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‘Sonogram’ Spots Bridge Rebar Threats

Wednesday, April 25, 2012

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Engineers at the Swiss Federal Institutes of Technology (EPFL) have developed a sonogram-like technology that can reveal rebar corrosion inside concrete bridges.

The imaging technology is based on a technique that uses microwave radiation to penetrate a bridge’s insides, yielding precise images that offer “a quick and easy diagnosis” of corrosion in a steel rebar skeleton, according to a release by EPFL.

 The EPFL imaging system reveals humidity and ion chlorides in a bridge structure.

 EPFL

The EPFL imaging system reveals humidity and ion chlorides in a bridge structure.

Rebar corrosion is the primary weakness in bridges and other structures made of reinforced concrete, the university notes.

Non-invasive detection of corrosion’s effects or corrosive conditions below the surface could help catch problems at an early stage, heading off the need for major rehabilitation down the road, the research team says.

‘Georadar’

The process was developed as part of PhD research by Alexis Kalogeropoulos and involves experts in materials science, measurement methods and signal processing from several organizations. The work is being directed by Professor Eugen Brühwiler, head of EPFL’s Structural Engineering Group.

The technology is based on a technique known as “georadar,” which uses microwave radiation to penetrate a bridge interior.

Numerical treatment of the data gathered by the georadar produces images that are precise and easy to interpret,” according to EPFL.

Currently, examining rebar commonly requires drilling core samples from the bridge deck. However, only 10 samples at most may be taken from a 100-meter bridge—a scattershot approach that cannot provide meaningful, comprehensive data before problems begin, says Brühwiler.

“This method is too random to be able to precisely predict the magnitude and duration of rehabilitation, and thus to optimize the intervention,” he explains.

Rust and Rebar

Rebar is subject to corrosion when water infiltrates concrete, a porous material. Worse, the water typically carries a variety of corrosive chemicals, including chloride ions from de-icing salt. Gradually, the steel rebar deep inside the concrete structure begins to rust.

“Even the best concretes have this defect,” explains Brühwiler.

(Approaching the corrosion problem from another angle, Brühwiler has also been among the researchers developing Ultra High Performance Fiber Reinforced Concrete, a newer type of concrete material that replaces gravel with a select proportion of short, thin metal fibers. This increases load resistance and makes the concrete virtually impervious to water and gases.)

Dances with Data

Researchers have used georadar for a decade to view interiors. Mounted on a vehicle, a device sends radiation into the structure, then records the reflected signal, according to EPFL.

The method gives engineers a look inside the bridge and allows them to diagnose construction defects, water pockets, the presence of chloride ions, and other conditions that signify corrosion, says Brühwiler. (Engineers don’t detect the corrosion itself, he noted.)

 Henley Street Bridge, Knoxville, TN

 Bridgehunter.com / Calvin Sneed

Concrete bridges like the Henley Street Bridge, in Knoxville, TN, could get a boost from the new imaging technology in spotting developing problems below the surface, developers say.

The drawback, the university says, is that an expert must read and interpret the results.

“In Switzerland, there are only two or three people who are capable of interpreting this data, with at best an accuracy of 70%,” Brühwiler says while showing one bridge image. “I would be completely at a loss.”

The new data processing and analysis method makes the process “much more precise and accessible,” providing clear, readable images from the reflected signal, according to EPFL.

‘A World First’

A 100-meter bridge can be scanned in just a few hours,  and the diagnosis had an accuracy rate of more than 90 percent in its first field test, EPFL said. The technology will soon be tested again as part of a project commissioned by the Swiss Federal Roads Office, the institute said.

“It’s a world first,” said Brühwiler. “With our method, bridge rehabilitation will be easier to plan, and the costs will be easier to determine.”

   

Tagged categories: Bridges; Concrete; Corrosion; Inspection; Rebar

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