Two different methods exist for assembling and prestressing steel tendons:10 (i) pre-tensioning, where the prestressed tendons are surrounded directly by con- crete, and (ii) post-tensioning, where the prestressing steels run along ducts which are then filled by injection with mortar. A third method, unbonded tendons in greased ducts, is a new technique that has several advantages over traditional post-tensioning with bonding.
Post-tensioned construction has been a very important technique for many years, especially in the construction of bridges and storage tanks, due to its decisive technical and economic advantages. The most important advantages offered by post-tensioning are:
· slimmer designs which provide a considerable saving in concrete and steel compared with reinforced concrete
· smaller deflections than with steel and reinforced concrete
· good crack behaviour, which tends to protect the steel against corrosion · almost unchanged serviceability even after considerable overload, since temporary cracks close again after the overload has disappeared and
· high fatigue strength, since the amplitude of the stress changes in the prestressing steel under alternating loads are quite small.
For the above reasons, post-tensioning is used in many aspects of building construction. In addition to the general features of post-tensioned construction, post-tensioned slabs have several advantages over reinforced concrete slabs, including:
· structures are more economical due to the use of prestressing 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 on the columns and foundations, reducing the overall height of buildings or enabling additional floors to be incorporated
in buildings of a given height
· very good behaviour under permanent load in terms of deflections and cracking
· higher punching shear strength with appropriate layout of tendons and · considerable reduction in construction time due to use of pre-formed elements.
