Dynamic Behavior of Concrete: Experimental Aspects - Dynamic Behavior of Concrete and Seismic Engineering - page 43

Such observations are consistent with the rate effect being a function of the
cement phase only. Before cracking occurs, fibers have no influence, and the
linearity-limit increase (matrix cracking) is similar to that observed in a non-fiber
reinforced concrete. After cracking however, the anchoring provided by fibers on
both sides of the crack assists in the maintenance of strength. As for reinforcement
anchoring, the strength of concrete mini-connecting rods confined by the presence of
the other fibers shows a rate effect, as the anchorage is limited by the strength of the
concrete around the fibers. Moreover, the wider the crack opening, the more micro-
cracked the concrete where anchoring has to take place will be, which limits the
effect of the viscous mechanisms underlying the strength increase.

A beneficial “synergy” could be observed by comparing the shock strength of
reinforced and fiber reinforced concrete pieces to those of fibered or reinforced
pieces. Considering the limited crack openings permitted by fibers for a given load,
once cracking has started, some confinement seems possible around the
reinforcements, which is not the case for reinforced concrete where adherence has
been damaged because of alternating dynamic stresses.

1.5.3.2. Anisotropy and its consequences

a) Opening rate 0.15 μm/s b) Mean opening rate 3.2 m/s

Figure 1.21a and b. Direct traction characterization tests on very high performance
concrete, at low and high rates, on pre-slotted specimens. Direction A specimens.
Low efficiency of the fibers (from [TOU 99b])