Abstract
Styrene-butadiene-styrene (SBS)-modified asphalt has been widely used in high-grade pavement due to its excellent road performance. However, environmental factors will lead to continuous deterioration of SBS-modified asphalt; thus, it is difficult for asphalt pavement to meet the demands of long-term service. In recent years, graphene oxide (GO) has attracted the attention of scholars due to its excellent compatibility and modification effect when used as an asphalt modifier. Based on previous studies, this study explored the characteristic evolution of GO/SBS composite-modified asphalt during aging. To achieve this goal, GO and the SBS polymer were blended to prepare GO/SBS composite, which was then used to prepare the modified asphalt. Pure SBS and the GO/SBS composite were investigated by gel permeation chromatography (GPC), Fourier transform infrared spectroscopy (FTIR), and thermogravimetry (TG). In addition, the GO/SBS-modified asphalt and SBS-modified asphalt were studied by FTIR, fluorescence microscope (FM), dynamic shear oscillatory (DSR) test, and multiple-stress creep recovery (MSCR) test. The results indicated that the incorporation of GO inhibited the deterioration of the asphalt binder and relieved the degradation of the SBS modifier during the thermo-oxidative aging and photo-oxidative aging processes. Based on the results extracted from the curves of the original samples in the FTIR analysis, it was concluded that GO was combined by physical blending in SBS and SBS-modified asphalt. The TG test results verified that the GO powder could ameliorate the thermal stability of SBS. Moreover, GO improved the antirutting performance of the original SBS-modified asphalt, while the GO/SBS-modified asphalt had relatively poorer antirutting performance compared to the SBS-modified asphalt after the same aging tests. The reason was that the increment of antirutting performance of asphalt binder by aging process was inhibited by GO.
1. Introduction
Styrene-butadiene-styrene (SBS)-modified asphalt has attracted immense interest due to its excellent road performance. In the field of pavement engineering, SBS-modified asphalt has been widely used in the surface and intermediate layers of high-grade pavement. However, throughout the preparation and application periods, the organic binder continuously deteriorates due to multiple factors that occur simultaneously, such as vehicle loading, sunlight, and temperature variations. Thus, to guarantee the superior performance of SBS-modified asphalt in the service life, numerous studies have been conducted on its aging behavior under various conditions [1–4].
In general, the aging of SBS-modified asphalt includes the aging of the asphalt binder and the degradation of the SBS modifier [5, 6]. During the aging process, the SBS polymer will decompose gradually, generating relatively small molecules due to chain scission. In addition, volatile substances in the asphalt continuously evaporate, and lightweight components irreversibly associate and form macromolecules through the absorption of oxygen. Therefore, the colloid structure of the asphalt binder is transformed into gel type, and the performance of the asphalt pavement severely deteriorates [7, 8].
Studies have verified that SBS-modified asphalt will exceed conventional asphalt in terms of aging resistance [9]. The cause lies in that the SBS modifier can inhibit the aging process of the asphalt matrix. In addition, the asphalt matrix can protect the SBS modifier due to the coating action of the binder [10]. Consequently, mutual protection can effectively delay the overall aging rate of SBS-modified asphalt. Of note, seriously degraded SBS polymers will lose their modification ability in composite materials [11].
Over the past decades, the incorporation of nanomaterials (such as layered double hydroxides, montmorillonite, and nano-ZnO) within asphalt binders have been considered an efficient approach to ameliorate the performance of asphalt binder and prolong its service life [12–16]. In this aspect, graphene oxide (GO) has shown outstanding potential for improving the performance of asphalt binder.
As a precursor of graphene, GO features a similar quasi-two-dimensional layered structure and inherits the excellent physical properties of graphene [17, 18]. In addition, GO contains abundant oxygen-containing functional groups (such as carboxyl, hydroxyl, epoxy, and ester groups) in the layers and lamella edges, endowing GO with outstanding interfacial compatibility toward inorganic and organic materials [18–20]. When GO is incorporated into asphalt binders, these oxygen-containing functional groups can weaken the van der Waals forces in the GO molecules, benefiting the uniform dispersion of GO [21]. In addition, these groups can be used as connection sites between GO and the asphalt binder, forming hydrogen bonds to significantly enhance the bonding strength of the GO-asphalt interface [22, 23]. According to the available literature, the modification mechanism of GO-modified asphalt is still under debate. Liu et al. [24, 25] postured that the modification mechanism of GO to virgin asphalt entailed both physical blending and chemical reactions, while only physical blending occurred in the SBS-modified binder. Interestingly, Zeng et al. [23] found that the peaks in the FTIR spectra of asphalt binders changed inconspicuously after the addition of GO, and conjectured that only physical bonding may occur between GO and the asphalt binder.
