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Construction of the California Precast Concrete Pavement Demonstration Project
Chapter 2. Precast Concrete Pavement Concept
As mentioned in chapter 1, the California precast pavement demonstration project was based on a concept developed through a feasibility study completed by the Center for Transportation Research. While several different concepts for precast pavement have been developed in the past several years, the focus of these new FHWA demonstration projects is the prestressed precast pavement concept developed by CTR.(1) This concept was successfully implemented in a precast pavement pilot project constructed near Georgetown, Texas in 2002.(2) While several improvements have been made to the concept based on the Texas pilot project, the overall concept is essentially the same for the California demonstration project, as described below.
The precast pavement concept utilizes full-depth precast panels. Full-depth panels are a very efficient solution in that the pavement can be opened to traffic almost immediately after installation of the precast panels. Additional construction time is not required for a hot-mix asphalt or thin bonded concrete overlay wearing course. Full-depth panels were demonstrated to provide acceptable ride quality without the need to overlay or diamond grind for the Texas pilot project.(2) While it is anticipated that surface diamond grinding will be required to achieve more stringent ride-quality standards, full-depth panels can still be opened to traffic in the interim, prior to diamond grinding.
Full-depth panels require careful consideration of both base preparation and vertical classment between panels. Based upon the conclusions from the CTR feasibility study and the Texas pilot project, it is apparent that either a hot-mix asphalt or portland cement base can be placed smooth and flat enough to serve as a leveling course beneath the precast panels. As demonstrated by the Texas pilot project, a hot-mix asphalt leveling course can actually deform under the weight of the precast panels, conforming to the bottom surface and minimizing voids.(2)
Vertical classment between adjacent precast panels is ensured by casting continuous shear keys into the edges of the panels. The keyways have been demonstrated to “lock” the precast panels together vertically, providing load transfer prior to post-tensioning, and ensuring satisfactory ride quality of the finished surface. The keyways have also been shown to expedite panel installation by eliminating the requirement to “level” adjacent panels after installation.
The concept for precast pavement incorporates prestressing. As discussed in chapter 1, prestressing not only improves the durability of the pavement, but also permits a significant reduction in slab thickness by inducing a precompressive stress in the pavement that must be overcome before tensile stresses that lead to cracking can occur.
Prestressing in both the longitudinal (in the direction of traffic flow) and transverse (normal to traffic flow) directions is essential for prestressed pavements. Previous (cast-in-place) prestressed pavements constructed in the United States that had only longitudinal prestressing developed longitudinal cracking over time.(10) A cast-in-place prestressed pavement constructed on I-35 near West, Texas, which had both longitudinal and transverse prestress, has only recently developed minor cracking after 19 years of heavy truck traffic.(5)
Bi-directional prestressing is incorporated into the precast pavement through both pretensioning and post-tensioning. The precast panels are pretensioned in the transverse direction (long axis of the panel) during fabrication, and are post-tensioned together in the longitudinal direction after installation on site. Transverse pretensioning not only provides the necessary prestress for long-term durability, but also prevents cracking during lifting and handling of the panels. Likewise, longitudinal post-tensioning not only provides the necessary prestress for long-term durability, but also provides load transfer between panels.
Figure 1 shows a typical precast panel assembly. The panels are installed transverse to the flow of traffic, incorporating both traffic lanes and shoulders if possible. The three types of panels that make up a precast prestressed pavement—base, joint, and central stressing panels—are shown in figures 2, 3, and 4. As described previously, all of the panels are pretensioned lengthwise (transverse to the flow of traffic), and monostrand post-tensioning ducts are cast into each panel widthwise (parallel to the flow of traffic) for longitudinal post-tensioning after the panels are all assembled. Note also the continuous shear keys cast into the edges of the panels to ensure vertical classment and temporary load transfer, as described previously.
After each section of panels is installed (from joint panel to joint panel), the post-tensioning strands are fed into the post-tensioning ducts from the pockets in the central stressing panels (described below). The strands are pushed or pulled through all of the panels to the post-tensioning anchors cast into the joint panels. Post-tensioning is then completed from the pockets in the central stressing panels. Each post-tensioned slab acts independently of the adjacent slab in terms of expansion and contraction movements. Expansion joints are cast into the joint panels (described below) to permit adjacent slabs to move independently. The length of each post-tensioned slab can be adjusted by increasing or decreasing the number of base panels between the joint panels and central stressing panels.
The base panels, shown in figure 2, are the most basic of the three panels and are the majority of the panels in each post-tensioned slab. Typical details of the base panels include continuous shear keys along the panel edges, post-tensioning ducts, pretensioning spaced evenly to provide uniform transverse prestress, and lifting anchors, located approximately 0.2L from each edge of the panel.
The joint panels, shown in figure 3, contain both the expansion joint and the post-tensioning anchorage. The expansion joint is designed to accommodate the significant amount of horizontal slab movement during daily and seasonal temperature cycles while providing load transfer across the joint.
The pockets cast into the joint panels provide access to the post-tensioning anchors. The pockets are used to manually install the post-tensioning chuck and wedges around the post-tensioning strand after the strands have been fed through the ducts. The two elongated pockets in the joint panel are used for feeding temporary post-tensioning strands into the ducts. The temporary post-tensioning strands are used to pull the panels together incrementally as each one is installed. Grout inlets/vents are also cast into the joint panels just in front of the post-tensioning anchors to facilitate grouting the ducts after the strands are tensioned.
