Dynamic effects in porous building materials
01.02.2019 - 31.01.2021
Funded by: Austrian Science Fund (FWF), Erwin Schrödinger Fellowship
Project leader: Michele Bianchi Janetti
Host institute: KU Leuven (BE), Building Physics and Sustainable Design Section
Summary:
Building moisture can have a direct impact on the durability and sustainability of buildings and an indirect impact on the health and comfort of occupants. The accurate understanding and reliable assessment of moisture transport in building materials is therefore very important to design damage-free buildings and optimise materials accordingly.
The use of numerical models to predict moisture risk can be considered state of the art today, nevertheless, the basic kinetics of moisture storage and transport in porous building materials is usually only approximated. Available models based on continuum theory assume moisture transport in porous materials to be a purely diffusive process, with the assumption that the moisture transport potential (capillary pressure) is independent of the process rate (drying or wetting rate). However, several experimental results show that this assumption is inaccurate in some cases. It was found that the process speed can significantly influence the capillary pressure. This phenomenon is referred to in the literature as the "dynamic effect".
Based on the results obtained at the Department of Building Physics of the KU Leuven (the host institute for this project), dynamic effects are investigated here by means of modelling on the pore scale. The results obtained show that dynamic effects can have a considerable influence on both the absorption and drying process and allow the causes of this phenomenon to be better recognised and evaluated. Finally, analytical models are proposed that describe the extent of dynamic effects as a function of process speed, taking into account the influence of pore size distribution and wettability of the pore surface.
(a) Phase field simulation of the moisture distribution in an exemplary pore structure. The colours red and blue indicate the humidifying and non-humidifying fluid respectively; (b) Exemplary moisture storage functions for a stationary and transient humidification process. The deviation between the steady-state and transient curves is due to dynamic effects.
Publications
Bianchi Janetti M, Janssen H. (2020). Impact of the drying rate on the moisture retention curve of porous building materials. Construction and Building Materials 258:1-9. https://doi.org/10.1016/j.conbuildmat.2020.119451.
Bianchi Janetti M, Janssen H. (2020). Pore scale modelling of moisture transfer in building materials with the phase field method. E3S Web of Conferences, 172. https://doi.org/10.1051/e3sconf/202017204004
Bianchi Janetti M, Janssen H. (2019). Characterisation of the diffusivity through water-uptake tests, MATEC Web of Conferences, 282. https://doi.org/10.1051/matecconf/201928202040