Determination of the Isothermal Moisture Transport Properties of Porous Building Materials

Abstract

A multiscale network approach is proposed as a practical method for estimating the isothermal permeability of porous building materials covering hygroscopic and overhygroscopic ranges. The multiscale approach is based on the concept of examining the porous space at different levels of magnification, and it allows modeling of combined liquid water and water vapor transfer over a wide saturation range. The proposed method simply requires the knowledge of common moisture properties, such as the capillary pressure curve, the capillary absorption coefficient and water vapor permeability, which can be determined from standard experiments. Hysteresis between drainage and wetting due to air entrapment can be adequately modeled by considering only those pores that become filled by water. The calculated network permeability is validated comparing experimental and simulation results of isothermal capillary absorption and drying processes in ceramic brick and calcium silicate. It is shown that transient moisture processes of capillary absorption from a dry sample and the isothermal drying from an initially capillary saturated sample can be accurately modeled by a single nonlinear permeability and capillary pressure curve. The capillary pressure curve to be used is the main wetting curve, which describes the moisture content relationship from the dry state to the capillary moisture content, correctly taking into account air entrapment phenomena. The description of the absorption or drying moisture processes by a single or a double exponential diffusivity relation was found to be less accurate.

Keywords

moisture transport properties permeability diffusivity multiscale network model capillary absorption,drying

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References

Brocken, H., 1998. “Moisture transport in brick masonry: the grey area between bricks,” Ph.D. thesis, Eindhoven University of Technology, the Netherlands.

Brutsaert, W. , 1979. “Universal constants for scaling the exponential soil water diffusivity?,” Water Resources Research; 15(2): 481–483.

Carmeliet, J. , Descamps, F. and G. Houvenaghel , 1999a. “A multiscale network model for simulating moisture transfer properties of porous media,” Transport in Porous Media, 35, 67–88.

Carmeliet, J. , Roels, S. and K. Van Den Abeele , 1999b. “Multiscale network approach for estimating the imbibition permeability of porous building materials showing air entrapment,” submitted for review, Water Resources Research.

Carmeliet, J. and S. Roels , 1999. “Non-isothermal moisture flow in porous materials,” Proc. of the 10th. Bauklimatisches Symposium, ed. Tech. Univ. Dresden, 1, 221–230.

Crank, J. , 1989. “The mathematics of diffusion,” Oxford University Press, Oxford.

Descamps, F., 1997. “Continuum and discrete modelling of isothermal water and air transfer in porous media,” Ph.D. Thesis, Catholic University of Leuven, Leuven.

Durner, W. , 1994. “Hydraulic conductivity estimations for soils with heterogeneous pore structure,” Water Resources Research, 30, 211–223.

Künzel, H.M., 1995. “Simultaneous heat and moisture transport in building components, one-and two-dimensional calculation using simple parameters,” Dissertation, IRB-Verlag Fraunhofer-Institut für Bauphysik, Stuttgart, Germany.

10.

Krus, M., 1995. “Feuchtetransport-und Speicherkoeffizienten poröser mineralischer Baustoffe. Theoretische Grundlagen und neue AU 15: Messtechnieken,” Dissertation, Universität Stuttgart, Germany.

11.

Krus, M. and A. Holm , 1999. “Simple methods to approximate the liquid transport coefficients describing the absorption and drying,” Proceedings of the 5th Symposium on Building Physics in the Nordic Countries, Chalmers University of Technology, Göteborg, Sweden, 241–248.

12.

Mulaem, Y. , 1976. “Hysteretical models for prediction of the hydraulic conductivity of unsaturated porous media,” Water Resources Research, 12(6), 1248–1254.

13.

Pel, L., 1995. “Moisture transport in porous building materials,” Ph.D. thesis, Eindhoven University of Technology, the Netherlands.

14.

Philip, J.R. , 1955. “Numerical solution of equations of the diffusion type with diffusivity concentration-dependent,” Transactions of the Faraday Society, 51, 885–892.

15.

Roels, S. , Carmeliet, J. and H. Hens , 1999a. “Mesh adaptive finite element formulation for moisture transfer in materials with a critical moisture content,” International Journal for Numerical Methods in Engineering, 46(7), 1001–1016, 1999.

16.

Roels, S. , Elsen, J. , Carmeliet, J. and H. Hens , 1999b. “Pore volume distribution determination of limestone combining mercury porosimetry and micrography,” accepted for publication, Materials and Structures (RILEM).

17.

Roels, S., 2000. “Moisture transfer in heterogeneous porous media,” Ph.D. Thesis, Catholic University of Leuven, Leuven, 2000.