Chemical, rheological and aging characteristic properties of Xinjiang rock asphalt-modified bitumen

Construction and Building Materials 240 (2020) 117908

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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat

Chemical, rheological and aging characteristic properties of Xinjiang rock asphalt-modified bitumen Long Cheng a,⇑, Jiang Yu b, Qun Zhao b, Jinshi Wu a, Lei Zhang a,⇑ a b

Intelligent Transportation System Research Center of Southeast University, Nanjing 210096, China Civil Construction College of Xinjiang University, Urumqi, Xinjiang 830047, China

h i g h l i g h t s

g r a p h i c a l a b s t r a c t


The aging process exerted more

distinct impacts on the neat bitumen compared with the XRA-modified binder.
The optimum XRA addition ratio to modified neat bitumen was 12%.
The addition of XRA could reduce the formation of carbonyl in bitumen.
The introduction of XRA could improve the working temperature range and enhance the thermal stability of the K64.

a r t i c l e

i n f o

Article history: Received 27 July 2019 Received in revised form 15 December 2019 Accepted 18 December 2019

Keywords: XRA FTIR Rheological master curve Aging indexes PAC Selection sort algorithm

a b s t r a c t Recently, rock asphalt has been widely used to modify bitumen binder; however, limited studies have been conducted to comprehensively evaluate the chemical, rheological and aging characteristics of bitumen modified with Xinjiang rock asphalt (XRA) from China. In this study, base bitumen (K64) was added with 0%, 8%, 12%, 16% and 20% XRA by mass respectively to prepare different modified binder, and rolling thin-film oven (RTFO) and pressure aging vessel (PAV) tests were performed to simulate short-term and long-term aging, respectively. The evolution of the chemical and rheological characteristics of the binders before and after aging were monitored with Fourier transform infrared (FTIR), rotational viscosity, dynamic shear oscillatory, frequency sweep, multiple stress creep recovery (MSCR) and bending beam rheometer (BBR) tests. Furthermore, several empirical indexes were tracked with softening point, ductility and penetration tests. The FTIR results indicated that with the introduction of the XRA modifier, the carbonyl functional group of the binders was reduced after aging, and the carbonyl index was more stable than the sulfoxide index to characterize the aging extent of the XRA-modified bitumen binders. Moreover, the rheological and empirical indexes suggested that the XRA could reduce excessive plastic deformations at high temperatures and broaden the final performance grade (PG) of the binder, while a high XRA addition ratio could compromise the low-temperature stress relaxation properties of the bitumen. Additionally, principal component analysis (PCA) suggested that ten parameters employed in this paper (such as stiffness, PG grade, percent recovery and etc.) have similarity and can be explained by two variables: PG grade and carbonyl index. Generally, based on the above evaluations and selection sort algorithm, 12% XRA was the optimum dose for modifying K64 to exhibit the best performance. Ó 2019 Elsevier Ltd. All rights reserved.

⇑ Corresponding authors. E-mail addresses: [email protected] (L. Cheng), zhanglei1905seu@163. com (L. Zhang). https://doi.org/10.1016/j.conbuildmat.2019.117908 0950-0618/Ó 2019 Elsevier Ltd. All rights reserved.

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L. Cheng et al. / Construction and Building Materials 240 (2020) 117908

1. Introduction Rapid economic development and frequent occurrences of extreme climate result in severe distresses of bitumen pavement, particularly intermediate-temperature fatigue cracking and hightemperature permanent deformation [1]. Binder modification technologies offer a viable solution to overcome these deficiencies [2]. The best-known practice in modification technologies is the utilization of polymer modifiers to resist excessive plastic deformations at high-temperature conditions by means of increasing the binder stiffness while maintaining its flexibility enough to prevent brittle fracture at low service temperatures [3]. However, polymers such as styrene–butadiene–styrene (SBS) are not quite compatible with neat bitumen, arising from its rather large molecular weight (200,000–300,000 g mol1) [4]. On the other hand, the polymers are expensive and may subject to the varying degrees of degradation when the polymer-modified binder exposed to environment air and mechanical stress [5,6]. These reasons have eventually motivated road engineers to modify neat bitumen properties using more compatible and economical rock asphalts to replace polymers [7,8]. However, compromising the performance of certain aspects of bitumen for rock asphalt addition is unacceptable. Thus, it is essential to comprehensively investigate the chemical, rheological and aging resistance characteristics of Xinjiang rock asphalt (XRA)-modified bitumen. Rock asphalt is a kind of natural asphalt formed by petroleum impregnated into rock fractures followed by the combined action of heat, pressure, oxidation, catalysis, and bacteria over hundreds of millions of years [9]. The most commonly utilized rock asphalt sources can fall into four categories, including Gilsonite rock asphalt (GRA) from the USA, Buton rock asphalt (BRA) from Indonesia, UM rock asphalt (URA) from Iran and Qingchuan rock asphalt (QRA) from China. The physical and chemical properties of rock asphalt depend on its source and purity. GRA contains 80–95% pure bitumen and is known for its good affinity with asphalt and easy use. Kim [10] et al. found that GRA contained abundant polar functional groups and highly condensed polycyclic aromatic rings with long alkyl chains. Other studies indicated that the addition of 10% GRA to neat bitumen (PG 58-34) achieved the desired performance grade (PG 70-34). Moreover, the 10% GRAmodified binder mixture presents better stability, tensile strength, and resistance to performance deformation and fatigue than 3.5% SBS [11]. Kök et al. [7] suggested that approximately 3–4% of GRA was needed to replace 1% of SBS when the two modifiers were mixed. For URA, its pure bitumen content is approximately 90%. Ameri et al. [12] determined that the high-temperature performance of two neat binders varied from PG 58 and PG 64 to PG 76 and PG 82, respectively, when 12% UM was introduced to the binder. BRA contains approximately 20–28% pure bitumen and 70–75% mineral ash [13]. To prevent segregation, engineers commonly used ‘‘dry progress” to produce BRA-modified bitumen [14]. In this progress, rock asphalt powder was incorporated into bitumen mixture for partially replacing the mineral aggregates in the grading and mixing prior to addition of the bitumen binder. Zha et al. [15] studied BRA ash containing a large proportion of calcium carbonate, which enhanced adhesion and anti-stripping properties between the binder and aggregate. Additionally, the BRA-modified mixtures showed better permanent deformation and stiffness and creep modulus than neat bitumen mixtures [16]. Li et al. [17] concluded that the BRA-modified bitumen mixture revealed a lower rate of aging compared to the control mix. A recent study indicated that QRA increased the stiffness and viscosity of the binder at high temperature but compromised the ductility at low temperature [18]. However, an opposite conclusion was obtained in another study where the low-temperature performance of the mixture was not deteriorated as long as the addition

