Tài liệu Flexural performance and bond characteristics of frp strengthening techniques for concrete structures

FLEXURAL PERFORMANCE AND BOND CHARACTERISTICS OF FRP STRENGTHENING TECHNIQUES FOR CONCRETE STRUCTURES By Tarek Kamal Hassan Mohamed A Dissertation Submitted to the Faculty o f Graduate Studies In Partial Fulfilment o f the Requirements for the Degree o f DOCTOR OF PHILOSOPHY Structural Engineering Division Department o f Civil and Geological Engineering The University o f Manitoba Winnipeg, Manitoba, Canada © May 2002 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. National Library of Canada Bibliotheque nationale du Canada Acquisitions and Bibliographic Services Acquisitions et services bibliographiques 395 Wellington Street Ottawa ON K1A0N4 Canada 395, rue Wellington Ottawa ON K1A0N4 Canada Your file Votre reference OurBle Notre reference The author has granted a non­ exclusive licence allowing the National Library o f Canada to reproduce, loan, distribute or sell copies o f this thesis in microform, paper or electronic formats. 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UNIVERSITY OF MANITOBA FACULTY OF GRADUATE STUDIES FINAL ORAL EXAMINATION OF T H E PHD THESIS The undersigned certify that they have read, and recommend to the Faculty o f Graduate Studies for acceptance, a PhD thesis entitled: FLEXURAL PERFORMANCE AND BOND CHARACTERISTICS OF FRP STRENGTHENING TECHNIQUES FOR CONCRETE STRUCTURES BY TAREK KAMAL HASSAN MOHAMED In Partial fulfillment o f the requirements for the PhD Degree |U 2 s l Dr. S. H. Rizkalla, Advisor External Examiner Dr. Kenneth Neale Department de Genie Civil, Faculte de Genie Universite de Sherbrooke Sherbrooke, Quebec Dr. A. Mufti ........................................ t i " Dr. D. Polyzois Dr. C. Wu Date o f Oral Examination:.......................................... May 10,2002................................ The Student has satisfactorily completed and passed the PhD Oral Examination. iSk. Dr. S. H. Rizkalla, Advisor Dr. K. Neale Dr. A. Mufti Dr. D. Polyzois ! I Dr. C. Wu ......... 3 / ................. i d wJL^ Chair o f PhD Oral (The signature of the Chair does not necessarily signify that the Chair has read the thesis.) Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. THE UNIVERSITY OF MANITOBA FACULTY OF GRADUATE STUDIES COPYRIGHT PERMISSION FLEXURAL PERFORMANCE AND BOND CHARACTERISTICS OF FRP STRENGTHENING TECHNIQUES FOR CONCRETE STRUCTURES BY TAREK KAMAL HASSAN MOHAMED A Thesis/Practicum submitted to the Faculty of Graduate Studies of The University of Manitoba in partial fulfillment of the requirement of the degree of DOCTOR OF PHILOSOPHY TAREK KAMAL HASSAN MOHAMED © 2002 Permission has been granted to the Library of the University of Manitoba to lend or sell copies of this thesis/practicum, to the National Library of Canada to microfilm this thesis and to lend or sell copies of the film, and to University Microfilms Inc. to publish an abstract of this thesis/practicum. This reproduction or copy of this thesis has been made available by authority of the copyright owner solely for the purpose of private study and research, and may only be reproduced and copied as permitted by copyright laws or with express written authorization from the copyright owner. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Acknowledgments The author would like to express his deepest gratitude to his supervisor Dr. Sami Rizkalla. In addition to his support and friendship over the past three years, he has provided the unwavering source o f inspiration, determination, and leadership that was so essential for the successful execution o f this research project. The author would like to thank Dr. Aftab Mufti, Dr. Dimos Polyzois and Dr. Christine W u for their constructive comments and encouragements throughout the research. The support provided by the Network o f Centres o f Excellence on the Intelligent Sensing of Innovative Structures (ISIS Canada) is greatly acknowledged. The author also expresses his thanks to Mr. M. McVey, Mr. G. W hiteside and Mr. S. Sparrow for their valuable assistance during the fabrication and testing o f the specimens. Special thanks are extended to Dr. A. Kamiharako, Dr. A. Sultan, Dr. M. Mohamedien and Dr. N. Hassan for their assistance during the experimental phase o f this study. The support provided by Vector Construction Group and Concrete Restoration Services is greatly appreciated. Finally, the love, patience and support of my parents, my wife and my daughter cannot be praised enough; to them this thesis is dedicated. ii Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Abstract Strengthening o f reinforced concrete structures using FRPs has emerged as a potential solution to the problems associated with civil infrastructure. Many researchers have reported significant increases in strength and stiffness of FRP retrofitted concrete structures. Nevertheless, possible brittle failures o f the retrofitted system could limit the use o f the full efficiency o f the FRP system. These brittle failures include premature debonding of the FRP, which could occur at load levels significantly less than the strength o f the FRP material used in the retrofitted system. Therefore, there is a need for an improved understanding o f various failure mechanisms of FRP strengthened concrete structures as a basis for a reliable retrofit design. Innovative structural detailing is also needed to utilize the FRP system more effectively. The work reported in this thesis deals with the development of a comprehensive approach towards understanding the flexural behaviour o f FRP strengthened concrete structures. This study presents a comparison among various FRP strengthening techniques and develops fundamental criteria governing the choice o f a specific technique. The applicability o f cracked section analysis as well as non-linear finite element simulations for the analysis o f concrete structures strengthened with FRP reinforcement is enumerated. Design guidelines regarding the use of FRP in retrofitting applications are provided. Mathematical models are proposed to quantify the bond characteristics and load transfer mechanisms of various FRP strengthening schemes using bars and strips for near surface mounted configurations as well as externally bonded sheets and strips. iii Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. A two-phase experimental program was conducted at the University o f M anitoba to