Hybrid fiber reinforced polymer (FRP) system of carbon FRP laminate and sprayed glass FRP for strengthening reinforced concrete beams

vi Spraying Process ...34 Effects of Specimen Orientation and Thickness...36 Specimen Preparation and Testing Setup ...36 Results and Discussion ...39 Effects of Fiber Length...39 Specim

LAMINATE AND SPRAYED GLASS FRP FOR STRENGTHENING REINFORCED CONCRETE BEAMS

By NINGFENG LIANG

A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL

OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT

OF THE REQUIREMENTS FOR THE DEGREE OF

DOCTOR OF PHILOSOPHY UNIVERSITY OF FLORIDA

2006

3228772 2007

UMI Microform Copyright

All rights reserved This microform edition is protected against unauthorized copying under Title 17, United States Code.

ProQuest Information and Learning Company

300 North Zeeb Road P.O Box 1346 Ann Arbor, MI 48106-1346

by ProQuest Information and Learning Company

Copyright 2006

by NINGFENG LIANG

To my parents and Hong

iv

First, I would like to thank Dr Andrew Boyd, my advisor, for his constant support and guidance in the entire course of this research The assistance by all the other

committee members to form the research content is truly appreciated as well

Second, without the collaboration with those lab and department staffs, it would be impossible for this research to be finished Specifically, my sincere thanks go to JJ, Chuck, Danny, George and Tony I really enjoyed working with all of them

Third, I really appreciate the generous donation of the CFRP laminates by Sika Ron made sure the materials arrived on time and in good shape The same appreciation also goes to MVP for donating the spraying equipment, resin and glass fiber Gregg provided invaluable technical assistance in operating the spraying equipment

Last, but not least, the friendship shown by Yanjun, Huamin, Xiaoyan, Yu Chen and Jeff will keep me cherishing the time I spent at the University of Florida for the rest

