Pine moisture. Determination of moisture content and drying of wood

Wood is a capillary-porous material (heterocapillary system), which consists mainly of hydrophilic components, and therefore it constantly contains more or less water. A living tree needs water to keep it alive. The water content is characterized wood moisture. Humidity is one of the main characteristics of wood.

wood moisture is the amount of water it contains. Wood moisture s affects the properties of wood and on the suitability of wood for building purposes. Under humidity wood refers to the ratio of the mass of water to the dry mass of wood, expressed as a percentage. Wood moisture- the ratio of the mass of moisture contained in wood to the mass of absolutely dry wood, expressed as a percentage.

Wood moisture and the interaction of wood and its components with water are important for the mechanical and chemical technology of wood, for example, for the impregnation of wood with solutions of chemical reagents, antiseptics, fire retardants, etc., when floating and storing timber in water.

Water plays a role in the activation of cellulose before chemical reactions take place. The interaction of cellulose with water in the pulp during grinding and the subsequent removal of water during the formation of a paper sheet determine the formation of strong interfiber bonds in the paper.

There are two concepts - relative humidity wood and absolute humidity wood.

- mass fraction of water, expressed as a percentage in relation to the mass of wet wood.

Absolute moisture content of wood (moisture content) - mass fraction of water, expressed as a percentage in relation to the mass of absolutely dry wood. absolute humidity wood is the ratio of the mass of moisture in a given volume of wood to the mass of absolutely dry wood. According to GOST, the absolute humidity of parquet should be 9% (+/- 3%).

Absolutely dry wood conventionally called wood dried to constant weight at a temperature of (104 ± 2) ° C. The values of the relative humidity of wood are needed for the analysis of wood when calculating the mass fractions of its components as a percentage in relation to absolutely dry wood. The absolute moisture content of wood (moisture content) is used to quantify wood samples when comparing them by water content.

According to the degree of moisture, wood is distinguished into the following types:

Wet wood. Its humidity is over 100%. This is possible only if the wood has been in the water for a long time.

Freshly cut. Its humidity ranges from 50 to 100%.

Air dry. Such wood is usually stored in the air for a long time. Its humidity can be 15-20%, depending on climatic conditions and seasons.

Room dry wood. Its humidity is usually 8-10%.

Absolutely dry. Its humidity is 0%.

Wood moisture schedule: 1 - hot water; 2 - saturated steam; 3 - cold water

The water in the tree is distributed unevenly: the roots and branches contain more water than the trunk; butt and top - more than the middle part of the trunk; coniferous sapwood is more than heartwood and mature wood. In hardwood, water is distributed more evenly over the cross section of the trunk, and in some tree species (for example, oak), the moisture content of the core is much higher than in coniferous species. In the bark, the moisture content of the bast is significantly (7–10 times or more) higher than that of the crust.

Freshly cut wood has a moisture content of 80 - 100%, moisture content of driftwood reaches up to 200%. In coniferous species, the moisture content of the core is 2–3 times lower than that of the sapwood.

In construction practice, according to the moisture content, wood is usually distinguished:

freshly cut wood with an average absolute moisture content of 50 to 100%, depending on the time of felling (water content is much higher in spring and least in winter), as well as tree species and growing conditions;

air-dry wood is wood dried in air to an equilibrium state of its moisture content with the relative humidity of the air; the absolute humidity of such wood depends on the relative humidity of the air and is usually 16 ... 21%;
room-dry wood - wood aged in a heated room and having an absolute humidity of 9 ... 13%; wet wood, obtained by prolonged exposure to water, with an absolute humidity above 100% (up to 200% or more).

There are two forms of water in wood - bound (hygroscopic) and free (capillary). They add up to the total amount of moisture in the wood. Bound (or hygroscopic) moisture is contained in the cell walls of wood, and free moisture occupies half of the cells and intercellular spaces. Free water is removed more easily than bound water and affects the properties of wood to a lesser extent.

Free (capillary) moisture is contained in cell cavities, and bound moisture is contained in the walls of wood cells. The gradual saturation of dry wood with water initially occurs due to bound moisture, and only when the cell walls are completely filled, a further increase in moisture occurs due to free moisture. Therefore, it is obvious that it is the change in bound moisture that affects the processes of shrinkage and warpage of wood, as well as its strength and elastic properties. The increase in free moisture practically does not affect the properties of wood.

Water absorption of wood- the ability of wood to absorb water in direct contact with it.Wood is a natural material that is susceptible to fluctuations in temperature and humidity.Its main properties include hygroscopicity, that is, the ability to change humidity in accordance with environmental conditions.

It is said that wood “breathes”, that is, it absorbs air vapors (sorption) or releases them (desorption), reacting to changes in the microclimate of the room. Absorption or release of vapors is carried out due to the cell walls. With a constant state of the environment, the moisture content of wood will tend to a constant value, which is called the equilibrium (or stable) humidity.

In wood, moisture is contained in the cells of wood, in the intercellular space, in the channels of blood vessels, and it is called free moisture.. AT the lag contained in the cell membranes is called hygroscopic (bound) moisture.

Hygroscopicity of wood- the ability of wood to change humidity depending on changes in the temperature and humidity state of the surrounding air. Hygroscopicity for most rocks is 30% at 20°C.

