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Wood: Moisture Content, Hygroscopicity, and Sorption

Abstract

Wood is a hygroscopic material that has the ability to adsorb or desorb water in response to temperature and relative humidity of the atmosphere surrounding it. This affinity of wood for water is caused by hydroxyl groups accessible in the cell walls of wood. As a consequence, the moisture content of wood is one of the most important variables affecting its physical and mechanical properties. The present paper focuses on moisture content in wood and presents an account of various theories of moisture sorption such as unimolecular sorption, multilayer theories, solution theory, and cluster theory.
Citation: Hartley I., and Hamza M.F., Wood: Moisture Content, Hygroscopicity, and Sorption. In:
Saleem Hashmi (editor-inchief), Reference Module in Materials Science and Materials Engineering.
Oxford: Elsevier; 2016. pp. 1-7.
The full text can be found at:
http://www.sciencedirect.com/science/article/pii/B9780128035818022190
... • The moisture buffering capacity of timber structures [158,159] and the relatively low thermal FIGURE 5.14. Wind blowing beneath the palafitic timber building. ...
... Timber structures are very hygroscopic, and thus they have the ability to exchange water vapour with the surrounding air continually, i.e. gaining and losing moisture until reaching an equilibrium point (equilibrium moisture content -EMC) [158,159]. The EMC is influenced by relative humidity and temperature, and any change in these parameters forces the material to adjust to a new equilibrium point [159]. ...
... Timber structures are very hygroscopic, and thus they have the ability to exchange water vapour with the surrounding air continually, i.e. gaining and losing moisture until reaching an equilibrium point (equilibrium moisture content -EMC) [158,159]. The EMC is influenced by relative humidity and temperature, and any change in these parameters forces the material to adjust to a new equilibrium point [159]. In the study of Silva et al. [158], from the three softwood species analysed, Maritime Pine (Pinus Pinaster) had the highest EMC (12.8%) and also the lowest ability to shrink and swell. ...
Thesis
Vernacular architecture is characterised by embodying and expressing a plurality of factors - geographic, climatic, economic and cultural - of the places in which it is located. In its long evolution, and inserted in a context of scarcity, a range of pragmatic strategies and building techniques of adaptation to the surrounding environment were developed. These strategies/materials are usually simple, low-tech and have a low potential environmental impact. From a sustainability point of view, several studies highlight them as having the potential to reduce the environmental impacts of buildings. In Portugal, there are many expressions of vernacular architecture, whose specificities are an identity factor of several regions. However, there is a lack of quantitative studies on the thermal and environmental performance of vernacular buildings and materials in the Portuguese context. In this sense, this research work presents a qualitative and quantitative study of i) climate-responsive strategies; ii) the thermal performance and comfort conditions of different Portuguese vernacular buildings throughout the different seasons; and iii) the environmental performance of vernacular materials. The research focused on the study of three case studies, with specific features and located in three different zones of mainland Portugal. The thermal performance and comfort conditions of the case studies were evaluated through in situ monitoring of hygrothermal parameters, surveys on occupants’ thermal sensation, and the data analysed according to an adaptive model of comfort. To compare the influence of some strategies on the annual energy demand for heating and cooling, simulations under dynamic conditions for different scenarios were carried out. In the case of vernacular materials, although these are seen as ecological, the quantitative studies available are scarce and that allow establishing an equative comparison with conventional materials. Thus, the life cycle assessment of two earthen materials, rammed earth and compressed earth blocks (CEBs), was carried out and based on specific life cycle inventory values obtained from a producer company, following the guidance provided by the standard EN15804. From the results, in general, it was found that the case studies have shown a good thermal performance by passive means alone and that the occupants feel comfortable, except during winter when there was a need to use heating systems. In the case of materials, in a cradle-to-gate analysis of different walls, the use of earthen building elements can result in reducing the potential environmental impacts by about 50%, when compared to the use of conventional ones. Additionally, earthen materials have the advantage that they can be recycled/reused in a closed-loop approach.
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... According to Equation (2), MC may range from 0% for oven-dry wood to over 100% when the water in the wood weighs more than the wood substance. The maximum MC depends of tree species and may achieve 140-150% [28]. The only direct method to determine MC in wood material is to measure the water content in a wood sample and the weight of oven-dry sample. ...
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... At a certain point, equilibrium is attained between moisture in the leaf and that in the surrounding atmospheric environment, and this situation is the best for the tea transport conditions. This is where the tea leaves will not gain or lose any moisture with time and it is known as the equilibrium moisture content of the tea [Hartley and Hamza, 2016]. Simulation models for dryer design, dryer optimization and control for several agricultural products use the difference between the actual moisture content and the equilibrium moisture as a measure for the driving force for drying. ...
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Acoustic properties of wood are best recognized when listening to the tone and clarity of wood instruments such as the violin, piano, xylophone, or guitar. Less recognized, however, are the major roles acoustic properties play in helping to minimize sound transmission from one room to the next and in providing sound barriers to highway noise. Acoustic waves are also used to evaluate strength properties in wood, evaluate processing variables during manufacturing, and elucidate anatomical characteristics of wood. The acoustical properties of wood are discussed in details within this article.
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