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Permissible moisture content of wood used in construction. Wood moisture content and dry lumber. Induction or electromagnetic drying of wood

From logs of what moisture content can reliable and warm log houses be created? Experts believe that for construction it is necessary to choose only material with natural moisture. Why?

Secrets of the ancient wooden house construction, They say that a 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 from the point of view of modern data. Although it must be said that durability log houses built in antiquity, which have been standing for several centuries, is in itself proof that the ancient masters were right. But still, why is the natural moisture content of wood so important?

From properties building material, in particular, now we are talking about wood, they strongly depend strength characteristics Houses. When the air humidity is unbalanced, the tree takes or gives away moisture, and 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 causes cracking and deformation. To avoid this, it is necessary to control the moisture content of the lumber during production (at all stages).

There are the following types of lumber:

• Wet wood. Its humidity is almost one hundred percent. This happens when lumber for a long time stayed in the water. Such material is never used in construction.
• Wood of natural moisture, recently felled. The moisture content of such lumber depends on the type of tree, the time of felling and ranges from fifty to eighty percent.
• Air-dried lumber. This tree has been stored outdoors 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 wood, after forced drying in special chambers, has a moisture content of no more than two percent.

Experts say that if logs for a house kit are subjected to forced drying or stored unassembled for a long time, then they can become deformed.

The tree loses its moisture until its humidity reaches a certain level. The level depends on atmospheric conditions. The same process occurs when moisture is absorbed (sorption).

If completely dry lumber is taken outside, brought to the construction site, it will begin to absorb moisture from the air, which will cause it to swell or bend. Practice has shown that a house built from dry wood does not shrink evenly.

Unlike natural moisture log construction, A log house made from forced-dried logs does not decrease in size (dries out), but increases (swells). In this case, the walls bend outward, and the crowns and roof may diverge. To completely remove the consequences of this process is not easy, expensive and often unrealistic. Even pine, which is considered a strong wood, loses its strength by up to six percent, while its toughness increases by an average of ten percent.

If the house set is made from logs of natural moisture, then the decrease in humidity 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 blown out less, which means you will need to spend less effort and money on insulation.

When constructing structures made of wood with natural humidity You cannot do a rigid fixation. For about two years, the main shrinkage of the house structure takes place, which must not be interfered with. Shrinkage can range from three to seven percent - the amount depends on the type of wood and conditions environment.

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

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

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

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

There are many secrets to building log houses and only experienced craftsmen can build a warm and reliable home, which will last for several centuries.

Wood is a rather porous material containing a large number of capillaries filled with moisture. In practice, wood moisture content is defined as the ratio of the weight of water contained in the tree to the weight of absolutely dry wood. There is a concept of “free” and “bound” moisture. “Free” moisture is contained in the pores and capillaries of the tree. “Bound” moisture is that contained directly in the cells of the tree.

When drying, the tree shrinks - it decreases in size (volume). In this case, there is practically no decrease in size along the fibers (along the length of the board), but in the direction transverse to the grain, there is a significant change in size (along the thickness and width of the board). The magnitude of this change depends on the type of wood and the specific value of the change in wood moisture content. In life, the most unpleasant surprises are associated with changes in the width of the board.

For example, if you lay a floor with a board that has natural moisture, then the decrease in its width over time can be so significant that two adjacent boards will lose their grip on each other. In this case, to remove the cracks, you will have to tear off all the boards from the joists and lay them again, fitting them end to end.

“What humidity should the board have?” you ask. It’s simple - any wooden product, during its operation, tends to the so-called “equilibrium humidity”. “Equilibrium humidity” is determined by the temperature and humidity of the air in the environment where the board will be located. You can see the values ​​of this humidity in the table. For a residential premises it averages 8-10%, for a street it averages 12-14%. It logically follows from this that a damp board will dry out indoors, losing its width, on the other hand, a dry board will be moistened outdoors, expanding.

