Paul Fossier, Louisiana Department of Transportation and Development
When the Charenton Canal Bridge on LA 87 in St. Mary Parish opened on November 4, 1999, the occasion marked the completion of Louisiana’s first high performance concrete (HPC) bridge. HPC was used in all components of the bridge. Completion of the project marked the implementation of HPC that began with research work in Louisiana in the early 1980’s.
The project consisted of the replacement of a 55-year old reinforced concrete bridge with a 365-ft (111-m) long continuous prestressed concrete structure using Type III AASHTO girders. Each 73-ft (22.3-m) span consists of five girders that are spaced at 10-ft (3.1-m) centers and support an 8-in. (203-mm) thick cast-in-place concrete deck. The substructure for the bridge consists of cast-in-place concrete bent caps supported on 24- and 30-in. (610- and 762-mm) square precast, prestressed concrete piles. Specified compressive strength of the girders and piles was 10,000 psi (69 MPa) no later than 56 days. The bridge deck and bent caps had a specified concrete compressive strength of 4200 psi (29 MPa) at 28 days. A rapid chloride permeability of 2000 coulombs or less at 56 days was specified for concrete used in all members.
The use of HPC enabled the bridge to be designed with one less line of girders than would be required when using 6000 psi (41 MPa) compressive strength concrete. The additional strength in the precast piles increased their resistance to compressive and tensile driving stresses and allowed the casting and shipping of longer lengths. We anticipate a 75- to 100-year service life for the bridge instead of the normal 50-year service life for concrete structures.
The use of match-cure cylinder molds was a requirement for the precast fabricator. The internal concrete temperature of all precast members was limited to 160°F (71°C). The contractor elected to use fly ash in the precast members and slag in the cast-in-place members. As part of the project, material testing, bridge instrumentation, and bridge monitoring are being performed by Tulane University in cooperation with the Louisiana Transportation Research Center. The instrumentation was used to measure girder curing temperatures and prestressing forces during fabrication and is being used to determine prestress losses, strains in the bridge deck, and girder deflections.
Completion of the Charenton Canal Bridge proves that it is feasible to construct an HPC bridge in Louisiana with local materials and local contractors. The success of the project has now enabled the Department to plan for three more HPC bridge projects in the next two years.