Concrete Repair and Structural Strengthening

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Abstract
 The extent of deterioration to concrete structures globally is occurring at an alarming rate, which challenges engineers throughout the world on a daily basis. This includes damage to bridges, buildings, parking structures, environmental facilities, as well as other structures. Unfortunately, repair costs can be staggering. Delaying repairs usually results in much more costly repairs later. Furthermore, if concrete deterioration or damage is not timely addressed, some of these structures eventually may cease to be serviceable and worse yet, failures could occur. The micro-cracks and porosity of concrete structures are very common problems due to the fact that this material has a high permeability which allows water and other aggressive media to enter thus leading to deterioration. The use of traditional organic polymer based crack sealers is a common way of contributing to concrete durability. However, the most common organic polymers have some degree of toxicity and are not environmental friendly. Recent investigations in the field of biotechnology show the potential of bio-inspired materials in the development of low toxic solutions. Calcium carbonate is one of the most well known mineral that bacteria deposit by the phenomenon called bio-cementation or microbiologically induced calcite precipitation (MICP). An alkalophilic aerobic soil bacterium Bacillus subtilis JC3 was incorporated into concrete at different cell concentrations with the mixing water. The study showed that a 30 % increase in 28 days compressive strength of concrete was achieved with the addition of 105 bacterial cells per ml of mixing water. This paper presents the research findings to suggest the potential use of the microbial calcite precipitation process in remediation of the surface cracks and pores present in concrete.

Introduction
Concrete is most used construction material in the world today. There are two probable reasons for deterioration of concrete; they are human inflicted causes such as low strength concrete, inadequate concrete cover, poor construction techniques etc and environment related causes freezing and thawing, early-drying shrinkage, chemical attack, carbonation etc. Concrete crack repair could be done to accomplish the following objectives such as restoring and increasing strength and stiffness, to improve structural integrity, to provide water tightness, improve appearance of the concrete crack surface area, to improve durability and to prevent corrosion in steel. So there are many different forms of causes for cracking to deal with on a daily basis. When ignored can lead to more serious problems with corrosion. Before you begin to repair these cracks in concrete, identifying the nature of cause and method of repair is chosen based on whether to achieve structural integrity, focus on aesthetics, seal against intrusion of foreign materials, protect against carbonation, protect against chloride and water ingress; and/or seal in preparation for additional topping material. Hairline cracks are often ignored because they are thought of as nonstructural and, therefore, not a threat to the integrity of the structure. If left untreated, hairline cracks will eventually become larger and lead to more costly repairs. To minimize future deterioration of concrete, cracks exposed to a moist or corrosive environment should be sealed by using various available engineering solutions for structural crack repair. An innovation based on biomimicry and biotechnology has lead to the method of sealing up of micro cracks in concrete by itself using microorganisms as a sustainable alternative to other available chemical methods of crack repair such as epoxy treatment etc.
Bio-deposition Versus Natural Carbonation
Compared to natural carbonation of concrete, bio-deposition is a relatively quick process. Natural carbonation occurs from the dissolution of atmospheric CO2 in the pore solution and formation of CaCO3 from CSH or portlandite. In the bio-deposition treatment however, calcium ions are also provided by an externally added calcium source, while the carbonate ions result from the microbiological hydrolysis of amino acids. As a result of the rapid hydrolysis of amino acids (under optimal conditions), the majority of the calcium ions added to the specimens are precipitated within a couple of days.
Mechanism of Bio-based Concrete Crack Repair

In nature, microorganisms can induce calcite mineral precipitation through nitrogen cycle either by ammonification of amino acids/ nitrate reduction/ hydrolysis of urea. Bacillus subtilis JC3 is able to precipitate calcium carbonate (CaCO3) in its micro-environment by the ammonification of amino acids into ammonium (NH4+) and carbonate (CO32-) ions. The precipitated bio-CaCO3 has a great potential ability to heal concrete cracks because it is natural, environmentally friendly and compatible with the concrete matrix. Biomineralization in concrete is the process by which micro organisms that produces minerals is uses as a possible method to promote concrete crack remediation. But the challenge was finding the bacteria that would be active in concrete’s environment of high alkalinity and low oxygen. Bacillus subtilis JC3 a non-pathogenic alkalophilic microorganism commonly found in soil and is known to deposit the calcite minerals when it is supplied with nutrients and right conditions to grow. The bacteria introduced into the concrete during mixing process will form spores in the highly alkaline environment of concrete. Once a crack forms, the pH level at the cracked surface will drop due to the exposure to air. The combination of the pH drop and a flow of oxygen, moisture and carbon dioxide at the crack face will activate the microorganisms and will provide the conditions favorable for growth. The microorganisms will deposit calcium carbonate, and as the crack fill up, the supply of oxygen and carbon dioxide will be interrupted, causing the microorganisms to hibernate again, ensuring the continual effectiveness of the microorganisms in filling up cracks at the same location. Bio-mineralization by Ammonification (Ammo acid degradation) is mediated by Bacillus subtilis JC3. Ammonification usually occurs under aerobic conditions (known as oxidative deamination) with the liberation of ammonia (NH3) or ammonium ions (NH4) when dissolved in water. The ammonia liberated will provide the conditions favorable for growth and also maintains the pH of concrete.

 

 

CONCLUSION
Improvement in compressive strength reaches a maximum at about 105/ml cell concentration. SEM examination reveals the growth of fibrous filler material within the pores due to the presence of such microorganisms. This growth is beneficial by the modification of the porosity and pore size distribution of cement mortar which it generates. The presence of biogenic calcite crystals in pores of concrete resulted in a decrease of its permeation properties. As a result, an increased resistance towards chloride migration and freezing and thawing was noticed.  Precipitation of these crystals inside the gel matrix may enhance the durability of concrete significantly. Bacteria incorporated concrete specimens reports an increase in ultrasonic pulse velocity, indicating that pore structure is modified.

 

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