of my life Their encouragement and help made the days of hard work enjoyable

v

TABLE OF CONTENTS

page

ACKNOWLEDGMENTS iv

LIST OF TABLES viii

LIST OF FIGURES ix

LIST OF ACRONYMS xii

ABSTRACT xiii

CHAPTER 1 INTRODUCTION 1

General 1

Research Objectives 4

Thesis Structure 5

2 LITERATURE REVIEW 6

FRP Materials 6

Bond Strength of FRP-concrete Joint 9

Flexural Strengthening of RC Beams 12

Brief History 12

Failure Modes 15

Flexural Strength Models 18

Shear Strengthening of RC Beams 20

Introduction 20

Failure Modes 23

Shear Strength Models 23

Sprayed FRP 24

Design Guideline 26

Flexural Strengthening 26

Shear Strengthening 28

3 MATERIAL PROPERTIES 30

Preparation of Sprayed FRP 31

vi

Spraying Process 34

Effects of Specimen Orientation and Thickness 36

Specimen Preparation and Testing Setup 36

Results and Discussion 39

Effects of Fiber Length 39

Specimen Preparation 39

Results and Discussion 40

Effects of Fiber Content 43

Specimen Preparation 43

Results and Discussion 43

Concrete 44

Steel 45

Carbon Fiber Reinforced Polymer (CFRP) Laminate 45

4 BEAM STRENGTHENING WITH CFRP/SPRAYED GFRP HYBRID SYSTEM.48 Introduction 48

Preparation of Reinforced Concrete Beams 48

Beam Formwork 48

Beam Casting 49

Beam Testing Program 51

Concrete Beam Design 51

Strengthening Plan 52

Program Objectives 53

Strengthening Details 54

Simulated Steel Corrosion 56

FRP Strengthening of Beams 58

Load Test 62

Test Setup and Procedure 62

Instrumentation 64

Results and Discussion 66

Failure Modes 70

Energy Absorption 76

Beam Deflection Profile 79

Steel Strain 83

CFRP Strains 85

Sprayed GFRP Strains 87

Effects of Reinforcement 89

5 THEORETICAL ANALYSIS 91

Classical Beam Analysis 91

Material Models 92

Moment-Curvature Analysis 94

Calculation of Mid-span Deflection 101

vii

Load and Deflection 104

Material Strains 110

Steel strains 111

CFRP strains 115

Sprayed GFRP strains 117

6 DESIGN GUIDELINES 123

Notation 124

Design Procedure 126

Design Example 128

7 REPAIR OF A CONCRETE BRIDGE WITH CFRP/SPRAYED GFRP HYBRID SYSTEM 133

Introduction 133

Background 133

Bridge Conditions Before Repair 135

Strengthening Design 137

Load Test before Repair 137

Materials 138

Flexural Strength 138

Shear Strength 139

Installation of CFRP-sprayed GFRP System 141

Environmental Issues and Protection 141

Repair Work 144

8 CONCLUSIONS AND RECOMMENDATIONS 151

APPENDIX MATHCAD PROGRAM FOR THE CLASSICAL BEAM ANALYSIS 154

LIST OF REFERENCES 165

BIOGRAPHICAL SKETCH 173

viii

2-1 Typical tensile properties of fibers used in FRP systems 7

2-2 Typical mechanical properties of GFRP, CFRP and AFRP composites 8

3-1 Material properties of resin 31

3-2 Material properties of glass fiber 32

3-3 Typical laminate mechanical properties (Ortho polyester) .32

3-4 Effect of specimen orientation 39

3-5 Effect of sprayed FRP thickness 39

3-6 Effect of fiber length on material properties of sprayed FRP 40

3-7 Effects of fiber content on material properties of sprayed FRP .43

3-8.Tensile properties of reinforcing steel 45

3-9 Properties of CFRP 46

4-1 Beam configuration .52

4-2 Summary of beam test results 67

5-1 Theoretical beam results 105

6-1 Design nominal load-carrying capacity of strengthened beams 132

7-1 Materials properties-bridge repair 138

7-2 Nominal strength of bridge girders 141

ix

1-1 Typical flexural strengthening scheme for RC beams with FRP laminate 2

2-1 Bond test .10

2-2 A Typical flexural strengthening scheme for a RC beam with FRP laminate .12

2-3 Failure modes of beams FRP strengthened in flexure 16

2-4 Shear strengthening .20

3-1 Spraying equipment 33

3-2 Geometry of testing specimen .36

3-3 Specimen layout on sprayed FRP panel .37

3-4 Sprayed FRP tensile test setup 38

3-5 Typical stress-strain curve from sprayed FRP tensile test 38

3-6 Effects of fiber length .41

4-1 Partial assembly of a beam form .49

4-2 Beam casting site layout 50

4-3 Concrete truck delivering concrete to wooden forms 51

4-4 Concrete beam details 52

4-5 Beam strengthening schemes .55

4-6 Simulated damages .57

4-7 Sandblasting concrete beam surfaces .58

4-8 Setup for CFRP laminate bonding 59

4-9 Application of sprayed GFRP .61

x

4-11 Instrumentation on reinforcement steel .65

4-12 Instrumentation on beams strengthened with CFRP / sprayed FRP hybrid system 65

4-13 Undamaged beam failure modes .72

4-14 Damaged beam failure modes 74

4-15 Load–deflection curves of undamaged beams 77

4-16 Load-deflection curves of damaged beams .78

4-16 Load-deflection curves of damaged beams .78

4-17 Deflected shapes at beam failure .79

4-18 Variation of support rotation of undamaged beams .81

4-19 Variation of support rotation of damaged beams .82

4-20 Steel strains 84

4-21 CFRP strains in A-2 and A-4 86

4-22 CFRP strains in B-2, B-4 and A-4 86

4-23 SGFRP strains in A-3 and A-4 .88

4-24 Sprayed GFRP strains in B-2, B-4 and A-4 .89

5-1 Material models .93

5-2 Flexural analysis of RC section retrofitted with the hybrid FRP system .97

5-3 Graphic illustration for mid-span deflection calculation 101

5-4 Geometric representation of two-dimensional finite element model .102

5-5 Meshed ADINA model 104

5-6 Beam load-deflection curves .107

7-1 University boulevard bridge (NBI # 724214) 134

7-2 Vehicle weight limit for University boulevard bridge 134

7-3 A beam after removal of loose concrete cover .136

xi

7-5 Cross section of strengthened bridge 139

7-6 Containment for resin drum 142

7-7 Work platform .143

7-8 Turbidity barrier and plastic sheeting 144

7-9 Patched beam 145

7-10 Cleaning of CFRP laminate surface 146

7-11 Two installed CFRP laminates on a beam 147

7-12 Spray of another layer on hardened GFRP 149

7-13 Rollout of newly sprayed GFRP 149

xii

ACI: American Concrete Institute

CFRP: Carbon Fiber Reinforced Polymer

FRP: Fiber Reinforced Polymer

GFRP: Glass Fiber Reinforced Polymer

SGFRP: Sprayed Glass Fiber Reinforced Polymer

Cochair: H R Hamilton

Major Department: Civil and Coastal Engineering

This work is aimed to combine the potential of CFRP laminates in flexural

strengthening of reinforced concrete (RC) beams and that of sprayed glass FRP (GFRP)

in shear strengthening to form a better hybrid FRP system Only the flexural

strengthening effects of the hybrid system were investigated in this study

The sprayed GFRP also contributes to the flexural capacity of the strengthened beams Tension tests were performed to determine its tensile properties, in terms of tensile strength and stress-strain relationship These properties were shown to be affected

by glass fiber length and content, but not by the orientation and thickness of the

xiv

including the hybrid system and tested in lab by a three-point loading setup The results indicate that the hybrid system improved the load-carrying capacity of the beams but failed mostly in CFRP laminate debonding, a premature failure mode

A classical RC beam analysis and a 2-D finite element analysis were performed to predict the behavior of the strengthened beams The results agree well with the

experimental results in terms of load-carrying capacity

With the ‘plane section’ assumption and the ACI design guide for CFRP flexural strengthening, the effects of the sprayed GFRP were developed for possible incorporation into the existing design guide The resulting design of the hybrid system was shown to be conservative

