Effect of Nanoclay on the Performance Characteristics of SBS-Modified Asphalt Concrete Mixtures

Abstract

This study examined the synergistic effects of Styrene–Butadiene–Styrene (SBS) polymer and nanoclay on asphalt concrete mixture performance through a systematic experimental program using 4.5% SBS with varying nanoclay concentrations (0–8%). Performance evaluation included Indirect Tensile Strength (ITS), Indirect Tensile Resilient Modulus (E_RI), and Hamburg Wheel Tracking Tests (HWTT), along with novel quantitative analysis of visco-plastic and moisture resistance indices. Results demonstrated that 4.5% SBS with 6% nanoclay (4.5S6N) yielded optimal performance, achieving 38% increase in dry ITS, 68% improvement in wet ITS, and enhanced moisture resistance with Tensile strength Ratio (TSR) improving from 79.53% to 97.14%. The E_RI value increased by 39%, while rutting resistance improved by 39.3%. At this optimal concentration, nanoclay’s uniform dispersion and layered silicate structure created an effective reinforcement network, enhancing stress distribution and interfacial bonding with the SBS polymer network and asphalt components. However, exceeding 6% nanoclay content led to performance deterioration due to particle agglomeration. These findings demonstrate that optimized SBS–nanoclay modification effectively addresses both mechanical and moisture-related performance requirements for modern pavement applications.

1. Introduction

Asphalt concrete remains a cornerstone material in pavement construction due to its cost-effectiveness, durability, and ability to provide a smooth, impermeable surface. However, the increasing demands of modern transportation infrastructure, including higher traffic volumes, heavier loads, and more extreme climate conditions, have exposed the limitations of traditional asphalt mixtures [1]. To address these challenges, various modifications to asphalt binders have been investigated, among which SBS-modified asphalt has emerged as one of the most promising solutions.

SBS-modified asphalt is a polymer-modified binder that incorporates SBS, a thermoplastic elastomer, to enhance the mechanical and rheological properties of asphalt concrete. The unique biphasic microstructure of SBS, composed of polystyrene (PS) end-blocks and polybutadiene (PB) mid-blocks, creates a three-dimensional network structure within the asphalt matrix. This network imparts elasticity, strength, and resistance to deformation to the asphalt mixture [2,3,4]. The polystyrene domains act as physical crosslinks, providing strength and stability, while the polybutadiene segments contribute elasticity and flexibility to the modified binder.

The benefits of SBS modification are extensively documented in the literature and span a wide range of performance characteristics. At high temperatures, SBS-modified asphalt demonstrates superior resistance to permanent deformation (rutting), a critical property for pavements in regions experiencing intense summer heat [5]. The enhanced elasticity provided by the PB blocks significantly improves the binder’s resistance to low-temperature cracking, making it particularly suitable for cold climate applications [6]. Furthermore, SBS modification substantially enhances fatigue resistance under repeated loading conditions, a crucial factor in extending pavement service life [7]. The modified binder also exhibits improved aging resistance and superior adhesion to aggregates, contributing to increased pavement durability and reduced maintenance requirements [8].

Traditional asphalt pavements face numerous challenges in service, including thermal cracking, fatigue damage, moisture-induced deterioration, and rutting. These distresses are particularly pronounced under extreme weather conditions, heavy traffic loads, and prolonged exposure to environmental factors such as ultraviolet radiation and oxidation [9]. While SBS modification effectively addresses many of these issues, certain challenges persist. The high cost of SBS polymer and its potential for phase separation during storage and handling pose practical limitations for widespread implementation [2,10,11]. Moreover, moisture susceptibility remains a significant concern, as water infiltration can weaken the adhesive bond between the binder and aggregates, leading to premature pavement failure [12].

To address these limitations, researchers have explored various complementary modification techniques, with nanomaterial modification emerging as a particularly promising approach [13]. Among the various nanomaterials studied, including nano-silica, carbon nanotubes, and graphene, nanoclay has garnered significant attention due to its unique properties and potential for synergistic effects when combined with polymer modifiers [14]. Nanoclay, particularly montmorillonite-based materials, possesses a layered silicate structure with high surface area, aspect ratio, and cation exchange capacity, making it highly effective in modifying the rheological and mechanical properties of asphalt binders [15,16,17].

The mechanism through which nanoclay enhances asphalt properties is multifaceted and involves both physical and chemical interactions. When properly dispersed within the asphalt matrix, nanoclay forms a nanocomposite structure that can significantly improve the binder’s performance characteristics [18,19,20]. Through its layered structure, nanoclay absorbs maltene fractions from the binder, effectively reducing flow and enhancing stiffness [21]. This interaction particularly benefits rutting resistance at high temperatures and helps mitigate fatigue damage under cyclic loading. Additionally, the exfoliation of nanoclay layers creates a barrier effect that enhances the material’s resistance to moisture infiltration and oxidative aging [22,23].

The potential for enhancing asphalt performance through multiple modification strategies has led to increased interest in hybrid modification systems. Studies have explored various combinations of polymers, nanomaterials, and other additives to develop optimized binder formulations. For instance, researchers have investigated the combined effects of SBS with various types of nanomaterials, including nano-silica, carbon nanotubes, and graphene, reporting improvements in multiple performance indicators [20,22,24,25,26]. Similarly, the incorporation of anti-stripping agents and adhesion promoters alongside polymer and nano-modifications has shown promise in addressing moisture susceptibility issues [27].

Despite these advances, several knowledge gaps remain in understanding the complex interactions between SBS and nanoclay in modified asphalt systems. While individual studies have demonstrated improvements in specific properties, comprehensive research examining the combined effects on critical performance indicators, such as indirect tensile strength, moisture susceptibility, resilient modulus, and rutting resistance, remains limited. Furthermore, the optimization of modifier content and the development of effective modification protocols require additional investigation.

