Cross-laminated timber (CLT) panels are recognized as an engineered building product worldwide and, given that they are made from solid wood, they can absorb moisture from or emit moisture to their surrounding environment when placed in either wet or dry conditions. This absorption and desorption processes creates a moisture gradient in the wood material, causing stress. This study investigates the moisture distribution and gradients in the CLT panels made from fir wood layers that have been heat-treated at 170°C. The panels were subjected to wet-dry cycles (from relative humidity of 88% to 32% and then from 32% to 88%). The paired heat-treated CLT panels showed less moisture and more uniform moisture compared to the untreated panels, with a less uniform moisture distribution and different moisture pockets. The moisture gradients between the treated and untreated the CLT panels indicated that heat-treatment affected the moisture gradient change. The treated panels had lower moisture gradients than the untreated panels.
Research Article
Moisture Distribution in Cross Laminated Timber (CLT) Made from Heat Treated Wood
https://doi.org/10.21203/rs.3.rs-2906320/v1
This work is licensed under a CC BY 4.0 License
Version 1
posted
You are reading this latest preprint version
Cross Laminated Timbers
Thermal Treatment
Moisture Distribution
Moisture Gradient
Cross Laminated Timber (CLT)" panels are products of engineered wood layers made from massive wood in which the layers are attached to each other perpendicular at 90 degrees (Gereke et al., 2010). Such panels are made of massive wood, which has its own advantages and disadvantages because of its natural structure. One of the important and fundamental drawbacks is its moisture-absorbing or hygroscopic property. When wood is exposed to a humid or dry environment, it can absorb moisture from/release moisture to the surrounding atmosphere. As a result of this moisture transfer (absorption and desorption), a moisture gradient is created in the wood-based materials, which can cause stress and even cracks in the structure (Hassani et al., 2015; Angst and Malo, 2012; Fragiacomo et al., 2011; Gereke et al., 2009; Jönsson, 2005; Jönsson and Thelandersson, 2003). The moisture gradient occurs when the moisture changes within wood go beyond the balance of the surrounding environment (Fragiacomo et al., 2011). Therefore, by utilizing methods that can reduce moisture absorption and desorption, the moisture gradients and resulting stresses can be reduced. One of the methods that can be considered for this purpose is likely to be the use of wood modification treatments of the thermal type that reduce the equilibrium moisture content of wood (Borysiuk et al., 2014; Mirzaei et al., 2012; Mirzaei et al., 2018; Mohebby et al., 2008). Therefore, the subject of this study is to investigate the moisture distribution and moisture gradients in the CLT panels.
Rough sawn fir timber blocks with the dimensions of 50 (R) × 150 (T) × 550 (L) were used to make the CLT panel samples. Wooden blocks were subjected to hydrothermal thermal modification treatment at 170°C for 30 minutes. Then, they were air dried and then vacuum dried at 60°C for 72 hours to reach an equilibrium moisture content of 10%.
For the manufacture of the CLT panels, wooden layers with dimensions of 20 (thickness) × 50 (width) × 550 (length) mm were prepared from the treated timber blocks and laid up according to Table 1. A two-component polyurethane (PU) adhesive was used for bonding the wooden layers at a rate of 200 g/m2. And finally, the CLT panels were pressed by a hydraulic press with a pressure of 10 bars. The CLT panels with dimensions of 10 (thickness) × 550 (width) × 550 (length) mm were obtained in the end. Three types of samples were prepared; a) the CLT panels from untreated wood, b) the panels with only one layer of the hydrothermally treated wood on both surface sides, c) the CLT panels with two layers of the heat-treated wood on both sides.
Table 1. Layup of CLT samples according to treated laminations
To determine the moisture distribution across the layers of the CLT panels, after the completion of the moisture conditioning steps of drying and wetting cycles, some of the CLT panels were cut along the glue line according to Figure 1. Then, wooden layers were cut from each other, and samples with dimensions of 20 (thickness) × 450 (width) × 450 (length) mm were prepared throughout each layer of the CLT panels. The samples were weighed using a balance and then were dried in the oven at a temperature of 103 ± 2°C. Afterward, they were weighed again, and the moisture content for each sample was calculated separately using equation 1.
