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Influence of graphite on the thermal characteristics and anti-ageing properties of asphalt binder
Construction and Building Materials 68 (2014) 220–226
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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Influence of graphite on the thermal characteristics and anti-ageing properties of asphalt binder Pan Pan, Shaopeng Wu, Yue Xiao ⇑, Peng Wang, Xing Liu State Key Laboratory of Silicate Materials for Architectures, Wuhan University of Technology, Wuhan, Hubei 430070, China
h i g h l i g h t s Graphite improves anti-ageing properties of asphalt. Thermal conductive and diffusivity increases with increasing of graphite content. Specific heat decreases with the increasing of graphite content. Thermal properties of asphalt depend largely on the amount of graphite particles. Graphite is a potential material to improve energy efficiency of hydronic pavement.
a r t i c l e
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Article history: Received 11 April 2014 Received in revised form 16 May 2014 Accepted 29 June 2014
Keywords: Asphalt binder Graphite Ageing Thermal characteristic Rheological property
a b s t r a c t Hydronic asphalt pavement (HAP) is an emerging technology for the purpose of harvesting solar energy in the summer and deicing the pavement in the winter. Increasing the thermal conductivity of pavement material is a fundamental technology to improve the operation efficiency of such novel system. In this paper, the influences of graphite on the thermal characteristics and anti-ageing properties of asphalt binders were experimentally investigated. A control asphalt binder (CAB) sample was prepared by the same weight ratio of asphalt and mineral filler. Experimental results indicated that the thermal conductivity and diffusivity increased linearly with the increasing of graphite content, while the specific heat presented a descending trend correspondingly. Although the storage stability of asphalt binders with graphite were better than the CAB sample, binders with mineral filler or graphite showed bad high temperature storage stability. Differences between the physical and rheological properties of the original asphalt binders and the aged samples illustrated that graphite improved the anti-ageing properties of asphalt binders. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Asphalt pavement surface temperature can reach to 70 °C in summer due to its high absorption coefficient to solar radiation [1]. On the one hand, this phenomenon consequently degrades the durability of asphalt pavement. High temperature will induce the permanent deformations of asphalt pavement with the effect of traffic. It can also accelerate the thermal oxidative, which will result in degradation of the pavement performances [2]. One the other hand, higher surface temperature on the pavement leads to environment problems. The high surface temperature plays an important role on contributing to the development of urban heat island effect (UHIS), which means the temperature of urban area ⇑ Corresponding author. Tel./fax: +86 27 8716 2595. E-mail address: [email protected] (Y. Xiao). http://dx.doi.org/10.1016/j.conbuildmat.2014.06.069 0950-0618/Ó 2014 Elsevier Ltd. All rights reserved.
close to asphalt pavement is higher than the temperatures of surrounding suburban and rural areas [3]. Therefore, in order to prolong the service life of asphalt pavement and mitigate the UHIS problem, it is necessary to develop a method to cool the asphalt pavement during the hot period. In addition, cold weather with the accumulation of snow and the ice formation on the road surface can lead to the transportation safety problems, especially on some sections like bridges and ramps. How to remove such snow/ice effectively and keep the asphalt pavement with acceptable pavement behaviors are the primary concerns by the transportation managers. The use of deicing agent is a traditional method to melt the snow and ice. Unfortunately, it can result in concrete corrosion and environmental pollution [4,5]. Some mechanical method, which can be used to remove the snow and ice, will surely lead to the surface damages and high maintenance cost for mechanical devices [6]. In recent decades,
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many studies have been conducted on exploiting a most efficient, convenient, safe and environmental conservative method to remove snow and ice. To solve these problems, one of the most promising methods is the application of hydronic asphalt pavement (HAP). HAP consists a series of serpentine or parallel pipes embedded in the pavement, below the surface layer. The HAP system has two main parts: cooling the warm pavement by the circulated groundwater and deicing (and/or heating) the pavement by the extracted energy from the pavement [7,8]. What makes this type of pavement more enticing is that HAP can extract solar energy in the summer and provide heat for snow melting in the winter. Numerous researches have been investigated in the hydronic asphalt pavement with laboratory measurements and numerical simulations [9–11]. It is commonly believed that improving the thermal conductivity of asphalt concrete can facilitate the energy transfer process occurring in the asphalt concrete. Previous studies testified that the addition of graphite powder is helpful to improve the thermal conductivity and electrical conductivity of asphalt concrete [12–14]. Compared to conventional asphalt concrete, graphite modified asphalt concrete shows better permanent deformation resistance and fatigue resistance, but worse moisture stability [15]. However, the long-term performance of graphite modified asphalt concrete is still not reported. Factors that can influence the long term performance of asphalt mixture can be classified in three main categories, including realistic traffic loading, environmental loading, material properties and compositions [16]. It is well known that the HMA behaviors subject to the asphalt mastic, which consists of an intimate homogeneous mixture of fine aggregate, filler and bitumen. The asphalt can be easily get aged under the heat, sunlight, oxygen or combination of these factors. The ageing of the asphalt binder would cause serious degradation in the performance of asphalt binder, and hence attenuates the durability of pavement [17,18]. The influence of ageing on the rheological and physical properties of asphalt binder with graphite is not reported as well. Moreover, although the incorporation of graphite can improve the thermal conductivity of asphalt concrete/binder, the effect of graphite on the other related thermal characteristics of asphalt concrete/binder is still unknown. The other related thermal characteristics of asphalt binder are thermal diffusivity and specific heat. The thermal diffusivity indicated the heat transfer ability of asphalt binder. Pavement materials with high thermal diffusivity could improve the heat transfer efficiency of hydronic asphalt pavement. For the same amount of heat, pavement materials with low specific heat implied large temperature variation. This may result in bigger temperature gradient between the pavement and heat transfer pipes, which improve the heat transfer behavior of hydronic asphalt pavement. Therefore, thermal diffusivity and specific heat are as important as the thermal conductivity. This paper aims to provide some insight into assessing the antiageing properties of graphite modified asphalt binder. At the same time, the effect of graphite on the thermal characteristics of asphalt binder was investigated by Thermal Constants Analyzer. The changes on rheological and physical properties of asphalt binders with or without graphite were investigated through short-term ageing test (thin film oven test, TFOT), long-term ageing test (pressurized ageing vessel, PAV) and ultraviolet radiation ageing test (UV). 2. Materials and experimental 2.1. Materials AH-70 paving asphalt, obtained from the Hubei Guochang Hi-tech Material Co., Ltd., with a softening point of 47.2 °C (ASTM D36) [19], a ductility of 156 cm (25 °C, ASTM D113) [20], a penetration of 73 dmm (deci-millimetre, 25 °C, ASTM D5) [21] and a thermal conductivity of 0.17 W/(m K), was used for this research. Limestone
