PSI - Issue 42

5

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

759

a

c

b

Blast holes

1,050 mm

Larger tension

150 mm

Slits

5.0  10 -4

Cartridges

300 mm

Removed main fragment

5.0  10 -5

Fig. 3. (a) The concrete slab of thickness 150 mm without reinforcing steel bars and a typical rectangular section (1,050 mm  300 mm) bounded by pre-set slits of depth 70 mm. The rectangular section has six blast holes along the center line that are simultaneously pressurized with EDI at central depth of 70 mm (modified after Sakaguchi et al. (2018)). (b) Contours of numerically obtained volumetric strain at time 140  s after the start of simultaneous application of EDI, illustrating the regions of larger tension in the right half part of the rectangular section. (c) Typical final fracture network (top) generated by the simultaneous application of EDI. Surprisingly, a cuboidal space (1,050 mm  300 mm  some 70 mm) has appeared after the removal of the main fragment (bottom) (modified after Sakaguchi et al. (2018)). 2.4. Steel-concrete interpaces Lastly, use of interfaces between concrete and steel for efficient disintegration of reinforced structures is considered. Figure 4(a) shows a steel-concrete composite structure consisting of a reinforced concrete slab on top of a steel girder and stud dowels (diameter 22 mm, height 130 mm). In the slab, four blast holes that can hold the cartridges are drilled and covered by a stemming material. The horizontal spatial distance between each stud dowel in the direction of the axis of the specimen  L is 200 mm. As above, the development of waves and its relation with dynamic fracture can be traced numerically in light of the volumetric strain developing in the specimen (Fig. 4(b)). In the finite difference calculation, for simplicity, the steel girder is not included except for the upper plate (flange) with a thickness of 20 mm. This steel flange is considered to be perfectly bonded to the concrete slab above (welded interface). The concrete material as well as the steel is presumed to be homogeneous, isotropic and linear elastic, and the density, Young’s modulus and Poisson’s ratio are 2,320 kg/m 3 , 34.2 GPa and 0.25 for the concrete, and 7,800 kg/m 3 , 200 GPa and 0.3 for the steel, which gives the longitudinal and shear wave speeds as c P 1  4,200 m/s and c S 1  2,400 m/s for the concrete, and c P 2  5,900 m/s and c S 2  3,100 m/s for the steel, and the constant time step is now  x /(2 c P 2 ). Although it can be incorporated technically quite easily, for visual clarity, the reinforcing steel bars and fracture criteria are not incorporated in the calculations (Uenishi et al., 2022). The simulation indicates that because of the reflection of the initially compressive waves at the free surfaces of the specimen, dynamic tensile regions can develop rather independently above the cartridges but the regions below the cartridges remain in compression due to the existence of welded interface between concrete and the flange. However, since waves in the stud dowels can travel at a supersonic speed with respect to concrete, shock waves or Mach waves, carrying concentrated kinetic energy and propagating from the flange upwards along the stud dowels, can be formed. In fact, at 140  s after the start of action of EDI in Fig. 4(b), such tensile Mach waves can be seen and the Mach fronts (blue lines) form parts of the boundaries between the cup-shaped compressive regions around the cartridges and the regions of larger tension (indicated by the black broken lines). Fracture may develop along these boundaries. In the field observation (Fig. 4(c)), the specimen has indeed cup-shaped fractures connecting the cartridges to the heads of the stud dowels and indicated by black lines, and the numerical speculation seems to well explain the experimental finding. Moreover, the concrete material above the heads of the stud dowels with the four “cups” can be lifted and removed as one block(see the block lifted in Fig.