Assessing the Viability of Waste Plastic Aggregate in Stone-Modified Asphalt Concrete Mix for Bus Rapid Transit Pavement Maintenance

Abstract

This research takes on a scientific problem originating from the pervasive deterioration observed in the pavements of Bus Rapid Transit (BRT) systems, which presents formidable challenges to their durability and imposes significant financial burdens on BRT organizations. While wear and tear on BRT pavements is a widely recognized concern, there exists a pronounced deficiency in sustainable solutions to address this issue comprehensively. This study endeavored to bridge this scientific gap by exploring the option of incorporating waste plastic aggregate (WPA) and recycled asphalt pavement (RAP) into the pavement material. The series of comprehensive investigations commenced with an assessment of modified binders. We identified a 25% extracted RAP binder as the most suitable candidate. Our research next determined that a 4% WPA content offers optimal results when used as an aggregate replacement in a stone-modified asphalt concrete mix, which is further refined with a 13 mm nominal maximum aggregate size (NMAS) gradation, resulting in superior performance. Under double-load conditions of the Hamburg Wheel Tracking test, rutting in the 10 mm NMAS mixture rapidly increased to 9 mm after 12,400 HWT cycles, while the 13 mm NMAS mixture showed a more gradual ascent to the same critical rutting level after 20,000 HWT cycles (a 61% increase). Real-world application at a designated BRT station area in Seoul reinforced the findings, revealing that the use of 13 mm NMAS with 4% WPA and RAP significantly improved performance, reducing rutting to 75 µm and enhancing pavement resilience. This configuration increased Road Bearing Capacity (RBC) to 5400 MPa at the center zone, showcasing superior load-bearing capability. Conversely, the 10 mm NMAS mixture without RAP and WPA experienced severe rutting (220 µm) and a 76% reduction in RBC to 1300 MPa, indicating diminished pavement durability. In general, this research highlights the need for innovative solutions to address BRT pavement maintenance challenges and offers a novel, environmentally friendly, and high-performance alternative to traditional methods.

1. Introduction

Pavement infrastructure is the lifeblood of modern transportation systems, connecting cities and regions [1], and facilitating the movement of people and goods [2]. In densely populated urban environments like Seoul, maintaining robust and sustainable pavements is paramount to operating efficient public transportation systems such as Bus Rapid Transit (BRT) [3,4]. However, these high-traffic zones, crucial for daily commuting and urban connectivity, often face substantial wear and tear, necessitating frequent maintenance, which demands significant financial resources [5].

BRT systems have emerged as an essential mode of public transportation in urban areas worldwide, offering efficient and sustainable solutions to growing mobility demands [6,7]. However, the pavement conditions at BRT bus stops have been a recurrent concern, as they experience substantial wear and tear, affecting passenger safety, ride comfort, and overall system performance [8,9]. The unique challenges encountered by BRT pavements set them apart from conventional asphalt pavements, primarily attributable to the substantial directional pressure, particularly at bus terminals. The constricted wheel path and the dimensions of buses necessitate meticulous weight distribution to ensure structural integrity [5]. Moreover, the frequent deceleration and acceleration of buses contribute significantly to the formation of substantial ruts, posing a potential risk to the durability of the road surface [8,9]. These pavements often exhibit various forms of distress, including rutting, cracking, fatigue, and loss of structural integrity [10,11].

In the operational scenario of BRT roads, the swift development of rutting and cracking, particularly in station areas, poses challenges shortly after deployment [4]. Intense rutting, influenced by frequent bus braking, triggers premature pavement failure. Previous research advocates the use of high-modulus asphalt concrete to tackle these challenges [12]. Nevertheless, variables such as axle weight, bus characteristics, and environmental temperature have a substantial impact on BRT asphalt pavement. Urban bus loads, characterized by distinctive weights and speeds [13], demand specialized analysis. Wang et al. [14] observed stress concentration in asphalt pavement, and Hajj et al. [9] established load equivalency factors for BRT buses under various conditions in Nevada. The primary reasons for this deterioration are the high-frequency loading imposed by frequent stops and the heavy axle loads of BRT vehicles [12]. Exposure to environmental factors such as temperature variations, rainfall, and humidity can exacerbate these issues, leading to a more rapid decline in pavement quality [15,16].

