Performance Evaluations of Warm-Mix Reaction-Rejuvenated SBS Modified Asphalt Mixtures Incorporated with Wax-Based Additive

Abstract

The high-performance, cleaner rejuvenation of aged SBS-modified asphalt mixtures (ASBSMAMs) has been a hotspot in asphalt research. Currently, the most popular rejuvenation method still involves hot-mix asphalt with a commonly used oil as the rejuvenator for recycling. However, high-quality, cleaner warm-mix rejuvenation technology for ASBSMAMs is still needed to enrich this field. This study considered adopting a polyurethane (PU) prepolymer and 1,4-butanediol diglycidyl ether (BUDGE) as reactive rejuvenators to achieve warm-mix reaction–rejuvenation to enhance the properties of ASBSMAMs with the use of a wax-based additive, Sasobit. A series of tests were conducted to realize this, including the viscosity–temperature correlation of the rejuvenated binders, as well as tests of the moisture-induced damage, high-temperature stability, low-temperature cracking resistance, and fatigue resistance of the rejuvenated mixtures. The results showed the following: through reaction–rejuvenation, Sasobit could reduce the viscosity of the rejuvenated SBSMA (RSBSMA) below 150 °C for warm mixing and slightly decrease the viscosity–temperature susceptibility; warm-mix reaction–rejuvenation helped to improve the resistance to water-immersion-induced damage and freeze–thaw damage in ASBSMAMs; the addition of Sasobit showed benefits in improving their resistance to permanent deformation, with the dynamic stability values exceeding 5700 pass/mm as more than 1% Sasobit was added; the flexural damage resistance of ASBSMAMs at low temperatures could be enhanced after warm-mix reaction–rejuvenation; and, under reaction–rejuvenation conditions, Sasobit did not reduce the fatigue resistance of the RSBSMAM and, conversely, at limited higher dosages, it worked more effectively. Overall, the studied warm-mix reaction–rejuvenation technology has been proven to be effective for the environmental recycling and reuse of ASBSMAMs at high quality.

1. Introduction

The poly(styrene-butadiene-styrene)-modified asphalt mixture (SBSMAM) is one of the most commonly used high-quality paving materials due to its high- and low-temperature properties. Nevertheless, it will age severely and decline in performance over time, so measures need to be taken to improve its safety and comfort [1,2,3]. Therefore, end-of-life pavement materials should be reclaimed and renewed for reapplication under the consideration of resources and the environment.

As for reclaimed asphalt pavement (RAP), the hot-mix asphalt rejuvenation method is generally adopted in practice to improve the engineering properties of rejuvenated mixtures through the incorporation of common oil-type rejuvenators [4,5,6]. Although this method has clear advantages in quality assurance in rejuvenated mixtures, some negative issues, especially the significant energy consumption and toxic gas emissions, raise ecological and environmental concerns [7,8,9]. In cleaner rejuvenation, aged asphalt mixtures are reprocessed at lower temperatures with reduced emissions during rejuvenation. Considering cleaner production, the warm-mix asphalt rejuvenation method is promising as long as the rejuvenated mixtures exhibit properties similar to those achieved by the hot-mix method [10,11,12].

In recent years, some relevant publications have shown that warm-mix rejuvenation methods are reliable in improving the overall performance of aged asphalt mixtures. For instance, Wang et al. [13] prepared a new warm-mix rejuvenator (WR) composed of acer oil, surfactants, plasticizers, and anti-aging agents for the rejuvenation of asphalt mixtures, and they found that the WR was capable of reducing the mixing and compaction temperatures, while the rejuvenated mixture exhibited good low-temperature performance and a long fatigue life. Hasan et al. [14] found that Sasobit additives could enhance the fatigue resistance compared to control blends, for both fresh and aged mixtures. These studies demonstrate that warm-mix rejuvenation is the best rejuvenation method for aged asphalt mixtures as the chemical composition of the rejuvenator is appropriate.

The warm-mix rejuvenation of aged SBSMAMs differs significantly from that of aged virgin asphalt mixtures [15,16]. It is necessary to consider not only the rejuvenation of aged SBSMA binders but also the warm-mix conditions. Due to the decreased mixing temperature, it is recognized that the quality of rejuvenated SBSMA mixtures will generally decline [17,18,19]. To overcome this, it is necessary to improve the overall performance in aged SBSMA mixtures under warm-mix conditions. Some investigations indicate that the high-performance rejuvenation of aged SBSMA mixtures is mostly attributed to their rejuvenated binders, where they include the recovery of both SBS and virgin binders [20,21]. To achieve this, the hot-mix reaction–rejuvenation method was proposed to chemically repair the structure of the aged SBS and binders for the full restoration of aged SBSMA binders. However, it is not clear whether the warm-mix reaction–rejuvenation method can also show similar effects on rejuvenated SBSMA mixtures when compared to the hot method.

