Microstructural Evolution and Rheological Enhancement of Asphalt–Rubber Binders: Unveiling the Role of Morphology in Performance

Understanding the development of an asphalt binder’s internal network structure is essential when interacting asphalt and crumb rubber. Thus, the focus of this study was to reveal the development of asphalt–rubber binders’ (A-RBs) network structures at different interaction times and their correlation with performance. Atomic force microscopy (AFM) was utilized to image the morphologies of the binders, and the binders’ performances were explored rheologically with a dynamic shear rheometer. Extending the interaction time to 8 h and utilizing a soft binder altered the network structures from agglomerated dispersoids—minuscule distributed phase zones embedded in the continuous matrix of the asphalt binder—to well-organized lamellar structures. At 8 h, using a softer binder increased stiffness by 25% and elasticity by 15%, accelerating early rubber dissolution. Extending the interaction time from 4 to 8 h increased rubber dissolution by 5–23%, depending on the binder type. The 150% increase in stress overshoot for A-RBs with the soft binder versus those with the stiff one reflects the development of the network structure. At 8 h, the soft binder reduced the AFM mean phase angle by 10% and the standard deviation by 64%, indicating a more homogeneous and stable network than that obtained with the stiff binder. Thus, the 8 h interaction time and soft binder facilitated rubber swelling and enhanced component diffusion, aiding in the formation of a homogeneous network.