Feasibility of Carnauba Wax Rejuvenators for Asphalt Concrete with Vacuum Tower Bottom Binder

Abstract

This study addresses the need for effective rejuvenators in asphalt concrete mixtures containing Vacuum Tower Bottom (VB) binder, a by-product of petroleum refining. We investigated the use of a softening rejuvenator, comprising Carnauba (5.5%), Soybean oil (3%), water (81%), surfactant (1.5%), and additive (3%) from a Korean refining company, to mitigate the brittleness of VB binder. Laboratory experiments were conducted to compare the performance of the modified binder with the original hardened binder. The results showed that adding the rejuvenator improved the properties of the VB binder. Optimal asphalt grades were achieved with a 2% content of the softening additive in the VB binder. The rejuvenator enhanced moisture resistance, leading to settlements comparable to the control asphalt. Settlements after 20,000 load repetitions were 11.49 mm for the modified mixture, which were slightly better than the control material at 12.44 mm. Moisture stripping points occurred at around 16,000 cycles for the modified mixture, while the control material experienced them at approximately 13,000 cycles. Under freeze-thaw cycles, the modified mixture exhibited enhanced durability compared to the control mixture. The control mixture experienced a significant increase in rutting value of approximately 59.7% (from 12.4 mm to 19.7 mm), while the modified mixture showed a relatively lower increase of approximately 37.4% (from 11.5 mm to 15.8 mm). Additionally, the modified VB mixture demonstrated approximately 7.8% higher dynamic modulus at lower temperatures, indicating improved mechanical properties. It also displayed superior fatigue crack resistance, with a fatigue life of 18,385 cycles compared to 15,775 cycles for the control asphalt. Field results confirmed that the VB asphalt mixture with the rejuvenator achieved comparable site compactness to the control mixture, indicating successful compaction performance. These findings highlight the rejuvenator’s efficacy in mitigating binder stiffening and restoring the original state of aged asphalt binders.

1. Introduction

Petroleum products derived from light crude oil have long been extensively used in various industries [1]. However, due to the limited reserves of light crude oil, estimated to be around one trillion barrels, and its declining production since the peak in 2010, alternative sustainable energy sources have gained significant attention [2]. Exploration and research efforts are focused on unconventional fossil fuels, such as oil sands and oil shale, found in the sea to ensure a continuous supply of crude oil [3].

In line with global advancements, South Korea has been actively developing new-concept, advanced heavy oil upgrading technologies to produce high-quality synthetic crude oils [4]. These technologies involve upgrading refining facilities, which traditionally rely on petroleum energy [5]. As the processes for upgrading heavy oil continue to evolve, the utilization of the by-products, including asphalt (bitumen) [6,7], vacuum residue (VR) [8], and vacuum tower bottom (VB) [9], presents unique challenges. These by-products possess complex physical and chemical properties that differ considerably from conventional bitumen products traditionally used in construction [10]. Overcoming these differences and effectively harnessing the potential of these materials requires innovative approaches and a deep understanding of their distinct characteristics [11].

Asphalt, a vital material in the road construction industry, must possess sufficient durability and workability to ensure optimal performance under field conditions [6]. At lower temperatures, asphalt exhibits hardness and brittleness, while extreme heat causes it to flow in the manner of a viscous liquid [6,12]. Its composition primarily consists of two components: asphaltene and maltene. Asphaltene, comprising 5%–25% of asphalt, is a high molecular weight compound with polar functional groups, such as carboxyl, alcohol, ketone, amine, and sulfur [13]. Maltene, on the other hand, has a relatively lower molecular weight and contributes to the workability of asphalt, being soluble in heptane [14].

However, the final stages of heavy oil refining result in the excessive removal of maltene from by-products such as VR and VB due to advanced refining technologies. This excessive removal hampers the workability of these by-products, leading to higher asphaltene content [4]. Consequently, their utilization as binders for asphalt mixtures becomes limited. Similar to waste asphalt concrete or recycled aggregates, these by-products exhibit brittleness [15]. Moreover, the ongoing advancements in crude oil refining technologies contribute to the further degradation of the quality of asphalt binders [16].

In order to maintain the desired workability and constructability of asphalt used in road pavements, the inclusion of functional oils or rejuvenators is of utmost importance [17]. These oils play a critical role in ensuring that the asphalt material is suitable for industrial processing and a reliable component for pavements [18]. However, with the advancement of crude oil refining technologies, there is a growing concern about the potential decline in the quality of road pavement materials [19]. Thus, there is a need to address this issue and find innovative solutions to preserve the performance and integrity of asphalt for sustainable road construction [15,16].

Recent studies have delved into the effects of mineral presence on the oxidative degradation of bitumen [20] and have explored the kinetics of oxidation thermolysis in asphaltenes with varying chemical prehistory. Specifically, Vasilyev et al. investigated the oxidative destruction of bitumen in the presence of minerals, highlighting the intricate processes that can occur within asphalt mixtures [20]. Similarly, work by Sharikov et al. delved into the kinetics of oxidation thermolysis, offering insights into the chemical transformations of asphaltenes under different conditions [21]. These findings collectively underscore the multifaceted nature of asphalt performance, prompting further exploration and inquiry into the complex interplay between bituminous materials and their surrounding environment.

Despite the increasing interest in VB, limited research has been conducted on its application in pavements and the rejuvenation of VB through the use of additives. Therefore, conducting comprehensive research to explore the practical utilization of VB after appropriate treatment is vital for the development of sustainable pavement solutions. Furthermore, the exploration of incorporating biomaterials in the construction industry remains largely unexplored in South Korea, warranting further investigation [22]. The utilization of rejuvenators in asphalt concrete mixtures containing VB binder presents a significant research need in the field of road pavement materials [8,9,10]. Direct application of VB binder to road pavements could cultivate distress due to its inherent brittleness. Therefore, there is a pressing need to investigate and develop effective rejuvenators that can address the stiffening effect of VB binders and restore their original properties.

The novelty of this research lies in the exploration of a softening rejuvenator comprising Carnauba and Soybean oil, along with other additives, obtained from a Korean refining company. The use of natural oils as rejuvenators in asphalt mixtures is a novel approach that offers potential benefits in terms of enhancing the performance and durability of the pavement. By incorporating these rejuvenators, it is expected that the brittleness of the VB binder can be mitigated, leading to improved moisture resistance, viscoelastic behavior, fatigue crack resistance, and resistance against long-term rutting.

