Flexural members - Reinforced Concrete with FRP Bars: Mechanics and Design - page 112

Cohn, Ghosh, and Parimi [19] reported an increase on the strength and
ductility of under-reinforced steel RC beams with the increase of compression
reinforcement. Such behavior, particularly important in seismic regions or if
moment redistribution is desired, is due to the reduction of the depth of the
compression stress block caused by the presence of compression reinforce-
ment that allows for the neutral axis to shift toward the extreme compression
fiber to maintain force equilibrium. As such, the strain in the tension reinforce-
ment at failure increases, resulting in more ductile behavior. These authors
also conducted tests on over-reinforced beams and demonstrated that the
brittle failure associated to the crushing of the concrete before yielding of the
steel can be mitigated by the presence of compression reinforcement.
In the case of FRP reinforced flexural members, over-reinforced sections
exhibit a more ductile behavior than their under-reinforced counterparts. The
presence of compression FRP reinforcement (assumed to have a higher stiffness
than that of concrete) would increase the strain in the FRP tension reinforce-
ment as seen in the case of over-reinforced steel RC, but the beneficial effect
on the member ductility would not be as relevant since FRP is linear-elastic to
failure. The addition of compression reinforcement could, instead, cause the
section failure mode to shift from concrete crushing to FRP rupture.
4.9 SERVICEABILITY
The serviceability limit states for FRP reinforced concrete flexural members
generally include crack width, maximum deflections, and maximum FRP
stress levels to avoid FRP creep-rupture and fatigue. In many instances,
serviceability criteria (crack width and deflections) may control the design
of FRP reinforced one-way slabs or beams because of the relatively smaller
stiffness of these members after cracking.
COMMENTARY
Replacing the steel reinforcement of a flexural member with an equal area
of FRP reinforcement results in larger deflections and wider cracks [20,21].
However, the serviceability criteria for crack width and deflections are gen-
erally satisfied when a section is designed to achieve concrete crushing failure
[22]. A section designed to reach a failure controlled by FRP bar rupture, vice
versa, is almost unattainable as it would require a very small amount of rein-
forcement that, in turn, would not satisfy the service condition requirements.