Mechanical Characterization of Tensile Behaviour of Concrete Under High Strain Rates

The use and development of concrete has seen a continuous evolution since its first use 2000 years ago. Manufacturing processes, new mix designs, innovative ingredients and combinations with other materials have make concrete a key building material for all type of modern structures. Even though the knowledge around concrete properties has reached the maximum standards, the use of this material for the most challenging infrastructures has introduced new loads and variables in the projection of structures. Bridges, military structures, dams, off-shore platforms, train structures, all of them subjected not only to the classical quasi-static loads but also to different dynamic actions. As opposite to quasi-static loads, the characterization of concrete under dynamic actions is still a field of scarce development, with few experimental data and without a clear experimental proposed methodology. The importance of having a deeper knowledge in the dynamic behavior of concrete is justified over the well known fact that, several mechanical properties of concrete show a notorious increase under dynamic actions. In the recent years some authors, noting the relevance of this change in the properties, have proposed different experimental techniques to estimate mechanical parameters defining the behavior of concrete. Thanks to the development of the experimental device designed by Hopkinson, a significant increase in the study of concrete subjected to impact loading has taken place during the last decades under compressive and tensile configurations. However, the nature of the dynamic actions in concrete, has focused the problem and interest on the tensile properties, due to the responsibility of these actions in the final collapse of structures subjected to spalling failure. In an effort to increase the existing data, experimental methodologies using the Hopkinson Bar and based on the spalling phenomenon have tried to recreate the transmission of pulses inside concrete material, in an effort to estimate dynamic mechanical properties governing the fracture of concrete such as the tensile strength and the fracture energy. Despite the work carried during the last decades, there is still some dispersion among published results with not a clear or reliable proposed methodology to predict the dynamic behavior of concrete. In the present work an important effort has been carried to design an easy experimental methodology to estimate dynamic fracture properties of concrete. Experimental configurations based on the modified Hopkinson Bar are proposed both for estimating the tensile strength and fracture energy. Moreover, a validation experimental campaign has been carried out, comparing results with an alternative approach. Finally, the developed methodology has been used to characterize and study two different concrete mixes under tensile spalling tests