|FHWA > Engineering > Pavements > Concrete > FHWA-IF-06-010 > Chapter 5|
Construction of the California Precast Concrete Pavement Demonstration Project
Chapter 5. Panel Fabrication
The precast panels for the California demonstration project were fabricated by Pomeroy Corporation of Perris, California. The fabrication plant was located approximately 97 km (60 mi.) from the test site in El Monte. Because of the unique shape of the side forms (change in cross slope cast into the panel surface) and the small number of panels to cast, it was cost-prohibitive for the precast fabricator to purchase side forms for more than two panels, limiting production to two panels per casting. In total, 31 panels were cast, including 3 joint panels, 4 central stressing panels, and 24 base panels.
The panels were fabricated on a long-line casting bed, with two panels cast end to end. The pretensioning strands extended through both panels, anchored at permanent stressing abutments at either end of the casting bed. After each pair of panels was cast and had reached the specified release strength of 27.6 MPa (4,000 lbf/in²), the pretensioning strands were de-tensioned and cut at the ends of the panels and the panels were removed from the casting bed to a storage area in preparation for shipment. Depending on set-up time for the casting bed, which varied by panel type, one set of panels was generally cast every other day.
Because the panels were not match-cast, tolerances were particularly critical to ensure that adjoining panels would fit together and provide a satisfactory riding surface. Table 6 summarizes the tolerances for the precast panels, as specified in the project drawings. Based upon previous experience with the Texas pilot project, a “squareness” tolerance was added to ensure that a “curve” was not built into the pavement by panels that were out-of-square. The position and straightness of the post-tensioning ducts were also critical to ensuring that the ducts would line up between adjacent panels and that wobble would be minimal over the length of the entire duct.
It should be noted that table 6 is not an exhaustive list of tolerances for the precast panels. Tolerances such as clear cover for reinforcement were dictated by Caltrans Standard Specifications for Portland Cement Concrete Pavement (section 40), Prestressing Concrete (section 50), Concrete Structures (section 51), Reinforcement (section 52), Portland Cement Concrete (section 90), and others.(14)
Tolerances were checked at the precast plant by Caltrans inspectors, and any dimensions out of tolerance were generally handled on a case-by-case basis. The precast fabricator indicated that there were no problems with meeting the required tolerances.
The majority of the details for the different panel types were essentially the same with the exception of the expansion joints in the joint panels and stressing pockets in the central stressing panels, as described below.
As discussed previously, the primary purpose of the keyways along adjoining panel edges was to ensure vertical classment between panels as they were assembled and to provide temporary load transfer until post-tensioning was completed. Because the panels were not match-cast, keyway dimensions were specified such that there would be a slight amount of “play” in the keyways. As figure 15 shows, the nose of the “male” keyway was tapered slightly more than the “female” keyway to prevent a wedging action in the keyway as the panels were assembled. The nose of the male keyway was also shortened to ensure that the vertical faces above and below the keyway would contact and the nose of the male keyway would not “bottom out.” Figure 16 shows the end of a keyway for two panels assembled at the fabrication plant (without post-tensioning).
A tapered opening was cast into the panels at the ends of the post-tensioning ducts, as shown in figure 15. The purpose of this taper was to help guide the post-tensioning strands into the ducts as they passed between panels. Additionally, a recess was cast into the female keyways at each post-tensioning duct to receive a foam gasket. The foam gasket helped to seal the post-tensioning ducts at the joints between panels.
Although the thickness of the panels varied from 250 mm (10 in.) at the ends to 330 mm (13.1 in.) at 3 m (10 ft) from one end, the keyways remained parallel to the bottom of the panel, resulting in a variable-depth vertical face above the keyways (figures 10, 15). Likewise, the vertical center of the post-tensioning ducts was maintained parallel to the bottom of the panel, at the center of the keyways. This helped to greatly simplify the formwork and assembly of the panels.
Figure 15. Illustration. Keyway dimensions for the precast panels.
Figure 16. Photo. Keyway of two assembled panels at the fabrication plant.
Grout channels were cast into the bottom of each panel to allow for underslab grouting after completion of post-tensioning. Two “half-round” channels with a 13-mm (1/2 in.) radius were cast into the bottom of each panel parallel to the long axis of the panel. Each channel was located approximately 0.5 m (20 in.) from the edge of the panel and was stopped 0.3 m (1 ft) from the ends of the panel. Grout inlets/ports for the channels were spaced at 2.4 m (8 ft) along the length of the channel, extending vertically to the top surface of the panel.
The base panels were the most basic of the three panel types. Six pretensioning strands, 13 mm (0.5 in.) in diameter, were spaced evenly across the 2.4-m (8 ft) width of the panel, parallel to the bottom of the panel, alternating just above and just below the post-tensioning ducts. Twelve galvanized steel post-tensioning ducts, 25 mm (1 in.) in diameter (inside), were spaced at 1 m (3 ft) on center across the length of the panel, beginning 0.6 m (2 ft) from each end. Non-prestressed reinforcement was minimal, with 13-mm (0.5 in.) (ASTM A706) deformed bars around the top and bottom perimeter of each panel. A minimum of 50 mm (2 in.) of concrete cover was provided for all of the reinforcement. Lifting anchors were located approximately 0.2L from the edges of the panels. Intermediate grout vents/inlets for the post-tensioning ducts were cast into approximately every fourth base panel.
