Incorporation of E-Waste Plastics into Asphalt: A Review of the Materials, Methods, and Impacts

Abstract

This paper presents a comprehensive review of the environmentally friendly management and reutilization of electronic waste (e-waste) plastics in flexible pavement construction. The discussion begins with an overview of e-waste management challenges and outlines key recycling approaches for converting plastic waste into asphalt-compatible materials. This review then discusses the types of e-waste plastics used for asphalt modification, their incorporation methods, and compatibility challenges. Physical and chemical treatment techniques, including the use of free radical initiators, are then explored for improving dispersion and performance. Additionally, in situations where advanced pretreatment methods are not applicable due to cost, safety, or technical constraints, the application of alternative approaches, such as the use of low-cost complementary additives, is discussed as a practical solution to enhance compatibility and performance. Finally, the influence of e-waste plastics on the conventional and rheological properties of asphalt binders, as well as the performance of asphalt mixtures, is also evaluated. Findings indicate that e-waste plastics, when combined with appropriate pretreatment methods and complementary additives, can enhance workability, cold-weather cracking resistance, high-temperature anti-rutting performance, and resistance against moisture-induced damage while also offering environmental and economic benefits. This review highlights the potential of e-waste plastics as sustainable asphalt modifiers and provides insights across the full utilization pathway, from recovery to in-field performance.

1. Introduction

Rapid global economic growth and technological advancements have led to the continuous introduction of new electrical and electronic products, rendering older devices obsolete at an accelerating rate. In recent years, growing urbanization, increased mobility, further industrialization, and higher levels of disposable incomes have led to a rapid increase in the consumption of electronic equipment worldwide. With the ongoing expansion of the global electronic equipment market, the lifespans of equipment are getting shorter. After the usage and disposal of electronic equipment, a complex waste stream emerges, comprising both toxic substances and recoverable resources. This waste stream, which includes common electronic products such as computers, televisions, monitors, VCRs, copiers, fax machines, hard drives, and other similar products, is referred to as electronic waste (e-waste). E-waste is growing at an accelerating pace annually and is now recognized as the fastest-growing form of waste globally [1,2,3,4,5].

End-of-life processing for this waste stream is an extremely challenging problem worldwide owing to its high consumption rate, large volume, short lifespan, and non-biodegradable structure. Moreover, the rapid growth of this technology is resulting in the obsolescence of many electronic products each and every day. According to the Global E-waste Monitor 2020 report, an estimated 53.6 million metric tons (Mt) of e-waste were generated globally in 2019 alone. This marks an increase of 9.2 Mt since 2014, with projections indicating that the total volume could reach 74.7 Mt by 2030. In stark contrast, only 9.3 Mt of e-waste, representing just 17.4% of the total generated, was formally documented as collected and recycled in 2019. The data reveal that e-waste recycling efforts are falling behind the rapid global increase in its generation. Rising e-waste levels present significant threats to environmental and public health, since many hazardous substances, including toxic metals such as barium (Ba), beryllium (Be), cadmium (Cd), cobalt (Co), chromium (Cr), copper (Cu), iron (Fe), lead (Pb), lithium (Li), and mercury (Hg), as well as persistent organic pollutants (POPs) such as dioxin, brominated flame retardants (BFRs), and polybrominated dibenzo-p-dioxins and dibenzofurans (PBDD/Fs), are associated with these streams [2,6]. On the other hand, e-waste contains various valuable materials, including plastics that can be recovered and reused. However, recycling plastics from e-waste, which can account for up to 20% of the total stream, poses greater challenges than conventional plastic recycling, primarily due to the presence of BFRs [5,7]. Therefore, it is vital to not only increase the collection and recycling rate of e-waste and its environmentally friendly management but also identify alternative applications where e-waste can be utilized in a sustainable manner [2,8].

At the same time, the use of different waste products in asphalt binders and mixtures has become widespread due to their environmental and economic benefits. In recent years, researchers in the asphalt industry have placed a special emphasis on the utilization of different waste materials, such as waste plastics, waste crumb rubber, or bio-oils from different sources, in road pavements to not only alleviate the landfilling problems of these waste materials but also turn them into value-added new materials that can enhance the performance and durability of the pavements [9,10,11,12,13].

The use of waste plastics in road pavements is receiving growing attention, driven by the enhanced performance of asphalt binders and mixtures modified with plastic waste, as well as the associated economic and environmental benefits [9]. The use of e-waste plastics in road pavement applications has been the subject of growing interest among researchers. Reutilizing these non-degradable and potentially hazardous materials in pavement construction not only helps alleviate the burden on landfills and reduce environmental impacts but also contributes to significant resource savings [8,14].

This study explores how e-waste plastics can be integrated into asphalt production by examining and reviewing each stage of their reuse process, from the recovery stage of the plastics to their performance. After briefly discussing multiple perspectives on e-waste generation, handling, reuse, and the associated problems and challenges with these procedures, e-waste plastic recycling methods, as well as the available e-waste plastics for utilization in the asphalt industry and their incorporation methods, are reviewed.

Another part of the discussion also focuses on several key stages with the potential to significantly impact binder-plastic compatibility and the overall performance of e-waste plastic-modified asphalt binders. The study then evaluates the impact of e-waste plastics on the fundamental and rheological characteristics of asphalt binders, along with the mechanical performance of the resulting modified mixtures.

A review of the different stages in the utilization process of e-waste plastics can provide a better understanding of how to successfully incorporate these waste materials into road construction.

