Performance Assessment of Eco-Friendly Asphalt Binders Using Natural Asphalt and Waste Engine Oil

Abstract

The depletion of petroleum reserves and increasing environmental concerns have driven the development of eco-friendly asphalt binders. This research investigates the performance of natural asphalt (NA) modified with waste engine oil (WEO) as a sustainable alternative to conventional petroleum asphalt (PA). The study examines NA modified with 10%, 20%, and 30% WEO by the weight of asphalt to identify an optimal blend ratio that enhances the binder’s flexibility and workability while maintaining high-temperature stability. Comprehensive testing was conducted, including penetration, softening point, viscosity, ductility, multiple stress creep recovery (MSCR), linear amplitude sweep (LAS), energy-dispersive X-ray spectroscopy (EDX), Fourier transform infrared (FTIR) spectroscopy, and scanning electron microscopy (SEM). The results reveal that WEO effectively softens NA, improves ductility, and enhances workability, with the 20% WEO blend achieving the best balance of physical and rheological properties. Chemical analysis indicates that WEO increases carbon content and reduces sulfur and impurities, aligning NA’s composition closer to PA. However, excessive WEO (30%) compromises thermal stability and deformation resistance. The findings underscore the potential of WEO-modified NA for sustainable pavement applications, with 20% WEO identified as the optimal content to achieve performance comparable to conventional petroleum asphalt while promoting environmental sustainability.

1. Introduction

The depletion of petroleum reserves and growing environmental concerns have intensified the search for sustainable alternatives to conventional asphalt binders [1]. Global climate initiatives, such as the COP28 recommendations, emphasize the urgent need to reduce dependency on fossil fuels and transition to renewable energy solutions [2]. These initiatives aim to significantly reduce greenhouse gas emissions and limit global temperature rise to 1.5 °C by 2030 [3]. This push for sustainability aligns with developing eco-friendly asphalt binders, such as waste engine oil (WEO)-modified natural asphalt, which reduces reliance on petroleum-based materials while promoting waste recycling and contributing to climate goals.

Natural asphalt, sourced from rock formations or asphalt lakes [4], presents a promising alternative due to its natural composition and durability. Depending on the source, it appears in either solid form (rock asphalt) or semi-solid or liquid form (lake asphalt). Natural lake asphalt is formed when crude oil, rich in bitumen, migrates through the Earth’s crust and becomes trapped in depressions or basins. Over time, the lighter components of the crude oil evaporate, leaving behind the heavier bitumen, which hardens to form natural asphalt [5]. This material is abundant in various locations globally, including the Middle East, North America, and South America, making it an attractive alternative to petroleum-based asphalt. In regions such as western Iraq, natural asphalt offers a viable eco-friendly alternative for road construction, but its inherent stiffness and brittleness make it prone to cracking and deterioration over time [6].

To overcome these limitations, rejuvenators have been widely employed to restore the flexibility of natural asphalt and extend its service life [7]. WEO has emerged as a particularly effective rejuvenator for aged asphalt and recycled asphalt pavement (RAP) [8]. WEO replenishes the maltenes lost during the aging process, restoring the rheological properties of aged binders [9,10]. By improving the binders’ mechanical properties, WEO enhances the performance of RAP mixtures, allowing for higher RAP content in new asphalt mixes without compromising performance [11,12]. The use of WEO not only enhances the durability of asphalt but also promotes the recycling of waste materials, making it a valuable addition to sustainable construction practices [13].

However, while WEO has been extensively studied in RAP and petroleum-based asphalt, limited research has focused on its application in natural asphalt. Natural asphalt has a unique chemical composition and aging behavior, and understanding how WEO interacts with it is crucial for optimizing binder performance and long-term sustainability [14]. The rejuvenation of natural asphalt with WEO presents an opportunity to address both performance challenges and environmental concerns, as WEO shares a similar molecular structure with asphalt and possesses physical and chemical properties close to those of conventional asphalt binders [15]. Other bio-materials, such as bio-oils, have also shown promise as sustainable alternatives to petroleum-based binders [16]. Bio-oil, produced from renewable sources, has been used as a binder modifier and rejuvenator in various asphalt applications, restoring aged asphalt to a state comparable to virgin bitumen [17]. Studies have shown that bio-oils can balance fundamental performance properties, such as resistance to rutting and cracking, in asphalt mixtures [18,19]. Lignin, a byproduct of bioethanol production, and vegetable oils are additional bio-based alternatives that have been explored for their potential to improve the sustainability and performance of asphalt binders [20,21,22]. Other researchers have confirmed the synergistic effects of rejuvenators combined with materials such as styrene-butadiene-styrene (SBS), which improve fatigue endurance and thermal stability while reviving aged modified asphalt [23,24,25].

These previous efforts to develop eco-friendly asphalt cement using materials such as WEO, bio-oils, lignin, and calcium lignosulfonate are summarized in

Table 1. Previous efforts to produce eco-friendly asphalt cement.

