Viscoelastic and Fatigue Performance of Modified Bitumen Using Polymer and Bio-Based Additives: A Comparative Study

Abstract

This study investigates the performance and viscoelastic characteristics of unmodified and modified bitumen using Performance Grading, Frequency Sweep, and Linear Amplitude Sweep tests. The bitumen modifications include styrene-butadiene–styrene at 4% and 5%, animal bone powder at concentrations of 4%, 5%, and 6%, and waste cooking oil at 3%, 4%, and 5%. Performance Grading tests were conducted to evaluate the high-temperature performance of bitumen samples. Frequency Sweep tests were used to analyze the complex shear modulus and phase angle, providing insights into stiffness and elasticity. The Linear Amplitude Sweep tests assessed fatigue resistance by monitoring the degradation of the complex shear modulus under cyclic loading. Styrene-butadiene–styrene and animal bone powder significantly enhanced stiffness, elasticity, and fatigue resistance, with styrene-butadiene–styrene-modified samples achieving the highest performance grades and fatigue resistance. Waste cooking oil-modified bitumen reduces stiffness and fatigue resistance, indicating it primarily acts as a plasticizer. Styrene-butadiene–styrene and animal bone powder are effective modifiers for improving bitumen’s mechanical and fatigue properties and are suitable for demanding applications. In contrast, waste cooking oil compromises structural performance despite its environmental benefits, making it less suitable for high-performance use.

1. Introduction

1.1. Background

The global demand for sustainable infrastructure development has brought attention to innovative materials that can enhance the performance and durability of construction components [1,2,3]. Bitumen is primarily valued because of its critical importance as a binder in asphalt mixtures concerning the structural endurance of road networks [4,5]. Indeed, maintaining durable and sustainable roadways now poses more serious challenges amid expanding transportation systems with high-pressure usage demands due to growing populations. Bitumen modification has emerged as a key area of research to address these challenges by improving its resistance to deformation, fatigue, and environmental aging [6].

Although widely used, traditional bitumen mostly fails to meet modem transportation network requirements that pass through harsh environmental conditions with high traffic loads [7]. The limitation was overcome by adding various additives to modify the properties of bitumen. One such established solution is the category of polymer modifiers, in which styrene-butadiene–styrene (SBS) has been one of the better-known modifiers for its enhancement in elasticity and high-temperature performance [8]. However, dependence on synthetic polymers has brought in cost and environmental sustainability issues, hence the interest in possible alternatives [9].

Organic waste materials, such as animal bone (AB) and waste cooking oil (WCO), are two promising renewable resources that have emerged recently to modify bitumen [10]. Besides economic solutions, these materials contribute to global waste management by reusing organic by-products [11]. For example, AB is rich in calcium and may thus improve the mechanical properties of bitumen, while WCO has been found to provide improved fatigue performance [12,13]. The integration of such materials is in line with the global push to reduce the environmental footprint of construction materials.

The rheological properties of bitumen are among the most important parameters for defining the material’s performance under different loading conditions [14]. These characteristics, describing the flow and deformation behavior of the material, are directly related to the material’s resistance to high temperatures, elasticity, and fatigue [15]. In this respect, SBS-type modifiers tend to be effective in improving such characteristics, which enable asphalt mixtures to perform well under high traffic and temperature conditions [16]. Correspondingly, AB and WCO introduce added benefits, with cost savings and a shift toward sustainability, though the combined interaction of these two will have to be further researched.

Despite significant developments, researchers still face challenges balancing performance and sustainability for bitumen modification [17]. For instance, although WCO improves fatigue resistance, it may compromise on high-temperature performance such as rutting resistance (G*/sin δ) [18]. Similarly, AB can enhance the mechanical strength of the samples; however, a detailed investigation into its effects on the viscoelastic behavior in dynamic conditions is needed. Understanding such trade-offs is the foundation for optimizing performances in modified bitumen.

The construction industry has started prioritizing sustainable practices; hence, developing high-performance asphalt mixtures with a balance between durability and environmental responsibility is one of the key areas of critical focus [19]. With the combined potential of SBS, AB, and WCO, researchers are looking toward developing bitumen formulations that can meet the demands of modern infrastructure while being aligned with sustainability goals [20]. Therefore, this background lays the platform for studying the rheological and fatigue performance of bitumen modified with these innovative materials.

1.2. Problem Statement

The transportation networks are supposed to carry increasingly heavy traffic loads and a wide range of environmental conditions as they expand [21]. Traditional bitumen, while serviceable, usually lacks resilience in the face of such evolving challenges and deteriorates prematurely, which is costly [22]. Although modifiers like SBS have proved their worth in improving high-temperature and elastic performance, reliance on synthetic polymers raises concerns regarding cost and environmental sustainability [23]. Conversely, organic waste materials like AB and WCO present a promising solution by leveraging waste recycling to produce cost-effective and eco-friendly modifiers [24].

However, all these organic modifiers have their corresponding drawbacks: for instance, the fatigue performance of WCO can be partly frustrated by reduced G*/sin δ at high temperatures, whereas AB provides both mechanical and sustainability improvements; however, the knowledge about its overall rheological effects is scarce under a variety of conditions. Matters become even more complex when combinations of various modifiers are considered because the modifier–modifier interaction may result in synergistic benefits and unforeseen trade-offs. Addressing these issues requires a deeper understanding of these modifiers’ individual and combined effects on bitumen’s rheological and fatigue performance.

1.3. Objective

The focus of this research paper will be the evaluation and comparison studies on the rheological properties and fatigue performance with SBS, AB, and WCO. By employing advanced testing methods such as Performance Grading (PG), Frequency Sweep, and Linear Amplitude Sweep (LAS) tests, this research aims to do the following:

• Assess the effectiveness of SBS, AB, and WCO in enhancing bitumen’s performance under various loading conditions.
• Identify the trade-offs between different modifiers regarding high-temperature resistance, elasticity, and fatigue life.
• Explore the combined effects of these modifiers to provide insights into their suitability for sustainable and high-performance asphalt mixtures.

2. Literature Review

Modified bitumen application is important in further development to raise the performance and sustainability level of asphalt mixes in infrastructure projects of current importance [25]. Hundreds of investigations have explored modifiers to improve rheological and mechanical properties, varying from synthetic polymers and waste material to bio-based additives [26]. The presented section includes a review of the current state of selected research issues concerning the viscoelastic and fatigue performance of modified bitumen; attention was focused on the latest findings and trends.

One of the significant studies on low-temperature and elastic properties of SBS-modified bitumen proved that SBS enhances fatigue resistance better than crumb rubber-modified bitumen. The results indicated that resistance to crack initiation and propagation was higher, which is important for extending asphalt pavement life [27].

Research has also explored incorporating WCO as a modifier. The chemical modification of WCO improves the viscoelastic performance of asphalt binders. WCO enhances resistance to fatigue cracking, but slightly compromises G*/sin δ [28]. Additional studies have emphasized the importance of evaluating rheological properties parallel to fatigue analysis for long-term performance [29,30]. WCO has also been identified as an eco-friendly rejuvenator for reclaimed asphalt pavement, effectively restoring aged bitumen properties [31]. Reviews on WCO-modified asphalt mixtures highlighted the trade-offs between enhanced fatigue resistance and compromised high-temperature properties [32]. Structural modification of WCO has yielded environmental benefits and performance improvements in asphalt production [33].

