Study on the Influence of ZM Modifier on the Rheological Properties and Microstructural Characteristics of Asphalt

Abstract

As traffic load continuously rises and climatic conditions increasingly vary, the performance of conventional base asphalt can no longer satisfy the needs of modern road engineering in low-temperature cracking resistance, high-temperature stability, and long-term durability. Therefore, the development of novel and efficient asphalt modifiers holds significant engineering value and practical importance. In this study, modified asphalt was prepared using varying dosages of ZM modifier (direct-injection asphalt mixture modified polymer additive). A series of experiments was executed to assess its influence on asphalt properties. First, fundamental property tests were implemented to determine the regulating effect of the ZM modifier on basic physical performances, like the softening point and penetration of the base asphalt. Penetration tests at different temperatures were performed to calculate the penetration index, thereby assessing the material’s temperature sensitivity. Subsequently, focusing on temperature as a key factor, tests on temperature sweep, and multiple stress creep recovery (MSCR) were implemented to delve into the deformation resistance and creep recovery behavior of the modified asphalt under high-temperature conditions. In addition, bending beam rheometer (BBR) experiments were introduced to attain stiffness modulus and creep rate indices, which were applied to appraise the low-temperature rheological performance. Aside from Scanning Electron Microscopy (SEM), Fourier Transform Infrared Spectroscopy (FTIR) was utilized to explore the mechanism by which the ZM modifier influences the asphalt’s functional group composition and microstructure. Our findings reveal that the ZM modifier significantly increases the asphalt’s softening point and penetration index, reduces penetration and temperature sensitivity, and enhances high-temperature stability. Under high-temperature conditions, the ZM modifier adjusts the viscoelastic balance of asphalt, hence enhancing its resistance to flow deformation and its capacity for creep recovery. In low-temperature environments, the modifier increases the stiffness modulus of asphalt and improves its crack resistance. FTIR analyses reveal that the ZM modifier does not introduce new functional groups, indicating a physical modification process. However, by enhancing the cross-linked structure and increasing the hydrocarbon content within the asphalt, it strengthens the adhesion between the asphalt and aggregates. Overall, the asphalt’s performance improvement positively relates to the dosage of the ZM modifier, providing both theoretical basis and experimental support for its application in road engineering.

1. Introduction

Utilized as a binding material in road engineering, asphalt is an important player in gauging the durability of pavement structures, their overall service performance, and the comfort and safety of driving. As traffic load is on the rise and climate conditions are becoming complex, traditional petroleum asphalt has revealed several performance limitations during long-term service. For instance, it is prone to permanent deformation and rutting at escalated temperatures, and to brittle cracking at low temperatures—issues that severely compromise the service life and reliability of roadways [1,2]. Therefore, ameliorating the asphalt’s high- and low-temperature stability and its overall environmental adaptability has become a key focus in the field of pavement material research.

To address the performance limitations of base asphalt, extensive research has been devoted to asphalt modification technologies. Among these, polymer modification has attracted particular attention due to its remarkable enhancements in the mechanical performances and environmental adaptability of asphalt, making it one of the key directions in current research and practical application. Common asphalt modifiers can be broadly categorized into three types: First, polymer-based modifiers—such as SBS (styrene-butadiene-styrene block copolymer), PE (polyethylene), and EVA (ethylene-vinyl acetate copolymer)—enhance the elasticity and toughness of asphalt, thereby significantly improving its rutting resistance at high temperatures and cracking resistance at low temperatures [3,4,5]. Second, inorganic modifiers—such as crumb rubber, mineral fibers, nano-clay, and graphene—primarily reinforce the internal skeleton or form composite systems to enhance the mechanical strength and thermal stability of asphalt [6,7]. Third, bio-based modifiers—including natural rubber, vegetable oils, and lignin—contribute to the sustainability and environmental friendliness of asphalt materials [8,9]. Among them, SBS-modified asphalt has achieved large-scale application in many countries due to its well-balanced performance. However, challenges such as insufficient thermal stability at high temperatures, poor long-term aging resistance, and price volatility have hindered its wider adoption, particularly in regions with extreme climates. Consequently, the progress of novel, eco-friendly, cost-effective, and high-performance modifiers has become an urgent need in material innovation and engineering applications.

