Materials science in construction. Theme: Building materials

Modern production offers a wide choice for construction. The markets are full of a huge number of products from both foreign and domestic manufacturers.

Building materials vary greatly in their properties.

To make the right choice in this area, you need to have an idea not only about what materials are used, say, in the construction of a bath, but also about what is included in the selected materials, as well as about the properties that they possess.

When using any of the existing materials in private construction, it is necessary to take into account all the physical and mechanical properties. This will allow you to choose the most suitable building material, the quality of which will meet the necessary requirements. The basic properties of building materials can be classified into several basic types.

The first type of properties are physical properties, which include: total volumetric weight, actual specific gravity, density and its possible porosity. It is on these properties that the ratio of one or another building material and its belonging to individual types of construction depends.

The second type of properties includes those properties that determine the effect of moisture on the material itself and the possible consequences when this moisture freezes. These properties include: moisture absorption, initial moisture, as well as the return of this moisture to the environment, resistance to moisture absorption and freezing resistance.

The third type of properties are mechanical properties such as wear, strength and density. The fourth type of properties includes all those properties of a building material that are associated with thermal effects.

porous brick

In more detail, they can be characterized as total heat conductivity, initial thermal capacity and fire resistance, as well as fire resistance. In addition, there are some thermal properties that are unique to a certain type.

Some building materials have a rather rare ability to resist destruction, which can be caused by exposure to various acids, gases, salts and alkalis. Such properties refer to corrosive or, as they are commonly called, chemical properties.

Technological type properties belong to a separate type of properties. These properties include the ability that contributes to the processing of the mechanical type of a single building material.

For example, lumber can be easily machined with manual or automatic tools. All of these properties must be taken into account before choosing one or another building material for private construction of any type.

The main characteristics of physical and chemical properties building materials

Specific gravity- this is the total weight of an individual building material, which is indicated in a volume unit. In this case, the state of the material itself should be as dense as possible, excluding any pores. Accordingly, volumetric weight is the total weight of the material in its given state, taking into account any level of graininess and pores.

Volumetric weight has another fairly common type - bulk weight. This weight is the total weight of fillers (sand or gravel), which does not subtract the weight of voids that form between large particles of bulk building material.

Density is the total degree of filling of the volume of an individual material with those solid particles of which the material itself is composed. Porosity is the ratio of the total volume of porous parts of a material to its total volume.

Due to the different size of air pores, it can be divided into large-porous and fine-porous. Such pores are calculated in hundredths and tenths of a millimeter. If there are larger pores in building materials, for the most part this applies to bulk options, then such pores are usually called voids.

As a rule, porosity is indicated as a percentage. For example, metal has 0 percent porosity, while mineral wool board has 90 percent porosity. As a rule, building materials with maximum porosity play the role of a good heat-insulating material, which is used both in outdoor and indoor construction.

Palette of building materials

Water absorption is the maximum degree of filling of a free volume with moisture. The difference in reliability and strength of an individual building material in its dry state and saturated with moisture can be called the softening coefficient of the material.

Knowing this coefficient is necessary in order to calculate the strength in conditions of high humidity. AT otherwise the reliability of the constructed structure will be questionable. This coefficient can vary from 0 to 1 for different building materials. As a rule, the use of a stone in conditions of high humidity is unacceptable if its softening is 0.8.

All building materials that have a softening factor higher than 0.8 can be used in high humidity conditions. These are called waterproof.

Moisture release by a building material is a rare ability of a material, in which, under the conditions of climate change, a certain percentage of moisture, which was contained in the building material itself, is released into the environment.

This ability can be determined by how quickly the material dries at elevated air temperatures, as well as the weight of moisture given off, which can be found from the total weight of the building material. The moisture content of a building material is the amount of liquid contained in the building material in its standard form.

Moisture permeability is a separate ability in which, under the influence of artificial pressure, a liquid passes through a building material.

Frost resistance is a separate ability of a material in a wet state to withstand sudden changes in temperature. In this case, the structure should not be destroyed. Those building materials that do not differ in high saturation with moisture can be safely considered frost-resistant.

In order for the building material to have good frost resistance, its softening coefficient should not be lower than 0.9. A rather important property of the building material used for the construction of load-bearing walls is gas permeability. This property of a building material is responsible for the ability to pass gas or air through its structure.

In order to reduce the gas permeability, products made of such material must be coated with oil-type paints, bituminous or simple cement plaster.

Thermal conductivity is the ability of a material to transmit thermal energy through its structure. This happens when the air temperature on both sides of the structure, which is built from this building material, has different indicators.

Knowing such properties of building materials is simply necessary in order to provide a high-quality surface for load-bearing walls, interfloor floors or insulating structures. Otherwise, a house that is built from building materials with high heat conductivity in winter will be quite difficult to heat.

And in the summer, it will be quite hot inside the house, which will negatively affect the microclimate of the living space. For the highest quality construction, it is necessary to know the coefficient of heat conductivity of the building material, which is equal to the total amount of thermal energy, indicated in kilocalories, that passes through a structure whose thickness is 1 meter and with a total area of ​​​​1 square meter for a period of time of 1 hour. At the same time, the air temperature on both sides of the structure should differ by only 1 degree Celsius.

Construction of a log house

The degree of thermal conductivity can be determined by taking into account the level of porosity of the material, its type and weight, as well as the minimum heating temperature at which thermal energy is released or conducted. The maximum conductivity of thermal energy has those building materials that have a minimum amount of air pores.

The fact is that the air itself has a rather low thermal conductivity. For this reason, building materials that have increased porosity have a minimum percentage of thermal conductivity. Enough great importance It also has the type of pores in the building material.

For example, a finely porous material has a lower percentage of thermal energy conductivity than a coarsely porous one. In addition, a material whose pores are isolated from each other are also not considered highly conductive building materials, unlike those whose pores intersect. This can be explained by the fact that air is transported in large pores, during which there is an insignificant conductivity of thermal energy.

Thermal capacity is the ability of an individual material to absorb a certain amount of thermal energy when a heat source acts on the material. In order to determine the coefficient of thermal capacity of a building material, it is necessary to calculate the total amount of heat, determined in kilocalories, which will be required in order to heat a separate building material to 1 degree.

This coefficient varies from 0 to 1. Stone building materials have a thermal capacity coefficient of 0.20. lumber has 3 times the value of this coefficient. Metal building materials cannot boast of a high value of this coefficient. For example, for steel, such a coefficient is 0.11.

Thermal stability is considered quite an important feature. This is the ability to maintain the temperature given to it for as long as possible. This is especially important in the construction of load-bearing walls, partitions or floors. The longer these building elements retain heat on their surface, the easier it will be to heat the living space in the winter season.

Fire resistance is the ability of a building material to resist the effects of elevated temperatures for as long as possible, which has a direct effect on the building material itself.

Such properties are quite useful in the construction of structures that are at high temperatures for a long time. Such structures include stoves, heat pipes or fireplaces. For all these building structures a building material with increased fire resistance is required.

Corrosion or chemical resistance is considered one of the rarest qualities of building materials. One of the types that successfully resists chemical attack is ceramics. Such a building material can resist such exposure for quite a long time, which most cannot do. Thus, it becomes clear that knowledge of the various features of the building material is quite important in the construction of a residential building or outbuildings.

For example, the main physical properties that you need to pay attention to are fire resistance, heat capacity, air permeability, water absorption, porosity, radiation resistance, sound absorption, and others.

More about some of them.

fire resistance

This is the ability to maintain its main characteristics (hardness, strength, etc.) even when exposed to high temperatures. Depending on the degree of fire resistance, materials are divided into:

• combustible;
• flame retardant;
• Fireproof.

For example, steel, concrete and brick are fireproof building materials, as they do not smolder or ignite when exposed to open fire. All that can happen to these materials in such cases is their deformation.

Asphalt concrete and fiberboard are related to slow-burning. When directly exposed to a flame, they smolder and char, but their smoldering stops after the source of high temperature is removed. The most unstable to fire are plastics, wood and roofing material. These continue to burn even after the ignition source is removed.

Thermal conductivity

Filling device

This feature implies the transfer of heat outside or inside the building at a temperature difference. Structure, porosity and humidity are the main properties of building materials on which thermal conductivity depends.

The greater the density, the better it will transfer heat. The presence of moisture in the building material also allows you to increase its thermal conductivity.

Air resistance

Due to this property, the building material can withstand repeated wetting and drying without losing shape and reducing strength over time. In order to increase air resistance, water-repellent additives are introduced into building materials.

When building a house, you should pay attention not only to the physical, but also to the environmental features that you decide to choose for work. For interior decoration it is better to choose a material that is not only resistant to all kinds of adverse effects, but also environmentally friendly. People will live in the house, so it’s worth considering what impact it will have on health.

Option for interior decoration

Many people do not pay due attention to the material intended for finishing work inside the house. The fact is that the walls occupy most of the area of ​​\u200b\u200bthe living space, so the look at the entrance to an apartment or house immediately stops at them. Regardless of the nature and position in society, people want one thing: that the walls in their house are beautiful and reliable.

Glass magnesite is a material that is not afraid of fire

In the decoration of residential premises, it is best to choose plaster, wood or textile materials. Now they are called "breathable". In small rooms, wall decoration with ceramics, stone and wood will look very expressive. Such a finish will give not only the effect of naturalness, but also make the walls in the room bright and catchy. If you do not like this perspective, then it is better to use such materials in the decoration of additional rooms (for example, a balcony or a loggia).

When decorating walls, one cannot exclude the fact that the finish natural materials you might get bored. It would be a pity to change the scenery, in the installation of which not only a lot of money was invested, but also their own work. Therefore, painted surfaces and wallpapers have an advantage over natural “products”.

Interior decoration plays a significant role in the design of housing. But what material to choose when building? Practice shows that often many questions arise during the construction of walls. After all, everyone knows that walls must be strong, provide protection from various atmospheric influences, and also have good thermal insulation. There are many materials that are used for this purpose. Of course, when choosing, knowledge about what are the main properties of building materials will be useful?

Brick: advantages and disadvantages

First of all, this material is durable and resistant to high temperatures. A brick of good quality is not afraid of rot, water or fire. The disadvantage is its high thermal conductivity, so all brick houses- cold. Thanks to modern technologies options for porous bricks appear, but even it cannot be compared with wood. There are two types: silicate and ceramic bricks.

Sand-lime brick is made from water, lime and sand, so it has a grayish-white color. It is heavier and denser than clay bricks, but absorbs water easily and cannot be used in foundation construction.

Variety of stone for decoration

In the production of ceramic bricks, fired clay-containing mixtures are used. This is divided into construction and facing brick. During the construction of internal and external walls building brick is used because it is highly resistant to frost and also holds the load well. To finish the facades, they take a facing one, which not only performs a decorative function, but also improves the thermal insulation of the walls.

When choosing for construction, you need to pay attention to its color, strength qualities, frost resistance. For example, the pale pink color of a brick speaks of its underburning. Accordingly, when building walls from such material, your house will absorb all the water like a sponge. You should not buy a brick of a dark brown color, as it is absolutely waterproof, which means that it will not be held together with mortar during construction. It is good to use this for the construction of paths - it will be pleasant and dry to walk along them.

The use of concrete in the construction of houses

Concrete is a stone material that is characterized by fire resistance, durability, low hygroscopicity and high strength. In modern construction, the leader is cellular concrete, which has good thermal insulation. Due to this quality, this material allows you to build cottages and houses with fairly thin walls, the weight of which is small. The varieties of cellular concrete include foam concrete, aerated concrete and gas silicate, which differ from each other in the way cells are formed and in composition.

The main disadvantage is its high hygroscopicity, therefore, when creating a base during construction, good waterproofing is needed. Regarding other characteristics, cellular concrete is an environmentally friendly material with low thermal conductivity, fire resistance, and resistance to frost. In addition, it has a relatively small weight, which makes it so popular.

Wood in construction

If brick or concrete is used mainly in the construction of industrial buildings and multi-storey buildings, then for building your own home there is nothing better and better than wood. It is easy to handle and is one of the most durable, yet lightweight materials that retain heat and pleasant smell for a long period of time. Wood is not inferior in plasticity to plasticine and clay, as it takes the necessary shape in its raw form. Color, texture, smell and shine are the main properties of wood building materials.

Wood in construction

The color of wood depends on the composition of the soil, the age of the tree and the climate. The wood contains various tannins, which give it a certain color. Texture refers to the natural pattern that wood fibers have. Trees belonging to decorative species have a very beautiful texture: oak, mahogany, walnut. Among all varieties, dense and light wood has the greatest brilliance.

The most commonly used in construction is pine. Due to the high resin content, it is resistant to decay and various atmospheric influences. Pine has a soft structure, which allows it to easily absorb various varnishes and dyes. During shrinkage, such material almost does not warp.

Spruce is inferior to pine in many ways. It is more difficult to process, the resin content in its wood is low, so the resistance to weathering is low.

Materials used in road construction

These are subject to various influences. environment much more often than the materials that are used in interior decoration. Mechanical influences include various loads from vehicles, as well as the effects of wind and water. Atmospheric precipitation and temperature fluctuations are physical and chemical factors. Over time, road structures are gradually destroyed, so suitability for any conditions is determined by their properties.

Physical properties determine the attitude to the processes occurring in the environment. Specific gravity, bulk density, moisture, shrinkage, light and fire resistance - all these are the main physical properties of road building materials.

For road construction, mainly natural stone materials are used. Their properties depend on the composition of the rock, as well as on its condition. Rocks located in earth's crust in the form of arrays. Boulder stone, sand and gravel are clastic rocks. These can be used in construction without special processing. For example, sand is used to prepare various solutions, as well as in the construction of underlying layers.

The properties that characterize the work of the material in various elements of the road structure are called operational. They determine the durability of the road structure, i.e. her performance. Without knowledge of these properties, it is impossible to build and operate roads. In some cases, you have to pay attention to the biochemical, thermal insulation and decorative properties.

The right choice of materials for construction and decoration will allow you to get only a positive result from the work.

The wide scope of construction in the Soviet Union is accompanied by an expansion in the production of local materials and the introduction of new types of materials into construction practice, as well as an increase in building parts and semi-finished factory-made products. The main building materials include: forest materials, natural stone, ceramic, mineral binders, concrete and products made from them, artificial stone materials, bituminous and heat-insulating materials, metal products, etc.

Forest materials- pine, spruce, fir, cedar and larch are widely used in construction. These materials are divided into roundwood (logs, bollards and poles) and sawn timber (plates, quarters, boards, slabs, beams and bars). In construction, wood with a moisture content of not more than 20% is used. To protect the wooden structures of buildings from moisture and decay, they are coated or sprayed with antiseptics (tar, creosote, etc.)

natural stone materials used in construction both without processing and after preliminary processing (splits, hewing and sawing). The volumetric weight of natural stones ranges from 1100 to 2300 kg / m3, and their thermal conductivity coefficient ranges from 0.5 to 2. Therefore, rubble and cobblestones are used mainly for laying foundations, paving roads and for processing into crushed stone. Rocks are also used to make lime, gypsum, cement and bricks. Materials such as sand, gravel and crushed stone are used as aggregates for the preparation of concrete.

Ceramic materials and products- These are artificial stone products that are obtained by molding and subsequent firing of the clay mass. These include porous ceramic products (ordinary clay brick, porous brick, hollow brick, facing tiles, roof tiles, etc.) and dense ceramic products (clinker and floor tiles). Recently, it has been widely used in construction new material- expanded clay. it lightweight material in the form of gravel and crushed stone during accelerated firing of fusible clays. During firing, the clay swells and a porous material with a bulk density of 300-900 kg/m3 is obtained. Expanded clay is used for the manufacture of concrete and reinforced concrete.

Mineral binders- these are powdery materials, when mixed with water, form a pasty mass, which gradually hardens and turns into a stone-like state. There are air binders that can harden only in air (building gypsum, air lime, etc.), and hydraulic ones that harden not only in air, but also in water (hydraulic lime and cements).

concretes and products from them - artificial stones obtained as a result of hardening a mixture of binder, water and aggregates (fine sand and coarse gravel or crushed stone). Concrete can be heavy (volume weight above 1800 kg/m3), light (volume weight from 600 to 1800 kg/m3) and heat-insulating or cellular (volume weight less than 600 kg/m3). Cellular concrete includes foam concrete and aerated concrete.

foam concrete obtained by mixing cement paste or mortar with a special, stable foam. To obtain aerated concrete, gas-forming substances are introduced into the cement paste containing sand, slag and other aggregates. Concrete structures and parts into which a steel frame is introduced - reinforcement consisting of steel rods interconnected by welding or connected by wire, are called reinforced concrete.

Artificial stone non-fired materials- these are gypsum and gypsum-like products (slabs and panels for partitions and sheets of dry plaster, magnesite) used for flooring and the manufacture of fibrolite, silicate products (silicate brick, etc.) and asbestos-cement products, smooth roofing slabs and corrugated sheets (slate) .

Bituminous materials contain natural bitumen or tar oils, pitches, raw tars in their composition. A mixture of bitumen and sand is called asphalt mortar, used as a base for laying tile floors, asphalt floors, and for waterproofing. Bituminous materials include roofing material, glassine, hydroisol, borulin, roofing felt. These materials are used for roofing, waterproofing and vapor barrier.

Thermal insulation materials used to protect rooms or individual structures from heat loss or from heating. These materials have high porosity, low bulk density and low thermal conductivity up to 0.25. There are thermal insulation materials of organic and mineral origin. Organic include: fibreboard (hardboard) from crushed wood fiber; straw and reeds - slabs pressed from straw or reeds and stitched with wire; fibrolite - plates pressed from wood shavings bound with a magnesian binder solution. Of the mineral heat-insulating materials, foam concrete and aerated concrete, mineral wool, foam silicate, etc. have become widespread. Recently, products based on plastics have been introduced into construction practice. This is a large group of materials, which is based on natural artificial high-molecular compounds. For sheathing the interior surfaces of the room, you can use aluminum sheets that reflect thermal radiation from animals and heaters.

