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24th Street Bridge, Council Bluffs, IA.
Photo: Keith Philpott
HPC Precast Panels Provide Accelerated Construction of 24th Street Bridge
Ahmad Abu-Hawash, Iowa Department of Transportation and Hussein Khalil, HDR Inc.The Council Bluffs Interstate System project affects 11 interstate-to-local road interchanges as well as three interstate-to-interstate interchanges along an 18-mile (29 km) stretch of I-80 and I-29 in Council Bluffs, IA—a corridor that is carrying more than double its originally intended traffic capacity. The first segment scheduled for construction was the 24th Street interchange, an important arterial serving several major attractions and businesses.
For the project to be successful, it was essential to maintain three lanes of traffic throughout construction of the new interchange. Consequently, each half of the bridge was replaced in separate phases. It also was determined that traffic restrictions should be limited to a single construction season (April through October) as opposed to the two seasons it typically would take to complete a project of this scope. As a result, maintaining traffic flow and the accelerated construction schedule became the driving forces in the bridge’s design.
HPC Precast Panels Prevail
Because of the need to construct the bridge quickly and avoid closures to the interstate highway below, the design team selected full-depth, full-width precast deck panels in lieu of a traditional cast-in-place concrete deck. Each panel was 10 ft x 52 ft 4 in. x 8 in. (3.05 m x 16.0 m x 200 mm) and transversely pretensioned with twenty 0.5-in. (13-mm) diameter low relaxation strands initially tensioned to 31 kips (138 kN) each. Twenty-three flat polyethylene ducts were embedded near the top of each panel for longitudinal post-tensioning after erection.
The panels were placed in two primary phases of 35 panels each, with a cast-in-place longitudinal closure joint between each half. A 2-in. (50-mm) thick low slump, high density concrete overlay riding surface provided an additional layer of corrosion protection and provided the final profile. This project was the first Iowa Department of Transportation (IaDOT) project to use high performance concrete (HPC) in the western part of Iowa. Due to material suitability, use of HPC in western Iowa had been at an impasse.
Methods to level the panels and form the slab build-up below the deck panels were left to the contractor; however, the plans included optional details that could be used by the contractor to aid in setting the panels to the correct elevations. The contractor elected to level the panels with leveling bolts and to form the slab build-up using lumber power nailed to the bottom of the deck. The transverse joints were filled with conventional high-strength, non-shrink grout.
A female-to-female transverse joint between panels eliminated the need for match-casting and reduced the risk of damaging panel edges during erection and post-tensioning. Experiences on other projects showed this type of joint tended to perform better than other joint types, especially where longitudinal post-tensioning had been utilized. Details of the transverse joint and a plan view layout are provided in the reference below.
The longitudinal post-tensioning consisted of four 0.6-in. (15.2-mm) diameter strands in each of the 23 ducts. Each strand was initially tensioned to 41 kips (182 kN). Longitudinal post-tensioning force was applied after the grout in the transverse joints had attained the required strength of 6000 psi (41 MPa). The amount of post-tensioning force was computed in order to eliminate any tension in areas where auxiliary reinforcement was not provided. Tension in the panels was caused by the composite dead load, live load, and impact in the negative moment region near the pier. This criterion controlled the post-tensioning design at the transverse joints between the panels. After post-tensioning, the deck panels were made composite with the steel girders using steel studs installed through pockets in the panels.
Analysis to determine the needed amount of post-tensioning showed that both the age and concrete strength of the panel at the time of post-tensioning have a significant effect on the amount of losses. For example, a 6000 psi (41 MPa) strength panel would be required to be 100 days old before the post-tensioning force could be transferred to the concrete, while a 12,000 psi (83 MPa) panel would need to be 28 days old before post-tensioning. With an October letting date and an expected erection of the panels in June, this would require an accelerated winter fabrication schedule and storage of the panels—resulting in an economic disadvantage to the project. To avoid this situation and to provide as much flexibility as possible during construction, the precasting contractor was given the option of designing a concrete mix that would yield the required design strength while accommodating the construction contractor’s accelerated schedule and minimizing fabrication costs. This resulted in a specified compressive strength of 11,000 psi (76 MPa) for post-tensioning at 28 days, 10,000 psi (69 MPa) at 40 days, 9000 psi (62 MPa) at 70 days, or 8000 psi (55 MPa) at 100 days. The contractor selected a target strength of 9500 psi (66 MPa). The panel ages ranged from 30 to 75 days for Phase 1 and 60 to 100 days for Phase 2. All panels in Phase 2 were erected in less than 1 day.
The concrete for the deck panels was specified to have a target rapid chloride permeability of 1500 coulombs at 28 days. The test specimens were wet cured at 73°F (23°C) until age 7 days followed by water curing at 100°F (38°C) for 21 days. The concrete mix design contained 865 lb/yd3 (513 kg/m3) of cementitious materials consisting of 65% portland cement, 20% Grade 120 ground granulated blast-furnace slag, and 15% Class C fly ash at a water-cementitious materials ratio of 0.28. The panels were steam cured after removal from the formwork and achieved concrete compressive strengths ranging from 9140 to 12,470 psi (63.0 to 86.0 MPa). The average strengths were 11,490 and 10,160 psi (72.2 to 70.1 MPa) for Phases 1 and 2, respectively. This project was partially built with funding from the FHWA Highways for LIFE (HfL) initiative and the Innovative Bridge Research and Construction (IBRC) program. The project was substantially completed in 179 days.
The design incorporated details from past projects, as well as the latest in research in the areas of deck panels. Coordination among the designer, the owner, local contractors, and fabricators was key to developing an economical design that could be constructed under an accelerated timeframe, while minimizing disruption to the traveling public and enhancing safety during and after construction. To date, no reflective cracking in the overlay above the joints has been observed.
The authors would like to thank Norm McDonald, James Nelson, and Kimball Olson, of the Iowa Department of Transportation; Brent Phares, Iowa State University; and Phil Rossbach, HDR Inc. for their contributions to this article.
For further information about this project, please contact the second author at firstname.lastname@example.org. or see the following reference.
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 59, Jan/Feb 2010