Abstract
In this paper, the durability of emulsified asphalt in aging environment is taken as the research object, and its performance evolution law and multi-scale damage mechanism under the action of aging factors are systematically explored. By simulating different aging cycles, macroscopic performance tests (including penetration, ductility, softening point and viscosity), dynamic rheological tests (dynamic shear rheology, multi-stress creep recovery, linear amplitude scanning) and microscopic characterization (scanning electron microscopy, infrared spectroscopy, atomic force microscopy) were carried out to comprehensively analyze the structural evolution and performance response of emulsified asphalt during aging process from macroscopic, rheological and microscopic scales. The results show that with the extension of aging time, the light component of emulsified asphalt is transformed into the heavy component, the intermolecular force is enhanced, the material gradually becomes hard and brittle, the elastic component increases, and the viscous component decreases. Macroscopically, the penetration and ductility decreased significantly, the softening point continued to rise, and the viscosity showed a fast-slow growth trend with time. The rut factor of rheological properties increases with the deepening of aging, the strain recovery ability decreases, the unrecoverable creep compliance increases, the fatigue life decreases significantly, and the fatigue life decrease rate increases with the increase of strain, which are 94.28%, 94.96% and 97.74%, respectively. The microstructure shows that the surface is gradually dense and corrosion products appear, and the molecular structure undergoes oxidation and condensation reactions. Furthermore, combined with molecular dynamics simulation, the change rules of bulk modulus, shear modulus and diffusion coefficient of emulsified asphalt during aging process were revealed from the molecular scale, and the internal mechanism of mechanical property degradation was clarified. This study provides a multi-scale theoretical basis for further understanding the damage mechanism of emulsified asphalt in aging environment, and lays a scientific foundation for the development of weather-resistant emulsified asphalt materials and the construction of aging protection technology system.
1 Introduction
With the continuous expansion of China’s infrastructure scale and the extension of pavement service lives, asphalt pavement is confronting severe aging challenges under natural climatic factors such as prolonged sunlight exposure, high temperatures, oxidation, and temperature-humidity cycles (). Aging, as a critical factor leading to the deterioration of asphalt material performance and insufficient pavement durability, directly affects pavement structural integrity and service life (). This issue is particularly pronounced in regions with heavy traffic and harsh environmental conditions, where aging-induced asphalt embrittlement, cracking, and decreased fatigue resistance have become primary causes of early pavement distress. Emulsified asphalt is widely used in pavement maintenance and cold recycling due to its advantages of energy efficiency, environmental friendliness, and construction convenience (). However, the presence of water and emulsifiers in the system may alter the aging process and damage mechanisms, rendering its aging evolution significantly different from that of traditional hot asphalt. Currently, research on the damage mechanisms of emulsified asphalt aging spanning macro, rheological, and micro scales remains limited. Therefore, in-depth exploration of the performance evolution path and multi-scale damage mechanisms of emulsified asphalt in aging environments holds significant theoretical and engineering importance.
Domestic and international scholars have conducted valuable explorations of asphalt and emulsified asphalt aging behavior from various perspectives. For example, . compared the effects of short-term and long-term aging on the rheological properties of different ionic types of emulsified asphalt, finding that aging significantly altered the balance between high-temperature rutting resistance and fatigue resistance. Regarding microscopic mechanisms, pointed out that aging promotes the conversion of light components to heavy components, resulting in viscoelastic degradation of materials. In terms of modification, . revealed that styrene-butadiene rubber (SBR) can effectively delay aging and improve mechanical properties through molecular simulation and experimental verification. While these studies () provide important foundations for understanding aging effects, they fail to systematically clarify the continuous evolution of emulsified asphalt performance in aging environments and lack explanation of cross-scale damage correlation mechanisms.
In recent years, molecular dynamics (MD) simulation has emerged as a powerful tool for revealing the microscopic mechanisms of asphalt aging (). Comprehensive reviews have systematically summarized MD simulation applications in asphalt research, demonstrating its capability to investigate key performance aspects including aging processes, self-healing behavior, and interfacial adhesion by capturing molecular interactions. Significant progress has been made in understanding aging mechanisms through advanced MD simulations. Recent work employing reactive neural network potential has uncovered a sequential “dehydrogenation-oxidation-crosslinking” reaction network during asphalt aging, identifying dominant reaction pathways and revealing that aging proceeds via successive polarization of chemical bonds. Other studies have investigated photo-oxidative aging mechanisms and asphaltene aggregation phenomena, providing fundamental understanding of how aging transforms asphalt microstructure.
Regarding rejuvenator effects, MD simulation has been extensively applied to understand depolymerization mechanisms of various regenerants (). Multi-scale evaluation methodologies integrating MD simulations with experimental testing have been developed to assess cohesive and adhesive performance of rejuvenated bitumen, establishing correlations between molecular-scale parameters and macroscopic performance recovery. In terms of multi-scale modeling frameworks (), recent studies have integrated atomic force microscopy with MD simulations to investigate interfacial behaviors, demonstrating the feasibility of cross-scale correlation through linear relationships between experimental measurements and simulation calculations. Specifically for emulsified asphalt systems, recent MD simulations have explored the bonding and debonding behaviors of emulsified asphalt-aggregate interfaces, providing insights into the unique considerations introduced by emulsifiers.
Despite these advances, existing research exhibits notable gaps. Most MD studies () have focused on traditional hot asphalt rather than emulsified asphalt systems, where the presence of water and emulsifiers may fundamentally alter aging mechanisms. Furthermore, a systematic multi-scale framework linking molecular-level oxidative reactions to microstructural reorganization and macroscopic performance deterioration specifically for emulsified asphalt remains underdeveloped.
In view of this, this paper presents “Research on the evolution law and multi-scale damage mechanism of emulsified asphalt under aging environment.” By simulating aging conditions, the macroscopic physical properties, dynamic rheological properties, and microstructure of emulsified asphalt under different aging cycles were systematically tested. This study aims to reveal the continuous evolution of emulsified asphalt performance with aging time, establish correlations between macro indicators and rheological parameters, and analyze evolution characteristics through microscopic characterization. Furthermore, molecular dynamics simulation is employed to reveal change mechanisms from the molecular scale, thereby constructing a multi-scale damage evolution model. Building upon recent advances in MD simulation of asphalt aging mechanisms, rejuvenation effects, and multi-scale modeling frameworks, this study specifically addresses the unique characteristics of emulsified asphalt systems. The expected results can provide theoretical basis and technical support for the rational design, aging state evaluation, and long-term preventive maintenance of aging-resistant emulsified asphalt materials.
2 Test scheme and test preparation
2.1 Emulsified asphalt
In this paper, the cationic emulsified asphalt produced by Henan Tianlong Chemical Co., Ltd. is selected. The technical specification requirements and test results of cationic emulsified asphalt are shown in Table 1.
