  Based on the fact that for an isotropic material model the elastic predictor
and the projected stress tensors have the same eigenvectors, it is shown that
the scalar damage can be obtained directly from the projection algorithm. This
eliminates the difficulty of a proper definition of equivalent strain which
serves as a driving force for evolution of damage in concrete. Moreover, if
eigenvectors are known it is not more nec-essary to use invariants of the
stress tensor for the formulation of loading surface. In the present model the
loading surface is represented in the polynomial form. This has two advantages:
(i) it automatically fulfils C1 continuity and (ii) plane stress formulation is
achieved by simply setting the third stress to zero. The perform-ance of the
model is illustrated on example of a mixed-mode fracture of concrete. It is
shown that for the pre-sent example the model prediction strongly depends on
the choice of the stress degradation law.
