Research into the internal flaw evolution mechanism and the constitutive model of recycled aggregate concrete under monotonic and cyclic loading

This paper investigates the influence of internal flaws in recycled aggregate concrete (RAC) on its mechanical properties. RAC is an effective material for repurposing waste concrete; however, its flaw evolution process under different loading conditions remains unclear. Monotonic and cyclic loading tests were performed on RAC specimens with different recycled coarse aggregate (RCA) replacement rates. These tests revealed that the flaws in RAC can be divided into two types: porous defects (gradually compacting and strengthening the RAC as the stress increases) and induced cracks (continuously propagating and damaging the RAC). Based on the competition evolution of these two types, the stress-strain curve of RAC under monotonic loading is divided into four stages: porous defect-dominated, induced crack-dominated, rapid induced crack extension, and continuous damage stage. In addition, two more stages occur under cyclic loading: the unloading stage and the reloading stage. Based on the competition effect and evolution of two flaw types, a constitutive model, which describes flaw evolution with stress, is proposed. This model can reveal the flaw evolution laws of RAC with different RCA contents for each stage, which can describe the stress-flaw strain relationship of RAC with varying RCA content. The proposed models overcome the inherent limitations of empirical models, which lack physical parameter significance and rely on ambiguous mechanical theories for cyclic loading.