Investigation on the Performance and Modification Mechanism of High-Modulus Asphalt Binder: Effect of Micronized Composite Modifier Agent

High-modulus asphalt binders (HMABs) are critical for heavy-duty pavements but are often constrained by low-temperature brittleness and poor dispersion of conventional granular modifiers. This study develops a novel micro-pulverized composite modifier (ZY) integrating hard asphalt, an ethylene–propylene copolymer, and a plasticizer system via high-shear melt blending and centrifugal atomization. The modification effects and underlying mechanisms were systematically investigated through rheological characterization (DSR, BBR, LAS), thermodynamic analysis (DSC), chemical functional group evaluation (FTIR), and microscopic morphological observation (FM), with two commercial high-modulus agents (PR and JK) as benchmarks. The results demonstrate that ZY significantly enhances high-temperature deformation resistance, elevating the Performance Grade from PG 64-22 to PG 82-10, with the complex modulus (G*) consistently exceeding those of PR and JK across the entire temperature sweep range (46–82 °C). At low temperatures, the synergistic toughening effect of the elastomeric copolymer and plasticizer enables a creep stiffness S of 240 MPa and an m-value of 0.298 at −12 °C, satisfying Superpave requirements and ensuring superior stress relaxation capability. The LAS test reveals a fatigue life of 245,000 cycles at 2.5% strain level, representing an approximately 60% improvement over the JK-modified binder. Microscopic characterization (DSC, FTIR, and FM) confirms that the modification mechanism is dominated by physical blending, forming a highly uniform micro-scale multiphase dispersion system: the hard asphalt component integrates into the matrix to achieve viscosity enhancement and stiffening, while the elastomeric copolymer forms finely dispersed spherical microspheres that effectively impede crack propagation and dissipate strain energy. This synergistic design achieves a favorable balance between high-temperature modulus and low-temperature flexibility, offering a promising solution for durable and rut-resistant pavement applications.