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 is more expensive but is more durable in difficult sites such as crossing and roundabout. Moreover, it also has side benefits of good acoustic quality. Anti-skid dressing has normally a different colour to the road surface and used to be red in colour. Anti-skid dressing is commonly used in areas which require higher friction values such as approach to roundabouts and traffic lights. The ability of the road surface to resist kidding is a combination of the surface texture of the road surface and the Polished Stone Value of the aggregate in the road surface. The higher the Polished Stone Value the higher resistance the aggregate has to polished, and hence the greater the capability the aggregate of retaining its own texture. Surface texture of the road surface is often referred to as macrotexture while texture actually on the surface of the aggregate is known as microtexture. Bleeding forms a sticky and shiny surface which results in the loss of skid resistance for vehicles in wet weather. 24. Is the skid resistance of bituminous pavement derived from microtexture of aggregates or texture depth of road surface? The skid resistance of road surface is of paramount importance in enhancing road safety. The chance of occurrence of skidding is reduced with an increase of skid resistance. At low traffic speed (i.e. 50km/hr), the skid resistance is mainly controlled by microtexture of bituminous aggregates. For road speed exceeding 50km/hr, the macrotexture of road surface comes into play which is characterized by texture depth of pavement. Hence, surface texture becomes an important parameter when designing high speed roads and expressway. The typical texture depth for concrete carriageway is 0.7mm. However, to maintain skid resistance provided by 0.7mm texture depth of concrete carriageway, 1.5mm texture depth is required for bituminous pavement in high speed road. 252
A Self-Learning Manual Mastering Different Fields of Civil Engineering Works (VC-Q&A Method) Vincent T. H. CHU Level One (Core FAQs) Part II: Concrete Road 1. Why is the slump specified in concrete carriageway comparatively low (30mm) when compared with normal concrete (75mm)? (C1) The slump of concrete carriageway is purposely specified to be a relatively low value, i.e. 30mm. For concrete carriageway, traffic loads directly act on concrete pavement surface and therefore the surface strength is detrimental to its future performance. In freshly placed concrete, segregation (may be in the form of bleeding) occurs within the mixture of cement paste and aggregates. The degree of resistance to segregation is related to workability of concrete. If substantial segregation is allowed to take place, then the relatively porous and weak laitance layer will be formed on the carriageway surface and the aggregates will concentrate in the bottom. Hence, concrete which has insignificant bleed possesses a stronger surface layer and is more abrasion resistant. Consequently, a small slump value is specified to increase the wearing resistance of concrete and to achieve a suitable surface texture of concrete pavements. Moreover, a low-slump concrete facilitates the use of slipforms when constructing the concrete pavement. With concrete of a low slump value, it still remains its compacted shape and is not liable to deform when the paving machines go away. However, if a high slump concrete is used instead, the pavement surface would drop and the edges may deform readily. 2. How can unreinforced concrete pavement function without mesh reinforcement? (PS1) For concrete carriageway, it is normally classified into two types: reinforced and unreinforced concrete pavement. The reinforcement in reinforced carriageway (in the form of mesh) is used for controlling cracking. Then one may query how unreinforced pavement can control cracking without the use of mesh reinforcement. To answer this question, one should pay attention to the features of unreinforced concrete pavement. In accordance with Highways Standard Drawing No. H1109, an approximately 3mm wide groove with a depth of about one-third to one-fourth of slab thickness is designed with a regular spacing (normally 5m). The grooves are designed to be too narrow for stones to fall into when the cracks are open due to concrete contraction. The sectional area in which the groove is located is a 253
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A Self-Learning Manual - Institution of Engineers Mauritius

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