SELF HEALING CONCRETE

Bacterial concrete or Self Healing concrete fills up the cracks developed in structures by the help of bacterial reaction in the concrete after hardening. 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 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 cracks. 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

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 0.05-0.1 mm act as capillary and the water particles seep through the cracks. These water particles hydrate non or partially reacted cement and the cement expands, which in turn fill 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. 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 with a calcium based nutrient with the help of bacteria to heal the cracks appeared on the building. Special type of bacteria known as Bacillus is used along with calcium nutrient known as Calcium Lactate. While preparing concrete, this products are added in the wet concrete when the mixing is done. This bacteria can be in dormant state for around 200 years. When the cracks appear in the concrete, the water seeps through 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 aid. The other advantage of this process is that 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.

Bacterial Concrete or Self Healing Concrete For Crack Repairs

Preparation of Bacterial Concrete

Bacterial concrete can be prepared in two ways,

  1. By Direct application
  2. 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 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 the 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. About6% of the clay pellets are added for making bacterial concrete. The concrete structures are made with bacterial concrete when the cracks occur in the structure then the 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 is costlier than direct application.

Chemical process of 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.

CaO+H2O——>Ca(OH)2

Ca(OH)2+CO2——>CaCO3+H2O

The limestone then hardens itself and seals the cracks in the concrete.

Tests on Bacterial Concrete

Standard test were conducted on normal concrete and self healing concrete. Tests conducted were Compressive test and Flexural strength test on a concrete cube for 7 and 28 days.

SL. No.DAYSNORMAL CONCRETE [N/mm2]BACTERIAL CONCRETE [N/mm2]
1720.8527.10
22830.0038.95
COMPRESIVE STRENGTH TEST RESULT FOR 7 AND 28 DAYS FOR BACTERIAL CONCRETE
SL.No.DAYSNORMAL CONCRETE [N/mm2]BACTERIAL CONCRETE [N/mm2]
173.904.60
2287.057.80
FLEXURAL STRENGTH TEST RESULT FOR 7 AND 28 DAYS FOR BACTERIAL CONCRETE

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

ADVANTAGES OF BACTERIAL CONCRETE

  • Self repairing of cracks without any external aid.
  • Significant increase in compressive strength and flexural strength when compared to the normal concrete.
  • Resistance towards freeze-thaw attacks.
  • Resistance in permeability of concrete.
  • Reduces the corrosion of steel due to cracks formation and increases 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 the 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 in any IS code or other code.
  • Investigation of calcite precipitate is costly.