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Physical, rheological and chemical characterization of aging behaviors of thermochromic asphalt binder
Fuel 211 (2018) 850–858
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Full Length Article
Physical, rheological and chemical characterization of aging behaviors of thermochromic asphalt binder
MARK
⁎
Henglong Zhang , Zihao Chen, Guoqing Xu, Caijun Shi Key Laboratory for Green & Advanced Civil Engineering Materials and Application Technology of Hunan Province, College of Civil Engineering, Hunan University, Changsha 410082, China
G RA P H I C A L AB S T R A C T
A R T I C L E I N F O
A B S T R A C T
Keywords: Thermochromic asphalt binder Physical properties Rheological properties Aging resistance Aging indices Grey relational analysis
As an innovative road material, thermochromic asphalt binder has great effect on keeping the surface temperature of pavement within relatively reasonable range. The objective of this paper was to investigate the antiaging properties of asphalts containing 0, 2, 4, 6, 8% content of black thermochromic powders (named as blank sample, 2, 4, 6, 8% BTP binder for convenience). Three aging methods, including thin film oven test, pressure aging vessel test and ultraviolet radiation, were applied to simulate thermal and photo oxidation aging of asphalt, respectively. Physical and rheological properties of binders with and without thermochromic powders were measured before and after three aging methods. The results show that the introduction of black thermochromic powders could improve thermal stability and low-temperature cracking performance of asphalt binder. After three aging methods, all of thermochromic asphalt binders exhibit better aging resistance than blank sample by physical, rheological aging indices, and 4% is the optimal content in which thermochromic asphalt binder synthetically exhibits the best aging resistance. In addition, based on grey relational analysis (GRA), the aging indices of complex modulus aging index (CAI) and viscosity aging index (VAI) are more reliable in aging evaluation.
1. Introduction Asphalt, a viscoelastic material with mechanical and rheological properties appropriate for its performance as road paving binder, is
⁎
broadly applied in pavement construction [1]. However, the black appearance of asphalt binder brings about substantial solar absorption, which leads to high surface temperature of asphalt pavement, and hence results in acceleration of various high-temperature pavement
Corresponding author. E-mail address: [email protected] (H. Zhang).
http://dx.doi.org/10.1016/j.fuel.2017.09.111 Received 28 July 2017; Received in revised form 15 September 2017; Accepted 27 September 2017 Available online 09 October 2017 0016-2361/ © 2017 Elsevier Ltd. All rights reserved.
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Fig. 1. Color of thermochromic powder below and above transition temperature: left is below 31 °C, right is above 31 °C.
produces the oxidation components, and finally causes the increment of stiffness and brittlement of asphalt binder [21]. Generally, thin film oven test (TFOT), pressure aging vessel (PAV) test and ultraviolet (UV) radiation are adopted to simulate short-term, long-term thermal oxidation and photo oxidation aging of asphalt, respectively. In present paper, physical and rheological properties of asphalts containing different contents (0, 2, 4, 6, 8%) of black thermochromic powders were analyzed. Anti-aging properties of thermochromic asphalt binders were evaluated by variation amplitude of physical, rheological properties and carbonyl index before and after three aging methods. Additionally, correlations between chemical aging index and various physical and rheological aging indices were also discussed.
diseases (i.e., rutting, shoving, aging, fatigue damage) and environment-unfriendly issues (i.e., heat island effects, volatile gas emission) [2,3]. Developed by using materials with fixed high reflectivity and emissivity to solar radiation, cool pavements can indeed reduce surface temperature of asphalt pavement in summer, while the cooling effect on pavement also aggravates the distress of low-temperature cracking during the cold weather, which compromises the service life of such pavement in general [2,4–8]. To compensate for the weakness of cool pavement, an innovative thermochromic asphalt binder, which implies adding thermochromic materials into conventional asphalt binder, has been gradually investigated in recent years. Thermochromic materials are substances that can reversibly change their colors in accordance with temperature. Above certain temperature, they predominantly reflect solar energy (mainly infrared radiation); conversely, under that temperature, they mainly absorb solar energy by lowering reflectivity [9]. Thermochromic materials have been widely used in building materials due to its change of optical and thermal properties in such a dynamic way [10–13]. Hu et al. made pioneering researches on thermochromic asphalt binder as a road material. They discovered that in comparison with conventional asphalt binder, the surface temperature of asphalt concretes containing thermochromic asphalt binder are much lower during a typical summer and higher under cold weather conditions [14]. Besides, they analyzed the mechanisms by optical and thermal properties, the results showed that compared with conventional asphalt binder, thermochromic asphalt binder possessed higher reflectance in the near-infrared range and heat capacity and lower thermal conductivity [15,16]. Although thermochromic asphalt binder has great effect on keeping the surface temperature of pavement within relatively reasonable range that could mitigate the high- and lowtemperature distresses of pavement, aging of asphalt is one of the key factors causing the deterioration of pavements [17], so it’s necessary to further investigate the anti-aging behaviors of thermochromic asphalt binder for its comprehensive application. The aging of asphalt can be categorized into two types: thermal oxidation and photo oxidation aging (the latter mainly refers to ultraviolet irradiation) [18,19]. Short-term thermal oxidation aging occurs when asphalt is exposed to heat and air in the process of asphalt mixture production and paving, which is primarily due to the oxidation and loss of volatile components at high temperatures. Long-term thermal oxidation aging proceeds in the time of service life of pavement as a result of continuous oxidation [20]. In terms of ultraviolet irradiation aging, the structure of asphalt molecule changes with the absorption of ultraviolet energy that induces the cleavage of chemical bond and
2. Materials and methods 2.1. Materials The 60/80 pen grade asphalt was used as base asphalt. And its measured physical properties are shown as follows: penetration, 64.2 dmm at 25 °C; softening point, 45.5 °C; ductility, 56.8 cm at 10 °C; viscosity, 436 mPa·s at 135 °C. Black thermochromic powder with transition temperature around 31 °C was chosen for this research. The specific gravity and average particle size of powder are 0.25 (water = 1) and 3-10um, respectively. Below the transition temperature, powder exhibits black color, while powder becomes colorless above the transition temperature, which can be seen in Fig. 1. Its chemical ingredients and Fourier transform infrared (FTIR) spectrum are presented in Table 1 and Fig. 2, respectively. As shown in Table 1, black thermochromic powder contains ingredients of melamine-formaldehyde resin, bisphenol a, methyl stearate and 3-diethylamino-6methyl-7-2,4-xylidinofluoran which can be demonstrated by FTIR spectrum. The appearance of absorption bands centered around 3355 cm−1 and 1509 cm−1 corresponds respectively to hydroxyl (eOH) and benzene skeleton vibration derived from bisphenol a. The characteristic peaks of methyl stearate lie in 2925 cm−1, 2850 cm−1 and 1740 cm−1 indicating methyl (eCH3), methylene (eCH2) and Table 1 Chemical ingredients of black thermochromic powder.