This study focuses on the utilization of a specialized softening rejuvenator composed of Carnauba (5.5%), Soybean oil (3%), water (81%), surfactant (1.5%), and additive (3%) obtained from a renowned Korean refining company. The objective is to address the inherent brittleness of the vacuum tower bottom, which is a by-product of the petroleum refining process. Furthermore, this study contributes to the knowledge base by conducting comprehensive laboratory experiments, including dynamic modulus, Hamburg wheel tracking, and fatigue tests, to evaluate the quality of the asphalt concrete mixture. These tests provide a thorough assessment of the rejuvenated mixture’s mechanical properties, moisture resistance, rutting resistance, and cracking resistance. The investigation of these key performance indicators provides valuable insights into the effectiveness of the rejuvenator in improving the properties of the VB binder and optimizing the asphalt mixture. By addressing these research objectives, this study contributes to the advancement of sustainable pavement technologies and explores the potential benefits of incorporating biomaterials in construction practices.

2. Materials and Methods

This study aimed to examine the fundamental characteristics of the vacuum tower bottom (VB) material and compare it with the conventional asphalt binder (PG 64–22 from SK2 company, Seoul city, South Korea) through laboratory experiments. Additionally, a novel laboratory approach was introduced to restore the performance of the VB material. By incorporating rejuvenator additives, an optimized VB binder was developed. A visual summary of the research findings is depicted in Figure 1.

In this research, the aggregate used in the asphalt mixture was sourced from a local supplier, while the binder consisted of VB obtained from a petroleum refining facility. Additionally, a rejuvenator comprising carnauba wax and soybean rejuvenator (CS rejuvenator) was incorporated into the asphalt mixture. The CS rejuvenator, along with its specific composition and content, was obtained from a Korean refining company. These carefully selected materials and additives formed the basis for investigating the performance and characteristics of the asphalt mixture in various laboratory tests and field applications.

2.1. Materials

2.1.1. Fundamental Properties

Fundamental property tests, including performance grade (PG) tests, were conducted to evaluate the fundamental characteristics of VB.

Table 1. General properties of tested binders.

Testing Type	Testing Temperature	Reference Binder	Vacuum Tower Bottom Binder
Penetration	25 °C	64	57
Rotational Viscosity, cP	135 °C	429	792

DSR: Dynamic Shear Rheometer; RTFO: Rolling Thin Film Ovens.

provides an overview of the properties of the control sample and VB. Analysis reveals that the high-temperature grades of VB were greater compared to the control sample. This difference can be attributed to advancements in the refining process, which resulted in an increased asphaltene content and a subsequent decrease in properties associated with softness, ultimately leading to a more brittle material.

The comprehensive analysis of the basic properties presented in Table 1 provides a detailed understanding of the distinct characteristics exhibited by the control and VB samples. The penetration test results at 25 °C revealed a slightly lower penetration value of VB (57) compared to the control sample (64), indicating a relatively firmer consistency of the VB material. Moving on to the rotational viscosity measurements, it was observed that VB displayed higher viscosity values at 135 °C (792 cP) compared to the control sample. These elevated viscosity levels in VB can be attributed to the increased asphaltene content resulting from advancements in the refining process, which subsequently contributes to a reduction in softness-related properties and an overall transition towards a more brittle material.

2.1.2. Enhancing VB: Strategies for Improvement

General Concept

The exploration of VB involved a comprehensive analysis of its SARA component ratios and fundamental physical properties. The findings revealed notable distinctions when compared to conventional asphalt binders. VB exhibited elevated softening points and DSR results, indicating increased resistance to plastic deformation. However, these materials displayed reduced flexibility and stiffness at lower temperatures, making them prone to cracking. Additionally, their high viscosity posed challenges during processing. These characteristics bear a resemblance to those observed in aged asphalt binders, which can be modified or revitalized through the integration of specialized additives.

Traditionally, regeneration additives incorporating aromatic-oil-based materials have been employed to address these challenges by reducing viscosity levels and enhancing flexibility, thereby improving elongation properties [23,24]. Nevertheless, the cost implications of such additives have driven researchers in the field of bio-engineering to explore alternative materials, including bio-oils and bio-polymers, to facilitate the regeneration of aged asphalt [18,19].

Among these alternatives, natural oils, particularly vegetable oils, have emerged as promising candidates for enhancing asphalt performance. Vegetable oils, such as soybean oil, castor oil, palm oil, rapeseed oil, and sunflower oil, consist of triglyceride molecules. Notably, soybean oil exhibits favorable double-bond characteristics, making it an attractive option for asphalt modification [25,26]. Hence, the present study focuses on the application of carnauba and soybean oil to fabricate an optimized rejuvenator for VB binders, aiming to evaluate their suitability for road pavement applications.

The specific composition of the liquid emulsifier employed in the study is referred to as a modified binder. This formulation includes 8.5% wax (carnauba), 3% vegetable oil (soybean), 5.5% mineral oil, 1.5% surfactants (fatty acid (C8∼C20) amine surfactant), 0.2% additive, and 81.3% water, totaling 100%. The properties of the rejuvenator components were summarized in

Table 2. Properties of oil components in the rejuvenator.

Property	Carnauba Oil	Soybean Oil
Viscosity	75 cP	45 cP
Density	0.88 g/cm³	0.93 g/cm³
Flash Point	260 °C	220 °C
Acid Value	0.35 mg KOH/g	0.45 mg KOH/g
Iodine Value	48 g I₂/100 g	132 g I₂/100 g
Saponification Value	195 mg KOH/g	187 mg KOH/g

and

Table 3. Properties of surfactant.

Property	Surfactant
Chemical Structure	Sodium dodecyl sulfate
Surface Tension	38 mN/m
Critical Micelle Concentration	4 mM
pH Range	7
Solubility	Soluble in water
Emulsifying Power	High
Foaming Ability	Strong
Biodegradability	Biodegradable

, respectively. By incorporating carnauba and soybean oil as key components, the objective is to optimize the properties of VB, opening new avenues for advancements in road pavement materials.

Regarding the surfactant used in this research, antioxidants are additives used in rejuvenators to combat oxidative aging of the asphalt binder. They provide excellent protection against aging, improving the binder’s durability and extending its service life. Commonly used antioxidants include phenolic compounds, such as butylated hydroxytoluene and hindered phenols, which exhibit high antioxidant activity and inhibit binder oxidation.

