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Applying graphene, a two-dimensional carbon nanomaterial with special structure and excellent performance, to civil engineering can comprehensively improve the performance of engineering materials and expand the application field of graphene.By reviewing the research status and development trends of graphene-asphalt composites in the field of civil engineering at home and abroad in recent years, it is found that the incorporation of graphene into asphalt improves its road performance, but there is a lack of relevant research, such as fine characterization of graphene-asphalt structure, The microscopic action mechanism of the unique behavior of graphene-asphalt, the change law of key behavioral characteristics of graphene-asphalt and asphalt mixture, and the graphene-asphalt composite material system.Graphene was incorporated into pitch 70, and the graphene pitch mixture was initially explored using both macroscopic and microscopic research methods.The research results show that graphene has an affinity for asphalt and can be intercalated or exfoliated by hot asphalt to form a supramolecular structure with graphene sheets as the basal plane; the incorporation of graphene significantly improves the behavior of asphalt, especially low-temperature performance.
Graphene is a honeycomb-like two-dimensional nanomaterial formed by hybridization of C atoms with sp2 electron orbitals.In recent years, graphene has become a research hotspot in the fields of electronics, information, energy, materials, environmental protection and biomedicine due to its unique structure, excellent properties and broad application prospects [1−11].Positive progress has been made in graphene preparation technology, which provides raw material guarantee for the basic research and application development of graphene. Whether in theoretical research or engineering application, graphene and its derived graphene composite materials have shown great scientific significance and application. potential.In China, graphene and related derivatives with carbon atom layers between 1 and 10 layers are collectively referred to as graphene materials. Among them, graphene has a quasi-two-dimensional structure formed by simple carbon atoms. At the same time, the carbon layer of graphene can be combined with -NH2, -COOH , -OH, -C=O and -O- and other functional groups form functionalized graphene, which currently mainly include graphene oxide (GO) and aminated graphene (TNRGON).Applying graphene materials to the field of civil engineering and conducting research on graphene-asphalt composite materials can enable engineering materials in different environments to obtain better performance and longer life, reduce service costs, and also provide graphene materials with functional properties. Chemical applications provide a new development direction.At present, the research on graphene composite modified asphalt materials in the engineering field at home and abroad is still in the preliminary exploration stage, and there are many problems in theory and practical application that need to be solved urgently.In this paper, the research status and development trends of graphene-modified asphalt materials in engineering fields at home and abroad in recent years are discussed, and the graphene-modified asphalt composite materials are discussed.
1. Research status of carbon nanomaterials and graphene composite pitches
1.1 Current status of foreign research
In the field of carbon nanomaterial-modified asphalt research, MICHELL et al.  studied one-dimensional carbon nanotube-modified road asphalt, and believed that carbon nanotube composite asphalt could improve the high-temperature performance of asphalt.YAO et al.  used multilayer graphite nanosheets to improve the high temperature and low temperature performance of asphalt; BRCIC  studied the effect of graphite nanosheets on asphalt behavior through SHRP-PG classification, and believed that the durability of modified asphalt was improved, and the elastic modulus was improved. [15–16] believed that graphite nanosheets can improve the temperature sensitivity of asphalt, increase the strength, significantly shorten the compaction process of asphalt mixtures, and be suitable for lower temperature environments.ZHOU et al.  used molecular simulation to evaluate the thermomechanical properties of graphene/carbon nanotube modified asphalt, and believed that the thermomechanical properties of the asphalt were significantly improved after adding graphene or carbon nanotubes; ZENG et al.  believed that graphite oxide Graphene can improve the high temperature, low temperature and anti-aging properties of different types of asphalt; LAM  believes that a higher content of graphene oxide can improve the strength, consistency, deformation and softening point of asphalt.HABIB et al.  believe that graphene oxide asphalt helps to improve the deformation resistance of the pavement and reduce rutting in the pavement.LIU et al.  found that the addition of graphene and carbon fiber composite modified asphalt mixture can lead to a certain