Dynamic Behavior of Concrete: Experimental Aspects - Dynamic Behavior of Concrete and Seismic Engineering - page 40

and difficulties in ensuring validation on the scale of the structure, as it has already
been enhanced during test interpretation.
1.5.2. Data available for reinforced concrete
1.5.2.1. Dynamic behavior of reinforcements
To design structures, the dynamic characteristics of the steel framework are as
crucial as those of concrete; indeed, the ductile character of a failure due to
accidental dynamic strain can be investigated, owing to failure due to R-bars.
Technical data concerning high-speed behavior of reinforced concrete framework
proved to be rare, and information about pre-stressed frameworks was non-existent.
A recent synthesis is available in [MAL 97]. The results are exploited to determine a
relative increase in traction strength (DIF) or elastic limit. Owing to the idealization
of framework behavior that is general in calculations and to the fact that no elasticity
modulus variation seems to be revealed at the strain rates considered, rate effects on
the elastic limit and strength are sufficient to ensure that a consistent behavior
description is obtained using standard calculation methods for reinforced concrete
sections.
The expression proposed by Malvar for describing the relative increase in steel
strength as a power of the strain rate (value imposed in the monotonic identification
tests) is consistent with the usual descriptions given for concrete ([MAL 98] for
example). For about 1/s, relative increases ranging from 10 to 50% of the elastic
limit can be expected, depending on the nature of the steel considered. Taking into
account the mainly one-dimensional feature of strain in frameworks, it must be
possible to directly calibrate a strain-hardening elasto-plastic model for frameworks
from this data - with the physical meaning of the variable driving the increase of
strength still to be determined.
1.5.2.2. Identification of steel-concrete adherence in dynamics
The R-bar-concrete adherence enables us to consider the strains of the
surrounding framework and concrete as identical, and is at the root of the operation
and calculation of reinforced concrete structures. The permanence of this property in
dynamics and the evolution of limit shear stress are basic questions; however,
experimental identification of adherence properties is relatively complex, because
measuring techniques used at the interface disrupt the phenomenon itself. In
practice, most of the sliding strength of reinforcement is provided by setting locks
on concrete, which disconnect when traction strength is reached in the transverse
direction. Adherence can then be considered to be directly linked to concrete traction
strength, as stipulated in most calculation regulations. Nevertheless, transverse