BACTERIA BASED SELF HEALING CONCRETE


1. 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)

























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2. INTRODUCTION

Concrete which forms major component in the construction Industry as it is cheap, easily available and convenient to cast. But drawback of these materials is it is weak in tension so, it cracks under sustained loading and due to aggressive environmental agents which ultimately reduce the life of the structure which are built using these materials.  This process odamage occurs in the early life of the  building  structure  and  alsduring its life time. Synthetic materials like epoxies are used for remediation. 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. the concept was first introduced by Ramakrishna. Journal publication on self-healing concrete over the last 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 bio- mineralization.  MICP  is  highldesirable  because  the  Calcite  precipitation induced as  a result  omicrobial  activities  is pollution  free and natural. The technique can  be used to improve the compressive strengtand stiffnesof cracked concrete specimens. Research leading to microbial Calcium Carbonate precipitation and its ability to heal  crackof construction  materials has led to many applications like crack remediation of concrete,     sand     consolidation, applications.so it can be define 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














CONCRETE:



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Concrete is a composite material composed of coarse granular material (the aggregate or filler) embedded in a hard matrix of material (the cement or binder) that fills the space between the aggregate particles and glues them together. We can also consider concrete as a composite material that consists essentiallof  a  binding  medium  within which are embedded particles or fragments of aggregates. The simplest representation of concrete is:

Concrete = Filler   + Binder.

According to the type of binder used, there are many different kinds of concrete. For instance, Portland cement concrete, asphalt concrete, and epoxy concrete. In concrete construction, the Portland cement concrete is utilized the most. Thus, in our course, the term concrete usually refers to Portland cement concrete. For this kind of concrete, the composition can be presented as follows

Cement  +

(+ Admixture)  →   Cement paste
+ Water                           +            →     mortar
Fine aggregate           +              concrete
Coarse aggregate

Here we should indicate that admixtures are almost always used in modern practice and thus become an essential component of modern concrete. Admixtures are defined as materials other than aggregate (fine and coarse), water,   fibre   an cement which   are   adde int c o n c r e t e    b a t c h
i m m e d i a t e l y  b e f o r e  or  d u r i n g  m i x i n g .    The w i d e s p r e a d  use  of admixture  is  mainly  due  to  the  many  benefits  made  possible  by  their
application.  For instance, chemical admixtures can modify the setting and hardening characteristic of cement paste by influencing the rate of cement hydration. Water-reducing admixture can plasticize fresh concrete mixtures by
reducing surface tension of water, air-entraining admixtures can improve the durability of concrete, and mineral admixtures such as pozzolanas (materials containing reactive silica) can reduce thermal cracking. A detailed description
of admixtures will be given in latter sections.

Advantages:

a)       Economical Concrete is the most inexpensive and the most readily available material. The cost of production of concrete is low compared with other engineered construction materials. Three major components: water, aggregate and cement. Comparing with steel, plastic and polymer, they are the most inexpensive materials and available in every corner of the world. This enables concrete to be locally produced anywhere in the world, thus avoiding the transportation costs necessary for most other materials.





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b).     ambient temperature hardened material: Because cement is a low temperature bonded inorganic material and its reaction occurs at room temperature, concrete can gain its strength at ambient temperature.

c)       Ability to be cast: It can be formed into different desired shape and sizes right at the construction site.

d)      Energy efficiency: Low energy consumption for production, compare with steel especially.     The energy content of plain concrete is 450-750 kWh / ton and that of reinforced concrete is 800-3200 kWh/ton, compared with 8000 kWh/ton for structural steel

e)       Excellent resistance to water. Unlike wood and steel, concrete can hardein water and can withstand the action of water without serious deterioration. This makes concrete an ideal material for building structures to control, store, and transport water.   Examples include pipelines (such as the Central Arizona Project, which provide water from Colorado River to central Arizona. The system contains1560 pipe sections, each 6.7 m long and 7.5 m in outside diameter 6.4 m inside diameter), dams, and submarine structures. Contrary to popular belief, pure water is not deleterious to concrete, even to reinforced concrete: it is the chemicals dissolved in water, such as chlorides, sulphates, and carbon dioxide, which cause deterioration of concrete structures.

f).      High temperature resistance: Concrete conducts heat slowly and is able to store considerable quantities of heat from the environment (can stand 6-8 hours in fire) and thus can be used as protective coating for steel structure.

g).     Ability to consume waste: Many industrial wastes can be recycled as a substitute for cement or aggregate. Examples are fly ash, ground tire and slag.
h).     Ability to work with reinforcing steel: Concrete and steel possess similar coefficient  of  thermal  expansion  (steel  1.2  x  10-5;  concrete
1.0-1.5  x  10-5). Concrete also provides good protection to steel due to
existing of CH (this is for normal condition). Therefore, while steel bars
provide  the  necessary  tensile  strength,  concrete  provides  a  perfect environment for the steel, acting as a physical barrier to the ingress of
aggressive species and preventing steel corrosion by providing a highly
alkaline environment with pH about 13.5 to passivate the steel.

i)       Less  maintenance  required:  No  coating  opaintinis  needed  as for  steel structures.


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Limitations:

a)       Quasi-brittle failure mode: Concrete is a type of quasi-brittle material. (Solution: Reinforced concrete)

b)      Low  tensile  strength:     About  1/10  of  its  compressive  strength. (Improvements: Fibre reinforced concrete; polymer concrete)

c)       Lo t o u g h n e s s :      Th ab i l i t  t ab s o r en erg  i l o w. (Improvements:  Faber reinforced concrete)

d)      Low s t r e n g t h /BSG r a t i o  ( specific s t r e n g t h ):        Steel ( 300-
600)/7.8.  Normal concrete      (35-60)/2.3.    Limited    to    middle-rise buildings.        (Improvements: Lightweight concrete; high strength concrete)

e)       Formwork  is   n e e d e d :     Formwork  f a b r i c a t i o n  is  l a b o u r
i n t e n s i v e a n d t i m e consuming, hence costly (Improvement: Precast concrete)

f).      lon curing   time:   Ful strengt developmen need  month. (Improvements: Steam curing)


g).     working with cracks: Most reinforced concrete structures have cracks under service load. (Improvements: Pressurised concrete).

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