Moisture in Timber
Equilibrium Moisture Content
Timber is hygroscopic, that is it will absorb moisture from the atmosphere if it is dry and correspondingly yield moisture to the atmosphere when wet, thereby attaining a moisture content that is in equilibrium with the water vapour pressure of the surrounding atmosphere.
Thus, for any combination of vapour pressure and temperature of the atmosphere there is a corresponding moisture content of the timber such that there will be no inward or outward diffusion of water vapour; this moisture content is referred to as the equilibrium moisture content (EMC). Generally, it is more convenient to use relative humidity rather than vapour pressure.
Relative humidity is defined as the ratio of the partial vapour pressure in the air to the saturated vapour pressure, expressed as a percentage. The fundamental relationships between moisture content of timber and atmospheric conditions have been determined experimentally, and the average equilibrium moisture content values are shown graphically in Fig. 1.
A timber in an atmosphere of 20°C and 22% relative humidity will have a moisture content of 6% (see below), while the same timber if moved to an atmosphere of 40°C and 64% relative humidity will double its moisture content. It should be emphasised that the curves in Fig. 1 are average values for moisture in relation to relative humidity and temperature, and that slight variations in the equilibrium moisture content will occur owing to differences between timbers and to the previous history of the timber with respect to moisture.
Determination of Moisture Content in Timber
It is customary to express the moisture content of timber in terms of its oven-dry mass using the equation:
where minit = initial mass of timber sample (g), mod = mass of timber sample after oven-drying at 105°C (g) and µ = moisture content of timber sample (%).
Expression of the moisture content of timber on a dry-mass basis is in contrast to the procedure adopted for other materials, where moisture content is expressed in terms of the wet mass of the material. Determination of the moisture content of timber is usually carried out using the basic gravimetric technique above, though it should be noted that at least a dozen different methods have been recorded in the literature. Suffice it here to mention only two of these alternatives.
First, where the timber contains volatile extractives, which would normally be lost during oven drying, thereby resulting in erroneous moisture content values, it is customary to use a distillation process, heating the timber in the presence of a water-immiscible liquid such as toluene, and collecting the condensed water vapour in a calibrated trap.
Second, where ease and speed of operation are preferred to extreme accuracy, moisture content is assessed using electric moisture meters. The type most commonly used is known as the resistance meter, though this battery-powered hand-held instrument actually measures the conductance or flow (the reciprocal of resistance) of an electric current between two probes. Below the fibre saturation point (about 27% moisture content; see later) an approximately linear relationship exists between the logarithm of conductance and the logarithm of moisture content.
However, this relationship, which forms the basis for this type of meter, changes with species of timber, temperature, and grain angle. Thus, a resistance-type meter is equipped with a number of alternative scales, each of which relates to a different group of timber species; it should be used at temperatures close to 20°C. with the pair of probes inserted parallel to the direction of the grain. Although the measurement of moisture content is quick with such a meter, there are, however, two drawbacks to its use.
First, moisture content is measured only to the depth of penetration of the two probes, a measurement that may not be representative of the moisture content of the entire depth of the timber member; the use of longer probes can be beneficial though these are difficult to insert and withdraw. Second, the working range of the instrument is only from 7 to 27% moisture content.
Moisture Content of Green Timber
In the living tree, water is to be found not only in the cell cavity, but also within the cell wall. Consequently the moisture content of green (newly felled) wood is high, usually varying from about 60% to nearly 200%, depending on the location of the timber in the tree and the season of the year.
However, seasonal variation is slight compared to the differences that occur within a tree between the sapwood and heartwood regions. The degree of variation is illustrated for a number of softwoods and hardwoods in Table 1; within the former group the sapwood may contain twice the percentage of moisture to be found in the corresponding heartwood, while in the hardwoods this difference is appreciably smaller or even absent. However, pockets of ‘wet’ wood can be found in the heartwood. Green timber will yield moisture to the atmosphere with consequent changes in its dimensions.
At moisture contents above 20% many timbers, especially their sapwood, are susceptible to attack by fungi; the strength and stiffness of green wood are considerably lower than for the same timber when dry. For all these reasons it is necessary to dry or season timber following felling of the tree and prior to its use in service.
Removal of Moisture from Timber
Drying or seasoning of timber can be carried out in the open, preferably with a top cover. However, it will be appreciated from the previous discussion on equilibrium moisture content that the minimum moisture content that can be achieved is determined by the lowest relative humidity of the summer period.
In the UK it is seldom possible to achieve moisture contents of less than 16% by air seasoning. The planks of timber are separated in rows by stickers (usually 25–30 mm across) that permit air currents to pass through the pile; nevertheless it may take from two to ten years to air-season timber, depending on the species of timber and the thickness of the timber members.
The process of seasoning may be accelerated artificially by placing the stacked timber in a drying kiln, basically a large chamber in which the temperature and humidity can be controlled and altered throughout the drying process; control may be carried out manually or programmed automatically.
