Dynamic Behavior of Concrete: Experimental Aspects - Dynamic Behavior of Concrete and Seismic Engineering - page 28

Figure 1.14. Approximate average behavior
At last, the same type of trial can be carried out in confinement, using
confinement cells coupled to a compression Hopkinson bar system [GAR 98]. It is
important to use pressure confinement with gas and a large enough chamber inside.
Using an incompressible fluid actually leads to interference confinement, because it
acts as a strain limiter. Typical results [GAR 98] show that the confinement effect is
the same kind as the strain speed effect to which it is added. Therefore, we can infer
that the main effect of strain rate in a concrete dynamic compression test is a
structure effect linked to the inertial confinement. This result was confirmed by
digital simulations, which are developed in [BAI 99].
1.3.2.2. Direct impact tests (shock cannon)
The Hopkinson bar test gives information on high-speed concrete compression
behavior, however the practical limitation of transversal dimensions (less than 10 cm
in general) works against good representativity of the concrete material. That is why
“block bar” devices that carry out direct impacts on a concrete cylinder have been
developed ([BIS 95], [DAR 95]). The impactor is guided and propelled either by
direct falling or a compressed-air canon. The quality of the contact between the