Performance of FRP strengthened beams subjected to elevated temperatures

PERFORMANCE OF FRP-STRENGTHENED BEAMS SUBJECTED TO ELEVATED TEMPERATURES ZHOU YUQIAN NATIONAL UNIVERSITY OF SINGAPORE 2010 PERFORMANCE OF FRP-STRENGTHENED BEAMS SUBJECTED TO ELEVATED TEMPERATURES ZHOU YUQIAN (B.Eng., WHUT) A THESIS SUBMITTED FOR THE DEGREE OF DOCTOR OF PHILOSOPHY DEPARTMENT OF CIVIL ENGINEERING NATIONAL UNIVERSITY OF SINGAPORE 2010 Acknowledgements ACKNOWLEDGEMENTS The author would like to express her sincere gratitude to her supervisor, Professor Tan Kiang Hwee, for the constant supervision, invaluable advice and patience throughout the research study. The help given by the staff of the Structural Engineering and Concrete Technology Laboratories in the experimental research is greatly appreciated. The author would like to thank Mr. Y. K. Koh, Mr. P. K. Choo, Mr. K. K. Yip and Mr. Ishak for their help in specimen preparation; Ms. Annie Tan, Mr. B. O. Ang and Mr. W. M. Ow for their assistance in test setup and instrumentation; and Mr. H. B. Lim for his kind support. The research works were supported by material suppliers. The author would like to thank Mapei, Shea Technology, Hilti, Unitherm, S&P Clever Reinforcement Company and Polymer Technologies Pte Ltd. The author would like to express her sincere appreciation to her family and friends for the continuous support during the study. The author wishes to express her gratitude for the Scholarship from National University of Singapore. i Table of Contents TABLE OF CONTENTS Acknowledgements i Table of Contents ii Summary v List of Notations vii List of Tables x List of Figures xi Chapter Introduction 1.1 Background 1.2 Objective and Scope of Study 1.3 Thesis Structure Chapter Literature Review 2.1 General 2.2 Fire Performance of FRP Systems 2.2.1 Polymeric Resin 2.2.2 Reinforcing Fibers 10 2.2.3 FRP Laminates 12 2.3 Fire Resistance of FRP Strengthened RC Members 15 2.3.1 Columns 16 2.3.2 Beams 18 ii Table of Contents 2.3.3 2.4 Slabs 24 Design of FRP Systems against Fire 26 2.4.1 FRP Fire Design Philosophy 26 2.4.2 Fire Design Approaches 27 Properties of Materials Subjected to Elevated Temperatures 48 Chapter 3.1 General 48 3.2 Concrete 48 3.3 Steel Reinforcement 50 3.4 Basalt FRP Laminates 51 3.4.1 Tensile Properties 54 3.4.2 Bond Strength 55 3.4.3 Summary 56 Carbon FRP Laminates 56 3.5.1 Tensile Properties 56 Behavior of FRP Strengthened Beams after Subjecting to Elevated Temperatures 68 3.5 Chapter 4.1 General 68 4.2 Investigation Using Small FRP-Strengthened Prisms 68 4.2.1 Test Program 68 4.2.2 Test Results and Discussion 73 4.2.3 Effect of Elevated Temperature on Ultimate Strength 79 4.3 Investigation on Prototype Beams 80 iii Table of Contents 4.4 4.3.1 Test Program 81 4.3.2 Fire Chamber 84 4.3.3 Test Results and Discussion 85 4.3.4 Comparison with Test Results on Prism Specimens 89 Summary Chapter 5.1 Analytical Considerations 90 119 Proposed Model 119 5.1.1 Assumptions 119 5.1.2 Flexural Capacity 120 5.1.3 Comparison with test results 122 Conclusions 133 6.1 Review of the Work 133 6.2 Summary of Findings 134 6.3 Recommendation for Future Works 136 Chapter List of publications 137 References 138 Appendix 142 iv Summary SUMMARY Fiber reinforced polymer (FRP) systems have been widely used for strengthening and rehabilitation of reinforced concrete structures. They can provide significant improvement in static load carrying capacity of concrete members. However, one main obstacle which hinders FRP from becoming more widely used is the very limited information on the behavior of FRP-strengthened members under elevated temperatures. This research presents test results regarding the structural behavior of FRPstrengthened RC beams after subjecting them to elevated temperatures. The investigation on different fire protection systems as well as the effect of sustained loadings serves as useful reference for future work. An analytical method is also proposed to predict the