Self-healing concrete, as the name suggests, has self-healing properties. In this blog, Gharpedia gives you insight into what self-healing concrete is, its types, advantages, disadvantages, and applications of self-healing concrete.
What is Self-healing Concrete?
Self-healing concrete is a type of concrete that can self-repair after being damaged. This concrete can mend itself when cracks emerge. Microbiologist Professor Henk Jonkers of Delft University of Technology in the Netherlands invented Self-Healing Concrete (SHC) as a new type of concrete in 2006.
The Need for Self-Healing Bacterial Concrete/Bioconcrete
Concrete is the most important building material for the construction industry, but, most concrete structures suffer cracks and other damages during their lifespan. Small cracks developing on the surface of the concrete make the whole structure appear vulnerable. That is because water seeps into the ordinary concrete and starts degrading the same and corroding the steel reinforcement, greatly reducing the lifespan or durability of any structure. It is a known fact that concrete can withstand compressive forces very well, but not tensile forces. It cracks when subjected to tensile force.
Structures built in high-water environments, such as underground basements and marine structures, are particularly vulnerable to corrosion of steel reinforcement. In most civil engineering structures, tensile forces can lead to cracks, which occur relatively soon after construction. Repair of conventional concrete construction involves applying a concrete mortar to the damaged surface, but these repairs are time-consuming and expensive because they are difficult to execute.
As repair work and regular manual maintenance of concrete constructions are costly and, in some cases, not at all possible, it becomes necessary to find a recovery option for it, and self-healing concrete or self-repairing concrete is one such promising option. A self-healing repair mechanism is highly advantageous because it reduces maintenance and increases material durability.
The biggest advantage of self-healing concrete is that it reduces the problem of concrete structures deteriorating well before the end of their durable service life. Self-healing concrete is also known as bio concrete, self-fixing concrete, bacterial concrete, self-repairing concrete, bacterial self-healing concrete, and bacteria-healing concrete.
Get more insights on why cracks develop in concrete, with our detailed article:
7 Reasons Why Cracks Occur in Various Parts of a Building
History of Self-Healing Concrete
The idea of bacteria healing concrete was first introduced by a US academician in the late 1990s by the research group of Professor Sookie Bang. Testing and application of the theory were not further investigated because there was a lack of interest from the commercial branch of the engineering sector for such a product.
However, later, inspired by the way bones in living beings healed through mineralisation, Hendrik Jonkers, a Microbiologist at the Deft University in the Netherlands, invented self-healing concrete. It was not an easy experiment. Since concrete is extremely alkaline, it was difficult to identify any bacteria that could survive its dry, stone-like properties and remain dormant till its activation by water. After daunting three-year-long research focusing on special qualities and conditions of concrete, Jonkers was selected as a finalist for the 2015 European Inventor Award.
How Self-Repairing Concrete Works?
SHC repairs itself by producing limestone biologically to heal cracks that appear on the surface of concrete structures. SHC is created by integrating certain materials or self-repairing agents into a concrete mixture, including repair treatments. When a crack occurs, these agents break, and the liquid within them distributes and cures the crack simultaneously. These agents are as follows:
- Super Absorbent Polymers (SAP), also known as hydrogels, can absorb and hold massive amounts of fluid (up to 500 times their weight) without disintegrating. When cracks form, SAP expand after being exposed to the atmosphere, partially sealing the cracks. After swelling, SAP particles desorb and transport the fluid to the surrounding matrix, where it cures internally, hydrates further, and precipitates calcium carbonate, thereby closing fissures completely.
- Microorganisms that precipitate calcium carbonate are buried in the concrete matrix after being immobilised on diatomaceous earth in microcapsules or SAP, and when a crack forms, they begin to precipitate calcium carbonate. During this process, the bacterial cell is coated with a layer of calcium carbonate, which results in fracture filling.
- Encapsulated polymers rupture upon breaking, releasing their contents. Because of capillary action, the agent enters the crack. After the reaction, the fracture faces link together, mending the crack.
Selection of Bacteria for Self-Healing Concrete
Specific bacteria that can survive an extreme alkaline environment are injected into self-healing concrete. Cement used in concrete may have a pH value of up to 13 when mixed. Usually, in an adverse environment, most organisms die in an environment with a pH value 10 or above. Therefore, the research was carried out on microbes that can remain alive in alkaline environments, which can be found in natural environments, such as alkali lakes in Russia, soda lakes in Egypt and carbonate-rich soils in the deserts of Spain.
