Building concrete often cracks during shrinkage, affected by freezing temperature and exceeding of the design load. Small cracks in beams and slabs are not dangerous in themselves as they are harmful to the internal steel reinforcement, which performs the main load-bearing function in the material. When water and air seep into the cracks, the steel begins to rust, which leads to the gradual destruction of the entire reinforced concrete structure. A method for a concrete self-restoration using a biomineral additive has been proposed by scientists from South Ural State University.
“We add special bacteria to the concrete composition, which during the process of their living, feed on organic food, calcium lactate, specially placed for them in the concrete system,” said Professor of the SUSU Department of Building Materials and Products Tamara Chernykh. “These bacteria eat it and create from it not an organic, but a mineral substance: calcite. Bacteria deposit calcite in a crack, the crack gets “healed” and the access of water and air gets blocked in it. This prevents rusting in the reinforced concrete and restores its continuity.”
The research team of the Department of Building Materials and Products, jointly with the SUSU Research Laboratory for Systemic Pathology and Advanced Medications, uses the Bacillus subtilis (hay bacillus) bacterium as a "medication" for concrete. The scientists place this bacillus in special granules in concrete, next to which they placed edible calcium lactate. The pozzolanic component (microsilica or fly ash) placed around them reduces the pH of the concrete, providing microorganisms with favourable conditions for existence and reproduction.
Up to a certain time, spores of aerobic bacteria are asleep. As soon as a crack in the concrete has reached the granule and violated its integrity, the bacteria, influenced by moisture and oxygen, wake up and start their "healing" activity. In the process of eating dissolved calcium lactate, the Bacillus subtilis, secreting calcite, gradually blocks the access of water and air both to the fittings (the main goal) and to itself, which is why it falls asleep again. When a new crack appears, other populated capsules, of which there are thousands in concrete, get activated. Thus, concrete is capable of endless "self-healing".
The advantage of the proposed method is that such regeneration of building material is achieved five times faster than under normal conditions (without bioadditives). If, without bacteria, concrete heals its cracks after 50 cycles of natural wetting and drying, then when a bacterial broth is added to its composition, complete self-restoration of concrete occurs after 10 cycles. By the way, the bacillus is quite viable: Bacillus subtilis feels comfortable at temperatures down to 40 degrees below zero and is able to live without food in a state of spores for more than two hundred years.
Now, at the SUSU laboratory, bacteria successfully heal cracks up to 0.5 millimetres wide. However, Chelyabinsk scientists are already working on microorganisms to "heal" wider damage in concrete, ensuring not only the sealing, but also the strength of the rehabilitated area.
Self-restoring concrete can be applied in the construction of bridges, tunnels, coastal and other buildings where there is access to water, which is a trigger for the life of hay bacillus in the building material. Also, the development by Chelyabinsk scientists will be of use in the construction of concrete roads and repair of external surfaces, for example, in the restoration of old buildings.