*2.6. Asphalt Mixture Tests*

#### 2.6.1. Water Damage Resistance Test

The Marshall test and splitting test were used to study the water damage resistance of the mixture; the loading rate was 50 mm/min. For the Marshall test, the unconditioned and the conditional Marshall specimens were held in a 25 ◦C water bath for 0 and 24 h, respectively; for the splitting test, the unconditioned specimens were held in a 15 ◦C water bath for 2 h, while the conditional specimens were first held in a 25 ◦C water bath for 22 h, and then in a 15 ◦C water bath for 2 h.

#### 2.6.2. Low-Temperature Cracking Resistance Test

The −10 ◦C semi-circular bending test was used to study the low-temperature cracking resistance of the mixture, using a universal testing machine (UTM-100, IPC Global, Alpharetta, GA, USA). This test was conducted according to the criterion of ASSHTO TP 105-13. The Marshall specimen (ϕ101.6 mm × 63.5 mm) was cut into two semicircles, and then a 5 mm deep and 3 mm wide groove was cut at the midpoint of the specimen. The specimen was conditioned at −10 ◦C for 2 h, at a loading rate of 0.5 mm/min.

#### 2.6.3. Fatigue Resistance Test

The fatigue resistance test of the mixture was carried out using a semi-circular bending test; the test was conducted using a universal testing machine ((UTM-100, IPC Global, Alpharetta, GA, USA)). The specimens of the fatigue test were manufactured according to ASSHTO TP 105-13. The stress ratios were 0.3, 0.4, 0.5, 0.6, 0.7, and 0.8. The specimens were conditioned at 25 ◦C for 6 h; the loading rate was 0.5 mm/min.

#### 2.6.4. Rutting Resistance Test

The rutting resistance of BRA-rejuvenated RAP was investigated with the wheel tracking test. The size of the rutting resistance test was 300 mm × 300 mm × 50 mm. Before the test, the specimens were conditioned at 60 ◦C for 6 h. The contact pressure of the loading wheel and the specimen surface was 0.7 MPa, and the rolling speed was 42 times/min. The dynamic stability was calculated by taking the test data from 45 min to 60 min.

#### *2.7. Technical Map*

The Technical map of this research is shown in Figure 1.

**Figure 1.** Technical map. (Where, BRA: Bio-rejuvenated additive; RAP: Reclaimed bituminous mixture; GPC: gel permeation chromatography; FTIR: Fourier transform infrared spectrometer).

#### **3. Results and Discussions**

#### *3.1. Regeneration Mechanism of BRA-Rejuvenated RAP*

#### 3.1.1. Molecular Weight of BRA

The microscopic molecular structure of a substance can significantly affects its macroscopic technical properties. The bitumen's molecular weight also affects its macroscopic technical properties. Normally, the higher the average molecular weight, the worse the low-temperature performance. A previous study has shown that the chromatogram of 70# bitumen can be divided into three parts: RT < 16.7 min, 16.7 < RT < 18.8 min, and RT > 18.8 min. These three parts correspond to the large molecular size (LMS), medium molecular size (MMS), and small molecular size (SMS), respectively. Their number-average molecular weight and weight-average molecular weight are each ~434 Da. The chromatogram of BRA is shown in Figure 2. The molecular weight of BRA is much lower than that of the 70# bitumen, with a number-average molecular weight of 500 Da, and a weight-average molecular weight of 512 Da.

The chemical composition of bitumen can be changed under the action of heating, oxygen, and ultraviolet light [28,29]. After aging, the molecules of bitumen tend to increase, and the light components of bitumen (saturates and aromatics) convert to heavy components (resins and asphaltenes) [30]. The changes in the chemical components in bitumen during aging damage the component balance of bitumen, resulting in an attenuation of the macroscopic physical and rheological properties of the bitumen. For instance, with respect to physical properties, the softening point and viscosity of bitumen increase, while the penetration and ductility decrease; in terms of the rheological properties, the complex modulus of bitumen increases, while the phase angle decreases. The molecular weight of BRA is similar to that of the light components of bitumen; it can increase the contents of the light components (saturates and aromatics) of bitumen and reduce the relative contents of resins and asphaltenes. Therefore, the BRA can rebalance the chemical components of the aged bitumen by decreasing the relative content of large molecules and heavy components.

