BIO-CONCRETE...THE NEXT GENERATION CONCRETE


ABSRACT:
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 successfully remediate cracks in concrete. A novel technique in remediating cracks and fissures in concrete by utilizing microbiologically induced calcite precipitation (MICP) is a technique that comes under a broader category of science called biomineralization. It is a process by which living organisms form inorganic solids. Bacillus Pasteruii, a common soil bacterium can induce the precipitation of calcite.
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.
AN INTRODUCTION TO CONCRETE:

Concrete can be considered as a kind of artificial rock with properties more or less similar to certain natural rocks. As it is strong, durable, and relatively cheap, concrete is, since almost two centuries, the most used construction material worldwide, which can easily be recognized as it has changed the physiognomy of rural areas. However, due to the heterogeneity of the composition of its principle components, cement, water, and a variety of aggregates, the properties of the final product can widely vary. The structural designer therefore must previously establish which properties are important for a specific application and must choose the correct composition of the concrete ingredients in order to ensure that the final product applies to the previously set standards. Concrete is typically characterized by a high-compressive strength, but unfortunately also by rather low-tensile strength. However, through the application of steel or other material reinforcements, the latter can be compensated for as such reinforcements can take over tensile forces.

Modern concrete is based on Portland cement, hydraulic cement patented by Joseph Aspdin in the early 19th century. Already in Roman times hydraulic cements, made from burned limestone and volcanic earth, slowly replaced the widely used non-hydraulic cements, which were based on burned limestone as main ingredient. when limestone is burned (or "calcined") at a temperature between 800 and 900°C, a process that drives off bound carbon dioxide (CO2), lime (calcium oxide: CaO) is produced. Lime, when brought into contact with water, reacts to form portlandite (Ca(OH)2) which can further react with CO2, which in turn forms back into calcite (CaCO3), or limestone, the pre-burning starting material. However, a major drawback of this non-hydraulic cement is that it will not set under water and, moreover, its reaction products portlandite and limestone are relatively soluble, and thus will deteriorate rapidly in wet and/or acidic environments. In contract, Portland cement produces, upon reaction with water, a much harder and insoluble material that will also set under water. For Portland cement production a source of calcium, silicon, aluminum, and iron is needed and therefore usually limestone, clay, some bauxite, and iron ore are burned in a klin at temperatures up to 1500°C. The cement clinker produced is mainly composed of the minerals composed of the minerals alite (3CaO.SiO2), belite (2CaO.SiO2), aluminate (3CaO.Al2O3), and ferrite (4CaO.Al2O3.Fe2O3), which all yield specific hydration products with different characteristics upon reaction with water. The contribution of these clinker minerals to the composition of general-purpose Portland cement in weight percentage is typically 50%, 24%, 11%, and 8% respectively. Important characteristics of clinker minerals are reaction rate and contribution to final strength of the product. For example, of the two calcium silicates, alite is the most reactive and contributes to early strength. Aluminate contributes to early strength as its hydration reaction is fast but it also generates much heat. The final properties of cement-based materials can thus vary widely as they strongly depend on the mineral composition of the cement used and therefore, different types of cement, each suitable for specific applications, are produced. Quantitatively most important hydration product of general-purpose Portland cement is calcium silicate hydrate (C-S-H), an amorphous mineral somewhat resembling the natural mineral tobermorite. A secondary reaction product is calcium hydroxide (portlandite), which together with the very soluble sodium and potassium oxides (Na2O and K2O) also present in Portland cement, contribute to the high alkalinity of the concrete's pore fluid (pH≈13). The high matrix pH is important in structural concrete as it protects the embedded steel reinforcement from corrosion. The protective oxidized thin layer of Fe3+ oxides and oxyhydroxides on the reinforcement steel (the passivation film) rapidly degrade when the matrix pH drops below 9, leading to further oxidation and deterioration of the concrete structure due to expansion reactions and loss of strength. Corrosion of the steel reinforcement is in fact one of the major causes limiting the durability, or lifetime, of concrete structures. For further and more detailed information on general concrete properties the reader is referred to Reinhardt (1958) and Neville(1996).


