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Abstract

The inherent characteristics of asphalt binder can pose challenges to achieving a flexible pavement with the desired properties. However, these challenges can be addressed through asphalt modification. While numerous additives are available, selecting the most suitable one is crucial due to issues such as cost, storage stability, and homogeneity. This study focused on evaluating the physical and rheological properties of an asphalt binder modified with a hydrocarbon-based thermoplastic resin. The investigation included conventional binder tests and rotational viscometer (RV) analysis, along with an in-depth rheological evaluation of the modified binders using a dynamic shear rheometer (DSR). Additionally, the thermal properties of the additive were analyzed through thermogravimetric analysis (TGA). The findings revealed that the modified binders exhibited reduced penetration, elevated softening points, and increased viscosity, all achieved without compromising workability or pumpability. Furthermore, the modification enhanced rutting resistance. Laboratory tests identified 5% as the optimal additive content, which improved the performance grade of the pure binder. Activation energy results derived from the complex viscosity master curves indicated that while the energy required for flow increased with the addition of 5% resin, the elevated viscosity mitigated the temperature sensitivity issue observed in the pure binder.

Keywords

Asphalt binder modification rheology thermoplastic resin activation energy

Introduction

Flexible pavements are typically constructed by laying and compacting a mixture of aggregate and asphalt binder, and they are widely preferred due to advantages such as driving comfort, ease of repair, and rapid traffic reopening.^1,2 These pavements derive their “flexible” nature from the viscoelastic asphalt binder, which, despite comprising only 4%–5% of the mixture, plays a critical role in determining overall road performance.^3–5 The majority of asphalt pavement deterioration can be attributed to the binder.^6,7 Issues such as thermal cracking from binder shrinkage in cold climates, fatigue cracking under traffic loads, oxidation from prolonged exposure to sunlight and air, permanent/plastic deformations (rutting) in hot conditions, and low-temperature fractures are all directly linked to the asphalt binder.^8–12 The unmodified binder often lacks the resilience required to withstand these forms of deterioration. Consequently, it must be adapted to endure various climatic, chemical, physical, and traffic-related challenges.

To achieve this, asphalt binders are modified with various additives, among which polymer-based additives are widely used. Polymer modification involves the incorporation of polymers into the binder through mechanical or chemical processes.^13,14 These polymer additives are broadly categorized into two groups: elastomeric and plastomeric. Plastomeric additives include materials such as polyethylene (PE), polypropylene (PP), ethylene-vinyl acetate (EVA), and ethylene-butyl-acrylate (EBA), while elastomeric additives, which are thermoplastic, include styrene-butadiene-styrene (SBS), styrene-isoprene-styrene (SIS), and styrene-ethylene-butylene-styrene (SEBS).^15–17 Generally, these polymeric additives enhance the elasticity of the binder, improve fatigue and crack resistance, increase the complex modulus, and bolster resistance to permanent deformation at high temperatures.^18–20 However, recent research has highlighted certain drawbacks of commonly used polymeric additives, particularly concerning cost and various performance parameters, prompting efforts to identify alternative additive solutions.^21,22

In the quest for alternative additives, a diverse range of materials has been explored, including carbon-based substances, resins, crumb rubber derived from waste vehicle tires, bio-based products, waste polymers, organic and inorganic waste materials, and oils. Quan et al. highlighted the increasing interest in bio-additives sourced from renewable raw materials by examining the potential of functionalized canola oil in asphalt modification. Their findings demonstrated that this additive reduced production costs by 28.3%, energy consumption by 21%, and CO₂ emissions by 20.5% compared to SBS modification.²³ Casado-Barrasa et al. investigated the use of carbon black to enhance the physical and rheological properties of asphalt binders, employing the Evotherm additive to counteract the negative effects of elevated production temperatures. The study revealed that carbon black-modified binders performed comparably to commercial polymeric additives, with carbon black mixtures delivering superior results in certain cases.²⁴ Geçkil et al. assessed the performance of asphalt mixtures modified with oak ash waste, a type of biomass waste. Laboratory tests on mixtures with varying percentages of this additive indicated that it improved nearly all mechanical properties, particularly enhancing resistance to plastic deformation at high temperatures. The optimal additive content was identified as 2%.² Erkuş et al. examined the combined use of crumb rubber (CR) and SBS, emphasizing its economic and rheological advantages. Their results indicated that several triple binder combinations offered superior high-temperature performance compared to SBS alone. However, the number of effective combinations decreased when low-temperature behavior was considered. Among these, binders modified with 6% CR, 2% SBS, and 2% Sasobit exhibited better overall performance than those with 6% SBS alone.²⁵ Another study explored the impact of polyethylene wax additives on asphalt binder and mixtures. Using three different wax types, the researchers conducted critical rheological evaluations, including performance grading (PG), storage stability, and rutting resistance tests. The findings showed that wax additives significantly enhanced high-temperature stability and moisture resistance but reduced low-temperature stability. As the molecular weight of the wax increased, viscosity and resistance to permanent deformation also improved, although some adverse effects on storage stability were noted.²⁶

