Evaluating the Effects of RA on the Rheological Properties and Aging Susceptibility of RAM Asphalt

Abstract

Recycling agents (RAs) can mitigate the adverse effects of reclaimed asphalt pavement (RAP) on recycled asphalt, which can result in more RAP being added. To explore the effects of four RAs on recycled asphalt, this study used asphalt rheological performance experiments, including the bending beam rheometer (BBR) test, the dynamic shear rheological (DSR) test, and the indirect tensile asphalt cracking test (IDEAL-CT), to study the performance grade (PG), cracking resistance, and cracking susceptibility of recycled asphalt. In addition, an aging model for asphalt was used, and short-term and long-term aging sensitivities were evaluated according to this model. Results showed that US Soybean significantly enhanced the high-temperature and low-temperature performance grade (PGLT) and cracking resistance and reduced cracking sensitivity of the recycled asphalt. In addition, the short-term aging sensitivity was the lowest. It showed the best regeneration effect. However, the recycled asphalt with US Soybean showed the highest long-term aging sensitivity, suggesting that longer aging time results in poorer performance. Ingevity’s PG, cracking resistance, and cracking sensitivity are slightly lower than that of US Soybean, and Ingevity demonstrated the second lowest short-term aging sensitivity. However, its prolonged aging sensitivity was much lower than that of US Soybean. Asphalt and Wax Innovations and Georgia Pacific agents slightly improved recycled asphalt properties. The long-term aging sensitivity of recycled asphalt with four Ras was higher than that of recycled asphalt without Ras. These results indicated that the recycled asphalt’s performance with Ras worsened as aging time increased compared to the original recycled asphalt.

1. Introduction

Reclaimed asphalt pavement (RAP) is commonly used for asphalt mixtures in pavement construction and is derived from crushing and reclaiming old pavements [1]. Large quantities of RAP are produced during road maintenance and repair, and RAP can be recycled and mixed with new asphalt and aggregate as part of new asphalt mixtures. When the RAP gets mixed in with the virgin asphalt binder, part of the aged asphalt from the RAP binder becomes reactivated and forms part of the ‘total binder.’ This approach not only solves the problem of RAP disposal but also saves new asphalt and aggregates and reduces environmental pollution. However, RAP’s chemical components and mechanical characteristics become poor because of exposure to oxygen, ultraviolet light [2], and moisture during use [3]. Generally, recycled mixtures containing over 40% RAP by total weight of asphalt mixture are harder than the original mixture, making it difficult to machine and reducing its service life significantly [4]. Moreover, the fusion degree of recycled asphalt varies, and the delamination phenomenon between recycled asphalt and RAP can quickly occur. The binders’ aging process has two stages: short-term and long-term aging [5,6]. The short-term aging stage happens because of heat during agitation and compacting [7]. The long-term aging phase is caused by parameters such as sunlight, oxygen, and longevity over the pavement life. The aging of asphalt is considered a critical issue during road engineering applications [8].

Therefore, the recycling agent (RA) is an essential means of restoring the recycled mixtures’ properties due to the deterioration of the aged asphalt’s rheological properties. In earlier studies, researchers added a soft binder to aged asphalt and found that it reduced its viscosity and positively affected its performance recovery [9,10]. The National Centre for Asphalt Technology has recently classified RAs into five broad categories based on the source: (1) paraffin oils, (2) aromatics, (3) naphthenic oils, (4) tall oils, and (5) fatty acids [11]. An appropriate method for selecting the optimal RA dosage should be based on the RA dosage’s effect on related properties of recycled asphalt and the recycled asphalt mixture, as well as the various states of aging that asphalt binders face during their lifetime.