degree of improvement in the Marshall stability, residual stability and rutting stability of the asphalt mixture, and it is beneficial to the resistance of asphalt concrete to strain. self-monitoring.LIU et al.  found that graphene oxide significantly improved the viscosity, high temperature elasticity and permanent deformation resistance of PG 64-22 asphalt. When using graphene oxide, warm mix asphalt binder and waste cooking oil to modify asphalt, the high temperature and Excellent low temperature performance.The research of MORENO-NAVARRO et al.  showed that graphene would produce a more effective response in asphalt binder, but its elastic recovery ability was not as good as other asphalt modifiers. In addition, in the composite modification, graphene could increase thermal conductivity by increasing the thermal conductivity. Transmissibility to reduce the thermal sensitivity of other adhesives.[24–25] showed that octadecylamine (ODA) was grafted onto the surface of graphitic nanosheets (GNPs) to generate ODA-GNPs complexes through covalent bonding, and then GNPs were combined with ODA-GNPs. The compound modified SBS modified asphalt can improve the ductility, softening point, permeability, viscoelasticity, high temperature rutting resistance and compatibility of the secondary modified asphalt. On the surface of graphite nanosheets, composites (PS-GNPs) were formed, and SBS-modified asphalt was modified with GNP and PS-GNPs, and the performance was improved.The research of Wang et al. [26−28] showed that the use of flake graphite and expanded graphite nanosheets to composite modified asphalt materials can lead to an increase in asphalt viscosity, a decrease in activation energy, an increase in absorbance and thermal conductivity, and can effectively enhance the recovery of asphalt mixture from fracture. The ability of energy and strength improves the healing ability of asphalt mixture, and the conductivity increases with the increase of graphite.The research of LI et al.  showed that when graphene oxide was mixed with asphalt binder, GO decomposed to release CO2 gas, there was no chemical reaction with the asphalt binder, and the GO structure was completely peeled off and dispersed in the asphalt binder. single layer.LIU et al.  have shown that adding a trace amount of graphene oxide can improve the paving temperature viscosity, high temperature elasticity and rutting resistance of unmodified/SBS modified adhesives. The modification mechanism of GO on unmodified adhesives has a chemical reaction and Physical reaction, but only physical reaction exists in SBS modified adhesives.GUO et al.  used mixed CNT graphite powder as asphalt binder, which could improve the mechanical properties of asphalt at high temperature.YAO et al.  used graphite nanosheets to modify asphalt, which reduced the activation energy of asphalt, and improved high temperature and low temperature properties, complex shear modulus, and asphalt mixture resistance to rutting and water damage.CHENG et al.  developed a technique to generate large sheets of graphene on asphalt surfaces.LIU et al.  used petroleum pitch as raw material to prepare 3D graphene networks by chemical vapor deposition, and proved that pitch is one of the suitable carbon sources for 3D graphene.
1.2 Domestic research status
Asphalt is an extremely complex polymer compound. Compared with polymer modification (such as SBS), using nanocomposite to modify asphalt can obtain better performance, more stable and durable asphalt binder [35−36] .The research of Sun Lu et al. [37−38] shows that the surface modification of nanoparticles has a significant impact on the asphalt performance, and it is believed that the nano-modified asphalt mixture has excellent comprehensive road performance.In May 2018, the team of Shen Peikang successfully installed graphene composite rubber modified asphalt on the main bridge deck of Nanning Bridge, and believed that the high temperature, low temperature performance, noise reduction performance, and bonding performance of the bridge deck pavement have been improved , However, the composite modification mechanism of the interaction between graphene and rubber asphalt is still unclear.Huang Jinyu et al.  mixed SBS modifier and graphene into the base asphalt, and found that graphene increased the consistency of SBS modified asphalt and improved the high temperature performance of asphalt, but the low temperature performance of asphalt was deteriorated.Liu Yu et al.  expounded the application of graphene in smart roads. By applying graphene energy harvesting system and road energy system, traffic accidents can be reduced, and development ideas and solutions for future smart roads can be provided.Li Qizhong  found that adding nano-graphite directly to asphalt can improve the temperature sensitivity and high-temperature stability of asphalt to a certain extent. Conclusion, and there is no clear research conclusion on which index to use to judge.Han Shan  found that when using graphene capsules to repair asphalt cracks, the repair time and effect are better than the repair ability of asphalt itself.
