Optimisation and prediction of compressive properties for concrete containing recycled aggregates and rice husk ash using response surface methodology (RSM)

Rice husk ash (RHA), an agricultural by-product, has been added as supplementary cementitious material (SCM) in concrete mixture to improve the compressive properties of recycled aggregate concrete (RAC) in recent years. This study aimed to optimise the mixture design of RAC considering two variables: the replacement ratio (wt.%) of recycled aggregate (RA) to natural aggregate (NA) with three levels (0%, 50% and 100%) and the replacement ratio (wt.%) of RHA to cement with three levels (0%, 10% and 20%). Compression test was implemented at concrete age of 28 days based on the full factorial experiment. By means of response surface methodology (RSM), the optimised RA replacement ratio and RHA replacement ratio can be calculated with respect to the compressive strength and E-modulus at 28 days, and vice versa the compressive strength and E-modulus at 28 days of RAC containing RHA can also be predicted. According to response surface modelling, the compressive strength reaches the maximum value when the RA replacement ratio is 0% and the RHA replacement ratio is 7%, and the E-modulus would reach the maximum when the RA replacement ratio is 17% and the RHA replacement ratio is 7%. The determination coefficient (R²) and adjusted coefficient (R²_adj) for the compressive strength model are 0.9632 and 0.9544 respectively, and for the E-modulus model are 0.9319 and 0.9157 respectively, showing that the models developed by RSM are relatively well correlated with the experimental results.