A layer of waterproof (or separation) membrane is normally placed between sub-base and concrete slab for the following reasons:
(i) It prevents the loss of water from cement paste which affects the strength of concrete slab.
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The surfaces of sub-base material for concrete carriageway should be constructed in a regular manner because of the following reasons:
(i) One of the main functions of sub-base in concrete pavement is to provide a smooth and even interface between concrete slab and subgrade so that a uniform support is established. A regular surface of
sub-base assists in reducing the frictional and interlocking forces between concrete slab and sub-base and allowing easier temperature and shrinkage movement.
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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.
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.
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There are several shortcomings of using granular sub-base in concrete carriageway:
(i) Since sub-base is permeable, water can seep through sub-base and soil particles will be pumped out through contraction/expansion joints when subject to traffic load. Consequently, voids are formed underneath the pavement structure and the concrete pavement may crack under severe traffic loading.
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When there is a delay between bituminous laying of different bituminous layers (i.e. roadbase, base course etc.), a tack coat is applied on top of the bituminous layers because it helps to enhance better bonding between bituminous materials. If there is insufficient bonding between adjacent bituminous layers, they behave as separate independent layers which can hardly resist the traffic loads. When applying the tack coat, it should be sprayed uniformly on the bituminous surface and allowed for sufficient curing.
Besides providing load distribution in post-construction stage and working platform in construction stage, sub-base can also serve as a drainage layer to remove water coming from the pavement.
In normal practice, joints are provided in road kerbs to cater for concrete expansion and contraction. However, the location of joints in kerbs is not arbitrary and they should match with joints in concrete carriageway.
Otherwise, it is very likely that cracks may form in concrete kerbs at location of pavement joints (Ministry of Transport (1955)).
In General Specification of Civil Engineering Works (1992 Edition), in Clause 10.09 it states “Fine aggregate for concrete shall be natural river-deposited sand consisting of at least 95% by mass of quartz grains”.
The aim of such specification is to control the quality of river sand.
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The main purposes of reinforcement in concrete roads are:
(i) to control the development and pattern of cracks in concrete pavement.
(ii) to reduce the spacing of joints. In general, joints and reinforcement in concrete structures are common design measures to cater for thermal and shrinkage movement. Hence, the inclusion of reinforcement allows
the formation of tiny cracks in concrete pavement and this allows wider spacing of joints.
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