Shear- and Tension-Type Tests to Evaluate Bonding of Hot-Mix Asphalt Layers with Different Tack Coat Application Rates

Bonding between adjacent pavement layers is an important factor affecting overlay service life. Poor bonding between adjacent layers of hot-mix asphalt can lead to distresses in the pavement system, such as slippage cracking, top-down cracking, premature fatigue, and, in some cases, complete delamination. Although interface behavior between different pavement layers is important, there is no consensus about the best testing technique for bonding. Interface bonding is usually assessed for shear strength. Considering that interface debonding can be caused by both shear and tension loading and that the optimum tack coats obtained from shear-type and tension-type interface tests may differ, a study to compare shear and tension testing results was conceived and carried out. When optimum tack coat rates as obtained from shear-type and tension-type interface tests were obtained and compared, a significantly higher tack coat rate was needed to create a strong adhesive bond between layers than the optimum rate suggested by shear type tests. In addition, shear- and tension-type tests produced difference rankings in tack coat types in their expected performance under shearing and tensile forces, respectively. It is recommended that tension-type tests, such as the interface bond test, be used along with shear-type tests in the design and control of tack coats and thin-bonded overlay systems and that additional studies be performed to develop a system that allows the holistic optimization of pavement bond on the basis of meeting functional requirements in both shear and tension for a given pavement system, traffic level, and environment.