Disturbance Effect on Time-Dependent Yield Stress Measurement of Bentonite Suspensions

Disturbance effect on time-dependent yield stress measurement of bentonite suspension was investigated using vane and cone and plate geometry. With cone and plate geometry, the disturbance occurs as a result of the large strains the suspensions are subjected to when placed on the lower plate and when the cone is lowered to the testing position before the actual shear stress is applied. As an alternative, vane geometry has been used to measure yield stress in bentonite suspension to (1) minimize the placing disturbance, and (2) eliminate the loading disturbance. Even though a minimal amount of disturbance was still induced by vane geometry, it was assumed that the amount of disturbance was small enough to be ignored compared to that with cone and plate geometry. Yield stresses of bentonite suspensions at different weight fractions and various resting times were determined from stress ramp tests. Linear relationships were developed between yield stresses measured with the different geometries. Disturbance rapidly increased up to approximately 50 % with resting times, then converged at long resting times (60 % at 10 days). This indicates that cone and plate geometry significantly underestimated the actual yield stress of the tested material because of disturbance. Based on the quantified disturbance, a method to predict the undisturbed yield stress from disturbed one was proposed. The observations were interpreted with structural kinetics.