Precast-prestressed concrete producers long ago proved our product as a method for providing durable high-performance structural elements for bridges and commercial buildings. In recent years, however, some of these producers have found a new market segment for their product: pavement construction.
It's only natural. Precast concrete provides a long-lasting solution for pavements, offering many economical benefits over conventional, cast-in-place construction. Perhaps most importantly, precast concrete pavement panels can be installed quickly, during short (preferably overnight or weekend) construction windows, minimizing lane closures and disruptions to motorists.
The concept has gained acceptance. Five projects in the last six years have utilized pre-cast-prestressed concrete pavement (PPCP) construction. Each presented unique challenges and demonstrated the adaptability of precast concrete pavement to variety of applications.
Road projects in Texas, California, Missouri, and Iowa were accomplished through a Federal Highway Administration (FHWA)-sponsored demonstration program intended to help familiarize state highway agencies and local contractors with PPCP construction. A fifth project in Alaska was designed and constructed by Central Pre-mix Prestress Co., a precast producer in Washington State for a private mine project.
“The use of precast-prestressed concrete for pavements is a tremendous opportunity for the industry to show owners the advantages our product offers in the way of long-term durability and performance,” says Jon Grafton, president of Pomeroy Corp., of Perris, Calif. Pomeroy worked on the I-10 project in El Monte, which used the construction method.
“As more demonstration projects are constructed, owners will have the opportunity to compare the versatility that precast-prestressed concrete has over other repair methods,” the Pomeroy president explains.The basics
The PPCP concept features full-depth precast concrete panels that are prestressed in both directions through either a combination of plant pretensioning and site post-tensioning, or simply bi-directional site post-tensioning. Pockets are generally cast into selected precast panels to provide access to the post-tensioning anchors.
These pockets are located either in panels at the middle of a section of post-tensioned panels or at the ends of the post-tensioning tendons. An expansion joint is cast into a panel at the ends of each post-tensioned section of panels to “absorb” the daily and seasonal expansion and contraction movements of the pavement.
Using full-depth precast panels, the top surface becomes the riding surface of the pavement, making production tolerances and surface finish critical aspects of panel fabrication. Generally, a light broom or turf drag finish is applied to the surface. Tongue-and-groove keyways are cast into mating edges of the precast panels to ensure vertical alignment of the panels as they are assembled onsite.
The panels are not match-cast to improve production rates. Epoxy is applied to the keyways before assembling the panels onsite to seal the joints between panels. The precast panels are installed over a prepared base. If necessary, standard undersealing procedures can be used to ensure full support beneath the panels.
The durability and performance benefits are the result of the quality from plant-produced products as well as incorporating prestressing. Plant precasting operations offer a tremendous degree of quality control over the types and the production of the concrete mixtures used, as well as the curing process for the products.
Also, restressing will further benefit durability and performance by inducing a compressive stress in the concrete, helping to minimize or even eliminate any cracking in the pavement. Prestressing also gives the pavement slab an ability to span voids or non-ideal base layers beneath the pavement. It also reduces the slab thickness required for traffic loading on the pavement.
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