# Acoustic Emission for Evaluating the Reinforcement Effectiveness in Steel Fiber Reinforced Concrete
## 4 Discussion
Thus, it is argued that the stress field is dynamic and changes continuously after each crack propagation event compared to the initial static field. This dynamic character is inevitably transferred  
Figure 10: (**a**) Correlation between AE parameters obtained from the elastic regime and toughness of SFRC under 3p-bending. (**b**) Correlation between RA value and max. load for SFRC under 4p-bending and different contents and types of fibers. into the AE signals, increasing the scatter and it is the reason that pattern recognition approaches have been effectively used in the field for classification of the signals based on different mechanisms [27; 28; 29]. This stochastic character can only be increased by the heterogeneity and propagation effects. The final parameters measured from the recorded waveform are influenced by several factors, including the wave propagation distance and conditions in general. Although the location of the sensors is constant throughout loading, the sources are not the same, meaning that an event close to a sensor will be recorded with a higher amplitude, while an event similar in mode and intensity further away would be recorded with a lower amplitude and frequency due to attenuation. Obviously, this influences the RA (rise time/amplitude) and frequency content values. In a material like concrete with numerous potential cracking sources (pores, voids, fiber/matrix interphases) the results carry a degree of distortion, so it is normal to expect overlap between different classes. However, the importance is not in accurately defining each one of the recorded hits. This would be an extremely difficult task (if not practically impossible in absolute terms) but also of doubtful significance, since as previously mentioned, the exact values are a function of many conditions apart from the crack itself (wave propagation, sensor sensitivity). Therefore, the importance lies on the comparisons between different cases, since the relative trends between specimens and the fluctuations during loading clearly indicate the correlations between AE and the fracture process in the early or later stages. RA and RT values have been known to increase throughout the loading of a component as the damage increases, because the mode of stresses shifts from tensile to shear as the material approaches the final failure [10; 11; 12; 13; 30; 31; 32]. Therefore, in the literature, high values of RA have been associated with a higher degree of damage. While this is a universal trend and holds in usual cases of fracture for each single component or specimen, the importance in this study is focused on the comparison between specimens with different level of reinforcement. The AE indices obtained at low load and before damage is manifested in any measurable way, reveal the effect of the reinforcing mechanism thanks to the development of the necessary additional shear stresses. Enhancement of the fiber content increases the occurrence of local shear stress fields, leaving its signature on the AE parameters by increasing durational characteristics of the waveform and slightly decreasing the frequency content.