According to the MC data obtained from each sample; the moisture distribution in each layer of the CLT panels was plotted using MiniTab software version 17.
To determine the moisture gradient of the panels, two groups of the panels were classified. The first group was selected for the wet-to-dry state (from 88% RH to 32% RH), and the second group was chosen for the dry-to-wet state (from 32% RH to 88% RH). After the moisture conditioning steps, the CLT panel samples were cut along the glue lines. In the next step, samples with dimensions of 20 (thickness) × 20 (width) × 20 (length) mm were prepared from the whole panels, and all samples were weighed. The prepared samples were dried in an oven at a temperature of 103 ± 2°C for 24 hours. Then, the samples were weighed again, and the moisture content for each layer of the CLT panels was measured using equation 1.
Eq. 1) MC=(Mh-Mo)/Mo
where MC = Moisture Content (%), Mh=Weight of the samples after the wet or dry stage (g), Mo=Weight of the dry sample (g)
Moisture Distribution in the CLT Panels
One of the important goals of this study was to investigate the uniformity of moisture distribution in each layer of the CLT panels in the moisture conditioning steps (from dry to wet and wet to dry). In addition to investigating the moisture profile of the panels, the moisture distribution in each layer was also examined to understand the relationship between moisture-induced stresses and the uniformity and non-uniformity of moisture during drying and wetting cycles. The moisture distribution in different layers of the CLT panels is shown in Figures 2 and 3 after drying-to-wet and wet-to-drying cycles. The moisture distribution in different panels is presented in Figure 2 during the drying-to-wet cycle. These figures demonstrate that there is a difference in moisture content between every five layers used in the constructed panels, especially in the control sample. By comparing layers, A to E, it is apparent that layer C is the driest layer in the control panel, while outer layers A and E are the moistest layers. On the other hand, layers B and D are at medium moisture content (Figure 2).
In treated panels, it can be seen that the outer layers A and E have lower moisture content compared to the control sample (Figure 2). This can be observed by comparing each layer in the treated panel with its corresponding layer in the control panel. Since in all panels, the middle layer of the constructed panels was untreated; it is almost possible to see uniformity in their moisture content (layer C in all panels). In general, in each treated layer in the treated panels, more uniformity can be seen in the moisture distribution compared to the untreated layers of the control panels. This can play an important role in preventing stresses caused by unequal moisture. In the paired-layer treated panels, less moisture and more uniform moisture distribution are observed in the second and fourth layers (i.e., B and D). By examining Figure 2 carefully, it can be seen that in the drying-to-wet step, the control sample layers have different non-uniformity and moisture pockets compared to the treated layers; it should be noted that non-uniformity reduces with increasing numbers of treated layers; also, the moisture distribution in the central layers becomes more uniform.
In the drying-to-wet step (from 88 to 32% relative humidity), the non-uniformity in moisture distribution increased more in the control panels compared to the treated panels (Figure 3). In the drying-to-wet step, the non-uniformity of moisture distribution of each layer of the control panels showed more dissemination than the drying-to-wet step in the same layers of the treated panels. However, the application of treated layers at 170°C, either as a single layer or paired layer, demonstrated a more efficient role in homogenizing and reducing moisture in the paired-layer treated panels compared to their corresponding control layer in the control panel (Figure 3). The middle layer in the treated and control panels has lower moisture content, and its moisture distribution is also less than its corresponding layer in the control panel. This indicates the effectiveness of the treatment in reducing moisture absorption by the middle layer. Low moisture dissemination in each layer can play a significant role in preventing stresses caused by moisture. Also, reducing stresses can contribute to reducing the risk of cracks.
