Application of carbonate precipitating bacteria for improving properties and repairing cracks of shotcrete

- Calcite producing bacteria improved the strength properties of shotcrete.
- Influence of bacteria on shotcrete were more noticeable in comparison with concrete.
- Use of bacteria leads to reduction in permeability and water absorption.
- Shotcrete crack healing was observed in bacteria-exposed shotcrete specimens.
- SEM and XRD analysis indicated the formation of calcite in bacterial shotcrete.

Shotcreting is a construction technique commonly used in many civil and mining engineering applications. Cracking is an unavoidable and inherent weakness of most cement-based construction materials including shotcrete. In this study, the effect of Bacillus Subtilis on healing and mechanical properties of shotcrete was evaluated. For this purpose, bacteria were introduced into mix design and curing solution in order to examine the effect of each approach on the compressive strength, tensile strength, permeability, porosity and healing of shotcrete specimens. The results of uniaxial compressive test showed up to 30% increase in the compressive strength of bacteria-exposed shotcrete specimens compared to control specimens. This strength improvement was 10% more in shotcrete specimens than in the conventional cast concrete specimens with the same mix design. The presence of bacteria, both in the mix design and curing solution, was found to enhance the tensile strength and decrease the water absorption and porosity of shotcrete. The maximum reduction in water permeability at 28 days was observed in the specimens cured in the bacteria-containing solution. The ability of bacteria to heal the cracks created in the specimens was also evaluated in the present work. Bacterial precipitation of calcium carbonate in the pores of specimens was confirmed by SEM/XRD analysis. The results give clear evidence supporting the utility of calcium carbonate precipitating bacteria for healing the cracks and improving the durability of shotcrete.