## 3 Results and Discussion  
The THz A-scanner has been tested to ensure that it is working properly. For this test, an aluminum mockup was made consisting of half a cylinder with the same diameter as that of the external tendons and a base plate (see Figure 3). The THz A-scanner was used to measure the THz A-scan signal reflected from the aluminum mockup. Figure 4 shows the measured frequency-domain THz data and the THz A-scan data obtained by FFT of the frequency-domain THz data. In Figure 4b, the peak at 10 ps represents the reflected signal from the surface of the aluminum mockup. The full width at half maximum (FWHM) of the peak is 1.75 ps, indicating that the A-scan resolution is 0.26 mm in free space. The A-scan resolution is limited by the measurable frequency bandwidth and could be enhanced by using a THz-Tx and THz-Rx with a wider bandwidth. The frequency band of 0.5-1.88 THz has a mean frequency of 0.88 THz when weighted with the amplitude. If the THz beam is assumed to be a Gaussian beam, the FWHM of the beam spot on the focal plane, that is, the lateral resolution, is estimated to be roughly 2.7 mm by \(\sqrt{\ln 2}(4/\pi)(f\lambda/D)\), where \(f\) is the focal length of the off-axis parabolic mirror, \(\lambda\) is the wavelength corresponding to the mean frequency, and \(D\) is the input beam diameter [36,37]. To test the performance of the THz A-scanner for detecting voids in external tendons, tendon specimens were constructed by inserting steel strands in an HDPE duct with outer and inner diameters of 110 and 100 mm, respectively, and injecting grout as a filler. As the grout hardened, voids were formed in the upper part of the duct interior. Photographs of one of the tendon specimens thus constructed are shown in Figure 5. Figure 5a,b shows the side and cross-sectional views of the tendon specimen, respectively. Figure 5b shows that there is a void between the duct and the grout at the top of the inner side of the duct. The tendon specimen shown in Figure 5 was inspected using the THz A-scanner. Figure 6a shows the THz A-scan data measured in the void-free area of the tendon specimen. The first peak is the reflected signal from the duct surface, and the second peak is the reflected signal from the interface between the duct and the grout. The magnitude ratio of the second peak to the first peak is about 70%. Figure 3: Photographs of the fabricated mobile THz A-scan unit. Figure 6b,c shows the THz A-scan data measured in the void area of the tendon specimen. In Figure 6b,c, the first peak is the reflected signal from the duct surface, and the second peak is the reflected signal from the interface between the duct and the void. The third peak in Figure 6c, which is not visible in Figure 6b, is the reflected signal from the interface between the void and the grout. The third peak may or may not be present because the reflected signal from the interface between the void and the grout is measured only when the interface is perpendicular to the THz wave propagation. In Figure 6c, the time delay of 14.7 ps between the second and third peaks indicates that the void is 2.2 mm deep. The magnitude ratio of the second peak to the first peak is about 30%, as shown in Figure 6b,c. Real-time THz A-scan measurements on the tendon specimen using the mobile THz A-scanner are demonstrated in Video S1. Indoor and outdoor THz A-scan measurements gave almost identical results, and all the results shown here were measured outdoors. Figure 4: (**a**) Frequency-domain THz data measured with the aluminum mockup. (**b**) THz A-scan data obtained by FFT of the frequency-domain THz data. Figure 5: (**a**) Side and (**b**) cross-sectional views of the constructed tendon specimen. In (**b**), A and B represent void-free and void areas of the tendon specimen, respectively. Reflectivity is determined by the refractive indices of two materials at their interface.