Based on the findings, it can be clearly seen that the non-uniformity of moisture in the control panel layers is higher and more visible than the treated samples. This also indicates the effect of thermal treatment in enhancing the uniform distribution and dispersion of moisture. The presence of moisture pockets with higher or lower moisture levels than the surrounding wood in each layer represents the non-uniformity of moisture, especially in the outer layers that can lead to severe stress and visible cracking. Although some cracks were observed in the glue line between outer blocks in the control panel samples (Figure 4), the occurrence of cracks in the wood and glue lines was also reported in the control samples by Hassani et al. (2015), Gereke et al. (2009), Angst and Malo (2012).
The moisture gradient in the layered panels has been illustrated in Figure 5. The panels with treated layers had lower moisture than panels with untreated layers. The application of one or two layers of treatment on the outer layers of the CLT panels also played a decreasing role in moisture gradient. In the wet to dry moisture step, the control panels had a limited moisture gradient, but they had more moisture than the treated panels overall (Figure 5a and 5b). The use of treated layers also led to a decrease in moisture in the panels. In the single-layered samples, there was not much difference between the two treatments. However, in the samples with two treated layers used in the outermost layers of the panel, the moisture decreased with an increase in temperature. Moreover, the difference between the two treatment temperatures was more prominent. Especially in the comparison of the outer and inner layers, it was seen that in the treated samples, the wood moisture was lower than the control samples. In the wet to dry moisture step (from 32 to 88% relative humidity), more severe moisture gradients were visible in the control samples (Figure 5C and 5D). An interesting point in all panels was that there was no difference in the moisture content of the intermediate layers, which were all uniform and higher in moisture. This indicates that the thermal treatment affects the moisture absorption of the wood. The reason for the decrease in moisture in the treated layered panels and the treated layers compared to the untreated panels is indicative of the effectiveness of the thermal and hydrophobic treatment. Just as previous researches have also shown the same effectiveness in various wood species (Kesik et al., 2014; Korkut et al., 2013; Mohebby and Sanei, 2005).
The decrease in the moisture gradient of treated panels can be attributed to the decrease in equilibrium moisture content of the treated wood. Previous studies have reported a decrease in equilibrium moisture content of thermally treated wood (Mohebby and Broushakian, 2022; Gereke et al., 2009; Hassani et al., 2015; Mirzaei et al., 2017). One reason for this behavior is that in thermally modified wood, hemicelluloses are degraded into furfural, which is a non-hydrophilic polymer. Hemicelluloses are one of the polymers present in wood that undergo rapid hydrolysis and as the temperature and treatment time increase, their degradation gets worse (Poncsák et al., 2006; Olarescu et al., 2014). The treatment also leads to an increase in crystalline regions of cellulose (Yildiz and Gümüşkaya, 2007), and due to the esterification reaction, the hydroxyl groups on the hemicellulose molecules are decreased, and cross-linking between hemicellulose and lignin occurs, which creates inhibiting cross-links for water absorption, eventually leading to a decrease in equilibrium moisture content of the wood (Poncsak et al., 2006; Weiland and Guyonnet, 2003; Cao et al., 2012; Olarescu et al., 2014).
The distribution of layer-by-layer moisture content and moisture gradient in cross-laminated timber (CLT) panels made of thermally modified wood was examined. Based on the findings of this study:
- Heat treatment led to a uniform distribution of moisture in different layers of the panels.
- In the treated samples, there were no moisture pockets that form the basis for creating moisture gradients and non-uniformity of moisture. However, the moisture pockets were observed in the control samples, which led to an increase in moisture gradients and, consequently, moisture-induced stresses.
- No cracks were observed in the glue line or panel structure in the heat-treated panels. However, the cracks were observed in the glue line of the control panels due to severe moisture gradients and non-uniform moisture distribution.
- Heat treatment reduced moisture gradients. Moisture gradients decreased with an increase in the number of treated layers, and moisture distribution in the panel structure decreased and became more uniform.
- Due to the equilibrium moisture content in thermally modified wood, which resulted in a reduction in moisture gradients, moisture-induced stresses decreased as well.
- In conclusion, it is safe to use thermally modified wood to create panels without worrying about moisture gradients and moisture distribution
Author contributions VB took primary responsibility for collecting the literature, and writing the original manuscript draft. BM planned, instructed, and supervised the research process as well as reviewed and edited the final version of the manuscript.