A potential method for synthesizing the rejuvenator described in the laboratory experiment can be achieved through the following steps [27,28,29]:

-

Measure and prepare the required ingredients: Gather the necessary components for the rejuvenator synthesis, including carnauba (5.5%), soybean oil (3%), water (81%), surfactant (1.5%), and additive (3%) obtained from SK2 refining company, Seoul city, South Korea.

-

Heat and melt the wax: Begin the synthesis process by heating the carnauba wax to a temperature of 70 °C for 2 h, which is sufficient to melt it. This can be achieved using a suitable heating apparatus or a water bath.

-

Add soybean oil: Once the wax has melted, gradually introduce the soybean oil into the mixture while stirring continuously. Ensure that the oil is evenly distributed throughout the molten wax. Maintain the temperature at 70 °C and continue stirring for an additional 30 min.

-

Incorporate mineral oil: Next, introduce the mineral oil into the mixture and continue stirring. The mineral oil should blend with the wax and soybean oil, forming a homogeneous mixture. Stir at 70 °C for 1 h.

-

Introduce surfactants: Add the fatty acid (C8∼C20) amine surfactant to the mixture while maintaining stirring. The surfactant aids in emulsifying and stabilizing the rejuvenator formulation. Continue stirring for 30 min at 70 °C.

-

Include the additive: Incorporate the designated additive into the mixture, continuing to stir to ensure uniform dispersion. The additive serves a specific purpose in enhancing the rejuvenating properties of the final product. Stir at 70 °C for an additional 30 min.

-

Dilute with water: Gradually add the pre-measured amount of water to the mixture while stirring continuously. This step helps achieve the desired consistency and facilitates the emulsification of the rejuvenator. Stir for 2 h at 70 °C.

-

Continue stirring and cooling: Maintain stirring the mixture as it cools down. This ensures that all components are well blended, and the rejuvenator attains a stable, uniform composition. Allow the mixture to cool to room temperature over a period of 2 h while stirring intermittently.

-

Characterize and evaluate: Once the rejuvenator has cooled and solidified, assess its physical properties and performance characteristics through laboratory tests. This includes analyzing its rheological properties, compatibility with VB, and its potential for enhancing the desired properties of road pavement materials.

2.1.3. Mix Design

To ensure consistency and compliance with industry standards, a mix design process was implemented in this study, following the guidelines set forth by the Korea Ministry of Land, Infrastructure, and Transport for 13-mm (nominal maximum aggregate size) NMAS asphalt mixtures used in surface layers. A granitic aggregate was selected for this purpose. The particle size distribution of the aggregate employed in the mix design is depicted in

Table 4. Gradation of aggregate.

Sieve Size (mm)		20	13	10	5	2.5	0.6	0.3	0.15	0.08
Percent Passing (%)	13 mm	100.0	96.8	81.2	16.5	6.1	-	-	-	-
	Crushed sand	100.0	100.0	100.0	97.8	76.1	41.3	22.7	11.8	4.6
	Filler	100.0	100.0	100.0	100.0	100.0	100.0	98.8	97.2	88.5

. The mix design was carried out using the Superpave mix design method, utilizing a compactor for compaction purposes. The asphalt binder used in this research is 6%. By adhering to established protocols, the study aimed to create a well-designed asphalt mixture that meets the required specifications and performance criteria.

2.1.4. Compaction Process

To prepare the asphalt mixture, the aggregates were accurately weighed according to the mix design specifications. The granitic aggregate, with the particle size distribution outlined in Table 4, was placed in a mixing bowl. The rejuvenator, synthesized as per the laboratory experiment, was then added to the mixture in the prescribed ratio. The components were thoroughly mixed using a mechanical mixer until a homogeneous blend was achieved. After the mixing process, the asphalt mixture was compacted using the Superpave Gyratory compaction method [30]. The compaction procedure involved placing the mixture in a Superpave Gyratory compactor (Matest, Treviolo BG, Italy) and subjecting it to the specified number of gyrations under controlled conditions. This compaction method simulates the actual field conditions more accurately, resulting in improved compaction and performance of the asphalt specimens. Following compaction, the specimens were cured to promote binder stiffening and achieve the desired properties. The curing process involved placing the compacted specimens in an oven set at a specific temperature for a predetermined duration. The specimens are allowed to cure at 60 °C for a period of 24 h after being removed from the molds. This allowed the rejuvenator to penetrate and react with the asphalt binder, leading to the desired modifications and improved performance. The meticulous addition of the rejuvenator and the careful curing of the specimens ensured the optimal incorporation and activation of the rejuvenating properties, contributing to the overall performance enhancement of the asphalt mixture.

The compacted specimens were targeted to achieve an air void content of 4.0 ± 0.5%. Following compaction, the specimens were cured at 60 °C for 24 h and then subjected to an additional curing period at room temperature for 24 ± 3 h. Regarding aging conditions, short-term oven aging was conducted to simulate accelerated aging effects, while long-term oxidization aging was not performed in this study.

2.2. Methods

2.2.1. Assessment of Viscoelastic Characteristics of VB Mixtures

To assess the viscoelastic properties of VB mixtures, an extensive experiment was conducted using the advanced MTS 810 testing machine as shown in Figure 2. The experiment aimed to capture the intricate variations in the modulus of elasticity under different temperature and load conditions. Meticulous attention was given to multiple loading cycle parameters, including a wide range of frequencies from 20 to 0.1 Hz and temperatures spanning from −10 °C to 54 °C. The experiment strictly followed the guidelines outlined in the AASHTO TP 62 standard [31], employing the load control test method to ensure precise and dependable outcomes. The reported test results were obtained by averaging the data from three replicates for each mix design. This approach of replicating the tests enhances the reliability and precision of the obtained results.