powder, with a particle size of lesser than 0.075 mm, a density of 2.699 g/cm3 and a thermal conductivity of 2.92 W/(m K), was used as the mineral filler. Graphite was used as thermal conductive filler. It has a density of 2.1 g/cm3 and a thermal conductivity of 59.32 W/(m K), consists of carbon (98.9%), ash (0.2%), and iron (0.03%) by weight. Its particle size was less than 150 lm. 2.2. Preparation of modified asphalt binders Table 1 shows the composition of graphite modified asphalt binders. For each type of modified binder, the weight sum of mineral filler and graphite were the same with that of asphalt. As shown in Table 1, the weight fractions of graphite were 0 wt.% (CAB), 10 wt.% (GMAB-10), 20 wt.% (GMAB-20), 30 wt.% (GMAB-30) and 40 wt.% (GMAB-40) respectively. The asphalt binders were mixed using a high shear mixer (made by Weiyu Machine Co., Ltd., China). Asphalt binder (500 g ± 5 g) was first heated to 165 ± 5 °C in an oil-bath heating container. Secondly, the mineral filler and graphite were separately added slowly within 10 min, while the shear speed was kept at 2500rp/m. After all the mineral filler and graphite were added, the asphalt binder was sheared for another 30 min to make sure the homogenously dispersing of additive in the asphalt. 2.3. Storage stability test The storage stability of modified asphalts was specifically used to evaluate the high temperature storage stability of modified asphalt binders. Due to the differences in the density of asphalt and graphite or mineral filler, additives sedimentation would take place in the graphite modified asphalt binder during storage at high temperatures. Therefore, it is necessary to investigate the storage stability of graphite modified asphalt binders. There were simply three steps to conduct the storage stability test. Firstly, a certain amount of specimen was poured into an aluminum toothpaste tube (32 mm in diameter and 160 mm in height). The tubes were then sealed and stored vertically in an oven at 163 °C for different storage time of 0.5 h, 1 h, 4 h, 8 h, 24 h and 48 h. Secondly, the tubes were taken out of the oven, cooled at 5 °C for 4 h ± 5 min and cut into three equal sections. Thirdly, the differences in the softening point of the samples, which were taken from the top and bottom sections, were used to evaluate the storage stability of the asphalt binders. Lower differences in softening point indicates better high-temperature storage stability. 2.4. Thermal characteristics analysis The thermal properties of asphalt binders were measured by Thermal Constants Analyzer (TPS 2500S, Hot Disk, Sweden). In this study, three test repetitions were tested and the average value was used. The thermal conductivity and thermal diffusivity were measured by the thermal constant analyzer. And the heat specific is calculated by the following equation:
cv ¼
k
ð1Þ
a 3
where cv is the specific heat, MJ/(m K), k is the thermal conductivity, W/(m K),
a is the thermal diffusivity, mm2/s. Rapid cooling of asphalt binders could mitigate the sedimentation of graphite and mineral filler. The effect of graphite on the thermal conductivity, thermal diffusivity and specific heat were investigated. Fig. 1 illustrates the schematic representation of the thermal properties measurement for asphalt binders. The test sensor connected to the thermal constant analyzer. The test temperature was 20 °C. The modified asphalt binders were heated to 150 °C and poured into the container with a dimension of 120 60 60 mm. As soon as the test sensor was vertically immersed into the asphalt binder, the container was put in the oil-bath. The temperature of asphalt binder was 145 °C and the temperature of oil bath was 0 °C. When the asphalt binder became a solid state, the temperature of oil-bath was set to 20 °C. Then, the container should be kept in the oil-bath for 4 h, to make sure the temperature in asphalt binder remaining uniform. 2.5. Standard ageing procedure In this study, accelerated ageing of asphalt binders were performed by the thin film oven test, TFOT (ASTM D1754), pressurized ageing vessel, PAV (ASTM D6521), and ultraviolet radiation (UV) ageing test. The TFOT simulates short-term ageing
Table 1 Composition of graphite modified asphalt binders. Asphalt binder
Asphalt Mineral filler Graphite
Composition (g) CAB
GMAB-10
GMAB-20
GMAB-30
GMAB-40
501.1 501.1 0.0
502.4 452.2 50.2
499.1 399.3 99.8
500.5 350.4 150.1
501.8 301.1 200.7
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Fig. 1. Schematic representation of thermal properties measurement for asphalt binders.
that takes place during the hot mixing of asphalt binder with aggregates and during the pavement construction phase. In the TFOT test, 50 g of asphalt binder was placed in a pan. The oven was kept at 163 °C and the pans were rotated in the oven at a rate of 15 rpm for 5 h. The PAV procedure simulates long-term ageing that occurs during the in-service life of asphalt pavement. As soon as the ageing time of TFOT was over, the pans of each sample were immediately placed in the PAV for another 20 h ± 10 min, where the temperature was 109 °C and the air pressure was 2.1 ± 0.1 MPa. UV simulates ultraviolet radiation ageing that occurs during the in-service life of asphalt pavement. UV ageing procedure were also begun after the TFOT ageing procedure. The pans containing the asphalt binders after TFOT ageing were then kept in the ultraviolet radiation chambers for another 20 d ± 0.5 h, where the temperature was 50 °C and radiation intensity of the ultraviolet lamps was 35 W/m2. When the ageing procedures were completed, the aged asphalt binders were poured into containers for further physical and rheological properties tests. 2.6. Physical properties test The physical properties of asphalt binders, including softening point and penetration (25 °C), were studied in accordance with ASTM D36 and D5 respectively. 2.7. Dynamic shear rheological test Rheological properties of asphalt binders were measured with dynamic shear rheometer (DSR, Anton Paar Inc. Germany). Temperature sweeps (from 0 to 30 °C) with 1 °C increment were applied at a fixed frequency of 10 rad/s. Frequency sweeps, with a frequency region from 0.1 to 400 rad/s, were applied at 0, 10, 20, 30, 40, 50 and 60 °C. The principal rheological parameters obtained from the DSR were complex modulus (G⁄) and phase angle (d) which will be discussed later on.
3. Results and discussion 3.1. Storage stability The high temperature storage stabilities of the modified asphalt binders with varying proportions of mineral filler and graphite were tested and the results are shown in Fig. 2. Obviously, the differences in softening point temperature decreases as the content of mineral filler substituted by graphite increases. While the content of graphite increases from 0% to 40%, the differences in softening
Fig. 2. Effect of graphite on the storage stability of modified asphalt binders.
point between the CAB and GMAB samples are 0.6 °C for 0.5 h storage and 2.2 °C for 48 h storage, respectively. On the one hand, the density of mineral filler is 2.699 g/cm3, which is much heavier than the density of 2.1 g/cm3 for graphite. Therefore, the mineral filler is more easily to settle down in the asphalt at a high temperature, which leads to a larger difference in softening points between the top and bottom sections of the asphalt binder. On the other hand, graphite shows better compatibility with asphalt and higher oil-absorption compared to mineral filler. However, Fig. 2 shows that the sedimentation did happened. Although the difference in softening point of GMAB-40 asphalt binder is only 3.4 °C after 48 h storage period, which is still higher than the technical specification required value of 2.5 °C. It indicated that the graphite modified asphalt binder could not be regard as storage stable blend. It is unreasonable to prepare and store the graphite modified asphalt
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binder in practical application. Therefore, it is recommended that graphite should be added at the same time as adding mineral filler for preparing thermal conductive asphalt concrete in practical application.
