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Physical and rheological evaluation of aging behaviors of SBS modified asphalt with thermochromic powders
Construction and Building Materials 193 (2018) 135–141
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Physical and rheological evaluation of aging behaviors of SBS modified asphalt with thermochromic powders Lu Li, Henglong Zhang ⇑, Zihao Chen, Chuanwen Wei Key Laboratory for Green & Advanced Civil Engineering Materials and Application Technology of Hunan Province, College of Civil Engineering, Hunan University, Changsha 410082, China
h i g h l i g h t s Thermochromic powders had a slight effect on high-temperature properties of SBS modified asphalt. Thermochromic powders could improve low-temperature performance of SBS modified asphalt. Thermochromic powders could significantly improve the aging resistance of SBS modified asphalt.
a r t i c l e
i n f o
Article history: Received 15 July 2018 Received in revised form 19 September 2018 Accepted 23 October 2018
Keywords: SBS modified asphalt RTP binder Physical properties Rheological properties Anti-aging performance
a b s t r a c t Styrene-butadiene-styrene copolymer (SBS) modified asphalt is widely utilized in road construction. However, aging effect on SBS modified asphalt is still a serious problem that needs to be resolved. It has been demonstrated that thermochromic materials can-not only adjust the road surface temperature, but also improve the aging resistance of base asphalt. This paper mainly investigated the effects of different content (0, 2, 4, 6, 8%) of red thermochromic powders (RTP) on the thermal oxidizing aging of SBS modified asphalt. Thin film oven test (TFOT), pressure aging vessel test (PAV) and ultraviolet radiation (UV), were used to simulate thermo-oxidative aging and ultraviolet aging of RTP binders, respectively. Physical and rheological properties of RTP binders before and after three aging methods were discussed. The results displayed that the red thermochromic powders had a slight effect on high-temperature stability of SBS modified asphalt, while the low-temperature performance grade of the SBS modified asphalt could be improved with the introduction of RTP. Aging behaviors of RTP asphalt binder could be effectively prevented in comparison with blank sample, indicating its good aging resistance which is in accordance with changes in physical and rheological properties of the binders before and after aging. In addition, 2% and 4% RTP binders showed superior aging resistance than 6% and 8% RTP binders. Ó 2018 Elsevier Ltd. All rights reserved.
1. Introduction Styrene-butadiene-styrene copolymer (SBS) modified asphalt is widely utilized in road constructions as a binder [1–3]. Nevertheless, similar to traditional asphalt, aging effect on SBS modified asphalt is still a serious problem, which results in premature failure and reduces the service life of asphalt pavement [4–7]. Researchers founded that the structure of SBS modified asphalt was destroyed during thermo-oxidative aging process due to break of SBS polymer chain and oxidation of base asphalt [8–12]. In addition, the black appearance of asphalt leads to the absorption of larger amounts of solar energy, accelerating various pavement diseases and causing environmental issues [13–16]. Hu et al. dis⇑ Corresponding author. E-mail address: [email protected] (H. Zhang). https://doi.org/10.1016/j.conbuildmat.2018.10.197 0950-0618/Ó 2018 Elsevier Ltd. All rights reserved.
