Fracture energy of concrete at high loading rates in tension

The effect of high loading rates in tension on the failure energy and strength of concrete is reported in this paper. High loading rates exceeding 5000 GPa/s corresponding to strain rates higher than ∼120 s −1 can be applied by use of Hopkinson bar set-up designed to produce spall. Tension tests were performed on cylindrical specimens made of micro-concrete. At high loading rates, or strain rates, the failure energy of micro-concrete, as well as the strength, was found to substantially increase. It was also found that the material humidity has only a small effect on the failure energy of micro-concrete in the range of high loading rates. On the other hand, post-mortem optical analysis of the fractured surfaces of specimens indicated that almost all the population of aggregates, and not only the matrix, was fractured on the same surface perpendicular to the specimen axis. Such observations suggest that the substantial increase of the failure energy and strength in dynamic tension is caused by multiple micro-cracks, which propagate at the same time on the fracture plane. The mechanism of multiple micro-cracking triggered within the short loading time characteristic of high loading rates needs a high energy input in order to create the fracture surface, leading to the observed increase in strength.