According to the Federal Highway Administration’s (FHWA) National Bridge Inventory, 147,870 of the 607,751 bridges in the country are deficient. This includes 63,522 structurally deficient and 84,348 functionally obsolete bridges. Structurally deficient bridges are bridges with critical load-carrying elements that are in poor condition due to damage or deterioration. Functionally obsolete bridges are bridges that no longer meet current standards for bridge construction such as load capacity or lane width.
With nearly a quarter of all bridges in the country labelled deficient, some state DOTs have turned to accelerated bridge construction (ABC) programs to reduce the number of structurally deficient and functionally obsolete bridges they maintain. Accelerated bridge construction involves using innovative engineering, materials and construction methods to deliver cost-effective solutions to improve project delivery times and work zone safety while at the same time reducing onsite construction time and traffic impacts.
The Bridge-in-a-Backpack system involves bridge technology that can meet the demands of an ABC program. The Bridge-in-a-Backpack system was developed at the University of Maine’s Advanced Structures and Composites Center. The company Advanced Infrastructure Technologies was created to license and commercialize the technology. The system uses carbon fiber reinforced polymer (FRP) composite tubes to form arch spans. The corrosion resistant tubes provide external reinforcement while adding strength and durability to the concrete superstructure.
The name Bridge-in-a-Backpack came about because the composite tubes were designed to be rolled up and stuffed in duffel bags in order to be easily transported to the construction site. Once at the site, the tubes would be rolled out, inflated and then set into a frame that bends it into an arch. After the tubes are shaped to the required geometry they are infused with resin using a vacuum-assisted transfer molding process and allowed to cure.
Once cured, the lightweight arches are placed into the abutments and covered with corrugated FRP decking panels. The arches are then embedded into the concrete foundation and the arch tubes are filled with an expansive concrete. Headwalls are installed and dirt fill is then compacted on top of the FRP decking and paved over with asphalt. Total construction time from the demolition of the existing bridge to the completed Bridge-in-a-Backpack can be completed in a matter of days.
The first Bridge-in-a-Backpack system was constructed in 2008 and since then has been used for a number of bridges in Maine, New Hampshire, Michigan and Trinidad. The system has also been used as part of Massachusetts’ and Vermont’s Accelerated Bridge Programs. The system can be used for single span bridges from 25 to 75 ft. as well as multi-span designs that exceed 800 ft. In addition to lower construction costs and shorter project completion times, the system is designed to last 100 years with little maintenance required.