Leo L Pel

Interlaboratory comparison of hygric properties of porous building materials

The precision of methods used for the determination of hygric properties of porous building materials was investigated. The study was performed in the framework of the EU-initiated HAMSTAD-project. Six laboratories measured the selected hygric properties of three porous building materials. While the most measured properties show acceptable agreement, yet, it was found that some of the existing standards or commonly accepted measurement methods need improvement. Most striking were large variations in the results of the vapour transmission tests performed in accordance to the existing European Standard.

One-dimensional scanning of moisture in porous materials with NMR

A versatile and modular nuclear magnetic resonance (NMR) instrument is described that is particularly suited for the study of moisture transport in porous media such as various building materials in which moisture can give rise to several kinds of damages. Quantitative measurements of one‐dimensional moisture profiles and their time evolution can be performed on cylindrical samples having a diameter up to 20 mm with a spatial resolution better than 1 mm. Water absorption and drying experiments on various building materials demonstrate that the instrument can also be used for materials containing relatively large amounts of magnetic impurities, which until now were hardly accessible by NMR techniques.

Determination of moisture diffusivity in porous media using scanning neutron radiography

A technique for measuring moisture concentration profiles based on scanning neutron radiography is presented. With this technique moisture concentration profiles have been measured during drying of brick and kaolin clay. The isothermal moisture diffusivity as a function of moisture content could be determined directly from these profiles. An error analysis shows that the minimum spatial resolution for scanning should be 0.5 mm. If this condition is not satisfied, it is not possible, for example, to detect the minimum in the moisture diffusivity in the case of a receding drying front.

Ion transport and crystallization in inorganic building materials as studied by nuclear magnetic resonance

Salt weathering is a major cause of deterioration of buildings and monuments of cultural heritage. We have determined the underlaying moisture and ion transport within a representative building material by measuring the time evolution of NaCl-saturated samples during one-sided drying using nuclear magnetic resonance. The measured NaCl concentration profiles reflect the competition between advection to the surface and redistribution by diffusion. By representing the measured moisture and NaCl profiles in an efflorescence pathway diagram (EPD) the crystallization is also taken into account. The pathways followed in the EPD indicate that for historical objects in general, crystallization at the surface cannot be avoided.

A Comparison of Different Techniques to Quantify Moisture Content Profiles in Porous Building Materials

Several advanced non-destructive techniques are available to measure the evolution of content profiles with time, allowing the analysis of unsaturated flow and the determination of the moisture diffusivity of porous building materials. The reliability of six different techniques is investigated: the NMR-technique, the MRItechnique, the γ-ray attenuation technique, the capacitance method, the X-ray projection method and the TDR-technique. All of them were applied to measure the moisture content evolution during free uptake experiments on two building materials. Considering the limitations of some of the techniques, a good overall agreement is obtained. The work presented is an outcome of the EU-initiated HAMSTAD-project.

Determiination of moisture diffusivity in porous media using moisture concentration profiles

A procedure is presented to determine the moisture diffusivity for drying from measured moisture concentration profiles. From error analysis it is shown that the applied space grid for scanning and the one-dimensional resolution of the NMR (nuclear magnetic resonance) method used for the measurements had a minor influence on the accuracy of the determination of the moisture diffusivity. The inaccuracy was dominated by experimental noise and inhomogeneities in the porosity of the materials under investigation. A receding drying front method is presented to determine the moisture diffusivity at low moisture contents: here the moisture transport is dominated by vapour transport, which limits the overall drying rate. The method uses the velocity of the receding drying front to approximate the moisture diffusivity.

Determination of the liquid water diffusivity from transient moisture transfer experiments

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.

How ions distribute in a drying porous medium: A simple model

Salt crystallization at surfaces is an important problem for buildings and monuments. We do not consider the formation of salt crystals as such, but focus on transport properties of ions in a drying porous medium. We deal with the first phase of the drying process, where the water is still uniformly distributed throughout the medium. An approximate model is presented, which accounts for both convection and diffusion. It is shown that the key parameter is the Peclet number at the evaporating surface, Pe≡hL/eD, where h, L, e, and D are the drying rate, sample size, porosity, and diffusion constant, respectively. When Pe≪1 (diffusion dominates over convection) the ions remain uniformly distributed throughout the system. Strong accumulation at the evaporating surface occurs for Pe≫1 (convection dominates over diffusion). Crossover behavior is found for Pe≈1. Therefore, it is likely that the first crystals will be formed both in the bulk and at the interfaces of the material when Pe≪1. For high values of Pe th...

Combined NMR cryoporometry and relaxometry

Both cryoporometry and relaxometry probe the surface-to-volume ratio of a porous material. Nuclear magnetic resonance (NMR) relaxometry uses the random motion of molecules, whereas cryoporometry uses the melting-point depression of a confined liquid. An NMR setup has been built to simultaneously perform cryoporometry and relaxometry measurements. Using materials with a well-defined pore size, i.e. silica gels, both methods are compared with the standard N2-adsorption technique, and a good correlation is found. The methods can be used in the pore size range between 1 and 100 nm. By performing NMR relaxometry during cryoporometry, more information about the pore-size distribution can be obtained. Besides for silica gels, this is demonstrated for mortar, which has a complicated pore structure.

Salt transport and crystallization in porous building materials.

Salt weathering is a major cause of deterioration of porous building materials. To obtain information about the mechanisms underlying these damage processes we have studied the moisture and ion transport. We measured the time evolution of NaCl saturated samples of fired-clay brick during one-sided drying using Nuclear Magnetic Resonance. The moisture content and amount of dissolved Na ions could be measured quantitatively as a function of position. The NaCl concentration profiles obtained from these data reflect the competition between advection to the surface and redistribution by diffusion. By representing the measured moisture and NaCl profiles in an efflorescence pathway diagram (EPD) also the crystallization can be taken into account.