ratio of QRA was less than 8% by mass of neat bitumen [19]. In 1958, a rock asphalt was discovered in Karamay of China. However, a rock asphalt has been used as a binding agent and pigment in paints, inks and enamels industries [20]. Recently, researchers used it as a binder modifier to improve the performance of bitumen pavement. For example, Sun et al. [21] showed that the use of XRA to modify bitumen leads to an increase in the softening point and a reduction in the penetration, indicating an overall enhancement in the elastic response of the binder. In addition, the moisture damage resistance, fatigue properties and tensile strength of bitumen mixtures were improved with the addition of XRA [22]. Although the studies on the physicochemical characteristics of GRA and BRA modified bitumen can be found in some references, the modification mechanism and the rheology behavior of XRA modified binder still remain unclear. Additionally, high temperatures and oxidation during the construction and service periods cause bitumen aging, which further complicates and deteriorates the properties of XRA-modified binder [23]. Therefore, a comprehensive investigation of the physicochemical characteristics of XRA-modified bitumen before and after aging is of significant interest for the bitumen industry to expand the application of XRA modifiers. 2. Objectives In this study, binders with five XRA contents were prepared and assessed. Rolling thin film oven aging (RTFO) and pressure aging vessel (PAV) tests were performed to simulate the short-term and long-term aging conditions, respectively. The chemical and rheological properties of XRA-modified bitumen were tracked by Fourier transform infrared (FTIR), frequency sweep, multitude stress creep recovery (MSCR) and bending beam rheometer (BBR) tests. Moreover, several empirical indexes were employed to measure the physical properties of the binders. Ten parameters obtained from chemical and rheological tests before and after aging were statistically analyzed by the Principal Component Analysis (PCA) to investigate the influential parameters for evaluation of the aging extend of bitumen binders. In the end, the regression analysis was conducted to describe the significance of changes in carbonyl functional groups as relating to critical property parameters of binders. And, the selection sort algorithm was employed to rank the aging resistance characteristics of the specimens. The schematic diagram of steps in this research work was shown in Fig. 1 and the main objectives of this paper are shown as follows: 1. To comprehensively understand the chemical, rheological and aging resistance characteristics of XRA-modified bitumen binders. 2. To find the optimum XRA addition ratio used in XRA-modified bitumen. 3. Analyze the applicability of different aging indexes to evaluate the aging extent of XRA-modified bitumen and rank the aging resistance of the binders based on the selection sort algorithm. 3. Materials and methods 3.1. Materials One neat base bitumen and one Xinjiang rock asphalt (XRA) modifier were selected to prepare modified bitumen in the laboratory. The neat binder was Karamay bitumen (K64), whose performance grade (PG) was 64-22. The basic properties of K64 are summarized in Table 1. The penetration of K64 was 87.65 (0.1 mm), which seems to be high than normal bitumen that

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L. Cheng et al. / Construction and Building Materials 240 (2020) 117908

3.2. Preparation of the XRA-modified bitumen The schematic diagram of the XRA-modified bitumen preparation and two aging processes are shown in Fig. 2. First, crush the block XRA, pass the square hole sieve with a diameter of 1.18 mm to obtain XRA powder used for preparing modified bitumen binders. To evaluate the influence of thermal oxidation aging on the properties of the XRA-modified bitumen, three aging states, namely, virgin, short-term and long-term aging, were considered in this study. As shown in Fig. 2, according to ASTM D2872, the RTFOT approach was applied to simulate the short-term aging of the samples. Then, the RTFOT residue was subjected to a pressurized aging vessel (PAV) test to simulate long-term aging in the bitumen service life according to ASTM D6521. Five concentrations of XRA were prepared, including 0%, 8%, 12%, 16%, and 20% XRA (by weight of neat bitumen), which were coded with K64-VG, 8% XRA-VG, 12% XRA-VG, 16% XRA-VG and 20% XRA-VG, respectively. After the RTFO or RTFO + PAV tests, the residues were coded with corresponding names, such as 8% XRA-RT and 12% XRA-PAV. The specific codes of samples in different aging stages were shown in Table 4. 3.3. Fourier transform infrared (FTIR) test To quantitatively analyze the changes in the chemical functional groups of the bitumen binders before and after laboratory aging, a sensitive and accurate index to monitor the aging extent was used. Related literature [26,27] indicate that aging was regarded as a result of the formation of oxygenated functional groups, for example, sulfoxides, and of ketones that finally yielded carboxylic acids. Given this favorable characteristic, sulfoxide groups, carbonyl groups and the functional groups that do not significantly change with oxidation were defined as characteristic peaks and reference peaks, which is shown in Fig. 3 [28]. In this study, the indexes based on the band area ratio of the characteristic peaks and the reference peaks were employed to quantify the aging severity of the bitumen [29]. The sulfoxide index (IS¼O ) and carbonyl index (IC¼O ) were calculated by formulas (1), (2), and (3) as follows:

Fig.1. The schematic diagram of steps in this research work.

PG upper grade is PG64. Since the rough correspondence of PG 64-22 is penetration 60-70. To verify the PG grade and penetration testing results of (at 25 °C) K64, this paper performed repeated tests on different batches of bitumen samples. As shown in Table 2, the test result of PG grade of all batches of K64 remains 64-22. Meanwhile, we also referred to the relevant literature [24,25] of K64 and contacted the bitumen supplier, and their conclusion supported the author’s finding that the PG grade of K64 was 64-22. The basic properties of XRA are summarized in and Table 3. A hot solvent extraction test was conducted to analyze the bitumen content of the XRA, and the results showed that the content of pure asphalt in the XRA was 99.2%, which was one of the natural asphalts with the highest asphalt content [21]. Furthermore, the density of the XRA (in a normal atmospheric temperature) was 1.03 g/cm3, which was roughly consistent with the bitumen density, which typically lies between 1.01 and 1.04 g/cm3.

