Multiscale Evaluation of Asphalt Aging Behaviour: A Review

: The performance of asphalt pavement will deteriorate gradually due to oxidation, also named the ageing of asphalt binders. In this study, the research progress on the multiscale evaluation of asphalt aging behaviour in the past decade was reviewed to further analyse the evolution law of asphalt aging behaviour and determine the asphalt aging mechanism. Firstly, artiﬁcial aging methods were introduced, the factors affecting asphalt aging behaviour were analysed, and the changes of asphalt properties before and after aging were identiﬁed. Secondly, the methods and research progress in terms of the evaluation of asphalt aging degrees were summarised from the macro-, mesoscopic-, micro-, and nanoscales. Finally, considering extensively studied rheological properties as an example, the correlations among the rheological properties of asphalt in the aging process and the conventional physical properties, chemical composition, microscopic properties, nano-molecular dynamics, and other parameters were analysed. The results show that various scales are interrelated, and the multiscale evaluation of asphalt aging can provide a more comprehensive prediction of the extent of asphalt aging. The correlation between multiple scales enables a thorough analysis of the mechanism of asphalt aging and the evaluation of asphalt aging behaviour at multiple scales.


Introduction
Asphalt is a by-product of petroleum refining.Because of its excellent viscoelasticity, it can be widely used in the road construction industry [1,2].Approximately 70% of existing pavement structures were built using asphalt as the binders [3].However, due to complex factors such as high temperature, moisture, oxygen, solar radiation, reactive oxygen species such as NO x and O 3 , etc., the asphalt will suffer ageing during its service life [4][5][6][7].In this process, the asphalt undergoes chemical component migration, microstructure transformation, and performance change [8,9]; it is transformed from the original "soft" structure to a "rigid and brittle" structure after aging [10].This triggers and aggravates damage to the asphalt pavement structure because of other processes, such as fatigue and cracking, and severely affects the usability of asphalt pavement [11][12][13].
Asphalt aging mainly occurs in three stages: asphalt storage and transportation, mixing and paving, and operation and service [14].Researchers and practitioners have gradually formed a unified understanding of aging through long-term aging research [15], indicating that asphalt aging is not only a physical process, but also a chemical process [16,17], including the volatilization of light components in asphalt materials, polarization transformation [18], and chemical reactions with oxidation, etc., under conditions of high temperature and UV radiation [19].Therefore, according to the cause of the aging phenomenon, asphalt aging can be divided into thermal oxidative and photo-oxidative aging, as shown in Figure 1 [20].Previous studies have mainly focused on the thermal oxidative aging of asphalt under high-temperature conditions [21] and have developed unified indoor standard aging test methods based on rolling thin film oven tests (RTFOT) and pressure aging vessels (PAV) as well as Vienna Binder Ageing (VBA) to simulate the short-term and long-term aging occurring in the long-term service process [22,23].Compared with that on thermal oxidative aging, research on photo-oxidative aging is more recent, and a unified indoor aging test specification has not been established yet.However, researchers have also achieved certain results by simulating the form of photo-oxidative aging through a variety of self-made test chambers [24,25].For example, it was found that the degree of photo-oxidative aging mainly depends on the type of asphalt and the wavelength of UV radiation.In addition, to study the complex external environment of asphalt, some researchers developed bespoke equipment to simulate the coupling effect of heat, oxygen, oxynitride, UV, and other factors [26][27][28].
Sustainability 2023, 15, 2953 2 of 26 transformation [18], and chemical reactions with oxidation, etc., under conditions of high temperature and UV radiation [19].Therefore, according to the cause of the aging phenomenon, asphalt aging can be divided into thermal oxidative and photo-oxidative aging, as shown in Figure 1 [20].Previous studies have mainly focused on the thermal oxidative aging of asphalt under high-temperature conditions [21] and have developed unified indoor standard aging test methods based on rolling thin film oven tests (RTFOT) and pressure aging vessels (PAV) as well as Vienna Binder Ageing (VBA) to simulate the shortterm and long-term aging occurring in the long-term service process [22,23].Compared with that on thermal oxidative aging, research on photo-oxidative aging is more recent, and a unified indoor aging test specification has not been established yet.However, researchers have also achieved certain results by simulating the form of photo-oxidative aging through a variety of self-made test chambers [24,25].For example, it was found that the degree of photo-oxidative aging mainly depends on the type of asphalt and the wavelength of UV radiation.In addition, to study the complex external environment of asphalt, some researchers developed bespoke equipment to simulate the coupling effect of heat, oxygen, oxynitride, UV, and other factors [26][27][28].The evaluation of asphalt aging behaviour can provide a clear control of the pavement usage during the asphalt aging service period and guidance for choosing the right time for maintenance and repair.Previous studies mainly focused on the macro-level, such as the performance evolution law, to evaluate the aging levels of asphalt, for example, residual penetration, differences between softening points before and after aging, differences between the viscosity before and after aging, and other conventional basic physical indicators [29,30].In addition, the Strategic Highway Research Program (SHRP) proposed multiple rheology indexes such as phase angle and shear modulus.Research results have provided scientific support for engineering practices [1,31].Recently, some advanced machines were applied to analyse the chemical properties of bitumen, for example the rod thin-layer chromatography-flame ionization detector (TLC-FID) was employed to detect the SARA properties of asphalt [32,33], the Fourier infrared spectrometer [34] was employed to get the functional groups of asphalt and gel chromatography tester, etc. [35].Moreover, in the micro-characterization scope, fluorescence microscopy (FM) [36], scanning electron microscopy (SEM) [37][38][39], and atomic force microscopy (AFM) [40] were also employed in the industry of asphalt materials.The introduction of a series of microinstruments such as the spectrophotometer [41] has considerably enriched asphalt aging The evaluation of asphalt aging behaviour can provide a clear control of the pavement usage during the asphalt aging service period and guidance for choosing the right time for maintenance and repair.Previous studies mainly focused on the macro-level, such as the performance evolution law, to evaluate the aging levels of asphalt, for example, residual penetration, differences between softening points before and after aging, differences between the viscosity before and after aging, and other conventional basic physical indicators [29,30].In addition, the Strategic Highway Research Program (SHRP) proposed multiple rheology indexes such as phase angle and shear modulus.Research results have provided scientific support for engineering practices [1,31].Recently, some advanced machines were applied to analyse the chemical properties of bitumen, for example the rod thin-layer chromatography-flame ionization detector (TLC-FID) was employed to detect the SARA properties of asphalt [32,33], the Fourier infrared spectrometer [34] was employed to get the functional groups of asphalt and gel chromatography tester, etc. [35].Moreover, in the micro-characterization scope, fluorescence microscopy (FM) [36], scanning electron microscopy (SEM) [37][38][39], and atomic force microscopy (AFM) [40] were also employed in the industry of asphalt materials.The introduction of a series of micro-instruments such as the spectrophotometer [41] has considerably enriched asphalt aging research.In this regard, the nanoscale molecular dynamics research of asphalt materials is in the early stage of exploration, but it has considerable potential for development.The molecular dynamics method is used to study the molecular dynamics behaviour of each component of the asphalt system [42], which can disclose the movement law of molecules in terms of time range that cannot be observed in conventional tests, predict the macroscopic properties of asphalt materials, and propose measures to improve pavement performance according to the molecular structure of asphalt.In this process, researchers have gradually deepened their understanding on the asphalt aging mechanism and the characterisation of the degree of asphalt aging.
However, the use of only a single angle, macro-characterization, chemical composition, or micro/nano-perspectives is not sufficient to evaluate the asphalt aging process.According to materials science, the macroscopic properties of materials are affected by their microscopic properties, which are determined by the chemical compositions of materials [43].For this reason, researchers have studied asphalt aging from multiple perspectives, established the correlation between different scales, and explored the evolution law and action mechanism of asphalt aging behaviour more comprehensively [44,45].Thus, the present study summarises the research achievements of asphalt aging in the past few decades and provides a review of the research progress of asphalt aging behaviour evaluation from macro-, mesoscopic-, micro-, and nano-scales.Based on the rheological properties of asphalt aging, this study introduces correlation studies between different scales performed by researchers and practitioners.In addition, the results of the asphalt aging process considering rheology and the correlation between different scales are introduced.This literature review can give a comprehensive understanding about the aging mechanism and behaviour of asphalt, and therefore can provide further information about recycling waste asphalt materials.

