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Concrete creep at variable humidity: constitutive law and mechanism

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Abstract

The previously formulated rate-type aging creep law based on Maxwell chain is generalized to variable humidity and is calibrated by extensive comparisons with test data from the literature. The main object of attention is the Pickett effect, i.e., the apparent increase in creep due to drying simultaneous with loading. This effect is shown to have four sources, in their decreasing order of importance: (1) stress-induced shrinkage, (2) tensile strain softening due to progressive cracking, (3) irreversibility of unloading contraction after tensile strainsoftening, and (4) increase of material stiffness due to aging (hydration). The model, which is a special case of a previously advanced thermodynamic theory, depends on only one hypothesis about the microscopic physical mechanism of creep: The creep rate depends on the magnitude of the flux of microdiffusion of water between the macropores (capillary pores) and the micropores in the cement gel. By assuming this microdiffusion to be infinitely fast, the effect is reduced to a dependence of creep viscosities on the time rate of pore humidity, and this is further shown to be equivalent to stress-induced shrinkage, in which the shrinkage coefficient defining the ratio of the increments of shrinkage strain and pore relative humidity depends on stress. In three dimensions, the shrinkage coefficient thus becomes a tensor. For thermodynamic reasons, there must also exist stress-induced thermal expansion. Although tensile cracking is found to make significant contribution to the Pickett effect, it is far from sufficient to explain in fully. The theory agrees with test data on basic creep, creep of specimens with reduced water content at hygral equilibrium (predried), shrinkage, swelling, and creep at drying under compression, tension, or bending. The strainsoftening model used for tensile cracking is the same as that used previously to fit test data from fracture tests, direct tensile tests, and deflection tests of reinforced beams.

Résumé

La loi de fluage de type différentiel d'après une chaîne de Maxwell et relative au béton en cours de vieillissement, qui avait été formulée précédemment, est généralisée à une humidité variable et vérifiée par de nombreuses comparaisons avec les données d'essai disponibles dans la littérature. On porte une attention particulière à l'effet de Pickett, c'est-à-dire l'augmentation apparente du fluage due à un séchage simultané avec le chargement. Il apparaît que cet effet a quatre origines qui sont les suivantes par ordre décroissant d'importance: (a) retrait causé par la contrainte; (b) adoucissement en traction due à une fissuration progressive; (c) irréversibilité de la contraction au déchargement après la diminution de la contrainte en traction, et (d) augmentation de la rigidité du matériau due au vieillissement (hydratation). Le modèle, qui est un cas particulier de la théorie thermodynamique précédemment avancée, repose sur une seule hypothèse concernant le mécanisme physique microscopique du fluage; le taux de fluage dépend de la grandeur du flux de microdiffusion de l'eau entre les macropores (pores capillaires) et les micropores du gel de ciment. En supposant que cette microdiffusion soit infiniment rapide, l'effet se réduit à ce que les viscosités du fluage dépendent d'un facteur temps de l'humidité interstitielle relative; on montre ensuite que ceci équivaut à un retrait provoqué par la contrainte, dans lequel le coefficient de fluage définissant le rapport des accroissements de fluage, déformation et humidité interstitielle relative dépend de la contrainte. En trois dimenstons le coefficient de retrait devient ainsi un tenseur. Pour des raisons thermodynamiques, il doit exister aussi une dilatation thermique causée par la contrainte. Bien que la fissuration en traction contribue notablement à l'effet de Pickett, elle est bien insuffisante à l'expliquer entièrement. La théorie est en accord avec les données d'essai du fluage de base, du fluage d'éprouvettes avec une teneur en eau limitée à l'équilibre hygrométrique (préséché), du retrait, du gonflement et du fluage au séchage en compression, traction ou flexion. Le modèle d'amollissement de la déformation utilisé pour la fissuration en traction est le même que celui utilisé précédemment pour ajuster les données d'essai à partir des essais de rupture, de traction directe et de fléchissement de poutres renforcées.

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Bazant, Z.P., Chern, J.C. Concrete creep at variable humidity: constitutive law and mechanism. Materials and Structures 18, 1–20 (1985). https://doi.org/10.1007/BF02473360

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