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.
‘With a cost factor of materials and labor as high as five [times more] in the early stages of this technology, using precast slabs for highways will never replace traditional concrete paving,” said Sam Tyson, P.E., Federal Highway Administration (FHWA) task manager in the Concrete Pavement Technology Program. “However, precast systems are worth investigating for rapid overnight repairs where pavements cannot be closed.”
Currently, DOT and tollway authorities are installing precast replacements of concrete highway slabs using three different systems—Super-Slabs on engineered grades supplied by New York-based Fort Miller Co. Inc.; precast slabs undersealed with Texas-based Uretek USA's polyurethane foam; and post-tensioned, precast-slab demonstration projects sponsored by FHWA.
“While rapid set concrete can accomplish overnight repairs, engineers need more options—more tools in the tool box—to tackle the problem of replacing destroyed pavement on busy freeways within a small construction window,” said Tom Pyle, California Transportation Department (Caltrans) chief, office of rigid pavement. He extols the potential advantages of precast because it is durable, high-quality concrete with a high cement content cured under controlled conditions and has additional steel to provide the strength needed to transport and suspend the panels.Slab-On-Engineered-Grade
Peter Smith, Fort Miller vice president of market development and product engineering, estimates his company has 270,000 square feet of precast slabs—an amount equal to 4.3 lane miles—in service in New York and New Jersey, and 85% of these lane miles carry more than 100,000 vehicles daily.
For intermittent repairs, Fort Miller produces a menu of sizes, 12-foot-wide by 8-, 10-, 12-, and 14-foot-long, and has developed a hand-operated laser for preparing individual, engineered subgrades. The patented Super-Slab system relies on standard dowel bars for load transfer. Dowel bars are cast in one slab and corresponding dovetailed slots with two grout ports at the top are cast in the adjoining slab. For single intermittent repairs, the panels have slots cast into both ends to receive dowels epoxied into the existing pavement. After installing the slabs the contractor comes back and pumps structural grout into each dowel port until grout exits the opposite port. Although the engineered grade is accurate to within 1/8 of an inch, each 12-foot-wide Super-Slab still has three grout channels to allow for distribution underneath the slab. After setting the slab on the grade, the contractor pumps in bedding grout to fill any voids.
“One feature of our system that's unique is we cast cross slopes in the precast,” said Smith. When Fort Miller engineers lay out a job, they calculate the XYZ coordinates on every corner of every slab. Then the XYZ data are used to cast slabs with the proper cross section and profile for each particular section of the road. The same data are used to prepare a matching subgrade surface.
Fort Miller's largest intermittent repair project to date involved placement of 514 slabs on Interstate 90 near Albany, N.Y., in the summer of 2004. The prime contractor, Fahs Rolston out of Binghamton, N.Y., bid the project using rapid-set concrete and then brought in the Super-Slab system in a value-engineering proposal. The company explained that with rapid-set concrete, the contractor has only a four-hour work window in the eight-hour closure. The remaining four hours provide cure time for the concrete. Installation of 378 precast slabs on a critical section of the heavily traveled six-lane highway required 47 night closures—about half the time estimated for rapid-set concrete. By sawcutting the panels to be removed the night before and grouting the panels the night after, the contractor averaged placement of 15 slabs of 10-foot lengths each night.
Other significant projects using this system include replacement of highway slabs at the New Jersey portal of the Lincoln Tunnel in July 2003 and a taxiway repair at Dulles International Airport in November 2002. This May, Caltrans District 8 will perform a heavy vehicle simulator (HVS) test at Fontana, Calif., on Fort Miller Co.'s precast highway panels. Based on the results of the HVS testing, replacement with precast may be included in a rehabilitation project on Interstate 15 in San Bernadino in the spring of 2006, according to David Thomas, Caltrans design engineer.Undersealed Precast Slabs
The Uretek USA method of precast replacement began in Colorado in 2000 when a group of Colorado DOT highway maintenance supervisors were brainstorming a way to repair heavily traveled concrete highways with fewer interruptions to the traveling public and fewer safety concerns. They questioned, “Can we use precast concrete? And if Uretek's polyurethane foam can lift and support a sunken slab, could it anchor a precast slab?”
On Highway 287 north of Fort Collins, Colo., during the first week of December 2000, a Colorado highway team successfully installed Colorado's first precast concrete replacement panels within a critical time schedule. This team effort cut a normal 16-hour or more repair job in half.
On the first Colorado projects, Colorado Precast Concrete of Loveland manufactured each precast panel to exacting shop drawings then cast the panel to within a 1/8-inch tolerance. The panels included slots for the Uretek Stitch-in-Time fiberglass load-transfer devices and nickel-sized ejection holes for the Uretek 486 foam that anchors and levels the panel. The injection-hole pattern ranges from 18 inches to 3 feet on center with 15 to 17 holes per panel. Two mats of Grade 60 #4 black rebar are used in each panel. Four rebar span in both the longitudinal and the transverse directions. Each slab weighs up to 25,000 pounds and attains a strength in excess of 6000 psi in 28 days of curing.
On all of the Colorado projects, the contractor sawcut the pavement the day or night ahead of installation, both to expedite the project and to give the water time to evaporate. The contractor also cut a slot into adjacent panels for the 3-foot-long by 5-inch-deep by ¼-inch-thick load-transfer devices. Prior to installation, Concrete Stabilization Technologies of Denver injected the foam under adjacent slabs for sealing and stabilizing purposes allowing the slabs to better support the load-transfer devices.
To install the panel, the contractor excavates at least 2 inches below final grade and prepares the subgrade with compaction and a Class 6 road base. A crane lowers the panel into place, and the workers inject foam bringing the panel up to grade. The foam reaches 90% of its strength in just 15 minutes. Next, the workers insert the fiberglass load-transfer devices and seal with Uretek's joint sealer material.
In August 2004 TLM Constructors Inc. of Greeley and its subcontractors applied the lessons learned in four years of Colorado projects to the intermittent placement of 250 panels on Interstate 25 in the Mead to Fort Collins area. Last year TLM installed 150 precast panels, and this month the contractor plans to install 60 linear feet—or about 10 panels—each night to complete the project.