High temperature rheological characteristics of plasma-treated crumb rubber modified binders

Construction and Building Materials 236 (2020) 117614

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High temperature rheological characteristics of plasma-treated crumb rubber modified binders Jin Li, Feipeng Xiao ⇑, Serji N. Amirkhanian Key Laboratory of Road and Traffic Engineering of Ministry of Education, Tongji University, 4800 Cao’an Highway, Jiading District, Shanghai 201804, China

h i g h l i g h t s
Cold plasma treatment resulted in a significant enhancement of high temperature performances.
The plasma-treated binders with -40 mesh crumb rubber were considered the most desirable.
Plasma treatment somehow improved the thermal storage stability of modified binders.
A strong relationship between rotational viscosity, test temperature and crumb rubber size.
A short-term aging caused an improvement in both elasticity and viscosity of treated binders.

a r t i c l e

i n f o

Article history: Received 25 September 2019 Received in revised form 9 November 2019 Accepted 12 November 2019

Keywords: Cold plasma surface treatment Crumb rubber modified asphalt High temperature performance Statistical analysis Coupling effect

a b s t r a c t This study investigated the effect of cold plasma surface treatment of crumb rubber on the high temperature performances of CRM binders at unaged and RTFO-aged states, which were produced with two base asphalts and three crumb rubber sizes. The high temperature performance tests conducted on the rubberized binders included rotational viscosity, failure temperature, rutting factor, phase angle, storage modulus, and loss modulus at different temperatures. According to the result of performance tests and statistical analyses, cold plasma surface treatment on crumb rubber significantly enhanced the high temperature performances of CRM binders, regardless of base asphalt source, crumb rubber size and aging state. The effect of base asphalt source was mainly reflected in determining the high temperature performances of RTFO-aged binders. With respect to the crumb rubber size, the -40 mesh plasma-treated CRM binders were the most desirable. In addition, the RTFO aging caused an enhancement in elasticity and viscosity of plasma-treated CRM binders as expected. Finally, the Gray Relational Analysis (GRA) determined the coupling effect of these variables, indicating that cold plasma surface treatment of -40 mesh crumb rubber and base asphalt A was the most effective combination in terms of high temperature rheological properties of CRM binders. Ó 2019 Elsevier Ltd. All rights reserved.

1. Introduction With the expansion of needs for automobile worldwide, the ever-increasing scrap tires has been a notable environmental issue [1,2]. As an environmentally friendly technology of recycling the waste tire rubber, Crumb Rubber Modified (CRM) asphalt has been studied considerably [3,4]. Nowadays, an increasing number of countries have widely adopted the CRM binders in the construction of highway and airfield pavements [3]. There are evidently many advantages for adopting scrap tires to the polymer modified asphalt field, such as the improved fatigue and thermal cracking resistance, reduced traffic noise and cost, as

⇑ Corresponding author. E-mail addresses: [email protected] (J. Li), [email protected] (F. Xiao). https://doi.org/10.1016/j.conbuildmat.2019.117614 0950-0618/Ó 2019 Elsevier Ltd. All rights reserved.

well as combined recycling with reclaimed asphalt pavement [4,5]. However, due to the poor compatibility between crumb rubber particles and asphalt matrix [6,7], CRM binder shows poorer thermal storage stability when compared with other polymer (e.g. Styrene-butadiene-styrene) modified asphalts in terms of asphalt fractions and polymer types [8,9]. Therefore, the investigation on a low-cost, high-efficiency and time-saving surface treatment for crumb rubber has been a hotspot in this field. Kocevski et al. performed a surface treatment on Ground Tire Rubber (GTR) particles by grafting acrylic acid [10]. The results indicated that this treatment enhanced surface roughness of GTR and high temperature performances of GTR modified binders. Chen et al. adopted motor oil and zinc stearate to chemically activate crumb rubbers and investigated its impact on the rheological properties of CRM binders [11]. They found this rubber treatment caused a decrease in high temperature viscosity, which promoted the work-

