Performance-Based Evaluation of Nanomaterials for Enhancing Moisture Damage Resistance in Asphalt Concrete

Abstract

Moisture-induced damage is one of the primary causes of premature distress in asphalt pavements, leading to reduced service life and increased maintenance costs. Although nanomaterials have shown potential in enhancing asphalt performance, the underlying composite interaction mechanisms among nanomaterials, asphalt binder, and aggregate phases under moisture exposure are still not fully understood. In addition, comparative evaluations under consistent experimental conditions remain limited. This study investigates the influence of five nanomaterials: nano-silica (NS), nano-alumina (NA), nano-titanium dioxide (NT), nano-zinc oxide (NZ), and carbon nanotubes (CNT) on the physical and mechanical properties of asphalt binders and mixtures, with particular emphasis on moisture damage resistance. The nanomaterials were incorporated at dosages of 1.5%, 3.0%, 4.5%, and 6.0% by binder weight. Binder performance was evaluated using conventional and performance grading (PG) tests, while mixture performance was assessed through Marshall properties and moisture susceptibility indicators, including the tensile strength ratio (TSR) and the index of retained strength (IRS). Fluorescence microscopy (FM), scanning electron microscopy (SEM), and Fourier transform infrared spectroscopy (FTIR) were employed to investigate nanomaterial dispersion characteristics, microstructural morphology, and physicochemical interactions within the asphalt composite system. The results indicate that nanomaterial modification reduced penetration and increased softening point and Marshall stability, reflecting enhanced stiffness and thermal resistance, although ductility decreased at higher dosages. Significant improvements in moisture resistance were observed, particularly under conditioned states. The TSR increased from 81.2% for the control mixture to 92.4% for NS and 91.7% for NA, while the IRS improved from 72.7% to 88.5% for NS. Statistical analysis indicated that both nanomaterial type and dosage significantly affected TSR and IRS performance, with dosage exhibiting comparatively greater influence on moisture resistance improvement. FM and SEM analyses revealed comparatively better dispersion and lower agglomeration tendency for NS and NA, which corresponded to their superior moisture resistance performance. FTIR analysis indicated that the modification process was predominantly physical, with no major formation of new chemical functional groups. Among the investigated nano materials, NS at 6% dosage exhibited the most pronounced improvement, followed by NA at similar dosage levels. Overall, the findings suggest that nanomaterial modification can considerably improve the moisture resistance and mechanical performance of asphalt mixtures under laboratory conditions. However, higher nanomaterial dosages may adversely affect binder workability due to increased viscosity, particularly in CNT-modified binders.

1. Introduction

Moisture-induced damage is one of the most critical and persistent challenges affecting the performance and longevity of asphalt pavements [1,2,3,4]. It primarily arises from the progressive loss of adhesion between the asphalt binder and aggregates, as well as cohesion within the binder matrix, under the influence of water, traffic loading, and environmental aging [5,6,7,8]. This degradation mechanism leads to stripping, rutting, fatigue cracking, and structural weakening, significantly reducing pavement service life and increasing maintenance costs [9,10,11,12]. Conventional asphalt mixtures often fail to provide adequate resistance to such damage, particularly under cyclic moisture exposure and freeze–thaw conditions, necessitating the development of advanced modification strategies [13,14,15]. In this context, nanotechnology has emerged as a transformative approach in pavement engineering, offering the potential to enhance asphalt performance at the microstructural level. Nanomaterials (NMs), typically characterized by particle sizes below 100 nm and high specific surface areas, exhibit enhanced physicochemical reactivity and improved interaction with asphalt constituents [16,17]. When incorporated into asphalt binders or mixtures, these materials improve stiffness, thermal stability, and resistance to moisture-induced damage. Previous studies have demonstrated that nanomodification enhances binder–aggregate adhesion and increases tensile strength and durability, often reflected in improved tensile strength ratio (TSR) and index of retained strength (IRS) values [18,19]. Despite these promising findings, most previous studies have focused on individual nanomaterials evaluated under different experimental conditions, including variations in binder source, aggregate type, dosage range, and testing procedures. Consequently, direct comparison between nanomaterials remains difficult, and the relative effectiveness of different nanomaterial types in improving moisture damage resistance is still not fully understood. In particular, systematic evaluations using consistent moisture resistance indicators such as the TSR and IRS under unified laboratory conditions remain limited. NS has demonstrated superior performance due to its high surface energy, strong chemical reactivity, and ability to fill micro-voids within the binder–aggregate matrix. This results in improved adhesion and reduced moisture susceptibility, as confirmed by both mechanical testing and microstructural observations [20,21]. NS has also shown effectiveness in recycled asphalt mixtures, improving durability even at high reclaimed asphalt pavement (RAP) contents [22,23]. NA contributes to increased stiffness and enhanced adhesion, particularly at elevated temperatures, while NT improves moisture resistance, aging durability, and rutting performance, although its effectiveness is highly dependent on dosage and stability [24,25,26]. NZ enhances binder cohesion and aging resistance and has been shown to improve fatigue life and moisture resistance, particularly when used in combination with other modifiers [27,28]. CNTs, due to their high aspect ratio and exceptional tensile strength, act as reinforcing elements that improve mechanical performance; however, their effectiveness is often limited by dispersion challenges [29,30]. The distinctive characteristics and performance contributions of these nanomaterials are summarized in

Table 1. Summary of nanomaterials and their reported effects on asphalt concrete performance.

References	Nanomaterial	Key Properties	Reported Effects on Asphalt Concrete
[19,21,23,31,32]	NS	High surface energy; high chemical reactivity; fills micro-voids in binder–aggregate matrix	Improves tensile strength, stiffness, and moisture resistance; enhances binder–aggregate bonding
[19,21,24]	NT	UV resistance; self-cleaning properties; thermal stability	Improves rutting resistance and thermal durability; reduces oxidative aging; enhances moisture damage resistance
[33,34]	NA	Ultra-hardness; excellent thermal stability	Increases mixture stiffness; reduces moisture sensitivity, especially at high temperatures
[35,36]	CNT	Exceptional tensile strength; high aspect ratio	Acts as micro-reinforcement; improves cracking resistance, stiffness, and indirect tensile strength
[37,38]	NZ	Anti-oxidative properties; aging resistance	Enhances binder cohesion and long-term durability; modest effect on moisture susceptibility

, which provides a comparative overview of their key properties and reported effects on asphalt concrete performance.

From a mechanistic perspective, the improvement in moisture damage resistance is primarily attributed to enhanced binder–aggregate adhesion and improved dispersion of nanomaterials within the binder matrix. Advanced characterization techniques, such as surface free energy measurements and atomic force microscopy (AFM), have confirmed that nanomodification increases interfacial bonding forces, thereby reducing moisture susceptibility [39,40,41]. In addition, microstructural analyses (SEM, FESEM) indicate that well-dispersed nanomaterials form interconnected networks that enhance cohesion and stress transfer within the asphalt mastic [42,43]. However, achieving uniform dispersion remains a critical challenge, particularly for CNT and NT, where agglomeration can significantly reduce performance benefits [17,44].

Recent studies have also explored composite nanomaterial systems, combining nanoparticles with polymers, fibers, or other modifiers to achieve synergistic improvements. For example, combinations of TiO₂ and ZnO have been shown to enhance adhesion, aging resistance, and moisture durability through improved interfacial interactions [45,46]. Similarly, nano-SiO₂/SBS and nano-TiO₂/SBS systems have demonstrated improved fatigue performance and resistance to moisture damage, although challenges related to workability and stability remain [17,47,48,49]. Previous studies have employed a wide range of nanomaterial dosages depending on nanomaterial type, binder grade, and blending conditions, with reported contents generally varying between 1% and 10% by binder weight [19,21,50]. However, the selected dosage ranges often differ substantially among studies, making direct comparison difficult. Therefore, the present study adopts a unified dosage framework consisting of 1.5%, 3.0%, 4.5%, and 6.0% by binder weight to evaluate both moderate and relatively high modification levels under consistent experimental conditions. Accordingly, the main objective of this study is to systematically evaluate the influence of five nanomaterials (NS, NA, NT, NZ, and CNT) at multiple dosage levels on the physical, mechanical, microstructural, and moisture resistance characteristics of asphalt binders and mixtures under identical laboratory conditions. The experimental program integrates binder-level characterization with mixture-level performance assessment, including Marshall properties, TSR, and IRS, complemented by FM, SEM, and FTIR analyses to evaluate nanomaterial dispersion characteristics, microstructural morphology, and binder–nanomaterial interactions. The experimental program observation is further validated with statistical analysis discussed ANOVA, Tukey HSD, and Dunnett multiple comparison tests, conducted to quantitatively evaluate the significance of nanomaterial type and dosage on moisture resistance performance. By systematically linking nanomaterial type, dosage, dispersion behavior, chemical interaction characteristics, and statistical performance evaluation with moisture resistance indicators, this study establishes a consistent comparative framework for assessing nanomaterial effectiveness under unified laboratory conditions. These outcomes provide a reliable basis for identifying suitable nanomaterial types and dosage levels for improving the durability and moisture resistance of asphalt mixtures.

