Non-destructive Inspection of Concrete Surface Crack Using Near-Field Scattering

Abstract

This paper presents non-destructive inspection of sub-millimeter wide concrete surface cracks covered by paper using near-field scattering. One of the problem for nondestructive imaging using near-field scattering is low millimeter-wave (MMW) image contrast at the surface crack, because the decrease of received power due to near-field scattering is about 1 dB. This paper presents the nondestructive inspection technologies that improve the MMW image contrast at the surface crack by using standing wave between the probe antenna and sample surface. We have found that the black and white of the MMW image pixels that correspond to the concrete cracks is inverted according to the paper thickness or antenna height, and the image contrast can be improved by calculating the difference of two MMW images obtained from different paper thickness. When we measured the frequency characteristics of the reflected MMW signal, a sharp spectral notch was observed in the reflected MMW signals, and the spectral notch frequency depends on the presence or absence of crack, paper thickness, and antenna height. We have achieved the improvement of MMW image contrast at surface crack by using these MMW spectroscopy technologies.