Résumé Nous examinons dans cette étude l'influence du temps de durcissement du mortier avant la mise en charge sur son comportement sous chargement harmonique monoaxial de faible fréquence. Les essais effectués ont porté sur une période de durcissement allant de 28 jours à 1 an, le nombre de cycles de chargement étant de 10 000. Nous avons suivi l'évolution des déformations respectivement parallèle et transversale à la direction de chargement. Nous avons comparé le comportement mécanique du mortier—module de déformation longitudinale, coefficient de Poison—avant et après le chargement harmonique pour les différents âges à la mise en charge. Enfin nous avons déterminé les variations volumiques de nos éprouvettes. A partir de l'ensemble de ces informations nous avons essayé de dégager des hypothèses sur les modifications structurales intervenant lors des cycles de chargement et, parallèlement, lors du durcissement du mortier avant la mise en charge.

Summary This investigation studies the influence of initial hardening of mortars at the time of loading on the mechanical behaviour of test pieces subjected to low-cycle fatigue. The test pieces are subjected to 10,000 cycles of monoaxial harmonic loading between a minimum of 25% of ultimate load and a maximum of 50% of ultimate load. Hardening times before loading are 28 days, 3 months, 6 months and 1 year. Research focused on the development during cycling of transverse and longitudinal deformation of the test piece from which were deduced the volume variations. Using quantitative analysis of test results, hypotheses are formulated on the structural modifications of mortars subjected to such types of loading. It becomes evident that these structural modifications include progressive closing of non-saturated pores and pressurizing of free pore water bringing about penetration into the capillaries and acceleration of discharge into surrounding air. The mortar development is therefore linked to the presence of free water and non-saturated pores. Deferred longitudinal and transverse deformations under the effect of cyclic loading decrease exponentially as a function of the mortar hardening time. The values tend towards a limit from the 12th month of hardening. Cycling also causes consolidation of the mortar which, from the mechanical point of view, corresponds to artificial ageing. So it is that, after being subjected to harmonic cycling, 28 day mortars show behaviour similar to that of mortars tested by direct loading after 1 year hardening: similar modulus of elasticity and Poisson ratio, etc. To conclude, at the level of loading and with the number of cycles previously defined, harmonic loading contributes to hardening mortar. The linearization of stress/strain curves, the increase in test piece stiffness tend to prove that the initial non-linearities of stress/strain curves are due essentially to a closing of porosity and not to microcracking.