Shear Strengthening of Concrete Members - Strengthening Design of Reinforced Concrete with FRP - page 152

6 Shear Strengthening of

Concrete Members

6.1 OVERVIEW
Experimental studies have shown that shear strengthening of concrete members is pos-
sible to accomplish by applying the fibers transverse to the member axis or perpendicular
to the shear cracks (Triantafillou 1998; Triantafillou and Antonopoulos 2000). Increasing
the shear strength may cause the flexural failure to dominate the behavior, which gen-
erally provides more ductile response than that of shear failure (ACI 440.2R-08 2008).
Chapter 11 of ACI 440.2R-08 provides a model to compute the extra shear
strength furnished by FRP to concrete beams and columns. This extra shear strength
depends on the concrete strength, the type of FRP wrapping scheme, the geometry
of the concrete member, and the amount of steel shear reinforcement provided. For
external FRP shear stirrups in the form of discrete strips, the center-to-center spac-
ing between the strips should not exceed d /4 plus the width of the strip. In other
words, the clear distance between the strips should not exceed d /4.

6.2 WRAPPING SCHEMES
For beams and columns of rectangular sections, the FRP wrapping schemes are as
illustrated in Figure 6.1.

1. Complete wrapping : FRP systems wrapped around all four sides of the sec-
tion represent the most efficient scheme. This scheme is typically used in
columns, since all four sides of the section are accessible (Figure 6.1a).

2. U-wrapping : FRP systems wrapped around three sides of the section in
beams are used to improve shear strength where it is impractical to com-
pletely wrap the section due to the existence of the slab attached to the beam
from both sides (interior beams, Figure 6.1b) or from one side (exterior or
spandrel beams, Figure 6.1c). This technique is less efficient than the com-
plete wrapping scheme.

3. Side bonding : FRP systems bonded along the two opposite sides of the web
of the beam are used to improve the shear strength where it is impractical to
U-wrap the section due to the existence of section enlargement such as the bulb
tee (Figure 6.1d). This technique is the least efficient of the three schemes.

For all three wrapping schemes, it is possible to install a continuous sheet along
the span of the member as well as install discrete strips. The first case of fully

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