Laboratory Investigation of Aging Resistance for Rubberized Bitumen Modified by Using Microwave Activation Crumb Rubber and Different Modifiers

Different modification methods, such as adding modifiers and pretreating crumb rubber, have been developed to achieve decent engineering properties and reduce the viscosity of rubberized bitumen. This study evaluated the influence of the modification methods on the aging resistance for rubberized bitumen. Two types of crumb rubber—a 40-mesh crumb rubber and a microwave-pretreated crumb rubber—and two kinds of modifiers—Sasobit and Trans-polyoctenamer—were selected to prepare rubberized bitumen. The samples were subjected to a Thin-Film Oven Test for the simulation of the short-term aging condition, while a Pressure-Aging-Vessel test was used to simulate the long-term aging condition. The indexes of rubberized bitumen, including softening point, elastic recovery ratio, maximum load, ductility, fracture energy, phase angle, and dynamic modulus, were tested before and after aging. The result showed that trans-polyoctenamer displayed the best resistance to short-term aging, while Sasobit significantly improved the fracture energy of rubberized bitumen after short-term aging. Microwave pretreated partially destroyed the internal structure of crumb rubber, leading to a decrease of short-term aging resistance for rubberized bitumen. Compared with short-term aging, the changing trends of various indexes were basically same, except the discrepancy of properties indexes was reduced after long-term aging.


Introduction
Rubberized bitumen was extensively used in the bitumen pavement because of its excellent performance [1,2]. It has been proved that crumb rubber (CR) effectively improved the resistance to the rutting, low temperature and fatigue cracking, and thermal oxygen aging of pavement [3,4]. However, the differences of density and polarity are the main causes for incompatibility between CR and bitumen, which results in the lack of storage stability. The swelling of CR also leads to an excessively high viscosity of rubberized bitumen. To resolve the limitation of rubberized bitumen, different modification methods have been proposed to reduce the incompatibility and high viscosity of rubberized bitumen [5][6][7][8][9][10]. Warm mix additives (WMA) and pre-degradation CR were two effect methods to decrease the high viscosity of rubberized bitumen. Another method is to add reactive modifiers such as trans-polyoctenamer (TOR), the purpose of which is to improve the compatibility between CR and bitumen [11]. Previous studies proved that the effects of these methods on the

Objective and Approach
The main objective of this study is to evaluate the influence of different modification methods on the aging resistance of rubberized bitumen with an identical preparation process. The following subtasks were implemented to achieve the objective of this study.


The TFOT and PAV test were used for the simulations of the aging conditions.  Indexes including softening point, elastic recovery ratio, maximum load, ductility, and fracture energy were tested before aging, and after STA and LTA to evaluate the physical properties of rubberized bitumen.  Dynamic shear rheometer (DSR) tests were conducted on rubberized bitumen samples (before aging, subjected to TFOT, and subjected to TFOT + PAV) to obtain the indexes including phase angle and dynamic modulus.
The differences of the indexes before aging, and after STA and LTA were compared and analyzed to evaluate the effect of the modification methods on the aging resistance of rubberized bitumen. The flowchart of research steps is presented in Figure 1.  Table 1 presents the physical properties of SK#90 bitumen. CR was supplied by Xinyuda Environmental Protection Technology Co., Ltd., Wuwei, China. A household microwave oven (Midea Co. Ltd., Shunde, China) was used to apply microwave radiation to the rubber. The dried and dehydrated CR was subjected to microwave oven under a power of 800 W for 150 s. The microstructures of CR and pretreated CR are shown in Figure 2. As can be seen from Figure 2, the surface of CR is relatively flat and smooth. After microwave irradiation, the surface of CR became porous and uneven. This phenomenon can increase the reaction area and promote the swelling of CR in bitumen [11].

Materials and Samples Preparation
TOR is a solid polymer with a double bond structure that can crosslink the sulfur of the asphaltene and the sulfur on the CR's surface to form a ring and mesh structure composed of chain polymers [7]. It was supplied by EVONIK Company and its application rate was 4.5% by weight of CR.
The WMA used in this research was Sasobit, which was supplied by Sasol-Wax Co., Ltd., Johannesburg, South Africa. The application rate of Sasobit was 1.0% by weight of bitumen. It is a The TFOT and PAV test were used for the simulations of the aging conditions. Materials 2020, 11, x FOR PEER REVIEW 3 of 16 Vessel (PAV) test were used to simulate aging conditions. The comparison of various indexes before and after aging was discussed to evaluate the aging resistance of rubberized bitumen.

