Plastic-Waste-Modified Asphalt for Sustainable Road Infrastructure: A Comprehensive Review

Abstract

Plastic waste accumulation poses a critical environmental challenge, while the road construction industry continues to rely heavily on energy intensive, non-renewable binders. Integrating waste plastics into asphalt offers a dual solution to these issues by enhancing pavement performance and promoting circular economy principles. This review provides a comprehensive and data-driven synthesis of global research on plastic-waste-modified asphalt (PWMA), covering six major plastic types and both wet- and dry-processing technologies. Unlike prior reviews, this study employs a systematic PRISMA-based selection framework to evaluate 42 peer-reviewed experimental studies from 2000 to 2024, quantitatively comparing rheological, mechanical, and environmental outcomes. The review identifies polymer bitumen compatibility mechanisms, microstructural interactions revealed through microscopy, and the role of pre-treatment processes (glycolysis and pyrolysis) in improving dispersion and stability. Life Cycle Assessment (LCA) data reveal 20–35% reductions in carbon emissions and 10–12% life cycle cost savings compared to conventional and SBS-modified asphalt. The review proposes a strategic roadmap addressing performance variability, microplastic emissions, and compatibility challenges. By integrating material science, sustainability assessment, and field implementation data, this review advances a novel multidisciplinary perspective on waste plastic valorization in road infrastructure, bridging the gap between laboratory research and policy-ready, scalable applications.

1. Introduction

Plastic pollution has reached crisis levels, with global production exceeding 400 million tons annually and less than 10% effectively recycled [1]. Statistics indicate that the total amount of plastic waste in the environment has already surpassed a billion tons of plastic waste predicted to accumulate in the environment by 2040 [2].

The emission of single-use plastics may reach 5–10 percent of the global emissions by 2050 [3]. Even with the many positive advantages and multi-purpose usages of plastics in different industries, the intensive use of the material over the last decades has caused serious ecological problems [4]. The improper disposal of plastic waste [5], and its ability to continue existing within the environment, has resulted in a serious situation. Plastic pollution has now infected the natural water bodies, the landfills are filled up, and it is a contributor to air pollution as it emits dangerous, cancer-causing fumes unchecked [6].

This waste build-up highlights the imperative of finding sustainable ways of disposing of waste and waste management strategies to deal with the growing environmental crisis [7]. Thus, waste plastics are one of the most important kinds of solid wastes which need a lot of recycling. It is important to recycle plastics as much as possible, not only to ease the burden on landfills, but also to prevent the negative impact of plastic contamination of the environment [8,9,10,11].

Pakistan ranks among the world’s most plastic-polluted nations. In 2020, it generated 3.9 million tons of plastic waste, with 70% of it improperly managed due to insufficient waste management infrastructure [12]. This points out the situation in which, as a developing nation, Pakistan faces significant challenges in managing plastic waste disposal, which is subject to wide-scale environmental degradation. There are various strategies accomplished for using plastic waste, such as incorporating waste plastic in the construction industry, particularly as a substitute for aggregates or as a binder additive in asphalt pavements, which have been explored extensively [13].

By 2018, the United States generated about 27 million tons of plastic waste in landfills, which amounted to 18.5 of total municipal solid waste (MSW) dumped in landfills in 2018. This form of disposal has not only been used in the U.S. but also in other nations such as China and Japan, where plastic garbage disposal is based more on landfills and incineration methods [14,15,16,17,18]. The possibility of chemicals leaching through plastics into the soil and water bodies is a major environmental issue, and the burning of waste plastics has a serious environmental impact because it releases heavy metals like cadmium and lead. The metals can be released throughout the incineration in the form of smoke dust and residues, which contaminate the air and soil [19,20,21]. Recycling and proper use of plastic waste are the key measures to reduce the environmental impact and save natural resources [22,23,24].

Asphalt is a thermoplastic substance that exhibits viscoelastic properties in normal temperature of pavement operation, a factor critical to securing the best performance of pavement [25]. To improve the durability and reliability of asphalt pavements in relation to climatic conditions, traffic needs, and other demands, the application of modified asphalt has been recommended since the nineteenth century as a proven substitute to regular neat asphalt [26,27,28,29].

Directly derived feedstocks such as petroleum or natural gas are newly produced polymers, which are a pure, non-recycled form of polymer, known to improve asphalt performance, specifically high-temperature performance [30,31,32]. Nevertheless, they are frequently limited in quantity and too expensive to be utilized as asphalt modifiers [33,34,35]. The use of waste plastics as substitutes for newly produced polymers in asphalt production has been extensively studied, focusing on the characteristics of waste-plastic-modified asphalt, the modification process, and the environmental implications, such as waste reduction and pollution prevention

Numerous studies have demonstrated the potential of utilizing waste plastics in asphalt modification, offering a promising solution to both plastic waste management and the enhancement of road infrastructure. However, several scientific and technological challenges remain that hinder its widespread implementation. Existing reviews often summarize the findings without focusing on the critical aspects necessary for successful adoption in real-world applications, particularly the performance trade-offs, and the environmental and economic implications of using waste plastics in asphalt. This review aims to address these gaps by providing a comprehensive synthesis of the latest advancements in plastic-waste-modified asphalt. It highlights the contributions of plastic waste to the sustainability of road infrastructure, examining performance improvements, and assessing the environmental benefits and cost-effectiveness of using recycled plastics in asphalt production. In doing so, this review offers a strategic framework for researchers and practitioners to better understand and navigate the complexities of incorporating plastic waste into asphalt for the development of more sustainable and resilient road infrastructure.

2. Objective and Methodology

This study employed a structured and transparent systematic review approach to evaluate the influence of recycled plastics on asphalt modification, focusing on their effects on microstructural behavior, rheological properties, environmental impacts, and overall pavement performance. The review followed the PRISMA 2020 framework (Supplementary Material) [36], encompassing four main stages: identification, screening, eligibility, and inclusion to ensure reproducibility and minimize bias, shown in Figure 1. Comprehensive searches were conducted across six major databases: Web of Science, Scopus, CNKI, ScienceDirect, ASCE Library, and the Transportation Research Record (TRR), covering publications from 2000 to 2024. Supplementary records were also obtained through citation tracing and manual searches. The primary search query combined keywords such as “plastic waste”, “recycled plastic”, “polymer-modified asphalt”, and “road construction”. A total of 467 records were initially identified (184 from core databases, 62 from specific asphalt and pavement journals, and 221 through manual or reference-based searches). After removing 137 duplicates, 330 unique studies remained for title and abstract screening, of which 211 were excluded for not meeting the inclusion criteria. The remaining 119 full-text papers were assessed for eligibility, and 42 studies were ultimately included in the final synthesis. Inclusion criteria required peer-reviewed, English-language studies providing experimental or field-based data on asphalt modified with waste plastics, specifying the plastic type, incorporation method (wet or dry), dosage, and at least one performance indicator such as rutting, fatigue, or moisture resistance. Review-only papers, non-English publications, and studies lacking reproducible data were excluded. The final pool included studies on six major plastic types: polyethylene (PE, 12 studies), polypropylene (PP, 8), polyethylene terephthalate (PET, 7), polystyrene (PS, 5), polyvinyl chloride (PVC, 6), and high-density polyethylene (HDPE, 4). To enhance quantitative rigor, each study was coded by plastic type, modification method, dosage, testing parameters, and performance outcomes. A weighting system was applied based on dataset completeness, assigning higher weights (1.0) to studies reporting both rheological and mechanical data, moderate weights (0.7) to those with one complete dataset, and lower weights (0.4) to studies with limited quantitative data. Weighted averages were then used to derive the overall performance improvements of plastic-modified asphalt mixtures. This structured methodology provides a reproducible, data-driven synthesis of global research, ensuring transparency, balance, and statistical reliability in assessing the sustainable potential of waste plastics in asphalt pavement applications.

Figure 1. Systematic process of literature selection for content analysis.

The framework presented in Figure 2 provides a complete view of the integration of plastic waste into asphalt pavements, highlighting the interplay between plastic types, modification methods, asphalt properties, performance outcomes, environmental impacts, and future research directions. This framework serves as a strategic roadmap for addressing current challenges and advancing sustainable road construction practices.

Figure 2. Framework for the use of plastic waste in asphalt pavements.

3. Plastic Classification

Plastics are synthetic materials primarily derived from refined petroleum products obtained from crude oil [37,38,39]. Plastic waste originates from various sources, driven by the rapid rise in plastic production since the mid-20th century. This sharp increase, coupled with insufficient disposal and recycling systems, has led to a global plastic waste crisis that severely impacts both terrestrial and marine ecosystems [40,41].

In general, the primary sources of plastic waste in the environment include plastic containers, packaging materials, and various industrial plastic products commonly used in consumer and manufactured polystyrene PS, polyethylene terephthalate PET, polyvinyl chloride PVC, polypropylene PP, low-density polyethylene LDPE, and high-density polyethylene HDPE [42,43,44,45,46,47]. There are numerous types of plastics, each with distinct compositions and engineering properties. For asphalt modification, melting point is a crucial physical property, as plastics must first melt to effectively integrate with asphalt binders.

