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Materials and Structures

, 24:409 | Cite as

Improved prediction model for time-dependent deformations of concrete: Part 2—Basic creep

  • Zdeněk P. Bažant
  • Joong-Koo Kim
Article

Abstract

The second part of this series presents the formulae for the prediction of basic creep of concrete, i.e. creep at no moisture exchange. The formulae give the secant uniaxial compliance function which depends on the stress level, and, as a special case, the compliance function for linear structural analysis according to the principle of superposition. The formulae are based on the recently developed solidification theory for concrete creep which takes into account simultaneous ageing, satisfies all the basic thermodynamic requirements, and avoids divergence of creep curves. The formulae, which describe both creep and elastic properties, involve only four free material parameters. All four appear linearly, so that optimum data fits can be obtained by linear regression. For the frequent situations where no test data for the particular concrete to be used are available, empirical formulae for predicting these four parameters from the concrete mix composition and the standard compressive strength are given. These formulae, however, involve considerable error. To avoid it, one should, whenever possible, carry out measurements of the elastic modulus and, if possible, also the short-time creep of 7 to 28 days duration. With such measurements, greatly improved predictions can be achieved. The predictions are compared with 17 extensive data sets taken from the literature, and the coefficients of variation of the deviations are found to be smaller than with previous models.

Keywords

Creep Curve Compliance Function Cement Type Basic Creep Axial Compressive Stress 
These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

Resume

Dans le second volet de cette série on présente les formules de prédiction du fluage de base du béton, c'est à dire du fluage en confinement. Les formules donnent la fonction sécante de compliance uniaxiale qui dépend du niveau de contrainte, et comme cas particulier, la fonction de compliance pour l'analyse structurelle linéaire suivant le principe de superposition. Les formules s'appuient sur une théorie de la solidification récemment établie pour le fluage du béton, qui prend en compte le vieillissement simultané, satisfait à toutes les exigences thermodynamiques de base, et évite la divergence des courbes de fluage. Les formules, qui décrivent aussi bien le fluage que les propriétés élastiques, comprennent seulement quatre paramètres indépendants du matériau libre. Les quatre apparaissent de façon linéaire, en sorte qu'on peut obtenir les ajustements de données optimaux par régression linéaire. Dans les situations fréquentes où l'on ne dispose pas de données d'essai pour un béton particulier à utiliser, on donne des formules empiriques de prédiction de ces quatre paramètres à partir de la composition du mélange de béton et de la résistance à la compression normale. Cependant, ces formules entraînent une erreur importante. Pour l'éviter, il convient, autant que possible, de réaliser des mesures du module d'élasticité et, si possible, du fluage à court terme d'une durée de 7 à 28 jours. Avec ces mesures, les prédictions se trouvent considérablement améliorées. On compare les prédictions avec 17 séries importantes de données prises dans la littérature, et on trouve que les coefficients de variation des déviations sont plus petites qu'avec les modèles précédents.

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Copyright information

© RILEM 1991

Authors and Affiliations

  • Zdeněk P. Bažant
    • 1
  • Joong-Koo Kim
    • 1
  1. 1.Center for Advanced Cement-Based MaterialsNorthwestern UniversityEvanstonUSA

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