Crumb Rubber (CR) and Low-Density Polyethylene (LDPE)-Modified Asphalt Pavement Assessment: A Mechanical, Environmental, and Life Cycle Cost Analysis Study

Abstract

Due to the growing consumption of plastic and rubber products, effective waste management solutions are crucial. This study evaluates the use of crumb rubber (CR), low-density polyethylene (LDPE), and their combination (CR+LDPE), as asphalt binder modifiers for improving pavement performance and sustainability. The analyses covered two critical pavement layers: the wearing surface (WS) and the treated base (TB). The methodology included (1) Binder Development and Testing; (2) Superpave Mix Design; (3) mechanical testing, including Indirect Tensile Strength Testing and Semi-Circular Bending Testing; (4) life cycle cost analysis; and (5) carbon footprint analysis. The results revealed that CR+LDPE significantly enhanced the fatigue resistance of the TB mixes, with a fracture energy increase of 47%, and increased the flexibility index by 53% in the WS. CR increased the flexibility index by about 146% in the TB layer, while LDPE increased the fracture energy by 21% in the WS layer. The life cycle cost analysis demonstrated that using LDPE, CR, and CR+LDPE reduced the life cycle costs by about 16% in the WS layer. Meanwhile, the life cycle carbon footprint analysis showed that using LDPE and CR+LDPE reduced the carbon footprint by about 87% and 81% for the TB and WS layers, respectively. The study findings highlight the mechanical, economic, and environmental benefits of incorporating wastes into asphalt pavements.

1. Introduction

The asphalt pavement industry plays a critical role in the infrastructure industry. In the United States, there are over four million public centerline road miles, 63% of which are paved with asphalt [1]. It is worth noting that the terms bitumen and asphalt are often used interchangeably; however, bitumen refers to the binder, while asphalt refers to the composite of bitumen and aggregates (i.e., asphalt cement concrete mix). In the United States, the term asphalt refers to either the binder or the asphalt concrete mix.

Despite asphalt pavements’ utility and effectiveness in providing durable and reliable infrastructure, asphalt pavement poses significant environmental challenges throughout its life cycle due to the energy-intensive production of bitumen, high temperatures required for mixing, fuel use during transportation and construction, and emissions from maintenance and end-of-life processes [2,3]. Taking into consideration the scale and frequency of road construction on a global level, their environmental impact cannot be overlooked [2,3,4]. It also contributes to urban heat islands, global warming, and air quality degradation [5]. Moreover, the excessive use of resources to produce asphalt pavement depletes natural reserves [6,7]. The current warming phenomenon further intensifies these issues while accelerating pavement deterioration, leading to increased maintenance and resurfacing activities [8]. As such, if the pavement industry integrates sustainable approaches to mitigate some of the above-mentioned environmental impacts, it could significantly enhance sustainable infrastructure, reduce its carbon footprint, minimize resource depletion, and improve ecological resilience while maintaining its reliable functional role [9]. Additionally, such efforts would lead to life cycle cost savings in reduced maintenance and rehabilitation activities, all while promoting the development of eco-friendly construction materials and practices [10].

On another note, as of 2022, two billion tons of municipal solid waste (MSW) are generated annually, with a projection to reach 3.4 billion by 2055, a 70% increase [11]. Hence, waste management has become an issue of concern worldwide. Wastes filling landfills reduce the amount of livable space, contaminate the surrounding environment with its produced leachate, and pollute the air with its toxic airborne fumes [12]. As such, researchers are actively working to explore resilient means to manage, recycle, and repurpose waste materials in an attempt to mitigate environmental and health risks, as well as promote resilient development [13,14,15]. These efforts include investigating innovative recycling technologies, enhancing material processing methods, and identifying alternative applications for waste byproducts. Simultaneously, the pavement industry, with its massive and continuous production volume, has been identified as a prime candidate for integrating waste materials into its processes, offering an opportunity for enhancing resilience and resource efficiency [12,16,17].

In consequence, studies have explored the incorporation of sustainable alternatives to traditional asphalt modifiers due to their environmental merits and support for circular economy solutions. Among these are lignin, bio-oils, and other waste-based materials [18,19,20]. Two promising waste candidates, crumb rubber (CR) and low-density polyethylene (LDPE), have shown great potential as binder modifiers [21,22,23,24,25,26,27]. By utilizing crumb rubber from used tires and plastic waste from plastic bags, the industry can reduce its reliance on resources and minimize its carbon footprint while addressing waste management challenges [24,25,26,27]. These practices would minimize environmental impacts and enhance the resilience of pavement production by reducing landfill waste, conserving energy, and lowering greenhouse gas emissions, all while maintaining or even improving pavement performance.

Such studies have shown that waste materials can improve the rheological and mechanical properties of modified asphalt binders in conjunction with allowing a significant amount of industrial waste to be recycled. Crumb rubber and plastics have proven to enhance rutting, cracking, and aging performance [23,24,27,28]. Additionally, crumb rubber enables a reduction in tire/road noise emissions while reducing the binder WS layer thickness by 20–50 percent [29,30,31,32]. For instance, the authors of Ref. [3], after conducting laboratory and in situ testing on two specifically built field trials in Tuscany using crumb rubber-modified asphalt concrete for the WS, concluded that such mixes achieved optimal mechanical and functional performance, reduced tire noise, and provided greater durability when compared to conventional asphalt pavements.

However, despite extensive efforts and promising results in incorporating wastes into asphalt pavements, research gaps and challenges persist. One challenge is ensuring the optimal performance of these modified mixes under real-world conditions, specifically in terms of resistance to environmental factors such as temperature fluctuations and moisture [33]. Further investigation is needed to address issues such as the compliance of waste materials with the virgin binder in the mix, the interaction of more than one waste component with other mix components, and the potential for degradation over time [9,34,35]. The main challenge arises from the fact that waste materials alter different rheological properties of the base asphalt binder (e.g., viscosity and elasticity), which in turn affects the mechanical properties of the mixture (e.g., stiffness, tensile strength, and fatigue resistance). However, how these changes affect the overall performance of the pavement under different loading conditions and environmental factors is not yet fully understood [3,34]. In addition, the authors of Refs. [29,34] concluded that despite the efficiency of using CR and LDPE in HMA, certain conditions, such as their pre-treatment, could impair their physical properties, resulting in poor high-temperature performance. Hence, a more long-term performance investigation is required.

Moreover, there is a limited understanding of the combined effects of waste materials and mix design parameters on the mechanical and rheological properties of asphalt pavements [36]. While individual waste materials, such as CR, LDPE, and recycled aggregates, have been extensively investigated, their interactions with various mix design parameters, such as binder source, binder content, aggregate type/gradation, and compaction methods, are not fully explored [1,36]. For example, waste materials come from different sources and undergo different processing methods, which might result in significant variability in their field performance. Thus, without a thorough understanding of these material-specific variations, achieving predicted performance across different modified asphalt mixtures becomes challenging.

Another challenge in the waste material science field is understanding the complex interaction between the different waste materials and the hot asphalt mix components [37]. Interactions could be physical, chemical, or both, and this would alter the physical, chemical, and rheological characteristics of the output asphalt pavement, making it difficult to predict how they would behave under various traffic and climate conditions [38,39,40].

In the interim, there is a lack of standardized processing and testing procedures for incorporating different waste materials into hot asphalt mixes [1]. Published research mostly offers guidelines without any illustration or clear understanding of the combined effect. On the other hand, local agencies and Departments of Transportation (DOTs) often have their methods along with their predefined mix design based on their practice and expertise, which are difficult to adopt due to changed conditions such as the materials source, types, climate, and traffic conditions [41,42].

In addition, most previous studies on hot mix asphalt (HMA) research prioritized short-term performance analysis (i.e., Marshall stability or Superpave gyratory compaction) while overlooking long-term performance testing. Despite the importance of short-term testing, which gives insights into the mix properties and compatibility, it still lacks long-term performance testing, such as aging, fatigue, creep, and low-temperature cracking, accurately [43,44,45,46]. Most studies conducted on HMA often overlook crucial tests such as the Indirect Tensile Test (IDT) and the Semi-Circular Bending (SCB) test [47,48,49]. However, such tests are essential for evaluating the long-term performance of modified mixes, since they provide insights into the material’s resistance to cracking, tensile strength, moisture susceptibility, and overall durability [50,51,52,53,54,55].

In the meantime, while some agencies depend on using mechanistic–empirical (ME) methods to understand pavement’s long-term structural performance, they fail to gain detailed insights into the material’s cracking resistance and fracture toughness, as provided by IDT and SCB testing that focuses on thorough assessing of cracking, particularly thermal and fatigue cracking [30]. As such, it is essential to complement ME methods with fracture-based tests (e.g., IDT SCB) that more directly evaluate the cracking susceptibility of asphalt mixtures. It follows that the ME methods help design the pavement system with little understanding of the underlying factors. Conversely, IDT and SCB testing help evaluate the mix’s resilience to cracking and fracture behavior [50,52,56].

Most studies conducted on waste-modified HMA primarily focus on performance metrics (i.e., mechanical, rheological, chemical, and physical properties) and often neglect other significant assessments, such as environmental and cost [57,58,59,60,61]. For example, while selecting an optimum pavement material, life cycle cost analysis (LCCA) and greenhouse gas emissions quantification are essential, particularly in the context of resilient development. Integrating such methodologies provides a comprehensive understanding of waste-modified HMA pavement design, ensuring that decisions are made with long-term performance, cost-effectiveness, and resilience in mind [60,62]. Consequently, there is a need to integrate environmental and cost perspectives into innovative HMA research to aid decision makers in identifying materials that not only offer long-term durability and cost savings but also align with resilience goals. This approach ensures that infrastructure investments are economically and environmentally responsible, promoting the development of resilient and eco-friendly road networks.

Despite previous efforts to promote sustainability in the infrastructure industry, such as incorporating waste materials like CR and LDPE in asphalt mixes, key knowledge gaps remain regarding their understanding of performance and interactions of wastes with mix components. In addition, previous work lacks standardized methodologies, comprehensive mechanical testing, and integrated sustainability assessments. This study addresses these gaps by evaluating CR and LDPE-modified binders through rheological, mechanical, economic, and environmental analyses to support sustainable pavement design.