Table 1 provides a detailed summary of all these waste plastics that can be recycled in the accordance with ASTM D7611 [48,49,50]. Figure 3 shows the distribution of plastic types in asphalt mixtures from global field trials. LDPE dominates (40%) due to its low melting point and compatibility with bitumen, while PVC usage is limited (11.8%) due to chlorine emissions [51].

Table 1. Melting points and primary source of waste plastic [52,53,54,55,56].

Types	Density (g/cm3)	Melting Point (°C)	Source	Advantages	Disadvantages
PS	1.04–1.07	210–249	Disposable food containers, takeout containers, compact disc cases, laboratory test tubes, packaging materials, CD storage cases.	Rigidity and strength, thermoplastic nature, wide availability	Brittleness at low temperatures, poor UV resistance, compatibility issues
PET	1.16–1.58	250–260	Plastic beverage bottles, food packaging materials, soft drink bottles, and water bottles.	High strength and stiffness, chemical resistance, high melting point	Poor compatibility with bitumen, brittleness, hydrophilic nature
PVC	1.34–1.39	180–200	Plumbing fittings, pipes, window framing, structural profiles, cable insulation, and garden hose.	Flame retardant, high hardness, and durability	Critical thermal degradation, poor compatibility, environmental and health hazard
PP	0.90–0.91	145–165	Drinking straws, furniture, packaging materials, plastic containers, piping systems, and automotive components.	Chemical resistance, toughness and impact resistance, high melting point	Susceptibility to UV degradation, low-temperature brittleness, poor UV resistance
LDPE	0.91–0.94	110–125	Plastic shopping bags, storage trays, food containers, agricultural films, and water bottles.	Excellent flexibility, chemical resistance, easy blending	Low rigidity, weak UV resistance
HDPE	0.94–0.97	130–149	Plastic bottle packaging, children’s toys, shampoo bottles, piping, and household items.	Durable, moisture resistance, recyclable	Reduced flexibility, compatibility, and homogeneity

Figure 3. Distribution of plastic types used in asphalt mixtures based on global field trials.

3.1. Pre-Treatment of Waste Plastics

This involves physically processing waste plastics into reusable materials without changing their chemical structure. It typically includes steps such as grinding, washing, shredding, pulverization, and extrusion. The physical properties and dimensions of plastics were not always strictly required, making this method commonly used in construction, packaging, piping, and other related fields [57].

Waste plastics can be pre-treated using appropriate physical methods to transform them into various sizes, depending on their source and intended use. For instance, waste PET bottles can be processed into particles of varying sizes through crushing or grinding. Table 2 shows how different physical methods for processing waste plastics work in recycling and modification.

Table 2. Physical methods for processing waste plastics.

Type	Physical Method	Material Composition	Size	Reference
PET	Crushing	Particle	0.45–1.18 mm	[58,59,60,61]
	Extruding	Pellet	0.45–1.18 mm
	Grinding	Pieces	0.45–0.50 mm
PVC	Pulverization	Powder form	0.45–0.50 mm	[62]

3.2. Chemical and Thermal Degradation of Waste Plastics for Asphalt Modification

3.2.1. Chemical Degradation

Chemical degradation involves polymer chains splitting up via solvolysis reactions to generate lower-molecular-weight, more compatible modifiers. The most popular technique is glycolysis of PET, where ethylene glycol is used to break the ester bonds of PET at 180–200 °C to produce oligomeric compounds that are easily incorporated with asphalt binders [63]. Not only does this decrease the melting point of PET but also adds hydroxyl end-groups to react with polar components in asphalt such as (resins and asphaltenes) to enhance storage stability and rheological homogeneity. On the same note, ammonolysis and meta-analysis have also been discussed as a way of breaking down PET to monomers that can be used to wet-modify asphalt. Such chemically modified PET derivatives have been shown to have superior rutting resistance and compatibility in crumb rubber-modified asphalt (CRMA) systems [64].

3.2.2. Thermal Degradation

Thermal degradation (pyrolysis) is the heating of plastics under an oxygen-restricted condition to generate liquid oils, waxes, or gases. Bitumen can be partially substituted by the resulting plastic pyrolysis oil (PPO), or the oil serves as a rejuvenator to an aged asphalt. An example is to use oil obtained by pyrolysis of low-density polyethylene (LDPE) to soften rigid binders and make them workable, and PET-based oils have demonstrated the ability to improve the low-temperature workability. Nevertheless, thermal degradation must be properly controlled in temperature to prevent excessive charring, release of volatile organic compounds (VOCs), or production of dangerous by-products when working with PVC or PS. PPO presents a bright pathway despite these problems and helps to turn mixed or polluted streams of plastic waste into an asset that cannot be directly included in asphalt [65,66].

Both chemical and thermal degradation methods bridge the gap between incompatible raw plastics and functional asphalt modifiers, enabling the use of otherwise problematic waste streams. Their integration into the modification workflow is essential for achieving homogeneous, stable, and high-performance plastic-modified asphalt, particularly in wet-process applications where molecular compatibility is critical.

3.3. Integrating of Plastics in Asphalt Modification

Generally, recycled plastics can be incorporated into asphalt mixtures through two primary methods: the dry process and the wet process.

3.3.1. Asphalt Modified with Plastic Using the Dry Process

In the dry process, recycled plastics are directly added to the asphalt mixture, functioning either as a replacement for part of the aggregate or as a modifier to enhance mixture properties. The aggregate replacement approach is generally used with high-melting-point recycled plastics, such as PET and PS. Conversely, the mixture modifier approach is suitable for most types of recycled plastics (e.g., PE, PP, PET, and PS), except polyvinyl chloride (PVC), due to the risk of hazardous chloride emissions [67].

Most methods for incorporating PET into asphalt utilize dry processes, where PET bottles are shredded into smaller pieces to replace aggregates. Pre-mixing PET with hot aggregates is essential to ensure that the shredded PET effectively coats the aggregates. In the other study, 5 to 15% by weight of PET waste, sized between 2.36 and 4.75 mm, was added as a partial aggregate replacement in asphalt mixtures. Test results demonstrated that the Marshall stability of these mixtures was comparable to that of control samples, suggesting that waste PET can serve as an aggregate substitute in asphalt mixtures [68].

A study found that incorporating waste PVC can improve the rutting and fatigue resistance of the base binder; however, it may reduce the binder’s resistance to low-temperature cracking [69]. The chemical bonds between chlorine and carbon enhance the stiffness and hardness of PVC. Three types of PVC waste were investigated, including waste from cables, windows, and blinds. The test results concluded that PVC derived from waste blinds exhibited the highest brittleness, followed by waste from windows and cables. In other studies, it was observed that processed PETs with finer dimensions (No. 8 = 2.3 mm to No. 40 = 0.425 mm) exhibit better moisture resistance compared to PETs sized between passing No. 8 = 2.36 mm and No. 10 [70].

Although laboratory studies suggest that PVC may enhance rutting resistance, its practical use in asphalt is generally avoided due to the release of toxic chlorine compounds (e.g., HCl, dioxins) during high temperature mixing and compaction. Figure 4 and Table 3 explain the asphalt modification process using dry processing. A meta-analysis of 15 studies as shown in Figure 5 reveals that plastics like LDPE and PP improve rutting resistance by 25–35% and fatigue life by 20–30% on average, with wet-processed blends showing superior performance due to enhanced binder homogeneity. The dry process aligns with ASTM D6155 [71] for aggregate replacement, while wet processing follows AASHTO TP 129 [72] for binder modification [73,74,75,76].

Figure 4. Asphalt mixtures modified with plastic waste dry process.

Table 3. Research using the dry process for asphalt modification.

Plastic	Effect on Bituminous Mixtures	Reference
PP, PS	Bituminous mixtures with 10–15% waste plastic by weight (wb) showed no signs of stripping even after 72 h of water soaking. The aggregate impact value and the stability of the bituminous mixture (BM) both showed improvement.	[77]
LDPE, HDPE	A 2.3% increase in the tensile strength ratio (TSR) value was observed with the addition of 9% waste plastic by weight. An 8.2% increase in the tensile strength ratio (TSR) value was achieved with the addition of 9% waste plastic by weight.	[78]
PVC or HDPE pipes, plastic carry bags, disposable cups, and PET bottles.	The crushing value of plastic mix-coated aggregates was reduced by 40%, and the stability value of the bituminous mixture (BM) was improved with the addition of 10% waste plastic by weight.	[79]

Figure 5. Quantitative performance improvements of plastic-modified asphalt mixtures [80,81,82].

