Effect of Waste Mask Fabric Scraps on Strength and Moisture Susceptibility of Asphalt Mixture with Nano-Carbon-Modified Filler

Abstract

This research investigates the influence of waste mask fabric scraps (WMFSs) and nano-carbon-modified filler (NCMF) on the mechanical characteristics and durability of hot mix asphalt, aiming to improve pavement performance concerning tensile stress, fatigue, and moisture damage using recycled materials. Asphalt mixtures were created with aggregate and WMFS/NCMF at 0.3% and 0.5% weight percentages (relative to aggregate), with fiber lengths of 8, 12, and 18 mm, utilizing a ‘wet mixing’ method where fibers were incrementally added to aggregates during mixing. The samples underwent indirect tensile strength, moisture susceptibility, and Marshall stability testing. The results demonstrated that incorporating WMFSs and NCMF initially enhanced tensile strength, moisture susceptibility resistance, and Marshall stability, reaching an optimal point; beyond this, further fiber addition diminished these properties. Data analysis identified the sample containing 0.3% fibers at a 12 mm length as the superior performer, showcasing the highest ITS and Marshall stability values. Statistical t-tests revealed significant differences between fiber-containing samples and control groups, verifying the beneficial impact of WMFSs and NCMF. Design-Expert software (Design-Expert 12.0.3) was used to develop functional models predicting asphalt properties based on fiber percentage and length. The optimal combination—12 mm fiber length and 0.3% WMFS/NCMF—demonstrated a 33% increase in tensile strength, a 17% improvement in moisture resistance, and a 70% reduction in fatigue deformation. Safety protocols, including thermal decontamination of WMFSs, were implemented to mitigate potential health risks.

1. Introduction

One of the most pressing challenges in pavement engineering is enhancing the performance of asphalt mixtures through sustainable modifications. Recent studies have explored the use of waste materials as additives to improve asphalt quality, particularly in the context of environmental waste management [1,2]. Among these materials, disposable medical masks, whose global proliferation surged during the COVID-19 pandemic, have emerged as a critical waste management issue [3]. These masks, composed of non-degradable petroleum-based polymers such as polypropylene (PP), persist in the environment and contribute to soil and air pollution due to their hydrocarbon derivatives [4,5]. Concurrently, research on the role of filler materials in asphalt mixtures has been extensively studied, with micronized calcium carbonate (CaCO₃) and carbon-coated fillers identified as modifiers to enhance mechanical properties [6,7]. However, no prior research has investigated the synergistic effects of NCMF and WMFSs on asphalt performance [8,9]. The dual objectives of this study are (1) to repurpose COVID-19-generated mask waste (6.8 billion masks/day) into functional asphalt reinforcement, mitigating environmental pollution, and (2) to leverage NCMF’s nanoscale modification of bitumen–aggregate interfaces for enhanced moisture resistance and mechanical properties. While waste masks (WMFSs) and nano-modified fillers (NCMFs) have been studied independently, their synergistic interaction—a novel mechanism for improving moisture resistance, tensile strength, and Marshall stability—remains unexplored [8,9]. This gap is critical given masks’ non-degradable polypropylene content and asphalt’s vulnerability to moisture/fatigue damage. Here, we investigate how WMFSs and NCMF jointly enhance asphalt performance, addressing both environmental waste crises and infrastructure resilience.

Medical masks consist of non-woven layers, including spunbond and melt-blown PP, which exhibit high tensile strength and polymer compatibility with bitumen [10,11]. Their fibrous structure can mitigate asphalt’s inherent weakness in tensile resistance, reducing reflective cracking and rutting [12,13]. Studies confirm that fibers enhance bitumen absorption, optimize binder content, and form a 3D reinforcement network, improving mixture durability [14]. For instance, aramid and polyester fibers increase fatigue life by 1.6–2 times compared to conventional mixes [15], while polypropylene fibers reduce thermal cracking due to their chemical affinity with bitumen [16]. Critically, enhancing moisture resistance is paramount, as moisture damage accelerates rutting, raveling, stripping, and fatigue cracking, while long-term aging exacerbates moisture susceptibility through binder embrittlement [17]. Cost-effectiveness remains underexplored; a reported cost increase for fiber-modified mixes is offset by extended service life [18].

While prior research has examined mask fibers and modified fillers independently, none has addressed their combined application. Consequently, key unresolved questions stemming from this gap in the literature require investigation. First, how NCMF and shredded mask fibers (SMFs) jointly influence Marshall stability must be examined. Second, understanding their impact on moisture susceptibility and fatigue behavior is crucial for optimizing asphalt mixtures.

Therefore, this study evaluates three key aspects: It assesses Marshall stability of mixes containing limited dosages (0.3–0.5%) of shredded mask fibers (SMFs) by aggregate weight combined with 5–7% NCMF; examines fatigue life under controlled-stress loading (25–40 °C); and evaluates moisture damage using tensile strength ratio (TSR) tests.

Table 1. Key characteristics and research focus for WMFSs and NCMF in asphalt modification.