Using the performance grade (PG) system, researchers evaluated variations in RAP binder stiffness due to the addition of RAs and variations in RAP binder stiffness due to RA’s addition [12]. A combined chart was developed for this approach to establish the suitable RA dosage to satisfy the required PG values for low-temperature cracking resistance and high-temperature rutting [13]. Zaumanis et al. [14] studied effects of the nine types of RA products on asphalt mixtures using RAP. In addition, it was found that four RAs decreased the low-temperature cracking susceptibility of asphalt mixtures. Furthermore, they found that the penetration index was an excellent RA indicator. Li et al. [15] investigated the low-temperature rutting and cracking properties of SBS-modified asphalt mixtures containing RAP. The findings suggested that appropriate recycling agents enhanced the low-temperature properties of RAP without significantly adversely affecting the compaction of asphalt mixtures with RAP [16]. The use of RA in the RAP restoration process is becoming more and more frequent. However, some studies have found that although RA can improve the performance of aged asphalt, its effectiveness may decrease as the aging time increases [17,18].

Several researchers have utilized semicircular bending and indirect stretching tests to assess mixtures’ cracking susceptibility at middle-range temperatures [19,20,21]. The large quantity of RAP leaves asphalt mixtures susceptible to fracture in moderate temperatures. Fracture susceptibility at low temperatures can be assessed using the SCB, bending beam rheometer (BBR), disc compression tensile, and IDT tests [22]. Researchers found that high RAP mixtures were more sensitive to cracking [23,24,25].

In this paper, four types of recycled asphalt with four RAs were prepared. Rheological tests were implemented to measure the recycled asphalt’s crack resistance and cracking sensitivity. Rheological tests involved DSR and BBR experiments. The cracking resistance of different recycled asphalt mixtures was assessed using IDEAL-CT, and cracking resistance indicators were investigated. In addition, the asphalt aging model was used to analyze the sensitivity of six recycled asphalts to short-term and long-term aging. Based on the analysis results of the asphalt performance parameters, four RAs were comprehensively evaluated and recommended.

The technology roadmap for this paper is depicted in Figure 1.

2. Materials and Method

2.1. Materials

The unmodified asphalt utilized in this research was PG58-28, sourced from a petroleum supplier in Minnesota, USA. It is a common base asphalt material. A total of four RAs were used in this study. RA1 is US Soybean, an unsaturated fatty acid commonly used in asphalt modification. RA2 is a warm mix agent named Evotherm that is produced by Ingevity (North Charleston, CS, USA). The main chemical components are hexadecyl imidazoline and sodium dodecyl sulfate. RA3 is SPA140, a wax produced by Asphalt and Wax Innovations (Kapalama, MS, USA), which is used to recycle reclaimed asphalt pavements. RA4 is a saturated fatty acid produced by Georgia Pacific, Inc. (Augusta, GA, USA) and serves as a saturated phenol modifier. The RAP material was obtained from sampled aggregates in Minnesota.

2.2. Preparation

2.2.1. Aged Asphalt Recovery

The binder in the RAP was extracted according to the standard outlined in ASTM D2172 [26]. The asphalt binder extracted was then recycled according to the standard reported in ASTM D1856 [27]. Toluene was selected as the solvent in this process.

2.2.2. Aging of Asphalt

Asphalt was exposed to high temperatures and air during the mixing process. Thus, the short-term aged asphalt (STA) utilized in the study was considered recycled asphalt after mixing. Long-term aged asphalt was prepared using pressure aging. Here, 50 g (±0.5 g) blended recycled asphalt was added to each RTFO vessel, and the container was placed in a pressure aging machine for 20, 40, and 60 h. For convenience, PAV20, PAV40, and PAV60 denote asphalt binder aged for 20, 40, and 60 h, respectively.

2.3. Laboratory Testing of Rheological Properties of Recycled Asphalt

2.3.1. DSR Experiments

To evaluate the influence of various RAs on the rheological properties of asphalt at elevated temperatures, DSR tests were performed according to the standard described in ASTM D4402 [28] at (15 °C, 25 °C, 35 °C, 45 °C, and 55 °C) and various frequencies (16 frequencies ranging from 0.1 rad/s to 100 rad/s) [29]. Figure 2 shows the DSR tester. The DSR test was applied to determine the rheological parameters and performance grades, including R, G − R, and PGHT (PG high temperature). Here, R is determined to be the deviation of the logarithmic glassy modulus and logarithmic equilibrium modulus of the recycled asphalt binder. Larger R values indicate higher cracking sensitivity of the asphalt. R is expressed using Equation (1):

$$R=Log {\left|G*\right|}_{glassy}-Log {\left|G*\right|}_{crossover}$$

(1)

where $Log {\left|G*\right|}_{glassy}$ is the logarithm of the complex shear modulus at the glassy asymptote, and $Log {\left|G*\right|}_{crossover}$ is the logarithm of complex shear modulus at the crossover frequency.

