The typical post-tensioned strands are comprised of seven high tensile strength steel wires that are coiled together to create a ½-inch-diameter strand. The wires are high tensile steel having a yield strength of 270 kips per square inch (ksi), which is substantially larger than the standard 40- and 60-ksi rebar. In contrast to conventional reinforcing, the strands have no direct bond to the concrete along their length. The strands are encased in grease and covered by an extruded plastic sheathing. The sheathing prevents a bond between the strand and the concrete, while the grease allows the strand to slide inside the sheathing. The ability of the strand to move freely is critical because each tendon will be stretched by a hydraulic jack after the concrete has been placed. Each strand will be loaded to 33,000 pounds at stressing. Unlike rebar that is activated only once the concrete begins cracking, the force from the strands is always present in the slab to resist applied loading and minimize cracking.

The force in the strands is transferred to the concrete by an anchor at each end that is embedded into the slab. The anchor is a ductile iron casting with a plan dimension of 5¼ x2 inches and a tapered hole in the center. Two small wedges placed around each side of the strand physically clamp the steel wires to the anchor. The anchor and wedges may appear small, but this system has been used for decades and is the same assembly that is used in elevated concrete construction.

Bryan Allred Seven high strength steel wires are wrapped together to form a half-inch-diameter post-tensioning strand. The strands are placed through the tapered hole in anchor and are clamped to the anchor with wedges.

The Post-Tensioning Institute recommends that the tendons be stressed between 3 to 10 days after the concrete has been placed and has obtained a minimum compressive strength of 2000 psi. The time between placing and stressing is an issue because the tendons are “unbonded” to the concrete and are effectively useless for crack control as the slab cures and shrinks. Until the tendons are stressed, the slab is essentially un-reinforced, and, if left in this condition, noticeable cracking is to be expected. I have unfortunately witnessed numerous slabs that have experienced substantial cracking simply because the contractor waited 3 to 4 weeks to stress the tendons. A standard fix is to rout out the cracks and inject epoxy, but the fix is not aesthetically pleasing and has left many a homeowner unhappy.

There is no difference in the concrete used between post-tensioned and conventional foundations. I recommend a minimum compressive strength of 3000 psi to allow the mix to reach 2000 psi within a few days and the tendons to be stressed within a week of pouring the slab. Since a post-tensioned foundation design is based upon allowable stresses, higher strength concrete (up to 5000 psi) has been used instead of increasing the slab or beam depths. In most cases, the compressive strength is based upon the sulfate content of the soil. Per ACI table 4.3.1, moderate and above levels of sulfate require between 4000 to 4500 psi concrete with a maximum water cement ratio of 0.45.

Bryan Allred Once the hydraulics are activated, the jack will push against the embedded anchor and stretch the tendon to apply a large force into the foundation.

The stressing procedure involves a large application of force and should be performed only by qualified personnel. The jacking system is typically operated by a two-person crew. One person handles the jack and the other operates the hydraulics. An experienced crew can stress a single-family home in less than an hour. Once the jack is attached to the tendon and is bearing against the anchor, the hydraulics are activated. When the appropriate gauge pressure is achieved, the jack will hold the force while the wedges are pushed into the tapered hole, locking the strand to the anchor and their force to the foundation. To achieve the 33,000 pounds of load, each strand is stretched approximately 0.08 inch for every foot of length. For a 50-foot-long strand, the required elongation will be 4 inches. After the tendons are stressed, the deputy inspector will measure the elongation and as long as they are within 7% of the calculated value, the tails of the strand can be removed. A rust-resisting spray is applied to the exposed anchor and tail, and the stressing pocket is filled with a grout to provide cover to the end of the assembly.

For additional information, review the publications available at www.post-tensioning.org and use its discussion forum to post questions.

— Bryan Allred is a licensed structural engineer and vice president of Seneca Structural Engineering in Laguna Hills, Calif. He can be reached at Bryan@SenecaStructural.com