Aspect	Conventional Asphalt Filler	Waste Mask Fabric Scraps (WMFSs)	Nano-Carbon-Modified Filler (NCMF)	Target Synergistic Effect
Primary Composition	Limestone/Portland cement	Polypropylene (PP) polymers	CaCO3 + carbon nanostructures	Hybrid reinforcement
Dominant Mechanism	Void filling	3D fiber reinforcement	Bitumen activation	Interfacial bond strength
Critical Parameter	Particle size (µm)	fiber length (mm)	Nano-carbon concentration (%)	Fiber–filler interaction
Waste Integration	Not applicable	129 billion masks/year recycled	Industrial byproduct utilization	Circular economy pavement
Performance Gap	Limited crack resistance	Unquantified durability impact	Unknown moisture inter	Fatigue–moisture balance

[19,20] summarizes the fiber types used.

This research employs a materials approach to enhance asphalt matrix performance. Shredded mask fibers (2–6 mm length) reinforce the matrix, while NCMF (1–5% nano-carbon by weight) activates bitumen–filler reactions. Moisture susceptibility and fatigue performance are quantified via AASHTO T 283 and four-point bending tests. The novelty lies in optimizing filler–fiber synergy to enhance mechanical performance (e.g., ≥20% Marshall stability increase) while advancing waste recycling (repurposing 129 billion masks/year [21]). Results aim to provide actionable data for sustainable pavement design, supporting Sustainable Development Goals (SDGs) for infrastructure resilience [22]. The incorporation of WMFSs in asphalt mixtures, coupled with the addition of NCMF, presents a novel multifaceted approach to enhancing moisture susceptibility, tensile strength, and Marshall stability of asphalt pavements through their synergistic interaction—a mechanism unexplored in the existing literature on individual additives [8,9]. However, long-term aging, weathering effects, and environmental implications (e.g., fiber degradation or leaching) require further investigation to assess real-world feasibility. From a mechanistic perspective, the WMFS fabric serves as a reinforcing agent, distributing tensile stresses more uniformly within the asphalt matrix and thereby reducing crack propagation [22]. The enhanced ductility provided by the polymer component of the fabric allows the asphalt to deform more significantly before failure, increasing its resistance to fatigue cracking caused by repeated traffic loading. Furthermore, the woven structure of the fabric creates a physical interlock with the aggregate particles, improving aggregate retention and reducing the potential for raveling, a major contributor to pavement deterioration [23]. In relation to (1), the adhesion energy between the asphalt binder and aggregate, denoted as W_ad, is commonly modeled by considering the surface free energy components of both materials using the following form:

$$W_{ad}={\gamma }_A+{\gamma }_B-{\gamma }_{AB}$$

(1)

where γ_A is the surface energy of asphalt (mJ/m²), γ_B is the surface energy of aggregate (mJ/m²), and γ_AB is the interfacial tension (mJ/m²) [24]. The presence of WMFS fabric alters γ_A due to the incorporation of rubber particles, leading to a modified adhesion energy and subsequently influencing moisture susceptibility. An increase in TSR indicates improved resistance to moisture-induced damage. The fabric’s presence effectively enhances the mixture’s cohesive strength, counteracting the weakening effect of water infiltration. The NCMF significantly modifies the surface chemistry of the aggregate. Nano-carbon materials, such as graphene or carbon nanotubes, possess a high surface area and can strongly interact with the asphalt binder, forming a more robust interfacial bond [25]. This enhanced adhesion is due to van der Waals forces and potential chemical reactions between the carbon nanostructure and the asphalt components, particularly the polar fractions. The presence of nano-carbon can also inhibit oxidative aging of the asphalt, thereby improving its long-term durability. The complex modulus of the asphalt mixture, G, is influenced by the properties of the filler. In this context, NCMF contributes to a higher G value, particularly at elevated frequencies, indicating increased stiffness and resistance to permanent deformation [8]. To theoretically assess the impact of a stiff filler like NCMF on mixture properties, a model based on the principles of the Hirsch model can be considered, leading to the following relation (2):

$$G_{mix}^{\ast }=v_a⌈1-\left({\displaystyle \frac{1}{1+\left(\frac{G_f}{G_b}\right)\left(\frac{1-v_a}{v_f}\right)}}\right)⌉+(1-v_a)G_b$$

(2)

where v_a is the air void content (%), v_f is the filler content (%), G_f is the complex modulus of NCMF (Pa), and G_b is the complex modulus of asphalt binder (Pa). Furthermore, the coating may create a hydrophobic surface, reducing the aggregate’s affinity for water and thereby potentially enhancing resistance to moisture damage [26]. The surface energy of the NCMF, particularly the polar component, can be significantly lowered by the nano-carbon coating, thereby minimizing the disruption of the asphalt–aggregate bond by water molecules [27,28]. The combination of WMFSs and NCMF affects the overall structure and morphology of the asphalt mixture.