Standard Indoor Aging Method
The aging of asphalt pavement takes approximately 10-20 years from completion to loss of service performance [46].Studies focusing only on field asphalt aging would take a long time and occur in uncontrollable research conditions, obtaining a large dispersion of results.Researchers and pavement practitioners found that the main cause of asphalt aging is the significant influence of asphalt heating and oxygen conditions [47,48].Therefore, it was proposed to replace natural aging with an indoor simulated aging test.The test conditions were designed according to the needs of the researchers, so as to considerably reduce the aging test period and eliminate interference from non-test factors.The thin-film oven aging test (TFOT) [24], RTFOT, and PAV [49] have been used to simulate shortterm and long-term aging behaviour indoors (as shown in Table 1).A unified standard specification has been gradually formed in long-term studies [50,51], providing a solid research foundation for the study of the thermal oxidative aging of asphalt.The bespoke aging test method is mainly intended for evaluating the photo-oxidative aging of asphalt.As research on the photo-oxidative aging of asphalt is still fairly recent, a unified test specification has not been established yet, and no standard procedures for UV aging tests is available [20].Thus, researchers have designed various aging test methods to obtain asphalt aging samples after photo-oxidative aging.In the preparation process, factors such as light-wave wavelength, test sample thickness, and radiation duration have been considered [52,53].Hu et al. [54] employed UV light belonging to wavelength ranges of 260-300 nm, 300-350 nm, and 350-400 nm to investigate the effects of radiation on asphalt with a fixed film thickness (1.13 mm) for 2 days, 4 days, and 8 days; they also prepared asphalt aging samples to study the influence of wavelength and aging cycles on such samples.Zeng et al. [55], Mouillet et al. [56], Xiao et al. [57], and Xu et al. [58] obtained aging samples using an indoor homemade UV aging chamber (Figure 2), although some differences existed in the aging parameters.In addition, some researchers [59] adopted a climate simulation system (Figure 3), comprehensively considering the coupling effect of light, heat, water, and other environmental factors on asphalt, and simulated the aging process of asphalt binder during the actual service live of asphalt pavement to obtain asphalt samples resembling actual aged asphalt more closely.The bespoke aging test method is mainly intended for evaluating the photo-oxidative aging of asphalt.As research on the photo-oxidative aging of asphalt is still fairly recent, a unified test specification has not been established yet, and no standard procedures for UV aging tests is available [20].Thus, researchers have designed various aging test methods to obtain asphalt aging samples after photo-oxidative aging.In the preparation process, factors such as light-wave wavelength, test sample thickness, and radiation duration have been considered [52,53].Hu et al. [54] employed UV light belonging to wavelength ranges of 260-300 nm, 300-350 nm, and 350-400 nm to investigate the effects of radiation on asphalt with a fixed film thickness (1.13 mm) for 2 days, 4 days, and 8 days; they also prepared asphalt aging samples to study the influence of wavelength and aging cycles on such samples.Zeng et al. [55], Mouillet et al. [56], Xiao et al. [57], and Xu et al. [58] obtained aging samples using an indoor homemade UV aging chamber (Figure 2), although some differences existed in the aging parameters.In addition, some researchers [59] adopted a climate simulation system (Figure 3), comprehensively considering the coupling effect of light, heat, water, and other environmental factors on asphalt, and simulated the aging process of asphalt binder during the actual service live of asphalt pavement to obtain asphalt samples resembling actual aged asphalt more closely.A unified indoor standard aging test method must be established based on the sufficient knowledge of the researchers.With an in-depth study of UV aging, the standard method of indoor accelerated photo-oxidative aging tests will be gradually unified, and  The bespoke aging test method is mainly intended for evaluating the photo-oxidative aging of asphalt.As research on the photo-oxidative aging of asphalt is still fairly recent, a unified test specification has not been established yet, and no standard procedures for UV aging tests is available [20].Thus, researchers have designed various aging test methods to obtain asphalt aging samples after photo-oxidative aging.In the preparation process, factors such as light-wave wavelength, test sample thickness, and radiation duration have been considered [52,53].Hu et al. [54] employed UV light belonging to wavelength ranges of 260-300 nm, 300-350 nm, and 350-400 nm to investigate the effects of radiation on asphalt with a fixed film thickness (1.13 mm) for 2 days, 4 days, and 8 days; they also prepared asphalt aging samples to study the influence of wavelength and aging cycles on such samples.Zeng et al. [55], Mouillet et al. [56], Xiao et al. [57], and Xu et al. [58] obtained aging samples using an indoor homemade UV aging chamber (Figure 2), although some differences existed in the aging parameters.In addition, some researchers [59] adopted a climate simulation system (Figure 3), comprehensively considering the coupling effect of light, heat, water, and other environmental factors on asphalt, and simulated the aging process of asphalt binder during the actual service live of asphalt pavement to obtain asphalt samples resembling actual aged asphalt more closely.A unified indoor standard aging test method must be established based on the sufficient knowledge of the researchers.With an in-depth study of UV aging, the standard method of indoor accelerated photo-oxidative aging tests will be gradually unified, and A unified indoor standard aging test method must be established based on the sufficient knowledge of the researchers.With an in-depth study of UV aging, the standard method of indoor accelerated photo-oxidative aging tests will be gradually unified, and future studies may focus on the simulation of actual outdoor asphalt pavement environment in real indoor environments.

Analysis of Influencing Factors of Asphalt Aging Behaviour
Asphalt is a type of complex high-viscosity polymer dispersed phase.In the process of construction and service, environmental factors such as light, water, and the action of vehicle loads change the asphalt chemical composition and molecular structure, which leads to asphalt aging [52].

Environmental Factors
Many studies [53][54][55] found that environmental factors have the most significant influence on asphalt aging behaviour, generally including heat, light, oxygen, water, and other factors.Heat refers to high-temperature conditions.As a viscoelastic material, asphalt is significantly affected by temperature.Sensitive groups such as double bonds in the internal molecules of asphalt break under high-temperature conditions and generate free radicals [56].Light refers to solar radiation (UV light).Similar to high-temperature conditions, under constant exposure to sunlight, the internal molecules of asphalt obtain energy in an excited state and easily react with oxygen in the air.The degree of asphalt aging under the action of solar radiation is generally higher than that under thermal aging [24,57].Regarding oxygen, regardless of high-temperature or solar radiation conditions, the internal molecules of asphalt are in an unsaturated and high-energy state and can easily undergo oxidation reactions with oxygen in the air and generate hydroperoxides.Therefore, asphalt aging is often divided into thermal oxidative aging [58,59] and photo-oxidative aging [24] according to the causes of asphalt aging.In contrast to the evident promoting effects of high temperature and UV light on asphalt aging, the effect of moisture is still controversial [60,61], and studies focusing on this aspect are scarce.Some researchers have found that water-based media can affect the characteristic functional groups of asphalt aging and that more carbonyl and sulfoxide are generated after asphalt is soaked in an aqueous solution.The longer the soaking time, the higher the index of carbonyl and sulfoxide, and the higher the asphalt aging degree [62].

Asphalt Film Thickness
As asphalt aging is a process of gradual diffusion, the thickness of the asphalt film is also one of the factors affecting the degree of aging [17,63,64].Fang [65] designed styrenebutadiene-styrene (SBS)-modified asphalt with five different film thicknesses for TFOT aging and found that the thinner the asphalt film is, the higher the degree of aging.Thus, the thickness of the SBS-modified asphalt film was reduced to improve the asphalt film oven test method.When the thickness of the SBS-modified asphalt film was 2.568 mm, that is, when the sample mass of the SBS-modified asphalt in the TFOT test was 41 g, the TFOT test could accurately evaluate the short-term aging performance of SBS-modified asphalt [66].In addition, through further research on UV aging in recent years, scholars have observed that the depth of UV irradiation of asphalt is limited [67].Katsuyuki et al. [68] studied indoor UV aging of asphalt films with different thicknesses and found that when an asphalt film is thin, the elastic modulus, viscosity, and carbonyl content of the asphalt significantly increase after UV aging.In the design of asphalt mixtures, the thickness of the asphalt film is approximately 4-19 µm [68][69][70].Therefore, the thickness of the asphalt film is also one of the influencing factors to be considered in the prediction of the aging degree of asphalt pavement.

