REMOTE SENSING: Fiberoptic chloride sensors monitor Vermont bridge

Chloride-based deicing agents, otherwise known as road salts, are used on roads and bridges located in winter climates because the agents lower the melting point of ice. But, after mixing with the melted ice, the chloride can find its way deep into cracks within the surface of a bridge, corroding metal reinforcement bars (rebars) and other supporting structures. Such corrosion can lead to degradation of the bridge structure, necessitating periodic inspections and core sampling, which are labor-intensive and destructive procedures. The Federal Highway Administration estimates that 39% of US bridges are structurally deficient or functionally obsolete from a variety of different causes.

To combat this problem, researchers at the University of Vermont (Burlington, VT) have developed a fiberoptic chloride sensor that can be embedded into the concrete fabric of a bridge to help monitor its structural integrity. Led by Peter Fuhr, associate professor in the university`s department of electrical and computer engineering, and Dryver Huston, associate professor in the mechanical engineering department, the group installed 36 such sensors in a steel truss bridge over the Winooski River in Waterbury, VT—a 70-year-old bridge that was renovated recently. These chloride-sensitive fiberoptic sensors join a host of other fiberoptic sensors now being used to monitor the strength and integrity of such structures (see Laser Focus World, Sept. 1997, p. 107).

A smoking gun

"Chloride is the smoking gun for corrosion," explains Fuhr, whose group determined that chloride-ion penetration in roadway surfaces is a precursor to corrosive conditions that ultimately cause a bridge to fail. The group then developed a fiberoptic sensor to detect the concentration of chloride ions in an aqueous solution. The sensor head consists of a porous throat on top of a case that contains a silver nitrate (AgNO₃) solution with dichlorofluoroscein, an anionic dye. Fiberoptic leads emerge from the bottom of the case.

In the presence of a chloride-containing aqueous solution, chloride migrates through the porous throat of the sensor into the AgNO₃ solution. The precipitate of the resulting reactions changes color from pink to white when the concentration of chloride has exceeded the known concentration of silver. This color change provides a threshold for detection of a preset level of chloride. Fiber readings are then converted to an electrical signal and transmitted to a remote computer by a phone line.

The sensors were designed to be robust enough to be installed in bridge decks—they are embedded in wet concrete and the optical fiber is laid along the metal rebar. The sensor`s "nemesis," says Fuhr, is the agitating vibrator used by construction crews after the concrete is poured, the purpose of which is to remove any air bubbles in the wet cement. "We go to great lengths to position fibers in such a way that they won`t get destroyed by this vibrator machine," he says. About 87% of installed sensors of all types survive and are able to provide data. The sensors are of modular design so that the detecting unit can be extracted from the bottom and a new unit, perhaps with a different chemical threshold, put back into place.

The Waterbury bridge has other fiberoptic sensors installed, including nine fiberoptic Bragg-grating strain sensors and two fiberoptic multimode vibration sensors. While the fibers were donated for this particular project, the group estimates that the cost of the materials inside the Waterbury bridge is about $50,000, with another $50,000 required for readout and transmission equipment. For a "reasonable amount" of sensors, said Fuhr, about 1.5% of the total cost of a construction site would need to go toward the embedded sensors. The total cost of renovating the Waterbury bridge was $2.4 million.

Fuhr said the project has generated a great deal of interest, especially from overseas. The group is now working on determining the longevity of sensors involved in such projects and improving remote interrogation of embedded fiber sensors.