IS¼O ¼

A1030cm1 P A

ð1Þ

IC¼O ¼

A1700cm1 P A

ð2Þ

X

A ¼ A1700cm1 þ A1600cm1 þ A1460cm1 þ A1376cm1 þ A1030cm1 þ A864cm1 þ A814cm1 þ A743cm1 þ A724cm1

ð3Þ

P

where A1030cm1 , A1700cm1 and A represent the area of sulfoxide peaks, the area of carbonyl peaks and the total area of reference peaks, respectively. The infrared spectra of specimens were collected by attenuated total reflectance Fourier transform infrared spectroscopy (ATRFTIR). The test procedures of ATR-FTIR were as follows: opened the OPUS software and tested the background spectra to reduce the signal-to-noise ratio for reflected light to the background. After that, approximately 1 g of bitumen binder was placed on the surface of the ATR diamond and fixed by a metal indenter to maintain

Table 1 Basic properties of neat bitumen. Items

Penetration (25 °C, 0.1 mm)

Softening point (oC)

Ductility (15 °C, cm)

Viscosity (Pasec)

Measured results Test method

87.65 ASTM D5

48.5 ASTM D36

150 ASTM D113

0.483 ASTM D4402

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L. Cheng et al. / Construction and Building Materials 240 (2020) 117908

Table 2 The PG upper grade of different batches K64. Samples

K64-A K64-B K64-C K64-D K64-E K64-F K64-G

Virgin

RTOF

Fail temp

G*/sin (delta)

PG grade

Fail temp

G*/sin (delta)

PG grade

74.3 74.8 71.6 73.4 74.7 75.8 72.4

876.91 816.23 1081.42 972.46 831.07 696.40 1440.29

70 70 70 70 70 70 70

67.3 67.9 65.4 66.7 67.8 69.5 65.2

2076.91 1876.62 2037.46 1580.34 1672.09 1456.96 2100.41

64 64 64 64 64 64 64

To show the final performance grade of samples. Table 3 Basic physical properties of XRA. Items

Asphalt contents (wt%)

Density (g/cm3)

Ash contents (wt%)

Flash point (oC)

Moisture content (wt%)

Measured results

99.2

1.03

0.53

>230

0.1

Fig. 2. Schematic diagram of the XRA-modified bitumen preparation and aging processes.

Table 4 The codes of samples in different aging stages. XRA content (%)

0 8 12 16 20

Different aging stages Virgin

Short term aging

Long term aging

K64-VG 8% XRA-VG 12% XRA-VG 16% XRA-VG 20% XRA-VG

K64-RT 8% XRA-RT 12% XRA-RT 16% XRA-RT 20% XRA-RT

K64-PAV 8% XRA-PAV 12% XRA-PAV 16% XRA-PAV 20% XRA-PAV

full contact between the bitumen and ATR diamond. Thirty-two scans from the wavenumber range of 4000 to 400 cm1 were collected and averaged for each sample. Three scan tests were per-

formed for each specimen to minimize the operating error. After scanning, band normalization and baseline correction were applied in Thermo-Scientific OMNIC to analyze the spectra. To obtain more accurate results, the computational code developed in the MATLAB environment was executed to obtain carbonyl and sulfoxide indexes automatically. 3.4. Conventional physical properties test In this study, the softening point, ductility (@ 10 °C), and penetration (@ 25 °C) of the specimens were tested according to ASTM D36, ASTM D113 and ASTM D5, respectively. The Brookfield viscometer was used to measure the rotational viscosity of the binders at 135 °C according to ASTM D4402. 3.5. Frequency sweep tests and construction of the master curve

Fig. 3. The fingerprint and reference areas in the absorbance spectra.

For many years, master curves of both the complex modulus and the phase angle have been widely interpreted to evaluate the influence of modifiers and aging on the rheology properties of bitumen [30]. A dynamic shear rheometer (DSR) (Malvern Kinexus) was employed to perform frequency sweep tests at eight temperatures (5, 15, 25, 35, 45, 55, 65, and 75 °C) and at frequencies ranging from 0.1 to 30 Hz. The 8 mm parallel plate geometry with a 2 mm gap was used at a temperature range from 5 to 25 °C, while the 25 mm-diameter with a 1 mm gap was used from 35 to 75 °C. Based on the time-temperature superposition principle and the frequency sweep test results, the complex modulus and phase angle master curves were constructed with a sigmoidal function

L. Cheng et al. / Construction and Building Materials 240 (2020) 117908

model at a reference temperature of 25 °C. The shift factor was adjusted in the function model to obtain the best fitting effect. The shift factors and shift procedures for construction of the phase angle master curve were similar to those used for the complex modulus master curve. The sigmoidal and DL function model is as follows [31]:

logjG  j ¼ v þ

a

ð4Þ

1 þ eðbþclogf r Þ

where G* is the complex shear modulus, m is the lower asymptote, a is the difference between the value of the upper and lower asymptotes, b and c are two shape parameters, and f r is the reduced frequency.





0

d ¼ dp  dp  H f r  f p  @1  e 0  @1  e

  2 1

SR log

fr fp

fp fr

A

ð5Þ

where d is the phase angle value, dP is the phase angle plateau, f r is the reduced frequency, f p is the frequency where dP occurs, SR and SL are the shape parameter phase angle master curves on the right and left sides of the plateau, dL stands for the left side of the phase angle     plateau, and H f r  f p and H f p  f r are two Heaviside step functions. 3.6. MSCR test According to ASTM D7405, an MSCR test was performed to obtain percent recovery (R) and nonrecoverable creep compliance (Jnr) of the bitumen binders at 0.1 and 3.2 kPa. The specimens were subjected to 10 cycles of continuous creep loading and recovery tests at 64 °C, which is a typical high-temperature performance grade of bitumen in China. Each cycle was creep loaded for 1 s followed by 9 s of recovery. 3.7. Continuous performance grade (PG) temperature test To investigate the influence of the XRA addition on the continuous grade temperature of the bitumen binder, a dynamic oscillatory test was conducted according to ASTM D2872. The G*/Sin d of the RTFO residue and G*Sin d of the PAV residue were obtained to characterize the performance grade temperature of the binders. Furthermore, the stiffness (S) and creep rate (m-value) of the PAV residue were determined by a BBR test at the low temperatures of 12, 18 and 24 °C according to ASTM D6648. 3.8. Aging indexes In this study, changes in the amplitudes of the chemical, physical and rheology performance parameters before and after aging were employed as aging indexes to evaluate the influence of XRA addition on the aging susceptibility of bitumen binders. The expressions of the 6 aging indexes are shown in formulas (6)– (11). The larger the changing amplitude of the aging indexes between the virgin and aging specimens is, the more serious the bitumen is aging [32].

