Thermal-Oxidative Aging Behavior of Waste Engine Oil Bottom-Rejuvenated Asphalt Binder

Abstract

Incorporating waste engine oil bottoms (WEOBs) as rejuvenators into reclaimed asphalt pavement offers a sustainable solution to reduce the consumption of non-renewable resources. To explore the effect of WEOBs on aged asphalt, WEOB-rejuvenated asphalt (WEOB-asphalt) with different thermal-oxidative aging times was prepared. Subsequently, viscosity, double-edge-notched tension (DENT), temperature sweep, linear amplitude sweep (LAS), and Fourier transform infrared spectroscopy (FTIR) tests were conducted to investigate the performance of WEOB-asphalt. The results indicate that WEOB-asphalt shows acceptable thermal-oxidative aging ability within 180 min. The WEOB-asphalt experiences a small decrease in critical crack tip opening displacement within a 180 min aging time. Additionally, the temperature sensitivity of WEOB-asphalt is low, and the rutting factors at temperatures of 46 °C and 52 °C can significantly distinguish the thermal-oxidative aging performance of asphalt at different aging degrees. The fatigue life of WEOB-asphalt decreases compared to the original asphalt after 540 min of aging when the strain exceeds 0.04%. Furthermore, WEOB-asphalt displays increased carbonyl and sulfoxide groups, indicating poorer thermal-oxidative aging resistance than the original asphalt. Based on these results, it is suggested that WEOB-asphalt should be used in areas with mild climate conditions to avoid its rapid secondary aging.

1. Introduction

The large-scale construction of asphalt pavement entails significant consumption of asphalt, a non-renewable material [1,2]. Simultaneously, substantial quantities of waste engine oil are produced globally each year, accounting for tens of millions of tons globally [3,4]. Waste engine oil recycled from vehicles can be repurposed as lubricating oil through distillation and purification, leaving behind a residual solid/liquid coexistence residue known as waste engine oil bottom (WEOB) [5]. Despite comprising 20–30% of the total, WEOB cannot be effectively recycled due to numerous impurities [6,7]. Meanwhile, the annual highway maintenance mileage is more than 5 million kilometers worldwide. A large amount of recycled asphalt pavement (RAP) is produced during maintenance. The application of RAP can provide economic and environmental benefits [8]. After distillation treatment, WEOB contains significant amounts of hydrocarbon oil and asphaltene. The light components in WEOB can soften asphalt. Meanwhile, the asphaltene can promote the compatibility of the WEOB and asphalt, thus improving the homogeneity of the WEOB-asphalt system during production [9,10]. Consequently, incorporating WEOBs as rejuvenators into reclaimed asphalt represents a sustainable approach to enhancing binder quality or reducing the consumption of new asphalt [11,12,13,14,15]. Furthermore, its economic and environmentally friendly features, alongside excellent performance and waste treatment capabilities, have attracted global attention. The life cycle assessment conducted by Jiang’s group reveals that WEOB-rejuvenated asphalt pavement can reduce global warming potential by 79.6% and non-biotic resource consumption by 90.8% compared to traditional asphalt pavement [16].

The use of WEOB in reclaimed asphalt has attracted considerable attention in recent years. Zamhari et al. investigated the impact of WEOB as a recycling agent on aging asphalt properties, revealing its ability to restore viscosity, permeability, and rheological properties of aging asphalt binder [17]. Qui et al. demonstrated WEOB’s effectiveness in recovering penetration, softening point, and ductility of aged SBS-modified asphalt. Compared to the commercial rejuvenator XT-1, WEOB exhibited a more significant positive influence on the fatigue life of aged SBS-modified asphalt and better recovery of recycled asphalt’s viscoelasticity, thus improving SBS modifier degradation due to aging [18]. Zaumanis et al. introduced WEOB into reclaimed asphalt mixtures, finding that it effectively maintained or increased low-temperature creep compliance, indirect tensile strength, and fracture energy, thereby enhancing the mixture’s low-temperature performance [19]. Liu et al. examined the impact of WEOB on chemical compounds and rheological properties of WEOB-rejuvenated rubber + SBS asphalt binders, observing a new characteristic peak in the infrared spectrum indicating a chemical reaction during modification [20]. Additionally, WEOB incorporation enhanced the high-temperature elasticity and low-temperature viscosity of modified bitumen. Li et al. conducted accelerated loading tests to assess the long-term performance of recycled asphalt pavement with WEOB, concluding that WEOB-reclaimed asphalt pavement exhibited approximately 10% higher long-term anti-rutting ability than base asphalt pavement, although its long-term anti-sliding force and anti-fatigue performance were poor [21]. The introduction of SBS to WEOB-rejuvenated asphalt is an effective strategy to improve its usability. Li’s group investigated the effect of SBS dosage, and shear time and rate during preparation to WEOB-rejuvenated asphalt, and found that the addition of SBS can improve the elastic recovery performance and high/low temperature performance of the reclaimed asphalt [22,23].

