Dynamic Behavior of Concrete: Experimental Aspects - Dynamic Behavior of Concrete and Seismic Engineering - page 3

implemented are around 10 -10 s -1 , as compared to “static” test standard velocities that
range from 10 -6 to 10 -5 s -1 , and the strain velocities reached during “hard” shocks on
civil engineering structures, which are usually in the range of 0.1 and 10 s -1 .
These elementary considerations understood, it appears that a critical factor in
the experimental characterization of concrete behavior is discarding the inertia
terms. The problem is more delicate with concrete (a brittle material) than it is with
metals. As a matter of fact, the first manifestation of inertial effects on a sample
submitted to dynamic loading is the transient response observed when the time taken
by elastic waves to pass through the sample (the transfer time) is significant relative
to the test's time duration. When studying this problem, the pertinent time-
dependent parameter is not the strain velocity (which, in any case, is not well-
defined in the transient phase), but the loading time relative to the transfer time. If
sufficient strain levels are reached in very short periods of time, the sample could
fail before a homogenous stress and strain state, measurable as an average, could be
reached. In fact, low amplitude traction strains (ranging from 100 to 150 x 10 -6 ) lead
to material failure. Test analysis is generally difficult. For common sized samples
(centimeter scale), we cannot go beyond 1/s average strain velocities when
conducting a quasi-static test analysis. This feature of brittle materials can be
exploited advantageously, and is used in scabbing tests (see section 1.3.1).
This limitation is far less a problem with metals, where important local strains
arise, but do not cause failure. Such a situation can only occur in concrete if
particular conditions that guarantee mechanical field homogenity exist to prevent
cracking. This is the case when tests are conducted in strong confinement (under
which circumstances, concrete behavior is described by plasticity-type models). As
far as metals and most polymers are concerned, it is also important to take thermo-
mechanical coupling into account, due to the adiabatic feature of dynamic tests. This
effect can only be neglected when failure occurs under low strain for which the
dissipated heat remains low: with concrete, it can also be neglected in confinement
tests, since we can presuppose a low thermo-mechanical coupling.
1.1.2. Reminders about dynamic experimentation
1.1.2.1. Specificity of dynamic tests
As far as statics and dynamics are concerned, it is reasonable to consider sample
analysis in a separate section, along with the overall measures it involves (generally
carried out on the peripheral part of the material). This is the second aspect
mentioned in the introduction.