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The paper presents a method to calculate the moisture penetration depth and the undisturbed moisture level, inside the region of periodic variation, in a porous material subjected to a periodic relative humidity variation at the boundary. The moisture properties may be highly non-linear. The method is strictly valid for a semi-infinite material. The theory to determine the undisturbed moisture level is given in detail, and a calculation method to determine the penetration depth is presented. A specially designed computer program has been developed. Results for six different building materials subjected to daily and annual periodic relative humidity at the boundary are given.
Introduction
A building material is normally exposed to moisture conditions that are variable with time. Natural variations in moisture state can often be described as a periodic variation. A material exposed to such periodic moisture conditions during a long time will take up or loose humidity in a periodic way.
Moisture transport calculation in building components and constructions would be rather time consuming if we take all variable parameters occurring in a natural situation into account. For each material we have moisture equilibrium curves and moisture transport coefficients, which very often are strongly dependent on moisture state. We also have the surrounding climate with day and night fluctuations and variations according to the season. Simplifications have to be made in most cases in order to be able to handle a specific problem on a reasonable level of complication. A good simplification is one that reduces the problem without loosing to much accuracy in the result. One possible simplification is to use mean values of a variable condition, another is to consider the transport coefficients constant (Kerestecioglu 1990 and Cunningham, 1992). If a constant transport coefficient is used it is even possible to solve the problem analytically. |