Study on Bonding Performance of Double-Layer Ultra-Thin Friction Course of Asphalt Pavement

ABSTRACT To reduce the material consumption and resource waste associated with conventional milling and repaving of aged ultra-thin friction courses (UTFCs), this study proposes a rehabilitation strategy in which a new UTFC is overlaid on an existing UTFC to form a double-layer UTFC and investigates its interlayer bonding performance. Interlayer shear and pull-off tests were conducted to evaluate the bonding behavior of the double-layer system. In addition, image analysis techniques, including computed tomography scanning and three-dimensional reconstruction, were used to characterize the porosity distribution and tack coat migration within the interlayer zone of double-layer UTFC structures with different surface texture depths. Owing to the inherently high porosity of UTFC mixtures, the tack coat can migrate both upward and downward through interfacial voids during specimen preparation and subsequent loading. The results indicate that the interlayer bonding performance is jointly controlled by the surface texture depth of the existing UTFC and the tack coat dosage, with the tack coat dosage exerting a more pronounced influence. The interlayer shear modulus can serve as an effective indicator for evaluating interlayer bonding quality and shows a strong correlation with both interlayer shear strength and pull-off strength. Under different tack coat dosages, an under-compacted zone with varying spatial extent consistently formed near the interlayer interface. Further analysis of the tack coat migration mechanism showed that part of the tack coat penetrated downward into the pores of the underlying aged UTFC, whereas another part migrated upward during subsequent construction and wheel loading, thereby filling the pores of the newly placed UTFC. This bidirectional migration process plays a critical role in establishing effective interlayer bonding. These findings provide a theoretical basis for the structural design of double-layer UTFCs and for the optimization of relevant construction parameters.