John Horn, Volkert Construction Services
In August 2005, Hurricane Katrina decimated the Gulf Coast and nearly destroyed the 5.5-mile (8.9-km) long twin-span, I-10 bridges that connect South Mississippi with New Orleans across Lake Pontchartrain. The existing structures were quickly repaired under an emergency project but were not expected to last more than 5 to 10 years. As a result, the Louisiana Department of Transportation and Development (LADOTD) designed a near “Hurricane Proof” replacement structure. It incorporates foundations capable of withstanding tremendous wave impact, heavily reinforced concrete restraining walls that allow expansion and contraction but prevent uplift and lateral shifting, and high performance concrete (HPC) to ensure the longevity of the structure. The parameters for the new structure required a 100-year service life. In order to provide the necessary service life in Lake Pontchartrain’s moderately aggressive environment, the designers had to all but eliminate salt intrusion into the concrete. HPC was chosen for its extremely low permeability and long-term durability. In addition, concrete cover to the uncoated reinforcement was specified as 4 in. (100 mm) for the footings; 3 in. (75 mm) for the piles, piers, and pier caps; 1.5 in (40 mm) for the girders; 1 in. (25 mm) for the bottom steel of the deck with galvanized metal deck pans; and 2-3/8 in. (60 mm) for the top steel in the deck.
High Performance Concrete
The HPC mixes specified for this project are considered structural class concrete per the LADOTD Standard Specifications for Roads and Bridges. The contractor was required to use fly ash or ground granulated blast-furnace slag (GGBFS) in the concrete mixes when using Type II portland cement. This required either fly ash at a content of 20 to 30% or GGBFS at a content of 30 to 50% by weight of the total cementitious materials. Only Class F fly ash or Grade 100 or 120 GGBFS slag was allowed. In addition to fly ash or GGBFS, the cementitious materials used for all structural concrete mixes required 5 to 10% silica fume by weight of the total cementitious materials. Table 1 gives the target parameters for the HPC utilized on the project.
Table 1. Concrete Mix Parameters
| Bridge Component | Compressive Strength at 28 days,(1) psi | Minimum Cementitious Materials, lb/yd3 | Maximum Water- Cementitious Materials Ratio | Slump, in. |
| Deck | 4400 | 600 | 0.40 | 2 to 4(2) |
| Girders | 8500 | 700 | 0.35 | 2 to 10 |
| Substructure | 4400 | 550 | 0.40 | 2 to 4(2) |
| Piles | 6000 | 700 | 0.35 | 3 to 5(2) |
1. Or at 56 days, per the project specification.
2. For mixes containing a water-reducing admixture, the slump could not exceed 8.5 in.
Construction
The project budget was established at approximately $800 million with two separate construction contracts executed. One contract for approximately 4-1/2 miles (7.2 km) of the twin structures was won by Boh Brothers Construction and the other for the 1-mile (1.6-km) sections crossing the channel was won by Traylor, Kiewit, and Massman (TKM), a Joint Venture. Piles were precast by Gulf Coast Pre-Stress and Prestress Services Industries. The girders were cast by Gulf Coast Pre-Stress, Prestress Services Industries, and Boykin Brothers.
The project included 433,500 linear ft (132,100 m) of 36-in. (915-mm) square precast, prestressed concrete piles, 496 concrete pile caps, 32 concrete piers, 29,500 linear ft (9000 m) of AASHTO Type III girders, 317,500 linear ft (96,800 m) of BT-78 girders, and 3,770,000 ft2 (350,000 m2) of concrete bridge deck. The pile caps were precast for the BT-78 spans on the low-level portion. All other nonprestressed concrete was cast in place.
Acceptance of concrete both at the precasting plants and at the project site was based on initial slump, initial air content, compressive strength, and permeability at 56 days with compressive strength and permeability being the primary indicators of the quality of the final product. Compressive strength tests were run on every structural concrete placement and at 200 yd3 (153 m3) intervals on large placements. Permeability tests were scheduled so that each span would have separate results representing the piles, bent cap, girders, and deck. Table 2 shows the results of the compressive strength and permeability testing performed to date.
Table 2. Concrete Test Results
| Bridge Component | Average Compressive Strength, psi | Average Permeability, coulombs |
| Deck | 7100 | 350 |
| Girders | 9000-10,000(1) | 240 |
| Substructure | 6900 | 230 |
| Piles | 8000-9000(1) | 260 |
Currently, the TKM contract has been completed and the Boh contract is about 85% complete. Compressive strength results have been very consistent throughout the projects resulting in only one concrete penalty to date. Permeability test values have been extremely low and have resulted in no penalties to date. The mixes have performed very well in the field with only minor issues associated with delivery times up to 2 hours. Portions of the project required mixes to be delivered by mixer truck to a barge mounted agitator and barge-mounted pump truck. These issues were overcome by using water-reducing and set-retarding admixtures. Workability has been very good and rapid strength gain has helped move these massive projects along well ahead of schedule. It is expected that all work on the new structures will be completed by the end of 2011. The new twin-span bridges constructed entirely of high performance concrete will stand well into the next century as a symbol of New Orleans recovery from one of the worst natural disasters in history.
Further Information
For more information about this project, contact the author at [email protected] or 251-591-3121.