Scientists Know Why A Flawed Bridge Sings

on October 24 2012 5:48 PM
Flickr via Creative Commons/dok1

It turns out Gene Kelly isn’t the only one who’s singing in the rain. Brigham Young University engineers recently discovered that water drops falling on concrete make distinct sounds that can be used to detect underlying flaws in the material on which they land. The insight could lead to cheaper, easier ways to spot problems in highway bridges.

A water droplet hitting flawed concrete “is like hitting a drum,” says BYU researcher Brian Mazzeo. “It’s quite audible.”

Mazzeo and his colleagues published their work in the latest issue of the journal Non-Destructive Testing and Evaluation International.

America’s infrastructure is in exceedingly poor shape. In 2009, more than a quarter of the nation’s bridges were either structurally deficient or functionally obsolete, according to the American Society of Civil Engineers. One particular problem that plagues reinforced concrete bridges is delamination, which is caused by the corrosion of metal reinforcements. As the rebar in a bridge rusts, it expands, causing breaks that can lead to dangerous potholes.

Typically, engineers or technicians check for signs of delamination by dragging a steel chain across the concrete and listening for a dull hollow sound. But this method takes a long time and can cause lane closures that snarl traffic.

Mazzeo and his colleagues were looking for a simpler way to detect concrete flaws. Some of Mazzeo’s preliminary tests weren’t exactly what you might think of as high-tech -- he was dropping ball bearings and rock salt on the concrete floor of his lab, located in a warehouse.

Then, one late night, “I just took a beaker of water and poured it on the concrete, and lo and behold, you could a hear difference between the impacts,” Mazzeo said in a phone interview.

The reason a droplet makes such a striking sound on concrete is related to the "water hammer" effect, which occurs when a fluid in motion is suddenly stopped. Sometimes you can hear a larger version of this effect if you turn off a faucet and hear a distinct "thump" in the pipes.

Coauthor W. Spencer Guthrie says that while their research is still preliminary, “we envision a day in some years when this approach could be embodied in form of attachment on the back of truck.”

Microphones could also be used in an apparatus to pick up the sound of the striking droplets, and a computer could pinpoint the telltale frequencies that spell future potholes, allowing maintenance crews to swoop in and treat the problem area.

SOURCE: Mazzeo et al. “Acoustic impact-echo investigation of concrete delaminations using liquid droplet excitation.” Non-Destructive Testing and Evaluation International 51: 41-44, October 2012.