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: