GFRP I Beton [PDF]

Zitiervorschau

THESIS FOR THE DEGREE OF LICENTIATE OF ENGINEERING

Durability of FRP Reinforcement in Concrete -Literature Review and Experiments Valter Dejke

Department of Building Materials CHALMERS UNIVERSITY OF TECHNOLOGY Göteborg, Sweden 2001

Key words alkali degradation durability fibre fibre reinforced polymers FRP GFRP glass fibre reinforced polymers non metallic reinforcement prediction reinforcement resin service life ISSN 1104-893X © Valter Dejke Publication no P-01:1 Department of Building Materials Chalmers University of Technology SE-412 96 Göteborg Sweden Telephone - 46 (0)31-772 1000 Reproservice, Chalmers Göteborg, Sweden 2001

ABSTRACT

Abstract During the latest decade there has been an important increase in the use of FRP (Fibre Reinforced Polymers) as concrete reinforcement in the construction industry. The most obvious benefit of using FRP for concrete reinforcement is that it does not corrode in the same way as steel, which makes it an interesting reinforcement option for concrete structures in severe environments. However, FRP is prone to deteriorate due to other degradation mechanisms than those for steel. The high alkalinity of concrete, for instance, is a possible degradation source. Other potentially FRP aggressive environments are sea salt, de-icing salt, freeze-thaw action, UV-light and fresh water/moisture. This licentiate thesis includes an update of knowledge regarding durability of FRP reinforcement in relevant environments and an overview of current research activities in this field. Although a great deal of research has been addressing the 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 to deteriorate in the environment of concrete. However, of the FRP types available GFRP is the cheapest and consequently has the highest potential of being cost effective. Hence, there is a need for reliable estimations of the rate of deterioration of GFRP in the environment of concrete. An important part of the work described in this thesis have been to gain a better understanding of the degradation mechanisms of GFRP reinforcement in concrete and to make a quantitative service life prediction for this material in real applications. The work includes a literature review of degradation mechanisms of GFRP in concrete, a theoretical discussion of possible degradation modes, 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 were examined, using several analysis methods, to investigate the environmental effects on mechanical and physical properties. Altogether approximately 1400 specimens were included 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 after approximately 1.5 year in moisture saturated concrete at 60°C and the ILSS retention after 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 exposure time under accelerated conditions to time in a real application. Thus 1.5 years at 60°C correspond to approximately 50 years in outdoor conditions in the south of Sweden (mean annual temperature, 7°C). The other predictive model takes account of any differences in the influence of the temperature on the rate of transport mechanisms within the composite and on the chemical reactions leading to degradation.

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ABSTRACT

A general conclusion from this work is that the use of FRP reinforcement can be recommended for concrete structures of arbitrary required service lives provided a proper strength reduction factor is used to take account of the deterioration of the material. Such a strength reduction factor should be separately determined for every application, and based the deterioration rate controlling factors including moisture conditions, temperature, stress level and required service life.

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PREFACE

Preface This work was mainly carried out at the Department of Building Materials, Chalmers University 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, both on a professional and a personal level. His good relations with researchers around the world enabled many journeys and the establishment of new contacts, which contributed a great deal to make my PhD studies a very fruitful and positive period in my life. I also want to express my appreciation to my main supervisor Professor LarsOlof 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 the experimental work. Special thanks also to Alf Andersen, Anders Lindvall and Juhan Aavik for their help with experiments and analysis work, and to Professor Johan Claesson 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 University of Sheffield during my stay there (August 1999 - January 2000). I also wish to thank Hughes Brothers Inc. and Fiberkonst AB who have supplied GFRP bar specimens for the experiments conducted. Representatives from FoU Väst (R&D West) have acted as the reference group and are 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 for her patience during my long and late spells of work at Chalmers.

Göteborg, January 2001 Valter Dejke

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DISPOSITION OF THE REPORT

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, AFRP and CFRP) are discussed with respect to their durability characteristics in all environmental conditions of relevance. In addition, the current research in this field is summarised. Part B focuses on the durability of GFRP in alkaline environments. This part includes a literature review on possible degradation mechanisms of GFRP in alkaline environments, discussion of experimental work conducted in the project and the 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 applications for FRP used as concrete reinforcement. Chapter 2 is aimed to provide the reader with general information about the FRP composites 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 the influence of various environmental conditions are discussed. Environmental conditions focused on are: alkali, water/moisture, freeze/thaw, UV–radiation and thermal action . In addition the approaches to service life predictions which have been found 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 documents are summarised and discussed in Chapter 4, with special emphasis on how the durability issue is handled. In the literature reviewed, a number of durability related topics have been pointed out for 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 resin subjected to different chemicals Appendix 2 summarises a number of research projects conducted in recent years. The aim has been to give an overview of how durability research is generally performed.

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DISPOSITION OF THE REPORT

Part B Durability of GFRP reinforcement in concrete -Literature review, experiments and service 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 alkaline environments and water. This chapter is based on a literature review and also includes a discussion on transport mechanisms, of alkali and water, within GFRP In Chapter 3 a theoretical discussion of durability and degradation mechanisms of GFRP in concrete is given. The relationship between transport properties within the GFRP material and possible failure modes is discussed. Furthermore, two models for strength retention prediction, formulated within the project, are described. Chapter 4 describes the experiments conducted within this project. Exposure conditions, test methods and results are discussed. In Chapter 5 strength retention predictions are made using one of the strength retention prediction models described in Chapter 3 and the experimental results from Chapter 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 for the Grey bar. Appendix 3 gives the tensile strength and ILSS (inter laminar shear strength) data for the Yellow bar. Appendix 4 gives the tensile strength and ILSS (inter laminar shear strength) data for the Green bar. Appendix 5 gives the tensile strength and ILSS (inter laminar shear strength) data for FIBERBAR.

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ABBREVIATIONS

Abbreviations AFRP AGE AGFRP AR CFRP CTE DMA DMTA DSC EPMA EDX FEM FRP FTIR GFRC GFRP GRC ILSS LA-ICPMS MAT MTI PVA RH RTE SBS SEM SIC TGA TS UTS VFRP TSF

aramid fibre reinforced polymer relative age at the temperature T (°C) compared with conditions similar to those in Des Moines, Iowa (applies to Porter and Barnes, 1998). aramid-glass hybrid alkali resistant carbon fibre reinforced polymer coefficient of thermal expansion dynamic mechanical analysis dynamic mechanical thermal analysis differential scanning calorimetry electron probe microanalysis electron disperse X-ray finite element method fibre reinforced polymer Fourier transform infrared glass fibre reinforced concrete glass fibre reinforced polymer glass fibre reinforced cement inter-laminar shear strength laser ablation inductive coupled plasma mass spectroscopy mean annual temperature mode of temperature influence polyvinyl alcohol relative humidity reverse thermal effect short beam shear (a flexural test of a specimen having a low test span to thickness ratio, such that failure is primarily in shear) scanning electron microscopy strand in cement thermo gravimetric analysis tensile strength ultimate tensile strength vinylon fibre reinforced polymer time shift factor

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CONTENTS

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 ....................................................................................................5 1.2 Objective and scope .......................................................................................5 1.3 Applications of FRP.......................................................................................6 2 Material characteristics.......................................................................................9 2.1 Introduction....................................................................................................9 2.2 Fibres..............................................................................................................9 2.2.1 2.2.2 2.2.3 2.2.4

2.3 2.3.1 2.3.2

2.4 2.4.1 2.4.2 2.4.3 2.4.4

3

General considerations ........................................................................................................9 Carbon Fibres ......................................................................................................................9 Aramid fibres.....................................................................................................................11 Glass fibres ........................................................................................................................12

Resin ............................................................................................................13 General consideration........................................................................................................13 Degradation of resin ..........................................................................................................14

FRP concrete reinforcement ........................................................................17 Manufacture.......................................................................................................................17 Classification .....................................................................................................................17 Physical and mechanical properties ..................................................................................17 Durability...........................................................................................................................20

Review of research activities and results .........................................................23 3.1 Introduction and general considerations ......................................................23 3.2 Effects of water and moisture on FRP .........................................................25 3.2.1 3.2.2 3.2.3 3.2.4

3.3 3.3.1 3.3.2 3.3.3 3.3.4

3.4 3.4.1 3.4.2 3.4.3 3.4.4

3.5 3.5.1 3.5.2 3.5.3 3.5.4

3.6 3.6.1 3.6.2

Introduction .......................................................................................................................25 Experimental methodology ...............................................................................................26 Experimental results and discussions................................................................................26 Conclusions .......................................................................................................................30

Influence of salt on FRP ..............................................................................30 Introduction .......................................................................................................................30 Experimental methodology ...............................................................................................30 Experimental results and discussions................................................................................31 Conclusions .......................................................................................................................33

Effects of Alkali on FRP..............................................................................34 Introduction .......................................................................................................................34 Experimental methodology ...............................................................................................34 Experimental results and discussions................................................................................35 Conclusions .......................................................................................................................39

Influence of freeze-thaw cycles ...................................................................40 Introduction .......................................................................................................................40 Experimental methodology ...............................................................................................40 Experimental results and discussions................................................................................40 Conclusions .......................................................................................................................42

Degradation caused by ultraviolet rays........................................................42 Introduction .......................................................................................................................42 Experimental methodology ...............................................................................................42

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CONTENTS 3.6.3 3.6.4

3.7 3.8 3.8.1 3.8.2 3.8.3 3.8.4 3.8.5

4

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

Thermal actions............................................................................................43 Other degradation sources............................................................................45 Deterioration by heat and fire............................................................................................45 The effect of galvanic coupling.........................................................................................45 Gaseous Mixtures ..............................................................................................................46 Pressure..............................................................................................................................46 Exposure to diesel..............................................................................................................46

Durability approach in existing design guidelines ..........................................47 4.1 Introduction..................................................................................................47 4.2 Japanese Society of Civil Engineers ............................................................47 4.2.1 4.2.2 4.2.3 4.2.4

4.3 4.3.1 4.3.2 4.3.3 4.3.4

4.4 4.4.1 4.4.2 4.4.3 4.4.4

4.5 4.5.1 4.5.2

4.6 4.6.1 4.6.2 4.6.3

Introduction .......................................................................................................................47 Material coefficients and member factor ..........................................................................47 Creep rupture .....................................................................................................................49 Durability test method .......................................................................................................49

Canadian Highway Bridge Design Code .....................................................50 Introduction .......................................................................................................................50 Strength reductions and stress limits.................................................................................50 Prestressed reinforcement .................................................................................................51 Restrictions in the use of FRP as concrete reinforcement................................................51

American Concrete Institute ........................................................................52 Introduction .......................................................................................................................52 Environmental reduction factor.........................................................................................52 Creep rupture stress limits.................................................................................................53 Fatigue ...............................................................................................................................53

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

Norwegian standard .....................................................................................55 Introduction .......................................................................................................................55 Strength reduction factors .................................................................................................55 Prestressed reinforcement .................................................................................................56

4.7 Conclusions..................................................................................................56 5 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 ..................................................................................................89 1.2 Objective and scope .....................................................................................89 2 Degradation mechanisms of GFRP in concrete ..............................................91 2.1 General considerations.................................................................................91 2.2 Stress rupture and stress corrosion...............................................................91 2.3 Degradation of glass fibres ..........................................................................93 2.3.1 2.3.2 2.3.3 2.3.4 2.3.5

2.4

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

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

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CONTENTS 2.4.1 2.4.2

2.5 2.6 2.6.1 2.6.2 2.6.3 2.6.4

3

Degradation in fibre/resin interface ...........................................................102 Transport properties in GFRP....................................................................103 General discussion of the transport properties of fibre composites ...............................103 Water transport in GFRP.................................................................................................103 Alkali transport in GFRP.................................................................................................105 Calculation of diffusivity and concentration profiles in plates and cylinders................106

2.7 Conclusions................................................................................................109 Theoretical discussion of durability of GFRP reinforcement in concrete ..111 3.1 Structural requirements of GFRP concrete reinforcement.........................111 3.2 Possible degradation mechanisms..............................................................111 3.2.1 3.2.2

3.3 3.3.1 3.3.2 3.3.3 3.3.4

4

General considerations ....................................................................................................101 Degradation mechanisms ................................................................................................102

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

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

Experiments......................................................................................................131 4.1 Introduction and overview .........................................................................131 4.2 Specimens ..................................................................................................132 4.2.1 4.2.2 4.2.3 4.2.4 4.2.5 4.2.6 4.2.7

4.3 4.3.1 4.3.2 4.3.3 4.3.4 4.3.5 4.3.6 4.3.7

4.4 4.4.1 4.4.2 4.4.3 4.4.4 4.4.5

4.5 4.6 4.6.1 4.6.2 4.6.3 4.6.4 4.6.5 4.6.6 4.6.7

4.7 4.7.1 4.7.2 4.7.3 4.7.4

GFRP bars........................................................................................................................132 Matrix material ................................................................................................................132 Exposure condition..........................................................................................................133 Concrete...........................................................................................................................133 Alkali solution .................................................................................................................134 Tap water .........................................................................................................................135 Test plan...........................................................................................................................135

Test methods ..............................................................................................135 Bond strength...................................................................................................................136 Tensile strength................................................................................................................137 Modulus of elasticity .......................................................................................................137 ILSS .................................................................................................................................138 Weight gain measurements .............................................................................................140 TGA .................................................................................................................................140 SEM/EDX and LA-ICPMS.............................................................................................141

Results: Unexposed specimens ..................................................................141 Tensile strength................................................................................................................141 Modulus of elasticity .......................................................................................................142 ILSS .................................................................................................................................143 Density of composites and matrix materials...................................................................143 TGA .................................................................................................................................144

Results: Matrix material.............................................................................145 Results: Grey bar........................................................................................146 Tensile strength................................................................................................................146 Modulus of elasticity .......................................................................................................149 ILSS .................................................................................................................................149 Weight gain measurements .............................................................................................149 SEM .................................................................................................................................150 LA-ICPMS.......................................................................................................................151 Discussion........................................................................................................................152

Results: YELLOW bar...............................................................................154 Tensile strength................................................................................................................154 Modulus of elasticity .......................................................................................................155 ILSS .................................................................................................................................156 Weight gain measurements .............................................................................................157

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CONTENTS 4.7.5 4.7.6 4.7.7

4.8

SEM .................................................................................................................................158 LA-ICPMS.......................................................................................................................158 Discussion........................................................................................................................158

Results: GREEN bar ..................................................................................159

4.8.1 4.8.2 4.8.3 4.8.4 4.8.5 4.8.6 4.8.7 4.8.8

4.9

Tensile strength................................................................................................................159 Modulus of elasticity .......................................................................................................160 ILSS .................................................................................................................................160 Weight gain measurements: Water .................................................................................162 Weight gain measurements: Alkaline solution ...............................................................162 SEM .................................................................................................................................163 LA-ICPMS.......................................................................................................................163 Discussion........................................................................................................................164

Results: FIBERBAR ..................................................................................165

4.9.1 4.9.2 4.9.3 4.9.4 4.9.5 4.9.6 4.9.7 4.9.8 4.9.9 4.9.10

Visual inspection of surface layer ...................................................................................165 Tensile strength................................................................................................................165 Modulus of elasticity .......................................................................................................166 ILSS .................................................................................................................................167 Bond strength...................................................................................................................167 Weight gain measurements .............................................................................................167 SEM .................................................................................................................................169 EDX .................................................................................................................................170 LA-ICPMS.......................................................................................................................170 Discussion...................................................................................................................172

4.10 Summary of results ....................................................................................175 4.11 General considerations regarding the exposure conditions .......................176 5 Prediction of strength retention......................................................................179 5.1 Approach involving time shift factor.........................................................179 5.1.1 5.1.2 5.1.3 5.1.4 5.1.5 5.1.6

Description of execution .................................................................................................179 Grey bar ...........................................................................................................................181 FIBERBAR......................................................................................................................185 Yellow bar .......................................................................................................................185 Green bar .........................................................................................................................186 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

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Part A

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

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Part A

SUMMARY

Summary During the latest decade there has been a substantial increase in the use of FRP (Fibre Reinforced Polymers) as concrete reinforcement in the construction industry. FRP consists of continuous fibres, usually aramid, carbon or glass, in a polymer matrix, usually consisting of polyester, vinyl ester or epoxy. The mechanical properties are quite different from those of steel. Generally, FRP has lower weight; lower Young’s modulus but higher tensile strength than steel. In addition, the stress-strain curve is straight until failure, giving the material a brittle failure mode. The most important benefit of FRP reinforcement is perhaps the fact that it does not corrode in the same way as steel, which makes it an interesting reinforcement option for concrete structures in severe environments. Other applications are structures where reinforcement with non-metallic properties is required (for example in the surroundings of some medical equipment) or where a member must have a high strength to weight ratio. Another important advantage of this material is that it is easy to handle, which reduces the application time and total cost. This is a great benefit especially in repair/retrofit works. However, the FRP material is prone to deteriorate due to other degradation mechanisms and durability is probably the most important criterion for the implementation of FRP reinforcement for concrete in the construction industry. Structural engineers have to be convinced that FRP reinforcement will have an acceptable lifetime before this kind of reinforcement can be used on a large scale. The durability of FRP for concrete reinforcement has therefore been a pressing issue in recent years and has been the subject of many research activities now in progress in several countries all over the world. In the present part of this report (Part A), the durability of FRP used as concrete reinforcement is discussed. The objective has been to summarise the current research and knowledge on the subject, and to identify areas where more research is needed to clarify the uncertainties associated with the durability of FRP. Examples of potentially aggressive environments and conditions discussed in this part of the report are: water and moisture, salt, alkali, freeze-thaw actions, ultraviolet rays, thermal actions. In addition, different service life prediction methods that have been proposed are discussed. 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 using elevated temperatures in the exposure environments. From the literature review it can be concluded that more knowledge is needed at different levels. There are some specific topics where further research is required. These topics are discussed in the report. Furthermore, there are some aspects which are appropriate for research activities in a general sense. An important issue is that no “standard durability test method” exists. This makes the results obtained by different researchers difficult to compare. However, the most important general problem is -3-

Part A

SUMMARY

probably the uncertainty associated with the transformation of accelerated test results into real exposure time. The “real” lifetime is sometimes determined by extrapolation of deterioration data from real exposure, or by using a shift factor determined by relating deterioration under accelerated conditions to deterioration under real application environments. Nevertheless, the need of verification by following up the real ageing and related deterioration is always emphasised. To achieve more reliable lifetime data, the degradation mechanism for different FRP systems must be surveyed and more closely connected to the lifetime prediction. This consideration is valid for the majority of durability areas, but the resistance of FRP to alkali could be said to be the most basic issue since alkali will always be present when FRP are used as concrete reinforcement.

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Part A

Chapter 1: Introduction

1 Introduction 1.1

Background

During the latest decade there has been an important increase in the use of FRP (Fibre Reinforced Polymers) as concrete reinforcement in the construction industry. FRP consists of continuous fibres, mostly aramid, carbon or glass, in a polymer matrix, usually polyester, vinyl ester or epoxy. The mechanical properties are quite different from those of steel. The properties are dependent on the fibre and resin type. Generally, FRP has lower weight, lower Young’s modulus but higher strength than steel. In addition, the stress-strain curves are straight up to failure, giving the material a brittle failure mode. The most obvious benefit of the FRP material is perhaps the fact that it does not corrode in the same way as steel, which makes it an interesting reinforcement option for concrete structures in severe environments. Other applications are structures where reinforcement with non-metallic properties is required (for example in the surroundings of some medical equipment) or where a member must have a high strength to weight ratio. Another important advantage of this material is that it is easy to handle, which reduces the application time and total cost - a great benefit for example 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 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 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 pore solution, for example, is a potential durability threat for FRP reinforcement. Glass fibres in particular are susceptible to the high alkaline environment in the pore solution and must be well protected by the matrix to prevent rapid degradation. Sea salt, de-icing salt, freeze-thaw action, UV-light and even fresh water and moisture are other possible degradation sources that have to be taken into consideration when using FRP reinforcement in concrete. Durability is probably the most important criterion for the implementation of FRP reinforcement for concrete in the construction industry. The durability of FRP has therefore been a pressing issue in recent years and research activities 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 identify areas in the field of FRP durability where more knowledge is needed. The subject of the research reported in Part B was chosen based on these findings.

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Part A

Chapter 1: Introduction

In this context, durability refers to FRP related degradation as a result of the environmental conditions in the surroundings of the FRP reinforcement. Fatigue and creep, unless considered in conjunction with environmental influence, are disregarded. 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 be discussed 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 relatively low stiffness of the fibre composite material, it is often the deformation requirements which determine the amount of reinforcement needed. For this reason FRP reinforcement may only be competitive in environments aggressive to steel, such as chemical industries and road bridges subjected to de-icing salts, or when special demands are placed on the reinforcement, such as low weight or non-metallic characteristics. FRP reinforcements exhibit brittle failure, which has to be taken into consideration to guarantee a safe design. Prestressed concrete FRP cables and strands have been used in many prestressed concrete members around the world. In contrast to reinforced concrete, the low Young’s modulus of FRP is a benefit here because the creep and shrinkage of the concrete do not cause as great a reduction in the prestressing force as in the case of traditional prestressing steel. The FRP unit on its own should have no creep. Anchorage can be a problem, but there are several anchorage methods. For example, the cables can be fixed by conventional anchorage 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, but also because of changed load capacity requirements. Applying steel plates on the concrete 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 plates is the fact that they are prone to corrode, especially near the steel/epoxy interface. In recent years, FRP laminate strips have been used as a substitute for steel plates in many objects. The strips can relatively easily be put and glued in place without any -6-

Part A

Chapter 1: Introduction

external support. Another advantage is that FRP can be delivered in long lengths on rolls, eliminating the need for joints. Such FRP reinforcement increases the material cost, but it is a lot easier to place, which often reduces the total repair/retrofit cost and time. 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 conjunction with the wrapping. This technique is also used to repair reinforced concrete columns where steel corrosion has started. After application of the FRP laminates, the composite material is cured and a appropriate surface treatment is applied, for example painting or rendering.

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Part A

Chapter 2: Material characteristics

2 Material characteristics 2.1

Introduction

The term "composite" can be applied to any combination of two or more separate materials which have an identifiable interface between them. In this report the term composite refers to FRP, which consists of continuous fibres arranged and manufactured in different ways and embedded in a polymer matrix. The properties of FRP are governed by the properties of the fibres, the polymer matrix, the interface region and the orientation of the fibres. FRP reinforcement may degrade in the concrete environment. The most important degradation source is probably the alkalis present in the concrete, but other factors such as UV radiation and water may also cause deterioration in the properties of the FRP material. It is also known that when FRPs are subjected to tensile stress, creeprupture 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 is given together with a brief discussion regarding their durability characteristics. Only the 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 more rarely used in recent years are polyvinyl alcohol fibres, polyethylene fibres, and silicon fibres. Carbon, aramid, and glass fibres have tensile strength higher than that of steel and are elastic up to tensile failure without showing any yield. There are large differences regarding 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 and orientation of the crystals determine the characteristics of the fibres. There are pitch type and PAN type fibres. Both are made of HT fibres (HT denotes high tension) of high strength and lower modulus of elasticity than HM fibres (HM denotes high modulus), which have higher modulus of elasticity but lower strength. The pitch fibres have generally 20% lower qualities than the PAN fibres. However, due to the high modulus of elasticity and small ultimate elongation they have poor toughness and are susceptible to

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Part A

Chapter 2: Material characteristics

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

Table 1

Qualitative assessment of fibres with respect to chemical resistance. Notation: A=excellent, B=good, C=passable and D=poor. Modified version of table in Machida (1993).

