The irreversible plastic deformation and degradation of unloading stiffness are the most typical nonlinear mechanical behaviors of concrete materials in steady environments, which can be described by the plasticity damage model that couples plastic mechanics and damage mechanics. In this chapter, a 3D non-orthogonal plasticity damage model for concrete in steady environments is established based on the combination of the effective stress method and geotechnical non-orthogonal plasticity theory. The proposed model consists of the damage part driven by plastic strain and the plastic part based on effective stress. In the damage characterization, the degeneration law of elastic stiffness is described by an S-type damage variable. In the plastic characterization, the direction of plastic strain increment is obtained directly by the non-orthogonal gradient of the yield surface, and the need for the construction of the plastic potential function is bypassed. The magnitude of plastic strain increment is calculated based on the effective hardening function and the closed-form yield function, where the effective hardening function corresponding to the undamaged material is derived by both the nominal hardening/softening function corresponding to the damaged material and the damage variable. The ability of the model to capture the complex mechanical behavior of concrete is assessed by the monotonous and cyclic test data from the literature.