Construction of a Precast Prestressed Concrete Pavement Demonstration Project on Interstate 57 Near Sikeston, Missouri
Chapter 2. Precast Prestressed Concrete Pavement Concept
Key Features of PPCP
The basic concept for the Missouri PPCP demonstration project was developed through the original FHWA feasibility study completed by the Center for Transportation Research.(1) Certain refinements were made to the original concept based on the projects constructed in Texas(2) and California.(3,4) Although additional modifications to the original concept were necessary to meet the project requirements for the Missouri demonstration project, the basic elements remained the same and are described in more detail below.
The PPCP concept incorporates prestressing in both the transverse and longitudinal direction in the form of pretensioning, post-tensioning, or both. As discussed in the previous chapter, prestressing provides numerous benefits for long-term pavement performance. Prestressing induces a compressive stress in the slab, helping to reduce or even eliminate the occurrence of cracking, while also reducing the required slab thickness.
In general, the precast panels are pretensioned along the long axis of the panel (transverse pavement direction) during fabrication, and post-tensioned together along the short axis (longitudinal pavement direction) after installation, as will be described below. Transverse pretensioning not only provides the necessary permanent prestress in the pavement slab, but also permits longer and thinner precast panels to be used as it helps counteract lifting and handling stresses. Likewise, the longitudinal post-tensioning not only provides the necessary permanent prestress, but also provides load transfer between the panels.
The post-tensioning system used for the longitudinal tendons is a bonded post-tensioning system. After the longitudinal post-tensioning tendons are tensioned, grout is pumped into the ducts to bond the strands to the precast panels. A bonded post-tensioning system provides continuity between the prestressing strand and concrete, reducing the amount of nonprestressed steel required in the panels. This continuity also permits individual panels to be sawcut and removed from the pavement, if necessary, without compromising the integrity of the entire longitudinal post-tensioning system. Grouting also provides an additional layer of corrosion protection for the post-tensioning tendons, which is critical in colder climates where deicing salts are used.
The PPCP concept utilizes full-depth precast panels. Full-depth panels are an efficient solution in that the pavement can be opened to traffic almost immediately after installation of the precast panels. Additional steps such as placement of a thin hot-mix asphalt or bonded concrete overlay wearing course are not required prior to opening to traffic. Full-depth panels were demonstrated to provide acceptable ride quality without the need to overlay or even diamond grind for the Texas demonstration project.(2) Although diamond grinding was required for the California demonstration project to meet stringent smoothness requirements for an interstate pavement, opening to traffic temporarily, prior to grinding, would have been acceptable.(3,4)
Using full-depth panels requires careful attention to base preparation to minimize vertical misalignment of the panels and voids beneath the panels as they are installed. Strict tolerances on the smoothness of the underlying base material are critical for helping to ensure alignment of the panels and minimization of voids. Flexible base materials, such as hot-mix asphalt or bituminous-treated bases, have been shown to actually conform to the bottom surface of the precast panels under the weight of the panels.
Keyed Panel Joints
Vertical alignment of full-depth precast panels is achieved through the use of keyways along the adjoining edges of the panels. These keyways facilitate rapid installation of the precast panels by helping to ensure vertical alignment during installation even if the underlying base is not perfectly flat. The keyways also provide temporary load transfer between panels prior to post-tensioning.
Figure 1 shows the typical precast panel assembly used for the Missouri demonstration project. The panels are installed transverse to the flow of traffic, incorporating two traffic lanes and inside and outside shoulders. Three types of precast panels make up each post-tensioned section of PPCP: base panels (Figure 2), joint panels (Figure 3), and anchor panels. Each of the panels are pretensioned in the transverse direction (long axis of the panel), and post-tensioned in the longitudinal direction (short axis of the panel) through ducts cast into the panels. Keyways are cast into the edges of each panel, as described previously, to provide vertical alignment as the panels are assembled.
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 joint panels (described below). The strands are pushed or pulled through all of the panels to the post-tensioning anchors in the joint panels at the other end of the section. Post-tensioning is then completed from the pockets in the joint 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, with dowels across the expansion joints for load transfer. The length of each post-tensioned slab can be adjusted by increasing or decreasing the number of base panels between the joint panels.
Figure 1. Illustration. Typical PPCP panel layout.
The base panels, shown in Figure 2, make up the majority of each post-tensioned slab. Figure 2 shows the typical components of the base panels, including the transverse pretensioning strands, longitudinal post-tensioning ducts, and keyways along the edges of the panels. Approximately every fifth base panel contains grout inlets/vents for the longitudinal post-tensioning tendons. For the Missouri demonstration project, a pavement crown was cast into the precast panels, resulting in panels with variable thickness, as described in Chapter 3. Because of the variable panel thickness, the keyway along the edge of the panel was only provided in the traffic lanes and discontinued in the shoulders.
