When it comes to larger-scale structural components, however, fiber-reinforced plastics have had less of an impact. They have mostly been used in bridge decking, where corrosion resistance is critical and the lighter weight allows for a higher “live” load of vehicles. Only a handful of bridges have major support beams made from them.

One reason F.R.P. components haven’t caught on, experts say, is that engineers and contractors have little experience with the materials, and full standards guiding their use in highway construction have not been developed.

Engineers “have to deal with life-safety issues,” said John P. Busel, director of the composites growth initiative of the American Composites Manufacturers Association. “They have a desire to understand how materials fully develop and how they fully last before they specify them.”

The materials also do not always interact well with others. One problem with F.R.P. bridge decks, for example, is that the road surface  asphalt or concrete, applied as an overlay  can wear out quickly, said Lijuan Cheng, an assistant professor of engineering at the University of California, Davis.

But the main argument against using fiber-reinforced plastics has been economic.

“No. 1 is the upfront cost issue,” said Paul Ziehl, an associate professor of engineering at the University of South Carolina. “That’s a tough one to get around.”

Dr. Ziehl, who helped design and test F.R.P. beams used on a small bridge in Texas, said the problem was that no two projects were the same. “If you’re going to design things that really make sense from an optimized engineering standpoint, they are going to be one-of-a-kind items at first, until economies of scale kick in,” he said. The beams for the Texas bridge, for example, were custom designed and built using a labor-intensive method.

“The construction industry is very persnickety about cost,” Mr. Busel said. With F.R.P. decks, he added, “we’re more expensive, sometimes twice as expensive,” as conventional ones. What contractors need to understand, he said, is that there are transportation, labor and equipment savings from using lighter components, and potential maintenance savings, too.