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SBS composite modified asphalt
Construction and Building Materials 75 (2015) 169–175
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Technical Note
Thermal oxidative aging mechanism of crumb rubber/SBS composite modified asphalt Li Xiang a,b,c,⇑, Jian Cheng b,c, Shunji Kang a a
The College of Post and Telecommunication of Wuhan Institute of Technology, Wuhan 430073, China Key Laboratory for Green Chemical Process of Ministry of Education, Wuhan Institute of Technology, Wuhan 430073, China c Hubei Key Laboratory of Novel Chemical Reactor & Green Chemical Technology, Wuhan Institute of Technology, Wuhan 430073, China b
h i g h l i g h t s The thermal oxygen aging mechanism of CR/SBSCMA has been discussed. IR and group components test have been used for the investigation of aging mechanism. The aging process of CR/SBSCMA was different from CRMA and SBSMA. The reaction process of group components of CR/SBSCMA was presented.
a r t i c l e
i n f o
Article history: Received 11 April 2014 Received in revised form 31 July 2014 Accepted 23 August 2014
Keywords: Crumb rubber SBS Composite modified asphalt Thermal oxidative aging Group components
a b s t r a c t Crumb rubber/SBS composite modified asphalt (CR/SBSCMA) technology is proposed to combine the advantages of crumb rubber (CR) and SBS. Then, the production cost of the modified asphalt could be reduced, the waste rubber resources could be made full utilization, and the performance of asphalt could be improved. Rotate thin film oven test was used for stimulating the thermal oxidative aging of matrix asphalt, crumb rubber modified asphalt (CRMA), SBS modified asphalt (SBSMA) and CR/SBSCMA. The influences of matrix asphalt, CR and SBS on the aging properties have been investigated based on the composition and structure analysis of asphalt and modified asphalt pre and post aging process. Then, the thermal oxidative aging mechanism of CR/SBSCMA has been discussed. The test results demonstrated that the carbonyl index and sulfoxide index could reflect the aging degree of asphalt well. Moreover, due to the presence of CR and SBS at the same time, the aging process of CR/SBSCMA was different from CRMA and SBSMA. In the aging process of CR/SBSCMA, the modifiers degradation products and asphalt secondary components reacted severely, which resulted in the decrease of aromatics and resin and the increase of asphaltenes and toluene insoluble. Meanwhile, the distribution and colloid structure of composite modified asphalt were changed by the modifiers degradation products, the asphalt secondary components and their reaction products, so were the macroscopic properties of CR/SBSCMA. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Crumb rubber/SBS composite modified asphalt (CR/SBSCMA) technology is proposed to combine the advantages of crumb rubber (CR) and SBS. Then, the production cost of the modified asphalt could be reduced, the waste rubber resources could be made full utilization, and the performance of asphalt could be improved [1–3]. The current studies about CR/SBSCMA are mainly focus in the asphalt mixture preparation, road construction and ⇑ Corresponding author at: The College of Post and Telecommunication of Wuhan Institute of Technology, Wuhan 430073, China. Tel.: +86 13545232145. E-mail address: [email protected] (L. Xiang). http://dx.doi.org/10.1016/j.conbuildmat.2014.08.035 0950-0618/Ó 2014 Elsevier Ltd. All rights reserved.
performance evaluation and so on. However, the stable ability of the composite modified asphalt is the first problem in order to promote a large-scale application of composite modified asphalt, which requires the study of the modification mechanism directly of composite modified asphalt. At present, the related studies are not well documented, which restricts the application and popularization of CR/SBSCMA technology. CR/SBSCMA has a good performance at high and low temperature. However, the aging of asphalt in construction and service will make the performance recession greatly of asphalt [4]. Asphalt aging resistance is very important to the use, maintenance and regeneration of paving asphalt. Paving asphalt would experience a series of physical and chemistry changes (such as evaporation,
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condensation, dehydrogenation, oxidation and decomposition) due to exposing to heat, oxygen, and ultraviolet light during storage, mixing, transport and laying down, as well as in service life. Then the asphalt pavement is prone to generate rut and crack, and could not perform its original bonding and sealing effect in the mixture. Aging is this kind of irreversible changes of the asphalt performance, physicochemical properties, colloidal structure and mechanical properties in this process. The asphalt aging is the main factor affecting its performance, and the asphalt aging resistance performance directly affects the service life of the paving asphalt [5,6]. In the aging process of CR/SBSCMA, besides the aging of matrix asphalt, the aging of the modifiers CR and SBS were also influence on the aging properties of composite modified asphalt. Rotate thin film oven test (RTFOT) was used for stimulating the thermal oxidative aging of matrix asphalt, crumb rubber modified asphalt (CRMA), SBS modified asphalt (SBSMA) and CR/SBSCMA. The influences of matrix asphalt, CR and SBS on the aging properties have been investigated based on the composition and structure analysis of asphalt and modified asphalt pre and post aging process. Then, the thermal oxygen aging mechanism of CR/SBSCMA has been discussed. 2. Materials and experimental 2.1. Materials The selected matrix asphalt was SK-70, whose 25 °C penetration was 7.43 mm, softening point was 48.0 °C, and 15 °C ductility was more than 150 cm. The Furfural extract oil (FEO) which is high in saturates and aromatics is an ideal compatibilizer for the production of modified asphalt. The group components of the FEO were as follows: the content of saturates was 38.54%; the content of aromatics and resins were 57.04% and 4.42%, respectively. Both the SK-70 and FEO were provided by Hubei Guochuang Advanced Material Co. Ltd. The general CR was tyre crumb rubber from Huangshi Second Rubber Factory. The size of CR was 40–60 mesh. YH-791 SBS was supplied by the Hunan Yueyang Changling Petrifaction Co. Ltd. And several kinds of modifying agents included stabilizer and activator. 2.2. Preparation of modified asphalt The preparation of modified asphalt was consisted of swelling, shearing and breeding. Firstly, mixture of modifier (CR, SBS or CR/SBS) and matrix asphalt with different proportions swelled at 163 °C for 1 h. After swelling, a given amount of fresh matrix asphalt was added to the mixture. The new mixture was high-speed shearing and stirring with 3000 r/min for 1 h at 180 °C. Then, adding 1‰ stabilizer by the total content of the mixture, the modified asphalt was made by breeding the mixture with 1000 r/min for 1 h at 170 °C.
into heptane solubles (saturates, aromatics and resins) and heptane insoluble (asphaltenes and toluene insoluble) as the standard test method GB/T 0618-1993. Then the heptane insoluble was heated with toluene to reflux for more than 6 h. Finally, the toluene solubles were just as asphaltenes, and the residue was toluene insoluble.
