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Article

Investigating the Rheological Impact of USP Warm Mix Modifier on Asphalt Binder

by
Yali Liu
1,2,
Jingfei Ping
3,
Hao Guo
4,
Yikai Kang
1,2 and
Yali Ye
1,2,*
1
School of Transportation and Civil Engineering, Shandong Jiaotong University, Jinan 250357, China
2
Digitalization of Transportation Infrastructure and Long Term Service Performance Technology Shandong Engineering Research Center, Jinan 250357, China
3
Shandong Hi-Speed Traffic Construction Group Co., Ltd., Jinan 250014, China
4
Shandong Hi-Speed Maintenance Group Co., Ltd., Jinan 250014, China
*
Author to whom correspondence should be addressed.
Coatings 2025, 15(7), 784; https://doi.org/10.3390/coatings15070784
Submission received: 15 May 2025 / Revised: 17 June 2025 / Accepted: 30 June 2025 / Published: 3 July 2025
(This article belongs to the Special Issue Surface Treatments and Coatings for Asphalt and Concrete)
Browse Figures
Versions Notes

Abstract

USP (usual temperature pitch)-modified asphalt optimizes its rheological properties through reactions between the modifier and the asphalt. This significantly enhances the high- and low-temperature adaptability and environmental friendliness of asphalt. It has now become an important research direction in the field of highway engineering. This article systematically investigates the impact of different dosages of USP warm mix modifier on asphalt binders through rheological and microstructural analysis. Base asphalt and SBS-modified asphalt were blended with USP at varying ratios. Conventional tests (penetration, softening point, ductility) were combined with dynamic shear rheometry (DSR, AASHTO T315) and bending beam rheometry (BBR, AASHTO T313) to characterize temperature/frequency-dependent viscoelasticity. High-temperature performance was quantified via multiple stress creep recovery (MSCR, ASTM D7405), while fluorescence microscopy and FTIR spectroscopy elucidated modification mechanisms. Key findings reveal that (1) optimal USP thresholds exist at 4.0% for base asphalt and 4.5% for SBS modified asphalt, beyond which the rutting resistance factor (G*/sin δ) decreases by 20–31% due to plasticization effects; (2) USP significantly improves low-temperature flexibility, reducing creep stiffness at −12 °C by 38% (USP-modified) and 35% (USP/SBS composite) versus controls; (3) infrared spectroscopy displays that no new characteristic peaks appeared in the functional group region of 4000–1300 cm−1 for the two types of modified asphalt after the incorporation of USP, indicating that no chemical changes occurred in the asphalt; and (4) fluorescence imaging confirmed that the incorporation of USP led to disintegration of the spatial network structure of the control asphalt, explaining the reason for the deterioration of high-temperature performance.

