Self-healing concrete-What do think it would be the term. Yes, it refers to the concrete that is self-regenerative and could repair the cracks. It sounds something magical!!! But it isn’t sorcery, its’ technology. These self-healing /bio-concrete plays a major role in overcoming the common defects of concrete such as crack formation, spalling and chemical attacks. These are prepared either by super-absorbant polymers (SAP) or by using bacteria with the nutrients.This was first developed by Dr. Henk Jonkers
Concrete is resistant to compressive force but not the tensile force. So the cracks in this concrete are natural. Water can enter through the cracks and causes corrosion of reinforcements. These cracks enlarge and then results in spalling. Hence, the strength of the building gradually decreases. At the same time, repair of the cracks would consume a lot of time and money. That’s when the microbiologist Dr: Henk Jonkers struck with an idea of developing a concrete that can heal its own cracks and succeeded in it.
A suitable bacterium has to be selected. The pH of concrete is 13 indicating its highly alkaline nature. Most of the micro-organisms die at such alkalinity. But bacteria like Pseudo firmus, Cohnii, Filla, Pasteurii live in the alkali lakes of Russia and soda lakes of Egypt sustaining the high alkalinity. Hence these bacteria are used. The bacteria should be alkali-resistant; spore-forming; sustaining more than 50years and concrete-compatible. The nutrient is Calcium Lactate. The nutrient and the bacteria are mixed with the concrete either by direct mixing or by making the mixture into fine capsules/ pellets made of glass or ceramic tubes of size 2-4 mm by vacuum technique.
Due to tensile strength, cracks occur on the surface of the concrete. The bacteria forms spores in the unfavorable condition.
When water seeps through the crack, bacterial spores gets activated and feeds on the nutrients and convert the nutrients into calcium carbonate. Therefore the lime formed fills the cracks in the concrete structure.
Cracks in the concrete structure
Action of the pellets filling the cracks
Calcium lactate is converted into Limestone as the bacteria intakes oxygen
Formation of limestone in the cracks
When strength regain is a more important issue, methyl methacrylate is encapsulated. For structures where the aesthetic aspect is important, water repellent agents can be encapsulated
The cracks are visible in the Concrete stucture. The cracks in the structures are filled by concrete.
Generally, cracks heal to a considerable amount by 7 days. After 100 days, it heals about 0.46 mm which is double the amount healed by self- healing.
COMPRESSIVE STRENGTH TEST OF THE CONCRETE
FLEXURAL STRENGTH OF THE CONCRETE
• It increases the durability of the concrete
• Reduces the corrosion of the steel as the bacteria consumes the oxygen
• Compressive strength increases by 14.92% thereby reducing the formation of cracks
• Bacteria used is safe
• Good for construction connected to groundwater
• Reduction in freeze-thaw cracks
• Cost of the concrete (Calcium lactate) is high.