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Rheological properties, low-temperature cracking resistance, and optical performance of exfoliated graphite nanoplatelets modified asphalt binder
Construction and Building Materials 113 (2016) 988–996
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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Rheological properties, low-temperature cracking resistance, and optical performance of exfoliated graphite nanoplatelets modified asphalt binder Hui Yao a, Qingli Dai a, Zhanping You a,⇑, Mingxiao Ye b, Yoke Khin Yap b a b
Department of Civil and Environmental Engineering, Michigan Technological University, 1400 Townsend Drive, Houghton, MI 49931, USA Department of Physics, Michigan Technological University, 1400 Townsend Drive, Houghton, MI 49931, USA
h i g h l i g h t s The xGNP nanoplatelets were used to modify the asphalt binder. The ABCD test was used to evaluate the low-temperature of the asphalt binders. The MCR was employed to characterize the rheological properties of the binders. The aging effect of the asphalt binders was evaluated by FTIR. The optical characterizations of the asphalt binders were tested.
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Article history: Received 19 October 2015 Received in revised form 18 March 2016 Accepted 23 March 2016
Keywords: Multiple-layer graphite Modular compact rheometer Asphalt binder thermal cracking test Fourier transform infrared spectroscopy FTIR Electromagnetic radiation absorption Asphalt
a b s t r a c t The purpose of this study is to use multiple-layer graphite nanoplatelets to modify the asphalt due to the special features of the graphite nanoplatelets. These include the high optical absorption, self-lubrication, and high thermal stability and conductivity. The graphite nanoplatelets with different weight contents (1% and 2%) were slowly added into the asphalt. The modified asphalt binder was mixed in the high shear mixer. Different tests were used to evaluate the properties of the modified asphalt binder, including the Rotational Viscometer (RV), Modular Compact Rheometer (MCR), Asphalt Binder Thermal Cracking (ABCD) Test, Fourier Transform Infrared Spectroscopy (FTIR) and electromagnetic radiation absorption tests. The test results demonstrate that 1) the viscosity of the modified asphalt binder increases at various temperatures and the activation energy of the modified asphalt binder decreases compared to the control one; 2) the complex shear modulus of the modified asphalt binder also rises at different temperatures and frequencies and the high-temperature of the modified asphalt binder improves; 3) the cracking temperatures, fracture stresses, and strains of the modified asphalt binder improve and the low-temperature performance of the modified asphalt binder is enhanced; 4) the aging groups increase in the modified asphalt binder and more aging groups in asphalt may help to increase the resistance to the rutting and moisture damage in asphalt mixtures; and 5) the absorption of lights with different wavelengths increases in the modified asphalt binder compared to the control one. Therefore, the resistance to rutting and cracking of graphite nanoplatelets modified asphalt binder is improved and they can be applied to the field for different purposes considering the advanced properties. Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction There are four main distresses existing in the current pavement, and these are rutting, moisture damage, thermal cracking and fatigue cracking [1]. The modified asphalt binder is designed to ⇑ Corresponding author. E-mail addresses: [email protected] (H. Yao), [email protected] (Q. Dai), [email protected] (Z. You), [email protected] (M. Ye), [email protected] (Y.K. Yap). http://dx.doi.org/10.1016/j.conbuildmat.2016.03.152 0950-0618/Ó 2016 Elsevier Ltd. All rights reserved.
improve these properties. In research and industry areas, different modifiers are used for the improvement of pavement performance, such as the styrene-butadiene-styrene (SBS), fibers, rubber and hydrated lime. The SBS material was projected to improve the low-temperature performance of asphalt mixtures. The performance tests of SBS asphalt binders and mixtures were conducted, and the tomography and chemical analysis of SBS modified asphalt binders were also examined [2]. The different fibers were used to enhance the rutting resistance in the asphalt mixtures, including
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polypropylene fibers, polyester fibers, asbestos fibers, cellulose fibers, carbon fibers, glass fibers and nylon fibers [3]. The dynamic modulus and moisture susceptibility of fibers modified asphalt mixtures improved. The rubber can also improve the resistance to rutting and fatigue cracking in the modified asphalt mixtures [4]. The hydrated lime was widely used to reduce the moisture susceptibility in the asphalt mixtures [5]. It is common that the utilization of carbon-based materials are applied to modify the asphalt, such as Pyrolytic Carbon Black (CBp), carbon fibers, carbon micro-fibers, nano-fibers and waste polyester fibers. The pyrolysis process of tires produces the oil and pyrolytic carbon black (CBp), and the carbon black produced under the vacuum pyrolysis is closer to the commercial carbon black than that processed at the condition of atmospheric pyrolysis [6]. The carbon black was used for modification of asphalt binders, and the temperature susceptibility and physical response characteristics were reduced. 5%–30% of carbon black in the asphalt binder increased the loss and storage moduli [7]. In the mixtures of carbon black and bitumen, a part of the asphaltene component in bitumen was absorbed on the carbon black (CBp) and not easily extracted by toluene. The asphaltene has the same ring size with the aromatics. In addition, it is not difficult to extract saturates and naphthene aromatics from the mixtures [8]. The carbon fibers were used to modify the asphalt, and the resistance to permanent deformation and fatigue characteristics were enhanced due to the rigidity and high tensile strength [9]. The carbon microfibers (MCFs) are also a good modifier for asphalt modification due to the size effect of the fibers, as well as nano-fibers. The different properties of the MCFs modified asphalt binders were tested and compared to the neat asphalt, such as micro-images, rheological properties, and high- and low-temperature performance [10,11]. The asphalt with the carbon nano-fibers (CNFs) was also used for the modification of the asphalt. The characteristics of the CNFs modified and neat asphalt were compared and discussed. The CNFs improved the resistance to rutting and increased the fatigue life after modification [12]. The waste polyester fibers enhanced the indirect tensile strength (ITS) in the modified asphalt mixture and improved the resistance to moisture damage [13]. Recently, the nanomaterials are brought into the hot spot due to their unique properties. The different nanomaterials were used for the improvement in the performance of asphalt, such as the nanosilica, nanoclay and carbon nanoparticles. When the nanosilica material was added into the asphalt, the surface of nanosilica was converted from hydrophilic to hydrophobic. The nanosilica improved the dynamic modulus and rutting susceptibility of the modified asphalt mixtures [14]. The layer structures of nanoclay were intercalated and exfoliated when the nanoclay was added and mixed in the asphalt matrix. The rutting resistance and moisture susceptibility were significantly meliorated in the nanoclay modified asphalt mixtures [10,11,15]. The different percentages of carbon nanoparticles were mixed with asphalt and the rheological properties of the modified asphalt binders were investigated. The carbon nanoparticles contribute to the improvements of the rutting resistance and the permanent deformation in asphalt mixtures [16]. Based on the literature review [17,18], the graphite and graphene oxide were tested for asphalt modification. However, there are few multiple-layer graphite applications in the asphalt modification. The special characteristics of multiple-layer graphite sheets include self-lubrication, high thermal stability, high optical adsorption, high thermal stability and electrical conductivity [19–22]. In this study, the graphite nanoplatelets were selected to improve the performance of asphalt. A series of tests were employed to evaluate the properties of the modified asphalt binder. The chemical aging groups and electromagnetic radiation absorption were analyzed in the asphalt.
