Performance Assessment of Asphalt Binder Modified with Batu Pahat Soft Clay as an Eco-Friendly Additive

Abstract

This study aims to evaluate the impact of incorporating Batu Pahat Soft Clay (BPSC) into conventional asphalt binder at varying proportions: 2-, 4-, 6- and 8%-BPSC by weight of asphalt binder. A comprehensive laboratory investigation was carried out, including consistency test, Fourier Transform Infrared Spectroscopy (FTIR), Dynamic Shear Rheometer (DSR), Scanning Electron Microscopy (SEM) tests. In terms of rutting, the parameter G*/sin δ increased significantly by nearly 839.25% at 45 °C and 196.67% at 75 °C for the 4%-BPSC binder compared to the base binder. The Multiple Stress Creep and Recovery (MSCR) test further confirmed the BPSC effectively reduce the residual strain by over 55%. FTIR analysis indicates a physical interaction between the BPSC and the binder, with no evidence of new chemical bond formation. Based on overall findings, the 4%-BPSC modification is identified as the optimal percentage for achieving balanced improvement in binder performance, contributing to more sustainable asphalt solutions.

1. Introduction

Asphalt binder plays a vital role in modern infrastructure, particularly in highways and airports pavements owing to its flexibility and ease of application. However, conventional asphalt binder is highly sensitive to temperature variations and repeated vehicular loading, which can lead to rutting and thermal cracking, thereby shortening pavement service life [1,2]. To address these issues, numerous modification techniques have been developed to improve the mechanical and rheological behavior of asphalt binders. Among these, polymers and nanomaterials modifiers are the most widely used [3,4,5,6]. Polymers such as Styrene-Butadiene–Styrene (SBS) and Styrene–Butadiene–Rubber (SBR) enhance elasticity, fatigue resistance, and deformation control, and overall suitability for asphalt pavement [7,8,9], while Polyethylene (PE) and Polyvinyl Chloride (PVC) improve stiffness and thermal stability under high-temperature conditions [10].

Nanomaterials, including nano-silica, carbon nanotubes, nano-calcium carbonate, and nano-bentonite, have shown great improvement in stiffness, moisture resistance, and aging characteristics [11,12,13,14,15,16]. Recent studies have demonstrated the effectiveness of nanoclays, particularly kaolin nanoclay (NKC), as natural modifiers for enhancing the physical and rheological properties of bitumen [17]. Incorporating about 5% of these nanomaterials into asphalt binders has consistently yielded optimal performance under both unaged and aged conditions, improving toughness, rutting resistance, and overall service life. These findings highlight the potential of natural minerals as efficient and environmentally compatible modifiers for asphalt applications. Regarding softening point and storage stability, research has shown that nano-kaolin addition increases bitumen hardness, raises the softening point, and reduces thermal sensitivity [18]. Laboratory tests revealed improved storage stability, as indicated by minimal differences between the upper and lower sections of stored samples. This enhancement is attributed to the homogeneous distribution of nanoparticles within the bituminous matrix, which preserves structural integrity during storage and prevents phase separation [19]. Moreover, the inclusion of the modifier slowed viscosity degradation during aging, enhancing long-term stability and maintaining desirable rheological behavior across varying service conditions [20]. Parallel studies have also explored other natural additives, such as mechanically and chemically activated shungite, to enhance bitumen characteristics [21]. The inclusion of 1% shungite increased the softening temperature and reduced penetration values, indicating higher stiffness and improved thermal resistance. Expansion capacity decreased correspondingly, with carbonaceous shungite outperforming rock-derived varieties. These findings further confirm the potential of natural mineral additives to improve the durability and overall performance of asphalt binder [21].

Nonetheless, concerns about their cost, long-term durability, and environmental safety remain. The production, which involves the mixing of polymer-modified and nanomodified binders, may emit volatile organics, which pose health risks and complicate recycling processes.

Given these limitations, the search for sustainable and environmentally benign asphalt modifiers has gained momentum. Natural clay-based additives derived from abundant, low-cost, and renewable resources, present a promising alternative due to their compatibility with asphalt and lower ecological footprint.

This study explores the incorporating various perorations of BPSC, a locally available natural clay, as sustainable additive for asphalt modification. The research aims to assess the influence of varying BPSC dosages on the physical and rheological properties of bitumen, with the goal of identifying an optimal formulation that balances performance enhancement with and environmental sustainability.

2. Materials and Testing Procedures

2.1. Materials

The 80/100 penetration-grade bitumen was used as the base binder in this study. Batu Pahat Soft Clay (BPSC) particles, ranging in size from 10 µm to 14 µm, were selected for asphalt modification due to their favorable morphology and particle size distribution. These fine particles exhibit mechanical properties and a relatively large surface area, which enable strong interaction with the asphalt matrix. The production of BPSC particles involved a series of sequential steps. BPSC was primarily extracted from a depth of approximately 4 m below the ground surface. The material was then dried in an oven at 155 °C to eliminate moisture. Following the drying process, the clay was then compacted and sieved to obtain a uniform particle size of 0.075 mm. The processed BPSC was incorporated into the asphalt binder at four different dosages: 2%-, 4%-, 6%-, and 8%-BPSC by weight of asphalt binder. A schematic diagram of the equipment used in the BPSC preparation process is shown in Figure 1.