3. Results and discussion 3.1. IR analysis of asphalt pre and post aging IR analysis of the matrix asphalt, CRMA, SBSMA and CR/SBSCMA pre and post RTFOT have been conducted to check the changes of the main functional groups absorption peak during the aging process, which was conducive for understanding the aging behavior. The IR spectra of asphalt before and after aging were shown in Figs. 1–4. As shown in the IR spectra, the characteristic absorption peak position of the matrix asphalt and modified asphalt was basically same pre and post aging. There was only a new absorption peak appearing at 1700 cm1. The new absorption peak and the absorption peak of 1030 cm1 have been enhanced during aging process. According to the characteristics absorption frequency of organic compound groups, the 1030 cm1 and 1700 cm1 absorption peak were carbonyl peak and sulfoxide peak, respectively. The appearance of carbonyl peak showed that there were a series of oxidation reactions of asphalt in aging process, which generated ester, ketone, aldehyde and carboxylic acids containing carbonyl peak. The enhancement of the sulfoxide peak showed that the sulfur oxidation occurred in the aging process and had an important influence. In addition, the enhancement degree of carbonyl peak and sulfoxide peak were different for the matrix asphalt and three kinds of modified asphalt, which could be calculated quantitatively. Because the sample thickness was different, the quantitative calculation may affect the absorption peak intensity. Internal standard method has been used to eliminate the effect of specimen thickness on absorbance in this study. The absorption peak area of CACH3 asymmetric bending vibration at 1460 cm1 as a benchmark was used to calculate the relative content of carbonyl and sulfoxide [7,8] for each sample. Then, the asphalt aging index has been characterized by the ratio between the carbonyl and sulfoxide absorption peak area and CACH3 asymmetric bending vibration absorption peak area. The carbonyl index (CI) and sulfoxide index (SI) could be calculated as following equations:
2.3. Thermal oxidative aging test of matrix asphalt and modified asphalt
2.4. FTIR analysis of matrix asphalt and modified asphalt The changes of main functional groups of matrix asphalt and modified asphalt pre and post thermal oxidative aging were analyzed by Impact 420 Attenuated total reflectance-Fourier transform infrared spectrophotometer (ATR-FTIR) (Nicolet, USA). The solid asphalt specimen was placed in the optical path to conduct IR analysis, which was convenient and accurate.
Absorbance
RTFOT was used for stimulating the thermal oxidative aging of matrix asphalt and modified asphalt during storage, mixing, transport and laying down, as well as in service life. RTFOT was conducted according to the standard test method GB/T 0610-2011. RTFOT 75 min (not including the 10 min heating time) and RTFOT 270 min (not including the 10 min heating time) were performed respectively for stimulating the thermal oxidative aging.
carbonyl peak
sulfoxide peak
RTFOT 270min RTFOT 75min
2.5. Measurement of group components of matrix asphalt and modified asphalt Asphalt is usually divided to four groups: saturates, aromatics, resins and asphaltenes. The group components of matrix asphalt and SBSMA pre and post thermal oxidative aging could be measured according to the standard test method GB/T 0618-1993. While CRMA and CR/SBSCMA contained the CR component, and there was only a few fraction of CR which is dissolved in toluene, therefore, CRMA and CR/SBSCMA which should be divided into five groups: toluene insoluble, saturates, aromatics, resins and asphaltenes. At first, the CRMA or CR/SBSCMA was divided
Oringinal sample
4000
3500
3000
2500
2000
1500
1000
-1
Wave number (cm ) Fig. 1. IR spectra of matrix asphalt pre and post RTFOT.
500
Absorbance
L. Xiang et al. / Construction and Building Materials 75 (2015) 169–175
carbonyl peak
sulfoxide peak
RTFOT 270min RTFOT 75min Oringinal sample
4000
3500
3000
2500
2000
1500
1000
500
-1
Wave number (cm )
Absorbance
Fig. 2. IR spectra of CRMA pre and post RTFOT.
carbonyl peak
sulfoxide peak
RTFOT 270min RTFOT 75min Oringinal sample
4000
3500
3000
2500
2000
1500
1000
500
-1
Wave number (cm )
Absorbance
Fig. 3. IR spectra of SBSMA pre and post RTFOT.
carbonyl peak
sulfoxide peak
RTFOT 270min RTFOT 75min Oringinal sample
4000
3500
3000
2500
2000
1500
1000
-1
Wave number (cm ) Fig. 4. IR spectra of CR/SBSCMA pre and post RTFOT.
500
171
CI ¼
AC@O ACCH3
ð1Þ
SI ¼
AS@O ACCH3
ð2Þ
where AC@O , AS@O and ACCH3 were the integral area of carbonyl, sulfoxide and CACH3 absorption peak, respectively. Baseline correction has been conducted for the IR spectra of matrix asphalt, CRMA, SBSMA and CR/SBSCMA before and after aging. Then integral calculated the carbonyl, sulfoxide and CACH3 characteristic absorption peak, and obtained the absorption peak area. Finally, the CI and SI could be calculated according to Eqs. (1) and (2), and then the results were listed in Table 1. As shown in Table 1, the matrix asphalt and modified asphalt CI and SI were increased gradually with the extension of RTFOT aging time, which demonstrated the aging degree deepened unceasingly. The CI of matrix asphalt sharply rose with the prolonging of aging time, and the CI value after RTFOT 270 min aging was about 3 times comparing with that post RTFOT 75 min aging. In contrast, the SI of matrix asphalt was significantly increased after RTFOT 75 min aging, and increased slightly after RTFOT 270 min aging. The CAC bond energy is less than the CAH bond energy for the asphalt molecular structure [9]. In the aging process of asphalt, the CAC bond of alkane or alkyl side chain would break for the effect of heat, then the oxidation reaction occurred with the heat and oxygen action, and formed a series of compounds containing carbonyl peak. The reaction continued with the aging time, the carbonyl content increased, and then the carbonyl absorption peak enhanced. The generation of sulfoxide has a connection with the sulfur element content of asphalt. The formation reaction of sulfoxide occurred constantly during the RTFOT 270 min, but the sulfur content was less and less with the aging time, and the reaction speed gradually slowed down until the sulfoxide content was no change. The internal cause of the increase of asphaltenes and viscosity was the increase of the carbonyl and sulfoxide polar functional groups content during the aging process. In the aging process of CRMA, SBSMA and CR/SBSCMA, the integral area of CRMA carbonyl peak was zero, which reflected that there was no or a little carbonyl generated; the SBSMA and CR/ SBSCMA CI increased with the extension of the aging time, but the increase degree of CI was smaller than the matrix asphalt. The CR particles contained carbon black, antioxidant and antiozonant which could hinder the oxidation reaction. Therefore, the aging resistant performance could improve when the asphalt was mixed with CR. Modifier CR and SBS in the modified asphalt system played a role of anti-aging agent. The SI of three kinds of modified asphalt was significantly increased with the prolonged aging time, and the increasing degree was larger than that of matrix asphalt. On the one hand, for the modified asphalt with CR, because CR is a kind of vulcanized rubber after aging; on the other hand, stabilizer has been used in the modified asphalt preparation process which prompted modifier and asphalt to combine stably, and the main components of the stabilizer were sulfur, vulcanizing agent, vulcanization accelerator and superfine rubber powder, etc. Therefore, the sulfur content in the modified asphalt was higher than that of matrix asphalt due to the added modifier and stabilizer. In addition, the vulcanization accelerator could accelerate the oxidation of sulfur. To sum up, in the aging process of asphalt, the carbon chain of asphalt broke firstly, and then occurred oxidation to produce carbonyl, and the sulfur turned into sulfoxide in the effect heat and oxygen. Mass production of these polar functional groups made the asphalt component transforming in the heavy direction. Moreover, the original colloidal structure of asphalt has also changed, which changed the physical and chemical properties of
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Table 1 The CI and SI of asphalt pre and post aging.
