Flexural Strengthening of Beams and Slabs - Strengthening Design of Reinforced Concrete with FRP - page 127

FRP debonding (see (b))

Longitudinal steel

Cover delamination (see (c))

(a) Behavior of exural member having bonded reinforcement on soffit

Flexural
crack

Inclined
cracks

Longitudinal steel

Debonding progresses
through cement matrix
or along adhesive layer

Delamination progresses
through cement matrix
or along adhesive layer

FRP pulls away
from substrate

FRP pulls away
from substrate

(b) Debonding initiated by
flexural and/or shear cracks

(c) Cover delamination initiated at
curtailment of bonded FRP reinforcement

FIGURE 5.19 Cover delamination and debonding failure modes of FRP-strengthened
beams. (Courtesy of ACI 440.2R-08.)

Accordingly, the simplified model implemented by FIB Task Group 9.3 FRP (2001)
will be studied here.
FIB 2001 presents the ultimate tensile strain approach in the FRP as a means
to guarantee FRP anchorage to concrete substrate prior to the cover delamina-
tion failure mode. This model was first developed by Holzenkämpfer (1994) and
was modified by Neubauer and Rostasy (1997). It is based on bilinear bond-shear-
stress-bond-slip law with a linear ascending part and linear descending part, as
seen in Figure 5.20.
In this model, the maximum FRP force that can be anchored, N fa, max , and the
maximum anchorage length, l b, max , are defined as follows:

N =α C kkbEtf

ff ctm (5.91)

fa ,max 1 cb

Et
Cf

ff

l =

(5.92)

b ,max

2 ctm

where

α is a reduction factor = 0.9 to account for the effect of inclined cracks on bond
strength (Neubauer and Rostasy 1997); α = 1 in slabs and beams with suf-
ficient internal and external shear reinforcements;
k c is a factor accounting for the state of compaction of concrete, where k c = 1.0 in
general for FRP bonded to well-compacted surfaces and k c = 0.67 if FRP is
bonded to surface not in contact with the formwork during casting;