High-Temperature Rheological Properties of Crumb Rubber Composite Modified Asphalt

Abstract

As a kind of environmentally friendly material, crumb rubber modified asphalt is widely used in highway engineering to accelerate the consumption of end-life tires. The objective of this study is to investigate the high-temperature rheological properties of crumb rubber composite modified (CRCM) asphalt. In this study, the same content of crumb rubber and different content of composite additives were added to prepare the CRCM asphalt, including CRCM-SBS, CRCM- Sasobit/BRF, and CRCM-RARX. The viscosity, phase angle, rutting factor, critical temperature, storage modulus, non-recoverable creep compliance, percent recovery, stress sensitivity, and the functional groups of all testing specimens were obtained by the rotational viscometer, dynamic shear rheometer, multiple stress creep recovery, and Fourier transform infrared spectroscopy tests to evaluate the high-temperature rheological properties of asphalt. In general, the results were favorable for enhancing the high-temperature performance and reducing the stress sensitivity of asphalt. It showed that the incorporation of crumb rubber and additives increased the viscosity of asphalt under different testing conditions. Additionally, the addition of crumb rubber and additives in the base asphalt could be used to increase elastic components and improve the permanent deformation resistance, and the performances are closely related to the types of additives. Thus, considering the high-temperature performance, the asphalt modified by crumb rubber and RARX additive was recommended to apply to asphalt pavement.

1. Introduction

An increasing number of vehicles on roads all around the world generates millions of used tires every year. The wide application of crumb rubber particles in asphalt pavement has been rapidly increasing in recent decades due to the cost-saving, material-reducing, noise-reducing, and environmentally-friendly characteristics of asphalt pavement [1,2,3,4]. A large number of studies have reported that the addition of crumb rubber could noticeably increase the viscosity of asphalt, and improve the low-temperature performances, skid resistance, durability, and fatigue properties in comparison to virgin asphalt. Additionally, the rubberized asphalt could also enhance moisture susceptibility, rutting resistance, and temperature susceptibility [5,6,7,8,9,10]. In the asphalt mixture, the crumb rubber could be incorporated into the mixes using two different methods, which were referred to as the dry process and the wet process [11,12]. For the wet process, the crumb was added to the asphalt and acted as a kind of modifier. First, the crumb rubber was added to the asphalt and blended to produce the crumb rubber modified asphalt (CRM) with a set time (30 to 90 min), high mixing temperature (usually around 176 to 226 °C), and high blending speed. Then the CRM asphalt was mixed with the mineral aggregate at a certain gradation to obtain the CRM mixture [6,13,14]. However, for the dry process, the crumb rubber particles were used to replace portions of fine aggregate in asphalt mixtures. The rubber particles with designed gradation were mixed with an aggregate before the asphalt was added to the mixtures. In general, the replacement percentage of crumb rubber was around 1% to 3% and the particle size was less than 30 mesh (0.6 mm) [15,16,17].

However, the utilization of rubber also had an adverse influence on the performance of asphalt, such as the reduction of workability due to the increased viscosity. Therefore, the performance of base asphalt was enhanced using composite additives [12,14,17,18,19,20,21]. As a kind of biodegradable, low-cost, environmentally friendly, and renewable material, bio-based fillers could be used as an alternative replacement for synthetic fillers to improve the mechanical, thermal, and rheological properties of asphalt [22,23]. Sasobit is one of the most commonly-used warm agents in highway engineering and the suggested amount is recommended around 1–3% by the weight of asphalt. With the relatively lower drop melting point, it could be easily dissolved into the asphalt to prepare a stable matrix [24]. Additionally, many studies were conducted to determine the optimal preparation process of SBS/CRCM asphalt through the comparison of the material type, swelling time, and shearing time [25]. The rheological properties and chemical characterization of reacted and activated rubber (RAR) modified asphalt were analyzed and the results indicated that the addition of RAR could improve the rutting resistance, fatigue cracking resistance, and low-temperature cracking resistance performances of base asphalt [9,12,26,27]. Additionally, the dominating influencing factor on the high-temperature performance of CRCM asphalt with different composite additives was not identified.

2. Objectives and Scope

The objectives of this study were: (1) to investigate the high-temperature rheological properties of CRCM asphalt; (2) to determine the most effective additive to improve the rheological and physical properties of CRCM asphalt in comparison to the virgin binder. The CRCM asphalt was prepared through the mixing of the virgin binder, different types of composite additives, and the same content of crumb rubber (15% by the weight of asphalt), respectively. Rotational viscometer (RV), dynamic shear rheometer (DSR), multiple stress creep recovery (MSCR), and Fourier transform infrared spectroscopy (FTIR) tests were used to evaluate the high-temperature rheological properties and functional groups of three different types of CRCM asphalt under different testing conditions.

