3.2.2. Storage Stability

Figure 10 shows the softening point difference of the three modified bitumen samples. The density of commonly used CR is about 1.13 g/cm<sup>3</sup> , while that of commonly used 70# base bitumen is about 1.03 g/cm<sup>3</sup> . The CR in CRMB sinks to the bottom of the bitumen over time [2]. Technical specifications for the construction of highway asphalt pavement require that the softening point difference of CRMB should be −5 ◦C to 10 ◦C [3]; the maximum softening point difference of the three modified bitumen samples was 6.85 ◦C, which obviously meets this requirement. With the continuous increase of CR dosage, the

segregation softening point difference of bitumen decreased, which is consistent with Li [35,45]. The softening point difference of 10% CRMB was 42.77% lower than that of 15% CRMB, and the softening point difference of 15% CRMB was only 19.39% lower than that of 20% CRMB. According to Stokes' sedimentation theory, 10% of CR was continuously segregated in the aluminum tube and most of the CR failed to form a stable system at the bottom of the aluminum tube. CR always maintained a certain sedimentation rate, resulting in a large softening point difference. Furthermore, 15% and 20% of CR segregated constantly in the aluminum tube; however, at this time a part of the CR (within 10% to 15%) was able to form a stable system at the bottom of the aluminum tube and the CR sedimentation rate slowed down, meaning that the difference in the softening point was small. It can be seen that there was a dosage value that led the change rate of the CR softening point difference to drop sharply within the dosage of 10% to 15%. *Materials* **2022**, *15*, x FOR PEER REVIEW 13 of 21

**Figure 9.** Viscosity temperature curves of bitumen. **Figure 9.** Viscosity temperature curves of bitumen.

of 20% CRMB. According to Stokes' sedimentation theory, 10% of CR was continuously segregated in the aluminum tube and most of the CR failed to form a stable system at the

ing in a large softening point difference. Furthermore, 15% and 20% of CR segregated constantly in the aluminum tube; however, at this time a part of the CR (within 10% to 15%) was able to form a stable system at the bottom of the aluminum tube and the CR sedimentation rate slowed down, meaning that the difference in the softening point was small. It can be seen that there was a dosage value that led the change rate of the CR

Figure 11 shows the results of the particle size distribution of CR and SCR. It can be seen that the peak position of the particle size curve of SCR is to the right compared with the CR, the peak of CR is at 400 µm, and the peak of SCR is at 502 µm, while the particle size distribution range of SCR is larger than that of CR. The particle size curves of CR and SCR intersect at 532 µm; 66.2% of the total number of CR particles are from 0 µm to 532 µm, and only 35.9 % of the total number of SCR particles are from 0 µm to 532 µm, indicating that the particle size of CR increased after dissolution. Table 6 shows the average particle sizes of CR and SCR. It can be seen that the D10, D50, and D90 of SCR are larger than those of CR. The area average diameter (D[3,2]) and volume average diameter (D[4,3]) of SCR are larger than CR as we, with D[3,2] being increased by 52.3% and D[4,3] increased by 49.7%. In addition, it can be seen that the volume of CR expanded by about 50% under this preparation condition, while its mesh size decreased. The above conclusions prove that CR undergoes a swelling reaction and its volume increases after CR ab-

softening point difference to drop sharply within the dosage of 10% to 15%.

sorbs the light components of bitumen via the action of the bitumen [9,46].

SCR

0 500 1000 1500 2000

Particle Size (祄 )

[35,45]. The softening point difference of 10% CRMB was 42.77% lower than that of 15% CRMB, and the softening point difference of 15% CRMB was only 19.39% lower than that **Figure 10.** Softening Point Difference of CRMB. **Figure 10.** Softening Point Difference of CRMB.

