A bridge of polymer
Eric Kid, Bedford Reinforced Plastics, UK, looks at the use of fiberglass-reinforced polymer in bridge construction, reinforcement and retrofitting.
Traditional building materials have their place, but for harsh, corrosive environments, fiberglass reinforced polymer (FRP) can be a useful alternative material. FRP can be used for long-standing bridges of any size due to its strength and ability to withstand corrosion. These features – among others – have helped FRP become a preferred alternative to traditional bridge materials such as steel, aluminum, concrete and wood.
FRP can have the same strength of steel at a lighter weight, and won’t corrode or rot the way steel or wood might. Additionally, FRP doesn’t attract insects of conduct electricity, giving it structural applications.
The corrosion problem
According to the World Corrosion Organisation, the annual cost of corrosion worldwide is US$22.2tln – more than 3% of global GDP. In the USA alone, a study conducted by the National Association of Corrosion Engineers estimated that corrosion cost more than US$42bln in 2013. The issue widely impacts bridges
and other infrastructures across the world. This is especially true in areas with highly corrosive environments where water, moisture, salt spray, chemicals and heat are a constant.
Steel is subject to electrochemical corrosion – when the metal rusts due to constant exposure to water or moisture – and aluminum is vulnerable to galvanic corrosion when exposed to saltwater or salt spray, while in contact with another metal. Similarly, when exposed to moisture, wood can warp, rot, mould, and mildew. Therefore, when dealing with seaside or coastal locations, salt spray creates an especially corrosive environment that can cause wooden bridges to break down more quickly over time.
In contrast, FRP is unaffected by salt spray, moisture, or prolonged immersion in water, and can withstand a broad range of corrosive chemicals and environments. This ability to resist corrosion is a big factor in the long-term cost-effectiveness of FRP.
The perks of an FRP bridge
Beyond corrosion resistance, the biggest selling point is FRP’s light weight. It can weigh up to 70% less than steel and is just as strong – if not stronger, pound-for-pound. When an FRP bridge is fully assembled, the deadload weight is much lighter compared with a steel bridge, making the FRP bridge easier to transport on site. This is especially advantageous for projects in remote locations or areas with limited transportation options. For example, FRP was chosen to replace the Eagle Creek Trail hiking bridge in Oregon, USA – a frequently traveled wood and steel bridge that took on damage over the winter. Because of the remote location and critical habitat surrounding the area, the FRP bridge had to be airlifted into place by helicopter.
Steel bridges typically require welding or cutting torches and heavy machinery to erect and install, whereas FRP bridges can be fabricated onsite using basic carpentry tools.
The material is also low-maintenance. To offer up a comparison, steel occasionally needs to be repainted or recoated with zinc to prevent corrosion, and wood bridges need frequent power washing, sanding, and staining, and inspections for signs of rot. Maintenance checks and replacements are hardly necessary for FRP bridges.
Historically, wood, and steel have been the go-to materials for the construction of pedestrian bridges. However, due in part to its lightweight durability, FRP has become the new material of choice for several pedestrian bridge applications.
One such application is the FRP decking to retrofit existing bridge structures for pedestrian traffic. A common example of this is dilapidated railroad bridges that have long been closed for use by trains and can now only support pedestrian loading. An instance of this type of application occurred with the Two Carson’s crossing bridges on the eight-mile extension of the ghost town trail in Pennsylvania, USA.
Both bridges were originally a part of the Cambria and Indiana railroad, which was in operation from 1904–1994. The abandoned railroad corridor was donated to the Cambria County Conservation & Recreation Authority in 1995 and retrofitted for pedestrian use in 2017.
The material can also be used in the construction or reconstruction of pedestrian bridges found along hiking trails where moisture, insects, remote locations and a delicate ecosystem can pose a variety of challenges. Environmentally harmful coatings are not needed to protect FRP from corrosion, which makes a more eco-friendly choice for minimal impact on the surrounding environments.
In another example, the United States Forest Service in Telluride, Colorado, needed a bridge that would be cost-effective, durable and easy to transport three-quarters of a mile up a trail. Because of its weight, steel would have cost tens of thousands of dollars to transport by helicopter, so that option was quickly ruled out. However, lightweight FRP could be delivered to the site on carts and once in place would not corrode in the harsh outdoor conditions, potentially saving the forest service maintenance and replacement costs further down the road.
Living in the city
FRP can also be a budget-friendly solution for city pedestrian bridges and walkways, which was the case for the construction of the West Street Bridge in New York City.
Architects and municipal leaders wanted a long-lasting decking material that would last and be resistant to rot and corrosion. They also did not want a material that could potentially shrink or swell, causing nails and screws to pop up and pose a tripping hazard. FRP was ultimately chosen for the job for its ability to meet all of the project criteria. Plus, the deck board quickly snapped together, saving time and labour costs during the fabrication process.
Vehicular and railroad bridges
Starting in 1996, the West Virginia Department of Transportation’s Division of Highways began a program to use FRP materials for vehicular bridge construction and rehabilitation. This work included the use of deck systems in lieu of conventional concrete or steel decks, rebar in place of steel, wraps to repair deteriorated concrete, and strengthening timber bridge components. Of the 25 FRP implementations on bridges, 92% were completed between 1996 and 2004, providing more than a decade of in-service data.
The first application explored for this type of bridge was the use of FRP deck systems in place of concrete or steel, which had been used in the Goat Farm Bridge in Jackson County, West Virginia, USA. FRP bridge decks consist of factory-manufactured panels that are assembled in field. After that, wearing surfaces composed of either polymer concrete or asphalt are installed over the FRP decking to prevent damage, provide UV protection and enhance traction for vehicles.
Following this was the use of FRP rebar as an alternative to steel to combat corrosion issues. When conventional steel rebar corrodes, it expands and fractures the concrete around the rebar, impacting the integrity of the bridge. FRP rebar will not corrode or disrupt the surrounding concrete, providing a strong advantage in areas where deicing salt is used extensively.
FRP wraps were also tested as a means to repair deteriorated concrete. They bond to the concrete surface to compensate for strength lost due to deterioration, corrosion, or fire damage. It was found that the use of the wraps enabled the rehabilitation of the existing concrete, resulting in a more economical repair project than substructure replacement, which generally involves replacing the entire bridge.
Lastly, FRP was used to strengthen timber bridge components commonly found in older vehicular and railroad bridges. The benefit in this instance is that the embedded FRP provides additional strength without compromising the natural appearance of the original wooden bridge. For example, the Barrackville covered bridge in Marion County, West Virginia, was closed to vehicular traffic in 1985, but has since been preserved as a pedestrian crossing. The timber bridge was rehabilitated using embedded FRP rods to reinforce the existing wood truss while maintaining the original iconic charm. The south branch valley railroad also used FRP wraps to repair over 50 timber piles and several bridge stringers on 100-year-old railroad bridges.
From pedestrian bridges to vehicular bridges, FRP has become a proven alternative to traditional steel, aluminum, concrete and wood materials. Its light weight, corrosion resistance, ease of transportation and assembly, and low maintenance qualities all make it a solid, long-term, cost-effective solution for both retrofitting and new-build bridge applications.