Evaluation of near surface mounting (NSM) carbon fiber reinforced polymer (CFRP) technique used in a concrete bridge slab

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(1)PONTIFICIA UNIVERSIDAD CATOLICA DE CHILE SCHOOL OF ENGINEERING. EVALUATION OF NEAR SURFACE MOUNTING (NSM) CARBON FIBER REINFORCED POLYMER (CFRP) TECHNIQUE USED IN A CONCRETE BRIDGE SLAB. ARIELA ASTORGA LÓPEZ. Thesis submitted to the Office of Research and Graduate Studies in partial fulfillment of the requirements for the degree of Master of Science in Engineering. Advisor: MAURICIO LÓPEZ CASANOVA. Santiago de Chile, March 2011 c MMXI, A RIELA A STORGA L ÓPEZ.

(2) PONTIFICIA UNIVERSIDAD CATOLICA DE CHILE SCHOOL OF ENGINEERING. EVALUATION OF NEAR SURFACE MOUNTING (NSM) CARBON FIBER REINFORCED POLYMER (CFRP) TECHNIQUE USED IN A CONCRETE BRIDGE SLAB. ARIELA ASTORGA LÓPEZ. Members of the Committee: MAURICIO LÓPEZ CASANOVA HERNÁN SANTA MARÍA OYANEDEL MANUEL RODRIGUEZ ......... MICHEL VAN SINT JAN FABRY Thesis submitted to the Office of Research and Graduate Studies in partial fulfillment of the requirements for the degree of Master of Science in Engineering.

(3) Santiago de Chile, March 2011 c MMXI, A RIELA A STORGA L ÓPEZ. iii.

(4) To my family.

(5) ACKNOWLEDGEMENTS. I would like to thank my advisor, Professor Mauricio López, for giving me the opportunity to work on this thesis, for supporting me during these past two years, for his patient guidance and for giving me inspiration to work in this project. I have been extremely lucky to have a supervisor who cared so much about my work, and so dedicated to his students. Also I would like to thank him for teaching me during the undergraduate courses. I would also like to thank Professor Hernán Santa Marı́a for his assistance in this thesis and guidance in getting my graduate career. I also wish to thank the research group of Professor López: Daniel Moreno, Gastón Espinoza, Alvaro Paul, Fernando Bustos, Juan Pablo Cancino, Franco Zunino and Ricardo Serpell for their helpful suggestions and critical comments. I acknowledge the help of Rodolfo Arguello who work with me during experimental work. I would also like to thank Cristián Maluk for developing DMA test and for his collaboration writing one of the papers, and to Manuel Rodriguez for his logistical support and guidance during the thesis. Thanks to the people without whom my Master would not have been possible, the laboratory technicians Manuel Rabelo, Atilio Muñoz, Armando Arellano and Mauricio Guerra for their support during my experimental work. I would like to thank the financial and logistical support provided by Basf Chile S.A, Sacyr, Sociedad Concesionaria Autopista Nororiente, Ingelab Ltda., Dictuc, S.A. and The Ministry of Public Works (MOP). The work in field would not have been possible without the assistance of Juan Pablo Rodrı́guez, Patricio Hauck, Claudio Morales, Daniel Fajardo, Patricio Garcı́a, Flavio Flores and other students of the University. Finally and most importantly, I would like to thank my family for their continuous support and encouragement I received while doing my thesis. I would like to dedicate my. v.

(6) thesis to my father, my mother and my sister. Thank you all for your care, love, understanding, trust and belief, which make me who I am.. vi.

(7) TABLE OF CONTENTS. ACKNOWLEDGEMENTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. v. LIST OF FIGURES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. x. LIST OF TABLES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. xiv. ABSTRACT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. xv. RESUMEN . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. xix. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 1. 1.. 2.. 3.. INTRODUCTION 1.1.. Characterizing Fiber Reinforced Polymers (FRP). . . . . . . . . . . . . .. 1. 1.2.. Near Surface Mounting (NSM) Reinforcement Techniques . . . . . . . . .. 7. 1.3.. Bond Behavior . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 8. 1.4.. Monotonic and Fatigue Performance of NSM reinforcement technique . .. 13. 1.5.. NSM Reinforcement on Centenario Bridge . . . . . . . . . . . . . . . . .. 16. SUMMARY OF CONDUCTED STUDY . . . . . . . . . . . . . . . . . . . .. 19. 2.1.. Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 19. 2.2.. Objectives and Methodology . . . . . . . . . . . . . . . . . . . . . . . .. 20. 2.3.. Hypothesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 20. 2.4.. Experimental procedure. . . . . . . . . . . . . . . . . . . . . . . . . . .. 21. Behavior of a concrete bridge cantilever slab reinforced using NSM CFRP strips. 24. 3.1.. Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 24. 3.2.. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 25. 3.3.. Research significance . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 29. 3.4.. Experimental Program and Strengthening Technique . . . . . . . . . . . .. 29. 3.4.1.. Experimental Program . . . . . . . . . . . . . . . . . . . . . . . . .. 30. 3.4.2.. Strengthening Technique . . . . . . . . . . . . . . . . . . . . . . .. 32. Material, Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 33. 3.5.. vii.

(8) 3.5.1.. CFRP strips and epoxy adhesive . . . . . . . . . . . . . . . . . . . .. 33. 3.5.2.. Concrete and Steel. . . . . . . . . . . . . . . . . . . . . . . . . . .. 33. Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 34. 3.6.1.. Stain Measurement on Concrete Bridge . . . . . . . . . . . . . . . .. 34. 3.6.2.. Load Tests in Concrete Bridge . . . . . . . . . . . . . . . . . . . . .. 37. 3.6.3.. Monotonic load tests in concrete beams . . . . . . . . . . . . . . . .. 40. 3.6.4.. Analytical strength on Centenario Bridge slab . . . . . . . . . . . . .. 52. 3.7.. Summary and Conclusions . . . . . . . . . . . . . . . . . . . . . . . . .. 54. 3.8.. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 56. 3.6.. 4.. Characterizing Bond Behavior of NSM CFRP strips under elevated temperatures 4.1.. Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 57. 4.2.. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 58. 4.3.. Research Significance. . . . . . . . . . . . . . . . . . . . . . . . . . . .. 61. 4.4.. Experimental Program and Strengthening Technique . . . . . . . . . . . .. 62. 4.4.1.. Experimental Program . . . . . . . . . . . . . . . . . . . . . . . . .. 62. 4.4.2.. Strengthening Technique . . . . . . . . . . . . . . . . . . . . . . . .. 67. Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 67. 4.5.1.. Concrete and Steel Reinforcement . . . . . . . . . . . . . . . . . . .. 67. 4.5.2.. CFRP Strips and Epoxy Adhesive . . . . . . . . . . . . . . . . . . .. 67. Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 68. 4.6.1.. Pull-out Tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 68. 4.6.2.. Dynamic Mechanical Analysis (DMA) - Modulus . . . . . . . . . . .. 73. 4.6.3.. Dynamic Mechanical Analysis (DMA) - Creep . . . . . . . . . . . .. 74. 4.6.4.. Monotonic Load Test on Concrete Beams Under High Temperatures .. 77. 4.7.. Summary and Conclusions . . . . . . . . . . . . . . . . . . . . . . . . .. 81. 4.8.. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 83. SUMMARY AND CONCLUSIONS . . . . . . . . . . . . . . . . . . . . . .. 84. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 84. 4.5.. 4.6.. 5.. 57. 5.1.. viii.

(9) Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 84. REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 88. APPENDICES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 94. 6.1.. CFRP Installation on Centenario Bridge . . . . . . . . . . . . . . . . . .. 94. 6.2.. Centenario Bridge instrumentation . . . . . . . . . . . . . . . . . . . . .. 98. 6.3.. Strain Measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 99. 6.4.. Load Tests on Centenario Bridge . . . . . . . . . . . . . . . . . . . . . . 104. 6.5.. Monotonic Load Test in RC beams . . . . . . . . . . . . . . . . . . . . . 108. 6.6.. Pull-out tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119. 6.7.. Tension test on CFRP strips . . . . . . . . . . . . . . . . . . . . . . . . . 129. 6.8.. Monotonic Load test on reinforced concrete beams under temperature . . . 129. 6.9.. DMA test of epoxy matrix for specimens mixed 1:1 in proportion (resin and. 5.2.. 6.. hardener) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132. ix.

