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
Prime coat is an asphalt which, when applied evenly to the surface of
sub-base or subgrade, serves to seal the surface to hinder the penetration
of moisture into subgrade. Vehicular traffic should be avoided on the
surface sprayed with prime coat because the traction and tearing action of
vehicles would damage this asphalt layer.
18. Should emulsified asphalts or cutback asphalts be selected as
tack coat in bituminous roadworks? (C5)
Emulsified asphalt is a suspension of asphalt in water by using an
emulsifying agent which imposes an electric charge on asphalt particles so
that they will join and cement together. Cutback asphalt is simply asphalt
dissolved in petroleum. The purpose of adding emulsifying agent in water
or petroleum is to reduce viscosity of asphalt in low temperatures.
The colour of emulsion for tack coat is brown initially during the time of
application. Later, the colour is changed to black when the asphalt starts to
stick to the surrounding and it is described as “break”. For emulsified
asphalts, when water has all evaporated, the emulsion is said to have “set”.
Cutback emulsion is described to have been “cured” when the solvent has
evaporated. There are several problems associated with cutback asphalts:
(i) Emulsified asphalt can be diluted with water so that a low application
rate could be achieved.
(ii) The evaporation of petroleum into atmosphere for cutback asphalt
poses environmental problem.
(iii) The cost of production of petroleum is higher than that of emulsifying
agent and water.
Fig. Position of application of tack coat
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