Lars-Olof Nilsson

CHLORIDE BINDING CAPACITY AND BINDING ISOTHERMS OF OPC PASTES AND MORTARS

A method for evaluating chloride binding capacity of concrete was proposed based on the adsorption from solution and chloride binding isotherms of OPC pastes and mortars with water-cement ratio 0.4, 0.6 and 0.8. The experimental results show that the chloride binding capacity of concrete strongly depends on the content of CSH gel in the concrete, regardless of the water-cement radio and the addition of aggregate. The relationship between the bound chloride and the free chloride can be described by Freundlich isotherm at high free chloride concentrations and Langmuir isotherm at low free chloride concentrations. (11 refs.)

Numerical simulation of multi-species transport through saturated concrete during a migration test - MsDiff code

A numerical model, called MsDiff (Multi-Species DIFFusion), based on a finite difference method and on the Nernst-Planck relation has been developed. The concentration profiles for several species (i.e., Cl-, Na+, K+ and OH-), the flux and the potential profile are computed. The simulations allow having a better understanding about the phenomena involved during the transport of different ions into concrete under the influence of an electrical field. The effect of various parameters as the pore solution composition, the non-ideality of the solutions or the chloride binding, is also discussed. Finally, it is shown how the effective diffusion coefficient of chlorides could be measured experimentally by using the LMDC test method.

Resistance of concrete to chloride ingress: Testing and modelling

Chloride ingress in reinforced concrete induces corrosion and consequent spilling and structural weakness, and it occurs world-wide and imposes an enormous cost. Yet it can be resisted by using test methods and relevant models for service life prediction. Resistance of Concrete to Chloride Ingress sets out current understanding of chloride transport mechanisms, test methods and prediction models. It describes basic mechanisms and theories, and classifies the commonly used parameters and their units which expressing chloride and its transport properties in concrete. Laboratory test methods and in-field applicable test methods, including precision results from inter-laboratory comparison tests, are then outlined. Some of the fundamentals of models are explained, and the different types of models are then analyzed theoretically and critically. Analytical and probabilistic approaches are used to analyze the sensitivity of various models and the results from a benchmarking evaluation of different models are presented and discussed. Guidelines for the practical use of test methods and models are given, including tests for in-situ applications, and test methods validated by the precision results are detailed. The book draws to a large extent on the Chlortest project, which involved seventeen partners from ten European countries, and serves as an authoritative guide.

Numerical simulation of multi-species diffusion

A numerical model has been developed to simulate the transport of several ionic species across a saturated concrete or mortar sample. The chloride binding as well as the electrical coupling between the different ionic fluxes are included in the model by using the Nernst-Planck system of equations. This model highlights which parameters affect substantially chloride penetration into reinforced concrete structures and then shows that the use of Fick’s first law in a predictive model for chloride penetration is strongly challenged. The simulations are in good agreement with diffusion-cell experiments and membrane potential measurements.RésuméUn modèle numérique a été développé afin de simuler le transport de plusieurs espèces ioniques dans une éprouvette de béton ou de mortier saturée. La fixation des ions chlorures ainsi que le champ électrique créé par les divers flux ioniques sont modélisés à l’aide d’un système d’équations de Nernst-Planck. Le modèle permet, premièrement, de mettre en évidence les paramètres qui influencent le plus la pénétration des ions chlorures et, deuxièmement, il montre que l’utilisation de la 1e loi de Fick dans un modèle de prédiction de la pénétration des ions chlorures est fortement remise en cause. Les résultats sont en accord avec les expériences de diffusion et les mesures expérimentales du potentiel de membrane.

Dielectric properties of cement mortar as a function of water content

We have studied the dielectric properties of cement mortars, with water/cement ratios between 0.5 and 0.78, in the frequency range 10−3≤f≤107 Hz. The dielectric response can be modeled by an equivalent circuit incorporating electrode polarization and bulk conduction. The bulk dc conductance and the amplitude of a constant phase element describing the dispersive part of the bulk response display characteristic dependencies on the water content of the samples. The power‐law exponent of the constant phase element is independent of water content and can be related to the fractal dimension of the pore surfaces.

