Investigation on rheological, microscopic properties and rejuvenation mechanism of rejuvenated asphalt undergoing multiple aging cycles

To investigate the rheological properties and rejuvenation mechanism of asphalt binders subjected to multiple aging-rejuvenation cycles, Grade 90 A asphalt was used as the base binder in this study. Aged binders were prepared via laboratory accelerated aging: short-term aging was conducted using a Rolling Thin-Film Oven (RTFOT) at 163 °C for 85 min, and long-term aging was performed with a Pressure Aging Vessel (PAV) at 100 °C under 2.1 MPa for 20 h. Then, an asphalt binder system with 1–3 aging-rejuvenation cycles was established through “rejuvenation-aging” cycles. For the asphalt samples experienced 1–3 aging and rejuvenation cycles, a series of experimental analyses were carried out: dynamic shear rheological (DSR) tests were used to characterize their rheological properties, differential scanning calorimetry (DSC) tests to analyze their thermal behavior, thermogravimetric analysis (TGA) tests to evaluate their thermal stability, gel permeation chromatography (GPC) tests to investigate their molecular weight distribution, Fourier transform infrared spectroscopy (FTIR) tests to analyze the evolution of their chemical functional groups, and atomic force microscopy (AFM) tests to observe their microstructural morphology. Moreover, a molecular fusion model for asphalt binders with different rejuvenation cycle numbers was established to explore the rejuvenation mechanism of asphalt subjected to multiple rejuvenation cycles. Results revealed that after asphalt aging, the complex shear modulus (G *) increased, the phase angle (δ) decreased, the viscosity reduced, and the resistance to shear deformation enhanced; the content of large molecular size (LMS) fractions, carbonyl (I _{C O}) and sulfoxide (I _{S O}) indices, and microscopic DMT modulus increased, while the content of small molecular size (SMS) fractions and aromatic index (I _Ar) decreased, with these changes becoming more pronounced as the number of aging cycles increased. After asphalt rejuvenation, G * decreased, δ increased, and thermal stability, heat storage capacity, and viscosity were restored; LMS, I _{C O,} I _{S O}, and Derjaguin-Muller-Toporov (DMT) modulus decreased, while SMS and I _Ar increased, with the viscosity recovery effect improving as the number of rejuvenation cycles and rejuvenator dosage increased, and all indices (except I _{S O}) showing greater magnitudes of change. Significant correlations existed between the DMT modulus and storage modulus, molecular dispersity and non-recoverable creep compliance, and chemical functional group indices and G * and δ of multiple-rejuvenated asphalt, indicating that microstructural parameters have a direct influence on macro-rheological properties. The molecular fusion model of rejuvenated asphalt can effectively characterize the evolution of nano-scale molecular behavior during asphalt rejuvenation, and the recovery efficiency of the diffusion coefficient was enhanced after asphalt rejuvenation, with the recovery effect becoming more obvious as the number of rejuvenation cycles increased. For asphalt that has undergone 1 aging cycle, with a rejuvenator dosage of 8 wt%, and for asphalt in pavements subjected to ≥ 2 aging cycles, adjusting the rejuvenator dosage to 10 wt%–12 wt% enables the asphalt to be basically restored to its original state.