What is Pre-Tensioning in Prestressed Concrete ?

Before pouring concrete, pre-tensioning is achieved by extending tendons—wires or strands—between two anchorages to a predefined tension, as seen in Figure 1. Tendons are then cemented to the concrete along their whole length once the concrete is poured. Tendons are severed at the anchorages and released once the concrete has solidified. Tendons tend to shorten and revert to their former length, imparting a compressive stress to the concrete through bonding. Using hydraulic jacks frequently results in stress on the tendons. Tendon ends are anchored to abutments spaced up to 200 metres apart, which maintains the tension in the tendons during the pouring and curing of concrete. The term prestressing bench or bed refers to the abutments and additional formwork required in this process

What is Post-Tensioning in Prestressed Concrete ?

Post-tensioning is an alternative to pre-tensioning. When concrete is cast and sufficiently strong to safely bear the prestressing force, the ends of the tendons in a post-tensioned beam are fastened to the concrete section and the tendons are stressed, as seen in Figure 2. Tendons are covered with grease or another bituminous substance in the post-tensioning technique to keep them from bonding to the concrete. Encasing the tendon in a flexible metal hose prior to inserting it into the forms is another technique used to stop the tendons from adhering to the concrete during the pouring and curing of concrete. The metal hose, sometimes known as a duct or sheath, is still within the building.

Grout is used to fill in the space created by the strained tendon and the sheath. In this way, the tendons adhere to the concrete and stop the steel from corroding. On-site post-tension prestressing is an option. In some circumstances, this method can become required or advantageous. Moving a member from a precasting facility to a construction site may be extremely challenging for structures with enormous loads and wide spans, such as skyscrapers or bridges. Conversely, pre-tensioning can be applied to both cast-in-place and precast construction. The ends of the tendons must be anchored to the concrete section during post-tensioning, which calls for the usage of certain devices. Usually, these gadgets are called end anchorages. A great deal of patents covering various kinds of anchorages are available. They might also vary in the specifics of construction.

A few common ways to post-tension prestressed concrete are as follows:

1.Freyssinet framework

2.Magnel framework

3.The Leonhardt system

4.The Lee-McCall system

5.System Gifford-Udall