Dynamic Behavior of Concrete: Experimental Aspects - Dynamic Behavior of Concrete and Seismic Engineering - page 42

necessary to resort to experiments on structure components (see [PAU 02] and [TOR
88]). In fact, at this level it is possible to underline the effectiveness of transverse
reinforcement, thereby ensuring the confinement of concrete beyond cracking,
allowing loads to travel after redistribution via the creation of “joints”. The amount
of experimental work in this field and its complexity will not be dealt with here.
It should be noted that the difficulties arise not because of the dynamic character
of stresses (some Hz frequencies with regard to the specific frequencies of the
elements - in the 100 Hz order of magnitude) or the dynamic reaction of the
material. The difficulties are linked to the intense and repeated feature of stresses
(incursions into the plastic field, stiffness damage, crack spreading), and even to the
interaction between the loading frequency and the specific frequency of the whole
structure (whether sound or progressively damaged).
1.5.3. Data about fiber-reinforced concretes
1.5.3.1. Post-cracking mechanisms and “rate effects”
Thanks to metallic fibers, the favorable influence of diffused reinforcement in
dissipating energy during a shock has been empirically proven [ROS 98], but is
difficult to quantify. The sensitivity to pealing caused by shocks during transport of
some prefabricated fiber reinforced concrete pieces, compared to corresponding
concrete or reinforced concrete pieces is admitted by professionals . Using fiber
reinforced concrete for particular applications where absorption of energy is
important has been shown to be interesting with regard to conventional reinforced
solutions [HAN 92]. Most of the time it was proved globally through experiments
allowing interpretations in terms of energy. However, complete and documented
experimental data on the dynamic behavior of such materials in traction for
characterizing after-peak behavior and revealing the contribution of fibers [ROS 98]
are quite rare.
The existing data [KÖR 88, TOU 99b] highlight major aspects of the rates of
behaviors of these materials: the increase in the linearity limit corresponding to
matrix traction strength, a phenomenon which can be directly compared to rate
effect of all cement materials in direct traction; also, a stress increase in the after-
peak phase (with regard to the load obtained in statics during this phase), the
increase being all the less important as a widespread range of crack openings is
considered. In other words, the relative increase of the absorbed energy and its peak
value is lower than the relative strength increase of the matrix, and it is even weaker
if we take crack openings into account.