Question

Which one of the following statements is the disadvantages of post-tensioning method?
1. The loss of pre-stress is less as compared to pre-tensioning system.
2. Post-tensioning method is costly as compared to pre-tensioning method.
3. Post-tensioning can be done in factories and at the site also.
4. Post-tensioning method is used for large spans and heavily loaded structures.

Answer 1

Correct Answer - Option 2 : Post-tensioning method is costly as compared to pre-tensioning method.

Explanation:

• Prestressing system: two type
  • Pre tensioning
  • Post-tensioning

​Post-tensioning:

• In the case of post-tensioning, the strands are enclosed within a duct and then concrete is cast. The process of tensioning the strands is carried out after the concrete attains its sufficient strength so in this method we can say first casting and then tensioning.
• Post-tensioning can be done in factories as well as on-site.
• In the case of post-tensioning, the size of the member is not restricted, long-span bridges can be constructed with the help of this method.
• The post-tensioning method is costly as compared to the pre-tensioning method.

Losses of prestressing:

	Pre-tensioning		Post-Tensioning
1.	Electric shortening loss	1.	No elastic shortening loss in cables if pulled simultaneously, however, losses will be there if cables are pulled are by one.
2.	Creep loss	2.	Friction loss
3.	Shrinkage loss	3.	Anchor slip loss
4.	Relaxation loss	4.	Creep loss
		5.	Shrinkage & Relaxation

Pre tensioning:

• In the case of pre-tensioning, the prestressing cables also called as strands are tensioned before casting the concrete. Latter concrete is cast enclosing the tensioned cables, so in this method, we can say first tensioning and then casting.
• Mostly pre-tensioning is done in factories hence it is suitable for precast construction works.
• Pre tensioning system is used in the manufacturing of railways sleepers.
• In the case of pre-tensioning, the size of the sections is limited due to transportation restrictions and also due to the availability of cranes capable of lifting the sections into place.
• pre-tensioning is more reliable and durable as compared to post-tensioning.