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The use of post-tensioned concrete floors in buildings has been consistently growing in recent years. Typical applications have included offices, car parks and apartments. The primary advantages of post-tensioned floors over conventional reinforced concrete in situ floors include increased clear spans, thinner slabs, reduced deflections and cracking, and significant reductions in the amount of conventional reinforcement. Secondary effects include lighter structures and reduced storey heights.
Post-tensioned floors can be constructed using either bonded or unbonded tendons. In bonded systems the strands are installed in galvanised steel or plastic ducts at the required profile. The shape of the duct depends on the number of steel strands required within it. Once the strands have been stressed the void around the strands is filled with a cementitious grout, which fully bonds the strands to the concrete. In an unbonded system the individual steel strands are encapsulated in a polyurethane sheath and the voids between the sheath and the strand are filled with a rust inhibiting grease.
As it is vital that the tendons are not damaged by subsequent fixings, their position is typically marked on the soffit with paint, see image.
Technical Report 43, Post-tensioned concrete floors – Design Handbook gives appropriate design methods and reviews aspects of the construction process.
UK CARES has published a Model specification for bonded and unbonded post-tensioned flat slabs. This model specification offers guidance to organisations that are producing specifications for bonded and unbonded post-tensioned flat slabs and slabs with beams not more than 1m deep containing monostrand or flat tendons. Where appropriate notes for designers are given, though these do not form part of the specification. Copies of the Model Specification may be obtained from UK CARES
Acknowledgement: The Concrete Society
Other references:Federation International du Beton (FIB), Bulletin 31, Post-tensioning in buildings, February 2005.
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