## 4 Discussion
### Acoustic Emission Results
#### 4.2.2 Investigation of Fracture Mode  
Previous studies have shown that RA and AF values are the key parameters to characterize the fracture mode of concrete materials. Comparing with shear cracks, tensile cracks led to higher AF and lower RA. The fracture mode of BFRPC under flexural condition was studied by the dynamic changes of parameters throughout the loading process.  
Figure 10a-c depicts the change of RA and AF vs. load level for specimens under three fiber contents. To reduce the scattering and show the trend clearly, each point on the curves is the moving average of the recent 50 hits. It can be seen in the figure that the specimen with fiber content of  
Figure 9: Distribution of amplitude: (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\({}^{3}\).  
6 kg/m\({}^{3}\) exhibited the lowest RA and the highest AF, respectively, within the range of 50-90 ms/V and 200-800 kHz. When the fiber content increased to 14 kg/m\({}^{3}\), RA and AF were obtained, respectively, in the ranges of 70-120 ms/V and 200-600 kHz. Due to the bridging effect of the fibers, the propagation of micro-cracks within the specimens was restricted, and the tensile crack split into different shear cracks. The inclusion of more fibers led to the increase of RA and the decrease of AF, which was associated with the dominance of shear mode in the fracture process. As can be seen in the figure, the moving average curves fluctuated several times in the loading process, which was related to the dynamic fracture of the specimen. The abrupt change of RA and AF values could significantly reflect the transformation of fracture mode, which is consistent with the results obtained by [PERSON] et al. (2018). As shown in Figure 10a,b, the average frequency had a sustained downward trend throughout the loading process. The RA rose sharply in the final failure stage, indicating that shear mode dominated the failure of specimens. In Figure 10c, RA and AF values were in a state of constant fluctuation, which meant that the further increase of fiber content led to the existence of shear failure throughout the fracture process. The shift from tensile mode to shear mode was expressed by the increase of RA to above 90 ms/V and the drop of AF to approximately 300 kHz.  
The variation trend of cumulative RA and AF vs. load level in flexural test is illustrated in Figure 11. It can be observed that RA and AF always maintained a similar growth trend during the loading process, indicating that the coupling effect of tensile mode and shear mode existed in the flexural test. For RA and AF with the basalt fiber content of 6 kg/m\({}^{3}\), AF fluctuated slightly with no obvious rise or fall during the test, while RA increased sharply within the load level of 0.8-1.0 until the failure of specimen. It revealed that the shear mode dominated the critical failure of specimen after entering the final failure stage. As for the fiber content of 10 kg/m\({}^{3}\), RA and AF changed more gently, which was attributed to the increased toughness of specimens by adding fiber. When the fiber content  
Figure 10: Moving average of RA and average frequency: (**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\({}^{3}\).  
was further increased to 14 kg/m\({}^{3}\), RA and AF showed basically the same variation trend and were correlated with the defined fracture stage. As shown in Figure 11c, several sharp rises existed within load level of 0.1-0.5 and the growth trend slowed down from load level 0.5 to 1. The addition of fibers made the tensile fractures generated in the early stage split into several shear fractures, leading to the increase of acoustic emission activities.