## 4 Performance of Waste-Plastic-Modified Bitumen Binder and Mixture
### Characteristics of Plastic-Modified Bitumen Binder
#### 4.1.1 Mechanical and Rheological Enhancements  
The use of waste polymers generally results in diminished penetration values, indicating enhanced stiffness and decreased vulnerability to deformation at elevated temperatures [39,40]. Reductions in penetration vary from 15% to 63%, depending on the type of polymer, its concentration, and the mixing technique employed [39]. The higher stiffness enhances rutting resistance but may result in brittleness at low temperatures [39,41]. The softening point rises in direct correlation with plastic content, hence improving the high-temperature performance of asphalt. LDPE-modified binders have softening points ranging from 57 to 68.5 \({}^{\circ}\)C, HDPE from 51 to 79 \({}^{\circ}\)C, and LLDPE from 50 to 67 \({}^{\circ}\)C [39]. This improvement is attributed to the formation of polymer networks within the binder, enhancing its resistance to flow [41]. The enhancement in high-temperature performance frequently results in decreased flexibility at low temperatures, thereby reducing resistance to low-temperature cracking. This trade-off arises from the enhanced stiffness imparted by plastics within the bitumen matrix, limiting its capacity to relax thermal stresses during cold condition [11,39]. Dynamic Shear Rheometer (DSR) tests typically demonstrate that incorporating waste plastics increases the complex shear modulus (G) and diminishes the phase angle (\(\delta\)) of bitumen binders, signifying improved elastic behaviour and resistance to rutting. For example, molecular simulations of photo-oxidized LDPE have demonstrated enhanced G/sin\(\delta\) and compatibility with bitumen [41], whereas experimental investigations suggest that HDPE modifications generally increase G\({}^{*}\), although compatibility limitations may marginally influence phase angle [39]. Bending Beam Rheometer (BBR) tests evaluating low-temperature performance yield variable findings based on the type of plastic. Blends of LDPE and rubber exhibit diminished creep stiffness (S) and an elevated m-value, signifying enhanced flexibility and resilience to thermal cracking [40]. However, for standalone LDPE or HDPE, this may result in heightened stiffness at elevated doses, which could diminish flexibility and potentially elevate cracking susceptibility [39]. Plastic-modified bitumen has enhanced viscosity, which improves coating and drainage resistance, although it raises issues regarding mixing and compaction [39,40]. The increased viscosity may surpass workability limits, particularly when plastic content exceeds 5 wt% [11,39,41]. The increase in viscosity is associated with the swelling of polymer particles and the establishment of a three-dimensional polymer network inside the bitumen [10,39,41]. Bitumen modified with 10% plastic waste and controlled particle size (0.063-4 mm) exhibited enhanced workability and consistent viscosity [39]. At 135 \({}^{\circ}\)C, the viscosity of this blend roughly approximated that of conventional polymer-modified bitumen, confirming its suitability for practical applications [39]. Thermal stability evaluations demonstrated that plastic-waste-modified binders maintained chemical stability at standard mixing temperatures (\(\sim\)150 \({}^{\circ}\)C), with no notable oxidation detected using FTIR (Fourier-Transform Infrared Spectroscopy) [16,39,41]. Superpave performance assessments validate improved rutting resistance (elevated G\({}^{*}\)/sin\(\delta\)) with plastic modification; nevertheless, fatigue resistance (G\({}^{*}\)-sin\(\delta\)) may diminish at higher plastic contents [11]. The incorporation of 5% PE considerably diminished ductility, underscoring an elevated risk of low-temperature cracking [39]. Hybrid systemsthat integrate LDPE and crumb rubber enhance both rutting and fatigue resistance, with LDPE contributing rigidity and crumb rubber enhancing elasticity [39; 40]. Excessive polymer content may result in poor low-temperature performance unless compatibilizers are included [39].