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ability of CRM binders regardless of base asphalt source and test temperatures. In addition, some researchers in other fields also employed other surface treatments to modify crumb rubber, such as UV-ozone and ultrasonic waves [12,13]. They reported that these surface treatments generally enhanced the surface properties of crumb rubber, such as surface polarity and roughness. On the other hand, the common surface treatment technologies fall into two categories: chemical and physical treatment [14]. As a representative method of physical treatment, plasma surface treatment has been widely applied to modify polymers now [15,16]. Plasma is also called the fourth state of matter except for solid, liquid and gas [17]. It consists of charge particles (i.e. electrons and ions), neutral particles, photons, free radicals and other activated molecules. Plasma surface treatment has been proved to be able to effectively improve the surface free energy, surface roughness and surface wettability of polymers [18–20]. The modification mechanisms of plasma surface treatment on polymers were shown in Fig. 1 [19]. Therefore, in order to improve rubber-asphalt compatibility and pavement performance of CRM binder, cold plasma was used to modify the surface of crumb rubber particles in this study. The objective of this present work was to investigate the high temperature performances of 12 CRM asphalts at two aging states, in terms of 2 plasma surface treatment conditions, 2 base asphalt sources and 3 crumb rubber sizes. With respect to these multiple variables, several statistical methods, including Gray Relational Analysis (GRA), significance testing and regression analysis, were adopted. These methods were used to verify if these variables had significant effect on high temperature properties of CRM asphalt, especially the cold plasma surface treatment. In addition, the coupling effect of these variables was also determined by means of GRA. 2. Materials and methods 2.1. Experimental design The detailed flowchart of experimental designs for all CRM binders was shown in Fig. 2. At least three replicates were performed in each test. In this work, the crumb rubber was obtained by means of ambient shredding of waste tires. In order to investigate the effect of crumb rubber size on the performance of plasma-treated CRM binder, the -30, -40 and -80 mesh crumb rubbers were adopted. The designed dosage of crumb rubber was 15% by weight of base asphalt in this work, which was a typical content to produce PG 76-22 binder. The passing percentage of each crumb rubber was shown in Fig. 3. Two PG 64-22 base asphalts from different crude oil sources were adopted to produce CRM binders in this study, referred to as A and B. The basic properties of base asphalts were presented in Table 1.

2.2. Materials testing A cold plasma machine (OPS-OMEGA-DT03 produced by Suzhou OPS plasma technology company, China) was employed to this research. Generally, the efficiency plasma surface treatment depends on discharge power and processing time [14]. In this work, the selected discharge power and processing time were respectively 250 W and 8 min, based on the result of previous study [14]. 20 g crumb rubber particles were first weighed and spread evenly on an 18*18 cm glass plate. Then the specimen was put into the reactor chamber. The vacuum degree was adjusted to 150 sccm by using a roots pump and the dry air gas was injected into the chamber at a flow rate of 50 sccm afterwards. Once the pressure became steady, the treatment started under the selected power until the preset processing time run out. In order to avoid the potential aging effect of plasma surface treatment [21,22], the crumb rubber particles were immediately used to produce CRM binders once treated. The CRM binders were produced by wet progress in this work, which was the most common method of modifying asphalt with crumb rubber particles. Firstly, 400 g base asphalt was weighed and preheated in a closed metal container at 177 °C until it flowed fully. Then the given particle size and dosage of crumb rubber were added and blended with base asphalt at 177 °C and 1000 rpm for 1 h. Finally, a total of 12 CRM binders (2 plasma surface treatment conditions * 2 base asphalt sources * 3 crumb rubber particle sizes) were produced and further tested. It was noted that in order to avoid the influence of high-speed shearing on the crumb rubber particle size, only the mechanical blending process was adopted in this work. The short-term aged asphalt samples were obtained from the residues of RTFO test according to AASHTO T240. Rotational viscosity of unaged CRM binders was measured by a Brookfield rotational viscometer at 3 different temperatures (135, 155 and 175 °C) in accordance with AASHTO T316. In this test, a sample of 10 g and a #27 spindle were used. High temperature rheological properties of unaged and RTFO-aged CRM asphalts were tested by a Dynamic Shear Rheometer (DSR) based on AASHTO T315 and AASHTO M320, including failure temperature, rutting resistance factor and so on. A 25-mm plate was used in these DSR tests and the working gap between two parallel plates was set as 2 mm. In addition, the test temperature started from 64 °C for all samples tested. 2.3. Statistical analyses Regression analysis is a common method to determine the interdependent quantitative relationship between two or more variables. In this study, the simple linear regression analysis was used. Significance testing is used to test whether there is a significant difference between the effects of the experimental group and con-

Fig. 1. Modification mechanisms of plasma surface treatment.

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J. Li et al. / Construction and Building Materials 236 (2020) 117614

Fig. 2. Flowchart of experimental design.

