The aim of the present project was to compare measurements from the five types of concrete rheometers to provide data to establish correlations among them. As a matter of fact, differences between the various rheometers were expected, due to the complex granular structure of concrete. Slip at the rheometer surfaces and coarse particle segregation are just two examples of such granular aspects of fresh concrete behavior. They are not accounted for in rheometer analyses, based upon classical fluid mechanics of homogeneous fluids. Comparison and correlation functions, which can relate the results obtained with the various rheometers, are essential to advance the science of concrete rheology and therefore provide a better characterization of concrete workability. As expected, it was found that the rheometers gave differing values of the Bingham constants of yield stress and plastic viscosity, even for those instruments that gives these directly in fundamental units. However, it was found that:1. All the mixes were ranked statistically in the same order by each rheometer. This result is valid for both the yield stress and for the plastic viscosity.2. The degree of correlation of both yield stress and plastic viscosity measurements between any pair of rheometers was quite high. Relationships with 95% confidence intervals have been proposed to relate measurement from one apparatus to another.3. The slump tests correlates well with the yield stress as measured with any of the rheometers.4. The slumping time measured with the modified slump test does not correlate with the plastic viscosity. Therefore, it was concluded that this test is not usable with concretes having a relatively narrow range of plastic viscosities such as the one presented in this report.The fact that (1) and (2) above were obtained from instruments operating on different principles and with a range of geometries was very encouraging and an important step forward for the subject as a whole. All the rheometers are therefore able to describe the rheology or flow of fresh concrete, and the correlations obtained will assist in the analysis and comparison of results from different rheometers in different laboratories on different concretes. There are, however, some consistent differences that cannot be unambiguously explained at this point. Some hypotheses were discussed but further research is needed.
Citation: NIST Interagency/Internal Report (NISTIR) - 6819
Issue: No. 8
NIST Pub Series: NIST Interagency/Internal Report (NISTIR)
Pub Type: NIST Pubs
concrete, rheology, rheometer