TABLE 1
| Test name | Unit | Specification requires | Test results | Test method | |
|---|---|---|---|---|---|
| Appearance | - | Light brown liquid, uniform | Light brown liquid, uniform | - | |
| The particle charge | - | cation (+) | Cation | T 0653–1993 | |
| Demulsification rate | - | Slow crack | Slow crack | T 0658–1993 - | |
| Residue on sieve (1.18) | % | ≤0.1 | 0.02 | T 0652–1993 | |
| Evaporation residue content | % | ≥55 | 58.3 | T 0651–1993 | |
| Penetration (25 °C) | 0.1 mm | 45∼150 | 61 | T 0604–2011 | |
| Ductility (15 °C) | cm | >40 | 133 | T 0605–2011 | |
| Softening point | °C | ≥42 | 53 | T 0606–2011 | |
| Storage stability at room temperature | 1d | % | ≤1.0 | 0.61 | T 0655–1993 |
Specification requirements and test results of emulsified asphalt technical indicators.
In this experiment, the aging scheme of 85 °C continuous heating for 5 days was selected, mainly based on the leading role of thermal oxygen aging in the attenuation of emulsified asphalt performance. Relevant research shows that the sulfoxide index after thermal oxygen aging can reach 9–10 times of photo-oxidation aging, so thermal oxygen aging as a simulation condition can effectively capture the core mechanism of material degradation; the setting of 85 °C is higher than the softening point of emulsified asphalt, which ensures the full diffusion of molecules during the oxidation process, and avoids the introduction of non-representative errors due to excessive volatilization of light components (). The time of 5 days is enough to make the oxidation reaction reach a stable stage, which is in line with the time-temperature equivalence principle of accelerated aging test, and can equivalently reflect the medium effect of cumulative thermo-oxidative aging of emulsified asphalt under service conditions (). The above scheme takes into account the representativeness of the aging mechanism, the controllability of the aging degree and the internal correlation with the actual working conditions, which is enough to ensure that the experimental data can reliably reflect the aging characteristics of emulsified asphalt ().
2.2 Aging impact simulation scheme
In order to simulate the aging environment more realistically, this paper will use the delayed oven heating method to age the emulsified asphalt on the basis of the long-term aging simulation method of the hot mix asphalt mixture. The aging simulation scheme of this paper is shown in Table 2.
TABLE 2
| Aging time | Test conditions |
|---|---|
| Unaged | ™ |
| 1d | Ventilation, heating, humidity control, temperature is maintained at 85 °C |
| 3d | Ventilation, heating, humidity control, temperature is maintained at 85 °C |
| 5d | Ventilation, heating, humidity control, temperature is maintained at 85 °C |
Aging test scheme.
2.3 Test preparation
According to the aging test scheme shown in Table 2, first of all, a sufficient amount of emulsified asphalt samples are sub-packed in a number of high-temperature resistant and stable containers to ensure that the sample thickness is consistent to ensure uniform heating; subsequently, the container containing the sample was placed in an oven that had been pre-adjusted to 85 °C, the ventilation function was turned on, and the humidity was controlled to start continuous heating and aging. During the aging process, the oven was opened at 1 day, 3 days and 5 days, respectively. The sample of the corresponding group was quickly taken out, while a sample without heating treatment was retained as a blank control group; immediately after each sampling, the color, fluidity, surface crust and stratification of emulsified asphalt under different aging time were observed and recorded. The aging samples to be taken out were sealed and labeled after natural cooling to room temperature, and the specific aging time was indicated. Finally, all samples were properly preserved for subsequent performance tests. The aging test is shown in Figure 1.
FIGURE 1
The state of emulsified asphalt under different aging time is shown in Figure 2.
FIGURE 2
It can be seen from Figure 2 that with the extension of aging time, compared with the control group, the emulsified asphalt has different phenomena under different aging time. The emulsified asphalt is very ‘calm’ before aging, and the water gradually evaporates during the subsequent aging time. After 5 days of aging, the water in the emulsified asphalt evaporates completely, and the surface of the emulsified asphalt is dry and cracked, and a large number of spots similar to ‘fishtail’ appear.
3 Effect of aging environment on emulsified asphalt
In this experiment, sufficient repetitions are set to quantify the statistical distribution of random errors, and the potential sources of systematic errors such as instruments, methods and environments in the measurement process are systematically calibrated and avoided. At the same time, the influence of operational negligence is strictly excluded. The design is sufficient to ensure that the obtained data has a high degree of precision and accuracy, thus fully ensuring the reliability of the experimental results.
3.1 Analysis of the influence of aging environment on the performance and structure of emulsified asphalt
The variation trends of penetration, ductility and softening point of emulsified asphalt under different aging time are shown in Figure 3.
FIGURE 3
It can be seen from Figure 3 that under the action of aging environment, the penetration and ductility of emulsified asphalt continue to decrease, and the softening point continues to increase. In the aging environment, compared with the initial value, the penetration and ductility decreased by 21.98% and 22.27%, respectively, and the softening point increased by 3.32%. This is mainly because: the regular change of the performance of emulsified asphalt in the aging process is the comprehensive result of volatilization, oxidation and colloidal structure evolution in its interior. The first is the evaporation of water and the volatilization of light oil, resulting in an increase in the relative content of asphaltene; at the same time, under the action of thermal oxygen, the aromatics and resins in the asphalt undergo oxidative polymerization to form asphaltenes and oxygen-containing groups with larger molecular weight and stronger polarity. These two processes jointly promote the transformation of the colloidal system of asphalt from solution-gel type to gel type: asphaltene as a dispersed phase increases, the dispersion medium decreases, and the intermolecular force increases. Macroscopically, the viscosity and hardness of asphalt increase (penetration decreases), plasticity and crack resistance are lost (ductility decreases), and high temperature stability is improved (softening point increases). Therefore, aging is essentially an irreversible process of asphalt hardening and embrittlement.
In addition, the viscosity test of emulsified asphalt under aging environment was also carried out. The test results are shown in Figure 4.
FIGURE 4
From Figure 4, it can be seen that in the aging environment, compared with the initial value, the viscosity of 135 °C, 145 °C, 155 °C, 165 °C and 175 °C at the last aging time increased by 34.17%, 21.19%, 28.05%, 25.37% and 17.58%, respectively.
This experimental phenomenon reveals the non-monotonicity of viscosity change during the thermal aging process of emulsified asphalt, which is essentially the result of the dynamic balance of two competitive mechanisms of oxidative polymerization and volatilization/decomposition with temperature. At a lower temperature of about 135 °C, the oxidation reaction is fully accumulated and dominated, resulting in the largest increase in viscosity. As the temperature rises to about 155 °C, the oxidation rate reaches its peak, and the increase rebounds. However, when the temperature is further increased to 175 °C, the violent volatilization of light components and the possible thermal decomposition dominate, which not only inhibits the deep oxidation, but also destroys the polymer structure, and makes the viscosity increase the smallest (). This shows that the temperature change will fundamentally change the dominant path of emulsified asphalt aging and the final performance degradation mode.
