Václav Kočí

Effective thermal conductivity of hollow bricks with cavities filled by air and expanded polystyrene

Effective thermal conductivity of hollow bricks with the cavities filled by either air or expanded polystyrene is analyzed using a hybrid experimental–computational approach. The experimental setup involves an application of a thermal insulation box and a set of temperature and heat flux probes placed at characteristic positions of the specimen-insulating box system. Using the measured heat fluxes and temperatures, the heat loss of the system is determined. A computer simulation tool based on the finite element principle is then used for modeling the temperature fields and heat losses in the studied system. Finally, the effective thermal conductivity is identified using an iterative procedure. Experimental results show that the application of expanded polystyrene as cavity filler instead of air leads to ∼30% decrease in the effective thermal conductivity of hollow brick blocks.

Computational analysis of hygrothermal performance of renovation renders

A combined computational-experimental approach for service life estimate of surface layers of historical building made from renovation renders is presented in the paper. The experimental part is aimed at estimation of the durability of two commercial systems of double layered renovation renders in terms of their frost resistance. The computational part includes calculation of number of freeze-thaw cycles that may occur in surface layers during a reference year. To achieve this, a diffusion-type model of coupled heat and moisture transport is used. The computations are performed on historical load-bearing structure made of sandstone, brick or arenaceous marl, finished by two different hydrophobic renovation render systems.

Service Life Assessment of Historical Building Envelopes Constructed Using Different Types of Sandstone: A Computational Analysis Based on Experimental Input Data

Service life assessment of three historical building envelopes constructed using different types of sandstone is presented. At first, experimental measurements of material parameters of sandstones are performed to provide the necessary input data for a subsequent computational analysis. In the second step, the moisture and temperature fields across the studied envelopes are calculated for a representative period of time. The computations are performed using dynamic climatic data as the boundary conditions on the exterior side of building envelope. The climatic data for three characteristic localities are experimentally determined by the Czech Hydrometeorological Institute and contain hourly values of temperature, relative humidity, rainfalls, wind velocity and direction, and sun radiation. Using the measured durability properties of the analyzed sandstones and the calculated numbers of freeze/thaw cycles under different climatic conditions, the service life of the investigated building envelopes is assessed. The obtained results show that the climatic conditions can play a very significant role in the service life assessment of historical buildings, even in the conditions of such a small country as the Czech Republic. In addition, the investigations reveal the importance of the material characteristics of sandstones, in particular the hygric properties, on their service life in a structure.

Parallel modeling of hygrothermal performance of external wall made of highly perforated bricks

Abstract Coupled heat and moisture transport in highly perforated bricks is indispensable part of design of energy efficient buildings. Geometry of the perforated bricks is very complicated which results in large number of nodes and elements in numerical analysis. Moreover, material model of the coupled heat and moisture transport leads after discretization to nonsymmetric systems of algebraic equations which need large computer memory. In order to reduce the computational time or to solve problems with many degrees of freedom, parallel computers are employed. Parallelization is based on the Schur complement method which is able to deal with nonsymmetric systems. Example of the coupled heat and moisture transport in a perforated brick of HELUZ company is showed. Real climatic boundary conditions for two different locations are used.

Computational prediction of hygrothermal conditions in innovated AAC-based building envelopes

Hygrothermal conditions in innovated AAC-based building envelopes are studied in the paper, using computational analysis with exactly measured hygric and thermal transport and storage properties as input parameters. The applied computational model of heat and moisture transport in multi-layered systems of porous materials is based on the Galerkin finite element approach. The computer code written in C++ is used in a series of computational simulations. Climatic data corresponding to the test reference year for Prague are used as boundary conditions, so that cyclic wetting and drying occurs in the envelope. The results of computer simulations of moisture and temperature fields are then utilized in the subsequent service life analysis. On the basis of changes of moisture and temperature, particularly the number of wetting-drying cycles and frost cycles the durability of the AAC envelopes is assessed.

