The basic concept of prestresssing the concrete consists in introducing the artificially the compressive stresses in a structure before it is loaded. The tensile stresses in the prestressed concrete structure may be reduced to a great extent or even entirely eliniminated depending upon the magnitude of prestressing. In a prestressed concrete structure, the cost of supporting structure and foundation is reduced, dead load of structure is reduced and cracking of concrete is avoided. The high strength concrete and high tensile steel should be used in a prestressed concrete member. According to Indian standards, the cube strength of the concrete used should bot be less than 35N/mm 2 . The ultimate strength of high tensile steel wires used in prestressing varies from 1500N/mm 2 for 8mm diameter bars to 2350N/mm 2 for 1.5 mm diameter bars. The various methods adopted in prestresssing are as follows: Pre-tensioning The method of providing desired amount of compressive stress in
When the steel bars are embedded in concrete, the concrete after setting, adheres to the surface of the bars and resist any force that tends to pull or push this rod. The intensity of the adhesive force is called bond stress . It is the longitudinal shear stress acting on the surface between the steel and the concrete. The term bond is used to describe the means by which slip between the steel and concrete is prevented. The bond is provided by anchoring the bars properly and extending the bars beyond the point of maximum shear. OR Bond stress is defined as the shear force per unit of Nominal surface area of a reinforcement bar acting parallel to the bar on the interface between the bars and surrounding concrete. The bond between steel and concrete is mainly due to pure adhesive resistance, frictional resistance and mechanical resistance. The bond stress developed in concrete around the steel reinforcement is given by Design bond stress For deformed bars these values s