To achieve the abovementioned goal, the following objectives were set: (1) study the performance of waste-modified asphalt binders in terms of rheology, rutting, and fatigue cracking resistance; (2) assess the environmental impact through life cycle carbon footprint analysis; and (3) determine their associated life cycle costs. The presented framework will aid decision makers at both policy and industry levels in making informed decisions about the most suitable resilient pavement design based on a balance of mechanical performance, environmental impact, and long-term cost efficiency, which can further promote wide improvements in resilience and performance standards.

2. Materials and Methods

2.1. Material Properties & Processing

The materials used in this study were mainly asphalt cement, aggregates, and waste products such as LDPE and CR.

Table 1. Materials’ chemical and physical properties.

Material	Properties
Asphalt Binder	Egyptian asphalt binder is sourced from the Alexandria Oil Company in Alexandria. Penetration Grade 60/70, with penetration depth of 6.5 mm at 25 °C. Softening point temperature of 51 °C. Rotational viscosity of 0.797 Pa.s at 135 °C, and performance grade of PG 64-22.
LDPE	Shredded LDPE from second-grade plastic bags of particle size passing sieve #50 and retained on sieve #100. The material has a density of 0.93 g/cm3, a melting point of 115 °C, a thickness of 0.024 mm, and a tensile strength of 10.2 MPa.
CR	Ground CR with particle size passing sieve #50 and retained on sieve #100. The material has a density of 1.15 g/cm, a thickness of 0.5 mm, and a tensile strength of 8 MPa.
Aggregates	Limestone coarse aggregates were obtained from a local limestone quarry in Egypt. The Bulk Specific Gravity of coarse aggregates is 2.439. The fine aggregates were obtained by crushing larger aggregates; no natural sand was used. Fine aggregates’ Bulk Specific Gravity is 2.535 (ASTM C128-22 [63]). Powder mineral filler used passes sieve #200.

shows the various materials used and their properties. All aggregates used in this study were provided by the same supplier, Emar El Delta Group, which extracts the aggregates from a local limestone quarry in Cairo. The fine aggregates were obtained by crushing larger aggregates; no natural sand was used. All aggregates were washed before use to remove fines (i.e., particles smaller than sieve #200) to avoid the associated bonding problems that result from their coating.

The CR and the LDPE wastes used in this study were obtained from waste collection centers in Cairo.

2.2. Research Methodology

The research methodology proposed herein comprises five phases, as shown in Figure 1: (1) Waste-Modified Binder Blends Development and Rheological Testing, using crumb rubber (CR), low-density polyethylene (LDPE), a combination of both wastes (CR+LDPE), and a virgin binder (control); (2) Superpave HMA design; (3) mechanical testing, including Indirect Tensile Testing (IDT) and Semi-Circular Bending Testing (SCB); (4) cost analysis (LCCA); and (5) environmental analysis using Life Cycle Carbon Foot Print (LCCFA). The study’s Experimental Characterization and Analysis part is presented in the first three phases, while the economic and Environmental analysis part is presented in the final two.

2.2.1. Waste-Modified Binder Blend Development and Rheological Testing

The first phase focuses on producing the various blends of the waste–asphalt binders. A CR-modified binder was developed by incorporating 7% crumb rubber (CR) by weight of the asphalt, while an LDPE-modified binder used 5% low-density polyethylene (LDPE). A hybrid-modified binder was formulated by combining 3% CR and 3% LDPE. Such percentages were selected based on previous laboratory trials with waste dosages (3%, 5%, 7%, and 10%) [27,28,64]. The optimum content was determined using rotational viscosity testing results and performance-grade (PG) evaluations to ensure compliance with viscosity specifications to achieve optimal binder performance. All prior studies were conducted using the same source materials and tested in the same laboratory setting [27,28,64].

In this study, CR and LDPE waste materials were added to a pre-heated virgin asphalt at 165 °C. A Silverstone L5M-A shear mixer was used to mix the blend gradually for two hours at 2000 rpm to reach a thoroughly mixed blend. Proceeding with this, a full asphalt binder rheological characterization was conducted to compare the binder blends to the control (virgin) asphalt binder. To simulate the transportation and laying of the asphalt pavement as well as the oxidization and moisture effect over 5–10 years, tests were conducted at three different aging levels: unaged, short-term aging after the Rolling Thin Film Oven (RTFO) test following (ASTM D 2872-70 [65], Humboldt Mfg. Co., Elgin, IL, USA) aging after the Pressure-Aging Vessel (PAV) test following (ASTM D6521-22 [66], Humboldt Mfg. Co., Elgin, IL, USA) standards. Additionally, rotational viscosity test (AASHTO T 316 [67], Humboldt Mfg. Co., Elgin, IL, USA), Dynamic Shear Rheometer test (AASHTO T 315 [68], Humboldt Mfg. Co., Elgin, IL, USA), and Bending Beam Rheometer test (AASHTO T 313 [69], Humboldt Mfg. Co., Elgin, IL, USA) were conducted to fully characterize the rheological properties of the developed blends.

