Dynamic Behavior of Concrete: Experimental Aspects - Dynamic Behavior of Concrete and Seismic Engineering - page 12

is rather constant during the test. The propagation speed of the waves within
concrete - about 4,000 m/s, the standard size of specimens (10 cm) - and the
traction failure stress (4 to 10 MPa) limit the quasi-static interpretation of this kind
of test, results typically showing a divergence about 10% between the specimen's
input stress and output stress.
The measurements typically carried out during this test are of the applied force
as a function of time, and of the average longitudinal strain at the center of the
specimen (extensometer gages or sensors for which we have to check that the inertia
will stay weak and the fixing will be ensured during the test). Taking into account
the small size and fixedness of the assembly, we can consider that there are no
differences between the measured force and the force applied to the specimen, so we
can assess traction uniaxial behavior by eliminating time. In such a test, the
specimen behavior corresponds quite well to brittle elastic behavior up to localized
cracking. Localization brings about loss of the homogenity of the strains, and an
almost instantaneous decrease of the load.
Going through these tests, which implies expressing the maximum measured
stress according to the “load build-up” parameter in a logarithmic diagram, typically
allows us to define a traction rate effect corresponding to the strength relative
increase.
1.2.1.2. High-speed press machines and compression tests
The second most conventional test that can be performed at high speed is the
compression test. It enables us to define a compression “rate effect” from the
measurement of the maximum strain reached [BIS 91]. The size of the test sample
necessary to free oneself from the size effect and to ensure the correct strain level
reached lead to strict constraints on press dimensions, unit power and the jack flow
rate. For this reason, a great number of the tests described in the literature were
carried out on mortar, cement pastes or micro-concrete [HAR 90]. As is the case in
traction, it has proved possible to look for a size compatible with the higher speed
test performed with Hopkinson bars [DAR 95].
As it is difficult to stop the jack when its speed has been stabilized, few test
reports have included extensometer measurements [BIS 91], measuring the load
obviously remains the main data. For standard size specimens (10 cm), considering
the wave propagation speed and the maximum stress reached, the load build-up rate
beyond which the sample cannot be deemed to be in a stationary process is about 10
times as important as it is in traction tests, which correspond to the strength ratio.
When expressed in terms of strain rate, the threshold is about 10 instead of 1 s -1
[MAL 98].