Using response surface method to optimise the mix design of waste cooking oil modified bituminous mixture incorporating steel slag

This study aims to determine the optimum bitumen content (OBC) for waste cooking oil (WCO) modified bituminous mixture incorporating steel slag using response surface method (RSM) for experimental design. A total of 17 combinations were selected from a range of WCO, steel slag and bitumen content, based on central composite design. The effect of WCO, steel slag and bitumen content on Marshall mix design parameters was found to be interdependent. Particularly, the influence of steel slag was prominent at higher bitumen and WCO dosages. Subsequently, a quadratic polynomial regression model was fitted to the test data for each test parameter. OBC was determined using the developed models and validated experimentally. The absolute difference between the predicted and observed OBC values was found to be less than 0.2% across all cases, confirming the accuracy and applicability of RSM in optimising bituminous mixtures with varying dosages of WCO and steel slag.