SELF-HEALING CONCRETE
The future of Construction
Before getting into this article, imagine that you have just entered a construction site that is 30 years old to identify possible flaws in that building.
The first thing that most of us would notice is the plethora of cracks that envelope the place. In technical terms, we identify this as the degradation of concrete. Concrete degradation is one of our time’s costliest problems and can be reduced with well-designed and well-constructed structures.
Concrete will continue to be an essential building material for infrastructure, but most concrete structures are prone to cracking. A crack may appear due to reasons such as:
- Expansion and shrinkage of concrete due to temperature differences
- Loss of water from the concrete surface
- Insufficient vibration at the time of laying the concrete
- A high water-cement ratio that makes the concrete workable
- Increased shrinkage potential in mixtures with improved strength gain performance and rapid setting time.
Tiny cracks on the concrete’s surface make the whole structure vulnerable as water seeps in to degrade the concrete and corrode the steel reinforcement. Concrete degradation of our infrastructure now requires high repair costs, which can be avoided if we use self-healing concrete in the first place.
Bacterial Concrete or Self-Healing Concrete.
Small cracks with a width in the range of 0.05 to 0.1mm get completely sealed in repetitive dry and wet cycles without adding any agents like bacteria.
But for cracks with a width greater than 0.1mm, other remedial work is required. One possible technique that is currently under investigation involves the addition of mineral producing bacteria in concrete.
The bacteria used for self-healing of cracks are acid-producing. These bacteria types can be in dormant cells, act as a catalyst in the cracks healing process, and are viable for over 200 years.
Various Types of Bacteria Used in Concrete
Particular types of bacteria known as Bacillus are used along with calcium nutrients known as Calcium Lactate. While preparation of concrete, these products are added to wet concrete during mixing.
The various types of bacteria used in constructions are:
- Bacillus pasteurizing
- Bacillus sphaericus
- Bacillus subtilis
- Bacillus cohnii
- Bacillus halodurans
- Bacillus pseudofirmus
Preparation of Bacterial Concrete
Bacterial concrete can be prepared in two ways,
- By direct application
- By encapsulation in lightweight concrete
In the Direct application method, bacterial spores and calcium lactate are added into concrete directly when mixing concrete is done. The use of these bacteria and calcium lactate doesn’t change the typical properties of concrete.
In the Encapsulation method, the bacteria and its food i.e., calcium lactate, are placed inside treated clay pellets, and concrete is prepared. About 6% of the clay pellets are added for making bacterial concrete. When cracks occur in the structures made with bacterial concrete, and the clay pellets break, the bacteria germinate and eat down the calcium lactate to produce limestone, that seals the crack.
Mechanism of Bacterial Concrete
Self-healing concrete results from the biological reaction of non-reacted limestone and a calcium-based nutrient with the help of bacteria to heal the cracks that appear in the building.
Bacteria plays two roles during the process of self-healing
- Acts as a healing material
When the cracks appear in the concrete, the water seeps in the cracks. The bacteria’s spores germinate and start feeding on the calcium lactate (Ca(C₃H₅O₂)₂ ), consuming oxygen. The soluble calcium lactate is converted to insoluble limestone (CaCO₃). The insoluble limestone starts to harden, thus filling the crack automatically without any external aid.
Ca(C₃H₅O₂)₂ + 7O₂ → CaCO₃ + 5CO₂ +5H₂O
The other advantage of this process is that as the oxygen is consumed by the bacteria to convert calcium into limestone, it helps prevent corrosion of steel due to cracks. This improves the durability of steel-reinforced concrete construction.
2. Acts as a Catalyst
When the water comes in contact with the non-hydrated calcium in the concrete, calcium hydroxide is produced by bacteria, which acts as a catalyst. This calcium hydroxide reacts with atmospheric carbon dioxide and forms limestone and water. This extra water molecule keeps the reaction going.
CaO(s) + H₂O → Ca(OH)₂(s)
Ca(OH)₂ + CO₂ →CaCO₃ + H₂O
The limestone then hardens itself and seals the cracks in the concrete.
Advantages of Bacterial Concrete
- Self-repairing of cracks without any external aide.
- Significant increase in compressive strength and flexural strength when compared to normal concrete.
- Resistance towards freeze-thaw attacks.
- Reduction in the permeability of concrete.
- Reduces the corrosion of steel due to the formation of the cracks and improves the durability of steel-reinforced concrete.
- Bacillus bacteria are harmless to human life and hence it can be used effectively.
Disadvantages of Bacterial Concrete
- The cost of bacterial concrete is double than of conventional concrete.
- The growth of bacteria is not good in any atmosphere and media.
- The clay pellets holding the self-healing agent comprise 20% of the volume of the concrete. This may become a shear zone or fault zone in the concrete.
- The mix design of the concrete with bacteria here is not available in any IS code or other code.
- Investigation of calcite precipitate is costly.
CONCLUSION: THE FUTURE CONCRETE
Self-healing concrete is a material that could revolutionize the construction industry shortly. The material is superior to traditional abiotic reinforced concrete in several areas. It is readily apparent that traditional concrete lacks long-term sustainability for several reasons. Conventional concrete has a short lifespan, deteriorating or needing replacement after just a few decades. Self-healing concrete lasts much longer than traditional concrete because it can heal cracks caused by water with no human intervention.
Moreover, standard concrete is expensive to produce and impossible to recycle efficiently, indicating it is neither economically sustainable nor environmentally friendly. Since self-healing concrete can renew itself, it requires much less concrete, therefore saving money on building costs in the long run and reducing the amount of waste associated with the demolition of traditional concrete structures.
According to Dr. Henk Jonkers (a microbiologist), “The results of the experiments on the bacterial concrete shows that immobilized bacteria and certain classes of needed food sources do not negatively affect concrete strength characteristics. Therefore, we can conclude that bacterially controlled crack-healing in concrete by mineral precipitation is potentially feasible”.
It can be extrapolated that there are few experimental limitations to the self-healing concrete in its current state. Still, the economic and environmental impact of the material is not yet fully understood. Further research will be required to reduce the cost of producing the bacteria so that the material may have a lower upfront cost and be accepted by contractors.
Until then, this innovative material will remain the “THE FUTURE CONCRETE”.
