Q.: We're going to be placing and finishing the concrete for a post-tensioned concrete slab on grade in a warehouse. It's the first time we've done a post-tensioned floor. How does a post-tensioned floor differ from a conventional floor, and where can I get information about construction methods for this kind of work? We want to avoid problems so we're not learning as we go at the jobsite.
A.: In a post-tensioned slab on grade, a grid of high-strength post-tensioning tendons replaces the welded-wire fabric or conventional rebar normally used. For a lightly loaded industrial floor, the slab is placed 4 inches thick with tendons in both directions at 2- to 5-foot intervals. After the concrete has reached sufficient strength, the tendons are stressed by hydraulic jacks to an effective force of about 25,000 pounds.
This force is permanently transferred from the tendons to the concrete through anchorage devices at the ends of the tendons. This produces an internal compressive force in the concrete that makes the floor more resistant to cracking under load or cracking due to concrete shrinkage or temperature variations. Other benefits of post-tensioning include elimination of most joints, reduced slab thickness, and reduced excavation costs.
You can get general information about post-tensioning from the Post-Tensioning Institute, 1717 W. Northern Ave., Suite 218, Phoenix, AZ 85021 (602-870-7540). There's an article on post-tensioned floors in the January 1987 issue of Concrete Construction magazine, and an article on post-tensioned residential foundations in the February 1991 issue of Concrete International published by the American Concrete Institute.
Find out who the post-tensioning subcontractor will be for the project. Ask them to take you to another one of their jobs already in progress. By touring a job and asking what's most likely to go wrong, you'll have a head start in avoiding most of the common problems.