Materials based on organic binders include polymer concrete, polymer cement concrete, wood-based products, tar-based and bitumen cement-based concrete.
10.1 Asphalt concretes and mortars
Asphalt concrete is an artificial building material obtained as a result of the hardening of compacted asphalt concrete mass consisting of thoroughly mixed asphalt binder, crushed stone (gravel) and sand. Asphalt without coarse aggregate called asphalt mortar .
Asphalt binder is a mixture of oil road bitumen with finely ground mineral powders (limestone, dolomite, chalk, asbestos, slag).
Bitumens are organic binders of amorphous structure, which include high-molecular-weight hydrocarbons and their derivatives. Bitumens are natural and artificial. Artificial bitumens are obtained by refining crude oil. Depending on the production technology, there may be: residual, obtained from tar (residue after the distillation of oil fractions from fuel oil) by further selection of oils from it; oxidized, obtained by the oxidation of tar in special devices (air blowing); cracking, obtained by processing residues resulting from the cracking of oil.
The chemical composition of bitumen is very complicated: carbon 70 ... 80%, hydrogen 10 ... 15%, sulfur 2 ... 9%, oxygen 1 ... 5%, nitrogen less than 2%. These elements are in bitumen in the form of hydrocarbons and their compounds with sulfur, oxygen and nitrogen. The whole variety of compounds forming bitumen can be reduced to three groups: the solid part (asphaltenes) with a molecular weight of 1000 ... 5000, a density of more than 1, resins with a molecular weight of 500 ... 1000, a density of about 1, and oils with a molecular weight of 100 ... 500, a density of less than one.
The groups of hydrocarbons, being a part of bitumens in various ratios and forming a complex dispersed system, predetermine their structure and properties. If there is an excess of dispersed medium in the dispersed system, then the complex particles - micelles do not contact each other, moving freely. This structure is characteristic of liquid bitumen at normal temperature and for viscous bitumen at elevated temperatures. When a large number of micelles are in contact with each other, forming a micellar spatial grid. Such a structure is characterized by high viscosity and hardness at high temperature. Having an amorphous structure, bitumens, in contrast to crystalline materials, do not have a specific melting point. The gradual transition from solid to viscous is reversible and occurs without changing the basic properties, therefore, bitumens belong to thermoplastic organic materials. The technology of obtaining materials and products with their use (solidification by a thermoplastic mechanism) is based on this property.
The chemical composition determines the properties of bitumen. Bitumens are hydrophobic (they are not wetted by water), they are water resistant, their porosity is practically zero, therefore they are waterproof and frost-resistant. These properties make it possible to widely use bitumens in the production of waterproofing and roofing materials. Since bitumens are absolutely dense materials, their average and true densities are numerically equal and fluctuate depending on the group composition from 800 to 1300 kg / m3. Thermal conductivity is characteristic of amorphous substances and is 0.5 ... 0.6 W / (m · ° C); heat capacity is 1.8 ... 1.97 kJ / kg · ° C.
Technological properties of bitumen, as a dispersed system, are determined by the ratio of its constituent parts: oils, resins and asphaltenes. There are three main indicators: the depth of needle penetration (viscosity, penetration), softening temperature and tensile properties (ductility), which are interrelated. Increasing the content of asphaltenes and resins leads to an increase in hardness, softening temperature and brittleness (low elongation) of bitumen.
Over time, during storage and under operating conditions under the action of sunlight and oxygen, the composition and properties of bitumens change: they increase the relative content of solid and fragile components and, accordingly, the amount of oily and resinous fractions decreases, and therefore brittleness and hardness increase (process aging). Materials based on bitumen cannot be used in conditions of exposure to hot water and liquid organic media (oil, solvents, petroleum products). The disadvantages of bitumen are also their low heat, frost and radiation resistance. However, the availability and relatively low cost of bitumen provided them with widespread use in construction.
In order to increase elasticity, heat resistance, mechanical strength, adhesion strength, polymer additives, surfactants are introduced into bitumens.
Mineral aggregate plays an active role in asphalt binder. Its surface is a substrate on which a monomolecular layer of bitumen is adsorbed. This layer is very dense and durable due to its structuredness. By itself, bitumen hardens by the mechanism of thermoplasticity, however, as we have already noted, this mechanism does not provide the heat resistance of the resulting solid. The introduction of the filler includes an adsorption mechanism for the formation of a solid phase state. It is he who determines the hardness and strength of asphalt concrete and the high temperature of its softening. Without filler, concrete would have plasticity, especially in heat, and would not cope with power loads.
