Differences of Pre-Tensioning vs Post-Tensioning | MOMENT

Pre-tensioning system vs Post-tensioning system

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With Pre-Tensioning, the tendons are tensioned against the abutments before the concrete is place. Once the concrete is hardened, the tension force is released and cut off after getting the required strength of the concrete to transfer the pre-stress on the concrete member. The tendon tries to shrink back to the initial length but the concrete resists it through the bond between them, thus, compression force is induced in concrete. Pre-Tension is usually done with precast. Wedge and barrel are the two components required to hold the tendons or pre-tensioning wires tightly when it is pulled out for inducing tensile force. The barrel will have tampered finish into which the wedge will get locked as the force increased.

Post-tensioning is a technique for reinforcing concrete. With Post-Tensioning, the tendons are tensioned after the concrete has hardened. Commonly, corrugated ducts are placed inside the concrete along the centroidal axis before casting. After the concrete has hardened and had enough strength, the tendon was placed inside the duct, stressed, and anchored against concrete. Space remains in the duct is filled with mortar by grouting. Post-Tensioning can be done either as precast or cast-in-place.

Post-tensioning vs Conventional

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Post-tensioned concrete is stronger and more flexible than conventional steel-reinforced concrete. Post-tensioning helps reduce cracking from shrinkage as the concrete dries, and holds together any cracks that do form.

Summary

Pre-TensioningPost-TensioningPre-tensioning is done in the factories thus suitable for precast construction works.Post-tensioning can be done in factories as well as on the site.In this type of concrete, the pre-stressing cables called the strands are tensioned before casting the concrete and then concrete is cast enclosing the tensioned cables.In this type of concrete, the strands are enclosed within a duct in the form and then concrete is cast. The process of tensioning the strands is carried out after the concrete attains its sufficient strength.Small sections are to be constructed.The size of a member is not restricted, long-span bridges are constructed by post-tensioning.Pre-tensioning members are produced in a mould.Cables are used in place of wires and jacks are used for stretching. The concrete is prestressed with tendons before it is placing in position.Prestressing is done after the concrete attains sufficient strength.Pre-tensioning is preferred when the structural element is small and easy to transport.Post-tensioning is preferred when the structural element is heavy.

There are two reasons why builders used post tensions slabs during the construction of a home or building.

A post tension slab is reinforced with stranded steel cables that are tensioned (tightened) after the concrete hardens. The cables are slid inside a plastic sheathing, like a wrapping. This prevents the cables from touching the concrete. When the concrete has sufficiently cured, the plastic sheathing is stretched. Stretching the post-tensioned cables applies significant force to the concrete system, lifting the slab into a compressed state, which reduces shrinkage cracks and cracks caused by difficult soil conditions.

Problems With Post Tensions Slabs

One of the biggest problems with a post tension slab is cracks. The cables laying in the concrete are generally not tightened until at least 7-10 days after the concrete is poured. Since the cables are not stretched or elongated, they cannot provide any crack control in those first few days.

Another reason for cracks is when a vehicle or another large object is placed upon the structure, and its concrete slab undergoes tension. To solve this problem, post tensioned steel tendons are placed when the concrete is poured and tensioned after with conventional reinforced bars.

A second issue with post tension slabs is that the footings anchor the foundation in place. The tension required to bind the additional concrete in the footings plus the dirt between the footings can&#;t be met.

Defects such as slab cracks can be costly to repair after construction is finished. Picture ripping up flooring or carpet to repair a small crack you may never notice. Trying to fix cracks can be time-consuming and expensive. Depending on the extent of damage, a family may have to leave their home while the work is being done. Talk about prices adding up.

Learn more about &#; Concrete slab cracks in Florida home.

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