2. Materials

2.1. Asphalt Cement

The asphalt binder utilized in this study was obtained from the Doura Oil Refinery, located southwest of Baghdad, Iraq. Its conventional physical properties were first evaluated using penetration grading tests, and the results are summarized in

Table 2. Physical properties of asphalt cement.

Test	ASTM	Results	Specification Limit [51]
Penetration at 25 °C, 100 gm, 5 s. (0.1 mm)	D 5 [53]	44	40–50
Softening point (°C)	D 36 [54]	50.2	-
Specific gravity at 25 °C	D 70 [55]	1.03	-
Flashpoint (Cleveland open cup) (°C)	D 92 [56]	295	Min. 232
Ductility at 25 °C, 5 cm/min. (cm)	D113 [57]	119	Min. 100
Viscosity at 135 °C, m Pa·s	D 4402 [58]	674	Max. 3000
Residue from Thin-Film Oven Test
Retained Penetration, % of original	D 5	62.8	≥55
Ductility, cm	D 113	60	≥25

. Based on these results, the binder satisfies the requirements for an AC 40–50 penetration grade in accordance with ASTM D946 [51]. In addition to conventional characterization, the rheological properties of the binder were evaluated using the Superpave Performance Grading (PG) system in accordance with AASHTO M320 [52]. The results, presented in

Table 3. Rheological properties of asphalt binder (performance grading).

Asphalt Cement	Properties	Temperature Measured °C	Measured Parameters	Specification Requirements, AASHTO M320 [52]
Original	Viscosity at 135 °C (m Pa·s)	-	753	3000 m Pa·s, max
	DSR, G*/sinδ at 10 rad/s (kPa)	58	7.3411	1.00 kPa, min
		64	3.1709
		70	1.4537
		76	0.7088
RTFO Aged	Mass Loss (%)	-	0.262	1%, max
	DSR, G*/sinδ at 10 rad/s (kPa)	64	6.5081	2.2 kPa, min
		70	3.4227	2.2 kPa, min
		76	1.5076
PAV Aged	DSR, G*.sinδ at 10 rad/s (kPa)	28	3414	5000 kPa, max
		25	5208
	BBR, Creep Stiffness (MPa)	−6	192	300 MPa, max
	Slope m-value	−6	0.346	0.3, min

, indicate that the binder complies with the requirements for PG 70-16 classification.

2.2. Aggregate

Crushed quartz aggregates were utilized in this study and sourced from the Al-Nibaie Quarry, located north of Baghdad, Iraq. The mineralogical composition of the aggregates was first evaluated using X-ray diffraction (XRD) analysis. The test was conducted using a diffractometer operating at a voltage of 40 kV, with Cu-Kα radiation as the X-ray source. Data were collected over a 2θ range of 5° to 65° with a scanning time of 1 s per step. The results, presented in

Table 4. Mineral composition of aggregates.

Mineral Compound	Content, %
Quartz	80.79
Calcite	8.92
Anhydrite	7.81
Dolomite	2.29
Total	99.81

, indicate that quartz is the dominant mineral phase (80.79%), followed by calcite (8.92%), anhydrite (7.81%), and dolomite (2.29%). The high quartz content confirms the predominantly siliceous nature of the aggregates. The aggregates were separated using standard dry-sieving procedures into size fractions of 19.0, 12.5, 9.5, 4.75, 2.36, 0.30, and 0.075 mm (including pan). The fractions were then recombined to conform to the gradation requirements of the D-5 mix type for wearing course applications, as specified in ASTM D3515 [59], as shown in

Table 5. Selected aggregate gradation and specification limit.

Sieve Size		Selected Gradation (% Passing)	Specification Limit [59]
(mm)	(in.)
19	3/4″	100	100
12.5	1/2″	94	90–100
9.5	3/8″	83	…
4.75	No. 4	63	44–74
2.36	No. 8	46	28–58
0.3	No. 50	13	5–21
0.075	No. 200	7	4–10

. The physical properties of the aggregates are presented in

Table 6. Physical properties of aggregate.

Test	ASTM Specification	Result	Specification Requirement
Coarse aggregate
Bulk specific gravity	C 127 [60]	2.583	––––
Apparent specific gravity	C 127	2.622	––––
Water absorption, %	C 127	0.582	––––
Los Angeles abrasion, %	C 131 [61]	16.7	Max. 30
Fine aggregate
Bulk specific gravity	C 128 [62]	2.501	––––
Apparent specific gravity	C 128	2.537	––––
Water absorption, %	C 128	0.745	––––

. The high quartz content confirms the predominantly siliceous nature of the aggregates, which are generally characterized by relatively hydrophilic surfaces and a weaker affinity toward asphalt binders compared with calcareous aggregates. Such characteristics increase the susceptibility of asphalt mixtures to moisture-induced stripping and interfacial debonding in the presence of water. Therefore, the selected aggregate type provides a suitable framework for evaluating the effectiveness of nanomaterials for improving binder–aggregate adhesion and enhancing resistance to moisture damage.

2.3. Mineral Filler

Limestone dust, obtained from the Karbala Governorate in Iraq, was used as the mineral filler in this study due to its local availability and economic advantage. The filler was defined as the material passing the 0.075 mm (No. 200) sieve in the aggregate structure. The physical properties of the limestone filler were determined in accordance with relevant ASTM D 242 standards and are summarized in

Table 7. Physical properties of mineral filler.

Property	ASTM Standard	Result
Passing No. 200%	D 854 [63]	97
Bulk specific gravity	C 117 [64]	2.72

.

2.4. Nanomaterials

Five nanomaterials were employed in this study as asphalt modifiers: NS, NA, NT, CNT, and NZ. These nanomaterials were selected based on their distinct physicochemical properties and their reported potential to enhance the performance of asphalt binders and mixtures. The nanomaterials were supplied by Hebei Suoyi New Material Technology Co., Ltd., Handan, China.

The nanomaterials were incorporated into the asphalt binder at four dosage levels of 1.5%, 3.0%, 4.5%, and 6.0% by weight of the binder to evaluate their influence on the moisture damage resistance of asphalt concrete mixes. Due to differences in particle size, surface area, and density, each nanomaterial was expected to interact differently with the asphalt binder, thereby affecting the overall performance of the mixtures. Prior to blending, the nanomaterials were oven-dried at 105 °C for 24 h to remove absorbed moisture and improve mixing consistency. The key physical properties of the nanomaterials are summarized in

Table 8. Key properties of NMs.

Nanomaterial	Chemical Formula	Particle Size (nm)	Bulk Density (g/mL)	Surface Area (m2/g)
NS	SiO2	25–35	0.080	190–250
NA	Al2O3	10–20	0.200	120–160
NT	TiO2	20–30	0.510	120–160
NZ	ZnO	15–20	0.331	30–60
CNT (multi-walled carbon nanotubes)	C	20 nm diameter; 10 $\mathsf{\mu }$m length	0.126	100–300

, while their physical appearance is illustrated in Figure 1.

2.5. Nano-Modified Asphalt Preparation

The base asphalt binder (AC 40–50) was modified by incorporating nanomaterials (NMs) at controlled dosages of 1.5%, 3%, 4.5%, and 6% by binder weight. A high-speed shear mixer (HSSM), as presented in Figure 2a,b and commonly applied in nano-modified asphalt studies [21,65], was used to ensure uniform dispersion of the NMs and to minimize particle agglomeration. Approximately 500 g of asphalt cement was preheated to 140–150 °C to achieve suitable fluidity. The nanomaterials were then introduced gradually at a rate of 4 g/min under continuous mixing at 500 rpm to reduce localized particle clustering during addition. Following complete incorporation, the mixture was sheared at 4000 rpm for 30 min while maintaining a temperature of 150 ± 5 °C throughout the process. During mixing, thermal imaging (Figure 2c) was continuously employed to monitor the blending operation. The relatively uniform thermal distribution observed during mixing indicates stable heating conditions throughout the binder matrix and suggests the absence of severe localized agglomeration during the modification process. The adopted mixing conditions were selected to promote consistent nanomaterial distribution within the binder matrix while minimizing excessive thermal aging and reducing agglomeration tendency, particularly at higher dosages.

3. Experimental Tests

The experimental program was designed to evaluate the influence of nanomaterials on the performance of asphalt binders and mixtures, with particular emphasis on moisture damage resistance. The study involved the preparation of nano-modified binders, followed by a series of conventional and rheological binder tests. Mixture performance was assessed through Marshall properties (stability, flow, and volumetric characteristics) and further evaluated for moisture susceptibility using the TSR and IRS. In addition, microstructural and chemical characterization analyses were conducted using FM, SEM, and FTIR to investigate nanomaterial dispersion and binder–nanomaterial interactions. A schematic representation of the overall experimental workflow is presented in Figure 3.

