Research on the Properties of Low Temperature and Anti-UV of Asphalt with Nano-ZnO/Nano-TiO2/Copolymer SBS Composite Modified in High-Altitude Areas

Strong ultraviolet light and low-temperature are the typical environmental characteristics in high-altitude areas. *e performance of SBS-modified asphalt in the above environmental characteristics needs further study. To improve the resistance ultraviolet (UV) ageing and low-temperature performance of copolymer(SBS-) modified asphalt, an SBS-modified asphalt containing nano-ZnO and nano-TiO2 is proposed. In this paper, nano-ZnO, nano-TiO2, and SBSwere used asmodifiers with the silane coupling agent (KH560) as the nanomaterial surface modification. *e orthogonal test table was used to analyse the effects of the three modifiers on the physical properties of modified asphalt at different dosages. On this basis, the physical properties, low-temperature properties, and ageing indices (carbonyl index and sulfoxide index) were studied for base asphalt, SBS-modified asphalt, nano-ZnO/SBS-modified asphalt, and nano-ZnO/nano-TiO2/SBS composite-modified asphalt before and after photoaging. *e content changes of characteristic elements (Zn and Ti) in the nano-ZnO/nano-TiO2/SBS composite-modified asphalt before and after ageing were studied by scanning electron microscopy with energy dispersive spectroscopy (SEM/EDS), and the UV ageing mechanism was revealed. *e results indicate that two nanoparticles show the best compatibility with asphalt after surface modification and can improve the binding ability between SBS and base asphalt.*e orthogonal test analysis shows that nano-ZnO has a highly significant effect on the lowand high-temperature performance of the nano-ZnO/nano-TiO2/SBS composite-modified asphalt, and nano-TiO2 has a significant effect on the high-temperature performance. *ree optimal composite-modified systems for base asphalt including 4% nano-ZnO/1.5% nano-TiO2/3.2% SBS were proposed and had the best antiaging ability. Compared with the sulfoxide index, the carbonyl index changedmost obviously before and after ageing. Additionally, the results reveal that nano-TiO2 has a good absorption effect at a wavelength of 365 nm (ultraviolet light), while nano-ZnO is liable to photolysis, and its activity decreases at this wavelength.


