where f, was the confinement strength, dg was the diameter of the CFRP grid tube andfg,

was the ultimate strength of the CFRP grid strands.

 By rearranging equation 3-3 the confinement strength (fI) was calculated using the

following:

 2-t
 fru = fg (3-4)


 Assuming that the confined concrete was in a triaxial stress state, the increase in

strength provided by the confinement was reflected in the maximum stress (f"cc) for a

cylindrical specimen, which was defined as (Mander et al. 1988):

 fl = f + k, f (3-5)

where k, was the confinement effectiveness coefficient. The confinement effectiveness

coefficient for concrete confined by steel is usually taken between 2.8 and 4.1. Campione

and Miraglia (2003) found that the above values overestimate the confinement

effectiveness coefficient for concrete wrapped with FRP. They found the confinement

effectiveness coefficient for FRP wrapped concrete to be two. Therefore, the confinement

effectiveness coefficient was assumed to be equal to two.

 The axial strain of CFRP grid confined concrete at the peak stress (eco) was

determined in a similar manner as unconfined concrete using the following expression

(MacGregor 1997):


 8 =1.8f (3-6)
 E,

 Equations 3-5 and 3-6 were combined with the modified Hognestad (Park and

Paulay 1975(a)) stress-strain equation as follows: