Bioconcrete: New Perspective of Self-Healing Concrete
As we all know that, concrete is still one of the main materials used for any construction project, ranging from the foundation of buildings to the structure of bridges and underground parking lots. Concrete construction is widely preferred all over the globe due to the convenience in working and promising durability. But like the old idiom that states – ‘Every coin has its two sides’; the concrete construction also has its share of problems.
Some issues arising in concrete constructions are listed here under:
- The formation of cracks in the concrete structure which ultimately affects its durability.
- As the size of cracks become more, it damages the structural integrity.
- The smaller millimeter-sized cracks might also result in durability problems.
- Similarly, few connected cracks increase matrix permeability.
- Ingress water and chemicals can cause premature concrete degradation and corrosion of steel reinforcement.
The Need of Self-Healing Bacterial Concrete / Bioconcrete
Concrete is the most important building material for the construction industry, but, the most concrete structures are affected by various types of cracking problems during its life span. Small cracks developing on the surface of the concrete make the whole structure appear vulnerable because water seeps into the ordinary concrete and starts degrading the same and corrode the steel reinforcement, greatly reducing the lifespan or durability of any structure. It is a well-known fact that concrete can withstand compressive forces very well but not tensile forces. When it is subjected to tensile force it starts to crack, this is why steel reinforcement is used to withstand the tensile forces and support concrete structures.
Structures built in high-water environment, such as underground basements and marine structures are particularly vulnerable to corrosion of steel reinforcement. Bridges are also affected by corrosion problems because salts used to de-ice the roads penetrates into the cracks in the structures and can accelerate the corrosion of steel reinforcement. In the majority of civil engineering structures, tensile forces can lead to cracks and these can occur relatively soon after the structure is constructed. Repair of conventional concrete construction is done by applying a concrete mortar which is bonded to the damaged surface. Sometimes, the mortar needs to be inserted into the existing structure’s cracks with metal pins to ensure that it does not fall away. Repairs are time-consuming and expensive because these are often very difficult to execute. Additional effort is required sometimes to gain access to the structure to make repairs, especially if these are underground or at a greater height.
As repair work and regular manual maintenance of concrete constructions are costly and, in some cases, not at all possible, hence it becomes necessary to find a recovery option for it, and the self-healing concrete is one such promising option. The adoption of an autonomous self-healing repair mechanism would be highly advantageous as it could both reduce maintenance and increase material durability. The cracks also create problems in brickwork. To know more about it, read Investigation & Repairing of Cracks in Brickwork.
Self-healing concrete used in construction can reduce the problem of concrete structures deteriorating well before the end of their durable service life. Self-healing concrete is also known as a bioconcrete, self-fixing concrete, bacterial concrete, self-repairing concrete, bacterial self-healing concrete and bacteria healing concrete.
History of Self – Healing Concrete
Self-healing concrete was invented by ‘Hendrik Jonkers’, microbiologist of the Delft University of Technology in the Netherlands. He was highly inspired by natural body processes in which bones heal through mineralization, and Jonkers implemented the idea whether this could be used in concrete at all.
Looking towards the unique properties and conditions of concrete, it was a challenging experiment that took Jonkers almost three years and saw him named as a finalist for the European Inventor Award 2015. We all know that concrete is an extremely alkaline material in the nature and finding some bacteria that could survive its dry, stone-like properties and remain dormant before being activated by water was not easy.
Also Read: Fibre Reinforced Concrete History
Self – Healing Concrete Mechanism
Self-healing concrete repairs itself by biologically producing limestone to heal cracks that appear on the surface of concrete structures. There are selected types of the bacteria called Genus Bacillus, along with a calcium-based nutrient known as calcium lactate nitrogen and phosphorus, are added to the ingredients of the concrete when it is being mixed.
These self-healing agents or self-healing concrete bacteria can remain alive in concrete up to 200 years. Whenever the concrete is damaged and water starts entering the cracks that appear in the concrete, the spores of the bacteria fertilize after coming in contact with water and nutrients will get activated. The bacteria start to feed the calcium lactate. As the bacteria consume oxygen, the soluble calcium lactate is converted to insoluble limestone. The limestone solidifies on the cracked surface, thereby sealing it up.
It copies the process by which bone fractures in the human body are naturally healed by osteoblast cells that mineralize to re-form the bone. The consumption of oxygen, during bacterial conversion of calcium lactate to limestone has an additional advantage. As we know that, the oxygen is a major element in the process of corrosion of steel, here in this process, the bacterial activity has consumed it all increasing the durability of steel reinforced concrete constructions.