(10) LIST OF FIGURES. 1.1 Stress-strain curves for FRP materials (De Paula 2005). . . . . . . . . . . . .. 2. 1.2 Stress-strain curve for FRP composites (De Paula 2005). . . . . . . . . . . . .. 5. 1.3 Schematics of the bond behavior of NSM FRP reinforcement (De Lorenzis 2007).. 8. 1.4 Broomlike failure on CFRP bars (Adimi et al. 2000). . . . . . . . . . . . . .. 9. 1.5 Reaction of an epoxy resin (Greene 2005). . . . . . . . . . . . . . . . . . . .. 11. 1.6 Centenario Bridge cross section. . . . . . . . . . . . . . . . . . . . . . . . .. 17. 3.1 Location of CFRP strips reinforcement on the concrete slab in Centenario Bridge. 30 3.2 ERSG location on CFRP strips. . . . . . . . . . . . . . . . . . . . . . . . . .. 31. 3.3 VWSG location on concrete slab. . . . . . . . . . . . . . . . . . . . . . . . .. 31. 3.4 Cross section of reinforced and unreinforced beams for monotonic load tests . .. 32. 3.5 Strain measurements in concrete (+ tension) and ambient temperature throughout time. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 34. 3.6 Strain measurements in CFRP strips thorough time (+ tension) . . . . . . . . .. 36. 3.7 Truck location according to instrumented strips and concrete. . . . . . . . . .. 37. 3.8 Strains in CFRP strips during second load test, after CFRP reinforcement. . . .. 38. 3.9 Strains in concrete strips during load tests 1 before CFRP reinforcement. . . .. 39. 3.10 Strains in concrete strips during load tests 2 after CFRP reinforcement. . . . .. 39. 3.11 Strain Gage location refereed to concrete beams. . . . . . . . . . . . . . . . .. 40. 3.12 Load-Deflection curves for reinforced and unreinforced specimens. . . . . . .. 41. 3.13 Rupture of CFRP strip on reinforced concrete beam. . . . . . . . . . . . . . .. 44. 3.14 Rupture of CFRP strip on reinforced concrete beam. . . . . . . . . . . . . . .. 45. 3.15 Crack distribution in bottom side of reinforced and unreinforced specimens. . .. 45. 3.16 Strains in CFRP strip for reinforced specimen R2. . . . . . . . . . . . . . . .. 46 x.

(11) 3.17 Stress and strain distribution for concrete beams. . . . . . . . . . . . . . . . .. 47. 3.18 Centenario Bridge Section. . . . . . . . . . . . . . . . . . . . . . . . . . . .. 52. 4.1 Pull-out test set-up. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 62. 4.2 Strain gauge distribution on concrete cubes for pull-out tests . . . . . . . . . .. 63. 4.3 Flexure beam tests set-up. . . . . . . . . . . . . . . . . . . . . . . . . . . .. 64. 4.4 RC beam cross- section and thermocouple location. . . . . . . . . . . . . . .. 65. 4.5 Strain gage distribution for concrete beams tested under flexure . . . . . . . .. 66. 4.6 Temperature throughout time in reinforced concrete beams . . . . . . . . . . .. 66. 4.7 Failure at the epoxy-CFRP interface for pull-out tests. . . . . . . . . . . . . .. 70. 4.8 Ultimate Load v/s bond length for pull-out test . . . . . . . . . . . . . . . . .. 70. 4.9 CFRP strain distribution in Specimen BT-450a. . . . . . . . . . . . . . . . .. 71. 4.10 Bond strength distribution in Specimen BT-450a . . . . . . . . . . . . . . . .. 72. 4.11 Load-slip response in Specimen BT-450a. . . . . . . . . . . . . . . . . . . .. 73. 4.12 DMA test results (1st and 2nd run). . . . . . . . . . . . . . . . . . . . . . . .. 75. . . . . . . . . . . . . . . . .. 76. 4.14 DMA test results for creep tests. . . . . . . . . . . . . . . . . . . . . . . . .. 76. 4.15 Load-deflection curves for monotonic load test on RC beams . . . . . . . . . .. 77. 4.16 Concrete surrounding CFRP strip detachment . . . . . . . . . . . . . . . . . .. 80. 4.17 CFRP strip bucking and failure at the concrete-epoxy interface . . . . . . . . .. 80. 4.13 DMA test results (1st run) and Tg-Tanδ values.. 4.18 Strain in CFRP strip v/s load applied for monotonic load test for specimen FT-SB-s1. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 81. 6.1 Cutting the grooves on Centenario Bridge slab. . . . . . . . . . . . . . . . . .. 94. 6.2 Cleaning the grooves using compressed air. . . . . . . . . . . . . . . . . . .. 95. 6.3 Painting CFRP strips using epoxy prior installation. . . . . . . . . . . . . . .. 96. 6.4 Filling the grooves with epoxy. . . . . . . . . . . . . . . . . . . . . . . . . .. 97 xi.

(12) 6.5 ERSG installation on CFRP strips. . . . . . . . . . . . . . . . . . . . . . . .. 98. 6.6 VWSG installation on concrete. . . . . . . . . . . . . . . . . . . . . . . . .. 98. 6.7 Strain measurements on CFRP strips. . . . . . . . . . . . . . . . . . . . . . .. 99. 6.8 Strain measurements on concrete. . . . . . . . . . . . . . . . . . . . . . . .. 99. 6.9 ERSG and VWSG location and identification. . . . . . . . . . . . . . . . . . 100 6.10 Load test on Centenario Bridge. . . . . . . . . . . . . . . . . . . . . . . . . 104 6.11 Monotonic load tests set up. . . . . . . . . . . . . . . . . . . . . . . . . . . 108 6.12 Failure mode control beam V1.. . . . . . . . . . . . . . . . . . . . . . . . . 109. 6.13 Load - deflection curve control beam V1.. . . . . . . . . . . . . . . . . . . . 109. 6.14 Failure mode control beam V2. . . . . . . . . . . . . . . . . . . . . . . . . . 110 6.15 Load - deflection curve control beam V2.. . . . . . . . . . . . . . . . . . . . 110. 6.16 Failure mode beam V3 reinforced with CFRP. . . . . . . . . . . . . . . . . . 111 6.17 Detail failure mode beam V3: strip buckling along longitudinal plane . . . . . 112 6.18 Load - deflection curve reinforced beam V3 . . . . . . . . . . . . . . . . . . 112 6.19 Failure mode beam V4 reinforced with CFRP. . . . . . . . . . . . . . . . . . 113 6.20 Detail failure mode beam V4.. . . . . . . . . . . . . . . . . . . . . . . . . . 113. 6.21 Detail failure mode beam V4: strip rupture along transverse plane.. . . . . . . 114. 6.22 Load - deflection curve reinforced beam F-SB-s2 . . . . . . . . . . . . . . . 114 6.23 Pull-out test set up. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119 6.24 Pull-out test instrumentation for embedment length 450 mm. . . . . . . . . . 120 6.25 Failure mode for embedment length 450 mm: strip fracture over the concrete surface. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121 6.26 Slip.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124. 6.27 Slip corrected. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125 6.28 Failure mode for embedment length 275 mm: strip fracture over the concrete surface. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 126 xii.

(13) 6.29 Broomlike failure mode for embedment length 200 mm. . . . . . . . . . . . . 127 6.30 Failure at the epoxy-CFRP interface for embedment length 100 mm. . . . . . 127 6.31 Strip rupture for embedment length 50 mm. . . . . . . . . . . . . . . . . . . 128 6.32Strip rupture for embedment length 50 mm. . . . . . . . . . . . . . . . . . . . 128 6.33 Failure mode CFRP strips under tension. . . . . . . . . . . . . . . . . . . . . 129 6.34 Load-deflection curve for beam FT-SB-s1 tested under temperature and corrected by blanket deflections. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 129 6.35 Load-deflection curve for beam FT-SB-s2 tested under temperature and corrected by blanket deflections. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 130 6.36 Load-strain curves for beam FT-SB-s1 subjected to temperature. . . . . . . . . 130 6.37 Load-strain curves for beam FT-SB-s2 subjected to temperature. . . . . . . . . 131 6.38 DMA test comparison for samples A and B. . . . . . . . . . . . . . . . . . . 132. xiii.