The synergistic effects of combining modifiers have gained attention as well. Studies on nano clay’s influence on SBS-modified bitumen highlighted improved compatibility and performance [34]. Recent investigations into animal bone ash (ABA) as a filler in asphalt mixtures demonstrated enhanced mechanical properties and sustainability potential.

The findings summarized here emphasize the need for comprehensive testing to balance performance, sustainability, and cost-effectiveness in modified bitumen applications. A detailed synthesis of relevant studies is presented in

Table 1. Summary of Key Studies on Modified Bitumen: Rheological and Fatigue Performance.

Reference	Year	Study Objective	Methodology/Tools Used	Key Findings
[35]	2018	Improvement in properties of bitumen using selected additives	Experimental measurements; adhesion testing between bitumen and aggregate	Additives improved adhesion, softening point, and viscosity. Sasobit and Licomont BS significantly increased softening point.
[36]	2019	Effect of polymer and functional additives on the properties of bitumen and asphalt-concrete	Use of copolymers and carbon nanotubes; rheological and performance tests	Copolymers and nanotubes improved heat resistance, elasticity, shear stability, and strength in asphalt concrete.
[37]	2020	Effect of soybean oil-derived additives on polymer-modified asphalt with RAP	Rheology experiments; Multiple Stress Creep Recovery (MSCR) testing	Soybean oil-derived additives improved fatigue and low-temperature performance. SESO maintained elastic recovery.
[38]	2020	Investigation of high-temperature rheological behaviors and fatigue performance	TFOT and PAV aging tests; RV, DSR, and LAS tests	TOR-activated CRMA improved high-temperature properties but negatively impacted fatigue performance compared to SBSMA.
[39]	2021	Effect of waste polyethylene and wax-based additives on bitumen performance	Analysis of viscoelastic properties; blending polyethylene and Fischer–Tropsch waxes	Long-chain waxes enhanced resistance to permanent deformations, while short-chain waxes negatively affected rutting resistance.
[40]	2021	Temperature susceptibility of aged bitumen rejuvenated by polymer-modified bio-derived rejuvenator	BBR, DSR, RV, and SARA analysis tests	PMBR showed superior long-term performance and preferable temperature susceptibility compared to conventional rejuvenators.
[41]	2021	Effectiveness of polymer-modified asphalt binders with RAS and RAP	Four-point bending-beam method for fatigue testing	Polymer-modified binders improved fatigue life of asphalt mixes containing RAS and RAP; RAS alone negatively impacted performance.
[42]	2022	Biomaterials as additives in bitumen technology for sustainability	Literature review of bio-based materials	Biomaterials offer eco-friendly approaches and improved bitumen properties for asphalt applications.
[43]	2022	Comparison of rutting, fatigue, and cracking resistance in CR and SBS modified binders	MSCR and LAS tests	CR + Sasobit improved rutting but reduced fatigue life at high strain values.
[44]	2022	Long-term aging effects on hybrid polymer-modified bitumen	PAV aging simulation; rheological and thermal analysis	SBS degraded, improving elastic response; RLLDPE showed minimal degradation under long-term aging.
[45]	2023	Bio-polymer modified bitumen	Gel Permeation Chromatography, FTIR, DSC, and DSR tests	PHBV improved bitumen’s physical and mechanical properties, demonstrating sustainability as a bio-polymer modifier.
[46]	2023	Aging impact on fatigue and thermal cracking performance of bio-blend bitumen	PAV system, LAS tests, BBR tests	Compound-modified bitumen exhibited better fatigue resistance and anti-aging performance.
[47]	2023	Fatigue evaluation of polymer-modified asphalt mixtures using S-VECD model	S-VECD and LAS tests	Polymer-modified mixtures demonstrated superior fatigue and mechanical performance with reduced viscous behavior.
[48]	2023	High-viscosity and high-elasticity bitumen modified with five additives	Rheological property evaluation; compounding modifiers	Additives significantly enhanced viscoelasticity; reclaimed rubber improved bitumen sustainably.
[49]	2023	High-elasticity anti-rutting additive effect on asphalt binder performance	RV, FTIR, AFM, FM tests	Additives improved viscosity, elasticity, and reduced temperature susceptibility; compatibility influenced performance.
[50]	2023	Properties of polymer-modified bitumen with various additives	Penetration, ductility, softening point, RV, and TFOT tests	Different PMBs improved asphalt properties; high SBS content increased viscosity, affecting workability.
[51]	2023	Durability of polymer additives in upgrading Nigerian bitumen	Systematic literature review; evaluation of waste-packaging polymers	Polymer additives offered sustainable alternatives, enhancing local bitumen performance.
[52]	2024	Performance study of SBS/CRMA with different composite crumb-rubber particle size ratios	Brookfield viscosity measurements; storage stability, rheology, LAS, FTIR, FM analysis	Finer CR particles improved asphalt performance; composite CR ratios enhanced SBS/CRMA significantly.
[53]	2024	Performance of SBS and crumb-rubber composite-modified asphalt using pretreatment	FEO pretreatment; DSR, FTIR, SEM, FM, penetration, softening-point tests	CR pretreatment enhanced compatibility and storage stability; there was mild impairment in high-temperature resistance.
[54]	2024	Influence of crumb rubber on temperature susceptibility and activation energy of CRM bitumen	Viscosity temperature susceptibility; activation energy; penetration index evaluations	Increased CRM content enhanced viscosity and reduced activation energy, improving temperature susceptibility.

.

Despite significant advancements in bitumen modification, the current literature highlights key areas requiring further exploration. Firstly, while polymers like SBS have been extensively studied for their high-temperature and elastic performance, there is limited research on their fatigue resistance under prolonged cyclic loading. The impact of varying SBS concentrations on stiffness and fatigue resistance, especially under realistic traffic conditions, remains insufficiently addressed. Similarly, bio-based modifiers such as AB powder have shown promise in enhancing bitumen performance. However, studies often neglect a comprehensive evaluation of their fatigue properties and the trade-offs between stiffness and long-term durability. Additionally, while WCO has been investigated as a sustainable modifier, the literature predominantly focuses on its rejuvenating effects on aged bitumen rather than its impact on fresh binders. There is a lack of understanding regarding how WCO concentrations influence high-temperature performance and fatigue resistance.

This study aims to bridge existing gaps by comprehensively evaluating unmodified bitumen and bitumen modified with SBS, animal bone powder (AB), and waste cooking oil (WCO) across multiple performance metrics. Through advanced rheological tests, including PG grading, Frequency Sweep, and LAS, the research systematically examines the impact of varying concentrations of these modifiers on stiffness, elasticity, rutting resistance, and fatigue performance.

