Fragment impacts into concrete

Övrigt konferensbidrag, 2004

Massive concrete withstand blast wave and fragment impacts effectively, and consequently, reinforced concrete is commonly used for protective structures. An important issue is how the fragment impacts influences the concrete. It is well known that the behaviour of concrete exposed to fragment impacts leads to damage in the form of severe cracking as well as spalling. When fragment penetrates concrete deeply, scabbing may occur at the reverse side of the wall, or even perforation. However, the knowledge of how the fragment impacts influences the material properties of concrete are not well known. To study this, experiments have been carried out, in order to examine to what extent the concrete, at various distances, is affected by the fragments. The fragments, which were spherical and 4 mm in radius, were shot against concrete blocks with dimensions of 750 x 750 x 500 mm3 at a speed of approximately 1 650 m/s. The depth of penetration and crater size was measured. Thereafter, the concrete blocks were cut in two halves and the global macro cracking observed. Uniaxial compressive and splitting tests were made from cylinders, which were drilled out from various positions of the block. Furthermore, specimen from the blocks were thin grinded and micro cracking was analyzed with microscope. Experiments presented here shows that the damage from fragment impacts is localized in the very front of the impact zone. The concrete strength below this zone, though, was hardly affected at all. This indicates that is possible to distinguish between the global load effects and the local damage effects, which is caused by the fragment impacts. Consequently, it might be possible to separate the loads from blast wave and fragment impacts, in design; and hence, the structure could be analyzed as a pre-damaged structure with decreased effective depth or width.