Difference between pre-tensioning and post-tensioning?

Pre-tensioning and post-tensioning are two methods used in prestressed concrete construction to introduce internal stresses into the concrete before or after it has hardened. While both methods achieve the same goal of enhancing the performance of concrete structures, they differ in their execution and application. Here are the main differences between pre-tensioning and post-tensioning:

Timing of Tensioning:
- Pre-tensioning: In pre-tensioning, the high-strength steel tendons or cables are tensioned before the concrete is poured. The tendons are usually anchored at both ends of the concrete formwork, and then the concrete is poured around them. Once the concrete has cured to the required strength, the tendons are released from their anchorages, transferring their tension to the concrete.
- Post-tensioning: In post-tensioning, the concrete is first poured without any tensioned tendons. After the concrete has hardened to a certain degree, ducts or tubes are installed within the concrete forms, and the high-strength steel tendons or cables are inserted through these ducts. Hydraulic jacks are then used to tension the tendons, applying forces that compress the concrete. Once the desired level of prestress is achieved, the tendons are anchored at both ends.
Location of Tendons:
- Pre-tensioning: The tendons in pre-tensioning are usually placed within the concrete forms before the concrete is poured. They are typically positioned along the length of the concrete member and are anchored at both ends of the formwork.
- Post-tensioning: In post-tensioning, the tendons are placed within ducts or tubes after the concrete has been poured and hardened. The tendons are positioned at specific locations within the concrete member, such as along the neutral axis of a beam or within the slab thickness.
Ease of Construction:
- Pre-tensioning: Pre-tensioning requires specialized equipment and facilities for tensioning the tendons before the concrete is poured. It typically involves prefabricating the prestressed concrete elements in a controlled environment, such as a casting yard or precast plant.
- Post-tensioning: Post-tensioning offers more flexibility in construction since the tendons can be installed after the concrete has been poured. This method is often used in cast-in-place concrete construction, where the tendons are installed on-site before tensioning.
Applications:
- Pre-tensioning: Pre-tensioning is commonly used for precast concrete elements such as beams, slabs, and railroad ties, where the concrete elements are fabricated off-site and transported to the construction site.
- Post-tensioning: Post-tensioning is frequently employed in cast-in-place concrete construction for structures such as bridges, parking structures, and buildings, where the concrete is poured on-site.

In summary, while both pre-tensioning and post-tensioning achieve prestressing in concrete structures, they differ in the timing of tendon tensioning, location of tendons, ease of construction, and applications. The choice between pre-tensioning and post-tensioning depends on factors such as project requirements, construction methods, and structural design considerations.