(14) LIST OF TABLES. 1.1 FRP properties ACI440 (2006). . . . . . . . . . . . . . . . . . . . . . . . . .. 4. 3.1 Load by axle. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 37. 3.2 Monotonic load tests results. . . . . . . . . . . . . . . . . . . . . . . . . . . .. 42. 3.3 Analytical strenght compared to experimenta results. . . . . . . . . . . . . . .. 50. 3.4 Analytical strenght compared to experimenta results. . . . . . . . . . . . . . .. 53. 4.1 Pull-out tests results for different embedment lengths. . . . . . . . . . . . . . .. 69. 4.2 Monotonic load tests results. . . . . . . . . . . . . . . . . . . . . . . . . . . .. 78. 6.1 Strain measurements on concrete. . . . . . . . . . . . . . . . . . . . . . . . . 101 6.2 Strain measurements on CFRP strips. . . . . . . . . . . . . . . . . . . . . . . 102 6.3 Strain measurements on CFRP strips. . . . . . . . . . . . . . . . . . . . . . . 102 6.4 Strain measurements on CFRP strips. . . . . . . . . . . . . . . . . . . . . . . 103 6.5 Strain measurements on CFRP strips. . . . . . . . . . . . . . . . . . . . . . . 103 6.6 Strain measurements on concrete on PC1 before CFRP installation. . . . . . . . 105 6.7 Strain measurements on concrete on PC2 after CFRP installation. . . . . . . . . 106 6.8 Strain measurements on concrete on PC2 after CFRP installation. . . . . . . . . 107 6.9 Reinforced beam dimensions and material properties. . . . . . . . . . . . . . . 115 6.10Analytical strength unreinforced beams. . . . . . . . . . . . . . . . . . . . . . 116 6.11Moment. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116 6.12Analytical strength reinforced beams using Hognestad. . . . . . . . . . . . . . 117 6.13Moment. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 118 6.14Strain Gages measurements for embedment length 450 mm.. . . . . . . . . . . 122. 6.15Load-slip measurements for embedment length 450 mm. . . . . . . . . . . . . 123. xiv.

(15) ABSTRACT. Near surface mounting (NSM) carbon fiber reinforced polymer (CFRP) strips is a novel and promising reinforcement technique for structures, used for rehabilitation and retrofit of existing structures. However, there are many aspects about their application and performance that are still unknown. The performance of the NSM CFRP strips under monotonic, cyclic and sustained loads, in concrete elements have been studied in the laboratory; however, field performance of this reinforcement technique used in existing structures still needs to be understood properly. Bond behavior has an influence on the ultimate capacity and serviceability of the reinforced structure and determines the stress transfer between concrete and CFRP reinforcement. NSM technique generally uses epoxy as groove filling material, and it’s mechanical properties are lower than CFRP and concrete. Hence, debonding failures, or failures at the epoxy-concrete or CFRP-epoxy interface usually occurred in the system. The last decade, bond performance of externally bonded FRP has been widely studied; however limited literature is available to date on bond performance of NSM CFRP strips; especially the influence of the materials involved in this application. This investigation has two parts: the fist part has a general approach and studies the NSM CFRP structural system performance on field, and the second part, developed in laboratory, has an specific focuss on assessing the behavior of the materials involved in NSM system (concrete, epoxy, CFRP). The evaluation of NSM CFRP strips performance in service conditions on the field,was performed through the study of a reinforcement of a concrete bridge cantilever slab. Strains in CFRP strips and in the concrete surrounding the strips were measured over time, for two years. Strains in four CFRP strips were measured using electrical resistance strain gauges (ERSG) and while strains in concrete were measured using vibrating wire strain gages. xv.

(16) (VWSG). Also,strain in CFRP and concrete were measured during two load tests that were performed in the bridge with a 245 kN truck placed at the cantilever section. The results at the field were complemented with laboratory tests focused on the behavior under ultimate conditions. The laboratory study were conducted in four RC beams (two reinforced with CFRP and two unreinforced with CFRP) tested under monotonic load. Additionally, in order to assess the influence of CFRP and temperature in the bond performance of the reinforcement system, CFRP reinforced beams were tested under flexure and subjected to elevated temperatures, near the glass transition temperature (Tg) of epoxy. The study of the bond behavior of the NSM technique was mainly assesses through pull-out test performed in concrete cubical specimens, with different embedment lengths, where strains and slip of the strips were measured. Also Thermo-mechanical analysis (TMA) and Dynamical-mechanical analysis (DMA) were performed to the epoxy, used as groove filling material to study its behavior under cyclic and creep load at high temperatures. During the 2-year monitoring program of the concrete slab in the field, strains measured in concrete increased approximately 40 µ demonstrating that concrete slab is undergoing creep produced by the superimposed loads applied during strengthening. However there was an impportant effect of the ambient temperature on the measurements. CFRP reinforcement increased the ultimate load of the concrete beams between 13 to 26% with respect to control beams while the yield load was also increased by 12%. CFRP strip tensile rupture was the failure mode observed at the laboratory, which proved an adequate load transfer between CFRP, epoxy matrix, and surrounding concrete. However, failure mode of concrete beams changed when temperatures approached to Tg of the epoxy; for those beams FT-SB-s1 and FT-SB-s2 failure occurred by cohesive shear failure in the concrete at the epoxy-concrete interface, and no rupture of the CFRP. xvi.

(17) was observed. Temperatures near Tg caused caused a degradation in the epoxy mechanical properties; therefore the effective stress transfer between concrete and CFRP diminished.Also, temperatures near Tg caused an increase in average yield load and yield deflections over control specimens without reinforcement (FT-CB-s1 and FT-CB-s2) of 24% and 50% respectively; on the other hand, stiffness in the elastic stage ans stiffness after yielding decreased by 17% and 13% respectively. The pull-out test revealed that the failure mode observed for all specimens was CFRP tensile fracture at different heights and broomlike tensile failure. The slips measured at the load end were negligible, so debonding failure was not considered. Even though failure at the epoxy-CFRP interface was observed for some specimens at the free end, the bond provided by the epoxy was enough to develop full capacity of CFRP strips for all the embedment lengths tested. Pull-out test failure occurred at ultimate loads similar to that reported by manufacturer of the CFRP, for embedment lengths higher than 75 mm. Nevertheless; for smaller bond embedment lengths the ultimate capacity of the CFRP could not been developed reaching values about 40% lower than those reached with higher embedment lengths. The average Tg measured on the epoxy using DMA Modulus in a first run was 49 ◦ C; however, the heating cycle during the test provided additional curing of the samples, which produced and increase in Tg between 2 ◦ and 6 ◦ when samples were tested in a second run. On the other hand when sustained loads are applied, samples reached its ultimate strains at temperatures near 35 ◦ C. Also, the strains measured in the samples were higher than those reported by manufacturer. In summary, NSM CFRP strips reinforcement technique improves reniforced concrete elements structural capacity. The bond performance of this technique is adequate under service temperatures, but its behavior under temperatures near Tg has to be monitored, because mechanical properties of the epoxy diminishes and effective stress transfer between CFRP and concrete is not necessarily guarantied.. xvii.

(18) Keywords: carbon fiber reinforced polymer, near surface mounting, reinforcement, strain, beams, concrete bridge, bonding.. xviii.

(19) RESUMEN. El uso de pletinas de polı́meros reforzados con fibras de carbono (CFRP) utilizando la técnica de colocación en la superficie del material, o near surface mounting (NSM) por sus siglas en inglés, constituye un desafı́o; ya que ciertos aspectos de su desempeño aún son desconocidos por la ciencia. Es una técnica nueva y prometedora, de gran aplicabilidad y se utiliza actualmente tanto como refuerzo, como para la reparación y retroadaptación de estructuras existentes. La técnica NSM ofrece diversas ventajas sobre la colocación tradicional de tela CFRP en forma externa; puesto que provee una mayor protección y mejora la adherencia entre los materiales. Actualmente se conoce el desempeño del refuerzo en laboratorio frente a cargas monotónicas, cı́clicas y de creep, utilizado en elementos de hormigón; pero se desconoce su comportamiento en terreno en estructuras reales. El estudio de la adherencia entre materiales que logra esta técnica es de suma importancia; puesto que determina la capacidad última, serviciabilidad y transferencia de esfuerzos entre el hormigón y el refuerzo CFRP. La técnica NSM incluye el uso de epóxico como material de unión entre el hormigón y las pletinas; el cual es un material con propiedades mecánicas menores que éstos últimos. Muchas veces la falla del sistema está determinada por la adherencia que se logre entre los componentes. En la última década se ha estudiado ampliamente la adherencia entre el hormigón y elementos de FRP colocados externamente; pero no existen investigaciones suficientes acerca del comportamiento de pletinas CFRP colocadas NSM; especialmente desde el punto de vista de la influencia de los materiales que componen el sistema. Esta investigación se divide en dos partes según su enfoque: la primera parte tiene un enfoque global a nivel de sistema estructural CFRP NSM en terreno, y la segunda parte,. xix.

