Rheological Properties and Antiaging Performance of Graphene Oxide-Modified Bio-Asphalt

To enhance the performance of bio-asphalt, an attempt was made to modify bio-asphalt using graphene oxide (GO). Two bio-asphalts were prepared and modified with different GO dosage levels of 0 to 0.08% to obtain GO-modified bio-asphalts (GOMBA). The two GOMBA were tested in the conventional properties (penetration, softening point, and ductility), high- and low-temperature rheological properties, and aging properties. The results showed that the penetration decreases and the ductility increases slightly with the increase in the dosage of GO, while the softening point increases stably to the maximum by 17.84%, and 15.99% for the two GOMBA, respectively. GO is beneficial to improve the high temperature performance and the recovery after the deformation of the GOMBA; on the contrary, it does not significantly affect the low temperature cracking resistance of those. The maximum residual penetration ratio is 10.58% and 8.84%, respectively, and the maximum residual ductility is 10.79% and 27.51%, respectively, for the two GOMBA after short-term aging. GO modification plays a role in the change of carbonyl index (CI) and sulfoxide index (SI) of the GOMBA before and after aging based on the FT-IR spectra, indicating that the GO can effectively improve the antiaging performance of bio-asphalt.


Introduction
Asphalt mixtures for road pavement are mainly produced by petroleum asphalt derived from fossil fuels, currently. As oil reserves dwindle and the price of oil rises, the cost of petroleum asphalt mixtures enhances, accordingly. Bio-oil has obvious advantages over fossil fuels, as it comes from waste biomass and can be blended into traditional petroleum asphalt to produce a bio-asphalt mixture. e bioasphalt mixture has good social and economic bene ts in terms of improving the service level and service life of the asphalt concrete pavements while protecting the environment [1][2][3][4].
A number of studies have tested the performance of bioasphalt from di erent sources. Fini developed a bio-asphalt by using pig manure and studied the rheological properties of the bio-asphalt blended with base asphalt (usually called blended asphalt) [5]. It was found that the complex modulus of the blended asphalts decreased as the content of the bioasphalt and the slope of the low temperature creep decreased. Wen et al. prepared bio-asphalts by blending bio-oil derived from waste edible fats at 10%, 30%, and 60% with PG58-28, PG82-16, and PG76-22 petroleum asphalts, respectively, and then determined the performance grade of the bio-asphalts [6]. Yang et al. used the bio-oil from wood pyrolysis to prepare bio-asphalts at a temperature of 120°C and a shear rate of 5000 r/min and then tested the hightemperature viscosity and rheological properties of the bioasphalts [7]. However, the performance of the bio-asphalt is not satisfactory and does not meet the requirements for the asphalt pavements, mainly in terms of its susceptibility to aging at high temperatures, poor low temperature cracking resistance, and poor resistance to water damage [8][9][10].
To improve and enhance all aspects of the performance of the bio-asphalt, one or more modifiers are adopted [11][12][13]. More and more nanomaterials are being used in the field of pavement materials currently, such as nano-clay, nano-SiO 2 , nano-TiO 2 , and nano-ZnO to enhance the performance of the asphalt and the mixture [14][15][16]. Compared to other nanomaterials, graphene oxide (GO) used as a modifier for asphalt binders has attracted more and more attention recently. Abdelrahman et al. [17] and Li et al. [18] found that the addition of small amounts of GO could increase the viscosity and the stiffness modulus of the asphalts and improve the high-temperature rutting resistance and the low-temperature cracking resistance of the asphalts. Zhu et al. studied the effects of GO dosage on the aging of asphalt and found that GO could hinder the passage of oxygen molecules and slow down the aging rate of the asphalt [19]. GO can significantly improve the performance of the asphalt, but there are few studies on the GO-modified bio-asphalt (GOMBA) at present.