covered that ultraviolet (UV) radiation from sunlight has a severer effect on low temperature properties of asphalt [17,18]. Moreover, repeated temperature fluctuation, lower temperature at night while higher daytime temperature, can cause damage in the asphalt concrete layer and contribute to the top-down longitudinal cracking [19,20]. Thermochromic materials are innovative, and thermochromic asphalt binder regulates the temperature of the road surface. The transformation temperature of thermochromic materials used in this paper is 31 °C. Above the temperature, it reflects solar energy. Under that temperature, it absorbs solar energy [21]. Thermochromic materials are also widely used in other fields because of that properties [22–24]. Hu et al. found that, compared with traditional asphalt, the surface temperature of thermochromic asphalt concretes were higher in cold weather conditions and much lower in hot weather [25]. The maximum decrease in the
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pavement surface temperature can be as high as 15 °C by thermochromic materials in a sunny day with high ambient temperature [26,27]. They also researched the mechanisms by thermal and optical properties, they found that thermochromic asphalt binder showed higher reflectivity and lower conductivity in the nearinfrared range than conventional asphalt [28–30]. Zhang et al. discovered thermochromic asphalt binder exhibit better thermal stability than base asphalt. Meanwhile, thermochromic powders do not affect the low-temperature performance of asphalt. Besides, they also came to the conclusion that the aging performance of binder was better than conventional asphalt [31–33]. It can be concluded through above researches that add powders to base asphalt not only reduced the diurnal temperature difference of pavement but also improved the aging performance of base asphalt. Therefore, it is essential to investigate the influence of thermochromic powders on properties of SBS modified asphalt. This paper mainly studied the effects of different content (0, 2, 4, 6, 8%) of red thermochromic powders (RTP) on the thermal oxidizing aging of SBS modified asphalt. They were named as blank sample and 2, 4, 6, 8% RTP binder. In addition, physical, rheological properties of RTP binders were also discussed. Physical parameters (softening point, ductility, penetration) were conducted. Rheological properties of RTP binder were measured by dynamic shear rheometer (DSR) and bending beam rheometer (BBR). Thin film oven test (TFOT), pressure aging vessel test (PAV) and ultraviolet radiation (UV), were used to assess thermo-oxidative aging of RTP binders, respectively. Aging indices, expressed by changing amplitude of some physical and rheological parameters unaged and aged, were utilized to assess aging degree of RTP binders.
thickness of RTP binder film was about 3 mm. Every sample has 50 g RTP binder that experienced TFOT. Then these samples were UV-aged for 6 days in a chamber and the intension of ultraviolet radiation is 800 lw/cm2. The temperature of aging was 60 °C. The anti-aging properties of the RTP binder can be assessed by the aging index, including softening point increment (SPI), viscosity aging index (VAI), complex modulus increment (CMI) and phase angle increment (PAI). These aging indices can be defined as follows:
2. Experimental section
2.5. DSR and BBR test
2.1. Materials The physical properties of SBS modified asphalt were shown as follows: softening point, 92.1 °C; penetration, 52.1 dmm at 25 °C; ductility, 35.4 cm at 5 °C; viscosity, 2896 mPas at 135 °C. The transformation temperature of red thermochromic materials used in this paper is 31 °C. Thermochromic powders are mainly composed of methyl stearate, urea formaldehyde resin and bisphenol a. The specific gravity of thermochromic powders is 0.25 (water = 1) and average diameter is 3–10 um. When the temperature of the powder is below 31 °C, powder shows red color. When the temperature is above 31 °C, powder shows light-colored. 2.2. Preparation of SBS modified asphalt with thermochromic powders Heated the SBS modified asphalt in an oil cylinder to 180 °C. Calculated ratio (2, 4, 6, 8%) of red thermochromic powders (RTP) were added into SBS modified asphalt and the mixing speed and time were 4000 rpm and 60 min respectively. Then the red thermochromic asphalt binder (named as 2, 4, 6, 8% RTP binder) was finished. Moreover, SBS modified asphalt without any RTP named as blank sample experienced the same processes. In general, there are five samples. 2.3. Aging procedures Short-term thermal oxidizing aging of RTP binder was assessed by TFOT test (ASTM D1754) [34]. This simulation was to put the RTP binder in the thin film oven and then it was heated for 5 h at 163 °C. RTP binder that experienced TFOT was aged for 20 h at 100 °C in air vessel with 2.1 MPa of air pressure to simulate longterm thermal oxidizing aging (ASTM D6521) [35]. Ultraviolet light aging was simulated using ultraviolet aging chamber. The
SPI ¼ Aged softening point Unaged softening point VAI ¼
Aged viscosity value Unaged viscosity value
ð1Þ ð2Þ
CMI ¼ Aged complex modulus Unaged complex modulus
ð3Þ
PAI ¼ Aged phase angle Unaged phase angle
ð4Þ
2.4. Conventional physical tests Physical parameters such as softening point, penetration (25 °C) and ductility (5 °C) of RTP binder, were measured using standard procedures (ASTM D36, ASTM D5 and ASTM D113) [36–38]. Brookfield viscometer was used to test the viscosity of RTP binders at 135 °C corresponding to ASTM D4402 [39].
