Last month, Maine was ranked ninth in the nation for percentage of bridges classified as deficient in a report by the Washington-based Transportation for America. The report used Federal Highway Administration data to determine nearly 15 percent of Maine’s bridges require maintenance or replacement.
Replacing, and even rehabilitating, all of the bridges at once is a large financial burden for the Maine Department of Transportation.
Hannah Breton Loring, a University of Maine graduate student in the Department of Civil and Environmental Engineering from Greenville, Maine, hopes to ease that burden by offering the MaineDOT a more affordable bridge retrofitting system than the current commercial options.
Loring’s system, engineered and tested at UMaine’s Advanced Structures and Composites Center, is a fiber-reinforced polymer flexural retrofit system made of carbon composites and glass to reinforce and strengthen concrete flat-slab bridges, many of which are 50 or more years old.
“There are multiple reports and report cards on bridge infrastructure, and the U.S. is doing very poorly,” Loring says. “What we’re trying to do is give Maine a little bit of a stepladder. We’re giving them a low-cost alternative for the short term that would increase the strength and durability of the bridge, prevent it from having to be weight posted, and allow the bridge to remain safe.”
The 2007 collapse of the I-35 Mississippi River bridge in Minneapolis, Minn., that killed 13 people and injured 145 served as a wake-up call across the nation, urging transportation departments to look at the condition of their own bridges, according to Loring.
After the collapse, the MaineDOT formed a panel to review its bridge inspection and improvement programs. Engineers on the panel, from the MaineDOT, UMaine and private consulting and construction sectors, released the report “Keeping our Bridges Safe” in November 2007.
According to the report, the MaineDOT is responsible for 2,772, or 70 percent, of the known bridges in the state. Of those bridges, 205 are more than 80 years old, 244 were considered in poor condition and 213 were found to be structurally deficient. The report also estimated that 288 bridges were at risk of closure or weight restrictions from 2007–17.
“A lot of these bridges have to be replaced or extensively repaired, so that’s asking for a lot of money from the Maine department and we’re already struggling,” Loring says. “If we space the cost out over time, it’s almost like self-financing.”
Loring has been working with her adviser Bill Davids on the MaineDOT- and Federal Highway Administration-funded project since June 2011 after earning her bachelor’s degree in civil and environmental engineering in May 2011. Davids, the John C. Bridge Professor and chair of the Civil and Environmental Engineering Department, approached Loring with the research opportunity after working with former graduate student Timothy Poulin, who now works for global engineering firm T.Y. Lin International Group in Falmouth, Maine, to develop software that allows existing flat-slab concrete bridges to be analyzed more accurately.
Loring says calculations are used to determine the strength of a bridge and if it needs to be replaced, but current calculations can be overconservative, calling for more replacements than what might be necessary. The software Davids and Poulin developed was designed specifically to assess the load rating of flat-slab bridges to determine which bridges can be repaired instead of replaced.
For the bridges that can last a few more years with reinforcing instead of replacing, a retrofitting system such as the one Loring engineered, could be applied to increase the bridge’s strength and weight limits.
Loring’s retrofitting system includes composite strips of high-tensile-strength, lightweight carbon fibers sandwiched between glass fibers. The strips are about 4 inches wide and 0.20 inches thick and can be as long as the bridge allows.
“The strips have strength comparable to steel but are light enough to be handled by a single person, which is not something you could do with a piece of steel of the same dimensions,” Loring says.
The composite strips are applied to bridges by drilling holes in the bridge’s concrete and placing threaded rods into an epoxy adhesive, which Loring also tested for durability.
The concrete on the underside of a bridge is weak in tension and is not responsible for supporting the bridge, but rather holding the internal reinforcing steel in place. The reinforcing steel is strong in tension and is the main component in keeping a bridge sturdy. Bridges that are more deteriorated may not be able to withstand the drilling and would have to be replaced or use a more extensive rehabilitation system, Loring says.
While developing this technology, Loring tested four different composite material systems. She tested two all-glass systems, one with a core fiber orientation at plus or minus 45 degrees and one at 90 degrees, and two glass-carbon hybrid systems with the same orientations.
“The fiber-reinforced polymer composites are really strong in the direction of the fiber,” Loring says. “If you have fibers that run in one direction and you pull on the composite in that direction, it takes tens of thousands of pounds to break it. What we end up doing is kind of combining the fiber orientations in different directions, giving it different properties. We looked at different fiber orientations for the core fibers in order to ensure the threaded rods can develop sufficient capacity.”
Loring used glass and carbon because they are lighter than steel. Glass is usually cheaper than carbon, but tends to deteriorate in the environment faster. The hybrid system was chosen because it would be cheaper — due to the glass — and durable enough for short-term use — because of carbon’s superior durability properties.
After conducting durability studies on effects of saltwater, freezing and thawing, the four systems were whittled down to the two glass-carbon hybrid systems.
“The performance of the glass-carbon system was much more superior so we had that manufactured in large strips so we could apply them to reinforced concrete beams,” Loring says.
Working with Kenway Corp. of Augusta, the strips were manufactured and tested on beams designed to mimic flat-slab bridges.
“There has been a big constructability focus with everything we’ve done,” Loring says. “The ability to make the materials, the ability of the materials to perform properly, the ease of installing on a bridge. Everything we’ve done for testing, we’ve done overhead, because you can’t just pick a bridge up and roll it over.”
Loring found the glass-carbon systems performed the best.
“We were able to get about a 47 (percent) to 49 percent increase in the flexural capacity of the beam compared to an unreinforced beam,” she says.
Loring says the system looks promising, although some fine-tuning could increase efficiency. Another student is planning to perform fatigue testing after Loring graduates this summer. Fatigue testing is essential before any field application.
Although Loring doesn’t yet have an exact dollar figure on how much using her retrofitting system would cost, she’s confident it is cheaper than what is available and could save the department tens of thousands of dollars per bridge compared to other methods of strengthening.
“There are commercially available systems out there for the same type of product that I’ve engineered from the ground up, but they’re proprietary systems,” Loring says. “Basically what that means is you pay for the product from the company at whatever price they say it’s worth.”
Loring’s main goal for the project is to be able to give the MaineDOT an alternative option. She wants to present the department with a comprehensive report on a low-cost retrofitting system they could have manufactured instead of defaulting to a proprietary option.
“A lot of the time MaineDOT puts out to bid its work and sees what companies can do,” Loring says. “With this they would be able to present the design specifications to a composite manufacturer and say, ‘Here’s what we want. How much can you make it for?’”
For Loring, working in an environment that forced her to apply what she learned in college was overwhelming at first, but she credits her department, adviser and the Advanced Structures and Composites Center with making her feel comfortable and capable throughout the process.
“The department’s awesome, there’s always been a really close-knit community with the Civil and Environmental Engineering Department,” Loring says. “Professors go by their first names. It’s just friendly, it’s welcoming. I come from a big family so having a family environment at school has just been great.”
Loring chose to study civil and environmental engineering after developing a love of buildings at an early age. Growing up visiting worksites with her father who is a carpenter, Loring knew she wanted to have a hand in creating buildings. Following in the footsteps of her father and several siblings, she decided to come to UMaine to pursue her goal of becoming an engineer.
This is Loring’s first project working with bridges.