Grist Mill Bridge Opens with New Composite Beams System

The bridge span was replaced with a new span consisting of five composite beams. Photo courtesy of AIT Bridges.

In early 2021, AIT Bridges (Brewer, Maine, USA) announced that—in conjunction with the Maine Department of Transportation (MaineDOT) (Augusta, Maine, USA)—the replacement of Hampden’s Grist Mill Bridge is complete, making the revamped road open to traffic.1 AIT Bridges is a division of Advanced Infrastructure Technologies (AIT), which designs and supplies composite bridge systems and related structural components.

The former span of the Grist Mill Bridge was constructed in 1950, and it was comprised of three bridges built one on top of the other. However, the bridge had suffered from concrete deterioration, owing to years of exposure to the elements along with the use of deicing chemicals to combat Maine’s harsh winter climate. The bridge spans over the Souadabscook Stream, which is an environmentally sensitive tributary of the Penobscot River.

In the recent project, the span was replaced with a new span consisting of five composite beams manufactured at AIT’s nearby facility. The raw materials for making the composites were locally produced, with the fiber sheets coming from other Maine companies. The technology was developed in cooperation with The University of Maine’s Advanced Structures and Composites Center (UMaine ASCC) (Orono, Maine, USA).

The 75-ft (22.9-m), single-span bridge is the first in the nation to use what AIT refers to as composite tub girders, which have no concrete reinforcement. Rather, they are comprised of fiber-glass reinforced polymers (FRPs). The FRP is described as a composite material consisting of glass fiber, carbon fiber, a foam core, and resin. According to MaineDOT, the FRP composites do not rust or deteriorate the way many conventional bridge materials do. The “tub” reference is because of the bathtub-like shape of the vertical girders.

Officially known as the AIT CT Girder, the manufacturer explains that the novel system was designed to provide an affordable, long-term solution to traditional steel and concrete medium-span-deck bridges. The U-shaped girders are supported on standard foundations, with either a precast concrete panel deck or a cast-in-place concrete bridge deck. Spans up to 100 ft (30.5 m) are easily achievable.

“This bridge system takes advantage of the unique properties of both composite materials and precast concrete, and it is designed with construction logistics in mind,” says Habib Dagher, executive director of the ASCC.2 “The bridge girders weigh only 1 to 2 tons for 40- to 80-ft [12.2 to 24.4 m] spans, so that they can be erected with locally-sourced common rental cranes, making them easy to deploy in most locations.”

“The unique girder shape was designed to be nesting and stackable,” Dagher explains. “As a result, three to four bridges can be transported on a single flatbed.”

The system is naturally corrosion resistant and it is made of materials that are custom designed to overcome environmental challenges, according to the company, which notes that the CT Girder is roughly 50 percent lighter than steel and 75 percent lighter than concrete girders. This reduction in weight allows for a decrease in large equipment on the construction site and significantly reduces transportation costs.

“Our composite tub girder is, ultimately, going to be a game changer in the bridge construction sector,” says Brit Svoboda, chairman and CEO of AIT Bridges. “It is versatile, strong but lightweight, affordable, and ultimately intended to replace concrete and steel girders in the marketplace. For anyone that knows of our arch system, this is a natural extension of what we already offer. It can be used as a highway, rail, or pedestrian bridge, even as part of a building structure, parking garage system, marine structure, and in many other places where steel and concrete support girders are used.”

As part of a long testing process, developers built a bridge prototype using shorter girders, and it withstood weight equivalent to the Statue of Liberty. According to developers, the system withstands more than five times the HL-93 design load under the bridge design specifications of the American Association of State Highway and Transportation Officials, and it exceeds “twice the collapse strength of steel and concrete girders.” Based on those types of results, MaineDOT approved the system for a real-world application.

Once the beams were set on the Grist Mill Bridge using a single crane, crews installed falsework floors between the girders and forms for concrete placement with stainless steel reinforcement. Finally, a concrete deck was installed on top. Approximately 20 sensors were put onto the system to measure strain, and more than 260,000 lb (117,934.0 kg) of load was initially applied to test its performance. AIT says it will use those sensors to continue evaluating how the bridge performs in the field, which could allow the company to further refine its design methodology in the years ahead.

According to the company, the composite bridge system is designed to last over 100 years, which is at least 30 years longer than the average steel bridge. Additionally, because the bridge system uses composites instead of steel, it is expected to need only minimal maintenance, if any, over the expected lifecycle.

The company is informally calling the tub girder “The 72-hour Bridge,” because it hopes the technology can make it possible to finish a bridge replacement—from installing the supports to the final paving—within that time. By contrast, bridge constructions or replacements using conventional materials can often take several months, according to the developers.

The Grist Mill Bridge replacement is part of a project that includes reconstruction and paving work on a nearly 2-mi-long (3.2-km) stretch of Route 1A in Maine. The total cost for the project was approximately $8.9 million.

Source: AIT Bridges,


1 “Grist Mill Bridge Open to Traffic with AIT Bridges Composite Beams,” Smart Connections PR for AIT Bridges, Jan. 5, 2021, (Feb. 17, 2021).

2 “UMaine Engineers Unveil and Test Innovative Bridge System,” UMaine News, July 12, 2018, (Feb. 17, 2021).