However, research conducted in Canada on WEOB reclaimed asphalt pavement found that cracks of varying degrees appeared in the pavement 2–3 years after completion [21]. Researchers believe this may be due to premature hardening of the asphalt caused by the added WEOB, leading to cracking of the pavement [3]. Moreover, the rates of carbonyl formation during both the spurt and subsequent steady-rate periods were significantly elevated. These detrimental effects are attributed to the presence of abundant metal catalysts (such as Fe, Cu, and Cr) and/or oxidized engine oil components in WEOB [24,25], which expedite the aging process of asphalt under thermal-oxidative conditions, thereby accelerating its physical and chemical hardening.

Given these considerations, it is necessary to conduct a comprehensive and systematic study on the thermal-oxidative aging performance of WEOB-rejuvenated asphalt. Initially, the original asphalt undergoes aging via a modified extended Rolling Thin Film Oven Test (ME-RTFOT) to acquire aged asphalt [26]. Subsequently, two rejuvenators, including WEOB and a commercial rejuvenator, are employed to rejuvenate the aged asphalt. Following this, the rejuvenated asphalt undergoes ME-RTFOT to achieve aged asphalt with varying aging durations. Ultimately, viscosity tests, double-edge-notched tension tests, temperature sweep tests, linear amplitude sweep tests, and FTIR tests are conducted to compare the thermal-oxidative aging performance of the two types of rejuvenated asphalt with that of the original asphalt.

2. Materials and Methods

2.1. Raw Materials

The original asphalt utilized in this study is a 70# base asphalt (SINOPEC, Beijing, China) with a performance grade of 70–28, featuring penetration, softening point, and ductility values of 75 (0.1 mm), 46 °C, and 25.0 cm, respectively. In addition, two rejuvenators were employed: a waste engine oil bottom (Shandong Hope Environmental Protection Technology Co. Ltd., Dezhou, China) and a commercial CA-3 rejuvenator (Ingevity Functional Materials (Zhuhai) Co., Ltd., Zhuhai, China). In China, the chemical compositions and properties of WEOB can vary significantly depending on the diverse sources of waste engine oil and the varying degrees of oxidation and wear during their use. WEOB is derived through pretreatment and atmospheric and vacuum distillation in a large waste engine oil treatment plant. For comparative analysis, the commercially available CA-3 rejuvenator, developed with mineral oil as the base oil, was selected. Their basic properties and chemical components are presented in

Table 1. Properties of and components of WEOB and CA-3.

Type	Unit	WEOB	CA-3
Color	/	Jet black	Light yellow
Density	g/cm3	0.9864	0.9752
Flash point	°C	237	235
Asphaltenes	%	7.58	1.7
Resins	%	16.0	8.8
Aromatics	%	49.8	63.2
Saturates	%	25.7	26.3
Mechanical impurities content	%	0.92	0

.

2.2. Preparation of the Aged Asphalt Binders

The oxidation process serves as the primary mechanism of asphalt aging under usage conditions. The simulated aged asphalt was derived from the base asphalt using the modified extended rolling thin film oven test (ME-RTFOT). According to ASTM D 2872, the conventional RTFOT protocol can only simulate the short-term aging of the original asphalt mixture during mixing and paving, but the asphalt at this time has not reached sufficient aging. Concurrently, research by Li et al. demonstrated that an RTFOT short-term aging test lasting 270 min is equivalent to a 20 h Pressure Aging Vessel long-term aging test. In this case, the ME-RTFOT was employed for 270 min to prepare the aged asphalt binders [27]. The penetration, softening point, and ductility values of aged asphalt are 26 (0.1 mm), 61.5 °C, and 0 cm, respectively.