a
Absorption peak areaa
Asphalt
Aging mode
CI
SI
C@O
S@O
CACH3
Matrix asphalt
Original sample RTFOT 75 min RTFOT 270 min
0 0.072 0.209
0.122 0.761 0.790
2.619 4.539 4.536
0 0.0159 0.0461
0.0466 0.1677 0.1742
CRMA
Original sample RTFOT 75 min RTFOT 270 min
0 0 0
0.084 0.751 0.641
1.330 3.853 2.315
0 0 0
0.0632 0.1949 0.2769
SBSMA
Original sample RTFOT 75 min RTFOT 270 min
0 0.051 0.188
0.157 0.322 0.889
1.757 2.750 4.152
0 0.0185 0.0453
0.0894 0.1171 0.2141
CR/SBSCMA
Original sample RTFOT 75 min RTFOT 270 min
0 0.062 0.114
0.271 0.369 0.704
4.157 4.171 3.426
0 0.0149 0.0333
0.0652 0.0885 0.2055
The integral range of C@O was [1640 cm1,1740 cm1]; the integral range of S@O was [980 cm1, 1080 cm1]; the integral range of CACH3 was [1390 cm1, 1500 cm1].
asphalt. The change of CI and SI could well reflect the aging degree of asphalt. 40
A series of physical and chemical changes have been appeared in the asphalt aging process, which are difficult to express with specific chemical structure. It is usually to analyze the change of asphalt group components. Generally, in the asphalt aging process, saturations is relatively stable and changes little; aromatics is more prone to turn into resins with oxidative polymerization reaction; resins containing polar functional groups is prone to turn into asphaltenes with aggregation and condensation reaction. Therefore, the asphalt components generally transforms as the heavy direction of aromatics–resins–asphaltenes [10,11]. RTFOT has been used to stimulate the aging behavior of matrix asphalt, CRMA, SBSMA and CR/SBSCMA in this study. The group components of four kinds asphalt were shown in Figs. 5–10. As can be seen from Fig. 5, in the aging process of matrix asphalt, the content of saturates decreased slightly; the content of aromatics decreased significantly; and the content of resins and asphaltenes exhibited remarkably increase. It showed that the group components of matrix asphalt changed according to the heavy direction of aromatics–resins–asphaltenes. As shown in Figs. 6 and 7, in the aging process of CRMA, the content of saturates and aromatics decreased, and the content of resins
Content (%)
3.2. Group components analysis of asphalt pre and post aging
30
20
10
0
Content (%)
40
Saturates
Aromatics
Resins
Fig. 6. The group components of CRMA pre and post RTFOT.
50
Content (%)
Oringinal sample RTFOT 75min RTFOT 270min
Asphaltenes
Toluene insoluble
40
50
Oringinal sample RTFOT 75min RTFOT 270min
Oringinal sample RTFOT 75min RTFOT 270min
30
20
10
30
0
20
Asphaltenes
Saturates
Aromatics
Resins
Fig. 7. The group components of toluene soluble of CRMA pre and post RTFOT.
10
0
Asphaltenes
Saturates
Aromatics
Resins
Fig. 5. The group components of matrix asphalt pre and post RTFOT.
and asphaltenes increased, while the toluene insoluble content significantly reduced. It showed that the CR particles degraded in the aging process, and the degradation product distributed as resins and asphaltenes in group components. The occurrence of the ‘‘hardening’’ of matrix asphalt portion of CRMA, while CR experi-
L. Xiang et al. / Construction and Building Materials 75 (2015) 169–175
50
Content (%)
40
Oringinal sample RTFOT 75min RTFOT 270min
30
20
10
0
Asphaltenes
Saturates
Aromatics
Resins
Fig. 8. The group components of SBSMA pre and post RTFOT.
Content (%)
40
Oringinal sample RTFOT 75min RTFOT 270min
30
20
10
0
Toluene insoluble
Asphaltenes
Saturates
Aromatics
Resins
Fig. 9. The group components of CR/SBSCMA pre and post RTFOT.
50
Content (%)
40
Oringinal sample RTFOT 75min RTFOT 270min
30
20
10
0
Asphaltenes
Saturates
Aromatics
Resins
Fig. 10. The group components of the toluene soluble of CR/SBSCMA pre and post RTFOT.
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enced ‘‘softening’’, and the two kinds action shifted in the aging process, which resulted in the macroscopic properties of CRMA changed in a small range [12,13]. Comparing Figs. 5 and 7, the content of resins of CRMA toluene soluble increased much more effectively than that of matrix asphalt in aging process. Resins could enhance the adhesion and plastic of asphalt, which could improve the ductility. Moreover, the content of asphaltenes of CRMA toluene soluble increased far smaller than that of matrix asphalt. Therefore, the low temperature ductility of CRMA after aging just experienced slightly influence. Changes in the performance index changes degree of CRMA caused by the varieties of group components also showed that CRMA has good anti-aging properties. As can be seen from Fig. 8, in the aging process of SBSMA, saturates basically unchanged, aromatics decreased obviously, resins and asphaltenes increased. These changes tendency were similar with the matrix asphalt, but the change degree of each group component was smaller. Comparing Figs. 5 and 8, it can be found that both saturated and resins were basically the same in the matrix asphalt and SBSMA, while the aromatics amount was smaller and the content of asphaltenes was higher of SBSMA than that of matrix asphalt. Therefore, it can be resumed the 4 wt% SBS of SBSMA existed mainly as the form of asphaltenes. The aromatics reduction was contributed for the increase of resins and asphaltenes in the aging process of matrix asphalt and SBSMA, while the changed amplitude of SBSMA was much smaller than matrix asphalt. On one hand, the SBS maybe hinder the components heavy process. On the other hand, a part of SBS were occurred thermal oxidative degradation in the aging process, and the SBS in SBSMA was mainly existed as resins and asphaltenes, while the degradation products of SBS were mainly existed in the form of aromatics and resins. Thus, the SBSMA aging properties were superior to the matrix asphalt. As shown in Figs. 9 and 10, in the aging process of CR/SBSCMA, saturates, aromatics and resins were gradually reduced, while the asphaltenes and toluene insoluble increased gradually. This kind of change trend was very different from that of the matrix asphalt, CRMA and SBSMA. In the aging process of CR/SBSCMA, aging of matrix asphalt, CR and SBS occurred at the same time. Secondary resin and secondary asphaltenes were generated from the condensation of the aromatics and resin of matrix asphalt [14,15], which demonstrated that the asphalt components of CR/SBSCMA also transformed as a heavy direction. The modifier CR and SBS experienced desulphurization and degradation with the effects of thermal and oxygen. The ‘‘hardening’’ and ‘‘softening’’ interacted and shifted. Moreover, the degradation products of modifiers may react with the secondary resin and secondary asphaltenes based on the decrease of aromatics and resin content and the significantly increase of toluene insoluble and asphaltenes content. Compared with the group components changes of CRMA and SBSMA during the aging process, because of the existence of CR and SBS in CR/ SBSCMA at the same time, the degradation products of both modifiers may have mutual influences and aggravated reaction between the degradation products and the secondary components of matrix asphalt, which resulted in a large amount of asphaltenes and toluene insoluble. In summary, the matrix asphalt components transformed as the heavy direction of aromatics–resins–asphaltenes, while the aging process of modified asphalt was much more complex. RTFOT has been used in this study to simulate the aging process of modified asphalt, which led to the long-term exposures of modifier in high temperature and oxygen enrichment condition. Therefore, this kind of condition contributed to the degradation of modifier CR and SBS, and the degradation products, asphalt and heavy components and their reaction products would redistributed the modified asphalt composition. For matrix asphalt, condensation was the mainly reaction in aging process, while the modifiers CR and SBS
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would be degraded and desulfurized in the long-term high temperature and oxygen enrichment state, which resulted in the significantly changes of modified asphalt composition after RTFOT.
