Effect of incorporation of metakaolin and recycled coarse aggregate on properties of concrete

The present research work investigates the utilization of metakaolin as a mineral admixture in improving the properties of concrete mixes, containing varying percentages of recycled coarse aggregates (RCA). For this, fifteen concrete mixes have been designed by substituting natural coarse aggregates (NCA) with 0%, 50% and 100% RCA and cement with 0%, 5%, 10%, 15% and 20% metakaolin. Nine different properties of concrete including workability, compressive strength at three different curing ages (3, 7 and 28 days), splitting tensile strength, flexural strength, ultrasonic pulse velocity, rebound number, water absorption, density and volume of voids have been experimented to examine the effect of metakaolin and RCA on the properties of concrete. The outcomes of the study reveal that the workability becomes higher with higher percentage of RCA; however it slightly reduces when metakaolin is used. The mechanical properties like compressive strength, splitting tensile strength, flexural strength and dry density get reduced when quantities of RCA increases. The non-destructive parameters like ultrasonic pulse velocity and rebound number of concrete follows similar pattern as that of compressive strength. The water absorption and volume of voids shows higher values for recycled aggregate concrete (RAC) mixes. However, the use of metakaolin in RAC significantly reduces the water absorption and volume of voids. The optimum replacement percentage of metakaolin is found to be 15% as the RAC mixes shows best performance at this replacement level. Furthermore, the properties of concrete mix containing 100% RCA and 15% metakaolin are similar to that of normal concrete. Therefore, it is feasible to produce sustainable concrete by using maximum waste concrete i.e. 100% RCA and 15% metakaolin without much affecting the strength criteria. (C) 2018 Elsevier Ltd. All rights reserved.