Table 1. The 80/100 asphalt binder properties.

Characteristics	Test Method	Requirement
Penetration at 25 °C	ASTM-D5	80–100
Softening Point °C	ASTM-D36	45–52
Ductility at 25 °C	ASTM-D113	Min. 100
Retained penetration after thin-film oven test, %	ASTM-D5	Min. 47.0
Loss on heating, % wt	AASHTO-T240	Max. 1.00
Original G*/sin δ at 64 °C @ 10 rad, kPa	AASHTO-TP5	Min. 1.00
RTFO G*/sin δ at 64 °C @ 10 rad, kPa	AASHTO-TP5	Min. 2.20
Specific Gravity at 25 °C	AST-D70	1.01–1.05

summarizes the properties of the base asphalt binder, whereas

Table 2. The physical characteristics of BPSC.

Parameters	Results
Bulk Density (Mg/m3)	1.36
Specific Gravity	2.66
Plastic-Limit (PL)%	29.70
Liquid-Limit (LL) (%)	37.55
Plasticity-Index (PI) (%)	7.85
Moisture Content (%)	29.35

provides the physical characteristics of the BPSC material.

2.2. BPSC Particles Mixing Process

A pre-measured quantity of asphalt binder was heated at 160 °C for half an hour to ensure proper flow conditions were achieved. The heated asphalt binder was then transferred to the mixing vessel, where the mixing process was implemented using a high shear mixer conducted at a constant temperature of 155 °C and an initial speed of 500 rmp. Once the desired fluidity was confirmed, the BPSC was gradually introduced into the blend. The mixing speed was gradually increased to 3000 rmp to ensure thorough dispersion of additives. The mixing process carried out according to the durations specified in

Table 3. BPSC particles mixing process.

Asphalt Weight (kg)	BPSC Percentages (%)	Total Weight (g)	Blending Speed (rpm)	Blending Temperature (C°)	Blending Time (min)
0.4	2	8.2	3000	155	60
0.4	4	16.7
0.4	6	25.5
0.4	8	34.8

[22].

2.3. Viscosity Test

The test was implemented to assess the flow properties of the asphalt binder and to determine its suitability for use in hot-mix asphalt production. The test involved measuring the binder’s viscosity at two standard temperatures: 165 °C and 135 °C, which correspond to the recommended mixing and compaction temperatures. Approximately 30 g of asphalt binder was heated until it reached a sufficiently liquid state to be poured into the viscometer’s sample chamber, after which an appropriate spindle was selected. The filled chamber was then placed inside a temperature-controlled thermostatic vessel. Upon reaching a 30 min thermal conditioning phase, the spindle (No. 27) was lowered into the sample, and the viscosity measurements were taken at the specified temperature. The test was performed in accordance with ASTM D-4402.

2.4. Storage Stability Test

Phase separation in the modified asphalt binder at elevated temperatures was conducted using a standard procedure [23]. For this test, the binder was poured into an aluminum foil tube with an inner diameter of 30 mm and a height of 300 mm. The tubes were then placed vertically in an oven set to 136 °C for 48 continuous hours. Subsequent to heating, samples were positioned vertically and cooled to −5 °C for 24 h. Each tube was then divided into three equal segments (upper, middle, and lower), and softening point tests were conducted on the upper and lower sections of each sample. The asphalt binder was considered stable if the difference in softening point between the parts was less than 2.5 °C. Greater differences indicated inadequate storage stability and the occurrence of phase separation.

2.5. Temperature Susceptibility

Higher ambient temperatures have direct effects on the performance of road surfaces, and increased traffic volume further strengthens these effects. In this perspective, the change in the rheological properties of asphalt binder due to temperature fluctuations are generally referred to as temperature susceptibility, and it is a critical characteristic that must be carefully examined. This parameter is important because the behavior of asphalt binder is strongly affected by both temperature and loading rate, which influence its elasticity, hardness, and resistance to deformation. Consequently, two commonly implemented methods have been developed to evaluate the sensitivity of asphalt binder to thermal variation: the Penetration Index (PI) and the Penetration Viscosity Number (PVN) [24]. The PVN is used to quantify the binder’s resistance to the effects of temperature changes. It is calculated using the following mathematical equation:

$$\mathrm{P}\mathrm{V}\mathrm{N}={\displaystyle \frac{\mathrm{L}\mathrm{o}\mathrm{g}\left(\mathrm{L}\right)-\mathrm{L}\mathrm{o}\mathrm{g}\left(\mathrm{X}\right)}{\mathrm{L}\mathrm{o}\mathrm{g}\left(\mathrm{L}\right)-\mathrm{L}\mathrm{o}\mathrm{g}\left(\mathrm{M}\right)}}(-1.5)$$

(1)

• L: Softening point penetration of asphalt binder. The consistency of the binder is essential for comprehending its performance at elevated temperatures.
• X: Penetration at 25 degrees Celsius. This serves as a standard for evaluating binder performance across various thermal conditions.
• M: The penetration of asphalt binder at elevated temperatures, typically at the upper limit of its practical temperature range. This value quantifies the sensitivity of binder temperature.

