|FHWA > Engineering > Pavements > Concrete > FHWA-IF-07-034 > Chapter 7|
Construction of the Iowa Highway 60 Precast Prestressed Concrete Pavement Bridge Approach Slab Demonstration Project
Chapter 7. Project Evaluation and Recommendations for Future Projects
The Highway 60 approach slab demonstration project provided many unique challenges for the design and construction of PPCP. Many new ideas were developed and many lessons were learned throughout the course of the project. Some of the more salient issues related to the overall project layout, design, fabrication, and construction are presented below with recommendations for future projects.
The precast panel layout for the Highway 60 project worked very well. Using the first two panels at the abutment to “remove” the skew from the approach slab minimized the number of specialty panels that had to be fabricated. Using rectangular panels of the same size for the majority of the approach slab greatly simplified panel fabrication and installation. Two-way post-tensioning also simplified fabrication by eliminating pretensioning.
The cast-in-place keyed longitudinal joint accommodated the crowned pavement section and simplified fabrication of the mating panel edges. Transverse post-tensioning will keep the longitudinal joint in compression and help ensure that it remains closed over time. Dowel sleeves cast into the ends of the precast panels facilitated construction of the expansion joint at the end of the approach slab and eliminated the need for any connection to tie or anchor the approach slab to the adjoining pavement. Future projects, which may not have a leave-out for cast-in-place pavement at the end of the approach slab, will require some technique for dowelling the approach slab to existing pavement.
A similar panel layout should be considered for future projects. This layout provides flexibility with varying skew angles, pavement cross sections (crowned or uniform cross slope), and approach slab length and width. Also, by using partial-width panels, lane-by-lane construction can be utilized when full closure of the approach slab is not possible during reconstruction. Two-way post-tensioning should also be considered for future projects, but larger panel sizes may necessitate pretensioning to compensate for lifting and handling stresses.
An alternative panel layout proposed by IADOT would use only a single panel on either side of the longitudinal joint. These panels would likely be pretensioned in the longitudinal direction for handling purposes and post-tensioned together across the longitudinal joint after installation. The ends of the precast panels would be skewed to match the bridge abutment skew. This panel layout would limit the length of the approach slab (and the distance the expansion joint is from the abutment) to the maximum length of precast panel that could be safely fabricated, handled, and shipped. This layout option will likely be considered for future approach slab reconstruction projects.
As discussed in chapter 4, no design procedures for precast prestressed bridge approach slabs have been established. The methodology employed for the Highway 60 approach slabs was a design for flexure, treating the approach slab as a simple span “slab bridge.” While this provided a sound procedure for approach slab design, it resulted in very high prestress requirements for the Highway 60 approach slabs, particularly if the contribution of the mild steel reinforcement to flexural strength is ignored. For future projects, other design procedures may be examined or standardized prestress requirements could be adopted based on the Highway 60 project.
While many of the basic design details stemmed from previous PPCP demonstration projects, most had to be modified for this project, and other unique details had to be developed.
Keyways—The keyway dimensions for the Highway 60 precast panels were based on dimensions used successfully for previous demonstration projects in Texas, California, and Missouri. Because of the increased panel thickness, however, the depth of the interior vertical face was increased to provide more surface area around the post-tensioning ducts to seal the duct openings better. The keyways helped align the panels vertically and resulted in a satisfactory finished surface with essentially no “faults” or lips at the joints between panels. Similar keyway dimensions should be considered for future projects.
Longitudinal Joint—The primary benefit of the cast-in-place keyed longitudinal joint was accommodation of the crowned pavement cross section, permitting the panels on either side of the joint to be set at opposing cross slopes. While a cast-in-place joint does add an additional step to the construction process, the amount of fill material needed was minimal, and if necessary, the joint could have been temporarily filled or covered and opened to traffic prior to filling. A similar longitudinal joint detail should be considered for future projects with crowned cross sections. For a uniform cross slope, a butt joint could potentially be used at the longitudinal joint.
