**1. Introduction**

Due to the mature production process, cost-effectiveness, and the outstanding performance in terms of rutting, cracking, and moisture resistance, styrene–butadiene–styrenemodified asphalt-binder (SBSMA) is one of the most commonly used asphalt-binders for pavement construction, particularly as a surfacing layer [1]. The principle idea is to take advantage of the good binding and hardening properties of the SBS polymer to achieve effective modification of the bituminous material [2]. Thus, SBSMA has broad market prospects and accounts for more than 50% of the modified asphalt-binder around the world [3].

**Citation:** Chen, X.; Ning, Y.; Gu, Y.; Zhao, R.; Tong, J.; Wang, J.; Zhang, X.; Wen, W. Evaluating the Rheological, Chemical and Morphological Properties of SBS Modified Asphalt-Binder under Multiple Aging and Rejuvenation Cycles. *Appl. Sci.* **2021**, *11*, 9242. https://doi.org/ 10.3390/app11199242

Academic Editors: Amir Tabakovic, Jan Valentin and Liang He

Received: 31 August 2021 Accepted: 1 October 2021 Published: 4 October 2021

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Nevertheless, SBSMA is inevitably prone to aging during the service life of the pavement as a result of the negative effects of oxygen, heat, and ultraviolet (UV) light, which ultimately lead to the hardening and embrittlement of the asphalt-binder [4,5]. These effects usually result in the degradation of the SBS polymer and oxidation along with the volatilization of light components and the polycondensation of the asphalt-binder, consequently causing a decay in the physico-rheological properties [6–8]. Many researchers have observed an increase in the complex modulus ( *G*\*), elastic modulus ( *G*) and creep recovery ( *R*), but a reduction in phase angle (*δ*) and non-recoverable creep compliance (*J*nr) with the deepening of the aging by DSR, which means that the viscous behavior of the asphalt-binder has partly moved to elastic behavior [9–11].

SBSMA's performance decay accelerates pavement distresses and shortens its service life under traffic loading and fluctuating environmental conditions. It is essential to make a proper decision to restore the physico-rheological properties of the asphalt-binder at an appropriate time during the life cycle of the pavement [12]. Therefore, rejuvenators are often added to aged asphalt-binders to soften them and make the rejuvenated asphalt-binders more fluid. Specifically, the effects of the rejuvenators on the asphalt-binders include an increase in the penetration grade, ductility, phase angle, m-value, and a corresponding decrease in the softening point, viscosity, shear complex modulus, rutting factor, and stiffness, respectively [13,14].

During the aging and rejuvenation processes, the change in the physico-rheological properties of the SBSMA can be attributed to the chemical composition and microstructures. Fourier transform infrared (FTIR), atomic force microscopy (AFM), environmental scanning electron microscope (ESEM), and other new technologies have been successfully applied to analyze the aging mechanism and rejuvenating process of SBSMA at a micro scale [11,15–25]. FTIR was used to investigate the aging and rejuvenation mechanism of SBSMA as well as the relationship between the chemical and rheological characteristics by identifying the differences between the absorption peaks which represent the functional groups and contents in the asphalt-binder based on Lambert–Beer's Law [15–18]. Yang et al. [20] found that the oxygen content of the asphalt-binder increased greatly after thermal aging, leading to an increase in the polar functional groups such as carbonyl and sulfoxide groups in the asphalt-binder. In terms of rejuvenation, rejuvenators regularly decreased the carbonyl and sulfoxide indices [21]. The AFM can obtain the microscopic morphology of the asphalt-binder surface [10]. Guo et al. [22] observed that the bee structures slightly increased after PAV aging, and rejuvenator made the microstructures larger. Ozdemir et al. [23] studied aging effects of varying processing parameters on the phase structure of SBSMA. Cong et al. [24] observed a decrease in the surface roughness with aging. Aghazadeh Dokandari et al. [25] observed that tiny bee structures appeared with the addition of the rejuvenator.

The rejuvenation of aged SBSMA and reuse of asphalt mixtures reduces the environmental impacts, such as land occupation and consumption of nonrenewable resources [4]. Rapid in-place pavement recycling at an appropriate time is a very promising technology for preventive maintenance, and has an advantage of time- and cost-efficiency [26]. However, one cycle of rejuvenation has become inadequate to meet the current environmental and traffic demands. Considering the availability of bitumen and its price uncertainty, the need for multiple instances of maintenance in road construction has become essential. Three or four instances of maintenance has been proposed to extend the service life by 10–15 years with over 50% cost savings [12].

With the above background, it is therefore imperative to conduct a laboratory study on the multiple aging and rejuvenation cycles of asphalt-binder where rejuvenated asphaltbinder is widely used. Furthermore, multiple-cycle rejuvenation is expected to be a common practice in the future. To explore the mechanisms of multiple cycle aging and rejuvenation, various technological methods including FTIR and AFM were applied in this study for quantitatively evaluating the chemical and morphological properties of SBSMA through repeated aging and rejuvenation laboratory testing. Another aim of this study

was to investigate the effects of multiple aging and rejuvenation cycles on the rheological parameters of SMSMA and the potential for repeated rejuvenation of flexible pavements, so laboratory experimentations were accomplished by using DSR for investigating.

### **2. Materials and Test Methods**

SBSMAs were separately blended with three types of rejuvenators at different dosages in multiple aging and rejuvenation cycles. Basic performance tests, namely DSR, FTIR, and AFM, were used to test the original, aged, and rejuvenated asphalt-binder samples. The high-temperature rutting parameters and intermediate-temperature fatigue parameters were obtained using the DSR test device [27]. The functional group changes of the asphaltbinder samples were obtained by analyzing the results from FTIR [20]. The microstructures of asphalt-binder samples were observed and characterized with AFM [10].

### *2.1. Materials*

The main materials used in this study were asphalt-binder and three types of rejuvenators, namely Type I, Type II, and Type III, respectively.