Reference, Year	Materials Used	Main Conclusions
[26], 2007	Reclaimed asphalt pavement (RAP), WEO	WEO can rejuvenate RAP, but excessive amounts reduce high-temperature stability.
[27], 2010	Waste engine oil (WEO), aged asphalt	WEO effectively rejuvenates aged asphalt, restoring flexibility and reducing brittleness.
[28], 2011	Bio-oil, recycled materials	Bio-oil improves binder properties and reduces environmental impact in asphalt production.
[29], 2016	Reclaimed asphalt pavement (RAP), WEO	WEO enhances RAP’s flexibility and cracking resistance, contributing to sustainable construction.
[30], 2018	Waste vegetable oil (WVO), modified asphalt, HMA	Rejuvenators act to gain better performance and workability for aged asphalt, endurance fatigue cracking, susceptible to rutting resistance
[31], 2018	Bio-oil (sawdust oil)	Bio-binders’ rheological properties are more susceptible to the RTFO aging condition than the reference binder. Also, there has been a significant increase in the complex shear modulus of modified asphalt binders.
[32], 2019	WEO, modified asphalt	Promising to the anti-aging phenomena, suppress fatigue cracking, mitigate oxidative-aging polymerization
[33], 2019	WEO, modified asphalt	Chlorination with WEO improves asphalt viscosity and durability, promoting sustainability.
[34], 2019	Lignin, bio-binder, RAP	Lignin can partially replace bitumen, improving stiffness and temperature susceptibility.
[35], 2020	Vegetable oil, RAP	Vegetable oil-based rejuvenators restore elasticity to aged asphalt and improve environmental sustainability.
[36], 2020	Tung oil, modified asphalts blended	Upgrade flexibility of aged asphalt, sustain high-temperature resistance, induce low-temperature mechanisms
[37], 2020	Bio-oil (mainly derived from wood waste)	Improves the workability, reduces the cracking sensitivity and stiffness of the mixture, and reduces its compaction temperature at the same time
[38], 2021	Calcium lignosulfonate (CLS), bitumen	CLS reduces aging effects in bitumen, improving durability and high-temperature performance.
[39], 2021	Waste cooking oil, aged asphalt	Waste cooking oil rejuvenates aged asphalt, improving flexibility and reducing the need for virgin binders.
[40], 2021	PBR (waste polybutadiene rubber), reclaimed asphalt binder	Promote LAS fatigue performance and endure extremely heavy traffic against rutting at 64 °C within MSCR
[41], 2022	Compound rejuvenator WS-rejuvenator (70% WEO + 30% SBS copolymer), 30% and 50% RAP	WS-rejuvenator has altered physical, chemical, and rheological properties, enhanced fatigue and rutting tolerance, and revived the aged binder.
[42], 2022	Bio-mass oil (bio-crude oil, pyrolysis oil/liquid, wood acetic acid, and wood liquid)	Better low-temperature crack resistance and water stability than petroleum asphalt mixture and better anti-aging properties
[43], 2023	WEO, aged asphalt, molecular dynamics simulations	WEO restores the physical properties of aged asphalt and reduces molecular aggregation.
[44], 2023	Pure engine oil (PEO), RAP	Boost Marshall stability, indirect tensile strength value, and lower susceptibility to moisture damage

. This table highlights the diverse range of materials explored by researchers and their contributions to improving asphalt sustainability and performance. Despite the progress made in using bio-materials and WEO as rejuvenators, the effect of WEO on natural asphalt remains underexplored. This study aims to fill this gap by investigating the performance of natural asphalt modified with WEO at 10%, 20%, and 30% by weight of asphalt cement. The objective is to determine the optimal blend ratio that balances rejuvenation with high-temperature stability, ensuring both performance and environmental sustainability.

2. Materials and Testing Methods

2.1. Materials

Materials used in this study include PA, which serves as a benchmark for performance comparison due to its traditional use in paving applications, and NA modified with WEO in varying percentages to develop eco-friendly binders. A comprehensive series of physical, rheological, and chemical tests were conducted to evaluate the performance properties of these materials and their suitability for use in asphalt concrete paving works.

2.1.1. Asphalt Cement and Rejuvenator

Two types of asphalt cement were used in this study (shown in Figure 1). The PA, with a penetration grade of 40/50, was produced by the Dora Refinery, located southwest of Baghdad, and refined from Basra Petroleum.

Table 2. Properties of asphalt cement.

Test	Result		Specification Limit [45]
	Petroleum Asphalt	Natural Asphalt
Unaged (original) bitumen
Penetration at 25 °C, 100 gm, and 5 s (ASTM D5), dmm	49	17	40–50
Softening point, R&B (ASTM D36), °C	50	68	----
Specific gravity at 25 °C (ASTM D70)	1.018	1.066	----
Flashpoint (ASTM D92), °C	296	334	Min. 232
Ductility (ASTM D113), cm	120	45	Min. 100
Aged residue from thin film oven test (ASTM D1754)
Retained penetration, % of original (ASTM D5)	59	70	Min. 55
Ductility at 25 °C, 5 cm/min, (ASTM D113), cm	60	16	Min. 25

presents the physical properties of this asphalt cement, which meet the requirements specified for AC 40/50 according to the guidelines provided by the State Corporation of Roads and Bridges (SCRB) [45]. The second type of asphalt cement is the NA, sourced from the natural asphalt lakes in the Heet region, west of Iraq, specifically from the Abu-Jeer Lake. The physical properties of NA are also shown in Table 2. From the results of the consistency tests (penetration and softening point), it is evident that NA has a hard grade, making it unsuitable for paving work. According to SCRB specifications, the appropriate asphalt cement grade for paving should have a penetration range between 40/50 and 60/70. To address this, a rejuvenator was used to modify the NA. WEO was employed as the rejuvenator, added at varying contents of 10%, 20%, and 30% by weight of asphalt cement. This modification alters the consistency of the asphalt by adjusting its constituents and rejuvenating the hard NA. The chemical composition of WEO, which includes sufficient aromatic content similar to the molecular structure of asphalt, aids in this rejuvenation. The specific gravity (at 25 °C) of the used WEO is 0.946 (ASTM D70), and its viscosity (at 38 °C) is 104 cSt (ASTM D4402).

