Concrete which forms major components in the construction Industry as it is cheap, easily available and convenient to cast. But drawback of these materials is it is weak in tension so, it cracks under sustained loading and due to aggressive environmental agents which ultimately reduce the life of the structure which are built using these materials. This process of damage occurs in the early life of the building structure and also during its life time. Synthetic materials like epoxies are used for remediation .But,they are not compatible, costly, reduce aesthetic appearance and need constant maintenance .Therefore bacterial induced Calcium Carbonate (calcite) precipitation has been proposed as an alternative and environment friendly crack remediation and hence improvement of strength of building materials
A novel technique is adopted in remediating cracks and fissures in calcium concrete by utilizing Microbiologically Induced Calcite Or Carbonate (CaCO3) Precipitation (MICP) is a technique that comes under a broader category of science called biomineralization . MICP is highly desirable because the Calcite precipitation induced as a result of microbial activities is pollution free and natural .The technique can be used to improve the compressive strength and stiffness of cracked concrete specimens. Research leading to microbial Calcium Carbonate precipitation and its ability to heal cracks of construction materials has led to many applications like crack remediation of concrete ,sand consolidation ,restoration of historical monuments and other such
applications. So it can be defined as “The process can occur inside or outside the microbial cell or even some distance away within the concrete .Often bacterial activities simply trigger a change in solution chemistry that leads to over saturation and mineral precipitation.Use of these Bio mineralogy concepts in concrete leads to potential invention of new material called –Bacterial Concrete.
SELF HEALING BACTERIAL CONCRETE
Autogenously crack-healing capacity of concrete has been recognized in several recent studies .Mainly micro cracks with widths typically in the range of 0.05 to 0.1mm have been observed to become completely sealed particularly under repetitive dry/wet cycles .The mechanism of this autogenously healing is chiefly due to secondary hydration of non or partially reacted cement particles present in the concrete matrix. Due to capillary forces water is repeatedly drawn into the micro cracks under changing wet and dry cycles, resulting in expansion of hydrated cement particles due to the formation of calcium silicate hydrates and calcium hydroxide. These reaction products are able to completely seal cracks provided that crack widths are small. Larger sized cracks can only be partially filled due to the limited amount of non – reacted cement particles present, thus resulting in only a thin layer of hydration products on the crack surface. For these reasons, alternative and more sustainable self- healing mechanism are therefore needed.
Bacterial concrete specimen
One possible technique is currently being investigated and developed was based on application of mineral producing bacteria in concrete. Although bacteria and particularly acid producing bacteria, have been traditionally considered as harmful organisms for concrete, recent research has shown that species such as ureolytic and other bacteria can actually be useful as a tool to repair cracks or clean the surface of concrete. In the latter studies bacteria were externally and manually applied on the concrete surface , while for autogenously repair an intrinsic agents as their spores , specialized thick –walled dormant cells, have been shown to be viable for over 200 years under dry conditions .Such bacteria would comprise of one of the two components needed for autogenously healing system .For bacteria would comprise of one of the two components, and bacteria can act as catalyst for the metabolic conversion of a suitable organic or inorganic component, the second component to produce this. The nature of metabolically produced filler materials could be bio minerals such as calcite (calcium carbonate) .These minerals are relatively dense and can block cracks, and thus hamper ingress of water efficiently . The development of a self-healing mechanism in concrete that is based on potentially cheaper and more sustainable material then cement could thus be beneficial for both economy and environment.
ADVANTAGES OF USING BACTERIA IN CONCRETE
Around five per cent of all man made carbon dioxide emissions are from the production of concrete, making it a significant contributor to global warming .Finding a way of prolonging the lifespan of existing structures means we could reduce this environmental impact and work towards a more sustainable solution.
· This could be particularly useful in earthquake zones where hundreds of buildings have to be flattened because there is currently no easy way of repairing the cracks and make them structurally sound
· Fills the crack in an efficient period of time so that the life period of a concrete structure can be expected over 200 years
· Prevents the use of cement in future used as a maintenance structure by drilling and grouting process ,so in this way ,less use of cement can be seen
· As we know more of cement content ,more will be carbon dioxide gases released causing global warming ,effecting the ozone layer .By using this bacteria ,the structure does not need to be repaired except for the less cases and so results in less use of cement
VARIOUS TYPES OF BACTERIA USED IN CONCRETE
There are various types of bacteria were used in construction area
· Bacillus pasteurii
· Bacillus sphaericus
· Escherichia coli
· Bacillus subtilis
· Bacillus cohnii
· Bacillus balodurans
· Bacillus pseudofirmus
VIABLE BACTERIA AS SELF HEALING AGENT
The bacteria to be used as self- healing agent in concrete should be fit for the job, i.e. they should be able to perform long-term effective crack sealing, preferably during the total constructions life time. The principle mechanism of bacterial crack healing is that the bacteria themselves act largely as a catalyst, and transform a precursor compound to a suitable filler material. The newly produced compounds such as calcium carbonate-based mineral precipitates should than act as a type of bio-cement what effectively seals newly formed cracks. Thus for effective self –healing, both bacteria and a bio-cement precursor compound should be integrated in the material matrix. However, the presence of the matrix-embedded bacteria and precursor compounds should not negatively affect other wanted concrete characteristics .Bacteria that can resist concrete matrix incorporation exist in nature, and these appear related to a specialized group of alkali-resistant spore-forming bacteria. Interesting feature of these bacteria is that they are able to form spores, which are specialized spherical thick-walled cells somewhat homologous to plant seeds. These spores are viable but dormant cells and can withstand mechanical and chemical stresses and remain in dry state viable for periods over 200 years