Fluage en traction des bétons ordinaires et des bétons légers

Résumé

Nous nous proposons d'étudier l'influence du temps de chargement sur les propriétés mécaniques d'un microbéton de calcite et un béton d'argile expansée soumis à des efforts de traction.

La mesure des déformations totales et résiduelles montre que l'allongement des éprouvettes sous la charge de traction est un phénomène irréversible qui provoque une modification des caractéristiques mécaniques des matériaux.

Après fluage, on observe une légère consolidation du matériau qui se traduit par une augmentation du module de déformation longitudinale et une meilleure réversibilité du béton.

Summary

Determination of mechanical properties of concretes subjected to tensile loading constant in time. The first research phase consisted of studying the deformations of calcite and expanded clay microconcrete test pieces. By measuring with strain gauges the elongation of the test pieces along the casting axis, it was possible to determine the modulus of longitudinal deformation, the degree of reversibility and the failure of materials under tensile load. It then becomes obvious that the concrete behaviour is practically linear and brittle, not being reversible even for very small loads. This seems due to the presence of microcracks and micropores existing in the material before application of loads.

When these tensile loads (=25 and 50% failure) are kept constant in time, the total deformation (under loading) and the residual deformation (after unloading) are seen to vary in a quite identical way and they tend towards a limit after about 60 days. Observation shows that creep deformation is practically equal to residual deformation. The tensile load has therefore produced irreversible elongation over time. This phenomenon arises from the redistribution of internal stresses which are modified by water migration in the material.

After creep a modification of concrete mechanical properties is observed. Indeed, once the creep deformation stabilized and after unloading of samples, resuming testing reveals an increase of modulus of longitudinal deformation and the reversibility of the materials. Tensile creep has therefore brought about concrete consolidation with respect to tensile deformation. On the other hand the stresses characterizing the failure of the test piece are practically not modified.

Creep deformation directly linked to water movements in the concrete are more extensive in lightweight aggregate concretes because their pore structure is more pronounced.