Environments Acid resistance Alkali resist. Organic solvent resistance

Table 2

Hydrochloric acid Sulphuric acid Nitric acid Sodium hydroxide Brine resistance Acetone Benzene Gasoline

GPgrade pitch B A B A A A A A

Carbon fibre HTHPtype grade PAN pitch A A A A A A A A A A A A A A A A

HMtype PAN A A A A A A A A

Aramid fibre Kevlar Tecch -49 nora D D D B B A A A

B B B B B B B

Glass fibre EARglass glass D D D C C A A A

B -

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

Tensile strength (MPa) Modulus of elasticity (GPa) Elongation (%) Density (kg/m3) Diameter (µm)

PAN HT

PAN HM

Pitch HT

Pitch HM

(High tension)

(High modulus)

(High tension)

(High modulus)

3500

2500-4000

780-1000

3000-3500

200-240

350-650

380-400

400-800

1.3-1.8

0.4-0.8

2.1-2.5

0.4-1.5

1700-1800

1800-2000

1600-1700

1900-2100

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 to galvanic corrosion problems (fib, TG 4.2, June, 1998). For carbon fibres stress corrosion is not considered a problem.

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Part A

Chapter 2: Material characteristics

2.2.3 Aramid fibres Aramid is a generic term for a class of aromatic polyamide fibres introduced commercially during the early 1960s. Aramid fibre is unusual in that it is technically a thermoplastic polymer. (ASM 1987). Examples of commercially available aramid fibre types are Kevlar, Twaron, Technora and Russian aramid fibres SVM. The properties of some aramid fibres are given in Table 3 Table 3

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

Tensile strength (MPa) Modulus of elasticity (GPa) Elongation (%) Density (kg/m3) Diameter (µm)

Kevlar 49 and Twaron

Technora

Russian aramid SVM

2800

3500

2500 - 3800

130

74

130

2.3

4.6

3.5

1450

1390

1430

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 elevated temperatures, large deterioration in the flexural strength of aramid/epoxy laminates may take place. As can be noted in Table 1 some of the characteristics do not compare favourably with carbon fibres. In addition, they are generally considered to compare poorly in regard to resistance against ultraviolet rays, requiring care depending on the environment in which they are used. (Machida 1993) Aramid fibres have been shown to have bad resistance to high temperatures compared with glass and carbon fibres. Figure 1 shows the tensile strength of AFRP, CFRP and GFRP bars at different temperatures. In the temperature range between 20 and approximately 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 with aramid-FRP (see Chapter 4, Part A).

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Part A

Chapter 2: Material characteristics

Tensile Strength (in % of strength at room temperature)

120 Carbon-FRP 100 Glass-FRP 80

60 Aramid-FRP 40

20

0 0

50

100

150

200

250

Figure 1 The tensile strength of different FRP at various temperatures (Machida 1993).

Temperature (C)

2.2.4 Glass fibres Glass is an isotropic material and is based on silica (SiO2) with additions of oxides of calcium, boron, sodium, iron and aluminum. Glass fibres are made by drawing molten glass through a 1-3 mm aperture and drawing it further until the fibre diameter is 3-20 µm. The fibre diameter is controlled by adjusting the head of the glass in the tank, the viscosity of the glass, the diameter of the holes and the winding speed. Several different glass compositions are available. E-glass (E for electrical grade) is the most widely used general-purpose form of composite reinforcement. It has good mechanical properties and is available at a relatively low price. Other glass fibres of interest are S-glass (S for Strength) and AR glass (AR for alkali resistant). S-glass is more expensive than E-glass but has a higher strength, Young’s modulus and temperature resistance. AR glass contains an amount of zirconia, which serves to prevent corrosion by alkali attacks. When compared with E glass fibres, they show a large improvement in regard to resistance against alkalis, while showing more or less identical density, tensile strength and elasticity (Machida 1993). Some properties of different glass fibres are shown in Table 4 and an indication of the alkaline resistance of E- and AR-glass is given in Table 5.

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Part A

Table 4

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

Tensile strength (MPa) Modulus of elasticity (GPa) Elongation (%) Density (kg/m3) Diameter (µm) Table 5

Chapter 2: Material characteristics

E-glass

S-glass

AR-glass

3500-3600

4100

1800-3500

74-75

85

70-76

4.8

2-3

2600

2500

2270

8-12

8-12

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

Diameter decrease (%)

NaOH (1.5 hours) Saturated Ca(OH)2 (4 hours)

AR-glass

E-glass

5

59

450 days)=σ0/2. Four different GFRP plates with E-glass fibres were subjected to 4% salt conditioning for 220 to 240 days and at room temperature or exposed to cyclic temperature variations between 0-70°F. Strength and stiffness reductions up to 17% were observed. Chin et al (1997) observed no significant reduction in the strength of vinyl ester and isophthalic polyester films after immersion in 3.5% NaCl solution for 1300 hours. Weight measurements show that, for the vinyl ester resin studied, the total salt-water uptake is higher than that of water or concrete pore solution after immersion in various exposure liquids for up to 400 hours. Arockiasamy et al (1998) did not find any reduction in CFRP cables analysed after exposure of specimens to sea water for up to six months. Chin et al (1998) conducted tests on different unreinforced polymeric resins. Specimens have been subjected to saline environment at ambient and elevated temperatures. The specimens were then analysed by DMTA, DSC, TGA, FTIR as well as tensile strength measurements. FRP reinforced beam specimens subjected to a saline environment Tests have also been conducted on FRP reinforcement embedded in concrete either in the form of pullout specimens or as FRP reinforced beam specimens. To investigate the reduction in pullout strength, CFRP and AFRP rods embedded in concrete were subjected to a simulated tidal zone (Sen et al 1998). The artificial tide was changed twice a week and the water temperature was 60°C to accelerate the test. The duration of the test was 18 months, which is said to correspond to 50 years ageing in Florida. After the exposure an increase in bond strength was observed. This result is explained by swelling induced increase in bond friction. Since this phenomenon is not observed for beam specimens subjected to similar conditions, it is considered unwise to use pullout specimens in accelerated durability tests. According to Sen et al, generally, AFRP and CFRP have been proved to have adequate resistance to alkalis and chlorides (Sen et al 1997). The durability of GFRP reinforced concrete beams was investigated by Saadamanesh and Tannous (1997). Beams were placed in artificial de-icing solutions, NaCl+CaCl2 (2:1, 7%, at 25°C) or NaCl+MgCl2 (2:1, 7%, at 25°C) and the durability was evaluated in terms of losses in the flexural capacity as a result of loss in tensile strength of GFRP rebars due to chloride ion contamination. After two years, the loss in tensile strength was 9% to 11% for an E-glass FRP in both solutions. The same GFRP system exhibited a reduction in tensile strength between 25% and 30% when immersed directly into the solutions. This is claimed to demonstrate the concrete's ability to limit the effect of exposure to de-icing salts. -32-

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Chapter 3: Review on research activities and results

To study the influence of salt on the bond resistance, a 3.5% NaCl solution has been used as concrete mixing water by Sheard et al (1997). In addition pullout specimens have been subjected to field exposure at five test sites with different environmental conditions, from arid to arctic and marine intertidal to inland. In general, an increase in bond strength was observed. This result is explained by the continued strength gain of the concrete in the specimens. Scheibe and Rostasy (Scheibe and Rostasy 1997 and Rostásy 1997) have evaluated the durability of AFRP (Arapree) tendons under high tensile load. Pretensioned slabs (α=0.7-0.85, α denotes stress-ultimate strength ratio of the reinforcement) were precracked at midspan and placed outdoors. They were periodically sprayed with CaCl2 solution, to simulate de-icing salts. After two years, no reduction in load capacity was found. AFRP tendons could be extracted from the beams after the bending test. These tendons were then tested, either for short-term tensile strength or stress rupture in an alkaline solution, to assess whether the preceding embedment in the concrete had caused any damage. From these test results no significant reduction in performance was observed. It is concluded that the lifetime of stressed AFRP bars in dense concrete by far exceeds the lifetime of AFRP subjected to alkaline solution. Spainhour and Thompson (1998) referred to an investigation where a glass/epoxy system was exposed to an artificial seawater solution in wet-dry cycling. The fibreglass prestressing strands suffered a complete loss of strength after an average of 6 months in beams cracked at midspan and after fifteen months in uncracked beams. Gangarao and Vijay (1997) subjected GFRP reinforced beams to a salt solution and measured an 18.4% reduction in ultimate moment capacity. This was, however, attributed to alkali attack or degradation in the concrete rather than to salt induced deterioration in the FRP material. Mukhopadhyaya et al (1998) noted deterioration on FRP plates after wet-dry cycles, in a 5% sodium (NaCl) solution, of externally FRP reinforced concrete beams. Adimi et al (1998) studied the fatigue behaviour of GFRP and CFRP reinforcement embedded in concrete with varying salinity. The conditions for the fatigue test were for the GFRP reinforcement: α=0.043 to 0.43 (R=0.1, at 4Hz) and the salinity 1 to 10% (NaCl). For the CFRPs two conditions were tested: condition 1): α=0.048 to 0.48 (R=0.1, at 4Hz) and the salinity 1 to 10% (NaCl), condition 2): α=0.044 to 0.44 (R=0.1, at 6Hz) and the salinity 5 to 30% (NaCl). The salinity seems to have only negligible effect on the fatigue lives of these materials.

3.3.4 Conclusions There are several results indicating considerable degradation of FRP when subjected to salt solutions. Often, the deterioration is not attributed to salt attack but to other reactions, such as reactions with alkali from the concrete pore solution or resin plasticization by water uptake (see for example (Steckel et al 1998), (Rahman et al 1998) and (Gangarao and Vijay 1997)). Even attack from alkali in natural seawater has been discussed (Toutanji and El-Korchi 1998). However, there are some indications that saline solution is a slightly more severe environment than fresh water. Saadamanesh and Tannous (1997) observed that the reduction in strength was less when FRP specimens were subjected to salt solutions than to water, and Chin et al -33-

Part A

Chapter 3: Review on research activities and results

(Chin et al 1997) reported higher moisture uptake of resins when immersed in a salt solution than in water. When FRPs are embedded in concrete, in the form of pullout specimens or reinforcement in beams, which are subjected to salt solutions, the reduction in mechanical properties is considerably less than when naked FRP specimens are subjected to the same environment. Increase in concrete strength, increase in friction by swelling of the FRP rods and reduced mobility of harmful ions in dense concrete are explanations given for this phenomenon. It appears as if vinyl ester has better protection against chloride attack than polyester and generally, CFRP and AFRP composites exhibit superior durability to that of GFRP composites (Saadatmanesh and Tannous 1997). Concrete slabs reinforced with AFRP (Arapree) tendons, under a load of 70 to 85% of ultimate strength, did not show any reduction in load capacity after two years while stored outdoors and periodically sprayed with CaCl2 solution (Scheibe and Rostary 1997 and Rostásy 1997).

3.4

Effects of Alkali on FRP

3.4.1 Introduction Degradation of FRP due to alkali is probably the most widely studied issue in the whole field of FRP durability. While the natural concrete environment offers protection against steel corrosion when using conventional reinforcement, the high alkalinity constitutes a degradation source for FRP material. Resins are known to be more or less susceptible to alkali attacks. A typical mode of corrosion is alkali induced hydrolysis reactions of ester (Machida 1993). As regards fibres, the susceptibility of glass fibres to alkali is well documented (Machida 1993 and Steckel et al 1998). It is therefore an important role of the resin, in addition to transferring load between the fibres, to protect the glass against alkali and other harmful agents. Aramid fibres are generally considered to be more resistant than glass to alkali attacks. Carbon fibres are noted to have high resistance to alkali (Machida 1993).

3.4.2 Experimental methodology GFRP is generally considered more susceptible to alkaline environment than CFRP or AFRP, but experiments have been performed for all types of FRP materials. To investigate the influence of alkali, FRP reinforcement has been subjected to accelerated as well as non-accelerated exposure. Often, results from an accelerated exposure and a “field exposure” are compared, and the relationship is used to develop a lifetime prediction model. Frequently, specimens are subjected to artificial concrete pore water for various periods and at different temperatures. Usually 20-60°C but even temperatures up to 80°C have been used to accelerate the tests. The composition of the simulated pore solution varies between the experiments, but often, NaOH, KOH and saturated Ca(OH)2 are used to create a solution with a pH of 12-13.5. In many investigations, FRP has been embedded in concrete to study the bond properties, and/or to investigate the effect of real concrete environment. The exposure -34-

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Chapter 3: Review on research activities and results

conditions differ considerably. Some FRP reinforced beams or slabs have been stored in non-accelerated outdoor environment for up to 3 years, others have been subjected to wet/dry cycles with water or alkaline solutions at elevated temperatures. To determine the environmental influence, changes in moment capacity and pullout strength have been measured for the FRP-concrete specimens. Often, the FRP specimens have been subjected to load during exposure to alkaline solutions or concrete, to evaluate the stress corrosion resistance. Mechanical properties such as residual strength, Young’s modulus and strain at failure are usually measured to evaluate the degradation, but equipment for physical analysis (for example DSC and DMA) and diffusion tests has also been used to study the aged material.

3.4.3 Experimental results and discussions FRP specimens exposed to alkaline environments without mechanical stress Exposure of the naked FRP material to various alkaline solutions has been used in many experiments to test the environmental influence. To accelerate ageing, elevated temperatures and high alkali concentrations are commonly used. To study the durability of E-glass/vinyl ester rods, Bank et al (1998) immersed rods in Ammonium hydroxide (NH4OH) solution, 0.3, 3 and 30% at 23°C, for up to 224 days. A decrease (of 12%) in tensile strength was measured only for the 30% solution. Furthermore, TGA analysis indicated deterioration in the matrix or the matrix-fibre interface and matrix material appeared to be missing in resin rich areas. The same FRP material showed surface degradation when immersed in water at 80°C, indicating a degradation mechanism different from that in the above exposure. Steckel et al (1998) have evaluated the “Environmental Durability of Composites for Seismic Retrofit of Bridge Columns” for 4 CFRP and 3 GFRP systems. FRP specimens were immersed in CaCO3 solution, simulating carbonised concrete, with pH=9.5 at 23°C for 3000 hours. Mechanical properties and Tg were measured. The systems were found to be fairly unaffected by the exposure except for a 10% reduction in Young's modulus for a GFRP system and a 30% reduction in short beam shear strength for another GFRP system. Tannous and Saadatmanesh (1998) tested 3 FRP tendons, 1 AFRP and 2 CFRP, by immersion in saturated Ca(OH)2 solution with pH 12 at 25 and 60°C. For the AFRP tendon after 12 months, a reduction of 4.3 and 6.4% in tensile strength was measured for 25 and 60°C respectively. The CFRP tendons, on the other hand, proved to be almost unaffected by the exposure. According to Tannous and Saadatmanesh: ”This clearly demonstrates that PAN carbon in conjunction with epoxy matrix has excellent durability in alkaline environments”. In a referred experiment CFRP and AFRP rods were subjected to 2 mole/l NaOH at 40°C, which resulted in little to no alkali penetration into the specimen. In another investigation, an equation was developed to transform accelerated time to real time for GFRP (Porter et al 1997). Thus 50 years real exposure was simulated by a 2-3 months exposure to pH 12.5-13 at 60°C .3 GFRP systems studied had a reduction of 72.6 to 55.6% in tensile strength after this exposure. Uomoto et al (1997) subjected GFRP, AFRP and AGFRP (aramid-glass hybrid, aramid in external layer and glass with central placement) to Na(OH)2 solution at

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Chapter 3: Review on research activities and results

40°C for up to 120 days. The tensile strength was reduced by 70% for the GFRP specimen, but AFRP and the AGFRP specimens were unaffected. According to EPMA (Electron Probe Micro Analysis) the Na intrusion was deeper in the GFRP material. A combined freeze-thaw and alkali test was performed by Gangarao et al (1997). Reductions of up to 49% in tensile strength and 37% in Young’s modulus were found for some E-glass GFRP systems (containing vinyl ester or polyester). According to Gangarao et al, “it is concluded that resins such as low viscosity, urethane modified vinyl ester are necessary for ensuring long-term durability of GFRP reinforced bars”. In a study conducted by Saadatmanesh et al (1997), CFRP, AFRP and GFRP specimens were subjected to saturated Ca(OH)2 solution at 25 and 60°C. The results indicated that CFRP and AFRP were more durable than GFRP in an alkaline environment. Furthermore, Fick’s law was found to be adequate to predict the loss in tensile strength for the specimens. In another investigation (Chin et al 1997), vinyl ester and isophthalic polyester were immersed in an artificial concrete pore solution, (1.8 % (by mass) KOH, 0.68% NaOH and 0.5% Ca(OH)2 in distilled water) at ambient temperature. No changes in tensile strength or Tg were found after 1300 hours in the solution. Chin et al (1998) conducted tests on different unreinforced polymeric resins. Specimens were subjected to an alkaline environment at ambient and elevated temperatures. The specimens were then analysed by DMTA, DSC, TGA, FTIR as well as tensile strength measurements. According to the results vinyl ester exhibited a better resistance to the environment then the iso polyester (80% and 40% respectively of the tensile strength remaining after the exposure). Bakis et al (1998) subjected three different GFRP rods to 28-days immersion in an 80°C saturated solution of Ca(OH)2. For all rods the tensile strength was reduced by the treatment. The rods with 100% vinyl ester in the matrix were shown to be less affected by the exposure than rods with partly vinyl ester, partly polyester in the matrix. Arockiasamy et al (1998), did not find any reduction on CFRP cables analysed after exposure of specimens to an alkaline environment for up to nine months. Durability studies of FRP reinforcement in alkaline environments are described also in (Coomarasamy 1998). GFRP specimens subjected to alkaline environment (pH 13.5, at 60°C) had a reduction in tensile strength of approximately 30% after 25 weeks exposure. Alsayed and Alhozaimy (1998) have conducted durability tests on two types of GFRP bars in alkaline solutions. For one GFRP type (with 40% unsaturated polyester and 60% urethane modified vinyl ester), coated with cement paste (W/C=0.5) and immersed in water, the weight and tensile strength of the GFRP bar decreased by 1% and 20% respectively over four months . When coated specimens were immersed in an alkaline solution (20 gm/l NaOH) the corresponding values were 3% and 30% respectively. For the other GFRP type (resin type not declared), the corresponding reductions were practically zero. It is concluded that resin type and manufacturing quality may be regarded as the key factors for the durability of the GFRP bars, and that GFRP bars should not be used in concrete structures that will be exposed to humidity unless the long-term performance for that particular type of bar is checked.

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Part A

Chapter 3: Review on research activities and results

FRP specimens exposed to alkaline environments under mechanical stress In real concrete structures, the reinforcement will be under some stress. The durability of FRP material subjected to a combination of environmental influence and mechanical load has been studied by a large number of researchers. Rahman et al (1998) subjected GFRP and CFRP containing a vinyl ester resin to a NaOH (58g/l) solution at up to 70°C for up to 370 days. A tensile load of α=0.3 and α =0.5 for the GFRP and CFRP specimen respectively was applied prior to the exposure. The GFRP specimen failed within 45 days. According to Rahman et al, "it appears that the diffusion of hydrated hydroxyl ions (OH-) through the resin is very rapid". However, the FTIR analysis conducted indicates that the bulk of the resin was not chemically affected by the exposure. Gangarao and Vijay (1997) also found considerable reduction in strength (up to 76%) for GFRP bars subjected to tension of approximately 30% in an alkaline solution with pH 13. However, some systems containing a vinyl ester resin showed much less reduction in tensile strength. Sheard et al (1997) report that the interlaminar strength was reduced for some GFRP and CFRP systems (resin type not declared) after immersion in solutions with pH 11.513.5, others were almost unaffected. In (Clarke and Sheard 1998), it is shown that CFRP specimens analysed exhibited a larger reduction than GFRP specimens in Inter Laminar Shear Strength, after exposure for 6 months to an accelerated test environment (pH 12.5, 5% ultimate bending stress at 38°C). Benmokrane et al (1998), have conducted durability tests on GFRP bars in an alkaline environment. Tests to determine residual strength after exposure to alkali under tensile stress, as well as stress rupture tests, were performed to evaluate the influence of resin types and manufacturing parameters. It is concluded that vinyl ester confers greater durability on GFRP bars. Besides resin type, the factors which also influence durability performance are fibre type and manufacturing parameters. Arockiasamy et al (1998) did not find any strength reduction on CFRP cables analysed after exposure under tension (α=0.65) of specimens to an alkaline environment (pH 13-14) for up to nine months. FRP embedded in concrete which is subjected to accelerated exposure Several researchers are subjecting FRP specimens embedded in concrete to various solutions to accelerate ageing. Scheibe et al (1998) used this technique to compare the lifetime, by stress rupture tests, for AFRP (Arapree with epoxy resin ) bars under different exposure conditions. Bars, embedded in concrete, were loaded to α=0.75 and were stored in air (65% RH, at 20°C) or immersed in an alkaline solution containing 0.4nKOH and saturated Ca(OH)2 at temperatures ranging from 20 to 60°C. The lifetime in concrete exposed to 20°C alkaline solution and 20°C air was 714 and 3308 hours respectively. Diffusion tests further indicated that the weight gain was slightly less for AFRP when embedded in concrete than when it was in direct contact with the solution. Porter and Barnes (Tucson 1998) immersed concrete embedded E-glass/vinyl ester rods in 60°C water and 60°C alkaline (pH 12) baths. According to "alkali resistant glass ageing curves" (Litherland et al 1981) one day in this environment is claimed to -37-

Part A

Chapter 3: Review on research activities and results

simulate 279 days of real environment ageing. Pullout tests were performed, but the rods appeared unaffected by the exposure. A possible explanation is that too thick a concrete cover was used. In an investigation performed by Pantuso et al (1998), glass fibre/polyester bars were embedded in concrete and subjected to wet-dry cycles in an alkali bath for 60 days. The tensile strength was found to decrease by up to 21% compared with 7% when the same treatment was repeated but with a water bath instead of an alkaline bath. To simulate tidal zone, pullout specimens, CFRP with an epoxy resin embedded in concrete, were subjected to wet-dry cycles for 18 months at room temperature and at 60°C (Sen et al 1998). This resulted in an increase in bond strength, attributed to swelling of the FRP material. However, flexural tests on reinforced beam specimens do not show similar improvements in strength, and these kinds of pullout tests in accelerated environments are therefore considered "unwise". Sheard et al (1997) reported that no mechanical or physical evidence of any deterioration was found after 12 months exposure of pullout specimens, one GFRP and one CFRP type (resin types not declared), to various alkaline solutions at 20 and 38°C. In a study conducted by Porter et al (1997), prestressed beams (α=0.4) were subjected to a high alkali environment. GFRP tendons containing a polyester resin almost completely lost their prestressing force whereas CFRP, also containing a polyester resin, appeared unaffected by the ageing. Conrad et al (1998) subjected AFRP, CFRP and GFRP rods to a saturated Ca(OH)2 solution at 80°C for 28 days. The rods were then cast in concrete and tested for pullout strength after 7-10 days curing. No significant reduction in bond strength was measured. Adimi et al (1998) studied the fatigue (tension-tension) behaviour of GFRP and CFRP reinforcement embedded in concrete with varying alkalinity. The conditions for the fatigue test were for the GFRP reinforcement: α=0.043 to 0.43 (R=0.1, at 4Hz). For the CFRPs two conditions were tested: condition 1) : α=0.048 to 0.48 (R=0.1, at 4Hz), condition 2) : α=0.044 to 0.44 (R=0.1, at 6Hz). This environment seems to have only negligible effect on the fatigue lives of these materials. FRP reinforced concrete in field conditioning Non-accelerated tests have been conducted to study the durability of FRP under field conditions or to determine the relationship between accelerated and non-accelerated ageing. To study long-term durability of AFRP (Arapree), prestressed (α=0.7-0.85) slabs were tested (Scheibe and Rostasy 1998). The slabs were precracked and stored in different non-accelerated conditions. After 2 years the slabs were tested in bending. No decrease in moment capacity was found for any of the slabs (however, all slabs showed a concrete crushing failure mode). AFRP reinforcement could be taken from the slabs after failure, and tested with respect to residual strength and stress rupture behaviour. According to the tests, no significant deterioration had occurred. In another investigation (Gangarao and Vijay 1997), GFRP reinforced beams were immersed in salt water for 240 days. A reduction in moment capacity of 18% was reported and attributed to alkali induced bond deterioration (i.e. not caused by the salt). Tomosawa and Nakatsuji (1997) exposed reinforced beams to tropical climate at a test platform outside Japan's coast. After 2 years, there was no reduction in flexural strength for beams reinforced with AFRP, CFRP and GFRP bars. However, a small reduction was reported for prestressed beams with AFRP and CFRP tendons (no GFRP prestressed -38-

Part A

Chapter 3: Review on research activities and results

beam was tested). Field exposure tests were conducted at four different sites for GFRP and CFRP pullout specimens (Sheard et al 1997). No reduction but, in fact, a slight increase could be seen for all specimens after 12 months exposure. This result was attributed to an increase in concrete strength. Similar results were obtained by Sen et al (1998) who performed exposure tests on pullout, CFRP epoxy rod specimens in an outdoor environment for 18 months. However, the increase in pullout strength was here attributed to swelling of the FRP material.