Currently, most academic studies tend to evaluate the effects of GO on the macroscopic properties of asphalt binders and asphalt mixtures [3, 26–30]. These studies have found that the incorporation of GO increased the penetration and ductility of the asphalt binder but decreased its softening point and viscosity. Moreover, GO could improve many properties of virgin asphalt and SBS-modified asphalt, such as its resistance to rutting and fatigue, resistance to thermo-oxidative aging and ultraviolet (UV) aging, moisture stability, and thermal stability. In addition, GO could enhance the internal microstate structures of SBS modifiers and asphalt binders. When GO was added into rubber asphalt, the asphalt binder exhibited better performance than virgin asphalt and SBS-modified asphalt at low and high temperatures. As for asphalt mixtures containing GO, GO could improve the adhesion of the asphalt-aggregate system. Based on these improving effects, this study explored the changes of microstructure and macroperformance of GO/SBS composite and its modified asphalt under different aging conditions, which has theoretical guiding significance for improving the durability of asphalt pavement.
2. Objectives
The overall objective of this study is to explore the characteristic evolution of GO/SBS-modified asphalt during aging. More specific objectives are the following:(i)To prepare a GO/SBS composite and evaluate its microstructure and thermal stability at four states (i.e., original, RTFOT, PAV, and UV) versus the pure SBS polymer(ii)To modify asphalt by SBS and GO/SBS composite respectively and disclose the effect of GO on the antiaging properties of GO/SBS-modified asphalt(iii)To determine the cause for the effect of GO on the microscale and verify the conclusions using rheological tests
3. Materials and Experimental Methods
3.1. Material Details
3.1.1. Asphalt Binder
The virgin asphalt (SK Corp., Korea) was selected as the base material in this study. Its basic properties were measured according to the ASTM standards (Table 1).
3.1.2. Graphene Oxide
GO powder, selected as a raw material in this study, was produced from graphene following chemical methods from the Sixth Element (Changzhou) Materials Technology Co., Ltd. Figure 1 shows the chemical structure of GO, and Table 2 lists its basic parameters.
3.1.3. Preparation of the GO/SBS Composite
Based on multistage dispersion, the GO and the SBS polymer were first blended to prepare the GO/SBS composite by solution blending method in this study and then incorporated into the asphalt as a modifier. The dosage of GO in the composite was set to 4% (based on the weight of the SBS) [25]. SBS-T6302, chosen as the raw material, was supplied by Dushanzi Petrochemical Co., Ltd., Xinjiang, China. The S/B ratio was 3:7. The physical characteristics of the SBS polymer are presented in Table 3.
The specific preparation steps were as follows. First, 25 g of SBS was added into cyclohexane for swelling to obtain an SBS solution. Then, the solution was blended with 1 g of GO powder and sonicated by an ultrasonic instrument for 1 h to ensure uniform dispersion. The ultrasonic treated solution was placed in a round-bottom flask at 50°C, and inert gas was passed through for 5 h to remove the cyclohexane. After partial evaporation of the cyclohexane, the remaining black viscous mixture was spread onto a horizontal glass mold. Furthermore, the mold was placed in a fume hood for 24 h to guarantee complete volatilization of the solvent. Finally, the mixture formed a uniform and smooth black film in the mold. A schematic diagram of the GO/SBS composite preparation process is shown in Figure 2.
3.1.4. Preparation of the Modified Asphalt
In this study, two modifiers, namely, the SBS and GO/SBS composites, were used to produce the modified asphalt binders (denoted as MA-SBS and MA-GO/SBS in this study). First, the virgin asphalt was heated and melted in an oven at 140°C to 150°C, then 5% SBS and 5.2% GO/SBS (by the weight of the asphalt binder) were introduced into two equal parts of the molten asphalt, respectively. After 10 to 20 min of mixing manually with a glass rod, two types of modified binders were obtained by shearing at a high speed of 5000 r/min for 30 min at 170 ± 5°C, to achieve homogeneous dispersion of the SBS and GO/SBS composite in the asphalt binder. Afterward, some of the SBS- and GO/SBS-modified asphalts were individually cast onto aluminum plates to prepare the samples for subsequent thermo-oxidative aging and photo-oxidative aging.
3.2. Aging Tests
3.2.1. Thermo-Oxidative Aging
In this study, short-term aging was implemented using a rolling thin film oven test (RTFOT). Following the standard aging procedure in ASTM D2872, the samples were heated in an oven with the airflow set to 4000 mL/min. The test temperature was maintained at 163 ± 0.5°C, and the heating process lasted for 85 min.
