The Effectiveness of Waste Tire Pyrolysis Oils (WTPOs) as Rejuvenating Agents for Asphalt Materials

Abstract

The continuous increase in solid waste materials, such as waste tires, underscores the critical importance of recycling them to mitigate environmental impact and promote sustainable resource management. This research study evaluated the effectiveness of utilizing waste tire pyrolysis oils (WTPOs) as recycling agents for asphalt materials. The chemical composition and thermal behavior of WTPO were analyzed using Fourier transform infrared (FT-IR) spectroscopy and thermogravimetric analysis (TGA). Mechanically, the prepared WTPO binders were assessed by measuring dynamic viscosity and changes in high- and intermediate-temperature performance grades. Additionally, the cracking susceptibility of the binders was evaluated using the Glover-Rowe (G-R) parameter. The findings indicated that WTPOs might contain water and light aromatics in varying percentages, depending on the pyrolysis process. Incorporating WTPOs enhanced the workability of asphalt mixtures and ensured a high degree of blending between recycled/aged asphalt and raw binder. A 12% WTPO dosage was identified as the most effective for enhancing fatigue and low-temperature cracking resistance, facilitating improved interactions between the virgin binder and recycled asphalt materials. Finally, utilizing WTPOs as rejuvenating agents in pavement construction supports sustainable practices by recycling waste materials and significantly improving the performance and durability of asphalt mixtures.

1. Introduction

The United States (US) produces approximately 4.45 million tons of scrap tires and 35 million tons of waste plastic materials, with 75.8% of the scrap tires and less than 10% of the waste plastic materials being utilized in the market [1,2]. Tires are composed of numerous components, including rubber, carbon black, accelerators, and inorganic fillers. A typical tire’s main composition percentages are 60–65% rubber, 25–35% carbon black (CB), and 3% inorganic fillers [3]. The primary concern with incorporating waste tires and plastics into asphalt binders is the poor storage stability of the modified binders. Consequently, the risk of phase separation between the asphalt binder and these modifiers is high due to differences in density, viscosity, and chemical incompatibility. Consequently, the effective utilization and recycling of municipal solid waste (MSW), including plastics, tires, and wood, are crucial for mitigating environmental impact and enhancing economic sustainability by reducing landfill use, conserving natural resources, and promoting the circular economy.

Pyrolysis has emerged as an effective solution for converting these wastes into valuable energy forms: solid, liquid, and gas. This thermal decomposition process occurs in the absence of oxygen at varying temperature ranges (300–900 °C). The pyrolysis of waste tires, in particular, produces three primary components: oil (light), char (carbon black), and gas. The pyrolysis temperature and heating rate significantly influence oil and gas yield; higher temperatures convert more oil into gas, while the solid component’s content changes insignificantly (±3%) [3]. Waste tire pyrolysis oil (WTPO) is a medium-viscosity, brown/dark-colored liquid with an aromatic odor. This complex liquid contains over a hundred components, including aromatic and aliphatic hydrocarbons, heteroatom (nonhydrocarbon) components, and polar fractions. WTPO has diverse applications: fuel (e.g., hydrogen or upgraded to higher quality fuel), chemicals (e.g., resins, adhesives, and fertilizers), heat generation (e.g., co-firing in boilers and furnaces), and power generation (e.g., diesel and turbines) [3]. The adaptability and utility of pyrolysis products highlight the potential of this technology to address growing MSW challenges and contribute to sustainable energy solutions.

Recently, the use of reclaimed asphalt pavement (RAP) and reclaimed asphalt shingles (RASs) in newly constructed flexible pavements has increased. The United States produces approximately 400 million tons of RAP annually from old pavements, with around 21.1% of this RAP being incorporated into new asphalt pavements [4]. Notably, utilizing these materials offers environmental and economic benefits by balancing the usage of virgin materials and conserving landfill space. However, concerns remain regarding the content of recycled materials in new asphalt mixes. The binder in recycled materials is significantly stiffer than standard paving-grade binders, reducing the blending efficiency between virgin and recycled asphalt and consequently degrading the long-term performance of the asphalt mix. Furthermore, research indicates heterogeneity in the blending of virgin and RAP materials, influenced by the content and preheating process of RAP materials and the stiffness of the RAP binders (high PG is around 85–90 °C) [5,6,7]. Therefore, to enhance the degree of blending (DoB) and address durability concerns, it is essential to incorporate softer asphalt binders, oils [8,9,10,11,12], or chemicals additives [13,14] into asphalt mixes containing high amounts of recycled materials (i.e., over 25% for RAP and 5% for RASs) [8,9,10,11,12]. Studies have reported improvements in moisture and rutting resistance [8,9,10,11]. However, Mogawer et al. [15] found that the addition of rejuvenators increased the susceptibility of asphalt mixes to rutting and moisture.

