By William N. Nickas, P.E.

During the last 40 years, precast/prestressed concrete pavement (PPCP) panels have been used in various forms, from coast to coast. Unfortunately the traveling public of 2010 has far too much time to appreciate this smooth work while sitting in bumper-to-bumper traffic. But while congestion rates have increased, forcing motorists to suffer through lane reductions necessary for service repair or replacement, new technologies and engineering techniques have continued to develop. Today, PPCP is among the advancements that allows overnight replacement of aging highway surfaces with long-lasting, highly durable concrete panels.

Post-tensioning has been applied to cast-in-place concrete pavements in a number of states and proven successful over several decades. A project completed in Texas nearly 25 years ago has continued in service showing excellent performance. More recently, a 2002 pilot project with the Texas Department of Transportation (TxDOT) and Federal Highway Administration (FHWA) in Georgetown, Texas, applied this prestressed pavement technology to precast concrete panels, resulting in a concept that subsequent demonstration projects have built on. On this project, the FHWA, worked in conjunction with the Center for Transportation Research, located at the University of Texas at Austin.

The first precast/prestressed concrete pavement project was installed in 2002 in Georgetown, Texas. The panels were as wide as 36 feet, providing two lanes and two shoulders in one panel. The workman is standing on a joint panel at the end of a post-tensioned section, and the central stressing pockets are visible in the distance. Center for Transportation Research, UT Austin

Three more projects — partially funded by FHWA — located in California, Missouri, and Iowa, used these techniques in a range of applications. The FHWA worked to provide guidance in conjunction with The Transtec Group in Austin, Texas, which served as the specialty pavement engineer.

Today, a second generation of projects using PPCP is being advanced with projects recently completed in Delaware and Virginia. Both were constructed under heavy traffic conditions, with the highways used the morning after night-time installations. Projects like these are pushing the technology further, and helping to standardize techniques for installations nationwide.

More projects will continue to be constructed in the years to come. As Kirsten Stahl of the California Department of Transportation (Caltrans) stated in 2005, “Precast concrete pavement is the future of pavement repair and rehabilitation.” The FHWA designated precast pavement as one of its “vanguard” technologies, to expand its use and promote rapid-deployment strategies. Precast pavement also is being promoted by AASHTO’s Technology Implementation Group.

Quick installation
To install PPCP, the underlying base is prepared using typical base materials, including hot-mix asphalt, lean concrete, crushed aggregate, or pervious concrete. Over the underlying base is laid a sheet of polyethylene or geotextile fabric, which reduces friction between the base and the panels so they can respond during post-tensioning.

Each section of pavement contains three types of panels, which are generally pretensioned in the transverse direction to reduce bending stresses in the panels during lifting and transportation. The panels have monostrand ducts cast through them for post-tensioning in the longitudinal direction after installation. The three types of panels are:

• a two-part joint panel, positioned at each end of a post-tensioned section. These panels feature doweled expansion joints as well as terminal anchorage for the post-tensioning tendons on either side of the expansion joint and pockets for accessing the anchors;
• base panels, making up the bulk of each section of roadway. These panels, as well as the others, feature continuous shear keys cast into their edges to ensure vertical alignment between adjacent panels, creating a tongue and groove joint; and
• either a central stressing or an anchor panel, placed at the middle of each post-tensioned section. Central stressing panels include open stressing pockets above each post-tensioning tendon’s location for tensioning. Anchor panels simply provide anchor sleeves for anchoring the middle of the post-tensioned slab to the underlying pavement structure.

Once all the panels are in place, post-tensioning strands are fed into the ducts, terminating at the anchors in the joint panels. The tendons are then tensioned either from the anchors in the joint panels or from the pockets in the central stressing panels.

Finally, the pockets in the joint panels and central stressing panels are filled, and the post-tensioning ducts are grouted full. Any voids beneath the panels are filled with grout through ports cast into the panels.

Multiple benefits result
Many benefits accrue to states, contractors, and highway users from PPCP. Foremost is the speed of construction, allowing segments to be completed each night so users can drive on the new panels — which join to the existing highway — the next morning. This ensures no disruptions to traffic during peak periods.

Because the panels are manufactured under factory-controlled conditions, cured fully, and then delivered to the site for each night’s work, no time is required for curing on site. The panels provide immediately upon placement the necessary strength, and exhibit outstanding long-term durability. This approach significantly reduces user delay costs and minimizes traffic exposure for workers.

The speed of installation can be impressive. In a recent project completed in Virginia, PPCP was installed at a rate of 120 lane feet (160 square yards) per night. This compared with rapid-setting cast-in-place concrete installed at a rate of 40 lane feet (53 square yards) per night. The precast concrete applications required later post-tensioning of panels and grouting of the tendons and beneath the pavement, but these activities can be accomplished during later nighttime closures concurrently with other activities.