Researchers and practitioners have recognized the need to address these challenges and have explored numerous strategies to mitigate pavement damage [17,18]. Recent studies in asphalt pavement have explored innovative approaches to enhance performance and sustainability. Hoy et al. [19] evaluated asphalt pavement maintenance using recycled materials, emphasizing the benefits of integrating recycled asphalt pavement (RAP) with asphalt binders. Zhang et al. [20] provided a comprehensive review of phase change materials for temperature regulation in asphalt pavements. Sang et al. [21] conducted a factor analysis to assess asphalt pavement performance, specifically structural strength and hidden cracks. Suebsuk et al. [22] examined variations in cement-stabilized reclaimed asphalt pavement in tropical climates. Xue et al. [23] focused on pavement performance based on the separation technology of asphalt and aggregate in RAP. Sohail Jameel et al. [24] assessed asphalt mixtures modified with renewable oils and RAP for improved sustainability. Zhao et al. [25] investigated the impact of crumb rubber and reclaimed asphalt pavement on the viscoelastic properties of asphalt mixtures. Additionally, recent research [26,27,28,29,30] has conducted various studies on green pavements, rejuvenating agents, and rubberized asphalt mixtures. These studies collectively contribute significantly to advancing research into asphalt pavement, addressing aspects of performance, sustainability, and material innovation. Despite these efforts, there remains a demand for more comprehensive research to develop sustainable, resilient, and cost-effective solutions that ensure the durability of BRT bus stop pavements in diverse urban contexts [31]. This paper aims to contribute to the ongoing dialogue by investigating a novel approach that combines modified binders, recycled materials, and optimized mix designs to enhance pavement longevity and performance in BRT systems, with a particular focus on the bustling streets of Seoul, South Korea.

RAP is a key element in modern sustainable infrastructure [32]. It involves recycling old asphalt materials to create new mixtures, offering economic and environmental advantages by reducing the need for new resources and minimizing waste disposal [33]. RAP has broad applications in road construction, from highways to runways, and its careful incorporation into mixtures demands attention to quality, processing, and binder modifications to ensure performance and durability [34].

WPA is gaining recognition in the construction and building materials industry as a sustainable and versatile resource [35]. With growing concern for environmental sustainability, WPA is being incorporated into various construction materials, such as concrete, mortar, and asphalt, to enhance their properties [36]. WPA not only reduces the consumption of natural aggregates but also helps manage plastic waste, addressing two critical issues simultaneously [37]. Its lightweight and insulating properties make it ideal for use in construction materials, offering energy-efficient solutions [35]. Furthermore, WPA can enhance the durability, thermal resistance, and ductility of these materials, which aligns with the growing emphasis on eco-friendly and resilient building practices [38]. The introduction of WPA in construction and building materials demonstrates its potential to contribute to greener and more sustainable construction practices, offering a promising avenue for innovative, eco-conscious building solutions [39].

The development of sustainable pavements for bus stations at BRT systems presents a pressing need, driven by several limitations in current pavement materials. The existing pavements at BRT bus stations often suffer from severe damage, primarily due to high-frequency loading and wear, leading to increased maintenance costs. Moreover, these pavements do not fully address the growing concern for sustainability and environmental impact. To address these limitations, there is a compelling need for pavement materials that offer enhanced durability, resistance to rutting, improved moisture resistance, and reduced maintenance requirements. The integration of innovative materials, such as stone-modified asphalt (SMA) mixtures with WPA, RAP, and optimized binders, holds the potential to overcome these challenges, offering superior performance, environmental benefits, and cost-effectiveness. This research endeavors to meet the need for more sustainable and resilient pavement solutions at BRT bus stations while addressing the limitations of current materials to create lasting, eco-friendly infrastructure. The novelty of this research lies in its comprehensive investigation of integrating SMA mixtures with WPA, RAP, and optimized binders to develop sustainable and high-performance pavement solutions for bus stations at BRT systems, addressing the critical issues of durability, moisture resistance, and cost-effectiveness.

This study endeavors to tackle the challenges associated with deteriorating pavements at bus stations within BRT systems by formulating a sustainable and high-performance pavement solution for BRT bus stops in Seoul, South Korea. The primary objectives encompass evaluating the compatibility of SMA mixtures enhanced with varied WPA content percentages (2%, 4%, 6%), 25% to 50% extracted RAP binder, and an optimized asphalt binder fabricated from extracted RAP binder, to improve the binder’s properties. Moreover, the study aims to discern the most suitable nominal maximum aggregate size (NMAS) for the mix, with a focus on both 13 mm and 10 mm options. Additionally, the research seeks to appraise the long-term performance of the developed SMA mixtures under real-world BRT station conditions. To achieve these objectives, the study employs a systematic approach, including rheology tests, the Indirect Tension Strength Test, Hamburg Wheel Tracking tests, dynamic modulus tests, and falling weight deflectometer tests. By fulfilling these aims, this research aspires to make a substantial contribution to advancing sustainable BRT pavement solutions, addressing issues related to pavement durability, moisture resistance, and cost-effectiveness.

2. Materials and Methods

2.1. Materials

2.1.1. Recovery of RAP Binder

In this research, the extracted RAP binder content was examined at two levels, specifically 25% and 50%. The primary objective is to encourage the utilization of reclaimed pavement materials, aligning with sustainability goals [32]. Once the optimal RAP binder content is determined, it will be incorporated into the final binder mixture, which will then be blended with the WPA, further promoting environmentally sustainable practices.

