Fracture behaviours of sustainable multi-recycled aggregate concrete under combined compression-shear loading

Multi-recycling of concrete waste presents a promising alternative for reutilization of construction and demolition wastes. This study aims to investigate the synthetic effects of normal stress ratio (σ/fc) and number of recycling cycles (n) on mechanical performance and damage evolution of 3 generations of multi-recycled aggregate concrete (Multi-RAC) under combined compression-shear loading. Both the σ/fc and n significantly affect the failure mode, mechanical strength, stress-displacement relation, and damage evolution characteristics of Multi-RAC. The shear strength increases with the increased σ/fc, but decreases with the increased n except for σ/fc= 0. When σ/fc increases from 0 to 0.8, the failure modes shift from typical shear failure mode to complex failure one with combined shear and axial collapse features. The incemental reductions of contact friction strength and aggregate interlock strength are the main contributors to the decrease of shear strength with increased n. However, with increased σ/fc, the contributor fraction of contact friction strength contributing to the degradation of shear strength is decreased, whereas the corresponding contributor fraction of aggregate interlock strength is increased. The proposed aggregate interlock model of multiple recycled coarse aggregate (Multi-RCA) unveils the mechanism for the incremental reduction of aggregate interlock strength with the increasing n. The distinctively reduced volume fraction of natural coarse aggregate (NCA) in Multi-RCA mainly contributes to the reduced aggregate interlock strength. The findings provide an insight into the mechanical behaviors and damage evolution of Multi-RAC and can promote its application as structural concrete. •Both normal stress ratio and number of recycling cycles affect the shear stress-displacement curves.•Reduction of aggregate interlock strength with increased n is studied by an aggregate interlock model.•Unified twin-shear strength theory can better model the strength envelopes up to 3 recycling cycles.•Characteristics of damage evolution are related to characteristic stress-displacement curves and failure modes.