Long-term durability provides another significant advantage. Precast manufacturers can produce very consistent concrete mixes with a high degree of quality control. This ensures a very durable pavement surface that will require minimal maintenance.

Pretensioning the panels in the plant and post-tensioning them on site also improves performance by inducing compressive stress in the panels. This prevents or greatly reduces cracking, reducing future maintenance and user-delay costs for such maintenance. Prestressing can also significantly reduce the thickness of the slabs, which not only saves materials, but also benefits applications with limited overhead clearance. The pilot project in Georgetown, Texas, used 8-inch-thick panels, which have a design life equivalent to a 14-inch-thick, continuously reinforced cast-in-place concrete pavement.

Another benefit in many parts of the country comes from extending the available construction season. Plant-controlled production conditions allow casting and installation to progress even as field conditions become adverse. PPCP also uses standard materials and well-known construction techniques.

The next generation
These recent projects focused on night-time installations, ensuring opening of the highway to unimpeded daytime traffic. The first, completed in July 2009 in Newark, Del., involved replacing the left and right turn lanes of an alkali-silica reaction (ASR) affected jointed concrete pavement along northbound Route 896 at the intersection with Route 40. Panels the width of a single lane (12 feet) and a double lane (24 feet) were used, with construction permitted only from 7:30 p.m. to 5:30 a.m.

In 2004, in El Monte, Calif., a 37-foot-wide by 8-foot-long panel incorporated a change in pavement cross-slope by tapering the thickness from 13 inches to 10 inches. The panels were installed at night. Pomeroy Corporation

A total of 130 panels were installed over a pervious-concrete base. Threaded bars were used for temporary post-tensioning during construction and as final tendons. As many as 16 panels, providing 160 feet of two-lane highway, or 320 lane-feet, were replaced each night.

In Sikeston, Mo., 10-foot-long by 38-foot-wide panels were installed on Interstate 57 in 2005. The panels provided two lanes and both shoulders. They varied from 7 inches thick at the inside shoulder to 11 inches thick at the crown to 5 5/8 inches thick at the outside shoulder. Transtec Group, Inc.

The most recent project, completed in November 2009, involved reconstruction of a portion of westbound I-66 and one of two lanes of a high-speed ramp from I-66 to U.S. 50 in Fairfax, Va. A “hybrid” precast pavement solution was utilized for this project. PPCP was employed for reconstruction of 1,020 feet of the four-lane mainline pavement on I-66, while a proprietary jointed precast pavement system (Fort Miller Super Slab) was used for reconstruction of approximately 3,550 feet of the right lane of the ramp from westbound I-66 to westbound U.S. 50. The jointed precast system provided an ideal solution for addressing the complex superelevated geometry of the ramp, while PPCP provided an ideal solution for the tangent section of heavily trafficked mainline pavement. A total of 306 panels were used for the PPCP portion of the project, and 224 for the ramp. The panels for the ramp were 9 inches thick to match the thickness of the existing pavement, while the panels for the mainline were 8.75 inches thick.

The work for both portions of the project was accomplished at night, with the panels laid on as much as an inch of a No. 10 stone leveling course placed over the existing sub-base. Only two lanes could be reconstructed at a time, leaving at least one lane open to traffic, and lane closures were restricted to 9 p.m. to 5 a.m.

On projects like these, contractors stress preplanning, as installations focus on tight night-time schedules. In Virginia, for instance, “rehearsals” prior to construction allowed contractor Lane Construction, of Chantilly, Va., to build up to a peak production of 70 linear feet per night of single and dual-lane slab installation.

In July 2009, both single- and double-lane panels were used for the nighttime replacement of the left and right turning lanes on Route 896 at the intersection with Route 40 in Newark, Del. Transtec Group, Inc.

Additional projects will continue with FHWA support together with other agencies. The Precast/Prestressed Concrete Institute’s (PCI’s) Pavement Committee, chaired by David Merritt of The Transtec Group, is compiling four key “guidance documents” for PPCP applications: project selection; pavement design, layout and maintenance; panel fabrication; and construction recommendations.

These efforts will ensure continuous improvement in panel production, delivery, and installation for PPCP projects. The goal remains to foster efficient, durable, and comfortable highway construction that can be completed in rapid time at a predicable budget.

William N. Nickas, P.E., is the managing director of transportation systems for the Precast/Prestressed Concrete Institute (PCI) in Chicago. He can be reached at wnickas@pci.org. For more information on PPCP, visit www.precastpavement.com