Dear Antonella,
I don't know precisely why there is that difference in your models.
To improve the response you should check better the model I sent you on Monday.
The mesh should be different than the one you applied. There should be more retained nodes than constrained elements, and you should try to avoid ovelapping between the rebar nodes and the concrete elements.
Also the boundary conditions I modified.
Please do look at that model more closely.
Francesca