Dynamic Behavior of Concrete: Experimental Aspects - Dynamic Behavior of Concrete and Seismic Engineering - page 29

impactor and the sample face is essential for a good repartition of loads. In practice,
the surface of the impactor and that at the rear of the reaction system can lead to a
limitation of the transverse strains at both ends of the specimen, upon which it is
necessary to use a specimen with sufficient slenderness (2 or more). If an efficient
anti-helical reinforcement system is used, we observe prismatic failure
corresponding to the cutting of angular sectors [MUR 86].
The rates reached are important, ranging from 1 to 100 s -1 . The strains can only
be measured by extensometer gages glued to the sample. An artifact is possible due
to confinement, which is different at the heart and at the periphery of the specimen.
Measuring the loads also requires particular precautions, considering the inertia of
the impactor. It is generally inaccurate to consider the acceleration measurement on
the impactor. Bischoff and Perry developed an ultra-flat pressure cell to minimize
the reflections of waves therein, the cell being inserted between the specimen and
the assumed motionless reaction body at the back of the specimen.
The results obtained with these devices (mainly the maximum average stress
reached) are along the same lines as those obtained on Hopkinson bars. Guidance
defects (centering loads) and surface evenness (contact hard spots) could be the
cause of dislocations when the results are in a continuity with those obtained with
presses. Besides which, the immobility of the reaction device has to be verified,
otherwise a correction for inertia becomes necessary. Anyway, at the speeds
reached, the direct transformation of the maximum effort recorded into “failure
stress” can only be considered as conventional, as the analysis reveals inertial effects
and the “inhomogenous” divergent feature of the axial and radial stresses inside the
specimen.
1.4. Other tests
1.4.1. Tests adaptable to an energetic approach
In all that has been said so far, we have noted the difficulties linked to finely
identifying the high-speed dynamic properties of concrete. Considering these
experimental difficulties, and the necessity to identify calculation parameters simple
enough for engineers, some means have been developed to enhance - in a
comparative way - the energy absorption properties of some concretes (especially
fiber reinforced concrete). Basically the approach involves adapting the resilience
test, which is standard for metals, to concrete, and which corresponds to a dynamic
bending loading, the load being applied via a pendulum ram impact testing machine
(Charpy test).