This research emphasizes the importance of comprehending how penetration levels at varying temperatures influence asphalt binder performance and durability by delineating this relationship. Engineers and researchers utilize this knowledge to select and create pavement binders that satisfy performance criteria. The results of the softening point test were used to measure the PI for base and modified binders. The conventional technique considers the penetration value at the softening point as equivalent to its value at 25 °C. However, relying on such an assumption might compromise the accuracy of the calculated PI. To calculate the PI more precisely to obtain reliable results, it is suggested to measure penetration at two different temperatures. The protocol stipulates that the log data for the penetration values be plotted against the test temperature. This often results in a linear relationship, and the slope of this line is used to determine the PI by the following equation:

$$\mathrm{P}\mathrm{I}={\displaystyle \frac{1952-500\left(\right(\log \mathrm{p}\mathrm{e}\mathrm{n})-20\mathrm{S}.\mathrm{P})}{50\left(\right(\log \mathrm{p}\mathrm{e}\mathrm{n})-\mathrm{S}.\mathrm{P}-120)}}$$

(2)

2.6. DSR Test Procedure

A Dynamic Shear Rheometer was used to investigate the performance of the asphalt binder under a range of thermal conditions, in accordance with AASHTO-T315 standard [25]. This device measures the complex shear modulus (G*), which refers to the stiffness of the material, and the phase angle (δ), which indicates elasticity, thereby providing an indication of the viscoelastic behavior of the asphalt binder. The DSR testing included various conditions, including frequency sweep testing with a time–frequency sweep from 1 to 100 rad/s (0.1 to 15 Hz), using a temperature range from 30 to 70 °C. The Multiple Creep Stress and Recovery (MSCR) test was carried out at a constant temperature of 64 °C. The MSCR test consists of 10 test cycles at two stress levels: 100 and 3200 Pa, to determine the recovery ability of the asphalt binder when subjected to repeated stresses.

2.7. FTIR Test Procedure

To explore the functional and chemical properties of the base and modified asphalt binder, FTIR testing was performed [26]. This test measures the infrared spectral absorption of chemical bonds within a given material, with each functional group producing its characteristic spectrum at specific wavelengths. FTIR is an effective instrument used to visualize structural changes in asphalt binder, and this effect can be seen in the formation of carbonyl (C=O) and/or hydroxyl (OH) functional groups, or in the disappearance or change in the intensity of peaks related to the original bitumen components [27]. FTIR has been widely used in previous studies to assess the modification process, with this utility specifically attributed to its fast detection capabilities, sensitivity, and reliable and repeatable methods. This test has also been used to assess the effects of various aging processes, including thermal, long-term, and short-term aging. Additionally, FTIR has been used to examine the chemical modifications to asphalt that occur upon the incorporation of additive materials such as polymers or different types of biomaterials. In this study, FTIR was used to examine the chemical interaction resulting from the addition of BPSC.

2.8. Surface Energy Test Procedure

The test was used to measure the effects of the asphalt binder’s surface energy on the moisture damage resistance of asphalt. In this study, the surface free energy is defined as the energy required to expand the surface phase of asphalt binder. This is an important measurement for understanding the adhesion behavior between bitumen and aggregate, as surface energy represents the measure of adhesion between solids and liquids. A conventional digital contact angle meter was used with DROP image analysis software version 2.0 to measure surface energy with a high rate of precision. A drop of distilled water was carefully placed onto a solid surface with a thin film of asphalt binder, and the contact angle observed from the liquid to the solid phases was recorded and measured. The surface free energy components of the asphalt binder and filler were measured using the sessile drop and column-wick tests as described by Tan and Guo [28]. The contact angle measurements were used to determine the material surface energy components.

2.9. SEM Test and Analysis of Procedure

The SEM was utilized to investigate the microstructure of modified asphalt binders, and to examine the distribution of particles as well as observe the differences in surface composition and microstructure due to the additive (BPSC). SEM is a specialized method of analysis to produce high-resolution images of surfaces and assesses the mechanical and rheological behavior of asphalt binders based on their internal structure. The SEM works by focusing a beam of electrons onto the surface of the sample at a magnification thousands of times greater than that of a traditional optical microscope. These electrons interact with the sample’s atoms, resulting in signals that are produced and analyzed to create detailed images of the surface with contrast. In this work, SEM analysis was used to evaluate the BPSC distribution in the asphalt and to analyze whether any agglomeration or structural changes were observed with the different additive dosages.