Thermal insulation materials include lightweight, usually porous materials with a low coefficient of thermal conductivity. For example, lightweight concretes on porous aggregates have a density of 500-1800 kg / m3 and have large quantity since. Products made of lightweight concrete have a rough surface. Their thermal insulation properties depend on the number and nature of the pores.
Heat transfer in lightweight concrete occurs through the stone core of the material due to thermal conductivity and through air-filled pores by convection. The smaller the pore size, the less mobility the air will have in them, transferring the minimum amount of heat, and the higher the heat-shielding performance of the concrete.
Lightweight concrete is produced on the basis of Portland cement. If the concrete is autoclaved, then lime-slag, lime-ash and other binders are used. As fillers, porous materials with a bulk density of 1000–1200 kg/m3 are used: granulated slag, slag pumice, agloporite, expanded clay, expanded perlite, etc.
When used as a filler of expanded clay, expanded clay concrete is obtained. If the filler is perlite, then perlite concrete is obtained, if agloporite is aggloporite concrete, etc.
Expanded clay is one of the main porous fillers used in construction. It is a durable and lightweight material with a density of 250-800 kg/m3. Expanded clay is produced in the form of sand, gravel and crushed stone.
obtained by firing low-melting intumescent clays at a temperature of about 1200 ° C. As a result, granules 5-40 mm in size are formed. The sintered shell on the surface of the granule gives it strength. In a fracture, the expanded clay granule has the structure of a hardened foam.
has grains up to 5 mm, it is obtained in the production of expanded clay gravel in small quantities. In addition, it can be obtained by crushing gravel grains with a diameter of more than 50 mm.
Slag pumice - an artificial porous filler with a cellular structure - is obtained from metallurgy waste - molten blast-furnace slag. When slags are rapidly cooled with air, water or steam, they swell. The resulting pieces of slag pumice are crushed and scattered into crushed stone and sand.
Granulated slag is a fine-grained porous material in the form of coarse sand with grains 5-7 mm in size.
expanded perlite- loose heat-insulating material in the form of small porous grains white color, which is obtained by short-term firing of granules from volcanic water-containing glassy rocks. At a temperature of 950-1200°C, water evaporates vigorously from the material, the steam swells and increases the perlite particles by 10-20 times. Expanded perlite is produced in the form of grains with a diameter of 5 mm or sand and is used for the production of lightweight concrete, thermal insulation products and fire-retardant plasters. For the production of concrete, the density of expanded perlite should be 150-430 kg / m3, for heat-insulating fillings - 50-100 kg / m3. The thermal conductivity coefficient is 0.04-0.08 W / (m "°C).
The addition of expanded perlite to mineral binders makes it possible to obtain products with good thermophysical characteristics.
Slabs of perlitoplast concrete are obtained as a result of hardening of a mass consisting of expanded perlite sand, resin and other substances. Plates are faced with foil, fiberglass, self-adhesive film. Plates with a density of 100-150 kg / m3 with a coefficient of thermal conductivity of 0.075-0.04 W / (m ° C) are produced as self-supporting structures.
expanded vermiculite- loose heat-insulating material in the form of scaly particles of silver color, obtained as a result of grinding and firing water-containing mica. With rapid heating, vermiculite splits into separate plates, partially connected to each other. As a result, its volume increases by 15-20 times. Bulk density of vermiculite is 75-200 kg/m3.
Expanded vermiculite is used for the manufacture of heat-insulating boards for lightweight insulation. wall panels and lightweight concrete as a heat-insulating backfill.
Fuel slags are porous lumpy materials formed in the furnace as a by-product during the combustion of anthracite, hard and brown coal and other solid fuels.
Agloporite is obtained by sintering granules from a mixture of clay raw materials with coal. The sintering of the granules occurs as a result of the combustion of coal. Simultaneously with the burning of coal, the mass swells. The bulk density of aggloporite crushed stone is 300-1000 kg/m3.
At present, expanded clay concrete is widely used in construction, from which single-layer and three-layer panels are made.
Cellular concretes are lightweight concretes. They are obtained by autoclave hardening of a pre-expanded mixture of binder, water and silica component. They have 85% of the pores of the total volume of concrete.
obtained from a mixture of cement paste with foam (whipped from rosin soap and animal glue or other component) having a stable structure. After hardening, the foam cells form concrete with a cellular structure. A number of products are produced from foam concrete.
Heat-insulating blocks made of foam concrete with a thermal conductivity coefficient of 0.1-0.2 W / (m ° C) are cast with a size of 0.5x0.5x1 m and more. After hardening, they are cut into slabs of the desired size, for example, 1x0.5x (0.05-0.12) m. They are used for thermal insulation of reinforced concrete coatings and partitions.
Structural and heat-insulating with a coefficient of thermal conductivity of 0.2-0.4 W / (m ° C) is used for wall fencing of two-three-layer structures.
Structural with a thermal conductivity coefficient of 0.4-0.6 W / (m ° C) is used in two-layer building fences.
Aerated concrete is obtained from a mixture of Portland cement, a silica component and a blowing agent (most often aluminum powder). Often, air lime or caustic soda is added to this mixture. The resulting mixture is poured into molds, to improve the structure, it is subjected to vibration and processed mainly in autoclaves. Aerated concrete products are molded in large sizes and then cut into elements.
Autoclaved gas silicate is obtained on the basis of a lime-silica binder, using local materials - airborne lime, sand, ash, metallurgical slag. At present, houses, the walls of which are made of gas silicate, are widely used in countryside. Gas silicate houses are built from blocks of sizes 0.2x0.3x0.6 m or 0.3x0.3x0.6 m. The wall thickness is usually 0.3 m. Compared to brick, the laboriousness of building gas silicate walls is much less. In addition, with a gas silicate density of 550-600 kg/m3, it has a thermal conductivity coefficient of 0.15 W/(m°C), which is four times lower than the thermal conductivity coefficient of a brick.
As a heat-insulating material, sand-free concrete is used, which includes Portland cement grades 300-400, gravel or crushed stone with a particle size of 10-20 mm. Sand is not added to concrete. The voids formed in the concrete, filled with air, make it possible to increase the heat-shielding quality of the walls. The surface of walls made of sandless concrete is plastered.
sawdust concrete also used to build houses. It consists of lime-cement dough, which is mixed with a mixture of sawdust and sand. The resulting concrete composition - binder: sand: sawdust - (1:1.1:3.2) - (1:1.3:3.3) (by volume) is a good heat-insulating material.
The highest thermal insulation characteristics are thermal insulation used to insulate walls, coatings and other elements of residential buildings. They are porous plastics obtained by foaming and heat treating polymers. Under the action of temperature, there is an intensive release of gases that expand the polymer. As a result, a material with pores evenly distributed in it is formed. In cellular plastics, the pores occupy 90-98% of the volume of the material, while the walls account for 2-10%. Therefore, it is very easy. In addition, they do not rot, are quite flexible and elastic. The disadvantage of heat-insulating polymers is their limited heat resistance and flammability.
divided into rigid and elastic. In construction, rigid insulation is used to isolate enclosing structures. easy to process, they can easily be given any shape. In addition, they can be glued together and with other materials: aluminum, asbestos cement * wood. For bonding, diphenol rubber, modified rubber and epoxy adhesives are used.
Porous plastics are produced on the basis of polystyrene, polyvinyl chloride, polyurethane, phenolic and urea resins.
(expanded polystyrene) is the most common heat-insulating material, consisting of sintered together spherical particles of expanded polystyrene.
is a rigid foam with closed pores. It is a tough material resistant to water, most acids and alkalis. A significant drawback of expanded polystyrene is its flammability. At a temperature of 80 ° C, it begins to smolder, so it is recommended to arrange it in structures closed on all sides with fire-resistant materials. It is used as a heater in laminated panels made of reinforced concrete, aluminum, asbestos cement and plastic. It is produced in the form of plates with a density of 40-60 kg / m with a thermal conductivity coefficient of 0.03-0.04 W / (m "°C). The most common size is 1.2x1x0.1 (0.05) m. In addition, from porous plastics based on polystyrene, slabs with a density of 50-200 kg / m3 with a coefficient of thermal conductivity of 0.04-0.05 W / (m ° C) are made, 0.5-1 m long, 40-70 cm wide, 2.5- 8 cm
Polyurethane foam is made rigid and elastic. Polyurethane foam is produced in the form of porous polyurethane mats with a thermal conductivity coefficient of 0.04 W / (m ° C) with a size of 2x1x (0.03-0.06) m, as well as hard and soft plates with a density of 30-150 kg / m3 and thermal conductivity 0.022-0.03 W / (m "° C). Ease of manufacture allows the production of plates from this material not only in the factory, but also at the construction site. With special additives, polyurethane foam does not support combustion.
- porous heat-insulating material of white color, produced on the basis of urea-formaldehyde polymer. Mipora is produced in the form of blocks with a volume of at least 0.005 m3 and a thermal conductivity coefficient of 0.03 W / (m "°C) or tiles with a thickness of 10 and 20 mm.
Mypora is not a combustible material. At a temperature of 200 ° C, it only chars, but does not light up. However, it has low compressive strength and is a hygroscopic material. Mipora is used as a lightweight filler for frame structures or voids where there are no requirements for moisture resistance.
refers to new high-performance heat-insulating materials and is a frozen foam with closed pores. Depending on the additives introduced into it, it can be rigid and elastic. When used as a filler, finely ground expanded clay sand, penoizol becomes a difficult-to-ignite heat-insulating material. Up to a temperature of 350°C, it is resistant to fire, at temperatures up to 500°C it does not emit toxic substances, except for carbon dioxide. has good adhesion to brick, concrete and metal surfaces. Used for insulation country houses, cottages, garages, hangars, pool covers.
For the production of penoizol, large areas and bulky equipment are not required, except for a gas-liquid installation weighing 80 kg, serviced by two workers. Due to the ability of penoizol to harden under normal conditions within 20 minutes, its production is easy to organize at the construction site during the construction of cottages, individual houses, as well as during repair and construction works for the insulation of wall structures, roofs, etc., it is made in the form of plates and blocks of any thickness and size, poured in the form of foam into the voids of plates, hollow profiles and volumes. Depending on the multiplicity of the foamed composition, penoizol has a density of 25-300 kg / m, a thermal conductivity coefficient of 0.03-0.07 W / (m "°C).
Honeycomb plastics produced in the form of corrugated sheets of paper, cotton or glass cloth impregnated with polymer and flame retardant. Honeycombs are regularly repeating cells of regular geometric shape (in the form of honeycombs). It is used as a heater in three-layer panels made of aluminum or asbestos cement. When the cells are filled with mipora crumbs, the heat-insulating characteristics of the honeycomb layer increase. Honeycomb plastics are used in the form of plates and blocks with a thickness of 350 mm.
"Heat-insulating penoizol". Specifications. TU 5768-001- 18043501. Date of introduction 1.01.1994. M .: 1993. - 11 p.
The most rational for construction are honeycombs made of kraft paper impregnated with phenol-formaldehyde resin with honeycomb sizes of 12 and 25 mm. Honeycomb plastics made from ordinary paper and impregnated with urea-formaldehyde resin are brittle and brittle. When cut, they crumble a lot.
Aluminium foil- one of the effective heaters. At the same time, it is a good air and vapor barrier. At present, the non-ferrous metallurgy industry produces foil with a thickness of 0.005-0.2 mm.
Aluminum foil has a shiny, silvery surface with great reflectivity. Most of the flow of radiant heat incident on a structure covered with foil is reflected, which reduces heat loss through the fences and increases their thermal protection.
Reflecting the radiant component of the heat flux, aluminum foil improves the heat-shielding characteristics of the structure. It is effective to arrange the foil on the wall surface near the radiator or inside the structure only in such a way that the foil is on the border with the air gap. In this case, the thermal protection of the wall can increase by 1.5-2.5 times. Perhaps the device of the foil on both sides of the air gap. It is not recommended to install the foil in the thickness of the structure, since in this case the heat-shielding ability of the foil is practically not used.
Aluminum foil can be hard - non-hardened and soft - annealed. The emissivity of hard and soft foils differ insignificantly and their values ​​do not depend on the web thickness. Therefore, when choosing aluminum foil, they are guided by the convenience of handling it and the cost. The most convenient for building enclosing structures is foil 0.01 mm thick with a smooth, clean, even surface, without folds and tears.
Aluminum foil for construction is produced in Rolls with a diameter of 8-43 cm, a web thickness of 0.005-0.02 mm and a width of 10-460 mm.
Mineral wool is a heat-insulating material consisting of the finest vitreous fibers obtained by spraying liquid melts of charge from metallurgical and fuel slags, rocks such as dolomites, marls, basalts. The length of the fibers is 2-60 mm. The heat-shielding properties of mineral wool are due to the air pores enclosed between the fibers. Air pores account for up to 95% of the total volume of the mineral wool skeleton.
Thermal properties depend on the density, fiber thickness, porosity, the content of the so-called pellets. Korolki called fibers of mineral wool spherical or pear-shaped. The thermal conductivity coefficient of mineral wool ranges from 0.042 to 0.046 W / (m ° C).
Mineral wool occupies a leading position among inorganic thermal insulation materials due to its ease of production, unlimited raw materials, low hygroscopicity and low cost.
The disadvantage of mineral wool for thermal insulation is that during storage it becomes compacted, clumped, some of the fibers break and turn into dust. Having very low strength, mineral wool laid in structures must be protected from mechanical influences. Therefore, products based on it are used in construction - mats, rigid and semi-rigid plates.
Mineral wool mats are used for thermal insulation of external fences, as well as structures, the temperature of which is not less than 400°C. At a density of 100-200 kg/m3 they have a thermal conductivity coefficient of 0.052-0.062 W/(m°C). Sewing mats are produced with a length of 2 m, a width of 0.9-1.3 m with a web thickness of 0.06 m. In construction, stitched mats are used on a metal mesh, on a fiberglass lining, on a starch binder with paper and fabric lining.
Mineral wool mats on a metal mesh are obtained by stitching a mineral wool carpet on a metal mesh with cotton threads. The mats are produced with a density of 100 kg/m3 with a thermal conductivity coefficient of about 0.05 W/(m°C) and a size of 3x0.5x0.05 m.
Mineral wool mats on a fiberglass lining are made by stitching a mineral wool carpet with glass-fibre, processed in a soapy solution. They are produced with a density of 125-175 kg / m3 with a thermal conductivity coefficient of 0.044 W / (m "°C) with a size of 2x06: x0.04 m and can be used to insulate structures with temperatures up to 400 ° C.
Mineral wool mats on a starch binder with a paper lining are produced with a density of 100 kg / m3 with a thermal conductivity coefficient of 0.044 W / (m ° C) 1-2 m long, 0.95-2 m wide, 0.04 to 0.07 m thick in steps of 0.01 m.
Heat-insulating semi-rigid slabs based on a synthetic binder are used for insulation of building structures, etc., mainly as an effective thermal insulation of coatings and roofs, including slate ones. Their use is possible in all cases where moisture and deformation of the insulation during operation are excluded.
Half-life plates consist of mineral fiber impregnated by spraying solutions of phenolic alcohols with subsequent cooling. Plates of the PP brand are produced with a density of 100 kg / m with a thermal conductivity coefficient of 0.046 W / (m "°C) 1 m long, 0.5 m wide, 0.03; 0.04 and 0.06 m thick.
Semi-rigid slabs on a synthetic binder are made from a mineral wool carpet impregnated with a synthetic binder (for example, carbamide resins) with subsequent heat treatment. They are produced with a density of 80-100 kg / m3 with a thermal conductivity coefficient of 0.031-0.058 W / (m ° C).
Semi-rigid slabs based on bitumen binder are produced in lengths of 0.5 and 1 m, widths of 0.45 and 0.5 m, thickness from 0.05 to 0.1 m. in appearance compared to products on a synthetic binder.
Heat-insulating rigid mineral slabs consist of mineral wool and a binder: synthetic, bituminous or inorganic - cement, clay, liquid glass. They are made by mixing mineral fibers with a binder. Products are formed from the resulting mass, which are then compacted and subjected to heat treatment. To increase the strength, short-fiber asbestos is introduced into the composition of the material of the plates.
Rigid mineral wool boards on a bitumen binder, having a thermal conductivity coefficient of 0.042 W / (m ° C), are produced in sizes of 1x0.5x0.06 m. They have low hygroscopicity, high water resistance and are little susceptible to damage by fungi and insects.
Rigid mineral wool boards of the PE type on a synthetic binder have a thermal conductivity coefficient of 0.04 W / (m "°C) and are produced in sizes of 1x0.05x0.06 m. They have increased strength and can be used for insulating combined roofs and large-panel enclosing structures.
Thermal insulation from mineral wool materials can be carried out in various buildings and structures to lighten brick walls, to insulate reinforced concrete structures, metal flooring, etc. In low-rise buildings, strength brickwork used by an average of 20%. Therefore, it is advisable to use mineral wool insulation, which has low strength and high thermal insulation characteristics.
Mineral wool inserts are used to increase the thermal protection of reinforced concrete wall panels, three-layer panels on flexible ties.
A high thermal insulation effect can be obtained by using mineral wool insulation in combination with sheathing made of asbestos-cement, aluminum, steel sheets or waterproof plywood.
Mineral wool soft boards called mineral wool. It is produced in the form of rolls packed in rigid containers or waterproof paper. Mineral felt sheets are produced in length 1; 1.5 and 2 m, 0.45 wide; 0.5 and 1 m, thickness 0-05-0.1 m in increments of 0.01 m. Soft mineral wool boards on a bituminous binder are used to insulate building structures. Their serious disadvantage is the ability of the felt to compact under slight loads, primarily from its own weight. In this case, there is a sharp increase in density, sometimes twice, which leads to a decrease in its heat-shielding qualities.
Construction felt obtained from low-grade animal wool, to which vegetable fibers and starch paste are added. The resulting panels are impregnated with a 3% sodium fluoride solution to protect against moth damage and dried. Construction felt- a good insulating and soundproofing material, used for plastering walls and ceilings, insulating gaps between door or window frames and the wall.
glass wool is a heat-insulating material obtained by drawing molten glass and consisting of silky, thin, flexible white glass filaments.
glass wool and glass fiber have been known since ancient times. Also in ancient egypt glass fiber was used for jewelry. At the beginning of the XIX century. fabrics and embellishments for dresses, ladies' hats and ties made of fiberglass came into fashion. The enthusiasm for these fabrics was very great, and in the forties of the last century, the production of glass fabric for vests, collars, watch chains, sultans, feathers, etc. was launched at the Imperial Plant in St. Petersburg. But Vienna was especially famous for these products.
Due to the fragility of glass fibers, which, when worn, crumbled into small fragments, got into the eyes and irritated the skin, fiberglass products soon fell out of fashion and began to be used in laboratories and in construction to protect walls and floors from dampness and in all cases, where it was possible to use low thermal conductivity, fireproofness and chemical resistance of the material.
Currently, glass wool, despite its low density (in a loose state 130 kg / m3) and low thermal conductivity in its natural (pure, natural) form, is practically not used. Mats and strips are produced from glass wool, which are made by stitching with asbestos or twisted fiberglass threads.
Fiberglass mats on a synthetic bond with a density of 350 kg / m3 with a thermal conductivity coefficient of 0.045 W / (m "°C) are produced 1-1.5 m long, 0.5; 1; 1.5 m wide, 0.03-0 thick .06 m
Stitched fiberglass mats with a density of 50 kg / m3 with a thermal conductivity coefficient of 0.044 W / (m "°C) are produced in sizes (1-3) x (0.2-0.7) x (0.03-0.05) m.
Fiberglass plates are produced with a density of 50-75 kg / m3 and a thermal conductivity coefficient of 0.046 W / (m "°C) with a size of 1x0.5 (1) x30 (40, 50, 60) mm.
Basalt superthin glass fiber BSTV is a highly efficient heat-insulating material with a low density of 17-25 kg/m3 and a thermal conductivity coefficient of 0.027-0.036 W/(m°C). Mats are made from it, which have good thermal protection and sound insulation.
Foam glass is a material made from cullet or quartz sand, limestone, soda, i.e. the same materials from which various types of glass are produced. Foam glass It is formed as a result of sintering of cullet powder with coke or limestone, which at high temperature release carbon dioxide. Due to this, large pores are formed in the material, the walls of which contain smaller closed micropores. The dual nature of the porosity makes it possible to obtain foam glass, which, depending on the density, has a low thermal conductivity coefficient of 0.058-0.12 W / (m "°C). It has water resistance, frost resistance, fire resistance and high strength. Foam glass used for insulation of walls, ceilings, roofs, for insulation of basements and refrigerators.
cement fiberboard is a good thermal insulation material, consisting of a mixture of thin wood shavings 20-50 cm long (wood wool), Portland cement and water. The resulting mass is formed, subjected to heat treatment and cut into separate plates. Wood shavings, prepared from non-commercial coniferous wood on special machines, play the role of a reinforcing frame in the slabs. Cement - fiberboard slabs are produced in grades of density M 300, 350, 400 and 500 with a thermal conductivity coefficient of 0.09-0.12 W / (m "°C), 2-2.4 m long and 0.5-0 wide, 55 m and thickness 5; 7.5 and 10 cm.
Wood concrete is made from a mixture of Portland cement, crushed chips and water. In addition to chips, you can use other types of short-fiber organic raw materials - wood chips, sawdust, fire. Slabs flow out of wood concrete with a dry density of 500 kg / m3 with a thermal conductivity coefficient of 0.12 W / (m "°C) and dimensions in length and width of 0.5; 0.6 and 0.7 m with a thickness of 5, 6 and 7 mm Arbolite slabs are used as heat-insulating, structural-heat-insulating and acoustic materials.
Chipboards are made by pressing specially prepared chips with liquid polymers. Shavings are made on machines from non-commercial wood, using waste from plywood and furniture production. The slabs are a kind of layered structure, the middle layer of which consists of thick chips about 1 mm thick, and the outer layers of thin chips 0.2 mm thick. To ensure the biostability of the plates, an antiseptic (borax, sodium fluoride, etc.), as well as flame retardants and water-repellent substances, are introduced into the mass of chips and polymers. The use of water repellents makes it possible to reduce the swelling of the plates under the action of air moisture.
The plates are finished on the outside with polymeric film materials, paper impregnated with resin, which also protects them from moisture and abrasion. Sometimes the surface of the plates is covered with waterproof varnishes.
Chipboards produce various densities from 350 to 1000 kg/m3. Slabs of medium (510-650 kg/m3) and high (660-800 kg/m3) densities are used as structural and finishing materials, and low-density (350 kg/m3) slabs are used as heat-insulating and sound-insulating materials. Plates are made 1.8-3.5 m long, 1.22-1.75 m wide, 0.5-1 cm thick.
Fibreboards are made from wood or vegetable fibers obtained from woodworking waste, non-commercial wood, as well as fires, reeds, and cotton. The most widely used boards based on wood waste. Fibreboards are produced in various densities - from 250 to 950 kg/m3. Solid slabs (density greater than 850 kg/m) are used for partition walls, ceiling filing, flooring, fabrics and built-in furniture.
Insulating wood fiber boards with a density of up to 250 kg / m3 with a thermal conductivity coefficient of 0.07 W / (m "°C) are used for heat and sound insulation of rooms. They have a length of 1.2-3 m, a width of 1.2-1, 6 m, thickness 0.8-2.5 mm.
Insulating finishing boards(density 250-350 kg / m3) have a front surface covered with a synthetic film with a paper lining to match the color and texture of wood, or a matte surface painted with various paints.
Hardboard is a heat-insulating fibreboard made of crushed and chemically treated wood. With a density of 150 kg / m3, they have a thermal conductivity of 0.055 W / (m "°C) and are used for thermal insulation of walls, roofs, etc.
Peat insulating boards are made by pressing from slightly decomposed peat, which has a fibrous structure. Peat slabs are produced with a density of 170 and 250 kg / m3 with a dry thermal conductivity coefficient of 0.06 W / (m "°C), 1 m long, 0.5 m wide, 30 mm thick and are used to insulate building envelopes.
Asbestos cardboard is obtained from asbestos of the 4th and 5th grades, kaolin and starch. It is produced on sheet-forming machines in the form of sheets with a length and width of 0.9-1 m, a thickness of 2-10 mm. The coefficient of thermal conductivity in the dry state is 0.157 W / (m "°C).
Wood sawdust is obtained as a result of wood processing, in furniture production, during sawing. Sawdust with a density of about 150 kg/m is used as an insulating backfill, as well as for the production of wood concrete, xylolite, in the manufacture of sawdust concrete and other building materials.
Tow is a short-fiber material obtained from hemp and flax waste, has a density of 160 kg / m3, a thermal conductivity coefficient of 0.047 W / (m "°C) and is used for caulking walls and gaps in window boxes.
Gypsum boards for partitions are fire-resistant, have high sound-proofing qualities, nails are easily hammered into them. Plates are used for partitions in rooms with a relative humidity of not more than 70%. Gypsum partitions are produced solid and hollow, 0.8-1.5 m long, 0.4 m wide, 80, 90 and 100 mm thick.
Plasterboard sheets are a finishing material made of building gypsum reinforced with vegetable fiber. The surface of the sheets is pasted over with cardboard on both sides. Dry plaster is easily cut, does not burn, and is well nailed. Drywall sheets burst when bent. Like all products based on gypsum, they are destroyed by moisture.
Dry plaster is produced in sheets 2.5-3.3 m long, 1.2 m wide, 10-12 mm thick and is used for interior decoration. It is glued to the surface of walls and ceilings with special mastics. The seams between the sheets are sealed with non-shrink putty.
Gypsum concrete stones are a local building material, they are used for the outer walls of low-rise buildings in areas where there are no other effective wall materials.
Gypsum concrete is made on the basis of building, high-strength gypsum or gypsum-cement-pozzolanic binder. Porous fillers are introduced into its composition - expanded clay gravel, fuel slags, as well as a mixture of quartz sand and sawdust. Depending on the filler, gypsum concrete has a density of 1000-1600 kg/m3. Solid and hollow slabs of partitions are made from it.
Gypsum-concrete panels of partitions are made of concrete with a density of 1250-1400 kg / m3, which provides excellent sound insulation of adjacent rooms. The panels are produced on rolling mills or cassettes by the method of continuous formation from a mixture consisting of equal parts of gypsum, sand and sawdust in volume. The panels are produced solid and with openings up to 6 m long, up to 3 m high, 80-100 mm thick. The use of gypsum concrete panels of non-bearing partitions is destroyed in buildings with a relative humidity of not more than 60%.

MAIN PROPERTIES OF MATERIALS: PHYSICAL, MECHANICAL, CHEMICAL

Physical properties

These properties characterize its structure or relation to the physical processes of the environment. These include mass, true and average density, porosity, water absorption and water loss, humidity, hygroscopicity, water permeability, frost resistance, air, gas and vapor permeability, thermal conductivity and heat capacity, fire resistance and fire resistance.

Weightis a collection of material particles (atoms, molecules, ions) contained in a given body. The mass has a certain volume, i.e. occupies part of the space. It is constant for a given substance and does not depend on the speed of its movement and position in space. Bodies of the same volume, consisting of different substances, have an unequal mass. To characterize differences in the mass of substances having the same volume, the concept of density has been introduced. The latter is subdivided into true and average.

True Density- the ratio of the mass to the volume of the material in an absolutely dense state, i.e. without pores and voids. To determine the true density r (kg/m 3, g/cm 3), it is necessary to divide the mass of the material (sample) m (kg, g) by the absolute volume V (m 3, cm 3) occupied by the material itself (without pores):

Often the true density of a material is related to the true density of water at 4 about C, which is equal to 1g/cm 3, then the determined true density becomes, as it were, a dimensionless quantity.

