for pre-tension members is 40 N/mm2 and for post-tensioned members is 30 N/mm2.

Type of Losses in Prestressing

There will be a continuous reduction of effective stresses in prestressed concrete due to various causes with time from stage of a transfer.

Such losses are termed as the loss of prestress.

Losses in Pre-tensioning

Given below different type of losses :

Elastic deformation of concrete

Relaxation of stress in steel i.e creep in steel.

Shrinkage of concrete

Creep of concrete

Losses in Post-tensioning

Given below different types of losses:

If there is simultaneous tensioning of all the wires, then no loss happens due to elastic deformation.

Due to elastic deformation, there will be Loss of prestressing if successively bars are tensioned.

Relaxation of stress in steel.

Shrinkage of concrete

Creep of concrete

Frictional losses

Anchorage slip.

1. Elastic deformation of concrete

When prestress is applied elastic shortening takes place in concrete which results in prestressed steel shortening.

In both post-tensioning and pre-tensioning members, this elastic deformation loss will be there.

This elastic deformation depends upon the stress in concrete at the level of steel and modular ratio (m).

Elastic deformation loss can be given by = m x Fc

where ,modular ratio (m )= Es/Ec

2.Loss due to shrinkage of concrete

Shrinkage is defined as a change in the volume of concrete members.

This shrinkage loss is dependent on the passage of time and humidity present in the atmosphere but is independent of the load’s applied.

Due to Shrinkage of concrete tensioned wire will get shortened and hence produces the prestressed loses.

In both post-tensioning and pre-tensioning members, this shrinkage loss will occur.

Factors affecting shrinkage

Type of cement and aggregate

Method of curing

There will be a reduction in shrinkage and consequent prestress loses if high strength concrete with low water-cement ratios is utilised.

Loss of prestress = ecs X Es

where :

ecs = Total residual shrinkage strain

= 0.0003 (for members with post-tensioned )

= 0.0002/ log 10 (t +2) ( for members with pre-tensioned )

Es = Youngs Modulus of the elasticity of steel

3.Loss due to creep of concrete

Under sustained loading, the progressive increase in the plastic deformation of concrete is termed as Creep of concrete.

The transferred prestress effects in the creep of concrete in this method.

The stress in high tension steel is reduced due to this.

In both post-tensioning pre-tensioning members this creep of concrete occurs.

There are two methods for estimation of creep.

Ultimate creep strain method

Creep -coefficient method

4. Relaxation of stress in steel

Under constant strain at a constant temperature, the loss of stress in steel members are termed as relaxation of steel.

This loss is similar to creep in concrete

Loss due to relaxation differs uniquely for steels.

In both post-tensioning pre-tensioning members this loss of stress in steel members occurs.

In general, the loss is about 2% to 8 % of the initial stress and this loss will be higher at a greater temperature.

This loss is generally referred to at 1000 hours at 27-degree Celcius.

5.Friction loss due to post-tensioned member

In the members with post-tensioned only this friction loss occurs.

There will be small frictional losses in Jacking, equipment.

The friction is not small between tendons and the surrounding materials and so it is considered party wobble or length effect and partly a curvature effect.

Due to the wobble effect, friction loss occurs in straight length.

in curved members, the friction loss occurs due to wobbles effect and curvature.

6.Loss due to Anchorage slip

Due to the strain or due to the slip in anchorage this loss occurs.

In the members with post-tensioned this loss generally occurs.

Conclusion :

The above mentioned prestressed concrete materials are used in high raised building constructions and prestressed concrete losses need to be considered during concrete design stages.

Ashwini Kumar is a graduate in civil engineering from NIT Warangal. He has 8+ years of experience in the construction field.
He is the author, founder of the Easy Home Builds blog

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