3.4.4 Conclusions Elevated temperatures are generally used to accelerate the tests. The fact that no standard methods are available for durability testing leads to difficulties in comparing test results obtained by different research groups. Usually, each research group designs its own test series with respect to materials, composition of simulated concrete pore solution, duration of exposure and the way the properties are tested after exposure. However, in general, alkaline environment has been proved to be considerably more severe than water and saline environment. The results indicate that most of the FRP specimens directly exposed to alkaline environments without mechanical stress showed some degradation after exposure to accelerated ageing. Generally, for the CFRP, only marginal reductions in mechanical performance were found. For AFRP the deterioration was slightly higher than for CFRP. In general, GFRP was found to have poor durability properties compared with FRP with carbon or aramid fibres. Sometimes large reductions in tensile strength and other mechanical properties were measured after exposure. Furthermore, vinyl ester type resins appear to offer better resistance to alkali attack than epoxy and especially polyester. The resistance to alkali attack seems to be badly affected by the combination of alkaline environment and applied load, compared with exposure in an unstressed state. Especially GFRP has been proved to be very susceptible to stress corrosion. Most results indicate considerable deterioration in mechanical properties after the test period or even failure during exposure, when GFRP is subjected to high tensile stress in an alkaline environment. However, there are tests in which GFRP specimens show a greater resistance than CFRP specimens to alkali under tensile stress (Clarke and Sheard 1998). Compared with exposure tests of GFRP immersed directly in an alkaline solution, alkali exposure of GFRP embedded in concrete does not seem to be that severe. Tests on prestressed AFRP specimens point in the same direction. Some pullout tests show no decrease or actually an increase in bond strength after exposure to alkaline environment. This is attributed to swelling of FRP rods or increase in concrete strength. In general, no dramatic loss in strength has been found for FRP specimens embedded in concrete, and placed in non-accelerating field conditions. It appears as if alkaline solution is far harsher than natural concrete environment. According to Scheibe and Rostary (1998) this effect could be explained by the fact that “...in dense cement stone, the availability and the mobility of water and therein dissolved cations is dramatically reduced”.

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3.5

Chapter 3: Review on research activities and results

Influence of freeze-thaw cycles

3.5.1 Introduction In many parts of the world concrete structures are subjected to a large number of freeze-thaw cycles each year, 100 cycles are not unusual. Consequently, a good frost resistance in such areas is an important issue. Both possible degradation of the FRP material itself and deterioration at the FRP-concrete interface have to be taken into consideration. The difference between the areas “Thermal actions” and “Influence of Freeze-thaw cycles” is not obvious. Freeze-thaw influence could be regarded as the influence of freezing water at the interfaces of the FRP-concrete system. “Thermal actions”, on the other hand, could be considered to be more associated with thermal mismatch between the constituents in the FRP-concrete system. In this report, papers presented as a study of freeze-thaw are treated in the chapter “Influence of freeze-thaw cycles”, and papers presented as studies of “thermal actions” are treated in the chapter “Thermal actions”, even if this is not necessarily in agreement with the “definition” given above.

3.5.2 Experimental methodology In the research projects here studied, both naked FRP specimens and FRP reinforced concrete (both embedded and as sheets/wraps/plates on concrete surface) specimens have been subjected to freeze-thaw cycles and then tested to evaluate the influence. There have also been differences in experimental design with respect to the number of freeze-thaw cycles and the temperature interval used.

3.5.3 Experimental results and discussions Steckel et al (1998) conducted freeze-thaw tests on FRP laminates for seismic retrofitting of columns. Four CFRP and three GFRP systems, in the form of laminate specimens, were conditioned at 100% RH at 38°C for two weeks prior to the initial exposure to the freezer at -18°C. The temperature alternated between -18 and 38°C in 20 cycles over 20 days. The laminates were then tested with respect to weight, Young’s modulus, tensile strength, strain at failure, short beam shear strength and Tg. No freeze-thaw induced deterioration was found for any FRP system after the 20 cycles. There was a significant reduction in short beam shear failure after the exposure, but this was attributed to moisture uptake. Tannous and Saadamanesh (1998) performed freeze-thaw tests on three tendons, two CFRP (Leadline and CFCC), and one AFRP (Arapree). Specimens were subjected to 1200 cycles at the most. Each cycle consisted of two hours at -30°C followed by two hours at 60°C. It has been suggested that thermal fatigue in this case could be induced

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Chapter 3: Review on research activities and results

through thermal incompatibility between the fibres and the matrix forming the composite. The test indicated that freeze-thaw cycles had limited to no effect on the material property of all three tendons. In another investigation (Toutanji and El-Korchi 1998), the freeze-thaw resistance of concrete cylinders, measuring 200 mm in length and 16 mm in diameter and wrapped with three different FRP sheets, was studied. Two different carbon fibre sheets and one glass fibre sheet, all combined with an epoxy resin, were tested using the guidelines of the ASTM C-666 procedure of the Standard Test Method for Resistance of Concrete to Rapid Freezing and Thawing. The test consisted of alternately lowering the temperature of the specimens from 4.4 to -17.8°C and raising it from -17.8 to 4.4°C in four hours. After 300 such cycles ( over 50 days) in water, the reduction in tensile strength (axial) of the wrapped cylinders was measured. The glass fibre system and one of the two carbon fibre systems sustained a 5% and 10% reduction in tensile strength respectively. Since carbon fibres are chemically inert materials, the reduction is suggested to be physical in nature. The other carbon fibre system did not show any reduction at all. When the same material systems were subjected to wet-dry exposure in seawater for five months, the change in tensile strength was the same as in the freeze-thaw test for the CFRP systems whereas the GFRP system had a 20% reduction in tensile strength. Gangarao and Vijay (1997) subjected GFRP bars with seven resin types to a combined freeze-thaw and alkali conditioning. After 141 freeze-thaw cycles in alkali, a reduction in strength and stiffness could be seen for all systems. Reductions in tensile strength between 6 and 49% were measured for the different GFRP systems. In the same research project, the resistance of GFRP reinforced concrete beams to freeze-thaw cycles was tested. A beam was precracked and then subjected to freezethaw cycles in an environmental chamber for 143 days. Each conditioning cycle, similar to ASTM D1183, consisted of temperature variations between -20°C and 49°C, whereas the relative humidity ranged between 0 and 95%. Each cycle lasted for 121 hours. A reduction in load capacity of about 17% was observed after the exposure period. Green et al (1998) have conducted freeze-thaw tests to evaluate the influence on the bond between FRP sheets and concrete. Concrete beams reinforced with GFRP and CFRP sheets were subjected to freeze-thaw cycles (freeze for sixteen hours at -18°C in air and thaw in water at 15°C for eight hours) and then tested in four-point bending. The sheets were unbonded in the middle to ensure that the beams failed by peeling of the FRP sheets. At the time the article was written only results from beams reinforced with Sika sheets (CFRP) were available. No deterioration was found after 150 cycles. Similar tests were performed by Tysl et al (1998). They concluded that the externally reinforced concrete beam system studied is robust for up to 30% unbonded areas and freeze-thaw cycling. Toutanji and Rey (1998) subjected CFRP and GFRP wrapped concrete columns to freeze-thaw cycles. Contrary to the results of Green et al and Tysl et al, a significant strength reduction was noted for the FRP material after the conditioning. Mukhopadhyaya et al (1998) also noted deterioration on FRP plates after freeze-thaw exposure of externally FRP reinforced concrete beams.

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3.5.4 Conclusions The resistance of FRP reinforcement to freeze-thaw conditioning has been studied by several research groups. However, the results obtained are not totally unanimous. This can, to some extent, be explained by the fact that different research teams have been using different experimental designs, which makes comparison of results difficult. However, some researchers have found significant reductions in tensile strength as a result of the freeze-thaw conditioning.

3.6

Degradation caused by ultraviolet rays

3.6.1 Introduction Exposure to natural sunlight exposes the composite to ultraviolet radiation. Ultraviolet rays are known to affect polymeric materials, although this effect is primarily at the surface of the composite (Bank and Gentry 1995). FRP bars in a concrete member are protected from UV rays by the concrete cover. However, UV rays may degrade FRP materials when stored outdoors unprotected from the sunlight, or when used as externally applied reinforcement. As regards fibres, it is also well known that aramid fibres are susceptible to ultraviolet rays, requiring care depending on the environment in which they are used (Machida 1993).

3.6.2 Experimental methodology Exposure tests have been performed both in laboratories and under field conditions. Usually the tensile strength of aged and virgin samples is compared to evaluate the degradation.

3.6.3 Experimental results and discussions To investigate the effect of ultraviolet rays, Kato et al (1997) have conducted tests on FRP material using an accelerator machine. AFRP, CFRP and GFRP rods have been subjected to up to 1250 exposure cycles. One cycle consisted of 102 minutes in dry conditions (RH 52±2%) and 18 minutes in wet conditions (RH 90±2%). During the cycle, the intensity of ultraviolet radiation was 0.2 MJ/m2 per hour and the temperature was 26°C. From the exposure test the AFRP rods show almost 13% reduction in tensile strength after 1250 cycles. The corresponding value for the GFRP rods was approximately 8% and for the CFRP rod no deterioration was found. Exposure tests were also performed on aramid, carbon and glass fibres for up to 1000 hours. The tendency for the reduction in tensile strength of fibres was almost the same as for FRP. The tensile strength loss of aramid fibres due to ultraviolet rays was shown to be predictable using “weakest-link theory”.

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13 different FRP reinforcement specimens, including GFRP, AFRP, CFRP and polyvinyl alcohol (PVA) FRP, were exposed to air and sunshine for two years at a marine station in a tropical zone (outside Japan's coast), (Tomosawa and Nakatsuji 1997). After the test period the tensile strength was tested. No particular changes in tensile strength were observed for any of the specimens after the two-year exposure. In another investigation the effect of sunlight was studied for six types of FRP (naked) cables; 2 CFRP, 2 AFRP, 1 GFRP and 1 VFRP (vinylon FRP) with epoxy and vinyl ester resins, (Sasaki et al 1997). The location of the test was in a material exposure facility on the Pacific seaboard of Japan. Specimens without prestress were exposed for 32 months, whereas specimens with prestress were exposed for 42 months. For specimens without prestress, no significant sunlight induced deterioration was found. For the prestressed specimens, on the other hand, a considerable effect of sunlight could be observed for some material systems. The relaxation loss was, usually, 50% higher for specimens exposed to sunlight. Two factors are assumed to cause the difference for one carbon/epoxy system; deterioration by ultraviolet rays and difference in thermal hysteresis by the sunlight. Furthermore, the tensile strength seems to be negatively affected by the exposure to sunlight, especially in the case of the GFRP system (with a vinyl ester resin) subjected to a prestress level of α=0.4 and exposed to sunshine, where the reduction in tensile strength was 40% after 42 months compared with 20% for specimens without exposure to sunshine. Chin et al (1997) found surface oxidation on 2 resin types, vinyl ester and isophthalic polyester, after ultraviolet exposure in an ageing chamber. Although surface erosion and cracking appeared to be similar for both resin samples, the chemical mechanism of oxidation appeared to differ. Bank and Gentry (1995) refer to several investigations where deterioration, of varying extent, of different FRP materials has been observed as a result of ultraviolet rays and sunshine.

3.6.4 Conclusions Significant deterioration in mechanical properties has been observed after exposure of FRP specimens to ultraviolet rays in laboratories and exposure to sunlight at field exposure sites. AFRP have been shown to be more susceptible to ultraviolet rays than CFRP and GFRP in accordance with earlier results. However, in one investigation (Tomosawa and Nakatsuji 1997), no degradation could be found for GFRP, AFRP, CFRP or polyvinyl alcohol (PVA) FRP reinforcement after two years exposure to sunlight at a marine test site.

3.7

Thermal actions

Degradation by thermal action may be expected in composite materials where the constituents have different coefficients of thermal expansion. FRP reinforced concrete exhibits thermal incompatibility between fibres and resin but also between the FRP -43-

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composite and the concrete. In terms of FRP reinforced concrete, the transverse thermal expansion of FRP rods is particularly important since this expansion can lead to cracks along the rods in the concrete cover and consequently to bond failure. The difference between the areas “Thermal actions” and “Influence of freeze-thaw cycles” is not obvious. Freeze-thaw influence could be regarded as the influence of freezing water at the interfaces of the FRP-concrete system. “Thermal action”, on the other hand, could be considered to be associated more with thermal mismatch between the constituents in the FRP-concrete system. In this report, papers presented as a study of freeze-thaw are treated in the chapter “Influence of freeze-thaw cycles”, and papers presented as studies of “thermal actions” are treated in the chapter “Thermal actions”, even if this is not necessarily in agreement with the “definition” given above. Sen et al (1997) have conducted an investigation into the combined effect of wet-dry and thermal cycling. AFRP and CFRP reinforced concrete beams were subjected to one artificial tide a week for up to three years with water temperature at 60°C at “high tide”. The results after 33 months indicated that the CFRP beams were largely unaffected by the treatment whereas a significant loss in strength, due to bond failure, was observed for the AFRP beams. However, FEM analysis indicated that moisture absorption, not mismatch in thermal expansion coefficients, played a more dominant role in the degradation of bond. Results showed that even 1% absorption level would cause significant cracking along the FRP rods. In contrast, the temperature ranges specimens were exposed to would not lead to commensurate damage (Sen et al 1997). In another project (Gentry et al 1998), a theoretical analysis has been performed to study cracks in the concrete caused by the thermal expansion of the FRP reinforcement. According to the study it appears likely that cracks will be generated due to this mechanism. Further, there are some indications that helical wrapping, used for some FRP bars (primarily to improve the bond), may to some extent reduce the thermal, transverse, expansion of the rod. El-Badry and Abdalla H (1998) did experiments on the influence of cracks in the concrete due to the transverse expansion of GFRP and CFRP bars. They found that these cracks lead to a reduction in the tension stiffening of the concrete and an increase in deflection for beams reinforced with GFRP. However, this effect was less pronounced for CFRP, and in fact, even less than for steel reinforced beams. Katz et al (1998) studied the influence of elevated temperatures on the bond between FRP bars and concrete. Bars were subjected to temperatures up to 250°C and tested for pullout strength. The results indicated that at temperatures up to 100°C, the loss in bond is quite similar to that observed in conventional deformed steel rebars, but at higher temperatures the bond strength of FRP rods decreases dramatically to a residual strength of approximately 10% at 200220°C relative to the bond strength at room temperature. The bond strength loss was associated with temperatures exceeding Tg for the resin in the FRP bar. Clement et al (1998) subjected concrete beams, reinforced with externally applied CFRP, to thermal cycling (from -20°C to 30°C). After 220 cycles, in dry air, no reduction in load capacity was shown for the beam specimens. Adimi and Boukhili (1998) conducted fatigue tests on 3 GFRP rods, with vinyl ester, epoxy and polyester respectively. They found that, for GFRP with epoxy and vinyl ester, the fatigue life can be reduced by a factor of 1000 at a test temperature of 100°C compared with a test temperature of 23°C. The GFRP rod with a polyester resin was not tested at high temperature because the Glass Transition Temperature (Tg) for this material was found to be very -44-

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low (around 80°C). Tg for the GFRP with vinyl ester and epoxy was found to be 145 and 165°C respectively.

3.8

Other degradation sources

Some degradation sources, only briefly discussed in the literature, are considered in the following.

3.8.1 Deterioration by heat and fire The Dutch Ministry of Transport has applied FRP materials in some infrastructure works (Noordzij 1998). Some fire resistance investigations were conducted to evaluate the properties of some possible materials to be used for a 15 metre span pedestrian bridge. Tests were carried out on four laminates: laminate and gelcoat made of a fire retardant polyester a vinyl ester laminate with a polyester gelcoat a vinyl ester laminate with a fire retardant polyester gelcoat a vinyl ester laminate with no gelcoat As regards flame propagation the test results of the first three laminates were very similar; but the forth laminate performed much better. The unexpected result was attributed to differences in colour (the gelcoat was dark blue). 3.8.2 The effect of galvanic coupling Carbon fibre composites can degrade as a result of galvanic coupling. This phenomenon may occur when carbon fibres are coupled to a less electropositive material such as steel. Galvanic coupling can cause durability problems, for example when steel reinforced concrete columns are repaired using CFRP wraps. In an investigation described in (Nanni et al 1997 and Cetin et al 1998) experiments have been conducted to determine whether the FRP or the steel reinforcement, or both, is degraded as a result of inadvertent galvanic coupling. Two degradation mechanisms have been discussed. One mechanism suggests that carbon fibres develop a negative surface charge when they are galvanically coupled to a less electropositive material. As a result, the bond between the fibre and the polymer weakens, and water can migrate into these debonded areas bringing elements such as oxygen and chloride. Another mechanism suggests that water will permeate into defective carbon/polymer interface areas, which always exist. For both these mechanisms, reduction of oxygen to hydroxyl ions takes place on the carbon surface according to the following equation: 2e- + 1/2(O2) + H+ → OH-

(2)

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As a result, a hydroxyl solution is formed. By nature, hydroxyl solutions are hygroscopic and will draw more water into these regions. The osmotic pressure that is built up may cause blistering or delamination. Potentiodynamic polarization scans, galvanic coupling tests, and electrochemical impedance spectroscopy experiments were performed on carbon fibres, fibre reinforced epoxies and reinforcing steel in concrete. These tests were performed in seawater and in artificial concrete pore water. According to the results the reinforcing steel showed accelerated corrosion in the presence of carbon rod material. However, when the carbon was in the form of sized fibres a significantly lower galvanic current was observed. Furthermore, no visible degradation of the polymeric material was found.

3.8.3 Gaseous Mixtures In a study, the creep rupture of Kevlar and nylon fibres and their composites in air with 0.1% to 1.5% NOx (much more than in the real environment) were tested (Bank and Gentry 1995). Creep rupture strengths were shown to be affected substantially due to the synergism of load and gaseous environment. The effect of NOx and SOx on the long-term properties of composites and their constituents may need to be studied further.

3.8.4 Pressure In a study, referred to in (Bank and Gentry 1995), graphite/polymer composites were submerged (in sea water) at 2000 feet to investigate the influence of pressure. It was shown that although the diffusion coefficients were unchanged by pressure, the equilibrium moisture contents were higher in the pressurised samples. This was attributed to activation of damage mechanisms due to the pressure exposure. Specimens subjected to pressure showed strength and stiffness losses.

3.8.5 Exposure to diesel To evaluate the effect of a fuel spill following a vehicular accident, an experimental programme was set up including immersion of several FRP systems for four hours in diesel fuel (Steckel et al 1998). However, no results were reported.

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Chapter 4: Durability approach in existing design guidelines

4 Durability approach in existing design guidelines 4.1

Introduction

Up to the present time, design guides have been drawn up in Japan, Canada and UK. In the USA and Norway, work on provisional design recommendations is now going on. A brief summary of the key approaches to durability, used in these design recommendations, will be given in the following section. In Table 9 a scheme is given which summarises the durability related strength reduction, or stress limit factors that have been used for (non pre-stressed) FRP reinforcement in the design guidelines. The intention with the table is to simplify the comparison between the various design guidelines regarding the approaches adopted concerning durability related aspects. The headings in the following subsections are chosen to facilitate the reading but are also a reflection on the essential parts focused on in the various design guidelines. Therefore the subsections differ somewhat between the design guidelines studied.

4.2

Japanese Society of Civil Engineers

4.2.1 Introduction The Japanese design guidelines are published in "Recommendation for Design and Construction of Concrete Structures Using Continuous Fiber reinforcing Materials" (JSCE 1997). The document is the result of the work by two research committees, set up by JSCE Concrete Committee. The work of the first committee started in 1989 and involved various aspects of FRP including reviewing research works and actual applications, design methods, durability and test methods. The work of the second committee started in 1993 and aimed at producing design guidelines. This was seen as necessary in order to make the use of FRP on a larger scale possible. The design recommendations were first published in Japanese in 1996 and were translated to English in 1997.

4.2.2 Material coefficients and member factor To calculate the design strength of CFRM (Continuous Fibre Reinforcing Materials) reinforcement, a material factor, γmf, is used. According to the recommendations this factor can generally be set to 1.15 for CFRM with carbon and aramid fibres, and 1.3 for CFRM with glass fibres, see Table 9. The following section is an extract from the design recommendation document commenting on the material coefficient.

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Table 9

Reduction factors used in existing design guidelines to take account of tensile strength reduction due to environmental and sustained stress influence (JSCE 1997, CHBDC 1996, ACI 2000, Thorenfeldt 1998 and Clarke et al 1996)⋅

Factor

ACI

NS3473

CHBDC

JSCE

Reduction due to environmentally caused deterioration

CE ”environmental reduction factor” GFRP: 0.70-0.80 AFRP: 0.80-0.90 CFRP: 0.90-1.00

ηenv "conversion factor"

∗ φFRP "resistance factor"

∗∗

GFRP: 0.50 AFRP: 0.90 CFRP: 1.00

GFRP: 0.75 AFRP: 0.85 CFRP: 0.85

GFRP: 0.77 AFRP: 0.87 CFRP: 0.87

ηlt "conversion factor" GFRP: 0.8-1.0 AFRP: 0.7-1.0 CFRP: 0.9-1.0

F "factor" GFRP: 0.8-1.0 AFRP: 0.5-1.0 CFRP: 0.9-1.0

GFRP: 0.40-0.50 AFRP: 0.63-0.90 CFRP: 0.90-1.00

GFRP: 0.60-0.75 AFRP: 0.42-0.85 CFRP: 0.76-0.85

Reduction due sustained stress

to

Total strength due to reduction environment and sustained stress

GFRP: 0.70-0.80 AFRP: 0.80-0.90 CFRP: 0.90-1.00

BISE

1/γfm "material factor" 1/γm "material factor" GFRP: 0.30 AFRP: 0.50 CFRP: 0.60

GFRP: 0.77 AFRP: 0.87 CFRP: 0.87

GFRP: 0.30 AFRP: 0.50 CFRP: 0.60

0.8 × "creep failure strength" not more than 0.7 Stress limits not specified Stress limits not specified GFRP: ≤ 0.7 AFRP: ≤ 0.7 CFRP: ≤ 0.7 ∗ φFRP "resistance factor" – it is not clear from the design guideline text what is included in this factor. Reduction due to environment is assumed to be one of the aspects affecting the size of φFRP. ** Material factor given takes account of r: deviation of test data, damage, differences in test strength and strength in real structure, effects of material characteristics on the limit state, service temperatures, environmental conditions etc. Specified upper tensile in stress limits reinforcement due to permanent load

GFRP: 0.14-0.16 AFRP: 0.16-0.18 CFRP: 0.44-0.50

GFRP: 0.60-0.75 AFRP: 0.42-0.85 CFRP: 0.76-0.85

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“The material coefficient γmf shall be determined allowing for the quantity and deviation of test data, possible damage to CFRM during transportation and construction, differences in material characteristics between test pieces and actual structures, the effect of material characteristics on the limit state, service temperatures, environmental conditions etc. It takes into account variations of the material as well as effects on the material characteristics from influencing factors including environmental conditions.”.

4.2.3 Creep rupture The potential risk of creep failure (creep rupture) when FRPs are used as pretensioned tendons is discussed. According to the recommendations, the limit value for tensile stresses in tendons may generally be taken as the creep failure strength characteristic value ffck, multiplied by a reduction factor of 0.8. The limit value shall not be more than 70% of the tensile strength. The creep failure strength shall be measured according to JSCE-E 533 “Testing method for Creep Failure of Continuous Fibre reinforcing Materials”.

