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 contact detonation are characterized by spalling, which is caused by tensile stress waves reflected from the back side of the slab. 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 spall damage is an important problem faced by the designers of blast-resistant RC structures.
In a previous study, the authors verified the good blast-resistant performance of fiber reinforced cement composite (FRCC) slabs under contact detonation. However, a designing method for obtaining the required blast-resistant performance of the FRCC members has not been developed yet; one of the reasons for this is that it is difficult to obtain dynamic mechanical properties of FRCCs corresponding to this problem where the dominant strain rate is of the order of 103–104/s. Hence, it may be convenient to consider the spall-suppressing performance of FRCC member as a material property of the FRCC. It can be obtained directly based on material factors such as fiber shape, water-binder ratio (W/B) of the matrix, and fiber volume fraction (Vf).
This study was conducted to evaluate the influence of various material factors on the local failure of FRCC slabs under contact detonation; therefore, contact detonation tests were carried out on polyvinyl alcohol fiber reinforced mortar (PVAFRM) slabs with four different shapes of fibers (Type I: φ0.1 × 12 mm, Type II: φ0.2 × 12 mm, Type III: φ0.2 × 18 mm, and Type IV: φ0.2 × 24 mm), four different values of W/B of the mortar matrix (50, 40, 33, and 25%), and three different values of Vf (2.0, 3.0, and 4.0%). After the tests, the fracture appearances of each specimen were observed in detail, and then the sizes of the local failure created in each specimen were measured and compared each other. The main results obtained are as follows:
(1) It is more effective to adapt longer fibers to suppress spall if the fiber diameter is constant.
(2) Regardless of whether the length or the aspect ratio of the fibers is constant, it is more effective to adapt finer fibers to suppress spall.
(3) The spall-suppressing performance is reduced when the W/B value is too high or too low, and there is an appropriate value of W/B to suppress spall.
(4) It is more effective to increase Vf value under the appropriate combination of fiber shape and W/B value to improve the spall-suppressing performance of the PVAFRM slabs.
(5) When combining the fibers of type I and the matrix of W/B = 33%, the scaled concrete thickness of the spall limit in a normal RC slab can be reduced by almost 49% by setting the Vf value to 3.0% or more. The same effect can be obtained by setting Vf value to 4.0% under a combination of the type IV fibers and the matrix of 33%.
(6) When finer fibers are used in combination with the appropriate W/B value, mentioned in (3) above, the spall depth tends to be reduced even if the spall occurs.
