Michael W. Beacham, Nebraska Department of Roads
Construction began on Nebraska’s first high performance concrete (HPC) bridge in the summer of 1995. The 225-ft (68.6-m) long bridge utilizes seven lines of pretensioned concrete girders, with three spans of 75 ft (22.9 m) each. The site was selected for two reasons. A conventional concrete bridge with identical geometry would be constructed less than a half mile (0.8 km) from the HPC bridge. The conventional bridge is used as a control structure to help evaluate the service life of the HPC bridge. In addition, the HPC bridge was already designed using conventional concrete. This allowed the Nebraska Department of Roads to establish incremental costs for design and construction with relative ease.
The success of this project centered on the partnership of numerous stakeholders, which was formed at the outset. Input from people in industry, academia, and local, state, and federal governments was invaluable in determining the project strategy. The shared goal of this team was clear: “In lieu of optimizing the design, implement a strategy that eliminates or reduces the fear of producing, placing, and curing HPC.” Because of this partnership, we developed a methodology that was realistic, achievable, and cost effective.
The project specifications required only one performance characteristic for the concrete girders – compressive strength of 12,000 psi (83 MPa) at 56 days. The girder design required only 8,000 psi (55 MPa). The deck concrete required two performance characteristics—strength and chloride permeability. A concrete strength of 8,000 psi (55 MPa) at 56-days was specified, while the required design strength was 4,000 psi (28 MPa). The specified strengths for the girders and deck were intentionally higher than required by design as part of the implementation strategy. A chloride penetration of less than 1800 coulombs at 56 days measured in accordance with AASHTO T277 was also required for the deck concrete.
On a warm July day in 1996, the Governor, State Engineer, local politicians, and distinguished guests gathered under a blue fabric tent. To many, the ribbon cutting ceremony announced that just another bridge was being opened to the public. To others, it symbolized the willingness of department managers to take the steps necessary to advance the philosophy of engineering concrete to enhance long-term performance.
An important outcome of this project was the initiation of a strategic plan for the implementation of HPC on a statewide basis. Strategic implementation plans will vary from state to state, but the plans should include forming partnerships. The plan goals must be realistic, achievable, and cost effective. Plan goals may include documenting current concrete practices for pavement, cast-in-place structures, and precast structures. These practices include design, batching, placing, finishing, and curing. The data representing current practice can then be evaluated for comparative analysis with other states using the HPC Lead States web site information. Existing materials, concrete mixes, and construction practices should be tested in order to determine their performance characteristics as defined by the FHWA HPC definition. The next step is to determine target performance characteristic values for each use of concrete and to identify specification changes that are needed to achieve those values. A critical partnering step is to develop and test prototype mix designs and construction practices to achieve the desired performance characteristics. During the entire implementation effort, it is essential to educate upper management, designers, contractors, producers, and construction inspectors on the new practices and benefits of HPC. This is critical for the acceptance of, and successful transition to HPC. At the present time, Nebraska is continuing to develop its strategic plan for HPC implementation.
Every visionary state that commits to using HPC expands the collective experience. The potential to extend the service life of bridges and pavements, while reducing maintenance and replacement costs, should far outweigh the concerns of resource allocation to implement HPC.