To fill this information gap, this study considers employing a commonly used wax-based additive called Sasobit to support the benefits of the warm mix in the rejuvenation of aged SBSMAMs. During rejuvenation, a polyurethane (PU) prepolymer and 1,4-butanediol diglycidyl ether (BUDGE) are adopted as the reactive rejuvenators, according to previous studies. For evaluation, the viscosity–temperature correlations and the molecular structure of the rejuvenated binders are mainly assessed, and the moisture-induced damage, high-temperature stability, low-temperature cracking resistance, and fatigue resistance of the rejuvenated mixtures are examined to determine the availability and applicability of the warm-mix reaction–rejuvenation method.

2. Materials and Experiments

2.1. Raw Materials

2.1.1. SBS-Modified Asphalt (SBSMA) Binder

The SBSMA binder used in this study was provided by Hanyang Municipal Construction Group Co., Ltd., Wuhan, China. As is typical, this binder was prepared in a plant by mixing virgin bitumen with 5% SBS by weight of virgin bitumen, at 175 °C for approximately 1.2 h, with lower shearing rates. According to ASTMs D5, D113, D36, and D4402 [22,23,24,25], the measured values for the penetration at 25 °C, ductility at 5 °C, softening point, and viscosity at 135 °C were 48 dmm, 41.5 cm, 60.4 °C, and 1.20 Pa·s, respectively.

2.1.2. Polyurethane (PU) Prepolymer

The PU prepolymer used in this study was from BASF polyurethane special products (China) Co., Ltd., Guangzhou, China, which is mainly repair the molecular structure of the aged SBS in the asphalt binders so as to improve the performance recovery in the SBSMA binders. It is a mixture that mainly consists of diisocyanate and polyether polyol, where the terminal groups of -NCO are very active and can chemically capture the -OH-based oxygen-containing groups for rigid connections. The molecular structure and basic physical properties of this prepolymer are presented in Figure 1. It is known that the prepolymer is a liquid substance with lower viscosity (25 °C) of 170~250 mPa·s and a higher boiling point of 330 °C, and it is safe and suitable for use in the warm-mix rejuvenation of asphalt mixtures.

2.1.3. 1,4-Butanediol Diglycidyl Ether (BUDGE)

BUDGE, purchased from Jinan Hexin Chemical Co., Ltd., Jinan, China, was used in this study to chemically interact with the -OH-based oxygen-containing groups from aged SBS for flexible connections. It is a transparent liquid with no visible mechanical impurities. Its molecular structure is C₁₀H₁₈O₄. Figure 2 shows its molecular structure and physical properties.

2.1.4. Warm-Mix Additive (Sasobit)

The warm-mix additive used in this study was the wax-based material called Sasobit, sourced from Sasol Performance Chemicals, Sasolburg, South Africa. which is very commonly used in warm-mix asphalt; it is shown in Figure 3. As a phase-changing material, it tends to liquify at heating temperatures exceeding 90 °C, which indicates that it can help to reduce the viscosity of asphalt binders during production. In addition, when the warm-mix asphalt mixture is produced and paved, this distributed additive will return to the original solid state to enhance the hardness of the asphalt binders after being cooled.

2.2. Research Flowchart

The flowchart of this research is displayed in Figure 4 and detailed later. For rejuvenated binders, the viscosity–temperature correlation and molecular structure are first considered if the workability meets the blending requirements and the reactions occur during warm-mix rejuvenation. After this, the main engineering performance of the rejuvenated mixtures at the mixture level is comprehensively evaluated.

2.3. Preparation of Aged and Rejuvenated SBSMA Binders

The aged SBSMA binders were prepared through the following procedures: first, the fresh SBSMA binders were heated to the molten state at 170 °C and then poured into aging-use plates, with approximately 50 g in each; second, they were placed into a 163 °C oven for 48 h for thermal oxidative aging, where the experimental conditions were determined by our repeated tests; and, finally, the aged binders were collected for rejuvenation. As this study aimed to investigate the effects of Sasobit on reaction-rejuvenated SBSMA binders under warm-mix conditions, the standard aging method, namely RTFO and PAV, was not adopted, to simulate the aging procedures of asphalt binders. After measurement, the values of the penetration at 25 °C, ductility at 5 °C, softening point, and viscosity at 135 °C were 26 dmm, 1.1 cm, 70.3 °C, and 3.07 Pa·s, respectively. From this, it can be concluded that fresh SBSMA binders exhibit a severe degree of aging.