Post-tensioning Anchor Pockets—In detailing the post-tensioning anchor pockets, the goal was to keep them as small as possible while making them large enough to permit the post-tensioning anchor wedges to be manually inserted in the anchors. While the contractor would have preferred larger pockets, the size was adequate for inserting the anchor wedges by hand, and it also minimized the amount of fill required. Similar pocket dimensions should be considered for future projects. It is important to note, however, that these smaller pockets can only be used for dead-end post-tensioning anchors and not for feeding the strands into the ducts.
In some situations it may be necessary to feed the post-tensioning strands from the bridge end of the approach slab rather than at the pavement end. In this situation, it may be possible to cast narrow slots into the surface of the panels, behind the anchor pockets, that are wide enough to receive the post-tensioning strands as they are fed into the anchors. This would allow the strands to be laid out on the bridge deck prior to feeding them through the panels.
Abutment Anchor—The pins drilled and grouted into the paving notch provided a simple solution for anchoring the approach slab to the abutment. Instrumentation on the south approach slab will provide an indication of how well the pinned connections perform. Future projects should consider a similar detail, but may also consider an option for post-tensioning the approach slab to the abutment. While post-tensioning to the abutment would add complexity to the construction process and would require very careful attention in matching the skew angle of the approach slab to the abutment, a post-tensioned connection may provide a tighter and more durable joint, reducing the probability of water infiltration at the joint.
Underslab Grouting—The ports cast into the precast panels for underslab grouting appeared to be adequate for ensuring that voids beneath the precast panels were filled. In most instances, grout pumped into one port could be seen flowing from an adjacent port, indicating full grouting beneath the slab. Similar grout port details should be provided for future projects.
Below are some of the key aspects of the panel fabrication process for the Highway 60 project and recommendations for future projects.
Tolerances—Other than problems with out-of-tolerance keyways, discussed in chapter 5, no problems were reported in achieving the specified tolerances. These tolerances should be used as a baseline for future projects.
Formwork—As discussed in chapter 5, with a very limited number of panels to fabricate, the precast producer opted to use wood formwork for the panels, and no problems were reported in achieving the tolerances specified in the contract documents. There were, however, problems with the keyway sideforms that occurred over the course of the fabrication process, as discussed in chapter 5. These problems could be mitigated on future projects with closer inspection of the sideforms on a daily basis. Alternatively, steel sideforms, which are produced to very precise tolerances and are less susceptible to problems that occur with continual reuse, may be used. Steel forms may not be cost effective for smaller projects with a very limited number of panels, however.
Reinforcement—No problems were reported with the reinforcement layout other than around the post-tensioning anchor access pockets where reinforcement was somewhat congested and difficult to install properly. For future projects, the reinforcement layout in the anchor region should be re-evaluated to ensure that there will not be any problems with placing the reinforcement and placing the concrete around the reinforcement.
Finishing—Neither the precast producer nor the IADOT inspectors reported any problems with the finishing operation. A uniform panel thickness kept the finishing process simple, with a vibratory screed used to level the top surface of the precast panels. A light broom texture was applied to the surface to provide “temporary” texture until diamond grinding was completed. Minimizing the number of protrusions from the panel surface also benefited the finishing operation. Only the tubes used to form the underslab grout ports and the post-tensioning anchor/instrumentation pockets were protruding from the surface.
For future projects, uniform thickness panels should be specified whenever possible, although previous demonstration projects have successfully used variable thickness panels. The precast producer should also try to minimize protrusions from the panel surface. When diamond grinding is anticipated, a light broom texture should provide adequate surface texture until diamond grinding is complete.
Curing—No problems were reported with the panel curing operation. A heavy coat of curing compound was applied to the panel surface after texturing, and the panels were then heat-cured overnight. Curing compound was applied to the edges of the panels after they were removed from the forms and later sandblasted from the keyway prior to shipment of the panels. This curing process produced the concrete strength required by the precaster and did not result in any noticeable shrinkage cracking in the panels. A similar process should be utilized on future projects, although other alternative curing processes such as steam curing and wet-mat curing have been successfully used elsewhere.
Handling and Storage—Although some minor damage was sustained during handling of the precast panels at the fabrication plant, no major problems with handling and storage were encountered. The threaded-insert lifting anchors were adequate for the project and left only a small hole in the panel surface to be filled.
Panel installation was a successful process, as discussed in chapter 6. There were, however, some issues that could be improved for future projects.
