U.S. Department of Transportation
Federal Highway Administration
1200 New Jersey Avenue, SE
Washington, DC 20590
|Accelerating Infrastructure Innovations|
Publication Number: FHWA-SA-96-016
Date: May 1996
Engineers attending the pilot high-performance concrete (HPC) for bridges showcase workshop, held March 25-27 in Houston, Texas, got a look at the first U.S. bridge being built with HPC, a technology evaluated under the Strategic Highway Research Program (SHRP). The Texas Department of Transportation (DOT) hopes that the bridge--the Louetta Road overpass--will have a useful life of 75 to 100 years, roughly double the average life of a standard bridge.
A working definition of HPC for bridges has been developed by a group of concrete experts representing State DOTs, academia, the highway industry, and the Federal Highway Administration (FHWA). The definition allows States and industry to design concrete mixes that will work well in a specific area, taking into account local materials, traffic loads, and other factors.
The purpose of the HPC for bridges showcase workshops is to introduce construction professionals and bridge, materials, and design engineers to the new guidelines and to show them how they are already being used to build more durable structures. A longer life span means not only lower life-cycle costs, but also lower user costs, as motorists will encounter fewer lane closures and other delays caused by maintenance work.
Presentations in Houston also focused on:
FHWA's mobile concrete laboratory and staff were also on site to demonstrate test procedures.
Texas is currently building two HPC bridges (see sidebar). Texas DOT has found that the longer life and increased durability expected of bridges built with HPC can come at no additional initial cost. Although HPC mixes cost more than conventional mixes, overall bridge costs can be lower because fewer beams and supports are needed, notes Mary Lou Ralls, bridge design engineer for Texas DOT. For example, HPC is making it possible for Texas DOT to design and build a 48-m (157-ft) bridge span on a section of U.S. Route 67 in San Angelo (see sidebar).
According to Susan Lane, research structural engineer with FHWA, the showcase served its purpose. "A number of States came away with a better understanding of HPC technology and very interested in the use of HPC in bridges," she says.
The showcase was sponsored by FHWA and Texas DOT, in cooperation with the Center for Transportation Research (CTR) at the University of Texas at Austin. It was developed by CTR and Texas DOT with help from the showcase steering committee, which includes representatives of the highway agency, the university, and FHWA.
The Houston showcase, like other FHWA showcase workshops, had a distinctly regional emphasis. This works particularly well for the HPC workshops, says Terry Halkyard, FHWA's project manager, because local differences in cements, aggregates, and prestressing fabricators have a considerable impact on the design and construction of concrete structures. "For example," he says, "Texas can get concrete with strengths of 90 MPa to 100 MPa (13,000 psi to 15,000 psi). In other areas, you can't get those strengths, but you can get better durability and lower permeability."
The HPC showcases will continue to reflect regional interests. States building HPC bridges are assembling reports and audiovisual materials that can be used as the basis for showcases on HPC. The same is true for States involved in building high-performance rigid pavements. "For now, we're working with local projects, training materials, and workshops based on the States' experiences," says Halkyard. FHWA, with the help of expert task groups (ETGs) on bridges and rigid pavements, will use the States' materials to develop formal showcases on HPC in structures and pavements.
The next HPC for bridges showcase will be held this November in Nebraska. A workshop has also been tentatively scheduled for spring 1997 in Virginia.
For more information, contact Terry Halkyard, FHWA, 202-366-6765 (fax: 202-366-7909).
Two of the six high-performance concrete (HPC) bridges under construction in the United States are in Texas.
The first HPC bridge to be built in Texas and the first bridge in the United States to fully utilize HPC in all aspects of design and construction is the Louetta Road overpass on State Highway 249 in Houston. The project, which is scheduled to be completed later this year, consists of two U-beam bridges carrying adjacent northbound and southbound lanes of traffic. The spans range from 37 m to 41.3 m (121.5 ft to 135.5 ft) long. Texas DOT specified a concrete mix about twice as strong as conventional mixes.
The Louetta Road overpass cost an average of $260/m2 ($24/ft2) of deck area, a price comparable to the 12 conventional concrete U-beam bridges on the same project.
Texas DOT's second HPC bridge, located in San Angelo, carries the eastbound lanes of U.S. Route 67 over the North Concho River, U.S. 87, and the South Orient railroad. The 291-m (950-ft) I-beam HPC bridge runs parallel to a 292-m (958-ft) I-beam bridge built of normal concrete, which carries the westbound lanes. The project is scheduled to be completed in February 1997.
Texas DOT chose HPC for the eastbound lanes because the span crossing the North Concho River was 48 m (157 ft) long. This distance exceeds the capability of Texas DOT's normal prestressed concrete when used in the agency's preferred 1370-mm (54-in) U-beam simple-span construction.
The San Angelo bridge presents an ideal opportunity for comparing HPC and normal strength concrete. The first spans of the two bridges are the same length and width, making it easy to compare the cost and performance of the two concrete mixes.
The San Angelo project has already provided one clear indication of the advantages of HPC. While the normal concrete lanes of the first span required seven beams with 1.7 m (5.6 ft) spacing, the HPC span required only four beams with 3.4 m (11 ft) spacing.
The San Angelo project was expensive for an I-beam bridge, but not unprecedented. The HPC eastbound bridge cost $452/m2 of bridge deck ($42/ft2), and the normal concrete westbound lanes cost $484/m2 of bridge deck ($45/ft2). The westbound bridge was more expensive because the bid cost was a flat $377 per linear meter ($115 per linear foot) for all beams on the project, whether HPC or normal concrete. Other prestressed I-beam concrete bridges built between 1989 and 1992 ranged in cost from $270/m2 to $484/m2 of deck area ($25/ft2 to $45/ft2).
High-performance concrete (HPC) is designed to provide greater durability and, if needed, higher strength than conventional concrete. HPC is composed of the same materials used in normal concrete, but proportioned and mixed so as to yield a stronger, more durable product.
HPC may be slightly more expensive than normal concrete initially, but its greater strength means that HPC bridges may require fewer supports, which could reduce overall costs. HPC structures should last much longer than those made with conventional concrete and should suffer less damage from traffic and climatic conditions. Using HPC should thus save money on repairs and reconstruction in the long run.
To promote the use of HPC in highway structures, a group of concrete experts has developed a working definition of HPC, which includes performance criteria and the standard tests used to evaluate performance when specifying an HPC mixture. The designer determines what levels of strength, creep, shrinkage, elasticity, freeze/thaw durability, abrasion resistance, scaling resistance, and chloride permeability are needed. The definition specifies what grade of HPC satisfies those requirements and what tests to perform to confirm that the concrete meets that grade. The definition was published in the February 1996 issue of the American Concrete Institute's Concrete International ("High Performance Concrete (HPC) Defined for Highway Structures," by C. Goodspeed, S. Vanikar, and R. Cook).
For more information, contact Suneel Vanikar, FHWA, 202-366-0120 (fax: 202-366-7909).