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