Recent years have witnessed a growing body of research on the incorporation of resins, thermosetting resins, and reactive polymers, either directly into pure asphalt or into asphalt modified with conventional polymers like SBS. Thermosetting resins offer notable advantages, including high strength, resistance to corrosion and chemicals, and enhanced aging durability.²⁷ When subjected to heat, these resins undergo a chemical reaction, resulting in an insoluble and infusible substance after the crosslinking-curing process. This reaction forms a rigid, durable network structure that provides a sealed, resilient barrier against external factors.²⁸ Epoxy resin is among the most widely used additives in thermoset polymer-modified asphalt binders. However, its compatibility issues with asphalt often lead to segregation, which is why it is frequently used in combination with SBS. Xu et al. investigated the modification of asphalt binder using epoxy resin and SBS, finding that the tensile strength, elongation at break, toughness, and mechanical properties of a 3% SBS-epoxy blend were strongly influenced by particle size.²⁹ Huang et al. explored the potential of phenol-formaldehyde resin and organo-montmorillonite (OMMT) as asphalt modifiers. Their comprehensive characterization revealed that the composite structure achieved high homogeneity, while the addition of OMMT enhanced the interaction between asphalt and resin. The results indicated improvements in high-temperature rutting resistance, thermal stability, and aging resistance with the additive.³⁰ Yuniarti et al. addressed the issue of asphalt-polymer incompatibility, particularly at high polymer usage levels, by evaluating pine resin as an asphalt additive. The study aimed to create a compatible asphalt matrix by incorporating pine resin into asphalt modified with waste polymers. Findings showed that the aromatic oils in pine resin facilitated the uniform distribution of the polymer within the asphalt matrix, effectively enhancing the performance grade of the asphalt with appropriate resin usage.³¹ Another study examined the performance of asphalt binders modified with lignin-based resin and phenol-formaldehyde resin. Both resins increased the proportions of large molecular structures and elastic components within the asphalt, improving its resistance to plastic deformation at high temperatures. Additionally, they enhanced fatigue performance and aging resistance, although both resins negatively impacted low-temperature behavior.³² Similarly, Li et al. studied the use of lignin-based synthetic resin as an asphalt modifier, reporting improvements in high-temperature strength, fatigue resistance, and aging durability. This research also expanded the application possibilities of biomass-derived materials.³³ Shape memory polymers, known for their ability to return to their original state when exposed to specific temperature changes, have also been explored as asphalt modifiers. Their thermal adaptability enhances high-temperature rutting resistance. Zhou and his team incorporated a shape memory epoxy resin into asphalt and conducted extensive laboratory evaluations. The results demonstrated that the resin met all performance specifications, with the previously observed disadvantage of low-temperature performance loss successfully mitigated. The study also confirmed improved resistance to moisture damage.³⁴

Some previous studies on bitumen modification have generally focused on high-molecular-weight polymers or composite additives. For example, Babagoli et al., and Ameli et al., tested rutting and fatigue resistance using SBR, PPA, and warm-mix additives together, but these studies focused more on multi-additive interactions and blend-level performance.^14,35 Hesami et al., confirmed chemical bond formation with a reactive polyurethane polymer using FTIR analysis, but their studies primarily focused on hardness and low-temperature fracture behaviour.³⁶ Ghoreishi et al., focused on viscoelastic behavior and thermal stability using EPDM rubber with hybrid nanoparticles such as CNTs and nanoclay.⁷ Unlike these studies, in our study, a novel thermoplastic resin was considered as a stand-alone additive, and a mechanistic framework was established not only with standard indices, but also with time-temperature superposition master curves, Christensen-Anderson and Carreau-Yasuda rheological modeling, and Arrhenius-type activation energy analyses.

The studies, researches and discussions mentioned above show that there is always a need for innovative additives in the asphalt industry and that researchers will always be interested in this issue. In this study, the usability of thermoplastic resin derived from petroleum feedstock in asphalt modification was investigated. Resins as additives can provide desirable pavement properties and regulate the viscoelastic-rheological properties of asphalt. It can provide high temperature resistance, structural stability by forming a stable asphalt matrix, and improve adhesion properties due to its nature as a resin. The additive is thought to exhibit a high compatibility modification mechanism behavior due to its similar chemical structure with asphalt. In this study, thermoplastic resin additive was added at 1%, 3%, 5% and 7% by weight of asphalt to obtain modified binders. Conventional binder tests, Superpave binder tests, RTFOT and PAV tests were performed on pure and modified binders. The test results were subjected to detailed rheological analysis.

The motivation of this study is to investigate the potential of a thermoplastic resin, sourced from a commercial supplier, as an alternative bitumen modifier. While polymeric additives such as SBS, EVA, and crumb rubber have been extensively studied, the independent use of thermoplastic resins has been scarcely addressed. These resins are attractive because they can be incorporated with conventional mixing processes, are cost-effective, and may improve compatibility with bitumen fractions. At the same time, industry adoption requires a rigorous evaluation framework that goes beyond routine indices. Therefore, this study aims to establish the performance contribution of a thermoplastic resin modifier, identify the optimum dosage window balancing workability and rutting resistance, and provide a mechanistic understanding through master curves, rheological modeling, and activation energy analysis.

Significance of the study: Beyond routine indices, we map an optimum processing-performance envelope for an aromatic hydrocarbon resin modifier by integrating PG-based thresholds with full-domain master curves and Arrhenius-type activation energies. This framework provides implementable dosage guidance for hot-mix plants targeting rutting resistance in warm climates, complementing prior reports on resins’ compatibilization and high-temperature benefits.

Materials and methods

Asphalt binder

In this study, an asphalt binder of the 70/100 penetration grade, sourced from TÜPRAŞ Batman Refinery in Turkey, was utilized. This penetration grade was chosen due to its prevalent use in the region, as it is commonly employed by highway authorities, municipalities, and road engineering firms. The 70/100 penetration grade asphalt is recognized as a suitable binder for the region’s climatic conditions. The asphalt binder’s specific gravity was measured as 1.038 g/cm³.

