Designing slab-on-ground foundations to include unbonded post-tensioning reinforcement is one good way to control cracking without cutting joints. Although the concrete for such slabs may be the same as that used for slabs with standard rebar, a post-tensioned slab is a complex, highly engineered structure. For continued high performance, post-tensioned slabs require careful attention to the details of design and construction as well as a continuing maintenance program.
Perhaps the most effective way to ensure the success of a post-tensioned slab-on-ground foundation is by thorough communication and coordination among all the associated construction trades. Only when all the players know up front that some aspects become far more important in post-tensioned construction will they understand how important their contributions are to the project overall.
The Post-Tensioning Institute, PTI (www.post-tensioning.org), offers good reference material in its Construction and Maintenance Procedures Manual for Post-Tensioned Slab-on-Ground Construction. First published in 1998, the manual is being updated and should be available later this year.
PTI's Construction and Maintenance Procedures Manual provides background and guidance on the various stages in the process, from fabrication of tendons to post-construction site considerations. The following items from the manual are of particular importance to concrete contractors. Additional material from Kenneth L. Douglass, P.E., of Suncoast Post-Tension LP is also included.
The post-tensioning material should be fabricated in a PTI-certified plant and delivered to the jobsite in precut and dead-ended bundles. Engineers should require this certification, and builders should enforce it.
The concrete contractor or a specialty subcontractor may be responsible for placing post-tensioning tendons. Either way, the installers should be PTI-certified.
Regardless of who places the tendons, the concrete contractor should be familiar with the plans and specifications and should verify that project requirements have been met before concrete placement begins.
Post-tensioning tendons are an assembly that includes the strand, the plastic sheathing, and the anchorages (typically one fixed end and one stressing). The tendons should be installed only when the slab edge formwork has been completed, excavations for interior and exterior beams have been dug, and all underground utilities and risers have been installed. Formwork must be adequately braced to resist movement during concrete placement as well as to hold the post-tensioning anchorages in place.
If a vapor barrier is used, it should also be in place before the post-tensioning material is installed. If conventional rebar is required in the slab or grade beams, it should be installed in coordination with the post-tensioning.
Anchorage location can tolerate more adjustment in the horizontal direction than in the vertical direction. The vertical location should be within ½ inch of the design while maintaining a minimum 2-inch cover. Where conflicts occur, anchorages can be moved horizontally as much as 12 inches in either direction. However, they should not result in any tendon being less than 6 inches from any slab corner. Also, watch out for plumbing risers or block-outs—they should not be within the 45-degree bearing cone behind the tendon anchor. There should also be at least 18 inches between the anchor and block-out. If a pipe penetration must be located within that zone, use of a schedule 40 steel pipe sleeve is required.