PSI - Issue 42

758 4

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

a

c

Blast holes

Desired rather straight crack

Blast holes

100 mm

b

Dummy holes

Dummy holes

Screw holes for crane carriage

Cartridges

8.0  10 -5

Tension

300 mm

 8.0  10 -5

Fig. 2. (a) Top and side views of the rectangular concrete specimen with empty dummy holes to control crack propagation [unit: mm]. (b) Contours of numerically generated volumetric strain in the lower quarter part of the specimen at time some 100  s after the start of simultaneous application of EDI. (c) Top view of the specimen after the dynamic experiment, showing a desired, rather straight crack that directly connects the three blast holes (modified after Uenishi et al. (2018)).

2.3. Slits Next, vertical slits are pre-set inside large, unreinforced concrete slabs to control wave motion and dynamic structural disintegration. Figure 3(a) shows a slab and a typical rectangular section to be fractured by EDI-induced waves. The section has six vertical blast holes (depth 110 mm) along its center line at intervals of 150 mm, and in each blast hole a cartridge (depth at its center 70 mm) is placed and covered by a stemming material. The above observation (Fig. 2) suggests that inside this section, a main crack connecting the blast holes as well as bifurcated cracks running from the blast holes at both ends to the slits appear due to the tension that is dynamically induced by waves reflected at the top horizontal free surfaces and the vertical slits. Concurrently, it is expected that a horizontal fracture plane (approximately) connecting the bottoms of the vertical slits (and centers of cartridges) may be developed due to wave diffraction around them. As a result, only the rectangular zone surrounded by the slits, with a depth comparable to that of the slits (70 mm), is fractured. Again, a three-dimensional wave simulation can reinforce this point of view. This time, a concrete slab (2,100 mm  2,100 mm  800 mm) made of linear elastic concrete with mass density 2,270 kg/m 3 , Young’s modulus 33.3 GPa and Poisson’s ratio 0.28 is studied, which gives c P  4,300 m/s and c S  2,400 m/s. Figure 3(b) shows the contours of volumetric strain with larger tension at time 140  s after the start of action of EDI. The regions of larger tension (and more prone to tensile fracture) appear only inside the cuboidal section bordered by the top horizontal free surface, pre-set slits (vertical free surfaces) and the rather flat horizontal plane near the bottoms of the slits. Actually, in the photograph showing the typical final fracture network generated by the simultaneous application of EDI to the slabs, in Fig. 3(c) top, a main crack with bifurcated ends is visible. In Fig. 3(c) bottom, the unique, surprisingly flat horizontal fracture plane near the bottoms of the slits is recognized, and it is located at a depth slightly larger than the slit depth (see also the immediately removed main fragment of concrete in the photograph). Thus, only a cuboidal part of concrete slabs can be efficiently and quickly removed (Sakaguchi et al., 2018).