Dynamic Behavior of Concrete: Experimental Aspects - Dynamic Behavior of Concrete and Seismic Engineering - page 46

traction maximum stress or equivalent plastic stress absolute increase, which are
comparable as they are linked to the cement matrix for the different directions
considered in a fiber concrete piece.
Nevertheless, the absolute values of these strengths and stresses proved to be
widely different according to the direction (Tables 1.1 and 1.2), due to preferential
orientating of fibers during manufacturing [TOU 99b]. Therefore, the problem of a
potential taking into account of the anisotropy as far as fiber-reinforced structure
modeling is concerned remains in dynamics as well as in statics. The questions
related to the dispersion of properties are also the same.
1.6. Conclusion
The accumulated knowledge available for understanding and describing the
high-speed behavior of concrete material remains at a complex overall stage and
leaves both the structural engineer and the mechanic dissatisfied. This can be
explained by several factors: experimental difficulties in accessing the intrinsic
behavior of materials in dynamics tests, difficulties linked to the heterogenity range
of the concrete “material”, its sensitivity to the water environment, its brittleness as
a geomaterial which involves crack propagation effects within the specimens, and
the wide range of materials actually corresponding to the generic term “concrete”.
Besides this, we also have to note that part of the difficulties reflecting on
mechanical modeling problems are also present in the usual quasi-static field, even
if a standardized corpus valid for engineering common needs often avoids having to
ask too many questions.
After recalling the different experimental techniques that allow us to explore
concrete dynamic behavior, and after taking a few precautions, we described the
main established facts, i.e. the noticeable increase in strength and slight apparent
increase in the Young's modulus, which can be explained by the viscosity of the
interstitial water present inside the nanopores (the finest pores within the cement
hydrates). This viscous inner phenomenon is inherent to porous solids, and can be
observed separately in direct traction tests over a standard range from 10 -6 to 1 s -1 . It
also explains the rate effects induced in other stresses (compression, adherence, fiber
concrete behavior) reasonably well. At higher rates, interpreting the tests involves a
transient analysis of the loading and failure phases of the specimen, as inertial
phenomena (in terms of measured loads) that oppose critical crack propagation
become predominant.
Different empirical description levels of the mechanisms have been developed,
together with the underlying theoretical support and its potential limits: the DIF,
which can vary widely depending on the rate and strength of concrete, absolute