Dynamic Behavior of Concrete: Experimental Aspects - Dynamic Behavior of Concrete and Seismic Engineering - page 16

Figure 1.4. Shock tube failure trials for a plain or reinforced
concrete slab (from [TOU 95a])
In addition to excellent loading control and a size adapted to a well controlled
trial on “realistic” concrete, the advantages of this test are the realistic
representativity (bending is obtained with maximum deformation speed typically
ranging from 0.01 s -1 and 1 s -1 , which corresponds quite well to the “hard” shock
range) and geometric simplicity (radial symmetry is preserved up to cracking) which
make it possible to validate a calculation model as well as for comparing various
materials. The relative ease of interpretation stems from the fast loading building up
(about 10 P s for a maximum deformation reached in about 1 ms) and from the
absence of pressure gradients on the loaded face. We can consider that the plate is
loaded instantly (vibration setting with a first deformation peak which is particularly
intense compared to static loading), but with a bearing constant loading, which
allows a stationary vibration rate to be set up before unloading. A “conventional”
modal analysis enables access to local stresses and strains, at least until cracking
starts.
In [TOU 95a], the details about the instrumentation implemented to characterize
strains in test samples in these types of trials are presented. We have seen that in a
series of plain or reinforced concrete plates with strength in the range of 35 to 120
MPa (Figure 1.4), we are able to show the progressive deterioration of the modal
response (frequency drop, increasing damping), the appearance of deflection,
plastification of the reinforcement, crack progression (which is sometimes delayed
with regard to the maximum strain rate) and the collapse mode type (shear
force/bending competition) the respective appearances of which can be justified by
limit analysis-inspired calculations [TOU 95a].