Thermoplastic resin additive

In this study, a thermoplastic resin additive containing aromatic hydrocarbons, structurally similar to asphalt binder, was employed. The additive was procured from Kempro Chemicals and Foreign Trade Limited Company. The bitumen modifier was a commercial C9 aromatic hydrocarbon (petroleum) resin (KROL-C series, Kempro/Kempropol, Tuzla-İstanbul, Türkiye). C9 resins are thermoplastic, low-molecular-weight polymers produced by catalytic polymerization of aromatic C9 monomers (commonly indene, vinyltoluenes, and styrenic species) and are used as tackifying/compatibilizing agents in coatings and adhesives. It was derived from the polymerization of aromatic petroleum feedstock and exhibited a clear, yellowish appearance. Soluble in both aromatic and aliphatic solvents, the additive is characterized by nearly zero acid and saponification numbers, a low drying index, excellent chemical resistance, and water repellency. Resins are generally non-hazardous in solid form; the primary safety consideration is avoidance of overheating, which can release fumes and cause burns when molten.

Class-level composition and handling of aromatic resins are described in the literature. These resins originate from aromatic feeds and commonly contain indene and vinyltoluenes, among other aromatics; they act chiefly as thermoplastic tackifiers/compatibilizers in viscoelastic matrices. Their benefit for asphalt systems has been evidenced in recent studies reporting enhanced high-temperature rheology and improved dispersion/compatibility.^37–39

Prior to incorporation into the asphalt binder, the additive was ground into a powdered form (Figure 1). The detailed properties of the additive are summarized in Table 1.

Figure 1.

Image of the additive used in the study.

Table 1.

Properties of the thermoplastic resin additive used in the study.

Property	Value	Specification
Softening point, °C	115 °C–125 °C	ASTM E 28
Color gardner	7	ASTM D 1544
Acid value, Mg KOH/H	Max. 0.1	ASTM D 974
Specific gravity, g/cm3	1.09-1.13	ASTM D 71
Iodine value, g/100g	Max 30	ASTM D 1959
Ash content, %	Max 0.05 %	ASTM D 1063

Preparation of modified asphalt binders

The procedure for preparing modified asphalt is outlined as follows, with Figure 2 illustrating a mechanical mixer equipped with adjustable temperature control and designed to meet the experimental conditions required for performance testing of the modified asphalts. Initially, the unmodified asphalt binder was melted in an oven at 150 ± 5°C for a predetermined duration. Once the binder reached a workable consistency, 500 g were transferred into a metal mixing container that had been preheated to 150°C. This temperature was maintained consistently throughout the modification process using a temperature-controlled system. Additive was incorporated into the binder at proportions of 1%, 3%, 5%, and 7% by weight of asphalt. The modified asphalts were prepared using a mechanical mixer operating at 1000 r/min for 60 minutes. To ensure that the aging effects induced during the preparation of modified asphalt did not influence the results, the pure binder was subjected to the same mixing process.

Figure 2.

Mechanical mixer and modified asphalt samples in experimental environment.

Conventional binders tests

The penetration and softening point tests are essential methods for assessing the hardness and thermal properties of asphalt binders. The penetration test, conducted in accordance with TS EN 1426,⁴⁰ measures the depth a needle penetrates into the binder under a standard load of 100 g at a specified temperature, typically 25°C. This test provides a reliable indication of the binder’s relative hardness or softness. On the other hand, the softening point test, performed following TS EN 1427,⁴¹ determines the temperature at which the asphalt binder transitions to a softened state under heat. This parameter reflects the binder’s behavior under high-temperature conditions. Together, these tests play a critical role in evaluating the thermal performance of asphalt binders and optimizing the performance of flexible pavements.

Rolling Thin Film Oven Test (RTFOT)

The RTFOT test is a laboratory test method that simulates the short-term aging of asphalt and is performed in accordance with the EN 12,607-1 standard. Short-term aging occurs during the time between the asphalt mix’s production phase and its spreading and compaction, accounting for the majority of the aging process. The RTFOT test was performed on both pure and modified asphalt binders under standard conditions of 163°C for 85 minutes. This test evaluated the effects of the thermoplastic additive used on the asphalt’s aging properties.

Rotational Viscometer (RV) test

Viscosity refers to the resistance of asphalt binder to deformation under shear and tensile stresses.⁴² Due to its thermoplastic nature, asphalt binder exhibits varying mechanical responses to temperature fluctuations. This characteristic is particularly critical given that asphalt mixtures experience diverse temperature conditions during mixing and compaction. Understanding their response to these variations is essential for achieving optimal mixing and compaction processes.⁴³

The RV test was performed using the Brookfield DV-III apparatus in compliance with ASTM D4402. Measurements were conducted at a constant rotational speed of 20 RPM at temperatures of 135°C and 165°C. The operating mechanism of the apparatus is depicted in Figure 3. In brief, the torque required to sustain a constant speed of 20 RPM is recorded and converted into dynamic viscosity values.

Figure 3.

Dynamic viscosity measurement mechanism.

Dynamic oscillatory shear testing via DSR

The Dynamic Oscillatory Shear Test, conducted in accordance with AASHTO T312, is used to evaluate the rheological properties (flow and deformation behavior) of asphalt binders at moderate to high temperatures. The primary parameters of this test are the complex shear modulus (G*) and the phase angle (δ). The complex shear modulus (G*) reflects the total stiffness of the binder, representing its resistance to deformation under applied loads. A higher G* value indicates greater stiffness and enhanced resistance to deformation. G* is composed of two components: the elastic modulus (G′) and the viscous modulus (G'').⁴⁴ The phase angle (δ) measures the lag between the applied stress and the resulting deformation. A phase angle of 0° signifies purely elastic behavior, where the material fully recovers its original shape after the load is removed.⁴⁵ Conversely, a phase angle of 90° indicates purely viscous behavior, resulting in permanent deformation. The viscoelastic properties of asphalt binders are defined within the intermediate range of these values. Additionally, the rutting resistance parameter, G*/sinδ, is derived from these measurements.

