One of the most widely used materials for construction is concrete.Cracks in concrete are a common phenomenon due to the relatively low tensile strength. Once cracks form, in concrete it may reduce the life span of the concrete structures. Various repairing techniques are available to repair the cracks but they are highly expensive and time-consuming processes We have heard of our body having self-healing abilities. Just imagine, if the concrete of our futuristic buildings will have capacity to heal its own gaps and cracks caused due to various reasons. You heard it right, in the conscious efforts of going green, the next thing that we can watch out for is self-healing concrete or Bioconcrete. Dr Henk Jonkers, a microbiologist at Delft University, who specialises in the behaviour of bacteria in the environment, has developed self-healing concrete in the laboratory. Self-healing of cracks in concrete would contribute to a longer service life of concrete structures and would make the material not only more durable but also more sustainable
Self-healing concrete is a product that will biologically produce limestone to heal cracks that appear on the surface of concrete structures. Specially selected types of the bacteria genus Bacillus, along with a calcium-based nutrient known as calcium lactate, and nitrogen and phosphorus, are added to the ingredients of the concrete when it is being mixed. These self-healing agents can lie dormant within the concrete for up to 200 years.
However, when a concrete structure is damaged and water starts to seep through the cracks that appear in the concrete, the spores of the bacteria germinate on contact with the water and nutrients. Having been activated, the bacteria start to feed on the calcium lactate. As the bacteria feeds oxygen is consumed and the soluble calcium lactate is converted to insoluble limestone. The limestone solidifies on the cracked surface, thereby sealing it up.
The consumption of oxygen during the bacterial conversion of calcium lactate to limestone has an additional advantage. Oxygen is an essential element in the process of corrosion of steel and when the bacterial activity has consumed it all it increases the durability of steel reinforced concrete constructions.
The starting point of the research was to find bacteria capable of surviving in an extreme alkaline environment. Cement and water have a pH value of up to 13 when mixed together, usually a hostile environment for life: most organisms die in an environment with a pH value of 10 or above. The search concentrated on microbes that thrive in alkaline environments which can be found in natural environments, such as alkali lakes in Russia, carbonate-rich soils in desert areas of Spain and soda lakes in Egypt. Samples of endolithic bacteria (bacteria that can live inside stones) were collected along with bacteria found in sediments in the lakes. Strains of the bacteria genus Bacillus were found to thrive in this high-alkaline environment.
Different types of bacteria were incorporated into a small block of concrete and the block would be pulverised and the remains tested to see whether the bacteria had survived. It was found that the only group of bacteria that were able to survive were the ones that produced spores comparable to plant seeds. Such spores have extremely thick cell walls that enable them to remain intact for up to 200 years while waiting for a better environment to germinate. They would become activated when the concrete starts to crack, food is available, and water seeps into the structure. This process lowers the pH of the highly alkaline concrete to values in the range (pH 10 to 11.5) where the bacterial spores become activated. Finding a suitable food source for the bacteria that could survive in the concrete took a long time and many different nutrients were tried until it was discovered that calcium lactate was a carbon source that provides biomass.
There are two key obstacles that need to be overcome if self healing concrete is to transform concrete construction in the next decade. The first issue is that the clay pellets holding the self healing agent comprise 20% of the volume of the concrete. That 20% would normally comprise harder aggregate such as gravel. The clay is much weaker than normal aggregate and this weakens the concrete by 25% and significantly reduces its compressive strength. In many structures this would not be a problem but in specialised applications where higher compressive strength is needed, such as in high-rise buildings, it will not be viable.
The second disadvantage is the cost of self-healing concrete is about double that of conventional concrete, which is presently about €80 euros per cubic metre. Jonkers says: “At around €160 per cubic metre, self-healing concrete would only be a viable product for certain civil engineering structures where the cost of concrete is much higher on account of being much higher quality, for example tunnel linings and marine structures where safety is a big factor – or in structures where there is limited access available for repair and maintenance. In these cases the increase in cost by introducing the self-healing agents should not be too onerous.” Added to this, if produced on an industrial scale it is thought that the self healing concrete could come down in cost considerably. If the life of the structure can be extended by 30%, the doubling in the cost of the actual concrete would still save a lot of money in the longer term.
While presently the trials of the self healing concrete is undergoing at various Costains Construction Sites located at South Wales Valley, the progress and dependability of this concrete is being scrutinized and studied by Cardiff University’s team of researchers. They along with teams from University of Cambridge and University of Bath have combined hands to give their valuable inputs about the project, which is named Materials for life.
These teams are not only conducting real time analysis, however, they are also in charge to handle the pilot programs of the three construction sites of the company that is using this form of concrete. Their role in the entire project is to see how effective this concrete is and to provide an indebt analysis and study of their findings and observations
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