Investigation of rejuvenation and modification of aged asphalt binders by using aromatic oil-SBS polymer blend

Construction and Building Materials 231 (2020) 117154

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Investigation of rejuvenation and modification of aged asphalt binders by using aromatic oil-SBS polymer blend Wei Hong a, Liantong Mo a,⇑, Changluan Pan a, Martin Riara a,b, Mi Wei c,d, Jizhe Zhang e a

State Key Laboratory of Silicate Materials for Architectures, Wuhan University of Technology, Wuhan 430070, Hubei Province, China Department of Physical Sciences, South Eastern Kenya University, P.O Box 170-90200, Kitui, Kenya c Guangxi Transportation Research & Consulting Co. LTD, Nanning 530007, Guangxi Zhuang Autonomous Region, China d Guangxi Key Lab of Road Structure and Materials, Nanning 530007, Guangxi Zhuang Autonomous Region, China e School of Qilu Transportation, Shandong University, Jinan 250061, Shandong Province, China b

h i g h l i g h t s
Compound rejuvenator enhanced softening point and ductility of aged bitumen.
Polymer modification effect was distinguished by phase angle master curves.
SBS polymer was a good supplement to balance the overall performance.
Aged asphalt binders can be rejuvenated and modified simultaneously.

a r t i c l e

i n f o

Article history: Received 11 January 2019 Received in revised form 10 August 2019 Accepted 4 October 2019

Keywords: Asphalt binder Long-term aging Rejuvenation Modification

a b s t r a c t In this paper, long-term aged base asphalt and SBS modified asphalt binders were rejuvenated and modified simultaneously using a compound rejuvenator containing 77% aromatic oil and 23% SBS polymer. Aromatic oil was used for rejuvenation purpose due to its softening effect while SBS polymer was used for modification purpose because of its potential to form a polymer network. Conventional bitumen tests, dynamic shear rheological (DSR) test and bending beam rheological (BBR) test were conducted to evaluate the rejuvenation and modification effects. Test results indicated that use of the compound rejuvenator enhanced the softening point and ductility for both aged base asphalt and SBS modified asphalt binders. The polymer modification effect of compound rejuvenator was well distinguished by the plateau region of phase angle master curves. Aromatic oil had a strong softening effect on aged asphalt binder which improved the performance at low temperatures, but weakened the performance at high temperatures. The incorporation of SBS polymer was a good supplement to balance the overall performance of rejuvenated asphalt binders. The compound rejuvenator can be greatly effective for hot recycling of reclaimed asphalt mixture by means of the simultaneous rejuvenation and modification. Ó 2019 Elsevier Ltd. All rights reserved.

1. Introduction By the end of 2017, the total length of highways in China open to traffic reached 477 million km with a highway density of 19.5 km per 100 km2. Among these highways, 136.5 thousand kilometers expressways have been built. Up to 90% of high-grade highways in China are typical three-layer asphalt pavements. Currently about 20% of the high-grade highway asphalt pavements have been in service for longer than their design lives of 15 years and thus ⇑ Corresponding author at: Room 517, Concrete Building, State Key Laboratory of Silicate Materials for Architectures, Wuhan University of Technology, Luoshi Road 122, Wuhan 430070, China. E-mail address: [email protected] (L. Mo). https://doi.org/10.1016/j.conbuildmat.2019.117154 0950-0618/Ó 2019 Elsevier Ltd. All rights reserved.

rehabilitation is needed to restore their structural integrity and improve driving safety and comfort. Asphalt pavement rehabilitation typically involves milling and resurfacing of the existing asphalt pavement to mitigate various pavement distresses. Pavement milling will produce a large account of reclaimed asphalt (RA) mixtures. The use of RA in pavement rehabilitation is of great interest to preserve the natural environment, reduce waste and provide a cost effective material for constructing highways [1–3]. In general, RA can be recycled by means of cold recycling [2–7] or hot recycling [8–13]. Foamed asphalt and emulsified bitumen are the commonly used binding material for cold recycling [6,7]. Cold in-place asphalt recycling is popular for old asphalt pavements that are in dire need for both repair and an asphalt overlay.

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In this case, cold recycling RA is used as base materials. Recently, an innovative way of horizontal recycling of porous asphalt was developed under the European Life + program. Towards 90% warm re-use of reclaimed porous asphalt using foaming technology was demonstrated to be feasible in the Netherlands [14]. Compared to cold recycling, hot recycling can produce hot asphalt mixture containing RA with better performance and increased durability [10]. Hence, this technology is promising for RA recycling industry. Hot recycling can be classified into central plant hot recycling and hot in-place recycling [8,9]. Central plant hot recycling usually has a limited RA application of about 10–30% to produce good-quality hot asphalt mixture, which is comparable to traditional virgin hot mix asphalt. High amount of RA can also be applied with the addition of various bitumen rejuvenators [9,15–19]. Hot in-place asphalt recycling is used to correct surface defects in an asphalt pavement. It is not the right treatment for a pavement that has a base course or a subgrade failure. Currently, RA incorporation in top asphalt layers is still limited. Most of them are being reused as base materials. The horizontal recycling is the best way to make full use of RA. The rejuvenation efficiency of aged RA and the influence of high RA content on long-term pavement performance are still issues of concern. Typical asphalt pavements in China usually consist of a threelayer structure comprising the surface wearing, intermediate and base course layers from top to down respectively. Polymer modified asphalt are commonly used for the surface wearing and intermediate courses, while base asphalt binder is used for the base course layer. Asphalt pavement rehabilitation will produce more RA containing polymer modified asphalt binder than those containing base asphalt binder. Furthermore, the aging of polymer modified asphalt in the surface wearing course is considered to be more severe than that of the other two asphalt layers since it is directly exposed to the outer environment. The aging of asphalt binder is strongly dependent on the type of asphalt binder. The aging of base asphalt binder is considered to reflect changes in its colloidal nature mainly due to the volatilization of the lighter fractions and the oxidation of the other fractions [20,21]. However, the aging of polymer modified asphalt binder is much more complex, and it is considered as the combination of two parallel reactions including oxidation of base asphalt binders and degradation of constituent polymers [22,23]. Aging tends to make an asphalt binder stiffer, more brittle, and thus more susceptible to fatigue damage due to traffic and thermal loadings. Various bitumen rejuvenators can be used to restore the properties of RA that are lost due to aging. A systematic review on the use of rejuvenating agents in production of recycled hot mix asphalt can be found in [10]. The softening effect of bitumen rejuvenators usually improves the lowtemperature properties of RA asphalt binder, but it has a negative effect on rutting resistance at high temperatures [24,25]. It is necessary to optimize the type as well as the content of bitumen rejuvenators to balance relevant binder properties. Cavalli used bio-based rejuvenators including natural seed oil, cashew nut shell oil and on tall oil to rejuvenate RA binder [26]. It was found that the degree of aging had a significant effect on chemical and rheological behavior of rejuvenated RAP binders. The mechanical changes due to rejuvenators were related to the rearrangement at higher molecular scale such as polar/nonpolar components. Cao reported that the workability and fatigue resistance of aged asphalt was increased by addition of 5% waste vegetable oil. The sulfoxide index and the large molecule size content decreased due to physical dilution [27]. Zhang found that when a bio-oil generated from sawdust was as a rejuvenator, it decreased the activation energy and rutting index of aged asphalt. Rejuvenated asphalt with 15% bio-oil had better crack resistance

