Preparation and Performance Analysis of High-Viscosity and Elastic Recovery Modified Asphalt Binder

Qu, Liangchen; Gao, Yingli; Yao, Hui; Cao, Dandan; Pei, Ganpeng; He, Bei; Duan, Kairui; Zhou, Wenjuan

doi:https://doi.org/10.1155/2019/6070685

Advances in Civil Engineering

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Abstract Introduction Results and Discussion Conclusions Data Availability Conflicts of Interest Acknowledgments References Copyright Related Articles

Research Article | Open Access

Volume 2019 | Article ID 6070685 | https://doi.org/10.1155/2019/6070685

Preparation and Performance Analysis of High-Viscosity and Elastic Recovery Modified Asphalt Binder

Liangchen Qu,¹Yingli Gao,¹Hui Yao,²Dandan Cao,²Ganpeng Pei,¹Bei He,¹Kairui Duan,¹and Wenjuan Zhou¹

Academic Editor: Hayri Baytan Ozmen

Received11 Aug 2019

Revised22 Oct 2019

Accepted13 Nov 2019

Published10 Dec 2019

Abstract

This study presented the preparation and performance of a kind of high viscosity and elastic recovery asphalt (HVERA) by using some modifiers. The performance of styrene-butadiene-styrene (SBS), rock asphalt (RA), crumb rubber (CR), and stabilizing agent (SA) for different modifiers was investigated by conventional binder test. Effects of modifiers on the high- and low-temperature properties of HVERA were investigated. The dynamic viscosity (DV) test, dynamic shear rheometer (DSR), and bending beam rheometer (BBR) analysis indicated that the high- and low-temperature rheological properties of asphalt were improved attribute to the addition of mixture of modifiers. Meanwhile, the short-term aging and long-term aging were simulated by rolling thin film oven (RTFO) and pressure aging vessel (PAV) tests. Furthermore, the Fourier transform infrared spectroscopy (FTIR) and scanning electron microscope (SEM) measurements were conducted for obtaining the mechanism and microstructure distribution of the modified asphalt binders. From the test results in this study, it was evident that the addition of SBS, RA, CR, and SA into a neat asphalt binder could both significantly improve the viscosity of the binder at high temperature and lower the creep stiffness at low temperature, which was beneficial to better both high-temperature stability and low-temperature cracking resistance of asphalt pavements. It was proved that the high temperature grade of HVERA could be increased by increasing of RA and a proper percentage of modifiers could be improved by the low temperature grade of HVERA.

1. Introduction

Asphalt, as an adhesive for mineral aggregates, has been widely used in pavement engineering because of its excellent cohesiveness [1]. However, many damages, such as low-temperature cracking, fatigue, and high-temperature rutting, occur with the number of vehicles, and traffic loads increase dramatically as well as the extremely atmospheric conditions [2, 3]. These unfavorable pavement distresses are closely related to cohesive strength and viscoelastic behavior of asphalt binders, which have a bad influence on the pavement performance and service life [4, 5]. With the development of the transportation industry, the vehicle loading on pavement has become more and more heavy; thus, requirements for the physical properties of asphalt are more strict. Simultaneously, seeking for efficiently and practically modified asphalts has gotten growing attention of researchers [6–9].

To prevent the asphalt mixture from deforming under heavy wheel loading in scorching weather and retaining the original structure inside, asphalt must own higher motive viscosity. With higher motive viscosity of asphalt, the aggregate cohered by thick asphalt is difficult to deform in enduring boiling weather and heavy loading [10, 11]. Therefore, the internal structure of the pavement was not easy to be broken and the durable ability of pavement would not be affected. Recently, different types of admixture have been applied to elevate the viscosity of asphalts, such as rubber, resin, polymer, and wax [12]. Styrene-butadiene-styrene (SBS), plasticizer, and crosslinker were used to prepare high-viscosity modified (HVM) asphalt; compared with single-doped SBS-modified asphalts, HVM asphalt owns better high-temperature properties, strong holding power, and deformation resistance [13]. In addition, as the disadvantages of SBS-modified asphalt, such as high initial cost, difficult processing, and poor storage stability, which greatly hinder the development and application of high viscosity-modified asphalt, it is significant to study composed modified asphalt or have some alternative modifiers such elastomers and waste tire to substitute for SBS in modified asphalt [14, 15].

