Tack coat - emulsified asphalts vs. cutback asphalts

Emulsified asphalt is a suspension of asphalt in water by employing an emulsifying agent that imposes an electric charge on asphalt particles so that they would join and cement together. Cutback asphalt is simply asphalt dissolved in petroleum. Purpose of adding emulsifying agent in water or petroleum is to decrease viscosity of asphalt in low temperatures. 

Colour of emulsion for tack coat is brown initially at the time of application. Afterwards the colour is changed to black when asphalt starts to stick to the surrounding and it's described as 'break'. For emulsified asphalts, when water has all evaporated, emulsion is said to have 'set'. Cutback emulsion is described to have been 'cured' when solvent has evaporated. There are numerous problems associated with cutback asphalts:

(i)  Emulsified asphalt can be diluted with water so that a low application rate could be achieved.

(ii) Evaporation of petroleum into atmosphere for cutback asphalt poses environmental problem.

(iii)Cost of production of petroleum is higher than emulsifying agent and water.  

Fig: Position of application of tack coat