Introduction
In highway pavement engineering at high altitudes, the styrene-butadiene-styrene (SBS) block copolymer-modified asphalt is mostly used as a binder in the surface layer. Due to the long-term exposure to intense ultraviolet light, the ultraviolet (UV) ageing of SBS-modified asphalt tends to be photooxidation ageing [1][2][3]. Compared with the UV ageing of the base asphalt, the SBS-modified asphalt is affected by the base asphalt and SBS modifier; for example, the content of asphaltene depends on the synergistic effect between them [4][5][6]. During the ageing process, butadiene in SBS copolymers is degraded, the base asphalt is oxidized, and the ageing indices (sulfoxide index and carbonyl index) gradually increase. SBS modifier degradation could slow down the aging reaction of matrix asphalt, and the matrix asphalt could also play a role in the protection of the SBS modifier. eir interaction can effectively delay the ageing rate of SBS-modified asphalt [5,7]. However, due to the considerable difference in physical properties between SBS copolymers and base asphalt [7,8], the compatibility and dispersion of SBS-modified asphalt are generally poor, which affects the ageing resistance of the modified asphalt.
Currently, nanomaterials have been widely used due to their unique properties in improving the properties of asphalt. For example, Li et al. reported the effect of nano-ZnO with modified surface on the physical and anti-UV ageing properties of base asphalt and found that the surfacemodified nano-ZnO obviously increased the softening point, viscosity, and ageing resistance of binder, as well as improving the ductility of bitumen [9]. Zhu investigated the effect of nano-ZnO on pavement performance of SK-70 base asphalt through differential scanning calorimetry (DSC), dynamic shear rheometer (DSR), and Brinell viscometer, and the results showed that nano-ZnO could improve significantly the low-temperature ductility, high-temperature rutting resistance, and ageing resistance of asphalt [10]. Another study by Liu et al. showed that the dispersion stability of nano-ZnO in bitumen was improved by surface modifiers and could improve the anti-UV properties of asphalt [11]. Fang et al. concluded that the low-temperature anticracking properties had been improved significantly by adding nano-ZnO particles into asphalt [12]. Zheng et al. study showed that addition of nano-TiO 2 could improve UV-aging resistance of asphalt, with no obvious influence on low-temperature rheology [2]. Xu et al. indicated that nano-ZnO as an additive could improve the softening point, ductility (5°C), rheological properties, and fatigue performance of base asphalt [13]. Wang et al. conducted research on the effect of inorganic nanoparticles (nano-SiO 2 , nano-TiO 2 , and nano-ZnO) combined with organic expanded vermiculite (OVEMT) on asphalt binder and showed that other multiscale nanocomposites made the continuous PG grading temperature lower than base asphalt on the low-temperature property, except for 1% OVEMT plus 3% nano-SiO 2 [14]. Kleizienė et al. reported that nanoparticles (nano-SiO 2 , nano-TiO 2 , and nano-ZnO) as modification additive have the potential to enhance the complex shear modulus and reduce the phase angle of base asphalt, and the excessive amount of nanomaterials might result in the lower asphalt binder resistant to fatigue [15]. Saltan et al. indicated that the addition of zinc oxide nanoparticle (ZnONP) was able to improve permanent deformation and the resistance to thermal cracking compared to the neat bitumen [16]. Nazari et al. illustrated that the addition of nano-TiO 2 nanoparticles improved the aging resistance of asphalt binder [17]. Zhang and Zhang suggested that the ductility at 10°C of modified bitumen with nano-ZnO surface modification increased about 12 cm and the anti-UV aging ability enhanced significantly compared to those of base asphalt [18]. Hamedi et al. showed that the nano-ZnO was highly improved on the adhesion of asphalt-aggregate in wet conditions [19]. Moreover, many researchers studied the effect of the content or types of nanomaterial on the performance of asphalt. Salton et al. evaluated the effect of the content of zinc oxide nanoparticles (nano-ZnO) on permanent deformation of modified asphalt; the results showed that 5% modification had the best performance [16]. e study of Zhu et al. illustrated that 3% nano-ZnO and 1% organic expanded vermiculite (OEVMT) could improve the shear deformation resistance and elastic behavior of SBR modified asphalt [20]. Azarhoosh et al. analysed hot mix (HMA) asphalt containing 1%, 3%, 5%, and 7% of nano-ZnO mixed with bitumen 85/100. e results showed that fatigue life of mixtures containing nano-ZnO was higher than those of the control mixtures due to improved cohesion energy and higher resistance to fatigue cracking in asphalt [21]. In order to examine the role of nanomaterial in