Fine-milled natural rocks (limestone, dolomite, chalk, asbestos), as well as man-made products (slags, ashes) are used as a mineral powder (filler).
The greatest strength of the asphalt binder provides the optimum ratio of components, in which all the bitumen is adsorbed in the form of thin continuous films on the surface of the filler particles.
Fine aggregate in mortar and concrete are pure natural and artificial sands with a content of silty-clay particles of not more than 3% by mass. Crushed stone or gravel is used from strong and cold-resistant rocks, as well as from metallurgical slags. Carbonate rocks (limestone, dolomite), which adhere well to bitumen (asphalt binder), are preferred from sedimentary rocks. By the form of coarse aggregate, asphalt concrete is divided into crushed stone and gravel.
The basic properties of asphalt concrete depend on the used asphalt binder, composition and porosity. Due to bitumen, the consumption of which is 4.5 ... 10% by weight, asphalt concrete has hydrophobicity and chemical resistance to aggressive substances that cause corrosion of cement concrete, metals and other building materials.
Due to this, asphalt concretes and mortars are better than cement, resist corrosion.
However, they are characterized by solubility in organic solvents, increased deformability, the ability to soften when heated to complete melting, susceptibility to aging under the action of sunlight and oxygen from the air due to an increase in the amount of hard brittle components due to a decrease in the content of tarry substances and oils - a slow change in the composition and properties, accompanied by an increase in brittleness and a decrease in hydrophobicity.
The composition of asphalt concrete is selected on the basis of the condition of optimal structure: the asphalt solution must fill the voids in the rubble with a slight (10 ... 15%) excess, and the voids in the sand were completely filled with asphalt binder with an excess (10 ... 15%) for enveloping sand grains.
The quality of asphalt concrete pavement is assessed by strength, wear resistance and water resistance. Technical properties of asphalt concrete vary considerably with temperature. At ordinary temperature (20 ... 25 ° С) it has elastic-plastic properties, at elevated viscous-plastic properties, and at low temperatures it becomes brittle. In this regard, testing of mechanical strength is carried out at temperatures of 0, 20, 50 ° C at a constant feed rate load. Depending on the temperature and grade of bitumen used, the strength of asphalt concrete to bend, respectively, is 1.0 ... 1.2; 2.5 ... 3 and 10 ... 15 MPa.
A distinctive feature of asphalt concrete is its ability to viscous resistance to impact and wear. It has been established that under the conditions of urban transport, the wear is 0.2 ... 1.5 mm per year. Since asphalt concrete is sensitive to fluctuations in the temperature of the external environment, structural changes constantly occur in it, leading to the destruction of the coating. Particularly intense destructive processes occur with a sharp change in temperature. This process is accelerated by the action of water and the aging of the organic binder itself.
Depending on the grade of bitumen used and the laying temperature, the concrete is divided into:
- hot (80 ... 110 ° С) on viscous oil bitumens of the brands BND-90/130, BND-60/90 and
BND-40/60;
- warm (50 ... 100 ° С) on bitumen of low viscosity BND-200/300 and BND-130/200 or liquid bitumen BG-70/130;
- cold (not lower than 5 ° С) on liquid bitumens of the SG-70/130 brands.
Hardening of cold asphalt concrete is based on thickening of liquid bitumen due to evaporation of volatile fractions, oxidation and other processes that last 20 ... 30 days. Cold concretes are simpler and cheaper to manufacture and easier to install, especially in wet and cold weather. Can be stored in the warehouse for 6 ... 8 months. Therefore, they are made in the winter and laid in the summer. Cold concretes are only fine-grained or sandy.
The technology for preparing asphalt concrete mix for hot asphalt provides for preheating bitumen to 150 ... 170 ° C (for converting to a liquid state), aggregates and mineral powder to 180 ... 200 ° C (firstly, to remove moisture, which degrades the adhesion of bitumen to mineral partly, and secondly, the components must be hot, so that the bitumen does not quickly cool down while mixing) and thoroughly mixing them in the mixer. For cold concrete, all components are heated to 110 ... 120 ° C. Sometimes it is prepared on bitumen emulsion, mixing binders and fillers without heating.
The prepared asphalt concrete mixture is transported by dump trucks and at the place of laying it is loaded into an asphalt paver, which evenly distributes 15 ... 20% more of the project thickness over the prepared base. After laying the mixture is compacted with rollers with a mass of 5 ... 14 tons or with vibrating motor rollers with a mass of 0.5 ... 4.5 tons. In the premises, the mixture is compacted with pad vibrators. Hot and warm concretes harden, gaining density and strength, after 1 ... 2 h, after cooling. Cold asphalt mix is cooled to 60 ° C, transported to the site and laid. hardens the mixture under the action of vehicles.
