Self Healing Concrete Synopsis Print.docx

 INTRODUCTION Concrete is a vital use building material that is an absolutely essential component of public infrastructure and most buildings. It is most effective when reinforced by steel bar, mainly because its tensile strength without reinforcement is considerably low relative to its compressive strength. It is also a very brittle material with low tolerance for strain, so it is commonly expected to crack with time. These cracks, while not compromising structural integrity immediately, do expose the steel reinforcement to the elements, leading to corrosion which heightens maintenance costs and compromises structural integrity over long periods of time. That being said, concrete is a high maintenance material. It cracks and suffers serious wear and tear over the decades of its expected term of service. It is not flexible and cannot handle significant amounts of strain. Self-healing concrete in general seeks to rectify these flaws in order to extend the service life of any given concrete structure. There is a material in the realm of selfhealing concrete in development, now, that can solve many of the problems commonly associated with standard concrete. This material is bacterial self-healing concrete. Selfhealing concrete consists of a mix with bacteria incorporated into the concrete and calcium lactate food to support those bacteria when they become active.

 SELF BACTERIAL HEALING CONCRETE

Fig1- Self-healing bacterial concrete 1

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 cementing particles present in the concrete matrix. Due to capillary forces water is repeatedly drawn into 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. 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. Although bacteria, and particularly acid-producing bacteria, have been traditionally considered as harmful organisms for concrete, recent research has shown that specific species such as ureolytic and other bacteria can actually be useful as a tool to repair cracks or clean the surface of concrete.

 BACTERIA USED Cement and water have a pH value of up to 13 when mixed together, usually a hostile Environment for life, most organisms die in an environment with a pH value of 10 or above. Different types of bacteria were incorporated into a small block of concrete. Each concrete block would be left for two months to set hard. Then the block would be pulverized and the remains tested to see whether the bacteria had survived. It was found that the only group of bacteria that were able to survive were the ones that produced spores comparable to plant seeds. They are namely Bacillus-cohnii, Bacillus-filla, Bacillus-parturii

Fig2- Bacillus cohnii

Fig3- Bacillus filla 2

Fig4- Bacillus parturii

 Self-healing bacterial concrete can be prepared in two ways. 

In the direct application method, bacterial spores and calcium lactate is added into concrete directly when mixing of concrete is done.



By encapsulation method the bacteria and its food i.e. calcium lactate, are placed inside treated clay pallets and concrete is prepared about 6% of the clay pellets are added for making bacterial concrete

 BIOCONCRETE MECHANISM When the concrete is mixed with bacteria (bacillus subtilus), the bacteria go into a dormant state, a lot like seeds. All the bacteria need exposure to the air to activate their functions. Any cracks that should occur provide the necessary exposure. When the cracks form, bacteria very close proximity to the crack, starts precipitating calcite crystals. 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 nutrient. Such spores have extremely thick cell walls that enable them to remain intact for up to 200 years while waiting for a better environment to germinate.

 ADVANTAGES OF SELF-HEALING CONCRETE 

Incorporation of the agent in the concrete will be relatively cheap as well aseasy when the aggregate is immobilized in porous light weight aggregate prior to addition to the concrete mixture.



The self-healing bacterial concrete helps in reduced maintenance and repair costs of steel reinforced concrete structures.



Oxygen is an agent that can induce corrosion, as bacteria feeds on oxygen tendency for the corrosion of reinforcement can be reduced.



Self-healing bacteria can be used in places where humans find it difficult to reach for the maintenance of the structures. Hence it reduces risking of human life in dangerous areas and also increases the durability of the structure.



Formation of crack will be healed in the initial stage itself thereby increasing the service life of the structure than expected life.

3

 DISADVANTAGES OF SELF-HEALING CONCRETE 

If the volume of self-healing agents (bacteria and calcium lactate) mixed becomes greater than 20%, the strength of the concrete is reduced.



Preparation of self-healing concrete needs the requirement of bacteria and calcium lactate. Preparation of calcium lactate from milk is costlier. Hence preparation of self-healing concrete costs double than conventional concrete.

 CURRENT RESEARCHES There will be full-scale outdoor testing of self-healing concrete structures. Structures will be fitted with some panels of self-healing concrete and others with conventional concrete so that the behaviour of the two can be compared. The research will test two systems. The first technique will see bacteria and nutrients applied to the structure as a self-healing mortar, which can be used to repair large-scale damage. The second technique will see the bacteria and food Nutrients dissolved into a liquid that is sprayed onto the surface of the concrete from where it can seep into the crack

 APPLICATIONS Self-healing bacterial concrete can be used for sectors such as tunnel-lining, structural basement walls, highway bridges, concrete floors and marine structures.

Fig5- Marine structure

Fig6- Base wall 4

Fig7- Concrete flooring

Fig8- Tunnel lining

Fig9- Highway Bridge

 CONCLUSION The bacteria which are known to be alkali-resistant, i.e. they grow in natural environments characterized by a relatively high pH (10-11). In addition, these strains can produce spores which are resting cells with sturdy cell walls that protect them against extreme environmental mechanical- and chemical stresses. Therefore these specific bacteria may have the potential to resist the high internal concrete pH values(12-13 for Portland cement-based concrete), and remain viable for a long time as well, spore viability for up to 200 years is documented. We hypothesized that concrete immobilized spores of such bacteria may be able to seal cracks by bio mineral formation after being revived by water and growth nutrients entering freshly formed cracks. Although the exact nature of the produced minerals still needs to be clarified, they appear morphologically

5

related to calcite precipitates. The mechanism of bacterially-mediated calcite production likely precedes via organic carbon respiration with oxygen what results in carbonate ion production under alkaline conditions. The produced carbonate ions which can locally reach high concentrations at bacterially active 'hot spots' precipitate with excess calcium ions leaking out of the concrete matrix. This microbial calcium carbonate precipitation mechanism is well studied and occurs worldwide in natural systems such as oceans, bio films, microbial mats and stromatolites. For an autonomous self-healing mechanism all needed reaction components, or self-healing agents, must be present inthe material matrix to ensure minimal externally needed triggers. To conclude we can state that the application of bacteria as a self-healing agent in concrete appears promising

6

 REFERENCES Antonopoulos, S. Self-healing in ECC materials with high content of different Microfibers and micro particles, MSc Thesis, Delft University of Technology, 2009 De Muynck, W., Debrouwer, D., De Belie, N., Verstraete, W., 2008. Bacterial Carbonate precipitation improves the durability of cementations materials. Cement & Concrete Res. 38, 1005–1014. Bang, S.S., Galinat, J.K., Ramakrishnan, V., 2001. Calcite precipitatioinduced by polyurethane-immobilized Bacillus pasteurii. Enzyme Microb. Technol. 28, 404-409 Jonkers, HM & Schlangen, E. (2009a). Bacteria-based self-healing concrete. International journal of restoration of buildings and monuments, 15(4), 255265. .

7