1. Introduction

In the field of asphalt pavement, hot mix asphalt technology has become a widely used method. But its production process has the problems of high energy consumption and large emissions of toxic and harmful flue gas, which pose a serious threat to human health [1,2,3]. In order to promote highway infrastructure to achieve the goal of carbon peak and carbon neutrality, the advancement of warm mixing technology aimed at lowering construction temperatures has emerged as a key strategy to foster environmentally friendly and low-carbon progress within the highway construction sector [4]. In recent years, researchers around the world have begun to explore warm mix asphalt technology [5,6]. Compared with hot mix asphalt, WMA can effectively reduce the construction temperature, reduce the gas and smoke generated during the construction process, improve the ability of asphalt to resist low-temperature cracks and high-temperature deformation, promote recycling of resources, reduce waste of resources [7,8,9], and significantly improve the performance of asphalt pavement. At present, WMA technology has been applied in all countries around the world and has good economic and environmental benefits. Technological research and the development of various warm mix agents have also been gradually utilized and upgraded.
At this stage, warm-mix technology, both domestically and internationally, primarily achieves its effects through the addition of warm-mix agents [10,11,12]. The characteristics of various warm mixing methods differ from one another. While achieving a cooling effect, they also have different impacts on the performance of the asphalt binder [13]. According to their mechanisms of action, existing warm-mix additives on the market can be classified into three main categories [14]. The first category consists of organic additives, such as Sasobit and others. By increasing dispersion between molecules in the asphalt colloid, the goal of enhancing fluidity is achieved [15]. The second category consists of surfactant-type additives, such as Evotherm [16]. As surfactants, these additives can reduce energy consumption during the asphalt production process. They can also be directly mixed with asphalt mixtures, enhancing the workability and ease of application of asphalt [17]. The third category involves the technology of mineral foaming, such as that described by Advera et al. The method, combining foaming technology and surfactants, can restore aged asphalt to a certain extent, and foamed Evotherm DAT significantly reduces the softening point and viscosity [18]. The rheological properties of warm-mixed asphalt can effectively reflect the performance of the pavement [19,20]. Yuan et al. [21] found that Sasobit promoted the compatibility of desulfurized rubber powder and asphalt. With the addition of Sasobit, low-temperature performance first increases and then decreases. When 2.5% Sasobit is incorporated, the modified asphalt mixture exhibits the best overall performance. Adwani et al. performed an extensive rheological assessment of the warm mix modifier asphalt binder, showing enhanced performance at elevated temperatures [22]. Ji et al. studied the viscosity reduction effect of surfactants when organic wax was below 145 °C through basic performance tests of four kinds of warm mix technology and six kinds of warm mix rubber asphalt [23]. Xu et al. [24] and Gui et al. [25] developed a fractional derivative model along with a generalized fractional viscoelastic model to assess the performance of W-CRABs under both high- and low-temperature conditions. The findings revealed that a combination of 0.6% surfactant and CRABs yielded the best resistance to cracks and wrinkles. Hai et al. statistically analyzed the effects of eight typical waxes on the low-temperature properties of asphalt. The results showed that F-T wax and amide wax significantly increased the thermal stress of asphalt [26]. Farsha et al. [27] and Khan et al. [28] found that RAP (reclaimed asphalt pavement) and rubber-modified asphalt have comparable performance and better fatigue resistance than traditional HMA (hot mix asphalt) in WMA technology. Kok et al. investigated the modified adhesion of SBS after mixing with Evotherm. It was found that the Evotherm modification could reduce the degree of stiffness increase [29]. Xiao et al. found that, in foamed warm mix asphalt, regardless of the foam water content and compaction temperature, the ITS value of the foam mixture increases with increased compaction temperature. It significantly outperforms biological regenerants in enhancing fatigue performance [30]. With increased age, the low-temperature crack resistance of asphalt gradually deteriorates. Zhang et al. studied the low-temperature performance of warm mix recycled asphalt. It was found that the crack resistance of asphalt after thermal aging and ultraviolet aging gradually deteriorated at low temperature. Addition of the warm mixing agent enhances the fluidity of the asphalt, further improving its low-temperature crack resistance [31]. Li et al. evaluated the regenerative warm mix effect on aged SMB using reactive warm mix regenerant (RWR) [32].
The viscosity of warm mix modifiers is reduced by different methods. Solving the problem of insufficient low-temperature performance has become the focus of research. USP is a new type of asphalt warm mix modifier, which is mainly prepared by mixing recycled waste rubber and waste plastics with a certain proportion of organic additives [33]. USP alters the molecular dynamics between adhesives through physical methods. The high temperature viscosity of asphalt can be greatly diminished, which helps in averting cracking of its components [34,35,36], effectively improving the low temperature cracking resistance of asphalt mixtures and stability under high temperatures, and other road performance of asphalt mixtures. It also achieves effective smoke suppression and gas purification during the paving process, leading to a noticeable reduction in environmental pollution [37].
At present, domestic and foreign scholars have produced little research on domestic warm mix modifiers, highlighting an urgent need for more comprehensive studies on both the macro and micro characteristics of these modifiers. It is essential to categorize warm mix agents and assess how they differently affect the performance metrics of asphalt mixtures [38,39,40]. Research on the performance assessment of warm mix-modified asphalt is sparse, particularly concerning its influence on fundamental properties, resistance to water damage, and overall durability [41]. So, diverse USP warm mix asphalt variants and USP/SBS composites were created via manipulation of the USP modifier. The investigation focused on analyzing the viscoelastic and rheological properties of the asphalt [42].

