1. Introduction
In recent years, high performance and multi-functional materials and structures have received much attention [
1,
2,
3,
4]. Among them, concrete-filled steel tube (CFST) columns, which have characteristics such as high stiffness and strength, large energy absorption capacity, and high ductility, are widely used in the construction of buildings. There are a number of studies on the CFST columns, such as Shams et al. [
5], Portolés et al. [
6], Gourley et al. [
7], Lee et al. [
8], Sakino et al. [
9], and Varma et al. [
10,
11,
12]. L-shaped concrete-filled steel tube (L-CFST) columns are increasingly used because of their architectural appearance and space saving at structural corners. Wang et al. [
13], Shen et al. [
14], and Zhang et al. [
15] investigated the seismic behavior of L-CFST columns. CFST structures are subject to damages under the repeated action of seismic loads. Internal structural damages will continue to accumulate with the cyclic action of loads and the structure will fail as the damages increase to a certain degree. The damage of concrete inside the steel tube will significantly reduce the bearing capacity and ductility of the CFST, however, the internal concrete damage cannot be directly observed. Therefore, it is very important to monitor the internal concrete damage of the structure.
At present, the methods of structural damage detection include X-ray, acoustic emission, and ultrasonic methods [
16,
17], among others. Carpinteri et al. [
18] used acoustic emission (AE) technology to evaluate visual cracks in reinforced concrete of a multi-story building. Behnia et al. [
19] reviewed the application of acoustic emission technology to the health monitoring of concrete structures. Rucka and Wilde [
20] studied the cracks of reinforced concrete structures under tensile stress by the ultrasonic method. Antonaci et al. [
21] used linear or nonlinear ultrasonic methods to detect the compressive damage of circular concrete columns. Yue et al. [
22] developed a damage detectability model of pitch-catch configuration using lamb waves for composite plates
In recent years, smart materials, such as optical fibers and piezoelectric materials [
23,
24,
25], have been successfully applied to the health monitoring of structures. Kerrouche et al. [
26] used embedded Bragg grating optical fiber sensors to monitor the strain on carbon fiber polymer reinforcement (CFPR) rods. Ho et al. [
27] proposed a smart anchor plate, a simple but effective device that uses a fiber Bragg gratings (FBG) sensor to monitor the load level of the rock bolt. DeäŸErliyurt et al. [
28] used FBG sensors to monitor the damage of the composite beam structure under bending. Tjin et al. [
29] placed FBG strain sensors in a reinforced concrete structure to monitor strain changes in loading and unloading tests at different locations within the structure. Piezoceramic transducers, including the piezoceramic smart aggregates, have features such as low cost and actuating-sensing functions, and are widely used in structural health monitoring research [
30,
31,
32,
33,
34]. Sharif-Khodaei at al. [
35] employed piezoelectric transducers to detect impact damages to a composite plate. Song et al. [
36] embedded piezoelectric smart aggregates into reinforced concrete beams to perform damage monitoring through destructive tests. Du et al. [
37] embedded the piezoceramic smart aggregates (SAs) into a quartz sand-filled steel tube column (SFSTC) to monitor the internal structural stress during impacts. Li et al. [
38] studied the damage of concrete beams under a three-point bending test, and compared the damages measured by acoustic emission sensors with the damages measured by smart aggregates. Du et al. [
39] used piezoceramic transducers to perform the damage detection of pipeline with multi-cracks. Feng et al. [
40] used embedded smart aggregates to monitor the internal damage of concrete piles, including cracks, partial mud intrusion, secondary pouring, and all mud intrusion to four kinds of damage. Kong et al. [
41] used SAs to study the early hydration characteristics of concrete, and then divided the early hydration of concrete into three states: fluid state, transitional state, and hardening state. Markovic et al. [
42] established a model of damage detection process for concrete beams based on piezoelectric smart aggregates. Du et al. [
43] investigated the pipeline corrosion pit with the time reversal method using piezoceramic transducers. Chalioris et al. [
44] used embedded and externally bonded piezoelectric transducers to evaluate a shear-critical reinforced concrete beam. Nestorović et al. [
45] proposed a numerical modeling of the damage detection process in a concrete beam with piezoceramic transducers. Olmi et al. [
46] studied the use of embedded piezoelectric sensors to monitor impact when an over-height truck collides with a reinforced concrete beam. Ghafari [
47] studies the feasibility of using piezoelectric sensors to characterize the compressive strength of cement paste mixed with additional cementitious materials.