In this study, RAP mixture compatibility was assessed using a RAP material supplied by select asphalt plants in Seoul, South Korea. RAP materials from these sources underwent detailed analysis to determine asphalt content and absolute viscosity through extraction sieving and ASTM-standardized tests (ASTM D 2172 [40] and ASTM D 5404 [41]).

Low-viscosity recycled asphalt aggregates with an absolute viscosity of around 50,000 Poise, were identified. The general extracted asphalt content of RAP was around 5.3%. This meticulous analysis enabled the precise selection of RAP materials compatible with subsequent modification processes, facilitating the development of sustainable pavement solutions for Seoul’s BRT systems.

2.1.2. Development of Combined Mixture

This research is dedicated to the development of SMA mixtures to enhance the sustainability of pavement solutions at bus stations in Seoul. The study explores the compatibility of two SMA mixtures with different nominal maximum aggregate sizes (NMAS): 13 mm SMA, commonly used in BRT lanes in Seoul, and 10 mm SMA, selected for its superior plastic deformation resistance based on preliminary laboratory tests.

To achieve this objective, a comprehensive series of laboratory experiments was conducted, including the dynamic modulus test and Hamburg Wheel Tracking test. The SMA mixtures require PG 76-22 asphalt binder (see

Table 1. Results of PG76-22 tests.

Test	Result	Standard
G*/sinδ; at 76 °C (Original) [44]	1.66 kPa	$\ge$1.0 kPa
G*/sinδ at 76 °C (after RTFO) [44]	2.48 kPa	$\ge$2.3 kPa
G* × sinδ at 76 °C (after PAV) [45]	1652 kPa	$\le$5200 kPa
Stiffness at −20 °C [46]	179 MPa	$\le$290 MPa
m-value at −20 °C [46]	0.34	Min. 0.35
Viscosity at 140 °C [47]	0.30 Pa․s	Max. 3.2 Pa․s
Flash Point [48]	327 °C	Min. 235 °C [43]
Density (20 °C) [49]	1037 kg/m3	[44]

), and in some cases, a rubber-based polymer modifier is added to enhance high-temperature performance. This modifier increases the viscosity of the asphalt binder, impacting production and construction temperatures. The bitumen used across all mixtures was PG 76-22, chosen for its practical characteristics related to rutting resistance. The aggregate used in the study adhered to predetermined gradation sizes, including coarse aggregate (4.75 to 12.5 mm), fine aggregate (0.075~4.75 mm), and lime filler (less than 0.075 mm). The aggregate properties and mix gradations for both 13 mm SMA and 10 mm SMA align with guidelines from the Ministry of Land, Infrastructure, and Transport (MOLIT) [42]. The general properties of aggregate [43] and the gradations are shown in

Table 2. Aggregate properties.

Properties	Properties Value
Aggregate Apparent density	2.67
Los Angeles abrasion value	24.1%
Aggregate crushed value	17.6%
Flakiness and elongation index	12.2%
Water absorption	0.20%

and

Table 3. Aggregate gradation of proposed mixtures.

Percent Passing (%)	20 mm	13 mm	10 mm	5 mm	2.5 mm	0.6 mm	0.3 mm	0.15 mm	0.075 mm
10 mm PSMA	100	100	100	33.2	22.6	16.0	14.3	11.9	9.5
13 mm PMA	100	84.8	61.8	42.7	31.2	17.0	11.8	6.8	4.2

, respectively.

As mentioned above, the main goals include assessing the suitability of SMA mixtures enriched with different percentages of WPA (2%, 4%, 6%), a 25% to 50% extracted RAP binder, and an optimized asphalt binder derived from the extracted RAP binder to enhance its properties. Additionally, the study seeks to identify the optimal NMAS for the mixture, considering both 13 mm and 10 mm alternatives. In general, the flow of the research is presented in Figure 1.

2.1.3. Binder Selection and Modification

Selecting the right binder is a critical component of developing PSMA mixtures that meet the demanding requirements of BRT systems in Seoul. To ensure the optimal performance and sustainability of these mixtures, the following binder options were evaluated:

-

Normal PG76-22: Examining the unmodified standard PG76-22 asphalt binder with a softer consistency to gauge its suitability for BRT pavement.

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Normal PG76-22 with 25% Extracted RAP + 3% waste cooking oil: Modifying the standard PG76-22 asphalt binder with 25% RAP and an additional 3% waste cooking oil with the intention of enhancing properties for sustainable pavement solutions. The properties of the waste cooking oil is presented in

Table 4. The general properties of waste cooking oil.