than virgin asphalt. 15% and 20% rejuvenator content was recommended to recycle the PAV (pressure aging vessel) aged asphalt PG 58-28 and PG 64-22, respectively [28]. Zeng evaluated the effect of a bitumen rejuvenator derived from the residue in castor oil production and found that about 20–30% of the rejuvenator could restore the rutting and thermal cracking properties of aged binders [29]. Chen reported that 5% of cotton seed oil and waste cooking oil resulted in the reduction of rutting parameter, viscosity and failure temperature of the rejuvenated asphalt binders [30]. Ali conducted a research on various rejuvenators including naphthenic oil, paraffinic oil, aromatic extracts, tall oil, and oleic acid. It was found that for all the rejuvenators, a 5% to 9% dosage had the same effectiveness in rejuvenating the aged RA binder regardless of the RA percentage (up to 45%). Paraffinic oil was the most effective at rejuvenating the aged RA binder [31]. Many researchers have reported that the incorporation of less than 20% RA had an acceptable influence on bitumen property, aggregate gradation and mixture property [32–34]. When RA content was higher than 30%, a bitumen rejuvenator was needed. High RA content led to higher void content and mixture modulus, which increased the risk of fatigue and low-temperature cracking [33]. Each increase of 20% RA could enhance up to one performance grade at high temperatures and weaken the performance level at low temperatures [35,36]. Toward 100% recycling of RA is very attractive and it has been reported to be feasible with the application of bitumen rejuvenators [32,37,38]. Zaumanis assessed the rejuvenation of 100% recycled hot mix asphalt lab samples modified with a 12% dosage of six rejuvenators including waste vegetable oil, waste vegetable grease, organic oil, distilled tall oil, aromatic extract residue and waste engine oil. The rejuvenated mixtures showed an increase on rutting and cracking resistance, and a reduction on workability and moisture resistance [37]. Elkashef found that 6% to 12% of soybean-derived rejuvenator could enhance the fatigue and low temperature properties of the extracted RAP binders. 100% RA mixtures rejuvenated using a soybean-modified binder showed an increase in fracture energy [38]. Farooq performed a study on RA recycling by means of warm mix asphalt (WMA). With no rejuvenator, up to 20% RA can be used to prepare RA WMA mix. With 17.5%-20% used-automobile engine oil rejuvenator, up to 60% RAP could be used to prepare RA WMA mix [33]. Cong reported that a good performance mixture was obtained by blending new SBS modified asphalt binder and 5– 10% rejuvenating agent into an asphalt mixture incorporating 15–30% reclaimed SBS modified asphalt mixture [39]. Because of the limitation of the single function of common bitumen rejuvenators, a compound rejuvenator containing crumb rubber, plasticizer and plastomeric polymers was used to simultaneously restore and modify aged asphalt binders. Cristina provided a comprehensive study on the thermo-mechanical properties of agricultural plastic waste/naphthenic oil blends. Recycled polymer/oil blends were thermo-rheologically complex materials when polymer content ranged from 5% to 40%. They show a predominant gel-like behaviour at low and intermediates temperatures. The oil acted as a lubricant between the polymeric chains and blend complex viscosities were well predicted using Lecyar’s mixing rule [40]. Dong used crumb tire rubber lightly pyrolyzed in waste cooking oil to modify asphalt binder [41]. Compared to rubberized asphalt binder, the storage stability, plasticity at low temperatures and workability of modified asphalt binder were greatly improved. However, the resistance to rutting and deformation at high temperatures became worse. Zhu used a bio-plasticizer based rejuvenator (by-product of cotton-oil and dibutyl phthalate) to restore long-term aged asphalt binder. A dosage of 10% modified rejuvenator restored the workability and rutting resistance of the

