Durability of fiber reinforced polymer composites under tropical climate

DURABILITY OF FIBER REINFORCED POLYMER COMPOSITES UNDER TROPICAL CLIMATE LIEW YONG SEONG (B Eng (Hons.), UTM) A THESIS SUBMITTED FOR THE DEGREE OF MASTER OF ENGINEERING DEPARTMENT OF CIVIL ENGINEERING MATIONAL UNIVERSITY OF SINGAPORE 2003 Acknowledgement The author is deeply indebted to his supervisor Associate Professor Dr Tan Kiang Hwee from the Department of Civil Engineering, National University of Singapore for his help, stimulating suggestions, encouragement and systematic supervision throughout the course of this research and writing of the thesis The research works were supported by all the staff of the Structural Engineering and Concrete Technology Laboratories The author would like to thank Mr Y K Khoo and Mr P K Choo for their invaluable help in setting up the weathering chamber; Ms Annie Tan, Mr B O Ang and Mr W M Ow for their assistance in test setup and instrumentation; Mr K K Yip, Mr Kamsan and Mr Ishak for their help in specimen preparation; and Mr B C Sit and Mr H B Lim for their kind support and constructive suggestions The author would like to thank his friends: K S Leong, H D Zhao, F L Yap, P L Wee, K H Kong, Kelvin and Kevin It is a great blessing for the author to have their timely assistance and companionship Lastly, the author would like to dedicate this thesis to his parents and Ai Ling, whose patient love enabled him to complete this work i Abstract A lack of in-depth knowledge of the long-term durability of fiber reinforced polymer (FRP) composite in real service condition restricts its extensive use in structural rehabilitation works The organic nature of the matrix of the FRP composites, and the reinforcing fibers, make them susceptible to attacks of various tropical weathering factors, namely, ultraviolet (UV) ray from sunlight, moisture and heat, when used externally Therefore, in the first part of this study, the tropical climate was characterized and reproduced in an in-house designed weathering chamber to induce accelerated weathering effects on FRP composites and FRP-strengthened structural elements In the second part, the observed glass fibers reinforced polymer (GFRP) mechanical properties variations in the accelerated weathering tests were incorporated in a proposed analytical model to predict the time-dependent behavior of FRPstrengthened beams under the weathering effects of tropical climate Comparison with weathering test results showed that the effects of tropical climate weather were reproduced well in the proposed accelerated weathering test scheme The tensile strength of the GFRP dropped over time when subjected to outdoor tropical climate, and the reduction of tensile strength of GFRP laminates is matrix dependent In addition to the tensile coupons, 48 beam specimens were fabricated, exposed to exposure conditions and tested to validate the applicability of the proposed model The failure modes and ultimate loads of small-scale GFRPstrengthened beams changed with weathering time, and can be well predicted using the proposed model incorporating the material properties after weathering ii Table of Contents Acknowledgement i Abstract ii Table of Contents iii Nomenclature vi List of Tables ix List of Figures x Chapter One: Introduction 1.1 General 1.2 Background 1.2.1 Fiber Reinforced Polymer Resins 1.2.2 1.2.2.1 1.2.2.2 1.2.2.3 1.2.2.4 1.2.3 1.2.4 1.2.5 Types Glass Transition Temperature Curing of Resin Curing Degree and Durability of Resin 4 Reinforcing Fibers 1.2.3.1 1.2.3.2 Types Influence of Resin on Mechanical Properties of Composites Weathering of Polymer 1.2.4.1 1.2.4.2 1.2.4.3 8 Weathering Factors and Degradation Mechanisms Ultraviolet Ray and Photo-Oxidation Process Weathering Tests Durability of FRP 12 1.2.5.1 1.2.5.2 1.2.5.3 1.2.5.4 12 13 15 16 Past Durability Studies on FRP Environmental Effects on Tensile Characteristics Environmental Effects on FRP-Concrete Bond Strength Environmental Effects on FRP-strengthened Structural Elements 1.3 Objectives of Study 18 1.4 Report Organization 18 1.5 Tables and Figures 20 iii Chapter Two: Simulation of Tropical Climate 2.1 2.2 2.3 Characterization of Tropical Climate 36 2.1.1 2.1.2 2.1.3 2.1.4 2.1.5 36 37 37 38 38 Solar Irradiance Ambient Temperature Relative Humidity Rainfall Sunshine Hours Weathering Chamber 39 2.2.1 2.2.2 2.2.3 2.2.4 