Friday, January 11, 2013

Sizing of the Slab.

In general floor slabs are designed by eye from experience and are made up of a sub-base layer of hardcore blinded with either sand or concrete and sealed with a slip membrane upon which the slab is cast. However, an alternative approach is to consider the make-up and performance requirements in more detail.

Floor slabs supported directly on the ground are subject to bending and shear forces resulting from differential movements in the ground support during loading. In addition they are subjected to thermal and moisture movements which can produce the critical stresses particularly in slabs on uniform support.

Typical reinforcement proportions of 0.1–0.25% will not significantly affect the crack width and distribution. Limiting crack width to say 0.3 mm would require significant amounts of reinforcement of the order of 0.4%. Current thinking is to saw the slab into panels approximately  6 m square, the thermal contraction and drying shrinkage cracks being induced by the saw cuts. The use of square mesh ‘A’-type fabrics is now more common than ‘B’-type fabrics and the ‘small panel’ approach is considered to result in lower risk of cracking than the use of heavy fabric and more widely spaced joints.

The design process therefore should be to calculate the rein- forcement required for ground support, and then decide on sawn or formed joint locations to minimise the thermal and shrinkage stresses in the panels. The analysis for ground support can be assessed by the adoption of a design based upon spanning or cantilevering over a depression similar to that adopted for crust rafts. Due to the relatively small loads applied to slabs, the likely settlement depressions tend to be of small diameter when compared with a similar crust raft condition (see Table 11.3).

Table 11.3 Ground floor slabs – typical assumed depressions
Ground floor slabs – typical assumed depressions

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