Introcduction of Post Tension Slab ?
Post-tension concrete is increasingly heard in the construction industry. This method enables the designer to take advantage of reinforcing concrete similar to prestressed concrete. While this method can be used to reinforce concrete in concrete structures by the cast-in-place method.
Concrete has a high strength property in compression. But is very weak in tension. Because it is brittle in tension properties, it can break suddenly after failure.
Therefore, reinforcement is provided in the tension zone of the slab to strengthen the concrete in tension and to prevent the sudden collapse of the concrete structure of the building.
But, nowadays tendons are provided in concrete slabs. So the strength of concrete is increased by using tendons with material like reinforcement. Tendons used in the post-tension slabs are groups of steel wire.
Post tension slab is a combination of conventional slab reinforcement and additional high-strength steel tendon. After the concrete is set, the slab steel tendon is stressed according to the design.
The use of post-tension concrete has become increasingly prevalent in the construction industry. It is a method of increasing strength with conventional slab reinforcement.
This type of slab method can reduce the thickness of the slab. The post-tension slab method can be used to increase the space of a building and floor space can be obtained in a long span.
We have given detailed information about Post Tension Slab in this article.
What Is Post Tension Slab?
Post tension slab is a method of providing strength and stability to the long-span slabs in building construction.
Post-tension slabs are usually the flat slab and the thinnest slab. Post tension slab is also known as pt slab.
The post-tensioning slab was first used by Eugene Frisinet in 1933 to establish a maritime terminal in France. The post-tensioning method is then widely used in bridges, elevated slabs (parking structures and residential or commercial buildings), residential foundations, walls.
PT slab gives the thinnest slab and long span. Steel tendon increases the strength of the concrete slab. The steel tendon in the PT slab is mostly kept in compression. Which usually resists the deflection of the slab.
PT slab gives a very effective structure in building construction. In addition, it reduces the consumption of materials. This type of slab is used in building construction to save money and time.
Require of Post Tensioning
Concrete has high compressive strength and steel has high tensile strength. Therefore, if a slab is prepared using this compound, the load-bearing capacity of the slab is increased. It also increases its efficiency.
Sometimes a heavy live load comes on the structure when heavy stress passes through its concrete slab. As a result, cracks and deformation occur in the concrete slab. To overcome this problem, post-tension steel tendons are placed in the slab before concreting.
When the post-tension slab is stressed, the concrete is squeezed. In other words, compacted concrete increases the compressive strength, and similarly, steel tendons increase the tensile strength of concrete. As a result, the strength of the concrete increases.
Components of Post-Tension Slabs:
The basic element used in post-tensioning systems is called tendons. Post-tensioning tendons are made up of more than one group of prestressing steel. Which has a protective coating for high strength. Which is placed inside a tube or casing in a concrete slab.
According to ASTM-416 TENDONS is used in the concrete slab. According to this, standard sizes 0.50 and 0.60 mm are kept. The normal steel tendon used for post-tensioning should have a minimum yield of 243,000 psi.
Claw coupling or welding overlap is supplied as required in thin sheet metal pipes in post-tension slabs of 5 and 6 m length respectively.
The tubes are connected to each other by an external screw coupling and the outer face is sealed with PE tape. Watertight, abrasion-free, and fatigue-resistant plastic tubes are also available in the market. Which can be used in the post-tension slabs.
Anchors are used to anchoring in the concrete. The main function of anchoring is to transfer the stress force through the tendons to the concrete after the stressful process is completed in the tendons.
Construction Process of Post Tension Slab
The installation of post-tensioning tendons in concrete requires skilled labor and personnel as required. Experienced in post-tension slab work.
The tendons are laid in concrete with conventional reinforcement. The position of the tendons is determined by the engineer according to the structure design. These tendons’ cable is held in plastic or steel tubes. So that the tendons are protected from contact with water in the concrete.
The post-tension slab is fixed at one end of the tendons using an anchor and the other end is kept open in a plastic pipe. Where tension is emphasized by the tension process. Couplers are used between two tendons if a construction joint is formed.
The alignment of the tendons is observed during concreting of the slab. So that their proper position does not change. Once 75% strength of the concrete slab is achieved, or after about 27 days, stress is applied using stressing jacks.
Tensioning is done with a strand with a force of 80% of the tensile strength above the tendons. For a 270 stard with a standard 3-inch grade, the strand is given a tensile strength of 33,000 pounds. The steel expands during the tensioning process. And the concrete is shrinking. When the proper tensioning force reaches the tendon, it is fixed by the anchor.
The tendons by anchor are designed to have a permanent mechanical connection in the concrete, to keep the concrete in compression and the steel in tension.
Extra protruding post-tension cable is cut in the concrete slab. Nonshrink type grouting is used in place of anchoring.
Application of Post Tension Slab
Post-tensioning or PT slabs have become an increasingly popular use over the last 30 years. Initially, there were problems with corrosion of the cables used in this method. But good materials and construction methods of tendons have removed most of the problems.
Post tension slabs are used in some common applications as follows:
- Bridge designers have used PT slabs for both precast segment and cast-in-place concrete in bridges. PT slab allows for a long span and reduces cracks.
- PT slabs are widely used for slabs on grades, especially where ground movements are likely to occur.
- PT slabs are used externally post-tensioning to reinforce recently developed structures. This method is especially used to resist seismic forces.
Advantages & Disadvantages of Post Tension Slab
Advantages of Post Tension Slab
1. Commercial Height
Due to the tendons cable of the pt slab, the thickness of the concrete slab can be reduced. Therefore, saving can be done from floor to floor height. Provision of the additional floor of a residential building can be made possible.
2. Reduces of Floor Dead Load
By reducing the thickness of the slab with the use of tendons, the use of concrete and reinforcement can be reduced by 20% -30% compared to conventional concrete slabs.
3. Architectural Advantage
The biggest advantage of a post-tensioned slab is that it can provide spaces without columns in large spans with thinner slabs. The post-tension slab creates a frugal base for floor design. It gives the architect the freedom to work freely for the design of the building.
4. Structural Durability
Post Tension slab has a low maintenance cost of cracking, durability, etc. In this type of slab, the amount of post-tensioning can be adjusted to balance the deflection of any part of the load applied immediately after the tension in the tendons.
The post-tensions slab provides ample space for architects, engineers, contractors, and users in commercial buildings. Therefore, the demand for the use of post-tension slabs is constantly increasing around the world.
Post tension slabs reduce the structure like beam and column. Which reduces the dead load in the building.
Disadvantage of Post Tension Slab
1. Skilled Labours
Post tension slabs require skilled workers. Usually, local workers may not have the required post-tension slab-related skills to prepare this type of slab.
2. Poor Workmanship
This type of slab requires special skills. If poor workmanship can reduce the life of the structure of the building. Adequate compaction of concrete is required in the post-tension slab.
3. Poor Compaction
A number of tendons used in post-tension slabs are prone to corrosion. Which largely depends on the quality of the material used for the tendons.