The asphalt industry utilizes diverse rejuvenators, including various types of super-soft asphalt binders (typically with low asphaltene content), waste-derived oils, plant oils, and engineered products. According to the ASTM D4552 standard, rejuvenators are classified into six groups based on their viscosity at 60 °C. Rejuvenators with lower viscosity tend to have better diffusivity and softening efficiency but exhibit poorer thermal stability [16,17]. The amount of rejuvenator required to revitalize the rheological properties of aged binders typically constitutes less than 10% of the weight of the asphalt binder. However, there are instances where higher percentages have been utilized. However, few studies have investigated the impact of WTPO on the rheological properties of recycled asphalt materials [18,19,20].

Table 1. Summary of previous studies on bio-oils as rejuvenating agents.

Study	Rejuvenating Agent	Recommended Dosage, %	Mixing Condition	Main Findings
Zargar et al. [24]	WCO	3–4% by binder wt. (laboratory-aged binder)	Mixing at 200 rpm for 30 min at 160 °C	Enhanced the rheological properties and aging susceptibility and decreased the viscosity of binders.
Ali et al. [25]	Naphthenic oil Paraffinic oil Aromatic extracts Oleic acid Tall oil	5 and 9% 5 and 9% 5 and 9% 3 and 5.4% 2.5 and 4% by binder wt., for 25% and 45% RAP content, respectively	-	Rejuvenated binders exhibited lower PGs than control binders. Paraffinic oil was especially effective at lowering both high- and low-temperature PGs.
Zaumains et al. [11]	Six rejuvenators: WVO, WVG, organic oil, Hydrogreen STM, distilled tall oil, aromatic extract, and WEO	12% of binder wt. for both extracted RAP binder and RAP mixtures	Mixing at 140 °C for 40 min	Enhanced the rutting resistance, extended the fatigue life, and reduced the critical temperature of RAP recycled materials.
Yang et al. [26]	Waste wood-derived bio-oil	5 and 10% by binder wt. (PG58-28 binder)	-	Observed moisture content was around 15–30% by wt. in the new derived oil and 5–8% by wt. in the processed oil. Improved high-temperature performance of rejuvenated binders.
Zhu et al. [27]	By-product in cotton-oil production	5 and 10% by total wt. of aged binder (laboratory-aged binder)	Shear mixer at 5000 rpm for 15 min at 130 °C and 160 °C for neat and SBS-binders	Results revealed that 10% revitalized the workability, fatigue, and thermal cracking resistance and reduced asphaltene content.
Elkashef et al. [28]	Soybean oil	0.75% by wt. of binder (original binders)	Shear mill at 3000 rpm for 60 min at 140 °C	Enhanced fatigue and low-temperature properties. A notable decrease in the shear modulus (|G*|) and an increase in the phase angle (δ) were observed, which essentially reversed the effects of aging.
Elkashef and William [29]	Soybean oil	6% and 12% by wt. of original binder, blended with extracted RAP binder at a 1:5 wt. ratio, resulting in effective dosages of 1% and 2% by total binder wt.	-	Improved fatigue life and thermal cracking resistance. Increased the fracture energy, as measured by the DCT test.
Cavalli et al. [30]	Natural seed, cashew nutshell, and tall oils	5% by binder wt. (extracted RAP binder)	Speed mixer at 3500 rpm for 1 min at 145 °C	The results revealed that although the bio-oils may enhance the mechanical and rheological properties in an unaged state, their impact was reduced with aging.
Cao et al. [31]	WVO	5, 10, 15, and 20% by binder wt. (laboratory-aged binder)	Stirred at 3000 rpm for 15 min at 135 °C	Enhanced workability and fatigue life. Based on FT-IR results, the carbonyl index (IC=O) increased due to the fatty acid component in WVO, and the sulfoxide index (IS=O) decreased. Additionally, there was no chemical reaction between WVO and the binder.
Farooq et al. [32]	WMEO	10–20% by wt. of RAP binder	-	Improved the mechanical performance, compatibility, and aggregate coating of RAP recycled asphalt mixtures.
Zhang et al. [33]	Sawdust-derived bio-oil	10, 15, and 20% by wt. of binder (laboratory-aged binder)	Mixed at 5000 rpm for 15 min at 135 °C	Decreased the viscosity and activation energy, increased the softening of aged binder, and enhanced the thermal cracking resistance.

WCO is waste cooking oil, WEO is waste engine oil, WVO is waste vegetable oil, WVG is waste vegetable grease, WMEO is waste mobile engine oil, and DCT is disk-shaped compact tension.

provides examples of various rejuvenators employed in the asphalt industry to enhance the thermo-mechanical and rheological properties of asphalt materials [21,22,23].

