Comparison of Concrete Rheometers: International Tests at LCPC (Nantes, France) in October 2000.
Comparison of Concrete Rheometers: International Tests
at LCPC (Nantes, France) in October 2000.
Ferraris, C. F.; Brower, L. E.; Banfill, P.; Beaupre,
D.; Chapdelaine, F.; deLarrard, F.; Domone, P.;
Nachbaur, L.; Sedran, T.; Wallevik, O.; Vallevik, J. E.
NISTIR 6819; 156 p. September 2001.
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concretes; rheometers; rheology
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.