The Coupling Effects of Thermosetting Polyurethane Crosslinking and an Aggregate Skeleton on the Mechanical Response of Asphalt Mixtures

To clarify the coupled effects of the thermosetting polyurethane (PU) crosslinked binder network and aggregate gradation on the multi-mode mechanical response of asphalt mixtures, three representative 13 mm gradations—stone mastic asphalt (SMA-13), dense-skeleton asphalt concrete (SAC-13), and dense-graded asphalt concrete (AC-13)—were used to prepare polyurethane-modified asphalt (PUA) mixtures, with an SMA-13 styrene–butadiene–styrene (SBS)-modified asphalt mixture employed as the control. Uniaxial penetration and dynamic modulus tests were conducted to systematically characterize strength, deformation, fracture, and viscoelastic responses under direct tension, crack propagation, localized compression–shear loading, and cyclic loading. Under the same SMA gradation and binder–aggregate ratio, SMA-PUA exhibited a 212.5% increase in peak tensile stress, a 29.2% reduction in peak strain, and increases of 76.3% and 92.8% in penetration strength and the penetration modulus, respectively, relative to SMA-SBS, demonstrating the enhanced load-bearing and stress transfer capacity provided by the three-dimensional PU chemical network. Gradation had a limited influence on tensile strength but a pronounced effect on penetration and fracture behavior. In terms of penetration strength and the penetration modulus, the mixtures followed the order SAC-PUA > SMA-PUA > AC-PUA, the SCB maximum fracture strength followed the same order and the maximum tensile strain showed the reverse trend. Dynamic testing showed clear temperature–frequency dependence for all PUA mixtures, with SAC-PUA generally maintaining higher dynamic stiffness. At the common test temperature of 35 °C, SMA-PUA exhibited a higher dynamic modulus and a smaller, more stable phase angle than SMA-SBS. Overall, the macroscopic mechanical response of thermosetting PUA mixtures is governed by the synergy between the high-strength crosslinked binder network and the aggregate skeleton: the PU network establishes the basic load-bearing level, while gradation further regulates stiffness, crack propagation, and deformation compatibility.