Objective and Approach
The main objective of this study is to evaluate the influence of different modification methods on the aging resistance of rubberized bitumen with an identical preparation process. The following subtasks were implemented to achieve the objective of this study.  The TFOT and PAV test were used for the simulations of the aging conditions.  Indexes including softening point, elastic recovery ratio, maximum load, ductility, and fracture energy were tested before aging, and after STA and LTA to evaluate the physical properties of rubberized bitumen.  Dynamic shear rheometer (DSR) tests were conducted on rubberized bitumen samples (before aging, subjected to TFOT, and subjected to TFOT + PAV) to obtain the indexes including phase angle and dynamic modulus.
The differences of the indexes before aging, and after STA and LTA were compared and analyzed to evaluate the effect of the modification methods on the aging resistance of rubberized bitumen. The flowchart of research steps is presented in Figure 1.  Table 1 presents the physical properties of SK#90 bitumen. CR was supplied by Xinyuda Environmental Protection Technology Co., Ltd., Wuwei, China. A household microwave oven (Midea Co. Ltd., Shunde, China) was used to apply microwave radiation to the rubber. The dried and dehydrated CR was subjected to microwave oven under a power of 800 W for 150 s. The microstructures of CR and pretreated CR are shown in Figure 2. As can be seen from Figure 2, the surface of CR is relatively flat and smooth. After microwave irradiation, the surface of CR became porous and uneven. This phenomenon can increase the reaction area and promote the swelling of CR in bitumen [11].

Materials and Samples Preparation
TOR is a solid polymer with a double bond structure that can crosslink the sulfur of the asphaltene and the sulfur on the CR's surface to form a ring and mesh structure composed of chain polymers [7]. It was supplied by EVONIK Company and its application rate was 4.5% by weight of CR.
The WMA used in this research was Sasobit, which was supplied by Sasol-Wax Co., Ltd., Johannesburg, South Africa. The application rate of Sasobit was 1.0% by weight of bitumen. It is a Indexes including softening point, elastic recovery ratio, maximum load, ductility, and fracture energy were tested before aging, and after STA and LTA to evaluate the physical properties of rubberized bitumen. Vessel (PAV) test were used to simulate aging conditions. The comparison of various indexes before and after aging was discussed to evaluate the aging resistance of rubberized bitumen.

Objective and Approach
The main objective of this study is to evaluate the influence of different modification methods on the aging resistance of rubberized bitumen with an identical preparation process. The following subtasks were implemented to achieve the objective of this study.  The TFOT and PAV test were used for the simulations of the aging conditions.  Indexes including softening point, elastic recovery ratio, maximum load, ductility, and fracture energy were tested before aging, and after STA and LTA to evaluate the physical properties of rubberized bitumen.  Dynamic shear rheometer (DSR) tests were conducted on rubberized bitumen samples (before aging, subjected to TFOT, and subjected to TFOT + PAV) to obtain the indexes including phase angle and dynamic modulus.
The differences of the indexes before aging, and after STA and LTA were compared and analyzed to evaluate the effect of the modification methods on the aging resistance of rubberized bitumen. The flowchart of research steps is presented in Figure 1.  Table 1 presents the physical properties of SK#90 bitumen. CR was supplied by Xinyuda Environmental Protection Technology Co., Ltd., Wuwei, China. A household microwave oven (Midea Co. Ltd., Shunde, China) was used to apply microwave radiation to the rubber. The dried and dehydrated CR was subjected to microwave oven under a power of 800 W for 150 s. The microstructures of CR and pretreated CR are shown in Figure 2. As can be seen from Figure 2, the surface of CR is relatively flat and smooth. After microwave irradiation, the surface of CR became porous and uneven. This phenomenon can increase the reaction area and promote the swelling of CR in bitumen [11].

Materials and Samples Preparation
TOR is a solid polymer with a double bond structure that can crosslink the sulfur of the asphaltene and the sulfur on the CR's surface to form a ring and mesh structure composed of chain polymers [7]. It was supplied by EVONIK Company and its application rate was 4.5% by weight of CR.
The WMA used in this research was Sasobit, which was supplied by Sasol-Wax Co., Ltd., Johannesburg, South Africa. The application rate of Sasobit was 1.0% by weight of bitumen. It is a Dynamic shear rheometer (DSR) tests were conducted on rubberized bitumen samples (before aging, subjected to TFOT, and subjected to TFOT + PAV) to obtain the indexes including phase angle and dynamic modulus.
The differences of the indexes before aging, and after STA and LTA were compared and analyzed to evaluate the effect of the modification methods on the aging resistance of rubberized bitumen. The flowchart of research steps is presented in Figure 1. Vessel (PAV) test were used to simulate aging conditions. The comparison of various indexes before and after aging was discussed to evaluate the aging resistance of rubberized bitumen.

