*2.3. Experimental Methods*

*2.3. Experimental Methods* This study introduced the external-adding method, namely, the mixing proportion is the mass ratio of BRA to base asphalt. The proportions of BRA 1–5 in corresponding BRA-MA were 10%, 20%, 30%, and 40%, respectively. Thus, 20 BRA-MA samples were prepared based on different particle sizes and contents. Figure 3 illustrates the outline of this study. BRA-MA's physical properties, storage-stability evaluation method, static stor-This study introduced the external-adding method, namely, the mixing proportion is the mass ratio of BRA to base asphalt. The proportions of BRA 1–5 in corresponding BRA-MA were 10%, 20%, 30%, and 40%, respectively. Thus, 20 BRA-MA samples were prepared based on different particle sizes and contents. Figure 3 illustrates the outline of this study. BRA-MA's physical properties, storage-stability evaluation method, static storage stability, and storage stability of transportation process were studied. *Materials* **2022**, *15*, x FOR PEER REVIEW 5 of 20

age was discussed in this study.

greater than 0.01 and less than 0.05, the difference is significant. If the *p*-value is less than 0.01, the difference is highly significant. Figure 4 implies the storage process of BRA-MA in a production plant and during transportation. The produced BRA-MA was contained in a soaking tank with temperature-controlling equipment, which can keep BRA-MA at a relatively stable high tempera-Firstly, penetration, softening point, ductility, and viscosity of base asphalt and BRA-MA were characterized. The impact of BRA content and particle size on physical properties

ture. Static storage at a high temperature of BRA-MA can save the time and cost of reheating, especially in short-term construction. Maintaining the high liquidity of BRA-MA fa-

tion plant is static. The static storage of BRA-MA refers to the hot-storage process [22,24,25] in a production plant before transportation. Then, BRA-MA is pumped into the vehicle container for transportation, the transportation time is usually less than 12 h. Both the storage stability of BRA-MA in the production plant and transportation process stor-

**Figure 4.** Storage process of BRA-MA in a production plant and during transportation.

The separating tube used in this study was composed of an aluminum sheet with a thickness of 0.4 mm. Therefore, the customized tube was easy to cut and separate. It should be averagely separated and noted as 3 parts by length, which were the top, middle, and bottom parts. Figure 5 expresses how the separating-tube method was conducted to evaluate the storage stability and determination of the evaluation indicator for segregation. At stage 1, the separating tube should be filled with BRA-MA on a specific shelf,

can be concluded. Secondly, this study introduced a separating-tube method to indicate the distribution uniformity of BRA in BRA-MA. Two indicators for evaluating storage stability were compared, so that more appropriate evaluation indicators can be obtained. Subsequently, the stability of BRA-MA in factory static storage were investigated. Finally, the storage stability of BRA-MA during the transportation process was simulated according to temperature decline in transportation.

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The storage stability of modified asphalt is the key technical requirement for its production and transportation, which may lead to modification failure and certainly affect the service performance of the modified asphalt mixture. In order to study the influence of particle size and BRA content on the storage stability of BRA-MA, the optimum indicator for storage-stability evaluation should be determined. As explained in stage 5 of the outline of this study in Figure 3, the softening-point difference and index of segregation (IS) based on the viscosity difference between the top and bottom part samples were used to characterize the segregation degree of BRA. The two evaluation methods were used separately to analyze the influence of particle size and BRA content on the storage stability of BRA-MA. The feasibility of the two indicators of the methods was discussed. The static significance of the results according to two indicators was analyzed by variance tests. If F is greater than F crit, then the difference between the results is proven. If the *p*-value is greater than 0.01 and less than 0.05, the difference is significant. If the *p*-value is less than 0.01, the difference is highly significant. **Figure 3.** Outline of this study. Figure 4 implies the storage process of BRA-MA in a production plant and during

Figure 4 implies the storage process of BRA-MA in a production plant and during transportation. The produced BRA-MA was contained in a soaking tank with temperaturecontrolling equipment, which can keep BRA-MA at a relatively stable high temperature. Static storage at a high temperature of BRA-MA can save the time and cost of reheating, especially in short-term construction. Maintaining the high liquidity of BRA-MA facilitates rapid loading onto a transport vehicle. Thus, the storage of BRA-MA in a production plant is static. The static storage of BRA-MA refers to the hot-storage process [22,24,25] in a production plant before transportation. Then, BRA-MA is pumped into the vehicle container for transportation, the transportation time is usually less than 12 h. Both the storage stability of BRA-MA in the production plant and transportation process storage was discussed in this study. transportation. The produced BRA-MA was contained in a soaking tank with temperature-controlling equipment, which can keep BRA-MA at a relatively stable high temperature. Static storage at a high temperature of BRA-MA can save the time and cost of reheating, especially in short-term construction. Maintaining the high liquidity of BRA-MA facilitates rapid loading onto a transport vehicle. Thus, the storage of BRA-MA in a production plant is static. The static storage of BRA-MA refers to the hot-storage process [22,24,25] in a production plant before transportation. Then, BRA-MA is pumped into the vehicle container for transportation, the transportation time is usually less than 12 h. Both the storage stability of BRA-MA in the production plant and transportation process storage was discussed in this study.

**Figure 4.** Storage process of BRA-MA in a production plant and during transportation. **Figure 4.** Storage process of BRA-MA in a production plant and during transportation.

The separating tube used in this study was composed of an aluminum sheet with a thickness of 0.4 mm. Therefore, the customized tube was easy to cut and separate. It should be averagely separated and noted as 3 parts by length, which were the top, middle, and bottom parts. Figure 5 expresses how the separating-tube method was conducted to evaluate the storage stability and determination of the evaluation indicator for segregation. At stage 1, the separating tube should be filled with BRA-MA on a specific shelf, The separating tube used in this study was composed of an aluminum sheet with a thickness of 0.4 mm. Therefore, the customized tube was easy to cut and separate. It should be averagely separated and noted as 3 parts by length, which were the top, middle, and bottom parts. Figure 5 expresses how the separating-tube method was conducted to evaluate the storage stability and determination of the evaluation indicator for segregation. At stage 1, the separating tube should be filled with BRA-MA on a specific shelf, exactly when the mixing of BRA and base asphalt was completed. Subsequently, the separating tubes should be kept vertically in a temperature-controlling box according to the established storage conditions, during which the segregation occurs. BRA would gradually settle at the bottom of the tubes due to its gravity. In the next stage, segregated samples were kept