Crumb rubber reacted and activated rubber (RAR), bio-based reinforcing filler (BRF), Sasobit, and SBS are selected as the two kinds of rubber and different types of additives. The viscosity, phase angle, rutting factor, critical temperature, storage modulus, non-recoverable creep compliance, percent recovery, stress sensitivity, and the functional groups of all testing specimens were obtained to evaluate the high-temperature rheological properties of asphalt.

3. Materials

3.1. Asphalt

The petroleum asphalt of penetration grade 70, provided by Shell Xinyue (Foshan) asphalt co. LTC, was chosen as the base binder. In

Table 1. Technical properties of virgin asphalt (petroleum asphalt 70 penetration grade).

Property	Unit	Result	Specification	Method
Penetration at 25 °C	0.1 mm	63.5	60–80	T0604-2011
Penetration index	-	−1.37	−1.5~+1.0	T0604-2011
Softening point (ring & ball method)	°C	48.9	≥46	T0606-2011
Density at 15 °C	g/cm−3	1.038	measured records	T0603-2011
Ductility at 10 °C	cm	31.2	>20	T0605-2011
Ductility at 15 °C	cm	>150	>100	T0605-2011
Dynamic viscosity at 60 °C	Pa.s	208	≥180	T0620-2011
Wax content	%	<2.1	<2.2	T0615-2011
Solubility (TCE)	%	>99.5	>99.5	T0607-2011
Flash point (COC)	°C	>260	>260	T0611-2011
After RTFO (163 °C, 85 min)
Mass loss	%	0.03	≤±0.8	T0610-2011
Residual penetration ratio (25 °C)	%	67	≥61	T0604-2011
Residual ductility (10 °C)	cm	6.9	≥6	T0605-2011
Residual ductility (15 °C)	cm	18	≥15	T0605-2011

, the technical properties are obtained according to Chinese Standard JTG E20-2011 “Standard Test Methods of Bitumen and Bituminous Mixtures for Highway Engineering” Technical Specification for Construction of Highway Asphalt Pavement 2011 and all test results meet the specification.

3.2. Crumb Rubber and Reacted and Activated Rubber

In this study, the crumb rubber and reacted and activated rubber (RAR) are the two types of rubber powders utilized. In order to ensure adequate swelling between the rubber and asphalt, the particle size of the rubber in this study is 60-mesh (0.25 mm). The crumb rubber is produced from the waste tire and the specific gravity is 1.30 and the chemical properties are shown in

Table 2. Properties of rubber used in this study.

Chemical Properties of Crumb Rubber (by Weight of Crumb Rubber)
Property	Percentage (wt. %)
Moisture content	0.6
Ash content	5.11
Carbon black content	28.43
Acetone content	9.85
Fiber content	0.0.1
Sulfur content	1.47
Technical properties of Reacted and Activated Rubber (RAR)
Item	Technique information
Physical state	Solid, Black/grey powder
Odor and appearance	Mild rubber, black/grey with brownish color granules
Bulk density (g/cm−3)	0.6 ± 0.03
Specific gravity	1.031 ± 0.03
Flash point (°C)	>300
Solubility	Insoluble in water
Chemical stability	Incompatible with strong oxidizing

. The reacted and activated rubber is a kind of pretreated rubber produced and named as RARX shown in Figure 1. RARX is an elastomeric asphalt extender developed by hot blending and activation of rubber granulate with a selected asphalt and activated mineral binder stabilizer [18]. The proportion of fine rubber granules, conventional bitumen, and mineral fillers is 62:22:16 (by weight). In some sense, the RARX acted as a kind of composite additive in this study, and the technical information is shown in

Table 3. Technical information of three composite additives used in this study.

Item	Technique Information
Technical information of BRF
Physical state	Black with brownish color powders
PH value	7.0~7.5
Moisture content	0~50
Particle size D90 (μm)	≤13
Organic content (%)	25~35
Inorganic content (%)	35~45
Technical information of Sasobit
Physical state/odor and appearance	White/yellowish prills (small pellets), odorless
Density at 25 ℃ (g/cm−3)	0.94
Flash point (℃)	≥285
Drop melting point (℃)	Around 115 °C
Chemical composition	Long-chain aliphatic hydrocarbon
Technical information of SBS
Structure type	Star-shaped SBS
Styrene/butadiene (S/B)	31/69
Molecular weight (×104)	23~28
Tensile strength (kg·cm−2)	330
Hardness (shore A)	76
Specific gravity	0.94

.

