Dynamic Behavior of Concrete: Experimental Aspects - Dynamic Behavior of Concrete and Seismic Engineering - page 34

Figure 1.15. Apparent strength of concrete in traction
dynamic loading (from [TOU 95a])
The free water present in the porous volume is stressed like a viscous fluid by the
(fast) motion imposed on the sides of the skeleton, which in the ideal case of a film
between two walls is known as the Stefan effect. The consequences of this can be
observed in high-speed traction or compression tests as an increase (low relative
value) of stiffness, and a more significant increase in the strength, called the rate
effect. The macroscopic stress increase can then be interpreted by partition between
the stresses borne by the skeleton and viscous stresses borne by the fluid (Figure
1.15). Things progress as if these viscous stresses cause pre-stress in the skeleton
and delay either its traction failure or the failure in the extension direction induced
by loading when the latter is not purely tri-axial. The partition and its effect on
material failure are at the root of the elasto-plastic viscous strain-hardening model
developed by Sercombe [SER 98b].
For higher-rate tests (over 1 to 10 s -1 ), even when the hydration state is well
controlled, the transient character of the test and the failure phase of the specimen
take precedence over the rate effect linked to the nature of the material, qualitatively
at least [WEE 98]. The relative increase in “strength”, compared to the static
reference value, can exceed a value of 2, even for specimens in which free water has
been eliminated [ROS 96]. In fact, we can notice that a dynamic failure mechanical
analysis (which takes critical crack propagation inside a material with non-zero
inertia into account) is consistent with the experimental observation, which is that
the relative strength increase (dynamic increase factor (DIF)) evolves with the strain