The rejuvenated SBSMA binders were prepared as follows: first, the reactive rejuvenators consisted of 1% PU and 4% BUDGE by weight of the aged binder and were prepared according to our previous research [26]; second, the aged binders were heated at 165 °C and then mixed with the fixed-weight rejuvenator and different dosages of Sasobit at 1%, 2%, and 3% by weight for 5 min; and the warm-mix reaction-rejuvenated asphalt binders were prepared. The rejuvenated binders are denoted as 1% Sasobit/RSBSMA, 2% Sasobit/RSBSMA, and 3% Sasobit/RSBSMA, respectively.

2.4. Preparation of SBSMA Mixtures (SBSMAMs)

In China, the AC-13 aggregate gradation is commonly used for road surface paving. Because of this, one type of AC-13 degradation, as shown in

Table 1. The passing percentages of the selected aggregates for AC-13 gradation.

Sieve Size, mm	16	13.2	9.5	4.75	2.36	1.18	0.6	0.3	0.15	0.075
Upper limit, %	100	100	85	68	50	38	28	20	15	8
Lower limit, %	100	90	68	38	24	15	10	7	5	4
Medium value, %	100	95	76.5	53	37	26.5	19.0	13.5	10	6
Job mix, %	100	94	83	51	30	24	20	14	8.7	5.4

, was selected for Marshall design and mixture preparation in this study. According to our previous study [27], bitumen content of 3.5, 4.0, 4.5, 5.0, and 5.5% was selected for the mix design, and the mixtures were prepared at 165 °C for 2 min. By evaluating the volume parameters and Marshall loads, the optimum asphalt content (OAC) was finally determined as 4.8%. After this, all of the SBSMA mixtures for the road performance tests were prepared accordingly.

2.5. Preparation of Aged SBSMAMs

Before rejuvenation, all SBSMAMs underwent short- and long-term aging simulation experiments. For the short-term aging, the mixtures were evenly spread on plates and then placed in a 135 ± 3 °C oven with forced ventilation for 4 h ± 5 min, and they were stirred with a shovel every hour.

With respect to long-term aging, all of the short-term-aged mixtures were moved to an 85 ± 3 °C oven under forced ventilation for 5 d. It should be noted here that the aged mixtures for the tests were pre-molded according to their relevant standards, while the others prepared for rejuvenation were not demolded.

2.6. Preparation of Warm-Mix Reaction-Rejuvenated SBSMA Binders

In previous studies, we determined that the combined use of 1% PU and 4% BUDGE could effectively restore the high- and low-temperature properties of aged SBSMA binders and be used to prepare the best reaction-rejuvenated SBSMA binders. Therefore, we first heated the aged SBSMA binders to the melting state at 165 °C, added 1.0% PU relative to the asphalt mass, and mixed and stirred them for 5 min. The reaction-rejuvenated SBSMA binders were prepared by adding 4.0% BUDGE again and mixing with PU/aged SBSMA binders for 5 min at the same temperature.

Then, on this basis, different doses of Sasobit (1.0, 2.0, 3.0 %) were added and the reaction-rejuvenated SBSMA binders were mixed and stirred at the same temperature at 400 rpm for 5 min, finally obtaining the warm-mix reaction-rejuvenated SBSMA binders. The rejuvenation process is presented in Figure 5.

2.7. Preparation of Warm-Mix Reaction-Rejuvenated SBSMAMs

Before the preparation, the residual asphalt content of the aged SBSMAMs was first determined using the combustion method. After this, 1% PU and 4% BUDGE were added. Further, to balance the binder content of the rejuvenated SBSMA with that of the OAC of the fresh SBSMA, the insufficient binders were added using virgin asphalt with Pen. 70 for hybrid rejuvenation. During this rejuvenation steo, RSBSMA binders with 1, 2, and 3% Sasobit were first prepared and then mixed with aggregates and supplemented virgin asphalt at 165 °C for 2 min [28]. Finally, the warm-mix reaction-rejuvenated SBSMAM specimens were prepared.

3. Test Methodologies

3.1. Fourier-Transform Infrared Spectroscopy Test (FTIR)

In this study, this test was used to detect the changes in the molecular structures of the SBSMA binders after aging and reaction–rejuvenation. The test conditions were set as follows: resolution, 4 cm⁻¹; scanning times, 64; and wavenumber range, 400~4000 cm⁻¹ [29].

3.2. Viscosity–Temperature Susceptibility Test (VTS)

In this study, a Brookfield viscosimeter was used to test the viscosity of the modified binders at five different temperatures (120, 135, 150, 165, and 180 °C), and viscosity–temperature curves were drawn according to the results.

