Björn Johannesson

Microstructural changes caused by carbonation of cement mortar

The change of specific surface area and pore size distribution due to carbonation of an ordinary Portland cement mortar is investigated. The adsorption of water vapor on noncarbonated and well-carbonated cement mortar is measured in order to evaluate the difference in specific surface area for the two samples using the BET theory. From the measured desorption the pore size distribution is calculated using the Kelvin formula. A sorption balance is used to measure the sorption characteristics for the two studied sample qualities. In this method dry and saturated air are mixed in desired proportion in a closed system. One of the benefits of the method is that the samples not are exposed to carbon dioxide during testing, i.e., undesired effects caused by carbonation on the sorption can be eliminated. The specific surface area for a noncarbonated sample was calculated, using the measured adsorption data, to be 8% higher than for the well-carbonated sample. The difference in pore size distributions was more marked than the difference in specific surface area for the two samples. The well-carbonated mortar had about twice as much volume attributed to small pores as the noncarbonated cement mortar.

A theoretical model describing diffusion of a mixture of different types of ions in pore solution of concrete coupled to moisture transport

A theoretical model is established for diffusion of different types of ions in pore solution of concrete and the coupling to moisture flow and moisture content. Mass exchanges between ions in pore solution and solid hydration products in the concrete are also considered. The basic concepts behind the so-called mixture theory are used. The development of a mass balance principle for ions in pore solution is established. This principle accounts for (i) diffusion caused by concentration gradients of ions and gradients of the so-called internal electrical potential, (ii) convection, i.e. the effect on the motion of ions due to a motion of the pore solution in concrete, (iii) the effect on the concentration due to changes in the moisture content, and finally, (iv) the effect of mass exchange of ions between solid hydration products and the bore solution phase. The model is general in the sense that all different types of ions appearing in pore solution phase can-be included and computed for during quite arbitrary boundary conditions.

Diffusion of a mixture of cations and anions dissolved in water

In service life modeling of concrete, the ion transport in the pore solution is crucial. The main deterioration phenomena associated with ions in the pore solution are (1) corrosion due to external chloride ions reaching the embedded reinforcement bars; (2) carbonation due to presence of dissolved carbon dioxide (which will form carbonic acid with water) supplied from the surrounding air; and (3) leaching of hydroxide ions from the pore solution to the surrounding environment. Models dealing with diffusion of ions are usually based on the mass balance equations for the individual diffusing ions together with constitutive relations for the mass density flows and for the mass exchange among the constituents. The important consequences of electroneutrality among the diffusing ions, however, is often omitted in models dealing with diffusion of ions in the pore solution of concrete. Here a method will be examined that allows diffusion of different ions in water, which satisfies both the electroneutrality requirement and the mass balance laws. For simplicity the effect of built-up electric double layers on the charged pore walls will not be treated.

Measurement of the moisture storage capacity using sorption balance and pressure extractors

This paper presents measurements of the moisture storage capacity for several different porous building materials. The storage capacity is measured by a sorption balance in the hygroscopic range and with pressure plate and pressure membrane extractors in the superhygroscopic range. The results are presented both as retention curves and sorption isotherms and indicate, among other things, that some materials have a large hysteresis between absorption and desorption in the superhygroscopic range. This is contrary to what sometimes has been postulated.

Nonlinear transient phenomena in porous media with special regard to concrete and durability

Concrete deteriorates due to many different mechanisms. Among the most important mechanisms is the reinforcement corrosion induced by deleterious substances reaching the embedded reinforcement bars. The external sources of deleterious materials may, for example, be deicing salts, sea water, and carbon dioxide. Research has sought to determine threshold values, in terms of concentration of deleterious substances in concrete, at which reinforcement corrosion will be induced, that is, at which concentration the passive condition close to the reinforcement turns to an aggressive state. To predict when this threshold value is reached, the flow properties of the pollutant in concrete must be known. Some of the most important phenomena governing the movement of pollutants in concrete are diffusion of substances in the pore water, adsorption (and desorption) of pollutants onto the pore walls, and hydrodynamic dispersion and convection of substances due to flow of the pore water. Here a set of equations will be presented based on mass and energy balance. These coupled equations cope with the above-mentioned phenomena. The migration of ions due to an electric potential is not considered as only the initiation stage of corrosion is of interest. The constituents considered in the model are a solute γ (e.g., chlorides), the pore water α, and the solid phase s of the concrete, which is restricted to be nondeformable. The governed equation system is solved using the Petrov-Galerkin scheme and finite elements (compare references 1 and 2). Some examples of the performance of the proposed model are given.

Determination of ice content in hardened concrete by low-temperature calorimetry

Low-temperature calorimetry has been used to determine the ice content in concrete at different temperatures when exposed to low-temperature environments. However, the analysis of the ice content from the measured data of heat flow is not straightforward. In this study, two important factors influencing the ice content calculation are discussed. The importance of the baseline determination for the calculation of the ice content is realized. Two different methods of generating the baseline are discussed. First, the ‘J-baseline’ is discussed which is a recently proposed extrapolation method based on the accumulated heat curves measured in the freezing and the melting process. Second, the ‘C-baseline’ is discussed in which a calculated baseline is used where the heat capacity of both water and ice and the phase changing behaviour under different testing temperatures are considered. It turns out that both the ‘J-baseline’ method and the ‘C-baseline’ method can be used to calculate the approximate baseline. The heat of fusion of the water confined in small pores is another important parameter to be considered in ice content calculation. This property must be carefully analyzed in order to accurately calculate the ice contents at different temperatures in the freezing and melting process. It should be noted that there is no general agreement on how to obtain the important temperature dependence of the heat of fusion of water confined in small pores. By performing comparison studies, the present study shows the influence of the different values of the heat of fusion commonly adopted on the calculated ice content for the studied concrete samples. The importance and necessity to use an accurate value of the heat of fusion is emphasized. Based on the calculation of the baseline proposed in this work and by carefully selecting the values for the heat of fusion, the ice content in a hardened concrete sample is expected to be estimated with an acceptable accuracy.

Estimation of Effective Diffusion Coefficients and Non-Linear Binding Parameters of Chloride by Reverse Analysis of Chloride Profiles

Instead of the conventional method for evaluating an apparent diffusion coefficient of chloride ions (Cl-) from experimental Cl- concentration profile, this study introduces a novel method to estim ...

Pore‐scale modeling of vapor transport in partially saturated capillary tube with variable area using chemical potential

Here we illustrate the usefulness of using the chemical potential as the primary unknown by modeling isothermal vapor transport through a partially saturated cylindrically symmetric capillary tube ...

Restrictions on the Rate of Absorption When Evaluating Sorption Isotherms Obtained Using a Micro-Calorimetric Technique

One interestingway of measuring sorption isotherms and heats of sorption is the newly developed micro-calorimetric method. It turns out, however, that the adsorption kinetics must be proven to be rapid enough to fulfill the assumptions behind the theory for the determination of the isotherm to be valid. Here, the lower limit of the accepted rate of adsorption will be analyzed by making calculations of the diffusion behavior in the calorimetric vessel. The evaluation of the adsorption isotherms is based on assumptions of a quasi-static nature. If the adsorption kinetics are slow the behavior in the calorimeter will turn to a more or less transient process. When the heat of adsorption is of interest and the sorption isotherm and kinetics of sorption can be determined by other methods, compensation for the transient effects can be made.