BACTERIAL CONCRETE OR SELF
HEALING CONCRETE FOR REPAIR OF
CRACKS
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,
o
Concrete expands and shrinks due to
temperature differences
o
Settlement of structure
o
Due to heavy load applied
o
Due to loss of water from concrete
surface shrinkage occurs
o
Insufficient vibration at the time of
laying the concrete
o
Improper cover provided during
concreting
o
High water cement ratio to make the
concrete workable
o
Due to corrosion of reinforcement steel
o
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:
o
Bacillus pasteurizing
o
Bacillus sphaericus
o
Escherichia coli
o
Bacillus subtilis
o
Bacillus cohnii
o
Bacillus balodurans
o
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.
Preparation
of Bacterial Concrete
Bacterial concrete can be prepared in
two ways,
- o
By direct application
- o
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.
Advantages and Disadvantages of Bacterial Concrete
Advantages of Bacterial Concrete
o
Self-repairing of cracks
without any external aide.
o
Significant increase in
compressive strength and flexural strength when compared to normal concrete.
o
Resistance towards
freeze-thaw attacks.
o
Reduction in permeability of
concrete.
o
Reduces the corrosion of
steel due to the cracks formation and improves the durability of steel
reinforced concrete.
o
Bacillus bacteria are
harmless to human life and hence it can be used effectively.
Disadvantages of Bacterial Concrete
o
Cost of bacterial concrete
is double than conventional concrete.
o
Growth of bacteria is not
good in any atmosphere and media.
o
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.
o
Design of mix concrete with
bacteria here is not available any IS code or other code.
o
Investigation of calcite
precipitate is costly.
