For a three-hour fire rating, according to Table 720.1 of the International Building Code, the minimum bottom cover for post-tensioning tendons is 2 inches, while it is only 1 inch for rebar. This significant difference in cover is frequently overlooked and is a direct code violation. One retrofit option is to have plaster or other fire-rated material applied to the slab or beam around the low point of the tendon to increase the fire rating. In post-tensioned slabs, the cover to the strands should typically be greater than the cover to the rebar in the end (unrestrained) bays.

Waterproofing

A common myth is that post-tensioning will create waterproof concrete. Unfortunately, this is not true. Concrete by nature is a porous material and adding some amount of compression does not suddenly make it non-porous. When designed and built correctly, post-tensioning will enhance the natural water tightness that concrete possesses, but it will never be waterproof.

In fact, one could argue that more post-tensioning actually makes the system less waterproof. For higher values of post-tensioning, the slab will experience larger movements, which can induce more restraint cracks. No matter how much reinforcement is put into the slab, a crack is never waterproof.

Concrete blowouts and tendon profiles

Whether in a small residential slab on grade or a 30-story hotel, each tendon will be loaded to roughly 33,000 pounds during stressing. This force is transferred to the concrete through the bearing of the ductile iron casting anchor against the concrete. If the concrete is not well vibrated near the anchors, if penetrations are located in front of the anchors, or if congested rebar prevents a uniform bearing surface, a concrete blowout is likely. The concrete will literally explode during tensioning as the anchors crush into the slab or beam, which then can cause the hydraulic jack to move suddenly and violently. For this reason, only trained professionals should operate or be anywhere near the jack during stressing. If penetrations can't be relocated away from the anchors, steel pipes should be placed around the openings to resist the anchorage force.

Blowouts can also occur away from the anchors in the middle of the slab. These blowouts typically are caused by extreme horizontal or vertical tendon curvatures that create localized forces in the slab. Horizontal curving of the tendons is often done to avoid penetrations, slab openings, and embedded items that prevent typical strand placement. During stressing, the tendons tend to straighten and the concrete in the curve is unable to resist the localized lateral thrust.

For horizontal curves, a good practice is to start curving the tendons in the middle part of the slab so sufficient concrete and rebar are present to resist any lateral thrust. Curving tendons near the top of the concrete has led to problems since the concrete has minimum cover and rebar can't be added. If extreme curves are unavoidable, hairpins are often used to “pull back” the thrust into the main slab.

These tendons will create a downward force that is nowhere near a band or a vertical support and that can cause cracking or a small blowout.

Vertical draping of the strands is one of the main benefits of post-tensioning, but this needs to be done smoothly and gradually. Tendon discontinuities will produce localized point loads on the slab. Tendons draped in a reverse curve over other tendons will create a downward force. Instead of picking up the concrete, this reverse curvature will push down into the lightly reinforced slab below, most likely causing cracks or a small blowout. Reverse curvatures are typically caused by the tendons being kicked off their chairs, wrong chairs being used, or the strand being tied off to a piece of rebar that has been moved after the tendon was secured.

Drilling into a post-tensioned slab

Another popular myth with post-tensioned slabs is that it is very difficult to drill into an existing slab because of the unknown location of the tendons and anchors. But as long the tendons and the concrete in front of the anchors are not damaged, drilling into a post-tensioned slab is a fairly routine issue.

This small blowout is a result of a localized low point in the tendons between two higher points. Note the gap beneath the tendons and the slab, indicating how much the tendons lifted.

Existing tendons can be located using a pacometer (handheld metal detector) or an X-ray device. With the X-ray in hand, a technician can mark the tendon locations directly on the concrete surface. I have used a $40 store-bought metal detector and located tendons in a 12-inch-wide beam. For buildings where tenants change frequently, we recommend marking one side of the slab so future tenants will know exactly where the strands and anchors were placed.

The above list of “issues” is nowhere near complete but represents common items that I frequently come across. For other questions, the Post-Tensioning Institute has recently completed the sixth edition of its Post-Tensioning Manual, which covers a large range of topics for post-tensioned buildings and is useful for contractors as well as engineers.

— Bryan Allred, S.E. is vice president of Seneca Structural Engineering, Inc. in Laguna Hills, Calif. Allred specializes in reinforced concrete buildings using post-tensioned floor systems, the retrofit of existing buildings using external post-tensioning, and post-tensioned slabs on grade. He can be contacted at bryan@senecastructural.com.