Bacteria based self healing concrete

For every person on earth, more than 1 m³ of concrete is produced every year. This hunger for concrete has far-reaching consequences, as typically 100 kg of CO₂ is emitted into the atmosphere for the production of one ton of concrete. This is mainly caused by the use of cement which constitutes concrete, together with water, aggregate (gravel, sand) and possibly steel reinforcement bars. This is not a bad figure compared to other common construction materials, such as masonry, steel or aluminum. Moreover, concrete, during its life, slowly absorbs CO₂ in the process of carbonation, by which Ca(OH)₂ in the concrete is converted into CaCO₃. Nevertheless, the amount of concrete that is produced still makes it a significant source of anthropogenic CO₂ emissions.

It is therefore no surprise that several strategies have been devised to make concrete more sustainable. The most straightforward method is to (partially) replace cement by greener alternatives, such as blast furnace slag or fly ash, which are by-products of the iron and coal industries, respectively. Another complementary solution is to increase the lifetime of concrete, thus reducing the need for replacement materials. Currently, this is achieved by inspecting and repairing the structure periodically, but this is expensive and time-consuming. Moreover, degradation of concrete usually begins with microcracks that are hardly visible to the naked eye, if at all. 

Figure 1

Now a day, biologically modifications of construction materials become more popular for strength improvement and long-term sustainability. An investigation has been conducted to study the compressive strength of concrete by using two different bacteria named Bacillus Cereus and Bacillus Subtilis with three different concentration (viz. 10³, 10⁵, 10⁷ cells/ml) of each bacterium. It has been found that there was an increase in compressive strength at both 7 days and 28 days. It was noticed that the optimum concentration was different for different bacteria. The optimum concentration of Bacillus Subtilis and Bacillus Cereus were found as 10⁵ cells/ml and 10³ cells/ml respectively. Moreover, more compressive strength was obtained by addition of Bacillus Subtilis at optimum concentration compared to the samples made with all three concentrations of Bacillus Cereus both at 7 and 28 days.

Cracks in concrete are inevitable and are one of the inherent weaknesses of concrete. Water and other salts seep through these cracks, corrosion initiates, and thus reduces the life of concrete. So there was a need to develop an inherent biomaterial, a self-repairing material which can remediate the cracks and fissures in concrete. Bacterial concrete is a material, which can successfully remediate cracks in concrete.

This technique is highly desirable because the mineral precipitation induced as a result of microbial activities is pollution free and natural. As the cell wall of bacteria is anionic, metal accumulation (calcite) on the surface of the wall is substantial, thus the entire cell becomes crystalline and they eventually plug the pores and cracks in concrete. This paper discusses the plugging of artificially cracked cement mortar using Bacillus Pasteurii and Sporosarcina bacteria combined with sand as a filling material in artificially made cuts in cement mortar which was cured in urea and CaCl₂ medium.

 https://www.spiedigitallibrary.org/conference-proceedings-of-spie/4234/0000/Bacterial-concrete/10.1117/12.424404.short

https://www.sciencedirect.com/science/article/abs/pii/S0950061817313752

https://www.sciencedirect.com/science/article/pii/S1369702111701936