Tack coats are relatively thin applications of asphalt emulsion between the layers of a pavement structure. Their main role is to provide adhesion between the pavement lifts, to prevent slippage, and to confer adequate layer bonding for consolidating the whole pavement structure. A wide variety of bitumen emulsions, spray rates and application techniques are specified worldwide for the construction of tack coats. There seems to be no consensus on how to design tack coat applications and there is not a lot of information about which parameters affect the performance of the finished product and to what degree. The current paper describes a newly developed simple test method for measuring the performance of tack coats in shear. Using a carefully controlled specimen preparation protocol, the performance of a number of cationic and anionic emulsions prepared from asphalt cements of various stiffnesses were evaluated. An array of application rates and different substrate types were also tested to detect and quantify main tendencies, as well as optimum ranges for all the parameters in question. The scope of the study is to better predict the impact of all the described parameters on the field performance of asphalt emulsion tack coats.
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