About half of the country’s bridges will celebrate their 50th birthday by 2020. And with age, comes deterioration. “We’re facing a baby boomer bridge problem,” says Jerome Lynch, director of the Laboratory for Intelligent Structural Technology at the University of Michigan, Ann Arbor.
The big wake-up call came with the collapse of a bridge carrying the I-35W highway over the Mississippi River in Minneapolis, MN, in August 2007. The collapse indicated the need for more stringent inspections. In the US, human inspectors certify bridges every two years, but they can miss small cracks, problems in hard-to-access locations and internal damage.
To address the aging bridge problem, the National Institute of Standards and Technology and several other companies are developing solutions to monitor bridges.
Many of the systems are based on piezoelectric sensors. These devices generate a small current when they vibrate to passively monitor vibrations in the structure, and they can also actively probe for faults. Just as a medical ultrasound scanner uses an acoustic signal to image internal organs, an active piezoelectric sensor can send an acoustic signal into a bridge’s interior. By listening for the returned signal, it can detect structural anomalies such as hairline cracks or areas of unusual strain.
Distributed Sensor Technologies plans to use a single optical fiber to do the work of hundreds of discrete sensors. The idea is to stretch the fibers taut and attach them to the bridge. Cracks and other imperfections will alter the vibrations picked up by the fiber, which in turn will alter the way a beam of laser light travels along it.
At the University of Michigan, Lynch has created a sensor of a different sort: a “skin” made of polymers and carbon nanotubes that changes its electrical resistance when deformed and that can be painted onto a bridge. The results allow a computer to generate a 2-D image that, like an X-ray, will reveal details of the internal structure, providing a map of any damage it may have sustained.
Victor Li, also at the University of Michigan, has mixed carbon black into concrete to make it more electrically conductive. Existing concrete is “very dumb,” he says. Li’s idea is that cracks and damage to a bridge made of his concrete will show up as interruptions in an electric current, indicating where and how bad the damage is.
With more engineers monitoring the country’s bridges, future maintenance can be predictive rather than reactive.
Source: Sujata Gupta, New Scientist Magazine