SELF HEALING CONCRETE

Concrete is the second most consumed material across the world only second to water. It is the most widely used and most important construction material as it is solid, strong, durable, easy to prepare and cheap. But the downside is, concrete is prone to cracking which could compromise the entire structure of a building. Concrete structures are very strong in compression but tend to crack when subjected to tensile forces. To overcome this defect of concrete, steel reinforcement bars are usually used together with concrete. But the problem is, steel reinforcements are also susceptible to corrosion when placed in moist environment and ultimately result in cracking. Similarly, owing to adverse conditions such as sea water, acidic environment, alternate wetting and drying condition etc. also crack formation occurs in concrete. Drying shrinkage (contracting of hardened concrete due to loss of moisture) is also another cause of crack formation in concrete.

         

         

  Fig: Behavior of concrete in tension, compression and bending

                                                                   

       

                                                                                                                                                                              fig. Corrosion in concrete

                                                                                                                                              Image courtesy: https://www.dailymotion.com

So, cracking has been a lifelong problem seen in building industry. Cracks are fine until they start affecting the structural strength of concrete and then reduce the overall life of structure.

 

So, how to repair cracking?

In convectional practice, concrete mortar is applied to the damaged surface such that the concrete bonds properly with the surface. But, it becomes difficult to fully penetrate concrete up to the desired depth and different expensive techniques like grouting are to be applied to ensure that concrete does not flow down. Also, repairing of concrete at greater height or underground becomes too difficult. So, these methods are particularly time consuming and very expensive.

However, it seems like this problem could finally be solved now by the invention of revolutionary technology called Self -healing Concrete.

History of self-healing concrete

  Long back in 1877, Ferdinand Cohn, a German biologist claimed that concrete could be healed with bacteria known as “Genus Bacillus”. But it took almost 130 years to prove this theory.

Henk Jonkers, a microbiologist and professor at Delft University of Technology in the Netherlands in 2006 proved this theory after his three years long experiment. He found that the bacteria –bacillus could survive the harsh environment of concrete by producing spores, a cell that can resist extreme environmental condition, to defend themselves. He added calcium lactate to the limestone concrete mixture in order to feed bacillus so that they could produce limestone to repair cracks in the concrete.

Later in 2011, an engineering student Michelle Pelletier in collaboration with her professor Arijit Bose at University of Rhode Island, found a new self-healing agent sodium silicate. They utilized a micro-encapsulated sodium silicate in concrete. As cracks form in concrete, these capsules burst and release sodium silicate which reacts with calcium hydroxide within the concrete to form calcium-silica-hydrate gel that repairs cracks.

In 2018, Ning Zhang of Rutgers University and Congrui Jin, Guangwen Zhou and David Davies of Binghamton Universiy also created a self-healing concrete based on fungus. They utilized Trichoderma reesei fungus, which produced spores that remain dormant until the first cracks begin to emerge and then start to fill the cracks.

What is a self-healing concrete?

Self-healing concrete is a product which by biological or chemical means would fill up the cracks in the concrete autogenously and preserve the structural integrity of a building and prolong its service life.

 

Possible advantages of self-healing concrete:

• Improves Structural Performance: Reducing cracks, it reduces permeability, porosity, corrosion, etc. and provides better durability and strength than conventional concrete. 
•  Economical: As the cost of repair and maintenance of building during its service life are greatly reduced, it becomes economical in long run 
•  Reduces Green House emission: Cement is one of the major cause of the green house emission. By the use of self-healing concrete, use of cement in repair and maintenance is reduced which ultimately reduces the green house emission.  
•  Can be used in repair works: They can be applied as spray and prevent further cracking of building. Also, it could be used on road pavements which requires frequent repair and maintenance, which would eliminate traffic obstructions and help in smooth vehicular and pedestrian movement.

Self-healing concrete is still a research topic for many academics and researchers. Several research and trails are going on to see the full scale application of this technology in real world. Also, studying few instances of small scale application, Self-healing concrete is found to be twice as expensive as normal concrete. Due to which it might be difficult for it to grab a meaningful share of normal market. However, scientists are conducting several studies in perfecting self-healing concrete. If perfected, a large scale production is could bring a greater reduction in its cost and induce a larger penetration in market which could be a total game-changer in building industry. With the introduction of self-healing concrete, wear and tear in different concrete structures would no longer be a worry. So, it won’t be wrong to say that this is the future of building industry and in near future we will be using a lot of self- healing concrete in our construction practices.