A real-world case study of the hybrid system revealed the issues that have to be addressed before the system could be used effectively It also showed some technical difficulties related to the spraying operation

dramatically over the years due to a deeper understanding of the behavior of structures under various loading conditions Exposure to aggressive environmental agents and accidental events throughout this long period of service life also caused severe damage and deterioration in many structures Moreover, functional changes to old buildings, or increased loading due to heavier vehicles on bridges, resulted in a demand for necessary structural modifications One obvious option, replacing the deficient structures with new ones, is usually economically prohibitive, time consuming and even unviable For

instance, the shutdown of an outdated but crucial bridge in a busy highway network for replacement may well cause more problems than it solves Therefore, the other option of rehabilitating or upgrading these structures to modern standards becomes the only

possibility under certain situations, especially for load-carrying components such as beams and columns where strengthening is necessary to provide much needed load-carrying capacity

In the early 80’s, steel plates were often used to strengthen flexural members by bonding such plates to the underside of the members Significant improvement in

ultimate flexural strength and ductility can be achieved in this way However, the

difficulty inherent in handling heavy steel sections and their susceptibility to corrosion disqualified steel as the preferred candidate for such strengthening in practical

applications To overcome these problems, researchers have been investigating the applicability and effectiveness of fiber reinforced polymers (FRP), in lieu of steel, in structural strengthening since the early 90’s Figure 1-1 shows a typical FRP flexural strengthening scenario where Carbon Fiber Reinforced Polymer (CFRP) plates (strips or sheets) are bonded to the underside of a beam using epoxy resin

Figure 1-1 Typical flexural strengthening scheme for RC beams with FRP laminate FRP can be divided into three groups by fiber types, namely carbon FRP (CFRP), aramid FRP (AFRP), and glass FRP (GFRP) Of the three, CFRP and GFRP are the most commonly used while AFRP is rare due to its relatively poor long term creep behavior CFRP is superior in terms of strength and stiffness but is also expensive, while GFRP, on the other hand, is relatively cheap

Lightweight and inert to the aggressive environments typically encountered by concrete structures, FRP complements the disadvantages of steel while still possessing adequate strength and stiffness In the forms of ready-made laminates (plates) or fabrics, FRP can be either bonded to the underside of beams using adhesives like epoxy (in the case of laminates) or firstly wrapped over beams and then impregnated with epoxy to

FRP Laminate

effectively bond FRP fabrics to beams In addition to the flexural strengthening of beams, FRP strips or fabrics can also be bonded to the sides of beams for shear strengthening Previous experimental results have shown significant improvements in ultimate strength (flexural and shear) and stiffness of beams strengthened with FRP By

comparison, CFRP laminates outperformed both AFRP and GFRP laminates in flexural strengthening However, the strength gain was usually accompanied by a loss in member ductility, especially in the case of CFRP strengthening; and the debonding between the applied FRP and the concrete substrate often became the prevalent failure mode,

preventing the FRP from reaching its ultimate capacity at failure Anchoring the

laminates or U-jacketing beams has been shown to help in this case

Sprayed GFRP, another form of FRP that has been used for building boats and automobiles, was introduced in the mid 90’s to the FRP strengthening arena for concrete structures Using a commercially available spray gun system, the automatically chopped short fibers and resin are concurrently sprayed onto concrete surfaces After curing of the resin, an FRP plate composed of randomly oriented discontinuous fibers encapsulated in resin is formed Unlike the FRP laminates or fabrics mentioned above, two characteristics

of Sprayed GFRP should be noted:

1 The fibers are discontinuous instead of continuous;

2 The fibers are oriented randomly due to the spraying process, instead of oriented in

a specific direction In other words, the sprayed plate behaves isotropically within its application plane, rather than anisotropically

Studies (Boyd and Banthia, 1999, Boyd, 2000) have shown that these properties of sprayed GFRP, when employed to strengthen beams, lead to more ductile behavior and higher energy absorption capacity than in CFRP fabric-wrapped beams, while also

significantly improving the ultimate load carrying capacity of the member Further,

reinforced concrete beams with sprayed GFRP applied to their sides easily gained higher shear strength than those with wrapped CFRP fabrics

However, no one has reported testing on concrete members strengthened with a combined CFRP/sprayed GFRP configuration up to capacity As the previous research has shown, the CFRP laminates are excellent in resisting tension when bonded to the underside of concrete beams whereas the sprayed GFRP is superior at resisting shear when applied to the sides of concrete beams The sprayed GFRP, as indicated in the previous research, also made significant contributions to the moment capacity of concrete beams The hybrid system should thus provide double benefits, improving the moment capacity as well as the shear capacity of concrete beams

The following work, however, will focus on the flexural strengthening effects of this CFRP/sprayed GFRP system The shear strengthening effects are left to future research programs

Research Objectives

• To evaluate the material properties of the sprayed GFRP material that is used in the strengthening, taking into account factors such as laminate thickness, fiber

orientation, fiber length, and fiber content

• To investigate how well the CFRP/sprayed GFRP will perform in flexural

strengthening, when compared to systems using CFRP only and sprayed GFRP alone

• To determine how well the currently available theoretical tools (classical analysis and finite element modeling for example) can predict the load-carrying capacity and deflection characteristics of the CFRP/sprayed GFRP strengthened concrete beams

• To derive design guidelines for the CFRP/sprayed GFRP flexural strengthening of concrete beams evolved from the current ACI FRP strengthening design code

