Concrete may be one of the toughest buildings materials in common use but it does develop cracks over time, and in the case of structures such as buildings or bridges, it is imperative that those cracks are noticed before they lead to a collapse. While visual inspections are useful, they are also time-consuming, and may miss tiny but structurally-significant cracks. Some technologies have been developed to automate the process, such as rust sensors for steel-reinforced concrete. Now, an international team of scientists is proposing a system of flexible crack-detecting skins, that could be applied to the surfaces of concrete surfaces.
Structures such as bridges would simply have pieces of the skin glued onto any areas where cracks were considered likely to occur. The skin would contain patterns of capacitive rectangular patches, arranged in different patterns within the skin, depending on the type of stress that was likely - patches oriented diagonally to one another would work best for detecting cracks caused by shear, whereas horizontally-joined patches would be better for detecting sag in horizontal beams, for instance.
The capacitance of each patch (the amount of energy it was storing) would be altered by any movement underneath it, as would be caused by the formation of cracks. Once a day, a computer that was attached to the skin would send an electrical current through it, measuring the capacitance of each patch. If sharp differences were noted between adjacent patches, the computer would note the exact location, and then notify a human inspector of the discrepancy. This ability to pinpoint the location reportedly sets the system apart from other stress-detecting technologies, that don't provide such specific coordinates.
So far, the largest piece of the skin tested in laboratory conditions has measured 8 by 4 inches (20 by 10 cm). While a stretchy silicon fabric with silver electrodes was used initially, it proved to be too thin and flexible for some applications. It has since been replaced with a thermoplastic elastomer mixed with titanium dioxide, with the capacitive patches made from painted-on black carbon.