Mechanical, durability and economic performance of concrete incorporating fly ash and recycled aggregates

This paper studies the impact of replacing natural coarse aggregates (NCA) with recycled coarse aggregates (RCA) and cement (OPC) with fly ash (FA), on properties and economy of concrete. RCA was obtained by crushing old concrete specimens. FA of low calcium type was used to partially replace OPC. A total of nine concrete mixes (including a reference mix) were prepared using 50% RCA, and 100% RCA with four different levels of FA (0% FA, 10% FA, 20% FA, 30% FA). Workability, fresh density, compressive strength, apparent porosity, and bulk density was measured to evaluate the performance of each mixture. Mixes with optimum levels of both RCA and FA reaching the compressive strength potential of reference mix at 90 days, were subjected to chloride migration testing. To ascertain the combined impact of RCA and FA on economic and strength performance of concrete, a cost to strength ratio (CSR) analysis was also performed on all mixes. Results of testing indicate that FA compensates the loss in workability of fresh concrete and improve the compressive strength of RCA concrete at 28 and 90 days. Negative influence of RCA on density and porosity is minimized by consumption of FA in concrete. Mixes with 50% RCA and 20-30% FA show better resistance against chloride penetration compared to that of the reference mix. CSR analysis suggests that FA must be used along with RCA in concrete in order to minimize the negative influence of RCA on both cost and strength.