3.3. Composite Additives

As shown in Figure 2, Bio-based reinforcing filler (BRF), Sasobit, and SBS are selected as the commercial additives to prepare the composite modified asphalt, and the technical information is shown in Table 3. Figure 2a shows that the BRF is a kind of black with brownish color powders developed by Shenzhen Bofulong new material technology co., LTD. With the addition of BRF, the tensile properties and thermal oxidation properties of rubber could be improved. In Figure 2b, the Sasobit (Sasol Wax Company, Sasolburg, South Africa) used in this study is a finely crystalline, long-chain aliphatic polyethylene hydrocarbon produced from coal gasification. As a kind of warm agent, Sasobit has good comprehensive benefits, such as decreasing the temperature of mixing and construction, effectively improving the high-temperature performance of asphalt, increasing the service life of the pavement, and reducing cost [28,29]. In Figure 2c, the styrene-butadiene-styrene (SBS) is a block copolymer applied as a kind of composite additive in this study, which could increase the elasticity of asphalt and the star-shaped SBS was utilized in this study [30].

3.4. Preparation of Composite Modified Asphalt

In this study, four testing groups were designed (referred to as testing groups ①~④) and the preparation process is shown in Figure 3. The wet processing was applied to prepare three types of CRCM asphalt [31]. Testing group ① Control was the base asphalt (70#), which was heated at 135 °C to prepare the test samples; Testing group ② (CRCM-SBS) is a kind of composite modified asphalt with the active crumb rubber and SBS (active crumb rubber: SBS: base asphalt = 15:2:100). Firstly, the crumb rubber particles were treated with NaOH at 0.4% concentrations for soaking periods of 30 min to obtain the active crumb rubber [14]. After washing and drying, the active crumb rubber and SBS were added into a 70 grade of base asphalt (preheat and melt at 135 °C). Then, the active crumb rubber-SBS composited modified asphalt was prepared by mixing at the temperature of 175 °C for 60 min by an open blade mixer at a blending speed of 10,000 rpm. Testing group ③ (CRCM- Sasobit/BRF) was a kind of composite modified asphalt with Sasobit and BRF (crumb rubber: Sasobit: BRF: base asphalt = 15:2:4.5:100). Firstly, crumb rubber, Sasobit, and BRF were added into a 70 grade of base asphalt (preheat and melt at 135 °C). Then, the temperature was increased to 175 °C and mixing them for around 60 min by an open blade mixer at a blending speed of 10,000 rpm to obtain the crumb rubber-Sasobit-BRF composited modified asphalt. Testing group ④ (CRCM-RARX) was a kind of composite modified asphalt with RARX. In order to make sure the rubber ratio of asphalt was the same as the other testing groups, the 25.5% additional proportion of RARX (by the weight of base asphalt) was applied in this study. Then, the RARX composited modified asphalt is heated to 175 °C and mixed by an open blade mixer at a blending speed of 10,000 rpm.

4. Experiment Design and Testing Methods

4.1. Design of Experiment

In this study, the rotational viscometer, dynamic shear rheometer, multiple stress creep recovery, and Fourier transform infrared spectroscopy tests were conducted to evaluate the high-temperature rheological properties of four testing groups. The viscosity, phase angle, rutting factor, critical temperature, storage modulus, non-recoverable creep compliance, percent recovery, stress sensitivity, and the functional groups were obtained, and the detailed design of the experiment is shown in Figure 4.

4.2. Rotational Viscometer (RV)

Four testing groups were tested by the Brookfield rotational viscometer (RV) to measure the binder viscosity, determine the flow features of modified asphalt, and obtain the workability parameters [29]. The experiments were conducted according to Chinese standard JTG E20-2011 (AASHTO T 316), and two replicates were conducted for each testing group [32,33]. In the viscosity test, two spindle sizes (21# and 27#), four testing temperatures (135, 155, 160, and 175 °C), and two viscometer speeds (50 rpm and 100 rpm) were selected. For the base binder, the RV tests were conducted with spindle size 21 and a rotational speed of 50 rpm. For the CRCM asphalt, because of the relatively higher viscosity, the spindle size of 27 and a rotational speed of 100 rpm were selected.

4.3. Dynamic Shear Rheometer (DSR)

The rheological properties of different CRCM asphalt could be evaluated by dynamic shear rheometer (DSR) through temperature sweep and frequency sweep [34]. Malvern Kinexus rheometer was used in this study. The experiments were conducted according to Chinese standard JTG E20-2011 (AASHTO T 315-16) with two replicates for each testing group. Both unaged and RTFO-aged asphalt of each group were conducted. The range of test temperature was chosen from 64 °C to 82 °C with an interval of 6 °C. Loading frequencies were 0.1, 1.0, 1.59, 3.0, 5.0, and 10 Hz. The phase angle, complex modulus, and rutting factor were obtained and calculated.

4.4. Multiple Stress Creep Recovery (MSCR) Test

According to AASHTO T 315, the non-recoverable compliance $J_{nr}$ and the recovery of four testing groups were obtained by the multiple stress creep recovery (MSCR) test [35]. Two stress levels (0.1 and 3.2 kPa) and four testing temperatures (64, 70, 76, and 82 °C) were selected to analyze and evaluate the behavior of CRCM asphalt. The test protocol included 20 cycles, and each loading cycle referred to one-second shear stress and a nine-second rest. For the first 10 cycles, the tests were conducted with a 0.1 kPa stress level; for the other 10 cycles, the tests were conducted with a 3.2 kPa stress level.