3.2. Thermal characteristics The thermal characteristics (including thermal conductivity, thermal diffusivity and specific heat) of asphalt binder containing varying content of mineral filler and graphite were analyzed and the results are shown in Fig. 3. As shown in Fig. 3(a), the thermal conductivity and thermal diffusivity of modified asphalt binders increased as the content of mineral filler substituted by graphite increases. When the content of graphite increased from 0% to 40%, the thermal conductivity of asphalt binder increased from 0.396 W/(m K) to 0.934 W/(m K) and the thermal diffusivity of asphalt binder increased from 0.232 mm2/s to 0.637 mm2/s. A higher thermal conductivity and thermal diffusivity means the heat could transfer much more quickly through the asphalt binder. Since asphalt binder is an important component of asphalt concrete, better thermal conductive property of asphalt binder is helpful to improve the related property of asphalt concrete, which could further improve the energy efficiency of the road energy system. As seen from Fig. 3(b), the relation between specific heat and graphite content is not similar to that of thermal conductivity and thermal diffusivity. Specific heat varies irregularly, but shows a descending trend with the increment of graphite. The grey lines was the trending line of the variation of heat specific. The reason might due to the low specific heat of graphite particles. The
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differences in the effect of mineral filler and graphite on thermal characteristics of asphalt binders could be explained by their thermal characteristics. Increasing the graphite content can improve not only the electrical performance but also the thermal properties of asphalt binder. However, the difference of asphalt, mineral filler and graphite in thermal conductivity is not as significant as the difference in electrical conductivity. Threshold phenomenon, which is known for conductive asphalt binder, is not suitable for graphite modified asphalt binder with aspect to thermal conductivity. As reported in reference [15], a sudden change of electrical conductivity in graphite modified asphalt concrete would take place in a certain critical value, called percolation threshold. Both asphalt and mineral filler are electrical insulating materials. The electrical conductivity of graphite is 10–12 order of magnitude higher than that of the asphalt or mineral fillers. The increase of electrical conductivity of asphalt binder is strongly dependent on the connected conductive path formed by conductive particles. But this phenomenon does not occur in the curve variation of thermal conductivity with respect to graphite content. The thermal conductivity of asphalt, mineral filler and graphite are 0.17 W/(m K), 2.92 W/ (m K) and 59.32 W/(m K), respectively. The difference in thermal conductivity is not as significant as that in electrical conductivity. The increase of thermal conductivity of asphalt binder strongly depends on the amount of graphite particle dispersed in the asphalt. Graphite particles, that do not form the connected conductive path, also contribute to improving the thermal conductivity of asphalt binder. Therefore, the thermal conductivity and thermal diffusivity of asphalt binder show a linear increasing and the specific heat show a descending trend with the increment of graphite. 3.3. High temperature physical properties The physical properties of graphite modified asphalt binder before and after ageing were studied and results are shown in Figs. 4 and 5, respectively. The difference in softening point and penetration before and after ageing are used to evaluate the antiageing property of asphalt binders. After TFOT, PAV and UV ageing, the softening point of asphalt binders increases (Fig. 4) and the penetration declines correspondingly (Fig. 5). The results indicated that ageing procedures would improve the high temperature properties of asphalt binder. The larger change of physical properties implied deeper ageing degree of asphalt binders. The change of physical properties of asphalt binders differed in the ageing procedures. For all the asphalt binders, the effect of PAV and UV ageing
Fig. 3. Effect of graphite on the thermal characteristics of modified asphalt binders. (a) Thermal conductivity and thermal diffusivity. (b) Specific heat.
Fig. 4. Increment of softening point of asphalt binders after ageing.
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Fig. 5. Decrement of penetration of asphalt binders after ageing.
on the physical properties of asphalt binder was much more significant than the TFOT. For CAB sample, the softening point increased by 7.6 °C, 14.2 °C and 12.3 °C after TFOT, PAV and UV ageing, respectively while the penetration declined correspondingly by 13.9 dmm, 21.4 dmm and 19.8 dmm after related ageing procedures. Compared to UV ageing, the effect of PAV ageing shows more noticeable effect with respect to the increment of softening point. This phenomenon were confirmed by the results of penetration. Additionally, the change in increment of softening point and penetration were different for the asphalt binder containing varying content of mineral filler and graphite. With the content of graphite increases, the increment of softening point and the decrement of penetration become smaller after the three kinds of ageing procedures. When the content of graphite was wt. 40%, the increment of softening point were 6.2 °C, 11.5 °C and 9.9 °C, and decrement of penetration were 11.5 dmm, 18.5 dmm and 17.3 dmm. The result implied that graphite could improve the anti-ageing property compared to the mineral fillers.
angle (d) are the most important parameters to provide information about the rheological properties of the asphalt binders during the shearing process. Fig. 6 shows the temperature dependence of the modulus (G*) and phase angle (d) at 10 rad/s for each original binder in a range of temperature 0–30 °C. The obtained results indicated that the incorporation of graphite can increase the complex modulus and decrease the phase angle of asphalt binder. This phenomenon was more evident with the increasing of temperature. One reason is that the graphite absorbs most the lightweight fraction of asphalt. This could result in worse flexibility of asphalt binder, even in low temperature, which increase the risk of lowtemperature cracking. In this study, the results of physical properties indicated that asphalt binder would become more ‘‘harder’’ after ageing process, which would degrades the low temperature property. Since the complex modulus (G*) is related to the strength of asphalt binder, the ratio of G* of the ageing sample to that of original sample can be used to evaluate the ageing degree of asphalt binder during the ageing process. Complex modulus ageing indexes are calculated according to the following equation. Lower ageing index can be related to better anti-ageing property of asphalt binder.
AI ¼
G of aged asphalt binder G of unaged asphalt binder
ð2Þ
The performance of asphalt binder is strongly dependent on the temperature and load frequency. Dynamic shear rheological (DSR) test is recognized as an effective method to assess the viscoelastic properties of asphalt binders. Complex modulus (G*) and phase
Figs. 7–9 illustrate the complex modulus ageing index of asphalt binders at 10 rad/s in a range of temperature 0–30 °C. After TFOT, PAV, UV ageing procedures, the ageing index become smaller with the content of graphite increases. It implied that the graphite can improve the anti-ageing property of asphalt binder. The increase in G* ageing index after PAV and UV ageing were significantly greater than the increment in G* ageing index after TFOT ageing due to the prolonging the ageing process. In Fig. 7, the ageing index of control sample was smaller than the modified asphalt binder with varying content of graphite in the low temperature region. For example, the ageing index of asphalt binder with wt. 10% graphite is larger than the control one when the temperature was below 17.1 °C. And this temperature was 10.1 °C, 8.6 °C and 10.6 °C for the GMAB-20, GMAB-30 and GMAB-40, respectively. The results indicated that graphite may has a negative effect on the property of asphalt binder in the low temperature region. In Figs. 8 and 9, the ageing index of asphalt binder with graphite was smaller than the CAB sample in the full temperature region. It can be concluded that the graphite improves the anti-ageing performance of asphalt binder during the PAV and UV process. This conclusion can be confirmed by the varying trend of physical
Fig. 6. Temperature dependence of complex modulus (G*) and phase angle (d) of asphalt binders.
Fig. 7. Complex modulus ageing index of graphite modified asphalt binder after TFOT ageing.
3.4. Dynamic rheological properties
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Fig. 8. Complex modulus ageing index of graphite modified asphalt binder after PAV ageing.
Fig. 10. Complex modulus master curves of original asphalt binders.
difference in the complex modulus indicated that the effect of loading frequency on the asphalt binder with different content of graphite is not significant. As has been reported by previous studies, the temperature dependency of viscoelastic materials can be represented well by an Arrhenius equation. For asphalt binder, the shift factor of asphalt may fit the Arrhenius equation and the parameter of activation energy (Ea) has a strong correlation with the thermal susceptibility. Generally, the asphalt binder with higher (or smaller) activation energy were found to be less (or more) susceptible to the temperature variation [22,23]. Within the temperature range studied in this work, the temperature dependence of the shift factor for all the asphalt binder before and after ageing is described by an Arrhenius equation fairly well:
aðTÞ0 ¼ exp
Fig. 9. Complex modulus ageing index of graphite modified asphalt binder after UV ageing.
