Concrete’s ability to heal reduces the need to detect and repair cracks that may otherwise lead to corrosion of reinforcement and deterioration of the concrete. This reduces costs, while boosting durability.
Many concrete elements crack without there being any concern; some are even designed to crack. But for some elements, the avoidance, repair, or self-healing of cracks is of benefit. This is particularly true when steel reinforcement is present, such as in much large-scale infrastructure. In such concrete, the presence of unplanned cracks may raise the risk of corrosion of the reinforcement and subsequent deterioration of the concrete.
Traditional concrete, if in contact with water, has a mechanism for self-healing called autogenous healing. It has this capacity because unhydrated cement remains present in the matrix. When water contacts the unhydrated cement, further hydration occurs and ‘heals’ the crack. Recent research has however aimed at furthering concrete’s self-healing properties. This includes the use of:
- Superabsorbent polymers (SAP), or hydrogels, which can take up a large amount of fluid (up to 500 times their own weight) and to retain it in their structure without dissolving. When cracks occur, SAP are exposed to the environment and swell, partly sealing the crack. After swelling, SAP particles desorb and provide the fluid to the surrounding matrix for internal curing, further hydration, and the precipitation of CaCO3. In this way, cracks may close completely
- Micro-organisms that precipitate calcium carbonate. These organisms are embedded in the concrete matrix after immobilisation on diatomaceous earth in microcapsules or in SAP and will start the precipitation of CaCO3 when a crack occurs. Through this process, the bacterial cell will be coated with a layer of calcium carbonate, resulting in crack filling
- Encapsulated polymers that break open during cracking, releasing their content. Due to capillary action, the agent will flow into the crack. After reaction, the crack faces are bonded together, healing the crack.
Self-healing concretes reduce the need to detect and repair cracks and are one strategy to address corrosion risk. This has social, economic, and environmental benefits, as overcoming/reducing the need for maintenance and/or increasing longevity reduces disruption, as well as the cost and use of materials.
Header photo by Paul Mocan on Unsplash