States apply sensors to bridge conditions

Maryland and others are funding research into developing systems that could flag structural damage

The Interstate 35 eight-lane bridge in Minneapolis that collapsed earlier this month had been showing signs of fatigue for several years, although the bridge did routinely pass safety checks. After the collapse, inspectors admitted that there are limits to today's safety inspection techniques.

Now the research and development community is looking into the idea of embedding sensors within the bridges themselves. Such sensor units would not act as last-minute alarms but would let inspectors know how the material is holding up after years of use.

With funding from the Maryland State Highway Administration, the Johns Hopkins University Applied Physics Laboratory in Laurel, Md., has developed a small, low-cost sensing package, called a Wireless Embedded Sensor Platform, that could be used with new bridges.

When multiple units are embedded within a bridge, the WESP can monitor the state of the structure. At a cost of less than $10 for each unit, such platforms could one day prevent disasters such as the one in Minneapolis.

'A few dollars today will save millions down the road,' said Ann Darrin, supervisor of APL's aerospace and materials science group.

Chained to the past

Current approaches for detecting stress on bridges are largely low-tech, and the results they produce can be uneven. For instance, the American Society for Testing and Materials recommends dragging a chain across a bridge to test its structural integrity. Any hollow, dull sounds would indicate where the concrete has delaminated, or split into multiple layers. A steel rod or hammer could substitute for the chain.

Maryland, being a mid-Atlantic coastal state that gets its fair share of snow each year, was particularly interested in better understanding how salt could damage its bridges. So it commissioned APL to develop a Chloride Ion Sensor, which measures chloride concentrations in concrete, said Rengaswamy Srinivasan, the chief APL researcher behind the technology.

The presence of chloride is a good indicator of how badly the steel bars girding the bridge are being corroded by salt from road crews and, in coastal areas, from the air.

APL's sensor is an inch in diameter and a quarter of an inch thick. The sensor's material allows only chloride to adhere to the polymer. The amount of chloride on the sensor affects the electronic signal it emits when probed by a wireless portable reading device.

When a new bridge is built, multiple sensing platforms may be placed throughout the bridge. Someone with a handheld device near the sensor could read the results, which would be powered by electromagnetic waves coming from the device. Srinivasan is working on software that would aggregate the data and summarize the results into one of three levels depending on the severity of the corrosion: green, yellow or red.

APL is not alone in the pursuit of embedded sensors and other advanced techniques for detecting bridge stress. Los Alamos National Laboratory, along with the University of California, is also working on sensor devices that could cost less than $1 apiece.

The University of Cincinnati Infrastructure Institute has been studying how bridges weaken over time, along with ways to test that stress.

Five years ago, the Federal Highway Administration took money pooled from a number of states and sponsored a study to develop a system called the High Speed Electromagnetic Roadway Measurement and Evaluation System. It was basically a handheld ground-penetrating radar that could look for delaminations.

Unfortunately, FHA failed to commercialize the technology after the original research was completed.

So far, Maryland has implanted two bridges with prototype versions of APL's technology.

The number of sensors required per bridge is still being investigated, but it could be as many as 10,000.

'It could be one every square meter or one every 10 meters,' said Bliss Carkhuff, an APL researcher who worked on the sensor.

Darrin warned that the sensor system would not provide a quick fix for the problem of failing bridges because they could only be deployed in new bridges, not retrofitted to older ones.

'We really need to have a longer horizon when we look at infrastructure support,' she said. 'When we build a bridge today, we have to think about what condition it will be in 40, 60, 80 years from now.'

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