Concrete Structures Subjected to Blast Loading

Licentiatavhandling, 2015

The interest for building safety with regards to highly dynamic events such as close by explosion has accelerated after the recent decades of terrorist attacks. The interest has expanded from civil defence shelters and pure military targets to also include civil buildings used for different civil functions. Due to the rather rapid shift of area of interest, the general engineering community lack much of the knowledge and tools with which to design and evaluate structures with respect to dynamic events with very fast transient and high magnitude of peak loads. In this thesis, two studies of concrete structures are presented. Both studies focused on describing concrete with a combined damage and plasticity model. The aim was to study how different numerical models perform during highly dynamic events. Two main studies are presented. In the first study the response in a concrete wall subjected to shock wave blast, leading to spalling failure was investigated. This situation is important since spalled-off fragments in protective structures may cause severe injury to the persons or equipment it is supposed to protect. Previous research indicates that spalling occurs when and where the tensile strength of a strain-softening material like concrete is reached. By using a simple uni-axial numerical model, this study shows that spalling instead occurs when the cyclic response from a blast wave gradually increase the inelastic strains in the concrete. This means that spalling takes place after several loading cycles and not necessarily at the depth where tensile strength is firstly reached. Furthermore, the study shows that the cyclic response in the material model used for numerical simulation has a decisive influence on the position and extent of the resulting spalling crack. In the second study the response of reinforced concrete structures subjected to blast loads was investigated. Numerical models are used to evaluate the numerical response of a simply supported reinforced concrete beam and a one-way supported slab with a combined damage and plasticity constitutive model for concrete, CDPM2. Previous research has shown that strain-rate de- pendent material parameters might be overestimated for higher strain rate. In this study, these features are evaluated for reinforced concrete structures where bending is the dominating response. The numerical analyses indicate that fracture energy during tensile fracture and how this value is chosen have larger effects on the deformations of the structure than whether or not the strain-rate dependency of the material properties are taken into account. It is also concluded that mesh size and modelling techniques may have a large impact on the resulting response of the structure in the numerical analysis.