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2. Objective The objective of this study is to utilize the graphite nanoplatelets to modify the asphalt due to the unique characteristics of the graphite materials. The various percentages of graphite nanoplatelets without further treatment are added to the control asphalt and the modified asphalt binders is prepared. Different properties of the modified asphalt binder are tested, including viscosity, the complex shear modulus, the thermal cracking temperature, the aging index, and electromagnetic radiation absorption. Through these tests, it is extrapolated that the addition of graphite nanoplatelets enhances the high- and low-temperature performance of the modified asphalt binder and it is meaningful to generalize or further apply the materials. 3. Materials and experimental plan The PG 58-28 control asphalt was used and the ‘‘exfoliated graphite nanoplatelets (xGNP)” were selected to modify the control asphalt. The xGNP grade H-25 was shipped from XG Sciences Inc. The Hitachi SU6600 field emission scanning electron microscope (FE-SEM) was used to test the microstructures of graphite nanoplatelets. The FE-SEM images are shown in Fig. 1. The density of the black granules is 0.03–0.1 g/cm3 and the oxygen content is also less than 1%. The surface area of xGNP H-25 (Fig. 1) is around 50–80 m2/g and the average diameter of the particles (multiple-layer graphite sheets) is 25 lm (from the datasheet in manufactory). The thickness of xGNP multiple-layer graphite sheets is around 15 nm. The zeta potential of xGNP nanoplatelets is around 32.33 mV, and the layer spacing is around 3.35 Ã [23]. The tensile modulus of xGNP nanoplatelets is around 1000 GPa, and the tensile strength is around 5 GPa. The 1% and 2% xGNP particles with no treatment were added into the control asphalt and mixed in the high shear machine at a temperature of about 145 °C and a rate of 2000 rpm. The xGNP modified asphalt binder was prepared after mixing for two hours. In accordance with the SuperpaveTM specification, these tests were used to evaluate the performance of xGNP modified asphalt binder, including the rotational viscometer (RV), modular compact rheometer (MCR), asphalt binder thermal cracking (ABCD), Fourier transform infrared spectroscopy (FTIR) and electromagnetic radiation absorption tests. The Brookfield DV-II plus viscometer was used to test the viscosity of xGNP modified asphalt binder. The Anton Paar MCR 302 was used to test the complex shear modulus of asphalt binders. The low thermal cracking temperature in the asphalt was tested by the ABCD device from EZasphalt Inc. The Jasco IRT 3000 FTIR spectrometer was used to analyze the aging groups in the asphalt. The optical characterization in the asphalt was examined by the UV-1800 UV-Vis Spectrophotometer with a 1 nm resolution.
4. Discussions of results 4.1. Viscosity measurement The viscosity is the basic consistent property of materials and the dynamic shear viscosity of the fluid describes the resistance to the shear force. The viscosity of asphalt relates to the mixing and compaction temperatures of asphalt mixtures and also determines the workability of asphalt binders. The viscosities of the control and xGNP modified asphalt binders were tested at different temperatures: 90 °C, 100 °C, 125 °C, 135 °C, 150 °C, and 175 °C. The viscosity results are shown in Fig. 1. When the liquid materials flow, the molecules of the material slide over each other. However, the intermolecular forces in the liquid materials resist to flow. Activation energy is required to break the barrier and it is the minimum energy to flow. The ‘‘Arrhenius” equation is used to calculate the energy and determine the viscosity-temperature dependency of the asphalt [24,25]. The energy formula is shown in Eq. (1). Ef
g ¼ AeRT ; ln g ¼
Ef þ ln A RT
ð1Þ
where g is the viscosity of the asphalt; A is the coefficient of regression; Ef is the activation energy for the asphalt flow; R is the universal gas constant (8.314 J/mol/K); and T is the temperature in degrees K.
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Fig. 1. The appearance of xGNP particles at room temperature and FE-SEM images of xGNP particles: a) black powder; b) the FE-SEM image of graphite nanoplatelets at the magnitude of 15,000; c) the FE-SEM image of graphite nanoplatelets at the magnitude of 2000.
Fig. 2a demonstrates that the viscosities of 1% and 2% xGNP modified asphalt binders are higher than those of the control asphalt at the different temperatures listed above. The addition of xGNP particles in the control asphalt increases the viscosity and it indicates that the high-temperature performance of the modified asphalt binder improves. The viscosity improvement in the modified asphalt binder may be caused by the high thermal stability of xGNP particles. The viscosities of xGNP modified asphalt binder at 135 °C are lower than 3 Pas and the workability of the modified asphalt binder in the field is not affected, in accordance with the SuperpaveTM specification. There is a potential that more xGNP particles in the asphalt increase viscosity. The activation energy results of the xGNP modified asphalt binder and the control are shown in Fig. 2b and c. Fig. 2b shows the regression method used to calculate the activation energy in the asphalt binders. The independent variable is the inverse of the temperature (1/T) and the dependent one is the viscosity (1n g). The activation energy equals the universal gas constant (UGC) multiplied by the slopes shown in Fig. 2b, and the energy results are shown in Fig. 2c. Fig. 2c shows that the activation energy of the xGNP modified asphalt binder is lower than that of the control asphalt. The amount of intermolecular forces against flowing in asphalt is the viscosity. Low activation energy means that the required energy of asphalt is low for flowing under the test temperature. The activation energy relates to the development of self-healing, and it is the minimum energy for asphalt flow. So, the xGNP modified asphalt binder is good for
workability and self-healing function compared to the control asphalt. The results indicate that the xGNP modified asphalt binder is stiffer than the control one at high temperatures. The low activation energy of the modified asphalt binder is caused by high thermal conductivity and the self-lubricating features of multi-layer graphite. The multi-layer graphite particles accelerate the thermal transfer and thermal stress relaxation in the modified asphalt binder, and the self-lubricating function helps the asphalt flow. Therefore, it is confirmed that the addition of xGNP in the asphalt improves the high-temperature performance of the modified asphalt binder.
4.2. Complex shear modulus test The MCR rheometer (Fig. 3a) was used to characterize the elastic and viscous properties of asphalt binders. The complex shear modulus and phase angle were tested with the round plate (diameter, 25 mm) using the oscillatory mode. The test temperatures include 46 °C, 52 °C, 58 °C, 64 °C, and 70 °C and the frequencies range from 0.01 Hz to 45 Hz. The complex shear modulus describes the resistance to deformation when the sample is sheared, and the phase angle is the lag between the shear stress and the resulting strain. The larger the phase angle is, the more viscous the sample is. The storage modulus is the elastic portion of the complex shear modulus and it determines the ability of resistance to rutting in the asphalt. The MCR test results are shown in Fig. 3.
