New Composite Girder Technology Debuts in Maine’s Grist Mill Bridge

Powered by AIT Bridges / April 26, 2021
Grist Mill Bridge drone shot

In the town of Hampden, Maine, the Grist Mill Bridge, which carries traffic over the Souadabscook Stream was on its last legs in 2019, showing signs of deterioration after decades of rehabilitation projects. When it was time to plan for replacement, the Maine Department of Transportation (MaineDOT) knew exactly where to look for a solution: Advanced Infrastructure Technologies (AIT) Bridges, just down the road in Brewer. 

Having developed a longstanding relationship with the composite bridge system manufacturer, MaineDOT saw this project as the perfect opportunity to put one of AIT’s new innovations to use. This composite girder superstructure, developed in conjunction with researchers at the nearby Advanced Structures and Composites Center at University of Maine, would allow for the construction of a bridge that could last at least 100 years with little maintenance.

Through this special partnership, the new Grist Mill Bridge ultimately came to fruition in late 2020 as the first of its kind in the country, featuring AIT’s cutting-edge GBeam technology.

Building the Bridge

The Grist Mill Bridge project was part of a larger construction contract spanning two miles worth of road work in Hampden. MaineDOT advertised the project in March 2019, specifying AIT’s composite girders on an experimental basis. Along with AIT, the department collaborated with engineering firm T. Y. Lin on the design of the project as a whole. Contractor T Buck Construction joined the team after a bidding process. Demolition began in the spring of 2020.

Dating back to the late 1930s, the Grist Mill Bridge consisted of three bridges built on top of one another over the years. Brian Emmons, project manager at T Buck Construction, compared the demolishing of the bridge to peeling back the layers of an onion. 

After demolition, construction on the new bridge began in July 2020, and the bridge ultimately reopened to the public in late December 2020. While the old bridge measured roughly 50 feet across, the new Grist Mill Bridge spans 75 feet across the river to allow for a wider hydraulic opening. 

MaineDOT Bridge Program Manager Wayne Frankhauser said the overall design of the bridge is essentially conventional, with a reinforced concrete deck and cast-in-place concrete substructure. 

Emmons notes that to the untrained eye, you’d never know what you’re driving over—a first-of-its-kind superstructure made up of five beams of AIT’s corrosion resistant girders.

load testing

Cutting-Edge Composite Technology

AIT’s GBeam concept, patented by the University of Maine, came to be after seven years of work at the university’s Advanced Structures and Composites Center. Executive Director Habib Dagher said that the lab set out to research and define the next generation of girder bridges, with a focus on extending a bridge’s life cycle.

“We wanted to have a composite girder that reduces cost, life-cycle impact and carbon footprint,” Dagher said. 

The aim and the final outcome was a girder solution that would be easier to fabricate, ship, install and maintain over a long period of time. 

Bill Davids, head of the civil engineering department at University of Maine, said one especially important aspect of the GBeam technology is how straightforward and  approachable it is for contractors.

“Whenever you introduce a new technology like this, contractors don't always jump on it because there's a fear that it’s going to be difficult to build. One of the beauties of this is it's built exactly the same way as you would build a steel girder or a concrete girder bridge,” he said.

One of the key benefits of the girders is their inclined webs, which allow for stacking and easier shipping. Dagher said three to four bridges worth of girders could fit on one flatbed truck. This can reduce transportation costs in addition to being better for the environment. A bolt system connects the girder to the deck, allowing for ease of removal and replacement and no need for a painstaking jackhammering process.

The composite material also makes the girders especially corrosion-resistant, which was a great fit for the Grist Mill Bridge due to the high level of salt the structure is exposed to as it stands over the brackish Souadabscook Stream.

The light weight of the girders is also a considerable benefit. Emmons said the lightweight quality allowed for more ease in moving the beams around on the job. Because the composite material was so light compared to steel or concrete, workers were able to safely and securely mount and suspend water utility pipes under the bridge from a pair of girders and fly them as one unit from a crane. This was much safer for the workers and also more efficient, he said. 

“A lot of our time-saving activities once we got to the superstructure were absolutely governed by the weight of the girders themselves and the ability to do more things at once,” Emmons said.

Frankhauser adds that the longevity and low-maintenance requirement of this solution translates to cost savings for taxpayers along with increased safety with less need for disruptive road closures. “Anytime we can have something that lasts longer and requires less work through its life, that certainly is going to benefit the public,” Frankhauser said. 

Grist Mill Bridge construction

The Future of Composite Beams

The future of AIT’s GBeam technology is looking promising beyond the Grist Mill Bridge project. MaineDOT has already specified the GBeam Composite Bridge System for construction on the Hampden Twin Bridge, set for completion in June 2022. Beyond Maine, the technology is drawing attention from DOTs across the country, including Washington, California, Florida, and Rhode Island.

University of Maine researchers and AIT have continued developing the technology with further improvements and additional applications.

AIT Bridges President Ken Sweeney said he values the partnership with MaineDOT, which has a longstanding dedication to innovation.

“MaineDOT has always tried to come up with new ways of doing things, and this is just one example of that. It’s part of their genetics to be able to try new things,” Sweeney said. 

After a career spent learning how to design bridges with composites, Dagher said it’s rewarding to see the technology commercially adopted. He attributes that success to the unique partnership between the university, AIT and MaineDOT.

“It's a combination of people who know how to build things, how to design things in the real world, plus our knowledge at the university in composite materials,” he said. “Putting all that together really helped us develop the very unique secret sauce that I think will change how composites will be built in the future.”