Concordia Researchers Use Ground Penetrating Radar to Track Bridge Health
Canada has an infrastructure problem. One-third of Canada's 75,000 bridges have a short remaining service life due to structural and functional deficiencies.
Fatal bridge collapses in the last few decades have prompted engineers to seek better surveillance methods. Salt and snow removal accelerate corrosion, and as winter continues to batter both Canada and the U.S., improving bridge safety has never been more urgent.
But the state of North America’s bridges could soon change for the better, thanks to Concordia’s Tarek Zayed, a professor in the Department of Building, Civil and Environmental Engineering. With the help of his graduate students, he is developing new visualization methods that could drastically improve the quality and reliability of bridge inspections.
“We still rely on visual inspections of bridges. Experts look for cracks and gauge their length and depth. This works well for things on the surface, but what about what’s inside?” says Zayed.
To look beneath the surface, the researchers are using ground penetrating radar (GPR). It sends electromagnetic waves below the surface of concrete and other materials, and measures how the waves are reflected.
When transmitted into the concrete slab of a bridge deck, the waves are reflected by the reinforcing rebars. A strong, clear signal indicates the rebar is in good "health". Fuzzy and weak signals show various states of probable corrosion.
The essential question is: At what point is the corrosion critical? Zayed and his students are proposing several innovative new ways of answering this question.
Tracking the health of a bridge over its lifespan
Zayed believes that measurements taken via GPR should be tracked and monitored several times over the lifespan of a bridge and compared with its initial measurements.
The measurements taken when the bridge is completely “healthy” establish a clear baseline. Any changes in those initial measurements indicate deterioration in progress. This method holds great promise for new bridges and is already in use in pilot projects in Quebec and New Jersey.
Mapping deterioration using colour visualization
But what about bridges that have been around for decades already? They require a different approach. Zayed’s team proposes a sophisticated new tool called “clustering-based threshold calibration.”
The researchers use complex math to analyze the information provided by ground penetrating radar, organizing it into three colour-coded categories for probable rebar corrosion – green for non-corroded, yellow for moderate corrosion, and red for critical corrosion. The thresholds between the different categories are clear and not subject to individual interpretation, which can vary from one expert to the next.
Looking at maps generated in this way, it immediately becomes clear which parts of the bridge deck are probably corroded and likely to need maintenance soon.
“We were able to make a major breakthrough in determining probable thresholds between good, moderate and critical corrosiveness of concrete,” Zayed says. “I think many other jurisdictions across North America will want to follow our lead in using this technique.”
Zayed presented and published his research at the Transportation Research Board conference, session 768, in Washington, DC, in January 2015.