Rheological Properties and Microscopic Mechanism of Nano-SiO2/SBS Composite Modified Asphalt

Asphalt serves as the core binder for heavy-load high-modulus pavements, and its viscoelasticity across a wide temperature range directly governs pavement-rutting resistance, low-temperature crack resistance and service life. Virgin asphalt contains abundant light fractions and exhibits insufficient stiffness at high temperatures. Modification with single styrene–butadiene–styrene block copolymer (SBS) fails to meet the anti-deformation requirements under heavy loads, while separate incorporation of nano-silica (nano-SiO2) aggravates low-temperature brittleness. Existing studies lack comprehensive investigations into the rheological evolution laws and synergistic microscopic mechanisms of asphalt modified by combined SBS and nano-SiO2. In this paper, virgin asphalt was adopted as raw material to prepare composite modified asphalt with gradient dosages. Integrated macroscopic performance tests and multi-scale microscopic characterizations were conducted for systematic analysis. High-temperature, low-temperature and fatigue performances were evaluated via conventional physical property tests, temperature sweep tests, multiple stress creep recovery (MSCR), linear amplitude sweep (LAS) and bending beam rheometer (BBR) tests. Fourier transform infrared spectroscopy (FTIR), gel permeation chromatography (GPC) and thin-layer chromatography–flame ionization detection (TLC-FID) were utilized to analyze variations in functional groups, molecular weight and four fractions, to elaborate the two-phase synergistic modification mechanism. The results demonstrate that the combined incorporation of SBS and nano-SiO2 synchronously optimizes the comprehensive performances of asphalt. Compared with single-SBS-modified asphalt, the sample with optimal dosages achieves elevated high-temperature modulus and rutting factor, reduced permanent deformation, improved low-temperature stress relaxation capacity and remarkably decelerated fatigue damage accumulation rate. Microscopic characterizations verify that only physical interactions occur during modification without generating new substances. The nano-filler facilitates the aggregation of small molecules and increases the proportion of macromolecules; meanwhile, it physically adsorbs light fractions and induces apparent redistribution of asphalt components, raising the relative proportion of resins and asphaltenes in the organic asphalt phase, realizing moderate heavy-fraction enrichment of the asphalt system. This study clarifies the internal correlation between molecular fraction evolution characteristics and macroscopic rheological performances of asphalt co-modified by nano-SiO2 and SBS, which can provide theoretical references for formula design and engineering application of modified asphalt materials.