This study stands out for its novelty in several key areas:

• Exploration of Animal Bone Powder (AB): Using calcium-rich AB as a bio-based modifier is a relatively new approach. This research identifies the optimal concentration (4%) to balance stiffness and fatigue resistance and provides new insights through advanced rheological evaluations such as Frequency Sweep and LAS tests.
• Investigation of Waste Cooking Oil (WCO): While WCO is known as a rejuvenator for aged bitumen, this study uniquely focuses on its effects on fresh binders. The findings reveal critical trade-offs between enhanced fatigue resistance and reduced structural stiffness, emphasizing the importance of its cautious application in high-performance contexts.
• Comprehensive Testing Across Multiple Metrics: The application of advanced techniques, including the creation of master curves, Cole–Cole plots, and G*/sin δ evaluations under varied frequencies and temperatures, provides a deeper understanding of the modifiers’ viscoelastic behavior and their impact on performance.
• Quantification of Trade-Offs and Synergy: The study quantifies the trade-offs between stiffness, fatigue resistance, and elasticity for each modifier. Additionally, it uniquely explores the potential synergy between SBS and AB, offering practical recommendations for optimal performance.
• Challenging Existing Generalizations: By showing the limitations of WCO due to its plasticizing effects, this research challenges existing generalizations about its benefits, presenting a more nuanced understanding of its impact on performance.

These unique aspects contribute to advancing the understanding of bitumen modification and developing practical guidelines for sustainable and high-performance applications in infrastructure. The study’s findings are valuable for the existing body of knowledge, addressing gaps and offering innovative perspectives on using bio-based and waste-derived modifiers in bitumen.

3. Materials and Methodology

3.1. Methodology Overview

The experimental workflow is illustrated in Figure 1, Experimental Workflow for Modified Bitumen Preparation and Testing. This flow chart provides a detailed overview of the preparation and testing phases, including raw material characterization, additive incorporation, and performance evaluation through standardized tests.

3.2. Materials and Sample Preparation

3.2.1. Bitumen

The base material used in this study is 40/50 penetration grade bitumen, which is noted for high viscosity and stiffness; hence, this grade of bitumen will be best suited to those places where the temperature is high. This type of bitumen is suitable under high-temperature conditions, since its ability to resist deformation and rutting will help preserve pavement durability and performance for a long time under harsh environmental and heavy-traffic loading conditions.

In all, basic physical properties of neat bitumen were obtained to create a background in comparison and ascertain that it was fit for modification. A summary of some of these properties is outlined in

Table 2. Physical properties of neat bitumen.

Specification	Unit	Typical Value	Test Method
Penetration	mm	49	40–50 (ASTM D5 [55])
Ductility	cm	>150	≥100 (ASTM D113 [56])
Softening Point	°C	50	-
Flash Point	°C	245	232 min (ASTM D92 [57])
Specific Gravity	-	1.02	-
Rotational viscosity	Pa.s.	0.488	-
Rolling Thin-Film Oven Test
Retained penetration	%	80	≥55 (ASTM D5 [55])
Penetration Index (PI)	-	−1.25	-

below: penetration, ductility, softening point, flash point, specific gravity, rotational viscosity, and under the Rolling Thin-Film Oven Test (RTFOT), are the critical characteristics that give the potential behavior of the bitumen under various loading modes and temperature fluctuations, among other factors of aging.

3.2.2. Styrene-Butadiene–Styrene (SBS)

SBS is a block copolymer known for significantly enhancing bitumen’s rheological and mechanical properties. This polymer modifier improves elasticity and flexibility, making it highly effective in enhancing the long-term serviceability of asphalt pavements. SBS-modified bitumen exhibits superior resistance to permanent deformation and cracking, especially under high-temperature and heavy-loading conditions, which makes it a popular choice for regions with extreme climatic variations and heavy traffic loads.

In this study, SBS was sourced from Istanbul Teknik, a trusted supplier of high-quality polymer modifiers. The SBS was incorporated into the neat bitumen at proportions of 4% and 5% by weight. These proportions were selected based on established recommendations in the literature and previous studies [58,59], highlighting their effectiveness in achieving the desired balance between flexibility and stiffness.

SBS was added under controlled conditions to ensure homogeneity and optimal interaction between the polymer and the bitumen. The resulting modified bitumen was expected to exhibit enhanced elasticity, higher softening points, and improved resistance to fatigue and rutting, all of which are critical for the durability and performance of asphalt pavements.

3.2.3. Waste Cooking Oil (WCO)

WCO, collected from various sources, was filtered and chemically treated to enhance its physical and rheological properties. The treated oil was heated to 160 °C and blended with bitumen at 130 °C using a high-speed mixer at 1000 RPM. WCO was incorporated at proportions of 3%, 4%, and 5% by weight, as described by Azahar et al. [28].

3.2.4. Animal Bones (AB)

Animal bones were cleaned, dried, and heated in a furnace at 800 °C for 90 min, resulting in calcium-rich ash. After cooling, the ash was ground and sieved through a No. 200 mesh. ABA was used as a filler in proportions of 4%, 5%, and 6% by weight, following methodologies outlined by [60,61].

3.3. Testing Methods

3.3.1. Performance Grading (PG) Test

The Performance Grade (PG) Test was conducted to evaluate the rheological properties of bitumen in accordance with the ASTM D6373 standard [62]. The test aimed to classify the bitumen based on its suitability for specific climatic conditions and traffic loads. Using a Dynamic Shear Rheometer (DSR), the complex shear modulus (G*) and phase angle (δ) were measured at the target high-performance temperature to calculate the rutting resistance parameter G*/sin δ. Testing was performed using a DSR with an 8 mm diameter parallel-plate geometry and a 2 mm gap, while the temperature was maintained at the performance grading value, such as 58 °C, in a controlled environment to ensure accuracy and repeatability.

The δ, representing the viscoelastic balance between elastic and viscous behavior, was recorded for each sample to determine the binder’s deformation resistance under high temperatures. To simulate short-term aging effects during mixing and construction, the bitumen samples were aged using the Rolling Thin-Film Oven (RTFO). These aged samples were then tested to verify compliance with the PG criteria. This method comprehensively evaluated the bitumen’s resistance to rutting, fatigue, and thermal cracking, ensuring compliance with the Superpave system’s standards for durable pavement materials.

3.3.2. Frequency Sweep Test

The Frequency Sweep Test was conducted to evaluate the viscoelastic properties of the bitumen samples over a wide range of ω and temperatures in accordance with ASTM D7175 [63]. The test was performed using a DSR equipped with parallel-plate geometry and a gap of 2 mm. Frequency sweeps were carried out at 15 °C and 25 °C, which represent typical service conditions for bituminous pavements. The ω range spanned from 0.1 to 100 rad/s.

The samples were subjected to sinusoidal shear stresses at varying angular frequencies during the test. Key parameters were measured, such as G*, storage modulus (G′), loss modulus (G″), and δ. The G* reflects the total resistance of the material to deformation, while the δ indicates the viscous- to elastic-behavior ratio. The following calculations and analyses were performed:

• G*/sin δ: The parameter G*/sin δ was calculated at each frequency and temperature to assess the binder’s ability to resist permanent deformation under high temperatures and repeated loading.
• Cole–Cole Plot: A Cole–Cole plot was generated by plotting the G′ against the G″. This provided a graphical representation of the transition between elastic (energy storage) and viscous (energy dissipation) behavior, characterizing the viscoelastic nature of the bitumen.
• Frequency-Dependent Behavior: The data were analyzed to construct master curves that illustrate the frequency-dependent behavior of the G* and δ. These curves predicted the material’s performance over a broader range of service conditions.

