|FHWA > Engineering > Pavements > Concrete > FHWA-IF-07-034 > Chapter 5|
Construction of the Iowa Highway 60 Precast Prestressed Concrete Pavement Bridge Approach Slab Demonstration Project
Chapter 5. Panel Fabrication
The precast panels for the Highway 60 approach slabs were fabricated by IPC, Inc., in Iowa Falls, Iowa. The fabrication plant was located approximately 280 km (175 miles) from the project site in Sheldon. The panels were fabricated on an indoor bed large enough to cast one panel at a time. Because of the limited number of panels for the project, a larger fabrication operation (e.g., long line fabrication) was not necessary. Also, because pretensioning was not used, the panels did not need to be cast on a bed set up for prestressing. The sideforms were specially made for this project in-house by IPC using laminate wood.
In general, bed setup took approximately a full day to complete, particularly for the abutment panels which had additional reinforcement and post-tensioning anchor pockets. Concrete placement for each panel was completed in approximately 30 to 45 minutes. Following surface finishing, curing compound was applied to the panel surface and the panel was then covered and heat-cured overnight. In general, the panels were removed from the forms the day after casting and stacked for storage in the yard. Additional details of the fabrication process will be discussed below.
As with all previous PPCP demonstration projects, the precast panels for the Highway 60 project were not match-cast. This required special attention to tolerances to ensure that adjoining panels would fit together and provide a satisfactory riding surface. Table 4 summarizes the tolerances for the precast panels as specified in the project plans. It is important to note that while most of these tolerances are tighter than normally specified for similar precast products and those recommended by the Precast/Prestressed Concrete Institute, they are based on experience from previous demonstration projects in which there were no problems in achieving these tolerances.
The Highway 60 approach slab was a very unique application for PPCP and required unique details to be developed. Below are some of the key details of the precast panels (see appendix A for all detailed panel drawings).
Keyways were used for both the transverse joints between individual panels, and for the longitudinal joint at the centerline of the pavement. The primary purpose of the transverse keyways was to help ensure vertical alignment between panels as they were assembled. This permitted the panels to be installed over a base that was not perfectly flat while still providing load transfer between panels prior to post-tensioning. Figure 15 shows the transverse keyway dimensions. These dimensions, which were based on experience with previous PPCP demonstration projects, ensured no more than a 6-mm (1/4 in.) vertical differential between panels, while also providing a keyway that was “loose” enough to accommodate minor irregularities in the keyway. The chamfer along the bottom edge of the panel helps to reduce the chance of spalling as the panel is removed from the forms.
Figure 15. Illustration. Transverse keyway dimensions for the precast panels (dimensions in inches). (Note: 1 in. = 25.4 mm)
An open keyway was specified for the longitudinal cast-in-place joint in order to accommodate the crowned pavement cross section. Figure 16 shows the dimensions of this keyway. Because the panels on either side of the pavement centerline were set at opposing cross slopes, the edges of the panels at the longitudinal joint would need to be battered to achieve full contact across the joint. The open keyway, which is grouted after the panels are installed, provides tolerance for imperfections in the panel edges and slight vertical and horizontal misalignment of the precast panels. Load transfer across the joint is established after grouting and post-tensioning across the joint. The keyway is filled with a low-slump mortar or grout with minimal shrinkage. Partial post-tensioning of the joint as soon as the fill material has developed strength helps to prevent shrinkage cracking in the joint.
Figure 6 (chapter 3) shows the layout of the post-tensioning tendons for the approach slabs. Monostrand post-tensioning tendons were spaced at approximately 610 mm (24 in.) on center in both the transverse and longitudinal directions. Longitudinal post-tensioning anchors were located near the abutment and at the far end of the approach slab, abutting the adjacent pavement. To provide access to the anchors near the abutment, 230-mm (9 in.) square pockets were cast into the panels behind the anchors. These pockets allowed the anchor wedges to be seated in the anchor by hand after the strands were inserted. Flared openings were provided at the end of each post-tensioning duct, at the joints between panels, to facilitate feeding the strands through the ducts, even with slight misalignment of the panels. Additionally, to prevent grout leakage, recesses were formed around each post-tensioning duct to receive compressible foam gaskets, as shown in figure 17.
Anchors for the transverse post-tensioning tendons were located at the outside (shoulder) edges of the approach slab for the non-skewed section of the slab. For the skewed section, the anchors were located at the same anchor access pockets used for the longitudinal tendons, as shown in figure 6. Using these pockets for the transverse tendons resulted in tendon spacing of approximately 305 mm (12 in.) on center for the skewed section of the approach slab.
