Introduction - Strengthening Design of Reinforced Concrete with FRP - page 3

Full composite action between the beam and FRP is usually assumed. However,
this perfect bond typically depends on the shear stiffness of the interface adhe-
sive (Rasheed and Pervaiz 2002). Most resin adhesives yield excellent bond char-
acteristics with concrete and FRP, leading to perfect composite action. On the
other hand, some resin adhesives have low lap shear stiffness, leading to bond
slip between FRP and the concrete beam, thus reducing the composite action
(Rasheed and Saadatmanesh 2002; Pervaiz and Ehsani 1990). With full compos-
ite action, glass FRP (GFRP) and aramid FRP (AFRP) do not increase the initial
stiffness of the beam due to their relatively low modulus along the fiber direction.
On the other hand, carbon FRP (CFRP) slightly increases the initial stiffness
of the beam due to its high modulus along the fiber direction. Accordingly, this
application is not used to stiffen the beams; instead, it is used to strengthen the
beam due to the high strength of FRP materials available in practice, as shown
in Figure 1.2.
Flexural failure modes may be classified as

1. FRP rupture failure after yielding of primary steel reinforcement. This fail-
ure mode typically takes place in lightly reinforced, lightly strengthened
sections (Arduini, Tommaso, and Nanni [1997], Beam B2).

2. Concrete crushing failure after yielding of primary steel reinforcement.
This failure mode typically occurs in moderately reinforced, moderately
strengthened sections (Saadatmanesh and Ehsani [1991], Beam A).

3. Cover delamination failure primarily occurring after yielding of steel rein-
forcement. This failure mode initiates at the FRP curtailment due to stress

30000

25000

20000

15000

10000

Control
FRP bottom
FRP U-wraps

5000

0

0.000 0.500 1.000 1.500 2.000 2.500 3.000 .500 4.000

Deection (in)

FIGURE 1.2 Response of unstrengthened and CFRP-strengthened identical beams show-
ing limited stiffening compared to strengthening.