid FRP systems with improved ductileresponse and its applications for strengthening the masonrybeams, columns and walls for developing low cost masonrystructural elements. Moreover, a design approach will also bedeveloped to design the FRP reinforced ECC beams andmasonry structures (beams and columns).

CONCLUSIONS (Contd..)

Generalized stress-strain relationships are developed for PE-ECC in tension and compression. Using developed stress-strain relationships, a unified design approach is presented for flexural strength prediction of FRP reinforced PE-ECC beams.

Design equations derived herein predict the balanced ratio, depth of neutral axis section, moment carrying capacity which are in close agreement with experimental results available in literature. Close agreement between analytical and experimental results shows the robustness of the developed stress-strain models.

In general, for a specific size configuration, it is observed that for a given load, deflection is higher for the lower reinforcement ratio.

CONCLUSIONS (Contd..)

Conventional definition of section being under-reinforced or over-reinforced based on balanced ratio is not consistent with experimental results for ECC beams. Hence, more experimental investigations are required to investigate the correlation between flexural crushing strain and crushing strain from uni-axial test

Acknowledgements

DST – New Delhi (ECC Project)

CSIR- New Delhi (PostbucklingStrength of Laminated Composite

Plates)

UGC – New Delhi (NSM Project)

Aditya Birla Group Companies

&

UKIERI Research Collaboration

KK Birla Academy Projects, BITS-Pilani

1 August 2012 95Dr S. B. Singh