Comparison of Concrete Rheometers

This paper describes how fresh concrete is a complex fluid consisting of a suspension of a high volume of water with high volume percentage of particulate solid that has a very wide particle size distribution. The rheological properties of fresh concrete control the flow behavior of the material in mixing, placement, consolidation and finishing. Test methods that measure flow using a single parameter (e.g. slump) cannot properly evaluate the rheological properties of concrete in all uses. The paper describes how the American Concrete Institute (ACI) committee 236A, with the support of the Concrete Research Council and industry, has tested four concrete rheometers that were specifically designed to evaluate rheological properties of concrete materials. The second test series expands the data from the first test series using the same approach of bringing the rheometers together at a common test site and testing the same concrete mixtures simultaneously. All of the rheometers can measure a flow curve for fresh concretes with slumps in the range from 100 mm to 250 mm or non-segregating concretes with slump flows in the range from 300 mm to 800 mm. Each rheometer evaluates yield stress and plastic viscosity by fitting a Bingham model flow curve to measurements of rotation rate and torque for each mix. All of the rheometers gave different absolute values for the Bingham constants of yield stress and plastic viscosity for each mix. But all of the rheometers ranked the mixes in the same order for both yield stress and plastic viscosity.