4.5. Fourier Transform Infrared Spectroscopy (FTIR) Test

FTIR has been widely applied to identify the functional groups in chemicals and to analyze the behavior of asphalt. In this study, the FTIR investigations were conducted on the testing samples to characterize the chemical functional groups and the interaction between the asphalt and additives. The Nicolet FTIR-5700 spectrometer was utilized to test the Control, CRCM-SBS, CRCM-Sasobit/BRF, and CRCM-RARX groups. Then about 0.5 mm thickness of asphalt was spread on the KBr slides. The spectrum range is from 4000 cm⁻¹ to 400 cm⁻¹ resolution of the FTIR test was 0.09 cm⁻¹.

5. Results and Discussion

5.1. Rotational Viscosity

Figure 5 shows the viscosity of virgin asphalt and CRCM asphalt at different temperatures. Rotational viscosity of different types of asphalt (Control, CRCM-SBS, CRCM-Sasobit/BRF, and CRCM-RARX), decreased with the increase in testing temperature. All the CRCM asphalt had a greater viscosity value than that of the Control group which could be explained by the effect of rubber particles in the matrix. Similar results were found in another study [36]. At a testing temperature of 135 °C, the values of rotational viscosity for Control, CRCM-SBS, CRCM-Sasobit/BRF, and CRCM-RARX were 483, 925, 643, 1660 mPa.s and met the requirement of the specification (rotational viscosity less than 3 Pa.s at 135 °C). It meant that the addition of composite additives (such as the SBS, Sasobit/BRF, and RARX) could still ensure adequately flowable for workability and mixability during the construction. Compared to the Control group, the values of viscosity for three types of crumb rubber modified asphalt at four testing temperatures have a significant increase due to the addition of rubber particles, especially for the asphalt with additional RARX. For example, at 135 °C, the increase in viscosity of CRCM-SBS, CRCM-Sasobit/BRF, and CRCM-RARX were 1.92, 1.33, and 3.44 times compared to the Control group, respectively. It could be also deduced that the stiffness of asphalt increased with the addition of the composite additive types. For three types of CRCM asphalt, at all testing temperatures, the viscosity values of CRCM-Sasobit/BRF were lower than the other two CRCM groups, while the CRCM-RARX group had the highest viscosity. One possible explanation is that the addition of RARX affected the viscosity of asphalt due to the activated mineral binder stabilizer (AMBS) [12,18,37]. Concurrently, Sasobit affected the asphalt to reduce the viscosity and drag. Due to the drop melting point of Sasobit being around 115 °C, when the temperature of the testing condition was higher than 115 °C, Sasobit began to melt into liquid and caused the viscosity of the asphalt to reduce further. Thus, the property of Sasobit could beneficially reduce construction temperature and emission of rubber asphalt pavement during mixing, paving, and compacting processes.

5.2. Temperature Sweep

For the temperature sweep of asphalt in the DSR test, the variation of phase angle versus temperature; variation of rutting factor versus temperature, and the critical temperature were obtained.

5.2.1. Variation Rule of Phase Angle

Figure 6a,b show the relationship between the phase angle and temperature with unaged and RTFO-aged specimens of four testing groups. The higher phase angle of the asphalt presents the higher percentage of the viscous component.

Figure 6a displays the values of the phase angle for four testing groups increased with the increasing temperature. It meant that when the temperature became higher and higher, the viscous component increased, and the asphalt tended to become viscous liquid. However, the values of the phase angle for three CRCM groups were lower than that of the Control group at all temperature conditions. It meant that fewer components existed in the asphalt with the addition of rubber and additives. When the temperature was around 64 to 70° C, compared with the Control group, the phase angle of CRCM groups decreased. For example, at 64 °C, the phase angle of the Control was 87.22°, and the phase angle of CRCM-SBS, CRCM-Sasobit/BRF, CRCM-RARX were decreased by 8%, 11%, and 16%, respectively. This displayed that the percentage of viscous components decreased in asphalt with the addition of rubber and additives, and it also indicated that the CRCM asphalt became less susceptible to temperature. For the phase angle of asphalt, the lower increase rate indicated a better temperature sensibility. When the temperature changed from 64 to 70 °C, the increments of Control, CRCM-SBS, CRCM-Sasobit/BRF, and CRCM-RARX were 0.97°, 1.84°, 0.27°, and 3.7°; when temperature changed from 70 to 76 °C, the increments of CRCM-SBS, CRCM-Sasobit/BRF, CRCM-RARX were 1.48°, 0.35°, and 3.14°; when the temperature changed from 76 to 82 °C, the increments of CRCM-Sasobit/BRF and CRCM-RARX were 0.71°, and 2.11°. Compared to all increments of all testing groups, the increment and increasing rate of phase angle for the CRCM asphalt adding with Sasobit and BRF had the smallest value at the same temperature range. It presented that the CRCM-Sasobit/BRF had the least sensitivity to temperature, which also meant that fewers elastic component changed into viscous components in the CRCM-Sasobit/BRF when the temperature increased. Variation rule of phase angle for CRCM-Sasobit/BRF might be a benefit to rutting resistance at a high temperature.

