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Construction of composite slabs with steel decking

The most common application for composite slabs on steel decking is in steel framed buildings. Composite slabs are generally a quick and economical form of construction, but there are aspects of their design, construction and performance that need to be considered carefully if problems in construction are to be avoided. There had been a number of cases of decking bending and collapsing during concreting, and in a few of these cases operatives had fallen through the resultant opening. Guidance on aspects of the construction of composite slabs is given in Concrete Society Technical Report 75, Composite concrete slabs with steel decking.

It is important that designers consider the performance of the decking under construction loads as well as that of the completed composite slab. Concrete should preferably be placed by a pump. The loads from the pump line should be adequately distributed by spreader boards to avoid local damage to the decking and subsequent potential collapse. The number of operatives at the position of the discharge of the pump hose should be limited, due to concerns over concentrated loads and overloading the steel decking. If skips have to be used, care must be taken to avoid rapid discharge from height which could locally overload the steel decking, giving a risk of decking failure.

Steel decking is likely to be more flexible than conventional soffit formwork (unless adequately propped) and hence the deflections during casting are likely to be greater. Thus finishing the top surface of the concrete to a fixed level to datum (rather than to a fixed slab thickness) can lead to ‘ponding’ in the middle of spans, which can lead to overloading.

Steel decking is a poor insulator and composite slabs are often cast with little protection to the elements. In cold weather there is a risk that the concrete could freeze at the steel interface, thereby compromising the composite action required for structural capacity. Precautions to prevent the concrete at the steel interface falling below 5oC should be made until the concrete has achieved at least 5N/mm2, e.g. heaters in enclosures on the floor area under the slab being cast.

Acknowledgement: The Concrete Society

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TR75 Composite concrete slabs using steel decking