Found this on Google.OVERVIEWPost-tensioning is a method of...

Found this on Google.

OVERVIEW
Post-tensioning is a method of reinforcing (strengthening)
concrete or other materials with high-strength steel strands or bars, typically referred to as tendons. Post-tensioning applications include office and apartment buildings, parking structures, slabs-on-ground, bridges, sports stadiums, rock and soil anchors, and water-tanks. In many cases, posttensioning allows construction that would otherwise be impossible due to either site constraints or architectural requirements.
Although post-tensioning systems require specialized
knowledge and expertise to fabricate, assemble and install,
the concept is easy to explain. Imagine a series of wooden
blocks with holes drilled through them, into which a rubber
band is threaded. If one holds the ends of the rubber band,
the blocks will sag. Post-tensioning can be demonstrated by
placing wing nuts on either end of the rubber band and
winding the rubber band so that the blocks are pushed
tightly together. If one holds the wing nuts after winding,
the blocks will remain straight. The tightened rubber band is comparable to a post-tensioning tendon that has been
stretched by hydraulic jacks and is held in place by
wedge-type anchoring devices.

BENEFITS
To fully appreciate the benefits of post-tensioning, it is
helpful to know a little bit about concrete. Concrete is
very strong in compression but weak in tension, i.e. it will crack when forces act to pull it apart. In conventional concrete construction, if a load such as the cars in a
parking garage is applied to a slab or beam, the beam will
tend to deflect or sag. This deflection will cause the
bottom of the beam to elongate slightly. Even a slight
elongation is usually enough to cause cracking. Steel
reinforcing bars (?rebar?) are typically embedded in the
concrete as tensile reinforcement to limit the crack widths.
Rebar is what is called ?passive? reinforcement however;
it does not carry any force until the concrete has already
deflected enough to crack. Post-tensioning tendons, on the
other hand, are considered ?active? reinforcing. Because it is
prestressed, the steel is effective as reinforcement even
though the concrete may not be cracked. Post-tensioned
structures can be designed to have minimal deflection and
cracking, even under full load.

ADVANTAGES/APPLICATIONS
There are post-tensioning applications in almost all facets of construction. In building construction, post-tensioning allows longer clear spans, thinner slabs, fewer beams and more slender, dramatic elements. Thinner slabs mean lessconcrete is required. In addition, it means a lower overall building height for the same floor-to-floor height. Posttensioningcan thus allow a significant reduction in building weight versus a conventional concrete building with the same number of floors. This reduces the foundation load and can
be a major advantage in seismic areas. A lower building
height can also translate to considerable savings in
mechanical systems and fa?ade costs. Another advantage of
post-tensioning is that beams and slabs can be continuous,
i.e. a single beam can run continuously from one end of the
building to the other. Structurally, this is much more efficient than having a beam that just goes from one column to the next.
Post-tensioning is the system of choice for parking structures since it allows a high degree of flexibility in the column layout, span lengths and ramp configurations. Post-tensioned parking garages can be either stand-alone structures or one or more floors in an office or residential building. In areas where there are expansive clays or soils with low bearing capacity, post-tensioned slabs-on-ground and mat foundations reduce problems with cracking and differential settlement.
Post-tensioning allows bridges to be built to very
demanding geometry requirements, including complex
curves, variable superelevation and significant grade changes.
Post-tensioning also allows extremely long span bridges to be constructed without the use of temporary intermediate
supports. This minimizes the impact on the environment
and avoids disruption to water or road traffic below. In
stadiums, post-tensioning allows long clear spans and very
creative architecture. Post-tensioned rock and soil anchors
are used in tunneling and slope stabilization and as tie-backs for excavations. Post-tensioning can also be used to produce virtually crack-free concrete for water-tanks.