In recent years, with the rapid advancement of functional materials and nanotechnology, the research on novel asphalt modifiers has gained momentum and achieved promising progress. For example, organosilicon-based modifiers are capable of improving the asphalt’s high-temperature properties and aging resistance by forming stable network structures [10]. It is shown that organically modified bentonite and polyamide-based modifiers ameliorate the asphalt’s cohesive strength and interfacial stability [11]. Moreover, the valorization of certain industrial by-products and agricultural waste has opened new avenues for the green modification of asphalt. For instance, nano-silica is able to efficaciously enhance the asphalt’s thermal stability and UV aging resistance [12], while Guo and coworkers have proved that the incorporation of functionalized rubber powder ameliorates asphalt’s low-temperature crack resistance [13]. These studies provide valuable references for the development of high-performance modifiers and promote the functionalization and greening of asphalt materials.

Against this backdrop, the ZM modifier, as an emerging multifunctional material, has attracted growing attention in the academic community due to its favorable thermal stability, molecular compatibility, and structural regulation capability in asphalt. Preliminary studies have shown that ZM modifiers can effectively increase the softening point and viscoelastic properties of asphalt, reduce its temperature sensitivity, and enhance internal cross-linked structures, thereby improving deformation resistance and structural stability [14,15,16]. The modification process is primarily physical in nature, without the introduction of new functional groups, yet it significantly increases the hydrocarbon content within the asphalt, enhancing the adhesion between asphalt and aggregates.

However, despite its promising application potential, systematic investigations into the mechanism of ZM modification remain limited. In-depth studies on its effects at varying dosages on the basic properties, rheological behavior, and microstructural evolution of asphalt are still lacking. In particular, key aspects such as its regulatory influence on temperature sensitivity, the mechanism underlying viscoelastic performance enhancement, and the evolution of microstructure during modification require further exploration.

The application of ZM modifiers in asphalt represents a groundbreaking innovation that significantly improves many of the properties of asphalt and pushes the technological boundaries in the field of sustainable pavement materials. Therefore, this study focuses on the ZM modifier and prepares modified asphalt with varying dosages. Through basic performance tests, rheological property analysis, Fourier Transform Infrared Spectroscopy (FTIR), and Scanning Electron Microscopy (SEM), a systematic investigation is conducted from both macroscopic and microscopic perspectives to elucidate the performance variations and underlying mechanisms. The aim is to present a theoretical basis and technological reference for the progress of high-performance road materials, while establishing a scientific foundation for the application of ZM modifiers.

2. Experimental Materials and Methods

2.1. Experimental Materials

2.1.1. Base Asphalt

In our work, the used asphalt is a 90# base asphalt produced in Panjin, Liaoning.

Table 1. Fundamental performance indicators of base asphalt.

Test Item	Measured Value	Specification Requirement
Penetration at 25 °C/0.1 mm	82.8	80–100
Ductility at 15 °C/cm	106	≥100
Softening Point/°C	47.5	≥44
Dynamic Viscosity at 60 °C/Pa·s	230	≥160
Flash Point/°C	260	≥245
Penetration Index PI	−0.49	−1.8~+1.0

displays its fundamental performance indicators.

2.1.2. Modifier

The modifier investigated in this study is the ZM modifier, a solid granular, directly mixed high-polymer additive designed for asphalt mixtures. It is primarily composed of high-performance polymers and resins and is produced in Shenyang, Liaoning Province. Due to its capability of effectively enhancing the properties of asphalt pavements and reducing construction complexity, the ZM modifier has been widely applied in road engineering projects across many regions in China in recent years, achieving favorable results.

In Figure 1, the ZM modifier appears as black granules.

Table 2. Technical specifications of ZM modifier.

Item	Technical Requirement
Appearance	Black solid granules
Main Components	45% polymer, 50% resin, 5% reactants
Particle Size	<5.8 mm
Softening Point	137 °C
Density	0.95 g/cm3

presents its technological specifications.