3. Results

3.1. Homogenization Evaluation

Figure 2 shows the relationship between softening point and the blending time (minutes) for asphalt binders modified with different concentrations of BPSC, namely 2%, 4%, 6%, and 8%. The softening point is utilized as an indirect indicator to measure the efficiency of the mixing process and the degree of homogeneity achieved in the blend. The results show that the softening point increased with blending time up to 60 min for all modified samples, followed by a slight decrease of up to 120 min. Among the different concentrations, the 4%-BPSC-modified binder exhibited the highest softening point, reaching a peak of 51.8 °C after 60 min of blending, indicating improved homogeneity. Similarly, 6%-BPSC showed an increase in softening point up to 50.5 °C, although slightly lower than that of the 4%-BPSC blend. In contrast, the 2% and 8%-BPSC samples displayed lower softening points compared to the 4% and 6%-BPSC blends; however, the trend remained consistent, with 60 min appearing to be the optimal time in both cases. Based on these findings, it can be concluded that the optimal mixing time for achieving effective dispersion and thermal interaction between the asphalt binder and the BPSC modifier is 60 min. Furthermore, the 4%-BPSC concentration demonstrated the most balanced performance in terms of thermal response and mixture homogeneity.

3.2. Effects of BPSC on the Consistency Test

The results of the penetration test for both base and BPSC-modified asphalt samples are presented in Figure 3. The penetration test is an important indicator of asphalt hardness: higher values suggest a softer binder, while lower values indicate increased stiffness. The base binder exhibited the highest penetration value at 94.93. Upon the addition of 2%-BPSC, the value decreased sharply to approximately 90.02 and further reduced to 90.02 at 4%-BPSC. A slight increase was observed at 6%-BPSC, with a penetration value of 92.02, followed by the lowest recorded value of 89.54 at 8%-BPSC.

This reduction in penetration values indicates that increasing BPSC content generally enhances the stiffness of the asphalt binder, contributing to reduced temperature susceptibility, a favorable outcome for improving resistance to permanent deformation. However, the trend was not entirely linear; the slight increase in penetration at 6%-BPSC suggests possible variability in the dispersion or interaction of the modifier within the asphalt matrix.

Ductility is a key indicator of the elasticity of asphalt binders and their ability to elongate without breaking. This property is important for resisting the cracks caused by thermal expansion and contraction. The test results (Figure 3) showed that the base binder exceeded the required ductility limit, achieving a value greater than 100 cm, which reflects its high flexibility. With the addition of BPSC, a gradual reduction in the ductility values was observed. Specifically, it measured at 109 cm (22.14% decrease) at 2%-BPSC, 107 cm (23.57% decrease at 4%-BPSC, 88 cm (37.14% decrease) at 6%-BPSC and 85 cm (39.29% decrease) at 8%-BPSC. These findings indicate that BPSC directly influences the binder’s flexibility, with higher concentrations leading to a notable decline in elongation. This behavior can be attributed to the presence of fine particles, which might restrict the movement of binder chains, resulting in the reduction in its length, although that may restrict the mobility of binder chains, thereby reducing the material’s ability to stretch. While this reduction in ductility may enhance the binder’s stiffness and resistance to permanent deformation, it could also compromise low-temperature cracking resistance. Therefore, controlling the concentration of BPSC in the binder is essential for achieving a balance between stiffness and flexibility. The results indicate that lower concentrations, specifically 2%- and 4%-BPSC, can provide a favorable balance of mechanical properties without significantly compromising ductility. Therefore, this adjustment clarifies the findings, enhancing audience comprehension and supporting the study’s conclusions about the performance of BPSC-modified asphalt binders.

The softening point test results (Figure 3) indicate that the neat binder value without any additive was 45.75 °C. Adding 2%-BPSC of the additive increased the softening point by nearly 9.3%, reaching 50 °C. The highest value was recorded with a 4%-BPSC additive, at 52 °C, representing an increase of roughly 13.6% over the base binder and indicating the best performance at this dosage. In contrast, the softening point dropped to 46.5 °C with a 6%-BPSC additive, showing only a minimal increase of 16% compared to the neat binder. With an 8%-BPSC additive, the value rose again to 49.5 °C, an increase over the baseline, but remained below the 4%-BPSC sample. Thus, reinstating the SP value to one decimal place enhances the clarity, precision, and uniformity of paper reporting. This enhancement elucidates and comprehends research outcomes, augmenting quality and professionalism.