2.3. Preparation of the Rejuvenated Asphalt Binders

After preparing the aged asphalt, WEOB and CA-3 rejuvenators were utilized at the ideal dosages of 4% (mass ratio of aged asphalt) to prepare the rejuvenated asphalt samples for testing. The specific preparation method for rejuvenated asphalt is outlined as follows: The aged original asphalt was heated to 140 °C by a thermal mechanical agitator and rotated for 5 min at a speed of 4000 rpm. Subsequently, 4 wt% of WEOB/CA-3 was added to the aged original asphalt slowly. The mixture was further stirred for 40 min to ensure the rejuvenator was dispersed into the aged asphalt homogenously to obtain the rejuvenated asphalt binder. The two asphalt binders were labeled as WEOB-asphalt and CA-3-asphalt. Finally, the ME-RTFOT of two types of rejuvenated asphalt was performed to obtain aged asphalt binders at different aging times. The aging times are 85 min, 180 min, 360 min, and 540 min.

2.4. Test Methods

2.4.1. Brookfield Viscosity Tests

Viscosity stands as one of the crucial indicators that reflect the rheological properties of asphalt materials. According to ASTM D4402, a Brookfield viscometer (DV-II) was used to measure the viscosity of three types of aged asphalt at 135 °C. All the tests were repeated three times and the average values were obtained. The viscosity aging indices (VAIs) were calculated according to Equation (1) based on the viscosity results. Furthermore, to quantitatively assess the differences in thermal-oxidative aging performance among the three types of asphalt (original asphalt, WEOB-asphalt, and CA-3-asphalt), a non-linear differential aging model based on Brookfield tests measuring asphalt viscosity was developed [27], which can be expressed by Equation (2). Then, the aging rates at each aging time were calculated using Equation (3) [28].

$$VAI=lglg({\eta }_t·1000)-lglg({\eta }_0·1000)$$

(1)

$${\displaystyle \raisebox{1ex}{${\eta }_t$}\!\left/ \!\raisebox{-1ex}{${\eta }_0$}\right.}={\displaystyle \frac{L}{[1+(L-1\left)e^{-rt}\right]}}$$

(2)

$${\eta }_t^{\prime }={\displaystyle \frac{rL{\eta }_0(L-1)e^{-rt}}{[1+(L-1\left)e^{-rt}\right]}}$$

(3)

where η_t is the viscosity of asphalt at time t; r and L are constants; ${\eta }_0$ is the initial value of η; ${\eta }_t^{\prime }$ is the aging rate. The parameter L in this model denotes the ratio between the maximum and the initial values of asphalt viscosity in the aging process. The parameter r is the rate of increase in η for a given value of L. These two parameters (L and r) can quantify the aging state and aging velocity, thus enabling effective evaluation of the aging process and behavior.

2.4.2. Double-Edge-Notched Tension Test

To evaluate the low-temperature cracking resistance of the three types of asphalt, double-edge-notched tension (DENT) tests were performed according to AASHTO TP113-15 (Standard Method of Test for Determining the Resistance of Asphalt Binder to Plastic Failure Using the Double-Edge-Notched Tension Test) [29]. DENT testing represents an advancement over the force-ductility test, which relies on the asphalt binder’s ability to relax under applied loads while maintaining shape elasticity. However, the conventional force-ductility test is essentially an empirical method, and undamaged or non-notched samples are typically used. By employing double-edge-notched sample geometry, the DENT test provides a more accurate simulation of microcracks that occur between coarse aggregates in asphalt mixtures. Before the DENT test, samples were poured into silicone molds and conditioned on the base plates at 25 ± 0.5 °C for 3 h ± 5 min prior to testing in a temperature-controlled bath under a minimum of 25 mm of water. Testing was conducted at a constant speed of 100 ± 2.5 mm/min in a bath maintained at 25 ± 0.5 °C until ductile failure is reached or a stroke length of 1000 mm (39.5 in.) is reached.

