Qualitative work to study water movement in wood

Transcription

1 Qualitative work to study water movement in wood Dominique Derome, assistant professor. Building Envelope Performance, Laboratory, Department of Building, Civil and Environmental Engineering, Concordia University, 1455 de Maisonneuve blvd West, Montreal, Qc, Canada, H3G, 1M8, Staf Roels, associate professor, Department of Civil Engineering, Laboratory of Building Physics, Katholieke Universiteit Leuven, Kasteelpark Arenberg 51, 3001 Leuven,Belgium; Jan Carmeliet, professor, Department of Civil Engineering, Laboratory of Building Physics, Katholieke Universiteit Leuven, Kasteelpark Arenberg 51, 3001 Leuven,Belgium; also at Faculty of Building and Architecture, Building Physics Group, T.U.Eindhoven, The Netherlands KEYWORDS: wood, X-ray projection, liquid water transport, water vapour transport SUMMARY: Wood in building wall assemblies may come in contact with liquid water. Rain, as the main source of water, is expected to be shed by the wood cladding. Improper detailing may result in water entering the wall, reaching the wood sheathing or even the wood frame cavity. Traditional methods like capillary uptake provide some indication of the process of liquid movement in wood but cannot convey the role of the wood structure in the water entry, redistribution and drying processes. This paper presents qualitative results using X-ray images to view moisture distribution within wood during liquid uptake. The microstructure of wood and the direction of grain are shown to influence the pattern and rate of moisture movement. 1. Introduction When it comes to study liquid movement, wood is a complex material. Capillary movement depends of the diameter of pores, on the connectivity of pores (through bordered pit pores) and the direction of grain. Cell diameter and the number of bordered pit vary from earlywood to latewood. Also wood is orthotropic and the movement of water in the longitudinal, radial and tangential directions varies in terms of mode of distribution and rate. Liquid movement is accompanied by redistribution by vapour diffusion. Diffusion in wood is also a complex phenomenon as diffusion occurs through air (in pore voids) and through cell walls, and the rate of diffusion is affected by local moisture contents and temperature gradients. Furthermore, moisture content change below the fibber saturation point is accompanied with change of volume. Therefore, there is still a need to better understand liquid water uptake by wood, moisture redistribution within wood and drying of wood. As the study of moisture movement in wood must therefore take into account the structure of wood, this paper first presents some basics information of the structure of two species of wood. The method of using X-ray to measure moisture content is presented and the qualitative results on two wood species are presented. A discussion on further work needed in this area completes the paper. 2. Structure of wood Wood has three material scales (FIG. 1):

2 At the macroscale, three orthotropic directions can be distinguished, namely longitudinal, radial tangential. The annual growth rings consist of earlywood and latewood. Wood rays oriented perpendicular to the longitudinal direction may cross the growth rings and consist of banded tissue. The micro- mesostructure of wood is cellular, consisting of small earlywood and long latewood longitudinal cells, called tracheid (for spruce, length of 3 to 5 mm and diameter of 20 to 80 x 10-6 m), and thin walled cells, called parenchyma, located in the radial direction. In hard wood, vessels are large longitudinal cells and can be plugged with tyloses. Vessels, tracheid and parenchyma cells can also be interconnected by extensive pitting. The cell walls show a layered structure, governing the physical properties of wood. Pitting between adjacent cells may provide intercellular transport. FIG 1. Microstructure of spruce on the left (1) and oak on the right (2) 3. X-ray method to visualise and measure moisture content The X-ray technique allows to visualise the internal features of non-transparent materials in a nondestructive way. The technique is explained in detail in (3). The measurements presented in this paper were performed with an AEA microfocus X-ray apparatus, at the Katholieke Universiteit Leuven (FIG. 2). FIG. 2. Experimental set-up of the X-ray projection method showing the wood sample in the pool of water When an object is irradiated with an X-ray beam, the beam is attenuated due to the interaction with the material. The attenuation depends of the attenuation coefficient of the material and the thickness of the object. When moisture is present in the material, the attenuation depends also on the attenuation coefficient of water and the thickness of a fictitious water layer in agreement with the water content of the material.

3 The moisture distribution is obtained by logarithmically subtracting an image of the dry sample from an image of the wet sample. 4. Water distribution measurements during water uptake In a first step, the applicability of the technique for water uptake in wood was investigated. A few adjustments were required to the test set-up originally developed for porous materials like brick, stone and concrete. A weight was put on the wood sample to prevent the sample to float on the water pool. As wood changes dimensions depending on the moisture content in the hygroscopic region, a very fine metallic wire was positioned to frame the sample and allow for tracking of dimensional changes. Specimens of spruce (most probably white spruce) were cut to be approximately 2 cm by 3 cm by 5 cm high. They were cut in such a way as to have the early and late wood layers visible on the image and that water movement be in the radial, tangential or longitudinal direction. Then, they were covered with an adhesive tape on 4 sides, starting at 3 mm from the bottom surface, to minimise water losses though the sides. A first image (average of 256 pictures) was taken with the specimens oven dried. Then water was poured into the dish until the surface of water was in good contact with the bottom of the wood specimen. Images were taken at every 5 minutes for one hour. The weight of the specimens was taken before and after the uptake of water. Table 1 shows that the amount of water absorbed by the wood longitudinally was twice the amount absorbed radially, while tangentially, the amount was 50% more than radially. Table 1. Dry weight and moisture content of specimens at end of test Weight dry [g] Weight of water after one hour [g] Moisture content at end of test [%] Longitudinal water up-take Tangential water up-take Radial water up-take Water distribution along the longitudinal direction during water uptake test of spruce In the longitudinal direction, it was observed that water moves up into the finest capillaries, i.e. in the late wood layer of each growth ring. The rate of wetting was rapid and, during the test, water was observed at the top of the specimen around 20 minutes after initial contact with water. In addition, figures 3 and 4 demonstrate how water is concentrated in the layers of latewood. This preferential wetting pattern is explained by the smaller diameter of the tracheid in latewood and a larger number of pit connection between cells in latewood compared to earlywood. 4.2 Water distribution along the tangential direction during water uptake test of spruce The wetting pattern in the tangential direction shows moisture uptake in both the latewood and earlywood layers, see Figure 5. The distribution is more uniform than what was observed in the longitudinal test but the rate of water uptake is slower as reflected by the final moisture content of the specimen.