The intriguing nature of asphalt mixtures lies within their thermo-rheologically simple (TRS) behavior when operating within the linear viscoelasticity range. This remarkable characteristic involves a reduced time or frequency, effectively combining the dynamic interaction between time and temperature. Employing the powerful time-temperature shift factor ($a_T$) as the basis for determining the reduced frequency, the experiment meticulously converted the measured frequency at specific temperatures to the corresponding frequency at a reference temperature. The outcome of this meticulous conversion process facilitated the construction of a highly informative and insightful master curve. The master curve, cleverly represented as a sigmoidal function, offered a comprehensive perspective on the viscoelastic properties of the asphalt mixture across a broad range of reduced frequencies. Additionally, delving deeper into the intriguing correlation between the mobility coefficient and temperature, an intriguing quadratic function (Equations (2) and (3)) elegantly captured the intricate interdependence between these variables. To ensure the utmost accuracy and precision in determining the sigmoidal coefficient ($a_T$), a meticulous minimum error method was employed, expertly supported by the powerful Solver tool integrated into the EXCEL application. Additionally, a is the sigmoidal coefficient, and b is a parameter in the quadratic function. This comprehensive and meticulous approach enabled a thorough assessment of the viscoelastic behaviors of VB mixtures, unraveling the complex dynamics within their properties. In this manner, the experimental assessment of VB mixtures’ viscoelastic behaviors embodied a comprehensive and meticulous approach, leveraging advanced techniques and precise calculations to unravel the intriguing complexities within their dynamic properties.

$$f_R=f\times a_T$$

(1)

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

(2)

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

(3)

where

• $f_R$ is the reduced frequency
• f is the frequency of loading at the relevant temperature
• $a_T$ is time-temperature shift factor
• a, b, d, g: represent function parameters
• ${\alpha }_1$, ${\alpha }_2$, and ${\alpha }_3$ are coefficients that characterize the temperature dependency

2.2.2. Assessment of Fatigue Crack Resistance in Mixtures

An in-depth examination of the fatigue crack resistance in VB mixtures was carried out using a standardized testing method outlined in AASHTO T 321–07 [32] (see Figure 3). The objective was to evaluate the material’s ability to withstand repeated loading and its subsequent impact on stiffness. Test specimens, meticulously fabricated to meet specified dimensions, featured dimensions of 380 mm in length, 50 mm in height, and 63 mm in width. The core essence of the fatigue test was to scrutinize the capacity of the asphalt mixture to endure prolonged and repetitive loading cycles. Fatigue failure was identified as the point where the material’s stiffness diminished to only 50% of its original value after undergoing repeated and continuous loading. The initial stiffness, used as a crucial reference, was established by subjecting the material to 50 repetitive load cycles at a controlled room temperature of 20 °C. The obtained outcomes represent the mean value of three replicates conducted for each mix design.

By subjecting the VB mixtures to this rigorous fatigue assessment, the evolution of stiffness was carefully tracked to gain valuable insights into the material’s resistance to fatigue-induced cracking. This meticulous approach facilitated a comprehensive evaluation of the mixture’s structural integrity when subjected to realistic traffic loading conditions, providing crucial information to inform decisions regarding its suitability for robust and long-lasting pavement applications.

2.2.3. Hamburg Wheel Tracking Test

To assess the moisture sensitivity of the VB asphalt mixture, a comprehensive Hamburg wheel tracking test was conducted in accordance with the prescribed guidelines outlined in AASHTO T 324 [33]. The purpose of this test was to determine the resistance of the mixture to moisture-induced damage. During the test, the VB asphalt mixture specimens were subjected to a constant wheel load of 705 ± 4.5 N while being immersed in water maintained at a controlled temperature of 50 °C. The test followed a predetermined number of loading cycles to simulate the anticipated traffic loads and environmental conditions as presented in Figure 4. The specimens were monitored for the occurrence of stripping points, which are defined as abrupt changes in settlement slope, indicating the susceptibility of the material to moisture damage. Each mixture was tested using three replicates, and the average result was calculated.

It is worth emphasizing that the appearance of stripping points should be carefully monitored after a minimum of 10,000 cycles, while the settlement should remain within the acceptable limit of 20 mm after 20,000 repetitions in the Hamburg wheel tracking test. In order to ensure consistent and precise results, meticulous attention was given to the fabrication of each sample. The specimens were meticulously prepared with precise dimensions, featuring a height of 60 mm and a diameter of 150 mm while maintaining a targeted porosity level of 7.0 ± 1.0%. These dimensions and porosity levels were selected to ensure the consistency and representativeness of the tested samples. By conducting the Hamburg wheel tracking test, valuable insights into the moisture sensitivity of the VB asphalt mixture can be obtained. This information will contribute to the understanding of the material’s performance under moisture-related conditions, aiding in the development of more durable and resilient asphalt pavements.

2.2.4. Assessment of Plastic Settlement

The capability of the VB mixture to withstand plastic deformation was evaluated utilizing the Model Mobile Load Simulator 3 device (MMLS3) accelerated pavement tester [34]. Originally developed in Stellenbosch, South Africa, the MMLS3 has gained widespread adoption for assessing plastic deformation resistance, moisture sensitivity, and crack resistance in asphalt pavements. This advanced testing device accurately replicates the real-world conditions experienced by road pavements. It has been employed in asphalt pavement research across various countries, including the United States, Switzerland, China, and others [34]. The MMLS3 operates by subjecting the asphalt mixture to continuous loops of traffic load simulation in a single direction. It has the remarkable capacity to deliver approximately 7800 traffic loads per hour, closely mimicking field conditions. The test data for each mixture was obtained by conducting three replicates and averaging the results. In this particular study, tests were conducted under dry conditions and at an elevated temperature of 50 °C to evaluate the plastic deformation resistance. By subjecting the VB mixture to rigorous testing using the MMLS3, this study aimed to assess its ability to resist plastic deformation. These experiments provided crucial insights into the material’s performance under high-stress conditions, enabling a comprehensive understanding of its suitability for long-lasting and resilient road pavement applications.

2.2.5. Durability Test (Freeze-Thaw Cycles)

Under normal conditions, it can be difficult to determine the effectiveness of the CS rejuvenator mixture. Therefore, the aim of this study is to investigate the durability of reinforced specimens after subjecting them to multiple freeze-thaw (F-T) cycles. The specimens were prepared by incorporating the CS rejuvenator into the HMA mixture at a predetermined content. Gyratory compaction was employed to achieve the desired density and compaction levels. To assess the durability, the specimens underwent two stages of testing. In the first stage, they were exposed to 5 freeze-thaw cycles, with each cycle consisting of freezing at −10 °C for 24 h, followed by thawing at room temperature (25 °C) for 24 h. Subsequently, in the second stage, a more extensive evaluation was conducted with ten additional freeze-thaw cycles under the same temperature and duration conditions. Each freeze-thaw cycle subjected the specimens to alternating freezing and thawing in a freeze-thaw chamber. The impact of the freeze-thaw cycles on the durability of the reinforced mixture was assessed through the HWT and dynamic modulus tests. These tests were conducted to determine the effectiveness of the CS rejuvenator mixture in enhancing the durability of the HMA specimens under freeze-thaw conditions.