3.1. Conventional Binder Tests

Conventional tests were conducted to evaluate the consistency and physical properties of the different asphalt binders used in this study. The penetration test (ASTM D5, [53]) was performed to assess binder consistency by measuring the depth of penetration of a standard needle under a 100 g load for 5 s at 25 °C (Figure 4e). The softening point test (ASTM D36, [54]) determined the temperature at which the binder softens sufficiently for a steel ball to fall a specified distance, using the Ring-and-Ball method with a heating rate of approximately 5 °C/min (Figure 4a). The rotational viscosity (RV) test presented in Figure 4b (ASTM D4402, [58]) was conducted at 135 °C to evaluate binder workability, which is essential for ensuring proper mixing, pumping, and compaction during asphalt mixture production. The ductility test (ASTM D113, [57]) was used to assess binder flexibility and cohesion by measuring the elongation prior to failure at 25 °C, with a pulling rate of 5 cm/min. Finally, storage stability (Figure 4d) was evaluated following procedures adopted in previous studies [10,16], in which specimens were stored at 163 °C for 48 h, and the difference in softening point between the top and bottom portions of the sample was measured to detect binder phase separation.

3.2. Binder PG Tests

High-temperature performance grading (PG) of the asphalt binders was evaluated using a Dynamic Shear Rheometer (DSR) in accordance with AASHTO T315 [66] shown in Figure 4c. This test characterizes the viscoelastic response of asphalt binders under oscillatory shear loading and provides the fundamental parameters required for classification within the Superpave PG system. Testing was conducted at a constant angular frequency of 10 rad/s over a range of temperatures. The procedure was initiated at 58 °C for the reference binder and at 64 °C for the nano-modified binders, with temperature increments of 6 °C. The test continued until the binder failure temperature was reached.

The DSR determines the complex shear modulus (G*) and phase angle (δ), which describes the combined elastic and viscous behavior of the binder. Within the PG framework, the parameter G*/sinδ is used for high-temperature grading and is required to be no less than 1.0 kPa for unaged binders, in accordance with AASHTO M320. In addition, this parameter provides an indication of binder stiffness, which plays a key role in the cohesive strength of the binder and influences its interaction with aggregate surfaces. This interaction is particularly relevant to moisture susceptibility, as it governs the quality of adhesion at the binder–aggregate interface [67]. The results obtained from this test were used to classify the binders and to assess the influence of nanomaterial incorporation on their rheological characteristics. Representative photographs of the physical and rheological testing procedures for the modified binders are presented in Figure 4.

3.3. Marshall Test for Mixtures

The Marshall test was conducted in accordance with ASTM D6927 [68] to evaluate the resistance of asphalt mixtures to plastic flow under confined loading. Marshall stability was defined as the maximum load that the specimen could sustain before failure, while Marshall flow represents the total vertical deformation (in mm units) corresponding to the maximum load. In addition to stability and flow, the test also provided volumetric parameters, including air voids (%VTM) and voids in mineral aggregate (%VMA). These values were calculated from the bulk specific gravity of the specimens (ASTM D2726, [69]) and the maximum theoretical specific gravity of the voidless mix (ASTM D2041, [70]).

A total of 26 Marshall specimens were prepared for this study: 5 for the mix design using neat asphalt binder and 21 incorporating different nanomaterial types and dosages. Three replicate specimens were produced for each mixture, and average results were reported. Each specimen was compacted with 75 blows per face, corresponding to heavy traffic conditions (>10⁶ ESAL). Prior to testing, the compacted specimens were conditioned by immersion in a water bath at 60 °C for 30 min before stability and flow measurements.

3.4. Indirect Tensile Strength Test

The moisture susceptibility of asphalt concrete mixtures was evaluated in accordance with ASTM D4867 [71]. Specimens for each mix were prepared using the Marshall procedure and compacted to achieve an air void content of 7 ± 1%. Six specimens were fabricated per mix and divided into two subsets:

• Unconditioned subset (three specimens): tested directly at 25 °C using indirect tensile testing.
• Conditioned subset (three specimens): subjected to one freeze–thaw cycle, consisting of freezing at −18 ± 2 °C for 16 h followed by immersion at 60 ± 1 °C for 24 h, and then indirect tensile testing at 25 °C in the same manner as the unconditioned subset.

During testing, a compressive load was applied along the vertical diametral plane of the cylindrical specimens at a rate of 50.8 mm/min until splitting failure occurred. The test parameters were calculated using Equations (1) and (2):

$$\mathrm{I}\mathrm{T}\mathrm{S}={\displaystyle \frac{2000\times \mathrm{P}\mathrm{m}\mathrm{a}\mathrm{x}}{\mathsf{\pi }\mathrm{t}\mathrm{D}}}$$

(1)

$$\mathrm{T}\mathrm{S}\mathrm{R},\mathrm{\%}={\displaystyle \frac{\mathrm{I}\mathrm{T}\mathrm{S}\mathrm{c}}{\mathrm{I}\mathrm{T}\mathrm{S}\mathrm{d}}}$$

(2)

where ITS is the indirect tensile strength (kPa), Pmax is the maximum tensile load (N), D is the specimens’ diameter (mm), and t is the specimens’ thickness (mm). The tensile strength ratio (TSR, %) is used to characterize the mixture’s resistance to moisture damage. According to ASTM D4867 [71], the mixtures should exhibit a TSR ≥ 80% to be considered resistant to moisture damage.

3.5. Compressive Strength Test

The effect of water on the compressive strength of asphalt mixtures was evaluated in accordance with ASTM D1075 [72]. Unlike the TSR test, which is based on indirect tensile stress, this method assesses moisture damage through compressive stress. Cylindrical specimens with dimensions of 101.6 × 101.6 mm were fabricated following ASTM D1074 [73]. The mixture was placed into the mold in two layers and subjected to an initial stress of 1 MPa, which was then gradually increased to 20.7 MPa for 2 min to achieve the target specimen height of 101.6 mm. After preparation, specimens were divided into two groups:

• Immersion in a 25 °C water bath for 4 h (dry condition).
• Immersion in a 60 °C water bath for 24 h, followed by conditioning at 25 °C for 2 h (wet condition).

Compressive strength was determined for both groups by applying an axial load at a rate of 50.8 mm/min until failure. The Index of Retained Strength (IRS) was calculated to assess moisture-induced strength loss using Equation (3),

$$IRS={\displaystyle \frac{CSw}{CSd}}\times 100$$

(3)

where $CSw$ is the wet compressive strength and $CSd$ is the dry compressive strength, both in kPa units. According to ASTM D1075, mixtures should exhibit an IRS ≥ 70% to be considered resistant to moisture damage.

3.6. Fluorescence Microscopy

Fluorescence microscopy (FM) was employed to evaluate the relative distribution and dispersion uniformity of nanomaterials within the asphalt binder matrix. Unlike conventional optical microscopy, FM utilizes excitation light and selective emission filters to enhance the visibility of dispersed particles within the binder phase. For sample preparation, a thin layer (1–2 mm) of preheated modified binder was placed on a clean glass slide and compressed gently to obtain a suitable thickness for imaging. Since metal oxides and carbon nanotubes (CNT) may not exhibit natural fluorescence, a small quantity of fluorescent dye diluted in ethanol was uniformly applied to improve particle visibility and contrast during imaging. The stained samples were allowed to dry at room temperature prior to observation. Images were captured at a magnification level of 10,000× using an excitation filter selected according to the emission characteristics of the applied dye, as shown in Figure 5. The obtained images were used as a comparative characterization tool to assess the relative dispersion behavior and compatibility of nanomaterials within the asphalt binder matrix. Similar approaches have been reported in previous studies for evaluating nanomaterial dispersion in asphalt-based composites [74,75].

3.7. Scanning Electron Microscopy

Scanning electron microscopy (SEM) was employed to investigate the surface morphology and microstructural characteristics of the modified asphalt binders. The analysis was performed using an FEI Inspect 50 microscope manufactured by FEI Company at a magnification level of 120 kX. SEM imaging provided detailed visualization of nanoparticle distribution, surface texture, and agglomeration behavior within the binder matrix, thereby offering further insight into the microstructural changes associated with nanomaterial incorporation.

3.8. Fourier Transform Infrared Spectroscopy

Fourier transform infrared spectroscopy (FTIR) was conducted using a Bruker Alpha II spectrometer to evaluate the chemical characteristics of the neat and nanomodified asphalt binders. The spectra were recorded within a wavenumber range of 400–4000 cm⁻¹ at a resolution of 4 cm⁻¹ using transmittance mode. The analysis was carried out to investigate the interaction between the nanomaterials and the asphalt binder and to identify any changes in the chemical structure or functional groups after modification. The oxygen-containing functional groups, represented by the carbonyl (C=O) and sulfoxide (S=O) indices, were quantitatively evaluated to assess the influence of nanomaterial incorporation on the chemical characteristics and oxidation-related behavior of the asphalt binders. In addition, variations in peak intensity and spectral behavior were examined to evaluate the effect of nanomaterials on binder interaction characteristics.

The carbonyl index (CI) and sulfoxide index (SI) were determined according to Equations (4) and (5), respectively, following the procedure reported in [76]:

$$CI= {\displaystyle \frac{Area at 1700 {\mathrm{c}\mathrm{m}}^{-1}}{Total area (600-2000 {\mathrm{c}\mathrm{m}}^{-1})}}$$

(4)

$$SI={\displaystyle \frac{Area at 1030 {\mathrm{c}\mathrm{m}}^{-1}}{Total area (600-2000 {\mathrm{c}\mathrm{m}}^{-1})}}$$

(5)

where the area at 1700 cm⁻¹ corresponds to the carbonyl absorption region, while the area at 1030 cm⁻¹ represents the sulfoxide absorption region. The total integrated area within the range of 600–2000 cm⁻¹ was used as a normalization baseline to reduce the influence of film thickness and concentration variations among samples.