Complex modulus (G*) and phase angle (d) are used to assess the resistance deformation and viscoelastic equilibrium of RTP binders. They were tested by DSR. Each binder was measured starting from 30 °C to 90 °C with an increment of 2 °C/min. In terms of unaged and TFOT aged RTP binders, the diameter and gap are 25 mm and 1 mm respectively. As for UV and PAV aged RTP binders, the diameter and gap become 8 mm and 2 mm. According to ASTM D6648, the residues of TFOT and PAV were used to be bending beam rheometer (BBR) tests. The sample should be insulated at the test temperature for 1 h before testing. Deformability and stress relaxation ability of RTP binder evaluated by creep stiffness (S) and creep curve slope (m-value). The smaller value of S indicates the RTP binder has better crack resistance at low temperature. For m-value the conclusion is just contrary to S.
3. Results and discussion 3.1. Conventional asphalt properties Fig. 1 shows the physical parameters (softening point, ductility, penetration) of five samples. The results showed that after addition of thermochromic powders, the softening point of RTP binder has a slight decline compared with blank sample. Moreover, with the content of powders increases, penetration of SBS modified asphalt correspondingly decreases. Thermochromic powder are mainly composed of methyl stearate, urea formaldehyde resin and bisphenol a. Methyl stearate, urea formaldehyde resin and bisphenol a are all small molecules substances. The reticular structure of SBS molecules could adhere to other small molecular substances, which could reduce the rheological properties of SBS modified asphalt. In addition, ductility has a small fluctuation with the change of the content of red thermochromic powders, and 4% RTP binder shows the best ductility.
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Fig. 1. Conventional asphalt properties of blank sample and different RTP binder: (a) softening point, (b) ductility at 5 °C, and (c) penetration at 25 °C.
3.2. Analysis of rheological properties 3.2.1. Analysis of DSR test G*, d and rutting factor of five samples, within temperature range between 30 °C and 90 °C, were presented in Fig. 2. It is noticed that, G* of RTP binder was relatively smaller than blank sample, and the G* of each sample was close. The phase angle of five samples has a small fluctuation. The tendency of rutting factor was consistent with G*. G* and rutting factor of 2% and 4% RTP binder decreased slightly compared with blank sample, which shows thermochromic powders have little effect on the shear resistance of SBS modified asphalt. The main component of thermochromic powder is methyl stearate. Methyl stearate is mainly used to make lubricants. The lubrication effect of methyl stearate made the RTP binder relatively poor shear performance. Considering G*, d and rutting factor, thermochromic materials weaken the binder ability to resistance to deformation of SBS modified asphalt, but the effect is small. 3.2.2. Analysis of BBR test The results of S and m-value at the test temperatures (6, 12, 18, 24 °C) are showed in Fig. 3. For each sample, m-value decreases with the temperature while the tendency of S with temperature were just contrary to m-value. The results demonstrated that the deformability and stress relaxation ability of RTP binder became worse when temperature goes down. In addition, under all testing temperatures, S of each RTP binder is much smaller than that of blank sample while m-value is just contrary to S. It reveals that the low-temperature properties of SBS modified asphalt have been improved. It’s worth noting that when the temperature is
below 18 °C, m-value of RTP binder is larger than 0.3 while mvalue of blank sample is less than 0.3 which doesn’t satisfy the specification. Low-temperature properties of SBS modified asphalt will be improved firstly with increment of content of thermochromic powder and then decreased when content is over 6%. Among all samples, the 4% RTP binder shows the best lowtemperature properties. 3.2.3. Result of performance grade (PG) PG testing can be used to estimate the applicable temperature range of asphalt. It can not only take into account the process of asphalt pavement construction and service, but also take into account the possible cracking, rutting and other distresses of asphalt mixture [40,41]. According to ASTM D7643, based on G⁄/ sin d, G⁄ sin d, S and m-value that acquired from rheological properties test, the PG results were determined, and the results were presented in Table 1. It can be known that RTP degraded the high temperatures grading, but all RTP binder exhibit lower low temperatures grading than blank sample. The results showed that RTP could reduce the high-temperature performance of SBS modified asphalt and improve low-temperature performance of SBS modified asphalt. Among all of thermochromic asphalt binder, 4% RTP binder showed the best low temperature performance. 3.3. Anti-aging performance evaluation 3.3.1. Evaluation of SPI and VAI The results of physical aging indices (SPI and VAI) of five samples are displayed in Fig. 4. It can be seen that the value of SPI and VAI of each RTP binder after TFOT is smaller than blank sample
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Table 1 The continuous PG of various asphalt binders. Sample
Blank sample
2%RTP binder
4%RTP binder
6%RTP binder
8%RTP binder
Critical temperature (G*/sind 1.0 KPa for original binder, °C) Critical temperature (G*/sin d 2.2 KPa for TFOT residue, °C) High PG Intermediate PG (G*sin d 5.0 MPa for TFOT + PAV residue, °C) Low PG (S 300 MPa and m-value 0.3) PG
90 82.6 82.6 26.8 24.7 PG82.6-24.7
90 81.6 81.6 25.5 30.1 PG81.6-30.1
90 81.6 81.6 25.2 38 PG81.6-38
90 80.6 80.6 25.2 34 PG80.6-34
90 80.3 80.3 25.3 29.2 PG80.3-29.2
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Fig. 4. SPI and VAI of five samples: (a) SPI, and (b) VAI.