Several strategies mainly proved that the incorporation of modifiers into SBS-modified asphalt could enhance binder viscosity and storage stability. The addition of crumb rubber into asphalt induces a significant increase in binder viscosity [16–19]. As the viscous property of asphalt rubber is critical to mixture compaction temperature and binder workability during storage and pumping process, the viscosity of asphalt rubber has been the central focus in previous research work [20]. Mo et al. [21] added 20 wt.% crumb rubber powder into SBS-modified bitumen to produce a blended binder. They found that bituminous plug expansion joint materials containing high content of crumb rubber (CR) and granules performed well to deal with thermal and traffic loads. Zhang and Hu [22] studied the addition of sulfur on the performance of CR/SBS-modified (CRSM) asphalt. The result indicated that the addition of sulfur improved high-temperature performance of modified binders and the susceptibility of vulcanized binder to dynamic shear.

Some researchers have also been devoted to conduct experiments on composed modified asphalt to get a more satisfactory pavement performance. Cai et al. [23] proposed an asphalt modified by nanosilica, rock asphalt, and SBS; it has been found that the nanosilica/rock asphalt/SBS-modified asphalt had higher temperature stability, low-temperature cracking resistance, moisture susceptibility, and durability than those of 5% SBS-modified asphalt, but the fatigue life was similar. Some scholars [24] also found that the combined effect of high-density polyethylene (HDPE) and SBS on properties of asphalt, and it was indicated that the phase transition temperature of HDPE-SBS-modified binder was improved. It is recommended that the optimal percentage of SBS and HDPE was 4.5% and 1.5% of modified asphalt totaling mass.

Some researchers prepared high-viscosity modified asphalt and investigated the optimum dosage of modifiers through a series of tests such as penetration test, softening point test, toughness test, tenacity test, 60°C dynamic viscosity test, 135°C rotational viscosity, elastic recovery test, and dynamic shear rheometry (DSR) test [25, 26]. The morphology of the samples was observed using scanning electron microscope (SEM), and the chemical compatibility was characterized by Fourier transform infrared spectroscopy (FTIR) test. Dynamic shear rheometer (DSR) and rotational viscosity (RV) were used to characterize three types of modified asphalt of rolling thin film oven (RTFO) aging [27, 28]. According to their research, the increase of aging time will lead to an increase of viscosity for the modified asphalts at high temperature. It should be noted however that there was no clear trend in the viscosity change for the rubber-modified binder with and without aging. The SEM techniques were carried out to capture the effect of modified binder characteristics, including rubber concentration, on crumb rubber-modified (CRM) binder viscosity [29]. Their tests proposed that the CRM type plays an obvious role in influencing the viscous properties of the CRM binder. Meanwhile, many laboratory testing and field observations were carried out to investigate the interaction effects such as blending time, temperature, and rubber content of CRM binders with the aid of the DSR and RV test [30, 31]. Their work illustrated that longer blending time and higher blending temperature result in a higher viscosity of CRM binders.

Based on the previous studies, this study provided an efficient way in preparing high-viscosity and elastic recovery asphalt (HVERA) by the addition of SBS, CR, rock asphalt (RA), and stabilizing agent (SA) and studying the effect of each modifier on the major physical properties and aging resistance. Further, the BBR and DSR tests were used for rheological behavior of modified asphalt with and without two aging methods, rolling thin film oven (RTFO) aging and pressure aging vessel (PAV). Finally, the mechanism of the modifier was characterized by FTIR and SEM. It is anticipated that the results can provide a composite modification technology for preparing HVERA and also give a reference for the solution of pavement distresses.

2. Modified Asphalt Binder Design

2.1. Materials

The AH-70 neat asphalt was used as the base asphalt. Table 1 presents conventional properties of neat asphalt. The radial styrene-butadiene-styrene (SBS) copolymer was supplied by Sinopec Maoming Petrochemical Crop. Ltd., China, containing 28.1 mass/% styrene, and the average molecular weight of SBS is 288,719 g/mol⁻¹. The crumb rubber (CR) with 60-mesh of particle size was from China Sichuan Zhongneng Rubber Co., Ltd. The major properties of CR are displayed in Table 2. Albania rock asphalt (RA) was used in this study. The appearance of RA is grayish black powder, and the major properties of RA are shown in Table 3. Specifically, stabilizing agent (SA) is commercially available as fine grey powder, and the active ingredients are more than 99.0%. The apparent density is 1.35 g/cm³, and the content of antioxidant and crosslinker is more than 20% and 65%, respectively. The residue of 80-mesh sieve and the water content are less than 1% and 0.2%, respectively.