The critical crack tip opening displacement (CTOD) obtained from the DENT test serves as a metric for the asphalt’s low-temperature cracking resistance. CTOD offers a precise measure of ductile strain tolerance under critical tensile stresses, preventing situations where asphalt remains unstressed in conventional force-ductility tests. A higher CTOD value indicates better asphalt cracking resistance at low temperatures. The CTOD can be obtained based on DENT test results according to the calculation method previously reported [30,31]. Generally, the total work of fracture (W_t) is composed of two parts, total essential work and total plastic work of fracture (W_e and W_p), which can be determined by integrating the areas under the force/displacement curve according to Equations (4) and (5):

$$W_t=W_e+W_p=LB{w}_e+\beta L^{2}B{w}_p$$

(4)

$$w_t={\displaystyle \frac{W_t}{LB}}=w_e+\beta w_{p}L$$

(5)

where w_t is the specific total work of fracture (J/m²), w_e is the specific essential work of fracture ratio (J/m²), w_p is the specific plastic work of fracture (J/m³), β is a scaling factor describing the shape of the plastic zone, L is the ligament length (m), and B is the specimen thickness (m). w_e and βw_p can be obtained by plotting a curve of w_t versus L, then performing linear fitting on it.

The CTOD value can be determined according to Equation (6):

$$CTOD={\displaystyle \frac{w_e}{{\sigma }_{net,5\mathrm{m}\mathrm{m}}}}$$

(6)

where ${\sigma }_{net,5\mathrm{m}\mathrm{m}}$ is the average net section peak stress for duplicate measurements with the 5 mm ligament specimen (N/m²). These values can be reported by the machine after DENT testing.

2.4.3. Temperature Sweep Test

To investigate the high-temperature performance of the three types of asphalt, temperature sweep tests within the linear viscoelasticity limit were conducted according to the test method outlined in AASHTO T315-12 [27]. The rutting factor was recorded as the evaluation indicator to examine the impact of aging on the rutting resistance of the three types of asphalt. The test temperature was incrementally increased from 58 °C, with intervals of 6 °C, until reaching 82 °C.

2.4.4. Linear Amplitude Sweep (LAS) Test

To assess the fatigue damage resistance performance of the three types of asphalt, LAS tests were conducted at 25 °C following the AASHTO TP101-14 standard [32]. The LAS test consists of a frequency sweep phase and an amplitude sweep phase. During the frequency sweep phase, a shear strain of 0.1% and a frequency range of 0.2–30 Hz were utilized. Subsequently, the amplitude sweep phase was carried out to determine the fatigue life using the Viscoelastic Continuum Damage model.

2.4.5. Fourier Transform Infrared Spectroscopy (FTIR) Test

To investigate the impacts of two rejuvenators on the chemical structure and aging behavior of thermal-oxidative aged asphalt under various oxidation durations, the FTIR tests were conducted. This study employed the TNSOR II FT-IR Spectrometer (Bruker Company, Ettlingen, Germany), scanning wavelengths ranging from 4000 cm⁻¹ to 400 cm⁻¹ at a resolution of 4 cm⁻¹. Solid bitumen was dissolved in a trichloroethylene solution, and a small amount was placed on the KBr beam splitter. Spectral analysis commenced upon sample drying. Band area and peak height are commonly used metrics for assessing bond intensity in FTIR spectra, enabling the identification of binder chemistry alterations resulting from rejuvenator inclusion.

Moreover, the carbonyl index I_C=O and sulfoxide index I_S=O, associated with oxidation functional groups, are widely employed to investigate the influences of rejuvenators on aging performance. Therefore, the characteristic peak intensities of the carbonyl group (C=O), sulfoxide group (S=O), and the sum of both functional groups were computed to assess the aging performance of various asphalt types, expressed by the following equations:

$$I_{C=O}={\displaystyle \frac{A_{C=O}}{A_{C-H}}}$$

(7)

$$I_{S=O}={\displaystyle \frac{A_{S=O}}{A_{C-H}}}$$

(8)

$$I_{CS}=I_{C=O}+I_{S=O}$$

(9)

where I_C=O is the carbonyl index and A_C=O is the carbonyl peak area; I_S=O is the sulfoxide index and A_S=O is the sulfoxide group peak area; I_CS is the sum of I_C=O and I_S=O; A_C-H is the absorption peak area of asymmetric stretching vibration of C-H in methylene. The peaks at 1030 cm⁻¹ and 1700 cm⁻¹ correspond to sulfoxide and carbonyl groups, respectively.

3. Results and Discussion

3.1. Brookfield Viscosity Analysis

The aging process of asphalt binders significantly impacts the mechanical properties of pavements, resulting from alterations in their rheological behavior and binder composition. In this study, the 135 °C viscosity of the three aged asphalt binders was tested, and the non-linear differential aging models based on the 135 °C viscosity aging indices (VAIs) of the three asphalt types exhibiting varying degrees of aging were established. Differences in aging performance were quantitatively assessed by examining the aging rate of each asphalt type.

