Bonded Post Tensioned Concrete
Bonded post-tensioned concrete is the descriptive term for a method of applying compression after pouring concrete and the curing process. The concrete is cast around a plastic, steel or aluminium curved duct, to follow the area where otherwise tension would occur in the concrete element. A set of tendons is fished through the duct and the concrete is poured. Once the concrete has hardened, the tendons are tensioned by hydraulic jacks that react against the concrete member itself. When the tendons have stretched sufficiently, according to the design specifications , they are wedged in position and maintain tension after the jacks are removed, transferring pressure to the concrete. The duct is then grouted to protect the tendons from corrosion. This method is commonly used to create monolithic slabs for structures being constructed in locations where expansive soils 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 a variety of structures including high rise residential and commercial complexes, shopping centres, mixed use developments, hospitals, universities, single and multi-storey car parks, airports, government buildings and various bridges.
More advantages of this system (over un-bonded post-tensioning) are :
- Large reduction in traditional reinforcement requirements as tendons cannot de-stress 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.