Thesis Structure

Chapter 2 provides an overall view of the previous research into the FRP

strengthening of concrete beams Chapter 3 describes the various tests conducted to determine the sprayed GFRP material properties and the material properties of the

concrete and steel used in this study Chapter 4 details the lab testing carried out on two groups of FRP strengthened concrete beams Chapter 5 presents the application of two theoretical methods, namely reinforced concrete beam theory and finite element analysis,

to estimate the load-carrying capacity and deflection of the concrete beams tested

Chapter 6 shows how to modify the current ACI FRP strengthening design guide to account for the sprayed GFRP component of the CFRP/sprayed GFRP system studied In Chapter 7, a real-world case study of the application of the hybrid CFRP/sprayed GFRP system to a deteriorated concrete bridge is presented Finally, Chapter 8 emphasizes the key points of knowledge acquired through this research, and closes with some

recommendations for future work

CHAPTER 2 LITERATURE REVIEW This chapter provides background information concerning the FRP strengthening of concrete beams It is divided into six main sections, namely FRP materials, the bond between FRP and concrete, flexural strengthening, shear strengthening, sprayed FRP strengthening and the ACI design code Since sprayed FRP strengthening plays a key role

in current research but has drawn little attention before, a separate section is devoted to it Furthermore, in the three strengthening sections, strengthening schemes, observed failure modes and analytical models will be discussed in order The ACI design code is the basis for developing design rules for the hybrid system in this research; therefore its general procedures will also be described

The reviewed work is the start point of the CFRP/sprayed FRP system studied; it provides a knowledge base of CFRP only and sprayed FRP only strengthening; it also provides research methodologies that were helpful in formulating the research program in the following chapters; the ACI design code provides the format and framework for the modified design code as a result of the addition of sprayed FRP

FRP Materials

Fiber reinforced polymers are composite materials containing fibers encapsulated

in hardened resins Continuous glass-fiber, carbon-fiber and aramid-fiber are the three fibers most widely used Table 2-1 (excerpt from ACI 440R-02）below lists their typical tensile properties In terms of

elastic modulus, carbon fiber easily outperforms aramid fiber, followed by glass fiber

Carbon fiber also has the least elongation at rupture while glass fiber has the highest It is

not as clear which performs the best with respect to ultimate strength Depending on fiber

type, the carbon category contains not only the strongest but also the weakest, while the

range in ultimate strength for glass and aramid fibers is basically similar

Table 2-1 Typical tensile properties of fibers used in FRP systems

Fiber type Elastic Modulus

(GPa)

Ultimate Strength (GPa)

Rupture strain, Minimum, %

Merriam-Webster Online defines polymer as a chemical compound or mixture of

compounds formed by polymerization and consisting essentially of repeating structural

units Polymeric resins used in FRP are all synthetic products resembling their natural

counterparts in terms of some physical properties (viscosity, etc.) Their functions in FRP

strengthening are two-fold: as the matrix for the FRP and as the bonding adhesive

between FRP and concrete The latter is responsible for achieving adequate bonding

between FRP and concrete and effective strengthening as a result In this regard, epoxy

resins perform best There are also polyester resins, vinyl ester resins and their variants

For instance, the sprayed FRP in the following research uses vinyl ester resin The tensile

properties of polymeric resins can be determined in accordance with ASTM D638M

All of the research to date on FRP strengthening uses commercially available FRP products They come either as separate components of woven fabric (or tow sheet) and resin or as an integral part of FRP plates formed by pultrusion Corresponding to these two forms of FRP products, the wet lay-up method and plate-bonding method are most commonly used in FRP strengthening applications The former is more applicable in the situations such as bonding FRP to curved surfaces or around corners while the latter provides more consistent FRP properties since it has better quality control during

production

In general, FRP materials are designated by the fiber used, i.e glass FRP (GFRP), carbon FRP (CFRP) and aramid FRP (AFRP) Each has been used in both research and practical applications FRP tensile properties can be determined following the ASTM D3039/D3039M method Table 2-2 (compiled by Head (1996)) shows their typical mechanical properties

Table 2-2 Typical mechanical properties of GFRP, CFRP and AFRP composites

Unidirectional

advanced

composites

Fiber content (% by mass)

Density (kg/m3)

Longitudinal tensile modulus (GPa)

Tensile strength(MPa) Glass

fiber/polyester

GFRP laminate 50 - 80 1600 – 2000 20 – 55 400 – 1800 Carbon/expoxy

CFRP laminate 65 - 75 1600 – 1900 120 – 250 1200 – 2250 Aramid/expoxy

AFRP laminate 60 - 70 1050 -1250 40 - 125 1000 - 1800

The fiber content indicates that a large part of all the composites is fiber, with more than fifty percent by mass Densities are similar and imply that all of these composites are very light considering the amount actually used in FRP strengthening (for example, 1.2mm x 50mm x length CFRP laminate) Compared to steel rebar (400MPa of tensile

strength and 200 GPa of elastic modulus), all the FRPs have more than 175% higher tensile strength with CFRP leading the way, followed by AFRP and GFRP Only CFRP, however, has comparable elastic modulus Light weight and high strength, two qualities resulting in easy installation and adequate for resisting heavy load, are crucial factors in the real-life FRP strengthening of typically large-scale structural components (i.e beams, columns) It should be noted, however, that the ranges given in the table are

representative only When designing an FRP strengthening, the FRP material properties should be obtained from the FRP manufacturer

