Study on Ultraviolet Aging Performance of Composite Modified Asphalt Based on Rheological Properties and Molecular Dynamics Simulation

In order to study the eect of UV aging on asphalt performance, the asphalt four-component test, dynamic shear rheology test, and exural creep stiness test were combined with molecular dynamics simulation to matrix asphalt and rubber powder/styrenebutadiene rubber/polymerization.e properties of phosphoric acid/epoxy resin modied asphalt before and after UV aging were analyzed, and the mechanism of UV aging was discussed. Research shows that modied asphalt has better high and low temperature performance and mechanical properties than matrix asphalt. After aging, the asphaltene content increases, and the gum and light components decrease. When unaged, the modied asphalt has more asphaltenes than the matrix asphalt, and the change after UV aging is less than the matrix asphalt. With the progress of UV aging, the rutting factor gradually increases, the phase angle gradually decreases, and the high-temperature performance of the asphalt is improved; the stiness modulus gradually increases, the creep rate gradually decreases, and the low-temperature capability of the asphalt is weakened; cohesive energy density increases, the intermolecular interaction becomes stronger, and the solubility parameter becomes smaller, which reduces the compatibility between the asphalt and the modier and increases the elastic modulus, bulk modulus, and shear modulus, improving the high-temperature mechanical properties of the asphalt. Based on the rheological properties, composition changes, and molecular model parameters of the modied asphalt, the anti-ultraviolet aging ability of the modied asphalt is better than that of the matrix asphalt.


Introduction
Asphalt is a good bonding material, so it is widely used in road pavement construction [1]. Adverse environmental conditions and vehicle loads will lead to high temperature rutting and low temperature cracking of asphalt pavement, and the base asphalt cannot well meet the requirements of use, so asphalt needs to be modi ed [2,3]. SBS [4], PPA [5], epoxy resin [6], and ber [7] can improve the high temperature performance of asphalt. However, the low temperature performance has not been improved or even decreased. Rubber powder [8], SBR [9], nanomaterials [10], and other modi ers can improve the low temperature performance of asphalt, but the high temperature performance is slightly insu cient. A single modi er can only improve some properties of asphalt, while the compound modi ed asphalt solves this problem. e aging of asphalt materials is one of the main reasons for the destruction of asphalt concrete pavement. erefore, it is necessary to conduct in-depth research on the UV aging characteristics of asphalt in order to improve the durability of asphalt and extend the service life of asphalt pavement [11]. Ye et al. [12] conducted UV aging and PAV aging on asphalt, compared the changes of the three indexes before and after aging, and found that UV aging has a far greater impact on asphalt performance than PAV aging. Tan et al. [13] evaluated UV aging and PAV aging of asphalt through rheological properties and reached the same conclusion. A large number of scholars added SBS, rubber powder, carbon black, biological modi ers, and montmorillonite to asphalt and evaluated the UV aging resistance of asphalt by comparing the changes of rheological properties, mechanical properties, and durability before and after aging.
Materials are composed of various molecules and atoms. Deformation and destruction are naturally inseparable from their structural forms; that is, the properties of materials depend on their molecular and microscopic structures [14]. At present, the understanding of deformation and failure of asphalt binders is usually limited to the phenomenological engineering method, neglecting the potential failure mechanism of molecular structure. erefore, molecular simulation is of great significance in material research. Molecular dynamics simulation is a powerful numerical method, which has been widely used to simulate asphalt molecular behavior at molecular scale. Xu [15] et al. studied the bond performance between asphalt binder and aggregate by means of molecular dynamics simulation. Wang et al. [16,17] simulated the preparation process of rubber powder and SBS modified asphalt and studied the influence of temperature on the compatibility between the modifier and asphalt. Most of the research focuses on the unaged asphalt, and there are few studies on the aged asphalt.
Based on asphalt four-component test, the dynamic shear rheological test, and bending creep stiffness test, combined with the molecular dynamics simulation technology from both macro and micro scale study of ultraviolet aging properties of asphalt, we dig deeper into the compound modified asphalt ultraviolet aging mechanism and master its macroscopic change rule, for the development of resistance to ultraviolet aging asphalt to provide some technical support.

