Improvement of Detection in Concrete Surface Cracks Covered with Paper by Using Standing Wave of 77-GHz-Band Millimeter-Wave

This paper presents non-destructive millimeter-wave (MMW) imaging of sub-millimeter wide cracks on concrete surface that are covered by paper. Measurement of near-field scattering of 76.5-GHz-MMW signal at cracks enables the detection of sub-millimeter wide cracks. The decrease of received power due to near-field scattering is small, therefore the MMW image contrast of fine surface concrete cracks is not clear. We found that standing wave occurs between the sample surface and the antenna, and received power increases by the near-field scattering caused by the surface crack in case the antenna locates around the node of the standing wave. The MMW image contrast of the cracks improved by up to 3 dB by calculating the difference of two MMW images that are obtained with different paper thickness or different antenna height.