The deformation and the fracture of porous solids from internal crystallization of salt is explored in the framework of the thermodynamics of unsaturated brittle poroelasticity. In the first place ...the usual theory of crystal growth in confined conditions is further developed in order to include both the deformation and the drying of the porous solid. The thermodynamics reveals the existence of a dilation coefficient associated with the crystallization process, and provides a solute–crystal equilibrium condition which involves the relative humidity, the supersaturation, and the salt characteristics. This thermodynamic condition and the mechanical equilibrium of the solution–crystal interface combine to give the current crystallization pore radius. Upscaling this information at the macroscopic scale, and taking into account the salt mass supplied by the invading solution, the approach leads to a quantitative analysis of the role of the pore size distribution on the crystal growth under repeated imbibition–drying cycles. The deformation and the fracture of the porous solid from drying-induced crystallization are then considered in the context of brittle poroelasticity. The current unsaturated macroscopic poroelastic properties are upscaled from the microscopic elastic properties of the solid matrix and from the current liquid, crystal and gas saturations. The adoption of a fracture criterion based on the elastic energy that the solid matrix can ultimately store finally leads to the determination of how long a stone can resist repeated cycles of drying-induced crystallization of salt.
Enhanced coal bed methane recovery (ECBM) consists in injecting carbon dioxide in coal bed methane reservoirs in order to facilitate the recovery of the methane. The injected carbon dioxide gets ...adsorbed at the surface of the coal pores, which causes the coal to swell. This swelling in confined conditions leads to a closure of the coal reservoir cleat system, which hinders further injection. In this work we provide a comprehensive framework to calculate the macroscopic strains induced by adsorption in a porous medium from the molecular level. Using a thermodynamic approach we extend the realm of poromechanics to surface energy and surface stress. We then focus on how the surface stress is modified by adsorption and on how to estimate adsorption behavior with molecular simulations. The developed framework is here applied to the specific case of the swelling of CO2-injected coal, although it is relevant to any problem in which adsorption in a porous medium causes strains.
In this paper some aspects of equilibrium and transfer moisture properties of high-performance materials are presented and compared with ordinary cement pastes and concretes. First, the equilibrium ...moisture properties of the hardened materials are described by means of water vapour sorption isotherms, which illustrate the hysteretical behaviour of the materials. Experimental results of drying shrinkage versus relative humidity (RH) are also reported here. These experimental data are in good agreement with the numerical results provided by a thermodynamic modelling based on capillary stresses and hygromechanical couplings. In particular the linearity of the strains-RH curve over a wide range is pointed out in both cases. Isothermal drying process at RH = 50% has experimentally and numerically been studied. After identification of the intrinsic permeability of the materials from experimental weight losses, numerical moisture profiles were compared with gamma-ray attenuation measurements. The influence of the initial moisture state of the materials that results from self-desiccation in particular was pointed out on the evolution of the moisture profiles as a function of time.
Poromechanics of freezing materials Coussy, Olivier
Journal of the mechanics and physics of solids,
08/2005, Volume:
53, Issue:
8
Journal Article
Peer reviewed
When subjected to a uniform cooling below the freezing point a water-infiltrated porous material undergoes a cryo-deformation resulting from various combined actions: (i) the difference of density ...between the liquid water and the ice crystal, which results in the initial build-up of an in-pore pressure at the onset of crystallization; (ii) the interfacial effects arising between the different constituents, which eventually govern the crystallization process in connection with the pore access radius distribution; (iii) the drainage of the liquid water expelled from the freezing sites towards the air voids; (iv) the cryo-suction process, which drives liquid water towards the already frozen pores as the temperature further decreases; (v) the thermomechanical coupling between the solid matrix, the liquid water and the ice crystal. We work out a comprehensive theory able to encompass this whole set of actions. A macroscopic approach first provides the constitutive equations of freezing poroelastic materials, including the interfacial energy effects. This approach reveals the existence of a thermodynamic state function—namely the liquid saturation degree as a function of the temperature only. The macroscopic ice-dependent poroelastic properties are then upscaled from the knowledge of the elastic properties of the solid matrix, of the pore access radius distribution, and of the capillary curve. The theory is finally illustrated by analysing quantitatively the effects of the cooling rate and of the pore radius distribution upon the cryo-deformation of water-infiltrated porous materials. The theory succeeds in accounting for the experimentally observed shrinkage of embedded air voids, while predicting the partial melting of the ice already formed when the cooling suddenly stops.
