# Feasibility Study on Grouting Compactness Detection in Sleeves Using Piezoelectric Transducers
## 1 Introduction  
Grouting sleeve connection (GSC) technology plays a major role in prefabricated concrete (PC) structures, which are among the most important structures in the construction industry. With GSC technology, steel bars are inserted into both ends of a sleeve, and high-strength grouting materials are injected. The mechanical properties of connection joints depend on the bonding force between the grouting materials and two small steel bars, as well as that of the inner wall of the sleeve [1; 2; 3; 4; 5]. Therefore, the reliability of the joined bar heavily depends on the compactness of the grouting materials in the sleeves. However, improper design and construction processes of structural components often lead to insufficient grouting. Because of the invisibility of the grouting sleeve inside the structure, nondestructive testing methods are used to determine the grouting compactness. For example, [PERSON] et al. [6] demonstrated the feasibility of using an ultrasonic method for grouting compactness detection. Later on, [PERSON] and [PERSON] [7] proposed an ultrasonic probability method based on the t-distribution. Experimental results showed that this improved method could reduce the detection of workloads and improve detection efficiency while maintaining the detection accuracy. [PERSON] et al. [8] proposed an ultrasonic detection method based on wavelet packet energy for grouted defects in GSC, the results demonstrated thatthe proposed method could effectively detect the grouted defects in a certain range. [PERSON] et al. [9] identified insufficient grouting sleeves using the impact-echo method. In general, for single-row sleeves, the aforementioned methods provide fast qualitative analysis of the grouting quality. However, in many complex cases, e.g., where sleeves are arranged in double rows, the grouting quality in the GSC cannot be detected immediately. Thus, many scholars have focused on detecting the grouting quality in the GSC in PC structures. For instance, [PERSON] et al. [10] used industrial X-ray computed tomography (X-CT) to test the grouting compactness of sleeves encased in both plain and reinforced concrete and to overcome the problem of interference from the steel bars, concrete, and sleeves. However, this method is only suitable for detecting parallel pieces and depends on the shielding conditions in the laboratory. Using X-ray digital radiography, [PERSON] et al. [11] studied the quality of the sleeve grouting of a precast shear wall. The experimental results showed that the proposed method was feasible for detecting the grouting compactness in both single-row and plum-pile arrangement sleeves. However, it requires extensive safety protection measures to prevent radiation, which limits the potential application of this technology. In summary, existing detection methods for the grouting compactness of sleeves, as described in the current literature, have several problems. For instance, it can only determine whether there are grouting defects, but not the defect rate. In addition, the detection accuracy is low, and long-term detection is not possible. Lead zirconate titanate (PZT), the most common piezoelectric materials, has been widely used in structural health monitoring (SHM) due to its low cost, high bandwidth, fast response, and dual capability of actuation and sensing [12; 13]. PZT-based stress wave method is an advanced active energy harvesting and sensing testing method [14]. The propagation distance between stress waves in the structure is large, and the detection range is broad. Additionally, many types of excitation signals exist, and many signal characteristics can be analyzed by using different modalities. Therefore, this method has been used for integrity assessment, health monitoring, and damage detection in civil engineering. [PERSON] et al. [15] first used a PZT transducer and proposed the stress wave method in structural health monitoring.