PT slab is an abbreviation for post-tension slab, which refers to flat slabs, banned beams, or ripped slabs. They are formed through an amalgamation of conventional slab reinforcement as well as additional protruding high-strength steel tendons that are further subjected to pressure on a simultaneous basis, following the setting of the concrete.
The major technique that builds immense strength of the slabs is the concrete’s compression that takes place in the beginning, minimising the economic span range and the material usage compared to reinforced concrete. The PT slabs, containing the concrete that has been tested and subjected to compression, are the thinnest slab types and are also known to have a longer span with spaces free of columns.
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PT slab: How do PT slabs work?
PT slabs, due to being subjected to high compressive strength and having steel in them, exhibit huge strength and are used to carry loads. Generally, when a concrete slab undergoes tension, it is subjected to the formation of cracks or deformation when a heavy load is lifted on the structure. The PT slabs work to solve this very problem by being inserted while concreting as well as then being concreted with conventional rebars.
To put this method into simple words for understanding, you can picture concrete being squeezed, and then you can visualise post-tension steel tendons being stressed as a result of the same. Therefore when the concrete experiences heavy pressure, the compressive strength of the same is increased. The tensile strength of the steel tendons also increases after being pulled, therefore, maximising the overall strength of the said concrete.
PT slab: What does the PT slab consist of?
1) Ducts
The first part of a PT slab is the ducts that are made from thin sheet metal pipes that have welded overlapping or consist of screw coupling. They are supplied in specific measurements of 5 and 6 m, respectively. This is the standard measurement that is used for PT slabs. These ducts are then put together with each other through an external screw coupling and, further with the use of PE tape, are sealed. Talking about the present-day scenario, ducts made out of plastic have also been made available in the market for their water-tight characteristic, making them frictionless as well as fatigue resistant.
2) Tendons
The next part that goes into the making of PT slabs is the tendons. These are also referred to as the basic element that is required for the functioning and building of a post-tension system. To prepare these post-tensioning tendons, one or more than one piece of prestressing steel is coated with a protective coating and then placed within a sheathing or duct.
- Notable characteristics of the tendons include the prestressing steel, which is used during the manufacture, being made precisely according to the requirements of the ASTM A-416, in addition to the typical strand sizes, which are 0.50 as well as 0.60 inches as measured in diameter.
- The yield of a typical steel stand that is used for post-tensioning is estimated to be around 43000 psi. Compared to a typical piece of rebar, the difference can be calculated when the famous yield is taken into account, which happens to be approximately 60000 psi.
3) Anchors
To explain the part of the anchors, they work to fix the tendons to the concrete. While doing so, the job of the anchors is to terminate or join two tendons. The main function of anchors comes after the stressing process is finished when the anchors have to transfer the stressing force toward the concrete.
PT slab: How are post-tension slabs constructed or installed?
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Through the description, it is clear that PT slabs require precise handling and accurate measurements to function properly and ensure their job is fulfilled without any failures. The process and the important points that are required to be kept in mind while installing PT slabs are as follows:
- Higher-skilled labour needs to install post-tensioning tendons inside the concrete and stress, so there is no space for failure. Usually, only personnel who have certifications are eligible to do the tensioning work.
- An engineer is required to determine the positions in which the tendons are supposed to be laid. He/she is also supposed to ensure that the strands are protected by ducts made out of plastic or steel, so they do not come near or in contact with water while being placed in the concrete or after. While being laid, the tendons are accompanied by conventional rebars.
- Tendons put to present-day use are 7 high-strength steel wires that are wound together and then placed in a duct made of plastic. It is strictly advised that when the stressing takes place, it is only done by qualified workers to prevent the anchors from being mislocated or tendons from being of improper measurements. These workers also make sure that the anchors are placed in an area with grout to prevent corrosion.
- While one end of the anchor is left open with the help of a plastic pocket former in the area where the tendons are stressed, the other end of the tendons are anchored. In case of a formation of a construction joint, couplers are put into use.
