## 3 Results and Discussion
### Test Results of Impact-Echo Technique  
The test results of the impact-echo technique for different grout filling degrees are shown in Figure 8. To ensure the accuracy of the experimental data, each set of grout filling degree was tested five times.  
Figure 8: Test results of impact-echo technique under different grouting fullness: (**a**) 10%, (**b**) 20%, (**c**) 30%, (**d**) 40%, (**e**) 50%, (**f**) 60%, (**g**) 70%, (**h**) 80%, (**i**) 90%, (**j**) 100%.  
The analysis of the results indicates that the reflected waveforms differ for different grout filling degrees. As the grout filling degree increases, the reflected waveforms received by the impact-echo detection instrument become stronger.  
The experimental data obtained using the impact-echo technique are summarized in Table 1. The analysis of the data shows that the standard deviations of the measured filling degree results are relatively small (<2.5), indicating the stability of the test data and the absence of significant errors. Additionally, the percentage deviations of the test data are within 5%, which is an acceptable range, demonstrating that the measured data can accurately reflect the actual grout filling quality. This suggests that the impact-echo technique can reliably evaluate the grout filling degree in sleeve connections.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Fullness Setting Value/\%** & **Fullness Detection Value/\%** & **Average Value/\%** & **Standard Deviation** & **Deviation/\%** \\ \hline \multirow{4}{*}{10} & 10 & & & \\  & 9 & 9.6 & 0.80 & 4.00 \\  & 9 & 11 & & \\ \hline \multirow{4}{*}{20} & 19 & & & \\  & 20 & & & \\  & 22 & 19.6 & 1.36 & 2.00 \\  & 18 & & & \\  & 19 & & & \\ \hline \multirow{4}{*}{30} & 28 & & & \\  & 29 & & & \\  & 32 & 29.4 & 1.50 & 2.00 \\  & 30 & & & \\  & 28 & & & \\ \hline \multirow{4}{*}{40} & 39 & & & \\  & 39 & & & \\  & 38 & 39.4 & 1.02 & 1.50 \\  & 40 & & & \\  & 41 & & & \\ \hline \multirow{4}{*}{50} & 50 & & & \\  & 49 & & & \\  & 51 & 49.4 & 1.02 & 1.20 \\  & 49 & & & \\  & 48 & & & \\ \hline \multirow{4}{*}{60} & 55 & & & \\  & 56 & & & \\  & 61 & 57.4 & 2.06 & 4.33 \\  & 57 & & & \\  & 58 & & & \\ \hline \multirow{4}{*}{70} & 69 & & & & \\  & 70 & & & \\  & 73 & 69.6 & 1.85 & 0.57 \\  & 68 & & & \\ \hline \multirow{4}{*}{80} & 78 & & & & \\  & 75 & & & \\ \cline{1-1}  & 76 & & 77.8 & 2.14 & 2.75 \\ \cline{1-1}  & 81 & & & \\ \cline{1-1}  & 79 & & & \\ \hline \hline \end{tabular}
\end{table}
Table 1: Test results of impact-echo technique.  
The impact-echo technique uses a multi-function rock anchor tester that is based on the principle of impact elastic wave propagation. When the stress wave encounters an interface with different acoustic impedance, it will be reflected, and the resonance phenomenon will cause changes in the surface displacement of the concrete. By placing receivers around the excitation point, the displacement signals can be collected and analyzed using the fast Fourier transform (FFT) to evaluate the internal defects and density of the structure [29; 30; 31].  
The impact-echo technique has the advantages of high energy, strong penetration, convenient operation, and fast testing speed, making it particularly suitable for precast structures with single-sided access [32]. It also demonstrates a high accuracy in detecting the grout filling degree of both large and small diameter corrugated sleeves. In the field cases, under specific conditions, the impact-echo technique's results for the grout filling degree of large diameter corrugated sleeves were found to be consistent with the core sampling results.