Comparison between optimum tack coat application rates as obtained from tension-and torsional shear-type tests

With the increased usage of HMA overlays in pavement rehabilitation, research on interface bonding between adjacent layers of HMA pavement has gained considerable attention. Pavement layers constructed with insufficient bonding can result in a number of pavement failure modes including slippage cracking, top-down cracking, premature fatigue cracking, and delamination. Despite the significance of interface bonding on pavement performance, selection of tack coat type and application rate is still based on experience and engineering judgment. Until now, most of the studies conducted to evaluate the bonding between different layers of HMA pavement have been based on shear-type interface tests. Considering that pavement interface failure can be attributed to both shear and tension modes, tension-type tests are also needed to truly optimize the process of selecting and designing the tack coat system. Recent studies have shown that the optimum tack coat rate determined by shear-type interface tests may be lower than the optimum tack coat rate determined through testing in tension. The purpose of this study is to make a comparison between optimum tack coat application rates as obtained from Torque Bond Test and a new tension-type interface test called the Interface Bond Test (IBT). The variables considered in this study include: test mode, tack coat type, and tack coat application rate. For the mixtures and bonding materials evaluated herein, the optimum tack coat application rate was found to be approximately twice as high for maximizing tensile bond fracture energy as compared to torsional shear. Implications and recommendations for further study are discussed.