Concrete is the world’s most widely used man-made material. It supports both your brownstone and the Pantheon. Yet most people don’t give a flying-cement-truck about it. That is, until the president told us recently that there are? 70,000 structurally deficient bridges in the United States. Now, everyone from politicians on down to taxpayers is concerned about the costs of concrete. Good thing we just taught it how to fix itself.
In a paper published last month in ACS Applied Materials & Interfaces, scientists explain how a protective layer containing polymer microcapsules could prevent concrete from breaking down on a microscopic scale. Why go micro? Well, as every reader of waiting room posters knows, it only takes a tiny seed to uproot a sidewalk and a drop of water to blow a crack into a chasm—and it’s actually these minute but gradual forces of nature that do the most damage.
This new technology simply looks like a spray-on coating, but it’s the microcapsules that make it special. If the coating were trail mix, they’d be the M&M’s. Each one bears an infinitesimal amount of a “healing agent” dubbed MAT-PDMS. (You will never, ever need to know this, but MAT-PDMS stands for methacryloxypropyl-terminated polydimethylsiloxane.) Any penetration of the protective layer breaks the capsules open and releases the healing solution, which then fills in on top of whatever damage has been done. The solution solidifies and serves as a protective barrier against further injury.
To some degree, this much has been done with microcapsules before. The difference here is that those other approaches require a coating with both the healing agent and its catalyst built in. But the Korean product has no partner compound dedicated to turning the solution into solid. Instead, it harnesses a catalyst found commonly in nature: the big ol’ sun. Its rays allow the polymer to harden.
“Our coating system is the first example of catalyst-free, photoinduced microcapsule-type self-healing system,” says Chan-Moon Chung in an email. Chung is the communicating author of the paper and professor of polymer chemistry at Yonsei University in South Korea.
To test their polymer, Chung and his fellow researchers used three piles of concrete: one group coated in the self-healing polymer, one group with the same coating devoid of healing capsules, and one boring group of untreated concrete for control. The researchers then knifed up the concrete with a razor blade, baked it in the sun to allow for photopolymerization, and submerged them in water. After 24 hours in the drink, each was weighed to determine how much water was absorbed and thus how much damage each block suffered.
In the end, the plain blocks absorbed 11.3 grams of water on average. The coating stripped of fun stuff came in at an average of 3.9 grams. Both were put to shame at the scale by the 0.4 grams absorbed by the self-healing coating. Jillian Michaels, eat your heart out.
Using such a system benefits us in a few ways. The paper states that catalyst embedded products have many drawbacks including “availability, cost, environmental toxicity, stability, and materials processing.” For instance, a few years ago in the Netherlands they added crack-plugging bacteria to concrete with some effectiveness. Just one problem: Living things have a habit of dying. This meant the coating was only effective for up to four months. In an interview with Mike Orcutt of MIT’s Technology Review, Chung says his coating remains viable for at least a year.
In more good news, it seems concrete is only the beginning. In an email, Chung says, “This coating can be applied in protection of metals with some modification of the coating matrix.”
Of course, you know it’s only a matter of time before this structural breakthrough is co-opted, commercialized, and sold to us for $99.95 as a safeguard against iPhone damage. But who will be able to afford the scanning electron microscope to know if it’s working?