*2.3. Test Methods*

#### 2.3.1. Rheological Property Test

The DSR test can measure the complex modulus (*G* ∗ ) and the phase angle (*δ*) of the asphalt, and the rutting resistance of asphalt pavement can be characterized by the rutting factor *G* ∗/ sin *δ*. In this study, high-temperature rheological properties were evaluated by temperature sweep and frequency sweep tests. A rotor with a diameter of 25 mm was selected for base asphalt and reclaimed asphalt, and its setting interval is 1 mm. However, an 8 mm rotor with an interval of 2 mm was chosen for the aged asphalt. The temperature range of the temperature sweep test is 42~72 ◦C. The temperatures of the frequency sweep are 28 ◦C, 40 ◦C, 52 ◦C, 64 ◦C, and 76 ◦C, and its frequency is in the range of 0.1–10 Hz at each temperature. The master curves of the complex modulus and phase angle were constructed by the time-temperature equivalence principle. Based on the time–temperature equivalence principle, the effect of the temperature and loading frequency on the asphalt

material was converted into a reduced frequency by using the displacement factor. The displacement factor can be obtained by the WLF empirical equation:

$$Loga\_T = \frac{-\mathbb{C}\_1 \left(T - T\_{ref}\right)}{\mathbb{C}\_2 + T - T\_{ref}}\tag{1}$$

where *α<sup>T</sup>* is the displacement factor at T; T<sup>R</sup> is the reference temperature; and *C*<sup>1</sup> and *C*<sup>2</sup> are empirical constants.

In addition, the low temperature cracking resistance of asphalt can be characterized by the BBR test. The BBR test can measure the creep stiffness (*S*) and creep rate (*m*), and the test temperature includes −12 ◦C, −18 ◦C, and −24 ◦C.

#### 2.3.2. Anti-Aging Performance Test

The aging resistance of reclaimed asphalt was analyzed by rheological properties after PAV aging and UV aging. The aging resistance of reclaimed asphalt was analyzed according to the effects of rheological indicators (CMAI, PMAI) on the rheological properties of aged asphalt. The calculation of Formulas (2) and (3) is shown below:

$$\text{CMAI} = \frac{\text{G}^\*}{\text{G}\_0^\*} \tag{2}$$

$$\text{PMAI} = \frac{\delta}{\delta\_0} \tag{3}$$

where *G* ∗ is the complex modulus of asphalt after aging; *G* ∗ 0 is the complex modulus of asphalt before aging; *δ* is the phase angle of asphalt after aging; and *δ*<sup>0</sup> is the phase angle of asphalt before aging.

#### 2.3.3. Micro Performance Test

Scanning electron microscopy (SEM) was used to compare the difference between the microstructure and morphology of reclaimed asphalt with the optimum content and base asphalt. The asphalt samples were sprayed with gold prior to SEM.

Fourier transform infrared (FTIR) spectroscopy was adopted to compare the differences in the composition of characteristic functional groups between the reclaimed asphalt with the optimum content and the base asphalt. FTIR spectroscopy studied the physical and chemical changes during the aging and regeneration processes of asphalt, and the information on chemical bonds or functional groups was obtained through the absorption peaks with FTIR spectroscopy. The changes in asphalt functional groups after adding the tung oil composite regenerating agent were analyzed. The FTIR test wavelength range is 500–4000 cm−<sup>1</sup> , and the number of scans is 32.

Gel permeation chromatography (GPC) analyzed the molecular weight distribution changes of asphalt during the aging and regeneration processes. The molecular weight distribution of asphalt measured by GPC is closely related to the macroscopic properties of asphalt [24]. In this study, the mobile phase is tetrahydrofuran (THF), the concentration of the asphalt sample is 2 mg/mL, and the flow rate is 10 mL/min.

#### **3. Orthogonal Test Design and Analysis**

We used orthogonal tables to analyze multi-factor and multi-level experiments [25]. Under the condition that the orthogonal test can ensure the level of each test factor, the same number of tests can simplify the test groups and improve the test efficiency. The components of the tung oil composite regenerating agent mainly composed of tung oil, DOP, C9 petroleum resin, and OMMT were taken as the main factors, and the factor levels of the orthogonal test design are shown in Table 3.


**Table 3.** Factor levels of orthogonal test design.

#### **4. Test Results and Analysis**

*4.1. Orthogonal Test Results of Tung Oil Composite Regenerating Agent*

In order to select the primary and secondary effects of each factor on each index, a 25 ◦C penetration and softening point, a 15 ◦C ductility, and 135 ◦C viscosity of reclaimed asphalt were used as evaluation indexes to discuss, analyze, and determine the optimal level of each factor. The orthogonal test results and preferred combinations are shown in Tables 4 and 5, respectively.

