Building a durable bridge
Fiber-reinforced polymer composites continue to gain industry acceptance
As the Federal Highway Administration (FHWA) and state highway agencies focus on using emerging high-performance structural materials and innovative designs to build more durable and reliable bridges, one of the technologies showing great success is fiber-reinforced polymer (FRP) composites. Typically made of such fibers as glass, aramid and carbon in a polyester or vinyl ester resin matrix, FRP composites are highly corrosion- and fatigue-resistant. Across the country, composites are demonstrating their potential for use in new bridge construction, as well as the repair and retrofit of the nation’s aging structures.
Over the past six years, FHWA’s Innovative Bridge Research and Construction (IBRC) Program has provided $108 million to advance the use of high-performance materials in bridge applications. Of the 246 projects funded, 127 involved the use of FRP composite materials. The projects covered a range of applications, including the construction of new FRP bridge deck systems and concrete decks with reinforcing elements and the strengthening and repair of existing structures.
FRP composite deck systems for new bridges have the advantages of being lightweight, resistant to chemicals and corrosion and high strength. They also offer easy construction and handling. The deck systems, which are pre-engineered and prefabricated off site, can be rapidly deployed and installed at a jobsite, reducing the congestion and inconvenience caused by work zones and improving safety. In New York state, a conventional concrete deck on a 60-year-old, load-restricted steel truss bridge was replaced with an FRP composite deck system in less than a month. At a cost of $876,000, the rehabilitation amounted to only one-third the cost of completely replacing the bridge. More importantly, the rehabilitated bridge with the lightweight FRP composite deck was able to carry the legal truck load, due to the reduction in dead-weight replacement.
Some challenges remain in using FRP composites for deck replacement. The design of an FRP deck system requires finite-element analysis, for example. Depending on how a deck panel is fabricated, consistency and quality may vary. And because parts of the deck panel can be inaccessible for inspection, nondestructive testing or evaluation devices should be incorporated into the panels to monitor short- and long-term performance and facilitate maintenance inspections. FHWA is working with the American Association of State Highway & Transportation Officials to develop a guide specification for testing and acceptance of FRP deck systems. The specification will include testing standards and protocols for establishing strength and stiffness requirements.
Another leading application of FRP composites is the use of surface-mounted composites for bridge strengthening and repair. Generally, a structural member repaired using FRP composites will prove to be stronger than it was in its original undamaged condition. These repairs are made using FRP laminates, rods and wet lay-up fabrics, which are fiber fabrics saturated with resin and cured in place. Such a repair program requires some preparation to ensure that the substrate is sound and of good structural integrity. For example, deteriorated concrete or delaminations must be removed and spalled surfaces must be built up to provide a level and flat surface for bonding the FRP laminates or fabric sheets.
An additional use of FRP composites is for seismic column retrofit. Since the Loma Prieta earthquake in Oakland in 1989, the California Department of Transportation (Caltrans) has retrofitted thousands of concrete pier columns using FRP composite materials. Retrofitting with FRP composite materials has undergone extensive testing and development in California and has been accepted by Caltrans as an established method for column strengthening.
The use of FRP composites is already changing the way bridges are designed, built and maintained. And with recent concerns about a shortage of steel supplies, FRP composite rod manufacturers are stepping up their production in anticipation of increasing demand. As applications of FRP composites continue to grow, and design and construction specifications for the technology are developed and adopted, highway agencies will be able to build more durable bridges and extend the service lives of existing structures at a fraction of what it would cost to replace them.