Materials—While the crushed limestone aggregate worked adequately for the base beneath the Highway 60 approach slabs, the coarse nature of the material made it difficult to grade precisely, leaving voids beneath the approach slabs. While underslab grouting filled these voids, ideally they should be minimized during grading of the base. The use of a finer material that can be graded more easily at the surface should be considered for future projects. Any finer material that is used, however, should be checked to see that it has the necessary drainage characteristics and that it does not have an excessive percentage of very fine (passing No. 200 sieve) particles.
Leveling Technique—The crushed limestone base was graded by hand and compacted with a portable plate vibrator. A rotating laser mounted on a tripod, which is commonly used for site grading, established the grade line and 2 percent cross slope for the surface of the aggregate base. This technique provides a very precise grade line and should be utilized for future projects whenever possible.
Alternative Techniques—An alternative technique proposed by the contractor for quickly establishing the proper elevation would be to set steel plates on the base at the proper elevation at the location of the corners of the precast panels. Setting the precast panels on these steel plates would ensure that the surface of the pavement was at the proper elevation and cross slope. The void beneath the pavement would then be filled with grout or flowable fill through underslab grout ports in the panel. While this technique would likely require a significant amount of grout/flowable fill beneath the approach slab, it would minimize time-consuming precision grading of the base. If this technique is utilized on future projects, however, it is essential that the grouting operation be completed before opening the pavement to traffic because the panels would only be supported at the corners prior to grouting.
Joint Treatment/Epoxy—High-viscosity, gel-paste epoxy was applied to the panel keyways as they were installed. This epoxy helped lubricate the keyways for installation and sealed the joints to prevent water intrusion. The epoxy also helped to compensate for slight unevenness and irregularities in the mating surfaces of the panels. The epoxy filled in any “low” areas of the keyway such that the panels were in complete contact across the joint. This type of epoxy material and assembly technique should be utilized for future projects, particularly if unevenness is observed in the keyways. If unevenness is observed, it is important that only partial post-tensioning force be applied to the joint until the epoxy has set, as stress concentrations from nonuniform contact could result in spalling of the keyway if the full post-tensioning force were applied. When using this two-stage post-tensioning process under strict time constraints (e.g., overnight construction), an epoxy with an appropriate set time should be used.
Temporary Post-Tensioning—The purpose of temporary post-tensioning is to close the joints between panels as much as possible prior to final post-tensioning. It has been used successfully on all previous demonstration projects, and was beneficial for the Highway 60 project as well. Depending on the size of the precast panels, no more than two panels should be installed between temporary stressing operations. If an epoxy with a short pot life is used, temporary post-tensioning should be applied after each panel is installed to ensure that the panels are fully bedded in the epoxy before it sets.
Panel/Duct Alignment—No problems were experienced with longitudinal post-tensioning duct alignment. A mark on the top surface of the precast panels at mating joints above a given post-tensioning duct should always be used for alignment, rather than aligning the ends of the panels. This will help to ensure alignment of the post-tensioning ducts even if the edges of the panels do not line up.
As discussed in chapter 7, there was slight misalignment of the transverse post-tensioning ducts across the longitudinal joint. While this minor misalignment did not cause any problems, provision should be made to allow for slight misalignment. For future projects with a longitudinal joint, flat post-tensioning ducts (25 mm [1 in.] by 76 mm [3 in.]) should be considered for the transverse tendons. These will permit as much as 50 mm (2 in.) of misalignment across the longitudinal joint.
Ducts—The post-tensioning ducts used for the Highway 60 project are ideally suited for bonded monostrand post-tensioning tendons. The polyethylene duct material is corrosion resistant, and the channels running along the length of the duct facilitate the flow of grout even if a strand is pressing against the duct. Although the precast producer reported some challenges with installing the ducts in the forms, this material is recommended for future projects. A larger diameter duct or flat duct of the same material will permit some misalignment of the panels.
Post-Tensioning Tendons—No problems were reported with the strand used for the post-tensioning tendons. Grade 270, 15 mm (0.6 in.) diameter, strand is commonly used in the post-tensioning industry and maximizes the prestress provided by each tendon, permitting almost 42 percent more prestressing force (when tensioned to 75 percent of ultimate strength) than comparable 13-mm (1/2 in.) diameter strand. If corrosion of the post-tensioning tendons (particularly at the joints between panels) is a concern, epoxy-coated strand and grit-impregnated epoxy-coated strand are available.
