Volume 6, Issue 4

The application of a general thermodynamical mass and energy transport model to the coupled heat and moisture transfer in porous materials results in a balance equation system and the related constitutive equations of the considered quantities. The constitutive equations describe moisture transport in a phase-separated manner leading into phase-divided hygric transport coefficients (liquid water permeability, water vapour diffusivity). A conceptual model is presented in the paper in order to circumvent the difficulties resulting from non-isothermal overlaying moisture transport processes. Since phase-divided hygric transport coefficients are not directly measurable, but moisture transport coefficients in distinct hygric ranges, moisture conductivities and a phase dividing function are introduced. The moisture conductivities include liquid water and water vapour transport. For a known phase dividing function, the phase-divided hygric transport coefficients of the balance equation system can be calculated from the measurable moisture conductivities. The influence of a variation of the introduced phase-dividing function on non-isothermal moisture transport processes is investigated by means of computer simulations.

Cement-based materials undergo a complex interaction whenever they are exposed to water or aqueous solutions. Differences of ion concentrations are the driving force for ion migration into or out of the material. In this contribution hydrolysis is dealt with as an example for the interaction of cement-based materials with water. It is shown that among other processes Ca 2+ -ions diffuse out of the porous structure of the material. As soon as the total amount of Ca(OH) 2 is dissolved locally CSH-phases are attacked. The second part deals with Na + and K + ion diffusion out of water saturated concrete. It is shown that a two-step model describes this process adequately. The effective diffusion coefficient of ion transport is a sensitive indicator of the durability of cement-based materials.

When a masonry loaded with salt is exposed to natural weather conditions there are occurring two interdependent mechanisms of transport: moisture- and salt transport. For the purpose of investigating these coupled processes, the Fraunhofer- Institute of Building Physics developed a measurement technique which allows a non-destructive determination of both water content distribution and salt content distribution in capillary porous building materials. This system includes two different nuclear physical measurement procedures: Nuclear Magnetic Resonance (NMR) and gamma-ray attenuation. The NMR unit produces a signal that depends just on the number of protons in the specimen. Because of this, the water content of the specimen is linked to the signal of the NMR-measurement. During a gamma-ray attenuation measurement salt and water content distribution are gauged simultaneously. The larger the density of the measured specimen is, the stronger is the attenuation of the gamma-rays when they pervade the matter. By combining the results of these two measurement methods it is possible to determine the salt and water content distribution simultaneously in a non-destructive way. From this profiles it is possible to draw conclusions in respect of the occurring transport processes and the correlative parameters such as for example capillary transport coefficients and water absorption coefficients of salt loaded building material. These results yield the preconditions for first concrete basic approaches to describe the coupled moisture and salt transport and furthermore the influence of the salt on the moisture transport. Lately numerous absorption tests on Sander Sandstone were carried out. The specimens sucked saline solutions of different salts and concentrations. After defined time steps the profiles of water and salt content were measured. Furthermore tests during the drying period were preformed on specimens loaded with defined saline solutions.

Besides the formation of Friedel salt the transport and binding of chlorides in concrete is mainly defined by the electrochemical double layer at the interface between cement matrix and pore solution. Due to the alkaline pore solution the surface of hardened cement paste is negatively charged which may change to positive values by the potential regulating calcium ions. Inverting of the surface charge leads to an attraction of anions and therefore, to an adsorption of chloride ions in the diffuse part of the electrochemical double layer. Influence from outside like sulphates and carbon dioxide may lead to a decomposition of Friedel salt. Apart from these effect temperature, pH-value and certain environmental conditions affects the electrochemical double layer as well. The chloride equilibrium is mainly controlled by adsorbed ions in the electrochemical double layer. The model presented here is relevant for the assessment of ion transport processes in mineral building materials. Continuing investigations may lead to optimize transport models and a better evaluation of the critical chloride threshold value in reinforced concrete.

The diffusive mass transport in materials is above all determined by the material structure. The experimental determination of diffusion coefficients is based on diffusion models, which results from special mathematical solutions of Fick's second law and its corresponding boundary conditions. The general usefulness of these diffusion models will be described using some examples, e. g. the diffusion of deteriorating salts in masonry materials, the oxygen ion diffusion in mixed conducting ceramics and the cation exchange in single crystals. The measurement of the diffusion coefficients results in a better comprehension of the transport mechanism as well as of the "morphology" of the transport medium, which allows to optimize the conditions of the mass transport and the material structure and composition, respectively.

It is reported about substance migration problems and their consequences, which have been investigated experimentally and which could be particularly relevant with regard to repair and restoration of historic buildings. One aim of the investigations was to find out, whether facades, treated with water repellent agents, showing non treated areas, can be affected in their long time behaviour. Such lacks can be due to non careful application of the repellent agent. In addition, investigations were made to find out. of coatings on salt containing mineral substrates, especially natural stones can reduce their life time. As a result of the experiments and the theoretical considerations, it follows, that a lack in water repellent treatment. could cause the risk of crust-development spallings in environment of non treated spots in the case that substrate is salt contaminated and moisture exchange takes place. All experiments with coatings, performed under the same conditions, show, that the coated surfaces have a more favourable behaviour than the same surfaces without coatings.

The baroque Dukes Chapel in Krzeszów, Poland, contains elaborate stucco marble decorations of great historic value and artistic quality. Cracks and losses appeared in the thin stucco marble layers as a result of layer deformation and detachment from the ground. The glossy polished surface has become locally eroded and rough, and the original material deformed and porous. The damages are located in a non-uniform way; they appear at the base of the walls and high on the vaulted ceilings. A year-long monitoring of air parameters has been conducted inside and outside the chapel. The micro climate of the chapel has been found to be strongly influenced by the outside climate via air exchange with the exterior. The unheated interior undergoes cooling during the winter period and warms up very gradually during the spring and summer. This thermal inertia of the building gives rise to periods of moisture condensation on the walls. Large amounts of magnesium sulphate, which leached out in the past from the dolomitic mortar, are present in the deteriorated decorations of the ceiling. The repeated periods of moisture condensation and evaporation lead to the damaging cyclic crystallization of the salt. Considerable swelling of the stucco marble on humidification is another cause of the deterioration observed. The conservation strategy should restrict the air exchange between the chapel and the exterior and include treatments removing magnesium sulphate from the material.