Study on the performance and mechanism of graphene oxide / polyurethane composite modified asphalt

Abstract

In order to enhance the aging resistance, high temperature stability and low temperature crack resistance of asphalt pavement materials, 0.06% oxidized graphene (GO) and 12% polyurethane (PU) were used as composite modifiers to modify the base asphalt. The RTFOT test was conducted to evaluate the anti-aging performance of the modified asphalt. The rheological properties of GO/PU composite modified asphalt were evaluated. SEM and FTIR tests were used to analyze the microstructure of GO/PU modified asphalt. The results show that the basic performance of composite modified asphalt meets the requirements and the anti-aging performance is improved. The improvement of rutting factor and frequency scanning master curve shows its better anti-rutting deformation ability. The strain recovery R-value for the composite modified bitumen increased by 20.7% at a stress level of 0.1 kPa, and the unrecoverable creep compliance J_nr decreased by 76.0%, showing excellent viscoelastic properties. The creep stiffness S of composite modified asphalt decreased by 31.2% at−12 °C, indicating that it had good low temperature rheological properties. The surface of the modified composite asphalt is relatively smooth, exhibiting a stable network structure, thus solving the PU segregation problem in the matrix asphalt.The change of infrared spectrum absorption peak shows that reaction between isocyanate groups in polyurethane and aromatic esters in bitumen, GO oxidizes with the-CH functional group in asphalt, and the content of C = C double bond changes during the modification process, indicating that the modification of asphalt by GO and PU is a composite modification based on chemical modification and supplemented by physical modification.

Introduction

With the rapid development of the national economy in recent years, road engineering technology has also made steady progress.By the end of 2022, the total mileage of China’s highways has exceeded 6 million kilometers¹. Nowadays, Chinese highway transportation construction industry has higher and higher requirements for road engineering construction. The road performance of traditional asphalt can no longer meet the actual requirements. Therefore, domestic and foreign scholars have invested a lot of research, and the types of asphalt modifiers have overlapped². At present, among many asphalt modifiers, SBS shows more excellent performance in high and low temperature stability than matrix asphalt, so it has a vast range of applications in China. While, according to research, although Styrene-butadiene-styrene block copolymer (SBS) has superior performance, there are also shortcomings. The modified material SBS has no obvious chemical change in the process of modifying asphalt, so it is prone to polymer segregation or degradation during use. The phenomenon makes the permeable type of asphalt pavement larger and the water stability worse, resulting in a reduction in the service life of the pavement³.

Polyurethane ( PU ) is an organic polymer material with a series of excellent properties such as wear resistance, high temperature resistance, aging resistance, low temperature flexibility and high tear strength⁴. Bazmara⁵ studied the chemical reaction between polyurethane and asphalt by Fourier transform infrared spectroscopy. Sun⁶ et al. Firstly incorporated polyurethane modifier into matrix asphalt, and prepared polyurethane modified asphalt by stirring with a circular saw-tooth mixer. Secondly, their performance was evaluated by ultraviolet aging test, rotary film oven test and dynamic shear rheological test. Lastly, Fourier transform infrared spectroscopy, differential scanning calorimetry, and other testing methods were employed for the microscopic analysis. This shows that the disc sawtooth mixer can better expose the hydroxyl group in the asphalt and combine better with the isocyanate group in the polyurethane so that it has a better effect on the modification. At 64 °C, the asphalt with polyurethane as modifier has higher stability and better ultraviolet aging resistance than SBS modified asphalt. Guo⁷ used polyurethane prepolymer to modify asphalt. It was found by fluorescence microscopy that the longer the curing time, the more stable the three-dimensional network structure formed by crosslinking and curing of polyurethane in modified asphalt. The combined action of physical chemistry leads to significant improvements in asphalt pavement performance. Fang⁸ focused on the rheological properties of the polyurethane-modified asphalt, and the optimal preparation process was determined by orthogonal testing. They found that the optimum content of the polyurethane modifier was 6%, this improved the high-temperature performance, effectively improving the temperature sensitivity.

Graphene is a two-dimensional carbon nanomaterial extracted from graphite. It is composed of carbon atoms according to certain rules and is hexagonal honeycomb lattice. Carbon atoms are tightly bound together, with high specific surface area, multiple active sites, high strength and excellent electrical and thermal conductivity, toughness and mechanical properties¹⁰. Wu S et al. prepared graphene oxide modified asphalt by fusing styrene-butadiene-styrene modified asphalt with 80/100 penetration grade asphalt by melt blending method, and studied its anti-aging performance by DSR test. The results show that the addition of graphene oxide improves the anti-ultraviolet aging ability of asphalt¹¹. Graphene oxide can be uniformly dispersed in matrix asphalt and is well compatible with asphalt. Graphene oxide modified asphalt has low creep compliance and high recovery percentage, indicating that it has excellent rutting resistance¹². Duan S¹³ et al. prepared C₄H₉-GO/SBS composite modified asphalt on the basis of graphene oxide as a separate modified material. It was found that the performance indices of the modified asphalt under the combined action of graphene oxide, bromobutane, and SBS were improved compared to the SBS-modified asphalt. Improvements were made in the softening point and ductility. DSR and MSCR testing showed that the high-temperature rheological properties of the modified asphalt C₄H₉-GO/SBS composite were significantly improved and the sensitivity to stress was reduced. Various contents (0.25%, 0.50%, 0.75%, 1.00%) of graphene oxide and SBS were combined in order to modify the asphalt of the matrix. High-temperature rheological properties were assessed by dynamic shear rheological testing and multi-stress recovery creep test.The temperature sensitivity characteristics were analyzed by displacement factor. The results show that the optimum content of graphene oxide is 0.75%, and the high temperature rutting resistance and creep recovery ability are improved¹⁴. Yan Kezhen¹⁵ modified the base asphalt with polyether amine grafted graphene oxide, and analyzed the performance of the modified asphalt through relevant tests. The results showed that the performance indexes of the modified base asphalt were improved. Based on the surface free energy theory, Zhao Y¹⁶ et al. calculated the peeling work and cohesion work by contact angle test, and then analyzed the adhesion ability of GO modified asphalt and aggregate. The calculation results show that GO has the ability to enhance the binding surface energy of asphalt and aggregate, and the peeling work, adhesion work and cohesion work have been significantly improved.