This research aims to bridge these knowledge gaps by conducting a comprehensive investigation of the synergistic effects of nanoclay and SBS on the performance characteristics of asphalt concrete mixtures. The study evaluates critical performance indicators through a series of tests, including the Indirect Tensile Strength Test (ITS), Resilient Modulus Test (E_RI), and rutting resistance using Hamburg Wheel Tracking Test (HWTT). Particular attention is given to moisture susceptibility assessment through the Tensile Strength Ratio (TSR) method, which quantifies the adverse effects of moisture on mixture strength. The findings are expected to contribute to the development of more durable, resilient, and cost-effective asphalt mixtures capable of meeting the demands of modern transportation infrastructure.

2. Materials and Methods

2.1. Base Asphalt Binders

The base asphalt binder used in this study was ARL 60/70 penetration grade asphalt, obtained from Attock Refinery Limited (ARL), Pakistan. This grade of asphalt is widely used in road construction throughout South Asia due to its suitable properties for the region’s climate conditions. The binder was characterized through standard testing procedures to establish its baseline properties. The penetration values at 25 °C were found to be 68.33 (0.1 mm), measured according to ASTM D5 [28]. The softening point, determined using the ring and ball method (ASTM D36 [29]), was 49.1 °C. The specific gravity of the base binder at 25 °C was measured as 1.02 following ASTM D70 [30] procedures, and its rotational viscosity at 135 °C was 0.35 Pa·s, determined using a Brookfield viscometer per ASTM D4402 [31].

Additional physical and rheological properties of the base binder included a ductility value of 125 cm at 25 °C (ASTM D113 [32]), a flash point of 233 °C (ASTM D92 [33]), and a fire point of 278 °C. The solubility in trichloroethylene was determined to be 99.3% (ASTM D2042 [34]), indicating the purity of the binder. These properties ensured that the base binder met the standard specifications for penetration-graded asphalt cement used in pavement construction.

2.2. SBS Polymer Modifier

The SBS polymer modifier selected for this study was YH-791H, whose basic properties are summarized in

Table 1. Technical properties of SBS polymer modifier.

Property	Value
Structure Type	Linear
Purity	>95%
Physical Form	White granules
Styrene/Butadiene Ratio	30/70
Solution Viscosity	2.24 Pa·s
Tensile Strength	20 MPa
Hardness	76 A

. The technical properties of the SBS polymer modifier were obtained directly from the manufacturer’s technical data sheet, as these values are provided for quality control and procurement purposes. The linear structure of this SBS polymer, combined with its high purity exceeding 95%, provides optimal conditions for polymer–asphalt compatibility. The styrene-to-butadiene ratio of 30/70 was specifically chosen as it represents an optimal balance between high-temperature performance (contributed by styrene blocks) and low-temperature flexibility (provided by butadiene segments). The solution viscosity of 2.24 Pa·s (measured at 25 °C in 5% styrene solution) ensures proper workability during the modification process, while the tensile strength is 20 MPa and Shore. The hardness of 76 A contributes to enhanced mechanical properties in the modified binder system.

2.3. Nanoclay

The nanoclay used in this study was a commercially supplied organo-modified montmorillonite powder, provided by the manufacturer with a specified platelet thickness of less than 100 nm and lateral dimensions in the submicron to several micron range. These dimensions are typical for nanoclay products employed in asphalt modification, where the performance enhancement arises from the dispersion of individual platelets or intercalated layers within the asphalt binder.

Prior to incorporation into the SBS-modified asphalt binder, the as-received nanoclay was passed through a No. 200 sieve (75 μm) to remove large agglomerates and ensure ease of handling. Under high-shear mixing, the layered silicate structure of the nanoclay is expected to exfoliate or intercalate, allowing for uniform dispersion of platelets at the nanoscale within the asphalt matrix.

Table 2. Technical properties of nanoclay.

Property	Value
Color	Greyish yellow
Montmorillonite Content	>75%
Moisture Content	Max. 10%
API Water Loss	Max. 15% cm3
pH	9.5
Particle Size Distribution	99% passing No. 200 sieve
Free Swell Index	600%
Liquid Limit	292%
Plastic Limit	48.55%
Shrinkage Limit	25.7%
Chemical Composition	Al2H2Na2O13Si4

summarizes the technical properties of nanoclay. The high montmorillonite content exceeding 75% is fundamental to its effectiveness as a modifier, as montmorillonite’s layered silicate structure provides extensive surface area for interaction with asphalt components. The material’s moisture content and consistency limits are carefully controlled to ensure stability during the modification process. The alkaline nature and chemical composition confirm its identity as sodium montmorillonite, making it particularly suitable for asphalt modification through its ability to form stable intercalated structures.

2.4. Aggregates

The aggregates used in this study were sourced from the central region of Khyber, Pakhtunkhwa, Pakistan, and consisted of both coarse and fine fractions. The aggregate gradation was followed by the NHA-B Class specifications, with a normal maximum aggregate size (NMAS) of 19 mm (

Table 3. Size gradation of aggregates and nominal maximum aggregates.

Sieve Size (mm)	19.0	12.5	9.5	4.75	2.38	1.18	0.075	Pan
Upper and lower boundary	100	75–90	60–80	40–60	20–40	5–15	3–8	-
Aggregate	100	82.5	70	50	30	10	5.5	-

). The physical properties of the aggregate were thoroughly evaluated to ensure compliance with AASHTO specifications (

Table 4. Physical properties of aggregate.