Nanning Bridge adopts graphene composite rubber modified asphalt technology for bridge deck pavement, which is the first time in the world.Nanning Bridge was completed in 2009 and is one of the landmark buildings in Guangxi.Because it is a steel box girder structure, after nearly 9 years of heavy traffic operation, the bridge deck has suffered major diseases.On March 27 this year, the Nanning Municipal Engineering Management Office began to carry out a comprehensive comprehensive renovation of the bridge. Among them, the graphene composite rubber modified asphalt technology jointly developed by the team of Professor Shen Peikang of Guangxi University and Guangxi Zhenglu Machinery Technology Co., Ltd. was used to carry out the bridge deck. Pavement solves many problems that are prone to occur on steel box girder bridge decks.
To sum up, in the field of transportation, there are few related researches on graphene (non-graphene oxide)/asphalt composite system at home and abroad. The main reasons are: 1) The few existing studies have used graphite nanosheets Or graphene oxide is the raw material of the modifier. The former cannot meet the number of carbon layers (<10 layers) required by the definition standard of graphene. Demonstrate the unique properties of graphene; at the same time, the material lacks fine structural characterization (number of carbon layers, thickness distribution, plane size, structure, etc.), and the influence of graphene scale on the properties of asphalt cannot be obtained.2) The spatial distribution state and interfacial interaction mode of graphene nanosheets in asphalt are unknown, the graphene/asphalt structure has no parameters, and the effect of dispersion (distribution) state on the properties of the composite system (structure-activity relationship) has no clear conclusion. The microscopic interaction mechanism of graphene-pitch has not been deeply studied.3) The research on the behavior characteristics of the composite material system is not systematic and comprehensive, and the change rules of the key behavior characteristics of graphene pitch and asphalt mixture have not been mastered, and the behavior model has not been established.Therefore, the current research is still limited to macroscopic properties and is in the preliminary stage of experimental exploration, and the key scientific issues such as the theoretical basis, mechanism of action, structural characteristics, preparation methods and behavioral characteristics of high-performance road graphene-asphalt composites are not clear. In the strict sense, the synergistic enhancement mechanism, structural state, physicochemical properties and behavioral properties of graphene/asphalt composite system need to be further studied in depth and systematically.
2. Preliminary study on graphene modified asphalt
2.1 Intercalation and exfoliation of pitch on graphene
In the literature [44-45], the blend of 2% expanded graphite (externally added, in terms of asphalt mass percentage) and Zhonghai 70#A grade asphalt was mixed at a temperature of 180 °C and a rotation speed of 5000 r/min for 3 h. After mechanical shearing, the expanded graphite in the hot pitch (sample A) was eluted from the pitch with a solvent, and the morphology changes were observed by KYKY2800B scanning electron microscope (SEM).Figure 1 shows the SEM morphology comparison of expanded graphite eluted from hot pitch.It can be seen from Figure 1(a) that the graphite sheets have been intercalated and peeled off by the asphalt, and the expanded graphite sheets are wrinkled and curled due to random thermal motion, and are in the shape of gauze. The thickness of the sheets is estimated to be between several layers to dozens of layers. Graphene is formed.The resulting graphene sheets have been partially dispersed in the asphalt, which is similar to the SEM structure and morphology of the chemically prepared graphene in literature  (as shown in Fig. 1(b)).
The same graphene-pitch blend samples were used to conduct X-ray diffraction (XRD) experiments. According to Bragg equation calculation, it was found that some pitch molecular chains were inserted between the sheets of expanded graphite, which expanded the interlayer spacing and formed intercalated nanostructures. (graphene sheet).Figure 2 shows the XRD pattern of the material.It can be seen from Figure 2(a) that after the hot asphalt was mixed with expanded graphite and melted and sheared for 3 h, the ordered structure of the expanded graphite was partially destroyed, and some of the sheets were effectively exfoliated into graphene sheets, forming a more ] (Fig. 2(b)) with a larger spacing between graphene sheets. This is because some components in the hot pitch enter between the exfoliated graphene sheets, which effectively prevent the graphene sheets through intercalation. Inter-π-π force.
The above studies have proved that graphene has an affinity for asphalt and can be intercalated or exfoliated by hot asphalt, which makes it more feasible to prepare graphene asphalt through material composite modification technology.If graphene is uniformly dispersed in asphalt, it is very likely to greatly change and improve various properties of asphalt.
