Acoustic Approach Targets Defects in Concrete Bridge Decks

The new detection system drags multiple strings of metallic balls along concrete. Photo courtesy of UNL.

University of Nebraska (Lincoln, Nebraska) (UNL) researchers have designed an acoustic approach to detecting defects in concrete bridge decks.

According to Jinying Zhu, a civil engineering professor, the system represents a faster, more accurate alternative to conventional methods. “When we find delamination, we should fix it,” Zhu says. Otherwise, she notes, the combination of road salt and precipitation in the winter could cause further corrosion.

The system consists of a pushcart that drags multiple strings of balls made of brass. These balls produce different acoustic frequencies when striking delaminated vs. pristine concrete. Small attached microphones record sound and send it to a laptop that processes the signals. A specialized GPS unit tracks the cart’s location.

By correlating signals with the position, the system’s software generates a color-coded map that illustrates the location and dimensions of defects. Deep-blue indicates pristine concrete, and dark red represents severe delaminations.

Bridge inspectors have traditionally sought out delaminations by dragging chains and listening for hollow spots, and marking them with chalk or paint. “It's a very slow process, and it's subjective,” Zhu says. As a result, her research team replaced chain links with metallic balls connected by nylon string. They found the ball-and-string combo enhanced the desired signals while reducing the noise.

They also found that the balls produced more data points. Unlike chain links, which slide along the concrete while being dragged, the balls hop into the air when encountering even slight rough patches. Each time they land, they produce another signal—lessening the chances of missing a delamination.

The researchers tested their system on multiple state bridges, adding that it takes about 20 minutes to assess bridge segments that would require roughly two hours with traditional chain-drag methods.

Source: UNL,