The paper presents a 3D model and simulations of corroding reinforcement bars in a concrete element. Electric potential distributions are calculated in the concrete matrix and on its surface for two rebars arrangements with one or three active (anodic) sites to assess the reliability and identify possible problems when standard test measurements for corrosion assessment in concrete structures are used and conclusion on the corrosion state is inferred. The values of the potential strongly depend on a concrete layer thickness and beyond the threshold of 5-7 cm it is hardly possible to detect the number of active sites on the rebar. Also conductivity – which is not constant in real world constructions – is an important factor. Thus without estimation of the state of concrete it is difficult to draw reliable conclusions on the corroding activity from shear potential measurements on the surface.