Post-tensioning tendons, which are prestressing steel cables inside plastic ducts or sleeves, are positioned in the forms before the concrete is placed. Afterwards, once the concrete has gained strength but before the service loads are applied, the cables are pulled tight, or tensioned, and anchored against the outer edges of the concrete. 

Post-tensioning is a form of prestressing. Prestressing simply means that the steel is stressed (pulled or tensioned) before the concrete has to support the service loads. 

Most precast, prestressed concrete is actually pre-tensioned : the steel is pulled before the concrete is poured. 

Post-tensioned concrete means that the concrete is poured and then the tension is applied, but it is still stressed before the loads are applied so it is still prestressed. 

A Post Tension solution can bring many advantages to the construction process and is one of the greenest forms of construction giving 25% savings in concrete and 75% savings in reinforcement for a similar RC construction. The knock on effects for labour materials and programme can be easily quantified.


Design Principles 

The design principles of post tensioning are very simple and the analogy that we use is to place a number of books on a table with the spine facing towards the ceiling and try to lift them all together. 

The easiest way to lift all the books together is by applying pressure at each end as shown by the arrows and lifting them as one whole unit. 


Advantages of Post Tensioning

Improved constructability Less Material to be delivered, handled and placed on site improving the BREEAM credits Earlier stripping of formwork allowing improved programme  Simpler formwork More Architectural freedom Larger column free spaces Flexibility in subdivision of office floors. Wide spanning or boldly cantilevering floors that leave generous space for lobbies and public areas. Slender elegant floors, roofs The same characteristics for demolition as reinforced concrete structures Flexibility for post forming holes for services Reduced construction periods Reduced Slab thickness by up to 25% on Piled or Poor ground conditions Lower storey heights giving savings on fa├žade, and vertical services. Reduced loads on foundations.


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