The maximum amount of bound moisture is called limit of hygroscopicity or fiber saturation limit. At a temperature of 20 ° C, the limit of hygroscopicity is 30%. When the temperature rises, part of the bound moisture turns into free and vice versa.

Free and hygroscopic moisture is removed from the wood by drying. Moisture can be contained in wood in the form of chemically bound moisture in the form of substances that make up wood, this type of moisture can be removed during the chemical processing of wood.

The maximum amount of hygroscopic moisture is almost independent of the type of wood. The percentage of the weight of water to the weight of absolutely dry wood, as a rule, is 30% at a temperature of 20 °. Such wood moisture, is called the saturation point of the cell membranes, or the saturation point of the fibers. A further increase in moisture occurs due to free moisture filling the voids in the wood.

When the humidity changes from zero to the saturation point of cell membranes, the volume of wood changes, it swells. When the humidity decreases, the wood dries out.

Dimensional changes are always observed in the transverse direction and almost do not appear in the longitudinal direction, denser wood has a higher volumetric weight, therefore, more shrinkage and swelling. Late wood is denser.

Wood contains free (in cell cavities and intercellular spaces) and bound (in cell walls) water. The saturation limit of cell walls Wn,H, is on average 30%. A decrease in the content of bound water causes shrinkage wood.

The ability to absorb moisture is affected not only by the microclimate of the room, but also by the type of wood. The most hygroscopic species are beech, pear, kempas.

They respond most quickly to changes in humidity levels.

In contrast, there are stable species, such as oak, merbau, etc. These include the bamboo stalk, which is very resistant to adverse climatic conditions. It can be laid even in the bathroom.

Different types of wood have different moisture levels. For example, birch, hornbeam, maple, ash have a low moisture content (up to 15%) and tend to crack when dried. Humidity of oak and walnut is moderate (up to 20%). They are relatively resistant to cracking and do not dry out as quickly. Alder is one of the most desiccation-resistant species. Its humidity is 30%.

When testing wood in order to determine the physical and mechanical parameters, it is brought to normalized humidity (on average 12%) by conditioning at a temperature of (20 ± 2) ° C and relative humidity<= (65±5)%.

WOOD MOISTURE DETERMINATION

Knowing the types of wood, its density and other physical properties, it is possible to determine the moisture content of wood by weight, by the presence of cracks at the end or along the fibers of the wood, by warping and other signs. By the color of the bark, its size and the color of the wood, one can recognize ripe or freshly cut wood and its degree of moisture. When processing with a p / m planer, its thin shavings, squeezed by hand, are easily crushed - which means the material is wet. If the chips break and crumble, this indicates that the material is dry enough. When transverse cutting with sharp chisels, they also pay attention to chips. If they crumble or the wood of the workpiece itself crumbles, this means that the material is too dry. Very wet wood is easily cut, and a wet mark is noticeable at the cutting site from the chisel. But it is unlikely that it will be possible to avoid cracking, warping and other deformations.

Wood moisture determined in various ways: by drying samples of wood, wood chips or sawdust to a completely dry state; distillation of water in the form of an azeotropic mixture with water-immiscible non-polar solvents; chemical methods (titration with Fisher's reagent); electrical methods.

The moisture content of lumber is determined by the formula

W \u003d (m s - m o) / m s,

where m c and m about the mass of the sample, respectively, in the initial and dried states.

In fact, the moisture content of wood is determined by control weighing or using an electric moisture meter.

Moisture content of driftwood - 200%, freshly cut - 100%, air-dry - 15-20%.

DRYING WOOD

FROM
abalone wood- the process of removing moisture from wood to a certain percentage of moisture.

Bwood line- the ability of the surface of wood to reflect light rays in a directed way.

Gloss depends on the wood species, the degree of surface smoothness and the nature of the lighting. Radial surfaces of maple, sycamore, beech, elm, oak, dogwood, white locust, ailanthus, i.е. rocks in which a significant part of the surface is occupied by medullary rays, consisting of small cells. The gloss of wood is a decorative property that is taken into account when determining species.

Dielectric properties of wood- properties that are characterized by the dielectric constant and the tangent of the dielectric loss angle.

Wood swelling coefficient- average swelling of wood with an increase in the content of bound moisture by 1% moisture content.

Wood shrinkage coefficient- average shrinkage of wood with a decrease in the content of bound moisture by 1% moisture content.
Deformability of wood (warping)- the ability of wood to change its size and shape under external influences of load, humidity, temperature.

lateral warping associated with different shrinkage (swelling) of wood in the radial and tangential directions. Its nature depends on the location of the annual layers, due to the shape of the cross-section of the assortment, as well as the place where it was sawn out of the log.

Longitudinal warpage associated with some wood defects, such as large knots, list, tilt of the fibers.

The consequence of warping is a defect in wood - warping (transverse, longitudinal along the face and along the edge, wingedness).

Transverse and longitudinal warping also arise due to the imbalance of residual stresses in dried lumber during mechanical processing: one-sided milling, edge division of thick boards into thin ones.

Longitudinal warping of the boards is observed during sawing, a change in the cross-sectional shape of samples cut from different places of the log during drying.

Dry wood has high strength, warps less, does not rot, easily glues, finishes better, and is more durable. Any wood of various species is very sensitive to changes in environmental humidity.

This property is one of the disadvantages of timber. At high humidity, wood easily absorbs water and swells, and in heated rooms it dries out and warps.