Property	Value/Description
Chemical Composition	Triglycerides, Free Fatty Acids, Impurities
Density	900–930 kg/m3
Viscosity (at 40 °C)	32–60 mm2/s (cSt)
Flash Point	220–330 °C (428–626 °F)
Pour Point	−10 to −20 °C (14 to −4 °F)
Acid Value	2–7 mg KOH/g
Iodine Value	80–120 g I2/100 g
Saponification Value	185–200 mg KOH/g
Cloud Point	−2 to −10 °C (28 to 14 °F)
Color	Light to Dark Brown
Odor	Varies (depending on source)
Solubility	Insoluble in water, soluble in organic solvents

.

-

Normal PG76-22 with 50% Extracted RAP + 3% waste cooking oil: Using 50% extracted RAP as the main binder; this was found to be excessively rigid and unsuitable for the intended purpose.

Rheological frequency sweep tests were conducted on these binder options to assess their performance and compatibility with PSMA mixtures. Significantly, the results indicated that the binder that combined 25% extracted RAP and 3% waste cooking oil with the PG76-22 binder displayed promising characteristics. These characteristics aligned well with the goals of sustainable BRT pavement development. This particular binder not only demonstrated improved performance but also contributed to the eco-friendly utilization of byproduct materials.

2.1.4. WPA Content Selection

To promote the sustainable utilization of byproduct materials, three different WPA levels, namely 2%, 4%, and 6%, were considered as replacements for a portion of the conventional aggregates in a stone-modified asphalt concrete mix. All mixtures were also modified with 25% RAP and the optimized binder from Section 2.1.3 (Normal PG76-22 with 25% Extracted RAP + 3% waste cooking oil). WPA, a crucial element in this research, is sourced from recycled plastics, replacing natural aggregates in asphalt mixture production (see Figure 2). General properties are outlined in

Table 5. General Properties of WPA.

Property	Value
Apparent Density	1.25 g/cm3
Water Absorption	0.5%
Softening Point (Ring & Ball)	65 °C
Particle Size Range	2.36 mm–0.075 mm

. The production involves key stages:

Beginning with manual sorting (Stage 1), different plastic waste types are categorized based on unique characteristics. This labor-intensive task, often carried out by individuals, involves assessing the waste’s source and color. Following this, Stage 2 involves post-sorting and cleaning to eliminate residual dirt and dust. In Stage 3, the cleaned plastic waste undergoes shredding and preparation, transforming it into small flakes, which are then soaked and dried to remove contaminants. Stage 4 focuses on the Low-Density Polyethylene (LDPE) formation and compounding, where plastic fragments are finely ground and, if necessary, color compounding is performed. The plastic resin is heated and extruded through a custom die, resulting in controlled-dimension WPAs. This extrusion process includes an initial treatment stage where additives like magnesium, fly ash, and steel slag powder are introduced to enhance WPA properties. This initial treatment serves as a critical step to modify the plastic resin, ensuring that the WPAs maintain structural integrity and resist softening, particularly under the high temperatures encountered in asphalt mixture production. The additives act as stabilizers, contributing to the overall durability and suitability of WPAs for use in sustainable asphalt mixtures, effectively addressing concerns regarding temperature sensitivity.

Stage 5 involves curing and quality assurance, ensuring the structural integrity of freshly formed WPAs for use in asphalt mixtures. The final stage, Stage 6, emphasizes the utilization of WPAs in various products, reinforcing the sustainable use of recycled plastic materials.

2.1.5. Selection of Optimized WPA Content

Selecting the optimal WPA content is a crucial stage in the development of SMA mixtures for sustainable pavement solutions within BRT systems in Seoul. The study conducted a thorough analysis of different WPA content levels to identify the most suitable concentration for achieving the desired pavement performance. Three key WPA content levels were investigated:

-

2% WPA: This relatively low WPA content mixture was examined to gauge its impact on the mixture’s properties and durability.

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4% WPA: Positioned as the mid-range WPA content, this mixture strikes a balance between property enhancement and cost-effectiveness.

-

6% WPA: Representing the higher end of the WPA content spectrum, this mixture was chosen to explore the effects of a greater WPA concentration on the mixture’s characteristics and performance.

Each of these content options underwent comprehensive laboratory testing, with a specific focus on the Indirect Tension Strength Test. These tests provided valuable insights into how varying WPA content levels influence the mixture’s tensile strength and overall performance.

2.1.6. Gradation Modification

Having determined that the optimal mixture was the one including 4% WPA, the research delved into the influence of NMAS variations, specifically 10 mm and 13 mm NMAS. These mixtures were systematically modified with 4% WPA, 25% RAP, and the customized binder optimized for sustainability and performance in the context of BRT pavement systems in Seoul. This comprehensive investigation into the impact of varying NMAS on the mixture’s properties further informed the development of environmentally friendly and durable SMA mixtures.

