Pre-Stressing | Pre-tensioning | Post-tensioning

 

What is Pre-stressing?

 

Pre-stressing – Pre-stressed concrete is a different types of concreting for normal concreting. It is a high strength concrete with high tensile steel wires embedded and tensioned, Prior to the upcoming external loads, By this concrete can be Pre-compressed to such a degree that, after the structure is loaded there is practically the concrete no resultant tension develop in the concreting. It is generally applicable for –

                       

Ø  Where long span are encountered.

Ø  Where crack of concrete are not permitted.

Ø  Where fatigue loading is encountered Etc.

 

Advantages of pre-stressed concrete –

 

(1) Pre-stressed concrete members are lighter than RCC members for same capacity.

(2) No tension cracks develop in tension zone.

(3) It possesses more resistance to shearing force.

(4) It has better ability of energy absorption under impact load.

(5) It has high moment of resistance since higher grade of materials (steel & concrete) are used.

 

 

Disadvantages of pre-stressed concrete –

 

(1) It requires high tensile steel and high strength concrete, so it becomes uneconomical for traditional building construction.

(2) The method of pre-stressing requires skilled and efficient supervision.

(3) For pre-tensioning system, large abutments are required.

(4) In post tensioning system, the jacks used are heavy and expensive.

(5) Losses of pre-stress affects stress distribution on the section of the member.

 

Methods of pre-stressing?

 

There are two methods of pre-stressing.

 

(i) Pre-tensioning method

(ii) Post-tensioning method

  

Q. What is Pre-tensioning?

 

(i) Pre-tensioning method - In this method the steel wires or cables or tendons (The tendon is a stretched element used is a concrete members of structure to impart pre stress to the concrete. Generally high tensile steel wires, bars, cables or strands are used as tendons.) are first tensioned and anchored temporarily to the abutments. Then the fresh concrete is placed and when it has fully hardened and perfect bond is developed between steel wires and concrete, the anchors are released. The pre-stressing force is transferred to the concrete through bond and compressive stresses are developed in concrete.

 

Q. What is Post-tensioning?

 

(ii) Post-tensioning method - In this method first concrete is placed and members are casted leaving ducts for placing the tendons. After the concrete attains the desired strength then tendons are inserted in the ducts, tensioned and anchored to the ends of the members. In this way, after hardening the stresses are introduced in the concrete hence it is called post tensioning method.

Advantages and disadvantages of Pre-tensioning method.

 

Advantages:-

(i) It is best suited for mass production of precast members.

(ii) A single pair of abutment can be used for a line of members several metres long.

 

Disadvantages:- 

(i) It requires heavy abutments for anchoring steel wires.

(ii) Large size members are difficult to transport and handle.

 

Advantages and disadvantages of Post-tensioning method.

 

Advantages:-

(i) Stage pre-stressing can be done which is suitable for bridge construction.

(ii) The transportation and handling is avoided by casting the large size members at or near the site.

Disadvantages:- 

(i) The cables are anchored by mechanical means by which there is large concentration of stresses at ends which is difficult to analyse and requires special attention in design.

 

Compare between Pre-tensioning & Post-tensioning?

 

 

Pre-tensioning Method		Post-Tensioning Method
01.	It is done in the factories thus suitable for precast	It can be done in factories as well as on the site.
02.	Small sections are to be constructed.	Size of member is not restricted. So long span bridges are constructed by post-tensioning.
03.	Loss of pre stress is more (about 18%).	Loss of pre stress is less (about 15%).
04.	It is cheaper because the cost of sheathing is not involved.	It is costlier because of use of sheathing
05.	It is more reliable and durable	The durability depends upon the two anchorage mechanism