ORONO, Maine — Bridges in Maine, and across much of the United States, are getting older and many are outdated or in rough shape, according to government data. Price tags for repairing or replacing them are growing.
A University of Maine graduate student says her research could help trim the costs of strengthening some of those tens of thousands of ailing bridges.
Hannah Loring, formerly Hannah Breton of Greenville, is a 23-year-old graduate student in the civil engineering program at UMaine. For the past two years, she has been working with her adviser, Bill Davids, on an effort to create an inexpensive system for retrofitting aging concrete flat-slab bridges, using thin strips of carbon composites and glass for fortification.
“What we’re trying to do is give Maine a little bit of a stepladder,” Loring said. “We’re giving a low-cost alternative for the short term that would increase the strength and durability of the bridge, prevent it from having weight [limits] posted, and allow the bridge to remain safe.”
Concrete flat-slab bridges are typically short spans built along two-lane state routes and secondary roads, according to Davids. Most were built between the 1920s and 1950s. Davids estimates that a few hundred of Maine’s more than 2,400 bridges are this style.
Because the trucks and loads transported on Maine roads were much smaller when the bridges were built, some Maine flat-slab spans are now taking a beating from today’s larger vehicles and are at risk of being posted with weight limits, Davids said.
A typical flat-slab bridge could cost about $420,000 to replace or $120,000 to replace just the deck, according to Loring. But her creation, called a fiber-reinforced polymer flexural retrofit system, could increase the strength of a bridge by 30 percent and cost closer to $70,000. That $50,000 savings could add up if the DOT were to adopt the system.
Loring said the system is easy and inexpensive to install. The lightweight carbon-glass strips, about a foot wide and made to length, are placed under the bridge with adhesive and then screwed into place. A two-person crew could do the job in a matter of days, whereas a bridge replacement or renovation would take weeks or months.
“The strips have strength comparable to steel, but are light enough to be handled by a single person,” Loring said, adding that durability studies are still planned, but the strips have held up well in testing for the effects of saltwater and freezing and thawing.
The strips were tested on large concrete beams at the university’s Advanced Structures and Composites Center. Without the composite strips, the beams failed under 15,000 pounds of force. With the strip’s support, they failed under 22,000 pounds.
The report used Federal Highway Administration data, which shows that 356 of Maine’s 2,408 bridges — about 15 percent — are considered “structurally deficient,” meaning they require significant maintenance, rehabilitation or replacement. Those bridges also must be inspected at least once every year because load-carrying elements were found to be in poor condition. It does not mean they’re considered unsafe. The data also show that 436 of Maine’s bridges are functionally obsolete, meaning they wouldn’t meet current building standards, such as lane width or load capacity.
Many of these bridges will continue to deteriorate over time without maintenance. The older concrete slab bridges weren’t designed to carry today’s truck loads, and if reinforcement isn’t provided, more of them will need to be posted with weight limits to prevent degradation.
The University of Maine also has been involved with load testing several Maine bridges, according to Davids. Recently, the I-95 bridge that crosses the Kenduskeag Stream was shut down for a few hours and heavily loaded dump trucks were used to test the effects the loads had on the bridge.
The condition of U.S. bridges has fallen under the microscope in recent years. In May, a bridge in Washington state collapsed, sending three people to the hospital. There were no fatalities.
After the I-35 bridge in Minneapolis, Minn., collapsed into the Mississippi River in 2007, killing 13 people and injuring 145, the Maine Department of Transportation assembled a panel that released a report, “Keeping our Bridges Safe.”
That report found MDOT was responsible for 2,772, or 70 percent, of known bridges in the state, 205 of which were more than 80 years old. Transportation officials estimated that 288 bridges would be at risk of closure or weight restrictions within a decade.
The cost of repairs for these bridges would be significant, but Maine has it easier than most. Maine has the 33rd most expensive backlog of bridge repairs and replacements — about $542 million — according to the report card. New York has the highest estimated cost of getting caught up at nearly $9.4 billion.
Transportation for America, a national safety advocacy group, found Maine had the ninth highest percentage of structurally deficient bridges in the county. Nationally, about 67,000 of the United State’s 605,000 bridges are considered structurally deficient.
Maine has made some progress, reducing its number of structurally deficient bridges by about 8.5 percent from 2011 to 2012. The state received $29 million from the Federal Highway Administration to fund bridge repairs in fiscal year 2011, according to the infrastructure report card.
The SAFE Bridges Act, introduced in the U.S. House in June, would provide $5.5 billion to start to reduce the backlog of the more than 150,000 structural deficient and functionally obsolete bridges across the country.
Loring and Davids said they hope MDOT, or even other DOTs, might be interested in using their system. Other similar commercially available retrofitting technologies exist in the U.S., but UMaine’s differs slightly and is not proprietary, Davids said. Loring plans to provide a comprehensive report on the system and the savings it could yield to MDOT.
Davids said the system would be “quite close” to implementation if MDOT took interest.
“It would be neat if it could actually get implemented, that would be a nice little feather [in my cap],” Loring said.