Investigating viability of alkaliphilic Bacillus species in biological self-healing concrete

Abstract

Background: Concrete is the most widely used building material in the world. However, cracks inevitably open up in concrete structures. Some bacteria can precipitate calcium carbonate crystals that fill the cracks. The bacteria incorporated in the concrete matrix must survive the harch alkaline conditions. Most studies investigate the suitability of calcium source and bacteria species for self-healing without testing the viability of bacteria in the concrete matrix. Objectives: For the formation of biological self-healing concrete specific alkaliphilic spore-forming bacteria and mineral precursor compounds such as calcium lactate are needed. In this study, the viability of three Bacillus species in a concrete matrix containing several types of cement with different pH and metal ion concentrations was investigated. Methods: The sporulation of Bacillus bacteria was investigated using the light microscopy. An expanded clay (EC) was used to impregnate the bacterial spores and calcium lactate under the vacuum; the concrete mix was obtained by mixing EC, cement, sand and water. Viability of endospores was measured using standard microbiological dilution-to-extinction method by CFU counting after plating on alkaline nutrient agar. Results: Out of four cement types commonly used in Lithuania, the best survival rate was obtained in a concrete mix using the white CEM I cement. After 3 days of incubation, only 3.3% of Bacillus pseudofirmus, 1.4% of Bacillus cohnii and 1.6% of Bacillus halodurans remained viable. These results demonstrate that additional coatings of EC aggregates are needed to improve the viability of bacteria in the concrete.