1. INTRODUCTION
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.
2. 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.
FIG. 2.1: 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.
3. 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
· 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
4. 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
5. VIABLE BACTERIA AS SELF HEALING AGENT
· Bacillus pasteurii
· Bacillus sphaericus
· Escherichia coli
· Bacillus subtilis
· Bacillus cohnii
· Bacillus balodurans
· Bacillus pseudofirmus
5. 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 (FIG 5.1).
FIG.5.1: ESEM photomicrograph (5000x magnification) of alkali-resistant spore forming bacterium
However, when bacterial spores were directly added to the concrete mixture, their lifetime appeared to be limited to one-two months. The decrease in life-time of the bacterial spores from several decades when in dry state to only a few months when embedded in the concrete matrix may be due to continuing cement hydration resulting in matrix pore-diameter widths typically much smaller than the 1-µm sized bacterial spores. Another concern is whether direct addition of organic bio-mineral precursor compounds to the concrete mixture will not result in unwanted loss of other concrete properties. In the preceding study it was indeed found that various organic bio-cement precursor compounds such as yeast extract, peptone and calcium acetate resulted in a dramatic decrease of compressive strength. The only exception appeared to be calcium lactate what actually resulted in a 10% increase in compressive strength compared to control specimen.
In order to substantially increase the lifetime and associated functionality of concrete incorporated bacteria, the effect of bacterial spore and simultaneously needed organic bio mineral precursor compound (calcium lactate) immobilization in porous expanded clay particles was tested. It was found that protection of the bacterial spores by immobilization inside porous expanded clay particles before addition to the concrete mixture (FIG. 5.2) indeed substantially prolonged their life-time. Currently running viability experiments show that still after 6 months concrete incorporation no loss of viability is observed, suggesting that their long term viability as observed in dried state when not embedded in concrete is maintained.
FIG.5.2:self- healing admixture composed of expanded clay particles(left)loaded with bacterial spores and organic bio mineral precursor compound (calcium lactate).when embedded in the concrete matrix(right).
6. PREPARATION OF BACTERIAL CONCRETE
Bacterial concrete can be prepared in two ways
· By direct application
· By encapsulation in light weight concrete
· By direct application
· By encapsulation in light weight concrete
By the method of direct application bacterial spores and calcium lactate are added directly while making the concrete and mixed .Here when the crack occurs in the concrete bacterial spores broke and bacteria comes to life comes to life and feed on the calcium lactate and limestone is produced which fill the cracks.
By encapsulation method the bacteria and its food , calcium lactate ,are placed inside treated clay pellets and concrete is made. 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 bacterial treatment occurs and hence the concrete s healed. 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 .Bacillus bacteria are harmless to human life and hence it can be used effectively.
7. MECHANISM OF BACTERIAL CONCRETE
Self-healing concrete is a product that will biologically produce limestone to heal cracks that appear on the surface of concrete structures .Specially selected types of the bacteria genus Bacillus, along with a calcium-based nutrient known as calcium lactate, and nitrogen and phosphorus, are added to the ingredients of the concrete when it is being mixed. These self-healing agents can lie dormant within the concrete for up to 200 years.
However, when a concrete structure is damaged and water starts to seep through the cracks that appear in the concrete, the spores of the bacteria germinate on contact with the water and nutrients. Having been activated, the bacteria start to feed on the calcium lactate. As the bacteria feeds oxygen is consumed and the soluble calcium lactate is converted to insoluble limestone. The limestone solidifies on the cracked surface, thereby sealing it up. It mimics the process by which bone fractures in the human body are naturally healed by osteoblast cells that mineralize to re-form the bone.
The consumption of oxygen during the bacterial conversion of calcium lactate to limestone has an additional advantage. Oxygen is an essential element in the process of corrosion of steel and when the bacterial activity has consumed it all it increases the durability of steel reinforced concrete constructions.
FIG.7.1: Process of fixing crack in concrete
In the crack fixing process the anaerobic type bacteria which can be using along with concrete can be fix that crack by step by step. At first germination of germs by spores and swarming themselves and quorum sensing and growing from proper medium in large amount in particular time and from the metabolism process leans glue is produce and making such type of filamentous cell formation and precipitation CaCO3.This both material combine with each other and making cementations material.
