Mark A. Leonard, Colorado Department of Transportation
The replacement bridge for Interstate 25 over Yale Avenue in Denver, Colorado, is an excellent example of using high performance concrete (HPC) to meet the demands of urban bridge replacement. In growing urban centers, designers need to replace deteriorating bridges without changing existing vertical alignments, while providing for wider roadway sections on and beneath the bridges. This calls for longer spans at reduced superstructure depths, and bridges that can be built quickly with little disturbance to traffic.
The new bridge replaced a four-span, cast-in-place T-girder bridge that was structurally deficient, largely due to deck deterioration. This necessitated traffic closures when portions of the deck fell to the roadway below. The Colorado Department of Transportation (CDOT) needed to build the new bridge without lane closures or grade changes to either I-25 or Yale Avenue because of the high traffic volumes and the restrictive urban setting. CDOT also wanted to improve the vertical clearance over Yale by 18 in. (460 mm). This resulted in a span for the new bridge of 112 ft (34 m) with a superstructure depth of only 3 ft (0.9 m).
A two-span replacement bridge was selected to reduce construction time and disturbance, and to improve sight distances below the bridge. After investigating precast, cast-in-place, and structural steel alternatives, a side-by-side precast, pretensioned box girder bridge, with a partial-depth composite deck, was selected as the optimum solution. The partial depth deck was used to minimize superstructure depth. Given the sensitivity of the composite partial-depth system to chloride intrusion and the severe deterioration of the old bridge deck, durability of the new deck concrete was a primary concern. HPC was the solution.
The use of 10,000 psi (69 MPa) concrete in the girders was necessary to achieve the desired span-to-depth ratio. In the deck, 5,000 psi (35 MPa) concrete was needed to provide the necessary compression block for the flexural strength of the superstructure. To provide enough prestressing, CDOT’s practice of using 0.5-in.(12.7-mm) strands at 1.75-in. (44-mm) spacing was inadequate—0.6-in. (15.2-mm) diameter strands spaced at 2 in. (50 mm) were needed. The prestressing density and the concrete strengths, however, exceeded CDOT’s limits at the time.
The Federal Highway Administration ( F H WA) HPC implementation program allowed CDOT to use the greater prestressing density and higher concrete strengths by funding testing and verification before construction. A partnership was formed between FHWA, the University of Colorado (CU), and CDOT’s Bridge, Materials, and Research offices to conduct the testing and verification. This work was continued through construction to provide field verification of the laboratory information. FHWA’s program also allowed CDOT to investigate different practices and materials to improve the durability of concrete decks in the state.
CU conducted scale model testing of the actual bridge girders and successfully verified the development and transfer lengths of the strands, and the ultimate flexural strength behavior at the high reinforcement index. The production of the test girders also demonstrated that the girders could be successfully fabricated with the high strength concrete and 0.6-in. (15.2-mm) diameter strands. CDOT’s Materials Office conducted a series of tests before and during construction to define the durability characteristics of CDOT’s standard bridge deck concrete and the concrete used for the Yale Avenue project. CDOT’s Research Office instrumented the completed bridge to provide the field performance cross reference to CU’s laboratory results.
The HPC concrete used in the deck was also specified for the piers. This provided high durability for the columns in the splash zone of Yale Avenue, and high strength for the economical reduction of the number of columns beneath the bridge; thereby, improving aesthetics and sight distance. The previous bridge was 215 ft (65.5 m) long by 110 ft (33.5 m) wide and had three piers with a total of forty-five columns. The new bridge is 215 ft (65.5 m) long by 138 ft (42 m) wide, and has one pier with four columns. This is a dramatic example of how deliberate use of current materials technology during design can yield significant benefits. The repeated use of the I-25/Yale Avenue solution at other sites in Colorado shows standard practice can be changed through the use of new technology.
Further Information
For further information about the I-25/Yale Avenue demonstration project or to obtain a copy of CU’s report, Study on Transfer and Development Length of Prestressing in HPC Girders, contact the author at [email protected].