Dynamic Behavior of Concrete: Experimental Aspects - Dynamic Behavior of Concrete and Seismic Engineering - page 35

rate in a logarithmic diagram, along a 1/3 slope straight line ([CHA 98], [KIP 80]),
beyond a certain threshold (typically 1 to 30 s -1 , depending on geometry and
loading) which corresponds to the limit beyond which the test has to be analyzed as
a transient state ([REI 91], [WEE 89]). An analogous model taking local inertia into
account [BAI 94] also justifies the “double state” obtained experimentally if the
transient character of the failure is interpreted as a local property.
The latest results obtained on quite large-size concrete samples [CAD 01] are
consistent with these two basic mechanisms causing the strength increases observed
during high-speed dynamic tests, with the participation and viscosity of water,
beyond a specific threshold, and the participation of inertia on both sides of the
failure origin.
1.5.1.2. Effect of the structure of the cement paste
The crucial part played by free water in determining the sensitivity of concrete
behavior to loading speed in a transient state has led to speculations about the
relevance of conventional parameters used to describe the dependence. Actually, it
appeared that the conventional definitions ([COL 88], [BIS 91], [MAL 98]) of
compression or traction DIFs, as well as those of ultimate strain or Young's
modulus, have led to values varying according to the static properties of concrete,
including compression strength ([COL 88], [JAW 87], [ROS 95]). This is apparently
responsible for the wide discrepancies in the diagrams used to describe strength
evolution (Figures 1.16 and 1.17), and interferes with them being taken easily and
reliably into account in a regulation context.
Figure 1.16. Concrete compression strength. “Rate effects” (from [BIS 91])