Raymond D. Cook, Utah Department of Transportation
The 17-mile (27-km) long, $1.59 billion I-15 Reconstruction Project in Salt Lake City, Utah, was the nation’s largest design-build highway project awarded to a single contractor. The project included the design and construction of 142 bridges with 1783 prestressed concrete girders and more than 445,000 cu yd (340,000 cu m) of structural concrete. The project’s scope and fast-track schedule, which involved complete design and construction within 4-1/2 years, required that the Utah Department of Transportation (UDOT) and Wasatch Constructors use a number of innovative engineering solutions including the following:
- Precast, prestressed concrete girders with high strength concrete
- Spliced post-tensioned concrete girders
- Cast-in-place (CIP) concrete bridge decks with 5 percent silica fume (SF) for all bridges
- Precast, prestressed concrete deck form panels topped with CIP SF concrete
- Girder spacings up to 20 ft (6 m) and transversely post-tensioned concrete decks to minimize the number of steel girders
Precast, prestressed concrete girders were the design-builder’s preferred bridge members due to their lower construction and maintenance costs. The designers developed modified Nebraska bulb-tee girders with depths of 41.3, 57.1, and 72.8 in. (1050, 1450, and 1850 mm). The top flange width for all girders was 50 in. (1275 mm) and the bottom flange width was 38.4 in. (975 mm). A concrete compressive strength of 7500 psi (52 MPa) at 28 days was used to minimize the number of girders. Multi-span girders were made continuous at intermediate supports.
Eight interchanges were reconstructed as single point urban interchanges. The ramp geometry at these locations required bridges with clear spans up to 230 ft (70 m). At these locations, Wasatch Constructors chose to use spliced post-tensioned concrete girders. The girders were modified Nebraska bulb-tees similar to the pretensioned concrete girders, but with a depth of 94.5 in. (2400 mm). Each girder was cast in three sections. Each section was pretensioned to support dead loads and construction loads applied prior to post-tensioning. Intermediate girder ends were temporarily supported by falsework bents at the splice locations until the splice diaphragms and concrete deck were placed. The girder and deck composite section was then post-tensioned and the falsework bents removed to provide a single span bridge.
All structures required a predicted 75-year service life. To reduce corrosion potential, deck expansion joints were eliminated wherever possible and all reinforcing steel was epoxy coated. In addition, UDOT required that all CIP concrete bridge decks include 5 percent SF by weight of cementitious materials or an initial overlay. The design-builder chose to use SF concrete. In addition to SF, the structural concrete mixes included air entrainment, Class F fly ash, and a low range water-reducing admixture. Post-tensioned concrete decks required a 28-day compressive strength of 5000 psi (35 MPa). All other CIP concrete required a 28-day compressive strength of 4000 psi (28 MPa). After finishing, curing compound was applied to the deck surface followed by a seven-day water cure. Concrete temperature was maintained above 50°F (10°C) for at least the first seven days after placement.
Early in the project, the SF concrete mix caused difficulties with workability and finishing due to the stickiness of the surface. Placement procedures and specifications were evaluated and modified to reflect the needs of the SF. The allowable slump was increased to improve the ability to work with and finish the concrete. Controlled fogging was allowed to increase the humidity above the fresh concrete and minimize moisture loss from evaporation until the deck could be finished and the curing compound added. Fogging helped prevent drying of the concrete surface and the formation of a “skin” on the concrete. This had occurred early in the project leading to difficulties with finishing. Whenever possible, decks were placed during the cooler temperatures at night to reduce the evaporation of surface water.
Precast, prestressed concrete deck form panels were used on most precast, prestressed concrete girder bridges. Panels were 3.5 to 5.5 in. (90 to 145 mm) thick and were topped with CIP SF concrete to make an 8- to 10-in. (205- to 255-mm) total deck thickness. Panels were temporarily supported on the girder flanges using medium density polystyrene or preformed joint filler until the CIP concrete provided permanent support to the edges of the panels.
A research project is currently underway to evaluate the deck cracking that occurred on some of the I-15 bridges and to identify potential causes and remedies.
Further Information
For further information, see “I-15 Project Paves Way for Hybrid Precast Girder,” ASCENT®, Spring 1999, pp 24-27; or contact the author at 801-951-1026 ext. 319 or [email protected].