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binder to the original level. An improvement on the fatigue resistance and low-temperature cracking resistance was also observed [42]. Zhao prepared a polymer-modified rejuvenator by using extract oil (40–50%), SBS polymer 40% petroleum resin (5%) and dispersing agent (0–10%). Aged asphalt binder treated with 9% of the polymer-modified rejuvenator had an obvious improvement on the performance at high and low temperatures The formation of polymer network structures increased the ductility at 5 °C and the resistance to deformation [43]. Bonicelli used rejuvenators and plastomeric polymers to increase the durability of 40% RA content asphalt mixtures. It was found that the use of rejuvenator softened the high RA content asphalt mixture to appropriate levels and also reduced the low temperature stiffness by 15%, while the use of plastomeric polymers increased the stiffness by up to 50%. The use of plastomeric polymer balanced the softening effect of the rejuvenator and guaranteed a consistent increase in resistance to permanent deformation with increase in polymer dosage [44]. Fernandes found that enhanced modified asphalt binder can be prepared by using waste engine oil and recycled engine oil bottoms combined with polymers (waste polyethylene, crumb rubber and styrenebutadienestyrene). The modified asphalt binder had similar penetration values and higher softening points. Rheological analysis showed low thermal susceptibility, high value of hightemperature performance grade and low non-recoverable creep compliance values, which indicated a promising performance using large amounts of waste materials [45]. Fernandes carried out a study on the performance of recycled SMA mixtures with high content of waste materials, such as, waste engine oil products (waste engine oil and recycled engine oil bottoms), polymers (waste HDPE, waste crumb rubber and SBS) and RA material. High penetration asphalt binder modified with waste materials was used to produce recycled SMA mixtures with 50% RA material. Generally, the recycled SMA mixtures showed a better mechanical performance than a conventional SMA mixture and a recycled SMA mixture produced with a straight run asphalt binder [46]. Ma prepared high modulus recycled asphalt concretes with 20% to 60% RA together with 0.3% synthetic low-molecular weight polyolefin polymers as high modulus agents [47]. Test results showed that high modulus asphalt concrete had more tolerance to the negative effects of RA on the engineering properties than normal asphalt concrete. The application of high RA content in high modulus recycled asphalt concrete was more promising than in normal recycled asphalt concrete. Traditional rejuvenating agents have been widely studied and their negative effect on high-temperature properties was reported. However, earlier researches on compound rejuvenators show that they could be better for RA recycling. For this reason, this paper aimed to investigate the rejuvenation and modification effect of a compound rejuvenator containing aromatic oil and SBS polymer on long-term aged asphalt binders. Aromatic oil was used for rejuvenation purpose while SBS polymer was used for modification purpose. The combination of these two components was expected to improve the overall performance at low and high temperatures by the softening and reinforcement mechanisms, respectively. In order to get insight into the rejuvenation and modification effects of the proposed compound rejuvenator, conventional bitumen tests, dynamic shear rheological (DSR) test and bending beam rheological (BBR) test were conducted. The improvement on penetration, softening point, ductility, rutting parameter (G*/sind), fatigue parameter (G*sind) and creep stiffness was evaluated. The aging index and rejuvenating index were proposed for a quantitative analysis of the effects of aging and rejuvenation. Finally, Fouriertransform infrared spectroscopy (FTIR) analysis was used to investigate the chemical changes on aged and rejuvenated asphalt binders by means of carbonyl index, sulfoxide index and SBS index.

2. Materials and methods 2.1. Materials In this study, one base asphalt binder 70# (Penetration grade of 60–80, abbreviated as BA) and one Styrene-Butadiene-Styrene (SBS) polymer modified asphalt (PMA) binder were used. Both asphalt binders were supplied by Panjin North asphalt Co. Ltd, China. The SBS content in the modified asphalt binder was about 4.0%. It should be noted that SBS modified asphalt binder is commonly used for the surface wearing course and intermediate binder layer, while BA is widely used in the base course layer of most three-layer asphalt pavements in China. Table 1 shows the basic properties of the two asphalt binders. Aromatic oil was used as a rejuvenator for aged asphalt binder. The content of aromatic hydrocarbon in the aromatic oil was 78% according to ASTM D2140. Its kinematic viscosity at 100 °C was 20 mm2/s as determined in accordance with ASTM D445. Linear SBS polymer with a molecular weight of 105,000 and containing 30% styrene content was used as a modifier in this study. Aromatic oil was blended with SBS polymer to produce a compound rejuvenator, which was expected to rejuvenate and modify aged asphalt binder simultaneously. The blend of aromatic oil and SBS polymer was prepared by high temperature melting and continuous shear blending. The content of SBS polymer was 30% by weight of aromatic oil. Aromatic oil was first heated to 170– 180 °C in an oil-bath. SBS polymer was then added slowly to the aromatic oil, and shear blended with a speed about 3000 rpm for 30–45 min until it was completely dissolved and well dispersed. In order to obtain aged asphalt binder, BA binder and SBS modified asphalt binder were subjected to rolling thin film over testing (RTFOT, T0609, JTG E20-2011) [48] and pressure aging vessel (PAV) testing according to T0630, JTG E20-2011 [48]. RTFOT was conducted at 165 °C for 85 min with an air flow of 4 L/min. It was used to simulate the short-term oxidation that occurs during the hotmixing process. PAV was used to simulate the long-term aging of asphalt binder after service for 5–10 years. The test conditions of PAV aging were set at a pressure of 2.1 ± 0.1 MPa and a temperature of 100 °C. The asphalt binders were exposed to high pressure and temperature for 20 h to simulate the effect of long term oxidative aging. The obtained aged BA binder and SBS modified asphalt binder were rejuvenated using the aromatic oil and the compound rejuvenator containing aromatic oil and SBS polymer as mentioned before. The content of rejuvenator that was blended in aged asphalt binder was 15% of the total weight of the mixture. In total, two types of virgin asphalt binder, two types of long-term aged asphalt binder and four types of rejuvenated asphalt binder were involved: (1) BA: virgin base asphalt binder without aging; (2) BA-LTA: Base asphalt binder after long-term aging; (3) BA-LTA + AR: Base asphalt binder after long-term aging containing 15% aromatic oil (Aromatic rejuvenator, AR);

Table 1 Basic properties of the asphalt binders. Index

Penetration (25 °C, 100 g, 5 s) Ductility (10 °C, 5 cm/mm) Viscosity (135 °C) Softening point *Ductility at 5 °C for PMA.