Objectives and Scopes
To improve the performance of the bio-asphalt, an attempt was made to modify the bio-asphalts using GO. Two bioasphalts were prepared and modified with different GO dosage levels of 0 to 0.08% to obtain the GOMBA. e conventional properties such as penetration, softening point, ductility, high-and low-temperature rheological properties, and aging properties of the two bio-asphalt types were investigated. e flowchart of this study is shown in Figure 1

Raw Materials
e base asphalt used in the study is ESSO 70# A-Class asphalt. e performance indexes are shown in Table 1. All indexes met the requirements of the specification in China.

Bio-Oil.
Two bio-oils, noted as bio-oil I and bio-oil II, used in the study were produced by a company in Shandong and were mainly produced from waste biomass such as straw and wood chips through a fast pyrolysis method. e main performance indexes of the two bio-oils are shown in Table 2.

GO.
e GO used in the study was produced by a new material company in Guangzhou. e GO sample is shown in Figure 2, and the main performance indexes are shown in Table 3.

Bio-Asphalts Preparation.
e base asphalt was heated and melted in an oven at 160°C. To melt the bio-oil in the base asphalt evenly, a set mass of bio-oil was added to the molten base asphalt in stages and sheared at 4500 r/min for 30 minutes by using a high-speed shear. e bio-asphalt was then stirred manually for 10 minutes to disperse the air bubbles in the asphalt after shearing. e test bio-asphalt sample was then obtained after standing. In this study, the amount of bio-oil blended was set at 15% of the mass of the base asphalt. e two bio-oils, bio-oil I and bio-oil II, were used to produce bio-asphalts as bio-asphalt A and bio-asphalt B, respectively.

Preparation of GO-Modified
Bio-Asphalts. GOMBA was also prepared by using the shear mixing method described above. e GO dosages used in this study were 0, 0.02%, 0.05%, and 0.08% by weight of the bio-asphalt. A diagram of the bio-asphalt preparation process is shown in Figure 3. For ease of presentation and to avoid ambiguity, the test asphalts were numbered as shown in Table 4. Fractions of the two bio-asphalts are shown in Table 5.

Basic Performance Index Test.
e test methods T0604-2011, T0605-2011, and T0606-2011 in the Test Procedure for Asphalt and Asphalt Mixtures for Highway Engineering (JTG E20-2011) (subsequently referred to as the test procedure) were adopted to evaluate the effects of GO on the conventional performance indexes of penetration, ductility, and softening point of the bio-asphalts. e test temperature of the penetration and ductility was 25°C and 10°C, respectively.

Dynamic Shear Rheology Test (DSR).
A high temperature rheological test was conducted on a DSR (Dynamic shear rheology SYD-0628, Anton Paar Trading Co., Shanghai, China) by using different test asphalts. e diameter of the parallel plates was 25 mm and the space between the parallel plates was 1 mm. e sweep frequency was set at 10 rad/s. e sweep temperature ranged from 46 to 76°C, with an interval of 6°C. e complex modulus (G * ) and phase angle (δ) were recorded to calculate the rutting factor G * /sinδ and to evaluate the high temperature rheological performance.

Bending Beam Rheometer Test (BBR).
After aging in a pressure aging vessel (PAV, pressure aging vessel-1, Hangzhou Hanghua Metal Container Co., Hangzhou, China), the different test asphalt samples were subjected to beam bending tests using a bending beam rheometer.
BBR and TE-BBR ( ermoFisher Scientific (China) Co., Shanghai, China) were used to evaluate the low temperature cracking resistance. e test temperatures were set −12°C and −18°C. e stiffness modulus (S) and the creep rate (m) at 60 s were recorded to evaluate the low temperature cracking resistance of the asphalt binders.

Asphalts Aging Test.
To study the effects of GO dosage on the aging resistance of bio-asphalts, the different test asphalt aging samples were prepared using the rolling thin film oven test (RTFOT) according to the test method T0610-2011. e aging temperature of the asphalt samples was controlled at (163 ± 1)°C and the aging duration was 75 minutes in the RTFOT aging test. A control group was also evaluated by using the same method. e change in mass, the change in penetration, and the residual ductility at 10°C were analyzed.

