# Actuating Performance Analysis of a New Smart Aggregate Using Piezoceramic Stack
## 1 Introduction  
Worldwide, the conditions for civil infrastructures are deteriorating [1] due to many adverse factors, such as aging [2], corrosion [3; 4; 5], fatigue [6; 7], etc. Structural health monitoring (SHM) [8; 9] is playing an increasingly important role in the safety and reliability of infrastructures by providing real-time early warnings of structural abnormalities [10; 11] based on the integrated sensors [12; 13] and advanced algorithms [14]. As one of the many transducers employed in the field of SHM, piezoelectric transducers possess the advantages of low cost, quick response [15; 16], wide bandwidth [17], sensing and actuation [18; 19], energy harvesting [20; 21], enabling many applications from passive applications, such as vibration sensing [22] and acoustic emission detection [23], to active applications, suchas SHM based on active sensing [24; 25] and electromechanical impedance [26; 27; 28]. Since the introduction of the piezoceramic-based smart aggregate (SA) [29; 30; 31] that provides an effective way to protect the fragile piezoceramic patch, piezoceramic transducers have been employed extensively in concrete structures. Some of them play the role of an actuator to transmit ultrasounds in concrete, and others work as sensors to detect the signals with the change of material or structure properties [32; 33; 34; 35]. After further processing the measured waves, various types of damages are detected or located, and the health state or lifespan of the structure is also estimated. Recently, cracks [36; 37; 38], blast or impact load [39; 40; 41], leakage [42], debonding or bond slip [43; 44], corrosion [45], and concrete strength or water percentage [46; 47; 48] in different types of concrete structures, such as pipes [42], beams [49; 50] and columns [51], are monitored by using SAs. In a typical structure of SAs, there are mainly piezoelectric wafers, a water-proof and insulation layer, a protective housing, and a shielded cable. Since the first SA design [24; 25; 29; 30], the structures of SAs have been improved to meet the new requirements of SHM in concrete structures. The earliest SA was protected by a concrete shell, and it was used to transmit an elastic stress wave in a fixed direction [29; 30]. To prevent structure destruction under impact load and increase the service time of SAs, a marble shell was used to replace the concrete one [52; 53] in the next generation of SAs, and a copper shield was also added to increase the signal to noise ratio. A new tubular SA was proposed by [PERSON] et al. [54; 55; 56], and a piezoelectric tubular cylinder was used in this SA to excite omni-directional stress waves in a two-dimensional space. Nearly at the same time, a spherical SA, where the traditional piezoelectric patch was replaced by a piezoelectric spherical shell, was proposed by [PERSON] et al. [13; 57] to excite ultrasonic waves in all the directions in a three-dimensional structure. The main advantage of the tubular and spherical SAs is to generate uniform stress waves in two-dimensional (2D) and three-dimensional (3D) structures, respectively. To decrease the labor cost due to cable management, a wireless SA sensor was proposed to replace the traditional wired SA by using the Zigbee protocol [58]. Moreover, the electromechanical performance of SAs with a marble shell was investigated theoretically and experimentally by [PERSON] et al. [59] by using the theory of piezoelasticity [60; 61; 62]. The recent focus of SAs mainly involves their structural improvement and practical applications. However, how to increase the output of SAs is usually ignored. In recent decades, the piezoelectric stack actuator is employed extensively for force or displacement control in precision engineering including optical instruments, robotics, and MEMS. In a stack actuator, several piezoelectric pieces are piled up electrically and mechanically, and therefore, its output of force and displacement is significantly increased. However, the piezoelectric stack has never been applied in SAs.