Ground tire rubber thermo-mechanically devulcanized in the presence of waste engine oil as asphalt modifier

Construction and Building Materials 222 (2019) 588–600

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

Ground tire rubber thermo-mechanically devulcanized in the presence of waste engine oil as asphalt modifier Yue Li a,⇑, Aiqin Shen a, Zhenghua Lyu a, Shifeng Wang b, Krzysztof Formela c, Guangtai Zhang d a

Key Laboratory for Special Region Highway Engineering of Ministry of Education, Chang’an University, Xi’an 710064, Shaanxi, China Department of Polymer Science and Engineering, Shanghai Jiao Tong University, 200240 Shanghai, China c ´ sk University of Technology, Gdan ´ sk, Poland Department of Polymer Technology, Gdan d Architectural Engineering Institute, Xinjiang University, Urumqi 830047, China b

h i g h l i g h t s
GTR was thermo-mechanically devulcanized in the presence of waste engine oil.
The sol fraction of DGTR increases with the increase of WEO content.
Extrusion barrel temperature at 280 °C obviously degraded GTR.
Storage stability and processability of the DGTR modified asphalt was improved.
Properties at low temperature were evaluated by DMA.

a r t i c l e

i n f o

Article history: Received 7 December 2018 Received in revised form 23 April 2019 Accepted 20 June 2019 Available online 26 June 2019 Keywords: Extrusion Waste engine oil Ground tire rubber Recycling Modified asphalt

a b s t r a c t Cross-linked elastomers network is main limitation for industrial usage of ground tire rubber (GTR) as asphalts’ and road pavements modifier. GTR was thermo-mechanically devulcanized via extrusion in the presence of waste engine oil (WEO) at temperature ranges from 150 to 280 °C. Combined impact of WEO content and extruder barrel temperature on the change of cross-linked structure of degraded GTR (DTGR) was investigated through sol fraction measurements and thermogravimetric analysis. Fourier-transform infrared spectroscopic analysis, storage stability, viscosity, temperature susceptibility and rheological properties of asphalt modified with DGTR were studied via infrared spectrometer, conventional tests, viscosity, dynamic shear rheology, dynamic mechanical analysis. Fluorescence microscopy was also applied to observe microstructure and the interfacial interaction between DGTR and asphalt. The results shown that the sol fraction of DGTR increases with the increase of WEO content, which results in low dynamic viscosity and storage modulus of the modified asphalts. Barrel temperature strongly affects the form of DGTR and final properties of modified asphalt. Modified asphalts with DGTR at barrel temperature in 180 °C and 210 °C show better rheological properties and broader operating temperature than that of at 280 °C, which is related to degradation of polymer main chain. In addition, 30 % wt. of DGTR content resulted in significant improvement rheological properties and storage stability of modified asphalt. Moreover, it was observed that higher content (above 40 %wt.) of DGTR causes the deterioration of performance properties of modified asphalts, due to the excessive quantities of undissolved rubber particles in asphalt. Ó 2019 Elsevier Ltd. All rights reserved.

1. Introduction The rapid development of modern society has driven the fast growth of automobile industry, which simultaneously generated serious environmental issues. Waste tires disposal is one of the major concerns, and it’s estimated that about 1000 million tires ⇑ Corresponding author. E-mail address: [email protected] (Y. Li). https://doi.org/10.1016/j.conbuildmat.2019.06.162 0950-0618/Ó 2019 Elsevier Ltd. All rights reserved.

are discarded annually [1]. The disposal of waste tires took up the land resources and caused soil and air pollution as well [2]. One of the promising solutions to alleviate the situation, is application of waste tires as a modifier for asphalt. As indicated in many studies, it improves the anti-cracking, flexibility and ageing resistance properties of virgin asphalt [3–5]. However, ground tire rubber (GTR) modified asphalt (GTRMA) also had limitations in industrial applications over worldwide. This is related to the poor storage stability, processability, the unpleasant odor emitted

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during devulcanization and thermal decomposition of GTR [5–7]. To obtain a stabilized and more compatible GTRMA, researchers are compelling to find the way to break down the cross-linked network of rubber. Thus, various approaches have been extensively investigated [8,9]. In the present studies, it is reported that compared to the vulcanizated rubber, asphalt binders with devulcanizated rubber show changes in its properties like increased viscosity, increased softening point and reduced elasticity. The chain scission in waste rubber is responsible for the improvement storage stability and processing ability of modified asphalt. However, significantly reduced the modification effect with the increasing degradation degree of rubber [10]. Recently, a new promising devulcanization method using twinscrew extruder has been adopted extensively to obtained reclaimed rubber with very high quality [11,12]. Mouri et al. [13] proposed that twin-screw extruder could selectively break the chemical bonds in rubber by setting different extrusion conditions, such as screw configuration, feeding rate, barrel temperature or screw speed. Further research by Formela et al. [14–16] found that barrel temperature is the crucial factor affecting the final form of obtained reclaimed rubber. Moreover, Saiwari et al. [17,18] pointed out that twin-screw extruder could be considered as an effective devulcanization method for passenger tire rubber, and developed the optimal setting for extruder setup and screw configuration. However, previous studies mainly focus on devulcanization of rubber in high extruder temperature (in the range of 200– 300 °C). The degraded rubber has an excellent processability and compatibility with asphalt. In our previous work [19], twin screw extruder was used to produce lightly pyrolyzed rubber (LPR) at different pyrolysis temperatures. We observed that increasing temperature during pyrolysis leads to decreasing of molecular weight, which results in a corresponding excellent compatibility with asphalt. Nevertheless, higher barrel temperature not only deteriorated the mechanical properties of reclaimed rubber, but also generated higher amount of toxic gases. Shi et al. [20] found that reclaimed rubber with higher sol faction has relatively higher glass transition temperature and lower elongation modulus. Formela et al. [14] indicated that production of 1 kg/h of reclaimed rubber at a barrel temperature of 300 °C will generate 62,400 mg/h of toxic gases. Gagol et al. [21] investigated the volatile organic compounds (VOCs) emitted during GTR extrusion process at different barrel temperature conditions. The results showed that degradation degree of rubber affecting the amount of VOCs, and the concentration of VOCs increased with increasing temperature. Saiwari et al. [22] revealed that low temperature tended to selectively break the bonds of S-S, C-S instead of undesired scission of CAC bond in polymer main chain. This phenomenon has beneficial influence on the mechanical properties of reclaimed rubber. Additionally, Tao et al. [23] analyzed the relationship between gel fraction and tensile strength of reclaimed rubber, the result indicated that extrusion temperature should perform at reasonable low level for obtaining an optimum mechanical properties of the reclaimed rubber. However, lower temperature leads to an increase of screw torque, which may cause technological problems with GTR processing (including damage of the extruder) [16]. Thus, the processing oil or plasticizers was adopted to lower the energy consumption [24] at lower barrel temperature. Waste engine oil (WEO) is rich in aromatics, which could improve compatibility between GTR and virgin asphalt. WEO has similar molecular structure with asphalt binder [25]. Therefore, several attempts have been made to use WEO as partial replaced of asphalt. Liu [26,27] reported that WEO decreased viscosity value and enhanced the fatigue resistance of asphalt. Jia et al. [25] found the addition of WEO made asphalt softer and would decrease the high temperature grade of asphalt.