3.9. Mix Design

The optimum asphalt content (OAC) for the asphalt mixture was determined using the Marshall mix design method in accordance with the Asphalt Institute’s MS-2 guidelines [77]. Five asphalt contents, ranging from 4.0% to 6.0% (by total mix weight) at increments of 0.5%, were tested. The OAC was calculated as the average of the three asphalt contents corresponding to maximum stability, maximum bulk density, and 4% air voids, which resulted in 5.06%. For ease of sample preparation and practical application, this value was rounded to 5.0% and used as the design asphalt content. At this content, as shown in

Table 9. Marshall mix design results.

Asphalt Cement Content, %	Stability (kN)	Flow (mm)	Gmb	VTM (%)	VMA (%)
4.0	6.3	2.7	2.218	7.62	16.52
4.5	8.8	3.1	2.302	5.38	15.49
5.0	11.2	3.5	2.324	4.50	15.30
5.5	10.5	4.1	2.313	3.55	16.02
6.0	9.2	4.9	2.209	3.20	16.80
Specification limit	8.0 Min.	2.0–4.0	Not limited	3.0–5.0	14.0 Min.

, the Marshall properties met specification requirements; stability exceeded the minimum of 8.0 kN, flow was within the permissible range of 2.0–4.0 mm, and VMA met the minimum limit of 14%. Thus, the selected OAC satisfied all relevant criteria (SCRB, [78]). Since the primary objective of this study was to evaluate the influence of nanomaterials on the moisture damage resistance of asphalt mixtures, the OAC of 5.0%, determined from the control mix design, was applied consistently to all modified mixtures rather than optimizing the mix design separately for each type or dosage of nanomaterial. This approach ensured a uniform basis for comparison and allowed the effect of nanomaterial type and dosage to be systematically assessed.

4. Results and Discussion

4.1. Asphalt Binder Tests

The conventional physical and rheological properties of the nano-modified asphalt binders are summarized in

Table A1. Conventional properties of nano-modified asphalt binders.

NM Type	Dosage (%)	Penetration (0.1 mm)	Softening Point (°C)	RV (mPa·s)	Ductility (cm)	ΔT (°C)
Control	0	44 ± 0.5	50.2 ± 0.2	753 ± 18	119 ± 2	0.5 ± 0.03
NS	1.5	40 ± 0.6	54.2 ± 0.3	1093 ± 28	106 ± 2	0.7 ± 0.04
NS	3	36 ± 0.7	59.9 ± 0.4	1233 ± 35	90 ± 3	0.9 ± 0.05
NS	4.5	31 ± 0.8	60.7 ± 0.5	1433 ± 42	83 ± 3	1.0 ± 0.05
NS	6	29 ± 0.9	61.2 ± 0.5	1376 ± 40	80 ± 3	1.2 ± 0.06
NA	1.5	41 ± 0.6	54.5 ± 0.3	1043 ± 26	112 ± 2	0.7 ± 0.04
NA	3	37 ± 0.7	58.4 ± 0.4	1196 ± 33	91 ± 3	0.8 ± 0.05
NA	4.5	33 ± 0.8	59.7 ± 0.5	1246 ± 35	89 ± 3	1.1 ± 0.05
NA	6	30 ± 0.8	60.5 ± 0.5	1253 ± 36	85 ± 3	1.3 ± 0.06
NT	1.5	41 ± 0.7	53.1 ± 0.3	723 ± 20	110 ± 2	0.8 ± 0.04
NT	3	38 ± 0.7	57.9 ± 0.4	708 ± 19	95 ± 3	1.0 ± 0.05
NT	4.5	34 ± 0.8	58.8 ± 0.5	723 ± 20	90 ± 3	1.2 ± 0.05
NT	6	32 ± 0.8	59.5 ± 0.5	713 ± 20	87 ± 3	1.4 ± 0.06
CNT	1.5	42 ± 0.7	52.8 ± 0.3	1213 ± 34	112 ± 2	0.9 ± 0.05
CNT	3	38 ± 0.8	55.6 ± 0.4	1398 ± 40	105 ± 3	1.1 ± 0.05
CNT	4.5	35 ± 0.8	58.0 ± 0.5	1633 ± 48	98 ± 3	1.4 ± 0.06
CNT	6	33 ± 0.9	59.2 ± 0.5	2858 ± 75	92 ± 3	1.7 ± 0.07
NZ	1.5	43 ± 0.6	52.3 ± 0.3	853 ± 24	116 ± 2	1.0 ± 0.05
NZ	3	40 ± 0.7	54.8 ± 0.4	1148 ± 32	109 ± 2	1.4 ± 0.06
NZ	4.5	38 ± 0.8	55.7 ± 0.4	1213 ± 35	106 ± 3	1.8 ± 0.07
NZ	6	35 ± 0.8	56.2 ± 0.5	1288 ± 38	103 ± 3	2.0 ± 0.08

Note: Values are presented as mean ± standard deviation (SD) based on three replicate measurements/tests.

(Appendix A), while the corresponding trends are illustrated in Figure 6. The reported values represent the arithmetic mean of three replicate measurements, and the associated variability is presented in the form of the standard deviation. The influence of nanomaterials on asphalt binder properties is evident from the obtained results. Penetration decreased progressively with increasing nanomaterial content (Figure 6a), indicating a clear stiffening effect. At 6% dosage, penetration reductions reached approximately 34% for NS and 31% for NA, while NT, CNT, and NZ exhibited more moderate decreases of about 27%, 25%, and 20%, respectively, compared to the control binder. This behavior is primarily attributed to the high surface area of NS and NA, which enhances physicochemical interactions with the binder matrix and restricts molecular mobility. In contrast, the relatively higher densities of NT (0.51 g/mL) and NZ (0.331 g/mL) may limit their effective surface interaction, resulting in a less pronounced stiffening effect. These trends are in agreement with the findings reported by Al-hamdou and Albayati [79].

The softening point results (Figure 6b) further confirm this trend, showing a consistent increase with nanomaterial incorporation. At 6%, the softening point increased from 50.2 °C for the control binder to 61.2 °C for NS and 60.5 °C for NA, while NT and CNT provided moderate improvements (≈59.2–59.5 °C) and NZ showed the lowest increase (56.2 °C).

The superior performance of NS and NA can be attributed to their low density and high surface area, which promotes better dispersion and generates stronger interaction with the binder phase. The rotational viscosity results (Figure 6c) provide further insight into binder workability and internal resistance to flow. In general, viscosity increased with nanomaterial content, consistent with the observed stiffening behavior. At 6% dosage, viscosity increased from 753 mPa·s for the control binder to 1376 mPa·s for NS (≈83% increase), 1253 mPa·s for NA (≈66%), and 1288 mPa·s for NZ (≈71%). In contrast, NT showed minimal variation, remaining close to the control value (≈713 mPa·s), indicating a comparatively lower influence on binder rheology. This behavior may be related to differences in particle characteristics and interaction efficiency within the binder matrix, resulting in less pronounced stiffening compared with NS and NA. CNT exhibited a markedly different behavior, with viscosity increasing sharply to 2858 mPa·s at 6% (≈280% increase), indicating a substantial increase in internal resistance to flow. This behavior may be associated with intensified particle interaction and possible localized clustering at higher CNT dosages, leading to increased internal friction within the binder system. A similar sharp increase in viscosity at higher CNT dosages has been reported by Alhamdo and Albayati [80], supporting the observed behavior. These results indicate that while nanomaterials generally enhance binder stiffness, excessive viscosity, particularly in the case of CNT, may adversely affect mixing and compaction processes. Therefore, moderate dosages appear more suitable for balancing performance and workability.

Ductility decreased with increasing nanomaterial dosage (Figure 6d), reflecting reduced binder flexibility due to the stiffening effect. At 6%, ductility decreased from 119 cm for the control binder to 80 cm for NS (≈33% reduction), 85 cm for NA (≈29%), 87 cm for NT (≈27%), and 92 cm for CNT (≈23%), while NZ retained the highest flexibility at 103 cm (≈13% reduction). This suggests that NS and NA impose the greatest restriction on binder deformation, whereas NZ maintains better elongation capacity due to its relatively lower surface activity. The storage stability results (Figure 6e) indicate a slight increase in softening point difference (ΔT) with nanomaterial addition, suggesting some degree of phase separation. At 6%, ΔT values reached 2.0 °C for NS and 1.7 °C for CNT, while values for NT, NA, and NZ remained below 1.5 °C, compared to 0.5 °C for the control binder. The higher separation observed in NS and CNT may be related to their lower densities, which can reduce gravitational stability within the binder matrix. The results obtained for NS, NA, and NT are consistent with those reported by [33]. Nevertheless, all values remained below the critical threshold of 2.5 °C, indicating acceptable storage stability. Overall, the results demonstrate that nanomaterials significantly modify the physical and rheological properties of asphalt binders. NS and NA were the most effective in enhancing stiffness and thermal characteristics, as confirmed by penetration, softening point, and viscosity results, although at the expense of reduced ductility. NZ preserved greater flexibility but provided limited enhancement in stiffness, while CNT offered strong stiffening at higher dosages but raised concerns regarding workability due to excessive viscosity. NT exhibited a relatively moderate influence across all properties. These findings highlight the importance of optimizing both nanomaterial type and dosage to achieve a balanced improvement in binder performance.

