Feasibility and Sustainable Performance of RAP Mixtures with Low-Viscosity Binder and Castor Wax–Corn Oil Rejuvenators

Abstract

The utilization of Recycled Asphalt Pavement (RAP) mixtures in pavement construction is an environmentally friendly approach that promotes sustainable development by reducing energy consumption and material waste. However, the high cost of conventional rejuvenators limits the widespread use of RAP mixtures. In this study, a novel approach is proposed to enhance the performance of RAP mixtures by incorporating a combination of high-penetration asphalt binder and rejuvenators, namely Castor wax and Corn oil. The newly developed rejuvenator consists of 8.5% Castor wax oil, 3% Corn oil, 3% fatty acid amine surfactant, 0.2% additive, and 79.8% water. The test results demonstrate that the modified mixture exhibits superior properties compared with conventional RAP mixtures. The Multiple Stress Creep Recovery test results showed a 20% reduction in cumulative strain rate for the RAP mixture with the new rejuvenators compared with that for the conventional ones. Furthermore, the Tensile Strength Ratio test indicated a notable 9.47% improvement in the rejuvenated RAP mixture’s resistance to moisture-induced damage compared with the conventional mixture. Evaluation of viscoelastic behaviors revealed a slight reduction in dynamic modulus for the rejuvenated binder, but a significant improvement in elastic behavior. In terms of rutting resistance, the Hamburg wheel tracking rut depths of the rejuvenated binder were significantly lower, representing reductions of 21.83% for specific binder compositions. Additionally, the absence of the stripping phenomenon further confirmed the superior moisture resistance of the modified mixture. The rejuvenated binder exhibited a remarkable 28.55% increase in fatigue load cycles to failure compared with the reference RAP binder, demonstrating substantial resistance to fatigue cracking. These quantitative comparisons not only confirm the superior performance of the modified mixture over conventional RAP mixtures, but also highlight the potential cost savings achieved through the utilization of Castor wax and Corn oil rejuvenators.

1. Introduction

The application of Reclaimed Asphalt Pavement (RAP) has gained considerable attention in the field of pavement engineering [1]. RAP is obtained by milling and crushing existing asphalt pavements, and it can be reprocessed and used as a valuable resource in new pavement construction [2]. The use of RAP offers numerous benefits, including reduced energy consumption, conservation of natural resources, and decreased waste generation [3,4]. Moreover, the incorporation of RAP in pavement construction aligns with the principles of sustainable development and supports the circular economy concept [5].

Over the years, several innovative technologies have emerged to enhance the utilization of RAP materials and improve the overall performance of recycled asphalt [6,7,8]. One notable technology is the use of warm-mix asphalt (WMA) additives in RAP mixtures [9]. WMA technologies enable the production and placement of asphalt at lower temperatures, reducing energy consumption and emissions during construction [10]. Studies have shown that incorporating WMA additives in RAP mixtures can enhance workability, reduce moisture susceptibility, and improve pavement durability [11]. This technology not only contributes to sustainable practices, but also offers economic advantages by lowering production costs [12,13]. Another promising technology is the introduction of recycling agents or modifiers specifically designed for RAP [14]. These additives, such as polymers, chemical agents, and bio-rejuvenators, aim to restore the aged binder in RAP materials and improve their rheological properties [15]. By rejuvenating the binder, these technologies can enhance the stiffness, rutting resistance, and fatigue performance of RAP mixtures [16]. Additionally, the use of bio-based rejuvenators offers an eco-friendly alternative to traditional petroleum-based products, aligning with sustainable development goals [17]. Advancements in milling and processing techniques have also contributed to the efficient use of RAP materials [18]. Innovations in milling machines, such as drum designs, automation features, and temperature control allow for better control and precision in RAP processing [19]. This results in a higher quality of recycled materials, improved particle size distribution, and enhanced compatibility with virgin binders. The observed increase in performance can be attributed to various factors, and recent studies by Xing et al. have highlighted the significant contribution of the degree of blending between virgin and RAP binders [20]. It has been noted that an effective blending of these binders noticeably contributes to the improvement in performance. This finding emphasizes the importance of achieving optimal blending between the two types of binders to enhance the overall performance of RAP mixtures. Furthermore, the integration of intelligent systems and artificial intelligence in RAP processing has the potential to optimize milling operations, reduce waste, and improve overall productivity [21].

The current status of expensive rejuvenators in RAP reflects the challenges and limitations associated with their use [4]. Traditional rejuvenators, which are commonly derived from petroleum-based products, can be costly and contribute to the overall expenses of RAP mixtures [22,23]. This high cost hinders the widespread adoption and application of RAP in pavement construction, particularly in regions with limited financial resources. The expense of these rejuvenators poses a significant barrier to the sustainable utilization of RAP materials, as it can undermine the economic viability and feasibility of using recycled asphalt in road projects [24]. As a result, there is a need to explore alternative and more affordable rejuvenator options that can maintain or enhance the performance of RAP mixtures while reducing overall project costs.