3.3. Effects of BPSC on Mass Loss

This test method is used to evaluate the characteristics of certain petroleum-based materials by measuring their mass loss under standardized heating conditions [29]. Figure 4 presents the results of loss on heating for base and modified asphalt binders. The base asphalt binder exhibited a mass loss of 0.257%, with the addition of 2%, 6%, and 8%-BPSC, the values increased to 0.369, 0.495 and 0.387, respectively, indicating a rise in volatile content or partial disintegration of some BPSC particles at these concentrations. In contrast, the binder modified with 4%-BPSC showed the lowest mass loss value of 0.0815%, reflecting a significant improvement in the thermal stability at this specific dosage. These findings indicate that the effectiveness of BPSC as a modifier in enhancing aging resistance of asphalt binder is not solely dependent on the quantity added. Rather, the performance is closely influenced by the specific concentration used. The 4%-BPSC dosage appears to achieve an optimal balance, minimizing the volatility of compounds and improving the thermal stability under elevated temperatures. Conversely, higher concentrations may produce adverse effects, potentially due to the decomposition or evaporation of certain components of the additive.

3.4. Effects of BPSC on Viscosity

Figure 5 illustrates the relationship between the temperature and viscosity of the base and BPSC-modified asphalt binders. Viscosity is a critical parameter used to assess the workability of asphalt binders, particularly their suitability for mixing, pumping, and compaction operational temperatures. All samples exhibited typical flow behavior, with viscosity decreasing as temperature increased—an expected trend due to the reduced internal resistance of bitumen at elevated temperatures. The viscosity of the base asphalt binder was measured at 0.39 Pa·s at 135 °C, decreasing to 0.15 Pa·s at 165 °C. With the addition of BPSC, an increase in viscosity was observed across all testing temperatures compared to the base binder. The binder modified with 8%-BPSC showed the highest viscosity values reaching about 0.47 Pa·s at 135 °C. This increase in viscosity indicates that BPSC enhances the internal structure of bitumen, thereby improving its resistance to flow. While higher viscosity generally indicates better resistance to deformation, an excessive increase can negatively affect the workability and compaction of asphalt mixtures. Nevertheless, all modified binders met the viscosity requirements, remaining below the standard limit of 3 Pa·s at 135 °C. Similar findings were reported by Djurekovic and Mladenovic [30], who observed that the addition of fly ash to bitumen also resulted in increased viscosity.

3.5. Effects of BPSC on Storage Stability

Figure 6 presents the temperatures recorded at the top and bottom sections of base and BPSC-modified asphalt binders. This analysis was conducted to assess material distribution and potential phase separation within the asphalt matrix, providing insights into the homogeneity of the blends. The results show that all binders met the required criterion: the temperature difference between the top and bottom section was less than 2.5 °C. Notably, the base binder exhibited the lowest softening point, while the 4% and 6%-BPSC-modified samples recorded the highest softening points in both sections. The findings indicate that no phase segregation occurred at elevated temperatures and the BPSC was uniformly distributed throughout the asphalt matrix. This confirms the stability and homogeneity of the modified binders under thermal conditioning.

3.6. Effects of BPSC on Temperature Susceptibility

Figure 7 illustrates the effect of varying BPSC concentration on the Penetration Index (PI) and Penetration Viscosity Number (PVN), in comparison to the base asphalt binder. The results show that the base binder exhibited the lowest values for both indices, −0.72 for PI and 0.671 for PVN, indicating poor resistance to thermal fluctuation and aging. With the addition of BPSC, both PI and PVN values gradually improved. The binders modified with 2%, 4%, and 6%-BPSC demonstrated notable enhancement in both indices. In particular, the 8%-BPSC achieved the most significant improvement, recording a PVN of −0.013 and highest PI value of −0.22. These results suggest that BPSC contributes to strengthening the asphalt binder’s internal structure, thereby reducing its sensitivity to heat and aging. Although all the modified binders exhibited better thermal performance, the 8%-BPSC formulation showed the highest thermal stability, with PI and PVN values approaching zero, an indication of improved long-term durability. Nevertheless, while the physical indices such as PI and PVN provide useful insights, they should not be considered highly precise standalone indicators of binder performance. Further rheological and chemical evaluations are necessary to fully understand the behavior of modified asphalt under real-world conditions.

3.7. SEM

Energy Dispersive Spectroscopy (EDS) is a popular and effective technique for elemental analysis, especially in the study of organic materials such as asphalt. It is used to identify key elements such as carbon, hydrogen, and nitrogen. Figure 8 shows an electron image of the base asphalt binder taken using SEM, which was subsequently suggested to EDS analysis to reveal the elemental composition. Elemental analysis plays a pivotal role in assessing the chemical structure and purity of modified or manufactured materials, as it helps determine the homogeneity of the elemental distribution and conformity to the expected properties. During the EDS test, ZAF correction (which takes into account the effect of atomic number Z, absorption A, and fluorescence F) was applied to improve the accuracy of the analysis results and determine the elements with high precision. The results showed that carbon (C) was the dominant element in the composition of the binding material, constituting 92.42% of the sample, while sulfur (S) ranked second with a percentage of about 4.84%. The high carbon content highlights the predominantly organic character of the binder and emphasizes its essential role in the structural and rheological composition of bitumen.