properties. Compared to results of PAV ageing, there was a significant difference in the G* ageing index between the five types of asphalt binders after UV. It might because the graphite can absorb, shield and prevent the ultraviolet due to its layer structure, which lead asphalt reducing irreversible ageing effect on asphalt. Frequency sweep test was conducted to investigate the frequency and temperature dependence of the viscoelastic behavior in the normally operating temperature range of asphalt pavement. The frequency region ranges from 0.1 to 400 rad/s on each binder at seven different test temperatures of 0, 10, 20, 30, 40, 50 and 60 °C. The master curve of complex modulus (G*) describes the frequency dependency of asphalt binder at a reference temperature. According to the time–temperature superposition principle (TTSP), the master curves of complex modulus of each binder could be obtained by utilizing the well-known Williams–Landel–Ferry (WLF) theory. Fig. 10 illustrates the master curves of complex modulus of the original asphalt binders at the reference temperature of 20 °C. The G* of asphalt binders increased with the increasing of graphite contents and frequencies. This might result from the high oil absorption of graphite, which leading graphite absorbing the lightweight fraction of asphalt. The strength of asphalt binder is consequently improved by addition of graphite. The unclear
Ea Rð1=T 1=T 0 Þ
ð3Þ
where a(T)0 is the shift factor relative to the reference temperature; Ea, the activation energy; R = 8.314 J/mol K; T, the temperature (K); and T0 is the reference temperature, 20 °C. The R2 values ranged from 0.9913 to 0.9976. Table 2 shows the values of Ea for the five asphalt binders before and after TFOT, PAV and UV ageing process. The active energy of asphalt binder with graphite was higher than that of CAB sample and it increased with the increasing of graphite content. The results indicated that control asphalt binder is easier to be degraded under the influence of heat or dynamic shear loading and graphite made the asphalt binder less temperature sensibility. The higher activation energy of asphalt binder after ageing (or with further ageing) can attribute to molecular structure change and a more interaction between the asphalt components. The increment of active energy for each asphalt binder after ageing become smaller with the content of graphite increased, especially for UV
Table 2 Activation energy Ea in G* of asphalt binders after ageing. Asphalt binders
Unaged TFOT aged TFOT aged – unaged PAV aged PAV aged – unaged UV aged UV aged – unaged
Ea (kJ/mol) CAB
GMAB-10
GMAB-20
GMAB-30
GMAB-40
165.05 169.21 4.16 180.02 14.97 174.89 9.84
167.87 171.98 4.11 181.98 14.11 176.56 8.69
169.25 173.29 4.04 182.76 13.51 177.46 8.21
171.02 174.95 3.93 184.21 13.19 178.85 7.83
173.95 177.86 3.91 186.87 12.92 180.86 6.91
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ageing. The reason is that the graphite can mitigate the negative effect of ageing on the properties of asphalt binder. 4. Conclusions The influence of graphite on the thermal characteristics and anti-ageing properties of asphalt binder was investigated in this study. Thermal constant analyzer (TPS 2500S, Hot Disk, Sweden) was employed to measure the thermal parameters of binders. The storage stability of five types of asphalt binders (CAB, GMAB10, GMAB-20, GMAB-30 and GMAB-40) were evaluated under different storage time at 163 °C. The physical and rheological properties of asphalt binders after TFOT, PAV and UV ageing processes were compared with that of the original samples. Based on the research results, the following conclusions can be drawn: 1. Experimental results indicated that the thermal conductivity and diffusivity increased linearly with the increasing content of graphite, while the specific heat presented a descending trend correspondingly. The increase of thermal conductivity of asphalt binder depends largely on the amount of graphite particle dispersed in the asphalt. 2. Although the storage stability of graphite modified asphalt binder was better than the mixture of binder and mineral filler, binders with mineral filler or graphite showed bad high temperature storage stability. It is recommended that graphite should be added with the same method as adding mineral filler. Therefore, it is unreasonable to prepare and store the graphite modified asphalt binder in practical application. 3. By comparing the physical and rheological properties of original asphalt binders with the aged samples, it could be concluded that graphite improved the anti-ageing properties of asphalt binders, especially for UV ageing. This may attribute to the layer structure of graphite, which mitigates the irreversible ultraviolet ageing effect on asphalt. The pavement materials with good thermal conductivity and long-term performance could improve the service behavior of the novel system-hydronic asphalt pavement. This study confirmed that the incorporation of graphite improves the thermal and antiageing properties of asphalt binder. However, it needs further study on the improvement mechanism of graphite modified asphalt binders. Further research should be also conducted on the durability of thermal property and pavement performance of asphalt concrete. Acknowledgements This work was supported by the National High Technology Research and Development Program of China (‘‘863’’, No. 2013AA
031602) and National Project of Scientific and Technical Supporting Programs funded by Ministry of Science & Technology of China (No. 2011BAE28B03) and National Natural Science Foundation of China (No. 51278392). References [1] Van Bijsterveld W, Houben LJ, Scarpas A, Molenaar AA. Using pavement as solar collector: effect on pavement temperature and structural response. Transport Res Rec: J Trans Res Board 2001;1778:140–8. [2] Pan TY, Lu Y, Wang ZY. Development of an atomistic-based chemophysical environment for modelling asphalt oxidation. Polym Degrad Stabil 2012;97:2331–9. [3] Gustavsson T, Bogren J, Green C. Road climate in cities: a study of the Stockholm area, south-east Sweden. Meteorol Appl 2001;8(4):481–9. [4] Löfgren S. The chemical effects of deicing salt on soil and stream water of five catchments in southeast Sweden. Water Air Soil Pollut 2001;130:863–8. [5] Sanzo D, Hecnar SJ. Effects of road de-icing salt (NaCl) on larval wood frogs (Rana sylvatica). Environ Pollut 2006;140(2):247–56. [6] Nixon WA. Improved cutting edges for ice removal, strategic highway research program report, SHRP-H-346. Washington, DC; 1993. p. 5–6. [7] Loomans M, Oversloot H, de Bondt A, Jansen R, van Rij H. Design tool for the thermal energy potential of asphalt pavements. In: Eighth international IBPSA conference, Eindhoven, Netherlands, August 11–14, 2003. [8] Eugster WJ, Schatzmann J. Harnessing solar energy for winter road clearing on heavily loaded expressways. new challenges for winter road service. In: XIth international winter road congress; 2002. [9] Chen MY, Wu SP, Wang H, Zhang JZ. Study of ice and snow melting process on conductive asphalt solar collector. Sol Energy Mat Sol C 2011;95(12):3241–50. [10] Mallick RB, Chen BL, Bhowmick S. Harvesting heat energy from asphalt pavements: development of and comparison between numerical models and experiment. Int J Sustain Eng 2011;5(2):159–69. [11] Wu SP, Chen MY, Zhang JZ. Laboratory investigation into thermal response of asphalt pavements as solar collector by application of small-scale slabs. Appl Therm Eng 2011;31(10):1582–7. [12] Liu XM, Wu SP, Ye QS, Qiu J, Li B. Properties evaluation of asphalt-based composites with graphite and mine powders. Constr Build Mater 2008;22(3):121–6. [13] Liu XM, Wu SP. Study on the graphite and carbon fiber modified asphalt concrete. Constr Build Mater 2011;25(4):1807–11. [14] Li B. Properties investigation of thermal conductive asphalt mixture [Master thesis]. Wuhan: Wuhan University of Technology; 2008. [15] Wu SP, Pan P, Chen MY, Zhang Y. Analysis of characteristics of electrically conductive asphalt concrete prepared by multiplex conductive materials. J Mater Civil Eng 2013;25(7):871–9. [16] Collop A, Cebon D. Parametric study of factors affecting flexible-pavement performance. J Trans Eng 1995;121(6):485–94. [17] Durrieu F, Farcas F, Mouillet V. The influence of UV aging of a styrene/ butadiene/styrene modified bitumen: comparison between laboratory and on site aging. Fuel 2007;86:1446–51. [18] Vargas XA, Afanasjeva N, Álvarez M, Marchal PH, Choplin L. Asphalt rheology evolution through thermo-oxidation (aging) in a rheo-reactor. Fuel 2008;87:3018–23. [19] Standard test method for softening point of bitumen (Ring-and-ball apparatus). ASTM D36-76. [20] Standard test method for ductility of bituminous materials. ASTM D113-99. [21] Standard test method for penetration of bituminous materials. ASTM D5-13. [22] Cong PL, Xu PJ, Chen SF. Effects of carbon black on the anti aging, rheological and conductive properties of SBS/asphalt/carbon black composites. Constr Build Mater 2014;52:306–13. [23] Zhang F, Yu JY, Wu SP. Effect of ageing on rheological properties of storagestable SBS/sulfur-modified asphalts. J Hazard Mater 2010;182:507–17.