In developing countries, the application of RAP has shown great promise in addressing the challenges of limited resources and infrastructure development [25]. These countries often face constraints in terms of budget, construction materials, and environmental sustainability [26]. The utilization of RAP provides a cost-effective solution by recycling existing asphalt pavements and reducing the reliance on virgin materials [27]. Additionally, RAP offers the opportunity to enhance the performance and durability of road pavements without compromising on quality [28]. The use of RAP in developing countries not only helps in conserving natural resources, but also contributes to the reduction of carbon footprint and waste generation. Despite the potential benefits, the widespread adoption of RAP in these countries is still in its early stages and requires further investigation, optimization, and implementation strategies to overcome technical, economic, and policy challenges. By exploring and promoting the application of RAP, developing countries can achieve sustainable and resilient road infrastructure development while addressing their unique socio-economic and environmental needs.

While the use of RAP has demonstrated numerous advantages in sustainable road construction, its application in developing countries is often hindered by cost-related challenges [29]. One significant limitation lies in the high cost associated with conventional rejuvenators and additives used in RAP mixtures [30]. These countries, already grappling with limited financial resources and infrastructure budgets, find it challenging to afford expensive rejuvenators that are commonly used in developed nations [31]. As a result, the utilization of RAP materials becomes constrained, hindering the potential benefits of recycling and sustainability that RAP can offer [26]. Addressing this limitation requires innovative approaches and cost-effective solutions that cater to the specific economic constraints of developing countries, enabling them to fully leverage the potential of RAP in their road construction and maintenance endeavors.

Given the current limitations and cost implications surrounding the application of RAP in developing countries, there is a pressing need for research to address these challenges and develops cost-effective solutions. This study aims to fill this research gap by investigating the effectiveness of low-cost rejuvenators, specifically Castor wax and Corn oil, in replenishing the stiffness and hardened properties of RAP mixtures. The novelty of this research lies in the formulation of a newly developed rejuvenator that combines a high-penetration asphalt binder with Castor wax and Corn oil. By quantitatively comparing the performance of the modified RAP mixture with conventional RAP mixtures through various laboratory tests, this study provides valuable insights into the feasibility and practical application of low-cost rejuvenators in enhancing the performance and sustainability of RAP pavements. The incorporation of a low-viscosity binder and Castor wax with Corn oil rejuvenators holds significant importance in the field of asphalt pavement rejuvenation. Traditional methods of rejuvenation often rely on petroleum-based additives, which not only raise environmental concerns but also face limitations in terms of availability and cost. By exploring alternative rejuvenation strategies, such as the use of a low-viscosity binder and Castor wax with Corn oil rejuvenators, the research can address these challenges and fulfill the growing need for sustainable and cost-effective solutions in the asphalt industry. These innovative approaches offer the potential to enhance the performance of recycled asphalt mixtures by improving their quality and durability. Furthermore, by reducing the dependence on virgin asphalt binders, the research can contribute to the preservation of natural resources and mitigate the environmental impact associated with traditional rejuvenation methods.

In this research, a comprehensive mixed-design approach was employed to evaluate the effectiveness of low-cost rejuvenators in RAP mixtures. The mixed-design process involved combining a high-penetration asphalt binder with Castor wax, Corn oil, and other additives in specific proportions. The newly developed rejuvenator composition consisted of 8.5% Castor wax oil, 3% Corn oil, 3% fatty acid amine surfactant, 0.2% additive, and 79.8% water. The mixture’s performance was assessed through a range of laboratory tests, including the Multiple Stress Creep Recovery (MSCR) test, the Indirect Tensile Strength (ITS) test, Hamburg wheel tracking (HWT) test, and dynamic modulus test. These testing methods allowed for a comprehensive evaluation of the modified RAP mixture’s stiffness, resistance to deformation, moisture sensitivity, and overall performance. The results obtained from these tests were compared with those of conventional RAP mixtures, providing valuable insights into the potential of the developed rejuvenators in enhancing the performance of RAP pavements. A general summary of the research is briefly presented in Figure 1.

2. Materials and Methods

2.1. Materials

The materials used in this study included RAP aggregates, asphalt binders, and rejuvenator additives. The RAP aggregates were obtained from local road construction projects and were processed to meet the required specifications [27]. The asphalt binder selected for the mix design was a high penetration grade binder, chosen for its compatibility with the RAP materials [4]. As for the rejuvenator additives, Castor wax and Corn oil were utilized as the primary rejuvenators due to their potential to replenish the aged binder and enhance the performance of the RAP mixture. These materials were sourced and prepared according to established guidelines to ensure consistency and accuracy throughout the testing and analysis phases.

2.1.1. RAP Aggregates

The RAP aggregate used in this study was obtained from local road construction projects (see Figure 2). It consisted of asphalt pavement materials that were milled and removed from existing road surfaces. The RAP material was processed to meet the required specifications for the experimental mixtures [27]. Initially, the RAP material underwent a thorough screening process to remove oversized particles and debris. Subsequently, it was crushed to achieve the desired nominal maximum aggregate size of 13 mm. To ensure the quality of the RAP aggregate, it was subjected to rigorous cleaning procedures. Any contaminants, such as soil, vegetation, or other foreign materials, were removed to minimize their adverse effects on the performance of the asphalt mixture. The resulting RAP aggregate exhibited satisfactory properties in terms of particle shape, size distribution, angularity, and texture, making it a suitable component for the experimental mixtures in this research. The gradation and general properties of the employed RAP aggregate in this research are presented in Figure 3 and

Table 1. RAP aggregate properties.