2.2. Preparation of ZM-Modified Asphalt

Our ZM-modified asphalt was fabricated by directly adding the ZM modifier to base asphalt at mass ratios of 3%, 5%, and 7% relative to the asphalt content. The modified asphalt was produced using a combination of a mechanical stirrer and a high-velocity shear mixer. The specific fabrication procedure is as follows, and Figure 2 shows the flowchart: ① The heated base asphalt changed into a molten state and was poured into a metal container. ② ZM modifiers at discrepant dosages were supplemented to the asphalt. The metal container containing asphalt was put onto one hot plate showing 165 °C. The asphalt stirrer was turned on. Synchronously, the speed gradually rose to 300 r/min. The mixture was stirred at this speed for 15 min. ③ The pre-mixed modified asphalt was then subjected to high-velocity shearing through a shear mixer at 160 °C, with a shear rate of 5000 r/min for 60 min. The above mixing method ensures that the materials are thoroughly mixed.

2.3. Test Methods

2.3.1. Basic Performance Tests of Asphalt

The penetration experiment primarily reflects the consistency of the asphalt specimen; a lower penetration value indicates higher consistency. The test was conducted at 25 °C, with a loading time of 5 s and a normal load of 100 g. The ultimate outcome was used as the average of three parallel measurements. The softening point experiment was executed through the standard ring-and-ball approach. Before testing, the specimens were placed in a constant temperature water bath at 5 °C ± 0.5 °C for 15 min. The metal support frame, steel balls, and ball centering rings were also placed in the same bath for thermal equilibrium. Each test was implemented in duplicate. Significantly, the average of the two values was regarded as the ultimate result. The ductility experiment reflected the asphalt’s extensibility under low-temperature conditions; higher values indicate a greater low-temperature crack resistance. Before testing, the molds were put within a constant temperature water bath for 1.5 h at 5 °C. The pulling speed of the ductilometer (SYD-4508G-1, Shanghai Changji Geological Instrument Co., Ltd., Shanghai, China) was set to 5 cm/min. The ultimate outcome was determined as the average of three parallel experiments.

2.3.2. Temperature Sweep Test

The test on temperature sweep yields three key parameters: intricate shear modulus (G*), phase angle (δ), and rutting factor (G*/sinδ), which are used to evaluate the asphalt’s viscoelastic properties. The intricate shear modulus (G*) exhibits the asphalt’s overall resistance to shear deformation and is defined as the proportion of maximal shear stress to maximal shear strain. At a given temperature, a higher G* suggests a stronger resistance to deformation at high temperatures. The phase angle (δ) works as a key parameter for describing the asphalt’s viscoelastic behavior. It indicates the ratio of the material’s viscous and elastic components. A smaller δ value means a higher percentage of recoverable elastic deformation during loading. The rutting factor (G*/sinδ), proposed under the U.S. SHRP (Strategic Highway Research Program), is a crucial indicator for appraising the asphalt’s high-temperature rutting resistance. It represents the material’s resistance to permanent deformation at elevated temperatures; if the value is greater, the rutting resistance will be better [17]. Figure 3 displays the rheometer (HTR 3000, Haake Thermo Fisher Scientific, Karlsruhe, Germany) and test samples.

2.3.3. Multiple Stress Creep Recovery (MSCR) Test

The MSCR test (MCR 302e, Anton Paar GmbH, Graz, Austria) is devised to simulate the irreversible deformation and delayed elastic recovery of asphalt under external loading conditions. It is primarily conducted at high temperatures, where asphalt is subjected to loading to induce deformation, followed by unloading to observe its delayed recovery behavior. Through repeated loading and unloading cycles, the test realistically reproduces the stress conditions experienced by asphalt pavements under traffic loads [18]. In this test, two stress levels are applied: 0.1 kPa and 3.2 kPa. At each stress level, the test comprises 10 cycles, with each cycle containing 1 s of loading and 9 s of unloading. The test is conducted at temperatures from 58 °C to 76 °C, with intervals of 6 °C. As to each loading cycle, the initial strain is recorded as ε₀, and the strain at the end of loading is documented as εc. The actual creep strain is calculated as ε₁, ε₁ = ε_c − ε₀. After the recovery period, the strain is recorded as ε_r, and the unrecoverable creep strain is defined as ε₁₀, ε₁₀ = ε_r − ε₀. The recovery rate R(%) and unrecoverable creep compliance J_nr (%) at discrepant stress levels are figured out using Equations (1) and (2), respectively. The symbol σ denotes the applied shear stress during the MSCR test, with units of kPa.

