Prestressed concrete is a method for overcoming concretes natural weakness in tension. It can be used to produce beams, floors or bridges with a longer span than is practical with ordinary reinforced concrete. Prestressing tendons (generally of high tensile steel cable or rods) are used to provide a clamping load which produces a compressive stress that balances the tensile stress that the concrete compression would otherwise experience due to a bending load. Traditional reinforced concrete is based on the use of steel reinforcement bars, rebar’s, inside poured concrete. Prestressing can be accomplished in three ways: pre-tensioned concrete, and bonded or unbonded post-tensioned concrete. What is Post-Tensioning? Post tensioning is a technique for reinforcing concrete. Post-tensioning tendons, which are prestressing steel cables inside plastic ducts or sleeves, are positioned in the forms before the concrete is placed. Afterwards, once the concrete has gained strength but before the service loads are applied, the cables are pulled tight, or tensioned, and anchored against the outer edges of the concrete. Post-tensioning is a form of prestressing. Prestressing simply means that the steel is stressed (pulled or tensioned) before the concrete has to support the service loads. Most precast, prestressed concrete is actually pre-tensioned-the steel is pulled before the concrete is poured. Post-tensioned concrete means that the concrete is poured and then the tension is applied-but it is still stressed before the loads are applied so it is still prestressed. PRE-TENSIONING AND POST-TENSIONING IN PRESTRESSED CONCRETE DESIGN The prestress in a structure is influenced by either of the two processes: 1. Pre-tensioning, and 2. Post-tensioning Pre-tensioning can be further classified into two categories: 1. Linear pre-tensioning 2. Circular pre-tensioning 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. POST-TENSIONING 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: 1. Freyssinet system 2. Magnel system 3. Leonhardt system 4. Lee-McCall system 5. Gifford-Udall system
Posted on: Wed, 08 Oct 2014 08:07:39 +0000
Trending Topics
Recently Viewed Topics
© 2015