Aging Period
The aging cycle of asphalt is another factor that influences the degree of aging.The longer the aging cycle, the more significant the degree of aging [29].Subbarao et al. [49] studied the influence of the RTFOT aging cycle on asphalt properties.The aging degree of asphalt was characterised by the changes in the characteristic functional groups and rheological parameters before and after aging (Figure 4), and the aging period was positively correlated with the aging degree of asphalt.Zhang et al. [71] tested the microstructure of asphalt with different aging cycles based on AFM, and found that the performance of asphalt started to be affected after aging for 5 h, whereas after 55 h, the asphalt aging started to stabilize, and after 75 h, the microstructure of asphalt did not change, and the asphalt lost its service performance.
The aging cycle of asphalt is another factor that influences the degree of aging.The longer the aging cycle, the more significant the degree of aging [29].Subbarao et al. [49] studied the influence of the RTFOT aging cycle on asphalt properties.The aging degree of asphalt was characterised by the changes in the characteristic functional groups and rheological parameters before and after aging (Figure 4), and the aging period was positively correlated with the aging degree of asphalt.Zhang et al. [71] tested the microstructure of asphalt with different aging cycles based on AFM, and found that the performance of asphalt started to be affected after aging for 5 h, whereas after 55 h, the asphalt aging started to stabilize, and after 75 h, the microstructure of asphalt did not change, and the asphalt lost its service performance.In summary, the asphalt aging phenomenon is closely related to environmental factors (such as heat, oxygen, light, and water), asphalt film thickness, asphalt aging duration, and other factors, in addition to its chemical composition and molecular structure.Among these factors, the research on environmental factors includes heat, oxygen, light, and water.Studies on single factors have provided phased achievements.However, the coupling effect of environmental factors on the asphalt aging process should be considered for more accurate asphalt aging results applied to actual engineering.In addition, the asphalt film thickness and aging cycle cannot be neglected in the practical application of aging results.

Characteristics Change after Asphalt Aging
At present, the mechanism of asphalt aging has not been determined yet, but a large number of researchers believe that asphalt aging is a consequence of a combination of both physical and chemical reactions [72].In this regard, physical aging is mainly manifested by the evaporation of light components in asphalt materials and the accumulation of polar substances, whereas chemical aging is a series of chemical reactions, leading to the change of molecular structures.Studies have found that asphalt aging is a very complex process, and the macroscopic properties of asphalt after aging indicate its "hardening", such as decreased penetration, increased softening point, decreased ductility, increased viscosity, and decreased rheological properties [29,30].From the perspective of chemical composition, Dhalaan et al. [73] found that the aging process of asphalt is accompanied by the transformation of chemical components, mainly the light components, In summary, the asphalt aging phenomenon is closely related to environmental factors (such as heat, oxygen, light, and water), asphalt film thickness, asphalt aging duration, and other factors, in addition to its chemical composition and molecular structure.Among these factors, the research on environmental factors includes heat, oxygen, light, and water.Studies on single factors have provided phased achievements.However, the coupling effect of environmental factors on the asphalt aging process should be considered for more accurate asphalt aging results applied to actual engineering.In addition, the asphalt film thickness and aging cycle cannot be neglected in the practical application of aging results.

Characteristics Change after Asphalt Aging
At present, the mechanism of asphalt aging has not been determined yet, but a large number of researchers believe that asphalt aging is a consequence of a combination of both physical and chemical reactions [72].In this regard, physical aging is mainly manifested by the evaporation of light components in asphalt materials and the accumulation of polar substances, whereas chemical aging is a series of chemical reactions, leading to the change of molecular structures.Studies have found that asphalt aging is a very complex process, and the macroscopic properties of asphalt after aging indicate its "hardening", such as decreased penetration, increased softening point, decreased ductility, increased viscosity, and decreased rheological properties [29,30].From the perspective of chemical composition, Dhalaan et al. [73] found that the aging process of asphalt is accompanied by the transformation of chemical components, mainly the light components, into asphaltene and other hard components.From the aspect of characteristic functional groups, in the aging process of asphalt, evident oxygen-containing characteristic peaks, such as those of the carbonyl and sulfoxide groups, are generated at 1600 cm −1 and 1698 cm −1 , respectively, indicating that the aging process is mainly an oxidation reaction [34].Luo et al. [74] analysed the mechanism of oxidation using a molecular dynamics model and found that oxidative aging leads to a reduction in free volume and an increase in the intermolecular force of asphalt.In addition, from a microscopic perspective, researchers have studied the changes in microscopic morphology before and after asphalt aging, using advanced techniques such as AFM; they have found that a "cellular structure" is generated during the aging process.Some scholars believe that this structure is the main reason for asphaltene generation during the aging process, which is in agreement with the results of chemical composition changes.In summary, the aging mechanism of asphalt can be considered to be due to the influence of external factors such as high temperature and light.The internal unsaturated bonds break and absorb oxygen to form carbonyl and sulfoxide oxygen-containing functional groups, and small molecules become macromolecules and light components.In this process, the intermolecular force is continuously strengthened, which leads to the gradual hardening and brittleness of the asphalt and a general decrease in performance.
In modified asphalt, the degradation of the modifiers [75][76][77][78], such as the SBS modifier, occurs in addition to the aging of the asphalt phase.Studies conducted by Fang et al. [79], Zhou et al. [80], Gao et al. [81], Yu et al. [82], and Wei et al. [83] have shown that aging under high temperature, water, and other external conditions simultaneously promotes the degradation of the SBS polymer and polymerisation of the small asphalt molecules (Figure 5).As the degree of aging continues to increase, the network structure of the SBS polymer modifier is gradually degraded until the modification function is lost, which significantly affects the physical and rheological properties of the asphalt.Zhou et al. [84] conducted an in-depth exploration of the aging of SBS-modified asphalt at the microscopic scale.SEM analysis revealed that the surface of the aged base asphalt and SBS/styrene-butadiene rubber (SBR)-modified asphalt exhibited a large number of wrinkles (bee-like structure) during the asphalt aging process, which directly led to a decline in the macroscopic road performance of asphalt.
groups, in the aging process of asphalt, evident oxygen-containing characteristic peaks, such as those of the carbonyl and sulfoxide groups, are generated at 1600 cm −1 and 1698 cm −1 , respectively, indicating that the aging process is mainly an oxidation reaction [34].Luo et al. [74] analysed the mechanism of oxidation using a molecular dynamics model and found that oxidative aging leads to a reduction in free volume and an increase in the intermolecular force of asphalt.In addition, from a microscopic perspective, researchers have studied the changes in microscopic morphology before and after asphalt aging, using advanced techniques such as AFM; they have found that a "cellular structure" is generated during the aging process.Some scholars believe that this structure is the main reason for asphaltene generation during the aging process, which is in agreement with the results of chemical composition changes.In summary, the aging mechanism of asphalt can be considered to be due to the influence of external factors such as high temperature and light.The internal unsaturated bonds break and absorb oxygen to form carbonyl and sulfoxide oxygen-containing functional groups, and small molecules become macromolecules and light components.In this process, the intermolecular force is continuously strengthened, which leads to the gradual hardening and brittleness of the asphalt and a general decrease in performance.
In modified asphalt, the degradation of the modifiers [75][76][77][78], such as the SBS modifier, occurs in addition to the aging of the asphalt phase.Studies conducted by Fang et al. [79], Zhou et al. [80], Gao et al. [81], Yu et al. [82], and Wei et al. [83] have shown that aging under high temperature, water, and other external conditions simultaneously promotes the degradation of the SBS polymer and polymerisation of the small asphalt molecules (Figure 5).As the degree of aging continues to increase, the network structure of the SBS polymer modifier is gradually degraded until the modification function is lost, which significantly affects the physical and rheological properties of the asphalt.Zhou et al. [84] conducted an in-depth exploration of the aging of SBS-modified asphalt at the microscopic scale.SEM analysis revealed that the surface of the aged base asphalt and SBS/styrenebutadiene rubber (SBR)-modified asphalt exhibited a large number of wrinkles (bee-like structure) during the asphalt aging process, which directly led to a decline in the macroscopic road performance of asphalt.