2.2. Methods

2.2.1. Frequency Sweep Test

The Frequency Sweep Test (FST) is a fundamental laboratory evaluation method used to assess the rheological properties of asphalt binders across a broad spectrum of temperatures and time conditions. It employs a dynamic shear rheometer (DSR) in adherence to the ASTM D7552-22 [50] standard to measure two key parameters: the complex shear modulus (G*) and the phase angle (θ). By scrutinizing G* and θ across diverse temperature scenarios, the FST serves as a vital tool for characterizing how asphalt binders respond to different environmental conditions, offering insights into their performance and durability in varying climate settings.

2.2.2. Indirect Tension Strength Test

The Indirect Tension Strength Test adhered to the recommended ASTM D6931 [51] protocol and involved meticulous preparation of cylindrical specimens with a diameter of 150 mm and a height of 63.5 mm, utilizing a gyratory compactor. Tensile Strength Ratio (TSR) served as the key method to assess moisture resistance in our study. Each RAP mixture, with a consistent porosity of 7.0 ± 0.5%, was formed into six specimens. Three underwent a freeze-thaw moisture treatment, and the remaining three served as reference specimens immersed in a 60 °C water bath for 2 h. Freeze-thaw-treated specimens experienced cycles of freezing at −18 °C, thawing in a 60 °C water bath, and 2-h water bath immersion. Tensile strength tests were conducted for both sets. This comprehensive approach provided crucial insights into the moisture resistance of RAP mixtures, informing their suitability for sustainable pavement development in Seoul’s BRT systems. The analysis determined the influence of varied WPA content levels (2%, 4%, and 6%) on tensile strength, aiding in selecting the optimal WPA concentration for pavement development.

2.2.3. Dynamic Modulus Test

The dynamic modulus test is crucial to understanding how different NMAS variations influence the viscoelastic behavior of SMA mixtures, contributing to the selection of optimal mixtures for sustainable BRT pavement solutions in Seoul. To conduct the dynamic modulus test, cylindrical specimens were prepared using a Superpave Gyratory compactor. These specimens featured a diameter of 100 mm and a height of 150 mm. The test apparatus, following ASTM D3497-19 [52], applied sinusoidal loading at varying temperatures, frequencies, and loading times. The key parameters examined in this test included complex shear modulus (G*), phase angle (θ), and other dynamic mechanical properties. The overall view of the dynamic modulus test is presented in Figure 3a.

2.2.4. Hamburg Wheel Tracking Test

The Hamburg Wheel Tracking test (HWT) was employed to quantify the extent of plastic deformation exhibited by the bus stop mixtures based on AASHTO 324 [53]. This test method primarily evaluates the susceptibility to settlement by immersing a compacted specimen in hot water at a temperature of 50 °C, followed by the repeated application of a wheel load, with each load totaling 7054.5 N. For testing purposes, a cylindrical specimen measuring 150 mm in diameter and 60 mm in height was prepared by slicing one side of a cylindrical sample [53]. In accordance with the guidelines set by the United States Department of Transportation, the settlement amount, measured after subjecting the specimen to 20,000 loading cycles, should not exceed 20 mm to ensure the optimal performance and durability of the Hot Mix Asphalt (HMA) mixture. The general view of the dynamic modulus test is presented in Figure 3b. This test played a crucial role in evaluating the rutting resistance and overall performance of the bus stop mixtures, providing vital data for the development of sustainable and robust pavement solutions in the context of BRT systems in Seoul.

2.2.5. Falling Weight Deflectometer Test in Field Application

A real-world testbed within a South Korean BRT station was chosen for the field application of the optimized mixture. The falling weight deflectometer test (FWD) was conducted on a 200-m-long pavement section near busy bus stops. The testbed was divided into two zones: the first from 30 to 50 m with a 13 mm NMAS, 4% WPA, 25% RAP binder, and 3% waste cooking oil; the second from 130 to 150 m with a 10 mm NMAS and a regular binder. This setup allowed a practical evaluation of the asphalt mixtures’ performance and durability under real-world BRT station conditions, ensuring the relevance of the study to Seoul’s BRT system.

The reinforced section, using the optimized mix, was compared to the control section after one year of service. The study utilized an FWD test to assess pavement conditions at a severely damaged bus stop location in Seoul (ASTM 4694 [54]). The general testing setup of the FWD is presented in Figure 4 while Figure 5 exhibits general results measured from the FWD test. FWD is a non-destructive testing method involving an impact load on the pavement surface and measurement of surface deflection using sensors.

The standard load applied is 4082 kgf with a 30 cm diameter load plate. The FWD test captures deflection profiles at specific offsets using geophones, and discrete measuring points allow the calculation of deflection parameters. The FWD test was conducted at 20 m intervals for each recycled asphalt mixture lane, following established standards to evaluate pavement conditions. This testing was instrumental in evaluating pavement performance and guiding the development of sustainable pavement solutions tailored to BRT systems in Seoul.