The Saal formula was used to linearly fit the viscosity–temperature curves. As shown in Equation (1), the temperature sensitivity of asphalt can be determined by fitting the slope of the curve.

$$loglog\left(\eta \times {10}^3\right)=n-mlog\left(T+273.15\right)$$

(1)

where $\eta$ represents the viscosity, Pa·s; $T$ represents the test temperature, °C; $n$ represents the intercept; and $m$ represents the curve slope, indicating that the VTS of the binders becomes more susceptible as $m$ increases.

3.3. Moisture-Induced Damage Tests

3.3.1. Marshall Tests

The target mixtures were evaluated in terms of their moisture-induced damage using the immersed Marshall and freeze–thaw splitting tests. For the former test, three standard Marshall specimens for different types were first placed into a 60 °C water bath for 48 h, and the other identical specimens were preconditioned for only 0.5 h, in accordance with ASTM D6927 [30]. After this, the mixtures were placed in a Marshall tester to determine the peak loads. Accordingly, the moisture-induced damage was evaluated by calculating the residual Marshall stability (MS₀):

$${MS}_0=\frac{M{S}_1}{MS}\times 100\%$$

(2)

where MS₀ refers to the residual Marshall stability, %; MS₁ refers to the Marshall load after moisture immersion for 48 h, kN; and MS refers to the Marshall load after moisture immersion for 0.5 h, kN.

3.3.2. Freeze–Thaw Splitting Tests

In this test, three standard Marshall specimens for different types were pretreated after first freezing them in a −18 °C refrigerator for 16 h and then thawing them in a 60 °C water bath for 24 h, in accordance with AASHTO T283 [31]. Afterwards, the treated and untreated specimens were both placed in a 25 °C water bath for no less than 2 h. Finally, the splitting test was carried out for every specimen to collect the peak load for the determination of the freeze–thaw splitting strength ratio (TSR), as per Equations (3)–(5):

$$R_{T1}=0.006287P_{T1}/h_1$$

(3)

$$R_{T2}=0.006287P_{T2}/h_2$$

(4)

$$TSR=\frac{{\overline{R}}_{T2}}{{\overline{R}}_{T1}}\times 100\%$$

(5)

where R_T1 and R_T2 refer to the splitting tensile strengths before and after one freeze–thaw treatment, MPa; P_T1 and P_T2 refer to the peak loads before and after one freeze–thaw treatment, N; h₁ and h₂ refer to the specimen heights before and after one freeze–thaw treatment, mm; and TSR refers to the freeze–thaw splitting strength ratio, %.

3.4. Wheel Tracking Test (WTT)

This test was conducted to characterize the permanent deformation of the asphalt mixtures at elevated temperatures. As per ASTM D8292 [32], the target mixtures were first processed into slabs with fixed dimensions of 300 mm × 300 mm × 50 mm, and they were then moved to and kept in the 60 °C test chamber of the rut device for 5 h. Subsequently, the test was carried out with a back-and-forth rolling speed of 42 pass/min and a wheel load of 0.7 MPa for 60 min. Finally, the rut depth data were recorded to calculate the dynamic stability (DS) as per Equation (6):

$$DS=\frac{\left(t_2-t_1\right)\times N}{d_2-d_1}\times C_1\times C_2$$

(6)

where DS refers to the dynamic stability, cycle/mm; d₁ and d₂ refer to the rut depths at 45 min (t₁) and 60 min (t₂), respectively; C₁ and C₂ refer to the coefficients of the device type and specimen, normally 1.0; and N refers to the round-trip wheel speed, 42 pass/min.

3.5. Low-Temperature Bending Test

The beam bending test was used to examine the low-temperature properties of the target asphalt mixtures. Before the test, the prepared asphalt mixtures were cut into small beams with a fixed size of 250 mm × 30 mm × 35 mm, and then they were placed into the test chamber for insulation at −10 °C for 5 h. Afterwards, the beam was placed across two fulcrums with a span of 200 mm. Finally, the test was carried out with a loading rate of 50 mm/min to collect relevant data for the calculation of the flexural strength, maximum flexural strain, and flexural stiffness modulus. They were calculated according to Equations (7), (8), and (9), respectively.

$$R_B=\frac{3\times L\times P_B}{2\times b\times h^2}$$

(7)

$${\epsilon }_B=\frac{6\times h\times d}{L^2}$$

(8)

$$S_B=\frac{R_B}{{\epsilon }_B}$$

(9)

where R_B refers to the flexural strength at failure, MPa; ${\epsilon }_B$ refers to the flexural strain at failure, με; S_B refers to the flexural stiffness modulus, MPa; b refers to the specimen width, mm; h refers to the specimen height, mm; L refers to the span of the specimen, mm; P_B refers to the maximum load at failure, N; and d refers to the mid-span deflection at failure, mm.