Multiscale Evaluation of Asphalt Aging Behaviour
Currently, there are many studies on asphalt aging performance evaluation methods.In the past, the basic physical and rheological properties of asphalt during the aging process were used to evaluate its aging performance.The research results of asphalt aging at the macro-level have provided scientific support for engineering practice.With the aid of advanced equipment such as rod TLC-FID, Fourier infrared spectrometer [34], gel chromatography tester [35], and other advanced equipment, the chemical components, characteristic functional groups, and molecular weights of asphalt before and after aging have been qualitatively and quantitatively analysed.In addition, with the rapid development of microscopic characterization techniques in recent years, the microscopic properties of asphalt have gradually been considered in the aging characterization, particularly using FM, Sustainability 2023, 15, 2953 8 of 25 SEM, AFM, spectrophotometer [37][38][39]41], etc.The introduction of a series of microscopic instruments has considerably enriched the content of asphalt aging research.In addition, although nanoscale molecular dynamics research on asphalt materials is in the nascent stage, it has a considerable potential for advancement.The molecular dynamics method is used to study the dynamic behaviour of the components of the asphalt system.It can disclose the movement of molecules within asphalt, which cannot be observed in conventional experiments [42].This chapter introduces the evaluation methods and indicators of asphalt aging from four aspects: macro-level, chemical composition, micro-characteristics, and nanoscale.

Basic Physical Properties
The fundamental physical properties include three basic indexes, namely needle penetration, elongation, Fraaß breaking point, and softening point, which are determined based on ASTM D36, ASTM D5, and ASTM D113 standard tests, respectively [85,86].In addition, viscosity represents the fluidity index of asphalt at a certain temperature, which can be measured using a Brookfield viscometer according to ASTM D4402 [87,88].Zhou et al. [84] had investigated how the conventional properties change with ageing using TFOT, RTFOT, and PAV, as shown in Figure 6.The results showed that aging has a significant effect on the basic physical properties of asphalt.
the macro-level have provided scientific support for engineering practice.With the aid of advanced equipment such as rod TLC-FID, Fourier infrared spectrometer [34], gel chromatography tester [35], and other advanced equipment, the chemical components, characteristic functional groups, and molecular weights of asphalt before and after aging have been qualitatively and quantitatively analysed.In addition, with the rapid development of microscopic characterization techniques in recent years, the microscopic properties of asphalt have gradually been considered in the aging characterization, particularly using FM, SEM, AFM, spectrophotometer [37][38][39]41], etc.The introduction of a series of microscopic instruments has considerably enriched the content of asphalt aging research.In addition, although nanoscale molecular dynamics research on asphalt materials is in the nascent stage, it has a considerable potential for advancement.The molecular dynamics method is used to study the dynamic behaviour of the components of the asphalt system.It can disclose the movement of molecules within asphalt, which cannot be observed in conventional experiments [42].This chapter introduces the evaluation methods and indicators of asphalt aging from four aspects: macro-level, chemical composition, micro-characteristics, and nanoscale.

Basic Physical Properties
The fundamental physical properties include three basic indexes, namely needle penetration, elongation, Fraaß breaking point, and softening point, which are determined based on ASTM D36, ASTM D5, and ASTM D113 standard tests, respectively [85,86].In addition, viscosity represents the fluidity index of asphalt at a certain temperature, which can be measured using a Brookfield viscometer according to ASTM D4402 [87,88].Zhou et al. [84] had investigated how the conventional properties change with ageing using TFOT, RTFOT, and PAV, as shown in Figure 6.The results showed that aging has a significant effect on the basic physical properties of asphalt.

Rheological Properties
The rheological properties of asphalt refer to its rheological and permanent deformation law, which is related to time factors, under the conditions of stress, strain, and temperature effects.Because asphalt materials have been widely used for more than 50 years, many scholars have conducted in-depth studies on the rheological properties of asphalt [89,90], and found that they essentially refer to the degree of hardness and thinness of asphalt.If the asphalt is too hard, it is easy to crack, whereas if it is too soft, rutting can be easily produced.To evaluate the rheological properties of asphalt at high temperatures, the dynamic shear rheometer (DSR) is the most common equipment [91].The DSR uses the dynamic mechanical analysis method to obtain the rheological parameters to describe the rheological properties of asphalt such as complex modulus G* and phase angle δ.To a certain extent, the use of traditional physical performance indicators overcomes difficulties related to the lack of experience.Among the indicators, the rutting parameter (G*/Sin δ) is believed to be an important parameter for characterising the high-temperature performance of asphalt.To evaluate the low-temperature rheological properties of asphalt, the bending beam rheometer (BBR) is the most commonly used machine to get the stiffness modulus (S) and creep velocity (m-value) of asphalt at low temperatures.In accordance with some literatures, 4-mm DSR is hopefully an ideal tool to replace BBR in the measure of low-temperature rheological properties of bituminous binders.We have added more information about it, as shown below and in the revised manuscript.
The literature has reported that the DSR with a geometry of 4 mm parallel plates is a potential substitute of BBR, which has similar results while using less materials.The new-developed approach was proposed by Sui et al. [92,93] and has been recently widely employed.
During aging, asphalt gradually becomes hard and brittle with an increase in the aging degree, and its rheological properties significantly change [94,95].Therefore, researchers often reflect the aging degree of asphalt with the help of the rheological property change rule [96].Several studies [97,98] have shown that aging significantly affects the rheological properties of asphalt.As the degrees of aging continues to increase, the complex modulus increases, the phase angle decreases, the fluidity decreases, and the binders are getting more elastic, which are resistant to rutting.The factor G*/Sin δ increases, and aging improves the high-temperature performance of asphalt.For the low-temperature rheological properties, aging leads to the stiffening of asphalt, resulting in an increase in the stiffness modulus and a decrease in the creep rate, and an increase in low-temperature fracture [99].It was reported [100] that the cracking susceptibility before and after asphalt aging by comparing rheological indicators and other parameters, such as the carbonyl index.The correlation between the Glover-Rowe parameter, ∆Tc, rheological index, and carbonyl index was analysed.The Glover-Rowe (G-R) parameter equals to G*•cos 2 δ/sinδ, which is related to the ductility of bitumen.Furthermore, this parameter can be used to characterize the ageing degrees of bitumen [101].Moreover, in addition to S and m, the critical temperatures were also put forward to evaluate the low-temperature performance of bitumen.The critical temperature includes T C,S which is controlled by S and T C,m which is controlled by the m value.The difference between these two critical temperatures was the so-called ∆Tc.After ageing, the ∆Tc will get more negative, inducing more thermal cracking issues [102].The results indicated that these parameters have considerable potential to characterise the asphalt aging degree.However, experimental data support is required.Hang et al. [103] and Bi et al. [104] also conducted similar studies, establishing the relationship between rheological index and asphalt aging degree, and evaluating the asphalt aging degree based on the rheological index before and after aging (Equation (1)).The higher the absolute value of the aging index, the more serious the degree of aging.In addition, Wang et al. [105] studied the correlation between the rheological properties of asphalt and its conventional physical properties, and found that the rheological property parameters, such as rutting factor G*/sin-δ, phase angle δ, complex shear modulus G*, and elastic modulus G", were more closely related to the softening point.

