Plasticity Strain-Stress Graph

[InfiniteEngineer]

What is the best strain-stress model to use with conrete?? im trying to use mises isotropic plasticity (rate independent) but it doesnt allow me to have a curve for strain-stess that resemble the one used in euro-code for concrete :-(

negative slopes are not allowed! how can i used input the negative slope for concrete??

 

[Busa20]

as you say, negative slopes are not allowed with concrete material.