its results are same to UV aging. Seen from the three aging modes, for anti-aging property, 2% RTP binder and 4% RTP binder are better than 6% RTP binder and 8% RTP binder. It is noted that agingresistant of 6% and 8% RTP after UV and PAV was worse than blank sample. The mixture mechanism of SBS molecules and asphalt is physical blending. The reticular structure of SBS molecules can bond to other small molecular materials, and the main component of thermochromic powders is small molecular materials. The phenomenon may be caused by the combination of thermochromic powders and SBS molecules.
values. The result of UV and PAV is same to TFOT, which indicates that the anti-aging performance of RTP binder is better than blank sample. SPI and VAI of 2% RTP and 4% RTP after each aging modes were relatively smaller than other asphalt binders. To sum up, as for TFOT and UV, the anti-aging property of all RTP binders is better than blank sample, and 2% and 4% are the ideal content in which RTP binder show the better anti-aging performance to short-term thermal oxidizing and ultraviolet aging. With respect to PAV, softening point of all five samples was reduced compared with unaged sample. In addition, when the content of thermochromic powders is large enough (i.e., 8%), the viscosity after UV or PAV is smaller than the viscosity of unaged asphalt binder. As the content of powder increases, the lubrication effect of methyl stearate is more obvious.
4. Conclusions This paper mainly investigated the effects of different content of RTP on the thermal oxidizing aging of SBS modified asphalt. In addition, physical, rheological properties of RTP binders before and after three aging methods were also discussed. The main conclusions are stated below.
3.3.2. Evaluation of CMI and PAI For each sample, their rheological aging indices (CMI and PAI) after three aging modes were showed in Fig. 5–7. As for TFOT, it can be known from Fig. 5, 2%, 4% and 6% RTP showed smaller CMI and larger PAI than blank sample. It showed that RTP binder had a better ability to resisted short-term thermal oxidizing. Based on analysis of CMI and PAI, 4% RTP binder shows the best antiaging performance to short-term thermal oxidizing. For UV aging, the 2% RTP binder and 4% RTP binder showed a better antiultraviolet aging than blank sample. Considering the PAV aging,
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Based on physical and rheological properties, thermochromic powders had a slight effect on high-temperature stability of SBS modified asphalt compared with the blank sample. Adding RTP to SBS modified asphalt could extend its lowtemperature performance grade. Additionally, 4% RTP binder represented the best low-temperature performance.
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Compared with blank ample, SBS modified asphalt with 2% and 4% of RTP showed the best anti-aging performance for all of three aging modes. Conflicts of interest There are no conflicts of interest for this paper. Acknowledgements This work was supported by the National Natural Science Foundation of China (No. 51678232), the Hunan Provincial Natural Science Foundation of China (No. 2017JJ3015), the Transportation Science and Technology Development and Innovation Project of Hunan Province (No. 201705), the Science and Technology Planning Project of Changsha (No. kc1703038, kq1706018). The authors gratefully acknowledge their financial support. References [1] L. Zhang, L. Michael, Effects of polymer modification on properties and microstructure of mode asphalt systems, Energy Fuels 22 (2008) 3363–3375.
