Post Tensioning

Sarchia Khursheed Soran University –
Civil Engineering Dpt.
30 November2015
Post Tensioning System in Concrete Slab
Introduction
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, rockand soilanchors, and
water-tanks. In many cases, posttensioning allows construction thatwould
otherwisebe impossibledue to either site constraints or architectural
requirements.
Although post-tensioning systems requirespecialized knowledgeand expertise to
fabricate, assembleand install, the concept is easy to explain. Imaginea series of
wooden blocks with holes drilled through them, into which a rubber band is
threaded. If oneholds 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-typeanchoring devices.
Advantages
Post-tensioning, which is a formof prestressing, has severaladvantages over
standard reinforcing steel (rebars):
 Itreduces or eliminates shrinkagecracking-thereforeno joints, or fewer
joints, are needed
 Cracks that do formare held tightly together
 Itallows slabs and other structuralmembers to be thinner

Sarchia Khursheed Soran University –
Civil Engineering Dpt.
30 November2015
 Itallows us to build slabs on expansive or softsoils
 Itlets us design longer spans in elevated members, like floors or beams
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 moreslender, dramatic elements. Thinner slabs mean less
concrete is required. In addition, it means a lower overall building height for the
same floor-to-floor height. Posttensioning can thus allow a significant reduction in
building weight versus a conventional concrete building with the samenumber of
floors. This reduces the foundation load and can be a major advantagein seismic
areas. A lower building height can also translateto considerablesavings in
mechanical systems and façade costs. Another advantage of post-tensioning is
that beams and slabs can be continuous, i.e. a single beam can run continuously
fromone end of the building to the other. Structurally, this is much more efficient
than having a beam that justgoes fromone column to the next.
Post-tensioning is the systemof choice for parking structures sinceit 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 oneor
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 reduceproblems with cracking and differential settlement. Post-
tensioning allows bridges to be built to very demanding geometry requirements,
including complex curves, variablesuperelevation 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 slopestabilization
and as tie-backs for excavations. Post-tensioning can also be used to produce
virtually crack-freeconcrete for water-tanks.

Post Tensioning

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Post Tensioning