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An efficient form of construction for suspended floors is to use precast concrete units with in situ concrete to form a composite slab. Various forms of precast unit are available (see Precast/buildings/lattice girder slabs, Precast/buildings/beam and block floors). The precast units may contain some or all of the main reinforcement required for the completed slab.
For composite action, adequate continuity between the precast and the in situ concrete will be required. In lightly loaded situations (e.g. domestic buildings) this will be provided by the bond between the two concrete. Some preparation of the surface of the precast unit will generally be carried out.
In more heavily loaded situations some reinforcing steel will project from the precast concrete and be cast into the in situ concrete. As an alternative to concrete precast units, profiled steel decking may be used as permanent formwork with in situ concrete (often lightweight aggregate concrete) to form a composite floor slab.
This construction method is mainly used in steel frame, high rise buildings. The profiled steel provides all the tensile reinforcement for the completed slab. A nominal fabric reinforcement is provided to control shrinkage and thermal cracks in the in situ concrete, and to give some continuity over supports. In some systems that have been approved by the Steel Construction Institute, steel or macro synthetic fibres are used to replace the fabric. Further information may be found in Concrete Society Technical Report 75, Composite concrete slabs with steel decking.
Acknowledgement: The Concrete Society
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