Units

0.1 mm cm Pa.s °C

Asphalt binder BA

PMA

62 17.5 0.463 49.0

50 28.3* 2.121 82.5

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(4) BA-LTA + MAR: Base asphalt binder after long-term aging containing 15% compound rejuvenator (Polymer modified aromatic rejuvenator, MAR). (5) PMA: virgin SBS polymer modified asphalt binder without aging; (6) PMA-LTA: SBS polymer modified asphalt binder after longterm aging; (7) PMA-LTA + AR: SBS polymer modified asphalt binder after long-term aging containing 15% aromatic oil; (8) PMA-LTA + MAR: SBS polymer modified asphalt binder after long-term aging containing 15% compound rejuvenator. 2.2. Test methods All the virgin, aged and rejuvenated asphalt binders mentioned above were subjected to conventional penetration, softening point and ductility testing in accordance with T0604, T0606 and T0605, JTG E20-2011 [48] respectively. A dynamic shear rheometer, DSR (MCR101, Anton Paar, Germany), was used to test the rheological properties of virgin, aged and rejuvenated asphalt binders. DSR tests were performed in strain control mode and a small strain level for each temperature was selected to ensure that testing was done in the linear region based on previous machine calibration according to T0628, JTG E20-2011 [48]. The bitumen samples were sandwiched between two circular parallel plates. The parallel plates with a diameter of 8 mm and 2 mm gap were used for tests at temperatures below 25 °C, while 25 mm parallel plates and 1 mm gap were used for tests at temperatures above 25 °C. Two types of DSR testing were carried out: temperature sweep test and frequency sweep test. For temperature sweep test, a constant frequency of 10 rad/s was used. The 8 mm parallel plates were used for temperature sweep from 10 °C to 30 °C, while 25 mm parallel plates were used for temperature sweep from 30 °C to 80 °C. Frequency sweep testing was performed at different temperatures ranging from 10 °C to 60 °C with an interval of 10 °C. For each test temperature, the test frequency ranged from 0.01 rad/s to 400 rad/s. The obtained rheological parameters including the phase angle (d) and the complex modulus (G*) were used to construct the master curves. Furthermore, the rutting parameter, G*/sind and fatigue parameter G*sind were also determined from these parameters. The temperature, TR, at which rutting parameter G*/sind is equal to 1 kPa, is defined as a criterion for acceptable high-temperature performance of asphalt binders. With respect to fatigue parameter G*sind, the fatigue temperature, TF is determined by the test temperature, at which G*sind is equal to 5000 kPa. In this study, bending beam test was carried out by using a bending beam rheometer, BBR (Model TE-BBR, Cannon instrument Company, PA, USA). The BBR test was conducted in accordance with T0627, JTG E20-2011[48] at a low temperatures of 10 °C. The creep stiffness S(t) at 60 s and the m value representing the rate of stiffness change versus time was analyzed. The mean values of S(t) and m were determined from three test samples for each asphalt binder. Fourier-transform infrared spectroscopy (FTIR) analysis is a commonly used technology to monitor the molecular changes associated with bitumen oxidation and aging [20,27,49]. Carbonyl groups and sulfoxides are the major functional groups formed during bitumen oxidative ageing. Both of them can be well identified by C@O at 1700 cm1 and S@O at 1030 cm1 respectively in FTIR spectroscopy. For SBS PMA binder, SBS polymer consists of a polybutadiene segment in the middle with polystyrene blocks at the ends, which contains the C@C bonds in the unsaturated carbon chains and benzene rings respectively. Two special absorption peaks at 966 cm1 and 699 cm1 can identify the polybutadiene (PB) and polystyrene (PS), respectively and thus can be used to

characterize the existence of SBS in its modified asphalt binder with FTIR spectroscopy. For this reason, the degradation of SBS polymer during binder aging can also be detected. In this study, FTIR was used to record the spectra of virgin asphalt, aged asphalt and rejuvenated asphalt binders. FTIR spectra were recorded by a Fourier transform infrared spectrometer (Nicolet 6700, Thermo Electron Scientific Instruments Co., USA) loaded with Omnic 8.2 software. A small pellet of bitumen sample was dissolved in a specific amount of carbon disulfide to prepare 5 wt% concentration solution. Three drops of the solution were put onto KBr slide, dried for the FTIR analysis and then scanned within the range from 4000 cm1 to 400 cm1. The carbonyl index, sulfoxide index and polystyrene PB index were determined by the ratio of their bands’ area to that of the total spectra areas from 2000 cm1 to 600 cm1 using Eqs. (1)–(3), respectively. For the SBS polymer, the polybutadiene (PB) is more chemically susceptible to degradation than polystyrene (PS) during aging. The area ratio of polybutadiene (PB) peak to polystyrene (PS) peak was further defined to explain the degradation of SBS polymer, see Eq. (4).

IC¼O ¼ P

Area of the carbonyl centered around 1700cm1 Area of the spectral bonds between 2000 and 600cm1

 100% ð1Þ IS¼O ¼ P

Area of the sulphoxide centered around 1030cm1 Area of the spectral bonds between 2000 and 600cm1

 100% ð2Þ IPB ¼ P

Area of the polybutadiene centered around 966cm1 Area of the spectral bonds between 2000 and 600cm1

 100% ð3Þ

IPB=PS ¼

Area of the polybutadiene centered around 966cm1 Area of the polystyrene centered around 699cm1 ð4Þ

3. Results and discussion 3.1. Conventional test results Figs. 1–3 give the conventional bitumen test results on penetration, softening point and ductility. These figures show that long term aging resulted in an obvious decrease in penetration and ductility and an increase in softening point of BA binder. The penetration dropped from 62 mm to 25 mm and the ductility reduced from 17.5 cm to 1 cm. The dramatic loss of ductility was also observed for PMA binder. This strongly indicated the severe effect of long term aging (LTA) on these two asphalt binders. Aging resulted in an increase of softening point for BA binder due to the increase in asphaltenes fractions but a reduction of PMA binder was due to the degradation of polymer as well as the damage of polymer network. The aged asphalt binder can be well rejuvenated by the addition of 15% AR. This was indicated by a significant increase on penetration and ductility. This effect became more distinct for aged BA binder. With the addition of 15% AR, the penetration, softening point and ductility were almost restored to the level of virgin asphalt binder. Furthermore, AR also showed an obvious rejuvenation effect on aged PMA binder. It exhibited a strong softening effect and thus resulted in a low softening point of 52.5 °C, which is not desired for a PMA binder commonly used in high

W. Hong et al. / Construction and Building Materials 231 (2020) 117154

Fig. 1. Penetration results of various asphalt binders.

Fig. 2. Softening point results of various asphalt binders.

Fig. 3. Ductility results of various asphalt binders.

temperature regions. It should be noted that two parallel reactions occur during the oxidation of SBS PMA binder; the oxidation of base asphalts and degradation of polymers. This requires that the rejuvenation of aged PMA binder should take these two reactions into account. The oxidation of BA binder could be well rehabilitated by using AR. However, additional SBS polymer is needed to restore the damaged polymer network due to the polymer degradation during aging. This idea was demonstrated by the data obtained from MAR containing AR and SBS polymer. The use of MAR was effective to further enhance the softening point and ductility for both aged BA binder and SBS modified asphalt binder. 3.2. Rheological analysis based on master curve Figs. 4 and 5 present the rheological master curves of virgin asphalt, aged asphalt and rejuvenated asphalt binders. These master curves are commonly established based on the

Fig. 4. Master curves of virgin, aged and rejuvenated BA binders.

