## 2 Types and Sources of Waste Plastics Used in Pavement  
The incorporation of waste plastics in pavement construction materials has attracted considerable attention as a sustainable method for mitigating plastic pollution and promoting the circular economy [10; 14; 27]. Plastics are often classified into thermoplastics and thermosetting polymers based on their molecular structure and thermal behaviour. Thermoplastics soften upon heating and solidify upon cooling, enabling them to be re-moulded multiple times without experiencing a significant chemical change. This property makes them suitable for asphalt modification as they can blend with bitumen at mixing temperatures. LDPE, HDPE, PP, PET, PS, EVA, and recycled acrylonitrile butadiene styrene (rABS) are a few examples [10; 11; 14; 27]. On the other hand, thermosetting plastics undergo irreversible chemical crosslinking during the curing process, resulting in rigid structures that cannot be remelted when heated,  
Figure 1: Research framework and structure of this review. limiting their recyclability for asphalt applications. Polyurethane (PU), epoxy resins, and phenol-formaldehyde resins are examples of common thermosetting plastics [10; 11]. While PU has been investigated for its strength properties, there are health and environmental risks associated with its thermal decomposition during mixing [28]. The majority of the plastics used to make asphalt pavements come from a variety of waste sources, such as industrial plastic remnants, agricultural plastic films, municipal solid waste, and marine debris [14]. The main sources continue to be municipal and industrial wastes, which consistently supply polyethylene (PE), PP, PVC, PS, and PET [27; 29]. A key characteristic affecting the suitability of plastic waste for pavement applications is the melting point of the plastic. The asphalt mixing process typically proceeds in temperatures ranging from 140 \({}^{\circ}\)C to 180 \({}^{\circ}\)C. Consequently, plastics possessing melting points within or beneath this range can efficiently melt and combine with bitumen. For instance, LDPE has a melting point of roughly 105-115 \({}^{\circ}\)C, HDPE at 120-130 \({}^{\circ}\)C, PP at 160-170 \({}^{\circ}\)C, PS between 210 and 250 \({}^{\circ}\)C, and PET at 250-260 \({}^{\circ}\)C. PVC lacks a fixed melting point, instead decomposing at around approximately 200 \({}^{\circ}\)C. The melting point range is essential, as plastics with elevated melting points (such as PET and PS) may necessitate increased energy input or may not fully melt during mixing, resulting in inadequate dispersion or environmental hazards from degradation by-products [10]. Consequently, LDPE, HDPE, and PP are preferred due to their melting points being more compatible with standard asphalt production temperatures [10; 11; 14; 30]. Various plastic types demonstrate innate advantages and disadvantages that extend beyond their mechanical properties in pavement applications. LDPE provides flexibility and ease of processing; yet it is prone to ultraviolet (UV) degradation and exhibits reduced strength. HDPE offers superior strength and moisture resistance, although it may exhibit brittleness at low temperatures. PP performs well at high temperatures and is resistant to chemicals, but shows poor adhesion to mineral materials in the absence of compatibilizers. PET exhibits significant tensile strength and chemical resistance, although it necessitates extremely high temperatures for melting. PS offers significant rigidity; nonetheless, it is intrinsically fragile and susceptible to fracturing. PVC provides chemical and flame resistance; however, it raises environmental issues due to the emission of harmful gases when subjected to heating at pavement mixing temperatures [10; 11; 14; 27; 30]. The worldwide output of various plastics indicates their relative availability for recycling. Relevant research indicates that polyethylene (LDPE and HDPE combined) constitutes around 36% of global plastic manufacturing, making it the most prevalent form of plastic waste.