Wood humidity up to 10% is sufficient indoors, and no more than 18% outdoors. There are many ways to dry wood.

The simplest and most affordable - natural type of drying - atmospheric, air. It is necessary to dry the wood in the shade, under a canopy and in a draft. When dried in the sun, the outer surface of the wood heats up quickly, while the inner remains damp.

Due to the difference in stresses, cracks form, the tree quickly warps. Boards, beams, etc. p / m are stacked on metal, wooden or other supports with a height of at least 50 cm. Boards are stacked with inner layers up to reduce their warpage. It is believed that the drying of boards placed on the edges is faster, since they are better ventilated and moisture evaporates more intensively, but they also warp more, especially material with high humidity.

A stack of p / m, harvested from freshly cut and live trees, is recommended to be compacted from above with a heavy load to reduce warpage. During natural drying, cracks always form at the ends, in order to prevent their cracking and preserve the p / m, it is recommended that the ends of the boards be carefully painted over with oil paint or soaked with hot drying oil or bitumen to protect the pores of the wood. It is necessary to process the ends immediately after the transverse cuts into the cut.

If the tree is characterized by high humidity, then the end is dried with a blowtorch flame, and only then painted over. Trunks (ridges) must be debarked (cleaned of bark), only at the ends they leave small belts-couplings 20-25 cm wide to prevent cracking. The bark is cleaned so that the tree dries out faster and is not affected by beetles. The trunk, left in the bark, in relative heat with high humidity quickly rots, is affected by fungal diseases. After atmospheric drying in warm weather, the moisture content of wood is 12-18%.

There are several other ways to dry wood.

Way evaporation or steaming has been used in Rus' since ancient times. The workpieces are sawn into pieces, taking into account the size of the future product, they are laid in ordinary cast iron, sawdust from the same workpiece is sprinkled, poured with water and placed for several hours in a heated and cooling Russian furnace “languishes” at t = 60-70 0 C.

In this case, “leaching” occurs - the evaporation of wood; natural juices come out of the workpiece, the wood is painted, acquiring a warm, thick chocolate color, with a pronounced natural texture pattern. Such a workpiece is easier to process, and after drying it cracks and warps less.

Way waxing. The blanks are dipped into melted paraffin and placed in an oven at t=40 0 C for several hours. Then the wood dries out for a few more days and acquires the same properties as after steaming: it does not crack, does not warp, the surface becomes tinted with a distinct texture pattern.

Way steaming in linseed oil. Tableware made of wood steamed in linseed oil is very water-resistant and does not crack even with everyday use. This method is still acceptable today. A blank is placed in a container, poured with linseed oil and steamed over low heat.

Warp: 1 - transverse; 2 - longitudinal along the face; 3 - longitudinal along the edge; 4 - winged logs due to residual internal growth stresses.

Linear shrinkage of wood- reduction in the size of wood in one of the directions when bound water is removed from it. Linear swelling of wood - an increase in the size of wood in one of the directions with an increase in the content of bound water in it.

Normalized wood moisture- equilibrium moisture content of wood, acquired at a temperature of 20 ± 2°C and a relative humidity of 65 ± 5%.

Volumetric shrinkage of wood- reduction in the volume of wood when the bound water is removed from it.

Volumetric swelling of wood- increase in the volume of wood with an increase in the content of bound water in it.

Relative humidity of wood- the ratio of the mass of moisture contained in wood to the mass of wood in a wet state, expressed as a percentage. Wood is a hygroscopic material, and the humidity to which it tends in given temperature and humidity conditions is called equilibrium. For example, at a temperature of 20 ° C and an air humidity of 100%, the equilibrium moisture content of wood is W = 30%.

A rapid change in bound moisture and uneven shrinkage in different directions leads to warping or, conversely, swelling of the wood.

Shrinkage cracks form in massive elements due to uneven drying. Therefore, in the production of lumber, great importance should be attached to the organization of drying, and during the operation of wooden structures, large and sharp changes in temperature and humidity should be excluded. Wood is characterized by the well-known inertia of moisture exchange processes.

Shrinkage of wood: 1 - shrinkage; 2 - cracking; 3 - transverse warping; 4 - the same, longitudinal

The amount of shrinkage is different in different directions: it is greater in the tangential (6 - 12%) and less in the radial (3 - 6%) direction of the trunk cross section. Due to such uneven shrinkage, warping of the boards appears during drying. As moisture increases above the saturation point of the fibres, no further swelling occurs.

With a sharp change in the temperature and humidity conditions in the room, internal stresses arise in the wood, which lead to cracks and deformations. The optimum temperature in a room with parquet flooring should be approximately 20 0 C, and the optimum air humidity should be 40-60%. Hydrometers are used to control the room temperature, and the relative humidity in the room is maintained with the help of air humidifiers.

Deformation of wood during drying

Wood for building parts (windows, doors, floors, etc.), especially for glued structures, should contain no more than 8-15% moisture. Hence the need to dry the wood. Natural drying takes a long time; so, for example, to dry a board 50 mm thick in the summer in central Russia to a humidity of 20%, 30-40 days are required. Artificial drying in conventional dryers reduces the drying time of such boards to 5-6 days, and drying at elevated temperatures (>100°) can be done in 3-4 hours.