Table 1 . Basic properties of two PG 64-22 base asphalts. Aging states

Test properties

Base asphalt sources A

B

Unaged

Viscosity at 135 °C (Pas) Rutting factor at 64 °C (kPa) Failure temperature (°C)

0.522 1.46 66.8

0.513 1.41 66.4

RTFO-aged

Rutting factor at 64 °C (kPa) Failure temperature (°C)

3.02 67.0

2.98 66.2

PAV-aged

Rutting factor at 25 °C (kPa) Stiffness at 12 °C (MPa) m-value at 12 °C

3659 244.5 0.303

3970 229.0 0.322

Notes: RTFO = Rolling Thin Film Oven; PAV = Pressure Aging Vessel. Fig. 3. Percentage passing of three crumb rubbers.

trol group. The student’s t test at 5% level of significance was conducted in this work. Correlation analysis refers to the analysis of two or more variables with correlation, so as to measure the correlation degree of these variables. This analysis was used to investigate the relationship of rotational viscosity of unaged binders and other high temperature performance indices in this present work. GRA is a quantitative description of development trends of a system. Its basic idea is to determine whether the reference series and several comparative series are closely related based on their

geometric similarity, which in essence reflects the degree of correlation between these curves. This method generally can be used to evaluate the influence degree of each factor on the results by calculating the gray correlation coefficient among reference series and comparative series. Its detailed process is shown below: 1) Identify the comparative objects (evaluation objects) and reference series (evaluation indicators). Assume that: There are m evaluation objects and n evaluation indicators. The reference series is x0 ¼ fx0 ðkÞjk ¼ 1; 2; :::; ng and the comparative series is xi ¼ fxi ðkÞjk ¼ 1; 2; :::; ng, i ¼ 1; 2; :::; m.

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J. Li et al. / Construction and Building Materials 236 (2020) 117614

2) Determine the weight coefficients wk of each indicator by using Analytic Hierarchy Process (AHP) or other methods. wk , k ¼ 1; 2; :::; n represents the weight of kth indicator. 3) Calculate the gray correlation coefficient ni ðkÞ according to Eq. (1):

ni ðkÞ ¼

min min jx0 ðt Þ  xs ðt Þj þ q max max jx0 ðt Þ  xs ðtÞj s

t

s

t

jx0 ðt Þ  xi ðt Þj þ q max max jx0 ðt Þ  xs ðt Þj s

ð1Þ

t

where ni ðkÞ is the correlation coefficient of the kth indicator with regards to the comparative series xi vs. the reference series x0 . min min jx0 ðt Þ  xs ðt Þj and max max jx0 ðt Þ  xs ðt Þj are the minimum s

t

s

t

and maximum absolute difference among the comparative series and reference series, respectively. q is the resolution ratio and its range is ½0; 1. Generally, the bigger the q value, the greater the resolution ratio. 4) Calculate the gray correlation value (degree) ri according to Eq. (2):

ri ¼

n X

wk ni ðkÞ

ð2Þ

k¼1

where wk is the gray weighted correlation value of the ith evaluation object. In this study, all evaluation object was considered to have the same weight. 5) Rank the evaluation objects based on the calculated correlation value. The bigger the correlation degree, the greater the evaluation result. 3. Results and discussions 3.1. High temperature performance 3.1.1. Rotational viscosity The high temperature viscosity of asphalt binder is generally used to evaluate its flow characteristic and to ensure that it can be pumped and handled at the hot mixing plants as well as to provide a guide for determining the mixing and compacting temperatures of asphalt mixtures. The rotational viscosity values of various unaged binders at 135, 155 and 175 °C were presented in Fig. 4, which clearly reported the influence of plasma treatment, base asphalt source, crumb rubber size and test temperature on the rotational viscosity of unaged binders. Obviously, the viscosity values decreased with an increased test temperature. The effect of cold plasma surface treatment on rotational viscosity was significant for all CRM binders, with the viscosity value of treated binders higher than those untreated ones. It was attributed to that the plasma enhanced the surface compatibility of crumb rubber particles and thus promoted the interactions between crumb rubber and asphalt matrix [14]. In terms of base asphalt source, the binders produced with asphalt A generally had higher rotational viscosity than asphalt B. Regarding the effect of crumb rubber size, the binders produced with -40 mesh crumb rubber shown the greatest viscosity regardless of treatment condition and asphalt source, followed by the -80 mesh binders and the -30 mesh binders had the lowest indices. Moreover, it was noted that the plasmatreated CRM binders produced with asphalt A and -40 mesh crumb rubber shown the highest rotational viscosity value, which implied that the highest construction temperature was needed for these binders. In order to further investigate the detailed impact of plasma surface treatment, base asphalt source, crumb rubber size and test

