Self-Healing Concrete

Self-healing concrete is a special type of concrete invented by a group of microbiology researchers under the head of Henk M Jonkers. Self-healing Concrete is also called Bio concrete or Bacterial Concrete. It’s specially made to increase the lifespan or the durability of concrete structure by the self-healing action of that concrete.

Nepal is a developing country; the consumption of concrete is increasing every day. Concrete is the most used material in the building industry but the strength of the concrete is decreasing due to the Stress and weather conditions which are the main factors that slowly break the concrete. behind that Concrete is still one of the main materials used in the construction industry, from the foundation of buildings to the structure of bridges and underground parking lots. Traditional concrete has a flaw, it tends to crack when subjected to tension. it is possible to repair cracks, but this takes time and money, especially when the construction has to be closed down while the reparations take place.

When a crack is developed in the concrete it will fail if we don’t apply any remedies. Water will enter the cracks, then in winter, there are chances that it freezes, causing the cracks to grow. When these cracks are deep enough to reach the steel reinforcement, they can corrode the reinforcement, causing serious damage. So some repair work has to be done in the cracked concrete. The repairs can be particularly time-consuming and expensive because it is often very difficult to gain access to the structure to make repairs, especially if they are underground or at a great height. This is a serious problem which must be solved within days. If it is ignored the water seeps in to degrade the concrete and corrode the steel reinforcement, greatly reducing the lifespan of a structure. Tensile forces can lead to cracks and these can occur relatively soon after the structure is built. Repair of conventional concrete structures usually involves applying a concrete mortar which is bonded to the damaged surface. For solving this problem which arises in conventional concrete we can use Mineral-producing bacteria which could help mend micro-cracking in concrete.

Self-healing concrete can repair itself within three weeks and makes manual repairs superfluous. For this, we just have to mix a genus Bacillus bacterium, capable of surviving in an extreme alkaline environment, with a healing agent. Generally, cement and water have a pH value of up to 13. In this, we mix a healing agent that works when bacteria embedded in the concrete convert nutrients into limestone. These bacteria are enclosed in pellets and can be dormant for 200 years. They start to work when they come into contact with water and oxygen (this happens when the concrete is cracked or deconstructed). Fed by nutrients in the pellets, the bacteria will multiply and start to produce calcium carbonate (limestone), filling the cracks. The bacteria consume oxygen during the bacterial conversion of calcium lactate to limestone, so the corrosion activity will decrease. Oxygen is an essential element in the process of corrosion of steel and when the bacterial activity has consumed it all, it increases the durability of steel-reinforced concrete constructions. Also when water seeps into the concrete, the bacteria germinate and multiply quickly.

The bio-concrete starts to produce limestone to heal cracks that appear on the surface of the concrete structure. Genus Bacillus, along with a calcium-based nutrient known as calcium lactate, and nitrogen and phosphorus, are added to the ingredients of the concrete when it is being mixed. When genus bacillus comes in contact with water & nutrients the bacteria gets activated or say it wake up. The activated bacteria feed on calcium lactate & consume oxygen. The calcium lactate is converted into the insoluble limestone. The limestone solidifies on the cracked surface brings back the cracks to the original cracked condition.

In the context of Nepal, we should use the bio-concrete to decrease the re-constructional work. The bio-concrete increases the life span of the structure approximately 4 times, the life span of conventional concrete is 50-70 years whereas the life span of self- healing concrete is about 200-300 years. For the healing action of the bacteria, the length of the cracks does not matter, only the breadth of the crack is concerned. At a time, the bacteria can fill up cracks up to 0.8 mm. The material is repaired within 3 weeks, no matter the length of the crack. The liquid form of the bio-concrete which contains bacteria that can make ordinary concrete self-healing have also been developed.

The clay pellets holding the self-healing agent comprise 20% of the volume of the concrete. That 20% would normally comprise harder aggregate such as gravel. The clay is much weaker than normal aggregate and this weakens the concrete by 25% and significantly reduces its compressive strength. While talking about cost, its initial cost is high (about double that of conventional concrete (€80 euros per cubic meter)) but if the life of the structure can be extended by 30%, the doubling in the cost of the actual concrete would still save a lot of money in the longer term.