Performance assessment of concrete incorporating recycled coarse aggregates and metakaolin: A systematic approach

The current research work has been performed to access the effect of different quantities of metakaolin on mechanical and durability characteristics of recycled aggregate concrete (RAC) containing varying quantities of recycled coarse aggregates (RCA) and to determine the optimum percentage of metakaolin for RAC. For this, eleven concrete mixes have been produced by replacing the natural coarse aggregates (NCA) with 50% and 100% RCA and cement with 0%, 5%, 10%, 15% and 20% metakaolin. In addition to above, a control concrete mix has been designed without RCA and metakaolin at a water-to-cement ratio of 0.43 for comparison of the experimental results. The mechanical properties examined in the study include compressive strength, splitting tensile and flexural strength, each evaluated after 28 days of curing. Apart from this, the durability characteristics like sorptivity, carbonation resistance, chloride penetration resistance, acid resistance and sulphate resistance of all concrete mixes have been investigated. The results indicated that the compressive strength, splitting tensile strength, flexural strength, carbonation resistance, resistance against sorptivity, chloride penetration resistance, acid and sulphate resistance of RAC decreased by about 20%, 15.2%, 15%, 25%, 224%, 20%, 82% and 80%, respectively, by incorporating 100% RCA in concrete. The utilization of metakaolin in RAC substantially improved the aforementioned properties to a maximum of 17.6%, 15.7%, 13.8%, 20%, 62%, 39%, 42% and 33%, respectively, due to the combined action of pozzolanic and filler effect of metakaolin. It is found that the optimum substitution level of metakaolin for RAC is 15% since it causes maximum improvement in both mechanical and durability properties. Moreover, the concrete mix having 15% metakaolin and 100% RCA exhibits comparable results with control concrete. Therefore, sustainable concrete can be produced by taking the above combination of RCA and metakaolin as maximum waste concrete has been utilized without affecting the properties of concrete. (C) 2019 Elsevier Ltd. All rights reserved.