Application of effective media theory for determination of thermal properties of cavity bricks in dependence on moisture content

E. Vejmelkova1, M. Jerman1, L. Fiala1, Z. Pavlik1 and R. Cerny1

1Czech Technical University in Prague, Czech Republic

Keywords: thermal properties
property: thermal conductivity
material: cavity bricks

Cavity brick blocks have found a widespread use in building industry during the last decades. The increasing requirements to the thermal insulation properties of building envelopes given by national standards in Europe led the brick producers to reduce drastically the production of common solid bricks. The brick blocks with complex systems of internal cavities replaced the traditional bricks and became dominant on the building ceramics market. However, the development of the new types of bricks was very fast and their thermal properties were improved in more or less intuitive way. Experiments in steady state conditions then either confirmed the expected values of apparent thermal conductivity or showed a failure in a trial-and-error process. The dependence of thermal conductivity of the cavity bricks on moisture content was not taken into account, assuming implicitly that moisture cannot be present in the bricks in significant amounts.

In this paper, an effective media treatment is applied for the determination of apparent thermal conductivity of cavity brick blocks in dependence on moisture content. At first, the thermal properties of the brick body are analyzed in the whole moisture range from dry state to full water saturation. Then, homogenization principles are applied in two steps. The first step consists in combination of the properties of the brick body and air cavities in dry state. The second includes the moisture content both in the brick body and cavities. Several different homogenization formulas are used and their applicability is verified using a steady-state experiment simulating heat transport in the brick block in difference temperature conditions.

This research has been supported by the Czech Ministry of Industry and Trade, under project No. FR-TI2/007.



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