Properties	Value
Concrete asphalt content [32]	$\ge$3.7%
Asphalt penetration [33]	$\ge$21 (25 °C, 1/10 mm)
The amount lost in the wash test [34]	$\le$4.8%
Moisture content [35]	$\le$5%

, respectively.

2.1.2. Extracted RAP Binders

The extracted RAP binder refers to the asphalt binder obtained from the reclaimed asphalt pavement (RAP) material through a solvent extraction process [3]. In this study, the RAP aggregate was crushed to expose the asphalt binder within the RAP particles. Then, a suitable solvent, such as trichloroethylene or chloroform, was used to dissolve and extract the binder from the RAP aggregate. The extraction process involved mixing the crushed RAP material with the solvent and agitating the mixture to facilitate the dissolution of the binder. After a specified extraction period, the solvent and dissolved binder were separated from the solid residue through settling and subsequent separation techniques. The solvent was further removed from the extracted binder using methods like vacuum distillation or filtration to obtain a purified binder. The extracted RAP binder served as an important component in this research, as it allowed for the evaluation of its properties and its potential for reuse in asphalt mixtures. By utilizing the extracted binder, this study aimed to assess its performance characteristics and determine its suitability as a sustainable alternative to virgin asphalt binder in pavement construction. RAP binder exhibited key properties, including a penetration of 84.1 (1/10 mm) at 25 °C, a softening point of 68.2 °C, and a ductility of 102 cm/min at 5 °C. It demonstrated minimal mass loss (0.02%) and penetration loss (67) after thin film oven aging. Rheologically, it showed G*/sinδ values of 1.72 kPa (original), 2.41 kPa (after RTFO), and 1486 kPa (after PAV) at 76 °C. At −22 °C, the binder exhibited a stiffness of 182 MPa and an m-value of 0.32. These properties contributed to its performance in various asphalt applications.

2.1.3. New Low-Viscosity Rap Binder

The new low-viscosity RAP binder used in this study was a recently developed asphalt binder specifically designed for the rejuvenation of reclaimed asphalt pavement (RAP) materials [4]. This binder exhibited a high penetration grade of PG 58$-$22, indicating its low stiffness and improved workability at higher temperatures. The low viscosity of the binder was achieved through the careful selection of asphalt feedstocks and modification techniques during the refining process. This resulted in a binder with enhanced flow properties, allowing for better coating and binding of RAP aggregates in asphalt mixtures. The use of a low-viscosity binder in RAP mixtures offered several advantages. It promoted easier mixing and compaction of the asphalt mixture, leading to improved workability during construction. Additionally, the low-viscosity binder enhanced the coating and adhesion properties, facilitating the incorporation of RAP aggregates and ensuring better overall performance of the asphalt pavement.

2.1.4. Castor Wax Oil and Corn Oil Rejuvenators

The rejuvenator used in this study was a specially formulated mixture comprising Castor wax oil, Corn oil, a fatty acid amine surfactant, an additive, and water. The rejuvenator was designed to replenish the aging and stiffened properties of the extracted RAP binder, promoting the restoration of its original performance characteristics. The composition of the rejuvenator consisted of 8.5% Castor wax oil, 3% Corn oil, 3% fatty acid amine surfactant, 0.2% additive, and 79.8% water. Castor wax oil, derived from Castor beans, is known for its rejuvenating properties, helping to soften and improve the workability of the aged binder. Corn oil, on the other hand, acted as a natural rejuvenator, enhancing the elasticity and rejuvenation potential of the binder. The fatty acid amine surfactant was included to enhance the compatibility and dispersion of the rejuvenator within the binder matrix. The additive was incorporated to optimize the rejuvenation process and improve the overall performance of the RAP mixture. The newly developed rejuvenator composition aimed to provide an effective and cost-efficient solution for the rejuvenation of RAP materials. The research will evaluate the performance of a RAP mixture treated with the rejuvenator through various laboratory tests, including the MSCR test, to assess its ability to restore the desired rheological properties and improve the long-term performance of the asphalt pavement.

2.1.5. Binder Content for RAP Mixture

The percentage of the binder for the RAP mixture involved considering both the aged binder present in the RAP and the low-viscosity binder content added to rejuvenate the mixture. After careful analysis and testing, it was determined that a suitable percentage for the aged binder in the RAP was 3.7% by weight. This percentage accounted for the residual binder present in the reclaimed asphalt pavement. In addition to the aged binder, a low-viscosity binder was incorporated as a rejuvenator to replenish the stiffness and enhance the performance of the RAP mixture. Through extensive research and experimentation, it was found that a percentage of 2.3% by weight for the low-viscosity binder was effective in achieving the desired properties. Therefore, the recommended total binder content for the RAP mixture was 6% by weight, comprising 3% aged binder and 2% low-viscosity binder. This proportion ensured the proper balance of binder materials to promote adequate coating, bonding, and improved performance of the RAP mixture. The selection of these percentages took into account the specific characteristics of the RAP, the desired performance goals, and the anticipated traffic and environmental conditions the pavement will experience.

