# Acoustic Emission for Evaluating the Reinforcement Effectiveness in Steel Fiber Reinforced Concrete
## 4 Discussion  
The above results based on experiments conducted in different laboratories, following different experimental protocols by different people, prove the sensitivity of AE to the fracture mode of concrete. The study was conducted in different setups (3p- and 4p-bending) to show the effect of the reinforcement on the stress field towards shear, even in  
Figure 8: (**a**) RA value and (**b**) average frequency AF vs. steel fiber content for different fiber shapes. Figure 9: Correlation plot between average frequency (AF) and RA for different steel fiber content. The error bars correspond to the standard deviation. the case that nominally there is no shear in the middle (4p-bending). Resonant sensors were used which despite being very sensitive, may also mask the original frequency content of the elastic waves. It is possible that broadband sensors allow better characterization in the frequency domain, decreasing the overlap between different groups. One of the points of the study that shows strong potential is the possibility to characterize the behavior of a fiber composite material by a low-level proof loading. As shown herein, the sensitivity of AE allows us to track the influence of fibers even during the elastic regime and before cracks are nucleated. This can help in examining or comparing the adhesion between different types of fibers and matrix and lead to an easy way to investigate the suitability for application, helping in material development. Although this would need further study, the correlations between the AE parameters obtained at low load and the final load capacity or toughness are certainly encouraging towards the final aim of predicting the mechanical properties of SFRC. For such a purpose large numbers of specimens should be tested, a project that is under way by the authors. Figure 10 includes correlations between early load AE and final mechanical properties. Figure 10a shows the RT and CF vs. the toughness of the SFRC measured in the 3p-bending test. A strong correlation is obvious, since a decrease of CF and increase of RT at the initial part of the loading is well correlated with an increase in the final toughness. Figure 10b exhibits correlations between early age AE parameters and the load bearing capacity of the beams under 4p-bending. The RA value obtained before the main cracking correlates in a monotonic way with the maximum load for both types of fibers. Therefore, it is implied that the AE during the elastic regime can give serious insight on the final mechanical properties. Although this has not been widely utilized, it is normal, as it expresses the bonding between the matrix and the fibers as mentioned above. Obviously, further study is needed to allow engineering predictions of mechanical properties based on data obtained before cracking. In any case concerning prediction of mechanical properties, one should be very cautious, as absolute values would be important. A standardized procedure should be in place to allow researchers to obtain reliable results, since as previously mentioned, many test parameters influence the absolute value of AE parameters. Another point which is noteworthy in experimental studies and in particular in AE is the relatively large error bars or the overlap in the parameters of different groups of specimens, as seen for example in Figures 8 and 9. However, this is inherent to the damage process. The reason for this is that AE is the consequence of fracture, which is an extremely random process. It is reminded that the source of AE in this case is fracture propagation events. Even though the loading pattern does not change, the new crack increment depends on the local stress conditions that have been developed after the previous crack incidents. From the moment the fracture starts, and especially in a heterogeneous material, the process zone is affected \"by the stress field generated by the fracture itself\" [26].