examine the structural performance o f concrete structures strengthened w ith various FRP systems. The experimental program was designed to ensure full utilization o f the strengthening schemes and to avoid possible premature failure o f the retrofitted system. Three half-scale models o f a typical concrete bridge slab were constructed and tested in the first phase o f the investigation. The performance o f near surface m ounted FRP bars and strips as well as externally bonded FRP sheets and strips was evaluated. The three specimens were used to perform a total o f nine tests in this phase. A cost analysis for each of the FRP strengthening techniques considered in this investigation was performed. Complementary to the experimental program, numerical simulations were performed using finite element analysis to predict the behaviour o f concrete members strengthened with near surface mounted FRP reinforcement. Based on test results, the investigation was extended to a second stage to provide fundamental data for the bond characteristics o f efficient FRP techniques. A total of 24 concrete T-beams were tested to characterize the load transfer mechanisms between FRP and concrete. Three different strengthening techniques were investigated. For each technique, different bond lengths were considered. Based on the experimental results, development lengths for various FRP strengthening techniques are proposed. The thesis also presents three analytical models, proposed to predict the behaviour of concrete structures strengthened with near surface mounted FRP bars, near surface mounted FRP strips and externally bonded FRP sheets. New methodologies are introduced as a basis for design. With the formulae proposed in this thesis, the risk o f iv Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. premature failure o f concrete structures strengthened with various FRP systems can be estimated. The entire investigation leads to simple design rules, with a profound theoretical basis, to allow an economical, safe and reliable FRP retrofit design for concrete structures and bridges. V Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Table of Contents Acknowledgments..........................................................................................................................ii A bstract................................................. iii Table o f Contents................................. ....:,................................................................................. vi Notation...................................................... xiv Chapter 1. Introduction 1.1 G eneral.......................................................................................................................................1 1.2 Research O bjective................................................................................................................. 3 1.3 Research A pproach................................................................................................................. 5 1.4 Outline o f the Thesis............................................................................................................... 8 Chapter 2. Strengthening of Reinforced Concrete Structures with FRPs 2.1 Introduction.............................................................................................................................10 2.2 Strengthening o f Concrete Structures .......................................................................... 11 2.2.1 Historical Background.................................................................................................. 11 2.2.2 Concrete Beams Strengthened with Steel P lates...................................................... 14 2.2.3 Concrete Beams Strengthened with F R P s................................................................. 20 2.3 Concrete — Steel/FRP Interface Bond Strength..............................................................24 2.3.1 Effect o f Surface Preparation on Bond Performance................................................28 2.3.2 Effect o f Adhesive on Bond Perform ance................................................................. 28 2.3.3 Effect o f FRP Stiffness on Bond Perform ance..........................................................29 2.3.4 Effect of Concrete Strength on Bond Perform ance..................................................29 2.4 Failure Mechanisms o f Concrete Beams Strengthened with F R P s............................... 30 2.4.1 G eneral.......................................................................................................................... 30 vi Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 2.4.2 Flexural Failures............................................................................................................ 31 2.4.3 Shear Failures................................................................................................................. 35 2.4.4 Debonding Failures........................................................................................................36 2.4.4.1 Delamination M odels .................................................................................42 2.4.4.2 Applicability o f Delamination M o d els............................................................... 59 2.5 Existing Design Procedures for Delamination...................................................................60 2.5.1 General............................................................................................................................ 60 2.5.2 Professional O rganizations...........................................................................................60 2.5.2.1 American Concrete Institute................................................................................. 60 2.5.2.2 The Canadian Network o f Centres o f Excellence..............................................61 2.5.23 German Institute for Construction Technology................................................. 62 2.5.2.4 Japan Concrete Institute and Jpan Society o f Civil E ngineers........................63 2.5.2.5 International Conference of Building O fficials................................................. 63 2.5.3 Retrofit System M anufacturers.................................................................................... 63 2.5.3.1 S ik a ................................................................................................ 