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Researchers at
the "Laboratoire d'analyse des interfaces et de nanophysique"
(LAIN, the Nanophysics and Interface Analysis Laboratory, CNRS-University
of Montpellier 2) have developed a simple, inexpensive method for
monitoring the rheological properties of mortar and concrete, based
on an acoustic near field sensor which is sensitive to changes in
viscosity and yield stress.
Previous ultrasound techniques for monitoring setting in mortar and
concrete, a crucial criterion when transporting and using such materials,
were based on measuring the longitudinal and transverse velocity of
acoustic waves using echographic methods. They are accurate and reliable
but unwieldy and are limited to the elastic region, i.e., post-setting.
However, the rheology of concrete and mortar prior to setting, i.e.,
in the viscoelastic region, is of vital importance, particularly when
adding plasticizers to allow the concrete to be transported.
LAIN's new acoustic near field sensor technique can continuously monitor
rheological properties from the liquid to the solid state. It monitors
changes in the resonance conditions for an acoustic horn or sonotrode
as the rheological properties of the medium in which it is immersed
change; the measurements are highly sensitive to changes in the viscosity
of the medium and to the yield stress.
The sensitivity of the method can be changed using different sensor
geometries. The role of plasticizers and the false set phenomenon
can be studied, while the set itself can be accurately characterized,
along with maturity phenomena.
The method can be applied to studying liquid to solid transitions
such as polymer cross linking, adhesive setting, paint or ink drying,
and varnish hardening. The tiny amplitudes involved, a few tens of
nanometers, also means that phenomena occurring in fragile media such
as gels or inorganic sol-gels can be monitored.
References
:
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B.
Cros, J. Y. Ferrandis. Characterization of the cement setting by
acoustic near field, Sensors & Actuators B: Chemical,
60 (1999), pp. 200-207.
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B.
Cros, J. Y. Ferrandis, M. Pauthe. Étude de la mise en ¦uvre
des polymères par champ proche acoustique. La Revue de
Métallurgie - Science et Génie des Matériaux,
96 (1999), pp. 1537-1545.
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