Concretes have relatively low tensile strength. Hence cracks generate in concrete. These cracks decrease the durability of concrete, because liquids and gases containing harmful substances find an easy path for transportation.
If micro cracks grow and somehow reach the reinforcement. Along with the concrete being damaged, even the reinforcement may be damaged. Thus, it is necessary to control the width of the crack and afterwards healing it as soon as possible.
Costs involved in maintenance and repair of concrete is usually high. Here comes the need of self healing concrete. Self healing of cracks in concrete will result in longer life of concrete structures. It would also make the material more sustainable along more durable.
Different healing mechanisms:
1. Concrete has an autogenous healing capacity. Unhydrated cement is present in the matrix. When water contacts the unhydrated cement, further hydration occurs.
Moreover, dissolved carbon dioxide reacts with calcium ion, as a result, calcium carbonate crystals are formed. These two mechanisms can heal small cracks.
2. Superabsorbent polymers (SAP), or hydrogels, are able to take up a large amount of fluid (up to 500 times their own weight). They retain the fluid in their structure, thus not letting them to dissolve.
When cracks occur, SAPs get exposed to the humid environment. As a result, they swell up which partially seals the cracks.
3. Cracks can be healed by using calcium carbonate precipitating micro-organisms. These organisms are embedded in the concrete matrix after immobilization on diatomaceous earth in microcapsules or in SAP.
This will start the precipitation of calcium carbonate. This process coats the bacterial cell with a layer of calcium carbonate, as a result, the crack will be filled.
4. Another method is the use of encapsulated polymers. When a crack appears, the capsule will break releasing all its contents. The contents will flow into the crack because of capillary action (upward force due to surface tension).
As a result, the crack faces get bonded together and the crack gets healed.