63 2.5.3.2 MBrace Composite Strengthening Systems........................................................64 2.5.3.3 S&P Composite Reinforcing System s................................................................ 64 2.5.4 Independent Researchers..............................................................................................65 2.6 Bond Characteristics o f FRP R ebars..................................................................................65 2.6.1 G eneral............................................................................................................................ 65 2.6.2 Bond M echanism ..................................... 66 2.6.3 Factors Affecting Bond Perform ance......................................................................... 67 2.6.4 Bond-Slip M odels..........................................................................................................67 vii Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 2.7 Strengthening o f Reinforced Concrete Structures Using N ear Surface............................ Mounted FRP Reinforcem ent.................................................................................................... 69 2.8 Field A pplications................................................................................................................. 75 2.9 Durability o f FRP Strengthening Techniques................................................................... 79 2.9.1 G eneral............................................................................................................................ 79 2.9.2 Wet— Dry Exposure...................................................................................................... 80 2.9.3 Freeze— Thaw Exposure....................................................... 81 2.9.4 Thermal Exposure..........................................................................................................82 2.9.5 Fatigue...................................................................... 83 Chapter 3. Experimental Program 3.1 General.................................................................................................................................... 85 3.2 Phase I o f the Experimental Program ...............................................................................87 3.2.1 Large-Scale Slab Specimens........................................................................................ 87 3.2.2 Fabrication o f the Specimens....................................................................................... 91 3.2.2.1 Preparation o f the Forms............................................................. 91 3.2.2.2 Stressing o f the Tendons....................................................................................... 92 3.2.2.3 G routing.................................................................................................................. 94 3.2.3 Strengthening Techniques.............................................................................................95 3.2.4 Instrum entation................... 105 3.2.4.1 Cantilever T e sts................................................................................................... 105 3.2.4.2 Simply Supported T ests...................................................................................... 109 3.2.5 Testing Schem e............................................................................................................ I l l 3.2.5.1 Cantilever T e sts ................................................................................................... 111 3.2.5.2 Simply Supported T ests...................................................................................... 113 viii Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 3.3 Phase II o f the Experimental Program ............................................................................. 115 3.3.1 Bond Specimens...................................................................... 115 3.3.2 Fabrication of the Specimens..................................................................................... 119 3.3.3 Strengthening Procedures .............................................................................. 119 3.3.4 Instrumentation............................. 123 3.3.5 Testing Scheme................................................................................................ 124 3.4 Materials ....................................................... 124 3.4.1 C oncrete........................................................................................................................124 3.4.2 Prestressing S teel.........................................................................................................125 3.4.3 Sheathing.......................................................................................................................127 3.4.4 Mild Steel......................................................................................................................127 3.4.5 Leadline......................................................................................................................... 127 3.4.6 S&P CFRP Strips........................................................................................................ 128 3.4.7 MBrace CFRP S heets................................................................................................. 130 3.4.8 C -B A R ................................................................................................:.........................130 Chapter 4. Experimental Results & Analytical Modelling: Large-Scale Slab Specimens 4.1 Introduction.......................................................................................................................... 131 4.2 Experimental R esults..........................................................................................................133 4.2.1 G eneral..................................... 133 4.2.2 Cantilever Test R esults............................................................................................... 135 4.2.2.1 Deflection.............................................................................................................. 135 4.2.2.2 Failure M odes...................................................................................................... 138 4.2.2.3 Tensile Strains...................................................................................................... 