MEMS sensors warn of potential structural failure

by John Williamson
Contributing Editor, MEMS Investor Journal

MEMS sensors embedded in concrete bridges and similar structures offer promise as a more economical solution to detect potential failure according to Dr. Mohamed Saafi, Associate Professor at North Dakota State University’s (NDSU) Department of Construction and Engineering Management. A typical cause of failure is environmental exposure when moisture penetrates the concrete due to freeze-thaw cycles. Often containing salt, the moisture attacks rebar, the results of which are only visually apparent when spalling occurs.

“Fiber optic sensors are being employed to measure degradation, but it is a costly process,” Dr. Saafi says. “Cost factors include packaging optical fibers in the concrete, a difficult process requiring skilled personnel. Monitoring is also costly, requiring optical spectrum analyzers and a light source to observe changes in the optical fiber wavelength due to stretching.”

A team comprised of Dr. Saafi along with colleagues at Alabama A&M and the National Institute of Applied Sciences in Tunisia decided to apply MEMS technology based on its success in the transportation, communication and medical fields. According to Dr. Saafi, “The MEMS sensor used in this study is composed of 4 silicon cantilever beams equipped with a sensitive polymer designed to retract and expand upon exposure to moisture. It is also equipped with an on chip temperature sensor.”

The MEMS sensors are packed into an epoxy embedded in the concrete. Moisture and temperature readings are picked up by wireless reading scanners. During a measurement, the reader sends an RF signal that is picked up by the sensor. The sensor's rectifier first transforms the RF signal into voltage to power the sensor then transforms the sensor's response into an RF signal that is sent back to the reader, which in turn transforms it again into voltage. By using calibration equations, the reader calculates the temperature and moisture content inside the concrete. This process is known as passive sensing.

In a companion study embedded nanotube sensors operate much the same way to measure crack propagation in a structure, Dr. Saafi says. When a crack develops the nanotube cracks and its electrical field is changed, signaling a problem.

Research continues in the NDSU labs to fully develop this technology for field application and to evaluate the process under actual environmental and load conditions. For example, a reader could be attached under a school bus to scan the bridge and send the data to a GPS satellite and then to the highway department. Work also continues on the process to improve the sensitivity of the nanotube sensors and developing a batch fabrication process.

According to Dr. Saafi the technology can be applied to composites, steel, concrete, plastics and wood structures. When the technology becomes widely adopted it could lower monitoring costs by half, he predicts.