temperature on the rotational viscosity of CRM binders, several regression analyses were conducted. As that it was unreasonable to consider the plasma surface treatment condition and base asphalt source as continuous variables, these statistical analyses were only applied to determine the rotational viscosity value as a function of test temperature and crumb rubber size. The regression equations were depicted in Table 2 and the fitting surfaces obtained from the regression analysis were drawn in Fig. 5. It was noted that the selection of fitting equation form or fitting model was based on the consideration of the correlation between rotational viscosity and each single variable, including test temperature and crumb rubber size, respectively. According to the rotational viscosity value of various binders at different test temperature in this study, it could be found that the viscosity could be seen an exponential function of test temperature and quadratic function of crumb rubber size. Therefore, the form of fitting equation in Table 2 was selected. As seen in Table 2, the R-square values of four regression equations were nearly 1.00. It reported a strong statistical relationship between rotational viscosity, test temperature and crumb rubber size, regardless of plasma surface treatment condition and base asphalt source. In addition, Fig. 5 presented the rank of rotational viscosity values for all tested CRM binders. Generally, the temperature susceptibility of asphalt binders was evaluated via the A-VTS relationship [23]. In this work, the regression coefficient t (i.e. test temperature) also could indicate the temperature susceptibility to a certain extent. A higher value of this coefficient indicated a greater temperature susceptibility and vice versa. Hence, it could be further concluded from the equations in Table 2 that the plasma treatment enhanced the temperature susceptibility of CRM binders. With respect to the crumb rubber size, it was noted that both oversized and undersized crumb rubbers were tended to weaken the performance. This finding could be explained by the particle effect of crumb rubber in asphalt matrix [7,24]. The -40 mesh crumb rubber particles could mutually bond and form an optimal skeletal structure in CRM binder. Additionally, according to these fitting formulas, test temperature and crumb rubber size could be considered to be completely independent of each other. It revealed that the effects of test temperature and crumb rubber size were not relevant, which namely illustrated that the test temperature during the test process generally did not affect the crumb rubber size. Furthermore, it was unfavorable that the increased viscosity might degrade the workability of CRM binders to some extent. However, it could be further induced that cold plasma surface treatment improved their thermal storage stability. Due to the differences in density and solubility between base asphalt and crumb rubber, phase separation generally occurred in CRM binders. During the storage period at elevated temperature, CRM binder could be approximately seen as a suspension system. Therefore, the falling velocity of crumb rubber particles could be determined by Stock’s law:

v¼

2g ðq1  q0 Þr 2 9g

ð3Þ

where: q1 and q0 are the density of crumb rubber and base asphalt, respectively; g is the gravitational acceleration; r is the crumb rubber size; g is the viscosity of CRM binder. Obviously, a higher falling velocity of crumb rubber particles leads to a higher degree of phase separation in CRM binder. As the falling velocity varied inversely with viscosity of CRM binder, the increased viscosity thus revealed the enhanced thermal storage stability.

J. Li et al. / Construction and Building Materials 236 (2020) 117614

5

Fig. 4. Rotational viscosity of various unaged binders, (a) Binder A, (b) Binder B.

Table 2 . Regression analysis of rotational viscosity, test temperature and crumb rubber size. Binder type

Regression equation

Base asphalt A Untreated V ¼ 68:855  e Treated V ¼ 105:53  e Base asphalt B Untreated V ¼ 110:61  e Treated V ¼ 461:28  e

R-square

- 0:0295t - 0:000439s2 þ0:0535s

0.9762

- 0:0285t - 0:000365s2 þ0:0449s

0.9698

- 0:0387t - 0:000649s2 þ0:0759s

0.9931

- 0:0385t - 0:000249s2 þ0:0277s

0.9957

Notes: V = rotational viscosity; t = test temperature; s = crumb rubber size.

3.1.2. Failure temperature Regarding an asphalt binder at unaged state, failure temperature is the critical temperature at which G =sind is equal to 1.0 kPa. Generally, high value of failure temperature is considered desirable attributes from the standpoint of rutting or permanent

deformation resistance at hot conditions. The failure temperature of CRM binders at unaged state were measured and the test results were summarized in Fig. 6. It was noticeable that cold plasma surface treatment was effective for improving the failure temperatures of unaged binders. This result could also be explained by the enhanced compatibility between crumb rubber and asphalt matrix after cold plasma surface treatment [14]. As observed from Fig. 6, most CRM binders produced with different base asphalts had identical performance grades, except for those modified by plasma-treated -40 mesh crumb rubber. It stated that there was only a slight effect of base asphalt source on failure temperature of unaged binders. In terms of crumb rubber size, the CRM binders produced with – 40 mesh crumb rubber generally had the highest failure temperature at unaged state. In addition, there was a negligible difference in failure temperature between the -30 mesh and -80 mesh ones.

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J. Li et al. / Construction and Building Materials 236 (2020) 117614

Fig. 5. Fitting surfaces of regression analysis.

Fig. 6. Failure temperatures of various unaged binders.

A student’s t test at the 5% level of significance was conducted on the failure temperature as a function of plasma surface treatment, base asphalt source and crumb rubber size. The two-tailed p-value was calculated and test results were presented in Table 3. It could be observed that in most cases the failure temperature of CRM binders had statistically significant improvement after cold plasma surface treatment. The conclusion about the effect of plasma surface treatment drawn for the binders produced with 80 mesh crumb rubber was non-significant, which might be due to the variability of CRM binders. Similar to the results presented in Fig. 6, there was no significant difference in failure temperature of unaged binders resulted from base asphalt source. Meanwhile, the CRM binders produced by -30 mesh and -80 mesh crumb rubber did not show statistically significant difference.

