The precast panels are fabricated at an established precast plant, preferably as close as possible to the installation site. For larger jobs, particularly in remote locations, it may be possible to set up a temporary precast bed near the jobsite. The concrete mixtures used are similar to those utilized for other precast prestressed elements and are not be restricted to "paving" mixtures. Steam curing, wet mat curing, or membrane curing are all options for precast pavement panels.
Panels are normally removed from the forms the day following casting and stored at the precast plant. Additional wet mat curing may be applied during the first few days of storage. Panels are handled and stored such that they will not be damaged prior to delivery to the jobsite.
Delivery of the precast panels to the site is a critical aspect of the installation process. Enough delivery trucks must be available to meet the required installation rate. Care must be taken during the delivery process to ensure that the panels are not damaged in any way during shipment.
The installation process will vary with each job. Temporary post-tensioning can be used to "snug" the panels together as they are installed. Both "full width" and "partial width" construction have been utilized to date. Full width panels span the width of the roadway and may include integral shoulders. Partial width or lane-by-lane construction may require additional transverse post-tensioning to tie the two halves of the pavement together.
Post-Tensioning After the panels are installed, the post-tensioning strands are threaded through the panels, anchored at the ends and tensioned. The post-tensioning tendons utilized to date have been 0.6-inch diameter 7-wire monostrand tendons. Generally, the strands are tensioned to 75% of their guaranteed ultimate tensile strength. After the tendons are tensioned, the stressing pockets or blockouts are patched. A fast-setting patch material can be used if necessary.
It should be emphasized that post-tensioning does not need to be completed prior to opening to traffic. While it is desirable to complete post-tensioning as soon as possible, time constraints on construction may not permit it.
The post-tensioning system used for precast prestressed pavement is a bonded post-tensioning system. Grouting not only provides an additional layer of corrosion protection for the post-tensioning strands, but also bonds the tendons to the pavement so that if it is necessary to remove a precast panel at a later time, the post-tensioning system will remain intact.
In addition to tendon grouting, underslab grouting may also be required to fill any voids beneath the pavement. Grout ports can either be cast into the panels or drilled on site. Underslab grouting is essentially a "gravity" fed process as applying pressure can actually lift the pavement.
Open to Traffic
Precast pavement can be opened to traffic as soon as the panels are installed. Post-tensioning and grouting operations can be completed during a subsequent operation if necessary. The surface will be smooth enough to open to traffic and can be diamond ground during a subsequent operation if necessary.