Preparing regular concrete scientists replaced ordinary water with water concentrate of bacteria Bacillus cohnii, which survived in the pores of cement stone. The cured concrete was tested for compression until it cracked, then researchers observed how the bacteria fixed the gaps restoring the strength of the concrete. The engineers of the Polytechnic Institute of Far Eastern Federal University (FEFU), together with colleagues from Russia, India, and Saudi Arabia, reported the results in Sustainability journal.
During the experiment, bacteria activated when gained access to oxygen and moisture, which occurred after the concrete cracked under the pressure of the setup. The "awakened" bacteria completely repaired fissures with a width of 0.2 to 0.6 mm within 28 days. That is due to microorganisms released a calcium carbonate (CaCO3), a product of their life that crystallized under the influence of moisture. After 28 days of self-healing experimental concrete slabs retrieved their original compressive strength. In the renewed concrete, the bacteria "fell asleep" again.
"Concrete remains the world's number one construction material because it is cheap, durable, and versatile. However, any concrete gets cracked over time because of various external factors, including moisture and repetitive freezing/thawing cycles, the quantity of which in the Far East of Russia, for example, is more than a hundred per year. Concrete fissuring is almost irreversible process that can jeopardize the entire structure." Says engineer Roman Fediuk, FEFU professor. "What we have accomplished in our experiment aligns with international trends in construction. There is pressing demand for such "living" materials with the ability to self-diagnose and self-repair. It's very important that bacteria healed small fissures-forerunners of serious deep cracks that would be impossible to recover. Thanks to bacteria working in the concrete, one can reduce or avoid at all technically complex and expensive repair procedures."
Spores of Bacillus cohnii capable of staying alive in concrete for up to two hundred years and, theoretically, can extend the lifespan of the structure for the same period. This is almost 4 times more than the 50-70 years of conventional concrete service life.
Self-healing concrete is most relevant for construction in seismically risky areas, where small fissures appear in buildings after earthquakes of a modest magnitude, and in areas with high humidity and high rainfall where a lot of oblique rain falls on the vertical surfaces of buildings. Bacteria in concrete also fill the pores of the cement stone making them smaller and less water gets inside the concrete structure.
Scientists have cultivated the bacteria Bacillus cohnii in the laboratory using a simple agar pad and culture medium, forcing them to survive in the conditions of the pores of the cement stone and to release the desired "repair" composition. Fissures healing was assessed using a microscope. The chemical composition of the bacteria repairing life product was studied via electron microscopy and X-ray images.
Next, the scientists plan to develop reinforced concrete, further enhancing its properties with the help of different types of bacteria. That should speed up the processes of material self-recovery.
A scientific school of the scientific school of geomimetics run at FEFU. Engineers follow the principle of nature mimicking in the development of composites for special structures and civil engineering. Concrete, as conceived by the developers, should have the strength and properties of natural stone. The foundations of geomimetics were laid by Professor Valery Lesovik from V.G. Shukhov BSTU, Corresponding Member of the Russian Academy of Architecture and Construction Sciences.