2. Recycling of E-Waste Plastics

The current pathway for plastic recycling primarily relies on mechanical processes. After being collected, the materials typically undergo a series of steps, such as sorting by physical or chemical properties (e.g., shape, density, or color), washing to eliminate contaminants, and grinding to reduce their size into flakes. Depending on the waste type and processing setup, these steps may vary in sequence or repetition. In many cases, plastics are compacted into bales for easier transport before further processing. Finally, the recycled flakes may be reprocessed through compounding and pelletizing to create a granulated material that is more suitable for manufacturing new products. This conversion of waste into usable raw material is commonly referred to as the “end-of-waste” phase [15]. On the other hand, the emphasis on e-waste recycling stems not only from the need for proper waste management but also from the recovery of valuable materials, such as polymers, which make up approximately 20% of e-waste by weight [16]. Generally, there are three major stages in the recycling of e-waste. The first stage involves the retrieval of electronic devices that have reached the end of their useful life. Next, the plastic residues and wastes undergo pretreatment procedures, such as washing and drying, sorting, and melt filtration, to reduce their size and remove their paint and coating. This stage plays a critical role in e-waste recycling, as plastic residues and wastes can only be reprocessed effectively when subjected to appropriate pretreatment methods. Removing the paints and coatings from the plastic residues can be a challenging procedure since it can potentially result in the degradation of the properties of recycled plastics. One of the promising paint removal methods involves chemical treatment with heated aqueous solutions, which can remove surface coatings without damaging the polymer substrates. In the last step of e-waste recycling, methods such as density-based sorting, advanced spectroscopic-based sorting, electrostatic sorting, and froth filtration are utilized for the sorting and separation of different e-waste components [8,17].

According to the [2] report, since 2014, the number of countries that have adopted legislation and regulations for the collection, treatment, recovery, reuse, and recycling of e-waste has increased from 61 to 78. This aims to reduce the landfill disposal of e-waste and increase the recovery and recycling rates. Alongside legislative pressures, other factors, such as rising costs, poor biodegradability, and the generation of explosive greenhouse gases such as methane, have turned the landfill disposal of e-waste into the last desirable resort. Considering the fact that the percentage of plastics in different electronic products ranges from 15% to 30%, it is obvious that the target of increasing the recycling rate of e-waste cannot be fulfilled solely by recycling glass or metals from the waste stream. Therefore, it is vital to develop efficient ways to recover and recycle plastics [2,16].

Around 20% of total e-waste consists of plastics, with flame-retardant types accounting for 5% and the remaining 15% being non-flame-retardant [7]. Broadly, the recovery of plastics from e-waste can be grouped into material-based and energy-based strategies. Materials recovery includes methods such as mechanical recycling and chemical recycling. The goal of energy recovery is to utilize the high energy content of polymers through the incineration of plastic waste. Chemical recycling, which includes methods such as pyrolysis and hydrothermal treatment, aims to convert e-waste plastics into fuel. Among the available methods, mechanical recycling is the most commonly employed technique for reprocessing plastics into similar products without altering their fundamental properties. However, this approach is generally limited to manually sorted polymers that are free of brominated flame retardants (BFRs), as their presence can lead to the formation of harmful PBDD/Fs during processing [7,18].

3. Application of Recycled E-Waste Plastics in Asphalt

3.1. Available E-Waste Plastics

E-waste plastics frequently considered for incorporation into asphalt pavements include acrylonitrile butadiene styrene (ABS), acrylonitrile butadiene styrene-polycarbonate (ABS-PC), high-impact polystyrene (HIPS), polycarbonate (PC), polystyrene (PS), polypropylene (PP), polyethylene (PE), styrene acrylonitrile (SAN), polyurethane (PU), polyamide (PA), and polyphenylene oxide (HIPS/PPO). The conventional mechanical recycling method is well-suited for widely available e-waste plastics such as ABS, HIPS, and PP, which are generated in substantial quantities. Certain brominated flame retardants commonly present in older e-waste plastics, such as ABS and HIPS, pose environmental and health risks during recycling. Any improper handling can lead to toxic emissions, environmental contamination, and human exposure, highlighting the need for safer processing methods and further toxicological assessment [19,20,21].

ABS, the most widely used plastic in electronic product manufacturing, is known for its excellent impact strength, chemical resistance, toughness, and rigidity. PC is known for its superior transparency, high thermal stability, excellent impact resistance, and strong tolerance to heat-induced deformation. The incorporation of PE and PP has been shown to significantly improve material rigidity and strengthen resistance to long-term deformation under load. However, it has also been reported that the addition of PP reduces the ductility of modified asphalt binders and has a negative effect on their resistance against fatigue. PP-modified asphalt demonstrates superior performance in high-temperature regions, making it a suitable choice for such environments. PU has also been reported to enhance the resistance against rutting, fatigue, and aging. Conversely, PS possesses a melting point above the standard processing temperatures for modified binders and is therefore suitable for dry process modification. PS is reported to bring high rigidity to asphalt mixtures, which can be problematic in terms of the resistance against thermal cracking in cold areas [8,9,21,22,23,24].

3.2. Incorporation Methods of E-Waste Plastics

There are two primary methods for incorporating e-waste plastics into asphalt pavements, the wet process and the dry process, both of which have been widely explored in research and practice. In the wet process, plastics are introduced directly into the asphalt binder, where they act as modifiers to enhance the rheological behavior and mechanical properties. In this method, plastics are melted into the binder under elevated temperatures, leading to better dispersion and interaction in the asphalt matrix. In contrast, the dry process involves adding plastics to the asphalt mixture during the mixing stage, as a partial replacement for aggregates, which is in contrast to chemically modifying the binder [8,9,21]. Since most existing studies involving e-waste plastics have employed the wet modification process, greater focus will be placed on that method. However, both approaches and their relevant applications will be discussed in detail in the following sub-sections.