Natural moisture content, final wood moisture content

Natural humidity- this is the moisture inherent in wood in a growing or freshly cut (sawn) state, without additional drying. Natural humidity is not standardized and can range from 30% to 80%. The natural moisture content of wood varies depending on growing conditions and time of year. Thus, the natural humidity of freshly cut trees in a “winter” forest is traditionally less than the humidity of freshly cut trees in a “summer” forest.

Initial humidity- the same as natural humidity. A freshly felled tree has maximum moisture content, which various breeds may even exceed 100%. Balsa wood can have a freshly cut moisture content of up to 600%. In practice, we deal with smaller values ​​(30-70%), because After cutting, some time passes before the tree is sawed and placed in the dryer, and it, of course, loses a certain amount of water. We take the initial moisture content to be the moisture content of the wood that it has before being sent to the drying chamber.

Final humidity- this is the humidity we want to get after full cycle drying In this case, the purpose of the product made from dried wood is taken into account.

First of all, wood drying is the process of removing moisture from wood by evaporation.

Drying wood is one of the most important operations in the wood processing process. The wood is dried after sawing, but before wood processing. The wood is dried in order to protect it from damage by wood-staining and wood-decaying fungi during its further storage and transportation. Drying prevents wood from changing shape and size during the manufacturing and use of products made from it, improves the quality of wood finishing and gluing. The humidity to which wood is dried depends on its area further application. The whole point is to bring the moisture content of the board to the same value that a product made from this board would reach over time during operation under these conditions. This humidity value is called “equilibrium humidity”; it depends on the humidity and temperature of the surrounding air. For example, the board from which parquet and other products used indoors will be made should have a humidity of 6-8%, since this is the humidity that will be equilibrium. For products that will be used in contact with the atmosphere (for example: wooden windows, outer skin at home) the equilibrium humidity will be 11-12%.

You ask: “What will happen otherwise?” We answer: Otherwise, what happens all the time in Russia will happen, namely, the consumer will face problems. Imagine that you bought lining in order to sheathe the walls inside your country house or dachas. If you buy clapboard made from raw boards from a careless manufacturer and cover the walls of your house with it, it will begin to slowly dry out naturally in its already installed state. Let us turn to the table of equilibrium humidity and experience. If you heat a room in winter to 25 degrees Celsius, then with a typical indoor air humidity of 35% for winter, the equilibrium humidity value for a board in such a room will be 6.6%. At bases and markets, lining can very often have a humidity of 14% or higher (we have encountered 30%). Next, imagine that your lining begins to dry out, losing water from its pores. When drying, a process called “shrinkage” occurs and is expressed in a reduction in the size of the wooden product. The amount of shrinkage depends on the type of wood, the direction of the fibers in the product, etc. The main shrinkage occurs across the fibers (according to the thickness and width of your lining). When your lining dries in the installed state to equilibrium moisture, you, in the worst case, risk not only seeing that the lining has come apart in places, but getting gaps between the boards, almost the width of a finger.

The industry uses various technologies for drying wood, differing both in the equipment used and in the characteristics of heat transfer to the dried material.
The classification of types and methods of drying is usually based on heat transfer methods, according to which four wood drying technologies can be distinguished:

• convective drying technology;
• conductive drying technology;
• radiation drying technology;
• electric drying technology;

Each type of drying can also have several varieties depending on the type of drying agent and the characteristics of the equipment used for drying wood. There are also combined technologies for drying wood, in which various types of heat transfer are simultaneously used (for example, convective-dielectric) or other characteristics are combined various technologies drying wood.

Independent drying technologies

Chamber drying

Chamber drying. This is the main industrial technology for drying wood, carried out in wood drying chambers various designs, where lumber is loaded in stacks. Drying occurs in a gaseous environment (air, flue gases, superheated steam), which transfers heat to the wood by convection. To heat and circulate the drying agent, drying chambers are equipped with heating and circulation devices.

