|FHWA > Engineering > Pavements > HIF-08-009 > Chapter 5. Panel Fabrication|
Construction of a Precast Prestressed Concrete Pavement Demonstration Project on Interstate 57 Near Sikeston, Missouri
Chapter 5. Panel Fabrication
The precast panels for the Missouri demonstration project were fabricated by CPI Concrete Products, Inc., of Memphis, Tennessee, approximately 240 km (150 mi) from the I-57 project site. Before fabrication began, detailed shop drawings were developed by CPI and submitted to MoDOT for approval. The shop drawings provided the details for the formwork, including the post-tensioning anchor blockouts, which had to be specially sized for each blockout. Panel fabrication began on October 1, 2005, and was completed by December 16, 2005.
The panels were fabricated on a flat steel casting table, 4.3 m (14 ft) wide, which was long enough to produce up to two panels end to end. Steel sideforms and bulkheads were custom made for the panels. The pretensioning strands extended through both panels, anchored at bulkheads at either end of the self-stressing casting bed. Bulkheads between the precast panels, bolted down to the casting bed, were used to hold the pretensioning strands at the proper elevation at the ends of the precast panels. Steel chairs were used to harp the strands within the precast panels to match the strand profile provided in the panel detail drawings. Figure 16 shows the pretensioning strands being threaded through the forms.
Figure 16. Photo. Precast panel forms during installation of the pretensioning strands.
In general, two base panels were cast each day. Panels were normally stripped from the forms in the morning and the bed was cleaned and set up for an afternoon pour. Joint panels required additional form set-up time due to the anchor blockouts and additional reinforcement and post-tensioning anchors. Joint panels were each cast in two halves on separate days. A separate sideform was used to hold the dowel bars in place and form the recess for the header material at the expansion joint. The first half of the joint panel was cast and steam cured, but the pretensioning strands were not released until after the second half had been subsequently cast and cured. The second half of the panel was set up and cast either the afternoon of the same day (if the first half was cast in the morning) or the following day. Figure 17 shows the fabrication of a base panel, and Figure 18 shows fabrication of a joint panel during concrete placement for the first half of the panel.
Figure 17. Photo. Concrete placement for a base panel.
Figure 18. Photo. Fabrication of the first half of a joint panel.
Tolerances specified in the Job Special Provisions were based on experience with previous demonstration projects in Texas and California.(2,3,4) Tolerances for the keyways, post-tensioning ducts, and overall panel dimensions were particularly critical. Keyway tolerances helped to ensure that adjoining panels would fit together and provide a satisfactory riding surface. Tolerances for the position of the post-tensioning ducts helped to ensure that the ducts lined up between panels, and tolerances for overall dimensions helped to ensure that the panels were "square" so that a "curve" was not built into the finished pavement as the panels were assembled on site. Table 7 summarizes the tolerances for the precast panels, as shown in the Job Special Provisions.
Minimal mild-steel (non-prestressed) reinforcement was provided in the base panels. As the panel detail drawings in Appendix A show, only perimeter steel at the top and bottom of the panel was provided for the base panels. Joint panels also had perimeter steel, but were heavily reinforced with stirrups around the stressing pockets and post-tensioning anchors, as discussed previously. Epoxy-Coated, Grade 60, No. 4, deformed bars were specified for all non prestressed reinforcement. For the joint panels abutting the existing cast-in-place pavement, the half of the panel not post-tensioned was reinforced with mild steel in the longitudinal direction. No. 6 epoxy-coated reinforcing bars, spaced at 200 mm (8 in.) on center, provided approximately 0.65 percent steel for this portion of the precast panel. Figure 19 shows the reinforcement for the non-post-tensioned half of a joint panel prior to concrete placement.
Figure 19. Photo. Reinforcement for the non-post-tensioned half of a joint panel prior to casting.
All pretensioning steel was 13-mm (0.5 in.) diameter, Grade 270, low relaxation, 7-wire strand. For the base panels, eight strands were required. The strands were spaced evenly across the width of the base panels and alternated above and below the post-tensioning ducts to minimize any prestress eccentricity. For the joint panels, the pretensioning strands were concentrated in the outer edges and along the expansion joint to avoid crossing through the post-tensioning blockouts. Twelve strands were specified for the joint panels for to provide additional flexural strength for handling panels with post-tensioning blockouts. For both the base panels and joint panels, the strands were maintained at a constant depth over the portion of the panel that was 200 mm (8 in.) or thicker (traffic lanes). However, in the shoulder regions of the panels, it was necessary to harp the strands slightly to follow the top surface of the panels.