However, most materials have pores, so their average density is always lower than the true density:

MaterialDensity, kg/m 3True Middley 7850-79007800-7850 Ghostly2700-28002600-2700 Rozer (dense) 2400-26001800-2400PESOK2500-26001450-1700-cement3000-3100900-1300-ceramic brick2600-2700-1900-2600-29001500-12200SNA1500-1200POLASNA

Only for dense materials (steel, glass, bitumen and some others) are the true and average densities equal, because their internal pore volume is very small.

Average density - this is a physical quantity determined by the ratio of the mass of a material sample to the entire volume occupied by it, including the pores and voids present in it. Average density r (kg/m 3, g/cm 3) is calculated by the formula:

r = m / V,

where m is the mass of the material in its natural state; V is the volume of the material in its natural state.

The average density is not a constant value - it varies depending on the porosity of the material. For example, artificial materials can be obtained with different porosity (heavy concrete has a density of up to 2900 kg/m 3, and light - up to 1800 kg / m 3). Density is affected by the moisture content of the material.

For bulk materials, an important characteristic is the bulk density - this includes not only the porosity of the material itself, but also the voids between the grains or pieces of the material.

Porositymaterial is the degree of filling it with pores. Porosity complements the density to 1 or to 100%. Porosity of various materials:

· glass, metal 0%;

· heavy concrete 5 - 10%;

· brick 25 - 35%;

· aerated concrete 55 - 85%;

· styrofoam 95%,

those. it fluctuates widely.

The properties of the material are also influenced by the size of the pores and their nature (small or large, closed or communicating).

Density and porosity directly affect such characteristics of materials as water absorption, water permeability, frost resistance, strength, thermal conductivity, etc.

Water absorption- the ability of the material to absorb water and retain it. The value of water absorption is determined by the difference in the mass of the sample in a saturated with water and in a completely dry state. Distinguish between volumetric water absorption, when the difference is related to the volume of the sample and mass water absorption - when the difference is related to the mass of the dry sample. Mass water absorption for some materials:

· granite 0.5 - 0.8%

· heavy concrete 2 - 3%

· ceramic brick 8 - 20%

· porous heat-insulating materials, for example, peat boards >100%.

Saturation of materials with water adversely affects their basic properties: it increases the density and thermal conductivity, and reduces strength.

Humidity- moisture content, referred to the mass of the material in a dry state. The moisture content of a material depends both on the moisture absorption properties of the material itself and on the environment in which the material is located.

Moisture return- the property of the material to give moisture to the surrounding atmosphere. It is determined by the amount of water (as a percentage by mass or volume of a standard sample) lost by the material per day at an ambient humidity of 60% and a temperature of 20 0C. Water evaporates until an equilibrium is established between the moisture content of the material and the humidity of the surrounding air.

Hygroscopicity- the property of materials to absorb a certain amount of water with increasing humidity of the surrounding air. This property is typical, for example, for wood - to avoid this, protective coatings are used.

Water permeability- the property of the material to pass water under pressure. It is characterized by the amount of water that passed in 1 hour after 1 cm 2area of ​​the material under test at constant pressure. Especially dense materials (steel, glass, bitumen) and dense materials with closed pores (for example, concrete of a specially selected composition) are waterproof.

Frost resistance- the property of a material saturated with water to withstand multiple alternate freezing and thawing without signs of destruction and a significant decrease in strength.

When water freezes, it increases in volume by 9%, and if it completely fills the pores, the ice will destroy the walls of the pores, but usually the pores are not completely filled, so destruction can occur during repeated freezing and thawing.

Dense materials that do not have pores, or materials with a slight open porosity, the water absorption of which does not exceed 0.5%, have high frost resistance. Frost resistance is of great importance for wall, foundation and roofing materials that are systematically subjected to alternate freezing and thawing.

Materials for frost resistance are tested in freezers. Samples saturated with water are cooled to a temperature of - 15-17 0C and, after which, they are thawed at a temperature of +20 0C. The material is considered frost-resistant if, after a given number of cycles, the weight loss of the samples as a result of chipping and delamination does not exceed 5%, and the strength decreases by no more than 25%. According to the number of withstand cycles of freezing and thawing (degree of frost resistance), materials are divided into grades M mrz 10, 15, 25, 35, 50, 100, 150, 200 and more.

If the samples during the testing process do not have signs of destruction, then the degree of frost resistance is established by determining the frost resistance coefficient:

To mrz = R mrz /R us ,

where R mrz - compressive strength of the material after the frost resistance test, MPa; R us - compressive strength of the material saturated with water, MPa. For frost-resistant materials K mrz must be at least 0.75.

Vapor and gas permeability- the property of a material to pass through its thickness under pressure water vapor or gases, including air. All porous materials in the presence of open pores are capable of passing steam or gas.

Vapor and gas permeability is characterized by a coefficient, which is determined by the amount of steam or gas in liters passing through a layer of material 1 m thick and 1 m2 in area 2within one hour at a difference in partial pressures on opposite walls of 133.3 Pa.

Thermal conductivity- the property of a material to transfer heat through the thickness in the presence of a temperature difference on the surfaces limiting the material. the thermal conductivity of the material is estimated by the amount of heat passing through the wall of the tested material with a thickness of 1 m, an area of ​​1 m 2for 1 hour at a temperature difference of opposite surfaces of the wall 1 0C. Thermal conductivity is measured in W/(m·K).

The thermal conductivity of a material depends on many factors: the nature of the material, its structure, porosity, humidity, and the average temperature at which heat is transferred. A crystalline material is generally more thermally conductive than an amorphous material. If the material has a layered or fibrous structure, then its thermal conductivity depends on the direction of the heat flow with respect to the fibers, for example, the thermal conductivity of wood along the fibers is twice as large as across the fibers.

Finely porous materials are less thermally conductive than large porous materials, even if their porosity is the same. Closed cell materials have lower thermal conductivity than communicating cell materials.

The thermal conductivity of a homogeneous material depends on the value of its average density. So, with a decrease in the density of the material, the thermal conductivity decreases and vice versa.

The thermal conductivity of the material is significantly affected by its humidity: wet materials are more thermally conductive than dry ones, since the thermal conductivity of water is 25 times greater than the thermal conductivity of air.

As the temperature rises, the thermal conductivity increases.

Heat capacity- the property of a material to absorb a certain amount of heat when heated and release it when cooled. The indicator of heat capacity is the specific heat capacity, equal to the amount of heat (J) required to heat 1 kg of material per 1 0FROM.

Specific heat capacity, KJ/(kg 0FROM):

· artificial stone materials 0.75 - 0.92;

· wood 2.4 - 2.7;

· steel 0.48;

· water 4.187.

The heat capacity is taken into account when calculating the heat resistance of walls and ceilings of heated buildings, as well as when calculating furnaces.

fire resistance- the ability of the material to withstand the action of high temperatures and water in fire conditions. According to the degree of fire resistance, materials are divided into: fireproof, hardly combustible and combustible.

Fireproof materials under the influence of fire or high temperature do not ignite, do not smolder or char (steel, concrete, brick).

Difficult-to-combustible materials ignite with difficulty under the action of fire, smolder or char, but after the source of fire is removed, their combustion and smoldering stop (wood-cement material fibrolite, asphalt concrete, some types of polymeric materials).

Combustible materials ignite under the influence of fire or high temperature and continue to burn after the source of fire is removed (wood, felt, roofing felt, roofing material).

fire resistance- the property of a material to withstand prolonged exposure to high temperatures without melting or deforming. According to the degree of refractoriness, materials are divided into refractory (for a long time they withstand temperatures above 1580 0C), refractory (1350 - 1580 0C) and fusible, softening at temperatures below 1350 0C (they also include ordinary clay bricks).

Mechanical properties

They characterize the ability of a material to resist the destructive or deforming effects of external forces.

Strength- the property of a material to resist destruction under the action of internal stresses arising from external loads. Strength is the main property of most materials used in the mining industry; its value determines the magnitude of the load that a given element can take for a given section.

Materials, depending on origin and structure, differently withstand various pressures. Materials of mineral origin ( natural stones, brick, concrete, etc.) resist compression well, are much worse in shear and even worse in tension. Other materials (metal, wood) work well in compression, bending and tension, so they are used much more often in bending structures.

The strength of the material is characterized by the tensile strength (in compression, bending and tension). Strength limit- stress corresponding to the load at which the destruction of the material sample occurs. Ultimate compressive and tensile strength R rast , MPa, calculated by the formula

szh (R rast ) = P/F,

where P - breaking load, N; F - cross-sectional area of ​​the sample, mm 2.

Flexural strength R izg :

.with one concentrated load and a sample-beam of rectangular section

R izg = 3Pl / 2bh 2;

.with two equal loads located symmetrically to the axis of the beam

R izg = P(l - a) / bh 2,

where l - span between supports, mm; a - distance between loads, mm; b and h - width and height of the cross section of the beam, mm.

The tensile strength of the material is determined empirically, testing specially made samples in the laboratory on hydraulic presses or tensile machines. To test materials for compression, samples are made in the form of a cube or cylinder, for tension - in the form of round rods or strips, and for bending - in the form of beams. The shape and dimensions of the samples must strictly comply with the requirements of GOST or technical specifications for each type of material.

Limits of strength of some materials, MPa

compression bend stretchGranite150 - 2503 - 5Heavy concrete10 - 502 - 81 - 4Ceramic brick7.5 - 301.8 - 4.4Steel210 - 600380 - 900Wood30 - 6570 - 12055 - 150GRP90 - 150130 - 25060 - 120

The strength of materials used in the construction industry is usually characterized by a grade that corresponds in magnitude to the compressive strength obtained by testing specimens of a given shape and size. For example, the following grades are established for stone materials: 4, 7, 10, 15, 25, 35, 50, 75, 100, 125, 150, 200, 300, 400, 500, 600, 800, 1000. Materials with tensile strength at compression, for example, from 20 to 29.9 MPa refers to grade 200.

Elasticity- the property of the material to deform under load and to take the original shape and dimensions after the load is removed. The highest stress at which the material has elasticity is called the elastic limit. Elasticity is in most cases a positive property of materials.

Plastic- the ability of a material to change its shape and dimensions under the action of a load without the formation of gaps and cracks and to retain the changed shape and dimensions after the load is removed. This property is the opposite of elasticity.

fragility- the property of the material to instantly collapse under the action of external forces without preliminary deformation. Brittle are natural stones, ceramic materials, glass, cast iron, concrete, etc.

impact resistance- the property of a material to resist fracture under impact loads. This type of load occurs, for example, in bunkers. Brittle materials usually do not resist impact loads well.

Hardness- the property of a material to resist the penetration into it of another material, more solid. The hardness of the material affects the complexity of its processing.

There are several ways to determine the hardness of materials. The hardness of wood, concrete, steel is determined by pressing a steel ball (Brinell hardness method), a diamond pyramid (Vickers) or both (Rockwell) into the samples. The value of hardness is judged by the depth of indentation of the ball, the diameter of the resulting print, or by the ratio of the load to the surface area of ​​the resulting spherical print.

The hardness of natural stone materials is determined according to the hardness scale (Mohs method), in which ten specially selected minerals are arranged in such a sequence that the next mineral leaves a line (scratch) on the previous one, but it does not draw:

.Talc or chalk.

.Rock salt or gypsum.

.Calcite or anhydride.

.Fluorspar.

.Apatite.

.Orthoclase (feldspar).

.Quartz.

.Topaz.

.Corundum.

.Diamond.

For example, if the test material is drawn with apatite, and itself leaves a line (scratch) on fluorspar, then its hardness is 4.5.

Abrasion- the property of a material to change in volume and mass under the influence of abrasive forces. The possibility of using the material for decking, lining of bunkers depends on abrasion, executive bodies loading machines. The abrasion of materials is determined in laboratories on special machines - abrasion circles.

wear and tearcalled the destruction of the material under the combined action of abrasion and impact. A similar effect on the material occurs during the operation of bunkers. Materials are tested for wear in special rotating drums.

Chemical properties

Chemical properties characterize the ability of a material to undergo chemical transformations under the influence of substances with which it is in contact. The chemical properties of materials are very diverse, the main ones being chemical and corrosion resistance.

Chemical resistance- the ability of materials to withstand the destructive effect of alkalis, acids, salts and gases dissolved in water.

Corrosion resistance- the property of materials to resist the corrosive effects of the environment.

Many materials used in the construction industry do not have these properties. So, almost all cements poorly resist the action of acids, wood is not resistant to both acids and alkalis, almost all metal products are subject to corrosion to one degree or another. Materials made of plastics or fiberglass are better resistant to acids and alkalis.

METALS IN CONSTRUCTION

Metals and their classification

Metals are widely used in all sectors of the national economy. This is facilitated by a number of valuable technical properties of metals that distinguish them favorably from other materials: high strength and plasticity of pressure treatment (rolling, stamping, etc.). Along with this, metals also have significant drawbacks: they have a high density, under the action of various gases and moisture they strongly corrode, and at high temperatures they are significantly deformed.

Metals are divided into two main groups: ferrous and non-ferrous.

Black metalsare an alloy of iron and carbon. In addition, they may contain more or less other chemical elements (silicon, manganese, sulfur, phosphorus). In order to give ferrous metals specific properties, improving or alloying additives (nickel, chromium, copper, etc.) are introduced into their composition. Ferrous metals, depending on the carbon content, are divided into cast irons and steels.

Cast iron- iron-carbon alloy with a carbon content of 2-4.3%. Depending on the purpose, there are foundry, conversion and special cast irons. Cast iron is used for casting various products, including building parts. Pig irons are used in the production of steel, and special cast irons are used as additives in the production of steel and iron castings. special purpose. The presence in cast iron of manganese, silicon, phosphorus, as well as alloying additives - nickel, chromium, magnesium, etc. - gives it high mechanical properties and provides high heat resistance and corrosion resistance. Cast irons with additions of nickel, chromium, magnesium and other elements are called alloyed. Ductile irons are obtained by modifying liquid iron with additives of Si, Ca, etc.

Steel- malleable iron-carbon alloy with carbon content up to 2%. Steels, depending on the method of production, are divided into: open-hearth, converter and electric steels. According to the chemical composition, depending on the chemical elements included in the alloy, steels are carbon and alloyed. Carbon steels include alloys of iron with carbon and impurities of manganese, silicon, sulfur and phosphorus. carbon steel produced different ways, according to the nature of solidification, it is customary to divide into: calm, semi-calm and boiling. Alloyed are called steels, which include alloying additives (nickel, chromium, tungsten, molybdenum, copper, aluminum, etc.). Depending on the introduced alloying additive, steel is called chromium-manganese, manganese-nickel-copper, etc. In addition, according to the total content of additives, steels are divided into: low-alloyed (with an alloying content of up to 2.5%), medium-alloyed (with an alloying content of 2.5% to 10%) and high-alloyed (with an alloying content of more than 10%) .

According to its purpose, steel can be: structural, used for the manufacture of various building structures and machine parts, special, characterized by high heat and wear resistance, as well as corrosion resistance, and tool steel.

By quality, steel is divided into: ordinary (ordinary), high-quality, high-quality and extra high-quality.

Non-ferrous metalsin its pure form is rarely used. Non-ferrous metal alloys are used much more often, which are divided into light and heavy according to their true density.

light alloysbased on aluminum or magnesium. The most common light ones are aluminum-manganese, aluminum-silica, aluminum-magnesium and duralumin alloys. They are used for load-bearing (trusses, etc.) and enclosing (window frames, etc.) structures of buildings and structures.

heavy alloysobtained on the basis of copper, tin, zinc, lead. Among heavy alloys, bronze (an alloy of copper with tin or an alloy of copper with aluminum, iron and manganese) and brass (an alloy of copper with zinc) are used.

Fundamentals of iron and steel production

The production of ferrous metals from iron ore is a complex technological process that can be conditionally divided into two stages. At the first stage, cast iron is obtained, and at the second stage, it is processed into steel.

Cast iron is smelted in blast furnaces (Fig. 1). The starting materials for iron production are iron ores, fuels and fluxes. Iron ores are rocks containing iron in the form of chemical compounds with oxygen and other elements. The composition of iron ores, in addition, includes other compounds in the form of silica, alumina, limestone, etc. (united by the general concept - "waste rock"). Usually magnetic iron ore (Fe 3O 4) with an iron content of up to 70%, red iron ore (Fe 2O 3), containing up to 65% iron, and brown iron ore (2Fe 2O 32H 2O), containing up to 60% iron. The fuel in the blast furnace process is coke obtained by dry distillation (combustion without air access) of coking coal. Fluxes (fluxes) - limestones, dolomites, sandstones are used to lower the melting point of waste rock and transfer it and fuel ash into slag.

A blast furnace is a shaft, covered with a metal casing on the outside and lined with refractory bricks from the inside. The furnace through the upper part, called the top, is continuously loaded with charge, alternating layers of ore, flux and fuel. To maintain the combustion of fuel, heated air is supplied under pressure to the lower part of the furnace - horn through tuyeres.

Fig.1. Scheme blast furnace

Mine; 2 - top; 3 - boot device; 4 - metal casing; 5 - lining; 6 - cylindrical part of the furnace; 7 - shoulders; 8 - bugle; 9 - slag tap hole; 10 - cast iron; 11 - taphole for the release of cast iron; 12 - air supply pipe

Combustion of fuel - coke occurs in the upper part of the hearth due to oxygen in the air according to the reaction C + O2 = CO2. The resulting carbon dioxide rises up the furnace and, encountering red-hot coke on its way, passes into carbon monoxide CO2 + C = 2CO. Carbon monoxide reduces iron oxides to pure iron, and itself passes into carbon dioxide. The reduction of iron occurs according to the scheme: Fe 2O3 Fe 3O4 FeO Fe.

This process can be represented by the following chemical equations:

3 Fe 2O3 + CO = 2Fe 3O4 + CO2

2Fe3O4+ 2CO = 6FeO + 2CO2

6FeO + 6CO = 6Fe + 6CO

The reduction of iron from its oxides occurs during the movement of the charge under the action of its own mass from the top of the furnace to the bottom. In the lower part of the furnace at 900-1100°C, part of the reduced iron combines with carbon, resulting in iron carbide Fe3C. This process is called: carburization. At a temperature of about 1150 ° C, the carburized iron begins to melt, and the resulting liquid iron flows into the hearth of the furnace. Molten slag also flows here, which, as a lighter material, floats above the cast iron. Molten cast iron and slag are periodically discharged through special openings - cast iron and slag tapholes, and slag is released first, and then cast iron.

Cast iron in the molten state is fed to pouring machines for casting into "inks" or in special ladles delivered to steel shops, where it is processed into steel. Liquid slag from a blast furnace is used to produce slag pumice, granulated slag, stone casting, or dumped into a dump. A by-product of blast-furnace production is top gas, which is used for the needs of the metallurgical industry.

Steel production process consists in reducing the content of impurities present in pig iron (carbon, silicon, manganese, sulfur, phosphorus). These impurities burn out during steelmaking or pass into slag. The starting materials for steelmaking are: pig iron, steel scrap, ferroalloys, iron ore and fluxes.

Modern methods of steel production are converter, open-hearth and electric melting (in electric furnaces).

By converter method steel is obtained in furnaces - converters. The converter is a pear-shaped steel lined vessel that rotates around a horizontal axis on two trunnions. In the lower part of the converter there are tuyere holes for supplying air under pressure of 0.2-0.25 MPa (g). Liquid pig iron is poured from the ladle into the converter, after which oxygen-enriched air is passed through the tuyere holes. Under the influence of air, ferrous oxide FeO is formed in molten iron, which reacts with impurities (silicon, manganese, phosphorus), forming oxides that turn into slag or burn out, while ferrous oxide is reduced to pure iron. This process lasts only 15-30 minutes, which is a great advantage of this method. The capacity of modern converters reaches 600 tons. This method of steel casting is highly productive and most economical.

Converter steel is used for the manufacture of building profiles, sectional and sheet steel, wire, etc.

Fig.2. Converter circuit

Rotating pear-shaped vessel; 2 - lining; 3 - tuyere holes for air supply; 4 - rotary mechanism

open-hearth method obtaining steel is currently the most common. The open-hearth furnace is a unit, the working space of which has the shape of a chamber elongated in the horizontal direction. The lower part of the chamber, which looks like a bath, is called a hearth. It is made stuffed from refractory materials, and the walls and roof of the furnace are laid out from refractory bricks. In the upper part there are channels connecting the working chamber with gas and air regenerators. The capacity of modern open-hearth furnaces is up to 1000 tons.

Solid or molten cast iron with the addition of scrap (scrap metal) or ore is melted in open-hearth by burning fuel - a mixture of top gas or generator gas with air. To increase the thermal effect, gas and air are preheated in regenerators, oxygen blast is used. Impurities - silicon, manganese and phosphorus are oxidized by ferrous oxide FeO formed in the melt, pass into oxides and are removed in the form of slag, and ferrous oxide passes into pure iron. Sulfur is removed from the melt using limestone introduced as a flux. Carbon burns out at high temperatures. The slag formed during the steelmaking process accumulates on the surface of the liquid metal and is periodically removed.

Fig.3. Scheme of an open-hearth furnace 1 - under; 2 - vault; 3 - regenerators

During the smelting of steel, which lasts 4-8 hours, various additives are introduced into its composition - ferroalloys, such as ferrochromium, ferrovanadium, thereby obtaining alloyed steel. The chemical composition of the melt is controlled by systematic sampling for analysis. After receiving the steel predetermined chemical composition it is released into a ladle, and from it it is poured into molds - cast iron or steel molds.

Open-hearth steel differs from converter steel in higher quality. It is widely used for the manufacture of building structures (trusses, crane beams, bridges, rails, etc.), as well as for high-strength reinforcement.

electric smelting- the most perfect way of production of special and high-quality steels. Steel is smelted in electric arc or induction furnaces. The most common electric arc furnaces with a capacity of up to 200 tons.

Both steel scrap and iron ore, as well as liquid steel coming from an open-hearth furnace or a converter, are used as a raw charge for steel electric smelting. In addition, fluxes and alloying additives are introduced into the composition of the charge. The heat source is an electric arc formed between vertically mounted carbon electrodes and molten metal. In essence, the ongoing processes of electric smelting do not differ from the open-hearth method of steel production. However, a significant disadvantage of electric smelting is the low productivity and high cost of steel.

AT last years begin to use combined methods of steel production using sequential steel smelting in oxygen converters, and then in the main open-hearth furnaces, where steel of a given chemical composition is obtained. To reduce the consumption of electricity in the production of steel, an open-hearth furnace is first used for heating and melting, and then an electric furnace is used to finalize the steel to the desired properties.