Figure 9 shows the size distribution of BPSC particles, whose diameters range between 9.49 and 14 μm. The microscopic image clearly shows that the BPSC particles are regularly and homogeneously distributed within the asphalt matrix, indicating the quality of the mixing and homogeneity process. This uniform distribution is an important factor in enhancing the performance of the asphalt binder, as it contributes to achieving microstructure stability and reduces the possibility of weak areas or clusters forming. The relatively high surface area of the BPSC particles also enhances the interaction between the modifier and asphalt binder, resulting in stronger physical bonds and improved integration into the asphalt mixture. Therefore, the uniformity of BPSC particle distribution is a positive indicator of modification effectiveness, and directly contributes to improving the rheological and mechanical properties of modified binders, especially in terms of deformation resistance and thermal stability.

3.8. Surface Energy of Asphalt Binder

The interface energy between the binder and water is denoted by γBW, the solid surface energy between the binder and air by γBA, the liquid surface energy between air and water as γAW, as shown in Figure 10. Figure 11 presents the surface energy results for base binder and those modified with varying BPSC contents. The data reveal a gradual decrease in surface energy values as the BPSC content increases. While the base binder exhibited the highest surface energy, all modified binders showed lower values. Specifically, the binder containing 4%-BPSC showed a reduction of approximately 23.32%, and the 6%-BPSC sample exhibited a decrease of about 25.32%, both relative to the base binder. Interestingly, the binder with 8%-BPSC displayed a slight increase in surface energy compared to the 6% sample, although it remained below the value of the base binder. The evaluation underscores that the incorporation of BPSC diminishes interface energy and enhances pavement durability. This enhancement corroborates the study’s conclusions that BPSC modifiers facilitate asphalt applications.

These findings signify that the incorporating BPSC can influence the binder’s surface characteristics, potentially enhancing its ability to coat and bond with aggregates, an important factor in improving the performance of asphalt mixtures. Previous research has also highlighted that surface free energy is a valuable indicator for evaluating the binder–aggregate compatibility, particularly in relation to resistance to moisture-induced damage [31].

3.9. Fourier Transform Infrared Spectroscopy

The FTIR results of base and BPSC-modified asphalt binders are presented in Figure 12. This analysis was conducted to identify the chemical structural changes resulting from the incorporation of BPSC by detecting the active functional groups in the material. All samples exhibited prominent peaks at characteristic wavelengths, indicating the presence of common functional groups. Distinct peaks were observed at 2920 cm⁻¹, 2851 cm⁻¹, and 2915 cm⁻¹, corresponding to the stretching vibration of the –CH₂ and -CH₃ groups, which represent the basic carbon skeleton of the asphalt binder. Additional peaks at 1606 cm⁻¹ and 1459 cm⁻¹ can be attributed to C=C bonds in aromatic rings and CH-bending vibrations. Peaks at 1376 cm⁻¹, 1017 cm⁻¹, and 721 cm⁻¹ are associated with C-O and C-H group vibrations and other components, indicating intercalation between the bitumen and additive. Comparing the spectra of the base and modified binders reveals slight variations in the absorption intensity of certain peaks, particularly as the BPSC content increases. These changes support the hypothesis of physical or chemical interaction between the BPSC and bitumen. While no entirely new bonds were formed, the shifts in absorption intensity in specific regions suggest alterations to the internal molecular structure, which may influence the binder’s rheological and mechanical properties. The FTIR analysis results thus indicate that BPSC interacts with bitumen at the molecular level without forming new compounds, supporting the notion that it functions as an effective physical modifier, enhancing performance without compromising the binder’s fundamental structure.

Additionally, minor peaks observed in the 675–900 cm⁻¹ range correspond to C–H vibrations associated with the benzene ring. Notably, the FTIR spectra of the BPSC-modified asphalt binders did not show any major new functional groups, except for a slight peak at 721 cm⁻¹. The incorporation of BPSC, which contains carbonyl functional groups, appears to contribute to an increased level of oxidation within the binder. Therefore, it is reasonable to infer that higher BPSC content may lead to a corresponding rise in carbonyl group concentration, potentially affecting the aging characteristics of the asphalt binder.

3.10. Effects of BPSC on the Rheological Properties of Asphalt

3.10.1. Failure Temperatures

Figure 13 illustrates the impact of BPSC on the failure temperature of bitumen. The failure temperature is a critical parameter used to assess the high-temperature performance of bitumen, representing the upper temperature limit beyond which the binder loses its ability to resist permanent deformation. The base binder exhibited the lowest failure temperature, recorded at 64 °C. In contrast, the BPSC-modified samples demonstrated clear improvement. The binder with 2%-BPSC reached 66 °C, marking a 3.12% increase, while the 6%-BPSC sample achieved 68 °C, an improvement of 6.25%. The 8%-BPSC formulated recorded of 67 °C, corresponding to 4.69% enhancement. Notably, the 4%-BPSC sample showed the best performance, with a failure point of 70 °C, representing a 9.38% improvement over the base binder. This result indicates significant enhancement to the bitumen’s resistance to high-temperature failure. These findings align with the isochronal plots, where the addition of BPSC, in particular, to 4%-BPSC level, was shown to enhance the thermal performance of the binders and increase their resistance to permanent deformations. Moreover, the slight decline in the performance at 6% and 8%- BPSC, compared to 4%-BPSC sample, indicates that 4% represents an optimal additive concentration. Beyond this point, the increased dosage may yield diminishing returns, potentially due to material saturation or alterations in the internal structure of the binder.