2.2. Conventional Binder Test

The basic properties of asphalts were characterized by ring-and-ball softening point (ASTM D36), penetration (25°C, ASTM D5), ductility (5°C, ASTM D113), storage stability test (ASTM D6930), dynamic viscosity test, and elastic recovery test.

The dynamic viscosity test was conducted to evaluate the apparent viscosity of the asphalt binders by vacuum capillary tube viscometers at 60°C (ASTM D2171). In most countries, the high-temperature performance of high-viscosity asphalt was evaluated by the value of dynamic viscosity at 60°C. The time was measured for a fixed volume of the asphalt binder to be drawn up through a capillary tube by means of vacuum, under closely controlled conditions of vacuum and temperature. The viscosity in Pascal-seconds was calculated by multiplying the flow time in seconds by the viscometer calibration factor. Each sample was tested three times at least for accuracy, and the final result is averaged.

The elastic recovery was evaluated by the Infra Test 20-2346 digital ductility tester [32]. The three parallel binder samples were stretched at a speed of 5 cm/min simultaneously. The test temperature was constant at 25°C. When the binder samples were elongated to 10 cm ± 0.25 cm, stretching was stopped and the binder samples were cut from the middle quickly with scissors. The appearance of HVERA samples before stretching and 2 s, 30 min, and 1 h after being cut was recorded by the camera. Then, standing binder samples in water at 25°C for 1 h, the length of the binder samples were measured, and the elastic recovery rate was calculated using the following equation:where D denotes elastic recovery rate (unit: %) and X denotes residual length (unit: cm).

2.3. Rheological Characterization

In this study, dynamic shear rheometer (DSR) (MRP301, Anton Paar, Ostfildern, Germany) and a bending beam rheometer (BBR) (Cannon Instrument Company, State College, Pennsylvania) were used to investigate the rheological behaviors of unaged and aged asphalt binders. The aged asphalt binders were obtained by conducting the rolling thin film oven (RTFO) and pressure aging vessel (PAV) tests on the asphalts in the lab. In the RTFO test, each asphalt sample with a weight of 35 g ± 0.5 g is heated in an oven at 163°C for 85 min (AASHTO T240). In the PAV test, the asphalt binder from the RTFO is then placed in pans and aged at 100°C with a pressurized air of 2.10 MPa for 20 h (AASHTO R28).

Temperature sweep test was conducted at an angular frequency of 10 rad/s and oscillation strain of 12% using a 25 mm diameter DSR plates with a 1 mm gap. The test temperature incrementally increased from 58 to 90°C. The low-temperature creep test was conducted on the basis of BBR according to AASHTO T313-12, and the test was performed at −12°C, −18°C, −24°C, and −30°C, respectively.

2.4. Characterization

2.4.1. Infrared Spectrum Test

The Fourier transform infrared spectroscopy (FTIR) test was performed by using Fourier transform infrared absorption spectrometer of TENSOR27 type. The test range was 4000∼400 cm⁻¹, the resolution was 4 cm⁻¹, and the number of scans was 32 times.

2.4.2. Scanning Electron Microscope Test

The SEM test was used to investigate the microstructure of asphalt by using Zeiss tungsten filament scanning electron microscope (EVO MA10). The magnification factor is 0∼30000 times, and the maximum resolution is 3 nm.

2.5. Preparation and Mix Design

In this study, three modifiers including SBS, RA, CR, and SA were used to prepare HVERA asphalt. To study the effect of each modifier, based on a large number of preliminary experimental exploration and relevant research experience, the dosage of SBS was 5%, and the range of RA and CR dosage were determined to be 5%∼10% and 10%∼15% of modified asphalt totaling mass, respectively. Mix proportions of HVERA are listed in Table 4. After preparation, the mixture properties were characterized by the ring-and-ball softening point, penetration, ductility storage stability, and dynamic viscosity tests.