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Fig. 2. The viscosity and activation energy results of xGNP modified asphalt binder and control.
Fig. 3b shows the trends of the complex shear modulus with different frequencies and temperatures in asphalt binders. The complex shear modulus and G⁄/sin d values (Fig. 3c) of xGNP modified asphalt binder are higher than that of the control one, as well as the storage modulus (Fig. 3d). The addition of xGNP particles enhances the complex shear modulus and resistance to rutting in asphalt binders. When the xGNP particles were added into the asphalt, it is deduced that the layer structures of the particles were exfoliated considering that the interlayer spacing is around 3.35 Å. The asphalt was intercalated in these layers, and the ‘‘sandwich” structure reinforced the resistance to deformation and helped for the stress relaxation in the modified asphalt binder. Meanwhile, the high thermal conductivity function [26] of the xGNP particles/graphite may accelerate the heating transfer in the asphalt, and it may be helpful for temperature stability.
Therefore, the resistance to permanent deformation improves in the modified asphalt binders. 4.3. Thermal cracking test The thermal cracking at low temperatures is one of the four main distresses (rutting, moisture damage, thermal and fatigue cracking) in the asphalt pavement. Currently, there are many ways to determine low temperature properties of an asphalt binder, such as penetration, DTT, TSRST, and BBR tests. Based on the references [1], considering the accuracy and simplicity of the test, the Asphalt Binder Thermal Cracking (ABCD) test was employed in this study and there are a few advantages: 1) No elaborate assumptions; 2) fast measurement and easy to determination of the potential for thermal cracking; and 3) minimal errors and simple procedures.
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The fundamental theory of ABCD is to utilize the different contractions of an asphalt binder and aluminum rings and introduce the superposition principle to standardize the dimensions of rings and asphalt samples. The thermal strains of asphalt under the different temperatures were monitored and the sudden reduction of strains means that the thermal cracking of asphalt was at this low temperature. The thermal stress at this low temperature is calculated from Eq. (2). The sample and mold of the ABCD test are shown in Fig. 4a. The ABCD test results of unaged asphalt binders in this study are shown in Figs. 4b and 5.
rb ¼
F ABCD ecorr EABCD AABCD ¼ Ab Ab
ð2Þ
where rb is the thermal stress at the cracking temperature in the sample; F ABCD is the thermal force due to a temperature difference; Ab is the cross-sectional area of the sample; ecorr is the strain corrected for temperature; EABCD is the elastic modulus of the aluminum ring; and AABCD is the cross-sectional area of the aluminum ring. Fig. 4 shows microstrains in the asphalt samples and the cooling rates in the test chamber under the different temperatures. The xGNP modified asphalt binder fractures at the lower temperatures with a higher cracking strain than the control one. The 2% xGNP modified asphalt binder has a higher cracking strain at a lower temperature than the 1% xGNP modified asphalt binder. There is a potential that more xGNP particles in the asphalt lead to the lower cracking temperature and the higher fracture strain. It is noticed that the cooling rates of xGNP modified asphalt binder are still higher than that of the control one. The cooling rate in the test indicates that the pavement materials varies from different field conditions and it also reflects the accommodation of the different pavement areas. It is likely that the high cooling rate may accelerate the thermal cracking in the asphalt. That’s why the different cooling rate was used in this study. However, based on the previous study [25], the cooling rate does not affect the cracking temperature much. From this figure, the addition of xGNP particles in the asphalt enhances the low-temperature performance, especially the thermal shrinkage. Fig. 5 displays the fracture stresses at the cracking strains and the temperatures in the control and xGNP modified asphalt binders under the temperature/cooling setting. The xGNP particles in the asphalt decrease the cracking temperatures and increase the fracture stress and strain. It implies that the xGNP modified asphalt binder can withstand more thermal stress induced by the change in temperature relative to the control one. It is probably due to the layer structure of xGNP particles. After the homogenous mixing in the high shear mixer, the added particles were well scattered in the asphalt matrix. It is probably that the graphite layer helps the asphalt to relax the thermal stress due to the high thermal stability [21] of graphite layers. It is possible that the self-lubricating property (loose interlamellar coupling [22]) of multiple-layer graphite sheets in asphalt increases the cracking strain and also enhances the strain relaxation ability in the modified asphalt binder. The exfoliated structure of the xGNP particles also helps relax the thermal or mechanical strain in the modified asphalt binder. 4.4. Chemical group analysis
Fig. 3. MCR results of the control and xGNP modified asphalt binders.
The aging is one of common phenomena in the nature of materials and the aging leads to a life span reduction of the materials. However, the mild aging in asphalt mixtures is helpful to improve the resistance to the moisture sensitivity and rutting [27,28].
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Fig. 4. The mold, sample and results of the ABCD test.
Fig. 5. The cracking temperature, fracture stress and strain jump results of the control and xGNP modified asphalt binder.
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Therefore, it is necessary to detect the aging group distributions in the asphalt. The FTIR spectrometer was used to obtain the infrared absorption spectrum and the Fourier transform was used to convert the raw data to the testing spectrum. In this study, the FTIR spectra of asphalt were tested by the accumulation of 512 scans with a resolution of 4 cm1. All tested samples are coated on individual Si chips. These Si chips are cut from high purity Si wafers (resistivity > 10,000 Xcm) so that they are transparent to the infrared light source. All samples are then characterized in a transmission/absorbance mode. It is know that the main aging groups in asphalt are carbonyl (C@O) and sulfoxide (S@O) bonds. The wavenumber peaks of 1700 cm1 and 1030 cm1 in the spectra represent the carbonyl and sulfoxide [10,29], respectively. The FTIR test results are shown in Fig. 6.
Area of the carbonyl band between 1830 cm1 and 1670 cm1 IC@O ¼ P Area of the spectral bands between 2000 cm1 and 600 cm1 ð3Þ Area of the sulfoxide band between 1050 cm1 and 985 cm1 IS@O ¼ P Area of the spectral bands between 2000 cm1 and 600 cm1 ð4Þ Fig. 6 demonstrates the spectra images of xGNP modified asphalt binder and the control, and these trends are similar. From the data shown, there is no obvious new peak appearing in the images. The CAH bond characteristic of carbon rings in the xGNP particles is overlapped with the aromatic components of the asphalt. It is most likely that the xGNP particles were aged more or less during the mixing procedure of the asphalt binders due to the features of the nanomaterials. The carbonyl groups in xGNP materials are also overlapped with those in asphalt. This is probably the reason that no new peak is shown in the FTIR images of xGNP modified asphalt binder compared to the control one. There is a low probability that chemical reactions occurred between the asphalt and the xGNP particles since the graphite is a stable material under standard conditions. However, the aging components in these asphalt binders should be different and the average ratios of carbonyl and sulfoxide bonds in the unaged asphalt binders were calculated using Eqs. (2) and (3). The ratio results are shown in Fig. 7. Fig. 7 reveals the carbonyl and sulfoxide indexes (relative ratios) in the unaged asphalt binders and the smallest ratios of these bonds are in the control asphalt. The ratios of carbonyl and sulfoxide bonds in the 2% xGNP modified asphalt binder are higher than those of the 1% xGNP modified asphalt binder and the control one. This indicates that more aging groups are detected in the modified asphalt binder relative to the control asphalt. There is
Fig. 6. FTIR results of xGNP modified asphalt binder and the control.