2.1.2. NA Modification Process

The preparation procedure for the eco-friendly asphalt binder was refined through several laboratory trials. The blending parameters, including temperature, speed, and shear rate, were optimized to achieve a visually homogeneous blend. The NA was heated to a temperature of 140 °C and held constant for 30 min. The WEO was preheated to 60 °C and added at specific dosages of 10%, 20%, and 30% by weight of the asphalt cement into the hot NA. The blend was then continuously mixed using a high-speed shear mixer with a Jiffy head at a rotation speed of 3000 rpm for 15 min. These mixing conditions were initially selected based on established practices for blending high-viscosity materials, as referenced in [46], and were slightly adjusted through trial experiments to ensure a more homogeneous blend. A photograph illustrating the mixing process is presented in Figure 2. For ease of reference, the abbreviations used for the modified samples in this research correspond to the percentage of waste engine oil (WEO) added. For example, “10% WEO” refers to natural asphalt modified with 10% WEO by weight, “20% WEO” refers to 20% WEO modification, and “30% WEO” refers to 30% WEO modification.

2.2. Experimental Test Methods

A series of conventional, rheological, and analytical tests were conducted to assess the performance of the eco-friendly asphalt binders. Figure 3 illustrates the experimental methodology adopted for this study.

2.2.1. Physical Properties

Although conventional physical tests are becoming less common for asphalt binder characterization, they still offer a quick assessment of binder stiffness. To facilitate comparison, the penetration and softening points of the eco-friendly binders were determined following ASTM D5 and ASTM D36 standards, respectively. Additionally, the ductility test at 25 °C, conducted in accordance with ASTM D113, was used to evaluate the tensile properties of the binders. The rotational viscosity test (AASHTO TP48) was performed to measure the binder’s viscosity at 135 °C, a crucial parameter for assessing workability during mixing and compaction. Given that WEO has distinct properties compared to traditional asphalt binders, its addition could alter the density of the modified binder, potentially affecting the mix design of asphalt mixtures. Consequently, the specific gravity of the eco-friendly binders was measured according to ASTM D70. Although WEO is inherently insoluble in water, its interaction with components of asphalt cement might produce compounds with some degree of water solubility. This raises concerns about the potential separation of the binder in the presence of moisture. To address this, a simple water immersion test was conducted as per the suggested method by [1]. A fixed amount of the modified binder (2 g) was submerged in a water-filled beaker, and the solution was visually inspected for any color change after 24 h to evaluate possible dissolvability.

2.2.2. Rheological Properties

Rheological characterization was performed using a dynamic shear rheometer (DSR) (Anton Paar—SmartPave 102e, shown in Figure 4) to determine the high-temperature grades of asphalt for resistance to permanent deformation following AASHTO T315. For unaged bitumen, the critical parameter was a G*/sin δ value that should be greater than 1 kPa. These measurements were conducted using a 25 mm testing plate with a 1 mm gap between the plate and the specimen. However, concerns have been raised about AASHTO T315, mainly because the test employs very low shear strain levels: 12% for the original binder, 10% for RTFO-aged samples, and 1% for PAV-aged samples, which may not fully capture the behavior of asphalt, especially modified binders, under real-world conditions. Therefore, further tests were conducted: the multi-stress creep recovery (MSCR) test for assessing permanent deformation and the linear amplitude sweep (LAS) test for evaluating fatigue resistance.

2.2.3. MSCR Test

The MSCR test is performed in accordance with the AASHTO T350 standard. A sample of RTFO-conditioned asphalt binder is tested using a dynamic shear rheometer (DSR) at the high PG temperature, employing 25 mm parallel plate geometry with a 1 mm gap setting. The test subjects the sample to a creep and recovery process at two stress levels of 0.1 kPa and 3.2 kPa. During each stress level, the sample undergoes a 1 s creep period followed by a 9 s recovery, with no rest periods between cycles or changes in stress level. Initially, the sample is conditioned by running ten cycles at 0.1 kPa. Following conditioning, ten cycles are conducted at 0.1 kPa, and ten additional cycles are performed at 3.2 kPa, resulting in a total of 30 cycles and a total test duration of 300 s. The loading scheme, as well as a typical output for six cycles (three at 0.1 kPa and three at 3.2 kPa), is shown in Figure 5. Based on the applied shear stress (τ), the strain values—peak strain (εp), recovered strain (εr), and non-recoverable strain (εnr)—are measured. The MSCR test uses three parameters to characterize the viscoelastic properties of an asphalt binder: percent recovery (R), non-recoverable creep compliance (Jnr), and stress sensitivity (Jnr diff.). These parameters are calculated using the following equations:

$$\% R={\displaystyle \frac{\mathsf{\epsilon }r}{\epsilon p}}\times 100$$

(1)

$$Jnr={\displaystyle \frac{\epsilon nr}{\tau }}$$

(2)

$${Jnr}_{diff}={\displaystyle \frac{Jnr 3.2-Jnr0.1}{Jnr0.1}}\times 100$$

(3)

The %R and Jnr values are calculated for the applied stresses of 0.1 kPa and 3.2 kPa. These parameters are averaged over ten cycles to obtain the final values.

Figure 5. Loading scheme and typical strain for MSCR test.

Figure 5. Loading scheme and typical strain for MSCR test.

