Investigating the Cracking Processes and Bearing Performance of Fissured Concrete SCB Specimens via DEM-Based Mesoscopic Modeling Considering Fissure Angle, Aggregate Content and Porosity

To reveal the mesoscopic fracture mechanism of fissured concrete, this study employed the discrete element method (DEM) and adopted the parallel bond model (PBM) within the two-dimensional particle flow code (PFC2D) to construct a mesoscopic model of concrete semi-circular bending (SCB) specimens with prefabricated fissures. Three sets of schemes were designed by varying prefabricated fissure angles (0-45°), aggregate contents (30-45%), and porosities (3-6%), and numerical simulations of three-point bending loads were conducted to explore the effects of each parameter on the crack propagation law and load-bearing performance of the specimens. Validation was performed by comparing the simulated load-displacement curves with the typical quasi-brittle mechanical characteristics of concrete (exhibiting "linear elastic rise-pre-peak stress fluctuation-nonlinear decline") and verifying that the DEM could accurately capture the entire process from microcrack initiation at the aggregate-mortar interface, crack deflection/bifurcation induced by pores, to macroscopic fracture penetration-consistent with the known mesoscopic damage evolution law of concrete. The results indicate that the crack propagation mode evolves from straight extension to tortuous branching as parameters change. Moreover, the peak strength first increases and then decreases with the increase in each parameter: when the fissure angle is 15°, the aggregate content is 35%, and the porosity is 4%, the specimens achieve an optimal balance between crack propagation resistance and energy dissipation, resulting in the best load-bearing performance. Specifically, the prefabricated fissure angle dominates the stress type (tension-shear transition); aggregates regulate crack resistance through a "blocking-diverting" effect; and pores, acting as defects, influence stress concentration. This study verifies the reliability of DEM in simulating concrete fracture behavior, enriches the mesoscopic fracture theory of concrete, and provides reliable references for the optimization of concrete material proportioning (e.g., aggregate-porosity ratio adjustment) and anti-cracking design of infrastructure (e.g., pavement, tunnel linings) in engineering practices.