Multiscale Study and Numerical Simulation of Freeze–Thaw Damage in Polypropylene Fiber-Modified Recycled Concrete

The incorporation of recycled aggregates (RA) adversely affects the performance of concrete, whereas the addition of fibers mitigates these effects and enhances performance. This study combined experimental methods with finite-element modeling to investigate the damage mechanisms of polypropylene fiber–reinforced recycled concrete at varying RA replacement ratios (0%, 30%, 60%, and 100%) and fiber contents (0%, 1%, and 2%). The evolution of damage was determined by examining the macroscopic performance and microstructural changes of polypropylene fiber concrete under freeze–thaw cycles. An Abaqus-based model was developed to simulate the experimental outcomes. The results indicate that at an RA replacement ratio of 30% and a fiber content of 1%, the concrete achieved an optimal balance in performance, and Sample R30P1 had the best results in the experiments. The finite-element model effectively predicted the flexural strength of concrete after freeze–thaw cycles with various aggregate and fiber combinations and simulated the failure processes of polypropylene fiber–reinforced concrete. This study provides critical insights into enhancing the applicability of recycled concrete in cold regions, offering theoretical guidance and technical support for resource-efficient utilization.