Aged softening point  Virgin softening point Virgin softening point

ð6Þ

Aged v iscosity v alue  Virgin v iscostity v alue Virgin v iscostity v alue

ð7Þ

SPI ¼ VI ¼

DIS¼O ¼ Aged sulfoxide  Virgin sulfoxide

ð8Þ

DIC¼O ¼ Aged carbonyl  Virgin carbonyl

ð9Þ

Aged recov er percent  Vigin recov er percent Vigin recov er percent

ð10Þ

Aged Jnr  Vigin Jnr Vigin Jnr

ð11Þ

RAI ¼

JAI ¼

4. Results and discussion 4.1. Chemical characterization analysis

  A þ dL  H f p  f r

  2 1 SL log

5

The typical FTIR spectra of the XRA, K64 and XRA-modified bitumen binders under virgin states are shown in Fig. 4. It can be seen that the spectra of the XRA are roughly consistent with those of the K64 in most parts of the wavenumber ranges, indicating that the chemical characteristics of the XRA are similar to those of the neat bitumen. Hence, it can be speculated that the modified binder has excellent storage stability when the XRA is introduced [33]. Notable absorption bands are detected at approximately 2920 cm1, which is attributed to alkane –CH2– and C–H strong stretching vibrations [34]. Another pronounced absorption peak at approximately 1460 to 1376 cm1 is ascribed to deformation of methyl groups and asymmetric scissor vibration of the methylene groups. In addition, two slight bands are detected at 2000 and 2185 cm1 in the XRA, while they are not detected in the K64. This can be attributed to the XRA containing heteroatoms, including metal elements and nitrogen, which leads to mononuclear metal carbonyl complex C–O stretching vibrations and azide compound –N@N@N asymmetric stretching vibrations [35]. Due to a low proportion, however, the bands at approximately 2000 and 2185 cm1 are relatively short and are therefore easily neglected in low-content XRA-modified bitumen. The absorption bands at approximately 1600 and 1375 cm1 for the XRA are more intense and wider than those of the K64, which can be explained by the abundant asphaltene contained in the XRA, resulting in more significant C@C stretching vibrations [35]. The bands at 1060 to 1030 cm1 are attributed to the silicates contained in the XRA ash, leading to S@O stretching vibrations. It is quite interesting to see a unique carbonyl peak for virgin K64 sample. Generally, the formation of carbonyl groups attributed to the reaction of bitumen with oxygen. Different batches of K64 were selected for FTIR, the same results were observed for those samples. Furthermore, the production process of the neat binder was straight-running, which may the reasons that result in a slight carbonyl group presents in the FTIR spectrum for the unaged neat binder. Nevertheless, as a matter of fact, it does not disrupt the use of the carbonyl growth index difference values in different aging states of the specimens to quantitatively evaluate the aging extent of the bitumen. 4.2. Physical characterization analysis Fig. 5 displays the physical test results of the binders. An increasing trend is observed in the softening point and viscosity when the XRA is introduced to the neat bitumen. This is because the XRA contains more fused aromatic rings and more polar groups, which constitute the relatively rich asphaltenes. The light components are absorbed by these asphaltenes and become associated to form asphaltene micelles, which improves the glassy state conversion temperature of the bitumen [36]. Thus, the XRAmodified bitumen has greater rutting resistance properties and thermal stability at high temperatures.

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L. Cheng et al. / Construction and Building Materials 240 (2020) 117908

Fig. 4. FTIR spectra of K64, XRA and XRA-modified bitumen binders.

Fig. 5. Physical properties of different binders: (a) penetration @ 25 °C, (b) softening point, (c) ductility @10 °C and (d) viscosity @135 °C.

As shown in Fig. 5 (a) and (d), a downward trend is observed in ductility and penetration as the XRA content increases. It can be speculated that the XRA may compromise the low-temperature shrinkage crack resistance of the bitumen mixture. In addition, another notable phenomenon is that with the introduction of XRA, the ductility of the binders is subjected to the most severe deterioration in all the physical indexes. However, the addition ratio of the XRA only has a slight influence on the ductility of the modified bitumen. Therefore, the ductility retention of the binder

after aging should not be used as an index to quantify the influence of the XRA dose on the binder aging resistance. 4.3. Rheological characteristics analysis 4.3.1. The master curve of the XRA-modified bitumen To investigate the rheological characteristics of the XRAmodified bitumen binders in the linear viscoelastic range, the rheological master curves are constructed. The complex shear

L. Cheng et al. / Construction and Building Materials 240 (2020) 117908

Fig. 6. Complex modulus and phase angle master curves for binders: (a) complex modulus and (b) phase angle.

modulus (|G*|) and phase angle (d) master curves for the K64 and XRA-modified bitumen are presented in Fig. 6. The test results of the low-frequency and high-frequency regions correspond to high service temperatures and low service temperatures, respectively. It can be observed in Fig. 6 (a) that all |G*| master curves for the binders follow a similar S-shaped trend, although the curvatures and slopes of those curves are different. At low frequency (at approximately 105 to 101 Hz), the |G*| of the binders increases markedly along with the addition of the XRA. However, with an increase in the test frequency, the influence of the XRA addition ratio on |G*| gradually weakens until it approaches a constant and is close to