$$R(\%)={\displaystyle \frac{{\epsilon }_1-{\epsilon }_{10}}{{\epsilon }_1}}\times 100\%$$

(1)

$$J_{\mathrm{nr}}(\%)={\displaystyle \frac{{\epsilon }_{10}}{\sigma }}\times 100\%$$

(2)

2.3.4. Low-Temperature Bending Beam Creep Test

The asphalt’s low-temperature fracture resistance was assessed using a bending beam rheometer (BBR) (SYD-0627, Zealchon Electronic Technology Co., Ltd., Xi’an, China). The test was executed at −12 °C and −18 °C to determine the asphalt’s low-temperature property indices. The evaluation was based on two key parameters measured at 60 s: the stiffness modulus S and the creep rate m. The creep rate m reflects the stress relaxation capacity and time-dependent sensitivity of the asphalt’s stiffness; a more elevated m-value indicates greater resistance to low-temperature cracking. The stiffness modulus S corresponds to the asphalt’s capability of resisting cracking; a lower S-value suggests better low-temperature performance [19]. Figure 4 presents the BBR and test specimens.

2.3.5. Fourier Transform Infrared Spectroscopy Test

In this study, FTIR spectroscopy (Fourier Transform Infrared Spectroscopy) (TENSOR II, Bruker AXS GmbH, Karlsruhe, Germany) was utilized to delve into the impact of the ZM modifier on the asphalt’s functional groups. Infrared spectra are divided into three regions based on wavenumber: near-infrared, mid-infrared, and far-infrared. Among them, the mid-infrared area is the most pervasively applied to organic structural analysis [20]. The mid-infrared region covers a wavelength range of 2.5 μm to 25 μm, corresponding to a wavenumber range of 400–4000 cm⁻¹. The mid- and far-infrared areas are key players in spectral analysis and could generally be segregated into two sections: One is the functional group area (4000–1300 cm⁻¹), which is primarily used to identify the absorption peaks of various functional groups within a substance, thereby enabling the differentiation of distinct chemical moieties. The second is the fingerprint region (600–300 cm⁻¹), which is highly sensitive to changes in molecular structure. This region captures both simple variations in bond lengths and vibrational frequencies, as well as complex absorption peaks resulting from alterations in molecular conformation.

2.3.6. Scanning Electron Microscopy Test

In our research, SEM (GeminiSEM300 Carl Zeiss AG, Oberkochen, Germany) was introduced to examine the microstructural influences of the ZM modifier on asphalt. Asphalt binder is a complex polymeric material composed of compounds with varying molecular sizes and polarities, resulting in a diverse and intricate surface microstructure. These microstructural features significantly influence the performance of asphalt materials. By using the SEM experiment to examine the surface morphology, valuable insights into the structural characteristics and the modification mechanisms could be obtained. The SEM experiment operated at an accelerating voltage of 5 kV. Prior to observation, the asphalt specimens were covered by a thin layer of gold via continuous sputtering to enhance conductivity, and subsequently dried under an infrared lamp.

3. Results and Discussion

3.1. Basic Performance Test Results of Asphalt

Figure 5 presents the results of the fundamental performance tests for ZM-modified asphalt showing divergent modifier dosages.

In Figure 5, the incorporation of ZM modifier induces a reduction in the penetration value and a rise in the softening point of the base asphalt. Specifically, when the ZM dosage is 3%, the penetration decreases from 8.3 mm to 6.3 mm, and the softening point varies between 48 °C and 56 °C. This corresponds to a penetration reduction of approximately 25% and a softening point increase of about 16% relative to the base asphalt. With an increase in the ZM dosage, the modification effect is more pronounced. At a 7% dosage, the penetration further decreases to 4.5 mm, while the softening point increases to 80 °C. These changes indicate that the incorporation of ZM modifier ameliorates the asphalt consistency, thereby enhancing its high-temperature property. Such enhancement can be ascribed to the absorption of light fractions in the asphalt by the polymeric components of the ZM modifier. This results in an increased proportion of asphaltenes and resins, and a corresponding reduction in light constituents, which contributes to the observed decline in penetration and rise in softening point. Overall, the addition of ZM modifier enhances the asphalt’s resistance to deformation at high temperatures.

