Wireless sensors to monitor bridges' health

Published 16 January 2008

There are about 597,000 bridges exceeding 20 feet in length on public roads in the United States; more than 50,000 of them were found to be deficient in load-bearing ratings; wireless sensors embedded in the bridge’s concrete will monitor structure’s health

Arnold, Maryland-based Applied Sensors Research and Development Corporation (ASR&D) is working on a Maryland Industrial Partnerships (MIPS) contract with the University of Maryland’s civil engineering professor Dr. Dimitrios Goulias to test sensors which measure the temperature of concrete in structures such as bridges during the lifespan of the concrete. 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, $60 million was spent on general repairs that included repaving with concrete. Some reports suggest that the new pavement began to crack shortly afterwards 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 labour intensive,” said Jacqueline 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 thirty or forty 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 from 2005 indicated that of the roughly 597,000 bridges exceeding 20 feet in length on public roads in the United States, 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 said 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.