I drew some analytic stress-strain curves using Chang and Mander 1994 model.
It is quite obvious the confined concrete should have always a higher stress. However, when I plot my results the confined concrete has less stress than the unconfined concrete in the initial slope of the curve! In other words: Before the strain reaches epsilon'c (unconfined concrete strain) the unconfined curve is slightly above the confined concrete curve.
Does anyone have gotten the correct unconfined and confined stress-strain curves using Chang and Mander 1994 equations?
I just tested Concrete07 and I was getting what you just explained if the initial modulus of elasticity was the same for both confined and unconfined concrete. This is due to the parabolic equation for the concrete before the fc' or fcc' is reached. If you want to have confined concrete above the unconfined the initial modulus of elasticity has to be bigger for confined then for unconfined.
In other words: Before the strain reaches epsilon'c (unconfined concrete strain) the unconfined curve is slightly above the confined concrete curve.