Introduction - Strengthening Design of Reinforced Concrete with FRP - page 5

In other words, cover delamination and FRP debonding are less likely to occur
with NSM technology. Highly strengthened sections may suffer from splitting of
concrete cover through NSM bars.
The need to use a high ratio of strengthening reinforcement lends itself
to combining the two techniques described previously. While no more than
three to five layers of FRP sheets or one layer of prefabricated FRP plate may
be used as externally bonded reinforcement, combining these external plates
or sheets with NSM bars furnishes high strengthening ratios, especially for
lightly reinforced sections. This combination also helps the unstrengthened
design capacity, since the loss of external strengthening reinforcement still
offers higher capacity than the completely unstrengthened section (Rasheed et
al. 2013; Traplsi et al. 2013).

1.4 BEHAVIOR OF STRENGTHENED REINFORCED
CONCRETE BEAMS IN SHEAR
Concrete beams are typically strengthened in shear by external full wrapping,
U-wrapping, or side bonding FRP sheets or fabrics around or along the sides of
beams where fibers make 90° or 45° angles with the beam axis along the side profile
of the beam, as shown in Figure 1.4. Research and practical applications have shown
that this shear strengthening is a highly effective technique. The design of shear-
strengthened members with FRP is treated the same way as the design of steel stir-
rups used as shear reinforcement in beams. The only different design requirement is
that the effective FRP strain at failure needs to be identified as opposed to using the
yielding strength in the case of steel stirrups.
Failure modes range from FRP fracture for fully wrapped beams to shear
debonding for beams with U-wrapped or side-bonded FRP. The effective FRP
strain at fracture or debonding is significantly lower than the ultimate tensile strain
of FRP due to spots of high stress concentrations, which lead to premature frac-
ture of FRP or peeling off through the concrete near the FRP-concrete interface
(Triantafillou 1998). As a result of analyzing the findings of several investigators
on this subject, Triantafillou concluded that the effective FRP strain decreases
with increasing axial stiffness of the FRP (ρ f E f ). Triantafillou also reported that
FRP contribution to shear capacity does not increase beyond ρ f E f = 0.4 GPa (58
ksi). ACI 440.2R-08 is used in this text for FRP shear-strengthening design, and

(a) (b)

FIGURE 1.4 External shear strengthening of concrete beams: (a) 90° U-wrap sheets and
(b) 45° side sheets.