Influence of compaction and paving methods on the interlayer structure and mechanical properties of double-layer bitumen mixtures

Double-layer bitumen mixtures are increasingly used in pavement construction for their improved structural performance and interlayer bonding. However, the effects of paving and compaction methods on internal structure and mechanical behavior remain insufficiently understood, particularly under environmental loading. Most studies emphasize macroscopic mechanical properties, with limited attention to meso-scale structure-performance relationships. In this study, digital image processing techniques were applied to evaluate the interlayer structure of mixtures produced with four methods: continuous/discontinuous paving combined with single/double-sided compaction. Structural uniformity and continuity were quantified using the particles (pores) and cracks analysis system and image processing and analysis in Java (Image J) software. Mechanical behavior was examined through splitting shear, direct shear, splitting tensile, and uniaxial compression tests under three conditions: room temperature (25 °C), low temperature (−10 °C), and freeze–thaw cycles. Results showed that continuous paving with double-sided compaction substantially improved vertical aggregate distribution, mesostructural uniformity, and interlayer bonding. Compared with discontinuous paving or single-sided compaction, shear strength increased up to 4.0 times, splitting tensile strength 1.8 times, and compressive resilience modulus 15%–20%. Under freeze–thaw cycles, mixtures with weak bonding lost up to 70% strength, while optimized mixtures demonstrated markedly enhanced durability. These findings clarify the role of construction methods in governing mesostructure and mechanical response of double-layer bitumen mixtures, offering practical guidance for reliable design and quality control in pavement engineering.