Influence de chargements cycliques de faible fréquence sur le comportement mécanique d'éprouvettes de mortier

Résumé

Nous examinons le comportement du mortier soumis à des cycles de chargement de faible fréquence. Ceux-ci sont plus représentatifs des sollicitations que subit une structure que les essais habituels de fatigue.

Dans cette étude le chargement eset monoaxial et le suivi des déformations longitudinales et transversales nous permet de définir le comportement rhéologique du mortier tant durant l'applicátion du chargement harmonique qu'après sa cessation.

Nous déterminons les modifications dans le comportement mécanique du mortier que l'application du chargement cyclique entraîne.

Enfin, nous comparons l'évolution du mortier soumis à des cycles de chargement avec celle obtenue par un chargement stationnaire.

Summary

This research aims at defining the typical behaviour of a mortar subjected to a reference cyclic loading and comparing it with behaviour under constant load.

Testing is carried out under monoaxial compression and low-cycle harmonic loading: 2 cycles per minute. Compared with routine fatigue testing, these conditions are more representative of load effects normally applied to structures. The minimum and maximum bounds of harmonic loading are respectively equal to 25% and 50% of ultimate load. The reference loading comprises 10 000 cycles.

Studying the rheological behaviour of mortar subjected to harmonic loading has shown that at the beginning of cyclic loading the development of longitudinal deformation is twice as pronounced as that of transverse deformation. This difference tends to lessen when the number of cycles increases. The structural explanation of this phenomenon is linked to the progressive closing of free pores and the pressurizing of pore water. For a 6-month mortar the overall development of longitudinal deformation after the cycles is 17% of instantaneous deformation. Recovery stabilizes at 6% after 300 hours rest.

After cycling we found that the mortar had undergone consolidation leading to linearisation of stress/strain curves, apparent shakedown, an increase in the modulus of longitudinal deformation and test piece volume decrease.

Comparing the harmonic loading test and the constant load test underlines a much more pronounced development of longitudinal deformation in the latter and inversely a slighter deformation of transverse deformation. The loss of volume during creep is therefore greater than during cyclic loading of the same duration.