Experiments and numerical analyses of blast and fragment impacts on concrete

Concrete structures are commonly used as protective structures. An important issue is how the blast wave and fragment impacts from an explosion affect the concrete. It is well known that the fragments penetrate or even perforate the structure. Moreover, spalling occurs in the impact zone and scabbing may occur on the reverse side of a wall that receives an impact. However, knowledge of how the blast wave and fragment impacts influence the material properties of concrete is quite limited. Experiments and numerical analyses were carried out to examine the extent to which the concrete, at various distances, is affected by the blast wave and fragment impacts. The fragments, which were spherical, were shot against thick concrete blocks by using the explosives octol or hexotol; the fragment velocity was approximately 1650 m/s. After the concrete blocks were shot, the depths of penetration and spalling were measured. Next, the concrete blocks were cut into halves, and the global macro-cracking could be observed. To study how the material properties of concrete were influenced, uniaxial compressive and splitting tensile tests were carried out on cylinders drilled from selected positions in the block. Furthermore, specimens from the blocks were thin-ground to facilitate analysing the micro-cracking with a microscope. The experiments and numerical analyses presented here showed that the damage in the concrete, from the blast wave and fragment impacts, is localized in the impact zone. The concrete below this zone, at a depth of approximately twice the depth of the maximum penetration, was hardly affected at all by the blast wave and fragment impacts. This indicates that it is possible to distinguish between the global load effects and the local damage effects that are caused by the fragment impacts. Consequently, it may be possible to separate the loads, at the design stage, from a blast wave and fragment impacts.