Flexural Behavior of Concrete Beams Strengthened with Near Surface Mounted CFRP Strips

In-service steel-reinforced concrete flexural members may
require strengthening due to material decay of the internal
reinforcement and surrounding concrete, errant design and
construction practice, increased service loads, and unforeseen
settlement and structural damage. These conditions require
structural retrofit to increase the flexural strength of the
section. A popular method of increasing the flexural strength
of beams, walls, and slabs is through external bonding of
fiber-reinforced polymer (FRP) plates and sheets. FRP materials
are characterized by high tensile strength and low unit
weight, and they are noncorrosive when exposed to chloride
environments. An excellent summary of research in this area
is available by Teng et al. (2002) and ACI has published a
design guide for strengthening concrete structures with
externally-bonded FRP materials (ACI Committee 440 2002).
Premature failure of externally-bonded FRP plates and
sheets can occur before the ultimate flexural capacity of the
strengthened section is achieved. This is typically due to
bond failure between the FRP and concrete or tensile peeling
of the cover concrete. Available research documenting this
behavior is abundant. Brena et al. (2003) reported debonding
of longitudinal carbon FRP (CFRP) sheets at deformation
levels less than half the deformation capacity of control
specimens. Nguyen et al. (2001) observed only a limited
increase in flexural capacity for beams strengthened with
partial length longitudinal CFRP sheets due to premature
delamination, or ripping, of the concrete cover surrounding
the steel reinforcement. For beams strengthened with CFRP
plate and fabric systems, Grace et al. (2002) identified brittle
failure by shear tension and debonding, respectively. Shin
and Lee (2003) reported failure of beams held under
sustained load and strengthened with CFRP laminates due
to rip-off type failure of the CFRP at loads well below the
ultimate
flexural capacity of the sections. Similar results have been
reported by Rahimi and Hutchinson (2001), Bencardino et al.
(2002), Arduini and Nanni (1997), Sharif et al. (1994),
Saadatmanesh (1994), and Mukhopadhyaya and Swamy
(1999). In addition to problems associated with bond failure,
external FRP plates are vulnerable to mechanical, thermal,
and environmental damage. It should be noted, however, that
mechanical anchors can be used to improve the peel resistance
of externally bonded FRP.
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