AbstractIn this study, the mechanical properties and microscopic structure of concretes containing natural coarse aggregates (NCAs), recycled concrete aggregates (RCAs), and recycled asphalt pavement (RAP) aggregates are investigated. The replacement percentage of NCA with RCA and RAP was equal to 0%, 50%, and 100%. The study results showed that using RCA and RAP decreased the concrete’s mechanical properties, including a 14%–30% reduction in compressive strength and a 43%–53% loss in the modulus of elasticity. The target compressive strength can be obtained if a modified mix design is used, but the modulus of elasticity is still reduced. Also, the stress-strain curves for the uniaxial compressive test on RCA and RAP concretes showed that the strain at the maximum stress, ε0, increased by 22% and 27% for RCA and RAP concretes, respectively, in comparison with the NCA concrete. In addition, it can be concluded that the ascending branch of the stress-strain curves had a lower slope for both RCA and RAP concretes, and the ultimate strain reached, εcu, for RCA concrete decreased and for RAP concrete was up to two times higher than the NCA concrete. The toughness of specimens showed that RCA concretes had higher brittleness, while RAP concretes had higher toughness than NCA concretes. Also, analyzing the microscopic structures of specimens using scanning electron microscopy images showed that the interfacial transition zone is remarkably affected when RCA and RAP are utilized, which was the main reason for changing the behavior of RCA and RAP concretes.