Search

The durability of porous building materials largely depends on the transport of liquids and dissolved salts into the material. When the moisture dries out, salts will crystallise introducing important forces in the solid matrix of the porous material. These interaction forces may result in cracking of the material. To study the salt degradation of porous building materials it is necessary to analyse in detail salt transport in (cracked) porous material. In general, salt transport modelling includes different processes: diffusive and convective salt transport, ion adsorption at the pore walls, salt crystallisation and dissolution. In this paper, we analyse in detail the experimental determination of the salt diffusion coefficient of Na2SO4 in fully water saturated ceramic brick by using a natural diffusion test. The diffusion coefficient is assumed to depend on the salt concentration. However, it appears that a rotational convective flow gives rise to a three dimensional combined diffusion convection transport. Keywords: Salt transport, natural diffusion test, salt diffusion coefficient, rotational convective flow, convection. Bestimmung des Koeffizienten der Salzdiffusion in porösen Werkstoffen Die Beständigkeit poröser Werkstoffe des Bauwesens hängt weitgehend vom Transport von Flüssigkeiten und darin gelösten Salzen in den Werkstoff hinein ab.Wenn ein Werkstoff wieder austrocknet, kristallisieren Salze im porösen Gefüge aus. Dieser Vorgang kann im Feststoffgerüst bedeutende Kräfte hervor rufen. Die Wechselwirkung mit diesen Kräften kann zur Rissbildung im Werkstoff führen. Um die zerstörende Wirkung von Salzen in porösen Werkstoffen des Bauwesens untersuchen zu können, ist es notwendig, den Salztransport in (gerissenen) Werkstoffen in seinen Einzelheiten zu analysieren. Im allgemeinen werden beim Modellieren des Salztransportes unterschiedliche Prozesse berücksichtigt: diffusiver und konvektiver Salztransport, Ionenadsorption auf den Wänden der Poren, Kristallisieren und Auflösen von Salzen. In diesem Beitrag analysieren wir Ergebnisse von Versuchen zur Bestimmung des Koeffizienten der Salzdiffusion von Na2SO4 in vollständig wassergesättigten Proben aus gebranntem Ziegelstein unter Verwendung einer natürlichen Diffusionszelle. Es wird dabei angenommen, dass der Diffusionskoeffizient von der Salzkonzentration abhängt. Es stellt sich jedoch heraus, dass rotierendes konvektives Fließen zu einem dreidimensionalen kombinierten Transport mit diffusiven und konvektiven Anteilen führt. Stichwörter: Salztransport, natürlicher Diffusionstest, Koeffizient der Salzdiffusion, rotierendes konvektives Fließen, Konvektion. In: International Journal for Restoration of Buildings and Monuments, Vol. 9, No 6, 663–682 (2003)

In this paper we investigate the influence of microheterogeneity on the phenomenological macroscopic material properties. The material analysed is Savonnie?res: a French layered oolitic limestone. Due to the complex microstructure of the material the moisture permeability cannot uniquely be determined based on indirect measurement data. To overcome this problem we present an upscaling technique based on the modelling of transport phenomena on the mesoscale. The technique allows to determine an equivalent macroscopic permeability based on the geometric configuration and the permeability of the composing constituents.

In a PhD study on the heating of monumental churches one of the problems, which was encountered, was the damage to wooden interior parts due to the changing of the indoor climate. Particularly warm air heating frequently leads to sudden changes in relative humidity, which turned out to be dramatically hard for monumental organs during wintertime. The warm air inlet conditions and their result on the airflow and air conditions were examined experimentally and by CFD-simulation. These results provided boundary temperature and humidity conditions, to which the monumental organ was exposed. In an experimental study the response of wooden parts to changing indoor conditions was tested. In a Nuclear Magnetic Resonance (NMR) test setup the moisture content of samples was measured as a function of time and place. These moisture content changes were related to the free mechanical deformation of samples. Together with the measurement of typical material properties of wood, a two-dimensional model was developed in FlexPDE: a finite element program to solve partial differential equations simultaneously.

The paper presents the characteristics of the Walloon Church in Delft (Netherlands) and a description of constraints for the indoor climate, giving criteria for the indoor air temperature and relative humidity with the focus on the preservation of the monumental organ. The setpoint operation of the HVAC system is evaluated by simulation using MatLab, FemLab and Simulink models. The next main model components are presented and combined in a single integrated SimuLink model: 1) a WaVo Simulink building model for simulating the indoor temperature and relative humidity, 2) a FemLab PDE model for simulating detailed dynamic moisture transport in the monumental wood (organ) and 3) a Simulink controller model. The building model is validated with measurements. The main advantage of the integrated model is that it directly simulates the impact of HVAC control setpoint strategies on the indoor climate and the organ. Two types of control strategies are discussed. The first type is a limited indoor air temperature changing rate. The second type is a limited indoor air relative humidity changing rate. Recommendations from international literature suggest that 1) a changing rate of 2 K/h will preserve the interior of churches and 2) a limited drying rate is important for the conservation of monumental wood. This preliminary study shows that a limitation of indoor air temperature changing rate of 2 K/h can reduce the peak drying rates by a factor 20 and a limitation of the relative humidity changing rate of 2 %/h can reduce the peak drying rates by a factor 50. The second strategy has the disadvantage that the heating time is not constant

