Dynamic Behavior of Concrete: Experimental Aspects - Dynamic Behavior of Concrete and Seismic Engineering - page 11

Combining both types of information in statics enables a definition of failure or
plasticity criteria closed on the hydrostatic compression axle, as opposed to the
“intrinsic curves” (Coulomb criterion), the validity of which is preferentially
ensured when an extension direction is possible.
1.2.1. Tests involving deviatoric behavior
1.2.1.1. High-speed press machines and traction tests
Because of the difficulties connected with carrying out dynamic tests, most
authors use privileged uniaxial tests. Owing to the basic feature, traction behavior
identification stands out, and has given rise to a great number of tests. In order to
ensure continuity in the geometry of test samples, by controlling the loading
application speed and considering its limited artifacts, a direct traction test on a
cylindrical specimen has become essential. This is detailed in [HOR 87], [REI 82]
and [TOU 95a]. With particular precautions, this test can actually be carried out on
conventional servo-controlled machines with load build-up speeds ranging from
about 0.05 MPa/s (which is the standard loading rate for standard identification
tests) to about 10,000 times this load, with identification at still higher speeds of the
order of 50 GPa/s possible on the same specimen type thanks to the modified
Hopkinson bar (SHB).
The necessary precautions particularly involve:
- choosing to glue the specimen in place with centering and a rigid (without
spherical pairs) mounting onto the press to limit looseness which is a source of
interfering moments;
- choosing aluminum hard supports to limit the transversal strain divergence at
both ends of the specimen;
- controlling the hydration state of the specimen [TOU 95a];
- choosing a not too important slenderness ratio (1 to 1.5) to limit potential
bending;
- gauge extensometer or extensometers fixed in the middle of the specimen to
avoid the deformations due to the glue joint;
- using specimens with adequately sized diameters considering the maximum
size of the aggregates, and if possible core cylinders for better homogenity of the
material and to avoid scaling effects [ROS 92a].
With a sufficient automatic control and oil flow unit, and potentially using a
preload to carry out high velocity tests, we can consider that the load build-up speed