INFLUENCE OF PLUG FLOW WHEN TESTING SHEAR THICKENING POWDER TYPE SELF-COMPACTING CONCRETE IN A WIDE-GAP CONCENTRIC CYLINDER RHEOMETER

摘要

When testing powder type self-compacting concrete (SCC) in a wide-gap concentric cylinder rheometer, sometimes a plug state arises, introducing an error in the obtained rheological flow parameters. In this paper, the classification of plug state inside a wide-gap concentric cylinder rheometer is illustrated for a nonlinear (Herschel-Bulkley) flow behaviour, which is not seldom observed in the case of powder type SCC. For a linear (Bingham) flow behaviour, the classification of plug inside a concentric cylinder rheometer is already well described in literature. The applied methodology is adapted to the nonlinear case. With a plug state, a solid state arises inside the sheared test material, so that it is rotating as a rigid body. When applying a stepwise decreasing rotational velocity sequence, plug will begin at the outer, rotating cylinder and propagates towards the inner, stationary cylinder as the velocity of the outer cylinder will further decrease. This means that, with a plug state and assuming no slippage in the transition zone from the viscoplastic to the solid state, the outer boundary condition of the integration equations of the Couette inverse problem solution must be corrected to the rigid body velocity at the boundary between the viscoplastic and the solid state (i.e. the plug radius R_p). For each rotational velocity of the outer cylinder N-p, the corresponding plug radius R_p can be calculated. However, these calculations are based on the assumption that the calculated rheological parameters are correct to begin with. Nevertheless, it was found that even if plug was occurring in some of the measurements when testing powder type SCC, it did not introduce a large error to the rheological parameters. In fact, the error generated by plug flow on the rheological flow parameters always remained within their 95% confidence intervals in case of the shear thickening powder type SCC mixes tested.