Determination of the Liquid Water Diffusivity from Transient Moisture Transfer Experiments

Abstract

The Boltzmann transformation method is used to determine the liquid water diffusivity from moisture content profiles as measured in a capillary water absorption experiment. An inter-laboratory comparison for analyzing the reliability of the determination method showed that the inaccuracy in the liquid water diffusivity is caused by scatter in the transformed data and by uncertainty in the boundary conditions at the intake surface and ahead of the steep moisture front. A methodology is proposed based on (1) the evaluation of the validity of the diffusion approach, (2) a simplified handling of the boundary conditions, (3) smoothing of the scattered data and (4) the evaluation of the quality of the determined liquid water diffusivity. For HAM (Heat-Air-Moisture transport) calculations values of the liquid water diffusivity for moisture contents higher than the capillary moisture content are disregarded. The liquid water diffusivity can be described by an exponential function limited at a lower moisture content bound.To describe the moisture diffusivity including liquid water and water vapour transports, a new parametric description of the moisture diffusivity is presented, which shows sufficient flexibility both in the hygroscopic and overhygroscopic ranges. When permeability is calculated from diffusivity, the permeability should monotonically increase with decreasing capillary pressure. In the hygroscopic region it should coincide with the measured water vapour permeabilities.

Determination of hygrothermal parameters of experimental and commercial bio-based insulation materials

“…Thermal diffusivity is reported to be sensitive to moisture content as well. Carmeliet et al [8] reported a reduction of 5 to 13% when the moisture content was doubled for brick and calcium silicate respectively. Jerman et al [9] reported a rate variation of water vapour permeability of 0.13 10 -6 m 2 /s for each 1% of change in RH for an aerated concrete with a water vapour permeability (dry cup) of 2.17·10 -6 m 2 /s.…”

Characterization of fracture patterns and hygric properties for moisture flow modelling in cracked concrete

“…This can be done using gravimetric methods [17] or techniques such as nuclear magnetic resonance, c-ray attenuation, capacitance method or X-ray projection [2]. Several purposes can be served by these methods: extrapolation of the liquid conductivity from transient one-dimensional moisture uptake tests [18], observation of local variations of the flow speed due to fractures [19] or of the infiltration of chemicals, studies of reinforcement corrosion in concrete [8], etc. In the recent years, these techniques have been increasingly used in material sciences, and new operating methods were presented for their optimization and for a more efficient interpretation of their results.…”

“…As explained in [18], D w can be derived from transient 1D moisture profile measurements through a Boltzmann transform…”

“…As stated in [18], a boundary moisture content w B must be assigned at the inflow surface in order to satisfy the Boltzmann boundary conditions. This value can be considered equal to the capillary moisture content of the material, i.e.…”

“…The permeability was measured following the Boltzmann technique [18], which requires the monitoring of moisture concentration distributions in the material during capillary uptake.…”