A promising technology for obtaining sponge iron directly from ores by blowing them under pressure with hydrogen or a mixture of hydrogen and carbon monoxide, followed by separation of iron.

Cast iron

Properties and grades of cast iron. Depending on the content of impurities and the cooling rate, two main types of cast iron are obtained: white and gray. These names correspond to the color of cast iron. White cast iron has a high hardness, but it is very brittle; it is used to produce ductile iron and steel. Gray cast iron in the molten state has good fluidity and is easy to fill molds, has low solidification shrinkage, and is easy to machine. Gray cast iron is used for casting a variety of building products. A variety of gray cast iron is modified black cast iron. It is obtained by introducing additives (modifiers) into liquid cast iron. This cast iron has improved mechanical properties.

Gray, as well as modified cast iron, are marked with the letters SCH, for example, SCH12-28, SCH18-36, SCH28-48 and SCH32-52. The first digit of the cast iron grade shows the permissible tensile strength, and the second - in bending (in kgf / mm °). Gray cast iron used for casting products that work mainly in compression (columns, support pads, sewer pipes, tubing, etc.) is characterized by a tensile strength of 120 - 210 and a bending strength of 280 - 400 MPa. High-strength and alloyed cast irons are used much less often in construction.

Cast iron products. Cast iron products are produced in various ways, among which the simplest is casting into molds. Progressive forms of cast iron casting - under pressure and centrifugal. By casting from gray cast iron, elements of building structures operating in compression (columns, support pads, arches, vaults, subway tubing, floor slabs for industrial buildings, etc.) are obtained. Gray cast iron is used for casting furnace appliances (furnace doors, valves, grates, grates), as well as architectural and artistic products.

Types and properties of steels

Steels for building structures are divided into types and marked with symbols, which reflect the composition and purpose of the steel, mechanical and chemical properties, methods of manufacture and deoxidation.

Steel marking. According to the standard, the grade of carbon steel of ordinary quality is denoted by the letters St and numbers from 0 to 7. High-quality carbon steels are marked with two-digit numbers showing the carbon content in hundredths of a percent (0.8; 25, etc.). In the designation of boiling steel grades add<кп>, semi-calm -<пс>, calm -<сп>, for example St3sp, St5ps, St2kp.

In contrast to the marking of carbon steels, the letters in the grade of low-alloy steels show the presence of alloying impurities in the steel, and the numbers show their average percentage. The numbers preceding the letters show the carbon content in hundredths of a percent. For marking steel, each alloying element is assigned a specific letter:

C - silicon, B - tungsten, G - manganese, Yu - aluminum, X - chromium, D - copper, H - nickel, K - cobalt, M - molybdenum.

The first digits of the brand indicate the average carbon content (in hundredths of a percent for tool and stainless steels. The letter indicates the alloying element and the subsequent numbers indicate its average content, for example, steel 3X13 contains 0.3% C and 13% Cr, grade 2X17H2 - 0, 2% C, 17% Cr and 2% Ni. When the content of the alloying element is less than 1.5%, the numbers behind the corresponding letter are not put: 1G2S, 12XH3A. The letter A at the end of the grade designation indicates that the steel is high-quality, the letter W - especially high quality For example, alloyed structural steel grade 1G2S contains 0.1% carbon, 2% manganese and 1% silicon.

Carbon steels. Carbon steel of ordinary quality is an alloy of iron and carbon. It also contains a small amount of impurities: silicon, manganese, phosphorus and sulfur, each of which has a certain effect on the mechanical properties of steels. In ordinary quality steels used in construction, carbon contains 0.06-0.62%. steel with low content carbon are characterized by high ductility and impact strength. The increased carbon content makes the steel brittle and hard.

To improve the quality of building steels, impurities are added to the alloys - manganese and silicon. The manganese content is usually 0.25 - 0.9%; it increases the strength of steel without significantly reducing its ductility. Silicon, whose content in ordinary steels does not exceed 0.35%, does not significantly affect the properties of steel. Phosphorus and sulfur are harmful impurities. Phosphorus makes steel brittle (cold-brittle), in connection with this, its content in building steels should not exceed 0.05%. The presence of sulfur in an amount of more than 0.07% causes red brittleness of steel, and also reduces its strength and corrosion resistance. The main characteristics of the quality of carbon steel are the yield strength and tensile strength, as well as the relative elongation. All these indicators (except for relative elongation) increase with increasing steel grade.

The most widely used steel in construction is StZ grade, which is used for the manufacture of metal structures of civil and industrial buildings and structures, power line supports, tanks and pipelines, as well as reinforced concrete reinforcement. High-quality structural carbon steels are used, as a rule, in mechanical engineering, and tool carbon steels are used for the manufacture of various cutting tools.

Alloy steels. Low-alloy steels are most often used in construction. The carbon content in them should not exceed 0.2%, since with its increase, ductility and corrosion resistance decrease, and the weldability of steel also deteriorates. Alloy additives affect the properties of steel as follows:

· manganese increases the strength, hardness and wear resistance of steel;

· silicon and chromium increase strength and heat resistance;

· copper increases the resistance of steel to atmospheric corrosion;

· Nickel improves toughness without compromising strength.

Low-alloy steels have higher mechanical properties than low-carbon steels. Steels containing nickel, chromium and copper are highly ductile, well welded, they are successfully used for welded and riveted structures of industrial and civil buildings, superstructures of bridges, oil tanks, pipes, etc.

Low-alloy steel grades 10KhSND, 15KhSND, 10G2SD, etc. have received the greatest application in construction for the manufacture of metal structures.

Medium and high alloy steels are used in construction only when it is necessary to provide structures with high corrosion resistance. For this, structures are made of special stainless steel, for example, chromium-nickel and chromium-nickel-manganese.

Steel properties.Among the physical properties of steels, the true density, melting point, heat capacity, thermal conductivity, thermal expansion coefficient (some of the listed properties have already been considered) are of the greatest importance.

Melting point - the temperature at which steel changes from a solid state to a liquid state. The melting point of iron is 1535 ° C, but when carbon and other elements are introduced into its composition, it changes. For example, cast iron containing 4.3% carbon melts at about 1130°C.

The coefficient of thermal expansion - the index of relative elongation of the steel sample with a temperature increase of 1 ° is (11 - 11.9) 10-6 °C. The mechanical properties of steels are characterized by tensile strength, yield strength, relative elongation, hardness and impact strength.

A tensile test of steel, with a simultaneous assessment of its elasticity, is carried out on samples in the form of a rod of a round or rectangular section. For this, tensile machines are used, equipped with a device for recording the tensile diagram of the sample (Fig. 4). On the vertical axis of the diagram, the tensile load is plotted, and on the horizontal axis, the corresponding increment in the length of the sample. In the tension diagram, the straight section (from the origin to point 1) shows that the elongation l of the test specimen is directly proportional to the applied load P1. The maximum stress at which direct proportionality is maintained between the elongation of the sample and the applied load is called the proportionality limit pr. The deformations of the sample, in which the stresses do not exceed the proportionality limit, are elastic, and when the load is removed, the original length of the sample is restored. With a slight increase in load to P2 (point 2), the sample begins to stretch (steel<течет>), although the load remains constant, which corresponds to a horizontal area in the diagram. The stress at which steel yields appears is called the yield strength m. The sample acquires residual deformations, i.e., deformations that remain in the sample after the load is removed.

Fig.4. Steel Tensile Diagram

With a further increase in the load to P, the sample breaks (point 3). The maximum stress achieved in this case in the sample is called the tensile strength of steel p, MPa, which is calculated by the formula

p \u003d P / Fo,

where P is the maximum load, N; Fo is the initial cross-sectional area of ​​the sample, mm2.

The relative elongation of the sample during the tensile test characterizes the ductility of the steel, i.e., the ability to acquire significant residual deformations without ruptures and cracks. Relative elongation b,.%, is determined by the formula

b = (l1 - l0)/l0,

where l0 - estimated (initial) sample length, mm; l1 - sample length after rupture, mm.

The tensile test is the main one in assessing the mechanical properties of steels used in construction.

Hardness - the ability of steel to resist being pressed into it by other, harder bodies, such as a diamond cone or a steel ball.

Impact strength - the property of steel to withstand dynamic (shock) loads. Its value is determined by the amount of work required to destroy a steel sample on a pendulum impact tester.

Among the chemical properties of steel, the most important is corrosion resistance, which characterizes the ability of steels to resist the damaging effects of the environment.

Technological properties show the ability of steels to be processed by pressure, cutting, casting, welding, etc.

The main technological test of steel is the test of its specimens for bending in a cold state under the influence of a uniformly increasing load. The following types of tests are distinguished: bending to a certain angle, bending around the mandrel until the sides are parallel, bending until the sides are in full contact (close). A sign that the sample has passed the test is the absence of cracks, delaminations or fracture in it after bending.

Heat treatment improves the physical and mechanical properties of steel. There are the following types of heat treatment: hardening, tempering, annealing, normalization.

Hardening consists in heating the steel to 800-900 ° C and slowly cooling it in water or oil. Hardening increases the strength and hardness of the steel, but reduces the toughness. Tempering of hardened steel - its slow heating up to 200 - 350°C, exposure at this temperature, followed by slow cooling in air. When steel is tempered, the hardness decreases, but the toughness increases. Annealing - heating steel to a certain temperature, holding and slow cooling in a furnace. Steel is annealed to reduce hardness and increase its toughness. Steel normalization is a type of annealing, consisting of heating it to a temperature below the hardening temperature, holding it at this temperature and cooling it in air. Normalization increases the hardness, strength and toughness of the steel. To increase the strength and hardness of the surface layers of steel products, surface hardening is carried out with high-frequency currents, as well as steel cementation, i.e., carbon saturation of its surface layer when heated in a carbonaceous medium.

Steel products

Manufacture of steel products. In the manufacture of steel products, molten steel is poured into molds. The steel ingots taken out of them are subjected to pressure treatment. The pressure treatment is based on the high plastic properties of the steel. This changes not only the shape of the steel ingot, but also its properties. There are the following methods of processing steel ingots by pressure: rolling, drawing, forging, stamping, pressing.

Rolling is the most common method of manufacturing profiled steel products. During rolling, a steel ingot is passed between the rotating rolls of a rolling mill, as a result of which the workpiece is compressed, stretched, and, depending on the profile of the rolling rolls, acquires a given shape (profile). Cold rolled steel. Assortment of hot-rolled steel - round, square, strip, equilateral and unequal angle steel, channels, I-beams, sheet piles, pipes, reinforcing steel of a periodic profile, etc.

During drawing, the workpiece is sequentially pulled through holes (dies) smaller than the cross section of the workpiece, as a result of which the workpiece is compressed and stretched. When drawing, a so-called work hardening appears in steel, which increases its hardness. Steel drawing is usually carried out in a cold state, thus obtaining products of precise profiles with a clean and smooth surface. The drawing method produces wire, pipes of small diameter, as well as rods of round, square and hexagonal sections.

Forging is the processing of red-hot steel by repeated blows of a hammer to give the workpiece a given shape. Forging produces a variety of steel parts (bolts, anchors, brackets, etc.).

Stamping is a type of forging in which steel, stretching under the blows of a hammer, fills the shape of a stamp. Stamping can be hot or cold. In this way, products of very precise dimensions can be obtained.

Pressing - the process of extrusion of steel in the container through the outlet (point) of the matrix. The starting material for pressing is casting or rolled blanks. In this way, profiles of various sections can be obtained, including rods, pipes of small diameter and various shaped profiles.

Cold profiling - the process of deformation of sheet or round steel in rolling mills. Bent profiles with various configurations in diameter are obtained from sheet steel, and hardened cold-flattened reinforcement is obtained from round rods on cold profiling machines by flattening.

Types of steel products. The metalworking industry produces an extensive range of various steel products.

Rolled angle steel is produced in the form of equilateral and unequal angles with a flange width of 20-250 mm; channels - 50-400 mm high with a shelf width of 32 - 115 mm; I-beams - both ordinary and wide-shelf. The height of ordinary I-beams is 100-700 mm, wide-shelf - up to 1000 mm. The ratio of the width of the shelves to the height ranges from 1:2 (at a low height) to 1:3 (at a high height).

Profile steel is used for the manufacture of various steel building structures by welding or riveting (frameworks and trusses of industrial and civil buildings, span structures of bridges, floor beams, power line poles, building lighting lanterns, etc.). In addition, window casings of industrial and public buildings are made from rolled and stamped steel of special profiles.

Rolled steel of square section, as well as strip steel, is used for various purposes. Round steel is mainly used as reinforcement for reinforced concrete.

Rolled sheet steel has a number of varieties: rolled plate with a width of 600 - 3800 and a thickness of 4 - 160 mm; rolling sheet with a width of 600 - 1400 and a thickness of 0.5 - 4 mm; sheet roofing, including galvanized; 510 - 1500 wide and 0.5 - 2 mm thick, as well as corrugated and corrugated sheets.

Rolled steel for sheet piles is produced in a variety of profiles; it is used for hydraulic engineering construction.

Rice. 5. Range of rolled steels

a - isosceles corner; b- unequal corner; in - channel; g - I-beam; d - crane rail; e - round; g - square; h - strip; and - sheet pile; to - sheet; l - corrugated; m - wavy

Seamless and welded steel pipes with a diameter of 50 - 1620 mm are used for main gas and oil pipelines, water supply, heating and other purposes.

Small steel products in the form of bolts, nuts, washers, rivets are widely used in the manufacture of various designs from rolled steel profiles.

Steel reinforcement is the most important component of reinforced concrete and is designed to work reliably together with concrete throughout the entire service life of the product or structure. The reinforcement is located mainly in those places of the product or structure that are subjected to tensile forces, and it must perceive these forces.

Reinforcing steel is classified according to the method of manufacture, the profile of the rods and the scope. According to the manufacturing method, reinforcing steel is divided into hot-rolled rod and cold-drawn wire. Depending on the profile of the rods (the nature of their surface), rod and wire reinforcement can be smooth and periodic profile. Depending on the conditions of use, reinforcing steel is divided into non-stressed and prestressed, i.e., used, respectively, for conventional and prestressed reinforced concrete structures.

Rice. 6. Types of reinforcing steel

a - smooth rod; b - hot-rolled periodic profile, class A-I I; in - the same, class A-III; g - cold flattened on four sides; d - the same, on both sides; e - twisted)

Rod reinforcement is produced by hot-rolled ordinary, cold-strengthened and heat-strengthened hood.

Depending on the mechanical properties, bar reinforcement is divided into classes with the symbol A. Conventions classes of hot-rolled reinforcing steel: A-I, A-II, A-III, A-IV, etc. When designating a class of thermally hardened reinforcing steel, the index "t" is added to index A, for example, At-III. Draw-strengthened steel is designated according to the class of the original hot-rolled steel, but at the same time, the index "c" is added, for example Av-III.

Reinforcing steel of class A-I is made from carbon steel grades St3, St3ps and St3kp, class A-II with a diameter of 10 - 40 mm - from carbon steel grade St5, with a diameter of 40 - 90 mm - from low-alloy steel grade 18G2S; class A-III with a diameter of 6 - 40 mm - from low-alloy steel grade 25G2S, with a diameter of 6 - 8 mm - from low-alloy steel grade 18G2S; class A-IV - from low-alloy steel grade 20KhG2Ts (for structures with prestressing reinforcement). Reinforcing steel rods of class A-1 are supplied round, rods of class A-II, A-III, A-IV - with a periodic profile.

Wire reinforcement is divided into reinforcing wire and reinforcing wire products. Reinforcing wire can be cold drawn class B-I(low carbon) for non-stressed reinforcement and class B-II (carbon) for prestressed reinforcement. It is produced with a smooth and periodic profile with a diameter of 3 - 8 mm.

Reinforcing wire products can be used in the construction and production of reinforced concrete products in the form of non-twisting steel reinforcing strands, steel reinforcing ropes, welded reinforcing meshes, as well as woven and welded wire meshes intended for reinforced cement structures.

Reinforcing steel with a diameter of less than 10 mm is produced in coils (bays), with a diameter of 10 mm or more - in bars 6-12 m long.

Non-ferrous metals and their alloys

In modern construction, non-ferrous metals in their pure form are used quite rarely. Mostly, alloys of some non-ferrous metals are used, such as aluminum, copper, zinc, lead, tin, manganese, which are characterized by low density, high ductility and corrosion resistance, as well as good decorative qualities.

Aluminum and its alloys. Aluminum is a light silver-white metal with a density of 2.7 g/cm3. It is plastic, well rolled and cast, melting point 657°C. Aluminum has a high corrosion resistance in air due to the formation of a protective oxide film on the surface. Aluminum in its pure form is used for casting parts, making foil, in the form of the finest powder used in aluminum paint, and also as a blowing agent for cellular concrete.

Aluminum alloys are obtained by adding copper, manganese, magnesium, silicon to aluminum. These alloys have increased strength, ductility and corrosion resistance compared to aluminum. Among aluminum alloys, aluminum-manganese, aluminum-magnesium, duralumin alloys are most often used [aluminum alloy with copper (up to 5.5%), magnesium (up to 0.8%), silicon (0.8%) and manganese (up to 0. 8%)] and alvil, which has the same components as duralumin alloys, but in slightly different proportions.

Various types of rolled products are made from aluminum alloys: angles, channels, I-beams, flat and corrugated sheets, pipes, etc. At present, the scope of aluminum alloys has been significantly expanded. Alloys are recommended for use in the construction of structures of large-span structures, structures of chemical plants with aggressive environments, in prefabricated lightweight structures, for showcases and window casings, as well as for enclosing structures, for example, three-layer hinged panels with aluminum alloy sheathing and a middle layer of heat-insulating material, roof panels, suspended ceilings, balcony railings, etc.

Structural elements made of aluminum alloys are connected by rivets, bolts, as well as by welding or gluing.

Copper and its alloys. Copper is a soft, ductile, reddish metal with a density of 8.9 g/cm3, a melting point of 1083°C, and a tensile strength of 200 MPa. Copper has high thermal and electrical conductivity. In its pure form, it is practically not used, but in various alloys it is the main component.

An alloy of copper and zinc (up to 40%) is called brass. This alloy has high mechanical properties and corrosion resistance, and lends itself well to hot and cold working. Brass is used in the form of sheets, rods, wire, pipes, as well as products for architectural decoration of building interiors.

An alloy of copper with tin, aluminum, manganese or nickel is called bronze. It has high mechanical, antifriction, casting, decorative properties, as well as corrosion resistance. Bronze is used in the form of a variety of products for the internal equipment of buildings (sanitary fittings, fittings, etc.).

Zinc- bluish-white metal. It has high corrosion resistance, therefore it is used for galvanizing steel products (roofing steel, embedded parts, bolts, etc.).

Lead- heavy metal of a grayish-blue color. It pours and rolls well, is resistant to sulfuric and hydrochloric acids, and has high protective properties from exposure to X-rays. In construction, special pipes, corrosion-resistant coatings, special types waterproofing (seams between tubings in trunks are minted with lead), etc.

In recent years, some non-ferrous metals and their alloys have been successfully replaced by plastics, glass, chemically treated wood, and other cheap and less scarce materials.

Protection of metals from corrosion and fire

Corrosion is the destruction of metal under the influence of the environment. As a result of corrosion, about 10 - 12% of the annual production of ferrous metals is irretrievably lost.

Types of corrosion. Depending on the mechanism of the metal destruction process, corrosion can be chemical and electrochemical.

Chemical corrosion occurs when the metal is exposed to dry gases or liquids of organic origin, which are not electrolytes. An example of chemical corrosion is the oxidation of a metal at high temperatures, as a result of which an oxidation product, scale, appears on its surface. This type of corrosion is rare.

Electrochemical corrosion is formed as a result of exposure of the metal to electrolytes (solutions of acids, alkalis and salts). Metal ions go into solution, while the metal is gradually destroyed. This type of corrosion can also occur when two dissimilar metals come into contact in the presence of an electrolyte, when a galvanic current passes between these metals. In a galvanic pair of any two metals, the metal that is lower in the series of electrochemical voltages will be destroyed. For example, iron in a series of voltages is located above zinc, but below copper, therefore, when iron contacts zinc, zinc will be destroyed, and when iron contacts copper, iron will be destroyed. In metals, due to the presence of heterogeneous structural components, microcorrosion can occur. Spreading along the boundaries of metal grains, it causes intercrystalline corrosion.

Depending on the nature of the environment, electrochemical corrosion can be atmospheric underwater and soil, as well as caused by stray currents. Steel structures are often exposed to atmospheric corrosion. Carbon dioxide and sulfur dioxide in the atmosphere form an electrolyte with air moisture that acts on steel. In this case, the degree of destruction of steel depends on the type and concentration of the electrolyte. Underwater corrosion is possible in metal immersed in water. Soil corrosion occurs when metal structures interact with soil. Pipe metal corrosion is quite common, metal frame underground structures from the effects of stray currents that occur when underground cables are located close to each other, and rails of tram or railway tracks.

Protection of metal from corrosion. There are various methods of protecting metals from corrosion, including the protection of the base metal with paint, non-metal and metal films, as well as the introduction of alloying elements into the metal.

Paint coating is the most common type of anti-corrosion protection of metal. As film-forming materials, nitroenamels, petroleum, coal-tar and synthetic varnishes, paints based on vegetable oils, etc. are used. A dense film formed during coating on the surfaces of structures isolates the metal from the effects of its surrounding humid environment.

Non-metallic coatings are quite diverse. These include enamelling, coating with glass, cement-casein composition, sheet plastic and tiles, spraying of plastics, etc. These coatings are quite resistant to external aggressive environments and reliably protect the metal from corrosion.

Metal coatings are applied to metals by galvanic, chemical, hot, metallization and other methods. In the galvanic method of protection, a thin layer of metal is created on the metal surface by electrolytic deposition from a solution of metal salts. protective layer any metal. In this case, the coated product serves as the cathode, and the deposited metal serves as the anode. An example is the galvanization of embedded parts for reinforced concrete structures. Chemical treatment of metal products ensures the creation of a protective film on the metal surface. With the hot coating method, the products are immersed in a bath with molten protective metal (zinc, tin, lead).

Metallization is a common way to protect metals. It consists in applying compressed air to the thinnest layer of atomized molten metal on the surface of a metal product to be protected from corrosion. For this purpose, devices - metallizers are used.

In alloying protection, alloying elements are introduced into the metal, which increase the resistance of the alloy to corrosion. For example, the introduction of copper significantly increases the corrosion resistance of steel. High-alloy stainless steels are highly resistant to corrosion.

Fire protection. For the protection of metal structures, the most promising are the so-called intumescent coatings or paints based on polymer binders, which, when exposed to fire, form a coked foamed melt that prevents the metal from heating.

