Post-tensioning systems

In the early fifties, post-tensioning systems for the prestressing of concrete were developed by specialists, such as VSL in Switzerland.

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Even though post-tensioning is a mature technology, it is still a fantastic tool for the design engineer as it enables the active definition of the internal load path in concrete structures by superposing a favorable state for the internal stresses. This enables deformations to be minimized, helps reduce the thickness of members, reduces reinforcement congestion, facilitates segmental construction without the need for wet joints and allows the use of high-strength steel.

In this blog, we will elaborate on the concept of Pre-stressed Concrete and Pre-tensioning-and-post-tensioning methods.

Concrete is known as one of the best materials to bear the compressive loading, however when it comes to tensile strength, unfortunately, it doesn’t stand anywhere. That’s why reinforcement in the form of steel wires are used in concrete beams, slabs and many other units to bear the tensile loading.

But what if we compress the concrete very heavily in advance so that when a tensile load is applied, the compressive load neutralise the tension and the unit sustain the load?

Isn’t an exciting concept? Yes, that concept is known as Prestressing in simple terms.

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What is Pre-stressed Concrete?

Pre-stressed Concrete or Prestressing of concrete is a unique concept, invented by French Engineer Eugène Freyssinet. Prestressing of concrete counterbalances the external tensile stress by advanced compression.

In a normal reinforced concrete beam, tensile loads are counterbalanced by the steel reinforcement. This is standard practice in building construction. In prestressed concrete systems, the beam is compressed from both end and when the vertical load of the structure is applied, it is neutralised by the compressive load. Pre-stressed Concrete structures are more resilient to shockwave and trembling than normal concrete.

San Franciscan engineer P.H Jackson had patented the Pre-stressed Concrete System in 1886, but it gained popularity in the 1930s due to the scarcity of steel and rapid rise in construction activity globally.

Pre-stressed Concrete is very advantageous in the case of concrete beams, railway sleeper, piles and many other bespoke concrete units that can be used as part of a structure.

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Also, prestressing of concrete assists engineers to play with longer span design because old-style reinforced concrete beams or units couldn’t be designed for a longer span just because of huge tensile load. Whereas prestressing of concrete allows them to go for longer span beams or units and significantly reduced the construction cost by minimizing the need for supporting columns.

Prestressing Concrete Systems are carried in two ways:

• Pre-tensioning Method
• Post-tensioning Method

Pre-tensioning Method

In the Pre-tensioning method, the steel wires are stretched out by hydraulics jacks and concrete are placed as per the required size (L x W x H). Once the concrete settles and obtains the shapes, the jacks are released, and metals plates are placed at both ends, then wires are fastened to the concrete & plates with the help of bolts.

The pre-tension method is very useful for off-site and factory production. As a result, mass production of pre-stressed concrete beams or units is developed in the factory and transported to the site. This method significantly reduces construction time and cost. It also helps to project planner to fix the construction sequencing and transportation of the unit’s way in advance and so mitigate the risk of construction delay.

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Post-tensioning Method

In the post-tensioning method, the concrete is cast first and plastic ducts or sleeves are placed during the process of concrete casting. The ducts or sleeves are placed exactly in the same location where wires are supposed to place.

Once the concrete is settled, plastic ducts or sleeves are removed, and steel wires are inserted from one end to the other. With the help of hydraulics, jacks wires are stretched out from both ends and then anchored with the help of steel plates and bolts.

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Comparison of Pre-tensioning and Post-tensioning

1. In Pre-tensioning, steel wires are tensioned before concrete is placed and hardened. In Post-tensioning, steel wires are stretched out once the concrete is settled and hardened.
2. In post-tensioning, the longitudinal profile of the steel wires can be placed at any desired profile. In Pre-tensioning, the profile of the steel wires mostly stays horizontal.
3. Pre-tensioning units are mostly developed in factories. Post-tensioning units can be placed in factories as well on-site, so provide more flexibility.

Hope, you would like the blog, please feel free to comment or raise any query.

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