Bonded post-tensioning
In this method, the prestressing steel is placed in ducts and, after stressing, is bonded to the surrounding concrete by grouting with cement suspension. Round corrugated ducts are normally used. For the relatively thin floor slabs of buildings, however, round ducts would reduce possible eccentricity of the prestressing steel too much, particularly at cross-over points, so flat ducts are better. They normally contain tendons comprising four strands of a nominal 13mm diameter, a design which has proved effective in most construction applications.
Unbonded post-tensioning
In the early stages of development of post-tensioned concrete in Europe, post- tensioning without bonding was also used in a few structures (for example, in 1936/37 in a bridge constructed in Aue/Saxony in Germany according to a design by Dischinger, and in 1948 for the Meuse Bridge at Sclayn in Belgium, designed by Magnel). After a period without any substantial applications, unbonded post-tensioning has once again been used for some important structures in recent years.
In the first applications in building work in the USA, prestressing steel was greased and wrapped in paper, to facilitate its longitudinal movement during stressing. More recently, the following method for sheathing the steel has generally become common. The strand is first given a continuous film of permanent corrosion-preventing grease in a continuous operation, either at the manufacturer’s works or at the prestressing firm. A plastic tube of polyethylene or polypropylene of at least 1mm thickness is then extruded over this. The plastic tube forms the primary and the grease the secondary form of corrosion protection. Strands sheathed in this manner are known as monostrands. The nominal diameter of the strands is 13mm or 15 mm, the latter becoming more common in recent years.
Bonded or unbonded tendons
The question of whether bonded or unbonded tendons are preferable was and still is debated frequently. The subject will not be discussed in detail here, but the most important arguments for and against will be listed. Arguments in favour of post-tensioning without bonding include the following:
· Maximum possible tendon eccentricities, since tendon diameters are minimal; of special importance in thin slabs.
· Prestressing steel protected against corrosion as soon as it leaves the factory.
· Simple and rapid placing of tendons. Very low losses of prestressing force due to friction.
· Grouting operation is eliminated.
· In general more economical.
Arguments for post-tensioning with bonding include the fact that local failure of a tendon (due to fire, explosion, earthquakes, etc.) has only limited effects on the overall performance of the slab.
In the USA, post-tensioning without bonding is used almost exclusively, whereas post-tensioning with bonding is employed more widely in Australia and Europe. Among the arguments for bonded post-tensioning, the better perform- ance of the slabs in failure conditions is frequently emphasised. It has, however, been demonstrated that equally good structures can be achieved with unbonded post-tensioning by suitable design and construction techniques. Local circumstances (such as prevailing national standards) may become the decisive factor in the choice of one type of post-tensioning over the other.