Concluding the materials and methods section, the following segment shifts focus to the presentation and analysis of results. The subsequent section interprets the outcomes, exploring the influence of varied parameters on the performance of the developed asphalt mixtures. Through an in-depth discussion of these results, our aim is to provide an understanding of their implications for sustainable BRT pavement development in Seoul. This shows a detailed exploration of the findings within the context of the research objectives and overarching goals of enhancing pavement durability, sustainability, and performance.

3. Results and Discussion

3.1. Frequency Sweep Test Results

The rheology test results are integral to the assessment of binder modifications suitable for SMA mixtures in the context of Seoul’s BRT systems. As shown in Figure 6, three distinct binder options were examined, encompassing a modified PG76-22 binder with 25% extracted RAP and 3% waste cooking oil, a standard PG76-22 binder known for its relatively softer consistency, and 50% extracted RAP binder, noted for its excessive stiffness. Employing dynamic shear rheometry, the study measured complex shear modulus (G*) across varying temperature and time conditions, providing insights into the binders’ performance under diverse climate scenarios. The rheology test results highlight the promising performance of a mixture with 25% RAP and 3% waste cooking oil, characterized by a balanced G* modulus, rendering it well-suited for integration into the subsequent WPA mixtures in the upcoming phase of the study. This finding underscores the feasibility of employing a mix including 25% RAP and 3% waste cooking oil in the formulation of WPA mixtures, paving the way for further investigations in the research.

These findings are aligned with related research due to the underlying mechanism of the modified binder’s performance [55,56]. The integration of 25% extracted RAP and 3% waste cooking oil has contributed to the balanced G* modulus of the binder. The reclaimed asphalt and cooking oil have likely acted as modifiers, enhancing the flow and deformation characteristics of the binder.

The findings in the frequency sweep test highlight the performance of the modified binder with 25% extracted RAP and 3% waste cooking oil. The balanced G* modulus observed in the study aligns with the underlying mechanism identified in previous research [57,58], where the integrated reclaimed asphalt and cooking oil serve as modifiers, enhancing the flow and deformation characteristics of the binder. This supports the results showcasing the viability of the modified binder with improved flow and deformation characteristics. The compatibility of this binder with sustainable BRT pavement development, attributed to its incorporation of RAP and waste cooking oil, sets the foundation for binder selection and modification considerations in the study’s subsequent phases.

3.2. Indirect Tension Strength Test Results

The Indirect Tension Strength (ITS) test results demonstrate the significant influence of WPA content on the mechanical properties of SMA mixtures. The ITS result is shown in Figure 7. As the WPA content increases from 0% to 6%, the ITS values exhibit a notable trend. Without WPA (0%), the ITS value is the highest of the tested materials at 1.15 MPa. However, the introduction of 2% WPA lowers the ITS value to 0.8 MPa, indicating a slight decrease in tensile strength. Interestingly, at 4% WPA content, the ITS value increases to 0.9 MPa, suggesting a positive impact on the mixture’s tensile strength. Subsequently, at 6% WPA content, the ITS value slightly decreases to 0.84 MPa. The stiffness of the SMA mixtures, measured in terms of stiffness modulus (kN/mm), follows a similar trend. Mixtures with 0% and 4% WPA exhibit the highest stiffness values, with 2.8 kN/mm and 2.6 kN/mm, respectively. The 2% WPA mixture displays a lower stiffness of 2.2 kN/mm, and the 6% WPA mixture registers a stiffness of 2.3 kN/mm.

These findings align with prior research due to the fundamental relationship between WPA content and the mechanical characteristics of SMA mixtures. The mechanical response of SMA mixtures to varying WPA content is rooted in the changes induced by WPA on the mixture’s internal structure and interactions between the asphalt binder and aggregates [35,37]. The proper addition of WPA enhances the strength of HMA through multiple mechanisms. WPA particles reinforce the mixture, reducing the risk of cracks. The particles‘ irregular shapes create interlocking effects, boosting cohesion. WPA acts as a filler, improving aggregate packing and densifying the mixture. Additionally, WPA’s chemical properties, such as its high melting temperature, enhance HMA’s performance. This combination of factors results in a stronger and more durable asphalt mix, capable of withstanding the stresses of road pavements, including high traffic and heavy loads. These results emphasize the intricate relationship between WPA content and the mechanical properties of SMA mixtures, providing essential insights for optimizing sustainable pavement solutions for BRT systems.