Compliance with ethical standards
Conflict of interest The authors declare no conflicts of interests. The founding sponsor had no role in the design of the study; in the collection, analysis, or interpretation of data; in the writing of the manuscript, and in the decision to publish the results.
- Angst V., Malo K. A., 2012. Moisture-induced stresses in glulam cross sections during wetting exposures, Wood Sci Technol, 47 (2): 227-241.
- Borysiuk P., Boruszewski P., Radoslaw A., Marcin G., 2014. Dimensional stability of layered elements made of thermally modified wood, Trieskové a beztrieskové obrbánie dreva, 9 (1):191-196.
- Cao Y., Lu J., Huang R., Jiang J. 2012. Increased dimensional stability of Chinese fir through steam-heat treatment, Eur J Wood Prod, 70 (4): 441-444.
- Fragiacomo, M., Fortino, S., Tononi, D., Usardi, I., Toratti, T., 2011. Moisture-induced stresses perpendicular to grain in cross-sections of timber members exposed to different climates. Engineering Structures 32 (11): 3071-3078.
- Gereke T., Schnider T., Hurst A., Niemz P. 2009. Identification of moisture-induced stresses in cross-laminated wood panels from beech wood (Fagus sylvatica L), Wood Sci Technol, 43 (3): 301-315.
- Gereke, T., Hass P., Niemz P., 2010. Moisture-induced stresses and distortions in spruce cross-laminates and composite laminates, Holzforschung, 64 (1): 127-132.
- Jönsson J. H., 2005. Internal stresses in glulam due to moisture gradients in the grain direction. Holzforschung, 59 (1): 18-22.
- Jönsson J., Thelandersson S., 2003. The effect of moisture gradients on tensile strength perpendicular to grain in glulam, Holz Roh-Werkst, 61 (5): 342-348.
- Hassani M. M., Wittel F. K., Ammann S., Niemz P., Herrmann H. J., 2015. Moisture-induced damage evolution in laminated beech, Wood Sci Technol, 50 (5): 917-940.
- Kesik H. I., Korkut S., Hiziroglu S., Sevik H. 2014. An evaluation of properties of four heat treated wood species. Industrial Crops and Products, 60: 60-65.
- Korkut D S., Hiziroglu S., Aytin A., 2013. Effect of heat treatment on surface characteristics of wild cherry wood, BioResources, 8 (2): 1582-1590.
- Mohebby B., Broushakian V., 2022. Moisture-induced stresses in cross laminated timber (CLT) made from hydrothermally modified wood. Eur J Wood Prod, 43
- Mirzaei G., Mohebby B., Tasooji M., 2012. The effect of hydrothermal treatment on bond shear strength of beech wood, Eur J Wood Prod, 70 (5): 705-709.
- Mirzaei G., Mohebby B., Ebrahimi G., 2017. Glulam beam made from hydrothermally treated poplar wood with reduced moisture induced stresses, CONSTR BUILD MATER, 135: 386-393.
- Mirzaei G., Mohebby B., Ebrahimi G., 2018. Technological properties of glulam beams made from hydrothermally treated poplar wood. Iranian Journal of Wood and Paper Industries (IJWP) 13(1):36-44.
- Olarescu M. C., Campean M., Ispas M., Cosereanu C., 2014. Effect of thermal treatment on some properties of lime wood, Eur J Wood Prod, 72 (4): 559-562.
- Poncsák S., Kocaefe D., Bouazara M., Pichette, A., 2006. Effect of high temperature treatment on the mechanical properties of birch (Betula papyrifera). Wood Sci Technol, 40 (8): 647-663.
- Weiland J. J., Guyonnet R. 2003. Study of chemical modifications and fungi degradation of thermally modified wood using DRIFT spectroscopy, Holz Roh-Werkst, 61 (3): 216-220.
- Yildiz S., Gümüşkaya E. 2007. The effects of thermal modification on crystalline structure of cellulose in soft and hardwood, Build Environ, 42 (1): 62-67.
No competing interests reported.