GFRP i Beton

8/10/2019 GFRP i Beton

1/225

THESIS FOR THE DEGREE OF LICENTIATE OF ENGINEERING

Durability of FRP Reinforcement in Concrete-Literature Review and Experiments

Valter Dejke

Department of Building MaterialsCHALMERS UNIVERSITY OF TECHNOLOGY

Gteborg, Sweden 2001


2/225

Key wordsalkalidegradationdurabilityfibrefibre reinforced polymersFRPGFRPglass fibre reinforced polymersnon metallic reinforcement

predictionreinforcementresinservice life

ISSN 1104-893X

Valter Dejke

Publication no P-01:1Department of Building Materials

Chalmers University of TechnologySE-412 96 GteborgSwedenTelephone - 46 (0)31-772 1000

Reproservice, ChalmersGteborg, Sweden 2001


3/225

ABSTRACT

iii

Abstract

During the latest decade there has been an important increase in the use of FRP (FibreReinforced Polymers) as concrete reinforcement in the construction industry. Themost obvious benefit of using FRP for concrete reinforcement is that it does notcorrode in the same way as steel, which makes it an interesting reinforcement optionfor concrete structures in severe environments. However, FRP is prone to deterioratedue to other degradation mechanisms than those for steel. The high alkalinity ofconcrete, for instance, is a possible degradation source. Other potentially FRPaggressive environments are sea salt, de-icing salt, freeze-thaw action, UV-light andfresh water/moisture. This licentiate thesis includes an update of knowledge regardingdurability of FRP reinforcement in relevant environments and an overview of currentresearch activities in this field. Although a great deal of research has been addressingthe durability of FRP reinforcement, very few quantitative predictions of material

property deterioration have been reported.

This thesis particularly focuses on GFRP reinforcement. GFRP is known todeteriorate in the environment of concrete. However, of the FRP types availableGFRP is the cheapest and consequently has the highest potential of being costeffective. Hence, there is a need for reliable estimations of the rate of deterioration ofGFRP in the environment of concrete. An important part of the work described in thisthesis have been to gain a better understanding of the degradation mechanisms ofGFRP reinforcement in concrete and to make a quantitative service life prediction forthis material in real applications. The work includes a literature review of degradationmechanisms of GFRP in concrete, a theoretical discussion of possible degradationmodes, durability experiments and formulation of service life prediction models.

The experimental work involves exposure of four GFRP types in concrete, alkaline

solution and water at 20, 40, 60 and 80 C. After exposure the specimens wereexamined, using several analysis methods, to investigate the environmental effects onmechanical and physical properties. Altogether approximately 1400 specimens wereincluded in the experimental programme.

Of the GFRP bars tested, systems containing E-glass and vinyl ester appear to have better overall durability than the other systems. The tensile strength retention afterapproximately 1.5 year in moisture saturated concrete at 60 C and the ILSS retentionafter approximately one year under the same conditions were 57% and 96%respectively for the bar having the best environmental resistance.

Two models for strength retention predictions have been formulated . One of them

assumes that the rate of strength retention at different temperatures can be described by the Arrhenius equation. Using this approach it is possible to transform exposuretime under accelerated conditions to time in a real application. Thus 1.5 years at 60 Ccorrespond to approximately 50 years in outdoor conditions in the south of Sweden(mean annual temperature, 7 C). The other predictive model takes account of anydifferences in the influence of the temperature on the rate of transport mechanismswithin the composite and on the chemical reactions leading to degradation.


4/225

ABSTRACT

iv

A general conclusion from this work is that the use of FRP reinforcement can berecommended for concrete structures of arbitrary required service lives provided a

proper strength reduction factor is used to take account of the deterioration of thematerial. Such a strength reduction factor should be separately determined for everyapplication, and based the deterioration rate controlling factors including moistureconditions, temperature, stress level and required service life.


5/225

PREFACE

v

Preface

This work was mainly carried out at the Department of Building Materials, ChalmersUniversity of Technology. I wish to express my gratitude to my assistant supervisor

Professor Ralejs Tepfers, for his great commitment to my work and for his care, bothon a professional and a personal level. His good relations with researchers aroundthe world enabled many journeys and the establishment of new contacts, whichcontributed a great deal to make my PhD studies a very fruitful and positive period inmy life. I also want to express my appreciation to my main supervisor Professor Lars-Olof Nilsson for his support and encouraging attitude to my work.

I am grateful to all my colleagues at the department for their support. Special thanks goes to Marek Machovski. His effort and positive attitude were invaluable for theexperimental work. Special thanks also to Alf Andersen, Anders Lindvall and Juhan

Aavik for their help with experiments and analysis work, and to Professor JohanClaesson for his help with diffusion modeling.

I would also like to acknowledge the support from Dr Kypros Pilakoutas, Dr Ewan Byars, Dr Peter Sheard, Professor Jones and the doctoral students at The Universityof Sheffield during my stay there (August 1999 - January 2000).

I also wish to thank Hughes Brothers Inc. and Fiberkonst AB who have suppliedGFRP bar specimens for the experiments conducted.

Representatives from FoU Vst (R&D West) have acted as the reference group andare acknowledged for their contribution. The support of SBUF (The Development

Fund of the Swedish Construction Industry), BFR (Swedish Council for Building Research), NCC (Nordic Construction Company), ConFibreCrete (Training and

Mobility of Researchers, EU project) are also gratefully acknowledged.

Finally I would like to express my great appreciation to Karin for her support and forher patience during my long and late spells of work at Chalmers.

Gteborg, January 2001

Valter Dejke


6/225


7/225

DISPOSITION OF THE REPORT

vii

Disposition of the report

This licentiate thesis consists of two parts. Part A deals with durability of FRP in a broad perspective. All FRP types available for concrete reinforcement (GFRP, AFRPand CFRP) are discussed with respect to their durability characteristics in allenvironmental conditions of relevance. In addition, the current research in this field issummarised. Part B focuses on the durability of GFRP in alkaline environments. This

part includes a literature review on possible degradation mechanisms of GFRP inalkaline environments, discussion of experimental work conducted in the project andthe service life prediction models which have been formulated.

A brief description of the different chapters in Part A and Part B is given below.

Part A Durability of FRP reinforcement in concrete -A literature review

Chapter 1 is an introductory chapter that gives the reader the background to the topic,the objective and scope of this part of the thesis and a brief description of applicationsfor FRP used as concrete reinforcement.

Chapter 2 is aimed to provide the reader with general information about the FRPcomposites and their constituents regarding environmental resistance as well as other

properties.

Chapter 3 summarises durability related research work conducted in recent years.Experimental methodology and results from research projects investigating theinfluence of various environmental conditions are discussed. Environmentalconditions focused on are: alkali, water/moisture, freeze/thaw, UVradiation and

thermal action . In addition the approaches to service life predictions which have beenfound in the literature are reviewed.

Design guidelines for the design of concrete structures using FRP reinforcement have been drawn up in Japan, Canada, USA, Great Britain, and Norway. These documentsare summarised and discussed in Chapter 4 , with special emphasis on how thedurability issue is handled.

In the literature reviewed, a number of durability related topics have been pointed outfor which further research is needed. In Chapter 5 these topics have been put together.

Chapter 6 is a concluding chapter and the references are given in Chapter 7 .

Appendix 1 shows the environmental resistance of epoxy and polyester resinsubjected to different chemicals

Appendix 2 summarises a number of research projects conducted in recent years. Theaim has been to give an overview of how durability research is generally performed.


8/225

DISPOSITION OF THE REPORT

viii

Part B Durability of GFRP reinforcement in concrete -Literature review, experiments andservice life prediction

Chapter 1 includes the background and the objective and introduces Part B.