8. CHEMICAL PROCESS TO REMEDIATE CRACKS BY
BACTERIA
Crack –penetrating water would not only dissolve calcite (CaCO3) particles present in mortar matrix ,but would also react together with atmospheric carbon dioxide with not fully hydrated lime constituents such as calcium oxide and calcium hydroxide according to the following reactions:
The freshly produced minerals from the above stated reactions and from dissolved and re-crystallized calcite mineral, precipitated on the surface of cracks what resulted in crack-sealing and concomitant reduction in permeability of the mortar. The healing potential of this system was directly related to the amount of nonreacted lime particles within the set mortar. Calcium carbonate precipitation is a straight forward chemical process governed mainly by four key factors.
1. Calcium concentration
2. Concentration of dissolved inorganic carbon(DIC)
3. The pH
4. Availability of nucleation sites
The concentration of calcium carbonate ions is related to the concentration of DIC and the pH of a given aquatic system. The precipitation of Calcium carbonate crystals occurs by heterogeneous nucleation on bacterial cell walls once super saturation is achieved .The fact that hydrolysis of urea is a straight forward microbial process and that a wide variety of microorganisms produce urease enzyme and makes it ideally suited for crack remediation for building material applications. This precipitation forms a highly impermeable layer which can be used as crack remediation for concrete or any other building material. The precipitated calcite has a coarse crystalline structure that readily adheres to the concrete surface in the form of scale. In addition it has the ability to continuously grow upon itself and it is highly insoluble in water.
9. STUDY ON STRENGTH AND DURABILITY OF BACTERIAL
CONCRETE
CONCRETE
The strength and the total durability of the concrete can be increased by introducing bacteria (Bacillus pasteurii). This microorganism is a soil bacterium. As we know Bacillus pasteurii exhibits a phenomenon known as bio-calcification as a part of its metabolic activity. Bio-calcification is a process through which the microorganism externally secretes calcium precipitate, which in the presence of a carbonate ion forms CaCO3 which fills up the voids in the concrete texture thus making it more compact. This in turn improves the strength in concrete due to growth of the filler material within the pores of the concrete mixer. A comparison study was made with concrete cubes and beams subjected to compressive and flexural strength tests with and without the bacterium. It was found that there was high increase in strength and healing of cracks subjected to loading on the concrete specimens.
9.1.1 PREPARATION OF BACTERIAL SOLUTION
· Primarily 12.5g of Nutrient broth (media) is added to a 500ml conical flask containing distilled water.
· It is then covered with a thick cotton plug and is made air tight with paper and rubber band.
· It is then sterilized using a cooker for about 10-20 minutes. Now the solution is free from any contaminants and the solution is clear orange in colour before the addition of the bacteria.
FIGURE 9.1: Solution without bacteria (only media)
· Later the flasks are opened up and an exactly 1ml of the bacterium is added to the sterilized flask and is kept in a shaker at a speed of 150- 200 rpm overnight.
· After 24 hours the bacterial solution was found to be whitish yellow turbid solution. FIGURE9.2: Solution with bacteria
9.1.2 COMPRESSION STRENGTH TEST
· The cubical Moulds of size 150mm x 150mm x 150mm were cleaned and checked against the joint movement. A coat of oil was applied on the inner surface of the Moulds and kept ready for the concreting operation.
· Meanwhile the required quantities if cement, fine aggregate and coarse aggregate (passing through IS sieve of 20 mm size and retained on 4.75 mm) for the particular mix are weighed accurately for concreting.
· Fine aggregate and cement were mixed thoroughly in a hand mixer such that the colour of the mixture is uniform.
· Now weighed quantity of coarse aggregate is added to the mixer and then it rotated till uniform dry mixture is obtained.
· Then, calculated quantity of bacterial solution and water was added and mixing was continued for about 3 to 5 minutes to get a uniform mix.
· The wet concrete is now poured into the Moulds and for every 2 to 3 layers and compacted manually. After concreting operations, the upper surface is levelled and finished with a mason’s trowel.
· The corresponding identification marks were labelled over the finished surface and they were tested for 7 and 28 day strengths in a compressive strength testing machine.
· COMPRESSIVE STRENGTH = TOTAL FAILURE LOAD/AREA OF THE CUBE
FIG. 9.3: Concrete cube subjected to compression
9.1.3 FLEXURAL STRENGTH TEST
· Moulds of 10cm x 10cm x 50cm is used and the Moulds are cleaned and the joints between the sections of Moulds shall be thinly coated with Moulds oil and a similar coating of Moulds oil shall be applied between the contact surfaces of the bottom of the Moulds and the base plate in order to ensure that no water escapes during the filling.