851
Ingredient
Melamineformaldehyde resin
3-Diethylamino-6methyl-7-2,4xylidinofluoran
Bisphenol A
Methyl stearate
Content (%)
1–5
2–10
5–15
50–80
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complex modulus aging index (CAI) and phase angle aging index (PAI). They can be defined as following equations.
SPI = Aged softening point −Unaged softening point
(1)
VAI =
Aged viscosity value−Unaged viscosity value × 100 Unaged viscosity value
(2)
CAI =
Aged complex modulus Unaged complex modulus
(3)
PAI =
Aged phase angle Unaged phase angle
(4)
The larger value of SPI, VAI and CAI denotes aging of binder is more serious. Conversely, the smaller value of PAI, the more serious aging degree is. 2.4. Physical properties test Fig. 2. The FTIR spectrum of black thermochromic powder.
The physical properties of binders, such as penetration (25 °C), softening point and ductility (10 °C), were tested corresponding to ASTM D5, ASTM D36 and ASTM D113, respectively [27–29]. The viscosity of the binders at 135 °C was measured by Brookfield viscometer according to ASTM D4402[30].
carbonyl (C]O) group stretching vibration, separately. The peaks centered around 1552 cm−1 and 813 cm−1 attributed to thiotriazinone stretching and melamine skeleton bending vibration are related to melamine-formaldehyde resin. Three component organic thermochromic composite consists of leuco dye, developer and solvent. The reaction between dye and developer prevails at lower temperature where the solvent exists in its solid form and causes colored dye-developer complexes; as temperature rises, the solvent melts making the solvent-developer interaction dominant which converts the system into its colorless state [22–24]. This is the mechanism that thermochromic composite can change its color with temperature. In terms of black thermochromic powder used in this research, bisphenol a, methyl stearate and 3-diethylamino-6-methyl-7-2,4-xylidinofluoran function as developer, solvent and leuco dye separately. Melamine-formaldehyde resin is used as a shell material to microencapsulate thermochromic composite and hence protect it from unwanted influences by external factors such as heat, acid and alkali.
2.5. Rheological properties test As paramount dynamic shear parameters, complex shear modulus (G∗) and phase angle (δ), indicating binder total resistance to deformation and viscoelastic balance of behavior separately, were measured with dynamic shear rheometer (DSR). Temperature sweeps between appropriate ranges with 2 °C increments were conducted at a fixed frequency of 10 rad/s and under controlled strain condition. As for un-aged and TFOT aged asphalt binders, the plate applied for sweep test was 25 mm in diameter and the gap between parallel plates was 1 mm, while as for UV aged and PAV aged asphalt binders, values changed to 8 mm and 2 mm, respectively. As for high and intermediate continuous grading temperature testing, smart applications (i.e., Original Binder Grading, RTFO Grading, PAV Grading) contained in software of RHEOPLUS/32 V3.40 were applied directly to obtain results. The bending beam rheometer (BBR) tests were conducted on (TFOT + PAV) residues after 60 min in testing temperature bath according to ASTM D6648. The stiffness (S) and creep rate (m-value), used to evaluate deformability and stress relaxation capability of binders in low temperature, were determined at loading time of 60 s. The smaller S and larger m-value implies the ability of binder to resist cracking at low temperature is stronger. Moreover, Low continuous grading temperatures were obtained according to ASTM D7643.
2.2. Preparation of thermochromic asphalt binder The specific preparation procedures of thermochromic asphalt binders were as follows: base asphalt was heated to 150 ± 5 °C in an oilbath heating vessel until it was fully fluid. Then a certain proportion (2, 4, 6, 8%) of black thermochromic powders were blended into base asphalt and then the mixture was mixed at 4000 rpm at 150 °C for 60 min to produce thermochromic asphalt binders. As a blank sample, base asphalt without any additives underwent the same processes.
2.6. FTIR test 2.3. Aging procedures It is acknowledged that primary cause of aging in asphalt binder during its service time is oxidation by oxygen from the air. This oxidation leads to the creation of highly polar and strongly interacting oxygen functional groups like carbonyl group (C]O) [31–33]. Therefore, it is viable to evaluate the aging degree of asphalt binder quantitatively by tracking content of carbonyl group. In this work, carbonyl index (CI) is utilized to quantify the amount of carbonyl group and its detailed definition is as follows [31]:
The short-term thermal oxidation aging of binders was simulated using TFOT according to ASTM D1754[25]. The long-term thermal oxidation aging was simulated by PAV test according to ASTM D6521[26]. Photo oxidation aging was performed mainly using ultraviolet irradiation vessel. To ensure the thickness of asphalt film was about 3 mm, the residue from TFOT weighing 50 g was placed on a Φ140 ± 0.5 mm iron pan. Then the samples had been UV aged at 60 °C for 6 days in a draft oven with a UV lamp of 500 W, the intensity of ultraviolet irradiation of 8w/m2. The anti-aging properties of binders could be evaluated by aging index, namely the ratio or difference value of a performance parameter of the aged asphalt to that of the un-aged asphalt. The aging indices adopted in this paper were obtained from the measurement of physical and rheological properties of binders before and after aging, which were softening point increment (SPI), viscosity aging index (VAI),
CI =
AC=O ∑A
(5)
where AC]O is area of the absorption band represented by carbonyl functional group (C]O) centered around 1700 cm−1 and the sum of the area represents: ∑A = A1700cm-1 + A1600cm-1 + A1460cm-1 + A1376cm1 + A1030cm-1 + A864cm-1 + A814cm-1 + A743cm-1 + A724cm-1. The larger CI difference (ΔCI) between aged and un-aged samples, the deeper 852
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Fig. 3. Physical properties of different asphalt binders: (a) softening point, (b) penetration at 25 °C, (c) ductility at 10 °C and (d) viscosity at 135 °C.
3.2. Rheological properties analysis
aging of binder is. Tracking carbonyl group can be achieved by means of FTIR [34]. The asphalt samples, dissolved in carbon disulfide, were laid on a thin potassium bromide (KBr) plate. Then spectrometer was conducted on samples after evaporation. The spectra were recorded with are solution of 4 cm−1 and an accumulation of 32spectra. The spectra were analyzed with the OMNIC3.1 software.
3.2.1. Complex modulus and phase angle The complex modulus (G∗) and phase angle (δ) of asphalt binders with and without thermochromic powder, within temperature range between 40 °C and 90 °C, are shown in Fig.4. Compared with blank sample, G∗ of thermochromic asphalt binders is obviously larger in whole temperature range. Although the difference of G∗ among diverse thermochromic asphalt binders is relatively narrow, the trend that G∗ increases with the increment of thermochromic powder content also exists. As for phase angle, contrary to results of G∗, δ declines with the growth in powder content after 60 °C. Considering both G∗ and δ, thermochromic material enhances the elastic response of asphalt binder and intensifies the binder ability to resist deform in high temperature, manifesting effect of thermochromic material in improving thermal stability of asphalt binder which corresponds to the results of physical properties.