Chapter 2 treats degradation mechanisms of GFRP, glass fibres and resin in alkalineenvironments and water. This chapter is based on a literature review and also includesa discussion on transport mechanisms, of alkali and water, within GFRP

In Chapter 3 a theoretical discussion of durability and degradation mechanisms ofGFRP in concrete is given. The relationship between transport properties within theGFRP material and possible failure modes is discussed. Furthermore, two models forstrength retention prediction, formulated within the project, are described.

Chapter 4 describes the experiments conducted within this project. Exposureconditions, test methods and results are discussed.

In Chapter 5 strength retention predictions are made using one of the strengthretention prediction models described in Chapter 3 and the experimental results fromChapter 4.

Chapter 6 is a concluding chapter for Part B.

In Chapter 7 topics for future research are discussed.

The references are listed in Chapter 7 .

Appendix 1 shows a photograph of Grey bars with a spiral wrapping of different

tightness. This difference has been shown to have a big influence on the mechanical properties of the bar.

Appendix 2 gives the tensile strength and ILSS (inter laminar shear strength) data forthe Grey bar.

Appendix 3 gives the tensile strength and ILSS (inter laminar shear strength) data forthe Yellow bar.

Appendix 4 gives the tensile strength and ILSS (inter laminar shear strength) data forthe Green bar.

Appendix 5 gives the tensile strength and ILSS (inter laminar shear strength) data forFIBERBAR.


9/225

ABBREVIATIONS

-ix-

Abbreviations

AFRP aramid fibre reinforced polymerAGE relative age at the temperature T ( C) compared with conditions similar

to those in Des Moines, Iowa (applies to Porter and Barnes, 1998).AGFRP aramid-glass hybridAR alkali resistantCFRP carbon fibre reinforced polymerCTE coefficient of thermal expansionDMA dynamic mechanical analysisDMTA dynamic mechanical thermal analysisDSC differential scanning calorimetryEPMA electron probe microanalysisEDX electron disperse X-rayFEM finite element methodFRP fibre reinforced polymerFTIR Fourier transform infraredGFRC glass fibre reinforced concreteGFRP glass fibre reinforced polymerGRC glass fibre reinforced cementILSS inter-laminar shear strengthLA-ICPMS laser ablation inductive coupled plasma mass spectroscopyMAT mean annual temperatureMTI mode of temperature influencePVA polyvinyl alcoholRH relative humidityRTE reverse thermal effectSBS short beam shear (a flexural test of a specimen having a low test span

to thickness ratio, such that failure is primarily in shear)SEM scanning electron microscopySIC strand in cementTGA thermo gravimetric analysisTS tensile strengthUTS ultimate tensile strengthVFRP vinylon fibre reinforced polymerTSF time shift factor


10/225


11/225

CONTENTS

-xi-

Contents

Abstract........................................................................................................................iii Preface...........................................................................................................................v Disposition of the report ............................................................................................vii Abbreviations ..............................................................................................................ix Contents .......................................................................................................................xi

Part A ..............................................................................1 Summary.......................................................................................................................3 1 Introduction..........................................................................................................5

1.1 Background....................................................................................................51.2 Objective and scope .......................................................................................51.3 Applications of FRP.......................................................................................6

2 Material characteristics.......................................................................................9 2.1 Introduction....................................................................................................92.2 Fibres..............................................................................................................9

2.2.1 General considerations .............................................................................. ..........................9 2.2.2 Carbon Fibres ......................................................................................... .............................9 2.2.3 Aramid fibres.....................................................................................................................11 2.2.4 Glass fibres .............................................................................................. ..........................12

2.3 Resin ............................................................................................................132.3.1 General consideration........................................................................................................13 2.3.2 Degradation of resin ...................................................................................... ....................14

2.4 FRP concrete reinforcement ........................................................................172.4.1 Manufacture.......................................................................................................................17 2.4.2 Classification ............................................................................................. ........................17 2.4.3 Physical and mechanical properties ............................................................... ...................17 2.4.4 Durability...........................................................................................................................20

3 Review of research activities and results .........................................................23

3.1 Introduction and general considerations ......................................................233.2 Effects of water and moisture on FRP .........................................................25

3.2.1 Introduction ............................................................................................... ........................25 3.2.2 Experimental methodology ............................................................................ ...................26 3.2.3 Experimental results and discussions................................................................................26 3.2.4 Conclusions ................................................................................... ....................................30

3.3 Influence of salt on FRP ..............................................................................303.3.1 Introduction ............................................................................................... ........................30 3.3.2 Experimental methodology ............................................................................ ...................30 3.3.3 Experimental results and discussions................................................................................31 3.3.4 Conclusions ................................................................................... ....................................33

3.4 Effects of Alkali on FRP..............................................................................343.4.1 Introduction ............................................................................................... ........................34

3.4.2 Experimental methodology ............................................................................ ...................34 3.4.3 Experimental results and discussions................................................................................35 3.4.4 Conclusions ................................................................................... ....................................39

3.5 Influence of freeze-thaw cycles ...................................................................403.5.1 Introduction ............................................................................................... ........................40 3.5.2 Experimental methodology ............................................................................ ...................40 3.5.3 Experimental results and discussions................................................................................40 3.5.4 Conclusions ................................................................................... ....................................42

3.6 Degradation caused by ultraviolet rays........................................................423.6.1 Introduction ............................................................................................... ........................42 3.6.2 Experimental methodology ............................................................................ ...................42


12/225

CONTENTS

-xii-

3.6.3 Experimental results and discussions................................................................................42 3.6.4 Conclusions ................................................................................... ....................................43

3.7 Thermal actions............................................................................................433.8 Other degradation sources............................................................................45

3.8.1 Deterioration by heat and fire............................................................................................45 3.8.2 The effect of galvanic coupling.........................................................................................45 3.8.3 Gaseous Mixtures .......................................................................................... ....................46 3.8.4 Pressure..............................................................................................................................46 3.8.5 Exposure to diesel..............................................................................................................46

4 Durability approach in existing design guidelines ..........................................47 4.1 Introduction..................................................................................................474.2 Japanese Society of Civil Engineers ............................................................47

4.2.1 Introduction ............................................................................................... ........................47 4.2.2 Material coefficients and member factor ..........................................................................47 4.2.3 Creep rupture .................................................................................................. ...................49 4.2.4 Durability test method ................................................................... ....................................49

4.3 Canadian Highway Bridge Design Code .....................................................504.3.1 Introduction ............................................................................................... ........................50 4.3.2 Strength reductions and stress limits.................................................................................50 4.3.3 Prestressed reinforcement .............................................................................. ...................51

4.3.4 Restrictions in the use of FRP as concrete reinforcement................................................51 4.4 American Concrete Institute ........................................................................524.4.1 Introduction ............................................................................................... ........................52 4.4.2 Environmental reduction factor.........................................................................................52 4.4.3 Creep rupture stress limits.................................................................................................53 4.4.4 Fatigue .............................................................................................. .................................53

4.5 British Institution of Structural Engineers ...................................................534.5.1 Introduction ............................................................................................... ........................53 4.5.2 Characteristic strength and safety factors .........................................................................54

4.6 Norwegian standard .....................................................................................554.6.1 Introduction ............................................................................................... ........................55 4.6.2 Strength reduction factors ............................................................................. ....................55 4.6.3 Prestressed reinforcement .............................................................................. ...................56

4.7 Conclusions..................................................................................................565 Topics of interest for further research.............................................................57 6 Conclusions and discussion ...............................................................................61 7 References...........................................................................................................63 APPENDIX 1..............................................................................................................73 APPENDIX 2..............................................................................................................74

Part B ............................................................................85 Summary.....................................................................................................................87 1 Introduction........................................................................................................89

1.1 Background..................................................................................................891.2 Objective and scope .....................................................................................89