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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Physical and rheological evaluation of aging behaviors of SBS modified asphalt with thermochromic powders Lu Li, Henglong Zhang ⇑, Zihao Chen, Chuanwen Wei Key Laboratory for Green & Advanced Civil Engineering Materials and Application Technology of Hunan Province, College of Civil Engineering, Hunan University, Changsha 410082, China
h i g h l i g h t s Thermochromic powders had a slight effect on high-temperature properties of SBS modified asphalt. Thermochromic powders could improve low-temperature performance of SBS modified asphalt. Thermochromic powders could significantly improve the aging resistance of SBS modified asphalt.
a r t i c l e
i n f o
Article history: Received 15 July 2018 Received in revised form 19 September 2018 Accepted 23 October 2018
Keywords: SBS modified asphalt RTP binder Physical properties Rheological properties Anti-aging performance
a b s t r a c t Styrene-butadiene-styrene copolymer (SBS) modified asphalt is widely utilized in road construction. However, aging effect on SBS modified asphalt is still a serious problem that needs to be resolved. It has been demonstrated that thermochromic materials can-not only adjust the road surface temperature, but also improve the aging resistance of base asphalt. This paper mainly investigated the effects of different content (0, 2, 4, 6, 8%) of red thermochromic powders (RTP) on the thermal oxidizing aging of SBS modified asphalt. Thin film oven test (TFOT), pressure aging vessel test (PAV) and ultraviolet radiation (UV), were used to simulate thermo-oxidative aging and ultraviolet aging of RTP binders, respectively. Physical and rheological properties of RTP binders before and after three aging methods were discussed. The results displayed that the red thermochromic powders had a slight effect on high-temperature stability of SBS modified asphalt, while the low-temperature performance grade of the SBS modified asphalt could be improved with the introduction of RTP. Aging behaviors of RTP asphalt binder could be effectively prevented in comparison with blank sample, indicating its good aging resistance which is in accordance with changes in physical and rheological properties of the binders before and after aging. In addition, 2% and 4% RTP binders showed superior aging resistance than 6% and 8% RTP binders. Ó 2018 Elsevier Ltd. All rights reserved.
1. Introduction Styrene-butadiene-styrene copolymer (SBS) modified asphalt is widely utilized in road constructions as a binder [1–3]. Nevertheless, similar to traditional asphalt, aging effect on SBS modified asphalt is still a serious problem, which results in premature failure and reduces the service life of asphalt pavement [4–7]. Researchers founded that the structure of SBS modified asphalt was destroyed during thermo-oxidative aging process due to break of SBS polymer chain and oxidation of base asphalt [8–12]. In addition, the black appearance of asphalt leads to the absorption of larger amounts of solar energy, accelerating various pavement diseases and causing environmental issues [13–16]. Hu et al. dis⇑ Corresponding author. E-mail address: [email protected] (H. Zhang). https://doi.org/10.1016/j.conbuildmat.2018.10.197 0950-0618/Ó 2018 Elsevier Ltd. All rights reserved.
