Methods of Prestressing

There are two methods of :

Pre-tensioning: Apply prestress to steel strands before casting concrete;

Post-tensioning: Apply prestress to steel tendons after casting concrete.

 

Pre-tensioning

 

This is the most common form for precast sections. In Stage 1 the wires or strands are

stressed; in Stage 2 the concrete is cast around the stressed wires/strands; and in

Stage 3 the prestressed in transferred from the external anchorages to the concrete,

once it has sufficient strength:

040213 1930 MethodsofPr12 Methods of Prestressing

In pre-tensioned members, the strand is directly bonded to the concrete cast around it.

Therefore, at the ends of the member, there is a transmission length where the strand

force is transferred to the concrete through the bond:

040213 1930 MethodsofPr22 Methods of Prestressing

At the ends of pre-tensioned members it is sometimes necessary to debond the strand

from the concrete. This is to keep the stresses within allowable limits where there is

little stress induced by self with or other loads:

040213 1930 MethodsofPr32 Methods of Prestressing

Post-tensioned

 

In this method, the concrete has already set but has ducts cast into it. The strands or

tendons are fed through the ducts (Stage 1) then tensioned (Stage 2) and then

anchored to the concrete (Stage 3):

040213 1930 MethodsofPr41 Methods of Prestressing

The anchorages to post-tensioned members must distribute a large load to the

concrete, and must resist bursting forces as a result. A lot of ordinary reinforcement is

often necessary.

A typical tendon anchorage is:

040213 1930 MethodsofPr51 Methods of Prestressing

And the end of a post-tensioned member has reinforcement such as:

040213 1930 MethodsofPr6 Methods of Prestressing

Losses

From the time the prestress is applied, the prestress force gradually reduces over time

to an equilibrium level. The sources of these losses depend on the method by which

prestressing is applied.

In both methods:

The member shortens due to the force and this relieves some of the prestress;

The concrete shrinks as it further cures;

The steel ‘relaxes’, that is, the steel stress reduces over time;

The concrete creeps, that is, continues to strain over time.

In post-tensioning, there are also losses due to the anchorage (which can ‘draw in’ an

amount) and to the friction between the tendons and the duct and also initial

imperfections in the duct setting out.

For now, losses will just be considered as a percentage of the initial prestress.

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