Despite its operational simplicity and compatibility with a wide range of plastic types, the dry process presents several critical limitations. Because plastics are not fully melted or chemically bonded to the asphalt binder, they remain as discrete particles within the mixture, which can lead to poor interfacial adhesion, phase separation, and reduced long-term durability under repeated traffic loading. The findings of recent research show that pavements treated through the dry method are prone to shedding microplastic during the wear of the surface, particularly in the high-traffic or high-stress areas. These microplastics may be carried through stormwater run-offs into water bodies, which are potentially hazardous to the environment and human health. Also, the fact that post-consumer plastic waste is heterogeneous in nature (applied to polymer type, contamination, water content, and particle size, among other factors) makes quality control a difficult challenge to ensure, potentially leading to uneven pavement performance. The dry method, in contrast to the wet one that enables a more effective homogenization, provides less possibility to modify rheological characteristics that may result in insignificant fatigue and low-temperature cracking resistance improvements.

3.3.2. Asphalt Modified with Plastic Using the Wet Process

Wet-processed technologies can be employed for recycling a wide range of plastic waste materials, including ground tire rubber and various types, sizes, and shapes of plastic. According to the Michigan Tech Asphalt Lab, wet-processed plastic asphalt is time-consuming and requires more energy for blending, as well as powerful mechanical tools and additional cooling. Polyethylene, making up 34% of the total plastics market, is the most extensively produced plastic. It is mainly classified into two types: LDPE, with a density ranging from 0.91 to 0.94, and HDPE, with a density exceeding 0.94. LDPE and HDPE are produced through polymerization processes conducted at low and high pressures, respectively [83].

Improper incorporation of asphalt binder may result in air pockets in the pavement. It is important to note that the typical maximum number of plastic powders or particles that can be added to asphalt binders is around 8% by weight [84]. PE is generally a thermoplastic material that can be shaped at high temperatures and solidifies as it cools. However, it can be transformed into a thermosetting material, such as cross-linked polyethylene. Initial studies have primarily explored the use of PE for asphalt modification via the wet process [85,86,87]. The mixing temperature for polyethylene (PE)-modified asphalt typically ranges from 150 °C to 180 °C, with PE additions varying from 1% to 10% by weight of binder. The inclusion of PE in asphalt generally leads to an increase in the softening point and a decrease in the penetration of the binder blends. These changes result in a more rigid asphalt mixture with improved resistance to permanent deformations, which enhances the pavement’s performance under heavy traffic and high temperatures.

This modification contributes to the overall durability and stability of the pavement [88]. Polypropylene is the second most widely produced plastic, accounting for approximately 21% of the total global plastic market. It is a versatile material used in a wide range of applications, including packaging, automotive parts, and textiles, due to its durability, chemical resistance, and low production costs [89,90]. Due to its higher melting point, polypropylene (PP) is more difficult to blend uniformly with asphalt compared to polyethylene (PE) when using the wet process.

The typical mixing temperature for PP-modified asphalt ranges from 160 °C to 190 °C, with PP content usually between 3% and 5% by weight of binder. These higher temperatures help achieve a uniform mixture, which enhances the asphalt’s properties, such as resistance to deformation and cracking. However, the process requires careful temperature control to ensure proper dispersion of the PP within the asphalt. Figure 6 and Table 4 explain the asphalt modification process using wet processing, and Figure 7 shows the effect of plastic waste additive on the viscosity of modified asphalt using wet-processed technology. Furthermore, at elevated temperatures, the wet-processed mixtures demonstrated increased stiffness and enhanced resistance to permanent deformation compared to the control mixes. Several studies have explored converting PET into a liquid state via a glycolysis reaction to modify base asphalt through the wet process.

Figure 6. Asphalt mixtures modified with plastic waste, wet process.

Table 4. Research using the wet process for asphalt modification.

Plastic	Effect on Bituminous Mixtures	Reference
PP, PS	Softening point increased from 49.9 °C to 62–65 °C; penetration reduced by 30–35%	[91]
LDPE, HDPE	Softening point increased from 49.9 °C to 68 °C; penetration reduced by 41.8%; TSR increased by 11.7%	[92]
PVC and HDPE	The addition of 10% waste plastic by weight increased the softening point from 52 °C to 80 °C and reduced the penetration value from 79 mm to 67 mm	[93]

Figure 7. Effect of plastic waste additive content and mixing temperature on the viscosity of modified asphalt using wet-process technology: PE = Polyethylene; CR = Crumb rubber, PET = Polyethylene terephthalate.

Table 5 offers a detailed description of the enhanced performance of waste plastics incorporated into asphalt through wet and dry processes.

Table 5. Summary of waste plastics recycled into asphalt through wet- and dry-process technologies [94,95].

Symbol		Dry Method Processing Performance	Wet Method Processing Performance
	[96]	No stripping after 72 h soaking, ↑ impact resistance, ↑ mix stability	Not commonly used
	[97,98]	Comparable Marshall stability at 5–15% aggregate replacement	↑ Rutting and fatigue resistance (via glycolysis-modified PET)
	[99]	↑ Stiffness but ↓ low-temp cracking resistance; rarely used due to emissions	Reduced thermal susceptibility
	[100]	No stripping after 72 h soaking, ↑ impact resistance	↑ Softening point (49.9 → 68 °C), ↓ penetration by 41.8%
	[101]	↑ Marshall stability, ↑ moisture resistance (TSR 88–92%)	↑ Rutting resistance (35–40%), ↑ stiffness, ↓ penetration
	[102]	↑ Marshall stability, improved moisture resistance	↑ Softening point, ↓ penetration, ↑ rutting resistance, ↑ TSR (11.7%)

Note: ↑ = increase/improvement; ↓ = decrease/reduction.

Despite its advantages in enhancing binder homogeneity and high-temperature performance, the wet process entails significant drawbacks. It is energy-intensive, requiring prolonged mixing at elevated temperatures (160–190 °C) and specialized high-shear equipment, which increases both production costs and carbon footprint. The high processing temperatures can also degrade thermally sensitive plastics, particularly PVC and PS, leading to the release of toxic emissions such as hydrochloric acid (HCl), dioxins, and volatile organic compounds. Additionally, many waste plastics, especially non-polar types like HDPE, exhibit poor compatibility with asphalt binders, resulting in phase separation during storage unless stabilizers are added, further complicating the process, and raising costs. Although often assumed to be more durable, wet-processed pavements are not immune to microplastic shedding mechanical wear, and UV aging can cause the polymer-modified binder to fragment, releasing microplastics into stormwater systems and posing ecological risks. Moreover, the effective dosage is limited typically ≤8% by weight as higher plastic content drastically increases viscosity, reduces workability, and may trap air pockets, compromising compaction and long-term pavement integrity. These environmental, technical, and economic constraints highlight the need for careful trade-off analysis when selecting the wet process for large-scale implementation [103].

4. Microscopic Analysis of Recycled Plastic in Asphalt

Microscopy techniques are commonly employed to examine the microstructural interactions between asphalt and polymers, including waste plastics, in modified asphalt and asphalt mixtures [104,105]. Fluorescence microscopy is frequently used to analyze polymer distribution in asphalt. In the other study, recycled waste packing polymers, such as waste milk bags, were used to replace conventional polymer modifiers, like waste rubber powders, to modify asphalt. The microstructures of the modified asphalt were then observed using a fluorescence microscope [106,107].

Researchers utilized fluorescent microscopy to analyze the structural network and polymer distribution in dried emulsified asphalt samples, which were prepared through different methods of latex addition [108,109]. Recent studies utilized scanning electron microscopy (SEM) to explore the modification mechanism of asphalt by adding 10% to 12% (wt.) PET, highlighting the most effective mixture compositions [110]. The ideal range of PET waste for modifying asphalt is between 10% and 12% (wt.), with the modified mixtures showing superior performance compared to unmodified asphalt. In addition, the modification process of waste polyethylene (PE) and polyvinyl chloride (PVC), sourced from recycled beer shrink film bags, was studied.

This analysis focused on the microstructure, properties of the waste plastics, and their effects on crack pinning and silver shear yield during the decentralized processing of the asphalt [111]. They concluded that the modification improved the composite performance of the membrane and helped mitigate the issue of white pollution caused by waste packaging polymers. In addition to fluorescence microscopy, techniques like gel permeation chromatography (GPC), atomic force microscopy (AFM), and scanning electron microscopy (SEM) are used to analyze the molecular weight distribution, microstructural, and surface morphology of plastic-modified asphalt at various depths. Researchers have found that GPC, AFM, and SEM effectively reveal the dispersion and microstructural behavior of polymer-modified rejuvenators in aged asphalt [112].

Some studies used optical microscopy (OM) to examine the morphology of asphalt with polystyrene (PS). The images revealed that the white PS particles were coarsely and uniformly dispersed within the black asphalt matrix. The PS particles reduced in size, indicating a significant improvement in the compatibility of waste PS-modified asphalt. However, it is important to note that optical microscopy (OM) cannot analyze the dispersion mechanism of PS within the asphalt matrix. Table 6 provides a detailed overview with regions of microscopic techniques applied in recycling waste plastics for asphalt modification.

Table 6. Summary of research on the use of microscopy technologies in recycling waste plastics for asphalt.

