One of the nation’s greatest challenges is how to effectively address the problems resulting from our aging transportation infrastructure. Last year, the American Society of Civil Engineers (ASCE), Reston, Va., updated its 2005 “Report Card for America’s Infrastructure” presenting the country’s bridges with an overall grade of C. The report stated that more than 26% of the nation’s 600,000 bridges are either structurally deficient, meaning they could be closed or restrict traffic due to limited structural capacity, or functionally obsolete, meaning they cannot accommodate current traffic volumes, vehicle sizes, and weights.
According to the U.S. Department of Transportation’s 2006 Conditions and Performance Report, $8.7 billion in annual capital investment is needed to maintain bridge conditions at current levels, while another $12.4 billion would be needed to actually improve conditions to a level that would help relieve congestion and reduce accidents.
Individuals from the University of Maine, Orono, Maine; the Maine Department of Transportation (MaineDOT); and Advanced Infrastructure Technologies, Orono, Maine, believe they have a technology that could play a role in alleviating some of the country’s transportation infrastructure needs. The technology is known as a bridge-in-a-backpack and was used for the first time in the country to construct the 34-foot-long Neal Bridge in Pittsfield, Maine, and the 28-foot-long McGee Bridge in North Anson, Maine. MaineDOT also is currently building five new bridges using the technology, with the first being the 38-foot-long Royal River Bridge in Auburn, Maine.
The technology received its name from the portability of its materials. According to creator Habib Dagher, director of the University of Maine-based Advanced Engineered Wood Composites Center, the fiber-reinforced polymer (composite) tubes that form the structural spine of the bridge could fit inside a backpack. When compared with traditional concrete and steel projects, the bridge-in-a-backpack technology offers comparable construction costs, reduced maintenance, shorter build schedules, and less harmful impacts to the environment.
How it works
The composite tubes, typically 12 inches in diameter, are inflated and formed into arches. Using a vacuum pump, the tubes are treated with an epoxy resin, causing them to stiffen into shape. Then the tubes are installed spaced apart—in the case of the Royal River Bridge, 13 were needed—and filled with concrete. Covered with a composite deck form topped with concrete and compacted soil, the tubes can support a standard gravel-and-asphalt roadway.
Concrete became a choice material in filling the tubes because of its relatively low cost; major support beams made from a solid composite member would be too expensive. With the bridge-in-a backpack design, a small volume of fiber-reinforced material encases the concrete, adding significant strength. It also acts as a protective shell for the concrete, increasing durability, and keeping out chemicals and moisture.
One of the benefits of the product is that arch elements are highly resistant to corrosive elements such as chloride. Prolonged exposure to chloride is a major deterioration factor for concrete structures, including bridges. Chloride initiates the corrosion of embedded reinforcement, producing signs of deterioration on the concrete surface, such as rusting, cracking, and spalling. Once these signs appear, it may be too late to prevent further deterioration through repair. Corroded reinforcement also significantly reduces the load-carrying capacity of concrete bridges.
The chloride comes from either marine exposure or the use of deicing salts for snow and ice removal. The technology is designed to create longer-lasting bridges, particularly in a state like Maine, where concrete often falls victim to the aforementioned deicing chemicals used in winter.
Another benefit is that large trucks are not needed to haul the heavy beams typically found in traditional concrete-and-steel bridges. Before they are filled with concrete, the composite tubes are light enough to be installed quickly, making large cranes or other heavy equipment unnecessary. In fact, the only heavy vehicles required are concrete trucks for filling the inflated tubes and installing the concrete deck topping. These factors make the technology friendlier to its natural setting.
Just as crucial is how the technology can speed up construction times. The McGee Bridge took just two weeks to complete. Also the life expectancy of your typical bridge built in this manner is estimated to be 75 to 100 years, comparable to or even greater than most concrete, wood, and steel bridges.
Paving the way to the future
The technology could redefine the current thinking of how bridges are designed. Currently, steel, concrete, or possibly timber is thought of in regard to bridge design—materials that have been available for many years. Today, a new material, fiber-reinforced polymer, is rapidly becoming one of the most popular structural applications. The material is frequently used in airplanes, watercraft, sports equipment, and for many other purposes.
With the cost and availability of steel becoming a significant issue for many local governments, as well as the negative impact bridge projects can have on the environment, selective use of this technology could lift the strain of our nation’s already stressed transportation systems.
Pamela Hetherly, PE, is a technical advisor to Kleinfelder/S E A Consultants in the areas of bridge and structural design.
Lisa Dickson, PG, heads up Kleinfelder/S E A Consultants’ Augusta, Maine, office.