To increase the fire resistance limit (600°C) of metal, including aluminum structures asbestos-cement, asbestos-perlite, asbestos-vermiculite coatings applied by pneumatic spraying are also used. The new kind fire protection - a phosphate coating 20 - 30 mm thick, which is a monolithic light mass resistant at 1000 ° C. Traditional ways to increase the fire resistance limit are the use of facings and plasters made of fireproof fireproof materials (brick, hollow ceramics, gypsum boards, mortars, etc.).

MINERAL BINDERS

Basic information about mineral binders and their classification

mineral binders called artificially obtained powdery finely dispersed materials, which, when mixed with water (aqueous solutions), form a plastic dough that can harden as a result of physicochemical processes, i.e., pass into a stone-like state. This property of mineral binders allows them to be widely used for the preparation of mortars and concretes, as well as for the production of various unfired artificial stone materials, products and parts, adhesives and paint compositions. This is the largest in terms of nomenclature, the most common and significant group of building materials in terms of application.

Mineral binders are divided into air and hydraulic. Air binders are substances that are able to harden, retain and increase their strength for a long time only in air. Air binders include air lime, gypsum and magnesia binders, liquid glass, etc.

Hydraulic binders are substances that are able to harden, retain and increase their strength for a long time not only in air, but also in water. Hydraulic binders include hydraulic lime, roman cement, Portland cement and its varieties, aluminous cement, waterproof expanding and non-shrinking cements, etc.

Building air lime

Building air lime is a binder obtained by moderate firing (not to sintering) of limestone containing no more than 6% clay impurities. As a result of roasting, a product is formed in the form of white lumps, called quicklime lump (boiler). Depending on the nature of the subsequent processing, the following types of air lime are distinguished: quicklime ground, slaked hydrated (fluff), lime dough, milk of lime.

Air lime production. Limestone, chalk, dolomitic limestone, etc., consisting mainly of calcium carbonate CaCO3, as well as a small amount of impurities - dolomite, gypsum, quartz and clay, are used as raw materials for the production of air lime.

The technological process for the production of air lime consists of the extraction of carbonate rock (limestone or chalk) in a quarry, crushing and sorting it and subsequent firing in shaft or rotary kilns, where, due to the combustion of fuel, the temperature rises to 1000 - 1200 ° C and decomposition (dissociation) of limestone occurs: CaCO3 = CaO + CO2. Magnesium carbonate MgCO3 present in limestone also decomposes during firing: MgCO3 = MgO + CO2.

When further lowering into the cooling zone, the burnt lime is cooled by air, and then unloaded into the lower furnace by a special mechanism.

By using rotary kilns it is possible to produce lime from any carbonate rocks, including fine crushed limestone and loose wet chalk, which cannot be fired in shaft kilns.

High-quality lump lime can be obtained by uniform firing of limestone until CO2 is completely removed from it. The oxides of calcium and magnesium (CaO + MgO) remaining after firing are the active components of lime; their quantity determines the quality of the resulting material as a binder. In addition, lump lime usually contains a certain amount of underburning and overburning. Underburning - undecomposed calcium carbonate is obtained when too large pieces of limestone are loaded into the kiln or the firing temperature is not high enough. The underburnt has almost no astringent properties and therefore is a ballast. The burn is obtained as a result of the fusion of calcium oxide with impurities - silica, alumina and iron oxide - under the influence of too high a temperature. Burnt grains are extinguished very slowly. The presence of overburning in lime is dangerous, since unextinguished particles can begin to be extinguished in hardened lime mortar and cause cracks in plaster, silicate products, etc.

Quicklime lump consists of porous pieces with a density of 900 - 1100 kg / m3 and is an intermediate product, which is then crushed or quenched to be converted into commercial products.

When grinding in ball mills pre-crushed pieces of lump lime-boilers will receive quicklime, which, unlike slaked lime, has the ability to quickly set and harden. In the process of grinding boiled lump lime, various additives can be introduced: slag, ash, sand, pumice, limestone, which improve its properties and reduce the cost. In this way, for example, carbonate lime is obtained, consisting of 30 - 40% quicklime and 70 - 60% raw limestone. This lime is used for the preparation of self-heating mortars used in winter conditions.

Extinguishing lime. When quicklime is treated with water, calcium oxide is converted into a hydrate according to the following formula: CaO + H2O = Ca(OH)2. This process is called "lime slaking" and is accompanied by the release of a large amount of heat and intense vaporization (this is why quicklime is usually called a boil).

Depending on the amount of water taken during quenching, hydrated lime (fluff), lime dough or lime milk are obtained.

hydrated lime(fluff) is obtained when 6O - 70% of water is taken to quench boiling lime. In this case, 32% of the water is involved in a chemical reaction, and the rest of the water evaporates during the quenching process. As a result of quenching, the volume of lime obtained increases by 2–3 times compared to the original. The resulting hydrated lime is a white powder consisting of tiny particles of calcium hydroxide.

lime dough is a white plastic mass with a density of up to 1400 kg/m3. When slaking lime - boiling water in lime dough, the water consumption is increased to 2 - 3 parts by weight per 1 part of lime. By using more water, milk of lime. The volume of the resulting lime paste is 2 - 3.5 times higher than the volume of the initial boiled lime.

Depending on the quenching speed, lump lime is divided into quick-extinguishing with a quenching period of up to 20 minutes and slow-extinguishing - over 20 minutes. The higher the activity of lime, the faster it is extinguished and the greater the yield of lime dough.

Hardening of lime. Lime, as a rule, is used in construction in the form of a solution, that is, mixed with sand. Lime mortar in air gradually hardens, turning into fake diamond. During the hardening of a lime mortar prepared on slaked lime, several processes take place simultaneously. As a result of evaporation of excess moisture from the lime solution, the smallest particles of Ca (OH) 2 approach each other, crystallize, and then form strong crystalline aggregates that bind sand grains into a monolithic body. Along with this, due to the interaction of calcium hydroxide with carbon dioxide in the air, a carbonization process occurs with the release of water:

Ca(OH)2 + CO2+ nH2O = CaCO3 + (n+1)H2O.

As a result of this reaction, calcium carbonate is formed, which has high strength. However, the carbonization process is very slow, since a dense crust of calcium carbonate forms on the surface of the lime mortar layer, making it difficult for carbon dioxide to penetrate inside. This explains the extremely slow increase in the strength of lime mortars.

Applications, transportation and storage. Air lime is used for the preparation of lime-sand and mixed mortars used for masonry and plaster, in the production of silicate products, and also as a binder for painting paint compositions. In addition, ground and fluffy air lime is used in the production of lime-pozzolanic and lime-slag cements, which have hydraulic properties.

Solutions and products made with air-lime should not be used in damp rooms and foundations, as they are not waterproof. Plaster mortars on ground quicklime are recommended to be used both at positive and at negative outside temperatures. In this case, due to the fact that during the preparation and application of the solution a large amount of heat is released, excess moisture evaporates, and the solution itself quickly gains strength.

Quicklime lump lime is transported in bulk in railway cars or dump trucks, covering the bodies with tarpaulin to protect the lime from moisture. Tightly closed metal containers and bituminized paper bags serve as containers for the transportation of fluff lime and ground lime. Lime dough is transported in dump trucks with specially adapted bodies, and lime milk is transported in tank trucks.

From boiled lime entering the construction site, lime paste should be prepared, which, with small amounts of work, can be in the creation pits for a long time. Fluffy lime can be stored for a short time in dry bags. warehouses. Ground lime should not be stored for more than a month, as it is gradually quenched by air moisture and loses its activity.

When transporting, storing and using air lime, precautions must be taken, since lime dust irritates the respiratory system and wet skin.

Gypsum binders

Gypsum binders are materials consisting of semi-aqueous gypsum or anhydrite and obtained by heat treatment of finely divided raw materials.

The raw materials for the production of gypsum binders are: natural gypsum dihydrate CaSO4 H2O, called gypsum stone, natural anhydrite CaSO4 and some industrial waste containing dihydrate or anhydrous calcium sulfate (phosphogypsum, borogypsum, etc.).

Gypsum binders, depending on the processing temperature of raw materials, are divided into two groups: low-firing and high-firing. Low-firing gypsum binders are obtained by heat treatment of gypsum dihydrate at 110 - 180°C. They consist mainly of finely ground semi-aqueous gypsum CaSO40.5H2O and are characterized by rapid hardening. High-firing gypsum binders are fired at 600 - 1000°C. They mainly include anhydrous gypsum - anhydrite CaSO4, they are characterized by slow hardening. Low-firing gypsum binders include: molding, building and high-strength gypsum, as well as gypsum binders from materials containing gypsum. High-firing binders include: anhydrite binder (anhydrite cement) and high-firing gypsum (extrich gypsum),

Production of building gypsum. When firing lumpy gypsum stone in a drying drum (rotary kiln), hot flue gases come into direct contact with slowly moving crushed gypsum stone. After firing, the gypsum is ground in a ball mill.

Joint firing of gypsum stone and its grinding is carried out in ball mills. In them, gypsum stone is crushed, its small particles are picked up by the flow of hot flue gases entering the mill. While in suspension, gypsum stone particles are dehydrated until they turn into semi-aqueous gypsum and are carried out by flue gases from the mill to dust settling devices.

Hardening of building plaster. When semi-aqueous gypsum is mixed with water, a plastic dough is formed, which quickly thickens and turns into a stone-like state. The process of hardening semi-aqueous gypsum occurs as a result of hydration of semi-aqueous gypsum, i.e. adding water to it and turning it into dihydrate gypsum: CaSO4 0.5H2O + 1.5H2O = CaSO4 2H2O.

Further drying of the hardening mass leads to a significant increase in the strength of the gypsum. To accelerate hardening, artificial drying of gypsum products is used at a temperature not exceeding 60-65 ° C. At higher temperatures, the process of decomposition of gypsum dihydrate may begin, accompanied by a sharp decrease in strength. During hardening, gypsum increases in volume up to 1%, filling the molds well when casting gypsum products.

Properties of building plaster. Building gypsum is a white powder. Its density in a loose state ranges from 800 - 1100 kg / m3, and in a compacted state - 1250 - 1450 kg / m3, the true density is 2.6 - 2.75 g / cm3. It is a fast-setting and fast-hardening binder, the main properties of which include water demand, setting time, fineness of grinding, and compressive and flexural strength.

The normal density of a gypsum dough is characterized by the amount of water (in%) at which a dough of a given mobility is obtained. Building gypsum has a high water demand. To obtain a test of normal density, 50-70% of water by weight of gypsum is needed.

The setting time of the gypsum dough (i.e., the density of the dough at which mechanical mixing is difficult or impossible) is determined on the Vicat device by the depth of the needle immersion in the gypsum dough. According to the setting time, gypsum dough is divided into three groups: A - quick-setting (the beginning of setting is 2 minutes and the end of setting is 15 minutes); B - normally setting (respectively 6 minutes and 30 minutes); B - slow-setting (the beginning of setting is not earlier than 20 minutes from the moment the gypsum dough is mixed).

The rapid setting of gypsum makes work difficult, therefore, if necessary, setting retarders (animal glue, sulfite-yeast mash - SDB) are added to the gypsum dough in an amount of 0.1 - 0.3% by weight of gypsum. In the production of gypsum concrete products, it may be necessary to accelerate the setting of gypsum, then gypsum dihydrate and table salt are added to it in a small amount.

The strength of gypsum is characterized by the compressive strength of beam specimens 40x40x160 mm in size from gypsum dough of normal density, tested 1.5 hours after manufacture.

According to the compressive strength, 12 grades of gypsum were established: G-2, G-3, G-4, G-5, G-6, G-7, G-10, G-13, G-16, G-19, G-22, G-25, while the minimum bending strength for each grade must correspond to a value of 1.2 to 8 MPa, respectively.

Due to the relatively high solubility of gypsum dihydrate, the strength of gypsum products when wet is sharply reduced (by 40–70%) and plastic deformations are detected. The water resistance of gypsum is increased by adding ground granulated blast-furnace slag. In addition, the water resistance of gypsum products is increased by coating their surfaces with various compositions that form waterproof films.

Application of building plaster. Building gypsum is used for products and parts used in the construction of buildings and structures at a relative humidity of not more than 60%. Gypsum and lime-gypsum plaster mortars, decorative, heat-insulating and Decoration Materials, as well as various architectural details by casting.

High-strength gypsum called a binder, consisting mainly of calcium sulfate hemihydrate, obtained by heat treatment of gypsum dihydrate in an autoclave under steam pressure or boiling in aqueous solutions of certain salts, followed by drying and grinding into a fine powder. It has a lower water requirement (about 45%), which makes it possible to obtain gypsum products with high density and strength.

The compressive strength of high-strength gypsum is not less than 25 - 30 MPa. The setting time of high-strength gypsum is approximately the same as that of building gypsum.

High-strength gypsum is used for the manufacture of architectural details and building products with increased strength requirements.

Magnesian binders

Magnesia binders are finely ground powders containing magnesium oxide and hardening when mixed with aqueous solutions of magnesium chloride or sulfate. Magnesia binders, depending on the raw materials used, are divided into two types: caustic magnesite and caustic dolomite.

Caustic magnesite- powder, consisting mainly of magnesium oxide. It is obtained by roasting magnesite rock MgCO3 - in shaft or rotary kilns at 700 - 800 ° C, followed by grinding the roasting product into a fine powder. When fired, magnesite decomposes according to the reaction

O3 \u003d MgO + CO2.

The finished binder is packed in steel drums or paper bags and sent to the place of application. Due to its high hygroscopicity, caustic magnesite is not subject to long-term storage.

Caustic magnesite is closed not with water, but with aqueous solutions of magnesium chloride or sulfate. Caustic magnesite hardens relatively quickly. Its setting should occur no earlier than 20 minutes, and the end - no later than 6 hours from the moment of mixing. Caustic magnesite grades - 400, 500 and 600.

Caustic dolomite- a powder consisting of magnesium oxide and calcium carbonate, obtained by roasting natural dolomite CaMg (CO3) 2, followed by grinding into powder. Due to the content of inert CaCO3, caustic dolomite is inferior in quality to caustic magnesite. Caustic dolomite grades - 100, 150, 200 and 300.

Magnesia binders have the ability to adhere strongly to sawdust, shavings and other organic fillers, which do not decompose and rot in products. These binders are used for the manufacture of heat-insulating materials (fibrolite, etc.), for the installation of warm and wear-resistant xylolite floors, steps, and tiles.

Liquid glass and acid-resistant cement

Air binders include liquid glass and the acid-resistant cement it mixes.

Liquid glass is sodium Na2nSiO2 or potassium silicate K2OnSiO2 yellow color, which is obtained by fusing in glass furnaces at a temperature of 1300 - 1400 ° C crushed pure quartz sand with Na2CO3 soda or K2CO3 potash. The transparent pieces and lumps of bluish, greenish and yellowish color formed after rapid cooling of the melt dissolve under the action of steam (in an autoclave) under a pressure of 0.4 - 0.6 MPa, turning into a viscous solution, usually called liquid glass. For construction, liquid glass (mainly sodium, as cheaper) comes with a true density of 1.32 - 1.50 g / cm3. It only hardens in air. The hardening process of liquid glass is significantly accelerated by the introduction of a catalyst - sodium silicofluoride Na2SiF6.

Liquid glass is used to obtain silicate fire-retardant paints, to protect natural stone materials from weathering, compaction (silicification) of soils, as well as to obtain acid-resistant cement and heat-resistant concrete.

acid resistant cement- finely ground mixture of quartz sand and sodium silicofluoride, closed with liquid glass. Setting and hardening of acid-resistant cement occurs at a temperature not lower than 10 ° C, while the beginning of setting should occur no earlier than 30 minutes, and the end no later than 6 hours from the moment of mixing. Acid-resistant cement is not waterproof and is relatively quickly destroyed by the action of water and weak acid solutions.

Mortars and concretes prepared on acid-resistant cement are highly resistant to a number of mineral and organic acids, but are destroyed in alkalis, as well as in phosphoric, hydrofluoric and hydrofluorosilicic acids. They are used for the lining of chemical equipment, the construction of tanks and other structures of the chemical industry.

hydraulic lime

Hydraulic lime is a product of moderate firing of marly limestones containing 6-20% clay and fine sandy impurities. These limestones are fired in shaft kilns at 900 - 1100°C. At this temperature, calcium carbonate decomposes and part of the calcium oxide combines with oxides of silicon and aluminum, which are contained in clay. As a result, silicates and calcium aluminates are formed, which give hydraulic lime the ability to harden in water.

Hydraulic lime, slightly moistened with water, is completely or partially quenched and crumbles into powder, and filled with a sufficient amount of water forms a dough, which, having begun to harden in air, continues to harden in water, while the physical and chemical processes of air hardening are combined with hydraulic ones.

Quicklime hydraulic lime is a powder. The compressive strength of hydraulic lime after 28 days is from 1.7 to 10 MPa.

Hydraulic lime is used for the preparation of masonry and plaster mortars used in both dry and wet environments, as well as for low-grade concrete. Solutions and concretes on hydraulic lime in the first day of hardening must be protected from water, as they are easily washed away.

Hydraulic lime should be stored in dry enclosed spaces and protected from moisture during transportation.

Portland cement

Portland cement and its varieties are the main binders in modern construction. In Ukraine, its production is over 65% of the output of all cements.

Portland cement is a hydraulic binder obtained by finely grinding Portland cement clinker with gypsum, and sometimes with special additives.

Portland cement clinker is a product of firing before sintering a finely dispersed homogeneous raw material mixture consisting of limestone and clay and some other materials (marl, blast-furnace slag, etc.). During firing, the predominant content of highly basic calcium silicates in the clinker is ensured.

To regulate the setting time of Portland cement, gypsum dihydrate is introduced into the clinker during grinding in an amount of 1.5 - 3.5% (by weight of cement in terms of SO3).

The composition distinguishes Portland cement without additives, Portland cement with mineral additives, Portland slag cement, etc.

The raw materials for the production of Portland cement are rocks - marls, limestone (limestone, chalk, shell rock, calcareous tuff, etc.) and clay rocks. With limestone in the composition of cement. basic oxide CaO is introduced; with clay - oxides of silicon, aluminum, iron; with marl - all the necessary oxides.

In nature, rocks are rarely found, the chemical composition of which would ensure the production of Portland cement clinker of the required quality after firing, so the raw mixture is made up of two or more components. The ratio of the components of the raw mixture is chosen so that the Portland cement clinker obtained during firing has the following chemical composition; 63 - 68% CaO; 4 - 8% Al2O3; 19 - 24% SiO2, 2 - 6% Fe2O3. Typically, the raw mix consists of 75 - 78% limestone and 25 - 22% clay.

The production of Portland cement consists of the following main processes: extraction of raw materials and preparation of the raw mixture, roasting the mixture before sintering to obtain clinker, grinding the clinker into a fine powder together with additives.

Depending on the properties of raw materials and the type of kilns, raw materials are prepared for production by a wet or dry method. In the wet method, the components are crushed and mixed in the presence of water, and the mixture in the form of a liquid mass (slurry) is fired; in the dry method, the raw materials are crushed, mixed and dry-fired.

Portland cement wet production. Soft rocks (clay and chalk) used as raw materials are pre-crushed in roller crushers and crushed in special basins-talkers in the presence of 36-42% water by weight. Suspensions of clay and chalk in predetermined proportions enter ball mills for fine grinding. If solid limestone is used as a lime component, then it is subjected to two-stage crushing in jaw and hammer crushers, and then crushed in ball mills together with a clay suspension obtained in talkers.

Fig.1. Technological scheme for the production of Portland cement by the wet method

Receiving hopper for limestone; 1 - crusher for limestone; 3 - trolley with clay; 4 - water dispenser; 5 - basin-talker; 6 - raw mill; 7 - sludge pools; 8 - rotary kiln; 9 - fuel injector; 10 - clinker warehouse; 11 - warehoused gypsum stone; 12 - crusher for gypsum stone; 13 - ball mill; 14 - silos for cement; 15 - wagons with cement)

Ball multi-chamber mill - a steel cylinder with a length of 8 - 15 and a diameter of 1.8 - 3.5 m, the inner surface of which is lined with steel plates. The mill rotates on hollow pins, through which, on the one hand, it is loaded, and on the other hand, it is unloaded. A mixture of limestone, clay and water passes through all the chambers of the mill and, crushed under the impact of steel balls and cylinders, leaves it in the form of a creamy mass - sludge.

The sludge is pumped into cylindrical sludge pools to adjust its composition. When adjusting, the chemical composition of the sludge is established (mainly the content of calcium carbonate is determined) and, in accordance with the data obtained, a strictly defined amount of sludge of a different composition (enriched or depleted in limestone) is added to it. The sludge adjusted in this way is pumped into sludge pools for storage. In these pools, the sludge is constantly stirred. As needed, the sludge is pumped for roasting.

The raw mixture is fired in rotary kilns (Fig. 2), which are a welded cylinder with a diameter of 4 - 5 and a length of 150 - 185 m, lined from the inside with a refractory material. The furnace is located at a slight slope to the horizon and slowly rotates around its axis. Dosing feeders feed the sludge to the upper end of the furnace. Due to the rotation of the furnace and its inclination to the horizon, the fired material moves to the lower end of the furnace. Hot flue gases are moving towards it, formed during the combustion of fuel (pulverized coal, fuel oil, gas) supplied through the nozzle at the bottom of the furnace.

Fig.2. Cement Clinker Rotary Kiln

smoke exhauster; 2 - feeder for sludge supply; 3 - drum; 4 - drive; 5 - fuel injector; 6 - refrigerator)

The sludge is washed by hot gases and dried, forming clods. As the material advances at 500 - 750 ° C, they burn out organic matter and dehydration begins - the release of chemically bound water from the clay component, accompanied by a loss of plasticity and binding properties. Lumps of material disintegrate into mobile powder. At 750 - 800°C and higher, reactions in the solid state between its constituents begin in the material. Their intensity increases with increasing temperature. There is an adhesion of individual particles of the powder and the formation of granules of different sizes. When passing through a zone with a temperature of 900 - 1000 ° C, dissociation of calcium carbonates occurs with the release of calcium oxide and carbon dioxide, which is carried away with combustion products. Calcium oxide CaO enters into chemical interaction with alumina, iron oxide and silica. CaO chemical bonding reactions proceed quite intensively in the solid state at 1200 - 1250 ° C, with the formation of the following chemical compounds: 2CaOSiO2 (dicalcium silicate), 3CaOAl2O3 (tricalcium aluminate) and 4CaOAl2O3Fe2O3 (tetracalcium aluminoferrite). At temperatures above 1300°C, 3CaOAl2O3 and 4CaOAl2O3Fe2O3 pass into a melt, in which CaO and 2CaO SiO2 partially dissolve until the solution is saturated; in the dissolved state, they react with each other, forming tricalcium silicate 3CaO SiO2 - the main mineral of Portland cement. The process of formation of tricalcium silicate, which separates from the liquid phase in the form of crystals capable of growing, usually occurs around 1450 ° C. When the temperature drops to 1300°C, the liquid phase solidifies, and the sintering process ends.

