Research on high and low temperature rheological properties of TPU/SBR composite modified asphalt

In road engineering, SBS modifier has been widely applied in China to enhance the quality of asphalt pavement. However, without considering the physical and chemical properties of SBS modifier and the asphalt substrate, alternative polymerization methods for asphalt modification that offer better performance and economic benefits have not been explored. In this study, an approach was taken to compound modify the base asphalt by incorporating Thermoplastic polyurethane (TPU) and Styrene Butadiene Rubber (SBR). The high and low temperature rheological properties of the modified asphalt were examined using tests such as Rotational Viscosity (RV) test, Dynamic Shear Rheological test (DSR), Multiple Stress Creep Rheological test (MSCR), and Bending Beam Rheological test (BBR). The modification mechanism of the mixed modified asphalt, combining SBR and TPU, was investigated through fluorescence microscope testing. The RV test revealed that the inclusion of TPU and SBR enhanced the viscosity of the asphalt to a certain extent, affording it favorable workability during construction and improved resistance against high temperature deformation. DSR and MSCR tests demonstrated that the incorporation of TPU significantly bolstered the external load resistance, deformation recovery, rutting resistance, and overall stability of the SBR modified asphalt under high temperature conditions; The BBR test proves that the composite modified asphalt has excellent low temperature crack resistance. The FM tests have shown that TPU are effective in reducing the segregation that may occur in SBR-modified asphalt, thus improving the performance of the composite modified asphalt.


Introduction
In recent years, with the increasing demand for transportation and the growing number of vehicle ownership, asphalt pavement has become a widely used material in road construction due to its advantages such as a smooth surface and comfortable driving experience [1,2].However, this widespread application has also brought about a series of issues, including the frequent occurrence of distresses such as ruts, cracks, and potholes, which significantly compromise the safety of driving and also diminish the service life of the road network [3,4].To tackle these challenges, researchers have turned to asphalt modification as a means to enhance its performance.By improving the properties of asphalt, it becomes possible to significantly enhance the serviceability of asphalt pavements and extend their lifespan [5].
Currently, polymer modifiers commonly used for asphalt include thermoplastic elastomers, rubber-based modifiers, and thermoplastic resin modifiers [6].Among these, the Styrene-Butadiene-Styrene (SBS)modifier is widely utilized and the addition of an appropriate amount of SBS enhances the performance of asphalt.Despite the widespread use of SBS, there are some drawbacks to its use.These limitations include the relatively high cost of SBS, lower resistance to UV radiation, oxidation, and heat compared to some other asphalt modifiers, and segregation problems with SBS-modified asphalt [7][8][9][10].These disadvantages have resulted in a significant amount of money being spent to transport SBS modified asphalt [11].
Polyurethane materials have prominent advantages in terms of chemical corrosion resistance, tear resistance, adjustable softness and hardness [12][13][14].In the last few years, they have gradually become a hot spot for research in the field of modified asphalt.A special network structure, more stable than those formed by other modifiers and asphalt, is formed by the chemical reaction between polyurethane and asphalt.This enhanced stability effectively improves the storage stability of asphalt and ensures that the performance of modified asphalt remains favorable over the long term [15].Furthermore, studies conducted by respected scholars have demonstrated that incorporating polyurethane into asphalt leads to significant improvements in resistance against high-temperature rutting, aging, and water damage, as well as various other benefits.However, it should be noted that the chemical reaction between polyurethane and asphalt may adversely affect its low temperature cracking resistance [16].Lack of low-temperature crack resistance can cause brittle cracks to form in modified asphalt pavements, especially in cold regions during winter.These cracks not only impact driving comfort but also result in a shortened service life for the asphalt pavement.Consequently, it becomes crucial to explore alternative modifiers that are readily available and cost-effective to incorporate into polyurethane modified asphalt.
For this reason, the search for an alternative modifier with abundant sources and low costs to enhance the low-temperature crack resistance of polyurethane-modified asphalt becomes imperative.Butadiene rubber, a high-quality synthetic rubber, is characterized by its wide availability and affordable price [15,17].Research has shown that the addition of a small amount of butadiene rubber can significantly enhance the low-temperature performance of asphalt and improve the low-temperature crack resistance of asphalt mixtures [17][18][19].The addition of butadiene rubber does not significantly enhance the high-temperature stability of asphalt.Moreover, chemical interaction between butadiene rubber and asphalt is insufficient, hindering the formation of a stable thermodynamic system [20].This could negatively impact the storage stability of modified asphalt.Incorporating butadiene rubber can potentially disrupt the asphalt's structure, resulting in reduced storage stability of the modified asphalt [21][22][23].Consequently, this can somewhat constrain the extensive use of butadiene rubber.
Considering the advantages and disadvantages of the two polymer modifiers, polyurethane (TPU) and styrene-butadiene rubber (SBR), this study aims to investigate the effects of these modifiers on asphalt properties.Specifically, thermoplastic polyurethane (TPU) and styrene-butadiene rubber (SBR) are selected as the modifiers to prepare composite modified asphalt.The high-temperature rheological characteristics of the modified asphalt are evaluated using the Brookfield rotational viscosity and dynamic shear rheological tests.Furthermore, the low-temperature rheological performance of the modified asphalt is assessed through the bending beam rheological test.By comprehensively evaluating the performance of the modified asphalt under different temperature conditions, both high and low, this study intends to provide a thorough understanding of how TPU and SBR modifiers influence the properties of asphalt.In order to reveal the reasons for the changes in the properties of the composite modified asphalt, this study also analyzed the microscopic phase structure of the modified asphalt by fluorescence microscopy test.The research framework and plan for this study are illustrated in figure 1.

