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The mixed mode I-II crack propagation process in concrete subjected to quasi-static conditions has been extensively investigated. However, in engineering practice, many concrete structures are subjected to various loading rates. The rupture process is closely related to the loading rate. Therefore, the knowledge of loading rate effect on the mixed mode I-II crack propagation process in concrete is of significant application value. The purpose of this study is to predict the crack propagation in concrete under different loading rates and to evaluate the effect of loading rate on the characteristics of crack propagation in concrete by laboratory tests and numerical analyses. A total of 60 four-point shear concrete specimens with four different initial crack locations are investigated in the study. The mixed mode I-II crack propagation process in concrete is evaluated for loading rates of 0.012, 0.12, 1.2, 12 and 120 mm/min. The main parameters, such as the tensile strength, elastic modulus, Poisson’s ratio, mode I initial fracture toughness and fracture energy of concrete, are measured. The initial fracture toughness is used as a parameter of the crack propagation criterion. The finite element method (FEM) is used to simulate the load-crack mouth shear displacement (P-CMSD) curves and crack propagation paths for each loading rate, and the numerical simulation results are compared with the test results. It is shown that the tensile strength, elastic modulus, mode I initial fracture toughness and fracture energy all increase significantly with loading rate. The P-CMSD curves and crack propagation paths of four-point shear specimens under each loading rate agree well with the test results. It is also shown that the mixed mode I-II fracture process zone length of concrete at a given load increases with loading rate.

A new test method for measuring the complete load-displacement curve of concrete under mixed mode I-II fracture is presented in this paper. Three-point bending beams with a multi-segment notch in the midspan were selected for this purpose. The ratio of the stress intensity factors at the notch tip between mode II and mode I, KII/KI, varied with the notch-depth ratio of the first segment and the inclination angle of the last segment. Compared with the available testing methods, it was less challenging to follow the proposed procedure to obtain the entire load vs. displacement curves for the KII/KI ratio up to 1.23. By introducing the finite element method, the entire crack propagation process was reproduced based on the crack propagation criterion for a mixed mode I-II crack in concrete. The predicted crack trajectories, the load vs. crack mouth opening/shear displacement curves, the initial cracking loads, and the peak loads were validated against the experimental results. The results agreed remarkably well.