The Glover-Rowe parameters (G − R) are utilized to evaluate the fracture susceptibility, with lower G − R parameters indicating better durability of the asphalt to resist cracking. G − R parameters are usually converted to a temperature of 15 °C and frequency of 0.005 rad/s [30,31,32]. When the asphalt’s G − R produces 180 kPa, it is considered crack-producing. When the G − R value is greater than 600 kPa, substantial fracture is created. The G − R value is calculated as shown in Equations (2)–(5):

$$\left|G^{*}\right|\left(f_c\right)=\frac{G_a^{*}}{{\left(1+{\left(\frac{f_c}{f_a}\right)}^k\right)}^{\frac{m_e}{k}}}$$

(2)

$$f_a=f\times {\alpha }_T$$

(3)

$${\alpha }_T={10}^{\frac{-c_1(T-T_R)}{c_2+(T-T_R)}}$$

(4)

$$G-R=\frac{\left|G^{*}\right|{\left(\cos \delta \right)}^2}{\sin \delta }$$

(5)

where δ is the phase angle, $\left|G^{*}\right|$ is the complex shear modulus, $G_a^{*}$ is the glassy complex modulus of asphalt, $f_c$ is the conversion frequency, $k$ and $m_e$ are the shape parameters of the fitted curve, $c_1$ and $c_2$ are fitting parameters,$f_a$ is the reduced frequency, and ${\alpha }_T$ is a time-temperature transformation parameter.

2.3.2. BBR Experiments

BBR experiments were performed to analyze changes in asphalt rheological characteristics at low temperatures [33]. Tests were performed according to ASTM D6648 [34], and the test temperatures were set at −6 °C, −12 °C, and −18 °C and the Figure 3 shows the DSR tester. For this test, asphalt binder samples were heated, poured into a silicon mold to form beams, and trimmed as they cooled. These beams were then soaked in an ethanol bath at a specific temperature for one hour. The specimens were then mounted on a support, and a 3-point load of 980 mN ± 50 mN was applied. The loading times were 240 s each. The creep rate m and creep strength S were measured automatically using the instrument to obtain the graphs of load and deflection as a time function.

The ΔT_c and PGLT (PG low temperature) obtained from BBR experiments were applied to evaluate the asphalt’s low-temperature rheological characteristics. For the BBR experiment, creep flexibility as a function of intrinsic viscoelasticity is applied to determine the bending creep rate (m) and creep stiffness (S) of recycled asphalt binders. The S and m values can be calculated using Equations (6) and (7).

$$S(t)=\frac{P{L}^3}{4b{h}^3\delta \left(t\right)}$$

(6)

$$m\left(t\right)=\left|\frac{dlgS\left(t\right)}{dlgt}\right|$$

(7)

The rheological index, including the ΔT_c parameter, was designed as the critical temperature discrepancy between the S and m values obtained from the BBR test and expressed noted in as Equation (8).

$$\Delta T_c=T_s-T_m$$

(8)

The low-temperature threshold for T_s corresponds to a temperature where S(60) is 300 MPa. The low-temperature threshold for Tm is noted when m(60) is 0.3. The lower the ΔT_c, the more susceptible the asphalt is to non-load-induced cracking problems. When ΔT_c is greater than 0, the asphalt grade is stiffness controlled (S-controlled). If ΔT_c is less than 0, the asphalt grade is relaxation rate controlled (m-controlled) [35]. The Asphalt Institute recommends using −2.5 °C as the cracking alert limit and −5.0 °C as the cracking limit.

