Credit: Photo: Fort Miller Co. Inc.; Colorado DOT
Far left: Replacing deteriorated concrete pavement with precast panels at the New Jersey portal of the Lincoln Tunnel took just a few hours during weekend closures. Left: A double layer of Grade 60 #4 rebar strengthens precast panels that were produced at Colorado Precast Concrete in Loveland for a northern Colorado project that used Uretek USA under-sealing.
Credit: Photo: Transtec Group
A worker surveys a 124-foot, post-tensioned section of precast pavement placed on Interstate 10 in California.
Post-Tensioned Precast Slabs
In a March 2002 pilot project for a post-tensioned system, Granite Construction out of Watsonville, Calif., installed 340 precast, prestressed panels on a frontage road along Interstate 35 near Georgetown, Texas. Caltrans sponsored the next pilot project in April 2004, modifying several design features and requiring nighttime construction on Interstate 10 just outside Los Angeles.
Both the Texas and California projects are part of FHWA's ongoing Concrete Pavement Technology Program Task 58. This program also has granted funding ranging from $100,000 to $400,000 and design and onsite support from The Transtec Group of Austin, Texas, on two additional demonstration projects in Texas and Missouri.
The post-tensioned system consists of three different panel types: joint panels, central stressing panels, and base panels. Joint panels located at the end of each post-tensioned section contain dowelled expansion joints to absorb horizontal slab movements. Central stressing panels at the middle of each post-tensioned section contain large pockets or block-outs where the post-tensioning strands are fed into the ducts and stressed. Base panels make up the majority of the pavement and are placed between the joint panels and stressing panels.
In the Georgetown project workers placed 8-inch-thick panels on a thin, hot-mix asphalt leveling course topped with a polypropylene liner to prevent the precast from bonding with the base. Then the contractor post-tensioned the panels in the longitudinal direction in 250-foot sections. Shear keys along the edges—comparable to tongue and groove joints—ensure alignment during installation and interlock the panels to prevent differential vertical movement between joints.
In the Caltrans project, California-based Yeager Skanska placed 31 panels in two nights on a lean concrete base and post-tensioned the panels in the longitudinal direction in 124-foot sections. Pomeroy Corp. of Peris, Calif., manufactured the panels to span 37 feet—the full width of two lanes and the shoulder. Caltrans specified that the top surface of the precast panels vary in cross slope from 2% on the driving lanes to 5% on the shoulders. Caltrans officials expect the precast panels, placed in one of the country's heaviest traffic areas, to last more than 50 years.
Texas DOT, with FHWA and Transtec, is planning a second precast demonstration project on Highway 175 southeast of Dallas for fall 2005 or spring 2006. These panels will have pits precast to accommodate weigh-in-motion scales. On a rural section of Interstate 57 near Sikeston, the Missouri DOT will install five 8-inch-thick precast sections to replace 45-year-old slabs in a heavy truck traffic area.
Caltrans has worked with all three methods for replacing highway concrete with precast. Pyle said, “This option has potential to be used in both short work windows and in weekend closures when we will close a portion of the freeway for 55 hours and work to achieve literally weeks of work, during a continuous, focused work window. The goal is that the public, who commuted on the freeway on Friday morning, will see a completely new pavement, for potentially up to a couple of miles, on the following Monday.”
— Carder is a freelance writer based in Denver.