Robert Cerny

Interlaboratory comparison of hygric properties of porous building materials

The precision of methods used for the determination of hygric properties of porous building materials was investigated. The study was performed in the framework of the EU-initiated HAMSTAD-project. Six laboratories measured the selected hygric properties of three porous building materials. While the most measured properties show acceptable agreement, yet, it was found that some of the existing standards or commonly accepted measurement methods need improvement. Most striking were large variations in the results of the vapour transmission tests performed in accordance to the existing European Standard.

A Comparison of Different Techniques to Quantify Moisture Content Profiles in Porous Building Materials

Several advanced non-destructive techniques are available to measure the evolution of content profiles with time, allowing the analysis of unsaturated flow and the determination of the moisture diffusivity of porous building materials. The reliability of six different techniques is investigated: the NMR-technique, the MRItechnique, the γ-ray attenuation technique, the capacitance method, the X-ray projection method and the TDR-technique. All of them were applied to measure the moisture content evolution during free uptake experiments on two building materials. Considering the limitations of some of the techniques, a good overall agreement is obtained. The work presented is an outcome of the EU-initiated HAMSTAD-project.

Determination of the liquid water diffusivity from transient moisture transfer experiments

The Boltzmann transformation method is used to determine the liquid water diffusivity from moisture content profiles as measured in a capillary water absorption experiment. An inter-laboratory comparison for analyzing the reliability of the determination method showed that the inaccuracy in the liquid water diffusivity is caused by scatter in the transformed data and by uncertainty in the boundary conditions at the intake surface and ahead of the steep moisture front. A methodology is proposed based on (1) the evaluation of the validity of the diffusion approach, (2) a simplified handling of the boundary conditions, (3) smoothing of the scattered data and (4) the evaluation of the quality of the determined liquid water diffusivity. For HAM (Heat-Air-Moisture transport) calculations values of the liquid water diffusivity for moisture contents higher than the capillary moisture content are disregarded. The liquid water diffusivity can be described by an exponential function limited at a lower moisture content bound. To describe the moisture diffusivity including liquid water and water vapour transports, a new parametric description of the moisture diffusivity is presented, which shows sufficient flexibility both in the hygroscopic and overhygroscopic ranges. When permeability is calculated from diffusivity, the permeability should monotonically increase with decreasing capillary pressure. In the hygroscopic region it should coincide with the measured water vapour permeabilities.

System for Testing the Hygrothermal Performance of Multi-Layered Building Envelopes

A measuring system for determination of temperature, moisture, relative humidity, capillary pressure and salinity profiles in building envelopes is presented in the paper. The system makes it possible to analyze the hygrothermal performance of either particular materials or the whole building envelopes in the conditions of difference climate. Both the spatial dimensions of the specimens and the applied exterior and interior climatic data are chosen to be as close as possible to the conditions in the particular building site, so that the system can simulate the hygrothermal processes in the envelopes in quite a realistic way. A sophisticated technology for the determination of moisture, relative humidity, capillary pressure, temperature and salinity in the analyzed specimens based on the application of timedomain reflectometry, mini-tensiometry and mini-hygrometry is employed, which guarantees obtaining reliable data for validation of sophisticated computational models.

Thermal and Hygric Parameters of Carbon-fiber-reinforced Cement Composites after Thermal and Mechanical Loading

The basic thermal and hygric parameters of two different types of carbon-fiber-reinforced cement composites are analyzed in this article. The thermal conductivity, specific heat capacity, moisture diffusivity, and water vapor diffusion resistance factor are determined as functions of thermal load and tensile load applied before the measurement as well as of the combination of both types of load. The tensile load up to failure is found to be not a very significant factor for all material properties analyzed except for the moisture diffusivity. On the other hand, the thermal load is observed to result already at 600 C in considerable changes in all investigated thermal and hygric properties except for the specific heat capacity. The combinations of thermal and tensile loads lead to similar results as the effect of the thermal load alone so that the domination of the thermal load is apparent. This is supposed to be due to the positive effect of randomly distributed carbon fibers that can reduce the damage of the pore structure by the tensile stress. The resistance of the materials studied to high temperatures expressed by the change of hygric and thermal properties after thermal load is found to be positively affected by the application of the high alumina cement and in the case of the Portland cement-based composite also by using the autoclaving procedure in the production process.