in a freezer at −4 ◦C in 4 h. The segregation rate of BRA would decrease to a very low level due to the low temperature, which can be considered that the segregation of BRA-MA was terminated. As shown in stage 4, separating tubes should be cut and separated into 3 parts by an electric saw. Subsequently, BRA-MA from the top and bottom parts should be characterized by the softening-point and viscosity tests, respectively. Softening point and viscosity of the top and bottom samples were different, as a result of the BRA content difference of the two samples owing to BRA segregation. In the final stage, the softening-point difference and index of segregation based on viscosity difference can be acknowledged, respectively. The optimum evaluation indicator can be therefore be determined. The storage stability of BRA-MA can consequently be calculated and obtained. a very low level due to the low temperature, which can be considered that the segregation of BRA-MA was terminated. As shown in stage 4, separating tubes should be cut and separated into 3 parts by an electric saw. Subsequently, BRA-MA from the top and bottom parts should be characterized by the softening-point and viscosity tests, respectively. Softening point and viscosity of the top and bottom samples were different, as a result of the BRA content difference of the two samples owing to BRA segregation. In the final stage, the softening-point difference and index of segregation based on viscosity difference can be acknowledged, respectively. The optimum evaluation indicator can be therefore be determined. The storage stability of BRA-MA can consequently be calculated and obtained.

exactly when the mixing of BRA and base asphalt was completed. Subsequently, the separating tubes should be kept vertically in a temperature-controlling box according to the established storage conditions, during which the segregation occurs. BRA would gradually settle at the bottom of the tubes due to its gravity. In the next stage, segregated samples were kept in a freezer at −4 °C in 4 h. The segregation rate of BRA would decrease to

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**Figure 5.** Separating-tube method of index of storage stability. **Figure 5.** Separating-tube method of index of storage stability.

#### **3. Results and Discussions 3. Results and Discussions**

*3.1. Physical Properties of BRA-MA*

#### *3.1. Physical Properties of BRA-MA* 3.1.1. Softening Point

of BRA-MA.

3.1.1. Softening Point Figure 6 presents the results of the softening point of base asphalt and BRA-MA. BRA-MA samples were labeled with BRA type and BRA content. The yellow bars present the data of base asphalt, whose BRA content is 0%. The softening point of BRA-MA showed an obvious rising tendency, along with the increase in BRA content. The softening-point value of BRA-MA with 40% BRA-1 was 54.05 °C, which was 5.3 °C higher than that of base asphalt. On the other hand, the softening point of BRA-MA with the same BRA content decreased from BRA-1 to BRA-5. The results of BRA-MA can meet the standard limits of JTG F40-2004. It proved that the softening point of BRA-MA can be enhanced by a decline in BRA particle size. The results also illustrate that the softening point is positively correlated to BRA content. The high-temperature stability and temperature sensitivity of asphalt is generally evaluated by its softening point [26,27]. The temperature sen-Figure 6 presents the results of the softening point of base asphalt and BRA-MA. BRA-MA samples were labeled with BRA type and BRA content. The yellow bars present the data of base asphalt, whose BRA content is 0%. The softening point of BRA-MA showed an obvious rising tendency, along with the increase in BRA content. The softening-point value of BRA-MA with 40% BRA-1 was 54.05 ◦C, which was 5.3 ◦C higher than that of base asphalt. On the other hand, the softening point of BRA-MA with the same BRA content decreased from BRA-1 to BRA-5. The results of BRA-MA can meet the standard limits of JTG F40-2004. It proved that the softening point of BRA-MA can be enhanced by a decline in BRA particle size. The results also illustrate that the softening point is positively correlated to BRA content. The high-temperature stability and temperature sensitivity of asphalt is generally evaluated by its softening point [26,27]. The temperature sensitivity of asphalt can be reduced by the addition of BRA. Thus, the increase in BRA content and decline in particle size would help to enhance the high-temperature performance of BRA-MA.

sitivity of asphalt can be reduced by the addition of BRA. Thus, the increase in BRA content and decline in particle size would help to enhance the high-temperature performance

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**Figure 6.** Softening point of base asphalt and BRA-MA. **Figure 6.** Softening point of base asphalt and BRA-MA.

#### 3.1.2. Penetration 3.1.2. Penetration 3.1.2. Penetration

Figure 7 shows the penetration of base asphalt and BRA-MA. Penetration indicates the hardness of asphalt. A higher penetration value means a lower hardness. The yellow bars present the data of base asphalt without BRA. Penetration of BRA-MA decreased as BRA content increased, while base asphalt presented the highest penetration value. Penetration of BRA-MA containing 40% BRA-1 was 4.85 mm, which decreased by 31.9% compared to that of base asphalt. Additionally, the penetration of BRA-MA showed an approximativelyincreasing trend from BRA-1 to BRA-5 at the same BRA content. Therefore, the penetration of BRA-MA increased with the increment in particle size. The results suggest that BRA content negatively affects penetration, while the particle size of BRA shows Figure 7 shows the penetration of base asphalt and BRA-MA. Penetration indicates the hardness of asphalt. A higher penetration value means a lower hardness. The yellow bars present the data of base asphalt without BRA. Penetration of BRA-MA decreased as BRA content increased, while base asphalt presented the highest penetration value. Penetration of BRA-MA containing 40% BRA-1 was 4.85 mm, which decreased by 31.9% compared to that of base asphalt. Additionally, the penetration of BRA-MA showed an approximativelyincreasing trend from BRA-1 to BRA-5 at the same BRA content. Therefore, the penetration of BRA-MA increased with the increment in particle size. The results suggest that BRA content negatively affects penetration, while the particle size of BRA shows a contrary effect. Therefore, the addition of BRA can improve the hardness of asphalt. Figure 7 shows the penetration of base asphalt and BRA-MA. Penetration indicates the hardness of asphalt. A higher penetration value means a lower hardness. The yellow bars present the data of base asphalt without BRA. Penetration of BRA-MA decreased as BRA content increased, while base asphalt presented the highest penetration value. Penetration of BRA-MA containing 40% BRA-1 was 4.85 mm, which decreased by 31.9% compared to that of base asphalt. Additionally, the penetration of BRA-MA showed an approximativelyincreasing trend from BRA-1 to BRA-5 at the same BRA content. Therefore, the penetration of BRA-MA increased with the increment in particle size. The results suggest that BRA content negatively affects penetration, while the particle size of BRA shows a contrary effect. Therefore, the addition of BRA can improve the hardness of asphalt.

