by Some say there are two types of concrete – cracked and about to crack. But what if when concrete is cracked, it could heal itself?
We are part of a team of materials scientists and microbiologists who have harnessed the power of bacteria to create biological fibers that initial results suggest can heal cracks in concrete. We are working on a technology that could extend the life of concrete if we work out the connections and one day bring it to market.
Concrete cracking
A picture of a bridge exposed to snow, rain, temperature changes and trucks carrying heavy loads. The concrete on the bridge will gradually develop cracks from 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 authorities have to pay for repairs which are not only expensive but also disrupt traffic and drain public resources.
Now, consider a medical patient recovering from a serious injury. As the patient’s cells recognize the damage, they release tiny healing agents – like microscopic repair teams. These agents target the wounded area, repairing tissue and restoring cell functionality. What if concrete had the same kind of self-healing ability as human tissue?
A self-healing concrete
Our team in the Advanced Materials Laboratory at Drexel University was inspired by self-healing tissue in the human body. We have developed an additive to concrete that we call BioFiber.
BioFiber has three essential functions: it heals itself, stops cracks from growing wider, and remains intact within the concrete when there are no cracks.
Each BioFiber has three main components: a tough core fiber made of a polymer called polyvinyl alcohol, a porous layer of hydrogel infused with Lysinibacillus sphaericus bacteria, and an outer shell that responds to damage. When cracks hit the BioFiber, its outer shell breaks and releases the bacteria into the crack, which starts the self-healing process.
The strong central fibers in BioFiber bridge the cracks and stop them from growing wider during the healing process.
Around the core fiber, the hydrogel layer consists of a mesh of polymer chains at the molecular level that attract water. Its spongy structure can absorb and retain large amounts of water. During the production process, we add calcium to help the hydrogel solidify.
The hydrogel itself is made up of a natural polymer found in seaweed called alginate, which has special properties that allow it to trap bacteria. Alginate is not toxic and is even safe for biomedical applications such as drug delivery and tissue engineering.
The hydrogel houses spores, which are dormant bacteria. As soon as the outer scales split and the inspores wake up from their dormant state, they facilitate self-healing.
Activating BioFiber
The endospore needs water to activate. Fortunately, the middle hydrogel layer absorbs water well. When the concrete cracks, and water from rain, humidity or street runoff seeps in, the spores awaken.
The carbon spores we specifically add into the concrete mix, as well as calcium in the concrete itself. With these materials, the bacteria facilitate a chemical reaction known as microbially induced calcium carbonate precipitation, or MICCP. This reaction creates calcium carbonate crystals, which rise and fill cracks in the concrete.
The crystal shape varies, from sphere to needle shape, and each shape is strong enough to heal the cracks. We can change the type of crystals produced by the bacteria by changing the pH level, calcium source and the type of bacteria.
Concrete acts as a solid, hard substance because it is a mixture of cement, sand, gravel and water. We throw the BioFibers into the mix and spread them out as the concrete is mixed, making sure they are evenly distributed throughout the mix.
When the self-healing process stops and the bacteria die, the BioFiber is activated – it can no longer be cured. But since there are many BioFibers scattered throughout the concrete, another fiber can repair the next crack. At the moment, we don’t know how many cracks BioFiber concrete can heal, and we are doing more research to find out.
To feed the bacteria, we add the amount of food it needs to stay alive and we heal the cracks, depending on how many cracks we hope they will have to repair. When the bacteria run out of food, the process stops. The bacteria can survive for about a few weeks during the healing process.
Although BioFiber shows initial promise, there are shortcomings, which could make manufacturing on a larger scale challenging. The manufacturing process and the materials used are specialized and not always affordable and practical. Although our first tests suggest that BioFiber extends the life of concrete, we will need more testing, including field trials, to verify these early results.
We hope to eventually commercialize and manufacture the fibers at larger production scales, and in the meantime we continue to run tests and study how to improve BioFiber’s self-healing ability. We want to one day introduce these fibers into roads and sidewalks to prevent cracking in aging concrete.
This article is republished from The Conversation, a non-profit, independent news organization that brings you reliable facts and analysis to help you make sense of our complex world. It was written by: Mohammad Houshmand, Drexel University and Jacob Farnam, Drexel University
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Mohammad Houshmand works for Drexel University. It receives funding from the National Science Foundation.
Yahoob Farnam receives funding from the National Science Foundation. In addition to his role as an associate professor at Drexel University, he is co-founder and senior technical advisor for SusMaX Inc.