Central Stressing Panels
The central stressing panels, shown in figure 4, contain large pockets where post-tensioning is completed. The pockets, approximately 1.2 m (48 in.) long by 200 mm (8 in.) wide (full depth), are cast into the panels at every post-tensioning duct. The pockets are split into two separate panels, with the pocket for each duct alternating between panels, to prevent weakening of the panel, as shown in figure 1. After stressing has been completed, the pockets are filled with a fast-setting concrete or temporarily covered to allow traffic onto the pavement immediately. As in the joint panels, grout inlets/vents are cast into the panels on either side of the stressing pockets for grouting the strands after post-tensioning.
Base preparation consists of providing a smooth, flat surface to support the precast panels as well as providing a bond-breaking, friction-reducing material. As mentioned previously, the base preparation technique consists of using a hot-mix asphalt or cementitious material as a leveling course to support the precast panels. Care must be taken to ensure that high spots in the leveling course are minimized to prevent the panels from resting on the high spots, creating voids beneath the panels.
Over the leveling course, a friction-reducing material is required. The friction-reducing material prevents the precast pavement from bonding to the underlying base (leveling course), while also reducing the sliding friction between them. Because prestressed pavements are generally constructed as long, post-tensioned sections of pavement with no intermediate joints, a significant amount of horizontal expansion and contraction movement will take place. If this movement is restrained by friction between the pavement and leveling course, detrimental tensile stresses can develop in the slab, causing premature cracking and pavement failure.
A single layer of polyethylene sheeting (0.15 mm [0.006 in.] thickness, minimum) has proven to be an economical and effective friction-reducing material for prestressed pavements. This material was used successfully for the West, Texas, cast-in-place prestressed pavement(5) as well as for the Texas precast pavement pilot project.(2) Care must be taken during construction to prevent rips and tears and bunching of the sheeting between and beneath the panels.
As discussed previously, the purpose of longitudinal post-tensioning is to improve durability, reduce slab thickness, and provide load transfer between the precast panels. Longitudinal post-tensioning is applied through central stressing. Central stressing permits a more continuous pavement placement operation by eliminating the need for gap slabs to tension the strands and also reduces frictional losses during post-tensioning by reducing the effective length of the tendons. The post-tensioning strands coming into the central stressing pockets from either side of the slab are spliced in the pocket and tensioned using a “dogbone” or “ring anchor” coupler similar to that shown in figure 5. A monostrand stressing ram is used to tension the strands through the coupler. The post-tensioning sequence begins with the tendons at the middle of the slab moving outward, alternating to either side of the middle tendons.
Post-tensioning does not have to be completed before the pavement is opened to traffic. Post-tensioning can be completed during a subsequent construction operation if time constraints do not permit post-tensioning immediately after panel installation. Although post-tensioning is the primary mechanism for providing load transfer between panels, the keyways will provide some degree of load transfer prior to post-tensioning.
Post-tensioning ducts are located as close to mid-depth of the panel as possible. It is essential that the ducts line up between panels. It is also essential that the ducts are kept as straight as possible during panel fabrication to prevent post-tensioning losses due to “wobble.”
Grouting consists of both post-tensioning tendon grouting and underslab void grouting. After the post-tensioning strands have been stressed and the pockets in the central stressing panels and joint panels are filled, the post-tensioning tendons are grouted. The primary purpose for grouting is to provide an extra layer of corrosion protection for the post-tensioning strands. This protection is particularly critical at the joints between precast panels where the post-tensioning duct is not continuous across the joint. However, post-tensioning also permanently bonds the strand to the pavement. This will prevent a loss of prestress if a strand is inadvertently cut or if a section of the pavement is cut out and replaced. Tendon grouting is accomplished by pumping grout into the ducts at inlets/vents located at the ends of each post-tensioning tendon in the joint panels and central stressing panels. Intermediate vents cast into the base panels also provide additional vent points if necessary. It is essential to ensure a tight seal around the post-tensioning ducts between panels to prevent grout leakage. A foam or neoprene gasket or thick epoxy can be used for this purpose.
Underslab grouting helps to ensure full support beneath the precast panels, filling any voids that may be present after panel installation. Underslab grouting is accomplished by pumping grout into grout channels cast into the bottom of the panels through inlets/vents at the surface of the panel. Grout is pumped into one inlet until it flows out of the vent at the other end of the grout channel. Care should be taken to seal/backfill the edges of the slab prior to underslab grouting to prevent grout leakage from beneath the slab.
Similarly to post-tensioning, grouting can be completed during a subsequent construction operation if time constraints do not permit grouting immediately following panel placement. Grouting should not be completed, however, until all post-tensioning tendons are stressed and the pockets in the joint panel and central stressing panels are patched.
Figure 6 shows the overall construction process. Beginning with removal of the existing pavement, the base is then prepared by either smoothing the existing base or by placing a leveling course. The precast panels are then installed, followed by post-tensioning, patching of the stressing/access pockets, and grouting of the post-tensioning ducts. If needed, underslab grouting and diamond grinding can then be completed.
As mentioned above, not all of these steps need to be completed before the pavement can be opened to traffic. If necessary, the pavement can be opened to traffic after placement of the precast panels; the keyways will provide load transfer between panels in the interim. The pavement can also be opened after post-tensioning; the stressing pockets and access pockets can be temporarily covered for traffic. The pavement can be opened prior to grouting also, provided that grouting is completed within a reasonable amount of time after post-tensioning and any water that may have entered the ducts is forced out prior to grouting. Likewise, the pavement can be opened to traffic prior to underslab grouting (if required) if the support beneath the pavement is deemed adequate to withstand traffic loading.