7

the glassy complex modulus. Specifically, from low frequency to high frequency, the spacing for the |G*| master curves of the binder with different XRA contents increasingly diminishes and eventually overlaps (at approximately 103 to 105 Hz). These phenomena can be attributed to the fact that the asphaltene in the XRA absorbs light components to form a network micelle structure, which restricts the movement of bitumen molecules, but the effects of the network micelle structure on the rheological characteristics of the bitumen are less pronounced at high frequencies. The d value is a time lag between the stress and strain of viscoelastic materials. The larger the d value tends to be, the smaller the proportion of elastic response shares. As shown in Fig. 6 (b), at a wide frequency range of 105 to 104 HZ, the spacing of the d master curves for the binders presents an increasing trend as the XRA addition ratio increases. This implies that XRA enhances the elastic response of the neat binder. In terms of the shape of the d master curves for the binders, K64 and 8% XRA follow a simple S-shaped trend. In contrast, for high-content modified bitumen (12% to 20% XRA), the d master curve consists of a downward trend toward lower values at high frequency, a slightly intermediate plateau and a rise toward 90° at low frequency. This is because the binder with a high XRA addition ratio contains sufficient asphaltenes, which form an asphaltenes micelle network structure by means of absorbing light components [37]. However, the higher the test temperature is, the less stable these asphaltenes micelle network structures. Therefore, even in the d master curves of binders with high XRA addition ratios, the intermediate plateau zones are also less distinct. In addition, as the XRA dose grows, the required temperature for the modified binders to transform from gel-like to sollike also rises, which leads the frequency of the d plateau zone of the bitumen to shift to lower values [36].

4.3.2. MSCR percent recovery and nonrecoverable creep compliance To further investigate the deformation resistance performance of the XRA-modified bitumen binder at high temperature, the MSCR test was performed at 64 °C. Fig. 7 presents the timestrain response values of the K64 and modified bitumen at shear stress levels of 0.1 and 3.2 kPa, respectively. The cumulative strain values of the XRA-modified bitumen are lower than those of the K64. It is interesting to note that a larger-amplitude delayed elastic response (tooth-shaped response) is observed in the XRA-modified bitumen, while the neat bitumen only presents a small recovery

Fig. 7. MSCR Time-strain response of binders at two stress levels: (a) 0.1 kPa and (b) 3.2 kPa.

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L. Cheng et al. / Construction and Building Materials 240 (2020) 117908

Fig. 8. MSCR results of binders at 64 °C: (a) R values and (b) Jnr values.

Fig. 9. The results of the BBR test for bitumen binders: (a) creep stiffness, and (b) creep rate.

(staircase response). These indicate a favorable influence of XRA addition on the permanent deformation resistance. R0.1 and R3.2 represent the MSCR percent recovery values of specimens at 0.1 and 3.2 kPa, respectively. As shown in Fig. 8 (a), an increasing trend is observed in R0.1 and R3.2 of the modified bitumen binders as the XRA addition ratio grows. For the contribution of the XRA addition ratios to the growth of the R0.1 and R3.2 values, the 20% XRA dose benefits the elastic recovery remarkably; 8% XRA has certain benefits, but they are less than those of 12% XRA and 16% XRA. Meanwhile, it should be noted that the 12% XRA contribution to the MSCR percent recovery values is comparable to that of 16% XRA. The nonrecoverable creep compliance value (Jnr) is an indicator to characterize the rutting resistance ability of bitumen binders;

the lower Jnr is, the better the rutting resistance performance [38–41]. As shown in Fig. 8 (b), as the XRA addition increases, Jnr0.1 and Jnr3.2 become lower. This implies that the modified binder is less susceptible to plastic deformation at high temperatures. Moreover, it should be mentioned that there is almost no difference between the Jnr0.1 and Jnr3.2 values of 12% XRA (1.25 and 1.42 kPa) and 16% XRA (1.24 and 1.44 kPa). Therefore, 12% XRA addition is the optimal content to improve the binder’s elastic response and rutting resistance. 4.3.3. Low-temperature rheological properties analysis BBR test results have been broadly used to evaluate the rheological characteristics of bitumen binder at low ambient temperatures [42]. According to Fig. 9 (a), the stiffness values of the modified

Fig. 10. The critical temperatures and the final PG temperature for binders: (a) low-temperature, and (b) high-temperature.

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bitumen and neat binder increase significantly when the test temperatures decline. Additionally, with an increase in the XRA concentration, the stiffness values present a growing trend. Moreover, at the same test temperature, the stiffness value of 12% XRA is roughly consistent with that of 8% XRA but lower than the samples with 16% XRA or 20% XRA additions. This result means that a low XRA (8% and 12%) addition ratio only has a slightly negative effect on the low-temperature performance of the neat binder. The creep rate (m-value), which can be obtained by fitting the log of the stiffness values versus the log of the loading time curve, indicates the stress relaxation properties of bitumen binder at a low temperature. As shown in Fig. 9 (b), from 12 °C to 24 °C, a downward trend is observed in the m-values of the modified bitumen as the XRA addition ratio increases. This implies that the introduction of XRA compromises the binder’s lowtemperature stress relaxation performance. This can be attributed to, at least in part, the high average molecular weight and high asphaltene content of the XRA, leading to its modified bitumen showing more pronounced brittle characteristics at low ambient temperatures. However, the small difference in m-values between binders with low XRA addition ratios (8% and 12%) verifies that the adverse effect of the XRA on creep ability can be minimized by controlling the XRA content. 4.3.4. Performance grade (PG) analysis According to ASTM-D6648, the low-temperature PG of the bitumen binder was defined by the critical temperature, which was calculated based on the stiffness (S = 300 MPa) and m-value (m = 0.3). The results of the low-temperature PG samples are presented in Fig. 10 (a). The low-temperature PGs of K64 and four XRA-modified bitumen binders are 22, 22, 22, 22 and 16 °C, respectively. Theoretically, the XRA does not have a negative effect on the low-temperature PGs of binders, except for 20% XRA content. Fig. 10 (b) shows that, compared with the neat binder, the XRA-modified bitumen binders exhibit higher PG temperatures. This result is favorable for the previous MSCR and softening point tests, which implies that the XRA could improve the rutting resistance performance of the binder. Furthermore, the working temperature ranges for the K64, 8% XRA, 12% XRA, 16% XRA and 20% XRA are 86, 92, 98, 98 and 98 °C, respectively. Although the critical low temperature of the binders increases as the XRA dose increases, the modified bitumen presents a broader working temperature range than the K64. Moreover, to leave intact the desirable high-temperature deformation resistance of the modified bitumen and reduce the risk of cracking of the bitumen mixture at low ambient temperatures, 12% XRA is recommended as an optimum content. 4.4. Aging resistance property analysis 4.4.1. Chemical aging index evaluation Fig. 11 shows the infrared spectra of binders in different aging states at approximately 600~2000 cm1. It can be observed that the carbonyl peaks present an increasing trend when the binders are subjected to RTFOT and PAV aging. This can be attributed to molecular interactions and chemical composition changes of the binders under high air pressure and tempereature. The changes in the chemical composition include the conversion of nonpolar fractions to the polar fractions and a loss of resin and aromatics [43]. However, the change amplitude for the area of the sulfoxide groups is not as pronounced as that of the carbonyl groups in both the K64 and the XRA-modified bitumen, particularly after longterm aging. This may be because of the dynamic reciprocal reaction of sulfoxide group formation and sulfoxide group deoxygenation

