Post Tension Slab

A Post Tension Slab generally refers to a section of cured concrete, which has tensile forces introduced into the slab after the concrete has set.

This is usually through the use of high-tensile wires sheathed in a plastic or steel duct to allow the stresses to distribute evenly through the slab.

The strand is profiled typically in parabolic shapes to provide upward and downward forces to account for the loads applied to the slab, it includes thinner slab depths, greater spans and quicker prop stripping time.

Pre-tensioned slabs are typically used for pre-cast elements where the strand or bar is stressed before the concrete being poured over it (no ducts used).

Post tension slab is used when spans are large and normal RCC slabs become unformed.

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It has a lower thickness than RCC slabs because their deflection can be controlled by post-tensioning; the slab has less reinforcement due to post-tension.

Due to the reduced thickness of the slab, its weight is also reduced which in turn controls the design of the columns and mass of the entire structure is reduced, it results in reducing base shear due to seismic loads.

Post tension slab definition:

The post tension slab is a combination of traditional slab reinforcement and additional high-strength steel tendons, which are subject to stress after the concrete is set.

Working of post-tension slabs:

Steel has very high tensile strength and Concrete has high compressive strength.

When they are together used to bear the load, the strength will increase.

Whenever a heavy live load is applied on a structure, the concrete slab experiences stress which causes cracks and deformation.

To overcome this problem, post-tensioned steel tendons are used during concreting.

In other words, when the concrete is compacted, the compressive strength of the concrete will increase in addition to the steel will likely be drawn and the tendon strength will increase.

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As an outcome, the final strength of concrete will increase.

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Components of post-tension slabs:

1. Nozzles:

Thin sheet metal pipes with the claw coupling or welded overlapped seams of 5 and 6 meters length are offered.

The nozzles are attached by an outer screw coupling and wrapped with the PE tape.

In recent days, plastic tubes are also available in the market, they are watertight, frictionless and fatigue resistant.

Nozzles
Type of Ducts used to encase steel tendons

2. Tendons:

The fundamental element of a post-tensioning system known as the tendon.

Many steel objects are constructed of post-tensioning tendons, which are coated with a protective coating and are located inside the duct or sheathing.

Prestressed metal is manufactured according to the necessities of ASTM A-416 and the everyday strands are 0.50 and 0.60 inches in diameter.

A characteristic steel strand used for post-tensioning can be around 243,000 psi.

In distinction, a particular piece of rebar will yield approximately 60,000 psi.

Tendons
Steel tendons used in Post Tensioning of Slab

3. Anchoring to post-tension slab:

Anchors are used to anchor the tendons with the concrete.

The main function of anchoring is to transfer the stressing force into the concrete after the tensioning process is completed.

Anchoring to post-tension slab
 Slab Anchor

Advantages of post tension slab:

1. Vastu Benefits:

Post-tensed slabs have an advantage over others because of varieties a real environment-friendly base for flooring design with thin slabs and column fewer spaces in larger slabs.

This provides an architect with the freedom to work independently with their designs.

2. Commercial Location:

Subsequent stress results in a slab of thin concrete due to the valuable financial savings available from flooring top to extra flooring.

This might present an additional rentable house inside a similar general building height.

3. Reduces dead load:

Since the thickness in the post-tension slabs is reduced, the amount of concrete and reinforcement used has been reduced by 20% –30% compared to conventional concrete slabs.

4. Structural Durability:

Post-stress slab reduction improved durability and decreased maintenance prices.

Their deflection can be managed immediately after stressing different amounts to balance any part of the applied load.

5. Popularity:

The demand for stressed slabs has increased worldwide because it has countless benefits for builders, civil engineers, architects, contractors and end-users.

Disadvantages of Post tension slab:

  1. Pre-tensioned members are fabricated off-site.
  2.  Delay in delivery or production will cause a delay in the schedule but the installation is easier but also uses an enormous time in the crane.
  3. Post-tensioned member, however, is installed on-site; once the structure is concreted the stressing is no more critical in the schedule.
  4. The good thing about pre-tensioned members is that it eliminates the loss of strength due to friction of tendons against the tendon duct.
  5. Post-tensioned members are subject to friction loss.

Uses of Post Tension Slab:

Post-tension Slab is now extensively used in bridges, elevated slabs, residential foundations, walls, and columns.

Also, read 1. Stair Stringer 2. Moment Frame 3. Moment Resisting Frame

Frequently Asked Questions:

Why use post tension slab?

Post-tensioned concrete is stronger and more resilient than conventional steel-reinforced concrete.

Can you drill into a post tension slab?

The post tension slab on the ground can be placed and stamped like any other concrete slab.

What is the benefit of a post tension slab?

The post stress tensionin is done to keep the concrete structure under compression in areas where the load causes tensile stress.

Can post tension slabs crack?

Post stress concrete slabs are generally expected to have less cracks than other slabs.

Conclusion:

Post tension slab is a slab where the reinforcement goes through ducts and is made of steel wires (not bars).

The wires have to be made taut by pulling them after the concrete has hardened and before the scaffolding is taken away.

This means that the slab bend upwards, so all the concrete works in compression through the slab. It’s usually called post-stressed concrete, once the mix has hardened, you release the wires.

The effect is the same as the slab is bent upwards by the tension and there are no sections of the slab where the concrete is “pulled”.

The slab with wires in the conduit ready to be stressed, this is done after the concrete has hardened, so it is the post-stressed slab.

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