Property	Test Method	Coarse Aggregate	Fine Aggregate	Specification Limits
Bulk Specific Gravity	AASHTO T85 [35]	2.65	2.62	-
Water Absorption (%)	AASHTO T85 [35]	1.8	2.1	Max 2.5
Los Angeles Abrasion (%)	AASHTO T96 [36]	25.5	-	Max 30
Aggregate Crushing Value (%)	BS 812-110 [37]	22.3	-	Max 25

).

2.5. Mix Design and Sample Preparation

The asphalt mixture design followed the Marshall method (ASTM D1559 [38]), targeting 4% air voids, which is optimal for dense-grade asphalt mixtures in highway applications. The control mixture (ARL 60/70) and SBS–nanoclay modified binders were prepared using a two-stage process: (1) blending SBS polymer (4.5% by weight of base bitumen) into the base binder using a high-shear mixer at 5000 rpm for 60 min at 160 °C, and (2) incorporating nanoclay at 0%, 2%, 4%, 6%, and 8% by weight of bitumen. This mix design resulted in five distinct modified binder compositions, namely, 4.5S0N, 4.5S2N, 4.5S4N, 4.5S6N, and 4.5S8N. In these designations, the first number (4.5) indicating the SBS percentage, ‘S’ representing SBS, the final number indicating nanoclay percentage, and ‘N’ representing nanoclay. Aggregates were heated to the mixing temperature and proportioned to meet the selected gradation, mixed thoroughly with the binder, and compacted using a Marshall compactor (75 blows per face). Key parameters and results for the control mixture are presented in

Table 5. Technical properties of nanoclay.

Marshall Variables	Results	Specification Criteria	Status
Optimum Binder Content (OBC)	4.3%	-	Achieved
Bulk Specific Gravity	2.36	-	-
Voids in Mineral Aggregate (VMA)	13.6%	Min. 13%	Pass
Voids Filled with Asphalt (VFA)	70.3%	65%–75%	Pass
Stability	15.2 kN	Min. 8.006 kN	Pass
Flow	2.6 mm	2.0–3.5 mm	Pass

.

The 4.5% SBS content by weight of base bitumen was selected based on prior research findings, where it provided an optimal balance between high-temperature rutting resistance, low-temperature flexibility, and cost-effectiveness for this base asphalt [39]. In this study, maintaining a fixed SBS dosage allowed us to control the polymer variable and directly assess the synergistic effect of nanoclay with a proven, field-practical SBS level. While varying SBS content in combination with nanoclay may field additional insights, such factorial optimization is recommended for future research.

The nanoclay concentrations were systematically varied from 0% to 8% by weight of bitumen, based on previous studies [13,14] indicating that optimal performance is typically achieved between 4% and 6%, while contents exceeding 8% may cause particle agglomeration and performance deterioration. This range enables the identification of the peak performance level while maintaining practical feasibility in terms of cost and processing.

3. Experimental Testing Program

3.1. Indirect Tensile Strength (ITS) Test and Tensile Strength Ratio (TSR)

The indirect tensile strength (ITS) test was performed to evaluate the tensile strength of asphalt mixtures and their ability to resist cracking under tensile stress. This test is widely used to determine the performance and durability of asphalt mixtures, especially in assessing their resistance to thermal and fatigue cracking.

Cylindrical specimens were compacted using a Superpave Gyratory Compactor to achieve a target air void content, as recommended by ASTM D6931 [40]. Each specimen has a diameter of 100 mm and a thickness of 63.5 mm. The specimens were divided into two groups: dry group (unconditioned) and wet group (moisture-conditioned). For dry conditions, specimens were stored at room temperature (25 °C) without any additional moisture conditioning. For wet conditions, specimens were subjected to moisture conditioning to simulate the effects of water on the asphalt mixture’s performance. These specimens were submerged in a water bath maintained at 60 °C for 24 h. After water immersion, the specimens were air-dried at room temperature (25 °C) for at least 1 h to stabilize their temperature before testing.

The ITS test was conducted at a controlled temperature of 25 °C using an indirect tensile testing machine equipped with a loading fixture designed for cylindrical specimens. A vertical compressive load was applied along the diameter axis of each specimen at a constant deformation rate of 50 mm/min. The ITS value was determined using the following formula:

$$ITS={\displaystyle \frac{2P}{\pi tD}},$$

(1)

where P is maximum load at failure (N), t is thickness of the specimen (mm), and D’ is diameter of the specimen (mm). Three samples from each mixture group were tested, and the ITS values were averaged for the two groups, i.e., dry and wet conditions.

To evaluate the resistance of asphalt mixtures to moisture-induced damage, the tensile strength ratio (TSR) was calculated:

$$\mathrm{T}\mathrm{S}\mathrm{R}\left(\mathrm{\%}\right)={\displaystyle \frac{{\mathrm{I}\mathrm{T}\mathrm{S}}_{\mathrm{w}\mathrm{e}\mathrm{t}\_\mathrm{c}\mathrm{o}\mathrm{n}\mathrm{d}\mathrm{i}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}}}{{\mathrm{I}\mathrm{T}\mathrm{S}}_{\mathrm{d}\mathrm{r}\mathrm{y}\_\mathrm{c}\mathrm{o}\mathrm{n}\mathrm{d}\mathrm{i}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}}}}\times 100,$$

(2)

A TSR value greater than 80% indicates satisfaction resistance to moisture-induced damage in accordance with AASHTO T283 [41]. Specimens failing to meet this criterion are considered susceptible to moisture damage and may require modification through anti-stripping agents or alternative binder modification.