Materials and Tests
2.1. Material. In the experiment, Panjin 90# matrix asphalt was selected, and Shandong Taian 60-mesh waste tire rubber powder was selected as rubber powder, while Jinan Shanhai Chemical Technology Co., Ltd. provided SBR latex, McLean Company provided PPA, and Shandong Deyuan Epoxy Technology Co., Ltd. provided E-44 resin and 650 low-molecular polyamide as epoxy resin and curing agent. According to the preliminary study of the team [18], the composite modified asphalt was prepared with the rubber powder content of 7.1%, the SBR content of 4.1%, the PPA content of 1.7% and the epoxy resin and curing agent content of 4.9%. Dry the matrix asphalt in an oven at 140 deg c for 1 h, weighing 500 g of the matrix asphalt and continuously heating on an experimental furnace, wherein the speed of a high-speed shearing machine is 3000 r/min; when the temperature reaches 160 deg c to 180 deg c, set the speed of the high-speed shearing machine to 5000 r/ min, add rubber powder for shearing for 30 min, and sequentially add SBR latex, polyphosphoric acid, epoxy resin, and curing agent, continuously shearing for 45 min after all the components are added, and put the components into the oven at 140 deg c for standing and development for 30 min to obtain the composite modified asphalt.

Ultraviolet Aging Test.
e ultraviolet aging test was conducted in an ultraviolet weather chamber (model LRHS-NZY, Shanghai Forest Frequency). e ultraviolet radiation wavelength was 340 nm, and the irradiance was 0.68 w/m 2 . e molten asphalt was placed in a tray with a diameter of 14 cm to form a film with a thickness of about 3 mm, the distance between the sample and the lamp tube was 50 mm, and the working temperature was controlled at 60 C. e ultraviolet aging test of asphalt was performed for 0-468 h (equivalent duration: 12 months in the eastern part of Inner Mongolia). e samples were taken once every 4 months for performance testing.

Four-Component Test.
Test the chemical composition of asphalt according to "T0618-1993" in "Test Specification for Asphalt and Asphalt Mixture" (JTGE 20-2011).

DSR Test.
In a dynamic shear rheometer (AR2000 model, TA Co, USA), the dynamic rheometer of all samples before and after aging was measured in parallel plate mode. e parallel plates have a diameter of 25 mm, a spacing of 1 mm, a frequency of 10 rad/s, and a temperature range of 42°C∼82°C, and they are scanned every 4°C

BBR Test.
In the experiment, the bending beam rheometer made by CANNON Company of USA was used to conduct rheological tests on composite modified asphalt and matrix asphalt at different aging stages at the test temperature of −18°C, and the stiffness modulus under the loading time of 60 s was obtained. Table 1 lists the changes of four components of base asphalt and composite modified asphalt before and after UV aging for 12 months. It can be seen that, after UV aging, the four components of the two kinds of asphalt show the same change trend, with asphaltene increasing, saturated content and colloid decreasing less, and aromatic content decreasing significantly. Ultraviolet light, as a light wave with short wavelength and high energy, can cut off some chemical bonds in asphalt. Because of its low energy, aromatic components are most easily cut off, producing a large number of free radicals, which are recombined and converted into colloid. e change of gum content depends on the speed at which saturated and aromatic components generate gum and the speed at which gum is transformed into asphaltene, and the gum content decreases; obviously, the latter is larger than the former. For base asphalt, the content of asphaltene increased by 14.7%, the content of saturated and gum decreased slightly, and the content of aromatic decreased by 11.65%. However, the change of the content of each component in the composite modified asphalt is less than that of the base asphalt. is is because the rubber powder in the modified asphalt and SBR contains part of carbon black, which is a good ultraviolet absorber and part of ultraviolet rays, which reduces the ultraviolet irradiance, thus slowing down the transformation rate of asphalt components. Comparing the unaged base asphalt with the compound modified asphalt, it is found that the compound modified asphalt has more asphaltene, and other components are less than the base asphalt. When rubber powder and SBR are mixed into asphalt, it will absorb the oil in asphalt and swell, which will lead to the increase of asphaltene content. PPA, as a kind of phosphoric acid, will esterify with alcohol in asphalt to form phosphate, which will be converted into asphaltene after recombination with colloid.

