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.
Self-healing concrete also known as Bio-Concrete or Bacterial concrete or living concrete can be produced by adding bacteria in concrete along with its nutrient to keep it alive for production of calcite to fill crack after precipitation. Bacteria was added in concrete along with calcium lactate to repair cracks. The focus is not only to keep bacteria alive but also to generate much calcite to fill the cracks.
WHICH BACTERIA IS USED?
The bacteria from Bacillus family is chosen for self-healing in concrete. Following are those bacteria:
- Bacillus Pastuerii
- Bacillus Sphearicus
- Bacillus Subtili
- Bacillus balodurans
- Bacillus pseudofirmus
- Bacillus Pseudofirmu
- Bacillus cohnii
Which are found naturally in highly alkaline lakes near volcanoes, and are able to survive for up to a staggering 200 years without oxygen or food. These are activated when they come into contact with water and then use the calcium lactate as a food source, producing limestone that, as a result, closes up the cracks.
Preparation of Bacterial Concrete
BY DIRECT APPLICATION
The bacteria and the chemical precursor(calcium lactate) are added directly while making concrete
BY ENCAPSULATION LWA
By encapsulation method the bacteria and its food i.e. calcium lactate, are placed inside treated clay pellets and concrete is prepared. About 6% of the clay pellets are added for making bacterial concrete.
Advantages of Bacterial Concrete
- Eco- friendly nature, self-healing abilities.
- Self-repairing of cracks without any external aide.
- Reduction in permeability of concrete surface.
- Significant increase in compressive strength and flexural strength when compared to normal concrete.
- By using Bacterial concrete the carbon dioxide emissions are reduced significantly.
- Reduction in permeability of concrete.
- Reduces the corrosion of steel due to the cracks formation and improves the durability of steel reinforced concrete. According to research and experimentation bacteria-based SHC is denser and more durable than concrete.
- Reduction in reinforcement corrosion.
- Bacillus bacteria are harmless to human life and hence it can be used effectively.
- It can prevent water to percolate into reinforcement steel concrete and hence it does not comes in contact with reinforcement.
- Resistance towards freeze-thaw attacks.
Disadvantages of Bacterial Concrete
- It is obvious that the initial cost of construction using self healing concrete is higher. Cost of bacterial concrete is double than conventional concrete.
- The clay pellets holding the self-healing agent comprise 20% of the volume of the concrete.
- Growth of bacteria is not good in any atmosphere and media.
- Design of mix concrete with bacteria here is not available any IS code but few researches found the mix ratio for that particular environment.
- Investigation of calcite precipitate is costly.
- Self-healing concrete can only repair cracks up to 0.8 mm wide and as such are not helpful in repairing large cracks or potholes in roads.
Applications of Bacterial Concrete
- Self healing concrete can be used for sectors such as
- Tunnel-lining, structural basement walls, highways, bridges, concrete floors and marine structures.
- This is new technology can provide ways to durable roads.
- High strength buildings with more bearing capacity.
- Long lasting river banks.
- Erosion prevention of loose sands.
Probable reason for the massive precipitation of calcium carbonate near the crack rim (Fig. A) is that concentration of both reactants calcium hydroxide and carbon dioxide are
relatively high here due to the opposing diffusion gradients of the respective reactants.
Calcium hydroxide diffuses away from the crack interior towards the overlying bulk water while carbon dioxide diffuses from the bulk water towards the crack interior where it is scavenged by high concentrations of calcium hydroxide.
The process of chemical calcium carbonate reaction from dissolved calcium hydroxide occurs according to the following reaction:
CO2 + Ca(OH) 2 → CaCO3 + H2O
The amount of calcium carbonate production inside the crack in control concrete
specimens is likely only minor due to the low amount of CO2 present in the limited.
The self healing process in bacterial concrete is much more efficient due to the active metabolic conversion of calcium lactate by the present bacteria:
Ca(C3H5O2) 2 + 7O2 → CaCO3 + 5CO2 + 5H2O
This process results in the precipitation of substantially higher amounts of calcium carbonate inside the crack as calcium carbonate is in this case not only directly produced from the conversion of calcium lactate in equimolar amounts of calcium carbonate, but also indirectly via the chemical reaction of metabolically produced CO2. As latter carbon dioxide is produced at the surface of the crack interior it will directly react with portlandite particles still present in the crack interior. In the latter case, portlandite does not dissolve and diffuse away from the crack surface, but instead reacts directly on the spot with local bacterially produced CO2 to additional calcium carbonate. The process of bacterial calcium lactate conversion thus results in the production of in total six calcium carbonate equivalents, resulting in efficient crack sealing as can be seen in Figure B. In this study the potential effect of only calcium lactate addition (without addition of bacterial spores) on crack healing was not considered. In order to establish the purely chemical effect of calcium lactate additions on crack-healing potential, experiments under completely sterile conditions have to be performed.