Michigan isn’t alone in the struggle to repair crumbling roads and bridges.
The American Society of Civil Engineers has given America's infrastructure a grade of "D" based on years of underfunding and delayed maintenance.
Victor Li may have the key to solving this nationwide struggle.
The University of Michigan civil and environmental engineering professor has invented self-healing concrete. It can bend, and if it cracks, it can repair itself.
In order to understand how the self-healing concrete works, Li says it’s important to understand a little about the regular stuff we’ve been using.
Concrete, Li explains, is a fundamentally brittle material.
“Concrete is brittle like glass. So if you try to press on it, bend it, it will fracture,” he says. “And once cracked, then everything starts to go bad.”
This is why concrete is reinforced with steel.
Those big cracks and potholes we’re all unfortunately accustomed to start off as tiny fractures and chips in the surface, and Li says that those allow water, salt, and oxygen to penetrate the concrete, creating the perfect environment for the corrosion of steel.
When the steel laid in the concrete corrodes, it expands, placing more pressure on the concrete surrounding it and leading to further cracking.
“Sometimes we call those ‘spallings,’ whole chunks of concrete spall out and expose the corroded reinforcements,” Li tells us.
Li says that the first step to creating self-healing concrete is to figure out how to control and minimize the size of the cracks.
According to Li, the inspiration for this piece of the puzzle comes from examining the iridescent surface inside the abalone shell.
Li says that this surface, called nacre, is also fundamentally brittle, but the secret lies in its structure.
“If you look at [nacre] under the microscope, you see that there are lots of layering of this material,” he says. “Some people call it ‘brick-layering’ techniques.”
He explains that this structure allows the layers to move and slide over one another, giving this very brittle material the ability to behave like a ductile, or flexible one.
Li says that this same sort of behavior is accomplished in the self-healing concrete using tons of coated microfibers that allow the concrete to shift and deform slightly when overloaded.
“It will cause these very small, minute motions inside that makes it … look like a metallic material,” he explains. “It’s very resilient.”
Instead of turning into larger cracks with more pressure, the extra force is distributed throughout the material so that they stay small.
So that’s step one. Step two is actually mending those small cracks.
Li says from there it’s just simple chemistry.
Water and carbon dioxide get in those little gaps, he explains, and react with unused cement particles that remain under the surface.
“It forms a glue, basically, and connects the two sides of the micro cracks and basically heals it,” Li says.
He tells us that the material has been tested in a patch on Curtis Road in Ann Arbor, and has performed favorably against traditional patching material.
The Michigan Department of Transportation says that bridges are typically designed to reliably provide 30 years of service. Li is optimistic that his self-healing concrete could drastically change that figure.
“I think it’s fair to expect, let’s say, 100 years … with very little repair requirements,” he says.
Self-healing concrete is still making its way down the road from research to practice, but Li says that he and his team are almost on the cusp of making the material commercially available.
Victor Li is the E.B. Wylie Collegiate Chair Professor of Civil and Environmental Engineering at the University of Michigan.