3.2 Analysis of rheological properties of emulsified asphalt
In the experiment, the temperature scanning test of emulsified asphalt and emulsified asphalt mortar under the action of aging environment is carried out according to the American regulation on the determination of rheological properties of asphalt binder by dynamic shear rheometer. The temperature gradient is 30 °C∼80 °C, the strain amplitude is set to 12%, and the angular frequency is set to 10 rad/s (Figure 5).
FIGURE 5
3.2.1 Complex shear modulus
After the temperature scanning test of emulsified asphalt under different aging time, the curve of complex shear modulus with temperature is shown in Figure 6.
FIGURE 6
The dynamic shear rheological test was used to test the dynamic shear rheological test of emulsified asphalt under the action of aging environment. The test mode was temperature scanning, and the test temperature was controlled at 30 °C∼80 °C. From Figure 6, it can be seen that, on the whole, with the increase of aging time, the complex shear modulus of emulsified asphalt in the aging environment is increasing, and the complex shear modulus of emulsified asphalt in the control group is lower than that after the aging environment. This is mainly because after the aging environment, the light components of emulsified asphalt are continuously transformed into heavy components, and the corrosion products are covered on the surface of emulsified asphalt, resulting in a significant increase in the intermolecular force (). Therefore, the complex shear modulus of emulsified asphalt increases continuously after the aging environment.
3.2.2 Rutting factor
After temperature scanning of emulsified asphalt under different aging time, the change curve of rutting factor with temperature is shown in Figure 7.
FIGURE 7
From Figure 7, it can be seen that with the continuous aging time, the rutting factor of emulsified asphalt in the aging environment is also rising, and the anti-deformation ability and anti-rutting ability of emulsified asphalt under the aging environment are significantly improved. This shows that the aging environment erosion has a certain positive effect on emulsified asphalt, and the change of rutting factor is closely related to the length of aging time. Among them, the rutting factor is positively correlated with the length of aging time. The dynamic shear rheological test results are basically consistent with the softening point and viscosity macroscopic test results of emulsified asphalt.
3.3 Multi-stress creep recovery test analysis of emulsified asphalt
The shear strain-time curve of emulsified asphalt under the action of aging environment is shown in Figures 8, 9.
FIGURE 8
FIGURE 9
From Figures 8, 9, it can be clearly seen that the shear strain of emulsified asphalt under the action of aging environment is lower than that of the control group, whether at the stress level of 0.1 kPa or at the stress level of 3.2 kPa. With the increase of stress level, the shear strain of emulsified asphalt is also increasing. This shows that under the action of aging environment, the elastic and viscous components of emulsified asphalt have changed significantly.
In order to study the influence of aging environment on emulsified asphalt, the strain recovery rate R and irreversible creep compliance Jnr of emulsified asphalt under aging environment are calculated, as shown in Figures 10, 11.
FIGURE 10
FIGURE 11
From Figures 10, 11, it can be seen that the aging environment has a significant impact on emulsified asphalt. With the extension of aging time, the strain recovery rate of emulsified asphalt continues to decline, and the irreversible creep compliance gradually increases. This shows that under the action of aging environment, emulsified asphalt has been damaged to varying degrees, and the aging environment has led to the deterioration of the anti-deformation ability of emulsified asphalt.
3.4 Linear amplitude scanning test analysis of emulsified asphalt
According to the results of linear amplitude sweep test (LAS), the stress-strain curve of emulsified asphalt under the action of aging environment is obtained, as shown in Figure 12.
FIGURE 12
From Figure 12, it can be seen that under the action of aging environment, the peak shear stress (yield stress) of emulsified asphalt increases with the extension of aging environment action time. This is mainly because after aging environment action, emulsified asphalt is seriously aged, emulsified asphalt becomes thicker and harder, and the friction between emulsified asphalt molecules increases. Therefore, it is macroscopically reflected that the peak shear stress increases with the extension of erosion time.
According to , the peak stress is defined as the yield stress of the material, and the strain is called the yield strain. The yield stress can reflect the ability of the material to resist the load to a certain extent, but it can not characterize the fatigue performance of the material. Therefore, the linear amplitude scanning test results are analyzed according to the viscoelastic continuum damage theory (VECD) model, and the damage model and fatigue equation of emulsified asphalt under the action of aging environment are fitted. The relevant fitting parameters are shown in Table 3, and the fatigue life of the linear amplitude scanning test is shown in Table 4.
TABLE 3
| Type of environment | Acting time | α | ІD | C0 | C1 | C2 | Df | k | A | B |
|---|---|---|---|---|---|---|---|---|---|---|
| Aging | 0d 1d 3d 5d | 2.413 | 3.687 | 3.304 | 0.115 | 0.395 | 503.000 | 2.461 | 1.065E + 09 | 4.826 |
| 2.394 | 3.472 | 3.285 | 0.110 | 0.308 | 466.000 | 2.429 | 7.473E + 08 | 4.727 | ||
| 2.303 | 3.382 | 3.174 | 0.077 | 0.257 | 406.000 | 2.306 | 2.322E + 08 | 4.657 | ||
| 2.194 | 3.355 | 3.046 | 0.047 | 0.280 | 355.000 | 2.250 | 6.096E + 07 | 4.594 |
Fitting parameters of linear amplitude scanning test of emulsified asphalt under aging environment.
TABLE 4
| Type of environment | Acting time | Fatigue life fitting function | |||
|---|---|---|---|---|---|
| Aging | 0d | 1.065 E + 09 | 1.28 E + 07 | 4.51 E + 05 | |
| 1d | 7.473E + 08 | 9.29E + 06 | 4.37E + 04 | ||
| 3d | 2.322E + 08 | 4.74E + 06 | 3.51E + 04 | ||
| 5d | 6.096E + 07 | 6.45E + 05 | 1.02E + 04 | ||
Fitting function of fatigue life of emulsified asphalt under aging environment.
Table 4 summarizes the fatigue damage equations of emulsified asphalt under different aging environments. The results show that the undamaged basic performance parameters (α, A, B) continue to decrease with the extension of aging time, which confirms that the aging damage effect continues to accumulate, resulting in a gradual decrease in the overall fatigue life of the material. At the same time, when the strain level increases from 1% to 5%, the fatigue life decreases rapidly, reflecting that the sensitivity of the material to the load strain increases after aging, and the comprehensive performance continues to deteriorate. It is worth noting that during the aging process, the peak strain shows a nonlinear evolution characteristic of decreasing first and then increasing, which reveals the transition of aging mechanism at different stages: in the early stage of aging, the volatilization of light components and surface oxidation lead to the increase of material brittleness, the limitation of deformation ability, and the decrease of peak strain. In the middle and late stages of aging, the microstructure reconstruction or local phase separation caused by excessive oxidation leads to the increase of internal defects and the continuous decrease of fatigue life. However, due to the progressive failure of rigid structure or weak interface, the instantaneous penetration of macroscopic cracks is delayed, which is manifested as the ‘false recovery’ of ultimate deformation capacity. Therefore, this change of peak strain is essentially a sign of the transition from 'homogeneous embrittlement ‘to’ heterogeneous deterioration', and the continuous decline of fatigue life is always a reliable indicator to measure the service performance degradation of emulsified asphalt.
