Posttension Concrete, What is posttension concrete, Why posttension concrete
Posttension concrete speaks to what is posttension concrete, whats is posttension concrete and explain posttension concrete.
It is almost always done in place on the building site. The high strength steel strands (tendons) are covered with steel or plastic tube to prevent them from bonding to the concrete and are not tensioned until the concrete has cured. Each tendon is anchored to a steel plate embedded in one end of the beam or slab.
A hydraulic jack is inserted between the other end of the tendon and a similar steel plate in the other end of the member. The jack applies a large tensile force to the tendon while compressing the concrete with an equal but opposite force that is applied through the plate. The stretched tendon is anchored to the plate at the second end of the member before the jack is removed. For very long members, the tendons are jacked from both ends to be sure that frictional lossees in the tubes do not prevent uniform tensioning.
The net effect of posttensioning is identical to that of pretensioning. The difference is that in posttensioning, abutments are not needed because the member itself provides the abutting force to tension the steel. When the posttensioning process is complete, the tendons may be left unbounded, or if in a steel tubes they may be bonded by injecting grout to fill the space between the tendons and the tube.
This method is commonly used to create monolithic slabs for house construction in locations where expansive soils (such as adobe clay) create problems for the typical perimeter foundation. All stresses from seasonal expansion and contraction of the underlying soil are taken into the entire tensioned slab, which supports the building without significant flexure.
Post-tensioning is also used in the construction of various bridges, both after concrete is cured after support by falsework and by the assembly of prefabricated sections, as in the segmental bridge.
Among the advantages of this system over unbonded post-tensioning are:
Large reduction in traditional reinforcement requirements as tendons cannot destress in accidents.
Tendons can be easily "woven" allowing a more efficient design approach.
Higher ultimate strength due to bond generated between the strand and concrete.
No long term issues with maintaining the integrity of the anchor/dead end.
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