Influence of end friction confinement on impact tests of concrete material at high strain rate

The compressive strength of concrete material increases with the strain rate. The dynamic compressive strength of concrete material is usually obtained by conducting laboratory tests such as split Hopkinson pressure bar (SHPB) test or drop-weight test. It is commonly agreed now that the dynamic increase factor (DIF) obtained from impact test is affected by lateral inertia confinement. In addition, friction at specimen-bar interfaces also constrains the lateral deformation of the specimen under high-speed impact and thus might influence the testing results as well. However, no systematic study that devotes to investigating the influence of end friction on the dynamic compression test of concrete specimen can be found in the literature. Moreover, owing to the complication of including aggregates in concrete specimens in high-speed impact tests, and complexity of including aggregates in numerical simulation of high-speed impact tests of concrete materials, coarse aggregates are usually neglected in both the laboratory tests and numerical simulations, which may not give accurate concrete material dynamic properties. In the present study, a mesoscale concrete model with distinctive consideration of different components in a concrete specimen is developed to simulate SHPB tests and to study the influence of the confinement due to end friction between specimen and pressure bars on impact tests of dynamic concrete material properties. The commercial software AUTODYN is used to carry out the numerical simulations of SHPB tests. The friction coefficient between steel bar and concrete specimen is varied from 0.0 to 0.5 in the simulation. The results confirm that the end friction confinement does affect the testing results, and its influence depends on the L/D ratio (specimen length to diameter). This observation is also verified by experimental tests. The influences with L/D ratio and friction coefficient on stress and strain distributions and failure process of mesoscale concrete specimen under different strain rates are discussed. Based on the results from numerical simulations, an empirical formula is proposed to remove the influence of end friction confinement on dynamic strength increment of concrete material obtained in SHPB tests.