A tack coat is a thin application of asphalt (typically emulsified) used to bond pavement layers together so that they act monolithically. Lack of bonding can lead to fatigue cracking, delamination, slippage and other distresses. This study was undertaken to explore the impacts of various tack coat materials, application rates, and other variables on tack coat performance. The ultimate original goal was to develop a tack coat quality acceptance system. As the study progressed, some of the original objectives and tasks were modified or dropped. Laboratory testing of lab- and field-fabricated specimens using a monotonic direct shear test was used to evaluate the factors of interest. The findings showed that the most commonly used tack materials in Indiana, AE-NT and SS-1h, can perform, with the AE-NT exhibiting somewhat better performance overall. INDOT’s tack specifications could be clarified. The current applications rates are reasonable but could be refined to provide more guidance for use on different types of surfaces, as widely recommended nationally. The use of spray pavers and alternate tack materials should be further explored. Planned spray paver trial projects could provide the opportunity to expand on the results of this project, to explore other test methods, gain more experience with shear testing, and assess typical tack applications on non-experimental projects to assess the state of the practice. Additional implementation studies may help to refine a performance test and criteria for use to assess tack coat quality. The importance of tack coats should be emphasized to contractors and field personnel.

Report Number



tack coat, specifications, spray paver

SPR Number


Performing Organization

Joint Transportation Research Program

Publisher Place

West Lafayette, Indiana

Date of this Version