Monitor Warns of Bridge Corrosion

The new device cuts maintenance costs and traveler delays,
while offering digital transmission of corrosion data.

by Robert Ross and Marc Goldstein

A Virginia company has developed the Embedded Corrosion Instrument — an electronic monitor that provides early warning of conditions that damage steel reinforcement and lead to cracking, spalling, and delamination of concrete bridge decks and support structures.

The ECI continuously measures five critical factors in rebar decay, and transmits these to bridge owners by cellular modem. A non-destructive evaluation device, it gathers and delivers all data without requiring inspectors to cut samples from the bridge, interrupt traffic, or even visit the site.

In a major advance, the ECI integrates processing electronics with its sensors, enabling it to use digital, not analog, communications. This eliminates data corruption by electro-magnetic interference from power lines, cellular telephones, and other sources. It also makes it possible to connect multiple ECI monitors to a single data logger, saving potentially tens of thousands of dollars in support electronics per bridge.

Bob Meline of the California Department of Transportation applauds this approach. “Measuring the chemical changes of a bridge deck over time provides improved maintenance programming,” he says. “It also aids in further development of improved materials and procedures for future construction.”

Device benefits

Virginia Technologies of Charlottesville developed the ECI with University of Virginia scientists, under a grant funded by the Virginia Transportation Research Council and the Federal Highway Administration. The company built its first prototypes in early 2002. It installed them in the new Pleasant Valley interchange bridge in Lynchburg, Virginia last March.

Last June, the California Department of Transportation placed its first order for the ECI. A number of other transportation agencies are also reviewing the technology.

The ECI helps address a multi-billion-dollar problem. The Road Information Program estimates that 14% of U.S. bridges are now structurally deficient, showing

significant deterioration to their decks and other major components. A further 1% become deficient each year, according to Steven B. Chase and Glenn Washer of the FHWA.

The annual direct cost of corrosion in highway bridges is roughly $8.3 billion, including maintenance, repair, replacement, and cost of capital, according to estimates reported by CC Technologies and NACE International in a study funded by the FHWA. Indirect costs, including traffic delays and lost productivity, may run 10 times that number, according to the study.

Meline says that, “Corrosion of bridge deck reinforcement is a major concern in many parts of California. Repairing structures is expensive in terms of direct costs and costs associated with lane closures, such as traveler delays and safety issues.”

Past tragedies have demonstrated the dangers of steel corrosion in concrete bridges. In June 1983, a 100-foot section dropped out of the Mianus River Bridge in Greenwich, Connecticut, killing three motorists and critically injuring three others. The steel pins that joined sections of the bridge had decayed.

In May 2000, in Concord, North Carolina, more than 100 people were injured when steel strands corroded in a pre-stressed concrete pedestrian bridge, and the structure collapsed onto the highway below.

The problem

When a bridge is first built, concrete protects its steel reinforcement by providing both a physical shield and an alkaline environment. This causes a passive film of iron oxide to form over rebar, preventing further corrosion.

Over time, however, chlorides from deicing salts and sea water permeate the concrete. These depassivate the steel. They react with and penetrate the iron-oxide film. They set up corrosive electric circuits within the bridge, in which chloride concentrations serve as anodes, chloride-free areas as cathodes, steel bars as conductors, and concrete as the electrolyte.

Carbon dioxide also poses a threat. It reduces concrete’s alkalinity, weakening the protection for embedded rebar.

Once corrosion begins, it’s self-sustaining. As steel rusts away, its corrosion products occupy three to six times the volume of the original rebar. This stresses the concrete, resulting in cracks, delaminations, and spall. These, in turn, provide new means for water and chlorides to reach the steel, which then corrodes even faster.

None of this is visible until late in the process, however, when cracks form. This is unfortunate, as visual inspection has long been the mainstay of bridge maintenance.

Supplementing the human eye, some maintenance engineers have cut samples from bridge decks for analysis. But this is destructive, expensive, and disruptive to traffic.

Others have installed probes in the concrete. But, these have generally measured only one or two factors in the corrosive environment. This has limited their usefulness and reliability.

Traditionally, probes have also faced challenges in delivering data. Many require inspectors to walk the bridge, plugging a reader into each probe by hand. Others transmit their readings to a data logger, but use old-fashioned analog signals, which are vulnerable to electro-magnetic interference. This has largely precluded probe transmissions greater than 30 feet. Worse, it’s obliged bridge owners to install a multi-thousand-dollar data logger to receive the readings from each probe.

One solution

Virginia Technologies’ ECI raises the state of the art to a new level, as it measures five corrosion factors — open-circuit potential, linear polarization resistance, resistivity, chlorides, and temperature. This reveals correlations among the causes and signs of corrosion, yielding a fuller, more certain picture of the threat.

The ECI also integrates processing electronics with its sensors and electrodes. It transmits all readings digitally, making it possible to connect numerous ECI units to a single data logger, hundreds of feet away. This can save tens of thousands of dollars. It also helps protect the data logger, which can now be housed in a more sheltered location.

From the data logger, corrosion readings can be downloaded to a computer or transmitted to an office by a wireless transceiver and cellular modem. Already digital, ECI data can readily feed into a bridge management system, preventing safety crises and optimizing the use of maintenance funds, as mandated by the Intermodal Surface Transportation Efficiency Act.

Each ECI is housed in an epoxy-potted, water-tight plastic case.

The system is powered by a gel-cell battery, which is charged by a small solar panel. This eliminates any need to connect to the regional power grid.

Reprinted from Better Roads Magazine
August 2003

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