It should be noted that the pretensioning strands were only tensioned to 75 percent of their ultimate strength, rather than the 80 percent assumed during the design process. While this will reduce the effective prestress in the panels, the lump-sum prestress loss assumed in the design calculations is likely very conservative anyway. Based on actual lifting and handling of the precast panels, the pretensioning level was clearly adequate to prevent cracking.
Monostrand post-tensioning ducts with an inside diameter of 23 mm (0.9 in.) were used for the longitudinal post-tensioning tendons. The ducts were developed specifically for monostrand post-tensioning and are made of corrosion-proof polyethylene material. The ducts are ideal for monostrand bonded tendons, with ribs to facilitate bond with the concrete and two continuous channels along the length of the duct to facilitate the flow of grout. The ducts are flexible and required adequate chairing and the use of bar stiffeners inserted in the ducts during concrete placement to prevent sagging and bowing of the duct under the mass of fresh concrete.
Encapsulated monostrand anchors were used for the post-tensioning anchors. The anchors were cast into the joint panels and bolted to the post-tensioning blockout formers. Grout ports were located just in front of the anchors and in every fifth base panel along the length of each post-tensioned section of pavement. Trumpeted openings and recesses for the foam gaskets were formed at the ends of the post-tensioning ducts in each panel using machined steel recess formers. Figure 20 shows the post-tensioning duct, anchor, and blockout former used in the joint panels.
Figure 20. Photo. Post-tensioning duct, anchor, and blockout former in a joint panel.
Based on experience with previous demonstration projects, threaded coil inserts were specified for the lifting anchors. These anchors leave only a small hole in the surface of the precast panel to patch, and could potentially be left unpatched in a nonfreezing environment. The lifting anchors were recessed below the surface of the panel slightly to minimize protrusions from the panel surface during screeding of the fresh concrete. The lifting inserts were made of galvanized steel to minimize the risk of corrosion of the anchors over time. The final location of the lifting inserts was determined by the precast fabricator such that the weight on each lifting line would be balanced.
To produce a set of panels every day, a concrete mixture was needed that would provide the necessary release strength within 12 to 15 hours after casting and steam curing. The concrete mixture also needed to be one that could be finished to the necessary tolerances required for the pavement driving surface. The compressive strength specified for the concrete mixture was 24.1 MPa (3,500 lbf/in2) at release of prestress and 34.5 MPa (5,000 lbf/in2) at 28 days. As presented in Chapter 7, actual 7-day strengths averaged 41.9 MPa (6,070 lbf/in2), and actual 28-day strengths averaged 49.6 MPa (7,190 lbf/in2). A 5 percent total air content and 160-mm (6 in.) slump were also required for the mixture.
The mixture used for the panels consisted of Type I portland cement with a 0.326 water-to-cementitious materials ratio, fine and coarse (limestone) aggregate, air-entraining admixture, and two water-reducing admixtures. The very low water-cement ratio helped to ensure that release strength could be achieved within 12-15 hours, while the water-reducing admixtures provided the necessary workability for placing the concrete in the forms. CPI utilizes a very advanced concrete batch plant for producing concrete mixtures. Proportioning is fully automated and the mixture components are combined using a twin-shaft mixer requiring only 30 seconds of mix time. Table 8 shows the concrete mixture used for the precast panels.
Concrete Placement, Finishing, and Curing
The concrete mixture was transported only a short distance (< 100 m) from the batch plant to the forms. Concrete was placed in the forms in two lifts, the first filling the forms to the level of the post-tensioning ducts and the second, immediately following the first, filling the forms to the top surface, as shown in Figure 21. The flowable nature of the mixture required only minimal vibration to consolidate the concrete around the reinforcement, ducts, and blockouts.
A hand screed was used to initially strike off the concrete surface, flowed by a vibratory screed to achieve a uniform, smooth surface. An intermediate curing compound (Confilm®) was sprayed over the surface between the hand screed and vibratory screed to minimize moisture loss from the large surface area of the fresh concrete. Immediately following the vibratory screed, a light broom texture was applied along the length of the panels (transverse to the direction of traffic flow). A carpet drag texture was specified in the original Job Special Provisions, but was changed to a broom finish after problems were experienced with the carpet drag. Figure 22 and Figure 23 show the screeding and texturing operations, respectively.
Figure 21. Photo. Concrete placement operation showing the two-lift procedure.
Figure 22. Photo. Vibratory screeding of the panel surface.
Figure 23. Photo. Application of the light broom texture to the panel surface.