Regardless of fiber type and product form, all FRP composites have a common stress-strain behavior: linear up to failure Unlike mild-steel, there is no yielding or strain-hardening This implies the brittleness of FRP composites, reducing the ductility of those concrete components strengthened with FRP On the other hand, the simple linear stress-strain relationship allows relatively easier incorporation of FRP strengthening into current structural design codes

Bond Strength of FRP-concrete Joint

In FRP strengthening of concrete beams, either flexural or shear, FRP resists

tension, force that is transferred from the concrete through the bond between the concrete and FRP Adequate bond strength, therefore, is crucial for effective FRP strengthening and to make full use of the FRP’s strength

In fact, a great deal of experimental and theoretical work has been conducted on the bond strength of concrete-FRP or concrete-steel plate joints Several generic bond testing setups have been used in the past, including single shear tests (Figure 2-1a) (Täljsten

1994, 1997, Chajes et al 1995, 1996, Bizindavyi and Neale 1999), double shear tests

(Figure 2-1b) (van Gemert 1980, Swamy et al 1986, Kobatake et al 1993, Autocon 1994, Brosens and van Gemert 1997, Fukuzawa et al 1997, Hiroyuki and Wu 1997, Maeda et

al 1997, Neubauer and Rostasy 1997), and modified beam tests (van Gemert 1980,

Ziraba et al 1995)

Figure 2-1 Bond test (a) single (b) double (c) plan view

Various methods have been used to develop bond models including fracture

mechanics (Triantafillou and Plevics 1992, Holzenbämpber 1994, Täljsten 1994, Yuan and Wu 1999, Yuan at al 2001), regression of experimental data or derivations from

simplistic assumptions (Van Gemert 1980, Chaalal et al 1998, Khalifa et al 1998) The theoretical work, combined with experimental investigations (Täljsten 1994, Chajes et al

1996, Maeda et al 1997), led to one major finding on FRP-to concrete joints, i.e the

bond strength does not always increase with increases in bond length L ( see Figure 2-1)

L

L

L

bc Concrete

This fact is summarized in the concept of effective bond length, beyond which further increase of bond strength is impossible

However, it was also observed that longer bond length increases the ductility of the failure process The phenomenon of effective length of FRP-to-concrete joints is so different from steel reinforcement in concrete, where the steel tensile strength can always

be achieved if sufficient bond length is available, that it has to be accounted for in any bond strength models for FRP-to-concrete joints

There have been several bond strength models developed in the past Based on their distinct approaches as mentioned in last section, they can be categorized as empirical

models (Tanaka 1996, Hiroyuki and Wu 1997, Maeda et al 1997), fracture mechanics

models (Täljsten 1994, Holzenkämpfer 1994, Neubauer and Rostásy 1997, Yuan and Wu

1999, Yuan et al 2001) and simplistic models (van Gemert 1980, Chaallal et al 1998, Khalifa et al 1998)

Each category of bond strength model has its own advantages and disadvantages Some empirical models (e.g Hiroyuki and Wu 1997) are simple but do not take the

effective bond length into account, others (e.g Maeda et al 1997) consider the effective

bond strength but are not complete; most fracture mechanics models (e.g Yuan 2001) incorporate the effective bond length but are rather involved in their derivations and thus relatively hard to understand; some simplistic models are based on simple and clear assumptions but either miss the key effective bond length (e.g van Gemert 1980) or

ignore some necessary factors such as concrete strength (e.g Chaalal et al 1998) For

details on the comparison between these bond strength models, a good review paper was presented by Chen and Teng (2001)

Flexural Strengthening of RC Beams Brief History

A typical flexural strengthening scheme for beams is shown in Figure 2-2, where

an FRP laminate (or fabric or strip) is bonded to the soffit of the beam

Figure 2-2 A Typical flexural strengthening scheme for a RC beam with FRP laminate Meier et al (1991) was the first to investigate the FRP strengthening concept, a program that included twenty-six 2m beams and one 7m beam strengthened with CFRP laminates (Figure 1) and tested under four-point bending The bonded CFRP laminates significantly improved the strength and stiffness of the beams More cracks appeared in the tension zone than in unstrengthened beams, but the total width decreased by almost 30% In the course of testing, four failure modes were observed, namely,

1 Tensile failure of the CFRP laminate

2 Classical concrete failure in the compression zone of beam

3 Continuous peeling-off of the CFRP laminates due to an uneven concrete surface

4 Sudden peel-off during loading due to the development of shear cracks in concrete

FRP Laminate

Enlarging the testing scope, Ritchie et al (1991) used GFRP and AFRP laminates along with CFRP laminates and steel plates Similar results as those of Meier were observed, which further established the feasibility of this FRP strengthening technique

Thereafter a good deal of research effort followed and extended to many design factors that might be crucial to strengthened beam behavior Virtually every investigator mentioned the critical importance of proper concrete surface preparation to ensure a good bond between the applied FRP laminates and the concrete substrate Ehsani et al (1990) observed that stiffer epoxies produced a better bond, which was manifested by the more favorable shear failure of the concrete sandwiched between the tension reinforcement and the FRP plate, rather than failure of epoxy bond itself