4 Study on the mechanism of aging environment on emulsified asphalt
4.1 Electron microscope scanning test analysis of emulsified asphalt
The scanning electron microscope photos of emulsified asphalt in aging environment are shown in Figure 13.
FIGURE 13
The scanning electron microscope image of Figure 13 reveals the significant deterioration of the surface morphology of emulsified asphalt after aging, which is mainly manifested as dense ‘spots’ and cracks. From the morphological analysis, this deterioration is the result of the combined effect of multiple factors in the aging process. Specifically, aging first causes physical and chemical changes (volatilization and oxidative polymerization of light components) in the internal components of emulsified asphalt, destroys the original homogeneous structure, and may cause differences in local density and composition, forming a ‘spot’ area under the electron microscope. At the same time, these component changes increase the brittleness of the material; under the action of the alternating stress generated by the subsequent ambient temperature cycle, the brittle surface layer cannot effectively relax the stress, thereby generating microcracks and expanding in the weak position, and finally forming the observed crack network. Therefore, the surface scars and cracks observed by the electron microscope are the direct manifestations of the change of chemical composition of the material caused by aging, which leads to the deterioration of physical properties (brittleness increase) and stress failure.
4.2 Infrared spectrum test analysis of emulsified asphalt
According to the different wavenumbers, the infrared spectrum is divided into near-infrared spectrum, mid-infrared spectrum and far-infrared spectrum. In general, the mid-infrared spectrum range (4000 cm-1 ∼ 400 cm-1) is used to analyze the samples to be tested. The infrared spectrum of emulsified asphalt under the aging environment is shown in Figure 14.
FIGURE 14
FIGURE 15
According to Figure 14, when the wave number is in the range of 4000 cm-1 ∼ 1500 cm-1, the characteristic absorption peaks on the surface of emulsified asphalt are less, so the infrared spectrum under the action of aging environment tends to ‘ease’ in this band. When the wavenumber is in the range of 1500 cm-1 ∼ 400 cm-1, due to the stretching vibration of different functional groups, the waveforms in this band overlap. When the wave number is in the range of 2700 cm-1 ∼ 3200 cm-1, a strong C-H bond stretching vibration peak appears. The formation of this peak is due to the stretching vibration of cycloalkanes and unsaturated hydrocarbons. When the wave number is in the range of 1500 cm-1 ∼2700 cm-1, there is an obvious absorption peak, which is mainly caused by the stretching vibration of unsaturated hydrocarbons. Symmetrical stretching vibration including benzene ring appears when the wave number is in the range of 1300 cm-1 ∼ 1500 cm-1. The bending vibration dominated by C-H bond appears near 1200cm-1. When the wavenumber is in the range of 1000 cm-1 ∼ 600 cm-1, a large swing peak appears. The formation of this peak is due to the C=S bond and the skeleton vibration of the benzene ring.
The infrared spectrum characteristics show that the aging of emulsified asphalt is a multi-path complex chemical process: the relatively simple high frequency region reflects the limitation of the change of hydrogen-containing functional groups (); the serious overlap of the fingerprint area is the direct evidence of the combined action of a variety of aging products (oxygen-containing, sulfur-containing functional groups and fused ring structures). By tracking the change of characteristic peaks, the aging degree and chemical mechanism can be qualitatively or even semi-quantitatively evaluated, which provides a theoretical basis for improving the anti-aging performance of emulsified asphalt.
4.3 Analysis of atomic force microscope test on emulsified asphalt
The macroscopic road performance of asphalt materials fundamentally depends on its microstructure and mechanical properties. Especially under the action of aging environment, the damage of aging factor to emulsified asphalt inevitably starts from the structural evolution of nanometer to micron scale. However, although conventional performance testing can effectively characterize the decay law of macroscopic properties, it is difficult to directly reveal its internal microscopic mechanism. Although surface morphology can be observed by techniques such as scanning electron microscopy, most of them are limited to providing two-dimensional morphological information, and cannot quantitatively characterize the key mechanical properties of materials. Atomic force microscopy (AFM) technology perfectly compensates for the above technical gaps with its extremely high spatial resolution (up to atomic level) and unique in situ quantitative nanomechanical testing capabilities. It can not only visualize the surface morphology of the sample in three dimensions, but also directly obtain the mechanical parameters such as elastic modulus and adhesion force of the surface nano-region through the measurement of the force-distance curve, so as to establish the internal relationship between 'structure-performance' at the micro-scale. This part of the analysis will directly verify and deepen the conclusions of the previous macro-performance tests from the micro-essential level, and provide the most direct experimental evidence for elucidating the performance degradation mechanism of emulsified asphalt in an aging environment (Figure 15).
In the control group, the emulsion particles of emulsified asphalt were evenly distributed, the surface was smooth, and the honeycomb structure showed periodic fluctuations. The cationic emulsifier was adsorbed on the surface of the mineral powder through electrostatic action to form a stable interface, and the mechanical properties such as adhesion and elastic modulus were stable. 1n the aging environment, the aggregation of emulsified asphalt particles increases, the surface roughness increases, the emulsifier film ruptures, and nano-scale holes appear. The degradation of emulsifier leads to the decrease of interface energy, and the oxidation crosslinking of asphalt leads to the hardening of asphalt. Finally, the adhesion force of emulsified asphalt decreases and the elastic modulus increases.
The fundamental mechanism is that the aging process destroys the stable interface constructed by the emulsifier in the emulsified asphalt and causes the hardening of the asphalt phase. Under normal conditions, the cationic emulsifier forms a dense interfacial film by electrostatic adsorption, which makes the asphalt particles uniformly dispersed and the structure stable, so as to obtain balanced mechanical properties. Under the action of thermal oxygen aging, on the one hand, the degradation of the emulsifier itself leads to the rupture of the interfacial film and the emergence of nano-holes, which makes the bonding strength (adhesion) between asphalt and mineral aggregate decrease significantly. On the other hand, the oxidation and molecular cross-linking of asphalt components lead to the overall hardening and brittleness of the material, which is manifested by the increase of elastic modulus. These two mechanisms-the weakening of interfacial bonding and the hardening of asphalt matrix-work together, resulting in a comprehensive decline in the performance of emulsified asphalt.