3.2.1. Dry Modification Method

In the dry modification process, waste plastics with melting points higher than typical asphalt production temperatures are introduced directly into the hot aggregate stream during production as an aggregate replacement. Generally, the dry method involves adding waste plastics to the hot aggregates first, followed by the base asphalt, after which all components are then mixed to produce the final plastic-modified asphalt mixture [25,26,27]. The effects of the dry process can vary depending on the type and properties of the plastic used. For example, certain plastics can seal the surface of aggregates, resulting in plastic-coated aggregates that exhibit reduced water absorption and improved surface characteristics. Additionally, the dry process has been reported to have a notable operational flexibility by allowing higher dosages of plastics without requiring major changes to the asphalt mixture production process and plant setup. According to Miranda et al., regardless of the plastic’s type, form, dosage, or its role as binder additive or aggregate replacement, the dry method remains a viable alternative for pavement construction and rehabilitation [28].

In addition, the dry process has been reported to offer simpler production, easier transportation, and improved energy efficiency. However, due to the limited interaction time between the plastic and binder, and the potential for non-uniform dispersion, the dry process may result in less effective modification compared to the wet method [27,29,30]. While numerous studies have investigated the use of recycled plastics from different sources, such as household items, only a limited number have focused specifically on e-waste plastics. For instance, Kumar et al. examined the non-metallic fraction of electronic printed circuit boards, in which 10 mm chips were used as a substitute for coarse aggregates in asphalt mixtures [31]. Despite a performance evaluation, the study did not specify the exact types of polymers present in the non-metallic fraction. Similarly, in a study by Murugan based on particle size analysis, it was reported that e-waste plastic particles could be used as partial replacements for coarse aggregates in asphalt mixtures [32]. While these studies reflect the use of the dry process, insufficient detail regarding polymer characterization limits the ability to draw conclusive insights for practical implementation.

3.2.2. Wet Modification Method

Wet modification of asphalt binders with e-waste plastics includes several key stages that can potentially play a vital role in the compatibility of the two materials and the performance of the final e-waste plastic-modified asphalt binder. Generally, the first step includes the crushing, shredding, and pulverizing of the bulk e-waste plastics to reduce their size. Before this process, e-waste containing materials such as lead, lithium, copper, and aluminum is separated, and the rest is placed in an industrial shredder or pulverizer to start the process of size reduction [31,33]. Promoting a more brittle behavior of plastics in the grinding process can also result in smaller particles. This can be achieved by using liquid nitrogen to lower the temperature below the glass transition point of the plastics, thereby freezing and embrittling the particles. In other words, when the ambient temperature of the plastics is below Tg, the molecular chains of the plastics are frozen in place and behave like solid glass. As a result, grinding a plastic that is in a solid glassy state can yield smaller particles. Following their size reduction, the smaller particles can be sieved to obtain a plastic powder for wet modification [8,21,34,35]. Despite performance benefits, which will be discussed in detail in Section 4, certain limitations, including the potential for phase separation, complex preparation procedures, the need for specialized mixing equipment, and increased energy consumption, have been identified as the main limitations for the wet modification of asphalt with plastics [27,36,37]. A summary of the processing methods of different e-waste plastic powders, including plastic types, the powder size, and the processing method, has been presented in

Table 1. Processing methods for different e-waste plastic powders. Data from [8,21,31,33,35,38,39,40].

Researcher	E-Waste Source	Plastic Type	Powder Size	Processing Method(s)
Colbert and You, 2012 [35]	Waste recycled computers	ABS HIPS	<300 µm	Before sieving, an industrial-grade plastic shredder was used to reduce the size of the plastics. Then, a pulverizer was used to pulverize the smaller particles into plastic.
Colbert, 2012 [21]	Waste recycled computers	ABS PC HIPS	<300 µm	Prior to the final sieving, a two-step size reduction process was employed. First, an industrial shredder was used to break down bulk plastics; this was followed by secondary grinding of the smaller e-waste plastic fragments, which had been treated with liquid nitrogen to facilitate brittleness and improve grinding efficiency.
Mohd Hasan et al., 2016 [8]	Waste recycled computers	ABS PC HIPS	-	Before the final sieving stage, a two-step process was used: bulk plastics were initially shredded using an industrial shredder, followed by treatment of the smaller fragments with liquid nitrogen and subsequent grinding in an industrial mill.
Shahane and Bhosale, 2019 [38]	-	-	<300 µm	E-waste powder was directly acquired from a plastic recycling company.
Singh et al., 2020 [39]	-	ABS	<375 µm	-
Santhanam et al., 2020 [33]	PC boards, phones, and other electronic appliances	-	-	After separating e-waste components that contain heavy metals and other potentially hazardous materials, the remaining plastic-rich fraction was crushed into a fine e-waste powder.
Kumar et al., 2021 [31]	-	-	-	After the e-waste from electronic printed circuit boards was collected and separated from the e-waste containing harmful metals such as lead or lithium, it was crushed into powder.
Mohammadi et al., 2025 [40]	Printed circuit boards	ABS	<250	ABS plastics were pretreated with liquid nitrogen and ground multiple times in an industrial grinder. Shredded plastics were sieved using a No. 60 sieve.

. As shown in the table, the final size for most of the e-waste plastic powders has been below 300 µm.

After obtaining the powder, e-waste plastic powders are blended and melted into neat asphalt binders using a high-shear mixer under a suitable temperature and with a proper mixing time and speed. The selection of a suitable mixing temperature is an important factor in the mixing process of e-waste plastics and the asphalt binder, since the melting temperature of different polymers can vary significantly depending on the polymer source and thermal history. For example, the melting point for HIPS is reported to be in the range of 180 to 260 °C [8]. Overall, it has been reported that waste plastics, which have melting temperatures below the preparation temperatures of asphalt binders in a hot-mix asphalt plant, are more suitable for the wet process. On the other hand, the dry method, in which the waste plastic particles are incorporated prior to the addition of the asphalt binder, is more suitable for hard and rigid plastic types that have higher melting points [41]. A summary of the main preparation parameters in the modification procedure of asphalt binders with e-waste plastics is presented in

Table 2. Preparation parameters for the modification of asphalt binders with e-waste plastics. Data from [8,31,33,35,38,39,40,42].