S. No	Technique	Plastic Types and Process	Key Findings	Region (Reference)
1	Scanning electron microscope (SEM)	LDPE (lubricant bottles), mixed plastics (Dry)	Both LDPE and HDPE aggregates reduced the density of asphalt mixture due to porous nature and increased air voids content.	Pakistan [113]
2	Scanning electron microscope (SEM)	Recycled polyethylene Terephthalate (Dry)	PET is uniformly dispersed within the asphalt binder matrix.	India [114]
3	Scanning electron microscope (SEM)	LDPE and HDPE sourced from bottles and bags at 0%, 2%, 4%, and 6% by weight of asphalt.	No microcracks could be observed on the surface of 6% HDPE-modified asphalt.	Russia [115]
4	Optical microscopy	The concentration of the linear low-density polyethylene grafted with maleic anhydride (LLDPE-g-MAH) was 0.13, 0.27, and 0.4 of asphalt weight. PET (milk bottles)	Small quantities of LDPE-g-MAH are incapable of forming a homogeneous system.	China [116]
5	Optical microscopy	PE/PP at 2%, 5%, and 10% by asphalt weight. (Dry)	Birefringent, fibrillar structures were observed in the 2% sample. In the 5% sample, insoluble, crystalline fibrils remained after extraction with CH2Cl2.	USA [117]
6	Optical microscopy	PE was added to asphalt in the range of 1 to 13 wt%, with a step size of 1–2%.	Separate particles of the polymer phase are distinguishable, while at high content, the polymer phase becomes harder to distinguish due to its greater integration into the asphalt.	Russia [118]
7	Optical microscopy	Waste plastic (PC) added at 0.5%, 1%, 2%, 3%, 4%, and 4.5% of the asphalt’s weight.	A 2% PC concentration in the base asphalt exhibits a more uniform dispersion compared to higher proportions. With an increase in PC content, the bitumen becomes stiffer as the polymer chains expand.	India [119]
8	Optical microscopy	4% HDPE by weight of asphalt.	The most effective way of decreasing the size of plastic particles in asphalt that will make them easier to disperse is pre-mixing with rubber.	USA [120]
9	Optical microscopy	Recycled polyethylene at 2%, 5%, 15%, and 25% by weight of asphalt.	At low content, RPE is evenly distributed throughout the continuous asphalt phase. At 15% content, the RPE phase is nearly continuous, with a larger dispersion area of the asphalt phase.	Spain [121]
10	Fluorescence microscopy (FM)	5 wt.% of the PE/asphalt blends, HDPE, LDPE.	The percentages of larger polymer phase grew with the increase in time.	China [122]
11	Fluorescence microscopy (FM)	HDPE, medium density polyethylene and LDPE at 5% of the PE/asphalt mixtures.	The separation and development of the polyethylene phase are primarily caused by particle agglomeration, resulting in an increase in particle size.	China [123]
12	Fluorescence microscopy (FM)	(PE) from milk bags (3 mm granules) is added to asphalt in 2%, 4%, 6%, 8%, and 10% by weight.	2wt% polyethylene (PE), scattering spots are dispersed in the binder, while at 4, 6, and 8 wt%, the PE forms a filamentous structure and eventually a complete net-like structure.	China [124]
13	Environmental scanning electron microscopy (ESEM)	HDPE and LDPE plastic pellets at 5% by weight of asphalt.	High-Density Polyethylene (HDPE) particles can generally be distinguished from the asphalt phase. LDPE-modified asphalt forms a typical binary blend, with distinct phases of asphalt	China [125]
14	Environmental scanning electron microscopy (ESEM)	PE pellets and PE shreds from packaging at 5% by weight.	PE particles exhibited a distinct, non-blended phase. Fibrils surrounding the particles indicated partial blending of PE with the binder.	Switzerland [126]

Mechanisms of Micro-Level Modification

The integration of recycled plastic into asphalt mixtures represents a transformative approach to enhancing pavement performance and sustainability. Recent studies using molecular dynamics (MD) simulations have elucidated key mechanisms such as oxidative aging, self-healing, compatibility, and interfacial adhesion between polymers and bitumen [127,128]. Experimental evidence supports these findings: microscopy and spectroscopy techniques have demonstrated that plastic modifiers significantly influence microstructural behavior. Fluorescence microscopy reveals that LDPE forms a continuous net-like structure at 8% dosage, which enhances elastic recovery and rutting resistance. Molecular dynamics simulations further confirm that non-polar polyolefins interact weakly with asphaltene clusters, explaining the phase separation observed in HDPE-modified binders unless compatibilizers are introduced.

Scanning electron microscopy (SEM) has shown improved dispersion of recycled polymers within the binder, reduction in microcracks, and enhanced fracture morphology, contributing to better viscoelasticity and thermal resistance [129,130]. The microscopic image as shown in Figure 8 would indeed provide a detailed view of the dispersion of modifiers in a material. By examining, it can be observed how well the modifiers are spread throughout the matrix and their interaction with the base material. This can reveal key insights into the material’s structural integrity, performance, and how modifications like additives enhance properties such as elasticity or thermal resistance in the case of asphalt mixtures.

Figure 8. SEM images: (a) Neat binder, (b) HDPE-modified bitumen (c) LDPE-modified bitumen (d) Base asphalt (e) HDPE (f) LDPE (g) PE (h) PET (i) PE [131].

The microstructural observations can be further explained through molecular-level and thermodynamic interactions between polymers and bitumen. During mixing, polymer chains diffuse into the lighter maltene fractions of bitumen at elevated temperatures, forming a semi-homogeneous blend. The degree of miscibility is governed by the solubility parameter difference (Δδ) between polymer and bitumen; a smaller Δδ indicates better compatibility, as seen in LDPE and PP systems. The blending process is entropy-driven, and the Gibbs free energy of mixing (ΔG_mix = ΔH_mix − TΔS_mix) becomes negative at higher temperatures, promoting polymer dispersion and interfacial adhesion. Crystalline polymers such as HDPE form ordered lamellae within the amorphous bitumen matrix, which enhances stiffness and rutting resistance, while amorphous polymers (e.g., LDPE) form elastic networks that improve flexibility. These molecular diffusion and entanglement mechanisms explain the network-like morphology observed in fluorescence microscopy and the enhanced viscoelastic properties of plastic-modified asphalt.

5. Road Performance of Plastic-Modified Asphalt

Due to the diverse types of plastics and their varying compositions, incorporating waste plastics into asphalt pavement substantially alters its performance. Below is a summary of the characteristics of several commonly used waste-plastic-modified asphalts and their mixtures, focusing on aspects such as thermo-rheological properties, and mechanical performance.

5.1. Rutting and High-Temperature Performance

The mechanical performance of asphalt paving mixtures is heavily dependent on the thermo-rheological performance of the materials used, and whether they are modified or not. Polypropylene (PP) and polyethylene (PE), two of the most common thermoplastics utilized for asphalt modification, are partially crystalline polyolefins. Although they can enhance high-temperature performance, their non-polar nature results in limited compatibility with bitumen, often leading to phase separation and storage instability without the use of compatibilizers or pre-treatment; as a result, they greatly improve the asphalt’s qualities and make up a large amount of recyclable plastic waste. The crystalline segments of thermoplastics provide enhanced strength to modified asphalt, improving its blend properties and overall performance in service conditions [132,133,134].

Exploring the influence of the crystalline behavior of recycled polyethylene (PE) on the chemical properties of asphalt binder is essential. X-ray diffraction (XRD), a non-destructive technique for examining crystalline structures, allows for the analysis of crystal phases and can reveal the chemical composition of asphalt binders modified with crystalline additives [135]. Along with XRD, differential scanning calorimetry (DSC) and nuclear magnetic resonance (NMR) can also be employed to assess crystallinity [136]. HDPE has a higher viscosity, which makes it less compatible with asphalt. In contrast, the crystalline structure of PE can improve the chemical interactions and elastic properties of the bitumen binder. Research has demonstrated that the crystalline behavior in modified asphalt, across high, medium, and low temperatures, notably decreases permanent deformation and enhances resistance to fatigue and thermal cracking [137]. Recycled LDPE with lower molecular weights tends to achieve a more uniform dispersion, minimizing phase separation in the bitumen. However, gel permeation chromatography (GPC) is more commonly used in research on asphalt aging, with limited studies focusing on the changes in molecular weight of PE-modified asphalt [138].

Dynamic shear rheometer (DSR) tests demonstrate that 10% LDPE increases the rutting parameter (G*/sinδ) by ≥30% at 64 °C, aligning with field observations of reduced deformation under load. Table 7 provides detailed insights into the thermos-rheological and mechanical properties of various waste-modified plastics. As shown in Figure 9, LDPE and HDPE generally offer the most balanced improvement across all four properties, particularly enhancing high-temperature stability and anti-fatigue performance. In contrast, PET improves high-temperature stability but may compromise low-temperature crack resistance, while PVC shows limited compatibility, despite some benefits in stiffness.

Table 7. Summary of the thermo-rheological and mechanical properties of various common waste-plastic-modified asphalts and their mixtures.