 

CONCRETE DURABILITY, DETERIORATION, AND SELF HEALING PROPERTIES:

    A variety of additives or replacements of cement can be applied in order to improve the durability of the final concrete product. Also certain industrial waste or recycled materials can be used to improve the sustainability, or environmental friendliness, of concrete and some even improve certain properties. The production of cement is high-energy consuming as raw materials are burned at 1500°C, a process that contributes to a significant amount of atmospheric CO2 release worldwide. Thus, for both economical and environmental reasons, cement production and use should be minimized. Examples of industrial waste products, which can partly replace and even improve cement properties, are fly ash, blast furnace slag, and silica fume. Fly ash, a waste product from coal-burning power plants, is a source of reactive silica and can substitute 35-75% of cement in the concrete mix. Application of fly ash increases concrete strength as it reduces the required water/cement ratio and also improves resistance against chemical attack as it decreases the matrix permeability. Similarly, silica fume from the silicon industry and blast furnace slag from steel industries can partially replace cement in the concrete mix, as these are source of reactive silica and both reactive silica and calcium respectively. Other commonly applied additives that improve or change certain concrete characteristics needed for specific applications are air-entraining agents to improve freeze/thaw resistance, setting or retarding agents and plasticizers to enable a lower water/cement ratio to increase concrete strength.
    A number of processes negatively affect the durability and result in the unwanted early deterioration of concrete structures. One major cause that initiates various mechanisms of concrete deterioration is the process of cracking what dramatically increases the permeability of concrete. The microstructure of hardened cement paste is porous as it contains isolated as well as interconnected pores. Specifically the connected pores determine permeability, as these allow water and chemicals to enter the concrete matrix. As cracking links both isolated and connected pore systems, this results in a substantially increased permeability. In most concrete-deterioration mechanisms permeability plays a major role. Intrusion of sulfate ions into the matrix may result in ettringite formation, a conversion reaction in which a high-density phase is transformed into a low-density phase, causing expansion and further cracking of the material. Chloride ions penetrating the matrix through the connected pore system will destabilize the passivation film of the steel reinforcement and by doing so accelerate further corrosion. Similarly, in a process called carbonation, CO2 diffusing through the pore system will react with alkaline pore fluid components such as Ca(OH)2 which will result in a lowering of matrix pH and again depassivation of the protective film on the steel reinforcement. These examples make clear that cracking of concrete should be minimized and that a potential healing mechanism should ideally result in the sealing or plugging of newly formed cracks in order to minimize increases in matrix permeability. An active self healing mechanism in concrete should be ideal as it does not need labor-intensive manual checking and repair what would save an enormous amount of money.
    A self healing mechanism or self healing agent in concrete should comply ideally with all, or at least with some, of the following characteristics:
  • Should be able to seal or plug freshly formed cracks to reduce matrix permeability.
  • Must be incorporated in the concrete matrix and able to act autonomously to be truly "self-healing".
  • Must be compatible with concrete, i.e. its presence should not negatively affect material characteristics.
  • Should have a long-term potential activity, as concrete structures are build to last typically for at least 50 years.
  • Should preferably act as a catalyst and not be consumed in the process to enable multiple healing events.
  • Must not be too expensive to keep the material economically competitive.
Different types of potential self healing mechanisms or agents for autonomous concrete repair can be thought of. One series of mechanisms could involve the secondary formation of minerals which are compatible with the material matrix, i.e. will not negatively affect but rather increase concrete durability by sealing freshly formed cracks and so decrease matrix permeability. A chemical agent such as the inclusion of still non-reacted cement particles in the concrete matrix is feasible as it complies with at least some of the listed self healing properties. Besides this, other agents could work equally well or can contribute to the self healing property of concrete in concert with the previous one. Next to chemicals one could think of an agent of biological origin, and in the next part the possible application of bacteria as healing agent will be considered.