3. Results and discussion 3.1. Physical properties analysis Various physical parameters of blank sample and thermochromic asphalt binders with different black powder contents (2, 4, 6, 8%) are presented in Fig. 3. It can be known that with the increment of powder content, softening point and viscosity gradually increase while penetration and ductility correspondingly decrease, manifesting that the introduction of black thermochromic powder could enhance the thermal stability of base asphalt and the effect of improvement is more obvious with the growth in content of thermochromic powder. The reason for this phenomenon is as follows: According to specification of product, thermal decomposition of thermochromic powder mainly occurs when temperature is above 200 °C that is higher than thermochromic asphalt preparation temperature. So powder is acting as filler rather than being soluble into the phase of asphalt. Microparticles dispersed in asphalt could hinder molecular motion which consequently increases consistency and yet decreases flow ability of asphalt.
3.2.2. Creep stiffness modulus and creep curve slope The creep stiffness (S) and creep rate (m-value) under different temperatures (-6, -12, -18 °C) are displayed in Fig. 5. For every asphalt binder, S grows with the decline of temperature. However, the change of m-value with temperature is opposite to S. This indicates that deformability and stress relaxation capability of binders deteriorate as temperature descends. Additionally, S of thermochromic asphalt binders are much smaller than that of blank sample under all testing temperatures whereas m-value are larger than blank sample’s, revealing 853
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Fig. 4. Complex modulus and phase angle of different asphalt binders between 40 °C and 90 °C: (a) complex modulus, and (b) phase angle.
and low temperature cracking [37,38]. Based on parameters (G∗/sinδ, G∗sinδ, S and m-value) obtained from DSR and BBR, the continuous PG results of various asphalt binders were determined according to Superpave binder specifications that is ASTM D7643, and corresponding results are listed in Table 2. Compared with blank sample, all thermochromic asphalt binders exhibit upper and lower continuous grading temperatures respectively when performing high and low PG test, which demonstrates that thermochromic powders could improve rutting and low temperature cracking resistance of asphalt. Furthermore, 6% BTP binder shows the best improving effect among them. It can be known that the introduction of thermochromic powders do not degrade the performance grade of asphalt but extend.
thermochromic powder could improve the low- temperature properties of asphalt binder enduring long-term aging. It deserves to be mentioned that the difference of results between ductility at 10 °C and S, m-value arises from the different testing samples—un-aged and TFOT + PAV, respectively. With extended aging, low-temperature anti-cracking property of asphalt deteriorates representing decrease in ductility and rate of stress relaxation but increase in thermal stresses [35,36]. The result that low-temperature properties of blank sample is superior than that of thermochromic asphalt binders before any aging methods whereas inferior after long-term aging, just demonstrates effect of thermochromic powder on improving long-term aging resistance of asphalt. Comparing thermochromic asphalt binders with different powder contents indicates that low-temperature properties improve along with increment of powder content first and then decline, and optimum content is 6%.
3.3. Anti-aging property analysis 3.3.1. Physical aging indices evaluation Physical aging indices (SPI and VAI) of various asphalt binders after three aging methods are displayed in Fig. 6. Both SPI and VAI, for all asphalt binders, increase as aging method varies from TFOT, UV to PAV, illustrating the aging degree of asphalt binders deteriorates as aging method shifts from TFOT, UV to PAV. Besides, as for three aging methods, variation trend of SPI of all asphalt binders is consistent with their VAI in general. In terms of TFOT and UV, SPI and VAI decrease
3.2.3. Performance grade (PG) analysis Performance grade (PG) testing has been widely used to evaluate performance of asphalt binders because it not only considers the entire range of temperature a pavement may experience (high, intermediate and low temperature) and three stages of asphalt suffering (manufacture, construction and service), but also focuses on controlling three specific types of HMA pavement distresses: rutting, fatigue cracking,
Fig. 5. Creep stiffness (S) and creep rate (m-value) of different asphalt binders at different temperatures: (a) creep stiffness, and (b) creep rate.
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Table 2 The continuous PG results of various asphalt binders. Sample
Blank sample
2% BTP binder
4% BTP binder
6% BTP binder
8% BTP binder
Critical temperature (G*/sinδ ≥1.0 kPa for original binder, °C) Critical temperature (G*/sinδ ≥2.2 kPa for TFOT residue, °C) High continuous grading temperature Intermediate continuous grading temperature (G*sinδ ≤5.0 MPa for TFOT + PAV residue, °C) Low continuous grading temperature (S ≤300 MPa and m-value ≥0.3) Continuous grade
67.6 69.6 67.6 20.6
69.1 69.6 69.1 19.2
69.6 70.1 69.6 19.5
70.1 71.1 70.1 20.5
70.1 68.6 68.6 20.8
−25.0 67.6–25.0 (20.6)
−26.1 69.1–26.1 (19.2)
−27.8 69.6–27.8 (19.5)
−29.2 70.1–29.2 (20.5)
−28.0 68.6–28.0 (20.8)
UV radiation [2], so it can protect asphalt from absorbing UV energy and hence mitigate aging degree.
