Pre-Tensioning in Civil Engineering.
Introduction : The idea of pre tensioning has been around for centuries and has been applied to many forms, such as: wagon wheels, wooden barrels etc. In these cases heated 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 concrete has been around since the late 19th century, but its use was limited by the quality of materials available at the time. It wasn’t until the 1920s that materials of a suitable quality were available in sufficient quantity to allow pretensioned concrete to be used with confidence. The pioneers of this field were Freyssinet, Magnel and Hoyer. Description: In pre-tensioned concrete tensile elements such as cables, ribbons, or rods are 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 transferred within the rigid concrete (stage 3).
As long as a load is applied within the design limit, the concrete structural element will never be subjected to tensile stress of sufficient force to cause failure. Pre-tensioning can be used in pre-cast as well as in cast-in-place construction. Most of the pre-tensioning construction techniques are patented although the basic principle used in all of them is common and is well known. Hard drawn steel wires which are indented or crimped are preferred for pre-tensioned elements because of their superior bond characteristics. Small diameter wires of 2 to 5 mm are mostly used in the form of strands comprising two, three or seven wires.
High Strength Concrete mix: Pre-stressed concrete requires concrete which has a high compressive strength, with comparatively higher tensile strength. Low shrinkage, minimum creep characteristics and a high value of Young’s modulus are generally deemed necessary for concrete used...