The WMFS fabric provides a continuous network within the mixture, acting as a crack arrestor and preventing the propagation of microcracks that can lead to premature failure. The NCMF, due to its small size and high surface area, can effectively fill the voids within the asphalt matrix, reducing the air void content and improving the density of the mixture [29]. This reduction in air voids minimizes the ingress of water and further enhances the mixture’s resistance to moisture damage. The Marshall stability (MS) and flow (F) values, critical indicators of pavement performance, are significantly influenced by this synergistic effect. The increased interlocking between the fabric, aggregate, and NCMF contributes to a higher MS value, indicating improved resistance to deformation under load. Simultaneously, the enhanced ductility provided by the WMFS fabric prevents the mixture from becoming brittle and cracking prematurely. Based on Equations (3) and (4), a potential mathematical model relating MS and F to the percentage of WMFS fabric (F_%) and NCMF (N_%) can be formulated as follows:

$$MS=\alpha +\beta \left(F_{\%}\right)+\gamma \left(N_{\%}\right)+\delta \left(F_{\%}\right)\left(N_{\%}\right)$$

(3)

$$F=\epsilon +\zeta \left(F_{\%}\right)+\eta \left(N_{\%}\right)+\theta \left(F_{\%}\right)\left(N_{\%}\right)$$

(4)

where α, β, γ, δ, ε, ζ, η, and θ are empirically determined coefficients derived from experimental data. The interaction term (F%) (N%) is included to account for a potential synergistic effect of combining WMFS fabric and NCMF [30]. The optimized combination of these materials produces an asphalt mixture with enhanced moisture susceptibility, improved tensile strength, and superior Marshall stability, making it a viable option for constructing durable and long-lasting pavements. Studies on the addition of shredded mask fibers to asphalt mixtures and their effects on asphalt properties have primarily focused on mixtures containing conventional fillers that act solely as space-fillers. There has been less emphasis on the impact of reactive fillers on the properties of asphalt mixtures [31]. As noted, numerous studies have examined various additives and their influence on asphalt performance; however, no study has simultaneously investigated the use of waste medical mask fibers and modified fillers [32]. From a mechanistic perspective, the WMFS fabric serves as a fibrous reinforcing agent, distributing tensile stresses more uniformly within the asphalt matrix and thereby reducing crack propagation [22]. This study aims to quantitatively evaluate the synergistic effects of waste mask fabric scraps (WMFSs) and nano-carbon-modified filler (NCMF) on the mechanical and durability properties of asphalt mixtures. The specific objectives are as follows:

-

To assess the impact of WMFS fiber length (8, 12, 18 mm) and content (0.3%, 0.5% by aggregate weight) combined with NCMF (5–7%) on Marshall stability, moisture susceptibility (TSR), and indirect tensile strength (ITS).

-

To investigate the fatigue resistance of WMFS/NCMF-modified asphalt under controlled-stress loading (300–500 kPa) at intermediate temperatures (25–40 °C).

-

To develop predictive models for key performance indicators (stability, TSR, fatigue life) using Design-Expert software, establishing optimal fiber–filler parameters.

Mechanistically, WMFS fibers reinforce the asphalt matrix through crack-bridging and stress redistribution [14,22], while NCMF enhances bitumen–aggregate adhesion via nanoscale surface modifications that increase hydrophobic character and interfacial bond strength [27,33,34]. Their synergy creates a dual-scale reinforcement system where fibers mitigate macrocracking while nano-carbon-modified fillers improve micromechanical properties—an interaction previously unexplored for waste-derived composites.

2. Research Methodology

In this research, a systematic approach was employed to investigate the effects of adding WMFSs to a nano-carbon-modified asphalt mixture. Initially, medical mask waste was processed using an ordinary office paper shredder to create fibers with lengths of 8, 12, and 18 mm. Fibers were oven-dried at 60 °C for 2 h to remove moisture. This thermal treatment also served to neutralize potential biological contaminants, ensuring safe handling during subsequent processing. Subsequently, graded limestone aggregates (MS-2 gradation; bulk specific gravity 2.64) were blended with preheated NCMF (105 °C, 4 h) in a mechanical mixer for 90 s. Asphalt mixtures were compacted using a Marshall hammer (75 blows/side) at 150 °C. These dimensions were selected to balance enhancing strength and preventing clumping of the shredded fabric during mixing. Subsequently, graded aggregate materials conforming to MS-2 specifications were blended with NCMF, as shown in Figure 1. The incorporation of nano-carbon into the filler aimed to enhance the rheological properties of the bitumen and improve the adhesion between the bitumen and aggregate. This strategy sought to increase shear resistance and decrease the temperature susceptibility of the asphalt mixture. Subsequently, PG 64-22 bitumen was heated to 160 ± 5 °C and blended with preheated NCMF filler (105 °C) at 5–7% by aggregate weight. Fiber dimensions (8, 12, and 18 mm) and content (0.3% and 0.5%) were selected based on aspect ratio optimization (length/diameter ≈ 120–270) derived from fiber-reinforcement literature [35,36]. This range balances optimal stress transfer and workability while preventing fiber clustering. During mixing, WMFSs were incrementally added to aggregates over 15 s intervals to ensure uniform dispersion. Fiber dimensions (8–18 mm) and content (0.3–0.5%) were selected based on aspect ratio optimization (length/diameter ≈ 120–270) to ensure dispersion and prevent clumping [37,38]. During the asphalt mixing process, the WMFSs were introduced at concentrations of 0.3% and 0.5% by weight of the aggregate. To ensure uniform dispersion and prevent clumping—critical for reliable fiber reinforcement—fibers were incrementally added over 15 s intervals during aggregate blending. Subsequently, mixing employed a high-shear mixer (Silverson L5M-A) at 2000 rpm for 3 min to achieve homogeneous distribution of both WMFSs and nano-carbon.