With chamber wood drying technology, the drying time for lumber is relatively short (from tens of hours to several days), the wood dries to any given final moisture content at the required quality, and the drying process can be reliably regulated.

Atmospheric drying

The second most important and widespread method at sawmills is the industrial drying of wood, carried out in stacks placed in a special open area (warehouses), washed by atmospheric air without heating. The advantage of atmospheric wood drying technology is its relatively low cost. In addition, this method is the most gentle. Disadvantages: seasonality (drying practically stops in winter); long duration; high final humidity. Atmospheric wood drying technology is used mainly for drying lumber at sawmills to transport moisture and at some woodworking enterprises for drying and leveling the initial moisture content of lumber before drying in wood drying chambers.

Drying in liquids

Drying in liquids is carried out in baths filled with a hydrophobic liquid (petrolatum, oil) heated to 105-120 °C. Intensive heat transfer from liquid to wood allows the drying time to be reduced by 3-4 times compared to chamber drying, all other conditions being equal. This method is used in wood preservation technology to reduce its moisture content before impregnation. Attempts to dry lumber in petrolatum at woodworking enterprises have not yielded positive results due to the fact that lumber after such drying does not meet the requirements for wood for furniture and joinery and construction products.

Conductive drying technology

Conductive (contact) wood drying technology is carried out by transferring heat to the material through thermal conductivity upon contact with heated surfaces. It is used in small quantities for drying thin wood materials - veneer, plywood.

Radiation drying

Radiation drying of wood occurs when heat is transferred to the material by radiation from heated bodies. The effectiveness of radiation drying is determined by the flux density of infrared rays and their permeability in solid wet bodies. The intensity of the radiant energy flow weakens as it goes deeper into the material. Wood is classified as low-permeable infrared radiation materials (penetration depth 3-7 mm), therefore this method is not used for drying lumber. It can be used for drying thin-sheet materials (veneer, plywood), in addition, this method is widely used in the technology of finishing wood products for drying paint and varnish coatings. Electric stoves, electric heating elements, gas (flameless) burners, and incandescent lighting lamps with a power of 500 W and above are used as emitters.

Rotary drying

Rotational drying of wood is based on the use of the centrifugal effect, due to which free moisture is removed from the wood when it is rotated in centrifuges. Mechanical removal free moisture is achieved at a centripetal acceleration value of at least 100-500g (g is the acceleration of free fall). Such accelerations have not yet been achieved in practice due to the difficulty of accurately balancing a centrifuge with a stack; only experimental development of corresponding devices is underway. In known industrial rotary dryers, the centripetal acceleration does not exceed 12g. Under these conditions, mechanical dehydration occurs to a small extent. However, intensification of the drying process in the humidity range above the hygroscopic limit is observed.

When installing a carousel in a drying chamber, the technology for drying lumber is the same as in conventional batch chambers. The duration of drying at the first stage (from the initial moisture content to the hygroscopic limit) is reduced several times depending on the thickness, species and initial moisture content of the wood compared to conventional convective drying under the same conditions. Although rotary dryers are economical and provide high quality drying, the rotary method has not yet found industrial use for drying lumber.

Vacuum drying

Vacuum drying at reduced pressure in special sealed drying chambers. Due to the complexity of the equipment and the impossibility of obtaining low final moisture content of wood, vacuum drying has no independent significance. It is used in combination with other drying methods and as an auxiliary operation in preparing wood for impregnation.

Dielectric drying

Dielectric drying is the drying of wood in an electromagnetic field of high frequency currents, in which the wood is heated due to dielectric losses. Due to the uniform heating of wood throughout its entire volume, the emergence of a positive temperature gradient and excess pressure inside it, the duration of dielectric drying is tens of times less than convective drying. Due to the complexity of the equipment, high power consumption and insufficient High Quality drying Dielectric drying itself is not widely used.

