Losses In Prestressing Concrete

Different Types of Losses In Prestressing System | Losses In Prestressing Concrete

The different reductions of the prestressing force from a casted structure have termed the losses in prestressing Concrete.

For prestressed concrete applications, the most important variable is the prestress force. In the early days, It has been observed that the prestressing force does not stay constant, but decreases with time.

Even during the prestressing of the tendon and the transfer of the prestress to the concrete member, there is a reduction of the prestressing force from the value recorded in the jack gauge.

Losses In Prestressing Concrete

The losses are broadly classified into two groups, immediate and time-dependent prestress.

The immediate losses occur during prestressing of the tendons and the transfer of prestress to the concrete structure.

The time-dependent prestress losses are found during the working life of the prestressed member.

The total losses are around 15 to 20% of the initial prestressing.

The different types of losses in prestressing are shown in Table

ImmediateTime-Dependent
a. Elastic Shorteninga. Creep
b. Frictionb. Shrinkage
c. Anchorage Slipc. Relaxation

Different types of losses in Pre-tensioning and Post-tensioning

Losses in Pretensioning

Losses in Pretensioning is due to the following reason.

  1. Elastic deformation of concrete
  2. Due to the relaxation of steel
  3. Due to the shrinkage of concrete
  4. Due to the creep of concrete
  5. Due to sudden changes in temperature

Explanation of Losses in Pre Tensioning is described below.

1. Loss of Prestress due to elastic deformation of concrete in pre-tensioned members

When the tendon is cut and the prestressing force member is transferred, the prestress causes the concrete to shrink immediately. The tendon is also reduced in the same amount. Which leads to loss of reputation.

2. Loss of Prestress due to the relaxation of steel in pre-tensioned members

Relaxation of Steel is defined as the reduction in stress over time under constant stress. Due to the loosening of the Relaxation of steel, the pressure in the tendon decreases over time. The relaxation depends on the type of steel, initial prestress, and temperature.

To calculate the loss in prestress force we use various test data which is done while keeping the concrete, in the case of the absence of data, we have a table as Indian Standard Code IS: 1343-1980.

Also Read: Grade of Concrete Table | Concrete Grade as per IS 456

3. Loss of Prestress due to Shrinkage of Concrete in pre-tensioned members

The shrinkage of concrete in prestressed members results in a reduction of tensioned wires and hence contributes to the loss of stress. 

In the case of pre-tensioned members, generally wet curing is practised in order to prevent shrinkage until the time of transfer of load. 

As a result, the total residual shrinkage stress will be larger in the prestressed members after the transfer of pressure, where a portion of the shrinkage will have already occurred by the time the stress is transferred.

4. Loss of Prestress due to creep of Concrete in pre-tensioned members

Concrete creep is defined as the deformation of the structure under constant load. basically in the form of long-term pressure or stress.

Concrete can change its shape. This deformation usually occurs in the direction in which the force is being applied.

Age of LoadingCreep coefficient
7 days2.2
28 days1.6
14 years1.1

Losses in Post-Tensioning

Losses in Post-tensioning is due to the following reason.

  1. Due to the elastic deformation of concrete
  2. Due to relaxation of steel
  3. Due to shrinkage of concrete
  4. Due to the creep of concrete
  5. Due to Frictional losses
  6. Due to Anchorage Slip

1. Loss of Prestress due to elastic deformation of concrete in Post-tensioned members

If there is only one tendon, there is no loss as the applied prestress is lodged after the elastic shortens of the member. For more than one tendon, if the tendons are stretched sequentially, loss to one tendon occurs during subsequent stretching of the other tendon.

2. Loss of Prestress due to the relaxation of steel in post-tensioned members

Most of the codes provide for the loss of stress due to the relaxation of steel as a percentage of the initial stress in steel. The Indian Standard Code recommends a value of 0 to 90 N/mm2 for tension in wires ranging from 0.5f to 0.84f (f=Stress)  of steel recommended in British and Indian codes compiled in IS 1343.

emporary over-stressing by 5% to 10% for a period of 2 min is sometimes used to reduce this loss as in the case of drawn wires.

3. Loss of Prestress due to the shrinkage of concrete in post-tensioned members

The shrinkage of the concrete in the prestressed members makes the tensioned wires shorter and therefore contributes to stress loss. The shrinkage of concrete is affected by the type of cement and aggregate and the curing method used.

4. Loss of Prestress due to the creep of concrete in post-tensioned members

Concrete creep occurs as a result of constant pressure in the concrete of a prestressed member which effectively reduces stress in high tensile steel.

The stress loss in steel due to concrete creep can be estimated if the magnitude of the final creep stress or creep coefficient is known.

The magnitude of the Creep Coefficient depends on the following factor,

  1. Humidity
  2. Concrete quality, 
  3. Duration of applied loading and 
  4. The age of the concrete when loaded.

5. Loss of Prestress due to Frictional losses in post-tensioned members

In the case of post-tension members, tendons are placed in tubules made of concrete. The ducts are either straight or follow a curved profile, depending on the design requirements.

As a result, when straining the curved tendon, there is a loss of tension in the post-tension members due to friction between the tendon and the surrounding concrete ducts.

The bad impact of this  loss is of the following types:

(a) Loss of Prestress due to the curvature effect

Which generally follows a curved profile along the length of the beam, depending on the tendon form or alignment.

(b) Loss of Prestress due to the wobble effect

which depends on the local deviation in the alignment of the cable. Waver or ripple effects are the result of accidental or unavoidable misalignment.

Due to ducts or sheaths may not be perfectly positioned to follow a predetermined profile along with the entire span of the beam.

6. Loss of Prestress due to Anchorage Slip in post-tensioned members

When the cable is tensioned and the jack is released to transfer the prestress to the concrete, friction is reduced, employed to grip the wires, slip over a small distance before the wires are firmly housed between the wedges.

The impact of Anchorage slip depends upon the kinds of the wedge and the stress in the wires. In systems where the tendons arc looped around concrete anchorage blocks.

Related FAQ

Q. What are the various types of losses in prestressing concrete systems?

1. Due to elastic deformation of concrete
2. Due to relaxation of steel
3. Due to shrinkage of concrete
4. Due to the creep of concrete
5. Due to Frictional losses
6. Due to Anchorage Slip
7. Due to Sudden changes in Temperature

Q. What is the effect of Shrinkage in Prestress Concrete?

Shrinkage of the concrete in the prestressed members makes the tensioned wires shorter.

Q. Why is Anchorage slip loss found in Prestress?

nchorage slip loss of prestressing is caused by a definite total amount of shortening, the percentage loss is higher for short members than for long ones.

Q. Name the Major portion losses of Prestress.

Loss of prestress due to anchorage slip, which forms a major portion of the total loss.

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