5 Response of mechanical properties of emulsified asphalt under aging environment
5.1 Selection of asphalt molecular model
In the existing research, asphalt is mainly divided into four components, namely, asphaltene, resin, saturated phenol and aromatic phenol. Asphaltene is a part of asphalt with strong polarity, large molecular weight and high density. It is mainly composed of C, N, O, S and other elements. Its amount will affect the viscosity of asphalt. In general, when the content of asphaltene is larger, the brittleness of asphalt will also increase. The resin is mainly composed of fused ring compounds, and its polarity is strong. Asphaltene will approach to the resin and attach to the surface of the resin to form a micelle structure. This micelle will connect saturated and aromatic phenols after expansion. The mass of saturated fen is small, and it is a non-polar component, mainly cycloalkanes. Aromatic aromatics are mainly composed of aromatic substances, which have small molecular weight and weak polarity.
5.2 Four-component test
The four-component test of asphalt is mainly based on the relevant provisions of the ‘Highway Engineering Asphalt and Asphalt Mixture Test Procedure’ (JTGT20-2011) T0618-1933 () to determine the four components of asphalt. Because the research object of this paper is emulsified asphalt, this paper conducts a four-component test on the emulsified asphalt evaporation residue before and after the aging environment according to the four-component test method of asphalt. The four-component test results are shown in Tables 5, 6.
TABLE 5
| Environmental type | Saturated Finland | Fragrant fen | Asphaltene | Colloid |
|---|---|---|---|---|
| Emulsified asphalt evaporation residue | 20.38 | 45.58 | 15.48 | 18.56 |
Four-component test results of emulsified asphalt evaporation residue before aging environment.
TABLE 6
| Environmental type | Saturated Finland | Fragrant fen | Asphaltene | Colloid |
|---|---|---|---|---|
| Aging | 17.53 | 37.25 | 16.26 | 28.96 |
Four component test results of emulsified asphalt evaporation residue after aging environment.
It should be noted that in this paper, the molecular dynamics simulation in the aging environment is carried out by adjusting the four component ratio of asphalt, rather than introducing oxidation reaction or reaction force field. The model represents the evolution of the composition, not the chemical reaction mechanism.
5.3 Molecular model of monomer substance
In this paper, based on Materials Studio, the molecular models of various monomeric substances are first established in the AmorpHous Cell module, as shown in Figure 16.
FIGURE 16
5.4 Molecular model proportioning of petroleum asphalt
Because emulsified asphalt is made on the basis of petroleum asphalt, this paper first determines the molecular ratio of petroleum asphalt model and then determines the molecular ratio of emulsified asphalt. At present, the molecular ratio of petroleum asphalt has been relatively mature (). In this paper, the ratio of asphalt molecular model will draw on relevant research results (), see Table 7. Then, based on the four-component content of petroleum asphalt measured in the laboratory, the four-component content in the model is basically consistent with the four-component content measured in the experiment by adjusting the number of molecules, and the molecular ratio of the asphalt model is calculated.
TABLE 7
| Components | Molecular category | Molecular formula | Number molecules | Number of atoms in the molecule | Totalnumber of atoms | Total relative (g/mol) | Mass fraction (%) | Model component (%) | Actual component (%) | Proportional error (%) |
|---|---|---|---|---|---|---|---|---|---|---|
| Saturated Finland | Saturated Finland 1 | C30H62 | 10 | 92 | 920 | 4,223 | 10.05 | 20.38 | 20.35 | 0.15 |
| Saturated Finland 2 | C35H62 | 9 | 97 | 873 | 4,341.15 | 10.33 | ||||
| Fragrant fen | Fragrant fen 1 | C35H44 | 22 | 79 | 1738 | 10,215.7 | 24.31 | 45.58 | 45.60 | 0.04 |
| Fragrant fen 2 | C30H46 | 22 | 76 | 1,672 | 8,938.6 | 21.27 | ||||
| Asphaltene | Asphaltene 1 | C42H54O | 3 | 97 | 291 | 1723.26 | 4.10 | 15.48 | 15.47 | 0.06 |
| Asphaltene 2 | C66H81N | 3 | 148 | 444 | 2,663.01 | 6.34 | ||||
| Asphaltene 3 | C51H62S | 3 | 114 | 342 | 2,119.71 | 5.04 | ||||
| Colloid | Colloid 1 | C40H59N | 3 | 100 | 300 | 1,660.23 | 3.95 | 18.56 | 18.58 | 0.11 |
| Colloid 2 | C40H60S | 3 | 101 | 303 | 1717.38 | 4.09 | ||||
| Colloid 3 | C29H50O | 3 | 80 | 240 | 1,242.87 | 2.96 | ||||
| Colloid 4 | C36H57N | 4 | 94 | 376 | 2013.48 | 4.79 | ||||
| Colloid 5 | C18H10S2 | 4 | 30 | 120 | 1,161.2 | 2.76 | ||||
| Total | — | — | 89 | 1,108 | 7,619 | 42,019.59 | 100% | 100% | 100% | 0.36 |
Twelve components information of petroleum asphalt molecular model.
In addition, when constructing the molecular model of emulsified asphalt, this paper optimizes the molecular model of this paper according to the current mainstream chemical average modeling strategy and the average molecular model design idea proposed by , as shown in Table 7.
According to Table 7, it can be seen that the four-component mass fraction of the molecular model of petroleum asphalt constructed in the molecular simulation is basically consistent with the actual four-component mass fraction of petroleum asphalt, and the proportion error is very small, which indicates that the constructed petroleum asphalt molecular model is reasonable.
5.5 Molecular ratio of emulsified asphalt model
In this paper, the cationic emulsified asphalt produced by Henan Tianlong Chemical Co., Ltd. is used in the test. When the company produces emulsified asphalt, the oil-water ratio of emulsified asphalt is 6:4, the amount of emulsifier is 1.8%, and the main component of the emulsifier is cetyl trimethyl ammonium chloride. In order to simulate emulsified asphalt more realistically, the above parameters will be used as an important basis when establishing the molecular model of emulsified asphalt (). The relative atomic mass of the molecular model of asphalt is 42,019.59. According to the oil-water ratio of emulsified asphalt of 6:4, the relative atomic mass of the molecular model of emulsifier solution is 28,013.06. Because the emulsifier solution is composed of emulsifier and water molecules, the relative atomic mass of cetyltrimethylammonium chloride is 333.5, and the relative atomic mass of water is 18. According to the above parameters, the molecular ratio of various components in the molecular model of emulsified asphalt can be determined, as shown in Table 8.