2.1.6. Mixing Process

The mixing process for the RAP mixture was conducted using the Superpave compaction gyratory method, which includes careful attention to temperature control at each step to ensure an effective blend of the materials [36]. Firstly, the RAP aggregates were preheated to a temperature of 160 °C to remove any moisture content. Next, the extracted RAP binder and the new low-viscosity RAP binder, along with the rejuvenator consisting of 8.5% Castor wax oil, 3% Corn oil, 3% fatty acid amine surfactant, 0.2% additive, and 79.8% water, were heated to their respective mixing temperatures. The preheated RAP aggregates were then combined with the binders and rejuvenator in a mechanical mixer, following the Superpave compaction gyratory method, which involved multiple cycles of mixing and compaction to simulate the field conditions. Throughout the mixing process, the temperature was maintained at a specified level, as specified by the Superpave compaction gyratory method, to facilitate proper blending and optimal workability of the mixture. After completion of the mixing, the RAP mixture was allowed to gradually cool to a suitable temperature for subsequent testing and analysis. The adherence to the Superpave compaction gyratory method during the mixing process ensured the proper compaction and performance of the RAP mixture. To conduct various laboratory tests, specimens of appropriate sizes were prepared from the RAP mixture. The specific specimen sizes were utilized for each specific laboratory test.

2.2. Methods

2.2.1. Multiple Stress Creep Recovery (MSCR) Test

The MSCR test was conducted to evaluate the performance of the asphalt binder used in the RAP mixture (see Figure 4). The test was performed in accordance with the standard AASHTO T350 procedure [37]. The MSCR test provided valuable insights into the rutting and healing potential of the binder under varying stress and temperature conditions [38,39,40]. The test involved subjecting the asphalt binder to a series of controlled stress and temperature cycles. The binder was loaded into a dynamic shear rheometer (DSR) equipped with a parallel plate geometry. The test was conducted at temperatures ranging from 40 °C to 70 °C and stress levels of 100 Pa, 200 Pa, and 300 Pa. During the test, the binder was subjected to a stress load for a specified period, followed by a recovery period. The creep compliance and recovery compliance of the binder were measured to assess its ability to resist deformation and recover its original properties. The MSCR test provided crucial data on the rutting potential, rutting resistance, and permanent deformation characteristics of the asphalt binder. The results of this test played a vital role in understanding the performance of the binder and its suitability for use in the RAP mixture.

2.2.2. Indirect Tensile Strength Test

The indirect tensile strength test, following the ASTM D6931 standard [41], was conducted to evaluate the mechanical properties of the RAP mixture. Cylindrical specimens with a diameter of 150 mm and a height of 75 mm were prepared from the compacted RAP mixture. The test was performed at a controlled temperature of 25 °C, simulating typical ambient conditions. Prior to testing, the specimens were conditioned in a controlled environment for 24 h to ensure moisture equilibration and consistent test results. During the test, the specimens were loaded at a constant displacement rate until failure occurred, and the load and deformation values were recorded. The indirect tensile strength was calculated based on these measurements. To compare the performance of the RAP mixture with that of the control, which consisted of virgin asphalt and aggregate without RAP or rejuvenators, the Tensile Strength Ratio (TSR) was determined. The TSR was calculated as the ratio of the average wet Indirect Tensile (IDT) strength to the average dry IDT strength. Higher TSR values indicated improved resistance to cracking and enhanced tensile strength in the RAP mixture. By conducting the indirect tensile strength test and evaluating TSR ratios, the effectiveness of the rejuvenators and their impact on the mechanical properties of the RAP mixture was assessed.

2.2.3. Dynamic Modulus Test

To assess the viscoelastic properties of RAP mixtures, an extensive experiment was conducted using an advanced MTS testing machine, as shown in Figure 5. The dynamic modulus test, based on AASHTO TP 62 [42], was performed to evaluate the viscoelastic properties of the RAP mixture. The test was conducted using a Universal Testing Machine (UTM) equipped with a dynamic modulus apparatus. Specimens in a cylindrical shape, with a diameter of 100 mm and a height of 150 mm, were prepared from the compacted RAP mixture. The test was performed at various temperatures ranging from $-$10 °C to 54 °C to assess the temperature-dependent behavior of the mixture.

The test involved subjecting the specimens to sinusoidal loading at different frequencies ($f$) ranging from 0.1 Hz to 20 Hz. The resulting complex modulus ($E^{*}$) was calculated using the equation:

$$log\left|E^{*}\right|=a+\frac{b}{1+\frac{1}{e^{d+g\ast \mathrm{l}\mathrm{o}\mathrm{g}\left(f_R\right)}}}$$

(1)

where $f_R$ is the reduced frequency, calculated as

$$f_R=f\times a$$

(2)

To determine the temperature dependency of the dynamic modulus, the logarithm of the time-temperature shift factor ($a_T$) was plotted against temperature. The relationship was modeled using a quadratic equation:

$$\log \left(a_T\right)={\alpha }_1{T}^2+{\alpha }_{2}T+{\alpha }_3$$

(3)

where ${\alpha }_1$, ${\alpha }_2$, and ${\alpha }_3$ are coefficients that characterize the temperature dependency.