Saadatmanesh et al (1991) found that lower reinforcement ratios helped the beams strengthened with GFRP plates gain much ultimate strength Chajes et al (1994)

compared the effects of equivalent amounts of AFRP, GFRP, CFRP and internal steel that would provide the same tensile strength The beams strengthened with AFRP, GFRP and CFRP exhibited similar magnitudes of strength and stiffness increase as those with added internal steel reinforcement At failure, AFRP strengthening induced fabric

debonding, though anchorages at plate ends forced the failure mode to concrete crushing GFRP and CFRP plating without anchorage led to FRP tensile failure The authors

believed the discrepancy in failure modes was due to the higher ultimate strain capacity

of AFRP than those of GFRP and CFRP

Arduni (1997) and Sharif (1994) observed that FRP stiffness, fiber orientation relative to beam axis and plate thickness (or the number of plies for FRP fabrics) were

key factors in determining the ultimate flexural strength, stiffness and failure modes of strengthened beams

The most notorious failure mode of FRP flexural strengthening is the debonding of FRP laminates This type of failure not only undermines the flexural capacity of

strengthened beams but also reduces their ductility because of the violently sudden failure inherent in this mode Debonding could result from crack propagation through the FRP-concrete interface, FRP peeling-off due to shear cracks, or delamination of the concrete cover between steel reinforcement and FRP plate Extremely high normal and shear stresses at FRP laminate ends were identified as the causes for debonding failure The stress magnitudes were related to many design variables (Malek et al, 1998)

To avoid debonding failure, a number of researchers (Spadea et al, 1999; Sharif, 1994) used anchorages to secure FRP plate, wherein most of the plate ends were bolted or wrapped Wrapping the entire FRP plate (U-jacketing) was found to be the most effective anchorage (Arduni et al, 1997) In addition to preventing bond failure, anchorage also increased the ductility of strengthened beams compared to those without such anchorage (Spadea et al, 1999)

While most of the abovementioned studies used intact beams, in real applications most of the beams that need strengthening are damaged or structurally deficient to some extent To investigate the effectiveness of FRP strengthening on damaged beams, Sharif (1994), before applying GFRP plates, pre-loaded a series of RC beams to 85% of their flexural capacity Subsequent test results then showed that the GFRP strengthening, with different anchorages to exclude premature failures, significantly improved the flexural capacity of the damaged beams, even beyond that of the undamaged beams

Arduni et al (1997) applied CFRP plates to the soffit of beams while those beams were under load This application method simulated the real-life situation wherein a beam

is strengthened under dead load conditions The results indicated that the ultimate

strength and stiffness of the beams could still be successfully recovered even under loaded conditions

At the same time of experimental research, theories for FRP strengthening were also proposed One notable piece of work produced by Triantafillou et al (1992) They employed the strain compatibility method, concepts of fracture mechanics and a simple model for the FRP peeling-off debonding mechanism due to shear cracks to derive four equations quantifying the conditions for the four failure mechanisms of FRP strengthened beams: steel yield-FRP rupture, steel yield-concrete crushing, concrete crushing, and debonding of FRP plates

Failure Modes

Previous experiments on beams with FRP bonded to their soffits revealed various

failure modes (Ritchie et al 1991, Saadatmanesh and Ehsani 1991, Triantafillou and Plevris 1992, Chajes et al 1994, Sharif et al 1994, Heffernan and Erki 1996, Shahawy et

al 1996, Takeda et al 1996, Arduni and Nanni 1997, Garden et al 1997, 1998, Grace et

al 1998, Ross et al 1999, Bonacci and Maalej 2000, Nguyen et al 2001, Rahimi and

Hutchinson 2001) Figure 2-3 shows schematically the typical failure modes observed

Figure 2-3 Failure modes of beams FRP strengthened in flexure

Concrete crushing (b)

Shear crack (c)

Crack propagation

High-stress zone (d)

High-stress zone

Crack propagation (e)

Load

Crack

High-stress zone

Crack propagation (f)

(g) High-stress zone Crack

Crack propagation

Load

They basically fall into seven categories as the figure shows, namely, (a) flexural failure by FRP rupture, (b) flexural failure by crushing of concrete, (c) shear failure, (d) concrete cover separation, (e) plate-end interfacial debonding, (f) intermediate flexural crack-induced interfacial debonding, and (g) intermediate flexural shear crack-induced interfacial debonding In general, (d) and (e) can be simply called as plate-end debonding failures and (f) and (g) as intermediate crack-induced interfacial debonding failures

If the ends of FRP plates are properly anchored, preventing plates from debonding, the strengthened beams can usually fail by rupture of FRP plates or by concrete crushing The rupture of FRP plates indicates a brittle behavior Unlike normal RC beams, this means that the strengthened beams fail suddenly without much warning If a sufficient amount of FRP is used, the strengthened beams could fail by concrete crushing, similar to ove-reinforced concrete beams However, this failure mode is also much more brittle for the FRP-strengthened beams For both failure modes, the ultimate strength is typically increased Actually, increased ultimate strength and reduced ductility are two main

characteristics of FRP-strengthen RC beams

Strengthened beams can also fail in shear (Buyukozturk and Hearing 1998, Lopez

et al 1999) This can occur when the beams are over strengthened in flexure compared to their shear capacity, such as when excessive FRP plates are bonded to the beam soffit without any shear strengthening to offset it’s effect Therefore, when strengthening a beam in flexure, considerations of shear strengthening is also necessary to ensure a flexural failure mode, which is preferred to shear failure which tends to be more brittle and wastes the strength reserve of the strengthened beams