After the short-term aging process at 163 °C for 85 min, the failure temperature of RTFO residuals were also measured and the results were illustrated in Fig. 7. According to the Superpave specification, the G =sind value shall not less than 2.2 kPa when the test temperature is under failure temperature. Similar to Fig. 5, it could be concluded that cold plasma surface treatment still improved the failure temperature as shown in Fig. 7. It revealed the stability of its effect on compatibility enhancement between crumb rubber and asphalt matrix during aging process. It was also seen that the base asphalt sources played an important role in determining failure temperature at RTFO-aged state, with the CRM binders produced with asphalt A generally showing higher failure temperature values than those made by asphalt B. Moreover, the binder with the highest failure temperature was higher than that with the lowest by two PG grades (12 °C). With regard to the crumb rubber size, the failure temperature of RTFO-aged CRM binders decreased with the deceasing crumb rubber size, regardless of plasma surface treatment condition and base asphalt source. These abovementioned changes between two aging states were attributed to the different thermal oxidation extent of base asphalt and level of rubber-asphalt interactions during the aging process. A student’s t test at 5% level of significance was also repeated at RTFO-state for CRM binders and the test results were shown in Table 4. After cold plasma surface treatment, the binders produced with -30 and -40 mesh crumb rubbers had significant differences in failure temperatures at RTFO-aged state. Though the mean failure temperature values of other CRM binders also increased, the differences resulted from treatment could not be considered statistically significant. However, it did not deny the effect of cold plasma surface treatment, the poor test results mainly were due to small sample numbers. Additionally, this test verified that both base asphalt source and crumb rubber size had significant influences on the dependent variable.

7

J. Li et al. / Construction and Building Materials 236 (2020) 117614 Table 3 . Significance test on failure temperature as a function of plasma surface treatment condition, base asphalt source and crumb rubber size at unaged state (a ¼ 0:05). Binder type

A

B

Untreated

A

Untreated

Treated

B

Untreated

Treated

30 40 80 30 40 80

mesh mesh mesh mesh mesh mesh

30 40 80 30 40 80

mesh mesh mesh mesh mesh mesh

Treated

Untreated

Treated

30 mesh

40 mesh

80 mesh

30 mesh

40 mesh

80 mesh

30 mesh

40 mesh

80 mesh

30 mesh

40 mesh

80 mesh

...

S ...

N S ...

S S S ...

S S S N ...

S S N S N ...

N ... ... ... ... ...

... S ... ... ... ...

... ... N ... ... ...

... ... ... N ... ...

... ... ... ... N ...

... ... ... ... ... N

...

N ...

N S ...

S N N ...

N S S N ...

N S N N S ...

Notes: N = no statistical difference between two means; S = statistical difference between two means.

prepared with plasma-treated crumb rubbers had higher G =sind value than those untreated ones, indicating a relative greater permanent deformation resistance. In addition, the binder produced by untreated -30 mesh crumb rubber had the lowest rutting factor value regardless of base asphalt source. It revealed that these binders were more susceptible to rut at a high pavement temperature than other binders. The rutting factor values of RTFO-aged binders were also presented in Fig. 9. It could be seen that all CRM binders met this requirement at 76 °C. In addition, the 85-min RTFO aging also had a notable effect on the rutting factor of CRM binders. This improvement could be attributed to the thermal oxidation of asphalt matrix and further swelling and decomposition of crumb rubber particles during short-term aging process [25]. Similar to unaged state, the plasma-treated CRM binders at RTFO-aged state also had higher G =sind values than those untreated, illustrating an enhanced aging resistance and deformation recovery capacity.

Fig. 7. Failure temperatures of various RFTO-aged binders.

3.1.3. Rutting factor Rutting factor (G =sind) reflects the non-recoverable deformation resistance of asphalt binder during the repeated loading process. In general, asphalt binder with a higher G =sind value at high temperature shows a lower flow deformation, namely a greater rutting resistance. Meanwhile, it also acts as a criterion to determine the high temperature Performance Grade (PG) of asphalt binder. The rutting factors at five PG tested temperatures were presented in Fig. 8. It could be observed that the CRM binders

3.1.4. Phase angle In the Superpave specification, phase angle is defined as the time lag between the shear stress applied on asphalt binder and its corresponding strain. It can be calculated as the angle between storage modulus and loss modulus, indicating the viscoelastic characteristic of asphalt binder. In this study, the phase angle of all CRM binders was tested at five temperatures. As shown in Fig. 10, the plasma surface treatment resulted in a drop in phase

Table 4 . Significance test on failure temperature as a function of plasma surface treatment condition, base asphalt source and crumb rubber size at RTFO-aged state (a ¼ 0:05). Binder type

A

B

Untreated

A

Untreated

Treated

B

Untreated

Treated

30 40 80 30 40 80

mesh mesh mesh mesh mesh mesh

30 40 80 30 40 80

mesh mesh mesh mesh mesh mesh

Treated

Untreated

Treated

30 mesh

40 mesh

80 mesh

30 mesh

40 mesh

80 mesh

30 mesh

40 mesh

80 mesh

30 mesh

40 mesh

80 mesh

...