Phase transformations induced in CdTe single crystal by ruby laser pulsed irradiation

Phase transitions induced by laser irradiation in CdTe wafers have been studied in situ and experimental and computer simulation methods. The samples were irradiated by ruby laser with pulse duration 80 ns in energy density range from 0.02 to 0.5 J/cm2. Time-resolved reflectivity (TRR) measurements were carried out at the wavelengths of λ1=1.064 μm and λ2=0.532 μm. Dynamics of transmissivity was studied at λ1. Photoluminescence of CdTe, excited by the ruby laser single pulse, was also investigated. The character of TRR transients changes with the increase of irradiation energy density. The changes are more considerable at λ1 than at λ2. The time dependencies of reflectivity are explained by the changes of optical parameters of CdTe in course of laser-induced melting, solidification and evaporation. The experimental data obtained from transmissivity and photoluminescence measurements correlate with those from TRR transients. Laser-induced melting, crystallization and evaporation processes were studied on the basis of the computational solution of a two-phase moving boundary problem with two moving interfaces. The calculated dependency of melt duration on energy density is in a reasonable agreement with experimental data. From our investigation it follows that in the molten state CdTe is characterized by constant or weakly changing reflectivity in the temperature range from Tm to 3000 K.

Calorimetry of building materials

Classical adiabatic methods for measuring specific heat in a wide temperature range are very suitable for homogeneous, well defined materials. However, measurements on inhomogeneous materials require large samples, which makes using of adiabatic treatments extremely expensive. In this paper, a nonadiabatic method for determining the specific heat of inhomogeneous building materials within the wide temperature range of −30 to 1200°C is presented. The method is relatively simple, very cheap, and sufficiently accurate for use with building materials. Tests of the new developed method on two typical building materials, concrete and basalt fibreboards, demonstrate the methods applicability to practical measurements.ZusammenfassungKlassische adiabatische Methoden zur Messung der spezifischen Wärme in einem breiten Temperaturebereich sind für homogene, gut definierte Substanzen sehr geeignet. Dagegen fordern Messungen an inhomogenen Stoffen umfangreiche Proben, was denn Einsatz von adiabatischen Behandlungen äu\erst teuer macht. vorliegend wird eine nichtadiabatische Methode zur Ermittlung der spezifischen Wärme von inhomogenen Baustoffen in einem Temperaturbereich von −30 bis 1200°C vorgestellt. Diese Methode ist relativ einfach, sehr billig und ausreichend genau für den Einsatz bei Baustoffen. Die Erprobung der neu entwickelten Methode an zwei typischen Baustoffen wie Beton und Basaltfiberplatten zeigen die Einsetzbarkeit der Methode für praktische Messungen.

Thermal properties of alkali-activated aluminosilicates with CNT admixture

Material properties of electrically conductive cement-based materials with increased attention paid on electric and thermal properties were often studied in the last years. Both electric and thermal properties play an important role thanks to their possible utilization in various practical applications (e.g. snow-melting systems or building structures monitoring systems without the need of an external monitoring system). The DC/AC characteristics depend significantly on the electrical resistivity and the electrical capacity of bulk materials. With respect to the DC/AC characteristics of cement-based materials, such materials can be basically classified as electric insulators. In order to enhance them, various conductive admixtures such as those based on different forms of carbon, can be used. Typical representatives of carbon-based admixtures are carbon nanotubes (CNT), carbon fibers (CF), graphite powder (GP) and carbon black (CB). With an adequate amount of such admixtures, electric properties significa...