Figure 8 presents the 15 °C ductility of base asphalt and BRA-MA, whose yellow bars are the data of base asphalt. It is obvious that the ductility of base asphalt is about 150 cm,

Figure 8 presents the 15 °C ductility of base asphalt and BRA-MA, whose yellow bars are the data of base asphalt. It is obvious that the ductility of base asphalt is about 150 cm,

**Figure 7.** Penetration of base asphalt and BRA-MA. **Figure 7.** Penetration of base asphalt and BRA-MA. **Figure 7.** Penetration of base asphalt and BRA-MA.

3.1.3. Ductility

3.1.3. Ductility

#### 3.1.3. Ductility

Figure 8 presents the 15 ◦C ductility of base asphalt and BRA-MA, whose yellow bars are the data of base asphalt. It is obvious that the ductility of base asphalt is about 150 cm, while ductility values of BRA-MA are almost lower than 60 cm. The ductility value of BRA-MA with 40% BRA-5 was 18.5 cm, whose decline rate reached 84.6% compared to base asphalt. This result indicates that the addition of BRA leads to remarkable ductility loss. The ductility of BRA-MA also shows a downward trend as the increment in BRA content and particle size. Thus, BRA content and particle size negatively determine the ductility of BRA-MA. It is believed that it was the inorganic mineral particles in BRA that played a critical role in reducing the ductility of BRA-MA. Although its ductility is much lower than that of base asphalt, whether its low-temperature performance is poor is still hard to define due to the lack of corresponding standard for BRA. while ductility values of BRA-MA are almost lower than 60 cm. The ductility value of BRA-MA with 40% BRA-5 was 18.5 cm, whose decline rate reached 84.6% compared to base asphalt. This result indicates that the addition of BRA leads to remarkable ductility loss. The ductility of BRA-MA also shows a downward trend as the increment in BRA content and particle size. Thus, BRA content and particle size negatively determine the ductility of BRA-MA. It is believed that it was the inorganic mineral particles in BRA that played a critical role in reducing the ductility of BRA-MA. Although its ductility is much lower than that of base asphalt, whether its low-temperature performance is poor is still hard to define due to the lack of corresponding standard for BRA.

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#### 3.1.4. Viscosity and Viscosity-Temperature Susceptibility (VTS) 3.1.4. Viscosity and Viscosity-Temperature Susceptibility (VTS)

Viscosities of 20 BRA-MA at 115 °C, 135 °C, 155 °C, and 175 °C were measured by a Brookfield viscometer. Figures 9–12 illustrate the viscosity of BRA-MA by temperature and particle-size dependency. The viscosity of base asphalt was appended for comparison. The 135 °C viscosity results of BRA-MA can meet the JTG F40-2004 standard limits for modified asphalt. The viscosity of base asphalt was significantly lower than that of BRA-MA. BRA-MA with BRA-1 showed the highest viscosity at the same temperature and BRA content. BRA-MA with 40% BRA-1 showed the highest viscosity, and its growth rate reached 114% compared to that of base asphalt. The viscosity difference between base asphalt and BRA-MA gradually decreased with the increase in temperature. The larger particle size of BRA led to lower viscosity. However, the effect of BRA particle size of viscosity was not very significant when the temperature was over 135 °C. Therefore, the Viscosities of 20 BRA-MA at 115 ◦C, 135 ◦C, 155 ◦C, and 175 ◦C were measured by a Brookfield viscometer. Figures 9–12 illustrate the viscosity of BRA-MA by temperature and particle-size dependency. The viscosity of base asphalt was appended for comparison. The 135 ◦C viscosity results of BRA-MA can meet the JTG F40-2004 standard limits for modified asphalt. The viscosity of base asphalt was significantly lower than that of BRA-MA. BRA-MA with BRA-1 showed the highest viscosity at the same temperature and BRA content. BRA-MA with 40% BRA-1 showed the highest viscosity, and its growth rate reached 114% compared to that of base asphalt. The viscosity difference between base asphalt and BRA-MA gradually decreased with the increase in temperature. The larger particle size of BRA led to lower viscosity. However, the effect of BRA particle size of viscosity was not very significant when the temperature was over 135 ◦C. Therefore, the viscosity of asphalt apparently increases with the addition of BRA and a smaller particle size.

viscosity of asphalt apparently increases with the addition of BRA and a smaller particle size. Figure 13 explains the effect of BRA content on the viscosity of BRA-MA, which uses a logarithmic longitudinal axis. The viscosity result of BRA-MA with 40% BRA-1 was used for an instance, and corresponding fitting curves of the data were presented. As the BRA-1 content increased from 10% to 40%, the viscosity of BRA-MA showed an obvious rising trend. This result proves that raising BRA content contributes to promote the viscosity of Figure 13 explains the effect of BRA content on the viscosity of BRA-MA, which uses a logarithmic longitudinal axis. The viscosity result of BRA-MA with 40% BRA-1 was used for an instance, and corresponding fitting curves of the data were presented. As the BRA-1 content increased from 10% to 40%, the viscosity of BRA-MA showed an obvious rising trend. This result proves that raising BRA content contributes to promote the viscosity of BRA-MA. To conclude, viscosity had a negative correlation with the particle size of BRA, but it was positively affected by BRA content.

BRA-MA. To conclude, viscosity had a negative correlation with the particle size of BRA,

Figure 14 demonstrates the straight lines of BRA-MA fitting with BRA-1. According to

VTS [28] refers to the slope of the viscosity–temperature curve with the iterated log-

but it was positively affected by BRA content.