2.2 Preliminary molecular dynamics simulation of the interaction between asphaltenes and graphite
Molecular dynamics methods are powerful tools for studying condensed matter systems and are widely used in materials science, biophysics, and drug design.Using the materialsstudio software, the asphaltene model was optimized, and the NPT system was used to simulate the 2ns relaxation time condition, and the molecular dynamics calculation of the interaction between the asphaltene and the graphite surface was carried out.Figure 3 shows the molecular dynamics (MD) computational model of the interaction of asphaltenes with the graphite surface.The results show that under the same simulation calculation conditions, for the adsorbed material system with an atomic number of 3000, the adsorption energy of asphaltene and graphite surface is equivalent to that of epoxy resin (E51) molecule, and is the difference between the adsorption energy of water molecule and graphite surface. It is 94% of that of nitrogen methyl pyrrolidone (NMP, a commonly used graphene dispersion solvent), indicating that there is a strong interaction between asphaltenes and graphite sheets, and graphene has an impact on the structure of asphalt colloids.Asphalt itself is a colloidal structure with solid asphaltenes as dispersions. Since asphaltenes are flake-like structures of condensed aromatic hydrocarbons, these flake molecules have π-π interactions of aromatic rings, resulting in some asphaltenes also stacked in layers. Structure exists.Figure 4 shows the simulation of the supramolecular structure formed by the interaction of asphaltenes and graphite.Through computational analysis, it is speculated that the interaction between the asphaltene sheet and the giant graphene sheet (relative to the asphaltene sheet) is stronger than the interaction between the individual sheets of the asphaltene itself.Therefore, by adsorbing the monolayer molecules of asphaltenes on the graphene surface, it is very possible to separate the stacked structure of asphaltenes to form a supramolecular structure with graphene sheets as the basal plane.
2.3 Preliminary study on the properties of graphene pitch
Taking Zhonghai No. 70 base asphalt as the original asphalt reference, using graphene produced by Deyang Carbon Technology Co., Ltd., setting the exact same test conditions, using strong mechanical shearing method (5000r/min, 160℃, 30min), high-speed shearing Graphene pitch mixtures with different graphene content (0.5%, 1.0%, 2.0% and external doping) were prepared, and the penetration, dynamometric ductility, softening point and Brookfield viscosity at 135°C were tested [48–49]. Multiple parallel experiments were performed, and the data were summarized after normal analysis, as shown in Tables 1-3.
Table 1 shows the effect of graphene incorporation quality and test temperature on the penetration and softening point of graphene pitch mixture.It can be seen from Table 1 that with the increase of the graphene content, the change law of the penetration of the graphene pitch mixture is not significant between 5 and 25 °C, but at 30 °C, the penetration of the mixture shows a downward trend, indicating that graphene The asphalt mixture can harden the asphalt when the temperature is higher. At the same time, the softening point of the graphene-asphalt mixture is higher than that of the base asphalt, which proves that the incorporation of graphene can improve the high-temperature performance of the asphalt. The changing law of softening point needs further study.
Table 2 shows the effects of graphene incorporation quality and test temperature on the dynamometric ductility of graphene-pitch mixtures.It can be seen from Table 2 that at the same temperature, with the increase in the amount of graphene doping, the tensile strength of the mixture increases continuously, indicating that graphene can greatly improve the ability of the asphalt to resist deformation; when the test temperature is 10~15 °C, the graphene pitch The ductility of the mixture is smaller than the ductility of the matrix asphalt, but the tensile force increases, which proves that the incorporation of graphene has an effect on the viscoelasticity of the asphalt, and its regularity still needs to be further studied.More importantly, when testing the ductility at 5°C, the matrix pitch produces brittle fracture, while the ductility and tensile strength of the graphene-pitch mixture increase with the increase of the graphene content. When 2.0% graphene is added, the The ductility of asphalt increases significantly, the ductility reaches 7.73 cm, which is 33.6 times that of the base asphalt, the maximum tensile force is 2.4 times that of the base asphalt, and even more than 1.5 times that of the SBS modified asphalt , and the required fracture energy is the largest.Studies have shown that graphene can greatly improve the fracture toughness and tensile strength of asphalt under low temperature environment, and has a significant enhancement effect on the mechanical properties of asphalt.