Raw materials 2.1.1. Base asphalt
To analyze the impact of TPU and SBR modifiers on the performance of the base asphalt thoroughly, we used Kunlun brand A-70# road petroleum asphalt as the base asphalt in this study.Based on the specifications outlined in JTG E20-2011, the essential engineering performance indicators of the base asphalt were evaluated and presented in table 1.This indicates that the performance of the 70# base asphalt satisfies the specification requirements.

Thermoplastic polyurethane(TPU)
The thermoplastic polyurethane used in this study is TPU-801 from Dongguan Guangye Plastic Raw Materials Co., Ltd, Guangdong, China.Its color is white and it is a powder solid.The properties of TPU are shown in table 2.

Styrene butadiene rubber (SBR)
The selected SBR for this study is in solid powder form, specifically SBR1502.It appears as a white to slightly yellow powder at room temperature.It is produced by Shanghai Yuanxiang Industrial Co., Ltd The technical property indicators of SBR powder are shown in table 3.

Preparation of modified asphalt
To guarantee optimal high and low-temperature performance properties of TPU/SBR, the group identified that the ideal percentage of TPU to be used is 15%, while the optimum SBR percentage is 3.5%.
Heat the base asphalt to the molten state at 150 °C, adjust the shear temperature to about 140 °C.Then add SBR powder until it is completely incorporated into the asphalt, shear at 1000 r min -1 for 10 min.Keep the shear  temperature and shear rate unchanged, add a small amount of TPU powder several times.And adjust the shear rate to 4000 r min −1 , and continue to shear for 60 min, until SBR powder and TPU powder is completely and evenly dispersed in asphalt.And then adjust the shear rate of the high-speed shear to 800 r min −1 , keep the shear temperature and shear rate unchanged, and cut for 10 min to remove bubbles.The process for preparing composite modified asphalt is illustrated in figure 2.

RV test
According to the requirements of the asphalt rotational viscosity test specified in JTG E20-2011 [24], this study utilized a NDJ-1F type Brinell viscometer to measure the apparent viscosity of the asphalt at temperatures of 135 °C, 150 °C, and 175 °C.First, place the flowing asphalt in a beaker and keep it in an oven for 45 min to remove any bubbles.Based on the research experience of relevant scholars, estimate the viscosity range of different types of asphalt.Select an appropriate rotation speed and rotor according to the specifications.Finally, pour the specified asphalt into the viscosity meter sample container.Place the rotor and the filled container together in the oven and keep it warm for 1.5 h before conducting the viscosity test.By conducting three consecutive tests with a 60-second interval between each test, the average viscosity at different temperatures can be determined.The experimental data makes it possible to analyze the viscosity and temperature sensitivity of asphalt at various temperatures.This helps evaluate how asphalt performs when exposed to high temperatures.

DSR test
In order to investigate the changes in the rheological properties of asphalt under actual ambient temperature conditions, this study examined the changes in the rheological properties of asphalt materials using DSR according to the ASTM D7175 (AASHTO T31509) specification [25][26][27].The oscillatory stress applied by this method better corresponds to the load conditions that asphalt experiences during actual service, thus providing a more accurate characterization of the asphalt's real-world performance.In the Temperature Sweep Test, the temperature range was set from 34 °C to 82 °C, with a temperature gradient of 6 °C.A single vibration frequency mode was used, with a fixed frequency of 1.59 Hz.The loading method employed was strain-controlled mode, with a set strain of 1%.
To investigate the changes in the complex modulus master curve of asphalt materials at different vibration frequencies, simultaneous Frequency Sweep Tests were performed.The vibration frequency range was controlled between 0.1 Hz and 10 Hz.The loading method remained as strain-controlled mode, but the set strain needed to be changed to 12%.Other parameters remained unchanged.Using the principle of timetemperature equivalence, this study explores the viscoelastic behavior of asphalt in relation to changes in frequency and temperature.The 64 °C dynamic complex modulus master curve is employed to extend the frequency and temperature range, providing in-depth analysis of the viscoelastic properties of asphalt materials across a wide frequency and temperature domain.Williams-Landel-Ferry (WLF) equation [28] and Sigmoid equation [29] are shown in equations (1) and (2) to calculate the αT and G * master curvein this study.

