The Maine Department of Transportation (DOT) is excited about a potential bridge repair technique that could save the state $50,000 per bridge. Engineers at the University of Maine created a retrofitting system that applies composite strips underneath deteriorating flat slab concrete bridges, which could extend their lives up to 20 years. The DOT and university hope this system will provide a low-cost, long-lasting alternative to traditional repairs or bridge replacements.
The importance of fixing dilapidated bridges hit home in 2007, when the I-35 bridge in Minneapolis collapsed into the Mississippi River killing 13 people and injuring 145 others. Many state DOTs began to look carefully at the conditions of their own bridges. That year, the Maine DOT, along with the University of Maine and engineers from other organizations, created the Advanced Bridge Safety Initiative to determine ways to repair or replace deteriorating bridges.
“The main part of that initiative was to look at how we can use composites to strengthen some of our bridge structures,” says Dale Peabody, director of transportation research for the Maine DOT, who frequently works with the University of Maine’s Advanced Structures and Composites Center. Maine has more than 2,700 bridges: DOT officials estimate more than 450 of them are in poor condition or structurally deficient.
In 2011, Hannah Loring, a graduate student at the University of Maine, created a retrofitting system for aging concrete flat-slab bridges alongside her advisor Bill Davids, professor of civil and environmental engineering. The fiber-reinforced polymer (FRP) flexural retrofitting system – featuring carbon fiber and fiberglass hybrid strips – can increase the strength of a bridge by 30 percent.
The retrofitting system is an inexpensive option, according to Davids. A flat-slab concrete bridge could cost $420,000 to replace or $120,000 for deck repair. The university’s flexural retrofitting system would cost closer to $70,000. “Our goal is to provide a cheaper alternative for repairing bridges,” says Davids. The Maine DOT funds the university’s retrofitting project and works with the Advanced Structures and Composites Center to test, analyze and provide new technologies for retrofitting and construction.
At the heart of the system are FRP sheets manufactured by Kenway Corporation in Augusta, Maine. The university cuts the sheets into 4-inch-wide strips, while the length varies based on needs of each repair project. Jacob Marquis, senior project manager at Kenway, worked with Davids and Loring to design and produce FRP strips capable of achieving the necessary bolt-bearing capacity and required tensile strength. The strips consist of woven carbon fiber fabric core laminate layers sandwiched between unidirectional fiberglass outer laminate layers.
The team at the University of Maine opted for FRP because of its lightweight properties. This benefits construction workers, who lift the strips over their heads when applying them to the underside of bridges. “Although having a corrosion-resistant material is also a major benefit, the weight is the biggest advantage,” says Davids.