‘‘hardening’’ of matrix asphalt and the ‘‘softening’’ of modifiers interacted and shifted. CR/SBSCMA has an excellent anti-aging performance, and the modifiers CR and SBS played roles of anti-aging agent in the aging process.
3.3. Aging mechanism of CR/SBSCMA 4. Conclusions In the aging process of CR/SBSCMA, CR, SBS and matrix asphalt aged simultaneously, and then the CR/SBSCMA aging properties depend on the synergistic effects and mutual influences of CR, SBS and matrix asphalt aging. The IR spectra of CR/SBSCMA was similar with that of matrix asphalt, which showed that the modifier and matrix asphalt were physical mixing [16–18], and this three had not formed an effective chemical combination. So, although the matrix asphalt was composite modified with CR and SBS, the modification mechanism of the composite modifiers was similar with independent modification mechanism. The composite modifiers could combine the advantages of CR and SBS, and get better modification effects. In the aging process of CR/SBSCMA, the matrix asphalt component aged along the heavy direction of aromatics–resin–asphaltenes. A part of the aromatics occurred condensation and secondary resin was generated, which could be further transformed into secondary asphaltenes. At the same time, a part of primary resin also occurred condensation to produce secondary asphaltenes. Secondary resin and secondary asphaltenes were highly active compared with their protogenesis components. Moreover, CR and SBS would generate some desulfurization and degradation products in heat and oxygen condition. Then, the desulphurization degradation products, secondary resin and secondary asphaltenes with strong activity would react and some new substances were generated, which resulted in the increasing of asphaltenes and toluene insoluble content in CR/SBSCMA after RTFOT. The reaction process of group components of CR/SBSCMA was presented in Fig. 11. The reaction products which were generated by the modifiers degradation products and the asphalt secondary components existed as resin, asphaltenes and toluene insoluble in composite modified asphalt. However, their chemical composition was significantly different from the protogenesis components. Therefore, the asphalt composition distribution and colloidal structure have been obviously changed, and Gel index has already been no longer suitable for evaluation of the stability of composite modified asphalt colloid structure. In addition, the effects of the degradation products of modifiers and the secondary components on the composite modified asphalt performance were unclear, which need the more detailed analysis of chemical composition and structure. However, some researches showed that [19–21] the conventional properties CR/SBSCMA after aging declined slightly. To sum up, in CR/SBSCMA aging process, the matrix asphalt was hardening, and the CR and SBS modifiers experienced desulphurization and thermal oxidative degradation. Therefore, the
Aromatics
Condensation
PR
Resin
Condensation
SR
PD
Asphaltenes
SA
PA
Condensation
Asphaltenes
Toluene insoluble
PR—protogenesis resin; SR— secondary resin; SA— secondary asphaltenes; PA— protogenesis asphaltenes; PD— the degradation products of CR and SBS. Fig. 11. The reaction process of group components of CR/SBSCMA.
RTFOT has been used to simulate the thermal oxygen aging process of matrix asphalt, CRMA, SBSMA and CR/SBSCMA in this paper. The influences of matrix asphalt, CR and SBS on the aging properties have been investigated based on the composition and structure analysis of asphalt and modified asphalt pre and post aging process. Then, the thermal oxygen aging mechanism of CR/SBSCMA has been discussed. Some conclusions have been drawn as follows. IR analysis results showed that in the asphalt aging process, carbonyl was generated from the oxidation and break of the carbon chain, and sulfur in asphalt transformed into sulfoxide under the thermal oxygen condition. Then, the asphalt components transformed along the heavy direction with those generated polar functional groups, and the original colloid structure of asphalt was changed, which resulted in the decline of the asphalt properties. Moreover, the CI and SI could reflect the aging degree of asphalt well. Based on the change of group components in the asphalt aging process, the matrix asphalt component aged along the heavy direction of aromatics–resin–asphaltenes. For CRMA and SBSMA, the thermal oxidative degradation of modifiers occurred simultaneously with the heavy matrix asphalt components. The ‘‘hardening’’ of matrix asphalt and the ‘‘softening’’ of modifiers interacted and shifted. The composition distribution of CR/SBSCMA was changed significantly by the degradation products of modifiers and the reaction products from the modifiers degradation products and asphalt components. In the aging process of CR/SBSCMA, the matrix asphalt was hardening, and the modifiers CR and SBS experienced desulphurization and thermal oxidative degradation. Therefore, the ‘‘hardening’’ of matrix asphalt and the ‘‘softening’’ of modifiers interacted and shifted. CR/SBSCMA has an excellent anti-aging performance, and the modifiers CR and SBS played roles of anti-aging agent. References [1] Tan Yiqiu, Guo Meng, Cao Liping, Zhang Lei. Performance optimization of composite modified asphalt sealant based on rheological behavior. Constr Build Mater 2013;47:799–805. [2] Francisco de Almeida Jr Adão, Aparecida Battistelle Rosane, Stolte Bezerra Barbara, de Castro Rosani. Use of scrap tire rubber in place of SBS in modified asphalt as an environmentally correct alternative for Brazil. J Clean Prod 2012;33:236–8. [3] Xiang Li, Cheng Jian, Que Guohe. Microstructure and performance of crumb rubber modified asphalt. Constr Build Mater 2009;23:3586–90. [4] Wu SP, Pang L, Mo LT, Chen YC, Zhu GJ. Influence of aging on the evolution of structure, morphology and rheology of base and SBS modified bitumen. Constr Build Mater 2009;23:1005–10. [5] Arega Zelalem A, Bhasin Amit, De Kesel Tom. Influence of extended aging on the properties of asphalt composites produced using hot and warm mix methods. Constr Build Mater 2013;44:168–74. [6] Shenoy Aroon. Prediction of high temperature rheological properties of aged asphalts from the flow data of the original unaged samples. Constr Build Mater 2002;16:509–17. [7] Jun Lu. Study on aging behavior and recycling technique of asphalt pavement. Dissertation for the Doctoral Degree. Xi’an: Chang’an University, 2008, p. 78– 82. [8] Larsen Diego O, Alessandrini José Luis, Bosch Alejandra, Susana Cortizo M. Micro-structural and rheological characteristics of SBS-asphalt blends during their manufacturing. Constr Build Mater 2009;23:2769–74. [9] Liu Guiyang, Wang Wenming, Song Chao. Study on preparation technology of energy saving of rubber asphalt. J Highw Transport Res Dev (Appl Technol) 2011;6:140–3. [10] Lee Soon-Jae, Amirkhanian Serji N, Shatanawi Khaldoun, Kim Kwang W. Shortterm aging characterization of asphalt binders using gel permeation chromatography and selected Superpave binder tests. Constr Build Mater 2008;22(2):220–7.
