J. Marchand

Calculation of ionic diffusion coefficients on the basis of migration test results

Migration tests are now commonly used to estimate the diffusion coefficients of cement-based materials. Over the past decade, various approaches have been proposed to analyze migration test results. In many cases, the interpretation of test data is based on a series of simplifying assumptions. However, a thorough analysis of the various transport mechanisms that take place during a migration experiment suggests that some of them are probably not valid. Consequently, a more rigorous approach to analyze migration test results is presented. The test procedure is relatively simple and consists in measuring the evolution of the electrical current passing through the sample. Experimental results are then analyzed using the extended Nernst-Planck-Poisson set of equations. A simple algorithm is used to determine for each experiment the tortuosity factor that allows to best reproduce the current curve measured experimentally. The main advantage of this approach resides in the fact that the diffusion coefficients of all ionic species present in the system can be calculated using a single series of data. Typical examples of the application of this method are given. Results indicate that the diffusion coefficients calculated using this approach are independent of the applied voltage and depends only slightly on the concentration level and the chemical make-up of the upstream cell solution.RésuméLes essais de migration sont maintenant couramment utilisés pour estimer les coefficients de diffusion des matériaux cimentaires. Récemment, différentes approches ont été proposées pour analyser les résultats de l’essai de migration. Dans la plupart des cas, l’analyse des mesures est basée sur une série d’hypothèses simplificatrices. Cependant, une étude détaillée des mécanismes de transport des ions présents durant l’essai de migration révèle que certaines de ces hypothèses sont probablement incorrectes. Une approche plus rigoureuse de l’analyse des résultats de l’essai de migration est donc présentée. La méthode consiste à mesurer les courants électriques traversant l’échantillon durant l’essai. Ces résultats sont ensuite analysés à l’aide du système d’équations Nernst-Planck—Poisson. Un algorithme numérique permet de trouver pour chaque essai le facteur de tortuosité permettant de reproduire au mieux la courbe de courant mesurée expérimentalement. L’avantage principal de cette méthode est qu’elle permet de calculer le coefficient de diffusion de chacune des espèces ioniques présente dans le matériau sur la base de cette seule mesure de courant. Des exemples d’utilisation de la méthode sont décrits. Les résultats montrent que les coefficients de diffusion évalués selon cette approche sont indépendants du voltage appliqué au cour de l’essai et qu’ils ne dépendent que très légèrement du niveau de concentration et du type de solution utilisé dans le bac amont du montage.

Influence of cracking on the diffusion properties of cement-based materials: Part I: Influence of continuous cracks on the steady-state regime

The influence of traversing cracks on the steady-state diffusion properties of concrete was studied. The effect of both anisotropic and isotropic crack networks was first theoretically assessed using an analytical approach. To simplify the transport equations, cracks were assumed to be of uniform size and evenly distributed on a one- or two-dimensional grid. Results of the theoretical analysis were then compared to experimental data. Both series of results indicate that cracking can markedly alter the diffusion properties of the material and favor the penetration (or the leaching) of drifting species. A simple method to predict the effect of cracking on the concrete diffusivity is proposed. Predictions are made on the basis of two parameters: the crack density and the mean crack aperture. This method can provide a first estimate of the diffusion properties of severely damaged concrete elements.

Influence of the interfacial zone on the chloride diffusivity of mortars

Three different series of mortars with variable sand volume contents were cast in two different laboratories to study the influence of the interfacial transition zones (ITZ) on the transport coefficient of chloride ions. The first series was prepared and tested at Laboratoire Materiaux et Durabilite des Constructions (LMDC, Toulouse, France). The water/cement (w/c) ratio was 0.38 and the sand volume fractions were 0, 19, and 57%. The transport properties were investigated using a diffusion test. The two other series were prepared and tested at Centre de recherche interuniversitaire sur le beton (CRIB, Quebec, Canada). The w/c ratios were 0.25 and 0.45 and the sand volume fractions were 0, 30, and 50%. The transport properties were assessed using a migration test. The test results indicate that aggregates modify the microstructure and the transport properties of mortars. The transport coefficient of chloride ions was found to decrease with an increase of the sand volume fraction. At the same time, the transport coefficient of the corresponding paste fraction was found to increase. The increased tortuosity of the matrix induced by the presence of aggregates thus appears to be more important than the influence of ITZ. The interconnection of ITZ was not found to lead to a rapid increase of the chloride ion transport coefficient.