Combined wood drying technologies

It is more effective to use combined wood drying technologies, for example convective-dielectric and vacuum-dielectric. For mass drying, the use of these methods is uneconomical, but in in some cases, especially when drying expensive, critical lumber and blanks made from difficult-to-dry wood species, these methods can be used.

Convective-dielectric drying

With a combined convective-dielectric technology for drying wood, high-frequency energy from a special high-frequency generator is also supplied to a stack loaded into a chamber equipped with thermal and fan devices through electrodes located near the stack.
The heat consumption for drying in the drying chamber is mainly compensated by the thermal energy of steam supplied to the heaters, and high-frequency energy is supplied to create a positive temperature difference across the cross section of the material. This difference, depending on the characteristics of the material and the rigidity of the given mode, is 2-5°C. The quality of convective-dielectric drying of lumber is high, since drying is carried out with a small difference in humidity across the thickness of the material.

Vacuum dielectric drying

This is another way of drying wood using high-frequency energy. This technology uses the advantages of both vacuum and dielectric drying. By heating wood in a high-frequency field at reduced pressure, boiling of water in wood is achieved at low wood temperatures, which helps preserve its quality. The movement of moisture in wood during vacuum-dielectric drying of wood is ensured by all the main driving forces of moisture transfer: moisture content gradient, temperature, excess pressure, which reduces the drying time.

During vacuum-dielectric drying, a stack of lumber is placed in an autoclave or a sealed chamber, where a vacuum pump creates a reduced pressure of the environment (1-20 kPa). The lower the environmental pressure, the lower the evaporation temperature of moisture and wood during drying. Heat consumption for drying is provided by the supply of high-frequency energy to the wood. When using this wood drying technology, operational difficulties also arise - the complexity of the equipment, especially the setup and operation of high-frequency generators, and the high energy consumption for drying. Therefore, when deciding on the use of vacuum-dielectric chambers, it is necessary to first develop a feasibility study based on the conditions of a particular enterprise.

Induction or electromagnetic drying of wood

The method is based on the transfer of heat to the material from ferromagnetic elements (steel mesh) stacked between rows of boards. The stack, together with these elements, is located in an alternating electromagnetic field of industrial frequency (50 Hz), formed by a solenoid mounted inside the drying chamber. Steel elements (mesh) are heated in an electromagnetic field, transferring heat to wood and air. In this case, a combined transfer of heat to the material occurs: by conduction from the contact of heated meshes with wood and convection from circulating air, which is also heated by the meshes.

Tree - living plant, which naturally absorbs water to support life. Recently cut, it contains a significant percentage of moisture, different breeds it is individual. However, this value must be taken into account when purchasing material in order to understand whether the wood is suitable for further processing and use or still needs to be dried.

Types of humidity

Absolute - the ratio of the mass of liquid in a volume of wood to the mass of a dry sample of the same size.

Relative shows the amount of moisture relative to the mass of the same sample of wood in the same condition.

In natural materials, bound and free moisture coexist:

• The free one is located in the intercellular space and makes up the arteries of the tree. This is easily removed during drying and does not affect the quality of the workpiece;
• Bound is contained in the cells of the tree.

Norms

The moisture content of wood species is regulated by regulatory documents:

• SNiP II-25-80;
• SP 64.133330.2011;
• GOST 16483.7-71*;
• GOST 17231-78.

There are moisture standards for wood according to the class of use of the material. For each individual species, the value is difficult to determine: in what conditions the plant grew, what season it was, in what area the felling was done - all factors are reflected in the amount of moisture in one individual. Average values:

Wood moisture content: the norm according to GOST is determined for ready-made structures V different conditions operation.

Humidity determination

To determine what percentage is the natural moisture content of wood, you should use one of the methods:

1. The simplest and most common is the use of a moisture meter - electrical appliance, whose work is based on measuring the electrical conductivity of the measured raw materials. 3 needle electrodes are inserted into the wood, the device sends a discharge through them and displays the desired value on the screen.
2. Four-needle chip probes are based on the same operating principle.