Fig. 11. Infrared spectra of binders in different aging states.

during long-term aging; therefore, a significant sulfoxide group increasing trend cannot be monitored. As shown in Section 4.1, due to the presence of the K64, the carbonyl groups of bitumen are discernable in the spectra of the XRAmodified bitumen even in the virgin state. To more accurately evaluate the influence of the modification level and aging process on the chemical characteristics of the XRA-modified bitumen, the carbonyl growth index difference value (D IC=O) of different aging states was employed to eliminate the effect of the inherent carbonyl of the K64.

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As shown in Fig. 12 (a), a lower D IC=O value (both after RTFOT and PAV aging) is observed in the modified binder. This may be attributed to the XRA containing a variety of organic chains that promote the cross-linking polymerization of reactive groups (carboxyl, aldehyde, naphthalene, etc.) in the bitumen, thereby leading to an improvement in the arrangement mode and network structure (node and strength) of the binder. On the other hand, the macromolecular chain contained in the XRA improves the reaction activation energy of the modified bitumen, which increases resistance to the aging reaction. In terms of D IC=O values (IC=O RT - IC=O VG) of the binders between the RTFOT and virgin states, 12% XRA exhibits the smallest growths, which reveals that 12% XRA presents the best short-term aging resistance, followed by 8%, 20%, and 16% XRA. Additionally, as the XRA concentration increases, a declining trend is observed in the D IC=O values (IC=O PAV - IC=O RT) of the binder between the PAV and RTFOT states. The same method was employed to calculate the differences in the sulfoxide groups in different aging states. According to Fig. 12 (b), the changing trend of D IS=O values is quite unstable, and even negative growth is detected after long-term aging. Therefore, it is not suitable to use D IS=O to evaluate the aging extent of the XRA-modified bitumen binder. 4.4.2. Physical aging index evaluation The softening point index (SPI) and viscosity index (VI) of the binders after short-term and long-term aging are shown in Fig. 13. The more aging the binder endures, the stiffer it becomes, resulting in higher SPI and VI values. However, regardless of the

type of aging method used, a downward trend was detected in the SPI and VI values of the binder after the incorporation of XRA. This indicates that the XRA-modified binders have better aging resistance performance than the K64. After short-term aging, 8% XRA exhibits the smallest SPI and VI values, followed by 12%, 20% and 16% XRA. With respect to long-term aging, the physical indexes of the binders show a completely different trend and decrease with increasing XRA content. Furthermore, the VI is more sensitive than the SPI for evaluation of the degree among K64 and XRA-modified binders aging. 4.4.3. Rheological aging index evaluation Complex modulus master curves for specimens at three aging stages are displayed in Fig. 14. As a consequence of bitumen oxidizing and hardening, general increases in |G*| values are observed for both the K64 and the XRA-modified bitumen, while the changes that occur to the modified binders are less pronounced compared with the K64. Specifically, under short-term aging conditions, XRA-RT and XRA-VG revealed overlapping modulus master curves; however, the noticeable space of K64-RT and K64-VG was detected. On the other hand, although the modulus curves further ascend when the binders are subjected to long-term aging, the ascent occurs when XRA-PAV is reduced when the high-ratio XRA is added to the binder. These indicate that the introduction of the XRA appears to enhance the aging resistance of the XRAmodified bitumen. Phase angle master curves for the binders are shown in Fig. 15. Aging and oxidizing lead to the transformation from maltene to

Fig. 12. Chemical aging indexes of samples in different aging states: (a) D IC=O and (b) D IS=O.

Fig. 13. Physical aging indexes of binders at different aging methods: (a) SPI and (b) VI.

L. Cheng et al. / Construction and Building Materials 240 (2020) 117908

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Fig. 14. Complex modulus master curve of binders at different aging stages: (a) K64, (b) 8% XRA, (c) 12% XRA, (d) 16% XRA, and (e) 20% XRA.

asphaltene, which induces a decline in the d master curve and a stiffer response. Moreover, for the K64, as the extent of the aging goes deeper, the descending amplitude of d values at intermediate frequencies (approximately 102 Hz to 102 Hz) is more pronounced than at low frequency and high frequency, which increases the probability of binder fatigue cracking. A consecutive descending in the d master curves of the XRA binders is detected from high frequency to low frequency, indicating that the effects of aging on the d values of the modified bitumen are mainly focused at low frequency. 4.4.4. MSCR aging indexes The MSCR percent recovery aging indexes (RAIs) for the binders in different aging states are shown in Fig. 16 (a). With the improvement in the oxidation level, the R3.2 values increase irrespective of the content of XRA added, suggesting that oxidation and high temperatures increase the deformation recovery potential of the samples. On the other hand, except for 8% XRA, the change amplitude of the RAI values of the modified bitumen binders after aging obviously decreases with an increase in the XRA concentration. This

indicates that the presence of XRA alleviates the aging susceptibility magnitude of the bitumen. According to Fig. 16 (b), a significant downward trend in Jnr3.2 is observed after RTFOT and PAV aging. Moreover, an unexpected result is detected insomuch that, irrespective of the concentration of XRA, the decline ratio of JAI after aging for the modified bitumen is larger than that for the K64. This phenomenon can be attributed to the MSCR test being performed at 64 °C, which is lower than the high-temperature PG of the XRA-modified bitumen, particularly in the case of the high XRA addition ratio. Thus, the Jnr3.2 values of the modified binders are inherently small at the 64 °C test temperature. Therefore, even if the Jnr3.2 of the XRA is reduced marginally after aging, a large change ratio is also exhibited. In other words, although the JAI values of the modified binders are larger than those of the K64, it does not mean that the XRA cannot contribute to mitigating the hardening effect that yields by the aging of the neat bitumen. Furthermore, in the author’s laboratory, several other cases of test temperatures and stress levels in the MSCR test were performed, and the specific mechanism was still under study and thus will not be specifically discussed in this paper.

