Investigating the Flow Curve of the Flowcyl Test as a Measure of Yield Stress

The Particle-Matrix Method (PMM) is a two-phase model for proportioning and controlling the rheological properties of concrete. The PMM has been used in Norwegian industry, teaching and research for approximately 25 years. The method considers concrete as particles dispersed in matrix, defined as the filler modified cement paste including all fluids and particles < 125 microns. Matrix flow-properties are measured with the FlowCyl apparatus in a gravity-driven flow experiment, from which the flow resistance ratio, λQ, is calculated based on time vs mass-flow of approximately 1.5 litres of matrix. The PMM uses λQ as input to estimate concrete workability. Previous research has shown that λQ mainly relates to the plastic viscosity, while the correlation to the yield stress of the matrix is poor. This was partly attributed to high shear rates at the narrow outlet nozzle of the FlowCyl. In this paper, experimental and numerical analysis are performed to investigate whether different parts of the time vs volume flow curve correlate to yield stress. The results show that the correlation to the yield stress is substantially improved when either considering the time it takes for a given volume loss through the FlowCyl or the volume loss for a given time span, compared to the correlation to the traditionally calculated λQ. The best correlation to yield stress is found when considering the time it takes for a limited part of the flow of matrix to pass through the FlowCyl.