"TRB's National Cooperative Highway Research Program (NCHRP) Report 712: Optimization of Tack Coat for HMA Placement presents proposed test methods for measuring the quality and performance characteristics of tack coat in the laboratory and the field, and includes a training manual presenting proposed construction and testing procedures for tack coat materials."--

NCHRP Project 9 40, Optimization of Tack Coat for HMA Placement, was conducted by the Louisiana Transportation Research Center, Baton Rouge, Louisiana, with major participation by the Texas Transportation Institute, College Station, Texas, and consultant James A. Scherocman, Cincinnati, Ohio. The objectives of this research were to determine optimum application methods, equipment type and calibration procedures, application rates, and asphalt binder materials for the various uses of tack coats and to propose new or revised AASHTO methods and practices related to tack coats. In accomplishing these objectives, both present and emerging technology in the United States and worldwide was evaluated. In the research, the contractor developed two new test methods and associated criteria for characterizing the quality and performance of tack coat materials: the Louisiana Tack Coat Quality Tester (LTCQT) and the Louisiana Interlayer Shear Strength Test (LISST). The LTCQT is a small test unit that can measure the bond strength of a tack coat in the field. The LISST is a test fixture fitted into a universal testing machine to measure the interface shear strength (ISS) of a tack coat in a field or laboratory specimen. With the LISST, the effects of pavement surface types and conditions, tack coat material types, and tack coat application rates and methods on tack coat performance can be assessed. The research demonstrated a strong direct relationship between the ISS and the residual application rate of a wide range of tack coat materials, including a PG 64 22 asphalt binder, and trackless, CRS 1, SS 1, and SS 1h emulsions. Similarly, the LISST results show that for a given tack coat material the ISS is directly related to the pavement surface roughness. Finally, the research established a proposed minimum laboratory measured ISS to provide acceptable tack coat performance in the field as well as optimal tack coat residual application rates for different pavement surface types.--