2.2.4. LAS Test

The LAS test is used to evaluate the resistance of asphalt binders to fatigue damage through cyclic loading with linearly increasing load amplitudes, as per AASHTO T391. It is conducted using a dynamic shear rheometer at the intermediate pavement temperature corresponding to the binder’s performance grade (PG), following AASHTO M 320. The test is applied to binder samples aged with T 240 (RTFOT) and R 28 (PAV) to simulate in-service aging of asphalt pavements, using 8 mm plates and a 2 mm gap as specified in AASHTO T315. The LAS test consists of two consecutive stages: the frequency sweep and the amplitude sweep. In the frequency sweep, an oscillatory shear load at a constant amplitude of 0.1% strain is applied over a range of frequencies from 0.2 to 30 Hz to capture the rheological properties of the binder. These data are used to determine the damage analysis parameter, alpha (α). The amplitude sweep is then conducted in strain-control mode at a frequency of 10 Hz, where the loading amplitude linearly increases from 0% to 30% strain over 3100 loading cycles, as shown in Figure 6. Key parameters, including peak shear strain, peak shear stress, phase angle (δ), and complex shear modulus (G*), are recorded every 10 cycles. The data from this amplitude sweep provide insights into the asphalt binder’s damage characteristics and fatigue resistance. For further analysis, the viscoelastic continuum damage (VECD) model is applied to interpret the LAS test results and predict fatigue performance. The frequency sweep data are used to calculate the undamaged material property parameter α. Using the complex shear modulus (G*) and phase angle δ, the storage modulus G′(ω) is computed as per Equation (4):

$${\mathrm{G}}^{\prime }\left(\omega \right)=G^{*}\left(\omega \right)\times cos \delta \left(\omega \right)$$

(4)

A best-fit line is applied to the log-log plot of G′(ω) versus frequency ω, with slope m, allowing α to be calculated as follows:

$$\mathsf{\alpha }={\displaystyle \frac{1}{\left(1+m\right)}}$$

(5)

In the amplitude sweep portion of the test, G*, phase angle δ, and applied strain γ are recorded at each cycle. Damage accumulation D(t) is determined iteratively with the following equation:

$$D\left(t\right)={\sum }_{i=1}^N{\left[\pi I_D{\gamma }_o^2\left(G^{*}\sin {\delta }_{i-1}-G^{*}\sin {\delta }_i\right)\right]}_{1+\alpha }^{\alpha } {\left(t_i-t_{i-1}\right)}^{{\displaystyle \frac{1}{1+\alpha }}}$$

(6)

where: I_D is the initial value of G* from the 1.0 percent applied strain interval, in MPa; γ_o is the applied strain for a given data point, in %; G* is the complex shear modulus, in MPa; α is the value calculated from the frequency sweep data; and t is the testing time, in seconds.

The parameters A and B are subsequently derived for the binder fatigue performance model:

$$\mathrm{A}={\displaystyle \frac{{f\left(D_f\right)}^k}{{k\left({\pi C}_{1 }{C}_2\right)}^{\alpha }}}$$

(7)

where Df is the accumulated damage at failure, f is the loading frequency (typically 10 Hz), and $k=1+\left(1-C_2\right)\alpha$.

B = 2α

(8)

The number of cycles to failure N_f is then computed as follows:

$$N_f={A\left({\gamma }_{max}\right)}^{-B}$$

(9)

where γ_max represents the maximum expected strain on the pavement. This model provides a predictive measure of binder fatigue performance under anticipated pavement conditions.

Figure 6. Loading scheme for amplitude sweep test.

Figure 6. Loading scheme for amplitude sweep test.

2.2.5. Chemical Composition and Morphology Analyses

The microstructural and compositional characteristics of the asphalt binders were analyzed by utilizing energy-dispersive X-ray spectroscopy (EDX), Fourier transform infrared spectroscopy (FTIR), and scanning electron microscopy (SEM). EDX is a technique used in conjunction with SEM to analyze the elemental composition of materials. It provides information about the types and quantities of elements present in a sample, offering essential insights into any chemical changes resulting from the incorporation of WEO. This analysis helped identify and map the distribution of key elements within the binder matrix. FTIR analysis was performed using a BRUKER Alpha II device (shown in Figure 7) to identify the functional groups present in the binder, giving insights into the chemical interactions between the asphalt components and the rejuvenator. By detecting functional groups, FTIR revealed the chemical modifications induced by the addition of WEO. Lastly, SEM analysis, conducted at a magnification level of 20,000× using an FEI Inspect 50 instrument shown in Figure 8, was employed to observe the surface morphology and microstructural changes in the asphalt binders. This analysis allowed for a detailed visualization of the binder structure, which is crucial for understanding the physical alterations caused by the modification process. Together, these tests provide a comprehensive understanding of the effects of WEO on the binder’s microstructure and composition.

3. Results and Discussion

3.1. Physical Properties

Figure 9 illustrates the penetration values for different asphalt binders and the percent change compared to PA. The NA exhibits a significantly lower penetration value, with a 65.31% reduction compared to PA, indicating its higher stiffness. However, as WEO content increases, the penetration values of NA-modified binders rise, showing a softening trend. Specifically, 10% WEO increases penetration by 32.65%, 20% WEO by 51.02%, and 30% WEO by 69.39%, reflecting the impact of WEO’s paraffinic components that enhance binder fluidity and reduce stiffness. The thermal stability of the asphalt binders, as indicated by the softening point, is presented in Figure 10, along with the percent change relative to PA. The results align well with the penetration findings. Natural asphalt (NA) significantly increases the softening point, 36.60% higher than PA, indicating greater thermal stability. However, as the WEO content increases, the softening point decreases, with 10% WEO showing a 4.80% increase, 20% WEO almost stabilizing at 0.60%, and 30% WEO decreasing by 6.00%. This trend highlights the predominant softening effect of WEO on NA, reducing the binder’s thermal stability.

Figure 11 illustrates the ductility values of various asphalt binders, highlighting the impact of WEO modification on NA. PA shows a ductility of 120 cm, meeting the specification requirement of being greater than 100 cm, which is necessary for paving applications. In contrast, NA exhibits a significantly reduced ductility, with a 62.50% decrease compared to PA, indicating excessive stiffness. The incorporation of WEO progressively enhances the ductility of NA. At 10% WEO, the ductility increases to 80 cm, showing a substantial improvement, though it still falls short of the specification. With 20% WEO, the ductility reaches 105 cm, meeting the paving work requirement. At 30% WEO, the ductility improves further to 140 cm, exceeding PA and demonstrating a 16.67% increase. This enhancement can be attributed to the molecular interactions within asphalt: asphalt cement consists of asphaltenes, which contribute to rigidity, and maltenes, which provide fluidity. WEO disrupts the interactions between these fractions, diluting the asphaltenes and enhancing the influence of maltenes, thereby significantly increasing ductility and making the modified binders more suitable for paving applications.

