## 3 Experimental Study
### Test Set-Up  
Experimental program consisted of two main stages: (i) Conducting 3D-PCT to identify attenuation properties and (ii) conducting flexure tests to correct AE signals and to cluster different damage mechanisms. As seen in Figure 3, 150 \(\times\) 100 \(\times\) 100 mm meshes were created at flexure spans of the specimens and tests were conducted on these volumes which were divided into 50 \(\times\) 100 \(\times\) 100 mm grids. 1 V, 5 V, 10 V, and 24 V artificial sine wave sources (frequency of 50 kHz) were generated on each test node by PCI-5412 Signal Generator, which can produce different signal forms such as sine and triangle with 100 MS  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Experiment** & **Type** & **Length** & **Tensile Strength** \\ \hline BetonFiber HE 0735 Hooked Steel Fiber & Cold-drawn & 35 mm & 1400 MPa \\ KraTos Micro Synthetic Fiber & Polyamide 2.6 & 12 mm & 900 MPa \\ KraTos Macro Synthetic Fiber & Polypropylene & 54 mm & 550 MPa \\ \hline \hline \end{tabular}
\end{table}
Table 3: Properties of fibers.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Experiment** & **Statement** & **Tensile Strength** \\ \hline REF & Conventional plain concrete & & & \\ S05 & Steel fiber-reinforced concrete (0.5\% of the volumetric fraction) & & & \\ S10 & Steel fiber-reinforced concrete (1.0\% of the volumetric fraction) & & & \\ M105 & Micro polyamide fiber-reinforced concrete (0.5\% of the volumetric fraction) & & & \\ M10 & Micro polyamide fiber-reinforced concrete (1.0\% of the volumetric fraction) & & & \\ Ma05 & Macro polypropylene fiber-reinforced concrete (0.5\% of the volumetric fraction) & & & \\ Ma10 & Macro polypropylene fiber-reinforced concrete (1.0\% of the volumetric fraction) & & & \\ CM & Cement mortar & & & \\ \hline \hline \end{tabular}
\end{table}
Table 1: Descriptions of the test specimens.  
Figure 2: Fibers added into concrete mixtures.  
data per second between \(-6\) V and \(+6\) V interval. A piezoelectric AE sensor connected to the signal generator was used as a transmitter, and it was moved on each point to allow the elastic waves to propagate within the material. All artificial sources were recorded by an 8-channel AE system with six AE sensors by Mistras Group. All sensors were amplified with preamplifiers having 40 dB gain and the threshold was set as 40 dB to eliminate ambient noise. In addition, Hsu-Nielsen Calibration Tests were conducted at the close points of the sensors to check the quality of the detected signal. This procedure was carried out to create 3D attenuation contour maps of the specimens.  
In the second stage of the study, flexure tests were conducted on each specimen in accordance with TS EN 12390-5 standard [45] with a net span of 500 mm. The load was applied with a speed of 0.2 mm/min at the center of the mid-span by Shimadzu AG-IS 100 kN Universal Testing Machine (Figure 4). Simultaneously, AE data were recorded during the tests. Afterwards, by using correction relations for the relevant sensor from 3D contour maps (see Section 4.1 for details), localized AE signals were corrected.  
To classify different damage mechanisms of the specimens, AE activities were clustered based on unsupervised pattern recognition using their frequency characteristics (WPF vs. PP). PP is divided into equal ranges considering the higher frequency of the fiber activities [37; 38; 39; 40; 41; 42]. Accordingly, PP was divided into four ranges as 0-75 kHz (PP1), 75-150 kHz (PP2), 150-275 kHz (PP3), and 275-300 kHz (PP4) considering peak frequency values of the spectrums. K-means clustering was conducted on these distributions and almost the same results were obtained for each PP value.  
Figure 3: 3D-PCT test setup (Unit: m).