Workers cover hardened, concrete-filled tubes with a composite deck form.

Some are labeling it the “corrosion of America.” Congestion on the nation’s roads is increasing and the cost to improve them is rising. Aging drinking water facilities are nearing the end of their useful life; leaking sewage systems are creating health hazards as they contaminate rivers and streams. In its 2009 report card on the state of America’s infrastructure, the American Society of Civil Engineers (ASCE) gave the country’s overall infrastructure system a D.

Bridges in particular are in need of attention. According to the ASCE report card, more than 26 percent of the nation’s 600,000 bridges are either structurally deficient (they could be closed or restrict traffic due to limited structural capacity) or functionally obsolete (they cannot accommodate current traffic volumes, vehicle sizes, and weights).

It is imperative that engineers find cost-friendly solutions to address such issues. A technology colloquially known as a Bridge-in-a-Backpack is one such solution. Developed by Habib Dagher, director of the University of Maine-based Advanced Engineered Wood Composites (AEWC) Center in Orono, Maine, and offered by Advanced Infrastructure Technologies (AIT), the Bridge-in-a-Backpack offers many economic and ecological benefits.

When compared with traditional concrete and steel projects, the Bridge-in-a-Backpack technology can offer comparable construction costs, reduced maintenance, shorter build schedules, and less harmful impacts to the environment. Simply put, the technology could redefine the current thinking of how bridges are designed and constructed.

How the technology got its name
The Bridge-in-a-Backpack earned its name from the materials that make up the structural spine of the bridge. The fiber-reinforced polymer (composite) tubes are relatively light and portable, and could be transported to the construction site in large bags.

During the construction phase, these composite tubes, which typically measure 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; installed several feet apart; 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.

The first-ever structure completed using this technology was the Neal Bridge in Pittsfield, Maine. Completed in the fall of 2008 and replacing a 70-year-old structure, the 34-foot-long, two-lane bridge is comprised of 23 of these arches. A corrugated, fiber-reinforced plastic (FRP) composite decking was installed on top; the head walls were constructed with a FRP sheet pile system. Total cost of the bridge was roughly $600,000 — comparable to the cost of a precast bridge.

The project was a success, spurring the Maine Department of Transportation (MaineDOT) to utilize the technology in the 2009 construction of the 28-foot-long McGee Bridge in North Anson.

Based on these initial successes, Gov. John Baldacci established an initiative that set aside funds to bring this technology to a commercially viable level. Through this initiative, MaineDOT received funding to construct another five bridge projects across Maine and retained Kleinfelder/S E A to serve as the engineer of record. While the superstructure elements had received significant analysis, additional research and “real world” testing had to be done with respect to the substructure elements and headwall design. The last year of collaboration among the private and public sectors has resulted in a product that now meets those criteria.

The benefits
One of the potential benefits of the Bridge-in-a-Backpack technology is cost. It’s expected that the costs will be reduced further as more of these structures are built.

Also, the composite tubes are relatively portable, meaning there is less need to hire large trucks to deliver building materials or large cranes to put large sections of the bridge into place. In fact, the only heavy vehicles needed to construct the superstructure are concrete trucks, used to fill the inflated tubes and install the concrete deck topping. This portability also means the bridge project could have less impact upon natural areas and wildlife.

Quicker construction times are another cost saver. The superstructure can be built in weeks instead of months. The McGee Bridge, for example, took just two weeks to complete.

Greater life expectancy
Prolonged exposure to chloride is a major deterioration factor for concrete structures, including bridges. Chloride initiates the corrosion of embedded reinforcement. This can produce 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 work. Corroded reinforcement also significantly reduces the load-carrying capacity of concrete bridges.

The chloride typically comes from either the use of de-icing salts for snow and ice removal or through marine exposure. De-icing chemicals are prevalent in a state like Maine, where they are used during the winter months.

Structures built using the Bridge-in-a-Backpack technology are highly resistant to corrosive elements such as chloride, resulting in a longer-lasting bridge. In fact, the life expectancy of a typical Bridge-in-a-Backpack is estimated to be 75 to 100 years, comparable with or even greater than most concrete, wood, and steel bridges.

A vital component
Across the country, thousands of bridges are structurally deficient or functionally obsolete. One of the nation’s greatest challenges is how to address these problems effectively. Widespread use of this Bridge-in-a-Backpack technology could help reverse the crumbling being witnessed today and ensure bridges remain a vital component of our country’s infrastructure system.

View from beneath a completed Bridge-in-a-Backpack shows the epoxy resin-treated, concrete-filled, fiber-reinforced polymer tubes spaced several feet apart.

Pamela Hetherly, P.E., currently serves as a technical adviser to Kleinfelder/S E A Consultants. She can be contacted at pam.hetherly@seacon.com. Lisa Dickson, P.G., heads up Kleinfelder/S E A Consultant’s Augusta, Maine, office and currently is working on her second book, “Historic Bridges of Maine: 350 years of Bridge and Roadway Design.” She can be contacted at lisa.dickson@seacon.com.