Effects of Tack Coat Application on Interface Bond Strength and Short-Term Pavement Performance

The objectives of this study were to evaluate the effects of pavement surface type, tack coat material, and application rate on the interface bond strength between a hot-mix asphalt overlay and underlying pavement layers in the field. The effects of interface bonding on short-term pavement performance were also investigated. Three field projects that included 14 in-service test sections were constructed with four types of emulsified tack coats applied at different residual application rates. Specimens were cored from the test sections, and the interface shear strength (ISS) was measured at different service times with a direct shear test device, the Louisiana interlayer shear strength tester. The results of the study showed that, with respect to surface type, the ISS was largely dependent on the type of pavement surface receiving tack coat materials and surface texture. With respect to tack coat material type, the use of a nontracking (rapidly setting) tack coat resulted in a greater ISS than the use of slowly setting (SS-1 and SS-1H) tack coats, a result that was primarily attributed to the stiffer base asphalt cement used in the nontracking tack coat material. With respect to the effects of service time, the interface bonding strength increased with service time in all field projects and for all surface types. This phenomenon was primarily attributed to tack coat curing, which was more pronounced with slowly setting tack coat materials. Laboratory ISS test results correlated well with short-term field performance. All test sections except those that did not meet the minimum ISS threshold of 40 psi, recommended by NCHRP Project 9-40, exhibited satisfactory cracking performance.