Bacteria-based self-healing concrete

Abstract

The degradation of concrete has brought about major concerns because of its negative impact on the economy, the environment and the building life cycle. Degradation requires the need for regular maintenance and repair to extend a building's life span. Extensive research has recently been carried out on the use of bioorganic agents or naturally occurring organisms such as bacteria which produces microbially-induced calcite precipitation in the presence of a calcium source and urea to heal micro cracks in concrete and reduce its permeability. l he aim of this research was to investigate the potential for bacteria-based self-healing concrete by bio-deposition and to study the effects of this treatment on the permeability of the concrete. The research process consisted of two preliminary studies and one main study. The Pilot Study was carried out to investigate the conditions for the formation of a microbiologically-induced calcium precipitate (MICP) and the Characterisation Study was carried out to determine the morphology, crystal size and mineralogical composition of the precipitate using stereoscopic microscopy, SEM, EDS and FTIR. The Surface and Crack Treatment Study was carried out on cracked and un-cracked mortar samples to determine the degree of crack-healing due to the treatment and the effect the treatment has on the permeability of the samples using digital microscopy, UPV analysis and absorption tests. A set list of treatment combinations was created to test the different parameters chosen, and these were applied to both cement paste samples and mortar samples throughout the 3 studies. The parameters chosen were: the species of bacteria chosen, namely Lysinibacillus sphaericus and Lactobacillus pasteurii, the frequency of treatment which was applied once or twice, the calcium substrates used, being calcium chloride, calcium acetate, calcium lactate and calcium nitrate, and the type of medium supplied. These media were the suspension of bacteria in nutrient broth, bacterial inoculum immobilised in a manufactured mannose-glactose solgel polymer (solgel 1) and bacterial inoculum immobilised in a natural solgel derived from dry Opuntia ficus-indica mucilage (sol gel 2). Results obtained proved that the calcite formed as a white precipitate on the sample surface was sufficient in healing cracks and plugging voids within the concrete matrix, therefore decreasing the permeablllty of the samples too. All parameters proved to be successful, but the optimum parameters were found to be the use of Lysinibacillus sphaericus, the double application of treatment, sol gel 2 and calcium nitrate. These proved to be the most successful for both the crack treatment study and the surface treatment study. This confirms that the immobilisation of bacteria in a solgel results in a higher amount of MICP produced.