*3.3. Swelled Mechanism of CR* 

0

1

2

3

4

Volume (%)

5

6

7

8

9

CR

**Figure 11.** Particle size distribution curve of CR and SCR.

#### *3.3. Swelled Mechanism of CR* Figure 11 shows the results of the particle size distribution of CR and SCR. It can be

0

1

2

3

4

Softening point difference (℃)

5

6

7

8

*3.3. Swelled Mechanism of CR*

#### 3.3.1. Particle Size Distribution of CR seen that the peak position of the particle size curve of SCR is to the right compared with

3.3.1. Particle Size Distribution of CR

**Figure 10.** Softening Point Difference of CRMB.

Figure 11 shows the results of the particle size distribution of CR and SCR. It can be seen that the peak position of the particle size curve of SCR is to the right compared with the CR, the peak of CR is at 400 µm, and the peak of SCR is at 502 µm, while the particle size distribution range of SCR is larger than that of CR. The particle size curves of CR and SCR intersect at 532 µm; 66.2% of the total number of CR particles are from 0 µm to 532 µm, and only 35.9 % of the total number of SCR particles are from 0 µm to 532 µm, indicating that the particle size of CR increased after dissolution. Table 6 shows the average particle sizes of CR and SCR. It can be seen that the D10, D50, and D90 of SCR are larger than those of CR. The area average diameter (D[3,2]) and volume average diameter (D[4,3]) of SCR are larger than CR as we, with D[3,2] being increased by 52.3% and D[4,3] increased by 49.7%. In addition, it can be seen that the volume of CR expanded by about 50% under this preparation condition, while its mesh size decreased. The above conclusions prove that CR undergoes a swelling reaction and its volume increases after CR absorbs the light components of bitumen via the action of the bitumen [9,46]. the CR, the peak of CR is at 400 µm, and the peak of SCR is at 502 µm, while the particle size distribution range of SCR is larger than that of CR. The particle size curves of CR and SCR intersect at 532 µm; 66.2% of the total number of CR particles are from 0 µm to 532 µm, and only 35.9 % of the total number of SCR particles are from 0 µm to 532 µm, indicating that the particle size of CR increased after dissolution. Table 6 shows the average particle sizes of CR and SCR. It can be seen that the D10, D50, and D90 of SCR are larger than those of CR. The area average diameter (D[3,2]) and volume average diameter (D[4,3]) of SCR are larger than CR as we, with D[3,2] being increased by 52.3% and D[4,3] increased by 49.7%. In addition, it can be seen that the volume of CR expanded by about 50% under this preparation condition, while its mesh size decreased. The above conclusions prove that CR undergoes a swelling reaction and its volume increases after CR absorbs the light components of bitumen via the action of the bitumen [9,46].

10% CRMB 15% CRMB 20% CRMB

*Materials* **2022**, *15*, x FOR PEER REVIEW 14 of 21

**Figure 11.** Particle size distribution curve of CR and SCR. **Figure 11.** Particle size distribution curve of CR and SCR.



#### 3.3.2. Micromorphology of CR

Figure 12a,b shows the microscopic morphology of CR. It can be seen that the surface smoothness of CR is better and angularity is prominent; this is the result of high-strength shear tires, the surface of which have a visual "hardness feel". Figure 12c,d shows the microscopic morphological results of SCR. It can be seen that after the swelling effect of bitumen, the swelling effect on the surface morphology of CR is more obvious; more holes and obvious grooves appear on the surface of SCR, the smoothness is reduced and the complexity is higher. The surface of SCR has a soft flocculent edge, in contrast to the hard angularity of CR, and intuitively the surface of SCR has a "delicate feel". Under the effect of swelling, the admixture in CR falls off, leading to more holes and grooves, and in turn

led to the larger specific surface area and stronger interface sense of SCR, which make SCR more stable in the bitumen. led to the larger specific surface area and stronger interface sense of SCR, which make SCR more stable in the bitumen.

