Weak-phase competition and damage redistribution in shale ceramsite–basalt mixed-aggregate concrete: experimental observations and discrete element simulations

The compressive failure of shale ceramsite–basalt mixed-aggregate concrete involves competition among matrix cracking, interfacial bond breakage, and intra-aggregate fracture. This study combines experimental peak responses and representative fracture observations for concrete with shale ceramsite replacement ratios of 0%, 25%, 50%, 75%, and 100% with calibrated two-dimensional discrete element method (DEM) analyses. Mortar, shale ceramsite, and basalt were represented as distinct material phases, and five contact categories were distinguished to track phase-specific bond breakage. The absolute relative deviations between the experimental calibration targets and simulated peak compressive stresses were 0.04%–1.38%, whereas the corresponding peak-strain deviations were 2.38%–14.66%. At the simulated peak-strain state, the contribution of intra-ceramsite fracture to the cumulative bond-breakage population increased from 19.99% in C25 to 34.12% in C100, while the matrix contribution decreased from 83.78% in C0 to 57.40% in C100 but remained the largest single contribution. Coupled enhancement of the ITZ parameters from 0.1 to 0.2 of the corresponding mortar values increased peak compressive stress by 34.87%, 28.31%, and 18.14% for C0, C50, and C100, respectively, with generally smaller additional gains at higher ratios. Increasing intra-aggregate cohesion and tensile strength to 300% of the baseline values increased peak stress by approximately 0.65% for C0 but by 18.01% for C100. These results indicate a composition- and resistance-dependent redistribution of phase participation rather than a discrete transfer of failure control among individual phases. The findings provide a mesoscale interpretation of damage competition within the investigated mixtures and calibrated two-dimensional numerical framework.