the ageing process of asphalt, Zhang and Zhang further found that the improved anti-UV aging performance could be attributed to the shielding function of inorganic nanoparticles to UV radiation [5,18,22]. In addition, many researchers extensively studied the properties of nano-TiO 2 modified asphalt. For example, Azarhoosh et al. found that nano-TiO 2 could increase the surface-free energy of asphalt, which had greater adhesion energy with the aggregate [21]. Marinho et al. evaluated the effect of nano-TiO 2 as modifiers to improve the rutting resistance and alleviate the fatigue cracking of asphalt mixtures [23]. e results showed that 3% nano-TiO 2 effectively increased the fatigue life and resistance to permanent deformations. Qian et al. reported that the 5% nano-TiO 2 asphalt mixture exhibited an NO 2 degradation productive effect under UV irradiation to reduce environmental pollution [24]. Ye and Chen reported the effect of nano-TiO 2 particles on the temperature properties of AH-70 base asphalt and found that the softening points of the modified asphalt with 0.5% nano-TiO 2 increased by 5°C, and the ductility at 5°C decreased by approximately 22.5 mm compared to those of base asphalt [25]. Sun et al. showed that nano-TiO 2 modified asphalt had more stable ductility and a smaller softening point changing ratio than the base asphalt, and thus, nano-TiO 2 particles actively enhanced the photooxidation resistance of asphalt, and the nano-TiO 2 modified asphalt could resist UV ageing [26]. In a word, adding appropriate amount of nanomaterials can effectively improve the physical properties and rheological properties of base asphalt, but the special environmental characteristics in high-altitude areas need the higher performance of nanomodified asphalt. e compatibility between base asphalt and copolymer SBS has a strong effect on the low-temperature performance of the modified asphalt [8]. However, nanomaterials can significantly improve the compatibility between polymer and base asphalt. erefore, many researchers have pay attention to the performance of nanomaterial/polymer modified bitumen. For example, Kang et al. [27], Xiao and Li [28], and Fang et al. [29] found that the nano-ZnO could promote even dispersion of SBS in asphalt, which reduced the free energy of SBS fine particles and improved the adhesion ability of SBS and asphalt interface. Zhang et al. studied the effects of nano-ZnO, nano-TiO 2 , and copolymer SBS on the high-and low-temperature properties for SK-70 base asphalt and the dispersion role of nanomaterials between copolymers and base asphalt. eir results indicated that nanomaterials could effectively improve the dispersion of copolymer SBS and significantly enhanced the high-and low-temperature properties of modified asphalt [30]. Zhang et al. discussed the influence of the multidimensional nanomaterials composed of 1% organic expanded vermiculite (OEVMT) plus 2% nano-TiO 2 , 2% nano-SiO 2 , or plus 2% nano-ZnO on rheological performance of SBS-modified asphalt (SBSMA) and found that other two multidimensional nanomaterials improved rutting performances and increased thermal cracking resistance of SBSMA except for 1% OEVMT plus 2% nano-SiO 2 [31]. Fang et al. discussed the influence of nanomaterials (nano-TiO 2 , nano-SiO 2 , and nano-ZnO) and isocyanate on performance of base asphalt [12]. Wang et al. showed the influence of the multidimensional nanomaterials composed of 1% organic expanded vermiculite (OEVMT) plus 2% nano-TiO 2 , 2% nano-SiO 2 , or plus 2% nano-ZnO on rheological performance of asphalt and asphalt mixture and found that the multidimensional nanomaterials increased the high-temperature properties and decreased the low-temperature continuous grading [14].
erefore, the addition of nanomaterials (nano-TiO 2 or nano-ZnO) has improved the physical performance and rheological performance; however, the research on the effect of the interaction between nano-TiO 2 and nano-ZnO on the properties of modified asphalt is not perfect.
In the above studies, many researchers focused on the effect of nanomaterials or copolymer SBS on the properties of asphalt. ere has been no perfection of nanomaterial/copolymer SBS in high-altitude areas, especially with respect to the UV antiageing properties of SBS-modified asphalt with the same dimensional different nanomaterials compound modified. erefore, nano-ZnO, nano-TiO 2 , and copolymer SBS were used to modify AH-90 base asphalt. e optimum content nanomaterial/copolymer SBS for AH-90 base asphalt was selected through physical property tests. en, the effect of the nanomaterial on the durability properties was studied by beam bending rheometer (BBR) tests and UV ageing test. Furthermore, the morphology, modification mechanism, and UV ageing mechanism of nanomaterial/copolymer SBS were studied by optical microscopy, scanning electron microscopy (SEM), and Fourier transform infrared (FTIR).