Central Stressing Panels
The central stressing panels were similar to the base panels with the addition of the stressing pockets for post-tensioning, as described in chapter 2. The pockets were 200 mm (8 in.) wide by 1.2 m (4 ft) long, through the panel, large enough to accommodate a monostrand stressing ram and the “movement” of the strand coupler from elongation of the post-tensioning strands.
As shown in the panel assembly diagram in chapter 2, the stressing pockets were divided into two panels to prevent weakening of the panels from having too many pockets. The pockets for adjacent tendons were alternated between the two panels. The corners of each of the pockets were rounded to reduce stress concentrations and associated cracking from the corners of the pockets. Tendon grout inlets/vents were located on either side of the stressing pockets.
The six pretensioning strands, 13 mm (0.5 in.) in diameter, were spaced evenly on either side of the stressing pockets. Non-prestressed reinforcement (A706, 13 mm) was provided around the top and bottom perimeter of the panels, similar to the base panels. Two additional bars were provided through the bottom of each panel to help hold the concrete for the pockets in place, and 90-degree angle bars were provided at the ends of each stressing pocket to arrest any cracks that may propagate from the corners of the pockets. Figure 17 shows a central stressing panel at the fabrication plant during storage.
Figure 17. Photo. Typical central stressing panel at the fabrication plant prior to shipment.
The joint panels contained the expansion joints for “absorbing” the horizontal expansion and contraction movements of the post-tensioned slabs. The expansion joint detail is shown in figure 18. While an armored expansion joint detail was used for the Texas pilot project,(2) a plain dowelled joint was specified for the California project due primarily to the fact that diamond grinding was anticipated, and armored expansion joints cannot be ground very easily. Fortunately, the anticipated expansion joint movement (< 25 mm) is much less than that of the Texas project, eliminating the need for such a robust armored joint.
As figure 18 shows, stirrups (A706, 13 mm) spaced approximately every 100 to 150 mm (4 to 6 in.), were provided to transfer the prestress from the face of the post-tensioning anchors back to the expansion joint, ensuring the entire joint panel is prestressed. Non-prestressed reinforcement was also provided in front of the post-tensioning anchors and at the corners of the stirrups. Stainless-steel-clad dowel bars and expansion sleeves were spaced at 0.3 m (1 ft) on center, parallel to the bottom of the panel, 130 mm (5 in.) from the bottom. The pockets behind the post-tensioning anchors were provided for manual fitting of the wedges around the strands. The majority of the pockets were 150 mm (6 in.) by 200 mm (8 in.), through the panel, but as mentioned previously, two larger pockets (150 mm by 0.6 m), through the panel) were provided for feeding the temporary post-tensioning strands into the ducts during panel assembly (figures 3 and 19).
Two of the six pretensioning strands (13 mm [0.5 in.] in diameter) were located behind the anchor access pockets (between the pockets and expansion joint), and the other four were located in front of either of the post-tensioning anchors. Additional non-prestressed reinforcement was provided around the top and bottom perimeter of each “half” of the joint panel.
For the two joint panels abutting the new cast-in-place pavement at either end of the precast pavement test section, slots were cast into the joint panel to receive tie bars extending from the existing pavement. The tie-bar slots were cast into the bottom half of the joint panels at 0.3 m (1 ft) on center, with grout ports extending to the surface of the panel for grouting the tie-bars in place. For the non-post-tensioned half of the joint panels abutting the existing pavement, 13-mm (0.5 in.) A706 bars were provided at mid-depth at 0.3 m (1 ft) on center over the length of the panel.
Joint panels were fabricated by casting one half at a time. Dowels were left protruding from the face of the expansion joint, and a bond-breaker was applied to the hardened concrete prior to casting the other half of the joint panel. This helped to ensure a clean joint between the two halves of each joint panel. Figure 19 shows one of the joint panels at the precast plant prior to shipment to the installation site. The anchor access pockets, grout vents, and lifting devices are visible in this picture. The strongbacks used to hold the two halves of the panel together during handling are also visible.
Figure 19. Photo. Typical joint panel at the fabrication plant prior to shipment.
To remain productive, precast fabricators must continually keep their casting beds in use. This usually requires a concrete mixture design that allows them to remove a product from the forms the day after casting. Fortunately, experience has led to mixture designs that will meet this requirement without compromising quality or durability. For the precast pavement panels, a mixture was needed that would not only give the precast fabricator the required release strength the day after casting, but would also meet the finishablity requirements for a pavement surface.
The mixture design used for the panels consisted of 340 kg (8 sacks) of Type II cement (Prestress/Mojave) with 15 percent Type F fly ash replacement, a water-to-cementitious materials ratio of 0.37, superplasticizer, fine aggregate, and 13-mm (1/2 in.) maximum size granitic coarse aggregate. The mixture had a design release strength of 27.6 MPa (4,000 lbf/in²) and 28-day strength of 41.4 MPa (6,000 lbf/in²). The maximum pouring slump was 100 mm (4 in.), and unit weight was 2,355 kg/m³ (145 lb/ft3 ). The precast fabricator indicated that there were no problems with this mixture design for pavement panels. Figure 20 shows placement of concrete for one of the base panels at the fabrication plant.
