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 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 precompression may be arranged symmetrically however more common it is applied eccentric to the centroid of the section axis enabling additional efficiencies in construction to be made. 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.