Post-tensioning is a method of reinforcing concrete. High-strength steel tendons are positioned in ducts or sleeves before the concrete is placed.
1. Pretension is the technique in which we are imparting tension in strands before placing the concrete. Post tensioning is done by forming a duct in which strands are pulled (tensioned) after the concrete gains it's full strength.
Post-tensioning is a method of reinforcement employing high-strength steel strands or bars, which enable the existence of longer clear spans, thinner slabs and in many cases of structural systems that would otherwise be impossible due to either site constraints or architectural requirements.
Post-tensioned (PT) slabs are typically flat slabs, band beam and slabs or ribbed slabs. PT slabs offer the thinnest slab type, as concrete is worked to its strengths, mostly being kept in compression. Longer spans can be achieved due to prestress, which can also be used to counteract deflections.
Advantages of prestressed concrete:Prestressed concrete sections are thinner and lighter than RCC sections, since high strength concrete and steel are used prestressed concrete. In prestressed concrete, whole concrete area is effective in resisting loads, unlike RCC where concrete below the neutral axis is neglected.
A post tension slab is a concrete slab with steel cables running through it that have been placed under 33,000 +/- pounds of tension. This tension makes the concrete slab and foundation much stronger than concrete without reinforcement and helps reduce cracking.
Due to simplified structural layout of floor and reduced materials used, construction can be carried out faster with less material wastages. Steel laying is a very labour intensive activity, and by reducing the quantity of steel in PT design, the slab can be ready for concreting earlier.
Post tension (PT) tendon systems are primarily used for large slab construction projects. Since the early 1950's, post tension tendons have been used in the design of concrete structures. For such critical structural element, the repair should only be completed using a certified, experienced repair contractor.
Post-tensioned concrete slabs are typically expected to crack less than other slabs because post-tensioning produces a 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.
It will shoot out the plugs around the slab edge. If it hit someone it could break a leg. Although you might be able to locate the tendon location looking at the slab edge, the problem is that the tendons are not run in straight lines.
To create a slab using this method, plastic sleeve-covered steel cables are laid in a grid running in both directions. Then concrete is poured and allowed to harden to 75% of its design strength. The cables are then post-tensioned or pulled by a hydraulic jack and the concrete is allowed to cure completely.
The cable may still be live if only one strand has broken. The cable should be de-tensioned (which can be very dangerous) and removed. Cable failures can be as uneventful as a “pop,” or as catastrophic as “exploding concrete,” capable of severe damage and bodily injury.