How to determine wood moisture content without a moisture meter

An accurate calculation with a measurement error of no more than 1% is described by GOST 17231-78. Simplified algorithm:

1. Take a piece of wood and weigh it.
2. The sample is placed in a drying chamber with an air temperature of +103°C until a constant mass is obtained.
3. The dried section is cooled until room temperature and weighed again.
4. The moisture content of the source is determined using the formula:
   • W=(m-m0)/m0*100%, where m and m0 are the mass of wood before and after drying.

Determination by instruments can give an error of up to 10%:

• The electrodes penetrate to a shallow depth, and the water in the barrel or product is unevenly distributed. As the top layers dry, they become drier.
• Each breed deals with moisture differently.
• Specific electrical resistance different types trees individually; universal devices do not take this into account.

The methods described by GOST are cumbersome and time-consuming; determining the value can take several days and requires special equipment.

Experienced craftsmen determine the moisture content of wood by its appearance:

• According to the color of the breed;
• According to availability;
• For warping, etc.

You can determine the readiness of wood for processing yourself:

• According to the quality of the bark: in a damp tree it is rich and soft, during the drying process the bark becomes coarser;
• At the place of cutting: the fresh plant is damp to the touch;
• Regarding the chips: when the bundle is compressed, they should not stick together - such chips are not ready for processing, the product will shrink significantly.
• The presence of evenly distributed shallow cracks indicates the maturity of the material, deep cracks indicate drying out and heterogeneity.

Wood moisture content is a value that shows the level of moisture exchange between wood and air. Since the humidity of the environment is variable, it is constantly changing in wood. The level of this indicator affects the quality and use of lumber. Products with high humidity more susceptible to mold and mildew, rot and deformation. Therefore, dry raw materials are chosen for the construction and manufacture of wooden products.

Humidity can be absolute or relative. The first represents the ratio of the mass of moisture in wood to the mass of absolutely dry wood. A more commonly used, convenient and easy-to-calculate quantity is relative humidity. This is the ratio of the moisture mass of wood to the total mass of wood, which actually reflects the water content in the product. This value is decisive, so it is used in practice.

Types of wood by moisture content

The initial figure is usually 50-60%. With natural drying, after 1.5-2 years it drops to 20-30%, after which it does not change much and remains at approximately 25%. There are wet wood with a moisture content above 35%, semi-dry wood with a moisture content of 25-35% and dry wood with a moisture content of less than 25%. To reduce the moisture level to room-dry 7-18%, lumber is dried in special drying chambers.

The following types are distinguished:

• Floating wood that has been in water for a long time or a well-damp and wet log - over 60%;
• The initial rate of a freshly cut tree is 45-60%;
• Air-dry reaches 20-30%. This result is typical for logs and beams that have been in storage for a long period of time. outdoors or located in a dry, well-ventilated area;
• Room-dry is dried in rooms with heating and good ventilation or in special chambers. Amounts to 7-18%;
• Absolutely dry wood with a moisture content of 0% is dried to constant weight at a temperature of 100-105 degrees above zero.

To build a house or bathhouse, dry lumber with a moisture content of up to 20% is used. Today the safest and effective method, which allows you to achieve such indicators, is condensation drying. When drying, both hot and cold air. This prevents the appearance of defects and defects on the log or timber, reduces the number of cracks and allows you to obtain high-quality, durable and dry lumber. Read more about condensation drying.

Logs and beams for building a house

In “MariSrub” you can order construction wooden houses only from high-quality and carefully selected dried materials. We independently produce logs and beams for the design of a wooden house, bathhouse, gazebo or garage. Choose individual design or revision already ready-made version which you will see in .

During production we use condensation drying, we comply with GOST requirements, storage and transportation standards. We carry out antiseptic treatment and control every stage of production, which allows us to obtain durable and high-quality lumber. Due to this treatment, the wood retains its original color and natural properties for a long time. It does not rot or darken or become moldy. The materials are resistant to moisture and cold, wind and insects, and temperature changes.