· The interior faces of the assembled Moulds shall be thinly coated with Moulds oil to prevent adhesion of the concrete.
· Meanwhile the required quantities of cement, fine aggregate and corresponding coarse aggregate for the particular mix are weighed accurately for concreting.
· Fine aggregate and cement were mixed thoroughly in a hand mixer such that the colour of the mixture is uniform.
· Then, weighed quantity of coarse aggregate is added to the mixer and then it rotated till uniform dry mixture is obtained.
· Then, calculated quantity of water and bacterial solution was added and mixing was continued for about 3 to 5 minutes to get a uniform mix.
· The wet concrete is now poured into the Moulds in 2 to 3 layers and compacted manually.
· After concreting operations, the upper surface is levelled and finished
with a mason’s trowel.
· The corresponding identification marks were labelled over the finished surface and the beams were tested for 7 and 28 days strengths.
· FLEXURAL STRENGTH=Pl/b
P – Failure load of the specimen
l - Length of the specimen
b - Breadth of the specimen
d- Depth of the specimen
9.1.4 RESULT AND ANALYSIS OF TEST
The test results showed a significant difference in the specimens tested, with and without bacteria. Here are the following tables and charts which will give clear information about the compression and flexural strength test results.
TABLE .9.1: compressive strength test result for 7 and 28 days
TABLE .9.2: flexural strength test result for 7 and 28 days
FIG.9.4: graph shows comparison of compressive strength test results for 7 and 28 days
FIG.9.5: graph shows comparison of flexural strength test results for 7 and 28 days
9.1.5 SEM OR SCANNING ELECTRON MICROSCOPY EXAMINATION
To determine whether there is increase in compressive strength of the bacterial concrete with Bacillus pasteurii bacteria with microbial calcite precipitation in the crack sample was examined in SEM . The sample showed the presence of calcite crystals grown all over the surface of the crack and also the presence of Bacillus pasteurii bacteria is the evidence, that suggests microbial remediation properties of bacterial concrete.
FIG.9.6: SEM for conventional concrete shows that more voids in micro structures
FIG.9.7: SEM for bio concrete shows that more dense in micro structures as compare to conventional concrete, which implies that it enhance strength to the concrete
10. STUDY ON PERMEABILITY OF BACTERIAL CONCRETE Permeability is the most crucial internal factor in concrete durability .The durability of a concrete is closely related to its permeability. The permeability dictates the rate at which aggressive agents can penetrate to attack the concrete and the steel reinforcement. Water penetrability is defined as the degree to which a material permits the transport gases, liquids or ionic species through it. Water can be harmful for concrete, because of its ability to leach calcium hydroxide from the cement paste, to carry harmful dissolved species such as chlorides or acids into the concrete, to form ice in large pores in the paste, and to cause leaching of compounds from the concrete. Bacterial concrete works on the phenomenon of microbiologically induced calcite precipitation. Calcite crystals formed, due to microbial activities of bacteria Bacillus , seals the cracks and pores in concrete and enhances the strength and durability of concrete by making concrete impermeable to transport different fluids or gases, like water, chlorides, sulfates or oxygen.
10.1 DETERMINATION OF PERMEABILITY OF CONCRETE AS PER IS 3085
Concrete water permeability test is conducted as per IS3085:1965.
· The permeability tester used was a 3-cell tester comprising of three test cells, a pressure chamber and an air compressor supplying water to the test samples under required pressure.
· Bacteria treated concrete cylindrical specimens and controlled cylindrical concrete specimens (without bacteria treatment) of diameter 150mm and height 150mm are casted and cured for 28 and 90 days.
· They are loaded in the specially designed cells, and the sealing compound is used to fill the annular space between the specimen and the cell comprised of two parts of resin and one part of wax by volume.
· All surfaces of the sample except the top one through which water was to be supplied were painted with the hot sealing compound.
· The specimen was placed centrally in the cell.
· The testing of seal for any leakage was done after allowing the sealing compound to harden for 24 hours.
· The specimen was subjected to hydrostatic pressure so that water should percolate from above the specimen’s top surface and collected in the bottles kept below the cell with funnel arrangements. It is essential that the seal is leak-proof.This may be checked very conveniently by bolting on the top cover plate, inverting the cell and applying an air pressure of 1 to 2 kg/ from below
· A constant air pressure of 15 kg/ is maintained by using air compressor throughout the experiment for a given interval of time.