By conducting the dynamic modulus test and analyzing the obtained data, the viscoelastic behavior of the RAP mixture under varying temperatures and frequencies was assessed. This information provided insights into the material’s stiffness and ability to resist deformation under different loading conditions, contributing to the understanding of its performance in pavement applications.

2.2.4. Fatigue Crack Resistance in Mixtures

An extensive investigation into the fatigue crack resistance of RAP mixtures was conducted using a standardized testing method outlined in AASHTO T 321-07 [43]. The primary objective of this dedicated fatigue cracking test was to assess the RAP asphalt mixture’s ability to withstand repeated loading cycles and resist crack formation under cyclic loading conditions. Test specimens, prepared according to standard procedures, were subjected to repeated loading cycles at a predetermined strain level, using a beam subjected to bending action with a continuous 10 Hz haversine or sinusoidal loading. The specimens were fabricated to adhere to precise measurements, boasting dimensions of 380 mm in length, 50 mm in height, and 63 mm in width. Fatigue failure was determined when the material’s stiffness was reduced to 50% of its original value after enduring repeated and continuous loading. Three replicates were employed in this test to generate the average result.

To establish a crucial baseline for stiffness, the material underwent 50 repetitive load cycles under controlled conditions at a constant room temperature of 20 °C. Throughout the test, the specimens’ response was continuously monitored, meticulously observing any visible crack initiation and propagation. The fatigue life of the RAP asphalt mixture was determined by analyzing several performance indicators, including stiffness degradation, the number of cycles required for crack initiation, and the extent of crack propagation. These parameters enabled the evaluation of the resistance of the RAP asphalt mixture to fatigue cracking, comparing it against established performance criteria. The ultimate aim was to replicate the cumulative effects of traffic loading and environmental factors on the fatigue performance of the RAP asphalt mixture. By comprehensively assessing stiffness degradation and crack development, the test results provided valuable insights into the material’s resistance to fatigue cracking.

2.2.5. Hamburg Wheel Tracking Test

In order to assess the vulnerability of the RAP asphalt mixture to moisture-induced damage, a comprehensive Hamburg wheel tracking test was conducted following the guidelines specified in AASHTO T 324 [44], as shown in Figure 6. The main purpose of this test was to evaluate the resistance of the mixture against potential issues caused by moisture. During the testing process, the RAP asphalt mixture specimens were submerged in water at a precisely controlled temperature of 50 °C while being subjected to a constant wheel load of 705 ± 4.5 N. The test consisted of a predetermined number of loading cycles designed to simulate real-world traffic loads and environmental conditions. It should be noted that the test results from each mix design were averaged from three replicates. Throughout the test, close attention was paid to the appearance of stripping points, which are indicative of the material’s susceptibility to moisture-related damage.

The determination of the stripping point in the HWT test involved analyzing the relationship between the stripping inflection point and stripping slope, which are indicators of the moisture resistance of HMA. The stripping inflection point represented the number of passes at the intersection of the creep slope and stripping slope curves. It served as a critical parameter in assessing the susceptibility of the HMA to moisture damage, providing valuable insights into its resistance to the detrimental effects of water infiltration and potential stripping phenomena. To ensure accurate and reliable results, certain criteria were set. Specifically, the detection of stripping points was carefully observed after a minimum of 10,000 cycles. Additionally, the settlement of the specimens was monitored, and it was ensured that the settlement did not exceed 20 mm after 20,000 repetitions of the test. These specific thresholds were established to provide valuable insights into the performance of the RAP asphalt mixture under moisture-induced conditions. The fabrication process of each sample was carried out with great care to ensure consistency and precision. The specimens were meticulously prepared, adhering to strict dimensional requirements. Specifically, they were crafted to possess a height of 60 mm and a diameter of 150 mm. Furthermore, a targeted porosity level of 7.0 ± 1.0% was maintained during the fabrication process. These meticulous measures were taken to ensure the reliability and validity of the test results, allowing for a comprehensive assessment of the moisture sensitivity of the RAP asphalt mixture. The data obtained from the Hamburg wheel tracking test provided valuable insights into the performance of the RAP asphalt mixture under moisture-induced conditions. By examining the occurrence of stripping points and monitoring settlement, the test results shed light on the mixture’s resistance to moisture-induced distress. Ultimately, this information contributed to enhancing the understanding of the RAP asphalt mixture’s behavior and aided in the development of more durable and resilient pavement materials.

3. Results and Discussion

3.1. MSCR Test Results

The MSCR test results for the RAP mixture utilizing rejuvenators are depicted in Figure 7. It is evident from the data that the RAP mixture with the newly developed rejuvenators exhibited a significant reduction in cumulative strain rate under the applied creep load, outperforming the conventional rejuvenators. Moreover, the strain recovery slope of the RAP mixture with the new rejuvenators displayed superior characteristics across different creep load levels. For instance, at an MSCR level of 3.2 kPa and 160 s, the strain value for the original RAP mixture was about 670%, whereas for the RAP mixture with the new rejuvenators, it reached around 530%. These results indicate the improved rejuvenation effect achieved by the new rejuvenators in the RAP binder, leading to enhanced stiffness and minimized strain accumulation.

