Self-healing or self-repairing concrete is right now one of the hottest topics in the field of sustainable architecture and green buildings. The target of self-healing concrete is to create a system where healing agents are released if cracks appear and then the cracks mend without human intervention. This not only increases safety in buildings and structures like roads, bridges or stadiums, but also saves billions of dollars in maintenance costs.
How can concrete heal itself?
There are several approaches to creating self-healing concrete including organic, inorganic and biochemical solutions. And there are also several approaches currently being experimented on as to the best method of applying those solutions. Approaches range from using biological microorganisms to using polymers, and from including the solution within the concrete mixture itself to applying surface coatings to the concrete.
The fundamental premise behind the creation of self-healing concrete is that fine hair cracks appear on the surface of the concrete or weak portions of structures much before actual damage occurs by severe or accidental stress. If, the fine cracks could be treated at the time of their appearance, then further damage could be prevented. For this, scientists are experimenting with different agents which can be released when cracks first start appearing, and which can set off reactions leading to the creation of sealants that can bond the concrete together to repair the cracks.
Five healing mechanisms for self-healing concrete
While the number of healing mechanisms that scientists are experimenting with is too numerous to consider here, the principal self-healing mechanisms of concrete that have already been accepted by researchers include five methods:
1. Autogenous healing augmented by microfibers: Concrete exhibits self-healing capacities wherever unhydrated cement is present. When water contacts unhydrated cement small cracks can be repaired by formation of CaCO3 and the hydration of cement. The process is augmented by adding microfibers to the concrete mixture so that when concrete cracks, multiple smaller cracks occur instead of one big fissure.
2. Superabsorbent Polymers (SAP): Superabsorbent polymers or hydrogels also use the autogenous healing capacity of concrete. Hydrogels can absorb and retain up to 500 times their own weight of fluid. When mixed with concrete, if and when SAP is exposed to the environment due to cracking, it absorbs moisture and swells – the swelling initially seals the crack and prevents external elements from entering and then SAP particles release the fluid to the surrounding matrix thus helping in internal curing and autogenous healing.
3. Calcium Carbonate Precipitating Microorganisms: Certain microbes or microorganisms that thrive in dry weather are used in the concrete matrix embedded in diatomaceous earth. Once a crack happens, these microorganisms start precipitating calcium carbonate. The microorganisms die, but the calcium carbonate they leave behind help in autogenous healing of concrete and bond together the faces of the crack.
4. Encapsulated Polymers: Encapsulated polymers are mixed with concrete so that when a crack appears the capsules release the polymers. The liquid polymer flows into the crack due to capillary action and bonds together the faces of the crack. Different materials are used for encapsulating the polymers including glass or ceramic tubes.
5. Alkali Activators: Sodium Hydroxide, Potassium Hydroxide and silicate solutions are used in the matrix of concrete made of fly-ash and blast furnace slag. The low hydration of slag and fly-ash particles increase the possibility of self healing in such concrete and unreacted particles can be activated upon the formation of cracks and help to close the crack.
A different approach to applying reagents to self-healing concrete
The MIT Technology Review drew attention of the academia in 2013 to a method of applying reagents on the surface of concrete rather than in the mix or matrix to make the structure self-healing. The method proposed by researchers from the Yonsei University, South Korea, uses photopolymerization, or a method where polymers act as healing agents when exposed to sunlight. Researchers studied the behavior of a methacryloxypropyl-terminated polydimethylsiloxane (MAT-PDMS) agent that acts as a self-healing coating for concrete.
The Korean researchers claim that while most other methods of creating self-healing concrete focuses on restoring strength to damaged concrete, their method focuses on protecting the surface, sealing cracks near the surface and preventing the entry of substances like water, chloride ions or carbon dioxide that could lead to further deterioration of the structure.
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