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Technical Note
Thermal oxidative aging mechanism of crumb rubber/SBS composite modified asphalt Li Xiang a,b,c,⇑, Jian Cheng b,c, Shunji Kang a a
The College of Post and Telecommunication of Wuhan Institute of Technology, Wuhan 430073, China Key Laboratory for Green Chemical Process of Ministry of Education, Wuhan Institute of Technology, Wuhan 430073, China c Hubei Key Laboratory of Novel Chemical Reactor & Green Chemical Technology, Wuhan Institute of Technology, Wuhan 430073, China b
h i g h l i g h t s The thermal oxygen aging mechanism of CR/SBSCMA has been discussed. IR and group components test have been used for the investigation of aging mechanism. The aging process of CR/SBSCMA was different from CRMA and SBSMA. The reaction process of group components of CR/SBSCMA was presented.
a r t i c l e
i n f o
Article history: Received 11 April 2014 Received in revised form 31 July 2014 Accepted 23 August 2014
Keywords: Crumb rubber SBS Composite modified asphalt Thermal oxidative aging Group components
a b s t r a c t Crumb rubber/SBS composite modified asphalt (CR/SBSCMA) technology is proposed to combine the advantages of crumb rubber (CR) and SBS. Then, the production cost of the modified asphalt could be reduced, the waste rubber resources could be made full utilization, and the performance of asphalt could be improved. Rotate thin film oven test was used for stimulating the thermal oxidative aging of matrix asphalt, crumb rubber modified asphalt (CRMA), SBS modified asphalt (SBSMA) and CR/SBSCMA. The influences of matrix asphalt, CR and SBS on the aging properties have been investigated based on the composition and structure analysis of asphalt and modified asphalt pre and post aging process. Then, the thermal oxidative aging mechanism of CR/SBSCMA has been discussed. The test results demonstrated that the carbonyl index and sulfoxide index could reflect the aging degree of asphalt well. Moreover, due to the presence of CR and SBS at the same time, the aging process of CR/SBSCMA was different from CRMA and SBSMA. In the aging process of CR/SBSCMA, the modifiers degradation products and asphalt secondary components reacted severely, which resulted in the decrease of aromatics and resin and the increase of asphaltenes and toluene insoluble. Meanwhile, the distribution and colloid structure of composite modified asphalt were changed by the modifiers degradation products, the asphalt secondary components and their reaction products, so were the macroscopic properties of CR/SBSCMA. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Crumb rubber/SBS composite modified asphalt (CR/SBSCMA) technology is proposed to combine the advantages of crumb rubber (CR) and SBS. Then, the production cost of the modified asphalt could be reduced, the waste rubber resources could be made full utilization, and the performance of asphalt could be improved [1–3]. The current studies about CR/SBSCMA are mainly focus in the asphalt mixture preparation, road construction and ⇑ Corresponding author at: The College of Post and Telecommunication of Wuhan Institute of Technology, Wuhan 430073, China. Tel.: +86 13545232145. E-mail address: [email protected] (L. Xiang). http://dx.doi.org/10.1016/j.conbuildmat.2014.08.035 0950-0618/Ó 2014 Elsevier Ltd. All rights reserved.
performance evaluation and so on. However, the stable ability of the composite modified asphalt is the first problem in order to promote a large-scale application of composite modified asphalt, which requires the study of the modification mechanism directly of composite modified asphalt. At present, the related studies are not well documented, which restricts the application and popularization of CR/SBSCMA technology. CR/SBSCMA has a good performance at high and low temperature. However, the aging of asphalt in construction and service will make the performance recession greatly of asphalt [4]. Asphalt aging resistance is very important to the use, maintenance and regeneration of paving asphalt. Paving asphalt would experience a series of physical and chemistry changes (such as evaporation,
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condensation, dehydrogenation, oxidation and decomposition) due to exposing to heat, oxygen, and ultraviolet light during storage, mixing, transport and laying down, as well as in service life. Then the asphalt pavement is prone to generate rut and crack, and could not perform its original bonding and sealing effect in the mixture. Aging is this kind of irreversible changes of the asphalt performance, physicochemical properties, colloidal structure and mechanical properties in this process. The asphalt aging is the main factor affecting its performance, and the asphalt aging resistance performance directly affects the service life of the paving asphalt [5,6]. In the aging process of CR/SBSCMA, besides the aging of matrix asphalt, the aging of the modifiers CR and SBS were also influence on the aging properties of composite modified asphalt. Rotate thin film oven test (RTFOT) was used for stimulating the thermal oxidative aging of matrix asphalt, crumb rubber modified asphalt (CRMA), SBS modified asphalt (SBSMA) and CR/SBSCMA. The influences of matrix asphalt, CR and SBS on the aging properties have been investigated based on the composition and structure analysis of asphalt and modified asphalt pre and post aging process. Then, the thermal oxygen aging mechanism of CR/SBSCMA has been discussed. 2. Materials and experimental 2.1. Materials The selected matrix asphalt was SK-70, whose 25 °C penetration was 7.43 mm, softening point was 48.0 °C, and 15 °C ductility was more than 150 cm. The Furfural extract oil (FEO) which is high in saturates and aromatics is an ideal compatibilizer for the production of modified asphalt. The group components of the FEO were as follows: the content of saturates was 38.54%; the content of aromatics and resins were 57.04% and 4.42%, respectively. Both the SK-70 and FEO were provided by Hubei Guochuang Advanced Material Co. Ltd. The general CR was tyre crumb rubber from Huangshi Second Rubber Factory. The size of CR was 40–60 mesh. YH-791 SBS was supplied by the Hunan Yueyang Changling Petrifaction Co. Ltd. And several kinds of modifying agents included stabilizer and activator. 2.2. Preparation of modified asphalt The preparation of modified asphalt was consisted of swelling, shearing and breeding. Firstly, mixture of modifier (CR, SBS or CR/SBS) and matrix asphalt with different proportions swelled at 163 °C for 1 h. After swelling, a given amount of fresh matrix asphalt was added to the mixture. The new mixture was high-speed shearing and stirring with 3000 r/min for 1 h at 180 °C. Then, adding 1‰ stabilizer by the total content of the mixture, the modified asphalt was made by breeding the mixture with 1000 r/min for 1 h at 170 °C.
into heptane solubles (saturates, aromatics and resins) and heptane insoluble (asphaltenes and toluene insoluble) as the standard test method GB/T 0618-1993. Then the heptane insoluble was heated with toluene to reflux for more than 6 h. Finally, the toluene solubles were just as asphaltenes, and the residue was toluene insoluble.
