The contribution of de Larrard, Ferraris and Sedran in Materials and Structures was one of the first to show that the rheological behavior of fresh concrete was not always linear. Similar to Tattersall making the case in 1970 that we need two data points in our flow curve to determine both yield stress and plastic viscosity, de Larrard et al. demonstrated 25 years later that two points are insufficient, and that not all mixtures obey the Bingham (linear) law. In fact, de Larrard et al. showed that the application of the wrong rheological model, namely the Bingham model in this case, could lead to a negative estimate of the yield stress, which is physically impossible.