Ultimate wood moisture should correspond to the humidity in the operating conditions.

With prolonged drying, water evaporates from the wood, which can lead to significant deformations of the material. The process of moisture loss continues until the level of moisture in the wood reaches a certain limit, which directly depends on the temperature and humidity of the surrounding air. A similar process occurs during sorption, that is, the absorption of moisture. The decrease in the linear volumes of wood when bound moisture is removed from it is called shrinkage. Removal of free moisture does not cause shrinkage.

Shrinkage is not the same in different directions.On average, the total linear shrinkage in the tangential direction is 6–10%, and in the radial direction it is 3.5%.

With complete shrinkage (that is, one in which all bound moisture is removed), the moisture content of the wood decreases to the limit of hygroscopicity, that is, to 0%. With an uneven distribution of moisture during the drying of wood, internal stresses can form in it, that is, stresses that occur without the participation of external forces. Internal stresses can cause changes in the size and shape of parts during the mechanical processing of wood.

Schemes for the development of deformations during convective drying

The process of convective drying of wood is accompanied by an uneven distribution of moisture in its volume. This causes its uneven shrinkage, which in turn causes the formation of internal stresses.

Let us consider how internal stresses arise and develop in wood, not yet taking into account its anisotropic structure, i.e., assuming the shrinkage in the tangential and radial directions to be the same. For simplicity, we will also assume that the movement of moisture in the material occurs only along its thickness. This will allow us to plot moisture distribution curves on a cross-sectional drawing of the assortment to be dried.

Let's consider curves of distribution of humidity on thickness for the most characteristic moments of process: 0 - the moment of the beginning of drying; 1 - the moment when the moisture content of the surface layers fell below the saturation limit of the cell walls Wn, and free water is still contained inside the assortment; 2 - the moment when the moisture content over the entire section became lower than WH, but there is still a significant difference in moisture content across the thickness; 3 - the moment of the end of the process, when the humidity became approximately the same throughout the section, close to stable humidity.

At the initial moment of the process, there is still no shrinkage and, obviously, there are no stresses. After some time, the moisture content of the surface layers will drop below Wn (moment) and they will tend to shrink. However, this desire cannot be fully manifested due to the counteraction of the inner layers, the shrinkage of which has not yet begun. It is possible to identify the beginning shrinkage by cutting out an end plate, the so-called section, from the dried assortment along its entire cross section, and dividing it into a number of layers in thickness.

Wood is a “living” material, structures made of it breathe and can change their humidity. This is its main difference from brick, concrete, metal ... This feature causes some problems when using building materials made of wood, especially when it comes to changing the percentage of humidity.

Humidity: the concept of free and bound moisture

In wood, the main part of the water is in the cell cavities, intercellular spaces, in channels, voids, cracks - this is free moisture. A certain amount of water is present in the thickness of the cell membranes - bound moisture.

Free (capillary) moisture in the tree is retained by simple physical and mechanical bonds, it easily evaporates during normal drying. This is the water that wood can absorb and release. When it comes to the moisture content of lumber, it is the amount of free moisture that is meant.
Bound (microcapillary) moisture is retained in the tree by complex physical and chemical processes, and its removal is associated with huge energy costs. In nature, it evaporates from wood during combustion or natural aging, that is, when the cells are completely destroyed.

The percentage of moisture content of lumber is one of the most important technical characteristics that affect the quality and scope of products. In addition, the moisture indicator can translate a board or beam into one of five grades. So, GOST 8486-86 states that the selected and first-third grades include lumber with a moisture content of up to 22 percent (dry) or from 22% (raw, treated with an antiseptic), and only the 4th grade is not standardized for this indicator.

Separate absolute and relative humidity. In construction practice, attention is mainly paid only to the absolute, which is defined as the ratio of the mass of moisture contained in the tree to the mass of dry wood.
It is customary to distinguish several types of humidity:

wet wood (floating) - 100 percent or more;
freshly sawn - from 50 to one hundred percent;
air-dry - up to 20 percent;
room-dry - 7-10 percent;
absolutely dry - 0 percent.

There are also the concepts of "semi-dry" lumber, wood products "transport moisture" - about 22%.

Why you need to know the percentage of moisture in lumber

Wood moisture content is unstable. Wood is hygroscopic, it always strives to enter into equilibrium with the environment. Moisture exchange occurs constantly, with an increase in air humidity, lumber absorbs water from it, and when it decreases, they give it back. This interaction causes a number of processes to change the structure and shape of the material, such as:

swelling;
shrinkage;
warping;
cracking.

During shrinkage, lumber decreases in volume due to the evaporation of moisture from the wood. Moreover, the volume decreases in direct proportion to the decrease in humidity. The tree dries out differently in different directions, least of all along the fibers (by 0.1-0.3%), in the radial direction - 4-8%, in the tangential direction - 6-10%. Volumetric shrinkage can average 12-15 percent. In order for the manufactured lumber to have the desired dimensions after drying, when sawing logs, for example, into a beam or a board, allowances are made. At the same time, it is taken into account that shrinkage depends on the density of wood - the higher it is, the more it dries out. By the way, different breeds react differently to changes in the humidity regime, they distinguish among them more or less resistant ones, and some are characterized by carpenters and joiners as “nervous”.