Researcher	E-Waste Type	E-Waste Contents (%)	Control Binder	Mixing Temperature (°C)	Mixing Time (min)	Mixing Speed (rpm)
Colbert and You, 2012 [35]	ABS HIPS	2.5, 5, 15	USA: PG 58–28	-	30	-
Xia et al., 2012 [42]	-	2, 4, 6, 8, 10	-	-	-	-
Mohd Hasan et al., 2016 [8]	ABS PC HIPS	5, 15	USA: PG 1 58–28	-	15 45	3000 5000
Shahane and Bhosale, 2019 [38]	-	2, 2.5, 4, 5, 6	India: VG 2 30	175–180	40	150
Singh et al., 2020 [39]	ABS	1, 2, 3, 4, 5	India: VG 30	165	30	2000
Santhanam et al., 2020 [33]	-	5, 10, 15, 20	India: VG 30	-	-	-
Kumar et al., 2021 [31]	-	6, 12, 18	Pen. Grade 3 60/70	-	-	-
Mohammadi et al., 2025 [40]	ABS	2, 5	PG 58–28	200	15	5000

¹ Performance grade. ² Viscosity grade. ³ Penetration grade.

.

3.3. Compatibility Between E-Waste Plastics and Asphalt

One of the key challenges in incorporating e-waste plastics into road pavements is achieving adequate compatibility with asphalt binders. This compatibility is essential for the successful performance of plastic-modified asphalt. However, a major limitation in this process arises from the inherent differences between plastics and asphalt binders, particularly in terms of density, molecular weight, polarity, and solubility. These differences often result in poor interactions between the two materials. Consequently, during the blending process, no chemical reaction occurs between the asphalt binder and the plastic, leading to potential phase separation and storage stability issues [34,43]. Studies have suggested that the compatibility between these two groups of materials can be enhanced with the aid of chemical methods, such as grafting the functional groups, which can form reactions between plastics and the asphaltene organic functional groups in asphalt, leading to improved compatibility between these materials [44,45,46].

A common manifestation of incompatibility between recycled e-waste plastics and asphalt binders is the development of a dense plastic film on the binder surface, which impedes uniform dispersion and can contribute to early binder degradation or failure. The molecular weight of e-waste plastics is also a critical factor influencing their compatibility and overall performance. For example, it has been reported that fine particles of plastics with high-weight molecules can improve the creep properties of asphalt binders and lead to higher resistance against rutting in asphalt mixtures [8,21].

Poor compatibility between plastics and asphalt binders, which can adversely affect storage stability, is largely influenced by the chemical structure and reactivity of the plastics. To address this issue, certain pretreatment methods are applied prior to modification. These methods aim to improve compatibility by promoting stronger bonding and interactions between the plastic materials and the asphalt binder [44,45]. In the past, studies on the application of different waste plastics in asphalt, chemical pretreatment methods, such as functionalization and grafting, irradiation, or the addition of certain crosslinking agents, have been successfully applied to improve the compatibility between waste plastics and asphalt binders [46,47,48,49].

Similarly, the chemical treatment of e-waste modifiers through the inclusion of free radical initiators such as cumene hydroperoxide (CHP) has also been proposed prior to the blending of e-waste powders with neat asphalt binders to facilitate better interactions between materials and enhance intermolecular interactions and the effective molecular weight of e-waste-modified asphalt binders. According to the study conducted by Mohd Hasan et al., the chemical treatment of e-waste powders can enhance the molecular bonding between e-waste plastics and the asphalt binder and improve the rheological performance of the e-waste-modified asphalt binders [8]. The inclusion of free radical initiators, such as potassium persulfate, sodium peroxide, and pentaerythritol, was also investigated by Xia et al. in research that focused on the optimization of asphalt binder behavior in terms of softening point and ductility. Their results showed that with the addition of potassium persulfate as a free radical initiator at a blending content of 6% for e-waste modifiers, the ductility and softening point of the e-waste-modified asphalt binders can be optimized [42].

While these studies highlight notable progress in improving compatibility through chemical pretreatment, several practical challenges still remain for the successful modification of e-waste plastics, particularly when considering field- and plant-level implementation:

• Despite promising research, there is a lack of simple, cost-effective, and scalable pretreatment methods suitable for asphalt plant operations and field applications;
• The proposed methods, specifically the use of radical initiators, require specialized equipment, trained personnel, and strict safety protocols, limiting their suitability for asphalt plant operations;
• These limitations, when combined with the known complexity of wet mixing operations, pose significant limitations for scaling up the use of e-waste plastics in flexible pavement construction.

To overcome these key limitations, Mohammadi et al. proposed a practical approach to incorporate ABS plastics into asphalt by focusing on two key elements: the cryogenic grinding of plastic particles and the addition of carbon black derived from post-consumer waste tires as a complementary additive. This combined strategy aimed to reduce the density differences between plastics and the asphalt binder and improve rheological performance while maintaining feasibility for plant-scale implementation [40]. While the proposed approach offers a significant step toward practical implementation, several critical research gaps still need to be addressed to ensure broader applicability and long-term performance. In particular, the role of complementary additives such as compatibilizers should be further investigated across different types and forms of e-waste plastics to determine their effectiveness in enhancing compatibility, especially given that the majority of existing studies have predominantly focused on ABS plastics. Additionally, more comprehensive laboratory evaluations are needed to assess phase separation tendencies and the storage stability of e-waste-modified binders. Techniques such as fluorescence microscopy can provide valuable insight into the dispersion and homogeneity of plastic modifiers within the asphalt binder [50], which are critical parameters for ensuring long-term performance and processing reliability.

4. Performance Evaluation

This section summarizes how different e-waste plastics and their types and blending contents can impact the performance at the asphalt binder and mixture levels. The performance assessment of the e-waste-modified asphalt binders mainly include a summary of their conventional properties, such as penetration, softening point, and ductility, and their rheological performance in terms of viscosity, rutting resistance, and cold-weather cracking resistance. Moreover, in the last part of this section, a review of their performance at the mixture level is presented.