The pressure aging vessel (PAV) test was conducted to simulate the long-term aging process. In accordance with ASTM D6521, the samples after RTFOT were heated in the pressurized vessel for 20 h ± 10 min. The air pressure was 2.1 ± 0.1 MPa, and the internal temperature was controlled within 100 ± 0.5°C.
The letters “R” and “P” were added following the codes of samples subjected to RTFOT and RTFOT + PAV, respectively, such as MA-SBS-R and MA-GO/SBS-P.
3.2.2. Photo-Oxidative Aging
Photo-oxidative aging, or UV aging, was carried out to simulate the aging process of asphalt exposed to UV radiation from sunlight, mainly using a straight-pipe high-pressure Mercury lamp. The melting residue obtained from RTFOT was poured into a square iron plate until the sample flowed and formed a 1 mm thick film. Then, the film was placed in an oven at 60°C for a period of time and subjected to photo-oxidative aging with a straight-pipe high-pressure Mercury lamp. The UV wavelength was 365 nm, the detection intensity of UV irradiation was 85 W/m2, and the duration of aging was up to 32 days. The sample mass of photo-oxidative aging was 20 ± 0.1 g, and the test temperature was kept constant at 60 ± 0.5°C. The letter “U” was added after the codes of samples subjected to RTFOT + UV, such as SBS-U and MA-GO/SBS-U.
3.3. Analytical Methods
3.3.1. Gel Permeation Chromatography
Gel permeation chromatography (GPC) was applied to collect the information regarding the molecular weight and molecular weight distribution of SBS and the GO/SBS composite, to assess the effect of the GO powder on SBS under various aging conditions. In this work, 1515 GPC from the US Waters Company was employed to determine the molecular weight distributions of the samples, according to ASTM D3593. Each sample was dissolved in tetrahydrofuran (THF) and then injected into the manual sample injector. Then, the solution passed through columns full of special porous fillers for 70 min at a flow rate of 1 mL/min. The columns were maintained at 40°C throughout the test.
3.3.2. Fourier Transform Infrared Spectroscopy
Evolutions of the prominent functional groups in SBS, GO/SBS, and the two corresponding modified binders at various aging degrees were monitored by attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR). A Bruker FTIR spectrometer was employed for functional group analysis. The wavenumber ranged from 4,000 to 400 cm−1. The spectra were collected at 25°C with a 4 cm−1 wavenumber resolution after 32 continuous scans, and three replicates were tested for each group.
3.3.3. Thermogravimetry
Thermogravimetry (TG) was used to detect the mass variations of SBS as a function of the experimental time under a nitrogen atmosphere in this study. The range of the test temperature was 25°C to 800°C, and the heating rate was 20°C/min. Three replicates were tested for each group.
3.3.4. Fluorescence Microscope
Fluorescence microscope (FM) was an effective tool that can intuitively reflect the micromorphology evolution of asphalt with the aggravation of aging. Polymer swelled after absorbing the aromatic oils from the asphalt and then emitted fluorescence under UV light. The asphalt samples had to be heated until melted in advance. Then, a drop of asphalt was extracted, placed on a clean hot glass slide (120°C), and covered by a coverslip. Subsequently, the glass slide was placed on a stage and observed by an FM (LW300LFT). Under observation, the polymer-rich phase was dark or black in appearance, while the asphalt-rich phase was yellow-green.
3.4. Rheological Tests
3.4.1. Dynamic Shear Oscillatory Test
In accordance with ASTM D7175, the dynamic shear oscillatory test was adopted on a dynamic shear rheometer (DSR) to measure the rheological parameters of the asphalt binders at various aging degrees. The frequency sweep test was conducted from 40°C to 70°C with an increment of 6°C in temperature. At each temperature to be measured, the frequency varied from 0.1 to 10.0 rad/s. The test was run in parallel plate geometry with a diameter of 25 mm, and the height of the sample was 1 mm. The strain amplitude was limited to 0.1% to guarantee that asphalt binders were tested in the linear viscoelastic (LVE) domain. The test was repeated three times for each group of samples, and the average value was reported.
The complex modulus () was selected as rheological parameter, and the data of all samples were plotted in master curves. The reference temperature was 40°C. The shift factor (αT) was obtained according to the (1), and a sigmoidal model is used to construct the master curves, as given by the following equation:where is the reduced frequency at the reference temperature in Hz; is the frequency at the tested temperature in Hz; and , , , and are coefficients.
