## 4 Discussion
### Acoustic Emission Results  
#### 4.2.1 Total AE Hits  
It is known that the fracture process of concrete could be confirmed by the analysis of AE characteristic parameters [31]. The AE data for the three groups BF9, BF8, and BF5 were selected for analysis to study the characteristics of BFRPC fracture process with different basalt fiber content. Figure 7: Failure morphology of BFRPC: (**a**) compressive mode; and (**b**) flexural mode. The ultimate flexural strength loads were normalized and divided into ten load levels. In Figure 8, the cumulative AE hits and energy vs. load level of three groups are depicted. As can be seen in the figure, the cumulative AE hits and the cumulative energy have positive correlation with basalt fiber content. This is because the AE activities associated with fiber pull-out and the pulling out events will increase with the increase of fiber content. As shown in Figure 8a,b, when the fiber content was 6 kg/m\({}^{3}\) and 10 kg/m\({}^{3}\), the fracture process of specimens could be divided into three stages according to the variation trend of the cumulative curves. In Figure 8a, the explosion of cumulative energy was recorded at 80% load level; the value of cumulative energy grew from 6000 to 20,000 during the third stage. However, the steady increase of cumulative AE hits indicated that the signal strength generated in the final failure stage was relatively large. When the fiber content was 10 kg/m\({}^{3}\), it can be seen in Figure 8b that the critical point of the third stage was at the load level of 70%. At that moment, the cumulative energy value increased by nearly 100%, and then mutation occurred again when it increased to 20,000, representing the ultimate failure of the specimen. In Figure 8c, it is shown that addition of basalt fibers up to 14 kg/m\({}^{3}\) led to considerable different trend in fracture process, which could be divided into four stages according to the slope change of curves. Most AE events were activated by measurement noise, which explained the small increase in cumulative energy at load level below 0.2. It can be seen that many AE hits were recorded at load level ranging from 0.2 to 0.25, accompanied by an abrupt increase in energy. This behavior was linked to the production of initial cracks that released great energy, which was reported by [PERSON] [17]. When the loading was further increased above 0.25 threshold, the growth trend of cumulative parameters tended to be gentle, indicating that the fracture inside the specimen had entered the stable expansion stage. In contrast to the above two cases, the cumulative  
Figure 8: History curves of load level versus cumulative AE features: (**a**) basalt fiber content of 6 kg/m\({}^{3}\); (**b**) basalt fiber content of 10 kg/m\({}^{3}\); and (**c**) basalt fiber content of 14 kg/m\({}^{3}\). parameter curves tended to be horizontal in the final stage, ranging from load level 0.8 to the collapse of specimen, related to the addition of basalt fibers, which led to an enhancement of toughness. The AE amplitude value of BFRPC specimens is shown in Figure 9. It can be seen in the figure that the AE amplitude value has a great correlation with the damage stage, which was divided according to cumulative hits and cumulative energy. Sudden increases in amplitude and number of AE signals were observed at the boundaries of each stage. This behavior is also reported in the literature for structure using the AE technique [32]. As shown in Figure 9a,b, at the stage of stable fracture propagation (Stage II), AE amplitude ranged 40-60 dB; at the boundary of Stages II and III, the amplitude reached more than 70 dB; when macro fracture occurred, the amplitude reached a peak of more than 90 dB. As shown in Figure 9c, most of the signals had high amplitude values above 55 dB, which fluctuated with the increase of load. At the critical points of the damage stages, the peak amplitude of more than 75 dB was observed, which was accompanied by the increase of signal quantity.