Semi-rigid polypropylene ducts with a 23-mm (0.9 in.) inside diameter were used for all post-tensioning tendons. The ducts used were specifically designed for bonded monostrand tendons, with corrugations to provide better bond with the concrete and a rib along the length of the duct to facilitate the flow of grout through the duct. Post-tensioning anchors were coated or encapsulated to provide corrosion protection. Grout ports were located just in front of each post-tensioning anchor. The grout port inlets were located on the edges of the precast panels and faces of the anchor access pockets to minimize protrusions from the top surface of the precast panels.
Sleeves for the anchor pins used to tie the approach slab to the paving notch were cast into the skewed panels at 610 mm (24 in.) on center across the width of the approach slab. Anchor sleeves, 50 mm (2 in.) in diameter, were specified to receive stainless steel anchor bars, 25-mm (1 in.) in diameter. The larger sleeves provided enough room for a core bit to be used to drill the holes into the paving notch.
As discussed previously, a dowelled expansion joint was constructed at the end of the approach slab. Sleeves with a 41-mm (1 5/8 in.) inside diameter were cast into the ends of the precast panels to receive dowel bars with a 38-mm (1 1/2 in.) diameter for the expansion joint.
In addition to post-tensioning, mild steel reinforcement was provided in all of the precast panels. A double mat of epoxy-coated reinforcement, similar to that normally used for the doubly-reinforced section of cast-in-place approach slabs, was provided. For the skewed panels (1A and 1B), this consisted of 25-mm (No. 8) bars at 305 mm (12 in.) on center in the bottom of the panel and 19-mm (No. 6) bars at 610 mm (24 in.) on center in the top of the panel in the longitudinal direction, and 16-mm (No. 5) bars at approximately 305 mm (12 in.) on center in the top and bottom of the panel in the transverse direction. For the square panels (2A thru 4B), mild reinforcement consisted of 19-mm (No. 6) bars at 610 mm (24 in.) on center in the top and bottom of the panel in the longitudinal direction, and 16-mm (No. 5) bars at 610 mm (24 in.) on center in the top and bottom of the panel in the transverse direction. Additional mild steel reinforcement was provided through and around the post-tensioning anchor pockets, around the anchor pin sleeves, and in front of the post-tensioning anchors.
Threaded coil inserts were specified for the lifting anchors. Coil inserts leave only a small recess in the surface of the panel, which can be easily patched. A four-point lifting anchor arrangement was used, with the anchors positioned to minimize lifting stresses in the precast panels.
Precast fabrication plants use concrete mixtures and curing methods that will permit maximum productivity while still meeting the materials requirements and strength criteria for the final product. In general, precast plants turn over beds every day or every other day, depending on the amount of time required to set up the bed. The Highway 60 panels were produced in such a manner, using a concrete mixture and curing process that would allow IPC to cast a panel at least every other day.
The mixture design used for the panels was required to reach a compressive strength of 24 MPa (3,500 lbf/in2 ) at form removal and 34 MPa (5,000 lbf/in2 ) at 28 days. The mixture contained 19-mm (3/4 in.) crushed limestone coarse aggregate and sand, had a water/cement (Type I) ratio of 0.4, and included an air-entraining admixture, water-reducing admixture, and set-retarding admixture. The target air content was 6.5 +/- 1 percent before vibration.
A higher strength mixture was provided by IPC, and IADOT inspectors reported average compressive strengths of 38 MPa (5,500 lbf/in2 ) at 12 hours and 59–62 MPa (8,500–9,000 lbf/in2 ) at 28 days. No problems with the concrete mixture were encountered during the fabrication process. Figure 18 shows placement of concrete for one of the abutment panels at the fabrication plant.
Figure 18. Photo. Placement of concrete for an abutment panel.
Finishing and Curing
Based on experience with previous demonstration projects, it was anticipated that diamond grinding would be required to achieve the level of smoothness required for mainline highway pavements. For this reason, only a light broom surface texture was applied to the precast panels to provide a “temporary” surface texture until grinding was completed. Figure 19 shows the light broom texture being applied to the panel surface.
Following application of the surface texture, a heavy coat of curing compound was applied to the surface of the precast panels to minimize moisture loss from the panel surface during curing. Following the application of curing compound, the panel was covered with a curing tent and heat cured until the required compressive strength was reached. Heat curing was used in lieu of steam curing to minimize any swelling of the wood forms used for the precast panels. Figure 20 shows the curing tent being lowered over a precast panel after application of the curing compound.