2.3. Evaluation of the Practical Application of VB Asphalt Mixture

2.3.1. Field Test Construction Overview

To examine the real-world suitability of the VB asphalt mixture as a road pavement material, a comprehensive field test construction was conducted. The aim was to assess its compliance with national standards in South Korea and verify its performance under actual operating conditions. The field test site selected was located at an industrial park in South Korea, spanning a total length of 300 m (150 m per lane).

2.3.2. Construction Process Utilizing VB Asphalt Mixture

The field test involved the construction of a control pavement using the same aggregate gradation, binder content, and target air void as the modified mixture. The only difference was that the control pavement utilized Hot Mix Asphalt (HMA) with a PG64-22 binder (SK2 company, Seoul city, South Korea). This approach was adopted to assess the construction feasibility and performance of the modified mixture in comparison to the established HMA standard. The construction procedure for the VB-modified asphalt mixture involved precise temperature management and meticulous compaction practices. Prior to unloading the mixture from the transport truck, the temperature was carefully monitored and recorded within a specific range of 155 ± 5 °C. These temperature measurements were also documented during the installation process, particularly just before initiating the initial compaction. Throughout the construction progress, temperature readings were continuously monitored at various stages: approximately 150 ± 5 °C for the first compaction using a Macadam roller, followed by 130 °C for the next compaction performed with a tire roller, and finally, achieving a temperature of lower than 100 °C for the following compaction utilizing a tandem roller. A comprehensive depiction of the construction process, outlining the procedures employed, is presented in Figure 5 for reference, while Figure 6 presents the pavement quality checking.

3. Results and Discussions

3.1. PG Test Results

The PG test was conducted to evaluate the performance of the asphalt mixture based on different rejuvenator content ratios.

Table 5. Selection of optimized rejuvenator content.

Test Type	Rejuvenator Content (%)
	1%	2%	3%
Original DSR G*/sinδ ≥ 1.0 kPa [35]	0.86	1.29	1.35
RTFO DSR G*/sinδ ≥ 2.2 kPa [35]	3.28	3.48	3.51
Stiffness < 300 MPa [36]	281	272	267
M-value > 0.3 [36]	0.279	0.307	0.318

G*: the complex modulus.

summarizes the selection of the optimized rejuvenator content based on various test types. The data presented shows the performance of the original DSR, RTFO DSR, stiffness, and M-value for different rejuvenator contents of 1%, 2%, and 3%. The findings revealed that varying the rejuvenator content from 1% to 3% with a constant VB binder had a significant impact on the mixture’s performance. A rejuvenator content of 3% met the general criterion, indicating favorable workability and consistency. However, it may not meet the stiffness grade requirements. On the other hand, the use of 1% rejuvenator content resulted in a binder that was too stiff, with negligible improvement observed. The most promising results were obtained with a rejuvenator content of 2%. This content not only fulfilled the criterion but also satisfied the specifications for high-temperature and low-temperature grades. Therefore, it can be concluded that a softening additive content of 2% yields an optimal asphalt mixture that meets the required penetration, high-temperature, and low-temperature performance grades.

Based on the suggestion from the test results, the equivalent content of the CS rejuvenator used in all tests conducted, including the Hamburg rutting test, viscoelastic performance analysis, and fatigue life analysis, was consistent with the optimal content of 2% determined in this section.

3.2. HWT Test Results

3.2.1. Before Freeze-Thaw Cycles

The HWT was conducted to assess the moisture sensitivity of the VB mixture and specifically investigate the impact of the carnauba wax and soybean rejuvenator (CS rejuvenator). Figure 7 provides a clear illustration of the test findings, revealing the behavior of the mixtures when subjected to repeated loading and moisture exposure. Upon subjecting the control material to 20,000 wheel load repetitions, it experienced a settlement of 12.44 mm and demonstrated moisture stripping points after approximately 13,000 cycles. In contrast, the VB mixture exhibited a settlement of 11.49 mm under the same test conditions, with moisture stripping points occurring around 16,000 cycles. Remarkably, both the control material and the VB mixture exhibited comparable settlements after 20,000 load repetitions. The precision of the Hamburg Wheel Tracking (HWT) test was determined based on three replicates measurements performed on the same mixtures under the same testing conditions. The average rut depth measurements were calculated, and the precision of the HWT test was found to be within an acceptable range of ±5% based on the replicates conducted in this study. The test method precision ensures reliable and consistent measurements of rut depth, allowing for the accurate evaluation of the performance of the asphalt mixtures under repeated wheel loading conditions.

It is important to note that the inclusion of the CS rejuvenator played a crucial role in the slight improvement observed in the performance of the modified mixture. The presence of the CS rejuvenator in the VB mixture contributes to its overall resistance against moisture-induced distress, as evidenced by the delayed occurrence of the stripping issue. While both mixtures demonstrated some susceptibility to moisture-related problems, the presence of the CS rejuvenator holds promise in mitigating these concerns and enhancing the moisture resistance of the VB mixture. The combination of carnauba wax and soybean rejuvenator provides an effective solution for achieving performance comparable to that of the control asphalt, even under challenging moisture conditions.

3.2.2. After Freeze-Thaw Cycles

The behavior of the asphalt mixture after subjecting it to freeze-thaw cycles in the Hamburg Wheel Track test reveals significant reductions in strength for both the control and modified mixtures, as presented in Figure 8. The control mixture experienced a substantial increase in rutting value, rising by approximately 59.7% from 12.4 mm to 19.7 mm. This pronounced increase can be attributed to the detrimental effects of freeze-thaw cycles on the control mixture’s ability to resist permanent deformation. In contrast, the modified mixture demonstrated a relatively lower rate of increase in rutting value, with measurements ranging from 11.5 mm to 15.8 mm, representing an increase of approximately 37.4%.