The DSR test was performed for each of the unaged and aged asphalt binders. A 25 mm diameter plate was used in the test, and the gap between the plates was set at 1 mm in accordance with SHRP. Test temperatures were set at 52°C, 58°C, 64°C, 70°C, and 76°C, with a constant frequency of 10 rad/s. In addition to the DSR-PG test, a frequency sweep test was applied to investigate viscoelastic behavior under different temperatures and frequencies. This test is performed by applying oscillatory shear under constant stress. Literature indicates that this test can simulate vehicle speed, and that a 10 Hz frequency corresponds to a vehicle speed of approximately 60-65 km/h.⁴⁶ Frequency sweep tests were performed at a frequency of 0.01-10 Hz at temperatures between 40°C and 70°C, with 10°C increments.

The viscoelastic parameter (G*) obtained from the frequency sweep test was converted to a master curve format, adhering to the Time-Temperature Superposition Principle (TTSP). Master curves allow for the evaluation of the viscoelastic properties of asphalt binders at a wide frequency range while also revealing the role of modifiers. Master curves were analyzed using the Christensen-Anderson (CA) model, an empirical analytical approach developed in 1992 as part of the Strategic Highway Research Program (SHRP). The CA model is given in equation (1). Numerous studies in the literature have reported that the CA model successfully represents the viscoelastic properties of pure and modified asphalt binders.^47–49 A typical CA model curve and the meanings of the parameters are shown in Figure 4. Glassy modulus (Gg), crossover frequency (ωc), and rheological index (R) are model parameters used to characterize the viscoelastic behavior of asphalt binders. ωc represents the frequency at which viscous and elastic moduli are equal, and a decrease in this value indicates that the material is approaching the elastic region. R is considered a shape factor for the master curve.

| G^{*} | = G_{g} {[1 + {(\frac{⍵_{c}}{⍵})}^{\frac{\log 2}{R}}]}^{\frac{- R}{\log 2}}

(1)In addition to the complex modulus (G*) value, complex viscosity values can also be obtained in the frequency sweep test. This value follows the basic definition of viscosity and varies as a function of angular frequency (ω) (equation (2)).

η^{*} = G^{*} / ω

(2)

Figure 4.

Definition of Christensen-Anderson model.

Master curves for complex viscosity were constructed similarly to those for the complex modulus. Using the shift factors derived from these master curves, the flow activation energies of both pure and modified binders were calculated based on the Arrhenius model. This analysis provides a quantitative measure of the energy required for the binders to flow under varying temperatures, offering insights into their thermal sensitivity and performance characteristics. The overall experimental workflow of the study is schematically presented in Figure 5. All tests were repeated at least three times.

Figure 5.

Experimental flowchart of the study.

Results

Thermogravimetric analysis (TGA) results

The thermogravimetric analysis (TGA) results of the thermoplastic resin additive are presented in Figure 6. As shown, a significant mass loss begins at approximately 192°C. However, since asphalt modification typically occurs at 150°C—well below this threshold—no chemical deterioration of the additive is anticipated. The steep slope in the graph, accompanied by a marked decrease in resin weight, signifies the onset of thermal instability, which is generally attributed to the decomposition of volatile compounds or the thermal degradation of the main structure. This phenomenon is observed in the temperature range of approximately 200 °C–500 °C in Figure 6. Since these temperatures are not encountered during the asphalt modification process, the resin remains thermally stable throughout. At temperatures above 500°C, the weight loss ceases, reaching a stable plateau. This point corresponds to the carbon residue, or char yield, left after the additive undergoes complete decomposition.

Figure 6.

TGA result of additive.

Overall, the TGA analysis indicates that the thermoplastic resin additive possesses excellent thermal stability for asphalt modification. With 150°C being significantly below the decomposition temperature, the additive can integrate into the asphalt matrix without undergoing degradation. This highlights its potential to enhance the performance of modified asphalt. Furthermore, the additive’s thermal durability at elevated temperatures can contribute to the long-term stability and durability of asphalt pavements.

Test results of unaged pure and modified asphalt binders

Conventional test results for asphalt binder are presented in Figure 7. The influence of systematically added aromatic hydrocarbon additive at varying rates on the consistency and high-temperature resistance of the asphalt binder was assessed through penetration and softening point tests.

Figure 7.

Conventional binder test results of pure and modified binders.

The penetration test is an indicator of the stiffness of asphalt binders, where stiffness is defined as the ratio of stress to strain. In this study, penetration tests were conducted in accordance with the EN 1426 standard. As illustrated in Figure 7, the penetration values of the pure binder steadily decreased with the incorporation of additives at rates of 1%, 3%, and 5%. The lowest penetration value was observed in the binder containing 5% additive. This reduction in penetration values suggests that the inclusion of additives enhances the hardness of the asphalt and improves its resistance to deformation. Based on the chemical structure of the additive, the increase in hardness is attributed to the proportional reduction in light fractions, as the additive is composed of relatively heavy molecules. Beyond the 5% additive rate, however, the downward trend in penetration values reversed, with a slight increase observed at a 7% additive ratio. This suggests that at this higher concentration, the additive failed to achieve the desired hardening effect and negatively influenced penetration performance. Nonetheless, the penetration values for the 7% additive binder remained lower than those of the pure binder. Specifically, penetration values decreased by 9.97%, 12.50%, 19.66%, and 9.27% for additive rates of 1%, 3%, 5%, and 7%, respectively, compared to the pure binder. Notably, increasing the additive concentration from 5% to 7% led to an approximately 10% increase in penetration values.