(20) desarrollada en laboratorio, tiene un enfoque especı́fico a nivel de materiales utilizados en el sistema NSM (CFRP, epóxico, hormigón). Para evaluar el desempeño de las fibras de carbono en condiciones de servicio en terreno, se estudió el Puente Centenario, cuya losa en voladizo fue reforzada utilizando la técnica mencionada. Se midieron las deformaciones en pletinas CFRP en el tiempo, en un perı́odo de 2 años, en 4 pletinas CFRP ubicadas en la sección central del puente utilizando transductores de resistencia eléctrica (electrical resistance strain gauges, ERSG); mientras que las deformaciones del hormigón ubicado entre ellas se midió utilizando cuerdas vibrantes (vibrating wire strain gauges, VWSG). Además las deformaciones en las pletinas CFRP y hormigón fueron medidas durante dos pruebas de carga con un camión de 245 kN, ubicado en la pista en voladizo. Los resultados en terreno se complementaron con ensayos de laboratorio enfocados en el comportamiento último del refuerzo. Los ensayos de laboratorio se realizaron en 4 vigas de hormigón armado (dos vigas de control y dos reforzadas con pletinas CFRP) frente a carga monotónica. Adicionalmente, para estudiar la influencia de la temperatura en el desempeño de la técnica en términos de adherencia. Dos vigas reforzadas con CFRP se ensayaron frente a carga monotónica y sometidas a temperaturas cercanas al punto de transición vı́trea (Tg) del epóxico. Para estudiar la adherencia del sistema se realizaron ensayos pull-out en cubos de hormigón reforzados con pletinas CFRP NSM, embebidas en distintas longitudes, y se medieron las deformaciones de las pletinas y su deslizamiento. Además se realizaron ensayos Dynamical-mechanical analysis (DMA) al epóxico para estudiar su compotamiento frente a cargas cı́clicas (DMA - Modulus test) y de creep (DMA - Creep) a altas temperaturas. Durante el perı́odo de monitoreo de la losa de hormigón en terreno, las deformaciones en el hormigón aumentaron aproximadamente en 40 µ; lo cual demuestra que la losa en voladizo está sufirendo creep debido a las cargas muertas superimpuestas durante el refuerzo.. xx.

(21) El refuerzo CFRP NSM en vigas de hormign incrementó su carga última entre 13 y 26% sobre las muestras de control, y la carga de fluencia se incrementó en un 12%. El modo de falla observado en laboratorio fue rotura de la pletina; lo cual indica que la transferencia de esfuerzos entre la pletina y el hormigón que la rodea es adecuada; puesto que es posible desarrollar la capacidad última del CFRP. Sin embargo, el modo de falla observado en vigas reforzadas con CFRP cambió cuando la temperatura se acercó al Tg del epóxico; para dichas vigas se produjo falla por adherencia por corte en la interfaz epoxico-hormigón, y no se observó ruptura de la pletina. Temperaturas cercanas a Tg generaron una diminucin de la rigidez elstica de las vigas de un 17%y en la resistencia del hormign. Además frente a temperaturas cercanas a Tg se produjo un incremento de la carga de fluencia de un 24% y las deformaciones de fluencia de un 50%, respecto de las viguetas de control sin refuerzo a temperatura ambiente. En los ensayos de adherencia, o pull-out, la falla ocurrió a una carga similar a la reportada por el fabricante, para longitudes embebidas mayores a 75 mm. Sin embargo, para menores longitudes sólo se desarrolló un 60% de la capacidad última de las pletinas. El modo de falla observado para todas las muestras fue rotura de la pletina a diferentes alturas medidas desde la superficie del hormigón. Además se observó ”falla como escoba” (”bromlike failure”) en algunas probetas, lo cual es señal de falla en la matriz que une las fibras dentro del sistema CFRP. El deslizamiento medido en la superficie del hormigón, descontando deformaciones elásticas de la pletina fue insignificante; por lo cual fallas por adherencia fueron descartadas. Aunque se observó falla en la interfaz epóxico - hormigón, en el extremo libre de algunas muestras, se concluyó que la adherencia que provee el epóxico es suficiente para desarrollar la capacidad última de las pletinas. La temperatura de transición vı́trea promedio medida fue 49 ◦ C durante el desarrollo de la primera serie de DMA; sin embargo, el ciclo de calentamiento al cuál fueron sometidas las muestras durante el ensayó brindó curado adicional; generando un incremento de Tg entre 2 ◦ y 6 ◦ .. xxi.

(22) Cuando las muestras fueron ensayadas bajo carga sostenida (DMA-creep) alcanzaron su deformación máxima a temperaturas cercanas a 35 ◦ C. Además presentaron deformaciones mayores a las reportadas por el fabricante. En resumen, la técnica de refuerzo NSM utilizando pletinas CFRP permite incrementar la capacidad estructural de elementos de hormigón reforzados. La adherencia que provee este refuerzo es adecuada para temperaturas de servicio, pero bajo temperaturas cercanas a Tg, las propiedades mecánicas del epóxico disminuyen y la transmisión efectiva de esfuerzos entre CFRP y hormigón no se garantiza.. Palabras Claves: fibras de carbono, near surface mounting, refuerzo, deformación, vigas, puente, hormigón, adherencia. xxii.

(23) 1. INTRODUCTION. 1.1. Characterizing Fiber Reinforced Polymers (FRP) Over the past decade the concepts of retrofitting and reinforcement of existing structures have gained more importance. The main reasons are: the resources available are limited, the concept of recycling are becoming more recognized by the population, the high costs of reconstruction, time and space constraints, preserve the heritage represented in existing structures. All over the world there is highway infrastructure aging and deteriorating over time. There are many causes of deterioration, from environmental degradation and lack of maintenance, increase in service loads, to errors made during design or construction phases. Also population growth of cities has increased the demand for transportation facilities. To solve these problems, new techniques for existing structures rehabilitation must be developed and understood properly. Fiber reinforced polymer (FRP) is a composite material, the mayor constituents are the reinforcing fibers and a matrix, which acts as binder for the fibers. To manufacture these composites a large number of fibers, in a unidirectional orientation for strips, are incorporated in a matrix to form a lamina (Mallik, 1993). The fibers occupy the largest volume fraction in the composite, and is the structural element. The primary function on the fibers is to carry tensile stresses to provide strength and stiffness, depending on their orientation. The matrix’s function is to to hold the fibers together, to ensure shear stress transfer between CFRP and concrete. The ultimate strain provided by the matrix must be higher than the ultimate strain of the fibers, to ensure that matrix could carry loads after fiber failure (De Paula, 2005). In FRP materials the fibers have a brittle behavior, while the matrix exhibits a ductile behavior, as shown in Figure 1.1. FRP is an anisotropic material, and the characteristics of the strips depend on many factors as fiber volume, type of fiber, type of resin matrix, fiber orientation, dimensional 1.

(24) F IGURE 1.1. Stress-strain curves for FRP materials.. effects, and quality control during manufacturing (ACI440, 2006). The type, amount and orientation of fibers influences the specific gravity, tensile strength and modulus, compressive strength, fatigue strength and fatigue failure mechanisms, electric and thermal conductivities and cost (Mallik, 1993). The matrix used in FRP is commonly made of polyester, epoxy or nylon. Commercially available, the fibers are made of aramid (AFRP), glass (GFRP) and carbon (CFRP). The name CFRP is given by the arrangement of hexagonal molecule located between the layers of the graphite. If the layers had 3D arrangements, the fiber material is defined as graphite. If the bonding between the layers is weak and has two-dimensional arrangements, the material is carbon (De Paula, 2005). Two types of CFRP are fabricated: high modulus of elasticity CFRP (near 200 GPa), named as Type 1, and high resistance CFRP, named as Type 2 (De Paula, 2005). The CFRP strips used in this investigation has a modulus of elasticity of 131 GPa, and tensile strength of 2068 MPa; hence, they are Type 2 CFRP. 2.

(25) The Table 1.1 from ACI440 (2006). shows some properties for FRP bars made from different fiber materials.. 3.

(26) TABLE 1.1. FRP properties ACI440 (2006).. Densities. CTE x10-6/C. Nominal. Tensile. Elastic. Yield. Rupture. strength. Modulus. strain. strain. (Mpa). (Mpa). (Gpa). Longitudinal (L) Transverse (T) Yield Stress (gr/cm3) Steel. 7.9. 11.7. 11.7. 276 to 517. 483 to 690. 200.0. 0.14 to 0.25 6.0 to 12. GFRP 1.25 to 2.1. 6.0 to 10.0. 21.0 to 23.0. N.A. 483 to 1600. 35.0 to 51.0. N.A. 1.2 to 3.1. CFRP. 1.5 to 1.6. -9.0 to 0.0. 74.0 to 104.0. N.A. 600 to 3690. 120.0 to 580.0. N.A. 0.5 to 1.7. AFRP 1.25 to 1.4. 6.0 to -2.0. 60.0 to 80.0. N.A. 1720 to 25400. 41.0 to 125.0. N.A. 1.9 to 4.4. 4.