Subsequently, various researchers started to investigate the WEO as rejuvenator in improving performance of aged asphalt [28,29]. In this study, the function of WEO could be summarized in three aspects. Firstly, the pre-added WEO could plasticize GTR and cause partial scission of S-S chain, which is beneficial for low temperature devulcanization and recyclability improvement. Secondly, it plays a key role on lowering machine load, as plasticizers WEO improves polymer chains mobility with enhanced the processing of GTR. Thirdly, the addition of WEO reduces the viscosity of asphalt, which extend the potential applications for GTRMA. In this paper, one of our purpose is to increase the utilization of GTR and WEO by finding a pathway for more effective devulcanization of GTR. Another is to obtain an environmental friendly reclaimed rubber modified asphalt with balanced processability and mechanical properties based on recycled materials. The GTR was devulcanized by twin-screw extruder at low temperature with the presence of WEO. The combined impact of WEO content and barrel temperature during extrusion process on devulcanization degree and thermal stability of degraded GTR (DGTR) was investigated. Through the conventional, storage stability and rheological test, we also studied the DGTR modified asphalt (DRMA) properties as function of WEO content, barrel temperature and DGTR modifier content. Furthermore, for better understanding asphalt-DGTR interfacial interactions, the compatibility of modified asphalts was evaluated by fluorescence microscopy analysis. The presented work is a continuation of the authors’ research about industrial methods of waste tires recycling [19]. 2. Experimental 2.1. Materials GTR obtained by grinding of passenger car tires rubber, with average particles size 60 mesh was produced by Yinge Asphalt Co. (Jiangsu, China). It is well known that passenger car tires rubber is more difficult to dissolve in asphalt due to it contains more styrene-butadiene rubber compared with truck tire rubber [11,30]. In order to enhance the devulcanization process of passenger car tires rubber and lower the machine load, WEO was selected as plasticizer. WEO was obtained from vehicle service station, and the physical properties were recorded as following: dynamic viscosity at 25 °C was 56 mPa.s, density was 0.906 g/cm3, and flash point was 230 °C. Base asphalt (penetration grade AH-50) was provided from China Oil Corp. (Shanghai, China). Characteristics of base asphalt are presented in Table 1. 2.2. Preparation of DGTR and the modified asphalt 2.2.1. Preparation of degraded GTR (DTGR) The preparation of degraded GTR includes two steps: Firstly, WEO and GTR were mixed at mass ratio of 25%, 35%, 45%, and the mixture was put in the oven at 100 °C about 1 h for obtaining pre-treated GTR (PGTR). Subsequently, the mixture was poured out to feeding bin and started to extrude by using co-rotating twin screw extruder model ZE25A (Bernstorff GmbH, Germany). The extruder has 9 heating/cooling zones with a screw diameter of 25 mm and an L/d ratio of 41. Barrel temperature was selected as 150 °C, 180 °C, 210 °C, and 280 °C, the screw speed and throughput Table 1 Properties of asphalt AH-50 grade. Penetration (25 °C, 100 g, 5 s, 0.1 mm)

Softening point (°C)

Ductility (5 cm/ min, 25 °C, cm)

Viscosity (135 °C, Pa.s)

48.5

52.2

＞100

0.31

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remained constant at 150 rpm and 3 kg/h. DGTRs extruded at different barrel temperatures were coded as DGTR150, DGTR180, DGRR210 and DGTR280, respectively. Obtained DGTRs with different WEO content were denoted as W25DGTR, W35DGTR and W45DGTR, respectively. 2.2.2. Preparation of asphalt modified with GTR and DGTR Modified asphalt samples were prepared by melt-blending process. The asphalt was heated in an iron container at 170 °C until fluid, then added modifier to asphalt. High shear mixer was used to mix rubber and virgin asphalt at the speed of 3000 rpm for 50 min. GRT and DGTR modified asphalt were denoted as GTRMA and DRMA, respectively. According to the difference of DGTR, the DRMAs were named as follows. The three WEO content of DGTR modified asphalt were denoted as W25DRMA, W35DRMA and W45DRMA; the four barrel temperature of DGTR modified asphalt were named as DRMA150, DRMA 180, DRMA 210 and DRMA 280; the three amount of DGTR modified asphalt were coded as in DRMA20, DRMA30 and DRMA40. Abbreviations list is presented in Table 2, and detailed data of rubber and modified asphalt samples listed at Table 3. 2.3. Measurements To evaluate the devulcanization degree and thermal stability of different DGTRs and the properties of modified asphalt, various tests were conducted. Fig. 1 summarized the experimental program of this study. 2.3.1. Sol fraction Sol fraction was used to evaluate the devulcanization degree of DGTRs with different extrusion recipe and parameters. Sol fraction was determined from a swelling test. Samples of DGTRs (around 0.2 g) were swollen in toluene for 72 h (room temperature). Sol fraction was calculated as mass difference of samples before swelling (m1), then extracted samples were dried in a vacuum oven at 80 °C until the weight (m2) became constant. The sol fraction was calculated according to Eq. (1):

Table 2 Abbreviations list of samples. Abbreviations

Full titles

GTR WEO DGTR GTRMA DRMA PGTR

Ground tire rubber Waste engine oil Degraded ground tire rubber Ground tire rubber modified asphalt Degraded ground tire rubber modified asphalt Pre-treated ground tire rubber

Sol fraction ¼

m1  m2  100% m1

ð1Þ

2.3.2. Thermogravimetric analysis Thermogravimetric analysis (TGA) was used to evaluate the changes of chemical structure of GTR and DGTRs. TGA was performed in a nitrogen atmosphere by using Q5000IR from TA Instruments, Inc. Test temperature ranged from 30 to 800 °C at a heating rate of 10 °C/min. 2.3.3. Fourier transforms infrared (FTIR) The changes in chemical functional groups of asphalt with different DGTRs were acquired by using FTIR Spectrometer (Avatar370, Nicolet, USA) with the wavenumber ranges from 500 cm1 to 4000 cm1 at a resolution of 4 cm1. 2.3.4. Conventional tests The conventional tests which conducted in modified asphalt were as follows: penetration test, softening point, ductility, viscosity and storage stability, which were carried out according to the Chinese specification of GB/T0604-2011, GB/T0606-2011, GB/ T0605-2011, GB/T0625-2011 and GB/T0661-2011, respectively. The storage stability of modified asphalt was carried out by tube test. After preparing the modified asphalt, poured blend into a standard aluminum tube (25 mm diameters and 140 mm heights) in an amount of about 50 g and sealed tube with pliers. Then placed the tubes vertically in the oven at 163 °C for 48 h. Further, took out tubes and cooled in the refrigerator more than 4 h. Subsequently, cut frozen tubes into three equal sections and tested the softening point of asphalt from top and bottom sections. 2.3.5. Dynamic shear rheological property test Dynamic shear rheological property test was performed to evaluate the high temperature rheological properties of modified asphalt by using dynamic shear rheometer (Gemini 200HR, Bohlin Instruments, UK). Test temperature ranged from 40 to 90 °C with the heating rate of 3 °C/min. Loading frequency and strain were 10 rad/s and 1.0%, respectively. 2.3.6. Dynamic mechanical analysis The low-temperature performance of modified asphalt was characterized by the storage modulus, loss modulus and tan d from dynamic mechanical analysis at temperature ranged from 60 °C to 5 °C. The DMA apparatus (Q800, USA) was manufactured by TA Instruments, Inc. Samples were measured at frequency of 10 Hz with a strain of 0.01% in single cantilever mode. The modified asphalt was heated to 170 °C until fluid, then slowly dumped on the oil paper. After about 5 min for fluid solidification, flatten asphalt into 2 mm sheets with wooden strips and

Table 3 Detailed data of samples. Sample Code

WEO content

Processing method

Barrel temperature

Corresponding modified asphalt sample

Rubber content

GTR PGTR W25DGTR W35DGTR W45DGTR DGTR150 DGTR180 DGTR210 DGTR280 DRMA20 DRMA30 DRMA40

– 35% 25% 35% 45% 35% 35% 35% 35% 35% 35% 35%

Neither P nor E P P and E P and E P and E P and E P and E P and E P and E P and E P and E P and E

– – 150 °C 150 °C 150 °C 150 °C 180 °C 210 °C 280 °C 150 °C 150 °C 150 °C

GTRMA – W25DRMA W35DRMA W45DRMA DRMA150 DRMA180 DRMA210 DRMA280 DRMA20 DRMA30 DRMA40

20%

P represents the pre-treatment process, E represents extrusion process.

