**1. Introduction**

Asphalt pavements are subject to deterioration by light, heat, and oxygen during service, resulting in the deterioration of pavement performance [1,2]. The critical factor in the regeneration of asphalt pavements is the regeneration of aged asphalt. Springback agents are generally used to restore the performance of aged asphalt during the reuse of asphalt pavements. The literature shows [3] that the main types of regeneration agents include vegetable oils, waste-derived oils, engineering products, and various refinery base oils. Previous studies on asphalt regeneration have focused more on the effects of regeneration agents on asphalt and the macroscopic properties of asphalt mixtures [4]. Still,

**Citation:** Yang, J.; Luo, L.; Gao, J.; Xu, J.; He, C. Study on the Effect of Regeneration Agent on the Viscosity Properties of Aged Asphalt. *Materials* **2022**, *15*, 380. https://doi.org/ 10.3390/ma15010380

Academic Editor: Simon Hesp

Received: 26 November 2021 Accepted: 2 January 2022 Published: 5 January 2022

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in recent years, scholars at home and abroad have turned their attention to studying the mechanism of asphalt regeneration and have achieved specific results.

Huang et al. [5] explored the interaction between new asphalt, old asphalt, and regeneration agent by Fourier-transform infrared spectroscopy (FTIR) and gel permeation chromatography (GPC). Moreover, the results showed that the changes of the functional group index and the molecular weight distribution could explain the regeneration effect of the regeneration agent. The parameters of the sulfoxide index and the GPC of qiFTIR can be used as reliable indicators to predict the macroscopic properties of asphalt. Lin et al. [6] studied the changes of the characteristic functional group content of asphalt before and after regeneration using FTIR. The results showed that the addition of the regeneration agent made the characteristic functional group content of regenerated asphalt decrease. Zhao et al. [7] studied regeneration agent changes on asphalt's relative molecular weight distribution using gel permeation chromatography. The results showed that the regeneration agent reduced the macromolecular content of the aged asphalt, which effectively reduced the viscosity of the asphalt. Cui et al. [8] used atomic force microscopy (AFM) tests to obtain Young's modulus (DMT) and adhesion indexes and explored the changes of regeneration agents on asphalt viscoelastic properties. From the study results, we can see that the DMT of the regenerated asphalt was reduced after the addition of the recycler. As a result, the viscoelastic properties were well recovered, and the adhesion properties of the asphalt–aggregate interface were enhanced. Li et al. [9] analyzed the effect of regeneration agents on the microstructure of aged asphalt using FTIR and AFM. The results showed that the regeneration agents made the absorption intensity of some specific functional groups decrease, enhanced the asphalt molecular motility and decreased the asphalt viscosity. He et al. [10] used infrared spectroscopy to explore the mechanism of reagents on SBS-modified asphalt. The results showed that a layer of interfacial film was formed between asphaltene and a soft pitch due to the addition of reagents, which further played a role in lubricating the asphalt, thereby reducing the viscosity of the aging asphalt. Yao [11] studied the mechanism of the effect of regeneration agents on aged asphalt, and the results showed that the regeneration agent could regulate the molecular weight distribution of the aged asphalt.

In summary, the current research on asphalt regeneration mainly focuses on the mechanism of asphalt regeneration and the prediction of post-regeneration performance, and fewer studies combined microscopic morphology and macroscopic performance on the change law of the viscosity performance of aged asphalt after regeneration. In this study, indoor simulated aging tests were conducted on the matrix asphalt, from which aged asphalt with a needle penetration of 30 was obtained, and the doses of a springback agent were 2%, 4%, and 6%. GPC, FTIR, and AFM tests, as well as activation energy, were used to analyze the changes of the relative molecular weight, chemical components, microstructure, and viscosity properties of asphalt before and after regeneration and to explore the change law of the microscopic properties of the regenerated asphalt and its influence on the viscosity properties.

#### **2. Experimental Methods**

#### *2.1. Raw Materials*

Asphalt's primary performance test results are shown in Table 1, and its main indexes met the specification requirements [12]. The test materials included 70# matrix asphalt and a regeneration agent. Figure 1 illustrates a sample of rejuvenators, and Table 2 describes specific technical properties.


**Table 1.** Basic properties of 70# base asphalt.

**Figure 1.** Regeneration agent.

**Table 2.** Basic properties of the regeneration agent.


#### *2.2. Test method*

2.2.1. Aging Asphalt

The rotating film oven (RTFOT; Figure 2) was used to age the 70# matrix asphalt into the asphalt with a needle penetration (0.1 mm) of 30. The aging time required for the asphalt was determined by relating the aging time to the degree of aging [13], and the results are shown in Figure 3. One hundred and thirty-five minutes were the aging time required for the 70# matrix asphalt to meet the performance index of 30# asphalt after aging.

**Figure 2.** Rotating film oven.

**Figure 3.** Needle penetration depths at different aging times in the RTFOT.
