Pre-Tensioning in Civil Engineering

Introduction The idea of pre tensioning has been most for centuries and has been apply to many forms, such(prenominal) as wagon wheels, wooden barrels and so on In these cases modify metal is made to just fit an object. When the metal cools it contracts and induces prestress into the object thereby strengthening it. The idea of pretensioned cover has been around since the late 19th century, exactly its use was limited by the quality of satisfyings available at the time.It wasnt until the 1920s that materials of a suitable quality were available in commensurate mensuration to allow pretensioned concrete to be utilise with confidence. The pioneers of this field were Freyssinet, Magnel and Hoyer. commentary In pre-tensioned concrete tensile elements such as cables, ribbons, or rods argon clamped under calculated tensile stress (Stage 1). Concrete is cast around these elements and allowed to cure( Stage 2). When fully hardened the clamps are released and the stress is transf erred within the severe concrete (stage 3).As long as a load is applied within the design limit, the concrete structural element will neer be subjected to tensile stress of sufficient force to cause failure. Pre-tensioning fanny be utilise in pre-cast as well as in cast-in-place construction. Most of the pre-tensioning construction proficiencys are patented although the basic principle apply in all of them is common and is well known. Hard drawn stigma wires which are indented or crimped are preferred for pre-tensioned elements because of their superior bond characteristics. depleted diameter wires of 2 to 5 mm are mostly used in the form of strands comprising two, three or seven wires.High talent Concrete mix Pre-stressed concrete requires concrete which has a heights compressive strength, with comparatively higher tensile strength. Low shrinkage, minimum creep characteristics and a high value of Youngs modulus are generally deemed necessary for concrete used for prestresse d members. Uses Common uses include Railway Sleepers, Communications Poles, Pretensioned precast Hollowcore slabs, Pretensioned Precast Double T units for very long spans, Pretensioned precast inverted T beams for short-span bridges, Pretensioned precast PSC piles, Pretensioned precast portal frames.Alternatives 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. In the posttensioning method, tendons are encased in a duct or suit or coated with grease or a bituminous material before placing them in the formwork to prevent them from becoming bonded to concrete.Advantages Prestressed concrete has the following advantages 1. Since the technique of prestressing eliminates cracking of concrete under all stage of loading, the entire section of the structure takes part in r esisting the external load. In contrast to this, in reenforce concrete, only the portion of the concrete above neutral bloc is effective. 2. Since the concrete does not crack, the possibility of steel to rust and of concrete to decay is minimized. 3. Absence of cracks results in higher capacity of the structure to bear throwback of stresses, impact, vibration and shock. 4. In prestressed concrete beams, dead loads are practically neutralized. The reactions required are therefore much smaller than required in reinforced concrete. The reduced dead load weight of the structure results in savings in the cost of foundations. The neutralization of dead load is of brilliance in large bridges. 5. The use of curved tendons and the pre-compression of concrete helps to resist shear.6. The quantity of steel required for prestressing is about 1/3 of that required for reinforced concrete, although the steel for the former should have high tensile strength. 7. In prestressed concrete, precas t blocks and elements can be assumed and used as one unit. This saves in the cost of shuttering and centering for large structures. 9. Prestressed concrete can be used with advantage in all those structures where tension develops, such as necktie and suspender of a bow string girder, railway sleepers, electric poles, etc. 10. Prestressed concrete beams usually have low deflection.