Water Absorption in Two-Layer Masonry Systems - Properties, profiles and predictions

Sammanfattning: The main impetus for this research was the desire to verify models for determining transient moisture profiles in the capillary range. For the evaluations, transient moisture profiles were both simulated and measured, after which the results of the two methods were compared. The study was performed for on specimens exposed to continuous water absorption, using specimens of both single and combined materials. The materials used for the study were two mortars and a lime silica brick. For testing, we chose materials with moisture properties corresponding to those of common inorganic façade materials.

As input for the simulations, data such as sorption isotherms and moisture diffusivities were determined for the studied materials. Sorption isotherms were experimentally determined over the complete moisture range, two separate methods having to be used for the hygroscopic and the capillary parts of the moisture range. In the hygroscopic moisture range, both absorption and desorption isotherms could be determined, but in the capillary moisture range only desorption isotherms could be determined. To allow measurement of absorption isotherms in the capillary moisture range, the measuring equipment had to be modified; however, despite the modifications, reliable absorption measurements were still not possible. The desorption isotherms determined by the two different methods generally agreed well for all tested materials.

Moisture diffusivities were determined by evaluating a series of water absorption tests using a Boltzmann transformation principle. To determine moisture diffusivities over the complete moisture range, the water absorption tests were performed on specimens conditioned to different initial moisture contents, ranging from completely dry to the vacuum saturation point. The variation in the results was significantly reduced when the water content was expressed as the degree of vacuum saturation; thus, moisture diffusivity was determined as a function of moisture content, expressed as a degree of vacuum saturation.

Different methods for measuring the transient moisture profiles were considered. The moisture profile measurements were supposed to be performed on specimens of both single and combined materials. Two of the most interesting methods were the nuclear magnetic resonance (NMR) technique and the slice and dry method. The NMR technique displayed excellent precision in terms of both moisture content measurement and the spatial resolution. However, the method was severely limited, in that even an insignificant amount of iron in the specimen disturbed the measurements. Another limitation was restricted access to the measuring equipment.

For the larger-scale measurements of the moisture profiles, the slice and dry technique was chosen. This method offered high and conclusive precision in terms of moisture content measurement, and only relatively simple laboratory equipment was needed. One drawback of the method was that the specimens had to be sliced for testing, meaning that a particular specimen could not be tested repeatedly. Other drawbacks were limited spatial resolution and the destruction of the specimen.

Moisture profiles were measured after a continuous wetting phase, for both the specimens of three single materials and the two material combinations. The combinations were intended to simulate a façade consisting of a layer of mortar on masonry: the outer layer of mortar was exposed to continuous wetting, and the moisture profiles were then measured for the material combination.

Both the thickness of the outer mortar layer and the properties of the mortar were varied. The measurements showed that the moisture properties of the outer layer of mortar depended more significantly on both the moisture penetration depth and moisture levels in the underlying material than on mortar thickness.

Moisture profile simulations were performed using off-the-shelf software, using as inputs the absorption isotherms and moisture diffusivities determined in this work. The simulations were intended to reflect the larger-scale experimental portions of this research. To this end, the boundary conditions and wetting durations used as inputs corresponded to the conditions of the larger-scale moisture profile measurements.

The simulated and measured results for the combined materials largely agreed with each other, in terms of both moisture levels and depth of moisture front. However, the simulated moisture profiles of the two mortars as single materials significantly diverged from the measured values. The simulated profiles lacked a clear front in the moisture levels and moreover were overestimated after the front. Alternative simulations were carried out in which the deviation between the simulations and the measurements was clarified, and was shown to depend on errors in the technique for evaluating diffusivities at low moisture levels.

Comparison between the simulated and measured moisture profiles indicated that moisture profiles can be predicted with considerable precision in the high moisture range. However, by determining additional material data pertaining to moisture diffusivity in the low moisture range, the precision of the predictions can be improved. A primary conclusion of the research is that moisture profiles can be predicted for combined materials using off-the-shelf software.