Pre-stressed concrete is something which is further to normal RCC.
So , story of concrete starts with Plain cement concrete then it is developed into Reinforced cement concrete . Reinforced cement concrete is further strengthened to have Pre-stressed concrete.
As you know , PCC is good in compression but weak in tension.
We provide reinforcement bars in concrete to care of this tension and this is known as Reinforced cement concrete. The RCC is stressed before actual loading with compression force so that when the tension comes in the element , it is balanced by the compression.
As a result , we get Pre-stressed concrete.
In a commercial complex , there is a basement .The span between columns is 10.5 m.
Now if a normal RCC beam of span 10.5 m was provided, its depth would be approximately 750mm to 800 mm. Imagine there is a parking in the basement. That means a minimum clearance of 2500 mm was needed below the beams. Thus , the slab bottom has to be placed at 2500 +800 = 3300 mm. Accordingly , the columns would have to be designed .
When the Pre-stressed concrete Post tensioned slab was designed by the consultants , there were no beams protruding out of the slab and , the slab thickness was just 250mm.
Thus , the actual basement height was kept at 2500 mm only.
Look at this photograph.
Detailed Story :
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 Pre-stressing ?
In short :
If a opposite stress [ compression ] is introduced into the concrete member , prior to the actual loading on the element , it balances the tension. This is the theory of prestressing.
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 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
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 :
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 [ Sleeves ]: 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 pre-stressing:
By virtue of the fact as to whether the pre-stressing is done before or after casting of concrete, the pre-stressing is known as Pre-tensioned or post- 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.
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 :
- Economical to use pre-stressed concrete in longer spans
- Reduced depth of the RCC section