In summary, GO modified asphalt and PU modified asphalt have their own unique advantages and disadvantages. GO may be stably dispersed in asphalt as a result of its unique physical structure. The purpose of asphalt modification is to improve the anti-ultraviolet aging performance of asphalt and reduce stress-sensitivity characteristics, but its effect at low temperatures on matrix asphalt is not readily apparent. Asphalt matrices with PU have lower penetration, higher softening point, and higher viscosity, which promotes high temperature performance. Furthermore, the addition of PU can reduce the sensitivity of the matrix asphalt to temperature changes, enhancing its flexibility in a low-temperature environment, and improving its ability to resist cracking at low temperatures. For this reason, in this paper, GO and PU were used as composite modifiers to modify the asphalt of the matrix, and the high-and low-temperature rheological and viscoelastic properties of the modified asphalt composite were comprehensively evaluated to compensate for their deficiencies as a single admixture. Simultaneously, the benefits of both modifiers were fully utilized to achieve a new high quality, environmentally friendly modified asphalt composite material.

Raw materials

Asphalt

The asphalt A grade 70# asphalt used in this paper is provided by Zhengzhou Zhengfa Municipal Construction Co., Ltd. According to the relevant procedures¹⁷ to detect the basic performance of asphalt, the results of the test are presented in Table 1, in line with the technical requirements of the specification¹⁸.

Table 1 Basic performance index of 70# asphalt.

Polyurethane

The two-component polyurethane cementing agent was provided by Guangzhou Jibisheng Technology Industry Co., Ltd. According to the relevant specifications¹⁹, the polyurethane related performance indicators are determined as shown in Table 2.

Table 2 Properties of polyurethane adhesive.

Compatibility agent

Due to the poor compatibility between polyurethane and asphalt, segregation will occur after mixing, so adding compatibilizer is the best way to improve the compatibility between the two. Compatibilizer can increase the adhesion between the two, thus forming a stable structure. In this study, maleic anhydride was used as a compatibility agent, which was provided by Dongguan Youxin New Materials Co., Ltd. The relevant performance indicators are shown in Table 3.

Table 3 Compatibility agent performance index.

Diluent

Polyurethane is composed of component A and component B. The combination of the two components will cause the curing reaction, which will accelerate the viscosity growth rate of the asphalt system, resulting in the premature hardening of the modified asphalt during the preparation process or the asphalt mixture during the transportation process. Thus diluent must be added to inhibit the polymerization rate of polyurethane and prevent the viscosity of the asphalt system from increasing too rapidly.This article uses advanced environmental protection diluent, the main component is xylene, provided by Shanghai Muxuan Building Materials Co., Ltd., its technical indicators are shown in Table 4.

Table 4 Related performance indicators of diluent.

Graphene oxide

Graphene oxide is generated by oxidation of graphene. Graphene oxide contains oxygen groups, which will be more active and better attached to the material to prevent its agglomeration. The high-purity, high-performance graphene oxide powder supplied by Suzhou Carbon Technology Co., Ltd.is was used in this paper.The basic performance indicators are shown in Table 5.

Table 5 Basic performance indexes of graphene oxide.

Test methods

Analysis of influencing factors of graphene oxide / polyurethane composite modified asphalt

The selection of modifier and its dosage, preparation time and temperature will directly affect the performance of modified asphalt. In this paper, 70 # A grade matrix asphalt was modified by GO and PU modifiers, and the modified asphalt was prepared by EM 100 L high-speed shearing machine. On the basis of previous scholars ' research, the effects of GO content, PU content, shear time and shear temperature on modified asphalt were analyzed.

Modifier dosage

The most direct factor affecting the pavement performance of polyurethane modified asphalt is the amount of polyurethane. The addition of polyurethane can significantly improve the low temperature crack resistance and ductility of asphalt. Graphene oxide has a large specific surface area and rich functional groups. Graphene oxide modified matrix asphalt can improve the high temperature characteristics and anti-aging ability of matrix asphalt. In order to study the optimal blending ratio of graphene oxide and polyurethane, combined with the relevant experimental research and analysis, the content range of GO is 0.02% ~ 0.08%, and the content range of PU is 10% ~ 16%.

Shearing time

The appropriate shear time can make the modified asphalt have better modification performance. The shear time of PU after adding matrix asphalt affects the dispersion degree of PU in matrix asphalt. Studies have shown that after a long time of mixing at higher temperatures, the high temperature stability of PU modified asphalt will deteriorate, and the modification effect of PU cannot be exerted. Too short shear time will affect the compatibility of modifier and asphalt²⁵. The shear time range selected in this test is 30 min ~ 60 min.

Shearing temperature

The shear temperature is also a major factor affecting the performance index of modified asphalt. The performance index of asphalt is very different under different shear temperatures. Because PU is a thermosetting material, when the shear temperature is too high, the curing speed of PU will be too fast, which will affect the modification effect of asphalt. When the shear temperature is too low, reducing the fluidity of asphalt will affect the dispersion effect of the modifier. Therefore, this test selects the temperature range of 135 °C ~ 165 °C for asphalt shearing.

Preparation of graphene oxide / polyurethane composite modified asphalt

The modified asphalt was prepared using a high-speed shearing method. Here is the specific process:

• 1. The matrix asphalt was heated in an electric drying oven at 163 °C for 80 min until it melted, and the graphene oxide powder was dried in an oven at 100 °C. A certain amount of melted matrix asphalt was placed on the heater for constant temperature heating to keep it in a molten state, and the temperature was monitored by a thermometer to keep the temperature stable at about 155 °C.

• 2. In order to reduce the error of GO mass weighing, a high-precision electronic balance is used. Start the high-speed shearing machine and adjust its speed to 1500r/min, add the weighed GO to the matrix asphalt for high-speed shearing for 10 min, and slowly adjust the speed to 3000r/min. During this shearing process, the corresponding amount of diluent, compatibilizer, component A and component B were weighed.

• 3. The speed of the high speed shearing machine is set to 1000r/min, and the weighed diluent and compatibilizer are alternately added to the asphalt.The two-component polyurethane mixed by component A and component B is uniformly stirred at 5:1 and slowly added to the asphalt in batches. GO/PU composite modified asphalt can be prepared by adjusting the speed of high-speed shearing machine and shearing for corresponding time.

• 4. After the shear is completed, the GO/PU composite modified asphalt with smooth surface and no bubbles is initially prepared and cured in an oven at 130 °C for 1 h to complete the preparation of GO/PU composite modified asphalt. In order to discharge the air in the asphalt, the asphalt sample is stirred every 15 min.

Orthogonal test design

In this study, the orthogonal test method was used to optimize the best combination of GO / PU composite modified asphalt by designing different factors and different levels. Among them : A represents the GO content ; b represents the amount of PU ; c represents the shear time ; d represents the shear temperature. Factor A In this experiment, four levels of 0.02%, 0.04%, 0.06% and 0.08% were selected. Factor B in this test selected 10%, 12%, 14%, 16% four levels of content, factor C in this test selected 30 min, 40 min, 50 min, 60 min four levels of time ; factor D This experiment selected 135 °C, 145 °C, 155 °C, 165 °C four levels of temperature, according to the factors and the corresponding level of design of orthogonal test table as shown in Table 6.