3. Results and discussion 3.1. IR analysis of asphalt pre and post aging IR analysis of the matrix asphalt, CRMA, SBSMA and CR/SBSCMA pre and post RTFOT have been conducted to check the changes of the main functional groups absorption peak during the aging process, which was conducive for understanding the aging behavior. The IR spectra of asphalt before and after aging were shown in Figs. 1–4. As shown in the IR spectra, the characteristic absorption peak position of the matrix asphalt and modified asphalt was basically same pre and post aging. There was only a new absorption peak appearing at 1700 cm1. The new absorption peak and the absorption peak of 1030 cm1 have been enhanced during aging process. According to the characteristics absorption frequency of organic compound groups, the 1030 cm1 and 1700 cm1 absorption peak were carbonyl peak and sulfoxide peak, respectively. The appearance of carbonyl peak showed that there were a series of oxidation reactions of asphalt in aging process, which generated ester, ketone, aldehyde and carboxylic acids containing carbonyl peak. The enhancement of the sulfoxide peak showed that the sulfur oxidation occurred in the aging process and had an important influence. In addition, the enhancement degree of carbonyl peak and sulfoxide peak were different for the matrix asphalt and three kinds of modified asphalt, which could be calculated quantitatively. Because the sample thickness was different, the quantitative calculation may affect the absorption peak intensity. Internal standard method has been used to eliminate the effect of specimen thickness on absorbance in this study. The absorption peak area of CACH3 asymmetric bending vibration at 1460 cm1 as a benchmark was used to calculate the relative content of carbonyl and sulfoxide [7,8] for each sample. Then, the asphalt aging index has been characterized by the ratio between the carbonyl and sulfoxide absorption peak area and CACH3 asymmetric bending vibration absorption peak area. The carbonyl index (CI) and sulfoxide index (SI) could be calculated as following equations:
2.3. Thermal oxidative aging test of matrix asphalt and modified asphalt
2.4. FTIR analysis of matrix asphalt and modified asphalt The changes of main functional groups of matrix asphalt and modified asphalt pre and post thermal oxidative aging were analyzed by Impact 420 Attenuated total reflectance-Fourier transform infrared spectrophotometer (ATR-FTIR) (Nicolet, USA). The solid asphalt specimen was placed in the optical path to conduct IR analysis, which was convenient and accurate.
Absorbance
RTFOT was used for stimulating the thermal oxidative aging of matrix asphalt and modified asphalt during storage, mixing, transport and laying down, as well as in service life. RTFOT was conducted according to the standard test method GB/T 0610-2011. RTFOT 75 min (not including the 10 min heating time) and RTFOT 270 min (not including the 10 min heating time) were performed respectively for stimulating the thermal oxidative aging.
carbonyl peak
sulfoxide peak
RTFOT 270min RTFOT 75min
2.5. Measurement of group components of matrix asphalt and modified asphalt Asphalt is usually divided to four groups: saturates, aromatics, resins and asphaltenes. The group components of matrix asphalt and SBSMA pre and post thermal oxidative aging could be measured according to the standard test method GB/T 0618-1993. While CRMA and CR/SBSCMA contained the CR component, and there was only a few fraction of CR which is dissolved in toluene, therefore, CRMA and CR/SBSCMA which should be divided into five groups: toluene insoluble, saturates, aromatics, resins and asphaltenes. At first, the CRMA or CR/SBSCMA was divided
Oringinal sample
4000
3500
3000
2500
2000
1500
1000
-1
Wave number (cm ) Fig. 1. IR spectra of matrix asphalt pre and post RTFOT.
500
Absorbance
L. Xiang et al. / Construction and Building Materials 75 (2015) 169–175
carbonyl peak
sulfoxide peak
RTFOT 270min RTFOT 75min Oringinal sample
4000
3500
3000
2500
2000
1500
1000
500
-1
Wave number (cm )
Absorbance
Fig. 2. IR spectra of CRMA pre and post RTFOT.
carbonyl peak
sulfoxide peak
RTFOT 270min RTFOT 75min Oringinal sample
4000
3500
3000
2500
2000
1500
1000
500
-1
Wave number (cm )
Absorbance
Fig. 3. IR spectra of SBSMA pre and post RTFOT.
carbonyl peak
sulfoxide peak
RTFOT 270min RTFOT 75min Oringinal sample
4000
3500
3000
2500
2000
1500
1000
-1
Wave number (cm ) Fig. 4. IR spectra of CR/SBSCMA pre and post RTFOT.
500
171
CI ¼
AC@O ACCH3
ð1Þ
SI ¼
AS@O ACCH3
ð2Þ
where AC@O , AS@O and ACCH3 were the integral area of carbonyl, sulfoxide and CACH3 absorption peak, respectively. Baseline correction has been conducted for the IR spectra of matrix asphalt, CRMA, SBSMA and CR/SBSCMA before and after aging. Then integral calculated the carbonyl, sulfoxide and CACH3 characteristic absorption peak, and obtained the absorption peak area. Finally, the CI and SI could be calculated according to Eqs. (1) and (2), and then the results were listed in Table 1. As shown in Table 1, the matrix asphalt and modified asphalt CI and SI were increased gradually with the extension of RTFOT aging time, which demonstrated the aging degree deepened unceasingly. The CI of matrix asphalt sharply rose with the prolonging of aging time, and the CI value after RTFOT 270 min aging was about 3 times comparing with that post RTFOT 75 min aging. In contrast, the SI of matrix asphalt was significantly increased after RTFOT 75 min aging, and increased slightly after RTFOT 270 min aging. The CAC bond energy is less than the CAH bond energy for the asphalt molecular structure [9]. In the aging process of asphalt, the CAC bond of alkane or alkyl side chain would break for the effect of heat, then the oxidation reaction occurred with the heat and oxygen action, and formed a series of compounds containing carbonyl peak. The reaction continued with the aging time, the carbonyl content increased, and then the carbonyl absorption peak enhanced. The generation of sulfoxide has a connection with the sulfur element content of asphalt. The formation reaction of sulfoxide occurred constantly during the RTFOT 270 min, but the sulfur content was less and less with the aging time, and the reaction speed gradually slowed down until the sulfoxide content was no change. The internal cause of the increase of asphaltenes and viscosity was the increase of the carbonyl and sulfoxide polar functional groups content during the aging process. In the aging process of CRMA, SBSMA and CR/SBSCMA, the integral area of CRMA carbonyl peak was zero, which reflected that there was no or a little carbonyl generated; the SBSMA and CR/ SBSCMA CI increased with the extension of the aging time, but the increase degree of CI was smaller than the matrix asphalt. The CR particles contained carbon black, antioxidant and antiozonant which could hinder the oxidation reaction. Therefore, the aging resistant performance could improve when the asphalt was mixed with CR. Modifier CR and SBS in the modified asphalt system played a role of anti-aging agent. The SI of three kinds of modified asphalt was significantly increased with the prolonged aging time, and the increasing degree was larger than that of matrix asphalt. On the one hand, for the modified asphalt with CR, because CR is a kind of vulcanized rubber after aging; on the other hand, stabilizer has been used in the modified asphalt preparation process which prompted modifier and asphalt to combine stably, and the main components of the stabilizer were sulfur, vulcanizing agent, vulcanization accelerator and superfine rubber powder, etc. Therefore, the sulfur content in the modified asphalt was higher than that of matrix asphalt due to the added modifier and stabilizer. In addition, the vulcanization accelerator could accelerate the oxidation of sulfur. To sum up, in the aging process of asphalt, the carbon chain of asphalt broke firstly, and then occurred oxidation to produce carbonyl, and the sulfur turned into sulfoxide in the effect heat and oxygen. Mass production of these polar functional groups made the asphalt component transforming in the heavy direction. Moreover, the original colloidal structure of asphalt has also changed, which changed the physical and chemical properties of
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Table 1 The CI and SI of asphalt pre and post aging.
