Performance and Structural Interpretation of SBS Composite-Modified Asphalt Incorporating a Liquid-Rich Fraction Derived from Subcritical Acetic Acid Degradation of Waste Wind Turbine Blades

To explore the high-value utilization of liquid recovery products from waste wind turbine blades in road binders, a liquid-rich fraction obtained from subcritical acetic acid degradation and subsequent vacuum distillation of waste wind turbine blade epoxy composites was introduced into styrene–butadiene–styrene (SBS)-modified asphalt to prepare composite-modified asphalt. Conventional property tests, dynamic shear rheological tests, bending beam rheological tests, steady shear tests, master curve analysis, Fourier transform infrared spectroscopy (FTIR), and gel permeation chromatography (GPC) were conducted to systematically evaluate the influence of the liquid-rich fraction on the properties and structural characteristics of the composite-modified asphalt. The results showed that, with increasing liquid-rich fraction content, the softening point increased, while penetration and ductility decreased, and the rotational viscosity at 135 °C increased, indicating enhanced overall stiffness and high-temperature flow resistance. High-temperature rheological results showed that the liquid-rich fraction increased the storage modulus, loss modulus, and rutting factor, while decreasing the phase angle improved the high-temperature deformation resistance of the composite-modified asphalt. Low-temperature rheological results indicated that the creep stiffness S increased, the m-value decreased, and the S/m ratio increased, suggesting weakened stress relaxation capacity and reduced cracking resistance at low temperature. Fatigue factor and steady shear results revealed that the liquid-rich fraction enhanced structural stability and flow resistance but also increased fatigue damage sensitivity at intermediate temperature. Master curves, Black diagram, and Cole–Cole plots further demonstrated that the liquid-rich fraction increased the modulus level over a wide frequency domain and strengthened the structural stability of the asphalt system. FTIR and GPC results indicated that the introduction of the liquid-rich fraction increased the relative contents of aromatic structures and polar oxygen-containing groups and promoted molecular association and increased the apparent molecular weight level of the system. Overall, the liquid-rich fraction acted as a structure-enhancing modifier in SBS-modified asphalt, improving its high-temperature performance while causing a certain trade-off in low-temperature and fatigue properties. Therefore, the dosage should be selected by balancing high-temperature stability, low-temperature cracking resistance, and fatigue durability, and the practical sustainability of this recycling route still requires dedicated economic and environmental evaluation.