5

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for rutting resistance. MAR also resulted in lower complex modulus at high frequencies, that is, low temperatures. The above analysis indicated that MAR performed better than the AR. The former rejuvenated and modified the aged BA binder and thus it enhanced the overall performance. Fig. 5 gives the master curves of complex modulus and phase angle for virgin, aged and rejuvenated PMA binders. Long-term aging resulted in higher complex modulus and lower phase angle. The addition of 15% AR seemed to over-rejuvenate the aged PMA binder. This is indicated by lower complex modulus of rejuvenated PMA binder together with higher phase angle. Aged PMA binder rejuvenated using 15% MAR exhibited rheological properties comparable to those of virgin PMA in terms of complex modulus and phase angle. Again, MAR provided two effects of rejuvenation and modification when compared to AR.

Fig. 5. Master curves of virgin, aged and rejuvenated PMA binders.

time-temperature superposition principle by using DSR frequency sweep data at different temperatures. In this study, the reference temperature was selected at 30 °C. Test data at other temperatures were shifted horizontally to the data at 30 °C to obtain a smooth curve. The shift factors at various temperatures are described by using the Arrhenius equation.



aðTÞ ¼ exp

  Ea 1 1  R T T0

ð5Þ

where: aðTÞis the shift factor relative to the reference temperature; Ea is the activation energy, J/mol; R = 8.314 J/(mol.K); T is the test temperature, K; T0 is the reference temperature, K. The activation energy was determined by fitting the shift factors at various temperatures using the method of least squares. Table 2 gives a summary of the regression analysis results of the Arrhenius equation for all of the studied asphalt binders. The data indicates clearly that long-term aging resulted in a higher value of activation energy, however the addition of AR tended to reduce the activation energy. Compared with AR, MAR showed a higher activation energy. This indicated that the SBS polymer incorporated MAR could reduce the temperature susceptibility of the rejuvenated asphalt binders. Fig. 4 gives the master curves of complex modulus and phase angle for virgin, aged and rejuvenated BA binders. As can be seen, aged BA binder showed increased complex modulus together with lower phase angle when compared with virgin BA binder. These phenomena became more obvious at the low-frequency range, which is equivalent to high-temperature range. When the aged BA binder was rejuvenated using 15% AR, its master curves for complex modulus and phase angle almost overlapped the corresponding ones of the virgin BA binder. This indicated that 15% AR was very effective to rejuvenate the rheological properties of long-term aged BA over a wide range of frequency/temperature. The rejuvenation effect of MAR seemed to be more promising. The master curve of phase angle exhibited a plateau region, which is an indicator of the existence of SBS polymer network [50,51]. Higher complex modulus together with lower phase angle at low frequencies (which is equivalent to high temperatures) is good Table 2 The fitting results of activation energy for various asphalt binders. Type of binders

Ea (KJ/mol)

R2

BA BA-LTA BA-LTA + AR BA-LTA + MAR PMA PMA-LTA PMA-LTA + AR PMA-LTA + MAR

193.7 195.1 190.4 198.3 186.6 194.2 185.4 193.8

0.9956 0.9988 0.9965 0.9933 0.9969 0.9974 0.9967 0.9956

3.3. Fatigue parameter and low-temperature properties Figs. 6 and 7 illustrate the relation of fatigue parameter (G*sind) and temperature for various asphalt binders. The fatigue temperature, TF is determined by the test temperature, at which G*sind is equal to 5000 kPa. In general, lower values of G*sind and TF are desired for fatigue resistance [52]. Aging usually results in higher G*sind as well as TF, which indicate decreased fatigue resistance. With respect to the series of BA binders, the addition of 15% AR into aged BA binder restored its fatigue parameter to the one of virgin BA binder. MAR also showed an obvious decrease on fatigue parameter, especially at temperatures below 10 °C. For the series of PMA binder, 15% AR seemed to reduce the fatigue parameter of aged PMA binder significantly, while 15% MAR could properly restore the fatigue parameter to a value close to that of virgin PMA. In order to make a quantitative analysis on the effects of aging and rejuvenation, the change percentages of complex modulus and phase angle after aging and rejuvenating were calculated by using the Eqs. (6)–(9).

AIðGÞ ¼

AIðdÞ ¼

dA dV

RIðGÞ ¼

RIðdÞ ¼

GA GV

GR GV

dA dV

ð6Þ

ð7Þ

ð8Þ

ð9Þ

where: AI and RI are aging and rejuvenating index; GA*, GV* and GR* are the complex modulus for aged, virgin and rejuvenated asphalt binders. dA, dV and dR are the phase angle for aged, virgin and rejuvenated asphalt binders. Fig. 8 presents the results of aging index (AI) and rejuvenation index (RI) of complex modulus for aged and rejuvenated BA binder. As indicated by aged BA binder, the aging index increased with the increase in temperature. The aging index at low temperature was relatively small, however, the aging effect was critical for lowtemperature properties, as indicated by the previous ductility test results. After long-term aging, BA binder lost its ductility. With the addition of 15% AR, the rejuvenating index of BA binder remained constant and close to 1 over a wide range of temperature. This indicated the strong rejuvenating ability of AR. For MAR, the rejuvenating index were obviously smaller than aging index at every temperature. The change of rejuvenating index can be divided into two phases: below 5 °C, the rejuvenating index was smaller than 1; above 5 °C, the index was larger than 1. The above analysis

W. Hong et al. / Construction and Building Materials 231 (2020) 117154

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Fig. 6. Relation between fatigue parameter (G*sind) and temperature for virgin, aged and rejuvenated BA binders.

Fig. 7. Relation between fatigue parameter (G*sind) and temperature for virgin, aged and rejuvenated PMA binders.

