A Comparative Study For Mode?Crack Propagation Criterion Of Concrete

In the fracture analysis of concrete structures, it is crucial to use a crack propagation criterion with high accuracy and wide range of application to describe the whole fracture process of concrete. The crack propagation criterion reflects the relationship between the crack tip stress field and extension resistance of concrete during the crack extension process, which can be helpful to analyze and describe the overall crack propagation process. In this paper, mode I crack propagation criterion of concrete is studied by numerical simulation, experimental measurement and analytical calculation. A comparative study is carried out to investigate the fracture properties of concrete with different strength grades and initial crack depth ratios. The main work is as follows:(1) An experimental investigation on the double-K fracture parameters and fracture energy of ultra high strength concrete is presented. The compressive strength is up to 115.8MPa. The dimension of three-point bending concrete beam specimens is 480mm ×120mm x 60mm. The initial fracture toughness KiniIC, unstable fracture toughness KunIC and fracture energy Gf were calculated from the initial cracking load Pini, the maximum load Pmax and critical crack mouth open displacement CMODc, etc. The result indicates that there is no obvious relationship between the double-K fracture parameters and the initial crack-length ratios for ultra high strength concrete. When the initial crack-depth ratio is identified, the double-K fracture parameters and fracture energy increase as the compressive strength increases.(2) In order to analyze mode I crack propagation of concrete, three crack propagation criteria, which characterized by initial fracture toughness, maximum tensile stress and nil stress intensity factor at the tip of the crack, are introduced to derive the load-crack mouth opening displacement curves (P-CMOD) of three-point bending beams. The calculated P-CMOD curves calculated are compared with the experimental results for concrete with different strengths including C20, C40, C60, C80 and C100. The results indicate that peak load and the P-CMOD curves calculated using the crack propagation criterion characterized by initial fracture toughness have the best agreement with the experimental ones. With the increasing of concrete strength, the P-CMOD curves calculated by the crack propagation criteria characterized by maximum tensile stress and nil stress intensity factor at the tip of the crack show an obvious deviation from the experimental curves; while the P-CMOD curves calculated by the crack propagation criterion characterized by initial fracture toughness are close to the experimental results. The results indicate that the crack propagation criterion characterized by initial fracture toughness is more suitable to analyze the fracture process of concrete beams with different strengths.(3) To analyze the crack extension resistance, the KR curves based on cohesive force and the KR curves based on crack propagation criterion are two widely adopted types of calculation methods. The above two methods are applied to calculated the KR curves of three point bending beams of different strength grades and initial crack depth ratios. The results show that for the concrete of low compressive strength, there is little difference between the KR curves calculated based on cohesive force or based on crack propagation criterion. But for the high strength concrete, the differences become apparent. This results from using different ways to determine the initial fracture toughness and length of fracture process zone (FPZ). Generally speaking, the KR curves based on crack propagation criterion needs less parameters, saving the computation time and improving the computational efficiency, which makes it have a wider foreground of applications.