Concrete Structures Subjected to Blast and Fragment Impacts - Numerical Simulations of Reinforced and Fibre-reinforced Concrete

Licentiate thesis, 2008

Concrete is widely used in design of protective structures due to its good energy-absorbing characteristics under high pressures and, when properly reinforced, ductile behaviour. Nevertheless, the response of concrete structures subjected to severe dynamic loading differs from their static behaviour, on a structural level but also on a material level. The addition of steel fibres in the concrete may improve the energy-absorbing characteristics of plain concrete, which is especially true for the tensile behaviour. The fracture energy for steel-fibre reinforced concrete may be many times higher already for low dosages of fibres compared to plain concrete. In design of protective structures it is important to identify the possible threats and their risk of occurrence to be able to characterise the design loads. Often this involves the effects of cased charges, i.e. combined blast and fragment loading. While the structural behaviour for blast load and single fragment impacts is relatively well understood, the response under combined loading, including the blast and multiple impacts of fragments, is not yet clear. The theoretical bases for concrete material behaviour, weapon load characteristics, and their effect on the structural response are treated in this licentiate thesis. In addition, three numerical studies are presented, whose aim is to increase the understanding of impact and impulsive loading and the subsequent response of a concrete element. The first numerical study was a comparative investigation of the relative effect on the impact resistance when adding steel fibres to concrete. It was concluded that the depth of penetration of the striking projectile was only slightly influenced by the addition of fibres, while the sizes of the front- and rear-face craters were decreased. The second numerical study involved combined blast and fragment loading of a reinforced concrete wall strip, and it was seen that the total damage of the wall strip subjected to the combined loads was highly related to the damage caused by the fragment impact alone. Furthermore, the mid-point deflection in combined loading was larger than the sum of mid-point deflections in blast and fragment loading, indicating synergetic effects of the two loads. In the third numerical study the effect of reinforcement on the projectile impact resistance was studied. It was concluded that the presence of reinforcement may improve the impact resistance of the concrete if a suitable reinforcement detailing is used.