Previous studies [13, 33, 34] have illustrated some modification methods about composite modified asphalt, including temperature condition and stirring speed, which could provide a good demonstration. The compatibility of each modifier is different, so the order of adding modifiers should be taken into consideration in the preparation process. In this study, the order of adding modifiers was RA, SBS, and CR. The specific synthesis procedure of HVERA was as follows:(1)Heat the neat asphalt in an oven at 135°C to melt it completely and then putting certain dried RA, mixing with a glass rod for 5 min(2)Heating asphalt to 180°C, then continue adding SBS and CR and mix the mixture with the glass rod for 5 min(3)Stirring at the speed of 2,500 r/min by using an impeller-type stirrer for 25 min to ensure that modifier particles become smaller and evenly disperse in asphalt(4)Shearing mixture at 5,500 r/min by the high-shear dispersing emulsifying machine at a temperature of 180°C for 50 min(5)Keeping at 160°C for 1 h to make CR grow or swell in the asphalt, and then slowly stirred with a glass rod to expel the air

The flowchart of the synthesis of HVERA is shown in Figure 1.

3. Results and Discussion

3.1. Conventional Properties

3.1.1. Effect of SA on Storage Stability of HVERA

In order to evaluate the performance of HVERA for technical improvement, the asphalt binders which add different dosages of CR, RA, and SA were used for experimental investigation. The performance of different asphalt binders was evaluated in terms of conventional binder test, DSR test, and BBR test. Three replicates were conducted in all tests, and the average test values were applied in the subsequent discussion.

A kind of SA [29–31] was introduced to improve the storage stability of HVERA. Because of the great difference in density, structure, and molecular weight between asphalt and SBS modifiers, the thermodynamic system formed between them is unstable, which could lead to segregation during storage or transportation. Adding SA into asphalt is one of the effective and thorough methods to solve the storage stability of asphalt. Thus, the influence of SA on HVERA was discussed in this section. Figures 2(a)∼2(c) show the effect of SA dosage on performance of HVERA.

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The results showed that with the increase of the amount of SA, the changes of penetration, ductility, softening point, elastic recovery, and dynamic viscosity of unaged HVERA were not significant, while there has already been a definite drop of softening point difference between the bar graph of four samples. This revealed profoundly that SA could improve the thermal storage stability of modified asphalt to a certain extent and reduce its segregation softening point difference. This was due to the cross-linking reaction of some components of SA, neat asphalt, and SBS, which increases the affinity and facilitates the formation of a thick phase boundary between the polymer phase and neat asphalt phase, and as a result, the high-temperature viscosity of modified asphalt was increased. For aged binders, there was remarkable decreasing in toughness of HVERA as well as the tenacity. Compared with the original binders, the tenacity value of #6, #7, #8, and #9 after short-term aging procedures has decreased by 52.4%, 50%, 46.5%, and 48.9%, respectively. The toughness value of #6, #7, #8, and #9 was decreased by 20.7%, 26.5%, 26.7%, and 29.4%, respectively, by RTFO aging.

3.1.2. Effect of CR and RA on HVERA

The conventional properties of the asphalt binder with various modifier dosages after laboratory tests were tested, as shown in Figures 2(d)∼2(f). It was worth noting that the dynamic viscosity of neat asphalt before and after aging was 298 Pa·s and 564 Pa·s, respectively. Moreover, the values of ductility at 5°C and tenacity of neat asphalt before and after aging were all 0. It could be seen that the physical properties of HVERA asphalt were improved greatly compared with neat asphalt. HVERA asphalt had better high- and low-temperature properties compared with neat asphalt, as shown by the greatly improved softening point, dynamic viscosity, and low-temperature ductility. Moreover, the higher toughness and tenacity of HVERA asphalt showed better holding power and deformation resistance. It could be seen that with the dosage of CR increased from 10% to 15%, the dynamic viscosity increased from 112,561 Pa·s to 203,965 Pa·s; this indicated that the dynamic viscosity of the asphalt binder could be improved dramatically by CR. In addition, a relatively slighter improvement of the penetration and softening point as well as decreasing of the ductility could be found. As could be seen from Figure 2(e), as similar to the CR, the RA had a significant influence on the dynamic viscosity. With the dosage of RA increased, a slighter increasing of the penetration could be observed, and the ductility decreased in contrast.

