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Wireless Sensors Being Developed to Monitor Concrete in Bridges and Other Structures

The rush-hour collapse of the Interstate 35W bridge in Minneapolis -- which killed 13 people and sent hundreds of motorists careening into the Mississippi River -- highlighted not only the precarious state of U.S bridges, but also the systems used to monitor infrastructure.

But a Maryland company wants to help by monitoring public structures -- such as bridges, dams and roads -- with tiny wireless sensors.

Annapolis-based Applied Sensors Research and Development Corporation is working on a Maryland Industrial Partnerships contract with Civil Engineering Professor Dr. Dimitrios Goulias to test sensors that measure temperature, a key factor in concrete's stability throughout its lifespan.

Hardening at the wrong temperature can cause concrete to become unstable and crack, creating safety hazards and expensive repairs. During the Chesapeake Bay Bridge renovation in 2002, officials spent $60 million in general repairs that included re-paving with concrete. According to The Washington Post, the new pavement began to crack shortly thereafter, and an additional $7 million was put into the project. Later, inspections proved that the cracking was a result of low curing temperatures.

"Many current structural heath inspection processes, particularly in the U.S., are completely manual and labor intensive," says Jacqueline H. Hines, president of ASR&D. "Teams of engineers spend hours or even days climbing up and down scaffolding. The sensors we are developing will provide a continuous wireless monitoring capability."

The wireless sensors -- lasting 30 or 40 years or longer -- also monitor temperature changes throughout the lifespan of a structure due to environmental factors, or incidents such as vehicle fires or explosions.

"If there is a vehicle fire or an explosion in a tunnel or a bridge, our sensors can monitor how hot the concrete gets. Concrete exposed to extremely high temperatures can become almost sand-like and lose its structural integrity," said Hines.

Workers embed the sensors in the structure before the concrete is poured. The sensors then relay temperature measurements to a software program that provides strength predictions and thermal gradients. Depending on the readings, construction workers may insulate or pour cold water on the structure as it hardens to keep it at optimal temperature.

The team will study how the sensors work in differing concrete mixes on current construction sites either on or nearby campus. Goulias and ASR&D will also study how construction equipment, such as cranes and front loaders, may affect the wireless signal of the sensors.

"I really do not foresee any difficulties in the testing phase," said Goulias. "This technology will make it much easier and much cheaper to monitor concrete maturity."

According to the American Society of Civil Engineers, in 2003 more than 27 percent of all U.S. bridges were rated structurally deficient or functionally obsolete. Federal Highway Administration surveys as of 2005 indicated that of the roughly 597,000 bridges exceeding 20 feet in length on public roads in the U.S., more than 50,000 were found to be deficient in load-bearing ratings, with many more lacking adequate safety margins for superstructure, substructure, and bridge decks.

Temperature is not the only indicator for concrete failure; other stresses in the form of corrosion of reinforcing steel, vibrations and traffic loads are important instigators as well. Hines says her company is looking into other types of sensors to monitor these factors, including using these devices to provide a wireless link to interface with other sensor technologies.

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