Dynamic Behavior of Concrete: Experimental Aspects - Dynamic Behavior of Concrete and Seismic Engineering - page 8

concrete, for example), a metal cylinder can be used [GAR 99]. In this case,
confinement pressure is not studied, but can be measured during the test by
assuming the (most often elastic) response of the confinement ring is known (as in
an oedometric test). Another way to carry out high confinements involves using the
“plate on plate” test developed to study the high-speed spherical behavior of metals.
It is a plane strain-loading test, the inverse analysis of which is based on behavior
modeling. High confinement there is associated with very high strain speeds.
1.1.2.2.5. Traction tests
A conventional traction test can be carried out with a Hopkinson bar [REI 86]. If
we consider only global measures, the main difficulty is due to keeping the sample
in contact with the bars. To avoid having to resort to assemblies leading to
impedance failures, it is reasonable to glue the sample to the bars. Some authors
[TED 93] have had the idea of using the Brazilian test again in dynamics. In this
case we have to check that the conditions of strain homogenity are compatible with
the assumptions. Finally, the spalling test [DIA 97] allows an accurate measurement
of the average stress just before failure, but its interpretation is difficult as it is
between the classical traction test and the fracture test (toughness measurement).
1.1.3. Identifying the behavior of concrete under fast dynamic loadings
When identifying the dynamic behavior of concrete, we are confronted with a
series of typical problems for each high-speed behavior identification test. Some of
these problems are increased by the nature of concrete, which is the reason why we
prompt the reader to be very cautious when using experimentation signals or results.
Due to its structure in aggregates, where it is mixed with sand and hardened
cement paste, concrete can be a highly heterogenous material, and the size of a
representative sample is not always an easy thing to state. As far as statics is
concerned, a 2 slenderness cylinder, over five times as big in diameter as the
aggregates, is the lowest volume necessary to obtain stable properties representing
the material in these tests, particularly as far as strength is concerned, otherwise
“scale effects” will be observed. Such a constraint raises several types of problems:
- for standard concretes, in which the maximum size of aggregates ranges from
20 to 25 mm, the dimensions of test samples (diameter over 10 or 12 cm, mass over
5 kg) involve resorting to important energies, particularly for high speed tests, which
involves sometimes tricky technological arrangements;
- to avoid this difficulty, tests are often carried out on micro-concrete, mortar or
cement paste samples. Transposing these results to structure concrete requires a
critical analysis, mainly because the volume fraction of cement paste (generally