## 5 Discussion  
Track condition is important because it significantly affects track performance and maintenance requirements. Currently there is no proper NDT method to detect a gap between track slab and track foundation soil. In this study, a nondestructive test method (NDT) named LRAW (leaky Rayleigh acoustic wave) method is proposed based on a wavelet time-frequency concept for the practical purpose of detecting gaps under track slabs. The LRAW method uses a microphone sensor to catch leaked Rayleigh acoustic waves in the air and an accelerometer to measure surface Rayleigh waves on the slab. In order to  
Figure 15: Leaky Rayleigh acoustic waves detected in the air transmitted from the model concrete surface in the field. (**a**) Without gap. (**b**) With gap. investigate the possibility of developing the test system to use leaky Rayleigh waves in the air to detect gaps between track slabs and track bed soil, a FEA using ABAQUS software was performed to simulate Rayleigh wave generation on the concrete surface and in the air. A test system module composed of a microphone and an accelerometer, DAQ, and an analyzer program was also assembled for a small backyard pilot test. From the performed LRAW test results and important findings of the FEA in this study, the following implications and conclusions can be made:  
1. From computed results by FEA simulation of NDT in cases of with and without a gap, considerable waves were transmitted into the air to be detected after the waves were reflected at the boundaries around the gap in the soil ground. Characteristics of wave transmission were investigated in detail by analyzing wavelet distribution and autospectrum density. 2. At first, considering the computed results of FEA, based on comparison of acceleration level in frequency domain for two cases of with and without a gap, it was found that the leaked Rayleigh waves into air provided a much lower level than those captured on the concrete surface. However, in the case of gap existence, the peak values of the case with a cavity were two times larger than those of the case without a gap. 3. On the concrete surface, the predominant resonant frequency was generated around 170 Hz. On the other hand, the resonant frequency of the Rayleigh waves transmitted into air in the case of gap existence was generated at 7 to 12 Hz, which was much lower than the range at concrete surface. Compared to the concrete surface, relatively weak reflected waves were transmitted into the air in the cases of with or without the gap. However, a considerable number of waves were transmitted into the air to be detected after the waves were reflected at the boundaries around the gap in the soil ground. Wave amplitude generated in the gap case provided a more prominent peak and was distributed over a wider range of frequency than that in the non-gap case. 4. After applying an impact load on the concrete slab surface, the maximum acceleration amplitude generated in the air was investigated. Much higher maximum amplitudes were generated intensively near the impact loading point in the case of with a gap as opposed to without a gap. These phenomena were found also in contours of acceleration level in the slab and the air. 5. Based on the autospectral density of the acceleration, it was found that a few modes of resonance were generated due to reflections of waves at the boundaries around the gap in the soil ground. A sharper and higher peak of autospectral density at a resonance frequency is observed in the case of with a gap. 6. In case of gap existence, longer and higher wavelets of leaky Rayleigh acoustic waves were obtained than without case. Resonance frequency is higher also than without case. Thus, it could be confirmed from the analyzed results of the FEA for the model railway track slab that the presence of a gap below the slab is more easily identified using the LRAW method than using an accelerometer installed on the surface. 7. A long rectangular model concrete slab with three different sizes of gap was constructed in a rural field site.