3.4.2. Multiple Stress Creep Recovery Test
The multiple stress creep recovery (MSCR) test was performed at 76°C according to ASTM D7405. Two stress levels of 0.1 kPa and 3.2 kPa were set up for continuous testing. Subsequently, 20 cycles were operated at a stress level of 0.1 kPa, and then 10 cycles were conducted at a stress level of 3.2 kPa, totaling 30 cycles. Each cycle lasted 10 s, including a 1 s creep phase and a 9 s unloading recovery phase. The first 10 cycles at 0.1 kPa were used for adjustment. The total time required to complete the two-stage recovery and creep tests was 300 s. Two characteristic indices, average percent recovery (R), and nonrecoverable creep compliance (Jnr), were obtained in the tests. The R and Jnr at 0.1 and 3.2 kPa were reported as R0.1, R3.2, Jnr0.1, and Jnr3.2. The detailed calculation methods of these indices were reported by Huang and Tang [31]. Two replicates were tested for each group, and the average value was reported.
4. Results and Discussion
4.1. Characteristic Evolution of the GO/SBS Composite during Aging
4.1.1. GPC Analysis
To ascertain the variation of molecular weight of SBS and GO/SBS composite preaging and postaging, the samples at four different aging states (i.e., original, RTFOT, PAV, and UV) were tested by GPC. Various substances were separated as a consequence of different flow rates in the mobile phase. High molecular weight species eluted first, followed by molecules with decreasing molecular weights. The retention time in gel columns was correlated with the signal intensity for plotting the curves, as shown in Figure 3.
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Figure 3 shows the most visible feature of the GPC chromatogram for the aged SBS and GO/SBS, specifically, the peak relative to the original samples shifted significantly to the right and slumped in height. This feature indicated that the SBS polymer subjected to the aging tests degraded into smaller molecules as a result of chain scission, and the corresponding retention time in the columns was prolonged. This occurred as a result of the oxidation reaction that happened to SBS during the aging tests and was further intensified by the test conditions (i.e., high temperature, pressurized condition, and UV radiation). As shown in Figure 3(a), it was apparent that the curve of GO/SBS-O showed almost no differences from that of SBS-O. This finding illustrated that the combination between the SBS and GO powder was possibly physical in form, and the incorporation of GO did not alter the molecular weight of SBS. Using the original samples as references, it was found that the samples with GO showed relatively smaller changes than pure SBS under the same aging conditions. The reason was that GO powder was impenetrable to oxygen, due to the high potential energy barrier; thus, it inhibited thermo-oxidation and photo-oxidation of the composite during the aging tests.
Every polymer possesses its own molecular weight distribution. Common average molecular weight includes the number average molecular weight, Mn, and the weight average molecular weight, , as given by equations (3) and (4) by Zhang et al. [9]. The polymer molecular weight distribution width, a quantitative description of polymer dispersing degree, can be described by the dispersion coefficient (d) and derived based on the previous two indices, as given as equation (5) by Zhang et al. [5].where n is the mole number of the molecule in mol, is the weight of molecule in , M is the molecular weight in kD, N is the mole fraction, and W is the weight fraction. The molecular weight distribution will be wider when the value of d is greater. The , , and d can be automatically calculated by the computer program on the basis of the leaching volume and leaching net mass at the different time.
For quantitative analysis, the detailed values (i.e., , , and d) of all samples are displayed in Table 4. The and of SBS-O exceeded those of GO/SBS-O, which was possibly attributed to the occurrence of oxidation during the preparation of GO/SBS composite, or potentially was a sampling error. For the same material at different aging degrees, it could be found apparently that the trends of and were identical. Using SBS as an example, the index order of the samples was SBS-U > SBS-P > SBS-R > SBS-O. The finding illustrated that the degradation of the polymer molecules after UV aging was most severe, and SBS-U potentially lost its ability to modify asphalt. The SBS polymer was synthesized with polystyrene (PS) and polybutadiene (PB) blocks. The PB blocks endowed the SBS polymer with high sensitivity to heat and light; thus, SBS was prone to oxidation under high temperature and intense radiation. Furthermore, the d values of the aged SBS polymer and GO/SBS composite increased, indicating that chain scission and crosslinking occurred simultaneously during the aging process. According to the molecular weight values of the two materials under the same aging condition, it was observed visually that GO could hinder the aging of SBS.
4.1.2. FTIR Analysis
The FTIR spectra recorded for SBS and the GO/SBS composite are displayed in Figures 4(a) and 4(b). The broad peak around 3,440 cm−1 corresponded to the typical hydroxyl group stretching vibrations. The peak observed at 3,006 cm−1 was characteristic of the stretching vibrations of C–H in the phenyl ring. The double peaks attained at 2,845 cm−1 and 2,916 cm−1 were caused by the asymmetric and symmetric stretches of –CH2–, respectively. The absorption peak at 1,710 cm−1 was assigned to the stretching vibration of the carbonyl group. The prominent peak near 1,445 cm−1 was ascribed to the deformation vibrations of the C–H bond in –CH2–. The peak at 966 cm−1 presented the bending vibration of C=C in the polybutadiene blocks. The presence of a peak at 753 cm−1 was related to the C–H bending vibrations on the phenyl ring. The peak at approximately 699 cm−1 was associated with the C–H bending in styrene.
