New experimental insights on concrete plasticity under triaxial loading. From the theoretical point of view, nonlinear material models for concrete are suitable for failure simulations of concrete structures. From the practical point of view, however, they remain limited to verification calculations of known experiments, where it is necessary to adapt many non-physical model parameters to the existing test results. Hence, a prognosis of the behavior of building components is impossible. Already in the simulation of a simple uniaxial compression test, the cause for the limitations of existing concrete material models becomes obvious. In the direction of the load, a realistic force-deformation diagram is predicted. However, the corresponding lateral deformations are simplified to a large extent. In the direction of the load, concrete failure due to compression is suggested, although the failure in the experiment occurs due to transverse tensile cracks. This contribution provides the so far missing experimental observations to expand and improve the basic assumptions of the plasticity theory for concrete material models. The performed compression tests contain the required multiple un- and reloading steps, and deformations lateral to the direction of the main load are documented carefully. Based on the new insight, the evolution laws for the multiaxial behavior of concrete are developed. With the resulting model approach, the simulation of various multiaxial compression tests becomes possible without the adaption of the model parameters to respective test results.