**Figure 2.** Gel permeation chromatography (GPC) analysis diagram of BRA.

3.1.2. Chemical Structure of BRA-Rejuvenated RAP

FTIR spectroscopy was used to investigate the chemical structure of the bitumen, as shown in Figure 3, where the absorption bands at around 1169 cm−<sup>1</sup> and 1740 cm−<sup>1</sup> belong to the characteristic absorption bands of BRA, which can only be found in the FTIR spectra of BRA and BRA-rejuvenated RAP; they cannot be found in the FTIR spectra of unaged bitumen or aged bitumen without BRA, showing that in the BRA-rejuvenated RAP, the BRA was successfully mixed with the aged bitumen. In addition, the FTIR spectrum of the BRA-rejuvenated RAP does not show any absorption band at a new wavenumber, but only a simple superposition of the bitumen and BRA absorption bands, indicating that no chemical reaction between the aged bitumen in the RAP and BRA occurred during the blending process of BRA and RAP. The mixing process of RAP and BRA is a physical process; it takes place mainly due to the penetration and diffusion of BRA into the aged bitumen, after which the BRA can gradually rebalance the chemical components and restore the performance of the aged bitumen covering the surface of the RAP.

**Figure 3.** FTIR spectra of BRA and bituminous binders.

#### *3.2. Design and Optimisation of BRA-Rejuvenated RAP*

3.2.1. Effect of BRA Dosage on the Mechanical Properties of Bituminous Mixture

The early-stage mechanical properties of BRA-rejuvenated RAP were evaluated by the Marshall strength; the results are shown in Figure 4. As shown in Figure 4, when the BRA dosage was lower than 1.5%, the Marshall stability of the BRA-rejuvenated RAP increased with the increase in the BRA dosage; when the BRA content was higher than 1.5%, the Marshall stability of the BRA-rejuvenated RAP decreased with increasing BRA content. The mixture with 1.5% BRA had the highest Marshall stability, indicating that the BRA-rejuvenated RAP with 1.5% BRA had the best mechanical properties. This is because, on the one hand, BRA can activate the aged bitumen and rebalance its components, while on the other hand, the BRA can soften the aged bitumen, and offer the workability of RAP; the softening effect of BRA on the aged bitumen is also good for the compaction of BRArejuvenated RAP. When the BRA dosage is higher than 1.5%, the softening and viscosity reduction effects of BRA on the aged bitumen are dominant, being more obvious than the benefits in terms of workability, and decreasing the strength of the BRA-rejuvenated RAP. Thus, 1.5% BRA was suggested to be the optimal dosage for the rejuvenation of RAP.

**Figure 4.** Effects of BRA dosages on the mechanical properties of BRA-rejuvenated RAP.

3.2.2. Effects of Low Dosages of New Bitumen on the Mechanical Properties of BRA-Rejuvenated RAP

The results of the Marshall stability and splitting strength of BRA-rejuvenated RAP with low dosages of new bitumen are shown in Figure 5. As shown in Figure 5, the Marshall stability of BRA-rejuvenated RAP can satisfy the requirement of cold-recycled RAP (>3 kN) in the "Technical Specifications for Highway Asphalt Pavement Recycling" (JTG/T 5521—2019). After adding 0.7% new bitumen, the unconditional and conditional Marshall stability of BRA-rejuvenated RAP increased by 50.2% and 52.5%, respectively. Meanwhile, the unconditional and conditional splitting strength each increased by 195.5%. The results indicate that the conditioned and unconditioned Marshall stability and splitting strength of the BRA-rejuvenated RAP can be significantly increased by the addition of low dosages of new bitumen, and the improvement in mechanical properties was more significant with higher content of new bitumen.

**Figure 5.** Effects of new bitumen on the mechanical properties of BRA-rejuvenated RAP: (**a**) Marshall stability; (**b**) splitting strength.