After mixture preparation, asphalt samples were fabricated (including NCMF/WMFS composites and controls). Triplicate samples (n = 3) were tested for Marshall stability, flow, TSR, ITS, and fatigue to ensure statistical reliability. These samples underwent Marshall stability tests, indirect tensile strength tests, moisture susceptibility evaluations, and fatigue testing. The Marshall stability test was performed to assess the compressive strength and stability of the asphalt mixture under traffic loads. The indirect tensile strength test was conducted to determine the tensile strength and elastic modulus of the asphalt mixture, evaluating its ability to withstand tensile stresses caused by temperature variations and repeated traffic loads. Moisture susceptibility evaluation was carried out to assess the reduction in asphalt mixture strength due to water infiltration and to evaluate the adhesion between bitumen and aggregate in the presence of water. Fatigue testing was performed to determine the lifespan of the asphalt mixture under repeated traffic loads and to evaluate its resistance to fatigue cracking.

The indirect tensile test, conducted in accordance with ASTM D3967-95a, aims to determine the tensile strength of asphalt samples. This is a diametric compression test in which a cylindrical sample is subjected to lateral compressive force, inducing tensile stresses within the sample; thus, this test serves as a reliable measure for predicting cracking in asphalt mixtures, according to ASTM International (2025) [39]. To perform this test, the sample is positioned within the jaws of the testing machine and loaded at a temperature of 26 °C with a rate of 50 mm/min until failure occurs.

The moisture susceptibility test, carried out according to AASHTO T283 [39], is the most widely used method for assessing the moisture susceptibility of asphalt mixtures. In this test, two groups of samples are evaluated. The first group is wrapped in plastic and immersed in water at 25 degrees Celsius for 4 h before testing. The second group is first vacuum saturated at a pressure of 13 to 67 kPa for five minutes, followed by submersion in water for 5 to 10 min to achieve saturation. The saturated samples are then placed in a plastic bag containing 10 mL of water and frozen at −18 °C for 16 h. Subsequently, the samples are immersed in a water bath at 60 °C for 24 h, as shown in Figure 2. Finally, both groups of samples undergo an indirect tensile strength test at a constant loading rate of 50 mm/min, and the force required to break the samples is recorded. After determining the force needed to break the samples, the indirect tensile strength is calculated, followed by the calculation of the moisture susceptibility of the sample, or the TSR.

Fatigue testing was performed using a Universal Testing Machine (UTM) under a controlled-stress loading mode according to AASHTO T 321. Cylindrical specimens (100 mm diameter × 63.5 mm height) were subjected to repeated haversine axial compression at specific stress levels, including 300 kPa, 400 kPa, and 500 kPa. The loading frequency was set at 10 Hz, and the tests were conducted at temperatures of 20 °C ± 0.5 °C, which are representative of intermediate pavement temperatures. The failure criterion was defined as a 50% reduction in initial stiffness or specimen rupture [36]. While stiffness reduction was monitored, vertical deformation (mm) was selected as the primary analysis parameter for this study. This is because it serves as a direct and unambiguous physical measure of cumulative damage and rutting potential under repeated loading, providing a clear indicator of the mixture’s resistance to permanent deformation and fatigue cracking. Each test continued until sample failure, with vertical deformation recorded at 100-interval increments. Fatigue life (Nf) was defined as the number of load cycles at which stiffness dropped to 50% of its initial value. To ensure statistical reliability, triplicate samples were tested per stress level [36]. Therefore, assessing the fatigue performance of asphalt mixtures is particularly important. The experimental results were meticulously analyzed using Design Expert and SPSS software 26 to thoroughly evaluate the effects of incorporating shredded mask fibers and nano-carbon on the mechanical and performance characteristics of the asphalt mixture. This analysis aimed to develop statistical models capable of predicting the behavior of the asphalt mixture under various loading and environmental conditions. Graded aggregates (Figure 3) were oven-dried at 175 ± 5 °C for 4 h, then mixed with NCMF-modified bitumen for 90 s in a mechanical mixer.

Aggregates conforming to MS-2 specifications (Figure 3) were utilized. Graded aggregates were oven-dried at 175 ± 5 °C for 4 h, then mixed with NCMF-modified bitumen for 90 s. The aggregates, sourced directly from the factory, underwent comprehensive characterization to confirm their suitability for the intended application. All assessed properties, including abrasion loss (determined via the Los Angeles test), soundness in sodium sulfate solution, flat and elongated particle content, fractured faces, sand equivalent value, and fine aggregate angularity, were found to be within acceptable limits. Furthermore, the specific gravity and water absorption for both the coarse and fine aggregates were also well within the permissible ranges, indicating the high quality and compliance of the materials.

