Failure behaviour and damage evolution of multi-recycled aggregate concrete under triaxial compression

In this paper, the stress-strain behaviour, mechanical properties, characteristic stress, and dam-age evolution of multi-recycled aggregate concrete (Multi-RAC up to 3 recycling cycles) under triaxial compression were investigated, considering the effects of confining pressure and number of recycling cycles. Furthermore, a parametric sensitivity evaluation of key parameters using grey correlation theory to analyse the peak stress, peak strain, and elastic modulus of Multi-RAC. The results indicate that both peak stress and peak strain increase with rising lateral confining pressure. While the peak stress of Multi-RAC is lower than that of natural aggregate concrete (NAC), the deformation capacity of Multi-RAC is greater. The deformation and failure of Multi-RAC under triaxial loading can be categorized into five stages. Notably, crack initiation stress (& sigma;ci) and crack expansion stress (& sigma;cd) demonstrate nonlinear growth as lateral confining pressure increases. In particular, the second generation of Multi-RAC (RACII) exhibits superior performance in resisting crack initiation and expansion, while the third generation of Multi-RAC (RACIII) performs relatively less favourably. The energy evolution characteristics and division of failure stages of Multi-RAC are effectively characterized by the evolution of elastic strain en-ergy (Ue) and dissipation energy (Ud). The degree of progressive damage aligns with the increase in dissipation energy. Among the five key parameters, the number of recycling cycles shows the most significant grey correlation effect on the peak stress, peak strain, and elastic modulus of Multi-RAC. These findings contribute to a deeper understanding of the mechanical properties and failure behaviours of Multi-RAC under the complex triaxial loading conditions, thus promoting the application of Multi-RAC as a structural concrete.