Features	Types	Major Finding	Region (Reference)
Thermo-rheological performance	LDPE, HDPE	These polymer additives greatly enhanced asphalt’s rheological properties. Adding 10% (wt.) low-density polyethylene (LDPE) to asphalt achieves the optimal rutting resistance across various temperatures.	Saudi Arabia [139]
Thermo-rheological performance	PET	PET-based additives in crumb rubber-modified asphalt (CRMA) enhance rutting and fatigue resistance while increasing the rotational viscosity of the modified binders. The PET modifier enhances rutting resistance and reduces asphalt’s susceptibility to cracking and deformation at high temperatures.	Hong Kong, China [140] Australia [141]
Mechanical performance	Recycled polypropylene (PP) and rubber	Plastic–rubber asphalt (PRA) mixtures and styrene–butadiene–styrene (SBS) asphalt mixtures demonstrate similar performance in high-temperature stability, low-temperature flexibility, and water resistance. Plastic–rubber asphalt (PRA) is a promising material for enhancing various engineering properties of asphalt mixtures, offering significant improvements in performance and durability.	China [142]
Mechanical performance	Low-density polyethylene (LDPE) and high-density polyethylene (HDPE)	High-density polyethylene with the wet mixing method offers superior adhesion properties. Modified Lottman and Hamburg wheel track tests outperform the Marshall stability test for assessing moisture damage.	Pakistan [143]

Figure 9. Comparative performance impact of major plastic waste types (PET, LDPE, HDPE, PP, PS, PVC) on asphalt properties. Radial segments indicate direction and magnitude of change.

Quantitative analysis of key performance metrics reveals substantial improvements in plastic-modified asphalt mixtures, as shown in Figure 10. The data demonstrate 35–40% enhancement in rutting resistance for LDPE/HDPE-modified mixes, correlating with increased stiffness observed in rheological testing. Similarly, moisture resistance improves by 35–40% for PVC/PS formulations, while PET/PP additives extend fatigue life by 15–20%.

Figure 10. Performance improvements of plastic-modified asphalt mixtures. Key metrics include 35–40% higher rutting resistance (LDPE/HDPE), 35–40% better moisture resistance (PVC/PS), and 15–20% extended fatigue life (PET/PP).

Adding waste modifiers had a significant impact on the rheological properties of asphalt, improving its strength and flexibility. Specifically, it increased the complex modulus while reducing the phase angle. The performance of asphalt modified with LDPE, HDPE, and CR was closely studied, and it was found that these polymer additives greatly enhanced the material’s rheological characteristics. Notably, adding 10% LDPE (by weight) to asphalt improved its resistance to rutting across a range of temperatures, outperforming both HDPE and CR. This modification also raised the high-temperature performance grade of the asphalt from 64 °C to 70 °C, demonstrating its effectiveness in improving the material’s overall durability.

Similarly, the addition of 4% (wt.) HDPE yielded the highest rutting parameter at all temperatures, with the performance grade improving progressively from 64 °C to 70 °C when modified with 4%, 8%, and 10% (wt.) HDPE. CR-modified asphalt also exhibited enhanced performance at the maximum tested temperature of 70 °C. Furthermore, the feasibility of using waste PET additives, processed through ammonolysis, was investigated to improve the storage stability and rheological properties of CR-modified asphalt [144]. Researchers also investigated asphalt modified with waste plastic fibers, examining variations in its conventional and rheological properties before and after aging at different temperatures. Their study accounted for both short- and long-term aging. Using test results, the optimal fiber content was determined, and the properties of unmodified and modified asphalt mixes were compared through the Marshall test. The findings revealed an optimum binder content (OBC) of 5.4% by weight of total mix, with the modified mix demonstrating significantly higher Marshall stability compared to the unmodified mix.

5.2. Fatigue, Cracking, and Low-Temperature Behavior

Plastic-modified asphalt low-temperature performance is important in cold weather conditions where cracking can be caused by thermal contraction. Bending Beam Rheometer (BBR) test is also an important test that is applied to assess the behavior, and the key parameters in this test are the m-value (stress relaxation rate) and S(t) (creep stiffness). Greater m-value (>0.30) and reduced S(t) (<300 MPa) at −12 °C and −18 °C will generally reflect an enhanced resistance to thermal cracking. The use of polyethylene (PE)-based modifiers, especially LDPE, increases the flexibility of the material because it is semi-crystalline but ductile, which leads to a higher m-value at 12 °C and lower brittle nature. By contrast, high-melting-point plastics such as PET are more likely to raise binder hardness, reducing the m-value (0.24 at 10 °C) and increasing the brittleness temperature, increasing the risk of cracking at thermal stresses. Likewise, the performance of PVC-modified binders at low temperatures is low since they have rigid molecular structure and embrittlements caused by chlorine.

Fatigue resistance defined as the number of load cycles to failure (N_f)is also greatly dependent on the type of plastic and the dispersion quality. LDPE and PP at 4–6 percent dosage rate can be well dispersed and could raise fatigue life by 15–20 percent over neat asphalt and this is due to the fact that this has improved elasticity and the ability to heal cracks. Nevertheless, PET that is poorly dispersed or in excess doses (>8%) can serve as stress concentrators and promote the speed of microcrack propagation and decrease N_f.

The aging index (retained ductility after RTFOT/PAV) also verifies that PE-modified binders have better fatigue behavior after aging, but PVC-modified systems have significant losses. Together, these results highlight a trade-off that is very important; most plastics can enhance high-temperature stability, but their effect on low-temperature and fatigue performance is highly sample-dependent in terms of compatibility, dosage, and processing technique.

5.3. Moisture Resistance and Durability

Premature pavement failure caused by moisture action, including stripping, raveling, and pothole formation, is a significant factor in pavement failures, especially in high-rainfall or freeze–thaw areas. The hydrophobic waste plastics such as LDPE, HDPE, and PP play a significant role in increasing moisture resistance by reducing the water intrusion at the binder–aggregate interface and adhesion. This is measured by Tensile Strength Ratio (TSR), whereby a score above 80 percent (according to AASHTO T283) [145,146,147,148], means the material is not overly moisture resistant. Experiments indicate that physical modification of binder by 9 percent LDPE or HDPE weight gain has been found to enhance TSR by 8–12 percent, and in fact, TSR values are 88–92 percent—far above the 8-percent minimum. Equally, asphalt containing 10–15% PP or PS showed no stripping following 72 h of water cansoaking, and this indicated outstanding cohesion and aggregate coating. The improved performance is attributed to the non-polar nature of polyolefins, which repel water and form a more cohesive binder film around aggregates.

Long-term exposure to the environment is also very important in terms of durability. Pakistan (Islamabad and Karachi) field trials using LDPE-modified asphalt indicated no evidence of moisture damage, raveling, or potholes, after two seasons of the monsoon season. In Indonesia, a 600 m LDPE-modified section of pavement gave better results on Hamburg wheel tracking and moisture conditioning tests than the control, and forensic examination showed better binder–aggregate bonding. Similarly, 2500 km of plastic roads in India which are primarily made of dry-process PE/PP have outlasted more than 10 years without any reported distress caused by moisture.

Nevertheless, PVC and poorly processed PET can also suffer moisture resistance unless coated or compatibilized, because they have polar groups or surface roughness which can entrap water. Thus, consistency and sufficient pre-coating of plastic with hot aggregates (notably in dry-process work) is necessary to achieve the maximum number of ideal durations. Taken together, these results prove that the plastic modification, when implemented effectively, improves the short-term moisture resistance and long-term field durability of pavement, which is why it can be regarded as the effective approach to sustainable construction of pavements in rainy climate or humid environment.

5.4. Compatibility and Storage Stability of Plastic-Modified Asphalt

One of the most serious problems that complicate the general application of plastic-modified asphalt, especially in wet-process, is the question of compatibility and storage stability, which is the capacity of the plastic modifier to create a single and stable mixture with the base bitumen without segregating into phases in the high temperature storage. Most of the plastics are thermodynamically incompatible with bitumen, making the resultant physical system multiphasic and prone to segregation [149].

The main reason for incompatibility is the fact that the chemical nature and solubility parameters of the non-polar polyolefins (PE, PP) are different than that of the complex, colloidal structure of bitumen which consists of the non-polar (saturates, aromatics) and the polar (resins, asphaltenes) components. The incompatibility is caused by polarity, molecular weight, and crystallinity differences between plastic and bitumen. Highly crystalline materials such as HDPE and PP often exhibit phase separation when held at high temperatures but lower crystallinity and branching, thus, LDPE and EVA are more compatible. The result of this discrepancy may be the migration and agglomeration of plastic particles, which results in a stiff and polymer-enriched layer at the surface of the storage tank and a binder-enriched layer at the bottom. This segregation counteracts the performance objectives and results into unreliable mix production.

There are many basic aspects that define the compatibility of polymers with bitumen. Polarity differences influence miscibility; polyolefins like polyethylene and polypropylene are non-polar and do not chemically interact with bitumen, which separates in phases unless compatibilizers are utilized. Increased molecular weight and crystallinity (as HDPE) reduce chain mobility and diffusion into the bitumen, and polymers with lower crystallinity, including LDPE, and branched structure increase entanglement and dispersion.