Table 2. Properties of PG 64-22 asphalt binder according to ASTM standards [39].

Test Type (ASTM Standard [39])	Result	Unit
Flash Point (ASTM D92)	292	Degrees (°C)
Viscosity (ASTM D4402)	0.33	Pascal-seconds (Pa·s)
Dynamic Shear (ASTM D7175)	1.174	kPa
Specific Gravity (ASTM D70)	1.03	-
Penetration Grade (ASTM D5)	64	0.1 mm
Softening Point (ASTM D36)	51	Degrees (°C)

outlines the properties of the PG 64-22 asphalt binder used, including flash point, viscosity, dynamic shear, specific gravity, penetration grade, and softening point.

The nano-carbon-modified filler (NCMF) consisted of micronized calcium carbonate (CaCO₃, particle size ≤ 75 µm) coated with multi-walled carbon nanotubes (MWCNTs). The MWCNTs (purity > 95%, outer diameter 10–20 nm, length 10–30 µm, surface area 220 m²/g) were uniformly dispersed onto CaCO₃ via ultrasonic processing in ethanol (30 min at 40 kHz), achieving a nano-carbon coating of 5 wt.% of the filler. The technical specifications for WMFS material properties and description are provided in

Table 3. Technical specifications for WMFS material properties and description.

Property	Value/Description	Unit
Material Type	100% spunbond	-
Color	White	-
Specific Gravity	0.91	-
Thickness	79	Microns (µm)
Melting Point	168	Degrees (°C)
Tensile Strength	3.86	-
Tear and Abrasion Resistance	High	Qualitative description
Acid and Alkali Resistance	High	Qualitative description

. After solvent evaporation, the composite was dried at 105 °C for 4 h. NCMF replaced the mineral filler fraction (passing sieve #200) in the asphalt mixture. Standard test methods, units, and purposes are summarized in

Table 4. Standardized material properties and performance test parameters for WMFS/NCMF-modified asphalt mixtures according to ASTM standards [39].

Material Properties	Standard Code [39]	Unit	Purpose
Binder Flash Point	ASTM D92	°C	Safety during heating
Binder Viscosity	ASTM D4402	Pa·s	Flow resistance measurement
Binder Penetration	ASTM D5	0.1 mm	Consistency evaluation
Binder Softening Point	ASTM D36	°C	Temperature susceptibility
WMFS Thickness	-	μm	Fiber dimensional property
WMFS Tensile Strength	-	MPa	Reinforcement capacity
Performance Tests
Marshall Stability	ASTM D6927	kN	Load-bearing capacity
Indirect Tensile Strength	ASTM D3967	kPa	Cracking resistance
Moisture Susceptibility	AASHTO T283	TSR (ratio)	Water damage resistance
Fatigue Life	AASHTO T321	Nf (cycles)	Durability under repeated load
Dynamic Shear	ASTM D7175	kPa	Rutting resistance

.

3. Results Analysis

Following the determination of the optimal bitumen content, asphalt mixture samples were prepared for subsequent testing. A total of 90 samples were fabricated for the experimental program. The indirect tensile strength (ITS) test was employed as a valuable tool to evaluate the tensile resistance of asphalt mixtures and predict the onset of cracking. Furthermore, this test was utilized to assess moisture susceptibility and the fatigue life of the asphalt mixtures. In addition to ITS testing, resilient modulus and fatigue performance were characterized using a Universal Testing Machine (UTM). It is important to note that the UTM facilitates a range of pavement laboratory applications, including dynamic tension tests, asphalt fatigue analysis, and indirect tensile strength determination. Asphalt mixture samples were systematically labeled to denote the length and percentage of WMFSs incorporated, based on the aggregate weight. The nomenclature utilized consisted of an “A” prefix, followed by the fiber length in millimeters, e.g., A₈, A₁₂, and A₁₈, and a suffix indicating the fiber content as a percentage, e.g., 0.3 and 0.5. For example, “A8-0.3” represents a mixture containing 8 mm fibers at a content of 0.3% by weight of aggregate. A total of 90 samples covering seven mix types (

Table 5. Sample acronyms and composition details.

Sample ID	Fiber Length (mm)	WMFS Content (% by agg. Weight)	NCMF Content (% by agg. Weight)	Nano-Carbon in NCMF (%)
Control	0	0	6	5
A8-0.3	8	0.3	6	5
A8-0.5	8	0.5	6	5
A12-0.3	12	0.3	6	5
A12-0.5	12	0.5	6	5
A18-0.3	18	0.3	6	5
A18-0.5	18	0.5	6	5

Note: NCMF content fixed at 6% (midpoint of 5–7% range); nano-carbon coating = 5 wt.% of NCMF.

) were fabricated for the experimental program.

The optimum bitumen content was determined as 4.4% using the Marshall method, corresponding to Marshall stability = 1250 kN, theoretical specific gravity (G_mm) = 2.38 g/cm³, voids in mineral aggregate (VMA) = 13.6%, air voids (V_a) = 3.5%, and flow = 2.95 mm. These values were derived by averaging the bitumen percentages at peak Marshall stability, maximum G_mm, and minimum Va, while ensuring all parameters met standard specifications.