3.2.3. Effects of Low Dosages of Epoxy Resin on the Mechanical Properties of BRA-Rejuvenated RAP

Figure 6 shows the results of the mechanical properties of the BRA-rejuvenated RAP with different dosages of epoxy resin. As shown in Figure 6, the epoxy resin can significantly improve the mechanical properties of BRA-rejuvenated RAP, and the improvement increases gradually with the increase in the epoxy resin dosage. The 0.5%, 0.7%, 1.0%, and 1.5% epoxy resin increased the unconditional Marshall stability by 54.0%, 66.0%, 87.8%, and 110.7%, respectively, and increased the unconditional splitting strength by 315.5%, 321.8%, 389.8%, and 394.4%, respectively. Compared with the same content of new bitumen, the beneficial effects of the epoxy resin on the mechanical properties of the BRA-rejuvenated RAP are more obvious. The reason for this is that, after curing, the epoxy resin has a highstrength and high-elasticity effect, and acts as a skeleton stiffener in the BRA-rejuvenated asphalt concrete, resulting in improvement of the early-stage mechanical properties of the recycled RAP.

Taking the above results of the mechanical properties of the RAP into consideration, the epoxy resin and BRA play different roles in the mixture. Due to the low molecular weight of BRA, the function of BRA is to soften the aged bitumen and decrease its viscosity. Another main function of BRA is to rebalance the chemical components of aged bitumen covering the surface of RAP, and to restore the technical performance of aged bitumen. Therefore, the RAP can be compacted at room temperature. The results show that all of the RAP samples could be compacted without heating. However, due to the low viscosity of bitumen, after curing, the mechanical strength of the RAP mixtures was not high. In order to enhance the mechanical strength of BRA-rejuvenated RAP, the epoxy resin was added to the mixture as a kind of reinforcing agent. After being mixed evenly in the mixture, the epoxy resin acted as a skeleton reinforcement, significantly improving the strength of BRA-rejuvenated RAP.

#### *3.3. Road Performance of BRA-Rejuvenated RAP*

#### 3.3.1. Water Damage Resistance of BRA-Rejuvenated RAP

The water damage resistance of BRA-rejuvenated RAP was investigated by using the water-immersed Marshall test and the freeze–thaw indirect tensile strength test [31]. The water-immersed residual strength (IRS) and freeze–thaw indirect tensile strength ratio (TSR) can be used to evaluate the water damage resistance of BRA-rejuvenated RAP [32]; the results are shown in Tables 5 and 6. According to the "Technical Specifications for Construction of Highway Asphalt Pavement" (JTG F40-2004) and "Technical Specifications for Highway Asphalt Pavement Recycling" (JTG/T 5521—2019) in China, in order to satisfy the water damage resistance requirement, the IRS and TSR values of hot-mixed bituminous concrete should be no less than 80% and 75%, respectively. As shown in Tables 5 and 6, the IRS and TSR values of BRA-rejuvenated RAP were 96.8% and 84.4%, respectively. These values are even higher than the water damage resistance requirements of hot-mixed bituminous concrete; therefore, the BRA-rejuvenated RAP has a good water damage resistance. In addition, the IRS values of BRA-rejuvenated RAP with low dosages of new bitumen or epoxy resin generally increase, and eventually approach 100%. With the addition of low dosages of new bitumen or epoxy resin, the TSR values of BRA-rejuvenated RAP increase significantly—0.7% new bitumen and 1.5% epoxy resin improve the TSR values of BRA-rejuvenated RAP by 18.0% and 12.8%, respectively. The results indicate that low dosages of new bitumen or epoxy resin can further improve the water damage resistance of BRA-rejuvenated RAP.

**Table 5.** Effects of new bitumen on the water damage resistance of BRA-rejuvenated RAP.




3.3.2. Low-Temperature Cracking Resistance of BRA-Rejuvenated RAP

The −10 ◦C SCB test was conducted to investigate the low-temperature cracking resistance of BRA-rejuvenated RAP, and the results are shown in Figure 7 and Table 7. From Figure 7 and Table 7, we can see that the maximum loadings of the BRA-rejuvenated RAP with 0.4% new bitumen, 0.7% epoxy resin, and 1.5% epoxy resin were 511.9 N, 758.6 N, and 1234.2 N, respectively, indicating that the 1.5%-epoxy-resin-reinforced BRA-rejuvenated RAP has the maximum bending and tensile strength. The differences in the the fracture work and fracture energy of BRA-rejuvenated RAP with 0.4% new bitumen and 0.7% epoxy resin was not significant; compared with them, the 1.5%-epoxy-resin-reinforced BRA-rejuvenated RAP showed an increase of 78.3% and 84.5%, respectively, indicating that the low-temperature performance of the 0.4%-new-bitumen- and 0.7%-epoxy-resinreinforced RAP was essentially the same, while the 1.5%-epoxy-resin-reinforced RAP showed a significant increase.