4.2. PG Results

The effects of nanomaterial type and dosage on the rheological performance of asphalt binders are summarized in

Table A2. G*/sin δ (kPa) at different temperatures (mean ± SD).

NM	Dosage (%)	58 °C	64 °C	70 °C	76 °C	82 °C
Neat	0	7.3411 ± 0.007	3.1709 ± 0.004	1.4537 ± 0.002	0.7088 ± 0.001	…
NS	1.5	5.3851 ± 0.006	2.7216 ± 0.003	1.3983 ± 0.002	0.7728 ± 0.001	…
	3	6.6501 ± 0.007	3.5521 ± 0.004	1.8527 ± 0.002	0.9811 ± 0.001	…
	4.5	8.4517 ± 0.009	4.1809 ± 0.005	2.3638 ± 0.003	1.2327 ± 0.002	0.7152 ± 0.001
	6	9.3327 ± 0.010	4.5222 ± 0.005	2.7711 ± 0.003	1.4227 ± 0.002	0.7483 ± 0.001
NA	1.5	4.4167 ± 0.005	2.3337 ± 0.003	1.1522 ± 0.002	0.5907 ± 0.001	…
	3	4.2781 ± 0.005	2.0537 ± 0.003	1.0743 ± 0.001	0.6141 ± 0.001	…
	4.5	5.9176 ± 0.006	2.8247 ± 0.003	1.5529 ± 0.002	0.8331 ± 0.001	…
	6	7.8511 ± 0.008	3.6608 ± 0.004	1.8571 ± 0.002	0.9497 ± 0.001	…
NT	1.5	3.7847 ± 0.004	2.0122 ± 0.002	1.0273 ± 0.001	0.4817 ± 0.001	…
	3	4.5271 ± 0.005	2.3624 ± 0.003	1.1274 ± 0.001	0.6338 ± 0.001	…
	4.5	5.7272 ± 0.006	2.9136 ± 0.003	1.4417 ± 0.002	0.7166 ± 0.001	…
	6	6.3344 ± 0.007	3.1591 ± 0.004	1.6627 ± 0.002	0.8433 ± 0.001	…
CNTs	1.5	12.0852 ± 0.012	5.1341 ± 0.006	2.3972 ± 0.003	1.1524 ± 0.002	0.5845 ± 0.001
	3	12.2252 ± 0.012	5.2411 ± 0.006	2.4117 ± 0.003	1.1475 ± 0.002	0.5989 ± 0.001
	4.5	12.0534 ± 0.012	5.2171 ± 0.006	2.4431 ± 0.003	1.1822 ± 0.002	0.6605 ± 0.001
	6	11.6254 ± 0.011	4.9664 ± 0.005	2.2843 ± 0.003	1.1208 ± 0.002	0.5668 ± 0.001
NZ	1.5	12.4874 ± 0.013	5.1451 ± 0.006	2.3147 ± 0.003	1.0817 ± 0.002	0.5427 ± 0.001
	3	11.9247 ± 0.012	5.1837 ± 0.006	2.3846 ± 0.003	1.1234 ± 0.002	0.5728 ± 0.001
	4.5	11.3327 ± 0.011	5.1900 ± 0.006	2.4542 ± 0.003	1.1637 ± 0.002	0.5919 ± 0.001
	6	11.4145 ± 0.011	5.6324 ± 0.007	2.6474 ± 0.003	1.2264 ± 0.002	0.6356 ± 0.001

(Appendix A) and illustrated in Figure 7. The results indicate that the incorporation of nanomaterials generally enhanced the high-temperature grading characteristics of the binder, as reflected by the increase in G*/sinδ values and the corresponding true failure temperatures. For the neat binder, the critical temperature corresponding to the PG criterion (G*/sinδ ≥ 1.0 kPa) was approximately 73.1 °C. The addition of nanomaterials shifted this threshold to higher temperatures, indicating improved resistance under loading conditions. Among the tested nanomaterials, CNT and NZ exhibited the highest G*/sinδ values across all temperatures, resulting in the greatest increase in true failure temperature, reaching approximately 76.8 °C at 6% dosage. This behavior is consistent with the significant increase in rotational viscosity observed for these materials, particularly CNT, indicating a substantial rise in binder stiffness.

NS also showed notable improvement with increasing dosage, where the true failure temperature increased progressively from about 71.8 °C at 1.5% to 76.1 °C at 6%. This enhancement can be attributed to its high surface area and effective dispersion within the binder matrix. This trend is in agreement with the softening point results, where NS exhibited one of the highest increases, confirming improved thermal stability. NA and NT showed comparatively moderate improvements, with failure temperatures generally ranging between 70 and 75 °C, depending on dosage. The relatively lower influence of NT may be associated with its higher density and reduced surface activity, which limits its interaction with the binder phase.

Overall, the results demonstrate that nanomaterials enhance the rheological properties of asphalt binders within the PG framework. The increase in G*/sinδ and true failure temperature reflects higher binder stiffness, which contributes to improved cohesion and stronger binder–aggregate interaction [81,82]. This behavior is particularly relevant to moisture damage resistance, as improved cohesion and adhesion at the binder–aggregate interface, which reduces the potential for stripping under environmental and loading conditions.

4.3. Marshall Properties

The effects of nanomaterials on the Marshall properties of asphalt mixtures are presented in Figure 8. Marshall stability increased consistently with nanomaterial dosage (Figure 8a). At 6%, stability rose from 11.2 kN for the control mixture to 14.0 kN for NS (≈25% increase) and 13.0 kN for NA (≈16% increase), while CNT and NT exhibited moderate improvements (≈12.5 kN, ≈11% increase), and NZ showed the lowest increase (12.1 kN, ≈8%). These enhancements can be attributed to improved binder–aggregate adhesion and increased mixture stiffness, particularly for NS and NA, whose high surface areas promote better stress transfer within the asphalt mastic.

Flow values decreased with nanomaterial content (Figure 8b), indicating a lower deformation under constant load. At 6%, flow was reduced from 3.5 mm (control) to 2.3 mm for NS (≈34% reduction) and 2.5 mm for NA (≈29% reduction). NT, CNT, and NZ showed more moderate reductions (2.7–3.0 mm). This reduction confirms the stiffening effect of nanomaterials, particularly NS and NA, though excessive flow reduction may imply a reduction in mixture flexibility. Bulk density trends (Figure 8c) showed only slight changes for most nanomaterials, remaining close to the control value (2.324 g/cm³). NA, NT, CNT, and NZ increased marginally to about 2.33 g/cm³, while NS decreased to 2.30 g/cm³ at 6%, likely due to its very low density (0.080 g/mL), which reduces the compacted mixture’s overall density despite its reinforcing role. Air voids (VTM) decreased with increasing nanomaterial content (Figure 8d). At 6%, VTM decreased from 4.5% for the control mixture to 3.1% for NS and approximately 3.8–3.9% for the other nanomaterials. This reduction indicates improved binder coating and enhanced filling of void spaces, particularly in the case of NS, where ultrafine particles can occupy micro-pores within the mixture [83]. Voids in mineral aggregate (VMA) generally increased with nanomaterial incorporation (Figure 8e). From a control value of 15.5%, VMA rose to 16.2% for CNT and 16.1% for NT, while NA and NZ reached about 15.8–15.9%. NS showed less consistent behavior, stabilizing near 15.6% at 6%. The increase in VMA value indicates greater binder demand, which is consistent with the higher surface areas of the nanomaterials [84].

Overall, the Marshall test results confirm that nanomaterials, particularly NS and NA, significantly enhanced mixture stability and reduced flow, reflecting improved resistance to plastic deformation. CNT and NT provided balanced performance, while NZ produced only modest improvements. The density-related behavior of NS (lower bulk density and VTM) underscores the role of nanomaterial density in governing compacted mixture characteristics. These findings are consistent with those reported by [19].

4.4. Indirect Tensile Strength Test Results

The effects of nanomaterials on the ITS and moisture resistance of asphalt mixtures were presented in Figure 9. The results demonstrate a consistent improvement in both dry and wet ITS with increasing nanomaterial dosage, indicating enhanced mechanical performance and resistance to moisture-induced damage.

Under dry conditions (Figure 9a), ITS increased progressively for all nanomaterials, reaching peak values at 6% dosage. The ITS improved from 1155 kPa for the control mixture to 1513 kPa for NS (≈31% increase) and 1393 kPa for NA (≈21% increase), while NT, CNT, and NZ exhibited more moderate gains of approximately 8%, 5%, and 4%, respectively. These improvements are consistent with the trends observed in the conventional binder tests, particularly the reduction in penetration and increase in rotational viscosity, which indicate enhanced binder stiffness and internal cohesion. The superior performance of NS and NA can be attributed to their finer particle size and higher specific surface area, which promote stronger physicochemical interaction with the asphalt binder and improve stress transfer within the asphalt mastic. In contrast, the relatively lower improvement observed for CNT and NZ may be associated with less effective interaction within the binder matrix and a reduced ability to uniformly reinforce the asphalt mastic.

