ABSTRACT
A rigid concrete pavement comprises the concrete slab supported by its foundation which may be a subbase constructed directly over the subgrade. In the case of low strength subgrades, the foundation may include a capping layer separating the subbase from the subgrade. Capping layer material usually comprises locally available low-cost material with a California Bearing Ratio (CBR) of 15 per cent or more. It may be crushed concrete, hardcore or crushed rock of insufficient strength or stability to function as a subbase. When a capping is specified, for mechanistic design purposes it is assumed that the subgrade has been strengthened so that its CBR is 5 per cent and the effect of the capping is otherwise ignored. The stresses in a rigid slab are influenced by the properties of the subgrade and the subbase, as now described.
16.2 Subgrade The subgrade is the naturally occurring ground or imported fill at formation level. Homogeneity of the subgrade strength is particularly important and avoiding hard
and soft spots is a priority in subgrade preparation. Any subgrade should be suitable material of such grading that it can be well compacted. Material containing variable piece sizes often proves difficult to compact, giving rise to settlement and early failure of the pavement. On very good quality subgrades, such as firm sandy gravel the subbase material may be omitted.