3.1. Indirect Tensile Strength Testing

ITS testing was conducted on asphalt mixtures incorporating NCMF, carbon black, and WMFSs to evaluate cracking susceptibility. This test, a dependable indicator of tensile resistance, predicted cracking potential via recording the ultimate failure load and subsequent ITS calculation. The results, detailed in Figure 4, initially demonstrated an increase in ITS with augmented fiber length and content, followed by a decline at elevated concentrations. Specifically, the A12-0.3 sample exhibited the highest ITS value (Figure 4). This peak performance is attributed to optimal fiber–matrix bonding and stress distribution, while higher fiber contents caused agglomeration and reduced workability. Fibers acted as stress transfer agents, bridging microcracks and impeding crack propagation, thus enhancing tensile resistance. Conversely, the subsequent ITS reduction at higher fiber contents likely stemmed from fiber agglomeration or altered matrix stiffness, hindering optimal stress distribution. Excessive fiber content potentially reduced workability and caused incomplete aggregate coating, creating stress concentrations and failure points. The superior performance of A12-0.3 likely represented an optimized balance between fiber reinforcement and matrix bonding, minimizing stress concentrations and maximizing tensile resistance. Optimal fiber content is crucial for enhancing cracking resistance without compromising asphalt mixture integrity. Further investigations, including microscopic fracture surface analysis, would elucidate underlying mechanisms of ITS variations.

3.2. Moisture Susceptibility Testing

To investigate the influence of moisture on the adhesive bond between the bitumen and aggregate surfaces, moisture susceptibility testing was performed. The ITS test was conducted on both dry and saturated asphalt mixture samples, and the TSR was calculated. The results are presented in Figure 5. Figure 5A shows that the tensile strength of the saturated asphalt mixture samples was lower compared to the dry samples. The TSR, calculated as the ratio of the average tensile strength of the saturated group to that of the dry group, is shown in Figure 5B. The TSR initially increased with greater fiber length and content, followed by a subsequent decrease. This trend mirrors the variations observed in the ITS results. As shown in Figure 5A, the tensile strength of both dry and saturated asphalt mixture samples initially increased before decreasing as the percentage and length of waste facemask fibers increased. This indicates that sample A12-0.3 demonstrates the highest resistance to moisture damage.

3.3. Fatigue and Marshall Stability Testing; Analysis

As previously noted, fatigue failure is a common mode of pavement distress, posing a significant concern for pavement engineers. This research examined the fatigue performance of asphalt specimens using a UTM, with the results displayed in Figure 6. According to Figure 6, specimen A12-0.3, containing 0.3% of 12 mm fibers, demonstrated the lowest deformation (0.224 mm), while specimen A18-0.5, containing 0.5% of 18 mm fibers, exhibited the highest deformation (0.319 mm) under loading. This indicates a superior fatigue cracking performance of specimen A12-0.3 compared to the others.

Marshall stability and flow tests were performed on triplicate specimens (n = 3 per mix). Results, including average load capacity and flow values, are shown in Figure 7. The Marshall stability results indicated that as the percentage and length of WMFSs increased, the compressive strength initially rose, reaching a peak of 1741 kN. Subsequently, the strength of the specimens decreased by approximately 8% to 1607 kN. As shown in Figure 7, the highest and lowest Marshall stability values were recorded for specimens A12-0.3 and A8-0.3, respectively, while the highest and lowest flow values corresponded to A8-0.3 and A18-0.5, respectively.

3.4. Effects of WMFSs on Asphalt Properties

As shown in past Section 3.1, Section 3.2 and Section 3.3, adding WMFSs initially enhanced tensile strength, moisture resistance, fatigue life, and Marshall stability (peaking at 12 mm/0.3% WMFSs). Beyond this threshold, properties declined due to bitumen absorption by WMFSs (estimated at 0.5–1.5% by fiber weight based on polypropylene fiber literature [14,17]) and consequent reduced workability. Critically, optimum binder content was maintained at 4.4% for all mixtures, indicating no significant increase was required despite fiber absorption—though this may contribute to workability challenges at higher WMFS doses (

Table 6. t-Test results for key asphalt properties among different groups.

Parameter	Comparison	Sig.	0.05>	Result
Fatigue	Group 1 vs. 2	0.00	Yes	Significant difference
	Group 1 vs. 3	0.00	Yes	Significant difference
Marshall Stability	Group 1 vs. 2	0.048	Yes	Significant difference
	Group 1 vs. 3	0.005	Yes	Significant difference
Flow	Group 1 vs. 2	0.00	Yes	Significant difference
	Group 1 vs. 3	0.002	Yes	Significant difference
Moisture Susceptibility	Group 1 vs. 2	0.00	Yes	Significant difference
	Group 1 vs. 3	0.00	Yes	Significant difference
Tensile Strength	Group 1 vs. 2	0.00	Yes	Significant difference
	Group 1 vs. 3	0.00	Yes	Significant difference

).