Humidification is sometimes required in order to keep the humidity of the surrounding air at a desired level when insufficient moisture is coming out of the timber; it is frequently required towards the end of the drying run, and is achieved either by the admission of small quantities of steam or by the use of water atomisers or low-pressure steam evaporators. Various designs of kiln are used, which are reviewed in detail by Pratt (1974).
Drying of softwood timber in a kiln can be accomplished in from four to seven days, the optimum rate of drying varying widely from one timber to the next; hardwood timber usually takes about three times longer than softwood of the same dimensions. Following many years of experimentation, kiln schedules have been published for different groups of timber, which provide wet- and dry-bulb temperatures (maximum of 70°C) for different stages in the drying process, and their use should result in the minimum amount of degrade in terms of twist, bow, spring, collapse and checks (Pratt, 1974).
Most timber is now seasoned by kilning, and little air drying is carried out. Dry stress-graded timber in the UK must be kiln-dried to a mean value of 20% moisture content with no single piece greater than 24%. However, UK and some Swedish mills are now targeting 12% (‘superdried’), as this level is much closer to the moisture content in service. Recently, solar kilns have become commercially available and are particularly suitable for use in developing countries to season many of the difficult slow-drying tropical timbers.
These small kilns are very much cheaper to construct than conventional kilns and are also much cheaper to run. They are capable of drying green timber to about 7% moisture content in the dry season and about 11% in the rainy season.
Influence of Structure
Water in green or freshly felled timber is present both in the cell cavity and within the cell wall. During the seasoning process, irrespective of whether this is by air or within a kiln, water is first removed from within the cell cavity; this holds true down to moisture contents of about 27–30%.
Since the water in the cell cavities is free, not being chemically bonded to any part of the timber, it can readily be appreciated that its removal will have no effect on the strength or dimensions of the timber. The lack of variation of the former parameter when moisture content is reduced from 110 to 27% is illustrated in Fig. 2.
However, at moisture contents below 27% water is no longer present in the cell cavity, but is restricted to the cell wall, where it is chemically bonded (hydrogen bonding) to the matrix constituents, to the hydroxyl groups of the cellulose molecules in the noncrystalline regions and to the surface of the crystallites; as such, this water is referred to as bound water.
The uptake of water by the lignin component is considerably lower than that by either the hemicellulose or the amorphous cellulose; water may be present as a monomolecular layer, though frequently up to six layers can be present. Water cannot penetrate the crystalline cellulose since the hygroscopic hydroxyl groups are mutually satisfied by the formation of both intra- and intermolecular bonds within the crystalline region. This view is confirmed by X-ray analysis, which indicates no change of state of the crystalline core as timber gains or loses moisture.
However, the percentage of noncrystalline material in the cell wall varies between 8 and 33% and the influence of this fraction of cell-wall material as it changes moisture content on the behaviour of the total cell wall is very significant. The removal of water from these areas within the cell wall results first in increased strength and second in marked shrinkage. Both changes can be accounted for in terms of drying out of the water-reactive matrix, thereby causing the microfibrils to come into closer proximity, with a commensurate increase in interfibrillar bonding and decrease in overall dimensions. Such changes are reversible, or almost completely reversible.
Fibre Saturation Point
The increase in strength on drying is clearly indicated in Fig. 2, from which it will be noted that there is an approximately three-fold increase in strength as the moisture content of the timber is reduced from about 27% to zero. The moisture content corresponding to the inflexion in the graph is termed the fibre saturation point, where in theory there is no free water in the cell cavities while the walls are holding the maximum amount of bound water.
In practice this situation rarely exists; a little free water may still exist while some bound water is removed from the cell wall. Consequently, the fibre saturation ‘point’, while a convenient concept, should really be regarded as a ‘range’ in moisture contents over which the transition occurs.
The fibre saturation point therefore corresponds in theory to the moisture content of the timber when placed in a relative humidity of 100%; in practice, however, this is not so since such a situation would result in total saturation of the timber (Stamm, 1964). Values of EMC above 98% are unreliable.
It is generally found that the moisture content of hardwoods at this level is from 1 to 2% higher than for softwoods. At least nine different methods of determining the fibre saturation point are recorded in the literature; the value of this parameter depends on the method used.
Timber, assumes with the passage of time a moisture content that is in equilibrium with the relative vapour pressure of the atmosphere. This process of water sorption is typical of solids with a complex capillary structure, and this phenomenon has also been observed in concrete.
The similarity in behaviour between timber and concrete with regard to moisture relationships is further illustrated by the presence of S-shaped isotherms when moisture content is plotted against relative vapour pressure.
Both materials have isotherms that differ according to whether the moisture content is reducing (desorption) or increasing (adsorption), thereby producing a hysteresis loop (Fig. 3).
For more information on sorption and diffusion, especially on the different theories of sorption, you should consult the comprehensive text by Skaar (1988).