Évolution du comportement rhéologique d'une pâte de ciment fraîche suivant la distance à une source vibrante

Résumé

L'efficacité d'une vibration dans une pâte de ciment fraîche est avant tout liée à l'amplitude communiquée aux grains par la source. Il faut en effet qu'elle soit suffisamment grande pour que les forces de liaisons existant entre les grains puissent être diminuées notablement, entraînant ainsi une meilleure fluidité.

Par contre, la fréquence semble être le paramètre essentiel régissant la propagation de la vibration; plus elle est élevée, plus l'amortissement est faible.

Pour un vibrateur de puissance donnée, l'augmentation de l'amplitude entraîne la diminution de la fréquence; l'amélioration de l'efficacité se fait donc au détriment du rayon d'action.

Summary

During placing, fresh concrete should retain flowability whilst remaining homogeneous. To ensure this, casting is usually carried out under vibration.

In order to determine the parameters which improve the efficiency of a poker vibrator, the authors have undertaken investigations on the rheological behaviour of cement pastes and following their development not only as affected by the distance from the vibrating source but also the type of vibration. Guided by practical requirements, they sought essentially to define the areas where the behaviour is made exclusively viscous by vibrating (yield value suppressed) and those where vibration is ineffective since the material remains viscoplastic (with a yield value).

After describing the test apparatus and method used, the paper shows that there is never really a sudden change in the rheological behaviour of cement pastes, so the existence of any shear threshold linked to measuring equipment sensitivity becomes an arbitrary concept. The efficiency of a vibrator is therefore defined conventionally starting from the moment when the yield stress can be considered no longer negligible in the range of normal practical applications.

Vibration efficiency in fresh cement paste is known to be above all related to the amplitude the source exerts on the grains. It must be sufficient especially to lower the forces of bonding between the grains significantly, in this way ensuring improved flowability.

On the other hand, frequency seems to be the essential parameter governing the propagation of vibration; the higher the frequency, the lower the damping.

For a vibrator of given output, increase in amplitude implies lowering of frequency. Any improvement in efficiency is therefore achieved to the detriment of the radius of action.

This paper helps to define areas of efficiency but the development of material rheological properties within within these areas is not dealt with. This will be the subject of a forthcoming article.