Raw Materials.
e base asphalt AH-90 in this paper was provided by Pengsheng Asphalt Co. Ltd. (Henan province, China), and its physical properties are shown in Table 1. e physical properties of nano-ZnO and nano-TiO 2 are shown in Table 2. e properties of copolymer SBS are shown in Table 3.
Methacryloxypropyltrimethoxysilane (C 9 H 20 O 5 Si; KH-560) is a type of surface modification in the field of composite materials [9,22,32]. In this study, methacryloxypropyltrimethoxysilane was diluted with distilled water and methanol. e concentration of methacryloxypropyltrimethoxysilane was 20% of the mixing suspensions by weight.

Preparation of Modified Asphalt.
Considering the related research results [9,13,28,30,32,33], the modified asphalt was prepared as follows. Firstly, the nanomaterials were placed in a vacuum drying chamber at 80°C for 240 minutes, and the base bitumen was heated to 140°C. Secondly, nanomaterial was added in the mixing suspension according the mass fraction and blended at 25°C for 30 minutes; the rotating speed gradually reached 2000 rpm, and the modified nanomaterials were placed in an air blast drying chamber at 130°C for 120 minutes. irdly, in order to give full play to the characteristics of nanomaterial and improve the dispersion effect of SBS in base asphalt [22,28,32], appropriate amounts of modified nanomaterials were added into the melt base asphalt and mixed for 120 min with the precision magnetic mixer at 4000 rpm; then, the nanomaterial-modified asphalt was obtained. Fourthly, appropriate amounts of copolymer SBS were added into the nanomaterial-modified asphalt, and the temperature was increased to 180°C for 40 minutes using a high-speed shearing machine with the rotational speed of 4500 rpm. en, the composite-modified asphalt was put into an oven for 120 minutes to well mix the nanomaterial/copolymer SBS-modified asphalt.

Experimental Design
e orthogonal experimental design (OED) can strongly decrease the experiment quantity and increase the work efficiency [30,34]. Hence, the OED method was used to analyse the effect of these modifiers. According to the existing research results [2,5,9,10,13,22,30,31,33], the nano-ZnO content in the base asphalt was selected as 3 wt%, 4 wt%, and 5 wt%, and the nano-TiO 2 content in the base asphalt was selected as 0.5 wt %, 1.5 wt%, and 3 wt%; then, the copolymer SBS content in the base asphalt was selected as 2.7 wt%, 3.2 wt%, and 3.7 wt %. is experimental scheme is shown in Table 4.

UV Aging Test.
To simulate UV irradiation in highaltitude areas, the UV ageing environment oven was designed. e 1000 W high-pressure mercury lamp was a simulated UV light source, and its performance parameters are shown in Table 5.
e main spectrum of the highpressure mercury lamp is 365.0 nm wavelength ultraviolet light, and its wave had the most obvious ageing effect on the asphalt binder [35,36]. To prevent the asphalt from thermal ageing and maintain it at 35°C in the UV ageing oven, the 200 W axial flow blower was used to supply sufficient cold air, and the 100 W exhaust fan in the top of the oven was used to discharge warm air [37,38]. e average UV density on the sample surface was approximately 370 W/m 2 . e UV ageing samples were prepared as follows. Firstly, the thin-film oven test (TFOT) was implemented according to the Chinese Standard Test Methods of Petroleum Asphalt Film Oven Test (JTG E20-2011) [39]. Secondly, the asphalt specimens aged by the TFOT were put into the preheated silica gel abrasive tool with a 140 mm inner diameter and a 1 cm depth to prepare a thin asphalt film with the thickness of approximately 3.2 mm, which was cooled to the sample temperature. Finally, the asphalt specimens were put into the UV ageing oven for 10 days.

Physical Property and BBR Test.
e physical properties of all tested asphalts in this paper, including softening point, penetration (5°C), and ductility (5°C), were tested according to the standard test methods in China test Advances in Materials Science and Engineering specification JTG E20-2011 [39]. According to AASHTO T313-12 [40], the low-temperature performance of the modified asphalt was evaluated at −12°C, −18°C, and −24°C in the bending beam rheometer (BBR) test.
In order to further evaluate the UV ageing resistance of modified asphalt, combining the results of softening point test, ductility (5°C), and BBR test, the increases in softening point (ΔT), ductility retention rate (K D ), and creep stiffness ageing index (SAI) are often used to evaluate the UV ageing degree, which can be calculated using equations (1)- (3):