4.2.4 Durability test method A test method is given for evaluation of alkali resistance of CFRM. In summary the method is as follows: Test pieces, with sealed ends, are to be immersed in alkaline solution having the same composition as the pore solution found in the concrete. The specified temperature of the solution is 60°C, however temperatures in the range of 20-60°C may be selected depending on the expected application conditions and the properties of the CFRM. The specified immersion duration is one month, variable in the range of 7 days to 1 year. No tension will normally be applied during exposure, however if the CFRM reinforcement is going to be used as pre-stressing tendons, immersion under tension is desirable. After the immersion period, the tensile strength of the specimens shall be tested and related to the strength of unexposed specimens. In addition, the mass change and the changes in external appearance, such as colour, surface condition, and shape shall be evaluated. Comment: Nothing is mentioned on how to interpret the results from the suggested test. For example, no guidance is given regarding how large strength reduction should be considered acceptable.

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4.3

Chapter 4: Durability approach in existing design guidelines

Canadian Highway Bridge Design Code

4.3.1 Introduction The Canadian design recommendations for bridges and structures were produced by a technical committee created by The Canadian Society of Civil Engineers in 1989. The work by this committee resulted in a State-of-the-Art report in 1991 (Mufti A. A. et al.) and in the publication of design recommendations, "Canadian Highway Bridge Design Code (CHBDC) section 16 for FRP", in 1998 (CHBDC 1998). The following guideline principles are given to take account of the durability of FRP used as concrete reinforcement.

4.3.2 Strength reductions and stress limits To take account of the fact that some FRPs lose strength under sustained loads, the maximum stress in non-prestressed reinforcement is limited to φFRP⋅F⋅fpu. Here, fpu is the "specified tensile strength" (which is the 5 percentile) of an FRP bar, grid or tendon. φFRP is the resistance factor. However, nothing is mentioned about what is taken into account by the use of this factor. φFRP are set to 0.75, 0.85 and 0.85 for GFRP, AFRP and CFRP respectively (CHBDC 1998). The factor F shall be obtained from Table 10, in which R is the ratio of the stresses due to factored dead loads to the stresses due to factored live loads in the FRP bars or grids (CHBDC 1998). Table 10

Stress limiting factor (CHBDC 1998) R= F for GFRP F for AFRP F for CFRP

0.5 1.0 1.0 1.0

1.0 0.9 0.6 0.9

2.0 or more 0.8 0.5 0.9

Environmentally caused deterioration is not explicitly treated in these design guidelines. The values of strength reduction factors as well as of stress limits are in the ranges of 0.60-0.75, 0.42-0.85 and 0.76-0.85 for GFRP, AFRP and CFRP respectively. Values depend on the ratio between dead and live load.

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4.3.3 Prestressed reinforcement The maximum permissible stresses in FRP tendons at transfer for pre-tensioning and post-tensioning are given in Table 11. fpu is the characteristic (5 percentile) tensile strength of the tendons. Table 11

Maximum permissible stresses in FRP tendons at transfer (CHBDC 1998) GFRP AFRP CFRP

Pre-tensioning Not applicable 0.38fpu 0.6fpu

Post-tensioning 0.48fpu 0.35fpu 0.6fpu

4.3.4 Restrictions in the use of FRP as concrete reinforcement For FRP bars and grids, when used as primary reinforcement in concrete, and for FRP tendons, only thermoset resins are accepted for the matrices. This is basically because thermoplastics are generally less stable under high temperatures and aggressive environments. To avoid improper use of FRP, a table is given showing where FRP bars, grids and tendons are permissible/inadmissible (CHBDC 1998), see Table 12. Table 12

Conditions of use for tendons and primary reinforcement (CHBDC 1998).

Applications Pre-stressed Concrete Beams and Slabs Post-tensioned Grouted Cement- Ungrouted Ungrouted NonInternal External Based alkaline Pregrout Grout tensioned I P I P P P P P P P P P P P P N/A N/A N/A P P

Material: GFRP CFRP AFRP Aramid rope P=permissible, I=inadmissible, N/A=not applicable

Deck Slabs I P P N/A

Stressed Wood Decks P I P P

Barrier Walls

Furthermore, a concrete beam or slab with FRP tendons shall also contain supplementary reinforcement capable of sustaining the unfactored dead loads. Such reinforcement can be steel or FRP reinforcement or even FRP tendons (having minimal prestressing force at the time of installation.

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P P P N/A

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Other measures prescribed are that anchors for aramid fibre ropes and FRP tendons in concrete shall be of suitably durable materials and that exposed tendons which are deemed to be susceptible to damage by UV rays or moisture shall be protected accordingly. For FRP as secondary reinforcement, thermoplastic resin can be used as matrices, provided that the matrix is not susceptible to degradation from alkali (as for polyester).

4.4

American Concrete Institute

4.4.1 Introduction The American Concrete Institute (ACI) Committee 440 Fiber Reinforced Polymer Reinforcement was formed in 1991. In 1996 it published a State-of-the-Art report (ACI 440R-96) addressing FRP for concrete reinforcement. For the time being provisional design recommendations, "Guide for the Design and Construction of Concrete Reinforced with FRP Bars" are being created, the last version was drafted in January 2000 (ACI 2000).

4.4.2 Environmental reduction factor An “environmental reduction factor” is introduced to take account of the deterioration of tensile strength due to long-term environmental influence. This factor shall be multiplied by the characteristic strength, given by the manufacturer (mean strength minus three times the standard deviation), to obtain the design ultimate tensile strength for an FRP reinforcing bar, as in Equation 2. ffu=CE ⋅ ffu* where:

ffu CE ffu*

(2)

design ultimate tensile strength environmental reduction factor guaranteed ultimate tensile strength of an FRP bar as reported by the manufacturer

The environmental reduction factor to use depends on fibre type and exposure condition. Two environmental classes are introduced, “Enclosed Conditioned Space” and “Unenclosed Conditioned Space”. The reduction factors suggested are shown in Table 13.

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Table 13

Chapter 4: Durability approach in existing design guidelines

Environmental reduction factor for various fibres and exposure conditions (ACI 2000). Exposure Condition Enclosed Conditioned Space Unenclosed Conditioned Space

Fibre Type CFRP GFRP AFRP CFRP GFRP AFRP

Environmental Reduction Factor, CE 1.00 0.80 0.90 0.90 0.70 0.80

4.4.3 Creep rupture stress limits To take into account of the risk of creep rupture, stress limits are given,. These limits are different for different FRP types and apply for the force due to the sustained stress (dead load and the sustained part of the live load). The limits suggested are shown in Table 14 and are based on test results published by Yamaguchi et al (1997) with an imposed safety factor of 0.6. Table 14

Creep rupture stress limits for FRP reinforcement where ffu is the design strength obtained according to Section 3.4.2 (ACI 2000). Fibre Type GFRP AFRP CFRP

Creep rupture stress limit 0.20ffu 0.30ffu 0.55ffu

4.4.4 Fatigue Fatigue is considered as an important long-term performance indicator of FRP rods especially when used in structures such as bridge decks, and it is pointed out that the fatigue capacity is affected by various parameters including temperature and moisture. The fatigue stress limits suggested are those presented in table 14.

4.5

British Institution of Structural Engineers

4.5.1 Introduction EUROCRETE is a pan-European project that started in 1993. One of its aims was to create design guidelines for FRP reinforcement in concrete. As a result the document "Modification of Design Rules to Incorporate Non-ferrous Reinforcement" (Clarke et -53-

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al 1996) was produced and finished in 1996. Much of this work is included in "Interim guidance on the design of reinforced concrete structures using fibre composite reinforcement" which is an interim guidance published by the British Institution of Structural Engineers (1999). Both these documents will be referred to in the current section.

4.5.2 Characteristic strength and safety factors The definition of characteristic strength is suggested to be the mean value minus 1.67 standard deviations for non-prestressed reinforcement and the mean value minus 3 standard deviations for prestressed reinforcement. This is equal to 5 percentile (given in Table 9) and 0.13 percentile respectively. (Clarke et al 1996). In this guideline the material factor γm is introduced. This factor includes the uncertainty regarding the location of the bar in the cross-section of the member (Clarke et al 1996), differences between actual and laboratory values, local weaknesses, long-term effects and inaccuracies in assessment of the resistance of sections (BISE 1999). According to Clarke et al (1996) γm can be expressed as 1.1 ⋅ γenv where γenv ( this notation introduced here) is the safety factor for environmental influence and longterm effects and 1.1 is a factor to take into account the uncertainty regarding the location of the bar in the cross-section of the member. For CFRP a γenv value of 1.67 is recommended. Hot wet environments, attacking the fibre/matrix bond, are here considered as the main durability problem. For AFRP the corresponding value is set to 2. The concrete environment is not anticipated to cause any specific problem. However, there is a lack of relevant data to confirm this. The sensitivity of aramid to moisture is also noted. For GFRP a γenv value of 3.3 is given. The recommendation is based on the susceptibility of GFRP to OH ions present in water and alkaline solutions and the stress rupture process in air. These partial safety factors are given also in Table 9 (note that the inverse values are given in that table). Recommended values are considered as initial and may be modified according to materials developments and the provision of validated test data. In the British design guidelines (British Institution of Structural Engineers, 1999) only the γm values are declared. These values are given in Table 15. Table 15

Partial safety factors for strength of composite material (Clarke et al 1996). Material GFRP (E-glass) AFRP CFRP

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If FRP reinforcements are only subjected to short term loads early in the life of the concrete structure (such as reinforcements for early shrinkage and thermal effects) the factor applied to the strength may be reduced to 1.25 for all materials. 4.6

Norwegian standard

4.6.1 Introduction The Norwegian Standard NS3473 Concrete structures –Design Rules, has been complemented with regard to the use of non-ferrous reinforcement. The main purpose has been to identify all sections that will need modification to incorporate FRP reinforcement. The work has been carried out as a EUROCRETE sub-project and is documented in "EUROCRETE Modifications to NS3473 when using fiber reinforced plastic (FRP) reinforcement" (Thorenfeldt, 1998).

4.6.2 Strength reduction factors One conversion factor, ηlt, takes into account “the decreased strength of FRP reinforcement subjected to long time load in dry air at room temperature (stress rupture)”. These values are said to be dependent on load duration and fibre type. Suggested values are given in Table 16. However, they can be increased to 1.0 if "the quasi permanent part of the load effects is not dominating". The possible ranges of the ηlt values are given in Table 9. Deterioration of tensile strength of reinforcement due to the concrete environment is taken into account by the conversion factor ηenv. (Thorenfeldt 1998). The suggested values of ηlt and ηenv are given in Table 16 as well as in Table 9. However, they are considered tentative and should be further evaluated. Table 16

Conversion factors for various FRP types (Thorenfeldt, 1998). FRP types GFRP AFRP CFRP

ηlt** 0.8 0.7 0.9

ηenv 0.5* 0.9 1.0

*The conversion factor for GFRP due to moist alkali environment may be taken equal to 0.9 if the reinforcement is protected e.g. by rubber hose in unbonded pre-stressing systems. **The conversion factors for long-term load may be increased to 1.0 when the quasi-permanent part of the load effects is not dominating.

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Chapter 4: Durability approach in existing design guidelines

4.6.3 Prestressed reinforcement Long term/durability properties are considered also for FRP used as prestressed reinforcement. The different resistances to long-term loading are taken into account by use of the stress limits suggested by CHBDC. The values can be found in Table 11. Because of the uncertainty regarding the resistance to alkali, GFRP is considered not applicable for pre-tensioning (with direct contact between tendon and concrete).

4.7

Conclusions

This section has discussed how durability related aspects of FRP used as concrete reinforcement are treated in existing design guidelines. In these guidelines durability is basically dealt with by using restrictions (on some FRP types or constituents in some applications) and various strength reduction factors. From this review the following conclusions can be drawn •

The JSCE design guidelines uses one factor that incorporates several uncertainty aspects including environmental durability. Stress limits for sustained stress are used.

•

The British design guideline deals with environmental degradation of FRP by using one factor that takes account of the influence of environment and sustained stress and a few other uncertainties.

•

The Norwegian and the ACI design guidelines have a factor taking account exclusively of the environmentally caused deterioration.

•

The Canadian design guideline uses a slightly different approach than the others. Rather liberal stress limits/design strengths are given but these are complemented by restrictions on the use of some FRP types in some applications, for example GFRP are stated inadmissible as primary reinforcement in deck slabs. There are also restrictions on the use of resin types for FRP reinforcement.

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Part A

Chapter 5: Examples of topics of interest for further research

5 Topics of interest for further research During the literature investigation some specific durability related areas were identified where more research is needed. Such areas are briefly treated in the following. The topics set out below are not to be considered exhaustive, but rather as examples of interesting areas mentioned in relevant papers in recent years. Low-temperature characteristics According to (Machida 1993), no data on the low temperature characteristics of FRP materials are available. It is probably temperatures below -10°C that are referred to. Freeze-thaw cycles Tests are needed to evaluate the behaviour of AFRP and CFRP when subjected to a combination of freeze-thaw cycles and initial tensile stresses, concrete and salts (Tannous and Saadatmanesh 1998). Durability of AFRP in concrete Direct exposure of AFRP specimens to artificial concrete pore solutions has been shown to be much more severe than exposure to the real concrete environment. Further research is needed to model the behaviour of AFRP in concrete elements stored outdoors (Scheibe and Rostasy 1998). Adhesive systems in severe environments According to (Cousins et al 1998), one unanswered question is how FRP adhesive systems respond to severe environmental conditions when used in the form of plates for strengthening concrete beams. Durability in “high pressure” environments According to Gangarao and Vijay (1997), the effects of pressure head on the degradation rate of composites needs to be established for offshore and deepwater marine applications, even though some data on this topic exist through aerospace research studies. The influence of various gases The effect of NOx and SOx ( present to some extent in the surroundings of all concrete structures) on the long-term properties of composites and their constituents may need

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Chapter 5: Examples of topics of interest for further research

to be studied further. The synergistic effect of load and NOx has been shown to reduce the creep rupture strength substantially (Bank and Gentry 1995). Bond between concrete and FRP According to (Nanni et al 1998), a closer analysis of the concrete-FRP interface, using SEM, is needed after conditioning to assess degradation mechanisms acting on the bond. Durability topics of interest for further research according to ACI 440 Research areas of fairly general nature are emphasized here as important durability issues Creep rupture The nature of this behaviour must be understood to provide adequate safety against its occurrence. Thermal expansion Studies are needed of the relative coefficients of thermal expansion of fibre and resin and their effect on mechanical properties of reinforcing bars and tendons. Low temperature and freeze-thaw cycles Research is needed to determine the effect of low temperature on FRP, as well as to determine any loss of strength under freeze-thaw cycles Fire Research is needed to determine the endurance limit of FRP during fire. Further, tests are needed to establish a minimum cover to achieve a desired fire rating for concrete members reinforced with commonly used FRP bars and tendons. Alkaline attack GFRP is generally susceptible to alkaline attack. There is a need to consider impervious alkali resistant matrices and alkali resistant glass or other types of alkali resistant fibres as alternative fibre materials. Long-term studies are also required to examine the effect of alkalinity on the composite. Acids Long-term studies are required to examine the durability of FRP bars and tendons under acidic environments. Salts and de-icing chemicals The influence of salts and de-icing chemicals on the durability of FRP bars and tendons needs to be evaluated since these agents are present in the environments of sea bridges and bridges in cold regions.

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Chapter 5: Examples of topics of interest for further research

Ultraviolet radiation Polymers generally degrade with time when subjected to UV light. Studies are therefore needed to determine the effect of UV light and to evaluate methods to protect FRP against the deteriorating effect of UV light (additives or UV proof coatings).

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Part A

Chapter 5: Examples of topics of interest for further research

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Part A

Chapter 6: Conclusions and discussion

6 Conclusions and discussion In this report, the durability of FRP used as concrete reinforcement has been discussed. The objective has been to summarise the current research and knowledge on the subject, and to identify areas where more research is needed to clear away the uncertainties associated with the durability of FRP. The information in this 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 determined in a reasonable time the experiments are generally accelerated to a great extent by using elevated temperatures in the exposure environments. The durability topics most frequently treated in recent years are: Effects of water and moisture, Influence of salt, Effects of Alkalis, and Influence of freeze/thaw actions. Usually, the tensile strength and Young’s modulus are measured before and after an exposure period to determine the change due to environmental influence. The composites most frequently studied are : aramid, carbon or glass fibres in a matrix of epoxy, polyester or vinyl ester resin. The durability results obtained from the different durability studies are generally not totally unanimous. In some tests, specimens seem to be unaffected by the exposures while, in other studies, a considerable degradation is observed for similar analysed materials. However, in general terms, carbon fibres have been shown to be almost unaffected by environmental exposure, while glass fibres often exhibit some deterioration after an exposure period. Especially alkaline conditioning under tensile stress has proved to be a very severe condition for GFRP. As regards resins, vinyl ester generally has better durability properties than polyester in an alkaline or moist environment. During the literature review on which this report is based, it became obvious that more knowledge of FRP durability is needed at different levels. There are some specific topics where further research is required (these topics are discussed below) Furthermore, there are some aspects that are appropriate for research activities in a general sense. An important issue is the fact that there exists no “standard durability test method”. This makes the results obtained by different researchers difficult to compare. However, the most important general problem is probably the uncertainties associated with the transformation of accelerated to real exposure time. Different methods have been used to predict the lifetime of FRP reinforcement. Predictions based on diffusion measurements (Katsuki and Uomoto 1995), extrapolation of nonaccelerated exposure data (Sheard et al 1997) and development of accelerated factors (for example Porter and Barnes 1998) are some examples of methods that have been used. However, for each method described, the need of verification by following up the real ageing and related deterioration is emphasised. Therefore, accelerated ageing of FRP should be regarded as a tool that can give indications, but hardly exact answers, of the long-term behaviour of GFRP composites. Frequently, the aim of the research work studied in this paper has been to compare FRP materials and to evaluate which is the best, among the composites analysed,

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Part A

Chapter 6: Conclusions and discussion

regarding one or several durability properties. More rarely has the purpose been to determine the lifetime of the FRP reinforcement in an expected concrete environment. To obtain more reliable lifetime data, the degradation mechanisms for different FRP systems must be surveyed and more closely connected to the lifetime prediction. This consideration is valid for the majority of durability areas, but the resistance of FRP to alkali could be said to be the most basic issue since alkali will always be present when FRP is used as concrete reinforcement. Design guidelines for FRP concrete reinforcement have been prepared, or are under development, in Japan, Canada, USA, Norway, and UK. In these guidelines the environmental influence on the FRP material is dealt with by introducing reduction factors. These factors take account of the time dependent deterioration in tensile strength and the susceptibility to stress-rupture. Some design guidelines (Norwegian and American) use one reduction factor that takes account exclusively of environmental effects while others (Japanese, British and Canadian) use more general reduction factors that take account of several uncertainty aspects including environmental durability.

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Part A

Chapter 7: References

7 References Adimi M R, Boukhili R, ”Influence of Resin And Temperature On the Interlaminar Shear Fatigue of Glass Fibre Reinforced Rods”, Proceedings from the First International Conference on Durability of Fibre Reinforced Polymer (FRP) Composites for Construction, Sherbrooke, August, 1998, pp. 681-690. Adimi M R, Rahman A H, Benmokrane B, ”Durability of FRP Reinforcements under Tension-Tension Axial Cyclic loading”, Proceedings from the First International Conference on Durability of Fibre Reinforced Polymer (FRP) Composites for Construction, Sherbrooke, August, 1998, pp. 635-647. Alsayed S, Alhozaimy A, ”Effect of High temperature and Alkaline Solutions On the Durability of GFRP Bars”, Proceedings from the First International Conference on Durability of Fibre Reinforced Polymer (FRP) Composites for Construction, Sherbrooke, August, 1998, pp. 623-634. American Concrete Institute, ”State-of-the-Art Report on Fiber Reinforced Plastic Reinforcement for Concrete Structures”, ACI 440R-96, American Concrete Institute, 1st printing, Farmington Hills, Michigan, p 68., February 1996. American Concrete Institute, “Provisional Design Recommendations For Concrete Reinforced with FRP Bars”, ACI Committee 440, Draft 2, September 21, 1998. American Concrete Institute, “Provisional Design Recommendations For Concrete Reinforced with FRP Bars”, ACI Committee 440, Draft, January 17, 2000. Arockiasamy A, Amer A, Shahawy M, Environmental and Long-Term Studies on CFRP Cables and CFRP Reinforced Concrete Beams”, Proceedings from the First International Conference on Durability of Fibre Reinforced Polymer (FRP) Composites for Construction, Sherbrooke, August, 1998, pp. 599-622. ASM International, ”Composites, Engineered Materials Handbook”, Volume 1, 1987. Bakis C E, Freimanis A J, Gremel D, Nanni A, Effect of Resin Material on Bond and Tensile Properties of Unconditioned and Conditioned FRP Reinforced Rods”, Proceedings from the First International Conference on Durability of Fibre Reinforced Polymer (FRP) Composites for Construction, Sherbrooke, August, 1998, pp. 525-535. Bank L C, Gentry T R, "Accelerated Test Methods to Determine the Long-Term Behavior of FRP Composite Structures: Environmental Effects", Journal of Reinforced Plastic and Composites, Vol. 14, June 1995, pp. 558-587. Bank L C,. Gentry T R, Barkatt A, Prian L, Wang F, Mangla S R, "Accelerated Aging of Pultruded Glass/Vinyl Ester Rods", Fiber Composites in Infrastructure, Proceedings of the Second International Conference on Fibre Composites in Infrastructure ICCI’98, Vol. 2, Tucson, 1998, pp. 423-437.

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Chapter 7: References

Beaudoin Y, Labossière P, Neale W, ”Wet-Dry Action on the Bond Between Composite Materials And Reinforced Concrete Beams”, Proceedings from the First International Conference on Durability of Fibre Reinforced Polymer (FRP) Composites for Construction, Sherbrooke, August, 1998, pp. 537-546. Benmokrane B, Rahman H, Ton-That M-T, Robert J-F, ”Improvement of the Durability of FRP Reinforcements For Concrete Structures”, Proceedings from the First International Conference on Durability of Fibre Reinforced Polymer (FRP) Composites for Construction, Sherbrooke, August, 1998, pp. 571-585. Case S W, Lesko J J, Cousins T E, ”Development of a Life Prediction Scheme for the Assessment of Fatigue Performance of Composite Infrastructures”, Proceedings from the First International Conference on Durability of Fibre Reinforced Polymer (FRP) Composites for Construction, Sherbrooke, August, 1998, pp. 69-80. Cetin K M, Shaw B A, Bakis C, Nanni A, Boothby T, "Environmental Degradation of Repaired Concrete Structure", Fiber Composites in Infrastructure, Proceedings of the Second International Conference on Fibre Composites in Infrastructure ICCI’98, Vol. 2, Tucson, 1998, pp. 488-500. CHBDC (Canadian Highway Bridge Design Code), Final Draft July, 1996. Chin J W, Haight M R, Hughes W L, Hguyen T, ”Environmental Effects on Composite Matrix Resins Used In construction”, Proceedings from the First International Conference on Durability of Fibre Reinforced Polymer (FRP) Composites for Construction, Sherbrooke, August, 1998, pp. 229-241. Chin J W, Nguyen T, Aouadi K, "Effects of Environmental Exposure on FiberReinforced Plastic (FRP) Materials Used in Construction", Journal of Composites Technology and Research, Vol 19, No 4, Oct 1997, pp. 205-213. Clarke J L, O’Regan D P and Thirugnanendran C, “EUROCRETE Project, Modification of Design Rules to Incorporate Non-ferrous Reinforcement “, EUROCRETE Project, Sir William Halcrow & Partners, London, 1996. Clarke J L, Sheard P, ”Designing Durable FRP Reinforced Concrete Structures”, Proceedings from the First International Conference on Durability of Fibre Reinforced Polymer (FRP) Composites for Construction, Sherbrooke, August, 1998, pp. 13-24. Clément J L, Fuzier J P, Lacroix R, Luyckx J, ”Durability of Composites For Construction”, Proceedings from the First International Conference on Durability of Fibre Reinforced Polymer (FRP) Composites for Construction, Sherbrooke, August, 1998, pp. 561-569. Conrad J O, Bakis C E, Boothby T E, Nanni A, ”Durability of Bond of Various FRP rods In Concrete”, Proceedings from the First International Conference on Durability of Fibre Reinforced Polymer (FRP) Composites for Construction, Sherbrooke, August, 1998, pp. 299-316.

