George Schuler, Coreslab Structures (Omaha) Inc.
The Pacific Street Bridge over I-680 in Omaha, NE, was constructed as the culmination of ongoing research to test the impact of using 0.7-in. (17.8-mm) diameter strands in NU I-girders. The objective of the project is to develop the quality control and design criteria required to introduce 0.7-in. (17.8-mm) diameter strands at 2-in. (50-mm) horizontal and 2.5-in. (64-mm) vertical spacing in NU I-girders. Compared to 0.5- and 0.6-in. (12.7- and 15.2-mm) diameter strands, only half and three quarters, respectively, of the total number of strands are needed. This results in immediate labor savings in precast concrete product costs. More importantly, having the ability to introduce almost twice the prestressing force, compared to 0.5-in. (12.7-mm) diameter strands and 135% of the prestressing force compared to 0.6-in. (15.2-mm) diameter strands, could result in a significant increase in the span capability of the current Nebraska Department of Roads (NDOR) NU I-girder without having to modify the sections or acquire new forms.
This research project was funded under the Innovative Bridge Research and Deployment (IBRD) program as a collaborative effort between the Federal Highway Administration (FHWA), NDOR, University of Nebraska-Lincoln, and the Transportation Research Board. As such, the FHWA reviewed the proposal, approved it, and provided the funding. In the proposal, the 0.7-in. (17.8-mm) diameter strand was to be demonstrated as an innovative idea. Hawkins Construction Company was the General Contractor for the project, and Coreslab Structures (Omaha) Inc. produced the precast, prestressed concrete bridge girders.
Twenty 98-ft 4-in. (30.0-m) long NU 900 bridge girders were constructed for the project. Each girder weighed 90,000 lb (41 metric tons) and included a partially thickened top flange. The girders were specified to include high performance concrete (HPC) with a compressive strength of 10,000 psi (69 MPa) at 28 days, and thirty 0.7-in. (17.8-mm) diameter strands per girder.
Full-scale testing performed prior to this project had shown that no unusual web cracking was observed at the girder ends, and the current provision for end zone reinforcement of the AASHTO LRFD Bridge Design Specifications was adequate. However, additional bottom flange reinforcement was needed to enclose the strands and confine the bottom flange concrete.
High Strength Concrete
“We used a standard HPC mix design and were confident that we could deliver the specified compressive strengths for the project,” stated Michael Wilson, who serves as Coreslab Structures (Omaha) Inc.’s Quality Assurance Manager. Wilson, a 45-year veteran of the industry, is familiar with the special measures that must be employed to ensure the necessary workability of high strength, low water-cementitious materials ratio concrete mixes. “We used 865 lb/yd3 (513 kg/m3) mix with 65% Type III cement, 20% slag, and 15% Class C fly ash at a 0.28 water-cementitious materials ratio,” said Wilson. “We also employed a three-stage mixing strategy to increase the consistency of the batches.” According to Wilson, all the water, cementitious materials, and half the aggregates were placed in the mixer in the first stage, followed by the admixtures in the second stage, and finally the remaining aggregates were added for the third stage of mixing. Wilson went on to say, “Due to the extremely low water-cementitious materials ratio, we had to make sure we properly sequenced the addition of materials to allow the mixer to work efficiently.”
The average concrete compressive strength was 11,000 psi (76 MPa) at 28 days, exceeding the specified minimum strength of 10,000 psi (69 MPa). Overnight release strengths averaged approximately 7000 psi (48 MPa).
Production Challenges
When asked if the combination of HPC and 0.7-in. (17.8-mm) diameter strands changed the normal dynamics of typical casting procedures, Wilson replied, “Not at all. The larger strand diameter didn’t seem to have an effect on our normal casting procedures. The biggest challenges had more to do with the handling of the strand itself.” Wilson referred to challenges associated with getting the strands out of the coils and flexibility issues when feeding the strands through the bulkheads. He went on to say, “This was a learning process for everyone involved. I’m sure once the use of 0.7-in. (17.8-mm) diameter strands becomes more common, many of these issues will be addressed and easier ways of handling the strands will be developed. It may be as simple as using a larger coil.” When asked about the aspects of the project that went smoother than anticipated, Wilson responded, “The tensioning process. We didn’t experience any tolerance issues, everything elongated properly as planned.”
The flame cutting process to release the strands went as normal. Despite the larger tensioning force, the 0.7-in. (17.8-mm) diameter strands seemed to have less reaction when cut than smaller diameter strands. There were no unusual cracks in the beams with the reinforcement provided. Dennis Drews, Project Consultant with Coreslab Structures, agreed with Wilson’s remarks regarding the larger strand size. “Strand availability was definitely a concern,” stated Drews, “We also experienced challenges acquiring the larger hold-down devices necessary to execute the draped strand pattern.” Drews went on the say, “Due to the fast-track nature of this project, the bridge designers responded immediately to this challenge and engineered an alternative strand pattern, eliminating the need to employ the draped strand approach.” “Everyone worked together to get the job done,” added Drews. “There was a great team in place for this project. We look forward to more opportunities to serve in the future and are excited to have played a role in producing the first bridge in the United States to utilize this unique strand size.”
More Information
For more information regarding this research project, please visit http://rip.trb.org/browse/dproject.php?n=13599.