2.2.5 39 40 40 40 41 UV Light Heaters Water Atomizers Weathering Cycles Acceleration by Intensified UV-A Ray Verification Tests 42 2.3.1 Patterns of Weathering Factors 42 2.3.1.1 2.3.1.2 2.3.1.3 42 43 43 2.3.2 Temperature and Humidity Measurement Outdoor Solar Irradiance Measurement Results and Discussion Weathering Effects on FRP Tensile Coupons 45 2.3.2.1 2.3.2.2 2.3.2.3 2.3.2.4 2.3.2.5 45 46 47 47 48 Materials Fabrication of Tensile Coupons Weathering of Specimens Test Setup and Instrumentation Test Results and Discussion 2.4 Summary 51 2.5 Tables and Figures 52 Chapter Three: Time-Dependent Behavior of FRP-strengthened Beams 3.1 General 75 3.2 Proposed Model 76 3.2.1 Assumptions 76 Failure Mode and Flexural Capacity 77 3.2.2.1 3.2.2.2 77 80 3.2.2 Flexural Analysis Time-dependent Behavior iv 3.3 Test Program 82 3.3.1 Specimen Designation 3.3.2 Specimen Details 3.3.3 Material Properties 3.3.4 Fabrication 3.3.5 Exposure Conditions 3.3.6 Test Setup and Instrumentation 82 83 84 85 86 86 Results and Discussion 87 3.4.1 Visual Inspection on GFRP Laminates 3.4.2 Ultimate Load and Failure Mode 3.4.3 Ductility 3.4.4 Crack Width 3.4.5 Strains in Concrete, Steel Reinforcements and GFRPs 3.4.6 Comparison of Test and Predicted Results 87 88 91 92 93 94 3.5 Summary 96 3.6 Tables and Figures 97 3.4 Chapter Four: Conclusions 4.1 Review of Work 132 4.2 Conclusions 132 4.3 Recommendations for Future Research 134 References 135 Appendix A: ASTM-G151-97: Table A-1 v Nomenclature As area of internal longitudinal tensile reinforcement As’ area of internal longitudinal compression reinforcement Ap cross section area of FRP laminate b width of beam c depth of neutral axis ds’ distance from extreme compression fiber to the centroid of compression steel ds distance from extreme compression fiber to the centroid of tension steel dp distance from extreme compression fiber to FRP laminates Ec elastic modulus of concrete Es’ elastic modulus of compression steel reinforcement Es elastic modulus of tensile steel reinforcement Ep elastic modulus of FRP laminate E p, X (t ) elastic modulus of Type X FRP laminate at age t * E p, X (t ) elastic modulus of Type X FRP laminate at accelerated age t fc(x) compression stress in concrete fiber at distance x away from neutral axis fc' cylinder compressive strength of concrete fcu cube compressive strength of concrete fs stress in internal longitudinal tensile steel reinforcement fs' stress in internal longitudinal compression steel reinforcement fpu rupture strength of FRP laminate fsy yield strength of internal longitudinal tensile steel reinforcement fsy' yield strength of internal longitudinal compression steel reinforcement vi h overall beam depth ka weathering acceleration factor L FRP bond length Le effective FRP bond length Mcc ultimate moment resistance of strengthened flexural members failing by concrete crushing Mfr ultimate moment resistance of strengthened flexural members failing by rupture of FRP Mdb ultimate moment resistance of strengthened flexural members failing by debonding of FRP Mu ultimate moment of resistance Pu ultimate load for flexural members Tam,ch ambient temperature in chamber Tam,ou outdoor ambient temperature Tex,ch surface temperature in chamber Tex,ou outdoor surface temperature tch elapsed chamber time tou elapsed outdoor time thickness of FRP laminate wp width of FRP laminate x distance from the top concrete fiber to the centroid of compression stress block α bond strength calibration factor βL bond length coefficient βp bond width coefficient δu beam deflection at failure δy beam deflection at yield of internal steel reinforcement vii ε c (x ) concrete strain at distance x from neutral axis ε co concrete strain corresponding to fc’ ε cu ultimate compressive strain of concrete εp strain in FRP laminate ε pu , X (t ) rupture strain of Type X FRP composite at age t * ε pu , X (t ) rupture strain of Type X FRP composite at accelerated age t ε pdb debonding strain of FRP laminate εs strain in internal tensile steel reinforcement εs' strain in internal compression steel reinforcement ε sy yield strain of tensile reinforcement ε sy ' yield strain of compression steel reinforcement φ X (t ) residual value of X at age t µ ductility index ρb balanced steel reinforcement ratio ρmax maximum steel reinforcement ratio ρmin minimum steel reinforcement ratio σ dbic