2.4. Experiments with Asphalt Mixtures

In this study, the IDEAL-CT test was used to assess the fracture tolerance in asphalt following short-term aging. The test was implemented according to the recommended test method outlined in the ASTM D8225-2019 American Society of Testing Materials [36]. The Figure 4 shows the UTM tester. The test specimens were loaded along the diameter at 25 °C. Before the test started, the samples were heated in an environmental chamber at 25 °C for two hours. The test was performed at a load speed of 50 mm/min and was stopped at a loading force of less than 0.1 kN. The cracking tolerance index was determined using derived load-displacement curves, a more extensive index demonstrating that asphalt is more resistant to cracking. The cracking tolerance index was calculated using Equation (9):

$$I_{CT}=\frac{t{l}_{75}{G}_f}{62D\left|m_{75}\right|}\times {10}^6$$

(9)

where G_f is fracture energy, which is computed via integration of the stress-displacement curve; m₇₅ is the stress-displacement curve ramp at 75 percent of maximum; and l₇₅ is the displacement associated with 75 percent of the maximum load after the maximum load.

2.5. The Asphalt Aging Model

The G − R parameter of asphalt was chosen and calculated as an aging index at a combination of 20 °C and 5 Hz to model changes in asphalt performance with aging time. The primary benefit of utilizing the G − R parameter is the use of the combination of stiffness and relaxation capacities to evaluate the asphalt mixtures’ fracture properties [37]. This was mainly because A, which was developed by SHAP, could not predict the cracking properties of asphalt mixtures. Instead, Glover derived rheological parameters using Maxwell’s model containing springs and sticky pots, which proved helpful as a surrogate for tensile strain damage. This parameter uses DSR results measured at 15 °C and a frequency of 0.005 rad/s. Rowe has simplified the procedure for calculating the Glover parameter, and the G − R parameter can be expressed using Equation (10) as follows:

$$G-R=\left|G^{*}\right|*{(\cos \delta )}^2/\sin \delta$$

(10)

where δ is the phase angle, and $\left|G^{*}\right|$ is the complex shear modulus.

The aging process of asphalt materials includes a period of rapid reaction. This phase is characterized by a rapid increase in the G − R parameter. It is then followed by a slow reaction phase with an almost steady rate. Both of these reaction phases represent essentially separate chemical processes [38]. For rapid reactions, sulfoxide is the main oxidation product, leading to an increase in viscosity [39]. During slow reactions, the ketone is the primary product, leading to an increase in viscosity. The fast constant oxidation kinetics model is shown in Equation (11):

$$Log\left(G-R\right)=Log\left(G-R_{ST}\right)+K\left(1-\exp \left(-R_{a}t\right)\right)+R_{b}t$$

(11)

where G − R_ST is the G − R value after short aging; R_a, R_b, and K are model coefficients; and T is aging time.

The parameters K and R_a are related to the rapid reaction phase, with larger values of K and R_a indicating a faster short-term aging rate of the asphalt during the pavement’s life. The long-term aging factor, R_b, was obtained from a steady state reaction phase calculation and indicates the long-term aging rate. R_b was obtained from a logical regression of the rise data’s rate, and the value of Rb is the line ramp over the steady reaction phase.

3. Results and Discussion

3.1. DSR Results

3.1.1. PGHT

The PGHT of asphalt with different levels of aging is shown in Figure 5. The PGHT results showed that the recycled asphalt with the addition of RA1 had the lowest PGHT among the asphalt samples, both for the STA and long-term aging asphalt, with a difference of approximately 8 °C. This demonstrated that RA1 had a significant regeneration effect. The PGHT of 40% RAP with RA1 was significantly lower than that of 30% RAP and recycled asphalt with other RAs. The PGHT of the RA1 sample with 60 h of aging was the same as that of the 30% RAP sample after 40 h of aging. Recycled asphalt with the addition of RA2 had a slightly lower PGHT than the other four groups. RA2 also significantly affected PGHT, and its regeneration ranked second among the four RAs tested. RA3, RA4, 30% RAP, and 40% RAP had comparable PGHT values. Adding RA3 and RA4 to recycled asphalt did not significantly improve its PG compared to 40% RAP. Otherwise, the PGHT of the asphalt continued to increase as it aged. The PGHT of the asphalt increased the fastest from STA to 20 h of aging, and the PGHT increased more slowly from 20 h of aging to 60 h of aging in all groups, except for RA4.