VTS [28] refers to the slope of the viscosity–temperature curve with the iterated logarithm of viscosity and logarithm of temperature as the coordinate axes [29]. For instance, Figure 14 demonstrates the straight lines of BRA-MA fitting with BRA-1. According to ASTM D 2493, the smaller absolute value of VTS implies the lower temperature sensitivity of asphalt [30]. The basic equation of VTS is: ASTM D 2493, the smaller absolute value of VTS implies the lower temperature sensitivity of asphalt [30]. The basic equation of VTS is: ASTM D 2493, the smaller absolute value of VTS implies the lower temperature sensitivity of asphalt [30]. The basic equation of VTS is:

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$$\text{VTS} = \frac{\lg\left[\lg(\eta\_{\text{T}\_2})\right] - \lg\left[\lg(\eta\_{\text{T}\_1})\right]}{\lg(\text{T}\_2) - \lg(\text{T}\_1)} \tag{1}$$

where the T<sup>1</sup> and T<sup>2</sup> = temperatures (centigrade) of the binder at two known points, and ηT<sup>1</sup> and ηT<sup>2</sup> 5 = viscosities of the binder at the same two points. The viscosities of BRA-MA at 115 ◦C, 135 ◦C, 155 ◦C, and 175 ◦C were used to fit a straight line according to Equation (1). ɳ<sup>భ</sup> and ɳ<sup>మ</sup> 5 = viscosities of the binder at the same two points. The viscosities of BRA-MA at 115 °C, 135 °C, 155 °C, and 175 °C were used to fit a straight line according to Equation (1). ɳ<sup>భ</sup> and ɳ<sup>మ</sup> 5 = viscosities of the binder at the same two points. The viscosities of BRA-MA at 115 °C, 135 °C, 155 °C, and 175 °C were used to fit a straight line according to Equation (1).

**Figure 9.** Viscosity of BRA-MA with 10% BRA. **Figure 9.** Viscosity of BRA-MA with 10% BRA. **Figure 9.** Viscosity of BRA-MA with 10% BRA.

**Figure 10.** Viscosity of BRA-MA with 20% BRA. **Figure 10.** Viscosity of BRA-MA with 20% BRA. **Figure 10.** Viscosity of BRA-MA with 20% BRA.

**Figure 11.** Viscosity of BRA-MA with 30% BRA. **Figure 11.** Viscosity of BRA-MA with 30% BRA. **Figure 11.** Viscosity of BRA-MA with 30% BRA.

**Figure 12.** Viscosity of BRA-MA with 40% BRA. **Figure 12.** Viscosity of BRA-MA with 40% BRA. **Figure 12.** Viscosity of BRA-MA with 40% BRA.

The absolute value of BRA-MA VTS values can be observed in Table 3, which involves the VTS value of base asphalt for comparison. The addition of BRA can reduce the VTS of asphalt. It is clear that the absolute VTS value decreased with the increase in BRA content, which indicates that BRA can reduce the temperature susceptibility of asphalt. Additionally, the smaller particle size also leads to a lower absolute VTS value. Therefore, VTS is negatively correlated to BRA content, but positively determined by particle size. These results indicate that the higher BRA content and smaller particle size reduce BRA-MA's temperature susceptibility, which may improve the BRA-MA mixture's stability at high temperatures. The absolute value of BRA-MA VTS values can be observed in Table 3, which involves the VTS value of base asphalt for comparison. The addition of BRA can reduce the VTS of asphalt. It is clear that the absolute VTS value decreased with the increase in BRA content, which indicates that BRA can reduce the temperature susceptibility of asphalt. Additionally, the smaller particle size also leads to a lower absolute VTS value. Therefore, VTS is negatively correlated to BRA content, but positively determined by particle size. These results indicate that the higher BRA content and smaller particle size reduce BRA-MA's temperature susceptibility, which may improve the BRA-MA mixture's stability at high temperatures. The absolute value of BRA-MA VTS values can be observed in Table 3, which involves the VTS value of base asphalt for comparison. The addition of BRA can reduce the VTS of asphalt. It is clear that the absolute VTS value decreased with the increase in BRA content, which indicates that BRA can reduce the temperature susceptibility of asphalt. Additionally, the smaller particle size also leads to a lower absolute VTS value. Therefore, VTS is negatively correlated to BRA content, but positively determined by particle size. These results indicate that the higher BRA content and smaller particle size reduce BRA-MA's temperature susceptibility, which may improve the BRA-MA mixture's stability at high temperatures.

**Figure 13.** Viscosity of BRA-MA with 40% BRA with logarithmic axis. **Figure 13.** Viscosity of BRA-MA with 40% BRA with logarithmic axis. **Figure 13.** Viscosity of BRA-MA with 40% BRA with logarithmic axis.

**Figure 14.** Viscosity–temperature curve of BRA-MA with BRA-1 for VTS. **Figure 14.** Viscosity–temperature curve of BRA-MA with BRA-1 for VTS. **Figure 14.** Viscosity–temperature curve of BRA-MA with BRA-1 for VTS.

**Table 3.** Absolute value of BRA-MA VTS values. **Table 3.** Absolute value of BRA-MA VTS values. **Table 3.** Absolute value of BRA-MA VTS values.


#### *3.2. Storage-Stability Evaluation Determination 3.2. Storage-Stability Evaluation Determination*

#### *3.2. Storage-Stability Evaluation Determination* 3.2.1. Softening Point Difference

3.2.1. Softening Point Difference Softening-point difference [31] is one of the most commonly used indicators in the storage-stability evaluation of polymer-modified asphalt. Figure 6 proves that BRA content is positively correlated to the softening point of BRA-MA; thus, a higher softening point means a higher BRA content. Through the difference of the softening point between 3.2.1. Softening Point Difference Softening-point difference [31] is one of the most commonly used indicators in the storage-stability evaluation of polymer-modified asphalt. Figure 6 proves that BRA content is positively correlated to the softening point of BRA-MA; thus, a higher softening point means a higher BRA content. Through the difference of the softening point between Softening-point difference [31] is one of the most commonly used indicators in the storage-stability evaluation of polymer-modified asphalt. Figure 6 proves that BRA content is positively correlated to the softening point of BRA-MA; thus, a higher softening point means a higher BRA content. Through the difference of the softening point between the top and bottom samples, the segregation of BRA content between the top and bottom of the

separating tube can be determined. If the softening-point difference between the top and bottom is less than 2.2 ◦C, according to ASTM D 5976, this indicates that the segregation is not serious and the modified asphalt has acceptable storage stability. The equation for calculating the softening-point difference is: rating tube, SP<sup>୲</sup> and SP<sup>ୠ</sup> are the softening points of the top and bottom of the separating tube, respectively. Figure 15 illustrates the softening-point difference of BRA-MA. The softening-point difference of BRA-MA with more than 20% BRA-5 was over 2.2 °C, which indicates their severe segregation. Softening-point difference of BRA-MA with BRA-1~4

where ∆SP refers to the softening-point difference between the top and bottom of sepa-