Softening point values for pure and modified binders, measured according to the EN 1427 standard, are also presented in Figure 7. The softening point represents the temperature at which asphalt binders begin to lose their rigidity and serves as a critical parameter for evaluating high-temperature performance. As shown in Figure 8, the softening point of the pure binder, initially measured at 53.1°C, increased progressively with higher additive concentrations up to a certain limit. The softening point values for binders with 1%, 3%, and 5% additives increased by 9.4%, 11.11%, and 15.25%, respectively, compared to the pure binder. These higher softening point values indicate improved resistance to high temperatures, suggesting that pavements constructed with such binders would exhibit greater resistance to permanent deformation. However, when the additive concentration reached 7%, the upward trend in softening points reversed, likely due to homogeneity issues. Beyond a certain additive threshold, the inability to form consistent physical bonds between the additive and the asphalt, potentially caused by inadequate mixing, was reflected in the experimental outcomes.

Figure 8.

Rotational viscometer (RV) test results of asphalt binder samples.

The results of the rotational viscosity (RV) test for all binder samples are presented in Figure 8. The viscosity values were determined according to the ASTM-D4402 standard, where viscosity is defined as the ratio of applied shear stress to shear strain rate.⁵⁰ Across all additive ratios and test temperatures, the modified binders exhibited higher viscosity values compared to the pure binder. Importantly, all binder samples satisfied the Superpave specification limit of 3000 cP at 135°C, indicating no limitations regarding workability or pumpability. As depicted in Figure 8, viscosity values decreased with increasing temperature, a phenomenon attributed to enhanced molecular mobility at elevated temperatures, which reduces the binder’s resistance to flow. Conversely, the use of additives led to an increase in viscosity at the same temperature. This increase can be attributed to the higher molecular weight introduced by the additives, as well as their structural compatibility with asphalt. Elevated viscosity values suggest that the use of additives can improve the resistance of asphalt pavements to permanent deformation. Specifically, at 135°C, viscosity values increased by 37.78%, 40%, 46.67%, and 31.11% for additive ratios of 1%, 3%, 5%, and 7%, respectively, compared to the pure binder. At 165°C, these increases were 23.8%, 30.77%, 61.54%, and 23.08%, with the binder containing 5% additive showing the most pronounced viscosity increase (61.54%) at the highest test temperature.

The results of the dynamic shear rheometer (DSR) temperature sweep test for pure and modified binders, conducted at 52°C, 58°C, 64°C, 70°C, and 76°C, are shown in Figure 9 and summarized in Table 2. The parameter G∗/sinδ, derived from the DSR test, represents the binder’s resistance to permanent deformation, and by extension, the pavement’s rutting resistance, at a given temperature. This parameter combines the complex shear modulus (G*) and phase angle (δ), where higher G* values and lower δ values indicate enhanced elastic properties and improved rutting resistance. Figure 9 reveals that G∗/sinδ values decreased with increasing test temperatures, reflecting the reduced stiffness of binders at elevated temperatures. Among the samples, the binder containing 5% additive consistently exhibited the highest G∗/sinδ values across all temperature ranges, while the pure binder displayed the lowest values. The improved G∗/sinδ values observed at all additive ratios and temperature levels highlight the effectiveness of the modification process in enhancing the binders’ resistance to rutting.

Figure 9.

G*/sinδ values obtained from the temperature scan of pure and modified binders.

Table 2.

G*/sinδ results (Pa).

G*/sinδ (Pa)	52	58	64	70	76
Pure	10,284	4684	2619	1079	-
1%	13,654	6123	3269	1363	-
3%	14,236	6881	3364	1408	-
5%	19,564	9165	4667	2193	1005
7%	12,983	6007	2930	1208	-

According to the Superpave binder performance specifications, the G*/sinδ value must exceed 1000 Pa to ensure adequate resistance to rutting. If the binder’s G*/sinδ value falls below this threshold at a given temperature, it indicates a higher susceptibility to rutting, and the highest temperature at which this criterion is satisfied defines the binder’s performance grade. At 70°C, the lowest G∗/sinδ value was recorded for the pure binder, yet all binder samples, including the pure binder, met the 1000 Pa specification limit. However, at 76°C, only the binder modified with 5% additive met this requirement. These results suggest that the performance grade of the 70/100 penetration-grade bitumen was enhanced from 70°C to 76°C through the addition of 5% additive. Overall, it was concluded that the additive significantly improved the asphalt binder’s resistance to permanent deformation. Specifically, the addition of 5% additive is expected to produce asphalt pavements with superior resistance to plastic deformation.

The phase angle (δ) values of the binders are presented in Figure 10. The phase angle is a critical parameter for characterizing the behavior of viscoelastic materials, as it represents the time lag between the application of stress and the resulting strain. A shorter time lag, indicated by a lower phase angle, reflects a more elastic material. Asphalt binders exhibiting behavior closer to the elastic region are generally more resistant to permanent deformation. As shown in Figure 10, phase angle values increase with rising temperature, a trend attributed to the thermoplastic nature of the material, which is accompanied by a decrease in viscosity. However, the addition of additives mitigates this effect. At all temperatures, the pure binder exhibits the highest phase angle values, while the modified binders demonstrate lower phase angles, indicating enhanced elasticity. Among the modified binders, the lowest phase angle values are observed in the sample containing 5% additive. Up to this additive level, the phase angle decreases consistently, but with the addition of 7% additive, an increase in phase angle is noted. These findings suggest that the addition of 7% additive disrupts the regular improvement in the elastic properties of the binder. Consequently, the optimal additive content was determined to be 5%, beyond which the elastic performance begins to diminish.

Figure 10.

Phase angle values obtained as a result of temperature sweep test.