Under wet conditions, a more pronounced enhancement was observed, where ITS increased from 938 kPa for the control mixture to 1398 kPa for NS (≈49% increase) and 1277 kPa for NA (≈36% increase). NT, CNT, and NZ showed improvements of approximately 20%, 13%, and 11%, respectively. The greater improvement under moisture conditioning highlights the effectiveness of nanomaterials in strengthening the binder–aggregate interface and reducing the moisture-induced stripping. This behavior suggests that nanomaterials contribute more significantly for preserving both adhesion and cohesion under moisture exposure than merely increasing dry tensile strength.

The TSR, shown in Figure 9b, further confirms the beneficial influence of nanomaterials on moisture resistance. The control mixture exhibited a TSR of 81.2%, slightly above the minimum requirement of 80% specified in ASTM D4867. In contrast, nanomaterial-modified mixtures showed significant improvements, reaching 92.4% for NS, 91.7% for NA, 90.4% for NT, 87.8% for CNT, and 86.6% for NZ at 6% dosage. These findings are in agreement with previous studies [17,19], which reported enhanced TSR values with the incorporation of NS, NA, and NT. The higher TSR values obtained from NS and NA are closely related to their ability to enhance binder stiffness while maintaining effective interaction with aggregate surfaces. Furthermore, higher surface activity may contribute to forming a denser and more cohesive asphalt film, which reduces moisture-induced damage and limits stripping susceptibility. Conversely, although CNT significantly increased binder viscosity at high dosages, the TSR improvement remained comparatively lower, suggesting that excessive stiffening does not necessarily translate into proportional improvement in moisture resistance.

From a mechanistic perspective, the observed improvements can be attributed to the modification of binder rheological properties and interfacial characteristics. An increase in the highest G*/sinδ value reflects higher binder stiffness, which is a key factor governing cohesive strength within the binder matrix [85]. This enhanced cohesion improves the resistance of the asphalt film towards moisture-induced weakening [86]. In addition, the increase in binder viscosity enhances aggregate coating and reduces moisture damage, leading to stronger adhesion at the binder–aggregate interface and reduced stripping potential [67]. The superior performance of NS and NA is primarily attributed to their high surface area and strong physicochemical activity, which promotes better dispersion and stronger interaction within the binder–aggregate system.

Overall, the ITS and TSR results demonstrate that nanomaterials significantly enhance the resistance of asphalt mixtures to moisture damage. The improvement is governed by a combination of increased binder stiffness, enhanced cohesion, and improved adhesion, confirming that binder-level modifications are effectively translated into improved mixture-level performance.

4.5. Index of Retained Strength Test Results

Figure 10 shows the influence of nanomaterials on compressive strength (CS) and the IRS. The results indicate a clear improvement in both dry and moisture-conditioned compressive strength with increasing nanomaterial dosage, suggesting enhanced load-bearing capacity and better resistance to moisture-related deterioration. Under dry conditions (Figure 10a), compressive strength increased noticeably with nanomaterial incorporation, reaching its highest value at 6% dosage. The CS increased from 5820 kPa for the control mixture to 8381 kPa for NS (≈44% increase), 7715 kPa for NT (≈33%), and 7200 kPa for NA (≈24%), while CNT and NZ showed comparatively smaller improvements. The superior performance of NS and NT indicates their greater ability to improve the internal cohesion and structural integrity of the asphalt matrix under compressive loading. NS, in particular, benefits from its fine particle size and high surface activity, which enhance interaction within the binder–aggregate system and contribute to a denser and stronger mastic structure.

The IRS results (Figure 10b) further support these observations. While the control mixture achieved only 72.7%, all nanomaterial-modified mixtures exceeded the minimum requirement of 70% specified in ASTM D1075. The IRS increased to 88.5% for NS, 86.2% for NT, 83.9% for NA, 77.9% for CNT, and 77.1% for NZ at 6% dosage. These results are consistent with those reported by Adwar and Albayati [19], who observed that incorporating NS and NT led to higher dry and wet compressive strength values, resulting in notable improvements in the IRS, particularly at 6% dosage. The overall ranking (NS > NT > NA > CNT > NZ) reflects the relative efficiency of each nanomaterial in maintaining strength after moisture conditioning. The comparatively lower performance of CNT and NZ suggests that increasing stiffness alone is insufficient to maximize moisture resistance unless accompanied by effective interaction and reinforcement within the binder matrix.

The higher improvement observed in both IRS and TSR moisture indicators can be explained by the difference in how the mixtures respond to loading. Under compressive loading, the response is governed by a combination of mastic cohesion, aggregate interlock, and binder–aggregate adhesion, allowing the reinforcing effect of nanomaterials to be more fully utilized. In contrast, tensile loading is more sensitive to interfacial debonding caused by moisture, which limits the improvement observed in the TSR. These trends are also consistent with the binder test results, where increases in rotational viscosity and G*/sinδ indicate a stiffer binder with improved cohesion. This contributes to a stronger asphalt matrix, better resistance to moisture damage, and improved retained strength after conditioning. Among the nanomaterials, NS showed the most pronounced improvement, likely due to its high surface area and good dispersion, which enhance both cohesion and adhesion. NT also performed well, while NA showed moderate but consistent gains. CNT and NZ exhibited smaller improvements, possibly due to dispersion limitations and lower interaction with the binder matrix. Overall, the results confirm that nanomaterials can significantly improve the resistance of asphalt mixtures to moisture damage. The improvements are mainly related to increased binder stiffness, enhanced cohesion within the mixture, and stronger bonding at the binder–aggregate interface, all of which help to reduce strength loss under moisture exposure.

4.6. FM Image Analysis

The FM images presented in Figure 11 provide insight into the dispersion characteristics of nanomaterials within the asphalt binder at 6% dosage. Clear differences in distribution, particle morphology, and agglomeration behavior can be observed among the different nanomaterials.

For NS (Figure 11a), the images show a fine and relatively homogeneous distribution of bright particles throughout the binder matrix, indicating effective dispersion. This uniformity can be attributed to its ultrafine particle size and high surface area, which promote strong interaction with the binder phase. NA (Figure 11b) exhibits a similar but slightly less uniform distribution, with the presence of small, localized clusters. This behavior may be associated with its lower density, which can lead to mild particle aggregation during mixing. In contrast, NT (Figure 11c) shows more pronounced agglomerated regions, suggesting reduced dispersion efficiency compared to NS and NA. NZ (Figure 11d) displays distinct bright clusters with limited spatial distribution, indicating poor dispersion, likely due to its relatively lower surface area and weaker interaction with the binder. The CNT-modified binder (Figure 11e) is characterized by elongated and randomly oriented fibrous structures, reflecting their high aspect ratio. While these structures can contribute to reinforcement, the formation of bundles indicates incomplete dispersion.

These microstructural features are consistent with the mechanical and durability performance observed in previous sections. The more uniform dispersion observed for NS and NA corresponds to their superior performance in the ITS, TSR, and IRS results, where improved moisture resistance was recorded. A well-dispersed nanomaterial enhances binder–aggregate adhesion and increases cohesion within the mastic, thereby reducing moisture-induced debonding. In contrast, the presence of agglomerates, as observed in NT, NZ, and CNT, can create localized weak zones that limit stress transfer and reduce the effectiveness of moisture resistance improvement. This explains the comparatively lower gains in the TSR and IRS for these nanomaterials.

Overall, the FM analysis confirms that dispersion quality plays a critical role in governing the effectiveness of nanomaterials. Materials with better distribution within the binder matrix (such as NS and NA) provide more efficient interaction with the asphalt phase, leading to enhanced mechanical performance and improved resistance to moisture damage. These observations support the conclusion that microstructural characteristics directly influence the macroscopic behavior of nano-modified asphalt systems.

4.7. SEM Image Analysis

The SEM images presented in Figure 12 provide further insight into the surface morphology and dispersion characteristics of the investigated nanomaterials within the asphalt binder matrix at 6% dosage. Compared with the fluorescence microscopy observations, SEM analysis provides clearer visualization of particle morphology, agglomeration tendency, and interaction with the binder phase at the microstructural level. The neat asphalt binder (Figure 12a) exhibited a relatively smooth and homogeneous surface without visible particulate structures, indicating the absence of solid dispersed phases within the binder matrix. In contrast, the nanomodified binders displayed noticeable microstructural alterations associated with nanoparticle incorporation. NS-modified binder (Figure 12b) exhibited relatively fine and widely distributed particles embedded within the asphalt matrix, although limited small-scale agglomerated regions were also observed. This behavior indicates generally good dispersion and compatibility with the binder phase despite the presence of minor localized clustering. Similarly, the NA-modified binder (Figure 12c) showed relatively homogeneous particle distribution with small, localized agglomerations, indicating dispersion behavior generally comparable to that observed for NS.

