## 1 Introduction  
In recent years, the production of automobiles has led to the accumulation of numerous waste tires that are difficult to dispose of, resulting in a large amount of space occupied and a high fire hazard. The impact of waste tires on the environment and how to effectively deal with them has become an important issue. Therefore, scholars in different fields have proposed several methods to recycle waste tires. In addition to the method of retreading and reusing [1], waste tires can also be converted into other degradable or usable polymeric materials through a series of chemical methods [2]. [PERSON] et al. [3] pyrolyzed waste tires and applied the products to energy storage devices such as batteries. [PERSON] et al. [4] found that in wastewater treatment systems, the use of waste tires as carriers of organisms is a promising application. Recently, in the direction of civil engineering, the application of grinding waste rubber tires into particles as aggregates has become increasingly widespread [5; 6; 7; 8; 9; 10; 11; 12]. Many scholars have investigated the mechanical properties of rubber concrete. According to the experimental results, they have found that the addition of rubber particles has significantly improved the ductility and durability of the concrete, but it results in a significant decrease in strength [13; 14; 15; 16; 17]. Based on the current urgency of finding ways to improve strength defects and increasing the recycling rate of waste tires, scholars in related fields have stimulated in-depth research. Follow-up scholars have also studied the fatigue and impermeability behaviors of rubber concrete [18; 19; 20; 21; 22]. Currently, crumb rubber concrete has been widely used in tennis courts, park areas, and bridge road constructions. Therefore, it is important to monitor the structure of these materials to avoid large losses due to large damage inside or outside the structure. For the study of material microcracking and fracture monitoring methods, [PERSON] et al. [23] proposed a new computational method for the simulation of mesh division of composite materials, which allows accurate prediction of crack propagation and fracture load when damage occurs in the material. [PERSON] et al. [24] proposed a damage model for describing material fracture, which achieves the adaptive and capture of arbitrary crack propagation in solid materials. [PERSON] et al. [25] analyzed the connection between mechanical properties and waveforms and frequencies of four rocks during fracture to improve the accuracy of crack prediction in rocks. Recently, in order to understand the development of microcracks and the monitoring of fracture-causing behaviors, [PERSON] et al. [26] have conducted experimental studies based on acoustic emission (AE) technology. For crumb rubber concrete, there have been some studies on its microcrack and fracture characteristics [27; 28; 29; 30; 31], but there are few studies on the monitoring of microcrack propagation and fracture behavior under different damage modes [32; 33; 34]. Hence, if the acoustic emission signals of the microcracks at various stages in different modes can be monitored and a reasonable analysis made, it will bring benefits to practical engineering applications. For this reason, this paper used AE technology to conduct damage monitoring research on crumb rubber concrete. To investigate the different damage patterns of crumb rubber concrete under a four-point bending test, it is critical to categorize different AE signals. In recent years, a variety of different clustering methods have been adopted for the classification of AE signals obtained in the damage and failure process of concrete materials [35; 36; 37; 38; 39]. Although the types of these methods have increased, their inherent limitations have not been studied in depth. At the same time, no one used the clustering method to classify the AE signals for crumb rubber concrete.