## 3 Results and Discussion
### AE Hits Analysis of Concrete with Different Fillers  
AE hit counts are often regarded as an important parameter associated with crack events, and the number of AE counts has a positive correlation with both micro- and macro-crack occurrences [30, 31]. Specimens at 7-day curing ages were selected to analyze the effect of fillers on AE characteristics, as shown in Figure 7, where the compressive stress is also provided. To highlight different cracking states during the cracking process of specimens, four points were marked on the stress-time curve. Points a, b, and c were determined by the method described in Section 3.2.  
Based on the growth rate, the cumulative AE count curves can be classified into two categories. The first category includes plain and CB-reinforced samples, where the growth rate of cumulative counts is lower at first and then increases. The second category comprises CF- and SF-reinforced samples, both reinforced with fiber materials, whose growth rate remains almost linear throughout the loading process. The lower growth rate in the first category corresponds to the compaction and elastic-like stage of the stress-time curves, indicating that the closing process of micro pores in these samples is easier than in fiber-reinforced samples, which is consistent with results in the literature [32]. The cumulative count growth rate of the CB-reinforced sample is significantly lower at the  
Figure 6: Mechanical characteristics of samples during loading: (**a**) D7 Re; (**b**) D7 CB; (**c**) D7 CF; (**d**) D7 SF; (**e**) D14 Re; (**f**) D14 CB; (**g**) D14 CF; (**h**) D14 SF; (**i**) D28 Re; (**j**) D28 CB; (**k**) D28 CF; (**l**) D28 SF.  
beginning of loading compared to plain samples, likely due to the denser microstructure of the CB-reinforced sample [33].  
Figure 7 also shows the distribution of AE counts during the loading process (red line). It can be seen that the plain sample exhibits more AE events with a large number of counts compared to the filler-reinforced samples. Notably, the CB-reinforced concrete shows a more uniform distribution of AE events, with only a few large-count events occurring around point a and after point c, attributed to the filling effect of CB particles on the matrix's micro pores [33]. The AE strength of CF-reinforced samples is lower during the compaction stage, but some high-strength AE events appear in stages III and IV, which is quite different from the plain and CB-reinforced samples. This is due to the bridging effect of CFs, where the pull-out events can be detected by AE signals. A similar phenomenon is observed in SF-reinforced samples, with high-strength AE events appearing in stages II and III, indicating that AE signals can reflect the reinforcing effect of fillers.  
Moreover, there are notable differences between CF- and SF-reinforced samples. AE events in SF-reinforced samples are higher in stage I but significantly lower in stage IV. Generally, rigid fibers (SFs) and flexible or soft fibers (CFs) have different effects because rigid fibers do not deflect to fill voids within the concrete [31,32]. During the compaction stage, CF-reinforced samples are more easily deformed due to the flexibility of CFs. After peak loading, SFs exhibit a better reinforcing effect on the matrix, leading to fewer AE events. This is further demonstrated by comparing the stress-time curves of CF- and SF-reinforced samples: the stress decline rate of SF-reinforced samples is slower, indicating that the reinforcing effect of rigid fibers is better than that of flexible fibers [34].