Pre stressed concrete
- 2. Prestressed concrete is a method for overcoming concrete's natural weakness in tension. In 1904, Freyssinet attempted to introduce permanent acting forces in conc. to resist elastic forces under loads and was named “Pre Stressing”. It can be used to produce beams, floors or bridges with a longer span than is practical with ordinary reinforced concrete.
- 3. Pre-stressing is a method in which compression force is applied to the reinforced concrete section. Pre-stressing 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 member would otherwise experience due to a bending load.
- 4. This classification is based on the method by which the prestressing force is generated. They are Hydraulic Prestressing Mechanical Prestressing Electrical Prestressing Chemical Prestressing
- 5. Pre-stressed concrete refers to a procedure whereby tensile rods are put in place first and tightened, followed by concrete pouring. Compression can be applied after pouring concrete using bonds. They are tightened once the concrete is dry Main disadvantage is that a cable can burst out of the slab, if the anchoring system fails.
- 6. Even without a load, the ordinary concrete beam must carry its own weight. An upward force is created which in effect relieves the beam of having to carry its own weight.
- 7. There are two types of prestressed concrete. They are Pre- tensioned concrete Post tensioned concrete
- 8. Pre-tensioned concrete is cast around already tensioned tendons. This method produces a good bond between the tendon and concrete, which both protects the tendon from corrosion and allows for direct transfer of tension. The cured concrete adheres and bonds to the bars and when the tension is released it is transferred to the concrete as compression by static friction.
- 9. 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. They are classified into two types, Bonded post tensioned concrete Unbonded post tensioned concrete
- 10. Bonded post-tensioned concrete is the descriptive term for a method of applying compression after pouring concrete and the curing process (in situ). The concrete is cast around a plastic, steel or aluminium curved duct, to follow the area where otherwise tension would occur in the concrete element.
- 12. Unbonded post-tensioned concrete differs from bonded post-tensioning by providing each individual cable permanent freedom of movement relative to the concrete. To achieve this, each individual tendon is coated with a grease (generally lithium based) and covered by a plastic sheathing formed in an extrusion process.
- 15. Take full advantages of high strength concrete and high strength steel Need less materials Smaller and lighter structure No cracks Use the entire section to resist the load Better corrosion resistance Very effective for deflection control Better shear resistance
- 16. Bridges Slabs in buildings Water tank Concrete pile Thin shell structures Offshore platform Nuclear power plant Repair and rehabilitations
- 17. Thus, pre-stressed concrete increases the quality, strength, span of the structure. Since it is cost effective, it is used widely on recent days.