We are building wooden houses from timber and logs from the manufacturer. Own production- quality assurance and low prices for lumber! We offer turnkey construction, which includes the creation of a project and installation of timber or log house, construction of the foundation and roof, finishing inside and outside the house, installation and commissioning utility networks. We guarantee quality construction on time!

Wood is 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 ensure its life. Water content is characterized wood moisture. Humidity is one of the main characteristics of wood.

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

Wood moisture content 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 rafting and storing timber in water.

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

The properties of wood directly determine the properties of wooden products. When there is excess or insufficient humidity, wood usually absorbs or releases moisture, increasing or decreasing in volume accordingly. With high humidity in the room, wood can swell, and with a lack of moisture, it usually dries out, so everything wooden crafts, including floor coverings, require careful maintenance. To prevent deformation flooring the room must be maintained at a constant temperature and humidity.

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

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

Absolute humidity of wood (moisture content) - mass fraction of water, expressed as a percentage relative to the mass of absolutely dry wood. Absolute humidity wood is the ratio of the mass of moisture contained 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 referred to as wood dried to constant weight at a temperature of (104±2)°C. Values relative humidity wood are needed for analyzing wood when calculating the mass fractions of its components as a percentage relative to absolutely dry wood. The absolute humidity of wood (moisture content) is used to quantitatively characterize wood samples when comparing them by water content.

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

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


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


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


Room-dried 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

Water in a tree is distributed unevenly: roots and branches contain more water than the trunk; butt and top - larger than the middle part of the trunk; sapwood coniferous species- more than sound and mature wood. In wood hardwood water is distributed more evenly across the cross-section of the trunk, and in some tree species (for example, oak) the core moisture 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%, while the moisture content of driftwood reaches up to 200%. In conifers, the moisture content of the core is 2-3 times lower than the moisture content of the sapwood.

In construction practice, wood is usually classified according to its moisture content:

freshly cut wood with an average absolute humidity of 50 to 100% depending on the time of felling (water content is significantly higher in spring and lowest in winter period), and tree species and growing conditions;


• air-dried wood is wood dried in air until its moisture content is in equilibrium 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 kept in a heated room and having an absolute humidity of 9...13%; wet wood, resulting from prolonged exposure to water, with an absolute humidity above 100% (up to 200% or more).

There are two forms of water found in wood - bound (hygroscopic) and free (capillary). These 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 the cells and intercellular spaces. Free water is removed more easily than bound water and has less effect on the properties of wood.

Free (capillary) moisture is contained in the cavities of the cells, and bound moisture is contained in the walls of the 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 does a further increase in moisture occur due to free moisture. Therefore, it is obvious that it is the change in bound moisture that affects the processes of shrinkage and warping of wood, as well as its strength and elastic properties. An increase in free moisture has practically no effect on the properties of wood.

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

They say that wood “breathes,” that is, it absorbs air vapor (sorption) or releases it (desorption), reacting to changes in the microclimate of the room. The absorption or release of vapors is carried out due to the cell walls. If the environmental condition remains unchanged, the moisture level of wood will tend to a constant value, which is called equilibrium (or stable) moisture.

In wood, moisture is contained in wood cells, in the intercellular space, in the channels of blood vessels and it is called free moisture. IN Lag contained in 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 breeds is 30% at 20°C.

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

Free and hygroscopic moisture is removed from 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 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 is usually 30% at a temperature of 20°. Such wood moisture, is called the saturation point of cell membranes, or the saturation point of fibers. A further increase in humidity occurs due to free moisture filling the voids in the wood.

When humidity changes from zero to the point of saturation of the cell membranes, the volume of wood changes and it swells. When humidity decreases, 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 bound water content 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 include beech, pear, and kempas.

They respond most quickly to changes in humidity levels.

