Rheological measurements of a concentrated suspension can be used to describe the flow of concrete. The rheological constants (yield value and plastic viscosity) of the mortar can be determined with a co-axial viscosimeter, and this technique is applied in this study. If the efficiencies of the mortar phase with respect to the cohesion, fluidity, bleeding and the friction of the concrete with the pipe are taken into consideration, this approach can be recognized as beneficial and helpful. Rheological tests on mortars were carried out with a Mettler RM 180 Rheomat co-axial viscosimeter. The angular deformation rate ((gamma)over dot) and shearing stresses (tau) were determined, and (gamma)over dot-tau diagrams were drawn. All the mortars showed a tixotropic behavior conforming to the Bingham model. A linear regression of these parameters gave the yield value (tau(0)) and the plastic viscosity (eta(pl)) of the mortars.