Tack coat can be defined as an application of bituminous material to an existing relatively non-absorptive surface to provide a thorough bond between old and new surfacing. Normally, hot bituminous binders, cutback bitumens (solvent based) and/or bitumen emulsions (water based) are used as tack coat materials. Bitumen emulsions are the most widely used materials for tack coats instead of cutback asphalt or hot bituminous binders due to the fact that: (a) they can be applied at lower temperatures compared to cutback asphalt or hot bituminous binder, (b) they are environmentally friendly as they do not contain harmful volatile solvents and (c) they are safer to use as they are not flammable and pose a health risk to workers. Tack coats are sprayed in a thin film on the existing layer surface before the construction of the next pavement layer. After the tack coat wets the surface of the old pavement and fills the tiny pores, it solidifies and develops what is called interlocking. This interlocking is strongly affected by the liquid’s viscosity at installation. Low viscosity tack coat can penetrate and follow the surface irregularities better than the high viscosity tack coat and, hence results in a better interlocking. There are many other factors affecting tack coat performance and interlayer bond strength including: tack coat type, application rate, temperature, normal pressure application methods and surface roughness.
The most common cationic emulsion bitumens used for tack coat are CRS-1 and CRS-2, whose application temperature is 10-60 ˚C and the amount of the used bitumen is 250-500 g/m2. Also, asphalt cutbacks RC-70 and RC-250 are used with application temperature of 50-70°C and the amount of the used bitumen 200-400 g/m2.