# Feasibility Study on Grouting Compactness Detection in Sleeves Using Piezoelectric Transducers
## 5 Conclusion and Future Work  
Sleeve grouting compactness is a key factor in ensuring the performance of nodes in prefabricated concrete structures. Because the sleeve is invisible in the structure and thus not easily examined, only the slurry discharge state of the outlet hole of the mud has been used as a standard for grouting compaction, which is not a reliable method to indicate the compactness of sleeve grouting. Herein, a PZT-based detection method was proposed for monitoring the grouting compactness of sleeves in PC structures. The proposed method employs a stress-wave-based active sensing approach using PZT transducers. In the experiment, PZT transducers acting as an actuator and a sensor were bonded to opposite sides of a sleeve along the outer surface of the ring. The transducers were wrapped in epoxy resin to prevent the effects of the external environment. The WPE and HHT signal processing methods were used to analyze the signals received by the PZT sensor. Five grouting stages were  
Figure 8: Relationship between the grouting compactness and the proposed indices: (**a**) wavelet packet energy (WPE) method, (**b**) Hilbert–Huang transform (HHT) method. \begin{table}
\begin{tabular}{c c c c c c c c c c c} \hline \hline \multicolumn{2}{c}{**Methods**} & \multicolumn{5}{c}{**WPTEVs**} & \multicolumn{5}{c}{**HEPVs**} \\ \hline
**Grouting compactness of sleeve** & **0\%** & **30\%** & **50\%** & **70\%** & **100\%** & **0\%** & **30\%** & **50\%** & **70\%** & **100\%** \\ \hline \multirow{8}{*}{Serial} & 1 st group of A1S1 & 663.23 & 388.93 & 388.43 & 388.60 & 325.43 & 0.154 & 0.0485 & 0.0551 & 0.0641 & 0.0439 \\  & 2 nd group of A1S1 & 611.60 & 111.81 & 533.27 & 303.12 & 577.06 & 0.163 & 0.0013 & 0.1200 & 0.0358 & 0.1500 \\  & 3 rd group of A1S1 & 714.09 & 510.44 & 418.36 & 400.41 & 288.51 & 0.196 & 0.114 & 0.0616 & 0.0645 & 0.0350 \\ Number & 1 st group of A2S2 & 549.37 & 314.57 & 259.85 & 240.05 & 234.02 & 0.122 & 0.0452 & 0.0283 & 0.0194 & 0.0188 \\  & 2 nd group of A2S2 & 455.73 & 435.46 & 382.89 & 254.81 & 250.92 & 0.072 & 0.0666 & 0.0625 & 0.0212 & 0.0207 \\  & 3 rd group of A2S2 & 567.11 & 481.14 & 336.66 & 306.49 & 259.84 & 0.126 & 0.0897 & 0.0443 & 0.0357 & 0.0266 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Grouting-compactness indices. studied experimentally in the grouting range of 0% to 100%. The experimental results revealed that the signal index recorded by the PZT transducer decreases with respect to the grouting compactness, proving the feasibility of the method. The HHT method was more suitable than the WPE method on the grouting compactness detection in sleeves. Compared with the existing sleeve compactness detection methods, the PZT-based method is not affected by the double-row arrangement of the sleeve and is more practical to be used in the filed than the X-CT or X-ray methods. Although the proposed method is effective for detecting the grouting compactness, further quantitative investigations are required to establish correlations between WPTEV and HEPV signals and grouting compactness. Furthermore, future work should also focus on boundary conditions that affect results and detection of the grouting compactness of the sleeve inside the component under stress. Nevertheless, this study has demonstrated the feasibility of using PZT sensors to detect grouting compactness, and it lays a solid foundation for future studies. Conceptualization, C.W. and S.M. ; provision of sensing technologies, X.X. ; design of experiments, C.W. and C.Y. ; execution of experiments and analysis of data, C.Y., C.W., and S.M. ; writing of manuscript, C.Y. ; revision of manuscript, C.W. All authors have read and agreed to the published version of the manuscript. This work was supported by the National Natural Science Foundation of China with grant No. 51808119; the Fujian Industry-university Cooperation Project with grant No. 2019Y4011; the Fujian New Century Excellent Talent Support Project with grant No. GYZ-160144; and the Fujian University of Technology Research and Development Fund with grant No.