with the increment of thermochromic powder amount except for a little deviation of 6% BTP binder, and 8% BTP binder exhibits the best resistance to short-term thermal oxidation and photo oxidation aging and the next is 4% BTP binder. With respect to PAV, 6% BTP binder with the smallest value of SPI and VAI is superior to other asphalt binders in long-term thermal oxidation aging resistance and the next is also 4% BTP binder. It is necessary to mention that SPI and VAI values of 8% BTP binder after PAV are obviously higher than 4% and 6% BTP binders, not following the same trends as TFOT, UV results. This phenomenon could be attributed to particle agglomeration at higher concentration of thermochromic powder, which causes negative impact on long-term thermal oxidation aging resistance of 8% BTP binder. Moreover, regardless of what aging method is, aging resistance of thermochromic asphalt binders is better than that of blank sample. As we know, petroleum asphalt consists of three parts: oils, resins and asphaltenes. With asphalt undergoing thermal oxidative aging, two processes occur: one is that light component such as oil volatilizes under heat influence, and another is that oil transforms into heavier components such as resins and asphaltenes influenced by heat and oxygen [39,40]. According to UV–VIS-NIR reflectance spectra of asphalts with and without black thermochromic powder made by Hu et al., thermochromic asphalt binder is more reflective in the near-infrared range in comparison with base asphalt binder in high temperature [2]. This means that thermochromic powder could reduce heat absorption of asphalt and hence mitigate influences by thermal aging. Moreover, methyl stearate, one of chemical ingredients of thermochromic powder, is a substance similar to oil that has low density, melting point and small molecular weight. Methyl stearate, flowing from a small numbers of damaged thermochromic microcapsules due to being heated for long time at high temperature, will compensate for oil loss during thermal aging process and retard aging rate as well. The reason for better UV aging resistance of thermochromic asphalt binders lies in black thermochromic powder has relatively high reflectance to
3.3.2. Rheological aging indices evaluation Rheological aging indices (CAI and PAI) of various asphalt binders after three aging methods are presented in Fig. 7. In concern of results after TFOT, all thermochromic asphalt binders exhibit smaller CAI and larger PAI within whole sweep temperature in comparison with blank sample, indicating the better resistance to short-term thermal oxidation aging of thermochromic asphalt binders. In addition, through synthetical consideration of CAI and PAI, 8% BTP binder shows the best shortterm thermal oxidation aging resistance, followed by 4%, 2% and 6% BTP binder. Combining CAI with PAI (except for 6% BTP binder due to abnormity of PAI after 55 °C) after UV, the sequence in which photo oxidation aging resistance of binders are ranked is 4%, 6%, 2%, 8% BTP binder and blank sample. Regarding PAV test, the outcomes of ranking is the same as the sequence in UV aging and 4% BTP binder reveals the strongest ability to resist long-term thermal oxidation aging as well. The reasons that thermal and photo oxidation aging resistance of thermochromic asphalt binders are better than that of blank sample have been presented as section 3.3.1. Although 6% and 8% BTP binders sometimes exhibit deviant aging indices after a certain aging method due to particle agglomeration at higher concentration of thermochromic powder, 0–4% BTP binders rigorously follow the trend that with the increment of powder content, thermochromic asphalt binder shows better aging resistance for all of three aging methods. So 4% is the optimal content to avoid influence by particle agglomeration. 3.3.3. FTIR analysis Based on comprehensive analysis of physical and rheological aging indices, 4% BTP binder is selected to contrast with blank sample in terms of FTIR discussion. Their spectra before and after three aging methods are presented in Fig. 8 as well as the corresponding CI are
Fig. 6. Physical aging indices of various asphalt binders after three aging methods: (a) SPI, and (b) VAI.
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Fig. 7. Rheological aging indices of various asphalt binders after three aging methods: (a) CAI after TFOT, (b) PAI after TFOT, (c) CAI after UV, (d) PAI after UV, (e) CAI after PAV, and (f) PAI after PAV.
quantitatively evaluate aging extent, the difference of CI between aged and un-aged samples is applied. In accordance with Table 3, as for blank sample, the ΔCI after TFOT, UV and PAV aging is 0.0047, 0.0081, 0.0223, separately, and as for 4% BTP binder, 0.0009, 0.0062, 0.0187,
listed in Table 3. As illustrated in Fig. 8, the appearance of absorption band centered around 1700 cm−1 corresponds to the formation of carbonyl functional group. It can be noted that peak area of carbonyl group gradually increases as aging degree deteriorates. To
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Fig. 9. Relational degree between CI increment and corresponding physical and rheological aging indices. Fig. 8. FTIR spectra before and after three aging methods.
Table 3 Carbonyl index (CI) of blank sample and 4% BTP binder after different aging methods. Sample
Un-aged TFOT aging UV aging PAV aging
Blank sample
4% BTP binder
CI
ΔCI
CI
ΔCI
0.0120 0.0167 0.0201 0.0343
– 0.0047 0.0081 0.0223
0.0074 0.0083 0.0136 0.0261
– 0.0009 0.0062 0.0187
• •
respectively. It is clear that with the deterioration of aging degree, ΔCI is gradually magnified for both blank sample and 4% BTP binder, the results which correspond with trend of physical aging indices in Fig. 6. In addition, ΔCI of 4% BTP binder is smaller than that of blank sample after all three aging methods, further demonstrating the superior antiaging properties (including short- and long-term thermal oxidation and photo oxidation aging) of thermochromic asphalt binders.
•
viscosity and G∗ while decreasing penetration, ductility, and δ, and the enhancing effect is more obvious with the increment of powder content. Based on analysis of S and m-value, thermochromic powder could improve low-temperature cracking performance of binder, especially at 6% powder content. According to the continuous PG results, it can be known that the introduction of thermochromic powders do not degrade the performance grade of asphalt but extend. For three aging methods, the anti-aging properties of thermochromic asphalt binders are superior to blank sample by physical and rheological aging indices, and 4% is the optimal content in which thermochromic asphalt binder exhibits the best aging resistance synthetically. By FTIR analysis, ΔCI of 4% BTP binder is smaller than that of blank sample after all three aging methods, further demonstrating the anti-aging properties of thermochromic asphalt binders. Based on GRA, the aging indices of CAI and VAI could be more reliable than others in aging evaluation.
5. Recommendations for future work 3.3.4. Correlation analysis As previous researches present, the growth in carbonyl content has been used as an indicator of the oxidation extent in asphalt [41–43]. Therefore, the aging index which has tight correlation with ΔCI can be used to characterize the aging degree of asphalt. Correlations between ΔCI after three aging methods and corresponding physical and rheological aging indices (SPI, VAI, CAI at 55 °C, PAI at 55 °C) are discussed by grey relational analysis (GRA) [44,45]. And their detailed relational degrees (r) are illustrated in Fig. 9. It can be seen that, for blank sample, CAI at 55 °C has the tightest correlation with ΔCI, followed by VAI, SPI and PAI at 55 °C, while rank is VAI, CAI at 55 °C, SPI and PAI at 55 °C for 4% BTP binder. It is also worth noting that differences among r(ΔCISPI), (-VAI), (-CAI) and (-PAI) of blank sample are more obvious than that of 4% BTP binder. Based on above analyses, the conclusion could be drawn that in terms of different samples, though, ranks of correlations between ΔCI and various physical and rheological aging indices are different, CAI and VAI have tighter correlations with ΔCI than other aging indices synthetically. So the aging indices of CAI and VAI could be more reliable in aging evaluation.
This work mainly focused on the anti-aging properties of thermochromic asphalt binders and initial investigation of their anti-aging mechanisms was made as well. More researches are needed to further explore their anti-aging mechanisms in future.
• To evaluate the effect of three aging methods on thermochromic • •
microcapsules, observing micro morphology of black thermochromic powder filled in asphalt before and after aging by scanning electron microscope (SEM) is needed. Introducing thermochromic powder into different asphalts explores whether the anti-aging effect of thermochromic powder is asphalt dependent. The durability of thermochromic powders in asphalt binders needs further research.
Acknowledgments This work was supported by the Hunan Provincial Natural Science Foundation of China (No. 2017JJ3015), the Transportation Science and Technology Development and Innovation Project of Hunan Province (No. 201307) and the Key Project of Science and Technology Plan of Hunan Province (No. 2015SK2063-1). The authors gratefully acknowledge their financial support.
