Comportement mécanique du mortier soumis à des chargements harmoniques à fréquence faible

Résumé

Cette étude s'inscrit dans le cadre de travaux ayant pour objectif de mieux connaître le comportement des mortiers et bétons sous l'effet de chargements harmoniques monoaxiaux de faible fréquence([1] et[2]).

Nous abordons ici l'influence des caractéristiques des chargement cycliques. Nous analysons successivement l'influence d'une variation de l'amplitude du chargement harmonique pour une même charge maximale puis celle de la variation de cette dernière.

Les essais comportent un nombre de cycles de chargement égal à10 000. La borne maximale des chargements cycliques varie entre12,5 et90% de la charge de rupture sous essai direct.

Nous avons suivi l'évolution, sous l'effet des cycles de chargement, des déformations longitudinales et transversales. Nous avons pu ainsi comparer les variations des diverses caractéristiques mécaniques du mortier: module de déformation longitudinale, coefficient de Poisson, degré de réversibilité, etc.

A partir de l'ensemble de ces informations nous avons émis des hypothèses sur l'évolution structurale des mortiers soumis à des cycles de chargement de faible fréquence d'intensité variable et sur une prévision de leur comportement sous un plus grand nombre de cycles.

Summary

This study deals with the influence of characteristics of cycles on the behaviour of mortars and concretes subjected to repeated low frequency harmonic loading in monoaxial compression.

The number of cycles used throughout the set of tests was10,000 the loading cycle lasted30 seconds.

We first examined the influence of loading amplitude for an identical maximum cyclic load. Two tests were carried out under harmonic loading with respectively maximum and minimum bounds of0 and80% and40 and80% ultimate load under direct testing. Experimental results showed that, provided the loading amplitude is non negligible, the differences of amplitude for identical maximum cyclic loads have only a slight influence.

We then studied the changes in mechanical behaviour of mortar subjected to cycles where the intensity of the maximum cyclic load is different. We took successively as maximum bound in the cyclic loading12.5, 25, 50, 60, 80 and90% of the ultimate load under direct testing.

This experimental study made it possible to identify a change in behaviour of mortar for harmonic loads whose maximum value reaches50–60% of the ultimate load. Beyond this limit we can consider that the effect of cycles is on the whole positive in compression. Indeed, the increase of deformation during cycling remains moderate, the rate of development decreasing as a function of the number of cycles. Moreover the modulus of longitudinal deformation increases and the Poisson ratio remains stable, therefore the material is consolidated.

Beyond this limit the development of the longitudinal deformation as a function of the maximum cyclic load is very rapid and the damping of the increase in deformation as the cyclic load is applied makes itself felt much more faintly. The consolidation of the mortar ceases to increase as a function of the maximum load applied. All this gives ground for questioning the durability of mortars loaded by a higher number of cycles with these characteristics. Indeed, in the case of the10 000 cycles carried out, for loads above80% of the ultimate load, the mortar behaviour deteriorates, bringing about failure after900 loading cycles in the case of the test specimen subjected to cycles of maximum load equal to90% of the ultimate load.

The study closes with some assumptions on the structural modifications in the mortar as a function of the intensity of the cyclic loading to which it was subjected.