Intelligent Machines

Bridge to Tomorrow

Materials

It doesn’t look much different from California’s other arroyo crossings, but peek under the deck of the Kings Stormwater Channel Bridge on Route 86 in Riverside County and you’ll find something surprising: plastic, carbon and glass where steel girders and reinforced concrete ought to be. The California Department of Transportation’s recent completion of the bridge could mark the beginning of a move away from traditional construction materials in favor of lightweight, rugged composites.

Though “polymer matrix composites” (fibers like carbon or glass encased in plastic) have found their way into a smattering of smaller structures in the past decade, the new bridge is the first to face the test of highway traffic and long-haul trucks. It boasts a fiberglass composite deck with a thin veneer of special concrete supported by carbon-fiber composite tubes just 355 millimeters in diameter. The materials are so lightweight two men can do what normally requires a crane. That translates into faster construction, which is critical when a busy bridge is out of commission, says the Federal Highway Administration’s Eric Munley, a composites expert. “If you can replace the thing in a week, you’re going to be very interested in composites,” Munley says.

In fact, the Kings Stormwater bridge went up in one-third the usual time-but at twice the cost, because composites are relatively pricey. Still, says California Department of Transportation chief deputy director Jim Roberts, the materials’ superior durability could make them worth the expense, not only for bridges but also for highway decking in areas where ice and salt readily do in steel and concrete. “All you have to do is get 20 to 30 years out of it, and you’re way ahead of the game economically,” Roberts says. His department is planning to begin another composite bridge in San Diego in mid-2002.

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Jim Cooper, the director of bridge technology for the Federal Highway Administration, is also optimistic: “It’s my belief that composites will form a major construction material in the future.” But until the cost of composites comes down, the gap between present and future may be a difficult one to bridge.

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