Optimizing blended cement concrete strength using the Box-Behnken design technique

A properly optimized concrete mix design yields the required workability and strength for the fresh and hardened concrete to sustain desired loads and stresses over time, preventing premature failure. Thus, it is imperative to investigate the behavioural sensitivity of blended cement concrete to mix design variations. The research uses the Box-Behnken design of the response surface method to optimize the slump and compressive strength of blended cement concrete incorporating Shea nutshell ash (SNA). SNA was partially utilized as a Portland limestone cement (PLC) substitute at 5-15 wt% replacement levels using C25, C30, and C40 MPa mix design proportions and tested for compressive strength after 7-90 curing ages. Binder (SNA-to-PLC) ratio, water-to-binder ratio, binder-to-aggregate ratio, and curing age were engaged as continuous (independent) variables to optimize the response (dependent) variables (slump and compressive strength). The slump and compressive strength responses were optimized by the Box-Behnken design. The results exhibited a minimized slump and a maximized compressive strength with approximately 40-63% reduction and 10% increment. The correlations between the optimized and experimental variables were accurate and strong, with 98.89% and 98.44% R² for slump and compressive strength. Ultimately, this response model is beneficial in determining the optimum mix design proportions to achieve the desired compressive strength of blended cement concrete incorporating repurposed waste materials.