The application of pyrolysis oil (PO) in pavement construction offers significant sustainability and resource conservation benefits by recycling waste materials and reducing the carbon footprint associated with traditional pavement materials. WTPO is an effective recycling agent or rejuvenator for asphalt binders, enhancing their performance and durability. Research indicates that bio-oils improve asphalt binders’ rheological and physical properties, leading to better resistance to fatigue and overall pavement performance. However, limited studies have investigated the applicability of WTPO in pavement construction. Consequently, evaluating the effectiveness of utilizing WTPO in pavement construction is crucial to ensure its practical application and to optimize its benefits. The utilization of WTPO in pavement construction contributes to a circular economy by transforming waste products into valuable resources, thereby promoting environmental stewardship and economic efficiency. This innovative approach aligns with sustainable development goals and paves the way for more resilient and long-lasting road infrastructure.

2. Objective and Research Approach

The primary goal of this research study was to evaluate the effectiveness of oil derived from scrap tires as a recycling agent for asphalt pavement construction. Consequently, a systematic approach was undertaken, blending and testing two generic WTPO batches with an asphalt binder at various dosages, up to 16% by the asphalt’s weight, as displayed in Figure 1. The assessment of the modified binders encompassed the workability, rheological properties, and chemical characterization of asphalt. Workability was evaluated by measuring the viscosity of the modified asphalt binder using a rotational viscometer (RV) and by assessing the compatibility of the asphalt mixtures. Rheological properties were measured at different stress and temperature conditions. Chemical characterization was performed using Fourier transform infrared (FT-IR) spectroscopy to identify the chemical composition of the WTPO and to understand the WTPO–asphalt interaction mechanism.

A WTPO was blended with the original asphalt binder at 100 ± 5 °C using a high-shear mixer operating at 500 rpm for 4 h. Binder samples were collected at various intervals (1, 15, 30, 60, 120, and 240 min) during the mixing process to evaluate the interaction mechanism between the WTPO and the asphalt binder. This was done by assessing changes in rheological properties and chemical composition throughout the mixing process. Notably, during the mixing process, foaming (bubbles) was observed for the first 15–30 min, depending on the pyrolysis oil content. This indicates that the WTPO contains water and/or light aromatics, which contribute to bubble formation. The WTPO–binder interaction curve consisted of three main regions, indicated by the G*/sinδ parameter, as illustrated in Figure 2. Initially, after 1 min of mixing, the rutting parameter (G*/sinδ) dropped significantly by almost seven times, continuing to decrease up to 30 min (Region I). However, between 30 and 60 min (Region II), G*/sinδ increased, suggesting that the WTPO binder lost weight due to the volatilization of light aromatics. Finally, after 60 min of interaction (Region III), the G*/sinδ parameter increased slightly, indicating the further stabilization of the binder’s properties. Consequently, an interaction time of 60 min at 100 °C was selected to ensure an optimal interaction time and to avoid the premature aging of the binder.

3. Materials and Testing Methods

3.1. Materials

This research study utilized two types of asphalt binders: an original PG 64-22 source, which is commonly used in Missouri, and an aged PG 64-22 binder. Following the asphalt institute recommendation, the long-term aging (LTA) of the asphalt binder was simulated using a forced-draft oven, where the binder was aged for 24 h at 135 °C. This process approximates 7 to 10 years of aging in field conditions for asphalt pavements [34,35]. The rheological and physical properties of this asphalt binder are detailed in

Table 2. Physical and rheological properties of the proposed asphalt binder.

Parameters	PG 64-22	Aged Binder	Limit	Standard
Viscosity at 135 °C, Pa.s	0.401	1.33	≤3 Pa.s	ASTM D4402 [36]
RTFO mass loss, %	0.041	-	≤1.0	ASTM D2872 [37]
G*/sinδ (original case)	1.41 1	-	≥1.0	ASTM D7175 [38]
G*/sinδ (RTFO residue)	3.34 1	17.21 2	≥2.2
G*.sinδ at 25 °C, kPa	4056.97 *	3763.41	≤5000	ASTM D7175 [38]
S at 12 °C, MPa	172.87 *	197.85	≤300	ASTM D6648 [39]
m-value at 12 °C	0.336 *	0.31	≥0.30

¹ Measured at 64 °C and ² measured at 76 °C. * These values measured at the PAV condition.

. Additionally, one commercial rejuvenator, coded as E; two generic WTPO batches, coded as B1A and P4; and a reference rejuvenator, coded as E, were incorporated to evaluate the effectiveness of WTPO as a recycling agent in pavement construction.

Table 3. Details of the received WTPOs and the applied dosage.

Code	Description	Flash Point, F	Dosage, %
B1A	Collected at 400 °F with 38.8 cP dynamic viscosity	265	2, 4, 6, 8, and 12
P4	Plant-treated batch of combined trays (570 °F) for 4 hrs with 45 cp dynamic viscosity.	259	4, 8, 16

summarizes the available data for these WTPO batches provided by the supplier. The WTPOs were blended at varying dosages, up to 16% by weight of the asphalt binder, to identify the optimal dosage needed to enhance the performance of the asphalt binder.

