As we know we have to find the solution to repair concrete crack manually in the past. Now we have another alternative method to replace this process called Self Healing Concrete.
In this article I will explain self healing concrete in detail.
Table of Contents
Definition
Concrete that can fix its cracks on its own, either autogenously or autonomously, is characterized as self-healing concrete.
What Is Self Healing Concrete?
As the name suggests it heal itself when it comes in contact with air and water and due to this process it produces lime and water in the outer surface of the concrete.
So the condition to heal is it should come in contact with air and water. When it comes in contact with air and water biological reaction starts to heal the concrete.
In the conventional concrete mixture 25-30% cement remains unhydrated. Due to concrete cracking this unhydared cement exposed to moisture to penetrate the crack.
Technical Development
Self-healing concrete has undergone significant development since its concept was introduced in the 1990s.
Delft University of Technology in the Netherlands filed the first patent for self-healing concrete in 1996. The university’s researchers discovered that the addition of bacteria to concrete could help it heal itself. The bacteria would remain dormant in the concrete until a crack formed. Then, they would become active and produce calcium carbonate, which would fill the crack and heal the concrete.
In the early 2000s, researchers at the University of Michigan developed self-healing concrete that utilized microcapsules filled with a healing agent. When a crack formed, the capsules would rupture, releasing the healing agent, which would fill the crack and heal the concrete.
Many other developments in the field of self-healing concrete have taken place since then. Researchers have experimented with adding nanoparticles to concrete, which can react with moisture and carbon dioxide to create a self-healing process. Additionally, concrete has been developed that uses shape-memory polymers that can return to their original shape when heated to fill in cracks.
The development of self-healing concrete holds promise for the future of construction by significantly reducing maintenance costs and increasing the lifespan of concrete structures. While the technology is still in the research and development phase, it has the potential to revolutionize the construction industry.
Autogenous Self Healing In Cement based Materials
Autogenous healing of cement-based materials plays a crucial role in the self-closure of cracks, thereby enhancing the durability and physical-mechanical performance of composites. This phenomenon has been identified as one of the main reasons for extending the life of ancient structures and buildings. The French Academy of Science first observed the autogenous self-healing in cement-based composites in 1836 when cracks in pipes and water-retaining structures self-repaired. Over time, significant theoretical and experimental research has demonstrated that autogenous self-healing processes are predominantly related to physical, mechanical, and chemical processes occurring within the cementitious matrix.
When cracking occurs, the so-called “surface-controlled crystal development” takes place, allowing calcium ions to be immediately accessible from the fracture faces, thereby accelerating crystal growth. Once an initial layer of calcite is formed on the crack walls, the surrounding concrete matrix becomes less rich in calcium ions, and the transition to the “diffusion-controlled crystal growth” phase occurs. During this phase, the Ca2+ ions must diffuse through the concrete and the CaCO3 layer to reach the crack surface and ensure the precipitation of healing products, which is slower than the first phase. In the case of composite cement, which includes pozzolanic additions, a portion of the calcium hydroxide, identified as a primary source of Ca2+ ions, is used in the pozzolanic reaction for CSH formation, leading to delayed and weaker precipitation of calcium carbonate.
Additional mechanisms depicted in the scheme include the swelling of hydrated cement paste along the crack walls due to water absorption by calcium silicate hydrates and mechanical crack blocking by means of debris and fine concrete particles, a direct result of the cracking process, or impurities in the water entering the crack. Autogenous healing mechanisms are only effective for small cracks, with a maximum healable crack width of 10–100 µm, sometimes up to 200 µm but less than 300 µm, only in the presence of water. It is challenging to control and predict autogenous healing mechanisms due to their usually scattered outcomes and dependence on a variety of factors and variables, including the age and composition of the concrete, the presence of water, and the thickness and form of the concrete fracture.
Stimulated Healing In Concrete
Autogenous healing of concrete is a process where the material is capable of repairing its own cracks without external intervention. Various methods can promote or enhance autogenous healing, such as restricting crack width, providing water, or boosting hydration or crystallization. Mineral additions like blast-furnace slag and fly ash, which remain unhydrated even at older ages, can promote autogenous healing. Crystalline admixtures are permeability-reducing admixtures that can block pores and cracks, thereby improving cementitious system density and resistance to water penetration. Superabsorbent polymers (SAPs) can be added to cementitious systems to limit self-desiccation shrinkage and improve freeze-thaw resistance while inducing self-sealing and self-healing properties. SAPs absorb mixing water during concrete mixing and leave behind macropores that operate as weak matrix sites, encouraging multiple cracking and promoting crack closure by allowing cracks to cross SAP macropores and generate narrower cracks. However, the inclusion of SAPs may lead to strength loss, depending on the kind of SAP utilized, particle size and shape, the number of SAPs, the w/c ratio of the mix, the addition of water to compensate for the loss in workability, and the mixing technique.
