The almost ubiquitous construction material we know as concrete has high compressive strength but low tensile strength. In order to overcome this problem, reinforced concrete was developed. Unfortunately, reinforced concrete more readily succumbs to corrosion particularly from water ingress so there is a need to develop ways to improve the formulation of reinforced concrete and perhaps to develop additives that allow the self-healing of cracks and fissures that grow so that a structure might be saved from complete deterioration.
Writing in the International Journal of Structural Engineering, a team from the National Institute of Technology, in Raipur, India, explain that there are two major causes of deterioration: carbonation-induced corrosion and chloride-induced corrosion. “Through the random distribution of pore spaces in concrete, aggressive substances, such as carbon dioxide, chloride, moisture, and oxygen may penetrate the structure,” the team explains. This, in turn, can break down the protective layer around reinforcing steel bars within the structure leading to their corrosion and ultimate failure.
In terms of the chemistry of the initial corrosion process involving carbonation. The initial alkalinity arising from the hydration process of cement protects the concrete formed from corrosion. However, carbon dioxide ingress leads to reactions with calcium compounds in the concrete which generates calcium carbonate and lowers the alkalinity making the material more acidic, unstable, and thus susceptible to degradation.
Other researchers have already shown that adding Bacillus subtilis bacteria to the cement formulation can have a protective effect. The team has now shown that calcium lactate can boost the benefits of the microbes by reducing the carbonation rate. It also improves the compressive strength of the concrete. Moreover, the living bacteria can refill and repair microscopic cracks within the structure to a degree allowing concrete to self-heal. This was observed in the laboratory by the team using scanning electron microscopy.
Vijay, K. and Murmu, M. (2020) ‘Effect of calcium lactate and Bacillus subtilis bacteria on properties of concrete and self-healing of cracks’, Int. J. Structural Engineering, Vol. 10, No. 3, pp.217–231.