2. Material and Methods

2.1. Material

2.1.1. Asphalt

Finished 70# matrix asphalt and SBS-modified asphalt were used. The basic technical indicators of the two are shown in Table 1.

2.1.2. USP-H Warm Mix Modifier

The H-type USP warm mix modifier is a polymer elastomer crafted through a specialized process. It originates from the USP low-temperature modified asphalt additive of Zhongyou Lu Zhixing New Materials Co., Ltd. Zhengzhou, China, utilizing raw materials such as recycled tire rubber powder, polymer resin, coupling agents, waste plastics, and surfactants. This innovative compound significantly reduces asphalt viscosity, enhancing the workability of asphalt mixtures during construction. Additionally, it allows for notably lower mixing and rolling temperatures compared to traditional hot mix asphalt. Detailed technical specifications for the USP warm mix modifier can be found in Table 2.

2.2. Preparation of Modified Asphalt

The USP additive was heated in a 120 °C oven for 20 min to liquefy it. Different amounts of USP were added to matrix bitumen and SBS-modified bitumen.

2.2.1. USP-Modified Asphalt (USP-A)

First, the matrix asphalt was heated to 145 °C and placed in a constant-temperature heating jacket for insulation. Then the melted USP was poured into the asphalt (3.0%, 3.5%, 4.0%, 4.5%, and 5.0%) according to the predetermined dosage and stirred evenly. Then it was placed in a 145 °C shear machine and subjected to 700 r/min shear for 45 min. Finally, the sheared samples were placed in an oven at 145 °C for 30 min to obtain the USP-modified asphalt (USP-A).

2.2.2. USP/SBS Composite-Modified Asphalt (USP/SBS-A)

USP was poured into SBS-modified asphalt (4.0%, 4.5%, 5.0%, 5.5% and 6.0%) according to the same steps and stirred evenly. Then it was cut at 900 r/min for 45 min in a 145 °C shear machine. Finally, the sheared samples were placed in an oven at 145 °C for 30 min to obtain the USP/SBS composite-modified asphalt (USP/SBS-A). The steps are shown in Figure 1.

2.3. Asphalt Aging

Short-term aging is simulated by Thin Film Over Test (TFOT) for short-term aging of four types of bitumen. Each type of asphalt shall be weighed to 50 g ± 0.5 g and placed into the aging turntable and then placed into a film oven to maintain at a temperature of 163 °C ± 1 °C for 5 h for the preparation of short-term aging samples. For long-term aging, at the end of short-term aging, the sample tray is placed into the PAV (pressurized-aging vessel) pressure-aging instrument. The air pressure is adjusted to 2.1 MPa, the test temperature is set to 100 °C, and a test is conducted for 20 h ± 10 min to prepare long-term aging sample.

2.4. Test Scheme

2.4.1. Basic Performance Testing

The basic performance tests primarily include penetration, softening point, and ductility tests. The needle penetration test measures the distance a standard needle penetrates the sample under a load of 100 g at a temperature of 25 °C for 5 s. The drop temperature of the asphalt sample indicates the softening point. The ductility test is conducted under fixed conditions, stretching the sample at a speed of 5 cm/min until fracture occurs, at which point the change in length is measured.

2.4.2. Temperature Sweep Test

The original and short-term aged samples of four asphalt samples were tested using a dynamic shear rheometer. In this experiment, 46 °C is set as the starting temperature, with a temperature gradient of 6 °C. Based on the temperature scan, the ratio of the complex modulus (G*) to the phase angle (δ), denoted as the rutting factor (G*/sin δ), can be derived.

