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Mixing test facility
The mixing of granular building materials determines their
resultant quality and thus the performance of the road
or civil engineering structures; this impact is also felt
in the field of geotechnical engineering, in areas such
as soil treatment. In that context, the LCPC has developed
a mixing test facility in order to study the mixing processes
representative of work site conditions. The facility has
been equipped with a platform for accommodating, depending
on the nature of the study conducted, various types of
discontinuous industrial mixers used in cold-mixing processes.
This platform enables assessing the efficiency of a given
mixing process with respect to both the average value
and homogeneity of the mix's use characteristics. A number
of parameters may be examined herein, including: mixer
type and geometry, sequences of components introduction,
mixing time, residence time, and type of mix.
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The mixer platform and sampling
conveyor (general view) |
The mixer platform and sampling
conveyor (close-up) |
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· Unit assigned responsibility for equipment operations:
Division for Sustainable Approaches in Civil Engineering (DDGC)
· Sectors of activity:
- Civil engineering
structures
- Roads
- Geotechnical engineering
Contacts :
Division for Sustainable Approaches in Civil Engineering (DDGC)
· An exemplary application:
Optimization of mixing time
for high-performance concretes and self-compacting
concretes
(D. Chopin "Malaxage des bétons à
hautes performances et des concretes autoplaçants",
Ph.D. dissertation, Ecole centrale de Nantes and
the University of Nantes, 2001)
Over the past twenty years, concrete has become
more than a simple mix of four basic components.
The use of liquid admixtures (superplasticizers,
viscosity agents, etc.) along with various mineral
additions (silica fume, filler, fly ash, etc.) is
now increasingly widespread and scientific approaches
towards concrete mix design are developed. In that
context, it seems necessary to give now serious
consideration to the fabrication process. More specifically,
the industrial use of both high-performance concretes
(HPC) and self-compacting concretes (SCC) has revealed
problems associated with longer mixing times than
those for conventional concretes.
Thanks to important experimental campaigns, the
mixing behavior of these new-generation concretes
could be partially interpreted. This study mainly
focused on the analysis of the stabilization times
of the power consumed by mixers during the fabrication
process.
We have first demonstrated the strong correlation,
on a number of mixers, between the stabilization
times of identical concrete. This finding then makes
it possible to predict, at the laboratory stage,
the behavior of a given mix design with respect
to mixing time at the industrial scale. At the same
time, the comparison between mixers with an identical
capacity (1 m3) yet operating with different equipment
(rotating paddles, or high speed agitators) indicates,
on average, shorter power curve stabilization times
for the high speed agitators mixers. This finding
implies that the minimum times imposed by current
regulations do not always correspond with the particular
equipment chosen. Exceptions to these regulations
could be envisaged in the form of performance validation
testing.
Afterwards, we studied the influence of mix design
parameters on the power curve stabilization time.
This phase has allowed highlighting the dominant
role played by the relative solid concentration
of a concrete on the time required for its homogenization.
Both the porosity of the granular skeleton and the
total quantity of water in the mix constitute, from
this perspective, the major parameters.
A third experimental campaign, aiming at identifying
the evolution of material use characteristics with
respect to mixing time, has revealed a clear-cut
relationship between mixing time and compressive
strength. Moreover, a mixing period of less than
the power curve stabilization time does not necessarily
lead to a crippling degradation of characteristics
such as resistance to deicing salts scaling or gas
permeability. In our opinion, the practical implications
of this result are noteworthy.
Lastly, the analysis of experimental results has
shown a clear relationship between the rheology
of concrete, assimilated with a Bingham material,
and the active power consumed by the mixer during
fabrication. This result is encouraging in the perspective
of the determination of an average shearing rate
within mixers.
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· Technical characteristics:
This facility has been rehabilitated for the end
of 2003; it will be operational in September 2004.
This retrofit program should lead to a more precise,
more reliable tool and especially much more versatile.
The station will then make it possible to vary
the types of mixer used but also the sequences
of production.
The mixing test facility will have the following
components:
- an assembly for
setting the mix proportions of components and
coordinating their discontinuous supply at the
industrial scale;
- a test platform
that enables accommodating, depending on the type
of study, a wide panel of discontinuous industrial
mixers (up to 1 m3);
- a conveyor belt
located at the mixer output and fitted with a
sampler to allow extracting a multitude of mix
samples semi-automatically and practically bias-free.
Mixing efficiency can then be evaluated by means
of examining not only the average value and dispersion
of the composition (especially the particle size
distribution), but also sample performance (workability,
mechanical strength, durability, etc.);
- a particle grading
test bench. This piece of equipment serves both
to prepare (washing and drying) approximately
45 test specimens a day and to measure the grading
curve (down to an 80-µm). The majority of
these operations have been automated in order
to limit operator-induced bias;
- a variety of sensors
(e.g. wattmeters to allow monitoring mixer power
consumption, etc.).
- a program package
for monitoring and data acquisition allowing to
control the whole production cycle with, when
necesary, a control of measures made in real time
in the mixer (wattmeter for example);
· Operational since: 1981, with a first rehabilitation in 1987. The
station is right now in rehabilitation and modernization
since end 2003 and will be again operational in
September 2004.
· Application examples:
- Influence of mixing
time on the strength and permeability of self-compacting
concretes. Study conducted within the scope of
the National B@P Project (2001).
- "Homogénéisation
des bétons en centrale de fabrication discontinue
: influence du temps de malaxage et du mode d'introduction
des additions minerals" (Concrete homogenization
in discontinuous batch plant: influence of mixing
time and mineral addition introduction sequence")
by P.O. Vandanjon, F. de Larrard, B. Dehousse,
G. Villain, R. Maillot and P. Laplante.
Ref.: Bulletin des Laboratoires des Ponts et Chaussées,
N° 208, September-October, 2000.
- "Modélisation
de l'écoulement des constituants dans un
malaxeur industriel : interprétation physique
et capacités prédictives" (Modelization
of components flow in an industrial mixer: physical
analysis and prediction ability) by P.O. Vandanjon,
M.-L. Gallenne and J. Terrière.
Ref.: Bulletin des Laboratoires des Ponts et Chaussées,
N° 208, September-October, 2000.
· For more information:
Division
for Sustainable Approaches in Civil Engineering
(DDGC) |
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