Post-Tensioning

What is Post Tensioning?

Post-Tensioning Methods

Essentially, Post-Tensioning is a method of reinforcing concrete, masonry, and other structural elements. Post-Tensioning is a method of Pre-Stressing. Pre-Stressed concrete or masonry is made up of internal stresses (forces) induced into it during the construction phase with the purpose of counteracting the anticipated external loads that it will encounter during its lifecycle.

There are two methods of Pre-Stressing
Pre-Tensioning & Post-Tensioning

Pre-Tensioning

This method consists of stressing and reinforcing the inside of large steel buttresses, and then casting the concrete around the reinforcement. This method can only be done at a precast manufacturing facility. The completed Pre-Stressed concrete members will then be trucked out to the job site and assembled.

Post-Tensioning

Instead of stressing the reinforcing inside of large steel buttresses at a manufacturing plant, the high tensile cables are simply installed on the job site after the contractor forms up the slabs or constructs the walls and columns. The cables are housed in sheathing or duct that prevents the steel from bonding to the concrete so that it can be stressed after the concrete cures (hardens). Using the Post-Tensioning method of Pre-Stressing enables a builder to capitalise on all of the advantages of Pre-Stressed concrete or masonry while still having the flexibility to construct the member on the job site.


Why does concrete and masonry need to be reinforced?

Resist compressive forces

Concrete, masonry, and most cement based products are very strong in compression as they have a high capacity to resist compressive forces. Compressive forces can be described as crushing forces. Concrete has a very high compressive strength that can be anywhere from 17.24 N/mm2 and in most residential foundations to 27.58 N/mm2 in suspended slabs and walls in buildings, to even higher strengths in bridges. However, concrete is relatively weak in tension as it does not resist tensile forces very well. Tensile forces are the forces that pull an element apart and cause cracking in concrete.

Crack Proof

Conversely, steel is very strong in tension. It has a high capacity for resisting the forces that pull apart or bend it. Combining specially produced high tensile woven wire cables as the reinforcement with concrete or masonry, results in a product that can resist both compressive forces and tensile forces. In addition, substantial benefits can be achieved by using tendons to "squeeze the concrete together" or place it in compression. Compressing the concrete increases the bending strength of the member without which cracking will occur. By increasing the tensile strength of the member (making the concrete slab or masonry wall stiffer), a designer can achieve longer spans with thinner concrete sections.

Less Shrinkage

Putting the concrete into compression also helps to resist the development of shrinkage cracks. Shrinkage cracks, while typically not detrimental to the flexural performance of the structure, can be unsightly and can allow the passage of moisture or termites. Generally, shrinkage cracks will develop in most cement based products as the water combines with the cement and the concrete cures (hardens). The more the concrete is "squeezed together", the less likely it is that shrinkage cracks will develop or open.


COMPRESSION

TENSION

Bends

Tensile forces pull apart the bottom of this concrete slab when it bends

What kind of materials are used in Post-Tensioning ?

Post-Tensioned reinforcement consist of very high strength steel strands. Typically, strands are used in horizontal applications like foundations, slabs, beams, and bridges; and bars are used in vertical applications like walls and columns. A typical steel strand used for Post-Tensioning has a tensile strength of 1861 N/mm2. In comparison, a typical non-prestressed piece of reinforcing (rebar) has a tensile strength of 413 N/mm2. Strands commonly have a diameter of 12.7mmØ or 15.2mmØ and are stressed to a force of 228 N/mm2 using a hydraulic jack.

The Pre-Stressing steel is housed in sheathing or duct to allow it move as the tensioning force is applied after the concrete cures. The steel stretches as it is tensioned, and it is locked into place using an anchoring component that forms a mechanical connection and keeps the force in the strand for the life of the structure. After the tendons have been stressed and engineering approval given, they are cut off and permanently sealed. A free flowing cementitious grout is then pumped into the ducts to lock the cables in position permanently. This is classified as bonded Post-Tensioning.

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