2.4.3. Multiple Stress Creep Recovery

Twenty cycles of cyclic loading are conducted under a pressure of 0.1 kPa. The last 10 occurrences are selected for data processing, conducting 10 cycles at a pressure of 3.2 kPa. A load of 10 rad/s is applied at 64 °C. Ultimately, the non-recoverable creep compliance (Jnr) and recovery rate (R) serve as evaluation indicators for polymer-modified asphalt.

2.4.4. Bending Beam Rheometer (BBR) Test

An asphalt sample beam with a length of 127 mm × width of 6.35 mm × height of 12.7 mm is fabricated. The prepared asphalt beam is placed into the BBR constant-temperature domain for 1 h. A load of 980 ± 50 mN is applied to the sample within 1 s to conduct a low-temperature bending beam rheological test. The creep stiffness (S) and creep rate (m) are obtained.

2.4.5. Micro Performance Test

Fourier infrared spectrometer was used to test different asphalts. The scanning range was set at 4000 cm−1~400 cm−1, and the temperature was set at 25 °C to explore changes in the internal functional groups of the modified asphalt and the evolution of characteristic peaks for testing and analysis. A fluorescence microscope was used to observe whether the mixing state was the best after USP was mixed with two kinds of asphalt, observe trend change in the internal phase state after magnification 100 times, and analyze its internal phase structure.

3. Results and Discussion

3.1. Basic Performance Analysis

As shown in Figure 2a, USP increases the penetration values of both types of asphalt. As the USP increases, the penetration of USP/SBS-A becomes more pronounced. The increase in USP will result in softening of the asphalt and a reduction in its viscosity. As shown in Figure 2b, adding USP will decrease asphalt’s softening point by roughly 5% to 10%. Moreover, the proportion of USP is inversely related to the softening point; the higher the doping concentration, the lower the softening point. The increase in USP will enhance the fluidity of the asphalt, while reducing its high-temperature performance. Under three different conditions, as aging increases, the softening point gradually rises. In Figure 2c, the addition of USP improves asphalt ductility, which declines progressively with advanced aging. The explanation indicates that the addition of USP will cause the matrix asphalt and SBS-modified asphalt to soften, reduce their viscosity, and decrease their high-temperature stability. The low-temperature ductility of USP-A and USP/SBS-A has been significantly improved. In a study by Dong et al. it was found that the penetration and ductility of the matrix asphalt decreased to varying degrees after adding Sasobit and Evotherm-DAT to the matrix asphalt [43]. Both characteristics were inversely proportional to the penetration and ductility of the USP warm mix modifier. The results show that USP warm mix modifier has a strong ability to plastically deform asphalt at low temperature.

3.2. Analysis of Temperature Scanning Test Results

3.2.1. G* and δ

According to Figure 3a, it can be observed that the complex modulus of the matrix asphalt, the SBS-modified asphalt, and the modified asphalt after incorporating USP modification gradually decreases with increasing temperature. Higher temperatures reduce asphalt’s resistance to deformation. In Figure 3a, the phase angle of the matrix asphalt gradually increases compared to modified asphalt. The phase angle of SBS-modified asphalt and its composite modified asphalt increases and then stabilizes after reaching 70 °C. The overall complex modulus of the asphalt after the addition of USP is lower than that of matrix asphalt and SBS-modified asphalt. With the increase in USP, the complex modulus gradually decreases, while the phase angle gradually increases. This indicates that the addition of USP changes the viscoelastic properties of the two basic asphalts, resulting in increased viscosity.
The trend of two indicators after aging of the asphalt mixed with USP warm mix modifiers is roughly similar to that of the unaged asphalt, as shown in Figure 3b. The complex modulus has increased, while the phase angle has decreased. This indicates that aging can significantly enhance the deformation resistance of asphalt.

