Pushing The Boundaries
Demonstration projects around the country continue to explore the feasibility of using precast prestressed concrete in pavement applications.
|TxDOT had nearly 340 precast panels fabricated and installed on the frontage road near I-35 in Georgetown, TX, as part of a demonstration project in 2002. Here, workers guide a partialwidth precast panel into place.|
Already a staple in the bridge designer's toolbox, precast prestressed concrete is finding new applications in the paving world. For years engineers have designed bridges, buildings, and other vertically oriented infrastructure using precast concrete, but only recently have researchers begun exploring the feasibility and potential benefits of applying this technology ongrade in pavement construction.
The July/August 2002 issue of PUBLIC ROADS showcased a demonstration constructed by the Texas Department of Transportation (TxDOT) that investigated the feasibility of using precast concrete for a paving project ("Texas Tests Precast for Speed and Usability," page 30). The article recounts an earlier feasibility study conducted by the Center for Transportation Research at The University of Texas at Austin for the Federal Highway Administration's (FHWA) Office of Infrastructure, Research, and Development (R&D), which showed that precast prestressed concrete panels could be a viable option for reconstructing pavements quickly. The TxDOT demonstration, which was completed in March 2002 on a frontage road near Georgetown, TX, proved that precast works and opened the door to future research and implementation.
Since that time, the California DOT (Caltrans) took the concept of precast construction a few steps forward by modifying some of the design features and requiring nighttime construction.
Both the Texas and California projects are part of the Federal Highway Administration's (FHWA) ongoing Concrete Pavement Technology Program (CPTP) Task 58, "The Use of Precast Concrete Panels to Expedite Highway Pavement Construction." The CPTP consists of research, development, and technology-delivery activities to foster innovation, improve the performance and cost effectiveness of concrete pavements, and ultimately reduce user delays.
Following the success of the pilot projects in Texas and California, Missouri, Texas, and Indiana are planning new demonstrations within the scope of Task 58, which will continue to add layers of complexity and push the boundaries of precast pavement.
Anatomy of a Precast Prestressed Concrete Pavement
Three specific types of panels make up a precast prestressed concrete pavement: joint panels, central stressing panels, and base panels.
Joint panels are located at the ends of each posttensioned section and contain dowelled expansion joints to absorb horizontal slab movements.
The central stressing panels are placed at the middle of each posttensioned section and contain large pockets or blockouts where the posttensioning strands are fed into the ducts and stressed.
The base panels make up the majority of the pavement, placed between the joint panels and stressing panels. During a single construction operation, workers must place at least one complete section of panels from joint panel to joint panel.
The range of panel types requires special attention when transporting the precast panels to the construction site, as each type is needed for a complete section. Because the panels are not match-cast, however, they are interchangeable if problems are encountered during shipment. As a result of the logistics involved in the prestressing, this technology is more suited to large-scale replacement projects rather than small, isolated patches.
Why Precast Prestressed Concrete?
The obvious benefit of using precast concrete is that agencies can reopen a pavement to traffic almost immediately after placing the panels, rather than having to wait for a cast-in place concrete to reach sufficient strength in the onsite curing environment. Precast pavement, therefore, permits overnight and weekend reconstruction projects.
Less obvious, but equally important, is the benefit of improved durability. Conventional concrete paving is prone to certain quality problems, such as built-in curl and warp (due to moisture and temperature gradients, respectively), strength loss (due to insufficient curing), and inadequate air entrainment. Precast panels, on the other hand, are cast in a controlled environment using high-quality concrete mixes and optimal curing conditions, thereby reducing the probability of encountering such problems. The result is a more durable pavement that will meet the design-life requirements placed upon it.
Agencies can realize further benefits by incorporating prestress (in the form of pretensioning or posttensioning) into the precast panels. Prestress not only benefits durability by reducing tensile stresses that lead to cracking, but it also reduces the required slab thickness. Because prestressing induces a compressive stress in the pavement, tensile stresses caused by wheel loads in a thinner slab can be reduced to that which would be expected in a much thicker pavement slab. Engineers can therefore design precast panels that are only 200 millimeters (8 inches) thick but are essentially equivalent to a 355- millimeter (14-inch)-thick nonprestressed pavement, thereby reducing the concrete volumes and the probability of overhead clearance conflicts, which typically occur at bridge structures with the construction of thicker pavements.