failure load and failure mode for FRP-strengthened RC beams. The experimental investigation composed of two main test programs. The first program was carried out using small prism specimens strengthened with glass FRP systems with various fire protection systems and basalt FRP systems without any protection. The specimens were subjected to elevated temperatures in a small electrical furnace. Subsequently a second program was carried out on prototype beams strengthened with carbon or basalt FRP systems using a larger chamber. The effects of elevated temperatures and sustained loading were investigated. Two other protection systems were examined in the test program. Subjecting the beam specimens to elevated temperatures of up to about 600oC led to a decrease in ultimate strength. For carbon FRP strengthened beams, the ultimate strength decreased but the initial beam stiffness is not affected after subjecting to temperatures ranging from about 300oC to 600oC. The failure mode changed from flexural debonding to flexural rupture after subjecting to elevated temperatures due to the v Summary deterioration of the materials. Sustained loading applied on prototype beam specimens during heating did not however affect in the beam stiffness and strength. Among all the protective systems, mortar overlay had limited effectiveness on prototype beams. Other coating systems were effective in protecting the FRP systems but further improvements are needed if the specimens are subjected to elevated temperatures higher than 600oC. The analytical model is based on strain compatibility and force equilibrium, and predicts the ultimate strength and failure mode of FRP-strengthened reinforced concrete beams using the deteriorated material properties. The analytical predictions compared with test results well. However further improvement is needed before the model can be used in a fire design of FRP strengthened beams. vi List of Notations LIST OF NOTATIONS Af cross section area of FRP laminate As area of internal longitudinal tensile reinforcement ' As area of internal longitudinal compression reinforcement b width of beam c depth of neutral axis df distance from extreme compression fiber to FRP laminates ds distance from extreme compression fiber to the centroid of tension steel ' ds distance from extreme compression fiber to the centroid of compression steel Ef elastic modulus of FRP laminate Es elastic modulus of tensile steel reinforcement Es ' elastic modulus of compression steel reinforcement f c ( x ) compression stress in concrete fiber at distance x away from neutral axis fc ' cylinder compressive strength of concrete f cu cube compressive strength of concrete f fs strength of FRP laminate fs stress in internal longitudinal tensile steel reinforcement fs ' stress in internal longitudinal compression steel reinforcement h overall beam depth L bond length vii List of Notations Le effective bond length Mu ultimate moment of resistance M u ,cc ultimate moment of resistance corresponding to concrete crushing M fr ultimate moment of resistance corresponding to FRP rupture M db ultimate moment of resistance corresponding to debonding of FRP laminate tf thickness of FRP laminates wf width of FRP laminates x distance from the top concrete fiber to the centroid of compression stress block α calibration factor βp bond width coefficient βL bond length coefficient εc concrete strain in extreme concrete compressive fiber ε co concrete strain corresponding to f c ε c (x ) concrete strain at distance x from neutral axis ε cu ultimate compressive strain of concrete εf strain in FRP laminates ε fu ultimate tensile strain of FRP laminates ε fdb FRP debonding strain εs strain in internal tensile steel reinforcement εs ' ' strain in internal compression steel reinforcement viii Chapter 2. There was slight difference between the reduced material properties obtained in the current test data and those in existing literature. For FRP systems, material test is recommended since the types of FRP systems and properties can be very different. 