Figure 12a,b shows the microscopic morphology of CR. It can be seen that the surface smoothness of CR is better and angularity is prominent; this is the result of high-strength shear tires, the surface of which have a visual "hardness feel". Figure 12c,d shows the microscopic morphological results of SCR. It can be seen that after the swelling effect of bitumen, the swelling effect on the surface morphology of CR is more obvious; more holes and obvious grooves appear on the surface of SCR, the smoothness is reduced and the complexity is higher. The surface of SCR has a soft flocculent edge, in contrast to the hard angularity of CR, and intuitively the surface of SCR has a "delicate feel". Under the effect of swelling, the admixture in CR falls off, leading to more holes and grooves, and in turn

**Figure 12.** Micromorphology: (**a**) CR 1000×; (**b**) CR 5000×; (**c**) SCR 1000×; (**d**) SCR 5000×. **Figure 12.** Micromorphology: (**a**) CR 1000×; (**b**) CR 5000×; (**c**) SCR 1000×; (**d**) SCR 5000×.

#### 3.3.3. Relative Molecular Weight of CR 3.3.3. Relative Molecular Weight of CR

*Materials* **2022**, *15*, x FOR PEER REVIEW 15 of 21

**Table 6.** Statistics of average particle size for CR and SCR.

3.3.2. Micromorphology of CR

**Sample D10 (µm) D50 (µm) D90 (µm) D[3,2] (µm) D[4,3] (µm)**  CR 137.976 322.548 592.279 214.524 345.878 SCR 175.095 449.226 969.408 326.616 517.881

> Figure 13 shows the molecular weight distribution of CR and SCR. After swelling, the molecular weight distribution curve of CR shifts to the left, indicating that CR mainly undergoes the transformation from large molecules to small molecules during the swelling reaction. Table 7 shows the average molecular weight (Mn), heavy average molecular weight (Mw), and polydispersity (pD) of CR and SCR. It can be seen that after CR Figure 13 shows the molecular weight distribution of CR and SCR. After swelling, the molecular weight distribution curve of CR shifts to the left, indicating that CR mainly undergoes the transformation from large molecules to small molecules during the swelling reaction. Table 7 shows the average molecular weight (Mn), heavy average molecular weight (Mw), and polydispersity (pD) of CR and SCR. It can be seen that after CR swelling, the pD increased by 71.83%, indicating that CR swelling made the distribution of CR significantly wider and the concentricity of molecular weight distribution worse. It can be seen that Mn decreased by 35.80% and Mw increased by 10.25%. The decrease in the value of Mn was 3.5 times the increase in the value of Mw, indicating that the molecular weight of CR became smaller after swelling, and there was a trend of small molecules turning into large molecules [47].

turning into large molecules [47].

swelling, the pD increased by 71.83%, indicating that CR swelling made the distribution of CR significantly wider and the concentricity of molecular weight distribution worse. It can be seen that Mn decreased by 35.80% and Mw increased by 10.25%. The decrease in the value of Mn was 3.5 times the increase in the value of Mw, indicating that the molec-