debonding stress of FRP laminate *τ pu , X (t ) interfacial bond strength between Type X FRP and concrete at accelerated age t viii List of Tables Table 1.1 Glass transition temperature of moisture free resins (Pritchard, 1999) Table 1.2 Typical chemical composition of commercial glass fibers (Leggatt, 1984; ACI, 1996) Table 1.3 Wavelength regions of UV (Sharman et al., 1989) Table 1.4 Maximum photochemical sensitivity for different plastics (Sharman et al., 1989) Table 1.5 Summary of weathering effects on FRP Table 2.1 Outdoor weathering factors for Singapore (1987-1997) Table 2.2 Properties of FRP constituents Table 2.3 Environmental tensile strength reduction factors for GFRP (Byars et al., 2001) Table 3.1 Test matrix Table 3.2 Geometrical and reinforcements details of test beams Table 3.3 Concrete cube compressive strength post to various exposure conditions Table 3.4 GFRP laminate properties Table 3.5 Ultimate load and failure mode of type C (unbonded) specimens Table 3.6 Ultimate load and failure mode of type G1 specimens Table 3.7 Ultimate load and failure mode of type G2 specimens Table 3.8 Ductility indices for AB, OB and CB series specimens Table 3.9 Maximum crack widths at 60% Pu Table 3.10 Comparison of predicted and test results ix Chapter Four: Conclusions (d) the tensile strength of GFRP laminates reduce as a result of outdoor or equivalent in-chamber weathering, (e) the forecast total reduction in tensile strength of GFRP laminates after tropical weathering exposure seems to exceed the proposed environmental reduction factors of various national/international standards Also, the test program on 48 beam specimens subjected to different exposure conditions suggested that: (a) the changes in failure modes and ultimate loads of FRP-strengthened beams over time are predicted reasonably well by the proposed model, (b) GFRP-strengthened beams sustain all the initial strength gain and exhibit the same design failure mode over time when protected from weathering effects, (c) short-term (less than month) outdoor exposure improves the properties of epoxy and delays the lateral splitting rupture of unidirectional G1 laminates post to crushing of concrete, and (d) the design ductile failure mode of GFRP-strengthened beams changed to brittle GFRP rupture after to months of outdoor weathering with marginal drop in the enhanced strength, while longer period of weathering (up to years) causes substantial drop in the enhanced strength 133 Chapter Four: Conclusions 4.3 Recommendations for Future Research The reproducibility of the tropical weathering effects and the time-dependent behavior of small-scale GFR-strengthened beams under the weathering effects were focused in this study Further works are recommended in the following areas: (a) A study on the time-dependent behavior of large-scale beams strengthened with GFRP under tropical weathering effects to verify the general validity of the proposed model (b) A study on the effects of weathering on loaded GFRP-strengthened beams (c) A study on the effects of weathering on GFRP-strengthened beams with different pre-designed failure modes, that is, different steel reinforcement, GFRP ratios and concrete strengths (d) A study on the effects of weathering on beams 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Canada 146 Appendix A: ASTM-G151:Table Appendix A ASTM-G151-97: Table TABLE Spectral Global Irradiance (condensed from Table of CIE Publication No 85 – 1989) * Wavelength (nm) Irradiance (Wm-2) Percent Total Percent of UV and (300-2450 nm) Visible (300-800nm) 300-320 4.1 0.4 0.6 320-360 28.5 2.6 4.2 360-400 42.0 3.9 6.2 74.6 11.0 300-400 6.8 400-800 604.2 55.4 89.0 300-800 678.8 62.2 100.0 800-2450 411.6 37.8 … 300-2450 1090.4 100.0 … * Source: ASTM-G-151-97 A-1 ... fiber- reinforced polymer (FRP) composites, either by wet lay-up of fiber sheets or adhesive bonding of composite strip/panel, gained popularity in structural retrofitting and rehabilitation of. .. behavior of FRPstrengthened beams under the weathering effects of tropical climate Comparison with weathering test results showed that the effects of tropical climate weather were reproduced well... Elements 1.3 Objectives of Study 18 1.4 Report Organization 18 1.5 Tables and Figures 20 iii Chapter Two: Simulation of Tropical Climate 2.1 2.2 2.3 Characterization of Tropical Climate 36 2.1.1 2.1.2