3.2. Test Methods

3.2.1. Thermogravimetric Analysis (TGA)

The thermal behavior of the three WTPO batches was assessed using the TGA. Approximately 50 mg of WTPO sample was placed in an alumina crucible, and changes in sample mass with temperature were measured using an internal mass balance. Nitrogen gas with a flow rate of 20 mL/min was used as a purging gas. The B1A WTPO batch was heated from room temperature (23 °C) to 500 °C at a ramp rate of 5 °C/min. The same WTPO source was also tested using a stepwise isothermal method to differentiate the WTPO compounds. In parallel, the P4 batch was heated from room temperature to 800 °C. The degradation process was conducted in three stages: (1) a ramp from room temperature to 100 °C, (2) an isothermal hold at 100 °C for 15 min, and (3) a ramp from 100 °C to 800 °C at a rate of 10 °C/min.

3.2.2. Laboratory Aging Techniques

In this research study, the rolling thin-film oven (RTFO) and pressure-aging vessel (PAV) techniques were employed to simulate the short-term aging (STA) and long-term aging (LTA) of the investigated binders, respectively. These procedures were carried out per ASTM D2872 for RTFO [37] and ASTM D6521 for PAV [40].

3.2.3. Fourier Transform Infrared (FT-IR) Spectroscopy

This study utilized an FT-IR spectrometer to perform FT-IR analysis on the investigated binders. The FT-IR spectra for each sample were recorded over a wavenumber range of 4000 to 400 cm⁻¹ at room temperature, with a resolution of 4 cm⁻¹ and 16 scans per sample. After analyzing each sample, the diamond crystal was meticulously cleaned using a dust-free tissue and ethanol to ensure accuracy and prevent contamination. FT-IR spectroscopy was conducted to identify the functional groups present in the WTPOs and asphalt binders and to evaluate the interaction mechanism between asphalt binders and recycled agents. Additionally, the carbonyl, sulfoxide, and chemical aging indices were computed for the investigated binders, which included neat and rejuvenated binders with effective dosages. These computations were carried out across three aging conditions, unaged, STA-aged, and LTA-aged states, to comprehensively evaluate the aging susceptibility of the binders [41].

3.2.4. Brookfield Rotational Viscometer (RV)

The rotational viscometer (RV) test measured the dynamic viscosity (η) of asphalt binders at elevated temperatures ranging from 120 °C to 165 °C. This test was performed using spindle No. 21, rotating at an RPM that keeps the torque within 2 and 98 percent, following ASTM D4402 standards to evaluate the pumpability, mixability, and workability of the asphalt binders.

3.2.5. Temperature Sweep Test (TST)

A dynamic shear rheometer (DSR) was utilized to perform temperature sweep tests at ranges of 5–25 °C and 40–70 °C, with a frequency of 10 rad/sec, to estimate the intermediate and high failure temperatures of all investigated binders following ASTM D7175 [38]. Two different plate geometries were used: 8 mm plates with a 2 mm gap for lower temperature ranges and 25 mm plates with a 1 mm gap for higher temperature ranges. The critical intermediate and high temperatures of investigated binders were defined in accordance with ASTM D7175 [38].

3.2.6. Frequency Sweep Test (FST)

The frequency sweep test was conducted across three logarithmic spans of frequency (100 to 0.1 rad/sec) at temperatures ranging from 5 °C to 25 °C, with five °C intervals. Complex modulus master curves were created using the frequency sweep data. The Christensen–Anderson (CA) model (Equation (1)) was employed to construct the master curves of the investigated binders due to the physical significance of its parameters, including cross angular frequency (ω_c) and the R–value [42]. In contrast, Equation (2) was employed to construct the master curve of phase angle. Additionally, the Williams-Landel-Ferry (WLF) time–temperature superposition model was employed to determine the shift factor at a reference temperature of 15 °C. Subsequently, the Glover-Rowe (G-R) parameter, which represents the age-related cracking resistance of asphalt binders at low temperatures [43,44], was computed using Equation (3). The G-R parameter was employed to evaluate the aging and cracking susceptibility of the binders.

$$\mathrm{R}\mathrm{A}\mathrm{I}={\displaystyle \frac{{\left(\raisebox{1ex}{$\mathrm{G}\mathrm{*}$}\!\left/ \!\raisebox{-1ex}{$\mathrm{s}\mathrm{i}\mathrm{n}\mathsf{\delta }$}\right.\right)}_{\mathrm{R}\mathrm{T}\mathrm{F}\mathrm{O} \mathrm{a}\mathrm{g}\mathrm{e}\mathrm{d} \mathrm{c}\mathrm{a}\mathrm{s}\mathrm{e}}}{{\left(\raisebox{1ex}{$\mathrm{G}\mathrm{*}$}\!\left/ \!\raisebox{-1ex}{$\mathrm{s}\mathrm{i}\mathrm{n}\mathsf{\delta }$}\right.\right)}_{\mathrm{o}\mathrm{r}\mathrm{i}\mathrm{g}\mathrm{i}\mathrm{n}\mathrm{a}\mathrm{l} \mathrm{c}\mathrm{a}\mathrm{s}\mathrm{e}}}},$$

