PSI - Issue 68

Koji Uenishi et al. / Procedia Structural Integrity 68 (2025) 547–553 Uenishi et al. / Structural Integrity Procedia 00 (2025) 000–000

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1. Introduction As mentioned in our earlier paper (Uenishi et al., 2024), the number of old reinforced concrete (RC) structures that need renovation for longer usage has been increasing, but partial removal of such structures for the renovation work can become very difficult, especially in densely populated urban areas where the influence to the surroundings must be minimized. In order to lessen the burden to the surrounding environments, we have been developing several techniques to quickly and efficiently remove only parts of concrete structures (e.g. blocks and slabs) as desired by using dynamic waves that are induced by safely controlled electric discharge impulses (EDI) (e.g. Uenishi et al., 2014, 2018, 2024). Among a variety of dissimilar ways utilizing electric energy (e.g. Walsh and Vogler, 2020; Che et al., 2021; Peng et al., 2022; Shi et al., 2022; Wang et al., 2023), we have selected and been using the electric discharge impulse crushing system (EDICS) developed by Kanadevia Engineering Corporation (former Nichizo Tech, Inc.) where quick release of the electric energy in a self-reactive liquid inside a cartridge swiftly evaporates the liquid and generates high pressure or EDI. So far, by performing a series of field experiments and corresponding finite difference numerical simulations, we have identified typical structural inhomogeneities that can be used for precisely controlled dynamic disintegration of concrete structures. The controlling inhomogeneities include outer free surfaces, drilled blast holes for EDI sources (cartridges containing the self-reactive liquid) and empty dummy holes, slits as well as preexisting planes of weakness (Uenishi et al., 2024). During the study, we have also found that reinforcing steel bars (rebars) in RC slabs can mechanically serve as a virtual interface and if blast and empty dummy holes are geometrically properly set, they do have strong influence on the dynamic behavior of fracture caused by EDI-induced waves. As a consequence, the fracture inside the RC slab can be limited only to the specific small region between an outer free surface and a virtual interface formed by the nearest rebars, and that small region may be cut off as if the slab were “sliced”. Thus, the corresponding rebars can be exposed very efficiently, and in practice, once exposed, they can be cut mechanically by an electric sander, etc., for later renovation work. In Uenishi et al. (2024), we have investigated mainly the horizontal cutoff along a horizontal virtual interface formed by the topmost rebars through experimental and numerical observations. There will be a brief summary in the next chapter. Then, we show in some detail that, even without the existence of empty dummy holes, if the relative positions of the blast holes, free surfaces and rebars are rightly considered, the foremost rebars can function as a vertical virtual interface in a RC block, and the block can be vertically cut off along this vertical virtual interface and only the foremost rebars can be exposed efficiently. 2. Horizontal cutoff Now let us review the horizontal “virtual interface” effect induced by the topmost rebars in a RC slab that is subjected to EDI (Fig. 1). Figure 1(a) shows a preliminary experimental example where in addition to the screw hole prepared for the carriage of the slab, three blast holes and four empty dummy holes are drilled. Inside each blast hole, an EDI source (cartridge) containing the self-reactive liquid and connected to the control unit of EDICS is placed and covered by a stemming material. The extra empty dummy holes are set to guide the dynamic interaction of EDI-induced waves and ensuing fracture development precisely as desired in the three-dimensional framework. The photograph taken after the simultaneous application of EDI to each blast hole, Fig. 1(b), indicates that, in comparison to the depth where the EDI sources are placed (some 150 mm), the depth of fracture is very shallow (about 30 mm) and it is equal to the depth of the horizontal virtual interface corresponding to the topmost rebars around the blast and empty dummy holes. After the planned application of EDI from 9 blast holes with 24 empty holes in total (Fig. 1(c, d)), the rebars are clearly exposed on top and bottom horizontal surfaces of the RC slab. In addition, the empty dummy holes induce the “sandwich effect”, which concentrates the EDI-induced wave energy inside the rows of dummy holes and keeps the regions well outside the rows undamaged. That is, when the exposed rebars are mechanically cut, the RC slab is quickly disintegrated into two smaller and almost identical blocks. Finite difference calculations show that the above experimental observations can be well explained by considering the dynamic interaction between blast holes, empty dummy holes, rebars and EDI-induced waves (Uenishi et al., 2024).