Concrete Structures: Types of pre-stressing

Concrete Definition: Concrete in which there have been introduced internal stresses of such magnitude and distribution that the stresses resulting from given external loadings are countered to a desired degree - ACI

Types of pre-stressing

1 Pre-tensioning & Post-tensioning

In pre-tensioning the tendons are tensioned before the concrete is placed. The tendons are temporarily anchored to abutments or stressing beds. Then the concrete member is cast between and over the wires. After the concrete has attained the required strength, the wires are cut from the bulkhead and  pre-stress is transferred to the concrete member.

In post-tensioning the concrete member is cast with ducts for the wires. After concrete has attained  sufficient strength, wires are threaded into the ducts, tensioned from both or one end by means of jack/jacks and at the precise level of pre-stress the wires are anchored by means of wedges to the anchorage plates at the ends.

2 Bonded & Un-bonded tendon

In post-tensioned members, the wires are either left free to slide in the ducts or the duct is filled with grout. In the former, the tendon is un-bonded and in the latter it is bonded.

Stages of loading

Initial stage

The member is under pre-stress but is not subjected to any superimposed external loads. Further

subdivision of this stage is possible.

1. Before pre-stressing: Concrete is weak in carrying loads. Yielding of supports must be prevented.

2. During pre-stress:

a. Steel: This stage is critical for the strength of tendons. Often the maximum stress to which the wires will be subjected throughout their life may occur at this stage.

b. Concrete: As concrete has not aged at this stage, crushing of concrete at anchorages is possible, if its quality is inferior or the concrete is honeycombed. Order of pre-stressing is important to avoid overstress in the concrete.

3. At transfer of pre-stress: For pre-tensioned members, where transfer is within a short period, and for post-tensioned members where transfer may be gradual, there are no external loads on the member except its own weight. n

4. De-shuttering: The removal of form-work must be done after due consideration

Thus the initial pre-stress with little loss imposes a serious condition n the concrete and often controls the design of the member.

Final stage

This is the stage when actual working loads come on the structure. The designer must consider various combinations of live loads on different parts of the structure with lateral loads such as wind and earthquake forces and strain loads produced by settlement of supports and temperature. The major loads in this stage are:

1. Sustained load: It is often desirable to limit the deflection under sustained loads sue to its own weight and dead loads.

2. Working load: The member must be designed for the working load. Check for excessive stress and deflection must be made. But this design may not guarantee sufficient strength to carry overloads.

3. Cracking load: Cracking in a pre-stress member signifies a sudden change in bond and shearing stresses. This stage is also important

4. Ultimate load: This strength denotes the maximum load the member can carry before collapse.