covered that ultraviolet (UV) radiation from sunlight has a severer effect on low temperature properties of asphalt [17,18]. Moreover, repeated temperature fluctuation, lower temperature at night while higher daytime temperature, can cause damage in the asphalt concrete layer and contribute to the top-down longitudinal cracking [19,20]. Thermochromic materials are innovative, and thermochromic asphalt binder regulates the temperature of the road surface. The transformation temperature of thermochromic materials used in this paper is 31 °C. Above the temperature, it reflects solar energy. Under that temperature, it absorbs solar energy [21]. Thermochromic materials are also widely used in other fields because of that properties [22–24]. Hu et al. found that, compared with traditional asphalt, the surface temperature of thermochromic asphalt concretes were higher in cold weather conditions and much lower in hot weather [25]. The maximum decrease in the
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pavement surface temperature can be as high as 15 °C by thermochromic materials in a sunny day with high ambient temperature [26,27]. They also researched the mechanisms by thermal and optical properties, they found that thermochromic asphalt binder showed higher reflectivity and lower conductivity in the nearinfrared range than conventional asphalt [28–30]. Zhang et al. discovered thermochromic asphalt binder exhibit better thermal stability than base asphalt. Meanwhile, thermochromic powders do not affect the low-temperature performance of asphalt. Besides, they also came to the conclusion that the aging performance of binder was better than conventional asphalt [31–33]. It can be concluded through above researches that add powders to base asphalt not only reduced the diurnal temperature difference of pavement but also improved the aging performance of base asphalt. Therefore, it is essential to investigate the influence of thermochromic powders on properties of SBS modified asphalt. This paper mainly studied the effects of different content (0, 2, 4, 6, 8%) of red thermochromic powders (RTP) on the thermal oxidizing aging of SBS modified asphalt. They were named as blank sample and 2, 4, 6, 8% RTP binder. In addition, physical, rheological properties of RTP binders were also discussed. Physical parameters (softening point, ductility, penetration) were conducted. Rheological properties of RTP binder were measured by dynamic shear rheometer (DSR) and bending beam rheometer (BBR). Thin film oven test (TFOT), pressure aging vessel test (PAV) and ultraviolet radiation (UV), were used to assess thermo-oxidative aging of RTP binders, respectively. Aging indices, expressed by changing amplitude of some physical and rheological parameters unaged and aged, were utilized to assess aging degree of RTP binders.
thickness of RTP binder film was about 3 mm. Every sample has 50 g RTP binder that experienced TFOT. Then these samples were UV-aged for 6 days in a chamber and the intension of ultraviolet radiation is 800 lw/cm2. The temperature of aging was 60 °C. The anti-aging properties of the RTP binder can be assessed by the aging index, including softening point increment (SPI), viscosity aging index (VAI), complex modulus increment (CMI) and phase angle increment (PAI). These aging indices can be defined as follows:
2. Experimental section
2.5. DSR and BBR test
2.1. Materials The physical properties of SBS modified asphalt were shown as follows: softening point, 92.1 °C; penetration, 52.1 dmm at 25 °C; ductility, 35.4 cm at 5 °C; viscosity, 2896 mPas at 135 °C. The transformation temperature of red thermochromic materials used in this paper is 31 °C. Thermochromic powders are mainly composed of methyl stearate, urea formaldehyde resin and bisphenol a. The specific gravity of thermochromic powders is 0.25 (water = 1) and average diameter is 3–10 um. When the temperature of the powder is below 31 °C, powder shows red color. When the temperature is above 31 °C, powder shows light-colored. 2.2. Preparation of SBS modified asphalt with thermochromic powders Heated the SBS modified asphalt in an oil cylinder to 180 °C. Calculated ratio (2, 4, 6, 8%) of red thermochromic powders (RTP) were added into SBS modified asphalt and the mixing speed and time were 4000 rpm and 60 min respectively. Then the red thermochromic asphalt binder (named as 2, 4, 6, 8% RTP binder) was finished. Moreover, SBS modified asphalt without any RTP named as blank sample experienced the same processes. In general, there are five samples. 2.3. Aging procedures Short-term thermal oxidizing aging of RTP binder was assessed by TFOT test (ASTM D1754) [34]. This simulation was to put the RTP binder in the thin film oven and then it was heated for 5 h at 163 °C. RTP binder that experienced TFOT was aged for 20 h at 100 °C in air vessel with 2.1 MPa of air pressure to simulate longterm thermal oxidizing aging (ASTM D6521) [35]. Ultraviolet light aging was simulated using ultraviolet aging chamber. The
SPI ¼ Aged softening point Unaged softening point VAI ¼
Aged viscosity value Unaged viscosity value
ð1Þ ð2Þ
CMI ¼ Aged complex modulus Unaged complex modulus
ð3Þ
PAI ¼ Aged phase angle Unaged phase angle
ð4Þ
2.4. Conventional physical tests Physical parameters such as softening point, penetration (25 °C) and ductility (5 °C) of RTP binder, were measured using standard procedures (ASTM D36, ASTM D5 and ASTM D113) [36–38]. Brookfield viscometer was used to test the viscosity of RTP binders at 135 °C corresponding to ASTM D4402 [39].
