**1. Introduction**

The study of a crumb rubber–asphalt (CR) mixture proved that it had good road performance and diverse functions [1–6]. However, in the service process of crumb rubber– asphalt pavement, the looseness, spalling, cracks and other diseases have seriously affected the service performance and service life of crumb rubber–asphalt pavement [7–9]. The reason is that many issues are related to moisture. The performance of the interface between CR and aggregate decreases under the condition of moisture, resulting in moisture damage to the CR mixture, thus affecting the service life of asphalt pavement [10,11]. Therefore, clarifying the interaction mechanism between the CR and the aggregate in the presence of moisture is the core issue to ensure the service durability of a CR mixture and provides a reference for the application and promotion of a crumb asphalt mixture.

The interface behavior of asphalt mixtures under the action of moisture is usually evaluated by macro-indicators such as the crack strength and spalling rate [12–15]. In addition, some scholars have used microcharacterization technology to study the asphalt mixture, including the surface morphology, structure, mechanical properties and other

**Citation:** Wang, L.; Liu, Y.; Zhang, L. A Multiscale Study of Moisture Influence on the Crumb Rubber Asphalt Mixture Interface. *Appl. Sci.* **2022**, *12*, 6940. https://doi.org/ 10.3390/app12146940

Academic Editor: Leonid Burakovsky

Received: 14 June 2022 Accepted: 6 July 2022 Published: 8 July 2022

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microproperties of the material [16–20]. Molecular simulation [21,22] is a computer numerical simulation test, which is not limited by the detection means, specimen preparation or other conditions. It effectively complements the shortcomings of macro tests and can reveal the molecular-scale mechanism of many macro tests. In recent years, it has also been more widely applied to the research of asphalt mixtures. From the current research, the research on the interface effect of asphalt mixtures mainly focuses on single-scale research and analyses of macroperformance and materials' microcharacteristics, but the multiscale and cross-scale research idea can better solve the mechanism problem of asphalt mixture performance. Many scholars have also carried out multiscale studies on different properties of asphalt and asphalt mixtures, including asphalt aging performance [23,24], the combination of old and new materials in recycled asphalt mixtures [25–27], the diffusion of modifiers [28,29], and the interaction between asphalt and aggregates [30,31]. A large number of studies have confirmed that the multiscale idea is feasible in the research of asphalt and asphalt mixtures.

Therefore, in order to study the interface adhesion and failure mechanism of CR mixtures under the action of moisture using the multiscale method, this study carried out a cracking resistance test of a CR mixture at the macroscale and compared the effects of different cracking forms on the CR mixture. At the microscale, the surface free energies of asphalt and aggregate were calculated and analyzed by the surface energy theory, and the damage from moisture on the surface adhesion of asphalt and aggregate was studied. At the molecular scale, the interaction mechanism between CR and aggregate under the action of moisture was studied by constructing a 12-component CR model, a representative molecular aggregate model, and an interface model of CR and aggregate. Finally, the interface adhesion mechanism and failure behavior of the CR mixture under the action of moisture was revealed by correlating the interface properties under different scales.

### **2. Materials and Models**

### *2.1. Preparation of Test Materials and SCB Specimens*