Prestress also allows the precast panels to span small voids in the base layer, thereby accommodating placement on a base material that may not be perfectly flat. And with prestress, the pavement stays in compression, which prevents cracks from opening up, minimizing the potential for corrosion.
The Texas Pilot
Completed in 2002, the earliest project, near Georgetown, TX, demonstrated the viability of constructing a pavement using precast prestressed concrete. TxDOT placed approximately 700 meters (2,300 feet) of two-lane pavement (plus shoulders) on the frontage road along Interstate 35.
The precast, pretensioned panels were placed over a hot-mix asphalt leveling course and then posttensioned together in the longitudinal direction in 76-meter (250-foot) sections. Shear keys along the edges of the panels (similar to "tongue and groove" joints) ensured satisfactory vertical alignment during installation and good ride quality of the finished surface by interlocking the panels, preventing differential vertical movement between panels.
|Cars travel on the newly completed frontage road of I-35 in Georgetown, TX, paved with precast prestressed concrete.|
The precast panels were 200 millimeters (8 inches) thick, but because they were prestressed, TxDOT expects them to have a design life equivalent to that of a 355-millimeter (14-inch)-thick continuously reinforced concrete pavement. As the contractor became more familiar with the construction process, approximately 25 panels—equaling 76 meters (250 feet) of pavement—were installed in a 6-hour period. TxDOT found that posttensioning, completed after the panels were installed, generally took just a few hours for each section of pavement.
Aside from demonstrating the efficacy of placing the slabs on grade, the Georgetown project demonstrated the viability of posttensioning the panels together in place. The project also showed that match-casting, which is commonly used for precast segmental bridge construction, was not required to ensure a tight fit between panels and to align the posttensioning ducts. Because matchcasting is not necessary, the manufacturer can fabricate the panels faster, and shipment and installation are simplified.
TxDOT officials report that after nearly 3 years in service, the pavement shows no signs of distress. In fact, the agency expects the panels to last up to 40 years before any rehabilitation is required.
"Precast concrete pavement will be an excellent solution to provide long-term, high-quality pavement replacement and repairs with minimal impact on the traveling public," says Mark Herber, P.E., transportation engineer from the Georgetown Area Office of TxDOT. "[The material] could be especially useful when the pavement on urban freeways in downtown areas needs replacing, and the existing facility cannot be rebuilt due to adjacent development."
California's Project Adds Complexity
More recently, in April 2004, Caltrans upped the ante by constructing a precast prestressed concrete pavement at night on an interstate with a low tolerance for lane closure. The agency placed approximately 76 meters (250 feet) of two-lane (plus shoulder) pavement on Interstate 10 in El Monte, CA. The precast panels were placed over a lean concrete base and posttensioned together in the longitudinal direction in 38-meter (124-foot) sections.
|Workers place concrete for one of the panels used in the California demonstration project.|
In addition to conducting the operation overnight, another innovative feature is that Caltrans specified that the top surface of the precast panels vary in cross-slope from 2 percent on the main lanes to 5 percent on the shoulders. This requirement varied the panel thickness from 330 millimeters (13 inches) at the point of cross-slope change to 250 millimeters (10 inches) at the ends of the panels. Incorporating the crowned design, which will facilitate drainage to the outside edge of the pavement, demonstrated the viability of casting panels with more complicated shapes and features.
Sixteen, or just over half, of the precast panels were placed in approximately 5 hours during the first night of construction (between the hours of 12 midnight and 5 a.m.), while the remaining 15 panels were placed in just over 3 hours during the second night. The contractor completed posttensioning in a few hours the morning following installation of the panels.
Although the project was not constructed under the more stringent time constraints anticipated for future projects, it further demonstrates the viability of this technology for fast-track projects.
"Because the durable concrete panels are placed overnight or on weekends, construction is expedited, costs are reduced, and the freeway remains open to the motoring public during peak commute periods," says Douglas R. Failing, P.E., director of Caltrans District 7, which includes Los Angeles and Ventura Counties.
Furthermore, Caltrans officials expect the installation to last more than 50 years, under extremely high traffic volumes in one of the most heavily-trafficked corridors in the United States.
"Being on the cutting edge of new technology and techniques for freeway pavement reconstruction and rehabilitation is exciting," Failing adds. "This is a perfect example of a public-private partnership working as a team for the public benefit-a win-win situation all around."