Figure 6b reflects that the variation law of phase angles for four testing groups after RTFO aging was consistent with that of the unaged testing groups. After aging, all the CRCM asphalt had lower phase angles than that of the Control group, which indicated that less viscous and more elastic components existed in the three CRCM testing groups. It also implied that CRCM asphalt might be easier to recover after loading compared with the base asphalt and had better rutting resistance. Overall, the RTFO-aged asphalt had a smaller phase angle generally compared with the unaged asphalt at the same temperature. It might be due to the volatilization of light components in asphalt after the aging condition. For both unaged and RTFO-aged conditions, the increasing rates of phase angle for CRCM-Sasobit/BRF within the whole temperature range were lower than that of three CRCM groups. It meant that the least volatilization of the light component was lost in the CRCM-Sasobit/BRF testing group with the addition of Sasobit and BRF. It also implied that the composite additive of Sasobit and BRF could retard the light component to volatilize, improve the high-temperature performance, and become less susceptible to temperature.

5.2.2. Variation Rule of Rutting Factor

The rutting factor (G*/sinδ) of asphalt was chosen to characterize the permanent deformation resistance at high temperatures and determine the performance grading of the binder [38,39,40]. Figure 7a,b show the relationship between rutting factor and testing temperature with unaged and RTFO-aged four testing groups. According to the definition in the Superpave, the rutting factor indicated the elastic component at high temperatures. The higher rutting factor of the asphalt implied a higher percentage of elastic components existed in asphalt with better deformation resistance. In the function of the rutting factor, it could be calculated on the basis of complex shear modulus (G*) diving to sinδ. It also meant that asphalt would present better rutting resistance with the higher complex shear modulus and lower phase angle.

In Figure 7a, with the temperature increased, the rutting factor of all testing groups decreased significantly. Because higher temperature would make the asphalt softer and the viscous part increase, further leading to the weakening of the rutting resistance. Additionally, the temperature dependency of the rutting factor was significant. Values of the rutting factors for all testing groups decreased with temperature increase. It could also be found that three CRMC testing groups had a larger rutting factor than that of the Control group. That is to say, the addition of crumb rubber could provide the elastic component to asphalt, and different composite additives have different degrees of improvement. Furthermore, the rate of decrease for rutting factors became lower as the temperature increased. Moreover, the addition of crumb rubber increased the rutting factor and different additives had a different degree of improvement. Improving the level of rutting resistance for the testing groups from high to low was CRCM-RARX, CRCM-Sasobit/BRF, CRCM-SBS, and Control group. For example, when the temperature was 70 °C, compared to the Control group, the rutting factor of CRCM-SBS, CRCM-Sasobit/BRF, and CRCM-RARX increased 94%, 181%, and 264%, respectively.

In this study, RTFO was used to obtain the aged asphalt and the rutting factors of four groups are shown in Figure 7b. The variation law of the rutting factor was consistent with that of the unaged groups. Compared to the unaged groups, all values of rutting factors were increased dramatically at the same testing temperature. For instance, rutting factors at 64 °C of Control, CRCM-SBS, CRCM-Sasobit/BRF, and CRCM-RARX were increased by 146%, 187%, 177%, and 211%, respectively. It might be because the light component in the asphalt decreased during the aging processing.

It could also be found from Figure 7a,b, that the rutting factors of three CRCM testing groups were higher than that of the Control at any testing temperature. In particular, the CRCM-RARX testing group had the highest rutting factor value regardless of unaged and RTFO-aged conditions, suggesting a relatively strong permanent deformation resistance. This meant that it was less susceptible to the rutting of CRCM-RARX at high temperatures than the other testing groups. Rutting resistance of these four testing groups from high to low was CRCM-RARX, CRCM-Sasobit/BRF, CRCM-SBS, and the Control group.