Table 3 shows the effect of graphene incorporation quality on the Brookfield viscosity of graphene pitch mixtures.It can be seen from Table 3 that with the increase of graphene incorporation quality, the viscosity of the mixture gradually increases, which once again proves that the incorporation of graphene leads to changes in the viscoelasticity of the asphalt, which greatly improves the high temperature performance of the asphalt.
The rheological parameters of the graphene-asphalt mixture were tested by dynamic shear rheological test, and the change rule of viscoelasticity was found. The relevant test was completed on the BohlinDSRI dynamic shear rheometer. The strain control mode was used, the strain was set to 12%, and the diameter was 25mm large rotating shaft, 1000μm small gap; the test oscillation speed is 10.0rad/s, and the SHRP-PG classification test data of graphene pitch mixtures with different contents are shown in Table 4.
It can be seen from Table 4 that due to the large temperature range of SHRP high-temperature PG classification, there are cases where the graphene-pitch mixture with different graphene content and the matrix pitch are at the same temperature level, and the high-temperature level is PG64, but the actual When the graphene content was 1.0% and 2.0%, the high temperature grade of the asphalt mixture was close to PG70.That is, the anti-rutting factor G*/sinδ increases significantly with the increase of the graphene content, indicating that the elastic recovery ability and anti-rutting ability of asphalt are continuously enhanced, and the high-temperature performance has been significantly improved.
The effect of graphene on asphalt properties was continued to be evaluated using a multi-stress repetitive creep recovery test (MSCR).The test was performed on a BohlinDSRI dynamic shear rheometer in stress control mode with stresses of 0.1 and 3.2 kPa and a test temperature of 64°C.Each creep cycle is loaded for 1 s and unloaded for 9 s, and the number of repetitions is 100 . The test results are shown in Table 5.
According to Table 5, the properties of graphene pitch mixtures with different incorporation qualities in terms of deformation recovery can be distinguished significantly.With the increase of graphene content, the non-recoverable creep compliance under 0.1 and 3.2 kPa stress showed a decreasing trend, and the elastic recovery rate showed an increasing trend, which proves that the incorporation of graphene pitch has a great influence on the rheological properties of pitch. .Under low stress, the elastic recovery ability of the 0.5% graphene-asphalt mixture is significantly higher than that of the matrix asphalt, and the elastic recovery ability of the 1.0% and 2.0% graphene-asphalt mixture is very significant; at high At the stress level, the average recovery rate of the three-dosage graphene-asphalt mixtures is high, and the non-recoverable creep compliance is low, which proves that the incorporation of graphene makes the asphalt elastic recovery ability good, and the residual permanent plastic deformation is small.
From the above analysis, it can be seen that the incorporation of graphene has a significant impact on the behavior of asphalt and can improve the characteristics of asphalt. However, due to the imperfect preparation method of composite materials in the study, the unique properties of graphene cannot be fully demonstrated, and graphene asphalt cannot be revealed. The mechanism of action of material composites and the establishment of guiding theories, for the selection and action of surfactants, the evaluation of the homogeneity of composite materials, the intercalation efficiency, the characteristics of nanoscale changes, the correlation and influence of factors, behavioral characteristics and their changing laws and other key scientific issues Both require in-depth research.
1) The research status and development trends of graphene-modified asphalt-based composite materials in the field of civil engineering at home and abroad are reviewed, and it is believed that the current related research is still in the early stage of exploration.
2) Graphene materials can improve various properties of asphalt materials; however, key issues such as the material composition system and preparation method, the working mechanism of synergy and theoretical support system, and the change law of key behavioral characteristics need to be further studied.
3) Graphene has an affinity for asphalt, and graphene can be intercalated or exfoliated by hot asphalt to form a supramolecular structure with graphene sheets as the basal plane, and significantly improve the behavior of the mixture.
4) The research results provide a research basis and evidence for the use of graphene to comprehensively enhance asphalt materials, and have enlightenment and promotion effects on the preparation and application of graphene asphalt using composite modification technology.
Zhang Xia, Huang Gang, Zhou Chao, Yuan Xiaoya, He Junxi, Feng Manman, Liu Zhao (National and Local Joint Engineering Laboratory for Transportation and Civil Engineering Materials of Chongqing Jiaotong University, Chongqing, 400074)
Source: Journal of Central South University (Natural Science Edition) July 2019
Author: Huang Gang, Ph.D., professor, engaged in road structure and material research, E-mail: hg_2004 126.com