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When dealing with porous media, the liquid-gas phase-change is generally considered instantaneous, while a retardation time is observed in the case of hygroscopic soils. So far, little research has been done to characterize the non-equilibrium behaviour of water phase change. Therefore, we propose a macroscopic model of the liquid-gas phase-change rate in porous media, based on the difference of chemical potentials between the liquid and its vapour, which is taken as the driving force. It introduces a phenomenological coefficient that must be determined experimentally. An original experiment able to create a macroscopic non-equilibrium between the liquid and its vapour is described. Analysing the return to equilibrium leads to the determination of the phenomenological phase-change coefficient. Depending on the range of partial vapour pressure, two different behaviours are observed: a linear domain close to equilibrium and a nonlinear one far from equilibrium. The results emphasize the relation between water retention properties in hygroscopic porous media and these phase-change characteristics.

The phenomenological relation of non-equilibrium liquid-gas phase change in a porous medium is described at the macroscopic level using the difference in chemical potentials between the liquid and its vapor. The experiments conducted consisted in lowering the partial pressure of water vapor in the pores of a hygroscopic soil and analyzing the return to equilibrium by two measurements: the macroscopic temperature and the partial pressure of vapor. The central hypothesis of the study is that the characteristic time associated with thermal equilibrium is much lower than the characteristic time associated with mass transfers. From these measurements, it is possible to determine the relation that links phase change rate to the logarithm of the ratio of partial vapor pressure divided by the equilibrium pressure (RH). The representation of this relation according to RH reveals two regimes in the return to equilibrium. The characteristics of these regimes are analyzed according to water content, temperature, and total gas phase pressure.

Interpreting the drying kinetics of a soil using a macroscopic thermodynamic non-equilibrium of water between the liquid and vapour phase A. Chammari (a) , B. Naon (b) , F. Cherblanc (a) , B. Cousin (a) and J.C. Bénet (a)

This paper presents the results of using distributed Brillouin fibre sensors to detect crack formation in a simply supported reinforced concrete beam subjected to four-point loading. A Brillouin multiple-peak fitting method was used to enhance the spatial and strain resolutions of the measurements. By doing this, the distributed strain profile along the beam was determined with a 5 cm read-out resolution in comparison with the 15 cm spatial resolution of the fibres. The location of the cracks was identified by locating the positions in the strain profile where the strain suddenly changes, by searching for the maximum compressive or tensile peaks in the Brillouin frequency spectrum, as opposed to conventional strain reading, which focuses solely on the maximum Brillouin peak. The amplitude of the Brillouin peak for the suddenly changed strain (crack) was found to be smaller than half of the amplitude of the maximum Brillouin peak at the maximum strain location corresponding to the average strain of the material, which would have been neglected by standard peak or area fitting methods, especially for fine cracks or the initial crack build-up period.