Working Principle, Components and Construction

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Post Tension slab is a combination of conventional slab reinforcement and additional protruding high-strength steel tendons, which are consequently subjected to tension after the concrete has set. This hybridisation helps achieve the formation of a much thinner slab with a longer span devoid of any column-free spaces.

In this article we study about the working principle, components, construction and advantages of post tension slab.

Working Principle of Post Tensioning 

We all know that concrete has a high compressive strength and steel has a high tensile strength, and when their combination is used to bear loads, the efficiency increases manifold.

Fig 1: Typical Details of Post Tension Slab

When a heavy live load is brought upon a structure, its concrete slab undergoes tension, which leads to the formation of cracks and ultimately deformation occurs. To mitigate this problem, post tensioned steel tendons are inserted at the time of concreting and tensioned after concreting with conventional rebars.

When these post tensioned steel tendons are stressed, the concrete is squeezed, in other terms, the concrete is compacted which increases the compressive strength of the concrete and at the same time the steel tendons that are pulled increase the tensile strength. As a result, the overall strength of the concrete increases.

Components of Post Tensioning Slab 

1. Ducts 

Thin sheet metal pipes with claw coupling or welded overlapped seam supplied in lengths of 5 and 6 m respectively are used as a standard. Ducts are connected to each other by an external screw coupling and sealed with PE tape. Plastic ducts are also available in the market these days which are water tight , frictionless and fatigue resistant

Fig 2: Type of Ducts used to encase steel tendons.

2. Tendons

The basic element of a post-tensioning system is called a tendon. A post-tensioning tendon is made up of one or more pieces of prestressing steel, coated with a protective coating, and housed inside a duct or sheathing. 

Fig 3: Steel tendons used in Post Tensioning of Slab.

 The prestressing steel is manufactured as per the requirements of ASTM A-416 and typical strand sizes are 0.50 and 0.60 inch in diameter. A typical steel strand used for post-tensioning will yield about 243,000 psi. In contrast, a typical piece of rebar will yield about 60,000 psi.

3. Anchors

Anchors are used to anchor the tendons into the concrete while terminating or joining two tendons. Main function of anchorage is to transfer the stressing force to the concrete once the stressing process is completed.

Fig 4: Slab Anchor.

Construction of Post Tensioned Slab

1. The installation of post tensioning tendons in the concrete and stressing it requires skilled labour and a personnel who are certified in doing the tensioning works.
2. The tendons are laid down along with the conventional rebars. The position of laying of the tendons is decided by the engineer. These tendons are encased in plastic or steel ducts so that they do not come in contact with the water in concrete.
3. One end of the tendons are anchored with the help of anchor and the other end is left open with plastic pocket former, where the tendons are stressed. Couplers are used in between if  any construction joint is formed.
4. Concrete is poured and the alignment of these tendons are taken care of so as to let their positions unaltered.  Once after the concrete has achieved its 75% of strength , that is around 20 - 23 days, these tendons are stressed with the help of  stressing jacks.
5. The tensioning is done to a force equal to 80% of a strand's tensile strength. For a typical ½-inch grade 270 strand, the strand is tensioned to a force of 33,000 pounds. As the tensioning comes into effect, the steel gets elongated, and the concrete is compressed.
6. When the proper tensioning force is reached, the prestressing steel is anchored in place. The anchors are designed to provide a permanent mechanical connection, keeping the steel in tension, and the concrete in compression.
7. The extra tendons that are left out at one end are trimmed and non shrink grouting is put in the anchor pocket.

Advantages of Post Tension Slab

1. Architectural Benefits

Post-Tensioned Slab has an advantage over others as it makes a very efficient base for floor design with thin slabs and columnless spaces in larger spans. It provides an architect the freedom to work freely with his designs.

2. Commercial Spaces

Post-tensioning results in thinner concrete slabs making the valuable savings in floor to floor height available as additional floors.This can provide extra rentable space within the same overall building height.

3. Reduces Deadload

As the post-tensioned slabs have lesser thickness, the quantity of concrete and reinforcement used is reduced upto 20% - 30% when compared to conventional concrete slabs.

4. Structural Durability

Post-Tensioned slabs show reduced cracking, improved durability and lower maintenance costs. Their deflection can be controlled by varying the amount of post-tensioning to balance any portion of applied loads immediately after stressing.

5. Popularity

The demand for Post-Tensioned slabs, throughout the world, continues to increase because of the significant benefits for developers, architects, engineers, contractors and end users.

Read More : Pre-Tensioning and Post-Tensioning in Prestressed Concrete Design