دانلود رایگان مقاله تحولات زمان غیر پیری نسبت ویسکوالاستیک پواسون بتن و مفاهیم برای خزش C-S-H

ABSTRACT

The viscoelastic Poisson’s ratio of concrete is an essential parameter to study creep and loss of prestress in biaxially prestressed structures. Here we first aim to scrutinize the various existing definitions of this ratio. We then analyze all creep data of concrete available in literature that make it possible to compute the evolutions of this viscoelastic Poisson’s ratio, which, for mature concrete, is found to remain roughly constant or slightly decrease over time, such as to reach a long-term value always comprised between 0.15 and 0.2. Then, the long-term viscoelastic Poisson’s ratio of concrete is downscaled to the level of calcium silicate hydrates (noted C-S-H) with micromechanics. The long-term viscoelastic Poisson’s ratio of the C-SH gel is found to range between 0 and 0.2. Finally, the identification of this range is used to discuss various potential creep mechanisms at the level of the C-S-H particles.

6. Conclusions

We analyzed the long-term viscoelastic Poisson’s ratio of concrete from creep experiments from the literature. The results were used to compute the long-term viscoelastic Poisson’s ratio of C-S-H gel by downscaling with the help of elastic homogenization schemes extended to viscoelasticity. Several conclusions can be drawn. For what concerns creep of concrete, the analysis of all experimental results shows that: • The time-dependent behavior of concrete is isotropic, as expected from the theory of linear viscoelasticity. • The long-term creep of concrete is not only deviatoric, but also volumetric. • The long-term viscoelastic Poisson’s ratio of concrete is equal to or smaller than its elastic Poisson’s ratio, and comprised between 0.15 and 0.20. • When the elastic Poisson’s ratio of mature concrete is significantly greater than 0.20, the variation of its viscoelastic Poisson’s ratio over time is non-negligible. • When the elastic Poisson’s ratio of mature concrete is comprised between 0.15 and 0.20, for practical applications, considering that its viscoelastic Poisson’s ratio is constant over time, as proposed in particular by Bažant [2,40], is a very reasonable assumption. For what concerns downscaling of the viscoelastic Poisson’s ratio of concrete, if the aggregates, portlandite, calcium sulfoaluminates hydrates and clinker can be considered as spherical: • The long-term viscoelastic Poisson’s ratio of the C-S-H gel has little effect on the long-term viscoelastic Poisson’s ratio of concrete. • The interface between aggregates and cement paste can be considered adhesive for downscaling or upscaling the longterm viscoelastic Poisson’s ratio. • The interface between portlandite, calcium sulfoaluminates hydrates and clinker on one hand, and the mixture of C-S-H with the capillary porosity on the other hand, has little effect on the relation between the viscoelastic Poisson’s ratio of concrete and that of the C-S-H gel. For what concerns creep of the C-S-H gel, if we consider that the experimental data at the concrete scale are sufficiently reliable, downscaling of all experimental results obtained at the scale of concrete shows that: • The long-term viscoelastic Poisson’s ratio of the C-S-H gel is comprised between 0 and 0.2. • The long-term creep of C-S-H gel in concrete is both deviatoric and volumetric. • If creep of the C-S-H gel is due to creep of the C-S-H particles themselves, evolutions of the creep Poisson’s ratio observed experimentally cannot be explained if one considers that the CS-H particles are spherical and that they creep by sliding of its C-S-H layers over each other: either the C-S-H particles need to be considered aspherical, or the interlayer distance between neighboring C-S-H layers must be considered to vary in the long term. • If creep of the C-S-H gel is due to creep of the contact points between C-S-H particles, and if one considers that C-S-H particles are spherical, he/she cannot consider that C-S-H particles can only slide over each other: in the long term, the C-S-H particles must also be allowed to get closer to each other, i.e., to interpenetrate each other.