Concrete made with hybrid blends of crumb rubber and metakaolin: Optimization using Response Surface Method

• Hybrid blends of crumb rubber and metakaolin in concrete production is used.
• Response Surface Method is utilized to design the experiments.
• The mathematical equations are obtained by regression analysis and then verified.
• An optimization is made to maximize the strength minimizing water absorption.
• The correlation between the responses is illustrated.

The purpose of this study was to optimize the concrete made with crumb rubber (CR) as waste material in conjunction with metakaolin (MK). The study seeks to use CR as partial replacement for fine aggregates and use MK as partial replacement for cement. To this end, prior to conducting the tests, the experiments were designed by Response Surface Method (RSM). Upon execution of the experimental program, regression analysis was used to obtain the equations. Then, optimization was achieved when as the compressive strength maximized, the water absorption of the mixture was minimized. Compressive strength of the concrete decreased with increased rubber content. It can be seen that the negative effect of CR on compressive strength of concrete can be relatively mitigated by incorporating a certain amount of MK. The water absorption decreased with increased CR content up to about 9 vol.% but increased when CR content exceeded this ratio. Based on the results obtained, an optimum mixture can be achieved with a 3.3 vol.% replacement of sand with CR and 19.5 vol.% replacement of cement with MK to have a structural clean concrete.