In order to achieve the 100-year service life, NCDOT prescribed robust concrete mix designs in the contract, which included extensive use of fly ash, ground granulated blast furnace slag, silica fume, a low water-cement ratio, and calcium nitrite corrosion inhibitor. In addition, for the first time in the state’s history, the department also prescribed cast-in-place concrete to be reinforced with stainless steel to provide additional corrosion protection and reduce future maintenance costs.
One component that the bridge’s design-build team understood as a key advantage to the project’s success was the substantial use of precast concrete. “We keyed in pretty early on the extensive use of precast concrete to provide structural elements that were a higher quality, because they were prefabricated off-site under controlled conditions rather than having to cast concrete out there in the marine environment with saltwater spray and the [harsh] weather,” Coletti explained. The precast elements also proved to be more economical for the project and more reliable than trying to deliver cast-in-place concrete to such a remote project location.
At the north approach span of the bridge—which covered about a mile and a half of the structure and rested in fairly shallow water at shallower scour depths—the team took the innovative approach to implement an all-precast pile bent system, with precast cylinder piles and a precast bent cap that made for easier construction. At the deeper sections of the inlet, where the structure would face more significant scour and higher vessel collision forces, the team opted for a two-column bent with a pile cap and multiple battered piles. That bent cap and the columns were all precast, and the columns were post-tensioned. This approach was fairly unusual for a U.S. bridge construction project.
One of the most significant challenges the team faced to complete the work on the Basnight Bridge was to address the deep scour and high lateral forces associated with the Oregon Inlet, including wind, wave action, and vessel collision. These concerns placed heavier demands on the bridge foundations, particularly regarding overturning effects. Understanding this, the design team performed extensive, highly refined soil structure analyses, often using multiple models to capture zero scour, full scour, and partial depth scour conditions. These analyses provided detailed pictures of the foundation performance for the design team to optimize each foundation design. This approach provided the contractor with the most constructible design for each bent.
