Post Tension Slab
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.
Post-tension slab 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.
What Is Post Tension Slab?
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 a Post Tensioning Slab?
Post tensioning slab is a method of providing strength and stability to the long-span slabs in building construction. Post-tensioning 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-tensioning steel tendons are placed in the slab before concreting.
When the post-tensioning 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.
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Components of Post-Tension Slabs:
1. Tendons
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.
2. Ducts
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.
3. Anchors
Anchors are used to anchor 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
- Commercial Height: Due to the tendon cable of the pt slab, the thickness of the concrete slab can be reduced. Therefore, savings can be done from floor to floor height. Provision of the additional floor of a residential building can be made possible.
- Reduces 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.
- 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 on the design of the building.
- 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.
- Popularity: 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.
Also Read :Â What Is Monolithic Slab? | What Is Floating Slab? | Difference Between Floating Slab and Monolithic Slab
Disadvantage of Post Tension Slab
- 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.
- 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.
- 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.
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Frequently Asked Questions (FAQ)
Post-Tension Slab
Post tension slab is a combination of conventional slab reinforcement and additional high-strength steel tendon.
Post Tension Foundation
Post-tension foundation is slab-on-grade foundations. Concrete is reinforced with threaded steel cables that are tensioned after curing.
Post Tension Cables
Post-tensioning cables are used in construction. It allows structures and features such as fewer beams, longer clear spans, thinner elements, and thinner slabs. It helps in reducing the quantity of concrete required for construction.
Post Tensioned Concrete Slab
Post Tensioned Concrete Slab is a method of providing strength and stability to long-span slabs in building construction. Post-tensioning slabs are usually the flat slab and the thinnest slab. Post tension slab is also known as pt slab.
Post Tension Cable Repair Cost
Post Tension Cable Repair Costs can typically range from $800 to $1,200. And that doesn’t include the cost of other concrete repairs. In post tension slab systems, cables are spaced approximately 30 to 36 inches apart.
How Long Does Post-Tension Slab Last?
Conventional concrete & rebar foundations last about 20-30 years before failure begins to occur. Post-tension slabs are lasting about 15-20 years and are failing at a much higher rate. Post-tension slabs are created with concrete trenches around the perimeter of the house where the external walls will be placed.
Post Tension Slab Foundation
A post-tensioned foundation is a concrete slab foundation reinforced with stranded steel cables. which goes into tension after the concrete hardens. The stranded cable is slid inside the plastic sheathing, which prevents the stranded steel tendons from touching the concrete.
Does Post-Tension Slab Crack?
Post-tensioned concrete slabs are typically expected to crack less than other slabs because post-tensioning produces compressive stress that must be exceeded before enough tensile stress develops to cause cracking. For a 100,000 ft2 post-tensioned slab, this would result in an estimated 900 lineal ft of cracks.
Can I Drill into a Post-Tension Slab?
PT slabs on the ground can be placed and stamped just like with any other concrete slab. The only concern is to always remember not to cut or drill into post-tensioned concrete slabs since once a tendon has been cut, it is very difficult to repair.
What Are the Principles of Post-Tensioning?
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.
What Are the Methods of Post-Tensioning?
Some of the popular methods of post-tensioning in prestressed concrete are:
- Freyssinet system.
- Magnet system.
- Leonhardt system.
- Lee-McCall system.
- Gifford-Udall system.
Why Post-Tensioning Is Done?
Designers use post-tensioning as a way to reinforce concrete by prestressing it. In prestressed members, compressive stresses are introduced into the concrete to reduce tensile stresses resulting from applied loads including the self-weight of the member (dead load).
How Much Does a Post-Tension Slab Cost?
The cost of the post-tensioned slab is $30.52 per square foot which is approximately the average cost of the four-floor systems. At the interior supports, a substantial amount of reinforcement was required for ultimate strength.
Where Is Post-Tension Slab Used?
Post-tensioning now is used extensively in bridges, elevated slabs (parking structures and residential or commercial buildings), residential foundations, walls, and columns. Jim Rogers Bending and flexing creates high tensile forces that can cause the concrete floor slab to crack.
What Are the Disadvantages of Post-Tensioning?
Here are a few disadvantages of post-tension slabs:
- Since there are a number of tendons and wires spread inside the post-tension slab, it can result in corrosion.
- The complexity of work: The post-tension slab can be made only by skillful professionals.
How Thick Are Post Tension Slabs?
A post-tension slab foundation can range anywhere from 4 to 12 inches thick.
What Is Post Tension Slab Construction?
Post-tension concrete slabs are used to create a monolithic (single pour) slab that is stronger than a traditional slab without reinforcement. This is achieved by laying out high-strength steel cables in a crisscrossed grid pattern throughout the foundation prior to pouring the concrete.