· The standard test pressure head to be applied to the water should be 10 kg/cm2.
· The quantity of percolate and water collected is measured at periodic intervals.
· In the beginning, the rate of water intake is larger than the rate of outflow. As the steady state of flow is approached, the two rates tend to become equal and the outflow reaches maximum and stabilizes.
· With further passage of time both inflow and outflow generally register a gradual drop.
· Permeability test shall be continued for about 100 hours after the steady state of flow has been reached and the outflow shall be considered as average of all the outflows measured during the period of 100 hours.
· Then the coefficient of permeability (k, in m/sec) based on Darcy’s law for a falling water head ,which is applicable at steady state flow conditions, can be computed using the following formula.
· The permeability tester used was a 3-cell tester comprising of three test cells, a pressure chamber and an air compressor supplying water to the test samples under required pressure.
· Bacteria treated concrete cylindrical specimens and controlled cylindrical concrete specimens (without bacteria treatment) of diameter 150mm and height 150mm are casted and cured for 28 and 90 days.
· They are loaded in the specially designed cells, and the sealing compound is used to fill the annular space between the specimen and the cell comprised of two parts of resin and one part of wax by volume.
· All surfaces of the sample except the top one through which water was to be supplied were painted with the hot sealing compound.
· The specimen was placed centrally in the cell.
· The testing of seal for any leakage was done after allowing the sealing compound to harden for 24 hours.
· The specimen was subjected to hydrostatic pressure so that water should percolate from above the specimen’s top surface and collected in the bottles kept below the cell with funnel arrangements. It is essential that the seal is leak-proof.This may be checked very conveniently by bolting on the top cover plate, inverting the cell and applying an air pressure of 1 to 2 kg/ from below
· A constant air pressure of 15 kg/ is maintained by using air compressor throughout the experiment for a given interval of time.
· The standard test pressure head to be applied to the water should be 10 kg/cm2.
· The quantity of percolate and water collected is measured at periodic intervals.
· In the beginning, the rate of water intake is larger than the rate of outflow. As the steady state of flow is approached, the two rates tend to become equal and the outflow reaches maximum and stabilizes.
· With further passage of time both inflow and outflow generally register a gradual drop.
· Permeability test shall be continued for about 100 hours after the steady state of flow has been reached and the outflow shall be considered as average of all the outflows measured during the period of 100 hours.
· Then the coefficient of permeability (k, in m/sec) based on Darcy’s law for a falling water head ,which is applicable at steady state flow conditions, can be computed using the following formula.
Where K= coefficient of permeability in m/sec Q = Quantity of water collected in millilitres over the entire period of test
T=Time in seconds over which Q is measured =100×60×60 sec=360000sec
A= area of the specimen face in = 0.01767
Water pressure =10 kg/ = Pa
Pressure head = 100m
H/L = ratio of pressure head to thickness of the specimen both expressed in metre = 100/0.15=666.67
TABLE 10.1: coefficients of permeability for controlled and bacterial treated concrete specimens of age 28 days.
TABLE 10.2: coefficients of permeability for controlled and bacterial treated concrete specimens of age 90 days
TABLE 10.3: coefficients of permeability ranges as per IS:3085-1965
Fig 10.4: Graph Show Variation of Coefficient of water permeability with Grade of Concrete at 28 days age
Fig 10.5: Graph Show Variation of Coefficient of water permeability with Grade of Concrete at 90 days age
11. APPLICATION OF BACTERIA IN CONSTRUCTION AREA
The use of microbial concrete in Bio Geo Civil Engineering has become increasingly popular .From enhancement in durability of cementations materials to improvement in sand properties, from repair of limestone monuments, sealing of concrete cracks to highly durable bricks, microbial concrete has been successful in one and all. This new technology can provide ways for low cost and durable roads, high strength buildings with bearing capacity, long lasting river banks, erosion prevention of loose sands and low cost durable housing. Another issue in conventional building materials is the high production of greenhouse gases and high energy consumed during production of these materials and these greenhouse gases leads to global warming. High construction cost of building materials is another drawback in such cases. These drawbacks have lead to use of novel , eco-friendly ,self-healing and energy efficient technology where microbes are used for remediation of building materials and enhancement in the durability characteristics.