3.1.2. R Value

Figure 6 shows the R values of various types of recycled asphalt. The R values of six asphalt binders were similar. The cracking sensitivity increased as aging increased. Among them, RA1 had the lowest R value compared to other asphalt samples. The results showed the recycled asphalt with RA1 had the lowest cracking sensitivity. The R value of RA2 slightly decreased compared with 40% RAP and 30% RAP. The R value of recycled asphalt with the addition of RA3 was marginally lower than that of 40% RAP but higher than that of 30% RAP, which indicated that RA3 did not improve the R value of recycled asphalt to the same level as that noted for 30% RAP. In contrast, the R value of asphalt was slightly different from 40% RAP with the addition of RA4. After 40 h and 60 h of aging, the R value of asphalt was higher than 40% RAP. This demonstrated that after prolonged aging, the cracking sensitivity of recycled asphalt with the addition of RA4 was higher compared to 40% RAP. The R value increased faster at the beginning of the aging and slowed down later.

3.1.3. G − R

As the Figure 7 showed, analysis of unaged recycled asphalt revealed the G − R of the asphalt decreased with the addition of all four RAs. These four RAs enhanced the recycled asphalt’s crack resistance. The asphalt’s G − R value increased as aging increased, and the growth trend of G − R was exponential. After 60 h of aging, the asphalt with RA1 showed better cracking resistance. The values are less than 180 kPa, which are lower than the threshold for cracking. The results showed that the cracking resistance of the recycled asphalt with RA1 was substantially enhanced. There was no risk of cracking in the recycled asphalt after long-term aging. The G − R values of the recycled asphalt with RA2 were less that 180 kPa until 40 h of aging. After aging for 60 h, the G − R values of the asphalt exceeded the critical value for cracking, and at this time, the reclaimed asphalt was at risk of cracking. It was suggested that RA2 effectively reduced the likelihood of cracking in recycled asphalt, but cracking still occurred after long-term aging. The G − R values of the other four asphalt binders were less than 180 kPa after aging for 20 h. After aging for 40 h, the G − R values were between 180 kPa and 600 kPa. After aging for 60 h, the G − R value was greater than 600 kPa, indicating that all four asphalt samples were at risk of fracture after prolonged aging. The G − R values of the two recycled asphalts with RA3 and RA4 were less than that of 40% RAP and slightly greater than that of 30% RAP. It was worth noting that the recycled asphalt with the addition of RA3 and RA4 showed better resistance to cracking with a short aging time. The G − R values of the two RAs were not improved to the same level as that of 30% RAP. However, after a long aging period, the cracking was more severe than that noted in the recycled asphalt without the addition of recycling agents.

3.2. BBR Results

3.2.1. PGLT Value

As PGLT results showed in

Table 1. PGLT results.

PGLT	Unaged	20 h PAV	40 h PAV	60 h PAV
RA1	−38.4	−33.8	−30.2	−26.1
RA2	−33.7	−28.3	−26.0	−23.7
RA3	−30.5	−25.8	−22.9	−19.2
RA4	−33.2	−28.4	−24.3	−21.5
30% RAP	−31.5	−28.1	−24.5	−21.5
40% RAP	−31.6	−26.6	−24.0	−22.1

, the PGLT results showed that the addition of RA1, RA2, and RA4 made the PGLT colder. All three asphalt had colder PGLT values than 30% RAP. It suggested that these three RAs could enhance the recycled asphalt’s low-temperature properties, and the effect of the improvement was better than that noted for 30% RAP. Among them, RA1 had the best improvement effect, and the low-temperature performance of recycled asphalt with RA1 was reduced by one grade. This finding suggested that RA1 can significantly improve the low-temperature performance of recycled asphalt. The improvement effect of RA2 ranked second, slightly better than RA4, but there was no increase in grade. RA3 increased the PGLT compared to 40% RAP. RA3 adversely affected the low-temperature performance of recycled asphalt. The PGLT of asphalt increased as the aging time increased, and the effect rate was proportional to the aging time. In addition, with the increase in aging time, the impact of RA1 and RA4 on PGLT of recycled asphalt gradually became smaller compared to RA2 and RA3.

