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July/August 2002
Texas Tests Precast for Speed and Usability
by David K. Merritt, B. Frank McCullough, and Ned H. BurnsRapidly gaining attention in the transportation industry, precast pavement uses a thinner prestressed slab than conventional concrete, while providing equivalent durability and high-performance. Precast pavement enables construction crews to work overnight or on weekends, and is ready to carry traffic almost immediately after placement. Economically, precast concrete can lower roadway user costs, such as fuel consumption and lost work time, due to delays caused by construction activities.
Finished set of base panels at the precast plant. Panels were cast on a casting-bed approximately 122 meters (400 feet) long.
Perhaps even better, precast concrete will minimize or even eliminate some problems common to conventional concrete paving, such as built-in curl (due to moisture gradients), strength loss (due to insufficient curing), and inadequate air entrainment. Eliminating these problems will help roadway engineers stretch the lives of their pavements, contributing to a decrease in long-term operational costs and work zone congestion.
Given precast concrete's success in the bridge and building industries, the Federal Highway Administration (FHWA) and Texas Department of Transportation (TxDOT) decided to sponsor a study that investigates the feasibility of using precast concrete for pavements. At the conclusion of the study, done by the Center for Transportation Research (CTR) at The University of Texas at Austin, researchers developed a concept for precast concrete pavement and recommended testing the concept through pilot projects. In March 2002, TxDOT completed the first pilot project on a frontage road near Georgetown, TX.
Looking down the side form of the casting bed at the precast plant.
Concept Features Interlocking Slabs
Several projects throughout the United States and abroad prove the benefits of post-tensioned pavement. Most notably, a 150-millimeter (6-inch) thick, post-tensioned overlay on Interstate 35 near West, TX, requires little maintenance and is in excellent condition after 17 years, despite heavy volumes of truck traffic (27 to 30 percent).
The concept for precast pavement calls for full-depth precast, post-tensioned concrete panels. The panels are both pretensioned during fabrication and post-tensioned together after setting them in place. Post-tensioned panels provide the same design life as thicker conventional concrete pavements using thinner slabs; therefore, a 200-millimeter (8-inch) post-tensioned pavement would have the same life as that of a 355-millimeter (14-inch) conventional pavement. Post-tensioning also increases durability by minimizing or even eliminating cracking, and ties the individual panels together, promoting load transfer between the panels.
Placement of concrete in the forms at the precast plant.
The downside of using a long, post-tensioned pavement is the need to accommodate significant expansion and contraction movements. Consequently, armored expansion joints, similar to those used in bridge decks are cast into several of the precast panels and located approximately every 76 meters (250 feet) along the length of the pavement.
A finished precast pavement essentially consists of three types of panels. Joint panels contain the armored expansion joint detail, mentioned previously, and the post-tensioning anchorage. Central stressing panels contain large pockets 122 centimeters by 20 centimeters (48 inches by 8 inches) for completing the post-tensioning process. Base panels are essentially "filler" panels placed between the joint and central stressing panels, making up the majority of the pavement.
The post-tensioning anchorage fastens to the armored expansion joint detail and is encapsulated in the joint panels. Self-seating (spring-loaded) wedges allow the post-tensioning strands to be fed blindly into the anchors. The strands are fed into the post-tensioning duct from the pockets in the central stressing panel, threaded through the panels, and inserted into the anchors in the joint panels. The strands then are tensioned from the pockets in the central stressing panels. Although this post-tensioning method appears labor-intensive, central stressing actually facilitates a more continuous pavement placement operation, because strand tensioning can be done without accessing the post-tensioning anchorage.
The study postulated that a thin layer of asphalt pavement might be smooth and flat enough as the all-important base for supporting precast panels. Over the asphalt course, a single layer of polyethylene sheeting is placed, prior to setting the panels. The polyethylene sheeting not only serves as a bond-breaker, preventing the panels from bonding to the leveling course; it decreases the frictional resistance between the bottom of the slab and the leveling course, greatly reducing the prestress losses during post-tensioning and the stresses that build up in the slab during normal expansion and contraction.
This schematic of a typical three-panel precast pavement assembly shows the keyed edges on all panels. The keyways enable the individual panels to interlock at the transverse joints, ensuring vertical alignment between panels and facilitating rapid assembly.
TxDOT Pilots Project in Georgetown
The first precast pavement pilot project near Georgetown, TX, was constructed on the northbound frontage road of Interstate 35, which TxDOT closed to traffic during construction. Although the ultimate application for precast pavement will be urban freeways and intersections facing extreme construction time constraints the frontage road provided an ideal environment for testing and fine-tuning precast paving techniques, without the hindrance of anticipated construction time restrictions.
