Dynamic Behavior of Concrete: Experimental Aspects - Dynamic Behavior of Concrete and Seismic Engineering - page 10

It is important to note that because of the weak growths withstood by the
material at high velocities , experimental precautions have to be taken - especially
because of transient effects, in dynamic experiments where limit conditions are
difficult to control. In experiments where an “energetic” approach is privileged, this
aspect is also important: the inertia of the test sample cannot always be neglected
with regard to that of the test machine, and the energy dissipation through damage
on the support, or through contact with the impact separator can be important
compared with the energy supposedly dissipated by the “normal” cracking expected
in bending.
Finally, a delicate feature of concrete is its porosity: it has such a tortuous
network that water exchange times with the environment are quite long (about 10
years for the representative volumes considered above). We can consider the
hydration state of the sample as constant during dynamic tests, which is not the case
for shrinkage or plastic flow tests. However, important relative pore moisture and
mechanical state coupling, together with frequent cracking due to the stress levels
reached when desiccated, begins at the sample's surface and/or their environment as
soon as they are fabricated. In at least one stress and velocity field ([DAR 95] [TOU
95a]), researchers have shown that the partly water-saturated feature of the porous
network explains the modification of apparent mechanical properties: these are
generally called “velocity effects” in the literature.
In following sections (1.2 to 1.4), we will detail the arrangements, test type by
test type, used to analyze the results and infer the indications and modifications
required to calculate and understand the behavior of fast dynamic loading concrete
structures. The actual and measured behaviors are summarized in a rational way in
section 1.5.
1.2. Tests in which the transient rate has little influence
In this chapter, we will deal with behavior identification tests that, for reasons
developed in section 1.1 can have a “quasi-static” interpretation.
Two test families can be distinguished. The first is derived from typical concrete
characterization tests and emphasizes growth or cracking failures. This is called
deviatoric behavior, and is the failure kind that is also, indirectly, the cause of
collapse observed in compression and even in biaxial compression. The second test
type corresponds to “volumic” behavior, which can seldom be observed in ordinary
structures, except in relatively confined areas where specific reinforcement by the
surrounding material ensures tri-axial confinement at high velocity: concrete areas
directly submitted to impact and those close to an explosive charge or perforating
projectile are examples.