3.6. Semi-Circular Blending (SCB) Test

This test was conducted to characterize the fatigue life of the target asphalt mixtures for the evaluation of the reaction–rejuvenation contribution under the warm-mix conditions. All target SCB specimens were obtained from the half-cutting of standard Marshall specimens, which were cut at the bottom with a seam of 10 mm in length and 2 mm in width. Prior to the test, all specimens needed to be placed into the chamber at 25 °C for insulation for more than 5 h. During the test, a semi-sinusoidal loading mode was adopted, and the loading period, loading time, and loading frequency were set to 0.5 s, 0.25 s, and 2 Hz, respectively. Based on this, the maximum fatigue load for each group was first determined at 25 °C, and the fracture strength (K_IC) was calculated using Equations (10) and (11). After this, the test was carried out at 25 °C with loads at 30, 40, 50, and 60%, using the K_IC of the specimen, for each group. As the data were collected, the relationship between the fatigue life (N_f) and the stress strength factor (k_I) could be established using Equations (12) and (13). Among them, if the value of n is higher, the fatigue susceptibility of the mixture is higher.

$${\sigma }_0=\frac{P_c}{2rt}$$

(10)

$$K_{IC}=Y_I\times {\sigma }_0\sqrt{\pi a}$$

(11)

$$N_f=K\times k_I^{-n}$$

(12)

$$lg{N}_f=lgK-nlg{k}_I$$

(13)

where $K_{IC}$ refers to the fracture strength, MPa × m^0.5; $Y_I$ refers to the test constant, no dimension; $P_c$ refers to the critical load, N; $r$ refers to the radius of the SCB specimen, m; $t$ refers to the specimen thickness, m; $a$ refers to the cut-seam length, m; $N_f$ refers to the fatigue life, no dimension; $k_I$ refers to the stress strength factor; $K$ refers to the ability of the material to resist fatigue, no dimension; and $n$ refers to the fatigue susceptibility with a load.

4. Results and Discussion

4.1. Effect of BUDGE/PU Reactive Rejuvenation on Molecular Structures of Aged SBSMA Binders

Figure 6 shows the effect of BUDGE/PU reactive rejuvenation on the molecular structures of the aged SBSMA binders. After adding 4% BUDGE, the peak signal at 1267 cm⁻¹ was significantly weakened, indicating that a chemical reaction occurred between the epoxy system in the BUDGE and the molecules of the oxidative degradation products of the aged SBS. After the collaborative reaction of PU and BUDGE in the aged SBSMA binders, the peak signal of the C-N stretching vibration belonging to -NH-CO at 1307 cm⁻¹ was significantly enhanced, and a new peak signal caused by the C=C stretching vibration was generated at 1506 cm⁻¹. This showed that the reactive rejuvenation system composed of PU and BUDGE underwent a chemical reaction with the SBS oxidation degradation products in the aged SBSMA binders, and its molecular structure was repaired.

4.2. Viscosity–Temperature Correlation of Warm-Mix Reaction-Rejuvenated SBSMA Binders

Figure 7 shows the influence of warm-mix reaction–rejuvenation on the viscosity–temperature characteristics of the ASBSMA binders. Based on this, the fitted viscosity–temperature and relevant parameters are summarized in

Table 2. Fitted equations and relevant parameters of viscosity–temperature curves of target binders.

Item	Viscosity–Temperature Equation	m	R2
ASBSMA	$lglg\left(\eta \times {10}^3\right)=3.665-0.26lg\left(T+273.15\right)$	0.26	0.999
1% Sasobit/RSBSMA	$lglg\left(\eta \times {10}^3\right)=3.532-0.25lg\left(T+273.15\right)$	0.25	0.973
2% Sasobit/RSBSMA	$lglg\left(\eta \times {10}^3\right)=3.417-0.23lg\left(T+273.15\right)$	0.23	0.960
3% Sasobit/RSBSMA	$lglg\left(\eta \times {10}^3\right)=3.214-0.20lg\left(T+273.15\right)$	0.20	0.935

. The correlation coefficients (R²) of the fitted curve equations are all clearly greater than 0.930, indicating that the fitted curves can accurately reveal the viscosity–temperature correlations of the studied asphalt binders between 120 and 180 °C. In addition, it is believed that the viscosity of Sasobit/RSBSMA binders is lower than that of ASBSMA binders, while the viscosity further decreases and the m value becomes slightly smaller as the dosage of Sasobit increases from 1 to 3%. These results show that Sasobit will significantly reduce the viscosity of RSBSMAs at warm-mix temperatures below 150 °C and slightly decrease their susceptibility to the temperature, which can help to improve the warm-mix effect as it is added at higher dosages. This is because the selected Sasobit is a wax-based material with a melting point of around 100 °C, and it can be formed into an organic liquid to lubricate the asphalt molecules under warm-mix conditions to improve the viscosity reduction.

