An eight-component DEM model for carbonated recycled aggregate concrete

• Developed an eight-component model for carbonated recycled aggregate concrete (C-RAC) • Incorporated porous mortar and carbonation-induced densification • Quantified multi-factor effects on C-RAC mechanical behaviours • Investigated micro-scale stress distribution and bond failure • Linked macro-properties to micro-mechanics . Carbonation of recycled aggregates (RAs) is an effective approach to improving the performance of recycled aggregate concrete (RAC). However, the extent to which carbonation degree, replacement ratio (RR), and old mortar content (OMC) influence the mechanical performance of RAC remains insufficiently understood due to the lack of mechanistic microscale models. This study develops a novel discrete element method (DEM) model for carbonated recycled aggregate concrete (C-RAC), conceptualising it as an eight-component composite comprising new and old aggregates, new and old mortar, three interfacial transition zones (ITZs), and a carbonated component. A bond-removal algorithm is introduced to reproduce the intrinsic porous structure of RAs, followed by a bond-restoration algorithm for carbonation-induced densification. The validated model systematically quantifies the combined effects of carbonation degree, RR, and OMC on the mechanical behaviours of C-RAC. Fully carbonated RAs enhance the elastic modulus and compressive strength by up to 6% and 11%, while increasing the mean stress in carbonated mortar by 20%. Nevertheless, carbonation cannot fully offset the detrimental influence of excessive old mortar, with reductions of up to 8% in both elastic modulus and compressive strength observed at OMC = 0.6. Furthermore, a micro–macro linkage is established to interpret mechanical behaviour in terms of stress distribution and crack growth.