Objective and Approach
The main objective of this study is to evaluate the influence of different modification methods on the aging resistance of rubberized bitumen with an identical preparation process. The following subtasks were implemented to achieve the objective of this study.


The TFOT and PAV test were used for the simulations of the aging conditions.  Indexes including softening point, elastic recovery ratio, maximum load, ductility, and fracture energy were tested before aging, and after STA and LTA to evaluate the physical properties of rubberized bitumen.  Dynamic shear rheometer (DSR) tests were conducted on rubberized bitumen samples (before aging, subjected to TFOT, and subjected to TFOT + PAV) to obtain the indexes including phase angle and dynamic modulus.
The differences of the indexes before aging, and after STA and LTA were compared and analyzed to evaluate the effect of the modification methods on the aging resistance of rubberized bitumen. The flowchart of research steps is presented in Figure 1.  Table 1 presents the physical properties of SK#90 bitumen. CR was supplied by Xinyuda Environmental Protection Technology Co., Ltd., Wuwei, China. A household microwave oven (Midea Co. Ltd., Shunde, China) was used to apply microwave radiation to the rubber. The dried and dehydrated CR was subjected to microwave oven under a power of 800 W for 150 s. The microstructures of CR and pretreated CR are shown in Figure 2. As can be seen from Figure 2, the surface of CR is relatively flat and smooth. After microwave irradiation, the surface of CR became porous and uneven. This phenomenon can increase the reaction area and promote the swelling of CR in bitumen [11].

Materials and Samples Preparation
TOR is a solid polymer with a double bond structure that can crosslink the sulfur of the asphaltene and the sulfur on the CR's surface to form a ring and mesh structure composed of chain polymers [7]. It was supplied by EVONIK Company and its application rate was 4.5% by weight of CR.  Table 1 presents the physical properties of SK#90 bitumen. CR was supplied by Xinyuda Environmental Protection Technology Co., Ltd., Wuwei, China. A household microwave oven (Midea Co. Ltd., Shunde, China) was used to apply microwave radiation to the rubber. The dried and dehydrated CR was subjected to microwave oven under a power of 800 W for 150 s. The microstructures of CR and pretreated CR are shown in Figure 2. As can be seen from Figure 2, the surface of CR is relatively flat and smooth. After microwave irradiation, the surface of CR became porous and uneven. This phenomenon can increase the reaction area and promote the swelling of CR in bitumen [11]. granular and opaque pellet polymer, which has long chain aliphatic hydrocarbon obtained from coal gasification [37].  The six rubberized bitumens used in this study adopted the same preparation process. The details and labels of samples are shown in Table 2. CR and modifiers were gradually added to bitumen at a certain temperature and blended under high shear condition. The specific processes are given as follows.

Materials and Samples Preparation
(1) Blend the CR (or pre-treated CR) modifiers with the virgin bitumen previously conditioned at 150 °C by a 300 rpm stirring for 30 min.   TOR is a solid polymer with a double bond structure that can crosslink the sulfur of the asphaltene and the sulfur on the CR's surface to form a ring and mesh structure composed of chain polymers [7]. It was supplied by EVONIK Company and its application rate was 4.5% by weight of CR.
The WMA used in this research was Sasobit, which was supplied by Sasol-Wax Co., Ltd., Johannesburg, South Africa. The application rate of Sasobit was 1.0% by weight of bitumen. It is a granular and opaque pellet polymer, which has long chain aliphatic hydrocarbon obtained from coal gasification [37].
The six rubberized bitumens used in this study adopted the same preparation process. The details and labels of samples are shown in Table 2. CR and modifiers were gradually added to bitumen at a certain temperature and blended under high shear condition. The specific processes are given as follows.
(1) Blend the CR (or pre-treated CR) modifiers with the virgin bitumen previously conditioned at 150 • C by a 300 rpm stirring for 30 min.

Aging Methods
The TFOT at 163 • C for 5 h was used to simulate the STA of rubberized bitumen during bitumen mixture mixing, transportation, and paving progress, while the LTA samples were obtained by PAV tests for 20 h at 100 • C and 2.1 MPa [38]. In addition, the samples used in PAV test were aged by TFOT at 163 • C for 5 h first.

