Concrete is a Very Popular Construction Material, Which Can Be Regarded as a Suspension of Solids of Several Orders of Magnitude, Suspended in Water. from a Rheological Point of View, the Fresh Fluid Concrete Mostly Obeys the Bingham Law, Showing a Yield Stress and a Plastic Viscosity. Due to the Presence of Cement, a Permanent Process of Coagulation and Dispersion Occurs, Causing the Concrete to Be Thixotropic. This Paper Describes the Influence of Thixotropy on the Measured Pressure Losses during High Speed Pumping. Apparently, the Concrete Becomes More Fluid with Increasing Discharge, Increasing the Risk for the Loss of Internal Stability, Which Can Lead to a Very Low Quality Product. © 2008 American Institute of Physics.