Future Demonstration Projects
The success of the first two demonstrations generated significant interest within the highway industry, spawning additional projects and new applications for the technology. Whereas the Texas and California projects demonstrated the viability of the concept of precast prestressed pavements, three new projects will focus on constructing these pavements under more complex conditions and under even more demanding time constraints.
|This photo of a precast panel shows the crown shape used for the California project. The panel tapers from 330 millimeters at the high point down to 250 millimeters at the ends.|
On a rural section of Interstate 57 near Sikeston, the Missouri Department of Transportation (MoDOT) will use precast prestressed concrete to replace a vintage jointed reinforced concrete pavement. Although the existing pavement lasted an impressive 45 years, increasingly heavy truck traffic has taken its toll on the roadway over the past 8 to 10 years. Pumping and joint faulting have led to severe pavement distresses, requiring patching and now full-depth replacement. The goal of the project is to test precast on a rural section of interstate that carries heavy traffic loads to determine whether similar applications could be implemented in more heavily congested areas like downtown St. Louis.
MoDOT, with assistance from FHWA and its contractor, will replace approximately 0.4 kilometer (0.25 mile) of pavement with about five posttensioned sections. The project will replace the existing pavement with 200 millimeters (8 inches) of precast prestressed pavement, which will be approximately 100 millimeters (4 inches) thinner than the jointed plain concrete pavement that will be used for the majority of the Missouri rehabilitation project. Because the elevation of the finished pavement surface will not be increased by the new precast pavement, MoDOT will avoid the need to excavate base material for vertical clearance at bridges or to taper down to the existing pavement surfaces at either end of the project.
The "rooftop crown" slope of the existing pavement surface will be cast into the surface of the precast panels, with each panel spanning the full width of the roadway (two lanes plus shoulders). The panels will be placed over a stabilized base and posttensioned together in 76-meter (250-foot) sections. To minimize disruption to traffic, construction will take place during weekend operations, ensuring that the project will not impede heavier weekday traffic.
"MoDOT anticipates gaining an alternate treatment option for high-density urban arterials like those in St. Louis that require an absolute minimum of traffic disruption during construction and virtually no maintenance through a long design life," says John P. Donahue, P.E., research and development engineer with MoDOT in Jefferson City. "The field experience gained on an interstate setting and the performance data acquired through an instrumentation investigation conducted by the University of Missouri at Columbia will combine to shape the State's expectations for this technology."
Construction is tentatively scheduled for the spring of 2005.
Adding to its track record of innovative applications for precast concrete in the pavement environment, TxDOT, along with FHWA and its contractor, is developing plans for a second demonstration project. This time, TxDOT will use precast panels that can accommodate weigh-in-motion (WIM) scales, which Texas and other States use to gather weight data for traffic.
TxDOT plans to construct several WIM sites around the State, and precast offers a way to minimize traffic disruptions associated with construction. Because the standard practice in Texas is to install the scales in a section of concrete pavement, the department selected a future WIM site on U.S. 175 southeast of Dallas as the location for its latest demonstration.
A 152-meter (500-foot) section of the existing asphalt pavement will be removed and replaced with precast concrete panels that have blockouts (or pits) for the WIM scales cast into them. Preforming the blockouts into the pavement panels during the casting operation will eliminate the usual field operations of sawcutting and jackhammering the pits for the scales, facilitating faster installation.
Because of the high volume of truck traffic on these pavements, TxDOT aims to minimize disruptions during construction. The original design called for a 355-millimeter (14-inch) continuously reinforced concrete pavement, which would have required several weeks of lane closures to construct. However, using a precast prestressed pavement will permit TxDOT to complete construction during a weekend closure.
"Our goal is to 'get in, get out, and stay out' of the traveling public's way," says Pavement Engineer Abbas Mehdibeigi, P.E., with the Dallas District of TxDOT. "We envision minimizing the construction time, and thus the closing time of the roadway. By utilizing precast panels, part of the construction can be undertaken offsite, as opposed to the conventional method where everything is done onsite."
Also, because the new pavement will be prestressed, the panel thickness will be only 250 millimeters (10 inches), minimizing the amount of excavation required during removal of the existing pavement. Similar to the panels used in the first Texas project, these precast panels will be placed over a thin hot-mix asphalt leveling course. The panels for both the east- and westbound pavement sections will be posttensioned together in 152-meter (500-foot) lengths, and the panels will span the full width of each roadway (two lanes plus shoulders).
