Paul B. Fossier, Louisiana Department of Transportation and Development and Harold R. Paul, Louisiana
Transportation Research Center
Louisiana’s interest in high strength concrete (HSC) and high performance concrete (HPC) was initiated in the mid-1970s by the Louisiana Department of Transportation and Development’s (LADOTD) Research Section. The early efforts to develop and produce 10,000 psi (69 MPa) compressive strength concrete were accomplished with inhouse research.
Feasibility Investigation
In the late 1980s, the feasibility of using high strength concrete in the design and construction of highway bridges in Louisiana was examined by a research group consisting of LADOTD, Louisiana Transportation Research Center (LTRC), Tulane University, and Construction Technology Laboratories, Inc. (CTL). The feasibility investigation stipulated that 10,000 psi (69 MPa) concrete had to be produced with local materials using existing local production methods.
The investigation included the design, fabrication, and testing of HSC prestressed concrete piles and prestressed concrete bridge girders. A prestressed concrete pile 130 ft (40 m) long was tested under driving conditions, while other pile lengths were tested statically in flexure. Five full-size, 70-ft (21-m) long, prestressed concrete bulb-tee girders were fabricated and tested as part of this project. As a result of the feasibility investigation, the research group recommended that high strength, prestressed concrete girders having a concrete compressive strength of 10,000 psi (69 MPa) should be used in the design and construction of a prototype bridge, and that the bridge should be instrumented and monitored to determine long-term behavior. While the focus was on concrete strength, the project resulted in HSC having many properties of HPC. The project received the Engineering News Record Medal of Excellence in 1992.
Implementation
LADOTD accepted the recommendation to utilize high strength concrete in a prototype bridge, and, in 1997, launched a major implementation effort that led to the design and construction of Louisiana’s first bridge designed and constructed entirely of HPC — HSC was used in the piles and
girders and HPC for durability was used in the other superstructure and substructure members. Two efforts, one research and one design/construction, proceeded concurrently over a four-year period. The prototype bridge was built across the Charenton Canal at Charenton, Louisiana.* The overall effort was successful, gaining international recognition and receiving awards from the American Concrete Institute. The Charenton Canal Bridge continues to be monitored for camber growth, concrete strains, and prestress losses.
Fatigue and Shear Behavior
In 2000, LTRC launched a new research investigation into the fatigue and shear behavior of 72-in. (1.83-m) deep, 96-ft (29.3-m) long, prestressed HSC concrete bulb-tee girders. Gulf Coast Pre-Stress Inc. fabricated the girders in Pass Christian, MS, and shipped them to CTL in Skokie, IL, for testing.
The five girders had a design concrete compressive strength of 10,000 psi (69 MPa) and incorporated 0.6-in. (15.2-mm) diameter Grade 270 low-relaxation prestressing strands. The shear reinforcement quantities were selected to evaluate the applicability of the shear strength provisions of the AASHTO Standard Specifications for Highway Bridges and AASHTO LRFD Bridge Design Specifications. Shear reinforcement consisted of conventional bars or deformed welded wire reinforcement. Measured shear strengths of six bulb-tee girder ends consistently exceeded the strengths calculated by both AASHTO specifications, using both design and measured material properties.**
The intentionally pre-cracked bulb-tee girders performed satisfactorily under five million cycles of flexural loading when the design tensile stress in the extreme fiber of the bottom flange was limited to a maximum value of 610 psi (4.21 MPa). When the concrete design tensile stress was 750 psi (5.17 MPa) or larger, fatigue fractures of the prestressing strand in the cracked girders occurred, and the fatigue life was reduced. However, two uncracked girders performed satisfactorily under five million cycles of flexural fatigue loading when the design tensile stress was 600 and 750 psi (4.14 and 5.17 MPa).
Other Applications
The results of the fatigue and shear investigation were utilized in the design of the Rigolets Pass Bridge on U.S. 90 east of New Orleans. The original design used 130-ft (40-m) long, 72-in (1.83-m) deep, HPC bulb-tee girders spaced at 7.87 ft (2.40 m). A redesign of the bridge uses the same girders spaced at 12.6 ft (3.83 m). The bridge is presently under construction. One of the HPC spans will be instrumented and monitored to determine bridge behavior.
Two other projects have used HPC girders in their design and construction. The Union Pacific Railroad Overpass on U.S. 165 uses 54-in. (1.37-m) deep AASHTO Type IV girders with a span length of 115 ft (35 m). The LA 27 Overpass in Calcasieu Parish uses AASHTO Type IV girders with a span length of 112 ft (34 m).
*See HPC Bridge Views Issue No. 8, March/April 2000.
**See HPC Bridge Views Issue No. 21, May/June 2002.