A multi-scale study of compatibility in SBS- and EVA-modified asphalt and its impact on optimisation of binder performance

Polymer-based modifications have emerged as an effective solution to enhance asphalt binder performance at high, low and intermediate temperatures. However, the poor compatibility between asphalt and polymers often causes phase separation, thereby limiting their performance benefits. This paper provides a comprehensive review of styrene-butadiene-styrene (SBS)- and ethylene-vinyl acetate (EVA)-modified asphalt, with particular emphasis on compatibility mechanisms and their implications for long-term binder performance. The influence of polymer type and dosage, chemical interactions, swelling behaviour, processing conditions and microstructural evolution were assessed using Fourier transform infrared (FTIR) spectroscopy and molecular weight distribution (MWD). In addition, improvement strategies, including sulphur vulcanisation, nano-clay incorporation and polymerisation are reviewed for their effectiveness in enhancing compatibility. The findings demonstrate that SBS and EVA achieve optimal compatibility at 4–6 wt.%, resulting in enhanced rutting resistance, cracking performance, fatigue life and ageing stability. Moreover, integrating polar functional group grafting such as maleic anhydride (MAH) in combination with nano-clay additives, such as montmorillonite (MMT < 3 wt.%), enhanced compatibility by 77% and reduced phase separation. Consequently, future PMA formulations should integrate chemical functionalisation with optimised polymer dosage to ensure balanced durability under varying climatic conditions and traffic loadings.