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.



Longer Spans

Post-tensioning allows longer spans to be used. This means the number of columns per floor is reduced, which opens up the floor area and increase the flexibility of the structure.

Overall Structural Cost

Post-tensioning becomes more economical than plain reinforced concrete slabs for spans greater than 7 metres. This is due to the reduced cost of materials, labour and formwork.

Reduced Floor to Floor Height

The slab thicknesses can be reduced when using post-tensioning. This results in savings for facade costs and allows more floors to be constructed within the original building envelope.

Deflection Free Slabs

Slab deflections can be eliminated with post-tensioning.

Rapid Construction

Post-tensioning allows for earlier removal of formwork and reduced back-propping requirements. This means faster construction cycles with the quick reuse of formwork.

Column and Footing Design

Reducing the slab thickness has the benefit of reducing the floor dead loads, which results in an economical design of the concrete columns and footings.

Waterproof Slabs

Post-tensioned slabs can be designed to be 100% crack free, which means the slabs are essentially waterproof. In order to construct a waterproof slab, the choice of concrete mix, curing methods, quality workmanship and detailing are key factors.