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Part A

Chapter 7: References

Coomarasamy A, A.K.C Ip, ”Evaluation of Fibre Reinforced Plastic (FRP) Materials For Long Term Durability In Concrete Structures”, Proceedings from the First International Conference on Durability of Fibre Reinforced Polymer (FRP) Composites for Construction, Sherbrooke, August, 1998, pp. 325-336. Cousins T E, Lesko J J, Carlin B, ”Tailored performance and Durability of Reinforced Concrete Beams Strengthened with FRP Plates", Fiber Composites in Infrastructure, Proceedings of the Second International Conference on Fibre Composites in Infrastructure ICCI’98, Vol. 2, Tucson, 1998, pp. 399-409. Devalapura R K, Gauchel J V, Greenwood M E, Hankin A, Humpherey T, "LongTerm Durability of Glass-Fiber Reinforced Polymer Composites in Alkaline Environments", ", Non-Metallic (FRP) Reinforcement for Concrete Structures: Proceedings of the Third International Symposium, Vol 2, Sapporo, Oct. 1997, pp. 83-90. Devalapura R K, Greenwood M E, Gauchel J V, Humphrey T J, ”Evaluation of GFRP Performance Using Accelerated Test methods”, Proceedings from the First International Conference on Durability of Fibre Reinforced Polymer (FRP) Composites for Construction, Sherbrooke, August, 1998, pp. 107-116. El-Badry M, Abdalla H, ”Experimental Studies On Thermal Cracking in Concrete Members Reinforced With FRP”, Proceedings from the First International Conference on Durability of Fibre Reinforced Polymer (FRP) Composites for Construction, Sherbrooke, August, 1998, pp. 669-680. Gangarao H V S, Vijay P V, "Aging of Structural Composites under Varying Environmental Conditions", Non-Metallic (FRP) Reinforcement for Concrete Structures: Proceedings of the Third International Symposium, Vol 2, Sapporo, Oct. 1997, pp. 91-98. Gentry T R, Husain M, ”Thermal Compatibility of Concrete and Composite Reinforcement: Analytical and Numerical Predictions”, Proceedings from the First International Conference on Durability of Fibre Reinforced Polymer (FRP) Composites for Construction, Sherbrooke, August, 1998, pp. 203-215. Green M F, Bisby A, Beaudoin Y, Labossiere P, ”Effects of Freeze-Thaw Action on the Bond of FRP Sheets to Concrete”, Proceedings from the First International Conference on Durability of Fibre Reinforced Polymer (FRP) Composites for Construction, Sherbrooke, August, 1998, pp. 179-190. Hayes M D, Garcia K, Verghese N, Lesko J J, "The Effects of Moisture on the Fatigue Behavior of a Glass/Vinyl Ester Composite", Fiber Composites in Infrastructure, Proceedings of the Second International Conference on Fibre Composites in Infrastructure ICCI’98, Vol. 1, Tucson, 1998, pp. 1-13. Institution of Structural Engineers, "Interim guidance on the design of reinforced concrete structures using fibre composite reinforcement", Published by SETO Ltd, 116 pages, August 1999.

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Part A

Chapter 7: References

Japan Society of Civil Engineers, “Recommendation for Design and Construction of Concrete Structures Using Continuous Fiber Reinforcing Materials”, October, 1997. Karlsson M., ”Fiberkompositarmering”, Literature review, NCC Teknik, Göteborg, Sweden, 1998. pp. 19-21. Kato Y, Yamaguchi T, Nishimura T, Uomoto T, "Computational Model for Deterioration of Aramid Fiber by Ultraviolet Rays", Non-Metallic (FRP) Reinforcement for Concrete Structures: Proceedings of the Third International Symposium, Vol 2, Sapporo, Oct. 1997, pp. 163-170. Katsuki F, Uomoto T, " Prediction of Deterioration of FRP Rods Due to Alkali Attack", Non-Metallic (FRP) Reinforcement for Concrete Structures, 1995, pp. 82-89. Katz A, Berman N, Bank L C, ”Effect of Cyclic Loading and Elevated temperature On the Bond Properties of FRP Rebars”, Proceedings from the First International Conference on Durability of Fibre Reinforced Polymer (FRP) Composites for Construction, Sherbrooke, August, 1998, pp. 403-413. Litherland K L, Oakley D R, Proctor B A, "The Use of Accelerated Ageing Procedures to Predict the Long Term Strength of GRC Composites", Cement and Concrete Research - An International Journal- Vol 11, No 3, May 1981, pp. 455-466. Machida A, ”State-of-the-Art Report on Continuous Fiber Reinforcing Materials”, Society of Civil Engineers (JSCE), Tokyo, 1993. Mukhopadhyaya P, Swamy R N, Lynsdale C J, ”Durability of Adhesive bonded Concrete-GFRP Joints”, Proceedings from the First International Conference on Durability of Fibre Reinforced Polymer (FRP) Composites for Construction, Sherbrooke, August, 1998, pp. 373-389. Nanni A, ”Fiber-Reinforced-Plastic (FRP) Reinforcement For Concrete Structures: Properties and Applications”, Elsevier Science Publishers B.V., 1993. Nanni A, Bakis C E, Mathew J A, ”Acceleration of FRP Bond Degradation” Proceedings from the First International Conference on Durability of Fibre Reinforced Polymer (FRP) Composites for Construction, Sherbrooke, August, 1998, pp. 45-56. Nanni A, Boothby T, Cetin K M, Shaw B A, Bakis C, "Environmental Degradation of Repaired Concrete Structure", Non-Metallic (FRP) Reinforcement for Concrete Structures: Proceedings of the Third International Symposium, Vol 2, Sapporo, Oct. 1997, 155-162. Nguyen T, Byrd E, Alshed D, Aouadi K, Chin J, ”Water at the Polymer/Substrate interface and its Role in the durability of polymer/Glass Fiber composites”, Proceedings from the First International Conference on Durability of Fibre Reinforced Polymer (FRP) Composites for Construction, Sherbrooke, August, 1998, pp. 451-462.

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Part A

Chapter 7: References

Noordzij R, "Design and Manufacturing of a Full Composite Pedestrian Bridge", Fiber Composites in Infrastructure, Proceedings of the Second International Conference on Fibre Composites in Infrastructure ICCI’98, Vol. 1, Tucson, 1998, pp. 443-455. Pantuso A, Spadea G, Swamy R N, "An Experimental Study on the Durability of GFRP Bars", Fiber Composites in Infrastructure, Proceedings of the Second International Conference on Fibre Composites in Infrastructure ICCI’98, Vol. 2, Tucson, 1998, pp. 476-487. Porter M L, Barnes B A, "Accelerated Aging Degradation of Glass Fiber Composites", Fiber Composites in Infrastructure, Proceedings of the Second International Conference on Fibre Composites in Infrastructure ICCI’98, Vol. 2, Tucson, 1998, pp. 446-459. Porter M L, Barnes B A, “Accelerated Durability of FRP Reinforcement For Concrete Structures”, Proceedings from the First International Conference on Durability of Fibre Reinforced Polymer (FRP) Composites for Construction, Sherbrooke, August, 1998, pp. 191-201. Porter M L, Mehus J, Young K A, O´Neil E F, Barnes B A, "Aging For Fiber Reinforcement in Concrete", Non-Metallic (FRP) Reinforcement for Concrete Structures: Proceedings of the Third International Symposium, Vol 2, Sapporo, Oct. 1997, pp. 59-66. Proctor B A, Oakley D R, Litherland K L, "Developments in the assessment and performance of GRC over 10 years", Composites, April 1982, pp. 173-179. Rahman A H, Kingsley C, Richard J, Crimi J, "Experimental Investigation of the Mechanism of Deterioration of FRP Reinforcement for Concrete", Fiber Composites in Infrastructure, Proceedings of the Second International Conference on Fibre Composites in Infrastructure ICCI’98, Vol. 2, Tucson, 1998, pp. 501-511. Rostásy F S, "On Durability of FRP in Aggressive Environments", Non-Metallic (FRP) Reinforcement for Concrete Structures: Proceedings of the Third International Symposium, Vol 2, Sapporo, Oct. 1997, pp. 107-114. Saadatmanesh H, Tannous F, "Durability of FRP Rebars and Tendons", Non-Metallic (FRP) Reinforcement for Concrete Structures: Proceedings of the Third International Symposium, Vol 2, Sapporo, Oct. 1997, pp. 147-154. Sasaki I, Nishizaki I, Sakamoto H, Katawaki K, Kawamoto Y, "Durability Evaluation of FRP Cables by Exposure Tests", Non-Metallic (FRP) Reinforcement for Concrete Structures: Proceedings of the Third International Symposium, Vol 2, Sapporo, Oct. 1997, pp. 131-137. Scheibe M, Rostasy F S, "Stress-Rupture Behavior of AFRP-Bars in Concrete and Under Natural Environment", Fiber Composites in Infrastructure, Proceedings of the Second International Conference on Fibre Composites in Infrastructure ICCI’98, Vol. 2, Tucson, 1998, pp. 138-151.

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Part A

Chapter 7: References

Seible F, "US Perspective of Advanced Composites Bridge Technology in Europe and Japan" , Fiber Composites in Infrastructure, Proceedings of the Second International Conference on Fibre Composites in Infrastructure ICCI’98, Vol. 2, Tucson, 1998, pp. 605-636. Sen R, Shahawy M, Rosas J, Sukumar S; "Durability of AFRP & CFRP Pretensioned Piles in a Marine Environment", Non-Metallic (FRP) Reinforcement for Concrete Structures: Proceedings of the Third International Symposium, Vol 2, Sapporo, Oct. 1997, pp. 123-130. Sen R, Shahawy M, Sukumar S, Rosas J, "Effects of Tidal Exposure on Bond of CFRP Rods", Fiber Composites in Infrastructure, Proceedings of the Second International Conference on Fibre Composites in Infrastructure ICCI’98, Vol. 2, Tucson, 1998, 512-523. Sentler L, ”Fiberkompositer som armering Materialegenskaper”, Statens råd för byggforskning, Rapport R10:1992, Stockholm, 1992. Sheard P, Clarke J, Dill M, Hammersley G, Richardson D, "Eurocrete - Taking Account of Durability for Design of FRP Reinforced Concrete Structures", NonMetallic (FRP) Reinforcement for Concrete Structures: Proceedings of the Third International Symposium, Vol 2, Sapporo, Oct. 1997, pp. 75-82. Spainhour L K, Thompson I, "Effect of Carbon Fiber Jackets on Reinforced Concrete Columns Exposed to a Simulated Tidal Zone", Fiber Composites in Infrastructure, Proceedings of the Second International Conference on Fibre Composites in Infrastructure ICCI’98, Vol. 1, Tucson, 1998, pp. 426-439. Steckel G L, Hawkins G F, Bauer J L, "Environmental Durability of Composites for Seismic Retrofit of Bridge Columns", Fiber Composites in Infrastructure, Proceedings of the Second International Conference on Fibre Composites in Infrastructure ICCI’98, Vol. 2, Tucson, 1998, pp. 460-475. Tannous F E, Saadatmanesh H, "Durability and Long-Term Behavior of Carbon and Aramid FRP Tendons", Fiber Composites in Infrastructure, Proceedings of the Second International Conference on Fibre Composites in Infrastructure ICCI’98, Vol. 2, Tucson, 1998, pp. 524-538. Tepfers R, fib 4.2, ”Bond models”, State-of-the-Art-report Chapter 7, “Bond of nonmetallic reinforcement”, June 1998, Gothenburg. Thorenfeldt E, “EUROCRETE Modifications to NS3473 When Using Fiber Reinforced Plastic (FRP) Reinforcement, April, 1998. Tomosawa F, Nakatsuji T, "Evaluation of ACM Reinforcement Durability By Exposure Test", Non-Metallic (FRP) Reinforcement for Concrete Structures: Proceedings of the Third International Symposium, Vol 2, Sapporo, Oct. 1997, pp. 139-146.

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Part A

Chapter 7: References

Toutanji H A, Rey F, ”Performance of Concrete Columns Strengthened With Advanced Composites Subjected To Freeze-Thaw Conditions”, Proceedings from the First International Conference on Durability of Fibre Reinforced Polymer (FRP) Composites for Construction, Sherbrooke, August, 1998, pp. 351-360. Toutanji H, El-Korchi T, "Tensile Durability Performance of Cementitious Composites Externally Wrapped with FRP Sheets", Fiber Composites in Infrastructure, Proceedings of the Second International Conference on Fibre Composites in Infrastructure ICCI’98, Vol. 2, Tucson, 1998, pp. 410-421. Tysl S R, Imbrogno M, Miller B D, ”Effects of Surface delamination on the Freeze/Thaw Durability of CFRP-Reinforced Concrete Beams”, Proceedings from the First International Conference on Durability of Fibre Reinforced Polymer (FRP) Composites for Construction, Sherbrooke, August, 1998, pp. 317-324. Uomoto T, Nishimura T, "Development of New Alkali Resistant Hybrid AGFRP Rod", Non-Metallic (FRP) Reinforcement for Concrete Structures: Proceedings of the Third International Symposium, Vol 2, Sapporo, Oct. 1997, pp. 67-74. Verghese N E, Hayes M, Garcia K, Carrier C, Wood J, Lesko J J, "Effects of Temperature Sequencing During Hygrothermal Aging of Polymers and Polymer Matrix Composites: The Reverse Thermal Effect", Fiber Composites in Infrastructure, Proceedings of the Second International Conference on Fibre Composites in Infrastructure ICCI’98, Vol. 2, Tucson, 1998, pp. 720-739. Zheng Q, Morgan R J, "Synergistic Thermal-Moisture Damage Mechanisms of Epoxies and Their Carbon Fiber Composites", Journal of Composite Materials, Vol 27, No 15, 1993, pp. 1465-1478.

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Chapter 7: References

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Part A

APPENDIX

Appendix Part A

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Part A

APPENDIX

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APPENDIX

APPENDIX 1: Resistance of epoxy and polyester resin subjected to different chemicals (Machida 1993).

APPENDIX 1

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APPENDIX

APPENDIX 2: Brief summary of some research activities on durability of FRP as reinforcement in concrete APPENDIX 2 In this appendix brief summaries are given of some research activities studied in this review. One purpose of these summaries is to present projects from which durability results have been referred to, but not always put into context, in this paper. The purpose is also to give examples of how FRP durability research is generally performed, regarding: number of specimens analysed, quantity of different environmental influences studied and duration of test periods. Another aim is to give a picture of how different, potentially harmful, environmental influences on the FRP material in the real structural application are experimentally handled. Most of the research projects reported in this appendix have been presented at FRP related conferences from the year 1997 and later. For each project the ambition has been to cover: the experimental method used, the materials analysed and the most important results obtained. However, in some cases only a selection of interesting results is presented. . The references are related to Chapter 5 “References” Porter et al (1997): Ageing for Fibre Reinforcement in Concrete In this project, long term durability testing has been performed on commercial FRP products. Materials used were three GFRP rods with E-glass fibre and three types of polyester resins and CRFP composites. The tensile strength of rods directly exposed to accelerating solution simulating high alkaline environment has been evaluated. Rods both with and without applied load during the exposure have been analysed. Prestressed concrete sections subjected to high alkalinity have also been studied. Earlier developed equations for conversion between real and accelerating time have been used. The different specimens were immersed in a simulated pore water solution (pH 12,5-13, 60°C) for 2-3 months. This is claimed to simulate approximately 50 years of real weather ageing. The exposure has been shown to be very harmful for the GFRP reinforcement investigated, especially the combined effect of sustained stress and a corrosive environment. However the CFRP material did not appear to be affected by the exposure. Uomoto and Nishimura (1997): Development of New Alkali Resistant Hybrid AGFRP rod In order to obtain a cheap and alkali resistant FRP material, a hybrid fibre composite bar has been created. The bar consists of a GFRP core surrounded by an AFRP material. In that way, the alkali sensitive glass fibres are protected by the aramid fibre layer. The resin used was a vinyl ester. Specimens were subjected to an accelerated alkaline environment (NaOH, 1 mole/l, at 40°C) for up to 120 days. This is said to be similar to 100 years duration in an “ordinary“ concrete environment (pH 12,5, 20°C). The hybrid bars are compared with GFRP and AFRP bars subjected to similar exposure conditions, with respect to tensile strength and the depth of Na concentration within the rod (EPMA analysis). Results show that AFRP and AGFRP have higher resistance to Na intrusion than GFRP. The results further indicate that the AGFRP rods may possess their original strength even after 100 years of exposure in concrete. -74-

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APPENDIX

Sheard et al (1997): EUROCREATE-Taking Account of Durability for Design of FRP Reinforced Concrete Structures A durability research programme has been established by the Eurocrete consortium to provide the necessary scientific data and the design framework for determining appropriate and reliable design allowables. Five parallel research programmes were initiated, the overall objective being to develop a model for predicting FRP reinforcing bar behaviour over time. The five phases of the durability programme were as follows: Level 1) Material optimisation, Level 2) Accelerated rebar durability testing, Level 3) Accelerated reinforced concrete testing, Level 4) Field exposure testing, Level 5) Case study analysis. The rationale behind the programme was to identify composite materials most likely to perform well in the concrete environment and to test these through a series of accelerated laboratory trials using high alkali fluids. The results were compared with data from reinforced concrete samples placed in accelerated exposure environments to provide a benchmark between the real concrete environment and the high alkali fluids. The final two levels of testing were concerned with capturing information from in-service environments; test samples in extreme conditions and structures in real working environments. This approach has enabled the accelerated laboratory data to be interpreted and benchmarked against performance in in-service environments. The extrapolations performed are therefore said to be more reliable and can also be checked for accuracy with the passing of time on the case study structures. This in turn will enable safe design factors to be derived which can be continually upgraded on the basis of in-service performance. Test data from the durability programme can be plotted on the basis of residual strength v. logarithmic time. The residual strength for the required design life can then be determined using extrapolation with a straight line. When the long term properties of the FRP reinforcement have been determined, a safety factor can be established to take the deterioration effect into account. As regards the experiments , investigations have been conducted on carbon and glass fibre bars as follows: Level 2), specimens were subjected to an accelerated environment including pH 10-13,5, Ionic content of solution (K, Ca, Na) as well as temperature from 21 to 80°C and stress levels of 5 to 75% (of ultimate tensile stress). The deterioration was checked using interlaminar shear strength test. For specimens subjected to 5% of ultimate tensile strength, the maximum reduction was found to be 29% for the GFRP specimens and 18% for the CFRP specimens (for the case with α=75% no results were declared), Level 3) specimens were subjected to accelerated conditions including alkali (38°C), wetting/drying (cycles: 2 days in room temperature water followed by 5 days of drying at 38°C), chloride in concrete (3% per mass of cement) and carbonated concrete. The FRP bars were tested with respect to: pullout tests, visual and SEM examination and microanalysis using EDAX facility in conjunction with SEM. According to the results, there was no evidence of any deterioration of the FRP bars after 12 months. Level 4) Five exposure sites were selected. As in level 3, there was no evidence of any deterioration of the FRP bars after 12 months. It is however concluded that simulated pore fluid is a more severe environment than concrete itself. Regarding Level 5), no results were presented in the current article.

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APPENDIX

Gangarao and Vijay (1997): Ageing of Structural Composites Under Varying Environmental Conditions Accelerated ageing response of GFRP bars and plates as well as concrete beams reinforced with GFRP and concrete beams wrapped with CFRP fabrics have been investigated. Ageing factors include sustained stress, high pH, as well as temperature and humidity variations. The GFRP materials studied consisted of five different types of resins. For all the bars a reduction in tensile strength and stiffness could be measured after the exposure. A bar consisting of a vinyl ester resin showed the smallest reduction in strength. GFRP plates were subjected to 4% salt conditioning (at room temperature or temperature cycles 0-70°C) for 220 to 240 days. Some of the specimens were tested at room temperature, and some at temperatures ranging from 74-200°F (equal to 23-93°C). Strength reductions of 17% were observed. For salt environment exposure of composites, strength degradation rate was found to be of the form σt=σ0⋅e-λt, where t=time in days (450)= σ0/2. Guimaraes and Burgoyne (1997): Thermal Expansion of Kevlar 49 Yarns An investigation has been conducted to study the relationship between the coefficient of thermal expansion (CTE) of Kevlar 49 yarns (i. e. aramid material) and applied stress. The investigation consisted of tests at stress levels between 3% and 45% of the ultimate tensile strength, while temperature varied from 5 to 75°C. Test results indicated that the material contracts longitudinally as the temperature increases and that the absolute value of CTE increases with an increase in applied stress. The relationship between initial strain and CTE was found to be α=-4.0⋅ε0-3.7, where α=coefficient of thermal expansion expressed in 10-6/°C and ε0 =initial strain caused by loads (expressed in %). Tomosawa and Nakatsuji (1997): Evaluation of ACM Reinforcement Durability by Exposure Test An investigation has been performed to evaluate the durability of FRP reinforcement subjected to non-accelerated testing in the air at a marine structure-like site in the tropical zone. The different reinforcements used for the experiment included FRP bars with carbon, aramid glass and PVA fibres. After the exposure, the specimens were tested with respect to tensile strength. FRP bars were placed as reinforcement in concrete beams, with and without prestressing, which were tested in bending following the exposure. After two years of exposure no reduction in strength could be observed. Sasaki et al (1997): Durability Evaluation of Cables by Exposure Test Non-accelerating experiments have been performed on six types of FRP cables. Material systems used were; aramid/vinyl ester, aramid/epoxy, E-glass/vinyl ester, Vinylon/epoxy, two types of carbon/epoxy systems. Specimens have been subjected to different exposure conditions including atmospheric, splash zone and submerged zone conditions. Some cables were prestressed during the exposure. After 3.5 years the specimens were tested. For the submerged cables without prestressing, a major reduction in tensile strength (29-100%) was observed except for the CFRP cables. A possible explanation of these results is inadvertent fatigue load caused by waves. For GFRP cables exposed to air without prestress the reduction in tensile strength was 20% for specimens not subjected to sunshine and 10% for specimens subjected to sunshine. For CFRP, AFRP and CFRP cables exposed to air under prestress (α=0.60-76-

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0.80, α denotes stress-ultimate strength ratio), all cables except the E-glass/vinyl ester composite retained their original strength after the exposure period. For the GFRP, however, the strength was reduced by 20 to 40% after the exposure period for cables subjected to α=0.40 prestressing level. For cables prestressed at α=0.25, no reduction in tensile strength was found. Kato et al (1997): Computational Model for Deterioration of Aramid Fiber By Ultraviolet Rays To investigate the effect of ultraviolet rays, FRP rods have been exposed in an accelerating machine for up to 2500 hours. During exposure, specimens were subjected to climate cycles composed of 102 minutes of dry condition (52±2% RH) and 18 minutes of wet condition (90±2% RH). Intensity of ultraviolet radiation was almost 0.2 MJ/m2 per hour and the temperature in the climate chamber was 26°C. Experiments were conducted on AFRP, GFRP and CFRP rods as well as aramid, Tglass and carbon fibres. After the exposure test AFRP, GFRP and the CFRP rods show 13% (after 2500 hours), 8% (after 500 hours) and no (after 2500 hours) reduction in tensile strength respectively. It has been shown that for the aramid fibres, the tensile strength loss can be predicted using the so called weakest link theory Saadatmanesh and Tannous (1997): Durability of FRP Rebars and Tendons The durability of 8 types of GFRP, 2 types of CFRP and one AFRP composite has been investigated using accelerated and non-accelerated tests. Specimens were immersed in seven different environments including water, alkaline, acid and saline conditions (a total of 450 reinforcing bars and tendon specimens have been analysed). By measuring the increase in weight due to moisture diffusion, and using some physical relations, including Fick's law, the reduction in strength was calculated. The following conclusions were drawn: Fick’s law may be used to simulate the changes in moisture content and to predict the tensile strength for a period of time during which Fickian diffusion is valid. Diffusivity is dependent on temperature and the type and concentration of solution, vinyl ester has lower diffusivity and better resistance to chemical attack than polyester. Glass displayed durability problems whereas Arapree demonstrated excellent durability under the accelerated exposure. Also, 20 concrete beams reinforced with different kinds of GFRP reinforcements were tested in flexure to examine the durability of GFRP in concrete when exposed to de-icing salts. The results indicated that adequate protection could be provided by increasing the concrete cover and reducing the width of concrete cracks.