The influence of macro-scale heterogeneities on the imbibition process is investigated for Savonnie?res, a French layered limestone. Free uptake experiments are performed both parallel and perpendicular to the bedding. It is found that the position of the different layers, and the exact material properties inside each layer can significantly influence the imbibition process. The experimental results are compared with numerical simulations. For the flow simulations, moisture permeability of the different layers is obtained with the upscaling technique presented in Part 1. Good agreement between simulations and experiments validate the proposed upscaling from meso to macroscopic scale.

This paper adresses the hygric behaviour of ceramic and calcium silicate brick treated with a water repellent oligomeric alkyl-alkoxysilane. Although the hydrophobic treatment succesfully blocks capillary effects, diffusion-adsorption processes of water molecules into the hydrophobic treated material still can take place. This process determines the particular hygric behaviour of hydrophobic treated materials. First, the contact angle of a drop deposited on an hydrophobic surface changes continuously over time due to absorption of water from the droplet into the hydrophobic material. This absorption process is found to be drop-size dependent. As a result large droplets, that remain on the hydrophobic surface for a longer time, are preferentially used to accurately measure the contact angle. Second, the water vapour permeability can decrease substantially due to clogging of connective pores caused by the hydrophobic treatment. Porous building materials showing an important midsize pore system within the length of the polymer chains are very sensitive to clogging of connective pores and therefore susceptible to an important increase in water vapour transmission resistance. Due to micro-droplet and water island formation on the hydrophobic network, the water vapour permeability increases with relative humidity. Finally, liquid water break-through in hydrophobic treated cracks is explained in view of microdroplet and water forming process in the crack. When liquid water islands become connected in the crack, capillary forces arise leading to the break-through.

A multiscale network model is presented to model unsaturated moisture transfer in hygroscopic capillary-porous materials showing a broad pore-size distribution. Both capillary effects and water sorption phenomena, water vapour and liquid water transfer are considered. The multiscale approach is based on the concept of examining the porous space at different levels of magnification. The conservation of the water vapour permeability of dry material is used as scaling criterion to link the different pore scales. A macroscopic permeability is deduced from the permeabilities calculated at the different levels of magnification. Each level of magnification is modelled using an isotropic nonplanar 2D cross-squared network. The multiscale network simulates the enhancement of water vapour permeability due to capillary condensation, the hysteresis phenomenon between wetting and drying, and the steep increase of moisture permeability at the critical moisture saturation level. The calculated network permeabilities are compared with experimental data for calcium silicate and ceramic brick and a good agreement is observed.

The damage process in quasi-brittle materials is a complex phenomenon in which heterogeneity plays an important role. This heterogeneity may imply that the exact failure mode can be highly dependent upon the precise spatial distribution of initial imperfections. To model this inhomogeneity stochastic material properties must be assumed in numerical simulations. However, the use of a stochastic approach does not resolve the issue of the change of character of the governing differential equations during progressive damage. As a consequence, the initial value problem becomes ill-posed at the onset of strain localisation and the finite element calculations suffer from a severe mesh dependence. A simulation technique that describes the true failure process must incorporate both a regularisation of the standard continuum during progressive damage and a stochastic description of the random continuum. This statement will be substantiated in this contribution. To do so, we will present finite element analyses of direct tension tests with a standard local damage model and with a nonlocal damage model. The randomness in the damage process will be introduced by considering the initial damage threshold of the continuum damage model as a random field, characterized by a relevant distribution and autocorrelation function. The response statistics calculated by the Monte Carlo technique will be presented for two different levels of discretisation. The non local and random field formulations rely both on the introduction of length parameter: the internal length of the nonlocal continuum and the correlation distance of the random field. The effect of the relative variation of the correlation distance and the internal length will also be investigated.

Continuum approaches are reviewed which can properly model localised deformations that act as a precursor to final fracture in quasi-brittle materials. Next, one such higher-order damaging continuum model is combined with a stochastic approach to describe the heterogeneity in quasi-brittle materials.

Continuum approaches are reviewed which can properly model localised deformations that act as a precursor to final fracture in quasi-brittle materials. Next, one such higher-order damaging continuum model is combined with a stochastic approach to describe the heterogeneity in quasi-brittle materials.

Probabilistic finite elements in conjuncture with continuum damage mechanics are used to model the stochastic damage behaviour of strain-softening materials. The material properties are modeled as random fields, describing the spatial distribution and interdependence. It is shown that a stochastic model that describes the true failure process must incorporate both a regularisation of the standard continuum during progressive damage and a stochastic description of the random continuum. To substantiate this statement, we will present finite element analyses of direct tension tests with a standard local damage model and with a nonlocal damage model for different degrees of discretisation. The effect of the number of random variables and their crosscorrelation will also investigated.