PSI - Issue 42

Koji Uenishi et al. / Procedia Structural Integrity 42 (2022) 755–761 Uenishi et al./ Structural Integrity Procedia 00 (2022) 000–000

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1. Introduction In order to effectively and more accurately disintegrate old structures for partial renovation or total reconstruction in populated urban areas, instead of using chemical energy by detonating explosives, we have been trying to use more safely handleable and more controllable electric energy. Although different technical ways to use electrical energy have been proposed (see e.g. Andres (1989), Weise and Loeffler (1993), Hofmann and Weiss (1997), Rim et al. (1999), Lisitsyn et al. (1999), Bluhm et al. (2000), Narahara et al. (2007) and Kencanawati and Shigeishi (2011)), we have employed the electric discharge impulse crushing system (EDICS) developed by Nichizo Tech, Inc. In EDICS, the electric energy stored in a capacitor is released in several hundreds of microseconds in a cartridge containing a self reactive liquid through an electronic switch and high pressure or an electric discharge impulse (EDI) is generated by the rapid evaporation of the liquid (Uenishi et al., 2022). So far, by considering geometrical settings of blast holes that emit EDI and waves, empty dummy holes, free surfaces and interfaces and predicting three-dimensional wave motion with the aid of the dynamic theory of waves and fracture, we have developed several techniques to efficiently dismantle (parts of) concrete structures. This contribution summarizes some of these techniques by briefly introducing the geometrical settings, numerical speculations and real field observations. 2. Controlled dynamic disintegration of concrete structures 2.1. Basic observation: pree surpaces Firstly, let us mention the very basic effect of three-dimensional wave motion and free surfaces on the dynamic fracture of a cylindrical concrete specimen without reinforcing steel bars. The specimen has a blast hole where a cartridge containing self-reactive liquid and connected to the control unit of EDICS is placed and covered by a stemming material (Fig. 1(a)). The model is symmetric with respect to the cartridge except for the stemming section. Since the cracks due to the application of EDI have been found to be initiated and guided directly by EDI-induced waves and not by gas pressurization, in Fig. 1(b), wave motion in a homogeneous, isotopic, linear elastic cylindrical specimen is numerically traced by our three-dimensional finite difference code with the second-order spatiotemporal accuracy and the material properties, density 2,320 kg/m 3 , Young’s modulus 34.2 GPa and Poisson’s ratio 0.25 (longitudinal and shear wave speeds c P  4,200 m/s and c S  2,400 m/s), constant grid spacing  x = 10 mm and time step  x /(2 c P ). For simplicity, it is assumed that the stemming material has the same mechanical properties as concrete and the temporal variation of the pressure due to the action of EDI, P ( t ), has a very simplified form P ( t ) = A sin 2 (  t / T ) (for 0  t  T ) and 0 (otherwise) with A = 1 GPa and T = 260  s. Figure 1(b) left shows the contours of volumetric strain (a strain invariant) at 80  s after initiation of pressurization by EDI where due to the effect of the free surfaces, tensile parts develop in an initially compressive outbound wave, and some concentrically-shaped tensile region on the top surface as well as in the middle part of the edge is recognizable. That is, outbound tensile cracks may propagate from the center on the top surface and the specimen may be cut along the horizontal plane located at a height of some 250 mm from the bottom. Later at time 100  s (Fig. 1(b) right), a three-dimensionally reflected inbound tensile wave can be observed on the top surface that may guide inbound cracks propagating toward the center. Indeed, although in the snapshot taken by a normal home video camera (Fig. 1(c)) the cracks on the top surface seem to have run unidirectionally from the central blast hole to the edge, the photograph recorded by a high-speed camera (Photron FASTCAM SA5) (Fig. 1(d)) clearly shows that the final fracture is formed by the combination of outbound and inbound tensile cracks guided by three-dimensional direct and reflected waves as speculated. Also, a horizontal tensile crack plane is visible in the very middle section of the specimen at a height of about 250 mm in Fig. 1(c), as numerically predicted (Uenishi et al., 2014). Thus seemingly simple three-dimensional cracks do have rather complex history of dynamic development. 2.2. Empty dummy holes Secondly, empty dummy holes are introduced in a more realistic, larger-scale rectangular concrete specimen without reinforcing steel bars to check the controllability of the crack propagation by waves due to EDI (Fig. 2(a)). The specimen has three blast holes situated on a virtual central vertical plane. Each blast hole is surrounded by four