We keep hearing about the perilous state of our infrastructure. But we continue to use our bridges, roads and tunnels every day without ever thinking much about them.
There are worries, too, about state of repair of such things as the facilities where we store hazardous waste, including, in some cases, nuclear waste.
There have been developments in self-healing concretes that, when used, should alleviate many of our worries. But acceptance of that technology has been slow. Besides, retrofitting a building using self-healing concrete can be pretty complex. Now, though, scientists from North Carolina State University and the University of Eastern Finland, have developed a paint-on system that would detect small problems before they become big problems, that can be used in both new and older structures, and is almost as easy to apply as a coat of paint.
In fact, it's more like paint than anything else. Its developers call it "sensing skin" and it can quickly detect cracks and other damage in concrete structures.
Researchers say it is designed to serve as an early-warning system for anything made of concrete, although the impetus for the work was concern about the integrity of such critical infrastructure as nuclear waste storage facilities.
Mohammad Pour-Ghaz, a professor of civil, construction and environmental engineering at North Carolina State, said the system utilizes electrical impedance tomography, which is an off-shoot of a scanning system first developed for diagnostic work in the world of medicine. The sensing skin consists of a thin layer of electrically conductive copper paint that is applied to the surface of the concrete. Cracking of the concrete substrate results in a rupture of the sensing skin, which decreases its electrical conductivity. And it is the decreased conductivity that the tomography picks up.
Researchers used paint containing copper, although other conductive materials could be used.
To install the system, electrodes are applied around the perimeter of a structure. The sensing skin is then painted onto the structure, over the electrodes. In use, a computer program then runs a small current between two of the electrodes at a time, cycling through a number of possible electrode combinations. A small area would have only a few electodes; a large structure would have a lot, and the more — sometimes many more — pairs would be included in the scan.
Every time the current runs between two electrodes, a monitor records the electrical potential of all the electrodes on the structure. These data are then used to calculate the conductivity of the entire skin. If the conductivity decreases, it means the structure has cracked, or been damaged in some other way. The system also pinpoints the location of the damage.
Aku Seppänen, of the University of Finland, said that pinpointing the damage posed "a challenging mathematical problem," but added that he believes the researchers work represents an advance over previous developments in the field.
At the moment, the system has been accurate when used on a small scale, using concrete beams less than a metre wide.
"Our next step is to extend this to large geometries," Pour-Ghaz said. "We want to show that this will work on real-world structures."
If the research team brings that off, the sensing skin could provide owners of infrastructure — public and private — with a valuable management tool that, by sounding an early warning, would enable early repair and thus substantially extend the life of the structure.
A paper describing the system, "Electrical impedance tomography-based sensing skin for quantitative imaging of damage in concrete" was published online last month in the journal Smart Materials and Structures. You can find it at http://iopscience.iop.org although if you want to read the whole paper, you'll have to buy a copy.
Korky Koroluk is an Ottawa-based freelance writer. Send comments to firstname.lastname@example.org