Dynamic effects in porous building materials

The research project Dynamic Effects in Porous Building Materials aims at determining potential causes of so-called dynamic effects concerning the moisture transfer behaviour of construction materials. The research is carried out through experimental methods and pore- scale investigation. An accurate modelling of moisture transfer in building materials is decisive for predicting potential damages and hence for achieving a correct hygrothermal design. This statement concerns particularly refurbishment of existing buildings (e.g. by applying internal insulation), where the choice of proper solutions has an essential practical relevance: on the one hand extended building refurbishment has to contribute for a reduction of the overall energy consumption and CO2 emissions. On the other hand, moisture-safe solutions are required for conservation of valuable listed and historical buildings. For these reasons, a great effort has been made in recent years in modelling the hygric behaviour of porous materials. Nerveless existing models still present important limitations concerning mainly the common assumption of local equilibrium i.e. the instantaneous equilibrium assumed between capillary pressure and water content inside the pores. Recent results have shown that this assumption is not always verified. Indeed, deviations may occur, which are referred to as dynamic effects in the literature. An advanced investigation on this phenomenon is urgently required and represents the main motivation of this project. Moving on from early results obtained at the building physics section of KU Leuven (the host institution of this project proposal), dynamic effects will be investigated using a pore scale approach. To this aim, the different phenomena affecting the hygric dynamic behaviour will be implemented in characteristic pore-networks for a set of exemplary materials. An experimental campaign will be carried out in order to gain the data necessary for the model validation. To this aim, sophisticated experimental techniques based on both absorption and desorption tests are developed and/or improved. The validated models will be used for explaining the potential causes of dynamic effects and for quantifying the impact of this phenomenon for a set of different boundary conditions and problem parameters. The proposed project presents important scientific originality concerning both modelling and experimental issues. On the one hand, to the knowledge of the authors, dynamic effects in building materials have never been investigated trough pore scale modelling until now. Some results concerning this topic are already available in soil science; however the extension to building materials requires additional modelling effort. On the other hand, experimental investigation of dynamic effects is at a very early stage. The rare studies on this topic concern a restricted capillary pressure range and are limited to few materials. Nevertheless, such early results clearly indicate that dynamic effects have a decisive influence under certain boundary conditions. For this reason, the contribution of the proposed research is essential for further development of moisture transfer modelling.

Accurate modelling of moisture transfer in porous materials is crucial in numerous fields on science and engineering, as for instance concrete technology, hydrology and building physics. Moisture affects the durability and sustainability of built structures as well as the health and comfort of building occupants. The corrosion of reinforcement bars in concrete due to chloride ingress and the degradation of timber elements via fungal attack are just two examples of potential moisture-related damages. On the other hand, excessive humidity levels at the internal surfaces may induce mould formation and depreciate air quality. Moisture is hence often the critical factor when judging built structures' durability and sustainability as well as building occupants' health and comfort, for which the accurate understanding and reliable assessment of moisture transfer in building materials are crucial. Although the use of numerical models for prediction of the moisture risk is well-established, fundamental kinetics of moisture storage and transfer in porous building materials are not fully understood yet. Available numerical models based on the continuum approach generally accept moisture transfer in porous materials to be a purely diffusive process, by assuming the moisture transfer potential (capillary pressure) to be independent of the (de)saturation rate (speed of local saturation changes over time). Several experimental findings put this assumption into question however, having been noted that sufficiently high (de)saturation rates may significantly influence imbibition and drying processes. This phenomenon is addressed in the literature as "dynamic effects". The investigation of potential causes of dynamic effects and the assessment of their magnitude in different moisture transfer processes constitute the object of this research. Moving on from early results obtained at the building physics section of KU Leuven (the host institution of this project), numerical pore scale models are developed and applied to investigate the (dynamic) moisture transfer and storage behaviour of porous materials. To this aim, the different phenomena affecting moisture transfer and storage (capillary absorption, vapour diffusion, condensation and evaporation processes) are implemented in suitable algorithms for simulation of capillary wetting/drying processes. In addition, an experimental campaign is carried out on exemplary materials. The obtained results reveal that considering dynamic effects is crucial for reliable modelling of imbibition/drying processes and enables better insight on the causes of this phenomenon. Finally, analytical models are proposed to describe the magnitude of dynamic effects as a function of the (de)saturation rate, by taking into account the influence of pore size distribution and pore surface wettability.