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 Fig. Position of application of prime coat 19. Why is it necessary to ensure cleanliness of bituminous pavement before applying tack coat? (C5) Surface preparation of bituminous pavement is essential in proper application of tack coat. The entire surface has to be cleared of debris, dust and soils. Otherwise, the tack coat would stick to the debris left on the pavement instead of adhering to the pavement. When delivery or placement equipment is driven over tack coat, it tends to stick to the tire’s of the equipment instead of pavement. 20. What are the differences between anionic emulsions and cationic emulsions? Bitumen emulsions consist of particles of bitumen dispersed in water by using emulsifying agent. When the emulsion breaks, it represents a change from a liquid to a coherent film with bitumen particles coagulating together. The sign of breaking is the change of colour from brown to black as the colour of emulsion and bitumen is brown and black respectively. There are in common two broad types of emulsions, namely anionic emulsions and cationic emulsions. The breaking of anionic emulsions is dependent on the evaporation of water from bitumen emulsion. As such, it poses difficulty in wet weather condition. However, for cationic emulsions, instead of relying on the evaporation of water the breaking is achieved by chemical coagulation. Hence, cationic emulsions are particularly useful in wet weather conditions [66]. 250
A Self-Learning Manual Mastering Different Fields of Civil Engineering Works (VC-Q&A Method) Vincent T. H. CHU 21. Why are stone mastic asphalt used in heavily trafficked roads? (RS5) Stone mastic asphalt (SMA) developed in Germany in late sixties. It is characterized by having a high proportion of coarse aggregates that interlock to form a strong aggregate skeleton. Typical SMA composition consists of 70-80% coarse aggregates, 10% filler and 6% binder. The concept of SMA design is that severe traffic loads is carried by the stone skeleton, and the mastic (a combination of fine aggregates, filler and binder) is introduced in the mix to fill out the remaining void spaces with an aim in achieving durability. To ensure that the course aggregate stone contact function, it is essential to prevent immediate sizes of aggregate to hold course aggregates apart. Owing to high binder content, drainage inhibitor is added to prevent binder drainage during transport and placing. In essence, SMA provides a textured, durable and rut resistant wearing course and hence they are commonly used in heavily trafficked roads with high traffic stress. 22. Why is it preferable for SMA layer to be cooled below 40 o before opening to traffic? (RS5) SMA contains high binder content and extreme care should be taken during the placing process of SMA. Multi-tyred rollers are not used in paving SMA because of the possible working of binder to the surface of the SMA leading to flushing and pick-up. By the same reason, trafficking of newly-placed hot SMA also produces the same effect of flushing the binder to the surface. As such, it is preferable to have the SMA cooled below 40 o before opening to traffic. 23. Why is anti-skid dressing sometimes applied to pavement surface? (RS2) Skid resistance of road pavement plays an importance role when vehicles take a bend or brake. The use of anti-skid dressing increases the friction to reduce the possibility of skidding. Anti-skid dressing refers to the road surface treatment which includes high-friction calcined bauxite as the high Polished Stone Value artificial aggregate, together with a resin (epoxy, polyurethane) instead of bitumen to tie the aggregate to the road surface. It 251
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A Self-Learning Manual - Institution of Engineers Mauritius

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