Mechanical properties and fracture parameters of sustainable concrete produced with recycled aggregates

In this study, the mechanical properties and fracture parameters of sustainable concrete mixtures produced with different recycled aggregates at a w/c of 0.50 were investigated. To produce sustainable concrete mixtures, recycled concrete aggregates (RCA), recycled brick aggregates (RBA), and recycled fly ash aggregates (FAA) were used in total replacement, by volume, of natural crushed stone coarse aggregates (CSt). The recycled aggregates were utilized in six different sustainable concrete mixtures as plain (RCA, RBA, FAA) and surface treated (TRCA, TRBA, TFAA) by employing ground granulated blast furnace slag (GGBFS) slurry during concrete production as a multi-step concrete mixing method. Microstructural investigations were also carried out on samples taken from recycled aggregates and hardened concrete specimens to examine the effect of recycled aggregate treatment during the concrete mixing procedure with slag slurry on the mechanical properties and fracture parameters of the recycled aggregate concrete. The results of the experiments conducted in this study revealed that complete replacement of the crushed stone coarse aggregate with recycled aggregates reduced the unit weight of fresh concrete by up to 17%, while the compressive strength values were in line with the limitations for structural use. Besides, the treatment of recycled aggregates with GGBFS slurry improved the mechanical properties and fracture parameters of recycled aggregate concrete mixtures with increased statistical reliability. For example, treating recycled aggregates with slag slurry not only increased the bond strength between the reinforcement bars and the concrete mixtures, but also decreased the minimum required anchorage length for the reinforcement to carry the load. The findings of the microstructural investigations confirmed the formation of secondary hydration products as a result of the pozzolanic activity of fine slag grains, which penetrated and filled the voids and cracks on the surface of the treated recycled aggregates and in the interfaces between these aggregates and the concrete matrix, improving the mechanical properties and fracture parameters of the recycled concrete mixtures.