In order to explore and analyze the influence of modifier blending ratio, shear temperature and shear time on the performance of modified asphalt, the penetration, softening point, 5 °C ductility, elastic recovery test and Brinell rotational viscosity test under different blending ratio, temperature and time were carried out. In order to avoid errors, three groups of parallel tests were carried out in each design scheme and the average value was taken as the experimental result.

Table 6 Orthogonal design table of GO/PU composite modified asphalt.

Rotating film oven aging test

In this experiment, according to the specification¹⁷, the base asphalt, PU - modified asphalt and GO/PU composite - modified asphalt were subjected to short - term aging by the rotating thin - film oven test. The indexes such as penetration, ductility and softening point of the three kinds of asphalt before and after aging were compared and analyzed. The aging performance of the modified asphalt was evaluated by the residual penetration ratio, ductility retention rate and softening point increment²⁰. The rotary thin film oven test first needs to heat the asphalt to the liquid and then put it into the asphalt sample bottle, each bottle can be filled with 35 g of asphalt, and then put the sample bottle filled with good asphalt into the oven that has been preheated for 16 h in advance. After that, 8 bottles can be put in each time, so that it can be heated at 163 ± 0.5 °C for 85 min. Finally, the aging asphalt obtained will be tested for related properties.

Dynamic shear rheology test

The dynamic shear rheometer is the fundamental test instrument for investigating the viscoelasticity of asphalt²¹. The dynamic shear rheology test was performed to investigate the high temperature rheological properties of asphalt.The DHP−1-like dynamic shear rheometer was used. The test temperature were 46℃~88℃, and the specifications of the parallel plate fixture and the plate spacing are uniform 25 mm in diameter and 1 mm thick. A strain level of 1%, an angular frequency of 10 rad/s, and a frequency sweep range of 0.1 to 100 rad/s.When scanning the temperature of matrix asphalt, PU modified asphalt and GO / PU modified asphalt before and after aging, the temperature scanning form in DSR test is used to analyze the complex shear modulus G * and phase angle δ of matrix asphalt, PU modified asphalt and GO / PU modified asphalt before and after aging. The development trend of the two indexes at different temperatures ; in the frequency scanning, the test results of matrix asphalt, PU modified asphalt and GO / PU modified asphalt were applied to the principle of time-temperature equivalence. By fitting and translating the complex modulus-angular frequency curve under five temperature environments, the viscoelastic master curve was drawn, and the variation law of G * of three kinds of asphalt was compared and evaluated.

Multi-stress recovery creep test

Multi-stress recovery creep test is a test method based on the simulation of the repeated loading and unloading process of real asphalt pavements, which can comprehensively assess the viscoelastic properties of asphalt. The J_nr represents the unrecoverable creep compliance, that is, the deformation of the asphalt that cannot be automatically recovered after the high temperature cyclic loading. R represents the strain recovery rate. The smaller the J_nr value and the larger the R value indicate that the asphalt has better resilience. The test process experienced 20 creep cycles, and the total cycle time was 200 s. Each cycle is divided into two stages: loading and unloading. The loading time is 1s and the unloading time is 9s. The matrix asphalt, PU modified asphalt and GO / PU modified asphalt were carried out at two stress levels of 0.1 kPa and 3.2 kPa.The size of MSCR sample is 25 mm in diameter and 1 mm in thickness, and the test temperature is 64 °C.

Low temperature bending rheological test

The SHRP program specified that the relevant parameters (S and m) can be obtained by BBR test to evaluate the rheological properties of asphalt at low temperature and load²². The creep stiffness modulus S may represent the ability of asphalt to withstand deformation under different temperatures and loadings. The creep curve slope m is the rate of change of S with temperature under load, which may reflect the stress-relaxation capacity of asphalt. The best crack resistance performance at low temperatures is the smaller value of the stiffness modulus S and the larger value of creep rate m^23,24. The purpose of this paper was to use the BBR test to test the low temperature stiffness of modified GO/PU composite asphalt and to compare it with PU modified asphalt and matrix asphalt to investigate whether the low temperature crack resistance of PU modified asphalt can be further improved after the addition of modified GO material. Three kinds of asphalt before and after ageing were prepared in trabecular flexure specimens in this experiment. The specimens measured 127 mm×12.7 mm×6.35 mm in diameter. The trabecular bending specimens were subjected to continuous stress loading at three low temperatures of−24 °C,−18 °C and−12 °C, respectively. The total loading time was 240s. The computer automatically collected the corresponding S value and m value at 15s, 30s, 60s, 120s and 240s, respectively. According to the requirements of the specification, when the test is 60 s, the S ≤ 300 MPa, m ≥ 0.3.

Road performance test of asphalt mixture

On the basis of previous experiments, this chapter studies the road performance of GO/PU composite modified asphalt mixture at the optimal dosage, further demonstrating the advantages and disadvantages of modified asphalt. AC−13 mineral aggregate gradation is selected, and the oil stone ratio of GO/PU composite modified asphalt mixture and PU modified asphalt mixture are 5.3% and 5.1%, respectively. Comparing GO/PU composite modified asphalt mixture with PU composite modified asphalt mixture, conducting rutting test, low-temperature creep bending test, immersion Marshall test, freeze-thaw splitting test, and four point bending fatigue test to analyze their high-temperature stability, low-temperature crack resistance, water stability, and fatigue performance.

Scanning electron microscope

To investigate whether the addition of GO modified material effectively improves the compatibility between the modified material and the matrix asphalt, the matrix asphalt was magnified to 500 times, and the PU modified asphalt and GO/PU composite modified asphalt were magnified to 500 times and 5000 times for electron microscopy scanning. The scanning electron microscope model used in this experiment is JSM−7500 F, provided by Jieolu Company. The steps for preparing asphalt SEM samples based on the characteristics of asphalt materials are as follows: (1) Put the modified asphalt into an oven at 130 degrees Celsius, and after melting, use a glass rod to dip a small amount of asphalt and drop it on a glass carrier sheet, the size is about 10 mm×10 mm×1 mm. (2) Put the prepared asphalt sample together with the glass slide into a constant temperature box at−10 °C for low temperature cooling treatment, and remove it after it is completely solidified, and gently remove the asphalt sample from the glass slide. (3) The asphalt sample was taken out and placed in the ion sputtering instrument (model SBC−12) for vacuum gold plating, and the gold plating was adjusted to the appropriate gold plating strength for 2 min. (4) Put the gold-plated sample into the scanning electron microscope instrument, adjust the sample in a suitable position and scan the image.

