Pre-stress Concrete – Design of Concrete Structures

Pre-stress Concrete – All You Should Know

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Pre-stress concrete

After reading this post you will be able to explain what is pre-stress concrete various materials used for its construction and the different prestressing methods.

What is prestressing?

Pre-stress concrete
Pre-stress concrete

So before we start with the pre-stress concrete I shall explain the history behind this prestressing method, So as we all know in a reinforced concrete structure as shown in the figure they are well capable of taking all kinds of compressive stresses.

But it has so many drawbacks such as this very weak in tension and also it is brittle in nature so I will design this RC reinforced concrete structures two things are kept in mind.

The first one is the ultimate strength and the second one is deflection.

Before the failure of any structure, one thing is always common, that is none other than the development of cracks there will be numerous cracks developed on the surface of the reinforced concrete structures.

And the steel reinforcement kept inside the concrete structures will get corroded when the cracks are exposed to the humid atmospheric conditions.

Pre-stress concrete
Pre-stress concrete

So this steel reinforcement no longer takes up any kind of tensile stresses and as a result of this, that leads to the reduction of strength.

Gradually the collapse of the structure can be expected, So in order to avoid cracks in and in order to increase strength and to reduce deflections actually we are going for this pre-stressed concrete method or PSC.

The basic concept of pre-stress concrete

When we apply the vertical Lord on a concrete member what actually happens is a deflection will be observed especially at the mid-span of the structure.

That means the concrete will experience a sort of compression at the top and tension at the bottom.

As you all know the concrete can’t take up so many tensile stresses at this portion and that will result in the development of cracks, that’s why we are providing the steel reinforcement or tendons at the bottom of the reinforced concrete structures.

So what happens in pre-stressed concrete is these tendons will be get stressed by making use of tensioning devices and these high tensile bars have the capacity to prevent cracking on the concrete.

This is what happening in the pre-stressed concrete structures, that is in prestressed concrete actually what is happening is tendons are stretched along the ends and the concrete is poured.

After the concrete achieved the required strength, we will release the tendons from its support conditions.

Pre-stress concrete
Pre-stress concrete

So, this makes a transfer of pre-stress from the tendons to the concrete that is near to the RC member.

So we will get a prestressed deflection as shown in this figure in the upward direction.

So a lot of compressive stresses will be obtained in this bottom portion and this will act in this direction.

So what is actually happening is the stresses that are formed due to the service load rather we will call it as Live Load will be contracted with this compressive stress, that is formed from the prestressing of tendons

Different materials used in pre-stress concrete

High tensile steel

  • Wires: Individually drawn wires having 7mm dia.
  • Strand: A collection of wires (usually 7) spun together.
  • Tendon: A collection of strands wound together and encased in ducts.
  • Bars: High tensile steel bars of dia more than 20mm

High-strength concrete

  • IS code recommending a minimum mix of m40 for pre-tensioned and M30 for the post-tensioned system.
  • High compressive strength
  • The greater value of modulus of elasticity and fewer deflections
  • Result in small sections
  • Offers resistance in tension, shear, and bearing.

Advantages of pre-stress concrete

  • Pre-stress concrete members are thinner and lighter in C.S
  • Less self-weight and overall economical.
  • Structures with longer unsupported span can be constructed.
  • Shows less deflection compared to RC structures.
  • Concrete doesn’t crack in PCS – Steel rusting avoid.

Prestressed concrete is mainly used in bridges, High rise buildings, railway sleepers, and storage tanks.

Different prestressing methods

  • Pre-Tensioning Method
  • Post-Tensioning Method

Pre-Tensioning Method

pre-tensioning method
pre-tensioning method

In the pre-tensioning method, the important feature is that pre-stress is induced before the concrete is placed.

In fig, you can see a prestressing bed and it’s 2 ends you have the end abutments where the tendons are supported and you have a tensioning device that’s called Jack.

And the first thing you are doing is you will stretch these tendons apart and are supported on the end abutment, then we will apply all kinds of tensioning by using Jack like devices.

After that the concrete is poured, after the curing time, we have to release or cut the tendons so that is the transfer of prestressing occurs in between tendons as well as the concrete

So this is happening in a pre-tensioning method that is first we have to apply tension to the tendons then casting of the concrete and at last the transferring of pre-stress.

Features of Pre-tensioning.
  • Per-tensioning is mostly done in factories
  • It is more reliable and durable.
  • Used for smaller sections-heavier and longer.
Applications of Pre-tensioning
  • Foundation pile
  • Railway sleepers
  • Lighting poles
  • Floor slab
  • Beam
  • Pipe
  • Partition wall

Post-Tensioning Method

post-tensioning method
post-tensioning method

In this method, pre-stress is induced only after the concrete has hardened, So in this figure, you can see the green color tube, So this represents the tubes through which the tendons are placed later.

So first the concrete is poured and is set to harden once it had achieved sufficient strength the tendons are stressed using tensioning devices as it is shown in the figure.

After the application of prestressing force, the tendons are cut and released slowly, So this will result in the transfer of pre-stress from the tendons to the nearby concrete.

So this is actually happening in the post-tensioning method, then the void spaces between the tendons and the duct are closed by making use of cement grout.

Features of Post-Tensioning
  • Post-tensioning can be done in factories as well as in site too
  • The Loss of pre-stress is very less compared to pre-tensioned concrete structures.
  • Sleek and slender concrete members are constructed-less self-weight consumption of material.
  • Widely used in heavily loaded – large-span structures.
  • Crack free and fire-resistant.
  • The compressive strength of concrete and tensile strength of the steel is used to its fullest.
Application of Post-tensioned concrete
  • Roads
  • Bridges
  • Railways
  • Tunnels
  • Dams
  • Containment Tanks
  • Underground construction
  • Special structures.

Limitations

  • PSC constructions require very good quality control and supervision
  • Cost of the materials used in PSC is very high
  • Tensioning equipment and devices are very expensive.
  • Needs skilled laborers and supervision.
  • It is un-economical for structures with short span and light loads

 

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