S ...

S S ...

S S S ...

S S S S ...

S S N S S ...

S ... ... ... ... ...

... S ... ... ... ...

... ... S ... ... ...

... ... ... S ... ...

... ... ... ... S ...

... ... ... ... ... S

...

N ...

S S ...

N S S ...

N S S S ...

S S S S S ...

Notes: N = no statistical difference between two means; S = statistical difference between two means.

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J. Li et al. / Construction and Building Materials 236 (2020) 117614

Fig. 8. Rutting factors of various unaged binders, (a) Binder A, (b) Binder B.

Fig. 9. Rutting factors of various RTFO-aged binders, (a) Binder A, (b) Binder B.

angle of all unaged binders, implying an enhancement in elasticity of CRM binders. It was clearly seen that the binders produced with plasma-treated -40 mesh crumb rubber had the lowest phase angle value, regardless of base asphalt source. As expected, these binders took the least time to deform after a certain stress was applied and

to recover after the stress was removed. In addition, there was no obvious trend with respect to base asphalt source. In general, a higher phase angle value suggests the asphalt binder with more viscous component. Therefore, this study conducted a correlation analysis to investigate the relationship between

J. Li et al. / Construction and Building Materials 236 (2020) 117614

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Fig. 10. Phase angle of various unaged binders, (a) Binder A, (b) Binder B.

phase angle and rotational viscosity of unaged CRM binders. As shown in Fig. 11, these two indices exhibited a fair correlation at three test temperatures with relative high R-square values. In addition, it could also be found from Fig. 11 that the increase of phase angle generally resulted in a decrease of rotational viscosity for all test temperatures and this decrease was more remarkable when the rotational viscosity was tested at higher temperature. Phase angle is another indicator of high temperature viscoelastic properties for RFTO-aged binders. The phase angle values of RTFO-aged binders at five test temperature were presented Fig. 12. Apparently, the variation trend of phase angle in terms of base asphalt source and crumb rubber size at RTFO-aged state remained the same as that at unaged state. However, the phase angle value was still higher than 45° at 64 °C, indicating that the CRM binder showed more viscosity rather than elasticity. Moreover, plasma surface treatment reduced phase angle of RTFOaged binders in most cases, regardless of base asphalt source, crumb rubber size and test temperature.

Fig. 11. Correlations between phase angle and rotational viscosity of unaged binders.

3.1.5. Storage modulus and loss modulus Unlike Newtonian liquid or purely elastic substances, asphalt binder is a kind of viscoelastic material. It is namely composed by both viscous and elastic component, characterized by loss modulus and storage modulus. The storage modulus and loss modulus of unaged binders at five test temperatures were shown in Figs. 13 and 14. It was clearly found that both the storage and loss modulus decreased as the test temperature increased. In addition, these two modulus values of plasma-treated CRM binders were greater than those untreated. Meanwhile, the ratio of these two modulus was the tangent of phase angle which had the same variation trend as phase angle presented in Fig. 10. Therefore, though both storage and loss modulus increased after plasma surface treatment, the proportion of storage modulus increased while the proportion of loss modulus decreased. Similar to the findings of phase angle, the binder produced by plasma-treated -40 mesh crumb rubber had the highest storage and loss modulus values, regardless of base asphalt source. There was a significant relationship between storage modulus of CRM asphalt and crumb rubber size. These -40 mesh binders performed best, followed by the -80 and -30 mesh binders orderly. The elasticity and viscosity of RTFO-aged CRM binders could also be evaluated by the values of storage and loss modulus. The results were presented in Figs. 15 and 16. Both storage and loss modulus of RTFO-aged binders significantly increased at all test temperatures when compared with unaged binders. It indicated that the elasticity and viscosity of CRM binders improved after a short-term aging process. In addition, the effect of plasma surface treatment on RTFO-aged binders was identical to that on unaged binders. The storage and loss modulus values of CRM binders produced by plasma-treated crumb rubbers were greater than those produced by untreated crumb rubbers. Meanwhile, the proportion of storage modulus increased while the proportion of loss modulus decreased with plasma surface treatment being applied.