The 135 °C viscosity curves of the three asphalt binder are shown in Figure 1a. It can be seen that the viscosity of the three binders gradually increased with the increase in aging time. The viscosity of WEOB-asphalt is the largest among the three binders whether before or after aging. The VAI values of the three binders were calculated according to Equation (1), which can be used for evaluating the anti-aging performance [28]. Figure 1b shows the VAI values. One can see that the WEOB-asphalt exhibited the highest VAI value, corresponding to the weakest anti-aging ability. The viscosity-based aging rate can be obtained according to Equation (4). Firstly, the viscosity data were fitted using numerical analysis software (Origin) based on the least squares method with the non-linear differential Equation (2) mentioned earlier. The fitted curves are presented in Figure 1c and the non-linear dynamic parameters L and r for different asphalt types are summarized in

Table 2. Non-linear dynamic parameters L and r for the different types of asphalt.

Asphalt Binder	L	r	R2
Original asphalt	14.987	0.2681/h	0.997
WEOB-asphalt	12.956	0.3391/h	0.988
CA-3-asphalt	14.325	0.2766/h	0.996

. Additionally, the relationship between aging time and η_t/η₀ is depicted in Figure 1c. Then, the aging rate of three asphalt binders at each aging time can be calculated according to Equation (3). The results are depicted in Figure 1d.

From Figure 1c,d, it can be observed that the viscosity of asphalt gradually increases with aging time, while the growth rates of viscosity exhibit significant differences, indicating a decrease in the aging rate over time. This variation reflects differences in the aging process under various conditions. Table 2 reveals that during the aging process of ME-RTFOT, the parameter r values of the original asphalt are consistently smaller than those of WEOB-asphalt and CA-3-asphalt. This suggests that the original asphalt exhibits stronger resistance to aging compared to the two types of rejuvenated asphalt. Notably, the aging rate of WEOB-asphalt is notably faster than that of both the original asphalt and the CA-3-rejuvenated asphalt, recorded as 0.071/h and 0.062/h, respectively. This indicates that WEOB-asphalt undergoes the most intense oxidation reaction and exhibits the fastest aging rate throughout the aging process.

Additionally, it is evident from Figure 1d that the initial aging rates of original asphalt, WEOB-asphalt, and CA-3-asphalt are 5.30 Pa·s·h⁻¹, 3.95 Pa·s·h⁻¹, and 2.97 Pa·s·h⁻¹, respectively. Within the first 85 min of ME-RTFOT aging, the aging rate of original asphalt was the highest. These findings suggest that the short-term thermal-oxidative aging resistance performance of the two types of rejuvenated asphalt surpassed that of the original asphalt. However, after 85 min of aging, the aging rate of the original asphalt sharply declined, signifying a significantly weaker oxidation reaction compared to the two types of rejuvenated asphalt. From 85 min until the completion of the aging reaction, the aging rate of WEOB-asphalt remained higher than that of CA-3-asphalt and the original asphalt. This indicates that WEOB-asphalt exhibited the highest corresponding aging rate, and the poorest thermal-oxidative aging resistance. Furthermore, it can be also seen from Figure 1d that the aging rate of the three asphalt binders decreases with the extension of aging time. This is because the number of active molecules that react with oxygen on the surface of the asphalt is decreased [33].

3.2. DENT Tests

The CTOD of the three types of asphalt obtained from the DENT test is presented in Figure 2a,b. The analysis reveals that short-term aging has the least impact on the low-temperature cracking resistance of WEOB-asphalt. However, the long-term thermal-oxidative aging resistance of WEOB-asphalt is inferior to that of the original asphalt and CA-3-asphalt. For instance, after aging ME-RTFOT for 180 min, the CTOD decreased by 21.8%, 22.1%, and 17.63%. Extending the aging time to 540 min resulted in a CTOD decrease of 44.7%, 40.8%, and 67.2% for the original asphalt, CA-3-asphalt, and WEOB-asphalt, respectively. This could be attributed to the heavy metal impurities in WEOB, which accelerate asphalt aging under prolonged thermal-oxidative conditions, leading to a reduction in asphalt’s low-temperature ductility. The impact of short-term aging on the low-temperature crack resistance of WEOB-asphalt appears relatively minor. This could be attributed to the lower catalytic rate of heavy metals during shorter short-term aging periods, resulting in a smaller change in asphalt’s low-temperature ductility. In essence, prolonged thermal oxidation aging is essential for accurately characterizing the low-temperature performance of WEOB-asphalt.