Piezoelectric transducer based structural damage detection methods include two major categories: the electromechanical impedance method [
48,
49,
50] and the active sensing method. Yang et al. [
51] used the structural mechanical impedance (SMI) extracted from the PZT (Lead Zirconate Titanate) electro-mechanical (PZT EM) admittance signature as the damage indicator. Providakis and Voutetaki [
52] investigated a statistical metamodeling utilization of electro-mechanical admittance approach to the damage identification. Xu et al. [
53] applied the impedance method to monitor the damage of a structure by using both embedded and surface bonded piezoelectric transducers. Madhav et al. [
54] reviewed and prospected the application of electromechanical impedance technique in engineering structures. Karayannis et al. [
55] proposed a method for monitoring the potential damage of reinforced concrete members by electromechanical admittance method, which used a bonded piezoelectric sensor. Wang et al. [
56] proposed a new detection method based on an inverse impedance method to study the damage detection of plain concrete beams. Interestingly, Zou and Aliabadi (2015) developed a piezoelectric sensor for damage detection with self-diagnosis capacity using electro-mechanical impedance.
The active sensing method using piezoceramic smart aggregate is commonly used for monitoring concrete structural damage. The active sensing method uses a smart aggregate as an actuator and another smart aggregate as a sensor. Using the inverse piezoelectric effect, the smart aggregate actuator generates a stress wave, which propagates along the interior of the structure to the piezoelectric smart aggregate sensor. Using the direct piezoelectric effect, the sensor converts the received stress wave into an electrical signal effect. Once there are cracks inside the structure, the signal received by the piezoelectric smart aggregate sensor will attenuate. Therefore, the internal damage of the structure can be analyzed. Gu et al. [
57] used SA enabled active sensing monitor concrete early age strength development. Divsholi and Yang [
58] used a combination of embedded and surface bonded piezoceramic transducers to detect the damages of a concrete beam, and the results show that this combination provides an effective way to assess both the local and overall damage conditions of the structure. Zou et al. [
59] used SA enabled active sensing to monitor the degree of water seepage of concrete structures. Feng et al. [
60] proposed an active sensing method based on SAs to monitor cracks and further leakage of concrete pipes. Kong et al. [
61] studied the presence of internal moisture in concrete structures and used active sensing methods based on embedded piezoelectric sensors to monitor the presence of cracks and moisture in concrete structures. Xu et al. [
62] used a smart aggregate embedded in the concrete and PZT patches bonded on the surface of the steel tube to monitor the debonding between the steel tube and the concrete. Jiang et al. [
63] presented a stress wave based active sensing approach using piezoceramic transducers to monitor grouting compactness in real time.
From the above applications, it can be found that the existing smart aggregate-based damage studies are mostly conducted under static loading, however, less experimental studies are carried out under dynamic loading. Gu et al. [
64] embedded smart aggregates inside circular reinforced concrete columns to study the internal damages of structures under seismic actions. The research results show that smart aggregates have great potential for application in the health monitoring of mass concrete structures. Liao et al. [
65] used smart aggregates to monitor the damage of reinforced concrete frame structures under earthquake excitations and compared the index of smart aggregate monitoring damage with the calculated displacement ductility demand of structural components. The results show that the two are consistent. Kong et al. [
66] used smart aggregate to monitor the internal damage of reinforced concrete bridge columns under pseudo-dynamic loading. The results verified the effectiveness of smart aggregates in the health monitoring of reinforced concrete column. Liao et al. [
67] conducted tests of concrete columns using smart aggregates for structural health monitoring.
At present, the study of structural damage detection under dynamic loads using SAs is mainly about reinforced concrete structures, and there is little research on the internal concrete damages of concrete filled steel tube structures. CFST structure is a structure filled with concrete inside the steel tube, and its bearing capacity is shared by the steel tube and the internal concrete. The damage of the concrete core will greatly impact the bearing capacity of a CFST. In this research, three L-CFST columns with different wall thicknesses of steel tube were used in this experiment. Piezoelectric smart aggregates were embedded inside the L-CFST columns. The active sensing method based on SA was used to monitor the concrete damage inside column base of L-CFST columns under low-frequency cyclic loading, and wavelet packet analysis was used to establish a damage index, which was then used to analyze and process the monitoring data. The experimental results find that the structural damage index under the low-frequency cyclic loading is basically consistent with the results of the hysteretic curves and the skeleton curve of the specimen. The use of smart aggregate enabled active sensing can directly and clearly reflect the damage process of the concrete core in the L-CFST specimens.