Lg C T T C
T T 1 where αT is the shift factor; T is the measured temperature; T1 is thereference temperature which is chosen to be 35 °C; Ca and Cb are the model coefficients.
where Log |G * | is the log-frequency; θ is the lower asymptotic line; α is the difference between the upper asymptotes and the lower asymptotes; fr is reducing frequency; β and γ define the location of the asymptote and inflection points [30].
To comprehend asphalt's performance fluctuations at high temperatures, seven distinct asphalt materials underwent MSCR Tests.In this study, the MSCR tests were conducted at different temperatures according to the AASHTO T350 specification [31][32][33].According to the high summer temperatures and road surface temperature variations in China, the test temperature is set at 64 °C.During the experiment, stress levels of 0.1 kPa and 3.2 kPa, which are commonly used, are selected.Various types of asphalt materials undergo 10 seconds of loading-induced deformation and 9 seconds of attempted stress recovery following 1 s of this same loading.During the creep loading phase, the asphalt undergoes deformation under stress.The unloading recovery phase allows the deformed asphalt to partially recover.This process is repeated 10 times.

BBR test
In this study, the low-temperature properties of asphalt materials were evaluated using the Bending Beam Rheometer (BBR) test proposed by the U.S. SHRP program, and the BBR test was carried out according to the AASHTO T313 protocol.The performance is evaluated by analyzing the creep stiffness (S) and the creep rate (m).These parameters offer insights into how asphalt behaves in terms of viscoelasticity and its capability to withstand deformation under low temperatures.The rheological and stress relaxation properties of lowtemperature TPU/SBR modified asphalt under fixed loads will be investigated through experiments conducted using bending beam rheometers.According to the temperature variations in winter in China, the test temperature will be set at −12 °C.Under creep loading, three parallel experiments will be conducted on asphalt samples with different dosages.The final test result will be the average of these experiments, providing valuable insights into the performance of TPU/SBR-modified asphalt under low-temperature conditions.

FM test
Polymer asphalt modifiers SBR and TPU exhibit fluorescence when excited by high-energy ultraviolet light.By utilizing this principle, the fluorescent microscope has been employed to observe the microstructure of polymer modifiers in asphalt, and analysis and research have been conducted on the modification mechanism of polymer-modified asphalt.This study performed FM tests on five different types of asphalt using the IMAGER.Z2 fluorescence microscope provided by Carl Zeiss Optical Ltd, Germany.The sample preparation was conducted using the hot-droplet cover glass method, and the observation was conducted at a magnification of 100×.From figure 3, it can be observed that the viscosity of all the asphalt samples falls within the range of 3 Pa.sspecified by the Superpave specification under the temperature condition of 135 °C.This result indicates that the asphalt exhibits good flowability.At the same temperature, the viscosity of 70# base asphalt is the lowest.Compared to the base asphalt, the viscosity of the four types of modified asphalt increased by 0.203 Pa.s, 0.316 Pa.s, 0.234 Pa.s, and 0.388 Pa.s, respectively.Compared to the SBR modified asphalt, the viscosity increased by 0.09 Pa.s, 0.203 Pa.s, 0.121 Pa.s, and 0.275 Pa.s, respectively.This phenomenon indicates that the viscosity of the asphalt improves when TPU or SBR modifiers are added to the base asphalt.This means that the modified asphalt has a higher viscosity, which makes it more adhesive and cohesive.When compared to SBRmodified asphalt, TPU-modified asphalt shows a higher increase in viscosity, indicating that TPU has the potential to improve asphalt's high-temperature performance.TPU modifiers offer improved resistance against deformation and aging under high-temperature conditions, ultimately enhancing the performance of the asphalt.Therefore, both TPU and SBR modifiers can thicken the asphalt and enhance its performance under different conditions.