2.2.2. Superpave HMA Design

The second phase of the methodology entails the HMA design using the previously produced binder blends. Two aggregate structures, corresponding to two pavement layers, WS and TB, were selected, following the Superpave Mix Design, as per the Asphalt Institute SP-2 [11], identifying the optimum asphalt content (OAC) of each HMA along with the different volumetric properties. Hence, four asphalt binders—three modified with CR, LDPE, and a combination of CR+LDPE, along with one unmodified (virgin) binder as the control—were used to design the two pavement layers. Each pavement layer (WS and TB) was designed using the four asphalt binders independently, resulting in a total of eight different combinations of mixtures.

2.2.3. HMA Mechanical Testing

In the third phase, the various HMAs designed in phase 2 were characterized using the Indirect Tensile Strength (IDT) test, as per (ASTM D6931-17 [70], Humboldt Mfg. Co., Elgin, IL, USA), and the Semi-circular Bending Test (SCB), as per (ASTM D8044-16 [71], Humboldt Mfg. Co., Elgin, IL, USA). First, the IDT test was conducted on the various HMA mixes. Three samples were fabricated at the OAC of each HMA mix and then tested to determine their dry and wet indirect tensile strengths along with their Tensile Strength Ratios (TSR). Hence, 48 samples were used (3 samples × 8 mixes × 2 conditions; dry/wet) to conduct the IDT test, according to (ASTM D6931-17 [70], Humboldt Mfg. Co., Elgin, IL, USA). For each HMA, three samples were tested in their dry condition, while the other three were tested after being soaked in water for 24 h. During testing, a vertical compressive ramp load was applied to the samples until failure; then, the obtained values were averaged to calculate the IDT strength as an indicator of the mix strength, rutting, and crack resistance.

After calculating the indirect tensile strength for dry and soaked conditions, the Tensile Strength Ratio (TSR) was calculated by dividing the wet indirect tensile strength by the dry indirect tensile strength, evaluating the HMA’s moisture susceptibility. A higher TSR value, typically greater than 80%, indicates the asphalt mixture has good resistance to moisture-induced damage and is less susceptible to stripping or loss of adhesion. A lower TSR value suggests that the mixture is more prone to moisture-related distress.

The SCB cylindrical samples were cut to a half-disk with an engineered single crack (notch) of 38 mm depth, and the test was conducted using an MTS-universal testing machine with a three-point bending setup. Following the test, a sample output graph (Figure 2) was plotted, representing load versus displacement. Two metrics were assessed from this test, which include the fracture energy and the flexibility index.

Fracture energy is calculated using Equation (1), where G_f is the fracture energy (J/m²); W_f represents the work of fracture and is computed using the area under the load–displacement curve; and Area_Lig is the ligament area, which is computed by multiplying the ligament length by the thickness of specimen, where the ligament length describes the portion of the specimen ahead of the pre-cut notch.

$$\mathrm{G}\mathrm{f}={\displaystyle \frac{\mathrm{W}\mathrm{f}}{{\mathrm{A}\mathrm{r}\mathrm{e}\mathrm{a}}_{\mathrm{L}\mathrm{i}\mathrm{g}}}}$$

(1)

The flexibility index (FI) is calculated to characterize resistance to crack propagation, using Equation (2) [72], where $\left(m\right)$ is the absolute value of the slope of the unloading curve at the point of inflection, and A is a conversion factor used in the study, which is 0.01.

$$\mathrm{F}\mathrm{I}={\displaystyle \frac{\mathrm{G}\mathrm{f}}{\mathrm{m}}}\times \mathrm{A}$$

(2)

While the first three phases investigate the experimental characterization of the modified asphalt binders and mixes, the last two phases (phases 4 and 5) concentrate on economic and environmental analysis, respectively.

2.2.4. Cost Analysis

Life cycle cost analysis (LCCA) was implemented in phase 4 and showcases the relationship between costs, timing of costs, and discount rates [60]. Upon identifying the lifetime analysis of 40 years, the future cost was discounted to the base year and added to the initial costs to determine the net present value (NPV) for each LCCA alternative. The basic NPV formula for discounting discrete future amounts at various points in time back to the base year is shown in Equation (3), as per [57].

$$\mathrm{NPV}=\text{initial cost} +\sum \left[\text{Maintenance costs}{\displaystyle \frac{{\left(1+\mathrm{i}\right)}^{\mathrm{n}}-1}{\mathrm{i}{\left(1+\mathrm{i}\right)}^{\mathrm{n}}}}\right]$$

(3)

The initial costs are determined per square meter of each asphalt pavement alternative (including both WS, WS, and TB, TB), and include the unit pricing of sourced resources, such as aggregates, asphalt, waste materials, transportation, labor, and equipment. These costs were obtained from the local Egyptian market as of January 2024. The cost calculation in this study is categorized into four primary components: materials, labor, equipment, and maintenance costs. Each of these components is calculated based on specific criteria, such as material volumes, labor requirements, and equipment usage. The price of one cubic meter of each material within the pavement layer is determined based on the market price units. Hence, the cost of each pavement layer per square meter is determined by multiplying the total price of the mix per cubic meter by the layer thickness (i.e., 50 mm for WS and 60 mm for the TB).