**Table 4.** Orthogonal test results.


**Table 5.** Preferred combinations of orthogonal tests.


It can be seen from Tables 4 and 5 that the performance of aged asphalt can be restored by selecting 70% or 75% of tung oil; however, when the content of tung oil increases from 70% to 75%, the change trend of the penetration is relatively small, while the change trend of the ductility, viscosity, and softening are relatively large. Therefore, the optimal content of tung oil is 75%. With the increase in DOP content, the indexes of the softening point and viscosity first decrease and then increase, while the indexes of the penetration and ductility first increase and then decrease, indicating that the DOP starts to have adverse effects after improving the aging asphalt maximally. As a result, the optimal content of DOP is 15%. To meet the principle whereby the softening point is the minimum while the penetration and ductility are the maximum, the optimal contents of the C9 petroleum resin and OMMT are 6% and 9%, respectively.

According to the comprehensive analysis of the orthogonal test, the best combination of the tung oil composite regenerating agent is A1B2C3D3, namely tung oil: DOP: C9 petroleum resin: OMMT = 25:5:2:3.

*4.2. Effects of Composite Regenerating Agent on the Rheological Properties of Reclaimed Asphalt* 4.2.1. Complex Modulus

Figures 1 and 2 show the effect of the tung oil composite regenerating agent on the complex modulus and phase angle of aged asphalt. The range of the test temperature is 42–72 ◦C, and its increase rate is 2 ◦C/min; the loading frequency ω is 10 rad/s, and the strain control is 12%.

**Figure 1.** Test results of complex modulus (*G* ∗ ) of aged asphalt after the regeneration.

**Figure 2.** Test results of phase angle (*δ*) of aged asphalt after the regeneration.

It can be seen from Figure 1 that the complex modulus *G*\* of all asphalt samples decreases gradually with the increase in temperature. For instance, at the initial test temperature, the *G*\* of aged asphalt is nearly 6 times higher than that of base asphalt, indicating that the aging makes the asphalt harder. The addition of the tung oil composite regenerating agent can reduce the complex modulus of reclaimed asphalt, because the tung oil can dissolve macromolecular substances and supplement the light components of asphalt, softening the asphalt and reducing the complex modulus. As the content of the tung oil composite regenerating agent increases, the *G*\* of each reclaimed asphalt gradually decreases, which has a negative influence on the deformation resistance of the asphalt. However, the appropriate content of the tung oil composite regenerating agent can restore the fluidity of the aged asphalt. The *G*\* of the R-8% reclaimed asphalt is close to or even higher than that of base asphalt, partly because the C9 petroleum resin in the composite regenerating agent is favorable to high temperatures. YAN [22] et al. used tung oil as a regenerating agent to restore the high-temperature rheological properties of aged asphalt

only to the level of base asphalt. However, the composite regenerating agent of tung oil in this paper caused the high-temperature performance of R-8% asphalt to be better than that of matrix asphalt. Therefore, the tung oil composite regenerating agent can restore and improve the deformation resistance of aged asphalt.

#### 4.2.2. Phase Angle

In Figure 2, it is shown that after the asphalt is aged, the phase angle *δ* decreases and the deformation resistance increases. With the addition of the tung oil composite regenerating agent, the phase angle *δ* of reclaimed asphalt gradually decreases and is still smaller than that of base asphalt, indicating the elastic recovery ability of reclaimed asphalt is better than that of base asphalt.

#### 4.2.3. Rutting Factor

The test results of the rutting factor of reclaimed asphalt are shown in Figure 3. As the temperature increases, the *G* ∗/ sin *δ* of all asphalts gradually decreases. Moreover, as the content of the tung oil composite regenerating agent increases, it also declines, indicating that the addition of the regenerating agent and the increase in temperature reduce the deformation resistance of the asphalt. The *G* ∗/ sin *δ* of aged asphalt is the largest, indicating its rutting resistance is the best. As the content of the tung oil composite regenerating agent increases, the *G* ∗/ sin *δ* of reclaimed asphalt gradually decreases and is close to that of base asphalt. The addition of too much of the tung oil composite regenerating agent can lead to poorer rutting resistance of reclaimed asphalt. Therefore, the proper content of the regenerating agent ensures that the reclaimed asphalt has sufficient rutting resistance. The *G* ∗/ sin *δ* of R-8% reclaimed asphalt is very close to or even better than that of base asphalt. Thus, the content of the tung oil composite regenerating agent should not exceed 8%.