∆SP = |SP<sup>୲</sup> − SP<sup>ୠ</sup>

the top and bottom samples, the segregation of BRA content between the top and bottom of the separating tube can be determined. If the softening-point difference between the top and bottom is less than 2.2 °C, according to ASTM D 5976, this indicates that the segregation is not serious and the modified asphalt has acceptable storage stability. The equa-

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tion for calculating the softening-point difference is:

$$
\Delta \text{SP} = |\text{SP}\_{\text{l}} - \text{SP}\_{\text{b}}| \tag{2}
$$


where ∆SP refers to the softening-point difference between the top and bottom of separating tube, SP<sup>t</sup> and SP<sup>b</sup> are the softening points of the top and bottom of the separating tube, respectively. Figure 15 illustrates the softening-point difference of BRA-MA. The softening-point difference of BRA-MA with more than 20% BRA-5 was over 2.2 ◦C, which indicates their severe segregation. Softening-point difference of BRA-MA with BRA-1~4 was lower than 2.2 ◦C, suggesting that their segregation is slight. However, it was found that the segregation of BRA-MA with BRA-2, -3, and -4 was also severe by manual inspection, which is contrary to the evaluation standard of the softening-point difference test. Additionally, even if the softening-point difference of two kinds of asphalt is equal, their segregation degrees are thought to be different since their original softening points are different. Therefore, the softening-point difference method may not quantitatively indicate the degree of segregation of BRA-MA. tion, which is contrary to the evaluation standard of the softening-point difference test. Additionally, even if the softening-point difference of two kinds of asphalt is equal, their segregation degrees are thought to be different since their original softening points are different. Therefore, the softening-point difference method may not quantitatively indicate the degree of segregation of BRA-MA. The variance analysis results show that the F value of the softening-point difference by BRA-content dependency was lower than F crit, proving that BRA content did not show a statistical effect on the softening-point difference. In addition, F-value by particlesize dependency was higher than F crit and the *p*-value was less than 0.05, which implies that particle size can significantly affect softening-point difference. Therefore, the softening-point difference test failed to reveal how the BRA content influenced the storage stability, nor can it reveal the quantitative segregation of BRA-MA.

**Figure 15.** Softening-point difference of BRA-MA. **Figure 15.** Softening-point difference of BRA-MA.

3.2.2. IS Based on the Viscosity Difference IS based on the viscosity difference was used as an indicator for the purpose of developing the susceptibility of detecting the segregation of BRA-MA. As shown in Figure 5, this evaluation method is also based on the separating-tube method. The viscosity of the top and bottom part of separating-tube should be tested at 135 °C. Higher viscosity signifies the corresponding higher BRA content, according to Figure 13. Additionally, IS The variance analysis results show that the F value of the softening-point difference by BRA-content dependency was lower than F crit, proving that BRA content did not show a statistical effect on the softening-point difference. In addition, F-value by particle-size dependency was higher than F crit and the *p*-value was less than 0.05, which implies that particle size can significantly affect softening-point difference. Therefore, the softeningpoint difference test failed to reveal how the BRA content influenced the storage stability, nor can it reveal the quantitative segregation of BRA-MA.

#### 3.2.2. IS Based on the Viscosity Difference

IS based on the viscosity difference was used as an indicator for the purpose of developing the susceptibility of detecting the segregation of BRA-MA. As shown in Figure 5, this evaluation method is also based on the separating-tube method. The viscosity of the top and bottom part of separating-tube should be tested at 135 ◦C. Higher viscosity signifies the corresponding higher BRA content, according to Figure 13. Additionally, IS

was used to illustrate the segregation of BRA-MA. The calculation equation is shown in Equations (3) and (4). ∆ɳ = |ɳ<sup>୲</sup> − ɳ<sup>ୠ</sup> | (3)

was used to illustrate the segregation of BRA-MA. The calculation equation is shown in

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Equations (3) and (4).

$$
\Delta \mathfrak{η} = |\mathfrak{η}\_{\mathfrak{t}} - \mathfrak{η}\_{\mathfrak{b}}| \tag{3}
$$

$$\mathbf{IS} = \Delta \mathbf{\eta} / \eta\_{\mathbf{O}} \tag{4}$$

$$\mathbf{IS} = \begin{bmatrix} \Delta \mathbf{\eta} / \eta\_{\mathbf{O}} \\ \vdots \\ \vdots \\ \mathbf{O} \end{bmatrix} \tag{5}$$

where ∆η refers to the viscosity difference, η<sup>t</sup> and η<sup>b</sup> are the viscosities of the top and bottom of the separating tube, η<sup>O</sup> is the original viscosity of BRA-MA before segregation, and IS is the index of segregation based on the viscosity difference. A higher IS value means a more serious segregation of BRA. Not only the viscosity difference, but also the original viscosity of the modified asphalt had been considered in the calculation of IS. The segregation of asphalt with different original viscosities can be semi-quantitatively compared through the IS method. bottom of the separating tube, ɳ is the original viscosity of BRA-MA before segregation, and IS is the index of segregation based on the viscosity difference. A higher IS value means a more serious segregation of BRA. Not only the viscosity difference, but also the original viscosity of the modified asphalt had been considered in the calculation of IS. The segregation of asphalt with different original viscosities can be semi-quantitatively compared through the IS method. Figure 16 presents the IS of BRA-MA after the segregation process, which kept BRA-

Figure 16 presents the IS of BRA-MA after the segregation process, which kept BRA-MA at 165 ◦C for 48 h. The value of IS showed an almost rising trend, along with a the BRA content increases. Moreover, the IS value illustrated an increasing tendency with the increment in BRA particle size. The F-value is the statistic of the F-test, which is used to indicate the statistical significance of the two methods. The variance analysis results indicate that both the F-values of IS by BRA content and particle-size dependency are higher than the corresponding F crit. The *p*-values proved that the discrepancy of IS by BRA content and particle-size dependency were significant. Consequently, IS based on the viscosity difference had more static significance in the evaluating influence of BRA content and particle size on segregation compared with the softening difference evaluation. IS was then determined as the method to indicate the storage stability of BRA-MA in the following study. MA at 165 °C for 48 h. The value of IS showed an almost rising trend, along with a the BRA content increases. Moreover, the IS value illustrated an increasing tendency with the increment in BRA particle size. The F-value is the statistic of the F-test, which is used to indicate the statistical significance of the two methods. The variance analysis results indicate that both the F-values of IS by BRA content and particle-size dependency are higher than the corresponding F crit. The *p*-values proved that the discrepancy of IS by BRA content and particle-size dependency were significant. Consequently, IS based on the viscosity difference had more static significance in the evaluating influence of BRA content and particle size on segregation compared with the softening difference evaluation. IS was then determined as the method to indicate the storage stability of BRA-MA in the following study.

