3.1. Evaluation of Complex Modulus G*
Figure 1 presents the complex modulus G* of different bitumen samples before and after long-term aging at 58 °C, 64 °C, 70 °C, 76 °C and 82 °C, respectively. It can be seen from the figure that as the temperature increases, the complex modulus of bitumen sample tends to decrease. The development of complex modulus is attributed to the increase in temperature intensifying the irregular movement of bitumen molecules. As such, less stress is required for the same strain response, reducing the resistance of bitumen to external forces, which finally expresses a reduction in the complex modulus. In addition, the addition of waste plastic increases the complex modulus of the base bitumen. Using 58 °C as an example, the addition of 4.5%, 6% and 7.5% plastic modifier to the base bitumen increases the complex modulus by 205.26%, 619.63% and 1127.55%, respectively. The increase in complex modulus is more pronounced with increasing amounts of plastic incorporated. The incorporation of SBR modifier does not only increase the complex modulus of plastic-modified bitumen. The above results suggest that the integrated modification enhances the permanent deformation resistance of bitumen.
After long-term aging, the complex modulus of different modified bitumen increases substantially. In existing studies, the resistance of bitumen to aging is usually evaluated via complex modulus aging index (CAI), which is the ratio of complex modulus of bitumen after aging to that of bitumen before aging. In this study, to compare the aging resistance of different bitumen samples, the CAI of base bitumen, plastic-modified bitumen and plastic/SBR composite-modified bitumen were calculated; results are shown in
Table 4. As can be seen from the table, the CAI of base bitumen is the highest, indicating that after long-term aging, the complex modulus of base bitumen varies most significantly. In contrast, the aging index CAI of bitumen gradually decreases with the incorporation of plastic modifier. Using 58 °C as an example, the addition of 4.5%, 6% and 7.5% plastic modifier to the base bitumen decreases the CAI by 12.7%, 16.7% and 38.3%, respectively. The decrease in CAI becomes more pronounced as the amount of plastic modifier is increased. This indicates that the addition of plastic modifier has improved the aging resistance of bitumen. In addition, the integrate modification of plastic and SBR resulted in the smallest CAI for composite-modified bitumen. The CAI of SBR/plastic composite-modified bitumen is far less than that of plastic-modified bitumen, indicating that compared with plastic, the application of SBR is more effective in improving the aging resistance of bitumen. For plastic/SBR composite-modified bitumen with 6% and 7.5% plastic inclusion, the aging index CAI is only around 2.5. As such, 6% and 7.5% plastic inclusion are recommended to produce plastic/SBR composite-modified bitumen to obtain the bitumen with excellent aging resistance.
3.2. Evaluation of Phase Angle δ
In addition complex modulus, the phase angle of bitumen samples also can be obtained via TS test, which reflects the hysteresis of strain to stress.
Figure 2 presents the phase angle of bitumen samples before and after aging at 58 °C, 64 °C, 70 °C, 76 °C and 82 °C, respectively. It can be concluded from the figure that as the temperature rises, the phase angle of bitumen increases, but the increase is not obvious. The addition of plastic modifier reduces the phase angle of bitumen and the decrease becomes apparent with increasing plastic content. Similarly, the addition of SBR modifier further lowers the phase angle of the modified bitumen. This is achieved due to the addition of modifiers creating a support network in the modified bitumen, which leads to an enhanced elastic response for modified bitumen.
After long-term aging, the phase angle of the bitumen is substantially reduced. This is due to the content of polar components such as asphaltenes increasing as the bitumen ages and the elastic response of bitumen is higher after aging. This pattern also applies to plastic-modified bitumen and plastic/SBR composite-modified bitumen with different plastic content, indicating that the aging of modified bitumen results in an increase in elastic response of composite-modified bitumen.
