The fracture properties of concrete are rate dependent. In this thesis the results on tensile tests at static, moderate and high loading rate are presented. The results show the influence of the loading rate not only the tensile strength, but also on the fracture energy, stress-defromation relation and fracture parameters, like fracture lengths and width of the fracture zone. The failure mechanisms are reconstructed and the dominant mechanisms behind the rate dependency are identified. By using basic principles of fracture mechanics and a simple model based on the Stefan effect, the loading rates at which the mechanisms have significant effect have been determined. The dominant mechanisms found in the research can be implemented in dynamic models and the acquired data set can be used to validate numerical models. @en

Dynamic tests demonstrate an extensive rate effect on the tensile strength as well as the post-peak behaviour beyond loading rates of about 50 GPa/s. One of the possible explanations for the observed rate effects on the fracture behaviour is enhanced resistance by moisture in the pores. To study the influence of the moisture content and pore structure on the rate dependency, different moisture contents and concrete types are used and tested at three loading rates. From the test results it is concluded that the moisture volume, porosity and pore structure play an important role for tensile strength as well as the fracture process. The NMR tests showed that the water in the capillary pores causes the strength increase and not the water in the gel-pores. From the analysis of the experimental results it is concluded that for loading rates < 50 GPa/s, the main cause for the observed strength increase is the viscous behaviour of concrete. For loading rates beyond 50 GPa/s, also rate effects due to limitations on crack propagation contribute to the observed strength increase for all moisture contents and concrete types. Concerning the post peak response for rates > 50 GPa/s, the additional resistance is due to additional micro cracking, the moisture in the capillary pores and the limited crack propagation velocity.@en