High Temperature Rheology. Complex modulus (G * )
indicates the ability of asphalt to resist shear deformation, and phase angle (δ) can evaluate the proportion of viscous and elastic components of asphalt. As can be seen from Figure 1, the G * of the two kinds of asphalt increases with the increase of temperature, while the δ decreases gradually, which indicates that the elastic component of asphalt gradually transforms into viscous component with the increase of temperature, so that the asphalt is softer, and the antideformation ability is worse. Comparing the two kinds of asphalt, the δ of modified asphalt is always higher than that of base asphalt, which indicates that the elastic component in modified asphalt is more than that in base asphalt. Because rubber powder and SBR are excellent elastic materials, the antideformation ability of asphalt is improved. In American SHRP plan, the rutting factor (G * /sinδ) is proposed to evaluate the rutting resistance of asphalt. e larger the G * /sinδ, the better the rutting resistance of asphalt. It can be seen from Figure 2 that the G * /sinδ of modified asphalt is always higher than that of base asphalt, which indicates that modified asphalt is used for better rutting resistance. Rubber powder and SBR in modified asphalt swell and decompose after absorbing light components to form a network structure. Epoxy resin and curing agent act to form an epoxy system. After PPA reacts with asphalt, the content of macromolecules in asphalt increases. All these factors will increase the antideformation ability of asphalt. When external stress is applied, the deformation is small, thus improving the rutting resistance of asphalt. Epoxy system is formed after the action of resin and curing agent, and the content of macromolecule in asphalt is increased after the esterification reaction between PPA and asphalt. All these factors will increase the antideformation ability of asphalt. When external stress is applied, the deformation is small, thus improving the rutting resistance of asphalt. Figures 3 and 4 show the changes of G * and δ of asphalt with aging time at 62°C. With the aging time increasing, G * gradually increases, while δ gradually decreases, which shows that aging improves the high-temperature deformation resistance of asphalt. e increase of asphaltene changes the viscoelastic ratio of asphalt, which makes the G * of asphalt increase and δ decrease. After 12 months of aging, the G * of base asphalt increased from 1107.31 Pa to 2943.18 Pa, an increase of 165%, and δ decreased from 86.56 to 80.49, a decrease of 7%. G * of modified asphalt increased from 5229.84 Pa to 6540 Pa, only increased by 25%, and δ decreased from 68.28 to 65.36, only decreased by 4.2%. Figure 5 shows the change of G * /sinδ of asphalt with aging time at 62°C. e change of G * /sinδ shows the same trend as that of G * . Compared with base asphalt, the indexes of modified asphalt are all smaller than those of base asphalt, which shows that modified asphalt has better antiaging  Advances in Materials Science and Engineering ability. Rubber powder and SBR contain carbon black, which absorbs part of ultraviolet rays and weakens the aging rate, thus making the index of asphalt change less. By observing the growth of G * /sinδ in different aging periods, it can be seen that it increased the most in the first four months and then gradually decreased. With the gradual aging, the asphalt on the surface layer aged seriously and gradually carbonized and hardened, which hindered the aging process.

Low Temperature Rheology.
Bending stiffness modulus (S) is an index to evaluate the ability of asphalt to resist deformation at low temperature, and creep rate (M) can evaluate the stress relaxation ability of asphalt under low temperature load. Figure 6 shows the changes of S and M of asphalt with aging time at −18°C. e S of base asphalt is always larger than that of modified asphalt, which shows that, under the same load, the strain of base asphalt is larger than that of modified asphalt, and the asphalt is more brittle and easier to crack. However, the M of the base asphalt is larger than that of the modified asphalt, which indicates that the stress in the base asphalt is longer, and the stress relaxation ability is worse, which increases the possibility of cracking at low temperature. Although epoxy resin shows brittleness at low temperature, the toughness of rubber powder and SBR at low temperature is far greater than the influence of brittleness of epoxy resin on asphalt. e network structure formed by rubber powder and SBR in asphalt still maintains high performance at low temperature, making its deformation smaller, and the network structure can disperse stress faster, avoiding stress concentration from damaging asphalt, reducing the possibility of cracking, thus having better low temperature crack resistance. After aging, the S of asphalt gradually decreases, which indicates that it    becomes brittle at low temperature, and its deformation is larger, and its antideformation ability decreases. M gradually increases, indicating that the stress stays in the interior longer, and the stress relaxation ability decreases. Overall, aging reduces the low-temperature performance of asphalt and increases the possibility of cracking at low temperature. On the one hand, after aging, asphalt components change, asphaltene increases, and colloid and light components decrease, which increases the brittleness of asphalt; on the other hand, ultraviolet rays also crack the modifier, which reduces the performance of asphalt. Compared with base asphalt and modified asphalt, whether the amount of S decreases, or the amount of M increases, modified asphalt is less than base asphalt, indicating that the antiaging ability of modified asphalt is better than that of base asphalt, which is consistent with the results under high temperature conditions.

