Post Tensioning Concept

PT – Post tensioning is a method of reinforcing (strengthening) concrete with high-strength steel strands or cables, typically referred to as tendons. Post-tensioned concrete is a variant of prestressed concrete where the tendons are tensioned after the surrounding concrete structure has been cast. The essence of prestressed concrete is that once the initial compression has been applied, the resulting material has the characteristics of high-strength concrete when subject to any subsequent compression forces, and of ductile high-strength steel when subject to tension forces. This can result in improved structural capacity and/or serviceability compared to conventionally reinforced concrete in many situations.

Types of PT Systems -

Bonded & Unbonded

There are basically two types of Post Tensioning Systems; namely, Bonded Post Tensioning and Unbonded Post Tensioning. Conceptualizing the framing for PT layout & specifying the right system is a vital stage in the entire process & it requires sound engineering consideration in order to maximize the benefits of optimized structure for all the stakeholders in a project.
img

Bonded PT System

This is a multi-strand system where one or more tendons are inserted into a metal or plastic duct that is embedded in the concrete. By filling the duct with cementitious grout, the tendons are 'bonded' with the surrounding concrete. This system is more suitable for bridges, tanks and silos, as well as heavily loaded beams in buildings.

Unbonded PT System

This is a mono-strand system wherein the prestressing steel is coated with special grease and so, not actually bonded to the concrete that surrounds it, except at the anchorages. This system is more suitable in slabs and slabs-on-ground for buildings and parking structures, as well as structures where inspection and replacement of the tendons is required.

img

Advantages of PT

  • Post Tensioning allows longer clear spans, thinner slabs, fewer beams & more slender elements.
  • Post Tensioning can thus allow a significant reduction in building weight as compared to a conventional concrete structure with the same number of floors, reducing the foundation load and can be a major advantage in seismic areas.
  • Thinner slabs enable to achieve a lower overall building height for the same clear floor-to-floor height. A lower building height translates to considerable savings in mechanical systems & facade costs.
  • Strength & Durability of structure is enhanced due to use of high strength concrete & also gives better control on deflection & cracks.
  • Post Tensioned elements for long spans are more economical than structural steel & RCC elements.
  • The conventional reinforcement in the concrete member is reduced providing easy overall laying work & reduction in reinforcement leads to easy pouring of concrete without congestion.
  • Longer spans with reduced depths are possible in post tensioning much to the delight of Architects.
  • Speed of construction is considerably increased due to less shuttering & de-shuttering period.