In contrast, there are stable species, for example, oak, merbau, etc. These include a bamboo stem, which is very resistant to adverse climatic conditions. It can even be installed in the bathroom.

Different types of wood have different levels humidity. For example, birch, hornbeam, maple, and ash have low humidity (up to 15%) and, when dry, tend to form cracks. The moisture content of oak and walnut is moderate (up to 20%). They are relatively resistant to cracking and dry less quickly. Alder is one of the most drying-resistant species. Its humidity is 30%.

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

DETERMINATION OF WOOD MOISTURE

There are several ways to determine wood moisture content. At home, they use a special device called an electric moisture meter. The operation of the device is based on changes in the electrical conductivity of wood depending on its humidity. The needles of an electric moisture meter with electrical wires connected to them are inserted into the wood and an electric current is passed through them, while the moisture content of the wood is immediately marked on the scale of the device in the place where the needles are inserted.

Knowing the types of wood, its density and other physical properties, it is possible to determine the moisture content of wood by mass, by the presence of cracks at the end or along the wood fibers, by warping and other signs. By the color of the bark, its size and the color of the wood, you can recognize ripe or freshly cut wood and the degree of its moisture content. When processing a semi-finished plane with a plane, its thin shavings, compressed 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 making transverse cuts with sharp chisels, also pay attention to the shavings. If they crumble or the wood of the workpiece itself crumbles, this means that the material is too dry. Very wet wood is easy to cut, and a wet mark from the chisel is noticeable at the cutting site. But it is unlikely that cracking, warping and other deformations will be avoided.

Wood moisture content determined in various ways: by drying samples of wood, wood chips or sawdust until completely dry; distillation of water in the form of an azeotropic mixture with non-polar solvents that are immiscible with water; chemical methods (titration with Fischer reagent); electrically.

The moisture content of lumber is determined by the formula

W = (m s - m o) / m s,

where m c and m o are the mass of the sample in the original and dried states, respectively.

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

Humidity of drift wood is 200%, freshly cut wood is 100%, air-dry is 15-20%.

DRYING WOOD

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

Bwood line- the ability of the wood surface to directionally reflect light rays.

Gloss depends on the type of wood, the degree of smoothness of the surface and the nature of the lighting. The radial surfaces of maple, sycamore, beech, elm, oak, dogwood, white acacia, ailanthus wood are distinguished by their shine. rocks in which a significant part of the surface is occupied by medullary rays consisting of small cells. The shine of wood is a decorative property and is taken into account when determining species.

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

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

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

Transverse warpage associated with various shrinkage (swelling) of wood in the radial and tangential directions. Its character depends on the location of the annual layers, determined by the shape cross section assortment, as well as the place where it is cut from the log.

Longitudinal warping associated with certain wood defects, such as large knots, leaning, and tilting of the fibers.

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

Transverse and longitudinal warping also occurs due to an imbalance of residual stresses in dried lumber during machining: 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 parts of the log during drying.

Dry wood has high strength, warps less, is not susceptible to rotting, is easy to glue, is better finished, and is more durable. Any wood of various species reacts very sensitively to changes in environmental humidity.

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

Indoors, wood moisture content is sufficient up to 10%, and outdoors - no more than 18%. There are many ways to dry wood.

The simplest and most accessible - natural type of drying - atmospheric, airy. Wood should be dried in the shade, under a canopy and in a draft. When dried in the sun, the outer surface of the wood quickly heats up, but the inner surface remains damp.

Due to the difference in stress, cracks form and the wood quickly warps. Boards, timber, etc. are stacked on metal, wooden or other supports with a height of at least 50 cm. The boards are stacked with their inner layers facing up to reduce their warping. It is believed that boards placed on the edges dry faster, since they are better ventilated and moisture evaporates more intensely, but they also warp more, especially material with high humidity.