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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Influence of graphite on the thermal characteristics and anti-ageing properties of asphalt binder Pan Pan, Shaopeng Wu, Yue Xiao ⇑, Peng Wang, Xing Liu State Key Laboratory of Silicate Materials for Architectures, Wuhan University of Technology, Wuhan, Hubei 430070, China
h i g h l i g h t s Graphite improves anti-ageing properties of asphalt. Thermal conductive and diffusivity increases with increasing of graphite content. Specific heat decreases with the increasing of graphite content. Thermal properties of asphalt depend largely on the amount of graphite particles. Graphite is a potential material to improve energy efficiency of hydronic pavement.
a r t i c l e
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Article history: Received 11 April 2014 Received in revised form 16 May 2014 Accepted 29 June 2014
Keywords: Asphalt binder Graphite Ageing Thermal characteristic Rheological property
a b s t r a c t Hydronic asphalt pavement (HAP) is an emerging technology for the purpose of harvesting solar energy in the summer and deicing the pavement in the winter. Increasing the thermal conductivity of pavement material is a fundamental technology to improve the operation efficiency of such novel system. In this paper, the influences of graphite on the thermal characteristics and anti-ageing properties of asphalt binders were experimentally investigated. A control asphalt binder (CAB) sample was prepared by the same weight ratio of asphalt and mineral filler. Experimental results indicated that the thermal conductivity and diffusivity increased linearly with the increasing of graphite content, while the specific heat presented a descending trend correspondingly. Although the storage stability of asphalt binders with graphite were better than the CAB sample, binders with mineral filler or graphite showed bad high temperature storage stability. Differences between the physical and rheological properties of the original asphalt binders and the aged samples illustrated that graphite improved the anti-ageing properties of asphalt binders. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Asphalt pavement surface temperature can reach to 70 °C in summer due to its high absorption coefficient to solar radiation [1]. On the one hand, this phenomenon consequently degrades the durability of asphalt pavement. High temperature will induce the permanent deformations of asphalt pavement with the effect of traffic. It can also accelerate the thermal oxidative, which will result in degradation of the pavement performances [2]. One the other hand, higher surface temperature on the pavement leads to environment problems. The high surface temperature plays an important role on contributing to the development of urban heat island effect (UHIS), which means the temperature of urban area ⇑ Corresponding author. Tel./fax: +86 27 8716 2595. E-mail address: [email protected] (Y. Xiao). http://dx.doi.org/10.1016/j.conbuildmat.2014.06.069 0950-0618/Ó 2014 Elsevier Ltd. All rights reserved.
close to asphalt pavement is higher than the temperatures of surrounding suburban and rural areas [3]. Therefore, in order to prolong the service life of asphalt pavement and mitigate the UHIS problem, it is necessary to develop a method to cool the asphalt pavement during the hot period. In addition, cold weather with the accumulation of snow and the ice formation on the road surface can lead to the transportation safety problems, especially on some sections like bridges and ramps. How to remove such snow/ice effectively and keep the asphalt pavement with acceptable pavement behaviors are the primary concerns by the transportation managers. The use of deicing agent is a traditional method to melt the snow and ice. Unfortunately, it can result in concrete corrosion and environmental pollution [4,5]. Some mechanical method, which can be used to remove the snow and ice, will surely lead to the surface damages and high maintenance cost for mechanical devices [6]. In recent decades,
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many studies have been conducted on exploiting a most efficient, convenient, safe and environmental conservative method to remove snow and ice. To solve these problems, one of the most promising methods is the application of hydronic asphalt pavement (HAP). HAP consists a series of serpentine or parallel pipes embedded in the pavement, below the surface layer. The HAP system has two main parts: cooling the warm pavement by the circulated groundwater and deicing (and/or heating) the pavement by the extracted energy from the pavement [7,8]. What makes this type of pavement more enticing is that HAP can extract solar energy in the summer and provide heat for snow melting in the winter. Numerous researches have been investigated in the hydronic asphalt pavement with laboratory measurements and numerical simulations [9–11]. It is commonly believed that improving the thermal conductivity of asphalt concrete can facilitate the energy transfer process occurring in the asphalt concrete. Previous studies testified that the addition of graphite powder is helpful to improve the thermal conductivity and electrical conductivity of asphalt concrete [12–14]. Compared to conventional asphalt concrete, graphite modified asphalt concrete shows better permanent deformation resistance and fatigue resistance, but worse moisture stability [15]. However, the long-term performance of graphite modified asphalt concrete is still not reported. Factors that can influence the long term performance of asphalt mixture can be classified in three main categories, including realistic traffic loading, environmental loading, material properties and compositions [16]. It is well known that the HMA behaviors subject to the asphalt mastic, which consists of an intimate homogeneous mixture of fine aggregate, filler and bitumen. The asphalt can be easily get aged under the heat, sunlight, oxygen or combination of these factors. The ageing of the asphalt binder would cause serious degradation in the performance of asphalt binder, and hence attenuates the durability of pavement [17,18]. The influence of ageing on the rheological and physical properties of asphalt binder with graphite is not reported as well. Moreover, although the incorporation of graphite can improve the thermal conductivity of asphalt concrete/binder, the effect of graphite on the other related thermal characteristics of asphalt concrete/binder is still unknown. The other related thermal characteristics of asphalt binder are thermal diffusivity and specific heat. The thermal diffusivity indicated the heat transfer ability of asphalt binder. Pavement materials with high thermal diffusivity could improve the heat transfer efficiency of hydronic asphalt pavement. For the same amount of heat, pavement materials with low specific heat implied large temperature variation. This may result in bigger temperature gradient between the pavement and heat transfer pipes, which improve the heat transfer behavior of hydronic asphalt pavement. Therefore, thermal diffusivity and specific heat are as important as the thermal conductivity. This paper aims to provide some insight into assessing the antiageing properties of graphite modified asphalt binder. At the same time, the effect of graphite on the thermal characteristics of asphalt binder was investigated by Thermal Constants Analyzer. The changes on rheological and physical properties of asphalt binders with or without graphite were investigated through short-term ageing test (thin film oven test, TFOT), long-term ageing test (pressurized ageing vessel, PAV) and ultraviolet radiation ageing test (UV). 2. Materials and experimental 2.1. Materials AH-70 paving asphalt, obtained from the Hubei Guochang Hi-tech Material Co., Ltd., with a softening point of 47.2 °C (ASTM D36) [19], a ductility of 156 cm (25 °C, ASTM D113) [20], a penetration of 73 dmm (deci-millimetre, 25 °C, ASTM D5) [21] and a thermal conductivity of 0.17 W/(m K), was used for this research. Limestone