a
Absorption peak areaa
Asphalt
Aging mode
CI
SI
C@O
S@O
CACH3
Matrix asphalt
Original sample RTFOT 75 min RTFOT 270 min
0 0.072 0.209
0.122 0.761 0.790
2.619 4.539 4.536
0 0.0159 0.0461
0.0466 0.1677 0.1742
CRMA
Original sample RTFOT 75 min RTFOT 270 min
0 0 0
0.084 0.751 0.641
1.330 3.853 2.315
0 0 0
0.0632 0.1949 0.2769
SBSMA
Original sample RTFOT 75 min RTFOT 270 min
0 0.051 0.188
0.157 0.322 0.889
1.757 2.750 4.152
0 0.0185 0.0453
0.0894 0.1171 0.2141
CR/SBSCMA
Original sample RTFOT 75 min RTFOT 270 min
0 0.062 0.114
0.271 0.369 0.704
4.157 4.171 3.426
0 0.0149 0.0333
0.0652 0.0885 0.2055
The integral range of C@O was [1640 cm1,1740 cm1]; the integral range of S@O was [980 cm1, 1080 cm1]; the integral range of CACH3 was [1390 cm1, 1500 cm1].
asphalt. The change of CI and SI could well reflect the aging degree of asphalt. 40
A series of physical and chemical changes have been appeared in the asphalt aging process, which are difficult to express with specific chemical structure. It is usually to analyze the change of asphalt group components. Generally, in the asphalt aging process, saturations is relatively stable and changes little; aromatics is more prone to turn into resins with oxidative polymerization reaction; resins containing polar functional groups is prone to turn into asphaltenes with aggregation and condensation reaction. Therefore, the asphalt components generally transforms as the heavy direction of aromatics–resins–asphaltenes [10,11]. RTFOT has been used to stimulate the aging behavior of matrix asphalt, CRMA, SBSMA and CR/SBSCMA in this study. The group components of four kinds asphalt were shown in Figs. 5–10. As can be seen from Fig. 5, in the aging process of matrix asphalt, the content of saturates decreased slightly; the content of aromatics decreased significantly; and the content of resins and asphaltenes exhibited remarkably increase. It showed that the group components of matrix asphalt changed according to the heavy direction of aromatics–resins–asphaltenes. As shown in Figs. 6 and 7, in the aging process of CRMA, the content of saturates and aromatics decreased, and the content of resins
Content (%)
3.2. Group components analysis of asphalt pre and post aging
30
20
10
0
Content (%)
40
Saturates
Aromatics
Resins
Fig. 6. The group components of CRMA pre and post RTFOT.
50
Content (%)
Oringinal sample RTFOT 75min RTFOT 270min
Asphaltenes
Toluene insoluble
40
50
Oringinal sample RTFOT 75min RTFOT 270min
Oringinal sample RTFOT 75min RTFOT 270min
30
20
10
30
0
20
Asphaltenes
Saturates
Aromatics
Resins
Fig. 7. The group components of toluene soluble of CRMA pre and post RTFOT.
10
0
Asphaltenes
Saturates
Aromatics
Resins
Fig. 5. The group components of matrix asphalt pre and post RTFOT.
and asphaltenes increased, while the toluene insoluble content significantly reduced. It showed that the CR particles degraded in the aging process, and the degradation product distributed as resins and asphaltenes in group components. The occurrence of the ‘‘hardening’’ of matrix asphalt portion of CRMA, while CR experi-
L. Xiang et al. / Construction and Building Materials 75 (2015) 169–175
50
Content (%)
40
Oringinal sample RTFOT 75min RTFOT 270min
30
20
10
0
Asphaltenes
Saturates
Aromatics
Resins
Fig. 8. The group components of SBSMA pre and post RTFOT.
Content (%)
40
Oringinal sample RTFOT 75min RTFOT 270min
30
20
10
0
Toluene insoluble
Asphaltenes
Saturates
Aromatics
Resins
Fig. 9. The group components of CR/SBSCMA pre and post RTFOT.
50
Content (%)
40
Oringinal sample RTFOT 75min RTFOT 270min
30
20
10
0
Asphaltenes
Saturates
Aromatics
Resins
Fig. 10. The group components of the toluene soluble of CR/SBSCMA pre and post RTFOT.
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enced ‘‘softening’’, and the two kinds action shifted in the aging process, which resulted in the macroscopic properties of CRMA changed in a small range [12,13]. Comparing Figs. 5 and 7, the content of resins of CRMA toluene soluble increased much more effectively than that of matrix asphalt in aging process. Resins could enhance the adhesion and plastic of asphalt, which could improve the ductility. Moreover, the content of asphaltenes of CRMA toluene soluble increased far smaller than that of matrix asphalt. Therefore, the low temperature ductility of CRMA after aging just experienced slightly influence. Changes in the performance index changes degree of CRMA caused by the varieties of group components also showed that CRMA has good anti-aging properties. As can be seen from Fig. 8, in the aging process of SBSMA, saturates basically unchanged, aromatics decreased obviously, resins and asphaltenes increased. These changes tendency were similar with the matrix asphalt, but the change degree of each group component was smaller. Comparing Figs. 5 and 8, it can be found that both saturated and resins were basically the same in the matrix asphalt and SBSMA, while the aromatics amount was smaller and the content of asphaltenes was higher of SBSMA than that of matrix asphalt. Therefore, it can be resumed the 4 wt% SBS of SBSMA existed mainly as the form of asphaltenes. The aromatics reduction was contributed for the increase of resins and asphaltenes in the aging process of matrix asphalt and SBSMA, while the changed amplitude of SBSMA was much smaller than matrix asphalt. On one hand, the SBS maybe hinder the components heavy process. On the other hand, a part of SBS were occurred thermal oxidative degradation in the aging process, and the SBS in SBSMA was mainly existed as resins and asphaltenes, while the degradation products of SBS were mainly existed in the form of aromatics and resins. Thus, the SBSMA aging properties were superior to the matrix asphalt. As shown in Figs. 9 and 10, in the aging process of CR/SBSCMA, saturates, aromatics and resins were gradually reduced, while the asphaltenes and toluene insoluble increased gradually. This kind of change trend was very different from that of the matrix asphalt, CRMA and SBSMA. In the aging process of CR/SBSCMA, aging of matrix asphalt, CR and SBS occurred at the same time. Secondary resin and secondary asphaltenes were generated from the condensation of the aromatics and resin of matrix asphalt [14,15], which demonstrated that the asphalt components of CR/SBSCMA also transformed as a heavy direction. The modifier CR and SBS experienced desulphurization and degradation with the effects of thermal and oxygen. The ‘‘hardening’’ and ‘‘softening’’ interacted and shifted. Moreover, the degradation products of modifiers may react with the secondary resin and secondary asphaltenes based on the decrease of aromatics and resin content and the significantly increase of toluene insoluble and asphaltenes content. Compared with the group components changes of CRMA and SBSMA during the aging process, because of the existence of CR and SBS in CR/ SBSCMA at the same time, the degradation products of both modifiers may have mutual influences and aggravated reaction between the degradation products and the secondary components of matrix asphalt, which resulted in a large amount of asphaltenes and toluene insoluble. In summary, the matrix asphalt components transformed as the heavy direction of aromatics–resins–asphaltenes, while the aging process of modified asphalt was much more complex. RTFOT has been used in this study to simulate the aging process of modified asphalt, which led to the long-term exposures of modifier in high temperature and oxygen enrichment condition. Therefore, this kind of condition contributed to the degradation of modifier CR and SBS, and the degradation products, asphalt and heavy components and their reaction products would redistributed the modified asphalt composition. For matrix asphalt, condensation was the mainly reaction in aging process, while the modifiers CR and SBS
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would be degraded and desulfurized in the long-term high temperature and oxygen enrichment state, which resulted in the significantly changes of modified asphalt composition after RTFOT.
