Published on February 29, 2016
1. PRESTRESSING IN CONCRETE A PRESENTATION ON Anuj Singhal Divyansh
2. Pre-stressed concreteDefinition: • Prestress is defined as a method of applying pre- compression to control the stresses resulting due to external loads below the neutral axis of the beam tension developed due to external load which is more than the permissible limits of the plain concrete. • In other words it is a combination of steel and concrete that takes advantages of the strengths of each material. Advantages • Concrete remains un-cracked • Reduction of steel corrosion • Increases durability • Good for pressure vessels • High span to depth ratio (ex: 45:1 vs. 28:1) • less dead load • More economical
3. 1. Tendon: A stretched element used in a concrete member of structure to impart prestress to the concrete. 2. 2. Anchorage: A device generally used to enable the tendon to impart and maintain prestress in concrete. TERMINOLOGY
4. 2 Main Types of Internal Prestressed Concrete In which the tendons are tensioned before the concrete is placed, tendons are temporarily anchored and tensioned and the prestress is transferred to the concrete after it is hardened. • Pre tensioned Concrete • Pre tensioned concrete • Post tensioned concrete
5. In post-tensioning the concrete unit are first cast by incorporating ducts or grooves to house the tendons. When the concrete attains sufficient strength, the high-tensile wires are tensioned by means of jack bearing on the end of the face of the member and anchored by wedge or nuts. The forces are transmitted to the concrete by means of end anchorage and, when the cable is curved, through the radial pressure between the cable and the duct. The space between the tendons and the duct is generally grouted after the tensioning operation. • Post tensioned Concrete
6. Disadvantages of Prestressed Concrete 1. The availability of experienced builders is scanty. 2. Initial equipment cost is very high. 3. Availability of experienced engineers is scanty. 4. Prestressed sections are brittle 5. Prestressed concrete sections are less fire resistant.
7. Pre-tensioning Method Stage 1 Stage 2 Stage 3 Stage 4 Tendons and reinforcement are positioned in the beam mould. Tendons are stressed to about 70% of their ultimate strength. Concrete is cast into the beam mould and allowed to cure to the required initial strength. When the concrete has cured the stressing force is released and the tendons anchor themselves in the concrete.
8. Post-tensioning Method Stage 1 Stage 2 Stage 3 Stage 4 Cable ducts and reinforcement are positioned in the beam mould. The ducts are usually raised towards the neutral axis at the ends to reduce the eccentricity of the stressing force. Concrete is cast into the beam mould and allowed to cure to the required initial strength. Tendons are threaded through the cable ducts and tensioned to about 70% of their ultimate strength. Wedges are inserted into the end anchorages and the tensioning force on the tendons is released. Grout is then pumped into the ducts to protect the tendons.
9. 1. In pretensioned prestress concrete, steel is tensioned prior to that of concrete. It is released once the concrete is placed and hardened. The stresses are transferred all along the wire by means of bond. 2. Suitable for short span and precast products like sleepers, electric poles on mass production. 3. In pretensioning the cables are basically straight and horizontal. Placing them in curved or inclined position is difficult. However the wire’s can be kept with eccentrically. Since cables can not be aligned similar to B.M.D. structural advantages are less compare to that of post-tensioned. 4. Prestress losses are more compare to that of post-tensioned concrete. 1. Concreting is done first then wires are tensioned and anchored at ends. The stress transfer is by end bearing not by bond. 2. Suitable for long span bridges 3. The post tensioning cables can be aligned in any manner to suit the B.M.D due to external load system. Therefore it is more economical particularly for long span bridges. The curved or inclined cables can have vertical component at ends. These components will reduce the design shear force. Hence post-tensioned beams are superior to pretensioned beams both from flexural and shear resistances point. 4. Losses are less compare to pre-tensioned concrete Comparative Study: Pretension Vs Post-tensioned Member Pretension member Post-tensioned member
10. Forms • Wires • Strands • Tendons • Cables • Bars Source of Force • Mechanical • Hydraulic • Electrical • Chemical Prestressing Steel (High Strength steel) tendons Mechanical jacks tendons wires
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