Creep of concrete under multiaxial compressive stresses
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AbstractThe object of this investigation was to study the creep behaviour of concrete under multiaxial compressive stresses, for obtaining certain basic information regarding (i) the fundamental nature or creep as manifested in its behaviour under combined stresses, (ii) the validity or otherwise of the concept of superposition of creep strains under multiaxial states of stress. First, the mechanism of creep in hydrated cement paste is studied, since it is believed to determine the creep behaviour under complex states of stress. It is suggested that creep mechanism involves 'oriented’ and 'random' internal moisture diffusion as well as viscous deformation of adsorbed water and that the elastic skeleton of gel structure plays a significant role. Based on these concepts and supporting experimental data, the influence of multiaxial compressive stresses on creep is deduced. As a second step, creep mechanism in concrete, as influenced by the aggregate-matrix interaction, is studied analytically with the help of a two-phase structural model of concrete. It is shown that the interaction of the constituent phases of concrete, possessing dissimilar mechanical properties, during creep, results in secondary stresses in the lateral direction, under uniaxial compression. It is shown that the state of stress in the specimen, through its effect on the secondary stresses and secondary creep, mentioned above, influences the magnitude of Poisson creep in any principal direction, depending on whether the net strain in that direction is extension or contraction, and thus vitiates superposition of creep strains. For the experimental study a testing technique, incorporating some new features, is developed for studying creep under multiaxial states of stress, in which all the three principal stresses are independently controlled and hence can be different magnitudes. The efficacy of the testing technique is demonstrated by proving trials. The creep tests were conducted, under negligible moisture exchange by maintaining reasonable equilibrium between the internal and external humidities of the specimen, at 98± 1% R.H. A total of thirteen multiaxial (biaxial and triaxial) tests were conducted using a companion specimen under uniaxial compression in each test. The only variables were the relative magnitudes of principal stresses. The results, in general, point to the fact that creep strains in concrete are not superposable, under all states of stress. The parameters influencing the creep and creep recovery of concrete under multiaxial stresses are established and creep Poisson's ratio is expressed as a function of the principal stresses. Explanation and theoretical justification for the observed behaviour is offered on the basis of hypothetical study of creep mechanism in cement paste and the analytical work on the aggregate-matrix interaction. Based on this, a rational correlation between creep of concrete under uniaxial and multiaxial compressive stresses is developed which incorporates the influence of the state of stress on Poisson creep.
Bibliography: p. 301-307.