Analysis Methods
The softening point test and elastic recovery test were conducted in accordance with ASTM D36 and ASTM D6084, respectively [39,40]. The recovery test was conducted at 25 • C. The force ductility test (FDT) was conducted at 5 • C by using the samples apply for elastic recovery test. The complex shear modulus (G*) and phase angle (σ) were measured through temperature sweep test and frequency sweep test in accordance with ASTM D7175 [41]. The details of the experimental settings are shown in Table 3. The effect of aging on the high-temperature stability of rubberized bitumen can be evaluated with softening point, which is also suggested to relate to the interaction between CR and bitumen [42]. Figure 3 shows the softening points of different samples before and after STA and LTA. The softening point of RBT was the highest before the aging process. It has reported that TOR could promote the distribution of CR, which increased the high-temperature performance of rubberized bitumen [9].
The unaged RBM showed the lowest softening point, as the microwave destroyed the vulcanization of CR partially. The softening point of unaged RBS improved slightly compared with unaged URB. In addition, for all samples, the softening point increased after aging to varying degrees. Liu [43] indicated that the softening point increment could be an effective index to evaluate the aging resistance of rubberized bitumen. Rubberized bitumen has a smaller softening point increment indicating an excellent resistance to the high temperature deformation. It was calculated following the equations below, where T 1 , T 2 , and T 3 are average softening point before aging, after 5 h TFOT, and after 5 h TFOT + 20 h PAV, respectively.
Materials 2020, 11, x FOR PEER REVIEW 6 of 16 advance [36,44]. Therefore, the main reactions happening in STA can be divided into the aging of bitumen and the swelling and degradation of CR. The microwave irradiation broke down the external and internal chemical bonds of the CR partially [7,11]. The destruction of chemical bond accelerated the desulfurization and degradation of CR particles, and the light component in the bitumen absorbed by the swelling action was released partly. This process weakened the aging resistance of rubberized bitumen, which resulted in a higher softening point increment of RBM. According to Figure 4, the softening point increment of rubberized bitumen had a further increase after LTA. RBM had the highest ∆T2, and RBMT showed the lowest ∆T2 after LTA. Compared with URB, the addition of modifiers reduced the softening point increment of rubberized bitumen after LTA. ∆T2 of URB and RBM was impaired more heavily, indicating there relatively poor performance in LTA. RBMT and RBMS showed lower ∆T2, and this indicated that the combined effect of microwave activation and the modifiers had a favorable influence on the LTA.   Figure 5 presents the effects of STA and LTA on the elastic recovery ratio of different rubberized bitumens. As can be seen, the elastic recovery ratio decreased obviously after LTA. RBT still had the highest elastic recovery ratio after STA, while RBM showed the lowest elastic recovery ratio. This behavior was consistent with the unaged samples. It has been proved that virgin bitumen was viscous and barely exhibited recovery at 25 °C [30]. The presence of CR with higher elasticity endows the rubberized bitumen with resilience. The STA generally stiffened the bitumen because of the components and the volatilization of light fractions [3]. The hardened bitumen made the CR particles face a greater resistance when its recover.