3.4. Evaluation of Percent Recovery and Non-Recoverable Creep Compliance
The percent recovery of different bitumen samples at 0.1 and 3.2 kPa at five temperatures are measured via MSCR test, which characterizes the elastic deformation capacity of bitumen; results are shown in
Figure 4. It can be concluded from the figure that for base bitumen, the percent recovery is low at both 0.1 and 3.2 kPa stress. Modification with plastic or SBR modifier leads to improved percent recovery of bitumen. In comparison, the modification of SBR plays a more vital role in improving the percent recovery of bitumen. Following the long-term aging process, the percent recovery of all base bitumen, plastic-modified bitumen and plastic/SBR composite-modified bitumen increases at five temperatures. For base bitumen, the recovery rate of bitumen after aging is still at a low level. For plastic-modified or plastic/SBR composite-modified bitumen, the improvement of recovery rate at 0.1 and 3.2 kPa after long-term aging is pronounced. Especially for 7.5R-PAV, the percent recovery of bitumen is close to 100%.
Figure 5 shows the non-recoverable creep compliance of bitumen at 0.1 and 3.2 kPa. It is generally accepted that the non-recoverable creep compliance of bitumen correlates well with the resistance to rutting of modified bitumen at high temperature. The lower the non-recoverable creep compliance, the better the high-temperature performance of the bitumen. As concluded from the figure for base bitumen, the non-recoverable creep compliance values J
nr0.1 and J
nr3.2 are high, indicating the high-temperature performance of base bitumen is weak. In contrast, the addition of the plastic modifier reduces the non-recoverable creep compliance of bitumen. The decrease in non-recoverable creep compliance is evident as the amount of plastic modifier content increases. The additional incorporation of SBR further reduces the non-recoverable creep compliance of plastic-modified bitumen. Especially for 7.5% R-VIR, the non-recoverable creep compliance at 0.1 and 3.2 kPa is close to 0. As such, the higher plastic and SBR content is recommended to improve the high-temperature performance of bitumen. After long-term aging, the J
nr0.1 and J
nr3.2 of all base bitumen, plastic-modified bitumen and plastic/SBR composite-modified bitumen decreases dramatically. Especially for plastic-modified bitumen and plastic/SBR composite-modified bitumen, the J
nr0.1 and J
nr3.2 of bitumen after aging are close to 0. As such, for modified bitumen, the high-temperature rutting resistance of bitumen after long-term aging is not a concern. To improve the performance of plastic-modified bitumen, existing studies have tried adding recycled crumb rubber to plastic bitumen. The reported findings in their studies suggested the incorporation of 11% content crumb rubber to 6% plastic-modified bitumen increased non-recoverable creep compliance at 3.2 kPa of bitumen at 58 °C by 20% [
30]. The high-temperature performance of bitumen decreased after composite modification with crumb rubber and plastic. For comparison, the incorporation of 3% SBR to 6% plastic-modified bitumen reduced non-recoverable creep compliance at 3.2 kPa of bitumen at 58 °C by 96.3%. The above results show that the composited modification of SBR is more beneficial for the high-temperature performance of bitumen compared to crumb rubber.
3.5. Evaluation of Molecular Weight Distribution
Figure 6 shows the molecular weight distribution chromatograms of bitumen before and after aging. As can be seen from the figure for bitumen samples, there is a rising peak in the chromatogram around 21 to 22 min (corresponding to molecular weights of about 17,800 to 11,000 Daltons). The elevation of the peak at 21 to 22 min is higher in the chromatogram of the modified bitumen compared to the base bitumen. This indicates that a molecular aggregation process takes place during the modification process. The ongoing aggregation of small molecules into large molecules leads to an increase in the content of large molecular weight substances. After long-term aging, the higher peaks at 21 to 22 min for different bitumen chromatograms are found, suggesting the small molecule aggregation process also occurs during the long-term aging of bitumen.
To quantify the bitumen chromatogram, Li et al., [
31,
32] divided the bitumen chromatogram into large, medium and small molecules based on molecular weight. A schematic diagram of the division is shown in
Figure 7. Using this chromatogram division method, the large molecule size (LMS) percentage of bitumen can be calculated from the following Equation (3).
where
LMSP represents the large molecule size percentage in bitumen, %.