3.2.2. ΔT_c

In general, the asphalt cracking resistance deteriorated with as aging time increased. Based on the changing trend, the decay of ΔT_c was directly proportional to the aging degree. Figure 8 shows that the addition of RA1 to virgin asphalt increases ΔT_c, and in the long-term aging process, RA1 can alleviate the aging asphalt cracking phenomenon. RA1 can substantially improve the anti-cracking effect of recycled asphalt. The impact of its improved post-cracking resistance was better than that of 30% RAP, and the aged recycled asphalt reached the crack warning limit after 60 h of aging. The remaining five groups of recycled asphalt all exceeded the crack warning limit after aging for 40 h. In two groups of recycled asphalt, namely, recycled asphalt with RA3 and RA4, the recycled asphalt’s ΔT_c was less than that noted for 40% RAP, especially after 60 h of aging, when the ΔT_c had exceeded the fracture limit with the addition of the two RAs. After prolonged aging, the two recycled asphalts with the addition RA3 and RA4 showed severe deterioration in their cracking resistance. RA3 and RA4 negatively affected the cracking resistance of asphalt. The asphalt with the addition of RA2 was not aged, and its ΔT_c decreased slightly. The cracking resistance of recycled asphalt with RA2 was better than that of 40% RAP as the aging time increased. However, RA2 did not optimize the cracking resistance of the recycled asphalt to the extent of 30% RAP, with the exception of values obtained after aging for 60 h.

3.3. Mixture Test Results

Figure 9 shows the cracking resistance index of six recycled asphalt using the IDEAL-CT. It was demonstrated that RA3, RA4, 30% RAP, and 40% RAP showed minimal differences in cracking resistance index values. RA3 and RA4 did not increase the asphalt’s cracking resistance index effectively. RA1 and RA2 were far superior to the other asphalt samples with approximately two-fold greater cracking resistance index values compared to the other asphalt binders. It suggested that RA1 and RA2 increased the cracking resistance of recycled asphalt mixtures. In comparison, RA3 and RA4 did not significantly affect the cracking resistance of the asphalt mixtures. The asphalt mixture with RA2 showed the best cracking resistance performance.

3.4. Aging Model Result

Figure 10 shows the aging prediction model for six types of recycled asphalt, and

Table 2. Parameters of the aging models.

G − R	RA1	RA2	RA3	RA4	30% RAP	40% RAP
RS	0.347	0.060	0.102	0.130	0.407	0.493
K	0.316	1.367	1.571	1.795	2.962	1.596
Ra	0.060	0.067	0.105	0.055	0.025	0.067
Rb	0.038	0.020	0.021	0.016	0.002	0.012

shows the different parameters included in the aging model. The LOG(G − R) of asphalt showed a rapid increase followed by a gentle increase. Twenty hours was the point separating the fast increase phase and the gentle increase phase. During the stages of the injection process, RA1 had the lowest K and Rs, which indicated that RA1 had the best tolerance after 20 h of aging. RA3 displayed the worst resistance to short-term aging, with the highest Rs and the most rapid increase in the G − R index after 20 h of aging. The G − R parameter of recycled asphalt with RA3 exceeded that of 40% RAP at approximately 20 h of aging, which indicated the detrimental effect of RA3 on the cracking resistance of recycled asphalt during long-term aging. Recycled asphalt with the addition of RA2 showed insignificant improvements in resistance to short-term aging compared to 40% RAP and 30% RAP. The recycled asphalt with RA4 had Rs and K parameters less than that noted for 30% RAP and 40% RAP. RA4 had a weakening effect on the resistance of recycled asphalt to short-term aging.