3.3.3. Linear Amplitude Sweep (LAS) Test

The Linear Amplitude Sweep (LAS) Test was conducted to evaluate the fatigue resistance of bitumen under controlled cyclic loading conditions, following AASHTO TP 101 [64]. The test used a DSR with an 8 mm diameter parallel-plate geometry and a 2 mm gap at a constant temperature of 25 °C, representing typical service conditions for bitumen pavements. The procedure began with a frequency sweep at low strain levels to ensure the material remained within the linear viscoelastic range (LVR). This step determined the G* and δ over a frequency range of 0.1 to 100 rad/s, establishing baseline viscoelastic properties for subsequent fatigue analysis.

During the LAS test, sinusoidal shear stress was applied at a constant frequency of 10 Hz, with the strain amplitude (γ\gamma) linearly increasing from 0.1% to 30% over approximately 300 s. This simulated the progressive damage accumulation under traffic loading. Key parameters, including the G*, shear stress (τ), and shear strain (γ), were continuously recorded to assess the material’s stiffness loss and the transition from elastic to viscous behavior. The data were analyzed using the Viscoelastic Continuum Damage (VECD) model, which relates the dissipated pseudo-strain energy to the applied strain amplitude. The fatigue life (Nf) was estimated using the equation Nf = $A·{{(\gamma }_o)}^{-B}$, where A and B are material-specific parameters, and ${\gamma }_o$ is the applied strain amplitude. The fatigue life was determined by identifying the strain level at which the modulus reduction exceeded a critical threshold, indicating significant material damage.

4. Results

4.1. Performance Grade (PG) Results

Table 3. PG Results Before RTFOT.

Sample	Temperature (°C)	G* (Pa)	Phase Angle (δ °)	G*/sin(δ) (kPa)	Strain Amplitude (%)
Unmodified Bitumen	52.0	25,000.0	79.4	25.438	12.17
4% SBS	76.0	2135.9	81.6	2.1593	12.37
5% SBS	76.0	3584.9	75.4	3.7041	12.48
4% AB	64.0	3779.9	85.0	3.7945	12.28
5% AB	76.0	1001.9	85.7	1.0048	12.20
6% AB	76.0	1001.6	86.4	1.0036	12.04
3% WCO	52.0	9470.3	82.5	9.5524	12.14
4% WCO	52.0	7498.8	83.0	7.5552	12.33
5% WCO	52.0	5171.7	83.8	5.2017	12.13

summarizes the PG results of the bitumen samples before the RTFOT. The unmodified bitumen exhibited a high G*/sin(δ) value of 25.438 kPa at a temperature of 52.0 °C, indicating good resistance to rutting. Modified samples with SBS (4% and 5%) showed reduced G*/sin(δ) values of 2.1593 kPa and 3.7041 kPa, respectively, at a higher testing temperature of 76.0 °C. These modifications enhanced high-temperature performance but slightly lowered stiffness, compared to unmodified bitumen. AB-modified bitumen demonstrated varied results. At 4% AB, the G*/sin(δ) value was 3.7945 kPa at 64.0 °C, while 5% and 6% AB yielded lower values of 1.0048 kPa and 1.0036 kPa at 76.0 °C. These results indicate that increasing AB content reduced the material’s G*/sin δ at higher temperatures. WCO-modified bitumen improved performance at 3% WCO, achieving a G*/sin(δ) value of 9.5524 kPa at 52.0 °C. Higher proportions of WCO (4% and 5%) reduced the G*/sin(δ) values to 7.5552 kPa and 5.2017 kPa, respectively, maintaining sufficient resistance to rutting.

Table 4. PG Results After RTFOT.

Sample	Temperature (°C)	G* (Pa)	Phase Angle (δ °)	G*/sin(δ) (kPa)	Strain Amplitude (%)
Unmodified Bitumen	52.0	21,213.0	78.9	21.615	12.01
4%SBS	76.0	3428.3	76.0	3.533	12.12
5%SBS	76.0	4982.0	63.9	5.306	12.31
4% AB	64.0	3640.4	84.9	3.655	11.83
5% AB	76.0	1278.1	86.4	1.280	12.23
6% AB	76.0	1234.5	86.5	1.236	12.06
3% WCO	52.0	13,098.0	80.0	13.300	12.24
4% WCO	52.0	8937.1	81.2	9.043	12.46
5% WCO	52.0	11,509.0	80.3	11.674	12.16

presents the performance grade results after the RTFOT, reflecting the aging effects on the bitumen samples. The unmodified bitumen retained a G*/sin(δ) value of 21.615 kPa at 52.0 °C, slightly lower than the pre-aging value. SBS-modified samples demonstrated significant improvements, with 4% SBS R and 5% SBS R achieving G*/sin(δ) values of 3.5333 kPa and 5.3061 kPa, respectively, at 76.0 °C. For AB-modified samples, aging slightly reduced G*/sin(δ) values. The 4% AB R sample exhibited 3.6552 kPa at 64.0 °C, while 5% and 6% AB R resulted in 1.2806 kPa and 1.2368 kPa, respectively, at 76.0 °C. WCO-modified bitumen retained good performance after aging. The 3% WCO R sample achieved the highest post-aging G*/sin(δ) value of 13.3 kPa at 52.0 °C, followed by 9.0435 kPa and 11.674 kPa for 4% and 5% WCO R samples, respectively.

The data present different influences of each modifier on bitumen performance before and after aging. The SBS-modified samples significantly improve stiffness and high-temperature performance; 5% SBS still shows excellent G*/sin(δ) values, even after aging. Diminishing returns in stiffness with increasing SBS concentrations bring the requirement for striking a balance between rigidity and flexibility into focus. The AB-modified bitumen exhibited a significant reduction in G*/sin(δ) values at higher concentrations of 5% and 6%, which means losing stiffness and resistance to rutting. This implies that while AB presents a sustainable alternative, this additive must be made with precision in concentration for optimal performance without compromising structural integrity. WCO-modified bitumen showed excellent performance in fatigue resistance, especially for the lower concentration of 3% WCO. After aging, the potential as a rejuvenator was reflected in the fact that 3% WCO reached the highest G*/sin(δ) values among all modifiers. However, higher concentrations of WCO at 4% and 5% had reduced stiffness—a trade-off that may need to be considered for different applications.

4.2. Frequency Sweep Results

4.2.1. Phase Angle vs. Angular Frequency

The relationship between δ and angular frequency (ω) was analyzed for unmodified bitumen and modified samples with SBS, AB, and WCO at 25 and 15 °C. The δ provides insight into the balance between elastic and viscous behavior, with higher δ values indicating more viscous behavior, while ω reflects the rate of oscillation applied during testing. At 25 °C, the results demonstrated distinct trends across the frequency range of 0.1 to 100 rad/s for the different modifiers, as shown in Figure 2. For unmodified bitumen, the δ increased gradually, from approximately 52.82° at higher angular frequencies (100 rad/s) to 68.38° at lower frequencies (0.1 rad/s), reflecting a shift toward more viscous behavior as the loading frequency decreased. For SBS-modified samples, the 5% SBS sample demonstrated a more pronounced elastic behavior at higher frequencies, with δ values ranging from 57.08° to 73.27°. The 4% SBS sample showed δ values between 51.45° and 67.73° across the same frequency range. AB-modified samples, incorporating 4%, 5%, and 6%, exhibited δ that increased consistently with decreasing frequency. Among these, the 4% AB sample displayed slightly higher elasticity compared to the 5% and 6% AB samples, with δ values of 71.5°, 73.92°, and 72.54° at lower frequencies, respectively. In contrast, WCO-modified samples showed higher δ than those modified with SBS or AB, indicating a more viscous nature. The 5% WCO sample exhibited the highest δ values across all frequencies, ranging from 62.64° at high frequencies to 80.46° at low frequencies.