Analyzing the non-recoverable compliance under a shear creep load of 3.2 kPa, it was observed that the RAP mixture with the new rejuvenators exhibited behavior similar to that of the conventional rejuvenators in the initial stages (between 100 and 120 s). However, as the testing duration increased, the RAP mixture exhibited significant strain accumulation, resulting in a wider disparity between the two mixtures. This observation highlights the superior performance of the RAP mixture with the new rejuvenators, demonstrating enhanced resistance to deformation and reduced susceptibility to strain accumulation.

These results validate the effectiveness of incorporating Castor wax and Corn oil rejuvenators in enhancing the overall performance and longevity of the RAP mixture. The observed improvements in the MSCR test results can be attributed to the unique properties of Castor wax and Corn oil. This type of rejuvenator properly enhanced the flexibility and resilience of the binder, resulting in improved resistance to the MSCR test. The synergistic effects of these rejuvenators created a modified binder with superior properties, addressing the limitations associated with conventional rejuvenators. In general, the MSCR test results clearly demonstrated the positive impact of a low-viscosity binder incorporating Castor wax and Corn oil as rejuvenators in the RAP mixture. The rejuvenated binder exhibited enhanced properties.

In Figure 8, the dataset presents the percent difference of recovery and non-recovery for two types of RAP binders: the control RAP binder and the modified RAP binder, which incorporated a rejuvenator. In terms of recovery, the control RAP binder exhibited a percent difference of 4.92%, while the modified RAP binder showed a slightly higher percent difference of 5.87%. These values indicate a small variation in the recovery performance between the two binders, with the modified binder showing a slightly higher difference.

Regarding non-recovery, the control RAP binder displayed a percent difference of 42.27%, indicating a significant deviation from the expected non-recovery value. Conversely, the modified RAP binder, with the rejuvenator, showed a lower percent difference of 35.26% in non-recovery. These findings suggest that the addition of the rejuvenator in the RAP binder resulted in a reduction in non-recovery variability compared with the control binder.

The observed differences in recovery and non-recovery between the control and modified RAP binders highlight the positive influence of the rejuvenator on the binder’s rheological behavior. The lower percent difference in non-recovery for the modified binder indicates a reduced tendency for permanent deformation under applied stress. These findings underscore the effectiveness of the rejuvenator in improving the recovery and non-recovery characteristics of RAP binders, contributing to the development of more resilient asphalt mixtures with enhanced long-term performance and durability.

3.2. TSR Test Results

The TSR test results indicated a significant improvement in the moisture susceptibility of the rejuvenated RAP mixture. The TSR values were calculated as the ratio of the average wet IDT (Indirect Tensile Strength) strength to the average dry IDT strength. Comparing the TSR values of the conventional RAP mixture to the rejuvenated RAP mixture based on Figure 9, there was an observed increase of around 9.47% in the TSR ratio. This suggests that the rejuvenated mixture exhibited enhanced resistance to moisture-induced damage compared with the conventional RAP mixture. The improved TSR ratio can be attributed to the beneficial effects of the low-viscosity binder, Castor wax, and Corn oil rejuvenators. The low-viscosity binder facilitated better coating and adhesion of the binder to the RAP aggregates, resulting in improved bond strength and reduced susceptibility to moisture damage. Additionally, the Castor wax and Corn oil rejuvenators enhanced the binder’s flexibility, further contributing to its moisture resistance properties. These findings align with related research studies that have demonstrated the positive influence of rejuvenators in enhancing the TSR values of asphalt mixtures. The observed increase in the TSR ratio is consistent with the desired goal of improving the moisture resistance and durability of asphalt pavements.

3.3. Dynamic Modulus Test Results

The evaluation of viscoelastic behaviors for the RAP mixtures aimed to assess their performance characteristics. The dynamic modulus test results provide valuable insights into the mechanical behavior and stiffness characteristics of the RAP mixture. Figure 10 illustrates the dynamic modulus values of the RAP mixture at various testing frequencies and temperatures. For instance, at a testing temperature of 25 °C and a frequency of 10 Hz, the dynamic modulus of the RAP mixture with the rejuvenated binder was found to be 25.5 MPa, whereas the reference RAP binder exhibited a dynamic modulus of 28.7 MPa. This represents a slight reduction of 12.7% in the dynamic modulus for the rejuvenated binder, indicating a softer behavior under cold regions to counter the thermal cracking in winter. Moreover, the percentage reduction in dynamic modulus values was observed across different temperatures and frequencies; however, the gap between the two mixed-design scenarios was negligible.

As shown in Figure 11, the incorporation of rejuvenators in the RAP mixture resulted in a notable enhancement in the elastic behavior, as evidenced by the phase angle results. The modified mixture exhibited a significantly lower phase angle value (approximately 3 degrees) compared with that of the controlled mixture. This reduction in the phase angle confirmed the reinforcement of the ductile behavior of the RAP mixture under service conditions. It demonstrated that the rejuvenated mixture possessed improved resistance to deformation and a better ability to withstand stress and strains, thereby enhancing its overall performance and durability.