Fourier infrared spectroscopy test

To analyze the absorption peaks and functional group changes of matrix asphalt, PU modified asphalt, and GO/PU composite modified asphalt, and explore their modification mechanisms. The Frontier Fourier transform infrared spectrometer was used for infrared spectroscopy experiment scanning analysis, as shown in Fig. 6. The wave number range is 4000–400 cm ^− 1, the resolution is 4 cm ^− 1, and the number of scans is 32 times. Infrared spectrum tests of matrix asphalt, modified polyurethane asphalt, and GO/PU modified asphalt were performed, respectively.In this experiment, the liquid film method was used to prepare the asphalt: (1) 5 g CCl₄ solution and 0.5 g asphalt sample (CCl₄: asphalt sample = 10:1) were taken in a centrifuge tube and shaken until the asphalt was completely dissolved. Then, an appropriate amount of CCl₄ asphalt mixture solution was dripped onto the potassium bromide substrate by a dropper, and the infrared spectrum sample was detected after the CCl₄ was completely volatilized. According to the different parameters such as the position, quantity and intensity of the absorption peak, the molecular structure and group type of the substance can be distinguished.

Experiment results and analysis

Orthogonal test

According to the orthogonal experimental design table described in Sect. 3.1, graphene oxide/polyurethane composite modified asphalt with 16 different design schemes was prepared according to the experimental procedure¹⁷. In order to explore and analyze the effects of modifier blending ratio, shear temperature, and shear time on the performance of modified asphalt, the performance was determined through needle penetration, softening point, 5℃ ductility, elastic recovery test, and Brinell rotational viscosity test at different blending ratios, temperatures, and times. The experimental process is shown in Figs. 1, 2, 3, 4 and 5. To avoid errors, three parallel experiments were conducted for each design scheme and the average value was taken as the experimental result, as shown in Table 7.

Table 7 Orthogonal test results.

Based on the orthogonal experimental results of penetration degree, softening point, 5 ℃ ductility, and elastic recovery number under different factors in Figs. 1, 2, 3, 4 and 5, and in order to reduce the influence of experimental errors, variance analysis was conducted on the experimental data, and variance trend charts were drawn to evaluate the influence of the four factors on the performance of modified asphalt. The optimal preparation scheme of GO/PU composite modified asphalt was selected through comprehensive analysis.

Fig. 1

Penetration variance trend chart.

Fig. 2

Softening point variance trend chart.

Fig. 3

Ductility variance trend chart.

Fig. 4

Elastic recovery variance trend chart.

Fig. 5

Rotational viscosity variance trend chart.

The five combination schemes obtained from the three major indicators, elastic recovery, and Brinell rotational viscosity test data are as follows:

• A₃B₃C₄D₂: 0.06% graphene oxide + 14% polyurethane + 60 min shear time + 145 ℃ shear temperature;

• A₃B₂C₃D₃: 0.06% graphene oxide + 12% polyurethane + 50 min shear time + 155 ℃ shear temperature;

• A₃B₄C₄D₄: 0.06% graphene oxide + 16% polyurethane + 60 min shear time + 165 ℃ shear temperature;

• A₄B₄C₄D₂: 0.08% graphene oxide + 16% polyurethane + 60 min shear time + 145 ℃ shear temperature;

• A₃B₂C₄D₃: 0.06% graphene oxide + 12% polyurethane + 60 min shear time + 155 ℃ shear temperature.

From the above five schemes, it can be concluded that the optimal doping amount for graphene oxide is 0.06%; Considering the economic cost issue, the optimal level of polyurethane is 12%; Regarding the cutting time, four out of the five schemes have an optimal level of 60 min, so the cutting time is selected as 60 min; After comprehensive analysis of the influence of shear temperature, a temperature of 155 ℃ was selected as the optimal level.

In summary, the optimal preparation parameters for GO/PU composite modified asphalt are A₃B₂C₄D₃, with a graphene content of 0.06%, a polyurethane content of 12%, a shear time of 60 min, and a shear temperature of 155 ℃.

Ageing test

This chapter compares and analyzes the penetration, elongation, and softening point indicators of three types of asphalt before and after aging, and evaluates the aging performance of modified asphalt using residual penetration ratio, elongation retention rate, and softening point increment. Better aging performance is characterized by a larger residual penetration ratio and ductility retention rate and a smaller increase in the softening point. Figure 6 shows the results of the performance tests.

Fig. 6

The fundamental indicators of asphalt.

It can be seen from Fig. 6(a) that the matrix asphalt, PU modified asphalt and GO/PU composite modified asphalt are all reduced after aging compared with before aging. However, from the results of residual penetration ratio, it can be seen that the residual penetration ratio of matrix asphalt is 71.6%, the residual penetration ratio of PU modified asphalt is 78.1%, and the residual penetration ratio of GO/PU composite modified asphalt is 81.9%. As the modifier is added, the residual penetration ratio increases, indicating that the addition of PU into the asphalt improves the anti-ageing performance of the matrix asphalt.The residual penetration ratio continues to increase with the addition of GO on the basis of PU modified asphalt, indicating that the addition of GO further improves the aging performance of asphalt. Thus, the modified GO/PU asphalt has the best anti-ageing capability.

Figure 6(b)shows that the softening point of the matrix asphalt, The amount of asphalt modified with PU and modified with GO/PU composite increases to varying degrees after aging. From the value of softening point increment, the softening point increment of matrix asphalt is 4.2 °C, the softening point increment of PU modified asphalt is 2.1 °C, and the softening point increment of GO/PU composite modified asphalt is 1.1 °C. Matrix asphalt > PU amended asphalt > GO/PU amended composite asphalt. The softening point increment decreases after the addition of the modified material to the asphalt, indicating that the addition of polyurethane results in an improvement in the anti-ageing performance of asphalt, and the polyurethane-graphene oxide composite improves the aging performance of asphalt.

As can be seen in Fig. 6(c), the ductility retention rate of PU modified asphalt after aging is higher than that of the matrix asphalt after the aging process, and the ductility retention rate of the GO/PU modified asphalt composite after aging is higher than that of the PU modified asphalt alone. The ductility retention rate of PU modified asphalt is 17.8% higher than that of matrix asphalt, and the ductility retention rate of GO/PU composite modified asphalt is 17.2% higher than that of PU modified asphalt. It shows that the addition of PU can effectively inhibit the ductility reduction of asphalt in the aging process, and after adding GO and PU composite to modify asphalt, asphalt obtains better aging resistance.

In summary, PU supplementation can improve asphalt aging performance, but following modification of GO/PU composite, the residual penetration ratio increases, the softening point increment decreases, and the retention rate of ductility increases. It can be seen that the addition of GO and PU mixed modified materials effectively inhibits the aging of asphalt and plays a role in improving the aging performance of asphalt.

