Pre-stressed concrete ? Don’t be stressed .

Overview:

The concrete is strengthened by adding reinforcement bars. But RCC has its own limitations such as higher depths for increased spans. Pre-stressed concrete is a modern day marvel which is a boon to the construction technology .Whenever we think of a large span of beam or slab, invariably , we think Pre-stressed sections.

this article is here to introduce you with this wonderful topic. 

What’s prestressing ?

Background:
We already know that any flexural member shall have both compression and tension . Whereas,the concrete is quite strong in compression, it is quite weak in tension and hence steel bars are provided in concrete to take care of the tensile forces. This combination is known as Reinforced cement concrete.
Prestressing means inducing compression in the concrete members so that when that element is subjected to tensile stress , the tension gets neutralized by the compression induced beforehand.

Analogy to pre-stressing:

All of us appreciate the Arch in a building.Beauty of the arch is beyond words. An arch is a flexural member which has only compression in the entire cross section  and most important , there is no tension. 

You shall ask how is this feat achieved ?

Let us start from the basics.

Process of building a stone arch:
A temporary semicircular formwork is first built at the opening where the arch needs to be built. Well dressed stones are then joined with cement mortar .The masonry is started from the lower ends and is slowly taken higher up on the curves.
When both sides reach the apex point,a stone which is known as the key stone or crown is hammered in the narrow gap .
This stone is in the shape of a wedge  .The forceful insertion of the crown stone in the narrow gap induces compression in the entire cross section.
Now the temporary staging is gradually removed and we have a fully
self- supporting Arch in position.

LOOK AT THE BEAUTIFUL STONE ARCH
LOOK AT THE BEAUTIFUL STONE ARCH
Analogy :

Hammering of crown in its position induces compression which is prestressing and this compression neutralizes the tension arising due to dead load & live loads on the arch.
Conceptually,Same process is done in concrete while prestressing.

Some important terms :
1.Tendon :
It is the bunch of  steel wire which is subject to measured tension.This tension when transferred to concrete , creates an equal & opposite compression in the concrete. 
2.Sheathing: It is the sleeve in which the tendons are located. This is used in post-tensioned elements
3.Jacket: This is where both ends of the  tendons are secured .
4.Jack : The pulling mechanism which induces measured tension in the tendon
5.Grouting : the high strength cement slurry which is injected under pressure in the sheathing.

Types of prestressing:
By virtue of the fact as to whether the prestressing is done before or after casting of concrete, the prestressing is known as Pre-tensioned or post- tensioned.
Pre-tensioned:

The tendons are laid lengthwise ,at the specific location in the member to be pre-stressed. the measured tension is applied to the tendon. Casting is done in the member with the requisite grade of concrete. As the concrete sets, the tendon is released from one end . The tendon tends to go back to its original length as it is released. The surrounding concrete resists this movement and in turn gets compressed.

Post-tensioned :

The tendons are laid in the sheathing ,at the specific location in the slab or beam to be pre-stressed. Casting is done in the member with the requisite grade of concrete. After the final setting of concrete ,the measured tension is applied to the tendon by pulling from one end . After pulling, the stretched tendon is secured in the jacket. The tendon tends to go back to its original length as it is released and hence exerts pressure on the Jacket .jacket in turn compresses the concrete.

Advantages of pre-stressed concrete over RCC :

Process of prestressing
Case study of mall
Span 12 m.
RCC beam 1.2m
Prestressed beam 0.25 m
Headroom is available

Author’s notes :

Dear Reader

Need some more time to complete this article .

Some photographs are to be added and descriptions to be completed.

Just bear with me for the time being.

Thank you so much

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