Mechanistic insights into modification, ageing, rheology, and intrinsic self-healing of hybrid recycled HDPE/crumb rubber-modified binders

• Ageing and modification were assessed at binder and polymer network levels. • Polymer phase was extracted and its morphology assessed via SEM. • CR/RHDPE blends showed up to 70 % damage recovery after ageing. • SEM confirmed finer, stable networks with more CR content. • RHDPE crystallinity dropped with CR, improving compatibility. This study evaluates compatibilised hybrid asphalt binders combining crumb rubber (CR, up to 15 %) and recycled high-density polyethylene (RHDPE, up to 6 %) by innovatively coupling binder recovery and polymer-network extraction with attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR), high-temperature rheology (frequency sweeps for modification efficiency (ME) and the multiple stress creep and recovery (MSCR) test), scanning electron microscopy (SEM) of extracted networks, and multi-cycle linear amplitude sweep (MLAS) tests with healing intervals; ageing was imposed by ultraviolet (UV) exposure and pressure-ageing-vessel (PAV) conditioning. FTIR results of recovered binders show that the hybrids markedly suppress UV-induced carbonyl formation and reduce PAV-ageing sulfoxides relative to neat binder while mitigating aromaticity loss, indicating sequestration and barrier effects from the polymer network. Spectra of extracted modifiers indicate rubber-phase crosslink growth (reduced polybutadiene double bonds with emergent sulfoxides) and, at 15 % CR, disruption of RHDPE crystallinity consistent with finer dispersion; by contrast, high shear redirects sulphur from sulfoxide oxidation toward crosslink formation yet leaves polyethylene backbones more vulnerable to ageing—identifying a compatibility window of higher CR with moderated shear. SEM micrographs reveal two compatibility pathways: fibrillation-assisted interlocking in blends with 7.5 % CR and intermediate–RHDPE under moderated shear, and sub-micron RHDPE dispersion with partial interfacial masking in 15 % CR systems; excessive shear produces finer domains but weaker interfaces that later cluster and crack. Frequency sweeps show that modification efficiency decreases after PAV ageing in low-shear blends but can be retained or recover in high-shear blends as oxidised RHDPE becomes more compatible; UV generally increases modification efficiency, especially with higher CR. MSCR on recovered binders shows unaged gains driven by migrated, oxidised polyethylene oligomers potentially grafted via polyethylene-graft-maleic-anhydride; after PAV ageing the advantage shifts to CR-rich systems, whereas UV penalises CR-rich high-shear blends. MLAS healing tests show styrene–butadiene–styrene (SBS)-modified binder heals rapidly but loses healing capability to some extent after PAV ageing, whereas the hybrids heal more slowly yet retain healing capability during PAV ageing; combined with higher stiffness this implies slower rutting–cracking progression and longer intervention intervals in hot, oxidative service.