**Figure 13.** Molecular weight distribution of CR and SCR. **Figure 13.** Molecular weight distribution of CR and SCR.



SCR 407 2259 5.55 Figure 14 show the chromatogram of CR and SCR. It can be seen that after swelling, the strong peak of CR at the combination of MMS and SMS region shifts to the small molecule region after the dissolution; only one peak of the original two strong peaks of CR remains and is completely attributed to the small molecule region. This indicates that breakage of the medium molecular main Chain of CR occurs, producing small molecular weight molecules. The macromolecular region did not change significantly, and only the lifting of the peaks was somewhat slowed down, which did not affect the macromolecules significantly under this preparation condition. Table 8 shows the relative proportions of LMS, MMS, and SMS for CR and SCR, respectively; it can be seen that the proportion of MMS decreases by 40%, while the proportion of SMS increases by 50.5%. The CR swelling process is characterized by the transition from large and medium molecules to small molecules. The small increase in the proportion of LMS is due to the transition from large and Figure 14 show the chromatogram of CR and SCR. It can be seen that after swelling, the strong peak of CR at the combination of MMS and SMS region shifts to the small molecule region after the dissolution; only one peak of the original two strong peaks of CR remains and is completely attributed to the small molecule region. This indicates that breakage of the medium molecular main Chain of CR occurs, producing small molecular weight molecules. The macromolecular region did not change significantly, and only the lifting of the peaks was somewhat slowed down, which did not affect the macromolecules significantly under this preparation condition. Table 8 shows the relative proportions of LMS, MMS, and SMS for CR and SCR, respectively; it can be seen that the proportion of MMS decreases by 40%, while the proportion of SMS increases by 50.5%. The CR swelling process is characterized by the transition from large and medium molecules to small molecules. The small increase in the proportion of LMS is due to the transition from large and medium molecules to small molecules as well. A number of the original medium molecules can be classified as large molecules after lysis, i.e., "medium molecules" were medium molecules in CR and large molecules in SCR. Furthermore, when the CR was swollen, only a small amount of large molecules were transformed into small and medium molecules, which is consistent with the above conclusion and confirms that large molecules were less susceptible to CR interactions with bitumen than small molecules [48].

medium molecules to small molecules as well. A number of the original medium molecules can be classified as large molecules after lysis, i.e., "medium molecules" were medium molecules in CR and large molecules in SCR. Furthermore, when the CR was swollen, only a small amount of large molecules were transformed into small and medium molecules, which is consistent with the above conclusion and confirms that large molecules were less susceptible to CR interactions with bitumen than small molecules [48].

**Figure 14.** Chromatogram of CR and SCR. **Figure 14.** Chromatogram of CR and SCR.

**Table 8.** Relative proportion of three regions of CR and SCR.


#### SCR 16.0% 32.7% 46.8% 3.3.4. Chemical Structure of CR

3.3.4. Chemical Structure of CR Figure 15 shows the infrared spectra of CR and SCR. For convenience, Figure 15 only shows the infrared spectra from 3000 cm−1 to 500 cm−1. It can be seen that the infrared spectra of CR have absorption peaks at 2923 cm−1, 1427 cm−1, 1253 cm−1, etc. These absorption peaks are caused by the vibration of hydrocarbon bonds, and it is known that hydrocarbons are the main components of CR. The presence of absorption peaks at 2923 cm−1, 970 cm−1, 814 cm−1 to 657 cm−1, 575 cm−1 to 540 cm−1, and 521 cm−1 to 477 cm−1 indicates that the CR contained unsaturated bis- and benzene ring-conjugated olefins linked by sulfide bonds [8]. Comparing the infrared spectra of CR and SCR, the two are obviously different, and it can be concluded that the chemical structure of CR changes when it swells in the bitumen. Specifically, the original CR has an absorption peak at 1427 cm−1 and a strong absorption peak at 1297 cm−1 to 1037 cm−1, indicating the presence of ester groups, while only a few ester groups are present in SCR after CR swelling. The original CR has an absorption peak at 1190 cm−1, indicating the presence of amide, while this amide disappears after CR swelling. CR added a =C-H stretching vibration on the benzene ring at 2657 cm−1, indicating the emergence of a new benzene ring structure in SCR, while SCR added a =C-Figure 15 shows the infrared spectra of CR and SCR. For convenience, Figure 15 only shows the infrared spectra from 3000 cm−<sup>1</sup> to 500 cm−<sup>1</sup> . It can be seen that the infrared spectra of CR have absorption peaks at 2923 cm−<sup>1</sup> , 1427 cm−<sup>1</sup> , 1253 cm−<sup>1</sup> , etc. These absorption peaks are caused by the vibration of hydrocarbon bonds, and it is known that hydrocarbons are the main components of CR. The presence of absorption peaks at 2923 cm−<sup>1</sup> , 970 cm−<sup>1</sup> , 814 cm−<sup>1</sup> to 657 cm−<sup>1</sup> , 575 cm−<sup>1</sup> to 540 cm−<sup>1</sup> , and 521 cm−<sup>1</sup> to 477 cm−<sup>1</sup> indicates that the CR contained unsaturated bis- and benzene ring-conjugated olefins linked by sulfide bonds [8]. Comparing the infrared spectra of CR and SCR, the two are obviously different, and it can be concluded that the chemical structure of CR changes when it swells in the bitumen. Specifically, the original CR has an absorption peak at 1427 cm−<sup>1</sup> and a strong absorption peak at 1297 cm−<sup>1</sup> to 1037 cm−<sup>1</sup> , indicating the presence of ester groups, while only a few ester groups are present in SCR after CR swelling. The original CR has an absorption peak at 1190 cm−<sup>1</sup> , indicating the presence of amide, while this amide disappears after CR swelling. CR added a =C-H stretching vibration on the benzene ring at 2657 cm−<sup>1</sup> , indicating the emergence of a new benzene ring structure in SCR, while SCR added a =C-NH stretching vibration on the benzene ring at 1956 cm−<sup>1</sup> . After the swelling of CR, its characteristic peak at 1427 cm−<sup>1</sup> is significantly weakened, and it is known that bitumen can break the methyl conjugate bond of CR. SCR has a C=C bond absorption peak at 1528 cm−<sup>1</sup> , while the C–C bond of CR disappears at 873 cm−<sup>1</sup> and its weak C–S bond disappears at 640 cm−<sup>1</sup> and 574 cm−<sup>1</sup> . It can be seen that the C–C and C–S of bonds of the CR were broken after swelling, promoting the formation of the C=C bond to a certain extent.