2 Degradation mechanisms of GFRP in concrete ..............................................91 2.1 General considerations.................................................................................912.2 Stress rupture and stress corrosion...............................................................912.3 Degradation of glass fibres ..........................................................................93

2.3.1 Glass types of interest........................................................................................................93 2.3.2 General overview of glass corrosion.................................................................................94 2.3.3 Degradation mechanisms in alkaline environments .........................................................95 2.3.4 Degradation mechanisms in water ................................................................. ...................99 2.3.5 Stress rupture ................................................................................... ..................................99

2.4 Degradation of resin...................................................................................101


13/225

CONTENTS

-xiii-

2.4.1 General considerations .............................................................................. ......................101 2.4.2 Degradation mechanisms .......................................................................... ......................102

2.5 Degradation in fibre/resin interface ...........................................................1022.6 Transport properties in GFRP....................................................................103

2.6.1 General discussion of the transport properties of fibre composites ...............................103 2.6.2 Water transport in GFRP.................................................................................................103 2.6.3 Alkali transport in GFRP.................................................................................................105 2.6.4 Calculation of diffusivity and concentration profiles in plates and cylinders................106

2.7 Conclusions................................................................................................1093 Theoretical discussion of durability of GFRP reinforcement in concrete ..111

3.1 Structural requirements of GFRP concrete reinforcement.........................1113.2 Possible degradation mechanisms..............................................................111

3.2.1 Possible transport modes of concrete pore solution in GFRP material..........................111 3.2.2 Possible degradation modes of GFRP reinforcement in concrete ..................................115

3.3 Service life prediction modelling...............................................................1183.3.1 Introduction ............................................................................................... ......................118 3.3.2 Methods for service life prediction suggested by various researchers...........................118 3.3.3 Prediction of strength retention using time shift factors.................................................119 3.3.4 Strength retention prediction separating chemical reactions and transp. mechanisms..122

4 Experiments......................................................................................................131 4.1 Introduction and overview .........................................................................1314.2 Specimens ..................................................................................................132

4.2.1 GFRP bars........................................................................................................................132 4.2.2 Matrix material .......................................................................................... ......................132 4.2.3 Exposure condition..........................................................................................................133 4.2.4 Concrete...........................................................................................................................133 4.2.5 Alkali solution ............................................................................................. ....................134 4.2.6 Tap water ................................................................................... ......................................135 4.2.7 Test plan...........................................................................................................................135

4.3 Test methods ..............................................................................................1354.3.1 Bond strength...................................................................................................................136 4.3.2 Tensile strength................................................................................................................137 4.3.3 Modulus of elasticity ............................................................................... ........................137

4.3.4 ILSS .................................................................................................... .............................138 4.3.5 Weight gain measurements ............................................................................ .................140 4.3.6 TGA .................................................................................................... .............................140 4.3.7 SEM/EDX and LA-ICPMS.............................................................................................141

4.4 Results: Unexposed specimens ..................................................................1414.4.1 Tensile strength................................................................................................................141 4.4.2 Modulus of elasticity ............................................................................... ........................142 4.4.3 ILSS .................................................................................................... .............................143 4.4.4 Density of composites and matrix materials...................................................................143 4.4.5 TGA .................................................................................................... .............................144

4.5 Results: Matrix material.............................................................................1454.6 Results: Grey bar........................................................................................146

4.6.1 Tensile strength................................................................................................................146 4.6.2 Modulus of elasticity ............................................................................... ........................149 4.6.3 ILSS .................................................................................................... .............................149 4.6.4 Weight gain measurements ............................................................................ .................149 4.6.5 SEM .................................................................................................... .............................150 4.6.6 LA-ICPMS.......................................................................................................................151 4.6.7 Discussion........................................................................................................................152

4.7 Results: YELLOW bar...............................................................................1544.7.1 Tensile strength................................................................................................................154 4.7.2 Modulus of elasticity ............................................................................... ........................155 4.7.3 ILSS .................................................................................................... .............................156 4.7.4 Weight gain measurements ............................................................................ .................157


14/225

CONTENTS

-xiv-

4.7.5 SEM .................................................................................................... .............................158 4.7.6 LA-ICPMS.......................................................................................................................158 4.7.7 Discussion........................................................................................................................158

4.8 Results: GREEN bar ..................................................................................1594.8.1 Tensile strength................................................................................................................159 4.8.2 Modulus of elasticity ............................................................................... ........................160 4.8.3 ILSS .................................................................................................... .............................160 4.8.4 Weight gain measurements: Water .................................................................. ...............162 4.8.5 Weight gain measurements: Alkaline solution ...............................................................162 4.8.6 SEM .................................................................................................... .............................163 4.8.7 LA-ICPMS.......................................................................................................................163 4.8.8 Discussion........................................................................................................................164

4.9 Results: FIBERBAR ..................................................................................1654.9.1 Visual inspection of surface layer...................................................................................165 4.9.2 Tensile strength................................................................................................................165 4.9.3 Modulus of elasticity ............................................................................... ........................166 4.9.4 ILSS .................................................................................................... .............................167 4.9.5 Bond strength...................................................................................................................167 4.9.6 Weight gain measurements ............................................................................ .................167 4.9.7 SEM .................................................................................................... .............................169 4.9.8 EDX .................................................................................................... .............................170 4.9.9 LA-ICPMS.......................................................................................................................170 4.9.10 Discussion...................................................................................................................172

4.10 Summary of results ....................................................................................1754.11 General considerations regarding the exposure conditions .......................176

5 Prediction of strength retention......................................................................179 5.1 Approach involving time shift factor.........................................................179

5.1.1 Description of execution ...................................................................................... ...........179 5.1.2 Grey bar ................................................................................................... ........................181 5.1.3 FIBERBAR......................................................................................................................185 5.1.4 Yellow bar .............................................................................................. .........................185 5.1.5 Green bar .......................................................................................... ...............................186 5.1.6 Discussion and conclusion ..............................................................................................186

5.2 Approach separating chemical reactions and transport mechanisms.........188

6 Conclusions.......................................................................................................191 7 Future research ................................................................................................193 8 References.........................................................................................................195 APPENDIX 1............................................................................................................201 APPENDIX 2............................................................................................................202 APPENDIX 3............................................................................................................206 APPENDIX 4............................................................................................................208 APPENDIX 5............................................................................................................210


15/225

Part A

-1-

Part A Durability of FRP reinforcement in concrete

-A literature review


16/225


17/225

Part A SUMMARY

-3-

Summary

During the latest decade there has been a substantial increase in the use of FRP (FibreReinforced Polymers) as concrete reinforcement in the construction industry. FRPconsists of continuous fibres, usually aramid, carbon or glass, in a polymer matrix,usually consisting of polyester, vinyl ester or epoxy. The mechanical properties arequite different from those of steel. Generally, FRP has lower weight; lower Youngsmodulus but higher tensile strength than steel. In addition, the stress-strain curve isstraight until failure, giving the material a brittle failure mode.

The most important benefit of FRP reinforcement is perhaps the fact that it does notcorrode in the same way as steel, which makes it an interesting reinforcement optionfor concrete structures in severe environments. Other applications are structures wherereinforcement with non-metallic properties is required (for example in thesurroundings of some medical equipment) or where a member must have a highstrength to weight ratio. Another important advantage of this material is that it is easyto handle, which reduces the application time and total cost. This is a great benefitespecially in repair/retrofit works.

However, the FRP material is prone to deteriorate due to other degradationmechanisms and durability is probably the most important criterion for theimplementation of FRP reinforcement for concrete in the construction industry.Structural engineers have to be convinced that FRP reinforcement will have anacceptable lifetime before this kind of reinforcement can be used on a large scale. Thedurability of FRP for concrete reinforcement has therefore been a pressing issue inrecent years and has been the subject of many research activities now in progress inseveral countries all over the world.