Figure 12 presents the rotational viscosity values of the asphalt binders at 135 °C, which indicate the flow characteristics and workability of the binders. The hard consistency of NA, previously observed in the penetration results, is reflected in its significantly high viscosity value of approximately 3850 mPa.s, representing a 384.01% increase compared to PA. This high viscosity suggests that NA would be difficult to handle and mix during asphalt paving operations. The introduction of WEO progressively reduces the viscosity of NA, improving workability. At 10% WEO, the viscosity decreases to about 1120 mPa.s, an 83.31% reduction. Further reductions are observed with 20% WEO, where the viscosity drops to 620 mPa.s, only 15.86% higher than PA. At 30% WEO, the viscosity is comparable to PA, showing a decrease of 10.85% from the original NA viscosity. This trend confirms that WEO effectively reduces the viscosity of NA, making it more workable and suitable for asphalt paving operations by enhancing its flow properties.

Figure 13 shows the specific gravity results for the asphalt binders, highlighting how the incorporation of WEO affects NA. The specific gravity of NA is initially higher than that of PA, with a 4.72% increase, indicating a denser material. However, the addition of WEO, which has a lower specific gravity of 0.946, gradually reduces the blend’s specific gravity. With 10% WEO, the specific gravity decreases by 3.83%, and further reductions are observed as the WEO content increases: 2.46% at 20% WEO and 1.28% at 30% WEO. Compared to PA, the modified binders with higher WEO content exhibit specific gravity values closer to PA, reflecting the dilution effect of WEO. This reduction in specific gravity could influence the overall mix design of asphalt pavements, potentially improving workability and reducing material weight.

The results of the dissolvability test are presented in Figure 14. It is evident that no separation occurs between WEO and NA, regardless of the WEO dosage. This suggests a strong interaction between the components. The carbohydrates present in WEO interact with the phenolic groups in the asphalt binder [1], leading to the formation of compounds that are entirely insoluble in water. This stable blend indicates that adding WEO does not compromise the integrity of the asphalt in the presence of moisture.

3.2. Rheological Properties

The rheological properties of the asphalt binders, as characterized by the complex shear modulus (G*) and the phase angle (δ), provide essential insights into their viscoelastic behavior and suitability for high-temperature applications. The complex shear modulus (G*) measures the total resistance of the asphalt binder to deformation, combining both elastic and viscous components. A higher (G*) value indicates a stiffer material, which is beneficial for resisting permanent deformation or rutting at high temperatures. The phase angle (δ) reflects the delay between applied stress and resulting strain, where a lower δ value denotes a more elastic response, ideal for fatigue resistance, and a higher δ value indicates a more viscous, flow-like behavior.

As shown in Figure 15, the (G*) values decrease with increasing temperature for all samples, indicating reduced stiffness of the binders at high temperatures. PA consistently exhibits the highest (G*) values, reflecting its superior resistance to deformation across the temperature range. In contrast, NA demonstrates significantly lower (G*) values, highlighting its limited resistance to deformation at higher temperatures. Modifying NA with WEO enhances its (G*) values, particularly at 10% WEO, where the performance closely matches that of PA. This improvement can be attributed to the optimal balance achieved between stiffness and flexibility, as the WEO interacts effectively with the binder structure. However, at higher dosages of WEO (20% and 30%), the (G*) values gradually decline. This behavior could be attributed to the softening effect of excess WEO, which disrupts the asphaltene–maltene balance within the binder [47], reducing its overall stiffness.

As for the phase angle (δ), increasing WEO content results in higher δ values, indicating a shift towards a more viscous response. This trend aligns with the consistency test results (penetration) and thermal stability (softening point), which reflect a softer consistency with increasing WEO dosage and a lower softening point. Thus, the increase in δ values is consistent with the observed effects of WEO in softening the binder, enhancing its flexibility while reducing thermal stability at higher WEO contents. The rutting factor (G*/sin δ), depicted in Figure 16, decreases as temperature rises for all binders. PA starts with a high G*/sin δ value of 47.40 kPa at 46 °C but drops significantly at higher temperatures. NA shows lower rutting resistance, while the 10% WEO-modified binder achieves a balance, meeting the Superpave criteria. The 20% and 30% WEO binders exhibit reduced rutting resistance, with the latter falling short of performance expectations. Figure 17 illustrates the true failure temperature, where G*/sin δ equals 1 kPa, corresponding to the high-temperature performance grade (PG) for each asphalt binder at unaged conditions. The high-temperature PG for PA, NA, 10% WEO, and 20% WEO was consistently 70 °C, indicating their capacity to withstand higher temperatures before losing structural integrity. However, for the 30% WEO-modified binder, the high-temperature PG decreased to 64 °C. This reduction highlights the significant impact of higher WEO content on softening the binder, thereby lowering its thermal stability. This finding is crucial, as it shows that while moderate WEO modification can maintain adequate high-temperature performance, excessive amounts can reduce the binder’s ability to resist rutting at elevated temperatures. The percent change compared to PA emphasizes this effect, with 30% WEO showing a 6.60% decrease in fail temperature, further underscoring the trade-off between increased flexibility and reduced thermal resistance.

