Rheological and Micro Properties of High-Viscosity Binder–Incorporated Polyphosphoric Acid and Rock Asphalt

To enhance the performance of high-viscosity modified asphalt (HVMA), polyphosphoric acid (PPA) and North American rock asphalt (NA–RA) were incorporated into styrene-butadiene-styrene (SBS)/crumb rubber (CR) composite modified asphalt, resulting in two distinct types of HVMA. The two HVMA types were evaluated at various dosages using a dynamic shear rheometer (DSR), bending beam rheometer (BBR), fluorescence microscope (FM), and Fourier infrared spectroscopy (FT-IR) tests. These tests provided both macroscopic and microscopic insights into their properties. The findings indicated that both PPA and NA–RA modify the asphalt’s internal structure, enhancing its temperature resistance. PPA primarily improved high-temperature performance, while NA–RA enhanced low-temperature properties. Specifically, both modifiers improved rutting resistance, with higher dosages leading to further enhancements. However, excessive use of additives could reduce the cracking resistance. NA–RA at a 4% dosage provided the best low-temperature improvement. PPA and NA–RA also improved SBS dispersion and distribution, enhancing the asphalt’s morphology. NA–RA can form a finer flocculated structure within the asphalt. These structures enhanced compatibility more effectively than those formed by PPA. FT-IR analysis showed that PPA induced both physical and chemical changes in asphalt. By contrast, NA–RA caused only physical modifications with no new chemical peaks. The one-way ANOVA results indicated that both the type and dosage of additives significantly affected HVMA performance. These findings offer theoretical support and practical guidance for applying HVMA.