20% 20% 20% 20% 20% 20% 20% 20% 30% 40%

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Fig. 1. Flow chart of the experimental program.

2.3.7. Morphological analysis Fluorescence microscope can be used to observe the microstructure of polymer material without destroying the structure of polymer phase in asphalt [31,32]. In order to avoid the settlement of rubber particles in asphalt, freezing fracture method [33] was adopted to make observation of samples. The specific steps were as follows: modified asphalt was poured onto the oil paper and then rolled into a cylindrical shape. The samples were placed horizontally in the refrigerator at 25 °C for 5 h. Finally, the samples were broken into pieces quickly and put on the slide to observe the microstructure. The dispersion of GTR and asphalt was implemented by fluorescence microscopy Leica DM4500. 3. Results and discussion

of rubber, PGTR was compared with GTR and W35DGTR, respectively. Obviously, the sol fraction of PGTR has a sharp increment in comparison to GTR. Extrusion at 150 °C causes slight decrease of devulcanization degree of rubber. The sol fraction of W35DGTR is lower than that of PGTR at the same WEO content (35%). Above mentioned observations illustrate that as a plasticizer, WEO performs well in breaking bond in rubber. However, the shear force

WEO content

barrel temperature 45.67

40 Sol fraction(%)

placed the samples in the refrigerator at 20 °C about 2 h. Finally, cut the asphalt pieces into strips with the dimensions of 40  40  2 mm3.

24.09

24.46 23.77 22.91 23.77

26.23 27.25

20

3.1. Sol fraction of different DGTRs To investigate the effect of WEO content and barrel temperature on the devulcanization, the WEO content and barrel temperature were varied from 25% to 45%, and 150 °C to 280 °C, respectively. The sol fraction of GTR, PGTR and different DGTRs are presented in Fig. 2. PGTR is obtained at pretreatment process with 35 %wt. WEO content and without extrusion. Aiming to clearly understand the influence of plasticizers or extrusion process on the sol content

8.45

0 R GT

PG

TR 5 W2

80 50 10 80 TR GTR TR R1 R1 R2 R2 DG 35DG 5D DGT DGT DGT DGT 4 W W

Fig. 2. Sol fraction of GTR, PGTR and DGTRs.

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at 150 °C is insufficient for higher scission of cross-linked bonds in GTR, and some S-S bonds breaking down during the pretreatment process. Secondary cross-linking of DGTR occurs in the same time, which is confirmed by the decrease of sol fraction presents in W35DGTR. Further, increase content of WEO that applied during pretreatment of DGTR causes increase of the sol fraction. However, the increasing rate of sol content with WEO content from 25% to 35% is higher than that of 35% to 45%. The reason mainly due to WEO has positive effect on swelling GTR and breaking partial S-S bonds. But excessive WEO content reduces the friction between rubber particles and the screw, weakens the shear force and thus lowers the devulcanization degree of GTR [20]. As could be expected, DGTR obtained with higher barrel temperature shows higher sol fraction, which is related to the breakage degree of cross-linked structure of DGTR. Compared to other DGTRs, the sol fraction of DGTR280 shows a noticeable increase, indicating a destructive scission of main chain of rubber. 3.2. TGA of different DGTRs Thermogravimetric analysis (TGA) was used to evaluate the thermal stability of rubber with respect to different extrusion recipe and parameters. The results of TGA and derivative thermogravimetry analysis (DTG) of GTR, PGTR and DGTRs are shown in Figs. 3–5. Fig. 3 presents the TGA and DTG curve of GTR with different processing method at the same WEO content, and the consequence of WEO thermal degradation. Compared to W35DGTR, PGTR presents the different peak position in DTG nearly 200 °C and 600 °C, respectively. Through analyzing TGA and DTG curve of WEO and results from previous studies, it can be deduced that the first mass loss of PGTR at 200 °C supports to be connected with the presence of volatiles components in WEO, and the multiple peaks at temperature range from 550 °C to 650 °C probably due to the presence of oil degraded products [34] and emission organic compounds from carbon black (CB) [35,36]. From DTG curve of WEO, it is also worth noticing that the mass loss of WEO starts from 168 °C, and when temperature ranges from 168 °C to 366 °C, the rate of loss increasing with the increase of temperature. These phenomenons imply that WEO does not decompose when barrel temperature is 150 °C, and raising temperature could accelerate the thermal motion of WEO molecule, which is beneficial for rubber devulcanization in extruder. For DTG curve of DGTR, it presents three peaks position of mass loss. The first peak appears at temperature 390 °C (Tmax1) is related to the presence of natural rubber (NR). Second peak at 440 °C

100

(Tmax2) is due to the decomposition of styrene-butadiene rubber (SBR) [37]. The third peak at around 590 °C (Tmax3) can be attributed to the thermal degradation of CB. It can be further noted that the decomposition temperature of NR, SBR and CB are quite similar in PGTR and W35ATGR, which may due to the similar sol fraction content. In comparison to TGA curve presented in Fig. 4, found that all these TGA curves show similar trend regardless of WEO content. The Tmax1 and Tmax2 of W45DGTR are slightly lower than that of W25DGTR and W35DGTR. This indicates that the degradation degree of polymer chain in W45DGTR is higher than that of W25DGTR and W35DGTR, which confirms lower thermal stability of this sample. Furthermore, from Fig. 5(a) and (b), it can be deduced that an increment in barrel temperature leads to an increase of weight loss at test temperature ranges from 370 °C to 560 °C. However, the TGA curve of DGTR280 shows an opposite trend. The weight loss of DGTR280 is approximately 15.3 %wt., which is much lower than DGTR210 at 51.7 %wt. The sharp decrease of weight loss is related to the lower content of rubber matrix after thermal treatment in extrusion at relatively higher temperature. This also indicates that cross-linked network of DGTR280 is basically destroyed and the chains are mostly formed into soluble fraction, which could be extracted by toluene. This fact is consistent with sol fraction results of DGTRs.

3.3. Fourier-transform infrared spectroscopy analysis The FTIR spectra of WEO, GTRMA, PGTRMA and DRMA150 are shown in Fig. 6(a). As it depicted, the characteristic absorption bands of WEO, GTRMA and PGTRMA are basically consistent, which strongly indicates that there is mainly physical interaction occur between WEO and rubber at pre-treatment process. These similar chemical bonds at 2934 cm1 and 2854 cm1 for symmetric and asymmetric vibrations of C–H, 1624 cm1 for cC=C stretching, 1457 cm1 and 1384 cm1 for dC-H bending, 1115 cm1 for cC-O stretching. Meanwhile, compared to GTRMA and PGTRMA, modified asphalt with extruded rubber results in disappearance of peak position located at 1262 cm1 and 1030 cm1, corresponding to esters molecules [38] and S=O bonds. This phenomenon can be reasonable explained as the radicals generated by the decomposition of WEO re-combined with sulfur-containing radicals. Accordingly, the weaken absorption peaks at 791 cm1 are observed at DRMA150 in comparison with WEO and PGTRMA probably due to the partial decomposition of WEO.