6.3 RECOMMENDATION FOR FUTURE WORKS The effect of elevated temperatures on the residual structural behavior of reinforced concrete beams strengthened with glass, basalt or carbon FRP systems were investigated in this research using relatively small beams. Limited tests were conducted on fire protective systems. Further studies in following areas are recommended: 1. Performance of externally bonded FRP-strengthened beams partially exposed to gas fire scenario (ASTM E119 2000) may be investigated to better simulate the actual condition. 2. Further tests focusing on the improvement of protective coatings are to be carried out. So that recommendation on type and thickness of fire protective can be suggested in the future fire design. 3. More detailed temperature profiles at different locations within the beam specimens need to be further studied, so that temperature change of concrete and steel reinforcement during heating can be monitored and used to improve estimate of material properties. When the theory can accurately predict FRP strengthened RC members after subjecting to elevated temperature, it may can be adopted in fire design. 136 List of Publications LIST OF PUBLICATIONS Based on works presented in this thesis, the following technical papers were published / submitted for review / under preparation: 1. Zhou, Y.Q. and Tan, K.H. (2007), “Behavior of FRP strengthened beams subjected to elevated temperatures”, Proceedings of the 8th International Symposium on Fiber Reinforced Polymer Reinforcement for Concrete Structures (FRPRCS-8 2008), July 16-18, University of Patras, Patras, Greece, full text in CD. 2. Tan, K.H. and Zhou, Y.Q. (2007), “Fiber-reinforced cement composites under elevated temperatures”, Proceedings of the 5th High Performance Fiber Reinforced Cement Composites (HPFRCC5 2007), July 10-13, Mainz, Germany. 3. Tan, K.H. and Zhou, Y.Q. (2008), “Tensile and bond properties of BFRP laminates subjected to elevated temperatures”, Proceedings of the 3rd ACF International Conference-ACF/VCA, November 11-13, HoChiMinh City, Vietnam. 4. Tan, K.H. and Zhou, Y.Q. (2009), “Basalt FRP Strengthened Beams Subjected to Elevated Temperatures”, Proceedings of the 2nd Performance, Protection & Strengthening of Structures under Extreme Loading, August 19-21, Hayama, Japan. 5. Tan, K.H. and Zhou, Y.Q., “Performance of FRP-strengthened beams subjected to elevated temperatures”, ASCE Journal of Composite in Construction. (Under Review) 6. Tan, K.H. and Zhou, Y.Q., “Residual Strength and Failure Mode of FRPstrengthened Beams Subjected to Elevated Temperatures”, ACI Structural Journal. (Under Preparation) 137 Reference REFERENCES American Concrete Institute (ACI) (2001). “Guide for determining the fire endurance of concrete elements.” ACI 216-01, Farmington hills, Michigan, United States, pp. 15-21. American Concrete Institute (ACI) (2008). “Guide for the design and construction of externally bonded FRP systems for strengthening concrete structures.” ACI 440.2R-08. Farmington Hills, Mich. American Concrete Institute (ACI) (2007). “Report on Fiber-Reinforced Polymer (FRP) Reinforcement for Concrete Structures.” ACI 440R-07. Farmington Hills, Mich. ASTM (2000). “Standard test methods for fire tests of building construction and materials.” ASTM E 119-00a, West Conshohocken, Pa. ASTM (2002). “Standard test method for flexural strength of concrete (using simple beam with center-point loading.” ASTM C 293-02, West Conshohocken, Pa. Barnes, R. and Fidell, J. (2006). “Performance in fire of small-scale CFRP strengthened concrete beams.” J. Compos. Constr., 10 (6) 503-508. Bisby, L. A., Kodur, V. K. R. and Green, M. F. (2004). “Performance in Fire of FRP-confined Reinforced Concrete Columns.” 