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Fig. 15. Phase angle master curves of binders at different aging stages: (a) K64, (b) 8% XRA, (c) 12% XRA, (d) 16% XRA, and (e) 20% XRA.

Fig. 16. MSCR aging indexes of binders: (a) RAI and (b) JAI.

4.5. Correlation analysis Based on the front discussion, it should be noted that aging has apparent effects on both the rheological and chemical properties of

K64 and XRA modified bitumen binder. During the aging process, the carbonyl functional group content is more stable to characterize the aging extent of binder than sulfoxide. Therefore, it can be considered to construct the relationship among the carbonyl func-

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L. Cheng et al. / Construction and Building Materials 240 (2020) 117908

tional group with the binder properties. In this study, numerous indicators are used to comprehensively characterize the properties of XRA modified binder, however, there is an overlapping of information between those parameters. On the other hand, if the properties indexes of all bitumen samples with different aging states are regression analysis with IC=O, the obtained result will be over complicated, which is not conducive to exploring the essential impact of IC=O change on the binder properties. Hence, this paper uses Principal component analysis (PAC) to reveal the similarities and relationships between measured parameters, thereby reducing the original tests data dimension by establishing a set of variables called Principal Components (PCs). PCA was performed on the original data set of 15 samples and 10 parameters through Spss Statistics 25. The data set is listed in Table 5 and the loading plot of PC1 and PC2 is shown in Fig. 17. In the loading plot, the parameters occupying similar positions mean that they have similar scores and vice versa. According to Fig. 17, the PC1 and PC2 account for 85.93% total variance explained, which implies more than 85% of the initial data information can be successfully explained by translating 10 parameters to two virtual parameters (PC1 and PC2). Notably, PC1 holds a high correlation with PG-lower and PG-upper while PC2 is highly consistent with IC=O, indicating that PG grade and IC=O are the most significant parameters to characterize the aging resistance properties of XRA modified bitumen binder. Furthermore, PG-upper is close to PG-lower and the same is observed for Jnr3.2 and stiffness values as well as R0.1 with R3.2, indicating they are similar parameters. On the other hand, according to previous researches [29], the relationship between physical indicators such as viscosity and IC=O has been studied. Therefore, correlations between IC=O at different aging states and corresponding physical and rheological aging indexes (PGupper, stiffness values, RAI3.2 and VI) are investigated. Their detailed R-square is described in Fig. 18. It can be noted that, for neat bitumen, VI at 135 °C has the tightest correlation with IC=O, followed by PG-upper, stiffness, and RAI3.2 at 64 °C, whereas the rank is VI at 135 °C, RAI3.2 at 64 °C, stiffness, and PG-upper for 12% XRA. Although four regression has a high R-square (>0.9), VI has a tighter correlation with IC=O, indicating the VI is more reliable in the aging extent evaluation for K64 and XRA. 4.6. The aging resistance ranking of bitumen Selection sort algorithm was employed to rank the aging resistance characteristics of the specimens. As Table 6 shows, greater scores of the aging indexes imply that the aging extent of the binder goes deeper. It can be seen that the binders with 12% XRA additions present the lowest final scores after the RTFO test, followed

Fig. 17. Principal component analysis loading plot.

Fig. 18. Relational degree between.

by 8%, 20%, 16% XRA and K64. However, after long-term aging, the aging resistance ranking is 20%, 16%, 12%, 8% XRA and K64. These results indicate that XRA can dramatically enhance the aging resistance of the binder. However, considering that a high XRA addition ratio may adversely affect the low-temperature performance of the binder and combining the test results of the chemical, physical and rheological characteristics of samples pre- and postaging, it is suggested that 12% XRA should be the optimum addition

Table 5 data sets for PCA analysis. Samples

G*/sind

IC=O

R0.1

R3.2

Jnr3.2

PG-upper

PG-lower

XRA dose

m-value

S

K64 8% XRA-VG 12% XRA-VG 16% XRA-VG 20% XRA-VG K64-RT 8% XRA-RT 12% XRA-RT 16% XRA-RT 20% XRA-RT K64-PAV 8% XRA-PAV 12% XRA-PAV 16% XRA-PAV 20% XRA-PAV

777.42 816.90 690.40 831.03 792.41 1440.29 1836.71 1492.65 1704.61 1482.66 1976.24 1774.23 1425.21 1843.60 1674.46

51.03 52.88 49.49 51.71 49.05 77.63 71.65 63.47 64.23 65.97 152.37 142.65 116.3 126.69 91.65

0.80 5.52 11.86 13.53 22.66 6.88 18.01 28.06 29.29 41.47 20.71 35.30 44.19 48.55 50.57

0.29 2.13 5.26 5.51 13.7 1.75 10.22 18.76 20.04 36.64 3.05 26.47 38.05 43.9 46.75

5.59 3.62 1.25 1.24 0.61 3.1 0.95 0.53 0.56 0.2 1.64 0.52 0.21 0.15 0.13

67.6 74.3 81.99 82 87 72.3 74.4 82.02 82 88 77.4 80.6 85.8 87.3 91.4

27.9 25.9 25.2 –23.7 19.3 26.3 25.1 24.81 –22.72 18.57 24.6 24.52 –23.18 –22.27 17.72

0 8 12 16 20 0 8 12 16 20 0 8 12 16 20

0.297 0.276 0.289 0.281 0.258 0.283 0.265 0.284 0.259 0.24 0.264 0.255 0.263 0.262 0.225

366 392 405 447 494 397 461 410 463 507 456 475 416 464 526

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Table 6 The aging resistance ranking of bitumen. Sample

Aging method

D IC=O

SPI

VI

RI

Final score

K64

RTFOT PAV RTFOT PAV RTFOT PAV RTFOT PAV RTFOT PAV

5 5 2 4 1 3 4 2 3 1

5 5 1 4 2 3 4 2 3 1

5 5 1 4 2 3 4 2 3 1

5 4 4 4 2 2 3 3 1 1

20 19 8 16 7 11 15 9 10 4

8% XRA 12% XRA 16% XRA 20% XRA

ratio to improve the thermal-oxidation aging resistance of neat bitumen.