Figure 2. Illustration. Typical Base 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 for stressing the tendons with a monostrand stressing ram. The stressing pockets are also used for temporary post-tensioning during installation of the panels (described in Chapter 6). Grout ports are located just in front of each of the post-tensioning anchors for grouting the longitudinal tendons.
Figure 3. Illustration. Typical Joint Panel.
The anchor panels are located at the middle of each post-tensioned section. They contain sleeves cast into the panels for drilling and grouting anchor pins into the underlying base/subgrade (Figure 1). Anchoring the post-tensioned slab to the underlying base/subgrade is necessary so that the pavement expands and contracts outward from the center, helping to ensure uniform expansion joint widths, while also preventing the pavement slab from "creeping" or slowly moving in the direction of traffic over time.
Base preparation for PPCP consists of providing a smooth, flat surface to support the precast panels, as well as providing a bond-breaking, friction-reducing material. Both hot-mix asphalt and lean concrete base materials have been used for the prepared base on previous projects. Strict tolerances on the evenness of the base surface help to minimize high spots that could cause the panels to rest unevenly on the base and low spots that could create voids beneath the panels. As mentioned previously, flexible base materials permit the precast panels to settle into the base somewhat, helping to reduce voids and high points.
Because PPCP consists of long sections of pavement tied together through post-tensioning, significant expansion and contraction movement can be expected with daily and seasonal temperature cycles. If this movement is restrained by friction between the bottom of the precast panels and the surface of the base, tensile stresses can develop that are significant enough to cause cracking (similar to joint cracking in conventional jointed concrete pavement). Therefore, a bond breaker or friction-reducing material is required to minimize frictional restraint. A single layer of polyethylene sheeting has proven to be an effective and economical material for this purpose. Polyethylene sheeting was used successfully for the demonstration projects in Texas(2) and California,(3,4) as well as for a cast-in-place prestressed concrete pavement constructed near West, Texas, in 1985.(6)
As discussed previously, the purpose of longitudinal post-tensioning is to improve pavement performance, reduce slab thickness, and to provide load transfer between the precast panels. While the original PPCP concept utilized a technique know as "central stressing" for the longitudinal post-tensioning tendons, the Missouri demonstration project utilized end-stressing with larger stressing pockets in the joint panels, as described above. While central stressing has proved to be an efficient and effective post-tensioning technique, end-stressing eliminates the need for additional stressing pockets and central stressing panels in the post-tensioned slab.
Using the end-stressing technique, post-tensioning is completed from the stressing pockets in the joint panels. To help ensure that the full prestress force is attained along the length of the tendons, each is tensioned from both ends. The majority of the strand elongation will occur when stressing the first end of the tendon, with only a small amount of elongation occurring at the opposite end when it is tensioned.
To minimize the size of the stressing pockets in the joint panels, the monostrand stressing ram used for the Missouri project had a curved or "banana" nose that permitted the ram to protrude from the surface of the precast panel rather than sit inside the stressing pocket, as shown in Figure 4.
Post-tensioning ducts are located as close to mid-depth of the panel as possible to minimize any eccentric prestressing forces. It is essential that the ducts line up across panel joints and that the ducts are kept as straight as possible during panel fabrication to prevent post-tensioning losses due to "wobble."
It should be noted that post-tensioning does not need 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, as the keyways will provide some degree of load transfer prior to post-tensioning. Post-tensioning as soon as possible after panel installation will help to ensure the best pavement performance, particularly if epoxy is used to bond the panels together.
Figure 4. Illustration. Stressing ram with a curved nose used for longitudinal post-tensioning.
Grouting consists of both post-tensioning tendon grouting and, if necessary, underslab grouting. After the post-tensioning strands have been tensioned, the stressing pockets in the joint panels are patched to prevent grout leakage into the stressing pockets. At this point, grout can be pumped into the post-tensioning ducts to fully bond the post-tensioning strands to the pavement. Grout ports are located at each post-tensioning anchor and at approximately every 12-15 m (40-50 ft) along the length of the pavement.
Underslab grouting, when required, 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 beneath the slab through grout ports cast into or drilled through the panel. It is necessary to seal or backfill the edges of the pavement slab and the bottom of the expansion joints to prevent grout from leaking from the edge of the slab or into the expansion joints.
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 panels are filled.
Figure 5 shows a general flowchart for the construction process for PPCP. Although every project will have different design features and different constraints on construction, it should be noted that, when properly planned, many of the steps of the construction process can be completed independently, allowing the pavement to be opened to traffic between steps. Each project must be carefully planned from the beginning to help ensure minimal impact on the motoring public during construction, as this is the primary benefit of precast concrete pavement construction.
Figure 5. Illustration. Flowchart for the overall construction process.