4.1. Conventional Properties

Table 3. Conventional properties of e-waste-modified asphalt binders. Data from [31,33,38,39].

Researcher	E-Waste Type	E-Waste Content (%)	Penetration (0.1 mm)	Softening Point (°C)	Ductility (cm)
Shahane and Bhosale, 2019 [38]	-	0	69.0	52.2	78.0
		2	58.3	52.6	55.5
		2.5	56.3	52.6	50.3
		4	53.3	53.7	41.8
		5	50.7	55.0	40.7
		6	47.7	55.7	37.0
Singh et al., 2020 [39]	ABS	0	68.5	53.0	-
		1	53.0	73.5	-
		2	48.0	75.0	-
		3	31.0	76.5	-
		4	27.5	84.0	-
		5	40.5	80.0	-
Santhanam et al., 2020 [33]	-	0	-	54.0	38.0
		5	-	55.0	28.3
		10	-	66.7	25.6
		15	-	74.5	18.3
		20	-	85.0	15.4
Kumar et al., 2021 [31]	-	0	62.3	67.5	49.3
		6	63.0	69.0	46.7
		12	64.3	69.0	46.9
		18	60	70.5	46.0

presents a summary of the conventional properties, mainly in terms of penetration, softening point, and ductility. As shown in the table, the studies by Shahane and Bhosale and Singh et al. indicate that e-waste plastics have reduced the penetration value in comparison with the neat viscosity of the grade 30 asphalt binder [38,39]. For example, ABS powders with dosages of 1%, 2%, 3%, 4%, and 5% in the study by Singh et al. reduced the penetration of the VG 30 asphalt binder by 21.75%, 30.07%, 39.66%, 59.71%, and 41.90%. Moreover, the minimum penetration is observed at a blending content of 4%. On the contrary, test results by Kumar et al. show that there is an increase in the penetration of the 60/70 penetration-grade asphalt binder with the addition of the e-waste powder until the blending content of the e-waste powder reaches 12%. However, as the blending content of e-waste is increased to 18%, the e-waste-modified asphalt binder exhibits a lower penetration value than the base binder [31].

According to the softening point values, the addition of e-waste and the increase in its dosage led to higher softening point values. In the study conducted by Shahane and Bhosale, the softening point increased when the dosage of e-waste plastics exceeded 2.5% [38]. Singh et al. observed that as the ABS content increased, the softening point of the e-waste-modified asphalt binders also increased, with the highest value recorded at 4% ABS [39]. Similarly, the softening point test results by Santhanam et al. and Kumar et al. indicated that the addition of the e-waste plastic powder and the increase in its content resulted in higher softening point values [31,33]. Furthermore, as observed in Table 3, the ductility value of the e-waste-modified asphalt binders decreases with an increase in e-waste powder content. Overall, when compared to virgin binders, the e-waste-modified binders exhibited lower ductility values. Overall, the conventional test results highlight the stiffness of the binder and increase its brittleness.

4.2. Viscosity

A summary of the viscosity behavior of e-waste plastic-modified asphalt binders at different blending levels is presented in

Table 4. Summary of viscosity test results. Data from [21,31,33,35,38,39].

Researcher	E-Waste Type	E-Waste Contents (%)	Control Binder	Testing Temperature (°C)	Results
Colbert and You, 2012 [35]	ABS HIPS	2.5, 5, 15	USA: PG 58-28	135	Viscosity was increased
Colbert, 2012 [21]	ABS HIPS	2.5, 5, 15 (Untreated)   5 (Treated)	USA: PG 58-28	135	The addition of untreated e-waste powders increased the viscosity. However, the treatment of e-waste-modified binders reduced the viscosity to values that were comparable to the viscosity of the base binder.
Shahane and Bhosale, 2019 [38]	-	2, 2.5, 4, 5, 6	India: VG 30	60	Viscosity was increased.
Singh et al., 2020 [39]	ABS	1, 2, 3, 4, 5	India: VG 30	135, 145, 155, 165, 175	Viscosity was increased.
Santhanam et al., 2020 [33]	-	5, 10, 15, 20	India: VG 30	-	Viscosity was increased.
Kumar et al., 2021 [31]	-	6, 12, 18	Pen. Grade 60/70	-	Viscosity was increased.

. Overall, the addition of e-waste plastic powders has resulted in higher viscosity values compared to the base binders. According to the research conducted by Colbert and You, the addition of the ABS and HIPS powders with a blending content of 2.5% resulted in lower or similar viscosity values at 135 °C in comparison with the PG 58-28 asphalt binder. On the other hand, higher blending contents of ABS and HIPS resulted in higher viscosity values and mixing and compaction temperatures in comparison with the base PG 58-28 binder. Their results also showed that asphalt binders modified with HIPS exhibited higher viscosity, as well as elevated mixing and compaction temperatures, compared to those modified with ABS [35].

The test results of Singh et al. showed that the added e-waste powders increased the viscosity of the base binder. They also found that the sample with 4% ABS had the highest viscosity [39]. Similar results regarding the higher viscosity were also obtained in other studies [31,33,38].

Although the higher viscosity values of e-waste-modified binders can be an indication of their enhanced high-temperature properties, they can negatively affect the workability and pumpability and reduce the resistance against thermal cracking. In other research conducted by Colbert, it was reported that free radical initiators such as hydro peroxide can lower the viscosity and improve the workability and pumpability of the e-waste-modified asphalt binders. According to test results, treatment of the 5% ABS-modified asphalt binders reduced the viscosity by 89.1% compared to the untreated 5% ABS samples. Moreover, there was a 52% reduction in the viscosity values between 5% treated and untreated HIPS-modified asphalt binders. Studies reveal that although the addition of untreated e-waste powders can result in higher viscosities, the inclusion of free radical initiators can reduce the viscosity to values that are comparable with the viscosity of the base PG 58-28 asphalt binder and improve their workability and pumpability [21].