Chemical Composition of Asphalt
Asphalt is a complex mixture with a couple of organic compositions, which varies for different types of asphalt.So far, asphalt cannot be directly separated into its pure composition.To facilitate research on asphalt, scholars have proposed a series of theories, such as three-, four-, and five-component analyses, to classify asphalts, as shown in Table 2. Four-component analysis is the most widely used method, in which asphalt is divided into four components: saturated components, aromatic components, gum, and asphaltene (Table 3).The ratio of the SARA directly affects the performance of asphalt [106,107], and the SARA properties will change significantly when asphalt aging [108].
Table 2. Method for analysis of chemical components of bitumen [106,107].

Analysis Method Components
Marcusson-Ekman two-component method Soft asphaltenes; asphaltenes Hubbard-Steinfelmann three-component method Oil content; resin; asphaltenes Asphalt is a complex mixture with a couple of organic compositions, which varies for different types of asphalt.So far, asphalt cannot be directly separated into its pure composition.To facilitate research on asphalt, scholars have proposed a series of theories, such as three-, four-, and five-component analyses, to classify asphalts, as shown in Table 2. Four-component analysis is the most widely used method, in which asphalt is divided into four components: saturated components, aromatic components, gum, and asphaltene (Table 3).The ratio of the SARA directly affects the performance of asphalt [106,107], and the SARA properties will change significantly when asphalt aging [108].In the aging process of asphalt, the internalisation groups transform into each other, exhibiting different transformation laws under the influence of the external environment, but they mainly show the transformation law of soft to hard components.Feng et al. [110] compared and analysed the correlation between the TLC-FID method and the Corbett method for the detection of four components of three types of aging asphalt methods, including RTFOT, PAV, and UV, and proposed an appropriate expansion liquid method to accurately determine the content of chemical components in asphalt before and after aging.Dehouche et al. [21] used TLC-FID (Figure 7) to analyse the four components of asphalt before and after thermal oxidative aging and found that aging reduced the content of aromatic components while increasing the content of polar components such as resin and asphaltene.Moreover, the aforementioned researchers defined the colloid index to quantitatively characterise the degree of aging.Cheng et al. [111] obtained similar results by comparing the changes in the four components before and after aging of SK and ZH modified asphalts (Figure 8).
Asphalt is a complex mixture with a couple of organic compositions, which varies for different types of asphalt.So far, asphalt cannot be directly separated into its pure composition.To facilitate research on asphalt, scholars have proposed a series of theories, such as three-, four-, and five-component analyses, to classify asphalts, as shown in Table 2. Four-component analysis is the most widely used method, in which asphalt is divided into four components: saturated components, aromatic components, gum, and asphaltene (Table 3).The ratio of the SARA directly affects the performance of asphalt [106,107], and the SARA properties will change significantly when asphalt aging [108].In the aging process of asphalt, the internalisation groups transform into each other, exhibiting different transformation laws under the influence of the external environment, but they mainly show the transformation law of soft to hard components.Feng et al. [110] compared and analysed the correlation between the TLC-FID method and the Corbett method for the detection of four components of three types of aging asphalt methods, including RTFOT, PAV, and UV, and proposed an appropriate expansion liquid method to accurately determine the content of chemical components in asphalt before and after aging.Dehouche et al. [21] used TLC-FID (Figure 7) to analyse the four components of asphalt before and after thermal oxidative aging and found that aging reduced the content of aromatic components while increasing the content of polar components such as resin and asphaltene.Moreover, the aforementioned researchers defined the colloid index to quantitatively characterise the degree of aging.Cheng et al. [111] obtained similar results by comparing the changes in the four components before and after aging of SK and ZH modified asphalts (Figure 8).
Asphalt is a complex mixture with a couple of organic compositions, which varies for different types of asphalt.So far, asphalt cannot be directly separated into its pure composition.To facilitate research on asphalt, scholars have proposed a series of theories, such as three-, four-, and five-component analyses, to classify asphalts, as shown in Table 2. Four-component analysis is the most widely used method, in which asphalt is divided into four components: saturated components, aromatic components, gum, and asphaltene (Table 3).The ratio of the SARA directly affects the performance of asphalt [106,107], and the SARA properties will change significantly when asphalt aging [108].In the aging process of asphalt, the internalisation groups transform into each other, exhibiting different transformation laws under the influence of the external environment, but they mainly show the transformation law of soft to hard components.Feng et al. [110] compared and analysed the correlation between the TLC-FID method and the Corbett method for the detection of four components of three types of aging asphalt methods, including RTFOT, PAV, and UV, and proposed an appropriate expansion liquid method to accurately determine the content of chemical components in asphalt before and after aging.Dehouche et al. [21] used TLC-FID (Figure 7) to analyse the four components of asphalt before and after thermal oxidative aging and found that aging reduced the content of aromatic components while increasing the content of polar components such as resin and asphaltene.Moreover, the aforementioned researchers defined the colloid index to quantitatively characterise the degree of aging.Cheng et al. [111] obtained similar results by comparing the changes in the four components before and after aging of SK and ZH modified asphalts (Figure 8).
Asphalt is a complex mixture with a couple of organic compositions, which varies for different types of asphalt.So far, asphalt cannot be directly separated into its pure composition.To facilitate research on asphalt, scholars have proposed a series of theories, such as three-, four-, and five-component analyses, to classify asphalts, as shown in Table 2. Four-component analysis is the most widely used method, in which asphalt is divided into four components: saturated components, aromatic components, gum, and asphaltene (Table 3).The ratio of the SARA directly affects the performance of asphalt [106,107], and the SARA properties will change significantly when asphalt aging [108].In the aging process of asphalt, the internalisation groups transform into each other, exhibiting different transformation laws under the influence of the external environment, but they mainly show the transformation law of soft to hard components.Feng et al. [110] compared and analysed the correlation between the TLC-FID method and the Corbett method for the detection of four components of three types of aging asphalt methods, including RTFOT, PAV, and UV, and proposed an appropriate expansion liquid method to accurately determine the content of chemical components in asphalt before and after aging.Dehouche et al. [21] used TLC-FID (Figure 7) to analyse the four components of asphalt before and after thermal oxidative aging and found that aging reduced the content of aromatic components while increasing the content of polar components such as resin and asphaltene.Moreover, the aforementioned researchers defined the colloid index to quantitatively characterise the degree of aging.Cheng et al. [111] obtained similar results by comparing the changes in the four components before and after aging of SK and ZH modified asphalts (Figure 8).
In the aging process of asphalt, the internalisation groups transform into each other, exhibiting different transformation laws under the influence of the external environment, but they mainly show the transformation law of soft to hard components.Feng et al. [110] compared and analysed the correlation between the TLC-FID method and the Corbett method for the detection of four components of three types of aging asphalt methods, including RTFOT, PAV, and UV, and proposed an appropriate expansion liquid method to accurately determine the content of chemical components in asphalt before and after aging.Dehouche et al. [21] used TLC-FID (Figure 7) to analyse the four components of asphalt before and after thermal oxidative aging and found that aging reduced the content of aromatic components while increasing the content of polar components such as resin and asphaltene.Moreover, the aforementioned researchers defined the colloid index to quantitatively characterise the degree of aging.Cheng et al. [111] obtained similar results by comparing the changes in the four components before and after aging of SK and ZH modified asphalts (Figure 8).

Asphalt Characteristic Functional Groups
Many studies have shown that the internal characteristic functional groups of asphalt significantly change with aging.Therefore, the study of the characteristic functional groups of asphalt can also characterise the degree of aging of asphalt.Fourier transform infrared spectroscopy (FTIR) uses atomic vibration and rotation to identify the molecular structures [112] (Figure 9).The structure and types of the functional groups within asphalt can be determined in accordance with their position and the shape of the absorption frequency in the infrared spectrum.Moreover, the absorption intensity can also be captured.The test range is usually 4000 cm −1 -400 cm −1 .