Swelling is an increase in volume of wood products, that is, a process opposite to shrinkage. It occurs when the tree is operated in conditions of high humidity. The increase in volume in itself does not affect the strength of wood as such, however, it can lead to a violation of the shape / integrity of the structures assembled from it.

The property of wood to swell and shrink with a radical change in humidity is considered negative. However, in some cases, for example, swelling is useful - it ensures the tightness of the connection of parts, for example, in the manufacture of wooden boats, barrels, etc.

Street wooden structures undergo swelling and shrinkage processes periodically, with the change of seasons. Therefore, they are particularly susceptible to cracking and warping as a result of constantly changing humidity.

During shrinkage, an uneven distribution of water in the layers of wood can form, as a result of which stress arises in different directions, leading to the formation of cracks. Due to shrinkage, a curvature of the product is often observed - warpage. It can be longitudinal or transverse, appears unevenly in different directions. Curved lumber complicates installation and may lose load-bearing capacity. They even single out (GOST 2140 81) a specific visible defect of the board - “wingedness”, that is, a helical longitudinal bend.

Choosing lumber according to the percentage of moisture

Lumber of a certain moisture content is intended for certain purposes. It is allowed to use a board / beam with a moisture content of 20-30 percent for the construction of any street structures - fences, gazebos, sheds, animal fences, and the like. In addition, timber and a board of natural moisture are suitable for arranging some structural elements during the construction of houses and repair work. For example, for arranging a truss system or a lag for flooring. In this case, the reliability of the fastening will not allow cracks and warpage to appear during drying. And to protect against fungus, products are treated with special antiseptic compounds.

From dry lumber (room-dry humidity at the level of 7-10 percent) planed small and large moldings (lining, block house, skirting boards, corners, cashing) are made, external and internal door panels, window transoms and frames, parquet, furniture.

Woodworkers have such a thing as "transport moisture". Its indicator should not be more than 20-22 percent. If the moisture content of lumber is higher, then it is not permissible to transport them over long distances, since transportation will take a long time, during which the wood may rot.

Absolutely dry wood with an indicator of 0 percent does not occur in practice. This concept is used only when using one of the methods for determining moisture - weight.

Basic methods for determining moisture

To determine the moisture content of lumber, two main methods are used today - by weight and using a moisture meter.

weight method

Humidity is determined as follows - a small sample (control sample) 20-25 mm wide is cut out of a bar or board. It is very important to take it not from the very edge, but from the middle, since the end parts always have less moisture. The sample is cleaned of sawdust and weighed on technical scales capable of giving ultra-precise indicators (up to one hundredth of a gram). The resulting weight is recorded - this will be the initial mass of the sample (NMP).

Next, the sample is dried in a special oven at 100-105 degrees. After five hours, it is taken out and weighed, fixing the weight, dried again, checking the indicators every 1-2 hours. When the weight stops changing, absolutely dry wood is obtained - the final mass of the sample (CMP) is recorded. Further, the moisture content of lumber is determined as follows: the difference between the NMP and the KMP is divided by the KMP index, the resulting figure is multiplied by 100 - the initial moisture index is obtained.

The main advantage of the method is that it gives very accurate indicators (with an error of no more than 1 percent). Minuses:

analysis can take a long time;
you will need to cut a sample of the material, which is unacceptable for finished products.

Using a moisture meter

A moisture meter is an electrical device specially designed to measure humidity. There are two varieties of it:

contact (needle) - the work is based on the conductometric method;
non-contact - the work is based on the dielcometric method.

The needle moisture meter has two sharp metal needles that plunge into the wood. Then the button is pressed, closing the circuit. The device measures the electrical resistance, which changes depending on the level of moisture in the material. Further, according to a special formula embedded in the memory of the moisture meter, the percentage of moisture is calculated. Measurements in this case are carried out locally, so the procedure must be carried out in several places on the product to obtain accurate results.

The main working element of a non-contact moisture meter is a radio frequency generator. Measurements are carried out using built-in contact pads, therefore, unlike the needle model, the device does not leave traces of the product. The work is based on measuring the dielectric constant of wood - water itself is characterized by a high dielectric constant, which makes it possible to obtain accurate indicators of the percentage of moisture in the material.

The main advantages of using moisture meters are ease of use and the ability to quickly get results. Non-contact instruments are also excellent for measuring the moisture content of finished products. The main disadvantage is that the indicators of moisture meters are not very accurate, the error can be from 2 to 7 percent.

The main methods of drying lumber

Wood drying is the most important operation aimed at improving its technological and consumer properties. Even if the product has become waterlogged, the former dry lumber is much less likely to warp and crack, it is easier to process and install. A dry tree perfectly resists infection with harmful fungi. The weight of the products is less, while the strength and hardness increase, and the thermal insulation qualities are also noticeably improved.

Today, the woodworking industry uses two main drying methods - natural (atmospheric) and forced (chamber).

Natural drying

During atmospheric drying, lumber is stacked in piles. under a canopy outdoors. When forming a stack between rows of boards, beams, etc. put gaskets. Stacks are installed with gaps for air circulation. The ends of the products, in order to avoid the appearance of cracks, are treated with special compounds. In addition, hardwood lumber must be pre-treated before stacking.

The drying agent in this case is air, although, unlike the forced method, its parameters (temperature, humidity) cannot be controlled. It all depends on the weather and time of year. An important point is how tightly the stacks are stacked. The denser, the higher the relative humidity in them and the lower the temperature, which means that the wood will dry out more slowly.

