We report on systematic measurements of the viscosity of concentrated colloidal silica suspensions as a function of the volume fraction of particles and of the ionic strength. The experimental results are rescaled on a universal curve using a direct renormalization procedure which is described; the scaling ratios are interpreted in terms of an effective volume fraction which depends on the electrostatic repulsion between the particles. The master curve is compared with models existing for the viscosity of hard spheres suspensions.