Results and discussion
The modified asphalt has higher temperature sensitivity when compared to the base asphalt, as shown in figures 4 and 5.Moreover, the TPU-modified asphalt's viscosity increases at a 15% dosage, whereas the SBR-modified asphalt's viscosity increases at a 3.5% dosage.At 150 °C, the viscosity of TPU-modified asphalt is 0.483 Pa.s, whereas the viscosity of SBR-modified asphalt is 0.351 Pa.s.The increase in viscosity of TPU-modified asphalt compared to SBR-modified asphalt is 0.132 Pa.s, which corresponds to a 37.61% increase.When it comes to improving the high-temperature deformation resistance of asphalt, it is clear that TPU performs significantly better than SBR.At a test temperature of 175 °C, TPU-modified asphalt (at a 15% dosage) exhibits a viscosity increase of 0.074 Pa.s compared to the base asphalt, while SBR-modified asphalt (at a 3.5% dosage) shows a viscosity increase of 0.045 Pa.s compared to the base asphalt.These results demonstrate that modifiers improve the asphalt viscosity, resulting in better performance under hightemperature conditions.
The viscosity of TPU/SBR composite-modified asphalt exceeds that of both the base asphalt and SBRmodified asphalt.This indicates that the asphalt modified by the composite has excellent resistance to high temperatures.Furthermore, in the composite modified asphalt, the viscosity varies more noticeably with changes in TPU content, suggesting that TPU plays a dominant role.The reason for this could be that SBR and asphalt have difficulty in achieving complete compatibility.Even after high-speed shearing dispersion, uneven dispersion phenomenon may still exist due to their physical blending nature.In this way, changes to the internal structure and composition of the asphalt are prevented.Conversely, some TPU components combine with base asphalt and SBR-modified asphalt, resulting in improved intermolecular cross-linkage throughout the entire system.This process results in a more stable viscoelastic network structure, which enhances the viscosity of the  modified asphalt composite.This improvement is reflected in its good workability, ease of construction, and high-temperature performance at a macroscopic level.

Temperature sensitivity analysis
In order to analyze the effect of different temperatures on the viscosity of asphalt, researchers used a semilogarithmic function formula to fit the viscosity-temperature relationship and found a good linear relationship between the logarithm of asphalt viscosity and temperature [34].By studying the viscosity changes of TPU/SBR composite modified asphalt within a certain temperature range using the semi-logarithmic function formula, specific experimental results can be seen in figure 6 and table 4.
From figure 6, which shows the semi-logarithmic fitting curve, and table 4, which presents the fitting parameters, it can be observed that the fitting correlation coefficients (R2) are all greater than 0.9893.This indicates that the application of the semi-logarithmic function formula provides a good fit for the viscositytemperature characteristics of asphalt.All asphalt materials exhibit a linear decreasing trend in viscosity as temperature increases.However, among them, the 3.5% SBR + 15% TPU composite modified asphalt shows the most significant variation, indicating a higher temperature sensitivity.This suggests that the 3.5% SBR + 15% TPU composite modified asphalt is more responsive to changes in temperature.After analyzing the fitting parameter M, it was observed that the modified asphalt, with the modifier added, has higher M values compared  to the 70# base asphalt.Furthermore, the trend of the M values corresponds to the trend of asphalt viscosity changes.Next, analyzing the fitting parameter N, which represents the slope of the fitting curve, the slopes corresponding to the 7 types of asphalt are as follows: −0.01525, −0.01546, −0.01594, −0.01578, −0.01631, −0.01582, and −0.01703.All of these slopes are negative, indicating a negative correlation between viscosity and temperature for these asphalt samples.Additionally, the absolute values of N for the modified asphalt are all greater than that of the 70# base asphalt.The larger the absolute value of N, the better the asphalt's temperature sensitivity.As previously analyzed, the composite modified asphalt containing 3.5% SBR and 15% TPU has the greatest absolute value of N, indicating that it has good workability, excellent deformability, and superior resistance to deformation.

Dynamic shear rheological test 3.2.1. Complex modulus G * analysis
This study explores the variation pattern of the complex modulus G * of TPU/SBR modified asphalt at different high temperatures through temperature sweeping experiments.Figure 7 depicts the experimental results.
Figure 7 demonstrates that the complex modulus G * of the asphalt decreases significantly with an increase in temperature.The complex modulus G * of the seven asphalt samples exhibits a significant decline in the temperature range of 34 °C to 52 °C.The reduction rate of the complex modulus G * slows down relatively when the temperature falls within the range of 52 °C to 82 °C.Moreover, after reaching 64 °C, the difference in the complex modulus G * becomes insignificant, almost overlapping.This reveals that the high-temperature performance of asphalt is heavily dependent on temperature, and its resistance to shear deformation diminishes during hot weather.The reason for this trend may be that at higher temperatures, the molecular components within the asphalt become more active, leading to a decrease in internal energy and gradual softening of the asphalt.As a result, its high-temperature resistance to deformation weakens, making it more susceptible to shear deformation under external loads.Moreover, under equivalent temperature circumstances, the modified asphalt has a consistently higher complex modulus G * than that of the base asphalt.The addition of TPU, SBR, and TPU/SBR modifiers have been observed to enhance the asphalt's deformation resistance.Among them, the  TPU/SBR composite modifier shows the most significant effect.At temperatures lower than 64 °C, the composite modified asphalt of TPU/SBR exhibits a significantly higher complex modulus G * than the modified asphalt of SBR.This infers that TPU can significantly improve the high-temperature performance of SBRmodified asphalt and bolster its resistance to external loads.The addition of TPU to the SBR-modified asphalt further improves its resistance to deformation.

