Factors Affecting the Tracking Performance of Tack Coat Materials

Abstract

One commonly encountered problem with tack coats is the pickup of the tack coat material by haul trucks after placement, which is called tracking. In this study, the bitumen bond strength (BBS) test and loaded wheel tracking test (LWTT) are used to evaluate the tracking performance of the tack coat emulsion considering simulating the tire pressure in the field. The effect of critical factors including humidity, curing temperature and time, and residue application rate and dilution rate on the tracking performance of different tack coats were investigated. Results show that the tracking performance of the tack coat is dependent somewhat on the curing rate of the emulsion, but more importantly on the rheological properties of the residue. It is found that for all emulsions tested, water evaporation or curing happens in a very short time, as the applied layer is very thin in tack coat applications. However, if the rheological properties are not stiff enough, tracking will continue even after completion of water evaporation. In addition, both the modified BBS test (AASHTO T361) and LWTT (ASTM D6372) revealed that a minimum threshold for the G*/sinδ of the residue exists above which the tack coat become trackless. This minimum value is associated with a given temperature depending on the rheological properties of the emulsion residue. In other words, there is an equi-stiffness temperature at which all emulsions become trackless. A range of G*/sinδ values of 10–18 kPa was identified as the threshold above which an emulsion residue could transform to trackless.