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As shown in Figure 4, there existed no peak at 1,700 cm−1 in the spectrum of SBS-O. However, a peak appeared in the spectra of the aged samples along with a visible reduction of the peak at 966 cm−1, identifying the presence of aging. Considering the variations of the two absorption peaks, among the three types of aged SBS samples (i.e., RTFOT, PAV, and UV), SBS-U was severely oxidated due to the strong effect of UV radiation, while SBS-R showed slight aging. The finding was in agreement with the conclusion involving the aging degree of samples in the GPC analysis. In practical applications, the SBS modifier was suffer exposure to solar radiation and be degraded constantly. In particular, the –C=C– bonds in the PB block would be broken due to energy absorption from UV rays. Likewise, high temperature and pressure would accelerate the aging process of SBS by enhancing the activity of reactants. The weak and broad peak at 3,440 cm−1 was caused by the fragments of the SBS polymer. In Figure 4, the variations in the peaks found in the infrared spectra of GO/SBS composite with the aggravation of aging were consistent with those of the SBS.
The distinctions between the absorbance curves of SBS-O and GO/SBS-O were inapparent, namely the absence of new peaks or shifts in peak position. The results revealed that the interactions between the GO powder and SBS polymer were physical, not chemical. Another possibility was that the chemical reactions between GO and SBS were so weak that they could not be observed by an FTIR spectrometer. After the aging tests, the two peaks (–C=O and –C=C–) of GO/SBS varied, but at a relatively small level, compared to that of SBS. Accordingly, the incorporation of GO could improve the antiaging properties of SBS.
4.1.3. TG Analysis
TG is a frequently used technique to reflect the thermal decomposition behavior of a material. The TG curves of SBS and the GO/SBS composite under a nitrogen atmosphere are depicted in Figure 5, and some specific indices are summarized in Table 5.
In accordance with Figure 5, only single-step decomposition was observed in the curves of the samples. Decomposition was initiated when the temperature increased to a certain value (defined as initial decomposition temperature, T0) and terminated at a higher temperature. Among the eight groups, mass loss corresponding to the decomposition stage was at least 95.1%, and the maximum (97.8%) occurred in SBS-O. Except for GO/SBS-U, the other three groups of GO/SBS exceeded pure SBS for the same aging condition, in terms of residual mass, presumably because GO had a higher residual mass after thermal decomposition in nitrogen than pure SBS. The exception value of GO/SBS-U was potentially attributed to the fact that the distribution of GO in GO/SBS-U was inhomogeneous, resulting in a dosage of GO <5.0% in the test samples. The small mass loss in the low-temperature zone (from 25°C to T0) was caused by volatile matter, which was visible as a nearly horizontal straight line. In addition, the residue (<5%) above 500°C consisted of carbon deposits that were generated after decomposition. For an accurate comparison, the temperature corresponding to a mass loss of 5% was selected to evaluate the thermal stability of SBS and the GO/SBS composite, which was abbreviated as T5%. The T5% of SBS merited attention, as it varied along with the aging degree and the addition of GO. The T5% of SBS at different aging states (i.e., original, RTFOT, PAV, and UV) were 387°C, 378°C, 276°C, and 224°C, while the indices for the four types of GO/SBS composites were 394°C, 384°C, 370°C, and 330°C, respectively. The observed T5% shifted to a lower value after thermo-oxidative aging and photo-oxidative aging, indicating that the aging process had an adverse effect on the thermal stability of SBS; whereas, it turned to a higher value after the incorporation of GO, signifying that the GO powder in the composite material could ameliorate the thermal stability of SBS. This may have been because that GO possessed excellent thermal conductivity and a high specific surface area so that each portion of the tested material was heated evenly. In this instance, the thermal degradation of the GO/SBS composite did not occur prematurely.
4.2. Structural Characteristics of the GO/SBS-Modified Asphalt during Aging
4.2.1. FTIR Analysis
The FTIR spectra recorded for the SBS-modified asphalt and GO/SBS-modified asphalt are depicted in Figure 6.