Elastic Recovery Ratio
After LTA, the elastic recovery ratio of different rubberized bitumens decreased consistently. However, the magnitude of the decline was significantly reduced. It was because LTA conditions had a weak effect on the elasticity of CR particles. RBT had the highest elastic recovery ratio after LTA. TOR can effectively enhance the incorporation between CR and bitumen, and it stimulated The results of softening point increment are shown in Figure 4, in which the RBT was the smallest among the results of ∆T 1 , followed by RBS and RB. The rubberized bitumen contained CR with 40-mesh size had a smaller ∆T 1 compared with the rubberized bitumen contained CR activated by microwave. During the STA at 163 • C, the swelling reaction and degradation of CR will gradually advance [36,44]. Therefore, the main reactions happening in STA can be divided into the aging of bitumen and the swelling and degradation of CR. The microwave irradiation broke down the external and internal chemical bonds of the CR partially [7,11]. The destruction of chemical bond accelerated the desulfurization and degradation of CR particles, and the light component in the bitumen absorbed by the swelling action was released partly. This process weakened the aging resistance of rubberized bitumen, which resulted in a higher softening point increment of RBM.
Materials 2020, 11, x FOR PEER REVIEW 6 of 16 advance [36,44]. Therefore, the main reactions happening in STA can be divided into the aging of bitumen and the swelling and degradation of CR. The microwave irradiation broke down the external and internal chemical bonds of the CR partially [7,11]. The destruction of chemical bond accelerated the desulfurization and degradation of CR particles, and the light component in the bitumen absorbed by the swelling action was released partly. This process weakened the aging resistance of rubberized bitumen, which resulted in a higher softening point increment of RBM. According to Figure 4, the softening point increment of rubberized bitumen had a further increase after LTA. RBM had the highest ∆T2, and RBMT showed the lowest ∆T2 after LTA. Compared with URB, the addition of modifiers reduced the softening point increment of rubberized bitumen after LTA. ∆T2 of URB and RBM was impaired more heavily, indicating there relatively poor performance in LTA. RBMT and RBMS showed lower ∆T2, and this indicated that the combined effect of microwave activation and the modifiers had a favorable influence on the LTA.   Figure 5 presents the effects of STA and LTA on the elastic recovery ratio of different rubberized bitumens. As can be seen, the elastic recovery ratio decreased obviously after LTA. RBT still had the highest elastic recovery ratio after STA, while RBM showed the lowest elastic recovery ratio. This behavior was consistent with the unaged samples. It has been proved that virgin bitumen was viscous and barely exhibited recovery at 25 °C [30]. The presence of CR with higher elasticity endows the According to Figure 4, the softening point increment of rubberized bitumen had a further increase after LTA. RBM had the highest ∆T 2 , and RBMT showed the lowest ∆T 2 after LTA. Compared with URB, the addition of modifiers reduced the softening point increment of rubberized bitumen after LTA. ∆T 2 of URB and RBM was impaired more heavily, indicating there relatively poor performance in LTA. RBMT and RBMS showed lower ∆T 2 , and this indicated that the combined effect of microwave activation and the modifiers had a favorable influence on the LTA. Figure 5 presents the effects of STA and LTA on the elastic recovery ratio of different rubberized bitumens. As can be seen, the elastic recovery ratio decreased obviously after LTA. RBT still had the highest elastic recovery ratio after STA, while RBM showed the lowest elastic recovery ratio. This behavior was consistent with the unaged samples. It has been proved that virgin bitumen was viscous and barely exhibited recovery at 25 • C [30]. The presence of CR with higher elasticity endows the rubberized bitumen with resilience. The STA generally stiffened the bitumen because of the components and the volatilization of light fractions [3]. The hardened bitumen made the CR particles face a greater resistance when its recover.

Elastic Recovery Ratio
Materials 2020, 11, x FOR PEER REVIEW 7 of 16 rubberized bitumen to form an elastic structure. RBS had a similar elastic recovery ratio with URB whether it was unaged binder or after STA and LTA, which may be attributable to limit interaction between Sasobit and CR.

Force Ductility
Numerous studies have asserted that the cohesive strength of polymer-modified bitumen could be evaluated by FDT effectively [3,45,46]. The typical force-ductility curve of rubberized bitumen is shown in Figure 6. One of the biggest differences between rubberized bitumen and other binders is that it will present two yielding points in the force-ductility curve of rubberized bitumen. This phenomenon is associated with the elastic recovery ability of CR [3,47]. It can be seen that the force and ductility show a linear relationship until the maximum load (Fmax) is attained. In this region, the force is determined by the cohesive strength of bitumen ( Figure 6A) [45,48]. The force declines after reaching Fmax, and this phenomenon is due to the flow of bitumen and slight deformation of CR ( Figure 6B). Thereafter, the deformation of CR reaches a certain level, and the resilience of CR reacts to bitumen, resulting in a slight increase in force ( Figure 6C) [46,49]. Eventually, the bitumen was fractured and reached failure ductility ( Figure 6D). Indexes including Fmax, ductility, and fracture energy (W) were analyzed to evaluate the cohesive strength of rubberized bitumen.  After LTA, the elastic recovery ratio of different rubberized bitumens decreased consistently. However, the magnitude of the decline was significantly reduced. It was because LTA conditions had a weak effect on the elasticity of CR particles. RBT had the highest elastic recovery ratio after LTA. TOR can effectively enhance the incorporation between CR and bitumen, and it stimulated rubberized bitumen to form an elastic structure. RBS had a similar elastic recovery ratio with URB whether it was unaged binder or after STA and LTA, which may be attributable to limit interaction between Sasobit and CR.

