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.

During the planning of overlay construction activities, a contractor and/or state agency may be interested in selecting a tack coat material and application rate that would provide adequate interface bonding at the site. The objective of this study was to develop an interface shear strength (ISS) predictive model to estimate interface bonding between the hot-mix asphalt (HMA) overlay and the underlying pavement layer. To achieve this objective, 10 field rehabilitation projects were selected in six states that included 33 in-service test sections representing different traffic and climatic conditions. The overlay construction used different types of tack coat at various residual application rates. Field cores were obtained to measure ISS using a direct shear test device, referred to as the Louisiana Interlayer Shear Strength Tester (LISST). Influencing factors to be included in the model were pavement surface type and mean texture depth, residual application rate, and characteristics of tack coat residues. The experimental data were complemented with test results from a recently completed study for model development and validation. Two types of statistical regression models (linear and nonlinear) were evaluated. Results of the analysis showed that the predicted ISS obtained from the proposed models were in good agreement with the measurements. Further, the developed nonlinear model showed better prediction accuracy than the linear model. The nonlinear model can be used effectively for optimum selections of tack coats given the project conditions. The results presented herein were part of NCHRP Project 9-40A, Field Implementation of the Louisiana Interface Shear Strength Test.

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