4D CT–validated mesoscale finite-element modeling and coupled ITZ–fiber damage evolution in micro-steel-fiber-reinforced recycled aggregate concrete

A mesoscale finite-element (FE) model of micro-steel-fiber-reinforced recycled aggregate concrete (MSFRAC) was developed and experimentally validated through 4D X-ray computed tomography (CT) and digital volume correlation (DVC) to investigate the coupled damage evolution of the interfacial transition zone (ITZ) and steel fibers. The reconstructed four-phase geometry (matrix, aggregate, ITZ, and fibers) was implemented in an FE framework combining the Concrete Damaged Plasticity (CDP) model for the mortar matrix and cohesive elements for ITZ debonding. Model accuracy was quantitatively confirmed by comparing predicted and CT-measured crack-volume-fraction (CVF) evolution, with R² = 0.96. The analyses reveal that micro-steel fibers restrain early ITZ cracking and promote diffuse meso-crack propagation, reducing the CVF sensitivity coefficient from 1.099 to 0.629. Parameter sensitivity studies highlight that ITZ cohesion and fiber volume fraction dominate the post-peak softening and energy-absorption capacity. The validated model provides a physically interpretable and computationally efficient framework for designing fiber-reinforced recycled concretes with improved damage tolerance and interfacial performance.