Due to the development of Indian infrastructure, the production of concrete is increasing. As concrete is a porous material, there is always a susceptibility to ingress harmful chemicals which lead to degradation of the concrete structure. Usually, a lot of operation maintenance, time, and money are required for repairing pores and cracks through the surface coating of synthetic polymers to protect these concrete structures. These polymers are many times toxic, costly, and deteriorate the environment and hence an alternative is needed. Self-healing concrete is looked upon as an alternative to fill the pores and cracks present in concrete through biologically producing limestone (CaCO3). During the last decade, the biological healing of cementitious materials has attracted much attention from the research community as a promising tool towards a solution to reduce deterioration of the structure. In this phenomenon, bacteria are incorporated in cementitious materials. The addition of bacteria is responsible for the precipitation of calcium carbonate in the concrete matrix which fills the pores of the concrete and ultimately increases the strength and durability of concrete. This phenomenon reduces the cracks in the structure to work until the end of their service life without the problem of deterioration and also reduces the risks of toxicity. For the precipitation of calcium carbonate, it is essential that the bacteria should produce the urease enzyme. This urease enzyme produced by the ureolytic bacteria catalyzes the hydrolysis of urea. As aresult carbamateand ammonia are formed. This carbamate again hydrolyzes and produces one mole of ammonia and carbonic acid which further forms bicarbonate, hydroxide, and ammonium ions. Due to this reaction, the pH of the concrete increases, and carbonate ions are formed as expressed in Eqs.1 –6. Generally, bacteria surface has negatively charged and attracted Ca+ions present in cement which further combine with CO32-and precipitate CaCO3Due to the precipitation of calcium carbonate; the complete bacterial cell becomes encapsulated after a period. Lacking nutrient transfer leads to the death of the bacterial cell. The basic demonstration of the events happening throughout the calcium carbonate precipitation process by bacteria is shown in Figure1.
CO(NH2) + H2O →NH2COOH+NH3 (1)
NH2COOH+ H2O → NH3 +H2CO3 (2)
H2CO3 → HCO3–+H+ (3)
2NH3 + 2H2O → 2NH4+ + 2OH–(pHincrease) (4)
HCO3– + H++ 2OH– → CO32-+2H2O (5)
CO32-+ Ca+→CaCO3↓ (6)
This biological process is an environmentally safe and eco-friendly process. For this purpose, bacteria should be endospore-forming and urease-producing. A Bacillus species, isolated from soil improves the compressive strength and reduces the water absorption of concrete due to the precipitation of calcite. The bacterial strain for precipitating CaCO3should produce endospores because spores are nonproductive, dormant, tough, and very resistant structure and they allow bacteria to survive harsh, stressful, and abrasive surrounding of concrete. Figure 2 shows the endospores and vegetative cells of bacteria. It is observed that the inclusion of bacteria in concrete may fill the cracks up to 0.5 mm width. Biological self-healing of concrete could solve the problem of concrete structures deteriorating well before the end of their service life.
Dr. Sekhar Saxena
CE, FEAT, SGT University