Abstract
Remote-controlled, non-destructive testing is necessary to detect corrosion of the reinforced concrete structures at the Fukushima Daiichi Nuclear Power Plant (NPP) de-commissioning site. This work aims to demonstrate that laser-induced ultrasonic guided wave technology can be applied to achieve this task. Hence, accelerated electrolytic corrosion is performed on a reinforced concrete specimen fabricated by embedding a steel rod into mortar. Waveforms of the laser-induced ultrasonic guided wave on the rod are measured with a previously employed piezoelectric transducer (PZT) probe, for each fixed corrosion time. Based on the results of Fourier and wavelet transforms of the waveforms, issues concerning the detection and extent of rebar corrosion are discussed. It is exhibited that the changes in bonding strength due to corrosion are distinguishable in the frequency domain of the ultrasonic signal.
Highlights
We focus on the following point of view: When the bonding is strong, the ultrasonic wave propagated on the rebar can be modeled as an ultrasonic guided wave on a double-layer rod, where the first layer is the rod and the second layer is infinite concrete
The apparent difference is that only the intact specimen had sharp peaks in the low-frequency region, while all specimens had a sharp peak around 200 kHz in common, which was especially dominant in the naked steel rod and heated specimens
Through the discussion of the experimental results, we can draw the following conclusions: First, the behavior of the ultrasonic wave propagating on the rod is influenced by the bonding strength
Summary
De-commissioning the Fukushima Daiichi Nuclear Power Plant (NPP) is one of the most important challenges we must overcome, an effort that will take decades. Structures deteriorate over time; a de-commissioning site is no exception, making safe and continuous work difficult. The earthquake and subsequent tsunami damaged the site’s reinforced concrete structures, while seawater injection damaged those inside the Primary Containment Vessel. Such structures are the targets to be dismantled and comprise an important basis for future de-commissioning work, meaning it is imperative that these structures do not collapse during work. The current status of the Fukushima Daiichi NPP can be observed on the Tokyo Electric Power Company website [1,2]
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