- To make sure that the positions remain unaltered, the concrete is poured while also taking care of the alignment of these tendons. After around 20 to 23 days, the concrete is assumed to have achieved around 75% of its strength, following which these tendons are subjected to pressure through the machinery of stressing jacks.
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- This pressure or tensioning that is done is equal to a force of 80% of the strand’s tensile strength. This can be calculated as a force of 33000 psi being tensioned for a ½ inch grade 270 strand. When the tensioning comes into action, the concrete is then compressed, and the steel is elongated as a result.
- A post-tension concrete slab used for residential purposes is supposed to be of a typical thickness of 8 inches and uses 3000 psi concrete.
- The prestressing steel gets anchored to its place after a proper tensioning force is reached. Designed to give a permanent mechanical connection, these anchors keep the steel in tension as well as the concrete under pressure or compression.
- If any extra tendons are left out, they are trimmed on one end while, on the other hand, in the anchor pocket, non-shrink grouting is placed.
- The only aspect that one has to remember is not to make a cut or drill into post-tensioned concrete slabs, considering that a tendon is deemed very difficult to repair after the cutting of a tendon. For this very reason, many PT slabs have stamps on them so that owners or renovation contractors are aware of the post-tensioning of the slab.
PT slab: What are its advantages?
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Architecture
PT slabs play a major role in aiding an efficient base used for floor design with the help of thin slabs when column-less places are in question for larger spans. Post tension slabs provide a major advantage in comparison to others when floor design is concerned, as well as provide various options to the architect to work with all of his designs in mind, without hesitation.
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Commercial sectors
PT slabs provide thinner concrete slabs, therefore, resulting in savings on the floor-to-floor heights available, meaning the possibility of additional floors. Therefore this increases the space available for rent while also keeping the same height of the building overall.
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Dead load is reduced
In comparison to conventional concrete slabs that are used, PT slabs are thinner, therefore, making it clear that the quantity of concrete as well as reinforcements that go into the making see a reduction of up to 20 to 30%.
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The durability of structures
As PT slabs are made fresh, they show a reduction in cracking, an improvement in durability, as well as a dip in maintenance cost. To balance any part of the applied load after stressing, the deflection can be kept in control by using a varied amount of post-tensioning.
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PT slab: Major differences between normal and post-tension slabs
- One of the greatest differences that lie between commercial foundations and post-tension foundations is the amount of steel that is used in both. While in the case of residential construction slabs, tendons that are used for the centre are around 48 inches. On the other hand, commercial foundations contain a lot more steel than that.
- Compared to a flat slab, a PT slab requires more formwork. Even conventional slabs do not require the provision of column caps.
- A PT slab or post-tension slab contains a purse-stressed concrete structure, also known as PSC, while a normal slab contains a reinforced concrete structural element which is also known as RCC.
- In a PT slab, the reinforcement which is conducted to resist the compression is carried out with a replacement of cables or steel tendons, and therefore the slabs that have been tensioned or pressured after construction are known as post-tension slabs.
- Conventional slabs cannot work to overcome the weakness of the concrete that is present naturally, while post-tensioning removes all the natural weakness present in the concrete through tension, and they have a better advantage of strength in compression.
- Post-tension slabs provide a greater benefit when used in the construction of multi-storey buildings due to the greater support as well as, the more floor-to-floor ratio that is provided due to the thinnest of the slabs and the strength accumulated through the pressure.
- Unlike conventional slabs, post-tension slabs are compressed at the edge of the structure, which results in the strengthening of the concrete for resistance to tensile stresses. The placing of high tensile steel tendons in the structure is also unlike the conventional slabs and therefore has greater strength.
FAQs
Why are PT slabs used?
PT slabs are used to cover more floor ratio and better durability of structures that are constructed using them.
What does the abbreviation PT stand for?
PT stands for “Post Tension” slab.
How long can a PT slab last?
Post-tension slabs can last at least 15-20 years, owing to their durability and sturdiness.
