Launch Slideshow

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Paving the Way

Paving the Way

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    Transtec Group Inc.

    General precast panel layout for PPCP

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    Pomeroy Corp. manufactures a wide range of precast products, including bridge girders, crossties, parking structures, piles, segmental bridges, and prison cells.

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    The first precast-prestressed concrete pavement project in the U.S. was near George-town, Texas, in 2001-02. The panels were fabricated on a 400-foot-long casting bed.

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    Each precast panel for the I-10 project in El Monte, Calif., was 37 feet long and 8 feet wide.

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    The panels for the I-57 project near Sikeston, Mo., were post-tensioned onsite.

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Missouri

This project on I-57 near Sikeston included 1010 feet of mainline pavement. The panels spanned the full 38-foot width of the road, including two lanes and inside and outside shoulders. Each of the 101 panels was 10 feet wide. They were pretensioned in the transverse direction during fabrication using eight to 12 ½-inch strands per panel. They were post-tensioned onsite in 250-foot sections with 0.6-inch mono-strand tendons spaced at 2 feet on-center across the width of the road.

CPI Concrete Products of Memphis, Tenn., fabricated the panels, casting and steam-curing two panels at a time on a self-stressing casting bed. Panel fabrication was complex on this project as well, with variable thickness panels achieving crowned pavement cross-sections.

Panel thickness varied from 7 inches at the inside shoulder edge, to 11 inches at the pavement crown, to 5 5/8 inches at the outside shoulder edge. Also, because of the variation in thickness, the keyways along the edges of the panels were not continuous across the full width, terminating within the shoulder sections of the panels.

Plain, doweled expansion joints, with a recess provided for site-installed header material, were cast into the panels. The post-tensioning pockets were moved next to the expansion joints rather than at the middle of the pavement.

Iowa

This project was a unique application for PPCP, as it included bridge approach slab construction. Many states face the problem of how to reconstruct approach slabs that have failed because of a loss of support at the bridge abutment, while minimizing disruption to the traveling public. The Iowa DOT evaluated using PPCP for this purpose in Sheldon in 2006.

Both approach slabs at either end of the new bridge were constructed with PPCP panels. Eight precast panels were used for each approach slab—four for each lane on either side of the roadway centerline. The total length of each approach slab was about 77 feet at the centerline.

IPC Inc. of Iowa Falls fabricated the panels, with each panel cast and heat-cured separately on an indoor fabrication bed. The typical precast panel size was 14 feet wide by 20 feet long, and 12 inches thick. However, the panels abutting the bridge were trapezoidal in shape to accommodate the skewed bridge abutment.

In lieu of plant pretensioning, the panels for each approach slab were post-tensioned together onsite in both the transverse and longitudinal directions. The post-tensioning consisted of 0.6-inch monostrand tendons in both directions. Keyways were formed into both the transverse and longitudinal mating panel edges. The keyway for the longitudinal joint at the roadway centerline was an open keyway that was filled with a concrete mortar onsite just before completing transverse post-tensioning. Each approach slab was installed in two days.

Alaska

This project utilized PPCP for rebuilding a loading bay exit at Red Dog Mine. When 200-ton haul trucks making sharp turns caused problems by rutting and shoving the tundra material at the loading bay exit, a more durable, deformation-resistant material was necessary. Because this is a 24-hour mining operation, it was essential to minimize shut-down time of the loading bay for construction.

Central Pre-mix Prestress Co. of Spokane, Wash., fabricated eight precast panels. They had a uniform 8-inch thickness, but were each unique in shape to create a “fan-shaped” finished pavement around a horizontal curve. The panels varied in length from 18 to 28 feet, and from 5 to 7½ feet in width.

Crews pretensioned the panels during fabrication in the transverse direction with eight ½-inch strands per panel, and post-tensioned onsite with five four-strand (½-inch strand) tendons per panel.

The complex geometry of the precast panel layout and specific panel dimensions required strict tolerances and careful attention to the panel fabrication process, but still did not require match-casting. The panels were installed and mine traffic was open within 48 hours in fall 2002.

The author is project manager for The Transtec Group Inc., an engineering firm in Austin, Texas, specializing in pavements and pavement materials. E-mail dmerritt@thetrans tecgroup.com.

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