Evaluation of Tack Coating Practices for Asphalt Overlays in Nebraska

The strength of the bond between asphalt layers affects the lifespan of pavement structures. It is also a key factor in preventing major pavement distresses, such as slippage cracking and delamination. This research project evaluates and compares the effectiveness and performance of different tack coating approaches to ensure the proper bond strength is achieved in asphalt concrete (AC) interlayers through an experimental study. Various tack coat materials, including different types of emulsified asphalt and asphalt binders, at multiple application rates and dilution ratios were investigated. In the first part of this study, laboratory-prepared samples were used to evaluate the sensitivity and effectiveness of the direct shear testing (DST) method, which was selected for the characterization of the AC interlayers where different tack coats were treated. Then, emulsified asphalts and binders were applied to a field test section by varying application rates. The DST was performed under a monotonic loading condition at three different testing temperatures. Interlayer shear strengths were used to rank the performance of the tack coats. In addition, cyclic DST was conducted to investigate fatigue behavior of the interlayers treated with different tack coats. The parameters obtained from the monotonic DST were compared with the fatigue DST results. In general, the test results showed superior interlayer performance from CFS-1 and CRS-2P at double application rate (i.e., 0.16 gal/yd2 residual application rate) and CFS-1 at the standard application rate (i.e., 0.08 gal/yd2 residual application rate). Moreover, CRS-2P provided the shortest breaking time among all the emulsified tack coats. With regard to the correlation between the monotonic and cyclic DST results, the maximum shear force showed an acceptable correlation with the fatigue test results, and the interlayer bond energy, which can also be determined using a monotonic DST, is a good (or better) predictor of the fatigue-related shear resistance of the tack coats due to its higher correlation with the fatigue test results