Prestressed Concrete Structures A Brief Description With Example

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Prestressed Concrete Structure is the one of the most advancement in the Civil Engineering Construction Field, particularly in the Concreting Operation. In the modern day every heavily loaded structures such as Bridges, Flyovers, Railway Sleepers in all these structure where loads are so heavy that Normal RCC will not be enough there prestressed concrete is used. In this article I will share with you the main concept that is the backbone of the Prestressing technique and procedure for the Industrial manufacturing of Prestressed concrete, also after reading this article thoroughly you will understand why one should resort to Prestressed concrete, Advantages of Prestressed concrete over the normal RCC structure. First of all let us understand what the science is being the backbone of this prestressed concrete structures.

Prestressed Concrete Working Principle

Why Prestressed Concrete Structures becomes necessary ==>

As you and me already know that a structure under the action of different loads acting on it, the structure takes the shape of bending. The loads acting on a structure causes it to bend in between the supports with shape having convexity downwards that is what we called as positive bending,  and near the support the bending occurs in a reverse shape , that is the convexity is on a upward direction, this we called as the negative bending of the structure. Now due the nature of this kind of bending of the structure the one side of the structure about the Neutral Axis of the section the particles there goes on squeezing that is compressing, and the other side of the Neutral Axis the particles tries to go away from each other, hence tension between the each particles develops. Now the main problem arises because concrete is strong enough t withstand the compressive stresses but it cannot withstand the tensile stresses to a reasonable amount, hence we provided reinforcements to take up this tension.

But what if the loads are very high, I really mean very very high, as in the case of Bridges, Railways Sleepers? will it be able to withstand that high super high amount of tensile stresses?
Yes it can, but the sections will be huge and hence this will not be practical to construct normal RCC on these cases. Apart from this fact any RCC structure will defiantly have cracks (micro cracks) which may not be a matter of concern in case of normal structure, but in case of heavily loaded structure this cracks are not to be allowed, especially in case of water retaining structures. Due to all of these reasons we need to use Prestressed concrete structure.
If we study a Concrete structure we will notice that it is susceptible to failure mainly due to the Tensile stresses. Hence if we can make any arrangements by which there will be no Tensile stresses in the structure or a very little amount of tensile stress which is manageable then we can definitely the solve the problem, But How?
The main concept is that when a structure is loaded it results in bending and thus both the tensile and compressive stress develops now  as the Tensile and Compressive are of two exactly opposite nature of stress so we have to make arrangements to counteract this Tensile stresses, and I know you have guessed it right, Yes we will introduce Compressive Stress in the Tensile zone of the structure with atleast equal amount of that of the tensile stress, so this Compressive stress will counteract and omit the tensile stresses completely and in the whole section there will be no tensile stress and the whole section will become a compressive section. I know, you now know the conclusion don’t you? Yes now as the whole section becomes compressive therefore there is no risks of tensile failure, and about the compressive stress the concrete itself can handle it like captain America.

But Wait, How Can We Introduce Fully Compressive Sections? ==>

Simple, we can do this by Prestressing the section. So how to do so is what you want know now. For making prestressed concrete we have to use High Strength concrete made of OPC 53 grade, as the stresses are really high in section due to prestressing it is a must to use High Strength concrete of High Grade, otherwise the section will fail in high compressive stresses. Prestressing is done by means of inserting High Tensile Steel Tendons (similar to cable) in a bunch or in other form inside the section throughout its length about its periphery and exiting from the each end. This tendons are stressed within their Elastic Limit by pulling them at both ends of the structure under the action of machine., then from the both the ends of the structure at the exit points of the tendons arrangements are made by means of wedge so that the tendons are rigidly locked and if the machine releases the tendons, the tendons cannot slip away from the wedge and becomes fixed. As the tendons are pulled from both the ends, tensile stresses are developed, in the tendons, and after releasing the tendons  it tries to go back to its original normal position from its stretched condition, but due to the locking of the tendons at it both ends of the structure, the tendons cannot go back to its initial position, it tries squeeze itself but cannot, due to this squeezing or compression effect and as it cannot go back for being locked with the structure, it is always squeezing the structure along with itself, and hence the compressive stresses develops in that zone on which side Prestressing tendons are provided, and due to loading the amount of tensile stresses developed in that zone is counteracted by the compressive stress developed in the structure due to the prestressing. And thus the whole structure becomes compressive and no tensile stresses are remaining in the structure.

I hope this article helped you in understanding the Prestressed Concrete Structure concept. On the next article I will discuss about How the Prestressed concrete structures are manufactured and Advantages of the Prestressed concrete over RCC and link it to this post. For more Civil Engineering Info, Facts, Technological discussion keep visiting MyCivil – Civil Engineering Redefined. And do share this page in your circles on social network, make this blog large, as you know people like you are the only asset to this blog. Feel free to comment on this post, any comments will be highly appreciated.

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  1. Anonymous September 26, 2014
  2. Ranadip Chakraborty October 9, 2014

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