In the present part of this report (Part A), the durability of FRP used as concretereinforcement is discussed. The objective has been to summarise the current researchand knowledge on the subject, and to identify areas where more research is needed toclarify the uncertainties associated with the durability of FRP. Examples of potentiallyaggressive environments and conditions discussed in this part of the report are: waterand moisture, salt, alkali, freeze-thaw actions, ultraviolet rays, thermal actions. Inaddition, different service life prediction methods that have been proposed arediscussed. The information in this part of the report is mainly obtained from

proceedings from FRP related conferences from the year 1997 and later.

To evaluate the durability of FRP reinforcement, a lot of ageing experiments have been performed worldwide. Since the effects of natural ageing cannot be achieved in a

reasonable time, the experiments are generally accelerated to a great extent by usingelevated temperatures in the exposure environments.

From the literature review it can be concluded that more knowledge is needed atdifferent levels. There are some specific topics where further research is required.These topics are discussed in the report. Furthermore, there are some aspects whichare appropriate for research activities in a general sense. An important issue is that nostandard durability test method exists. This makes the results obtained by differentresearchers difficult to compare. However, the most important general problem is


18/225

Part A SUMMARY

-4-

probably the uncertainty associated with the transformation of accelerated test resultsinto real exposure time. The real lifetime is sometimes determined by extrapolationof deterioration data from real exposure, or by using a shift factor determined byrelating deterioration under accelerated conditions to deterioration under realapplication environments. Nevertheless, the need of verification by following up thereal ageing and related deterioration is always emphasised.

To achieve more reliable lifetime data, the degradation mechanism for different FRPsystems must be surveyed and more closely connected to the lifetime prediction. Thisconsideration is valid for the majority of durability areas, but the resistance of FRP toalkali could be said to be the most basic issue since alkali will always be present whenFRP are used as concrete reinforcement.


19/225

Part A Chapter 1: Introduction

-5-

1 Introduction

1.1 Background

During the latest decade there has been an important increase in the use of FRP (FibreReinforced Polymers) as concrete reinforcement in the construction industry. FRPconsists of continuous fibres, mostly aramid, carbon or glass, in a polymer matrix,usually polyester, vinyl ester or epoxy. The mechanical properties are quite differentfrom those of steel. The properties are dependent on the fibre and resin type.Generally, FRP has lower weight, lower Youngs modulus but higher strength thansteel. In addition, the stress-strain curves are straight up to failure, giving the materiala brittle failure mode.

The most obvious benefit of the FRP material is perhaps the fact that it does notcorrode in the same way as steel, which makes it an interesting reinforcement optionfor concrete structures in severe environments. Other applications are structures where

reinforcement with non-metallic properties is required (for example in thesurroundings of some medical equipment) or where a member must have a highstrength to weight ratio. Another important advantage of this material is that it is easyto handle, which reduces the application time and total cost - a great benefit forexample in repair/retrofit works.

The shape of FRP reinforcement can be widely varied, for example there are bars,cables (for prestressing purposes), profiles, wraps (to be used around concretecolumns), plates (for repair or retrofitting of concrete elements) and even as theformwork or mould for the concrete which the FRP is intended to reinforce.

While FRP reinforcement does not corrode in the same way as steel reinforcement, it

is prone to deteriorate due to other degradation mechanisms. The concrete poresolution, for example, is a potential durability threat for FRP reinforcement. Glassfibres in particular are susceptible to the high alkaline environment in the poresolution and must be well protected by the matrix to prevent rapid degradation. Seasalt, de-icing salt, freeze-thaw action, UV-light and even fresh water and moisture areother possible degradation sources that have to be taken into consideration when usingFRP reinforcement in concrete. Durability is probably the most important criterion forthe implementation of FRP reinforcement for concrete in the construction industry.The durability of FRP has therefore been a pressing issue in recent years and researchactivities are in progress in several countries all over the world.

1.2 Objective and scope

The aim of this work was to update knowledge on the subject: Durability and lifetime prediction of FRP as reinforcement for concrete . It was further the purpose to identifyareas in the field of FRP durability where more knowledge is needed. The subject ofthe research reported in Part B was chosen based on these findings .


20/225


-6-

In this context, durability refers to FRP related degradation as a result of theenvironmental conditions in the surroundings of the FRP reinforcement. Fatigue andcreep, unless considered in conjunction with environmental influence, aredisregarded.

The information given in this report is mainly obtained from the proceedings of FRP

related conferences and from various FRP State-of-the-Art Reports. In terms of possible FRP degradation mechanisms, only those treated in these sources will bediscussed here.

1.3 Applications of FRP

Reinforced concrete members

FRP bars have been used in several types of concrete structures. In these applications,the fact that FRP does not corrode in the same way as steel and its non-magnetic

properties have been important criteria for the material choice. Owing to the relativelylow stiffness of the fibre composite material, it is often the deformation requirementswhich determine the amount of reinforcement needed. For this reason FRPreinforcement may only be competitive in environments aggressive to steel, such aschemical industries and road bridges subjected to de-icing salts, or when specialdemands are placed on the reinforcement, such as low weight or non-metalliccharacteristics. FRP reinforcements exhibit brittle failure, which has to be taken intoconsideration to guarantee a safe design.

Prestressed concrete

FRP cables and strands have been used in many prestressed concrete members aroundthe world. In contrast to reinforced concrete, the low Youngs modulus of FRP is a

benefit here because the creep and shrinkage of the concrete do not cause as great areduction in the prestressing force as in the case of traditional prestressing steel. TheFRP unit on its own should have no creep. Anchorage can be a problem, but there areseveral anchorage methods. For example, the cables can be fixed by conventionalanchorage dowels, or they can be anchored in a cylinder filled with matrix materials.

Repair and retrofitting of concrete structures.

Concrete structures sometimes need to be strengthened, often due to degradation, butalso because of changed load capacity requirements. Applying steel plates on theconcrete surface can increase the moment capacity of concrete beams. To fix the

plates, epoxy adhesive is generally used. However, this is complicated work since the plates are heavy and need to be connected to the beam during the application by bolts,and a scaffold is also needed. An additional disadvantage associated with steel platesis the fact that they are prone to corrode, especially near the steel/epoxy interface. Inrecent years, FRP laminate strips have been used as a substitute for steel plates inmany objects. The strips can relatively easily be put and glued in place without any


21/225


-7-

external support. Another advantage is that FRP can be delivered in long lengths onrolls, eliminating the need for joints. Such FRP reinforcement increases the materialcost, but it is a lot easier to place, which often reduces the total repair/retrofit cost andtime.

Particularly in Japan and USA, FRP wraps have been used for seismic retrofit

purposes. The matrix material can be applied to the fibres before or in conjunctionwith the wrapping. This technique is also used to repair reinforced concrete columnswhere steel corrosion has started.

After application of the FRP laminates, the composite material is cured and aappropriate surface treatment is applied, for example painting or rendering.


22/225


23/225

Part A Chapter 2: Material characteristics

-9-

2 Material characteristics

2.1 Introduction

The term "composite" can be applied to any combination of two or more separatematerials which have an identifiable interface between them. In this report the termcomposite refers to FRP, which consists of continuous fibres arranged and manufacturedin different ways and embedded in a polymer matrix. The properties of FRP aregoverned by the properties of the fibres, the polymer matrix, the interface region and theorientation of the fibres.

FRP reinforcement may degrade in the concrete environment. The most importantdegradation source is probably the alkalis present in the concrete, but other factorssuch as UV radiation and water may also cause deterioration in the properties of theFRP material. It is also known that when FRPs are subjected to tensile stress, creep-rupture may occur, and the overall degradation generally takes place at a higher rate.