After application of the surface texture, the panels were covered with tarps and steam cured overnight, as shown in Figure 24. The steam was generally turned off by 5 a.m., at which time strength was checked. If the strength was adequate for release, the tarps were removed and the pretensioning strands de-tensioned. If strength was not adequate, the steam was turned back on for 1 to 2 hours until adequate strength had been achieved. While steam curing was used to achieve the necessary release strength overnight, some additional form of curing on the top surface and sides of the panels was required for a minimum of 72 hours after concrete placement or until the 28-day strength had been achieved. This requirement was satisfied by covering the panels with wet burlap and plastic sheeting for an additional 24 hours after they were moved from the forms to the storage area, as shown in Figure 25.
Figure 24. Photo. Casting bed during steam curing operation.
Figure 25. Photo. Wet burlap mat curing of the precast panels after removal from the forms.
De-tensioning, Handling, and Storage
Once the panels had reached the necessary release strength, the pretensioning strands were de-tensioned. De-tensioning was completed by flame-cutting the pretensioning strands at both ends of the casting bed. The same strand was cut simultaneously at each end in a specific order to ensure proper transfer of prestress into the concrete. De-tensioning began with the strands at the outside edges of the panels, moving towards the center of the panel, alternating between strands on either side of the centerline.
The precast panels were removed from the forms and transported to the storage area using a straddle-lift crane, as shown in Figure 26. Following the PCI Design Handbook recommendations, a lifting angle (angle between the top surface of the panel and the lifting line) of at least 60 degrees was required to minimize bending stresses in the panels when lifted.(17) Because a spreader beam was used for lifting the panels at the fabrication plant, this was not a concern. For the joint panels, steel strongbacks (angle steel) were bolted across the expansion joints, using the lifting anchors as bolt-down points, to ensure that expansion joints remained closed during handling and that the dowels were not subjected to bending.
Figure 26. Photo. Removal of a panel from the forms after curing.
All of the precast panels were stored at the fabrication plant until installation. Each panel was initially stacked individually until it was fully cured and inspected. The bar stiffeners were removed from the post-tensioning ducts, and the ducts were capped with a foam plug to ensure that nothing would get into the ducts prior to installation on site. The stubs of the pretensioning strands were cut back flush with the edge of the panel and patched with an epoxy mixture to prevent corrosion of the ends of the strands. The panels were stacked 10 high in the storage area, as shown in Figure 27. Careful attention to the thickness and location of the dunnage between precast panels was required due to the sloped top surface of the panels.
Figure 27. Photo. Panels stacked in the storage area after final curing and inspection.
As part of the Job Special Provisions, the precast fabricator was required to demonstrate the fit of the panels prior to beginning large-scale production. Two panels were initially fabricated and assembled at the precast plant. After proper fit of these panels was demonstrated, approval was given to continue with full-scale production of the remaining panels.
Repairs to panels damaged at the fabrication plant were addressed by MoDOT on a case-by-case basis. Of primary concern was any damage to the mating edges and keyways and to the top (riding surface) of the panels. Any major spalls or corner breaks were required to be cleaned, sawcut square, and patched with a MoDOT-approved patch material. Any minor hairline cracking in the top surface of the panels was required to be sealed with epoxy. Minor, shallow (< 6 mm [1/4 in.] deep) spalls in the shoulder regions of the panels were generally not required to be repaired.
Challenges and Issues Encountered
Overall, no major problems were encountered with the panel fabrication process, and no precast panels were rejected. The level of quality control and the quality of the finished precast panels was very good. Some of the issues that were encountered during the fabrication process are discussed below.
Longitudinal Cracking-The primary distress observed at the fabrication plant was hairline cracking along the long axis of the panel (Figure 28). Coring of these cracks from selected panels revealed that they generally extended the full depth of the precast panel. While the exact cause of this cracking has been difficult to determine, temperature and strain data collected in several of the panels during fabrication (described in Chapter 7) indicate very high and rapid changes in concrete temperature and strain during fabrication. This indicates that "thermal shock" could have caused stresses in the precast panels high enough to result in cracking. Fortunately, longitudinal post-tensioning in the finished pavement will help keep these cracks closed, and the epoxy injected in the cracks at the fabrication plant will prevent the ingress of water into these cracks.
Figure 28. Photo. Typical hairline longitudinal crack observed at the fabrication plant (photo taken after precast panel was delivered to the project site).
Transverse Cracking-A few instances of hairline cracking in the transverse (short) direction were also observed at the fabrication plant. The exact cause of these cracks is not known for sure, but is possibly related to harping of the pretensioning strands in combination with the relatively thin panel depth in the shoulder region. Fortunately, these cracks generally occurred in the shoulder regions of the precast panels and will be held tightly closed by the transverse pretensioning in the panels. These cracks were sealed with epoxy at the fabrication plant, as shown in Figure 29.
Figure 29. Hairline transverse crack in the shoulder after epoxy sealing (photo taken after panel was installed on site).