bacterial concrete

BACTERIAL CONCRETE – A NOVEL APPROACH A.SAI VIVEK, D.VARUN KUMAR – UNIVERSITY COLLEGE OF ENGINEERING, JNTUK KAKINADA ABSTRACT: Concrete structures are very susceptible to cracking which allows chemicals and water to enter and degrade the concrete, reducing the performance of the structures and also requires expensive maintenance in the form of repairs. Cracking in the surface layer of concrete mainly reduces its durability and also effects the reinforcement that is provided. To overcome all these problems a special concrete i.e., BACTERIAL CONCRETE which is a novel technique based on the application of bio mineralization of bacteria in concrete came into existence. Since synthetic polymers such as epoxy treatment etc. are currently being used for repair of concrete are harmful to the environment, hence the use of a biological repair technique in concrete is focused. This can be done by incorporating the bacteria like Bacillus Subtilis JC3 in the concrete matrix which contributes to the strength and durability of the concrete. Water which enters the concrete will activate the dormant bacteria which in turn will give strength to the concrete through the process of metabolically mediated calcium carbonate precipitation. Overview of development of bacterial concrete using the process of bio mineralization which can be served as a sustainable self-healing construction material will be discussed in this paper.

INTRODUCTION: Concrete is the most widely used component in the construction. But the main drawback is it is weak in tension. So, it cracks under sustained loading and due to environmental agents which reduce its durability. Synthetic materials like epoxies are used for remediation of these cracks. 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 re-mediating cracks and fissures in concrete by utilizing Microbiologically Induced Calcite or Calcium 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”.

HOW DOES BIO-CONCRETE WORK? 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. 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. The two self-healing agent parts (the bacterial spores and the calcium lactate-based nutrients) are introduced to the concrete within separate expanded clay pellets 24 mm wide, which ensure that the agents will not be activated during the cement-

mixing process. Only when cracks open up the pellets and incoming water brings the calcium lactate into contact with the bacteria do these become activated. Testing has shown that when water seeps into the concrete, the bacteria germinate and multiply quickly. They convert the nutrients into limestone within seven days in the laboratory. Outside, in lower temperatures, the process takes several weeks. Process of fixing cracks in concrete by bacteria in such a process can be shown in fig.

WORKING PRINCIPLE OF SELF HEALING PROCESS: In concrete the cracks up to 0.2 mm wide are healed autogenously. Such micro cracks are acceptable as these do not directly influence the safety and strength of the concrete. However, because of the variability of autonomous crack healing of concrete micro cracks can still occur. The inbuilt bacteria-based self-healing process was found to heal cracks completely up to 0.5 mm width. On the surface of control concrete, calcium carbonate will be formed due to the reaction of CO₂ present with calcium hydroxide present in the concrete matrix according to the following reaction: CO₂ + Ca (OH) ₂--> CaCO₃ + H ₂O

The self-healing process in bacteria incorporated concrete is much more efficient due to the active metabolic conversion of calcium nutrients by the bacteria present in concrete: Ca (C₃H₅O₂)₂ + 7O₂-->CaCO₃ + 5CO₂ + 5H₂O This process does not only produce calcium carbonate directly due to microbial metabolic process but also indirectly due to autogenously healing. This process results in efficient bio based crack sealing technique. Bacillus Subtilis JC3 can able to precipitate CaCO₃ in the high alkaline environment by converting urea into ammonium and carbonate. The ammonia degradation of urea locally increases the pH and promotes the microbial deposition of carbonate as calcite crystals in a calcium rich environment along with maintaining the pH of concrete. These precipitated crystals can thus seal the cracks. The enhancement of strength and durability properties of concrete due to bacteria induction is studied in this research by conducting water permeability tests, ultrasound transmission measurements.

TRADITIONAL CRACK REPAIR SYSTEMS: Repair of cracks in concrete structures usually involves applying a concrete mortar which is bonded to the damaged surface. Sometimes, the mortar needs to be keyed into the existing structure with metal pins to ensure that it does not fall away. Repairs can be particularly be time consuming and expensive because it is often very difficult to gain access to the structure to make repairs, especially if they are underground or at a great height. For crack repair, a variety of techniques is available but traditional repair systems have a number of disadvantageous aspects such as different thermal expansion coefficient compared to concrete and also have impact on environment and health. Therefore, bio based calcite precipitation has been proposed as an alternative and sustainable

environmental technique.

friendly

crack

repair

VIABILITY OF BACTERIA IN CONCRETE: 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 longterm 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. However, the presence of the matrix-embedded bacteria and precursor compounds should not negatively affect other wanted concrete characteristics. 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 50 years. 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. Studies have shown that various organic bio-cement precursor compounds such as yeast extract, peptone and calcium acetate resulted in a dramatic decrease of compressive strength. In order to substantially increase the lifetime and associated functionality of concrete incorporated bacteria, the bacterial spores should be protected by immobilization inside porous expanded clay particles before addition to the concrete mixture indeed substantially prolonged their lifetime.In subsequent experiments the expanded clay particles loaded with the two-component bio-chemical healing agent were applied as additive to the concrete mixture to test self-healing potential of bacterial concrete.

TABLE Strength Studies on Bacteria incorporated Concrete

Strength Studies @ 28 days age

Compressive Strength (MPa)

Bio Concrete Controlled Concrete M20 M40 28.18 52.01

M20 32.74

M40 61.06

Split Tensile 3.26 Strength (MPa)

4.51

3.73

5.13

Flexural Strength (MPa)

6.11

6.11

7.73

4.68

Courtesy: M.V. SeshagiriRao, V. Srinivasa Reddy, M. Hafsa, P. Veena and P. Anusha-Bioengineered Concrete- A Sustainable Self-Healing Construction Material, Research Journal of Engineering Sciences , Vol. 2(6), 45-51, June (2013).

ADVANTAGES AND DIS ADVANTAGES OF BACTERIAL CONCRETE: ADVANTAGES:

Fig:-ESEM photomicrograph (5000x magnification) of alkali-resistant spore forming bacterium (Bacillus strain B2-E2-1). Visible are active vegetative bacteria (rods) and spores (spheres), showing that spore diameter sizes are in the order of one micrometer.

1. Microbial concrete in crack remediation. 2. Improvement in compressive strength of concrete. 3. Better resistance towards freeze-thaw attack reduction. 4. Reduction in permeability of concrete. 5. Reduction in corrosion of reinforced concrete DISADVANTAGES: 1. Cost of bacterial concrete is double than conventional concrete. 2. Growth of bacteria is not good in any atmosphere and media. 3. Design of mix concrete in bacteria-no IS code is available.

CONCLUSION: Microbial 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. Enhancement of compressive strength, reduction in permeability, water absorption, and reinforced corrosion has been seen in various cementious and stone materials. 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. Hence looking into the advantages and disadvantages of this concrete, we can say that it is the best concrete which enhances the properties of the desired concrete.

REFERENCES: 1. M.V. SeshagiriRao, V. Srinivasa Reddy, M. Hafsa, P. Veena and P. Anusha-Bioengineered Concrete A Sustainable Self-Healing Construction Material, Research Journal of Engineering Sciences, Vol. 2(6), 45-51, June (2013). 2. H. M. Jonkers, Delft University of Technology, Microlab, Delft, the Netherlands Bacteria-based self-healing concrete, HERON Vol. 56 (2011). 3. Mayur Shantilal Vekariya, Prof. Jayeshkumar Pitroda, Civil Engg. Department, B.V.M. Engineering College, GujaratBacterial Concrete: New Era For Construction Industry, International Journal of Engineering Trends and Technology (IJETT) – Volume 4.