Concrete under biaxial dynamic compressive loading

A concrete type C20/25 was examined under biaxial dynamic compressive loading with several stress ratios and two different loading rates (75 and 150 1/s). The question to be answered is, if the two strength enhancing effects of biaxial compressive loading and elevated strain rate overlay under this combined loading condition. For these experiments a unique biaxial Split-Hopkinson-Pressure-Bar, developed at TechnischeUniversität Dresden, was used to load the cubic specimen with two compression pulses from two perpendicular directions, simultaneously. That means the pulses have to reach the specimen within a time span of maximum 25 μs to achieve a rising stress level in both spatial directions in the specimen. For comparison, further static uni- and biaxial trials, static-dynamic biaxial trials and dynamic uniaxial trials were conducted. After the trials, the particles of the fragmented specimens were analysed in size and shape. The results show clearly that the two examined strength increasing effects superimpose in case of a combined biaxial dynamic compressive loading, but not completely. Therefore the influence factor of each single effect was analysed, depending on the stress ratio and the strain rate. For the considered range of high loading velocities, the strain rate effect has the main influence on the strength enhancement. This effect is decreasing with a growing stress ratio, here the biaxiality gains more influence but still remains behind the strain rate effect.