Importance Of Self Healing Concrete In Construction
Self-healing concrete has the potential to revolutionize the construction industry by reducing the need for maintenance and repair, increasing the lifespan of structures, and improving sustainability. One promising approach to achieve self-healing concrete involves the use of carbon nanotubes (CNTs).
CNTs are known for their exceptional mechanical, electrical, and thermal properties, which make them ideal for reinforcing materials such as concrete. CNT-reinforced concrete (CNT-RC) has been shown to have significantly improved mechanical properties compared to traditional cement. The addition of CNTs can increase the compressive strength of cement paste by up to 23% and the flexural strength by up to 45% compared to plain cement paste. Moreover, the elastic modulus of CNT-RC can be increased by up to 85% compared to plain cement paste.
In addition to improving mechanical properties, CNT-RC has also been shown to have improved durability and resistance to environmental factors such as freeze-thaw cycles, chloride ion penetration, and sulfate attack. The addition of CNTs has been reported to reduce the diffusion coefficient of chloride ions in cement paste by up to 60%, indicating improved resistance to chloride-induced corrosion.
One potential application of CNT-RC is in subsurface wellbore cementation for oil and gas wells. The use of CNT-RC in wellbore cementation can improve the mechanical properties of the cement, which is crucial for preventing gas and fluid migration and ensuring the long-term integrity of the wellbore. A recent study investigated the reinforcement of wellbore cements with a CNT polymer nanocomposite additive, which resulted in improved compressive and flexural strength.
Another advantage of CNT-RC is its ability to heal after being subjected to fires and high temperatures. Research has shown that CNT-RC maintains its healing ability even after being exposed to temperatures up to 800 °C, which is a significant advantage in the event of a fire.
CNT-RC has the potential to revolutionize the construction industry by providing durable and sustainable concrete that can self-repair cracks and damage, leading to a reduction in maintenance costs and an increase in the lifespan of structures. While there are still challenges to overcome, such as the scalability and cost-effectiveness of producing CNTs, the future of self-healing concrete looks promising with the use of advanced materials such as carbon nanotubes.
Self Healing Concrete Methods
Autonomous self-healing in concrete refers to the ability of concrete to automatically repair cracks and other types of damage without the need for external intervention. There are various methods used for achieving autonomous self-healing in concrete, including microencapsulation, macroencapsulation, vascular healing, and self-healing bioconcrete.
Microencapsulation involves incorporating microcapsules containing a healing agent directly into the concrete matrix. Upon crack development, the microcapsules release the healing agent, which then reacts with a catalyst in the matrix to heal the crack. Macroencapsulation uses larger capsules that are embedded in the concrete and release the healing agent upon the creation of cracks.
Vascular healing is a biomimetic approach to self-healing that involves creating a network of channels within the concrete that can transport liquid healing chemicals to damaged areas. Finally, self-healing bioconcrete uses microbial activity to create calcium carbonate, which can fill cracks and improve the strength and durability of the concrete.
Necessity:
As we know concrete is very good to resist compressive force while it is not good to resist tensile forces. So to resist tensile force reinforcement is provided. When tensile force exceeded as per given reinforcement, and concrete does not withstand then concrete develops cracks.
So, to fill these cracks, self healing concrete is used.
Self- Healing Concrete Bacteria:
Bacteria contain an external layer of the thick wall which is capable to resist sunlight, chemical exposure, etc. We use these bacteria in mixed concrete.
Mixing Process:
Spores (organic bacteria), fine aggregate, coarse aggregate, and cement mix together to make the concrete mixture. This concrete mixture works as self-healing concrete.
Spores and cement should not mix with the help of clay palletes.
Working Concept:
Self-healing concrete is a type of concrete that biologically produced limestone to heal the crack that appears on the external surface of the concrete.
CA[C3H5O2]2 + 7O2 = CACO3 + 5 CO2 + 5H2O
(Calcium Lactate) (Lime)
Lime is our final product which fills the concrete cracks.
Specific types of bacteria Genus Bacillus, along with Calcium Lactate, Nitrogen, and phosphorous are added to the concrete ingredient when it is mixed.
Closing Thought:
Self Healing Concrete is crack resistant. It resists the concrete and reinforcement from cracks and corrosion. Self Healing Concrete also prevents reinforcement from water. It increases the strength of the concrete compare to conventional concrete.