Dynamic shear rheology test

Temperature scanning

The G * and δ data of matrix asphalt, PU modified asphalt and GO / PU composite modified asphalt before and after aging are shown in Figs. 7, 8, 9, 10, 11 and 12.

Fig. 7

G* graphs of the base asphalt before and after aging.

Fig. 8

δ graphs of the base asphalt before and after aging.

Fig. 9

G* graphs of the PU modified asphalt before and after aging.

Fig. 10

δ graphs of the PU modified asphalt before and after aging.

Fig. 11

G* graphs of the GO/PU modified asphalt before and after aging.

Fig. 12

δ graphs of the GO/PU modified asphalt before and after aging.

As a whole, it can be seen that the change trend of G * and δ before and after aging of the three asphalts is basically the same. With the increase of temperature, the complex modulus decreases and the phase angle increases. The main reason is that with the increase of temperature, the molecules in the asphalt will move faster, and the free volume of the material will increase with the increase of the speed of molecular movement, so that the asphalt will change from the high elastic state to the viscous flow state, resulting in the maximum shear stress that the asphalt can withstand in the experiment becomes smaller, so the higher the temperature, the smaller the complex shear modulus. As the temperature continues to increase, the viscous components in the asphalt will continue to increase, and the proportion of viscoelastic components will increase, which in turn will increase the phase angle of the asphalt as the temperature increases.

At 46 °C, the complex modulus of GO / PU composite modified asphalt is 38.8 kPa, which is 4.3 kPa larger than that of PU modified asphalt and 14.2 kPa larger than that of matrix asphalt. At 52 °C, the complex modulus of GO / PU composite modified asphalt is 17.7 kPa, which is 1.9 kPa larger than that of PU modified asphalt and 8 kPa larger than that of matrix asphalt. At 76 °C, the complex modulus of GO / PU composite modified asphalt is 0.4 kPa, which is 0.2 kPa larger than that of PU modified asphalt and 0.1 kPa larger than that of matrix asphalt. At 82 °C, the complex modulus of GO / PU composite modified asphalt is 0.2 kPa, which is 0.1 kPa larger than that of PU modified asphalt and 0.1 kPa larger than that of matrix asphalt. At the same temperature, the complex modulus of GO / PU composite modified asphalt is the largest, and the matrix asphalt is the smallest. It shows that the addition of graphene oxide GO effectively improves the compatibility between PU and matrix asphalt, makes the penetration of PU in asphalt better, improves the modulus of asphalt material, and then makes the strength of asphalt material significantly enhanced. Therefore, the high temperature performance of GO / PU composite modified asphalt is better than that of PU modified asphalt and matrix asphalt.

From 46°C to 82°C, the δ of matrix asphalt changed from 81.67 ° to 89.19 °. The δ of PU modified asphalt changed from 68.64 ° to 80.80 °. The δ of GO / PU composite modified asphalt changed from 64.35 ° to 75.47 °. It shows that the higher the temperature, the closer the asphalt is to the viscous body, and it is more difficult to return to the original state after the load is unloaded. At the same temperature, among the three kinds of asphalt, the phase angle of GO / PU composite modified asphalt is the smallest, and that of matrix asphalt is the largest, indicating that GO modifier can make the ' framework ' formed by PU in asphalt more stable, thus limiting the rheological properties of asphalt and effectively improving the ability of asphalt to withstand load. The addition of GO / PU composite increases the proportion of elastic components in asphalt components, indicating that GO / PU composite modified asphalt has stronger resistance to deformation than PU modified asphalt and matrix asphalt in high temperature environment.

The change trend of G * and δ of the three kinds of asphalt after aging is basically the same as that before aging. G * decreases with the increase of temperature, and δ increases with the increase of temperature. At the same temperature, the size relationship of G * is GO / PU composite modified asphalt > PU modified asphalt > matrix asphalt, and the size relationship of phase angle is GO / PU composite modified asphalt < PU modified asphalt < matrix asphalt. This shows that the shear deformation resistance of the aged composite modified asphalt is still higher than that of the aged PU modified asphalt and matrix asphalt when GO and PU composite modifiers are added to the asphalt, which indicates that the addition of GO / PU composite modified materials significantly improves the high temperature performance of asphalt before and after aging.

In summary, the complex shear modulus of matrix asphalt, PU modified asphalt and GO / PU composite modified asphalt after aging is increased compared with that before aging, indicating that during the aging process of asphalt, the light components in asphalt continue to decrease and other components are polymerized due to oxidation reaction, which makes asphalt continuously harden, and then makes the shear modulus relatively larger. The phase angle of matrix asphalt, PU modified asphalt and GO / PU composite modified asphalt after aging is smaller than that before aging, indicating that the elastic component of asphalt increases after aging. According to the data diagram, it can also be found that the phase angle of GO / PU composite modified asphalt after aging is greatly reduced compared with that of matrix asphalt and PU modified asphalt, indicating that the addition of GO / PU composite modified material makes the anti-metamorphic ability of asphalt after aging significantly improved, and the anti-fatigue characteristics of asphalt are improved.

Frequency scanning

The dynamic shear rheological test (DSR) has two scanning methods: temperature and frequency. In order to analyze the rheological properties bitumen after temperature scanning, the properties of bitumen under different shear frequencies can be studied by frequency scanning. In this experiment, the variation of complex modulus G * of matrix asphalt, PU modified asphalt and GO / PU composite modified asphalt under different loading frequencies of 0.1 ~ 100 rad/s was studied at 40 °C, 52 °C, 64 °C, 76 °C and 88 °C respectively, and then the complex modulus of asphalt at a certain temperature was compared to analyze the modification effect. Based on the three displacement factors of asphalt obtained from the experiment at 40 ℃, the lgG * - lg ω curves at other temperatures were shifted by the corresponding displacement factors towards the 40 ℃ curve, and the viscoelastic main curve was obtained as shown in Fig. 13.

Fig. 13

Summary of master curve of complex modulus of three kinds of asphalt.

Figure 6 is divided into high temperature low frequency region and low temperature high frequency region:

At high temperature and low frequency, the complex modulus matrix asphalt is the smallest of the three bitumen and shows weak resistance to weak high temperature deformation resistance. The G* curves of the two modified asphalts are approximately parallel to each other, which are higher than those of the matrix asphalt, and the G* of the GO/PU composite modified asphalt is the largest, indicating that the modification of the matrix asphalt has a certain effect on improving the high temperature performance, and GO and PU are used as composite modified materials to modify the matrix asphalt, which can significantly enhance its resistance to high temperature denaturation.

In areas with low temperatures and high frequencies, the complex modulus curve of the matrix asphalt shows there is a gradual shift to polyurethane-modified asphalt, and its difference gradually decreases, but the value is still the smallest among the three kinds of asphalt, this indicates that the resistance to high temperatures is not as high as that of modified bitumen.The G* curve of GO/PU composite modified asphalt in the three kinds of asphalt is still at the highest position, this suggests that the addition of GO modifier to polyurethane modified bitumen under low-temperature and high-frequency conditions significantly improves the high-temperature deformation resistance of the bitumen.