100 W25DGTR W35DGTR W45DGTR

1.2 80

1.0

40

0.4

Weight(%)

Weight(%)

0.6

WEO GTR PGTR W35DGTR

0.6 60

420

440

460

480

0.4 40

0.2 20

0.2 20

0.0 0

0

100

200

300

400

500 Temperature(°C)

600

700

-0.2 800

Fig. 3. TGA and DTG curve of GTR with different conditions of devulcanization.

0.0

0 0

100

200

300

400

500

600

700

800

Temperature(°C) Fig. 4. TGA and DTG curve of DGTRs with different WEO content.

Deriv.weight(%)

60

Deriv.weight(%)

0.8 410 420 430 440 450 460 470 480

80

0.8

593

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100

2934

20

0

200

400

600

800

Temperature(°C)

4000

3500

1115

WEO GTRMA PGTRMA DRMA150

DGTR150 DGTR180 DGTR210 DGTR280

1624 1457 1384

40

0

1030

1262

60

791

2854

Weight(%)

80

3000

2500

2000

1500

1000

500

Wavenumber(cm-1)

Fig. 5a. TGA curves of DGTRs with different barrel temperature. Fig. 6a. FTIR analysis of modified asphalt with different devulcanization process.

1.6 1.4

1.0

420

430

440

450

460

470

480

0.8

DGTR150 DGTR180 DGTR210 DGTR280

0.6 0.4 0.2

DRMA280 DRMA210 DRMA180 DRMA150

0.0 0

200

400

600

835

Deriv.weight(%)

1.2

800

Temperature(°C) 4000

3500

3000

2500 2000 1500 Wavenumber(cm-1)

Fig. 5b. DTG curves of DGTRs with different barrel temperature.

3.4. Convention tests The effects of GTR devulcanization conditions, such as WEO content, barrel temperature and DGTR modifier content on the properties of modified asphalt are presented and discussed as follows. 3.4.1. Penetration, softening point and ductility tests Penetration reflects the consistence and stiffness of asphalt. The higher penetration value means that the asphalt becomes softer and has lower possibility to crack. As can be seen in Fig. 7(a), the penetration of DRMAs increase continuously with an increase of WEO content. The sharp increase in asphalt penetration at 45 % wt. WEO content in DRMAs. As for barrel temperature, Fig. 7(a) clearly shows that changing temperature from 150 °C to 180 °C decreases the penetration value. However, with barrel temperature

500

Fig. 6b. FTIR analysis of modified asphalt DRMAs with different barrel temperature.

80

WEO content

barrel temperature DGTR content

70

67.1

60

Penetration(0.1mm)

The FTIR spectra of modified asphalts with GTR degraded at different barrel temperature are presented at Fig. 6(b). It can be noticed that the peak at 835 cm1 only appears in DRMA150 compared with other samples, which proves the presence of substituted benzene (–CH=CH2–) in asphalt. This result confirms that an increase in barrel temperature leads to an increased breakdown degree of crosslinks, which is associated with the degradation level based on the results of sol fraction.

1000

56.4

50 42.1 40

45.7

54.2 45.7

45.7 38.9

45 37.2

30 28.1 20 10 0

A A A 0 0 0 0 A 0 0 0 RM RM DRM RM A15 A18 A21 A28 MA2 MA3 MA4 5D 3 5 45D R M R M R M R M DR D R DR 2 D D D D W W W

GT

Fig. 7a. Penetration of GTRMA and DRMAs.

increases from 180 °C to 280 °C, the penetration increase. This implies asphalt binder with DGTR obtained at higher temperature

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is much softer than those prepared at lower temperatures. In addition, DRMAs with higher DGTR content show lower penetration. It may relate to the quantities of undissolved rubber particles. Softening point measures the high temperature deformation resistance properties of asphalt. The softening point of asphalt decreases with the addition of WEO. From Fig. 7(b), it’s also worth noticing that DRMA180 has the highest softening point, while DRMA280 yields the lowest softening point among the DRMAs. This indicates the scission degree of cross-linked chains in rubber, higher degradation makes a lower cross-link density of rubber and reduces its elastic proportion. Additionally, it can be observed that the softening point increases with the increase of DGTR content, which implies that higher rubber content strengthen the framework in asphalt. Ductility at 5 °C is often used to evaluate the anti-cracking properties of asphalt at low temperature. As shown in Fig. 7(c), the ductility of asphalt binder has been significantly improved with the addition of DGTR, especially with DGTR obtained at higher WEO content and barrel temperature. This results also indicate that increasing WEO content or barrel temperature both improve the plasticity of asphalt binder. Furthermore, the ductility of

90

WEO content

80

barrel temperature DGTR content

Softening point( )

70 69.7 60.9 60.1

60

56.7

60.1

64.6 65.2

63.2 61.2

60.1 52.3

50 40 30 20 10 0

GT

A A A 0 0 0 0 A 0 0 0 RM RM DRM RM A15 A18 A21 A28 MA2 MA3 MA4 5D 35 45D RM RM RM RM DR DR DR 2 D D D D W W W Fig. 7b. Softening point of GTRMA and DRMAs.

DRMA30 is higher than that of DRMA20, which confirms a stronger interconnection among DGTRs with higher rubber content. But further increment of DGTR content decreases the ductility, which caused by the weak network due to higher content of undissolved rubber particles. 3.4.2. Viscosity and viscosity-temperature sensitivity The relationship between viscosity and temperature of modified asphalt is presented in the Fig. 8. As can be seen in Fig. 8, for any given samples, the viscosity decreased with temperature increasing. As evident in Fig. 8(a), the addition of WEO leads to drop in viscosity due to the increase of light components in asphalt. Compared blend’s viscosity of DRMA150, DRMA210 and DRMA280, it’s can be found that barrel temperature increasing causes corresponding decrease in viscosity. However, DRMA180 is the exception for this trend. Elevated temperature accelerates the mobility of hydrocarbon molecules present in WEO, which has a positive effect on swelling rubber, and, consequently, an increment of viscosity achieved at DRMA180. Rising barrel temperature to 210 °C causes the decomposition of WEO and degradation of DGTR, resulting in viscosity reduction. The viscosity presents a noticing increase while rubber content comes up to 40%, which consequently leads to worse processing. Fig. 9 shows the viscosity of all modified asphalt measured at 180 °C. As can be seen, W45DRMA, DRMA210 and DRMA280 have viscosity lower than 1.0 Pa.s at 180 °C, which implying an excellent processing performances of rubber modified asphalt. Consequently, combined with conventional tests results, it is clear that DRMA210 is suitable for industrial production and application due to its balanced performance and processing ability, whereas W45DRMA and DRMA280 present a remarkable loss in hightemperature properties. This result must be related to the excessive WEO content and high degradation degree of DGTR. The temperature susceptibility is the crucial factor to evaluate the performance of asphalt binder and mixture. The viscoelastic properties of the asphalt are closely related to temperature. Lu and Isacsson [39] pointed out that the lower temperature susceptibility is beneficial for asphalt binder to resist the deformation and crack at low and high temperatures, respectively. To gain a better understanding of the relationship between viscosity and temperature of each DRMAs, the regression between viscosity and temperature for asphalt binder was studied based on the double-logarithmic linear fitting, which is presented in Eq. (2):

lgðgÞ ¼ a  blgT 18 16

WEO content

barrel temperature DGTR content 14.7

14.7

14

13.2

Ductility(cm)

12

10.8

10.1 10.2 10.1

10

10.1

9.1

8 5.8

6 4 2 1.1 0

GT

A A A 0 0 0 0 A 0 0 0 RM R M D R M R M A 1 5 A 1 8 A 2 1 A 2 8 M A 2 M A 3 M A 4 5D 35 45D RM RM RM RM DR DR DR 2 D D D D W W W Fig. 7c. Ductility of GTRMA and DRMAs.

