High-tech materials will make bridges safer and more durable
Christopher T. Freeburn
America’s bridges are in distress — suffering from the effects of aging and decades of neglect. Traffic on many is heavier than they were designed to carry. The Federal Highway Administration estimates that 42 percent of America’s bridges either are structurally deficient or functionally obsolete, some dangerously so. The cost of repairing them could reach $50 billion by 2010.
To counter this burgeoning problem, engineers are experimenting with new materials that promise lower maintenance, longer life and greater strength. For sheer toughness and durability, bridges fashioned from glass, carbon and plastics appear to outperform those made from steel and concrete. These “polymer composites” could be used to supplement or replace the traditional steel and concrete used in conventional bridges.
“One particular group of polymer composites called laminates is like cloth and can be glued or adhered to concrete structures, steel structures and masonry structures to reinforce them so that they will last longer and be stronger,” says John Scalzi, director of the National Science Foundation’s Large Structural and Building Systems Program.
Some of the composite products may be ready for application within five years, but Scalzi estimates that it may take 15 years before a wholly composite bridge is built. In civil construction, the problem is liability. “Who’s going to take the risk?” asks Scalzi. “We need what we call standards of acceptance — codes, for example. People are reluctant to use a product unless we have all this information with which codes and specific design criteria can be developed.”
Noting that Europe’s infrastructure “has been deteriorating faster than ours,” Scalzi says that Europeans and the Japanese long have been developing new materials. Canada has made a $20 million commitment to a consortium to develop products that could be sold overseas.
In fact, the United States lags behind other countries in the field. The National Science Foundation is supporting more than 40 research groups exploring the possibilities of polymer composites, according to Scalzi. “The aerospace and chemical industries that develop the fibers for the glass, carbon and aramids … are pushing hard to put these things into production, but we still need more information and research.”
Hota V.S. GangaRao, a professor of civil engineering and director of the Constructed Facilities Center at West Virginia University, is preparing to construct two bridges using glass-fiber composite materials. “The first bridge will be a concrete bridge with steel stringers in a traditional manner,” he says. “The difference will be in the concrete deck that will have glass-fiber reinforcing bars in lieu of steel reinforcing bars. The second bridge, with an all-composite deck system, will be a total replacement of the concrete deck with a composite deck.” They hope to build the latter bridge by the end of the year.
According to GangaRao, the glass-fiber composite decks will have several advantages: “They are approximately five times lighter than steel and about two to three times stronger,” he says, noting that the glass fiber is responsible for their great strength. “And if I used carbon fiber, the strength of the materials would be five or six times that of steel.”
The choice of materials is based upon their application. It may appear to be a good idea to reinforce a steel beam by gluing a carbon rather than a glass plate to it, because the former offers higher stiffness. But this may not be appropriate for certain applications: Gluing carbon and steel together can create a battery effect. “Suppose I am going to use this in some kind of railroad application,” says GangaRao. “It might cause a short-circuit.”
Another limitation of these composites: higher cost. Glass-fiber reinforcing bars are about 10 times more expensive than steel. But GangaRao believes that these composites have a performance rating 10 times better than steel. “Therefore, we are hoping we will come out even in terms of the cost-per-unit performance,” he says.
At California State University at Long Beach, a research group led by Joseph Plecnik, a professor of civil engineering, has developed cables made from composites of glass, kevlar (the trademark name of a synthetic fiber made by Du Pont) and graphite for a bridge that may be installed next year. “These are the cables that go from the main horizontal cable vertically down to support the deck,” explains Plecnik. “Not the main cables, but the so-called secondary or hanger cables, for which the capacities are smaller and they would appear to be feasible application.”
The cables cost far more than conventional steel cables. “Economically speaking, it takes a long time to justify the use of this type of material vs. conventional steel rods,” notes Plecnik.
At Catholic University in Washington, Lawrence Bank, a professor of civil engineering, is investigating the use of fiber-reinforced plastic-composite gratings and grid materials for reinforcing concrete. “We intend to build a project demonstrating this technology on a full scale in a real roadway situation,” says Bank. His team has designed a bridge deck 80 feet long and two lanes wide — a “pseudo bridge.” “We are excavating a hole in a roadway on the level of the road and building a laboratory underneath the superstructure over which this composite deck will be placed,” he says. “We will be able to observe, monitor and check the system under very controlled conditions over the next five to 10 years.”
Polymer composite materials already have been put into small-scale commercial use. E.T. Techtonics, based in Philadelphia, specializes in designing and developing short-span composite bridges and other structures, several of which are in state and national parks. “We developed a series of modular bridges for the Philadelphia Zoo, which could be configured in 10-, 20-, 30- and 40-foot lengths that would span over their moated areas to get forklifts and other heavy equipment in,” says G. Eric Johansen, president of the company. “We have also done equestrian bridges and light-vehicle-type bridges that can take vehicles up to about 20,000 pounds.”
Like others in the field, Johansen sees tremendous potential for polymer composite materials and views the current situation as an early stage in their development. “If you look at where we are in the composites field at this point, I think it is very analogous to the evolution of metals,” he says. “I sort of equate it to the cast-iron era — as we work with fiberglass we are on our way toward carbon, much as the cast-iron era moved to steel.”
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