4th International Conference on Advanced Composite Materials in Bridges and Structures, Calgary, Alberta, 1-8. (a) Bisby, L. A., Williams, V.R.K, Kodur, V. K. R., Green, M. F. and Chowdhury, E. (2005). “Fire Performance of FRP Systems for Infrastructure: A State-of-the-Art Report” Queen’s Universtiy and National Research Council, Kingston and Ottawa. (b) Bisby, L. A., Kodur, V. K. R. and Green, M. F. (2005). “Fire endurance of fiber-reinforced polymer-confined concrete columns.” ACI Struct. J., 102(6), 883-891. 138 Reference Blontrock, H., Taerwe, L. and Vandevelde, P. (2001). “Fire testing of concrete slabs strengthened with fibre composite laminates.” Proc., 5th Int. Conf. on Fibre Reinforced Plastics for Reinforced Concrete Structures, Telford, Cambridge, U.K., 547556. Blontrock, H., Taerwe, L. and Matthys S. (1999). “Properties of fiber reinforced plastics at elevated temperatures with regards to fire resistance of reinforced concrete memebers” Fourth International Symposium on Non-Metallic (FRP) Reinforcement for Concrete Structures, Baltimore, American Concrete Institute (ACI), 43-54. Bonacci, J.F. and Maalej. M. (2001), “Behavioral trends of RC beams strengthened with externally bonded FRP.” Journal of composites for construction, Vol. 5, No.2, pp. 102-113. British Standards Institution (BSI) (1987). “Fire tests on building materials and structures. Part 20: Method for determination of the fire resistance of elements of construction (general principles)” BS476-20, BSI, London. British Standards Institution (1997). “Structural use of concrete” BS 8110, BSI, London. Chowdhury. E. U., Bisby, L. A., Green, M. F. and Kodur, V. K. R. (2008). “Residual behavior of fire exposed reinforced concrete beams pre-strengthened in flexure with fiber reinforced polymer sheets.” J. Compos. Constr., 12(1), 61-68. Feih, S., Mouritz, A. P., Mathys, Z. and Gibson, A. G. (2007). “Tensile strength modeling of glass fiber polymer composites in fire” J. Compos. Materials, 41(19), 23872410. 139 Reference Han, L.H., Zheng, Y.Q. and Teng, J.P. (2006). “Fire resistance of RC and FRPconfined RC columns” Magazine of concrete research, 58(8), 533-544. Klamer, E.L., Hordijk, D.A. and Hermes, M.C.J. (2008). “The influence of temperature on RC beams strengthened with externally bonded CFRP reinforcement” Heron, 53(3), 157-185. Kodur, V. K. R., Bisby, L. A., Green, M. F. and Chowdhury, E. (2005). “Fire Endurance of Insulated FRPStrengthened Square Concrete Columns.” FRPRCS-7 (the 7th International Symposium on Fiber Reinforced Polymer Reinforcement for Reinforced Concrete Structures), 2005, 1253-1268. Kodur, V. K. R., Williams, B. K., Green, M. F. and Bisby, L. A. (2005). “Fire Endurance Experiments on FRP-Strengthened Reinforced Concrete Slabs and BeamSlabs Assemblies.” Research Report No. 175, Fire Research Program, Institute for Research in Construction and National Research Council Canada, Canada. Luise, R.R., (1997), “Applications of high temperature polymers”, CRC Press, Inc.(1997). Mallick, P.K., (1988), “Fiber-reinforced composites: materials, manufacturing, and design”, Marcel Dekker, Inc.(1988). Mouritz, A. P. and Gibson, A. G. (2006). Fire Properties of Polymer Composite Materials. Springer, Dordrecht, 398 pp. Nanni, A. (1993). “Fiber-Reinforced-Plastic (FRP) Reinforcement for Concrete Structures: Properties and Applications” Elsevier Science Publishers. Saafi, M. (2002). “Effect of fire on FRP reinforced concrete members.” Compos. Struct., 58(1), 11-20. 140 Reference Schneider, U. (1976). “Behaviour of Concrete under Thermal Steady State and Non-Steady State Conditions.” Fire and Materials, 1, 103-115. Teng, J. G., Chen, J. F., Smith, S. T. and Lam, L. (2002). FRP-strengthened RC Structures, John Wiley & Sons Ltd., England. Teng, J. G., Smith, S. T., Yao, J. and Chen, J. F. (2003). “Intermediate crackinduced debonding in RC beams and slabs.” Construction and Building Materials, 17, pp. 102-113. Wikipedia (2009), “Fibre-reinforced plastic” Wikipedia from internet. Williams, B., Kodur, V., Green, M. F. and Bisby, L. (2008). “Fire Endurance of Fiber-Reinforced Polymer Strengthened Concrete T-Beams.” ACI structural journal, 105(1), 60-67. 