The corresponding
LMSP of base bitumen, plastic-modified bitumen and plastic/SBR composite-modified bitumen before and after long-term aging is calculated via Equation (3); results are shown in
Figure 8. It is quite clear from the figure that the
LMSP of bitumen increases after the modification by plastic and SBR. The addition of 4.5%, 6% and 7.5% plastic modifier to the base bitumen increases the
LMSP by 4.3%, 7.8% and 19.8%. In addition, the addition of SBR to 4.5-VIR, 6-VIR and 7.5-VIR increases the
LMSP by 12.4%, 17.6% and 8.6%, respectively. In addition, the
LMSP of bitumen also increases with long-term aging. This indicates that the aggregation of small molecules occurs during the both modification and aging process of bitumen.
In addition to the LMSP of bitumen, the GPC test also gives the weight average molecular weight (
) of the sample, which is derived via the following equation.
is currently believed to reflect changes in large molecule size in the bitumen samples.
Figure 9 presents the weight average molecular weight of base bitumen, plastic and plastic/SBR composite-modified bitumen before and after aging. As can be seen from the figure, the
of bitumen slightly increases with the modification by plastic. The addition of 4.5%, 6% and 7.5% plastic modifier to the base bitumen increases the
by 0.5%, 4.5% and 4.7%, respectively. However, the lifting effect is not as effective as that with SBR modification. The addition of SBR to the 4.5-VIR, 6-VIR and 7.5-VIR increases the
by 11.8%, 7.4% and 7.5%,respectively. In addition, the
of bitumen increases after a long-term aging process, which is due to the aggregation of small molecules after aging.
where,
Mi represents the molecular weight;
wi represents the weight of molar mass
Mi.
To evaluate the aging resistance of bitumen in terms of the change in molecular size, two evaluation indexes, the large molecular size aging index (
LAI) and the molecular weight aging index (
MAI) (Equations (5) and (6)), are proposed.
Table 5 presents the
LAI and
MAI of base bitumen, plastic-modified bitumen and plastic/SBR composite-modified bitumen. It can be concluded from the table that the base bitumen has the highest
LAI and
MAI. For
LAI, the addition of 4.5%, 6% and 7.5% plastic modifier to the base bitumen decreases
LAI by 14.1%, 14.8% and 14.8%, respectively. For
MAI, the addition of 4.5%, 6% and 7.5% plastic modifier to the base bitumen decreases
MAI by 2.5%, 4.1% and 3.3%, respectively, indicating that the base bitumen is less resistant to aging. The
LAI and
MAI of plastic-modified bitumen and plastic/SBR composite-modified bitumen are lower, indicating that the addition of modifiers improves the aging resistance of the bitumen.
where
LAI,
MAI are the evaluation indexes;
LMSPvir is the
LMSP of bitumen before aging,%;
LMSPage is the
LMSP of bitumen after aging,%.
is the weight average molecular weight of bitumen before aging;
is the weight average molecular weight of bitumen after aging;
3.6. Correlation between Molecular Weight and Rheological Properties
To analyze the effect of molecular weight on the rheological properties of plastics and plastic/SBR-modified bitumen, correlations between
LMSP,
and rutting factor, J
nr3.2 at 58 °C of plastic and plastic/SBR-modified bitumen were established using Pearson correlation analysis.
Table 6 shows the correlation between
LMSP,
and rheological properties of bitumen. As can be seen from the table, the
and rheological properties of plastic and plastic/SBR-modified bitumen do not show a good linear correlation. This is due to the use of a 0.45-micron filter to filter the dissolved bitumen sample prior to the experiment. The size of the plastic particles is greater than 0.45-micron, so the plastic phase will still remain on the filter after filtration. Thus, the
measured via GPC does not provide a realistic representation of the
of the plastic-modified bitumen. In contrast, the
LMSP captures the molecular weight distribution of bitumen, rather than the actual molecular weight. Thus, the
LMSP exhibits a better correlation with the rheological properties of modified bitumen. This indicates that the molecular weight distribution of bitumen affects its rheological properties to some extent.