11-08-2014, 01:07 PM
Cracks
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ABSTRACT
Cracks in concrete reduce the durability of structures. The use of conventional sealing agents for repairing cracks has found to be environmentally unsafe and costly. The solution for an environmentally safe and cost efficient self healing material comes in the form of bacterial concrete.It is a concrete which can be made by embedding bacteria thus utilizing microbiologically induced calcite (CaCo3) precipitation.
This paper presents a review of bacterial concrete, its merits and applications relating to the strength and durability of concrete. Based on these studies, bacteria have found to be beneficial in enhancing the durabilty and strength of concrete. This technique has been found to improve the properties of concrete. Bacterial concrete can be used as a self- healing and repair material. It would lead to crack free and durable concrete structures in the future
INTRODUCTION
Crack formation in concrete is a phenomenon that can hardly be complete avoided due to for example shrinkage reactions of setting concrete and tensile stresses occurring in set structures. While larger cracks can potentially hamper a structures' integrity and therefore require repair actions, smaller cracks typically with a crack width smaller than 0.2 mm are generally considered un problematic. Although such micro cracks do not affect strength properties of structures they do on the other hand contribute to material porosity and permeability. Ingress of aggressive chemicals such as chlorides, sulfates and acids may result on the longer term in concrete matrix degradation and premature corrosion of the embedded steel reinforcement and thus hamper the structures' durability on the long term. In several studies indications have been found that concrete structures have a certain capacity for autonomous healing of such micro cracks. The actual capacity of micro crack healing appears primarily related to the composition of the concrete mixtures. Particularly mixtures based on a high binder content show remarkable crack-healing properties what is due to delayed (secondary) hydration of matrix embedded nonhydrated cement and binder particles upon reaction with crack ingress water. Autogenous self-healing of cracks in traditional but also high-binder content mixtures appear limited to cracks with a width smaller than 0.2 mm. This somewhat limited effectiveness appears largely due to the restricted expansive potential of the small non-hydrated cement particles lying exposed at the crack surface. Another limitation to application of highbinder content mixtures solely for the purpose of increasing self-healing capacities are current policies which advocate sparse use of cement in concrete for sustainability reasons as current cement production contributes about 7% to global anthropogenic CO2 emissions.For latter reasons, alternative and more sustainable self-healing mechanisms are therefore wanted. One possible mechanism is currently being investigated and developed in several laboratories, i.e. a technique based on the application of mineral-producing bacteria. E.g. efficient sealing of surface cracks by mineral precipitation was observed when bacteria-based mixtures were sprayed or applied onto damaged surfaces or manually inserted into cracks. As in those studies bacteria were manually and externally applied to existing structures, this mode of repair can not be categorized as truly self healing. In several follow up studies therefore, the possibility to use viable bacteria as a sustainable and concrete-embedded self healing agent was explored. In one study spores of specific alkali-resistant bacteria related to the genus Bacillus were added to the concrete mixture as self-healing agent. These spores germinated after activation by crack ingress water and produced copious amounts of crack-filling calcium carbonatebased minerals through conversion of precursor organic compounds which were also purposely added to the concrete mixture. However, in that study it was found that the bacteria-based self-healing potential was limited to relatively young (7-days cured) concrete only, as viability and related activity of bacterial spores directly (unprotected) embedded in the concrete matrix was restricted to about two months. The present study builds further on results reported in latter research paper. Here, bacterial spores and organic mineral precursor compounds are packed in porous expanded clay particles prior to addition to the concrete mixture. It is hypothesized that protection of bacterial spores within porous light weight aggregates extends there viability period and thus concrete selfhealing functionality when embedded in the material matrix.
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
Role Of Ureolytic Bacteria In Concrete
Microorganisms are incredibly diverse and include bacteria, fungi, archaea and some microscopic plants and animals such as plankton. Use of bacteria in concrete is recent thirst now a day in research. However a new approach that a microbial mineral deposit constantly occurs in natural environments. The primary role of bacteria in the precipitation process has been ascribed to their ability to create an alkaline environment through various physiological activities. Certain Negatively charged nature and specific functional groups of microbial cell walls favors the binding of divalent cations (Ca2+ and Mg2+), thereby making microorganisms ideal crystal nucleation site. Specific proteins present in biological extracellular polymeric substances cause the formation of different calcium carbonate polymorphs. Bacterial deposition of a layer of calcite on the surface of the specimens resulted in a decrease of capillary water uptake and permeability towards gas.
Autonomous Crack Repair Of Bacterial Self Healing Concrete
Concrete test specimens were prepared in which part of the aggregate material, i.e. the 2-4 mm size class, was replaced by similarly sized expanded clay particles loaded with the biochemical self-healing agent (bacterial spores 1.7x105 g-1 expanded clay particles, corresponding to 5x107 spores dm-3 concrete, plus 5% w/w fraction calcium lactate, corresponding to 15g dm-3 concrete). Before application, loaded expanded clay particles were oven-dried until no further weight loss due to water evaporation was observed (one week at 40ºC). Control specimens had a similar aggregate composition but these expanded clay particles were not loaded with the bio-chemical agent. Both types of expanded clay particles (empty for control specimens and loaded for bacterial specimens) were Composition of concrete specimens is shown in Table 1.
The amount of light weight aggregate applied in this case represents 50% of the total aggregate volume. Replacement of such a high fraction of sand and gravel for expanded clay has consequences for strength characteristics of the derived concrete. In this specific case a 50% decrease in compressive strength was observed after 28 days curing when compared to specimens of similar aggregate composition without replacement of sand and gravel fractions for expanded clay particles.
METHODOLOGY
The outcome of this study shows that crack healing of bacterial concrete based on expanded porous clay particles loaded with bacteria and calcium lactate, i.e. an organic bio-mineral precursor compound, is much more efficient than of concrete of the same composition however with empty expanded clay particles. The reason for this can be explained by the strictly chemical processes in the control and additional biological processes in the bacterial concrete. Non-hydrated cement particles exposed at the crack surface of concrete will undergo secondary hydration and in addition in control specimens carbon dioxide present in the bulk water will react with present portlandite (calcium hydroxide) particles to produce calcium carbonate-based mineral precipitates. Latter mineral precipitates will particularly form near the crack rim due to the relatively high solubility of calcium hydroxide. Here it is hypothesized that calcium hydroxide particles present at the surface of the crack interior will first scavenge all available carbon dioxide
from crack ingress water, where after remaining calcium hydroxide will dissolve and diffuse out of the crack into the bulk water. Once in the bulk water it will react with carbon dioxide present in close approximation to the crack rim resulting in the chemical production and precipitation of larger quantities of much lower soluble calcium carbonate.
Conclusions
The strength and durability characteristics was found to increase for bacterial concrete both as repair material for surface treatment and as self healing agent.So it has positive influence on concrete structures. The promising results in the earlier studies have attracted the attention of researchers and companies to investigate the performance of bioconcrete under real life conditions.If self healing concrete structures are built in the near future,maintenance will be less,durability will be increased and it would lead to cost efficient structures