Asphalt Characteristic Functional Groups
Many studies have shown that the internal characteristic functional groups of asphalt significantly change with aging.Therefore, the study of the characteristic functional groups of asphalt can also characterise the degree of aging of asphalt.Fourier transform infrared spectroscopy (FTIR) uses atomic vibration and rotation to identify the molecular structures [112] (Figure 9).The structure and types of the functional groups within asphalt can be determined in accordance with their position and the shape of the absorption frequency in the infrared spectrum.Moreover, the absorption intensity can also be captured.The test range is usually 4000 cm −1 -400 cm −1 .
can be determined in accordance with their position and the shape of the absorption frequency in the infrared spectrum.Moreover, the absorption intensity can also be captured.The test range is usually 4000 cm −1 -400 cm −1 .Currently, the FTIR is popular in identifying the types and contents of SBS within the SBS modified asphalt [113].This study discusses the application of FTIR in the field of asphalt aging.Furthermore, it can be used to determine the chemical change with asphalt as per the change of functional groups [113,114].Among them, the peak values corresponding to the 1700 cm −1 carbonyl and 1032 cm −1 sulfoxide eigenfunctions exhibited the most significant changes (Figure 10) [8].Researchers have proposed changes in the carbonyl index (C=O) and sulfoxide index (S=O) to evaluate the degree of asphalt aging, as shown in Equations ( 2) and (3) [49]; these equations can denote that carbonyl and sulfoxide groups are generated during ageing process due to oxidation.Zhao et al. [115] and Li et al. [116] studied changes in the functional group index of asphalt after aging using RTFOT, PAV, and UV; they found that both the carbonyl (C=O) and sulfoxide indexes (S=O) increase with the degree of aging and that the former is particularly correlated with the degree of aging, whereas the latter is not.In addition, through FTIR, Yao et al. [34] conducted a detailed analysis of functional groups with a wavelength of 1700 cm −1 in the asphalt aging process; they found that carboxylate and ketone groups are the main products in the asphalt aging process and that these functional groups, generated after aging, have a significant effect on the high-temperature related diseases such as rutting and fatigue performances of the asphalt mixture.

Asphaltenes
most significant changes (Figure 10) [8].Researchers have proposed changes in the carbonyl index (C=O) and sulfoxide index (S=O) to evaluate the degree of asphalt aging, as shown in Equations ( 2) and (3) [49]; these equations can denote that carbonyl and sulfoxide groups are generated during ageing process due to oxidation.Zhao et al. [115] and Li et al. [116] studied changes in the functional group index of asphalt after aging using RTFOT, PAV, and UV; they found that both the carbonyl (C=O) and sulfoxide indexes (S=O) increase with the degree of aging and that the former is particularly correlated with the degree of aging, whereas the latter is not.In addition, through FTIR, Yao et al. [34] conducted a detailed analysis of functional groups with a wavelength of 1700 cm −1 in the asphalt aging process; they found that carboxylate and ketone groups are the main products in the asphalt aging process and that these functional groups, generated after aging, have a significant effect on the high-temperature related diseases such as rutting and fatigue performances of the asphalt mixture.
Therefore, FTIR can be utilised to investigate the chemical composition changes of bituminous binders and to quantitatively define the aging degrees of asphalt.

Micro Scale
Advanced and micro-scale equipment have been recently applied to evaluate the ageing phenomena of asphalt such as SEM, AFM, and FM [117].Table 4 provides a performance comparison between AFM, SEM, and transmission electron microscopy (TEM).Among them, the bee-like structure obtained by an AFM is a hotspot in terms of the ageing of asphalt (working principle, Figure 11) [118].
Therefore, FTIR can be utilised to investigate the chemical composition changes of bituminous binders and to quantitatively define the aging degrees of asphalt.

Micro Scale
Advanced and micro-scale equipment have been recently applied to evaluate the ageing phenomena of asphalt such as SEM, AFM, and FM [117].Table 4 provides a performance comparison between AFM, SEM, and transmission electron microscopy (TEM).Among them, the bee-like structure obtained by an AFM is a hotspot in terms of the ageing of asphalt (working principle, Figure 11) [118].By the aid of AFM technologies, a lot of studies in terms of the microscopic morphology of asphalt were investigated [120].After considerable development, AFM has been widely used in research on bitumen micro-fields.In terms of the ageing behaviour of asphalt binders, it was found that the microstructure of asphalt detected by AFM could be termed the "bee-like structure" (Figure 12).This type of structure could be generated during the ageing of asphalt, as a result of the aggregation of asphaltenes [121].For the cognition of the "bee-like structure", most scholars have the same view that asphaltenes are formed during asphalt aging, and also some academics think that the "bee-like structure" is a consequence of crystalline paraffin contained in asphalt [122].There is not a unified opinion regarding the formation of the "bee-like structure", but the phase change of the By the aid of AFM technologies, a lot of studies in terms of the microscopic morphology of asphalt were investigated [120].After considerable development, AFM has been widely used in research on bitumen micro-fields.In terms of the ageing behaviour of asphalt binders, it was found that the microstructure of asphalt detected by AFM could be termed the "bee-like structure" (Figure 12).This type of structure could be generated during the ageing of asphalt, as a result of the aggregation of asphaltenes [121].For the cognition of the "bee-like structure", most scholars have the same view that asphaltenes are formed during asphalt aging, and also some academics think that the "bee-like structure" is a consequence of crystalline paraffin contained in asphalt [122].There is not a unified opinion regarding the formation of the "bee-like structure", but the phase change of the surface microstructure and micromechanical parameters during the aging process of asphalt can effectively help quantify the degree of aging.Moreover, researchers have defined the degrees of asphalt aging by introducing parameters such as the contents of the bee-like structure, as well as the adhesion properties and surface roughness [123].It was [40] considered that the coupling effects led to the oxidation of asphalt aging, therefore, using bespoke aging equipment to obtain the real situation of ageing is necessary.AFM can be used to observe the micromorphology of asphalt before and after aging; it was found that 50# matrix asphalt gradually deepened with ongoing ageing and the length of the single "bee-like structure" tends to get smaller (Table 5).It was reported that the number of "bee-like structures" on the surface of bitu- Moreover, researchers have defined the degrees of asphalt aging by introducing parameters such as the contents of the bee-like structure, as well as the adhesion properties and surface roughness [123].It was [40] considered that the coupling effects led to the oxidation of asphalt aging, therefore, using bespoke aging equipment to obtain the real situation of ageing is necessary.AFM can be used to observe the micromorphology of asphalt before and after aging; it was found that 50# matrix asphalt gradually deepened with ongoing ageing and the length of the single "bee-like structure" tends to get smaller (Table 5).It was reported that the number of "bee-like structures" on the surface of bituminous binders decrease while the areas increase compared with that before aging.Moreover, it was found that the correlation between the "bee-like structure" and the roughness of asphalt is relatively close.Therefore, the change of the bee-like structure can be used to identify the change of the roughness of the binder and thereby could be used as an indicator for the ageing degree evaluations [38].Research has still yet to disclose the specific molecular composition of asphalt owing to its complexity, which varies between different types of asphalt.It is believed that asphalt is an "extremely-complex fluid" with 10 5 to 10 6 types of non-reproducible molecular [124].Furthermore, it was reported that the aging of asphalt includes an occurrence of chemical reactions, through which the molecular weight increases and the distribution of molecules is rearranged, leading to the hardening of asphalt.Gel permeation chromatography (GPC) is an ideal tool to measure the changes in the molecular weight distribution of asphalt before and after ageing (Figure 13).

Molecular Dynamics Simulation of Asphalt Aging
As a novel digital tool, the molecular dynamic method was applied in the analysis of the physicochemical properties of asphalt materials [118,119,[126][127][128][129][130][131].Li et al. [132] utilised the asphalt molecular dynamics model to investigate the change in the physicochemical properties of asphalt caused by ageing.Xu et al. [58], Yang et al. [133], and Lei et al. [74] established an aging asphalt model by introducing oxygen-containing functional groups and using reactive molecular dynamics (MD) to simulate the oxidative aging behaviour of asphalt during long-term service.The changes in ketones, carbonyls, as well as

Molecular Dynamics Simulation of Asphalt Aging
As a novel digital tool, the molecular dynamic method was applied in the analysis of the physicochemical properties of asphalt materials [118,119,[126][127][128][129][130][131].Li et al. [132] utilised the asphalt molecular dynamics model to investigate the change in the physicochemical properties of asphalt caused by ageing.Xu et al. [58], Yang et al. [133], and Lei et al. [74] established an aging asphalt model by introducing oxygen-containing functional groups and using reactive molecular dynamics (MD) to simulate the oxidative aging behaviour of asphalt during long-term service.The changes in ketones, carbonyls, as well as sulfoxides during the ageing process were quantitatively analysed in the aspect of the nano-view (Figure 14), and the results essentially agree with laboratory testing results.Based on the MD model of AA-1 asphalt, Ding et al. [134] built an aged asphalt model by increasing the numbers of polar components such as asphaltene and reducing nonpolar components (such as saturates and aromatics).The diffusion behaviour of asphalt before and after oxidative aging was simulated.It was found that the ageing of asphalt increases the molecular interaction and glass transition temperature, slows down the molecular diffusion, and, as a consequence, the glass transition temperature increase during the ageing process.

