Blast and Impact Loaded Concrete Structures - Numerical and Experimental Methodologies for Reinforced Plain and Fibre Concrete Structures

Doctoral thesis, 2017

For decades the focus of civil protection has been on civil defence shelters to withstand the threat of warfare and protecting civilians. In latter years, the general focus has often shifted towards the protection of specific targets that host functions important to society. Direct attacks on the civil population also post a more common threat today than a few years ago. Numerical modelling of dynamic events has been more common in recent years and is to a large extent treated as an available complement to physical testing. Even so, the tools and techniques to evaluate structures with the help of numerical models are in many respects unexplored. Since some phenomena are not fully understood, it is difficult to determine the needed properties of these numerical models. The overall aim of the research presented in this thesis is to contribute to the knowledge of concrete structures subjected to extreme dynamic impulse actions. The scientific approach consisted of a combination of literature reviews, theoretical modelling, numerical analyses, and experimental investigations, including experimental design. Spalling in concrete structures was studied with a numerical model to show how spalling damage can develop gradually during non-monotonic loading and un-loading of the material and not by instantaneous fracture when the tensile strength is reached as described in the literature. The study also shows that the constitutive model for tensile fracture of concrete has a decisive influence on the results and position of spalling cracks. The response of reinforced concrete structures subjected to blast loads was investigated. Numerical models were used to evaluate the numerical response of a simply supported reinforced concrete beam and a one-way supported slab with combined damage and plasticity constitutive model for concrete, CDPM2. The numerical analyses indicate that fracture energy during tensile fracture and how this value is chosen have a larger effect on the deformations of the structures than whether or not the strain-rate dependency of the material properties are taken into account. Presented in this thesis is an experimental methodology to evaluate the bending response of reinforced plain and fibre concrete beams due to a drop weight impact. The minimum requirements for documented material parameters and the structural response for reinforced concrete structures have been defined when an experimental study aims to support calibration and validation of numerical models. This methodology was used to study the influence of adding steel fibres to the concrete mix in reinforced concrete beams. It was shown that the initial crack patterns, at impact, influenced the final crack pattern at the maximum deformation of the beams. These crack patterns were shown to be similar between the different concrete mixes with or without fibres.