Longitudinal shear behavior of composite slabs with recycled coarse and fine aggregate concrete

Steel-concrete composite slabs, as efficient modern structural components, can incorporate coarse recycled aggregates (CRA) and fine recycled aggregates (FRA) as green alternatives to sand and gravel. However, existing studies have not sufficiently investigated the effects of recycled coarse and fine aggregates as well as loading history on the longitudinal shear capacity of composite slabs, and the applicability of current design methods specified in existing codes for recycled aggregate concrete (RAC) composite slabs has yet to be systematically validated. Therefore, this study systematically investigates recycled aggregates (RAs) composite slabs through experimental testing and numerical modeling. Three full-scale specimens with closed steel decking were tested under varying RAs replacement ratios (coarse/fine aggregates) and sustained load durations (0–500 days). A validated finite element (FE) model incorporating interfacial bond-slip behavior was developed, enabling 136 parametric simulations to assess m-k and partial shear connection (PSC) approaches following Eurocode 4. The results show: (1) RAs incorporation reduces initial stiffness and cracking load by 14.3 % and 27.1 %, respectively, but negligibly affects ultimate shear capacity; (2) 500-day sustained loading enhances shear capacity, stiffness, and cracking load by 9.3 %, 26.3 %, and 16.1 % through concrete aging; (3) Both Eurocode 4 methods demonstrate satisfactory applicability for RAs composite slab design. These results provide critical validation for sustainable composite slab engineering using recycled materials.