At 15 °C, the unmodified bitumen exhibited a gradual increase in δ as the ω decreased, starting at 50.44° at 100 rad/s and reaching 64.18° at 0.1 rad/s, as shown in Figure 3. This behavior reflects a transition from more elastic to viscous dominance under slower loading conditions. SBS-modified samples demonstrated lower δ than unmodified bitumen at higher frequencies, highlighting their improved elastic performance. For example, the 4% SBS sample started at 49.84° at 100 rad/s and reached 60.1° at 0.1 rad/s, while the 5% SBS sample exhibited slightly higher values, ranging from 57.96° to 65.0°, over the same frequency range. The AB-modified samples exhibited intermediate δ, with values increasing as the frequency decreased. The 4% AB sample displayed a δ range of 55.61° to 64.97°, while the 6% AB sample showed a slightly more consistent increase, from 53.74° to 67.0°, indicating moderate improvements in elastic behavior compared to unmodified bitumen. In contrast, WCO-modified samples exhibited the highest δ, emphasizing their more viscous nature. The 5% WCO sample ranged from 57.89° at 100 rad/s to 71.85° at 0.1 rad/s, indicating a more pronounced shift toward viscous behavior under lower frequencies.

The frequency-sweep results reveal significant differences in the viscoelastic behavior of bitumen modified with SBS, AB, and WCO, compared to unmodified bitumen. SBS-modified samples exhibited improved elastic performance, particularly at higher frequencies, demonstrating the effectiveness of SBS in enhancing stiffness and resistance to deformation. However, the slight increase in δ at lower frequencies for higher SBS content (5%) indicates diminishing returns in elastic improvement, suggesting that 4% SBS may provide an optimal balance between elasticity and performance. AB-modified samples showed concentration-dependent trends, with lower AB content (4%) achieving higher elasticity and improved structural balance compared to 5% and 6%. This trend highlights the importance of optimizing AB content to prevent excessive softening, which can reduce deformation resistance under high loads. WCO-modified samples displayed the most viscous behavior, with δ values consistently higher than those of SBS- and AB-modified samples. While this reflects WCO’s plasticizing effect, enhancing fatigue resistance, it also highlights its limitations for applications requiring high stiffness. The pronounced shift toward viscous behavior at lower frequencies indicates WCO’s suitability for applications emphasizing flexibility and energy dissipation under slow loading conditions.

4.2.2. Complex Shear Modulus vs. Angular Frequency

The relationship between the G* and ω was evaluated for unmodified bitumen and bitumen modified with SBS, AB, and WCO. The G* measures the material’s stiffness and ability to resist deformation under oscillatory loading. Across all samples, G* decreased with decreasing ω, indicating a reduction in stiffness under slower loading conditions. At 25 °C, the unmodified bitumen displayed the steepest decline in G* as frequency decreased, with a modulus of 5.47 MPa at 100 rad/s, as shown in Figure 4, which dropped significantly to 0.054 MPa at 0.1 rad/s. This reduction suggests a rapid transition toward a more compliant behavior under lower loading rates. In contrast, SBS-modified samples maintained higher G* values across the frequency range, showcasing improved stiffness. For example, the 4% SBS sample retained a modulus of 6.87 MPa at 100 rad/s and 0.108 MPa at 0.1 rad/s, highlighting its superior elastic properties compared to the unmodified bitumen. The AB-modified samples demonstrated intermediate behavior, with G* values increasing slightly with higher AB content. For instance, the 6% AB sample exhibited a modulus of 4.99 MPa at 100 rad/s and 0.05 MPa at 0.1 rad/s, indicating enhanced stiffness compared to the 4% AB sample. However, the stiffness improvement was less pronounced than in SBS-modified bitumen. WCO-modified samples exhibited the lowest G* values across the frequency range, reflecting their more compliant and viscous behavior. The 5% WCO sample had a modulus of 2.02 MPa at 100 rad/s and 0.011 MPa at 0.1 rad/s, indicating a significant reduction in stiffness at lower frequencies.

At 15 °C, the unmodified bitumen gradually decreased G* as ω decreased, starting from 15.52 MPa at 100 rad/s to 0.24 MPa at 0.1 rad/s, as shown in Figure 5. This significant reduction highlights the material’s tendency to soften under lower loading frequencies. SBS-modified samples displayed significantly higher G* values across the frequency range, emphasizing their improved stiffness. The 4% SBS sample maintained a modulus of 25.77 MPa at 100 rad/s; at 0.1 rad/s, the value reduced to 0.80 MPa. Similarly, the 5% SBS sample demonstrated slightly lower values, ranging from 10.76 MPa at high frequencies to 0.40 MPa at low frequencies. AB-modified samples demonstrated intermediate stiffness, with G* values increasing as the proportion of AB increased. For instance, the 6% AB sample showed a G* of 16.42 MPa at 100 rad/s and 0.20 MPa at 0.1 rad/s, reflecting enhanced stiffness compared to unmodified bitumen but less improvement than SBS-modified samples. The 4% AB sample displayed slightly lower values, with G* ranging from 8.18 MPa to 0.24 MPa across the frequency range. WCO-modified samples exhibited the lowest stiffness among all modifiers. The 5% WCO sample started with a G* of 8.98 MPa at 100 rad/s, decreasing to 0.09 MPa at 0.1 rad/s. Similarly, the 4% WCO sample ranged from 5.47 MPa at high frequencies to 0.04 MPa at low frequencies, emphasizing its more compliant nature.

These results indicate the different influences of SBS, AB, and WCO on bitumen’s stiffness and viscoelastic properties. Among all frequencies, the G* values for SBS-modified samples were the highest, showing that these can enhance the rigidity and resistance to deformation of bitumen, especially under dynamic loading conditions. However, the higher stiffness at higher SBS content (5%) shows evidence of diminishing returns in the improvement of stiffness and, therefore, suggests that 4% SBS may provide an optimum solution to balance performance. The AB-modified samples had only fair- to middle-scale enhancements in stiffness; increasing the AB content shows an increase in G*, although less accentuated when compared to SBS. Such results unequivocally support the idea that AB works out as a bio-based modifier with mediocre performance but no embrittlement. Among the samples studied, WCO-modified samples showed the lowest G* values, indicating their more compliant and viscous nature. Such behavior is consistent with the known plasticizing effect from WCO, in which flexibility and fatigue resistance increase while stiffness decreases. These results hint that WCO is better suited for flexibility and energy-dissipation applications rather than rigidity and deformation resistance.

