POST-TENSIONED CONCRETE

        

        POST-TENSIONED CONCRETE

Post-tensioned concrete looks & acts just like other reinforced concrete. Post-tensioning is 

simply a way to reinforce in a more active way.

What is Post-Tensioning ?

Post tensioning is a technique for reinforcing concrete. Post-tensioning tendons, which are prestressing steel cables inside plastic ducts or sleeves, are positioned in the forms before the concrete is placed. Afterwards, once the concrete has gained strength but before the service loads are applied, the cables are pulled tight, or tensioned, and anchored against the outer edges of the concrete.

Post-tensioning is a form of prestressing. Prestressing simply means that the steel is stressed (pulled or tensioned) before the concrete has to support the service loads. Most precast, prestressed concrete is actually pre-tensioned-the steel is pulled before the concrete is poured. Post-tensioned concrete means that the concrete is poured and then the tension is applied-but it is still stressed before the loads are applied so it is still prestressed.

ADVANTAGES & APPLICATIONS OF POST-TENSIONING

Post-tensioning, which is a form of prestressing, has several advantages over standard reinforcing steel (rebars):

·         It reduces or eliminates shrinkage cracking-therefore no joints, or fewer joints, are needed

·         Cracks that do form are held tightly together

·         It allows slabs and other structural members to be thinner

·         It allows us to build slabs on expansive or soft soils

·         It lets us design longer spans in elevated members, like floors or beams

Some of the more common applications are:

• ·         Slabs on ground: Today, PT is used extensively for slabs on grade where soils are likely to move (expansive soils)-especially in the American southwest. Jim Rogers, editor and publisher of Post Tension Magazine, says that until housing construction ground to a halt last year, about half of all post-tensioning work was slabs-on-ground for homes.
• ·         Another good application for PT slabs is producing crack-free tennis courts.
• ·         A recently developed application of PT is external post-tensioning for strengthening of existing structures, especially as an upgrade to resist seismic forces. 

•        Bridge designers have used PT both for cast-in-place concrete and for precast segmental construction. PT allows longer spans and keeps cracks tight.
• ·         Concrete water tanks are often post-tensioned to reduce crack width and leakage. The companies that make prestressed concrete tanks are Crom, DYK, Natgun, and Preload.
• ·         Masonry walls can be post-tensioned-this is usually done with a solid steel bar fastened to the foundation and stressed with a nut at the wall's top.
• ·         One interesting application is for a concrete countertop that needed to span 6 feet and carry a heavy load.