Flexural Strengthening of Beams and Slabs - Strengthening Design of Reinforced Concrete with FRP - page 79

5 Flexural Strengthening

of Beams and Slabs

5.1 OVERVIEW
External bonding of carbon FRP reinforcement to the tension face of a concrete
beam to increase its flexural capacity has been introduced by Meier (1987). Since
then, a large volume of literature has been added to qualify this strengthen-
ing technique. The extension of this technique to near-surface-mounted FRP
bars and strips should also be noted (Alkhrdaji, Nanni, and Mayo 2000). These
strengthening techniques resulted in flexural capacity increases of 20%-200%
compared to the unstrengthened beams. However, the strengthening limits
imposed by ACI 440.2R-08 (2008) restrict the strengthening ratio up to about
40%-50%. More research on the insulation of FRP under fire is needed to waive
the need for these strengthening limitations and allow higher strengthening
ratios to be implemented.

5.2 STRENGTH REQUIREMENTS
It is widely established that the design nominal flexural strength of a strengthened
section reduced by the strength reduction factor should be equal to or greater than
the factored moment of the upgraded loads:

φ≥ MM

n (5.1) u

The nominal strengthened moment capacity M n

) with steel reinforcement and FRP is

(

to be determined using

• Strain compatibility

• Internal force equilibrium

• Internal moment equilibrium

• Expected failure mode

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