141 Appendix APPENDIX A APPENDIX A-1 Stress-strain characteristics of BFRP tensile specimens are: Series I 100 80 Stress (MPa) TB0A1 60 TB0A3 TB0A5 40 TB0A2 TB0A4 20 -2000 3000 8000 -6 Strain (*10 mm/mm) 13000 Figure A-1 (a) TB0A Stress (MPa) 80 60 TB035 TB031 40 TB032 TB033 20 TB034 0 2000 4000 -6 Strain (*10 mm/mm) 6000 8000 Figure A-1 (b) TB03 200 150 Stress (MPa) TB041 100 TB042 50 -1000 -50 1000 3000 5000 7000 9000 11000 -100 TB044 -150 -6 Strain (*10 mm/mm) Figure A-1 (c) TB04 142 Appendix 60 Stress (MPa) 50 TB052 40 TB054 TB053 30 20 TB051 10 0 2000 4000 6000 -6 Strain (*10 mm/mm) 8000 10000 Figure A-1 (d) TB05 (1) 50 TB054 Stress (MPa) 40 TB051 30 TB053 20 10 TB052 0 2000 4000 -6 Strain (*10 mm/mm) 6000 8000 Figure A-1 (e) TB05 (2) 143 Appendix Series II 120 TB0A1 100 TB0A5 TB0A4 Stress (MPa) 80 TB0A2 60 40 TB0A3 20 0 4000 8000 12000 16000 20000 -20 -6 Strain (10 mm/mm) Figure A-1 (f) TB0A 100 80 Stress (MPa) TB021 60 TB025 TB023 TB024 40 20 TB022 0 2000 4000 6000 8000 10000 12000 14000 16000 -6 Strain (10 mm/mm) Figure A-1 (g) TB02 100 TB034 Stress (MPa) 80 TB033 60 TB032 TB035 40 TB031 20 0 2000 4000 6000 8000 10000 12000 14000 16000 -6 Strain (10 mm/mm) Figure A-1 (h) TB03 144 Appendix 80 TB044 Stress (MPa) 60 TB043 40 TB042 TB041 20 TB045 0 2000 4000 Strain (10-6 mm/mm) 6000 8000 Figure A-1 (i) TB04 60 TB053 50 TB055 Stress (MPa) TB051 40 TB054 30 20 TB052 10 0 1000 2000 3000 4000 -6 Strain (10 mm/mm) 5000 6000 Figure A-1 (j) TB05 40 Stress (MPa) 30 TB0555 20 TB0552 -1000 TB0554 TB0553 10 1000 2000 3000 4000 5000 6000 -10 -6 Strain (10 mm/mm) Figure A-1 (k) TB055 145 Appendix Series III 120 TB3A4 Stress (MPa) 100 TB3A3 TB3A5 TB3A1 80 TB3A2 60 40 20 0 2000 4000 6000 8000 10000 12000 14000 16000 -6 Strain (10 mm/mm) Figure A-1 (l) TB3A 120 Stress (MPa) 100 TB4A3 TB4A5 80 TB4A1 TB4A2 60 TB4A4 40 20 0 2000 4000 6000 8000 10000 12000 14000 16000 -6 Strain (10 mm/mm) Figure A-1 (m) TB4A 146 Appendix Stress-strain characteristics of CFRP tensile specimens: 500 TCA1 Stress (MPa) 400 TCA3 TCA5 300 TCA2 TCA4 200 100 0 4000 8000 -6 12000 16000 Strain (10 mm/mm) Figure A-1 (n) TCA Stress (MPa) 500 TC24 400 TC23 TC22 300 TC25 TC21 200 100 0 4000 8000 12000 -6 Strain (10 mm/mm) 16000 Figure A-1 (o) TC2 500 Stress (MPa) 400 TC31 TC32 300 TC35 TC33 200 TC34 100 0 4000 8000 12000 Strain (10-6 mm/mm) 16000 Figure A-1 (p) TC3 147 Appendix Stress (MPa) 400 TC43 300 TC42 TC45 TC41 200 TC44 100 0 4000 8000 12000 Strain (10-6 mm/mm) 16000 Figure A-1 (q) TC4 Stress (MPa) 400 TC54 300 TC51 TC55 TC53 TC52 200 100 0 4000 8000 12000 -6 Strain (10 mm/mm) 16000 Figure A-1 (r) TC5 250 Stress (MPa) 200 TC61 150 TC62 100 TC6 50 0 4000 8000 12000 Strain (10-6 mm/mm) 16000 Figure A-1 (s) TC6 148 Appendix APPENDIX A-2 Load-strain characteristics of bond specimens: Figure A-2 (a) Strain gauge location Load (k N) 0 100 200 -6 300 400 500 400 500 Strain (*10 mm/mm) Figure A-2 (b) BB1A Load (kN) 0 100 200 300 Strain (*10-6 mm/mm) Figure A-2 (c) BB2A 149 Appendix Load (kN) -50 50 150 250 -6 Strain (*10 mm/mm) 350 450 Figure A-2 (d) BB12 10 Load (k N) 0 200 400 600 -6 Strain (*10 mm/mm) 800 1000 Figure A-2 (e) BB22 2.5 Load (k N) 1.5 0.5 -300 300 600 Strain (*10-6 mm/mm) 900 1200 Figure A-2 (f) BB14 150 Appendix Load (k N) 4 0 300 600 900 -6 Strain (*10 mm/mm) 1200 Figure A-2 (g) BB24 151 [...]