2. 2]
f = I a r (3-7)
f, = J; "[1- D, (, -o)] (3-8)
where ec was the concrete strain, eo was the strain at peak stress of unconfined concrete
and e,, was the ultimate strain. These equations were plotted in Fig. 3-15. The modified
Hognestad equations model the ascending branch (AB) with a parabolic relationship and
the descending branch BC with a linearly descending curve. The equation for region BC
is based on the deterioration constant (Dc) that controls the slope of the line.
Stressfc Region AB
c
fp___ B Region BC
I I
Sg Strain F,
A o en
Figure 3-15. Modified Hognestad stress-strain curve
The material properties of the CFRP grid strands were used to construct the stress-
strain curve of the CFRP grid confined concrete. The average strength of the control
cylinders tested in displacement control mode was taken as the strength of unconfined
concrete (f'c). An average CFRP grid tube diameter of 5.375 in. was used. The ultimate
concrete strain cu, was assumed to be 0.0061 in/in. The average stress-strain curve for the
CFRP grid confined concrete was calculated using data from all grid cylinders. The