The inclusion of the CS rejuvenator in the modified mixture provides reasons for its superior performance after freeze-thaw cycles. The CS rejuvenator likely contributes to enhanced adhesion and cohesion within the mixture, leading to improved resistance against rutting. Additionally, the CS rejuvenator may promote better elasticity and flexibility, allowing the modified mixture to withstand the stress and strain caused by freeze-thaw cycles more effectively. The results highlight the potential of the CS rejuvenator as a viable additive for mitigating the negative effects of freeze-thaw cycles on asphalt mixtures. However, further research is necessary to optimize the percentage of the CS rejuvenator and explore its long-term durability under repeated freeze-thaw cycles.

In general, the enhanced performance of the CS rejuvenator-modified VB mixture in terms of moisture sensitivity and freeze-thaw resistance can be attributed to the synergistic effects of the CS rejuvenator, which enhances binder adhesion and cohesion. This improved adhesion and cohesion contribute to the overall durability and moisture resistance of the asphalt mixture.

3.3. Analysis of Viscoelastic Behavior

3.3.1. Before Freeze-Thaw Cycles

The evaluation of viscoelastic behaviors for the VB mixtures aimed to assess their performance characteristics. Figure 9 illustrates the dynamic modulus master curve, showcasing a comparison between the control asphalt mixture and the VB mixture at a reference temperature of 25 °C. Examining Figure 9a, it becomes evident that the VB mixture, incorporating the CS rejuvenator, exhibited a notable improvement in performance compared to the control asphalt mixture. Specifically, at low and room temperatures, the dynamic modulus of the VB mixture was about 7.8% higher than that of the referenced mix. This outcome highlights the enhanced mechanical properties and structural integrity provided by the CS rejuvenator in the VB mixture.

However, it is important to note that this advantage was not observed at high temperatures. The effect of the CS rejuvenator in relieving the brittleness of the material seems to be limited in its ability to mitigate high-temperature stiffness. It is hypothesized that the CS rejuvenator’s functionality primarily lies in reducing brittleness through carbon molecule decomposition or dilution effects rather than directly influencing high-temperature performance.

As shown in Figure 9b, the evaluation of rutting resistance in asphalt concrete mixtures revealed a blurred distinction between the control mixture and the CS rejuvenator mixture at the low-frequency range. The test results indicated a negligible improvement in rutting resistance when using the rejuvenator. As a result, the utilization of the CS rejuvenator should be carefully optimized to achieve the desired balance between improved ductility and the preservation of the modulus of elasticity at high temperatures. By fine-tuning the CS rejuvenator content, the VB mixture can achieve superior performance, surpassing the limitations of the control asphalt mixture.

Overall, the limited effectiveness of the CS rejuvenator in enhancing high-temperature stiffness is hypothesized to stem from its primary role in mitigating brittleness through mechanisms such as carbon molecule decomposition or dilution effects, rather than exerting direct influence on high-temperature performance.

3.3.2. After Freeze-Thaw Cycles

The dynamic modulus test results after subjecting the specimens to freeze-thaw cycles provide valuable insights into the performance of the modified HMA in different temperature ranges. In the low-frequency range, the modified specimens exhibited notable improvements compared to the control specimens. After five freeze-thaw cycles, the dynamic modulus of the modified specimens increased by approximately 110%, reaching 80 MPa, whereas the control specimens had a modulus of 38 MPa, as shown in Figure 10a. This indicates that the inclusion of the CS rejuvenator significantly enhances the stiffness and resistance to freeze-thaw damage in the HMA. However, after ten freeze-thaw cycles, the dynamic modulus of the modified specimens decreased to 55 MPa, while the control specimens showed a further decrease to 18 MPa. These results suggest that the long-term durability of the modified mixture may be compromised when exposed to extended freeze-thaw cycles at high temperatures, resulting in reduced effectiveness of the CS rejuvenator.

In the high-frequency range, the results highlight the positive influence of the CS rejuvenator on the dynamic modulus of the modified HMA specimens, as shown in Figure 10b. After five freeze-thaw cycles, the modified specimens exhibited an approximately 11% increase in the dynamic modulus, with a value of 24,283 MPa, while the control specimens had a modulus of 21,808 MPa. This indicates an enhancement in stiffness and resistance to freeze-thaw damage at low temperatures due to the addition of the CS rejuvenator. After ten freeze-thaw cycles, the modified specimens maintained a higher dynamic modulus of 23,067 MPa, representing an approximate 17% increase, while the control specimens showed a slightly lower modulus of 19,643 MPa. These findings suggest that the CS rejuvenator consistently improves the dynamic modulus of the HMA specimens even after prolonged exposure to freeze-thaw cycles at low temperatures.

In summary, the dynamic modulus test results demonstrate the effectiveness of the CS rejuvenator in enhancing the mechanical properties and durability of the modified HMA specimens under freeze-thaw cycling conditions. The inclusion of the CS rejuvenator leads to significant improvements in the dynamic modulus at high temperatures and maintains enhanced stiffness at low temperatures. However, it is important to note that the modified specimens experienced a decrease in dynamic modulus after prolonged exposure to freeze-thaw cycles at high temperatures. Further research is necessary to explore the underlying factors influencing these trends and optimize the incorporation of the CS rejuvenator to ensure the long-term performance and durability of asphalt mixtures under various freeze-thaw conditions.

Overall, the observed changes in dynamic modulus after freeze-thaw cycles can be attributed to the interaction between the CS rejuvenator and the asphalt binder, where the rejuvenator enhances adhesion and cohesion, leading to improved stiffness and resistance to freeze-thaw damage, especially at low temperatures.

3.4. Analysis of Fatigue Life

The fatigue test results, as depicted in Figure 11, reveal valuable insights into the performance of the VB mixture in terms of fatigue crack resistance. The control specimen exhibited a fatigue life of 15,775 cycles, indicating its susceptibility to fatigue cracking under repetitive loading. In contrast, the VB mixture demonstrated a significantly improved fatigue life of 18,385 cycles, showcasing its enhanced resistance against fatigue-induced cracks. These findings underscore the beneficial effects of incorporating the CS rejuvenator in the VB mixture. By adding the CS rejuvenator, the VB mixture achieves better fatigue crack resistance compared to the control asphalt. This improvement can be attributed to the modified properties imparted by the CS rejuvenator, which contributes to the enhanced performance and durability of the VB mixture.

