Abstract
<p>This study evaluates the performance of structural grade concrete incorporating recycled concrete aggregate (RCA) and mixed recycled aggregate (MRA) as partial replacements of natural coarse aggregates at substitution levels of 10 %, 20 %, and 30 %. Two mix designs were prepared: conventional strength concrete (CSC, w/c = 0.46) and high-strength concrete (HSC, w/c = 0.30). Tests were performed on fresh and hardened states, including slump, density, compressive strength at 7, 14 and 28 days, ultrasonic pulse velocity (UPV), air permeability, and microstructural characterization through SEM, TGA, and XRD. Results showed that increasing RA content reduced density and workability, with RCA mixtures exhibiting lower slump than their corresponding MRA mixtures, due to the angularity of particles produced by crushing. In CSC, replacements up to 20 % maintained or slightly improved compressive strength (<= 1.4 %), while in HSC, reductions remained minor (<4 %) at 10-20 % but became significant at 30 %. Quality assessments confirmed that all mixtures achieved the requirements for structural applications, with UPV values indicating good concrete quality and permeability in accordance to ASTM C597. Microstructural analyses indicated that RCA mixtures exhibited higher calcium carbonate content due to carbonation occurring on the surface of RCA, improving matrix densification and contributing to strength retention. These findings demonstrate that both RCA and MRA can replace up to 20 % of coarse aggregates while maintaining mechanical properties comparable to those of NA, making them suitable for structural applications.</p>