This location and slab layout of the Georgetown precast pavement pilot project shows that each "slab" includes the joint panels, central stressing panels, and base panels between consecutive expansion joints.
The pilot research focused on a simple geometric layout to work out the basic construction techniques; therefore, the site contained no horizontal curves and very gradual vertical curves. Techniques for horizontal curves and super elevations will be worked out on future projects.
This project consisted of 700 meters (2,300 feet) of precast pavement on both sides of a new bridge. The precast panel orientation was transverse to the flow of traffic, requiring panels to span the full 11-meter (36-foot) roadway width of two 3.7-meter (12-foot) lanes, a 2.4-meter (7.9-foot) outside shoulder, and a 1.2-meter (4-foot) inside shoulder. Although not necessary, researchers used both full-width 11-meter (36-foot)and partial-width 5- and 6-meter (16- and 20-foot) panels to test the concept for both applications.
Researchers placed the full-width panels on the north side of the bridge, and the partial-width panels, tied together transversely through post-tensioning, on the south side. Additional flat, three-strand ducts were cast into each partial-width panel for the transverse post-tensioning. Transverse post-tensioning ensured a tight longitudinal joint between the 5-meter and 6-meter panels and load transfer across that joint.
Panel Fabrication and Casting
In addition to post-tensioning, the panels were pretensioned lengthwise, in the transverse pavement direction, during fabrication. The governing factor for pretensioning was the stresses generated from handling the panels.
The panels were fabricated on a 22-meter (400-foot) casting bed, accommodating production of 10 full-width panels and up to 20 partial-width panels at one time, end-to-end. The pretensioning strands extended continuously through all the panels, the full length of the casting bed. Long line casting required that engineers pay special attention to side forms, where imperfections or misalignments might prevent the keyed panel edges from matching up.
Panel Placement and Tensioning
While casting proceeded, the asphalt-leveling course was placed on the frontage road as flatly and uniformly as possible.
After a sufficient number of panels were cast, panel assembly began over the asphalt-leveling course. The single-layer of polyethylene sheeting (friction-reducing medium) was rolled out prior to the placement of each panel. A 578-kN (65-ton) capacity crane was used to lift each panel directly from the truck and set it in place. A slow-setting segmental bridge epoxy applied to the panel edges acted as an assembly lubricant and also bonded the panels together so they would act more like a continuous slab after post-tensioning.
At the start of the project, it took about 8 hours to place approximately 25 full-width panels. This placement rate varied depending on the number of workers available. Toward the end of the project, 25 panels could be placed in approximately 6 hours.
After assembly of a section of panels (between expansion joints), the post-tensioning strands were fed through the ducts from the central stressing panels, and inserted into the self-locking anchors in the joint panels. The strands were coupled in the stressing pockets and tensioned with a monostrand post-tensioning ram. After post-tensioning, the stressing pockets were filled and the post-tensioning strands were grouted in the ducts.
A full-width panel is lowered into place. Trucks carrying full-width panels line up to unload.
Placement of a partial-width base panel over the friction-reducing polyethylene sheeting.
Project Details and Panel Dimensions
Researchers selected a standard pavement length (between expansion joints) of 76 meters (250 feet) based on prior experience with post-tensioned pavement near West, TX. A longer length of 100 meters (325 feet) for the partial-width panels and a slightly shorter length of 68 meters (225 feet) for the full-width panels also were incorporated. A standard precast panel width of 3 meters (10 feet) was selected for all of the panels based on casting-bed width and transportation (weight limit) considerations. With this panel width, 26 precast panels were required for each of the standard 76-meter (250-foot) pavement sections, including 22 base panels, 2 central stressing panels, and 2 joint panels half of each joint panel at each end. In total, 123 full-width and 216 partial-width panels were required for the finished project.
A pavement thickness of 200 millimeters (8 inches) was chosen primarily on the basis of handling considerations. With post-tensioning, however, this pavement has an expected fatigue life equal to a 355-millimeter (14-inch) continuously reinforced concrete pavement. The compression that post-tensioning induces in the pavement allows for this reduction in pavement thickness and also should greatly reduce cracking. Although the equivalent of a 355-millimeter pavement is much thicker than necessary for the Georgetown frontage road, the design of the pilot section was to simulate the main lanes of an interstate pavement.
To achieve the 355-millimeter-equivalent pavement thickness, a maximum prestress of approximately 1.45 MPa (210 psi) was required at the ends of the slab. This translated into 15-millimeter (0.6-inch) diameter post-tensioning strands spaced at approximately 71 centimeters (28 inches) across the width of the pavement. However, for the purpose of standardizing strand spacing for future projects, a strand spacing of 61 centimeters (24 inches) was selected, which further increases the effective thickness of the pavement.