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J. Li et al. / Construction and Building Materials 236 (2020) 117614

Fig. 12. Phase angle of various RTFO-aged binders, (a) Binder A, (b) Binder B.

Fig. 13. Storage modulus of various unaged binders, (a) Binder A, (b) Binder B.

3.2. Gray relational analysis In this work, a GRA was further carried out to quantitatively analyze the coupling effects of cold plasma surface treatment,

base asphalt source, and crumb rubber size on the abovementioned high temperature rheological properties of CRM binders. The raw data for this analysis were obtained from the performance test results.

J. Li et al. / Construction and Building Materials 236 (2020) 117614

11

Fig. 14. Loss modulus of various unaged binders, (a) Binder A, (b) Binder B.

Fig. 15. Storage modulus of various RTFO-aged binders, (a) Binder A, (b) Binder B.

Generally, the first step was to normalize these data. With this regard, phase angle was considered as a ‘‘cost type” indicator and was commonly expected to be kept as small as possible. On the contrary, rotational viscosity, failure temperature, rutting resis-

tance factor and storage modulus were seen as ‘‘benefit type” indicators. Apparently, a higher value of these indicators was desirable. Then the correlation coefficients and degrees were calculated and finally ranked based on the factors evaluated. It was noted that

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J. Li et al. / Construction and Building Materials 236 (2020) 117614

Fig. 16. Loss modulus of various RTFO-aged binders, (a) Binder A, (b) Binder B.

the references series was by those indicators whose normalized value was 1.00. The specific correlation coefficients were calculated and presented in Table 5 and the rank result of correlation degrees was shown in Fig. 17. It was observed from Fig. 17 that the CRM binder produced with base asphalt A and plasma-treated -40 mesh crumb rubber had the highest correlation value under the couple effect of these variables on its high temperature properties. Moreover, the plasma-treated

CRM binders generally had higher correlation degrees than those untreated. Therefore, it was concluded that the plasma surface treatment on crumb rubber definitely resulted in enhanced high temperature performance of CRM binder. In terms of base asphalt source, the CRM binders produced with base asphalt A obviously had higher correlation values than those produced with B. The mean values of correlation degrees were 0.5915 for A and 0.4256 for B. It might be due to that base asphalt A had relative greater

Table 5 . Correlation coefficients. Binder type

A

B

Unaged

RTFO-aged

Rotational viscosity

Failure temperature

Rutting factor

Phase angle

Storage modulus

Failure Temperature

Rutting factor

Phase angle

Storage modulus

30meshuntreated 30meshtreated 40meshuntreated 40meshtreated 80meshuntreated 80meshtreated

0.5223

0.3333

0.3624

0.3731

0.3626

0.8898

1.0000

0.7085

1.0000

0.6425

0.4783

0.4948

0.5237

0.4906

1.0000

0.6815

1.0000

0.7629

0.7049

0.4968

0.6301

0.5136

0.5372

0.4892

0.4745

0.4381

0.4612

1.0000

1.0000

1.0000

1.0000

1.0000

0.5678

0.4984

0.5971

0.5167

0.7978

0.3362

0.3732

0.4210

0.3890

0.4397

0.4154

0.4335

0.4150

0.9007

0.3869

0.4274

0.4657

0.4327

0.4502

0.4275

0.4524

0.4286

30meshuntreated 30meshtreated 40meshuntreated 40meshtreated 80meshuntreated 80meshtreated

0.3333

0.3333

0.3333

0.3333

0.3333

0.5113

0.4672

0.3986

0.4436

0.4566

0.3649

0.3773

0.3822

0.3724

0.6532

0.6072

0.6162

0.6186

0.4432

0.4208

0.5437

0.4305

0.4534

0.3333

0.4004

0.4657

0.4088

0.4555

0.4611

0.6724

0.4793

0.5248

0.3657

0.4230

0.5078

0.4349

0.3993

0.3484

0.4267

0.3874

0.3913

0.3393

0.3333

0.3333

0.3333

0.4368

0.3684

0.4712

0.4055

0.4159

0.3587

0.3504

0.3694

0.3525

J. Li et al. / Construction and Building Materials 236 (2020) 117614

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4) In terms of aging state, a short-term aging generally caused an improvement in both elasticity and viscosity of plasmatreated CRM binders as expected. It was mainly attributed to the hardening of asphalt matrix under thermal oxidative condition and the further swelling and decomposition of crumb rubber particles at elevated temperature. 5) With respect to the result of GRA, the coupling effect of cold plasma surface treatment, base asphalt source and crumb rubber size was determined. It indicated that the plasma surface treatment on -40 mesh crumb rubber and base asphalt A were the most effective combination in terms of high temperature properties of CRM binders in this research work.