4. Conclusions
• The introduction of black thermochromic powder could enhance the thermal stability of asphalt binder by increasing its softening point,
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Full Length Article
Physical, rheological and chemical characterization of aging behaviors of thermochromic asphalt binder
MARK
⁎
Henglong Zhang , Zihao Chen, Guoqing Xu, Caijun Shi Key Laboratory for Green & Advanced Civil Engineering Materials and Application Technology of Hunan Province, College of Civil Engineering, Hunan University, Changsha 410082, China
G RA P H I C A L AB S T R A C T
A R T I C L E I N F O
A B S T R A C T
Keywords: Thermochromic asphalt binder Physical properties Rheological properties Aging resistance Aging indices Grey relational analysis
As an innovative road material, thermochromic asphalt binder has great effect on keeping the surface temperature of pavement within relatively reasonable range. The objective of this paper was to investigate the antiaging properties of asphalts containing 0, 2, 4, 6, 8% content of black thermochromic powders (named as blank sample, 2, 4, 6, 8% BTP binder for convenience). Three aging methods, including thin film oven test, pressure aging vessel test and ultraviolet radiation, were applied to simulate thermal and photo oxidation aging of asphalt, respectively. Physical and rheological properties of binders with and without thermochromic powders were measured before and after three aging methods. The results show that the introduction of black thermochromic powders could improve thermal stability and low-temperature cracking performance of asphalt binder. After three aging methods, all of thermochromic asphalt binders exhibit better aging resistance than blank sample by physical, rheological aging indices, and 4% is the optimal content in which thermochromic asphalt binder synthetically exhibits the best aging resistance. In addition, based on grey relational analysis (GRA), the aging indices of complex modulus aging index (CAI) and viscosity aging index (VAI) are more reliable in aging evaluation.
1. Introduction Asphalt, a viscoelastic material with mechanical and rheological properties appropriate for its performance as road paving binder, is
⁎
broadly applied in pavement construction [1]. However, the black appearance of asphalt binder brings about substantial solar absorption, which leads to high surface temperature of asphalt pavement, and hence results in acceleration of various high-temperature pavement
Corresponding author. E-mail address: [email protected] (H. Zhang).
http://dx.doi.org/10.1016/j.fuel.2017.09.111 Received 28 July 2017; Received in revised form 15 September 2017; Accepted 27 September 2017 Available online 09 October 2017 0016-2361/ © 2017 Elsevier Ltd. All rights reserved.
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Fig. 1. Color of thermochromic powder below and above transition temperature: left is below 31 °C, right is above 31 °C.
produces the oxidation components, and finally causes the increment of stiffness and brittlement of asphalt binder [21]. Generally, thin film oven test (TFOT), pressure aging vessel (PAV) test and ultraviolet (UV) radiation are adopted to simulate short-term, long-term thermal oxidation and photo oxidation aging of asphalt, respectively. In present paper, physical and rheological properties of asphalts containing different contents (0, 2, 4, 6, 8%) of black thermochromic powders were analyzed. Anti-aging properties of thermochromic asphalt binders were evaluated by variation amplitude of physical, rheological properties and carbonyl index before and after three aging methods. Additionally, correlations between chemical aging index and various physical and rheological aging indices were also discussed.
diseases (i.e., rutting, shoving, aging, fatigue damage) and environment-unfriendly issues (i.e., heat island effects, volatile gas emission) [2,3]. Developed by using materials with fixed high reflectivity and emissivity to solar radiation, cool pavements can indeed reduce surface temperature of asphalt pavement in summer, while the cooling effect on pavement also aggravates the distress of low-temperature cracking during the cold weather, which compromises the service life of such pavement in general [2,4–8]. To compensate for the weakness of cool pavement, an innovative thermochromic asphalt binder, which implies adding thermochromic materials into conventional asphalt binder, has been gradually investigated in recent years. Thermochromic materials are substances that can reversibly change their colors in accordance with temperature. Above certain temperature, they predominantly reflect solar energy (mainly infrared radiation); conversely, under that temperature, they mainly absorb solar energy by lowering reflectivity [9]. Thermochromic materials have been widely used in building materials due to its change of optical and thermal properties in such a dynamic way [10–13]. Hu et al. made pioneering researches on thermochromic asphalt binder as a road material. They discovered that in comparison with conventional asphalt binder, the surface temperature of asphalt concretes containing thermochromic asphalt binder are much lower during a typical summer and higher under cold weather conditions [14]. Besides, they analyzed the mechanisms by optical and thermal properties, the results showed that compared with conventional asphalt binder, thermochromic asphalt binder possessed higher reflectance in the near-infrared range and heat capacity and lower thermal conductivity [15,16]. Although thermochromic asphalt binder has great effect on keeping the surface temperature of pavement within relatively reasonable range that could mitigate the high- and lowtemperature distresses of pavement, aging of asphalt is one of the key factors causing the deterioration of pavements [17], so it’s necessary to further investigate the anti-aging behaviors of thermochromic asphalt binder for its comprehensive application. The aging of asphalt can be categorized into two types: thermal oxidation and photo oxidation aging (the latter mainly refers to ultraviolet irradiation) [18,19]. Short-term thermal oxidation aging occurs when asphalt is exposed to heat and air in the process of asphalt mixture production and paving, which is primarily due to the oxidation and loss of volatile components at high temperatures. Long-term thermal oxidation aging proceeds in the time of service life of pavement as a result of continuous oxidation [20]. In terms of ultraviolet irradiation aging, the structure of asphalt molecule changes with the absorption of ultraviolet energy that induces the cleavage of chemical bond and
2. Materials and methods 2.1. Materials The 60/80 pen grade asphalt was used as base asphalt. And its measured physical properties are shown as follows: penetration, 64.2 dmm at 25 °C; softening point, 45.5 °C; ductility, 56.8 cm at 10 °C; viscosity, 436 mPa·s at 135 °C. Black thermochromic powder with transition temperature around 31 °C was chosen for this research. The specific gravity and average particle size of powder are 0.25 (water = 1) and 3-10um, respectively. Below the transition temperature, powder exhibits black color, while powder becomes colorless above the transition temperature, which can be seen in Fig. 1. Its chemical ingredients and Fourier transform infrared (FTIR) spectrum are presented in Table 1 and Fig. 2, respectively. As shown in Table 1, black thermochromic powder contains ingredients of melamine-formaldehyde resin, bisphenol a, methyl stearate and 3-diethylamino-6methyl-7-2,4-xylidinofluoran which can be demonstrated by FTIR spectrum. The appearance of absorption bands centered around 3355 cm−1 and 1509 cm−1 corresponds respectively to hydroxyl (eOH) and benzene skeleton vibration derived from bisphenol a. The characteristic peaks of methyl stearate lie in 2925 cm−1, 2850 cm−1 and 1740 cm−1 indicating methyl (eCH3), methylene (eCH2) and Table 1 Chemical ingredients of black thermochromic powder.