3.2. Indirect Tensile Resilient Modulus

The indirect tensile resilient modulus test was conducted followed ASTM D7369 [42] to evaluate the elastic response of asphalt mixtures under repeated loading conditions, which is a key parameter in assessing the structural performance and durability of pavements.

The resilient modulus test was carried out using an indirect tensile machine equipped with a cyclic loading system. A haversine load was applied along the vertical diameter of each specimen in a repeated loading sequence. Each cycle consisted of a 0.1-s haversine load pulse followed by a 0.9-s rest period, resulting in a total cycle duration of 1 s. The applied load was adjusted to ensure horizontal deformations within the elastic range between 0.01 mm and 0.05 mm. During the test, the horizontal deformations (${∆H}_I$) of the specimen were recorded using a Linear Variable Differential Transformer (LVDT) affixed to the specimen, while the applied load (P) was measured using a load cell integrated into the testing apparatus. For each mixture type, three specimens were prepared and tested for the resilient modulus measurement. The resilient modulus (E_RI) was calculated using the following:

$$E_{RI}={\displaystyle \frac{P·\left(v_{RI}+0.27\right)}{t·{∆H}_I}},$$

(3)

where P represents the peak load (N), $v_{RI}$ is Poisson’s ratio (assumed to be 0.35 for asphalt mixture), t is the specimen thickness (mm), and ${∆H}_I$ is the instantaneous recoverable horizontal deformation (mm).

3.3. Hamburg Wheel Tracking Test (HWTT)

The Hamburg Wheel Tracking Test (HWTT) was conducted to evaluate the rutting resistance and moisture susceptibility of asphalt mixtures under simulated traffic and environmental conditions. Cylindrical specimens with a diameter of 150 mm and a height of 62 mm were prepared using a Superpave Gyratory Compactor to achieve a target air void content, as recommended by AASHTO T324 [43]. The HWTT was conducted using a wheel tracking device equipped with a steel wheel at the conditioned temperature of 40 °C. The wheel applied a load of 705 N at a frequency of 26.5 ± 1 passes per minute. Each test continued until one of the following criteria was met: (i) a maximum of 20,000 passes, (ii) a rut depth of 12.5 mm, or (iii) visible structural failure of the specimen. Rut depth measurements were recorded continuously using Linear Variable Differential Transformers (LVDTs) positioned to track vertical deformation. Triplicated specimens were tested for each mixture type in the HWTT evaluation.

3.4. Visco-Plastic and Moisture Resistance Quantification

A novel quantitative analysis of HWTT results was performed in this study, which is crucial for accurately evaluating asphalt mixture performance, particularly in distinguishing the effects of mechanical deformation and moisture-induced damage [44]. The HWTT results were analyzed following a three-step process to quantify both visco-plastic resistance and moisture susceptibility of asphalt mixtures, as illustrated in Figure 1.

This advanced analysis method utilizes a 6th-order polynomial fitting approach to smooth the rutting data and determine the stripping inflection point (SIP), where moisture-related deterioration begins to dominate the deformation process. The first step in the analysis is fitting the raw rutting curve using a 6th-order polynomial regression to minimize irregularities in the test data (Figure 1a). The SIP is determined by analyzing the first derivative of the polynomial curve, as illustrated in Figure 1b. The SIP is located at the local minimum of the first derivative, marking the transition from visco-plastic to moisture-dominated deformation. The deformation before this point is purely mechanical, consisting of post-compaction and visco-plastic rutting.

To comprehensively assess rutting performance, total deformation is divided into three distinct regions (Figure 1c). The initial densification phase, attributed solely to compaction effects, is defined as post-compaction deformation (R_p). Visco-plastic deformation (R_v) represents the gradual accumulation of shear deformation in the asphalt mixture under loading. The deformation beyond the SIP is attributed primarily to moisture-induced deformation (R_m) (stripping).

To provide a quantitative measure of an asphalt mixture’s ability to withstand mechanical and moisture-related rutting, two indices are defined.

Visco-plastic resistance (VR) is calculated using the following equation:

$$VR={\displaystyle \frac{N_f-500}{R_v\left(N_f\right)}},$$

(4)

where

• $VR$ represents the visco-plastic resistance index.
• $N_f$ is the failure cycle at 12.5 mm rut depth (or test termination at 20,000 cycles if the failure does not occur).
• $R_v\left(N_f\right)$ is the visco-plastic deformation depth at $N_f$.

Moisture resistance (MR) is calculated using the following equation:

$$MR={\displaystyle \frac{N_f-500}{R_m\left(N_f\right)}},$$

(5)

where

• $MR$ is the moisture resistance index.
• $R_m\left(N_f\right)$ is the stripping-induced rut depth at $N_f$.

A high VR value indicates that the asphalt mixture has good shear resistance and will perform well under heavy traffic loads. A high MR value indicates strong binder-aggregate adhesion, reducing the likelihood of moisture damage. On the other hand, a low MR value reveals potential stripping failure, necessitating modification using hydrated lime, anti-stripping agents, or polymer modification. This methodology, by isolating moisture damage from mechanical deformation, provides a more precise evaluation of asphalt mixture durability.

4. Results and Discussion

The results of the ITS test are presented in Figure 2, illustrating the comparison of the ITS values obtained under dry (unconditioned ITS) and wet (conditioned ITS) conditions for various asphalt mixtures. The ITS values are critical indicators of the asphalt mixture’s resistance to tensile stresses, which is directly associated with cracking and moisture susceptibility.