3.2.2. G*/sin δ

It can be seen from Figure 4a, the rutting factor of the base asphalt after adding USP begins to decrease. However, when the content is 3%~4%, the amplitude is not much different from that of matrix asphalt. When the dosage is further expanded, the rutting factor will be greatly reduced. This indicates that, when the content is 5%, the polymer in the USP will significantly disrupt the stability of the asphalt colloid structure, breaking the internal molecular chains at high temperatures and thereby increasing the free state in the asphalt. The rupture of molecular chains. eventually leads to a decrease in high-temperature stability. Similarly, according to Figure 4b, the rutting factor of USP/SBS-A with 4.5% USP content before and after aging is not significantly different from that of SBS itself. However, as the amount of USP warm mix modifier continues to increase, the rut factor will decrease significantly. The reason is that SBS itself is a chain network structure. The addition of USP in the early stage did not destroy the physical structure of SBS itself and had little effect on it. However, after increasing the amount of USP, the organic additives in the USP will accelerate destruction of the SBS network structure, so that its good stability will decrease with the addition of USP. Consequently, the impact of the USP on both matrix asphalt and SBS-modified asphalt remains consistent across various aging stages. It is evident that the USP warm mix modifier adversely affects the high-temperature stability of the asphalt itself. In a study by Wang et al., it was shown that Sasobit exists in the form of crystals in asphalt, which greatly improves the rutting resistance of asphalt [44]. However, Evo, ACMP, etc. are similar to USP, and Jun has a certain weakening effect on high-temperature performance.
Based on analysis of the figures, it can be observed that, when the USP warm mix modifier content in matrix asphalt is at 4%, and in SBS-modified asphalt is below 4.5%, the effect on high-temperature performance is minimal. When there are more than two kinds of dosage ratios, the complex modulus will have a greater degree of decline. Therefore, it can be concluded that the optimal content of the USP warm mix modifier for matrix asphalt is 4%, while that for SBS-modified asphalt is 4.5%.

3.3. Analysis of Creep Recovery Test Results

3.3.1. Stress and Strain

As shown in Figure 5, under a stress of 0.1 kPa, the strain variation of USP-A decreases with the increase in dosage. Under a stress load of 3.2 kPa, the difference in the response of USP warm mix-modified asphalt to strain changes continues to increase, indicating that, at higher stress levels, the accumulation of strain in asphalt is significantly impacted. Addition of the USP warm mix modifier can improve viscous deformation of the matrix asphalt at high temperatures, thereby reducing its viscous deformation. In the case of USP/SBS-A, the initial strain response is more substantial. This is attributed to the lower polymer content and higher oil content, which disrupt the SBS network structure, preventing multi-chain cross-linking reactions within the material. Consequently, the asphalt’s ability to resist strain diminishes when the USP warm mix modifier content is low. However, once the USP content reaches 4.5%, the strain changes in SBS-modified asphalt are significantly minimized, thereby optimizing its performance.

3.3.2. R and Jnr

As illustrated in Figure 6a, as the concentration of the USP warm mix modifier increases, there is a corresponding rise in the strain recovery rate. The USP warm mix modifier significantly boosts the asphalt’s ability to bounce back from strain, enabling it to swiftly rectify any deformation incurred at elevated temperatures. But the strain recovery rates at 3.2 kPa fall short of those recorded at 0.1 kPa. This discrepancy can be attributed to the asphalt’s partially viscous state at higher temperatures. Under substantial heat and stress, the molecular chains are more prone to rupture when subjected to external loads. Consequently, the recovery of viscosity in asphalt slows down, leading to irreversible plastic deformation, which ultimately results in a diminished strain recovery rate at 3.2 kPa.
According to Figure 6b, compared with the unrecoverable creep compliance Jnr of different asphalts, when different doses of USP warm mix modifier are added, the Jnr continues to decrease, and its value is close to 1, that is, the category of heavy traffic volume. The Jnr of SBS-modified asphalt falls into the range of 0 to 0.5, indicating that its traffic volume is in the category of heavy traffic. Similarly, with the addition of the USP warm mix modifier, the irreversible creep compliance continues to decrease. However, it will continue to increase after 4.5% USP content, indicating that excessive USP warm mix modifier will cause an extensive range of damage to the structure of SBS, thereby diminishing the structural stability of the composite-modified asphalt. Ultimately, the irreversible creep compliance rises with a USP content between 5% and 6%. In summary, these results show that the USP warm mix modifier not only bolsters asphalt’s resistance to deformation, but also enhances its creep recovery attributes.

