Frequently Asked Questions
Q: How will I know when my parking garage needs attention?
A: If your post-tensioned parking structure is more than 10 years old, or if there is visible evidence of broken tendons, or corroded or spalling concrete you should contact an registered engineer qualified in slab restoration.
Q: Why is it important to locate broken tendons?
A: Post-tensioned tendons are the primary reinforcing in the slab. With less than 10% loss in tendon area due to corrosion, stress concentration can develop causing the tendon to sever completely. This reduction of post-tensioning force will significantly reduce the load carrying capacity of the slab.
Q: Why are post-tension parking structures such a severe problem?
A: Primarily, because they generally appear to be in good condition, particularly when compared to mild reinforced parking decks. These post-tension decks display minimal amounts of spalling even when a majority of the tendons have released. Since these decks have very minimal observed deterioration, even professionals specializing in parking structure repairs are not identifying the extent of the structural deterioration, which may cause a collapse with virtually no warning.
Q: Why are post-tensioned tendons at risk for corrosion?
A: The basic problem with all parking structures is deterioration due to corrosion accelerated by heavy use of road salts. Unlike rebar where corrosion starts at the bars near the top surface, post-tensioned tendons can start corroding at any point along the drape of the tendon. Since, in all but the newer garages, the tendon sheathing is not watertight, water seeping through the concrete will pick up chloride ions readily available in the concrete and enter the tendon wrapping. This water can settle at the low point of the tendon profile and corrode the tendon.
Q: How can I identify a post-tensioned slab system?
A: A Structural Engineer can better asses your slab system, but these pictures show a typical “one-way” post-tensioned slab, and a typical “two-way” post-tensioned slab. These slabs are generally between 5 and 8 inches thick, and span over 18 feet.
One-way post-tensioned slab system
Two-way post-tensioned slab system