Bacterial Concrete or Self Healing Concrete For Repair of Cracks

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Bacterial concrete or self healing concrete fills up the cracks developed in structures by the help of bacterial reaction in the concrete after hardening. Types of bacteria, its mechanism and preparation of bacterial concrete is discussed. In modern days, the use of technology has taken the standards of construction to a new high level. Different types of procedures, methods and materials are used to attain a very good, sustainable and economic concrete construction. But due to human mistakes, incorrect handling and unskilled labors. An efficient building is hard to sustain its designed life. Many problems like weathering, cracks, leaks and bending etc., arises after the construction. To overcome this types of problems, many remedial procedures are undertaken before and after the construction. The common problem found in buildings is Crack. Crack may be due to many reasons. Some reasons are listed below,

• Concrete expands and shrinks due to temperature differences
• Settlement of structure
• Due to heavy load applied
• Due to loss of water from concrete surface shrinkage occurs
• Insufficient vibration at the time of laying the concrete
• Improper cover provided during concreting
• High water cement ratio to make the concrete workable
• Due to corrosion of reinforcement steel
• Many mixtures with rapid setting and strength gain performance have an increased shrinkage potential.

Bacterial Concrete or Self-Healing Concrete

This common problem of cracking in building has many remedies before and after the crack. One of the remedial process is Bacterial Concrete or Self-Healing Concrete. The process of self-healing of cracks or self-filling up of cracks by the help of bacterial reaction in the concrete after hardening is known as Self-Healing Concrete. It can be observed that small cracks that occur in a structure of width in the range of 0.05 to 0.1mm gets completely sealed in repetitive dry and wet cycles. The mechanism of this autogenously healing is, the width of range 0.05-0.1mm act as capillary and the water particles seep through the cracks. These water particles hydrate the non or partial reacted cement and the cement expands, which in turn fills the crack. But when the cracks are of greater width, need of other remedial work is required. One possible technique is currently being investigated and developed was based on application of mineral producing bacteria in concrete. The bacteria used for self-healing of cracks are acid producing bacteria. These types of bacteria can be in dormant cell and be viable for over 200 years under dry conditions. These bacteria acts as a catalyst in the cracks healing process.

Various Types of Bacteria Used in Concrete

There are various types of bacteria were used in bacterial concrete construction are:

• Bacillus pasteurizing
• Bacillus sphaericus
• Escherichia coli
• Bacillus subtilis
• Bacillus cohnii
• Bacillus balodurans
• Bacillus pseudofirmus

Mechanism of Bacterial Concrete

Self-healing concrete is a result of biological reaction of non-reacted limestone and a calcium based nutrient with the help of bacteria to heal the cracks appeared on the building. Special type of bacteria’s known as Bacillus are used along with calcium nutrient known as Calcium Lactate. While preparation of concrete, this products are added in the wet concrete when the mixing is done. This bacteria’s can be in dormant stage for around 200 years. When the cracks appear in the concrete, the water seeps in the cracks. The spores of the bacteria germinate and starts feeding on the calcium lactate consuming oxygen. The soluble calcium lactate is converted to insoluble limestone. The insoluble limestone starts to harden. Thus filling the crack, automatically without any external aide. The other advantage of this process is, as the oxygen is consumed by the bacteria to convert calcium into limestone, it helps in the prevention of corrosion of steel due to cracks. This improves the durability of steel reinforced concrete construction.

Fig: Process of fixing crack in concrete

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 is added into concrete directly when mixing of concrete is done. The use of this bacteria and calcium lactate doesn’t change the normal properties of concrete. When cracks are occurred in the structure due to obvious reasons. The bacteria are exposed to climatic changes. When water comes in contact with this bacteria, they germinate and feed on calcium lactate and produces limestone. Thus sealing the cracks. By 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 concrete structures are made with bacterial concrete, when the crack occurs in the structure and clay pellets are broken and the bacteria germinate and eat down the calcium lactate and produce limestone, which hardens and thus sealing the crack. Minor cracks about 0.5mm width can be treated by using bacterial concrete. Among these two methods encapsulation method is commonly used, even though it’s costlier than direct application.

Chemical Process of Self-Healing or Bacterial Concrete

When the water comes in contact with the unhydrated calcium in the concrete, calcium hydroxide is produced by the help of 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. The limestone then hardens itself and seals the cracks in the concrete.

Test and Result of Self-Healing or Bacterial Concrete and Normal Concrete

Compressive Strength Test result for 7 and 28 days for Bacterial Concrete

SL NO	DAYS	NORMAL CONCRETE (N/mm2)	BACTERIAL CONCRETE (N/mm2)
1	7	3.90	4.6
2	28	7.05	7.80

Flexural Strength Test result for 7 and 28 days of Bacterial Concrete

From the results we can see that both the compression strength and the flexural strength of the bacterial concrete is greater than that of normal concrete.

Advantages and Disadvantages of Bacterial 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 permeability of concrete.
• Reduces the corrosion of steel due to the cracks formation 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

• Cost of bacterial concrete is double than conventional concrete.
• 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.
• Design of mix concrete with bacteria here is not available any IS code or other code.
• Investigation of calcite precipitate is costly.