Fig. 7. The average ratios of carbonyl and sulfoxide groups in the asphalt.
also a possibility that more xGNP particles in the control asphalt result in more aging components in the modified asphalt binder. There are two reasons to explain the increase of aging components in the modified asphalt binder: 1) the xGNP particles were aged during the preparation of the modified asphalt binder and the partially aged xGNP particles increase the carbonyl index in the modified asphalt binder; and 2) the addition of xGNP particles in the control asphalt induces nano-aging effects during the open-air mixing process, seen from the sulfoxide index increase. The moderate aging in the asphalt increases the resistance to moisture damage and rutting in the pavement. Therefore, it is predicted that the xGNP materials are helpful in improving the performance of asphalt binders and mixtures.
4.5. Optical characterization The sun exposure is one of the working conditions for the pavement. The spectrum of the sunlight (all electromagnetic radiations) is from about 100 nm–1 mm. It consists of five regions: ultraviolet C (100 nm–280 nm), ultraviolet B (280 nm–315 nm), ultraviolet A (315 nm–400 nm), visible light (380 nm–780 nm), and infrared (700 nm–1 mm) [30]. In order to the test the absorptions of asphalt under different electromagnetic radiations, the UV-1800 UV-Vis Spectrophotometer was used to test with the radiation range of 300 nm–1100 nm. The test results of the control and xGNP modified asphalt binders are shown in Fig. 8. Fig. 8 shows the absorbance spectra of the control and the xGNP modified asphalt binders under the radiation range of 300 nm– 1100 nm. The control asphalt has a lower light absorption from 535 nm to 1100 nm compared to that of the xGNP modified asphalt binder. In the wavelength range from 300 nm to 535 nm, the radiant flux received in both the control and the modified asphalt binders is about 104 times than the radiant flux transmitted by these materials. The light absorption increases significantly with the
Fig. 8. The absorption of xGNP modified asphalt binder and the control under electromagnetic radiations (300 nm–1100 nm).
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addition of xGNP to the control asphalt for the wavelength range of 535 nm–1100 nm. Light absorbance is inversely proportional to the logarithm of the ratio between the transmitted and the i h i e ¼ log10 T received light flux to the base of 10 A ¼ log10 U Ut e
(where, Uie is the radiant flux received by the material, Ute is the radiant flux transmitted by the material, and T is the transmittance of the material). For example, when the light absorbance is measured as 2.33 for the 2% xGNP modified asphalt binder at a wavelength of 1100 nm, the ratio of the received light vs. the transmitted light flux is calculated as 213.80 (102.33). Similarly, when the light absorbance is measured as 1.15 for the 1% xGNP modified asphalt binder at a wavelength of 1100 nm, the ratio of the received light vs. the transmitted light flux is calculated as 14.13 (101.15). Thus, the absorbed light ratio (absorbed radiant vs. received radiant flux) is about 13.13/14.13 = 0.93. The ratio of the received radiant vs. the transmitted radiant flux is 1.48 (100.17) for the control asphalt and the absorbed ratio is around 0.48/1.48 = 0.32. The results indicate that the near infrared light absorbance (with a wavelength above 535 nm) increases significantly with the addition of small amounts of xGNP particles. It is likely that the exfoliated structure of the xGNP particles in asphalt also increases the surface area to absorb the test radiation. The improved visible and near infrared light absorptions can potentially provide new ways for pavement material heating with a visible/near-infrared light lamp or focused sunlight. The accelerated asphalt mixture healing was conducted by using the asphalt binder capillary Newtonian flow at the elevated temperatures (above transition temperatures) in our research group. The induction healing of asphalt mixtures with radio electromagnetic waves showed good healing performances [31]. Following this work, the healing performance of the xGNP modified asphalt binder mixture will be investigated with a visible/near-infrared light lamp or focused sunlight through lab testing and computation. It is anticipated that improved self-healing materials with high electric– magnetic wave absorption can potentially meet the challenge of efficient pavement maintenance and an elongated service life.
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susceptibility of the modified asphalt binders improve, due to the fact that the mild aging in the asphalt helps to resist rutting and moisture. The UV-Vis Spectrophotometer test measures the absorption under electromagnetic radiations with different wavelengths. The results display that significant difference exists between the control and the modified asphalt binders in the visible and near infrared regions, and the modified asphalt binder can absorb more electromagnetic waves relative to the control one. It also indicates that the xGNP modified asphalt binder absorbs more radiation and energy to maintain the performance of the asphalt, especially in low-temperature regions. In addition, the selfhealing effects or accelerated self-healing at elevated temperatures can potentially be enhanced with improved light absorption of the xGNP modified asphalt binder. Therefore, it is good to use the xGNP particles/multiple-layer graphite as an asphalt modifier to improve the properties of asphalt due to the improved rheological properties, lowtemperature performance, and high radiation absorption for accelerated self-healing at elevated temperatures. Future research would focus on the performance of the modified asphalt mixtures and the accelerated healing of these mixtures with a visible/nearinfrared light lamp or focused light.
Acknowledgements The authors appreciate Tristan Kolb and David Porter for help with laboratory testing, and Su Kong Chong’s assistance on FESEM characterization. The experimental work was completed in the Transportation Materials Research Center at Michigan Technological University. Any opinion, finding, conclusion, or recommendation expressed in this material are those of the authors and do not necessarily reflect the review of any organization.
References 5. Conclusions The multiple-layer graphite nanoplatelets (xGNP) were added into the asphalt as a modifier to improve the high- and lowtemperature performance of the modified asphalt binder. The various tests were used to evaluate the properties, including the RV, MCR, ABCD, FTIR and electromagnetic radiation absorption tests. Based on these test results, the following conclusions can be drawn: 1) Compared to the control asphalt, the viscosity of the modified asphalt binder increases, the mixing and compaction temperatures of the modified asphalt binder increase, and the activation energy of the modified asphalt binder decreases. This indicates that the high-temperature performance of the modified asphalt binder improves; the complex shear modulus and the rutting factor values of the xGNP modified asphalt binder increase, thus implying that the resistance to rutting is enhanced in the modified asphalt binder at high temperatures. The cracking temperature of the modified asphalt binder also improves, and the fracture stress and strain of the modified asphalt binder increase. This infers that the stress relaxation of the modified asphalt binder is enhanced and the resistance to low-temperature thermal cracking also improves. 2) The FTIR results show that the ratio of aging groups (C@O) in the modified asphalt binder significantly rises, and it causes that the high-temperature performance and moisture
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Contents lists available at ScienceDirect
Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Rheological properties, low-temperature cracking resistance, and optical performance of exfoliated graphite nanoplatelets modified asphalt binder Hui Yao a, Qingli Dai a, Zhanping You a,⇑, Mingxiao Ye b, Yoke Khin Yap b a b
Department of Civil and Environmental Engineering, Michigan Technological University, 1400 Townsend Drive, Houghton, MI 49931, USA Department of Physics, Michigan Technological University, 1400 Townsend Drive, Houghton, MI 49931, USA
h i g h l i g h t s The xGNP nanoplatelets were used to modify the asphalt binder. The ABCD test was used to evaluate the low-temperature of the asphalt binders. The MCR was employed to characterize the rheological properties of the binders. The aging effect of the asphalt binders was evaluated by FTIR. The optical characterizations of the asphalt binders were tested.