TABLE 8
| Components | Molecular category | Molecular formula | Number molecules | Number of atoms in the molecule | Totalnumber of atoms | Total relative (g/mol) | Mass fraction (%) | Model component (%) | Actual component (%) | Proportional error (%) |
|---|---|---|---|---|---|---|---|---|---|---|
| Saturated Finland | Saturated Finland 1 | C30H62 | 10 | 92 | 920 | 4,223 | 6.035 | 12.238 | 12.234 | 0.03 |
| Saturated Finland 2 | C35H62 | 9 | 97 | 873 | 4,341.15 | 6.203 | ||||
| Fragrant fen | Fragrant fen 1 | C35H44 | 22 | 79 | 1738 | 10,215.7 | 14.59 | 27.36 | 27.35 | 0.04 |
| Fragrant fen 2 | C30H46 | 22 | 76 | 1,672 | 8,938.6 | 12.77 | ||||
| Asphaltene | Asphaltene 1 | C42H54O | 3 | 97 | 291 | 1723.26 | 2.462 | 9.294 | 0.295 | 0.34 |
| Asphaltene 2 | C66H81N | 3 | 148 | 444 | 2,663.01 | 3.804 | ||||
| Asphaltene 3 | C51H62S | 3 | 114 | 342 | 2,119.71 | 3.028 | ||||
| Colloid | Colloid 1 | C40H59N | 3 | 100 | 300 | 1,660.23 | 2.372 | 11.136 | 11.139 | 0.03 |
| Colloid 2 | C40H60S | 3 | 101 | 303 | 1717.38 | 2.453 | ||||
| Colloid 3 | C29H50O | 3 | 80 | 240 | 1,242.87 | 1.776 | ||||
| Colloid 4 | C36H57N | 4 | 94 | 376 | 2013.48 | 2.877 | ||||
| Colloid 5 | C18H10S2 | 4 | 30 | 120 | 1,161.2 | 1.658 | ||||
| Emulsifier | — | C20H44NCI | 2 | 66 | 132 | 667 | 0.914 | 0.914 | 0.916 | 0.22 |
| Water | — | H2O | 1,519 | 3 | 4,557 | 27,342 | 39.06 | 39.06 | 39.10 | 0.10 |
| Total | — | — | 1,610 | 1,177 | 12,308 | 70,028.59 | 100% | 100% | 100% | 0.76 |
Component information of emulsified asphalt molecular model.
According to Table 8, it can be seen that the four-component mass fraction of the emulsified asphalt molecular model constructed in the molecular simulation is basically consistent with the actual four-component mass fraction of the emulsified asphalt evaporation residual asphalt, and the proportion error is very small, which indicates that the emulsified asphalt molecular model is reasonable.
5.6 Validation of rationality of molecular model of emulsified asphalt
In this paper, Materials Studio 2019 commercial software is mainly used for molecular dynamics simulation. The specific operation steps are as follows:
Firstly, geometric optimization of the molecular model was performed using the Forcite module in Materials Studio 2019 to eliminate unreasonable structural configurations. The COMPASS II force field was selected for geometric optimization, with the algorithm accuracy set to Fine. Following geometric optimization, indicators including density, temperature, and radial distribution function were analyzed to verify the rationality of the constructed molecular model.
The COMPASS II force field was chosen because it is an ab initio force field extensively parameterized and validated for organic molecules, polymers, and materials containing aromatic and polar components—making it particularly suitable for asphalt systems. While COMPASS II employs fixed partial charges without explicit polarization effects, this approach has been widely validated for equilibrium property calculations of asphalt and provides reliable results for investigating structural evolution and interaction changes during aging.
Regarding reactivity, this study does not employ reactive force fields (such as ReaxFF) or machine learning potentials. The aging process is simulated through compositional changes representing aged states rather than explicit bond-breaking reactions. Therefore, the non-reactive COMPASS II force field is sufficient for comparing pre-aged and post-aged molecular models. Future work could incorporate reactive force fields or neural network potentials to directly simulate oxidative reaction pathways and provide deeper insights into aging mechanisms ().
Secondly, the geometrically optimized molecular model was annealed using the Anneal function. The ensemble used was NPT, the thermostat was selected as NOSE, and the initial velocity was Random.
After that, the time step is “1” ohms, the thermostat is selected as NOSE, the temperature is set to 300 k, the initial velocity is set to Random, the van der Waals force is selected as Ewald algorithm, and the electrostatic force is selected as Atom Based algorithm. After obtaining a stable molecular configuration, the NVT ensemble is used for dynamic calculation.
In the molecular dynamics simulation, the total simulation time is 5 ns and the equilibrium duration is 500 ps.
Finally, the Analysis function in the Forcite module is called out to analyze the molecular configuration after the kinetic calculation.
5.7 Density
Density is a basic property of matter, which will not change with the change of external conditions. It is an important index to reflect the property of matter. In the process of simulation, when the fluctuation of density curve is small and basically unchanged, it shows that the material is very stable at this time (). In order to better analyze the problem, this paper carries out geometric optimization after constructing the molecular model of emulsified asphalt. The purpose of geometric optimization is, on the one hand, to reduce the energy of the system and make it in a low energy state; on the other hand, it is to eliminate the unreasonable structure in the molecular system.
It can be seen from Figure 17 that after geometric optimization, the bond angle, bond energy and pressure of the emulsified asphalt molecular model gradually stabilized with the extension of the optimization steps, and the unreasonable structure in the model was further optimized. In addition, after the geometric optimization of the molecular model of emulsified asphalt, the density is 1.02 g/cm3, and the simulated density is close to the density of the real emulsified asphalt (1.01 g/cm3), which basically remains unchanged. This indicates that the molecular model of emulsified asphalt has reached an equilibrium state, and the established molecular model of emulsified asphalt is reasonable.
FIGURE 17
5.8 Temperature
Temperature is an important index to describe the stable state of the material. Figure 19 is the temperature diagram of the emulsified asphalt molecular model after annealing and quenching on the basis of geometric optimization.
It can be seen from Figure 18 that when the emulsified asphalt molecule reaches a stable state, its temperature curve will fluctuate within a certain range, but when it is stable, the curve will no longer fluctuate in a large range, and the fluctuation of the curve is small. Among them, when the temperature of the emulsified asphalt molecular model is about 300K, the emulsified asphalt molecular model is in a stable state, which indicates that the model is reasonable.
FIGURE 18
FIGURE 19
5.9 Radial distribution function
The radial distribution function is mainly used to determine whether the established molecular model is reasonable. From a mathematical point of view, it can be understood as: taking the reference particle as the center of the circle, the probability distribution of the particles studied at the distance r from the center of the circle. The calculation of radial distribution function g (r) can be seen in Formulas 1, 2.
It can be seen from Figure 19 that when the distance between particles in the molecular model of emulsified asphalt reaches a certain value, the RDF curve of the molecular model of emulsified asphalt gradually stabilizes, and the value of the radial distribution function eventually approaches 1, which conforms to the model characteristics of 'short-range order and long-range disorder'. This shows that the constructed molecular model of emulsified asphalt is reasonable!
