In general, concrete is a highly nonlinear material with great dependence on
the confining stresses, a type of behaviour also common in other granular and quasi-brittle
materials. The CEB-FIP Model Code [1] recommends the use of a four-parameter failure
criterion to estimate the strength of concrete under multiaxial states of stress. This failure
criterion is also known as the Ottosen failure criterion, and it captures with high accuracy
the behaviour of these materials, as demonstrated by several researchers, performing
experimental test programs. The concrete strength estimation takes into account, with
great precision, the effect of the increase in the confining stresses. In order to simulate
the monotonic quasi-static multiaxial behaviour of concrete, one possible strategy is to
introduce in this failure criterion a hardening parameter and the corresponding evolution
law, under the isotropic behaviour framework. In the present work, the concrete compressive
strength in the Ottosen failure criterion is assumed as the hardening parameter,
and the CEB-FIP Model Code 90 law for the uniaxial nonlinear behaviour of concrete is
used to derive the hardening law. In this case, the loading surface is not explicitly defined
as a function of the hardening parameter, as in the other more common and simpler
isotropic models. As a consequence, some difficulties may emerge, mainly of a numerical
nature. In this context, the formulation of the model in a thermodynamically consistent
framework is presented. The general behaviour of this model is accessed by the simulation
of the monotonic multiaxial loading of concrete elements, and its numerical efficiency is
discussed