Phase angle δ analysis
The temperature sweep test results for seven distinct kinds of asphalt are displayed in figure 8.According to figure 8, it can be observed that the phase angle δ of asphalt is positively correlated with temperature.As the temperature increases, the phase angle δ also increases, indicating an increase in the viscous components.Asphalt becomes more susceptible to softening under high-temperature conditions, leading to permanent deformation.Under the same temperature conditions, the matrix asphalt shows the highest phase angle δ, indicating significant alterations in the viscoelastic fractions of the asphalt after the admixture of modifiers.Increasing the modulus of elasticity and decreasing the modulus of viscosity improves the elastic recovery of the asphalt, effectively increasing its resistance to high temperature rutting.Furthermore, at the same temperature, the phase angle δ of the four composite modified asphalts is lower than that of SBR modified asphalt.Among the composite modified asphalts, the addition of TPU has the most significant influence on the phase angle δ.The addition of TPU to SBR modified asphalt effectively reduces the proportion of viscous components while increasing the amount of elastic components.This, in turn, enhances the asphalt's resistance to high-temperature deformation.When TPU and SBR are used for composite modification of the matrix asphalt, there is a significant interaction between the two modifiers.The interaction improves the composite modified asphalt's capacity to recover from deformation and enhances its high-temperature resistance to rutting performance.

Rutting parameter G * /sinδ analysis
The DSR test instrument was used to perform temperature sweep tests on seven different modified asphalt samples with varying dosages.The purpose was to investigate the variation pattern of asphalt G * /sinδ within the temperature range of 34 °C to 82 °C.The experimental results are shown in figure 9.
As shown in figure 9, the rutting parameter G * /sinδ of the asphalt is significantly reduced as the temperature rises.The reduction rate is rapid within the temperature range of 34 °C to 52 °C, while it decreases more gradually within the range of 52 °C to 82 °C.This trend is consistent with the variation of the complex modulus G * with temperature.It shows that there is a significant change in the rut factor of the asphalt between 34 °C and 52 °C.The reason for this phenomenon may be attributed to the fact that within this temperature range, as the temperature increases, the molecular movement within the asphalt becomes more intense, thereby weakening the intermolecular crosslinking ability to some extent.As a result, the asphalt softens and undergoes a flow deformation under external forces, reducing its resistance to deformation in high-temperature environments.Under the same temperature conditions, the incorporation of modifiers into the base asphalt can enhance its G * /sinδ under the same test temperature.Additionally, in composite modified asphalt, the larger the TPU dosage, the more pronounced the increase in G * /sinδ.This means that TPU can effectively boost the rutting strength of both the base asphalt and the SBR-modified asphalt under high-temperature conditions.

Dynamic complex modulus primary curve analysis
The time-temperature equivalence principle was used to study the variation of the rheological properties of asphalt under high temperature conditions as a function of frequency.Additionally, the WLF equation and Sigmoidal function were combined to fit and process the frequency-modulus curves of seven types of asphalt at a reference temperature of 64 °C.The resulting complex modulus primary curves at 64 °C are shown in figure 10.
According to figure 10 it can be observed that at the test temperature of 64 °C, the asphalt exhibits a lower complex modulus G * at lower loading frequencies.The complex modulus G * of the asphalt increases as the loading frequency increases, demonstrating a positive correlation between the asphalt's complex modulus G * and frequency.This indicates that the resistance of the asphalt to deformation increases as the frequency of loading increases.Consequently, the asphalt pavement's ability to withstand rutting deformation during its service life is also improved.This is because at higher loading frequencies, the external load acts on the asphalt for a shorter period of time, resulting in a smaller range of stress diffusion within the asphalt or even insufficient  time for stress diffusion to occur.Additionally, the asphalt molecules are interconnected, providing overall stability and enabling it to withstand certain loadings.The base asphalt has a lower complex modulus G * than the other six modified asphalts under the same frequency conditions.This indicates that incorporating TPU, SBR, and TPU/SBR modifiers can enhance the high-temperature resistance of the asphalt.Furthermore, the complex modulus G * of the four composite-modified asphalts is greater than that of the SBR-modified asphalt.These results suggest that incorporating TPU modifiers can enhance the high-temperature performance of SBR elastomers.Furthermore, the study indicates that the improvement becomes increasingly significant with higher TPU content.This finding aligns with the conclusions drawn from the previous asphalt temperature sweep tests.

