Dynamic Mechanical Behavior and Energy Dissipation of Hybrid Fiber-Reinforced Recycled Aggregate Concrete Under Dry–Wet Cycling and Sulfate Erosion

To investigate the impact resistance of hybrid fiber-reinforced recycled aggregate concrete (RAC) under dry–wet cycles and sulfate attack, hybrid fiber-reinforced recycled aggregate concrete (RAC) was prepared. Dynamic impact compression experiments were conducted using an SHPB test device with a 50 mm diameter. The microstructure of recycled aggregate concrete (RAC) within dry–wet cycles and sulfate attack was examined using SEM. The results indicate that the dynamic compressive strength first rises and then declines with the rise in dry–wet cycles, and increases with the increase in the average strain rate. When the number of dry–wet cycles reaches 16, the dynamic compressive strength reaches its peak, with the B4S6 group achieving a maximum dynamic compressive strength of 59.02 MPa. The dynamic elastic modulus follows a good quadratic parabolic function distribution with respect to the number of dry–wet cycles. Both the incident energy and dissipated energy density initially rise and then reduce with increasing dry–wet cycles. The energy values of RAC with different fiber types follow the order: B4S6 > S6 > B4 > RAC. Under impact loading, the strain rate–strain time history curve of recycled aggregate concrete (RAC) exhibits the change of “increase–decrease–stable–decrease”. With increasing dry–wet cycles, the degree of fragmentation of recycled aggregate concrete (RAC) first increases and then decreases, the fractal dimension first decreases and then increases, and the average particle size first increases and then decreases. SEM results and microscopic reaction mechanisms reveal that in the early stage of dry–wet cycles, sulfate ions generate ettringite and gypsum within the recycled aggregate concrete (RAC), which fill internal cracks and pores, making the concrete denser and enhancing its mechanical properties. Towards the end of the dry–wet cycle, the amount of expansive ettringite and gypsum inside the recycled aggregate concrete (RAC) increases, leading to a sharp increase in pore wall stress, which induces new microcracks in the specimens, manifesting as a decline in mechanical properties at the macroscopic level.