Force Ductility
Numerous studies have asserted that the cohesive strength of polymer-modified bitumen could be evaluated by FDT effectively [3,45,46]. The typical force-ductility curve of rubberized bitumen is shown in Figure 6. One of the biggest differences between rubberized bitumen and other binders is that it will present two yielding points in the force-ductility curve of rubberized bitumen. This phenomenon is associated with the elastic recovery ability of CR [3,47]. It can be seen that the force and ductility show a linear relationship until the maximum load (F max ) is attained. In this region, the force is determined by the cohesive strength of bitumen ( Figure 6A) [45,48]. The force declines after reaching F max , and this phenomenon is due to the flow of bitumen and slight deformation of CR ( Figure 6B). Thereafter, the deformation of CR reaches a certain level, and the resilience of CR reacts to bitumen, resulting in a slight increase in force ( Figure 6C) [46,49]. Eventually, the bitumen was fractured and reached failure ductility ( Figure 6D). Indexes including F max , ductility, and fracture energy (W) were analyzed to evaluate the cohesive strength of rubberized bitumen.
reaching Fmax, and this phenomenon is due to the flow of bitumen and slight deformation of CR ( Figure 6B). Thereafter, the deformation of CR reaches a certain level, and the resilience of CR reacts to bitumen, resulting in a slight increase in force ( Figure 6C) [46,49]. Eventually, the bitumen was fractured and reached failure ductility ( Figure 6D). Indexes including Fmax, ductility, and fracture energy (W) were analyzed to evaluate the cohesive strength of rubberized bitumen.  The F max and ductility of rubberized bitumen before and after aging are shown in Figures 7 and 8. For the unaged rubberized bitumen, the ductility of RBM was largest and its F max was the lowest. This can be attributed to the desulfurization and depolymerization of CR, which can generate light components and modify the ductility of the binder. The ductility of RBT was the lowest and its F max was the highest. TOR had limited effect on the desulfurization and depolymerization of CR, and its main function was to link the CR surface and binders. The ductility of RBS was close to URB, and the difference was 5.5 mm. Sasobit primarily modified the molecular structure of free bitumen at high temperature, and the influence on the flexibility of rubberized bitumen at low temperature was influenced by the properties of CR and its reaction process with bitumen [11]. The Fmax and ductility of rubberized bitumen before and after aging are shown in Figures 7 and  8. For the unaged rubberized bitumen, the ductility of RBM was largest and its Fmax was the lowest. This can be attributed to the desulfurization and depolymerization of CR, which can generate light components and modify the ductility of the binder. The ductility of RBT was the lowest and its Fmax was the highest. TOR had limited effect on the desulfurization and depolymerization of CR, and its main function was to link the CR surface and binders. The ductility of RBS was close to URB, and the difference was 5.5 mm. Sasobit primarily modified the molecular structure of free bitumen at high temperature, and the influence on the flexibility of rubberized bitumen at low temperature was influenced by the properties of CR and its reaction process with bitumen [11].
After STA, the Fmax of different rubberized bitumens increased to various degrees. However, the ductility of different rubberized bitumens had no consistent trend with Fmax. The ductility of RBM and RBMS decreased after STA, and the others tended to increase. This phenomenon may be explained by the swelling and degradation of CR in aging process. Swelling can excite the elasticity of CR, while the degradation of CR can supplement the light component volatilized in the aging process. The constituents of matrix bitumen are volatilized and reconstituted during STA, which led to the stiffness of bitumen. Fmax was mainly affected by the hardness of the bitumen.
It also can be seen from Figures 7 and 8 that Fmax of different rubberized bitumens increased and their ductility decreased after LTA. LTA transforms the bitumen into a stiffer binder. Therefore, the variation tendency of the Fmax and ductility of rubberized bitumen was opposite. In addition, it was interesting that the results of ductility had only a small difference. The Fmax of rubberized bitumen contained microwave activation; CR had a more obvious increase compared with that of rubberized bitumen containing untreated CR. Figure 9 presents the W of rubberized bitumens. The inconsistency of W is associated with the swelling and disintegration of CR, and the aging stages of bitumen [3]. As shown in Figure 9, besides RBM, the W of rubberized bitumens increased after STA. Further, all samples had enhanced W after LTA compared with unaged binders. After STA, RBT and RBS showed a higher W and RBM showed the lowest W. After LTA, RBM showed a lower W than URB. Compared with the results of W after STA, the W of rubberized bitumens increased after LTA, except URB.