4 FIG. 3. X-ray view (where black is dry and white is wet) at 5 minutes (left) and 60 minutes (centre) from start of test and photo of top of specimen (right) at the end of the test, showing a dry plan between two latewood layers. FIG. 4 Microscopic view of wood wetted in the latewood section of the growth ring. FIG. 5. X-ray view (where black is dry and white is wet) at 5 minutes (left) and 60 minutes (centre) from start of test and photo of top of specimen (right) with tape and wire removed.

5 4.3 Water distribution along the radial direction for water uptake test of spruce Finally, the wetting pattern in the radial direction can be seen in Figure 6. The latewood layers become barriers to water movement perpendicular to their orientation. The rate of water uptake was the slowest of the three directions. FIG. 6. X-ray view (where black is dry and white is wet) at 5 minutes (left) and 60 minutes (centre) from start of test and photo of top of specimen (right) with tape and wire removed. 4.4 Water distribution along longitudinal direction for water uptake test of oak For comparison purposes, X-ray views of water uptake in a hard wood, namely oak, are presented (2). As explained above, the structure of oak is more complex than the one of a softwood like spruce. For the liquid water uptake experiment, the specimens are dried in the oven for two weeks at a maximum temperature of 60 C to prevent cracking. The lateral sides are sealed to prevent evaporation and subsequently immersed in water. The specimens used had a thickness of 17 mm in order to be able to distinguish the macroscopic structural elements as earlywood, latewood and rays. Figure 7 demonstrates that the water uptake is more important where the density of wood is lower. Furthermore, there is also a preferential wetting pattern in the latewood layers, although, due to the more complex structure, the pattern is not as specifically layered as the pattern observed with spruce. Figure 8 shows the role of rays, acting as barriers to the movement of water. 5. Moisture content distribution measurements during drying Finally, X-ray views are presented to illustrate the role played by the wood structure during drying (2). The method used was adapted to the slower process of vapour absorption and desorption. The specimens (300x300x200mm) are made vapour tight by painting five sides twice with epoxy, one side leaving unpainted to allow one-side drying. The specimens are oven dried first, subsequently brought to equilibrium within a container at 85% RH and then dried at a relative humidity of 12%. Care had to be taken to ensure that views were identical to allow subtraction, and the X-ray apparatus was properly calibrated from one test to the next. An experiment where oak is dried in tangential direction is illustrated in FIG. 9. Figure 9a shows the reference image where the upper surface is the drying surface, the darker vertical bands are the earlywood growth rings, the light coloured vertical bands are the latewood growth rings. The horizontal darker band is a ray (in the radial direction). Figure 9b-d shows that drying is preferentially in the latewood, which show a lower water vapour permeability. Rays allow for a 3D water vapour transport connecting in the radial direction. Figure 10 shows that drying in longitudinal is also preferentially in the vessels in the latewood. The non-uniform drying pattern is caused by inactive latewood vessels due to the formation of tyloses, which can very effectively inhibit flow in the longitudinal direction.

6 Fig 7. Longitudinal liquid uptake process, (a) reference image showing the density distribution (b) X-ray views (where blue is dry and red is wet) at 2 hours, (c) 4 hours, (d) 8 hours, the base of the image is 13 mm. FIG. 8. X-ray views (where blue is dry and red is wet) at 12 hours (left) and 23 hours (right) from start of water uptake test in the tangential direction

7 FIG. 9. X-ray density view of a dry oak specimen (a) and X-ray views (where blue is wet and red is dryer) of a specimen in equilibrium with 85 %RH at 5 hours (b), 10 hours (c) and 30 hours (d) after exposure at 12%RH, in the tangential direction FIG. 10 X-ray views (where blue is wet and yellow is dryer) of a specimen in equilibrium with 85%RH after at 5 hours at 12%RH, drying is allowed only to the top surface, to occur in the longitudinal direction. 6. Conclusions 1. Advantages of X-ray measurements: the method is non-destructive; it allows very clear view of moisture distribution versus time for the three directions. 2. The next step is to quantify, through appropriate measurements, the moisture content during wetting, redistribution and drying. 3. The X-ray method can play a role in the derivation of wood properties that takes into account the orthotropic nature of wood and

8 the presence of early and late wood. Once known these properties can be used for multi-scale modelling of wetting and drying of wood components with wall assemblies. 7. Acknowledgements The authors want to thank Hans Janssen for his contributions during measurements. 8. References [1] visited on Nov. 11, [2] Gaublomme J. 2004, Hygroscopic moisture transport and moisture dependent deformation of oak, MA.Sc. thesis, University of Technology Eindhoven. [3] Roels S. and J. Carmeliet, 2005, Analysis of Moisture Flow in Porous Materials Using Microfocus X- ray Radiography, submitted to Journal of Physics D: Applied Physics. FIG. 11. X-ray views in the three directions and at five times during the one-our water uptake test.