what is post tensioning & pre-stressed?

Answer 1

Post tensioning - cables are cast into the concrete, then when the concrete reaches the required strength the cables are pulled or "stressed" to place the concrete into compression.

Prestressed - normally in prefabricated concrete panels. The cables are placed in a forming jig and stressed, then the concrete is poured over the top. Once the jig is removed the load in the cables is transferred into the concrete.

The cables are accurately positioned within the concrete so that the compressive stress, from the cables, is in the zone and acting against the tension forces within the concrete when the concrete is placed under load.

Answer 2

In prestressed concrete, the cables are positioned below the centerline of the concrete because concrete is very strong in compression and weak in tension. So the lower half is post tenioned and the entire slab is prestressed.

These cables are pre tightened to several tons of force each and then the raw concrete is poured and cured around them.

The bending is actually due to the fact that the cables, as mentioned above, are BELOW the centerline of the slab and in so twist it upward archwise, or Post Tensioned. Upon installation, the cables then take the tensile load in the bottom 1/2 below the centerline (concrete being weak in tension) supporting the concrete. The compression load is above the centerline (concrete being strong in compression) and the prestress preloads the concrete to a greater compressive stress so it will take a higher compressive load.

So as it works out the beneficial compressive strength of concrete is used accordingly to a benefit whie post teniniioning the lower half which is in tension and weak as concrete is weak in tension.

Answer 3

Post tensioning is pulling on cables that are cast into a concrete structure during pouring. Prestressed concrete is cast with the cables in place. One indication of prestressed is that the pieces (which you may see on trailers on the road) are curved (arched up in the center), becoming level when the load is applied.