PSI - Issue 46

Koji Uenishi et al. / Procedia Structural Integrity 46 (2023) 136–142

137

2

Koji Uenishi et al. / Structural Integrity Procedia 00 (2019) 000–000

1. Introduction The number of surface and underground structures that are reaching their normal duration of life has been remarkably increasing, and in order to efficiently dismantle such aged structures, either partially for renovation or totally for reconstruction, dynamic technique of blasting by detonating explosives may be employed. Blasting demolition may be accomplished in a very short time span, but it is still planned by experience and may become extremely complex and difficult for reinforced structures that have been constructed very tightly. Hence, blasting tends to be carried out at an overcharge level of explosives that will cause complete destruction but at the same time may produce excessive energy for the operation. This may result in unwanted and/or dangerous failures, which is one of the main reasons blasting demolition is usually avoided in densely populated urban areas. Therefore, in our earlier study, instead of utilizing the energy produced by detonation of explosives, using more safely controllable electric energy and the dynamic theory of waves and fracture, we have studied more reliable methodologies for accurately controlled partial or total destruction of reinforced/unreinforced structures (Uenishi et al., 2014, 2016, 2018; Sakaguchi et al., 2018). Several different techniques for using electrical energy have been proposed (see, for example, 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)), but we have employed the electric discharge impulse crushing system (EDICS) developed by Nichizo Tech, Inc., where the electric energy stored in a capacitor (typically 3000 volts) 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. Hitherto, by applying these EDIs, we have effectively, in other words, as designed, dismantled brittle concrete specimens with/without reinforcing steel bars, ranging from relatively small rectangular and cylindrical ones to more realistically large concrete slabs. With a high-speed video camera, for instance, we have observed the development of dynamic fracture generated by EDIs from the cartridges placed in a specimen, and tried to find optimal geometrical and loading settings that can govern the propagation and interaction of waves and cracks and thus the final pattern of structural disintegration. Indeed, specific positioning of blast holes having cartridges, empty dummy holes and free surfaces as well as pre-existing planes of weakness can precisely control the dynamic wave-crack behavior and the disintegration pattern in a structure considered. Also, the field observations can be well reproduced by our three dimensional finite difference numerical code for a PC (Windows) that simulates wave interaction at/around the blast and dummy holes, surfaces, and planes of weakness. Here, by continuing the dynamic fracture experiments in the field and the numerical simulations in the laboratory, we observe the development of waves and fracture induced by EDIs in a steel-concrete composite structure (briefly mentioned in Wada et al. (2021)). The composite structure is a reinforced concrete slab placed on top of a steel girder and headed stud shear connectors (stud dowels) that literally reinforce the connection between the slab and the girder. By dynamically controlling wave-crack behavior, we try to crush only concrete material but keep the stud dowels and the steel girder undeformed for later renovation. 2. Controlled disintegration of a composite structure Figure 1 illustrates the plan, side and front views of the two steel-concrete composite structures that have been prepared for the field experiments of dynamic disintegration (and later partially modelled in the numerical simulations). As stated above, the structure is composed of a reinforced concrete slab on top of a steel girder and stud dowels having a diameter of 22 mm and a height of 130 mm. In the slab of dimensions 900 mm  300 mm  200 mm, blast holes that can hold the cartridges (energy sources) containing the self-reactive liquid are drilled and covered by stemming material. The distinct dissimilarity between the two types of specimens, A (Fig. 1 (a)) and B (Fig. 1(b)), is the horizontal spatial distance (  L ) between each stud dowel in the direction of the axis of the specimen. The distance  L is 200 mm for the specimen A and 100 mm for the specimen B. Accordingly, the cartridges set in the specimen B are double in number. In Fig. 2, the photographs of the specimens A and B before (top) and immediately after (bottom) the application of EDIs are shown. As seen in the top photographs, electricity, AC 100 volts, is supplied to the four (Fig. 2(a) top) and eight (Fig. 2(b) top) cartridges in the blast holes. Due to the action of EDIs, in the case when the stud dowels are less in number, in the specimen A (  L = 200 mm), cup-shaped fractures linking the cartridges to the heads of the stud