By measuring the Penetration Index (PI) of asphalt, its sensitivity to temperature variations can be evaluated. As demonstrated by a lower temperature sensitivity, the asphalt’s performance is less affected by temperature fluctuations [21].

Indeed, penetration values were measured at the test temperatures T = 15 °C, 25 °C, and 30 °C and analyzed using the specification JTG E20-2011 (T 0627) [22]. A regression analysis was conducted based on the logarithmic relationship between penetration (P) and temperature, as defined by Equation (3). Based on the regression results, the following parameters were calculated using Equations (4) to (7): Penetration Index (PI), Equivalent Softening Point (T₈₀₀), Equivalent Brittleness Temperature (T_1.2), and Plastic Temperature Range (ΔT). The results are summarized in

Table 3. Regression parameters of penetration and results of PI.

Dosage/%	Regression Equation	R2	PI	T800/°C	T1.2/°C	ΔT/°C
0	y = 0.0451x + 0.8102	0.9972	−0.78	46.78	−15.83	62.62
3	y = 0.0367x + 0.9020	0.9945	0.58	54.53	−22.42	76.95
5	y = 0.0362x + 0.8599	0.9961	0.68	56.44	−21.57	78.01
7	y = 0.0359x + 0.8020	0.9976	0.73	58.53	−20.13	78.66

.

$$\lg P=K+A_{\lg \mathrm{Pen}}$$

(3)

$$PI={\displaystyle \frac{20-500A_{\lg \mathrm{Pen}}}{1+50A_{\lg \mathrm{Pen}}}}$$

(4)

$$T_{800}={\displaystyle \frac{\lg 800-K}{A_{\lg \mathrm{Pen}}}}={\displaystyle \frac{2.0931-K}{A_{\lg \mathrm{Pen}}}}$$

(5)

$$T_{1.2}={\displaystyle \frac{\lg 1.2-K}{A_{\lg \mathrm{Pen}}}}={\displaystyle \frac{0.0792-K}{A_{\mathrm{lgPen}}}}$$

(6)

$$\mathsf{\Delta }T=T_{800}-T_{1.2}$$

(7)

In the equations: PI refers to the Penetration Index, K is the regression constant; T signifies the test temperature, and A_lgPen indicates the coefficient of the regression equation.

The linear regression correlation coefficient R² between penetration and temperature for all samples exceeds the specification threshold of 0.997, indicating a strong linear relationship [23]. The Penetration Index (PI) reflects the asphalt’s temperature sensitivity: a higher PI value stands for a lower sensitivity to temperature variations, while a lower PI signifies higher sensitivity. The regression equation represents the correlation between the penetration index and temperature. In the regression equation, y denotes lg P (logarithm of penetration), x denotes the test temperature (°C), and the numeric coefficients correspond to the intercept and slope obtained from the regression fitting.

As shown in Table 3, the PI values of all four asphalt types fall within the range of −1 ≤ PI ≤ 1. The base asphalt exhibits the lowest PI, at −0.78. With the ZM modifier added, the PI values of the modified asphalt increase and all exceed that of the base asphalt. Specifically, the PI increases from −0.78 to 0.58 as the modifier content increases. When the ZM dosage reaches 7%, the PI further rises to 0.73. This tendency validates that the incorporation of ZM modifier reduces the asphalt’s temperature sensitivity, thereby mitigating the effect of temperature fluctuations on its performance.

The equivalent softening point T₈₀₀ reveals the asphalt’s high-temperature performance; a more elevated value suggests a greater resistance to thermal deformation. As the ZM modifier dosage increases, T₈₀₀ rises from 46.78 °C to 58.53 °C, demonstrating that the ZM modifier effectively enhances the asphalt’s high-temperature performance. The equivalent brittleness temperature T_1.2 exhibits slight variations. As discussed in Section 3.1, the incorporation of the ZM modifier leads to relatively poorer low-temperature ductility. However, the ZM-modified asphalt shows a lower T_1.2 compared to the base asphalt, indicating that it retains a certain degree of low-temperature resistance. The plastic temperature range ΔT is associated with the asphalt’s temperature sensitivity. The incorporation of the ZM modifier heightens the ΔT value, thereby expanding the asphalt’s plastic deformation range and improving its adaptability to varying temperature conditions.

3.2. Temperature Sweep Test Results

Figure 6 and Figure 7 show the test results for the intricate shear modulus and rutting factor of ZM-modified asphalt with different modifier dosages, separately.

As shown in Figure 6 and Figure 7, both the asphalt’s intricate shear modulus and the rutting factor are reduced with increasing temperatures. Such a trend is ascribed to enhanced molecular mobility within the asphalt at elevated temperatures, which weakens intermolecular cross-linking and increases flowability, thereby reducing the material’s resistance to deformation.

The order of magnitude for both the intricate shear modulus and rutting factor at the same temperature is listed below: Base asphalt < SBS-modified asphalt < Base + 3% ZM < Base + 5% ZM < Base + 7% ZM.

At a given temperature, a higher complex shear modulus indicates better high-temperature performance. Therefore, the test results demonstrate that both SBS and ZM modifiers ameliorate the base asphalt’s high-temperature performance. Specifically, the complex shear modulus of asphalt with 3% ZM modifier is comparable to that of SBS-modified asphalt, while more escalated dosages of ZM show even greater enhancement.

At 64 °C, the intricate shear modulus (G*) of asphalt showing 3% ZM increases by 95% compared to the base asphalt, and with 5% ZM, G* increases by 127%. As the ZM dosage increases, G* continues to rise, indicating a progressively stronger effect in enhancing the asphalt’s high-temperature deformation resistance.

In Figure 7, the rutting factor is reduced with increasing temperature, indicating that the asphalt’s rutting resistance decreases as temperature rises. The reason is that higher temperatures lessen the asphalt’s elastic constituent, while increasing its viscous behavior. At the identical temperature, the rutting factors of modified asphalts markedly surpass that of the base asphalt. In accordance with the test results, at 64 °C, the base and ZM-modified asphalts meet the requirement of G*/sinδ > 1.00 kPa. However, the rutting factor of ZM-modified asphalt is substantially higher, illuminating a greater resistance to permanent deformation. When the temperature increases to 70 °C, all ZM-modified asphalts (at 3%, 5%, and 7% dosages) still meet the G*/sinδ > 1.00 kPa criterion, whereas the base asphalt falls short, with a value of only 0.54 kPa. This suggests that the ZM-modified asphalt forms a more stable internal structure capable of withstanding higher environmental temperatures. Figure 8 displays the results of the phase angle experiment.

In Figure 8, the phase angles of all asphalts vary continuously with increasing temperatures, suggesting a variation of the material’s viscoelastic balance. Within the temperature sweep range, the phase angle generally increases with temperature. This may be due to the reduction of elastic components and increase of viscous components in asphalt under the influence of heat. Additionally, elevated temperatures intensify molecular motion, leading to greater intermolecular friction. As a result, strain increasingly lags behind stress, which causes the phase angle to rise [24]. The base asphalt exhibits the highest phase angle across the temperature range, with its curve located at the top of the graph. Upon the addition of the ZM modifier, the phase angle decreases, and the corresponding curves shift downward. This indicates that the ZM modifier effectively delays the phase transition of asphalt. As the ZM dosage increases, the phase angle continues to decline, reflecting a higher proportion of elastic behavior at the identical temperature. Consequently, the modified asphalt demonstrates improved resistance to deformation at high temperatures.

3.3. Multiple Stress Creep Recovery Test Results

Figure 9 and Figure 10 exhibit the calculated values of unrecoverable creep compliance J_nr and creep recovery rate R for discrepant asphalt binders, respectively. J_nr and R values measured under stress levels of 0.1 kPa and 3.2 kPa.

As shown in Figure 9 and Figure 10, under both 0.1 kPa and 3.2 kPa stress levels, the creep recovery rate R of all five asphalt types decreases with increasing temperature, while the non-recoverable creep compliance Jnr shows a rising trend. Among the tested asphalts, the base asphalt exhibits the highest Jnr under both stress conditions. In contrast, ZM-modified asphalts consistently display lower Jnr values than the base asphalt, and Jnr decreases progressively as the ZM dosage increases at the same temperature-indicating enhanced elastic recovery capability.