3.3. Dynamic Modulus Test Results

The dynamic modulus test results reveal valuable insights into the mechanical properties and performance of SMA mixtures with varying NMAS and under different loading conditions, shedding light on their suitability for sustainable pavement solutions within BRT systems in Seoul. As displayed in Figure 8, the dynamic modulus (|E*|) values are consistently higher for the 13 mm NMAS mixtures compared to the 10 mm NMAS mixtures under both normal and double-load conditions. The 13 mm NMAS mixtures exhibit a superior ability to withstand the dynamic forces associated with BRT traffic, reflecting their enhanced structural integrity and resistance to deformation. Moreover, the results further emphasize the positive influence of the 13 mm NMAS on the mixtures’ performance, positioning them as the preferred choice for BRT pavement applications in Seoul. This comparative analysis contributes significantly to our understanding of the optimal NMAS and its role in achieving the desired pavement characteristics for sustainable BRT systems.

The corroborating evidence from related studies supports the findings obtained from the tests [5]. The superiority of the 13 mm NMAS mixtures over the 10 mm NMAS mixtures in terms of dynamic modulus values can be attributed to their enhanced resistance to deformation under the rigorous loading conditions typically encountered in BRT systems. This phenomenon is likely due to the greater interlock and mechanical stability provided by the larger aggregate particles. The 13 mm NMAS mixtures’ ability to distribute and dissipate applied loads more effectively leads to their higher dynamic modulus values. This aligns with existing research, emphasizing the significance of proper aggregate sizing for achieving optimal pavement performance and durability in BRT pavements like those in Seoul. The choice of 13 mm NMAS for sustainable BRT pavement applications is reinforced by these results and underscores the importance of addressing structural resilience and deformation resistance in pavement design.

3.4. Hamburg Wheel Tracking Test

The Hamburg Wheel Tracking (HWT) test results provide critical insights into the rutting resistance and overall performance of the SMA mixtures containing various NMAS. As shown in Figure 9, two NMAS options were explored, namely 10 mm and 13 mm, both incorporating 4% WPA, 25% RAP, and the optimized binder. In this test, the mixtures were subjected to repeated wheel loading under hot water immersion, simulating the harsh conditions experienced in real-world pavement applications.

In the HWT test, the initial 1000 cycles witnessed a notable and steep increase in rutting for all tested conditions. Following this initial phase, under normal load conditions, both the 10 mm and 13 mm NMAS mixtures exhibited improved stability, with rutting measurements stabilizing at approximately 3.3 mm. However, when subjected to double-load conditions, both mixtures experienced further increases in rutting. Notably, the 10 mm NMAS mixture displayed a steep and rapid increase, ultimately reaching 9 mm, while the 13 mm NMAS mixture demonstrated a more gradual ascent to the same critical rutting level. This divergence in performance underscores the significance of NMAS, with the 13 mm NMAS mixture displaying greater resistance to rutting under the double-load conditions, highlighting its superior suitability for sustainable pavement development in high-traffic BRT systems in Seoul.

The outcomes revealed that the SMA mixture with a 13 mm NMAS demonstrated superior resistance to plastic deformation under HWT conditions, particularly when subjected to a double-load condition simulating heavy traffic during peak hours. This remarkable performance was reflected in the reduced settlement and deformation observed in the specimens. In contrast, the 10 mm NMAS mixture exhibited comparatively higher levels of rutting under both normal and double-load conditions.

The results from the HWT test indicate that the 13 mm NMAS mixture outperforms the 10 mm NMAS mixture in terms of rutting resistance. This is attributed to several factors. The larger aggregate size in the 13 mm NMAS mixture provides a more robust and stable framework within the asphalt, better withstanding the repeated loading and high traffic stresses typical of BRT systems. The larger aggregates also allow for improved interlocking within the mixture, which enhances its resistance to deformation. Additionally, the 13 mm NMAS mixture effectively distributes stresses across the pavement, reducing localized rutting effects. The combination of these factors results in superior resistance to plastic deformation and rutting in the 13 mm NMAS mixture compared to the 10 mm NMAS mixture, making it a more suitable choice for sustainable pavement development within high-traffic BRT systems in Seoul.

Aligned with prior research [59,60], the HWT test outcomes substantiate the critical role of NMAS in the rutting resistance of SMA mixtures for BRT systems. The study’s results echo the findings of previous research, indicating that the 13 mm NMAS mixture, incorporating 4% WPA, 25% RAP, and the optimized binder, outperforms its 10 mm NMAS counterpart. This aligns with existing literature, in which larger aggregate sizes, such as 13 mm, have demonstrated superior resistance to rutting under heavy traffic conditions. The enhanced stability, interlocking, and stress distribution of the 13 mm NMAS mixture contribute to its superior performance, making it a more suitable and resilient choice for sustainable pavement development within high-traffic BRT systems in Seoul. These results underscore the significance of NMAS selection in achieving optimal rutting resistance for SMA mixtures in the context of BRT systems in Seoul. Furthermore, it emphasizes the potential of the 13 mm NMAS mixture with 4% WPA, 25% RAP, and the optimized binder as a promising solution for sustainable pavement development in high-traffic areas.

