Basically, sub-base for a concrete carriageway is provided for the following reasons:
(i) It provides a smooth and even surface between the subgrade and concrete slab. This avoids the problem of uneven frictional stresses arising from the uneven interface under thermal and shrinkage movement. It also improves the uniformity of support provided to concrete slab to enhance even distribution of wheel load to the subgrade.
(ii) For heavily trafficked carriageways with frequent occurrence of a high water table, it serves to prevent the occurrence of mud pumping on clayey and silty subgrade. The loss of these clayey soils through carriageway joints such as contraction and expansion joints will cause structural failure of concrete slab under heavy traffic load.
The stiffness of concrete slab accounts for the strength of rigid road structure. It is normally uneconomical to employ sub-base as part of the strength provider because a much thicker layer of sub-base has to be
adopted to reduce the thickness of concrete slab by a small amount. Hence, it is more cost-effective to increase the depth of concrete slab rather than to enhance foundation strength in order to achieve a higher
load-carrying capacity of the concrete pavement.
This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.
If you have a query, you can ask a question here.