5. Summary and conclusions This study aims to comprehensively evaluate the chemical, rheological and aging characteristic properties of XRA-modified bitumen binder. The chemical properties were monitored by FTIR spectra, while the rheological characteristics were measured by frequency sweep, dynamic shear oscillatory and MSCR tests. Furthermore, several empirical indexes in terms of softening point, ductility and penetration grading, were tracked with corresponding tests. To rank the aging resistance performance of the binders, the related parameters were analyzed using selection sort algorithm. It can be concluded as follows: Due to the more significant C@C stretching vibrations by abundant asphaltene in the XRA, the peaks at approximately 1600 to 1375 cm1 of the XRA were more intense and wider than those of the K64. The addition of the XRA could reduce the formation of carbonyl for binders after oxidation. Moreover, DIC=O was more stable than DIS=O to characterize the aging extent of the bitumen. The principal component analysis suggested that ten parameters employed in this paper (such as stiffness, PG grade, percent recovery and etc.) had similarity and could be explained by two variables: PG grade and carbonyl index. The construction of |G*| and d master curves demonstrated that the XRA improved the elastic response of the K64. Additionally, the |G*| and d master curves of the modified bitumen were less affected by oxidation compared with those of the base bitumen. Although the MSCR test indicated that the XRA addition enhanced the resistance to permanent deformation properties in the base bitumen at high temperatures, it might not be applicable using a JAI of 64 oC as an aging index to evaluate the aging resistance of the XRAmodified bitumen due to the sensitivity of the binders to the test temperature. The introduction of XRA could improve the working temperature range and enhance the thermal stability of the K64, while the low-temperature shrinkage crack resistance and stress relaxation properties of the bitumen were compromised when the XRA addition ratio exceeded 12%. The aging process exerted more distinct impacts on the neat bitumen compared with the XRA-modified binder. From the selection sort algorithm, the aging resistance ranking after the RTFOT was 12% XRA > 8% XRA > 20% XRA > 16% XRA > K64, and the aging resistance ranking after the PAV was 20% XRA > 16% XRA > 12% XRA > 8% XRA > K64. The experimental results showed that 12% XRA not only greatly enhanced the thermal stability and aging resistance performance of the K64, but also minimized the adverse effects of the XRA on the low-temperature performance. Thus, 12% XRA was recommended as the optimum dose for modifying base bitumen.

CRediT authorship contribution statement Long Cheng: Conceptualization, Methodology, Formal analysis, Data curation, Writing - original draft, Writing - review & editing. Jiang Yu: Funding acquisition, Project administration, Resources, Formal analysis. Qun Zhao: Formal analysis, Data curation. Jinshi Wu: Software, Formal analysis, Validation. Lei Zhang: Funding acquisition, Project administration, Resources, Formal analysis. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgment The authors gratefully acknowledge the financial support provided by the National Natural Science Foundation of China under grant numbers (51168044) and (51778142). References: [1] H. Kim, W.G. Buttlar, Multi-scale fracture modeling of asphalt composite structures, Compos. Sci. Technol. 69 (15–16) (2009) 2716–2723, https://doi. org/10.1016/j.compscitech.2009.08.014. [2] G.D. Airey, Rheological evaluation of ethylene vinyl acetate polymer modified bitumens, Constr. Build. Mater. 16 (2002) 473–487, https://doi.org/10.1016/ S0950-0618(02)00103-4. [3] G. Airey, Rheological properties of styrene butadiene styrene polymer modified road bitumensH, Fuel 82 (14) (2003) 1709–1719, https://doi.org/10.1016/ s0016-2361(03)00146-7. [4] C. Yan, W. Huang, F. Xiao, Q. Lv, Influence of polymer and sulphur dosages on attenuated total reflection Fourier transform infrared upon Styrene– Butadiene–Styrene-modified asphalt, Road Mater. Pavement Des. (2018) 1– 15, https://doi.org/10.1080/14680629.2018.1467336. [5] R. Chang, J. Pang, S. Du, Effect of dithiodimorpholine and tetraethylthiuram disulphide on ageing properties of SBS-modified asphalts, Road Mater. Pavement Des. 17 (1) (2015) 243–251, https://doi.org/10.1080/ 14680629.2015.1058849. [6] J. Wang, J. Yuan, K.W. Kim, F. Xiao, Chemical, thermal and rheological characteristics of composite polymerized asphalts, Fuel 227 (2018) 289–299, https://doi.org/10.1016/j.fuel.2018.04.100. [7] B.V. Kök, M. Yilmaz, M. Guler, Evaluation of high temperature performance of SBS+Gilsonite modified binder, Fuel 90 (10) (2011) 3093–3099, https://doi.org/ 10.1016/j.fuel.2011.05.021. [8] J. Liu, K. Yan, J. Liu, D. Guo, Evaluation of the characteristics of Trinidad Lake Asphalt and Styrene–Butadiene–Rubber compound modified binder, Constr. Build. Mater. 202 (2019) 614–621, https://doi.org/10.1016/ j.conbuildmat.2019.01.053. [9] J.R. Helms, X. Kong, E. Salmon, P.G. Hatcher, K. Schmidt-Rohr, J. Mao, Structural characterization of gilsonite bitumen by advanced nuclear magnetic resonance spectroscopy and ultrahigh resolution mass spectrometry revealing pyrrolic and aromatic rings substituted with aliphatic chains, Org. Geochem. 44 (2012) 21–36, https://doi.org/10.1016/j.orggeochem.2011.12.001. [10] N. Kim, Chemical Characterization of Gilsonite Bitumen, J. Petrol. Environ. Biotechnol. 05 (05) (2014), https://doi.org/10.4172/2157-7463.1000193. [11] M. Yilmaz, M.E. Celoglu, Effects of SBS and different natural asphalts on the properties of bituminous binders and mixtures, Constr. Build. Mater. 44 (2013) 533–540, https://doi.org/10.1016/j.conbuildmat.2013.03.036.

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