# Pulse Peak Delay-Total Focusing Method for Ultrasonic Tomography on Concrete Structure
## 4 Experiment  
The PPD-TFM algorithm was tested on a concrete cube specimen, as shown in Figure 7a. The concrete cube specimen is a plain concrete that has the dimension of 500 \(\times\) 500 \(\times\) 500 mm. Fine aggregates that are less than 5 mm in diameter, which is smaller than the wavelength of ultrasonic pulse, are used to avoid the interruption by aggregates. A 25-mm diameter anchor bolt is placed at (12.5, 100) mm in the X-Z plane. Unlike the simulation, the reflector is not a point, but a circular reflector. The ultrasonic array device A1040 MIRA, as shown in Figure 7b, was used to scan the specimen. The MIRA device consists of 4 \(\times\) 12 transducer array, and each 4 transducers acts as 1 transducer. The MIRA device resultantly operates as if 12 transducers array. Each transducer is a shear wave DPC transducer of which the tip is resistant-contact ceramic not requiring any coupling agent [29]. In addition, each one is independently braced by springs allowing to scan on rough surfaces. The MIRA device drives an ultrasonic pulse with a low frequency ranging from 25 to 100 kHz to avoid signal noise when the wavelength of the pulse is smaller than the size of concrete aggregates. In addition, the shear wave DPC transducer prevents the measured signal from being interrupted by different types of waves generated by mode conversion at interfaces. Figure 6: Array performance indicator (API) for Simulation by TFM (**left**) and PPD-TFM (**right**). Figure 7: (**a**) Scanning target concrete cube specimen and (**b**) ultrasonic array device A1040 MIRA. The MIRA device was placed at the \(z=0\) plane, as shown in Figure 2a, and the ultrasonic 1-cycle input pulse was driven at frequency of 30, 50, and 70 kHz. The raw data from the MIRA device was extracted to feed the TFM and PPD-TFM. B-scans by the PPD-TFM are compared with B-scans by the TFM and the MIRA device. Experimental Results and DiscussionFigure 8 shows the B-scans by the TFM, PPD-TFM, and SAFT-C embedded in the MIRA device. The values in B-scans are normalized to the maximum reflection values. As an ultrasonic pulse is reflected at the top boundary of the anchor bolt, a strong reflection needs to be formed at the top boundary of the circular anchor bolt. The PPD-TFM that calculates the PPDs for the different input frequencies shows the exact placement of the strong reflections regardless of the input frequencies. On the other hand, the TFM that does not deal with PPD formed strong reflections below the exact reflector location. But, the position of the strong reflection comes close to the exact location once the input frequency increases and the PPD length shortens. The SAFT-C algorithm calibrates the PPD in its own way, though the strong reflections were formed slightly below the exact location. Figure 8: B-scan for concrete cube specimen by TFM (**left**), PPD-TFM (**center**), and SAFT-C (**right**). The reflection surfaces and -6 dB API values are presented in Figure 9 and Table 1. The PPD-TFM shows better performance than the TFM, as well as the SAFT-C, embedded in the MIRA device. For the input frequency of 70 kHz that is out of the DPC transducer's frequency bandwidth range, the overall performance of all methods diminished. This shows that an input frequency higher than the bandwidth of a transducer does not provide adequate performance. One of the recorded signals is plotted in Figure 10. As the surface line is the shortest wave path, the surface wave is recorded at first, then the signal reflected at the anchor bolt \begin{table}
\begin{tabular}{c c c c c} \hline \hline  & & \multicolumn{3}{c}{**Image Reconstruction Algorithm**} \\  & & **TFM** & **PPD-TFM** & **SAFT-C** \\ \hline \multirow{3}{*}{Input signal frequency} & 30 kHz & 0.39 & 0.23 & 0.34 \\  & 50 kHz & 0.67 & 0.27 & 0.49 \\ \cline{1-1}  & 70 kHz & 1.42 & 0.56 & 0.89 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: API values for TFM, PPD-TFM, and SAFT-C (A1040 MIRA). Figure 9: API for concrete cube specimen by TFM (**left**), PPD-TFM (**center**), and SAFT-C (**right**).