4.2.3. G*/sin δ vs. Angular Frequency

The G*/sin δ and ω relationship were evaluated as unmodified bitumen and modified samples containing SBS, AB, and WCO. This parameter is critical for assessing the material’s performance under high temperatures and loading conditions, as higher G*/sin δ values indicate better resistance to deformation. At 25 °C, the unmodified bitumen exhibited a steady decline in G*/sin δ as the ω decreased, starting from approximately 6.86 MPa at 100 rad/s to 0.048 MPa at 0.1 rad/s, as shown in Figure 6. This behavior indicates a rapid reduction in the material’s resistance to deformation under slower loading conditions. Comparatively, SBS-modified samples demonstrated significantly higher G*/sin δ values across the entire frequency range, showcasing their enhanced resistance to rutting. For instance, the 4% SBS sample retained a G*/sin δ value of 8.25 MPa at 100 rad/s and 0.12 MPa at 0.1 rad/s, while the 5% SBS sample exhibited even greater values of 7.00 MPa and 0.08 MPa at the respective frequencies. AB-modified samples showed intermediate performance, with G*/sin δ values increasing slightly with higher AB content. At 100 rad/s, the 6% AB sample achieved 6.03 MPa, which decreased to 0.05 MPa at 0.1 rad/s. This trend suggests that AB-modified bitumen balances stiffness and flexibility, offering moderate improvements in G*/sin δ compared to the unmodified bitumen. WCO-modified samples, however, displayed the lowest G*/sin δ values across the frequency range, reflecting their more compliant nature. The 5% WCO sample started at 2.27 MPa at 100 rad/s and dropped to 0.008 MPa at 0.1 rad/s, indicating significantly reduced stiffness.

At 15 °C, the unmodified bitumen gradually reduced G*/sin δ as ω decreased. At high angular frequencies (100 rad/s), G*/sin δ reached approximately 20.1 MPa, as shown in Figure 7, while at low frequencies (0.1 rad/s), the value reduced to 0.27 MPa, highlighting the viscoelastic nature of the unmodified bitumen. The 4% SBS sample displayed G*/sin δ values ranging from 29 MPa at high frequencies to 0.92 MPa at low frequencies. Similarly, the 5% SBS sample exhibited slightly lower values, with G*/sin δ ranging from 12.7 MPa to 0.40 MPa. The higher resistance observed in SBS-modified samples is consistent with their improved stiffness and elasticity. AB-modified samples showed intermediate G*/sin δ values, with the resistance increasing as the proportion of AB increased. The 6% AB sample reached G*/sin δ values of 20.4 MPa at 100 rad/s and 0.20 MPa at 0.1 rad/s, providing enhanced resistance compared to unmodified bitumen but slightly lower than the SBS-modified samples. The 4% AB sample showed slightly lower values, with G*/sin δ ranging from 9.9 MPa to 0.27 MPa across the frequency range. The WCO-modified samples demonstrated the lowest rutting resistance among all modifiers, reflecting their more compliant and viscous nature. The 5% WCO sample had G*/sin δ values between 10.6 MPa at high frequencies and 0.09 MPa at low frequencies, while the 4% WCO sample showed a range from 6.5 MPa to 0.04 MPa.

The G*/sin δ analysis gives important information on modifiers’ ability to improve bitumen’s high-temperature performance. The SBS-modified samples had the highest G*/sin δ values in all cases, confirming their superior stiffness and resistance to rutting. These results make SBS the most effective modifier for applications requiring high deformation resistance under dynamic loading conditions. AB-modified samples gave moderate improvements in G*/sin δ, where increased AB content yielded a slight improvement in the resistance to deformation. However, improvements in G*/sin δ were not as dramatic as for SBS, indicating that the AB-modified bitumen achieves a balance between stiffness and flexibility, rather than achieving extreme stiffness. Although WCO increases bitumen’s flexibility and fatigue resistance, the lower G*/sin δ values obtained for WCO-modified samples indicate that it is less suitable for applications requiring high stiffness and rutting resistance.

4.2.4. Cole–Cole Plot

The Cole–Cole plots, which graph the G″ versus the G′, provide insights into the viscoelastic behavior of unmodified and modified bitumen samples at 25 °C and 15 °C. These plots help visualize the relationship between bitumen’s elastic and viscous components under oscillatory shear loading. At 25 °C, the Cole–Cole plot shown in Figure 8 reveals distinct behavior among the different modifiers. Unmodified bitumen exhibited a relatively linear relationship between G″ and G′, indicating a uniform distribution of viscous and elastic responses across the tested frequencies. The SBS-modified samples demonstrated higher G″ and G′ values, reflecting improved stiffness and elasticity, particularly at higher frequencies. Among the SBS samples, the 4% SBS modifier showed slightly higher G″ values than the 5% SBS, suggesting a more pronounced viscous response. AB-modified samples exhibited moderate G″ and G′ values, with the 6% AB sample demonstrating a steeper curve, indicating enhanced elasticity but reduced viscous dominance. In contrast, the WCO-modified samples displayed lower G″ and G′ values, indicating a less rigid structure and higher susceptibility to deformation under shear loading.

At 15 °C, the Cole-Cole plot shown in Figure 9 reveals increased values of both G″ and G′ across all samples, reflecting the higher stiffness of bitumen at lower temperatures. Unmodified bitumen retained a linear relationship, although the values shifted upward. The SBS-modified samples demonstrated superior elastic and viscous behavior, with the 4% SBS sample exhibiting the highest G″ and G′ values. AB-modified samples maintained moderate performance, with 6% AB showing the steepest curve, indicative of enhanced elasticity. The WCO-modified samples displayed similar trends to those observed at 25 °C, with lower G″ and G′ values, further confirming the dominance of viscous behavior.

The Cole-Cole plots give an idea of the viscoelastic behavior of modified bitumen samples, with each modifier contributing uniquely. In all cases, SBS-modified samples gave the highest values of G″ and G′, reflecting SBS’s ability to impart stiffness and elasticity over a wide temperature range, making it the most effective modifier for applications that require superior mechanical properties under dynamic loading conditions. AB-modified samples gave an intermediate performance, and steeper curves at higher AB concentrations indicate increased elasticity. These results confirm the suitability of AB as an environmentally friendly alternative for enhancing elastic properties in bitumen, particularly under cold conditions. The WCO-modified samples consistently showed the lowest G″ and G′, reflecting their compliant and viscous nature. Although this behavior enhances flexibility and energy dissipation, it limits the suitability of WCO for applications requiring high stiffness and resistance to deformation.

4.3. Linear Amplitude Sweep (LAS) Results

4.3.1. Frequency Sweep Output of Linear Amplitude Sweep (LAS) Test Results

The relationship between δ and frequency was evaluated for various bitumen samples under Linear Amplitude Sweep conditions at 25 °C. δ reflects the viscoelastic balance between elastic (solid-like) and viscous (liquid-like) behavior. Across all samples, δ decreased with increasing frequency, indicating a transition toward more elastic behavior at higher loading frequencies. Modified bitumen samples, such as those containing SBS and WCO, exhibited lower δ at high frequencies compared to unmodified bitumen, highlighting their enhanced elasticity. Among the modifiers, the 5% SBS sample showed the lowest δ, suggesting the most pronounced elastic response at high frequencies. These trends are illustrated in Figure 10.

The G* increased consistently with frequency, for all samples. The highest G* values were observed in 5% SBS-modified bitumen, indicating superior stiffness, especially at higher frequencies. Unmodified bitumen displayed the lowest G* values across the frequency range, reflecting its relatively lower stiffness compared to modified variants. Adding modifiers like AB and WCO enhanced bitumen stiffness, with SBS-modified samples demonstrating the most significant improvement. These observations are depicted in Figure 11.

G*/sin δ showed a marked increase in frequency across all samples. Modified bitumen, particularly that with SBS modifiers, exhibited higher G*/sin δ values compared to unmodified bitumen, indicating enhanced resistance to permanent deformation under repeated loading. The 5% SBS sample consistently showed the highest G*/sin δ values, highlighting its superior performance. This behavior is visualized in Figure 12.