Figure 20. Photo. Placement of concrete for a base panel.
Finishing and Curing
Surface finishing was a critical step as the finish greatly affects the ride quality of the finished pavement. Finishing the precast panels was somewhat challenging for two reasons. First, the change in cross slope cast into the panels formed a “peak” in the surface of the panel where the thickness was 330 mm (13.1 in.) versus 250 mm (10 in.) at the ends. Because fresh concrete tends to flow downhill, the mixture needed to be stiff enough to prevent sloughing at the high point on the surface. Secondly, grout vents and lifting anchor supports were protruding from the surface, forcing the fabricator to hand finish the surface of the panels around these embedments. After hand finishing the panel surface, an Astroturf drag texture was applied. Figure 21 shows workers screeding and hand finishing the surface of a joint panel around the grout ports protruding from the surface of the panel.
Following the finishing process, the panels were covered with tarps and steam cured in accordance with Caltrans Standard Specifications, section 90-7.04.(14) Steam curing was usually applied for 12 hours or until the panels reached the necessary release strength. Upon release of prestressing, the panels were removed from the forms and stored on site.
Figure 21. Photo. Screeding the surface of a joint panel.
Handling and Storage
One of the recommendations from the Texas pilot project was to use lifting anchors that would only leave a small hole or void to be patched.(2) Based on this recommendation, screw-type lifting anchors were used for this project. Although screw-type lifting anchors are slower for attaching lifting lines, the recesses left by these anchors are much smaller and easier to patch, and the pavement can be opened to traffic prior to patching them.
An important consideration with respect to lifting and handling such large precast panels is to ensure that the lifting lines are as near to vertical as possible to minimize any bending stresses caused by the lifting angle. The PCI Design Handbook recommends a lifting angle (angle between the top surface of the panel and the lifting line) of at least 60°.(3) Based on this recommendation, and assuming the four lifting lines come to a common point at the crane hook, the minimum length for the lifting lines for the panels used for this project was 6.7 m (22 ft). While the precast fabricator used a travel-lift with a spreader beam and nearly vertical lifting lines for handling at the plant, the 6.7-m (22 ft) minimum lifting line length was required for the crane used for on-site installation. Figure 22 shows a panel being removed from the forms at the precast plant after curing using a travel-lift with a spreader beam.
Figure 22. Photo. Removal of a panel from the forms after curing.
All of the precast panels were stored at the fabrication plant until installation. Because of the uneven surface of the panels, it was not possible to stack the panels on top of one another. The panels were stored at the plant with two supports at approximately the same location as the lifting anchors. The panels were continuously checked to ensure cracking did not occur during storage.
Several weeks prior to installation of the panels on site, the precast fabricator conducted a trial assembly of three panels at the precast plant. The panels were assembled over a prepared base material to simulate on-grade installation. The trial installation demonstrated excellent fit between the panels.
Panel repairs during fabrication were addressed on a case-by-case basis. The precast fabricator was required to repair any damage to the keyways, pockets, or surface of the panels that occurred during fabrication. Repairs to keyways were required to be such that the repaired area did not protrude (bulge) beyond the adjacent true edge of the keyway. Repairs to the surface of the panels required sawcutting a clean void around the repair area and patching with a Caltrans-approved patching material.
Only one repair was required due to damage that occurred during the trial assembly of the panels. During the trial assembly, the top lip of the female keyway of one of the panels ruptured at the outside edge (pavement shoulder) of the panel. To repair this, the precast fabricator was required to sawcut and remove the damaged area, drill and epoxy rebar into the existing concrete so that it would extend into the repair area, and patch the keyway such that it would not affect the fit of the panel. This repair was completed and did not affect assembly of the panel during the on-site installation.
Challenges and Problems Encountered
Corner Break—The only major problem encountered during fabrication was the corner break discussed previously, which occurred during the trial assembly at the precast plant. The reason for the break is not known, but it could have been caused by debris in the keyway during assembly or by a slight irregularity in the keyway that resulted in a wedging action when the panels were assembled. A similar problem encountered during construction is discussed in chapter 6.
Mid-panel Cracking—An issue that was encountered during the Texas pilot project was the formation of a hairline crack in the top surface of the panel approximately mid-way from the ends.(2) This crack was believed to have been caused by the temperature change experienced by the panel in the first 24 hours after casting. As the panel cools after reaching the peak hydration temperature and tries to contract, the movement is restrained by the casting bed, causing the crack to form. Similar mid-panel cracking was observed in some of the panels for the California project, most likely due to the same condition. Fortunately, these are only hairline cracks, which are essentially unnoticeable after release of prestressing, and they will be held closed by the pretensioning in the panels. Figure 23 shows one of these mid-panel hairline cracks, which are nearly invisible to the unaided eye.
Figure 23. Photo. Mid-panel hairline crack after curing.