Response Surface Methodology-Based Optimization of Bacterial Cell Concentration for Microbial Concrete

Microbial concrete has high potential for superior strength and durability, provided the optimal combination of bacterial cell concentration and water/cement (w/c) ratio is used. Despite there being a considerable volume of the literature on microbial concrete, no work has so far dealt with the prediction of compressive strength and water absorption, the most essential properties of concrete, at different levels of bacterial concentrations and w/c ratios. In addition, there has been no investigation on the optimization of bacterial concentration and w/c ratio with respect to these properties, which is very important from the microbial concrete mix design point of view. The present work addresses these issues and develops models to predict the compressive strength and water absorption of mortar mix within a range of commonly used w/c ratios, namely 0.40, 0.45 and 0.50, and cell concentrations of Bacillus subtilis, namely 10(3), 10(5) and 10(7) cells/ml of water, to circumvent detailed case-specific experimental investigation. This is achieved by applying response surface methodology using the Design-Expert software to experimental data obtained for nine combinations of w/c ratios and bacterial cell concentrations. Analysis of variance (ANOVA) is carried out to acquire simplified updated models. Optimal values of cell concentration and w/c ratio that maximize mortar compressive strength and minimize its water absorption are obtained, which lies between 10(5.719) and 10(5.8) cells/ml at w/c ratio of 0.40. Further, the optimal cell concentration for a preset w/c ratio is provided, which is highly pertinent from the practical application point of view. Finally, a microstructure analysis is carried out to offer insight into the obtained optimal cell concentrations.