Sub-base for concrete carriageway - non-strength provider

Essentially sub-base for a concrete carriageway is provided for the below reasons:

(i)  It provides aneven and smooth surface between the subgrade and concrete slab. This stops the problem of uneven frictional stresses arising from the uneven interface under shrinkage and thermal movement. It also enhances the uniformity of support provided to concrete slab to improve even distribution of wheel load to subgrade.

(ii) For heavily trafficked carriageways with frequent occurrence of a high water table, it serves to stop the occurrence of mud pumping on clayey and silty subgrade. Loss of these clayey soils by carriageway joints like contraction and expansion joints will cause structural failure of concrete slab under heavy traffic load.

Stiffness of concrete slab accounts for the strength of rigid road structure. It is generally uneconomical to use sub-base as part of the strength provider since a much thicker layer of sub-base has to be adopted to decrease the thickness of concrete slab by a small amount. Henceforth it is more cost-effective to increase the depth of concrete slab instead to enhance foundation strength in order to achieve a higher load-carrying capacity of concrete pavement.