Concept of Pretensioning and Post-Tensioning

CONCEPT OF PRETENSIONING AND POSTTENSIONING

Prestressed concrete is basically concrete in which internal stresses of a suitable magnitude and distribution are introduced so that the stresses resulting from external loads are counteracted to a desired degree. In 1904,freyssinet attempted to introduce permanently acting forces in concrete to resist the elastic forces developed under loads and this idea was later developed under the name of “prestressing”.

The various methods by which pre-compression is imparted to concrete are classified as follows:

1.Generation of compressive force between the structural element and its abutments using flat jacks.

2.Development of hoop compression in cylindrically shaped structures by circumferential wire winding.

3.Use of longitudinally tensioned steel embedded thin in concrete or housed in ducts.

4. Use of principle of distortion of a statically indeterminate structure either by displacement or by rotation of one part relative to the remainder.

5. Use of deflected structural steel sections embedded in concrete until the hardening of latter.

6. Development of limited tension in steel and compression in concrete by using

expanding cements.

The most widely used method for prestressing of structural element is longitudinal tensioning of steel by different tensioning devices like mechanical, hydraulic, electrical (thermal) and chemical. The prestressing process can be done in two systems. They are:

1. Pretensioning system and

2. Post tensioning system.

1.Pretensioning system:

In the pretensioning system,the tendons are first tensioned between rigid anchorblocks cast on the ground or in a column or unit mould type pretensioning bed,prior to the casting of concrete in the moulds.The tendons comprising individual wires or strands are stretched with constant eccentricity with tendon anchorageat one end and jacks at the other.With the forms in place,the concrete is cast around the stressed tendon.

High early-strength...