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The broad peak around 3,440 cm−1 corresponded to the typical hydroxyl group stretching vibrations. The major peaks appearing at 2,900 cm−1 and 2,850 cm−1 corresponded to the typical hydrocarbon stretching vibrations. The peak at 1,700 cm−1 was identified as the stretch vibrations of the carbonyl groups in the aldehydes, ketones, and acids. The absorption peak located at 1,603 cm−1 corresponded to the stretching vibrations of –C=C– in naphthene and benzene rings. The strong peak observed at 1,455 cm−1 was attributed to the C–H bond’s deformation vibrations in –CH2 and –CH3. The peak at 1,375 cm−1 was due to the symmetric vibrations of C–H in –CH3, while the peak at 1,030 cm−1 was caused by the vibrations of the sulfoxide groups. The appearance of a characteristic peak at 966 cm−1 represented the bending vibration of C=C in the polybutadiene blocks.
As shown in Figure 6, the FTIR spectra of the SBS-modified asphalt revealed that numerous absorption peaks, especially some oxygen-containing functional groups (e.g., the carbonyl and sulfoxide groups), were enhanced after the aging tests. The reason for the variations was that thermo-oxidative aging and photo-oxidative aging occurred to asphalt during the aging tests, and the aging processes were facilitated by the test conditions (i.e., high temperature, high pressure, and UV irradiation). This led to the variations in the molecular interactions and migration of components in the asphalt binder, along with the degradation of SBS. Besides, the other characteristic absorption peak positions in the SBS-modified asphalt were essentially unchanged preaging and postaging.
Regardless of whether the GO/SBS-modified asphalt was aged, there were no significant changes in the results of FTIR compared to the spectra of MA-SBS. In other words, none of the old peaks disappeared and no new peaks appeared, implying that modification by GO occurred through physical blending rather than chemical bonding. The peaks of MA-GO/SBS at different aging degrees exhibited similar variations to that of MA-SBS, the carbonyl group and the sulfoxide group in particular.
As discussed earlier, the differences between the FTIR spectra of the asphalt binders at different aging degrees were centered around the absorption bands involved in the oxygen-containing functional groups and unsaturated bonds, which were affected directly by the thickness of the film. To eliminate the effect of specimen thickness, an internal standard method was proposed to semiquantitatively analyze the changes in the functional groups of the modified binders at different aging states. The peak area presenting the asymmetric and symmetric stretching of –CH2– was selected as the benchmark, as it had no correlations with either the modification level or the aging states, statistically. Thus, the aging indices involving the area of the functional groups (i.e., carbonyl index (CI), sulfoxide index (SI), and butadiene index (BI)) were computed by the following equations [7]:where denotes the areas of 1,700 cm−1 centered around the carbonyl group absorption band, indicates the areas of 1,030 cm−1 centered the carbonyl group absorption band, denotes the areas of 966 cm−1 centered around the absorption band of carbon-carbon double bond, and indicates the reference area, which is set to the integral area ranging from 3,000 cm−1 to 2,800 cm−1 in the subsequent calculations.
Different calculated indices of the unaged, short-term aged, long-term aged and UV aged asphalt binder samples were compared, as shown in Figure 7. MA-SBS-P had the maximum CI and SI (0.074 and 0.065), substantiating that the SBS-modified asphalt after PAV underwent the most severe oxidative reaction. The CI and SI of the two asphalt binders exhibited identical regularities, with the roles of high temperature, air pressure, and UV radiation, in terms of various oxidation levels. Consequently, CI and SI grew by various extents after thermo-oxidative aging and photo-oxidative aging, correlating with increases in the content of most polar components that constituted those of larger molecular sizes entailed. During the aging process, the functional groups in the asphalt binder combined with oxygen, generating carboxyl, sulfoxide, and some other polarity groups, which enhanced the interactions among asphalt molecules and then led to the aged asphalt binder to become hard and stiff. In addition, the SBS polymer continuously degraded so that the C=C content in the PB block was reduced and C=O content increased. Using MA-SBS as an example, the BI of the original sample was 0.049 and then decreased to 0.038, 0.027, and 0.014 after RTFOT, PAV, and UV aging, respectively. This implied that the degradation of the SBS polymer in MA-SBS-U was more severe than that in MA-SBS-R or MA-SBS-P, which was consistent with the conclusions in the FTIR analysis of SBS and the GO/SBS composite.
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The two indices (CI and SI) for the SBS-modified asphalt decreased after blending with GO. For instance, the SI value of the unaged, short-term aged, long-term aged, and UV aged MA-SBS samples, compared to MA-GO/SBS under the same aging conditions, declined by 24.22%, 27.80%, 12.90%, and 19.85%, respectively. By contrast, the BI value of the aged asphalt binder increased by 5.94%, 8.86%, and 84.07%. The results were closely related to the GO powder and indicated that GO possibly restrained the formation of C=O and S=O, as well as the fracture of butadiene double bonds.