(27) Form Table 1.1 it can be seen that thermal expansion coefficient of CFRP is negative, which means when temperature increases the material contracts and when temperature decreases CFRP expands. CFRP bars have the highest tensile strength and elastic modulus; however they showed the stiffest behavior and the lower rupture strain. Also CFRP bars have a smaller cross-sectional area than GFRP bars and requires a smaller groove to be installed (De Lorenzis & Teng, 2007). Figure 1.2 shows the stress-strain curves for the FRP bars made from different types of fiber. It is reasonable to consider that the properties vary in similar proportions for FRP strips.. F IGURE 1.2. Stress-strain curve for FRP composites.. FRP’s offers advantages over conventional construction materials: its an excellent noncorrosive material, presents a high stiffness-to-weight ratio and strength-to-weight ratio, non-magnetic properties, ease of transportation and handling (Tljsten, n.d.; Barros & Fortes, 2005). Other advantages of FRPs include a low thermal expansion coefficient, good fatigue performance and damage tolerance, low energy consumption during fabrication (Tljsten, n.d.). Also has unlimited availability in size, geometry and dimension (Barros, Ferreira, Fortes, & Dias, 2006) . 5.

(28) Durability of FRP materials tends to be more complex than that of steel reinforcement, because degradation of the material could depend both on resin and bers and on their interface bond behavior. Also concrete could be an unfavorable environment due to alkali and moisture absorption (Ceroni, Cosenza, Gaetano, & Pecce, 2006). The characteristics of the resin that could reduce durability are: absence of cracks, absence of voids, resin’s degree of cure, resistance to alkali, toughness, impermeability and compatibility with fibers. Fibers provide stiffness and strength to composite materials; hence the performance of structural systems depends on their main mechanical properties and durability behavior (Ceroni et al., 2006). AFRP and GFRP bars are susceptible to changes in their properties due to environmental influences, but CFRP bars are less sensitive. Under service temperatures (-20 ◦ C to +60 ◦ C) a slight reduction in Youngs modulus occurs for AFRP and GFRP, and is negligible for CFRP. Also CFRP and AFRP are insensitive to chloride ions but GRFP can be seriously damage in marine environment (Ceroni et al., 2006). Silva and Biscaia (n.d.) tested concrete beams reinforced with externally bonded CFRP and GFRP strips after temperature cycles and immersion tests. For temperature cycles, ultimate capacity was reduced 31% and 20% for GFRP and CFRP beams respectively. Salt fog cycles caused considerable degradation in bond between FRP and concrete causing interface concrete-adhesive failure. Aldajah, Al-omari, and Biddah (2009) tested CFRP sheets under flexure after UV exposure at 60 ◦ C and obtained slight reductions in modulus of elasticity from 11.8% to 14.3%. However Ceroni et al. (2006) observed that UV exposure cause 13% and 8% of reduction in tensile strength for AFRP and GFRP respectively and no reduction for CFRP rods. Micelli tested CFRP bars under tension, after been subjected to the combination of alkaline environment, high temperature, humidity and UV exposure. Test results indicated that CFRP rods experienced a reduction of 8% in tensile strength and 30% of apparent horizontal shear strength; hence, CFRP specimens had a good retention of mechanical. 6.

(29) properties, and did not show weakness against alkaline solution; even if fluid penetration caused crack at fiber/matrix level. A number of research projects have studied the performance of bonded FRP strengthening systems under fire. The matrix polymer in a prefabricated FRP has a higher transition temperature (Tg) than the bonding adhesive, both below temperatures expected in a fire. Stratford et al. (2009) evaluated the behavior of a concrete slab reinforced with CFRP plates and CFRP rods, under fire test. The plates were completely separated from the concrete, even with an intumescent coating. The adhesive around the CFRP bar was burnt away, leaving the FRP partially exposed. In all cases bondline temperature rapidly exceeded the Tg of the adhesive.. 1.2. Near Surface Mounting (NSM) Reinforcement Techniques The most common method of reinforcement has been to place sheets or laminates externally bonded to the surface; however, further development has shown that it is favorable to place the FRP in the concrete cover, called near the surface mounting (NSM) technique. NSM strips installation starts by cutting 4 mm width and 20 mm depth-grooves in the concrete surface. The grooves are cleaned by using compressed air; after grooves are fully dried they were half-filled with an epoxy matrix used to bond CFRP strips into concrete. Due to I-shape cross section of the strips, some air voids were observed under the upper flange between the strip web and concrete during the trial stage. In order to avoid such effect, strips were coated with epoxy before installation. Then epoxy-coated strips were placed inside the groove half filled with the same epoxy and they were slightly pressed. Afterwards some more epoxy is applied from the top of the groove until the surface is leveled. Near surface mounting (NSM) application showed great advantages in terms of durability and flexural strengthening compared to other installation techniques: the surrounding concrete protects the FRP so that mechanical and thermal damage in unlikely. Other advantages include improved bond and bond transfer with surrounding concrete and the ability to 7.

(30) increase negative bending (Yost et al., 2007). Also NSM does not require extensive surface preparation compared with other techniques (Badawi & Soudki, 2009). NSM technique becomes attractive for flexural strengthening in the negative moment in slabs, because externally bonded reinforcement (EBR) would be subjected to mechanical and environmental damage and would require protective cover (Hassan & Rizkalla, 2003).. 1.3. Bond Behavior NSM CFRP strips have a strong and stiff behavior compared to CFRP bars embedded in concrete. In NSM technique, tensile forces are kept within the plane of the concrete, because the strips are thin compared o their width. In an embedded bar bond stresses create forces which push the bar out of the concrete surface (Blaschko, 2003), as shown in Figure 1.3. Also narrow strips maximize the surface area-to-sectional area ratio for a given volume and thus minimize the risk of debonding (Galati & Lorenzis, 2009).. F IGURE 1.3. Schematics of the bond behavior of NSM FRP reinforcement.. In flexural applications FRP is still likely to be governed by premature debonding mechanism, so the performance of this reinforcing system depends on the bond between CFRP and concrete, provided by the epoxy resin. Hence the properties of the epoxy used 8.

(31) as bonding material must be understood and pull-out test must be performed to characterize bond behavior. Three failure modes have been identified in previous bond studies: bond failure at the bar-epoxy interface (BE), bond failure at the epoxy-concrete interface (EC) and splitting of the epoxy cover (SP). BE failure mode may occur as either pure interfacial failure or as cohesive shear failure in the groove filler material. EC failure mode may occur as pure interfacial falure or as cohesive shear failure. SP failure mode is referred to longitudinal cracking of the groove filler material and/or fracture of the surrounding concrete along inclined planes (De Lorenzis & Teng, 2007). Another failure mode observed by Adimi, Habib, and Banmokrane (2000) was fracture by a ”broomlike” tensile failure of the rod, as shown in Figure 1.4. Iit can be seen that fracture in the CFRP strip in some cases occurs in the matrix polymer in a prefabricated CFRP composite. This failure mode is called broomlike failure.. F IGURE 1.4. Broomlike failure on CFRP bars.. 9.

(32) In bond test the fracture initiated at the loaded end in the bond zone propagates to the embedded length with increasing load (Chang et al., 2010; Teng et al., 2006, Blaschko, 2003) . Microcracking at the strip-epoxy interface and the slip of the FRP strip tend to produce a more even distribution of the bond stress (Teng et al., 2006). The failure modes obtained by Teng was failure at the CFRP-epoxy interface. When NSM technique is developed on field, the influence of some construction parameters must be considered to evaluate bond performance; such as groove depth and width. On field applications, the installation procedures are less carefully developed and the variables are not in control as in laboratory. De Lorenzis et al. (2004) studied the effect of the groove width, the bond length and the filling material in the ultimate load of the pull-out test; and obtained that ultimate load of the joint increases as the bond length increases. The effect of the groove size depends of the failure mode of the specimen: when failure is in the epoxy-concrete interface, average bond strength decreases as the groove size increases; for splitting failure bond strength increases for the increasing groove. For a given bonded length, the average bond strength generally increases for increasing groove size, as the bigger cover depth delays the occurrence of splitting (De Lorenzis & Teng, 2007). When the NSM technique is used to install CFRP strips, a third material must be analyzed to evaluate bond performance of the system: the groove filling material. The most common and best performing groove filler is a two-component epoxy (De Lorenzis & Teng, 2007). Other techniques have been developed with cement-filled grooves, but epoxy offers superior mechanical performance as groove filler (De Lorenzis et al., 2004). Cement mortar has inferior mechanical properties and durability, with a tensile strength one order of magnitude smaller than that of common epoxies (De Lorenzis & Teng, 2007). The adhesives are physically comprised by two essential components: an epoxy resin and a curing agent or hardener. The adhesive base or epoxy resin is the principal component of the adhesive and provides many of the characteristics such as wettability, curing properties, strength and environmental resistance. The curing agent or hardener is a substance. 10.

