# Acoustic Emission for Evaluating the Reinforcement Effectiveness in Steel Fiber Reinforced Concrete
## 3 Results
### Four-Point Bending AE Results  
Moving to the four-point bending, the mechanical results revealed the reinforcing effect of the fibers, as expected. Table 2 shows the average maximum load bearing capacity for all mixes. A volume content of 0.5% of fibers considerably increased the load capacity from less than 14 kN of the reference to 16-17 kN, while further addition of fibers to 2% increased the capacity further, especially for the hooked fibers (almost 25 kN). Figure 5: Cumulative AE curves for typical concrete specimens with different vol.% of steel fibers tested on three-point bending. Figure 6: Rise time for concrete specimens with different vol.% of steel fibers tested on 3\(p\)-bending. Indicative cumulative AE activity curves are shown in Figure 7. The trends are quite similar to the previous of three-point bending, with SFRC showing much higher activity than plain concrete. The most interesting trends come from the behavior of certain AE parameters before the main cracking, as seen in Figure 8. Figure 8a shows the RA value for different contents and types. While the RA value of reference specimens is between 200 and 600 us/V, this increases up to 1400 us/V for 0.5% of fibers and even higher for 2%. This shows the certain enhancement of the shear character by the addition of more fibers. At the same time, the effect of fiber shape does not seem similarly influential, as hooked and wavy fibers exhibit close averages. As reported above for the three-point bending case, here as well, the frequency shows an inverse trend but again the change is proportionally smaller than the one of RA. While the average AF for reference is 310 kHz, this drops to 250 kHz for 2% of fibers. \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Fiber Content** **(Vol.)**} & \multicolumn{2}{c}{**AE Parameters**} & \multicolumn{2}{c}{**Mechanical Test Results**} \\ \cline{2-5}  & **Rise Time (μs)** & **Central** **Frequency (kHz)** & **Secant** **Stiffness (N/mm)** & **Toughness** **(mJ)** \\ \hline
0\% & 39.5 & 165.6 & 5969 & 3195 \\ \hline
0.5\% & 48.8 & 155.3 & 7405 & 19,589 \\ \hline
1\% & 58.7 & 150.2 & 10,816 & 35,693 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Basic AE parameters and mechanical test results for different fiber contents after the three-point bending test. \begin{table}
\begin{tabular}{c c} \hline \hline \multirow{2}{*}{**Specimen Type**} & \multicolumn{2}{c}{**Flexural Load (St. Dev.)**} \\  & **(kN)** \\ \hline Plain & 13.82 (3.22) \\ \hline Waved 0.5\% & 16.22 (0.53) \\ \hline Hooked 0.5\% & 17.20 (3.48) \\ \hline Waved 2\% & 21.37 (1.58) \\ \hline Hooked 2\% & 24.94 (2.44) \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Load capacity for concrete specimens with a different type and content of fibers. Figure 7: Cumulative AE curves for typical concrete specimens with different vol.% of steel fibers tested on 4p-bending. Figure 9 shows a correlation plot between AF and RA, as proposed by the RILEM recommendation [2]. For each content area, it includes the data of both fiber types (wavy and hooked) since the results were close. As the fiber content increases from 0 to 0.5%, the RA exhibits a jump from 300 \(\upmu\)s/V to more than 1000 \(\upmu\)s/V. The further increase of fibers to 2% has a clear but relatively weaker push to higher RA values on average. This RA shift according to the recommendation shows the shift towards shear nature of the fracture. From the frequency point of view, there is a clear drop between 0% and 0.5% in AF values, while further addition of fibers does not seem to have an evident effect on further reducing the frequency content. It seems that the crucial change happens between 0 and 0.5% because in the first case there is no reinforcement at all and therefore, no influence on the early AE readings, while at 0.5%, the new shearing mechanism of fiber pull-out is already added.