Micromorphology Characterization.
e micromorphologies of the AH-90 base asphalt, copolymer SBS-modified asphalt, nanomaterial-modified asphalt, and optimal nano-ZnO/nano-TiO 2 /copolymer SBS-modified asphalt are evaluated by scanning electron microscopy (SEM). Both zinc and titanium percentages in the optimal nano-ZnO/nano-TiO 2 /copolymer SBS-modified asphalt were analysed by SEM and energy dispersive spectrometry (EDS). Its morphologies were observed using an optical microscope before and after the UV ageing. e preparation process of the micromorphology test sample is as follows: each sample to be tested of  Test number    Advances in Materials Science and Engineering fluid asphalt is put into the silica gel abrasive tool with a thickness of about 1 mm. e silica gel abrasive tool was cooled down to room temperature and then 3 samples of 5 × 5 mm 2 from different places of the silica gel abrasive tool to ensure repeatability of the result. e above samples are gold coated through the special machine, and then imaging from the section of base and modified asphalt is performed by SEM.

Microstructure Characterization.
Nano-ZnO and nano-TiO 2 were characterized by FTIR before and after the modification.
e modification mechanism of the composite-modified asphalt was also analysed; the scan was from 400 cm −1 to 4000 cm −1 at a resolution of 4 cm −1 . e preparation process of the FTIR test sample is as follows: firstly, a certain amount of dried potassium bromide (KBr) powder was pressed into sheet with good transparency through the table t machine. Secondly, the C 2 S solution with 5% asphalt was dropped onto the potassium bromide plate. Finally, the plate is placed into the apparatus for scanning, when the C 2 S solution in the tablet is completely volatilized. Each asphalt is prepared 3 samples, and each sample is scanned 3 times.
Carbonyl group C�O and sulfoxide group S�O were monitored to characterize the UV ageing degree of asphalt [41,42]. FTIR was further used to monitor these functional groups. e areas of the carbonyl and sulfoxide contents (approximately 1600 cm −1 and 1030 cm −1 ) were divided by the entire area between 2000 and 600 cm −1 to calculate the carbonyl index and sulfoxide index using formulas (4) and (5) [37,38,41,42]: Area of carbonyle band centered around 1600 cm − 1 Area of spectral bands between 2000 and 600 cm −1 , I S�O � Area of sulphoxide band centered around 1030 cm − 1 Area of spectral bands between 2000 and 600 cm −1 .

Analysis of Range (ANOR).
e effects of nanoparticles and polymer modifier on physical properties of AH-90 base asphalt are shown in Table 6. Compared with base asphalt, both the softening points and ductilities of nine formulations have also increased markedly, and the penetrations decrease significantly. It indicated that the base asphalt is enhanced in high-temperature performance and low-temperature ductility after adding nanoparticles and polymer modifier. Softening point used to describe the hightemperature performance of asphalt is one of the indexes in most countries. Compared with the base asphalt, the softening point of test number 5 modified asphalt is increased the most obviously, reaching 76.0°C, which is increasing by 30°C. Xu et al. indicated that the softening point value of modified asphalt increased about 56°C when the nano-ZnO dosage was 4.0% in base asphalt [13]. And Qian et al. showed that the rutting factor of SBS-modified asphalt with 1% nano-TiO 2 increased about 2.5 kPa at 76°C [24]. Zhang Hongliang et al. indicated that the rutting factor of 3.0% nano-ZnO/0.5% nano-TiO 2 /3.7% SBS-modified asphalt at 88°C is 2.2 kPa which is above the limit (1.0 kPa) provided by AASHTO [30]. erefore, the suggested asphalt formula meets the requirements of high-temperature stability. Compared with the base asphalt, the ductilities of 9 formulations increase by 16.1 cm at least. Qian et al. [24] and Ma et al. [43] showed that nano-TiO 2 had no significant effect on the low temperature of base asphalt before UV aging. is may be attributed to the addition of nano-ZnO and polymer SBS [13,24,30,43].
In general, a larger R j indicates a more significant factor level [34]. As presented in Table 6, the relationship of the effects of the three modifiers on the physical properties is nano-ZnO > copolymer SBS > nano-TiO 2 according to the climate characteristics in high-altitude areas. e blank column R j is much smaller than all factors, which implies that the above orthogonal test is reasonable.