Clinker - grayish-green granules 15 - 25 mm in size for cooling to 80 - 100 ° C is sent to the refrigerator, from where it goes to the warehouse, where it is kept for 1 - 2 weeks. As a result of aging, a small amount of free calcium oxide contained in the clinker is quenched by air moisture, and the hardness of the clinker grains also decreases, which, in turn, facilitates its grinding and ensures uniform changes in the volume of cement during hardening.

Clinker is ground in multi-chamber ball mills. In the process of grinding, 2-5% of gypsum stone is added to it to regulate the setting time of Portland cement and various additives provided for by the technological process. From ball mills, Portland cement is fed by pneumatic transport into silos - reinforced concrete cylindrical towers with a capacity of up to 6000 tons each, where the cement is aged for 10-14 days before being sent to the consumer. During this time, the cement heated during grinding is cooled and the free lime remaining in it is quenched, which improves the properties of the cement. From the silos, cement enters packaging machines for packaging in multi-layer paper bags of 50 kg each or is sent to specially equipped means of rail, road or water transport.

The dry method for the production of Portland cement is used when the raw materials are marls or mixtures of solid limestones and clays with a moisture content of 8–10%. According to this method, raw materials after preliminary crushing and drying are jointly crushed in ball mills. Dry raw flour with a residual moisture content of 1 - 2% is granulated into grains 20 - 40 mm in size or molded by adding coal ground on mechanical presses into briquettes.

Granules are fired in cyclone heat exchangers, conveyor calciners, rotary kilns, and briquettes - in mine. Further production operations are carried out in the same sequence as in the wet method.

With the dry method, much less fuel is consumed for firing clinker than with the wet method.

Along with the main production methods discussed above, a combined method has recently been used that combines the advantages of wet and dry methods. Its essence lies in the fact that the raw mixture is prepared by the wet method, after which the sludge is dehydrated in special installations and in the form of granules, as in the dry method, is fired in rotary kilns.

Mineralogical composition of clinker. Clinker consists of the following main clinker minerals: tricalcium silicate 3CaOSiO2 (alite), dicalcium silicate 2CaO. SiO2 (belite), tricalcium aluminate 3СаО. Al2O3, tetracalcium aluminoferrite 4CaOAl2O3 Fe2O3. Their abbreviation is often used: C3S, C2S, C3A and C4AF, respectively. The content of these minerals in Portland cement clinker usually ranges from: 40 - 65% C3S; 15 - 40% C2S; 2 - 15% C3A and 10 - 20% C4AF. With an increase in the content of the above minerals, Portland cement receives a special name. So, with a high content of C3S (more than 56%), it is called alitic; C2S (more than 38%) - belitic; C3A (more than 12%) - aluminate, etc. If the clinker contains an increased amount of two minerals, it is respectively called alitoaluminate, etc. Each of the clinker minerals has its own specific properties.

Tricalcium silicate (alite) is a reactive mineral, it has a decisive influence on the strength and hardening rate of cement. Its interaction with water occurs with a large heat release. Alit has the ability to quickly harden and gain high strength, therefore, the increased content of tricalcium silicate ensures the production of high-quality Portland cement from this clinker.

Dicalcium silicate (belite), mixed with water, hardens slowly in the initial period, while very little heat is released. The hardening product during the first month has a low strength, but then over the course of several years, under favorable conditions, its strength steadily increases.

Tricalcium aluminate is characterized by high chemical activity; on the first day of hardening, it releases the greatest amount of heat of hydration and quickly hardens. However, the product of its hardening has low durability and low resistance to the effects of sulfate compounds.

Tetracalcium aluminoferrite is characterized by moderate heat release, it hardens much more slowly than alite, but faster than belite. The strength of its hydration products is somewhat lower than that of alite.

Having data on the mineralogical composition of Portland cement clinker and knowing the properties of clinker minerals, it is possible to form an idea in advance about the main properties of Portland cement and the features of its hardening under various conditions.

Hardening of Portland cement. When portland cement is mixed with water, a plastic sticky cement paste is first formed, which then gradually thickens, turning into a stone-like state. Hardening is the process of turning cement paste into cement stone.

When Portland cement is mixed with water in the initial period, clinker minerals are dissolved from the surface of cement grains, minerals interact with water, and a solution saturated with respect to clinker minerals is formed. Upon reaching saturation, the dissolution of clinker minerals stops, but the reactions between them and water continue. The reactions of adding water to clinker minerals are called hydration reactions, and the reactions of decomposition of clinker minerals under the action of water into other compounds are called hydrolysis reactions.

In the second period, in a saturated solution, clinker minerals are hydrated in the solid state, i.e., water is directly added to the solid phase of the binder without its preliminary dissolution. The products of these reactions are hydrate neoplasms in colloidal form. The period of colloidation is accompanied by an increase in the viscosity of the cement paste, which causes the setting of the cement.

In the third period, the processes of recrystallization of the smallest colloidal particles of neoplasms take place, i.e. dissolution of the smallest particles and formations of large crystals. Crystallization is accompanied by hardening of the cement paste and an increase in the strength of the resulting cement stone.

The interaction of clinker minerals with water proceeds according to the following reactions:

CaO. SiO2 + (n+1)H2O = 2CaO. SiO2. pH2O + Ca(OH)2;

2CaO. SiO2 + nH2O = 2CaO. SiO2 . nH2O;

CaO. Al2O3. Fe2O3 + nH2O = 3CaO. Al2O3. 6H2O + CaO. Fe2O3(n - 6)H2O

The above chemical reactions show that as a result of the interaction of clinker minerals with water, new compounds are formed - hydrosilicates, hydroaluminates and calcium hydroferrites. Minerals C3S and C4AF, interacting with water, undergo hydrolysis, i.e. decomposition, and the minerals C2S and C3A are hydrated, that is, they add water.

According to the rate of interaction with water, clinker minerals are arranged in the following sequence: C3A, C4AF, C3S and C2S. The rate of hydration of clinker minerals largely determines the rate of their hardening. The faster the mineral hydrates, the faster it sets and hardens.

In the case of cement hardening in air, the above processes are supplemented by calcium hydroxide carbonization: Ca(OH)2 + CO2 = CaCO3 + H2O. It occurs mainly on the surface of the cement stone with the formation of a thin crust of calcium carbonate, which increases the resistance and strength of the cement stone.

As a result of the processes of colloidation, crystallization, compaction of hydrate neoplasms and carbonization, a strong cement stone is formed. The strength of the cement stone increases rather quickly during the first 3–7 days, then, in the range of 7–28 days, the increase in strength slows down. Further increase in strength is relatively small, but can continue for many years, especially in a humid and warm environment. In a dry environment or at negative temperatures, the processes of hardening of the cement stone stop and the growth of strength stops. Frozen cement stone has the ability to continue to gain strength after thawing.

The hardening of Portland cement can be accelerated by increasing the ambient temperature and introducing chemicals - hardening accelerators (calcium chloride, sodium chloride, etc.) in an amount of 1 - 2% by weight of cement.

The hardening of Portland cement is accompanied by the release of heat. This property of Portland cement is positive when concreting monolithic structures in winter conditions and negative in cases where the heating of massive concrete structures (dams, massive foundations, etc.) can lead to the appearance of thermal expansion cracks in them.

Properties of Portland cement. The main properties of Portland cement include average density, true density, fineness of grinding, water demand, setting time, uniformity of volume change and strength.

The average density of Portland cement in a loose state is 1000 - 1100 kg / m3, and in compacted - 1400 - 1700 kg / m3. The true density of Portland cement is 3.05 - 3.15 g / cm3.

The fineness of cement grinding is characterized by a residue on a sieve No. 008 (mesh size in the light of 0.08 mm) no more than 15% or a specific surface - the size of the surface of the grains (in cm) in 1 g of cement. The specific surface area of ​​Portland cement should be 2500 - 3000 cm2/g. With an increase in the fineness of cement grinding to 4000 - 4500 cm2 / g, the hardening rate increases and the strength of the cement stone increases.

The water requirement of Portland cement is determined by the amount of water (in%) that is necessary to obtain a cement paste of normal density, i.e., a given standard plasticity.

The normal density of the cement paste is its consistency, at which the needle of the Vicat device, plunging, does not reach the bottom (glass) of the ring by 5–7 mm. The water requirement of Portland cement usually ranges from 22 - 26% and depends on the mineralogical composition and fineness of grinding.

The setting time of cement paste of normal density is determined on the Vicat device according to the depth of penetration of the needle. The beginning of setting should occur no earlier than 45 minutes, and the end of setting - no later than 10 hours from the start of mixing. In Portland cement, usually the beginning of setting occurs after 1-2 hours, and the end - after 4-6 hours. The setting time of Portland cement is affected by its mineralogical composition, fineness of grinding and other factors. The uniformity of the change in the volume of cement is established on samples-cakes made from cement paste of normal density, by boiling them in water and keeping them over steam. The cement is considered to be of good quality if on the front side of the cakes subjected to tests there are no radial cracks reaching the edges or a network of small cracks visible through a magnifying glass or with the naked eye, as well as any curvature. One of the reasons for the uneven change in the volume of the cement stone during hardening is the presence of free CaO and MgO in the cement, which are hydrated with an increase in volume in the already hardened cement stone, destroying it.

The strength of Portland cement is characterized by its brand. The grade of cement is determined according to the flexural strength of samples of prisms with a size of 40x40xx160 mm and during compression of their halves, made from a cement-sand mortar with a composition of 1: 3 (by weight) on standard Wolsky sand with a water-cement ratio W / C = 0.4 and tested after 28 days. The compressive strength at the age of 28 days is called the cement activity, and the brand of cement is determined by its value. For example, if an activity of 43 MPa is established during testing of cement, then it is referred to grade 400.

Portland cements are divided into grades 400, 500, 550 and b00.

Corrosion of cement stone. Concrete structures erected using Portland cement may be subject to destruction (corrosion) under the influence of natural waters and aggressive liquids. Destruction usually begins with cement stone, as the most susceptible to corrosion.

There are three main types of corrosion of cement stone. Corrosion of the first type occurs when flowing fresh water (with low temporary hardness) acts on the cement stone of concrete. These waters dissolve and wash out the calcium hydroxide released during the hydrolysis of tricalcium silicate. As a result of this leaching action of water, the porosity of the cement stone increases and its strength decreases, which, in turn, leads to the gradual destruction of concrete.

To increase the resistance of cement stone in fresh waters, it is recommended to introduce hydraulic additives into Portland cement, which bind calcium hydroxide into poorly soluble compounds - calcium hydrosilicates.

Corrosion of the second type occurs when mineralized waters containing chemical compounds act on the cement stone of concrete, which enter into exchange reactions with the components of the cement stone. The resulting reaction products either dissolve easily and are carried away by water, or are isolated in the form of an amorphous mass that does not have binding properties.

Sea water, water of salt lakes and estuaries, as well as some groundwater containing MgCl2, MgSO4, NaCI and other salts, have a destructive effect on cement stone. So, when water containing magnesium chloride is exposed to cement stone, the latter interacts with calcium hydroxide of cement stone:

Ca(OH)2 + MgCl2 = CaCl2 + Mg(OH)2.

The calcium chloride formed as a result of the reaction has good solubility and is quickly washed out of concrete; the remaining magnesium hydroxide is an amorphous substance with no binding properties.

Natural groundwater usually contains free carbon dioxide CO2 and its salts, mainly Ca(HCO3)2. These salts are not dangerous for cement stone, but free (aggressive) carbonic acid destroys it. First, the dissolved carbon dioxide interacts with calcium hydroxide, forming sparingly soluble calcium carbonate, which densifies the surface of the cement stone. However, at a high content in water, free carbon dioxide reacts with calcium carbonate: CaCO3 + CO2 + H2O = Ca(HCO3)2. As a result, calcium bicarbonate, which is readily soluble in water, is formed, which is washed out of the concrete.

Thus, the main cause of this type of corrosion is the presence of free calcium hydroxide in the cement stone. Therefore, it is necessary to introduce active mineral additives into the composition of cement, which bind it into sparingly soluble compounds.

As active mineral additives to cement, tripoli, flasks, diatomites, as well as blast-furnace granulated slag, which is also capable of binding calcium hydroxide, are most often used.

Corrosion of the third type occurs when sulfate water acts on the cement stone of concrete. Sulphates CaSO4, MgSO4, Na2SO4, etc. are part of most natural soil, as well as Wastewater. As a result of the exchange reaction of sulfates with calcium hydroxide, calcium sulfate dihydrate (gypsum) is formed in the pores of the cement stone, which interacts with calcium hydroaluminate:

2(СаSO4 2H2О) + 3CaO Al2О3 6H2O + 19Н2О = ЗСаО Al2О3 3CaSO4 31Н2О.

The sparingly soluble calcium hydrosulfoaluminate formed in this case, crystallizing with a large amount of water, increases in volume by 2.5 times, which leads to concrete cracking. To prevent sulfate corrosion of concrete during its preparation, sulfate-resistant Portland cement should be used.

Salts of silicic, fluorosilicic and carbonic acids, weak solutions of alkalis, as well as oil, gasoline, kerosene and other petroleum products are harmless to cement stone if they do not contain residues of sulfuric acid and a significant amount of naphthenic acids.

The protection of cement stone from corrosion is carried out through the use of cements of a certain mineralogical composition, the introduction of the required amount of active mineral additives, the creation of dense concrete, as well as the use of protective coatings and linings. Bituminous insulation, coating with polymer films, glass and ceramic cladding should exclude the impact of an aggressive environment on concrete.

Application of Portland cement. Portland cement is used as a binder in the manufacture of monolithic and prefabricated concrete and reinforced concrete. Products and structures made on Portland cement can be used in aboveground, underground and underwater conditions, as well as in the case of alternating exposure to water and negative temperatures.

Portland cement of low grades is used for the preparation of masonry and plaster mortars. It should not be made of Portland cement structures exposed to sea, mineralized or even fresh water - flowing or under strong pressure. In these cases, it is recommended to use cement of siatic types - sulfate-resistant, pozzolanic Portland cement, slag Portland cement, etc.

Portland cement is a high-quality and scarce binder, it must be used sparingly, replacing, where technically possible, with other, cheaper, binders - lime, mixed cements, etc.

Varieties of Portland cement

Currently, along with ordinary Portland cement, a large number of its varieties are produced - fast-hardening, plasticized, hydrophobic and sulfate-resistant Portland cements. These cements are recommended only when their special properties can be used to their maximum effect.

Fast setting Portland cement(BTC) is characterized by a more intense increase in strength in the first 3 days of hardening. Rapid hardening of cement is achieved due to the content of active minerals in the clinker (C3S + C3A = 60 - 65%), as well as by increasing the fineness of the clinker grinding to a specific surface of 3500 - 4000 cm2 / g. When grinding BTC, it is allowed to introduce active mineral additives (no more than 15%) or blast-furnace granulated slags (up to 20% by weight of cement).

A variety of BTC is an especially fast-hardening Portland cement (OBTC), produced by fine grinding of clinker containing C3S up to 60 - 65% and C3A not more than 8%, together with the addition of gypsum to a specific surface of 4000 - 4500 cm2 / g and more. The introduction of mineral additives is not allowed. OBTC is characterized by high hardening speed and high grades 600 and 700.

Fast-hardening Portland cement grades 400 and 500 are advisable to use in the manufacture of prefabricated high-strength, conventional and prestressed concrete products and structures. Their use reduces the duration of heat and moisture treatment, accelerates the turnover of metal forms, and in some cases even allows you to abandon the heat and moisture treatment of products. Using fast-hardening Portland cement for the construction of structures from monolithic concrete, it is possible to significantly reduce the holding time of structures in the formwork. In addition, it should be used in repair and restoration work, where a rapid increase in the strength of concrete and mortar is required.

plasticized portland cement(PPC) is obtained by grinding Portland cement clinker together with gypsum and plasticizing additives SDB in an amount of 0.15 - 0.25% by weight of cement. The brands of this cement are 400 and 500. Plasticized Portland cement, in comparison with ordinary Portland cement, gives mortar and concrete mixtures increased plasticity, frost resistance and water resistance.

The use of plasticized Portland cement makes it possible, due to an increase in the mobility of concrete mixtures and a decrease in their water demand, to reduce the consumption of cement by an average of 5–8%. Plasticized Portland cement is recommended for the preparation of concrete used in road, airfield and hydraulic engineering construction.

Hydrophobic Portland cement(HPC) is obtained by introducing a water-repellent additive during grinding of Portland cement clinker in an amount of 0.1 - 0.3% by weight of cement. As a hydrophobic (water-repellent) additive, surface-active organic substances are used: soap naft, asidol, synthetic fatty acids, etc. These substances form the thinnest water-repellent films on cement grains that prevent moisture from penetrating to the grain, therefore, hydrophobic Portland cement retains flowability even during long-term storage and does not lose activity. Hydrophobic films of cement grains are easily removed during the mixing of mortar and cement mixtures, which ensures normal setting and hardening of cement.

Hydrophobic Portland cement increases the mobility of concrete mixtures, which in turn leads to an increase in water resistance, water resistance and frost resistance of concrete. Hydrophobic Portland cement is used in hydraulic, road and airfield construction, as well as in the transportation of concrete and mortar mixtures over long distances.

sulfate resistant portland cement(SPTs) are made by fine grinding from clinker of the following mineral composition: C3S - no more than 50%, C3A - no more than 5%, C3A + C4AF - no more than 22%, MgO - 5%. The introduction of inert and active mineral additives into cement is not allowed. With such a mineralogical composition of cement, the possibility of the formation of calcium hydrosulfoaluminate, a cement bacillus, in the cement stone (concrete) under the action of sulfate waters decreases.

Sulfate-resistant Portland cement is characterized by increased sulfate, frost and water resistance, reduced heat release during setting and hardening, as well as slow hardening intensity in the initial stages. Its grade 400 is produced. The remaining requirements for this cement are the same as for ordinary Portland cement.

Sulfate-resistant Portland cement is used for the manufacture of concrete and reinforced concrete structures of the outer zones of massive hydraulic structures operating under conditions of repeated freezing and thawing in fresh or low-mineralized water.

White and colored Portland cements are made from raw materials characterized by a low content of coloring oxides (iron, manganese, chromium), from pure limestones, marbles and white kaolin clays.

White Portland cement is produced in grades 400 and 500 and is divided into three grades according to the degree of whiteness: BTs-1, BTs-2 and BTs-Z.

Colored Portland cements are obtained by joint grinding of white Portland cement clinker with light and alkali resistant pigments (red lead, ocher, ultramarine, etc.).

White and colored cements are used in architectural and finishing works, to obtain a textured layer of wall panels, as well as for the manufacture of artificial marble and facing tiles.

Portland cements with active mineral additives

This group of hydraulic binders includes cements obtained by joint grinding of Portland cement clinker and an active mineral additive or by thorough mixing of these components after separate grinding of each of them.

Active mineral additives are substances containing mainly amorphous active silica, which easily enters into chemical interaction with calcium hydroxide to form sparingly soluble calcium hydrosilicates. Since Portland cement releases calcium hydroxide during the hardening process, which is soluble in water and therefore can be washed out of the cement stone, the presence of a mineral additive in the composition of Portland cement increases its water resistance.

Active mineral supplements have been known since ancient times. ancient rome to impart hydraulic properties to air lime, volcanic ash was added - pozzolana (named after the place of deposits near the city of Pozzuoli in Italy). Hence, they called active additives of volcanic origin "pozzolanic", and cements with these additives "pozzolanic".

Active mineral additives are divided into natural (diatomite, tripolite, flask, volcanic ash, pumice, trails, tuff) and artificial (blast-furnace granulated slag, ash from the combustion of brown coal, peat, oil shale, lightly burned clay, gliezh, ceramic production waste, etc. .).

Among the cements of this group, cement with mineral additives, pozzolanic Portland cement, slag Portland cement, sulfate-resistant Portland cement with mineral additives and sulfate-resistant slag Portland cement are distinguished.

Portland cement with mineral additives obtained by joint grinding of Portland cement clinker, mineral additives and gypsum. As additives, blast-furnace granulated slags or active mineral additives of sedimentary origin are introduced, but not more than 20% of the mass of cement. It is allowed to introduce plasticizing or water-repellent surface-active additives into the cement during its grinding, not more than 0.3% of the mass of cement. The setting of the cement proceeds somewhat slowly. In the early stages of hardening, the set of strength slows down a little. Portland cement with mineral additives is produced in grades 400, 500, 550 and 600.

This cement is successfully used in the preparation of concrete instead of Portland cement, except in cases where high frost resistance of concrete is required.

Pozzolanic portland cement is called a hydraulic binder obtained by joint fine grinding of cement clinker, gypsum and an active mineral additive or by thorough mixing of these materials, crushed separately.

Pozzolanic Portland cement is produced in grades Z00 and 400. The color of the cement is light; density in a loose state 800 - 1000, in a compacted state - 1200 - 1600 kg / m3, water demand 30 - 38%. The setting time, fineness of grinding and uniformity of change in the volume of pozzolanic Portland cement are the same as those of ordinary Portland cement.

Pozzolanic Portland cement is characterized by a slower increase in strength during the initial period of hardening compared to Portland cement made from the same clinker. However, after 3-6 months of hardening in a humid environment, concretes based on pozzolanic porland cement achieve the same strength as concretes based on Portland cement.

Pozzolanic Portland cement releases less heat during hardening than Portland cement. This circumstance makes it possible to widely use pozzolanic Portland cement when concreting large massifs, for example, hydraulic structures, where thermal deformations of structures are very dangerous. However, at temperatures below 10 ° C, its hardening slows down sharply and even completely stops. Conversely, at elevated temperatures, pozzolanic Portland cement hardens more rapidly than Portland cement. Therefore, it is advisable to subject concrete products based on this cement to heat and moisture treatment in steaming chambers and autoclaves.

Pozzolanic Portland cement concretes have higher water resistance and impermeability than Portland cements. However, pozzolanic Portland cement is not frost-resistant, therefore it is not recommended to use it in the construction of structures subjected to alternate freezing and thawing.

Pozzolanic Portland cement is used along with Portland cement for the manufacture of concrete and reinforced concrete products and structures (both prefabricated and monolithic). Due to the increased sulfate resistance, it is used for concrete and reinforced concrete structures of underwater and underground parts structures exposed to soft and sulphate waters. It should be borne in mind that in dry operating conditions, the hardening of concrete on this cement practically stops, therefore, during the first two weeks, concrete must be systematically moistened and protected from drying out.