After RTFO aging, the overall changing trend of the penetration of the six modified asphalts was reduced, and penetration ratio of #8 was significantly higher than that of the other five modified asphalts; this indicated that the aging degree of #8 was lower than that of the other five modified asphalts. At the same time, the ductility after RTFO aging was reduced; although the ductility of #8 was reduced, the ductility was still above 20 cm. This showed that #8 has better low-temperature anticracking properties. However, the softening point of the modified asphalt after aging had a slight increase, but the difference of softening point before and after aging was not obvious. It was indicated that #8 had better high-temperature stability than that of #5 attribute to the addition of SA of modifiers.

3.2. Elastic Recovery

The appearance of HVERA before stretching and 2 s, 30 min, and 1 h after being cut is shown in Figure 3. It was observed straightly that the elastic recovery state at different times within 1 h after the HVERA samples being cut has obvious differences. Also, the results showed that the elastic recovery which the test samples were cut for 2 s was stronger than that of the other binder samples. It was could be seen from Table 5 that the elastic recovery of each group of samples could meet the standard values, and the order of elastic recovery was #8>#3>#4>#5>#2>#1, and this indicated that #8 had a strong ability to restore the original state after being stretched by external force.

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3.3. High-Temperature Stability

3.3.1. Phase Angle (δ)

Figure 4 shows the phase angle (δ) on average of the unaged and RTFO-aged asphalts with different dosages of CR and RA. It could be seen that δ of #1, #2, and #3 decreased with the dosage of CR increased, with the same trend holding true for the RTFO-aged asphalts. It was indicated that the addition of the CR was conducive to the development of the elastic component in asphalt, which could increase the total resistance to deformation. It was consistent with the trend dependence on CR dosages in the literature [35]. Likewise, δ of #3, #4, and #5 increased with RA increased, which presented that RA transferred elasticity to the asphalt.

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It could be seen that δ of #8 was the lowest in Figure 4, which showed that #8 possessed the strongest elastic proportion. When the temperature rose from 58 to 70°C, δ of the six binders decreased gradually under the condition of unaged or RTFO-aged. δ of #1, #2, #3, #4, and #5 reached the lowest point at 70°C while it was at 84°C for unaged #8. Therefore, the modifiers used in HVERA provided a strong polymer continuous phase, which enabled elastic stability at high temperature.

Compared with the unaged asphalts, δ of RTFO-aging asphalts decreased as a whole. It should be also noted that with the decreasing of δ, it could induce the viscoelastic behavior of asphalt, gradually develops to elasticity, and this showed that the elastic resilience of asphalt after deformation was increased.

3.3.2. Dynamic Shear Modulus ()

The dynamic shear modulus () is used to evaluate the deformation resistance of the asphalt binder under repeated shear. A higher dynamic shear modulus corresponds to a better deformation resistance to shear. The master curve for on average of the modified binders with and without RTFO aging at different test temperatures is plotted in Figure 5. of #1, #2, and #3 increased with the increased dosage of CR ; similarly, of #3, #4, and #5 increased with RA increased, and these results showed that CR and RA could improve the deformation resistance of the asphalt binder.

(a)

(b)

of the six asphalts declined rapidly with the temperature increased. Rising temperature could have intensified the movement of asphalt molecular chains so as to weaken the intermolecular forces of asphalt binders, resulting in decreased stiffness, as well as . Compared with of each asphalt binder of two figures, there were remarkable higher values for aged binders that were RTFO #1>unaged #1, RTFO #2>unaged #2, RTFO #3>unaged #3, and RTFO #4>unaged #4, respectively. This illustrated that RTFO aging processing made the asphalt binder more stiff trend.

3.3.3. Rutting Index

/sin δ is a rutting index of the asphalt binder, which is applied to characterize the resistance to rutting at high temperature. Large rutting index (/sin δ) indicates that the asphalt binder had much higher stiffness and a higher ability to resist to deformation. The rutting index (/sin δ) of binders at 10 rad/s was obtained based on the dynamic shear modulus () and phase angle (δ).

Figure 6 displays /sin δ of the asphalts with and without RTFO aging. The asphalt binder with and without RTFO aging declined rapidly with the temperature increased. The asphalt binder had bigger /sin δ after being aged through RTFO test. When the temperature rose from 58°C to 90°C, /sin δ of HVERA presented higher than that of the other five binders. For unaged asphalt binders, when test temperature was at 58°C, /sin δ of #3 was about 1.4 times and 1.2 times than that of #4 and #1, respectively, which illustrated that the rutting resistance was improved significantly by the addition of the RA and CR. Moreover, when the temperature was at 88°C, #8 still had highest /sin δ which was 3.9 kPa in average. /sin δ of RTFO-aged asphalt was much higher than the unaged asphalt binders, regardless of the type of the base binder. The findings demonstrated straightly that #8 had strong deformation recovery property.

