Dynamic Behavior of Concrete: Experimental Aspects - Dynamic Behavior of Concrete and Seismic Engineering - page 14

centimeter thick test elements, therefore generally of mortar (or possibly fiber-
reinforced) rather than concrete.
The loading process links the level of the applied overpressure with its duration
and load build-up speed. However, interpreting the trial remains simple in so far as
the load build-up times can be considered as very short compared with the specific
period of the structure. Thus, we have what is called a pulse loading: the
overpressure time, which causes the structure to start vibrating, is very slightly
ahead of the latter's peculiar period, which is then in a free-vibration system. As the
probable area of maximum strain and even failure is known, the relevant section can
be instrumented in a preferential way. Therefore, we can measure the traction by
bending the final strain and the final bend. Note that shortly before failure, the strain
of the compressed side is slightly inferior to that of the opposite side (the start of
non-linearity which could be representative of micro-cracking). Since the structure
is undergoing free vibrations, the deformations should be linked to the stresses
generated from the by-pulse loading, which implies that a dynamic analysis can be
used to calculate the moments to link to the bends within the scope of behavior law
identification. Nevertheless as long as we stay at moderate loading levels and deal
with the behavior just before a brittle material fails, an elastic analysis is
satisfactory. The divergence from elastic behavior can be identified “at a quasi-static
speed”.
1.2.1.4. Shock tube tests on plates
The principle of a gas pressure by-pulse loading can also be applied by resorting
to a uniform loading the value of which is controlled thanks to a tube used as a wave
guide and called a shock tube. Using such a device is quite conventional for testing
industrial equipment in the defense field. Using the device for structure elements
was developed more recently [TOU 93]. Using explosives is limited and the loading
profile as well as its spatial repartition is better controlled than open-air explosions.
In so far as the conditions at limits can also be well controlled, we can directly
access to the behavior of a bending plate, which represents “basic” data for the
structure designer [KRA 93] or a simple basic situation to validate a behavior model
[PON 95, SER 98a].
The innovation of this trial was that it generated loading by means of a well-
controlled air shock wave (Figure 1.3). By using the closed tube, for the same plate
with the same support conditions, it is possible to carry out quasi-static loadings by
slowly inflating the whole tube. As an example¸ a 35 m long tube, 66.6 cm in
diameter, was used to compile an important experimental database about concrete
and reinforced concrete plate bending parameters [TOU 95a].