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Part A

APPENDIX

Hayes et al (1998): The Effects of Moisture on the Fatigue Behaviour of a Glass/Vinyl Ester Composite Experiments have been conducted on a commercial composite plate with glass fibres and vinyl ester resin. The study addresses the change in quasi-static and R =σmin/σmax =0.1 (tension-tension, at 10Hz) fatigue behaviour due to moisture. Specimens were conditioned by immersion in water bath for 30 days followed by a desorption period for 4 days. This procedure was repeated twice. The temperature during the whole cycle was 45°C. Quasi-static and fatigue tests were performed at different moisture conditions during the cycles described above. The results of the study suggest that the presence of moisture for the particular glass/vinyl ester system studied reduces the tensile strength properties. Quasi-static tensile strength is reduced by 26% at a moisture concentration of 0.95% by weight. The presence of moisture reduces the fatigue performance, but it appears to affect the fatigue damage mechanism uniformly, so that damage develops in a manner similar to the dry materials. The strength reduction of the composite occurs with the initial moisture exposure of the material system. The damage is fibre dominated and irreversible, and can be explained by moisture exposure of the E-glass fibres which leads to stress concentrations on the fibre surface. Spainhour and Thomson (1998): Effect of Carbon Fiber Jackets on Reinforced Concrete Columns Exposed to a Simulated Tidal Zone In an ongoing project the effect of wraps around concrete columns on the penetration of sea water is analysed. The research involves exposing scale model columns (square shaped cross section with side length 15.2 cm and column length 90 cm, externally wrapped with CFRP) in a salt water immersion tank for 3-4 years. The material system used is carbon fibre fabric with a marine-grade epoxy resin. To simulate the tide, the tanks are filled and emptied over one week, which is assumed to be enough for thorough saturation followed by thorough drying. The test is accelerated through elevated water temperature (40°C) and high salinity (5%). Furthermore a minimal concrete cover (2.5 cm) is used to accelerate the onset of corrosion. The specimens are preconditioned in different ways , some are wrapped prior to immersion to simulate a new structure, others will be exposed to the artificial tide for one year and then wrapped, to simulate repair of the column. There will also be concrete columns without jackets acting as control specimens. For each category, some specimens were precracked prior to wrapping and immersion. This was to simulate the effect of damage and to accelerate the exposure to salt water. Reducing the influence of wet/dry cycling by minimising surface drying of regions exposed to moisture through capillary action is a possible improvement accomplished by the wrapping. Samples of the fibre wrap will be analysed by scanning electron microscopy and by stiffness and strength measurements. Scheibe and Rostásy (1998): Stress-Rupture Behaviour of AFRP-Bars in Concrete and Under Natural Environment The goal of this investigation was to compare the stress rupture strength of AFRP bars embedded in pretensioned concrete elements with that of laboratory specimens where the aggressive solution acts directly on the surface of the bar. As part of the investigation experiments have been performed where the stress rupture strength of “naked” specimens exposed to air (20°C, 65% RH) and alkaline solution (saturated Ca(OH)2 + 0.4 n KOH, at 20°C) was compared with AFRP rods embedded in concrete and subjected to different environments. The following conclusions could be -78-

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drawn: The lifetime of AFRP in drying and sealed concrete was in the range of those attained in air. The lifetime of AFRP embedded in concrete and exposed to alkaline solution was much shorter , though longer than for the “naked” AFRP specimen exposed to alkali solution. To study the stress rupture behaviour of ARFP embedded in concrete, 36 slabs were cast and placed in different outdoor environments under constant bending forces (α=0.7 - 0.85 for the embedded bars). Some of the slabs were precracked prior to exposure. None of the slabs fell below the initial strength after one and two years of exposure (results from the third year are not available at the time of writing ). Following the bending test on the slabs after one and two years, AFRP bars were retrieved from each slab to measure the short term tensile strength as well as the stress rupture strength. From these tests no significant deterioration of the AFRP bars, caused by the environment, could be discerned. Bank et al (1998): Accelerated Ageing of Pultruded Glass/Vinyl Ester Rods In this investigation, which is part of a larger research project, the work focuses on the procedure for conducting ageing experiments; predictive modelling will be reported in the future. The material tested was E-glass/vinyl ester in the form of 6.23 mm diameter smooth cylindrical rods. The rod specimens were subjected to six different types of conditioning, and tested with respect to flexural strength, short beam shear strength (measured by a short beam shear strength test, a flexural test of a specimen having a low test span-to-thickness ratio, such that failure is primarily in shear, ASM International 1987), Thermo Gravimetric Analysis (TGA) and weight change and furthermore Differential Scanning Calorimetry (DSC) The exposure conditions, classified from A to F, and the results are summarised below. A) Immersion in deionized water with temperature ranging from 23 to 80°C for a period of 7 to 224 days. Result: The flexural strength is affected by the exposure time as well as the temperature. The water saturation level is never reached, indicating degradation of the material. B) Immersion in Ammonium oxide (0.3, 3, 30%) for 112 and 224 days. Result: only for the 30% solution is the degradation significant. This is indicated by the shear strength test and the TGA. C) Immersion in water (60°C) under two levels of pre-tension for 28, 112 and 224 days. Result: No appreciable difference of the results compared with those of “A)”, however some of the specimens ruptured during the exposure time. D) Storage in environmental chamber (23±0.3°C, 50% RH) for 1 to 8 months, to see if the properties would change. Result: No change in properties was detected. E). Storage in indoor climate for 1 to 8 months, to see if the properties would change in an ordinary storage climate. Result: No change in properties was detected. F) The specimens in this group were not exposed to any experimental environment, but were tested directly when delivered. Result: The results for these “base line” specimens ( i.e. reference specimens), are to be compared with those for the specimens in the other groups.

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Part A

APPENDIX

Steckel et al (1998): Environmental Durability of Composites for Seismic Retrofit of Bridge Columns A programme is being conducted to determine the environmental durability of 12 fibre composite overwrap systems. The composite material is potential casing for column seismic retrofit. The material system studied includes nine carbon/epoxy systems, one E-glass/epoxy system, one E-glass/vinyl ester system and one Eglass/polyester composite. The environmental exposures include 100% humidity (38°C), salt water (23°C), an alkaline solution (CaCO3, pH 9.5, 23°C), diesel fuel (4 h, 23°C), ultraviolet light (cycle: UV at 60°C and water condensation at 40°C, 4 h per condition, 100 cycles), elevated temperature (60°C) and a cyclic freeze/thaw test (cycle: 100% RH at 38°C and freezing at -18°C, 24 h per cycle, 20 cycles). Flat laminates are subjected to one of the environmental conditions mentioned above. Thus, synergistic effects are not being evaluated. The effect of the environmental exposure is being quantified by measurements of the composite panel mass, tensile modulus, strength, failure strain, interlaminar shear strength and glass transition temperature. Measurements are made after exposure intervals of 1000, 3000 and 10000 hours to allow estimates of the degradation over the projected service life. At the time of writing, the 1000 and 3000 hour exposure period tests have been completed for three glass fibre/polymer resin systems and four carbon fibre/polymer resin systems. According to the results, most of the systems exhibit excellent durability with no degradation in mechanical or physical properties. However, one glass-reinforced system showed a progressive reduction in strength after 3000 hours. Furthermore, one carbon/epoxy system had up to 35% reduction in short beam shear strength and a significant reduction in glass transition temperature associated with moisture absorption. All carbon/epoxy systems had a small reduction in tensile strength after 3000 hours in salt water. The significance of the findings will be easier to judge after 10000 hours of testing. Porter and Barnes, Tucson (1998): Accelerated Ageing Degradation of Glass Fiber Composites A research investigation to determine the effect of accelerated alkaline corrosion on the pullout resistance of E-glass/vinyl ester composite reinforcement embedded in concrete has been performed. Accelerated ageing was achieved by immersion of specimens, FRP rods embedded in concrete, in an alkaline solution (pH 12) under an elevated temperature of 60°C. A water bath at similar temperature was used to help in isolating the effect of the alkalis on the strength characteristics of the composite. To develop an accelerated ageing relationship for use in the investigation, a relationship between the bath temperature and the real temperature was developed . This relationship is expressed by the equation AGE=1.0235e0.0935(T), where AGE is the relative age at the temperature T (°C) compared with Des Moines, Iowa. Pullout tests are conducted at a simulated age of 5, 25, 50, 75 and 100 years. The results indicate that the effect of alkaline corrosion is minimal on the specimens analysed. However, a possible explanation of this result is the thickness of concrete cover and the time delay associated with moisture penetration.

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Part A

APPENDIX

Pantuso et al (1998): An Experimental Study on the Durability of GFRP Bars The effect of distilled water and alkaline environment on the durability of glass fibre/polyester pultruded rods has been investigated. The experiments consisted of one day's immersion of GFRP specimens in distilled water (23±2°C) followed by drying for one day; this treatment lasted for 60 days. To investigate the influence of alkaline environment, this procedure was repeated for specimens embedded in concrete (with W/C=0.6). After the exposure, measurements were made including: changes in weight, ultimate strength, tensile modulus and Poisson’s ratio. Furthermore, changes in the external surface of specimens were investigated with photomicrographs. From the results, the following conclusions could be drawn: Even after 60 days, the GFRP bars had not reached their saturated state, and were continuing to gain weight. The reduction in tensile strength was 1 to 7% and 6 to 21% for water and alkaline environment respectively. The treatments of the bars did not appear to have any significant effect on the fibre-matrix interface, though the electron micrograph showed a small change in the external surface of the rods. Cetin et al (1998): Environmental Degradation of Repaired concrete structure This study was conducted to determine whether CFRP or the steel reinforcement would degrade as a result of inadvertent galvanic coupling. This could be a potential problem if externally bonded FRP plates are used for repair purposes. A few possible degradation mechanisms are presented. Potentiodynamic polarization scans, galvanic coupling tests, and electrochemical impedance spectroscopy experiments were performed on carbon fibres, fibre reinforced epoxies and reinforcing steel in concrete. These tests were performed in artificial sea water and artificial pore water for both the reinforcing steel and the carbon fibres. The galvanic current between reinforcing steel and carbon fibres was measured by exposing the two materials to the artificial sea water and connecting a zero resistance ammeter between the two materials. Electrochemical impedance of the FRP coupled to reinforced concrete in artificial sea water was obtained. The specimens were immersed in artificial sea water up to 78 days. The FRP material analysed consisted of carbon fibre sheet and epoxy resin blend. According to the measurements, the reinforcing steel showed accelerated corrosion in the presence of CFRP rod material. However, when the carbon was in the form of sized fibres a significantly lower galvanic current was obtained. At the time this paper was written, no visible degradation of the polymeric materials had appeared. Rahman et al (1998): Experimental Investigation of the Mechanism of Deterioration of FRP Reinforcement for Concrete The durability of GFRP and CFRP under combined loading and exposure to different environments is analysed. A comprehensive research programme has been initiated including: experimental work to study the mechanism and causes of deterioration, accelerated testing, development of a provisional formula for predicting the service life, and field verification of the formula developed. Here, the experimental work is described. The composites studied were made in the test laboratory and consisted of glass fibre with vinyl ester resin and carbon fibre with vinyl ester resin. No sizing or primer was used. Specimens were exposed to: saline solution (NaCl, 58 g/l at 30, 50 and 70°C), alkaline solution (NaOH at 30, 50 and 70°C), water (70°C) and Air (24°C). During the exposure the load applied was 30% and 50% of ultimate load bearing capacity for the GFRP and CFRP specimens respectively. After the exposure, Fourier Transform Infra Red (FTIR) spectroscopy was used to identify changes in -81-

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functional groups that may indicate degradation. Surface degradation/change was examined by scanning electron microscopy (SEM). To determine changes in strength and stiffness of the material studied, tension tests were performed. The specimens were tested with the methods described after 45, 122 and 370 days. The GFRP specimens failed even before the first series of experiments (i.e. after 45 days), the CFRP specimen exposed to alkali solution showed signs of etching and spalling of the resin. Different crack patterns could be observed for different exposures. Furthermore, it could be concluded that the vinyl ester resin in a CFRP may undergo chemical deterioration when exposed to an alkaline environment. Tannous and Saadamanesh (1998): Durability and Long-Term Behaviour of Carbon and Aramid FRP Tendons Two CFRP tendons, of type Leadline and CFCC, and one AFRP tendon (270 specimens altogether) have been exposed to simulated field conditions. The CFRP composite consisted of carbon fibre of PAN type in an epoxy matrix. The AFRP is of Arapree type with Twaron filaments in an epoxy matrix. Eight different environments were simulated for accelerated exposure to aggressive elements. The environments were as follows: 1) Water (at 25°C); 2) Saturated Ca(OH)2 solution with pH of 12 (at 25°C); 3) Saturated Ca(OH)2 solution with pH of 12 (at 60°C); 4) HCl solution with pH of 3 (at 25°C); 5) NaCl 3.5% by weight solution (at 25°C); 6) NaCl+CaCl2 (2:1) 7% by weight solution (at 25°C); 7) NaCl+MgCl2 (2:1) 7% by weight solution (at 25°C); 8) Freeze-thaw cycles between -30°C and 60°C. The period of exposure was 6 and 12 months for 1) to 7) and the freeze test was run for 400, 800 and 1200 cycles (with a 4 h duration for each cycle). The specimens were oven dried prior to exposure. The effect of the exposure on: the ultimate strength, elastic modulus, Poisson’s ratio and, for the CFRPs also the relaxation and the fatigue characteristics, were measured. The following conclusions could be drawn: Exposure to chemical solutions had little effect on the durability of CFRP, the effect on AFRP was more pronounced. The effect of freeze-thaw cycles was limited. Relaxation losses for Leadline were lower than for CFCC. The CFRP tendons exhibited excellent fatigue strength. Sen et al (1998): Effect of Tidal Exposure on Bond of CFRP Rods An experimental study has been performed to determine whether pullout tests may be used to assess the integrity of the bond in FRP reinforced flexural members under environmental loads. The FRP specimens used were rods of a carbon/epoxy resin system. Pullout and beam specimens were each exposed to three different environments for 18 months. The three environmental conditions were: outdoor conditions, simulated tide condition (with salt water) and “accelerated” tide condition (with salt water at a temperature of 60°C). Every 6 months pullout and beam specimens were tested to determine and compare the bond resistance of reinforcement. The pullout specimens showed significant increase in bond resistance over the first twelve months of exposure, not observed in beams. The effect is attributed to swelling of the FRP material (hence, increase in friction between FRP rod and concrete) caused by moisture absorption. These results are claimed to suggest that pullout specimens may be unsuitable for use in accelerated exposure testing for evaluating the bond of FRP material under wet/dry cycles.

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Verghese et al (1998): Effects of Temperature Sequencing During Hygrothermal Ageing of Polymers and Polymer Matrix Composites: The Reverse Thermal Effect The effects of moisture absorption on vinyl ester (Derackane 441-440) resin and glass fibre/vinyl ester (EXTREN) composite have been investigated. Moisture uptake for the neat polymer under isothermal ageing conditions in water was found to obey Fick’s law. Information on moisture uptake at room temperature, 45, 66 and 84°C was used to compute diffusion coefficients and the Arrhenius activation energy. In this study Tg of the polymer was monitored as a function of moisture content using a DMA for 66°C ageing conditions. Also, chemical structure changes were studied for both non-aged and fully saturated specimens at 66°C using FTIR. Specific interactions in the form of hydrogen bonding with water were found to exist. The reverse thermal effect was also found to exist. The reverse thermal effect is briefly described in Section 3.2.3. Toutanji and El-Korchi (1998): Tensile Durability Performance of Cementitious Composites Externally Wrapped with FRP Sheets An experimental study has been performed on the tensile performance of cement and concrete materials wrapped with FRP sheets subjected to wet-dry and freeze-thaw cycles. Two carbon fibre sheets and one glass fibre sheet have been analysed. Over two hundred specimens were made and divided into three groups. The first group was used as virgin samples (room temperature exposure), the second was exposed to wetdry cycling (salt water and hot air at 35°C and 90% RH) and the third group was exposed to freeze/thaw cycling (300 hour cycles of temperature alternation between 4.4 and -17.8°C). All the specimens were tested regarding tensile strength. The specimens for the tensile test were cementitious cylinders wrapped with FRP sheets. The specimens were subjected to tension to failure using the ASCERA hydraulic tensile technique. The tensile force is obtained by hydraulic pressure in a pressure chamber, and the cylinders subjected to simulated field exposures were compared with the virgin samples. Test results showed that one of the CFRP wrapped specimens experienced no reduction in strength due to wet/dry or freeze/thaw exposure, whereas the other type of the CFRP wrapped specimens experienced a 10% reduction in strength. GFRP wrapped specimens subjected to wet/dry exposure exhibited about 20% reduction in strength, whereas specimens subjected to freeze/thaw exposure suffered about 5% reduction in strength.

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Part B

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

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Part B

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Part B

SUMMARY

Summary In Part B the focus is put on the durability of GFRP in concrete. The aims have been to gain a better understanding of the degradation mechanisms, and to make a quantitative service life (strength retention) prediction for GFRP reinforcement in concrete. The work has involved literature reviews as well as experiments and the development of service life prediction models. Four different GFRP bars have been exposed to concrete, alkaline solution and water, at 20, 40, 60 and 80°C to speed up the degradation. After exposure the specimens have been examined to investigate the environmental effects on mechanical and physical properties. Altogether approximately1400 specimens were included in the experimental programme. Tensile strength and pullout strength have been tested, as these two properties are fundamental for concrete reinforcement. In addition, ILSS (Interlaminar Shear Strength) tests have been conducted. Weight gain measurements were conducted for specimens exposed to water and simulated concrete pore solution. TGA (Thermo Gravimetric Analyses) were performed to determine the fibre volume fraction in the composites. Samples were also analysed using SEM/EDX (Scanning Electron Microscopy/Electron Disperse X-ray) and LA-ICPMS (Laser Ablation Inductive Coupled Plasma Mass Spectroscopy) to detect signs of degradation or alkali ingress taking place in the material. According to the experimental results the GFRP bars containing E-glass and vinyl ester appear to have better overall durability than the other systems tested. The tensile strength retention after approximately 1.5 year in moisture saturated concrete at 60°C and the ILSS retention after approximately one year under the same conditions were as follows: 41% and 90% for FIBERBAR (E-glass/vinyl ester), 57% and 96% for the Grey bar (Eglass/vinyl ester), 45% and 37% for the Yellow bar (AR-glass/vinyl ester) and 53% and 0% for the Green bar (AR-glass/polyester). 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 exposure time under accelerated conditions to time in a real application. 1.5 years at 60°C corresponds to approximately 50 years in outdoor conditions in the south of Sweden (mean annual temperature, 7°C). The other predictive model takes account of any difference in the influence of the temperature on the rate of transport mechanisms within the composite and on the chemical reactions leading to degradation. Thermosetting resins are known to be semi permeable allowing water but not alkali ions to penetrate. However, according to test results obtained within this project alkali penetration can actually be detected in GFRP composites (after exposure for 6 months in alkaline solution at 60°C), although no alkali penetration could be detected in the pure matrix material used in the composite after the same exposure. It is believed that alkali transport occurs through micro-cracks or possibly in the fibre/matrix interface. -87-

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SUMMARY

Furthermore, the rate of weight gain in the GFRP composites examined is higher than the theoretical, determined from the rate of weight gain in the polymer matrix and the fibre volume fraction. This is another indication that micro-cracks (or porous fibre/matrix interface regions) exist in the composites, in which the transport of alkali ions can take place. However, alkali ingress could not be detected in GFRP bars having an undamaged polymer surface layer. It therefore appears as if such polymer layers actually serve as an effective barrier to alkali ion penetration, and produce an improvement in durability. In this durability project the GFRP bars exposed under various environmental conditions have not been subjected to any mechanical stress during the exposure. Furthermore, the exposure conditions used in this project do not involve any cycling (in temperature or moisture level), and hence any effects caused by such cycling in exposure conditions are not covered by this investigation. Furthermore, in most real applications the concrete is not moisture saturated and hence, the deterioration probably takes place at a lower rate under real applications than under the experimental conditions. These aspects have to be taken into consideration when judging the results reported from this study. A general conclusion from this work is that the use of GFRP reinforcement can be recommended for concrete structures of arbitrary required service lives provided a proper strength reduction factor is used to take account of the deterioration of the material. Such a strength reduction factor should be separately determined for every application, and based the deterioration rate controlling factors including moisture conditions, temperature, stress level and required service life.

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Part B

Chapter 1: Introduction

1 Introduction 1.1

Background

During the last decade there has been an increase of interest in FRP (Fibre Reinforced Polymers) for concrete reinforcement. The most important benefit compared with steel is that it is not susceptible to chloride or carbonation initiated corrosion. Of the FRP types available GFRP is the cheapest. However, GFRPs are known to degrade due to other mechanisms than those for steel. The pore solution in concrete is highly alkaline with a pH of 13-14 (before leaching or carbonation has occurred) and both glass fibres and polymers are known to be more or less susceptible to alkali attack. Therefore, in order to ensure that design using GFRP reinforcement is safe, it is important to know the rate of deterioration in material properties due to environmental influence. FRP reinforcement for concrete should be able to serve its purpose for periods in the range of 50 to 100 years. A great deal of research worldwide has been addressing the durability of GFRP. However, there is a lack of quantitative predictions of strength retention for GFRP concrete reinforcement in field applications for the required service lives. Furthermore, to be able to improve the durability of GFRP reinforcement in concrete it is essential to better understand the degradation mechanisms. 1.2

Objective and scope

In this study the focus is put on the degradation of GFRP (Glass Fibre Reinforced Polymers) in water and alkaline environments. Degradation of GFRP due to other environmental degradation sources or stress conditions is discussed only very briefly. The objectives have been to make a quantitative service life (strength retention) prediction for GFRP reinforcement in concrete and to gain a better understanding of the degradation mechanisms involved. The work involves literature reviews as well as experiments and the development of service life prediction models. It is possible that the rate of degradation may be governed by the rate of transport, of various agents, within the composite. From a durability point of view it is therefore essential to understand the extent and rate of such transport. Therefore a great deal of attention, both regarding literature survey and experiments, is directed to the transport of water and alkali in GFRP.

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Part B

Chapter 1: Introduction

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Part B

Chapter 2: Degradation mechanisms of GFRP in concrete

2 Degradation mechanisms of GFRP in concrete 2.1

General considerations

Degradation of composites may take place in one or more of the constituents: fibres, matrix or the fibre/matrix interface. The source of deterioration may be physical as well as chemical, and reversible as well as irreversible. A list of possible degradation mechanisms in fibre/polymer composites is given in Table 1. Table 1 Component Resin

Possible deterioration mechanisms in polymer composites (Hull, 1981). Reversible changes

Irreversible changes

* Water swelling * Temperature flexibilising * Physical ordering of local molecular regions

* Chemical breakdown by hydrolysis * Chemical breakdown by UV-radiation * Chemical breakdown by thermal activation * Chemical break-down by stress induced effects associated with swelling and applied stress * Physical ordering of local molecular regions * Chemical composition changes by leaching * Precipitation and swelling phenomena to produce voiding and cracks * Non-uniform de-swelling to produce surface cracks and crazes * Chemical effect of thermoplastic polymer content on long term stability

Interface

Fibre

2.2

Flexibilising interface

* Chemical breakdown as for resin (see above) * Debonding due to internally generated stresses associated with shrinkage and swelling and the applied stress * Leaching of interface * Loss of strength due to corrosion * Leaching of fibre * Chemical breakdown by UV-radiation

Stress rupture and stress corrosion

If glass fibres are subjected to sustained tensile stress, stress rupture may occur. If the environment (apart from water) surrounding the glass has an accelerating effect on the failure process the phenomenon is named stress corrosion. This is a well-known problem associated with glass. In the existing design guidelines for GFRP reinforcement used as concrete reinforcement this problem is handled by using upper stress limits aimed at -91-

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Chapter 2: Degradation mechanisms of GFRP in concrete

preventing the occurrence of stress rupture during the service life of the concrete structure. See Chapter 4 in Part A.