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According to the Strategic Highway Research Program (SHRP) method, when evaluating the high temperature performance grade (PG) of binders, /sin δ of the original binder and the RTFO-aged binder after DSR tests should not less than 1.0 kPa and 2.2 kPa, respectively. From Figures 6(c) and 6(d), it was intuitively found that /sin δ of original neat asphalt at 70°C is less than 1.0 kPa and /sin δ of the residual asphalt after RTFO aging 70°C is less than 2.2 kPa. It can be concluded that the high temperature grade of neat asphalt was PG64. For all RTFO-aged modified binders, /sin δ of them was more than 2.2 kPa during the test temperature. At the same time for modified binders, /sin δ of unaged #1 was 1.03 kPa at 88°C, but the failure temperature was at 90°C; this indicated that the high temperature grade of #1 was PG88. It can be seen that /sin δ of unaged #4 was 1.36 kPa and 0.91 kPa at 82°C and 88°C, respectively. Thus, the high temperature grade of #4 was PG82. It can be captured that /sin δ of binders #2, #3, #5, and #8 did not be less than 1.0 kPa within the range of test temperature. It could be found that the high temperature grade of #4 was less than #3 and #5; this phenomenon was caused by the dosage of RA.

3.4. Low-Temperature Creep

The BBR is a test designed to measure the stiffness and the rate of stress relaxation of the materials. The creep stiffness obtained from BBR test could well characterize the cracking resistance of the asphalt binder at low temperature. The key reporting values were creep stiffness (S) at 60 s and the slope of the master stiffness curve at 60 s (commonly called the “m-value”). Three parallel samples were conducted and the average values were plotted as curves; the low-temperature creep index of six binders with and without PAV aging at 60 s is shown in Figure 7.

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As shown in Figure 7(a), it could be seen that the creep stiffness (S) decreased with the increased dosage of CR when the temperature was the same. The curves of #3, #4, and #5 showed that S of the binder could be decreased by the increase of RA dosage. Simultaneously, the m-value of the binder presented to increase with increasing CR and decrease with increasing RA. Superpave^™ specification (AASHTO M 320) requires the creep stiffness to be less than 300 MPa and m-value to be greater than 0.300 at the test temperature during the performance grading of the asphalt binder. All tested binders failed to meet the criteria but achieved at −12°C regardless of modifier proportions. However, all tested asphalts could meet the criteria at −18°C except #1. As a result, the increasing CR and RA dosage could induce lower creep stiffness for asphalt binders at low temperature.

From Figure 7(b), it could be seen that the creep stiffness (S) of the PAV-aged binder has been increased compared with the unaged binder. Take the binders tested at −18°C as an example, the average S of #1, #2, #3, #4, #5 and HVERA after aging was increased 31%, 36%, 31%, 32%, 30%, and 29%, respectively, than that of the unaged binder. It showed that the low-temperature crack resistance of the modified binder after long-term aging was reduced. The accumulated long-term aging may induce some potential aging on the binder, and due to the aforementioned decreasing of free aromatic and light fraction dosage, it results in the higher S of binders at the same temperature condition. After aging, the m-value of binders was lower than that of unaged binders. The smaller the m-value was, the worse the material relaxation ability was; as a result, the low-temperature performance of binders could be decreased by PAV aging. When the modified binder bore aging process, it was the easier to crack.

According to the SHRP method, when evaluating the low temperature PG of binders, the S and m-value of RTFO + PAV-aged binders at 60 s after BBR tests should meet the requirement that is the creep stiffness to be less than 300 MPa and m-value to be greater than 0.300 at the test temperature during the performance grading of the asphalt binder. Specific values of S and m are visually shown in Table 6. Table 6 indicates that at −12°C, the S and m-value of the neat asphalt and the modified binders meet the requirements. With the decrease of temperature, the S of neat asphalt and modified asphalts was lower than 300 MPa, but the m-value of neat asphalt, binder #1, and binder #2 was lower than 0.3, which was lower than the limit value. This indicated that the low temperature grading of neat asphalt, #1, and #2 was all classified as PG-22 and the low temperature grading of other modified binders #3, #4, #5, and #8 was PG-28, which was improved one grade than that of neat asphalt.