In the present chapter, a general description of the FRP materials and constituents isgiven together with a brief discussion regarding their durability characteristics. Onlythe most commonly used fibres (carbon, glass and aramid) and resins (epoxy,

polyester and vinyl ester) are considered.

2.2 Fibres

2.2.1 General considerations

The fibre materials most commonly used in FRP for concrete reinforcement are carbon,aramid and glass. The present section will focus on these fibres. Examples of fibres morerarely used in recent years are polyvinyl alcohol fibres, polyethylene fibres, and siliconfibres. Carbon, aramid, and glass fibres have tensile strength higher than that of steel andare elastic up to tensile failure without showing any yield. There are large differencesregarding environmental resistance between different fibres as can be seen in Table 1.

2.2.2 Carbon Fibres

Carbon fibre is made from either petroleum or coal pitch and polyacrylonitrile (PAN).The fibres are an aggregate of imperfect fine graphite crystals. The composition andorientation of the crystals determine the characteristics of the fibres. There are pitch typeand PAN type fibres. Both are made of HT fibres (HT denotes high tension) of highstrength and lower modulus of elasticity than HM fibres (HM denotes high modulus),which have higher modulus of elasticity but lower strength. The pitch fibres havegenerally 20% lower qualities than the PAN fibres. However, due to the high modulus ofelasticity and small ultimate elongation they have poor toughness and are susceptible to


24/225


-10-

impact (fib, TG 4.2, June, 1998). The properties of some carbon fibres are given inTable 2.

Table 1 Qualitative assessment of fibres with respect to chemical resistance.

Notation: A=excellent, B=good, C=passable and D=poor. Modifiedversion of table in Machida (1993).

Carbon fibre Aramid fibre Glass fibre

EnvironmentsGP-

grade pitch

HP-grade pitch

HT-typePAN

HM-typePAN

Kevlar-49

Tecchnora

E-glass

AR-glass

Acid Hydrochloric acid B A A A D B D -resistance Sulphuric acid A A A A D B D -

Nitric acid B A A A D B D -Alkali resist. Sodium hydroxide A A A A B B C B

Brine resistance A A A A B B C -Organic Acetone A A A A A - A -

solvent Benzene A A A A A B A -resistance Gasoline A A A A A B A -

Table 2 Properties of carbon fibres (fib, TG 4.2, June, 1998)

PAN HT(High tension)

PAN HM(High modulus)

Pitch HT(High tension)

Pitch HM(High modulus)

Tensile strength(MPa)

3500 2500-4000 780-1000 3000-3500

Modulus of elasticity

(GPa)200-240 350-650 380-400 400-800

Elongation(%) 1.3-1.8 0.4-0.8 2.1-2.5 0.4-1.5

Density(kg/m 3) 1700-1800 1800-2000 1600-1700 1900-2100

Diameter(m)

5-8 5-8 9-18 9-18

Carbon fibres do not absorb water and are resistant to many chemical solutions,making them particularly suited for environmental exposure (Bank and Gentry 1995).

The fibres are more or less completely free of problems in regard to chemical attacks.A high resistance to acid, alkali and organic solvents is noted (Machida 1993).Graphite transfers electrical current and, therefore, contact with steel might give rise togalvanic corrosion problems (fib, TG 4.2, June, 1998). For carbon fibres stresscorrosion is not considered a problem.


25/225


-11-

2.2.3 Aramid fibres

Aramid is a generic term for a class of aromatic polyamide fibres introducedcommercially during the early 1960s. Aramid fibre is unusual in that it is technically athermoplastic polymer. (ASM 1987).

Examples of commercially available aramid fibre types are Kevlar, Twaron, Technoraand Russian aramid fibres SVM. The properties of some aramid fibres are given in Table3

Table 3 Properties of aramid fibres . (fib, TG 4.2, June, 1998)

Kevlar 49 andTwaron Technora

Russian aramidSVM

Tensile strength(MPa) 2800 3500 2500 - 3800

Modulus of elasticity(GPa)

130 74 130

Elongation(%) 2.3 4.6 3.5

Density(kg/m 3) 1450 1390 1430

Diameter(m)

12 12 15

Aramid fibres are made of polymers and are susceptible to moisture (Bank and Gentry

1995). Studies have shown that in the saturated state, especially at elevatedtemperatures, large deterioration in the flexural strength of aramid/epoxy laminatesmay take place. As can be noted in Table 1 some of the characteristics do not comparefavourably with carbon fibres. In addition, they are generally considered to compare

poorly in regard to resistance against ultraviolet rays, requiring care depending on theenvironment in which they are used. (Machida 1993)

Aramid fibres have been shown to have bad resistance to high temperatures comparedwith glass and carbon fibres. Figure 1 shows the tensile strength of AFRP, CFRP andGFRP bars at different temperatures. In the temperature range between 20 andapproximately 60 C, however, no deterioration seems to occur (Machida 1993).

For aramid fibres stress rupture may take place at high tensile stresses over a long period of time. Therefore this has to be taken into account when designing witharamid-FRP (see Chapter 4, Part A).


26/225


-12-

0

20

40

60

80

100

120

0 50 100 150 200 250

Temperature (C)

T e

n s i

l e S t r e n g

t h ( i n

% o

f s t r e n g

t h a

t r o o m

t e m p e r a

t u r e

)

Glass-FRP

Aramid-FRP

Carbon-FRP

Figure 1The tensile strength of different

FRP at various temperatures(Machida 1993).

2.2.4 Glass fibres

Glass is an isotropic material and is based on silica (SiO 2) with additions of oxides of

calcium, boron, sodium, iron and aluminum. Glass fibres are made by drawing moltenglass through a 1-3 mm aperture and drawing it further until the fibre diameter is 3-20m. The fibre diameter is controlled by adjusting the head of the glass in the tank, theviscosity of the glass, the diameter of the holes and the winding speed.

Several different glass compositions are available. E-glass (E for electrical grade) isthe most widely used general-purpose form of composite reinforcement. It has goodmechanical properties and is available at a relatively low price. Other glass fibres ofinterest are S-glass (S for Strength) and AR glass (AR for alkali resistant). S-glass ismore expensive than E-glass but has a higher strength, Youngs modulus andtemperature resistance. AR glass contains an amount of zirconia, which serves to

prevent corrosion by alkali attacks. When compared with E glass fibres, they show alarge improvement in regard to resistance against alkalis, while showing more or lessidentical density, tensile strength and elasticity (Machida 1993).

Some properties of different glass fibres are shown in Table 4 and an indication of thealkaline resistance of E- and AR-glass is given in Table 5.


27/225


-13-

Table 4 Properties of glass fibres . (fib, TG 4.2, June, 1998; ASM 1987)

E-glass S-glass AR-glassTensile strength

(MPa) 3500-3600 4100 1800-3500

Modulus of elasticity

(GPa)74-75 85 70-76

Elongation(%)

4.8 2-3

Density(kg/m 3) 2600 2500 2270

Diameter(m)

8-12 8-12

Table 5 Alkali resistance of some glass fibre types (Machida 1993).

AR-glass E-glass NaOH

(1.5 hours) 5 59Diameter decrease (%)Saturated Ca(OH) 2

(4 hours)


28/225


-14-

Table 6 Typical properties of some FRP matrix materials (Nanni 1993 andSentler 1992).

Polyester Epoxy Epoxy- vinyl ester

Tensile strength(MPa) 20-100 55-130 81

Modulus of elasticity(GPa)

2.1-4.1 2.5-4.1 3.3

Elongation(%)

1-6 1-9 5

Density(kg/m 3) 1000-1450 1100-1300 1120

Resins used for anticorrosion FRP include phenol type epoxy resin, isophthalic acid polyester resin and novolac type vinyl ester resin. All these resins deteriorate over a period of years. Such deterioration may be due to physical effects, such as watersaturation, or it may be a result of chemical reactions. (Machida, 1993).