powder, with a particle size of lesser than 0.075 mm, a density of 2.699 g/cm3 and a thermal conductivity of 2.92 W/(m K), was used as the mineral filler. Graphite was used as thermal conductive filler. It has a density of 2.1 g/cm3 and a thermal conductivity of 59.32 W/(m K), consists of carbon (98.9%), ash (0.2%), and iron (0.03%) by weight. Its particle size was less than 150 lm. 2.2. Preparation of modified asphalt binders Table 1 shows the composition of graphite modified asphalt binders. For each type of modified binder, the weight sum of mineral filler and graphite were the same with that of asphalt. As shown in Table 1, the weight fractions of graphite were 0 wt.% (CAB), 10 wt.% (GMAB-10), 20 wt.% (GMAB-20), 30 wt.% (GMAB-30) and 40 wt.% (GMAB-40) respectively. The asphalt binders were mixed using a high shear mixer (made by Weiyu Machine Co., Ltd., China). Asphalt binder (500 g ± 5 g) was first heated to 165 ± 5 °C in an oil-bath heating container. Secondly, the mineral filler and graphite were separately added slowly within 10 min, while the shear speed was kept at 2500rp/m. After all the mineral filler and graphite were added, the asphalt binder was sheared for another 30 min to make sure the homogenously dispersing of additive in the asphalt. 2.3. Storage stability test The storage stability of modified asphalts was specifically used to evaluate the high temperature storage stability of modified asphalt binders. Due to the differences in the density of asphalt and graphite or mineral filler, additives sedimentation would take place in the graphite modified asphalt binder during storage at high temperatures. Therefore, it is necessary to investigate the storage stability of graphite modified asphalt binders. There were simply three steps to conduct the storage stability test. Firstly, a certain amount of specimen was poured into an aluminum toothpaste tube (32 mm in diameter and 160 mm in height). The tubes were then sealed and stored vertically in an oven at 163 °C for different storage time of 0.5 h, 1 h, 4 h, 8 h, 24 h and 48 h. Secondly, the tubes were taken out of the oven, cooled at 5 °C for 4 h ± 5 min and cut into three equal sections. Thirdly, the differences in the softening point of the samples, which were taken from the top and bottom sections, were used to evaluate the storage stability of the asphalt binders. Lower differences in softening point indicates better high-temperature storage stability. 2.4. Thermal characteristics analysis The thermal properties of asphalt binders were measured by Thermal Constants Analyzer (TPS 2500S, Hot Disk, Sweden). In this study, three test repetitions were tested and the average value was used. The thermal conductivity and thermal diffusivity were measured by the thermal constant analyzer. And the heat specific is calculated by the following equation:
cv ¼
k
ð1Þ
a 3
where cv is the specific heat, MJ/(m K), k is the thermal conductivity, W/(m K),
a is the thermal diffusivity, mm2/s. Rapid cooling of asphalt binders could mitigate the sedimentation of graphite and mineral filler. The effect of graphite on the thermal conductivity, thermal diffusivity and specific heat were investigated. Fig. 1 illustrates the schematic representation of the thermal properties measurement for asphalt binders. The test sensor connected to the thermal constant analyzer. The test temperature was 20 °C. The modified asphalt binders were heated to 150 °C and poured into the container with a dimension of 120 60 60 mm. As soon as the test sensor was vertically immersed into the asphalt binder, the container was put in the oil-bath. The temperature of asphalt binder was 145 °C and the temperature of oil bath was 0 °C. When the asphalt binder became a solid state, the temperature of oil-bath was set to 20 °C. Then, the container should be kept in the oil-bath for 4 h, to make sure the temperature in asphalt binder remaining uniform. 2.5. Standard ageing procedure In this study, accelerated ageing of asphalt binders were performed by the thin film oven test, TFOT (ASTM D1754), pressurized ageing vessel, PAV (ASTM D6521), and ultraviolet radiation (UV) ageing test. The TFOT simulates short-term ageing
Table 1 Composition of graphite modified asphalt binders. Asphalt binder
Asphalt Mineral filler Graphite
Composition (g) CAB
GMAB-10
GMAB-20
GMAB-30
GMAB-40
501.1 501.1 0.0
502.4 452.2 50.2
499.1 399.3 99.8
500.5 350.4 150.1
501.8 301.1 200.7
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Fig. 1. Schematic representation of thermal properties measurement for asphalt binders.
that takes place during the hot mixing of asphalt binder with aggregates and during the pavement construction phase. In the TFOT test, 50 g of asphalt binder was placed in a pan. The oven was kept at 163 °C and the pans were rotated in the oven at a rate of 15 rpm for 5 h. The PAV procedure simulates long-term ageing that occurs during the in-service life of asphalt pavement. As soon as the ageing time of TFOT was over, the pans of each sample were immediately placed in the PAV for another 20 h ± 10 min, where the temperature was 109 °C and the air pressure was 2.1 ± 0.1 MPa. UV simulates ultraviolet radiation ageing that occurs during the in-service life of asphalt pavement. UV ageing procedure were also begun after the TFOT ageing procedure. The pans containing the asphalt binders after TFOT ageing were then kept in the ultraviolet radiation chambers for another 20 d ± 0.5 h, where the temperature was 50 °C and radiation intensity of the ultraviolet lamps was 35 W/m2. When the ageing procedures were completed, the aged asphalt binders were poured into containers for further physical and rheological properties tests. 2.6. Physical properties test The physical properties of asphalt binders, including softening point and penetration (25 °C), were studied in accordance with ASTM D36 and D5 respectively. 2.7. Dynamic shear rheological test Rheological properties of asphalt binders were measured with dynamic shear rheometer (DSR, Anton Paar Inc. Germany). Temperature sweeps (from 0 to 30 °C) with 1 °C increment were applied at a fixed frequency of 10 rad/s. Frequency sweeps, with a frequency region from 0.1 to 400 rad/s, were applied at 0, 10, 20, 30, 40, 50 and 60 °C. The principal rheological parameters obtained from the DSR were complex modulus (G⁄) and phase angle (d) which will be discussed later on.
3. Results and discussion 3.1. Storage stability The high temperature storage stabilities of the modified asphalt binders with varying proportions of mineral filler and graphite were tested and the results are shown in Fig. 2. Obviously, the differences in softening point temperature decreases as the content of mineral filler substituted by graphite increases. While the content of graphite increases from 0% to 40%, the differences in softening
Fig. 2. Effect of graphite on the storage stability of modified asphalt binders.
point between the CAB and GMAB samples are 0.6 °C for 0.5 h storage and 2.2 °C for 48 h storage, respectively. On the one hand, the density of mineral filler is 2.699 g/cm3, which is much heavier than the density of 2.1 g/cm3 for graphite. Therefore, the mineral filler is more easily to settle down in the asphalt at a high temperature, which leads to a larger difference in softening points between the top and bottom sections of the asphalt binder. On the other hand, graphite shows better compatibility with asphalt and higher oil-absorption compared to mineral filler. However, Fig. 2 shows that the sedimentation did happened. Although the difference in softening point of GMAB-40 asphalt binder is only 3.4 °C after 48 h storage period, which is still higher than the technical specification required value of 2.5 °C. It indicated that the graphite modified asphalt binder could not be regard as storage stable blend. It is unreasonable to prepare and store the graphite modified asphalt
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binder in practical application. Therefore, it is recommended that graphite should be added at the same time as adding mineral filler for preparing thermal conductive asphalt concrete in practical application.