Fig. 8. Results of aging index(AI) and rejuvenating index (RI) based on complex modulus for aged and rejuvenated BA binder at low temperature range.

indicated that MAR showed a higher efficiency to improve the lowtemperature properties when compared to AR. Fig. 9 illustrates the variation of aging index (AI) and rejuvenation index (RI) of complex modulus over temperature for aged and

Fig. 9. Results of aging index (AI) and rejuvenating index (RI) based on complex modulus for aged and rejuvenated PMA binder at low temperature range.

rejuvenated PMA binders. After long-term aging, the aged PMA had an aging index close to 2. When rejuvenated with 15% AR, the rejuvenation index was generally smaller than 1, indicating that the aged PMA binder was over-rejuvenated. The addition of 15% MAR

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showed a proper rejuvenation for aged PMA binder. The rejuvenation index remained constant and was very close to 1 for the temperature range from 10 °C to 30 °C. Figs. 10 and 11 present the results of aging index (AI) and rejuvenation index (RI) based on phase angle for different aged and rejuvenated asphalt binders. In general, aging resulted in a reduction of phase angle, which is indicated by an aging index smaller than 1. For BA binder, the aging index remained constant but for PMA binder, it shows an increasing trend over the entire temperature range. The addition of 15% AR restored fully the phase angles of aged BA binder, while it over-rejuvenated the aged PMA binder. MAR was found to be more efficient at low temperatures for both types of asphalt binders. Tables 3 and 4 present a summary of aging index (AI) and rejuvenation index (RI) for various aged and rejuvenated asphalt binders. The AI and RI were determined by the complex modulus and phase angle at 10 °C. Furthermore, the fatigue temperature TF was also used to calculate the corresponding AI and RI by means of the TF value of aged asphalt binder or rejuvenated asphalt binder divided by the corresponding value for virgin asphalt binder according to Eqs. (10) and (11).

AIðT F Þ ¼

T FA T FV

ð10Þ

RIðT F Þ ¼

T FR T FV

ð11Þ

where:T FA , T FR and T FV are the fatigue temperature, TF value of aged asphalt binder, rejuvenated asphalt binder and virgin asphalt binder respectively. As indicated in Tables 3 and 4, AI and RI was determined based on complex modulus, phase angle and fatigue temperature. It should be noted that aging generally increase the complex modulus and reduce the phase angle. Therefore, the AI of complex modulus should be higher than 1 and a higher AI value indicates higher degree of hardness after aging. On the contrary, the AI of phase angle should be lower than 1 and a lower value indicates more brittleness at low temperature. Similarly, the meaning of AI and RI based on fatigue temperature, TF followed the same trend as those based on complex modulus. In order to obtain a same trend for these three parameters, the AI and RI based on phase angle were translated their corresponding reciprocals, 1/AI(d) and 1/RI(d). The data listed in Tables 3 and 4 show that the AI and RI values obtained from each parameter gave a consistent trend for aging and rejuvenating effects. For example, the aging index on average was 1.222 and 1.405 for long-term aged BA binder and PMA binder respectively. Addition of 15% AR resulted in a rejuvenation index of

Fig. 10. Results of aging index (AI) and rejuvenation index (RI) based on phase angle for aged and rejuvenated BA binders at low temperature range.

Fig. 11. Results of aging index (AI) and rejuvenation index (RI) based on phase angle for aged and rejuvenated PMA binders at low temperature range.

0.988 for aged BA binder and 0.907 for aged PMA binder. When 15% MAR was applied, the rejuvenation index was 0.994 for aged BA binder and 1.074 for aged PMA binder. Based on AI and RI, it was clear that 15% AR and MAR could efficiently rejuvenate the low-temperature properties of aged asphalt binders.

3.4. Rutting parameter and high temperature properties Figs. 12 and 13 show the relation between rutting parameter (G/sind) and temperature for different asphalt binders. The temperature, TR, at which G*/sind is equal to 1 kPa, is defined as a criterion for acceptable high-temperature performance of asphalt binders. Because the temperature sweep test were done at a temperature ranging from 30 °C to 80 °C, the available data was extrapolated in the case of high-temperature performance grade higher than 80 °C. Rutting parameter is of significant interest to correlate the rheological characteristics of asphalt binder with the rutting resistance of asphalt mixtures [45,53]. G* represents the total resistance to deformation under load while d represents the relative distribution between the response of the elastic and viscous components. An asphalt binder with high G* but low d is desired for rutting resistance. Higher value of TR indicates a better rutting resistance. According to Fig. 12, the TR of virgin BA binder was determined as 68.5 °C and after long term aging, the TR value increased to 81.5 °C. The addition of 15% AR was able to restore TR to its original value. MAR showed a higher value of TR about 83 °C indicating an increase on the thermal stability of BA binder at high temperatures. Softening point is typically used as a rutting parameter of asphalt binder. The results obtained from G*/sind were in agreement with those obtained for softening point. Both aging and addition of SBS polymer increased the softening point, and thus, it reduced the rutting susceptibility at high temperatures. Fig. 13 presents the relation between rutting parameter (G/sind) and temperature for virgin, aged and rejuvenated PMA binders. It was observed that aging increased the rutting parameter. Based on this parameter, AR had a stronger rejuvenation effect compared to MAR. However, this over-softening effect was not desirable for rutting resistance. The high-temperature performance grade determined at G*/sind = 1 kPa was 82.5 °C for virgin PMA binder. After long-term aging, this grade increased to 84 °C. The addition of 15% AR had a strong softening effect and it reduced the hightemperature performance grade to 71.8 °C. However, the aged PMA binder containing 15% MAR showed a high-temperature performance grade of 81 °C, which is close to that of virgin PMA binder. This increase could be attributed to the incorporation of SBS polymer in MAR.

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W. Hong et al. / Construction and Building Materials 231 (2020) 117154 Table 3 Summary of aging index (AI) and rejuvenation index (RI) for aged and rejuvenated BA binder at low-temperature range. Type of asphalt

BA

BA-LTA

BA-LTA + AR

BA-LTA + MAR

TF @ G*sind = 5000 kPa G* @ 10 °C [MPa] d @ 10 °C [°] AI or RI

18.9 216 24.6 – – – – –

24.5 241 19.6 1.296 1.116 0.797 1.255* 1.222

18.6 209 24.3 0.984 0.968 0.988 1.012** 0.988

21.6 160 22.4 1.143 0.741 0.911 1.098** 0.994

TF G* @ 10 °C d @ 10 °C 1/[AI or RI(d)] Average

Note: * the reciprocal to AI, 1/AI(d); ** the reciprocal to RI, 1/RI(d).