The long-term aging stage with more than 20 h of aging time was investigated, and it was found that the addition of RA weakened the recycled asphalt’s ability to resist long-term aging. Recycled asphalt with the addition of RA1 exhibited the weakest resistance to long-term aging. Following the aging model trend, the recycled asphalt with the addition of RA1 may exceed the G − R values of other recycled asphalt samples with longer aging times. This finding indicates that the recycled asphalt with the addition of RA1 became less resistant to cracking after longer periods of aging. The reclaimed asphalt with the addition RA2 had marginally higher LOG(G − R) values than that noted for RA1 during aging, and its values were the second lowest values obtained. RA2 and RA3 had approximately the same Rb values for resisting long-term aging, and these values were slightly worse than RA4. Here, 30% RAP and 40% RAP had the best resistance to long-term aging. This illustrated that the cracking resistance of recycled asphalt with the addition of RA worsened after a longer aging time.

4. Conclusions

This study compared the aging properties of reclaimed asphalt with the addition of four RAs with 30% RAP and 40% RAP. The high-temperature properties, cracking sensitivity, low-temperature properties, and cracking resistance of six recycled asphalt types were evaluated using the following tests: DSR, BBR, and IDEAL-CT. In addition, the aging model of asphalt was employed to predict the changes in the breaking resistance of recycled asphalt with aging time. Specific conclusions were obtained as follows:

(1)

DSR results indicate that asphalt performance levels increase with aging time. RA1 and RA2 improved the recycled asphalt’s high-temperature performance, while RA3 and RA4 had a lesser effect on the reclaimed asphalt. The R value increased as the aging time of asphalt increased. The R values of the six asphalt samples were similar. The R value of RA1 was the lowest, indicating that the recycled asphalt with RA1 had the lowest cracking sensitivity. The G − R increased with the asphalt’s aging time. The modification effect of RA1 was the best, and the G − R value still did not reach the cracking warning value of asphalt after aging for 60 h. The modification effect of RA2 ranked second, and the addition of R3 and R4 had a detrimental impact on the anti-cracking performance.

(2)

Based on the experimental results of the BBR test, the recycled asphalt’s PGLT continued to increase as the aging time increased. All four RAs made the PGHT lower, and RA1 obtained the lowest PGLT. Test results for ΔT_c showed that the ΔT_c continued to be reduced as the aging time increased. RA1 and RA2 improved the asphalt cracking resistance the most. Recycled asphalt with the addition of RA1 did not reach the crack warning limit for ΔT_c after aging for 40 h. RA3 and RA4 improved the cracking resistance of unaged recycled asphalt. Nevertheless, as the aging time increased, the cracking resistance of recycled asphalt with RA3 and RA4 was worse than that noted for 40% RAP.

(3)

The CT experimental results showed that RA1 and RA2 significantly increase the anti-cracking index Ict of asphalt mixtures. Its value increased approximately two-fold compared with that of 40% RAP recycled asphalt, and the addition of RA3 and RA4 had minimal influence on the crack-resistance index.

(4)

The recycled asphalt’s ability to resist short-term and long-term aging was better analyzed using an aging model. Analyzing the fitted parameters, it was found that RA1 had the best capability for short-term aging resistance, and RA3 had the worst ability to resist short-term aging. However, the ability of recycled asphalt with the addition of RA to resist long-term aging was worse than that noted for 30% RAP and 40% RAP. Among them, RA1 had the least resistance to long-term aging. The recycled asphalt with RA1 had the lowest G − R after 60 h of aging. However, according to the predicted trend, the G − R value of recycled asphalt with the addition of RA1 exceeded that of the other five asphalt samples as the aging time increased. This finding indicated that the crack resistance of recycled asphalt with the addition of RA1 would worsen after a longer aging time.

The results of all the experiments were combined to reveal that RA1 had the best regeneration effect and could substantially improve the properties of asphalt up to 60 h of aging. RA2 had the second-best modification effect, but its resistance to long-term aging was better than that of RA1.