## 3 Experimental Setup
### Detection of Corrosion in Steel Bars
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**The Mass Loss** & **The Magnitude of Focused** & **The Magnitude of Focused** & **The Magnitude of Focused** \\
**Ratio (\%)** & **Signal (50 KHz)** & **Signal (60 KHz)** & **Signal (70 KHz)** \\ \hline
0 & 0.2399 & 0.2311 & 0.2142 \\
0.88 & 0.2132 & 0.2132 & 0.1972 \\
2.11 & 0.2057 & 0.1997 & 0.1804 \\
3.19 & 0.1953 & 0.1846 & 0.1747 \\
4.10 & 0.1915 & 0.1776 & 0.1692 \\
5.54 & 0.1788 & 0.1703 & 0.1597 \\
6.61 & 0.1687 & 0.1601 & 0.1456 \\
7.80 & 0.1541 & 0.1505 & 0.1367 \\
8.80 & 0.1483 & 0.1403 & 0.1261 \\
9.90 & 0.1362 & 0.1284 & 0.1132 \\
10.89 & 0.1257 & 0.1173 & 0.1012 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: The data of specimen no. 1. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**The Mass Loss** & **The Magnitude of Focused** & **The Magnitude of Focused** & **The Magnitude of Focused** \\
**Ratio (\%)** & **Signal (50 KHz)** & **Signal (60 KHz)** & **Signal (70 KHz)** \\ \hline
0 & 0.5752 & 0.5578 & 0.5982 \\
1.03 & 0.5425 & 0.5148 & 0.5546 \\
1.99 & 0.5141 & 0.4822 & 0.5071 \\
2.76 & 0.4833 & 0.4495 & 0.4823 \\
3.90 & 0.4389 & 0.4153 & 0.4398 \\
5.09 & 0.4008 & 0.3863 & 0.3878 \\
6.18 & 0.3523 & 0.3455 & 0.3401 \\
7.47 & 0.3201 & 0.3067 & 0.3102 \\
8.52 & 0.289 & 0.2851 & 0.2648 \\
9.691 & 0.2524 & 0.2472 & 0.2254 \\
10.93 & 0.210 & 0.2113 & 0.1751 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: The data of specimen no. 3. The relationship between the mass loss ratio of the steel bar and the peak values of the focused signal were established, as shown in Figure 13a-c. Four different straight lines with cross, plus, square, and star signs (in Figure 13) express the experimental results of different steel bars (specimens no. 1, 2, 3, and 4, respectively). \begin{table}
\begin{tabular}{c c c c} \hline
**The Mass Loss** & **The Magnitude of Focused** & **The Magnitude of Focused** & **The Magnitude of Focused** \\
**Ratio (\%)** & **Signal (50 KHz)** & **Signal (60 KHz)** & **Signal (70 KHz)** \\ \hline
0 & 0.3881 & 0.3658 & 0.3718 \\
0.90 & 0.3628 & 0.3586 & 0.3603 \\
1.76 & 0.3510 & 0.3483 & 0.3481 \\
2.72 & 0.3201 & 0.3391 & 0.3261 \\
3.84 & 0.2962 & 0.3247 & 0.3099 \\
5.02 & 0.2722 & 0.3183 & 0.2812 \\
6.13 & 0.2431 & 0.3021 & 0.2632 \\
7.32 & 0.2259 & 0.2918 & 0.2504 \\
8.49 & 0.1952 & 0.2842 & 0.221 \\
9.69 & 0.1701 & 0.2728 & 0.2001 \\
10.84 & 0.1552 & 0.2572 & 0.1826 \\ \hline \end{tabular}
\end{table} 
Table 4: The data of specimen no. 4. Figure 13: The amplitude of focused signal with mass loss ratio of different specimens. The peak values of the focused signal in Figure 13 are decreased with the increase of the mass loss ratio for different steel bars. Because of the interference resulting from some inevitable human and external conditions, such as the process of pasting piezoceramic transducers at both the ends of the steel bar and the different thicknesses of the epoxy resin at both the ends, different specimens will obtain different signal amplitudes under an identical impressed current and emitted signal. In addition, it can be concluded that the peak values of the focused signal are linearly related to the severity of the corrosion of the steel bar.