PRESTRESSED CONCRETE STRUCTURES Reinforced concrete Concrete is strong

PRESTRESSED CONCRETE STRUCTURES

Reinforced concrete: • Concrete is strong in compression weak in tension. • Steel in strong in tension • Reinforced concrete uses concrete to resist compression and to hold bars in position and uses steel to resist tension. • Tensile strength of concrete is neglected (i. e. zero ) • R. C beams allows crack under service load.

Pre-stressed Concrete • What is Pre-stressed Concrete? : – – Internal stresses are induced to counteract external stresses. In 1904, Freyssinet attempted to introduce permanent acting forces in conc. to resist elastic forces under loads and was named “Pre stressing”.

Concept of pre-stressing: § i. The concept of pre stressing was invented years ago when metal brands were wound around wooden pieces to form barrels. invented § § § ii. The metal brands were tighten under tensile stress which creates compression between the staves allowing them to resist internal liquid pressure.

Principle of pre-stressing: • Pre-stressing is a method in which compression force is applied to the reinforced concrete section. • The effect of pre stressing is to reduce the tensile stress in the section to the point till the tensile stress is below the cracking stress. Thus the concrete does not crack. • It is then possible to treat concrete as a elastic material. • The concrete can be visualized to have two compressive force i. Internal pre-stressing force. ii. External forces (d. l , l. l etc ) • These two forces must counteract each other.

Principle of Pre-stressing: • Stress in concrete when pre stressing is applied at the c. g of the section

Principle of Pre-stressing: • Stress in concrete when pre stressing is applied eccentrically with respect to the c. g of the section.

Pre-stressed Concrete: Methods • There are two basic methods of applying pre-stress to a concrete member – Pre-tensioning – most often used in factory situations – Post-tensioning – site use

Types of pre-stressing: I. Pre-tensioning In Pre-tension, the tendons are tensioned against some abutments before the concrete is place. After the concrete hardened, the tension force is released. The tendon tries to shrink back to the initial length but the concrete resists it through the bond between them, thus, compression force is induced in concrete. Pretension is usually done with precast members

II. Post tensioning • In Post tension, the tendons are tensioned after the concrete has hardened. Commonly, metal or plastic ducts are placed inside the concrete before casting. After the concrete hardened and had enough strength, the tendon was placed inside the duct, stressed, and anchored against concrete. Grout may be injected into the duct later. This can be done either as precast or cast-in-place.

Post tensioning

Advantages: • 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 • Good for water tanks and nuclear plant • Very effective for deflection control • Better shear resistance

Disadvantages compared to RC: • • Need higher quality materials More complex technically More expensive Harder to re-cycle

Application: • Bridges • Slabs in buildings • Water Tank • Concrete Pile • Thin Shell Structures • Offshore Platform • Nuclear Power Plant • Repair and Rehabilitations

Definition: • 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.

Compressive stresses in concrete caused by pre stressing force F Stresses caused due to applied loads (without pre stressing)

Basic Concept • Prestressed concrete is basically concrete in which internal stresses of a suitable magnitude and distribution are introduced so that the stresses resulting from the external loads are counteracted to a desired degree.

• Tendon: A stretched element used in a concrete member of structure to impart prestress to the concrete. • Anchorage: A device generally used to enable the tendon to impart and maintain prestress in concrete.

• Pretensioning: A method of prestressing concrete in which the tendons are tensioned before the concrete is placed. In this method, the concrete is introduced by bond between steel & concrete. • Post-tensioning: A method of prestressing concrete by tensioning the tendons against hardened concrete. In this method, the prestress is imparted to concrete by bearing.

Advantage of Prestressed Concrete • Lighter and slender members • Free from tenslie loads, thus whole section is effective. • Capable to carry eccentric loads • Free from cracks • Factory products are possible. • Prestressed members are tested before use. • Fatigue strength is better.

• • Defection controlled. Smaller cross – section Improved resistance to repeating loads. Possibility to connect prefabricated members.

Disadvantages of Prestressed Concrete • The availability of experienced builders is scanty. • Initial equipment cost is very high. • Availability of experienced engineers is scanty. • Prestressed sections are brittle • Prestressed concrete sections are less fire resistant

Stages of Prestressed Concrete

Pre tensoning

Post-tensoning

Secondary categories • • External Prestressing Internal Prestressing Linear Prestressing Circular Prestressing Full Prestressing Limited Prestressing Partial Prestressing

• Uniaxial Prestressing • Biaxial Prestressing

Sources of Prestressing Force • • Mechanical devices Hydraulic devices Electrical devices Chemical devices

Materials for prestressing • • • Prestressing bed End abutments Shuttering / mould Jack Anchoring device Harping device (optional)

Pretension Vs Post-tensioned Member Pretension member Post-tensioned member 1. In pretensioned prestress concrete, steel is tensioned prior to that of concrete. It is released once the concrete is placed and hardened. 1. Concreting is done first then wires are tensioned anchored at ends. The stresses are transferred all along the wire by means of bond. The stress transfer is by end bearing not by bond. 2. Suitable for short span and precast products like sleepers, electric poles on mass production. 2. Suitable for long span bridges 3. In pretensioning the cables are basically straight and horizontal. 3. The post tensioning cables can be aligned in any manner like curved or inclined cables can have vertical component at ends. 4. Prestress losses are more compare to that of post-tensioned concrete. 4. Losses are less compare to pretensioned concrete

Losses in prestress • The initial prestressing concrete undergoes a gradual reduction with time from the stages of transfer due to various causes. This is generally defined as total “Loss of Prestress”.

Types of losses in prestress • Pretensioning 1. Elastic deformation of concrete 2. Relaxation of stress in steel 3. Shrinkage of concrete 4. Creep of concrete

• Post-tensioning 1. No loss due to elastic deformation if all wires are simultaneously tensioned. If the wires are successively tensioned, there will be loss of prestress due to elastic deformation of concrete. 2. Relaxation of stress in steel 3. Shrinkage of concrete 4. Creep of concrete 5. Friction 6. Anchorage slip

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