“Our goal is to get in and get out and stay out; composites look like they will give us the capability to not have to worry about continued maintenance of our substructure within the practical service years of the bridge,” says Nolan.
The Halls River Bridge piles will be constructed from CFRP pre-stressed concrete. Piles are traditionally pre-stressed with high-strength steel strands and spiral reinforcing to prevent cracking during pile driving. Cracks invariably still occur, as does long-term chloride ion diffusion from saltwater. “By using composites, we don’t have to worry about corrosion or degradation of the surrounding concrete if chlorides do penetrate,” says Nolan.
The piles will be manufactured by Gate Precast in Jacksonville, with Carbon Fiber Composite Cable (CFCC®) supplied by Tokyo Rope. A 15-millimeter diameter, seven-cord CFCC with six carbon fiber cords helically wound around a central straight cord will be used to pre-stress the piles longitudinally. A total of twelve CFCC strands will also be distributed along the perimeter of the rectangular pile, and a single 5-millimeter diameter CFCC cord will be helically wound around the length of the pile to provide spiral containment.
Each of the bridge’s five spans will be built with nine, 36-foot HCBs fabricated by authorized manufacturer Kenway Corporation in Augusta, Maine. According to Nolan, ordinary bridge beams are typically either concrete, pre-stressed concrete or steel. “We’re using a hybrid composite beam that contains all three – steel strand, concrete arch and composite shell,” he says. The GFRP shell will protect the beams from saltwater splash and spray, including from jet skis and boats. The deck will also include GFRP reinforcing bar rather than steel to further eliminate potential chloride-induced corrosion from the underside.
In addition, GFRP will be used in the bridge’s abutments, concrete caps, traffic railings, approach slabs and sheet pile retaining walls. “GFRP is used in almost everything but the piles and the beams,” Nolan notes. The sheet piles have GRFP transverse reinforcement around the perimeter and either a carbon or steel pre-stressed concrete core.
One of the main challenges of building a bridge with composites is a lack of familiarity with composites in the design community. In addition, efficient composite design tools aren’t readily available. Consequently, FDOT’s State Structures Design Office provided the regional FDOT office in Tampa with standards for the project’s piles, sheet piles, approach slab, traffic railing and gravity wall. Nolan says these standards can be used on future projects.