Post Tensioning

A brief overview of post-tensioning has been presented here, followed by its advantages and economics.

BRIEF

Post-tensioning is the technique of pre-loading the concrete in a manner that eliminates, or reduces, the tensile stresses that are induced by the dead and live loads; High-strength steel wires called strands are arranged to pass through the concrete floor. When the concrete has hardened, each set of strands is gripped by means of a hydraulic jack and stretched to a pre-determined force. At this stage the strands are locked in a purpose-made device called an anchorage, which has been cast in the concrete; this induces compressive stress in the concrete. The strand is thereafter held permanently by the anchorage. The non-jacking end of the strand may be bonded in concrete or may be fitted with a similar device on the other end as the case may be. In order to allow the strand to stretch in the hardened concrete under the load applied by the jack, the bond between the concrete and strand is prevented by a tube through which the strands pass. This tube is termed duct or sheathing and is made of galvanized iron. After stressing the duct is filled with grout material. Grout filling of ducts transfers the load to the entire concrete element and ensures corrosion protection.

ADVANTAGES

Post Tensioning offers various advantages, some of which have been listed below:-

  • Larger Clear Spans
  • Thinner Slabs (Spans of 7mm to 8mm = 210mm thick slab)
  • Lighter Structures
  • Reduced Cracking
  • Reduced Deflection
  • Rapid Construction
  • Better Water Tightness
  • Less Manpower
  • Reduced Passive Reinforcement

ECONOMICS

Conventional reinforced concrete slabs are best suited for spans up to 7m to 8m and also where the spans are uniform and formwork is simple so that farmworkers can work quickly. This is common in simple residential blocks up to three stories. Further, in the driveways often wider spans are required to provide parking access. More columns and thicker slabs are required for conventionally reinforced slabs, and this is where the labor costs come in. As in post-tensioned slabs, the slabs will be thinner, with larger spans and thereby reduced columns and formworks. The works can be carried out faster, and also the slab can be de shuttered quicker than conventional slabs.

GROWTH TREND/DEMAND FOR POST TENSIONING

With the above-mentioned advantages and economics becoming prominent, post-tensioning in buildings is becoming a trend in the region and world over. The economics are being harnessed in each and every possible way by the client and the contractors, by selecting post-tensioning for their projects. Thus, in all, there is a good scope of growth for post-tensioning ahead.

Post Tensioning