Project Overview
Steel and salt are generally not a great match. When bridges are built in harsh saltwater environments, internal components, including reinforcement and prestressing strands, are at risk for corrosion and eventual replacement, especially as structures age. On the eastern shores of North Carolina, such was the case for the Earl C. Davis Memorial Bridge (a drawbridge) and its companion structure, bridge no. 96. After more than a half century of service, both bridges, which connected Harkers Island with the mainland, had deteriorated beyond repair, creating transportation issues for vehicular traffic and commercial and recreational vessels attempting to navigate around Harkers Island. In partnership with precast concrete manufacturer Coastal Precast Systems and general contractor Balfour Beatty, the North Carolina Department of Transportation (NCDOT) turned to precast concrete and corrosion-resistant reinforcement for a new high-rise bridge solution that leverages innovative, resilient reinforcement in its design.
Salting Away Corrosion
According to Trey Carroll, PE, assistant state structures engineer with NCDOT, the agency’s standard policy for highly corrosive areas is to use precast concrete girder superstructures and precast concrete pile substructures with mixture designs that include silica fume, fly ash, and a calcium nitrite corrosion inhibitor. Therefore, the NCDOT team knew from the start that a structure designed with precast concrete components represented the best option for the new Harkers Island Bridge. Additionally, the NCDOT engineers chose glass-fiber-reinforced polymer (GFRP) bars and carbon-fiber-reinforced polymer (CFRP) prestressing strands to help ensure the bridge’s long-term success.
“An effective way to address durability challenges is to eliminate one of the sources resulting in corrosion, which in the case of precast concrete bridges is the internal steel reinforcing and prestressing strands,” Carroll said. “The use of noncorroding fiber-reinforced polymer is a demonstrated, viable alternative to steel. Typical maintenance and repair associated with steel-reinforced concrete is eliminated, increasing the useful service life and resilience of the structure.”
The new 3,200-ft-long, 28-span bridge has a cast-in-place sand lightweight concrete bridge deck reinforced with GFRP bars, which is supported by CFRP prestressed concrete Florida I-beam (FIB) girders with GFRP stirrups. The substructure includes more than two hundred 24-in.-square precast concrete piles manufactured with CFRP strands. NCDOT selected precast concrete piles for the bridge’s foundation because they are resilient in marine environments and can be erected efficiently. At the jobsite, there was a moratorium on in-water work from April 1 to September 30 to protect the marine environment, and the selection of precast concrete piles and girders allowed Balfour Beatty to move quickly within the appropriate work windows.
“By using precast, prestressed concrete piles, the contractor was able to immediately begin substructure installation during the initial six-month in-water construction window and get ahead of schedule,” Carroll said. “This contributed to the bridge opening to traffic in December 2023, 10 months ahead of the contract schedule.”
Travelers in the area are already benefitting immensely from the Harkers Island Bridge, which features 125 ft of horizontal clearance and 45 ft of vertical clearance. This allows both recreational and commercial vessel traffic to efficiently navigate through the area while also enabling motorists to pass without worrying of delays resulting from drawbridge openings.
Overall, the project represents a monumental accomplishment for all parties. It is the state’s first concrete bridge reinforced entirely with GFRP bars and CFRP prestressing strands. The bridge provides safe vehicular access for Harkers Island residents and visitors during both everyday travel and extreme weather events. And, thanks to the success of the effort, the project is now being used as a model for other projects where precast and prestressed concrete components with GFRP and CFRP reinforcement can improve long-term resilience and durability.
Mason Nichols is a Grand Rapids, Mich.-based writer and editor who has covered the precast concrete industry since 2013. |