(1)

$$\mathsf{\delta }=90-\left(90-{\mathsf{\delta }}_{\mathrm{m}}\right){\left[1+{\left({\displaystyle \frac{\left(\log \left(\frac{{\mathsf{\omega }}_{\mathrm{d}}}{{\mathsf{\omega }}_{\mathrm{r}}}\right)\right)}{{\mathrm{R}}_{\mathrm{d}}}}\right)}^2\right]}^{{\displaystyle \raisebox{1ex}{$-{\mathrm{m}}_{\mathrm{d}}$}\!\left/ \!\raisebox{-1ex}{$2$}\right.}},$$

(2)

where G* is the complex shear modulus (kPa), G_g is the glassy modulus (typically 1.0 × 10⁶ kPa), ω_r is the reduced angular frequency (rad/sec), ω_c is the crossover angular frequency at which phase angle equals 45° in rad/sec, and the R-value is the rheological index (shape factor) that represents the difference between the glassy modulus and the complex modulus at crossover angular frequency on a logarithmic scale. The ω_c indicates the overall hardness of binders, and the R-value signifies a gradual transition to steady-state flow from elastic behavior.

$$\mathrm{G}-\mathrm{R}=\left|\mathrm{G}\mathrm{*}\right|·{\displaystyle \frac{{\left(\cos \mathsf{\delta }\right)}^2}{\sin \mathsf{\delta }}},$$

(3)

where G* and δ are the complex shear modulus and phase angle of binders at 15 °C and 0.005 rad/s.

3.2.7. Low-Temperature Performance Grade

A bending beam rheometer (BBR) was utilized to measure the low-temperature performance grade parameters (flexural creep stiffness and m-value) at two temperatures: 12 and −18 °C. These measurements were conducted in accordance with ASTM D6648.

3.2.8. Evaluation of Binder Aging Susceptibility

The aging susceptibility of the binders was evaluated by measuring the %RTFO mass loss and rheological aging index (RAI) based on the rutting parameter (G*/sinδ). Equation (4) expresses the RAI formula in terms of the rheological parameters (complex modulus “G*” and phase angle “δ”), measured at 64 °C. Higher %RTFO mass loss and RAI values indicate that the binder is susceptible to aging.

$$\mathrm{R}\mathrm{A}\mathrm{I}={\displaystyle \frac{{\left(\raisebox{1ex}{$\mathrm{G}\mathrm{*}$}\!\left/ \!\raisebox{-1ex}{$\mathrm{s}\mathrm{i}\mathrm{n}\mathsf{\delta }$}\right.\right)}_{\mathrm{R}\mathrm{T}\mathrm{F}\mathrm{O} \mathrm{a}\mathrm{g}\mathrm{e}\mathrm{d} \mathrm{c}\mathrm{a}\mathrm{s}\mathrm{e}}}{{\left(\raisebox{1ex}{$\mathrm{G}\mathrm{*}$}\!\left/ \!\raisebox{-1ex}{$\mathrm{s}\mathrm{i}\mathrm{n}\mathsf{\delta }$}\right.\right)}_{\mathrm{o}\mathrm{r}\mathrm{i}\mathrm{g}\mathrm{i}\mathrm{n}\mathrm{a}\mathrm{l} \mathrm{c}\mathrm{a}\mathrm{s}\mathrm{e}}}}$$

(4)

4. Results and Discussion

4.1. Thermogravimetric Analysis (TGA)

A thermogravimetric curve illustrates the relationship between mass loss and temperature, as illustrated in Figure 3, offering insights into the thermal stability and composition of the initial sample. The TGA results showed that the WTPO undergoes thermal degradation beginning at around 100 °C with different residue percentages (1% to 16% depending on the pyrolysis process), as presented in

Table 4. TGA results of the WTPOs.

Criteria	* B1A	P4
Onset temperature, °C	106	-
%Mass change before the stepwise isothermal stage	5.05	1.24
%Loss at 100 °C after the isothermal step	25.26	22.20
%Mass loss at 163 °C **	35	45
%Residue	4.522	6.59

* Based on the ramp method with a five °C/min heating rate. ** The selected temperature was based on the RTFO condition temperature.

. The investigated WTPOs appeared to contain moisture and light volatiles condensed during the production process, as approximately 25% of the oil degraded at 100 °C. The investigated WTPOs began to degrade at approximately 100 °C, with around 40% of the WTPOs degrading at the typical mixing temperature of 163 °C. This observation highlighted the need for modification in the WTPO production process. Techniques such as distillation or vacuum drying can be employed to remove moisture and light volatiles from WTPOs before their use in asphalt mixtures, ensuring optimal performance and longevity.

