Post-Tension Slab

A post-tension slab consists of additional projecting high-strength steel tendons that are exposed to tension once the concrete has hardened, along with traditional slab reinforcement. The construction of a longer, much thinner slab without any column-free regions is made possible by this hybridization.

This article examines the benefits, construction, components, and operation of post-tension slabs.

Principles of Post-Tensioning

Concrete has a high compressive strength and steel has a high tensile strength, as is well known, and when these two materials are combined to support weights, efficiency rises dramatically.

A structure’s concrete slab experiences tension when a large live load is applied, which eventually causes fractures to form and deformation. Post-tensioned steel tendons are added during the concreting process and tensioned with traditional rebars afterwards to help reduce this issue.

The concrete is compressed or squeezed while these post-tensioned steel tendons are under stress, which raises the concrete’s compressive strength. At the same time, the tensioned steel tendons improve the tensile strength of the concrete. As a result, the concrete’s overall strength rises.

The post-tensioning slab’s Components

1. Ducts

Standard equipment is thin sheet metal pipes with welded overlapped seams or claw couplings, available in lengths of 5 and 6 meters, respectively. Ducts are sealed with PE tape and joined to one another using an exterior screw connector. These days, the industry also offers plastic ducts that are fatigue-resistant, frictionless, and waterproof.

2. Tendons

A tendon is the fundamental component of a post-tensioning system. One or more prestressing steel components are assembled into a post-tensioning tendon, which is then contained inside a duct or sheathing and protected with a covering.

The prestressing steel is produced in accordance with ASTM A-416 specifications, with typical diameter strand sizes being 0.50 and 0.60 inches. The normal yield of a steel strand used in post-tensioning is around 243,000 psi. A standard piece of rebar, on the other hand, will yield roughly 60,000 psi.

3. Anchors

When linking or terminating two tendons, anchors are utilized to secure the tendons into the concrete. After the stressing process is over, the primary purpose of anchorage is to transfer the stressing force to the concrete.

Construction of The Post-Tensioned Slab

1. Post tensioning tendons must be installed in concrete and stressed, which calls for professional labour and employees who are qualified to perform tensioning tasks.

2. The traditional rebars are placed down with the tendons in between. The engineer determines where the tendons should be laid. To prevent them from coming into contact with the water in the concrete, these tendons are enclosed in steel or plastic tubes.

3. Where the tendons are stressed, one end of the tendons is left open with a plastic pocket, while the other end is secured with an anchor. If there are any gaps created during construction, couplers are employed in between.

4. After the concrete is poured, care is required to ensure that these tendons remain in their original places.  Stressing jacks are used to put tension on these tendons once the concrete has reached its 75% strength, which should happen in 20 to 23 days.

5.Eighty percent of a strand’s tensile strength is applied force during the tensioning process. The tension applied to a standard ½-inch grade 270 strand is 33,000 pounds. The steel lengthens and the concrete is squeezed as the tensioning takes effect.

6. The Prestressing steel is fastened in place once the appropriate tensioning force is applied. The purpose of the anchors is to maintain a mechanical connection that is permanent, putting the concrete in compression and the steel in tension.

7. After trimming the excess tendons that protrude at one end, non-shrinking grout is placed in the anchor pocket.

Post Tension Slab – Advantages

1. Architectural Advantages

With its ability to create a particularly effective base for floor designs with thin slabs and columnless areas in bigger spans, Post-Tensioned Slab has an edge over other options. It gives an architect the flexibility to work on his designs without restriction.

2. Commercial Areas

Because post-tensioning produces thinner concrete slabs, the significant floor-to-floor height reductions become available as extra floors. This can offer more usable space while maintaining the building’s overall height.

3. Reducing the Deadload

Compared to typical concrete slabs, post-tensioned slabs require 20% to 30% less concrete and reinforcing due to their thinner nature.

4. Durability of Structure

Slabs that have been post-tensioned exhibit less cracking, increased durability, and less maintenance costs. By adjusting the post-tensioning amount to balance any part of the applied loads right after stressing, their deflection can be managed.

5. Popularity

Because post-tensioned slabs provide so many advantages for developers, architects, engineers, contractors, and end users, their demand is growing globally.