a r t i c l e
i n f o
Article history: Received 19 October 2015 Received in revised form 18 March 2016 Accepted 23 March 2016
Keywords: Multiple-layer graphite Modular compact rheometer Asphalt binder thermal cracking test Fourier transform infrared spectroscopy FTIR Electromagnetic radiation absorption Asphalt
a b s t r a c t The purpose of this study is to use multiple-layer graphite nanoplatelets to modify the asphalt due to the special features of the graphite nanoplatelets. These include the high optical absorption, self-lubrication, and high thermal stability and conductivity. The graphite nanoplatelets with different weight contents (1% and 2%) were slowly added into the asphalt. The modified asphalt binder was mixed in the high shear mixer. Different tests were used to evaluate the properties of the modified asphalt binder, including the Rotational Viscometer (RV), Modular Compact Rheometer (MCR), Asphalt Binder Thermal Cracking (ABCD) Test, Fourier Transform Infrared Spectroscopy (FTIR) and electromagnetic radiation absorption tests. The test results demonstrate that 1) the viscosity of the modified asphalt binder increases at various temperatures and the activation energy of the modified asphalt binder decreases compared to the control one; 2) the complex shear modulus of the modified asphalt binder also rises at different temperatures and frequencies and the high-temperature of the modified asphalt binder improves; 3) the cracking temperatures, fracture stresses, and strains of the modified asphalt binder improve and the low-temperature performance of the modified asphalt binder is enhanced; 4) the aging groups increase in the modified asphalt binder and more aging groups in asphalt may help to increase the resistance to the rutting and moisture damage in asphalt mixtures; and 5) the absorption of lights with different wavelengths increases in the modified asphalt binder compared to the control one. Therefore, the resistance to rutting and cracking of graphite nanoplatelets modified asphalt binder is improved and they can be applied to the field for different purposes considering the advanced properties. Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction There are four main distresses existing in the current pavement, and these are rutting, moisture damage, thermal cracking and fatigue cracking [1]. The modified asphalt binder is designed to ⇑ Corresponding author. E-mail addresses: [email protected] (H. Yao), [email protected] (Q. Dai), [email protected] (Z. You), [email protected] (M. Ye), [email protected] (Y.K. Yap). http://dx.doi.org/10.1016/j.conbuildmat.2016.03.152 0950-0618/Ó 2016 Elsevier Ltd. All rights reserved.
improve these properties. In research and industry areas, different modifiers are used for the improvement of pavement performance, such as the styrene-butadiene-styrene (SBS), fibers, rubber and hydrated lime. The SBS material was projected to improve the low-temperature performance of asphalt mixtures. The performance tests of SBS asphalt binders and mixtures were conducted, and the tomography and chemical analysis of SBS modified asphalt binders were also examined [2]. The different fibers were used to enhance the rutting resistance in the asphalt mixtures, including
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polypropylene fibers, polyester fibers, asbestos fibers, cellulose fibers, carbon fibers, glass fibers and nylon fibers [3]. The dynamic modulus and moisture susceptibility of fibers modified asphalt mixtures improved. The rubber can also improve the resistance to rutting and fatigue cracking in the modified asphalt mixtures [4]. The hydrated lime was widely used to reduce the moisture susceptibility in the asphalt mixtures [5]. It is common that the utilization of carbon-based materials are applied to modify the asphalt, such as Pyrolytic Carbon Black (CBp), carbon fibers, carbon micro-fibers, nano-fibers and waste polyester fibers. The pyrolysis process of tires produces the oil and pyrolytic carbon black (CBp), and the carbon black produced under the vacuum pyrolysis is closer to the commercial carbon black than that processed at the condition of atmospheric pyrolysis [6]. The carbon black was used for modification of asphalt binders, and the temperature susceptibility and physical response characteristics were reduced. 5%–30% of carbon black in the asphalt binder increased the loss and storage moduli [7]. In the mixtures of carbon black and bitumen, a part of the asphaltene component in bitumen was absorbed on the carbon black (CBp) and not easily extracted by toluene. The asphaltene has the same ring size with the aromatics. In addition, it is not difficult to extract saturates and naphthene aromatics from the mixtures [8]. The carbon fibers were used to modify the asphalt, and the resistance to permanent deformation and fatigue characteristics were enhanced due to the rigidity and high tensile strength [9]. The carbon microfibers (MCFs) are also a good modifier for asphalt modification due to the size effect of the fibers, as well as nano-fibers. The different properties of the MCFs modified asphalt binders were tested and compared to the neat asphalt, such as micro-images, rheological properties, and high- and low-temperature performance [10,11]. The asphalt with the carbon nano-fibers (CNFs) was also used for the modification of the asphalt. The characteristics of the CNFs modified and neat asphalt were compared and discussed. The CNFs improved the resistance to rutting and increased the fatigue life after modification [12]. The waste polyester fibers enhanced the indirect tensile strength (ITS) in the modified asphalt mixture and improved the resistance to moisture damage [13]. Recently, the nanomaterials are brought into the hot spot due to their unique properties. The different nanomaterials were used for the improvement in the performance of asphalt, such as the nanosilica, nanoclay and carbon nanoparticles. When the nanosilica material was added into the asphalt, the surface of nanosilica was converted from hydrophilic to hydrophobic. The nanosilica improved the dynamic modulus and rutting susceptibility of the modified asphalt mixtures [14]. The layer structures of nanoclay were intercalated and exfoliated when the nanoclay was added and mixed in the asphalt matrix. The rutting resistance and moisture susceptibility were significantly meliorated in the nanoclay modified asphalt mixtures [10,11,15]. The different percentages of carbon nanoparticles were mixed with asphalt and the rheological properties of the modified asphalt binders were investigated. The carbon nanoparticles contribute to the improvements of the rutting resistance and the permanent deformation in asphalt mixtures [16]. Based on the literature review [17,18], the graphite and graphene oxide were tested for asphalt modification. However, there are few multiple-layer graphite applications in the asphalt modification. The special characteristics of multiple-layer graphite sheets include self-lubrication, high thermal stability, high optical adsorption, high thermal stability and electrical conductivity [19–22]. In this study, the graphite nanoplatelets were selected to improve the performance of asphalt. A series of tests were employed to evaluate the properties of the modified asphalt binder. The chemical aging groups and electromagnetic radiation absorption were analyzed in the asphalt.
