Wireless Sensors Will Monitor Bridge Decks

Caltrans plans long-term evaluation of the 
Smart Pebble technology in selected bridge decks.

by David Watters

Many of 600,000 bridges in the United States are under attack by chloride, usually in the form of seawater spray and deicing salts. Chloride corrodes the reinforcing steel rebar in concrete bridge decks, requiring regular maintenance to ensure the health and safety of bridges and their travelers. Chloride’s diffusion rate is slow but relentless, and the resulting corrosion can expand the rebar’s volume and cause the surrounding concrete to crack. Unless preventive maintenance is performed regularly, bridge life spans can be significantly shortened.

Knowing where and when to perform such maintenance has traditionally been a costly and time-consuming process. Current methods for analyzing the levels of chloride in bridge decks involve extracting concrete core samples and analyzing them in a laboratory setting, which could take days, if not weeks. Many tests are required, since it typically takes several years for critical chloride concentration levels to be reached. During bridge inspection, lanes must be closed, and the bridge is usually sampled only at a few points along the deck. The costs add up. How could such tests be performed often, inexpensively, and with minimal effort?

In collaboration with the California Department of Transportation, researchers at SRI International, an independent nonprofit R&D institute in Menlo Park, California, have one potential solution. They are developing Smart Pebbles — long-life wireless sensors that continually monitor the health of the bridge from deep inside the bridge deck.

In less than a few seconds, the sensors check chloride ingress and can relay the information instantly and wirelessly to those who need it, providing an early-warning system for assessing damage before safety issues arise. The devices can help extend a structure’s life by prioritizing the need for preventive maintenance.

The sensors are powered remotely, so no lifetime-limiting batteries are required. The sensors are inserted in the bridge deck either during initial construction, during refurbishment, or in existing structures through a back-filled drilled core hole.

Researchers call the sensor devices pebbles since they’re roughly the size and weight of a typical piece of rock aggregate used in such structures. They’re smart in that they each contain a chloride sensor and radio frequency identification, or RFID, chip that can be queried remotely to identify it and indicate chloride concentration levels in that part of the bridge.

Corrosion of the reinforcing steel in highway infrastructure is one of several problems that require consideration when constructing structures with 75 years or greater design lives, according to Rob Reis, a Caltrans corrosion specialist collaborating with SRI on the Smart Pebbles project.

“Other problems, such as reactive aggregates, freeze-thaw damage, and sulfate attack on concrete, may also exist that can result in degradation of concrete, higher maintenance costs, and possible premature failure of highway structures,” Reis says.

Reis notes that existing methods for the health monitoring of reinforced concrete bridges include time-consuming activities such as chaining and coring the decks. “This work requires lane closures, results in delays for the traveling public, and involves elevated safety risks for maintenance crews.”

Reis says that the ability to non-destructively measure and monitor in-place chemical changes that occur within concrete over time can result in positive benefits that include increased safety, reduced delays to the traveling public, and substantial improvements to maintenance programming. “In other words, we could have better, safer, more cost-effective smart roads for the traveling public.”

The technology

The Smart Pebble reader can be either handheld or vehicle-mounted. Imagine a truck or van has the reader device mounted on its underside. While driving over the bridge, the reader picks up information from the Smart Pebbles embedded in the bridge deck and sends them to a collection point. Using a global positioning system, the van could automatically update the health status of the bridge in the bridge’s database.

For on-the-spot visual inspections, the reader indicates under-threshold conditions with a green light, and over-threshold chloride-concentration conditions with a red light.

The best sensor for the job appears to be an electrochemical one that generates an electric potential dependent on the concentration of chloride ions in the bridge deck. The circuit uses the Microchip Technology Inc.’s MCRF202 chip, which not only indicates when a threshold level has been exceeded by inverting the ID-code bit stream; it is also able to power the external electronics. The ID-code bit stream is converted by the reader to a unique ID code.

Smart Pebbles are in the early development stages. Laboratory tests have so far been very positive. Caltrans is formulating plans for a long-term evaluation of Smart Pebble prototypes in both the lab and in selected bridge decks.

Future work will include adding temperature compensation circuitry and integrating all the components into a rugged 1-inch-diameter package. SRI’s goal is to demonstrate the effectiveness of Smart Pebbles in real bridge decks. SRI is seeking an industrial partner for further development or technology licensing to mass-produce the Smart Pebbles devices. For more information, contact Dr. David Watters at 650-859-3753 or david.watters@sri .com.

Reprinted from Better Roads Magazine
February 2003

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