Dynamic tensile resistance of concrete - Split hopkinson bar test

The behavior of concrete structures is strongly influenced by the loading rate. Compared to quasi-static loading, on meso and macro-scale concrete loaded by impact loading acts in a different way. First, there is a strain-rate influence on strength, stiffness, ductility, and, second, there are inertia forces activated which influence the resistance and failure mode of concrete structure. The experimental and theoretical studies show that the influence of loading rate on tensile behavior of concrete is relatively strong. In dynamic testing the split Hopkinson bar (SHB) is used to measure concrete tensile resistance. The results of the experimental measurements show that after reaching some critical strain rate tensile resistance progressively increases with increase of strain rate. The questions discussed in the paper are: (i) what is the reason for progressive increase of tensile resistance? and (ii) can the resistance be attributed only to material strength or are there some other effects? To answer these questions the numerical analysis on a simple elastic-cohesive FE model is carried out. Moreover, simulation of the compressive pulse in a concrete bar, which is reflected from the free end-surface of the bar and causes tensile fracture, is carried out for different loading rates. The evaluation of the results clearly shows that the progressive increase of tensile resistance (apparent strength) can be attributed to structural inertia of the fracture zone, which is invoked by cracking of concrete and is not to the true material strength. It is shown that the size of the fracture process zone significantly influence apparent strength. Similar as the true strength it is also discussed that with the increase of strain rate concrete fracture energy does not increase progressively.