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2. Objective The objective of this study is to utilize the graphite nanoplatelets to modify the asphalt due to the unique characteristics of the graphite materials. The various percentages of graphite nanoplatelets without further treatment are added to the control asphalt and the modified asphalt binders is prepared. Different properties of the modified asphalt binder are tested, including viscosity, the complex shear modulus, the thermal cracking temperature, the aging index, and electromagnetic radiation absorption. Through these tests, it is extrapolated that the addition of graphite nanoplatelets enhances the high- and low-temperature performance of the modified asphalt binder and it is meaningful to generalize or further apply the materials. 3. Materials and experimental plan The PG 58-28 control asphalt was used and the ‘‘exfoliated graphite nanoplatelets (xGNP)” were selected to modify the control asphalt. The xGNP grade H-25 was shipped from XG Sciences Inc. The Hitachi SU6600 field emission scanning electron microscope (FE-SEM) was used to test the microstructures of graphite nanoplatelets. The FE-SEM images are shown in Fig. 1. The density of the black granules is 0.03–0.1 g/cm3 and the oxygen content is also less than 1%. The surface area of xGNP H-25 (Fig. 1) is around 50–80 m2/g and the average diameter of the particles (multiple-layer graphite sheets) is 25 lm (from the datasheet in manufactory). The thickness of xGNP multiple-layer graphite sheets is around 15 nm. The zeta potential of xGNP nanoplatelets is around 32.33 mV, and the layer spacing is around 3.35 Ã [23]. The tensile modulus of xGNP nanoplatelets is around 1000 GPa, and the tensile strength is around 5 GPa. The 1% and 2% xGNP particles with no treatment were added into the control asphalt and mixed in the high shear machine at a temperature of about 145 °C and a rate of 2000 rpm. The xGNP modified asphalt binder was prepared after mixing for two hours. In accordance with the SuperpaveTM specification, these tests were used to evaluate the performance of xGNP modified asphalt binder, including the rotational viscometer (RV), modular compact rheometer (MCR), asphalt binder thermal cracking (ABCD), Fourier transform infrared spectroscopy (FTIR) and electromagnetic radiation absorption tests. The Brookfield DV-II plus viscometer was used to test the viscosity of xGNP modified asphalt binder. The Anton Paar MCR 302 was used to test the complex shear modulus of asphalt binders. The low thermal cracking temperature in the asphalt was tested by the ABCD device from EZasphalt Inc. The Jasco IRT 3000 FTIR spectrometer was used to analyze the aging groups in the asphalt. The optical characterization in the asphalt was examined by the UV-1800 UV-Vis Spectrophotometer with a 1 nm resolution.
4. Discussions of results 4.1. Viscosity measurement The viscosity is the basic consistent property of materials and the dynamic shear viscosity of the fluid describes the resistance to the shear force. The viscosity of asphalt relates to the mixing and compaction temperatures of asphalt mixtures and also determines the workability of asphalt binders. The viscosities of the control and xGNP modified asphalt binders were tested at different temperatures: 90 °C, 100 °C, 125 °C, 135 °C, 150 °C, and 175 °C. The viscosity results are shown in Fig. 1. When the liquid materials flow, the molecules of the material slide over each other. However, the intermolecular forces in the liquid materials resist to flow. Activation energy is required to break the barrier and it is the minimum energy to flow. The ‘‘Arrhenius” equation is used to calculate the energy and determine the viscosity-temperature dependency of the asphalt [24,25]. The energy formula is shown in Eq. (1). Ef
g ¼ AeRT ; ln g ¼
Ef þ ln A RT
ð1Þ
where g is the viscosity of the asphalt; A is the coefficient of regression; Ef is the activation energy for the asphalt flow; R is the universal gas constant (8.314 J/mol/K); and T is the temperature in degrees K.
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Fig. 1. The appearance of xGNP particles at room temperature and FE-SEM images of xGNP particles: a) black powder; b) the FE-SEM image of graphite nanoplatelets at the magnitude of 15,000; c) the FE-SEM image of graphite nanoplatelets at the magnitude of 2000.
Fig. 2a demonstrates that the viscosities of 1% and 2% xGNP modified asphalt binders are higher than those of the control asphalt at the different temperatures listed above. The addition of xGNP particles in the control asphalt increases the viscosity and it indicates that the high-temperature performance of the modified asphalt binder improves. The viscosity improvement in the modified asphalt binder may be caused by the high thermal stability of xGNP particles. The viscosities of xGNP modified asphalt binder at 135 °C are lower than 3 Pas and the workability of the modified asphalt binder in the field is not affected, in accordance with the SuperpaveTM specification. There is a potential that more xGNP particles in the asphalt increase viscosity. The activation energy results of the xGNP modified asphalt binder and the control are shown in Fig. 2b and c. Fig. 2b shows the regression method used to calculate the activation energy in the asphalt binders. The independent variable is the inverse of the temperature (1/T) and the dependent one is the viscosity (1n g). The activation energy equals the universal gas constant (UGC) multiplied by the slopes shown in Fig. 2b, and the energy results are shown in Fig. 2c. Fig. 2c shows that the activation energy of the xGNP modified asphalt binder is lower than that of the control asphalt. The amount of intermolecular forces against flowing in asphalt is the viscosity. Low activation energy means that the required energy of asphalt is low for flowing under the test temperature. The activation energy relates to the development of self-healing, and it is the minimum energy for asphalt flow. So, the xGNP modified asphalt binder is good for
workability and self-healing function compared to the control asphalt. The results indicate that the xGNP modified asphalt binder is stiffer than the control one at high temperatures. The low activation energy of the modified asphalt binder is caused by high thermal conductivity and the self-lubricating features of multi-layer graphite. The multi-layer graphite particles accelerate the thermal transfer and thermal stress relaxation in the modified asphalt binder, and the self-lubricating function helps the asphalt flow. Therefore, it is confirmed that the addition of xGNP in the asphalt improves the high-temperature performance of the modified asphalt binder.
4.2. Complex shear modulus test The MCR rheometer (Fig. 3a) was used to characterize the elastic and viscous properties of asphalt binders. The complex shear modulus and phase angle were tested with the round plate (diameter, 25 mm) using the oscillatory mode. The test temperatures include 46 °C, 52 °C, 58 °C, 64 °C, and 70 °C and the frequencies range from 0.01 Hz to 45 Hz. The complex shear modulus describes the resistance to deformation when the sample is sheared, and the phase angle is the lag between the shear stress and the resulting strain. The larger the phase angle is, the more viscous the sample is. The storage modulus is the elastic portion of the complex shear modulus and it determines the ability of resistance to rutting in the asphalt. The MCR test results are shown in Fig. 3.