Multiple stress creep rheological (MSCR) test 3.3.1. The stress-strain relationship
The strain characteristics of the 7 different types of asphalt during the 10th and 1st cycle periods are shown in figures 11 and 12.
The following conclusions can be drawn based on the analysis of figures 11 and 12: a.The stresses of the seven asphalt types increase rapidly during the 0-1 s loading time and reach their maximum value at 1 s.Then, during the unloading phase of 9s, a rebound phenomenon occurs, indicating a certain degree of deformation recovery during the unloading stage.b.In the same cycle period, the strain of base asphalt is consistently greater than that of TPU-modified asphalt and SBR-modified asphalt at stress levels of 0.1 KPa and 3.2 KPa, suggesting that TPU and SBR modifiers enhance the high-temperature deformation resistance of asphalt.Of those, TPU shows a relatively superior enhancing effect.
c.In the first cycle period, the strains of the base asphalt and SBR-modified asphalt are higher compared to those of the composite-modified asphalt at different stress levels.Among them, the asphalt modified with 3.5% SBR and 15% TPU shows the lowest strain, indicating that TPU/SBR composite modifiers can significantly enhance the asphalt's resistance to external load deformation.
d.Over the course of 10 cycles, it is observed that all types of asphalt, when subjected to stress levels of 0.1 KPa and 3.2 KPa, exhibit the largest cumulative residual strain in the base asphalt and the smallest cumulative residual strain in the asphalt modified with 3.5% SBR and 15% TPU.Furthermore, it is found that the strains of all types of asphalt gradually increase.This indicates that after the loading phase, only partial recovery of deformation occurs during the unloading phase.However, with the addition of TPU and SBR, the recovery deformation of the asphalt increases.The synergistic effect of TPU and SBR significantly enhances the asphalt's elastic recovery and resistance to permanent deformation.

R and jnr analysis
R denotes the asphalt's capacity for elastic recovery under high-temperature conditions, while Jnr signifies the proportionality between the average residual strain of asphalt within a specified time period and the applied external force, thereby reflecting the asphalt's resistance to permanent deformation in elevated temperature environments.The values of R and Jnr for the seven distinct types of asphalt can be found in figure 13.By analyzing the data presented in and figure 13, the following conclusions can be drawn: a.In comparison to the stress level of 0.1 KPa, it is observed that at the higher stress level of 3.2 KPa, all types of asphalt exhibit a certain degree of decrease in R values.This indicates an increase in residual strain under higher stress conditions, suggesting a weakened ability of asphalt to resist deformation at elevated temperatures.This phenomenon can mainly be attributed to the limited capability of asphalt to withstand deformations, as repeated high-load applications can accumulate more damage and deformation, resulting in a limited capacity for immediate recovery.Consequently, the pavement made of such asphalt is prone to developing distresses such as rutting.b.When subjected to two different stress levels of 0.1 KPa and 3.2 KPa, the matrix asphalt exhibits the lowest R values, while the 3.5%SBR+15%TPU composite modified asphalt exhibits the highest R values.In the case of a 0.1 KPa loading stress, the R values of the six modified asphalts increased relative to the matrix asphalt by 0.3860%, 1.9766%, 0.4025%, 3.5352%, 0.4429%, and 3.7540% respectively.When subjected to a loading stress of 3.2 KPa, the R values of the six modified asphalts increased relative to the matrix asphalt by 2.3080%, 5.0917%, 2.5564%, 5.9797%, 2.9579%, and 6.1587% respectively.This indicates that the incorporation of TPU and SBR into asphalt results in an effective increase in the elastic component of the asphalt, thereby enhancing the elastic recovery performance of the modified asphalt.
c.The analysis of the Jnr values for the same asphalt reveals that Jnr3.2 is significantly higher than Jnr0.1, indicating a variation in the proportion of viscoelastic components within the asphalt as a result of different stress levels.This variation leads to changes in the performance of the asphalt, specifically, an increase in the viscous component and a corresponding decrease in the elastic component under higher stress levels.Consequently, the recoverable deformation during asphalt deformation decreases, while the accumulated residual deformation increases.This can cause the entire asphalt system to become deformed at high temperatures.
d.Under two different stress levels of 0.1 KPa and 3.2 KPa, the Jnr3.2 values of the six modified asphalts are significantly lower than that of the matrix asphalt, with the 3.5%SBR+15%TPU composite modified asphalt exhibiting the lowest Jnr3.2 value.Under a loading stress of 0.1 KPa, the Jnr values of the six modified asphalts decreased relative to the matrix asphalt by 0.7492%, 1.3797%, 1.1483%, 1.4656%, 1.2444%, and 1.7529%, respectively.Under a loading stress of 3.2 KPa, the Jnr values of the six modified asphalts decreased relative to the matrix asphalt by 0.8993%, 1.6884%, 1.3753%, 1.8030%, 1.5031%, and 2.2618%, respectively.Incorporating TPU, SBR, and TPU/SBR modifications improves the high-temperature performance of asphalt, with TPU/SBR modification exhibiting the most significant enhancement.These findings are consistent with the results of previous experimental studies, such as Rotational Viscosity tests, and temperature sweep tests.

