Influencing interface morphology and modified asphalt binders on interlayer bonding performance

Interlayer bonding strength is a critical factor influencing the durability and performance of asphalt pavement systems. This study investigates the combined effects of tack coat type, application rate, and substrate surface texture on the interfacial shear strength (ISS) between asphalt layers. Four surface conditions, new wear, binder, aged-worn, and milled, were simulated to represent varying field scenarios. The interface morphology was quantitatively analysed using both the sand patch method and 3D laser scanning. Seven tack coat formulations were evaluated, including a conventional tack coat, hard-grade cutback, and a novel hybrid material modified with Styrene-Butadiene-Styrene (SBS) and polylactic acid (PLA) labels MS4-70 and MSP5-70, respectively. The Tack coats were applied at three residual rates (0.1, 0.23, and 0.35 L/m2). Direct shear testing was conducted to assess ISS under various conditions. The results reveal that surface roughness and tack coat type significantly impact bonding performance. Milled surfaces showed the highest texture and yielded superior shear performance due to enhanced mechanical interlock and bonding area. Conversely, the wear surface, characterised by a dense gradation and smooth profile, exhibited the lowest ISS values. The hybrid MSP5-70 tack coat represents a new generation of modified bonding that consistently produces the highest ISS values compared to conventional tack coats (CRS-1, CSS-1h, and RC70). These findings underscore the importance of tailored tack coat formulations and texture-specific application strategies for enhancing pavement structural integrity. Z 2026 Published by the University of Al-Qadisiyah.