WHAT IS PRE-STRESSED CONCRETE AND CLASSIFICATIONS OF PRE-STRESSED CONCRETE




PRE- STRESSED CONCRETE

Normal concrete structure have high compressive strength and very low tension strength .in order to withdraw the weakness of concrete in tension pre-tensioning technique is used .pre-stressed concrete it mean the intentional creation of permanent compressive stresses in a structure for the purpose of improving its behavior and strength for many service situations.

CLASSIFICATION OF PRE-STRESSED CONCRETE

Pre stressed concrete structure may be classified based on their features of design and construction. they may be classified into three groups

1.     Location of construction
2.      Level of construction
3.      Method of pre-stressing

LOCATION OF CONSTRUCTION

For this type the pre-stressed structure may be precast cast, cast in situ, composite constrictions.

PRE CAST

Precast pre-stressed element are fabricated in a plant or near to the working site. Then they are transported into construction site .after transportation then they are erected and placed its position.

CAST IN SITU

Cost in situ is the another method this construction is done at the exact place or  construction adoption a suitable method of construction .so they don’t have transportation and erection process are required.

COMPOSITE CONSTRUCTION

Composite construction is the combination of both pre stressed and cast in situ methods are done. For example in a building structure beam is made by pre-stressed concrete and the top roof slab is constructed by the cast in situ method.

LEVEL OF PRE-STRESSING

Level of pre-stressing is classified into fully pre-stressed, limited pre-stressed, partially pre-stressed.

FULLY PRE-STRESSED

Fully pre-stressed concrete members do not have any tensile stress under the action of working or service loads.

LIMITED PRE-STRESS

Members are pre-stressed to a limited extended such that members may develop some tensile stress under working loads but the magnitude is less than the tensile strength of concrete. These members hence behave as a fully pre-stressed member and remain uncracked under working loads.

PARTIALLY PRE-STRESSED

Partially pre-stressed structure have both pre-stressing steel and reinforcing steel which resists working load. Partial pre-stressing have both advantages for pre-stressed concrete and reinforced concrete. And tensile stress is with in the limit .so the concrete crack like the reinforced concrete.

METHODS OF PRE-STRESSING

Under this classification pre-stressed concrete is based on methods are used . Two methods are used for conventional method and non-conventional method.

CONVENTIONAL METHOD.

For the conventional method of pre-stressing the concrete is pre compressed by stretching high tensile steel mechanically and transferring the forces to concrete by mean of bond or by end anchorage .for this method have four classifications.

CIRCULAR PRE-STRESSING

Circular pre-stressing is a term applied to pre-stressed circular structures generally used for pressure vessels and containment structures. The pressure vessels are pressure pipes, nuclear power pressure vessels. And containing elements are water tanks bunkers silos etc. in this case pre-stressing steel is in the form of continuous circles or endless loops spiraling all-round the concrete structure.

LINEAR PRE-STRESSING

Pre-stressing the concrete elements like beams , slabs ,post etc.is called linear pre-stressing. This type is used in building structure and bridges. Here the pre-stressing gables are not straight and also they go bent or curve but not circular.

PRE TENSIONED METHOD

In pre tensioned method tendons are pre stressed before the concrete is placed .in this method the tendons are temporarily anchored against some augments or stressing beds. The pre-stress is transferred to the concrete after it has set. this method is adopted in pre-casting plants or laboratories were permanents beds are provided for such tensioning it is also employed in the field where abutments can be economically constructed.

POST TENSIONED METHOD

For post tensioning method steel or cable or tendons are placed the  mold .but they pre-stressed after the concrete is placed and specified strength is achieved.
In this method the cables are enclosed in ducts or metal sheaths before concreting which prevent them form bonding to concrete. Alternatively holes are left in the concrete cast at site through which are subsequently threaded .

NON CONVENTIONAL METHOD

Non conventional method are prepared only for those which are adopted under special circumstances. This method have limited applications compared to conventional method. Non-conventional have the following classifications.

externally pre-stressed
Electro thermal pre-stressing
Chemical pre-stressing
Pre stressing with fibre composites.
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