Additionally, the slope of the Jnr temperature curve for ZM-modified asphalt noticeably comes short of that of the base asphalt. At a 5% ZM dosage, the slope becomes comparable to that of the SBS-modified asphalt. This suggests that the modifier effectively decreases the asphalt’s temperature sensitivity. Such improvement is attributable to the network structure formulated through the polymer components in the modifier, which acts as a barrier, limiting thermal softening and maintaining structural stability. The Jnr values at 0.1 kPa are lower than those at 3.2 kPa for all asphalt types, indicating that higher stress levels weaken the delayed elastic recovery and reduce deformation resistance. This also suggests that, in real-world scenarios, low-stress conditions (e.g., light traffic) cause minimal irreversible creep, emphasizing the importance of load control in traffic management. After the incorporation of the ZM modifier, the Jnr values drop significantly. For instance, at 0.1 kPa and 58 °C, the Jnr of asphalt with 7% ZM content is decreased by 90%, in marked contrast with the base asphalt, which confirms that the incorporation of ZM modifier markedly improves both the elastic recovery and deformation resistance of asphalt binders.

In Figure 10, the creep recovery rate R reflects the asphalt’s elastic recovery capacity. If R value is higher, it indicates a stronger elasticity and greater resistance to deformation. At all test temperatures, the recovery rate of the base asphalt remains below 10%, indicating that it behaves predominantly as a viscous material with poor elastic recovery at high temperatures. With the incorporation of the ZM modifier, the light fractions in asphalt decrease while the internal elastic components increase. The enhanced cross-linking effect introduced by the ZM modifier significantly improves the R value. For instance, at 0.1 kPa and 58 °C, the R value rises from 4% in the base asphalt to 20% with 3% ZM content, and further to 60% with 5% ZM content. This demonstrates a marked improvement in elastic recovery and high-temperature deformation resistance, and the enhancement becomes more pronounced with increasing ZM dosage. In some cases, during testing, negative R values were observed. This typically occurs under high temperature and high stress conditions, where the asphalt’s stress relaxation capacity is weakened. During loading, the rate of stress relaxation becomes slower than the rate at which stress is accumulated, resulting in continuously increasing deformation and an apparent negative recovery rate (i.e., R < 0).

3.4. Low-Temperature Bending Beam Creep Test Results

Figure 11 shows the stiffness modulus and creep rate of the modified asphalt at different temperatures.

As shown in Figure 11, for all modifier dosages, the stiffness modulus S of asphalt at −18 °C increases significantly compared to that at −12 °C, while the creep rate m shows a decreasing trend. This indicates that at lower temperatures, asphalt tends more to low-temperature deformation and cracking risks. A rise in stiffness and internal tensile stress, coupled with the reduction in stress relaxation capacity at lower temperatures, brings about a reduction in the asphalt’s resistance to cracking. Integrated with the incorporation of the ZM modifier, all m-values (creep rate) meet the specification requirements. At −18 °C and a 7% modifier dosage, the S-value slightly exceeds 300 MPa. At the identical temperature, as the ZM dosage increases, the S-value gradually rises while the m-value gradually decreases, reflecting a slight reduction in stress relaxation ability. However, the variation in S and m values caused by changes in ZM dosage is relatively small, indicating that the ZM modifier exerts a restrictive influence upon the asphalt’s low-temperature property.

3.5. FTIR Test Result Analysis

Figure 12 displays the FTIR spectra of asphalt showing divergent ZM modifier dosages.

Figure 12 exhibits the FTIR test results. In the spectrum, the horizontal axis indicates the wavenumber. Additionally, the vertical axis represents transmittance. A stronger absorption peak corresponds to a lower transmittance, meaning that the substance absorbs more infrared light at that specific wavenumber. After the addition of the ZM modifier, variations in functional groups lead to noticeable differences in the absorption peaks. From the comparison in Figure 12, multiple small absorption peaks are observed between 3400 and 4000 cm⁻¹. These peaks may be due to vibration coupling caused by highly polar asphalt components being too close together. Notably, two strong absorption peaks appear at 2925 cm⁻¹ and 2850 cm⁻¹ in the ZM-modified asphalt. These correspond to the symmetric and asymmetric stretching vibrations of –CH₂– and –CH₃– groups on saturated carbon chains, separately. Relative to the base asphalt, the enhanced intensity of these peaks indicates an increased content of saturated hydrocarbons due to the incorporation of the modifier.