Atmospheric drying allows you to get products with a moisture content of 18-20 percent at the output. The speed of the process will depend on the initial moisture content of the tree, the time of year, the type of wood and the cross section of the lumber.

The main advantage is the relative cheapness of the process. It does not require special equipment and costs for air heating. In addition, residual stresses during shrinkage will not be so strong, cracks will not form - this is especially true when drying wood with initially high humidity.

Minus - the lumber dries for a long time, it is impossible to control the temperature and humidity. With a prolonged increase in ambient humidity, there is a risk of fungal infection.

Forced drying

The most technologically advanced, highly efficient method that allows you to get wood with a moisture content of 7-12 percent in a short period of time. Its essence lies in the fact that specially formed stacks of lumber are placed in special chambers, in which the necessary temperature and humidity conditions are maintained for a certain time. The drying agent can be steam, heated air or flue gases, their movement can be both natural and forced.

The percentage of wood moisture is of great importance for understanding the further behavior of wooden products during operation. Wood is a living material made up of cells, and cells, as you know, cannot live without water. There are concepts like free moisture , which is contained in the pores and capillaries of the tree and bound moisture , which is contained directly in the cells of the tree. On the border of these concepts is fiber saturation point - this is the moisture content of wood at which all free moisture is removed from the tree, i.e. there is no water in the wood cavities, and all the bound moisture remains, i.e. cells are saturated with water. The moisture content of wood at the saturation point ranges from 22 to 35%, depending on the type of wood, and for practical calculations it is assumed to be 28%.

Moisture content of wood at fiber saturation point - 28%

Important to know: wood changes its physical dimensions from 0% humidity to its saturation point. Further moisture does not lead to a significant increase in dimensions. Moisture at fiber saturation point is higher than equilibrium humidity. Equilibrium humidity is established in the wood naturally during operation, depending on the temperature and relative humidity of the air, so the wood dries out.

For Minsk, the relative average annual air humidity is 78% , in autumn-winter 80-90%, in spring-summer 65-75%. Thus, when drying wood in a natural way, humidity is set in it in summer 12-15%, in winter 18-20%. It follows from this that when wood products are used outdoors, they change their geometric dimensions throughout the year depending on temperature and humidity. Getting wet from rain can increase the moisture content of the wood well above the saturation point. It is also worth noting that with humidity above the saturation point and warm weather, ideal conditions are created in the wood for the development of wood-destroying fungi. The moisture content of 22% is considered the limit of biostability of wood. . Therefore, when operating on the street in the conditions of the Republic of Belarus.

The main rule for the exploitation of wood: Before installation, the wood must have the humidity in which it will be used in the future. . This rule also applies to other materials - WPC, laminate, plywood, etc. Thus, before installation, the wood should be left for some time in the conditions of the environment in which it will be used.

The main types of wood used in construction in Belarus are pine and spruce. The volumetric shrinkage of these species from freshly cut to dry state is up to 15%, the change in size across the fibers is up to 10%. Knowing this rule is very important when installing lining, imitation timber, block house, floorboard, especially when using raw or very dry materials.

During atmospheric drying of wood in air, the season, wind direction, temperature regime, material cross-section and other factors are of great importance. Therefore, the period for achieving the equilibrium moisture content of wood during natural drying can only be predicted. According to these parameters, in the times of the USSR, Belarus was assigned to the 3rd conditional zone according to GOST 3808.1-80 and the following natural drying period for lumber was determined:

According to the table in Belarus, natural drying is possible only from April to September. This is not so, because shrinkage occurs even at 100% relative humidity and a temperature of 0 degrees. Thus, from October to March, being outside, the wood also dries.

Humidity of wood after autoclave impregnation

To understand what moisture wood has after autoclave impregnation, let's give some numbers and calculations.

Freshly sawn wood has a moisture content of 60-80%
At a humidity of 100%, the amount of water in the tree is 50% of the total mass
The weight of 1 cubic meter of wood with 100% moisture is conditionally equal to 1 ton (including 500 kg of water)
Wood with a moisture content of approximately 25% is supplied for impregnation.
When autoclaving 1 cubic meter. wood, it absorbs about 200 liters of antiseptic solution (conditionally equal to 200 kg)

Calculation of wood moisture content after impregnation

Weight of water (B1) in 1 cubic meter dry wood with a moisture content of 25%. B1=25х500/100=125 kg
Weight of water (B2) in 1 cubic meter impregnated wood. B2=125+200=325 kg
Humidity of wood after impregnation VP=325/500*100=65%

Thus, after autoclave impregnation, the moisture content of the wood is approximately 65%. This is the moisture content of freshly cut wood. Consequently natural drying times to operating humidity can be roughly determined from the second table at the top of the page.

Wood is a natural material that is susceptible to fluctuations in temperature and humidity. Its main properties include hygroscopicity , that is, the ability to change humidity in accordance with environmental conditions. It is said that wood "breathes", that is, it absorbs air vapor (sorption) or emits them (desorption), reacting to changes in the microclimate of the room. Absorption or release of vapors is carried out due to the cell walls. With a constant state of the environment, the moisture content of wood will tend to a constant value, which is called the equilibrium (or stable) humidity.