5.5. Performance Variability

In order to focus on the plastic variability of the different types of plastics and the study conditions, the performance data were further broken down into regional, material, and methodological factors. The improvements on the six primary types of plastics in asphalt modification in terms of rutting resistance, fatigue life, and moisture resistance have been summarized in Table 8. The data demonstrate that plastic materials with low melting point (LDPE and PP) have higher rutting resistance during wet and dry procedures because of greater dispersion and contact with the asphalt matrix. Conversely, plastics with high melting points, including PET and PVC, exhibit high fatigue and water resistance, especially during wet processing. Asphalt modified with LDPE was the highest in terms of average rutting resistance enhancement and HDPE was superior in the conditions of high load and high temperature. All these variations are since the standard deviations and are usually approximately 4–8 percent, which are caused by regional climate, mixture design, and lab versus field methods. This quantitative distinction offers a more moderated and situational analysis of international research findings on plastic-modified asphalt.

Table 8. Summary of average performance improvements and variability among major waste plastics used in asphalt modification.

Types	Process	Rutting Resistance Increase (%)	Fatigue Life (%)	Moisture Resistance (%)	Region Condition
LDPE	Wet/dry	35 ± 7	18 ± 4	22 ± 5	Warm climates
HDPE	Wet	30 ± 6	15 ± 3	25 ± 6	Hot regions
PP	Wet/dry	32 ± 6	20 ± 5	24 ± 4	Mixed climates
PET	Dry	28 ± 5	25 ± 6	27 ± 4	Tropical region
PVC	Wet	26 ± 5	21 ± 4	35 ± 8	Urban industrials zones
PS	Dry	24 ± 5	16 ± 4	30 ± 7	High-temperature environments

6. Environmental Concerns and Mitigation Strategies

Although plastic-modified asphalt has significant benefits to the environment due to landfill diversion and carbon emission, there is emerging evidence that highlights high ecological trade-offs which should be given due attention. The studies conducted recently in the field monitoring have measured the shedding caused by microplastic of plastic-modified pavements under real traffic conditions. According to Smyth et al. (2025) [150], these pavements emit 0.5–2.3 g of microplastics per square meter per year with particle sizes of between 1 and 500 µm, which is a size fraction that stormwater runoff can easily carry, and these are capable of infiltrating aquatic ecosystems. In line with this, Hao et al. (2024) [151,152,153] discovered that stormwater samples taken at 1 km of LDPE-modified roads in the state of Oregon had 18 times more microplastic concentrates versus the control areas, which points to the possible accretion of such contaminants in urban waterways and the subsequent effect on aquatic biota.

In addition to shedding microplastic, some chemicals enter a system through thermal treatment of specific plastics. Experimental evidence in laboratories shows that heating polyvinyl chloride (PVC) to temperatures above 180 °C (a typical temperature in wet-process asphalt modification) releases 50–200 ppm of hydrochloric acid (HCl) and small amounts of dioxin and furan, which are all persistent organic pollutants and known to be dangerous to humans and wildlife [154]. Equally, polystyrene (PS) also starts to thermally decompose above 200 °C and releases not only styrene monomers, a substance that is classified as a potential cancer-causative agent in humans by the International Agency of Research on Cancer (IARC), but also benzene and other volatile organic compounds (VOCs) [155]. The emissions have occupational health hazards when mixing and compaction is involved, and it may also be a cause of ambient air pollution when unchecked.

A study investigating four types of roads constructed using a combination of waste product PET, PU, and crumb rubber (CR) revealed several positive outcomes. The study found material costs were reduced by 8.5% to 33.9%, greenhouse gas emissions were reduced by 73% to 86%, and non-renewable energy use was decreased by 13.9% to 76.1%. These findings highlight the potential of waste-derived materials in road construction to improve both environmental and economic outcomes [156].

6.1. Comparative Life Cycle Assessment

A Life Cycle Assessment (LCA) was conducted to evaluate eight different plastic waste management scenarios. The analysis determined that the scenario that involved recycling 10.64% of the plastic waste and sending it to material recovery (MR), along with 2% for gasification and the remainder for waste-to-energy processes, was the most efficient option. This scenario showed the best balance of environmental and economic benefits in terms of waste reduction, energy recovery, and sustainability [157]. Figure 11 shows the environmental benefits of plastic-modified asphalt compared to conventional mixes. Data represent averages from 15 case studies, showing 30% reductions in energy use, landfill waste, and CO₂ emissions per ton of asphalt produced [158,159].

Figure 11. Environmental benefits of plastic-modified asphalt: energy savings, reduced landfill waste, and lower CO₂ emissions [160,161].

Multi-attribute analysis methods have been applied to evaluate the overall sustainability of using plastic waste in road construction. These analyses consider various factors, such as environmental impacts, economic costs, and the engineering properties of the asphalt mixtures. By considering these aspects together, the methods help identify the most viable options for incorporating plastic waste in road construction, ensuring optimal performance, cost-efficiency, and environmental benefits. This holistic approach allows for a more comprehensive understanding of the potential advantages and challenges of using recycled plastics in road pavements [162,163,164,165,166,167,168].

A growing body of Life Cycle Assessment (LCA) literature enables a more systematic comparison of the environmental footprint of plastic-modified asphalt systems. As summarized in Table 9, cradle-to-gate analyses consistently show that waste-plastic-modified asphalt particularly using LDPE, HDPE, or PP delivers lower global warming potential (GWP) and reduced cumulative energy demand (CED) compared to both conventional asphalt and SBS-modified alternatives. For instance, a comparative LCA found that waste PP-modified asphalt reduces GWP by 20% relative to SBS-modified asphalt, primarily because SBS is derived from energy-intensive petrochemical synthesis, whereas waste PP repurposes post-consumer material that would otherwise be landfilled or incinerated.

Table 9. Life Cycle Assessment (LCA) indicators for conventional and plastic-modified asphalt systems.

Types	GWP	CED	Landfill Diversion (KG)	AP (KG)	EP (KG)
Conventional asphalt	85–95	1200–1350	0	1.8–2.2	0.4–0.6
LDPE (6% wet)	62–70	980–1100	60	1.4–1.7	0.3–0.5
PET (10% dry)	68–75	1050–1180	100	1.5–1.9	0.4–0.6
PP (8% dry)	70–78	1080–1200	80	1.6–1.8	0.35–0.55
SBS (4.5% wet)	105–115	1450–1600	0	2.5–2.8	0.7–0.9
PVC (5% dry)	75–85	1100–1250	50	3.0–3.5	0.8–1.1

GWP = global warming potential; CED = cumulative energy demand; AP = acidification potential; EP = eutrophication potential.

Moreover, LDPE-modified asphalt (6% wet process) achieves a GWP of 62–70 kg CO₂-eq/ton, compared to 85–95 kg CO₂-eq/ton for conventional asphalt and 105–115 kg CO₂-eq/ton for SBS-modified asphalt. This positions waste plastics not only as performance enhancers but also as carbon-efficient alternatives to virgin polymer modifiers. However, this benefit is plastic-type dependent: systems incorporating PVC or untreated PS may offset GWP gains with higher acidification and ecotoxicity impacts due to emission risks during processing [169].

These findings underscore the importance of material-specific sustainability evaluation. While plastic-modified asphalt offers clear environmental advantages over conventional and SBS-modified mixes in terms of carbon and energy, a holistic assessment must also account for end-of-life impacts such as microplastic shedding and chemical leaching, which remain underrepresented in current cradle-to-gate LCAs. Future studies should adopt cradle-to-grave frameworks to capture the full environmental trade-offs of this promising technology [170].

Additionally, replacing 25% of bitumen with two types of plastic waste in highly modified asphalt mixtures resulted in both environmental and economic benefits. Specifically, this substitution led to a 17% reduction in environmental impact and an 11% reduction in economic costs, highlighting the potential for sustainable and cost-effective asphalt production using plastic waste [171,172,173,174,175,176].

While incorporating waste plastic in asphalt mixtures presents environmental advantages such as landfill reduction and lower virgin material demand, concerns persist regarding the release of microplastics and toxic emissions. Studies indicate that plastic-modified pavements may gradually shed microplastic particles through mechanical wear, especially in high-traffic areas. These particles can enter stormwater systems and ultimately affect aquatic ecosystems, raising public health concerns [177].

Furthermore, heating certain plastics, notably PVC and PS, at high temperatures can emit hazardous compounds, including dioxins, furans, and heavy metals like cadmium and lead. For instance, significant leaching and air emissions from plastic-incinerated residues were reported in poorly regulated setups [178,179,180,181,182].

6.2. Economic Impact

Using waste plastic in asphalt can significantly lower the need for virgin materials particularly bitumen. Research indicates that the cost of plastic is often less than the savings achieved by reducing bitumen usage. For example, one study found that incorporating waste plastic into road construction led to savings of approximately Rs. 28,600 (Indian Rupees) per kilometer, which is roughly $344.58 USD. While this unit cost saving may appear modest, it scales to substantial national savings exceeding $34 million across India, 100,000 km of plastic roads when viewed alongside the enhanced durability, and reduced long-term maintenance costs of the pavement [183,184,185,186,187,188].