5.2.3. Critical Temperature

According to the AASHTO M320, the performance grade (PG) of asphalt could be defined [41]. Within the PG, the high-temperature performance grade was usually based on a certain value of the rutting factor under the various testing condition at every 6 °C interval. In general, higher grades presented better high-temperature performance. Due to 6 °C being selected as the interval, so it was difficult to distinguish the performance of asphalt. Therefore, critical temperature based on the value rutting factor was used to distinguish the high-temperature performance of the four testing groups. As shown in Equation (1), the critical temperature was the minimum value among the temperatures when rutting factors were equal to 1.00 kPa and 2.20 kPa under the unaged and RTFO-aged conditions.

$$T_{G^{\ast }/sin\delta }=\min (T_{G^{\ast }/sin\delta }^{unaged}, T_{G^{\ast }/sin\delta }^{RTFO-aged})$$

(1)

where: $T_{G^{\ast }/sin\delta }$ is the critical temperature of asphalt binder, °C; $T_{G^{\ast }/sin\delta }^{unaged}$ is the temperature value of the unaged asphalt binder when the rutting factor is equal to 1.00 kPa, °C; $T_{G^{\ast }/sin\delta }^{RTFO-aged}$ is the temperature value of the RTFO-aged asphalt binder when the rutting factor is equal to 2.20 kPa, °C.

As displayed in Figure 7a,b, the red lines at the value of 1.00 kPa and 2.20 kPa for the unaged and RTFO-aged asphalt are labeled, and detailed critical temperatures of four testing groups are shown in Figure 8. For the CRCM asphalt, the critical temperatures of CRCM-SBS, CRCM-Sasobit/BRF, and CRCM-RARX were apparently higher than that of the Control group by 7.2, 9, and 15.3 °C, respectively. This indicated that the addition of rubber and additives improved the high-temperature performance of asphalt, and the effect degree was highly related to the type of composite additives. Compared to the other three testing groups, the temperatures of the CRCM-RARX testing group before and aging conditions were almost unchangeable (81.1 and 81.9 °C, rate of increase was less than 1%). This also implied that aging conditions had less effect on the critical temperature of asphalt with RARX. According to the performance-grade asphalt specification, the high-temperature of the Control, CRCM-SBS, CRCM-Sasobit/BRF, and CRCM-RARX testing groups reached PG64, PG70, GP70, and PG 76. Both CRCM-SBS and CRCM-Sasobit/BRF asphalt, the grade of samples, improved one level. At the same time, the performance grade of CRCM-RARX was raised by two levels.

5.2.4. Storage Modulus of Asphalt in Unaged and FTFO-Aged Condition

Asphalt acted as a viscoelastic material which could be expressed by two main parts: elastic and viscous parts. When it was borne to the load, the elastic part was presented with the recoverable deformation after unloading. Further, the energies were stored in the asphalt during the loading and unloading cycle [42,43]. As a real part of the complex modulus, the storage modulus was used as a parameter to characterize the elasticity of asphalt at various temperatures. The storage modulus of four groups is plotted in Figure 9a,b. It was found that all the storage modulus of all unaged asphalt decreased with temperature increase as shown in Figure 9a. However, in general, the decreasing rate was reduced gradually as the temperature increased. For example, when the temperature changed from 64 to 82 °C, storage modulus for unaged CRCM-RARX were 5.90, 3.08, 1.61, and 0.87 kPa and decreasing rates were 47.71%, 47.85%, 45.63%. When the temperature changed from 64 to 70 °C, the storage modulus for the unaged Control group increased to 2.00 and 0.89 kPa and the decreasing rate was 55.48%. Compared to the Control group, the storage modulus of asphalt was affected by the addition of rubber and additives, and the type of composite additive had a significant influence on the value of the storage modulus. It means that the elasticity of rubber-modified asphalt was found to be more enhanced than that of the virgin asphalt. Moreover, when the asphalt was conducted at the same temperature, the unaged asphalt modified with RARX had the largest storage modulus followed by Sasobit/BRF and SBS. Thus, improving the degree of additive from high to low was RARX, Sasobit/BRF, and SBS.

As mentioned above, storage modulus could be used to evaluate the asphalt elasticity at different testing conditions. Testing results of four asphalt after RTFO-aged are shown in Figure 9b. It could be found that the variation law of the storage modulus was consistent with that of the unaged testing groups and all storage moduli were larger than 1.00 kPa during the whole testing temperature range. Values of modulus for the aged testing groups increased more noticeably than that of asphalt without aging. For example, when the temperature was 64 and 70 °C, the storage modulus of the Control group increased by 45.35 and 44.61%; the storage modulus of CRCM-SBS increased by 83.74 and 87.14%. When the temperature changed from 64 to 76 °C, the values of the storage modulus for the CRCM-Sasobit/BRF group increased by 74.15, 71.77, and 67.38%. When the temperature changed from 64 to 82 °C, the values of the storage modulus for the CRCM-RARX group increased by 88.46, 104.27, and 131.08%.

5.3. MSCR Test

For the multiple stress creep recovery (MSCR) test of asphalt through DSR, values of non-recoverable creep compliance, percent recovery, and stress sensitivity could be obtained.