Multiple stress creep recovery test

In this section, the MSCR sample size is 25 mm in diameter × 2 mm in thickness, and the test temperature is 64 °C. The results are shown in Fig. 14.

Fig. 14

R and Jnr of Santong asphalt at 0.1 kPa.

According to Fig. 14, under the shear stress of 0.1 kPa, the R value of matrix asphalt is the smallest and almost 0, and the R value of GO/PU modified asphalt is the largest, which is 20.7% higher than that of PU modified asphalt. The results show that GO modifier can increase the elastic composition of asphalt in polyurethane modified asphalt, which reflects the best strain recovery rate of GO/PU modified asphalt. Compared to the Jnr values of the three bitumen types, matrix asphalt is the largest and GO/PU composite modified bitumen is the smallest, 76.0% lower than PU modified bitumen, reflecting the the best recoverable strain capacity GO/PU composite modified bitumen. GO/PU composite modified asphalt has excellent properties of high temperature creep recovery from both R and Jnr values, indicating that GO modified material plays a key role in the high temperature deformation resistance of composite modified asphalt and can effectively reduce high temperature rut damage during bitumen use.

Low temperature bending rheological test

Analysis of low temperature creep properties at different temperatures before aging

The stiffness modulus and creep rate obtained from this test were used to evaluate the performance of asphalt at low temperature. The S value and m value of the three asphalts at each temperature are shown in Fig. 15.

Fig. 15

The data of stiffness modulus and creep rate with temperature change.

From the analysis of Fig. 15(a), it can be seen that from−12 °C to−24 °C, the stiffness modulus of the three asphalts showed an increasing trend, reflecting that when the temperature decreased, the elastic stress of the asphalt also decreased, its brittleness increased and the asphalt gradually hardened. The value of the stiffness modulus S data of the three kinds of asphalt is matrix asphalt > PU modified asphalt > GO / PU composite modified asphalt. The S value of GO / PU composite modified asphalt is the smallest. At low temperatures of−12 °C,−18 °C and−24 °C, it is 64.2%, 55.1% and 67.3% lower than that of matrix asphalt, and 31.2%, 24.2% and 23.9% lower than that of PU modified asphalt. It shows that the addition of GO makes the brittleness of asphalt decrease and the flexibility increase, which can improve the low temperature performance of asphalt.

From the analysis of Fig. 15(b), it can be seen that the creep rate of the three kinds of asphalt is the largest at−12 °C, and the creep rate is the smallest at−24 °C. The asphalt material hardens with the decrease of temperature. At−12 °C, GO / PU composite modified asphalt increased by 17.1% compared with matrix asphalt, but decreased compared with PU modified asphalt, indicating that the addition of GO at this temperature improved the performance of matrix asphalt, but had a smaller negative impact than PU modified asphalt. At−18 °C and−24 °C, the creep rate of GO / PU composite modified asphalt is the largest, which is 0.556 and 0.421, respectively. In general, the addition of GO and PU composite modified materials makes the asphalt less prone to shrinkage deformation under low temperature conditions, and the low temperature crack resistance is better.

Analysis of low temperature creep properties at different temperatures after aging

The BBR test of three kinds of asphalt after short-term aging of RTFOT was also carried out at the temperature of−24 °C,−18 °C and−12 °C, and the S value and m value of asphalt after aging were obtained. The analysis of S and m can more accurately explore the influence of the incorporation of GO and PU composite modified materials on the performance of asphalt. The S and m data after aging are shown in Fig. 16.

Fig. 16

Aging stiffness modulus and creep rate with temperature change data map.

From Fig. 16(a), it can be seen that the change trend of the stiffness modulus of the three kinds of asphalt after aging is the same as that before aging, and it shows an increasing trend with the decrease of temperature, and the S value of GO / PU composite modified asphalt after aging is the smallest. At the low temperature of−12 °C,−18 °C and−24 °C, it is 69.2%, 59.7% and 56.3% lower than that of matrix asphalt, and 22.5%, 14.6% and 7.5% lower than that of PU modified asphalt. It shows that the GO / PU composite modified asphalt after RTFOT short-term aging still shows the best performance, which further shows that the low temperature crack resistance of GO / PU composite modified asphalt is the best.

From Fig. 16(b), it can be seen that the m values of the three kinds of asphalt at−12 °C,−18 °C and−24 °C after short-term aging of RTFOT are lower than those before aging to varying degrees, which is due to the aging test. The brittleness of asphalt increases. With the decrease of temperature, the creep rate m shows a downward trend, and at the three temperatures, the m value of the aged matrix asphalt is the lowest, and the m value of the GO / PU composite modified asphalt is the largest. It shows that on the basis of PU modified asphalt, the addition of GO material makes its low temperature crack resistance better.

The PG classification of matrix asphalt, GO modified asphalt, and GO/PU modified asphalt is summarized in Table 8.

Table 8 Summary of PG grading results.

Study on road performance of composite modified asphalt mixture

High-temperature stability

Rutting test results are shown in Table 9.

Table 9 Wheel tracking test results of asphalt mixture.

From Table 9, it can be seen that the dynamic stability of PU modified asphalt and GO / PU composite modified asphalt meets the technical requirements. The greater the dynamic stability, the better the high temperature stability of asphalt. The dynamic stability of GO / PU composite modified asphalt is 4972 times / mm, which is 28.9% higher than that of PU modified asphalt. It shows that the addition of GO modified material increases the viscosity of PU modified asphalt and enhances the binding force between asphalt and mineral aggregate, thus effectively improving the high temperature stability of modified asphalt. This is due to the physical reaction and chemical reaction during the modification of asphalt by GO and PU composite. GO and polyurethane form a stable three-dimensional network structure with asphalt molecules in the matrix asphalt, and the interaction between them is enhanced. The adhesion between asphalt and mineral aggregate is enhanced, so that the ability of asphalt to resist rutting deformation is improved²⁶.

Low temperature crack resistance

The results of low temperature bending test are shown in Table 10.

Table 10 Low temperature bending test results of two modified asphalt mixtures.

