GPI February 2009 Post tension slabs - National Property ...

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GLOBAL PROPERTY INSPECTIONS™

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This technical bulletin has been drafted to be general in nature and not technically exhaustive.

TECHNICAL BULLETIN
February, 2009

TOPIC: Post-Tension Cable Slabs

For several years the building industry has been incorporating concrete technologies by
using post-tension cables in slab construction. Post-tensioning is a method of
strengthening concrete using high-strength steel strands or cables, typically referred to as
tendons. Slabs using the post-tension method can be built thinner, which can cut down on
construction costs and curing time. The post-tensioning method is the best practice for
building stronger, more reliable foundation slabs.

Concrete is very strong in compression, usually rated
up to 2,500 PSI, but it's relatively weak in tension.
This means that a concrete slab can be prone to
cracking due to deflection, or bending, when the earth
under the slab sinks or becomes unstable. Expansive
soil is a type of soil that expands and contracts based
on its moisture level, and it can damage the structural
integrity of homes.

Post-tension concrete construction creates a tight grid of steel cables that actively help
support the slab. Unlike conventional "inactive" rebar, which only helps keep the slab
intact after cracking, post-tension tendons continually contribute to the structural integrity
of the home. Plastic sheathing is used to cover the cables to protect them from corrosion.

Here's how a post-tension slab is built and installed:
• Lay plastic-sheathed steel cables in a grid
where the slab will be poured.
• Pour the concrete, making sure that the cables
remain at the right depth (these are usually
held in place with positioning tools called
"chairs").
• After the concrete has cured to 75% of its
designed strength, pull the cable taut using a hydraulic jack. This is the 'post-
tension' put on the cables. Cables should be pulled to 25,000 PSI.
• Anchor the ends of the cables and let the concrete cure completely.
In addition to being used in residential construction, post-tension concrete construction is
widely used in commercial applications today. The combination of structural integrity and
thinner pads allows for many practical applications. Office buildings can have thinner
floors but retain comfortable ceiling heights to reduce the total height and weight load.
More interesting and creative structures can be used in event stadiums and ball parks.

GLOBAL PROPERTY INSPECTIONS™

_______________________________________________________________________

This technical bulletin has been drafted to be general in nature and not technically exhaustive.


Even parking garages can be made stronger, more
functional, and less expensive (to construct).
By employing post-tension concrete construction methods,
home builders can deliver a superior product at reduced
costs.


Inspection tip: The ends of the cables should
never be exposed to prevent rust and moisture
from attacking the ends of the cables which can
lead to degradation of the cable and failure of the
slab system. If you find the ends exposed,
recommend further evaluation and repair by a
qualified post-tension cable slab foundation
contractor.