Strength Retention

Hence, the stress condition for a glass fibre composite has a large impact on the degradation mode and service life. The typical degradation behaviour of composite materials subjected to some aggressive environments is shown in Figure 1 and Figure 2. Figure 1 sets out the strength retention for a specimen subjected to a combination of tensile stress and environmental influence. Figure 2 shows the behaviour of a specimen in a stress rupture test.

Exposure at a stress level below the ”Threshold” stress

Exposure at a stress level above the ”Threshold” stress

Time of Exposure

Figure 4

Strength retention ( schematic) for a specimen subjected to a combination of tensile stress and environmental influence (after Devalapura et al 1997).

Stress dominated failure

Stress

Crack initiation/propagation dominated failure

Diffusion dominated failure ”Threshold” level

Log Time to Failure

Figure 5

The behaviour of a specimen in a stress rupture test (after Devalapura et al 1997).

The “threshold stress level” indicated in Figure 1 and Figure 2 refers to the stress level at which micro cracks will allow a relatively fast rate of infusion of the environment and allow fibre degradation to occur. When the stress level is sufficiently low, the viscoelastic

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behaviour of the resin provides stress relief and the crack formation does not occur (Devalapura et al 1997). For GFRP used as concrete reinforcement it is essential to choose design stresses under the "threshold level". This is important both from a stress rupture and stress corrosion point of view. In the following discussion it is assumed that a load under the threshold level is chosen, so that the service life of GFRP in concrete is governed by alkali and water induced deterioration of the constituents without stress rupture taking place.

2.3

Degradation of glass fibres

2.3.1 Glass types of interest The glass types usually considered for GFRP concrete reinforcement applications are Eglass and AR-glass. However, the degradation of other glass types will also be briefly discussed in this section ("Degradation of glass fibres") where it is considered to be of general interest. E-glass (E for electrical grade) is the most widely used general-purpose form of composite reinforcement. It has good mechanical properties and is available at a relatively low price. E-glass contains boric acid (B2O3) and aluminate (Al2O3), which increase the resistance to water but also increase the susceptibility to acid and alkali degradation (Adams 1984). Hence corrosion of E-glass at a relative high rate can be expected outside the pH range 5 to 8 AR-glass (AR for alkali resistant) was developed to reinforce cement and concrete. It has been considered for FRP for concrete reinforcement because of its potentially high alkali resistance. One comersal product is named Cem-FIL glass. There are two versions : CemFIL I, which has received the most long-term durability studies and Cem-FIL II which is a surface treated version and has been found to have better durability (Jones 1984). The alkali resistance properties of AR-glass are due to the presence of zirconia (ZrO2) in the glass (Porter and Barnes 1998). However, the compatibility between Cem-FIL cement compatible sizing and polymeric matrix is known to be poor Table 2 shows the approximate composition of E-glass and AR-glass.

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Table 2

Chapter 2: Degradation mechanisms of GFRP in concrete

Example of E and AR glass composition (Veysel et al 1991 and Ghoch and Bose 1991). AR glass E glass (Cem-FIL) SiO2 54.1 62.5 Al2O3 15.2 0.5 CaO 17.3 5.5 8.0 B2O3 ZrO2 16.5 0.6 14.6 Na2O F 0.3 F2 0.1 MgO 4.7 -

2.3.2 General overview of glass corrosion Corrosion of glass can generally be classified into two processes: etching and leaching. These processes may occur separately or in combination (Adams 1984). Etching is usually caused by an alkali attack which destroys the silica network. If there is no accumulation of reaction products at the glass surface and no reaction influencing change in the composition of the attacking solution, the process proceeds at a rate proportional to time. Furthermore, in this case the temperature affects the corrosion rate according to the Arrhenius equation. (Adams 1984). Leaching usually occurs in the presence of an acid solution. This process is characterised by exchange of alkali ions or other positively charged ions in the bulk glass, and hydrogen (or more probably hydronium) ions from the solution. This ion exchange is generally considered to play an essential role in the stress corrosion mechanism of glass. The result is a "leached layer" which principally consists of only silica. The leaching of ions such as Na+, Ca2+, Al3+, Fe3+ and K+ may result in a weak shell (i.e. the leached layer) around a strong core of unleached glass (Jones, 1994). If the leached layer is not substantially transformed the reaction rate will be governed by the diffusion of exchanging ions resulting in degradation proportional to the square root of time. (Adams 1984). A combined leaching and etching (dissolution) process in acid solution is shown in Figure 3.

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Chapter 2: Degradation mechanisms of GFRP in concrete

New glass surface

Original glass surface

Leached layer

Na+ H+ Silica Bulk Glass

Figure 3

Schematic figure showing a combined leaching and dissolution process in water (Adams 1984).

In a real situation the reaction products frequently change the environmental conditions, which might result in a decrease in the rate of reaction. Important examples of such changes are: 1) when the reaction products form a "protecting" tight layer at the surface of the glass which will restrict the mobility of agents through the layer and hence lower the rate of reaction (Adams 1984 and Yilmaz and Glasser 1991). 2) when the reaction products accumulate in the solution (in case of silica saturation the reaction rate can be reduced to essentially zero), (Adams 1984).

2.3.3 Degradation mechanisms in alkaline environments In an alkaline environment glass is known to be attacked by hydroxide ions causing hydrolysis according to Equation 1 (Yilmaz and Glasser 1991, Veysel et al 1991, and Yilmaz 1992).

-Si-O-Si + OH- → -Si-OH (solid) + Si-O-

(solution)

(1)

In alkaline solutions (especially if pH is more than 10) HSiO3-, SiO32- and H2SiO3 may be leached from the glass surface as a result of this reaction (Yilmaz and Glasser 1991). Another important degradation mechanism for E-glass has been claimed to be the attack by alkali of the SiO2-CaO structure (Rostásy 1997). Yet another suggested mechanism is given in Equation 2 (Adams 1984): 2 x NaOH + (SiO2)X → x Na2SiO3 + x H2O

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(2)

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Chapter 2: Degradation mechanisms of GFRP in concrete

where Na2SiO3 is sodium silicate (water glass).

Network breakdown (% by weight)

Majumdar (referred to in Yilmaz and Glasser 1991) reports that the degradation of E glass in Portland cement extract is initially very rapid and levels off after a longer time. The diagram in Figure 4 shows the network degradation of E glass fibres measured as the quantity of SiO2, B2O3 and Al2O3 components released from the glass. 14 12 10 8 6 4 2 0 0

20

40

60

80

100

120

Time (days)

Figure 4

Network breakdown of E glass in Portland cement extract solution. Symbols: Squares=50°C, Triangles=60°C, Circles=80°C. Based on Yilmaz and Glasser 1991 (after Majumdar).

Chemical decomposition As a result of various reactions the chemical composition of the glass may change. Such changes have been observed in several experiments, mostly conducted on AR glass in alkaline solution or in cement matrixes. Makishima et al (referred to in Yilmaz and Glasser 1991) suggested a three-stage reaction of AR glass immersed in NaOH solution. The first stage is a rapid dissolution of glass and formation of a Zr-rich layer. The second stage involves further dissolution of the glass at a lower rate, a thickening of the layer occurs. In the third stage the layer cracks and becomes detached, exposing new glass surface to attack. The possibility of the development of a passivation layer in the glass fibres near the surface as a result of alkali induced chemical decomposition is discussed also by Yilmaz (1992). He found a Zr rich layer in AR glass embedded in concrete. However, it did not prevent further degradation of the glass. In cementitious environments the Na and Ca – ions are released from AR glass by the breakdown of the silicate network. Sodium silicate is formed and may react with Ca(OH)2 to form an insoluble calcium silicate hydrate gel (similar to the reaction product of hydrated cement), which precipitates at the glass cement interface (Jones 1984). The -96-

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possibility of the formation of such reaction products at the fibre surface is suggested also in Yilmaz and Glasser (1991). Ca was found to be released from E-glass fibres during the initial stages of reaction. The suggested mechanism is that calcium from the near-surface regions of the glass is exchanged by ions from the solution (e.g. alkali ions), (Larner et al, referred to in Yilmaz and Glasser 1991). In a study by Koshizaki (referred to in Yilmaz and Glasser 1991) the opposite reaction was seen, that is, after exposure in various alkali solutions Ca was found in AR glass although the glass did not initially contain any Ca. The outer layer of AR-glass, aged for one year in cement paste, underwent considerable change in chemical composition . Si and Na leached out which resulted in an increase in the Zr/Si and Ca/Si ratio up to a depth of 2 µm from the fibre surface. Examples of the results are shown in Figure 5. An increase of the Zr/Si ratio at the surface layer of AR glass fibre exposed to cement extract was reported also in Larner et al (referred to in Yilmaz and Glasser 1991). Similar results have been reported also by Chakraborty and Koshizaki (referred to in Yilmaz and Glasser 1991). Formation of various surface layers The formation of various layers on glass fibre surfaces has sometimes, but not always, been the result in the reviewed experiments . Such surface layers may act as a protective coating and the diffusion of ions in the layer can possibly be the rate dependent mechanism in the degradation process. (Yilmaz and Glasser 1991). However, the formation of surface layers has not always been found to slow down the degradation process. Scarinci et al (referred to in Yilmaz and Glasser 1991) noted that a formation of an adherent surface layer of reaction products can sometimes act as a protective layer limiting the rate of glass degradation. Larner et al (referred to in Yilmaz and Glasser 1991) observed that calcium from cement paste extract in which AR glass fibres were immersed, was removed from the solution and deposited on the fibre surface as calcium hydroxide or a more complex solid reaction product. Al-Cheikh and Murat (1988) subjected E glass fibres to a saturated calcium hydroxide solution. The possibility of the formation of a layer of hydrated calcium silicate was discussed. However the chemical reaction that took place did not essentially lead to such a layer. The formation of another solid deposit rich in calcium on the surface of E glass fibres was found instead. From a weight loss versus time curve, the layer formed did not appear to affect the fibre dissolution rate. Forkel et al (referred to in Yilmaz 1992) observed a thick "corrosion shell" around AR glass fibres in hot alkaline solution. However the corrosion products were found to be porous and did not uniformly adhere to the fibre surface.

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Atomic ratios

Part B

0,06 0,05 0,04 0,03 0,02

Na/Si

0,01 0 0

1

2

3

4

5

Distance from fibre surface (µm)

Atomic ratios

20 15

Ca/Si

10 5 0 0

1

2

3

4

5

Distance from fibre surface (µm)

Atomic ratios

12 10 8

Zr/Si

6 4 2 0 0

1

2

3

4

5

Distance from fibre surface (µm)

Figure 5

Changes in Na/Si, Ca/Si and Zr/Si atomic ratios of AR glass fibres in cement paste hydrated at 55°C for 360 days (based on Yilmaz 1992).

Notching A mechanical degradation mechanism, notching, may occur as a result of the growth of Ca(OH)2 crystals and other reaction products on the fibre surface. This mechanism has been observed for glass fibres in a cement or concrete matrix but not in solutions where crystals grow epitaxially on the fibres without notching them. Yilmaz and Glasser (1991).

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Chapter 2: Degradation mechanisms of GFRP in concrete

2.3.4 Degradation mechanisms in water The degradation of glass often involves both ion exchange and silica network destruction (Adams 1984). Degradation in the presence of water occurs mostly where the glass contains alkali ions (Comyn 1985). Water attack may become alkali attack if alkali is extracted into solution, especially in closed systems (Adams 1984). The first step in the reaction between water and an alkali silicate glass is generally considered to be an exchange between alkali ions from the glass and hydrogen ions from the water followed by base hydrolysis as shown in Equations 3-5 (Comyn 1985, Doremus et al 1983 and, Jones 1984). Si-ONa + H2O Si-O-Si + OHSi-O- + H2O

→ → →

Si-OH + NaOH Si-OH + Si-OSi-OH + OH-

(3) (4) (5)

The result is flaw formation at the fibre surface and strength reduction. This reaction is auto catalytic due to the gradual increase in the pH level (Comyn 1985). However, with regard to E glass fibres it has been claimed that little, if any, reaction with alkali will occur since they contain very small amounts of alkali (less than 1.0%) (McKinnis 1977). The surface of the glass also dissolves into the water according to Equation 6 (Perera and Doremus 1991 and Doremus et al 1983) H2O + Si-O-Si

→

SiOH + HOSi

(6)

as well as Equation 7 (Perera and Doremus 1991) SiO2 + 2H2O → H4SiO4

(7)

The reaction product is silicic acid.

2.3.5 Stress rupture Stress rupture (or static fatigue) is the process that leads to delayed failure in a material subjected to a constant load. Although stress rupture may cause fibre failure at stresses considerably lower than the ultimate tensile strength, the strength of the fibre (as measured in a short-term test) is usually completely retained until just a short time before stress rupture occurs. The failure is caused by the growth of a crack perpendicular to the direction of the fibre (Burgoyne 1999). Stress rupture of glass fibres has been shown to occur in shorter times if moisture and water are present in the surroundings of the glass -99-

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Chapter 2: Degradation mechanisms of GFRP in concrete

(Metclafe and Schmitz 1972). The mechanism leading to failure may be accelerated in the presence of an acid solution. In such cases the mechanism is called stress corrosion (or environmental stress corrosion cracking, ESCC (Jones 1998)) and deterioration takes place at a higher rate than in the presence of only water. GFRP used as concrete reinforcement can be expected to face an environment with high pH and consequently with low H+ concentration. However, basic solutions also contain hydrogen (or hydronium) ions to some extent. In addition the solution that penetrates the resin might have a less basic composition than the surrounding solution because of the semi-permeable properties of the resin. Therefore it is considered relevant here to discuss also the stress corrosion mechanism, based on reactions involving hydrogen (or hydronium) ions, even if stress corrosion is generally not considered a problem in alkaline environments (Jones 1998) The reactions which take place at a micro level and lead to stress corrosion failure are not exactly known (Tsui and Jones 1989). However it is frequently claimed that an ion exchange process plays an important role. The reaction in Equation 8 has been proposed for E glass (Metcalfe and Schmitz 1972) SiONa + H+ → SiOH + Na+

(8)

Hydrogen ions from a solution thus replace alkali ions in the glass resulting in a volume change which leads to tensile stresses at the glass surface balanced by a glass fibre core in compression. It is suggested that the flaw leading to creep rupture failure starts at an area of the glass weakened by ion exchange, possibly in an area with high sodium concentration. Conditions that make ion exchange difficult, such as low availability of sodium glass or low availability of hydronium , cause failure to be delayed. However, ion exchange can occur even in glass containing very small amounts of sodium (0.01%) and in solutions of high pH (e.g. NaOH of pH 14). However, immersion in liquid nitrogen and subjection to prolonged drying in high vacuum has been reported to inhibit ion exchange and stress corrosion. (Metcalfe and Schmitz 1972). Charles (1958) and McKinnis (1977) suggest another explanation for the stress rupture/stress corrosion mechanism (note that in the referred article the term "stress corrosion" is used although only water is involved in the reaction described). According to them reaction of water with alkali of the glass is not normally found with E-glass, and little, if any, alkali reaction is claimed to occur since it contains less than 1.0% of this constituent. Instead "disperse phases" in the glass fibres are believed to play the dominant role in the stress corrosion mechanism. The disperse phases create stress fields which at some places penetrate the fibre surface. Reaction with water at the surface of the fibre will take place at a higher rate in this stressed micro-area than in adjacent glass. The result is the initiation of a micro flaw, which finally results in fibre fracture. Stress corrosion occurs at intermediate stress levels when the rate of corrosion is similar to the rate of crack growth. This maintains the stress concentration at the crack tip which accelerates the hydrolysis of the network, causing the crack to propagate. At higher stress

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Chapter 2: Degradation mechanisms of GFRP in concrete

levels the mechanically applied load governs the lifetime and the environment does not play an important role. At low stress levels, on the other hand, ordinary corrosion around the crack tip occurs at a similar rate as at the crack tip resulting in a rounding of the tip. The consequence is a reduction in the stress concentration and decreased ability of the stress corrosion mechanism to continue. (Jones 1994, Jones 1998 and Charles 1958). AR glass has been proved to have better stress rupture resistance than E glass, probably because of its high alkali content Jones (1994) (however no further explanation is given). As previously mentioned, in general stress corrosion is not assumed to take place in alkaline environment (Jones 1998). Nevertheless, stress corrosion induced failure modes have been reported and attributed to the exchange of protons and/or sodium ions from an alkaline solution with larger cations on the glass fibre surface. Examples of stress rupture/stress corrosion values (obtained by extrapolation) for E and AR glass in air, water and basic solutions found in the literature are given below: The stress level leading to stress rupture (creep rupture) after 50 years for a GFRP bar at room temperature was reported to be 30% (Yamaguchi 1997). Stress rupture limit for an E glass/Isophtalic polyester system immersed in a cement extract was found to be 22% and 16.6% respectively after 114 years in room temperature and 60°C (Williams et al 1999). Results from another creep test conducted on a GFRP composite (Polystal) indicated that the long-term strength with respect to stress rupture in moderately moist air, R2 ⋅ 0.02 / D

(13)

2

where:

M(t) weight gain as a function of t M∞ maximum weight gain βn roots to J0(βn) = 0 D diffusivity in the composite t the time (the weight gain duration) R the radius of the cylinder

If a Fickian uptake takes place in a cylindrical specimen the diffusivity can be determined from equations 12 and 13 by applying curve fitting.

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Concentration of penetrant in a cylindrical specimen as a function of depth and time Provided that the diffusion of the penetrating liquid takes place according to Fick's law the concentration can be calculated for arbitrary values of time and depth. As for the weight gain calculation, different equations are preferable for different values of t. For t < R2 ⋅ 0.02 / D Equation 14 is most suitable and for t > R2 ⋅ 0.02 / D Equation 15 should be used. The equations apply for a cylinder with only radial diffusion. ∞

c(rp, tp ) = 1 − 2 ⋅ ∑ n =1

J 0 ( β n ⋅ rp ) −( β n ) 2 ⋅tp ⋅e β n ⋅ J1 (β n )

 1 − rp  + ⋅ erfc   rp  2 ⋅ tp  2  tp (1− rp )  1 − rp   1 − rp  + ⋅ ⋅ e 4⋅tp − (1 − rp ) ⋅ erfc    π 8 ⋅ rp ⋅ rp  2 ⋅ tp   

c(rp, tp ) =

(14)

1

(15)

where:

rp (r ) =

r R

(16)

and tp (t ) = D ⋅

Where:

c r R D t J0 J1

βn

t R2

(17)

the concentration in the material expressed as the fraction of concentration at saturation distance from the surface of the bar (variable) radius of the bar (constant) diffusivity of the bar time (duration of the penetration) Bessel function (of the zero order) Bessel function (of the first order) roots to J0(βn) = 0

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2.7

Chapter 2: Degradation mechanisms of GFRP in concrete

Conclusions

Degradation mechanisms for GFRP in moist and alkaline environment, suggested in the literature, have been compiled. Furthermore, the transport behaviour of water and alkali in fibre composites is treated in a separate section. It can be concluded from this review that several different degradation mechanisms have been suggested by different researchers. Important degradation mechanisms are rupture of the Si-O bonds by reaction with H2O molecules and OH molecules in water and alkaline solution respectively. Regarding the polymer matrix, base hydrolysis of ester bonds has been emphasised as the main degradation mechanism.

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Chapter 2: Degradation mechanisms of GFRP in concrete

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Chapter 3: Theoretical discussion on durability of GFRP reinforcement in concrete

3 Theoretical discussion of durability of GFRP reinforcement in concrete 3.1

Structural requirements of GFRP concrete reinforcement

For reinforcement to serve its purpose in a concrete element two basic requirement must be fulfilled: 1) the tensile strength must be sufficient to carry the tensile forces present in the concrete element and 2) the bond between reinforcement and concrete must be strong enough to transfer the tensile forces in the bar to the concrete. The technical service life of GFRP concrete reinforcement can therefore be defined as the time until either the tensile strength or the bond has deteriorated so that 1) or 2) is no longer satisfied. In a fibre composite different properties are governed by different components. Whether it is the bond properties or the tensile strength that is service life controlling depends on the rate at which different parts of the composite deteriorate. Hence, in a service life prediction both tensile strength and bond strength should be taken into consideration. Concrete reinforcement may be subjected to several different loads in a concrete member. Such loads can be mechanical, (for example as permanent or cyclic loads) or environmental (for example in a moist and high alkaline concrete) or a combination of these. The service life of GFRP reinforcement may therefore be controlled by deterioration of bond strength or tensile strength as a consequence of the different loads acting on the reinforcement. This relationship is illustrated in Figure 7. Areas focused on in the current project are indicates by black arrows.

3.2

Possible degradation mechanisms

3.2.1 Possible transport modes of concrete pore solution in GFRP material

The mode of degradation due to environmental influence will depend both on the rate of chemical reactions and the rate of transport of various agents within the composite. In the literature different assumptions regarding transport properties of water and alkali in GFRP have been made by different researchers, and it is not obvious which assumptions are correct and which are not. In this section possible modes of transport within the GFRP material will be discussed. Concrete pore solution mainly consists of water, sodium hydroxide, potassium hydroxide and calcium hydroxide. The exact composition differs between different types of cements as well as between different producers, depending on the composition of the clay and limestone from which the cement is made .

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Chapter 3: Theoretical discussion on durability of GFRP reinforcement in concrete

Sustained load, cyclic load

Loadenvironment synergistic effects

Bond strength

Environmental influence For example alkali and moisture

Tensile strength

Service life

Figure 7

Figure showing possible degradation sources causing deterioration in mechanical properties, which in turn control the service life. Black arrows indicate areas focused on in the current project.

The pH value in concrete is normally in the range of 13 to 14 (before leaching or carbonation has occurred) and all components in GFRP are known to be more or less susceptible to chemical attack in this highly alkaline environment. For this reason the transport properties of the concrete pore solution in the GFRP material play a central role. As previously discussed (see Subsection 2.6.3) thermosetting resin is generally considered to be a semi-permeable material that will allow water but not alkali ions to penetrate it. Nevertheless, as previously noted, some researchers have been able to measure ingress of alkali ions from simulated concrete pore solutions (Uomoto and Katsuki, 1997 and Gowripalan and Mohamed, 1999). However, the possibility that the transport took place in micro cracks cannot be excluded. Thus, the question of whether or not alkali ions can penetrate uncracked resin must be considered an issue. Another possibility is that some of the OH ions present in the solution might penetrate the resin although the alkali ions and Ca ions might not. In general, it is believed that OH ions and cations tend to go together for the charge neutrality to be satisfied. However, even if only a small fraction of the total amount of OH ions present in a basic solution may penetrate the resin, when compared to neutral water ingress, the concentration of OH -112-

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Chapter 3: Theoretical discussion on durability of GFRP reinforcement in concrete

ions may increase by several orders of magnitude. Under this hypothetical circumstance the rate of degradation should increase. Such OH ions will be termed "excess OH ions" from now on. Several researchers have reported larger deterioration in mechanical properties after exposure of GFRP to alkaline solutions compared with neutral solutions (for example Alsayed and Alhozaimy, 1998). Assuming that transport of alkali ions in thermosetting resin is not possible and provided that no micro-cracks can be found, the transport behaviour suggested in this paragraph might provide an explanation for such results. As the manufactures of GFRP reinforcements place great reliance on the ability of the resin to serve as a protection for the glass fibres against the alkaline environment, the importance of understanding the transport behaviour in GFRP for different agents in the concrete pore solution is obvious. It is obvious that the course of degradation will depend to some extent on the transport properties in the GFRP bar for different constituents in the concrete pore solution in contact with the GFRP reinforcement. A number of different transport behaviours for alkali solution in a GFRP bar is considered possible. Three cases are listed and described below and the result regarding ingress profiles of various agents is illustrated in Figure 8. In these cases it is assumed that the transport is governed by diffusion. Case a)

The matrix acts as a semi-permeable membrane allowing water but not alkali ions to penetrate the matrix. The charge neutrality requirements prevent penetration of OH ions (in addition to those present in neutral water) into the matrix.