The unconditioned ITS values demonstrate the tensile strength of the asphalt mixtures under optimal laboratory conditions, while the conditioned ITS values reveal the susceptibility of the mixtures to moisture-induced damage. As depicted in Figure 2, the ITS values for dry conditions are consistently higher than those for wet conditions, which is expected due to the adverse effects of water immersion on the adhesive bond between the asphalt binder and aggregates.

For all modified asphalt mixtures, there is a noticeable improvement in both dry and wet ITS values compared to the control sample. This enhancement can be attributed to the presence of modifiers such as SBS and nanoclay, which improve the cohesion and elasticity of the asphalt binder, thus strengthening the mixture’s resistance to tensile forces.

4.1. Effect of SBS Modifiers on ITS Values

Mixtures modified with 4.5% SBS alone (4.5S0N) showed a significant increase in ITS, particularly in dry conditions, indicating the role of SBS in improving the tensile strength and flexibility of the binder. This can be attributed to several key mechanisms associated with the unique properties of SBS.

SBS serves as a paramount polymer modifier in asphalt technology, fundamentally transforming the mechanical and rheological properties of asphalt mixtures through its sophisticated molecular architecture [27,39,45,46]. The complex interplay between its structural components creates a synergistic effect that enhances both the tensile strength and flexibility of modified asphalt systems [27,45].

At the molecular level, SBS exhibits a triblock copolymer configuration, where polystyrene (PS) domains occupy terminal positions, flanking a central polybutadiene (PB) segment [27,46]. This precise arrangement manifests a two-phase morphology that governs the material’s performance characteristics. The polystyrene end-blocks, characterized by their high glass transition temperature, undergo molecular association to form discrete, glassy domains. These domains function as thermally reversible physical crosslinks, establishing a three-dimensional network structure that significantly augments the mechanical stability of the modified binder.

Concurrently with the structural framework provided by the PS domains, the polybutadiene mid-blocks, distinguished by their low glass transition temperature, create a continuous elastomeric phase. This phase exhibits remarkable conformational flexibility, enabling rapid molecular reorganization in response to applied stresses. The rubber-like characteristics of the PB segments facilitate elastic deformation while maintaining structural integrity through the anchoring effect of the PS domains.

Therefore, the enhancement in ITS values can be attributed to this sophisticated molecular architecture. When subjected to tensile loading, the PS domains act as stress concentration points, effectively distributing applied forces throughout the polymer networks. Simultaneously, the flexible PB segments accommodate induced strains through reversible chain extension and reorientation, preventing localized stress accumulation that could lead to crack initiation and propagation. This molecular-level stain distribution mechanism results in superior macroscopic performance, characterized by enhanced tensile strength.

4.2. Effect of SBS–Nanoclay Modifiers on ITS Values

The incorporation of 4.5% SBS alone (4.5S0N) produced modest improvements in tensile strength. Dry and wet ITS values reached 5.23 kN and 4.28 kN, respectively, indicating a slight improvement over the control samples (Figure 2).

The addition of nanoclay to the SBS-modified mixture led to substantial improvement in tensile strength performance. The most significant enhancement was achieved with 6% nanoclay content (4.5S6N), reaching peak values of 7.12 kN in dry conditions and 6.94 kN in wet conditions, representing approximately 38% and 68% increases compared to the control mixture, respectively. However, increasing the nanoclay content beyond 6% (4.5S8N) showed a marginal decrease in ITS values (7.01 kN in dry conditions and 6.59 kN in wet conditions), suggesting that 6% represents an optimal nanoclay content for maximum tensile strength enhancement.

The introduction of nanoclay particles into the SBS-modified asphalt creates a more complex and effective reinforcement system. The progressive improvement in ITS values with increasing nanoclay content up to 6% can be explained through several mechanisms.

Previous studies have shown that nanoscale clay particles, such as organo-modified montmorillonite, possess a high surface area and layered structure that can create additional physical reinforcement within the asphalt matrix [45,46]. In this study, the observed performance improvements are consistent with the reinforcement mechanism reported in the literature, in which such particles act as stress distribution points, effectively dissipating tensile forces throughout the material structure. Nanoclay, particularly montmorillonite-based material, has a layered silicate structure with a high aspect ratio and surface area. When properly dispersed within the binder, nanoclay forms an intercalated or exfoliated nanostructure, creating a physical and chemical reinforcement mechanism [13,47]. This structure improves the stiffness of the binder by restricting the movement of bitumen molecules and reducing the flow of maltene fractions, which is particularly beneficial in high-temperature conditions.

Second, the chemical interaction between nanoclay surfaces and the polar components of the asphalt binder strengthens the overall matrix [48]. The surface chemistry of nanoclay particles promotes better adhesion between the binder and aggregate interface, contributing to enhanced tensile strength. Nanoclay particles have surfaces rich in hydroxyl and other polar functional groups, which can interact with the polar components of the binder through hydrogen bonding, electrostatic attraction, and van der Waals forces [49]. These interactions help integrate nanoclay particles into the binder matrix, creating a more cohesive and homogenous structure. The improved matrix cohesion enhances the tensile strength of the asphalt mixture, as the binder becomes more resistant to separation and deformation under tensile stress.

Third, the presence of nanoclay appears to complement the SBS polymer network. The nanoclay particles likely act as junction points within the polymer network, creating a more stable and resilient composite structure [13]. This synergistic effect explains why the combination of SBS and nanoclay achieves superior performance compared to either modifier alone.

To sum up, the SBS polymer alone creates an elastic network that enhances flexibility and tensile properties, but it lacks the stiffness and moisture resistance provided by nanoclay. Conversely, nanoclay alone adds stiffness and stability but cannot offer the elasticity and deformation recovery required to withstand dynamic traffic loads. When used together, these modifiers address each other’s limitations: (i) SBS provides the elasticity required for crack resistance, while nanoclay improves stiffness, preventing excessive deformation under high temperatures, and (ii) nanoclay’s moisture-resistant properties mitigate SBS’s susceptibility to water damage, enhancing the overall durability of the binder.