In 2019, the Oregon department of transportation reported that using waste plastic in asphalt mixtures can significantly reduce the demand for virgin materials, especially bitumen. The cost of waste plastic is generally lower than that of bitumen, resulting in direct savings. Research has indicated that replacing part of the bitumen with plastic waste can lead to savings of approximately $15,248 per kilometer in maintenance costs. Studies have found that recycling and eco-friendly road construction methods can reduce carbon emissions by up to 33%. To be considered eco-friendly, a construction project must incorporate waste materials and minimize the use of virgin resources [189]. It was also reported that incorporating PET plastic waste into road construction could reduce PET waste by millions of tons, which would lower greenhouse gas emissions and reduce plastic litter.

This approach would also extend landfill lifespans and conserve natural resources. Economically, it offers an additional revenue stream for waste managers and helps reduce the costs associated with flexible pavement construction [190]. Research demonstrated that coating aggregates with waste plastic polymers like PE, PP, and PS can offer notable environmental and economic benefits. In constructing a 1 km road with a 3.75 m width, nearly one ton of asphalt was saved, equivalent to approximately $462 in cost reduction for this single road section. Additionally, the enhanced durability of the road is projected to extend its service life by up to seven years before requiring maintenance, yielding even further cost savings [191,192,193,194,195]. Table 10 presents the estimated revenue from landfilled plastic waste in 2016 and summarizes the potential financial value of plastic waste disposed of in landfills during 2016.

Table 10. Estimated revenue from landfilled plastic waste in 2016.

Landfilled Plastic 2016	Weight%	Tons	Price/Ton $80	Price/Ton $160
PE	45	3,333,000	266,400,000	532,800,000
PP	19	1,406,000	112,480,000	224,960,000
PS	3	222,000	17,760,000	35,520,000
PE/PP	1	74,000	5,920,000	11,840,000
PVC	10	740,000	-	-
PET	3	222,000	-	-
Rubber	13	962,000	-	-
Others	6	444,000	-	-
Total	100	7,400,000	402,560,000	805,120,000

The economic viability of plastic-modified asphalt is best evaluated through a Life Cycle Cost (LCC) framework that accounts for material substitution, processing overhead, and long-term maintenance savings. Unlike conventional cost-per-km metrics, LCC analysis captures the full economic value of enhanced durability, particularly the 15–25% reduction in rehabilitation frequency due to improved rutting and fatigue resistance. The net savings are highly sensitive to regional bitumen prices, plastic feedstock costs, and the selected processing method (dry vs. wet). All values in this analysis are based on 2025 estimates, approximated from IMF and World Bank commodity and inflation data, and validated against recent field-based economic studies from India, USA, Indonesia, and the UK. As shown in Table 11, systems using PP and PET consistently deliver the highest LCC savings (10–12%) across global scenarios, while PVC yields the lowest due to high emission-control costs. This theoretical framework enables policymakers to identify context-specific optimal solutions, dry-process PP in low-income regions, wet-process LDPE in high-bitumen-price markets, thereby aligning economic efficiency with environmental sustainability.

Table 11. Life Cycle Cost (LCC) Sensitivity analysis of plastic-modified asphalt across bitumen price scenarios (2025 USD).

Types	Bitumen Price ($/Ton)	Processing Cost ($/Ton)	Substitution Rate (%)	Life Extension (%)	LCC Savings ($)	LCC Savings
LDPE	350	50	5	10	13,391	7.96
LDPE	350	50	8	20	17,733	10.54
HDPE	350	75	10	15	13,891	8.26
PP	350	50	8	25	18,233	10.84
PET	350	75	11	25	18,733	11.14
PVC	350	150	5	10	12,891	7.66
LDPE	400	100	8	20	17,483	10.39
HDPE	400	75	10	15	14,891	8.85
PP	400	50	8	25	19,733	11.73
PET	400	75	11	25	19,983	11.88
PVC	400	150	5	10	13,141	7.81
LDPE	450	150	5	10	13,141	7.81
HDPE	450	75	10	15	15,391	9.15
PP	450	50	8	25	20,233	12.03
PET	450	75	11	25	20,733	12.33
PVC	450	150	5	10	12,891	7.66

Substitution rate: % of bitumen replaced by plastic (wet process) or aggregate replaced (dry process). Life extension: Reduction in rehabilitation frequency due to improved durability. PVC shows lowest savings due to high processing costs and emission controls. PP and PET deliver highest LCC savings due to strong performance and moderate processing costs.

The total LCC savings (%) is computed as

$$\mathrm{L}\mathrm{C}\mathrm{C} \mathrm{s}\mathrm{a}\mathrm{v}\mathrm{i}\mathrm{n}\mathrm{g}\mathrm{s} \left(\mathrm{\%}\right)={\displaystyle \frac{(\mathrm{M}\mathrm{a}\mathrm{t}\mathrm{e}\mathrm{r}\mathrm{i}\mathrm{a}\mathrm{l} \mathrm{S}\mathrm{a}\mathrm{v}\mathrm{i}\mathrm{n}\mathrm{g}\mathrm{s}+\mathrm{M}\mathrm{a}\mathrm{i}\mathrm{n}\mathrm{t}\mathrm{e}\mathrm{n}\mathrm{a}\mathrm{n}\mathrm{c}\mathrm{e} \mathrm{S}\mathrm{a}\mathrm{v}\mathrm{i}\mathrm{n}\mathrm{g}\mathrm{s}-\mathrm{P}\mathrm{r}\mathrm{o}\mathrm{c}\mathrm{e}\mathrm{s}\mathrm{s}\mathrm{i}\mathrm{n}\mathrm{g} \mathrm{C}\mathrm{o}\mathrm{s}\mathrm{t})}{\mathrm{T}\mathrm{o}\mathrm{t}\mathrm{a}\mathrm{l} \mathrm{c}\mathrm{o}\mathrm{s}\mathrm{t} \mathrm{o}\mathrm{f} \mathrm{c}\mathrm{o}\mathrm{n}\mathrm{v}\mathrm{e}\mathrm{n}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}\mathrm{a}\mathrm{l} \mathrm{a}\mathrm{s}\mathrm{p}\mathrm{h}\mathrm{a}\mathrm{l}\mathrm{t}}}\times 100$$

7. Field Implementation of Waste Plastic in Asphalt Mixtures

Despite extensive laboratory research on asphalt mixtures incorporating waste plastics, there is still a lack of field validation. In response to the growing demand for sustainable practices, recent global pilot projects are being initiated to evaluate the real-world performance of plastic-modified asphalt. Field monitoring indicated that plastic-modified pavement is more suitable for heavy traffic due to enhanced binder properties, increased strength, and improved surface conditions of the asphalt mixtures [196].

7.1. USA

The University of California, San Diego (UCSD) built the first asphalt road in the USA using a recycled plastic binder, replacing the traditional petroleum-based bitumen binder [197]. Since 2018, approximately 19 field sections with asphalt layers containing waste plastic have been laid across various locations in the USA, using both dry and wet methods. For instance, in 2021, a 137 m long local road in Wisconsin was constructed using 450 kg of waste LDPE (0.5% by weight of aggregate, employing the dry method). Similarly, in 2020, a 2414 m² parking lot was built at the Cincinnati technology center in Ohio, incorporating 1945 kg of waste LDPE (0.5% by weight of aggregate, using the dry method). Additionally, in 2019–2020, two private roads in Texas were constructed with 1594 kg of LDPE-based waste plastic, using the wet method [198]. Figure 12 shows a trial section including an asphalt mixture that incorporates waste plastic from the University of California, San Diego (UCSD) complied with California DOT’s Interim Guidelines for Recycled Plastics in Asphalt.

Figure 12. Trial section with an asphalt mix containing waste plastic, University of California, San Diego (UCSD) [199,200,201,202,203,204,205].

7.2. Pakistan

The United Nations Environmental reports that Pakistan produced about 3.9 million tons of plastic waste in 2020. If current trends continue, this figure is expected to increase to around 6.12 million tons annually by 2050 [206,207,208]. Pakistan’s first plastic-paved road has been constructed in Islamabad, offering a sustainable solution for repurposing plastic waste into high-value materials. The one-kilometer stretch, located on Ataturk Avenue, is part of an innovative project aimed at exploring the potential of using waste plastics in infrastructure, contributing to both environmental and urban development goals [209]. Over 2.5 tons of discarded Shell lubricant bottles were effectively recycled to construct a road stretching 730 feet in length and 60 feet in width, located next to Shell House in Karachi. This initiative showcases the innovative use of recycled plastic materials for road construction, promoting sustainability and waste reduction in urban infrastructure projects. Figure 13 shows test sections with waste plastic in asphalt mixtures in Islamabad and Karachi, Pakistan.

Figure 13. Test section containing waste plastic in asphalt mixtures in (a) Islamabad and (b) Karachi, Pakistan [210,211].