3.5. Microstructure of the Modified Binders

3.5.1. FTIR Analysis

The FTIR spectra of modified asphalt with different modifiers are shown in Figure 8. As could be observed in Figure 8, the FTIR spectra of neat asphalt (Simple 1), SBS-modified asphalt (Simple 2), CR-modified asphalt (Simple 3), RA-modified asphalt (Simple 4), CR/RA/SBS composite-modified asphalt (Simple 5), and CR/RA/SA/SBS composite-modified asphalt (Simple 6) were complicated and there were tens of thousands of compound in the system. But, it could be found that the infrared spectra of Simple 5 were almost the superposition of those of Simple 1, Simple 2, Simple 3, and Simple 4. There was not any new characteristic peak in the region between 3500 cm⁻¹ and 1300 cm⁻¹; only the intensity of absorption peaks at 2358.82 cm⁻¹ and 1598.91 cm⁻¹ decreased slightly (the peak at 2358.82 cm⁻¹ was attributable to the P-H stretching vibration of phosphide in CR and carbon dioxide antisymmetric stretching, and the peak at 1598.91 cm⁻¹ was attributed to benzene ring skeleton conjugated double bond C=C stretching vibration). It was analyzed that P-H bond and carbon dioxide group of phosphate of the CR were interrupted with the addition of the other modifiers. It was noticed that there was no remarkable difference between the infrared spectral curves of Simple 5 and Simple 6, and the addition of SA did not induce new characteristic peak. This further indicated that the addition of SA was a process dominated by physical miscible modification.

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In the fingerprint region between 1300 cm⁻¹ and 600 cm⁻¹, it could be seen from the FTIR spectra of Simple 6 that the out-of-plane bending vibration of the olefin bond C-H (= C-H) weakens at 966.28 cm⁻¹ and the out-of-plane bending vibration of the cis-olefin bond C-H (= C-H) disappears at 698.34 cm⁻¹. And then, 966.28cm⁻¹ and 698.66 cm⁻¹ were the characteristic peaks of the SBS polybutadiene segment double bond and the SBS polystyrene segment, respectively. It was considered that the addition of SA causes the C-H bond of the SBS polystyrene segment and the partial polybutadiene segment double bond to be interrupted. The molecular structure of the SBS changes, the better structure which was more stable was formed with interweave of the matrix asphalt, CR, and RA, and the performance was improved greatly. It could be seen that chemical reactions were carried out with physical blending in this system and the reaction mainly acting on polystyrene segments in CR and SBS.

3.5.2. SEM Analysis of Modifiers

Microscopic morphologies of Simple 1, Simple 2, Simple 3, Simple 4, Simple 5, and Simple 6 were characterized, respectively, as shown in Figure 9. It could be seen from Figure 9(a) that the neat asphalt has a smooth surface with no particles and uniform texture, and the structure was homogeneous as a whole under microscopic conditions. As shown in Figure 9(b), the RA was evenly distributed in neat asphalt, which is mainly due to a large number of micelle particles and mineral particles of RA which were permeated and surrounded by the neat asphalt. Also, many particles show microporous structure under the microscope; adsorption of the small bitumen molecule makes the combination between RA with bitumen stronger. Therefore, RA modifier particles and neat asphalt were closely linked, which show better compatibility. As could be seen from Figure 9(c), the CR was dispersed in asphalt with the form of elastic particles, and there was a clear interface between some CR and asphalt, which also causes the phenomenon of nonuniform dispersion and aggregation of CR. But, the CR particles could absorb light components in asphalt, and then swelling, desulfurizing, and degrading, which make CR particles closely bond with the asphalt and form a gelatinous film, thereby improving the performance of the asphalt mortar. Figure 9(d) demonstrates that SBS could be fully dispersed in flocculent or strip form in asphalt under the action of high-speed shearing machine. The SBS segment diffused into asphalt was curled, further absorbs some light components in asphalt and swelling, and then a stable spatial network structure was formed with the SBS segment which was encapsulated by solvent components in asphalt, thus improving the performance of the asphalt binder.