3.3. MSCR Results

The results obtained from the MSCR test at a temperature of 70 °C, in terms of accumulated strain versus creep loading cycles (for the last 10 cycles under 3.2 kPa), are shown in Figure 18. It is evident that the harder asphalt type, NA, resulted in lower permanent strain compared to the other types of asphalt binder. The 30% WEO exhibited the highest permanent strain, reflecting lower resistance to rutting-type distress, which could be attributed to the softening effect of the waste engine oil rejuvenator on the consistency of natural asphalt. In other words, the higher amount of rejuvenator (30%) results in an asphalt binder with lower stiffness. Among the other types, NA modified with 20% WEO showed results very close to those of PA, whereas the 10% WEO exhibited approximately 35.3% lower permanent strain than PA at the end of the test.

The additional indices obtained from the MSCR test are presented in

Table 3. MSCR indices for different binders.

Binder Type	Jnr 0.1	Jnr 3.2	Jnr diff.	%R (0.1 kPa)	%R (3.2 kPa)
PA	1.1635	1.2677	8.96	3.90	0.41
NA	0.4231	0.4747	12.19	18.24	3.45
10% WEO	0.6539	0.8255	26.24	6.40	1.13
20% WEO	0.9516	1.2387	30.17	5.61	0.75
30% WEO	1.1333	1.5564	37.33	3.59	0.36

, offering a comparative analysis of the high-temperature performance of various asphalt binders: PA, NA, 10% WEO, 20% WEO, and 30% WEO assessed at 70 °C, which is the high PG temperature of PA. The Jnr3.2 values, representing the non-recoverable creep compliance under a high-stress level of 3.2 kPa, reveal that PA has a Jnr3.2 of 1.2677 kPa, indicating moderate resistance to permanent deformation with limited elasticity, as seen in its low recovery rate of 0.41%. NA, with a significantly lower Jnr3.2 of 0.4747 kPa and a recovery rate of 3.451%, demonstrates superior deformation resistance and elasticity, making it better suited for high-stress environments. The WEO-modified binders show a trend of increasing Jnr3.2 values with higher WEO content: 10% WEO has a Jnr3.2 of 0.8255 kPa, while 20% and 30% WEO exhibit higher susceptibility to permanent deformation, with Jnr3.2 values of 1.2387 and 1.5564 kPa, respectively. This increase in Jnr3.2 with higher WEO content indicates that greater WEO concentrations lead to increased stress sensitivity, as evidenced by their higher Jnr diff values, especially in the 30% WEO binder (37.33%).

In terms of elasticity, reflected by the percent recovery (%R) at 3.2 kPa, PA’s %R of 0.41%, and the declining %R values in the WEO-modified binders (1.128% for 10% WEO, 0.7542% for 20% WEO, and 0.3557% for 30% WEO), the results suggest reduced recovery potential with higher WEO content, making them less ideal for heavy deformation resistance. According to the AASHTO M332 classification (

Table 4. AASHTO M332 pavement asphalt grades.

Paving Grade	Test Temperature	Requirements
S	PG high temperature	Jnr3.2 ≤ 4.5 kPa, % R ≤ 75%
H	PG high temperature	Jnr3.2 ≤ 2.0 kPa, % R ≤ 75%
V	PG high temperature	Jnr3.2 ≤ 1.0 kPa, % R ≤ 75%
E	PG high temperature	Jnr3.2 ≤ 0.5 kPa, % R ≤ 75%

), which categorizes binders based on Jnr3.2 and %R, PA qualifies for the H grade, suitable for heavy traffic. At the same time, NA, with its lower Jnr3.2, meets the E grade requirements, making it ideal for extremely heavy traffic. The 10% and 20% WEO binders also fit the V grade, while the 30% WEO binder, due to its higher Jnr3.2 value, meets the criteria for the H grade but exhibits lower performance under high stress. In summary, NA and 10% WEO show the best high-temperature performance among the tested binders, with PA and higher WEO modifications (20% and 30%) displaying increased deformation and reduced recovery. The 20% WEO permanent deformation parameter Jnr is closely comparable to that of PA, which revealed the ability of this type of eco-friendly asphalt to mimic the performance of refinery-produced asphalt, i.e., PA.

3.4. LAS Results

Based on the linear amplitude sweep (LAS) test results at an intermediate temperature of 19 °C (AASHTO T 391), a comprehensive assessment of the fatigue behavior of various asphalt binders, including PA, NA, and natural asphalts modified with 10%, 20%, and 30% WEO, was conducted. The initial examination of the effective stress vs. effective strain curves recorded from the first phase of the LAS test, which includes a frequency sweep test using a very low strain amplitude of 0.1% to obtain undamaged material properties (as depicted in Figure 19), reveals substantial differences in the mechanical properties among the binders. Notably, NA exhibits the steepest initial slope, suggesting a higher initial stiffness but limited flexibility, possibly predisposing it to quicker fatigue failure under repeated loading. In contrast, the WEO-modified asphalts display progressively flatter slopes with increasing WEO content, indicating enhanced flexibility and ductility—attributes favorable for long-term fatigue resistance.

The LAS results are further analyzed using the viscoelastic continuum damage (VECD) model, which has been extensively used to model the complex fatigue behavior of asphalt binders by plotting the relationship between the strain level and the number of load cycles to failure based on both phases of the LAS test (frequency sweep and amplitude sweep). Using the plots presented in Figure 20, the Nf corresponding to two critical strain levels—2.5% for thicker asphalt concrete layers and 5% for thinner layers, which mimic real-world pavement stresses—are used to quantify the corresponding Nf. This method provides a more detailed evaluation of each binder’s capacity for enduring fatigue under realistic service conditions. The resulting data, detailed in

Table 5. Fatigue life (Nf) results based on LAS tests.