A weak absorption peak is also present from 2200 to 2400 cm⁻¹, corresponding to the stretching vibrations of C≡C bonds. This suggests the emergence of alkynes in the asphalt matrix. Additionally, the peak at 1599 cm⁻¹ is associated with the stretching vibration of –C=C–, reflecting the presence of aromatic or olefinic structures. The absorption peaks at 1450 cm⁻¹ and 1265 cm⁻¹ are associated with the bending vibrations of CH₃ (angular deformation) and the umbrella-type deformation of CH₂, respectively. These peaks are enhanced after the addition of the ZM modifier, confirming that the modifier increases the saturated hydrocarbon content in the asphalt. In the wavenumber range of 700 to 900 cm⁻¹, sharp absorption peaks appear, which are attributable to the out-of-plane bending vibrations of C–H bonds in aromatic compounds. Such peaks illustrate the emergence of benzene rings or adjacent hydrogen atom groups within the asphalt. Notably, the absorption intensities at 700 cm⁻¹ and 900 cm⁻¹ are significantly increased after the incorporation of the modifier, suggesting that ZM enhances the aromatic hydrocarbon content in the asphalt matrix. As unveiled by a comparative analysis of the FTIR spectra of base and ZM-modified asphalts, no new functional groups are introduced and no original groups are eliminated following the addition of the ZM modifier. Only the intensity of some characteristic peaks changes. The main increase observed in ZM-modified asphalt is in hydrocarbon-related functional groups. The elevated hydrocarbon content improves the overall polarity of the asphalt, thereby enhancing its adhesion to aggregates.

3.6. Scanning Electron Microscopy Results

Figure 13 and Figure 14 exhibit the SEM results of the base and modified asphalts, respectively.

The test images present microstructural features at 300 μm and 100 μm magnifications (the scale bar in Figure 13a, Figure 14a and Figure 13b, Figure 14b corresponds to 300 μm and 100 μm, respectively). A comparison of the two asphalt samples reveals that the ZM-modified asphalt (Figure 14) exhibits an interwoven, reticular cross-linked structure. This network results from the dispersion of the ZM modifier across the asphalt in the mixing process. The uniformly distributed cross-linked structure effectively limits the flowability of asphalt under high-temperature conditions and forms a stable internal framework. This network structure may originate from the uniform dispersion of ZM modifier during the asphalt mixing process. This uniformly distributed cross-linked structure effectively restricts the fluidity of asphalt under high temperature conditions and forms a stable internal framework.

4. Conclusions

Our research assessed the influence of ZM modifiers on the asphalt’s fundamental performances through three conventional index tests. The ZM-modified asphalt’s high- and low-temperature properties were analyzed in detail using a DSR and a BBR. FTIR was introduced to explore changes in functional groups prior to and subsequent to modification, and SEM was utilized to visualize microstructural alterations. The key conclusions are listed below:

(1)

According to the traditional index experiments, the ZM modifier significantly improves the asphalt’s penetration and softening point. At a 3% dosage, the penetration index (PI) increases from −0.78 to 0.58, indicating reduced temperature sensitivity of the asphalt.

(2)

Regarding high-temperature performance, the temperature sweep and MSCR tests show that the ZM modifier increases the rutting factor and creep recovery rate while reducing the unrecoverable creep compliance. This demonstrates enhanced elastic recovery and high-temperature deformation resistance. For low-temperature performance, the BBR results confirm that all indicators remain within specification limits after modification, suggesting that ZM has minimal adverse effects on asphalt’s low-temperature behavior.

(3)

FTIR and SEM analyses indicate that no new functional groups are generated after adding the ZM modifier; only quantitative changes in existing groups are observed. The increased presence of alkyl groups improves the asphalt–aggregate adhesion, while enhanced internal cross-linking contributes to a more stable microstructure in the modified asphalt.