There are two self – repairing agents:
- The bacterial spores
- The calcium lactate-based nutrients
There are added to the concrete with separate expanded clay pellets 2-4 mm wide. This ensures that the self-healing bacterias will not be activated during the cement mixing process. When cracks in a concrete open up the pellets and infiltrating water brings the calcium lactate into contact with the bacteria activating this self-healing process.
Testing data expressed that when water enters into the concrete, the bacteria fertilize and multiply quickly. The bacteria transform the nutrients into limestone within seven days in the laboratory.
Also Read: How to Avoid Cracks in Concrete?
Selection of Bacteria
The beginning of the research was to find bacteria capable of surviving in an extreme alkaline environment. Cement used in concrete may have pH value of up to 13 when mixed. Usually, in an adverse environment, most organisms die in an environment with a pH value of 10 or above. Therefore, the research is focused 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.
At ‘Delft University’ the similar bacteria were grown in a flask of water that would then be used as the part of the water mix for the self-healing concrete. Various types of bacteria were added into a small block of concrete. Each block of concrete would be left for two months to become hard. Then the block would be crushed into small particle and the remains tested to see whether the bacteria had survived during a specified time period. From test results, it was found that only some of the bacteria that were able to survive were the ones, that produced spores comparable to plant seeds. Such bacteria spores have extremely thick cell walls that enable them to remain intact for up to 200 years while waiting for a better environment to activate.
These bacteria would get activated when the concrete starts to crack, food becomes available, 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 different 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 Bacteria are identified as:
- Bacillus megaterium
- Bacillus pasteurii
- Bacillus sp. CT-5
- Bacillus subtilis
- Bacillus Aerius
- Sporosarcina pasteurii
- Shewanella Species
- Bacillus flexus
It is found, that Bacillus megaterium can precipitate the maximum amount of calcite when compared to other positive bacteria, which results in increase in compressive strength and higher efficiency of crack-healing.
Scope in Construction Industry
The idea of bacteria healing concrete was first introduced by US academician in the late 1990s by the research group of ‘Professor Sookie Bang’. Testing and application of the theory were not investigated further because there was a lack of interest from the commercial branch of engineering sector for such a product. But after recent research and development in bioconcrete, several big industries have created partnerships with Delft University to develop applications of self-fixing concrete. Investment funding from construction industry is now forthcoming. Delft University researchers are now therefore 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.
Testing and Comparison with Normal Concrete
For testing self-healing properties of concrete, the small concrete structure or part of a structure will be built with the self-healing material and observed over two to four years. The concrete structures will be attached to some panels of bacterial concrete and others with normal concrete so that the behavior of the two can be examined and compared. Cracks in concrete will be made much larger than the ones that have healed up in the laboratory to determine how well and fast they heal over the time. Commercial industrial partners have asked whether the process could be used to repair existing structures. To answer this question Delft University has just been given funding of €420,000 from the Dutch government. Two post-doctorate scientists will spend two years for developing a self-healing system to be applied to existing structures.
For finding out the solution to this problem, researchers are testing two systems. The first technique is to see that the structure gets bacteria and nutrients, as a self-healing mortar, so that they can be used to repair large-scale damage. The second technique will be to observe that the bacteria and food nutrients get dissolved into a liquid that is sprayed onto the surface of the concrete from where it can seep into the cracks after 200 years…
Advantages and Disadvantages
Advantages of Self – Healing Concrete
- Overall maintenance cost of this concrete becomes low. The use of bioconcrete significantly enhances the strength of the concrete.
- It has lower permeability when compared to conventional concrete.
- It has also lower water absorption when compared to conventional concrete.
- It offers great resistance against freeze and thaw attacks.
- The chances of corrosion or reinforcement are reduced to negligible.
- Redressing of cracks can be done efficiently.
- It basically increases durability of the structure to a large extent.
Disadvantages of Self – Healing Concrete
- Cost of this concrete is comparatively higher than conventional concrete; it’s about 10-30% more than conventional concrete.
- The germination of bacteria is not suitable in every possible environment.
- The investigations to observe calcite precipitation are costly.
- Bacteria that are used in concrete are not good for human health; hence its usage has to be limited to the structure.
- No design guideline for bacterial concrete are yet available in Codes of Design.
To sum up, self-healing concrete technology has proved to be better than many conventional technologies because of its eco- friendly nature, self-healing abilities and increase in durability of structure. Various researchers have successfully improved our understanding of the possibilities and limitations of biotechnological applications on building materials. Enhancement of compressive strength, reduction in permeability, water absorption, and reduced reinforcement corrosion have been seen in various cementations and stone materials. 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, more research work is required to be done to improve the feasibility of this technology from both economical as well as scalable practical view point.