# Feasibility Study on Grouting Compactness Detection in Sleeves Using Piezoelectric Transducers
## 1 Introduction
PZTs embedded in a structure emit and receive stress waves according to the direct and inverse piezoelectric effects of the piezoelectric materials. Structural damage is detected by establishing the relationship between the piezoelectric wave signals and the structural working states. [PERSON] et al. [16; 17] proposed the \"smart aggregate,\" which embed PZT pieces in marble to better adapt them to the high-salinity environment of concrete materials, and successfully applied it in the early-age strength monitoring of concrete, impact detection, and structural health monitoring. In SHM methods, establishing damage index is essential for diagnosis and identification of time, location and extent of structural damage. The commonly used damage indexes are amplitude, frequency, wave velocity and so on. However, there is no direct correlation between the damage of some structures and these indexes, and damage cannot be accurately described. Therefore, the signal processing method is vital, and wavelet packet energy (WPE) [18] method and Hilbert-Huang transform (HHT) [19] method are common signal processing methods. [PERSON] et al. [20] presented a stress wave-based method to detect the crack in FRP-reinforced concrete beams. The research validated damage characteristics extracted in PZT signal by the WPE method were able to effectively monitor the crack occurrence and development. [PERSON] et al. [21; 22] employed the stress wave method for monitoring the debonding damage in concrete-filled steel tubes. Using the WPE method, the damage index, which is known as the weighted energy ratio variation, was defined and used effectively to monitor the damage zone between the steel and concrete. [PERSON] et al. [23] investigated the nonlinear vibro-acoustic modulation technique for damage detection in metallic structures. PZT transducers were bonded on the surface of aluminum specimens. Instantaneous characteristics of the response were extracted from received signals by the HHT method and used for evaluating the damage location and degree of composite materials. [PERSON] et al. [24] used the nonlinear ultrasonic modulation method with piezoceramic transducers to detect the debonding in hidden frame supported glass curtain walls. The received signals were processed by the HHT method and the characteristics component containing the nonlinear damage information were selected. The experiments verified the feasibility and effectiveness of the nonlinear ultrasonic modulation method in debonding detection of hidden frame supported glass curtain walls. Currently, stress wave methods using PZT detection technology to monitor the grouting compactness of ducts have been developed [25; 26; 27]. The WPE analytical method was adopted to compute the total signal energy using PZT sensors. The results showed that the wavelet packet total energy value reflected the degree of grouting in the duct [25; 27]. However, the inner diameter of the tendon duct is usually greater than 50 mm, while the inner diameter of the grouting sleeve is generally less than 50 mm. Therefore, the internal environment of the sleeve is more complicated than that of the duct. As such, the influence of steel in the grouting sleeve cannot be ignored. At present, there is no method to complete the detection of sleeve grouting compactness at a low cost, and the detection of the sleeve in a double-row arrangement cannot be realized. PZT based detection has certain advantages in two aspects. In this paper, a stress-wave-based active sensing approach was adopted to detect the grouting compactness of sleeves. In the proposed method, PZT pieces covered in epoxy resin are attached to the opposite sides of the outside surface of a sleeve, and the sleeve is pre-filled with grouting materials. Fifteen samples with different grout degrees ranging from 0% to 100% grouting were experimentally studied. The WPE and HHT methods were used to analyze the signals received from PZT sensors.