Establishment of Asphalt Molecular
Model. According to many scholars' research on molecular models of asphalt components, four typical structures of asphalt components are selected [19][20][21][22][23], and according to the mechanism of asphalt molecular oxidation, four-component molecular models of asphalt before and after aging are constructed by using materials studio software, as shown in Figure 7. Rubber powder contains natural rubber, carbon black, styrene-butadiene rubber, and other components. Carbon black, as an ultraviolet absorber, has no effect in the simulation process. erefore, natural rubber molecules and styrene-butadiene rubber molecules are selected as the representatives of rubber powder molecules. e molecular model of modifier is also constructed by using software, as shown in Figure 8.
According to the ratio of four components of asphalt measured in the previous article, the model of matrix asphalt and modified asphalt aged for 12 months is constructed, as shown in Figure 9, in which the amount of modifier is added according to the ratio of molecular molar mass of asphalt to molar mass of modifier. e geometric structure of the constructed asphalt molecular model is optimized for 10000 fs under COMPASS II force field to eliminate the unreasonable conformation in the structure. Because the geometric structure optimization only finds the lowest energy point in the adjacent structure, but this is not accurate, it is necessary to conduct dynamic simulation to find the global optimal conformation and conduct NPT dynamic simulation at 298 K for 50 ps. From Figure 10, it can be seen that, after dynamic optimization, the density of asphalt at 298 K is stable between 0.98 and 1.0 g/ cm 3 , which is close to the actual asphalt density of 1.0 g/cm 3 , indicating that the four asphalt models are more accurate. It is not convincing to verify the accuracy of asphalt model only by density, so the radial distribution functions in four asphalt models are analyzed. As can be seen from Figure 10, the radial distribution functions of the four kinds of asphalt almost completely coincide. e radial distribution function is a characteristic physical quantity that reflects the microstructure of materials. When R tends to infinity, the value of g(r) tends to 1, which is in a disorderly state. When R is at a small value, the maximum value of g(r) will appear several times, which shows that there is a high probability of particles appearing here. In the figure, g(r) approaches to 1 at the beginning of 3 years and stabilizes at around 1 at the beginning of 5 years, which accords with the range of Van der Waals force and the state of real asphalt, indicating that the model is highly reliable.

Cohesive Energy Density and Solubility Parameters.
Cohesive energy density (CED) is the energy required for 1 mol of condensate in unit volume to overcome the intermolecular force of vaporization, and it can also be used to indicate the size of intermolecular force. For polymer compounds, the higher the cohesive energy density is, the more viscous the substance is. Solubility parameter (δ) is an important thermodynamic parameter representing the properties of polymer solution, and its physical meaning is the square root of cohesive energy density. e smaller the difference between the solubility parameters of the two substances, the better their compatibility. According to literature review, the solubility parameters of the four modifiers are all smaller than those of the base asphalt, so the smaller the solubility parameters of the asphalt, the better the compatibility between the modifier and the asphalt. Table 2 shows cohesive energy density and solubility parameters of four kinds of asphalt at 303 K temperature. e cohesive energy density of modified asphalt is higher than that of base asphalt, which increases by 110.3 J·cm −3 , indicating that modified asphalt has higher viscosity. Mainly rubber powder and SBR swell and decompose in asphalt, forming a network structure. Polyphosphoric acid reacts with asphaltene to make it more evenly dispersed in the network structure formed by modifier. Epoxy system composed of resin and curing agent makes the structure more stable and intermolecular interaction stronger, so cohesive energy density is higher. Whether it is modified asphalt or base asphalt, the cohesive energy density of aged asphalt will increase. Ultraviolet aging makes the chemical bonds of asphalt break, and it is easier to react with oxygen, thus generating more ketones and sulfoxide groups. ese groups have strong polarity, and the intermolecular force will also increase, resulting in an increase in cohesive energy density. Comparing the matrix asphalt and the modified asphalt before and after aging, it is found that the cohesive energy density of the aged matrix asphalt increases by 64.2 J·cm −3 , while that of the modified asphalt increases by 53.7 J cm −3 , which shows that the ultraviolet aging resistance of the modified asphalt is better than that of the matrix asphalt. By comparing the increment of the modifier and aging on the internal shaped energy density, it is found that the modifier has a greater influence on the internal shaped energy density. e solubility parameter of aged asphalt becomes larger, which indicates that the aging of asphalt reduces the compatibility between asphalt and modifier.