3.2. Thermal characteristics The thermal characteristics (including thermal conductivity, thermal diffusivity and specific heat) of asphalt binder containing varying content of mineral filler and graphite were analyzed and the results are shown in Fig. 3. As shown in Fig. 3(a), the thermal conductivity and thermal diffusivity of modified asphalt binders increased as the content of mineral filler substituted by graphite increases. When the content of graphite increased from 0% to 40%, the thermal conductivity of asphalt binder increased from 0.396 W/(m K) to 0.934 W/(m K) and the thermal diffusivity of asphalt binder increased from 0.232 mm2/s to 0.637 mm2/s. A higher thermal conductivity and thermal diffusivity means the heat could transfer much more quickly through the asphalt binder. Since asphalt binder is an important component of asphalt concrete, better thermal conductive property of asphalt binder is helpful to improve the related property of asphalt concrete, which could further improve the energy efficiency of the road energy system. As seen from Fig. 3(b), the relation between specific heat and graphite content is not similar to that of thermal conductivity and thermal diffusivity. Specific heat varies irregularly, but shows a descending trend with the increment of graphite. The grey lines was the trending line of the variation of heat specific. The reason might due to the low specific heat of graphite particles. The
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differences in the effect of mineral filler and graphite on thermal characteristics of asphalt binders could be explained by their thermal characteristics. Increasing the graphite content can improve not only the electrical performance but also the thermal properties of asphalt binder. However, the difference of asphalt, mineral filler and graphite in thermal conductivity is not as significant as the difference in electrical conductivity. Threshold phenomenon, which is known for conductive asphalt binder, is not suitable for graphite modified asphalt binder with aspect to thermal conductivity. As reported in reference [15], a sudden change of electrical conductivity in graphite modified asphalt concrete would take place in a certain critical value, called percolation threshold. Both asphalt and mineral filler are electrical insulating materials. The electrical conductivity of graphite is 10–12 order of magnitude higher than that of the asphalt or mineral fillers. The increase of electrical conductivity of asphalt binder is strongly dependent on the connected conductive path formed by conductive particles. But this phenomenon does not occur in the curve variation of thermal conductivity with respect to graphite content. The thermal conductivity of asphalt, mineral filler and graphite are 0.17 W/(m K), 2.92 W/ (m K) and 59.32 W/(m K), respectively. The difference in thermal conductivity is not as significant as that in electrical conductivity. The increase of thermal conductivity of asphalt binder strongly depends on the amount of graphite particle dispersed in the asphalt. Graphite particles, that do not form the connected conductive path, also contribute to improving the thermal conductivity of asphalt binder. Therefore, the thermal conductivity and thermal diffusivity of asphalt binder show a linear increasing and the specific heat show a descending trend with the increment of graphite. 3.3. High temperature physical properties The physical properties of graphite modified asphalt binder before and after ageing were studied and results are shown in Figs. 4 and 5, respectively. The difference in softening point and penetration before and after ageing are used to evaluate the antiageing property of asphalt binders. After TFOT, PAV and UV ageing, the softening point of asphalt binders increases (Fig. 4) and the penetration declines correspondingly (Fig. 5). The results indicated that ageing procedures would improve the high temperature properties of asphalt binder. The larger change of physical properties implied deeper ageing degree of asphalt binders. The change of physical properties of asphalt binders differed in the ageing procedures. For all the asphalt binders, the effect of PAV and UV ageing
Fig. 3. Effect of graphite on the thermal characteristics of modified asphalt binders. (a) Thermal conductivity and thermal diffusivity. (b) Specific heat.
Fig. 4. Increment of softening point of asphalt binders after ageing.
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Fig. 5. Decrement of penetration of asphalt binders after ageing.
on the physical properties of asphalt binder was much more significant than the TFOT. For CAB sample, the softening point increased by 7.6 °C, 14.2 °C and 12.3 °C after TFOT, PAV and UV ageing, respectively while the penetration declined correspondingly by 13.9 dmm, 21.4 dmm and 19.8 dmm after related ageing procedures. Compared to UV ageing, the effect of PAV ageing shows more noticeable effect with respect to the increment of softening point. This phenomenon were confirmed by the results of penetration. Additionally, the change in increment of softening point and penetration were different for the asphalt binder containing varying content of mineral filler and graphite. With the content of graphite increases, the increment of softening point and the decrement of penetration become smaller after the three kinds of ageing procedures. When the content of graphite was wt. 40%, the increment of softening point were 6.2 °C, 11.5 °C and 9.9 °C, and decrement of penetration were 11.5 dmm, 18.5 dmm and 17.3 dmm. The result implied that graphite could improve the anti-ageing property compared to the mineral fillers.
angle (d) are the most important parameters to provide information about the rheological properties of the asphalt binders during the shearing process. Fig. 6 shows the temperature dependence of the modulus (G*) and phase angle (d) at 10 rad/s for each original binder in a range of temperature 0–30 °C. The obtained results indicated that the incorporation of graphite can increase the complex modulus and decrease the phase angle of asphalt binder. This phenomenon was more evident with the increasing of temperature. One reason is that the graphite absorbs most the lightweight fraction of asphalt. This could result in worse flexibility of asphalt binder, even in low temperature, which increase the risk of lowtemperature cracking. In this study, the results of physical properties indicated that asphalt binder would become more ‘‘harder’’ after ageing process, which would degrades the low temperature property. Since the complex modulus (G*) is related to the strength of asphalt binder, the ratio of G* of the ageing sample to that of original sample can be used to evaluate the ageing degree of asphalt binder during the ageing process. Complex modulus ageing indexes are calculated according to the following equation. Lower ageing index can be related to better anti-ageing property of asphalt binder.
AI ¼
G of aged asphalt binder G of unaged asphalt binder
ð2Þ
The performance of asphalt binder is strongly dependent on the temperature and load frequency. Dynamic shear rheological (DSR) test is recognized as an effective method to assess the viscoelastic properties of asphalt binders. Complex modulus (G*) and phase
Figs. 7–9 illustrate the complex modulus ageing index of asphalt binders at 10 rad/s in a range of temperature 0–30 °C. After TFOT, PAV, UV ageing procedures, the ageing index become smaller with the content of graphite increases. It implied that the graphite can improve the anti-ageing property of asphalt binder. The increase in G* ageing index after PAV and UV ageing were significantly greater than the increment in G* ageing index after TFOT ageing due to the prolonging the ageing process. In Fig. 7, the ageing index of control sample was smaller than the modified asphalt binder with varying content of graphite in the low temperature region. For example, the ageing index of asphalt binder with wt. 10% graphite is larger than the control one when the temperature was below 17.1 °C. And this temperature was 10.1 °C, 8.6 °C and 10.6 °C for the GMAB-20, GMAB-30 and GMAB-40, respectively. The results indicated that graphite may has a negative effect on the property of asphalt binder in the low temperature region. In Figs. 8 and 9, the ageing index of asphalt binder with graphite was smaller than the CAB sample in the full temperature region. It can be concluded that the graphite improves the anti-ageing performance of asphalt binder during the PAV and UV process. This conclusion can be confirmed by the varying trend of physical
Fig. 6. Temperature dependence of complex modulus (G*) and phase angle (d) of asphalt binders.
Fig. 7. Complex modulus ageing index of graphite modified asphalt binder after TFOT ageing.
3.4. Dynamic rheological properties
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Fig. 8. Complex modulus ageing index of graphite modified asphalt binder after PAV ageing.
Fig. 10. Complex modulus master curves of original asphalt binders.
difference in the complex modulus indicated that the effect of loading frequency on the asphalt binder with different content of graphite is not significant. As has been reported by previous studies, the temperature dependency of viscoelastic materials can be represented well by an Arrhenius equation. For asphalt binder, the shift factor of asphalt may fit the Arrhenius equation and the parameter of activation energy (Ea) has a strong correlation with the thermal susceptibility. Generally, the asphalt binder with higher (or smaller) activation energy were found to be less (or more) susceptible to the temperature variation [22,23]. Within the temperature range studied in this work, the temperature dependence of the shift factor for all the asphalt binder before and after ageing is described by an Arrhenius equation fairly well:
aðTÞ0 ¼ exp
Fig. 9. Complex modulus ageing index of graphite modified asphalt binder after UV ageing.
