Design Decisions - Floating Slabs.

The design decisions relate to

(1) The loading anticipated on the slab.
(2) The ground conditions below the slab.
(3) The need to maintain specific levels and finishes for a normal design life within appropriate tolerances.
(4) The required durability.
(5) The control of shrinkage and other movements without excessive cracking.

The floating slab is chosen when the sub-strata or a hardcore layer over the sub-strata is suitable to allow a simple slab to adequately disperse the loads without excessive distortions or cracking. Where such conditions do not exist then a suspended slab may need to be adopted.

A floating slab can be of plain concrete or reinforced concrete depending on the quality of the sub-strata and the loading condition. Generally they are reinforced and while it can be argued that under their loading conditions positive and negative bending moments will be produced, it is common to only reinforce with one layer of reinforcement, usually using a mesh fabric. If one layer of reinforcement  is used it can be located in the bottom, top or middle of  the slab, depending on the designer’s requirements. However, generally a top mesh is usually considered the most suitable.

Cracking of concrete slabs is almost inevitable in some form either as a result of shrinkage or bending tensile stress.

Control over such cracking is usually more important on the top surface of the ground floor slab rather than on the underside and by providing the mesh in the top of the slab and accepting some cracking on the soffit the designer can economically control the condition for most ground slabs (see Fig. 11.46).
If, however, there is a need for the slab soffit to be protected then a bottom mesh can also be provided (see Fig. 11.47).

Fig. 11.46 Typical bending and reinforcement in
ground bearing slabs.

Fig. 11.47 Doubly reinforced ground slab.

In all cases one of the most important aspects of the design and construction is to maintain adequate cover for both wear and tear of the surface and to provide adequate durability.

The slab is generally sized and reinforced on the basis of experience. However, as with the crust raft, a calculated design can be adopted using nominal rules based upon the ground condition. For example, assumptions for variations in sub-strata and/or hardcore support can be made on the basis of expected diameter of any soft spot which may have to be spanned or cantilevered (see Fig. 11.48).

Fig. 11.48 Typical design depression.