On the other hand, labor costs are calculated based on the number of laborers required for specific tasks and the cost of labor per day. Once the number of laborers is established, the total labor cost is determined by multiplying the number of laborers by the daily wage and the duration of the task. Alternatively, equipment costs associated with laying down pavement and compaction using pavers and compactors are determined. Additional factors, such as transportation, are also integrated into the overall initial cost.

In the meantime, maintenance costs per pavement design alternative are calculated and discounted to present worth for the pavement study period of 40 years. Unmodified asphalt pavements in this study were expected to be rehabilitated using a 7-year replacement frequency, whereas modified pavements were expected to need a 12-year frequency maintenance plan, including 2% surface cracks sealing and patching, accounting for a total of 20% of their initial costs. This estimation of pavement performance is based on previous studies conducted on Egyptian road performance [73,74] and experience, considering factors such as traffic loading, climate conditions, material properties, and construction quality in Egypt. Finally, the discounted rate used in the study was 28 percent, as per the Central Bank of Egypt, reflecting the high cost of capital and economic conditions specific to the region.

2.2.5. Environmental Analysis

Aside from cost perspectives, this study found it essential to assess the environmental impact of each waste-modified pavement alternative. To investigate the ecological impact of each proposed pavement design, a life cycle carbon footprint (LCCF) analysis was conducted. LCCF considers materials (e.g., binder, aggregates, and additives), production, transportation, construction, and maintenance. Once the initial carbon emissions for each material were quantified, these values were summed to provide the total carbon footprint for each alternative pavement design along its period of study, as shown in Equation (4), where material quantity represents the amount of bitumen, aggregates, or wastes per square meters; emission factor presents the kg CO₂ per unit weight of material; and process emission reflects emissions from mixing, transportation, and construction. In addition, at each maintenance interval, the emissions generated, including those from repairs and rehabilitation, are added to the total emissions.

At each maintenance frequency, emissions incurred, including repairs and maintenance, are added to the initial sum. In this study, such data are acquired from [75].

$$\text{Total Emission}=\sum \left(\text{material quantity x emission factor}\right)+\text{Process Emissions}$$

(4)

3. Results

The results of this study are presented in the order of the proposed methodology phases. In this section, the study evaluates the performance of the control and modified binders in terms of their enhanced thermal stability, fracture properties, crack resistance, and sustainability.

3.1. Waste Modified Binder Blends Development and Characterizaton

The three waste-modified asphalt blends (CR, LDPE, and a hybrid of CR+LDPE) were developed using the above-mentioned methodology and were rheologically characterized and compared to the control (virgin) asphalt binder. The performance grade of each binder blend type was determined after conducting high-temperature DSR testing (unaged binders and RTFO-aged binders) and low-temperature BBR testing (PAV-aged binders). In addition, a rotational viscosity test was performed on all blends, and the compiled results are presented in

Table 2. Rheological testing results.

Binder	Performance Grade	Apparent Viscosity in cP @ 135 °C
Virgin Asphalt	64–22	796.7
CR-Modified Asphalt	64–12	315
LDPE-Modified Asphalt	70–18	650
CR + LDPE-Modified Asphalt	76–22	2361

.

Table 2 shows that LDPE enhanced the high-end performance grade of PG 70 compared to the reference virgin asphalt PG 64. Conversely, CR did not significantly affect the high-end performance grade but affected the low-grade performance grade, yielding a PG 64-12. It is worth mentioning that CR alone may not significantly enhance rutting resistance when its elasticity is insufficient at elevated service temperatures. This might be attributed to the fact that rubber particles soften when heated, which compromises their structural integrity. In contrast, other additives, such as polymers, are specifically engineered to maintain stiffness and enhance rutting resistance under high-temperature conditions.

In the meantime, adding a combination of CR and LDPE increased the binder’s high-performance grade from PG 64-22 to PG 76-22. Hence, LDPE+CR resulted in the most promising performance grade among all studied modified asphalt binders.

On the other hand, the rotational viscosity (RV) values of all asphalt binder types ascertain their good workability during construction, as all values passed the criterion of max 3000 cP as per (AASHTO A 316, Humboldt Mfg. Co., Elgin, IL, USA). The viscosity of CR+LDPE was recorded as the highest viscosity among all other asphalt binder types. Based on the measured binder properties, all modified asphalt binders had satisfactory performance grades and workability and hence can be used to design and produce various modified HMA mixes.

3.2. Superpave HMA Design Resuls and Analysis

An initial step in designing HMA according to Superpave is to evaluate the properties of the aggregates. The Los Angeles Abrasion Test (ASTM C 131-06 [76], Humboldt Mfg. Co., Elgin, IL, USA) yielded an abrasion loss percentage of 31% and aggregate flakiness and elongation indices of 14% and 20%, respectively, following the (ASTM D4791-19 [77], Humboldt Mfg. Co., Elgin, IL, USA). The bulk-specific gravities of each of the coarse and fine aggregates were determined to be 2.439 and 2.535, respectively, considering the ASTM C 127-24 [78], Humboldt Mfg. Co., Elgin, IL, USA) standards. The determined aggregate properties passed the specification requirements and hence can be used satisfactorily to produce hot asphalt mixes.