properties. Compared to results of PAV ageing, there was a significant difference in the G* ageing index between the five types of asphalt binders after UV. It might because the graphite can absorb, shield and prevent the ultraviolet due to its layer structure, which lead asphalt reducing irreversible ageing effect on asphalt. Frequency sweep test was conducted to investigate the frequency and temperature dependence of the viscoelastic behavior in the normally operating temperature range of asphalt pavement. The frequency region ranges from 0.1 to 400 rad/s on each binder at seven different test temperatures of 0, 10, 20, 30, 40, 50 and 60 °C. The master curve of complex modulus (G*) describes the frequency dependency of asphalt binder at a reference temperature. According to the time–temperature superposition principle (TTSP), the master curves of complex modulus of each binder could be obtained by utilizing the well-known Williams–Landel–Ferry (WLF) theory. Fig. 10 illustrates the master curves of complex modulus of the original asphalt binders at the reference temperature of 20 °C. The G* of asphalt binders increased with the increasing of graphite contents and frequencies. This might result from the high oil absorption of graphite, which leading graphite absorbing the lightweight fraction of asphalt. The strength of asphalt binder is consequently improved by addition of graphite. The unclear
Ea Rð1=T 1=T 0 Þ
ð3Þ
where a(T)0 is the shift factor relative to the reference temperature; Ea, the activation energy; R = 8.314 J/mol K; T, the temperature (K); and T0 is the reference temperature, 20 °C. The R2 values ranged from 0.9913 to 0.9976. Table 2 shows the values of Ea for the five asphalt binders before and after TFOT, PAV and UV ageing process. The active energy of asphalt binder with graphite was higher than that of CAB sample and it increased with the increasing of graphite content. The results indicated that control asphalt binder is easier to be degraded under the influence of heat or dynamic shear loading and graphite made the asphalt binder less temperature sensibility. The higher activation energy of asphalt binder after ageing (or with further ageing) can attribute to molecular structure change and a more interaction between the asphalt components. The increment of active energy for each asphalt binder after ageing become smaller with the content of graphite increased, especially for UV
Table 2 Activation energy Ea in G* of asphalt binders after ageing. Asphalt binders
Unaged TFOT aged TFOT aged – unaged PAV aged PAV aged – unaged UV aged UV aged – unaged
Ea (kJ/mol) CAB
GMAB-10
GMAB-20
GMAB-30
GMAB-40
165.05 169.21 4.16 180.02 14.97 174.89 9.84
167.87 171.98 4.11 181.98 14.11 176.56 8.69
169.25 173.29 4.04 182.76 13.51 177.46 8.21
171.02 174.95 3.93 184.21 13.19 178.85 7.83
173.95 177.86 3.91 186.87 12.92 180.86 6.91
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ageing. The reason is that the graphite can mitigate the negative effect of ageing on the properties of asphalt binder. 4. Conclusions The influence of graphite on the thermal characteristics and anti-ageing properties of asphalt binder was investigated in this study. Thermal constant analyzer (TPS 2500S, Hot Disk, Sweden) was employed to measure the thermal parameters of binders. The storage stability of five types of asphalt binders (CAB, GMAB10, GMAB-20, GMAB-30 and GMAB-40) were evaluated under different storage time at 163 °C. The physical and rheological properties of asphalt binders after TFOT, PAV and UV ageing processes were compared with that of the original samples. Based on the research results, the following conclusions can be drawn: 1. Experimental results indicated that the thermal conductivity and diffusivity increased linearly with the increasing content of graphite, while the specific heat presented a descending trend correspondingly. The increase of thermal conductivity of asphalt binder depends largely on the amount of graphite particle dispersed in the asphalt. 2. Although the storage stability of graphite modified asphalt binder was better than the mixture of binder and mineral filler, binders with mineral filler or graphite showed bad high temperature storage stability. It is recommended that graphite should be added with the same method as adding mineral filler. Therefore, it is unreasonable to prepare and store the graphite modified asphalt binder in practical application. 3. By comparing the physical and rheological properties of original asphalt binders with the aged samples, it could be concluded that graphite improved the anti-ageing properties of asphalt binders, especially for UV ageing. This may attribute to the layer structure of graphite, which mitigates the irreversible ultraviolet ageing effect on asphalt. The pavement materials with good thermal conductivity and long-term performance could improve the service behavior of the novel system-hydronic asphalt pavement. This study confirmed that the incorporation of graphite improves the thermal and antiageing properties of asphalt binder. However, it needs further study on the improvement mechanism of graphite modified asphalt binders. Further research should be also conducted on the durability of thermal property and pavement performance of asphalt concrete. Acknowledgements This work was supported by the National High Technology Research and Development Program of China (‘‘863’’, No. 2013AA
031602) and National Project of Scientific and Technical Supporting Programs funded by Ministry of Science & Technology of China (No. 2011BAE28B03) and National Natural Science Foundation of China (No. 51278392). References [1] Van Bijsterveld W, Houben LJ, Scarpas A, Molenaar AA. Using pavement as solar collector: effect on pavement temperature and structural response. Transport Res Rec: J Trans Res Board 2001;1778:140–8. [2] Pan TY, Lu Y, Wang ZY. Development of an atomistic-based chemophysical environment for modelling asphalt oxidation. Polym Degrad Stabil 2012;97:2331–9. [3] Gustavsson T, Bogren J, Green C. Road climate in cities: a study of the Stockholm area, south-east Sweden. Meteorol Appl 2001;8(4):481–9. [4] Löfgren S. The chemical effects of deicing salt on soil and stream water of five catchments in southeast Sweden. Water Air Soil Pollut 2001;130:863–8. [5] Sanzo D, Hecnar SJ. Effects of road de-icing salt (NaCl) on larval wood frogs (Rana sylvatica). Environ Pollut 2006;140(2):247–56. [6] Nixon WA. Improved cutting edges for ice removal, strategic highway research program report, SHRP-H-346. Washington, DC; 1993. p. 5–6. [7] Loomans M, Oversloot H, de Bondt A, Jansen R, van Rij H. Design tool for the thermal energy potential of asphalt pavements. In: Eighth international IBPSA conference, Eindhoven, Netherlands, August 11–14, 2003. [8] Eugster WJ, Schatzmann J. Harnessing solar energy for winter road clearing on heavily loaded expressways. new challenges for winter road service. In: XIth international winter road congress; 2002. [9] Chen MY, Wu SP, Wang H, Zhang JZ. Study of ice and snow melting process on conductive asphalt solar collector. Sol Energy Mat Sol C 2011;95(12):3241–50. [10] Mallick RB, Chen BL, Bhowmick S. Harvesting heat energy from asphalt pavements: development of and comparison between numerical models and experiment. Int J Sustain Eng 2011;5(2):159–69. [11] Wu SP, Chen MY, Zhang JZ. Laboratory investigation into thermal response of asphalt pavements as solar collector by application of small-scale slabs. Appl Therm Eng 2011;31(10):1582–7. [12] Liu XM, Wu SP, Ye QS, Qiu J, Li B. Properties evaluation of asphalt-based composites with graphite and mine powders. Constr Build Mater 2008;22(3):121–6. [13] Liu XM, Wu SP. Study on the graphite and carbon fiber modified asphalt concrete. Constr Build Mater 2011;25(4):1807–11. [14] Li B. Properties investigation of thermal conductive asphalt mixture [Master thesis]. Wuhan: Wuhan University of Technology; 2008. [15] Wu SP, Pan P, Chen MY, Zhang Y. Analysis of characteristics of electrically conductive asphalt concrete prepared by multiplex conductive materials. J Mater Civil Eng 2013;25(7):871–9. [16] Collop A, Cebon D. Parametric study of factors affecting flexible-pavement performance. J Trans Eng 1995;121(6):485–94. [17] Durrieu F, Farcas F, Mouillet V. The influence of UV aging of a styrene/ butadiene/styrene modified bitumen: comparison between laboratory and on site aging. Fuel 2007;86:1446–51. [18] Vargas XA, Afanasjeva N, Álvarez M, Marchal PH, Choplin L. Asphalt rheology evolution through thermo-oxidation (aging) in a rheo-reactor. Fuel 2008;87:3018–23. [19] Standard test method for softening point of bitumen (Ring-and-ball apparatus). ASTM D36-76. [20] Standard test method for ductility of bituminous materials. ASTM D113-99. [21] Standard test method for penetration of bituminous materials. ASTM D5-13. [22] Cong PL, Xu PJ, Chen SF. Effects of carbon black on the anti aging, rheological and conductive properties of SBS/asphalt/carbon black composites. Constr Build Mater 2014;52:306–13. [23] Zhang F, Yu JY, Wu SP. Effect of ageing on rheological properties of storagestable SBS/sulfur-modified asphalts. J Hazard Mater 2010;182:507–17.