Placement of a full-width central stressing panel.
For the partial-width panels, a section of 6-meter panels was set in place and post-tensioned, followed by the adjacent section of 5-meter panels. They were then post-tensioned transversely to tie them together.
No additional measures, such as surface-level diamond grinding, were required to improve the ride quality of the finished pavement.
Post-tensioning strands are fed by hand into the ducts at the pockets in the central stressing panels down to the anchors in the joint panels.
Benefits
According to Bill Garbade, district engineer for the Austin District of TxDOT, "There were a lot more pros than cons. It's certainly well worth the time and money to carry the experiment the next step." Mark Herber, graduate engineer at the project site for the TxDOT Georgetown Area Office, agreed, "Everything turned out as expected."
As mentioned throughout this article, precast pavement offers several benefits including:
- Expedited construction almost immediate exposure to traffic
- Possibility for overnight or weekend construction
- Roadway user cost savings such as fuel consumption and lost work time
- Greater control over concrete batching and curing
- Increased durability from post-tensioning
- Material savings through reduced pavement thickness
Although at this time the construction costs associated with precast pavement might be higher than conventional paving methods, the savings in user costs will far outweigh any additional construction costs.
Precast pavement panels can be cast and cured in a controlled environment at a precast plant, providing greater control in ensuring a consistent concrete mix, and properly curing the panels. Precast panels reduce or eliminate curling, strength, and air-entrainment problems that are common with conventional concrete paving.
Finally, post-tensioning not only reduces the required pavement thickness, but also greatly increases durability, lessening or even preventing cracking in the pavement. This increases the life of the pavement, contributes to a reduction in maintenance costs, and lessens the inconvenience to the motoring public.
Only the Beginning
Although this first pilot project did not include all of the intricacies anticipated in future precast pavement projects, it did demonstrate the viability of basic precast paving techniques. Not only are precast panels effective for rapid pavement construction, but also the incorporation of post-tensioning actually increases durability, which minimizes maintenance over the life of the pavement. Additionally, precast pavement is a construction technique that can be used for night and weekend construction, making it more "invisible" to roadway users.
As traffic volumes continue to expand on the Nation's deteriorating infrastructure, and as people become more frustrated with major traffic delays caused by conventional construction methods, expedited construction techniques will become critical. The precast pavement pilot project near Georgetown, TX, is only the beginning of precast pavement construction.
References
1. Chia, Way Seng, B.F. McCullough, and Ned H. Burns. Field Evaluation of Subbase Friction Characteristics. Research Report 401-5. Center for Transportation Research, The University of Texas at Austin, September 1986.
2. Mendoza Diaz, Alberto, Ned H. Burns, and B. Frank McCullough. Design of the Texas Prestressed Concrete Pavement Overlays in Cooke and McLennan Counties and Construction of the McLennan County Project. Research Report No. 555/556-1. Center for Transportation Research, The University of Texas at Austin, February 1986.
3. Merritt, David K., B. Frank McCullough, and Ned H. Burns. The Feasibility of Using Precast Concrete Panels to Expedite Highway Pavement Construction. Research Report No. 1517-1. Center for Transportation Research, The University of Texas at Austin, February 2000.David K. Merritt is a research associate with the Center for Transportation Research (CTR) at the University of Texas at Austin. He completed a master of science degree in civil engineering at The University of Texas at Austin in 2000 with an emphasis in structural engineering. After completion of his degree, he started full-time research at CTR, working on various concrete pavement research projects, specializing in precast and prestressed concrete pavement.
B. Frank McCullough is the Adnan Abou-Ayyash Centennial Professor Emeritus of Civil Engineering at The University of Texas at Austin and former director of the Center for Transportation Research. Dr. McCullough has a particularly strong interest and background in pavement design. During his career, he has supervised more than 50 research projects involving development of quality assurance and quality control specifications, planning, design, construction, rehabilitation, and maintenance of pavements.
Ned H. Burns is a Zarrow Centennial Professor Emeritus at The University of Texas at Austin, where he has been involved in teaching, research, and consulting in structural concrete for 40 years. He is an active member of ACI Committee 423Prestressed Concrete and is a Fellow of ACI. He is a member of the National Academy of Engineering.
Other Articles in this issue:
Taking Concrete to the Next Level
Fine-Tuning Innovative Technologies
Making Roads Better and Better
Texas Tests Precast for Speed and Usability
The Biggest Bang for Your Buck
New Software Promises to Put Whitetopping on the Map