4.3. Rutting Resistance

The rutting resistance performance of different e-waste-modified asphalt binders is summarized in

Table 5. Summary of rutting resistance performance. Data from [8,35,38,39,40].

Researcher	E-Waste Type	E-Waste Contents (%)	Control Binder	Test Equipment	Evaluation Indicators	Results
Colbert and You, 2012 [35]	ABS HIPS	2.5, 5, 15	PG 58-28	DSR	G*, G*/sinδ	The rutting resistance was improved.
Mohd Hasan et al., 2016 [8]	ABS ABS-PC	5, 15 (Untreated)   5, 15 (Treated)	PG 58-28	DSR	G*, δ, G*/sinδ	Enhanced rutting resistance, especially in treated e-waste-modified samples.
Shahane and Bhosale, 2019 [38]	-	2, 4, 5, 6	VG 30	DSR	G*/sinδ	The rutting resistance was improved.
Singh et al., 2020 [39]	ABS	1, 2, 3, 4, 5	VG 30	DSR	G*/sinδ	The rutting resistance was improved.
Mohammadi et al., 2025 [40]	ABS	2, 5 (with and without carbon black)	PG 58-28	DSR	G*/sinδ	The rutting resistance was improved, and the addition of carbon black further improved performance.

. Generally, some promising outcomes were reported, especially for the treated e-waste-modified asphalt binders from the dynamic shear rheometer (DSR) test. Colbert and You evaluated the anti-rutting performance of asphalt binders modified with ABS and HIPS powders from recycled computer plastics with blending contents of 2.5%, 5%, and 15%. The results indicated that the addition of e-waste modifiers and the increase in their blending content improved the rutting resistance performance of the base PG 58-28 binder by at least two high-temperature grades [35]. Mohd Hasan et al. investigated the anti-rutting performance of e-waste-modified asphalt binders and the influence of free radical initiators on their properties. Overall, the results indicated that e-waste-modified asphalt binders exhibited enhanced high-temperature properties in comparison with the base PG 58-28 asphalt binder. Moreover, the addition of free radical initiators significantly enhanced the rutting resistance performance in the treated ABS- and ABS-PC-modified asphalt binders [8]. Singh et al. studied the rutting resistance properties of ABS-modified asphalt binders with blending contents of 1%, 2%, 3%, 4%, and 5% and reported that the rutting resistance performance of asphalt binders was improved after the addition of the e-waste modifier. Their results also demonstrated that the addition of ABS with a blending content of 4% exhibited the best anti-rutting performance [39]. Shahane and Bhosale also found that the anti-rutting performance of the VG 30 asphalt binder was improved with the addition of e-waste plastic powders [38]. Mohammadi et al. also found that the combination of complementary additives, such as carbon black with e-waste ABS plastics, can further improve the anti-rutting performance of asphalt binders [40].

4.4. Cold-Weather Cracking Resistance

The cold-weather cracking resistance performance of different e-waste-modified asphalt binders is summarized in

Table 6. Summary of cold-weather cracking resistance performance. Data from [8,35,40].

Researcher	E-Waste Type	E-Waste Contents (%)	Control Binder	Test Equipment	Evaluation Indicators	Results
Colbert and You, 2012 [35]	ABS HIPS	2.5, 5, 15	PG 58-28	BBR	Stiffness, m-value	Thermal cracking resistance was reduced.
Mohd Hasan et al., 2016 [8]	ABS HIPS	5 (Both treated and untreated)	PG 58-28	BBR	Stiffness, m-value	Thermal cracking resistance was reduced under both treated and untreated conditions.
Mohammadi et al., 2025 [40]	ABS	2, 5 (with and without carbon black)	PG 58-28	ABCD	Cracking temperature	Addition of ABS plastics improved performance, and further enhancement was observed when carbon black was added.

. In general, the cold-weather performance results were comparable in terms of creep stiffness and m-value with the unmodified binders. According to the bending beam rheometer (BBR) test results from the study by Colbert and You, although e-waste-modified asphalt binders demonstrated higher stiffness and lower m-value versus the base asphalt binder over the tested temperatures, the samples with lower percentages of e-waste powders performed similarly to the base PG 58-28 binder. Overall, e-waste-modified binders with 5% of ABS or HIPS exhibited better resistance against thermal cracking in comparison with other e-waste-modified samples, and as the blending content of e-waste powders was increased, the binders became stiffer and failed to meet the Superpave low-temperature performance specifications. Moreover, it was also indicated that HIPS-modified asphalt binders had higher stiffness values in comparison with the ABS-modified samples [35].

The outcomes of the research by Mohd Hasan et al. also indicated that the creep stiffness of the base PG 58-22 binder was slightly increased with the addition of ABS and HIPS with a blending content of 5%. Furthermore, the addition of the treated ABS and HIPS also resulted in higher creep stiffness values than the base binder. Overall, it was indicated that the treated e-waste-modified asphalt binders do not necessarily improve the thermal cracking resistance of the conventional asphalt binders and their low-temperature performance [8]. These results point to the fact that the low-temperature performance of the e-waste-modified asphalt binder is still a primary issue that limits its successful application and has not been addressed despite the utilization of pretreatment methods such as the addition of radical initiators. Additionally, Mohammadi et al. used the asphalt binder cracking device (ABCD) test to assess the cold-weather cracking temperature of e-waste-modified binders and reported that the cryogenic size reduction of ABS plastics combined with complementary additives such as carbon black can lead to superior performance when compared to the base PG 58-28 asphalt [40].