These findings indicate that the incorporation of the low-viscosity RAP binder and the Castor wax and Corn oil rejuvenators had a significant positive impact on the ductile capacity of the RAP mixture. The comparisons highlight the superiority of the rejuvenated binder in terms of dynamic modulus performance compared with that of the reference RAP binder.

3.4. HWT Test Results

The HWT test was conducted to evaluate the moisture sensitivity and resistance to rutting of the RAP mixture. Figure 12 presents the results of the test, showing the extent of rutting in terms of the rut depth at different numbers of loading cycles. In general, all mixtures displayed a sharp increase in rutting depth in the initial 5000 cycles. At this stage, the referenced RAP mixture exhibited a greater rutting resistance compared with the modified mixes since the brittleness behavior of the aged RAP binder can considerably counter the permanent deformation. However, after the first stripping point at 12,800 HWT cycles, the results indicate that the RAP mixture with the rejuvenated binder exhibited superior resistance to rutting compared with the conventional RAP binder. For instance, the rut depth of the reference RAP binder was measured at 11.77 mm, while the rejuvenated binder showed a significantly lower rut depth of only 10.59 mm and 9.2 mm for the RAP mixture with a high penetration asphalt binder and the RAP mixture containing a low-viscosity binder with rejuvenator, respectively. This represents a reduction of 10.2% and 21.83% in rut depth, demonstrating the improved resistance to deformation and rutting for the rejuvenated binder.

Additionally, it is noteworthy that the stripping phenomenon was absent in the modified mixture, indicating its superior resistance to moisture-induced damage. In contrast, the RAP mixture exhibited signs of stripping at approximately 12,800 cycles. This stark contrast confirms the effectiveness of the rejuvenated mixtures in enhancing their moisture resistance. The incorporation of rejuvenators, such as the low-viscosity binder, Castor wax, and Corn oil, contributed to improved adhesion between the binder and aggregate particles, mitigating the detrimental effects of moisture infiltration. The absence of stripping in the modified mixtures demonstrates their ability to maintain structural integrity and durability even under prolonged exposure to moisture, further validating their suitability for long-lasting pavement applications.

The findings from the Hamburg wheel tracking test support the suitability of the rejuvenated RAP mixture for high traffic and challenging environmental conditions. The reduced rut depth and improved resistance to deformation are indicative of the superior quality and durability of the rejuvenated binder. These results emphasize the potential of using rejuvenation techniques to enhance the performance and longevity of RAP mixtures, making them a promising solution for sustainable and resilient pavement applications.

3.5. Fatigue Test Results

The fatigue test results, as depicted in Figure 13, reveal valuable insights into the performance of the RAP mixture in terms of fatigue crack resistance. The fatigue test was conducted to evaluate the resistance of the RAP mixture to repeated loading and its ability to withstand fatigue-induced cracking. It is important to note that the criterion for fatigue in the fatigue crack test results was set at a 50% reduction in the stiffness of the material compared with its original value. This criterion was used to determine the point of fatigue failure, indicating that the material’s stiffness had dropped to half of its initial stiffness after enduring repeated and continuous loading cycles.

Figure 13 illustrates the results of the fatigue test, showing the number of load cycles to failure for both the reference RAP binder and the rejuvenated binder. The results demonstrate a substantial improvement in fatigue performance with the rejuvenated binder compared with those of the reference RAP binder. The rejuvenated binder exhibited a 28.55% increase in the number of load cycles to failure, indicating a significant enhancement in resistance to fatigue cracking. For instance, the reference RAP binder failed at 16,215 load cycles, while the rejuvenated binder showed no signs of cracking and continued to endure the applied load even after 20,845 load cycles. These findings are consistent with related research studies that have highlighted the positive impact of binder rejuvenation on fatigue resistance.

The improved fatigue performance of the rejuvenated binder can be attributed to the unique properties of the Castor wax and Corn oil rejuvenators. Castor wax enhances the binder’s stiffness and resistance to rutting, while Corn oil improves flexibility and resilience, resulting in enhanced resistance to fatigue cracking. The synergistic effects of these rejuvenators contributed to the overall improved fatigue performance of the RAP mixture. The observed 28.55% increase in fatigue life with the rejuvenated binder provides strong evidence of its superior durability and potential for long-lasting pavement applications. These findings align with previous studies that have demonstrated the effectiveness of rejuvenation techniques in enhancing the fatigue resistance of RAP mixtures. The use of rejuvenation techniques presents a promising approach to extending the service life of RAP pavements and improving their overall sustainability.

3.6. General Discussions

The incorporation of a low-viscosity binder and Castor wax with Corn oil rejuvenators played a major role in improving the performance of the RAP binder. These additives addressed key challenges associated with RAP mixtures and enhanced their overall performance. The low-viscosity binder played a crucial role in improving the workability and blending characteristics of the RAP binder. Its high penetration grade facilitated better coating and adhesion between the RAP aggregates and binders, resulting in improved bonding and overall structural integrity. This improvement in bonding contributed to enhanced resistance against fatigue-induced cracking and improved durability of the RAP binder. Castor wax and Corn oil rejuvenators also played a significant role in the improved performance of the RAP binder. The regulator enhanced the flexibility and resilience of the binder, leading to improved resistance against fatigue cracking. The combination of these rejuvenators resulted in a synergistic effect, further enhancing the overall performance of the RAP binder. Comparisons with related research studies have shown consistent findings regarding the positive impact of bio-oil rejuvenators on RAP mixtures. Similar studies have reported improved workability, bonding, rutting resistance, and fatigue performance when these additives were incorporated. This consistency in findings supports the effectiveness and potential of these additives in enhancing RAP mixtures.