Table 4 Summary of aging index(AI) and rejuvenating index (RI) for aged and rejuvenated PMA binder. Type of asphalt

PMA

PMA-LTA

PMA-LTA + AR

PMA-LTA + MAR

TF @ G*sind = 5000 kPa G* @ 10 °C [MPa] d @ 10 °C [°] AI or RI

17.5 126 25.4 – – – – –

22.4 212 20.3 1.280 1.683 0.799 1.251* 1.405

14.1 126 27.8 0.806 0.999 1.094 0.914** 0.907

18.4 139 23.8 1.051 1.103 0.937 1.067** 1.074

TF G* @ 10 °C d @ 10 °C 1/[AI or RI(d)] Average

Note: * the reciprocal to AI, 1/AI(d); ** the reciprocal to RI, 1/RI(d).

Fig. 12. Relation between rutting parameter (G/sind) and temperature for virgin, aged and rejuvenated BA binder.

Fig. 13. Relation between rutting parameter (G/sind) and temperature for virgin, aged and rejuvenated PMA binder.

Figs. 14 and 15 present the results of aging index (AI) and rejuvenation index (RI) based on complex modulus G* at a temperature range from 30 °C to 80 °C. The aging index of long-term aged BA

Fig. 14. Results of aging index (AI) and rejuvenation index (RI) based on complex modulus G* for aged and rejuvenated BA binder at high temperature range.

Fig. 15. Results of aging index (AI) and rejuvenation index (RI) based on complex modulus G* for aged and rejuvenated PMA binder at high temperature.

ranged from 4 to 5. Addition of 15% AR to the aged BA binder reduced its AI to a value close to 1. The rejuvenation index of MAR increased continuously with increase in temperature. This can be explained by the combined rejuvenation effect of AR and

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W. Hong et al. / Construction and Building Materials 231 (2020) 117154

Fig. 16. Results of aging index (AI) and rejuvenation index (RI) based on phase angle for aged and rejuvenated BA binder at high temperature range.

the modification effect of SBS polymer. The existence of SBS polymer made a significant improvement on thermal stability. With respects to PMA binder, 15% AR reduced the complex modulus with a rejuvenation index of about 0.5. This made asphalt binder quite soft, and thus sensitive to rutting. The application of MAR showed a RI value of about 1.2, indicating a better rejuvenation effect for aged SBS modified asphalt binder when compared with AR.

Figs. 16 and 17 show the results of aging index (AI) and rejuvenation index (RI) based on phase angle for test temperature ranging from 30 °C to 80 °C. Long-term aging led to an aging index below 1 for BA binder, and a complex trend for PMA binder. In term of BA binder, AR could fully recover aged asphalt binder to its original state. However, MAR led to lower phase angle, and thus, it improved the elastic component. When aged PMA binder was rejuvenated using 15% AR, the RI value was larger than 1, indicating a strong softening effect. The rejuvenation index of MAR was found to be very close to the aging index, indicating a limited improvement on phase angle. Tables 5 and 6 present a summary of aging index (AI) and rejuvenation index (RI) for various aged and rejuvenated asphalt binders at high-temperature range. TR, complex modulus G* and phase angle d at 60 °C were used for the evaluation of aging and rejuvenation index as mentioned before. It should be noted that aging usually has a positive effect on rutting resistance, while the addition of rejuvenators softens asphalt binders, and increases their rutting susceptibility. Asphalt binders with higher TR and G*, but lower d are desired for rutting resistance at high temperatures, especially 60 °C. In order to obtain a same trend for these three parameters, the AI and RI based on phase angle were further translated to their corresponding reciprocals, 1/AI(d) and 1/RI(d). The aging index indicated that base asphalt binder was more susceptible to aging compared to PMA. When comparing AR with MAR, it was found that the former tended to have a lower RI based on TR and G*, and a higher RI based on d. This strongly indicated that AR was good for softening purposes, but not good for rutting resistance. In this respect, MAR could be a better choice since the rejuvenated asphalt binders generally showed a higher TR and G*. These results were desirable for rutting resistance at high temperature.

3.5. Low-temperature BBR test results

Fig. 17. Results of aging index (AI) and rejuvenation index (RI) based on phase angle for aged and rejuvenated PMA binders at high temperature range.

Figs. 18 and 19 present the BBR test results of various asphalt binders at a low temperature of 10 °C. The Creep stiffness and m-value were used to evaluate the thermal crack resistance at low temperatures after aging and rejuvenation. In general, an asphalt binder with low creep stiffness together with high m-value is desired to reduce the risk of thermal cracking at low

Table 5 Summary of aging index (AI) and rejuvenation index (RI) for aged and rejuvenated BA binder at high-temperature range. Type of asphalt

BA

BA-LTA

BA-LTA + AR

BA-LTA + MAR

TR @ G*/sind = 1 kPa G* @ 60 °C [kPa] d @ 60 °C [o] AI or RI

68.5 3.20 84.70 – – – – –

81.5* 18.95 75.00 1.190 5.922 0.885 1.129 2.747

68.60 3.14 84.90 1.001 0.981 1.002 0.998 0.993

83* 17.60 65.60 1.212 5.500 0.774 1.291 2.668

TR G* @ 60 °C d @ 60 °C 1/[AI or RI(d)] Average

Table 6 Summary of aging index(AI) and rejuvenation index (RI) for aged and rejuvenated PMA binder at high-temperature range. Type of asphalt

PMA

PMA-LTA

PMA-LTA + AR

PMA-LTA + MAR

TR @ G*/sind = 1 kPa G* @ 60 °C [kPa] d @ 60 °C [°] AI or RI

82.5* 7.18 63.3 – – – – –

84* 17.30 65.60 1.018 2.409 1.036 0.965 1.464

71.8 3.34 71.50 0.870 0.465 1.130 0.885 0.740

81* 9.00 63.4 0.982 1.253 1.002 0.998 1.078

TR G* @ 60 °C d @ 60 °C 1/[AI or RI(d)] Average

W. Hong et al. / Construction and Building Materials 231 (2020) 117154

11

of aged asphalt binder led to an obvious reduction of creep stiffness and an increase of m-value. The softening effect was more significant for AR. A very low creep stiffness was obtained on aged PMA after rejuvenation with AR. MAR exhibited a couple effect of rejuvenation and reinforcement due to the existence of AR and SBS polymer. The addition of SBS polymer was helpful to prevent the over-softening effect induced by aromatic oil. 3.6. FTIR analysis results

Fig. 18. Creep stiffness results of various asphalt binders.