As per the Superpave design methodology, the aggregate structure should be specified first at a fixed asphalt content, and then the OAC, by testing various asphalt contents, should be determined for the specified aggregate structure (gradation). Figure 3 and Figure 4 show the 0.45 power chart of the selected aggregate gradations for WS and TB layers. Both selected aggregate gradations satisfied the Egyptian code and the Superpave control points’ requirements for the aggregate Nominal Maximum Aggregate size (NMAS).

The volumetric properties of the designed HMA mixes were verified at the OAC, as depicted in

Table 3. OAC and volumetric properties of TB and WS layer mixes.

Mix Type	Virgin Asphalt Mix (Control Mix)		CR-Modified Asphalt Mix		LDPE-Modified Asphalt Mix		CR+LDPE-Modified Asphalt Mix
Layer Type	TB	WS	TB	WS	TB	WS	TB	WS
OAC, %	3.2	5	3.4	4.9	3.5	5.5	4.5	5
VMA % (<13%)	10.4	11.7	10.5	13	11	13	10.6	12
VFA % (65–75)	65	70	65	75	65	75	70	70
Gmm @Ni % (<=89)	88	89	88.4	87	88.8	87	88.5	88
Gmm @Nm % (<=98)	97.2	97.4	96.9	97.3	97	97	97.4	97.2

for both TB and WS. It is clear that for both the TB and the WS layers, the modified asphalt resulted in a slight increase in the Voids in Mineral Aggregates (VMA) due to the modified asphalt binders’ ability to change aggregate coating and compaction characteristics. It is also depicted that all the modified asphalt binder mixes had satisfactory volumetric properties within the specification requirements of the Superpave Mix Design method.

3.3. HMA Mechanical Characterization

All eight previously designed and produced HMA (three waste-modified asphalt mixes and one control mix for two asphalt layers; TB and WS) were subjected to two performance tests to characterize their behavior. Figure 5 and Figure 6 show the IDT results for TB and WS under dry and wet conditions, respectively.

The results in Figure 5 and Figure 6 indicate that CR and LDPE mixes had higher dry IDT than the control (virgin) mix for both the TB and WS. On the other hand, the hybrid use of CR+LDPE reduced the dry and wet IDT for the TB. The CR+LDPE mix achieved a lower dry IDT for the TB but a higher dry IDT for the WS. This can be justified by the aggregate combination structure used for each pavement layer. For example, the coarser aggregate gradation used for the TB provides a strong structural foundation yet provides ineffective filling of the voids, leading to weaker tensile strength when compared to the denser gradation with finer aggregates used for the WS, which allows for better binder coverage.

As for the wet IDT, all modified asphalt mixes had a reduced wet IDT for the WS, while only CR and LDPE mixes had a minor rise in the wet IDT for the TB. This means that the modified binders improved the mix’s dry strength but reduced its resistance to moisture damage. While modifications improved the dry tensile strength, the mix appeared overly stiff, which might have caused micro-cracking under wet conditions, as observed by the testing team. These cracks facilitate water infiltration, accelerating damage and reducing wet IDT values.

To profoundly study the moisture susceptibility of the different mixtures, TSR values were compared to the control (virgin) mix, as shown in Figure 7. For the TB, waste-modified asphalt mixes acquired acceptable moisture resistance, passing the specification requirements of a minimum of 80%. Conversely, when examining the WS, the waste-modified asphalt mixes had a significantly lower TSR than the control mix, failing the minimum required threshold value of 80%. This can be attributed to the effect of waste modification, which reduced the moisture resistance significantly. However, the insignificant effect of the modification on the TB layer may be attributed to the coarser aggregate configuration, which facilitates water drainage, reducing the moisture damage risk compared to the WS layer with a denser and finer aggregate structure.

A possible solution to improve the moisture resistance of the WS is to use liquid anti-stripping agents or hydrated lime. Based on the study findings, it can be noticed that the aggregate gradations (structure) of the different pavement layers had a significant effect on the modified asphalt mixes’ performance in terms of the wet IDT and the TSR results. Findings of the IDT mechanical testing confirm that all modified asphalt mixes are recommended to be used as a TB rather than a WS layer.

On the other hand, the SCB test was conducted to measure the fracture energy (FE) of each HMA. It is important to note that the more energy required to cause a complete fracture, the greater the fracture toughness (the ability of the asphalt mix to resist cracking under load). Additionally, the flexibility index (FI) offered a measure of the mixture’s resistance to cracking by evaluating both the fracture energy and the rate of crack propagation (the higher the FI, the more ductile and crack-resistant the asphalt mixture). Both parameters (FE and FI) were considered to evaluate the studied asphalt mixtures’ anticipated crack resistance.

Modifying the TB layer with CR+LDPE improved the FE by 47.4%, whereas using CR and LDPE individually reduced it by 21.1% and 42.1%, respectively. In the WS, the LDPE increased the FE by 20.7%, while CR and CR+LDPE reduced it by 17.2% and 20.7%, respectively. These results are reflected in Figure 8 and Figure 9, where CR+LDPE and LDPE mixes had the highest FE for the TB and WS layers, likely justified by LDPE’s presence, which typically provides higher toughness and cohesion due to its improved stiffness and elasticity properties.