4.2. Chemical Composition of the Investigated Modifiers and Binders

4.2.1. Chemical Composition of Rejuvenating Agents

The FT-IR analysis was conducted on the investigated rejuvenating agents, including the commercial rejuvenator and the B1A and P4 batches, to investigate their chemical composition. Qualitative observation of the FT-IR spectra of the WTPO batches revealed the presence of various functional groups, including water/alcohol (band between 3145 and 3435 cm⁻¹), alkanes (aliphatic C-H at 2868–2955 cm⁻¹ and C-H deformation vibrations at 1376–1451 cm⁻¹), alkenes, alkyls, carbonyl groups (C=O) at 1695 cm⁻¹, and aromatic groups (bands at 1604 cm⁻¹ and 695–800 cm⁻¹). Furthermore, the results revealed that all the WTPOs exhibited the same spectral features, with variations in the intensity of some functional groups, as displayed in Figure 4.

Table 5. Functional group of the WTPOs.

Wavenumber, cm−1	Functional Groups	Class of Components
3360	O-H stretching	Water and hydroxyl compounds (carboxylic acid, alcohols, or phenols)
3060	C-H stretching	Aromatic
2955	C-H asymmetric stretching (Methyl “CH3”)	Alkane
2923	C-H asymmetric stretching (CH3 and CH2)
2868	C-H symmetric stretching (CH3)
1695	C=O stretching	Carboxylic acid
1604	C=C stretching	Aromatic
1451	C=C stretching	Aromatic
	CH2-S	Sulfur-containing compounds
1376	-C-H bending	Alkane
	S=O asymmetric stretch	Sulfonyl chlorides
1267	C-O stretching	Ester or phenol
1111	C-H in-plane bending	Aromatic
697–880	C-H bending	Aromatic

presents the detected functional groups of the WTPOs.

4.2.2. Chemical Composition of the Binders

The FT-IR analysis was also conducted on various binders, including the neat and rejuvenated binders with 4% E, 12% B1A, and 16% P4, to investigate the mechanism of interaction between the rejuvenating agents and asphalt binder. Figure 5 illustrates the spectrum of the unaged neat binder, highlighting the predominant peaks.

Upon modification, changes in the neat asphalt binder are expected depending on how the WTPOs and other modifiers interact with asphalt. As demonstrated in Figure 6, the WTPO binders exhibited similar features to the neat one, with notable differences in peak intensity, especially aromatic C-H deformation groups (band between 700–900 cm⁻¹), and slight shifts, which could also be attributed to the mixing process. Additionally, the intensity of the carbonyl group (C=O) at around 1700 cm⁻¹ and C-O-C in epoxy/ester groups at around 1170 cm⁻¹ increased with the WTPOs. Conversely, the modified binder with the commercial rejuvenator exhibited a distinct peak at a wavenumber 1745 cm⁻¹, corresponding to the C=O stretching vibrations of the ester group. Since the asphalt binder and WTPOs have almost the same functional groups in the fingerprint region, no distinctive peaks were observed for WTPO-rejuvenated binders; hence, the WTPO–asphalt interaction is physical.

Furthermore, the carbonyl (I_C=O), sulfoxide (I_S=O), and chemical aging (CAI) indices were computed for the investigated binders, as demonstrated in Figure 7. The analysis revealed that the carbonyl and sulfoxide indices of the rejuvenated binders were lower than those of the neat binder under both short-term and long-term aging cases.

4.3. Mechanical and Rheological Characterization of Binders

Initially, the mechanical and rheological properties of rejuvenated tank binders containing 4% WTPO were assessed and compared with those of the commercial rejuvenator, coded as E. Subsequently, the WTPO batches B1A and P4 were mixed with tank binder at varying dosages, up to 16% by weight, to determine the optimal WTPO concentration for enhancing asphalt binder performance in comparison to that of the commercial rejuvenator. Finally, the aged binder was mixed with WTPOs at two dosages, 12% and 16%, for the B1A and P4 batches, respectively.

4.3.1. Analysis of Dynamic Viscosity

The measured dynamic viscosity of the rejuvenated binder at 135 °C and computed % reduction in viscosity upon incorporating the rejuvenator are illustrated in Figure 8a. It was observed that the WTPOs enhanced the binder’s workability, although with some variations. These differences are attributed to the distinct chemical compositions of the WTPO batches, which result from the varying pyrolysis conditions used during their production. Additionally, the percentage of reduction in the binder’s viscosity at 135 °C was also computed to evaluate the effectiveness of the WTPOs, as presented in Figure 8a.

Furthermore, it was observed that both WTPO batches (B1A and P4) significantly reduced the binder’s viscosity at 135 °C, demonstrating improved workability, following an exponential trend with increasing WTPO content, as displayed in Figure 8b. Additionally, Figure 8c,d illustrates the computed mixing and compaction temperatures of WTPO binders with varying WTPO content. The results showed a linear reduction in both mixing and compaction temperatures as WTPO content increased. This indicates that adding WTPO would enhance workability, promote better blending with recycled asphalt materials such as RAP, and result in energy savings. Consequently, this supports more sustainable and efficient pavement construction practices.