ð2Þ

Where g is viscosity, Pa.s, a is the intercept of the curve, b is the slope of the curve, T is temperature, °C. Fig. 10 gives the relationship between viscosity and temperature of DRMAs with different WEO content, barrel temperature and DGTR content. With increasing WEO content, the GTRMA has flatter slope than DRMAs. The slope of curve becomes more stepper, which suggests that DRMAs show higher temperature susceptibility than GTRMA. These results contribute to a change in the chemical component of asphalt. Addition of WEO has significant influence on the aromatic component of asphalt, hence DRMAs have a relatively lower viscosity at high temperature for a better workability. Additionally, the inclusion of DGTR obtained at high temperature leads to corresponding drop in slope of DRMAs. The b value of DRMAs are almost equal in the temperature range of 150–210 °C, whereas it reduces sharply when the temperature reaches 280 °C. According to Shi et al. [20], strong shear force leads to scission of main chain, and results in a dramatic decrease of viscosity of reclaimed GTR. Indeed, higher barrel temperature causes seriously broken degree of main chain in rubber, and the asphalt with highly degraded rubber has a relatively lower viscosity at

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Y. Li et al. / Construction and Building Materials 222 (2019) 588–600 12 GTRMA W35DRMA

10

DRMA150 DRMA210

10

W25DRMA W45DRMA

30

DRMA180 DRMA280

DRMA20

DRMA30

DRMA40

25

6 4

Viscosity (Pa.s)

Viscosity (Pa.s)

Viscosity (Pa.s)

8 8

6

4

2

2

1.0Pa.s Line

1.0Pa.s Line 135

15 10 5 1.0Pa.s Line

0

0 120

20

150

165

Temperature(

180

120

135

150

Temperature(

)

165

180

)

(b)Modified asphalt with different barrel temperature

(a) Modified asphalt with different WEO content

0

120

135

150

Temperature(

165

180

)

(c)Modified asphalt with different DGTR content

Fig. 8. Viscosity-temperature curves of modified asphalt.

14

180

Viscosity(Pa.s)

13.1

3.6

12 1.75 1.31

1.3 1.08

1.08

1

1.08

0.98

0.64 0.2 0

A 0 0 0 A 0 A 0 0 0 A RM RM DRM RM A15 A18 A21 A28 MA2 MA3 MA4 5D 35 45D DRM DRM DRM RM DR DR R 2 D D W W W

GT

Fig. 9. Viscosity of GTRMA and DRMAs at 180 °C.

high temperature, which also suggests less temperature susceptibility of asphalt. It is also worth noting that DRMA180 has the lowest b values in comparison with other DRMAs. Combined with the results of sol fraction and TGA, it could be analyzed that 180 °C is a little higher than thermal decomposition temperature of WEO, and promotes the molecular thermal motion of WEO. Hence further swelling occurs during extrusion process, which also results in a good interaction between DGTR modifier and asphalt. In other words, DRMA180 would exhibit better performance at low and high temperature than other DRMAs. Furthermore, with DGTR content increasing from 20% to 40%, the curves become much flatter. This indicates higher potential possibility in response to permanent deformation or crack of asphalt binder at increased DGTR content.

3.4.3. Storage stability The instability of waste rubber modified asphalt storage at high temperature is mainly related to the difference of composition and density between rubber and asphalt [40,41]. With the purpose of evaluating effect of pretreatment process, barrel temperature and DGTR modifier content on the improvement of storage stability of modified asphalt, tube test was adopted. Test results are presented in Fig. 11. It can be found that the value of softening points difference between top and bottom (4S) of W25DRMA and W45DRMA are higher than GTRMA, especially at high content of

WEO like W45DRMA, the value of 4S is twice times higher than that of GTRMA. The reason is related with content of plasticizer. Insufficient WEO content cannot fully swell rubbers and cause a poor effect on further improvement of degradation degree of rubbers. While excessive oils concentration leads to an increase in asphalt liquidity (reduced viscosity), which is convenient for rubber particles going down at high temperature. In terms of barrel temperature, we can see that higher barrel temperature leads to the decrease of 4S value. DRMA280 has the lowest 4S value than any other modified asphalt. It also can be attributed to the relationship between barrel temperature and rubber particles size. Navarro et al. [41] found that asphalt binder contains rubber particles with smaller size is more stable than this with larger one. Nevertheless, the rubber size is significantly influenced by extrusion temperature, higher temperature decreases rubber size into micro scale due to significant destruction of the cross-linked network of rubber [42]. Therefore, DRMA280 contains the smallest size of rubber, what reduces effectively phase separation of modified binder. As can be seen, higher DGTR modifier content helps for achieving better storage stability of modified asphalt. When the content of DGTR increasing at 40 %wt., the swollen rubber content and viscosity of asphalt increase accordingly. According to the Fig. 9, the viscosity of DRMA30 and DRMA40 which tested at 180 °C is far more than other DRMAs. Hence decrease in 4S value and increase in stability of asphalt could be expected. This results reveal that besides particle size and degradation degree of rubber, the storage stability of modified asphalt is greatly affected by viscosity of asphalt. Nevertheless, further increased DGTR is proved to have a counter-productive effects on improving storage stability of asphalt. This is in accordance with the trend dependence on rubber content described in the literature [43,44]. 3.5. DSR analysis The experimental test results of different modified asphalt are listed in Table 4. As can be observed, G*/sin d value of DRMAs gradually decrease with an increase of WEO content, and the G*/sin d value of W35DRMA, W45DRMA are lower than 1.0 kPa at 88 °C. Meanwhile, the phase angle of DRMAs are inversely related to the G*/sind value with respect to higher WEO content at various temperature. It is found that increasing WEO content leads to an increase of phase angle in whole range of test temperature. Low G*/sin d value and high phase angle are the evidence for unsatisfactory properties of DRMAs with higher WEO content at high temperature, which mainly due to the addition of WEO decrease the viscosity and softening point of asphalt binder [45].

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1.2 1.0

0.6 WEO content increase

0.4

0.6

lgη(Pa.s)

lgη(Pa.s)

DRMA180 DRMA280

0.8

0.8

0.4 0.2

0.2 0.0 -0.2

GTRMA

W25DRMA

W35DRMA

W45DRMA

a

10.431

11.358

12.134

12.616

b

4.523

5.011

5.411

5.687

R2

0.996

0.991

0.994

0.998

0.0 -0.2

DRMA150 DRMA210

1.0

GTRMA W25DRMA W35DRMA W45DRMA

-0.6

a

12.134

12.009

11.911

11.767

b

5.411

5.305

5.308

5.521

R2

0.994

0.998

0.995

0.996

-0.8 2.06 2.08 2.10 2.12 2.14 2.16 2.18 2.20 2.22 2.24 2.26 lgT( )

2.06 2.08 2.10 2.12 2.14 2.16 2.18 2.20 2.22 2.24 2.26 lgT(

DRMA150 DRMA180 DRMA210 DRMA280

-0.4

)

(a)

(b) rubber concent increase

1.4 1.2

lgη(Pa.s)

1.0 0.8

DRMA20 DRMA30 DRMA40

0.6 0.4

DRMA20

DRMA30

a

12.134

8.181

5.747

b

5.411

3.388

2.075

R2

0.994

0.994

0.987

0.2 0.0

DRMA40

-0.2 2.06 2.08 2.10 2.12 2.14 2.16 2.18 2.20 2.22 2.24 2.26 lgT( )

(c) Fig. 10. Regression analysis between viscosity and temperature of DRMAs with (a) different WEO content; (b) different barrel temperature; (c) different DGTR content.

decreasing, which indicates that high shear force could clearly reduce the elastic proportion of rubber. Particularly when temperature is 280 °C, G*/sin d value of DRMA280 even lower than 1.0 kPa at 80 °C, representing a greater rutting possibility. It should also be pointed out that, among the DRMAs with various barrel temperature, DRMA180 has the highest value of G*/sin d at temperature from 56 °C to 88 °C. This phenomenon could be explained by the highest viscosity of asphalt binder when compared to other DRMAs. With the increase of DGTR content, the G*/sin d of DRMA first increase at maximum and then decrease slightly, while phase angle show opposite change.

