Is concrete a poromechanics material? - a multiscale investigation of poroelastic properties

Abstract

There is an ongoing debate, in Concrete Science and Engineering, whether cementitious materials can be viewed as poromechanics materials in the sense of the porous media theory. The reason for this debate is that a main part of the porosity of these materials manifests itself at a scale where the water phase cannot be considered as a bulk water phase, but as structural water; in contrast to water in the gel porosity and the capillary porosity. The focus of this paper is two-fold: (1) to review the microstructure of cementitious materials in the light of microporomechanics theory by starting at the scale where physical chemistry meets mechanics, and which became recently accessible to mechanical testing (nanoindentation); (2) to provide estimates of the poroelastic properties (drained and undrained stiffness, Biot coefficient, Biot modulus, Skempton coefficient) of cementitious materials (cement paste, mortar and concrete) by means of advanced homogenization techniques of microporomechanics. This combined experimental-theoretical microporomechanics approach allows us to deliver a blueprint of the elementary poroelastic properties of all cementitious materials, which do not change from one cementitious material to another, but which are intrinsic properties. These properties result from the intrinsic gel porosity of low density and high density C-S-H, which yield a base Biot coefficient of 0.61 < b ≤ 0.71 and a Skempton coefficient of B = 0.20 - 0.25. While the base Biot coefficient decreases gradually at larger scales, because of the addition of non-porous solid phases (Portlandite, ..., aggregates), it is shown that the Skempton coefficient is almost constant over 3-5 orders of magnitude.