The ability to absorb moisture is affected not only by the microclimate of the room, but also by the type of wood. The most hygroscopic species are beech, pear, kempas. They respond most quickly to changes in humidity levels. In contrast, there are stable species, such as oak, merbau, etc. These include the bamboo stalk, which is very resistant to adverse climatic conditions. It can be laid even in the bathroom. Different types of wood have different moisture levels. For example, birch, hornbeam, maple, ash have a low moisture content (up to 15%) and tend to crack when dried. Humidity of oak and walnut is moderate (up to 20%). They are relatively resistant to cracking and do not dry out as quickly. Alder is one of the most desiccation-resistant species. Its humidity is 30%.

Humidity is one of the main characteristics of wood. Under humidity wood refers to the ratio of the mass of water to the dry mass of wood, expressed as a percentage.

absolute humidity wood is the ratio of the mass of moisture in a given volume of wood to the mass of absolutely dry wood. According to GOST, the absolute humidity of parquet should be 9% (+/- 3%).

Relative Humidity wood is the ratio of the mass of moisture contained in wood to the mass of wood in a wet state.

There are two forms of water in wood - bound and free. They add up to the total amount of moisture in the wood. Bound (or hygroscopic) moisture is contained in the cell walls of wood, and free moisture occupies half of the cells and intercellular spaces. Free water is removed more easily than bound water and affects the properties of wood to a lesser extent.

According to the degree of moisture, wood is distinguished into the following types:

Wet wood. Its humidity is over 100%. This is possible only if the wood has been in the water for a long time.

Freshly cut. Its humidity ranges from 50 to 100%.

Air dry. Such wood is usually stored in the air for a long time. Its humidity can be 15-20%, depending on climatic conditions and seasons.

Room dry wood. Its humidity is usually 8-10%.

Absolutely dry. Its humidity is 0%.

Shrinkage is not the same in different directions. On average, complete linear shrinkage in the tangential direction is 6-10%, and in the radial direction - 3.5%. With complete shrinkage (that is, one in which all bound moisture is removed), the moisture content of the wood decreases to the limit of hygroscopicity, that is, to 0%. With an uneven distribution of moisture during the drying of wood, internal stresses can form in it, that is, stresses that occur without the participation of external forces. Internal stresses can cause changes in the size and shape of parts during the mechanical processing of wood.

The properties of wood directly determine the properties of wooden products. With excess or insufficient moisture, wood usually absorbs or releases moisture, respectively increasing or decreasing in volume. When the humidity in the room is high, wood can swell, and when there is a lack of moisture, it usually dries out, so all wooden products, including floor coverings, require careful maintenance. To prevent deformation of the flooring in the room, it is necessary to maintain a constant temperature and humidity. This has a positive effect not only on the quality and durability of flooring and wooden furniture, but also on people's health. With a sharp change in the temperature and humidity conditions in the room, internal stresses arise in the wood, which lead to cracks and deformations. The optimum temperature in a room with parquet flooring should be approximately 20 0 C, and the optimum air humidity - 40-60%. Hydrometers are used to control the room temperature, and the relative humidity in the room is maintained with the help of air humidifiers.

WOOD MOISTURE DETERMINATION

There are several ways to determine the moisture content of wood. In domestic conditions, they use a special device-electric moisture meter. The operation of the device is based on the change in the electrical conductivity of wood depending on its moisture content. The needles of the electric moisture meter with electrical wires connected to them are inserted into the tree and an electric current is passed through them, while the moisture content of the wood is immediately noted on the scale of the device in the place where the needles are inserted. Many experienced carvers determine the moisture content of wood by eye. Knowing the types of wood, its density and other physical properties, it is possible to determine the moisture content of wood by weight, by the presence of cracks at the end or along the fibers of the wood, by warping and other signs. By the color of the bark, its size and the color of the wood, one can recognize ripe or freshly cut wood and its degree of moisture. When processing with a p / m planer, its thin shavings, squeezed by hand, are easily crushed - which means the material is wet. If the chips break and crumble, this indicates that the material is dry enough. When transverse cutting with sharp chisels, they also pay attention to chips. If they crumble or the wood of the workpiece itself crumbles, this means that the material is too dry. Very wet wood is easily cut, and a wet mark is noticeable at the cutting site from the chisel. But it is unlikely that it will be possible to obtain a high-quality thread as a result, since cracking, warping and other deformations cannot be avoided.

DRYING WOOD

Drying wood - the process of removing moisture from wood to a certain percentage of moisture.