In general terms, moisture reduces the glass transition temperature, T g, of the matrixdue to plastification of the matrix as a result of interruption of Van der Waals bond

between the polymer chains (Bank and Gentry 1995). This can cause considerablechanges in modulus, strength, strain to failure and toughness. These effects may bereversible, but the swelling stresses induced by moisture uptake can cause permanentdamage such as matrix cracking, hydrolysis and fibre-matrix debonding (Hayes et al1998). The diffusion coefficient of water in a neat resin or a composite is known to bedependent upon temperature and is commonly modelled using Ficks law (Hayes et al1998).

2.3.2 Degradation of resin

High molecular materials are not associated with the concept of rate of corrosion, asin the case of corrosion of metals, which makes systematic anticorrosion design ofFRP difficult (Machida 1993). However, some tests indicate the presence of corrosionmodes similar to those of metals. In general, these types of deterioration will followthe progress illustrated in Figure 2.


29/225


-15-

Environmentalsolvent

Dissolution section

Resin


Layer formation

Resin


Intrusion section

Resin

a) Surface reaction type corrosion

b) Layer formation type corrosion

c) Intrusion type corrosion

Figure 2 Types of resin deterioration progress (Machida 1993).

Corrosion can be further classified into the four different modes shown in Table 7,Surface reaction, Corrosion layer formation without strength, Corrosion layerformation with strength and Full penetration.

Table 7 Classification and examples of different corrosion modes (Machida1993).

Corrosion modeof resin

Chemicalstructure

Control factor ofcorrosion

Resintype

Solutiontype

Formula forcorrosion rate

Surface reaction Weak Reaction A NaOH solution x = k 1 tDiffusion B NaOH solution w = k 4(t t 0)1/2 Diffusion C NaOH solution x = k 2 t1/2

Weak layerformation Weak

Reaction C NaOH solution x = k 2 tStrong layer

formation Strong Diffusion D NaOH and KOH

solution x = k 4 t1/2

Diffusion B Boiling water w = k 7(t t 0)1/2

Intrusion -Diffusion E H 2SO 4 solution y = k 8(t t 0)1/2

A: Methyltetrahydrophthalicanhydride t: Immersion timehardening type epoxy resin t 0: Incubation period

B: Orthochromatic phthalic acid polyester resin x: x Corrosion depthC: Isophthalic acid polyester resin y: Intrusive depthD: Novolac type ester resin w: Eluded amount ofE: Menthan diamine hardening type epoxy resin resin

Surface reaction typeFigure 3 shows the relationship between the depth of corrosion and the time of

submersion of an epoxy resin type in NaOH solution. For NaOH=10% and T=50 C inthe figure the corrosion depth is about 15 m after 1000 hours. According to Table 7,x=k 1t (x=depth, k=constant and t=time) so depth is expected to be 1.32 mm in 10years and 6.6 mm in 50 years.


30/225


-16-

Corrosion layer formation type (weak)Figure 4 shows the relationship between the corrosion depth and the immersion timeof another epoxy type resin, in NaOH(10%) solution. For the 50 C immersion, thedepth is approximately 40 m after 1000 hours. Using the model proposed in Table 7,x2=k 2t, the corrosion depth is expected to be about 0.37 mm in 10 years and 0.82 mmin 50 years.

Corrosion layer formation type (strong) and full penetrationThe rate of corrosion is generally believed to be slower than in the above types.

0

0.01

0.02

0.03

0.04

0.05

0.06

0 200 400 600 800 1000 1200

Immersion time (h)

C o r r o s i o n

d e p

t h ( m m

) x = k 1 t

T=50 C

T=65 C

T=80 C

0.001

0.01

0.1

1

1 10 100 1000 10000

Immersion time (h)

C o r r o s i o n

d e p

t h ( m m

)

T=50 C

T=65 C

T=80 Cx = k 2 t1/2

Figure 3 Relationship between thecorrosion depth and theimmersion time for an epoxyresin in NaOH solution(Machida 1993).

Figure 4 As Figure 3 but for anotherepoxy resin (Machida 1993).

The table in Appendix 1, Part A, gives an idea of the chemical durability of epoxy and polyester resin subjected to different chemical solutions. It is clear from that table thatepoxy resin can show higher resistance against chemical attacks than polyester resin,and that the resins become more susceptible as temperature increases. Resinsgenerally have low resistance under high temperature conditions. Their resistance toheat, moreover, will further decrease when subjected to high temperatures for a longtime. The maximum temperature in service of epoxy and polyester resins forcontinuous use has been shown to be 120 and 105 C respectively (Machida 1993).

In general terms, vinyl ester and epoxy resin have been found to have goodenvironmental resistance under moist and/or alkaline conditions while polyester resinusually exhibits poor durability properties in such environments..


31/225


-17-

2.4 FRP concrete reinforcement

2.4.1 Manufacture

There are many ways to design an FRP bar. The fibres can be stretched and impregnatedin a pultrusion process, or mixed in a hybrid structure. The bar surface can be speciallyreinforced with extra hard corrosion resistant resin reinforced with chopped mineralfibres and the bond increased by siliceous grains. . It is not necessary for FRPreinforcement for concrete to have the same shape as steel reinforcement. In addition to

bars, the FRP reinforcement can be made in the form of cables, profiles, plates and evenas the formwork or mould for the concrete which the FRP is intended to reinforce. The

bars may also be provided with built in electronic deformation control and other moresophisticated systems.

Rods are usually manufactured by the pultrusion method. During this process the fibresare stretched to prevent twisting, thereby ensuring uniform stress distribution. Further, toensure good bonding with concrete, the rods are given surface treatment such as sandcoating or the fibre bundles are braided in a twill or spiral pattern. The surface and thedeformations (lugs) of the rods can be reinforced with chopped ceramic fibres and aharder type of resin resistant to environment influences (fib, TG 4.2, June, 1998). TheFRP reinforcement can be manufactured in a wide variety of shapes. For example,there are bars, cables (for prestressing purposes), profiles, wraps (for example, to beused around concrete columns), plates (for repair or retrofitting of concrete elements)and even as the formwork or mould for the concrete which the FRP is intended toreinforce. Generally, fibres are treated with a finish which contains a coupling agent toimprove the bond of resin to the fibres (ASM 1987).

2.4.2 Classification

FRP reinforcement can be classified in different ways. Figure 5 shows the classification by material. Figure 6 shows the classification by reinforcement shape.

2.4.3 Physical and mechanical properties

The mechanical properties are typically quite different from those of steel. The properties are dependent on the fibre and resin type, but generally, FRP has lowerweight, lower Youngs modulus but higher strength than steel. In addition, the stress-

strain curves of FRPs are straight up to failure, giving the material a brittle failuremode. Below, some important physical and mechanical properties will be furtherdiscussed. The information has essentially been obtained from (ACI committee 4401996).


32/225


-18-

Classification by material

Classification by

fibres

Classification by

matrix material

Inorganicfibres

Organicfibres

Carbonfibres

Glassfibres

AR-glass

E-glass

PAN-carbon

Pitch-carbon

Aramidfibres

Otherfibres

Inorganicmaterial

Organicmaterial

Epoxy Vinylester

Poly-ester

OthersCementetc

Classification by material

Classification by

fibres

Classification by

matrix material

Inorganicfibres

Organicfibres

Carbonfibres

Glassfibres

AR-glass

E-glass

PAN-carbon

Pitch-carbon

Aramidfibres

Otherfibres

Inorganicmaterial

Organicmaterial

Epoxy Vinylester

Poly-ester

OthersCementetc

Figure 3 Classification of FRP by material (Machida 1993) .