Test results of RTFOT-aged asphalt binders

The penetration test results following the RTFOT aging process are depicted in Figure 11. The pure binder exhibited the highest penetration value, while all modified binders showed reduced penetration values compared to the pure binder, demonstrating the impact of additive incorporation. Aging due to RTFOT led to changes in the asphalt’s functional groups and an increase in its hardness, primarily attributed to the loss of volatile components. The retained penetration value, calculated as the ratio of the penetration value of unaged samples to that of aged samples, serves as an indicator of the asphalt’s ability to retain its penetration capacity post-aging. Higher retained penetration values signify greater resistance to aging, indicating that the asphalt retains its flexibility and crack resistance more effectively. Conversely, lower retained penetration values suggest increased hardening and loss of flexibility after aging, which could lead to brittleness and a heightened risk of cracking. Based on the findings, all modified binders exhibited higher retained penetration values than the pure binder. Among the samples, the pure binder demonstrated the lowest retained penetration value, whereas the binder containing 7% additive achieved the highest, highlighting the positive influence of the additive on aging resistance.

Figure 11.

Penetration test results of RTFOT-aged binders.

The results of the softening point test conducted on RTFOT-aged asphalt binders are illustrated in Figure 12. The data reveal that the softening point values increased for all binder samples following short-term aging, a trend associated with the hardening effect of aging on asphalt. Among the binders, the sample containing 5% additive exhibited the highest softening point. A consistent increase in the softening point was observed with the additive content up to this percentage, after which a reversal occurred with the addition of 7% additive. As previously discussed, the incorporation of 7% additive may negatively influence binder compatibility. The softening point values after RTFOT showed increases of 10.02%, 12.73%, 16.30%, and 8.83% for the 1%, 3%, 5%, and 7% additive binders, respectively, compared to the pure binder.

Figure 12.

Softening point test results of RTFOT-aged binders.

The results of the DSR tests conducted on RTFOT-aged binders are presented in Figure 13. The temperature sweep was performed in 6°C increments, and the figure illustrates the variation in the rutting parameter, calculated using the G* and δ values, with respect to temperature and additive ratios.

Figure 13.

Rutting parameters of RTFOT-aged pure and modified binders.

Higher rutting resistance values were observed in the RTFOT-aged specimens compared to their unaged counterparts. This increase is attributed to the proportional increase in higher molecular weight components within the asphalt due to oxidation, which intensifies during short-term aging. The DSR tests were conducted at six different temperatures, with the pure binder consistently exhibiting the lowest G*/sinδ values across all temperatures. According to the Superpave performance specifications, RTFOT-aged specimens must meet a minimum G*/sinδ value of 2200 Pa. Analysis of Figure 12 shows that up to 70°C, all binders met this criterion. However, at 76°C, the pure binder and those with 1% and 7% additives failed to meet the specification, whereas the 3% and 5% binders satisfied the requirement. The binder with 5% additive exhibited the highest G*/sinδ values across all temperatures, indicating superior rutting resistance. Table 3 summarizes the G*/sinδ values (Pa) for the RTFOT-aged specimens.

Table 3.

G*/sinδ values (Pa) of RTFOT-aged binders.

G*/sinδ (Pa)	Pure	1%	3%	5%	7%
52°C	28,178	31,060	34,780	40,999	29,820
58°C	14,333	15,585	18,536	24,610	14,973
64°C	7136	7492	9064	12,880	7192
70°C	3615	3896	4399	6727	3740
76°C	1710	1925	2234	3190	1848
82°C	-	1030	1125	1386	-

Figure 14 displays the phase angle values of the RTFOT-aged binder samples. With increasing temperature, the binders shifted toward the viscous region, resulting in higher phase angle values. Additives were incorporated to mitigate and optimize this transition. As shown in Figure 14, the pure binder consistently exhibited the highest phase angle values at all temperatures except 82°C. The binder with 5% additive demonstrated the lowest phase angle values, suggesting enhanced elastic behavior. Overall, Figure 14 indicates that the binders with 3% and 5% additives exhibited distinct rheological behavior, with significantly improved elastic properties at these additive levels. Consequently, coatings prepared using these binders are expected to exhibit superior resistance to permanent deformation at elevated temperatures.

Figure 14.

Phase angle values of pure and modified binders after RTFOT.

Frequency sweep test results of pure and modified asphalt binders

In this study, frequency sweep tests were conducted at 10 different frequencies (0.01–10 Hz) and four temperatures (40°C, 50°C, 60°C, and 70°C) to examine the effects of varying loading rates and temperatures on the rheological behavior of modified asphalt binders. The results are presented in Figure 15.

Figure 15.

Complex shear modulus (G*) values of binders at different temperature and frequencies.

A detailed examination of Figure 15 reveals a pronounced decrease in complex shear modulus (G*) values with increasing temperature. At each temperature, the 5% binder exhibited the highest G* values, whereas the pure binder displayed the lowest. The G* values also increased with increasing frequency, reflecting the material’s rheological characteristics. At low frequencies (corresponding to long loading times), the asphalt exhibited reduced resistance to shear effects, while at high frequencies, the resistance increased. The additive’s role is to enhance the material’s shear resistance under high-temperature and low-frequency conditions. As shown in Figure 15, the 5% binder consistently exhibited the highest G* values across all frequencies. Using the data from Figure 16, Time-Temperature Superposition Principle (TTSP) master curves, which consolidate data from all frequencies and temperatures into a single representative curve, were constructed and are presented in Figure 16.

Figure 16.

TTSP master curves of pure and modified binders at 40°C reference temperature.

To construct the complex shear modulus master curves, a reference temperature of 40°C was selected, and the isothermal curves for the other binder samples were shifted to align with this reference. The curves were plotted on a logarithmic scale, and the behavior of binders with varying additive proportions is illustrated in Figure 16 using a magnified view. Figure 16 demonstrates that the master curves are smooth and continuous, as expected, indicating the successful application of TTSP and confirming that the binders exhibit “thermo-rheologically simple” behavior. While phase angle master curves could provide additional detail, the study scope was limited to temperature-sweep results combined with modulus master curves and rheological modeling, which sufficiently captured the binder’s viscoelastic response.