Analysis of Variance (ANOVA).
e ductility is the preferred indicator of the asphalt pavement performance in high-altitude areas in accordance with the Technical Specifications for Highway Asphalt Pavement Construction (JTJ F40-2004) [44]. As presented in Table 7, factor A (nano-ZnO) is highly significant, factor C (copolymer SBS) is significant, and factor B (nano-TiO 2 ) has no effect on the test results. us, the addition of nano-ZnO in copolymer SBSmodified asphalt has improved the ductility in high-altitude areas. In addition, the effect ranking of the ductility properties is nano-ZnO > copolymers SBS > nano-TiO2, and the effect ranking of the softening point properties is nano-ZnO > copolymers SBS > nano-TiO 2 . e optimal level of selection factors is related to the required indicators. e larger the indicator is, the better it is and the larger the indicators should be, namely, the largest level in each column R 1j , R 2j , and R 3j ; otherwise, if the smaller the indicator is, the better it is, the lowest level should be selected [45]. In high-altitude areas, asphalt binder is required to have good low-temperature performance and, at the same time, ultraviolet aging resistance. As presented in Tables 7 and 8, 4% nano-ZnO and 3.2% copolymers SBS have significant effects on the low-temperature properties of asphalt. And Zhang et al. [31] and Liang and Lv [46] indicated that the best content of nano-TiO 2 in improving the antiultraviolet aging ability of base asphalt or modified asphalt was 1%-2%. erefore, the optimal proportions of nano-ZnO, nano-TiO 2 , and copolymer SBS in the base asphalt are determined as 4% nano-ZnO, 3.2% copolymer SBS, and 1.5% nano-TiO 2 .
e results in Figure 1 reveal that the creep stiffness S of the modified asphalt decreases, while the creep slope m   us, the modified asphalt has better low-temperature performance than the base asphalt. Compared with modified asphalt 2#, modified asphalts 3# and 4# have better resistance to lowtemperature cracking, but 4# has no obvious change after the addition of nano-TiO 2 .
is result is consistent with the orthogonal test result analysis.
As presented in the nano-ZnO modified asphalt (Figures 2(a) and 2(b)), modified nano-ZnO is more evenly dispersed in the matrix asphalt, the particle size is approximately 32 nm, and the agglomeration phenomenon is obviously weakened. Compared with the morphology of the base asphalt (Figure 2(c)), the morphology of the copolymer SBS-modified asphalt (Figure 2(d)) exhibits floccules of particulate matter with small agglomerate because of the swelling effect of the copolymers SBS after it was absorbed into the oil of the base asphalt, which implies the poor compatibility of copolymer SBS and base asphalt. However, the morphology of copolymer SBS is improved with the addition of nanomaterial (Figures 2(e) and 2(d)), which shows that the copolymer SBS matter decreases in size and is more evenly distributed in the base asphalt. is result is caused by the nanometer size effect of nanoparticles. On one hand, nanoparticles with the active surface area have a strong binding ability to copolymer SBS and base asphalt. On the other hand, the interfacial structure of the nanomaterials, copolymer SBS, and base asphalt can effectively inhibit the movement of macromolecules to greatly improve the fracture toughness of the base bitumen [28,30,33]. Figure 3, the absorption peak at 450 cm −1 and the wide absorption peak at 700 cm −1 result from the Zn-O stretching vibration in the nano-ZnO lattice and the vibration of Ti-O-Ti in the nano-TiO 2 lattice, respectively. After the surface modification of KH-560, nano-ZnO and nano-TiO 2 have evident absorption peaks at 2941 cm −1 and 2864 cm −1 , which belong to the asymmetric stretching vibration and symmetric stretching vibration of -CH 2 with KH-560. us, KH-560 has been successfully connected to the surface of nano-ZnO and nano-TiO 2 . Figure 4 shows the characteristic peak from base asphalt (1#), 3.2% copolymer SBS-modified asphalt (2#), 4% nano-ZnO/3.2% copolymer SBS-modified asphalt (3#), and nano-ZnO/nano-TiO 2 /copolymer SBS-modified asphalt (4#). Compared with the FTIR spectrum of 1#, the 3.2% copolymer SBS-modified asphalt (2#) has no new characteristic peak, except for the characteristic peak of polystyrene at 698 cm −1 and butadiene at 966 cm −1 , and it is only a difference in transmission. erefore, the physical reaction plays the primary role between copolymer SBS and base asphalt. e FTIR of nanomaterials ( Figure 3) and modified asphalts 3# and 4# (Figure 4) shows that the absorption peak at 3440 cm −1 corresponding to the hydroxyl stretching vibration in these nanomaterials disappears, and the transmission of these characteristic peaks produces a larger deviation at approximately 1000 cm −1 and 400 cm −1 . Sun et al. indicated that chemical reactions occurred between different substances, and the spectra of the mixtures greatly deviated from those of several components [47]. erefore, the chemical reactions occurred between both nanomaterials and the base asphalt.