It is recommended to compact a stack of p/m, prepared from freshly cut and live trees, with a heavy load on top to reduce warping. During natural drying, cracks always form at the ends; to prevent cracking and preserve the quality of the boards, it is recommended to carefully paint the ends of the boards with oil paint or soak them in hot drying oil or bitumen to protect the pores of the wood. The ends must be processed immediately after cross-cutting into the cut.

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

There are several other ways to dry wood.

Way evaporation or steaming has been used in Rus' since ancient times. The blanks are cut into pieces, taking into account the size of the future product, placed in ordinary cast iron, sawdust from the same blank is added, filled with water and placed in a heated and cooled Russian oven for several hours, “languishing” at t = 60-70 0 C.

In this case, “leaching” occurs - 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 is less likely to crack and warp.

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 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. Wooden utensils steamed in linseed oil are very waterproof and do not crack even with everyday use. This method is still acceptable today. The workpiece is placed in a container, filled with linseed oil and steamed over low heat.

Warping: 1 - transverse; 2 - longitudinal along the surface; 3 - longitudinal along the edge; 4 – wingedness of logs due to residual internal growth stresses.

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

Normalized wood moisture content— equilibrium wood moisture content 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 bound water is removed from it.

Volumetric swelling of wood— an 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 under given temperature and humidity conditions is called equilibrium. For example, at a temperature of 20 o C and air humidity of 100%, the equilibrium moisture content of wood is W = 30%.

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

In massive elements, due to uneven drying, shrinkage cracks form. Therefore, in the production of lumber, great importance should be attached to the organization of drying, and when operating wooden structures, large and sudden changes in temperature and humidity should be excluded. Wood is characterized by a certain inertia of moisture exchange processes.

Wood shrinkage: 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 cross section of the trunk. Due to such uneven shrinkage, warping of the boards appears when drying. When humidity increases above the saturation point of the fibers, further swelling does not occur.

With a sharp change in the temperature and humidity conditions in a room, internal stresses arise in the wood, which lead to cracks and deformations. The optimal temperature in a room with parquet flooring should be approximately 20 0 C, and the optimal air humidity should be 40-60%. Hydrometers are used to control indoor temperature, and relative humidity in the room is maintained using 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; for example, to dry a board 50 mm thick in the summer in central Russia to a humidity of 20%, it takes 30 - 40 days. Artificial drying in conventional dryers reduces the drying time of such boards to 5 - 6 days, and drying at elevated temperatures (>100°) can be carried out in 3 - 4 hours.

Ultimate wood moisture must correspond to the humidity under operating conditions.

During prolonged drying, water evaporates from wood, which can lead to significant deformation of the material. The process of moisture loss continues until the moisture level 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, absorption of moisture. A decrease in the linear volume of wood when bound moisture is removed from it is called shrinkage. Removing free moisture does not cause shrinkage.

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 drying (that is, one in which all bound moisture is removed), the moisture content of the wood is reduced to the limit of hygroscopicity, that is, to 0%. At uneven distribution moisture when drying wood, internal stresses can form in it, that is, stresses that arise without the participation of external forces. Internal stresses can cause changes in the size and shape of parts during 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 throughout 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, without 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 depict moisture distribution curves on a cross-sectional drawing of the dried assortment.

Let's consider the moisture distribution curves along the thickness for the most characteristic moments of the process: 0 - the moment of drying begins; 1 - moment when humidity surface layers has dropped below the saturation limit of cell walls Wn, and there is still free water inside the assortment; 2 - the moment when the humidity across the entire section became below WH, but a significant difference in humidity across the thickness was still observed; 3 - the moment of the end of the process, when the humidity has become approximately the same throughout the entire cross-section, close to stable humidity.

At the initial moment of the process there is still no shrinkage and tension is obviously absent. After some time, the moisture content of the surface layers will drop below Wn (moment) and they will tend to dry out. However, this desire cannot fully manifest itself due to the resistance of the internal layers, the shrinkage of which has not yet begun. It is possible to identify the beginning of 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 according to thickness.