Furthermore, the dynamic modulus test findings align with the fatigue test findings. The VB mixture displayed a higher modulus of elasticity, approximately 7.8% greater than that of the control, indicating increased stiffness and structural integrity. This augmented stiffness is instrumental in reducing the likelihood of fatigue cracking, as it helps distribute the applied loads more effectively and mitigates stress concentrations within the mixture. In summary, the addition of the CS rejuvenator to the VB mixture enhances its fatigue crack resistance, as evidenced by the extended fatigue life observed in the VB mixture compared to the control asphalt. The improved performance is attributed to the modified properties, including increased stiffness and enhanced structural integrity, imparted by the CS rejuvenator. These findings highlight the potential of the VB mixture as a reliable and durable asphalt pavement material in applications where fatigue cracking is a concern.

On the whole, the improved fatigue crack resistance observed in the VB mixture, as evidenced by the extended fatigue life, can be attributed to the enhanced stiffness and structural integrity conferred by the CS rejuvenator. This increased stiffness distributes applied loads more effectively, reducing stress concentrations within the mixture and contributing to the VB mixture’s enhanced durability against fatigue-induced cracks.

3.5. Analysis of Plastic Deformation

The analysis of plastic deformation, as depicted in Figure 12 and Figure 13, provides insights into the behavior of the samples during the test and the resulting rutting sizes concerning the repeated load cycles. Figure 12 illustrates the characteristics of the specimen after the plastic deformation test, specifically highlighting the asphalt film that peeled off due to tire friction. On the other hand, Figure 13 presents the plastic deformation sizes observed at different loading numbers. Due to potential surface defects in the mixture, the control mixture exhibited relatively lower levels of rutting compared to the modified VB mixture during the early stages of testing. After applying traffic load for 100,000 cycles, both the VB mixture and the control suffered permanent rutting of 2.18 mm and 2.48 mm, respectively. Comparing the final plastic deformation resistance of the VB mixture and the control, it is evident that the VB mixture showed slightly higher resistance to plastic deformation. The difference in deformation between the two materials was relatively small (around 0.25 mm). This indicates that the two materials possess similar levels of plastic deformation resistance. However, it is important to consider the inherent behavior of VB with its high asphaltene content. Despite the small difference in plastic deformation resistance, the stiffness of the VB mixture demonstrated its ability to delay the onset of rutting compared to the conventional mixture. This suggests that the VB mixture offers improved resistance against long-term rutting, which can be attributed to the modified properties imparted by the inclusion of VR material.

Overall, the improved resistance to long-term rutting observed in the VB mixture, despite a small difference in plastic deformation compared to the control, can be attributed to the enhanced stiffness and modified properties introduced by the inclusion of VR material, showcasing the potential of the VB mixture to withstand repeated load cycles more effectively.

After a thorough evaluation of the mixture through a range of indoor tests discussed earlier, it is concluded that the VB mixture can be considered a viable option for an asphalt binder in road pavement applications, provided that an additive is introduced to enhance its functionality. However, it should be noted that the performance and dosage of the additive play a critical role in influencing the overall performance of the mixture. Therefore, careful consideration should be given to selecting an appropriate additive that can deliver the desired performance characteristics while also determining the optimal dosage to ensure the best results.

3.6. Field Compaction Test Results

Throughout the construction process of the actual pavement using the VB asphalt mixture, the in situ performance of the proposed mix was assessed by conducting basic quality tests. The samples of the asphalt mixture were collected from the surface in the form of three boxes, each weighing approximately 20 kg. These samples were then analyzed to determine the general density and compaction level, which serve as fundamental indicators of construction quality.

Furthermore, to evaluate the quality of the pavement, three core samples were extracted from both the referenced section and the VB section. These core samples were utilized to measure the level of compaction achieved and the presence of any porosity within the respective sections. Actual photographs of the core sampling process can be seen in Figure 14.

The assessment of site porosity and compactness was conducted by sampling cores from the test pavement sections, and the results are summarized in

Table 6. Summary of the field performance.

Classification	No.	Actual Density (g/cm3)	Compaction Level (%)
Control HMA mixture (PG64-22)	1	2.238	96.8
	2	2.197	96.3
	3	2.223	95.9
	Average	2.219	96.3
CS rejuvenator + VB	1	2.239	96.6
	2	2.233	96.3
	3	2.227	96.4
	Average	2.233	96.4

. The findings indicate that both the control asphalt mixture and the VB asphalt mixture displayed comparable levels of site compactness. This confirmed that the use of the VB mixture does not pose any significant challenges in terms of achieving sufficient compaction during field applications. Thus, the VB asphalt mixture demonstrates comparable compaction performance, indicating its viability as an option for field implementation. On the whole, the comparable levels of site compactness between the control and VB asphalt mixtures suggest that the incorporation of VB binder did not hinder achieving adequate compaction during field applications, affirming the viability of the VB asphalt mixture for practical implementation.

3.7. Field Observation Results and Comparison after Six Months of Service

The field observation conducted after a 6-month post-construction period revealed no confirmed breakage in both the control section or the reinforced section of the pavement. Visual inspection showed no visible damage, such as cracks or plastic deformation. Weak tire marks were observed along the wheel path; however, they did not indicate significant damage.

Table 7. General field test results.

Parameter	Control Section	Reinforced Section
Surface condition	Good	Excellent
Elastic modulus (MPa)	148.8	171.3
Rutting on section	$2\pm 0.5$ mm	$1.2\pm 0.5$ mm

summarizes the general findings of this stage. The overall surface condition was reported as good for the control section, while the reinforced section exhibited an excellent surface condition. Furthermore, the reinforced section displayed a higher elastic modulus of 171.3 MPa, a 15.4% increase compared to the control section’s value of 148.8 MPa, indicating improved stiffness and resistance to deformation. In terms of rutting, the reinforced section demonstrated a lower average rutting depth of 1.2 ± 0.5 mm, which was 40% less than the control section’s rutting depth of 2 ± 0.5 mm, showcasing enhanced resistance against permanent deformation. It is important to note that the endpoint at the 300 m mark exhibited a higher probability of plastic deformation due to frequent vehicle stops, highlighting the need for continuous follow-up and monitoring. Overall, the field observation results suggest that the reinforced section has shown promising performance in terms of surface condition, elastic modulus, and rutting resistance, indicating the potential effectiveness of the reinforcement in improving the long-term durability and performance of the pavement.

