Pre-tension Concrete
Pretension in concrete is a method that involves the use of tendons to prestress the concrete before it is placed. It is a suitable method for small structural elements and the pre-tensioning members are produced in the mold.

To pretension concrete, the steel is first tensioned in a frame or between anchorages that are external to the member. The concrete is then cast around the steel. After the concrete has developed sufficient strength, the tension is slowly released from the frame or anchorage, transferring the stress to the concrete to which the tendons have become bonded. The force is transmitted to the concrete over a certain distance from each end of a member, which is known as the transfer length.

Post-tension Concrete

Post-tensioning is a method used in concrete structures where the prestressing process is carried out after the concrete has gained sufficient strength. This technique is suitable for heavy structures and it was developed for better load-bearing capacity.

To create post-tensioned concrete, ducts are cast into the concrete through which tendons can be threaded. Alternatively, tendons can be encased in a plastic sleeve or greased and then cast in concrete.

Once the concrete has gained enough strength, the tendons are tensioned using portable jacks. The load is transmitted to the concrete through permanent anchorages embedded in the concrete at the ends of the tendons.

Later, the ducts are usually filled with grease or grouted to protect the tendons against corrosion. In some cases, post-tensioning tendons run alongside the concrete member.

Pre-tension Concrete vs Post-tension Concrete