The observed peak in performance at 6% nanoclay content (4.5S6N) represents an optimal balance point. At this concentration, the nanoclay particles are well-dispersed and effectively integrated with the SBS network. The subsequent decline in ITS values at 8% nanoclay content (4.5S8N) suggests that exceeding the optimal concentration leads to diminishing returns. This decline can be attributed to potential agglomeration of nanoclay particles at higher concentrations, which may create stress concentration points and reduce the overall effectiveness of the modification.

The improved performance under wet conditions with increasing nanoclay content is particularly noteworthy. This enhancement in moisture resistance can be attributed to the hydrophobic nature of modified clay particles and their ability to create stronger bonds between the binder and aggregate interface, effectively reducing water penetration and maintaining structural integrity under adverse conditions.

4.3. Moisture Susceptibility Analysis

Figure 3 presents the tensile strength ratio (TSR) of the unmodified and modified asphalt mixtures. It clearly indicates the improvement in moisture resistance with the incorporation of SBS–nanoclay modifiers in the asphalt mixtures.

The control mixture exhibited a TSR value of 79.53%, which falls below the commonly accepted minimum threshold of 80% for adequate moisture resistance in asphalt pavements. The addition of 4.5% SBS alone (4.5S0N) showed a modest improvement, raising the TSR to 81.86%, indicating that SBS modification provides some enhancement in moisture resistance.

A progressive increase in TSR values was observed with increasing nanoclay content in combination with SBS. The mixture containing 6% nanoclay (4.5S6N) achieved the highest TSR value of 97.14%, representing a significant improvement of approximately 22% compared to the control mixture. This substantial enhancement in moisture resistance can be attributed to the nanoclay’s ability to strengthen the adhesion between the aggregate and binder interface, thereby reducing the potential for moisture-induced damage.

However, increasing the nanoclay content to 8% (4.5S8N) resulted in a slight decrease in TSR to 93.56%. While this value still represents a substantial improvement over the control mixture, it suggests that exceeding the optimal nanoclay content may lead to diminishing returns in terms of moisture resistance.

The results demonstrate that the combination of SBS and nanoclay at optimal proportions creates a more moisture-resistant asphalt mixture, which is crucial for long-term pavement durability in regions experiencing significant moisture exposure. The enhanced moisture resistance can be primarily attributed to the improved interfacial bonding and reduced water sensitivity provided by the nanoclay modification.

4.4. Resilient Modulus Results

The resilient modulus (E_RI) results demonstrate a consistent improvement pattern with the incorporation of SBS–nanoclay modifiers in the asphalt mixtures (Figure 4). The control mixture exhibited a baseline MR value of 3383 MPa, while the addition of 4.5% SBS alone (4.5S0N) increased the resilient modulus to 3528 MPa, representing a modest enhancement in the material’s elastic response to loading.

The SBS polymer forms an elastic network structure through physical entanglements and chemical interactions with the asphalt components. This network provides essential viscoelastic properties, allowing the material to deform under loading while maintaining the ability to recover its original shape [50]. The styrene end-blocks of the SBS copolymer create rigid domains that act as physical crosslinks, while the flexible butadiene mid-blocks contribute to the material’s elasticity and recovery properties [51].

A systematic increase in resilient modulus values was observed with progressive additions of nanoclay content. The most significant improvement was achieved with 6% nanoclay content (4.5S6N), reaching a peak value of 4715 MPa. This represents approximately a 39% increase compared to the control mixture, indicating substantially enhanced load-bearing capacity and structural response under dynamic loading conditions.

The introduction of nanoclay particles creates a more complex composite system through multiple reinforcement mechanisms. The layered silicate structure of nanoclay, with its high aspect ratio and surface area, facilitates better stress distribution throughout the material [46,52,53]. These nanoscale particles can intercalate or exfoliate within the asphalt matrix, creating additional physical barriers to deformation and enhancing the overall stiffness of the mixture [19]. At the optimal concentration of the combined modifiers, the nanoclay particles achieve optimal dispersion and interaction with both the SBS network and the asphalt components. The surface chemistry of the nanoclay particles promotes stronger interfacial bonding between the various phases of the composite system, contributing to enhanced load transfer efficiency and improved elastic response under dynamic loading conditions [53,54,55].

However, when the nanoclay content was increased to 8% (4.5S8N), the resilient modulus decreased slightly to 4473 MPa. While this value still represents a significant improvement over the control mixture, it suggests that exceeding the optimal nanoclay content may lead to diminishing returns in terms of elastic performance. When the nanoclay content exceeds the optimal threshold, particle clustering may occur, creating localized stress concentration points that compromise the uniform distribution of applied loads throughout the material structure [56,57,58]. This behavior aligns with the trends observed in both indirect tensile strength and moisture susceptibility tests, further confirming that 6% represents an optimal nanoclay content for maximizing performance characteristics.

4.5. Hamburg Wheel Tracking Test Result

The rut depth results obtained from the Hamburg Wheel Tracking Test (HWTT), presented in Figure 5, demonstrate the rutting resistance of both unmodified and modified asphalt concrete mixtures under simulated traffic loading conditions. The control mixture exhibited the highest rut depth of 5.12 mm, indicating its relative susceptibility to permanent deformation under repeated loading.