80 Top

Bottom

S=Bottom-Top

Softening point (

)

70 60 50 15.5

10

7.8

8.8

7.4

7.4

7.4

5.7

3.6. DMA analysis

3

0

0.7

0.9

1.3

A A A 0 A 0 0 0 2 8 0 20 30 15 18 21 A A A4 RM RM RM M GT 25D 35D 45DR RMA RMA RMA RMA DRM DRM DRM D D D D W W W Fig. 11. Storage stability of GTRMA and DRMAs.

In addition, at a constant WEO (35%) and DGTR (20%) concentration, the various high temperature rheological properties of DRMAs as function of extruder barrel temperature are shown in Table 4. The G*/sin d value of DRMAs is found to almost increase while the phase angle decrease with barrel temperature

In order to assess the dynamical mechanical properties of different DRMAs at low temperature range, the DMA method was selected. The rheological parameters, namely, storage modulus, loss modulus and tand of all modified asphalts were obtained in the range of 60 °C 5 °C at 10 Hz. The obtained results are shown in the Fig. 12. 3.6.1. Storage modulus Storage modulus measures the stiffness of asphalt binder. As can be seen in Fig. 12(a), storage modulus decreases with the increase of temperature. Temperature increment decrease the inherent stiffness of binder. It is noteworthy that the storage

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Y. Li et al. / Construction and Building Materials 222 (2019) 588–600 Table 4 Results of DSR test for different modified asphalt.

Storage Modulus(Pa)

GTRMA W25DRMA W35DRMA W45DRMA DRMA150 DRMA180 DRMA210 DRMA280 DRMA20 DRMA30 DRMA40

G*/sin d (kPa)

phase angle (°)

56 °C

64 °C

72 °C

80 °C

88 °C

56 °C

64 °C

72 °C

80 °C

88 °C

40.835 21.998 16.803 11.811 16.803 27.966 13.041 7.767 16.803 20.834 18.808

17.233 10.598 7.17 4.998 7.17 11.702 5.338 2.891 7.17 10.528 10.108

8.55 4.761 3.527 2.209 3.527 5.851 2.644 1.336 3.527 6.042 6.138

4.381 2.251 1.629 1.144 1.629 2.775 1.243 0.584 1.629 3.289 3.612

1.994 1.14 0.828 0.573 0.828 1.504 0.648 0.292 0.828 1.941 2.186

58.51 60.71 62.39 66.33 62.39 60.05 68.22 76.23 62.39 54.75 50.79

60.34 63.17 64.86 67.98 64.86 63.06 69.41 77.91 64.86 55.67 51.95

63.55 66.81 68.76 70.69 68.76 66.03 71.34 79.54 68.76 57.63 53.95

66.61 70.07 72.63 73.28 72.63 66.79 73.29 80.19 72.63 59.76 55.81

67.92 72.04 75.27 74.95 75.27 64.64 74.59 79.17 75.27 61.14 56.94

(1)BA (2)GTRMA (3)W25DRMA (4)W35DRMA (5)W45DRMA

1E9

1E8

-60

-40

-20

Temperature( (a)

(1) DRMA150 (3) DRMA210

Storage Modulus(Pa)

Type of asphalt binder

1E9

1E8

-60

0

(2) DRMA180 (4) DRMA280

-50

-40

)

-30

-20

Temperature(

-10

0

)

(b )

Storage Modulus(Pa)

(1) DRMA20 (2) DRMA30 (3) DRMA40

1E9

1E8

-60

-50

-40

-30

-20

Temperature(

-10

0

)

(c) Fig. 12. Storage modulus of DRMAs with (a) different WEO content; (b) different barrel temperature; (c) different DGTR content.

modulus of GTRMA is much higher than that of base asphalt (BA) at the temperature ranges from 60 °C to 5 °C, implying that the addition of rubber greatly improves the stiffness of asphalt at low temperature. Meanwhile, the storage modulus of DRMAs are much lower than that of GTRMA. It is due to the WEO rich in aromatic components, which improve the flowability of asphalt. The storage modulus of DRMAs decrease with the increase of WEO content, especially when WEO content is 45 %wt. This demonstrates a better ductility and anti-cracking properties of asphalt at low temperature. Fig. 12(b) illustrates the storage modulus of DRMAs as a function of temperature. It is observed that DRMA180 shows higher

value of storage modulus than those of other DRMAs. This is related to the different degradation degree of DGTR. The residence time of rubber in the extruder is very short during extrusion process. Rubber with high degradation degree could hardly be obtained at lower barrel temperature. The temperature increasing from 150 °C to 180 °C accelerates WEO penetrating into rubber, which improves swelling effect and devulcanization reaction. Thus, DRMA180 containing more swollen rubber particles lead to an increment of elastic proportion and viscosity in asphalt. However, temperature at 210 °C and 280 °C could lead to main chain scission and result in an increase of asphalt plasticity. It also worth noticing that the storage modulus decreases with DGTR content increase at

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Y. Li et al. / Construction and Building Materials 222 (2019) 588–600

temperature ranges from 30 °C to 5 °C, which increases the flexibility of asphalt.

DGTR exhibits a relatively higher viscosity and better damping properties at low temperature.

3.6.2. Loss modulus and tan d of different DRMAs The glass transition temperature (Tg) is assigned to the maximum value of loss modulus as a function of temperature. Asphalt with lower Tg indicates a better low temperature performance. Tan d is a ratio of loss modulus to storage modulus, which indicates the viscoelastic proportion of asphalt binder. Fig. 13 shows that the loss modulus and tan d for BA, GTRMA and DRMAs of the various WEO content, barrel temperature and DGTR modifier content. The test results of BA and GTRMA appear that the inclusion of GTR greatly reduced the value of Tg, indicating a broader operating temperature range of asphalt. Compared to Tg and tan d of GTRMA, the presence of WEO increases the value of Tg and decreases the tand. Thus, it seems that asphalt with more WEO content has negligible effect on Tg and behaves more elasticity at temperature higher than Tg. Moreover, DRMA280 has noticeably higher Tg value than that of other DRMAs, which was caused by the large amount of main chain scission in rubber and the released carbon black in asphalt [35]. It also can be found from Fig. 13(c) that an increased DGTR modifier content results in an increase of Tg and tan d value, DRMA40 shows the lowest Tg value (39.8 °C) among all the DRMAs. The result indicates that asphalt with higher content of

