Cell Viability Studies on Bacillus sp. under Different Storage Conditions for Usage in Improving Concrete Compressive Strength

Bacterial concrete is a possible approach toward sustainability in concrete construction through crack-healing. Including a bacterial culture as an admixture in concrete can enhance the service life of a structure through the self-healing of cracks. Incorporating bacterial cells as an admixture in concrete is a major challenge as bacteria are living organisms with a limited shelf-life. It is essential to evaluate the shelf-life of bacterial cultures to encourage the inclusion of bacteria in concrete applications. Hence, the main focus of this study was to record the cell viability of these microorganisms before addition to cementitious systems. In the first stage, three different bacterial cultures of Bacillus subtilis, Bacillus cereus, and Bacillus licheniformis were stored in Luria Bertani broth under two different conditions of room temperature and refrigeration. These stored bacterial solutions were checked for viability based on cell count after 1 day, 3 days, 7 days, 15 days, and 20 days of storage. In the second stage, the fresh bacterial cultures and the 15-day stock were added to prepare bacterial concrete and cement paste samples to assess their compressive strengths and microstructural changes, respectively. It was observed that the cell viability in terms of cell count of the selected bacterial strains attained up to 15 days when stored at room temperature. It was also observed that the compressive strength of the bacterial concrete prepared with stored bacterial cultures increased by 6% and 11% at 7 and 28 days compared with the control Portland cement concrete mix, respectively. However, the compressive strength decreased by 6% to 12% compared with the bacterial concrete prepared with fresh cultures at the same ages. Additionally, the compressive strength results were validated using microstructural analyses.