broken after swelling, promoting the formation of the C=C bond to a certain extent.

NH stretching vibration on the benzene ring at 1956 cm−1. After the swelling of CR, its characteristic peak at 1427 cm−1 is significantly weakened, and it is known that bitumen

at 640 cm−1 and 574 cm−1. It can be seen that the C-C and C-S of bonds of the CR were

**Figure 15.** Infrared spectrum of CR and SCR. **Figure 15.** Infrared spectrum of CR and SCR.

#### **4. Conclusions**

crease of 42.8%.

33.16%.

**4. Conclusions**  To study the physicochemical properties of CR and SCR and the working properties of CRRB, we prepared CRMB, then SCR and CRRB were separated from CRMB and To study the physicochemical properties of CR and SCR and the working properties of CRRB, we prepared CRMB, then SCR and CRRB were separated from CRMB and tested. The working properties and high and low-temperature properties of CRMB and CRRB were investigated to characterize the physicochemical properties of CR before and after swelling in order to study the swelling mechanism.

tested. The working properties and high and low-temperature properties of CRMB and CRRB were investigated to characterize the physicochemical properties of CR before and after swelling in order to study the swelling mechanism. (1) The results of our high-temperature performance tests showed that CR significantly improved the high-temperature rutting resistance of bitumen. The high-temperature failure temperature of 10% CRMB was 30.62% higher than that of 70# base bitumen, and its high-temperature PG grade rapidly improved by four grades. Meanwhile, CR sig-(1) The results of our high-temperature performance tests showed that CR significantly improved the high-temperature rutting resistance of bitumen. The high-temperature failure temperature of 10% CRMB was 30.62% higher than that of 70# base bitumen, and its hightemperature PG grade rapidly improved by four grades. Meanwhile, CR significantly improved the elastic recovery of bitumen; the elastic recovery rate of 10% CRMB was improved by 339.89% compared to 70# base bitumen. After modification by CR, the dosage of CR was positively correlated with the low-temperature performance of bitumen; CR decelerated the failure temperature decrease rate of bitumen in an ultra-low temperature environment, and its ultra-low temperature crack resistance was enhanced.