Dry wood has high strength, warps less, does not rot, easily glues, finishes better, and is more durable. Any wood of various species is very sensitive to changes in environmental humidity. This property is one of the disadvantages of timber. At high humidity, wood easily absorbs water and swells, and in heated rooms it dries out and warps. Indoors, wood moisture content up to 10% is sufficient, and outdoors - no more than 18%. There are many ways to dry wood. The simplest and most affordable - natural type of drying - atmospheric, air . It is necessary to dry the wood in the shade, under a canopy and in a draft. When dried in the sun, the outer surface of the wood heats up quickly, while the inner remains damp. Due to the difference in stresses, cracks form, the tree quickly warps. Boards, beams, etc. p / m are stacked on metal, wooden or other supports with a height of at least 50 cm. Boards are stacked with inner layers up to reduce their warpage. It is believed that the drying of boards placed on the edges is faster, since they are better ventilated and moisture evaporates more intensively, but they also warp more, especially material with high humidity. A stack of p / m, harvested from freshly cut and live trees, is recommended to be compacted from above with a heavy load to reduce warpage. During natural drying, cracks always form at the ends, in order to prevent their cracking and preserve the p / m, it is recommended that the ends of the boards be carefully painted over with oil paint or soaked with hot drying oil or bitumen to protect the pores of the wood. It is necessary to process the ends immediately after the transverse cuts into the cut. If the tree is characterized by high humidity, then the end is dried with a blowtorch flame, and only then painted over. Trunks (ridges) must be debarked (cleaned of bark), only at the ends they leave small belts-couplings 20-25 cm wide to prevent cracking. The bark is cleaned so that the tree dries out faster and is not affected by beetles. The trunk, left in the bark, in relative heat with high humidity quickly rots, is affected by fungal diseases. After atmospheric drying in warm weather, the moisture content of wood is 12-18%.

There are several other ways to dry wood.

Way evaporation Or steaming has been used in Rus' since ancient times. The workpieces are sawn into pieces, taking into account the size of the future product, laid in ordinary cast iron, sawdust from the same workpiece is sprinkled, poured with water and placed for several hours in a heated and cooling Russian furnace “languishes” at t = 60-70 0 C. In this case, “leaching” - evaporation of wood; natural juices come out of the workpiece, the wood is painted, acquiring a warm, thick chocolate color, with a pronounced natural texture pattern. Such a workpiece is easier to process, and after drying it cracks and warps less.

Way waxing . The blanks are dipped into melted paraffin and placed in an oven at t=40 0 C for several hours. Then the wood dries out for a few more days and acquires the same properties as after steaming: it does not crack, does not warp, the surface becomes tinted with a distinct texture pattern.

What humidity logs can be used to create reliable and warm log houses? Experts believe that for construction it is necessary to choose material only of natural moisture. Why?

Secrets of ancient wooden housing construction, they say that a wooden log house should be built from logs of natural moisture. And this is not surprising, because such material has many advantages that can be justified in terms of modern data. Although it must be said that the durability of log houses built in the old days, which have been standing for several centuries, is in itself proof of the correctness of the ancient masters. But still, why is the natural moisture content of wood so important?

From the properties of the building material, in particular, now we are talking about wood, the strength characteristics of the house are highly dependent. With non-equilibrium air humidity, a tree takes or gives off moisture, while its diameter changes. So, with high humidity, logs absorb water and increase in volume, and with dry air, the tree dries out. If there is a sharp change in air humidity, stresses arise inside the log, which cause cracking and deformation. To avoid this, it is necessary to control the moisture content of lumber during production (at all its stages).

There are such types of lumber:

Wet wood. Its humidity is almost one hundred percent. This happens when the lumber has been in the water for a long time. Such material is never used in construction.
Wood of natural moisture, recently felled. The moisture content of such lumber depends on the type of wood, the cutting time and ranges from fifty to eighty percent.
Air-dry lumber. This tree has been stored outside for a long time, its humidity ranges from fifteen to twenty percent.
Room-dry lumber. This is a tree that has been stored in a warehouse for a long time, its moisture content is from eight to ten percent.
Completely dry lumber. This tree is after forced drying in special chambers, its moisture content is not more than two percent.

Experts say that if the logs for the house set are forced to dry or stored unassembled for a long time, then they can become deformed.

A tree loses its moisture until it reaches a certain level of moisture. The level depends on atmospheric conditions. The same process takes place during the absorption (sorption) of moisture.

If completely dry lumber is taken out into the street, brought to the construction site, it will begin to absorb moisture from the air, which will cause it to swell or warp. Practice has shown that a house cut down from dry wood does not shrink evenly.

In contrast to the construction of logs of natural moisture, a frame made of forcibly dried logs does not decrease in size (dries out), but increases (swells). In this case, the walls are bent outward, the crowns and the roof may diverge. To completely remove the consequences of this process is not easy, expensive and often unrealistic. Even pine, which is considered a durable wood species, loses its strength by up to six percent, while its impact strength increases by an average of ten percent.

If the house set is cut down from logs of natural moisture, then the decrease in moisture in the tree occurs gradually. At the same time, the volume of the house decreases, under the influence of its own weight, the crowns fit tightly into place. As a result of the tight fit of the logs, the walls are less blown through, which means that less effort and finance will have to be spent on insulation.

When erecting structures made of wood of natural moisture you can't do a hard fix. For about two years, the main shrinkage of the house structure has been going on, which should not be disturbed. Shrinkage can range from three to seven percent - the size depends on the type of wood and environmental conditions.

It is important to remember that shrinkage and swelling of lumber along and across the fibers goes at different speeds, therefore, all vertical elements of the house must be equipped with expansion joints.

With the help of compensatory devices, you can adjust the height of the house and geometric proportions. For reliability, the crowns are attached to each other using dowels made of wood, which are mounted in a checkerboard pattern one and a half meters apart.

Masters, when they create log houses, remember that shrinkage is an uneven process. It can go differently outside and inside the house.

Therefore, in the production of a house kit, technological gaps are made that prevent the initial snug fit along the groove, because this subsequently causes large gaps to appear.

There are many secrets of building log houses, and only experienced craftsmen can build a warm and reliable house that will last for several centuries.