Portland slag cement called a hydraulic binder obtained by joint grinding of Portland cement clinker and granulated blast-furnace slag with the addition of a small amount of gypsum, introduced to regulate the setting time and activate the hardening of the slag. Portland slag cement can also be produced by mixing the same starting materials, but crushed separately. The content of blast-furnace granulated slag in Portland slag cement should be at least 21 and not more than 60% by weight of cement.

Portland slag cement is produced in grades Z00, 400 and 500. It is grayish in color with a bluish tint, differs from other types of cement in that it contains a large amount of metal particles detected by a magnet. Its density in a loose state is 1000 - 1300, and in a compacted state - 1400 - 1800 kg / m3, the normal density of the cement paste is 26 - 30%; the fineness of grinding and the uniformity of the change in volume are the same as those of Portland cement.

The heat release of Portland slag cement during hardening is less than that of Portland cement, but it has greater heat, water and sulfate resistance. The frost resistance of Portland slag cement is somewhat lower.

In Portland slag cement, in comparison with Portland cement, the increase in strength is somewhat slower in the initial periods of hardening. In longer periods of hardening, the strength increases and after 2-3 months exceeds the strength of Portland cement of the same brand. The slowdown in hardening is especially pronounced at low temperatures, but this is not an obstacle to the widespread use of Portland slag cement, and an increase in temperature with sufficient ambient humidity sharply accelerates hardening. Concretes based on Portland slag cement, subjected to heat and moisture treatment at 80 - 95 ° C, gain higher strength than concretes based on Portland cement of the same grade, hardening under the same conditions.

A variety of Portland slag cement is a fast-hardening Portland slag cement, which differs from the usual one by a lower content of granulated blast-furnace slag (not more than 50%) and a higher fineness of grinding. Fast-hardening Portland slag cement grade 400 is characterized by an intensive increase in strength in the initial period of hardening, which is especially accelerated under conditions of heat and moisture treatment.

Portland slag cement can be successfully used for the manufacture of precast concrete products and structures that harden in curing chambers. It is advisable to use Portland slag cement in the construction of hot shops and in hydraulic structures subjected to sulfate aggression. From it, as well as from pozzolanic Portland cement, building masonry and plaster mortars are prepared. Portland slag cement is not recommended for structures that are subject to the systematic effects of alternate freezing and thawing or moistening and drying.

The mass production and widespread use of pozzolanic cements and slag Portland cements can be explained not only by the presence of a number of positive properties compared to Portland cement, but also by lower cost (by about 15–20%).

Among sulfate-resistant cements In addition to sulfate-resistant Portland cement, according to the material composition, sulfate-resistant Portland cement with mineral additives and sulfate-resistant Portland slag cement are also distinguished.

Sulfate-resistant Portland cement with mineral additives is obtained by grinding Portland cement clinker of normalized mineralogical composition, active mineral additives and gypsum. In cement, the content of granulated blast-furnace slag is allowed at least 10 - 20% of the mass of cement and active mineral additives of sedimentary origin (except for gliège) at least 5 - 10%.

Sulfate-resistant Portland slag cement is a product obtained by fine grinding of Portland cement clinker of a normalized mineralogical composition, slag of a normalized chemical composition (at least 21–60% by weight of cement) and gypsum.

The frost resistance of sulfate-resistant cements is lower than that of sulfate-resistant Portland cement, but the applications are the same.

Special cements

This group of hydraulic binders differs sharply from cements made on the basis of Portland cement clinker by the type of feedstock, production technology, chemical and mineralogical composition, properties, and applications. It includes - aluminous, expanding and non-shrinking cements, as well as gypsum-cement-pozzolanic binder.

Aluminous cement is a fast-hardening hydraulic binder obtained by fine grinding of a raw mixture rich in alumina fired to sintering or fusing. As raw materials for the production of aluminous cement, limestone or lime and rocks with a high content of alumina Al2O3, such as bauxite, are used. The mineralogical composition of aluminous cement is characterized by a high content of low-basic calcium aluminates, the main of which is single-calcium aluminate CaO Al2O3.

Aluminous cement has the appearance of a thin powder of gray-green, brown or black. Its density in a loose state is 1000 - 1300, and in a compacted state - 1600 - 1800 kg / m3, normal density is usually 23 - 28%. The fineness of grinding is slightly higher than the fineness of grinding Portland cement; when sieving aluminous cement through a No. 008 sieve, at least 90% of the sample (by weight) must pass. Setting time of aluminous cement: start - no earlier than 30 minutes, end - no later than 12 hours from the moment the cement is mixed with water.

The hardening process of aluminous cement is accompanied by significant heat release, which limits its use in massive concrete structures, but is very useful in construction work in winter.

Aluminous cement is produced in grades 400, 500 and 600. The grade of cement is set according to the compressive strength of cube samples at the age of 3 days after hardening under normal conditions. Cement is characterized by an intensive set of strength in the initial stages of hardening: after 24 hours it gains 80 - 90% of branded strength.

Concrete on aluminous cement is waterproof, resistant to fresh and sulphate water conditions, and frost-resistant. They harden well in a humid environment at 15 - 20%. When the temperature rises above 25 ° C, the strength of concrete decreases significantly, therefore concretes based on aluminous cement cannot be subjected to steaming and other methods of artificial heating. Aluminous cement cannot be mixed with Portland cement, as this reduces its strength.

The use of aluminous cement is limited by its high cost (it is 3-4 times more expensive than Portland cement). It is used for urgent repair and emergency work, work in winter conditions, for concrete and reinforced concrete structures exposed to highly mineralized waters, for the production of heat-resistant concrete, as well as for the manufacture of expanding and non-shrinking cements.

Expandable and non-shrink cements differ in the ability, when hardened in wet conditions, to slightly increase in volume or not to shrink. The industry produces waterproof expanding cement, gypsum-aluminous expanding cement, as well as waterproof non-shrinking cement.

Waterproof Expandable Cement (WEC) is a fast setting and hardening hydraulic binder obtained by co-grinding and thoroughly mixing ground aluminous cement, gypsum and highly basic calcium hydroaluminate. Cement is characterized by fast setting: the beginning - earlier than 4 minutes, the end - no later than 10 minutes from the moment of mixing. The linear expansion of samples from cement paste, hardening in water for 1 day, should be in the range of 0.3 - 1%. The physical and chemical essence of the cement expansion process lies in the fact that as a result of the interaction of calcium aluminates and gypsum, calcium hydrosulfate aluminate is formed, accompanied by an increase in volume.

Waterproof expanding cement (VRC) is used for caulking and waterproofing seams in tubings, socket joints, creating waterproofing coatings, sealing joints and cracks in reinforced concrete structures, etc. It cannot be used in structures operated at temperatures above 80°C.

Waterproof Non-Shrinking Cement (WBC) is a fast-setting and hardening hydraulic binder obtained by intimately mixing aluminous cement, semi-aqueous gypsum and slaked lime. The beginning of cement setting should occur no earlier than 1 minute, and the end - no later than 5 minutes from the moment of mixing. The value of the relative linear expansion of samples from cement paste after 1 day of their hardening in water should be in the range of 0.01 - 0.1%.

Cement is used for the installation of a waterproofing shotcrete shell of concrete and reinforced concrete underground structures operated in conditions of high humidity (tunnels, foundations, etc.).

Gypsum-cement-pozzolanic binder (GTsPV) is obtained by mixing 50 - 75% semi-aqueous (building or high-strength) gypsum, 15 - 25% Portland cement and 10 - 25% pozzolanic (hydraulic) additives. Instead of Portland cement, it is advisable to use pozzolanic Portland cement with the required amount of active additive, as well as Portland slag cement.

Gypsum-cement-pozzolanic binder is produced in grades 100 and 150. It is characterized by rapid hardening and increased water resistance. The strength of concrete on GTsPV 15 - 30 MPa, and already after 2 - 3 hours after their preparation, the strength reaches 30 - 40% of the brand, the softening factor is 0.6 - 0.8; frost resistance - 25 - 50 cycles. To accelerate the hardening of concretes on the HCPV, they are steamed at 70 - 80 ° C, while after 5 - 8 hours the concrete strength reaches 70 - 90% of the final one.

Gypsum-cement-pozzolanic binder is used for the manufacture of floor base panels, sanitary cabins, ventilation units and other products.

Transportation and storage of cements

Paper bags commonly carry white and colored Portland cements, as well as aluminous, watertight, expanding and non-shrinking cements.

Bulk cements are stored in silo or bunker warehouses separately by type, brand and batch from different plants. It is forbidden to mix cements during storage various kinds and stamps. Cement in paper bags is stored in closed storage sheds with a dense waterproof roof, walls and wooden floor, raised above the ground by at least 30 cm. During transportation and storage, it is necessary to protect the cement from moisture and clogging with foreign impurities.

During long-term storage of cement in a warehouse, usually due to the absorption of moisture from the air and premature hydration, its clumping and a decrease in activity occur. The activity of Portland cement decreases after 3 months by an average of 15-20%, after 6 months - by 20-30%, and finely ground fast-hardening Portland cements lose activity much faster, so large stocks of cement in warehouses of construction sites and construction industry enterprises are undesirable.

WOOD BUILDING MATERIALS

Basic information

Wood is an important material widely used in the construction industry, as it has high strength at low density, low thermal conductivity, and ease of machining. At the same time, there are also disadvantages in wood: the unequal nature of a number of properties in different directions, easy decay and flammability, high hygroscopicity, and the presence of a number of defects.

Since wood has recently risen in price significantly, it is necessary to use it economically and rationally. From wood waste - sawdust, shavings, wood chips and slabs, arbolite, fiberboard, fibreboard and chipboard, wood plastic products are made.

A tree consists of a trunk, crown and roots, and the trunk is the main and most valuable part of the tree. The quality of wood as a material depends on the structure of the trunk. The wood of the trunk has a heterogeneous structure in various directions. When studying the trunk in a cross section, the following parts of the trunk are distinguished: bark, cambium, wood and core.

Fig.1. End section of a tree trunk

1 - bark, 2 - cambium, 3 - sapwood, 4 - core, 5 - core)

Bark has an outer part - a peel, a middle part - a cork layer and an inner part - a bast.

Wood- the main mass of the trunk. On the cross section of the wood, growth rings can be distinguished, which are lighter towards the surface of the trunk and darker at the center. Each annual layer consists of two zones: an inner light one - early, formed in spring, and an outer dark one - late, formed by the end of summer, called early and late wood, respectively. Early wood is more porous and weaker than summer wood. The more layers of late wood, the stronger the material. On transverse sections of oak, beech, maple, and other species, narrow radial lines, the so-called core rays, are visible, directed from the bark to the wood. In coniferous wood there are resin passages located in the longitudinal and transverse directions, resin is concentrated in them. The light part of the wood is called sapwood, and the dark part is called the core. The core, unlike sapwood, consists of dead cells, it does not take part in physiological processes, but provides strength to the tree. Some tree species do not have a core (birch, aspen, alder, linden) - these are sapwood. The rest, for example, - pine, oak, larch, cedar - heartwood.

Cambium located in a single-row cylindrical layer (in the form of a ring in the cross section), forms with outer side bast, inside - wood.

Core located in the center of the trunk and runs along its entire length - this is a weak tissue of the primary formation, easily rotting.

tree species

conifers most commonly used in construction. The most commonly used are: pine, larch, spruce, fir and cedar.

Pine - has a pink or brown-red core and a yellowish-white sapwood, has enhanced physical, mechanical and operational properties, lends itself well to processing.

Larch - its wood according to appearance resembles pine wood, but has greater density and strength. It is very resistant to decay in conditions of variable humidity, so it is often used in hydraulic engineering, underground structures and for the manufacture of sleepers.

Spruce - its wood is not very resinous, therefore, when used in damp places, it quickly rots, so it should be used in dry conditions.

Fir has white wood, resembling spruce wood in appearance, but is distinguished by the absence of resin passages. It is even less resistant to decay than spruce.

Cedar has a strong and well-worked wood, so it is most often used in carpentry and furniture production.

hardwood are used much less frequently than conifers. Of these, the most common are: oak, ash, beech and birch.

Oak has a dense, hard and very durable wood of yellowish color and beautiful texture, it is well preserved in the air and under water.

Ash has a heavy, viscous, hard and durable wood in structure resembling oak wood, but lighter in color.

Beech - its wood is dense and durable with a reddish tint. It is used mainly for the manufacture of high-quality joinery and furniture.

Birch has a hard, strong and viscous wood with a yellowish or reddish tint, but it is short-lived in conditions of variable humidity and drying.

Physical and mechanical properties

Wood is an anisotropic material with very diverse physical and mechanical properties.

The color and texture (pattern) of wood are characteristic of a particular breed. The color depends on many factors, with the increase in the age of the tree, the intensity of the color of the wood increases. Tarnishing of wood, the appearance of gray, green, blue colors is a sign of disease.

The true density of wood of all species is approximately the same - 1.55 g / cm3.

The average density depends on the type of wood, growing conditions, humidity and other factors and ranges from 0.37 to 0.7 g/cm3.

Humidity. According to the degree of moisture, wood is distinguished: wet(floating), freshly cut(humidity 35% or more), air dry(humidity 15 - 20%), room-dry(humidity 8 - 12%) and absolutely dry, dried in the laboratory to constant weight at a temperature of 100 - 105 0 C. Conventionally, 12% humidity is considered standard, therefore, the indicators obtained when determining strength and density should be reduced to standard humidity. High moisture content of wood leads to warping, shrinkage and cracking. wooden structures and details and contributes to the defeat of wood by various fungi.

Hygroscopicity - as a result of changes in the humidity of the environment, the moisture content of wood changes all the time. The maximum amount of moisture in wood in the absence of free moisture is called fiber saturation point or limit of hygroscopicity. Its value for different breeds varies between 25 - 35%.

The amount of shrinkage and swelling of wood is not the same in different directions. Linear shrinkage along the fibers is 0.1 - 0.3%, in the radial direction - 3 - 6%, and in the tangential direction - 6 - 12%.

The thermal conductivity of dry wood is negligible - 0.171 - 0.28 W / (m 0 C), but with an increase in its humidity, thermal conductivity increases.

The resistance of wood to mechanical stress is not the same in different directions, in addition, it depends on the type of wood, its moisture content, and the presence of defects.

Average values ​​of the mechanical properties of wood at a moisture content of 12%:

Wood speciesAverage density, kg/m 3Tensile strength (MPa) along the fibers in: tension compression bending pine 5001104885 larch 66012562105 spruce 4501204480 fir 370704070 oak 70013058106 beech 67013056105 birch 63012555110

Wood well perceives compression across the fibers, when bending and stretching, it works well along the fibers.

With an increase in wood moisture content, its strength decreases, especially during static bending and compression.

The presence of defects in wood (knots, slant, etc.) also significantly impairs its mechanical properties.

With prolonged exposure to acids and alkalis, wood is slowly destroyed. The intensity of destruction depends on the concentration of solutions. In sea water, wood is less preserved than in river water.

open hearth steel cast iron corrosion

Wood defects

Wood defects are both deviations in wood associated with a violation of the external shape of a tree trunk, and various damages that affect its technical properties, etc. Wood defects reduce its grade and limit its scope. The following groups of defects are distinguished: knots, cracks, irregularities in the shape of the trunk and structure of wood, abnormal color, rot, insect damage.

Knots are the bases of branches enclosed in the wood of the trunk. They violate the homogeneity of wood, make processing difficult and worsen the mechanical properties of wood.

Knots can be intergrown (in whole or in part) and not intergrown (falling out hard, loose and tobacco).

Fig.2. Types of knots according to the degree of fusion with the surrounding wood (a - fused healthy, b - fused horny, c - falling out)

Cracks can be both on a growing tree and on a felled tree as a result of uneven compression of the wood during drying, sharp temperature fluctuations in winter, and other reasons. Cracks, in addition to reducing the grade and mechanical properties, contribute to the formation of rot. Cracks are of the following types: metic, peel, frost crack and shrinkage cracks.

Fig.3. Types of cracks (a - cross tag, b - arcuate peel, c - frost hole, d - shrinkage cracks_

Metic- one or more longitudinal cracks passing through the core and narrowing from the center to the periphery of the trunk. Metic can be simple - one or two cracks located along the diameter, and cross - cracks are located at an angle to one another, as well as consonant (with a crack in the same plane) and discordant, when the crack goes helically.

otlup- this is an annular crack (complete peel) or an arcuate crack (partial peel).

Morozoboina- an external longitudinal crack, wide on the outside of the trunk and tapering towards its center.

Shrinkage cracks often have a radial orientation and sharply reduce the grade of wood.

Deviations from the normal shape of the trunk

· curvature (unilateral and versatile);

· convergence (a sharp decrease in the thickness of the trunk from the butt to the top);

· buttiness (sharp thickening of the butt);

· oblique (helical arrangement of fibers in the trunk) - greatly impairs the mechanical properties of wood and contributes to its shrinkage and warping;

· tortuosity - a highly wavy or tangled arrangement of fibers.

Fungi attack on woodoccurs both on a growing tree, and in a warehouse and in wooden structures. Mushrooms develop well at high humidity of wood (20 - 60%), lack of ventilation and temperature 0 - 60 0 C. At negative temperatures, fungi do not develop, but they do not die either - they die only at temperatures above 60 0 With and when wood is under water.

The physical properties of building materials are studied in order to solve the practical issue of where and how to apply them in order to obtain the greatest technical and economic effect.
To physical properties include the weight characteristics of the material, its density, permeability to liquids, gases, heat, radioactive radiation, as well as the ability of the material to resist the aggressive action of the external operating environment. The latter characterizes the durability of the material, which ultimately determines the safety of building structures.

Under true density (kg / m³) understand the mass per unit volume of an absolutely dense material: ρ= m1/V1, where m1 is the mass of the material, kg; V1 is the volume of material in a dense state, m³.

The values ​​of the true density of some building materials are given in table-1.

Table 1. True density of some building materials

Under medium density (average density is also called simply density in many sources) ρ0=m1/V1, where m1 is the mass of the material, kg; V1-volume of material, m³. The average density of the same type of material may be different depending on the porosity and voidness.

Bulk materials (sand, crushed stone, cement, and others) are characterized by bulk density - the ratio of the mass of granular and powdery materials to the entire volume they occupy, including the space between the particles. Technical properties, such as strength, thermal conductivity, largely depend on the density of the material. These data are used in determining the thickness of the enclosing structures of heated buildings, the size of building structures, the calculations of vehicles, handling equipment, etc.

The values ​​of the average density of building materials are in a wide range (see table-2).

Table 2. Average density of some building materials

The density depends on the porosity and moisture content of the material. As the moisture content increases, the density of the material increases. The density index is also characteristic for evaluating efficiency.

Porosity (%) of material the degree of filling of its volume with pores is called: П=(1-ρ0/ρ)100,

where ρ0 is the bulk density of the material, kg/m³; ρ-density of absolutely dense material, kg/m³. Pores are small cells in a material filled with air or water. The pores are open or closed, large or small. Small pores filled with air give thermal insulation properties to building materials. By the value of porosity, one can approximately judge other important properties of the material: density, strength, water absorption, durability, etc.

For structures that require high strength or water tightness, dense materials are used, and for the walls of buildings, materials with significant porosity, which have good thermal insulation properties, are used. Open porosity is equal to the ratio of the total volume of all pores saturated with water to the total volume of the material: П0=[(m2-m1)/V] 1/ρ H2O

where m1, m2 is the mass of the sample in a dry and water-saturated state. Open pores communicate with the environment and can communicate with each other, they are filled with water when immersed in a water bath. The material usually has open and closed pores. In sound-absorbing materials, open porosity and perforation are specially created for greater absorption of sound energy.

Closed porosity in terms of size and distribution of pores is characterized by: a) an integral curve of the distribution of pore volume along their radii per unit volume (see Figure-1) and b) a differential curve of the distribution of pore volume along their radii (see Figure-2, a). Porosity, obtained using a mercury porometer, allows you to determine the size and volume of pores of each size and evaluate their shape. Mercury does not wet the pores of most building materials and penetrates into them when high blood pressure, which follows from the equation: Pd=-4σcosθ, where P is the applied pressure, d is the pore diameter; σ -surface tension mercury; θ is the wetting angle of mercury and the test material.

Picture 1. Integral curves of the distribution of pores along the radii (the dotted line shows the hysteresis curve)

It can be seen from the equation that at zero pressure, a non-wetting liquid will not penetrate into the pores. Figure-2, b shows the relationship between pressure and pore diameter. Figure -1 shows the integral curves of pore size distribution for four different materials. The pore radii are plotted along the x-axis, and the pore volume of a given size is plotted along the y-axis (it is equal to the volume of mercury that penetrated into the sample).

Curve-1 is typical for materials with a large volume of large voids (more than 10 µm). The dotted line shows the hysteresis curve. Curve-2 was obtained for a powder with a large volume of voids (4…6 µm) between grains. Curve -3 is typical for a material with fine porosity, and curve 4 is for a material with a homogeneous structure and pores of 0.02 ... 0.04 μm. Differential distribution curve of pore volume V by their size (see Figure-2, a)

Figure-2. a) Differential curve of the distribution of pores along the radii. b) Graph of the relationship between mercury pressure (in the poroser) and pore size.

dV/dr=fV(r), where dV/dr is the slope of the tangent to the integral curve. The area under the differential curve (shaded in Figure-2, a) is equal to the total pore volume per unit volume of the material. The specific surface of the pore space is determined using the average conditional pore radius or by adsorption methods (by adsorption of water vapor, nitrogen or other inert gas).

Specific surface area (cm² / g) is proportional to the mass of adsorbed water vapor (gas) required to cover the entire inner surface of the pores with a monomolecular layer (in 1 g per 1 g of dry material):

a \u003d a1 Na m1 / m2, where a1 is the surface covered by one adsorbed molecule, for a water molecule a1 \u003d 10.6 10 -16 cm²; Na-Avogadro number, Na \u003d 6.06 10 23; m1- mass and m2-molecular mass of adsorbed water vapor (gas). The properties of a building material are determined by its composition, structure and, above all, by the value and nature of porosity.

emptiness - the number of voids formed between the grains of loosely poured material (sand, crushed stone, and so on) or present in some products, for example, in hollow bricks, reinforced concrete panels. The voidness of sand and crushed stone is 35 ... 45%, hollow brick 15 ... 50%.

Hydrophysical properties of building materials

Hygroscopicity is the property of a capillary-porous material to absorb water vapor from humid air. The absorption of moisture from the air is due to the polymolecular adsorption of water vapor on the inner surface of the pores and capillary condensation. This physicochemical process is called sorption and is reversible. Wood, heat-insulating, wall and other porous materials have a developed inner surface of the pores and therefore a high sorption capacity.

