Performance optimisation of concrete with combinedly treated recycled aggregates via RSM

The use of recycled aggregate in structural concrete is limited by reduced workability, strength, and durability, primarily due to adhered mortar and a weak interfacial transition zone. This study proposes, for the first time, a systematic integration of a triple-stage aggregate treatment approach with response surface methodology (RSM), establishing a unified experimental-statistical framework for performance optimisation. The treatment involves sequential acetic acid soaking to weaken adhered mortar, controlled abrasion to remove loosened mortar and improve particle morphology and carbonation to densify the aggregate surface through calcium carbonate formation. Concrete containing 0%–100% combinedly treated recycled aggregate (CTRA) was evaluated for workability, strength, and durability indicators. Increasing CTRA content reduced performance of concrete; however, progressive abrasion significantly mitigated these effects. At 60% CTRA, compressive strength improved to 40.9 MPa, accompanied by reduced water absorption and enhanced surface resistivity. Quadratic RSM models demonstrated strong predictive capability with R² close to 0.97 and statistically significant terms with p-values below 0.0001. Multi-response optimisation identified an optimal condition at 60% CTRA and 500 abrasion revolutions, with a desirability value of 0.72, representing a balanced optimisation of workability, strength, and durability. The results demonstrate that higher recycled aggregate utilisation can be achieved in structural concrete while maintaining required engineering performance.