**Figure 3.** Test results of rutting factor (*G* ∗/ sin *δ*).

#### 4.2.4. Master Curve

The temperature range of the frequency sweep test is 28–76 ◦C (the temperature interval is 12 ◦C), and its sweep frequency is 0.1–10 Hz. According to the WLF equation [26], the master curve of the complex modulus and phase angle constructed at the reference temperature of 20 ◦C is shown in Figures 4 and 5.

**Figure 4.** Master curve of complex modulus of reclaimed asphalt.

**Figure 5.** Master curve of phase angle of reclaimed asphalt.

As shown in Figure 4, compared with the base asphalt, the aged asphalt shows a higher complex modulus, which is beneficial to the rutting resistance of RAP at a low frequency and high temperature; *G*\* has a great linear relationship with the frequency. As the content of the tung oil composite regenerating agent increases, the *G*\* of the asphalt shifts close to that of base asphalt and increases with the increase in frequency, which means that the asphalt has the advantage of road deformation resistance at a high-frequency state and a low temperature. As the loading frequency decreases and the temperature increases, the *G*\* of reclaimed asphalt decreases and the *G*\* of R-8% reclaimed asphalt is the closest to that of base asphalt.

It can be seen from Figure 5 that the aged asphalt has the smallest *δ* due to the loss of light components, which increases the proportion of elastic components in the asphalt; the addition of the tung oil composite regenerating agent can increase the *δ* of aged asphalt. As the content of the tung oil composite regenerating agent increases, the *δ* of the asphalt gradually increases and is close to that of base asphalt, indicating that the tung oil composite regenerating agent can increase the proportion of viscous components in the aged asphalt and improve the viscoelastic properties of aged asphalt; Moreover, when the content of the tung oil composite regenerating agent is 8%, the *δ* of R-8% asphalt is smaller than that of base asphalt, indicating that the elastic recovery performance of R-8% asphalt is better than that of base asphalt.

The black diagram used to evaluate the viscoelastic properties of asphalt is a diagram of rheological data for asphalt materials in the form of complex modulus and phase angle. Figure 6 shows a black diagram of base asphalt, aged asphalt, and reclaimed asphalt. The curve of aged asphalt is incoherent in the black diagram. However, because the tung oil composite regenerating agent can improve the molecular conformation of aged asphalt to a certain extent, the curve of reclaimed asphalt is smooth, which is basically a coherent curve. The phase angle of reclaimed asphalt is smaller than that of the base asphalt, indicating that the elastic response of reclaimed asphalt is stronger.

**Figure 6.** Black diagram of reclaimed asphalt.

#### 4.2.5. Creep Stiffness and Creep Rate

The results of creep stiffness *S* and creep rate *m* of different asphalt samples were shown in Figures 7 and 8. The test temperatures are −12 ◦C, −18 ◦C, and −24 ◦C. There is no test result for R-10% and R-12% reclaimed asphalts due to their excessive deformation at −12 ◦C.

**Figure 7.** S of different reclaimed asphalt samples.

**Figure 8.** m of different reclaimed asphalt samples.

It can be seen from Figures 7 and 8 that as the content of the tung oil composite regenerating agent increases, the *S* and *m* of reclaimed asphalt gradually decreases and increases, respectively, indicating that with the addition of the tung oil composite regenerating agent, the low-temperature flexibility and cracking resistance of reclaimed asphalt are gradually improved. YAN [22] restored the low-temperature performance of aged asphalt by using tung oil as a regenerating agent. At −18 ◦C, the S of the regenerated asphalt with 8% tung oil is 170 MPa, and its m is 0.38, while the content of the tung oil composite regenerating agent in this paper is 8%, the S of R-8% asphalt is 135 MPa, and its m is 0.375, indicating that the tung oil composite regenerating agent has better recovery ability compared to the low-temperature performance of aged asphalt. This is because the plasticizer in the tung oil composite regenerating agent can improve the flexibility, low-temperature ductility, and crack resistance of the asphalt. Compared with those of base asphalt, the *S* and *m* of R-8% reclaimed asphalt decrease by 60% and increase by 15.1% at −12 ◦C, respectively; at −18 ◦C, the *S* and *m* decrease by 57.7% and increase by 21.4%, respectively; and at −24 ◦C, the *S* and *m* decrease by 41.1% and increase by 23.2%, indicating that the tung oil composite regenerating agent can not only restore the *S* and *m* of aged asphalt to the level of base asphalt, but also improve the low-temperature crack resistance of reclaimed asphalt with the optimal content, which is better than that of base asphalt.