3.4. Analysis of Low-Temperature Bending Creep Test Results

3.4.1. S and m

According to the low-temperature performance characterized by the modulus of rigidity and creep rate, a smaller S value indicates stronger low-temperature toughness of asphalt, whereas a larger m value signifies a greater relaxation capability of asphalt, resulting in overall better low-temperature performance. Figure 7 reveals that, within a temperature range of −12 °C to −18 °C, both base asphalts exhibit elevated stiffness moduli alongside diminished creep. Adding the USP warm mix additive induces a decline in modified asphalt S with a rise in its creep rate, and the changes observed are consistent for both types of base asphalt. Figure 7b,c clearly demonstrate that, as the asphalt undergoes aging, thermally oxidized asphalt exhibits a noticeable rise in stiffness modulus alongside a corresponding decline in creep rate. The primary reason is that, after high-temperature aging, a substantial amount of the lighter components of asphalt will evaporate, resulting in a reduction of free substances within its internal space, which leads to transformation into a solid state.
As indicated in Figure 7c, after long-term aging, at −24 °C, the value of the creep rate m for SBS-modified asphalt has fallen below 0.3, indicating that it no longer meets the standard specifications. But with the increase of USP warm mix modifiers, the creep rate has shot up. Integrating USP markedly boosts resistance to low-temperature cracking in both the asphalt matrix and SBS asphalt. Additionally, it was also observed that the ductility of bitumen increased significantly after different degrees of aging.

3.4.2. S/m

The low-temperature crack resistance and ductility of different asphalts at various temperatures were studied using the S/m ratio. A reduced modulus S correlates with increased creep rate m, diminishing the S/m ratio. This shows that the low-temperature performance of the asphalt is better at this temperature, and a smaller S/m value across different conditions suggests that the asphalt exhibits greater stability in various aging states.
Figure 8 illustrates that the S/m ratio of asphalt varies greatly as the temperature drops. This indicates that lower temperatures and extended aging result in the evaporation of lighter components and oil within the asphalt. Consequently, this process stiffens the asphalt, shifting its viscoelastic properties toward partial elasticity. As the active components diminish, there is a reduction in molecular rheology, ultimately compromising performance at lower temperatures.
In its initial state, the S/m value declines as the USP content rises, indicating that incorporating USP markedly boosts the asphalt’s resistance to cracking and ductility at low temperatures. Following short-term aging, the S/m values for both the base asphalt and the SBS-modified asphalt saw increases of 41.1% and 29%, respectively, relative to their original values. In contrast, the asphalt blended with the USP warm mix modifier experienced increases of 23% and 30% post-aging. As the USP dosage escalates, the rate of change in the S/m value diminishes, with a correspondingly smaller increase. This highlights the substantial improvement in the asphalt’s low-temperature performance resulting from the USP addition. The underlying cause is that oil within the USP modifier amplifies the activity of its free components. Compared to asphalt lacking the USP modifier, the proportion of lighter components increases overall after aging, which reduces the asphalt’s hardness and thereby improves its performance at lower temperatures.