4.2.2. FM Analysis
In this study, the dispersion of the SBS modifier and GO/SBS composite in the asphalt matrix at different aging degrees were characterized by an FM. The fluorescence micrographs of MA-SBS and MA-GO/SBS are displayed in Figure 8.
As shown in Figure 8, many bright and clear spots of light were observed in the images of MA-SBS-O and MA-GO/SBS-O, which had small particle sizes and uniform distribution. These observations indicated that both SBS and GO/SBS possessed good compatibility with the asphalt matrix. In addition, the distribution densities of the two modifiers in the matrix were essentially the same, indicating that at the same dosage, the introduction of GO had no apparent influence on the distribution of SBS in the asphalt matrix. After RTFOT, the sizes of the light spots showed a downward trend while the quantity increased as a result of the degradation of SBS polymer. However, compatibility between the modifier and matrix was ameliorated by high temperatures, as reflected in the brighter background. Regarding MA-SBS-P and MA-GO/SBS-P, the SBS polymer was further degraded and dissolved in the asphalt as the aging process intensified. It was conspicuous in the image of MA-SBS-P that SBS in the modified asphalt without GO was severely degraded after long-term aging. As GO was capable of preventing degradation of the SBS polymer, the fluorescence emitted by the SBS polymer could still be easily observed in the image of MA-GO/SBS-P. Furthermore, among the three aging methods, the photo-oxidative aging process had the strongest effect on the decomposition of SBS. There were almost no light spots in the fluorescence image of MA-SBS-U, manifesting that SBS seemed to completely degrade after the UV aging process due to the lack of protection from GO.
4.3. Rheological Properties of the GO/SBS-Modified Asphalt during Aging
4.3.1. Master Curve Analysis
For all samples, the complex modulus () at six temperatures and 16 frequencies were plotted in a single curve (called as master curve) using the data from the frequency sweep test. The master curves for MA-SBS and MA-GO/SBS at a reference temperature of 40°C are presented in Figure 9. A larger complex modulus signified a better ability to resist rutting.
The master curves of the log frequency versus the log complex modulus were approximately linear. The complex modulus of the asphalt binder increased rapidly as the frequency increased. Within the range of parameters determined in this study, the values of the two modified binders were in the same order with the aging state. At any frequency, the value of the long-term aged samples was the largest, the short-term aged and UV aged samples had the median values, and the original sample had minimum values. Along with the aforementioned FTIR analysis, whether MA-SBS or MA-GO/SBS, the more serious the aging degree of the sample, the greater the value. The aging process caused the gradual transformation of asphalt binder into a gel structure, accompanied by an increment of stiffness and an improvement of high-temperature performance.
In contrast to MA-SBS-O, it appeared that MA-GO/SBS-O had nearly equal values in the high-frequency zone but relatively larger values in the low- and medium-frequency zones. The potential reason was that GO could form a dense Sandwich structure with the asphalt binder, which obstructed the movement of asphalt molecule chains, thereby increasing the value and improving the antirutting performance of the asphalt binder at medium and low frequencies. The obstructing effect of GO, however, was not as obvious at high frequencies. Conversely, after PAV or UV, the asphalt binder with GO/SBS was lower than the SBS-modified asphalt from the perspective of . As GO could effectively slow down the aging process of the asphalt binder. The two nearly coincident master curves corresponding to MA-SBS-R and MA-GO/SBS-R resulted from a counterbalance between the two aforementioned effects of GO.
4.3.2. MSCR Analysis
Relative to the master curves, the results of MSCR were more consistent with field pavement performance, especially for the polymer-modified asphalt binder. In this study, the MSCR tests were conducted at stress levels of 0.1 and 3.2 kPa on the two modified asphalt binders (i.e., MA-SBS and MA-GO/SBS) at four different aging states (i.e., original, RTFOT, PAV, and UV). The parameters R0.1, R3.2, Jnr0.1, and Jnr3.2 were calculated and plotted as histograms, as shown in Figure 10.
