High strength concrete was used in the U-beams of the North Boulevard Bridge.

Louisiana's First 10,000 psi Box Girder Bridge with U-Beams

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), 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 36.6 m for a total bridge length of 329 m. The bridge superstructure is a single 17.9 m wide horizontally S-curved structure with a radius of 350 m. All spans contain a total of five straight PPC beam lines with beam spacing ranging from 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 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 41 MPa at strand release and 69 MPa at 56 days. Ninety-two straight 12.7 mm diameter prestressing strands with a minimum ultimate tensile strength of 1.86 GPa were used. The total prestress force was 13.3 MN. There were 20 strands with different debonded lengths in the U-beam bottom flange. The U-beams are integral with a 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 $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 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 69 MPa at 56 days
	Slump	Maximum 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 m3)
Cement, Type III	410 kg
Fly Ash, Class C	176 kg
Fine Aggregate	647 kg
Coarse Aggregate(1)	1070 kg
Water	148 kg
High-Range Water-Reducing Admixture	10.6 L
Retarding Admixture	2.32 L
Water-Cementitious Materials Ratio	0.25

1. Limestone size No. 78

These mix proportions produced a concrete with a unit weight of 2450 kg/m³ and a 180 mm slump.

The measured concrete strengths based on test cylinders for 23 castings were as follows:

Age, days	Average, MPa	Range, MPa
1	47	28-60
2	68	63-76
3	64	27-74
28	94	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 36.6 m) long, 69 MPa, PPC U-beam weighed an average of about 73 metric tons. A previous bridge project in Louisiana with similar type and length of PPC U-beams and designed using 41 MPa concrete weighed an average of about 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.