To reveal the behavioral differences in the master curves, the curves were analyzed with the CA Model using equation (1). CA Model results for pure and modified binders are given in Table 4. In nonlinear regression analyses, the coefficient of determination (R²) was calculated according to the standard definition:

R^{2} = 1 - \frac{\sum {(y_{i} - ŷ_{i})}^{2}}{\sum {(y_{i} - y ˉ)}^{2}}

(3)where

y_{i}

are the observed values,

ŷ_{i}

are the predicted values from the nonlinear model, and yˉ is the mean of the observed values.

Table 4.

CA Model parameters of pure and modified binders.

Binder type	CA model parameters
Binder type	ωc (Hz)	R	Coefficient of determination (R²)
Pure	1358	2.43	0.998
1%	1062	2.52	0.989
3%	1003	2.56	0.997
5%	780	2.87	0.997
7%	1271	2.45	0.988

Table 4 demonstrates the successful application of the Christensen-Anderson (CA) model to the experimental master curves of the binders, with R² values exceeding 0.99. Previous research has consistently reported a glassy modulus (Gg) of approximately 1 GPa for asphalt binders under shear conditions, and it has been recommended to fix Gg at 10⁹ Pa for such analyses. In this study, the Gg value was set at 10⁹ Pa, while the other variables (ωc and R) were left free. The pure binder exhibited the lowest R value and the highest ωc value. After modification with additives, a significant reduction in ωc values was observed, indicating enhanced elastic properties of the asphalt binder at low frequencies and high temperatures due to the modification.

The rheological index, R, reflects the shape of the master curve and provides insight into the width of the relaxation spectrum, making it a valuable parameter for assessing changes in asphalt stiffness with respect to loading time or frequency.⁵¹ Small changes in binder stiffness due to aging or chemical modifications can significantly affect R values.⁵² An increase in R indicates a broader relaxation spectrum, associated with increased stiffness and reduced viscous behavior. In this study, the pure binder showed the lowest R value, while the 5% binder exhibited the highest R value, suggesting that the latter had the best rheological performance. The 5% binder also showed the lowest ωc value, demonstrating improved non-Newtonian behavior under low-frequency and high-temperature conditions, coupled with increased stiffness.

Figure 17 illustrates the complex viscosity curves obtained from the frequency sweep tests, showing the relationship between frequency and complex viscosity across temperatures ranging from 40°C to 70°C.

Figure 17.

Complex viscosity values obtained at four different temperatures and 10 different frequencies.

At each temperature, the pure binder displayed the lowest viscosity, while the 5% binder consistently exhibited the highest. Although temperature increases led to a marked reduction in viscosity for all binders, this effect was mitigated by the inclusion of additives. Notably, the 5% binder was the least affected by rising temperatures. A comparison of the complex viscosity curves at 40°C and 70°C highlights the material’s non-Newtonian behavior, as evidenced by the substantial increase in viscosity at low frequencies at 40°C. However, this non-Newtonian behavior diminished at higher temperatures for all binders. Nonetheless, the modification effect remained evident, particularly in the ability of the binders to retain elasticity at elevated temperatures.

Figure 18 illustrates the master curves derived from the complex viscosity values, with the reference temperature set at 60°C. These curves depict the rheological behavior of pure and modified asphalt binders across a wide range of frequencies, from very low to very high. The master curves were constructed by applying shift factors based on temperature, fitted to the Arrhenius model. The Arrhenius equation is presented in equation (4).⁵³

η = A \times e^{\frac{E_{a}}{R T}}

(4)Here,

η

is the viscosity of the binder (Pa.s), A is the regression coefficient, E_a is the activation energy (kJ/mol), T is the temperature of the test, and R is the universal constant of gas (8.314 J mol⁻¹. K⁻¹).

Figure 18.

Complex viscosity master curves for all asphalt binder samples.

In order to obtain viscosity as a function of temperature, through E_a, equation (5) was developed by mathematically editing equation (4),

η = η (T_{r e f}) \times e^{[\frac{E_{a}}{R} \times \frac{- c_{1} (T - T_{r e f})}{c_{2} + (T - T_{r e f})}]}

(5)where T is the absolute temperature, T_ref is the reference temperature (absolute), η(Tref) is the viscosity at T_ref, and E_a is the activation energy, C₁ and C₂ are two material coefficients and are obtained by the Williams–Landel–Ferry (WLF) equation.

This approach allows for the determination of the activation energy (Ea), which characterizes the temperature dependency of the binders’ viscoelastic behavior. The Ea values for pure and modified binders are presented in Figure 19.

Figure 19.

Activation energy values.

A reference temperature of 60°C represents a relatively high pavement temperature, at which molecular mobility accelerates, allowing asphalt molecules to move more freely over one another. This increased mobility reduces viscosity and predisposes the material to permanent deformation. Activation energy, a critical parameter in this context, reflects the minimum energy required to initiate molecular movement. During the hot mix asphalt production process, the binder must be liquefied, and higher activation energy implies greater energy consumption, which can result in adverse economic and environmental impacts.⁴³ Thus, activation energy is an essential parameter in pavement engineering.