The addition of SBS polymer (4.5S0N) indicated moderate improvement in rutting resistance, reducing the rut depth to 4.56 mm, approximately a 12.5% decrease compared to the control mixture. This enhancement can be attributed to the three-dimensional network structure formed by SBS within the asphalt matrix, which increases the material’s resistance to permanent deformation through improved elastic recovery and higher stiffness at elevated temperatures [27,51].

A systematic improvement in rutting resistance was observed with the progressive addition of nanoclay content in combination with SBS. The most significant enhancement was achieved with 6% nanoclay content (4.5S6N), which demonstrated the lowest rut depth of 3.16 mm, representing approximately a 39.3% reduction compared to the control mixture. This substantial improvement in rutting resistance can be attributed to two key mechanisms. The dispersed nanoclay particles create reinforcement points within the SBS-modified asphalt matrix, restricting molecular movement and increasing stiffness through their high surface area and layered structure [20,59]. Additionally, these particles serve as physical crosslinking points within the SBS network, creating a more stable composite system that effectively resists permanent deformation under repeated loading conditions [19,53,60].

However, increasing the nanoclay content to 8% (4.5S8N) led to a significant increase in rut depth to 5.01 mm, nearly approaching the rutting susceptibility of the control mixture. This deterioration in performance can be attributed to potential agglomeration of nanoclay particles at higher concentrations, which may create weak points within the material structure and compromise its resistance to permanent deformation. Similar findings have been reported in previous studies, where excessive nanoclay content resulted in poor distribution, microstructural defects, and diminished mechanical performance in modified asphalt systems [14,21,54]. The results imply that exceeding the optimal nanoclay content can lead to adverse effects on the mixture’s rutting resistance.

The HWTT results align with the trends observed in both the resilient modulus and indirect tensile strength tests, confirming that 6% nanoclay content represents an optimal level for maximizing the performance characteristics of SBS-modified asphalt mixtures. This optimization point reflects the balance between achieving adequate particle dispersion and avoiding the negative effects of excess modifier content.

These findings have significant implications for pavement design and construction, particularly in regions experiencing high traffic loads and elevated temperatures. The improved rutting resistance offered by the optimized SBS–nanoclay modification system suggests the potential for enhanced pavement durability and reduced maintenance requirements in challenging service conditions.

The SIP analysis provides critical insights into the moisture susceptibility of the asphalt concrete mixtures tested. As illustrated in Figure 6, the modification strategy demonstrated a clear positive impact on the mixtures’ resistance to moisture damage. The control mixture exhibited the earliest SIP at 800 cycles, indicating relatively low resistance to moisture-induced deterioration. With the incorporation of modifiers, a consistent improvement in moisture resistance was observed.

The initial modifications (4.5S0N and 4.52N) showed modest improvements, with SIP values of 900 cycles, representing a slight enhancement over the control mixture. A gradual improvement was found with 4.5S4N, which achieved a SIP of 1000 cycles. The 4.5S6N mixtures demonstrated superior performance with a SIP of 3000 cycles, representing nearly a fourfold increase compacted to the control mixture. The 4.5S8N mixture maintained this improved performance with an SIP of 2700 cycles, though slightly lower than 4.5S6N.

These results confirm that the modifier content significantly influences the moisture resistance of the asphalt concrete mixtures, with the optimal performance achieved at the 6% nanoclay modification level. The slight decrease in SIP value for the 4.5S8N indicates that increasing nanoclay modifier content beyond 6% may not provide additional benefits in terms of moisture damage resistance.

The performance of the asphalt concrete mixtures was further evaluated through quantitative analysis of their visco-plastic resistance (VR) and moisture resistance (MR) indices, as presented in Figure 7. These metrics provide comprehensive insight into the mixtures’ resistance to both mechanical deformation and moisture-induced damage.

The control mixture exhibited the lowest performance, with VR and MR values of 1923 and 2273, respectively, indicating baseline resistance to both deformation mechanisms. The introduction of modifiers generally improved both resistance parameters, though with varying degrees of effectiveness across different modifier contents.

A systematic improvement in both VR and MR values was observed up to the 4.5S6N mixtures. The VR values increased from 1923 for the control to 3309 for 4.5S6N, demonstrating enhanced resistance to visco-plastic deformation. Similarly, the MR values revealed consistent improvement, reaching a peak of 4546 with 4.5S6N, indicating superior moisture damage resistance.

However, the 4.5S8N mixture showed a notable decline in visco-plastic resistance, with the VR value dropping to 1896, slightly below that of the control mixture. The decline in visco-plastic resistance likely resulted from nanoclay agglomeration at higher concentrations. When nanoclay content exceeded the optimal threshold (6% in this study), the particles tended to cluster together rather than maintaining uniform dispersion. These agglomerates create weak points in the asphalt–polymer matrix. This phenomenon has been observed in other polymer–nanoclay modified asphalt studies, which found that excessive filler loading beyond the optimal threshold caused poor dispersion and created weak zones in the mastic matrix [14,21,54]. The agglomeration also interferes with the SBS polymer network, diminishing its effectiveness in providing elastic recovery and deformation resistance.

Interestingly, its MR value remained relatively high at 3995, implying that, while the higher modifier content may compromise mechanical resistance, it maintains enhanced moisture damage resistance. Even when agglomerated, nanoclay particles retain their hydrophobic characteristics and continue to provide a barrier against moisture penetration. The layer structure of nanoclay, even in clustered form, still creates tortuous paths that impede water movement through the mixture. Additionally, the surface chemistry of nanoclay particles continues to enhance aggregate-binder adhesion, maintaining resistance against moisture-induced damage.

This divergent behavior demonstrates that, while excessive nanoclay content can compromise the structural performance of the mixture, its beneficial effects on moisture resistance remain relatively intact. However, the overall performance optimization still requires achieving proper balance, which is better achieved at the 4.5% SBS and 6% nanoclay content level.