However, it is important to note the limitations of the current study. The research focused on laboratory-scale testing, and the findings need to be validated through field trials and long-term performance monitoring. Additionally, the study primarily examined the impact of the additives on specific performance characteristics, and further investigation is needed to explore their effects on other properties of the RAP mixture, such as moisture sensitivity and aging resistance. The results of this study provide a foundation for future research in this field. The next stage of this research could involve conducting field trials to evaluate the performance of the rejuvenated RAP binder under real-world traffic and environmental conditions. Long-term monitoring and performance evaluation would provide valuable insights into the durability and sustainability of RAP mixtures with a low-viscosity binder and Castor wax and Corn oil rejuvenators. Further investigation into the optimal dosage and combination of these additives can also be explored to maximize their benefits and improve the performance of RAP mixtures.

4. Conclusions

In this study, the effectiveness of incorporating a low-viscosity binder and Castor wax with Corn oil rejuvenators in improving the performance of Reclaimed Asphalt Pavement (RAP) mixtures was investigated. Through a comprehensive evaluation of various laboratory tests, the following conclusions can be drawn:

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The MSCR test results revealed that the RAP mixture with the newly developed rejuvenators exhibited a 20% reduction in cumulative strain rate under the applied creep load compared with that of the conventional rejuvenators. At an MSCR level of 3.2 kPa and 160 s, the strain value for the RAP mixture with the new rejuvenators was approximately 20% lower (around 530%) compared with the original RAP mixture (about 670%). The RAP mixture with the new rejuvenators initially exhibited behavior similar to the conventional rejuvenators in the early stages of testing (100–120 s) under a shear creep load of 3.2 kPa. However, as the testing duration increased, the RAP mixture with the new rejuvenators showed significantly reduced strain accumulation compared with the conventional mixture.

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The TSR test results demonstrate the effectiveness of the rejuvenated RAP mixture. The Tensile Strength Ratio (TSR) for the rejuvenated mixture showed a notable 9.47% improvement compared with that of the conventional RAP mixture. This improvement in TSR indicates enhanced resistance to moisture-induced damage, such as stripping and cracking, contributing to the longevity of the pavement structure.

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The evaluation of viscoelastic behaviors for the RAP mixtures revealed that the rejuvenated binder exhibited a slight reduction of 12.7% in dynamic modulus compared with the reference binder, indicating a softer behavior under cold temperatures. Furthermore, the incorporation of rejuvenators resulted in a significant improvement in the elastic behavior, as evidenced by the lower phase angle value (approximately 3 degrees) in the modified mixture. This indicates enhanced resistance to deformation and improved overall performance and durability of the RAP mixture.

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The results demonstrate that the RAP mixture with the rejuvenated binder exhibited superior resistance to rutting compared with the conventional RAP binder. The rut depth for the reference RAP binder was measured at 11.77 mm, while the rejuvenated binder showed significantly lower rut depths of only 10.59 mm and 9.2 mm for the RAP mixture with a high penetration asphalt binder and the RAP mixture containing a low-viscosity binder with rejuvenator, respectively. This represents a reduction of 10.2% and 21.83% in rut depth, indicating improved resistance to deformation and rutting. Additionally, the absence of the stripping phenomenon in the modified mixture further confirms its superior moisture resistance.

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The results demonstrate a significant improvement in the fatigue performance of the rejuvenated binder compared with the reference RAP binder. The rejuvenated binder exhibited a remarkable 28.55% increase in the number of load cycles to failure, indicating a substantial enhancement in resistance to fatigue cracking. In contrast, the reference RAP binder failed at 16,215 load cycles, while the rejuvenated binder showed no signs of cracking and continued to withstand the applied load even after 20,845 load cycles.

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In general, the incorporation of a low-viscosity binder, Castor wax, and Corn oil rejuvenators in the RAP binder improved its workability, bonding, and overall performance. The low-viscosity binder enhanced coating and adhesion between RAP aggregates and binders, resulting in improved structural integrity and resistance against fatigue-induced cracking. Castor wax and Corn oil may enhance flexibility and resilience, further enhancing the binder’s performance. The combined effect of these rejuvenators resulted in a synergistic improvement in the overall performance of the RAP binder.

It is essential to acknowledge the limitations of this study. Laboratory testing provides valuable insights, but real-world conditions and long-term performance may present different challenges. Therefore, further validation through extensive field trials and monitoring is recommended to confirm the findings and assess the rejuvenated RAP mixture’s performance in actual pavement applications. Future research should focus on optimizing the dosage and combination of rejuvenators to maximize their benefits and fine-tune the rejuvenated RAP mixture’s performance characteristics. Additionally, long-term performance monitoring of the rejuvenated RAP mixture in field applications will provide valuable data on its durability and sustainability.