## 5 Experiments and Results
The experimental detection process and the NU62 instrument are shown in Figure 8.  
The inspection program of the proposed ultrasonic CT is as follows: (1) determining the accuracy and the measurement points, calculating the required measurement number for damage detection; (2) selecting the paths based on a Bernoulli distributed matrix from the measurement net of the conventional ultrasonic CT technique; (3) capturing the travel time of ultrasonic waves in the concrete-filled steel tube by placing the ultrasonic probes on the two points in the selected measurement paths; (4) assembling the measured travel time vector after all the selected paths have been inspected; and (5) reconstructing the slowness vector and imaging the internal situation of the tube based on the proposed \(\ell_{1}\)-minimization algorithm of CS theory. For the conventional ultrasonic CT technique, the second step is removed; in addition, ART were employed in the fifth step. Figure 8: Ultrasonic inspection for the concrete-filled steel tube of the STS structure: (**a**) Schematic of damage detection; (**b**) NU62 equipment and the launched excitation signal. Figure 7: The Olympic Center Station in Shenyang Metro line 9: (**a**) Overview; (**b**) Cross-section; (**c**) Concrete-filled steel tubes of the STS structure. Based on the selected net mentioned previously, the travel time of the ultrasonic wave in the tube can be obtained according to the time-of-flight between the emission and reception, which is \(\mathbf{T_{m}}\) in Equation (8). After being measured by the instrument, the waveforms of ultrasonic waves through various selected paths are shown in Figure 9. The travel time of the ultrasonic wave in each path is measured by the arriving time of the first wave in the waveform, which is represented by a blue bar in Figure 9. Each travel time measured by the instrument is assembled as the order of paths to be the selected travel time vector \(\mathbf{T_{m}}\) in the Equation (8). Then the difference slownesses of the microstructures, \(\Delta\mathbf{s}\), representing the internal situation of the tube, can be reconstructed by using the \(\ell_{1}\)-minimization algorithm as expressed as Equation (10). Most inspected results are pristine, indicating that these indiscrete concrete-filled steel tubes of the STS system are without any defect. However, based on the reconstructed results, there are a few tubes containing holes, which should be attributed to construction issues. For instance, tube No. 19 was detected to have two significant flaws. The reconstructed results of that tube are shown in Figure 10. Note that the reconstructed damage vector obtained by the CS algorithm is sparse, indicating critical damage; for the purpose of pursuing a detailed damage report of the tube, conventional ultrasonic CT was employed. As we can see, the improved ultrasonic CT has great agreement with the conventional ultrasonic CT technique, which validates this proposed method via much fewer detections with reasonably accurate damage detection results, especially for the critical damage. There are two severe defects in this tube; the location of one is \((150,\,18^{\circ})\), and the other is located at about \((300,\,90^{\circ})\). Several small flaws, the difference slowness of which is larger than \(0.010\), are also identified by the proposed algorithm. The other concrete-filled steel tube of the STS system (No. 26 tube) was also found to contain one defect. The defect is located at \((300,\,90^{\circ})\). Its size and the location were first detected by the proposed ultrasonic CT and validated by the conventional one as well, as shown in Figure 11.  
Considering the dramatic reduction of the measurement number, the proposed ultrasonic CT improved by CS has a great potential for shortening the work time of damage detection, which provides a more effective approach with good accuracy for NDT. Figure 9: Waveforms of ultrasonic detection through various measurement paths. Figure 7:0 Reconstructed result of No.