Classification by shape

Bar, one-dimensional

Threedimensional

textile

Grid Shapes Others

Spiralhoop Stirrupetc

Twistedform

Straightform

Braidedform

Round bar

Polygonal bar

Withsurface

treatment

Withoutsurface

treatment

Withsurface

treatment

Withoutsurface

treatment

Withsurface

treatment

Withoutsurface

treatment

Withspiralfibre

Sandcovered

Others Sandcovered

Classification by shape

Bar, one-dimensional

Threedimensional

textile

Grid Shapes Others

Spiralhoop Stirrupetc

Twistedform

Straightform

Braidedform

Round bar

Polygonal bar

Withsurface

treatment

Withoutsurface

treatment

Withsurface

treatment

Withoutsurface

treatment

Withsurface

treatment

Withoutsurface

treatment

Withspiralfibre

Sandcovered

Others Sandcovered

Figure 6 Classification of FRP by reinforcement shape (Machida 1993) .


33/225


-19-

DensityThe specific gravity of FRP is nearly four times lower than that of steel, which leadsto easier handling and shorter installation time at the job site compared with steelreinforcement.

Thermal expansionReinforced concrete is itself a composite material where the reinforcement acts as thestrengthening medium and the concrete as the matrix. It is therefore desirable that thedifferential deformations of concrete and reinforcement are minimised. The thermalexpansion for FRP reinforcements ranges from -1.0 10 -6 C -1 to 9.9 10 -6 C -1. Forconcrete the corresponding values are 6 to 11 10 -6 C -1 and for steel 11.7 10 -6 C -1 .

Tensile strengthFRP reinforcements are elastic up to failure and consequently fail in brittle mode .The strength in the longitudinal direction is generally higher than that of steel.Furthermore, unlike steel, the tensile strength of FRP is a function of bar diameter.This effect is explained by shear lag, which means that fibres located near the surfaceare subjected to higher stress than those located at the centre. The shear lag and theinfluence of gripping devices for the fibres makes it difficult to determine the realtensile strength, with the consequence that it is difficult to use the tensile strength as areference for determination of FRP degradation due to environmental influence.

Tensile elastic modulusThe longitudinal modulus of elasticity for GFRP is approximately 25% that of steelwhile CFRP usually has a Youngs modulus just slightly less than that of steel.

Shear strength

The shear strength of FRP is very low. Therefore the FRP reinforcement membershave to be oriented in such a way that they will resist the applied loads through axialtension.

Creep and relaxationFor fibre composites, creep and relaxation are highly dependent on the load directionrelative to the fibre orientation. For CFRP and GFRP (at service load) the creep andrelaxation taking place have been reported to be equal to or less than that of "lowrelaxation steel". For AFRP on the other hand significant creep and relaxation mayoccur. Creep of 7% after 50 years at a load of 40% of the short-term strength has been

predicted for one AFRP product. For another AFRP product, the predicted relaxationafter 30 years and a load level of 50% of the short-term strength was 15%, which is

more than that of steel.

Creep ruptureFailure due to creep rupture may occur for most materials when they are subjected tohigh tensile stresses for a long period of time. Hence, it is essential to limit theallowable sustained stress limit in order to avoid this type of failure. Of the FRP typesused for concrete reinforcement GFRP is the most susceptible. Also for AFRP creeprupture may take place but at higher stresses than in GFRP. As an example it can bementioned that a sustained stress limit of 20% and 30% of the short-term strength has


34/225


-20-

been suggested for GFRP and AFRP respectively in the latest ACI draft of the designguideline for FRP used as concrete reinforcement (ACI 2000). For CFRP on the otherhand creep rupture is not considered a problem.

FatigueAFRP and CFRP generally exhibit good fatigue resistance (better than that of steel),

while GFRP performs slightly worse than steel regarding this property(fib TG 9.32000).

Typical values of mechanical properties for FRP materials and steel composites aregiven in Table 8.

Table 8 Mechanical properties of steel and FRP (ACI 440 1996, modified)

Steel bar(reinforcing)

Steeltendon

GFRPbar

GFRPtendon

CFRPtendon

AFRPtendon

Tensile strength(MPa)

450-900 1300-1900500-1200

1300-1700

1650-2400

1200-2000

Modulus ofelasticity

(GPa)200 185-200 40-55 45-65 150-165 50-75

Elongation(%) 10 4 3.5-5 3-4.5 1-1.5 2-2.6

Density(kg/m 3) 7900 7900

1500-2000 2400

1500-1600 1250

Coefficient ofaxial thermal

expansion(10 -6/C)

11.7 11.7 9.9 9.9 0.0 -1.0

2.4.4 Durability

MoistureAbsorption of water in FRP composites may result in swelling and change in other

properties. It is therefore important that the mechanical properties are determinedunder moisture conditions similar to those present in the real application. There are,however, resin types that are formulated to be moisture resistant.

High temperatureMost composites do not burn easily and many composites have good properties atelevated temperatures. For example, tests have shown that E-glass FRP bars mayretain 85% of their room-temperature strength after 30 minutes of exposure to 300 Cwhile stressed to 50% of their tensile strength. The resins are more susceptible to hightemperature than fibres. In the case of bars in concrete, the concrete serves as a barrierto protect the bars from direct contact with the flames and delays the rise intemperature.


35/225


-21-

The durability of FRP depends on the durability of its constituents; fibres, resins andtheir interface, but also on their interconnected behaviour within the composite.Durability properties of fibres and resins have been treated earlier in this report andwill not be further discussed here. Instead, the durability of composites will beconsidered in general terms.

The matrix usually has a larger ultimate tensile strain than the fibre material.However, when fibres are impregnated with resin, pores in the resin cannot beavoided. It is very difficult to get a compact resin-fibres composite. These pores giverise to stress concentrations in the resin under tensile load, which results in microcracking of resin. Therefore an FRP rod under tension sustains cracks in the matrix

before breakage of fibres in spite of the fact that the ultimate elongation of resin ismuch higher than that of fibres. Microcracks in thin outer matrix skin cause a loss oftightness against the ingress of fluids (Rostsy 1997).

Fibres are often given surface treatment such as sizing and finish. The finish increasesthe bond between fibres and matrix (ASM 1987), and can improve the durability

properties of the composite (Nguyen et al 1998).

The typical degradation behaviour of composite materials subjected to someaggressive environments is shown in Figure 7 and Figure 8. Figure 7 sets out thestrength retention for a specimen subjected to a combination of tensile stress andenvironmental influence. Figure 8 shows the behaviour of a specimen in a stressrupture test. The threshold stress level indicated in Figure 7 and Figure 8 refers tothe stress level at which micro cracks will allow a relatively fast rate of infusion of theenvironment through these open channels and allow fibre degradation to occur. Whenthe stress level is sufficiently low, the viscoelastic behaviour of the resin providesstress relief and the crack formation does not occur (Devalapura et al 1997).

Stress dominated failure

Crack initiation/propagationdominated failure

Diffusion dominated failure

Threshold level

Log Time to Failure

S t r e s s

Stress dominated failure

Crack initiation/propagationdominated failure

Diffusion dominated failure

Threshold level

Log Time to Failure

S t r e s s

S t r e n g

t h R e

t e n

t i o n

Time of Exposure

Exposure at a stress level below the Threshold stress

Exposure at a stress levelabove the Threshold stress S

t r e n g

t h R e

t e n

t i o n

Time of Exposure

Exposure at a stress level below the Threshold stress

Exposure at a stress levelabove the Threshold stress

Figure 7 Strength retention (schematic) fora specimen subjected to acombination of tensile stress andenvironmental influence (after

Devalapura et al 1997).