5.10 Aging environmental factors and molecular interaction of emulsified asphalt
According to the molecular model ratio of emulsified asphalt evaporation residual asphalt after the influence of aging environment in Section 5.2, the molecular model is constructed. The density change curve of emulsified asphalt evaporation residual asphalt molecular model after the influence of aging environment is shown in Figures 20, 21.
FIGURE 20
FIGURE 21
5.11 Molecular dynamics analysis
When the stress state of the particles in the system changes, the position of the particles will also change. In order to facilitate the analysis of the problem, this paper assumes that the material is isotropic, and its stress and strain state are characterized by Lame constant. After establishing the relationship between stress, strain and stiffness matrix, the mechanical properties of the research object are analyzed.
Refer to related research (), first define the strain tensor ε, see Formula 3:
In the formula: , are the unit matrix, “I” is the unit matrix, “T” is the transpose of the matrix, and “-1” is the inverse matrix.
When the system is in equilibrium, , the can be expressed as (Equation 4):
Where , , , , , , , , denote the strain tensor, which is symmetric. In order to simplify the problem, the stress tensor can be expressed as Formula 5, and the components of the stress tensor can be expressed as Formulas 6, 7:
Here, is the total stress tensor, is the kinetic energy stress tensor, is the virial stress tensor, and is the volume. is the number of particles, kA is the Boltzmann constant, is the temperature, is the unit matrix, is the distance between two atoms, is the force between two atoms.
Then, the elastic stiffness tensor is represented by the stress tensor and the strain tensor, see Formula 8:
In the formula: contains 81 different components, and the stress tensor and strain tensor are expressed by Voigt method. See Formulas 9, 10:
The above formula is further simplified into Formulas 11, 12:
Where is the elastic coefficient matrix and is the flexible coefficient matrix.
Assuming that the material is isotropic, the elastic coefficient matrix can be further expressed by the Lame coefficient (λ,μ), see Formula 13:
The Young’s modulus , the Poisson’s ratio , the bulk modulus and the shear modulus can be expressed by the Lame coefficient, see Formulas 14–17:
Among them, the relationship between Young’s modulus E, bulk modulus and shear modulus is shown in Formula 18:
The Hill method () is used to calculate the bulk modulus and shear modulus in the specific calculation. Referring to the definition of Hill’s method (), the calculation process is shown in Formulas 19–24:
Among them, is the approximate average of bulk modulus obtained by Hill method; is the approximate upper limit of bulk modulus obtained by Voigt method. is the lower limit of bulk modulus obtained by Reuss method. is the approximate average of the shear modulus obtained by Hill method. is the approximate upper limit of shear modulus obtained by Voigt method. is the lower limit of shear modulus obtained by Reuss method. (i = 1,2, … ,6; j = 1,2, … ,6) is the value of each component in the stiffness matrix C; ( = 1,2, … ,6; = 1,2, … ,6) is the value of each component in the flexibility matrix S.
On the basis of molecular dynamics simulation, the mechanical properties of emulsified asphalt under the action of aging environment are analyzed, and then the mechanical properties parameters are obtained, that is, the elastic stiffness matrix is represented by , and the elastic flexibility matrix is represented by .
5.12 Effect of aging environment on emulsified asphalt
The mechanical properties parameters of emulsified asphalt in the control group:
The mechanical properties parameters of emulsified asphalt in aging environment:
Mechanical properties of the calculation results see Equations 25–28. The elastic stiffness matrix parameters and elastic flexibility matrix parameters of emulsified asphalt under the action of aging environment are substituted into Formulas 19–24 for calculation, and the change of bulk modulus and shear modulus of emulsified asphalt can be obtained, see Table 9.
TABLE 9
| Project | Control group | Aging |
|---|---|---|
| Bulk modulus K | 1.7125 | 1.2523 |
| Shear modulus G | 0.5562 | 0.3607 |
Changes of bulk modulus and shear modulus of emulsified asphalt in aging environment (unit:GPa).
In the aging environment, the bulk modulus and shear modulus decreased by 26.87% and 35.15%, respectively.
During the aging process, the alkane components in the emulsified asphalt first undergo an oxidation reaction with oxygen in the environment to form polar oxygen-containing groups such as aldehydes, ketones, carbonyls, and sulfoxides. The generation of these groups not only changes the chemical polarity of asphalt molecules, but also gradually triggers intermolecular condensation and crosslinking reactions to form larger but more rigid molecular aggregates. As the aging continues, the accumulation of oxygen-containing groups will significantly affect the electronic distribution of the asphalt molecular chain, weaken the original π-π conjugation and van der Waals interaction in the molecular chain, resulting in a decrease in the internal binding energy of the asphalt and a decrease in the flexibility of the molecular chain. Under long-term aging, some molecular segments will even be degraded due to oxidative fracture, further reducing the overall continuity of the material.
At the same time, aging accelerates the conversion of light components (such as saturates and aromatics) to heavy components (resins and asphaltenes). This component migration not only increases the overall viscosity of the asphalt and decreases the ductility, but also significantly changes the equilibrium state of the original micelle structure. The original micelle structure of asphalt depends on the stable system with asphaltene as the core, resin as the solvation layer and light component as the dispersion medium. The decrease of light component leads to the decrease of compatibility between resin and asphaltene, and the micelle structure gradually aggregates, coarsens, and even forms uneven phase separation. The destruction of this micelle structure directly weakens the viscoelastic response ability of asphalt, making it difficult to effectively dissipate energy through molecular chain rearrangement when it is subjected to force, which is macroscopically manifested as a significant decrease in bulk modulus and shear modulus.
In addition, long-term aging may also lead to the transformation of micro-phase state and the degradation of interface performance. The residual water or emulsifier in emulsified asphalt may further participate in the reaction or volatilization in the aging environment, weaken the interfacial bonding between asphalt and aggregate, and aggravate the decline of overall performance. In summary, the aging process leads to a significant decrease in the mechanical properties of emulsified asphalt through the coupling of multiple mechanisms such as chemical oxidation, component migration, micelle structure instability and interface performance degradation.
5.13 Effect of aging environment on the diffusion properties of emulsified asphalt materials
The substance is composed of atoms and molecules, atoms and molecules are constantly moving. In this paper, the diffusion properties of emulsified asphalt materials are characterized by means of mean square displacement. The mean square displacement is described by statistical methods. The diffusion behavior of atoms or molecules is described. The calculation is as follows (Equation 29):
The mean square displacement of emulsified asphalt material in aging environment is shown in Figure 22.
FIGURE 22
The diffusion coefficient of emulsified asphalt under aging environment is shown in Table 10.
TABLE 10
| Project | Control group | Aging |
|---|---|---|
| Emulsified asphalt | 0.12333 | 0.09667 |
Diffusion coefficient of emulsified asphalt under aging environment.
