A precast concrete design helped designers with the Maine Department of Transportation replace a 60-foot-long bridge while closing the existing bridge for just 96 hours during the construction process. The new one-span bridge, which features 10 precast concrete box beams, eliminated a 55-mile detour. A joint effort between two Oldcastle Precast Inc. plants, Rotondo Precast in Rehoboth, Mass., and Superior Concrete in Auburn, Maine, supplied the precast concrete components.
This design was used for the Andover Dam Bridge to determine how well a fast-track design could work, explains Eric Calderwood, design engineer. "We know that some day, our hand will be forced by an emergency, so we wanted to figure out how to accomplish this design while we had the chance," he says. The bridge handled only 120 cars per day, mostly logging trucks, but the lengthy detour made it an ideal choice for the rapid-construction technique.
The bridge features new precast concrete abutments that are located behind the existing abutments, he explains. The bridge closures were needed four times during the construction to excavate boulders behind the existing abutments so new piles could be driven to secure the new construction.
The first closure, which required 13 hours, went smoothly, with excavation followed by driving four new piles. A frame was welded to the piles at the correct elevation to facilitate abutment placement. The second excavation ran into difficulties with remnants of past construction interfering with the pile locations. This interference required large rocks to be moved, and the bridge was reopened to traffic before a third closure to finally drive the piles. "It was a little disheartening to encounter this delay, but we'd scheduled as much as 192 hours of closures, so we were still in good shape," he says.
The abutments were cast in two pieces and post-tensioned after placement. Self-consolidating concrete was used to fill the voids in the abutments. The box beams featured precast curbs, which allowed guard rails to be placed directly on the curbs. This saved time in the final installation process, he notes.
The result was a quickly constructed bridge that minimized environmental impact because it eliminated the need for a temporary bridge to be built. "We wanted to see if we could use this format in the future, and it worked very well for us," Calderwood says.