## 4 Field Test on a Model Railway Track Slab with a Gap  
### Model Track Slab and LRAW Pilot Test System  
A test system module composed of a microphone (PCB-377C10) and an accelerometer (PCB-352C33), DAQ (NI model 9234 with cDNA9174), PCB482C15 signal conditioner, and an analyzer program written by using Matlab(r)[28] was assembled for a small backyard pilot test as shown in Figure 12. The microphones were encapsulated with the preamplifier (PCB model 426B03). Small model railway track slabs with a gap and without a gap were constructed separately in field. A detailed description for construction of the small model railway track slabs was explained by [PERSON] et al. [12]. The PCB-377C10 microphone was adapted to measure acoustic pressure in the field site without reflection environment. The microphone's open circuit sensitivity is 1 mV/Pa. Detecting frequency range at \(\pm\)2 dB is from 4 to 70 kHz. Dynamic range of upper and lower limit is 174 and 35 dB, respectively. These broad dynamic and frequency ranges are helpful to detect gaps existing under the concrete track slab. The PCB-352 CC accelerometer was adapted to measure acceleration in uniaxial (vertical) direction on the slab surface. Sensitivity was 100 mV/g, with a measurement range of 50\(\pm\) g peak. Frequency range at \(\pm\)5% is from 0.5 to 10,000 H\({}_{2}\). These two PCB sensors were combined to be a sensing module and located at a position on top of the slab where a gap was prepared artificially. Figure 11: Leaky Rayleigh acoustic waves detected in the air transmitted from the concrete surface. (a) Without gap. (b) With gap. Figure 12: Field test setup with test system module on a model concrete slab. Following the standard concrete mix and curing practice [29], a long rectangular concrete slab with three different sizes of gap was constructed alongside of bigger size slab in a rural field site. Table 2 summarizes the material properties of aggregate for concrete mix. The 28-day compressive strength of the cored sample of the concrete slab was 30 MPa. A mix design for the concrete slab is explained in Table 3.  
Leaky Rayleigh waves and stress waves were generated by the impact load applied at a location of the top surface of the model concrete slab and sensed by the test system module (Figure 12). The model concrete slab was constructed on compacted natural ground. The size of the model slab was 5.8 m long, 0.625 m wide, and 0.64 m high. The model slab was constructed alongside a big slab that has bigger cavities placed deeply in the ground below the slab. Water content of the natural ground was 9.78%. From the standard Proctor compaction test, it was decided that maximum dry density of the soil obtained from the site was 20.05 kN/m\({}^{3}\) at optimum moisture content (OMC) of 6.7%. The natural ground was compacted using a small compact by following the results of the compaction test. The natural ground soil was classified as SP in Unified Soil Classification System (USCS). Two impact loadings were carried out respectively to obtain and compare the FFT results and wavelet transforms as shown in Figure 13. Left wave source by impact #1 was detected by the first sensing module, which consisted of a microphone (MIC 1) and an accelerometer (Acc. 1) positioned d1 distance away from the excitation source. The gap was located exactly under the first sensing module. The second module consisted of a microphone (MIC 2) and an accelerometer (Acc. 2), which was positioned d1+d2 distance away from the excitation source. Right wave source by impact #2 was detected also by the first sensing module, which consisted of a microphone (MIC 1) and an accelerometer (Acc. 1) positioned d2 + d3 distance away from the right excitation source. The second module (MIC 2 + Acc. 2) was positioned d3 distance away from the excitation source.