In the low temperature creep bending test, the maximum bending strain εB is usually used to judge the performance of asphalt mixture in low temperature environment. The larger the εB value, the better the flexibility of asphalt mixture and the better the resistance to low temperature cracking. According to Table 8, the εB values of PU modified asphalt mixture and GO / PU composite modified asphalt mixture meet the specification requirements of more than 2500, indicating that the low temperature crack resistance of the two modified asphalts is better. Comparing the two kinds of asphalt, the flexural tensile strength, maximum flexural tensile strain and flexural stiffness modulus of GO / PU composite modified asphalt mixture are 10.687 MPa, 3922.7 and 2978.9 MPa, respectively, which are 20.47%, 13.79% and 14.22% higher than those of PU modified asphalt mixture, respectively. It shows that after adding GO, the tensile ability of PU modified asphalt mixture is enhanced, the stress relaxation performance is effectively improved, and the elastic recovery ability of asphalt is better played.The asphalt mixture combines the aggregate with the asphalt through the adhesion of the asphalt. The adhesion of the GO / PU composite modified asphalt is better, so it can make the combination between the aggregates closer, which can reduce the porosity and internal stress of the asphalt mixture, so that it has better low temperature crack resistance. Therefore, the low temperature performance of asphalt mixture is obviously improved by modifying asphalt with GO / PU composite modified material²⁷.

Water stability

Immersion Marshall test

The test results are shown in Table 11.

Table 11 Immersion Marshall test results.

According to Table 9, the residual stability of PU modified asphalt mixture and GO / PU composite modified asphalt mixture meets the specification technical index ≥ 80%. The stability of GO / PU composite modified asphalt mixture is 13.93% and 19.31% higher than that of PU modified asphalt mixture at 30 min and 48 h respectively. The residual stability of GO / PU composite modified asphalt mixture is 4.1% higher than that of PU modified asphalt mixture. This result can be seen that the addition of GO, to a certain extent, delayed the stability of asphalt mixture to reduce, so that it has better water stability ; the bonding performance between PU and matrix asphalt has been improved, the bonding force between asphalt and mineral aggregate has been enhanced, the water damage resistance of asphalt has been significantly improved, and the adverse effects of hydrodynamic pressure have been reduced.

Freeze-thaw splitting test

The test results are shown in Table 12.

Table 12 Freeze thaw splitting test results of two kinds of asphalt mixtures.

According to Table 10, it can be seen that the splitting strength ratio of PU modified asphalt mixture and GO / PU composite modified asphalt mixture meets the specification technical index ≥ 80%. The splitting strength of GO / PU composite modified asphalt mixture before and after freeze-thaw is greater than that of PU modified asphalt mixture, and the splitting strength before and after freeze-thaw is increased by 15.13% and 18.39% respectively. The splitting strength ratio is increased by 2.4%, and the change of strength after freeze-thaw is slowed down. In summary, the ability of GO / PU composite modified asphalt mixture to resist freeze-thaw damage is higher than that of PU modified asphalt, because the addition of GO modified material increases the thickness of asphalt film in asphalt mixture, thereby improving the water stability of asphalt mixture. The conclusion of this test is consistent with the immersion Marshall test.

Fatigue resistance

The test results are shown in Table 13.

Table 13 Fatigue test results of asphalt mixture.

It can be seen from Table 11 that the average fatigue number of GO / PU composite modified asphalt mixture is higher than that of PU modified asphalt mixture, which is increased by 21.3%. It shows that the addition of GO improves the fatigue resistance of asphalt mixture and can better bear the repeated load of driving. GO / PU composite modified asphalt mixture has better creep recovery ability, better flexibility and stability, and better fatigue resistance than PU modified asphalt.

Composite modified asphalt microstructure analysis

The microstructure of matrix asphalt, PU modified asphalt and GO / PU composite modified asphalt is shown in Figs. 17, 18 and 19.

Fig. 17

Baseasphalt scanning electron microscope.

Fig. 18

Sem of modified asphalt by PU.

Fig. 19

Sem of GO/PU composite modified asphalt.

As shown in Fig. 17, the surface of the matrix asphalt is very smooth and uniform texture, and its microstructure is uniformly distributed asphalt phase.

According to Fig. 18, the surface of PU modified asphalt is rougher than that of matrix asphalt, and PU modifier can be observed. It shows that PU modified materials are not fully dissolved in matrix asphalt and are distributed irregularly in asphalt. The compatibility between PU modified materials and matrix asphalt is poor. PU is not uniformly dispersed throughout the matrix asphalt, and there is a clear segregation phenomenon.

It can be seen from Fig. 19 that the surface of the modified GO/PU composite asphalt shows a slight wrinkling condition, and there are no obvious protrusions and particulates compared to PU-modified asphalt.It shows that adding GO modified material to PU modified asphalt can promote the dispersion state of PU modifier in asphalt and form a more stable framework. The surface morphology characteristics showed a relatively smooth morphology, and no free PU modified material particles were shown, suggesting that the segregation problem of PU modified asphalt was significantly improved by adding GO composite modified asphalt. In summary, GO/PU composite modified asphalt has good compatibility, which improves the storage stability of composite modified asphalt and has excellent road performance.

Infrared spectrum test analysis of composite modified asphalt

The infrared spectra of polyurethane are shown in Fig. 20, and the infrared spectra of matrix asphalt, PU modified asphalt and GO / PU composite modified asphalt are shown in Fig. 21.

Fig. 20

Infrared spectra of PU.

Fig. 21

Infrared spectra of asphalt.

According to Fig. 20, it can be seen that there is a small vibration absorption peak at 3125 cm ^− 1, indicating the presence of -CH in the polyurethane material ; the polyurethane material has a very obvious absorption peak at 2270 cm ^− 1, where is the stretching vibration absorption peak of the isocyanate group (-NCO), indicating that the polyurethane material has-NCO functional group ; the absorption peak at 2914 cm ^− 1 is the antisymmetric stretching vibration peak of methylene (-CH₂), indicating that there are saturated hydrocarbons in polyurethane materials. In the case of the carbamate, the group that usually appears in the wavenumber range of 1660 ~ 1760 cm ^− 1 is primarily the carbonyl group (C = O), and its vibration-absorption peak is located at 1717 cm ^− 1 in the infrared spectrum.The corresponding absorption peak at 1521 cm ^− 1 is the bending vibration peak of amino functional group in carbamate. The absorption peak at 720 cm ^− 1 is the bending vibration absorption peak of benzene ring (-C₆H₆) in polyurethane.

It can be seen from Fig. 21 that in the infrared spectra of matrix asphalt, Polyurethane modified asphalt and GO/PU composite modified asphalt showed strong absorption peaks in the range of 2800–3000 cm ^− 1., and the peaks appear at 2849 cm ^− 1 and 2922 cm ^− 1, which are the symmetrical and asymmetrical vibrational absorption peaks of methylene (-CH₂) indicate the presence of saturated hydrocarbons in the asphalt. The vibrational absorption peak of the weakly stretched C = C aromatic ring in matrix asphalt is 1600 cm ^− 1,and the appearance of carbon-carbon double bond indicates that there are aromatic compounds in asphalt. The antisymmetric vibration absorption peak of methyl functional group -CH3 appeared at 1455 cm ^− 1, and the absorption peak of wave number at 720 cm ^− 1 corresponded to -CH functional group, indicating that aromatic compounds existed in asphalt.

