Multiaxial stress states have a significant influence on the understanding of the strength of concrete. General information about the shape of the hypersurface of multiaxial strength exists and there is a range of models that depict these well-known boundaries.

In this study, a new model is presented that is formulated in a rotated principle direction coordinate system. The basic approach of the model satisfies the known boundaries and by modification of the approach, a more complex shape of hypersurface results. The modifications are made in such a way that by fitting the arbitrary parameters, the basic form of the model can be obtained. Based on a dataset of multiaxial strength values of an ultra-high-strength concrete, different modification of the model are investigated and discussed concerning prediction accuracy. As a result, the best approximation of the used dataset is obtained in the shape of a hypersurface, which points in the direction of hydrostatic tension strength non-convex sections.