3.3. Temperature Sweep Tests

The complex modulus (G*) and phase angle (δ) of the asphalt binders can be obtained based on temperature sweep tests. G* reflects the deformation resistance of the asphalt binder. δ is the time lagging angle of the stress peak and strain peak, reflecting the elasticity recovery ability of the asphalt binder. Therefore, sinδ can be employed to evaluate the proportion of irreversible deformation of the asphalt binder. The smaller the sinδ, the higher the elastic component, and the slower the rutting accumulation. So, G*/sinδ can reflect the ability of rutting resistance of the asphalt binder. The rutting factors (G*/sinδ) obtained from the temperature sweep tests of the original asphalt, CA-3-asphalt, and WEOB-asphalt under five different aging times are depicted in Figure 3. It is evident that the rutting factors of all three asphalt types gradually decrease with increasing temperature, indicating a transition from a viscoelastic to a viscous state as temperature rises. Moreover, as aging time increases, the rutting factors of all three asphalt types also gradually increase. This suggests an enhancement in asphalt’s ability to resist high-temperature shear deformation, consequently improving its rutting resistance at elevated temperatures.

In addition, from Figure 3, it is evident that the most notable changes in rutting factors for the three types of asphalt occur at 46 °C and 52 °C. To facilitate a better comparison of aging resistance among the asphalt types, the relationship between aging time and rutting factor at these two temperatures is established, as illustrated in Figure 3d. At 46 °C, the growth rate of the rutting factor curve for CA-3-asphalt closely mirrors that of the original asphalt, while the growth rate for WEOB-rejuvenated asphalt exceeds that of the other two types. As the temperature increases to 52 °C, the rutting factor for WEOB-rejuvenated asphalt significantly surpasses that of the other two types with increasing aging time. This indicates a lower temperature sensitivity for WEOB-asphalt and suggests a higher degree of aging compared to the other two types. The aging resistance of the three asphalt types varies from excellent to inferior, with the original asphalt demonstrating the highest resistance, followed by CA-3-asphalt, and then WEOB-asphalt.

3.4. LAS Test

The fatigue life curves of the three types of asphalt with varying aging times are depicted in Figure 4. It can be inferred that both the WEOB and CA-3 rejuvenator have the capability to enhance the fatigue resistance of asphalt, with initial fatigue life surpassing that of the original base asphalt. However, after prolonged thermal-oxidative aging, the fatigue life of WEOB-asphalt is lower than that of the original asphalt, and it notably decreases with increasing aging time.

To further assess the impact of aging time on asphalt fatigue life, the concept of cross-fatigue strain is introduced as an evaluation index. Cross-fatigue strain refers to the strain at which different types of asphalt exhibit the same fatigue life under identical aging conditions.

Table 3. Cross-fatigue strain of two types of rejuvenated asphalt and origin aged asphalt.

Asphalt Binder	Cross-Fatigue Strain with Aged Original Asphalt (%)
	85 min	180 min	360 min	540 min
CA-3-asphalt	3.2	5.6	6.8	0.06
WEOB-asphalt	/	22.7	2.09	0.04

presents the cross-fatigue strain between two types of rejuvenated asphalt and aged base asphalt. It can be concluded the cross-fatigue strain between CA-3 rejuvenated asphalt and the original base asphalt increases first and then decreases during the aging process. It indicates that within the aging time of less than 360 min and a small strain range, its fatigue life is better than that of the original asphalt. However, once the cross-fatigue strain threshold is exceeded, the fatigue life becomes inferior to that of the original asphalt. For instance, at 540 min of aging, the fatigue life of CA-3-rejuvenated asphalt is lower than that of the original base asphalt across all strain ranges except within the 0.06% strain level. Similarly, the cross-fatigue strain between WEOB-rejuvenated asphalt and original asphalt decreases with increasing aging time. Within 180 min of aging, the fatigue life of WEOB-rejuvenated asphalt exceeds that of the original asphalt. This may be because the hydrocarbon oil and asphaltene in the WEOB soften the aged asphalt and increase its toughness. However, as the aging time reaches 360 min, the cross-fatigue strain decreases to 2.09%, indicating that once the strain surpasses this value, the fatigue life becomes inferior to that of the original asphalt. When aging time reaches 540 min, the fatigue life of WEOB-rejuvenated asphalt is lower than that of the original asphalt across all strain ranges except within the 0.04% strain level.

