Self Healing concrete – Smart buildings for smart future

Cracks in concrete are a common phenomenon due to the relatively low tensile strength. Durability of concrete is impaired by these cracks since they provide an easy path for the transportation of liquids and gases that potentially contain harmful substances. If micro-cracks grow and reach the reinforcement, not only the concrete itself may be attacked, but also the reinforcement will be corroded. Therefore, it is important to control the crack width and to heal the cracks as soon as possible. Since the costs involved for maintenance and repair of concrete structures are usually high, this research focuses on the development of self-healing concrete. Self-healing of cracks in concrete would contribute to a longer service life of concrete structures and would make the material not only more durable but also more sustainable.

Self-healing concrete is a product that will biologically produce limestone to heal cracks that appear on the surface of concrete structures. … These self-healing agents can lie dormant within the concrete for up to 200 years.

Concrete has an autogenous healing capacity as unhydrated cement is present in the matrix. When water contacts the unhydrated cement, further hydration occurs. Furthermore, dissolved CO2 reacts with Ca2+ to form CaCO3 crystals. These two mechanisms, however, may only heal small cracks.

To enhance the healing mechanism, microfibres are added to the mixture. By mixing microfibres in the concrete, multiple cracking occurs. So, not one wide crack, but several small cracks are formed, which close more easily due to autogenous healing.

Superabsorbent polymers (SAP), or hydrogels, are able to 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 humid environment and swell. This swelling reaction partly seals the crack from intruding potentially harmful substances. 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.

Due to the fact that the pH in concrete drops from 12.8 to 9-10 when a crack occurs, it is useful to investigate pH sensitive hydrogels. These will only swell when a crack occurs and fresh water penetrates.

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, and will start the precipitation of CaCO3 once a crack occurs. Through this process the bacterial cell will be coated with a layer of calcium carbonate, resulting in crack filling.

One of the research programs considers the use of encapsulated polymers in order to obtain self-healing of concrete cracks. When a crack appears, the capsules break and the content is released. Due to capillary action, the agent will flow into the crack. After reaction, the crack faces are bonded together and the crack is thus healed.

Depending on the required regain in properties, different healing agents have been encapsulated. In order to reduce the water permeability of cracked concrete, polyurethane is provided inside the capsules. 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.

As encapsulation material brittle glass or ceramic tubes have been used. However, since capsules have to survive the mixing process in concrete, research is currently focusing on the development of capsules with suitable properties to survive the mixing process and to release the healing agent when cracks appear in the hardened matrix.