Considered by experts and officials alike as an ideal material, fiber-reinforced plastics (FRP) may be a more frequent specification in bridges in the near future.
It is because of FRP’s light weight, durability and resistance to fatigue, stress and corrosion that it is receiving increased scrutiny from the marketplace.
In all of these physical characteristics, FRP materials are competitive with the cast-in-place concrete decks and supporting steel that have traditionally been used for bridge applications. The FRP material has received favorable reports for bridge installations from the American Association of State Highway & Transportation Officials, as well as other federal, state and local officials and civil engineers.
Yet, despite the widespread use of FRPs in aerospace, marine and sports applications for many years, and their accelerated use in automotive and trucking applications (to replace metals that were once considered the standard for durability, safety and performance), these quality materials have only been able to capture a single-digit share of the bridge market to date. (FRP bridge decks have been used in the U.S. since the mid-1990s.) This situation may change in the years immediately ahead, as federal and state highway officials become more familiar with the long-term economic benefits delivered by FRPs for bridge installations.
The rapid aging of the country’s 600,000 bridges (and other infrastructure)—because of battering by daily stress, weathering and rusting—has resulted in a sharp increase in the funds needed for repair and maintenance.
Given the high price tag for maintenance and repair, we are nearing the point where the taxpayers will be paying more for bridge maintenance than for new bridge construction. This, in turn, has caused many transportation officials to look for alternative materials that will better stand the tests of time and stress.
In the long term, FRP materials have great staying power and economical advantages. The problem faced by proponents of greater use of FRPs in bridges is their significantly higher initial price tag versus concrete and steel.
The hesitation to increase the use of FRPs in bridge and other infrastructure applications stems from several factors, according to John Busel, director of the Composites Growth Initiative of the American Composites Manufacturing Association. “Almost all bridge owners in the U.S. continue to make the specification decision based on the lowest initial cost,” he said. Further, he added, “there is a reluctance of this generation of engineers to break a long tradition of doing business as usual with the standard steel and concrete being specified.”
The performance superiority of FRPs for bridge applications is clear. For example, FRP bridge decks are lightweight and can be installed quickly. Service life also is two to three times longer than other materials.
Furthermore, FRP bridges offer environmental benefits—e.g., they don’t corrode and therefore do not release pollutants into the environment. In addition, they have a stud-free surface for improved safety and noise reduction and require little maintenance.
Finally, FRP products in bridge applications, to date, have experienced such low levels of strain, fatigue and creep that component stress has not been an issue, when the proper design, testing and fabrication have taken place using a manufacturing process such as pultrusion.
Pultrusion production
Among the processes used to fabricate FRP structural decks and components for bridges—pultrusion; vacuum assisted resin transfer molding (VARTM); and hand lay-up/contact molding—pultrusion is the most popular. “Pultrusion can produce quantities of FRP bridge components and members at a lower per-unit cost than the other FRP production methods,” said Glenn Barefoot, director of marketing for Strongwell’s Bristol Division.
Barefoot noted a range of benefits for bridge owners and contractors available from fabricating carbon/e-glass fiber and other pre-engineered beam supports, bridge decks and superstructures using pultrusion. These include reduced maintenance; no painting required; light weight and ease of handling. “Over the long haul, these time, labor and money savings really add up,” Barefoot said.
With the benefits of FRP for bridge applications in mind, Nancy Teufel, technical support manager at Axel Plastics Research Laboratories, Woodside, N.Y., suggested that by subjecting pultruded bridge products to tests such as the Short Beam Shear Test, which measures performance under stress, mechanical properties can be built into the components for peak performance.
“Manufacturers often utilize mold releases in order to improve the resin-rich surface of the pultruded components and to release the parts more easily and more rapidly off the process line. With these specially designed mold releases, Teufel added, “performance characteristics of the structural members used in the bridge construction are further enhanced to withstand stress and enhance endurance.”
In need of a cling release
Despite the superior performance characteristics and much greater longevity of FRP components, the key impediment to accelerated use of these products is the initial cost of the components.
FRP members—decks, truss, etc.—will last 100 years or more.
Despite this wide disparity, Dustin Troutman, director of marketing and product development for Creative Pultrusion Inc., Alun Bank, Pa., said “many engineers cling to the notion that after 30 years, many roads and bridges may be redesigned—e.g., a two-lane bridge may become a four-lane bridge—so why be interested in longer-lasting materials?”
John Busel of the American Composites Manufacturing Association said a potential shift toward FRP components may require more than a comparison of the performance characteristics. First, he said, “today’s specifying engineers may not be specifying FRP because they don’t know much about it. Therefore, we need a skill-set manual to serve the entire marketplace, including the specifying engineers, so they understand the product and the advantages of FRP.”
He noted, however, that “newly minted engineers coming out of school today will probably now have had an opportunity to explore FRP products in school which were not really studied 20-25 years ago.”
A niche market
The most promising opportunities for greater use of FRPs in bridges is in “niche bridges,” where the other alternatives could end up costing more because of the bridge location, type or special environment requirements for a specific area, such as a public park. Such applications would include historical or pedestrian bridges, walkways and suspension bridges, according to Barefoot.
“If, in addition to these special considerations, you consider the reduced weight of shipping and fast erection time, FRP becomes an attractive alternative to consider for building certain types of bridges as well as the reconstruction, repair and maintenance of some of the existing bridge inventory,” Barefoot stated.
To sum up, FRP is today a viable alternative to conventional bridge decks or structural members. Owners, engineers and federal and state officials are becoming more familiar with the FRP process and the important role that it can play in repair, maintenance and new construction in this market. “With such a small percentage of market share of the total bridge construction market today, FRP is poised for rapid growth,” said Dustin Troutman of Creative Pultrusion.
