Lukáš Fiala

A material database for computational models of heat, moisture, salt and momentum transport: Construction of the code as an input module and example of application

Most currently used computational models of heat, moisture, salt and momentum transport can be considered appropriate for predicting the performance of building envelopes providing that the input data have a sufficient quality. However, the standard lists of parameters given by the producers or built-in packages for simulation tools often do not meet the demand of accurate numerical prediction (for instance, the dependency of heat and moisture transport and storage parameters on moisture or temperature is not included). In this paper, we present a material database suitable for application in most current computational models. It contains fundamental physical properties and heat, moisture and salt transport and storage parameters of building materials used in the contemporary buildings with some of them having the mentioned dependency. Selected mechanical parameters are included as well. The database is designed as an open system with secured online access.

Seebeck effect influence on joule heat evolution in electrically conductive silicate materials

In general, silicate building materials are non-conductive matters that are not able to evolve heat when they are subjected to an external voltage. However, the electrical conductivity can be increased by addition of electrically conductive admixtures in appropriate amount which leads to generation of conductive paths in materials matrix. Such enhanced materials can evolve Joule heat and are utilizable as a core of self-heating or snow-melting systems. In this paper, Joule heat evolution together with Seebeck effect in electrically conductive silicate materials was taken into consideration and the model based on heat equation with included influence of DC electric field was proposed. Besides, a modeling example of heating element was carried out on FEM basis and time development of temperature in chosen surface points was expressed in order to declare ability of such system to be applicable.

Thermal properties of alkali-activated aluminosilicates

The paper is focused on measurements and evaluation of thermal properties of alkali-activated aluminosilicates (AAA) with various carbon admixtures. Such composites consisting of blast-furnace slag, quartz sand, water glass as alkali activator and small amount of electrically conductive carbon admixture exhibit better electric and thermal properties than the reference material. Such enhancement opens up new practical applications, such as designing of snow-melting, de-icing or self-sensing systems that do not need any external sensors to detect current condition of building material. Thermal properties of the studied materials were measured by the step-wise transient method and mutually compared.

Modeling of heat evolution in silicate building materials with electrically conductive admixtures

Silicate building materials are electrically non-conductive, in general. However, a sufficient amount of electrically conductive admixtures can significantly increase their electrical conductivity. Consequently, new practical applications of such materials are available. Materials with enhanced electrical properties can be used as self-sensing sensors monitoring evolution of cracks, electromagnetic shields or cores of deicing systems. This paper deals with the modeling of heat evolution in silicate building materials by the action of passing electric current. Due to the conducting paths formed in the material’s matrix by adding a sufficient amount of electrically conductive admixture and applying electric voltage on the installed electrodes, electric current is passing through the material. Thanks to the electric current, Joule heat is successively evolved. As it is crucial to evaluate theoretically the amount of evolved heat in order to assess the effectiveness of such a system, a model describing the Joule heat evolution is proposed and a modeling example based on finite-element method is introduced.Silicate building materials are electrically non-conductive, in general. However, a sufficient amount of electrically conductive admixtures can significantly increase their electrical conductivity. Consequently, new practical applications of such materials are available. Materials with enhanced electrical properties can be used as self-sensing sensors monitoring evolution of cracks, electromagnetic shields or cores of deicing systems. This paper deals with the modeling of heat evolution in silicate building materials by the action of passing electric current. Due to the conducting paths formed in the material’s matrix by adding a sufficient amount of electrically conductive admixture and applying electric voltage on the installed electrodes, electric current is passing through the material. Thanks to the electric current, Joule heat is successively evolved. As it is crucial to evaluate theoretically the amount of evolved heat in order to assess the effectiveness of such a system, a model describing the Jo...

Correlation between dielectric and thermal properties of alkali-activated aluminosilicates with carbon fiber admixture