... Electrical chamber Fig 4.13 Temperature profiles Fig 4.14 Test set-up Fig 4.15 Effect of elevated temperature on BFRP strengthened beams Fig 4.16 Effect of sustained load Fig 4.17 Effect of elevated temperature on CFRP strengthened beams Fig 4.18 Effect of fire protection system Fig 4.19 Comparison of prototype beams with prism beams Fig 5.1 Section analysis of FRP- strengthened section Fig 5.2 Reduced material... research on all FRP types mentioned, which is believed to be of relevance to FRP laminates when subjected to elevated temperatures Figure 2.2 shows the influence of elevated temperatures on mechanical properties of glass FRP systems based on the survey done by Blontrock et al (1999) Figure 2.2 (a) presents the deterioration of tensile strength of glass FRP systems and a gradual decrease of up to 60% can... when GFRP systems were subjected to about 400oC Modulus of elasticity of GFRP systems on the other hand, remains rather stable when subjected to 300oC as shown in Figure 2.2 (c) Saafi (2002) further simplified the results of Blontrok et al (1999) and proposed a series of conservative equations to describe the deterioration of FRP systems when subjected to high temperatures For tensile strength of GFRP... the prediction of the failure mode and ultimate load-carrying capacity of FRP- strengthened beams after subjecting them to elevated temperatures 1.3 THESIS STRUCTURE There are six chapters in the thesis, including this chapter in which the need to study the behavior of externally bonded FRP strengthened RC beams after exposure to elevated temperatures is explained The objective and scope of the research... properties when subjected to elevated temperatures Several research and reviews (Blontrock et al 1999, Saafi 2002, Bisby et al 2005a, Feih 2007, Mouritz et al 2006) have summarized and discussed the performance of FRP systems subjected to elevated temperatures However, in these research and reviews, the types of FRP systems are not differentiated very clearly FRP rod, FRP grid, FRP bars and FRP laminates... the temperature of an FRP wrap below 100oC for up to 3 to 4 hours during exposure to the standard fire like ASTM E119 It was also demonstrated that appropriately designed FRP- wrapped reinforced concrete columns are capable of achieving the required fire endurance rating 2.3.2 Beams More research were available on performance of FRP reinforced or confined beams subjected to elevated temperatures (Barnes... results x List of Figures LIST OF FIGURES Fig 2.1 Reduction of tensile stress in E glass fibers as a function of time at various temperatures Fig 2.2 Temperature dependent mechanical properties of GFRP and AFRP systems Fig 2.3 Temperature dependent mechanical properties of CFRP composites Fig 2.4 Temperature reduction factor for FRP properties Fig 2.5 Variation of strength of various FRP systems with... strength of aramid FRP systems deteriorated to only about 10% of ambient value when heated to 400oC while the modulus of elasticity remains at 50% of ambient value when heated to 300oC Saafi (2002) suggested that the tensile strength of AFRP systems had no decrease till 100oC then followed by a linear decrease to zero when subjected to 400oC The modulus of elasticity followed the trend as glass FRP systems... negligible strength degradation to temperatures as high as 2000oC Thus carbon fibers are the most thermo stable fibers among commonly used fibers when subjected to elevated temperatures 11 Chapter 2 2.2.3 FRP Laminates It is crucial to know how mechanical properties of FRP laminates are affected by elevated temperatures Although there are a variety of different types of FRP systems, it is known that... carbon FRP systems to strengthen the beams But the sizes, types and heating temperatures are varied The sizes of the beam specimens were from small scale to full scale The types of the beam specimens include pre -strengthened and post -strengthened, square RC beams and T -beams The heating temperatures also range from temperatures which were close 18 Chapter 2 to the glass transition temperature to the . corresponding to FRP rupture db M ultimate moment of resistance corresponding to debonding of FRP laminate f t thickness of FRP laminates f w width of FRP laminates x distance from the top concrete. PERFORMANCE OF FRP- STRENGTHENED BEAMS SUBJECTED TO ELEVATED TEMPERATURES ZHOU YUQIAN (B.Eng., WHUT) A THESIS SUBMITTED FOR THE DEGREE OF. PERFORMANCE OF FRP- STRENGTHENED BEAMS SUBJECTED TO ELEVATED TEMPERATURES ZHOU YUQIAN NATIONAL UNIVERSITY OF SINGAPORE