5.3.1. Non-Recoverable Creep Compliance

Non-recoverable creep compliance ($J_{nr}$) could be used to evaluate the deformation resistance of asphalt at high temperatures, and the lower the better [12]. For the four testing groups, values of non-recoverable creep compliance under 0.1 and 3.2 kPa stress levels at 60 °C testing temperatures were obtained from MSCR tests, as shown in Figure 10. It could be found that values of non-recoverable creep compliance for all testing groups increased as the stress level increasing. For instance, when stress was 0.1 kPa, non-recoverable creep compliance of Control, CRCM-SBS, CRCM-Sasobit/BRF, and CRCM-RARX were 1.566, 0.483, 0.260, and 0.130, respectively. When stress increased to 3.2 kPa, values of the above testing groups increased by 9.5, 26.5, 74.6, and 82.3%. However, when the asphalt was conducted in 0.1 kPa at 60 °C, compared to the Control group, the non-recoverable creep compliance of asphalt reduced dramatically with the addition of rubber and additives. Furthermore, the decreasing amplitude was related to the different additive types and the influence of crumb rubber modified asphalt from low to high were SBS, Sasobit/BRF, and RARX. For example, the decreasing percentages of non-recoverable creep compliance for CRCM-SBS, CRCM-Sasobit/BRF, and CRCM-RARX were 69.2, 83.4, and 91.7% compared to the Control group. Thus, the addition of rubber and additives could reduce the value of non-recoverable creep compliance and improve elastic deformation capacity to enhance the non-recoverable deformation resistance under loading, especially for the asphalt with the addition of crumb rubber and RARX additive. It also could be obtained that RARX exhibited an anti-rutting additive to enhance the high-temperature performance of asphalt pavement. When the stress changed to 3.2 kPa, the variation law of non-recoverable creep compliance ($J_{nr}$) of four types of asphalt was the same as the 0.1 kPa level.

5.3.2. Percent Recovery

As an indicator to evaluate elasticity and deforming resistance, the value of percent recovery (R) could be obtained from the MSCR test and the larger the better. In Figure 11, the percent recovery of Control and the other three CRCM groups are presented. When tests were conducted under 0.1 kPa stress at 60 °C, it could be found that the percent recovery of the Control group was very small and equal to 3.5%. It implied that virgin asphalt presented strong viscous characteristics and had relatively poor deformation recovery capability under loading. Compared to the Control group, the percent recovery of asphalt increased notably with the addition of rubber and additives. Furthermore, the increasing amplitude was related to different additive types, and the influences of CRCM asphalt from high to low were RARX, Sasobit/BRF, and SBS. For example, the percent recovery of the Control, CRCM-SBS, CRCM-Sasobit/BRF, and CRCM-RARX was 3.5%, 23.7%, 36.7%, and 59.2%, respectively. Additionally, the increments of percent recovery for CRCM-RARX, CRCM-Sasobit/BRF, and CRCM-SBS were 55.7%, 33.2%, and 20.2% compared to the virgin asphalt. The change was more pronounced with a higher stress level. It was expressed that the addition of rubber and additives could obviously increase percent recovery and improve elastic capacity to enhance deformation resistance under loading, especially for the asphalt with the addition of RARX. When the stress changed to 3.2 kPa, the variation law of percent recovery (R) for four types of asphalt was the same as the 0.1 kPa level. The addition of rubber particles made the asphalt more elastic and thus improved its elasticity deformation restorability. Therefore, asphalt would be insusceptible to produce deformation when it was subjected to load at high temperatures. However, values of percent recovery under 3.2 kPa stress were smaller than that of asphalt under 0.1 kPa. It was consistent with the actual case in highway engineering that the heavy load would generate deeper deformation on asphalt pavement.

5.3.3. Stress Sensitivity

For stress sensitivity, the $J_{nrdiff}$ and $R_{diff}$ were defined as the indicators to present stress sensitivity of non-recoverable creep compliance ($J_{nr}$) and percent recovery ($R$) to stress. Values of $J_{nrdiff}$ and $R_{diff}$ between 0.1 kPa and 3.2 kPa for four testing groups are shown in Figure 12. As seen in Figure 11 and Figure 12, the CRCM asphalt had a high percent recovery, which was one benefit of having a lower value of $R_{diff}$.

Thus, the addition of crumb rubber particles in asphalt acted as a principal influence factor to $R_{diff}$. Concurrently, types of additives made fewer contributions to the $R_{diff}$. For example, values of $R_{diff}$ for Control, CRCM-SBS, CRCM-Sasobit/BRF, and CRCM-RARX groups were 66.71%, 43.04%, 40.87%, and 41.39%, respectively. When compared to $R_{diff}$, it was clear that the values of $R_{diff}$ for virgin asphalt and CRCM asphalt between 0.1 kPa and 3.2 kPa were less than 75%. It complied with limit values in the specification (AASHTO M332).

