Prestressed Concrete - Definition | Method | Advantages | Disadvantages


What is Prestressed Concrete?

Concrete can withstand a great amount of compressive stress but it has a very low tensile strength. Because of low tensile strength, concrete gets cracks when subjected to maximum load. 

As the name indicates prestressed concrete is a form of concrete in which is internal stresses are introduced before its application so that it can counteract the tensile stresses produced in concrete due to external load.

Prestressed Concrete - Definition | Method | Advantages | Disadvantages

When we design a concrete structure, we design it for ultimate-strength it means in the worst-case scenario the structure should not collapse.

Another, important design criteria is there should not be any moment or deflection when a load is applied to the structure.

Why prestressed concrete is used?

Before the failure of any concrete structure, cracks are formed in the concrete and then the structure collapses and cracks are formed due to deflection or moment in structure. When water comes in contact with these cracks the steel gets corroded. 

To avoid these cracks, to increase the strength of member and to reduce the deflection prestressing is done.

Prestressing means tensioning the reinforcement.

Prestressed Concrete - Definition | Method | Advantages | Disadvantages

for example, just take the case of a bridge over a river if we try to construct this bridge with traditional concrete then we will have to increase the depth of span so there will be no enough space under the bridge for ships to pass. So, to have a lower depth, longer span and more strength prestressing is done.

Prestressing

To understand prestressing see the image below when we apply load on the concrete member the member bends like this
Prestressed Concrete - Definition | Method | Advantages | Disadvantages

You can see compression develops on the top portion and tension develops at the bottom portion and due to this elongation at bottom concrete gets cracks. 

That is why we add some steel bars at the bottom section and stretch it so that it can resist most of the tension and save concrete from cracking. This is called as prestressing and this type of concrete is known as prestressed concrete.

In prestressing the tendons are stretched along the axis and concrete is poured. 

After that when the tendons are released the compression is generated at the bottom which tries to counterbalance the compression due to loading at the top part of the beam. 

The upward forces along the length of the beam counteract the service loads applied to the member. 

Prestressed Concrete - Definition | Method | Advantages | Disadvantages

Prestressing removes design limitations of conventional concrete and permits the building roofs floors, bridges and walls with longer unsupported spans. Because of this ability pre-stressed concrete is used in school auditoriums shopping-centres, gymnasiums, parking garages and cafeterias.

It is now commonly used for floor beams, pile and railway sleepers as well as structures such as water tanks and runways.

Method of Prestressing 
  • Pre-tensioning 
  • Post-tensioning
Pre-tensioning 

In the pre-tensioning process, the steel is stretched before the concrete is placed. High tensile steel (ultimate strength of 2100 N/mm2) wires or tendons are used between two ends and stretched to 70 to 80% of their ultimate strength. 

After that, the concrete is poured around the tendons and allowed to cure. Once the concrete gains desired strength, the stretching forces are released. 


Prestressed Concrete - Definition | Method | Advantages | Disadvantages

When highly stressed steel attempts to contract, the concrete gets compressed then the concrete will be in a permanent state of maintaining pre-stressed strength. 

Generally the pre-tensioning concrete elements are precast in a factory and must be transported to the construction site.

In place of tendons wire strands or cables, high tensile steel bars can also be used for pre-tensioning. examples of pre-tensioning concrete precast products are foundation pile railway, sleepers, electrical or lighting Pols, floor, slab beam, pipe partition wall etc.


Post-tensioning

In post-tensioning the steel is stretched after the concrete hardens unlike pre-tensioning work post-tensioning is usually carried out at the project site. In the case of post-tensioning, a duct is placed into the concrete structure. 


Prestressed Concrete - Definition | Method | Advantages | Disadvantages

Concrete is cast and allow to cure. When the concrete reaches its required strength the tendons are stretched and locked with anchors. The excess ends of tendons are then cut away and the duct will be grouted and covered with concrete for rust prevention. 

Examples - Roads, bridges, railways, tunnels, dams, foundations, buildings industrial facilities,  containment tanks, reservoirs, underground constructions, Airports and Seaports, special structures or any form of prestressed concrete structures etc.

Concrete used for Prestressed work


Concrete used for prestressed work should have a cube strength of 35 N/mm2 for post-tensioned system and 45 N/mm2 for the pre-tension system.

Advantages of Prestressed Concrete

With the help of pre-stressed concrete, sleek and slender concrete structures can be constructed. due to these dead load of the structure gets reduced. 
  • Consumption of materials like concrete, steel is reduced. 
  • Longer beams spans and girders can be constructed which gives the untroubled floor space and parking facilities. 
  • It has long-term durability. 
  • Possibility of steel corrosion and subsequent concrete deterioration are declined because of concrete is crack free. 
  • Pre-stressed concrete bridges are not easily damaged by fire they have excellent fire resistance and low maintenance costs in comparison to reinforced concrete. 
  • Pre-stressed concrete offers greater load resistance and shock resistance. 
  • The compressive strength of concrete and tensile strength of steel is used to their fullest.

Disadvantages of Prestressed Concrete

  • Pre-stressed concrete requires high-quality dense concrete of high-strength. 
  • High strength concrete in production, placement and compaction is required. 
  • It requires high tensile steel which is 2.5 to 3.5times costlier than mild steel. 
  • Prestressing process requires complicated tensioning equipment and anchoring devices which are very costly. 
  • Pre-stressed concrete construction requires very good quality control and supervisions.
  • Pre-stressed concrete needs skilled labourers. 
  • Prestressing is uneconomical for shorts spans and light loads.

Why high-strength Concrete is required for prestressing?

  • If the concrete is not strong enough it can be cracked or failed when it is stretched by tendons moreover high strength concrete is less subjected to shrinkage cracks. It has a higher modulus of elasticity and smaller creep strength.
  • Since large prestressing forces are applied to the member by the tendons, high bearing stresses are developed at the end by anchoring devices.
  • Bursting stresses liable to at the ends of the beam can not be satisfactory resisted by low strength concrete.
  • When the stress transfer to concrete has to take place by bond action, the concrete should have high bond stress which can be offered by high strength concrete.

Why high-strength Steel is required for prestressing?


The mild steel used in ordinary reinforced concrete has a yield point of 200 N/mm2 to 300 N/mm2. If such steel is used and if even it is subjected to stress say 200 N/mm2 at the stage of tensioning, we find that due to creep and shrinkage of concrete the net tensile stress left over will be extremely low. 
                                                            
                                                              In the design of prestressed concrete member, the estimated loss of prestress due to shrinkage of concrete and creep of concrete and steel is at the order of nearly 200 N/mm2. But high tension steel has an ultimate strength of 2100 N/mm2 and if initially, to say 1000 N/mm2 there will still be large stress in the reinforcement after making a deduction for the loss of prestress.


So, friends, I hope I have covered all the information about prestressed concrete in this article. If you learn something please be sure to share it with someone who might benefit from it. If you want to add any information which has missed in this article you can mention it in the comment section. 

Finally, Thanks for reading this article.

Also, Read - 

  1. Nominal Mix Concrete Vs Design Mix Concrete
  2. What are the Properties of Fresh Concrete?
  3. Water-Cement Ratio and its Significance
  4. What Is Workability of Concrete

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