Prestressed Concrete I.  *Unfavorable effects induced from the presence of cracking in reinforced concrete: To expose the reinforcement to corrosion; To decrease bending stiffness; To reduce shear strength; To increase deflection; To reduce watertightness; To detract from the appearance of the structure II. *Prestressing is a technique of introducing stresses of predetermined magnitude into a structural member to improve its behavior. III. *Purpose of prestressing To eliminate cracking in reinforced concrete To reduce deflections To increase shear strength 1 Tuesday, April 25, 2017

Prestressed Concrete IV. *Methods of prestressing by tensioned wires fixed at their ends (post-tensioning) by tensioned wires gripped by the concrete (pretensioning) by applying an external load (external prestressing) V. *Pre-tensioning The method in which the wires are stretched before concreting is referred to as pretensioning. The grip of concrete on wires increases slightly when the wires are released from the stretching device due to a slight shortening and swelling of the wires. VI. *Post-tensioning Process Cast the concrete in which a sheath or other wrapping is preembeded which allows the wires to move freely during stressing 2 Tuesday, April 25, 2017

Prestressed Concrete V. Post-tensioning Anchor the wires or tendons at the ends of the beam by fixing to an anchor plate, or other devices such as wedging with concrete cones Stress the wires or tendons Grout the sheath Disadvantage: the wires must be protected from corrosion by forcing cement grout into the sheath. Use of grout in post-tensioning: Protect wires from corrosion Provide a bond between the wires and the sheath and thus with the concrete Supplement the resistance of the wires to slip, without which the security of the wires would depend entirely on the permanence of the end anchorages 3 Tuesday, April 25, 2017

Prestressed Concrete VII. Applying external load: This method entails prestressing an arch slab or arch rib spanning between two unyielding abutments. A loaded column can be regarded as being prestressed against stresses due to bending moments, as also can an arch if it is uniformly compressed by a large dead load. VIII. Usage and Advantages of PC Examples of PC members: sleepers, hollow beams, concrete piles of precast blocks, cylindrical water tanks The steels are mostly in the shape of wires or strands but sometimes as bars. The prestressed concrete increases the shear strength and reduces radically both the deflections and the tensile cracks at service loads in such structures and thereby enables these high-strength materials to be used effectively. 4 Tuesday, April 25, 2017

Prestressed Concrete Prestressed concrete is particularly suited to prefabrication on a mass-production basis Prestressing increases the ultimate moment of resistance of the concrete and the resistance to shear forces and in some cases reduces the amount of steel required to resist shear. The concrete and the steel are strictly tested during the prestressing operation, and a lower factor of safety is justified. IX. *Disadvantages of PC Because of the large forces carried by the tendons, the concrete is vulnerable to crushing at points where tendons / wires are anchored or change direction. The high stresses in PC increase the axial and bending deformation caused by creep. 5 Tuesday, April 25, 2017