FI results shown in Figure 10 indicate that CR mix enhanced the performance for the TB, which witnessed a growth of 145.5%, but LDPE and CR+LDPE mixes experienced a drop of 45% and 9.1%, respectively. Meanwhile, all mixes used in the WS, except for the CR+LDPE mix, exhibited a decrease in their FI, with the LDPE mix decreasing by 62.3% and the CR mix decreasing by 19.9%; in contrast, CR+LDPE increased FI by 53.4% as illustrated in Figure 11.

These results highlight that LDPE enhances FE but compromises FI due to the higher stiffness and brittleness of LDPE. In other words, the LDPE mix absorbs a lot of energy before cracking, but once it starts cracking, it fails rapidly, like a tough but brittle material (e.g., glass). Alternatively, CR and CR+LDPE mixes had the highest FI for both layers due to the rubber particles’ nature, which increases flexibility. In other words, the CR mix has low toughness, yet fails gradually, not suddenly, and flexes well despite exhibiting low fracture energy. The synergistic effect of combining both LDPE and CR, especially for the WS, gave the most promising results, with higher FI and FE. Therefore, the CR+LDPE-modified binder is the most promising modified asphalt mix for WS applications based on the SCB test results.

Finally, based on the overall mechanical performance tests conducted in this study, it is evident that LDPE-modified mixes exhibited the highest strength concerning the dry IDT for both the TB and WS layers. While accounting for moisture exposure, it was found that the modified asphalt mixes were more susceptible to moisture and thus displayed reduced moisture resistance compared to the virgin asphalt mixes in terms of both the wet IDT and TSR values. This underscores the necessity for anti-stripping agents when using modified asphalt mixes to improve their moisture resistance. Conversely, the fatigue behavior analysis indicates that all modified asphalt mixes outperformed the virgin asphalt mixes in both the TB and WS layers. The CR+LDPE-modified mix was determined to be the most promising for the WS in terms of anticipated fatigue performance, highlighted by the significantly higher flexibility index.

3.4. Cost Analysis Results

Figure 12 illustrates the cost study findings, presenting the life cycle costs computed for the different combinations of pavement layer types (i.e., virgin pavement (with no modification), CR pavement, LDPE pavement, and a combination of crumb rubber and LDPE (CR+LDPE) Pavement). The cost analysis was performed for the WS and the TB in terms of EGP per square meter (EGP/m²) and was computed along the pavement’s life cycle, which is 40 years. It is worth noting that price differences between TB and WS within the same pavement type are attributed to variations in thickness, binder percent, and percentage of waste replacement.

A detailed examination of the results in Figure 12 indicates that virgin pavement had the highest cost, with the WS costing 1176 EGP/m² and the TB costing 1128 EGP/m². This suggests that conventional pavement materials incur higher costs over their life cycle compared to modified alternatives. This is expected and can be attributed to several key factors. First, traditional asphalt mixes rely on a higher percentage of bitumen, which is more expensive than the costs of other materials, especially when compared to waste materials. Second, the frequent need for rehabilitation over time in traditional pavements adds to the cost dynamics. In contrast, pavements that do not incorporate advanced or modified binders deteriorate more quickly under traffic and climate conditions, leading to a higher frequency of required maintenance and complete rehabilitation, reaching five rehabilitation cycles per pavement lifetime (i.e., 40 years), as explained previously. Therefore, based on the discussion presented, it can be stated that waste-modified mixes demonstrate an economic advantage when used as WS and TB compared to traditional pavement mixes.

Comparing the three waste-modified pavement types, it can be noted that although the LCC of the studied alternatives was close, the lowest LCC was depicted in the CR pavement in both the WS and TB, with values of 986 EGP/m² and 965 EGP/m², respectively. This CR-cost-effectiveness is linked to the relatively higher initial expense of LDPE wastes compared to CR, primarily due to their additional processing requirements that increase their upfront costs. Additionally, CR had a higher bitumen binder replacement (7%) compared to CR+LDPE (6%), leading to greater cost savings caused by reduced asphalt cement (AC) content.

3.5. Environmental Analysis Results

Environmental analysis using life cycle carbon footprint (LCCF) was conducted for each pavement alternative for the WS and TB, and the results are demonstrated in Figure 13. LCCF refers to the total amount of carbon emissions generated throughout the entire life cycle of the pavement in kg CO₂e/m² for the TB and WS, along with their service life, respectively.

It is clear in Figure 13 that for the TB layers, LDPE followed by CR+LDPE mixes had the lowest carbon emissions, with reductions of 87% and 81% of carbon footprint, respectively, when compared to the virgin asphalt pavement (control mix). Also, it is clear that all waste-modified mixes had significantly reduced carbon emissions compared to the control mix.

Additionally, Figure 13 illustrates that the WS LDPE and CR+LDPE mixes achieved the lowest carbon emissions of 1.25 and 1.4 kg CO₂e/m², with potential savings of 87% and 81%, respectively, compared to the control mix.