NT-modified binder (Figure 12d) exhibited more pronounced particle agglomeration and irregularly distributed clusters within the binder matrix. This behavior indicates reduced dispersion uniformity, which may limit the effectiveness of stress transfer and moisture resistance enhancement. The NZ-modified binder (Figure 12e) displayed relatively larger and more isolated agglomerated regions with less homogeneous distribution across the binder surface, suggesting weaker interaction between the nanoparticles and the asphalt matrix. The CNT-modified binder (Figure 12f) exhibited the most distinct microstructural morphology among the investigated nanomaterials, characterized by interconnected fibrous and entangled structures distributed within the binder phase. Although these fibrous structures may contribute to reinforcement, the formation of concentrated CNT bundles confirms incomplete dispersion and significant agglomeration behavior. The SEM observations are generally consistent with the FM analysis and the corresponding mechanical performance results. The comparatively better particle distribution observed for NS and NA correlates with their superior ITS, TSR, and IRS performance, indicating that improved dispersion enhances binder cohesion and binder–aggregate interaction. Conversely, the agglomeration behavior observed in NT, NZ, and particularly CNT-modified binders may create localized stress concentration zones and reduce the efficiency of moisture resistance improvement. These findings also explain the substantial increase in rotational viscosity observed for CNT-modified binders at higher dosages.

Overall, the SEM analysis confirms that dispersion quality and agglomeration behavior play an important role in governing the effectiveness of nanomaterial modification. Nanomaterials exhibiting finer and relatively more homogeneous distribution within the asphalt binder matrix, particularly NS and NA, demonstrated better compatibility and correspondingly higher enhancement in moisture damage resistance and mixture performance.

4.8. FTIR Spectral Analysis

The FTIR spectra of the neat and nanomodified asphalt binders are presented in Figure 13, while the calculated carbonyl index (CI) and sulfoxide index (SI) values are summarized in

Table 10. FTIR-Based CI and SI indices of asphalt binders.

Binder	Carbonyl Index (CI)	Sulfoxide Index (SI)
Neat	0.014175	0.054600
NS	0.013755	0.053655
NA	0.016065	0.037380
NT	0.012075	0.027090
NZ	0.012285	0.041265
CNT	0.016590	0.043155

. The spectra generally exhibit the typical absorption bands of asphalt binders, including the aliphatic C–H stretching vibrations within the range of approximately 2800–3000 cm⁻¹, together with several characteristic peaks associated with aromatic and oxygen-containing functional groups. No major new absorption peaks were observed after nanomaterial incorporation, indicating that the modification process was primarily physical rather than involving significant chemical reactions between the nanomaterials and the asphalt binder. The oxygen-containing functional groups, represented by the carbonyl (C=O) and sulfoxide (S=O) indices, were quantitatively evaluated to assess the influence of nanomaterial incorporation on the chemical characteristics and oxidation-related behavior of the asphalt binders. The neat binder exhibited CI and SI values of 0.014175 and 0.054600, respectively. Slight variations in these indices were observed after nanomodification, depending on nanomaterial type.

The NS-modified binder showed CI and SI values of 0.013755 and 0.053655, respectively, which are very close to those of the neat binder. This behavior suggests that NS incorporation did not significantly alter the oxidation-related functional groups of the asphalt binder. Similarly, the NT-modified binder exhibited the lowest CI and SI values among all investigated binders, reaching 0.012075 and 0.027090, respectively, indicating reduced intensity of oxygen-containing functional groups. The NZ-modified binder also exhibited lower CI and SI values compared with the neat binder, suggesting limited oxidation-related changes after modification. In contrast, NA and CNT modified binders exhibited relatively higher carbonyl index values of 0.016065 and 0.016590, respectively. The CNT-modified binder also showed a relatively elevated sulfoxide index compared with NT and NZ. This behavior may be attributed to the higher surface activity and interaction characteristics of these nanomaterials within the asphalt matrix, which could influence the absorption intensity in the oxygen-containing functional group regions.

The FTIR spectra also revealed differences in transmittance behavior among the investigated binders, particularly within the fingerprint region below 1500 cm⁻¹. CNT-modified binder exhibited the lowest transmittance values over a wide spectral range, which may be associated with its fibrous structure, higher light absorption behavior, and agglomeration tendency observed in the FM and SEM analyses. Conversely, NS and NA exhibited spectral behavior relatively comparable to the neat binder, which is consistent with their comparatively better dispersion characteristics and more homogeneous microstructure. Overall, the FTIR analysis indicates that nanomaterial incorporation did not introduce significant new chemical functional groups within the asphalt binder system. The observed changes in CI and SI values were relatively limited, suggesting that the improvement in moisture resistance and mechanical performance is primarily associated with physical reinforcement, enhanced particle distribution, and improved microstructural interaction within the binder matrix rather than major chemical modification.

5. Statistical Insights

The TSR and IRS results indicate that nanomaterial incorporation can enhance the resistance of asphalt mixtures to moisture-induced damage under laboratory conditioning conditions. However, due to the variation in nanomaterial type and dosage, statistical analysis was required to determine whether the observed differences were significant. Accordingly, a two-way analysis of variance (ANOVA) was performed considering nanomaterial type and dosage as independent variables, while TSR and IRS were considered dependent variables. The analysis was conducted at a 95% confidence level, where p-values lower than 0.05 indicate statistical significance. The null hypothesis assumed that neither nanomaterial type nor dosage significantly affects moisture damage resistance.

As summarized in

Table 11. ANOVA results.

Test Parameter	Source of Variation	df	SS	MS	F-Value	Fcrit	p-Value	Significance
TSR	Type	4	55.23	13.81	10.72	3.01	0.00020	Significant
	Dosage	3	257.08	64.27	49.90	3.01	<0.00001	Significant
	Error	12	20.61	1.29	-	-	-	-
	Total	19	332.91	-	-	-	-	-
IRS	Type	4	219.26	54.82	12.39	3.01	<0.0001	Significant
	Dosage	3	313.55	78.39	17.71	3.01	<0.00001	Significant
	Error	12	70.80	4.43	-	-	-	-
	Total	19	603.62	-	-	-	-	-

, the calculated F-values for both nanomaterial type and dosage exceeded the critical F-value (Fcrit ≈ 3.01), confirming that both factors significantly influenced TSR and IRS. For TSR, the F-values were 10.72 for nanomaterial type and 49.90 for dosage, while for IRS, the corresponding F-values were 12.39 and 17.71, respectively. In all cases, the associated p-values were substantially lower than 0.05, indicating that the observed variations were statistically significant rather than resulting from random experimental variability. The comparatively higher F-values associated with dosage, particularly for TSR, suggest that nanomaterial dosage exerts a stronger influence on moisture resistance than nanomaterial type under the investigated conditions.

To further identify the statistically significant differences among the investigated groups, Tukey’s HSD post hoc analysis was conducted for both TSR and IRS, as presented in

Table 12. Tukey HSD results for TSR.

Nanomaterial Type				Nanomaterial Dosage
Comparison	Mean Difference	p-Value	Significant?	Comparison	Mean Difference	p-Value	Significant?
CNT vs. NT	1.82	0.1027	No	0% vs. 1.5%	2.84	0.0005	Yes
CNT vs. NS	2.55	0.0475	Yes	0% vs. 3%	5.86	<0.001	Yes
CNT vs. NA	1.32	0.14	No	0% vs. 4.5%	7.99	<0.001	Yes
CNT vs. NZ	−1.61	0.12	No	0% vs. 6%	8.72	<0.001	Yes
NT vs. NS	0.72	0.1633	No	1.5% vs. 3%	3.02	0.0002	Yes
NT vs. NA	−0.5	0.1658	No	1.5% vs. 4.5%	5.14	<0.001	Yes
NT vs. NZ	−3.43	0.0113	Yes	1.5% vs. 6%	5.88	<0.001	Yes
NS vs. NA	−1.22	0.1458	No	3% vs. 4.5%	2.12	0.0168	Yes
NS vs. NZ	−4.16	0.0025	Yes	3% vs. 6%	2.86	0.0005	Yes
NA vs. NZ	−2.93	0.027	Yes	4.5% vs. 6%	0.74	0.798	No

and

Table 13. Tukey HSD results for IRS.