Molecular Dynamics Simulation of Asphalt Aging
As a novel digital tool, the molecular dynamic method was applied in the analysi the physicochemical properties of asphalt materials [118,119,[126][127][128][129][130][131].Li et al. [132] lised the asphalt molecular dynamics model to investigate the change in the physicoch ical properties of asphalt caused by ageing.Xu et al. [58], Yang et al. [133], and Lei e [74] established an aging asphalt model by introducing oxygen-containing functio groups and using reactive molecular dynamics (MD) to simulate the oxidative aging haviour of asphalt during long-term service.The changes in ketones, carbonyls, as we sulfoxides during the ageing process were quantitatively analysed in the aspect of nano-view (Figure 14), and the results essentially agree with laboratory testing resu Based on the MD model of AA-1 asphalt, Ding et al. [134] built an aged asphalt mode increasing the numbers of polar components such as asphaltene and reducing nonp components (such as saturates and aromatics).The diffusion behaviour of asphalt be and after oxidative aging was simulated.It was found that the ageing of asphalt increa the molecular interaction and glass transition temperature, slows down the molecular fusion, and, as a consequence, the glass transition temperature increase during the age process.To summarise, the investigation on the evolution of asphalt aging behaviour mainly experienced four stages.Firstly, the research focused on evaluating the impacts of aging on the fundamental performance of asphalt.The second stage was to reflect on asphalt aging behaviour through the changes of the chemical aspects such as the change in functional groups inside the asphalt.Thirdly, quantifying the aging degrees of asphalt in the aspect of the microscopic point of view was emphasized, for example, the microstructure of asphalt before and after ageing.The most recent stage was to reveal the aging mechanism of asphalt through nano-scale with the aids of molecular dynamics simulation.It has experienced the transformation from macroscopic scale to nanoscopic scale.In this process, researchers have gradually deepened the understanding and research of asphalt aging mechanisms and asphalt aging degrees.However, it is impossible to comprehensively evaluate the aging process of asphalt only from a single point of view, whether macro-, micro-or nano-scale, or considering its chemical composition.Future aging research is bound to be interrelated among all scales, and multi-scale evaluation of asphalt aging is an inevitable trend.

Multiscale Correlation Research
In-depth studies on asphalt aging have focused primarily on macroscopic properties and mesoscopic chemical composition.However, as the asphalt is a heterogeneous body, it presents different microstructures, which affect micro-mechanical properties and the macroscopic performance of asphalt.The microscopic properties are determined by their chemical composition.Therefore, in the research process of asphalt aging, macro-, mesoscopic-, micro-scales, and the nanoscopic scale are interrelated and influence each other.The macroscopic rheological properties that have been studied extensively can be used as a reference to assess the existing research results related to the correlation between rheological properties and other parameters.

Correlation between Rheological Properties and Basic Physical Properties
Both the rheological properties and conventional basic physical properties are macroscopic.A large number of studies have shown that during the gradual "hardening" process of asphalt under the action of aging, the softening point and viscosity increase, whereas the ductility and penetration degree decrease.The corresponding rheological parameters are manifested in a reduction in the phase angle and an increase in the complex shear modulus [135,136].In terms of their correlation, Wang et al. [105] found that the needle penetration index was negatively correlated with the elastic modulus and the complex shear modulus.The softening point was closely related to rheological parameters such as the rutting factor, phase angle, complex shear modulus, elastic modulus, and creep compliance Jnr, as listed in Table 6.Asphalt is a colloidal system (Figure 15), in which the asphaltene is encapsulated by the colloid and dispersed in the light components (saturated and aromatic).According to the colloid structure theory, the concentration of the dispersed phase (asphaltene) determines the rheology of asphalt [108].The aging effect in asphalt leads to the conversion of aromatic hydrocarbons and resins into asphaltenes, which leads to changes in the colloidal structure and affects the rheological properties [8].There is a good correlation between the ratio of the soft and hard components and rheological indices.Chen Huaxin et al. [137] established a grey correlation between asphaltenes, gums, aromatic components, saturated components, and rheological properties and proved that asphaltenes have the greatest impact on asphalt viscosity.In addition, Salehfard et al. [138] measured the relative content of internal chemical components before and after asphalt aging based on TLC-FID and calculated the colloidal instability coefficient (I C ); the results showed that the I C is closely related to the rheological properties of asphalt and this is the reason why the change in asphalt viscosity and the complex modulus based on I C . the ratio of the soft and hard components and rheological indices.Chen Huaxin et al. [137] established a grey correlation between asphaltenes, gums, aromatic components, saturated components, and rheological properties and proved that asphaltenes have the greatest impact on asphalt viscosity.In addition, Salehfard et al. [138] measured the relative content of internal chemical components before and after asphalt aging based on TLC-FID and calculated the colloidal instability coefficient (IC); the results showed that the IC is closely related to the rheological properties of asphalt and this is the reason why the change in asphalt viscosity and the complex modulus based on IC.Moreover, it was found that the rheological properties of asphalt are closely related to their characteristic functional groups [114,139].A significant linear correlation can be observed between the aging index of characteristic functional groups and the change in the rheological properties of asphalt (Figure 16).It was reported in [14,140] that the changes in the content of functional groups such as carbonyls and sulfoxides during the aging process has an extremely significant influence on the rheological properties of asphalt.Moreover, there is a possibility to predict the dynamic shear modulus of asphalt in terms of the aging degrees by the established models.Moreover, through the combination of FTIR and DSR, [141] found that there are multiple linear correlations between the rheological properties and the chemical properties of asphalt mixture after ageing.Moreover, it was found that the rheological properties of asphalt are closely related to their characteristic functional groups [114,139].A significant linear correlation can be observed between the aging index of characteristic functional groups and the change in the rheological properties of asphalt (Figure 16).It was reported in [14,140] that the changes in the content of functional groups such as carbonyls and sulfoxides during the aging process has an extremely significant influence on the rheological properties of asphalt.Moreover, there is a possibility to predict the dynamic shear modulus of asphalt in terms of the aging degrees by the established models.Moreover, through the combination of FTIR and DSR, Ref. [141] found that there are multiple linear correlations between the rheological properties and the chemical properties of asphalt mixture after ageing.Moreover, it was found that the rheological properties of asphalt are closely related to their characteristic functional groups [114,139].A significant linear correlation can be observed between the aging index of characteristic functional groups and the change in the rheological properties of asphalt (Figure 16).It was reported in [14,140] that the changes in the content of functional groups such as carbonyls and sulfoxides during the aging process has an extremely significant influence on the rheological properties of asphalt.Moreover, there is a possibility to predict the dynamic shear modulus of asphalt in terms of the aging degrees by the established models.Moreover, through the combination of FTIR and DSR, [141] found that there are multiple linear correlations between the rheological properties and the chemical properties of asphalt mixture after ageing.

Correlation between Rheological Properties and Micro Properties
During the aging process of asphalt, the internal microscopic morphology significantly changes.It was believed that the microscopic characteristics can help to build a bridge between the chemical properties and macro performance of asphalt.By coupling the changes and distribution of the molecular before and after the aging of asphalt, the mechanism of ageing could be better understood.By coupling the results obtained from AFM and DSR tests [43], the correlations between micromechanical and macro-rheological views during the aging of asphalt can be linked (Figure 17).AFM tests can effectively predict the macroscopic mechanical properties of asphalt at different temperatures and different acting frequencies, and overcome the limitation of DSR, which cannot directly test the rheological properties of asphalt under high-frequency action.It was observed that [143] a linear correlation existed between Young's modulus, complex modulus, and a carbonyl index, which confirmed that the influence of aging on macro-and micro-levels of asphalt is consistent.