3.5. Real-World Test Bed Results

The results of the falling weight deflectometer (FWD) test conducted at the actual BRT section in Seoul provide a practical assessment of the pavements’ performance. As depicted in Figure 10, the deflection bowl measurements, which quantify the extent of surface deflection under a dynamic load, were taken at various locations along the BRT section. The results indicate that the pavement reinforced with SMA mixtures incorporating 13 mm NMAS and 4% WPA displayed robust and resilient behavior. The deflection values recorded at different measuring points exhibited minimal deflection, signifying a high resistance to deformation and pavement distress. This finding underscores the effectiveness of the 13 mm NMAS and 4% WPA combination in enhancing the structural integrity and durability of the BRT pavement. In contrast, the 10 mm NMAS and 4% WPA mixture, while still offering satisfactory performance, demonstrated slightly greater deflection, implying a relatively lower capacity to withstand dynamic loads.

The application of 13 mm NMAS combined with 4% WPA and RAP demonstrated remarkable performance improvements. In this configuration, rutting reduced to an impressive 75 µm, reflecting a significantly enhanced pavement resilience. This result is further substantiated by the substantial increase in the Road Bearing Capacity (RBC) of the surface layer to an impressive 5400 MPa at the center zone, confirming the superior load-bearing capability of the pavement. In stark contrast, the 10 mm NMAS mixture without RAP and WPA exhibited severe rutting, with depths of up to 220 µm, indicative of diminished pavement durability. This observation is reinforced by a noticeable reduction in the RBC of the surface layer to 1300 MPa at the center zone, highlighting the reduced load-bearing capacity of this configuration. These findings underscore the pivotal role of 13 mm NMAS, RAP, and WPA (4%) in enhancing pavement performance, offering a promising solution for sustainable and resilient BRT systems in Seoul.

This comprehensive field evaluation of the SMA mixtures provides valuable real-world insights into their performance, reaffirming the preference for the 13 mm NMAS and 4% WPA combination for sustainable BRT pavement solutions in Seoul.

In conclusion, the comprehensive analysis presented in the “Results and Discussion” section sheds light on critical aspects of this study. As a transition to the “Conclusions” section, the synthesized insights from the results and discussions converge to offer a clear perspective on the implications of these findings. By summarizing key outcomes and their significance, the following conclusions will provide a succinct encapsulation of the study’s contributions, paving the way for future research in the realm of sustainable pavement development for BRT systems.

4. Conclusions

In this study, the challenges of deteriorating pavements at bus stations within BRT systems in Seoul, South Korea were addressed. The primary goals involved evaluating the compatibility of SMA mixtures with varying WPA content (2%, 4%, 6%), 25% RAP, and an optimized binder, as well as determining the most suitable NMAS—13 mm and 10 mm. The assessment of the long-term performance of the SMA mixtures under real BRT station conditions was also a key objective. The following conclusions can be drawn from the manuscript:

• This research demonstrated that the utilization of SMA mixtures incorporating 4% WPA, 25% RAP, and a specialized binder, results in a substantial reduction in rutting. The rutting decreased to an average of approximately 75 µm at the central zone, demonstrating the effectiveness of the developed mixtures in enhancing pavement durability under heavy loading conditions at bus stops in the BRT system.
• The reduction in rutting was verified by an increase in the Road Bearing Capacity of the surface layer to around 5400 MPa at the central zone of the BRT pavement. This significant enhancement in bearing capacity is a strong indicator of the mixture’s ability to withstand the repetitive loading from BRT buses.
• The research revealed that the use of 13 mm NMAS exhibited superior performance compared to 10 mm NMAS in terms of rutting. The 13 mm aggregate type proved more resilient to critical rutting, indicating that this size is better suited for the high-stress environment of BRT bus stops.
• The addition of 4% WPA in the SMA mixtures contributed to the improvement in moisture resistance, as demonstrated by the results of the ITS test.
• The use of sustainable materials like WPA and RAP in pavement construction effectively reduces the demand for new resources, minimizes environmental impact, and encourages responsible waste management. By promoting the eco-friendly utilization of byproduct materials such as waste plastic and reclaimed asphalt, this study aligns with broader waste reduction and responsible construction practices.
• While this research has shown promising results in enhancing pavement durability at BRT bus stops using SMA mixtures with 4% WPA, 25% RAP, and an optimized binder, there is room for further investigation. Future studies can delve into advanced binder modifications to fine-tune the performance of these mixtures under varying environmental conditions. Additionally, examining the long-term behavior of SMA mixtures with different WPA percentages and RAP content in actual BRT station settings can provide valuable insights into their real-world feasibility.