(a)
(b)
(c)
(d)
Figures 10(a) and 10(b) displays the R values of the different asphalt binders at 0.1 and 3.2 kPa. The R value represented the elastic component in the binder. Overall, the more severe aging that the asphalt samples endured, the more elastic they became, resulting in a higher R. However, distinguishable outliers were observed when MA-SBS-R was subjected to MSCR test at stress levels of 0.1 or 3.2 kPa. It was conspicuous that the R0.1 and R3.2 of MA-SBS-R (54.3% and 39.4%) were less than that of MA-SBS-O (56.5% and 40.7%). This phenomenon was a combined result of SBS degradation and asphalt oxidation. In the circumstance of short-term aging, the degradation of the SBS polymer was dominant and drove the binder to become more viscous. As for long-term aging and UV aging, the SBS polymer was almost degraded, and the oxidation reaction played a primary role, rendering the binder stiffer and more elastic. Consequently, MA-SBS-P and MA-SBS-U exceeded MA-SBS-O in the value of R under the two stress levels (i.e., 0.1 kPa and 3.2 kPa).
It could be detected that the presence of GO produced an increment in the R values of the original asphalt, indicating that the degree of deformation recovery of the modified asphalt binder was improved. This was explained by GO, which could absorb the light components of the asphalt binder, facilitate the crosslinking of the network, and effectively retard the destruction of the colloid structure. Thus, the elasticity of the asphalt binder increased. However, GO could also retard the aging process of modified asphalt; thus, the increment of elasticity was reduced. This reason accounted for the result that the GO/SBS-modified asphalt had a lower R than the SBS-modified asphalt after long-term aging or UV aging. In addition, the average percent recovery of the modified asphalt showed a downward trend with increasing stress levels (from 0.1 kPa to 3.2 kPa), signifying that the antideformation capacity of the asphalt binder turned worse. The potential reason was that the structure of the asphalt binder was damaged under high-stress levels. Hence the asphalt pavement was prone to rutting under heavy loads. In addition, the index difference among the groups was more distinct at high-stress level (3.2 kPa).
Figures 10(c) and 10(d) present the Jnr0.1 and Jnr3.2 of the SBS- and GO/SBS-modified asphalt binders. Jnr was defined as the ratio of the residual strain to the creep stress after every 10 s. The lower value of Jnr indicated less unrecoverable deformation. The nonrecoverable creep compliance of the asphalt binder exhibited an opposite regularity to R value. As shown in Figures 10(c) and 10(d), the Jnr0.1 of the samples containing GO/SBS versus the SBS-modified asphalt binders decreased by 15.7%, 20.4%, −25.9%, −16.3%, respectively. The Jnr values of the asphalt binders containing GO/SBS composite at 3.2 kPa decreased by 18.5%, 22.5%, −28.4% and −20.2%, respectively. These variations were attributed to the same reason as the variations in R value.
5. Conclusion
This study explored the characteristic evolution of GO/SBS-modified asphalt during aging. The following conclusions were obtained:(i)According to the results in GPC, the SBS polymer subjected to aging tests degraded into smaller molecules as a result of oxidation. GO powder could resist the thermo-oxidation and photo-oxidation of the composite during aging tests. These conclusions were also verified by FTIR analysis. In addition, the interactions between the GO powder and SBS polymer consisted of physical blending.(ii)In the TG tests, T5% shifted to a lower value after aging, illustrating that the aging process had an adverse effect on the thermal stability of SBS; however, it turned to a higher value after incorporating GO, signifying that the GO powder in the composite could ameliorate the thermal stability of SBS. The result was possibly attributed to the excellent thermal conductivity and high specific surface area of GO, which ensured that each part of the sample was heated evenly.(iii)Through the comparison of the infrared spectrum, it was concluded that GO was combined by physical blending in the SBS-modified asphalt binder. The variations in the three indices (i.e., CI, SI, and BI) identified that GO possibly restrained the formation of C=O and S=O, as well as the fracture of butadiene double bonding. Moreover, the degradation of SBS was most affected by UV aging, which was also verified by the FM test, while the aging of the asphalt binder was most affected by PAV.(iv)GO improved the antirutting performance of original SBS-modified asphalt, while MA-GO/SBS had a relatively poorer antirutting performance compared to MA-SBS after the same aging tests. The reason was as followed. Aging made the asphalt binder stiffer; thus, the antirutting performance of the aged asphalt binders was improved. However, the presence of GO retarded the aging of the asphalt binders, resulting in a variation range that diminished in the rheological indices of the asphalt binders.
Data Availability
The data used to support the findings of this study are included within the article.
Conflicts of Interest
The authors declare that there are no conflicts of interest for this work.
Acknowledgments
This work was financially supported by the Natural Science Foundation of Xinjiang Uygur Autonomous Region (grant no. 2019D01A44), the National Natural Science Foundation of China (grant nos. 51978070 and 51808051), the Basic Research Project of Natural Science in Shaanxi Province (grant no. 2019JQ-118), the Special Project of Technical Innovation Guidance in Shaanxi Province (grant no. 2019QYPY-145), and the Natural Science Foundation of Shandong Province (grant no. ZR2020QE072).