nificantly improved the elastic recovery of bitumen; the elastic recovery rate of 10% CRMB was improved by 339.89% compared to 70# base bitumen. After modification by CR, the dosage of CR was positively correlated with the low-temperature performance of bitumen; CR decelerated the failure temperature decrease rate of bitumen in an ultra-low temperature environment, and its ultra-low temperature crack resistance was enhanced. (2) CR raised the viscosity of bitumen, resulting in a significant increase in mixing and compaction temperatures, which can be mitigated by the addition of warm mixes. After modification with CR, the separation softening point difference of CRMB decreased with the increase in dosage, and its storage stability was better. The results in terms of softening point difference for the three dosages show that the softening point difference of 10% CRMB decreases more significantly than that of 15% CRMB, with a specific decrease of 42.8%.

(2) CR raised the viscosity of bitumen, resulting in a significant increase in mixing and compaction temperatures, which can be mitigated by the addition of warm mixes. After modification with CR, the separation softening point difference of CRMB decreased with the increase in dosage, and its storage stability was better. The results in terms of (3) When CR was swollen it absorbed light components of bitumen, increasing the relative content of the heavy component of bitumen and decreasing the relative content of oil. Therefore, CRRB maintained good high-temperature performance; its high-temperature performance was close to that of 10% CRMB, although its low-temperature performance

softening point difference for the three dosages show that the softening point difference of 10% CRMB decreases more significantly than that of 15% CRMB, with a specific de-

relative content of the heavy component of bitumen and decreasing the relative content of oil. Therefore, CRRB maintained good high-temperature performance; its high-temperature performance was close to that of 10% CRMB, although its low-temperature performance was poor, and there were obvious faults with the other four kinds of bitumen. Furthermore, due to its increased elastic component, CRRB had an elastic recovery rate of

(4) After CR swelling, the size distribution range of SCR was larger than that of CR

and the size of CR increased, as shown by the expansion of CR volume by about 50%. The surface of the swollen CR became more complex, with a larger specific surface area and a was poor, and there were obvious faults with the other four kinds of bitumen. Furthermore, due to its increased elastic component, CRRB had an elastic recovery rate of 33.16%.

(4) After CR swelling, the size distribution range of SCR was larger than that of CR and the size of CR increased, as shown by the expansion of CR volume by about 50%. The surface of the swollen CR became more complex, with a larger specific surface area and a stronger sense of interface. After the swelling reaction, the molecular weight distribution of SCR increased by 71.8% compared to CR and the percentage of MMS decreased by 40%, while the percentage of SMS increased by 50.5%. In addition, CR carried out the transformation process from large and medium molecules to small molecules. After swelling, the ester group content of CR decreased significantly and a new benzene ring structure appeared. Finally, the C–C and C–S bonds of the CR were broken to generate partial C=C bonds.

At this stage, although the performance of CRMB and CRRB and the physicochemical properties of SCR were studied, CR in a bitumen settling system needs to be further explored in conjunction with Stokes' sedimentation theory. This study investigated only the performance of CRMB; the next step should be from the perspective of CRMB research for green construction.

**Author Contributions:** Conceptualization, writing—original draft preparation, methodology, H.Z.; Conceptualization, M.Z.; Software, project administration, funding acquisition, writing—review and editing, Y.L.; validation, writing—review and editing, A.C.; formal analysis, Y.Z. and L.L.; investigation, D.G.; data curation, S.Z.; validation, supervision, F.W. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding. Financial support was provided by the transportation technology project of the department of transport of Hubei province (No. 2022-11-1-10), the scientific research fund project of the Wuhan institute of technology (No. K2021032), the strength formation mechanism and application of self-compacting asphalt pavement materials at ambient temperature for urban roads of science and technology planning project of Hubei Provincial Department of Housing and Urban-Rural Development (No. 202171), and test help from Shiyanjia Lab (www.shiyanjia.com).

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

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

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

#### **References**