Fourier Transform Infrared Spectroscopy Test (FT-IR).
During the aging process, the asphalt molecules were oxidized and the content of oxygen-containing functional groups, such as the carbonyl and sulfoxide groups, increased accordingly, so that the changes in the peaks of the carbonyl and sulfoxide functional groups can be used to characterize the changes in the chemical structure of the asphalt during aging. e infrared spectrum test was often used to evaluate the characteristics of the functional groups before and after aging of the asphalt. In this study, a FT-IR spectrometer (Fourier transform infrared spectroscopy-650S, Shanghai Haogang Optoelectronic Equipment Co., Shanghai, China) was adopted to analyze the chemical structure of the GOMBA before and after aging. e asphalt samples of A0, B0, A4, and B4 before and after aging were selected for the infrared spectroscopy test to compare and analyze the effects of GO on the functional groups of the two bio-asphalts before and after aging.

Effects on Conventional Property Indexes.
e three conventional property indexes of penetration, softening point, and ductility of the GOMBA with different GO dosages were evaluated first to clarify the effects of GO on the properties of bio-asphalts.   Advances in Materials Science and Engineering Figure 4 shows the results of the penetration tests of the modified bio-asphalts with different GO dosage levels. From Figure 1, it can be seen that the penetration of the bio-asphalt A1 and B1 increases after the addition of biooil I and II compared to the base asphalt A0. It was found that the bio-oil has more light components (saturated and aromatic fractions), which play a role in softening the base asphalt. e penetration of A4 and B4 decreases by 3.89%, and 4.26% compared to A1 and B1, respectively. e analysis suggested that the GOMBA samples were probably hardened by the volatilization of the light components during the preparation process and the adsorption of the light components by the GO powder with large specific surface area.  Figure 5, with the increase of GO dosage, the softening points of both bio-asphalt samples show an increasing trend. e softening points of the samples A2, A3, and A4, and B2, B3, and B4 increase by 11.45%, 16.52%, 17.84%, and 9.23%, 14.19%, 15.99%, respectively, compared with A1 and B1 with the increase of 0.08% from 0.02% GO. It shows that the softening points of the bio-asphalts increase substantially after GO addition, but the increase tends to slow down with increasing amount of GO. e analysis suggests that GO has good thermal conductivity to dissipate the heat inside the asphalt, resulting in the increasing of the softening point. When the dosage of GO reaches to a certain amount, the increase in thermal conductivity is not obvious, so the increasing trend of the softening point tends to slow down. Figure 6 shows the results of the GOMBA ductility tests with different GO dosage levels. e test results show that the ductility of the GOMBA increases slightly with the increase in the amount of GO, but the increase is not significant.
e ductility of GOMBA with bio-oil II is higher than that of GOMBA with bio-oil I. e main reason is that the components of bio-asphalt A and B are different, while GO does not play a significant role in increasing the ductility.