3.10.2. Isochronal Plots

Figure 14 illustrates the variation in the complex shear modulus (G*) with temperature for both the base and BPSC-modified asphalt binders. The isochronal plot is a key rheological indicator used to evaluate a binder’s resistance to permanent deformation under repeated loading, particularly at intermediate and high temperatures. The results show that all modified samples outperformed the base binder, indicating a notable enhancement in asphalt stiffness due to the addition of BPSC. Among the modified samples, the binder containing 4%-BPSC exhibited the highest G* values across the tested temperature range, followed by the 6% and 8%-BPSC blends. The 2%-BPSC samples also showed improved performance over of base binder, though its stiffness remained lower than that of the other modified formulations. As seen in Figure 14, all samples exhibited a gradual decline in G* with increasing temperature, a typical behavior of asphalt materials, resulting from the reduction in viscosity and deformation resistance to thermal fluidity increase. However, the consistently higher G* value observed for the 4%-BPSC binder across all the temperatures indicates a stronger resistance to structural breakdown under cyclic loading. These findings confirm that using BPSC as a modifier can significantly improve the rheological behavior of asphalt binder, particularly by enhancing resistance to rutting. This improvement serves as a positive indicator of better pavement performance under high-temperature conditions.

Figure 15 presents the phase angle (δ) results obtained through isochronal analysis across a range of temperatures. This phase angle is a key rheological parameter that reflects the balance between the elastic and viscous components of bitumen. A higher δ value indicates a more viscous, less elastic response, while a lower value suggests a more elastic material, which is desirable for resisting permanent deformation. The base binder exhibited consistently high phase angle values across all tested temperatures, indicating predominantly viscous behavior and reduced elasticity. This may indicate a weakness in its resistance to permanent deformities. Moreover, all modified samples showed lower phase angle values compared to the base binder, suggesting an improved ability to recover from deformation and enhanced resistance to rutting and other forms of permanent deformation under load.

In contrast, all BPSC-modified samples demonstrated lower phase angle values compared to the base binder. Notably, the binder with 4%-BPSC showed the most favorable performance, maintaining the lower δ values across the tested temperature range. This behavior indicates an improved rheological balance between elasticity and viscosity, contributing to enhanced resistance to rutting and flow under elevated temperature conditions.

3.10.3. Master Curve

The master curve of the complex shear modulus (G*) versus reduced frequency for both the base and BPSC-modified asphalt binders is presented in Figure 16. These master curves were developed from data obtained using a DSR, which measures the rheological properties of asphalt binders across a range of temperatures and loading frequencies. Using the Time–Temperature Superposition Principle (TTSP), the data were shifted into the frequency domain. Overall, the G* results show that all BPSC-modified samples maintained higher G* values compared to the base binder across all frequencies, indicating a clear improvement in the dynamic stiffness of the bitumen. Among the modified binders, the 4%-BPSC sample consistently showed the highest G* values, followed by 6% and 8%-BPSC samples. Although the 2%-BPSC sample showed smaller effects compared to the others, it still outperformed the base binder. The steady increase in G* with frequency indicates that the binder becomes stiffer under higher loading conditions, while maintaining more flexible behavior at lower loading rates. This is a typical of modified bituminous materials. These results demonstrate that BPSC enhances the bitumen’s resistance of permanent deformations under dynamic loading and improves pavement performance under varying traffic conditions. Therefore, the incorporation of BPSC improves the rheological behavior of asphalt binder across wide range of temperatures and loading times. Among the samples tested, the 4%-BPSC formulation provided the best balance between stiffness and elasticity. A similar observation was made by Djurekovic and Mladenovic [30], who reported that the adding fly ash as a filler improved rutting resistance due to its hydrodynamic interaction with the binder.