(33) added to an adhesive to promote the curing reaction by taking part in it by chemically combining with the base resin; which turns the epoxy resin into a solid, crosslinked network of molecules (Petrie, 2006). The epoxy are characterized by the presence of an oxyrane or an epoxy ring, an epoxy resin can be any molecule containing more than one of these epoxy groups. The amine curing agents are one of the most common types of curing agents of epoxy resins (Petrie, 2006). When these compounds are mixed together, the amine groups react with the epoxide groups to form a covalent bond. An example of an epoxy reaction is shown in Figure 1.5.. F IGURE 1.5. Reaction of an epoxy resin.. Often the curing agent becomes an integral part of the resulting compound. Its choice is a controlling influence on the curing properties of the mixture and on the performance properties of the cured adhesive. Polyamides have also been used to provide flexibility to epoxy base resin (Petrie, 2006). The advantages of epoxy resin used as bonding material are: low cure shrinkage, strength and durability, adhesion, corrosion and chemical resistance, and electric insulation. Also resin systems are capable of curing at either ambient or elevates temperatures and can be applied and cured under many adverse conditions including outdoors (Petrie, 2006). The performance of the NSM CFRP reinforcement technique depends of the time provided for the curing of the epoxy adhesive prior testing. Depending on the epoxy resin 11.

(34) and curing agent used, room temperature curing adhesive formulations can harden in several minutes at room temperature, but most systems require from 18 to 72h to reach full strength (Petrie, 2006). The behavior of the NSM system strongly depends on the properties of the adhesive, such as permissible shear stress of the epoxy and the glass transition temperature (Tg). The epoxies used for NSM reinforcement are thermoset polymers, their molecules are chemically joined together by cross-links, forming a rigid, three- dimensional network structure. The thermoset polymers cannot be reshaped by application of heat (Mallik, 1993); however, Tg is an important parameter to consider for the safety of the structures. Above Tg, the stiffness of a polymer decreases significantly and the rate of time dependent deformations (creep) under stress increases dramatically (Bakis, 2008). Ferrier, Michel, Jurkiewiez, and Hamelin (2011) studied creep behavior of several epoxy adhesives used as bonding material between CFRP and concrete, with Tg ranging from 45 ◦ C to 80 ◦ C. Test results showed a decrease of the shear modulus from 20% to 70%, hence creep behavior is correlated to Tg. Also demonstrated that beyond a shear stress corresponding to 40% of the ultimate bonded strength, creep is linear. Finally he recommended the use of polymer adhesives with high shear moduli (> 10GP a) and high Tg (> 55 ◦ C). Galati and Lorenzis (2009) evaluated the influence of the mechanical properties of the groove filling material in bond performance. The epoxy type-b with tensile strength and elastic modulus 20% and 210% respectively higher than epoxy type-a, changed the failure mechanism in bond test from failure at the bar-epoxy interface to failure by fracture in the concrete along inclined planes. Also the average bond strength increased significantly. Mix ratios and their precision are also considered critical for optimum performance. By over-or underusing a polyamide curing agent, the resulting formulation will have more or less characteristics of the curing agent, but a usable adhesive system usually results. Ten percent by weight in excess of polyamide in a two-part epoxy formulation will result in a system with greater flexibility and bond strength but lower temperature and environmental. 12.

(35) resistance due to the flexible nature of the polyamide molecule. Ten percent by weight below of polyamide will provide higher shear strength and temperature resistance but lower bond strength (Petrie, 2006). Also, consideration of environments to which the final adhesive will be exposed to must be analyzed; such as: service temperature and thermal degradation of the adhesive, influence of water on the adhesive and interface, and chemical degradation under ultraviolet (UV), radiation, solvents, and other environmental media (Petrie, 2006). According to MacGregor (1997) the compressive strength of concrete diminishes approximately 5% approximately at 100 ◦ C and there is no effect in strength when temperature is 80 ◦ C. The tensile strength of steel also diminishes in the same amount at 100 ◦ C and the reduction is negligible near 80 ◦ C. Under service temperatures (-20 ◦ C to +60 ◦ C) reduction in Youngs modulus is negligible for CFRP (Ceroni et al., 2006). Therefore any decrease measured in the mechanical properties and the bond performance of NSM technique should be caused by the degradation of the epoxy matrix or the CFRP strip. Hence, the performance of NSM technique, especially the behavior of the epoxy under service temperatures must be studied.. 1.4. Monotonic and Fatigue Performance of NSM reinforcement technique Among the studies concerning the NSM CFRP reinforcement performance in the field, the behavior of concrete beams under fatigue and monotonic loads has been reported previously (Barros & Fortes, 2005; Barros et al., 2006; Aidoo, Harries, & Petrou, 2006; Yost et al., 2007; Badawi & Soudki, 2009; Wang et al., 2008,Al-Mahamoud, Castel, Franoise, & Tourneur, 2009; Teng et al., 2006; Hassan & Rizkalla, 2003; De Lorenzis & Teng, 2007). The performance of concrete beams under cyclic loads recreate on field behavior of the concrete bridge under fatigue loads. Badawi and Soudki (2009) evaluated fatigue behavior of concrete beams strengthened with CFRP rods. The beams failed near 200.000 cycles under load 10% of the ultimate capacity. Test results of the reinforced beams were compared with control specimens, and obtained that CFRP reinforcement decreases steel 13.

(36) deformations and concrete deflections during cycles and increases 24% the amount of cycles without failure. Klowak and Mufti (2008) studied the performance of a cantilever bridge slab reinforced using CFRP rods and they did not observe failure in reinforced specimens until 600.000 cycles, under load 60% of the ultimate capacity . Aidoo et al. (2006) tested reinforced beams with NSM CFRP strips under cyclic load and demonstrated that 2.000.000 cycles did not affect ultimate capacity of the specimens compared to monotonic load test, but developed permanent deflections. Concrete beams reinforced using NSM CFRP and tested under monotonic load exhibit superior flexural behavior (yield load and ultimate load) compared to unreinforced specimens (Barros & Fortes, 2005; Barros et al., 2006; Yost et al., 2007; Wang et al., 2008; Al-Mahamoud et al., 2009). Other experimental results showed an increase in ultimate load and yield load (Badawi & Soudki, 2009; Wang et al., 2008; Teng et al., 2006). Yost et al. (2007) tested NSM CFRP reinforced beams and observed that ultimate and yield loads increased from 11% to 30% with respect to unreinforced beams. Barros and Fortes (2005),Barros et al. (2006) tested beams reinforced using CFRP NSM strips and observed that reinforced beams increased ultimate loads by 22% to 39% and yield loads by 78 to 91% with respect to unreinforced beams. Al-Mahamoud et al. (2009) increased the ultimate load of control beams using CFRP reinforcement 50% at least, and observed an increase in cracking load from 5.7% to 75%. Klowak and Mufti (2008) evaluated a concrete slab reinforced with different materials: steel, GFRP and CFRP, under monotonic load test. Test results showed similar ultimate loads between GFRP and steel, the percentage difference between the two specimens was about 5%, lower for the CFRP reinforced specimen. Also steel reinforced specimens showed lower deflections than GFRP and CFRP reinforcement, 12,7% and 38,6% lower respectively . The ductility achieved for concrete beams is an important issue when reinforcing using CFRP, because this material presents a brittle behavior compared to that of a steel. CFRP 14.

(37) under tension has a linear fragile stress-strain behavior until rupture occurs (De Paula, 2005). Ductility is even more important for statically indeterminate structures, such as continuous beams, as it allows for moment redistribution through the rotations of plastic hinges. Moment redistribution permits the utilization of the full capacity of more segments of the beam (Akbarzadeh & Maghsoudi, 2006). Studies carried out on ductility behavior of structures reinforced using CFRP materials have obtained different results: Yost et al. (2007) and Badawi and Soudki (2009) measured lower values for specimens reinforced with NSM CFRP of 6.6% to 22% and 30% respectively. On the other hand, Aidoo et al. (2006) observed an increase of 9.7% in ductility for NSM strips reinforced specimens compared with control specimens; however, a decrease in ductility of 35% to 50% was measured for beams reinforced with strips bonded externally. Hence, there is no agreement on whether NSM CFRP increase or decrease ductility. With respect to ultimate capacity, Barros et al. (2006) measured ultimate strains of CFRP strips in concrete beams reinforced with NSM CFRP technique tested under flexure. Strains ranged from 62% to 91% of the CFRP strips ultimate rupture strain showing this strengthening technique has high level of effectiveness. Different and combined failure modes have been reported for concrete beams reinforced with CFRP. The mode of failure observed by Aidoo et al. (2006) and Al-Mahamoud et al. (2009) was concrete crushing after yielding of the steel reinforcement followed by failure in the CFRP reinforcement. However Badawi and Soudki (2009) did not report any failure in the CFRP rod. There are many variables that affects reinforced concrete beams performance: AlMahamoud et al. (2009) studied the influence of strengthening length and obtained that a supplementary anchorage length led to an increase in deflection and bending moment. Failure mode observed was dependent on embedment length; if the strengthening length was sufficient to avoid the intersection between the shear cracks initiated by a bending crack and the CFRP rods end, the failure mode changed from failure by concrete peel-off to failure by pull-out of CFRP rods.. 15.

