PRE-TENSIONING AND POST-TENSIONING IN PRESTRESSED CONCRETE DESIGN
The prestress in a structure is influenced by either of the two processes:
- Pre-tensioning, and
Pre-tensioning can be further classified into two categories:
- Linear pre-tensioning
- Circular pre-tensioning
Pre-tensioning is accomplished by stressing wires or strands, called tendons, to predetermined amount by stretching them between two anchorages prior to placing concrete as shown in fig.1. the concrete is then placed and tendons become bounded to concrete throughout their length. After concrete has hardened, the tendons are released by cutting them at the anchorages. The tendons tend to regain their original length by shortening and in this process transfer through bond a compressive stress to the concrete. The tendons are usually stressed by the use of hydraulic jacks. The stress in tendons is maintained during the placing and curing of concrete by anchoring the ends of the tendons to abutments that may be as much as 200m apart. The abutments and other formwork used in this procedure are called prestressing bench or bed.
Fig.1: Section for Pre-tensioning
Most of the pre-tensioning construction techniques are patented although the basic principle used in all of them is common and is well known.
The alternative to pre-tensioning is post-tensioning. In a post-tensioned beam, the tendons are stressed and each end is anchored to the concrete section after the concrete has been cast and has attained sufficient strength to safely withstand the prestressing force as shown in fig.2. in post-tensioning method, tendons are coated with grease or a bituminous material to prevent them from becoming bonded to concrete. Another method used in preventing the tendons from bonding to the concrete during placing and curing of concrete is to encase the tendon in a flexible metal hose before placing it in the forms. The metal hose is referred to as sheath or duct and remains in the structure.
Fig.2: Section for Post-tensioning
After the tendon has been stressed, the void between the tendon and the sheath is filled with grout. Thus the tendons become bonded to concrete and corrosion of steel is prevented.
Post-tension prestressing can be done at site. This procedure may become necessary or desirable in certain cases. For heavy loads and large spans in buildings or bridges, it may be very difficult to transport a member from pre-casting plant to a job site. On the other hand, pre-tensioning can be used in pre-cast as well as in cast-in-place construction.
In post-tensioning it is necessary to use some types of device to attach or anchor the ends of the tendons to the concrete section. These devices are usually referred to as end anchorages. There are a large number of patents for different types of anchorages. They may also differ n the details of construction. Some of the popular methods are:
- Freyssinet system
- Magnel system
- Leonhardt system
- Lee-McCall system
- Gifford-Udall system