3.7. Fluorescence microscopy of different DRMAs Fluoresce microscopy was used to observe the dispersion state of DGTR in asphalt matrix and evaluate the compatibility between DGTR and asphalt. Morphologies of GTRMA and DRMAs are shown in Fig. 14. There are two separate phases presented in these micrographs, and the rubber particle appears black while asphalt appears yellow or green. DGTR is dispersed irregularly in a dark asphalt phase. Comparison of GTRMA with DRMAs, a clear change happened in rubber size and dispersion state of modified asphalt through thermo-mechanical devulcanization. The diameters of rubber particles containing in GTRMA are larger than 200 lm, which resulted in a severe sedimentation of DGTR modifier in asphalt. However, degradation enables a partial scission of chemical bonds, which facilitates DGTR dispersion in asphalt. From images of W25DRMA, W35DRMA and W45DRMA, it can be noted that there is no significant difference in size of DGTRs with various WEO content of DRMA. Meanwhile, as seen from images of DRMA with DGTR obtained at different barrel temperature, the size of DGTRs have a prominent decrease with increase of barrel temperature, indicating a better compatibility and

0.8

0.8 (1) DRMA150 (3) DRMA210

0.6

1E8

0.4

Tg(

-60

0.2

1 2 3 ) -16.7 -33.8 -30.3

-40

4 -30.7

5 -27.3

-20

Temperature(

1E8 Tg(

0.4

1 2 3 4 ) -30.7 -38.9 -37.4 -25.3

0.2

0.0 -60

0

-40

-20

Temperature(

)

(a)

0

0.0

)

(b)

(1) DRMA20

(2) DRMA30

1.0

(3) DRMA40

1E8

Tg(

1 ) -30.7

2 -38.2

0.6

3 -39.8

0.4

tanδ

Loss Modulus(Pa)

0.8

0.2

1E7 -60

-50

-40

-30

-20

Temperature(

-10

0

0.0

)

(c) Fig. 13. Loss modulus and tan d of DRMAs with (a) different WEO content; (b) different barrel temperature; (c) different DGTR content.

tanδ

(1)BA (2)GTRMA (3)W25DRMA (4)W35DRMA (5)W45DRMA

Loss Modulus(Pa)

0.6

tanδ

Loss Modulus(Pa)

2E8

(2) DRMA180 (4) DRMA280

599

Y. Li et al. / Construction and Building Materials 222 (2019) 588–600

W25DRMA

W35DRMA

W45DRMA

DRMA180

DRMA210

DRMA280

GTRMA

DRMA30

DRMA40

Fig. 14. Morphology of GTRMA and DRMAs.

superior storage stability of DRMAs. This confirms that compared with WEO content, barrel temperature plays a dominant role in decrease of rubber particles size, and this phenomenon is in accordance with the test results in storage stability of DRMAs. Furthermore, with DGTR modifier content increase, asphalt phase is gradually filled up with rubber particles and hard to observe. Higher rubber content causes particles size increase and deteriorates the dispersion of rubber in asphalt matrix. Rubber particles size of DRMAs with 40% DGTR are mostly close to 200 lm, which probably due to the limited swelling and agglomeration of DGTR. Consequently, DRMA40 exhibits an inferior storage stability.

4. Conclusion GTR particles from passenger car tires were devulcanized thermo-mechanically via twin-screw extruder. Three WEO contents were applied to pretreat passenger tire rubbers, and four barrel temperature were used to obtain DGTR with different devulcanization degree. The thermal stability of DGTR rubber and storage stability, viscosity temperature susceptibility, rheological properties of modified asphalt were characterized. Moreover, the interactions between rubber and asphalt were evaluated by fluoresce microscopy. Generally, the obtained results reveal that the devulcanization recipe and temperature strongly influence on the degree of crosslinked structure in DGTR and consequently properties of modified asphalt. Sol fraction and TGA measurements confirm that the

addition of WEO enhanced partial degradation of GTR, which improves the devulcanization degree of rubber effectively, and is beneficial for obtaining an expected properties of DGTR at low barrel temperature. An increase of WEO content achieves a better processing properties and decreases stiffness of modified asphalt, which results in well-performed cracking resistance ability at low temperature. However, WEO adversely affects the storage stability improvement due to an increase in flowability of asphalt, and worsens the anti-rutting properties at high temperature as well. Furthermore, microstructure analysis proves that barrel temperature is deeply affecting the rubber particles size and dispersion state in asphalt matrix, which also related to the workability, storage stability and rheological properties of modified asphalt. With the increase of barrel temperature, rubber particles size reduces significantly, thereby asphalt with DGTR obtained at higher temperature has better processing ability, storage ability and anti-cracking properties. Compared with DRMA280, asphalt with low-temperature DGTR attains higher softening point, modulus and broader operating temperature range. Moreover, the concentration of DGTR reaches more than 30 %wt. will decrease the homogeneity of blend, thereby cause deterioration of modified asphalt rheological properties.

Declaration of Competing Interest None.

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Y. Li et al. / Construction and Building Materials 222 (2019) 588–600

Acknowledgement The authors would like to gratefully acknowledge the financial supports from the National Natural Science Foundation of China (51568064). References [1] B.S. Thomos, R.C. Gupta, A comprehensive review on the applications of waste tire rubber in cement concrete, Renew. Sustain. Energy Rev. 54 (2016) 1323– 1333. [2] M. Miranda, F. Pinto, I. Gulyurtlu, I. Cabrita, Pyrolysis of rubber tyre wastes: a kinetic study, Fuel 103 (1) (2013) 542–552. [3] F. Xiao, S. Amirkhanian, C.H. Juang, Rutting resistance of rubberized asphalt concrete pavements containing reclaimed asphalt pavement mixtures, J. Mater. Civ. Eng. 19 (6) (2007) 475–483. [4] Q. Wang, S. Li, X. Wu, S. Wang, C. Ouyang, Weather aging resistance of different rubber modified asphalts, Constr. Build. Mater. 106 (2016) 443–448. [5] H.T. Zhang, M.Y. Gong, Study on durability of composite-modified asphalt mixture based on inherent and improved performance, Constr. Build. Mater. 179 (2018) 539–552. [6] M. Sienkiewicz, K. Borze˛dowska-Labuda, A. Wojtkiewicz, H. Janik, Development of methods improving storage stability of bitumen modified with ground tire rubber: a review, Fuel Process. Technol. 159 (2017) 272–279. [7] V. González, F.J. Martínez-Boza, F.J. Navarro, C. Gallegos, A. Pérez-Lepe, A. Páez, Thermomechanical properties of bitumen modified with crumb tire rubber and polymeric additives, Fuel Process. Technol. 91 (9) (2010) 1033–1039. [8] I. Mangili, M. Lasagni, M. Anzano, E. Collina, V. Tatangelo, A. Franzetti, P. Caracino, A.I. Isayev, Mechanical and rheological properties of natural rubber compounds containing devulcanized ground tire rubber from several methods, Polym. Degrad. Stab. 121 (2015) 369–377. [9] A.I. Isayev, T. Liang, T.M. Lewis, Effect of particle size on ultrasonic devulcanization of tire rubber in twin-screw extruder, Rubber Chem. Technol. 87 (1) (2014) 86–102. [10] D.L. Presti, Recycled tyre rubber modified bitumens for road asphalt mixtures: a literature review, Constr. Build. Mater. 49 (2013) 863–881. [11] H. Si, T. Chen, Y. Zhang, Effects of high shear stress on the devulcanization of ground tire rubber in a twin-screw extruder, J. Appl. Polym. Sci. 128 (4) (2013) 2307–2318. [12] H. Yazdani, M. Karrabi, I. Ghasmi, H. Azizi, G.R. Bakhshandeh, Devulcanization of waste tires using a twin-screw extruder: the effects of processing conditions, J. Vinyl Add. Technol. 17 (1) (2011) 64–69. [13] M. Mouri, H. Okamoto, M. Matsushita, H. Honda, K. Nakashima, K. Takeushi, Y. Suzuki, M. Owaki, De-vulcanisation conditions and mechanical properties of re-vulcanised rubber for EPDM. Continuous activation of rubber by shear flow reaction control technology (Part 2), Int. Polym. Sci. Technol. 27 (2000) 23–28. [14] K. Formela, M. Cysewska, J. Haponiuk, A. Stasiek, The influence of feed rate and shear forces on the devulcanization process of ground tire rubber (GTR) conducted in a co-rotating twin screw extruder, Polimery 58 (11–12) (2013) 906–912. [15] K. Formela, M. Cysewska, J. Haponiuk, The influence of screw configuration and screw speed of co-rotating twin screw extruder on the properties of products obtained by thermomechanical reclaiming of ground tire rubber, Polimery 59 (2) (2014) 170–177. [16] K. Formela, M. Cysewska, J.T. Haponiuk, Thermomechanical reclaiming of ground tire rubber via extrusion at low temperature: efficiency and limits, J. Vinyl Add. Technol. 22 (3) (2016) 213–221. [17] S. Saiwari, Post-Consumer Tires Back into New Tires: De-Vulcanization and ReUtilization of Passenger Car Tires, University of Twente, 2013 [PhD thesis]. [18] S. Saiwari, J.W. van Hoek, W.L. Dierkes, R. Leam, G. Heideman, A. Blume, N. Jwm, Upscaling of a batch de-vulcanization process for ground car tire rubber to a continuous process in a twin screw extruder, Materials 9 (9) (2016) 724. [19] X. Wu, S. Wang, R. Dong, Lightly pyrolyzed tire rubber used as potential asphalt alternative, Constr. Build. Mater. 112 (2016) 623–628. [20] J. Shi, H. Zou, L. Ding, X. Li, K. Jiang, T. Chen, X. Zhang, L. Zhang, D. Ren, Continuous production of liquid reclaimed rubber from ground tire rubber and its application as reactive polymeric plasticizer, Polym. Degrad. Stab. 99 (1) (2014) 166–175.