3.10.4. Rutting Parameter Performance

Rutting remains one of the primary concerns affecting the performance of asphalt pavement, particularly in high-temperature environments. Figure 17 illustrates the rutting factor (G*/sin δ) for both the base and BPSC-modified asphalt binders. This parameter, derived from the frequency sweep test using a DSR, serves as a key inductor for evaluating rutting resistance under elevated temperatures. The results follow a trend similar to that observed in the master curves, with all modified binders exhibiting higher G*/sin δ values compared to the base binder across the tested temperature range, which led to an improvement in resistance to rutting. Among the modified samples, the binder containing 4%-BPSC showed the best performance, maintaining the highest stiffness across all temperatures. It was followed by 6% and 8%-BPSC blends, which showed similar improvements and then by the 2%-BPSC sample, which presented a noticeable enhancement relative to the base binder. As shown in the curves, all samples exhibited a decreasing trend in the rutting factor with increasing temperature. This behavior is expected due to reduced stiffness and increased elasticity of bitumen at higher temperatures, which lower its resistance to thermal deformation. Nevertheless, the BPSC-modified samples retained higher stiffness across the temperature spectrum, indicating that the additive contributed to strengthening the internal structure of binder. Based on these findings, it is evident that incorporation of BPSC significantly improves the rutting resistance of asphalt binders, particularly at the 4%-BPSC dosage, which offered the most favorable balance between rheological strength and operational performance at elevated temperatures.

3.10.5. Creep and Recovery

Creep refers to the gradual, time-dependent deformation of the asphalt binder under constant stress, indicating the development of permanent strain within the material [32]. Similarly, the creep and recovery test assesses the extent of irreversible defamation after the applied stress is removed [33]. The elastic recovery ratio is calculated by subtracting the final strain from the peak strain and dividing the result by the peak strain [34]. In this investigation, the MSCR test was conducted at 64 °C using the DSR. Specimens were prepared with a 25 mm and a 1 mm gap and were subjected to a controlled loading sequence consisting of a 1 s creep phase and followed by a 9 s recovery phase. A total of 10 creep-cycles were applied at stress levels ranging from 100 to 3200 Pa. Figure 18 shows the compliance of base and BPSC-modified asphalt binders under a low-stress level at 100 Pa. This parameter reflects the number of deformations produced in a specimen when a constant load is applied; the higher the value, the lower the material’s resistance to deformation. The results indicate that the base asphalt binder exhibited the highest creep compliance, measured at 9.49 1/Pa. The addition of BPSC resulted in a gradual reduction in compliance: 8.49 1/pa for 2%-BPSC, 6.84 1/Pa for 6%-BPSC, and 5.93 1/Pa for 8%-BPSC. Notably, the 4%-BPSC binder recorded the lowest value, at approximately 4.26 1/Pa, indicating the greatest resistance to permanent deformation under low-stress conditions. Figure 19 also presents the result at high-stress level (3200 Pa), which simulates more severe loading conditions, such as those caused by heavy traffic. As observed under low stress, the base binder showed the highest accumulated strain, 381.36 1/pa, demonstrating a clear deficiency in deformation resistance. In contrast, the 4%-BPSC binder again showed the best performance, with a significantly lower strain value of 190.41 1/Pa. This was followed by 2%-BPSC (226.56 1/Pa), 8%-BPSC (275.19 1/Pa) and 6%-BPSC (334.28 1/Pa). These results confirm that the addition of BPSC enhances the rheological performance of asphalt binder under both low- and high-stress levels, which indicates the effectiveness of the additive in improving the rutting resistance. The consistent reduction in creep compliance with increasing BPSC content, particularly the optimal performance at 4%-BPSC, demonstrates the effectiveness of BPSC in improving rutting resistance and achieving a favorable balance of viscoelastic properties.

4. Conclusions

The findings of this study showed that the incorporation of BPSC into bitumen has a positive and significant impact on its physical and rheological properties, confirming the effectiveness of BPSC as a modifier in asphalt applications. Among the various concentrations tested, the addition of 4%-BPSC produced the most notable improvements across several key performance indicators when compared to the base binder. For instance, the penetration value decreased by 4.5% (from 94.93 to 90.67), and the elongation decreased by 22.1% (from 140 cm to 109 cm), indicating an increased stiffness while maintaining acceptable flexibility. In terms of thermal performance, the failure temperature increased from 64 °C to 70 °C, representing a 9.4% enhancement in high-temperature resistance and reduced susceptibility to permanent deformation. The heating mass loss test further confirmed a significant improvement, showing a 68.3% reduction at 4% BPSC, which reflects excellent thermal stability. The FTIR analysis revealed changes in peak intensities without the appearance of functional groups, confirming a physical interaction between the bitumen and BPSC rather than chemical bonding. Rheological evaluation supported these findings, as the rutting parameter (G*/sin δ) increased by more than 2.5 times at certain temperatures, indicating a substantial improvement in rutting resistance. Additionally, MSCR results showed a 55.1% reduction in non-recoverable compliance at 100 Pa and a 50.1% reduction at 3200 Pa, confirming enhanced resistance to permanent deformation even under high stress. Overall, the results indicate that 4%-BPSC concentration represents the optimal dosage, providing a well-balanced combination of stiffness, elasticity, thermal stability, and rheological performance. This makes the modified binder particularly suitable for asphalt paving in environments subjected to heavy traffic loads and elevated temperatures.