Temperature Sweep Test
The temperature sweep test of rubberized bitumen before and after aging was tested by DSR. According to Figure 10, the phase angles of the unaged binders decreased slightly before the temperature at 50 °C, and then increased with the temperature rising. From Figure 10d, after STA, the phase angle curves presented a same trend with the unaged samples. However, the difference between the phase angle curves was reduced. Moreover, the rising slope of the curve slowed down. In Figure 10f, phase angle generally increased with the temperature rising after LTA, and a plateau existed in the phase angle curves. The difference between the phase angle curves further decreased.
For the unaged rubberized bitumen, the binders modified by TOR had a smaller phase angle than others. RBM shown the largest phase angle, which could be explained by the degradation and desulfurization of CR. RBS shown a lower phase angle than URB, but the extent of the improvement was insignificant.
After STA, RBT, and RBMT still maintained small phase angles, but the difference between them increased. RBS had a lower phase angle than URB from 30 °C to 65 °C, but when the temperature beyond 65 °C, the phase angle of RBS was gradually larger than URB. This phenomenon may relate to the change of rheological property of Sasobit with temperature.
After LTA, RBT displayed the lowest phase angle and RBM showed the highest, but the difference between them significantly reduced. In Figure 10a,c,e, the regularity of difference of G* between six binders was basically uniform and consistent with the phase angle. As anticipated, RBT and RBM displayed the highest and lowest G ⁎ respectively before and after aging. After STA, the F max of different rubberized bitumens increased to various degrees. However, the ductility of different rubberized bitumens had no consistent trend with F max . The ductility of RBM and RBMS decreased after STA, and the others tended to increase. This phenomenon may be explained by the swelling and degradation of CR in aging process. Swelling can excite the elasticity of CR, while the degradation of CR can supplement the light component volatilized in the aging process. The constituents of matrix bitumen are volatilized and reconstituted during STA, which led to the stiffness of bitumen. F max was mainly affected by the hardness of the bitumen.
It also can be seen from Figures 7 and 8 that F max of different rubberized bitumens increased and their ductility decreased after LTA. LTA transforms the bitumen into a stiffer binder. Therefore, the variation tendency of the F max and ductility of rubberized bitumen was opposite. In addition, it was interesting that the results of ductility had only a small difference. The F max of rubberized bitumen contained microwave activation; CR had a more obvious increase compared with that of rubberized bitumen containing untreated CR. Figure 9 presents the W of rubberized bitumens. The inconsistency of W is associated with the swelling and disintegration of CR, and the aging stages of bitumen [3]. As shown in Figure 9, besides RBM, the W of rubberized bitumens increased after STA. Further, all samples had enhanced W after LTA compared with unaged binders. After STA, RBT and RBS showed a higher W and RBM showed the lowest W. After LTA, RBM showed a lower W than URB. Compared with the results of W after STA, the W of rubberized bitumens increased after LTA, except URB.

Temperature Sweep Test
The temperature sweep test of rubberized bitumen before and after aging was tested by DSR. According to Figure 10, the phase angles of the unaged binders decreased slightly before the temperature at 50 °C, and then increased with the temperature rising. From Figure 10d, after STA, the phase angle curves presented a same trend with the unaged samples. However, the difference between the phase angle curves was reduced. Moreover, the rising slope of the curve slowed down. In Figure 10f, phase angle generally increased with the temperature rising after LTA, and a plateau existed in the phase angle curves. The difference between the phase angle curves further decreased.

Temperature Sweep Test
The temperature sweep test of rubberized bitumen before and after aging was tested by DSR. According to Figure 10, the phase angles of the unaged binders decreased slightly before the temperature at 50 • C, and then increased with the temperature rising. From Figure 10d, after STA, the phase angle curves presented a same trend with the unaged samples. However, the difference between the phase angle curves was reduced. Moreover, the rising slope of the curve slowed down. In Figure 10f, phase angle generally increased with the temperature rising after LTA, and a plateau existed in the phase angle curves. The difference between the phase angle curves further decreased.

Frequency Sweep Test
As indicated in Figure 11, the phase angles declined smoothly while frequency rose for the unaged binders and binders suffered from STA and LTA, which was mainly because of the existence of CR [50]. It can be observed in Figure 10b that the phase angle of RBT was the lowest and that of RBM was the largest; the orderliness was consistent with the temperature sweep test. Figure 11c,d presents the phase angles and G* of rubberized bitumen after STA. RBT and RBMT showed lower phase angles than others, and RBT was the lowest. This demonstrated that in the frequency sweep test, TOR has a relatively massive impact on the rheological property after STA. RBM showed the highest phase angle and the lowest G* compared with others. This demonstrated that microwave activation has an adverse effect on the high temperature rheological property after STA. Figure 11e,f shows the results of phase angle and G* of rubberized bitumen after LTA. It can be seen that at low frequency, the difference of phase angles between different rubberized bitumen was evident, but with the frequency increasing, it was gradually decreasing.
From Figure 11a,c,e, it can be seen that the G* of rubberized bitumen increased as the frequency increased and that it showed the linear dependence of G* with the frequency in the double logarithmic coordinates. For the unaged rubberized bitumen, the binders modified by TOR had a smaller phase angle than others. RBM shown the largest phase angle, which could be explained by the degradation and desulfurization of CR. RBS shown a lower phase angle than URB, but the extent of the improvement was insignificant.
After STA, RBT, and RBMT still maintained small phase angles, but the difference between them increased. RBS had a lower phase angle than URB from 30 • C to 65 • C, but when the temperature beyond 65 • C, the phase angle of RBS was gradually larger than URB. This phenomenon may relate to the change of rheological property of Sasobit with temperature.
After LTA, RBT displayed the lowest phase angle and RBM showed the highest, but the difference between them significantly reduced. In Figure 10a,c,e, the regularity of difference of G* between six binders was basically uniform and consistent with the phase angle. As anticipated, RBT and RBM displayed the highest and lowest G* respectively before and after aging.