Figure 19. Photo. Application of the light broom texture to the panel surface.
Figure 20. Photo. Curing tent being lowered to cover a precast panel after casting.
Handling and Storage
After the required compressive strength was reached, the curing tent was removed and the forms were stripped from the precast panel. The panel was moved to a storage location at the precast plant, and a coat of curing compound was applied to the edges of the precast panels to minimize any moisture loss from the exposed surfaces. This curing compound was subsequently sandblasted off prior to shipment of the panels to the jobsite. Panels were stacked four high in groups according to the post-tensioned section each belonged to. Figure 21 shows a stack of panels stored at the precasting plant.
One of the requirements for the Highway 60 project was a trial assembly of the first “set” of precast panels prior to continuing fabrication. After the first four panels (1A through 4A) were fabricated, a trial assembly was conducted at the precast plant to test the fit of the panels. The panels were assembled on the rails of a casting bed to provide a level surface, as shown in figure 22. While some minor damage was sustained to one of the panels during this trial assembly (discussed below), the fit was good and approval was given to complete fabrication of the remaining panels. This trial assembly allowed inspectors and plant personnel to identify any issues with the precast panels that could affect construction on site prior to fabricating all of the panels.
Figure 21. Photo. Precast panels stored at the fabrication plant.
Figure 22. Photo. Trial assembly of precast panels at the fabrication plant (photo by Iowa DOT, reprinted by permission).
Based on experience with previous demonstration projects, minor damage to the precast panels at the fabrication plant was anticipated. Damage was addressed on a case-by-case basis by IADOT. For pavement panels, the keyways and the top surface are the critical areas, because damage to the keyways can adversely affect installation of the panels and damage to the top surface can affect the final ride quality of the pavement.
Based on damage sustained at the precast plant during the trial assembly and general handling, a repair procedure was established by IADOT. Any deep spalls, corner breaks, or keyway fractures were required to be removed, cleaned, and patched. One significant corner repair and one keyway repair were required from damage during the trial assembly, as shown in figures 23 and 24.
In addition to damage sustained during the trial assembly, additional grinding of the keyways was required prior to shipping the panels to the project site. As figure 25 shows, mortar trapped behind the keyway former on some of the sideforms and swelling of the forms due to repeated use resulted in battered top and bottom vertical faces on a few of the panels. This batter was removed by grinding the face of the keyway so that it was vertical and true. Figure 26 shows the keyway surface after grinding, prior to installation of the panel.
Figure 23. Photo. Damaged panel corner after trial assembly.
Figure 24. Photo. Damaged keyway after trial assembly.
Figure 25. Photo. Battered keyway after removal from forms.
Figure 26. Photo. Keyway after completion of grinding.
Fabrication Issues and Challenges
While the fabrication process was completely successful and no precast panels were rejected, several issues were discovered during the course of the process. Below are some of the key issues that need to be specifically addressed for future projects.
The post-tensioning ducts used for this project are ideal for monostrand post-tensioning tendons. However, because this duct material has some flexibility, it requires bar stiffeners to be inserted into the ducts prior to placing concrete in the forms. It also requires adequate chairing to prevent the duct from sagging.
As mentioned above, additional reinforcement was provided around the post-tensioning anchor access pockets. Combined with the reinforcement for the anchor pin sleeves, this added reinforcement created a very congested region around the pockets. This required a significant amount of time to tie all of the reinforcement and required special attention to concrete placement around the pockets to ensure that concrete had filled in around all of the reinforcement. For future projects, different options for reinforcing this region should be examined to reduce congestion.
As discussed above, the wood sideforms used for the precast panels accumulated mortar beneath the keyway former, resulting in vertical batter of the keyway faces for some of the panels. While wood forms provide an economical solution and can be manufactured to the tolerances required for this type of fabrication, they are probably not suitable for significant reuse. Although they can be much more costly, larger projects may necessitate the use of steel forms, which are more durable and less susceptible to the issues encountered with the Highway 60 panels.
The primary damage that occurred at the fabrication plant happened during the trial assembly. While this damage was adequately repaired, damage to precast pavement panels should be avoided at all costs as it can cause problems with the assembly of the panels and can also affect the smoothness of the riding surface. For future projects, damage tolerances and repair procedures should be prepared and approved in advance of the fabrication process so they can be addressed and resolved quickly.