Fifty-cent rheometer for yield stress measurements: From slump to spreading flow

The slump test, originally used to determine the "workability" of fresh concrete, has since been used in many industrial fields (e.g., mining and food industries). It offers a quick and easy way to measure the yield stress of suspensions or pasty materials. The model used for estimating the yield stress from the measured conical slump was first written by Murata [Mater. Struct. 98, 117-129 (1984)], corrected by Schowalter and Christensen [J. Rheol., 42, 865-870 (1988)] and adapted for a cylindrical geometry by Pashias [J. Rheol. 40, 1179-1189 (1996)]. However, a discrepancy between experimental and predicted slumps still appears in the case of conical slumps [Clayton et al., Int. J. Miner. Process. 70, 3-21 (2003)] and for high-yield stress materials. In the present paper, we extend the theoretical analysis of this simple practical test by including different flow regimes according to the ratio between the radius (R) and the height (H) of the slumped cone. We propose analytical solutions of the flow for two asymptotic regimes, namely H >> R and H << R. We finally compare the predictions of these solutions in terms of yield stress and previous expressions to three-dimensional numerical simulations and experimental data on a large range of yield stresses. This makes it possible to clarify the field of validity of the different approaches and provide further practical tools for estimating the yield stress of coarse materials. (C) 2005 The Society of Rheology.