A Self-Learning Manual - Institution of Engineers Mauritius

A Self-Learning Manual
Mastering Different Fields of Civil Engineering Works (VC-Q&A Method) Vincent T. H. CHU
9. Does sub-base of concrete carriageway provide strength support?
(PS3)
Basically, sub-base for a concrete carriageway is provided for the following
reasons [55]:
(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.
10. What are the differences between capping layer and sub-base?
(PS5)
For weak and poor quality subgrade, there is a need to increase the
thickness of pavement to compensate for it. In order to save cost for
sub-base which is relatively expensive, the concept of capping layer is
introduced in which capping materials of cheap but strong nature are used
to cap the weak subgrade. In this way, the thickness of expensive
subgrade is not required to be increased.
Crushed gravels and rockfill may be suitable options of capping material. In
essence, capping material should be readily available at low cost. The
capping layer not only serves to strengthen the subgrade, but also protect
the road formation during construction. It serves as haul road for
construction traffic during construction stage. Moreover, it protects the
subgrade from weathering such as wetting.
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