Binder Type	Strain Level (%)	Nf
PA	2.5	141,006
	5.0	1189
NA	2.5	36,512
	5.0	527
10% WEO	2.5	247,816
	5.0	8070
20% WEO	2.5	812,048
	5.0	48,431
30% WEO	2.5	502,011
	5.0	20,572

, highlight the remarkable fatigue resistance of WEO-modified binders. Notably, the 20% WEO binder demonstrates an exceptional increase in Nf values. At 2.5% strain, this binder achieves an Nf of 812,048, marking an impressive improvement over PA, which records an Nf of 141,014 with a 475% increase. At the more demanding 5% strain, 20% WEO records an Nf of 48,431 compared to PA’s 1189, showcasing a nearly 3975% improvement. These percentages underscore the substantial durability and fatigue resistance enhancement of WEO modification.

The 30% WEO and 10% WEO also show considerable improvements in fatigue resistance, but it is the 20% WEO modification that strikes an optimal balance between performance enhancement and eco-friendliness. This formulation significantly exceeds traditional performance metrics, particularly in terms of durability and resistance to fatigue. By incorporating waste engine oil, these modifications enhance the mechanical properties of the asphalt while contributing to a more sustainable approach in pavement engineering. This shift toward incorporating sustainable materials like WEO improves the lifespan and quality of asphalt pavements and aligns with broader environmental objectives by reducing waste and the consumption of virgin resources, setting a new standard in the pavement industry for eco-friendly innovations that do not compromise performance.

3.5. Interaction Mechanism and Morphology Results

3.5.1. EDX Analysis

The EDX analysis provides valuable insights into the elemental composition of both PA and NA, as well as the impact of WEO modification on NA. As exhibited in

Table 6. EDX elemental composition of asphalt.

Sample	Element	Atomic %	Atomic % Error	Weight %	Weight % Error
PA	C	94.6	0.6	86.9	0.6
	S	5.4	0.1	13.1	0.1
NA	C	91.0	0.6	79.3	0.5
	Al	1.1	0.0	2.2	0.1
	S	7.3	0.1	17.1	0.1
	Ca	0.5	0.0	1.5	0.1
10% WEO	C	92.7	0.4	82.6	0.4
	Al	0.2	0.0	0.3	0.0
	Si	0.3	0.0	0.7	0.0
	S	6.5	0.0	15.5	0.1
	Ca	0.3	0.0	0.9	0.0
20% WEO	C	93.7	0.5	84.7	0.5
	Si	0.3	0.0	0.6	0.0
	S	5.7	0.0	13.7	0.1
	Ca	0.4	0.0	1.1	0.0
30% WEO	C	97.6	0.6	93.9	0.6
	S	2.4	0.0	6.1	0.1

, carbon (C) is the predominant element in all asphalt samples, highlighting their hydrocarbon-rich nature. In unmodified NA, the carbon content is 91.0%, which increases with the addition of WEO—rising to 92.7% with 10% WEO, 93.7% with 20% WEO, and reaching 97.6% with 30% WEO. This increase is due to introducing hydrocarbon-rich WEO, which amplifies the overall carbon concentration. In comparison, PA exhibits an even higher carbon content of 94.6%, emphasizing its highly hydrocarbon-dominant composition. As NA is modified with increasing amounts of WEO, particularly at 30%, its carbon content surpasses that of PA, indicating that WEO effectively transforms NA’s composition to resemble PA. The sulfur (S) content exhibits a different trend. In unmodified NA, the sulfur content is 7.3%, which decreases as WEO is introduced, falling to 6.5% with 10% WEO, 5.7% with 20% WEO, and sharply to 2.4% at 30% WEO. In contrast, PA contains 5.4% sulfur, which is lower than unmodified NA but still considerable. The progressive decrease in sulfur with increasing WEO suggests that adding WEO dilutes sulfur-containing compounds in NA. At 30% WEO, the sulfur content in NA drops below that of PA, indicating a significant shift in composition as more WEO is added. Trace elements such as aluminum (Al) and calcium (Ca) in NA also decrease with increasing WEO content. In unmodified NA, Al is present at 1.1%, but this reduces to 0.2% with 10% WEO and is not detected at higher WEO levels. Similarly, Ca decreases from 0.5% in unmodified NA to 0.3% with 10% WEO and further with higher WEO contents. Silicon (Si), absent in unmodified NA, appears at 0.3% in both the 10% and 20% WEO-modified samples but is absent again at 30% WEO; this occurrence might be attributed to trace amounts of silicon introduced through WEO, potentially from wear and tear in engine parts where silicon-based materials (e.g., seals or gaskets) degrade over time. At higher WEO concentrations (30%), the silicon content may be diluted below the detection threshold. In contrast, PA shows little to no presence of these trace elements, suggesting that it is more refined compared to NA. In conclusion, the EDX results reveal that adding WEO significantly alters the elemental composition of NA, progressively increasing its carbon content while reducing the sulfur and trace elements. This brings NA closer to the chemical profile of PA, particularly at higher WEO modification levels.

3.5.2. FTIR Results Analysis

The FTIR spectrum, as exhibited in Figure 21, highlights essential differences in the chemical structures of all tested materials. One of the most notable differences in the unmodified NA spectrum compared to PA is the increased intensity in the region associated with O-H stretching (3200–3600 cm⁻¹), indicating a higher presence of hydroxyl groups and oxygenated compounds. This suggests that natural asphalt contains more polar components and moisture, which is typical for materials derived from natural sources, as they tend to have more complex and variable compositions than refined petroleum-based asphalts. Incorporating WEO into NA led to apparent changes in the spectrum. For instance, at 10% WEO modification, the peaks corresponding to polar compounds (such as O-H and C=O) became less pronounced, indicating that the waste engine oil helped reduce the presence of polar functional groups. This aligns with the expectation that WEO, rich in hydrocarbons, can dilute the polar nature of NA, making its chemical profile more similar to PA. The 10% WEO sample, as evidenced in the FTIR spectrum, closely aligns with PA in many key regions, particularly in the C-H stretching region (2800–3000 cm⁻¹) and the C=O stretching region (1650–1750 cm⁻¹), suggesting that this level of modification achieves a balance where the modified NA mimics the composition of PA effectively.