Effects on High Temperature Rheological Properties.
Complex modulus (G * ), phase angle (δ), and rutting factor (G * /sinδ) were used to evaluate the high temperature rheological properties. e complex modulus G * reflects the ability of asphalt to resist the deformation in shear, with larger values indicating greater resistance to the shear behavior [20,21]. e phase angle δ indicates the ratio of viscosity to the elasticity of asphalt, with higher values indicating greater viscosity and poorer recovery from deformation [22,23]. e rutting factor (G * /sinδ) indicates the ability of asphalt to resist permanent deformation, with higher values indicating greater resistance to permanent deformation.
e results of the complex modulus (G * ), phase angle δ, and rutting factor (G * /sinδ) measured at 46-76°C for the two GO-modified bio-asphalt A and B are shown in Figures 7, 8, and 9, respectively. e complex modulus test results on the GOMBA are shown in Figure 7. For both bio-asphalts, the complex modulus decreases with increasing temperature at different GO dosage levels. e higher the dosage, the greater the complex modulus of the GOMBA. It shows that the addition of GO plays a role in the increase of the complex modulus of the bio-asphalts, and the higher the dosage, the greater the complex modulus of the GOMBA within the tested dosage range. Overall, the complex modulus of the GO-modified bio-asphalt A is higher than that of the GO-modified bioasphalt B. It indicates that the fractions of the different biooils have a significant impact in the complex modulus of the prepared GOMBA.
As shown in Figure 8, the phase angle δ in both types of GOMBA increases in temperature at different GO dosages and decreases with increasing dosage at the same temperature conditions. It indicates that the addition of GO to bioasphalts can serve to reduce the viscous component of the viscoelastic ratio and facilitate the recovery of the asphalts after deformation. Figure 9 shows the results of the rutting factor calculations for the different dosages of GOMBA. As shown in    Comparing the e ects of GO on bio-asphalt A and B, it is found that GO increases the rutting factor of bio-asphalt A to a greater extent than bioasphalt B, and the e ects are more signi cant.

E ects on Low Temperature Cracking
Resistance. e sti ness modulus and the creep rate obtained from the BBR test are shown in Figure 10 and Figure 11. e lower the modulus of sti ness or the higher the creep rate for a given low temperature condition, the better the low temperature cracking resistance of the asphalt binder. From Figure 10, the increased sti ness modulus and the reduced creep rate of both bio-asphalt A and bio-asphalt B compared to the base asphalt A0 indicate that both the bio-asphalts show a decrease in low temperature cracking resistance relative after PAV aging to the base asphalt. e S and m values of the asphalt binders A2, A3, and A4 or B2, B3, and B4 vary within a small range (2% variation) at all temperatures; that is, the addition of GO does not signi cantly a ect the low temperature cracking resistance of the asphalt binders.

E ects on Aging Resistance.
e results of the RTFOT aging tests on the di erent asphalt samples are shown in Table 6. e results of the mass loss rate show that there is a certain increase in the mass loss rate for all samples of bioasphalt A and B relative to the base asphalt, with a decreasing trend in mass loss rate after GO modi cation.
In terms of the residual penetration ratio, both bio-asphalt A and B show a relatively large decrease compared to base asphalt after short-term aging by RTFOT. e residual penetration ratio of both GOMBA steadily increase by 3.88%, 8.47%, 10.58%, and 1.47%, 6.63%, 8.84%, respectively, with the increase of GO dosage. e residual ductility at 10°C of bio-asphalt A and B decrease by 53.2%, and 63.3% compared to the base asphalt, respectively, which indicates that the GO has a signi cant improvement on the bio-asphalts. e residual ductility at 10°C increases by 3.32%, 6.22%, 10.79%, and 14.29%, 17.99%, 27.51%, respectively, with increasing GO dosage.
On the one hand, according to the results of the mass loss rate, the residual penetration ratio, and the residual ductility at 10°C, it shows that the aging resistance of the two bioasphalts decreases largely relative to the base asphalt. On the other hand, GO modi cation steadily increases the aging resistance of the bio-asphalts with the increasing of GO dosage in the test dosage range.

