Some say there are two types of concrete: cracked and on the verge of cracking. What if concrete could heal itself when it cracked?
We are part of a team of materials scientists and microbiologists who are harnessing the power of bacteria to create biological fibers that initial results suggest can heal cracks in concrete. We are working on a technology that can extend the life of concrete if we manage to fix the problems and bring it to market one day.
concrete cracking
Consider a bridge exposed to snow, rain, temperature changes and trucks carrying heavy loads. Cracks will gradually form in the concrete on the bridge due to stress and wear. Over time, these cracks widen, allowing water and corrosive substances that weaken the concrete to penetrate further down.
At some point, local governments have to pay for repairs that are not only expensive but also disrupt traffic and consume public resources.
Now consider a medical patient recovering from a serious injury. When the patient’s cells detect damage, they secrete tiny healing agents like microscopic repair teams. These agents target the injured area, repairing tissues and restoring the functionality of cells. What if concrete had the same kind of self-healing ability as human tissue?
self-healing concrete
Our team at the Advanced Infrastructure Materials laboratory at Drexel University was inspired by the self-healing tissue in the human body. We have developed an additive to concrete that we call BioFiber.
BioFiber has three basic functions: It is self-healing, prevents cracks from growing, and remains intact within the concrete when there are no cracks.
Each BioFiber has three basic components: a tough core fiber made of a polymer called polyvinyl alcohol, a porous hydrogel layer infused with polyvinyl alcohol. Lysinibacillus sphaericus bacteria and an outer shell susceptible to damage. When cracks hit BioFiber, its outer shell breaks and releases bacteria into the crack, which begins the self-healing process.
BioFiber’s strong core fibers close the cracks and prevent them from widening further during the healing process.
The hydrogel layer surrounding the core fiber consists of a network of polymer chains at the molecular level that attract water. Their sponge-like structure can absorb and retain large amounts of water. We add calcium to help solidify the hydrogel during the manufacturing process.
The hydrogel itself consists of a natural polymer called alginate, which is found in seaweed and has special properties that enable it to trap bacteria. Alginate is non-toxic and safe even for biomedical applications such as drug delivery and tissue engineering.
The hydrogel harbors endospores, which are dormant bacteria. When the outer shell cracks and the endospores awaken from dormancy, they facilitate self-healing.
Enabling BioFiber
Water is needed for endospores to activate. Fortunately, the hydrogel layer in the middle absorbs water well. The spores are awakened when the concrete cracks and water from rain, moisture or street runoff seeps in.
Spores ingest the calcium in the concrete as well as the carbon we specially add to the concrete mixture. With these materials, bacteria facilitate a chemical reaction called microbially induced calcium carbonate precipitation, or MICCP. This reaction forms calcium carbonate crystals that accumulate and fill cracks in the concrete.
The shape of the crystal varies from sphere to needle shape, and each shape is strong enough to heal cracks. By changing the pH level, calcium source, and bacteria type, we can change the type of crystals the bacteria produce.
Since concrete is a mixture of cement, sand, gravel and water, it behaves like a solid and hard substance. We throw BioFibers into the mixture and spread them while the concrete is mixed, ensuring that they are distributed evenly throughout the mixture.
Once the self-healing process is over and the bacteria are dead, the activated BioFiber is finished; He can’t get better anymore. But since there are many BioFibers scattered throughout the concrete, another fiber can repair the next crack. We currently do not know how many cracks BioFiber concrete can heal and we are conducting further research to understand this.
To feed the bacteria, we add the amount of food it needs to survive and heal the cracks based on how many cracks we estimate need to be repaired. The process stops when the bacteria run out of nutrients. Bacteria can survive for roughly a few weeks during the healing process.
While BioFiber is promising at first, it has shortcomings that could make it difficult to produce on a larger scale. The manufacturing process and materials used are specialized and not always cost-effective and practical. Although our initial tests suggest that BioFiber extends the life of concrete, we will need further testing, including field trials, to confirm these early results.
We hope to eventually commercialize and manufacture the fibers at larger production scales; In the meantime, we continue to run tests and investigate how we can improve BioFiber’s self-healing abilities. We want to one day put these fibers on roads and sidewalks to potentially prevent aging concrete from cracking.
This article is republished from The Conversation, an independent, nonprofit news organization providing facts and authoritative analysis to help you understand our complex world. Written by: Muhammed Husmand, Drexel University and Yaghoob Farnam, Drexel University
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Mohammad Houshmand works at Drexel University. receives funding from the National Science Foundation.
Yaghoob Farnam receives funding from the National Science Foundation. In addition to his position as an associate professor at Drexel University, he is the co-founder and senior technical advisor of SusMaX Inc.