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Fig. 2. The viscosity and activation energy results of xGNP modified asphalt binder and control.
Fig. 3b shows the trends of the complex shear modulus with different frequencies and temperatures in asphalt binders. The complex shear modulus and G⁄/sin d values (Fig. 3c) of xGNP modified asphalt binder are higher than that of the control one, as well as the storage modulus (Fig. 3d). The addition of xGNP particles enhances the complex shear modulus and resistance to rutting in asphalt binders. When the xGNP particles were added into the asphalt, it is deduced that the layer structures of the particles were exfoliated considering that the interlayer spacing is around 3.35 Å. The asphalt was intercalated in these layers, and the ‘‘sandwich” structure reinforced the resistance to deformation and helped for the stress relaxation in the modified asphalt binder. Meanwhile, the high thermal conductivity function [26] of the xGNP particles/graphite may accelerate the heating transfer in the asphalt, and it may be helpful for temperature stability.
Therefore, the resistance to permanent deformation improves in the modified asphalt binders. 4.3. Thermal cracking test The thermal cracking at low temperatures is one of the four main distresses (rutting, moisture damage, thermal and fatigue cracking) in the asphalt pavement. Currently, there are many ways to determine low temperature properties of an asphalt binder, such as penetration, DTT, TSRST, and BBR tests. Based on the references [1], considering the accuracy and simplicity of the test, the Asphalt Binder Thermal Cracking (ABCD) test was employed in this study and there are a few advantages: 1) No elaborate assumptions; 2) fast measurement and easy to determination of the potential for thermal cracking; and 3) minimal errors and simple procedures.
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The fundamental theory of ABCD is to utilize the different contractions of an asphalt binder and aluminum rings and introduce the superposition principle to standardize the dimensions of rings and asphalt samples. The thermal strains of asphalt under the different temperatures were monitored and the sudden reduction of strains means that the thermal cracking of asphalt was at this low temperature. The thermal stress at this low temperature is calculated from Eq. (2). The sample and mold of the ABCD test are shown in Fig. 4a. The ABCD test results of unaged asphalt binders in this study are shown in Figs. 4b and 5.
rb ¼
F ABCD ecorr EABCD AABCD ¼ Ab Ab
ð2Þ
where rb is the thermal stress at the cracking temperature in the sample; F ABCD is the thermal force due to a temperature difference; Ab is the cross-sectional area of the sample; ecorr is the strain corrected for temperature; EABCD is the elastic modulus of the aluminum ring; and AABCD is the cross-sectional area of the aluminum ring. Fig. 4 shows microstrains in the asphalt samples and the cooling rates in the test chamber under the different temperatures. The xGNP modified asphalt binder fractures at the lower temperatures with a higher cracking strain than the control one. The 2% xGNP modified asphalt binder has a higher cracking strain at a lower temperature than the 1% xGNP modified asphalt binder. There is a potential that more xGNP particles in the asphalt lead to the lower cracking temperature and the higher fracture strain. It is noticed that the cooling rates of xGNP modified asphalt binder are still higher than that of the control one. The cooling rate in the test indicates that the pavement materials varies from different field conditions and it also reflects the accommodation of the different pavement areas. It is likely that the high cooling rate may accelerate the thermal cracking in the asphalt. That’s why the different cooling rate was used in this study. However, based on the previous study [25], the cooling rate does not affect the cracking temperature much. From this figure, the addition of xGNP particles in the asphalt enhances the low-temperature performance, especially the thermal shrinkage. Fig. 5 displays the fracture stresses at the cracking strains and the temperatures in the control and xGNP modified asphalt binders under the temperature/cooling setting. The xGNP particles in the asphalt decrease the cracking temperatures and increase the fracture stress and strain. It implies that the xGNP modified asphalt binder can withstand more thermal stress induced by the change in temperature relative to the control one. It is probably due to the layer structure of xGNP particles. After the homogenous mixing in the high shear mixer, the added particles were well scattered in the asphalt matrix. It is probably that the graphite layer helps the asphalt to relax the thermal stress due to the high thermal stability [21] of graphite layers. It is possible that the self-lubricating property (loose interlamellar coupling [22]) of multiple-layer graphite sheets in asphalt increases the cracking strain and also enhances the strain relaxation ability in the modified asphalt binder. The exfoliated structure of the xGNP particles also helps relax the thermal or mechanical strain in the modified asphalt binder. 4.4. Chemical group analysis
Fig. 3. MCR results of the control and xGNP modified asphalt binders.
The aging is one of common phenomena in the nature of materials and the aging leads to a life span reduction of the materials. However, the mild aging in asphalt mixtures is helpful to improve the resistance to the moisture sensitivity and rutting [27,28].
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Fig. 4. The mold, sample and results of the ABCD test.
Fig. 5. The cracking temperature, fracture stress and strain jump results of the control and xGNP modified asphalt binder.
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Therefore, it is necessary to detect the aging group distributions in the asphalt. The FTIR spectrometer was used to obtain the infrared absorption spectrum and the Fourier transform was used to convert the raw data to the testing spectrum. In this study, the FTIR spectra of asphalt were tested by the accumulation of 512 scans with a resolution of 4 cm1. All tested samples are coated on individual Si chips. These Si chips are cut from high purity Si wafers (resistivity > 10,000 Xcm) so that they are transparent to the infrared light source. All samples are then characterized in a transmission/absorbance mode. It is know that the main aging groups in asphalt are carbonyl (C@O) and sulfoxide (S@O) bonds. The wavenumber peaks of 1700 cm1 and 1030 cm1 in the spectra represent the carbonyl and sulfoxide [10,29], respectively. The FTIR test results are shown in Fig. 6.
Area of the carbonyl band between 1830 cm1 and 1670 cm1 IC@O ¼ P Area of the spectral bands between 2000 cm1 and 600 cm1 ð3Þ Area of the sulfoxide band between 1050 cm1 and 985 cm1 IS@O ¼ P Area of the spectral bands between 2000 cm1 and 600 cm1 ð4Þ Fig. 6 demonstrates the spectra images of xGNP modified asphalt binder and the control, and these trends are similar. From the data shown, there is no obvious new peak appearing in the images. The CAH bond characteristic of carbon rings in the xGNP particles is overlapped with the aromatic components of the asphalt. It is most likely that the xGNP particles were aged more or less during the mixing procedure of the asphalt binders due to the features of the nanomaterials. The carbonyl groups in xGNP materials are also overlapped with those in asphalt. This is probably the reason that no new peak is shown in the FTIR images of xGNP modified asphalt binder compared to the control one. There is a low probability that chemical reactions occurred between the asphalt and the xGNP particles since the graphite is a stable material under standard conditions. However, the aging components in these asphalt binders should be different and the average ratios of carbonyl and sulfoxide bonds in the unaged asphalt binders were calculated using Eqs. (2) and (3). The ratio results are shown in Fig. 7. Fig. 7 reveals the carbonyl and sulfoxide indexes (relative ratios) in the unaged asphalt binders and the smallest ratios of these bonds are in the control asphalt. The ratios of carbonyl and sulfoxide bonds in the 2% xGNP modified asphalt binder are higher than those of the 1% xGNP modified asphalt binder and the control one. This indicates that more aging groups are detected in the modified asphalt binder relative to the control asphalt. There is
Fig. 6. FTIR results of xGNP modified asphalt binder and the control.