The paper deals with the comparison of the thermal and dielectric properties of alkali-activated aluminosilicates containing the conductive admixture in the form of carbon fibers. To determine the thermal conductivity and thermal capacity of the system, a differential method based on the response to the step wise heating was used. The electrical conductivity and the capacities were determined by impedance spectroscopy. For both electrical and thermal properties, a model was created which allowed the determination of the characteristics of individual processes. The analysis of these models showed that the thermal conductivity of material decreases with increasing concentration of admixtures, probably because of greater porosity of material. For the concentration of 2.22% it is about three times smaller than for the concentration of 0.11%. The heat capacity is reduced at concentrations higher than 1.11%. The electrical conductivity decreases with increasing concentration of the admixture, which again seems to be related to greater porosity of material. For the concentration of 2.22% it is more than four times smaller than for the concentration of 0.11%.The paper deals with the comparison of the thermal and dielectric properties of alkali-activated aluminosilicates containing the conductive admixture in the form of carbon fibers. To determine the thermal conductivity and thermal capacity of the system, a differential method based on the response to the step wise heating was used. The electrical conductivity and the capacities were determined by impedance spectroscopy. For both electrical and thermal properties, a model was created which allowed the determination of the characteristics of individual processes. The analysis of these models showed that the thermal conductivity of material decreases with increasing concentration of admixtures, probably because of greater porosity of material. For the concentration of 2.22% it is about three times smaller than for the concentration of 0.11%. The heat capacity is reduced at concentrations higher than 1.11%. The electrical conductivity decreases with increasing concentration of the admixture, which again seems ...

Mechanical and Basic Physical Properties of High-Strength Concrete Exposed to Elevated Temperatures

In this paper, the effect of elevated temperatures on the mechanical and basic properties of two different newly-designed high-strength concretes is studied. The studied materials were prepared from Portland cement, steel fibers, reactive finely milled quartz powder and quartz sand, silica fume, plasticizer, and with a relatively low water/cement ratio of 0.24. The samples were stored in water environment for the first 28 days of hydration to achieve better mechanical properties. Then, after pre-drying at 105 °C to constant mass, the materials were exposed to elevated temperatures of 600 °C and 1000 °C where they were kept for 2 hours. The basic physical properties, such as matrix density, bulk density and open porosity were determined as a function of temperature. Mechanical properties (compressive and flexural strength) were also studied. The measured parameters exhibited a high dependence on temperature and the obtained results pointed to the structural changes of the studied materials. Spalling was not observed because of the pre-drying treatment.

Verification of Joule heat evolution model for silicate building materials with electrically conductive admixtures

Silicate building materials naturally exhibit electrically non-conductive behavior. However, a sufficient amount of electrically conductive admixtures leads to a significant increase of the electrical conductivity. This fact can be utilized in several practical ways, such as for development of self-sensing, electromagnetically-shielding or self-heating materials. In this paper, self-heating ability of chosen silicate material was tested and previously developed heating model was verified by means of comparison of calculated temperature evolution in time data with those experimentally determined by thermocouples placed on lateral sides. Sufficiently electrically conductive mixture with carbon black (CB) in amount of 8.89 % was used for DC experiment. Theoretical data were obtained by subsequent FEM calculations conducted on 3D model of the tested sample.

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...

Experimental and Theoretical Study of Heat Transport Parameters of Plasters Containing Pozzolanic Admixtures

Measurements of basic materials properties of building materials with pozzolanic waste admixture originated from grinding of thermally insulating bricks were performed by means of pycnometry method. Besides, the thermal conductivity dependence on the moisture content measurements were carried out by using a non-stationary pulse method. Obtained data were subsequently analyzed by simple Wiener’s bounds and sophisticated homogenization formula taking into account the shape of ellipsoidal pore inclusions. Validity of applied homogenization models were assessed by comparison of the measured and the calculated data. On the basis of experimental data and homogenization analysis, the shape effect on the thermal conductivity is discussed.

Application of the Lichtenecker’s mixing rule in modeling the thermal properties of autoclaved aerated concrete

Thermal conductivity is an important thermophysical parameter describing heat transport in porous building materials. It is significantly affected by the moisture content; higher water saturation degree leads to the deterioration of thermal insulation capability of building materials. The thermal conductivity is usually determined by repeated measurements for different water saturation levels. However, this procedure is very time consuming. Shortening the time necessary for the determination of thermal conductivity as a function of moisture content can be achieved by an application of convenient mixing formulas. Lichtenecker’s homogenization formula is widely used for its simplicity and the fact that the theoretical Wiener’s bounds can be expressed by choosing the Lichtenecker’s coefficient k equal to -1 and 1 k values, laying in close interval [-1, 1], express the geometric transition from parallel to serial connection of heterogeneous material components’ layers (matrix, air, water). This article is dealing with the assessment of an optimal coefficient k that identifies precisely the dependence of thermal conductivity of autoclaved aerated concrete (AAC) on moisture content. Data obtained by modeling is compared with measured data by means of RMSE. Under assumption of similar matrix, this approach can lead to an appropriate choice of coefficient k predicting the thermal conductivity dependence on moisture content for a wider group of AACs.