Chloride binding capacity of various hydrated cement paste systems

Abstract The interaction mechanisms of the cement paste hydration products with chloride were investigated for various systems. Test parameters included water/binder ratio (0.25 and 0.45), type of cement (ASTM type I, III, and V), use of silica fume (6%), and chemical composition of the chloride solution. Powdered cement paste samples were immersed in solutions of different chloride concentrations. The total amount of bound chlorides was determined by measuring the equilibrium chloride concentration of the solution after 3 weeks of immersion. The chemical interaction of chlorides with the hydrated cement paste was also studied by X-ray diffratometry. Test results show that the total amount of bound chlorides increases with the chloride concentration of the solution. This nonlinear relationship can be represented best by a Freundlich isotherm. When expressed on a unit mass of cement gel basis, the total amount of bound chlorides was found to be independent of the water/binder ratio and the type of binder in all cases but one. The chloride binding capacity and the formation of chloroaluminates appear to be attributable not only to the tricalcium aluminate content but also to the total aluminate content of the cement. The use of calcium chloride instead of sodium chloride, as well as the use of a lime solution instead of an alkaline solution, increases the amount of bound chlorides.

Influence of curing temperature on cement hydration and mechanical strength development of fly ash mortars

Abstract The influence of fly ash and curing temperature on cement hydration and compressive strength development of mortars was investigated. Test parameters included type of fly ash (two different Class F fly ashes were tested), the level of cement replacement (10, 20 and 30% by mass), and curing temperature (20°C and 40°C). The mortar physical and microstructural properties were determined by means of thermal analyses, compressive strength measurements and SEM observations. Test results confirm that fly ash tends to increase significantly the rate of cement hydration at early age. Data also demonstrate that an elevation of the curing temperature reduces the long-term compressive strength of the reference mortar mixture. In contrast, an increase of the curing temperature seems to have no detrimental effect on the long-term compressive strength of the fly ash mixtures.

Modeling chemical activity effects in strong ionic solutions

A new simple mathematical model for calculating the chemical activity coefficients of ions in electrolytic solutions is presented. The model was developed to account for the particular behavior of concentrated solutions for which short-range and long-range interactions between ions are important. The new model is essentially a modified version of the Davies equation. Given its simple mathematical form, it can be easily implemented in a numerical code aimed at modeling ionic transport phenomena in saturated porous materials. The transport equations of charged particles in concentrated solutions are also presented. Numerical results are shown and compared with experimental results. Applications of the model to ionic diffusion problems in saturated porous media are also shown.

Sem observations of the microstructure of frost deteriorated and self-healed concretes

Abstract The microcracking and self healing mechanisms of concrete exposed to rapid freezing and thawing in water and subsequently kept in water have been investigated by Scanning Electron Microscopy (SEM). Non air entrained concretes of water binder ratio 0.40 with 0 and 5 % silica fume were studied. Damage was measured as loss in resonance frequency and compressive strength. After frost exposure, concrete beams were stored three months in water. During this time resonance frequency largely recovered, whereas compressive strength showed smaller recovery. On Secondary Electron Images (SEI) of fractured surfaces hydration products mainly of the C-S-H type were seen traversing cracks at several locations after self healing, but not directly after freeze/thaw. Back Scattered Electron Images (BSEI) showed that the cracks due to freeze/thaw testing were of 1–10 μm width. The cracks traversed the paste and followed the interfaces of most larger aggregate particles. On BSEI self healing was seen on 300–1000 X magnification as partly closing of several cracks smaller than 5 μm. This was most clearly seen by switching between SEI and BSEI modes. In BSEI-mode the re-hydration products appeared less dense and the cracks appeared wider than in the SEI-mode.