**Figure 16.** IS of BRA-MA after the segregation process. **Figure 16.** IS of BRA-MA after the segregation process.

#### *3.3. Static Storage Stability in the Production Plant 3.3. Static Storage Stability in the Production Plant*

Storage temperature and time are the key factors affecting the static storage stability of samples. Therefore, how BRA content and particle size affect static storage stability at different temperatures and times was studied. The storage temperatures were 145 °C, 155 °C, and 165 °C, while the storage times were 24, 48, and 72 h. The aging of BRA-MA Storage temperature and time are the key factors affecting the static storage stability of samples. Therefore, how BRA content and particle size affect static storage stability at different temperatures and times was studied. The storage temperatures were 145 ◦C, 155 ◦C, and 165 ◦C, while the storage times were 24, 48, and 72 h. The aging of BRA-MA at high temperatures was negligible in a soaking tank, which cut off the outside air.

at high temperatures was negligible in a soaking tank, which cut off the outside air. Figures 17–19 present the IS results of BRA-MA after the segregation process by storage temperature and time dependency, respectively. The IS values illustrated an Figures 17–19 present the IS results of BRA-MA after the segregation process by storage temperature and time dependency, respectively. The IS values illustrated an obviously rising trend as the BRA content increased, which also showed an increasing tendency along with particle-size increase from BRA-1 to BRA-5. On the other hand, with the extension of storage time, the IS value also showed an increasing tendency It was evident that

*Materials* **2022**, *15*, x FOR PEER REVIEW 14 of 20

*Materials* **2022**, *15*, x FOR PEER REVIEW 14 of 20

heightening temperatures would result in higher IS values by comparison of Figures 17–19. Therefore, reducing the temperature, particle size, storage time, and BRA content would help to lower the segregation of BRA-MA in static storage in a static container. Figures 17–19. Therefore, reducing the temperature, particle size, storage time, and BRA content would help to lower the segregation of BRA-MA in static storage in a static container. content would help to lower the segregation of BRA-MA in static storage in a static container.

obviously rising trend as the BRA content increased, which also showed an increasing tendency along with particle-size increase from BRA-1 to BRA-5. On the other hand, with the extension of storage time, the IS value also showed an increasing tendency It was evident that heightening temperatures would result in higher IS values by comparison of

obviously rising trend as the BRA content increased, which also showed an increasing tendency along with particle-size increase from BRA-1 to BRA-5. On the other hand, with the extension of storage time, the IS value also showed an increasing tendency It was evident that heightening temperatures would result in higher IS values by comparison of Figures 17–19. Therefore, reducing the temperature, particle size, storage time, and BRA

**Figure 17.** IS of BRA-MA after the segregation process at 145 °C. **Figure 17.** IS of BRA-MA after the segregation process at 145 ◦C. **Figure 17.** IS of BRA-MA after the segregation process at 145 °C.

**Figure 18.** IS of BRA-MA after the segregation process at 155 °C. **Figure 18.** IS of BRA-MA after the segregation process at 155 °C. **Figure 18.** IS of BRA-MA after the segregation process at 155 ◦C.

**Figure 19.** IS of BRA-MA after the segregation process at 165 °C. **Figure 19.** IS of BRA-MA after the segregation process at 165 ◦C.

The correlation between the specific surface area of BRA and IS was analyzed. Figure 20 presents the IS results by temperature, specific surface area, and BRA content at 145 °C, 155 °C, and 165 °C, respectively, for 24 h' static storage, for instance. The IS value presented a decline as the increment in the specific surface area of BRA, since its particle size was negatively correlated to the specific surface area. Power function curves were used to fit the data point. The result of the fitting equation and R<sup>2</sup> of BRA-MA afterthe segregation process for 24 h of static storage is shown in Table 3. Corresponding complex correlation coefficients (R<sup>2</sup> ) were higher than 0.89, suggesting that fitting between the IS value and specific surface area was rational. It revealed that the functional relationship of IS and The correlation between the specific surface area of BRA and IS was analyzed. Figure 20 presents the IS results by temperature, specific surface area, and BRA content at 145 ◦C, 155 ◦C, and 165 ◦C, respectively, for 24 h' static storage, for instance. The IS value presented a decline as the increment in the specific surface area of BRA, since its particle size was negatively correlated to the specific surface area. Power function curves were used to fit the data point. The result of the fitting equation and R<sup>2</sup> of BRA-MA after the segregation process for 24 h of static storage is shown in Table 3. Corresponding complex correlation coefficients (R<sup>2</sup> ) were higher than 0.89, suggesting that fitting between the IS value and specific surface area was rational. It revealed that the functional relationship of IS and specific surface area can be fit as Equation (5):

$$\mathbf{IS} = \mathbf{s}\_{\mathbf{W}\_1} \mathbf{x}^{-\mathbf{a}} \tag{5}$$

20 y = 0.3852x − 5.58 0.9930 30 y = 0.6138x − 5.727 0.9672 40 y = 0.6268x − 5.505 0.9737

10 y = 0.2284x − 3.913 0.9935

30 y = 0.6213x − 4.967 0.9104 40 y = 0.7397x − 5.889 0.8968 (5)

IS = s<sup>୵</sup> x ିୟ where s<sup>୵</sup> refers to the segregation coefficients, which is positively correlated to the segregation of BRA-MA; x is the variable (specific surface area); and a is the power. This functional relationship of the IS and specific surface area can be used to indicate how particle size that is correlated to the specific surface area determines the segregation of BRAwhere sw<sup>i</sup> refers to the segregation coefficients, which is positively correlated to the segregation of BRA-MA; x is the variable (specific surface area); and a is the power. This functional relationship of the IS and specific surface area can be used to indicate how particle size that is correlated to the specific surface area determines the segregation of BRA-MA. Table 4 listed the Power function curve-fitting equation and R<sup>2</sup> of BRA-MA. It is obvious that the R 2 is mostly over 0.9, which suggested that the fitting is adequate.

