Concrete Structures Subjected to Fragment Impacts

Doctoral thesis, 2004

As concrete is commonly used for protective structures, how a blast wave and fragment impacts from an explosion affect the concrete is an important issue. Concrete subjected to explosive loading responds very differently from statically loaded structures. The compressive and tensile strengths and the initial stiffness increase due to the strain rate effects. When fragments penetrate, spalling occur at the impact zone and scabbing may occur on the reverse side of a wall, or even perforation, with a risk of injury to people inside the structure. The principal aim of this thesis is to improve the current knowledge of the behaviour of concrete structures subjected to blast and fragment impacts. The main focus is on numerical modelling of fragment impacts on plain concrete members. In addition, experiments in combination with numerical analyses were conducted to deepen the understanding of concrete subjected to blast wave and fragment impacts. In the experiments, both multiple and single fragments were shot at thick concrete blocks. To capture the response of the concrete material behaviour, both the fragment impacts and the blast wave must be taken into account. The damage in the spalling zone is caused by the fragment impacts, whereas the major stress wave that propagates is caused mainly by the blast wave. To predict the penetration depth of the fragment impacts, spalling and scabbing in concrete with numerical methods, material models that take into account the strain rate effect, large deformations and triaxial stress states are required. The depth of penetration depends mainly on the compressive strength of the concrete. However, to model cracking, spalling and scabbing correctly in concrete, the tensile behaviour is very important. The RHT model in AUTODYN was used for the numerical analyses. The RHT model does not describe the concrete behaviour in tension accurately: the softening is linear and the strain rate dependency does not fit experimental results. Hence, a bi-linear softening law and a strain rate law were implemented in the model. By parametrical studies it was shown that the tensile strength, fracture energy and the strain rate law influenced the cracking and scabbing of concrete. By implementing the bi-linear softening law and a modified strain rate dependent law, the results of the numerical analyses were improved for projectile and fragment impacts on concrete.