However, at higher modification levels, such as 20% WEO and 30% WEO, the chemical composition begins to diverge more noticeably from PA. The 30% WEO sample shows heightened absorption in the region around 1450–1600 cm⁻¹, likely due to an increased presence of long hydrocarbon chains or other byproducts introduced by the waste oil. This suggests that although low levels of WEO (such as 10%) bring the NA closer to PA in terms of chemical composition, higher WEO content may introduce excessive hydrocarbons that alter the chemical structure significantly. Thus, while the 10% modification appears optimal in terms of similarity to PA, excessive modification can lead to an imbalance, making the modified asphalt chemically distinct from PA. Overall, the FTIR analysis shows that WEO is an effective modifier, particularly at lower percentages, for adjusting the chemical composition of natural asphalt to resemble that of petroleum asphalt, with diminishing returns and potential divergence at higher WEO levels.

3.5.3. SEM Analysis

SEM images presented in Figure 22 reveal substantial differences in the surface morphology of various asphalt binders. Unmodified NA (Figure 22a) displays a rough, irregular surface typical of natural asphalt, attributed to its asphaltene content and heavy molecular components. This irregularity indicates a complex structure with higher impurity levels and a heterogeneous matrix compared to PA. The SEM image of PA (Figure 22b) shows smoother and more uniform surface characteristics of a refined asphalt product. This homogeneity reflects the controlled production process of PA, with fewer impurities and well-dispersed asphaltenes. Upon modifying NA with 10% WEO, as shown in Figure 22c, the surface becomes noticeably smoother, with smaller, well-dispersed particles, suggesting better asphaltene dispersion and improved workability. At 20% WEO (Figure 22d), the surface remains relatively smooth but starts showing larger aggregates, hinting at phase separation or clustering, potentially compromising uniformity under mechanical stress. With 30% WEO (Figure 22e), further aggregation is evident, likely due to excessive WEO addition, reducing homogeneity and performance.

The SEM findings align with the EDX and FTIR results, where the increased carbon content observed in EDX corresponds to the smoother morphology at higher WEO levels. FTIR data, showing chemical bond changes, support the idea that WEO alters NA’s chemical and structural properties. The smoother PA surface compared to NA emphasizes WEO’s refining effect, with 30% WEO modification bringing NA’s morphology closer to PA. These analyses collectively highlight the correlation between chemical modifications seen in EDX and FTIR and the physical changes observed in SEM, underscoring the transformative impact of WEO on NA.

3.6. Economic Benefits of Eco-Friendly Asphalt

The cost analysis highlights the economic advantage of using WEO-modified NA for eco-friendly asphalt production. Based on data from the Ministry of Industry and Minerals, the cost per metric ton of PA is $270, while NA costs $70 per metric ton. The collection and processing of WEO for mixing with NA amount to $30 per metric ton [48]. For 10% WEO-modified NA, the cost per ton is $66 [(0.9 × $70) + (0.1 × $30)]; for 20% WEO-modified NA, it is $62 [(0.8 × $70) + (0.2 × $30)]; and for 30% WEO-modified NA, it is $58 [(0.7 × $70) + (0.3 × $30)]. These results indicate that incorporating WEO reduces the cost of asphalt production compared to PA, offering significant economic benefits while utilizing waste materials and reducing environmental damage from improper disposal. By reusing WEO in asphalt production, this approach also helps mitigate pollution and protect water resources from contamination caused by improper WEO disposal into sewage systems.

4. Conclusions

This study evaluated the performance of eco-friendly asphalt binders produced by modifying NA with WEO at varying percentages (0%, 10%, 20%, and 30%) compared to conventional PA. The experimental program included a comprehensive analysis of physical, rheological, and chemical properties through tests such as penetration, softening point, viscosity, ductility, MSCR, LAS, EDX, FTIR, and SEM. The key findings are summarized below, emphasizing the potential of these eco-friendly binders for sustainable pavement applications:

• WEO significantly improved the physical properties of NA by increasing penetration and ductility while reducing the softening point and viscosity. At 20% WEO, the physical properties closely matched those of PA. EDX and SEM analyses demonstrated improved chemical composition and surface homogeneity with 20% WEO, while 30% WEO showed phase separation and potential performance issues;
• Rotational viscosity test results at 135 °C revealed that unmodified NA had high viscosity, making it difficult to handle. WEO addition progressively reduced viscosity, with 10% WEO achieving an 83.31% reduction and 20% WEO nearing PA’s viscosity, enhancing workability. At 30% WEO, viscosity matched that of PA, indicating improved flow properties;
• Rheological analysis confirmed that PA, NA, 10% WEO, and 20% WEO binders maintained a high-temperature PG grade of 70 °C, suitable for rutting resistance. The 30% WEO binder had a lower PG grade of 64 °C, indicating reduced thermal stability at high WEO content;
• MSCR and LAS tests indicated that the 20% WEO binder achieved the best balance of deformation resistance and fatigue performance, meeting Superpave criteria and surpassing PA’s fatigue life. The 10% WEO binder also improved performance, while 30% WEO exhibited reduced effectiveness under stress;
• The comprehensive analysis suggests that 20% WEO is the optimal content for producing eco-friendly asphalt using NA, balancing flexibility, thermal stability, and resistance to deformation and fatigue. Higher WEO levels (30%) should be avoided as they compromise performance. However, these results are based on controlled laboratory conditions, and further field studies are necessary to verify the practical performance and long-term sustainability of these findings.