(38) Concrete strength also has an influence in the behavior of reinforced specimens. The concrete strength does not influence the load-carrying capacity of the strengthened beam when the failure occurs by pull-out of the NSM system failure,but concrete strength less than 37.5 MPa led to concrete crushing failure mode without CFRP pull-out (Al-Mahamoud et al., 2009).. 1.5. NSM Reinforcement on Centenario Bridge Over the last decade several applications of FRP for reinforcing concrete bridges have been developed; some examples of GFRP embedded bars reinforcement are the Floodway Bridge in Winnipeg, Canada, the middle road over Crow Creek bridge in Iowa, USA, and the Arched Pedestrian bridge in Ottawa Canda. However a few applications of NSM CFRP strips on concrete bridge are known, an example is the Martin Springs Bridge in Missouri, USA. In Chile NSM CFRP strips reinforcement technique has been developed recently on Centenario Bridge concrete slab. Centenario Bridge was built in 1987 and had three spans of 61.1 m, 80.0 m, and 64.1 m each. The bridge structure was comprised of two postensioned concrete box girders with a common 27-m wide top flange with flexible pavement structure built atop. Box Girders cross section varied according to the flexural moment (Lopez, Santa Maria, & Rodriguez, 2008). Due to the expected traffic increase, the Ministry of Public Works decided to increase the lanes of Centenario Bridge from six to eight by eliminating the sidewalks and building a separate pedestrian bridge parallel to Centenario Bridge (see Figure 1.6). The new traffic lanes were placed over the cantilever slab (top flange of the box girders) and the existing metallic barrier was replaced by a New Jersey Barrier. The strength in the reinforced concrete cantilever slab was exceeded at some locations, and CFRP strips were considered to reinforce the cantilever slab. CFRP strips were preferred over steel reinforcing bars because of its relatively small dimensions of CFRP strips (2 x 16 mm) and shape that allowed for installation in the existing concrete cover. 16.

(39) F IGURE 1.6. Centenario Bridge cross section.. 17.

(40) Two lengths of CFRP strips were alternated (2750mm and 1150 mm), spaced 170mm apart. The reinforcement was placed on the surface of the slab, in the negative moment region, to improve flexure capacity.. 18.

(41) 2. SUMMARY OF CONDUCTED STUDY. 2.1. Background The existing knowledge of the NSM CFRP strips reinforcement technique is much more limited than that on the externally bonded FRP method, as reflected by the absence of provisions in the existing guidelines: ACI440 (2006). During the last decade the use of CFRP strips in existing structures using NSM technique has increased, some examples are Martin Springs Bridge in Missouri, USA, and Maple Leaf Meats fabric slab in Montreal, Canada. There is no quantitative data available for these applications. Strengthening concrete beams using NSM CFRP strip reinforcement has shown excellent performance compared to externally bonded laminates; due to its improved bond capacity and durability; however, the behavior of this technique on field in concrete structures has not been studied. The only study available was developed by Abdessemed et al. (2011); a concrete bridge reinforced using CFRP was evaluated using a dynamic analysis by ambient excitation, but the repaired techniques used were concrete jacketing of piles and flexural and shear strengthening for the beams using CFRP sheets and laminates externally bonded. The composite material increased the transverse rigidity of the structure and its modal frequency. Concrete bridges are subjected to potential fatigue damage due to vehicular loads; hence, it’s important to study the fatigue behavior of this reinforcement technique. Also the cumulative damage must be considered, so long term performance must be monitored. Also, more test need to be conducted to further clarify the ductility achieved by concrete specimens reinforced using this technique, compared to unstrengthened specimens. The epoxy used as filling material, is the most susceptible material to environmental degradation and mechanical damage within the NSM CFRP technique. In flexure and bond tests the most frequent failure mode reported was failure at the epoxy-concrete interface, therefore the main research need in this area are related to the performance of this material.. 19.

(42) The interaction between temperature and fatigue loads must be analyzed. The creep behavior also must be studied to evaluate the effect of permanent loads, this mean the weight of all superstructure elements.. 2.2. Objectives and Methodology The main objective is to evaluate the performance of the NSM CFRP strips reinforcement technique used on field application in a concrete structure. The specific objectives are: • To evaluate on field behavior of the NSM CFRP strips reinforcement technique under service loads (cyclic and permanent loads). • To evaluate the ultimate behavior of the NSM CFRP strips reinforcement technique using laboratory tests. • To evaluate bonding performance of the NSM technique and to quantify the development length, which develops full capacity of CFRP strips. • To study the properties of the epoxy resin used as bonding material between CFRP and concrete and to evaluate its behavior under permanent and cyclic loads and temperature. • To study the effect of high temperatures in bond behavior of NSM technique used in concrete elements. • To study the behavior of the reinforced concrete structure in a global approach and explain their behavior studying the properties of the materials involved in a specific level.. 2.3. Hypothesis The main hypothesis is that reinforcement of concrete structures using NSM CFRP strips improves structural performance and provides an adequate bonding between concrete and CFRP at service temperatures lower than 35 ◦ C. 20.

(43) This hypothesis comprised two specific issues: • The NSM CFRP reinforcement technique used in concrete structures subjected to fatigue loads will exhibit a creep behavior through time, this mean a initial stage of strain rate relatively high followed by steady state, produced by superimposed permanent loads. • The performance of the NSM technique is limited by bond provided by the epoxy used to bond CFRP and concrete; which is the most vulnerable material to mechanical, thermal and deterioration. • The temperatures near Tg of the epoxy decreases its mechanical properties and led to permanent strains.. 2.4. Experimental procedure This study investigates the performance of the NSM CFRP reinforcement in field and in laboratory. The results were analyzed in a global approach focused in the behavior of the structural element, and in a specific level referred to the properties of the materials involved in the reinforcement system (CFRP, epoxy and concrete). Also, the experimental results were compared to the results obtained from analytical models. 2.4.1 Field Investigation • Strains in CFRP and concrete The field portion aimed to assess service performance (dead and live loads) of the NSM CFRP reinforcement behavior on Centenario Bridge, and considered measuring stress and strain in CFRP reinforcement and concrete surrounding the strips on the cantilever slab. Strains were measured within two years period from July 2008, before CFRP installation, to August 2010. • Load Tests 21.

(44) Two load tests were performed using a 245 kN truck placed at the cantilever reinforced lane of the bridge. The first one was developed on July 2008, before strengthening the slab, and the other one was performed on August 2008, after CFRP installation. Strains in CFRP and concrete were measured using the Electrical Resistance Strain Gages (ERSG) and Vibrating Wire Strain Gauges (VWSG) installed previously for monthly measurements. 2.4.2 Laboratory Investigation • Monotonic Test Laboratory portion aimed to assess the behavior of CFRP reinforcement under ultimate conditions. Four simply supported full-scale concrete beams were tested in flexure and material characterization of the CFRP, and concrete. Two control beams were steel - reinforced, and two beams had steel reinforcement and one CFRP strip NSM placed in the bottom side. • Monotonic Test and Temperature Two simply supported full-scale concrete beams reinforced using NSM CFRP strips were tested in flexure under the combined effect of monotonic load and high temperature. • Bond Test In order to characterize the bond properties of the NSM technique, a series of bond tests was carried out. Twenty NSM CFRP strips were installed in concrete blocks and pull-out test were developed. The bond length varied from 50 mm to 450 mm. • DMA - Modulus A Dynamical mechanical analysis (DMA) was used to determine changes in the mechanical properties of the epoxy resin as a function of temperature. Changes in the retained modulus were measured and the glass transition temperature was estimated. Five epoxy samples were tested under fatigue tensile load, to measure the effect of temperature in tensile modulus, for temperatures ranging from 0 to 100 ◦ C. 22.

(45) • DMA - Creep Samples with the same dimensions as the ones used for assessing the modulus deterioration, where used for calculating the creep deformation at elevated temperatures. 2.4.2 Analytical Model • Monotonic load test The ultimate capacity of the reinforced concrete control beams, and the reinforced concrete beams additionally reinforced with CFRP was calculated using some design assumptions proposed in ACI440 (2008) , De Paula (2005) and using Hognestad equations. Also, the strains on concrete on the concrete slab prior reinforcing were calculated using Hognestad equations, and the ultimate strength of the concrete slab after CFRP reinforcement.. 23.