MA. Table 4 listed the Power function curve-fitting equation and R<sup>2</sup> of BRA-MA. It is ob-**Table 4.** Power function curve-fitting equation and R<sup>2</sup> of BRA-MA.


165 10 y = 0.2222x − 4.498 0.9928

145

155

**Table 4.** *Cont.*

**Figure 20.** IS of BRA-MA after the segregation process for 24 h static storage at 145 °C, 155 °C, and 165 °C. **Figure 20.** IS of BRA-MA after the segregation process for 24 h static storage at 145 ◦C, 155 ◦C, and 165 ◦C.

#### *3.4. Storage Stability during Transportation 3.4. Storage Stability during Transportation*

Transportation is a non-negligible process for storage stability and the aging of modified asphalt [32,33]. A simulated experiment was designed to investigate the storage stability variation of BRA-MA during its transportation. BRA-MA was pumped from a static container into a vehicle container that was in a delivery vehicle in a production plant. The original temperature of BRA-MA in the container was found to be around 145 °C due to heat loss in the pumping process. Then, the vehicle should transport BRA-MA to the corresponding construction site (as Figure 3 illustrated). The transportation time is usually less than 12 h, according to the on-the-spot investigation. The heat loss of BRA-MA during the transportation process can be alleviated by the insulation layer and heating equipment of the containers on the vehicles. The temperature decline in BRA-MA of 12 h during the transportation process was 20 °C, which means its temperature decreasing rate is averagely of 5 °C per 3 h. This experiment consequently simulated the segregation of BRA-Transportation is a non-negligible process for storage stability and the aging of modified asphalt [32,33]. A simulated experiment was designed to investigate the storage stability variation of BRA-MA during its transportation. BRA-MA was pumped from a static container into a vehicle container that was in a delivery vehicle in a production plant. The original temperature of BRA-MA in the container was found to be around 145 ◦C due to heat loss in the pumping process. Then, the vehicle should transport BRA-MA to the corresponding construction site (as Figure 3 illustrated). The transportation time is usually less than 12 h, according to the on-the-spot investigation. The heat loss of BRA-MA during the transportation process can be alleviated by the insulation layer and heating equipment of the containers on the vehicles. The temperature decline in BRA-MA of 12 h during the transportation process was 20 ◦C, which means its temperature decreasing rate is averagely of 5 ◦C per 3 h. This experiment consequently simulated the segregation of BRA-MA in consideration of its cooling process during transportation.

MA in consideration of its cooling process during transportation. Figure 21 explains the following stages of this simulated experiment. The original temperature of BRA-MA in the separating tubes was 145 °C. Then, BRA-MA should be Figure 21 explains the following stages of this simulated experiment. The original temperature of BRA-MA in the separating tubes was 145 ◦C. Then, BRA-MA should be placed into a temperature-controlling box, and the initial temperature is 145 ◦C when the

placed into a temperature-controlling box, and the initial temperature is 145 °C when the time is marked as 0 h. The temperature of the box should be switched to 140 °C when the

time at 3 h to confirm that its temperature has already decreased to 140 °C. Meanwhile, the IS value at 140 °C should be tested according to the viscosity difference test conducted as the temperature test ①. As Figure 21 illustrates, the temperature and IS value of BRA-MA should be tested when its temperature gradually reduces to 135, 130, and 125 °C, as the temperature test ②, ③ and ④, respectively. The temperature of BRA-MA finally

time is marked as 0 h. The temperature of the box should be switched to 140 ◦C when the storage time is 1.5 h. Then, the temperature of the BRA-MA should be tested for the first time at 3 h to confirm that its temperature has already decreased to 140 ◦C. Meanwhile, the IS value at 140 ◦C should be tested according to the viscosity difference test conducted as the temperature test <sup>1</sup> . As Figure 21 illustrates, the temperature and IS value of BRA-MA should be tested when its temperature gradually reduces to 135, 130, and 125 ◦C, as the temperature test <sup>2</sup> , <sup>3</sup> and <sup>4</sup> , respectively. The temperature of BRA-MA finally decreased to 125 ◦C after 12 h; the cooling-process simulation of the transportation was terminated. *Materials* **2022**, *15*, x FOR PEER REVIEW 17 of 20 decreased to 125 °C after 12 h; the cooling-process simulation of the transportation was terminated.

**Figure 21.** Outline of simulated experiment for the storage stability during transportation. **Figure 21.** Outline of simulated experiment for the storage stability during transportation.

The IS values of BRA-MA with BRA-1~4 during transportation are provided in Figure 22, while the IS value of BRA-5-based BRA-MA is shown in Figure 23. The abscissa is the temperature during transportation, and its corresponding transportation times are 3, 6, 9, and 12 h. The results show that the IS values of BRA-MA with BRA-1~4 are below 0.2, suggesting that their segregation is very slight. The IS value also showed an increasing trend as the particle size rises. However, BRA-5-based BRA-MA illustrated a high IS value during transportation, proving that its segregation was serious. In addition, the IS values of BRA-MA with BRA-5 presented an increasing tendency in the cooling process. However, IS change of BRA-1~4-based BRA-MA in the cooling process was not explicit, especially when BRA content was below 30%. Therefore, BRA-MA with BRA-1~4 showed low segregation and acceptable storage stability compared to BRA-5-based BRA-MA, although their IS values changed unreasonably. BRA-MA with BRA-1~4 whose d(0.5) particle sizes were lower than 13.6 μm showed low segregation. However, the segregation of BRA-MA with BRA whose d(0.5) particle size was over 106 μm was severe. The IS values of BRA-MA with BRA-1~4 during transportation are provided in Figure 22, while the IS value of BRA-5-based BRA-MA is shown in Figure 23. The abscissa is the temperature during transportation, and its corresponding transportation times are 3, 6, 9, and 12 h. The results show that the IS values of BRA-MA with BRA-1~4 are below 0.2, suggesting that their segregation is very slight. The IS value also showed an increasing trend as the particle size rises. However, BRA-5-based BRA-MA illustrated a high IS value during transportation, proving that its segregation was serious. In addition, the IS values of BRA-MA with BRA-5 presented an increasing tendency in the cooling process. However, IS change of BRA-1~4-based BRA-MA in the cooling process was not explicit, especially when BRA content was below 30%. Therefore, BRA-MA with BRA-1~4 showed low segregation and acceptable storage stability compared to BRA-5-based BRA-MA, although their IS values changed unreasonably. BRA-MA with BRA-1~4 whose d(0.5) particle sizes were lower than 13.6 µm showed low segregation. However, the segregation of BRA-MA with BRA whose d(0.5) particle size was over 106 µm was severe.

decreased to 125 °C after 12 h; the cooling-process simulation of the transportation was

**Figure 21.** Outline of simulated experiment for the storage stability during transportation.

BRA-MA with BRA whose d(0.5) particle size was over 106 μm was severe.