Frequency Sweep Test
As indicated in Figure 11, the phase angles declined smoothly while frequency rose for the unaged binders and binders suffered from STA and LTA, which was mainly because of the existence of CR [50]. It can be observed in Figure 10b that the phase angle of RBT was the lowest and that of RBM was the largest; the orderliness was consistent with the temperature sweep test.

Conclusions
The main objective of this paper is to evaluate the influence of different modification methods on the aging resistance of rubberized bitumen at an identical preparation process. The preliminary findings are concluded as follows.
(1) TOR and Sasobit improved the high-temperature stability and elastic recoverability of rubberized bitumen, but the effect of Sasobit was barely noticeable. However, microwaveactivated rubberized bitumen had an adverse effect on these properties. (2) STA had a significant effect on the high-temperature stability and elastic recoverability of rubberized bitumen. The TOR-modified rubberized bitumen showed the best aging resistance among the six rubberized bitumens. The CR activated by microwave has worse aging resistance compared with other modification methods. From the temperature sweep test and frequency sweep test, it can be seen that compared with unaged binders, the difference between the rheological properties of various rubberized bitumen after STA was reduced. (3) Among various modification methods, TOR modifier showed the best aging resistance to LTA, while microwave activation resulted in a weaker aging resistance due to the cracking of CR.
From the temperature sweep test and frequency sweep test, it can be seen that the difference between the rheological properties of various bitumen after LTA was further reduced. (4) This study evaluated the influence of the modification methods on the aging resistance for rubberized bitumen by analyzing the difference in its properties before and after aging. Future research is suggested to investigate the mechanism of different modification methods before and after aging.  Figure 11c,d presents the phase angles and G* of rubberized bitumen after STA. RBT and RBMT showed lower phase angles than others, and RBT was the lowest. This demonstrated that in the frequency sweep test, TOR has a relatively massive impact on the rheological property after STA. RBM showed the highest phase angle and the lowest G* compared with others. This demonstrated that microwave activation has an adverse effect on the high temperature rheological property after STA. Figure 11e,f shows the results of phase angle and G* of rubberized bitumen after LTA. It can be seen that at low frequency, the difference of phase angles between different rubberized bitumen was evident, but with the frequency increasing, it was gradually decreasing.
From Figure 11a,c,e, it can be seen that the G* of rubberized bitumen increased as the frequency increased and that it showed the linear dependence of G* with the frequency in the double logarithmic coordinates.

Conclusions
The main objective of this paper is to evaluate the influence of different modification methods on the aging resistance of rubberized bitumen at an identical preparation process. The preliminary findings are concluded as follows.
(1) TOR and Sasobit improved the high-temperature stability and elastic recoverability of rubberized bitumen, but the effect of Sasobit was barely noticeable. However, microwave-activated rubberized bitumen had an adverse effect on these properties. (2) STA had a significant effect on the high-temperature stability and elastic recoverability of rubberized bitumen. The TOR-modified rubberized bitumen showed the best aging resistance among the six rubberized bitumens. The CR activated by microwave has worse aging resistance compared with other modification methods. From the temperature sweep test and frequency sweep test, it can be seen that compared with unaged binders, the difference between the rheological properties of various rubberized bitumen after STA was reduced. (3) Among various modification methods, TOR modifier showed the best aging resistance to LTA, while microwave activation resulted in a weaker aging resistance due to the cracking of CR. From the temperature sweep test and frequency sweep test, it can be seen that the difference between the rheological properties of various bitumen after LTA was further reduced. (4) This study evaluated the influence of the modification methods on the aging resistance for rubberized bitumen by analyzing the difference in its properties before and after aging. Future research is suggested to investigate the mechanism of different modification methods before and after aging.