Wireless sensor systems for detecting corrosion in aging bridges

Steel Girder/Concrete Slab Bridge

Repair Methods

Steel Girder / Concrete Slab Bridge Repair Methods

America's roads, highways and freeways invariably require bridges to cross over canyons, rivers and other uneven terrain, and as strong as the bridges may seem to be at the time of their construction, the best of engineers have not yet been able to build corrosion-proof bridges. There are numerous studies that have been conducted as to how to best repair the damage to concrete slab / steel girder bridges, and their results, taken in their entirety, provide solid background for further research into these issues.

When is it time to apply repair technologies to an aging bridge? An example is provided by University of Illinois at Urbana-Champaign engineers. Bernhard, et al., propose the use of wireless technologies (a wireless embedded sensor system) to detect corrosion in concrete girders. The sensing mechanisms that are embedded into the girders are "active acoustic transducers" - and through the use of antenna sticking out of the girder, information about what is going on inside the girder can be transmitted to engineers on a constant basis.

There are currently over 600,000 highway bridges in the United States; and of those, "many" are "severely deteriorated" and in desperate need of major infrastructure repair, according to an article in the Journal of Structural Engineering (Enright, et al., 1998). Enright explains "Experience has demonstrated that highway bridges are vulnerable to damage from environmental attack," including freeze-thaw, salt corrosion, and "alkali-silica reaction." The Enright article provides research into "time-variant reliability methods" regarding "bridge life-cycle cost prediction."

Bridges are naturally expected to - and designed to - function safely over "long periods of time," Enright argues. And during those years of service the concrete bridges are fully expected to stay sturdy notwithstanding "aggressive" and "changing" environments. The bridge that was analyzed in this research is located near Pueblo, Colorado; it was a reinforced concrete T-beam (built in 1962; bridge L-18-BG). This bridge is made of three 9.1-meter (or 30 ft.) "Simply supported spans."

Each of the three spans has five girders "equally spaced" at 8.5 feet apart. The bridge provides two lanes of traffic heading north; the most intense random moment (pulse) for the bridge is when two heavily loaded trucks drive over it side-by-side. Once "corrosion initiation time" has begun, the reinforcing cross-sectional area begins to decrease; the rate of decrease depends upon the number of reinforcement bars that are indeed corroding. Also, the writer explains, after thorough examination, failure can happen when "the limit state of bending failure by yielding of steel of any one (or more) of the girders is reached." The need for competent repairs at this time is obvious and crucial.

Researchers M. Tavakkolizadeh and H. Saadatmanesh, in the Journal of Structural Engineering, suggest that conventional applications used in strengthening "substandard bridges" are not only "labor intensive," but they cost more and they take more time to apply to the deteriorating bridge. The National Bridge Inventory (NBI) showed that in 2003 there were 81,000 "functionally obsolete bridges" in the U.S. Moreover, of those 81,000 bridges more than 43% are constructed of steel. The main problems associated with steel bridges, the authors write, is that they corrode, are not maintained properly, and they suffer "fatigue" over the years.

What to do with these failing and faulty bridges? It is recommended that steel-concrete composite bridges be beefed up with concrete composite girders, and by introducing fiber-reinforced plastics (FRP) (made of glass, carbon, and Kevlar) "...placed in a resin matrix," the article continues. Using FRP materials means that when FRP is applied to a deteriorating girder, the laminates weigh "less than one fifth of the steel and are corrosion resistant."

The authors explain that traditional rehabilitation of bridges uses five techniques. One, simple strengthening of members; two, placing addition member (girder) in the bridge; three, "developing composite action"; four, "producing continuity at the support" structures; and five, "post-tensioning." The downside? "They do not eliminate the possibility of reoccurrence."

Tavakkolizadeh offers the example of welded steel cover plates used to beef up existing structures. The main problem with this kind of solution is that the welded plates are subject to the same fatigue that the original girder suffered. There may be "galvanic corrosion between the plate and existing member and attachment materials" (Tavakkolizadeh, et al., 2003).

The authors tested three large-scale girders that had been strengthened with "pultruded carbon fiber sheets." And then three identical girders were strengthened with "one, three, and five layers" of CFRP sheets. There was delamination in evidence so viscous epoxy was used to bond the laminate to the steel surface. Those girders were put through a series of tests at varying load levels. The conclusions they reached showed that when steel-concrete girders were retrofitted with epoxy-bonded CFRP laminates the ability of a concrete-steel composite girder to carry a heavy load is increased by 44% (one layer); 51% (three layers of epoxy-bonded CFRP applied); and with five layers the ability of the girder to handle a heavy load is raised by 76%.

The Journal of Structural Engineering author C.Q. Li sets out to develop new models of structural resistance deterioration used in "whole life performance assessment of corrosion-affected concrete structures." He mentions the lack of satisfaction regarding the results of 30 years of research. He points to "Concrete in the Ocean" in the UK; BRITE in Europe, and in North America, SHRP studies. These studies, Li claims, did not go deeply enough into the effect that corrosion has on structural deterioration.

Li's model goes beyond the existing Tuutti model ("the well-known" model that assesses and predicts the service life of corrosion-affected concrete structures). In his tests, Li used a total of 30 specimens that consisted of a variety of different concrete compositions (different water cement rations and cement types). He put the tested samples under "simultaneous loading and salt spray" conditions, which were simulated in a big "corrosive environmental chamber" that was constructed exclusively for Li's research purposes.

It was determined that the corrosion growth was directly related to "crack distribution" as well as the pattern within the test sample itself. What did this prove? Corrosion is "essentially a local activity at the cracked sections of RC members," Li indicates, which is "very important to structural engineers" concerned with the "cross-sectional capacity of structural members."

CONCLUSION

There are no simple one-size-fits-all solutions to repairing bridges once they have become corroded. Part of the answer lies in preventing corrosion in the first place, which this paper discusses. Another part of the repair dynamic is knowing when the corrosion will likely occur, which has also been reviewed in this paper. Meantime, Xion Corporation offers a coating for repair of bridge corrosion that is 85% zinc and 15% aluminum, and provides "good corrosion protection, and a hard polymer top coat" to resist weather attacks and wear and tear. This coating is used on concrete and on the rebar embedded in concrete and contains no volatile organic chemicals, the manufacturer insists. And the coatings cure "instantly." Now as far as future research, it would be appropriate for a research article to review Xion's product, and all other new devices, technologies, and products that can make America's bridges safer.

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