Categories FAQ

Question: When should you stress post tension cables?

Post tensioned slabs should be stressed when the concrete strength designated by the engineer’s plan is reached. The practice of MLAW has been to apply stress when the concrete reaches 2000 psi.

Can post tension cables break?

The cable may still be live if only one strand has broken. The cable should be de-tensioned (which can be very dangerous) and removed. Cable failures can be as uneventful as a “pop,” or as catastrophic as “exploding concrete,” capable of severe damage and bodily injury.

What happens when you hit a post tension cable?

Hazards of Post-Tensioned Slabs: Once the concrete is poured and sets, the cables are tensioned. Once tensioned, the cables cannot be de-tensioned. The hazard exists when workers cut into the slab and rupture the tensioned cable. People have been dismembered and killed when cables are cut and burst out of the concrete.

Are post tension cables bad?

3) Check if the building was built with post tension cables Steel cables are run through the concrete in order to increase the weight bearing ability of the concrete and to minimize cracking. The problem with post tension is that they are very expensive to repair if something goes wrong.

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Why Post-Tensioning is done?

Designers use post-tensioning as a way to reinforce concrete by prestressing it. In prestressed members, compressive stresses are introduced into the concrete to reduce tensile stresses resulting from applied loads including the self weight of the member (dead load).

How long does post-tension cables last?

Conventional concrete & rebar foundations last about 20-30 years before failure begins to occur. Post-tension slabs are lasting about 15-20 years and are failing at a much higher rate.

How much does it cost to fix post tension cable?

The cost of a single cable repair will typically range from $800 to $1,200 and does not include the cost of other concrete repair. In most slab systems, the cables are spaced about 30 to 36 inches apart. As a result, a significant expense for cable repairs can be experienced.

How deep are the cables in a post tension slab?

The plan maps show the rebar at 3″ depth, and the post-tension cables at 8″ depth. Discerning a post-tension cable from rebar generally requires scanning a larger area to properly understand the layout of structural elements in a slab. When in doubt, mark it out and avoid it!

Is post tension better than rebar?

Post-tensioning, which is a form of prestressing, has several advantages over standard reinforcing steel (rebars): It reduces or eliminates shrinkage cracking -therefore no joints, or fewer joints, are needed. Cracks that do form are held tightly together. It allows slabs and other structural members to be thinner.

When did post tension cables start being used?

The first post-tensioning in U.S. building construction was in the mid- to late 1950s in buildings using the lift- slab construction method.

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When did post tension slabs start residential?

At the same time, builders and designers were experimenting with post-tensioned reinforcing to construct residential slab-on-grade foundations. Officially accepted by the Federal Housing Administration in 1969, the use of this type of foundation exploded in popularity in the 1990s.

Why Post tensioning is used in bridges?

Post-tensioning makes possible the cost-effective construction of high-quality bridges over a wide range of conditions and span lengths including highway alignment. Further, post-tensioning can be used effectively to build bridges on alignments that are curved in plan.

What is the principle of post tensioning?

Post-tensioning is a special form of prestressed concrete in which the prestressing tendons are stressed after the concrete is cast. Post-tensioning utilizes high quality high strength steel such that 1 kg of post-tensioning strand may replace 3 or 4 kg of ordinary non- prestressed reinforcement.

Which is better pre tensioning and post tensioning?

Pre-tensioning is preferred when the structural element is small and easy to transport. Post-tensioning is preferred when the structural element is heavy.

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