Rdiff and jnr-diff analysis
The parameters Rdiff and Jnr-diff are widely used to measure the stress-sensitivity of asphalt, with the stresssensitivity of asphalt increasing as these parameters increase, leading to a decrease in asphalt stability.The Rdiff and Jnr-diff values for seven different types of asphalt are depicted in figure 14. Analysis of the experimental results presented in and figure 14 reveals that under the test temperature of 64 °C, the values of Rdiff and Jnr-diff for the matrix asphalt are higher than those of the six modified asphalts.This indicates that the matrix asphalt exhibits the highest stress-sensitivity and relatively poor high-temperature stability.It is susceptible to deformation under external loading at high temperatures.Incorporating TPU and SBR modifiers into asphalt can enhance its stress-sensitivity.Additionally, the Rdiff and Jnr-diff values for the four modified asphalts are lower compared to the SBR modified asphalt.These results suggest that TPU enhances the stress-sensitivity of SBR modified asphalt and contributes to improving the overall stability of the composite.

Bending beam rheological (BBR) test
Figure 15 shows the creep stiffness and creep rate of seven types of asphalt at a temperature of 12 °C.
According to relevant research studies, the creep stiffness S must not exceed 300 MPa after 60 seconds of creep loading, and the creep rate m should be a minimum of 0.3.Asphalt exhibits good resistance to cracking under low-temperature conditions [35]. Figure 15 shows that the creep stiffness S of all asphalt samples does not exceed 300 MPa and that the samples' creep rate m is not less than 0.3, which falls within the specified range required by the standards.The addition of SBR modifier to asphalt has been observed to significantly enhance its stress relaxation ability under low-temperature conditions, effectively improving its low-temperature performance.The creep stiffness (S) is lowest, and the creep rate (m) is highest for SBR modified asphalt.Conversely, TPU modified asphalt exhibits the highest creep stiffness S and the lowest creep rate m, indicating that the addition of TPU results in a relatively brittle characteristic under low-temperature conditions.The capacity of the asphalt to relax stress is decreased, making it susceptible to cracking at low temperatures.These findings suggest that TPU has a negative impact on the ability of asphalt to resist cracking in low-temperature conditions.In composite modified asphalt containing TPU and SBR, fixing the SBR content leads to an increase in creep stiffness S and a decrease in creep rate m of the asphalt as the TPU content increases.When the TPU content is held constant, an increase in SBR content results in reduced creep stiffness, represented by S, and increased creep rate, represented by m, of the asphalt.The incorporation of SBR greatly enhances the crack resistance of the asphalt at low temperatures and reduces the possibility of low-temperature cracking.Moreover, it efficiently mitigates the negative impacts of TPU on the low-temperature performance of asphalt.It improves the low-temperature deformation resistance of TPU-modified asphalt while also providing outstanding crack resistance to the modified composite asphalt, thus enhancing its service life.

