Rudy McLellan, Formerly HNTB Corporation
The major benefit of the proposed North Boulevard project was to improve access for highway traffic to either enter or exit the downtown area of the city of Baton Rouge without railroad train operations constantly obstructing highway traffic. Public meetings were held for the proposed improvements resulting in the desire to construct a bridge that was aesthetically pleasing and would complement the existing recreational parks, an historic Temple, and a church with arch-type structures. The City of Baton Rouge and the East Baton Rouge Parish Department of Public Works promised the community that a visually pleasing bridge would be built.
The solution became Louisiana’s first high performance concrete (HPC), 10,000 psi (69 MPa), precast, prestressed concrete (PPC) box girder bridge. Open to traffic in 2006, the new North Boulevard Bridge is a medium span bridge that is S-shaped in plan. The bridge aesthetics were provided by the smooth surfaces of the graceful and slender PPC U- beams, and the uniquely sculptured concrete arch-shaped piers. For this project, HPC was specified only for the concrete of the PPC U-beams.
HPC Girder Solution
The bridge consists of nine spans of 120.0 ft (36.6 m) for a total bridge length of 1080 ft (329 m). The bridge superstructure is a single 58.8-ft (17.9-m) wide horizontally S-curved structure with a radius of 1146 ft (350 m). All spans contain a total of five straight PPC beam lines with beam spacing ranging from 11.3 to 11.7 ft (3.4 to 3.6 m). The straight beams in combination with the S-curve result in a variable length deck overhang.
The superstructure PPC members are 54-in. (1.37-m) deep U-beam shaped box girders having the same dimensions as the standard Texas U54 beams. The PPC U-beams were designed for a concrete compressive strength of 6000 psi (41 MPa) at strand release and 10,000 psi (69 MPa) at 56 days. Ninety-two straight 0.5-in. (12.7-mm) diameter prestressing strands with a minimum ultimate tensile strength of 270 ksi (1.86 GPa) were used. The total prestress force was 3.0 million pounds (13.3 MN). There were 20 strands with different debonded lengths in the U-beam bottom flange. The U-beams are integral with a 7.5-in. (190-mm) thick cast-in-place concrete deck to create the box girders.
The box girder design using HPC materials resulted in an economical, much lighter, and thinner overall superstructure, which in turn required a smaller, less costly, substructure system and shorter approach lengths. The shorter approach lengths saved project costs, which included the reduced cost of expensive right-of-way acquisitions. The total construction cost of the bridge was $5.1 million or about $80/ft2 ($860/m2).
HPC Girder Fabrication
The high strength concrete PPC U-beams were fabricated in nearby Pass Christian, MS. The precaster had previous experience with fabrication of 10,000 psi (69 MPa) PPC girders, which proved to be vital.
The HPC performance requirements from the Louisiana Department of Transportation and Development specifications were as follows:
| Strength | Minimum 10,000 psi (69 MPa) at 56 days | |
| Slump | Maximum 10 in. (250 mm) | |
| Permeability | Maximum 2000 coulombs at 56 days | |
| Silica Fume | Maximum 10.0% by weight* | |
| Fly Ash | Maximum 35.0% by weight* | |
| * by weight of the total cementitious materials (cement, fly ash, and silica fume) |
The HPC mix proportions developed by the precaster were as follows:
| Materials | Quantities (per yd3) | Quantities (per m3) |
| Cement, Type III | 691 lb | 410 kg |
| Fly Ash, Class C | 296 lb | 176 kg |
| Fine Aggregate | 1091 lb | 647 kg |
| Coarse Aggregate(1) | 1803 lb | 1070 kg |
| Water | 250 lb | 148 kg |
| High-Range Water-Reducing Admixture | 275 fl oz | 10.6 L |
| Retarding Admixture | 60 fl oz | 2.32 L |
| Water-Cementitious Materials Ratio | 0.25 | 0.25 |
These mix proportions produced a concrete with a unit weight of 153 lb/ft3 (2450 kg/m3) and a 7 in. (180 mm) slump.
The measured concrete strengths based on test cylinders for 23 castings were as follows:
| Age, days | Average, psi | Average, MPa | Range, psi | Range, MPa |
| 1 | 6800 | 47 | 3990-8680 | 28-60 |
| 2 | 9900 | 68 | 9140-10,970 | 63-76 |
| 3 | 9300 | 64 | 3890-10,790 | 27-74 |
| 28 | 13,600 | 94 | 9790-15,840 | 68-109 |
The fabrication of the HPC high strength PPC U-beams did not pose any significant problems that could not be overcome with minor adjustments.
Each 120-ft (36.6-m) long, 10,000 psi (69 MPa), PPC U-beam weighed an average of about 80 tons (73 metric tons). A previous bridge project in Louisiana with similar type and length of PPC U-beams and designed using 6000 psi (41 MPa) concrete weighed an average of about 120 tons (109 metric tons) each because all cross-sectional dimensions had to be larger.
Conclusion
The North Boulevard Bridge project shows that HPC is an efficient, cost effective material, which can be successfully used in Louisiana for visually appealing, slender, and elegant bridges with challenging geometric conditions.
More Information
More information about this bridge is contained in the paper titled “Louisiana’s First 10,000 psi Precast, Prestressed Concrete Box Girder Bridge,” PCI National Bridge Conference 2007.