Case b)

Alkali and accompanying hydrogen ions diffuse into the matrix but the diffusivity is considerably lower than for water. In Case b1) the water transport is assumed to be unaffected by the alkali ingress. In Case b2) the material reached by alkali is assumed to be loosened up allowing water and more alkali to quickly penetrate that layer.

Case c)

The matrix acts as a semi-permeable membrane allowing water but not alkali ions to penetrate the matrix. However a significant amount of excess OH ions penetrate the matrix in spite of the charge neutrality requirement.

In Figure 8, short and long times refer to exposure times before and after moisture saturation has occurred .

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Chapter 3: Theoretical discussion on durability of GFRP reinforcement in concrete

After ”short” time

After ”long” time

Relative conc.

Relative conc.

1.0

1.0

Case a)

r Distance from bar surface

Relative conc.

r Distance from bar surface

Relative conc. 1.0

1.0

Case b1)

r Distance from bar surface

Relative conc.

r Distance from bar surface

Relative conc.

1.0

1.0

Case b2)

r Distance from bar surface

Relative conc.

r Distance from bar surface

Relative conc.

1.0

1.0

Case c)

r Distance from bar surface

Excess OH-ions

Figure 8

Water

r Distance from bar surface

Alkali ions

Hypothetical concentration profiles. For water the "relative concentration" refers to the concentration in relation to that at equilibrium. Relative concentration of alkali and OH ions refers to the concentration in the penetrating water, where 1.0 means that the water/alkali ion or water/OH ion ratio is the same in the composite as in the surrounding solution

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Chapter 3: Theoretical discussion on durability of GFRP reinforcement in concrete

A combination of b and c, with an alkali profile as in Case b1) or Case b2) but with the water carrying excess OH ions as in Case c, is also considered possible. In the presence of large amounts of micro-cracks in the composite quick penetration of water, alkali and/or OH ions may take place. In such a case agents from the concrete pore solution may reach the whole cross section of the GFRP bar immediately (in practical terms).

3.2.2 Possible degradation modes of GFRP reinforcement in concrete

The technical service life of a GFRP reinforcing bar in concrete can be either bond or tensile failure controlled, as previously concluded. The mode of failure depends on the mode of degradation, which in turn depends on the mode of environmental ingress. The hypothesis is now introduced that essentially three types of environmental attack will take place at essentially three different zones in a GFRP bar. Furthermore it is likely that one of these environmental attacks will dominate the overall course of deterioration of the GFRP bar, and consequently the corresponding zone constitutes the weakest link. Which zone is the weakest link can vary between different GFRP products and depends on the transport properties and the resistance to degradation of the constituents of each GFRP product. The three service life controlling degradation modes are suggested as follows (without specifying if the degradation takes place in the fibres or in the resin or in both): Mode A: Mode B: Mode C:

Attack by alkaline solution on the bond surface Attack by penetration of alkali- and OH ions on the outer layer of the bar Attack by water, (possibly at slightly higher pH value than neutral because of accompanying OH ions) on the whole bar cross section

Mode A The "bond surface" which is principally the surface of the bar where the stress in the reinforcement is to be transferred to the concrete, might be degraded by alkali attack. If this zone turns out to be the "weakest link" bond failure will occur. The configuration of the bar surface differs between different products. Some GFRP reinforcement products have resin nabs or a rough resin surface to provide good bond properties. Other products are made with sand of variable grading on the surface to obtain a bar with efficient bond capacity. This part of the bar faces the most severe environmental conditions as it will be in direct contact with the alkaline concrete pore solution. It is obvious that this part of the GFRP bar must be highly alkali resistant and retain sufficient shear capacity during the whole service life of the bar. Some GFRP bars are manufactured with a spiral wrap which gives the bar a deformed profile. Such surface configuration should make the bar to have a slightly less alkali sensitive bond zone, as the bar can be slightly deteriorated at the surface without losing much bond capacity (however this design brings about deterioration in tensile properties).

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Chapter 3: Theoretical discussion on durability of GFRP reinforcement in concrete

Mode A can be assumed to govern the overall deterioration if, upon exposure, large reductions in bond capacity are experienced while the tensile strength is unaffected. Furthermore, by examining a pullout specimen it can be determined whether the failure took place in the surface resin (Mode A) layer or at the depth of the fibres (Mode B). This mode does not involve transport within the composite and is thus probable for all three cases: a, b, and c given in Figure 8. Mode B Although thermosetting resin is generally considered to act as a semi-permeable membrane allowing water but not alkali ions to penetrate, some results point in the opposite direction as earlier discussed. However, any diffusion of alkali ions is believed to take place at a considerable lower rate than that of water as indicated in Figure 8, Case b. Furthermore, as E glass fibres are known to be susceptible to alkali attack, it is believed that degradation in the layer penetrated by alkali will take place at a higher rate than in parts of the composite affected by only water. Furthermore, it is possible that the degraded layer is somewhat loosened up by the alkaline attack allowing ingress at a higher rate. This might result in a progressive "alkali front" rather than a Fickian alkali profile. Hence, a degraded layer will grow and failure will occur once the layer reaches a critical thickness. Both bond failure and tensile failure are considered possible and depend on the tensile strength of the degraded layer as well as its ability to transfer shear forces to the inner undamaged core of the bar. In a real concrete structure the environmental conditions are likely to differ along the GFRP bar. Close to a crack in concrete for example, the moisture level is probably high while relatively low at other parts. In such cases the bar is likely to be degraded over a short length causing a tensile failure rather than a bond failure to occur.

This failure mode has been suggested (for a GFRP rod) by Katsuki and Uomoto (1997). They based a service life prediction model on Case b transport behaviour of the composite. In that model the "degraded layer" (i.e. e the layer reached by the alkali front) was assumed to completely lose its tensile strength. Mode B can be assumed to govern the overall deterioration if there are signs of degradation close to the surface with a sharp border between degraded material and undamaged core material. Such degraded surface layer can possibly be observed using light microscopy or SEM. Another sign of a Mode B degradation is if alkali elements can be detected, using proper analysis equipment, close to the surface of the bar, and if bond failure occurs with the fracture surface at a depth inside of the outermost fibres. Yet another indication is if the tensile strength decreases commensurate with the affected cross sectional area. This mode of degradation is probable for transport according to Case b in Figure 8.

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Mode C) In a composite where transport of water and OH ions takes place according to Case a or Case c, the degradation is likely to take place at a lower rate than in Case B. This may result in a degradation of the whole cross section at a rate depending on the concentration of water/OH ions. That is, it is the chemical reaction leading to deterioration rather than the transport mechanism that controls the rate of degradation. This degradation mode might result in a cross section where the composite strength is lower near the surface, where the penetrant has been present for a longer time and at higher concentrations than at locations closer to the centre. As indicated in Figure 8, Case c, some OH ions may accompany the water ingress and may cause any degradation to occur at a slightly higher rate. For this degradation mode both a tensile and a bond failure is possible. A tensile failure is most likely to occur if the environmental conditions vary along the GFRP bar as discussed for Mode B.

This mode is probable for transport according to Case a and Case c in Figure 8. The suggested failure modes are illustrated in Figure 9.

Mode A: Bond surface failure controlling

Mode B: Outer composite layer failure controlling

Bond failure Figure 9

Mode C: Whole cross section failure controlling

Tensile failure

Possible failure scenarios of a GFRP reinforcing bar in concrete. Dark areas indicate deterioration in bond surface, outer composite layer and whole cross section respectively.

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3.3

Chapter 3: Theoretical discussion on durability of GFRP reinforcement in concrete

Service life prediction modelling

3.3.1 Introduction

An always-present problem associated with accelerated durability testing is how to interpret the results. The requirements of service life are typically in the range of 50 to 100 years, and as GFRP concrete reinforcement has been used for only a few years there is a lack of long-term durability data. In durability investigations of FRP an elevated temperature is generally used to speed up the degradation. However, it is not at all obvious how to transform the time under accelerated conditions to time in a real application. In the majority of articles and papers dealing with durability of GFRP in concrete, a great deal of research projects with the aim of evaluating the durability of GFRP in alkaline environments have been presented at conferences and scientific journals in recent years. However, the approaches adopted in these projects are generally qualitative and no serious attempt is made to predict service life in a real application (Part A).

3.3.2 Methods for service life prediction suggested by various researchers

Although most of the research conducted in this field is qualitative in nature, methods to make quantitative assessments of the service life of GFRP reinforcement in alkaline environments have been suggested by a few researchers. Katsuki and Uomoto (1997) have used Fick’s law to simulate the deterioration of GFRP rods quantitatively. GFRP bars were immersed in alkaline solution and the sodium (Na) penetration depth in the GFRP material could be determined after different lengths of exposure using an EPMA (Electron Probe Microscope Analyser). The diffusion coefficient of alkali in GFRP was calculated using the same equation. By assuming that the strength in the GFRP material was totally lost in parts reached by the sodium front, the deterioration in tensile strength could be calculated from the ingress depth. Using this method, Katsuki and Uomoto predicted the reduction in tensile strength for GFRP in an accelerated test by immersion in 1.0 M aqueous NaOH at 40°C. The results were reported to be in agreement with the measured values. By using the Arrhenius law the diffusion rate at “non-accelerated” temperatures could be determined and consequently also the rate of deterioration in strength under non-accelerated conditions. The same approach was used by Saadatmanesh and Tannous (1997) to predict the reduction in tensile strength of various FRP tendons. In general, Fick’s law was considered to adequately predict the loss in strength of the tendons studied. This approach presupposes that the ingress of alkali ions in the GFRP material can be measured. However, it is doubtful if modern, high quality GFRP materials actually allow ingress of alkali ions in measurable quantities (see Subsection 2.6.3). Furthermore, a bar that loses all tensile strength in a layer penetrated by alkali ions probably also loses its

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Chapter 3: Theoretical discussion on durability of GFRP reinforcement in concrete

bond strength. Hence, for the assumptions made in that method, deterioration in bond strength may well control the service life rather than the tensile strength. Another approach that has been suggested (Porter and Barnes, 1998 and Vijay and GangaRao 1999) is to utilise the time-temperature relationship established for GRC (glass fibre reinforced concrete). This relationship is used to transform time under accelerated exposure of GFRP bars (in alkali solution or embedded in concrete) to time in a real application. The benefit of using this approach is that reliable time-temperature relationships for GRC are available for relatively long exposure times (at least 10 years). However, it is not obvious that the time-temperature relationship that applies for the degradation of GRC is valid also for GFRP. For instance movements of various agents are more restricted in thermosetting plastics than in concrete and, furthermore, the time dependence of degradation mechanisms taking place in the fibre/polymer interface is not included in the “time-shift factor” that applies for GRC.

3.3.3 Prediction of strength retention using time shift factors

In this project a new model for transforming time under accelerated conditions to time in a real application, which is considered to have a stronger scientific basis than those previously discussed, has been used. The model is based on the Arrhenius equation, and the approach is similar to that used for GRC (Glass-fibre Reinforced Concrete) in Litherland et al (1981). The Arrhenius equation gives the relationship between the temperature and rate of a chemical reaction or diffusivity (Katsuki and Uomoto 1997), and is shown in Equation 18. k = Ae-Ea/RT Where:

k A Ea R T e

(18)

is the rate constant is called the frequency factor is the activation energy is the gas constant is the absolute temperature is the base of natural logarithms

The frequency factor is related to the frequency of collisions with proper orientation, and is usually assumed to be independent of temperature (Hill, 1977). The activation energy is the minimum energy required for a reaction to occur between two molecules. The time for a certain reaction to occur must be proportional to the inverse of the rate of reaction according to Equation 19.

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treaction = c / k where:

(19)

treaction is the time required for a certain reaction to take place c is a constant k is the rate constant

Hence, an increase in the reaction rate by a factor of two means that a certain reaction occurs in half the time. The degradation of GFRP in concrete is likely to involve several different chemical reactions as well as transport of water and various agents in the GFRP material. However, if all important chemical reactions and transport mechanisms involved in the degradation of GFRP are affected at approximately the same extent the Arrhenius equation should accurately describe the time temperature relationship for the overall deterioration. This is the assumption made in Litherland (1981) for the determination of the strength retention of GRC at different temperatures. GRC specimens were exposed at various temperatures and the deterioration in strength (in bending) after different exposure durations was examined. It was found that the curves describing the strength retention versus the logarithm of time, established for different temperatures, had the same shape but were transformed in time. This was an indication that essentially the same degradation mechanisms took place at the different temperatures, and that the Arrhenius equation was applicable. The degradation mechanism of GRC probably involves both chemical reactions, between the concrete pore solution and the glass fibres, and transport of reagents and reaction products through the interface region. This method for the prediction of strength retention was shown to be successful for GRC (Litherland et al 1981), and is therefore believed to be useful also for GFRP in concrete. The time temperature relationship for the deterioration of GFRP in concrete can be obtained using mechanical property (for example tensile strength) deterioration data for a material after exposure in concrete at two different temperatures. If the shape of the curve describing the change in tensile strength as a function of the logarithm of time is similar for the two temperatures then an Arrhenius type relationship can be expected (Litherland et al, 1981). The ratio between the time required for a certain decrease in tensile strength at two different temperatures should be the same as the ratio of the inverse rate of deterioration for the two temperatures as shown in Equation 20 c E − a Ea  1 1  T2 ⋅ R ⋅ −  t1 k1 k 2 A ⋅ e R  T2 T1  TSF = = = = = e E c − a t2 k1 T1 ⋅ R A ⋅ e k2

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(20)

Part B

where:

Chapter 3: Theoretical discussion on durability of GFRP reinforcement in concrete

TSF t1, t2 c k1, k2 A e Ea R T 1, T 2

is the time shift factor is the time required for a certain decrease in some mechanical property at a temperature of T1 and T2 respectively (T in °K). is a constant is the rate constant at temperature T1 and T2 is the frequency factor is the base of natural logarithms is the activation energy is the gas constant are exposure temperatures in °K.

The ratio t1/t2 is from now on called the "time shift factor", and describes the level of acceleration of an environmental exposure obtained by increasing the temperature from T1 to T2. Knowing t1 and t2 for the exposure temperatures T1 and T2, the fraction Ea/R can be determined and hence, the time shift factor between any two temperatures within the temperature range in which the Arrhenius law is believed to be applicable, can be determined. The expected strength retention at the end of a required service life can be determined by measuring the strength retention for specimens exposed at an elevated temperature, in the environment of interest, for a period of time corresponding to the required service life in a real application The strength deterioration can be assessed by subjecting specimens to accelerated exposure at an elevated temperature for a period of time corresponding to the required service life in a real application. Hence, the strength retention after the accelerated exposure is the same as can be expected at the end of the service life in a real application. Comment: This approach can be used for all degradation modes discussed in Subsection 3.2.2, but is only valid if all mechanisms (chemical reactions and transport mechanisms) involved in the degradation are equally affected by a temperature increase. That is, the same course of degradation, but at a higher rate, must be accomplished by the accelerated exposure.

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Chapter 3: Theoretical discussion on durability of GFRP reinforcement in concrete

3.3.4 Strength retention prediction separating chemical reactions and transport mechanisms Theoretical description

The method for prediction of strength retention based on accelerated exposure, presented in Subsection 3.3.3, assumes that all mechanisms involved in the degradation process are affected to the same extent by a temperature increase. The mechanisms here referred to are transport mechanisms (of reagents and reaction products) as well as chemical reactions leading to degradation. In cases where such transport mechanisms and chemical reactions are differently affected by a change in temperature the method (suggested in Subsection 3.3.3) is not suitable, as a different course of degradation takes place. Within this work an effort has been made to develop a tensile strength deterioration model for prediction of retained tensile strength in concrete after an arbitrary period of time and temperature. The model takes into account any difference in the influence which the temperature has on the rate of transport and on the chemical reactions. For this model to apply a number of assumptions and simplifications must be made as listed below: The diffusivity depends on the temperature according to the Arrhenius law The rate of chemical reactions leading to degradation depends on the temperature according to the Arrhenius law. Different reactions are affected to the same degree by a temperature increase. The rate of chemical reactions leading to degradation is affected by the concentration of the liquid which has penetrated An effective exposure time teff can be calculated from the exposure time and the concentration history from time t=0 to t=t1 The mix of the penetrating agents does not change for different ingress depths, that is, the amount of penetrating agents (at different depths) causing the degradation can be determined from the weight gain. Any size effects caused by a decrease of effective composite cross section area can be disregarded Any influence of damage caused by the degradation (for instance crack initiations) apart from the "effective area reduction effect" can be disregarded Synergistic effects can be neglected when combining different components in the model (for example, the function drel is the same for different temperatures)

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Chapter 3: Theoretical discussion on durability of GFRP reinforcement in concrete

An overview of the calculation steps involved in the model is shown in Figure 10. Table 4 gives a description of the different components used in the model and the relationship between these .

T

Tref

AFd(T, Tref)

DTref

D(AFd,DTref)

C(t,r,D)

T

drel(C)

Tref

AFc(T,Tref)

teff(drel,AFc,t)

Sr(teff)

R

Sc(Sr,R)

Sc(t,…)

Figure 10

Overview of the calculation steps used in the strength retention prediction model.

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Part B

Table 4

Chapter 3: Theoretical discussion on durability of GFRP reinforcement in concrete

Description of the different components used in the model and the relationship between these

Sc

Is the strength retention of the whole composite bar. It decreases with time due to environmental degradation

Sr

Is the strength retention after a certain period of time for a material saturated with the environmental agents

teff

Is the “effective exposure time” and is obtained from the exposure time and the concentration of penetrating agents during the whole exposure time. That is, teff is the equivalent exposure time at saturation and at the “reference temperature”, and depends on the concentration level during the exposure time (through drel) and the real exposure time.

drel

Is the relative rate at which the degradation takes place at a certain concentration. Thus, drel depends on the concentration level of penetrating liquid, and is 1.0 at saturation

AFc

Is the acceleration factor for the reactions leading to chemical degradation. AFc depends on the current exposure temperature T and a reference temperature Tref, and gives the “time shift” between T and Tref.

C

Is the concentration of penetrating liquid in the composite and depends on exposure time, t, the distance from the centre of the bar, r, and the diffusivity D.

D

Is the diffusivity of the GFRP bar for the penetrating liquid, and depends on a reference diffusivity DTref, for the reference temperature Tref and the temperature T (through AFd).

AFd

Is the acceleration factor for the diffusivity at the current temperature T with respect to the reference temperature Tref.

DTref

Is the diffusivity at the reference temperature.

Tref

Is the reference temperature for which the rate of chemical reactions leading to degradation, and the diffusivity, are known.

T

Is the temperature for which the strength retention prediction is made.

r

Is the distance from the centre of the bar.

R

Is the radius of the GFRP bar.

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Chapter 3: Theoretical discussion on durability of GFRP reinforcement in concrete

To use the model, mathematical functions must be used to describe the relationships between the different components. More specifically, four mathematical functions must be known: 1) The strength retention (Sr) as a function of time (teff) for a material saturated with the environmental liquid. Sr = Sr(teff)

(21)

2) The influence of the concentration (c) of environmental liquid in the material on the chemical reaction rate (drel). drel = drel(c)

(22)

3) The influence of the temperature (T) on the chemical reactions leading to degradation (AFc). A reasonable assumption is that the the temperature dependence follows the Arrhenius law according to Equation 23, where Bc is a constant parameter.

AFc(T ) = e

Bc (

1 1 ) − T + 273.15 Tref + 273.15

(23)

4) The influence of the temperature (T) on the transport mechanisms involved in the degradation (AFd). A reasonable assumption is that the the temperature dependence follows the Arrhenius law according to Equation 24, where Bd is a constant parameter.. AFd (T ) = e

Bd (

1 1 ) − T + 273.15 Tref + 273.15

(24)

Using the assumptions (Equations 21-24)) made above, the change in tensile strength for the whole composite by time can be analytically determined. The calculation steps are shown below. The diffusivity (D) is calculated according to Equation 25. D = DTref ⋅ AFd

(25)

The concentration (c) is a function of time (t), depth (r) and the diffusivity (D), and is calculated using Equations 26-27. ∞

c(rp, tp ) = 1 − 2 ⋅ ∑ n =1

J 0 ( β n ⋅ rp ) −( β n ) 2 ⋅tp ⋅e β n ⋅ J1 (β n )

for t < R2 ⋅ 0.02 / D

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(26)

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Chapter 3: Theoretical discussion on durability of GFRP reinforcement in concrete

and  1 − rp  + ⋅ erfc  2 ⋅ tp  rp   2  tp (1− rp )  1 − rp   1 − rp  ⋅ ⋅ e 4⋅tp − (1 − rp ) ⋅ erfc +  2 ⋅ tp   8 ⋅ rp ⋅ rp  π    1

c(rp, tp ) =

(27)

for t > R2 ⋅ 0.02 / D where: rp (r ) =

r R

(28)

and tp (t ) = D ⋅

t R2

(29)

(Equations 26-29 are the same as Equations 14-17) The effective exposure time (teff) is calculated according to Equation 30. t

t eff (c, AFc) = ∫ d rel (c(t ,...))dt ⋅ AFc

(30)

0

Finally, the tensile strength retention of the bar (Sc) is calculated using Equation 31. R

Sc(t ,...) = ∫ [Sr (t , r ) ⋅ r ]dr ⋅ 0

2π Area

(31)

Comment: This approach for service life prediction assumes that the degradation takes place according to Mode 3 (Chapter 3.2.2). However, unlike the model discussed in Subsection 3.3.3 this model is valid even if the increased temperature (at an accelerated exposure) may affect the chemical reaction and the transport mechanisms differently.

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Chapter 3: Theoretical discussion on durability of GFRP reinforcement in concrete

Calculation example

As previously mentioned the approach discussed in this current subsection takes into account any difference in the influence of temperature on the chemical reactions and the transport mechanisms involved in the degradation. The following calculation example is intended to show the consequence of not taking such difference in temperature influence into consideration. A prediction of the decrease in tensile strength for a GFRP reinforcing bar after 50 years in concrete at 10°C, is required. The diameter is 20mm and the diffusivity (D) at 60°C is known to be 7⋅10-13 m2/s. The functions drel(c) and Sr(teff) are assumed to be known and to have the form shown in Equations 32 and 33. drel(c) = drel(c) =

Sr (teff ) =

for c ≤ 0.5 and for c > 0.5

0 1

(32)

4 − log(teff + 0.001) ⋅100 7

(33)

Furthermore, the strength retention after accelerated exposure for five years in concrete at 60°C is known and is shown in Figure 11.

Retained strength (%)

Accelerated exposure (60C) 100 80 60 40 20 0 0

1

2

3

4

5

6

Time (years)

Figure 11

Strength retention after exposure in concrete for 5 years at 60°C (Note, prepared for the calculation example, not based on real data).

In addition, the time period required for the strength to drop to 90% of the original value in concrete at 10°C is known. The task is now to predict the strength for these bars after 50 years in that environment.

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Chapter 3: Theoretical discussion on durability of GFRP reinforcement in concrete

For demonstration, three different "Modes of Temperature Influence" (MTI) will be used in this calculation: 1) The chemical reactions and the transport mechanisms involved in the degradation are equally affected by a temperature change. That is, Bc=Bd in Equations 23 and 24. 2) The temperature has a greater influence on the transport mechanisms than on the chemical reactions. That is, Bc>Bd in Equations 23 and 24. 3) The temperature has a greater influence on the chemical reactions than on the transport mechanisms. That is, BcBd

Retained strength (%)

MTI 2) 100 80 60 40 Real strength

20

Predicted strength

0 0

10

20

30

40

50

60

50

60

Time (years) MTI 3)

BcBd and on the safe side if Bc