Complex modulus (G*) and phase angle (d) are used to assess the resistance deformation and viscoelastic equilibrium of RTP binders. They were tested by DSR. Each binder was measured starting from 30 °C to 90 °C with an increment of 2 °C/min. In terms of unaged and TFOT aged RTP binders, the diameter and gap are 25 mm and 1 mm respectively. As for UV and PAV aged RTP binders, the diameter and gap become 8 mm and 2 mm. According to ASTM D6648, the residues of TFOT and PAV were used to be bending beam rheometer (BBR) tests. The sample should be insulated at the test temperature for 1 h before testing. Deformability and stress relaxation ability of RTP binder evaluated by creep stiffness (S) and creep curve slope (m-value). The smaller value of S indicates the RTP binder has better crack resistance at low temperature. For m-value the conclusion is just contrary to S.
3. Results and discussion 3.1. Conventional asphalt properties Fig. 1 shows the physical parameters (softening point, ductility, penetration) of five samples. The results showed that after addition of thermochromic powders, the softening point of RTP binder has a slight decline compared with blank sample. Moreover, with the content of powders increases, penetration of SBS modified asphalt correspondingly decreases. Thermochromic powder are mainly composed of methyl stearate, urea formaldehyde resin and bisphenol a. Methyl stearate, urea formaldehyde resin and bisphenol a are all small molecules substances. The reticular structure of SBS molecules could adhere to other small molecular substances, which could reduce the rheological properties of SBS modified asphalt. In addition, ductility has a small fluctuation with the change of the content of red thermochromic powders, and 4% RTP binder shows the best ductility.
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Fig. 1. Conventional asphalt properties of blank sample and different RTP binder: (a) softening point, (b) ductility at 5 °C, and (c) penetration at 25 °C.
3.2. Analysis of rheological properties 3.2.1. Analysis of DSR test G*, d and rutting factor of five samples, within temperature range between 30 °C and 90 °C, were presented in Fig. 2. It is noticed that, G* of RTP binder was relatively smaller than blank sample, and the G* of each sample was close. The phase angle of five samples has a small fluctuation. The tendency of rutting factor was consistent with G*. G* and rutting factor of 2% and 4% RTP binder decreased slightly compared with blank sample, which shows thermochromic powders have little effect on the shear resistance of SBS modified asphalt. The main component of thermochromic powder is methyl stearate. Methyl stearate is mainly used to make lubricants. The lubrication effect of methyl stearate made the RTP binder relatively poor shear performance. Considering G*, d and rutting factor, thermochromic materials weaken the binder ability to resistance to deformation of SBS modified asphalt, but the effect is small. 3.2.2. Analysis of BBR test The results of S and m-value at the test temperatures (6, 12, 18, 24 °C) are showed in Fig. 3. For each sample, m-value decreases with the temperature while the tendency of S with temperature were just contrary to m-value. The results demonstrated that the deformability and stress relaxation ability of RTP binder became worse when temperature goes down. In addition, under all testing temperatures, S of each RTP binder is much smaller than that of blank sample while m-value is just contrary to S. It reveals that the low-temperature properties of SBS modified asphalt have been improved. It’s worth noting that when the temperature is
below 18 °C, m-value of RTP binder is larger than 0.3 while mvalue of blank sample is less than 0.3 which doesn’t satisfy the specification. Low-temperature properties of SBS modified asphalt will be improved firstly with increment of content of thermochromic powder and then decreased when content is over 6%. Among all samples, the 4% RTP binder shows the best lowtemperature properties. 3.2.3. Result of performance grade (PG) PG testing can be used to estimate the applicable temperature range of asphalt. It can not only take into account the process of asphalt pavement construction and service, but also take into account the possible cracking, rutting and other distresses of asphalt mixture [40,41]. According to ASTM D7643, based on G⁄/ sin d, G⁄ sin d, S and m-value that acquired from rheological properties test, the PG results were determined, and the results were presented in Table 1. It can be known that RTP degraded the high temperatures grading, but all RTP binder exhibit lower low temperatures grading than blank sample. The results showed that RTP could reduce the high-temperature performance of SBS modified asphalt and improve low-temperature performance of SBS modified asphalt. Among all of thermochromic asphalt binder, 4% RTP binder showed the best low temperature performance. 3.3. Anti-aging performance evaluation 3.3.1. Evaluation of SPI and VAI The results of physical aging indices (SPI and VAI) of five samples are displayed in Fig. 4. It can be seen that the value of SPI and VAI of each RTP binder after TFOT is smaller than blank sample