Overall, the results across different aspects of rheological performance highlight the complex effects of e-waste plastics on binder performance, underscoring the need to summarize key benefits, trade-offs, and areas requiring further research. In summary, the incorporation of e-waste plastics has been shown to increase viscosity and significantly enhance resistance to rutting, with further improvements observed when pretreatment strategies such as cryogenic size reduction, chemical treatment using radical initiators, and the addition of complementary additives such as carbon black are employed. While the stiffening effect of e-waste plastics may raise concerns for cold-weather cracking performance, studies have found it to remain comparable to conventional binders, and recent approaches have even demonstrated enhancements in this regard with the aid of complementary additives. However, the associated increase in viscosity can pose challenges related to storage stability, phase separation, and elevated mixing and compaction temperatures, potentially requiring adjustments in plant equipment or handling practices. Moreover, there remains a need for comprehensive testing protocols, including the cigar tube separation test, multiple stress creep recovery (MSCR), and linear amplitude sweep (LAS) tests, to fully assess phase separation, rutting, and fatigue performance.

4.5. Performance at the Asphalt Mixture Level

Multiple researchers have also investigated the influence of e-waste plastics on the performance of asphalt mixtures. As mentioned before, the incorporation of e-waste into asphalt mixtures has been studied in two principal categories of binder replacement and aggregate replacement. The influence of e-waste on the performance of asphalt mixtures is summarized in

Table 7. Summary of performance at the mixture level. Data from [14,21,31,32,33,38,51].

Researcher	Type of Utilization		E-Waste Contents (%)		Tests	Result Highlights
	Binder Replacement	Aggregate Replacement	By Weight of Binder	By Weight of Aggregates
Colbert, 2012 [21]	Yes	No	2.5, 5 Untreated and treated	-	APA   Flow Number   Dynamic Modulus   TSR	Significant improvement in resistance against rutting, especially in the treated samples.   Slight improvement in intermediate- and low-temperature performance.   Improvement in moisture resistance, especially in the treated samples.
Ranadive and Shinde, 2016 [51]	No	Yes	-	5, 10, 15	Marshal	Increase in Marshal stability and flow when replacing up to 10% of aggregates.
Surya et al., 2017 [14]	Yes	No	5, 10, 15	10, 20, 30	Marshal	Marshal stability was improved.
Murugan et al., 2018 [32]	No	Yes	-	4, 8, 12, 16	Marshal	Increase in Marshal stability and flow with the inclusion of e-waste plastics and the increase in their content.
Shahane and Bhosale, 2019 [38]	Yes	No	5	-	Marshal   Dynamic Modulus	Marshal stability was increased.   Resistance against fatigue cracking and rutting was improved.
Santhanam et al., 2020 [33]	Yes	No	5, 10, 15, 20	-	Marshal	Increase in Marshal stability and flow when replacing the asphalt binder with 5 to 10% of e-waste.
Kumar et al., 2021 [31]	Yes	Yes	6, 12, 18	5, 10, 15	Marshal	Increase in Marshal stability and flow when replacing aggregates with 10% of e-waste.

. In research conducted by Surya et al., after the partial replacement of the asphalt binder and coarse aggregates with recycled HIPS and e-waste, the performance of the asphalt mixtures was investigated. The results of the study indicated that the stability of the mixtures increased with the increment of the HIPS content in the asphalt binder, which was mostly due to the adhesiveness of the HIPS. Furthermore, inclusion of 15% HIPS by the weight of the asphalt binder and 20% e-waste by the weight of the aggregates led to the most desirable performance in terms of both Marshal stability and cost effectiveness [14]. Shahane and Bhosale investigated the performance of asphalt mixtures that contained e-waste plastic-modified binders with dynamic modulus and Marshal tests. The dosage of e-waste plastic modifiers in this study was 5%. The researchers reported that the stability was increased, and the resistance of the asphalt mixtures against fatigue cracking was improved when e-waste plastic powder was used as a binder modifier, resulting in a higher dynamic modulus value and a lower phase angle value. Furthermore, the modified mixtures exhibited a more elastic behavior and higher resistance against rutting in comparison with unmodified mixtures [38]. Santhanam et al. assessed the performance of asphalt mixtures in which the binder had been partially replaced with 5 to 20% of e-waste plastics. The outcomes of the research indicated that the strength of the mixtures that contained 5 to 10% of e-waste plastics was significantly improved [33].

In an investigation conducted by Colbert, the performance of the e-waste-modified asphalt mixtures was evaluated using different tests, including the asphalt pavement analyzer (APA), dynamic modulus, flow number, and tensile strength ratio (TSR). In this study, the asphalt mixtures contained both treated and untreated ABS- and HIPS-modified asphalt binders with blending contents of 2.5% and 5%. The APA results showed that samples containing treated modified asphalt binders with 5% of ABS and HIPS had lower rut depths in comparison with the control mixture and demonstrated the best resistance against rutting. Furthermore, there was an improvement in the flow number performance after the treatment of ABS and HIPS for a given percentage of the modified mixture. According to the E* results at intermediate temperatures, E-waste-modified mixtures exhibited improved or similar performance in comparison with the control mixtures. Moreover, the addition of treated e-waste plastics enhanced the moisture resistance of the mixtures [21].

Several other researchers have also investigated the use of recycled e-waste plastics as an aggregate replacement in asphalt mixtures. Ranadive and Shinde evaluated the influence of e-waste and fly ash on the strength properties of asphalt concrete as a filler replacement. In their study, e-waste with percentages of 5%, 10%, and 15% was utilized for the partial replacement of filler. According to their results, incorporation of up to 10% of e-waste alongside a 5.5% asphalt binder led to higher Marshal stability and flow values [51]. Murugan studied the utilization of recycled e-waste plastic granules as a replacement for coarse aggregates and reported that Marshal stability and flow values were increased significantly with the addition of e-waste particles and an increase in their percentages by the weight of the aggregates. Moreover, it was indicated that the addition of 12% e-waste particles at the optimum binder content of 5.5% resulted in the highest Marshal stability, which was approximately three times higher than the conventional mix that was tested in the study [32]. In research conducted by Kumar et al., after modifying the asphalt binder with e-waste powders from electronic printed circuit boards and determining the optimal value of 12% by the weight of the asphalt binder for the e-waste modifiers, e-waste chips with percentages of 5%, 10%, and 15% were used as a replacement for coarse aggregates to assess the strength and stability of the mixtures. The outcomes of the study indicated that the addition of 10% e-waste chips by the weight of aggregates resulted in enhanced stability. The optimum binder content in their study was also 5% [31].

