抄録
When designing important structures such as industrial plants and public facilities, it is necessary to ensure their safety against accidental or intentional explosions, which happen rarely but can cause severe damage. In particular, the fracture modes of reinforced concrete (RC) slabs subjected to blast loadings are characterized by spalling, which is caused by a combination of reflected tensile stress waves and diagonal cracks created along the maximum shear stress surface, as shown in Photo. 1. To protect the lives of humans inside a structure under such conditions, it is necessary to prevent the launch of concrete fragments that accompanies spalling. Therefore, reducing spalling damage is the most important problem faced by the designers of blast-resistant RC structures.
In a previous study, we clarified that slurry infiltrated fiber concrete (SIFCON) exhibits better spall-reducing performance than normal concrete and other fiber-reinforced cementitious composites (FRCCs). However, it was also confirmed that the SIFCON, as well as the other FRCCs, had no effect on reducing crater damage during a detonation. Therefore, for more efficient blast-resistant strengthening of RC slabs using SIFCON, it may be necessary to adapt the SIFCON to a region in which the occurrence of the spalling damage inside the RC slab is expected.
The objective of this study was to develop an efficient blast-resistant strengthening method of RC slabs using SIFCON; to this end, experimental investigations were conducted to evaluate the blast resistance of RC slabs in which the concrete near the rear was replaced with SIFCON. The total thickness of the RC slabs was fixed at 100 mm, and the behaviors at different thicknesses of 0 (un-reinforcement), 25, 50, 75, and 100 (overall-reinforcement) mm of the SIFCON layer near the rear side was studied, as shown in Fig. 1. Contact detonation tests were carried out using two different amounts of explosives (SEP) 100 and 200 g. After the tests, the fracture behaviors of each specimen were observed in detail, and then the sizes of the local failures in all specimens were measured and compared. Further, the spall-reducing mechanism of the RC slabs with SIFCON cladding was qualitatively discussed, based on the propagation behavior of the stress wave in a one-dimensional wave model and the punching-shear strength of the SIFCON layer. The main results obtained are as follows:
(1) The SIFCON cladding effectively reduced spall damage of the RC slabs under conditions in which the failure modes of un-reinforced RC slabs with the same thickness were “spalling” and “perforation”.
(2) Both, the damage inside the SIFCON layer and the spall damage created in the rear side, decreased upon reducing the thickness of the SIFCON layer. However, even if the SIFCON layer was not perforated, the scale of the spalling damage in the normal concrete layer became almost as large as that in un-reinforced RC slabs.
(3) When the thickness of the SIFCON layer was sufficiently smaller than the wavelength of the stress wave, spalling damage in the rear side of the RC slabs with SIFCON cladding was reduced by a combination of the following three mechanisms: (a) Since the tensile stress attributed to the reflected tensile stress wave became small inside the SIFCON layer, serious spalling damage did not occur in the rear side of the slab. (b) Diagonal cracks, which formed in the normal concrete layer, did not penetrate the SIFCON layer and propagate along the interface between the normal concrete layer and SIFCON layer because the punching-shear strength of the SIFCON layer was considerably higher than that of the normal concrete layer. (c) The launching of the spall fragments formed inside the normal concrete layer was prevented by the SIFCON layer of high punching-shear strength.