3.8. Discussions

The findings of this research are in agreement with previous studies that have explored the effects of rejuvenators on asphalt mixtures. Several authors have reported similar outcomes when investigating the impact of additives on moisture sensitivity, rutting resistance, viscoelastic behavior, fatigue life, and plastic deformation of asphalt mixtures. For instance, the results of this study align with the findings of Zhang et al. (2012) [29], who also observed improved moisture resistance and delayed moisture stripping points in asphalt mixtures modified with rejuvenators. This consistency suggests that the inclusion of rejuvenators, such as the carnauba wax and soybean rejuvenator (CS rejuvenator), is an effective approach for enhancing moisture resistance in asphalt mixtures. Similarly, the improved fatigue crack resistance observed in the VB asphalt mixture with the CS rejuvenator is consistent with the findings of Ding et al. (2019) [37]. Their research demonstrated that the addition of rejuvenators increased the stiffness and resistance to fatigue-induced cracking in asphalt mixtures, supporting the present study’s findings.

Furthermore, the enhanced resistance to rutting observed in the VB mixture aligns with the results of several studies. Li et al. (2023) [18] reported that the incorporation of rejuvenators contributed to improved rutting resistance and delayed onset of deformation in asphalt mixtures, which is consistent with the findings presented here. These consistent findings across multiple studies provide further validation and support for the effectiveness of rejuvenators, particularly the CS rejuvenator, in enhancing the performance and durability of asphalt mixtures. The agreement with previous research highlights the potential of using rejuvenators as a viable solution for improving the properties and overall performance of asphalt mixtures in various applications.

Considering the long-term durability and sustained effectiveness of the modified asphalt mixture, it becomes crucial to address potential factors that might impact its performance. While the current study predominantly focused on assessing the mechanical attributes of the VB binder modified with carnauba wax and soybean Rejuvenator, an important consideration is the prevention of exudation.

The phenomenon of exudation, if not properly managed, could potentially lead to the loss of additives from the binder matrix, thereby affecting the material’s efficacy and potentially resulting in the formation of undesirable surface films. Although the present investigation did not explicitly delve into exudation, future research endeavors should comprehensively explore this aspect. This involves a detailed examination of additive-binder compatibility, potential additive migration, and the formulation of strategies to mitigate any adverse exudation effects. By addressing these factors, a more robust understanding of the material’s long-term stability and performance can be achieved. In forthcoming stages of this research, a deeper exploration of exudation and its implications is planned, thus contributing to a comprehensive assessment of the material’s behavior within real-world conditions.

4. Conclusions

This study aimed to enhance the performance of asphalt mixtures for road pavement by utilizing a softening additive, the carnauba wax and soybean rejuvenator (CS rejuvenator), derived from state-of-the-art heavy oil refining facilities. The findings from this study provide valuable insights into the feasibility and applicability of the CS rejuvenator in improving the performance of asphalt pavements.

• The PG test results indicate that a VB binder combined with a CS softening additive content of 2% yields an optimal asphalt mixture that meets the required performance grades, highlighting the importance of adjusting proportions for desired performance.
• The addition of the CS rejuvenator in the VB mixture slightly improved its resistance to moisture-induced distress, as evidenced by the Hamburg wheel tracking test. The VB mixture showed a settlement of 11.49 mm after 20,000 load repetitions compared to 12.44 mm for the control material. Moreover, the VB mixture exhibited delayed moisture stripping points at around 16,000 cycles, while the control material experienced stripping issues at approximately 13,000 cycles.
• The control mixture experienced a pronounced increase in rutting value, rising by approximately 59.7% from 12.4 mm to 19.7 mm. In contrast, the modified mixture demonstrated a relatively lower rate of increase in rutting value, with measurements ranging from 11.5 mm to 15.8 mm, representing an increase of approximately 37.4%. The inclusion of the CS rejuvenator in the modified mixture contributes to its durability by enhancing adhesion and flexibility, thereby improving resistance against rutting induced by freeze-thaw cycles.
• The evaluation of viscoelastic behaviors showed that the VB mixture, incorporating the CS rejuvenator, exhibited a slight improvement compared to the control asphalt mixture. At low and room temperatures, the dynamic modulus of the VB mixture was around 7.8% greater than the control, indicating enhanced mechanical properties. However, this advantage was not observed at high temperatures, suggesting that the CS rejuvenator primarily reduces brittleness rather than directly influencing high-temperature performance.
• After five freeze-thaw cycles, the dynamic modulus of the modified specimens increased by approximately 110%, reaching 80 MPa, while the control specimens had a modulus of 38 MPa. Additionally, in the high-frequency range, the modified specimens exhibited an approximately 11% increase in the dynamic modulus after five freeze-thaw cycles, with a value of 24,283 MPa, compared to the control specimens with a modulus of 21,808 MPa.
• The VB mixture, incorporating the CS rejuvenator, exhibited a fatigue life of 18,385 cycles, while the control asphalt had a fatigue life of 15,775 cycles. This indicates a slight improvement in fatigue crack resistance for the VB mixture. The addition of the CS rejuvenator enhances the VB mixture’s performance, providing increased durability and stiffness.
• After 100,000 cycles of traffic load, the VB mixture and control showed rutting of 2.18 mm and 2.48 mm, respectively. Although the difference in deformation was relatively small (approximately 0.2–0.3 mm), the stiffness of the VB mixture contributed to delaying the onset of rutting, indicating improved resistance against long-term permanent deformation compared to the reference mix.
• The field results confirmed comparable levels of site compactness among the VB asphalt mix and the control mixture, indicating good compaction performance. These findings highlight the viability of the VB asphalt mixture for field implementation.
• This research significantly advances the field by introducing a novel application of Vacuum Tower Bottom Binder as an effective modifier in asphalt mixtures, providing a sustainable and economical avenue for pavement design. This innovative approach addresses the underutilization of VB binder and aligns with global efforts towards greener infrastructure. Furthermore, the unique combination of VB binder with carnauba wax and soybean rejuvenator offers an inventive solution that enhances asphalt performance, distinguishing our study from the existing literature. The comprehensive evaluation, including field observations, underscores the practical significance of our findings and their potential impact on the construction industry’s practices and decision-making processes, contributing to more resilient and sustainable road pavement solutions.