3.5. Analysis of Microscopic Test Results

3.5.1. Analysis of Infrared Spectrum Results

From Figure 9, it can be observed that the absorption peaks at 2958 cm−1, 2916 cm−1, and 2847 cm−1 correspond to the C-H symmetric and asymmetric stretching vibration peaks of methyl (-CH3) and methylene (-CH2) groups. The absorption peak at 1602 cm−1 corresponds to the stretching vibration peak of the C=C bond in water adsorption. The explanation pertains to the structure of aromatic compounds. The absorption peak at 1453 cm−1 corresponds to the bending vibration peak of the methylene group. The absorption peak at 1375 cm−1 corresponds to the bending vibration peak of the methyl group. The appearance of the above absorption peaks indicates that the components in the matrix asphalt are primarily long-chain alkane compounds. After joining USP, no new peaks appeared, and there were no significant changes in the positions of the absorption peaks. The modification process of warm-mixed asphalt is a physical change; this process does not create new substances and does not involve any chemical changes.
In Figure 9, the spectrum of SBS asphalt shows two new peaks at 966 cm−1 and 699 cm−1 when compared to the matrix asphalt. These are the C-H and C=C vibrations. This indicates successful modification of matrix asphalt by SBS. Furthermore, after the addition of USP to the SBS-modified asphalt, there are no new absorption peaks in the spectrum compared to the SBS-modified asphalt. This further indicates that the modification process involving USP is a physical process.

3.5.2. Analysis of Fluorescence Microscopy Results

Figure 10a,b display fluorescence photographs of USP-A and USP/SBS-A at the optimal dosage of USP. Figure 10c,d illustrate conversion of the fluorescence images of the two types of asphalt into binary images using Image J 1.54f software. As shown in Figure 10a,c, USP exhibits a distinct granular structure in the matrix asphalt. This indicates that USP and the matrix asphalt exhibit an excellent state of blending. As shown in Figure 10b,c, the SBS-modified asphalt with the addition of USP exhibits uniform dispersion of fluorescent substances in the image. However, the original network structure is reduced, and there are partially uniform, long strips in the image. This is due to sufficient swelling of the waste tire rubber powder and resin in the USP under the action of surfactants, which alters the lightweight components of the asphalt within the asphalt. USP is dispersed within the asphalt system, resulting in a significant reduction in large particle sizes. Based on the above and previous test results, it is indicated that USP has good compatibility in both matrix asphalt and SBS-modified asphalt. However, the compatibility is better in the matrix asphalt.

4. Conclusions

To evaluate the flow and deformation properties of USP-A and USP/SBS-A across different aging scenarios, we performed comprehensive testing. The experimental protocol included fundamental performance assessments along with microscopic analyses, featuring temperature sweep experiments, repeated stress creep recovery measurements, and rheological examinations at low temperatures. The findings revealed significant insights into how USP influences the rheological behaviour of the materials, leading to the following key conclusions:
(1)
The optimal dosage of USP warm-mixed modifier in asphalt is 4.0% to 4.5%.
(2)
The experiments indicate that, with the addition of USP, the low-temperature flexibility of asphalt has significantly improved. At −12 °C, the creep stiffness of USP-A decreases by 38%. The creep stiffness of USP/SBS-A decreased by 35%.
(3)
The addition of the USP warm-mixing modifier can enhance the deformation resistance of asphalt. Furthermore, it can also enhance the creep recovery capability of asphalt. With the incorporation of USP, the deformability resistance of asphalt increased. At 0.1 kPa and 0.3 kPa, the modified asphalt R increases, while the Jnr decreases.
(4)
From a microscopic perspective, the incorporation of USP into the two types of asphalt did not result in any chemical reaction. Under a fluorescence microscope, USP is uniformly distributed within the asphalt. Furthermore, inclusion of USP will lead to collapse of the asphalt spatial network structure, which is the reason for the decreased high-temperature stability of asphalt.
This paper elucidates the modification mechanism of USP warm mix modifier on asphalt, based on analysis of its microscopic characteristics and rheological properties. A correlation between the micro index and macro performance has been established. There is an absence of detailed test evaluations on the performance decline of aged asphalt mixtures. In a follow-up study, the performance and degradation laws of aging asphalt mixtures can be further studied.