Debonding of FRP plates from concrete can initiate from the end of the plates It could be the separation of the concrete cover from the steel layer or the FRP plates from the beam soffit In reality, these two modes may be mixed Both modes are premature in terms of the ultimate strength that is reached and designed for It is believed that both failure modes are caused by high interfacial shear and normal stresses due to the abrupt termination of plates With quality adhesives and properly prepared concrete surfaces, the latter failure mode, plate-concrete interfacial debonding, can be prevented

Under bending, the soffit of beams at mid-span is the most highly stressed Thus, cracks, especially flexural cracks, always initiate in this region For FRP-strengthened beams, once a crack forms the stresses released will transfer to the FRP plates, creating a stress concentration between the concrete and the FRP As the crack propagates in the concrete, the stress will reach a certain magnitude where the crack will propagate along the bond layer until it reaches the plate ends, causing an abrupt concrete-FRP interfacial debonding failure Intermediate shear crack-induced interfacial debonding is similar, though in this case failure starts where the major shear cracks initiate

Flexural Strength Models

From the above discussion, it is clear that debonding failures are dominant modes unless FRP plate ends are sufficiently anchored Therefore, for strength models to give accurate predictions debonding must be taken into account Concrete cover separation and plate-end interfacial debonding have been extensively studied since they are

commonly observed in experiments Many strength models have resulted from previous research, which can be divided into three groups (a) shear-capacity models (Oehlers 1992, Jansze 1997, Ahmed and Van Gemert 1999), (b) concrete tooth models (Raoof and

Zhang 1997, Wang and Ling 1998, Raoof and Hassanen 2000), and (c) interfacial stress

based models (modelsⅠand Ⅱ of Ziraba et al 1994, Varastehpour and Hamelin 1997, Saadatmanesh and Malek 1998, Tumialan et al.1999) It should be noted that some

models (Oehlers 1992, Jansze 1997) were developed for steel-plated beams However, steel plates are only different from FRP plates in terms of material properties Therefore,

it is reasonable to assume that these models are also applicable to FRP-plated beams The model by Oehlers (1992) uses the moments and shear forces at plate ends at debonding as criteria to determine whether debonding failure occurs The expressions for debonding end moments and shear forces were derived with the help of testing data

Raoof and Zhang (1997) developed a concrete “tooth” model for concrete cover separation failure, where a concrete “tooth” is a piece of concrete between two adjacent cracks Under shear stresses between FRP plates and concrete, this tooth acts like a cantilevered beam Debonding is assumed to occur at the point when a tensile stress at the root of the “tooth” exceeds the tensile strength of concrete This model was originally devised for steel-plated beams, and further applied to FRP-plated beams With the

effective length of FRP plate end anchorage known the ultimate bond shear stress

between FRP plates and concrete can then be determined

The assumption that debonding is caused by high interfacial stresses (tensile and shear) at FRP plate ends is promising in view of previous theoretical research (e.g

Roberts 1989) Models of this type consider that the shear resistance of debonding is contributed by concrete and stirrups as well

In a review of available strength models, Smith and Teng (2002) discussed the advantages and disadvantages of the models they collected after comparing them with an

experimental data base they compiled As a result, they proposed a new model that they believe is better

Shear Strengthening of RC Beams Introduction

Flexural and shear failures are the two primary failure modes of RC beams Steel stirrups are used in RC beams where concrete alone is usually not enough for shear resistance Shear deficiency will occur if an RC beam is not adequately designed in shear

or flexural strengthening (e.g FRP) makes the beam relatively deficient in shear One promising character of FRP shear strengthening, other than its high strength-weight ratio and corrosion resistance, is its adaptability to different geometries of structural

components The following section is a brief description of FRP shear strengthening research that has been conducted

Shear strengthening of an RC beam usually consists of bonding FRP to the webs of the beams instead of the soffits as in flexural strengthening Various shear strengthening schemes, as investigated by a number of researchers (Berset, 1992; Chajes et al, 1995; Chaallal et al, 1998; Al-Sulaimani et al, 1994; Triatafillou, 1998; Miyauchi et al, 1997), are shown in Figure 2-4a ~ 2-4g

Figure 2-4 Shear strengthening a) FRP wings

FRP Laminate

Figure 2-4 Shear strengthening b) U-Jacket

Figure 2-4 Shear strengthening c) Vertical FRP strips

Figure 2-4 Shear strengthening d) Inclined FRP strips

Figure 2-4 Shear strengthening e) FRP U-strips

FRP Laminate

FRP Strips

FRP Strips

FRP Strips

Figure 2-4 Shear strengthening f) FRP O-strips

Figure 2-4 Shear strengthening g) Inclined FRP O-strips

The shear capacity was found to be increased after beams were strengthened in either of the above configurations, and the configuration with inclined FRP strips

outperformed that of vertical strips This could be explained as a consequence of the fiber

of the former system being oriented closer to the line that is perpendicular to the diagonal cracks, thus more effectively carrying the tension force caused by shear