851
Ingredient
Melamineformaldehyde resin
3-Diethylamino-6methyl-7-2,4xylidinofluoran
Bisphenol A
Methyl stearate
Content (%)
1–5
2–10
5–15
50–80
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complex modulus aging index (CAI) and phase angle aging index (PAI). They can be defined as following equations.
SPI = Aged softening point −Unaged softening point
(1)
VAI =
Aged viscosity value−Unaged viscosity value × 100 Unaged viscosity value
(2)
CAI =
Aged complex modulus Unaged complex modulus
(3)
PAI =
Aged phase angle Unaged phase angle
(4)
The larger value of SPI, VAI and CAI denotes aging of binder is more serious. Conversely, the smaller value of PAI, the more serious aging degree is. 2.4. Physical properties test Fig. 2. The FTIR spectrum of black thermochromic powder.
The physical properties of binders, such as penetration (25 °C), softening point and ductility (10 °C), were tested corresponding to ASTM D5, ASTM D36 and ASTM D113, respectively [27–29]. The viscosity of the binders at 135 °C was measured by Brookfield viscometer according to ASTM D4402[30].
carbonyl (C]O) group stretching vibration, separately. The peaks centered around 1552 cm−1 and 813 cm−1 attributed to thiotriazinone stretching and melamine skeleton bending vibration are related to melamine-formaldehyde resin. Three component organic thermochromic composite consists of leuco dye, developer and solvent. The reaction between dye and developer prevails at lower temperature where the solvent exists in its solid form and causes colored dye-developer complexes; as temperature rises, the solvent melts making the solvent-developer interaction dominant which converts the system into its colorless state [22–24]. This is the mechanism that thermochromic composite can change its color with temperature. In terms of black thermochromic powder used in this research, bisphenol a, methyl stearate and 3-diethylamino-6-methyl-7-2,4-xylidinofluoran function as developer, solvent and leuco dye separately. Melamine-formaldehyde resin is used as a shell material to microencapsulate thermochromic composite and hence protect it from unwanted influences by external factors such as heat, acid and alkali.
2.5. Rheological properties test As paramount dynamic shear parameters, complex shear modulus (G∗) and phase angle (δ), indicating binder total resistance to deformation and viscoelastic balance of behavior separately, were measured with dynamic shear rheometer (DSR). Temperature sweeps between appropriate ranges with 2 °C increments were conducted at a fixed frequency of 10 rad/s and under controlled strain condition. As for un-aged and TFOT aged asphalt binders, the plate applied for sweep test was 25 mm in diameter and the gap between parallel plates was 1 mm, while as for UV aged and PAV aged asphalt binders, values changed to 8 mm and 2 mm, respectively. As for high and intermediate continuous grading temperature testing, smart applications (i.e., Original Binder Grading, RTFO Grading, PAV Grading) contained in software of RHEOPLUS/32 V3.40 were applied directly to obtain results. The bending beam rheometer (BBR) tests were conducted on (TFOT + PAV) residues after 60 min in testing temperature bath according to ASTM D6648. The stiffness (S) and creep rate (m-value), used to evaluate deformability and stress relaxation capability of binders in low temperature, were determined at loading time of 60 s. The smaller S and larger m-value implies the ability of binder to resist cracking at low temperature is stronger. Moreover, Low continuous grading temperatures were obtained according to ASTM D7643.
2.2. Preparation of thermochromic asphalt binder The specific preparation procedures of thermochromic asphalt binders were as follows: base asphalt was heated to 150 ± 5 °C in an oilbath heating vessel until it was fully fluid. Then a certain proportion (2, 4, 6, 8%) of black thermochromic powders were blended into base asphalt and then the mixture was mixed at 4000 rpm at 150 °C for 60 min to produce thermochromic asphalt binders. As a blank sample, base asphalt without any additives underwent the same processes.
2.6. FTIR test 2.3. Aging procedures It is acknowledged that primary cause of aging in asphalt binder during its service time is oxidation by oxygen from the air. This oxidation leads to the creation of highly polar and strongly interacting oxygen functional groups like carbonyl group (C]O) [31–33]. Therefore, it is viable to evaluate the aging degree of asphalt binder quantitatively by tracking content of carbonyl group. In this work, carbonyl index (CI) is utilized to quantify the amount of carbonyl group and its detailed definition is as follows [31]:
The short-term thermal oxidation aging of binders was simulated using TFOT according to ASTM D1754[25]. The long-term thermal oxidation aging was simulated by PAV test according to ASTM D6521[26]. Photo oxidation aging was performed mainly using ultraviolet irradiation vessel. To ensure the thickness of asphalt film was about 3 mm, the residue from TFOT weighing 50 g was placed on a Φ140 ± 0.5 mm iron pan. Then the samples had been UV aged at 60 °C for 6 days in a draft oven with a UV lamp of 500 W, the intensity of ultraviolet irradiation of 8w/m2. The anti-aging properties of binders could be evaluated by aging index, namely the ratio or difference value of a performance parameter of the aged asphalt to that of the un-aged asphalt. The aging indices adopted in this paper were obtained from the measurement of physical and rheological properties of binders before and after aging, which were softening point increment (SPI), viscosity aging index (VAI),
CI =
AC=O ∑A
(5)
where AC]O is area of the absorption band represented by carbonyl functional group (C]O) centered around 1700 cm−1 and the sum of the area represents: ∑A = A1700cm-1 + A1600cm-1 + A1460cm-1 + A1376cm1 + A1030cm-1 + A864cm-1 + A814cm-1 + A743cm-1 + A724cm-1. The larger CI difference (ΔCI) between aged and un-aged samples, the deeper 852
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Fig. 3. Physical properties of different asphalt binders: (a) softening point, (b) penetration at 25 °C, (c) ductility at 10 °C and (d) viscosity at 135 °C.
3.2. Rheological properties analysis
aging of binder is. Tracking carbonyl group can be achieved by means of FTIR [34]. The asphalt samples, dissolved in carbon disulfide, were laid on a thin potassium bromide (KBr) plate. Then spectrometer was conducted on samples after evaporation. The spectra were recorded with are solution of 4 cm−1 and an accumulation of 32spectra. The spectra were analyzed with the OMNIC3.1 software.
3.2.1. Complex modulus and phase angle The complex modulus (G∗) and phase angle (δ) of asphalt binders with and without thermochromic powder, within temperature range between 40 °C and 90 °C, are shown in Fig.4. Compared with blank sample, G∗ of thermochromic asphalt binders is obviously larger in whole temperature range. Although the difference of G∗ among diverse thermochromic asphalt binders is relatively narrow, the trend that G∗ increases with the increment of thermochromic powder content also exists. As for phase angle, contrary to results of G∗, δ declines with the growth in powder content after 60 °C. Considering both G∗ and δ, thermochromic material enhances the elastic response of asphalt binder and intensifies the binder ability to resist deform in high temperature, manifesting effect of thermochromic material in improving thermal stability of asphalt binder which corresponds to the results of physical properties.