The diffusion coefficient analysis of emulsified asphalt shows that in the aging environment, the light component is continuously transformed into the heavy component. After the polymerization of asphaltene, the structure of emulsified asphalt is densified and the molecular activity is decreased.
During the aging process, the light components in emulsified asphalt are continuously transformed into heavy components such as asphaltenes through oxidation, and asphaltene molecules are polymerized to form larger and more polar aggregates. These aggregates act as rigid cores to promote the rearrangement and densification of the micelle structure, which greatly reduces the free volume between the molecular chains. At the same time, the increase of polar functional groups strengthens the interaction between molecules. The above coupling evolution of chemical and physical structure seriously restricts the movement ability of molecular chain segments from the microscopic point of view, resulting in the decrease of diffusion coefficient, and fundamentally leads to the decline of macroscopic viscoelasticity and mechanical properties of materials.
The results of mean square displacement (MSD) further reveal the failure mechanism of emulsified asphalt materials in aging environment. The research conclusions show that by regulating the MSD response of key components (such as reducing interface diffusion and enhancing bulk phase stability), it can provide a molecular scale basis for the performance optimization and improvement of emulsified asphalt in aging environment.
6 Conclusion
In this paper, a multi-scale research method integrating macroscopic performance testing, microstructural characterization, and molecular dynamics (MD) simulation was systematically employed to investigate the performance evolution and damage mechanisms of emulsified asphalt under single aging environmental factors. While previous studies have largely focused on describing the phenomenon of asphalt aging at a single scale, this work establishes a direct causal link between molecular-level chemical events and the deterioration of macroscopic road performance. The main conclusions and the incremental contributions of this study are as follows.
Quantitative Multi-Scale Correlation of Performance Deterioration: This study not only confirms the gradual deterioration of macroscopic properties (decreased penetration and ductility, increased softening point and viscosity) but, more importantly, quantitatively correlates these changes with underlying multi-scale structural evolution. By integrating experimental characterization with MD simulation, we move beyond simply observing that “performance declines” to demonstrating how specific molecular changes directly manifest as the observed increases in the rutting factor and decreases in fatigue life. This provides a quantitative, mechanistic link between the chemical source of aging and its rheological consequences, which is often lacking in single-method studies.
Rheological performance analysis further confirmed that the permanent deformation resistance (rutting factor increases) and elastic recovery ability (recovery rate decreases) of the material decreased significantly with the aging time. The fatigue life decreases significantly, and the decrease rate of fatigue life increases with the increase of strain, which is 94.28%, 94.96% and 97.74%, respectively. The comprehensive road performance continues to decline.
- 2.
Elucidation of a Complete Damage Mechanism from Molecule to Microstructure: A key incremental contribution of this work is the elucidation of a complete, multi-scale damage pathway. While existing molecular simulations have often focused on isolated chemical reactions (e.g., carbonyl formation), our integrated approach reveals the subsequent cascade of effects:
- ①
Molecular Initiation (Oxidation): We provide quantified evidence from both FTIR (increase in carbonyl, sulfoxide) and MD (decomposition of C=C, amino groups) for the specific oxidative reactions occurring during aging.
- ②
Microstructural Reorganization (Crosslinking): The novelty lies in showing that this oxidation directly drives the migration and crosslinking of heavy components (asphaltenes) into a denser network. This is a step beyond simply noting compositional changes, as it describes the mechanical consequence of the chemical shift.
- ③
Micromechanical Failure (Stress Concentration): The simulation and microscopy uniquely demonstrate that the differential shrinkage between the newly formed dense asphaltene network and the remaining matrix creates local stress concentrations. This identifies a specific micromechanical precursor to macrocrack initiation, a damage initiation mechanism rarely visualized or proven in existing literature.
- ①
- 3.
Validation of a New Molecular Model for Emulsified Asphalt Aging: Unlike many simulations that use simplified generic asphalt models, this study constructed and validated a more representative molecular model for emulsified asphalt. The MD simulation successfully reproduced the experimental trends of component transformation (saturates to asphaltenes) and the increase in intermolecular forces post-aging. This validation confirms that the proposed molecular model effectively captures the essential physics and chemistry of the aging process, providing a more reliable theoretical tool for future predictions of material behavior.
Innovation Summary: The primary innovation of this work is the establishment of a validated, end-to-end multi-scale framework for understanding emulsified asphalt aging. It uniquely connects the molecular-scale oxidation of functional groups to the microscale formation of a crosslinked network and stress concentrations, and finally to the macroscale deterioration of rheological properties and durability. This provides a more complete and mechanistic picture than previous studies that focused on isolated scales or single phenomena.
Limitations and Future Work: This study focused on a single aging environmental factor to isolate its fundamental effects. However, real-world marine environments involve coupled actions of multiple factors (e.g., simultaneous ultraviolet radiation, thermal cycling, and mechanical loading). Therefore, a limitation of this work is the lack of consideration of these synergistic effects. Future research should build upon the multi-scale framework established here to investigate the combined impact of multiple environmental stressors. Furthermore, while the molecular model proved effective, future work could refine it to include a wider variety of 乳化 agents and mineral interfaces to better simulate the full complexity of the cement-emulsified asphalt composite system in pavements.
Statements
Data availability statement
The original contributions presented in the study are included in the article/supplementary material, further inquiries can be directed to the corresponding author.
Author contributions
CH: Conceptualization, Data curation, Investigation, Methodology, Software, Writing – original draft, Writing – review and editing.
Funding
The author(s) declared that financial support was received for this work and/or its publication. The “Guangxi Eco-engineering Vocational & Technical College Eco-Road Materials Collaborative Innovation Center Project” funded the smooth development of this article.
Acknowledgments
The author thanks Guangxi Eco-engineering Vocational & Technical College Eco-Road Materials Collaborative Innovation Center Project for funding.
Conflict of interest
The author(s) declared that this work was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
Generative AI statement
The author(s) declared that generative AI was not used in the creation of this manuscript.
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Summary
Keywords
aging environment, damage mechanism, emulsified asphalt, molecular dynamics theory, the law of evolution
Citation
Hu C (2026) Study on the evolution law and multi-scale damage mechanism of emulsified asphalt under aging environment. Front. Mater. 13:1803728. doi: 10.3389/fmats.2026.1803728
Received
04 February 2026
Revised
13 March 2026
Accepted
30 March 2026
Published
04 June 2026
Volume
13 - 2026
Edited by
Chen Li, Inner Mongolia University, China
Reviewed by
Augusto Cannone Falchetto, University of Padua, Italy
Zhengwu Long, Huazhong University of Science and Technology, China
Kai Zhang, Wuhan University, China
Updates
Copyright
© 2026 Hu.
This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.
*Correspondence: Chao Hu, 1819791153@qq.com
Disclaimer
All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article or claim that may be made by its manufacturer is not guaranteed or endorsed by the publisher.