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In this article, we will discuss post tension slab.
Post tension slab is a variety of conventional slab reinforcement and additional stick-out high-strength steel tendons, which are therefore subjected to tension after the concrete has been placed. This hybridization helps accomplish the formation of a much thinner slab with a longer span devoid of any column-free spaces.
1. Working Principle
We all understand that concrete possesses high compressive strength and steel possesses high tensile strength, and when their variety is employed to bear loads, the efficiency improves manifold.
When a heavy live load is carried upon a structure, its concrete slab undergoes tension, which leads to the formation of cracks, and ultimately deformation happens. To reduce this issue, 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 pressed, in other terms, the concrete is compacted which improves the compressive strength of the concrete, and at the same time the steel tendons that are pulled boost the tensile strength. As a result, the whole strength of the concrete boosts.
2. Components of Post Tension Slab
Thin sheet metal pipes with claw coupling or welded overlapped seams provided in lengths of 5 and 6 m respectively are employed as a standard. Ducts are related to each other by an external screw coupling and sealed with PE tape. Plastic ducts are also unrestricted in the market these days and are water-tight, frictionless, and fatigue resistant
The basic part of a post-tensioning system is called a tendon. A post-tensioning tendon is constructed of one or more pieces of prestressing steel, coated with a protective coating, and housed inner a duct or sheathing.
The prestressing steel is manufactured as per the requirements of ASTM A-416 and typical strand sizes are 0.50 and 0.60 inches in diameter. A standard steel strand utilized for post-tensioning will produce about 243,000 psi. In difference, a specific piece of rebar will produce about 60,000 psi.
Anchors are utilized to anchor the tendons into the concrete while completing or joining two tendons. The main use of anchorage is to share the stressing force with the concrete once the stressing process is finished.
3. Construction of Post-Tensioned Slab
The construction of the Post-tensioned slab is as follows:
1. The installation of post-tensioning tendons in the concrete and stressing it needs skilled labor and personnel who are licensed in accomplishing the tensioning works.
2. The tendons are spread down along with the conventional rebars. The place of laying of the tendons is determined by the engineer. These tendons are covered in plastic or steel ducts so that they do not reach into contact with the water in concrete.
3. One end of the tendons is anchored with the use of an anchor and the other end is left open with a plastic pocket former, where the tendons are stressed. Couplers are employed in between if any construction joint is constructed.
4. Concrete is poured and the alignment of these tendons is taken care of so as to let their positions unaltered. Once the concrete has achieved 75% of strength, which is around 20 – 23 days, these tendons are stressed with the use of stressing jacks.
5. The tensioning is done to equal 80% of a strand’s tensile strength. For a specific ½-inch grade 270 strand, the strand is tensioned to a strength of 33,000 pounds. As the tensioning comes into effect, the steel gets extended, and the concrete is compressed.
6. When the appropriate tensioning force is advanced, the prestressing steel is anchored in position. The anchors are prepared to supply a permanent mechanical connection, maintaining the steel in tension, and the concrete in compression.
7. The additional tendons that are left out at one end are shortened and non-shrink grouting is placed in the anchor pocket.
4. Advantages of Post Tension Slab
The advantages of post tension slab are as follows:
1. Architectural Benefits
Post-Tensioned Slab possesses a benefit over others as it makes a very good base for floor design with thin slabs and columnless areas in more extensive spans. It feeds an architect the space to work willingly with his designs.
2. Commercial Spaces
Post-tensioning outcomes in thinner concrete slabs make valuable savings in floor-to-floor height available as extra floors. This can provide extra rentable space within the identical overall building height.
3. Reduces Dead load
As the post-tensioned slabs have a lesser thickness, the amount of concrete and reinforcement employed is decreased by up to 20% – 30% when compared to conventional concrete slabs.
4. Structural Durability
Post-Tensioned slabs offer decreased cracking, enhanced durability, and lower maintenance costs. Their deflection can be influenced by changing the quantity of post-tensioning to balance any portion of applied loads instantly after stressing.
The demand for Post-Tensioned slabs, whole the world, continues to advance because of the important advantages for developers, architects, engineers, contractors, and end users.
Hope you got an idea for Post Tension Slab.
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