Figure-3. Adsorption isotherm (at p>pa, rises steeply due to capillary condensation)

With an increase in water vapor pressure (i.e., with an increase in the relative humidity of the air at a constant temperature), the sorption moisture content of a given material increases (Fig. 3). According to the empirical Freundlich equation, the amount of adsorbed gas a=ℜp 1/n, where p is the pressure of the gas when equilibrium is reached; ℜ and n are empirical parameters that are constant for adsorbent and gas data at a certain temperature. In logarithmic coordinates, this equation is expressed by a straight line segment lga= lgℜ+ (1/n) lgp.
The curve expressing the dependence of the amount of adsorbed gas on pressure, after saturation of the inner surface of the pores, tends to a straight line parallel to the abscissa axis (point a in fig. 3).
A further increase in the hygroscopic moisture content of the material occurs due to capillary condensation. In narrow capillaries of a material that is well wetted by water (wood, brick, concrete, etc.), the meniscus will always be concave and the saturated vapor pressure below it will be lower than above a flat surface. As a result, vapor that has not reached saturation pressure with respect to a flat surface can be supersaturated with respect to the liquid phase in thin capillaries and will condense in them.

Due to the processes of adsorption and capillary condensation of water vapor from the atmosphere, the humidity of porous building materials, even after their long exposure to air, is quite high. So, the equilibrium moisture content of air-dry wood is 12 - 18%, wall materials 5 - 7% by weight. Humidification greatly increases the thermal conductivity of thermal insulation, therefore, they seek to prevent moisture by covering the insulation boards with a waterproofing film.

capillary suction water porous material occurs when part of the structure is in the water. So, groundwater can rise through the capillaries and moisten the lower part of the building wall. To avoid dampness in the room, a waterproofing layer is arranged that separates the foundation part of the wall structure from its above-ground part.

Capillary suction is characterized by the height of the rise of water in the material, the amount of absorbed water and the intensity of suction.

The height h of liquid rise in the capillary is determined by the Jurin formula: h=2σ cosθ/(rρg), where σ is the surface tension; θ-contact angle of wetting; r is the radius of the capillary; ρ is the density of the liquid; g is the free fall acceleration. Pores in concrete and other materials have an irregular shape and a changing cross section, so the above formula is only suitable for a qualitative consideration of the phenomenon; the suction height of water is determined using the method of "tagged atoms", or by changing the electrical conductivity of the material.

The volume of water absorbed by the material by capillary suction during time t, in the initial stage, obeys a parabolic law: V²=Kt, where K is the suction constant. A decrease in the suction intensity (that is, the K value) reflects an improvement in the structure of the material (for example, concrete) and an increase in its frost resistance .

Water absorption porous materials (concrete, brick, etc.) are determined according to the standard method, keeping the samples in water. The temperature of the water used must be 20 ± 2°C. Water absorption, determined by immersion of material samples in water, characterizes mainly open porosity, since water does not penetrate into closed pores. In addition, when samples are removed from the bath, water partially flows out of large pores, so water absorption is usually less than porosity. For example, the porosity of lightweight concrete can be 50 - 60%, and its water absorption is 20 - 30% by volume. Water absorption is determined by volume and mass.

Water absorption by volume W0 (%) - the degree of filling the volume of the material with water:

W0=((mb-mc)/Ve)100,

where mb is the mass of the material sample saturated with water, g; mc is the mass of the sample in a dry state, g; Ve- volume of material in its natural state, m ³.
Water absorption by mass Wm(%) is determined in relation to the mass of dry material: Wm=((mb-mc)/mc)100;

Dividing by members by W0/Wm, we get (%) W0=Wm ϒ, and the volumetric mass of dry material ϒ is expressed in relation to the density of water (dimensionless value). The water absorption of various materials varies widely: granite - 0.02 - 0.7%, heavy dense concrete - 2 - 4%, brick - 8 - 15%, porous heat-insulating materials - 100% or more. Water absorption by weight of highly porous materials can be greater than porosity, but water absorption by volume can never exceed porosity.

Water absorption is used to assess the structure of the material, using for this purpose the saturation coefficient of pores with water ℜn, equal to the ratio of water absorption by volume to porosity: ℜn=W0/P. The saturation coefficient can vary from 0 (all pores in the material are closed) to 1 (all pores are open), then W0=P.

A decrease in ℜn (with the same porosity) indicates a reduction in open porosity, which usually manifests itself in an increase in frost resistance.
Water absorption negatively affects the basic properties of the material: the bulk density increases, the material swells, its thermal conductivity increases, and strength and frost resistance decrease.

The softening coefficient ℜp is the ratio of the strength of the material saturated with water Rb to the strength of the dry material Rc.

ℜp=Rb/Rc.

The softening coefficient characterizes the water resistance of the material, it varies from 0 (soaking clays, etc.) to 1 (metals, etc.). Natural and artificial stone materials are not used in building structures in water if their softening coefficient is less than 0.8.

Water permeability - this is the property of the material to pass water under pressure. The filtration coefficient ℜf (m / h) characterizes the water permeability of the material: ℜf-Vb a/, where ℜf=Vb- the amount of water, m³, passing through the wall with area S=1m², thickness a=1m during the time t=1h with a difference in hydrostatic pressure at the boundaries of the wall р1-р2=1 m w.c. The filtration coefficient has the dimension of speed.

The water impermeability of a material (concrete) is characterized by a grade that indicates the one-sided hydrostatic pressure (in kgf / cm2), at which the concrete sample-cylinder does not pass water under the conditions of a standard test. There is a certain relationship between the filtration coefficient and the water resistance grade: the lower the coefficient, the higher the water resistance grade.

They struggle with water permeability in the construction of hydraulic structures, reservoirs, collectors, and in the construction of basement walls. They strive to use sufficiently dense materials with closed pores, arrange waterproofing layers, screens.

Moisture return - the ability of the material to release moisture. Materials, being in the air, retain their humidity only under the condition of a certain, so-called equilibrium relative humidity of the air. If the latter is below this equilibrium humidity, then the material begins to give off moisture to the environment (dry).

Secondly, the moisture loss is affected by the properties of the material itself, the nature of its porosity, the nature of the substance. Materials with large pores and hydrophobic materials release moisture more easily than hydrophilic and finely porous materials. Under natural conditions, the moisture loss of building materials is characterized by the intensity of moisture loss at a relative humidity of 60% and temperature 20°C.

Under natural conditions, the air always contains moisture. Therefore, the wet material is not completely dried under these conditions, but only to a moisture content called equilibrium. The state of the material in this case is air-dry. Wood in room conditions, where the relative humidity does not exceed 60%, has a moisture content of 8 ... 10%, the outer walls of buildings - 4 ... 6%. With a change in the relative humidity of the air, the moisture content of the materials also changes (if the latter are hydrophilic).

Air resistance

Air resistance is the ability of a material to withstand repeated systematic wetting and drying for a long time without significant deformations and loss of mechanical strength. Materials behave differently in relation to the action of variable humidity: they swell when moistened, shrink during subsequent drying, and sometimes warping of the material occurs.

Systematic moistening and drying cause alternating stresses in the material of building structures and eventually lead to their loss of bearing capacity (destruction). Concrete under such conditions is prone to destruction, since when dried, the cement stone shrinks, and the aggregate practically does not react.

As a result, tensile stresses arise in the cement stone, it shrinks and breaks away from the aggregate. Wood with a change in humidity is subjected to alternating deformations. It is possible to increase the air resistance of materials by introducing hydrophobic additives that give the material water-repellent properties.

Gas and vapor permeability. When a difference in gas pressure occurs near the surfaces of the fence, it moves through the pores and cracks of the material. Since the material has macro- and micropores, gas transfer can occur simultaneously by viscous and molecular flows, which obey the laws of Poiseuille and Knudsen, respectively.

The use of the Darcy-Poiseuille law at small pressure drops, when the change in gas density can be neglected, leads to a simplified formula for determining the mass of gas Vρ (density ρ) that has passed through a wall with area S and thickness a in time t, with a pressure difference on the faces of the wall Δp :

Vp=ℜgStΔp/a. From here it is possible to determine the coefficient of gas permeability [g/(m·h·Pa)]. ℜg= aVp/StΔp.

When determining the gas permeability coefficient, the volume of passing gas is brought to normal conditions.

The wall material must have a certain permeability. Then the wall will "breathe", i.e. through the outer walls will be natural ventilation, which is especially important for residential buildings in which there is no air conditioning. Therefore, the walls of residential buildings, hospitals, etc. are not finished with materials that retain water vapor.

On the contrary, walls and coverings are damp industrial premises needs to be protected from inside from water vapor penetration. In winter, inside warm rooms (textile factories, utilities, cowsheds, pigsties, etc.), 1 m³ of air contains much more water vapor than outside, so the steam tends to pass through the wall or coating.

Getting into the cold part of the fence, the steam condenses, sharply increasing the humidity in these places. Conditions are created that contribute to the rapid destruction of the material (lightweight concrete, brick) of the outer enclosing structure under the action of frost. Vapor barrier materials should be located on the side of the fence with the highest water vapor content in the air.

In some cases, almost complete gas impermeability is required; this applies to gas storage tanks, as well as to special structures, the interior of which must be protected from the penetration of contaminated air (for example, gas shelters). Vapor and gas permeability to a large extent depend on the structure of the material (bulk mass and porosity) (Table 3).

Table- 3. Relative values ​​of vapor and gas permeability (brick permeability is taken as 1)

Humidity deformations.

Porous inorganic and organic materials (concrete, wood, etc.) change their volume and dimensions when the humidity changes.

Shrinkage (shrinkage) is the reduction in the size of the material when it dries. It is caused by a decrease in the thickness of the water layers surrounding the particles of the material, and by the action of internal capillary forces tending to bring the particles of the material closer.

Swelling (swelling) occurs when the material is saturated with water. Polar water molecules, penetrating into the gaps between the particles or fibers that make up the material, sort of wedged them, while the hydration shells around the particles thicken, the internal menisci disappear, and with them the capillary forces.

The alternation of drying and moistening of a porous material, which is often encountered in practice, is accompanied by alternating shrinkage and swelling deformations. Such repeated cyclic impacts often cause the appearance of cracks, which accelerate the destruction. In similar conditions, there is concrete in road surfaces, in the outer parts of hydraulic structures.

Highly porous materials (wood, cellular concrete), capable of absorbing a lot of water, are characterized by high shrinkage:

Table-4. Shrinkage values ​​of some building materials

Shrinkage occurs and increases when water is removed from the material, which is in the hydrate shells of the particles and in small pores. Evaporation of water from large pores does not lead to the convergence of material particles and practically does not cause volumetric changes.

Frost resistance - the property of a water-saturated material to withstand alternate freezing and thawing without signs of destruction and a significant decrease in strength. Systematic observations have shown that many materials under conditions of alternate saturation with water and freezing are destroyed gradually.

Destruction occurs primarily due to the fact that water entering the pores of the material increases in volume by up to about 9% upon freezing. The greatest expansion of water upon transition to ice is observed at a temperature of -4°C. A further decrease in temperature does not cause an increase in the volume of ice. When the pores are filled with water and it freezes, the walls of the pores begin to experience significant stresses and can collapse.

Determination of the degree of frost resistance of the material is carried out by freezing samples saturated with water at a temperature of -15 to -17°C and their subsequent thawing. Such a low temperature of the experiment is accepted for the reason that water in thin capillaries freezes only at -10 °C. The frost resistance of the material depends on the density and degree of saturation of their pores with water. Dense materials are frost-resistant. Of the porous materials, only those that have mostly closed pores or water occupy less than 90% of the pore volume are frost-resistant.

The material is considered frost-resistant if, after a specified number of cycles of freezing and thawing in a state saturated with water, its strength has decreased by no more than 15%, and the loss in mass as a result of chipping has not exceeded 5%. If the samples after freezing do not have signs of destruction, then the degree of frost resistance is determined by the frost resistance coefficient: ℜf=Rf/Rb, where Rf is the compressive strength of the material after the frost resistance test, Pa; Rb is the compressive strength of the water-saturated material, Pa; For frost-resistant materials, ℜf should not be less than 0.75.

The frost resistance of the material is quantified by the frost resistance brand. For the brand of material in terms of frost resistance, the largest number of cycles of alternate freezing and thawing is taken, which the material samples can withstand without reducing the compressive strength by more than 15%; after testing, the samples should not have visible damage - cracks, chipping (mass loss - no more than 5%). The durability of building materials in structures exposed to atmospheric factors and water depends on frost resistance.

The frost resistance grade is set by the project, taking into account the type of structure, its operating conditions and climate. Climatic conditions are characterized by the average monthly temperature of the coldest month and the number of cycles of alternate freezing and thawing according to long-term meteorological observations. Frost resistance grades are determined by the number of withstand cycles of alternate freezing and thawing of a particular material (frost resistance), for example, grades -F 10, 15, 25, 35, 50, 100, 150, 200 and more.

Lightweight concrete, bricks, ceramic stones for the exterior walls of buildings usually have frost resistance of Mrz 15, Mrz 25, Mrz 35. However, concrete used in the construction of bridges and roads must have a grade of Mrz 50, Mrz 100 and Mrz 200, and hydraulic concrete - up to Mrz 500.

Let us consider the reasons for the destruction of a porous material under the influence of the combined action of water and frost on it. For example, let's take the material that is in the building envelope. In autumn, the outer part of the wall freezes through. At this time, there is a migration (movement) of steam "from heat to cold", i.e., steam tends to the outside, since its pressure at a negative temperature is lower than at a positive one.

Figure-4. Temperature distribution in the outer wall of the building (a) and filling the pore with water (b) isolated near the facade face

1-adsorbed water; 2-pore mouth; 3- rain water; 4- condensate

For example, the vapor pressure at +20°C is 2.33 kPa, and at -10°C it is only 0.27 kPa. In an effort to go outside, water vapor enters the zone low temperatures and condenses in the pores near the outer edge of the wall. Thus, the pores of the outer freezing part of the wall become watered (Fig. 4), and water comes here both from the outside (rain with wind) and from the inside (water vapor migration).

With the onset of even slight frosts (from -5 to -8 ° C), water in large pores freezes and, when it turns into ice, increases in volume by 9% (ice density 0.918). If the water saturation coefficient of at least some of the pores approaches 1, then large tensile stresses will arise in the walls of the pores. Destruction usually begins in the form of “peeling” of the concrete surface, then it spreads in depth.

Exposure of concrete to alternate freezing and thawing is similar to repeated exposure to repeated tensile loading, causing material fatigue.

The frost resistance test of the material in the laboratory is carried out on samples of the established shape and size (concrete cubes, bricks, etc.). Samples are saturated with water before testing. After that, the water-saturated samples are frozen in a refrigerator at -15 to -20°C to freeze the water in the fine pores. The samples removed from the cold chamber are thawed in water at a temperature of 15 - 20°C, which ensures the water-saturated state of the samples.

There is also an accelerated test method, according to which the samples are immersed in a saturated solution of sodium sulfate and then dried at a temperature of 100 ... 110 ° C. The crystals of decahydrate sulfate formed in this case in the pores of the stone (with a significant increase in volume) press on the walls of the pores even more than water during freezing. This test is particularly harsh. One cycle of testing in a solution of sodium sulfate is equivalent to 5 ... 10 and even 20 cycles of direct freezing tests.

Figure-5. Curve of the change in the strength of concrete during alternate freezing and thawing

To assess frost resistance, physical control methods are increasingly being used, and above all, the pulsed ultrasonic method. With its help, you can trace the change in the strength or elasticity modulus of concrete in the process of cyclic freezing (Fig. 5) and determine the brand of concrete by frost resistance in freezing and thawing cycles, the number of which corresponds to the allowable reduction in strength (ΔR) or modulus of elasticity (ΔE).

Thermal properties of building materials

Thermal conductivity is the property of a material to transfer heat from one surface to another. This property is the main one both for a large group of heat-insulating materials and for materials used for the construction of external walls and building coverings.

Figure-6. Dependence of thermal conductivity of inorganic materials on bulk density

1-dry materials; 2 and 3 - air-dry materials with different humidity; 4-materials saturated with water.

The heat flow passes through the solid "framework" and air cells of the porous material. The thermal conductivity of air [λ = 0.023 W / (m ° C)] is less than that of the solid substance that makes up the "frame" of the building material. Therefore, increasing the porosity of the material is the main way to reduce thermal conductivity. They strive to create small closed pores in the material in order to reduce the amount of heat transferred by convection and radiation.

In practice, it is convenient to judge the thermal conductivity by the volumetric mass of the material (Fig. 6). The formula of V.P. Nekrasov is known, which relates the thermal conductivity λ [W / (m ° C)] with the volumetric mass of stone material λ about, expressed in relation to water: λ = 1.16√ (0.0196 + 0.22ϒ²ob- 0.16). The exact value of λ is determined experimentally for a given material.

Moisture entering the pores of the material increases its thermal conductivity, since the thermal conductivity of water (0.58 W / (m ° C) is 25 times greater than the thermal conductivity of air.
Freezing of water in pores with the formation of ice increases λ even more, since the coefficient of thermal conductivity of frost is 0.1, and ice is 2.3 W / (m ° C), i.e. 4 times more than water. With increasing temperature, the thermal conductivity of most materials increases, and only a few (metals, magnesite refractories) decrease it.

Heat capacity

Heat capacity is characterized by specific heat capacity, s [J / (kg ° C)], which is determined by the amount of heat that must be imparted to 1 kg of a given material in order to increase its temperature by 1 ° C.

c=Q/, where Q is the amount of heat spent on heating the material from t1 to t2, J; m is the mass of the material, kg.

The heat capacity of inorganic building materials (concrete, brick, natural stone materials) varies from 0.75 to 0.92 kJ/(kg °C). The heat capacity of dry organic materials (for example, wood) is about 0.7 kJ / (kg ° C), water has the highest heat capacity - 1 kJ / (kg ° C), therefore, with an increase in the moisture content of materials, their heat capacity increases. Heat capacity indicators of different materials are needed for thermal engineering calculations. The heat capacity of the material is important in cases where it is necessary to take into account the accumulation of heat, for example, when calculating the heat resistance of walls and ceilings of heated buildings, in order to maintain the temperature in the room without sharp fluctuations when the thermal regime changes, when calculating the heating of the material for winter concrete work, when calculating furnaces and other structures.

fire resistance - the property of the material to withstand prolonged exposure to high temperatures (from 1580 ° C and above), without softening or deforming. Refractory materials are used for the internal lining of industrial furnaces.

Refractory materials soften at temperatures above 1350°C.

Fire resistance - the property of a material to resist the action of fire during a fire for a certain time. It depends on the combustibility of the material, that is, on its ability to ignite and burn.

Fireproof materials are concrete, brick, steel, etc. However, it must be taken into account that some fireproof materials crack (granite) or strongly deform (metals) during a fire at temperatures starting from 600 ° C. Therefore, structures made of such materials often have to be protected with more fire-resistant materials.

Slow-burning materials smolder under the influence of fire or high temperature, but after the cessation of the fire, their burning and smoldering stops (asphalt concrete, wood impregnated with fire retardants, fiberboard, some foam plastic).

Combustible organic materials that burn open flames must be protected from ignition. Widely used constructive measures that exclude the direct impact of fire on the material in a fire. Apply protective substances - flame retardants.

The coefficient of linear thermal expansion of concrete and steel is 10·10 -6 °С -1, granite - 10·10 -6 °С -1, wood - 20·10 -6 °С -1. With a seasonal change in the temperature of the environment and material by 50 ° C, the relative temperature deformation reaches 0.5-10 -3 or 1 10 -3, i.e. 0.5 - 1 mm / m. To avoid cracking, long structures are cut with expansion joints.

fire resistance - the ability of the material to withstand high temperatures without loss of bearing capacity (large reduction in strength and significant deformations).
This property is important in fires, and since water is used in the process of extinguishing fires, when assessing the degree of fire resistance of a material, the action of high temperature is combined with the action of water.
Construction Materials According to fire resistance, they are divided into fireproof, slow-burning and combustible. Fireproof materials under the influence of high temperature or fire do not smolder or char (natural and artificial inorganic materials, metals). However, some of these materials do not crack or deform under the influence of high temperature, for example, ceramic bricks, while others, in particular steel, are subject to significant deformations. Therefore, steel structures cannot be classified as fire resistant. Slow-burning materials under the influence of fire or high temperatures char, smolder or hardly ignite, but continue to burn or smolder only in the presence of fire (wood impregnated with flame retardants). Combustible materials burn and smolder when exposed to fire or high temperatures and continue to burn after the fire is removed (all organic materials that have not been impregnated with flame retardants).

Thermal resistance material is characterized by its ability to withstand a certain number of cycles of sudden thermal changes without destruction. Thermal resistance depends on the degree of homogeneity of the material, the temperature coefficient of expansion of its constituent parts. The lower the coefficient of thermal expansion, the higher the thermal resistance of the material. Thermally unstable materials include glass, granite.
Radiation resistance - the property of a material to retain its structure and physical and mechanical characteristics after exposure to ionizing radiation. The development of nuclear energy and the widespread use of ionizing radiation sources in various sectors of the national economy make it necessary to assess the radiation resistance and protective properties of materials.

Radiation levels around modern sources of ionizing radiation are so high that a profound change in the structure of the material can occur. The flow of radioactive radiation when it encounters structures made of this material can be absorbed to varying degrees depending on the thickness of the fence, the type of radiation and the nature of the shielding substance.

To protect against a neutron flux, materials containing a large amount of bound water are used; from y-radiation - materials with a high density (lead, especially heavy concrete). Bound water is contained in hydrated concrete, limonite ore (hydrous iron oxide), etc. The intensity of neutron radiation penetration through concrete can be reduced by introducing special additives (boron, cadmium, lithium) into it.

Chemical resistance - the ability of the material to resist the effects of acids, alkalis, solutions of salts and gases.
The most frequently exposed to aggressive liquids and gases are sanitary facilities, sewer pipes, livestock buildings, hydraulic structures (located in sea water, which has a large amount of dissolved salts).

Not able to resist the action of even weak acids carbonate natural stone materials - limestone, marble and dolomite; bitumen is not resistant to the action of concentrated solutions of alkalis. The most resistant materials with respect to the action of acids and alkalis are ceramic materials and products, as well as many products based on plastics.

Durability - the ability of the material to resist the complex action of atmospheric and other factors under operating conditions. Such factors can be: a change in temperature and humidity, the action of various gases in the air, or solutions of salts in the water, the combined effect of water and frost, sunlight.

In this case, the loss of mechanical properties of the material can occur as a result of a violation of the continuity of the structure (formation of cracks), exchange reactions with substances of the external environment, and also
as a result of changes in the states of matter (changes in the crystal lattice, recrystallization, transition from amorphous
to a crystalline state). The process of gradual change (deterioration) in the properties of materials under operating conditions is sometimes called aging.
The durability and chemical resistance of materials are directly related to the cost of operating buildings and structures. Increasing the durability and chemical resistance of building materials is the most urgent task in technical and economic terms.