Figure 8 The behaviour of a specimen ina stress rupture test(Devalapura et al 1997).


36/225


37/225

Part A Chapter 3: Review on research activities and results

-23-

3 Review of research activities and results

3.1 Introduction and general considerations

The durability of FRP, as reinforcement in concrete structures, has been a pressingissue in recent years. Research activity has been extensive, and a large number of

projects are now in progress in many parts of the world. Research activities are carriedout in different institutions. A lot of research has been performed at universities andgovernment departments and some are also performed by private companies andmanufacturers. Results from research conducted at private companies andmanufacturers are sometimes not available for the public (including researchers notconnected to the company) because of commercial interests.

Below, a survey is given of the character of research on FRP in Europe and Japan,(from US perspective, text is extracted from Seible, 1998):

In the UK , industry seems to take initiatives with product development, which is thensupported by application specific university testing. Furthermore large demonstration projects are early implemented subsequent to or sometimes in parallel with a fewlarge-scale laboratory validation tests. In Germany , the development of the GFRPPolystal post-tensioning system was industry led and supported by the governmentwith very systematic university research and validation testing. In Switzerland , themain research thrust comes from the Swiss Federal Materials Testing Laboratory(EMPA) with close ties to industry. Academic research is part of the development

programme. In Japan , most of the research and development is initiated, funded, andcarried out within the industry sector, and government research laboratories writespecifications and standards. Research in Japan is based on numerous large scaleexperimental tests followed by a large number of immediate prototype demonstration

projects with less emphasis on generic and theoretically supported research data.Demonstration projects are subsidised through the participating companies toshowcase their product development. In US , research is carried out by the universitieswith only limited industry support. More applied and fundamental research seems to

be conducted in the US prior to any large-scale prototype demonstration projects andthe US government seems to provide a large portion of the research and developmentfunding.

In Chapter 2 the durability of fibres, resins and the composite was discussed. Anotherway of looking at the durability problem is to take the environmental influencesleading to degradation as a starting point. As research activities are often categorisedaccording to such environmental topics, it is considered appropriate to divide the

evaluation into the following groups:


38/225


-24-

Effects of water and moisture on FRPInfluence of salt on FRPEffects of alkali on FRPInfluence of freeze-thaw actionsDegradation caused by ultraviolet raysThermal action

Other degradation sources (including fire and galvanic coupling)

Some research projects use models for lifetime predictions, based on the results ofvarious ageing methods. These kinds of activities are also discussed in the presentchapter.

A variety of exposure conditions and test methods have been used to investigate thedurability of FRP materials and their constituents. Generally, the properties studiedare measured before and after exposure to evaluate the environment-induced changes.Exposure to different solutions at elevated temperatures is mostly used to accelerateageing. However, non-accelerated tests in the laboratory or at field test sites are alsoused to evaluate the influence of real environmental conditioning.

The properties most frequently studied are changes in mechanical properties such astensile strength and tensile modulus. However, measurements of mechanical

properties after ageing are often complemented by studies of physical properties,which sometimes makes it possible to determine the failure mechanism at a microlevel.

In terms of mechanical properties: Young's modulus, tensile strength, interlaminarshear strength, strength and bond strength are examples of properties tested in theresearch projects studied in this investigation.

As regards physical properties: TGA (Thermo Gravimetric Analysis), Lightmicroscopy, SEM (Scanning Electron Microscopy), DMA (Dynamic MechanicalAnalysis), DSC (Differential Scanning Calorimetry), Potentiodynamic PolarisationScans, Galvanic Coupling test, FTIR (Fourier Transform Infrared Spectroscopy) areexamples of analysis methods used to evaluate changes in the materials.

Research results obtained from different experiments on the durability of FRPreinforcement are generally difficult to compare. There are several reasons for thisfact. There are no international standard FRP durability test methods, and as aconsequence each research team designs its own test programme. In durability tests,factors which vary between research groups are: accelerated/non-accelerated test,duration of exposure, temperature and composition of solutions for immersion of

specimens (for example, several different mix formulae have been used to simulateconcrete pore solutions), different levels of applied mechanical stress duringexposure, exposure of naked FRP specimen or FRP embedded in concrete, differenttest methods to evaluate the effect of exposure (bond strength, stress-rupture test,residual strength, physical tests etc ). In addition, different material systems andreinforcement shapes have different durability properties and results obtained for oneFRP system and shape are only valid for that particular FRP reinforcement. Further,the production methods are other important components, different manufacturers usedifferent production methods and some researchers create their own FRP material to


39/225


-25-

have full control of its constituents. All these aspects have to be taken intoconsideration when interpreting and comparing results from different durabilityexperiments. A standard durability test method for FRP concrete reinforcement, toovercome this problem, is frequently requested.

Another aspect which makes it difficult to draw any conclusions regarding FRP

durability based on durability data is that the materials analysed in differentexperiments are presented at different detailing levels. In some papers, a detailedmaterial description is given including for example type of: resin, monomer, catalyst,mould release, filler and name of the fibre product. Others only give the FRPabbreviation, for example AFRP which only indicates that the composite containsaramid fibres. Most frequently, however, the fibre type and the resin type aredeclared, for example in the form of E-glass fibre/vinyl ester composite. Often, themanufacturer is also mentioned.

The information reviewed in this chapter has mainly been obtained from varioustechnical papers, from FRP conferences or journals. In the present chapter, differentdurability topics are treated separately. Since a single technical paper may describe a

project dealing with a number of the durability topics listed above, one paper may bereferred to several times. Furthermore, some research projects are reported wherespecimens are at the same time subjected to more than one exposure type from thelist, to investigate synergistic effects, and therefore even the same test may be referredto several times.

It is important to note that the results reported in the proceedings are only valid for thespecific material system studied in the respective experiments, and under specificexperimental conditions.

3.2 Effects of water and moisture on FRP

3.2.1 Introduction

There is a large amount of durability data currently available from the aerospaceindustries. However, this data is largely restricted to carbon/epoxy composites. Muchless data is available for systems with glass fibre and vinyl ester or polyester, thecurrent materials of choice for construction and infrastructure (Hayes et al 1998).Furthermore, the design lives for these applications are much longer than for those inthe aerospace industries.

The effect of moisture on the performance of FRP for construction has been a widelystudied issue. Often the combination of moisture and elevated temperature is used toaccelerate the diffusion and reaction with the composite materials or to determine thesynergistic effect of moisture and temperature (Bank and Gentry 1995). As regardsdegradation by the influence of water, GFRPs are frequently studied because of thesusceptibility of glass fibres to water (Bank and Gentry 1995).

Some problems are known to be associated with the presence of water in resins.Moisture can act as a plasticizer in the resin and cause interruptions of Van der Waals


40/225


-26-

bonds between the polymer chains (Bank and Gentry 1995). This can causeconsiderable changes in modulus, strength, strain to failure and toughness. Theseeffects may be reversible, but the swelling stresses induced by moisture uptake cancause permanent damage such as matrix cracking, hydrolysis and fibre-matrixdebonding (Hayes et al 1998).

3.2.2 Experimental methodology

Specimens have been subjected to a variety of exposure conditions including air atdifferent RH and temperature as well as immersion in water at different temperaturesand for different periods. Temperatures in the range of 20-60 C have generally beenused but the exposure of specimens in boiling water has also been reported (Bank andGentry 1995). Furthermore some tendon specimens have been subjected to tensilestress during exposure, to investigate stress corrosion effects. To investigate theinfluence of moisture and thermal synergistic effects on the behaviour of FRP thetemperature has sometimes been varied during immersion periods. These studies areoften motivated by the fact that water itself may cause deterioration in the FRPmaterial (Steckel et al 1998) or reversible changes in the mechanical properties(Hayes

Documents

GFRP i Beton