4.3.2. Aging Sustainability

The percentage of RTFO mass loss and RAI of the investigated binders were computed to evaluate the aging susceptibility and thermal stability of the WTPOs in normal asphalt mixture production (163 °C). The results revealed that the commercial rejuvenator exhibited the smallest increase in RTFO mass loss, indicating better aging resistance. In contrast, the WTPOs showed high RTFO mass loss percentages and RAI parameters. The RTFO mass loss percentage and RAI correlated linearly with the rejuvenator content, as illustrated in Figure 9. This significantly high mass loss suggested the presence of light compounds and/or moisture, which evaporate at the RTFO conditioning temperature, which aligns the TGA results, leading to the increased aging susceptibility. It can be concluded that the WTPO batches were more volatile and more susceptible to aging, underscoring the need for modification in the WTPO production process. Additionally, these results highlight the potential benefit of applying WTPO in the production of warm asphalt mixtures and low-temperature applications.

Furthermore, the RTFO mass loss and RAI were evaluated for aged binder, as illustrated in Figure 9c. The results revealed that both parameters decreased significantly for the rejuvenated aged binder. This indicated that the addition of WTPOs to recycled asphalt materials enhanced their efficiency. The aged materials have a greater affinity for light component, thereby restoring their rheological properties. In contrast, tank binders are stable.

4.3.3. Temperature Sweep Test Results

High-Temperature Performance Grade (PG)

The rutting parameter (G*/sinδ) of all investigated binders was measured between 40 and 64 °C for unaged and RTFO-aged conditions, as illustrated in Figure 10. The rutting parameter of unaged binders was reduced significantly with the addition of 4% WTPOs, reducing the high-temperature PG by one grade. However, this influence diminished after RTFO conditioning for the WTPO-rejuvenated binders, indicating high aging susceptibility. In contrast, the high-temperature PG dropped by one grade for the binder with a 2% commercial rejuvenator, as demonstrated in Figure 10. Furthermore, the results for the rejuvenated aged binder showed that the addition of 16% of WTPO decreased the high-temperature grade by three levels, from 76 to 58, as illustrated in Figure 10e,f. However, after RTFO conditioning, the high-temperature PG increased by one level, suggesting the volatilization of light components during the asphalt mixture production process.

Furthermore, to determine the optimum WTPO content to drop the high PG by one grade, the critical high PG was computed for both B1A and P4 batches, as illustrated in Figure 11. It was observed that the critical high temperature for both unaged and RTFO-aged conditions showed a linear decrease with increasing WTPO content. The DSR results at the high-temperature range revealed no significant difference in the impact of WTPO batches, especially after the RTFO-aged state, on high-temperature PG. Additionally, a 4% WTPO dosage was sufficient to lower the high-temperature PG of the neat binder by one grade (to PG 58-XX).

Intermediate-Temperature Performance Grade

Given that long-term aging conditions are critical for assessing cracking susceptibility, the TST was conducted to determine the intermediate-temperature PG. Fatigue parameters (G*·sinδ) were measured for all investigated binders, as illustrated in Figure 12. The results revealed variations in the long-term performance of the WTPO batches among the WTPO batches. Notably, the plant-processed WTPO batch (P4) exhibited better long-term mechanical properties and higher fatigue cracking resistance, comparable to that of the commercial rejuvenator, as illustrated in Figure 12. These findings underscore the critical importance of the processing method in optimizing the performance and efficacy of WTPO as a sustainable and effective rejuvenator for asphalt materials.

Furthermore, Figure 12c illustrates the improvement in the intermediate-temperature PG of the aged binder with WTPO. The critical fatigue temperature decreased by 2.2 with the addition of 12% B1A and by 19.5% with the addition of 16% P4. Additionally, the critical intermediate PG temperature was computed for both B1A and P4 binders, as demonstrated in Figure 13. Notably, the fatigue resistance of the binders improved significantly upon increasing the WTPO dosage, particularly that of the P4 batch. This indicated that the low-temperature performance of the binders would be improved as well, matching the FT-IR analysis.

4.3.4. Complex Modulus Master Curve

The CA model was adopted to construct the dynamic shear modulus (|G*|) at 15 °C of the investigated binders, as illustrated in Figure 14. The results indicated that the inclusion of WTPOs softened the binder, allowing it to absorb and dissipate stresses more effectively. This softening effect would reduce the likelihood of cracks forming, thereby improving the pavement’s service life. Consequently, WTPO would enhance the durability of asphalt pavement by improving both low- and intermediate-temperature cracking resistance.