3. Results and discussion 3.1. Physical properties analysis Various physical parameters of blank sample and thermochromic asphalt binders with different black powder contents (2, 4, 6, 8%) are presented in Fig. 3. It can be known that with the increment of powder content, softening point and viscosity gradually increase while penetration and ductility correspondingly decrease, manifesting that the introduction of black thermochromic powder could enhance the thermal stability of base asphalt and the effect of improvement is more obvious with the growth in content of thermochromic powder. The reason for this phenomenon is as follows: According to specification of product, thermal decomposition of thermochromic powder mainly occurs when temperature is above 200 °C that is higher than thermochromic asphalt preparation temperature. So powder is acting as filler rather than being soluble into the phase of asphalt. Microparticles dispersed in asphalt could hinder molecular motion which consequently increases consistency and yet decreases flow ability of asphalt.
3.2.2. Creep stiffness modulus and creep curve slope The creep stiffness (S) and creep rate (m-value) under different temperatures (-6, -12, -18 °C) are displayed in Fig. 5. For every asphalt binder, S grows with the decline of temperature. However, the change of m-value with temperature is opposite to S. This indicates that deformability and stress relaxation capability of binders deteriorate as temperature descends. Additionally, S of thermochromic asphalt binders are much smaller than that of blank sample under all testing temperatures whereas m-value are larger than blank sample’s, revealing 853
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Fig. 4. Complex modulus and phase angle of different asphalt binders between 40 °C and 90 °C: (a) complex modulus, and (b) phase angle.
and low temperature cracking [37,38]. Based on parameters (G∗/sinδ, G∗sinδ, S and m-value) obtained from DSR and BBR, the continuous PG results of various asphalt binders were determined according to Superpave binder specifications that is ASTM D7643, and corresponding results are listed in Table 2. Compared with blank sample, all thermochromic asphalt binders exhibit upper and lower continuous grading temperatures respectively when performing high and low PG test, which demonstrates that thermochromic powders could improve rutting and low temperature cracking resistance of asphalt. Furthermore, 6% BTP binder shows the best improving effect among them. It can be known that the introduction of thermochromic powders do not degrade the performance grade of asphalt but extend.
thermochromic powder could improve the low- temperature properties of asphalt binder enduring long-term aging. It deserves to be mentioned that the difference of results between ductility at 10 °C and S, m-value arises from the different testing samples—un-aged and TFOT + PAV, respectively. With extended aging, low-temperature anti-cracking property of asphalt deteriorates representing decrease in ductility and rate of stress relaxation but increase in thermal stresses [35,36]. The result that low-temperature properties of blank sample is superior than that of thermochromic asphalt binders before any aging methods whereas inferior after long-term aging, just demonstrates effect of thermochromic powder on improving long-term aging resistance of asphalt. Comparing thermochromic asphalt binders with different powder contents indicates that low-temperature properties improve along with increment of powder content first and then decline, and optimum content is 6%.
3.3. Anti-aging property analysis 3.3.1. Physical aging indices evaluation Physical aging indices (SPI and VAI) of various asphalt binders after three aging methods are displayed in Fig. 6. Both SPI and VAI, for all asphalt binders, increase as aging method varies from TFOT, UV to PAV, illustrating the aging degree of asphalt binders deteriorates as aging method shifts from TFOT, UV to PAV. Besides, as for three aging methods, variation trend of SPI of all asphalt binders is consistent with their VAI in general. In terms of TFOT and UV, SPI and VAI decrease
3.2.3. Performance grade (PG) analysis Performance grade (PG) testing has been widely used to evaluate performance of asphalt binders because it not only considers the entire range of temperature a pavement may experience (high, intermediate and low temperature) and three stages of asphalt suffering (manufacture, construction and service), but also focuses on controlling three specific types of HMA pavement distresses: rutting, fatigue cracking,
Fig. 5. Creep stiffness (S) and creep rate (m-value) of different asphalt binders at different temperatures: (a) creep stiffness, and (b) creep rate.
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Table 2 The continuous PG results of various asphalt binders. Sample
Blank sample
2% BTP binder
4% BTP binder
6% BTP binder
8% BTP binder
Critical temperature (G*/sinδ ≥1.0 kPa for original binder, °C) Critical temperature (G*/sinδ ≥2.2 kPa for TFOT residue, °C) High continuous grading temperature Intermediate continuous grading temperature (G*sinδ ≤5.0 MPa for TFOT + PAV residue, °C) Low continuous grading temperature (S ≤300 MPa and m-value ≥0.3) Continuous grade
67.6 69.6 67.6 20.6
69.1 69.6 69.1 19.2
69.6 70.1 69.6 19.5
70.1 71.1 70.1 20.5
70.1 68.6 68.6 20.8
−25.0 67.6–25.0 (20.6)
−26.1 69.1–26.1 (19.2)
−27.8 69.6–27.8 (19.5)
−29.2 70.1–29.2 (20.5)
−28.0 68.6–28.0 (20.8)
UV radiation [2], so it can protect asphalt from absorbing UV energy and hence mitigate aging degree.
