A physically-based and code-aligned design method for longitudinal shear of RACCS

Concrete remains a cornerstone of structural engineering, but its production and lifecycle generate significant environmental pressures, primarily due to the extraction of natural aggregates, intensive energy use, and the increasing generation of demolition waste, underscoring the need for circular and sustainable alternatives. Recycled Aggregate Concrete (RCA) from Construction and Demolition Waste (CDW) constitutes a key strategy toward resource efficiency and waste minimization. Nonetheless, its integration into structural members remains challenging due to RCA’s inherent heterogeneity, higher porosity, and the consequent alteration of the bond behavior at the steel–concrete interface. This study investigates the longitudinal shear behaviour of Recycled Aggregate Concrete Composite Slabs (RACCS) with the aim of extending the applicability of the Eurocode 4 m-k design method to system incorporating recycled aggregates and providing an experimentally supported interpretation of the semi-empirical parameters m and k governing the steel-concrete interface. A dataset of 48 full-scale four-point bending tests covering six RCA replacement ratios (0–100%) and three span lengths was analyzed using linear least-squares regression, to derive the m and k parameters representing mechanical interlock and frictional resistance. Results indicate a non-linear response, with optimal interlock around 30% RCA and degradation beyond this threshold due to ITZ weakening, while frictional effects intensify at higher replacement ratios. A new set of experimentally calibrated correction factors α(rRCA) and β(rRCA) was introduced, recalibrating the m–k formulation as a function of the RCA replacement ratio. The proposed formulation is able to reproduce the experimental trend of longitudinal shear resistance across the investigated RCA range, with deviations generally within ±10% compared to the experimental results.