(46) 3. BEHAVIOR OF A CONCRETE BRIDGE CANTILEVER SLAB REINFORCED USING NSM CFRP STRIPS Ariela Astorga a , Hernán Santa Marı́a b , Mauricio López a∗ a. Department of Construction Engineering and Management, School of Engineering,. Pontificia Universidad Catlica de Chile. Vicua Mackenna 4860, Casilla 30, Correo 22, Santiago, Chile.. b. Department of Structural Engineering, School of Engineering, Pontificia Universidad. Catlica de Chile. Vicua Mackenna 4860, Casilla 30, Correo 22, Santiago, Chile.. ∗. Corresponding Author. 3.1. Abstract Strengthening concrete beams using near surface mounting (NSM) carbon fiber reinforced polymers (CFRP) have shown excellent performance. However, behavior on real structures has not been deeply studied yet. This paper presents results, taken over a 2-year period, of a field investigation of the behavior of NSM CFRP reinforcement in a concrete bridge under service conditions. Also, four RC beams were tested under monotonic load to study behavior under ultimate conditions. Strains in concrete and CFRP were 42% and 6% of the ultimate capacity according to maximum strain measured in concrete, service loads developed 9% of the concrete slab ultimate strength, confirming that CFRP improved bridge performance. Keywords: Near surface mounting, carbon fiber reinforced polymer, concrete bridge, reinforcement, flexural strengthening, beam test. 24.

(47) 3.2. Introduction Over the past decade, concepts of retrofitting and reinforcement of existing structures have gained great importance because all over the world, the existing highway infrastructure is ageing and deteriorating. There are many deterioration causes such as: environmental degradation and lack of maintenance, increase of traffic flow on major highways, mistakes during design or construction phases. Under this challenging situation, the alternatives of repairing and retrofitting structures are quite attractive because reconstruction involves higher costs and major demands of time and available space, not to mention the possible impact on users. To repair and retrofit structures in this scenario, new techniques shall be properly developed and understood. Fiber reinforced polymers (FRP’s) offer advantages over conventional construction materials as it is a non-corrosive material, it has high stiffness-to-weight ratio and strength-to-weight ratio, it possesses non-magnetic properties, and it is easy to transport and handle (Barros & Fortes, 2005; Tljsten, n.d.). Other advantages of FRPs include: low thermal expansion coefficient, good fatigue performance damage tolerance, and low energy consumption during fabrication (Tljsten, n.d.). Also FRPs have unlimited size, geometry and dimension availability (Barros et al., 2006). Durability of FRP materials tends to be more complex than steel reinforcement corrosion, because material degradation could depend both on resin and bers and also on the bond behavior of their interface. Aramid FRP (AFRP) and glass FRP (GFRP) bars are susceptible to property changes due to environmental influences, but CFRP bars are less sensitive. Under service temperatures (−20 ◦ C to 60 ◦ C) a slight reduction in Youngs modulus occurs for AFRP and GFRP, while the change is negligible for CFRP. Ultraviolet rays cause 13% and 8% reduction of tensile strength for AFRP and GFRP respectively and no reduction for CFRP rods. Also CFRP and AFRP are insensitive to chloride ions but GRFP can be seriously damaged by marine environment (Ceroni et al., 2006). The performance of FRP materials (AFRP, GFRP, CFRP) under the action of fire remains a serious concern due. 25.

(48) to vulnerability of the bonding adhesive to temperatures above glass transition temperature (Stratford et al., 2009). A method of FRP strengthening that has been widely used is externally bonded carbon fiber reinforced polymer (EBR CFRP), being laminates the most common method. In EBR applications, the surface of concrete is cleaned and prepared and epoxy is applied in order to bond CFRP sheets to concrete. This technique has been investigated by many researchers; e.g., (Aidoo et al., 2006; Chami et al., 2009; Arduini & Nanni, 1997; Meshgin et al., 2009; Choi, Meshgin, & Taha, 2007; Akbarzadeh & Maghsoudi, 2006). Among those, some have focused on the failure mode of EBR on concrete beams, and they concluded that under monotonic load, the main failure mechanism would be debonding at adhesive-concrete interface (Chami et al., 2009; Arduini & Nanni, 1997). The maximum strain recorded in both investigations was one third of its ultimate value; which indicates that the FRP used in the EBR technique did not reach full capacity. Additionally, Arduini and Nanni (1997) also demonstrated that effectiveness of EBR strongly depends on the techniques used for surface preparation. They obtained reductions of approximately 19% in ultimate loads when beams were strengthened by using a poor surface cleaning before EBR was installed. In near surface mounting (NSM) technique CFRP is bonded into grooves in the concrete surface filled with epoxy. NSM CFRP has shown great advantages in terms of durability and flexural strengthening with respect to EBR technique because the surrounding concrete protects FRP so that mechanical and thermal damages become unlikely. Other advantages include improved bonding and bond transfer with the surrounding concrete and the ability to increase bending capacity when they are in compression (negative moment) (Yost et al., 2007). Also, NSM does not require extensive surface preparation as compared to other techniques (Badawi & Soudki, 2009). Even though NSM CFRP reinforcement poses as a promising technique, existing knowledge is still limited and even less than externally bonded FRP method; in fact ACI440 (2008) proposes design guidelines only for NSM techniques only from 2008.. 26.

(49) Limited field studies on NSM CFRP reinforcement concrete beams behavior under fatigue and monotonic loads has been previously reported (Barros & Fortes, 2005; Tljsten, n.d.; Barros et al., 2006; Aidoo et al., 2006 Yost et al., 2007; Badawi & Soudki, 2009; Wang et al., 2008, Al-Mahamoud et al., 2009; Teng et al., 2006; Hassan & Rizkalla, 2003; De Lorenzis & Teng, 2007). Badawi and Soudki (2009) evaluated fatigue behavior on rectangular FRP rods. The beams failed near 200,000 cycles under a load equivalent to 10% of the ultimate capacity, and CFRP reinforcement decreased steel deformations and concrete deflections during cycles, and allowed more loading cycles without failure. Concrete beams reinforced using NSM CFRP and tested under monotonic load exhibited superior flexural behavior (yield load and ultimate load) compared to unreinforced specimens (Barros & Fortes, 2005; Barros et al., 2006; Yost et al., 2007; Wang et al., 2008; Al-Mahamoud et al., 2009). Barros and Fortes (2005) and Barros et al. (2006) tested reinforced beams by using CFRP NSM strips and observed that reinforced beams increased their ultimate loads 22% and 39% and their yield loads between 78 and 91% with respect to unreinforced beams. Al-Mahamoud et al. (2009) increased the ultimate load of control beams using NSM CFRP reinforcement at least by 50%, and observed an increase of cracking load of 5.7% to 75%. Klowak and Mufti (2008) evaluated a concrete slab reinforced with different materials: steel, embedded GFRP and CFRP, under monotonic load test. Test results showed similar ultimate loads between GFRP and steel, the percentage difference between the two specimens was of 2% lower for the GFRP reinforced specimen, and 5% lower for CFRP. Also steel reinforced specimens showed 12.7% and 38.6% lower deflections than GFRP and CFRP reinforced specimens, respectively. Studies carried out on ductility behavior of structures reinforced using CFRP materials have obtained different results: Yost et al. (2007) and Badawi (2007) measured lower values of ductility for specimens reinforced with NSM CFRP of 6.6% to 22% and 30% respectively, compared to control beams. On the other hand, Aidoo et al. (2006) observed an increase of 9.7% in ductility for NSM strips reinforced specimens compared to control. 27.

(50) specimens. Hence, there is no agreement on whether NSM CFRP increases or decreases ductility. Regarding ultimate capacity, Barros Barros et al. (2006) measured ultimate strains of CFRP strips in concrete beams reinforced with NSM CFRP technique and tested under flexural strength. Strains ranged from 62% to 91% of the CFRP strips ultimate rupture strain, proving that this strengthening technique has a high effectiveness level. Different failure modes have been reported for reinforced concrete beams under monotonic load: concrete crushing after yielding of steel reinforcement followed by failure of the CFRP reinforcement (Aidoo et al., 2006; Wang et al., 2008), and concrete crushing without failure in the CFRP (Badawi & Soudki, 2009). Over the last decade the use of CFRP strips in existing structures using NSM technique has increased significantly; some examples are Martin Springs Bridge in Missouri, USA, and Maple Leaf Meats fabric slab in Montreal, Canada. However, there are no quantitative data available for these applications. To the authors best knowledge, the only study on field performance of CFRP reinforced structures was carried out by Abdessemed et al. (2011), where a reinforced CFRP concrete bridge was evaluated by means of dynamic analysis by ambient excitation; however, repair techniques involved in the study were concrete jacketing of piles and flexural and shear strengthening for the beams using externally bonded CFRP sheets and laminates. Summarizing, behavior of reinforced concrete beams using the promising technique of NSM CFRP strips has been studied by a few researchers over the last decade. Available laboratory studies have focused on monotonic and fatigue loads and they have shown an improvement of beams structural capacity. However, field performance of concrete structures reinforced by this technique requires further investigations.. 28.