Dynamic Behavior of Concrete: Experimental Aspects - Dynamic Behavior of Concrete and Seismic Engineering - page 7

wave propagating in a single direction, which requires measurement of the incident
wave (propagating one way) separately from the reflected wave (which propagates
in the other direction). This limits the measuring duration to

' ' ,

C being the propagation velocity and L the length of the input bar. ' is thus a
function of the length of the bars. Consequently, for a behavior test, the total strain
cannot exceed the product of the average strain velocity and ' . For instance,
measuring duration will not exceed 400 μs (C | 5,000 m/s) for a 2 m long aluminum
bar, and the total strain will be limited to 4% for a test with a 100 s -1 average strain
velocity. Because of this limitation, even with concrete (for which high strains are
unlikely to be reached), the conventional Hopkinson bar system will not allow tests
at average strain velocities lower than 50 s -1 . On the other hand, for reasons
explained in section 1.2.1, traditional machines used without specific precautions do
not give reliable results at lower velocities. Besides, their superior limit is not clearly
established and is determined to an extent by the material being tested (the test
piece). The machine must be used in a particular way; it varies between 1 s -1 and
about 10 s -1 . However, a recent experimental technique using bars [BUS 02] that
covers this problem now exists.

TLC /

1.1.2.2.3. Difficulties inherent to dynamic measurements
The dynamic test facilities have numerous limitations, especially for stresses
other than simple compression or small strains. This limitation mostly affects low
strength stressed materials (impedance adaptation and high strain problems) and
brittle materials (low strain at failure).

The Hopkinson bar example illustrates the generic difficulties quite well. The
very short loading times do not enable us to carry out multi-axial dynamic loadings
easily, and it is not easy to synchronize loading with two (or three) orthogonal
Hopkinson bars. If synchronization is tricky in dynamics, it is all the more so when
piloting the test. Therefore, we cannot (for now) contemplate carrying out tests
under controlled multi-axial loading (deviatoric, for example), as is required in a
quasi-static mode. The need to control the loading and the difficulty in carrying out
dynamic displacement measurements limits the potential tests to a very small
number, which are described, along with their specific problems, in sections 1.2 and
1.3.

1.1.2.2.4. Compression tests with confinement
It is quite easy to superimpose quasi-static confinement on a dynamic
compression test. A cell in which a gas pressure confinement can be maintained
during the compression test is described in [GAR 99]. Some authors have proposed
a bi-axial loading scheme, where the secondary static stress is applied using a jack
[WEE 88]. For higher confinements (necessary if we want to study compaction of