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High strength, flowable concrete was used in the curved and straight girders.
High Strength Concrete for the Girders of Colorado's SH58 Ramp A Flyover
Gregg A. Reese, Summit Engineering Group, Inc. and Jim Fabinski, EnCon ColoradoThe $31 million State Highway 58 (SH58) Ramp A Bridge in Golden, CO, features a state-of-the-art design using curved, precast concrete bridge girders to overcome the serious challenges that arise when creating complex highway interchange projects. This latest project is the fifth of six projects to date to use this technique in Colorado. This project demonstrates the benefits of this approach in constructing cost-effective, complex, long-span structures in high-profile locations, where aesthetics and urban geometrics are significant design considerations.
The ramp, which connects eastbound I-70 traffic to westbound SH58, was value-engineered by the general contractor, Ames Construction, Inc., to use curved girders. The 10-span bridge crosses Clear Creek, a bike path, three traffic openings, east and westbound I-70, and eastbound SH58.
The roadway consists of a 38-ft wide deck designed for three traffic lanes, but currently accommodates one traffic lane and two wide shoulders. Its alignment consists of a spiral curve with a minimum radius of 809 ft (247 m), which transitions to a tangent section at the bridge’s end.
The superstructure consists of two lines of 86-in. (2.18-m) deep modified Colorado Department of Transportation U84 girders spliced near the quarter points of the typical span. The bridge begins in a spiral curve in Unit 1, which continues through Unit 2 and transitions in Unit 3 to a straight section. The first and last pairs of girders in the spiral curve were cast at varying radii to form the beginning and end of the spiral curve. The remaining girders in the central curve were cast with an 809-ft (247-m) radius for both girder lines. The straight girders in Unit 3 were precast in a conventional girder bed. The superstructure contained 30 curved and 8 straight precast concrete girders. Girders varied in length from 93 ft 2 in. to 119 ft 7 in. (28.4 to 36.5 m) and weighed from 220 to 265 kips (978 to 1179 kN).
The girders were designed for handling and erection loads with varying levels of prestressing in the bottom slab. Curved girders used post-tensioning tendons. Straight girders were designed with conventional pretensioning.
High Strength Concrete
The girder concrete was specified to have a compressive strength of 6500 psi (45 MPa) for initial post-tensioning or pretensioning and a 28-day strength of 8500 psi (59 MPa). Measured strengths ranged from 7500 to 10,000 psi (52 to 69 MPa) at 16 hours using match-cured cylinders and from 11,000 to 13,000 psi (76 to 90 MPa) at 28 days. The concrete mix proportions are given in the following table.
1. Set retarders and accelerators were used as weather conditions required.
2. 3/8 in. (9.5 mm) maximum size pea gravel.
The formwork for the curved girders used 10-ft (3.05-m) long chorded sections. Each 7 ½-in. (190-mm) thick web contains four 3-in. (75-mm) diameter post-tensioning ducts. The 8-in. (200-mm) thick bottom flange has significant nonprestressed reinforcement. All reinforcement was delivered straight and bent to the curve as it was tied in place. Welded wire reinforcement in 8-ft (2.4-m) wide sheets was used for the stirrups and bent to shape in one piece. All reinforcement embedded inside the girders was uncoated, whereas reinforcement protruding from the tops of the beams was epoxy coated.
The concrete was cast monolithically in the U-shaped forms. A flowable mix was designed that allowed the concrete to be placed first on one side such that it flowed across the bottom and approximately halfway up the other side. The second side was then topped off. Although the concrete had a spread of over 20 in. (510 mm), external vibration was required to help the concrete flow past the many obstacles in the girders. Special patented plastic duct chairs were developed to prevent hydraulic pressure from misplacing the post-tensioning ducts.
The use of a flowable concrete mix was essential to produce the girders. There were no signs of aggregate segregation although the concrete did require continuous monitoring. In addition to slump flow, quality control tests included air content, concrete temperature, and unit weight of the concrete. The beams were steam cured.
The success of this project and similar ones in Colorado has shown that curved, precast concrete girders are a viable option for long-span interchange construction.
For further information about this bridge, see ASPIRE™ Spring 2010.
Abu-Hawash, A., Khalil, H., Schwarz, P., Phares, B., and McDonald, N., "Accelerated Construction and Innovations, The 24th Street Bridge," Proceedings, The National Bridge Conference, Bridges for Life, October 4-7, 2008, Orlando, FL, Precast/Prestressed Concrete Institute, Chicago, IL, Compact Disc, 2008.
HPC Bridge Views, Issue 60, Mar/Apr 2010