It was also seen that values of $J_{nrdiff}$ for three types of CRCM asphalt dramatically increased with a combination of composite additives. It might be because the incorporation of rubber and additives enhanced the stiffness of asphalt, and additive types contributed definitively. For instance, when stress levels were changed from 0.1 kPa to 3.2 kPa at 60 °C, $J_{nrdiff}$ of CRCM-SBS, CRCM-Sasobit/BRF, and CRCM-RARX were 26.50%, 26.5%, 82.31%, respectively. However, according to the standard, except for CRCM-RARX asphalt, all the other testing groups had fulfilled the specification ($J_{nrdiff}$ ≤ 75%). It meant that asphalt with added rubber and additives might be at the creep failure stage. Additionally, three testing groups of CRCM asphalt presented higher elasticity. In general, the variation of $J_{nrdiff}$ was more notable than that of $R_{diff}$ between 0.1 kPa and 3.2 kPa at 60 °C as shown in Figure 12. For example, when stress levels were changed from 0.1 kPa to 3.2 kPa at 60 °C, various values of $J_{nrdiff}$ and $R_{diff}$ for CRCM-SBS, CRCM-Sasobit/BRF, and CRCM-RARX were 35% and 179%, 5% and 182%, 1%, and 10%, respectively.

5.4. FTIR Test

In addition, the Fourier Transform Infrared Spectroscopy (FTIR) test was adopted to evaluate chemical functional groups of different testing groups in this research. The testing range is from 400 cm⁻¹ to 4000 cm⁻¹. Specific peak values were observed and identified as different functional groups.

In Figure 13, the peak shapes, and positions of the infrared spectrum for Control, CRCM-SBS, CRCM-Sasobit/BRF, and CRCM-RARX are roughly similar. Due to stretching vibrations of C-H in -CH2-, strong absorption peaks at 2925 cm⁻¹ and 2851 cm⁻¹ are generated. Absorption peak at 1600 cm⁻¹ is caused by stretching vibrations of the benzene ring. Absorption peaks at 1462 cm⁻¹ and 1378 cm⁻¹ are produced by C-H in-plane bending vibration in -CH2- and -CH3, respectively. In the substituted range of benzene rings from 900 cm⁻¹ to 650 cm⁻¹, absorption peaks are caused by out-of-plane bending vibration of =C-H. Compared to the Control, an absorption peak of CRCM-SBS testing group generated by the vibration of the polystyrene segment in SBS is shown at 699 cm⁻¹. Without new absorption peaks’ appearance in the infrared spectrum of CRCM-SBS, the high-temperature performance of asphalt is mainly enhanced by the physical blending of active crumb rubber, SBS, and base asphalt. In the infrared spectrum of CRCM-Sasobit/BRF, an absorption peak is generated by ≡C-H out-of-plane bending vibration at 625 cm⁻¹. With large unsaturation of ≡C-H, the high-temperature performance of asphalt is enhanced according to the addition reaction and polymerization reaction. However, due to the peak intensity of ≡C-H being lower, the improvement effect is limited. Compared to the other three test groups, a strong absorption peak caused by the O-H stretching vibration in CRCM-RARX indicates the hydroxylation reaction of RARX. Therefore, a significant improvement in the high-temperature performance of asphalt is achieved due to the effectively increased activation degree of rubber, and the best high-temperature stability of CRCM-RARX is presented among all the testing groups.

6. Conclusions

In this study, different combinations of composite additive and rubber particles’ impact on the high-temperature rheological performances of asphalt were analyzed before and after RTFO-age. According to the testing results, the following conclusions could be obtained:

• The addition of rubber particles and composite additives increased the viscosity of the virgin asphalt at different testing temperatures and rotational speeds. However, the consequence was highly related to the types of composite additives. Concurrently, the viscosity of crumb rubber composite modified asphalt could still be adequately flowable for the workability and mixability during the construction.
• Crumb rubber composite modified asphalt presented lower non-recoverable creep compliance and higher percent recovery. Additionally, the crumb rubber composite modified asphalt reduced the stress sensitivity of percent difference in recovery, while improving the difference in the non-recoverable creep compliance compared with the virgin binder.
• For the evaluation of high-temperature performance for crumb rubber composite modified asphalt, the conclusions of the rutting factor and MSCR test were in agreement with each other, and the results in a diminishing sequence were: CRCM-RARX, CRCM-Sasobit/BRF, and CRCM-SBS.
• From the results of the microscopic test, a strong absorption peak caused by the O-H stretching vibration exists in CRCM-RARX. Due to the improving activation degree of rubber, the high-temperature performance of CRCM-RARX is effectively enhanced.

Furthermore, the low temperature and fatigue properties of crumb rubber composite modified asphalt with different combinations and contents of composite additive can be investigated in the future. The mechanism property will be further analyzed in the following research.