Fundamental research on non-destructive testing of reinforced concrete structures using sub-terahertz reflected waves

Abstract

Terahertz and sub-terahertz waves are unexplored waves range between infrared waves and microwaves. This range features with unique characteristics of both light straightness and electromagnetic wave transmission. Terahertz and sub-terahertz waves are attractively new diagnose method of inner objects because they are safer compared to normal non-destructive inspection methods involving high energy, such as X-rays. Hereby, a novel non-destructive inspection method for concrete and reinforced concrete structures is proposed using sub-terahertz reflection imaging. In Japan, where earthquakes and other natural disasters frequently occur, this technique is expected to be used to remotely inspect the deterioration of damaged RC structures that are difficult to access to. The results of this study confirm that various types of condition changes in concrete can be found by using this sub-terahertz imaging method. The presence of cracks/voids in concrete and under the finishing layers was successfully detected based on differences in reflectance. Moreover, the presence of metals inside/behind the concrete specimens was able to be identified. It was also confirmed that the decline rate of the mechanical properties of concrete could be evaluated according to the decrease in reflectance due to the existence of fine cracks.

Fundamental Properties of Sub-THz Reflected Waves for Water Content Estimation of Reinforced Concrete Structures

“…The frequencies employed in our study, 8.5 to 12.5 GHz, fall slightly below the conventional sub-THz range of 0.1 to 1 THz. However, our research group has been investigating the application potential across a broad frequency spectrum from approximately 0.01 to 10 THz, encompassing frequencies higher than those discussed in this paper [20][21][22][23][24][25]. Based on this foundation, we have classified this spectrum as the "sub-THz" range.…”

“…Figure 1 shows schematic diagrams of the (a) transmission and (b) reflection methods in the sub-THz wave optical system. There are two measurement methods for sub-THz wave optics: the transmission method [20] and the reflection method [21]. The transmission method requires the oscillator and detector to be placed in a straight line, which is difficult to align in measurements on real structures.…”