Fig. 19. m-value results of various asphalt binders.

temperature. For this reason, SHRP Superpave specification established a requirement for maximum creep stiffness as 300 MPa and the minimum m-value as 0.3 for effective release of thermal stress, and hence, avoid cracking [26]. As shown in Figs. 18 and 19, long-term aging resulted in higher creep modulus, but lower m-value. Both of these parameters indicated severe aging of BA binder and PMA. In general, addition of rejuvenator to both types

Figs. 20 and 21 present the FTIR spectra obtained from various asphalt binders. For the purpose of comparison, the FTIR spectra obtained from aromatic oil and pure SBS polymer are also presented. The analysis was focused on the sulfoxide (1030 cm-1), carbonyl (1700 cm1) and SBS polybutadiene (PB, 966 cm1) and polystyrene (PS, 699 cm1) bonds. The carbonyl index, sulfoxide index, PB index and the degradation index were calculated using the band area ratio as defined by Eqs. (1)–(4). Table 7 presents a summary of FTIR analysis results on carbonyl index, sulfoxide index, PB index and the degradation index. In FTIR spectra, the difference between BA binder and SBS PMA binder can be well distinguished by 966 cm1 and 699 cm1 wavenumbers, which are introduced by the addition of SBS polymer. Similarly, the addition of MAR can be detected by this method since it contained SBS polymer. After long-term aging, the amplitudes of the spectra of sulfoxide (1030 cm1), and carbonyl (1700 cm1) wavenumbers increased. However, the amplitudes at SBS C@C bonds (966 cm1 and 699 cm1) wavenumbers were reduced because of polymer degradation after aging. Based on the data listed in Table 7, a quantitative analysis on the change of carbonyl index, sulfoxide index, PB index and polymer degradation index was made. After longterm aging, the carbonyl index and sulfoxide index was 0.9% and 4.0% for aged BA, 1.4% and 2.9% for aged PMA, respectively. The sulfoxide index was found to be comparable to that of a 10-year field aged asphalt binder reported by Wu [49]. Due to the polymer degradation, the PB index reduced from 5.7% to 4.8%. The other polymer degradation index, IPB/PS also confirmed this point. The polybutadiene (PB) was degraded and thus resulted in a reduction of IPB/PS. The addition of 15% AR could further dilute the SBS content and thus weaken the polymer network. The SBS polymer incorpo-

Fig. 20. FTIR results obtained from virgin, aged and rejuvenated BA binder.

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W. Hong et al. / Construction and Building Materials 231 (2020) 117154

Fig. 21. FTIR analysis results obtained from virgin, aged and rejuvenated PMA binder.

Table 7 Summary of FTIR analysis results on carbonyl index, sulfoxide index, PB index and polymer degradation index. Type of materials

IS=O [%]

IC=O [%]

IPB [%]

IPB/PS [–]

AR SBS BA BA-LTA BA-LTA + AR BA-LTA + MAR PMA PMA-LTA PMA-LTA + AR PMA-LTA + MAR

2.115 1.348 2.104 4.085 3.395 3.084 1.770 2.907 2.635 2.587

0.698 – 0.012 0.908 0.772 0.330 0.005 1.410 1.235 0.746

– 25.506 – – – 3.744 5.705 4.846 4.070 5.194

– 1.251 – – – 0.944 1.128 1.058 1.001 1.139

rated in MAR was a supplement for the degraded polymer. On one hand, this re-balanced the chemical components in aged PMA after rejuvenation. On the other hand, the additional SBS polymer repaired the damaged polymer network and thus compensated for the limitations due to the softening effect of AR. 4. Conclusions In this paper, base asphalt (BA) binder and SBS polymer modified asphalt (PMA) binder were subjected to long-term aging. The aged BA and PMA binders were then rejuvenated using aromatic oil (AR) and a compound rejuvenator (MAR) containing 77% aromatic oil and 23% SBS polymer. Conventional bitumen tests, DSR rheological test, BBR test and FTIR analysis were performed to evaluate the rejuvenation and modification effects induced by the aromatic oil and SBS polymer. Based on the test results and the analysis presented, the following conclusions can be made: i) Penetration, softening point and ductility tests indicated that 15% aromatic oil was able to fully restore the longterm aged asphalt binders to their virgin state. MAR further improved the softening point and ductility of both aged BA and PMA binder.

ii) DSR rheological results demonstrated that 15% AR could restore the aged BA binders, but it over-softened the PMA binders. This was indicated by the overlap of the master curves of AR treated BA binders and the lower complex modulus master curves for AR treated PMA binders. The polymer modification effect of 15% MAR was well distinguished by the plateau region of phase angle master curves. MAR provided two effects of rejuvenation and modification when compared to AR and thus an improvement on the overall rheological properties. iii) The addition of AR and MAR reduced both the fatigue parameter (G*sind) and the fatigue temperature (temperature at which G*sind = 5000 KPa). The quantitative analysis based on rejuvenation index indicated that AR had a stronger softening effect than MAR, especially for aged PMA binder. This was also demonstrated by the BBR testing results on AR rejuvenated PMA binder, which showed the lowest creep stiffness and the highest m-value among all of the studied asphalt binders. iv) With respects to rutting parameter and high temperature properties, addition of AR reduced the performance grade at high temperatures. However, MAR could prevent the excessive softening effect at high temperatures due the formation of polymer networks. This allowed the MAR rejuvenated binder to maintain a balance between high and low temperature performance. v) FTIR analysis results showed a significant increase of carbonyl index and sulfoxide index after long-term aging, indicating that both BA and PMA binders were subjected to severe aging. The polymer degradation due to aging was indicated by reduced SBS index. However, addition of MAR increased the SBS index. Therefore, the SBS polymer incorporated in MAR introduced SBS polymer networks in aged BA binder, and supplemented for the lost SBS in the aged PMA binder to repair the degraded polymer network. vi) In general, MAR is a promising material that can improve the overall performance of aged base asphalt binders and polymer modified asphalt binders. This benefit should be further evaluated by hot recycling of reclaimed asphalt mixtures.

W. Hong et al. / Construction and Building Materials 231 (2020) 117154

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.

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Acknowledgements This research was funded by National Natural Science Foundation of China (No. U1733121), the Fundamental Research Funds for the Central Universities (WUT: 2019III152CG) and the China Scholarship Council. The authors are grateful for the cooperation between the People’s Republic of China and the Governments of Kenya and express their desire to see a prolonged and stronger cooperation between the two states.

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