**Figure 7.** *Cont*.

**Figure 7.** Displacement–load curve of −10 ◦C SCB test: (**a**) 0.4% bitumen; (**b**) 0.7% epoxy resin; (**c**) 1.5% epoxy resin.


**Table 7.** Average fracture energy of −10 ◦C SCB test.

#### 3.3.3. Fatigue Resistance of BRA-Rejuvenated RAP

Figure 8 shows the fatigue life results of BRA-rejuvenated RAP under different stress ratio conditions. As shown in Figure 8, compared with the 0.4%-new-bitumen-reinforced BRA-rejuvenated RAP, the fatigue resistance of the BRA-rejuvenated RAP was significantly improved by the addition of 0.7% and 1.5% epoxy resin. At a stress ratio of 0.4, the fatigue life of the 0.7%- and 1.5%-epoxy-resin-reinforced BRA-rejuvenated RAP was 1.6 times and 38.7 times that of BRA-rejuvenated RAP with 0.4% new bitumen, respectively. This indicates that the 0.7% and 1.5% epoxy resin are much more effective in improving the fatigue resistance of the BRA-rejuvenated RAP than the 0.4% new bitumen. The fatigue life of 0.7%-epoxy-resin-reinforced BRA-rejuvenated RAP is the least sensitive to load, which is beneficial to the fatigue performance of the road under large load adjustment. In addition, the fatigue curves can observe a change in the behaviour of epoxy-resin-reinforced BRA-rejuvenated RAP relative to the other two curves. The 1.5%-epoxy-resin-reinforced BRA-rejuvenated RAP has the highest sensitivity to stress ratio. The reason for this is that the mechanical strength of BRA-rejuvenated RAP is significantly enhanced by the 1.5% epoxy resin, and it is stiffer than the other two mixtures with 0% and 0.7% epoxy resin. The stiffness effect of 1.5% epoxy resin exerts a more obvious effect on the fatigue behaviour of

BRA-rejuvenated BRA, but at the 0.7 stress ratio, it is still greater than 0.7%-epoxy-resinreinforced BRA-rejuvenated RAP.

**Figure 8.** Fatigue life of the BRA-rejuvenated RAP.

#### 3.3.4. High-Temperature Rutting Resistance of BRA-Rejuvenated RAP

The lack of high-temperature rutting resistance is a typical problem for conventional cold-recycled and cold-mixed asphalt mixtures [33]. The high-temperature rutting resistance of BRA-rejuvenated RAP was investigated using the wheel tracking test, and the results are shown in Figure 9. The dynamic stability of the 1.5% BRA-rejuvenated RAP was only 451 cycles/mm, while after the addition of 1.5% epoxy resin and the combined addition of 0.4% new bitumen + 1.0% epoxy resin, the dynamic stability increased to 9545 cycles/mm and 30,000 cycles/mm, respectively. It can be seen that the rutting resistance of the BRA-rejuvenated RAP improved significantly with the addition of low doses of new bitumen and epoxy resin. The reason for this is that, after mixing and rejuvenation of the RAP with 1.5% BRA, there is still a very small amount of aggregate surface that is not covered with bitumen, and after compaction, this type of aggregate becomes the weak point in the BRA-rejuvenated bituminous concrete, resulting in insufficient rutting resistance. The addition of low doses of new bitumen enables the new asphalt to further coat the exposed aggregates and reduce the number of weak points in the bituminous concrete. Based on this, with the composite addition of epoxy resin, the epoxy resin acts as a skeleton reinforcement; therefore, the high-temperature rutting resistance of BRA-rejuvenated RAP can be improved significantly.