Results obtained from LAS draw significant attention to the superior performance of SBS-modified bitumen in terms of stiffness, elasticity, and rutting-resistance enhancement. The lower phase angle (δ) observed in SBS-modified samples confirms their enhanced elastic behavior, contributing to their superior resistance to deformation under repeated loading. Maximum G* and G*/sin δ values for the sample with 5% SBS indicate its best stiffness and stability, being the most effective modifier in high-performance applications under dynamic and high-temperature conditions. The AB-modified samples performed intermediately, exhibiting increased stiffness and resistance to deformation compared with unmodified bitumen. However, their lower G*/sin δ values relative to the SBS-modified samples suggest that such an increase in stiffness may involve some sacrifice of flexibility, and further indicate that AB might have potential as a bio-based modifier, satisfying moderate performance enhancements. The WCO-modified samples had the lowest stiffness and resistance to deformation, with their lower values of G* and G*/sin δ, respectively. This reflects a plasticizing effect of WCO, providing flexibility and enhancing energy dissipation, but reducing bitumen’s ability to resist permanent deformation under repeated loading.

4.3.2. Time-Dependent LAS Result

LAS results provide detailed insights into the time-dependent performance of unmodified and modified bitumen under cyclic loading conditions. The unmodified bitumen exhibits a steep decline in G* over time, as illustrated in Figure 13, indicating rapid degradation in stiffness under repeated stress cycles. The SBS-modified bitumen samples demonstrate a substantial improvement in both stiffness and fatigue resistance, as depicted in Figure 14 and Figure 15. These modifications result in higher initial G* values, with the modulus decreasing gradually over time, showcasing the enhanced durability imparted by the SBS polymer. Notably, the 5% SBS sample exhibits the slowest rate of degradation, maintaining higher modulus values throughout the test. However, the incremental improvement observed when increasing the SBS concentration from 4% to 5% is marginal, suggesting that the benefits plateau at higher polymer percentages.

AB-modified bitumen significantly improves fatigue resistance, especially at higher concentrations, as observed in Figure 16, Figure 17 and Figure 18. The 4% AB-modified sample shows a moderate decline in G*, with a distinct plateau, suggesting better structural integrity than unmodified bitumen. The 5% AB sample maintains a higher modulus over a prolonged period, indicating superior resistance to cyclic loading. However, the enhancements become less pronounced at 6%, implying a diminishing return in performance gains with increased concentration.

WCO-modified bitumen exhibits distinct behavior, characterized by lower initial G* values and a more rapid decline over time compared to other modifications, as depicted in Figure 19, Figure 20 and Figure 21. At a 3% concentration, the modulus decreases sharply, resembling the performance of unmodified bitumen and indicating minimal enhancement in fatigue resistance. Increasing the WCO content to 4% and 5% results in an even more significant reduction in stiffness, suggesting that WCO predominantly acts as a plasticizer, softening the bitumen rather than improving its structural durability under cyclic loading.

The time-dependent LAS results highlight the differential impacts of SBS, AB, and WCO on the fatigue resistance of bitumen under cyclic loading. SBS-modified bitumen exhibits the most significant enhancement in stiffness and durability, with outstanding resistance against degradation over time at 5% SBS. In contrast, marginal gains in performance at higher concentrations indicate an optimum modification level for achieving a balanced performance. AB-modified bitumen has improved fatigue resistance with particular improvements at 4% and 5% concentrations. At 6%, there is a diminishing return, which emphasizes the need for optimization in the AB content to avoid compromising workability and performance. WCO-modified bitumen exhibited limited improvements, while the stiffness and durability were reduced with higher concentration. These results reflected the role of WCO as a plasticizer, improving flexibility but compromising structural integrity under sustained loading.

4.3.3. LAS Result for Bitumen Fatigue

The evaluation of unmodified- and modified-bitumen samples reveals distinct performance trends in terms of stiffness (A-parameter) and damage sensitivity (B-parameter) (Figure 22 and Figure 23). Unmodified bitumen demonstrated relatively low initial stiffness (A-parameter) and a moderate B-parameter, as expected, due to the absence of additives that enhance mechanical properties. This lack of modification results in lower resistance to deformation underload and moderate susceptibility to fatigue damage, making it suitable only for light traffic conditions. Conversely, SBS-modified samples displayed significant improvements in stiffness, with the 5% SBS sample achieving the highest A-parameter, indicating excellent resistance to deformation. This enhancement is attributed to the ability of SBS to form a network structure within the bitumen, increasing elasticity and mechanical strength. However, the B-parameter indicates moderate damage sensitivity, suggesting room for further improvement in fatigue resistance. Animal bone-modified samples, particularly at 4%, exhibited the best overall performance, combining high stiffness and the lowest damage sensitivity among all tested materials. The organic and mineral composition of AB likely enhances the bitumen matrix, improving rigidity and crack resistance. A slight increase in animal bone content, to 6%, resulted in marginally lower stiffness but retained favorable fatigue resistance, indicating that 4% is the optimal concentration. WCO-modified samples, on the other hand, demonstrated reduced stiffness (low A-parameter) and higher damage sensitivity (high B-parameter) compared to SBS and AB modifications. While environmentally sustainable, the addition of WCO appears to act as a plasticizer, softening the bitumen matrix and compromising its structural integrity under cyclic loading.

The LAS fatigue analysis underlines the peculiar contribution of each modifier: SBS-modified bitumen is characterized by exceptional stiffness and good fatigue resistance; thus, it is highly suitable for applications requiring high durability under cyclic loading. However, the moderate values of the B-parameter suggest that future research could be directed toward further improving the fatigue performance of SBS. AB-modified bitumen was observed to be the most balanced, with high stiffness and low sensitivity to damage. The optimum performance of AB at 4% concentration indicates that AB could be an effective, sustainable modifier for improving stiffness and fatigue resistance. WCO-modified bitumen has limited structural improvements, and the low values of the A-parameter and high values of the B-parameter point toward the softening effect of the bitumen matrix. While its environmental sustainability is commendable, its mechanical limitations suggest its application in less demanding conditions.

5. Discussion

The results of this study provide a detailed evaluation of the performance of unmodified bitumen and bitumen modified with SBS, AB, and WCO. The findings cover key rheological properties, including stiffness, δ, fatigue resistance, and structural performance under varying conditions, such as temperature, frequency, and cyclic loading.