E ects on Functional Groups.
To analyze in depth, the di erences in the in uence of GO on the aging resistance of di erent bio-asphalts, four asphalts before and after RTFOT aging were evaluated and analyzed using FT-IR. e results of the FT-IR spectra for the four bio-asphalts are shown in Figure 11.
As can be seen from Figure 12, the peaks of all four asphalt species remain unchanged, at both 2932 cm −1 and 1460 cm −1 .
e wave peak at 2932 cm −1 represents the stretching vibration of the C-H bond and the wave peak at 1460 cm −1 represents the bending vibration of C-H bond. e main groups of these four bio-asphalts have not changed and are still dominated by the C-H bond.
FT-IR can e ectively distinguish the changes in the composition of the asphalts based on the changes in the wave peaks, therefore also suitable for observing the changes in the individual components during aging [24,25].
To analyze the magnitude of the aging resistance of the four asphalts in more depth, a comparative analysis of the FT-IR of the four bio-asphalts before and after aging was conducted. It can be seen from Figure 11 that the carbonyl group (C O) represented at 1700 cm −1 and the sulfoxide group (S O) peak at 1030 cm −1 change for the four asphalts after di erent degrees of aging. ese two sets of chemical bonds are produced by the absorption of oxygen by the unsaturated carbon chains and the sulphur elements in the asphalt during aging, respectively, indicating the dominance of oxygen-absorbed aging in the asphalt aging process. e carbonyl index (CI) and the sulfoxide index (SI) are de ned by using the area of the peak at the stretching Heat base asphalt up to 160 °C Add Bio-oil and GO Shear at 4500 r/min for 30 min Stir for 10 min Rest for 1 hour to obtain test asphalt sample Figure 3: Diagram of the bio-asphalt preparation process. Advances in Materials Science and Engineering 5 vibration of C-H bond (2923 cm −1 ) as a reference. e ratio of the absorption peak area of the carbonyl group (1700 cm −1 ) and the sulfoxide group (1030 cm −1 ) to the wave area at the C-H bond stretching vibration (2923 cm −1 ) is used to characterize the CI and SI, respectively [26,27]. e CI and SI are calculated as follows [28,29]:     where A c o is the carbonyl absorption peak area, A s o is the sulfoxide absorption peak area, and A c-H is the C-H bond stretching vibration absorption peak area. As can be seen from Figure 13, the CI and SI of the four bio-asphalts of A0, A4, B0, and B4 increase accordingly before and after aging. e change in CI before and after aging for samples is 63.29%, 61.73%, 59.52%, and 57.47%, respectively. Similarly, the change in SI before and after aging is 51.59%, 28.57%, 47.80%, and 35.37%, respectively.
e results indicate that GO modi cation plays a role in the changes of CI and SI of the bio-asphalts before and after aging, in other words, the addition of GO to bio-asphalt could e ectively improve the aging resistance of bioasphalts.       Advances in Materials Science and Engineering 9

Conclusions
To improve the performance of bio-asphalt, an attempt was made to modify the bio-asphalts using GO, and the conventional properties such as penetration, softening point, and ductility, high and low temperature rheological properties, and aging properties of two bio-asphalts with di erent GO dosage levels were studied. e following conclusions can be drawn: (1) With the GO dosage increasing from 0 to 0.08% for both the bio-asphalt A and B, the penetration at 25°C decreases slightly and the ductility at 10°C increases slightly with the increase in the amount of GO, while the softening point increases stably to the maximum by 17.84%, and 15.99%, respectively. (2) e modi cation of GO is bene cial to improve the high temperature performance of the bio-asphalts and to the recovery of asphalt after deformation. After PAV aging, GO could not signi cantly a ect the low temperature cracking resistance of the bioasphalts. (3) After short-term aging of RTFOT, the increase in GO dosage results in a decreasing trend in the mass loss rate and a steady increase in residual penetration ratio and residual ductility at 10°C. e maximum residual penetration ratio is 10.58% and 8.84%, respectively. e maximum residual ductility at 10°C is 10.79%, and 27.51%, respectively, for the two GOMBA. (4) GO modification plays a role in the change of CI and SI of bio-asphalts before and after aging. GO can effectively improve the antiaging performance of the bio-asphalt.

Data Availability
No data were used to support this study.

Disclosure
e results and opinions presented are those of the authors and do not necessarily reflect those of the sponsoring agencies.

Conflicts of Interest
e authors declare that there are no conflicts of interest regarding the publication of this paper.