Jaroslava Drchalová

The effect of compressive stress on thermal and hygric properties of portland cement mortar in wide temperature and moisture ranges

Abstract Basic thermal and hygric parameters of cement mortar — namely, the thermal conductivity, the linear thermal and hygric expansion coefficients, moisture diffusivity, and water vapor permeability — are determined in dependence on the applied mechanical load inducing compressive stress in the samples ranging from 0% to 90% of compressive strength. The measurements are performed in wide temperature and moisture ranges, from 20°C to 1000°C and from the dry state to the saturation water content. The measured results show that both thermal and hygric parameters of cement mortar are affected only by compressive stress of 90% of the compressive strength of the material and higher, in a significant way. Scanning electron microscope images and porosimetric measurements reveal that the most probable reason for the observed differences in thermal and hygric parameters is the appearance of cracks with a typical width of 1–2 μm.

THERMAL AND HYGRIC PROPERTIES OF PORTLAND CEMENT MORTAR AFTER HIGH-TEMPERATURE EXPOSURE COMBINED WITH COMPRESSIVE STRESS

Thermal conductivity λ, water vapor permeability δ, and liquid moisture diffusivity κ of cement mortar are measured on specimens subjected to four types of pretreatment, namely, unloaded, mechanically loaded to 90% of compressive strength, thermally loaded by subjecting to a temperature of 800 °C for 2 h, and loaded both mechanically and thermally. The values of λ and κ are found to depend very significantly on the loading mode. The maximum differences observed compared to the unloaded samples are almost one order of magnitude for λ and as much as three orders of magnitude for κ. In contrast, the values of δ are found to increase by only about 40% compared to the basic unloaded material. It is proposed that the observed large differences in λ and κ are due to the formation of cracks and the increase of total pore volume, which were shown by visual analysis and mercury porosimetry.

A simple gravimetric method for determining the moisture diffusivity of building materials

Abstract A simple gravimetric method for determination of moisture diffusivity which is able to give a moisture diffusivity vs. moisture relation point-wise is presented in this paper. The method can be employed in determining moisture diffusivity of anisotropic materials also in the case if one spatial dimension of the specimen is significantly smaller than the other two, which is the case of the most of lining materials, for instance. Practical application of the method is demonstrated for two fiber containing plate building materials produced in the Czech Republic, Dekalux and Dekalit P. Experimental results show that while for lighter Dekalit P, an order of magnitude difference in the moisture diffusivities κ for the two basic orientations, i.e. along and through the plate, is observed, for the heavier Dekalux the differences in κ are within the error bar of the experimental method. The method is tested by its application to rod samples, and a comparison with two commonly used methods is done with both mentioned lining materials and also with an extruded porcelain mixture which can be considered as a well homogeneous material. The results of this comparison show a reasonable agreement, taking into account the approximate character of the method.

The effects of thermal load and frost cycles on the water transport in two high-performance concretes

Abstract Moisture diffusivity of two high-performance concretes used in concrete containment buildings of nuclear power plants is measured as a function of temperature up to 80°C, and the effects of high-temperature exposure up to 800°C and freeze/thaw cycles on its value are analyzed. The temperatures in the range of 0–80°C are found to have a moderate effect on the moisture diffusivities of both concretes, the typical increase being about 100–200% over the whole region. The high temperature exposure results in a much more significant increase of moisture diffusivity, up to three orders of magnitude, and the freeze/thaw exposure of 400 cycles to an increase up to one order of magnitude. As the main reason for the mentioned moisture diffusivity increase, the crack formation is identified. Both thermal decomposition accompanied by release of substantial amount of gaseous substances and water freezing in the porous system can damage the internal microstructure of the matrix due to the significant pressure increase in the porous body. The magnitude of changes of moisture diffusivity is found to be affected by the quality of aggregates rather than by the quality of cement. High quality quartzitic aggregates with very low porosity and very good mechanical properties perform much better than other more porous siliceous aggregates such as opal, and than the calcareous aggregates such as chalk.

Thermophysical properties of concrete for nuclear-safety related structures

Abstract The exact knowledge of thermophysical properties of high-performance concrete for nuclear-safety related structures in wide temperature and moisture ranges is for realistic modeling of possible accidents in nuclear power plants of particular importance. In this paper, thermal conductivity, thermal diffusivity and linear thermal expansion coefficient of concrete from the French nuclear power plant Penly are determined in the temperature range from 20 °C to 200 °C, specific heat for −30 °C to 1000 °C, moisture diffusivity from 0 to 75% of maximum water saturation at room temperature, and water vapor diffusivity at room temperature. Comparison of measured results with the measurements of other authors for concretes with similar composition shows a reasonable agreement for most parameters.

Water and Water Vapor Penetration Through Coatings

Water and water vapor transport in two-layer systems where one layer acts as water retarder are analyzed. In the experimental work, four external paints supposed to protect the underlying structure against water under common service conditions are applied on glass fiber reinforced concrete substrates. The water absorption coefficient and the effective water vapor diffusion coefficient are chosen as evaluation parameters for the assessment of water and water vapor transport properties of the systems. In the water absorption experiments, two consecutive quasi-steady regimes are identified, the first with an almost constant water flux into the system, and the second obeying the square-root-of-time rule for cumulative mass of water in the system. The water protective qualities of all studied paints can be considered as satisfactory for common service conditions. The effective diffusion coefficients of all systems are found to be within the error range of the measuring method compared to the substrate itself, which is a very positive feature for external paints.

METHODS FOR EVALUATION OF WATER-PROOFNESS QUALITY AND DIFFUSION PROPERTIES OF COATING MATERIALS

A simple method for evaluating the water-proofness quality of coating materials on external linings is proposed. The method is based on the measurement of integral capillarity with dependence on time and on the comparison of its value with that determined for the basic lining material. Measurements of water vapor diffusion then provide the information on the risk of condensation, which may increase after applying the coating. The practical application of the method is performed with four types of lining materials, Dekalux 5, Dekalux 12, Dekalit P6 and Dekalit P10, and three types of the surface treatment, Rudicolor, Aquafob and Rudicolor-Aquafob combination. Measured results show that Aquafob is an effective water-repellent coating material, which keeps, at the same time, a high permeability for water vapor: Rudicolor is only effective for short times (≈ 1 h). The positive influence of Aquafob was observed to increase with decrease in the density of the underlying lining material, which corresponds to its hydrophobic quality, while the plaster material Rudicolor exhibited an opposite trend concerning the density.

Mechanical, Hygric and Thermal Properties of Flue Gas Desulfurization Gypsum

The reference measurements of basic mechanical, thermal and hygric parameters of hardened flue gas desulfurization gypsum are carried out. Moisture diffusivity, water vapor diffusion coefficient, thermal conductivity, volumetric heat capacity and linear thermal expansion coefficient are determined with the primary aim of comparison with data obtained for various types of modified gypsum in the future.