A Self-Learning Manual - Institution of Engineers Mauritius

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A Self-Learning Manual Mastering Different Fields of Civil Engineering Works (VC-Q&A Method) Vincent T. H. CHU 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. The hot bituminous material laid on top of the coat would soften it, enabling the tack coat to partly fill voids in the bituminous materials. For emulsified asphalt type tack coats, they are normally diluted with water in order to achieve a more uniform application without excessive usage of asphalt. After the subsequent compaction is carried out, the coat would be interlocked with the bituminous materials. On the other hand, care should be taken to ensure that excessive coat would not be laid, otherwise slippage or shear cracks in the bituminous material would occur due to the relative thick layer of the tack coat applied. However, for sub-base surface, priming coat instead of tack coat may be applied in the event of a delay in laying of bituminous layer on top of the sub-base layer. The function of the primer serves to maintain the existing surface condition for a longer period and it also provides an impermeable surface to prevent ingress of water or water loss by evaporation. Moreover, it fills the surface voids and protects the sub-base from adverse weather conditions. In addition, it also helps to promote adhesion between adjacent road layers and to harden the surface. 8. For rigid pavement, what are the advantages of using lean concrete sub-base instead of traditional granular sub-base? (PS3 & PS7) 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. (ii) Lean concrete increases the strength and renders the roads capable of carrying heavy traffic loads (David Croney and Paul Croney (1992)). (iii) Due to workmanship problem, it may have uneven distribution of sub-base and this results in cracking of concrete carriageway when subject to severe traffic loading. 256
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. 257
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A Self-Learning Manual - Institution of Engineers Mauritius

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