| Concrete-Filled Steel Tube (CFST) structure has recently been making unique prominent feature with the rise of the building heights. However, due to the existence of the clapboard in the CFST connection, the concrete below it in the CFST member is easy to become local void after pouring. In addition, due to the shrinkage of concrete itself, the interface between concrete and tube may separate or some uncompacted void space may appear. The compactness of concrete is the important precondition to ensure the reliability of the CFST structure, directly influencing the reliable performance of the total structure. Therefore, the detection of the concrete compactness will provide a reference for the evaluation of structure bearing capacity. This paper focuses on the study of concrete compactness using smart aggregates and the wave-based method to evaluate the states of the concrete compactness. The main contents are included as follows:In the first part, the concrete compactness for a CFST column in situ was experimentally researched by using piezoelectric ceramic to determine the void location and level. A group of piezoelectric ceramics (such as Lead Zirconate Titanate, PZT) was embedded into a small volume of concrete to make smart aggregates (SA) with dual function of actuating and sensing which were placed in the given locations in the column. The two adjacent units were used as a detecting pair. Launch-receive interactive method was employed to launch and receive signals. The detecting signal was passing through the void zone and carried the information of the void. Euclidean distance was applied as the measurement of compactness to decide the signals difference in a detecting pair to evaluate whether the specimen was defective. The result is agreed with traditional ultrasonic detection method. It showed that the proposed method was effective to detect the compactness of concrete, and it is feasible and effective to be used in civil engineering.In the second part, a time reversal concept of modern acoustics (TRA) has been adopted for guided-wave propagation to improve the detect ability of local defects in the CFST structure. For the description of the compactness level of concrete, two indices are proposed in the paper: time reversibility (TR) and symmetry (SYM) indices. Smart aggregates are used for both actuating and sensing the guided waves for detection. At the same time, the response signal is reversed in the time domain and applied back again. Six specimens are designed with artificial defects or not. The compactness of concrete can be effectively evaluated by choosing narrow-band actuating signal and signal processing. The results show the feasibility of this method, and the location and the range of the damage can be expressed well by the defined two indices. In the third part, the responses of a typical CFST column with artificial defects under different actuating frequencies are numerically and nonlinearly analyzed by using with using ANSYS11.0finite element software, and it will provide a reference for the choice of actuating signals. Through the application of the excitation waveform displacement with different frequencies, the FEA was performed by using transient analysis with change of the wave duration and MATLAB software for data processing. The numerical analysis results agree with the experimental results. |
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