‘‘hardening’’ of matrix asphalt and the ‘‘softening’’ of modifiers interacted and shifted. CR/SBSCMA has an excellent anti-aging performance, and the modifiers CR and SBS played roles of anti-aging agent in the aging process.
3.3. Aging mechanism of CR/SBSCMA 4. Conclusions In the aging process of CR/SBSCMA, CR, SBS and matrix asphalt aged simultaneously, and then the CR/SBSCMA aging properties depend on the synergistic effects and mutual influences of CR, SBS and matrix asphalt aging. The IR spectra of CR/SBSCMA was similar with that of matrix asphalt, which showed that the modifier and matrix asphalt were physical mixing [16–18], and this three had not formed an effective chemical combination. So, although the matrix asphalt was composite modified with CR and SBS, the modification mechanism of the composite modifiers was similar with independent modification mechanism. The composite modifiers could combine the advantages of CR and SBS, and get better modification effects. In the aging process of CR/SBSCMA, the matrix asphalt component aged along the heavy direction of aromatics–resin–asphaltenes. A part of the aromatics occurred condensation and secondary resin was generated, which could be further transformed into secondary asphaltenes. At the same time, a part of primary resin also occurred condensation to produce secondary asphaltenes. Secondary resin and secondary asphaltenes were highly active compared with their protogenesis components. Moreover, CR and SBS would generate some desulfurization and degradation products in heat and oxygen condition. Then, the desulphurization degradation products, secondary resin and secondary asphaltenes with strong activity would react and some new substances were generated, which resulted in the increasing of asphaltenes and toluene insoluble content in CR/SBSCMA after RTFOT. The reaction process of group components of CR/SBSCMA was presented in Fig. 11. The reaction products which were generated by the modifiers degradation products and the asphalt secondary components existed as resin, asphaltenes and toluene insoluble in composite modified asphalt. However, their chemical composition was significantly different from the protogenesis components. Therefore, the asphalt composition distribution and colloidal structure have been obviously changed, and Gel index has already been no longer suitable for evaluation of the stability of composite modified asphalt colloid structure. In addition, the effects of the degradation products of modifiers and the secondary components on the composite modified asphalt performance were unclear, which need the more detailed analysis of chemical composition and structure. However, some researches showed that [19–21] the conventional properties CR/SBSCMA after aging declined slightly. To sum up, in CR/SBSCMA aging process, the matrix asphalt was hardening, and the CR and SBS modifiers experienced desulphurization and thermal oxidative degradation. Therefore, the
Aromatics
Condensation
PR
Resin
Condensation
SR
PD
Asphaltenes
SA
PA
Condensation
Asphaltenes
Toluene insoluble
PR—protogenesis resin; SR— secondary resin; SA— secondary asphaltenes; PA— protogenesis asphaltenes; PD— the degradation products of CR and SBS. Fig. 11. The reaction process of group components of CR/SBSCMA.
RTFOT has been used to simulate the thermal oxygen aging process of matrix asphalt, CRMA, SBSMA and CR/SBSCMA in this paper. The influences of matrix asphalt, CR and SBS on the aging properties have been investigated based on the composition and structure analysis of asphalt and modified asphalt pre and post aging process. Then, the thermal oxygen aging mechanism of CR/SBSCMA has been discussed. Some conclusions have been drawn as follows. IR analysis results showed that in the asphalt aging process, carbonyl was generated from the oxidation and break of the carbon chain, and sulfur in asphalt transformed into sulfoxide under the thermal oxygen condition. Then, the asphalt components transformed along the heavy direction with those generated polar functional groups, and the original colloid structure of asphalt was changed, which resulted in the decline of the asphalt properties. Moreover, the CI and SI could reflect the aging degree of asphalt well. Based on the change of group components in the asphalt aging process, the matrix asphalt component aged along the heavy direction of aromatics–resin–asphaltenes. For CRMA and SBSMA, the thermal oxidative degradation of modifiers occurred simultaneously with the heavy matrix asphalt components. The ‘‘hardening’’ of matrix asphalt and the ‘‘softening’’ of modifiers interacted and shifted. The composition distribution of CR/SBSCMA was changed significantly by the degradation products of modifiers and the reaction products from the modifiers degradation products and asphalt components. In the aging process of CR/SBSCMA, the matrix asphalt was hardening, and the modifiers CR and SBS experienced desulphurization and thermal oxidative degradation. Therefore, the ‘‘hardening’’ of matrix asphalt and the ‘‘softening’’ of modifiers interacted and shifted. CR/SBSCMA has an excellent anti-aging performance, and the modifiers CR and SBS played roles of anti-aging agent. References [1] Tan Yiqiu, Guo Meng, Cao Liping, Zhang Lei. Performance optimization of composite modified asphalt sealant based on rheological behavior. Constr Build Mater 2013;47:799–805. [2] Francisco de Almeida Jr Adão, Aparecida Battistelle Rosane, Stolte Bezerra Barbara, de Castro Rosani. Use of scrap tire rubber in place of SBS in modified asphalt as an environmentally correct alternative for Brazil. J Clean Prod 2012;33:236–8. [3] Xiang Li, Cheng Jian, Que Guohe. Microstructure and performance of crumb rubber modified asphalt. Constr Build Mater 2009;23:3586–90. [4] Wu SP, Pang L, Mo LT, Chen YC, Zhu GJ. Influence of aging on the evolution of structure, morphology and rheology of base and SBS modified bitumen. Constr Build Mater 2009;23:1005–10. [5] Arega Zelalem A, Bhasin Amit, De Kesel Tom. Influence of extended aging on the properties of asphalt composites produced using hot and warm mix methods. Constr Build Mater 2013;44:168–74. [6] Shenoy Aroon. Prediction of high temperature rheological properties of aged asphalts from the flow data of the original unaged samples. Constr Build Mater 2002;16:509–17. [7] Jun Lu. Study on aging behavior and recycling technique of asphalt pavement. Dissertation for the Doctoral Degree. Xi’an: Chang’an University, 2008, p. 78– 82. [8] Larsen Diego O, Alessandrini José Luis, Bosch Alejandra, Susana Cortizo M. Micro-structural and rheological characteristics of SBS-asphalt blends during their manufacturing. Constr Build Mater 2009;23:2769–74. [9] Liu Guiyang, Wang Wenming, Song Chao. Study on preparation technology of energy saving of rubber asphalt. J Highw Transport Res Dev (Appl Technol) 2011;6:140–3. [10] Lee Soon-Jae, Amirkhanian Serji N, Shatanawi Khaldoun, Kim Kwang W. Shortterm aging characterization of asphalt binders using gel permeation chromatography and selected Superpave binder tests. Constr Build Mater 2008;22(2):220–7.
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