06-03-2013, 11:14 AM
SELF-HELING BACTERIAl CONCRETE
SELF-HELING BACTERIAl.docx (Size: 21.05 KB / Downloads: 287)
Abstract:
Cracks in concrete are inevitable and are one of the inherent weaknesses of concrete. Water and other salts seep through these cracks, corrosion initiates, and thus reduces the life of concrete. So there was a need to develop an inherent biomaterial, a self-repairing material which can remediate the cracks and fissures in concrete. Bacterial concrete is a material, which can successfully remediate cracks in concrete. This technique is highly desirable because the mineral precipitation induced as a result of microbial activities is pollution free and natural. As the cell wall of bacteria is anionic, metal accumulation (calcite) on the surface of the wall is substantial, thus the entire cell becomes crystalline and they eventually plug the pores and cracks in concrete. This paper discusses the plugging of artificially cracked cement mortar using Bacillus Pasteurii and Sporosarcina bacteria combined with sand as a filling material in artificially made cuts in cement mortar which was cured in urea and CaCl2 medium. The effect on the compressive strength and stiffness of the cement mortar cubes due to the mixing of bacteria is also discussed in this paper. It was found that use of bacteria improves the stiffness and compressive strength of concrete. Scanning electron microscope (SEM) is used to document the role of bacteria in microbiologically induced mineral precipitation. Rod like impressions were found on the face of calcite crystals indicating the presence of bacteria in those places. Energy- dispersive X-ray (EDX) spectra of the microbial precipitation on the surface of the crack indicated the abundance of calcium and the precipitation was inferred to be calcite (CaCO3).
INTRODUCTION:
Concrete is a vital building material that is an absolutely essential component of public infrastructure and most buildings. It is most effective when reinforced by steel rebar, 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. Standard concrete will bear up to approximately 0.1% strain before giving out [1]. 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 self-healing concrete in development, now, that can solve many of the problems commonly associated with standard concrete. This material is bacterial self-healing concrete. Self-healing concrete consists of a mix with bacteria (Bacillus subtilus) incorporated into the concrete and calcium lactate food to support those bacteria when they become active. The bacteria, feeding on the provided food source, heal the damage done and can also reduce the amount of damage sustained by the concrete structure in place [2]. This paper will also explain, in-depth, the processes that are behind bacterial self-healing in concrete and will describe the many components that are included in the process and how they work independently and collectively. This paper will also delve into practical applications of this self-healing method, including real-world integrations in currently standing structures. Sustainability and economics of these materials will also be discussed, as this new improvement on the staple building material, yet often taken for granted, of our modern civilization presents an opportunity to both reduce the environmental and financial impact of concrete production and its wide array of applications.
THE BIOLOGICAL SELF -HEALING PROCESS:
It is important to cover what kinds of bacteria will live in the concrete, how they work to improve the longevity of public infrastructure, what the catalyst will be that causes the chemical reaction in the bacteria, what happens to the specific kinds of specialized bacteria when exposed to the catalyst, and how they work together to not only heal cracks before they form, but also strengthen the overall structure they are incorporated into. When the bacteria are exposed to the air and the “food,” the bacteria go through a chemical process that causes them to harden and fuse, filling in the crack that has formed, strengthening the structure of the concrete, and adhering to the sides of the crack to seal the damage site. This process extends the lifespan of the structure while also fixing the damage caused. The process of healing a crack can take as little as a few days [3] . When we look at the crack sizes, we generally are looking in then micro- to nano meter range to maximize the healing potential. Concrete constructions are currently designed according to set norms that allow cracks to form up to 0.2 mm wide [4][5]. Such micro cracks are generally considered acceptable, as these do not directly impair the safety and strength of a construction. Moreover, micro cracks sometimes heal themselves as many types of concrete feature a certain crack-healing capacity. Research has shown that this so called ‘autonomous’ healing capacity is largely related to the number of non-reacted cement particles present in the concrete matrix[7]. On crack formation, ingress water reacts with these particles, resulting in closure of micro cracks. However, because of the variability of autonomous crack healing of concrete constructions, water leakage as a result of micro crack formation in tunnel and underground structures can occur. While self-healing of 0.2 mm wide cracks occurred in 30% of the control samples, complete closure of all cracks was obtained in all bacteria- based samples. Moreover, the crack sealing capacity of the latter group was found to be extended to 0.5 mm cracks. The basic concept behind our specific version of self-healing concrete is utilizing certain types of bacteria (in the present case bacillus subtilus) and how they function to seal microscopic cracks in the concrete before they grow into larger and harder to manage cracks and breaks. This biocalcification process involves several elements, working in unison, to complete these tasks. During the process the enzymatic hydrolysis of urea takes place forming ammonia and corbondioxide.
HOW DOES BACTERIA REMIDIATE CRACKS:
When the concrete is mixed with bacteria (bacillus subtilus), the bacteria go into a dormant state, a lot like seeds. All the bacteria need is 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. 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. 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. Tests all show that bacteria embedded concrete has lower water and chloride permeability and higher strength regain than the surface application of bacteria. The last, but certainly not least, key component of the self-healing concrete formula is the bacteria themselves. The most promising bacteria to use for self-healing purposes are alkaliphilic (alkali- resistant) spore-forming bacteria. The bacteria, from the genus Bacillus , subtilus is adopted for present study. It is of great concern to the construction industry whether or not these bacteria are “smart” enough to know when their task is complete because of safety concerns. Bacillus Subtilus which is a soil bacterium (isolated from JNTUH soil) is harmless to humans as it is non-pathogenic microorganism.
RESULTS AND DISCUSSION :
Table 1 summarizes the 3 days, 7 days and 28 days compressive strength of the mortar cubes containing different cell concentration of alkaliphilic microorganism (Bacillus subtilus). The greatest improvement in compressive strength occurs at cell concentrations of 10 5 cells/ml for all ages: this increase reaches to 16.15 % at 28 days. This improvement in compressive strength is due to deposition on the microorganism cell surfaces and within the pores of cement–sand matrix, which plug the pores within the mortar. The extra cellular growth produced by the microorganism is expected to contribute more to the strength of cement mortar with a longer incubation period and thus the strength improvement is found to be more at 28 days. Even the dead cells may simply remain in the matrix as organic fibers. Quantification and Characterization was done using Scanning Electron Micrograph analysis, only to be noted that cracks are sealed up by crystalline material grown over the surface due to microbial activity of the bacteria.
CONCLUSIONS:
1.Bacillus subtilis can be produced from lab which is proved to be a safe and cost effective.
2. The addition of bacillus subtilis bacteria improves the hydrated structure of cement mortar.
3. The compressive strength of cement mortar is maximum with the addition of bacillus subtilis bacteria for a cell concentration of 105 cells per ml of mixing water. So, bacteria with a cell concentration of 105 cells per ml of mixing water was used in the investigation.
4.The addition of bacillus subtilis bacteria increases the compressive strength of concrete. In standard grade concrete the compressive strength is increased upto14.92% at 28 days by addition of bacillus subtilis bacteria when compared to conventional concrete. The addition of bacillus subtilis bacteria showed significant improvement in the split tensile strength than the conventional concrete.