4. Discussion and Data Interpretation

This study thoroughly investigates the distinct and synergistic contributions of NCMF and WMFSs in asphalt mixtures. Mechanistically, NCMF primarily enhances bitumen–aggregate adhesion through its nano-carbon coating, which reduces interfacial tension (γ_AB in Equation (1)) and increases hydrophobicity [26,38,40], thereby improving moisture resistance. Concurrently, it elevates mixture stiffness (complex modulus G*) and inhibits oxidative aging [8,25]. In contrast, WMFSs act as fibrous reinforcement, redistributing tensile stresses, bridging cracks, and improving ductility through their 3D polypropylene network [14,22,41]. Their synergy manifests when NCMF-modified interfaces strengthen the bond between aggregates and the WMFS-reinforced bitumen matrix, while WMFS fibers prevent stress concentration at NCMF–aggregate contact points. Quantitatively, the optimal WMFS/NCMF combination improved moisture resistance by 17% (attributed primarily to NCMF’s hydrophobic effect) and tensile strength by 33% (driven dominantly by WMFS reinforcement), confirming complementary roles. Compared to control samples and carbon-modified mixtures [42,43], the synergistic system showed superior performance in all tested parameters.

NCMF’s nano-carbon coating (Figure 8A). The notable 410% increase in Marshall stability indicates a significant enhancement in the mixture’s resistance to permanent deformation under load. This can be ascribed to the fibers functioning as a network, distributing stress and preventing localized failure within the asphalt. Likewise, the 70% reduction in fatigue deformation suggests that WMFSs improve the mixture’s ability to endure repeated loading cycles before cracking. This enhancement likely results from the fibers bridging microcracks, inhibiting their propagation and prolonging the mixture’s fatigue life. The 33% increase in tensile strength further substantiates the strengthening effect of WMFSs, indicating that the composite material can withstand higher tensile stresses before failure. Lastly, the improved moisture sensitivity, evidenced by a 17% reduction in moisture-induced damage, may be due to the hydrophobic nature of the mask fibers, which decreases water absorption and minimizes the weakening of the asphalt–aggregate bond (

Table 7. t-Test results for key asphalt properties among sample groups.

Parameter	Comparison	Sig.	Result
Fatigue	Group 1 vs. 2	0.00	Significant difference
	Group 1 vs. 3	0.00	Significant difference
Marshall Stability	Group 1 vs. 2	0.048	Significant difference
	Group 1 vs. 3	0.005	Significant difference
Flow	Group 1 vs. 2	0.00	Significant difference
	Group 1 vs. 3	0.002	Significant difference
Moisture Susceptibility	Group 1 vs. 2	0.00	Significant difference
	Group 1 vs. 3	0.00	Significant difference
Tensile Strength	Group 1 vs. 2	0.00	Significant difference
	Group 1 vs. 3	0.00	Significant difference

Notes: Group 1 = WMFS composite mixtures; Group 2 = Control mixtures; Group 3 = carbon-modified mixtures; significance threshold: α = 0.05.

).

Furthermore, as noted, Design-Expert software is a tool for determining the relationship between independent and dependent variables. This research examines the relationship between independent variables, specifically fiber content and length, and dependent variables that represent the performance characteristics of the composite material. Experimental results were treated as dependent variables, while fiber content and length were defined as independent variables.

Table 8. Relationships between independent and dependent variables.

R-Squared	Relationship	Dependent Variable
0.72	Fatigue = 0.15038 + 0.0925 × Quantity + 0.00604 × Length	Fatigue
0.58	Marshall Stability = 1234.79167 + 68.33333 × Quantity + 24.96382 × Length	Marshall stability
0.92	Flow = 1.27683 − 0.12833 × Quantity − 0.00644 × Length	Flow
0.61	TSR = 0.96615 − 0.06923 × Quantity − 0.00377 × Length	Tensile strength ratio
0.57	Tensile Strength = 842.25 + 587.5 × Quantity + 23.553 × Length − 55.526 × Quantity × Length	Tensile strength

Note: quantity = fiber percentage, length = fiber length.

presents the established relationships between these variables, and Figure 8 illustrates these relationships through three-dimensional plots. As indicated in Table 8, the R-squared values for all relationships exceed 0.5, suggesting a reasonably high level of reliability. Therefore, the relationships presented can be utilized to predict experimental outcomes without the need for physical sample preparation and laboratory testing.

Further analysis was conducted to investigate the impact of the fibers. A factor termed “Fiber Amount” was defined as the product of the weight and length of the WMFS, serving as a new independent variable, as in Figure 9. Statistical analysis was performed using Design-Expert software to establish relationships between this independent variable and the previously defined dependent variables. The resulting relationships are presented in

Table 9. Relationships between independent and dependent variables (using “Fiber Amount”).

R-Squared	Relationship	Dependent Variable
0.75	Fatigue = 0.2421 − 0.00036 × X + 0.00001 × X2	Fatigue
0.60	Marshall Stability = 777.8541 + 23.9484 × X − 0.1510 × X2	Marshall stability
0.81	Flow = 1.2101 − 0.0011 × X	Flow
0.64	TSR = 0.9164 − 0.00016 × X − 0.000004 × X2	Tensile strength ratio (TSR)
0.65	Tensile Strength = 900.9919 + 6.8442 × X − 0.0513 × X2	Tensile strength

Note: X = product of weight and length of WMFSs.