The IS values of BRA-MA with BRA-1~4 during transportation are provided in Figure 22, while the IS value of BRA-5-based BRA-MA is shown in Figure 23. The abscissa is the temperature during transportation, and its corresponding transportation times are 3, 6, 9, and 12 h. The results show that the IS values of BRA-MA with BRA-1~4 are below 0.2, suggesting that their segregation is very slight. The IS value also showed an increasing trend as the particle size rises. However, BRA-5-based BRA-MA illustrated a high IS value during transportation, proving that its segregation was serious. In addition, the IS values of BRA-MA with BRA-5 presented an increasing tendency in the cooling process. However, IS change of BRA-1~4-based BRA-MA in the cooling process was not explicit, especially when BRA content was below 30%. Therefore, BRA-MA with BRA-1~4 showed low segregation and acceptable storage stability compared to BRA-5-based BRA-MA, although their IS values changed unreasonably. BRA-MA with BRA-1~4 whose d(0.5) particle sizes were lower than 13.6 μm showed low segregation. However, the segregation of

terminated.

**Figure 22.** IS of BRA-MA with BRA-1~4 during transportation. **Figure 22.** IS of BRA-MA with BRA-1~4 during transportation.

**Figure 23.** IS of BRA-MA with BRA-5 during transportation. **Figure 23.** IS of BRA-MA with BRA-5 during transportation.

#### **4. Conclusions 4. Conclusions**

susceptibility.

This study introduced BRA of different particle sizes and contents to prepare BRA-MA in order to determine how the factors affect their physical properties and storage stability. Storage-stability evaluation methods were also discussed. Based on the results, the following conclusions can be drawn. This study introduced BRA of different particle sizes and contents to prepare BRA-MA in order to determine how the factors affect their physical properties and storage stability. Storage-stability evaluation methods were also discussed. Based on the results, the following conclusions can be drawn.

(1) Softening point was positively correlated to BRA content, while the particle size of BRA showed a negative correlation. Penetration of BRA-MA increased as the increment in particle size, while BRA content negatively affected penetration. Both BRA content and particle size negatively determines the ductility of BRA-MA. A larger particle size of BRA resulted in lower viscosity, but a higher BRA content increased the viscosity of BRA-MA. The temperature susceptibility of BRA-MA decreased with (1) Softening point was positively correlated to BRA content, while the particle size of BRA showed a negative correlation. Penetration of BRA-MA increased as the increment in particle size, while BRA content negatively affected penetration. Both BRA content and particle size negatively determines the ductility of BRA-MA. A larger particle size of BRA resulted in lower viscosity, but a higher BRA content increased the viscosity of BRA-MA. The temperature susceptibility of BRA-MA de-

the increase in BRA content, but the smaller particle size led to lower temperature

by the softening-point difference test, and the segregation degree was not quantitively revealed. The IS value based on the viscosity difference test had a higher statistical significance when evaluating the influence of BRA content and particle size on segregation. The segregation of asphalt with different original viscosity could be quantitatively compared through the IS value based on the viscosity difference test. (3) Storage stabilities of static and transportation corresponded to storage in a production plant and during the vehicle transportation process. The segregation of BRA-MA illustrated a rising trend as the BRA coffintent and particle size increased. Both storage temperature and time were positively correlated to the segregation of BRA-MA. It was proved that the relationship between the specific surface area and segregation were power functional. This relationship can be used to understand how particle size, which is correlated to specific surface area, determines the segregation of BRA-MA. Based on the simulated experiment of transportation segregation, BRA-MA with BRA-1~4 whose d(0.5) particle sizes ere lower than 13.6 μm, showed low segregation. However, the segregation of BRA-MA with BRA whose d(0.5) particle

size was over 106 μm was severe.

creased with the increase in BRA content, but the smaller particle size led to lower temperature susceptibility.


**Author Contributions:** Conceptualization, Y.S., X.H. (Xiaodi Hu), J.W. and X.H. (Xing Huang); methodology, software, Y.S., X.H. (Xiaodi Hu) and J.W.; validation, Y.S., X.H. (Xiaodi Hu), J.W. and S.W.; formal analysis, Y.S., X.H. (Xiaodi Hu), J.W. and S.W.; investigation, Y.S., X.H. (Xiaodi Hu), J.W. and Z.L.; resources, X.H. (Xiaodi Hu), J.W., X.H. (Xing Huang) and S.W.; data curation, J.W. and Y.Z.; writing—original draft preparation, J.W., Y.Z. and Y.S.; writing—review and editing, S.W., Z.L. and X.H. (Xiaodi Hu); visualization, Y.S. and J.W.; supervision, X.H. (Xiaodi Hu) and S.W.; project administration, S.W., Z.L. and J.W.; funding acquisition, S.W., Z.L. and J.W. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by Technological Innovation Major Project of Hubei Province grant number (2019AEE023), Key R&D Program of Hubei Province grant number (2020BCB064), Technological Innovation Team in Universities of Hubei Province grant number (Project No. T2020010) And The APC was funded by Scientific Research Starting Foundation of Wuhan Institute of Technology grant number (No. K202021).

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Acknowledgments:** The authors are grateful for financial support from the Technological Innovation Major Project of Hubei Province (2019AEE023), Key R&D Program of Hubei Province (2020BCB064), the Plan of Outstanding Young and Middle-aged Scientific and Technological Innovation Team in Universities of Hubei Province (Project No. T2020010), and Scientific Research Starting Foundation of Wuhan Institute of Technology (No. K202021).

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**


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