4.3. Water Stability of Warm-Mix Reaction-Rejuvenated SBSMAMs

4.3.1. Residual Marshall Stability

Figure 8 illustrates the effect of the Sasobit dosage on the Marshall stability of the warm-mix RSBSAM before and after immersion. It is clear that the MS and MS1 values of the ASBSAM are 13.69 and 10.24 kN, respectively. After warm-mix rejuvenation, the MS and MS1 values of Sasobit/RSBSMAM are all increased. In addition, as the Sasobit dosage increases from 1 to 3%, the MS and MS1 values slightly decrease from 14.87 and 13.11 kN to 14.02 and 12.15 kN. These results indicate that warm-mix reaction–rejuvenation can help to improve the Marshall stability of ASBSMAMs before and after immersion, while the appropriate addition of Sasobit only has a slightly negative impact on the Marshall stability of RSBSMAMs.

Figure 9 reflects the effect of the Sasobit dosage on the residual Marshall stability of the warm-mix RSBSAM. It is very clear that the MS0 value of the ASBSAM is 74.8%, while, for the RSBSMAMs incorporated with 1, 2, and 3% Sasobit, the values become 88.2, 87.5, and 86.7%, respectively. These results imply that warm-mix reaction–rejuvenation contributes to improving the moisture-induced damage resistance of ASBSMAMs, although the addition of Sasobit has a slightly negative impact. This is because Sasobit, as a wax-based substance, has poorer adhesion to the material surface, leading to the increased stripping potential of its asphalt blend to aggregates when being immersed in water.

4.3.2. Freezing–Thawing Splitting Strength Ratio (TSR)

Figure 10 presents the effect of the Sasobit dosage on the splitting tensile strength of the warm-mix RSBSAMs before and after one freeze–thaw cycle. The splitting strength values of the ASBSAM before and after one freeze–thaw cycle are clearly 1.82 and 1.45 MPa, respectively. After warm-mix rejuvenation, the splitting strength values of Sasobit/RSBSMAM change slightly from 1.82 and 1.57 MPa to 1.76 and 1.49 MPa and to 1.67 and 1.38 MPa before and after one freeze–thaw cycle, as the Sasobit dosage increases from 1 to 2 and 3%. These results indicate that warm-mix reaction–rejuvenation will not significantly affect the splitting strength of RSBSMAMs, even if the Sasobit dosage reaches 3%.

Figure 11 shows the effect of the Sasobit dosage on the TSR of the warm-mix RSBSAMs after one freeze–thaw cycle. It can be found that the TSR value of the ASBSAM is 79.8%, while, for the RSBSMAMs incorporated with 1, 2, and 3% Sasobit, the values become 86.2%, 84.5%, and 82.6%, respectively. These results suggest that warm-mix reaction–rejuvenation shows benefits in improving the resistance of ASBSMAMs to freeze–thaw damage, but, with the greater incorporation of Sasobit, the resistance of the warm-mix RSBSMAM will slightly weaken. This is because a higher concentration of wax-based material leads to the weakened adhesion of the asphalt binders to the aggregates and thus causes reduced resistance to damage from freeze–thaw induction.

4.4. High-Temperature Performance of Warm-Mix Reaction-Rejuvenated SBSMAMs

Figure 12 shows the effect of the Sasobit dosage on the rut depth development of the RSBSMAMs at 60 °C. The rut depths of all mixtures clearly grow quickly until 1800 s, after which this growth slows down. This is because the freshly prepared mixtures will compact further in the first stage and then exhibit rutting behavior in the second stage. In the second stage, the rut depths of the warm-mix RSBSMAMs incorporated with different dosages of Sasobit are somewhat higher than that of the ASBSMAM; they are all less than 1.5 mm. In addition, with the increasing incorporation of Sasobit, the rut depths of the RSBSMAMs slightly decrease. These results show that the addition of Sasobit will not cause the warm-mix RSBSMAM to develop rut deformation, which offers some benefits in helping to improve the resistance to permanent deformation of the warm-mix RSBSMAM. This is dependent on whether the wax-based Sasobit presents a solid state at 60 °C, which is much more difficult under this condition as compared to asphalt binders.

According to the rut depth results of the target mixtures, the rut depths at 45 and 60 min and the calculated DS values are presented in

Table 3. Results of rut depth and calculated DS values of target mixtures.

