Performance Assessment of Interlayers with Different Tack Coats by Considering Loading Types and Failure Modes

A monotonic direct shear test (DST) is the most commonly used method to evaluate bond strength of interlayers; however, it does not consider the actual repetitive traffic loading that pavement undergoes during its service life. This study aims to examine the link between various parameters obtained from a monotonic DST and the results of a cyclic loading test that more realistically assesses the interlayer performance with tack coats. Furthermore, to investigate the possible effect of the interface failure mode (shear vs. tensile) on the performance assessment of the tack coat interlayers, a simple indirect tensile test was conducted to determine the tensile strength of the interlayers. Field core samples extracted from a test section constructed in Nebraska were used for various laboratory tests. The field test section included 14 subsections containing various types of tack coat materials with different application methods. Permanent shear deformation and interface stiffness were selected as the failure criteria to characterize the interface resistance associated with cyclic load repetition. Shear strength, shear stiffness, and interlayer bond energy were chosen as the failure criteria for the monotonic DST. Results show that the interlayer shear strength from the monotonic DST had an acceptable correlation with the cyclic test results, and interlayer bond energy is a good predictor of the shear resistance of the tack coats with a high correlation with the cyclic test. The simple indirect tensile test was capable of differentiating interlayer performance with different types of tack coating.