Optimum Tack Coat Application Rate and Curing Time from Direct Shear Test

To provide sufficient bonding between two layers of a pavement, a tack coat material is applied on top of one layer at an optimal application rate, and next, the tack coat material is subjected to curing prior to the placement of the next layer. Though different researchers have prescribed different application rates and curing times, neither a concise value of optimal application rate nor curing time can be found in asphalt literature. To that end, this study undertook a comprehensive laboratory investigation to determine the optimum application rate and curing time for a slow-setting anionic emulsified tack coat (SS-1h) using a direct shear test. The test results demonstrate that the shear strength initially increases as the tack application rate increases until it reaches a peak value. Subsequently, the shear strength decreases with the application rate. The decrease in shear strength can be attributed to the presence of an excessive amount of tack coat, resulting in the formation of a slip plane. A second-order polynomial model was employed to fit the shear strength vs % track coat. An optimum application rate is determined by setting zero value to the first derivative of the polynomial. The recommended application rate for the tested tack coat, in terms of residual asphalt content, is found to be 0.043 gallons per square yard. Regarding the curing time, it was observed that the shear strength exhibits a rapid initial increase as the curing time progresses, eventually reaching a plateau. This phenomenon can be attributed to the existence of a direct correlation between the quantity of water evaporation and shear strength. It was observed that complete evaporation to reach plateau value requires at least 2 hours after the initial application of the tack coat. Therefore, it is recommended that the curing process should be carried out for a minimum duration of 2 hours for this emulsion.