Correlation between Rheological Properties and Micro Properties
During the aging process of asphalt, the internal microscopic morphology significantly changes.It was believed that the microscopic characteristics can help to build a bridge between the chemical properties and macro performance of asphalt.By coupling the changes and distribution of the molecular before and after the aging of asphalt, the mechanism of ageing could be better understood.By coupling the results obtained from AFM and DSR tests [43], the correlations between micromechanical and macro-rheological views during the aging of asphalt can be linked (Figure 17).AFM tests can effectively predict the macroscopic mechanical properties of asphalt at different temperatures and different acting frequencies, and overcome the limitation of DSR, which cannot directly test the rheological properties of asphalt under high-frequency action.It was observed that [143] a linear correlation existed between Young's modulus, complex modulus, and a carbonyl index, which confirmed that the influence of aging on macro-and micro-levels of asphalt is consistent.In addition, Hou et al. [41,144] attempted to apply spectrophotometry to the study of asphalt aging by introducing asphalt into a "toluene-heptane" solution, obtaining the asphalt absorbance index by spectrophotometer, and establishing the correlation analysis with the existing rheological indexes.The results proved the feasibility of spectrophotom- In addition, Hou et al. [41,144] attempted to apply spectrophotometry to the study of asphalt aging by introducing asphalt into a "toluene-heptane" solution, obtaining the asphalt absorbance index by spectrophotometer, and establishing the correlation analysis with the existing rheological indexes.The results proved the feasibility of spectrophotometry in the characterisation of the degree of asphalt aging.The effect of aging on the rheological properties of asphalt could be predicted based on the absorbance of the asphalt.

Correlation between Rheological Properties and Molecular Structure
As for the molecular weight of asphalt, it was reported that the sizes of molecules increase continuously with ongoing ageing (Figure 18) [125,145], which is also strongly correlated with the rheological performance of asphalt.The polydispersity parameter obtained from GPC may be a useful tool to describe and predict the evolution of the rheological performance of asphalt.Some other research also shows that an increase in the high molecular weight of asphalt could increase the viscosity of bituminous binders [4].Moreover, You et al. [125] used a 12-component bitumen model in a molecular dynamics simulation, which considers an improved Amberer-Cornell force field.The moduli and phase angles were tested by laboratory experiments and simulated by molecular dynamics.The viscosity index obtained from both experimental and simulation approaches agree with each other (Figure 19).With the aids of molecular dynamic simulation, it is expected that the self-healing of asphalt and the adhesion behaviour between asphalt and aggregates could be further revealed in the future.As a viscoelastic material, the performance of asphalt is closely related to its rheological properties.By combining the existing research on the correlation between the rheological properties and chemical composition, microscopic properties, and molecular structure of the asphalt aging process, it was found that the correlation between multiple scales can help analyse the mechanisms of asphalt aging in depth and ensure its correctness and accuracy.In addition, other related scale results of asphalt can also be predicted by the aging degree or a certain scale result.Moreover, You et al. [125] used a 12-component bitumen in a molecular dynamics simulation, which considers an improved Amberer-Cornell force field.The moduli and phase angles were tested by laboratory experiments and simulated by molecular dynamics.The viscosity index obtained from both experimental and simulation approaches agree with each other (Figure 19).With the aids of molecular dynamic simulation, it is expected that the self-healing of asphalt and the adhesion behaviour between asphalt and aggregates could be further revealed in the future.Moreover, You et al. [125] used a 12-component bitumen model in a molecular dynamics simulation, which considers an improved Amberer-Cornell force field.The moduli and phase angles were tested by laboratory experiments and simulated by molecular dynamics.The viscosity index obtained from both experimental and simulation approaches agree with each other (Figure 19).With the aids of molecular dynamic simulation, it is expected that the self-healing of asphalt and the adhesion behaviour between asphalt and aggregates could be further revealed in the future.As a viscoelastic material, the performance of asphalt is closely related to its rheological properties.By combining the existing research on the correlation between the rheological properties and chemical composition, microscopic properties, and molecular structure of the asphalt aging process, it was found that the correlation between multiple scales can help analyse the mechanisms of asphalt aging in depth and ensure its correctness and As a viscoelastic material, the performance of asphalt is closely related to its rheological properties.By combining the existing research on the correlation between the rheological properties and chemical composition, microscopic properties, and molecular structure of the asphalt aging process, it was found that the correlation between multiple scales can help analyse the mechanisms of asphalt aging in depth and ensure its correctness and accuracy.In addition, other related scale results of asphalt can also be predicted by the aging degree or a certain scale result.

Conclusions
Multi-scale evaluation of asphalt aging behaviour can deeply explore the mechanisms of aging.This study compiled the research results of asphalt aging at different scales over the past ten years and summarised the multiscale research on asphalt aging.The conclusions are as follows: (1) At present, the research on asphalt aging has achieved phased results, and has reached a preliminary understanding of the mechanisms of asphalt aging.However, asphalt aging is a combination of physical changes and chemical reactions.In addition to its chemical components and molecular structure, it is also closely related to environmental factors (such as heat, oxygen, light, and water), asphalt film thickness, and asphalt aging time.It is necessary to comprehensively consider the coupling effect of multiple factors on the asphalt cement, close to the actual asphalt pavement aging environment, and apply the aging research results to the engineering practice.(2) The evaluation of the degree of asphalt aging has undergone a transition from a macro-scale to a meso-scale, and then to a micro-scale.In this process, researchers have gradually deepened their understanding and research on the mechanisms and characterisation of asphalt aging.However, the aging process of asphalt cannot be comprehensively evaluated from a single perspective, either macroscopic, through chemical composition, microscopic, or nano-view.In the future, aging research will be interrelated among different scales, and the multi-scale evaluation of asphalt aging is an inevitable tendency.(3) Based on the research of rheological properties, the correlation between rheological properties and other scales was determined and analysed.There are different degrees of correlation between rheological properties and various scales.The mechanism of asphalt aging can be thoroughly analysed through a correlation study between multiple scales to ensure accuracy.In addition, other related scale results of asphalt can also be predicted by the aging degree or a certain scale result.(4) Molecular dynamics simulation technology has been gradually introduced into research on asphalt materials.Although it is in the initial stage, it has considerable potential for development.In the future, multiscale research on asphalt aging and molecular dynamics methods can be used to rapidly establish correlations between scales.Through the combination of molecular dynamics and microstructure, which can reveal the profound changes in the aging process that cannot be determined via macro-and mesoscopic tests, the results can be used to predict the macro-performance of asphalt materials.

Figure 1 .
Figure 1.Schematic diagram of the aging process of asphalt during service.

Figure 1 .
Figure 1.Schematic diagram of the aging process of asphalt during service.

Figure 4 .
Figure 4. Relative content of carbonyl index and sulfoxide index corresponding to different RTFOT aging cycles [49].

Figure 4 .
Figure 4. Relative content of carbonyl index and sulfoxide index corresponding to different RTFOT aging cycles [49].

Figure 9 .
Figure 9. Schematic diagram of asphalt infrared spectrum test results.

Sustainability 2023 ,
15, 2953 14 of 26surface microstructure and micromechanical parameters during the aging process of asphalt can effectively help quantify the degree of aging.

Figure 19 .
Figure 19.Viscosity predictions for the present asphalt model at 374.15, 408.15, and 438.15 K. Comparison with experimental results shown with red lines for each temperature [146].

Figure 19 .
Figure 19.Viscosity predictions for the present asphalt model at 374.15, 408.15, and 438.15 K. Comparison with experimental results shown with red lines for each temperature [146].

Figure 19 .
Figure 19.Viscosity predictions for the present asphalt model at 374.15, 408.15, and 438.15 K. Comparison with experimental results shown with red lines for each temperature [146].