Multilinear Plasticity is intended for ductile materials and use tensile (positive) values of stress that start with the yield strength.

Concrete is modeled using material models under the Geomechanical catergory.

Go to the ANSYS Help and under the Mechanical APDL section, find the Material Reference, then go to the Geomechanical category.  Look at Drucker-Prager Concrete for example.