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Table 1 The continuous PG of various asphalt binders. Sample
Blank sample
2%RTP binder
4%RTP binder
6%RTP binder
8%RTP binder
Critical temperature (G*/sind 1.0 KPa for original binder, °C) Critical temperature (G*/sin d 2.2 KPa for TFOT residue, °C) High PG Intermediate PG (G*sin d 5.0 MPa for TFOT + PAV residue, °C) Low PG (S 300 MPa and m-value 0.3) PG
90 82.6 82.6 26.8 24.7 PG82.6-24.7
90 81.6 81.6 25.5 30.1 PG81.6-30.1
90 81.6 81.6 25.2 38 PG81.6-38
90 80.6 80.6 25.2 34 PG80.6-34
90 80.3 80.3 25.3 29.2 PG80.3-29.2
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its results are same to UV aging. Seen from the three aging modes, for anti-aging property, 2% RTP binder and 4% RTP binder are better than 6% RTP binder and 8% RTP binder. It is noted that agingresistant of 6% and 8% RTP after UV and PAV was worse than blank sample. The mixture mechanism of SBS molecules and asphalt is physical blending. The reticular structure of SBS molecules can bond to other small molecular materials, and the main component of thermochromic powders is small molecular materials. The phenomenon may be caused by the combination of thermochromic powders and SBS molecules.
values. The result of UV and PAV is same to TFOT, which indicates that the anti-aging performance of RTP binder is better than blank sample. SPI and VAI of 2% RTP and 4% RTP after each aging modes were relatively smaller than other asphalt binders. To sum up, as for TFOT and UV, the anti-aging property of all RTP binders is better than blank sample, and 2% and 4% are the ideal content in which RTP binder show the better anti-aging performance to short-term thermal oxidizing and ultraviolet aging. With respect to PAV, softening point of all five samples was reduced compared with unaged sample. In addition, when the content of thermochromic powders is large enough (i.e., 8%), the viscosity after UV or PAV is smaller than the viscosity of unaged asphalt binder. As the content of powder increases, the lubrication effect of methyl stearate is more obvious.
4. Conclusions This paper mainly investigated the effects of different content of RTP on the thermal oxidizing aging of SBS modified asphalt. In addition, physical, rheological properties of RTP binders before and after three aging methods were also discussed. The main conclusions are stated below.
3.3.2. Evaluation of CMI and PAI For each sample, their rheological aging indices (CMI and PAI) after three aging modes were showed in Fig. 5–7. As for TFOT, it can be known from Fig. 5, 2%, 4% and 6% RTP showed smaller CMI and larger PAI than blank sample. It showed that RTP binder had a better ability to resisted short-term thermal oxidizing. Based on analysis of CMI and PAI, 4% RTP binder shows the best antiaging performance to short-term thermal oxidizing. For UV aging, the 2% RTP binder and 4% RTP binder showed a better antiultraviolet aging than blank sample. Considering the PAV aging,
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Based on physical and rheological properties, thermochromic powders had a slight effect on high-temperature stability of SBS modified asphalt compared with the blank sample. Adding RTP to SBS modified asphalt could extend its lowtemperature performance grade. Additionally, 4% RTP binder represented the best low-temperature performance.
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Fig. 7. CMI and PAI of five samples after PAV: (a) CMI after PAV, (b) PAI after PAV.
Compared with blank ample, SBS modified asphalt with 2% and 4% of RTP showed the best anti-aging performance for all of three aging modes. Conflicts of interest There are no conflicts of interest for this paper. Acknowledgements This work was supported by the National Natural Science Foundation of China (No. 51678232), the Hunan Provincial Natural Science Foundation of China (No. 2017JJ3015), the Transportation Science and Technology Development and Innovation Project of Hunan Province (No. 201705), the Science and Technology Planning Project of Changsha (No. kc1703038, kq1706018). The authors gratefully acknowledge their financial support. References [1] L. Zhang, L. Michael, Effects of polymer modification on properties and microstructure of mode asphalt systems, Energy Fuels 22 (2008) 3363–3375.
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