Application of waste ceramic dust as a ready-to-use replacement of cement in lime-cement plasters: an environmental-friendly and energy-efficient solution

A potential application of waste ceramic dust as cement replacement in lime-cement plasters is studied using both experimental and computational approaches. A comprehensive experimental analysis of the material properties of lime-cement plaster and three lime-pozzolan plasters containing different amounts of waste ceramics is performed at first. The results show that compressive strength of ceramics-containing plasters can be up to three times higher as compared with the lime-cement plaster but their thermal conductivity is higher as well. In the second part of the study, the hygrothermal and energy performance of a characteristic building envelope provided with the four analyzed plasters as surface layers is analyzed. The results of numerical simulations reveal that the application of waste ceramic dust in lime-pozzolan plasters does not have a negative effect on both the hygrothermal and energy performance of the building envelope, as compared with the use of lime-cement plaster. Taking into account the energy demand and environmental load related to cement production, the application of waste ceramic dust as a ready-to-use replacement of cement in lime-cement plasters represents the right step toward sustainable development.

Determination of moisture-dependent moisture diffusivity using smoothed experimental data

Application of large sets of experimental data containing measurement errors in computational analyses without any preprocessing can lead to distortion of results. Therefore, transformation of these data from point-wise given values to smooth curves is one of the key factors allowing their proper utilization. In this paper, determination of moisture diffusivity of autoclaved aerated concrete as a function of moisture content using measured moisture profiles, which are processed by smoothing, is presented. For the processing of experimentally measured data smoothing splines are used.

Salt Damage and Rising Damp Treatment in Building Structures

Salt damage can affect the service life of numerous building structures, both historical and contemporary, in a significant way. In this review, various damage mechanisms to porous building materials induced by salt action are analyzed. The importance of pretreatment investigations is discussed as well; in combination with the knowledge of salt and moisture transport mechanisms they can give useful indications regarding treatment options. The methods of salt damage treatment are assessed then, including both passive techniques based on environmental control, reduction of water transport, or conversion to less soluble salts and active procedures resulting in the removal of salts from deterioration zones. It is concluded that cellulose can still be considered as the favorite material presently used in desalination poultices but hydrophilic mineral wool can serve as its prospective alternative in future applications. Another important cause of building pathologies is the rising damp and, in this phenomenon, it is particularly severe considering the presence of salts in water. The treatment of rising damp in historic building walls is a very complex procedure and at Laboratory of Building Physics (LFC-FEUP) a wall base hygroregulated ventilation system was developed and patented.

Effect of water-ice phase change on thermal performance of building materials

The effect of water ice-phase change on thermal performance of integrated building material is investigated in this paper. As a characteristic construction, simple external wall made of aerated autoclaved concrete was assumed which was exposed to dynamic climatic condition of Serak, Czech Republic. The computational modelling of hygrothermal performance was carried out using computer codes HEMOT and SIFEL that work on the basis of finite element method. The effect of phase change was taken into account by fixed-domain method, when experimentally determined effective specific heat capacity was used as a material parameter. It comprises also the effect of heat consumption and heat release that accompany the water-ice phase change. Comparing to the results with specific heat capacity, the effect of phase change on thermal performance could be quantified. The results showed that temperature fields can differ more than 6 °C. Additionally, the amount energy transported through the wall may be higher up to 4 %. ...

Determination of Radiative Heat Transfer Coefficient at High Temperatures Using a Combined Experimental-Computational Technique

Abstract The radiative heat transfer coefficient at high temperatures is determined using a combination of experimental measurement and computational modeling. In the experimental part, cement mortar specimen is heated in a laboratory furnace to 600°C and the temperature field inside is recorded using built-in K-type thermocouples connected to a data logger. The measured temperatures are then used as input parameters in the three dimensional computational modeling whose objective is to find the best correlation between the measured and calculated data via four free parameters, namely the thermal conductivity of the specimen, effective thermal conductivity of thermal insulation, and heat transfer coefficients at normal and high temperatures. The optimization procedure which is performed using the genetic algorithms provides the value of the high-temperature radiative heat transfer coefficient of 3.64 W/(m2K).