Chemo-physical rejuvenation of aged SBS-modified bitumen: Multiscale effects of oils, polymer replenishment, and reactive chain extender

Abstract

Styrene-butadiene-styrene (SBS)-modified binders in porous asphalt age rapidly, degrading both the viscoelastic bitumen phase and the crosslinked polymer network, hindering high-value recycling of reclaimed asphalt pavement (RAP). Conventional rejuvenation predominantly softens bitumen while overlooking polymer restoration. This study separates rejuvenation into two categories, what is repaired (bitumen vs SBS) and how it is repaired (physical vs chemical), to develop and compare four families: (1) oils targeting the bitumen phase, (2) oil–SBS blends supplying fresh polymer, (3) methylene diphenyl diisocyanate (MDI) promoting chemical reconnection of SBS chains, and (4) chemo-physical systems combining MDI with oil–SBS. Chemical properties were linked to performance using Fourier-Transformation Infrared (FTIR) spectroscopy, Gas Chromatography-Flame Ionization Detection (GC-FID), Dynamic Shear Rheometer (DSR) master curves, Multiple Stress Creep and Recovery Test (MSCRT), creep-relaxation, Linear Amplitude Sweep (LAS), fluorescence microscopy, and Environmental Scanning Electron Microscopy (ESEM). Results show that oils softened the binder and improved fatigue and low-temperature properties but diluted the SBS signal and failed to restore elasticity. Oil–SBS raised the SBS index and partially recovered elasticity, though microscopy revealed only discrete polymer domains. MDI on its own enhanced stiffness and elastic recovery but caused brittleness. Chemo-physical rejuvenators reconciled these trade-offs, yielding elasticity comparable to the unaged binder, creep-relaxation and LAS fatigue resistance approaching or exceeding the reference, and dense, well-dispersed polymer-rich domains. These findings highlight the need to combine bitumen softening, polymer replenishment, and chemical chain reconnection for effective SBS restoration and high-quality recycling of SBS-modified RAP, thereby advancing cleaner, resource-efficient pavement material systems.