Analysis of Physical Performance.
e experimental results of the physical properties (5°C ductility and softening point) of the base asphalt and modified asphalts are shown in Table 8 before and after UV ageing. e modified asphalts have smaller increases in softening point (ΔT) and larger ductility retention rates (K D ) than the base asphalt. Modified asphalt 4# had the lowest ΔTand highest K D . us, 4# has the lightest UV ageing degree. Compared to the 2# modified asphalt, K D of 3# and 4# increased from 79.20% to 87.72%; ΔT of 3# and 4# decreased from 6.9°C to 4.1°C. e addition of nano-ZnO and nano-TiO 2 to the modified asphalt improves the ability of UV-aging resistance, and nano-TiO 2 has a more significant improvement. All of these results prove that modified asphalt 4# has the best antiaging ability after UV irradiation. corresponds to higher susceptibility to low-temperature cracking of the asphalt, and higher m indicates the lower change rate of stiffness with the loading time [3]. erefore, modified asphalt 4# is less susceptible to low temperature among all asphalts. Figure 5(c) shows the UV ageing indices of the base asphalt and modified asphalt. Compared with the base asphalt, the stiffness ageing index (SAI) of the modified asphalt gradually reduced. Meanwhile, the SAI has no obvious difference between modified asphalts 2# and 3#. ere results imply that the 4# modified asphalt has more resistance to lowtemperature cracking, which is consistent with K D .

Morphological/EDS Analysis
(1) Optical Microscope Image Analysis. From the optical microscope test, images of base asphalt, pure nano-ZnO, pure nano-TiO 2 , and 4% nano-ZnO + 1.5% nano-TiO 2 + 3.2% copolymer SBS-modified asphalt (4#) are shown Figures 6 and 7. As shown in Figure 6(a), the morphological character of base asphalt is yellow-orange with a halogen lamp, the pure nano-ZnO is mostly light red (Figure 7(a)), and the pure nano-TiO 2 is dark brown due to the poor light transmission (Figure 7(b)). us, the dark substance is nano-TiO 2 in Figure 6(b), and the light red substance is nano-ZnO.
Compared to Figure 6(b), the number of nano-ZnO particles decreased, while nano-TiO 2 had no obvious change after the UV irradiation. Zhang et al. indicated that nano-ZnO had a photolysis reaction under UV irradiation, while nano-TiO 2 did not [48]. erefore, nano-ZnO in the 4# modified asphalt can cause photolysis.
(2) SEM/EDS Analysis. e micromorphology of modified asphalt #4 before and after UV ageing is shown in Figure 8. e number of the large particles decreased after UV ageing on a scale of 200 μm or 100 μm. Considering the particle size, that is, copolymer SBS > nano-ZnO > nano-TiO 2 , the copolymer SBS may degrade. To analyse the content changes of Zn and Ti elements before and after UV ageing, Figures 8(a) and 8(b) were further investigated using the energy dispersive spectrometry (EDS), and the results are shown in Table 9.
As shown in Table 9, the weight percentage of Zn and Ti elements in modified asphalt 4# changed before and after UV ageing. e weight percentage change rate of Zn decreased by 82.70%, and that of Ti decreased by 13.73%. e UV ageing degree of nano-ZnO is more than that of nano-TiO 2 . is result is consistent with the results of Zhang et al. [48]. Nano-ZnO is susceptible to UV photocorrosion, which causes the loss of Zn 2+ . erefore, the ductility retention rate Advances in Materials Science and Engineering 11 (K D ) of the nano-ZnO/copolymer SBS-modified asphalt (3#) increases less obviously than that of copolymer SBSmodified asphalt (2#), while the addition of nano-TiO 2 improves K D of modified asphalt 3#, that is, modified asphalt 4#.