Despite promising improvements at the binder-level performance evaluation, significant gaps remain in evaluating the performance of e-waste plastic-modified asphalt at the mixture and field scales. Field demonstration projects are particularly necessary to assess the long-term durability, constructability, and real-world behavior of these materials. In addition, future studies should explore key mixture-level tests, such as the disk-shaped compact tension (DCT) test for low-temperature cracking resistance, the Hamburg wheel tracking device (HWTD) test for rutting and moisture susceptibility, and field noise and distress evaluations [11].

5. Conclusions

This research aimed to review the application of e-waste plastics in the construction of flexible pavements. First, the approaches for the disposal and management of e-waste and the associated challenges were discussed. Then, the recycling methods for the e-waste plastics, available e-waste plastics for the modification of asphalt binders and mixtures, and their incorporation methods and compatibility issues were discussed. Finally, the performance of e-waste-modified asphalt binders and mixtures was assessed. Based on this research, the following conclusions can be drawn:

∘

The incorporation of e-waste plastics into asphalt binders and mixtures is a promising method for their environmentally friendly management that can relieve the pressure on landfills, lead to substantial savings in resources, and contribute towards the goal of increasing the recycling rate of e-waste streams.

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The successful incorporation of e-waste plastics into flexible pavements relies heavily on their compatibility with asphalt binders and mixtures. Chemical treatment of e-waste modifiers through the inclusion of free radical initiators such as cumene hydroperoxide has been one of the successful methods that can improve the intermolecular interaction and molecular weight of e-waste-modified asphalt binders and promote the compatibility between e-waste plastics and asphalt binders. Additionally, the use of carbon black as a complementary additive has shown potential in enhancing dispersion and improving overall compatibility.

∘

The addition of e-waste plastics increases the viscosity and enhances the asphalt binder’s resistance against rutting. Further improvements in terms of anti-rutting performance have been reported when pretreatment methods such as cryogenic size reduction and the incorporation of complementary additives such as carbon black are employed. Additionally, chemical treatment using radical initiators has also been shown to enhance performance.

∘

In addition to superior rutting performance, the chemical treatment of e-waste modifiers and the inclusion of free radical initiators such as cumene hydroperoxide have led to some promising results in the rheological performance of e-waste-modified asphalt binders. Enhanced workability and lower mixing and compaction temperatures in comparison with the untreated e-waste-modified samples have been observed with the addition of free radical initiators.

∘

Although the addition of e-waste plastic stiffens the asphalt binder under low-temperature conditions, the cold-weather cracking resistant performance of e-waste-modified asphalt binders is still comparable with that of conventional asphalt binders. Overall, without the addition of complementary additives, e-waste-modified asphalt binders with lower percentages of e-waste plastic powders exhibit better low-temperature performance. Moreover, the incorporation of complementary additives such as carbon black combined with proper cryogenic size reduction procedures has led to further enhancements in the cold-weather cracking resistance of e-waste plastic-modified asphalt binders.

∘

The incorporation of e-waste plastics as a binder or aggregate replacement into asphalt mixtures has increased the Marshal stability and flow of the mixtures and enhanced their resistance against rutting and fatigue cracking. Moreover, the treatment of the e-waste plastics and inclusion of free radical initiators before the addition of the binder to asphalt mixtures have improved the high- and intermediate-temperature performance of asphalt mixtures and increased their moisture resistance.

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Existing research has predominantly focused on ABS plastics, with limited exploration of other polymer types and forms commonly found in the non-metallic fraction of e-waste streams. Furthermore, despite recent efforts regarding the combined use of complementary additives and e-waste plastics, there remains a critical lack of simple, scalable, and cost-effective pretreatment strategies that are compatible with asphalt plant operations or field-level implementation.

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At the asphalt binder level, key performance concerns, including storage stability, phase separation, and the long-term durability of e-waste-modified binders, have not been sufficiently addressed. Additionally, advanced characterization methods such as fluorescence microscopy, which are critical for assessing dispersion and compatibility, remain underutilized in the current research.

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Important mixture-level performance assessments, such as the DCT test for low-temperature cracking resistance, the HWTD for rutting and moisture susceptibility, and post-construction assessments of surface noise and pavement distress, are notably underexplored. These tests are critical for establishing the field performance of e-waste-modified asphalt mixtures and should be prioritized in future investigations to ensure reliable implementation under real-world conditions.

6. Recommendations for Future Research

Although current research highlights the potential of e-waste plastics in road construction, several challenges must still be addressed to enable broader implementation. Future studies should prioritize the development of simple, cost-effective methods that are feasible at the asphalt plant scale. In addition, identifying additives or modifiers as compatibilizers to enhance the performance of plastic-modified asphalt remains a key research need. Importantly, future research should also include comprehensive life cycle assessments (LCAs) of e-waste plastic-modified asphalt, with specific attention to both background processes (e.g., plastic collection and preprocessing) and foreground processes (e.g., asphalt production and application). Special emphasis should be placed on evaluating greenhouse gas emissions and energy use, particularly given that plastic modification may require elevated production and compaction temperatures. Studying the leaching behavior of e-waste-modified asphalt, especially with concerns regarding the leaching of microplastics into soil and water, can also be another direction in future studies.