Notably, the fatigue life of WEOB-asphalt after 180 min aging increased to the maximum value. The cross-fatigue strain of WEOB-asphalt with aged base asphalt is 22.7%. However, when the aging time exceeded 180 min, the fatigue life of WEOB-asphalt sharply decreased. This result indicates that the light component in the WEOB may be volatilized during long-term aging of rejuvenated asphalt. Based on this result, it is suggested that the WEOB-asphalt is suitable for using in areas with short thermal-oxidative aging time.

3.5. FTIR Test

The infrared spectra of the three types of asphalt with varying thermal-oxidative aging times are illustrated in Figure 5. With increasing aging time, the absorption peak areas at 1600 cm⁻¹ and 1700 cm⁻¹ for all asphalt samples continue to rise. This phenomenon can be attributed to the chemical reaction between sulfur-containing and carbon-containing functional groups with oxygen during the thermal-oxidative aging process. This reaction leads to the formation of polar functional groups such as carbonyl and sulfoxide groups, consequently intensifying the absorption peak in this region. Throughout the thermal-oxidative aging, there was no notable alteration in the intensity of the C-H asymmetric stretching vibration absorption peak near 2926 cm⁻¹ for the original asphalt. However, both types of rejuvenated asphalt exhibit a decreasing trend in the intensity of this peak. This suggests that during the aging process, the rejuvenated asphalt experiences hydrogen bond cleavage within saturated C-H bonds, thereby accelerating the asphalt’s oxidation reaction.

The carbonyl index I_C=O, sulfoxide index I_S=O, and the sum of both functional groups ($I_{CS}$) of the three types of asphalt at different thermal-oxidative aging times are depicted in Figure 6. As aging time increases, both I_C=O and I_S=O show continuous increments, albeit with different trends. Sulfoxide groups primarily arise from the reaction of sulfur-containing compounds in asphalt with oxygen, whereas carbonyl groups result from the cleavage of C=C bonds and subsequent reaction with oxygen. From Figure 6a, it can be seen that the increase in I_S=O with aging time is smaller for the two recycled asphalts compared to the original asphalt. This disparity may stem from the original asphalt containing richer sulfur compounds. The sulfur compounds in the rejuvenated asphalts are mostly depleted before rejuvenation, resulting in fewer sulfoxide groups generated during aging.

Furthermore, it can be observed that in Figure 6b, the increase in carbonyl index within the two types of rejuvenated asphalt due to aging was greater than that observed in the original asphalt. In the case of CA-3-asphalt, this can be attributed to two primary factors: (1) carbonyl formation resulting from the original asphalt present within the rejuvenated asphalt, and (2) the carbon atom within the CA-3 rejuvenator itself, which produces oxygen-containing polar functional groups like esters, aldehydes, ketones, and carboxylic acids upon thermal-oxidative aging. Regarding WEOB-asphalt, besides the aforementioned reasons, the presence of metal impurities in WEOB could also play a role. These impurities might accelerate and catalyze reactions between asphalt components and oxygen, consequently increasing the carbonyl content.

From Figure 6c, it can be seen that as aging time increases, the rise in the sum of both functional groups of the two types of rejuvenated asphalt surpasses that of the original asphalt. Compared to I_C=O and I_S=O, I_CS provides a more comprehensive characterization of asphalt aging. Specifically, the increase in I_CS is most pronounced in WEOB-asphalt, followed by CA-3-asphalt, with the original asphalt showing the smallest increase. This suggests that the original asphalt exhibits the highest resistance to thermal-oxidative aging, followed by CA-3-asphalt, while WEOB-asphalt demonstrates the lowest resistance to thermal-oxidative aging.

3.6. Discussion

The viscosity, DENT, temperature sweep, LAS, and FTIR tests were employed to evaluate the aging behavior and performance of WEOB-rejuvenated asphalt. Some of the results are summarized in

Table 4. Summary of the results.