Author Contributions

Y.L.: data curation, conceptualization, validation, and writing. J.P. and H.G.: investigation and methodology. Y.K.: validation and conceptualization. Y.Y.: project administration and funding acquisition. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by Shandong Provincial Transportation Science and Technology Innovation Project (No. 2024B112-01).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

This article contains the original contributions presented in this study; further inquiries can be directed to the corresponding author.

Conflicts of Interest

Author Jingfei Ping was employed by the company Shandong Hi-Speed Traffic Construction Group Co., Ltd., while Hao Guo was employed by the company Shandong Hi-Speed Maintenance Group Co., Ltd. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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Coatings 15 00784 g001
Figure 1. Preparation steps for warm-mixed modified asphalt.
Figure 1. Preparation steps for warm-mixed modified asphalt.
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Figure 2. Basic testing of different asphalts.
Figure 2. Basic testing of different asphalts.
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Figure 3. Complex modulus and phase angle under different conditions.
Figure 3. Complex modulus and phase angle under different conditions.
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Figure 4. Rutting factor under different aging conditions.
Figure 4. Rutting factor under different aging conditions.
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Figure 5. Stress-strain diagram of different asphalts.
Figure 5. Stress-strain diagram of different asphalts.
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Figure 6. Strain recovery rate and nonrecoverable creep compliance of different asphalts.
Figure 6. Strain recovery rate and nonrecoverable creep compliance of different asphalts.
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Figure 7. Modulus of rigidity and rate of asphalt creep under aging scenarios.
Figure 7. Modulus of rigidity and rate of asphalt creep under aging scenarios.
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Figure 8. The S/m ratios of asphalts across varying conditions.
Figure 8. The S/m ratios of asphalts across varying conditions.
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Figure 9. Asphalt infrared spectroscopy.
Figure 9. Asphalt infrared spectroscopy.
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Figure 10. Fluorescence images of different asphalts.
Figure 10. Fluorescence images of different asphalts.
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Table 1. Pertinent performance metrics for matrix asphalt.
Table 1. Pertinent performance metrics for matrix asphalt.
Technical IndexesResultsRequirements
70#SBS
Penetration (0.1 mm)67.55140~60
Ductility (cm)4028.3≥20
Softening point (°C)48.467.6≥60
Density (g/cm−3)1.0211.030/
Solubility (%)99.699.3≥99
Boiling point (°C)289312≥230
Table 2. Technical index of the USP low-temperature modifier.
Table 2. Technical index of the USP low-temperature modifier.
Technical IndexUnitResultTest Procedure
Appearance/blackVisual estimation
Density (20 °C)g/cm30.98T 0603
Water content%0.2GB/T 260
Boiling point°C256T 0611
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Liu, Y.; Ping, J.; Guo, H.; Kang, Y.; Ye, Y. Investigating the Rheological Impact of USP Warm Mix Modifier on Asphalt Binder. Coatings 2025, 15, 784. https://doi.org/10.3390/coatings15070784

AMA Style

Liu Y, Ping J, Guo H, Kang Y, Ye Y. Investigating the Rheological Impact of USP Warm Mix Modifier on Asphalt Binder. Coatings. 2025; 15(7):784. https://doi.org/10.3390/coatings15070784

Chicago/Turabian Style

Liu, Yali, Jingfei Ping, Hao Guo, Yikai Kang, and Yali Ye. 2025. "Investigating the Rheological Impact of USP Warm Mix Modifier on Asphalt Binder" Coatings 15, no. 7: 784. https://doi.org/10.3390/coatings15070784

APA Style

Liu, Y., Ping, J., Guo, H., Kang, Y., & Ye, Y. (2025). Investigating the Rheological Impact of USP Warm Mix Modifier on Asphalt Binder. Coatings, 15(7), 784. https://doi.org/10.3390/coatings15070784

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