Abstract / Summary
Abstract The stage-dependent evolution of the fracture process zone (FPZ) in mixed mode I-II crack of concrete after low-level sustained loading was investigated. Concrete beams with five initial mode mixity ratios were subjected to sustained three-point bending or four-point shearing loading at two loading levels for 90 days. The specimens were subsequently loaded quasi-statically to failure. The digital image correlation technique was employed to monitor crack propagation behavior. A time-dependent finite element model incorporating concrete creep and cohesive crack behavior was further established for energy analysis. The results revealed that sustained loading exerted a clear stage-dependent influence on FPZ evolution. During the early post-initiation phase, the FPZ of creep specimens was generally activated earlier and exhibited slightly larger lengths than that of aging specimens. During the middle-to-late propagation phase, however, the FPZ length of creep specimens became smaller, and the reduction was generally more pronounced at the higher sustained loading level. Numerical analysis showed that creep deformation and associated energy evolution during sustained loading altered the structural energy state before subsequent fracture. During later crack propagation, sustained loading also modified the local crack-tip elastic strain energy state, which was closely associated with the reduced FPZ length near the peak load. These results demonstrated that sustained loading did not exert a uniform influence on FPZ evolution but induced a transition from early activation to later restriction during mixed mode I-II crack propagation. These findings improved the understanding of the time-dependent mixed mode I-II fracture behavior of concrete.