Multiscale Structure–Property Relationships in High-Viscosity Modified Asphalt with Different Polymer Contents

High-viscosity modified asphalt (HVMA) is widely used in porous and high-performance asphalt pavements because of its excellent high-temperature stability and elastic recovery. However, the relationships among modifier dosage, microscopic characteristics, and rheological performance remain insufficiently understood. In this study, HVMAs with different high-viscosity modifier dosages were characterized using conventional physical property tests, Fourier transform infrared spectroscopy, gel permeation chromatography, frequency and temperature sweeps, rheological master curves, and multiple stress creep recovery tests. Grey relational analysis was further conducted to evaluate multiscale structure–property associations. The results showed that increasing modifier dosage decreased penetration and non-recoverable creep compliance, while increasing softening point, low-temperature ductility, rotational viscosity, complex modulus, percent recovery, and viscoelastic transition temperature. Both 12% HVMA and 16% HVMA met the technical requirements for porous asphalt pavements, while 12% HVMA provided a more balanced compromise between performance and viscosity within the investigated range. Increasing modifier dosage also increased the weight-average molecular weight, polydispersity index, and polymer- and asphaltene-related molecular-weight fractions, while decreasing the maltene-related fraction. These changes may reflect the redistribution of asphalt components associated with polymer modification and the contribution of polymer-related species within overlapping apparent molecular-weight ranges. HVMA exhibited clear temperature- and frequency-dependent rheological responses. Under low-temperature and high-frequency conditions, the response was more strongly associated with the asphalt matrix, whereas the polymer-related elastic contribution became increasingly evident under high-temperature and low-frequency conditions, as reflected by the development of a phase-angle plateau. Grey relational analysis showed that the viscoelastic transition temperature had relatively high relational grades with SBS-related structural indicators, asphaltene-related characteristics, and molecular-weight parameters. Percent recovery and the phase-angle plateau were more strongly associated with polymer-related indicators, whereas non-recoverable creep compliance and complex modulus at low and intermediate temperatures showed stronger associations with maltene-related characteristics. At high temperatures, the complex modulus exhibited relatively high relational grades with both polymer- and asphalt-related parameters. These findings provide a basis for understanding dosage-dependent structure–property relationships and for selecting appropriate modifier contents in HVMA.