Crumb rubber modifier (CRM) is considered one of the most commonly used modifiers that enhances the rheological properties of asphalt binders. Optimizing the interaction process between CRM and asphalt binder to enhance the asphalt binder’s elasticity without additional additives is the main purpose of the article. Rheological properties were measured in this article for neat asphalt and crumb rubber modified asphalt (CRMA) binders. Two sets of interactions were selected. In the first interaction set, one interaction temperature (190°C), one interaction speed (3,000 rpm), and different interaction times (0.5, 1, 2, 4, and 8 h) were used. The used CRM percentage was 10 % by weight of the neat asphalt binder. Two sources of asphalt binder, one source and different percentages of CRM, one interaction temperature (190°C), one interaction speed (3,000 rpm), and different interaction times were selected for the second interaction set. This set was designated to confirm the rheological properties obtained for the first set. Thermogravimetric analysis (TGA) was performed on CRM particles before and after their interaction with asphalt binder after different interaction times. The CRMA binders that interacted for the entire 8-h interaction times had significantly enhanced properties, especially the elasticity, as compared to the neat asphalt binder. The 8-h interaction time showed the highest CRM dissolution percentage by dissolution and TGA testing. At this interaction time, more CRM components were released into the asphalt binder liquid phase, which was detected by observing Fourier-transform infrared peaks at 966 cm−1 for polypropylene and 699 cm−1 for polystyrene.

Asphalt binder modification by crumb rubber modifier (CRM) could enhance its rheological properties at high and intermediate temperatures by increasing its stiffness and elasticity. To obtain a modified binder blend with more enhanced intermediate- and low-temperature rheological properties, used motor oil (UMO) was introduced to the crumb rubber-modified asphalt (CRMA) binder. The enhanced high- and intermediate-temperature rheological properties of the modified binders were investigated using a temperature sweep test. UMO, used as a rejuvenator, can regulate the CRM role in the modified binder blend by creating a balance between the binder’s enhanced stiffness and fluidity. This was achieved by increasing |G*|/sin δ at high temperatures and decreasing |G*|.sin δ at intermediate temperatures. The mutual components between the CRM and the asphalt binder were explored by thermogravimetric analysis and Fourier transform infrared (FTIR) spectroscopy. UMO was found to have similar FTIR bands of the asphalt binder that helped the CRM particles to absorb more low-molecular-weight fractions at the beginning of the interaction time, which caused them to swell more and then dissolve, releasing their polymeric components in the asphalt binder’s liquid phase. Adding UMO with two and half percentage by the weight of the neat asphalt binder to the CRMA binder, interacted at 190°C–50 Hz–60 min, could increase the CRM dissolution and cause a greater release of CRM polymeric components into the asphalt binder matrix. This was assured by the presence of FTIR peaks at 911 and 966 cm⁻¹ that are related to the polybutadiene, synthetic rubber in CRM.

Asphalt–rubber binders (A-RBs) have a long and deep history of use; however, little is known regarding the interrelated chemical behaviors and miscibility of rubber with the asphalt fractions [saturates, aromatics, resins, and asphaltenes (SARA)]. This study comprehensively attempted to address this knowledge deficiency by employing Fourier transform infrared spectroscopy (FTIR) to investigate the chemical evolution of A-RBs. A-RB interacted at 190 °C and 3000 min⁻¹ for 8 h was deemed to have the optimal rheological performance. FTIR of the liquid fractions of A-RB 190–3000 showed a prominent chemical shift in the SARA fractions, with new peaks that showed rubber polybutadiene (PB) and polystyrene migration into asphaltenes. Meanwhile, decreases in peaks with C–H aromatic bending and S=O stretching for the A-RB 190–3000 saturates showed that the rubber absorbed low-molecular-weight maltenes during swelling. Peaks associated with C=C aromatic appeared in saturates and aromatics, respectively, emphasizing that unsaturated components migrated from the rubber into the asphalt. Thermal analysis showed that rubber dissolution for this sample reached 82%. While a PB peak existed in asphaltenes of A-RB 220–3000, its intensity was diminished by depolymerization, thus compromising the integrity of the migrated rubber structure and generating less rheological enhancement. This study concludes that FTIR characterization of SARA fractions offers valuable insights into the interactions between asphalt and rubber, and that regulated processing conditions are essential for enhancing binder performance.