Modeling the behavior of unsaturated cement systems exposed to aggressive chemical environments

The main features of a numerical model that predicts the mechanisms of ionic transport in unsaturated cement systems are described. The model, called STADIUM, is divided into four parts: ionic diffusion, moisture transport, chemical reactions, and chemical damage. The diffusion of all ions present in the system is modeled by solving the extended Nernst-Planck/Poisson set of equations. The model takes into account the electrical coupling between the various ionic species in solution and chemical activity effects. The transport of water by capillary suction is described by a diffusion equation. In the model, the transport phenomena and the chemical reactions are uncoupled. Phase dissolution and precipitation phenomena are modeled through a separate chemical equilibrium code. The model considers the influence of chemically induced microstructural alterations on the transport properties of the material. A brief description of the model is presented in the first sections of this paper. Typical applications of the model are given in the last section.RésuméCe document présente les principales caractéristiques d’un modèle numérique permettant de prédire le comportement de matériaux cimentaires exposés dans des conditions non saturées à des environements chimiquement agressifs. Le modèle est divisé en quatre parties traitant respectivement du transport d’ions par diffusion, du transport de l’eau, des réactions chimiques et de l’endommagement chimique. La diffusion des ions est décrite en résolvant l’équation de Nernst-Planck étendue. Le modèle prend en considération les effets reliés à l’activité chimique des ions en solution et le couplage électrique entre les différents flux ioniques. Le transport de l’eau par succion capillaire est décrit par une équation de diffusion. Les phénomènes de dissolution/précipitation sont modélisés en utilisant un module d’équilibre chimique. Les effets des réactions chimiques sur la structure poreuse du matériau et ses propriétés de transport sont également considérés. Une brève description du modèle est donnée dans la première partie du document. Des exemples d’application du modèle sont présentés dans la seconde partie.

FROST RESISTANT CONCRETE

Abstract There are two basic frost durability problems: internal cracking due to freezing and thawing cycles, and surface scaling, generally due to freezing in the presence of deicer salts. Although there are still parts of the problem which are not perfectly well understood and warrant further investigation, particularly with respect to the differences between laboratory tests and field exposure, the way to make concrete resistant to freezing and thawing cycles is very well known. It is simply to ensure that the hardened concrete has an adequate system of entrained air voids. Field experience as well as laboratory data has shown very conclusively that internal cracking due to frost in properly air-entrained concretes is almost non-existent. Scaling due to freezing in the presence of deicer salts is a much more complex problem than internal cracking for many reasons, but probably mainly because it is related to the microstructure of the very surface layer or ‘skin’ of concrete. Properly air-entrained and properly cured well-designed Portland cement field concretes are generally quite resistant to deicer salt scaling, but scaling still sometimes occurs unexpectedly after only a few years. Research in this area is therefore required. The ability of the commonly used deicer salt scaling tests to predict the performance of concrete under normal field exposure conditions must also be particularly investigated. In addition, research is needed to better understand the process of the formation of large air voids in air-entrained concrete, since the dosage of air-entraining admixtures is based on the total volume of air in the mixture and small dosages that yield an adequate air volume often do not yield an adequate air void spacing factor (and thus adequate frost protection).

Describing ion diffusion mechanisms in cement-based materials using the homogenization technique

The application of the homogenization technique to the mathematical description of the diffusion mechanisms in saturated cement-based materials is discussed. According to this approach, the transport and mass conservation equations are first written at the microscopic scale to describe the movement of particles in the fluid phase of the material. These equations are then averaged over the entire volume of the material. An example of the application of the homogenization technique is given. The homogenization technique is used to describe the diffusion of ions in cement-based systems. The various equations are written to consider the charged nature of the ionic particles and the coupling between the various ionic fluxes. The numerous advantages of this technique for the modeling of mass transport mechanisms in cement-based materials are discussed.