Comparing the infrared spectra of PU modified asphalt and PU modified materials, it can be found that the absorption peak of isocyanate group (-NCO) functional group with wave number of 2270 cm ^− 1 in PU almost disappeared in PU modified asphalt, indicating that the isocyanate group in PU reacted with the hydroxyl group in asphalt. A comparison of the infrared spectra of the polyurethane-modified asphalt and the initial asphalt showed that the polyurethane-modified asphalt had new characteristic absorption peaks at wavelengths 1662 cm ^− 1, 1540 cm ^− 1, and 1513 cm ^− 1.Among them, the absorption peak at 1662 cm ^− 1 represents urea carbonyl (-NHCONOOO), indicating that chemical changes have taken place in the modification of matrix asphalt by polyurethane, resulting in new functional groups.The chemical reaction mechanism can be seen in the following chemical formula :

$$\:\text{R-NCO+Ar-COOH→}\left[\text{RNHCOOCO-Ar}\right]\text{→RNHCO-Ar+C}{\text{O}}_{\text{2}}$$

$$\:\text{R-NCO+Ar-COOH→Ar-COOC-Ar+R-NHCONH-R}$$

The above equation indicates that the isocyanate (R-NCO) in PU reacts with the esters in the matrix asphalt to form an anhydride with weak stability. The unstable anhydride will decompose to form amide (-NHCO) and carbon dioxide gas (CO₂). Excessive isocyanate will continue to react with the aromatics in the asphalt, and eventually produce urea formate.

By comparing the infrared spectra of GO/PU composite modified asphalt and PU modified asphalt and matrix asphalt in Fig. 18, it can be seen that the infrared spectra of GO/PU composite modified asphalt and PU modified asphalt with matrix asphalt are in the range of 3100–3700 cm ^− 1. GO/PU composite modified bitumen curves have a wide absorption band, where is the hydroxyl (-OH) absorption peak band, indicating that after adding GO modified material, GO and methyl functional group (-CH) in asphalt were oxidized during the preparation of modified asphalt. The GO/PU modified bitumen exhibited a strong absorption peak at wavenumber 1615 cm ^− 1, which is due to the destruction of polymer chain segments and changes in C = C content, indicating excellent mechanical properties.

In summary, GO/PU composite modified asphalt is a modification method based on chemical modification and supplemented by physical modification. Chemical modification can make the modifier and the matrix asphalt better cross-linked together to form a stable network structure and improve the stability of the asphalt. Physical modifications improve the mechanical properties of asphalt and provide better performance on the road.

Conclusions

• 1. The residual penetration ratio of GO/PU composite modified asphalt increased by 4.8% and 14.4%%, softening point increment decreased by 1℃ and 3.1℃, and the ductility retention rate increased by 17.2% and 45, respectively, compared to polyurethane modified asphalt and matrix asphalt. The results show that adding modified GO material to polyurethane modified asphalt can effectively inhibit asphalt aging and improve asphalt aging performance. In practical application, road fatigue failure caused by aging can be effectively prevented.

• 2. In the range of 46℃~82℃, GO/PU composite modified bitumen had higher modulus and rut coefficient than PU composite modified bitumen, indicating no change in GO/PU composite modified bitumen. The basic properties of viscoelasticity enhance the ability of asphalt to resist plastic deformation. GO/PU composite modified bitumen further improves the high temperature stability and rutting deformation resistance of bitumen.

• 3. In the range of 40℃ to 88 ℃, at the same temperature and at the same loading frequency, the relationship between the complex modulus is GO/PU composite modified asphalt > PU modified asphalt > matrix asphalt. By fitting the complex modulus curve equation and calculating the displacement factor, the viscoelastic master curve diagram drawn. The results show that GO/PU modified asphalt has the highest curve position in both high and low frequency regions. The results show that GO/PU composite modified bitumen has the high temperature deformation resistance than PU modified bitumen and matrix asphalt. It is further proved that GO and PU composite modified materials can significantly improve the rut resistance of asphalt and the modification effect is excellent.

• 4. Matrix asphalt showed no significant strain recovery ability at temperatures of 64℃, 0.1 kPa and 0.3 kPa shear stress. In 10 cycles, the cumulative strain of matrix asphalt is much larger than that of two modified asphalts, and the cumulative strain of GO/PU composite modified asphalt is lower than that of PU modified asphalt. The R and Jnr values at 0.1 kPa were calculated based on experimental data. The results showed that the R value of GO/PU modified asphalt was the highest, 20.7% higher than that of PU modified asphalt. GO/PU composite modified asphalt had the lowest Jnr value, 76.0% lower than PU modified asphalt. The results show that GO modified material plays an important role in the high temperature deformation resistance of composite modified asphalt and can effectively improve the creep recovery of asphalt.

• 5. Under the low temperature state of−12 °C,−18 °C and−24 °C, GO/PU composite modified asphalt shows an overall increasing trend compared with the other two asphalts, and its creep stiffness modulus is the smallest, which is 31.2%, 24.2% and 23.9% lower than that of PU modified asphalt, indicating a significant improvement in the asphalt’s ability to relax at low temperatures.GO and PU as composite modified materials are added to asphalt to further improve its low temperature crack resistance.

• 6. Compared with PU modified asphalt, the dynamic stability of GO / PU composite modified asphalt mixture increased by 28.9% in high temperature rutting test. The low temperature creep bending test shows that the bending strength, maximum bending strain and bending stiffness modulus are increased by 20.47%, 13.79% and 14.22% respectively. The immersion Marshall test showed that the residual stability increased by 4.1%. The results of freeze-thaw splitting show that the splitting strength and splitting strength ratio before and after freeze-thaw are increased by 15.1%, 18.4% and 2.4%, respectively. The fatigue test results show that the average fatigue times are 21.3% higher. It shows that GO / PU composite modified asphalt has better high temperature stability, low temperature crack resistance, water stability, water damage resistance and fatigue resistance.

• 7. The microstructure of the two modified asphalts was analyzed by scanning electron microscopy. On the basis of PU modified asphalt, GO modified material was added to improve the segregation problem of PU modified asphalt and enhance the adhesion of asphalt. The modification mechanism of GO / PU modified asphalt was analyzed by infrared spectroscopy. The isocyanate group in PU reacted with the aromatic esters in asphalt to form ureidoformate. The oxidation reaction between GO and the-CH functional group in asphalt occurred. The content of C = C double bond changed during the modification process, indicating that the modification of asphalt by GO and PU modified materials is a mixed modification method based on chemical modification and supplemented by physical modification.