.

Regression models detailed in Table 9 reveal that “Fiber Amount” exerts a complex, often non-linear influence on asphalt mixture performance. Initial increases in “Fiber Amount” correlate with decreased fatigue resistance (negative linear coefficient), potentially stemming from matrix disruption or stress concentration. However, a positive quadratic term suggests that further increases could improve fatigue life through enhanced crack bridging. Marshall stability also exhibits a parabolic relationship, indicating an optimal “Fiber Amount” beyond which stability diminishes, possibly due to fiber agglomeration. Flow shows an inverse relationship with “Fiber Amount”, likely due to restricted binder mobility and tensile strength displaying quadratic relationships as well, suggesting a balance between fiber reinforcement and potential embrittlement [2]. While R-squared values highlight the significant contribution of “Fiber Amount,” the models’ limitations indicate the influence of other factors, such as fiber type, dispersion, and binder interaction.

Further investigation, including microstructural analysis, is warranted to clarify these relationships and validate assumptions regarding fiber entanglement and matrix disruption. Mechanistically, WMFS fibers reinforce the asphalt matrix through crack-bridging and stress redistribution [14,22], while NCMF enhances bitumen–aggregate adhesion via nanoscale surface modifications that increase hydrophobic character and interfacial bond strength [25,26,27] (Figure 10).

NCMF’s nano-carbon coating reduces aggregate surface energy (Equation (1)), increasing hydrophobicity and moisture resistance [26]. Concurrently, WMFS fibers form a 3D network that redistributes tensile stresses (Figure 10).

5. Conclusions

This research employed a comprehensive experimental methodology to evaluate the performance of asphalt mixtures incorporating nano-carbon modified filler (NCMF) and waste face mask scraps (WMFS). The methodology involved processing WMFSs into specific lengths (8, 12, 18 mm) and incorporating them at dosages of 0.3% and 0.5% by weight of aggregate into an NCMF-modified asphalt mixture. The samples were then subjected to a series of standardized performance tests—including Marshall stability, Indirect Tensile Strength (ITS), moisture susceptibility (AASHTO T283), and fatigue testing (AASHTO T321)—to statistically assess key mechanical properties. This systematic investigation yielded the following key outcomes:

• All properties peaked at 12 mm/0.3% WMFS, then declined due to fiber clustering.
• The WMFS-NCMF blend enhanced tensile strength by 33% versus control samples.
• Moisture resistance improved by 17%, and fatigue deformation was reduced by 70%.
• Marshall stability increased by 39% through optimized fiber–matrix synergy.
• The addition of WMFSs initially enhanced the indirect tensile strength by 9.9%, followed by a 3.9% decrease at higher fiber contents. The 12 mm and 0.3% WMFS sample exhibited the highest tensile strength.
• Similarly, moisture susceptibility initially improved by 2.3% with the addition of WMFSs, subsequently decreasing by 7% at higher concentrations. Again, the 12 mm and 0.3% WMFS sample demonstrated the best resistance to water damage.
• Fatigue performance peaked at 0.3% WMFS/12 mm length (lowest deformation: 0.224 mm), with higher contents increasing deformation.
• Marshall stability tests revealed an initial increase in compressive strength up to 1741 kN with increasing fiber content, followed by an 8% decrease to 1607 kN. The highest and lowest Marshall stability values were observed for the 12 mm and 8 mm samples at 0.3% WMFS, respectively. Maximum and minimum flow values corresponded to the 8 mm samples at 0.3% and 0.5% WMFS, respectively.
• Based on a thorough analysis of the test results, the 12 mm sample at 0.3% WMFS, which combines NCMF and WMFSs, emerged as the optimal blend. It demonstrated the highest tensile strength and Marshall stability, along with the lowest fatigue deformation and moisture susceptibility. The tensile strength of this sample increased by approximately 33% compared to control samples and 13% compared to those with only NCMF. Additionally, moisture susceptibility improved by approximately 17%, and fatigue deformation was reduced by 70% (to 30% of the control value) relative to control samples.
• WMFSs enabled macroscale reinforcement via stress distribution and crack bridging.
• NCMF enhanced bitumen–aggregate adhesion through hydrophobic nano-coatings.
• The 12 mm/0.3% threshold defines industrial optimization for sustainable asphalt.

This study tested WMFSs at 0.3% and 0.5% by aggregate weight. While these dosages identified a performance peak at 0.3%/12 mm, testing extended ranges (e.g., 0.1–0.7%) would enhance optimization robustness. Future studies should incorporate broader content variations to validate peak performance thresholds.

Future investigations need to perform microstructure analysis to understand the bonding between WMFSs, NCMF, and asphalt. Future research should investigate the synergistic impact of NCMF combined with WMFSs on porous asphalt (e.g., OGFC) mixtures, evaluate these additives in modified bitumen (e.g., SBS-modified), explore recycled concrete aggregate (RCA) alongside WMFSs, conduct life-cycle cost analysis (LCCA), and assess long-term durability via accelerated aging protocols (e.g., UV, moisture cycles).