with the increment of thermochromic powder amount except for a little deviation of 6% BTP binder, and 8% BTP binder exhibits the best resistance to short-term thermal oxidation and photo oxidation aging and the next is 4% BTP binder. With respect to PAV, 6% BTP binder with the smallest value of SPI and VAI is superior to other asphalt binders in long-term thermal oxidation aging resistance and the next is also 4% BTP binder. It is necessary to mention that SPI and VAI values of 8% BTP binder after PAV are obviously higher than 4% and 6% BTP binders, not following the same trends as TFOT, UV results. This phenomenon could be attributed to particle agglomeration at higher concentration of thermochromic powder, which causes negative impact on long-term thermal oxidation aging resistance of 8% BTP binder. Moreover, regardless of what aging method is, aging resistance of thermochromic asphalt binders is better than that of blank sample. As we know, petroleum asphalt consists of three parts: oils, resins and asphaltenes. With asphalt undergoing thermal oxidative aging, two processes occur: one is that light component such as oil volatilizes under heat influence, and another is that oil transforms into heavier components such as resins and asphaltenes influenced by heat and oxygen [39,40]. According to UV–VIS-NIR reflectance spectra of asphalts with and without black thermochromic powder made by Hu et al., thermochromic asphalt binder is more reflective in the near-infrared range in comparison with base asphalt binder in high temperature [2]. This means that thermochromic powder could reduce heat absorption of asphalt and hence mitigate influences by thermal aging. Moreover, methyl stearate, one of chemical ingredients of thermochromic powder, is a substance similar to oil that has low density, melting point and small molecular weight. Methyl stearate, flowing from a small numbers of damaged thermochromic microcapsules due to being heated for long time at high temperature, will compensate for oil loss during thermal aging process and retard aging rate as well. The reason for better UV aging resistance of thermochromic asphalt binders lies in black thermochromic powder has relatively high reflectance to
3.3.2. Rheological aging indices evaluation Rheological aging indices (CAI and PAI) of various asphalt binders after three aging methods are presented in Fig. 7. In concern of results after TFOT, all thermochromic asphalt binders exhibit smaller CAI and larger PAI within whole sweep temperature in comparison with blank sample, indicating the better resistance to short-term thermal oxidation aging of thermochromic asphalt binders. In addition, through synthetical consideration of CAI and PAI, 8% BTP binder shows the best shortterm thermal oxidation aging resistance, followed by 4%, 2% and 6% BTP binder. Combining CAI with PAI (except for 6% BTP binder due to abnormity of PAI after 55 °C) after UV, the sequence in which photo oxidation aging resistance of binders are ranked is 4%, 6%, 2%, 8% BTP binder and blank sample. Regarding PAV test, the outcomes of ranking is the same as the sequence in UV aging and 4% BTP binder reveals the strongest ability to resist long-term thermal oxidation aging as well. The reasons that thermal and photo oxidation aging resistance of thermochromic asphalt binders are better than that of blank sample have been presented as section 3.3.1. Although 6% and 8% BTP binders sometimes exhibit deviant aging indices after a certain aging method due to particle agglomeration at higher concentration of thermochromic powder, 0–4% BTP binders rigorously follow the trend that with the increment of powder content, thermochromic asphalt binder shows better aging resistance for all of three aging methods. So 4% is the optimal content to avoid influence by particle agglomeration. 3.3.3. FTIR analysis Based on comprehensive analysis of physical and rheological aging indices, 4% BTP binder is selected to contrast with blank sample in terms of FTIR discussion. Their spectra before and after three aging methods are presented in Fig. 8 as well as the corresponding CI are
Fig. 6. Physical aging indices of various asphalt binders after three aging methods: (a) SPI, and (b) VAI.
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Fig. 7. Rheological aging indices of various asphalt binders after three aging methods: (a) CAI after TFOT, (b) PAI after TFOT, (c) CAI after UV, (d) PAI after UV, (e) CAI after PAV, and (f) PAI after PAV.
quantitatively evaluate aging extent, the difference of CI between aged and un-aged samples is applied. In accordance with Table 3, as for blank sample, the ΔCI after TFOT, UV and PAV aging is 0.0047, 0.0081, 0.0223, separately, and as for 4% BTP binder, 0.0009, 0.0062, 0.0187,
listed in Table 3. As illustrated in Fig. 8, the appearance of absorption band centered around 1700 cm−1 corresponds to the formation of carbonyl functional group. It can be noted that peak area of carbonyl group gradually increases as aging degree deteriorates. To
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Fig. 9. Relational degree between CI increment and corresponding physical and rheological aging indices. Fig. 8. FTIR spectra before and after three aging methods.
Table 3 Carbonyl index (CI) of blank sample and 4% BTP binder after different aging methods. Sample
Un-aged TFOT aging UV aging PAV aging
Blank sample
4% BTP binder
CI
ΔCI
CI
ΔCI
0.0120 0.0167 0.0201 0.0343
– 0.0047 0.0081 0.0223
0.0074 0.0083 0.0136 0.0261
– 0.0009 0.0062 0.0187
• •
respectively. It is clear that with the deterioration of aging degree, ΔCI is gradually magnified for both blank sample and 4% BTP binder, the results which correspond with trend of physical aging indices in Fig. 6. In addition, ΔCI of 4% BTP binder is smaller than that of blank sample after all three aging methods, further demonstrating the superior antiaging properties (including short- and long-term thermal oxidation and photo oxidation aging) of thermochromic asphalt binders.
•
viscosity and G∗ while decreasing penetration, ductility, and δ, and the enhancing effect is more obvious with the increment of powder content. Based on analysis of S and m-value, thermochromic powder could improve low-temperature cracking performance of binder, especially at 6% powder content. According to the continuous PG results, it can be known that the introduction of thermochromic powders do not degrade the performance grade of asphalt but extend. For three aging methods, the anti-aging properties of thermochromic asphalt binders are superior to blank sample by physical and rheological aging indices, and 4% is the optimal content in which thermochromic asphalt binder exhibits the best aging resistance synthetically. By FTIR analysis, ΔCI of 4% BTP binder is smaller than that of blank sample after all three aging methods, further demonstrating the anti-aging properties of thermochromic asphalt binders. Based on GRA, the aging indices of CAI and VAI could be more reliable than others in aging evaluation.
5. Recommendations for future work 3.3.4. Correlation analysis As previous researches present, the growth in carbonyl content has been used as an indicator of the oxidation extent in asphalt [41–43]. Therefore, the aging index which has tight correlation with ΔCI can be used to characterize the aging degree of asphalt. Correlations between ΔCI after three aging methods and corresponding physical and rheological aging indices (SPI, VAI, CAI at 55 °C, PAI at 55 °C) are discussed by grey relational analysis (GRA) [44,45]. And their detailed relational degrees (r) are illustrated in Fig. 9. It can be seen that, for blank sample, CAI at 55 °C has the tightest correlation with ΔCI, followed by VAI, SPI and PAI at 55 °C, while rank is VAI, CAI at 55 °C, SPI and PAI at 55 °C for 4% BTP binder. It is also worth noting that differences among r(ΔCISPI), (-VAI), (-CAI) and (-PAI) of blank sample are more obvious than that of 4% BTP binder. Based on above analyses, the conclusion could be drawn that in terms of different samples, though, ranks of correlations between ΔCI and various physical and rheological aging indices are different, CAI and VAI have tighter correlations with ΔCI than other aging indices synthetically. So the aging indices of CAI and VAI could be more reliable in aging evaluation.
This work mainly focused on the anti-aging properties of thermochromic asphalt binders and initial investigation of their anti-aging mechanisms was made as well. More researches are needed to further explore their anti-aging mechanisms in future.
• To evaluate the effect of three aging methods on thermochromic • •
microcapsules, observing micro morphology of black thermochromic powder filled in asphalt before and after aging by scanning electron microscope (SEM) is needed. Introducing thermochromic powder into different asphalts explores whether the anti-aging effect of thermochromic powder is asphalt dependent. The durability of thermochromic powders in asphalt binders needs further research.
Acknowledgments This work was supported by the Hunan Provincial Natural Science Foundation of China (No. 2017JJ3015), the Transportation Science and Technology Development and Innovation Project of Hunan Province (No. 201307) and the Key Project of Science and Technology Plan of Hunan Province (No. 2015SK2063-1). The authors gratefully acknowledge their financial support.
4. Conclusions
• The introduction of black thermochromic powder could enhance the thermal stability of asphalt binder by increasing its softening point,
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