Effect of fly ash on the microstructure of cement mortar

A microstructural study of mortars prepared with a low-alkali, low-C3A cement and a Class F fly ash, both of Swedish origin, was carried out using the scanning electron microscopy-energy-dispersive X-ray analytical technique. Supplementary phase analyses were made by X-ray diffraction and thermogravimetry-differential thermal analysis. Normally, CH crystals in the transition zone grow with their c axis parallel (or the (0 0 1) cleavage plane perpendicular) to the aggregate surface. The encapsulation of the fly ash particles by the growing CH reduces the amount of orientated CH at the aggregate-paste interface. The growth mechanism of these crystals is discussed. The reduction of CH, most significant after 28 days of hydration, is mainly due to the reaction of CH with the fly ash glass phase. Initially, the replacement of cement by fly ash weakens the paste-aggregate interfacial zone due to reduction of contact points, and increases the local water-to-cement ratio. This, however, improves significantly when the fly ash has reacted. In order to enhance the reaction of fly ash, extra gypsum was added. The results show that gypsum can accelerate the fly ash reaction, but the products formed, and the beneficial effects of gypsum, are mainly determined by the total amount of gypsum in the paste.ResumeUne étude microstructurale de mortiers préparés à partir de ciment à faible teneur en alcali et en C3A ainsi que de cendres volantes de classe F (tous deux d’origine suédoise) a été menée en se servant du microscope électronique à balayage et de la technique d’analyse de rayons X à dispersion d’énergie. Les phases supplémentaires étaient étudiées par thermogravimétrie et analyse thermique différentielle.Normalement, les critaux de CH croissent dans la zone de transition en sorte que leur axe est parallèle à la surface du granulat (ou le plan de clivage (0 0 1) perpendiculaire à la surface). En croissant le CH recouvre les cendres volantes, ce qui réduit la quantité de CH orienté à l’interface pâte-granulat. La diminution de CH, très importante après 28 jours, semble dépendre surtout de la réaction entre les cristaux de CH et la phase vitreuse des cendres volantes.Au début de l’hydratation, l’addition de cendres volantes affaiblit la zone d’interface entre la pâte et le granulat en réduisant les points de contact. Le rapport eau-ciment local est aussi augmenté. Une fois que la réaction des cendres volantes commence, la situation s’améliore de façon significative. Afin d’augmenter la réactivité des cendres volantes, on a ajouté du gypse. Les résultats démontrent que, bien que le gypse puisse accélérer la réaction des cendres volantes, la formation de produits de réaction, ainsi que les avantages, dépendent de la quantité totale de gypse dans la pâte.

A STUDY OF THE QUANTITATIVE RELATIONSHIP BETWEEN PERMEABILITY AND PORE SIZE DISTRIBUTION OF HARDENED CEMENT PASTES

The fluid flow through hardened cement paste was theoretically analyzed by introducing the conception of long-range force and the quantitative relationship between permeability and pore size distribution was derived based on the model of intercepting cylindrical pores and the model of parallel porous bodies. Thirteen sets of data of various hardened cement pastes reported by other researchers were used to verify the derived relationship. The result show a good correspondence between the calculated permeability and the experimental permeability and the regressive square correlative coefficient is up to 0.97. The definition of impermeability coefficient was proposed and the difference between water and gas permeabilities was discussed. The derived relationship gives a better explanation to this difference. It might be accurate and convenient enough to evaluate or predict the impermeability of hardened cement paste by using the impermeability coefficient proposed in this paper instead of time consumptive and labour intensive permeability experiment.

Interaction between microclimate and concrete--a prerequisite for deterioration

This paper gives an overview of the present knowledge regarding the interaction between the climatic conditions at the concrete surface and the corresponding conditions inside the concrete. The phenomena dealt with are those involved in frost damage, frost and salt scaling and reinforcement corrosion, such as temperature and moisture conditions in the near surface regions, carbonation and chloride penetration. Data on the intensity, duration and frequency of the microclimatic conditions should be included. The interaction depends on material properties concerning various flow processes and binding of heat and substances transported in the concrete. It is of decisive importance to choose simplifications carefully depending on the required accuracy, available information and the effort to be put into the task.

A new approach to the determination of pore distribution by penetrating chlorides into concrete

In this paper, a new concept of pore distribution is discussed and consequently a new approach to the determination of pore distribution by penetrating chlorides into concrete is proposed. This approach involves 1) accelerating chloride penetration into concrete specimens by applying an electrical field, 2) determining the profile of total chloride content by using the dry-grinding technique and chemical analysis, and 3) calculating the pore distribution of concrete from the profile of total chloride content. A number of specimens have been tested. Preliminary results show that the pore distributions determined by using the new approach are qualitatively comparable with those determined by using conventional mercury intrusion technique.

Dielectric properties of porous cement mortar: Fractal surface effects

We have studied the electrical properties of cement mortars in the frequency range 10-4-107 Hz, for different water contents. The admittance scales as a power law with an exponent of about 0.7 in a wide frequency range. This behaviour is interpreted in terms of polarization and diffusion at fractal pore surfaces. We obtain a fractal dimension of 2.4, which is in good agreement with adsorption isotherm data for the mortar.