April 20th, 2012
LIGHT ROCKSPreserve Infrastructure, by Catherine Higgins.
Lightweight concrete proved to be a good solution for a deck replacement project in rural Utah.
Lightweight concrete is not commonly used for constructing bridge decks according to Joshua Sletten, Structures Design Manager at UDOT. Some of the barriers to using lightweight concrete are availability of aggregate and slightly higher cost. Additionally “many engineers simply aren’t familiar with it and may shy away from it for that reason.”
However, light weight concrete was a good choice for the Taggart Bridge. The twin structures that carry I-84 over the Union Pacific Railroad were originally built in 1967. Lightweight concrete allowed the bridge deck to accommodate a thicker deck and asphalt overlay to meet the adjoining freeway profile and not exceed the load capacity of the older, pre-stressed concrete girder bridges.
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The bridge geometry, along with the need to keep the freeway open during construction, presented challenges that were met by UDOT, Hanson Structural Precast and Granite Construction Company Inc. Design was the project’s first hurdle.
Both bridges are three-span structures on a curved alignment. A total of sixty individual deck panels were designed, and “no two panels were the same,” according to UDOT Design Engineer Robert Nash. He kept the outside dimensions the same where possible but “the location of shear blockouts and leveling devices were different for every panel.” Each panel was designed utilizing reinforcing bars grouted into the top flanges of the concrete beams.
Hanson created precise shop drawings for each pre-cast panel. An indoor pre-cast yard made Hanson immune to weather delay, and a rigorous internal quality control process eliminated fit issues at the construction site. Panels used concrete with Expanded Shale Lightweight Aggregates from Utelite Corporation, a local supplier.
Granite Construction achieved UDOT’s aggressive construction schedule requirements while keeping traffic moving during construction. Workers even kept pace during snow flurries and low temperatures that would have stopped a cast-in-place deck pour.
In addition to the deck replacement, Granite also completed extensive substructure repair work on columns, pedestals, bent caps, wingwalls beam ends and backwalls. The project was completed ahead of schedule and under budget and won Granite’s project of the year award among entries costing up to $5 million and UDOT’s Rural Project of the Year.
The lightweight concrete deck panels seem to be performing well, according to Sletten. “I think you will see it utilized more frequently in the future.”