[21] M. Ga˛gol, G. Boczkaj, J. Haponiuk, K. Formela, Investigation of volatile low molecular weight compounds formed during continuous reclaiming of ground tire rubber, Polym. Degrad. Stab. 119 (2015) 113–120. [22] S. Saiwari, W.K. Dierkes, J.W.M. Noordermeer, Comparative investigation of the devulcanization parameters of tire rubbers, Rubber Chem. Technol. 87 (1) (2012) 31–42. [23] G.L. Tao, Q.H. He, Y.P. Xia, G.C. Jia, H.C. Yang, W.Z. Ma, The effect of devulcanization level on mechanical properties of reclaimed rubber by thermal-mechanical shearing devulcanization, J. Appl. Polym. Sci. 129 (5) (2013) 2598–2605. [24] R. Scientists, I. Hygienists, Handbook of Plasticizers, William Andrew, 2012. [25] X. Jia, B. Huang, B.F. Bowers, S. Zhao, Infrared spectra and rheological properties of asphalt cement containing waste engine oil residues, Constr. Build. Mater. 50 (1) (2014) 683–691. [26] S.J. Liu, H.K. Meng, Y.S. Xu, S.B. Zhou, Evaluation of rheological characteristics of asphalt modified with waste engine oil (WEO), Pet. Sci. Technol. 36 (2) (2018) 1–6. [27] S.J. Liu, A.H. Peng, J.T. Wu, S.B. Zhou, Waste engine oil influences on chemical and rheological properties, Constr. Build. Mater. 191 (2018) 1210–1220. [28] I.A. Qurashi, A.K. Swamy, Viscoelastic properties of recycled asphalt binder containing waste engine oil, J. Clean. Prod. 182 (2018) 992–1000. [29] M.C. Cavalli, M. Zaumanis, E. Mazza, M.N. Partl, L.D. Poulikakos, Aging effect on rheology and cracking behaviour of reclaimed binder with bio-based rejuvenators, J. Clean. Prod. 189 (2018) 88–97. [30] X.J. Wang, C.P. Shi, L. Zhang, Y.C. Zhang, Effects of shear stress and subcritical water on devulcanization of styrene-butadiene rubber based ground tire rubber in a twinscrew extruder, J. Appl. Polym. Sci. 130 (3) (2013) 1845–1854. [31] D.O. Larsen, J.L. Alessandrini, A. Bosch, M.S. Cortizo, Micro-structural and rheological characteristics of SBS-asphalt blends during their manufacturing, Constr. Build. Mater. 23 (8) (2009) 2769–2774. [32] L. Xiang, J. Cheng, G.S. Que, Microstructure and performance of crumb rubber modified asphalt, Constr. Build. Mater. 23 (12) (2009) 3586–3590. [33] P. Frantzis, Crumb rubber-bitumen interactions: cold-stage optical microscopy, J. Mater. Civil. Eng. 15 (5) (2003) 419–426. [34] A. Bredin, A.V. Larcher, B.J. Mullins, Thermogravimetric analysis of carbon black and engine soot—Towards a more robust oil analysis method, Tribol. Int. 44 (12) (2011) 1642–1650. [35] S. Li, C. Wan, S. Wang, Y. Zhang, Separation of core-shell structured carbon black nanoparticles from waste tires by light pyrolysis, Compos. Sci. Technol. 135 (2016) 13–20. [36] S. Li, C. Wan, X. Wu, S. Wang, Core-shell structured carbon nanoparticles derived from light pyrolysis of waste tires, Polym. Degrad. Stab. 129 (2016) 192–198. [37] F. Chen, J. Qian, Studies of the thermal degradation of waste rubber, Waste Manage. 23 (6) (2003) 463–467. [38] G. Amir, 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. 2013. [39] X. Lu, U. Isacsson, Characterization of styrene-butadiene-styrene polymer modified bitumens—Comparison of conventional methods and dynamic mechanical analyses, J. Test. Evaluat. 25 (1997) 383–390. [40] M. García-Morales, P. Partal, F.J. Navarro, F.J. Martínez-Boza, C. Gallegos, Processing, rheology, and storage stability of recycled EVA/LDPE modified bitumen, Polym. Eng. Sci. 47 (2) (2007) 181–191. [41] F.J. Navarro, P. Partal, F. Martı´Nez-Boza, C. Gallegos, Thermo-rheological behaviour and storage stability of ground tire rubber-modified bitumens, Fuel 83 (14–15) (2004) 2041–2049. [42] R.T. Rasool, P. Song, S. Wang, Thermal analysis on the interactions among asphalt modified with SBS and different degraded tire rubber, Constr. Build. Mater. 182 (2018) 134–143. [43] V. González, F.J. Martínez-Boza, C. Gallegos, A. Pérez-Lepe, A. Páez, A study into the processing of bitumen modified with tire crumb rubber and polymeric additives, Fuel Process. Technol. 95 (3) (2012) 137–143. [44] F.M. Nejad, P. Aghajani, A. Modarres, H. Firoozifar, Investigating the properties of crumb rubber modified bitumen using classic and SHRP testing methods, Constr. Build. Mater. 26 (1) (2012) 481–489. [45] S.R.M. Fernandes, H.M.R.D. Silva, J.R.M. Oliveira, Developing enhanced modified bitumens with waste engine oil products combined with polymers, Constr. Build. Mater. 160 (2018) 714–724.