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Prestressed concrete

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/mm2. The ultimate strength of high tensile steel wires used in prestressing varies from 1500N/mm2 for 8mm diameter bars to 2350N/mm2 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 the concrete member before setting the concrete to the desired strength is known as pre-tensioning of concrete. In this method, the tendons ( wires of high tensile strength) are stretched to the desired amount. One end of the tendon is secured to an abutment, while the other end is stretched out with the help of a prestressing (distribution bars, stirrups etc.) are placed suitably and fresh concrete poured, compacted and properly cured. At the end of the curing period, concrete is hard enough to sustain the compression passed through the bond. The projecting steel wires are then cut. This method is particularly adopted in precast beams, posts and simply supported slabs.

Post-tensioning

The method of prestressing in which the prestressing force is applied to the tendons after the concrete has set or hardened and has acquired necessary strength is known as post-tensioning method of prestressing the concrete member. This method consists of pouring the concrete into the mould in which steel tendons are placed and then stretched after the concrete has hardened. The tendons are not bonded to the concrete before tensioning. The reinforcing steel may be in the form of single tendon, cables made of seperate wires of alloy bars. The hydraulic jacks acting against the ends of precast member are used for pulling or stretching the tendons or cables. The tendons are pulled through holes, ducts or grooves left for them in precast concrete member. When the required stretch has been given, the jack end  of the tendon is anchored and the duct is grouted with cement. It will save the steel from rusting as the cement grout forms a bond between tendons and concrete. Such type of beams are known as bonded post-tensioned beams.
The post-tensioning method of prestressing the concrete is used for either cast-in-situ or pre-cast construction.

Linear pre-stressing

The linear pre-stressing is done in straight members like beams,slabs, piles etc. These members are normally precast, using pre-tensioning technique.

Circular pre-stressing

The circular pre-stressing is adopted for circular structures like water or oil tanks, pipes, silos in which the tendons are in the form of a ring  and special device such as Merry-go-round equipment.

Losses in Pre-stressing

1. Loss of prestress due to creep of concrete

The creep of concrete causes loss of stress in tensioned steel. The creep coefficient of concrete for loss of prestress is defined as the ratio of creep strain to the elastic strain. The value of creep coefficient depends upon the humidity, concrete quality, duration of loading and age of concrete when load is applied. It varies form 1.5 for watery condition to 4.0 for dry condition.

2. Loss of prestress due to relaxation of steel

The loss of prestress due to relaxation of steel depends upon the quality of steel, initial prestress, age after prestressing and temperature. This loss should be taken 2 to 8% of the average initial stress.

3. Loss of prestress due to shrinkage of concrete

The loss of prestress due to shrinkage of concrete is taken as the product of modulus of elasticity of steel and the shrinkage strain of concrete. According to Indian Standards, the shrinkage strain for a pretensioned member is taken as 3 × 10-4.

4. Loss of prestress due to slip in anchorage

The loss of prestress due to slip in anchorage is of special importance with short members and is taken care of by providing additional stretch at the time of stretching the tendons. An average of about 2.5 mm is estimated as anchorage slip in wedge type of grips and for heavy strands, the slip may be 5 mm.

5. Loss of prestress due to shortening of concrete

The loss of prestress due to shortening of concrete depends upon the strength of concrete (modular ratio) and the initial prestress. In case of pre-tensioning tendon, the loss is calculated on the least modular ratio and the stress in the adjoining concrete. For post-tensioned tendon, the loss is calculated on the basis of half the product of stress in the adjoining concrete, averaged along their lengths and modular ratio. It is assumed that the tendons are located at the centroid of concrete.

6. Loss of prestress due to friction

There will be considerable movement of sliding of tendon relative to the surrounding duct during tensioning operation. Since the tendon is in direct contact with the duct or with spacers provided, therefore the friction will cause a reduction in prestressing force as the distance form the jack increases.

Notes: (1) At the time of tensioning, the maximum tensile stress behind the anchorage shall not exceed
(a) 80% of ultimate tensile strength of the wire or bar steel without a definite yield point, and
(b) The yield stress for steel with a guaranteed yield point.
2) After accounting for losses due to creep and shrinkage of concrete, friction effects and relaxation of steel, the maximum tensile stresses at the point of maximum bending shall not exceed
(a) 60% of the ultimate strength of wire or bar for steel without a definite yield point, and 
(b) 80% of the yield stress for steel with guaranteed yield point.

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