Comparative evaluation of acid, mechanical, and thermal pre-treatment of recycled coarse aggregate in concrete

This study investigates the mechanical and selected durability-related performance of concrete incorporating recycled coarse aggregate (RCA) subjected to three pre-treatment methods, namely acid treatment (RCAA), mechanical treatment (RCAM), and thermal treatment (RCAT), with untreated RCA (RCAU) and conventional concrete (CC) used as reference groups. RCA replacement levels of 25, 50, 75 and 100% were considered. Workability, compressive strength, splitting tensile strength and flexural strength were evaluated at curing ages up to 365 days. Water absorption and sorptivity were evaluated for the conventional and untreated-RCA concrete series. The results show decreasing workability and strength with increasing RCA replacement, mainly because of higher porosity, adhered mortar and weaker interfacial transition zones. At 28 days, the 100% untreated-RCA concrete showed a compressive-strength reduction of about 17.1% relative to the control, while the 25% replacement level showed smaller reductions. Among the pre-treatment series, acid treatment gave the highest reported compressive-strength retention, followed by mechanical treatment, whereas thermal treatment gave lower values than the corresponding untreated-RCA series in the reported dataset. The durability-related results available in this study show that increasing untreated RCA replacement increased water absorption and sorptivity, indicating greater susceptibility to water ingress. The findings support the need for controlled RCA replacement and appropriate pre-treatment, while the durability benefit of the pre-treatment methods requires further experimental verification.