Impact of parent concrete strength of recycled aggregates on the mechanical performance of RAC bricks

Natural aggregate and attached mortar are the two main components of the recycled concrete aggregates (RCA), therefore, compared to natural concrete aggregates (NCA), RCA generally have significant heterogeneity. Due to variation in their expected behavior as a result of heterogeneity, the use of RCA in concrete for various applications is still very limited. Since the properties of natural aggregates and adhered mortar in RCA are mainly dictated by the quality of their parent concrete, the careful evaluation of properties of RCA derived from waste concrete of different sources is always required before their possible use in concrete. Research studies in the past have revealed that the strength of parent concrete has always had a considerable impact on the physical and mechanical properties of RCA. In this study, RCA were produced from four classes of parent concrete with respect to their initial strength (i.e., 21 to 28 MPa, 28 to 35 MPa, 35 to 42 MPa, and above 42 MPa) and were used in the manufacturing of concrete bricks by replacing 100% NCA (fine and coarse) with RCA. Tests were performed to evaluate the effect of the parent concrete strength on the physical and strength properties of RCA, and the mechanical behavior of recycled aggregate concrete (RAC) bricks. For comparison, Natural Aggregate Concrete (NAC) and burnt-clay bricks were also tested. RAC and NAC bricks were manufactured using 10% cement and under a casting pressure of 35 MPa. Compressive, shear, and flexure strengths and impact resistance of bricks were determined using standard tests. The results showed that the physical and strength properties, and water absorption of RCA were improved with the increase in strength of their parent concrete. The water absorption of coarse RCA obtained from low-strength concrete (i.e., 21 to 28 MPa) was 4.8 times higher than that of NCA, whereas coarse RCA obtained from concrete with compressive strength above 42 MPa showed almost similar physical properties as those of NCA. For RAC bricks prepared with RCA obtained from high-strength parent concrete (i.e., > 42MPa), 11% improvement in compressive strength, 2.23% decrease in the flexure strength, and 6.6% increase in the shear strength were observed in comparison with NAC bricks. Compression, shear, flexure strength, and impact resistance of RAC bricks were found to satisfy the requirements of local and ASTM Standards and were higher than those of burnt-clay bricks. This study finally concluded that fully RAC bricks manufactured using RCA derived from old concrete having compressive strength greater than 21 MPa are not only structurally viable but are also environmentally friendly and could help to manage construction and demolition (CandD) wastes efficiently and beneficially.