The Fracture Properties Of CFRP Strengthened Cracked Concrete Beam

Fiber reinforced plastic (FRP) materials have been widely used in the strengthening of engineering structures. The regular service of FRP-strengthened concrete structures has been attracted much interest and many achievements have been made all over the world. However, owing to some unfavorable effects, there usually exist many initial cracks in concrete structures and needed to be reinforced. Thus, it is inadequate and unsuitable to evaluate the safety of this type of concrete structures based on the traditional classical strength theory. Therefore, based on the fracture mechanics theory, by the combination of experimental and analytical methods, the fracture properties of CFRP reinforced cracked concrete beams is studied in this paper. The failure mechanisms of the reinforced concrete structures with initial cracks are revealed. Furthermore, the contribution from steel bar or CFRP to the arresting of concrete crack and the effects of concrete crack propagation to the bond properties of CFRP/concrete or steel bar/concrete interfaces are all concerned. The details of the investigation in this dissertation are introduced as follows:(1) Three-point bending tests are respectively carried out on concrete beams and steel reinforced concrete beams with initial cracks at midspan. The effects of specimen size and relative initial crack length on the fracture properties of specimens are examined. By this way, the contributions from steel bar to load capacity and the capability of resisting crack growth of specimens are revealed. Furthermore, the stress intensity factors of feature points on the measured load versus crack mouth opening displacement curves (P-CMOD curves) are computed by use of the nonlinear fracture mechanics based method.(2) Considering the complex influences of concrete crack propagation and steel/concrete interfacial bond-slip properties, an analytical method is proposed to predict the whole fracture process of a steel reinforced concrete beam with an initial crack at midspan. Based on the fracture mechanics and linear elastic theory, the relations among the applied load, stresses in steel bar and concrete, concrete crack propagation, crack mouth opening displacement, shear stress in steel/concrete interface are derived. Then the fracture process of specimen is discussed considering two cases. Thus, the interrelations between concrete crack propagation and steel/concrete interfacial bonding characteristics are revealed. The results indicate that a good agreement is achieved between experimental and analytical results. In addition, the effects of specimen size, initial crack length, steel ratio on the fracture properties and load capacity of steel reinforced concrete beams are discussed in detail. (3) Based on the cohesive stress in concrete fracture process zone, the relative slip between steel/concrete interface, and the position of application point of tension force in steel bar, a method is proposed to compute the crack extension curve (KR resistance curves) of steel reinforced concrete beam with initial crack at midspan. A computing program is thus developed and the KR resistance curves of steel reinforced concrete beams with initial cracks at midspan are obtained. Then the effects of specimen size and initial crack length on the KR resistance curves are discussed by the proposed method.(4) A series of three-point bending tests are carried out to examine the fracture properties of cracked concrete beams externaly bonded with CFRP sheets. The failure process of specimen is described to reveal the comprehensive effects of CFRP/concrete interfacial debonding and concrete crack propagation. The contributions from CFRP sheet to resisting concrete crack propagation and bearing capacity of specimen are concerned by the tests. Based on the fictitious crack model, the effective crack length and the stress intensity factors of feature points on the measured P-CMOD curves are computed, and the size effect of the obtained stress intensity factors is analyzed. Simultaneously, the ductility evaluation method similar to that adopted for flexural toughness is used to evaluate the ductility characters of specimens based on the measured load deflection curves. The effects of specimen size and initial crack length on the fracture properties and ductility are discussed in detail.(5) Considering the complex influences of CFRP/concrete interfacial properties and concrete crack propagation, an analytical method is proposed to predict the whole fracture process of a cracked concrete beam externally bonded with CFRP sheet. Based on the measured material parameters in the test, some variables such as the maximum load, crack propagation length, CMOD, fracture process length, softening length and debonding length of CFRP/concrete interface, maximum interfacial stress and tension stress in CFRP sheet, et al. are determined. The comparisons between analytical results and experimental results, and corresponding discussion are carried out. Further investigation based on the numerical method is developed to verify the proposed analytical method. The failure mechanism of specimens is thus revealed. Moreover, the effects of specimen size, initial crack length, width and thickness of CFRP sheet on the fracture properties and load capacity are analyzed.(6) A method for computing the crack extension resistance curves of CFRP-strengthened cracked concrete beams is proposed. In this method, exponential constitutive relation is used to evaluate the effect of cohesive stress in concrete, and the bilinear constitutive relation is used to consider the bond-slip property in CFRP/concrete interface. Under monotonic loading, some critical variables, such as the effective crack length in concrete, tension force in CFRP sheet and the action point of resultant force, are determined. Based on the proposed method, a computing program for determining the KR resistance curves is developed. Substituting the measured parameters from tests, the KR resistance curves of CFRP strengthened cracked concrete beams are obtained. At last, the effects of specimen size and initial crack length on the KR resistance curves are discussed in detail.