Concrete is the most used and produced material in the world. Problems can be caused by cracks that may form in fresh or hardened concrete. Biological self-healing represents a great potential in the healing of these cracks. Bacteria and nutrients are needed in biologically self-healing concrete systems to produce calcium carbonate. These crystals fill cracks on the surface or inside of concrete. Choosing the optimal type of bacterium depends on what concrete we aim to make. Due to high alkalinity of concrete, it is of importance that bacteria are able to survive in it. Bacteria need oxygen for their metabolic activity. Therefore, the concentration of oxygen in the concrete is reduced. This increases the durability of reinforced concrete structures and reduces the corrosion of steel in concrete. Adding bacteria to concrete can be carried out through various means. They can be mixed directly into the concrete mix, whereas capsules are considered a more effective solution. When bacteria produce calcium carbonate, they also affect the properties of the concrete. Studies have shown that the addition of bacteria and nutrients can improve compressive strength, splitting tensile strength and flexural strength, and reduce water and chloride permeability. Ultimately, the self-healing process depends on the environment of the concrete as well. In a favourable environment, the cracks are filled with calcium carbonate and thus restore the tightness of the concrete.
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