**Figure 9.** Rutting depth of BRA-rejuvenated RAP.

#### **4. Conclusions**

The chemical structure and molecular weight distribution of BRA and bitumen were investigated by FTIR and GPC to reveal the regeneration mechanism of BRA-rejuvenated RAP. In addition, the mechanical and road properties of BRA-rejuvenated RAP were investigated, and technologies were proposed to improve the road properties of BRArejuvenated RAP. The main conclusions are as follows:

(1) The molecular weight of BRA is similar to that of the light components of bitumen; it can increase the contents of light components and reduce the contents of heavy components of aged bitumen. The mixing process of RAP and BRA is a physical process; it mainly takes place due to the penetration and diffusion of BRA into the aged bitumen, after which the BRA can gradually rebalance the chemical components and restore the performance of the aged bitumen covering the surface of the RAP;

(2) The mixture with 1.5% BRA has the highest Marshall stability, indicating that the bituminous mixture has the best cohesive and mechanical properties. The early-stage mechanical properties (Marshall stability and splitting strength) of BRA-rejuvenated RAP can be significantly improved by the addition of low dosages of new bitumen (0.4%) and epoxy resin (0.5–1.5%);

(3) The IRS and TSR values of BRA-rejuvenated RAP are even higher than the water damage resistance requirements of HMA (no less than 80% and 75%, respectively), indicating that the BRA-rejuvenated RAP has good water damage resistance. The 0.4% new-bitumen- and 0.7%-epoxy-resin-reinforced BRA-rejuvenated RAP had essentially the same low-temperature cracking resistance, while 1.5% epoxy resin reinforcement was significantly better. In comparison, the 0.7% and 1.5% epoxy resin reinforcements were much more effective in improving the fatigue resistance of the BRA-rejuvenated RAP than the 0.4% new bitumen. The rutting resistance of the BRA-rejuvenated RAP can be significantly improved by the addition of low dosages of new bitumen and epoxy resin;

(4) The epoxy resin and BRA play different roles in the BRA-rejuvenated RAP. Due to the low molecular weight of BRA, the function of BRA is to soften the aged bitumen and decrease its viscosity. Another main function of BRA is to rebalance the chemical components of aged bitumen covering the surface of RAP, and to restore the technical performance of aged bitumen. Therefore, the RAP can be compacted without heating; however, due to the low viscosity of bitumen, after curing, the mechanical strength of the RAP mixture is not high. The epoxy resin added to the mixture acts as a kind of

reinforcing agent, and forms a skeleton that can significantly improve the strength of the BRA-rejuvenated RAP;

(5) It is feasible to use BRA as a regenerating agent to achieve 100% regeneration of RAP. In addition, the addition of low dosages of new bitumen or epoxy resin can further improve the road performance of BRA-rejuvenated RAP.

**Author Contributions:** A.C.: conceptualisation, methodology, data curation, writing—original draft preparation, language editing; Y.Q.: methodology, data curation, writing—original draft preparation; X.W.: methodology, data curation, writing—original draft preparation; Y.L.: methodology, investigation, data curation, software, validation, writing—review and editing, funding acquisition, supervision; S.W.: methodology, writing—review and editing; Q.L.: data curation, software, validation, writing—review and editing. F.W.: data curation, writing—original draft preparation; J.F.: data curation, writing—original draft preparation; Z.L.: Visualization, Writing—Reviewing and Editing. All authors have read and agreed to the published version of the manuscript.

**Funding:** The authors acknowledge the financial support provided by the National Natural Science Foundation of China (No. 52108415), the National Key Research and Development Program of China (No. 2018YFB1600200), the Natural Science Foundation of China (No. 51778515), the Key Technical Innovation Projects of Hubei Province (No. 2019AEE023), the Plan of Outstanding Young and Middle-Aged Scientific and Technological Innovation Team in Universities of Hubei Province (No. T2020010), the Scientific Research Fund of Hunan Provincial Education Department (No. 18A117), and the Key R&D Program of Hubei Province (No. 2020BCB064); the project was also supported by the Science and Technology Projects Fund of Changsha City (No. kq2004065), and with help conducting the tests from Shiyanjia Lab (Available online: https://www.shiyanjia.com/all.html (accessed on 18 December 2021)).

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** All the data is available within the manuscript.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**