Some researchers also carried out theoretical work aimed at predicting shear

capacity of beams strengthened with FRP Triantafillou (1998) remarked that, based on limit states, the shear strengthening of beams could be designed in the way similar to that

of internal steel reinforcement, provided that an effective FRP strain is used in the

formulation Malek et al (1998) attacked the problem by first finding the upper bound value of the inclination angle of shear cracks Then a formula analogous to that for conventional RC beams was used to obtain the shear capacity for the FRP strengthened beams

FRP Strips

FRP Strips

Failure Modes

In previous research, several major failure modes were observed in experiments, namely shear failure with FRP rupture, shear failure without FRP rupture, shear failure due to FRP debonding and failure near mechanical anchors

Shear failure with FRP rupture usually initiates from a shear tension crack that originates from the tensile face and propagates from the support to the loading point The crack widens as the load increases, and the FRP will finally reach its ultimate strain and fail Existing test results indicate that beams strengthened with wrapped FRPs or U-jackets mostly failed in this mode (e.g Chajes et al 1995, Araki et al 1997, Funakawa

et al 1997, Kage et al 1997, Sato et al 1997, Chaalal et al 1998) Shear failure without FRP rupture is essentially the same as shear failure with FRP rupture

The difference is that in the former case the concrete fails first before the FRP has reached its ultimate strength and is still able to carry more loads This failure mode was observed in the two beams strengthened with an aramid FRP that has an ultimate strain of 2.25% (Chajes et al 1995) Shear strengthened beams can also fail by debonding This debonding is similar to the shear debonding in FRP flexural strengthening, which is due

to high stress concentration generated by shear cracking Experiments show most beams strengthened with side-bonded FRP only fail in this mode If anchors are used to secure FRPs on the sides of a beam, local failures could occur in the concrete or FRP around the anchors Sato el at (1997) observed these failure modes in experiments

Shear Strength Models

All of the available shear strength models assume the total strength of a

strengthened beam is the linear addition of the contributions of concrete, steel stirrups and FRP, similar to the assumed shear strength of RC beams The concrete and steel

contributions can be calculated in accordance with existing building codes But for FRP, different researchers proposed different formulae Chaallal et al (1998) used a similar equation as that for steel stirrups to calculate FRP’s contribution to the total shear

strength However, this model does not take into account the facts that FRP usually cannot reach its full strength at failure and the effective bond length for FRP is limited as has been discussed in the previous bond strength section Triantafillou (1998) used a semi-quantitative approach to accommodate various failure mechanisms However, this model suffers in its lack of distinction between different strengthening schemes and failure modes Khalifa et al (1998) modified Triantafillou’s effective strain model and used the ratio of FRP effective strain to its ultimate strain Still, this model

underestimates the usage of FRP in shear resistance, i.e no more than half of the FRP’s tensile strength can be used Further, this model employed a bond strength model that is unable to predict the effective bond length accurately Chen and Teng (2001) derived a model that incorporates both the FRP strip scheme and the continuous sheet scheme and also considers the effective bond length carefully However, this model is more complex and needs simplifications for the purpose of use in FRP shear strengthening designs

Sprayed FRP

Banthia et al (1996) first introduced Sprayed FRP to strengthening concrete

structures The results of their preliminary tests indicated, besides ease of application of Sprayed FRP, the fracture toughness, the load-carrying capacity and the fracture energy

of the tested beams could be remarkably increased though using only 8% fiber by volume content

Boyd (2000) conducted a relatively comprehensive research program on the

strengthening of RC beams with Sprayed FRP, both for flexure and for shear Various

parameters, including fiber length, fiber content, plate thickness, concrete strength and strengthening schemes, were investigated Reinforced concrete beams, with the

dimensions of 96mm x 125mm x 1000mm were used in this program In the shear

strengthening, the stirrups were removed to simulate shear deficiency All the beams were tested with four-point loading in the lab until failure A comparison between

strengthening with Sprayed FRP and a commercially available FRP wrapping system was also made As the last part of the program, several girders removed from a demolished bridge were strengthened with Sprayed FRP and load tested The collected data revealed

a number of outstanding features about the strengthening with Sprayed FRP;

1 While the flexural capacity was adequately increased, the beams strengthened for shear exhibited a strength gain as much as 200% or more than that of the control beams The author attributed this characteristic of Sprayed FRP to its in-plane isotropic property due to the randomly oriented fibers in the hardened plates

2 The energy absorption capacity of the strengthened beams was dramatically

increased, as compared to those strengthened with FRP having continuous fibers This discrepancy can be attributed to the less brittle failure mode of Sprayed GFRP than that of FRP with continuous fibers

3 U-jacketing or O-jacketing schemes effectively eliminated the undesired

debonding failure that frequently occurred in both the pure web bonding and soffit bonding schemes

4 The Sprayed FRP outperformed a commercially available composite strengthening system in the strengthening of full-scale damaged channel beams

In Japan, Furuta et al (2000) also pursued this line of research with Sprayed FRP However, they focused on how different anchoring schemes affected the behavior of strengthened T-beams Their results indicated that the FRP filled slit was an effective technique for anchoring Sprayed FRP to concrete