Fig. 17. Correlation values of various CRM binders.

high temperature performance than B according to Table 1, although both of them came from the same PG grade. With regard to crumb rubber size, the binders produced with -40 mesh crumb rubber shown the highest correlation value regardless of plasma surface treatment condition and base asphalt source, followed by the binders produced with -80 mesh crumb rubber and those binders produced with -30 mesh crumb rubber were the lowest. 4. Conclusions In this present work, a new surface treatment, namely cold plasma surface treatment, was applied on crumb rubber to produce CRM asphalt binders. The high temperature performances of 12 CRM binders, produced with 6 plasma-treated or untreated crumb rubbers and 2 base asphalts, were investigated at their unaged and RTFO-aged state. The following conclusions could be drawn according to the results obtained from performance tests and statistical analyses: 1) Cold plasma surface treatment on crumb rubber generally resulted in a significant enhancement of high temperature performances, regardless of base asphalt source, crumb rubber size and aging state. It could be attributed to the improved compatibility between plasma-treated crumb rubber particles and asphalt matrix. On the other hand, cold plasma surface treatment somehow degraded the workability of CRM binder, while it also improved its thermal storage stability as deduced. In addition, the phase angle and rotational viscosity of all unaged CRM binders exhibited a fair correlation based on the statistical analysis. 2) The effect of base asphalt source was mainly reflected in the comparison between two aging states. Concretely, the binders produced by base asphalt A tended to have similar high temperature performances to those produced by B at unaged states while performed better than the latter at RTFO-aged states. It could be attributed to the different thermal oxidation extent of base asphalt and level of rubber-asphalt interactions after the short-term aging. 3) With regards to crumb rubber size, binders produced by the oversized and undersized crumb rubbers were tended to have poorer high temperature performances than the medium-sized. The plasma-treated binders produced with -40 mesh crumb rubber were considered the most desirable, with the formation of optimal skeletal structure in CRM binder. In addition, there was a strong quantitative relationship between rotational viscosity, test temperature and crumb rubber size, based on the result of regression analyses.

Above all, it was found that the cold plasma surface treatment on crumb rubber modifier applied in this study has a significant effect on the improvement of high temperature performance of CRM binders. Furthermore, in this limited work, the combination of plasma surface treatment on -40 mesh crumb rubber modifier and base asphalt from source A were the most effective with respect to the high temperature rheological properties.

CRediT authorship contribution statement Jin Li: Data curation, Writing - original draft. Feipeng Xiao: Supervision, Investigation. Serji N. Amirkhanian: Methodology. 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. Acknowledgments This study was supported by the National Natural Science Foundation of China under grant numbers of 51578416 and 51861145402. The authors are also deeply grateful for the support from the Key Laboratory of Road and Traffic Engineering of Ministry of Education in Tongji University and National Institute of Clean-and-Low-Carbon Energy in Beijing. References [1] N.S. Mashaan, M.R. Karim, Waste tyre rubber in asphalt pavement modification, Mater. Res. Innov. 18 (suppl. 6) (2014). S6-6-S6-9. [2] X. Shu, B. Huang, Recycling of waste tire rubber in asphalt and portland cement concrete: An overview, Constr. Build. Mater. 67 (2014) 217–224. [3] D.L. Presti, Recycled tyre rubber modified bitumens for road asphalt mixtures: a literature review, Constr. Build. Mater. 49 (2013) 863–881. [4] T. Wang, F. Xiao, X. Zhu, B. Huang, J. Wang, S. Amirkhanian, Energy consumption and environmental impact of rubberized asphalt pavement, J. Cleaner Prod. 180 (2018) 139–158. [5] J. Li, F. Xiao, L. Zhang, S.N. Amirkhanian, Life cycle assessment and life cycle cost analysis of recycled solid waste materials in highway pavement: A review, J. Cleaner Prod. 233 (2019) 1182–1206. [6] M. Abdelrahman, S. Carpenter, Mechanism of interaction of asphalt cement with crumb rubber modifier, Trans. Res. Record: J. Trans. Res. Board (1661) (1999) 106–113. [7] A. Ghavibazoo, M. Abdelrahman, M. Ragab, Mechanism of crumb rubber modifier dissolution into asphalt matrix and its effect on final physical properties of crumb rubber-modified binder, Trans. Res. Record: J. Trans. Res. Board (2370) (2013) 92–101. [8] J. Wang, J. Yuan, F. Xiao, Z. Li, J. Wang, Z. Xu, Performance investigation and sustainability evaluation of multiple-polymer asphalt mixtures in airfield pavement, J. Cleaner Prod. 189 (2018) 67–77. [9] O. Xu, P.R. Rangaraju, S. Wang, F. Xiao, Comparison of rheological properties and hot storage characteristics of asphalt binders modified with devulcanized ground tire rubber and other modifiers, Constr. Build. Mater. 154 (2017) 841– 848.

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