141 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 4.2.2A Crack Patterns....................................................................................................... 144 4.2.2.5 Crack W idth...................... 147 4.2.3 Cost A nalysis................................................................................................................150 4.2.4 Test Results of the Simply Supported Specim ens.................................................. 153 4.2.4.1 General...................................................................................................................153 4.2.4.2 Deflection............................................ 154 4.2.4.3 Failure M odes....................................................................................................... 155 4.2.4.4 Tensile Strains....................................................................................................... 157 4.2.4.5 Crack Patterns....................................................................................................... 162 4.2.5 Deformability................................................................................................................166 4.3 Analytical M odelling........................................................................................................ 168 4.3.1 Introduction...................................................................................................................168 4.3.2 Cracked-Section Analysis........................................................................................... 169 4.3.3 Finite Element Sim ulation.......................................................................................... 173 4.3.3.1 Background.................................................................................. 173 4.3.3.2 Development of the Finite Element M odel...................................................... 174 4.3.3.3 Modelling of the Cantilever S la b s.....................................................................178 4.3.3.4 Modelling o f the Simply Supported Slabs........................................................183 4.3.3.5 Material M odelling.............................................................................................. 184 4.3.4 Results and D iscussion................................................................................................185 4.3.4.1 Cantilever Specimens................................. 185 4.3.4.2 Simply Supported Specim ens.............................................................................191 x Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter 5. Experimental Results & Analytical Modelling: Bond Specimens 5.1 Introduction.......................................................................................................................... 194 5.2 Near Surface Mounted FRP B ars...................................................................................... 197 5.2.1 Experimental Results (Series A )................................................................................197 5.2.2 Analytical M odelling..................................................................................................207 5.2.2.1 Significance o f the M odel...................................................................................207 5.2.2.2 ACI Approach for Steel B ars............................................................................. 207 5.2.2.3 Proposed Approach for NSM FRP bars........................................................... 209 5.2.2.4 Coefficient o f Friction (//).................................................................................. 216 5.2.2.5 Comparison with Experimental Results........................................................... 217 5.2.2.6 Comparison with ACI— 440.............................................................................. 218 5.2.2.7 Detailing G uidelines........................................................................................... 220 5.2.2.8 Maximum Stresses in NSM Bars (J f r p ) ............................................................225 5.3 Near Surface Mounted CFRP Strips................................................................................ 231 5.3.1 Experimental Results (Series B )............................................................................... 231 5.3.2 Analytical M odelling..................................................................................................235 5.3.2.1 General ............................................................................................................235 5.3.2.2 Derivation o f the M odel............... 235 5.3.2.2.1 Simply Supported Beam Subjected to a Concentrated L o ad ........... 235 5.3.2.2.2 Simply Supported Beam Subjected to a Uniform Load.................... 240 5.3.2.2.3 Simply Supported Beam Subjected to Two Concentrated Loads 241 5.3.2.3 Failure Criterion...................................................................................................241 5.3.2.4 Verification o f the Analytical M odel................................................................243 xi Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 5.3.2.4.1 Modelling o f Test Specim ens.....................................................................243 5.3.2.4.2 Comparison with Finite Element A nalysis...............................................247 5.3.2.4.3 Comparison with Experimental Results.................................................... 249 5.3.2.5 Parametric Study................ 251 5.4 Externally Bonded CFRP Sheets....................................................................................256 5.4.1 Experimental Results (Series C ).................................................................... 256 5.4.2 Analytical M odelling..................................................................................................260 5.4.2.1 Background...........................................................................................................260 5.4.2.2 Proposed Approach for Externally Bonded FRP Reinforcem ent.................261 5.4.2.3 Failure Criterion...................................................................................................263 5.4.2.4 Comparison with Experimental Results............................................................265 5.4.2.5 Parametric Study..................................................................................................268 Chapter 6. Summary & Conclusions 6.1 G eneral............................................................................... 