Nanomaterial Type				Nanomaterial Dosage
Comparison	Mean Difference	p- Value	Significant?	Comparison	Mean Difference	p-Value	Significant?
NS vs. NT	2.68	0.375	No	0% vs. 1.5%	3.91	0.026	Yes
NS vs. NA	4.83	0.014	Yes	0% vs. 3%	6.17	0.0001	Yes
NS vs. CNT	6.97	0.0001	Yes	0% vs. 4.5%	8.47	<0.001	Yes
NS vs. NZ	8.17	<0.001	Yes	0% vs. 6%	10.01	<0.001	Yes
NT vs. NA	2.15	0.594	No	1.5% vs. 3%	2.27	0.401	No
NT vs. CNT	4.29	0.039	Yes	1.5% vs. 4.5%	4.56	0.006	Yes
NT vs. NZ	5.50	0.004	Yes	1.5% vs. 6%	6.10	0.0001	Yes
NA vs. CNT	2.13	0.602	No	3% vs. 4.5%	2.30	0.386	No
NA vs. NZ	3.34	0.171	No	3% vs. 6%	3.84	0.030	Yes
CNT vs. NZ	1.21	0.925	No	4.5% vs. 6%	1.54	0.750	No

, respectively. For TSR, the analysis revealed that the differences among NS, NA, and NT were statistically insignificant ($p>0.05$), indicating comparable enhancement levels among these nanomaterials. However, statistically significant differences were observed between NS and NZ ($p=0.0025$), NA and NZ ($p=0.027$), NT and NZ ($p=0.0113$), and CNT and NS ($p=0.0475$). These findings indicate comparatively lower TSR improvement for NZ relative to NS, NA, and NT. Based on the mean TSR values, the effectiveness of the investigated nanomaterials can be ranked as: NS > NA > NT > CNT > NZ.

For the dosage effect on TSR, Tukey’s HSD analysis demonstrated that all modified mixtures produced significantly higher TSR values than the control mixture (0% nanomaterial). Significant differences were observed among most dosage levels, indicating progressive enhancement in moisture resistance with increasing nanomaterial content. However, the difference between 4.5% and 6% dosage levels was statistically insignificant ($p=0.798$), suggesting that TSR improvement begins to stabilize at higher dosages. Therefore, the effectiveness of dosage levels can be ranked as:

6% > 4.5% > 3% > 1.5% > 0%

Similarly, the Tukey HSD analysis for IRS confirmed that NS exhibited the highest retained strength values, followed by NT and NA, while CNT and NZ demonstrated comparatively lower performance. Significant differences were primarily observed between NS and CNT, NS and NZ, NT and CNT, and NT and NZ, confirming the superior moisture resistance performance of NS and NT mixtures. The IRS results also showed that increasing nanomaterial dosage significantly improved retained strength, particularly at dosage levels of 4.5% and 6%.

To further compare the modified mixtures directly with the neat binder, Dunnett’s multiple comparison test was performed for both TSR and IRS indicators, and the results are summarized in

Table 14. Dunnett comparison with the control mix for TSR and IRS.

Rank	Mixture	TSR Increase vs. Control	IRS Increase vs. Control	p-Value	Significance Level
1	NS-6%	10.6	15.53	<0.001	significant
2	NS-4.5%	9.6	14.13	<0.001	significant
3	NA-6%	9.57	10.32	<0.001	significant
4	NT-6%	9	13.53	<0.001	significant
5	NA-4.5%	8.3	7.82	<0.001	significant
6	NT-4.5%	8.17	10.93	<0.001	significant

. The analysis demonstrated that NS at 6% dosage produced the highest enhancement for both TSR and IRS relative to the control mixture, increasing TSR by approximately 10.60% and IRS by 15.53% ($p<0.001$). NS at 4.5% dosage ranked second, followed by NA at 6% and NT at 6%, all of which exhibited highly significant improvements in moisture resistance performance. For TSR, the effectiveness ranking obtained from the Dunnett comparison was:

$$\mathrm{NS}-6\%>\mathrm{NS}-4.5\%>\mathrm{NA}-6\%>\mathrm{NT}-6\%>\mathrm{NA}-4.5\%>\mathrm{NT}-4.5\%$$

Similarly, the IRS results confirmed the superior effectiveness of NS mixtures, particularly at higher dosages, with NT-based mixtures also demonstrating substantial improvement in retained strength. Overall, the Dunnett analysis suggests that nanomaterial dosage is a major factor influencing moisture resistance, while nanomaterial type governs the magnitude of improvement achieved. Among the investigated materials, nano-silica exhibited the highest overall effectiveness in improving both TSR and IRS, particularly within the dosage range of 4.5–6%, due to its superior ability to enhance asphalt–aggregate adhesion and reduce moisture-induced deterioration.

6. Conclusions

This study investigated the influence of five nanomaterials; nano-silica (NS), nano-alumina (NA), nano-titanium dioxide (NT), nano-zinc oxide (NZ), and carbon nanotubes (CNT) incorporated at dosages of 1.5%, 3.0%, 4.5%, and 6.0% by binder weight, on the physical and mechanical performance of asphalt binders and mixtures, with particular emphasis on moisture damage resistance. The experimental program included conventional binder tests, performance grading (PG), Marshall characteristics, tensile strength ratio (TSR), index of retained strength (IRS), fluorescence microscopy (FM), scanning electron microscopy (SEM), and Fourier transform infrared spectroscopy (FTIR). Based on the obtained results, the following conclusions are drawn:

• The incorporation of nanomaterials significantly influenced the physical properties of asphalt binders. Penetration decreased, and softening point increased with dosage, indicating improved stiffness and thermal stability; however, ductility was reduced, particularly at higher dosages.
• The Marshall test results showed enhanced stability and lower flow values with nanomaterial addition. Bulk density exhibited minor variations, while air voids (VTM) generally decreased, and voids in mineral aggregate (VMA) increased slightly, demonstrating favorable effects on mix compatibility and stability.
• All nanomaterials improved the ITS under both dry and wet conditions, with more pronounced improvements under moisture conditioning; for example, NS increased the ITS from 1155 to 1513 kPa (≈31%) under dry conditions and from 938 to 1398 kPa (≈49%) under wet conditions. TSR values increased from 81.2% (control) to 92.4% (NS), 91.7% (NA), and 90.4% (NT), confirming significant enhancement in resistance to moisture-induced stripping. These results suggest that nanomaterials are more effective in improving moisture-conditioned performance than only enhancing dry strength under the investigated laboratory conditions.
• Compressive strength and IRS results further confirmed the improved durability of nano-modified mixtures. The IRS increased from 72.7% (control) to 88.5% (NS), 86.2% (NT), and 83.9% (NA). The improvement in the IRS was consistently greater than that observed in the TSR, indicating that nanomaterials are more effective in preserving compressive strength under moisture exposure.
• FM and SEM analyses revealed that NS and NA exhibited comparatively better particle distribution and lower agglomeration tendency within the binder matrix, whereas NT, NZ, and particularly CNT showed more localized clustering and agglomerated structures. The improved dispersion characteristics of NS and NA corresponded well with their superior TSR and IRS performance, confirming the important role of nanomaterial dispersion quality in controlling moisture resistance.
• FTIR analysis indicated no significant formation of new chemical functional groups after nanomaterial incorporation. The relatively small variations in carbonyl and sulfoxide indices suggest that the observed performance improvement is mainly associated with physical reinforcement, improved microstructural interaction, and enhanced binder–aggregate adhesion rather than major chemical modification.
• Within the investigated dosage range, the highest TSR and IRS values were generally achieved at 6% dosage, particularly for NS and NA-modified mixtures. Statistical analysis (ANOVA, Tukey HSD, and Dunnett comparisons) confirmed that both nanomaterial type and dosage significantly influenced moisture resistance performance (p < 0.05), with dosage exhibiting comparatively greater influence. However, excessive nanomaterial contents may adversely affect binder workability; for example, CNT-modified binders at 6% dosage exhibited viscosity values exceeding 3000 mPa·s, indicating potential limitations for practical application.
• Overall, the findings demonstrate that nanomaterial modification can considerably improve the moisture resistance and mechanical performance of asphalt mixtures under laboratory conditions, particularly for NS- and NA-modified systems, which exhibited comparatively superior mechanical performance and more favorable microstructural characteristics. Nevertheless, additional long-term laboratory and field evaluations are required before generalizing these findings for practical pavement applications.

7. Limitations and Recommendations for Future Work

The present study focused primarily on the laboratory-scale evaluation of moisture damage resistance under short-term conditioning conditions. Although the obtained results demonstrated significant improvements in mechanical performance and moisture resistance, several limitations should be acknowledged. The experimental program considered only a single freeze–thaw conditioning cycle and did not include long-term aging procedures or repeated moisture exposure conditions. In addition, fatigue resistance, rutting performance, and field-scale validation were beyond the scope of the present work. The microstructural characterization conducted using FM and SEM provided comparative insight into dispersion behavior and agglomeration characteristics; however, more advanced quantitative techniques such as atomic force microscopy (AFM), surface free energy analysis, or particle size distribution analysis may provide further understanding of the interaction mechanisms between nanomaterials and asphalt binders.

In addition, the optimum asphalt content (OAC) was determined based on the control mixture and then applied uniformly to all nanomodified mixtures to maintain a consistent comparative framework. Although this approach facilitated direct evaluation of nanomaterial influence under identical conditions, it may not fully account for potential changes in workability, coating characteristics, and compactability resulting from nanomaterial incorporation. Therefore, future studies should consider determining mixture-specific OAC values for each nanomaterial type and dosage level to provide more comprehensive mixture optimization and performance evaluation. Accordingly, future research should investigate the long-term durability of nano-modified asphalt mixtures through repeated freeze–thaw cycles, aging simulations, fatigue and rutting evaluations, and field performance studies under actual traffic and environmental conditions. In addition, economic feasibility assessment and large-scale implementation studies are recommended to establish practical guidelines for pavement applications.