Self Healing Concrete

Self Healing Concrete 

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 gasses that potentially contain harmful substances. If micro-cracks grow and reach the reinforcement, not only the concrete itself may be attached, but also the reinforcement will be corroded. Therefore, it is important to control the crack width and to heal he cracks as soon as possible.

Since the cost involved for maintenance and repair of concrete structure are usually high, this research focuses on the development of self-healing concrete. 

There are different investigation done and the following were concluded:

1) 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 crystal. These two mechanisms, however, may only heal small cracks.

So to enhance and to make use of this mechanism, microfibers are added to the mixture. By mixing microfibers 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.

2) Super-absorbent Polymer SAP, or hydro-gels, are able to take up a large amount of fluid and to retain it in their structure without dissolving. When cracks occur, SAP are exposed to the humid environment and the swell. This swelling partly fills the cracks and the fluid absorbed by the particle is then available to the cement particle to form CaCO3 and fill cracks.

3) Cracks can be healed by using calcium carbonate precipitation micro-organisms. These organisms are embedded in the concrete matrix after immobilization on diatomaceous earth in mirocapsules 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. 

4) One of the research programs considers the use of encapsulated polymer in order to obtain self healing concrete crack. When a crack appears, the capsules is braked and the content is released. Due to capillary action, the agent will flow into the crack. After reaction, the crack face are bounded together and the crack is thus healed.

5) While Fly Ash and blast furnace slag concrete seem to be inferior with regard to early age micro-structure and strength development, their self healing capacity can be much higher, precisely because  of the low hydration degree of the slag and fly ash particles. Upon cracking the unreacted particles can be activated again in order to close the crack and to regain the water impermeability and strength. 

Besides this all methods the concrete is treated in extreme environment and high sulphate and chloride concentrations or in the case of carbonation.