Dynamic Behavior of Concrete: Experimental Aspects - Dynamic Behavior of Concrete and Seismic Engineering - page 13

The incidence of superfluous interference moments is generally less important
than it is in traction tests; however, the precautions to be taken to avoid restricting
transversal strains are as important as in statics, especially for specimens with
slenderness ratios below 2. To this end, we can mention lubricating the faces or
using aluminum. The quality of the stress transmission surfaces is essential to avoid
premature concentration of stresses.
We note that as in statics, and even for specimens that are simply laid, the
relative displacement (including interface crush and deformations at the ends of the
specimens) cannot result in a reliable indication of the strains of concrete in its
standard part, the error typically ranging from 30 to 100% [BOU 99]. As in statics,
failure obtained in compression tests begins with transversal extensions . The traction
rate effect results in an “inertial confinement”. However, the maximum stress is only
reached when the cracks parallel to loading direction meet, allowing either buckling
in the “small columns” formed inside the test body, or shear localization. As a
consequence, interpreting the strength evolution, where load build-up speed is the
only parameter, becomes complex.
We could not find any references to tests deriving from standard quasi-static
identification of multi-axial behavior with prevailing deviatoric behavior (bi-
traction, pure shear, bi-compression), at least not in areas where transient test
characteristic can be neglected. As a matter of fact, the most frequent cases of
dynamic multi-axial behavior identification use unidirectional loading with a
Hopkinson bar [GAR 98], [LOU 94] and [WEE 92], while confinement or loading
in the other direction is often “static”. These tests will be described in section 1.3.
Such a situation can indeed be explained by the difficulty in controlling and
synchronizing dynamic loadings, even at the “low” speeds reached by conventional
presses or jacks. Furthermore, taking the properties of concrete into consideration,
the regulations rarely take multi-axial behavior into account. As a result this lowers
the validation of high velocity dynamic models adapted to concrete, in situations
other than simple traction, uniaxial compression or compaction.
1.2.1.3. Tests with small plates or beams submitted to pressure loading
Considering the difficulty in carrying out dynamic loading with mechanical
application of the loads, some authors perform controlled loadings on mini-
structures (small rectangular plates, beams or small plates), using a pressure loading
generated by an explosion. The purpose is then to identify the bending behavior, the
bend-moment law being material information directly transposable to the calculated
structure, taking into account the similar nature of the tested material and the
geometric and energy similarities - called Hopkinson's - on the load. Detailed
experimentation of this kind will be described in [BAI 87] and [BAI 88]. The
limitation on the energies that can be used in a laboratory forces the use of