Positive results from field tests of a corrosion detection and remote monitoring technology for suspension bridge cables have raised researchers' hopes the tools could be used for testing the health of bridges worldwide.
The test confirmed that "we have the tools to reliably assess and quantify the level of corrosion on a suspension bridge for damage assessments," says Raimondo Betti, a civil engineering professor at Columbia University and lead project engineer on the $1.8-million collaborative research study. Before, the process involved subjective judgment based on visual inspection.
The five-year study, sponsored by the Federal Highway Administration and led by a research team including Columbia University; Parsons Transportation Group, a unit of Parsons Corp., Pasadena, Calif., and Princeton Junction, N.J.-based Mistras Group, ended on March 2, says Dyab Khazem, Parsons project manager.
The study developed after a major rehabilitation in 2001 of the Maine Waldo-Hancock Bridge, which involved use of acoustic monitoring and proved promising.
"The system was designed to detect the onset of corrosion as an early warning system by providing a corrosion rate for suspension-bridge cable so that the remaining service life and factor of safety can be reliably calculated," Khazem says. The system includes both direct assessments from sensors that provide quantitative measurements of corrosion, and indirect assessments from acoustic, humidity and temperature sensors that provide clues about the cable's internal environment.
First, the team selected the best sensor technologies available and bundled them inside 10,000 to 20,000 tightly compacted wires, Betti says.
The team then tested the technology on the strands of a New York City bridge—which researchers requested not be publicly identified. Finally, at Columbia in New York City, the researchers tested multiple types of bridge-cable sensors in a corrosion chamber that simulates corrosive conditions. The six-month test involved a cable mock-up tested under 1.2 million lb of tension.
"We subjected the cables to aggressive conditions simulating rain, heat, cool air and other conditions," Betti says.
Researchers in the U.S., Japan and Europe have addressed corrosion of main cables on bridges, but most work focuses on traditional structural questions, such as the effect of corrosion in bridge wires on cable strength, rather than monitoring for the extent of corrosion from inside the cables, Khazem says. "Now the ball is in the court of the bridge owners to test it out," he says.
Betti says the cost of the system is about 0.001% of a bridge's entire cost, depending on its size and condition and the number of sensor locations. Power needs are also small. The system can include a solar panel and has wireless capability via a sidewalk transmitter. The team chose an array of sensors expected to last at least 15 years.
Now the research team is launching a 15-month, $380,000 spin-off study to test dehumidification systems used for cable corrosion protection.
"This is a breakthrough because, due to tiny voids within cable, this validation of dehumidification systems was not possible before," according to Khazem.