7.3. China

China is the world’s largest producer of plastic waste, accounting for one-fifth of global plastic waste generation. The country generates about 18 kg of plastic waste per person annually. In 2019 alone, approximately 130 million tons of plastic waste were produced, largely consisting of food packaging and various single-use plastics. This immense volume underscores the significant challenge of managing plastic waste in China. In China, the first-ever plastic road project used recycled milk bottles to construct a road made with polymer-modified asphalt. The road, unveiled at the East China University of Science and Technology’s Xuhui campus, incorporated more than 6000 milk bottles along with other plastic waste, demonstrating an innovative approach to sustainable infrastructure development [212]. The company involved in this project has committed to recycling approximately 1 million tons of waste plastic by 2030. This ambitious goal aligns with global sustainability efforts to reduce plastic waste and promote recycling in various industries. By focusing on large-scale plastic recycling, the company aims to significantly contribute to reducing environmental impact while fostering a circular economy. Figure 14 shows asphalt mixture trial with waste plastic. Shanghai China ECUST.

Figure 14. Trial section containing waste plastic in asphalt mixtures Shanghai China ECUST.

7.4. India

India annual plastic waste production is approximately 3.5 million tons, a figure that has nearly doubled over the past five years [213]. In India, the use of waste plastics in road construction has become widespread. Since 2002, more than 2500 km of asphalt concrete pavements have been modified with polymer waste using the dry method. To assess the long-term performance of these roads, six sites were selected for further investigation. The results indicated that these modified roads have performed well, showing no significant issues such as potholes, raveling, or rutting. This approach not only helps in managing plastic waste but also enhances the durability of roads, contributing to both environmental sustainability and infrastructure improvement. In 2013, the Indian Highway Congress published guidelines for the incorporation of waste plastics into bituminous mixtures, aiming to enhance the durability and performance of road infrastructure. Additionally, the National Rural Highway Development Agency issued guidelines for using waste plastics in rural road construction. These efforts reflect India’s commitment to improving road quality and managing plastic waste sustainably. The guidelines help standardize the use of recycled materials in road construction, ensuring better performance while addressing environmental concern.

7.5. UK

In 2016, the UK generated about 1.53 million tons of plastic waste, reflecting a 24% increase compared to 2010 A project conducted at Carlisle Airport, UK, incorporated 200,000 plastic bags, 63,000 glass bottles, and over 4500 printer cartridges into asphalt pavement. The project demonstrated improved rutting and fatigue resistance compared to unmodified asphalt. Although costs were higher than standard asphalt due to limited production sources, they remained lower than those for other polymer-modified pavements [214].

7.6. Indonesia

Indonesia generates approximately 7.8 million tons of plastic waste annually, with more than half of this waste being improperly managed. A significant portion of this mismanaged plastic waste ends up in the ocean, posing a major threat to marine ecosystems. Indonesia ranks as the second-largest contributor to marine plastic waste globally. In response to this growing issue, the Indonesian government has implemented various waste management policies and is actively promoting the use of recycled plastics as part of its efforts toward sustainable development [215,216,217]. In 2017, an asphalt pavement with a total length of 700 m was constructed at the University of Udayana, Bali.

Figure 15 shows the Indonesian track containing waste plastic in asphalt mixtures. Of this, 600 m used recycled LDPE in the asphalt mixture, while the remaining 100 m was constructed without waste plastic for comparison. This project was part of an initiative to explore the benefits of incorporating recycled plastics in pavement construction, aiming to improve sustainability in infrastructure development.

Figure 15. (a) Test mixes, (b) spreading and compaction process.

In this project, 6% of virgin asphalt binder (penetration grade 60/70) was added using the dry method. The process began with laboratory investigations, including the Marshall mix design and subsequent mechanical performance evaluations, such as resilient modulus, wheel track rutting, and beam fatigue tests. The waste plastic was first added to heated aggregate to ensure proper surface coating before mixing it with hot asphalt. A forensic investigation of field samples showed that the section with waste plastic performed better than the control asphalt pavement, which did not contain waste plastic.

8. Critical Challenges and Research Gaps

Plastic-modified asphalt offers a promising approach for utilizing waste plastics in road construction, yet several significant scientific, environmental, and methodological challenges need to be addressed before this technology can be adopted on a large scale. A primary concern is the compatibility of waste plastics with bitumen. Plastics like PET and PVC are thermodynamically immiscible with bitumen, which leads to phase separation, storage instability, and inconsistent field performance. Unlike engineered modifiers such as SBS, which form stable elastomeric networks, common plastics often act primarily as fillers, sacrificing key performance attributes like low-temperature flexibility and fatigue resistance for high-temperature stiffness. This incompatibility remains a substantial limitation, hindering the performance and long-term viability of plastic-modified asphalts.

In addition, single-plastic systems present inherent limitations. Each type of plastic has its own defects, such as PET’s brittleness or PVC’s emission risks, which compromise their overall performance. To overcome these deficiencies, researchers have turned to composite modification, combining different plastic types to achieve a more balanced and optimized performance. For example, blends like PET-PE or SBS plastic hybrids show promise in improving asphalt properties. However, the full potential of composite modification remains underexplored in standardized testing frameworks, making it an area of active research that could unlock significant improvements.

Equally important is the issue of pre-treatment for high-melting-point plastics. Methods such as glycolysis (chemical degradation) and pyrolysis (thermal degradation) are often necessary to reduce the molecular weight of plastics and make them more compatible with bitumen. While effective, these processes add cost, energy demand, and potential emissions, raising questions about the true sustainability benefits of using waste plastics. Moreover, the need for standardized testing methodologies is another challenge. Current studies vary significantly in their use of dosages, particle sizes, aging protocols, and performance metrics, making cross-study comparisons difficult and unreliable. Without standardized protocols, the adoption of plastic-modified asphalt in engineering applications remains constrained.

Furthermore, long-term environmental impacts are often not adequately addressed in the literature. While Life Cycle Assessments (LCAs) suggest that plastic-modified asphalt can reduce CO₂ emissions and landfill use, they typically overlook the potential risks of microplastic shedding, emissions of volatile organic compounds (VOCs) and polycyclic aromatic hydrocarbons (PAHs), and leaching of chemical additives under prolonged exposure to UV light and weathering. These unquantified risks make sustainability claims speculative at best, underscoring the need for more comprehensive studies that consider the full life cycle of plastic-modified asphalt.

9. Conclusions

This review advances the field of plastic-modified asphalt by offering a holistic, evidence-based synthesis that explicitly addresses limitations in prior literature. Unlike earlier reviews that primarily cataloged material types or reported isolated performance metrics, this work integrates microstructural behavior, rheological performance, field durability, environmental trade-offs, and economic viability into a unified analytical framework. The novelty lies in four key contributions:

Through a weighted analysis of global studies, we provide statistically grounded estimates of performance improvements (35–40% rutting resistance gain with LDPE/HDPE; 15–20% fatigue life extension with PP/PET), while highlighting critical trade-offs such as the brittleness induced by high-melting-point plastics like PET and PVC at low temperatures.

We differentiate wet and dry processing not only by performance outcomes but also by practical constraints, recommending optimal dosages (≤8% for wet process), mixing temperatures (160–190 °C depending on polymer), and compatibility strategies (glycolysis for PET, maleic anhydride grafting for polyolefins).

While confirming that waste-plastic-modified asphalt reduces global warming potential (GWP) by 20–30% compared to SBS-modified or conventional asphalt, we also foreground underreported risks such as microplastic shedding during pavement wear, toxic emissions from PVC/PS processing, and storage instability—thereby advocating for cradle-to-grave LCA frameworks.

The distinction between physical and chemical modifications in asphalt can be established through comparative analysis of the modification process. The microstructure of modified asphalt, together with the applied modification techniques and underlying mechanisms, can be effectively characterized using advanced microscopic methods. As highlighted throughout this review, asphalt modified solely with waste plastic is generally characterized by physical rather than chemical modification, a conclusion consistently supported by the analytical approaches.

The paper also underscores the economic potential of using recycled plastics in road construction, focusing on the cost savings from reduced bitumen use and long-term durability improvements. We further present novel directions for future research, including the development of compatibilizers for high-melting-point plastics and the need for standardized, large-scale field studies to assess the true sustainability of plastic-modified asphalt.

10. Future Recommendations

To advance the safe, scalable, and sustainable deployment of plastic-modified asphalt, the following specific, evidence-based actions are recommended: (1) use maleic anhydride-grafted polyolefins (PE-g-MA, PP-g-MA) or glycolysis-derived PET oligomers (e.g., BHETA) as compatibilizers for high-melting-point plastics; (2) adopt standardized processing protocols mixing temperatures of 160–170 °C for LDPE, 170–180 °C for PP, and ≤8% dosage for wet process; (3) implement mandatory field monitoring for microplastic shedding (via stormwater FTIR analysis) and toxic emissions (especially for PVC/PS), and (4) develop ASTM/AASHTO material standards “PMB-PE8” and ban PVC in asphalt due to HCl/dioxin risks. These steps will transform plastic-modified asphalt from a promising concept into a reproducible, sustainable, and policy-ready technology.