FIG.11.1 :application of bacterial concrete in construction area
12. ADVANTAGES AND DISADVANTAGES OF BACTERIAL
CONCRETE
12.1 ADVANTAGES:
1. Microbial Concrete in Crack Remediation: Specimens were filled with bacteria, nutrients and sand. Significant increase in compressive strength and stiffness values as compared to those without cells was demonstrated. 2. Improvement in Compressive Strength of Concrete: Compressive strength test results are used to determine that the concrete mixture as delivered meets the requirements of the job specification .So the effect of microbial concrete on compressive strength of concrete and mortar was studied and it was observed that significant enhancement in the strength of concrete and mortar can be seen upon application of bacteria 2. Better Resistance towards Freeze- Thaw Attack Reduction: Application of microbial calcite may help in resistance towards Freeze –thaw reduction due to bacterial chemical process and also it can reduce the permeability than freezing process decreased. 4. Reduction in Permeability of Concrete: Effect of microbial concrete on permeation properties was studied by different researchers .Permeability can be investigated by carbonation tests as it is increasingly apparent that decrease in gas permeability due to surface treatments results in an increased resistance towards carbonation and chloride ingress .Carbonation is related to the nature and connectivity of the pores, with larger pores giving rise to higher carbonation depths. 5. Reduction in corrosion of reinforced concrete: application of microbial calcite may ingress and improves the life of reinforced concrete structures
12.2 DISADVANTAGES:
1. Cost of bacterial concrete is double than conventional concrete
2. Growth of bacteria is not good in any atmosphere and media
3. The clay pellets holding the self-healing agent comprise 20% of the volume of the concrete.
3. The clay pellets holding the self-healing agent comprise 20% of the volume of the concrete.
4. Design of mix concrete with bacteria here is no available any IS code or other code
5. Investigation of calcite precipitate is costly
13. COST COMPARISON OF CONVENTIONAL AND
BACTERIAL CONCRETE
The cost of self-healing concrete is about double that of conventional concrete, which is presently about €80 euros per cubic metre. At around €160 per cubic metre, self-healing concrete would only be a viable product for certain civil engineering structures where the cost of concrete is much higher on account of being much higher quality, for example tunnel linings and marine structures where safety is a big factor – or in structures where there is limited access available for repair and maintenance. In these cases the increase in cost by introducing the self-healing agents should not be too enormous.
Added to this, if produced on an industrial scale it is thought that the self- healing concrete could come down in cost considerably. If the life of the structure can be extended by 30%, the doubling in the cost of the actual concrete would still save a lot of money in the longer term. Research is currently working on the development of an improved and more economic version of the bacteria-based healing agent which is expected to raise concrete costs only by a few euros.
A second self-healing agent that will be much cheaper and also would result in much stronger concrete is being developed. Presently the majority of the extra cost comes from the calcium lactate which is very expensive. The process of embedding the bacteria and nutrients into the pellets is also expensive because it involves a vacuum technique. A sugar-based food nutrient would potentially bring down the cost of the self-healing concrete to €85-90 per cubic metre. But a sugar-based nutrient would not remain intact within expanded clay pellets as calcium lactate does. Much of the sugar would be dissolved and it would delay the setting time of the concrete. The new selfhealing agent being developed would immobilise the sugar-based nutrient during the mixing process. So the team has now developed an alternative self-healing agent with a new shape and form and the way that the bacteria and nutrients would be stored would be totally different. The new healing agent would comprise only 3-5% of the overall volume and the concrete would therefore be much stronger. The new selfhealing agent would be a viable product for most structural concrete applications .If the cost of the self-healing agent can be brought down sufficiently and the concerns over the long-term effects on the concrete performance properly addressed, then the product could have great potential.
14. CONCLUSION
1. Bacterial concrete technology has proved to be better than many conventional technologies because of its eco- friendly nature, self-healing abilities and increase in durability of various building materials.
2. Work of various researchers has improved our understanding on the possibilities and limitations of biotechnological applications on building materials.
3. Enhancement of compressive strength, reduction in permeability, water absorption, reinforced corrosion have been seen in various cementitious and stone materials.
4. In bacterial concrete interconnectivity of pores is disturbed due to plugging of pores with calcite crystals.Since interconnected pores are significant for permeability ,the water permeability is decreased in bacteria treated specimens.
5. Cementation by this method is very easy and convenient for usage. This will soon provide the basis for high quality structures that will be cost effective and environmentally safe but, more work is required to improve the feasibility of this technology from both an economical and practical viewpoints.
6. The application of bacerial concrete to construction may also simplify some of the existing construction processes and revolutionize the ways of new construction processes.
