[Image above] A new multifunctional, multilayer sensing skin applied to a polymeric substrate with a physical crack. Credit: Julie Williams Dixon
Engineers continue to work towards “greener” construction materials that require less energy to manufacture and exceed standards of strength and durability to ensure new roads and buildings stand the test of time.
Some scientists are returning to simpler times and investigating the ancient Roman secret to more ductile concrete. Geophysicists at the Stanford University School of Earth, Energy and Environmental Sciences (Stanford, Calif.) who discovered concrete-like rock deep within a dormant Italian volcano say this discovery could explain how ancient Romans invented the compound used to build structures like the still-standing Pantheon and Colosseum.
Meanwhile, materials scientists at Northwestern University’s Center for Sustainable Engineering of Geological and Infrastructure Materials in Evanston, Ill., are looking toward the future. They developed a method for making Martian concrete using materials that are available in generous supply on Mars and without using water—a resource that will be limited and precious on the planet.
And closer to home, researchers at Rice University in Houston, Texas, are leveraging concrete’s brittleness, saying the key to greener concrete manufacturing practices lies in the material’s defects and taking an atomic look at concrete’s structure to find out how to reduce the material’s carbon footprint.
But when it comes to infrastructure, fortifying existing roads and buildings is more cost-efficient than total replacement.
Enter “sensing skin”—a multilayered material that can be applied to a structure’s surface to detect corrosive or otherwise harmful substances in structures. The new skin, developed by researchers at North Carolina State University (Raleigh, N.C.), can also “detect cracks and other structural flaws that are invisible to the naked eye,” according to an NC State press release.
“We’ve created a skin that can be applied to the surface of almost any structure and be used to monitor the structure’s integrity remotely and in real time, identifying potential problems long before they become catastrophic,” says Mohammad Pour-Ghaz, assistant professor of civil, construction, and environmental engineering at NC State and co-author of the study.
This unique skin has three layers, the release explains, that can be applied like paint onto a structure’s surface or created separately and applied in sheets like wallpaper.
Each layer serves a specific purpose. “The first layer is electrically conductive and is used solely to detect cracks. The second layer acts as a buffer between the first and third layers. The third layer detects cracks, but is also engineered to detect specific chemicals of interest,” according to the release. The top (or third) layer uses metal nanoparticles that can be engineered to respond to the presence of certain chemicals.
To sense damage on a surface, “electrodes are applied around the perimeter of a structure and the sensing skin is applied to the structure (on top of the electrodes),” the release explains. “Using a computer program, a small current is run between two of the electrodes at a time, cycling through a number of possible electrode combinations,” and “the computer monitors and records the electrical potential at all of the electrodes on the structure for both the first and third layers of the sensing skin.”
Using a set of algorithms developed by the researchers, they measure changes in conductivity in the first and third layers of the skin “to detect and locate damage and the presence of target chemicals,” the release explains.
During a proof-of-concept study, the team put the sensing skin to the test on reinforced concrete, and exposed the concrete to corrosive elements—in this case chlorides—and strain to facilitate the type of fractures that occur to real-world structures.
“The skin performed really well,” Pour-Ghaz says. “We were able to detect cracks as small as a few hundred micrometers, and we could very accurately detect any instances in which chlorides came into contact with the skin.”
The goal, Pour-Ghaz says, is to detect these structural problems early enough to repair. And that’s not limited to concrete structures.
“We want people to be able to detect problems very early on,” Pour-Ghaz says. “And while this proof-of-concept looked at concrete, the technology—if properly applied—could be used on structural materials from metals to polymers.”
The paper, published in Structural Health Monitoring, is “A functionally layered sensing skin for the detection of corrosive elements and cracking” (DOI: 10.1177/1475921716670574).