Dynamic Behavior of Concrete: Experimental Aspects - Dynamic Behavior of Concrete and Seismic Engineering - page 20

The behavior law of the metallic ring is known. A thick enough ring to remain in
the elastic field allows the application of strong confinements. Using a ring made of
material that enters the plastic field (brass for example) will enable controlled
confinement to be applied. Thus, measuring the transversal strain of the ring allows
the confinement to be calculated, after which we can calculate the values that are
usually dealt with in geomechanics. As an example, Figure 1.7 shows evolution of
the volume-pressure relationship compared to the same relationship obtained using a
static trial.
1.3. Tests with transient phase conditioned interpretations
1.3.1. Tests involving mainly traction behavior
1.3.1.1. Modified Hopkinson bar
As explained in section 1.2.1.1, traction behavior is essential for characterizing
the failure of brittle geomaterials like concrete, which is why adapted tests have
been designed to obtain this data for high speeds, and has been widely studied.
The design has been achieved, mainly thanks to modified Hopkinson bar
configurations in which the specimen is glued between the input and output bars,
where it is submitted to traction produced by a shock to a retaining shoulder at the
end of the input bar. The main results with this technique were obtained on the
device of the University of Technology in Delft [REI 86] and [ZIE 82] between
1980 and 1995. The tested specimens are typically core sampling specimens 74 mm
in diameter (the same diameter as the bars), with a 1 to 1.5 slenderness. The duration
and energy of the shock which generates the traction wave depends on the mass
used, hydrostatic pressure and the number of dampers inserted between the masses
whose fall is triggered and the lower input bar shoulder.
In practice, as we want the shock to be intense enough to cause specimen failure,
and the loading build-up rate to be constant during the trial, the device allows
loading rates ranging from 4 to 200 MN/s, about 100 to 1,000 above the rates
reached with conventional press machines with similar specimen geometries.
The analysis of specimen loading uses the transient analysis described in section
1.1. The quality of glueing interfaces and the nature of the aluminum bars
contributes to impedance compatibility between concrete and the loaded material, so
an important part of the wave is transmitted to the specimen and the obstacles to
transversal strains are limited. We have verified that the transmitted-wave signal
gives a precise measurement of the average stress developed inside the sample -
after conversion into stress and calibration in time.