Flexural Strengthening of Beams and Slabs - Strengthening Design of Reinforced Concrete with FRP - page 81

5.4 FLEXURAL FAILURE MODES
For FRP-strengthened reinforced concrete section, the following failure modes are
likely to take place in flexure (GangaRao and Vijay 1998):

1. Ductile crushing of concrete: crushing of concrete in compression after
yielding of tensile steel (desirable failure mode)

2. Brittle crushing of concrete: crushing of concrete in compression before
yielding of tensile steel (undesirable failure mode)

3. Rupture of FRP: rupture of FRP reinforcement after yielding of tensile steel
(desirable failure mode)

4. Cover delamination: Shear/tension delamination of the concrete cover start-
ing at the FRP curtailment (not covered by ACI 440.2R-08)

5. Debonding of FRP: The intermediate crack-induced debonding of FRP
reinforcement (covered by ACI 440.2R-08)

5.4.1 D uCtile C ruShing oF C onCrete
This is the failure mode in which concrete reaches the limit of useful compressive
strain of 0.003 after yielding of tension reinforcement ε> s sy . It is a desirable fail-
ure mode because it involves a warning sign represented by the ductile yielding of
steel. It competes with the debonding failure mode, which alternatively takes place
in sections moderately strengthened with FRP. If the FRP is anchored with trans-
verse FRP U-wraps or stirrups, this failure mode would be the dominant one, since
debonding and cover delamination are prevented or delayed by the FRP U-wraps
(Rasheed et al. 2010).

5.4.1.1 Flexural Strengthening of a Singly Reinforced Section
For the concrete crushing failure mode after yielding of primary steel reinforcement
(Figure 5.2):

ε

= ε+ε

cu cu f

,where ε=ε+ε

Strain compatibility: (5.4) f fe bi

c d

f

ε cu

a

c

C

d

d f

M u

A s

ε s > ε y T s

ε f

T f

A f

b

FIGURE 5.2 Singly reinforced cross section with strain distribution and force profile.