 

HOW TO PRESERVE CONCRETE WITH BACTERIA:

    CONCRETE is one of the most commonly used building materials. It is cheap, strong and easy to work with. But, as a short walk through any city centre will prove, it cracks easily. The cracking of concrete pavements is merely a nuisance, but cracks in roads, bridges and buildings are a hazard. A way of making concrete that healed such cracks spontaneously would thus be very welcome. And a team led by Henk Jonkers at the Delft University of Technology in the Netherlands may have come up with one.
    The way to stop concrete cracking is to bung up small cracks before they enlarge. That process of enlargement is caused by water getting into a crack, then freezing in cold weather and thus expanding. This freeze-thaw cycle, a common form of erosion of natural rocks, too, weakens a structure directly and also exposes steel reinforcing rods to water, causing them to rust.
    When he began his research, Dr Jonkers knew that spraying mineral producing bacteria onto limestone monuments is often as effective way to stop freeze-thaw in its tracks. The mineral in question is calcium carbonate, the defining ingredient of limestone. He also knew, however, that when applied to concrete, this technique had proved to be just as time-consuming and, indeed, more expensive than traditional repair methods using sticky, water-repellent agents. That led him to wonder if the answer was to incorporate helpful bacteria into concrete from the start.
    To find out, he and his team selected various mineral-producing bacterial strains that can handle the highly alkaline environment found in liquid concrete. The added these bacteria, along with calcium lactate, an organic compound that such bacteria convert to calcium carbonate, to different samples and allowed those samples to set. At various intervals, the team powdered the solidified samples to set. At various intervals, the team powdered the solidified samples, created cultures to test for living bacteria, and ran calculations to determine the number of bacterial cells that had survived solidification. They also examined samples of the concrete for concrete for microscopic cracks and to see which minerals had formed.
    As they report in Ecological Engineering, Dr Jonkers and his team found that the mineral grains which formed in the cracks of samples of concrete that had been seeded with bacteria were often as large as 80 microns across. That would go a long way towards sealing those cracks and making them waterproof. The equivalent grains in control samples were rarely larger than 5 microns across.
    Unfortunately, this study also showed that the bacteria survive for only a few weeks. Beyond that period, the concrete falls to heal. But data from a second study, as yet unpublished, suggest that immobilizing the bacteria in particles of clay before they are added to the concrete allows them to live for months, and possibly years. The clay serves both a reservoir for the bacterial food and also as a haven for the bacteria while the concrete hardens. If the process can be scaled up, it may be prove that the best way to preserve concrete is to infect it.

 

THE SELF HEALING MECHANISM OF BIO-CONCRETE ALSO CALLED AS BACTERIAL CONCRETE:

    Smart structures are those that have the ability to sense certain stimuli and are able to respond to them in an appropriate fashion, somewhat like a human being. self healing comes under smart structures, which refers to structural materials to heal or repair itself automatically upon the sensing of damage. This ability enhances safety, which is particularly needed for strategic structures.
    Do bacteria exist which could potentially act as a self healing agent in concrete, and if so, what would be the healing mechanism? From a microbiological viewpoint the application of bacteria in concrete, or concrete as a habitat for specialized bacteria, is not odd at all. Although the concrete matrix may seem at first inhospitable for life, as it is a very dry and extremely alkaline environment, comparable natural systems occur in which bacteria thrive. Inside rocks, even at a depth of more than 1km within the earth crust, in deserts as well as in ultra-basic environments, active bacteria are found (Jorgensen and D'Hondt 2006; Fajardo-Cavazos and Nicholson 2006; Dorn and Oberlander 1981; Dela Torre et al. 2003; Pedersen et al. 2004; sleep et al. 2004). These desiccation and/or alkali-resistant bacteria typically form spores, which are specialized cells able to resist high mechanically and chemically induced stresses (Sagripanti and Bonifacino 1996). A low-metabolic activity and extremely along life-times also characterize spores, and some species are known to produce spores which are viable for up to 200 years (Schlegel 1993).
    In a number of recent studies the potential for application of bacteria in concrete technology was recognized and reported on, e.g. for cleaning of concrete surfaces (DeGraef et al. 2005) as well as for the improvement of mortar compressive strength (Ghosh et al.[53]). Moreover, bacterial treatment of degraded limestone, ornamental stone, and concrete structures for durability improvement has been the specific topic of a number of recent studies (Bang et al. 2001; Ramachandran et al. 2001; Rodriguez-Navarro et al. 2003; De Muynck et al. 2005; Dick et al. 2006). Due to bacterially controlled precipitation of dense calcium carbonate layers, crack-sealing, as well as significan decreases in permeability of concrete surfaces were observed in these studies. In these remediation and repair studies the bacteria and compounds needed for minereal precipitation were brought into contact with the structures surface after setting or crack formation had occurred, and were not initially integrated as healing agents in the material's matrix. The mechanism of bacterially mediated calcite precipitation in those studies was primarily based on the enzymatic hydrolysis of urea. In this urease-mediated process the reaction of urea (CO(NH2)2) and water yields CO2 ammonia (NH3). Due to the high pH value of the NH3/NH4+ system (about 9.2) the reaction results in a pH increase and concomitant shift in the carbonate equilibrium (CO2 to HCO3- and CO23-) which results in the precipitation of calcium carbonate (CaCO3) when sufficient calcium ions (Ca2+) are present.
    Precursor compounds were not initially part of the material matrix but rather externally applied, the remediation mechanism in those studies cannot be truly defined as self healing. Therefore, in order to investigate the potential of autonomous bacterially mediated self healing in concrete, a series of experiments were performed. Firstly, a number of potentially suitable bacterial species were selected. Four species of alkali-tolerant (alkaliphilic) spore-forming bacteria of the genus Bacillus were obtained from the Germany. These bacteria were cultivated and subsequently immobilized in concrete and cement stone (cement plus water in a weight ratio of 2:1 without aggregate addition) to test compatibility with concrete and bacterial mineral production potential respectively. As was listed above (paragraph 2), the ideal self healing agent should not negatively affect the material characteristics. To test this, a dense culture of sporosarcina pateurii was washed twice in tap water and the number of bacteria in the resulting cell suspension quantified by microscopic counting before addition to the concrete mix makes up water. Two parallel series of nine concrete bars (with and without bacteria) of dimensions 16X4X4 cm were prepared and triplicate bars of both series were subsequently tested for flexural tensile and compressive strength after 3,7 and 28 days curing. Table 1 shows the composition of the concrete mix and Figure 2 depicts the strength development of both types of concrete in time.

 

Table 1 Cement, Water, and aggregate composition needed for the production of nine concrete bars of dimensions 16X4X4cm. the washed cell suspension used for bacterial concrete was part of totally needed makeup water.
     
Fig.2 Flexural tensile (a) and compressive (b) strength testing after 3,7, and 28 days curing revealed no significant difference between control and bacterial concrete. The latter contained 1.14x 109 S. Pasteurii cells per cubic centimeter of concrete.The results of the concrete compatibility test show that the addition of bacteria to a final concentration of 109 cm-3 does not affect strength characteristics. Moreover, incubation of cement stone pieces in a medium to which yeast extract and peptone (3 and 5 g L-1 respectively) was added as a bacterial food source revealed that on the surface of bacteria-embedded specimen (Figure 3(B)), but not on control specimen (Figure 3(A)), copious amounts of calcite-like crystals were formed. From the latter experiment it can therefore be concluded that suitable bacteria, in this case alkali-resistant spore-forming bacteria, embedded in the concretes cement paste are able to produce minerals when an appropriate food source is available.