It could be seen from Figures 9(e) and 9(f) that the flocculent SBS in HVERA has basically disappeared, the interface between RA, CR particles, and neat asphalt was completely indistinguishable, almost in one, and presents phenomenon in which many pits and gullies were observed. This was because chemical reaction occurs at the polystyrene segment of SBS in HVERA by the addition of SA, changing the molecular structure of SBS, which the C-H bond of the polystyrene segment and the double bond of partial polybutadiene segment of SBS were interrupted. Grafting reaction between RA, CR, SBS, and base asphalt occurs, among which C-S-C bond plays a role of bridge. After crosslinking between them, a stable three-dimensional network structure of vulcanized macromolecules and vulcanized CR was formed, which restricts the flow and deformation of asphalt, making the bond between asphalt and mineral more close and thereby improving the overall performance dramatically.

3.5.3. SEM Analysis of HVERA

The morphology features of HVERA modified asphalts were analyzed by SEM, as shown in Figure 10. From Figure 10, it was apparent that neat asphalt was homogeneous under SEM, and the surface of samples was slightly wrinkled, which could be considered as homogeneous structure. Figures 10(b)∼10(d) showed that the agglomeration of materials occurred, the particle shape of RP was very irregular, and the surface was uneven. With the increasing of RP, the surface roughness of the samples increased. From Figures 10(d)∼10(f), it could be found that the particle size of RP decreases, and the network structure is formed inside the asphalt, which could enhance the properties of the modified binder.

(a)

(b)

(c)

(d)

(e)

(f)

(g)

(h)

(i)

Figures 10(g)∼10(i) presented that the particle surface was full of tiny folds and pores, and the network structure was better combined with rubber particles. This kind of structure could easily absorb the light components in the asphalt so that the RP particles and the neat asphalt could fuse well. The dispersion of RP particles in asphalt was very uniform, the surface of particles was uniformly wrapped by asphalt, and the surface morphology was fuzzy, which showed that RP particles and asphalt have very good blending effect. From Figure 10(i), significant variation in morphology, it was obvious that a large number of RP agglomerates were formed in the dosage of 0.3% of SA. It was beneficial to increasing the high-temperature viscosity, which was consistent with the results of dynamic viscosity test. The cross-linking reaction of some components of SA, neat asphalt, and SBS could increase the affinity and facilitate the formation of a thick phase boundary between the polymer phase and neat asphalt phase, and as a result, the high-temperature properties of the modified asphalt were increased.

4. Conclusions

In this study, high-viscosity and elastic recovery composite modified asphalt samples were prepared by using several modifiers, such as SBS, CR, RA, and SA. The performances were carried on high-viscosity and elastic recovery composite modified asphalt samples. The major conclusions were drawn as follows:(i)Considering the major properties of HVERA asphalt, the proportion of binder #8 (SBS 5%; RA 7.5%; CR 15%; SA 0.3%) is optimal and reasonable for the preparation of HVERA asphalt.(ii)It was indicated that the storage stability could be improved by SA. The conventional property test results showed that physical properties of HVERA such as softening point and dynamic viscosity could be improved dramatically by CR and RA.(iii)The rutting index of RTFO-aged asphalt was much higher than unaged asphalt binders, regardless of the type of the base binder. DSR test results showed that the high temperature grade of HVERA could be increased with RA increased. According to the SHRP method, low temperature PG of HVERA could be improved one grade by proper percentage of modifiers.(iv)FTIR analysis illustrated that chemical reactions were carried out with physical blending of the proposed modifiers in this system and the reaction mainly acting on polystyrene segments in CR and SBS.(v)The better structure which is more stable was formed with interweave of the neat asphalt, CR, and RA, and the performance was improved greatly according to the SEM analysis.

Data Availability

The data used to support the findings of this study are from previously reported studies, which have been cited. This manuscript does not contain previously published Figures, Tables, and Charts, so all Figures, Tables, and Charts of this manuscript are original.

Conflicts of Interest

The authors declare that they have no conflicts of interest.

Acknowledgments

The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China (Grant nos 51978080 and U1833127), the Nature Science Foundation of Hunan Province (Grant number: 2018JJ4016). The authors also appreciate the funding support from the Beijing Key Laboratory of Traffic Engineering, Beijing University of Technology (Grant number: 2018BJUT-JTJD007) and the Beijing Municipal Commission of Transport (Grant number: 40038003201805).

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Copyright © 2019 Liangchen Qu et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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