Mehdibeigi notes, "We expect to come up with a working design and specification that might be used for other sites across the State, with minor modifications." Construction is planned for the spring of 2005.
Depending on the success of casting the WIM blockouts into the panels, other unique applications may be possible down the road. Pavements in urban areas, for example, could feature drainage inlets or manholes cast into the panels.
|Precast panels delivered by truck to the California site could be lifted by crane over a construction barrier and lowered into place, as shown here.|
Bridge Underpasses In Indiana
A third demonstration currently in development in Indiana will focus on applying precast prestressed concrete at bridge underpasses. With a number of bridges that have been hit in the last few years by trucks that exceed the maximum clearance height, the Indiana DOT (INDOT) sees the need to be able to construct thinner pavement sections beneath these structures.
Although a new conventional cast-in-place pavement may increase the existing pavement thickness (and decrease overhead clearance) by several inches, precast prestressed concrete pavement will permit INDOT to place a much thinner pavement section, thereby preserving precious overhead clearance. Precast pavement also will permit INDOT to construct these pavements under strict time constraints (overnight or weekend) in urban areas, minimizing disruptions to traffic.
"Our expectation is that the use of precast pavement sections will help minimize traffic impacts as we eliminate these low bridge safety hazards and provide increased levels of service and safety," says Victor (Lee) Gallivan, pavement and materials engineer with FHWA in Indiana.
|The speed of placement of the precast panels used in the California demonstration improved significantly over the 2 nights of construction. Here, a crane lowers another panel into place.|
As pavements constructed during the interstate era reach their serviceability limits, highway agencies will need new technologies and strategies to repair and rehabilitate them. In urban areas, especially, the need to replace distressed pavements with minimal disruption to traffic will continue to drive decisionmaking.
"Traffic constraints will drive the market for this technology," says Failing from Caltrans. "The window for construction in places like California is getting smaller—maybe 4 hours per night. Precast is a known quantity, having proven its durability over the years."
|A worker surveys a 38-meter (124-foot) posttensioned section of precast pavement on the California jobsite.|
Researchers expect to see continued growth and greater momentum for precast pavement. Future research is necessary to determine the feasibility of casting horizontal curves or pavement sections with significant changes in superelevation. Another area for further study is the variety of base conditions upon which precast panels can be placed. For example, what is the minimum threshold for acceptable base conditions to ensure long-term performance of the finished pavement? Speed of construction is still an issue as well. Although the California project demonstrated the feasibility of overnight construction, the actual precast pavement section was not opened to traffic the next morning. Future demonstrations might involve shutting down a segment of road after rush hour and reopening it the next morning.
Although precast prestressed concrete is currently more expensive than conventional cast in place, proponents argue that the reduction in user costs—such as traffic congestion, fuel consumption, and lost work time—will validate this technology for certain situations. New, competitively bid demonstration projects, such as that in Missouri, should provide a more realistic estimate of the costs involved.
|The California demonstration project progressed smoothly. Shown here is the completed pavement.|
"This technology has tremendous potential as one of the many tools that highway agencies can use to achieve FHWA's Highways for LIFE goals of improved safety, reduced construction-related congestion, and improved quality," says FHWA Deputy Administrator J. Richard Capka.
Samuel S. Tyson is a concrete pavement engineer with FHWA in Washington, DC. He provides technical oversight and guidance for CPTP Task 65, a highly focused technology transfer activity for the numerous products resulting from some 30 projects within FHWA's CPTP. His areas of technical involvement for both jointed plain concrete pavements and continuously reinforced concrete pavements include all aspects of design, construction, repair, and rehabilitation. He earned his B.S. and M.S. degrees in civil engineering from the University of Virginia and is an active member of American Concrete Institute and Transportation Research Board (TRB) committees on concrete pavements. He is a registered professional engineer in the District of Columbia.
David K. Merritt is a project manager with The Transtec Group, Inc., in Austin, TX. He is currently the principal investigator for CPTP Task 58-C and was previously the principal investigator for Task 58-B while a research associate at the Center for Transportation Research at The University of Texas at Austin. Merritt specializes in precast and prestressed concrete pavements. He received his B.S. in civil engineering from Northern Arizona University and an M.S. from The University of Texas at Austin. He is an active member of the American Concrete Institute and the Precast/Prestressed Concrete Institute, and he is involved with TRB committees AFD70 and AFH50. Merritt is a registered professional engineer in Texas.