4.3.5. Cole–Cole Diagram

The modified Cole–Cole diagram, which depicts the components of the complex shear modulus, the elastic component (G′) against the viscous component (G″), is commonly used to characterize the elastic and viscous behaviors of asphalt binders, as illustrated in Figure 15. An equality line (G′ = G″) is also superimposed to differentiate between these behaviors: data points to the left of the line indicate a predominance of viscous properties, while those to the right represent more elastic behavior. At low temperatures, the elastic component of the complex modulus is predominant. As the temperature increases, the binder loses some of its elasticity, becoming more viscous. Adding rejuvenating agents to the control binder increased the viscosity, primarily by increasing the phase angle. Furthermore, all tested binders exhibited an almost linear relationship in the Cole–Cole diagram, indicating that the addition of either commercial rejuvenators or WTPO batches did not cause any significant structural changes.

4.3.6. Black Space Diagram and Glover-Rowe Damage Parameter

Figure 16 demonstrates the Glover-Rowe (G-R) results for the investigated binders, including both the neat and the WTPO binders, at PAV conditions. As the WTPO dosage increases, the rheological characteristics shift from the upper left towards the lower right on the black space diagram. This indicates better cracking resistance with aging and improved recovery properties. The binder with the P4 batch exhibited superior cracking resistance compared to the B1A binder. As illustrated in Figure 16b, increasing the WTPO dosage significantly enhanced the cracking resistance of both the B1A and P4 binders, as evidenced by the exponential decrease in the G-R parameter. The P4 binder, in particular, showed superior performance with lower G-R values at dosages similar to those of the B1A binder. The G-R parameter was reduced by almost 67% and 82% for the 12%B1A and 16%P4 batches, respectively, suggesting it may be more effective in improving binder durability.

4.3.7. Low-Temperature Performance Grade

Considering the critical importance of long-term aging conditions for assessing thermal cracking, the BBR test was conducted to determine the low-temperature PG. The flexural creep stiffness (S(T)) and m-value were measured for neat, 12%B1A, and 16%P4 binders at two temperatures, −12 and −18 °C, as illustrated in Figure 17. The results revealed that the plant-processed WTPO batch (P4) exhibited better long-term performance, as illustrated in Figure 17. These findings highlight the crucial role of the processing method in optimizing the performance and efficacy of WTPO as a sustainable and effective rejuvenator for asphalt materials.

5. Conclusions

Recycling waste tires to produce pyrolysis oil (WTPO) is crucial from an environmental perspective, as it enhances the incorporation of recycled asphalt materials, such as RAP and RASs, into pavement construction. This process reduces landfill waste and pollution and converts tires into valuable resources, conserving natural materials and supporting sustainable energy production. Consequently, this research study assessed the effectiveness of utilizing WTPOs as rejuvenating agents for recycled asphalt materials. An asphalt binder source (PG 64-22) was mixed with two generic WTPOs at various dosages, up to 16% by the asphalt’s weight, and their physical, rheological, and chemical properties were evaluated. The following key points were derived:

• Thermal and chemical analyses revealed variations in the chemical composition of the WTPOs, particularly in their aromatic content, depending on the pyrolysis process.
• The workability of the asphalt binder was enhanced significantly by the WTPOs, leading to significantly reduced mixing and compaction temperatures, thereby conserving energy and promoting more efficient pavement construction practices.
• The WTPOs demonstrated potential for enhancing the blending of raw binder and RAP materials, improving the overall performance and cohesion of the recycled asphalt mixture.
• The %RTFO mass loss indicated that WTPOs contain light aromatics and moisture that evaporate at standard mixing temperatures (163 °C). This finding underscores the need to optimize the pyrolysis process to enhance the thermal stability of WTPO, ensuring it remains effective and stable during asphalt production and application.
• The plant-processed WTPO batch exhibited better long-term mechanical properties and higher fatigue cracking resistance comparable to that of the commercial rejuvenator, suggesting the importance of optimizing the pyrolysis process.
• The fatigue cracking resistance and decreased cracking susceptibility of the asphalt binder were enhanced significantly with the incorporation of WTPO. This improvement is evidenced by the Glover-Rowe (G-R) parameter, which showed a reduction of up to 82% for a 16% WTPO content.
• The rheological and physical test results indicated that a WTPO dosage of 12% is the most effective for enhancing the durability of asphalt pavements by improving the cracking resistance.

Finally, incorporating WTPO would significantly increase the likelihood of effectively utilizing RAP materials, thereby advancing sustainability in pavement construction by enhancing the cracking resistance of asphalt pavements. However, it is recommended that the WTPO production process be optimized to eliminate light aromatics and unwanted compounds. These components should be extracted before utilizing the oil, thereby upgrading the WTPO and enhancing its effectiveness as a rejuvenating agent for asphalt binders. This optimization will improve the thermal stability and overall performance of WTPO, making it a more viable and efficient additive in pavement construction. Additionally, a thorough examination of the performance of asphalt mixtures modified with WTPO is necessary. Finally, the environmental benefits and potential impacts of using WTPO in asphalt mixtures should be thoroughly evaluated, including lifecycle analysis and carbon footprint reduction. This evaluation will help quantify the sustainability advantages of incorporating WTPO in pavement construction, ensuring that the use of WTPO not only enhances pavement performance but also contributes positively to environmental conservation and sustainable development.