An Introduction to Post Tensioned Construction

Post-tensioned construction has for many years occupied a very important position, especially in the construction of bridges and storage tanks. The reason for this lies in its decisive technical and economical advantages.

The most important advantages offered by post-tensioning may be briefly recalled here:

- By comparison with reinforced concrete, a considerable saving in concrete and steel since, due to the working of the entire concrete cross-section more slender designs are possible.

- Smaller deflections than with steel and reinforced concrete.

- Good crack behaviour and therefore permanent protection of the steel against corrosion.

- Almost unchanged serviceability even after considerable overload, since temporary cracks close again after the overload has disappeared.

- High fatigue strength, since the amplitude of the stress changes in the pre-stressing steel under alternating loads are quite small.

For the above reasons post-tensioned construction has also come to be used in many situations in buildings. The objective of the present report is to summarize the experience available today in the field of post-tensioning in building construction and in particular to discuss the design and construction of post-tensioned slab structures, especially post-tensioned flat slabs*. A detailed explanation will be given of the checks to be carried out, the aspects to be considered in the design and the construction procedures and sequences of a post-tensioned slab. The execution of the design will be explained with reference to an example. In addition, already built structures will be described. In all the chapters, both bonded and unbundled post-tensioning will be dealt with.

In addition to the already mentioned general features of post-tensioned construction, the following advantages of post-tensioned slabs over reinforced concrete slabs may be listed:

- More economical structures resulting from the use of pre-stressing steels with a very high tensile strength instead of normal reinforcing steels.

- larger spans and greater slenderness . The latter results in reduced dead load, which also has a beneficial effect upon the columns and foundations and reduces the overall height of buildings or enables additional floors to be incorporated in buildings of a given height.

- Under permanent load, very good behavior in respect of deflections and cracking.

- Higher punching shear strength obtainable by appropriate layout of tendons

- Considerable reduction In construction time as a result of earlier striking of formwork real slabs