Fig. 7. The average ratios of carbonyl and sulfoxide groups in the asphalt.
also a possibility that more xGNP particles in the control asphalt result in more aging components in the modified asphalt binder. There are two reasons to explain the increase of aging components in the modified asphalt binder: 1) the xGNP particles were aged during the preparation of the modified asphalt binder and the partially aged xGNP particles increase the carbonyl index in the modified asphalt binder; and 2) the addition of xGNP particles in the control asphalt induces nano-aging effects during the open-air mixing process, seen from the sulfoxide index increase. The moderate aging in the asphalt increases the resistance to moisture damage and rutting in the pavement. Therefore, it is predicted that the xGNP materials are helpful in improving the performance of asphalt binders and mixtures.
4.5. Optical characterization The sun exposure is one of the working conditions for the pavement. The spectrum of the sunlight (all electromagnetic radiations) is from about 100 nm–1 mm. It consists of five regions: ultraviolet C (100 nm–280 nm), ultraviolet B (280 nm–315 nm), ultraviolet A (315 nm–400 nm), visible light (380 nm–780 nm), and infrared (700 nm–1 mm) [30]. In order to the test the absorptions of asphalt under different electromagnetic radiations, the UV-1800 UV-Vis Spectrophotometer was used to test with the radiation range of 300 nm–1100 nm. The test results of the control and xGNP modified asphalt binders are shown in Fig. 8. Fig. 8 shows the absorbance spectra of the control and the xGNP modified asphalt binders under the radiation range of 300 nm– 1100 nm. The control asphalt has a lower light absorption from 535 nm to 1100 nm compared to that of the xGNP modified asphalt binder. In the wavelength range from 300 nm to 535 nm, the radiant flux received in both the control and the modified asphalt binders is about 104 times than the radiant flux transmitted by these materials. The light absorption increases significantly with the
Fig. 8. The absorption of xGNP modified asphalt binder and the control under electromagnetic radiations (300 nm–1100 nm).
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addition of xGNP to the control asphalt for the wavelength range of 535 nm–1100 nm. Light absorbance is inversely proportional to the logarithm of the ratio between the transmitted and the i h i e ¼ log10 T received light flux to the base of 10 A ¼ log10 U Ut e
(where, Uie is the radiant flux received by the material, Ute is the radiant flux transmitted by the material, and T is the transmittance of the material). For example, when the light absorbance is measured as 2.33 for the 2% xGNP modified asphalt binder at a wavelength of 1100 nm, the ratio of the received light vs. the transmitted light flux is calculated as 213.80 (102.33). Similarly, when the light absorbance is measured as 1.15 for the 1% xGNP modified asphalt binder at a wavelength of 1100 nm, the ratio of the received light vs. the transmitted light flux is calculated as 14.13 (101.15). Thus, the absorbed light ratio (absorbed radiant vs. received radiant flux) is about 13.13/14.13 = 0.93. The ratio of the received radiant vs. the transmitted radiant flux is 1.48 (100.17) for the control asphalt and the absorbed ratio is around 0.48/1.48 = 0.32. The results indicate that the near infrared light absorbance (with a wavelength above 535 nm) increases significantly with the addition of small amounts of xGNP particles. It is likely that the exfoliated structure of the xGNP particles in asphalt also increases the surface area to absorb the test radiation. The improved visible and near infrared light absorptions can potentially provide new ways for pavement material heating with a visible/near-infrared light lamp or focused sunlight. The accelerated asphalt mixture healing was conducted by using the asphalt binder capillary Newtonian flow at the elevated temperatures (above transition temperatures) in our research group. The induction healing of asphalt mixtures with radio electromagnetic waves showed good healing performances [31]. Following this work, the healing performance of the xGNP modified asphalt binder mixture will be investigated with a visible/near-infrared light lamp or focused sunlight through lab testing and computation. It is anticipated that improved self-healing materials with high electric– magnetic wave absorption can potentially meet the challenge of efficient pavement maintenance and an elongated service life.
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susceptibility of the modified asphalt binders improve, due to the fact that the mild aging in the asphalt helps to resist rutting and moisture. The UV-Vis Spectrophotometer test measures the absorption under electromagnetic radiations with different wavelengths. The results display that significant difference exists between the control and the modified asphalt binders in the visible and near infrared regions, and the modified asphalt binder can absorb more electromagnetic waves relative to the control one. It also indicates that the xGNP modified asphalt binder absorbs more radiation and energy to maintain the performance of the asphalt, especially in low-temperature regions. In addition, the selfhealing effects or accelerated self-healing at elevated temperatures can potentially be enhanced with improved light absorption of the xGNP modified asphalt binder. Therefore, it is good to use the xGNP particles/multiple-layer graphite as an asphalt modifier to improve the properties of asphalt due to the improved rheological properties, lowtemperature performance, and high radiation absorption for accelerated self-healing at elevated temperatures. Future research would focus on the performance of the modified asphalt mixtures and the accelerated healing of these mixtures with a visible/nearinfrared light lamp or focused light.
Acknowledgements The authors appreciate Tristan Kolb and David Porter for help with laboratory testing, and Su Kong Chong’s assistance on FESEM characterization. The experimental work was completed in the Transportation Materials Research Center at Michigan Technological University. Any opinion, finding, conclusion, or recommendation expressed in this material are those of the authors and do not necessarily reflect the review of any organization.
References 5. Conclusions The multiple-layer graphite nanoplatelets (xGNP) were added into the asphalt as a modifier to improve the high- and lowtemperature performance of the modified asphalt binder. The various tests were used to evaluate the properties, including the RV, MCR, ABCD, FTIR and electromagnetic radiation absorption tests. Based on these test results, the following conclusions can be drawn: 1) Compared to the control asphalt, the viscosity of the modified asphalt binder increases, the mixing and compaction temperatures of the modified asphalt binder increase, and the activation energy of the modified asphalt binder decreases. This indicates that the high-temperature performance of the modified asphalt binder improves; the complex shear modulus and the rutting factor values of the xGNP modified asphalt binder increase, thus implying that the resistance to rutting is enhanced in the modified asphalt binder at high temperatures. The cracking temperature of the modified asphalt binder also improves, and the fracture stress and strain of the modified asphalt binder increase. This infers that the stress relaxation of the modified asphalt binder is enhanced and the resistance to low-temperature thermal cracking also improves. 2) The FTIR results show that the ratio of aging groups (C@O) in the modified asphalt binder significantly rises, and it causes that the high-temperature performance and moisture
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