Fluorescent microscopy (FM) test
This study assessed the tolerance of the modifier to asphalt by examining the distribution of fluorescent markers in FM images.Additionally, the microstructural morphology of the modified asphalt was analyzed to gain insights into its microscopic phase structure.Figure 16(a) presents the fluorescence image of the pristine asphalt matrix, characterized by a uniform background color without any discernible fluorescent markers.This observation that the pristine asphalt possesses a homogeneous phase structure.Figures 16(b)-(e) display the fluorescence images of the modified asphalt, revealing the presence of dispersed fluorescent markers of varying sizes.This observation indicates that the modifier generates corresponding fluorescence under the ultraviolet illumination of the fluorescence microscope, thereby transforming the phase structure of the asphalt from a homogeneous phase to a dispersed phase.
Figure 16(b) illustrates the fluorescence image of SBR-modified asphalt, demonstrating an irregular distribution of fluorescent markers of varying sizes.Some fluorescent markers appear be aggregated, indicating a partial incompatibility between the incorporated SBR modifier and the asphalt matrix.This incompatibility is macroscopically manifested by a relatively poor storage stability of the SBR-modified asphalt.It is significant to note that certain fluorescent markers show more extensive shapes, which may be due to uneven shearing during the processing of SBR-modified asphalt.
The fluorescence image of TPU-modified asphalt is presented in figure 16(c), which demonstrates a lower density of fluorescent markers in comparison to SBR-modified asphalt.Moreover, the distribution of fluorescent markers in the TPU-modified asphalt appears to be more uniform.This observation suggests that TPU can effectively integrate into the asphalt system and become a constituent part of the asphalt's internal structure.This phenomenon can be attributed to the reaction between TPU and the asphalt matrix, which reduces the polarity difference between the heavy and light components within the modified asphalt.This results in significantly improved compatibility between the TPU and the asphalt matrix.
Figures 16(d) and (e) depict the fluorescence images of composite-modified asphalt with 3.5% SBR and 10% TPU, and 3.5% SBR and 15% TPU, respectively.Both figures demonstrate that, at a constant SBR content, the fluorescence markers in the composite-modified asphalt decrease in relative abundance with an increase in TPU content.Additionally, the fluorescence images of asphalt modified with composite materials show a higher quantity of fluorescence markers than asphalt modified with TPU when compared.In comparison to SBRmodified asphalt, the fluorescence images of the composite-modified asphalt indicate a reduction in the number of fluorescence markers.Moreover, the distribution of fluorescence markers in the composite-modified asphalt is more uniform without any observable clustering phenomenon.These observations indicate a good compatibility between TPU, SBR, and the asphalt matrix.The TPU/SBR composite-modified asphalt possesses a favorable phase structure, and TPU effectively mitigates the potential segregation issues that may arise in SBRmodified asphalt.Consequently, the TPU/SBR composite-modified asphalt can demonstrate improved overall performance.

Conclusions
To evaluate the modification effect of TPU and SBR composite modified asphalt, high and low temperature rheological tests were performed.Microstructure analysis was also performed.On this basis, we conclude that: (1) The addition of thermoplastic polyurethane (TPU) and styrene-butadiene rubber (SBR) to asphalt matrix substantially enhances its viscosity, which concurrently improves asphalt's resistance to high temperature deformation.
(2) The complex shear modulus (G * ) and rutting parameter (G * /sinδ) of TPU/SBR composite modified asphalt were found to be higher compared to that of SBR modified asphalt.Conversely, the change in the phase angle (δ) exhibited an opposite trend.These results indicate that TPU enhances the ability of SBR modified asphalt to resist external load.
(3) The TPU/SBR composite modified asphalt showed a significantly larger modulus enhancement trend in the main stress-strain curve.The differences were not significant in the low-temperature high-frequency range, indicating suitability for a wide frequency domain and temperature range.
(4) The incorporation of TPU contributes to the improvement of stress sensitivity in SBR modified asphalt, thereby enhancing the overall stability of the composite modified asphalt.
(5) TPU effectively minimizes the potential segregation in SBR modified asphalt, thus enhancing the overall performance of the composite modified asphalt.

Figure 1 .
Figure 1.The framework and research program.

Figure 2 .
Figure 2. The preparation process of composite modified asphalt.

3. 1 .
Rotational viscosity test 3.1.1.Viscosity analysis The study measured the viscosity of seven distinct types of asphalt under high-temperature circumstances.The experimental results are shown in figures 3, 4, and 5.

Figure 7 .
Figure 7. Curve of asphalt complex modulus with temperature.

Figure 8 .
Figure 8. Curve of asphalt phase angle with temperature.

Figure 9 .
Figure 9. Curve of asphalt rutting factor with temperature.

Figure 10 .
Figure 10.Master curve of complex modulus of asphalt at 64 °C.

Figure 13 .
Figure 13.Average creep recovery rate R and average irrecoverable creep compliance J nr of asphalt under the stress level of 0.1 KPa and 3.2 KPa: (a) R; (b) J nr .

Figure 15 .
Figure 15.Creep stiffness S and creep rate m of asphalt at a test temperature of −12 °C.

Table 1 .
Technical performance indicators of 70# base asphalt.

Table 3 .
Basic properties of SBR.

Table 4 .
Fitting parameters of asphalt viscosity.