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‘Smart Sensors’ Track Bridge Corrosion

Wednesday, March 7, 2012

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Engineers at Columbia University are developing “smart sensor” technology that could provide the world’s suspension bridges with the first real-time internal monitoring of a “silent killer”: corrosion.

The goal of the groundbreaking research is to develop an integrated methodology that uses state-of-the-art sensing capabilities and NDT direct and indirect technologies to assess bridge cable conditions.

 Columbia University

 Photos: Columbia University

A 20-foot-long mockup, built in a Columbia University lab, replicates the cables used on the Manhattan Bridge, where the system will be tested.

A smart sensor network that can monitor both external and internal environments, integrated with NDE that would map the entire length of the cable, is the only accurate method for reliably assessing the condition of suspension bridge cables, the research team says.

Cable Model: 20 feet, 9,100 wires

The heart of the research is a massive cable model and accelerated corrosion chamber painstakingly constructed in a university lab at Columbia’s Fu Foundation School of Engineering and Applied Science.

The cable mock-up—21 inches in diameter, 20 feet long, containing 9,100 wires, and fully instrumented—is exposed to the worst that nature can dish out in rain, heat and cooling cycles (courtesy of a sprinkler system, heating lamps and air conditioning unit) inside the six-by-five-by-16-foot chamber.

“This is the only cable mockup that you can see that is this long,” study director Raimondo Betti says in a video overview of the project. “It’s one of the largest ever built in the world.”

Sensor Technology

Along the cable’s diameter, seven strands contain 76 embedded sensors originally developed for the space industry.  The calibrated sensors incorporate a variety of NDT technologies (Main Flux method, Magnetostrictive technology and Acoustic Emission technology) that have been tested and validated for direct detection of corrosion damage on large suspension bridge cables.

As scientists pound the cable with various exposures, the sensors collect measurements of environmental variables such as temperature, relative humidity and pH. They also measure the corrosion rate of the cable’s zinc and steel.

 The system embeds calibrated sensors in seven strands along the cable diameter
The system embeds calibrated sensors in seven strands along the cable diameter. The system is subjected to 1.2 million pounds of actual force.

All of these measurements are then processed to provide estimates of remaining cable strength, which are updated hourly.

Researchers can also incorporate in the model pre-corroded and notched strands that can be pulled to failure, to simulate the behavior of a cable throughout its life cycle.

To simulate load bearing, the entire model is subject to 1.2 million pounds of actual force—greater force than typically is applied to actual bridges in service, says Betti.

Inspection Challenges

Current inspection of suspension bridge main cables mainly consist of visually inspecting the exterior covering every two years, the university notes.  An in-depth inspection is usually scheduled when necessary to assess the condition of the interior wires by wedging the cable at selected locations along its length.

But even wedging inspections allow review of only about 2 percent of a cable’s thousands of wires, notes Professor Andrew Smyth, a member of the research team.

The project, sponsored by the Federal Highway Administration, also includes researchers Parsons Transportation Group, and Physical Acoustics Corporation.

Field Testing

Once the cable mockup test has been concluded and the sensors’ performances have been evaluated, the sensor network will be installed on a panel of one of the main cables of the Manhattan Bridge. The network will collect information that will be transmitted in real time to a central unit that will provide an up-to-date evaluation of the cable strength.

 New York City is home to the highest density of suspension bridges in the world
New York City is home to the highest density of suspension bridges in the world.

“No systematic study like the one you see here was ever done,” says Betti, who calls corrosion a “silent killer” of suspension bridges. “So I think everybody in the world is looking at this experiment very carefully.”

View a video overview of the project.

View a video about the sensor technology.

   

Tagged categories: Bridges; Corrosion; Inspection; Research; Steel; Weathering

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