Drexel researchers are developing a structural fiber system that could one day enable damaged concrete structures to repair themselves. (Image: Drexel.edu)

Imagine a concrete surface that, if cracked or in need of repairs, can “heal” itself. Well, that’s exactly what a team at Drexel University in Philadelphia has engineered with BioFiber — a polymer fiber encased in a bacteria-laden hydrogel and a protective, damage-responsive shell with the entire assembly a little over a half-millimeter thick.

According to the Drexel team, a grid of BioFibers embedded within a concrete structure can improve its durability, prevent cracks from growing, and even enable self-healing — akin to human skin.

When a person cuts their skin, blood comes out and the tissue starts to heal. Of course, that’s not the case with infrastructure. So, the team based its idea on the human concept and bioengineered it for brittle material.

“When we have infrastructure that face damage or cracks, it’s static,” said Associate Professor Amir Farnam. “It doesn’t repair; it’s not living material. The major idea that we had was, ‘How could we make it a living structure, so it responds like our body?’ Such as when you get a cut and then you heal it.”

(Image: Drexel.edu)

The group used a strain of Lysinibacillus sphaericus bacteria as a bio-healing agent for the fiber. Typically found in the soil, it has the ability to drive a biological process called microbial-induced calcium carbonate precipitation to create a stone-like material that can stabilize and harden. When induced into forming an endospore, the bacteria can survive the harsh conditions inside concrete, lying dormant until ready for use.

Placed in a grid throughout the concrete as it is poured, the BioFiber acts as a reinforcing support agent. But its true talents are revealed only when a crack penetrates the concrete, enough to pierce the fiber’s outer polymer shell.

As water makes its way into the crack, it causes the hydrogel to expand and push its way out of the shell and up toward the surface of the crack. While this is happening, the bacteria are activated from their endospore form in the presence of carbon and a nutrient source in the concrete. Reacting with the calcium in the concrete, the bacteria produce calcium carbonate which acts as a cementing material to fill the crack all the way to the surface.

The healing time ultimately depends on the size of the crack and activity of the bacteria, but early indications suggest the bacteria could do its job in as little as one to two days.

“It’s the same thing [as skin healing],” said Caroline Schauer, the Margaret C. Burns Chair in Engineering. “When you crack the cement, water gets in and releases what's in there to then heal itself. We're making something dynamic. We're making something that actually will change once it’s damaged — it will change to heal itself.”

Manufacturing self-healing concrete is much greener than you might think; it’s an incredible way to reduce greenhouse gas emissions, as the ingredient-production process — burning minerals at upward of 2,000 °F — accounts for about 8 percent of all global greenhouse gas emissions. Concocting concrete that can survive longer would be a game changer.

Schauer added that the team’s next steps are two-fold: The group is considering manufacturing as the next step but also looking at what would be the environment that these fibers are going to experience.

“We developed these fibers. The next steps are to scale up this fiber-manufacturing process. And by scaling up, it's finding out how it's going to work in a real field application. So, we make concrete, we pour some pavements, and then we wait for few years to see those results,” said Farnam.

He added that in real-world applications the concrete very well could — and most likely will — be subjected to the elements: salt, snow, vehicles, and other variables. However, he’s hopeful the technology could be implemented sooner than later.

One potential application is to put the biopolymers down on the Philadelphia Airport runway. However, that is awaiting FAA permission.

“Based on my previous experience, about seven to 10 years of research and a scale of work needs to be done for new technologies,” Farnam said. “This is seven to 10 years from when you’re starting from scratch. We started the idea four years ago, so, hopefully, three or four more years.”

Andrew Corselli is Digital Content Editor at SAE Media Group. For more information, visit here .