What is Post-Tensioned Concrete?

The post-tensioning method is often used in in-situ concrete, in which the concrete is placed around ducts or sheaths containing unstressed strands.

The anchor, duct and strand combination is referred to as a tendon. Once the concrete has gained sufficient strength, the tendons are progressively stressed against the concrete and locked off by special anchors. A bonded system fills the tendons with grout, whereas an unbonded system is where the tendons are not grouted.

Post-tensioned concrete can most easily be defined as “pre-compressed” concrete.  A compressive stress is put into a concrete member before it begins its working life and is positioned in areas where tensile stresses will develop under a working load.

Concrete is strong in compression but weak in tension, so the introduction of precompression in areas that will become subject to tension means that the concrete will behave as if it had a tensile strength of its own.

The ACI code committee stated that Post-tensioned Concrete is concrete in which there have been introduced internal forces of such magnitude and distribution that the forces resulting from given external loadings are counteracted to a desirable degree.

The pre-compression may be arranged symmetrically; however, more commonly, it is applied eccentric to the section axis’s centroid, enabling additional construction efficiencies. Here, the post-tension tendons are installed to follow the bending moment profile.

This locally dumps load at peak negative moment areas over supports, and results in an uplift force at midspan to balance a portion of the slab self-weight.