Since there were four different mixes for each of the WS and the TB (virgin asphalt mix, CR-modified asphalt mix, LDPE-modified asphalt mix, and CR+LDPE-modified asphalt mix), there were sixteen (16) (4 WS × 4 TB) different combinations of these mixes that can be used in real-life applications. These sixteen (16) combinations were considered to be thoroughly analyzed in terms of their environmental impact (carbon footprint). The LCCF for all possible pavement cross sections, considering all different mix combinations, is shown in Figure 14.

The difference between the initial emissions and the LCCF is that the former refers to the emissions resulting from the pavement construction process, while the latter pertains to the maintenance and rehabilitation of existing pavement.

Figure 14 illustrates that the unmodified asphalt pavement, the traditional control option, had the highest carbon emissions when considering both the initial and life cycle carbon footprint (LCCF). Therefore, using modified asphalt pavement mixes can reduce a significant environmental burden. This burden was greatly reduced by including recycled materials, such as LDPE and CR, in the asphalt binders, whether used individually or in combination.

Taking a closer look at Figure 14, the traditional pavement (unmodified HMA on the TB and WS) was the highest carbon emitter in both the short and long term. In contrast, the CR+LDPE mix produced the lowest carbon emissions. Among the various combinations, the pavement with LDPE in both WS and TB layers, followed by LDPE in WS and CR+LDPE in the TB layer, provided the lowest life cycle carbon emissions. Other options that showed promising environmental results included CR in the TB and LDPE in the WS, and vice versa (LDPE in the TB and CR in the WS), as well as CR in the TB and CR+LDPE in the WS.

4. Conclusions

This research promotes sustainable development by using recycled waste in asphalt mixtures, reducing landfill waste, improving pavement durability, lowering maintenance costs, and reducing carbon emissions. Specifically, this study examined the use of waste-modified asphalt binders containing crumb rubber (CR), low-density polyethylene (LDPE), and their hybrid combination (CR+LDPE) to enhance the performance and resilience of pavements. Hence, these modified binders were developed according to SuperPave specifications, focusing on two key pavement layers: the wearing surface (WS) and treated base (TB). Rheological and mechanical testing were conducted to assess performance, whereas life cycle cost and carbon footprint analyses were conducted to evaluate their environmental and financial impacts.

Rheological testing highlighted that modifying the virgin asphalt binder (PG 64-22) with CR+LDPE and LDPE improved their high-temperature performance grade (PG 76-22) and (PG70-18), respectively, with a moderate improvement for CR (PG64-12). These improvements reflect enhanced elasticity and stiffness.

Mechanical testing demonstrated that LDPE increased the Indirect Tensile Strength (IDT) by 23.1% for TB and 29.8% for the WS. Although waste modification had no significant impact on the moisture susceptibility of the mixes in the TB layer, it notably influenced the performance of the WS layer. CR, LDPE, and CR+LDPE reduced the WS moisture resistance, with Tensile Strength Ratios (TSR) of 0.70, 0.58, and 0.55, less than the threshold of 0.80. This disparity was attributed to the differences in the aggregate structure (gradation) between the WS and TB layers. This underscores the necessity of using anti-stripping agents when using modified asphalt mixes to improve their moisture resistance.

In response to the low TSR values, a possible solution to improve the moisture resistance of the WS layer is to use liquid anti-stripping agents or hydrated lime. Hydrated lime, when incorporated into the asphalt binder, increases adhesion between the binder and the aggregates, thereby reducing the stripping potential [79]. Furthermore, the modification of the binder using hydrated lime can also reduce the oxidative aging of the binder, resulting in enhanced durability. Some of the liquid anti-stripping agents, such as amine or silane-based, also increase adhesion, especially when used with moisture-sensitive aggregates. Hence, future studies should consider the above-mentioned additives in future studies to account for the high potential of stripping action in the modified asphalt.

Modifying the TB layer with CR+LDPE improved the fracture energy by 47.4%, whereas using CR and LDPE individually reduced it by 21.1% and 42.1%, respectively. In the WS, the LDPE increased the fracture energy by 20.7%, while CR and CR+LDPE reduced it by 17.2% and 20.7%, respectively. On the other hand, CR-modified TB mix improved the flexibility index by 145.5%, while LDPE and CR+LDPE mixes reduced it by 45% and 9.1%, respectively. In the WS layer, the CR+LDPE showed an enhanced flexibility index, with an increase of 53.4%, whereas the LDPE and CR mix showed a reduction of 62.3% and 19.9%, respectively.

Focusing on the cost aspect, CR and/or LDPE resulted in life cycle cost reductions of about 14% to 16% compared to traditional (non-modified) pavements. Additionally, the environmental analysis results confirmed that LDPE and CR+LDPE reduced the life cycle carbon footprint by about 87% and 81%, respectively. These findings underscore that using CR and LDPE-modified binders enhances rheological, mechanical, and sustainability merits, with significant costs and carbon footprint savings, though attention to moisture resistance in the WS layers is needed.

Finally, this study was conducted under controlled laboratory conditions, which may not fully replicate field performance influenced by traffic and environmental factors. The results of this study indicate that future research should include field validation to confirm the same performance under service conditions. In addition, this study focused on short-term aging, which should be complemented by long-term investigations to assess the aging behavior.