The relationship between TSR and SIP values provides valuable insights into the moisture susceptibility of asphalt concrete mixtures modified with SBS polymer and nanoclay. Figure 8 illustrates a clear correlation between these two performance indicators across different modifier combinations.

The comparative analysis of TSR and SIP measurements provides crucial validation of the moisture damage resistance characteristics observed in the SBS–nanoclay modified asphalt mixtures. This dual-parameter approach offers several significant advantages for evaluating moisture susceptibility and validating the effectiveness of the modification strategy.

The complementary nature of these test methods strengthens the reliability of this research finding. While TSR provides a fundamental measure of strength retention after moisture conditioning through static testing, SIP captures the dynamic performance under repeated loading conditions that better simulate actual traffic loading. The consistent trends observed across both parameters, particularly the peak performance at 6% nanoclay content (4.5S6N) and subsequent decline at 8% (4.5S8N), provide comprehensive validation of the optimal modifier content determination.

The parallel improvements in both TSR and SIP values with increasing modifier content up to 4.5S6N (TSR: 97.14%, SIP: 3000 cycles) demonstrate the effectiveness of the combined SBS–nanoclay modification strategy. This consistency across different measurement approaches reveals that the enhanced moisture resistance observed in laboratory testing is likely to translate into improved field performance. The subsequent decline in both parameters for 4.5S8N (TRS: 93.56%, SIP: 2700 cycles) further validates the identification of 6% as the optimal nanoclay content.

Furthermore, this comprehensive assessment approach bridges the gap between laboratory characterization and field performance prediction. The TSR test, being a widely accepted industry standard, provides a reliable baseline measurement, while the SIP analysis offers insights into the material’s behavior under conditions more representative of actual service conditions. The strong correlation between these parameters confirms that the laboratory measured improvements in moisture resistance are likely to manifest as enhanced durability in field applications.

The relationship between TSR and the resistance indices (VR and MR) provides valuable insights into both the mechanical and moisture resistance properties of the modified asphalt mixtures. Figure 9 illustrates a coordinated improvement pattern across all three parameters with increasing modifier content, thought with notable variations in their relative responses.

The control mixture established baseline performance values with a TSR of 79.53%, VR of 1923, and MR of 2273, indicating its inherent vulnerability to both mechanical deformation and moisture damage. The introduction of 4.5% SBS polymer (4.5S0N) initiated improvements across all parameters, with TSR increasing to 81.86%, VR to 2737, and MR to 3028, demonstrating the fundamental benefits of polymer modification.

A systematic enhancement pattern emerged with increasing nanoclay content up to 6%. The 4.5S6N mixture achieved optimal performance with the highest TSR (97.14%), strong VR (3309), and peak MR (4546) values. This concurrent improvement across all parameters indicates that this modification level significantly addresses both mechanical stability and moisture susceptibility concerns.

The 4.5S8N mixture presented an interesting divergence in performance metrics. While maintaining a high TSR (93.56%) and MR (3995), it showed a significant decline in VR (1896). This pattern indicates that excessive nanoclay content may compromise the mixture’s resistance to mechanical deformation while largely preserving its moisture damage resistance properties.

The correlation between these three parameters provides a more comprehensive understanding of mixture performance than any single metric alone. The TSR results validate the moisture resistance improvements indicated by MR values, while the VR measurements offer complementary information about mechanical stability. This multi-parameter approach enhances confidence in identifying optimal modifier combinations and predicting overall field performance of modified asphalt mixtures.

The alignment of these performance indicators at the 6% nanoclay content level provides strong evidence for identifying this as the optimal modification level, balancing improvements in both mechanical properties and moisture resistance. This comprehensive evaluation approach offers a more reliable basis for material design decisions in practical applications. The enhanced performance of the modified mixtures demonstrates that targeted binder modifications can effectively improve pavement longevity, reduce maintenance costs, and enhance the overall sustainability of road infrastructure.

5. Conclusions

This research provides comprehensive insights into the synergistic effects of SBS polymer and nanoclay modification on asphalt concrete performance. The experimental investigation demonstrates that the combined use of these modifiers significantly enhances both mechanical properties and moisture resistance when optimally proportioned. The main conclusion can be drawn from this research as follows:

• The optimal modifier content was established at 4.5% SBS combined with 6% nanoclay, which maximized performance benefits across all evaluated parameters while maintaining economic feasibility.
• Significant improvements in mechanical properties were achieved, with indirect tensile strength increasing by 38% under dry conditions and 68% under wet conditions compared to control mixtures, demonstrating enhanced structural integrity and load-bearing capacity.
• Moisture resistance showed remarkable enhancement, with the Tensile Strength Ratio (TSR) improving from 79.53% in control samples to 97.14% in optimally modified mixtures, effectively addressing critical durability concerns in pavement applications.
• The quantitative analysis revealed strong correlations between performance indicators (TSR, SIP, VR, and MR), validating the comprehensive improvements and providing a rigorous methodology for evaluating modified asphalt mixtures.
• Uniform dispersion of nanoclay particles at optimal content created an effective reinforcement network, with their layered silicate structure facilitating improved stress distribution and stronger interfacial bonding within the asphalt matrix.
• Exceeding 6% nanoclay content resulted in performance deterioration due to particle agglomeration, establishing clear practical limitations for modification levels in pavement applications.
• Future research should focus on investigating long-term aging characteristics, performance under varying environmental conditions, cost-effectiveness analysis, and field validation through test sections to strengthen the applicability of these laboratory findings to real-world pavement construction.