Modeling of the drying of porous materials is often approached by assuming that the gas pressure of the vapor-air mixture remains constantly equal to the outer atmospheric pressure. Use of both ...experimental and theoretical results reveals that such an assumption is inadequate to account for the weight loss observed during the drying of weakly permeable materials. For such materials, the gas pressure cannot remain constant because no significant convective Darcean transport of the gas can actually occur. In contrast, the evaporation coupled with the diffusion of dry air generates a gas overpressure that propagates within the material and works actively toward a uniform vapor molar concentration. As a consequence, the diffusion of vapor becomes rapidly nonactive as a driving force of moisture transport. Paradoxically, the drying of weakly permeable materials is eventually achieved by the transport of moisture in its liquid form and its evaporation at the sample boundary in contact with the surrounding air. The analysis is carried out through a modeling of which the formulation is based upon macroscopic thermodynamic considerations. It involves a dry-air component and a water component, the liquid and the water-vapor phases being addressed separately.
Thermodynamics of open continua, when applied directly at the macroscopic engineering scale, allows one to extend unambiguously the principal concepts of continuum thermomechanics of solids to ...polyphasic porous materials, whose fluid constituents are subject to liquid-vapor phase change. This approach provides a consistent and relevant framework for the formulation of the constitutive equations of partially saturated deformable porous materials such as concrete. After recalling the general theory, the modeling is specified for porous materials that are partially saturated by a liquid water phase changing into its vapor phase, the latter forming with the dry air an ideal mixture. The nonlinear poroelasticity, as reference modeling, is then detailed. The theory is applied to the modeling of the drying shrinkage of concrete samples with isothermal sorption curves as only experimental data. The predicted shrinkage is found to be in close agreement with the observed one for a range of humidity greater than 50%. The drying shrinkage of a wall is finally examined. The analysis includes the study of the temperature variations due to latent heat effects. These variations are shown to be negligible.
A capacitive sensor-based experimental approach is worked out to study the ice/water phase change in cohesive porous media subject to freezing and thawing. This technique relies upon the dielectric ...properties of liquid water, ice, air, and mineral substrate in the radio-frequency range. A semi-empirical method based upon the Lichtenecker model and combining drying and freezing tests provides an accurate estimation of the liquid water content versus the temperature in freezing cement pastes. This estimation is further analysed with the help of thermoporometry concepts in order to characterize the pore size distribution and the specific surface area. The results range in the same order of magnitude as those assessed from gravimetric sorption/desorption isotherms.
The critical nature of the alkali-silica reaction (ASR) on premature concrete deterioration requires the quantitative assessment, in time and space, of the chemomechanical impact of ASR expansion on ...the dimensional stability of concrete structures. In particular, the coupled problem of heat diffusion and ASR kinetics can be critical, as the ASR is a thermoactivated chemical reaction. The quantitative analysis of these coupled effects on both material and structural level is the main objective of this paper. Starting from the governing micromechanisms of ASR expansion, a chemoelastic model is developed that accounts for ASR kinetics and the swelling pressure exerted by the ASR reaction products on the skeleton. This chemoelastic model is a first-order engineering approach to capture timescale and magnitude of ASR expansion. It is shown that the realistic prediction of ASR structural effects requires the consideration of two timescales: (a) A latency time associated with the dissolution of reactive silica; and (2) a characteristic time associated with the ASR product formation. In addition, a dimensional analysis of the governing equations reveals that the ASR deterioration of "massive" concrete structures is driven by the simultaneous activation of heat diffusion and reaction kinetics within a surface layer defined by a characteristic ASR heat diffusion length. In turn, in "slender" structures, it is the simultaneous activation of moisture diffusion and ASR kinetics that drives the surface layer delamination. This is illustrated through finite-element case studies of ASR effects in structures of different dimensions: a concrete gravity dam and a bridge box girder.
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