Mechanical Property.
e mechanical properties of materials refer to the mechanical characteristics of materials under different environments, which are loaded by external conditions. e Forcite module of MS software can be used to calculate the mechanical properties of the model. Its principle is to set six strains, such as the normal axis, to the molecular model through the program and then calculate the mechanical properties parameters after obtaining the stress values, which is just the opposite of our macro test. After calculation, we can get the elastic modulus (e), bulk modulus (k), and shear modulus (g) of the model. Elastic modulus is the ratio of stress to strain in the longitudinal direction of a material. e larger the elastic modulus, the stronger the ability to resist deformation. Volume modulus is the ratio of pressure difference to volume difference of materials. e larger the volume modulus, the smaller the volume change. Shear modulus is the ratio of shear stress to shear strain, which can characterize the shear resistance of asphalt.  Table 3 shows the change of mechanical properties of asphalt at 303 K temperature. Whether modified or not, the elastic modulus, bulk modulus, and shear modulus of asphalt increase after aging, which indicates that aging makes asphalt hard, which is mainly caused by the decrease of saturated content, aromatic content, and colloid of asphalt and the increase of asphaltene. e asphaltene molecules contain more benzene rings and less branched structures, and their molecules are relatively stable, so the elasticity of the molecular model is reduced. Compared with base asphalt and modified asphalt, the elastic modulus, bulk modulus, and shear modulus of modified asphalt are higher than those of base asphalt, with the elastic modulus increasing by 26%, bulk modulus increasing by 37% ,and shear modulus increasing by 32%. e molecular model of modified asphalt contains more natural rubber, styrene-butadiene rubber, and epoxy resin. ese molecules cross-link with asphalt component molecules inside the molecular model of asphalt, and the structural stability is stronger. e smaller the deformation under stress, the stronger the ability to resist deformation, which is consistent with the conclusion of DSR test.

Conclusion
(1) After UV aging, the components of asphalt change, the asphaltene content increases, the colloid and light components decrease, and the component change of modified asphalt is less than that of base asphalt, which is less affected by UV light. (2) Ultraviolet aging improves the high-temperature rutting resistance of asphalt but reduces the lowtemperature cracking resistance of asphalt. e ultraviolet aging rate gradually slows down with the increase of time. e high-temperature rutting resistance and low-temperature cracking resistance of modified asphalt are better than those of base asphalt. e increase of UV aging time, the increase of G * /sinδ and S, and the decrease of δ and M of modified asphalt are lower than those of base asphalt, which indicates that UV aging is less sensitive to modified asphalt than base asphalt.
(3) From the molecular level, the cohesive energy density and mechanical properties of modified asphalt are better than those of matrix asphalt. Aging improves the cohesive energy density and hightemperature mechanical properties of asphalt and reduces the compatibility between asphalt and modifier. (4) Considering the rheological property, component change, and molecular performance parameters of asphalt, the ultraviolet aging resistance of modified asphalt is better than that of base asphalt.
Data Availability e data that support the findings of this study are available from the corresponding author upon reasonable request.

Conflicts of Interest
e authors declare that they have no conflicts of interest.