Better Roads Staff
The replacement bridge was designed to have a 23-degree skew with four spans to carry three lanes of traffic, the authors write. All the major bridge elements including piers, abutments, I-girders and full-depth deck panels were prefabricated off-site. The superstructure is composed of PC-I Type III AASHTO girders, and the deck is composed of 48 9-inch-thick, partial-width, precast reinforced concrete panels.
Precast girders were pre-tensioned, and flared coil inserts were embedded in their top flanges, used to insert shear studs that facilitate girder-deck connection through shear pockets. Once the north and south panels were installed onsite, they were connected transversely using a reinforced cast-in-place closure.
“The north and south panels were cast to different widths; thus, the closure was about 5-feet offset from the bridge centerline,” Attanayake, Abudayyeh, Aktan and Cooper say. “Transverse connection between full-depth deck panels was established using grouted shear keys and longitudinal post-tension. The special provisions required the use of nonshrink grout.“
In the substructure, each abutment consists of two precast stems, they wrote. The stem splice was formed using cast-in-place concrete. Each abutment is supported on 16 H-piles. The plans specified placing of precast abutment on the grade level, maintaining pile embedment of 2.5 feet and filling the pile sleeve with grout. Each pier consists of four precast concrete columns that are mounted on a cast-in-place footing. The design specified connecting the pier columns to the footing using square pockets casted in the footing. The pier columns support a precast pier cap.
Special provisions to implement this new technology were prepared by Michigan DOT and were supplied to the contractor.
“Precast full-depth deck panels were placed in four days,” they write. “After placing the panels on the girders, longitudinal post-tensioning duct misalignment was noticed. This was due to a calculation error by the contractor during the prefabrication process. Bridge owner, design engineers and the contractor explored all potential solutions to salvage some or all of the deck panels that were already placed on the girders. Several options were considered, however the contractor chose to re-cast the deck panels after accurately incorporating the skew in the calculations. The ‘cast-match’ technique was used to ensure the correct alignment of post-tensioning ducts.”
Promoting the prefabrication at the jobsite or at a nearby location owned by the department may reduce construction costs and can minimize the impact of load restrictions, say Attanayake, Abudayyeh, Aktan and Cooper. However, setting up a certified plant at or near the site for a small bridge can be costly.
“Both the PCP and PPCP systems are performing satisfactorily after one year of traffic, and the contractor was also satisfied with the constructability.”
“Casting of large components such as abutment stems, pier columns and pier caps can be an option due to their weight and less-complicated details,” they add. “Fabrication of circular columns requires vertical formwork while pouring concrete and curing. Columns with rectangular sections can make the fabrication process easier, hence allowing onsite production. Language should be considered in the special provisions to complete prefabrication of components before demolition of the existing structure.”
Precast Slabs on I-66
A Virgina Interstate test and comparison to CIP
Both standard precast and precast/prestressed pavement slabs are performing in a high-level Interstate highway installation, say M. Shabbir Hossain, Ph.D., P.E., and Celik Ozyildirim, Ph.D., P.E., Virginia Center for Transportation Innovation & Research, Charlottesville, Va., in their paper, Precast Concrete Pavement on I-66 in Virginia.
“To expedite construction and reduce the associated traffic delay and provide longevity, precast concrete slabs have been used for more than 10 years with successive improvements in processes and systems,” the authors write. “The Virginia DOT has recently tried two precast systems along with conventional cast-in-place repairs on I-66 near Washington, D.C.”
There, the existing pavement was jointed, reinforced-concrete pavement. One precast system used reinforced slabs with doweled joints and is called Precast Concrete Pavement (PCP). The other system used transversely prestressed slabs, post-tensioned in the longitudinal direction, and is called Prestressed Precast Concrete Pavement (PPCP).
Three different mixtures were used for the cast-in-place (CIP) patches, PCP panels and PPCP panels, with varying cement contents of 846, 518 and 602 lb./cu. yd., respectively, and water/cement ratios of 0.32, 0.34 and 0.36, respectively, they report. PCP panel mixtures used 172 lb./cu. yd. of slag, and PPCP panel mixtures had 150 lb./cu. yd. of Class F fly ash. Cast-in-place cylinders were cured using a standard wet cure, but the cylinders for both precast systems were cured using radiant heat.
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