FITR.
To evaluate the UV ageing degree of the base asphalt and three modified asphalt, the carbonyl group index and sulfoxide group index were calculated according to formulas (4) and (5). Figure 9 shows that both indices of all asphalts are higher than those of unaged asphalt, especially   the carbonyl group index. is conclusion was proven by Hosseinnezhad et al., where the UV ageing affected the carbonyl group index of asphalt more than thermal ageing [49]. e base asphalt has the highest index after UV ageing. A lower index indicates better antiageing property [36,37]. erefore, the relationships of the antiageing degree of the base asphalt and modified asphalt are 4# modified asphalt >3# modified asphalt >2# modified asphalt >1# base asphalt. Compared with modified asphalts 2#, the addition of nano-ZnO and nano-TiO 2 in SBS-modified more effectively improves the UV ageing than SBS-modified asphalt.
is is mainly because that nanomaterials have the absorption effects on ultraviolet irradiation. Moreover, the addition of nano-TiO 2 in 3# modified bitumen shows the lowest carbonyl group index and sulfoxide group index among the two modified bitumens, which is in accordance with the physical properties test results and cryogenic performance analysis.

Conclusions
To improve the UV antiageing and low-temperature properties of AH-90 base asphalt, in this paper, nano-ZnO, nano-TiO 2 , and copolymer SBS were selected as modifiers.
e most appropriate amounts of the three modifiers were proposed to modify AH-90 base asphalt. e UV antiageing and low-temperature properties of nano-ZnO/nano-TiO 2 / copolymer SBS-modified asphalt were studied in optimal content. Both SEM/EDS test and FTIR test were used to explain the UV ageing mechanism of nano-ZnO/nano-TiO 2 / copolymer SBS-modified asphalt. e following conclusions are drawn: (i) e surface functionalization of nano-ZnO and nano-TiO 2 using KH-560 can turn nano-ZnO and nano-TiO 2 to a highly effective additive to enhance the UV ageing resistance and low-temperature performance of copolymer SBS-modified asphalt. (ii) e orthogonal test and BBR test show that the copolymer SBS significantly affects the low-temperature performance of the nano-ZnO/nano-TiO 2 / copolymer SBS-modified asphalt, and nano-ZnO has a highly significant effect on its low-temperature performance. e most appropriate amount of three modifiers to add is 4% nano-ZnO + 1.5% nano-TiO 2 + 3.2% copolymer SBS. (iii) e SEM test shows that the addition of nanomaterials can improve the dispersion of copolymer SBS in asphalt due to the encapsulation and dispersion of nanomaterials. e EDS test result shows that the nano-ZnO particles are prone to photocorrosion and reduce the activity under ultraviolet irradiation, so nano-ZnO/copolymer SBS-modified asphalt has less obvious antiaging performance than the copolymer SBS-modified asphalt. Furthermore, adding nano-TiO 2 in nano-ZnO/copolymer SBSmodified asphalt will improve its UV antiageing performance.
(iv) e FTIR test shows that the blending mechanism of nano-ZnO/nano-TiO 2 /copolymer SBS-modified asphalt is that the physical reaction is more dominant for both SBS copolymers and base asphalt, and the chemical reactions mainly occur between nanomaterials and base asphalt in the modification process. e carbonyl group index and sulfoxide group index results indicate that ultraviolet irradiation has the most significant effect on the carbonyl index of the nano-ZnO/nano-TiO 2 / copolymer SBS-modified asphalt, which has the best UV antiageing performance.

Data Availability
e data used to support the findings of this study are available from the corresponding author upon request.

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