Sensors and New Materials Build Smart Bridges
Brent D. Johnson
Armies of workers laid 650,000 miles of road and erected 75,000 bridges from 1935 through 1939 under the Works Progress Administration. The program was an unqualified success, and these structures remain a large part of the highway system in the US.
Nearly 70 years later, the roads and bridges show signs of age and may require major retrofitting over the next decade. As new technologies for rebuilding and extending their lifetimes come online, new means of testing their durability may further enhance construction methods.
Steve E. Watkins, director of the Applied Optics Laboratory at the University of Missouri-Rolla, and his team are developing a technique for real-time strain analysis of bridges that will demonstrate how fiber-reinforced polymer, or composite, materials can upgrade older bridges and defer the cost of new construction. They have completed a number of projects on the university campus.
One of these was the first all-composite bridge to be built in Missouri. It was a demonstration project of new construction technology that used a collection of composite tubes bonded together and assembled in modular sections, transverse and longitudinal to the direction of the bridge. The composite material is applied in layers like wallpaper, and the researchers made a small groove in each layer so that they could insert the fiber sensors.
In projects that involve repairing or upgrading reinforced concrete bridges, the optical sensors can be applied to the composite layers, to the concrete or to the steel rebar, depending on the application requirements.
Extrinsic Fabry-Perot fiber sensors are bonded to key locations with epoxy, and when strain or deformation of the bridge changes the cavity spacing between the fibers in a capillary tube, the reflected interference signal indicates that there is a strain event and translates into an electrical signal. Fiber coatings, whether on Fabry-Perot, Bragg or other types of sensors, influence the bonding characteristics.
The key feature of fiber optic sensors is that, as part of the building materials, they last a long time in the field. They have demonstrated millions of strain cycles without degradation and, because they also are directional and environmentally rugged, hold up over a wider range of strain.
Watkins said that deploying the sensors involves intelligent placement, and multiplexing is also an issue. The group uses both long-gauge and point sensors. The amount of instrumentation required per sensor is a balance of cost and complexity.
He said that, although the composite material and fiber optic sensors go together well, he sees persuading departments of transportation to incorporate the technology as a problem. The researchers are considering ways to make the system wireless with cell phones so that transportation departments could "call up the bridge and see how it's doing." He believes that critical structures can be monitored using this system not only for their structural health, but also for weather conditions, and temperature and traffic patterns so that officials know whether to send out the salt trucks when ice forms.
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