SELF HEALING CONCRETE
When we human beings would personally love ourselves being resilient enough at any situation, now envisage the same with concrete. Cracks are probably the compromising factor that concrete structures possess. Wouldn’t it be so nice if concrete could fix their own cracks? The answer to treat the cracks in concrete internally is by its self-healing properties.
The conventional concrete came into existence back in 200BCE by Romans which is still used as an inevitable ingredient in the world of construction. The raw materials for concrete include cement which itself has to go through a lot of process to be manufactured. Other required components include aggregates (gravel & sand) and water which are considerably the precious natural resources. Admixtures and air entraining agents are also used in concrete nowadays to obtain certain special requirements. Apart from procurement of raw materials, preparing of concrete has a series of steps involved which is then followed by laying with workmanship. When a structure is being built with such utmost care, energy of machines, efficiency of labor, cement which liberates CO2, aggregates and water which is in demand across the globe, imagine the efforts we should undertake to protect them from destruction.
IN A NUTSHELL:
Self-healing concrete is the one which autonomously produces limestone to repair when crack occurs. This is done with the help of microorganism like bacillus bacteria and hence called bio concrete. Bacteria along with required nutrients is mixed with the usual concrete to produce self-healing concrete. The nutrients serve as food for the bacteria when they come in contact with the moisture, which is mostly during crack formation.
THE IDEA BEHIND:
Many self-healing approaches including capsule based self-healing, vascular self-healing, self -healing through embedding Shape Memory Alloys (SMA) were proposed, among which microbial self-healing method stands out. Our bio-concrete is made of living materials and can heal itself like human skin or trees. The concept of self-healing concrete ages back to the early 1990’s when Carolyn Dry, an architecture professor in Urbana-Champaign developed this idea. Later inspired by natural body processes where mineralization helps in healing of bones, Hendrik Jonkers, a microbiologist in Netherlands wished to replicate the same idea in concrete using microbes. Following the idea of autonomously fixing cracks, repair mortar and liquid based repair system have been developed to fix the cracks in concrete that has already occurred.
MECHANISM:
Biologically at the occurrence of crack, the moisture disturbs the clay pellet which releases the bacteria and nutrients in it. These nutrients help in the growth and multiplication of bacteria forming limestone to fix the cracks. To describe on the chemical reaction that’s taking place, the water in contact with the unhydrated calcium in the concrete produce calcium hydroxide by the help of bacteria which acts as a catalyst. The calcium hydroxide then reacts with the carbon dioxide in the atmosphere forming limestone and water. This extra water molecule keeps the reaction going whereas the limestone hardens itself and seals the cracks.
CaO + H2O à Ca(OH)2
Ca(OH)2 + CO2 à CaCO3 + H2O
ALL ABOUT BACTERIA:
The bacterial growth and multiplication in times of healing require certain pH level for which nutrients are added with them. The pH required for bacterial growth on an average is 7 to 9 and so the nutrients with pH 4 to 12 are included. The bacteria can be added directly to the normal concrete during the mixing process or in the form of pellets by encapsulation method. Alkali resistant bacteria like Bacillus pseudofirmus which can resist the stresses occurring in concrete are induced in concrete. The bacteria are observed to heal cracks of size 0. and can exist up to 200 years inside the concrete. The tests on effects of bacteria have not shown any serious effects on animals as well as human beings and hence it is safe to be implemented.
TEST RESULTS:
Bio-concrete is neither flammable nor explosive and on conducting various tests it has proved its competency to be implemented in constructions. Compressive strength of self-healing concrete is found to be increased by 30–35% than the conventional concrete at the end of 28days whereas in case of split tensile strength, the strength of self-healing concrete increases by 15–20%. Bio-concrete has also proved to exhibit less scaling degradation which the concrete structures face during freeze and thaw cycles. The bacteria in new concrete allow lesser destruction to structure due to water by consuming them for their growth and multiplication.
PROS & CONS:
Introducing bacteria in concrete besides enhancing their strength, plays a significant role in increasing the durability of the reinforcement as well the concrete structure as whole. The bacteria fix the cracks internally without the need of an external aid thus reducing the need for adequate maintenance. They reduce the permeability of water into the concrete but require specific environment for their growth. The cost involved, labor requirements and lack of standard reference code for their design are the major drawbacks in implementing self-healing concrete in the construction world.
TAKE AWAY:
By introducing bacteria in concrete, we can obtain various favorableness among which durability and strength plays a key role. Also, self-healing concrete does not require regular inspections as bacteria themselves take care of the cracks occurred. There are various other options to heal cracks in concrete autonomously, still this way of using microorganisms would definitely be an eco-friendly approach. Since conventional concrete has been in use over several millenniums, it is hard to convince the mindset of people in production and construction. However, steps can be taken to include self-healing agents in concrete foreseeing its advantages in terms of sustainability. Why say no when there is a way to extend life? As sustainability is more important than cost, say yes to bio-concrete!
BY,
Mukila I
Civil Engineering
Batch of 2021.