As shown in Figure 19, the highest activation energy value was observed in the 5% binder, while the lowest was in the 1% binder. An increase in activation energy indicates that more energy is required to overcome yield deformation.⁵⁴ However, a sharp and significant reduction in activation energy was noted when the additive rate increased from 5% to 7%. Solomon and Zhai have reported that exceeding a critical additive concentration can lower the activation energy required for flow.⁵⁵ The activation energy of the pure binder was higher than that of all modified binders except the 5% binder, indicating greater sensitivity to temperature changes. This aligns with the observation that binders with lower viscosity (as seen in Figure 8) but higher activation energy (as shown in Figure 19) exhibit heightened temperature sensitivity.⁵⁶ The results suggest that this temperature sensitivity is mitigated through modification, enhancing the binder’s performance.

The phase angle master curves constructed using the time–temperature superposition principle (TTSP) at a reference temperature of 40°C are presented in Figure 20. As the reduced frequency increased, the phase angle (δ) exhibited a gradual decrease from approximately 85° toward 60°, indicating a clear transition from predominantly viscous to more elastic behavior across the investigated frequency domain. This trend is consistent with the expected viscoelastic response of bituminous binders, where molecular mobility is restricted under higher frequency (or lower temperature) conditions. It is observed that with the increase of the additive ratio, the phase angle values are obtained similarly to other test results. In general, the phase angle trends reveal a balanced viscoelastic response across the tested frequency range, suggesting that the resin additive enhanced the high-temperature performance of the binder while maintaining its typical rheological profile.

Figure 20.

Phase angle master curves for all asphalt binder samples.

Discussion

This study investigated the use of thermoplastic resin derived from the polymerization of unsaturated aromatic petroleum feedstock as an asphalt additive. The additive was incorporated into asphalt binders at proportions of 1%, 3%, 5%, and 7% by weight, and physical and rheological tests were conducted on the resulting modified binders. The findings of the study are summarized as follows.

• Results from conventional binder tests revealed that the aromatic hydrocarbon-based additive reduced the penetration values of pure asphalt across all additive levels. A consistent reduction was observed up to the 5% additive level, after which penetration values increased. The binder with 5% additive exhibited the highest softening point, while the pure binder had the lowest. The interaction between the additive—composed of relatively heavy molecules—and the asphalt binder resulted in an increased proportion of high molecular weight fractions, enhancing characteristics such as stiffness, viscosity, and high-temperature resistance. These findings suggest that the additive improves deformation resistance at elevated temperatures.

• Rotational viscometer tests showed that pure binder had the lowest viscosity values at both 135°C and 165°C, while the binder with 5% additive displayed the highest viscosity values. The results indicate that modifications using the aromatic hydrocarbon-based additive can be achieved without violating specification limits related to workability and pumpability.

• Rheological evaluation through the DSR test demonstrated that the additive significantly improved the rutting resistance of asphalt at high temperatures. Moreover, the performance grade of the binder with 5% additive increased from 70°C to 76°C. Phase angle results indicated that the additive shifted the binder’s behavior toward the elastic domain.

• Analysis of RTFOT-aged specimens revealed a significant decrease in penetration values after aging. However, retained penetration values improved with the addition of the modifier, suggesting enhanced aging resistance. This improvement indicates that the bonds formed between the additive and asphalt binder are more resistant to oxidative degradation.

• DSR tests on RTFOT-aged specimens showed that the specification limit of 2200 Pa was met at 76°C only for binders containing 3% and 5% additive. Across all temperatures, modified RTFOT-aged binders exhibited superior rutting resistance compared to pure binders.

• Frequency sweep tests demonstrated that the additive provided notable enhancements under varying loading rates and temperatures. The CA Model analysis applied to master curves suggested that the additive enabled broad relaxation behavior, allowing the binder to maintain elasticity even at high temperatures.

• Evaluation of activation energy revealed that the binder with 5% additive had the highest activation energy, while the binder with 1% additive had the lowest. Although the pure binder exhibited the lowest viscosity, it did not have the lowest activation energy. These results suggest that binders with low viscosity but high activation energy exhibit greater sensitivity to temperature variations.

The insights gained from this research provide a foundation for further exploration of innovative additives, fostering sustainable infrastructure development and reducing long-term maintenance costs. This study serves as a significant step toward more resilient and efficient asphalt technologies. A limitation of this study is that storage stability testing was not performed, since the main objective was to establish a mechanistic understanding of the thermoplastic resin’s effect on rheological behavior and high-temperature performance; this property will be addressed in future work to complement the present findings. Another limitation is that MSCR testing was not conducted, and although higher dosages were not explored beyond 7%, since performance already declined at this level, future work will incorporate MSCR and a finer dosage screening to complement the present finding.

Future studies will complement the present binder-level rheological findings with additional evaluations. These include storage stability testing under hot-storage conditions, long-term aging (PAV) and durability analysis, as well as low-temperature and fatigue performance assessments. Binder-level insights will be extended to asphalt mixture validation through rutting, fatigue, and moisture resistance tests, ensuring practical field applicability. Comparative studies with conventional modifiers and potential hybrid modification strategies are also planned, with the goal of optimizing both performance and cost-effectiveness.

Footnotes

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The authors received no financial support for the research, authorship, and/or publication of this article.

ORCID iD

Ahmet Münir Özdemir

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Optimizing high-temperature performance of asphalt binder using aromatic thermoplastic resin derived from petroleum feedstock

Abstract

Keywords

Introduction

Materials and methods

Asphalt binder

Thermoplastic resin additive

Preparation of modified asphalt binders

Conventional binders tests

Rolling Thin Film Oven Test (RTFOT)

Rotational Viscometer (RV) test

Dynamic oscillatory shear testing via DSR

Results

Thermogravimetric analysis (TGA) results

Test results of unaged pure and modified asphalt binders

Test results of RTFOT-aged asphalt binders

Frequency sweep test results of pure and modified asphalt binders

Discussion

Footnotes

Declaration of conflicting interests

Funding

ORCID iD

References