Ray Mumphrey and Kian Yap, Louisiana Department of Transportation & Development
The Rigolets Pass Bridge, with a total length of 5489 ft (1673 m), is part of U.S. 90 between eastern New Orleans and the Gulf Coast towns of Mississippi. The bridge crosses one of two waterways that connect Lake Ponchartrain, LA, with the Gulf of Mexico. The majority of the bridge superstructure consists of two-span continuous units that utilize BT-78 girders over a span length of 131.2 ft (40 m). The main channel crossing consists of a three-span continuous prestressed concrete spliced girder unit with a center span of 254.1 ft (77.5 m). The BT-78 girders used concrete with a specified compressive strength of 7500 psi (52 MPa) with the exception of one two-span continuous unit that used high performance concrete (HPC) with a specified compressive strength of 10,000 psi (69 MPa) at 56 days. The specified compressive strength of the deck concrete was 3400 psi (23 MPa).
HPC with a specified strength of 10,000 psi (69 MPa) was first used in Louisiana in 1999 on the Charenton Canal Bridge, a 365-ft (111.3-m) long structure that featured five lines of AASHTO Type III girders.* At the time of the final design of the Rigolets Pass Bridge project, the HPC specification was not part of the Louisiana Department of Transportation & Development standard specifications. Special provisions were created to supplement the standard specifications.
Special Provisions
The materials specification for the HPC girder concrete was a performance-based specification with the following requirements:
- Silica fume limited to a maximum of 10% by weight of the total cementitious materials (cement, fly ash, and silica fume).
- Fly ash (Class C or Class F) allowed to be used in combination with Type I, II, or III portland cement up to a maximum of 35% by weight of the total cementitious materials.
- An average minimum compressive strength of 10,000 psi (69 MPa) at 56 days.
- A maximum slump of 10 in. (255 mm).
- A maximum permeability (total charge passed) of 2000 coulombs at 56 days in accordance with AASHTO T 277.
In addition, the concrete was required to have a minimum compressive strength of 6670 psi (46 MPa) at strand release and the fabricator had to ensure that concrete did not segregate with the selected concrete mix design and slump. Fly ash was not used in the final approved concrete mix. About 10% by weight of silica fume was used.
The specifications required that the test cylinders for the HPC precast, prestressed concrete girders be match-cured under the same conditions as the corresponding members that they represented. Three cylinders were tested by the contractor at no later than 56 days after casting to determine if the required strength was achieved. Steam curing was done under an enclosure to minimize moisture and heat losses. The initial application of heat began after the concrete had reached its initial set as determined by ASTM C403. During application of steam, the concrete temperature increased at a rate not exceeding 40°F (22°C) per hour until the desired concrete temperature was achieved. The concrete temperature could not exceed 160°F (71°C). Test cylinder results showed that the HPC mix used for these girders achieved a compressive strength of 7500 psi (52 MPa) within a day and exceeded the 10,000 psi (69 MPa) requirement at an age of 28 days.
The fabricator was required to detension the strands before the internal concrete temperature had decreased to 20°F (11°C) less than its maximum temperature to avoid vertical cracking prior to release of the strands. The fabricator was permitted to add heat to maintain the internal concrete temperature within 20°F (11°C) of the maximum temperature. Two recording thermometers showing time-temperature relationships in the concrete were furnished for each 200 ft (61 m) of bed. One thermometer was located at the center of gravity of the top flange and one within 1 in. (25 mm) of the center of gravity of the bottom flange of the girder.
Permeability (total charge passed) of the HPC girder concrete was determined in accordance with AASHTO T 277 and limited to a maximum of 2000 coulombs at 56 days. The permeability samples were cut from 4×8 in. (100×200 mm) cylinders and tested at the Louisiana Transportation Research Center. The HPC test specimens were cured in a similar manner as the girders until the test age of 56 days. Measured permeabilities were less than 200 coulombs at 56 days.
Since the construction of Rigolets Pass Bridge, the HPC specification has been updated. The new 5.5-mile (8.9-km) I-10 Twin Span Bridges crossing Lake Pontchartrain, which are located 5 miles (8.0 km) west of the Rigolets Pass Bridge, utilize HPC for almost the entire bridge structures.
Further Information
For further information about the Rigolets Pass Bridge, please contact the second author at [email protected].