The unmodified bitumen exhibited relatively low stiffness and moderate damage resistance across all tests. The variation δ with ω at 25 °C and 15 °C (Figure 2 and Figure 3) revealed higher δ values, indicating a more viscous behavior. Similarly, the G* decreased significantly with decreasing frequency (Figure 4 and Figure 5), reflecting poor resistance to deformation underloading. The G*/sin δ values at both temperatures (Figure 6 and Figure 7) further highlighted the limited rutting resistance of unmodified bitumen, particularly at lower frequencies. The Cole–Cole plots (Figure 8 and Figure 9) also demonstrated a clear deviation toward the viscous region, indicating low G′ and dominance of the G″. Under the Linear LAS test at 25 °C, the unmodified bitumen displayed a steep decline in G* over time (Figure 13), confirming its susceptibility to fatigue damage and limited durability under cyclical loading. Several studies have reported similar results, where the unmodified bitumen exhibited relatively low stiffness and moderate damage resistance across various tests [65,66,67]

The SBS-modified bitumen samples demonstrated significant improvements in stiffness, elasticity, and resistance to deformation compared to unmodified bitumen. The superior performance of SBS-modified bitumen can be attributed to the polymer’s ability to form a three-dimensional network within the bitumen matrix. This network enhances the material’s elasticity and resistance to deformation, particularly under high-temperature conditions. The 5% SBS concentration showed the most significant improvement, as the higher polymer content facilitated stronger intermolecular interactions, leading to better mechanical properties. The δ results (Figure 2 and Figure 3) showed reduced δ values, indicating a shift toward more elastic behavior. The G* (Figure 4 and Figure 5) remained considerably higher across all frequencies, particularly in the 5% SBS sample, which maintained superior stiffness at both 25 °C and 15 °C. The G*/sin δ values (Figure 6 and Figure 7) highlighted the enhanced rutting resistance of SBS-modified bitumen, with the 5% sample outperforming the 4% modification. The Cole–Cole plots in Figure 8 and Figure 9 showed a balanced performance, while higher G′ can be seen for higher structural integrity. From the results of LAS, as shown in Figure 14 and Figure 15, it can be seen that there is a gradual reduction in the G* with time, while the sample with 5% SBS remained stiff during a longer time, proving to be more resistant against fatigue. These findings align with the studies by [16,48,68].

The performance of AB-modified bitumen was excellent, as it is especially high at 4% of modification. The enhanced performance of AB-modified bitumen is likely due to the calcium-rich composition of animal bone powder, which acts as a filler and reinforces the bitumen matrix. At 4% concentration, AB provided an optimal balance between stiffness and fatigue resistance, as the filler particles effectively distributed stress within the material. However, at higher concentrations (5% and 6%), the performance gains diminished, possibly due to particle agglomeration, which can weaken the bitumen structure. From δ, still, it can be seen to have lower values, which justifies the fact that the material is quite a lot stiffer and more elastic, compared with unmodified-bitumen varieties. The G* featured a significant increase with the 4% and 5% samples, where AB managed, for all frequencies, to maintain more elevated values of G*. Figure 6 and Figure 7 also reveal, from the results of G*/sin δ, that the enhanced rutting resistance is especially good at 4% AB content. The Cole–Cole plots (Figure 8 and Figure 9) demonstrated improved structural balance, with a significant increase in G′. In the LAS test (Figure 16, Figure 17 and Figure 18), the 4% AB sample exhibited superior fatigue resistance, as evidenced by a slower decline in G*, while the 5% and 6% samples showed slight decreases in performance, suggesting diminishing returns at higher concentrations. Overall, 4% AB modification provided the optimal balance between stiffness and fatigue resistance. These observations are consistent with findings by [69,70,71].

The WCO-modified bitumen showed reverse behavior, with lower stiffness and a greater tendency to be damaged. The reduced stiffness of WCO-modified bitumen is primarily due to the plasticizing effect of waste cooking oil, which softens the bitumen matrix. While this softening improves workability and fatigue resistance at lower temperatures, it compromises the material’s ability to withstand high-temperature deformation. This trade-off suggests that WCO is more suitable as a rejuvenator for aged bitumen, rather than a primary modifier for high-performance applications. The δ Figure 2 and Figure 3 showed higher values of δ for the WCO-modified samples compared to those obtained for SBS and AB-modified samples, indicating more viscous and flexible behavior. G* Figure 4 and Figure 5 showed a rapid decrease in G*, especially for the high content of WCO; it seems that the oil acted as a plasticizer, weakening the bitumen matrix. The results of the G */sin δ (Figs 6 and 7) did reveal reduced rutting resistance, more precisely when the samples were treated with 4% and 5% of WCO. The Cole–Cole plots for Figure 8 and Figure 9 exhibited lower values of G′ and featured a high viscous response, depicting poor structure. In the LAS tests conducted on WCO-modified samples, the plots in Figure 19, Figure 20 and Figure 21 show a sudden decline in G*, as even 3% contents declined rapidly, while at 4% and 5% modifications, G* improvement was marginal. These plots indicate that, with the flexibility gain offered by WCO-modified bitumen, its stiffness or resistance to fatigue becomes badly compromised. These findings align with the studies by [72,73].

The obtained general result has pointed out the better stiffness, rut resistance, and fatigue behavior of SBS- and AB-modified bitumen and, in every case, the best results were for 5% of SBS and 4% of AB. On the contrary, WCO-modified bitumen has shown worsening performances in different test conditions, and showed only low suitability for uses requesting high stiffness and good performance under repeated loads. The unmodified bitumen, while performing adequately under light loading, lacks the mechanical properties required for more demanding applications.

6. Conclusions

The paper comprehensively studied the performance and viscoelastic characteristics of unmodified and modified bitumen using PG, Frequency Sweep, and LAS tests. SBS and AB powder have shown better modifications which improved stiffness, elasticity, and resistance to fatigue, thus making them suitable for high-performance and heavy-traffic applications. Among these, SBS-modified bitumen showed the best performance in resisting deformation and fatigue, especially at a concentration of 5%. AB powder also proved to be a very effective modifier, and the optimal balance between stiffness and durability was achieved with 4% concentration. On the other hand, WCO-modified bitumen showed lower stiffness and higher sensitivity to damage, which means that it basically acts as a plasticizer, and thus its application in more structurally demanding applications is limited. From the results, it is deduced that SBS and AB powder emerged as promising additives to enhance the mechanical performance of bitumen. In the case of WCO, environmental benefits should be weighed against the negatives, regarding deterioration in structural properties. Future studies could consider long-term aging performance and thermal stability under field conditions. The additional studies in this area should focus on the process optimization for the blending and evaluation of the compatibility of such modifiers with various types of base bitumen, for even better performances. Regarding WCO, its blending with other modifiers probably produces results that minimize the effects on softening, while maintaining the environmental benefits. Above all, an appropriate process of development in an eco-friendly and economical way for the large-scale production of the AB powder and better exploitation of WCO will stand out as two great forward moves toward green bitumen applications.

7. Limitations and Future Research Directions

While this study provides valuable insights into the performance of bitumen modified with SBS, AB, and WCO, it has certain limitations that should be acknowledged. Firstly, the research focused on laboratory-scale experiments, which may not fully replicate real-world conditions such as varying traffic loads and environmental factors. Secondly, the study did not investigate the long-term aging effects of the modified bitumen, which is crucial for understanding its durability over time. Finally, the interaction between multiple modifiers (e.g., SBS with AB or WCO) was not explored, leaving room for further investigation into potential synergistic effects.

Future research should focus on addressing these limitations by carrying out the following:

• Conducting field trials to validate the laboratory findings under real-world conditions.
• Investigating long-term aging to assess the durability of modified bitumen over extended periods.
• Exploring the combined use of modifiers (e.g., SBS with AB or WCO) to identify potential synergistic effects that could further enhance bitumen performance.
• Evaluating the environmental impact of using bio-based additives like AB and WCO, including their carbon footprint and sustainability benefits.
• Optimizing the blending process to improve the compatibility of modifiers with different types of base bitumen.

By addressing these areas, future studies can build on the findings of this research to develop more sustainable and high-performance asphalt materials.