EXPERIMENTAL INVESTIGATIONS:

    In the south Dakota School of Mines, the effectiveness of microbiologically induced calcite precipitation in remediating cracks in concrete was evaluated by comparing the compressive strength and stiffness of cracked specimens treated with bacteria and with those of the control specimens (without bacteria).
    For studying the above, cement mortar beams of size 152X25.4X25.4 mm were prepared. The specimens were cured in water for 28 days and then kept exposed to air for another 3 months. Artificial cracks were cut. The width of the cracks were 302 mm for all the 10 specimens and the depth of cracks were 3.2 and 9.5mm. the first 5 specimens were used as control without any filling in the cracks and were left exposed to air. The cracks in the remaining 5 specimens were filled with a mixture of sand and B.Pasteurii Bacteria. The final concentration of bacteria in the sand is of the order of 602X1010 cells per ml. are forced into the crack by knife edge. Then the beams with bacteria in their cracks were placed in a tray containing urea-CaCl2 medium as food for bacteria and cured for 28 days. The medium was replaced after 14 days. Extreme care was taken not to disturb the precipitation of the calcium carbonate during change of the medium.
    The control beams and those of beams with bacteria were tested for their stiffness after 28 days. It was found that stiffness value of beams whose cracks were filled with bacteria and sand was higher than those of control specimens. This was also true for beams with both crack depths. But the beams with deeper cuts showed comparatively lower stiffness value than the beams with shallower cuts, meaning thereby the bacterial action and precipitation did not reach to the full depth of deeper cuts. Shallow cut beams showed improvements of stiffness by 23.9% while the deep cut beams showed an improvement of 14% over control specimens.
Appropriate and similar investigations were also conducted with respect to the following
  • The effect of microbial calcite precipitation to various depth of cracks on the compressive strength of cement mortar cubes.
  • The effect of different concentration of bacterial cells for cracks remediation, on the compressive strength of cement mortar cubes.
  • The effect of B.Pasturii with various concentrations on the modulus of rupture than that of cracked specimens without bacteria.
INVESTIGATIONS ON DURABILITY CHARACTERISTICS:

    Recently Ramakrishnan et al investigated the durability aspects of cement mortar beams made with different concentration of bacteria. The main objective was to determine whether the beams with bacteria performed better, when subjected to alkaline, sulphate and freeze thaw attack.    They used Scan Electron Microscope (SEM) which is one of the most versatile instruments available for examination and analysis of micro structural characteristics of solids.    From the detail investigations they concluded that microbial culture generated in the cracks of mortar beams increased the compressive strength, stiffness and modulus of rupture. It was also found that the durability characteristics improved with the addition of bacteria. SEM examination established the fact that the calcite precipitation inside cracks has been responsible for the improvement in mechanical properties and permeability characteristics for enhancing the durability.
    When bacterial concrete is fully developed, it may become yet another alternative method to replace OPC and its hazardous effect on environmental pollution.

CONCLUSIONS AND FUTURE PERSPECTIVES:

    Some previous studies reported on the successful application of bacteria for cleaning of concrete surfaces as well as concrete, limestone, and ornamental stone crack repair (DeGraef et al. 2005; Dick et al. 2006; Rodriguez-Navarro et al. 2003; Bang et al. 2001; Ramachandran et al. 2001). As the bacteria in these studies were brought into contact with the material only after damage had occurred, these examples can not be considered as truly, autonomous, self healing mechanisms. The experiments presented in this study, focused on the healing potential of concrete-immobilized bacteria, i.e. bacteria that are part of the concrete matrix. The results of the experiments show that immobilized bacteria mediate the precipitation of minerals and, moreover, the bacteria and certain classes of needed food sources do not negatively affect concrete strength characteristics. It can therefore be concluded that bacterially controlled crack-healing in concrete by mineral precipitation is potentially feasible. The concept, however, needs further developments on some areas. It should still be clarified whether bacterial mineral precipitation effectively seals cracks, i.e. significantly reduces the permeability of cracked concrete in order to protect the embedded reinforcement from corrosion and thus increases the durability of the material. Furthermore, bacterial species must be selected which, when part of the concrete matrix, remain viable for at least the expected lifetime of the construction. If so, the bacterial approach can successfully compete with other (a biotic) self healing mechanisms as such bacteria comply with all the listed characteristics of the most ideal self healing agent
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