These self-healing concrete bacteria activate when the concrete cracks and water seeps into the structure. This germination process lowers the pH of the highly alkaline concrete to values in the range (pH 10 to 11.5) where the bacterial spores become activated. For finding a suitable food source for the bacteria that could survive in the self-repairing concrete took a long time and many nutrients were tried until it was observed that calcium lactate was a carbon source that provides biomass. If it gets dissolved during the mixing process, calcium lactate does not interfere with the setting time of the concrete. The common bacteria used in bacterial concrete according to S.Dinesh, R.Shanmugapriyan and S.T.Namitha Sheen (Authors of research paper titled, “A Review on Bacteria – Based Self-Healing Concrete), are as follows:
- Bacillus pasteurii
- Bacillnesphaericus
- Escherichia colli
- Bacillus Subtilis
- Bacillus cohnii
- Bacillus pseodofirrius
- Bacillus balodurais
Difference Between Conventional Concrete And Self-Healing Bacterial Concrete
Some of the key difference between conventional concrete and self-healing bacterial concrete are:
- Self-healing concrete is more like metal than glass because it is more pliable than ordinary concrete. Traditional concrete is a ceramic material. It is extremely brittle and inflexible, and this property causes catastrophic failure in the event of an earthquake or regular usage. On the other hand, flexible self-healing concrete is flexible, which implies that it bends without breaking. Specially coated reinforcing strands firmly anchor the fabric.
- Bio concrete is safe, even at up to 5% of tensile strain rates. Traditional concrete fails to withstand a tensile strain of 0.01%.
- Today, steel bars support concrete constructions in an effort to minimise cracks. Water and de-icing salts can infiltrate the steel, creating corrosion, weakening it and making it even more vulnerable. But self-healing concrete reduces corrosion because it does not require steel reinforcing to restrict crack width.
Advantages of Self-Healing Concrete
The advantages of self-healing concrete over traditional concrete are as follows:
- One of the major advantages of self-healing concrete is that it reduces the chances of corrosion and eliminates the need to locate and repair cracks. As a result, this decreases the need for maintenance and extends a structure’s life, reducing the cost and consumption of materials. The chances of corrosion in reinforcement are negligible.
- The use of bio concrete significantly enhances the strength of the concrete. The research conducted by Partheeban Pachaivannan, C. Hariharasudhan, M Mohanasundram, M. Anitha Bhavani (Authors of the article titled “Experimental anaylsis of self-healing properties of bacterial concrete”), concluded that the compressive strength test conducted on the 28th day of curing shows that the strength increased up to 19.51% for 14- day bacterial concrete as compared to conventional concrete.
- It has lower permeability when compared to conventional concrete.
- It also has lower water absorption when compared to conventional concrete.
- It offers great resistance against freeze-and-thaw attacks.
- Repairing cracks can be done efficiently.
- It basically increases the durability of the structure to a large extent.
- There is no requirement for regular upkeep of the structure.
- It can function on interior levels, allowing it to penetrate even the tiniest cracks.
Disadvantages of Self-Healing Concrete
Along with advantages, self-healing concrete has the following disadvantages:
- The cost of this concrete is comparatively higher than conventional concrete; its price is about double compared to conventional concrete.
- Different environmental variables can have an impact on the growth of bacteria.
- Bacteria used in this concrete are not good for human health; hence, there is a need to restrict their usage to the structure.
- No design guidelines for bacterial concrete are yet available in the Codes of Design.
- There is a need for skilled workers while making self-healing concrete.
- Concrete’s matrix undergoes a shift that limits mixing and, as a result, weakens the concrete.
Applications of Self-Healing Concrete
Self-healing concrete is the best alternative for foundations, slabs, and columns. Most modest residential buildings rarely change their function, so self-healing concrete would be the most practical option.
Self-healing concrete is ideal for use in residential and public middle-sized structures. However, it is necessary to ensure that the designs are flexible enough to accommodate the extended lifespan of public buildings. This way, instead of bringing down a building when a particular service is no longer required, remodelling becomes an option. This approach can reduce CO2 emissions by avoiding the need to construct a new structure.
It is common for bridges, tunnel linings, structural basement walls, highways, concrete floors, maritime buildings, and all road structures to have minor fractures due to severe loads, calling for regular maintenance. Hence, these are major areas for applications of self-healing concrete. Because of the numerous advantages, it is wise to use self-healing concrete because it reduces maintenance costs and improves safety. Further, bacterial concrete applications include preventing minor cracks from expanding in the oil and gas sectors, reinforcing old and new structures, and working with frequent freezing and thawing cycles.
Future Scope of Self-healing Concrete in Construction Industry
After recent research and development in bio concrete, several big industries have joined hands with the Delft University to develop applications of self-fixing concrete. Investment funding from construction industry is now forthcoming. Delft University researchers are now developing self-healing concrete products for specific civil engineering markets that will not be in competition with one another. They are developing the product for sectors such as tunnel-lining, structural basement walls, highway bridges, concrete floors, and marine structures.
Conclusion
To sum up, self-healing concrete technology has proved to be better than many conventional technologies because of its eco-friendly nature, self-healing properties and ability to increase durability of structures. Various researchers have successfully improved our understanding of the possibilities and limitations of biotechnological applications on building materials Various cementations and stone materials have shown enhancement of compressive strength, reduction in permeability, water absorption and reduced reinforcement corrosion. Formation of cementation by this method is very easy and convenient for usage. This will provide the basis for high-quality concrete structures that will be cost-effective and environmentally safe. However, there is a need for more research in this area to improve the feasibility of this technology from both economical and scalable practical view point.
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