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Construction of the Iowa Highway 60 Precast Prestressed Concrete Pavement Bridge Approach Slab Demonstration Project
Chapter 2. Key Features of Precast Prestressed Concrete Pavement
The concept for PPCP that formed the basis for the Highway 60 demonstration project was developed through the original FHWA feasibility study.(1) While the bridge approach slab application differs somewhat from the original concept in terms of design features, many of the basic aspects of the project are the same. Below is a brief summary of these aspects.
The PPCP concept utilizes prestressing through either a combination of pretensioning and post-tensioning, or just post-tensioning. It is critical that prestress is provided in both directions, as previous experience with cast-in-place prestressed pavements has shown that prestress in only one direction can lead to cracking above the prestressing tendons.(10) The original PPCP concept featured precast panels that were pretensioned in the transverse direction during fabrication and post-tensioned together in the longitudinal direction after installation, generally in 76-m (250 ft) sections.(2) However, as will be discussed later, bi-directional post-tensioning can also be used, as was done for the Highway 60 demonstration project. Regardless of whether post-tensioning is in one or both directions, the post-tensioning system is a grouted or bonded system.
Another key aspect of the PPCP concept is the use of full-depth precast panels. Using full-depth panels, the top surface of the precast panels is the riding surface. Although diamond grinding may be required to achieve high-speed facility smoothness requirements, the pavement can generally be opened to traffic prior to diamond grinding. This was successfully demonstrated by the Texas demonstration project, which has not been diamond ground after 5 years in service.(2) Full-depth panels provide an efficient solution as they do not require a hot-mix asphalt or bonded concrete overlay for the final riding surface. Full-depth panels do require careful attention to base preparation, however, to ensure that the panels are set to the proper elevation and are not resting on high points, which could cause them to shift.
Another important feature of the PPCP concept, particularly for full-depth precast panels, is the use of continuous keyways along the edges of the panels. These keyways help to ensure vertical alignment between the panels as they are installed, minimizing the amount of grinding and surface correction required for the final driving surface. It is important to note that the precast panels are not match-cast. Match-casting is a more time-consuming and costly operation, and may only be viable for small projects. The dimensions and tolerances for the keyways are such that match-casting is not required to achieve a well-fitting joint between panels. Non-match-cast panels permit more efficient manufacturing operations such as long-line fabrication.
PPCP panels are installed over a prepared base. Materials used for the base on previous projects have included dense-graded hot-mix asphalt, permeable asphalt-treated base, and lean concrete base.(2,3,9) Regardless of the material used, strict tolerances on surface deviation of the prepared base will help to ensure full support beneath the panels and minimize any voids or stress concentrations from the panels resting on high points. Experience has shown that the precast panels tend to settle into flexible (hot-mix asphalt) base materials.(2) If voids are expected or observed during construction, it may be necessary to use underslab grouting to fill these voids after construction.
Over the prepared base it is necessary to place a friction-reducing material, such as a single layer of polyethylene sheeting. The polyethylene sheeting prevents the pavement from bonding to the base and also reduces the frictional restraint stresses that can accumulate in the pavement slab as it expands and contracts with daily and seasonal temperature cycles. Polyethylene sheeting has proven to be an effective and constructible material for both precast and cast-in-placed post-tensioned pavements.(2,11)
Grouting encompasses both post-tensioning tendon grouting and underslab grouting (if required). Grouting of the post-tensioning tendons provides an additional layer of corrosion protection for the strands. It also provides continuity between the prestressing steel and concrete to reduce the amount of non-prestressed reinforcement required, and to permit sections of the pavement to be cut out and removed in the future if necessary. Although prestress will be lost in the section that is removed, prestress in the rest of the pavement will remain intact. It is important that tendon grouting be done properly because improperly or poorly grouted tendons can result in premature failure of the tendons.(12) Proper grout materials, suitable for post-tensioning tendons, and trained workers should be part of the tendon grouting operation.
Underslab grouting is only necessary if significant voids are observed beneath the precast panels as they are installed. For most applications, this will be a necessary process because very precise grading of the prepared base material is often not possible within short construction windows. Ports for underslab grouting can be cast into the precast panels or drilled into the panels after installation. It should be noted that both underslab and tendon grouting can be completed separately from the panel installation process if time constraints so require. If significant voids are observed beneath the panels, however, underslab grouting should be completed prior to opening the pavement to traffic.
Figure 1 shows a generic flowchart for the construction process for PPCP. While every project will be different and may not require each of the steps shown, these are the most common processes for PPCP construction. It is important to note, as the flowchart shows, that many of the steps in the process can be completed independently of each other, allowing the pavement to be opened to traffic between steps, provided that necessary precautions are taken for issues such as temporarily filling or covering the stressing pockets. As with any project constructed under stringent time constraints, proper preconstruction planning is essential to ensuring the success of the project.