An alternative to post-tensioning strand would be high-strength post-tensioning bars. One advantage of bars is a significant reduction in anchor seating loss during stressing of the tendon. Bars would also permit the panels to be incrementally stressed together as they are installed. Sections of bar the length of each panel could be pre-installed in the ducts of the panel and coupled together with the bars from the adjoining panel as the panel is installed. This would require special attention to detailing of the ducts and anchors, but may provide a more efficient solution in certain cases.
Post-tensioning Anchors—Post-tensioning was completed from the live-end anchors at the edges and ends of the approach slabs. Dead-end anchors were located at either the anchor access pockets (longitudinal tendons) or at the edges of the approach slabs (transverse tendons). For future reconstruction projects, there may not be access to the edges and ends of the approach slab. This situation may require additional stressing pockets for the live-end anchors within the precast panels for the longitudinal and transverse tendons. Fortunately, this has been accomplished successfully with previous demonstration projects.(6,7)
Grouting Sequence—Based on the grouting operations for the Highway 60 project, it is recommended that underslab grouting be completed prior to tendon grouting. Underslab grouting will fill voids beneath the approach slab so that tendon grout cannot flow beneath the slab. This will help minimize loss of tendon grout, which is a more expensive material specially formulated for tendon grouting.
Joint Seal/Gasket—Significant leakage of grout during grouting of the longitudinal post-tensioning tendons indicated that a proper seal was not provided across all of the panel joints. The gaskets were carefully installed in recesses around the post-tensioning ducts as each panel was installed, so it is unlikely that the gaskets fell off or shifted. The gaskets likely leaked due to the pressure from the grouting operation. For future projects, the combination of gaskets and gel paste epoxy are still recommended. However, wider gaskets which provide greater bearing area (minimum of 25 mm [1 in.] wide) should be used, and a stiffer foam or neoprene material should also be considered.
Grout Vents—Tendon grout vents were “daylighted” at the edges of the panels and inside the anchor access pockets to minimize protrusions from the surface of the precast panels. For future projects, where access to the edges of the panels may not be possible, it may be necessary to daylight the vents at the surface of the pavement or within the stressing pockets only.
Smooth plastic tubes were used to form the underslab grout vents in the panels. This made the connection between the grouting hose and vent difficult even with the low grouting pressures that were used. For future projects, traditional ribbed or threaded grout vent tubing should be considered so that couplers can be screwed into the vents for attaching the grout hose.
Following installation and diamond grinding of the Highway 60 approach slabs, a visual condition survey was conducted to look for distresses that may have occurred during construction. The only visual distress observed was a hairline crack extending across Panel 2A of the south approach slab. The crack began at the edge of the approach slab and extended diagonally to the centerline joint, but did not cross the joint. This crack will be monitored over time to determine if it is indicative of a structural problem with the approach slab.
The instrumentation installed in the south approach slab and northbound bridge will be monitored for a minimum of 12 months after construction by Iowa State University. In addition to data collected from the instrumentation, regular visual surveys will be conducted to identify any distresses in the approach slabs over time.
The final unit cost for the Highway 60 PPCP approach slab demonstration project was approximately $890/m2 ($739/yd2 ). This cost includes all fabrication and installation costs, including grading of the base and anchoring the slab to the abutment. While this unit cost is significantly higher than that of conventional concrete pavement, the higher cost was not unexpected for several reasons. First, the project was experimental in nature, and neither the precast producer nor the installation contractor had any prior experience with this construction technique. As precast producers and contractors become more familiar with the construction technique, costs will likely decrease substantially. Second, the Highway 60 project was relatively small in size. As with any project, there are economies of scale, and the larger a project is, the lower the unit cost will likely be. Finally, this project was included as a field change order for a much larger project to realign Highway 60. Whenever unexpected items are added to an existing project, particularly when the change order involves something vastly different from the rest of the project, higher costs can be expected. Future projects, which will be competitively bid and will have improved design and construction details, will likely be substantially lower in cost.