Hybrid dry mixing of plastic-modified asphalt mixtures under superpave criteria: Mix design and performance considerations

The incorporation of recycled plastics into asphalt mixtures has gained increasing interest as a means to enhance pavement performance while reducing environmental burdens associated with plastic waste. This study examines the use of recycled High-Density Polyethylene (HDPE) and Polyethylene Terephthalate (PET) in asphalt concrete (AC) mixtures produced following the Superpave volumetric mix design. A Hybrid Dry Mixing (HDM) procedure was applied to improve dispersion and prevent clumping, in which aggregates were pre-coated with asphalt binder before introducing finely ground HDPE and PET at dosages of 0.5%, 1.0%, and 1.5% by weight of aggregates. A multi-scale experimental program was conducted to evaluate the thermal, chemical, rheological, volumetric, and compactability responses of plastic-modified mixtures. The thermal and degradation behavior of HDPE and PET were characterized using Differential Scanning Calorimetry (DSC) and Thermogravimetric Analysis (TGA), while chemical features were assessed through Fourier Transform Infrared Spectroscopy (FTIR). Short-term-aged binders were extracted and recovered using a closed-system asphalt analyzer and rotary evaporator, enabling evaluation of rheological properties through Dynamic Shear Rheometer (DSR) and rotational viscosity (RV) testing. Superpave volumetric properties were determined across multiple asphalt contents, and compactability was quantified using gyratory compaction parameters, specifically the Compaction Energy Index (CEI) and densification slope (K id ). Results show that HDPE softens at mixing temperatures and partially interacts with the binder phase, influencing binder availability, mixture stiffness, and compaction resistance. PET remains solid during mixing and acts as a rigid particulate component, affecting internal structure, void distribution, and densification behavior. These mechanisms explain the observed variations in binder demand, volumetric compliance, and compaction trends across all dosages. The HDM procedure produced uniform mixtures without clumping and is compatible with typical plant operations, supporting its practical relevance for integrating recycled plastics into asphalt pavement construction. • HDM achieved uniform dispersion of HDPE and PET without clumping. • HDPE softened in the mix, while PET acted mostly as a filler. • HDPE stiffened with binder; PET Softened the binder. • Both plastics lowered Gmm and increased the required asphalt content. • All plastic mixtures met Superpave volumetric limits.