272 6.2 Sum m ary.............................................................................................................................. 273 6.2.1 Experimental Investigation........................................................................................ 273 6.2.2 Analytical Phase...........................................................................................................274 6.3 Conclusions..........................................................................................................................275 6.3.1 Effectiveness o f FRP System s...................................................................................275 6.3.2 Bond Characteristics o f FRP Strengthening System s............................................ 278 6.3.2.1 General Conclusions........................................................................................... 278 6.3.2.2 Near Surface Mounted CFRP B ars....................................................................279 6.3.2.3 Near Surface Mounted CFRP Strips................................................................. 281 6.3.2.4 Externally Bonded CFRP Sheets.......................................................................282 xii Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 6.4 Recommendations for Future Research......................................................................... 283 References .............................................................................................................................. 284 xiii Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Notation tf/,2,3 = Coefficients used to determine the applied moment on a concrete beam A = Cross-sectional area o f a concrete section Ac = Area o f concrete in compression Ap = Area o f prestressed steel reinforcement A/ = Area o f CFRP reinforcement As = Area o f tension steel reinforcement A S’ = Area o f compression steel reinforcement b = Width o f concrete section b 1,2,3 — Coefficients defined by Equation 2.7. bf = Width o f externally bonded FRP sheets/strips bs - Width o f externally bonded steel plates c = Depth o f the neutral axis from the extreme compression fibres ccr = Neutral axis depth for the cracked transformed section; ceff = Effective neutral axis depth for the transformed section; cg = Neutral axis depth for the gross transformed section; Ci, C2 - Parameters in the solution o f differential equation; C = Clear cover o f reinforcing bars Cs’ = Compression force in steel reinforcement d - Depth from extreme compression fibre to the flexural reinforcement; diameter o f near surface mounted FRP bar df = Depth o f near surface mounted CFRP strips from compression fibre dp = Depth o f the prestressing steel from compression fibre xiv Reproduced with permission of the copyright owner. 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Depth of the internal steel reinforcement from compression fibre D Deformability index e = E Eccentricity o f tendons; edge distance Efficiency o f FRP strengthening techniques Ea = Modulus o f elasticity o f the adhesive Ec = Modulus o f elasticity o f concrete Ef = Modulus o f elasticity o f FRP reinforcement EP = Modulus o f elasticity o f the steel strands Es Modulus o f elasticity o f steel reinforcement j Bottom ~~ Concrete stress at bottom fibres fc Concrete stress in compression f t = Splitting tensile strength o f concrete fc = Compressive strength o f concrete after 28 days I - Maximum tensile stress in near surface mounted FRP bars fp s = Stress in steel strands ff = frp Stress in CFRP reinforcement fp u Ultimate stress in steel strands fs Stress in tension steel reinforcement fs ' = Stress in compression steel reinforcement Tensile stress in concrete after cracking t II ft fy G 1,2,2' Concrete stress at top fibres = Yield stress o f steel reinforcement = Coefficients for near surface mounted FRP bars Reproduced with permission of the copyright owner. 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Ga = Shear modulus of the adhesive Gf = Fracture energy h = Total height o f a concrete section h r (transformed) ~ Cracked moment o f inertia o f the transformed strengthened section Ieff = Effective moment o f inertia o f the transformed strengthened section If = Moment o f inertia o f the FRP sheets I g (transform ed) = Gross moment o f inertia o f the transformed strengthened section kn = Parameter defined by Equation 5.48 L = Embedment/bond length o f FRP reinforcement, length o f tendons L' = Total span o f the simply supported beam Ld — Development length o f reinforcement Le = Effective bond length Lg = Gauge length l0 = Unbonded length o f the CFRP reinforcement Ma = Applied moment on a concrete section Mcr = Cracking moment o f a concrete section Muve - Moment due to specified live load Mp = Applied moment at peeling failure Mu = Ultimate moment capacity o f a concrete section Ms - Moment corresponding to a concrete compressive strain o f 0.001 Msw = Moment due to self weight N = Normal force acting on the FRP at the two ends o f a segment n = Number o f cracks passing through a PI gauge; modular ratio; number o f xvi Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. FRP layers p = Applied load Per - Cracking load Pd = Debonding/delamination load Pe = Effective prestressing force after loses Po = Required prestressing force in the tendons Pj = Jacking force P« = Failure load o f the specimen q = Applied uniform load S : Clear spacing between grooves S = Section modulus o f the concrete ta = Thickness o f the adhesive layer tc = Thickness o f concrete in shear-type specimens = Thickness o f CFRP strip/sheet = Tensile force in CFRP reinforcement = Tensile force in prestressing reinforcement Ts = Tensile force in steel reinforcement u = Longitudinal displacement in the adhesive layer Ue = Elastic energy o f the system V = Vertical displacement in the adhesive layer Vc = Shear force in the concrete at cutoff points due to interfacial shear stresses = Shear force in the FRP sheets at cutoff points due to interfacial shear tf Tf TP Vf stresses xvii Reproduced with permission of the copyright owner. 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