Stay-in-place girder forms net MaineDOT world record for composite vehicular bridges

Source: Harbor Technologies, Brunswick, Me.

Harbor Technologies’ hybrid composite beams (HCB), consisting of fiber reinforced polymer forms bearing ready mixed concrete and conventional steel rebar placed on site, support the new two-lane, 540-ft. Knickerbocker Bridge in Boothbay, Me. With a 4-ft. vertical clearance and tidal water proximity, the Back River crossing has 33-in.-deep HCB box beams set on seven pile bents across six 70-ft. interior and 60-ft. end spans.

General contractor Wyman & Simpson was able to haul four 70-ft. HCB, weighing about 5,000 lbs. each, per truckload. As stay-in-place forms, they were placed side-by-side, eight across, eliminating separate formwork for a 7-in. concrete deck. Beam mixes were pumped to longitudinal openings along 4-ft. top flanges, with each 70-ft. member typically filled in 20-minute cycles. Auburn Concrete delivered about 200 yd. of self consolidating mixes to fill the 64 HCB.

Maine Department of Transportation approved the HCB as an alternative to initially proposed precast box beams. Their staging and handling would have been complicated due to weight limitations of the existing Knickerbocker Bridge, a 38-span timber structure dating to 1935. The composite beams’ weight allowed Wyman & Simpson to deploy a barge-mounted crane of lower capacity than precast box members would have required.

The beams combine the strength and stiffness of conventional concrete and steel with the lightweight and corrosion advantages of advanced composite materials, according to John Hillman, president of HC Bridge Co., HCB developer. The new Knickerbocker becomes the world’s longest composite vehicular bridge, he adds, and was completed at a cost competitive with a conventional concrete box beam structure. The Maine project follows HCB installations in New Jersey and Illinois, along with a Class 1 Railroad Bridge for BNSF. A Maine Composites Initiative, implemented in 2008 to support the state’s composites manufacturing base, has helped advance HCB development and commercialization.