Item	t1, min	t2, min	∆t, min	d1, mm	d2, mm	∆d, mm	DS, Pass/mm
ASBSMAM	45	60	15	0.891	0.980	0.089	7078
1% Sasobit/RSBSMAM				1.182	1.296	0.114	5733
2% Sasobit/RSBSMAM				1.061	1.173	0.112	6276
3% Sasobit/RSBSMAM				0.953	1.048	0.095	6631

. It can be found that the ∆d value of the ASBSMAM is only 0.089 mm, and the corresponding DS value is 7078 pass/mm. With the increase in the dosage of Sasobit, the ∆d value of the warm-mix RSBSMAM continues increasing. The ∆d value of the RSBSMAM increases from 0.114 to 0.112 mm and then to 0.095 mm as the Sasobit dosage increases from 1 to 2% and then to 3%, respectively. Correspondingly, the DS values change from 5733 to 6276 pass/mm and then to 6631 pass/mm. These results demonstrate that the addition of Sasobit can help to enhance the resistance of warm-mix RSBSMAMs to high-temperature deformation.

4.5. Low-Temperature Cracking Resistance of Warm-Mix Reaction-Rejuvenated SBSMAM

4.5.1. Flexural Strength

Figure 13 shows the effect of the Sasobit dosage on the flexural strength of the RSBSMAMs at −10 °C: the flexural strength of the ASBSMAM is 8.66 MPa, which changes to 11.44, 10.81, and 10.16 MPa as the RSBSMAM is combined with 1, 2, and 3% Sasobit, respectively. These results indicate that the flexural damage resistance of ASBSMAMs at low temperatures can be enhanced after warm-mix reaction–rejuvenation, which will gradually decrease to some extent with the increase in the dosage of Sasobit. The main reason is that, under low-temperature environments, the wax-based molecules dissolved in the asphalt will precipitate to reform the crystals, resulting in the brittleness and hardness of the rejuvenated asphalt binders and the poorer low-temperature performance of the corresponding RSBSMAM.

4.5.2. Flexural Strain at Break

Figure 14 shows the effect of the Sasobit dosage on the flexural strain of the RSBSMAMs at −10 °C: the maximum flexural strain of the ASBSMAM is 2122 με, which is less than the required 2500 με. As it is warm-mix-rejuvenated with 1, 2, and 3% Sasobit, the maximum flexural strain of the RSBSMAM changes to 3078 με, 2732 με, and 2515 με, respectively, which are all higher than the threshold value of 2500 με. These results suggest that with the use of Sasobit, the warm-mix RSBSMAM can still have good resistance to low-temperature cracking damage, but with an increase in the dosage of Sasobit, the flexural resistance at low temperatures will be somewhat decreased. This is verified in Section 4.5.1.

4.5.3. Flexural Modulus

Figure 15 shows the effect of the Sasobit dosage on the stiffness modulus of the RSBSMAMs at −10 °C: the stiffness modulus of the ASBSMAM at −10 °C is 4081 MPa, which changes to 3716, 3956, and 4039 MPa for the RSBSMAM as it is warm-mix-rejuvenated with 1, 2, and 3% Sasobit. It is noted that the low-temperature stiffness modulus of the RSBSMAMs can be increased towards that of the aged RSBSMAM when the addition of Sasobit is 3%. These results indicate that Sasobit can help to improve the resistance to flexural deformation of RSBSMAMs at low temperatures. However, in this context, it is still necessary to consider the flexural strength and strain to ensure that the warm-mix reaction-rejuvenated SBSMAM has good low-temperature properties.

4.6. Fatigue Resistance of Warm-Mix Reaction-Rejuvenated SBSMAMs

Figure 16 presents the effect of the Sasobit dosage on the fatigue life of the RSBSMAMs. Based on this, the fitted fatigue equations in different forms and the relevant parameters are summarized in

Table 4. Fitted fatigue equations and relevant parameters of target mixtures.