Asphalt Binder	Aging Time	Base Asphalt	WEOB-Asphalt	CA-3-Asphalt
135 °C Viscosity (Pa·s)	180 min	0.91	1.12	0.95
	540 min	3.03	4.51	3.61
Aging rate (Pa·s·h−1)	180 min	2.40	3.60	2.70
	540 min	0.49	1.65	1.61
CTOD reduction (%)	180 min	21.8	17.6	22.1
	540 min	44.7	67.2	40.8
46 °C Rutting factor (kPa)	180 min	103.2	147.9	124.6
	540 min	254.7	394.3	257.2
Fatigue life (Nf)	180 min	1.12 × 106	1.11 × 107	2.48 × 106
	540 min	1.51 × 106	3.01 × 105	3.01 × 105
ICS	180 min	0.075	0.175	0.151
	540 min	0.143	0.261	0.222

. It can be seen that both the aging rate and I_CS of WEOB-asphalt are greater than those of the other two binders, suggesting a good consistency in the viscosity test and FTIR test. That is, the aging process of WEOB-rejuvenated asphalt can be evaluated based on viscosity test results. Furthermore, when the aging time was less than 180 min, the WEOB-asphalt exhibited superior high/low temperature performance and fatigue resistance, which may benefit from the synergistic effect of hydrocarbon oil and asphaltene in the WEOB. These results are similar to those of previous publications [5,34].

4. Conclusions

In this study, WEOBs were recycled as asphalt rejuvenators via pretreatment and atmospheric and vacuum distillation in a waste engine oil treatment plant. Subsequently, the thermal-oxidative aging performance of WEOB-asphalt was evaluated through various laboratory tests, including viscosity tests, double-edge-notched tension tests, temperature sweep tests, linear amplitude sweep tests, and FTIR tests. The original asphalt and commercial CA-3-asphalt were selected as control groups. The key conclusions drawn from these investigations are outlined below:

• A non-linear differential aging model based on Brookfield tests measuring asphalt viscosity was developed. It can be concluded that WEOB-asphalt shows acceptable thermal-oxidative aging performance within the first 85 min. However, from 85 min until the completion of the aging reaction, the aging rate of WEOB-rejuvenated asphalt remained higher than that of CA-3-asphalt and the original asphalt. This indicates that WEOB-asphalt exhibited the highest aging rate and the poorest thermal-oxidative aging resistance.
• Short-term aging has the least impact on the low-temperature cracking resistance of WEOB-asphalt. However, the long-term thermal-oxidative aging resistance of WEOB-asphalt is inferior to that of the original asphalt and CA-3-asphalt. Therefore, prolonged thermal-oxidative aging is essential for accurately characterizing the low-temperature performance of WEOB-asphalt.
• The low-temperature sensitivity of WEOB-asphalt allows for significant differentiation in rutting factors at temperatures of 46 °C and 52 °C. This enables an effective assessment of the asphalt’s thermal-oxidative aging performance across various aging levels.
• Both the WEOB and CA-3 rejuvenators have the capability to enhance the fatigue resistance of asphalt. However, the fatigue life of WEOB-asphalt decreases with increasing aging time. Within an aging time of less than 180 min and a small strain range, its fatigue life is better than that of the original asphalt. However, once the cross-fatigue strain threshold is exceeded, the fatigue life becomes inferior to that of the original asphalt.
• As aging time increases, the rise in the sulfoxide index (I_S=O) of the two types of rejuvenated asphalt is smaller compared to that of the original asphalt. However, the increase in the carbonyl index (I_C=O) within the two types of rejuvenated asphalt due to aging is greater than that observed in the original asphalt. Therefore, compared to I_C=O and I_S=O, the sum of the sulfoxide and carbonyl indices provides a more comprehensive characterization of asphalt aging. Specifically, the rise in I_CS is highest in WEOB-asphalt, followed by CA-3-asphalt, indicating that the original asphalt offers the greatest resistance to thermal-oxidative aging, followed by CA-3-asphalt, with WEOB-asphalt displaying the least resistance.

The findings of this work proved that the secondary aging behavior of rejuvenated asphalt can be evaluated based on Brookfield viscosity test results. The viscosity aging index (VAI) and aging rate can directly indicate the aging degree and anti-aging performance of an asphalt binder. This work provides an example for researchers to study the aging evolution law of polymers. Furthermore, due to the poor thermal-oxidative resistance performance of WEOB-asphalt, it is suggested that the WEOB should be used in areas with mild climate conditions for aged asphalt pavement rejuvenation. In addition, this work focuses on the aging behavior of WEOB-rejuvenated asphalt binders based on the experimental investigation. The theoretical calculations, such as molecular dynamics and finite element analysis, are also very important for one to understand the internal process of aging WEOB-asphalt.