Item	Exponential Equation	Linear Equation	n	lgK	R2
ASBSMAM	$N_f=1812.7\times K_I^{(-1.900)}$	$lg{N}_f=3.258-1.900lg{k}_I$	−1.900	3.258	0.921
1% Sasobit/RSBSMAM	$N_f=7182.5\times K_I^{(-1.641)}$	$lg{N}_f=3.856-1.641lg{k}_I$	−1.641	3.856	0.948
2% Sasobit/RSBSMAM	$N_f=8910.0\times K_I^{(-1.627)}$	$lg{N}_f=3.950-1.627lg{k}_I$	−1.627	3.950	0.948
3% Sasobit/RSBSMAM	$N_f=11280.9\times K_I^{(-1.584)}$	$lg{N}_f=4.052-1.584lg{k}_I$	−1.584	4.052	0.957

. It is clear that the R² values of the fitted equations are all above 0.920, indicating that they can characterize the relationship between the fatigue life and stress intensity factors well. From Figure 16a, compared to the ASBSMAM, Sasobit/RSBSMAM can achieve a longer fatigue life, and, with the increase in the dosage of Sasobit, the fatigue life of Sasobit/RSBSMAM increases. This indicates that, under reaction–rejuvenation, the use of Sasobit can help to improve the resistance of RSBSMAMs to fatigue, which, at limited higher dosages, will work more effectively. From Figure 16b and Table 4, it can be found that the n value for the ASBSMAM is −1.900, and, for the RSBSMAMs with 1, 2, and 3% Sasobit, they are −1.641, −1.627, and −1.584. These results indicate that, compared to ASBSMAMs, the susceptibility of Sasobit/RSBSMAMs to fatigue is lower, and, with an increase in the dosage of Sasobit, the fatigue susceptibility of RSBSMAMs presents a slowly decreasing trend. A possible reason could be that as the Sasobit dosage increases during the mixing, sufficient blending can be reached at warm-mix temperatures to ensure that the reactive rejuvenators can be effectively dissolved into the molecules of the aged SBSMA to improve the chemical interactions, leading to a reduction in the risk of fatigue cracks.

5. Conclusions

This study considered employing a commonly used wax-based additive called Sasobit to provide warm-mix benefits in the rejuvenation of ASBSMAMs. During this rejuvenation process, a polyurethane (PU) prepolymer and 1,4-butanediol diglycidyl ether (BUDGE) were adopted as the reactive rejuvenators to achieve the reaction–rejuvenation of ASBSMAMs. The viscosity–temperature correlations of the rejuvenated binders were evaluated; furthermore, the moisture-induced damage, high-temperature stability, low-temperature cracking resistance, and fatigue resistance of the rejuvenated mixtures were examined to evaluate the applicability of the warm-mix reaction–rejuvenation method. Some main conclusions can be drawn as follows.

• The viscosity–temperature correlation results indicated that Sasobit can significantly reduce the viscosity of RSBSMAs at warm-mix temperatures below 150 °C and slightly decrease the viscosity’s susceptibility to the temperature, especially at a dosage of 3%, through the reaction–rejuvenation method.
• The Marshall test results indicated that, regardless of immersion, warm-mix reaction–rejuvenation can help to improve the Marshall stability and moisture-induced damage resistance of ASBSMAMs, and the limited addition of Sasobit only has a slightly negative impact on the Marshall stability of RSBSMAMs.
• The freeze–thaw test results demonstrated that warm-mix reaction–rejuvenation will not significantly affect the splitting strength of RSBSMAMs, even when the Sasobit dosage reaches 3%, which shows benefits in improving the resistance of ASBSMAMs to freeze–thaw damage. However, with the greater incorporation of Sasobit, the resistance of warm-mix RSBSMAMs will slightly weaken.
• The rut deformation test results suggested that the addition of Sasobit will not cause warm-mix RSBSMAMs to develop rut deformation but shows benefits in helping to improve their resistance to permanent deformation, reaching DS values of 5733, 6276, and 6631 pass/mm with the incorporation of Sasobit at 1, 2, and 3%, respectively.
• The low-temperature performance results showed that the flexural damage resistance of ASBSMAMs at low temperatures can be enhanced after warm-mix reaction–rejuvenation, which will, however, gradually decrease to some extent with the increase in the dosage of Sasobit. In addition, the maximum flexural strain of RSBSMAMs can still reach 2515 με to satisfy the application requirement when the dosage of Sasobit is 3%.
• The fatigue test results implied that, under reaction–rejuvenation conditions, Sasobit will not reduce the fatigue resistance of RSBSMAMs; in fact, at limited higher dosages, they will work more effectively. Compared to ASBSMAMs, the susceptibility of Sasobit/RSBSMAMs to fatigue is lower, and, as the Sasobit dosage increases, the fatigue susceptibility of RSBSMAMs decreases slowly.

Overall, the warm-mix reaction–rejuvenation technology is proven to be effective in environmentally improving the engineering properties of ASBSMAMs, including moisture-induced damage resistance, high-temperature deformation resistance, low-temperature resistance, and fatigue characteristics, for the purpose of the cleaner production and high-quality preparation of RSBSMAMs. Future studies should closely focus on not only the microstructural evaluation and durability of the binders and mixture properties, but also the environmental effects, cost-effectiveness, and social benefits.