Molded asphalt concrete is also used in construction practice (in cramped conditions or with small amounts of work). It is compacted in the hot state with irons, special rollers, light rollers (0.5 ... 1.5 tonnes) or does not condense at all.
Asphalt concrete and mortars are the most important materials for construction of road and airfield pavements, floors in industrial plants and warehouses, irrigation canals, flat roofs, screeds.
10.2 Polymer concrete
Polymer concrete is a cementless and anhydrous concrete, in which a binder from a liquid oligomer (polymer with a low molecular weight), a hardener and fine ground mineral filler is used instead of a mineral binder.
Polymers are high-molecular compounds whose molecules consist of several thousand or even hundreds of thousands of atoms. Artificial polymers are obtained by synthesis reactions from relatively simple in chemical composition of substances - monomers . Most often, macromolecules of such compounds are constructed by repeated repetition of certain structural units. The degree of polymerization refers to the number of structural units contained in a single macromolecule.
The starting materials for the production of polymers are natural gas and the so-called "associated" gas that accompanies oil outlets. In the gaseous products of oil refining contains ethylene, propylene and other gases that are processed at enterprises in polymers. Coal tar obtained by coking coal and containing phenol and other components also serves as raw material for polymers. In the production of synthetic materials, nitrogen and oxygen obtained from air, water and a number of other common substances are also used.
The internal structure distinguishes linear ones, which consist of long filamentous macromolecules linked together by weak intermolecular (Van-drevalsovymi) and hydrogen bonds (synthetic rubber), and spatial, in which strong covalent bonds between chains have the strength of the same order as the strength links within the chain and lead to the formation of a single spatial framework. Spatial structures are much worse deformed than the structures of linear molecules. When a solid spatial structure is formed, the polymer acquires the properties of a rigid elastic body.
According to the behavior of the heated polymers are divided into thermoplastic and thermosetting. Thermoplastic (thermoplastics) are polymers that are able to soften reversibly when heated and harden when cooled, while retaining their basic properties. These are linear polymers, since the intermolecular forces and hydrogen bonds between their chains are overcome with a relatively moderate increase in temperature. Thermosets (or thermosets) are polymers that, when cured, do not become plastic when heated. These are spatial polymers for which the breaking of bonds by thermal motion requires a high temperature, which can cause a break in the bonds not only between the chains, but also inside the chains, which leads to destruction (chemical decomposition) of the polymer. Decomposition products light up. Such a process is irreversible. Thermosetting polymers behave like wood when the temperature rises: they undergo degradation and light up when high-temperature heating occurs.
Thermosetting spatial polymers (phenolic-formaldehyde, polyester, furan, epoxy resins) serve as astringent for polymer concrete. The choice of synthetic binder depends on the operating conditions of the polymer and the types of aggressive media.
The hardener (initiator of the polymerization reaction) for phenolic and furan resins can be paratoluenesulfonyl chloride and benzenesulfonic acid, for epoxy resins - amines and polyamides, for polyesters - hydroperoxide and cobalt naphthenate. The optimal amount of hardener is chosen depending on the temperature of the medium, the type and amount of resin in the mixture, etc.
In addition, the quality of the binder is enhanced by the introduction of plasticizers (for epoxy resins, low molecular weight thiokols and polyamides, which are chemically bound to the resin, as well as divinyl nitrile rubber SKN-26).
Acid-resistant materials are used as fillers : quartz sand, andesite, diabase, basalt, granite and other silicate and aluminosilicate rocks, introduced in the form of ground powder with a particle size of 0.01 ... 0.1 mm. To reduce the fragility of polymer used fiber fillers - asbestos, fiberglass, etc. Mineral filler reduces polymer consumption and improves the properties of polymer concrete (as for bitumen).
Fillers are selected depending on the type of aggressive environment. The filler is sand, crushed stone or gravel from the same rocks as the filler. You can also use crushed battle of acid-resistant brick, quartzite, granite, coke, ceramics, expanded clay, anthracite, graphite. Carbonate aggregates (limestone, dolomite and portland cement) are permissible only for epoxy resins in stable environments. With acidic hardeners, the latter are neutralized, and the mixture does not harden.
Introduction to the composition of the valve allows to obtain high-strength armopolimerbeton. Depending on the material of the reinforcement, there are distinguished steel polymer concrete (steel reinforcement) and glass polymer concrete (fiberglass reinforcement). The reinforcement can be in the form of rods, wires or individual fibers, evenly distributed throughout the volume (dispersed reinforcement). If dispersed reinforcement is used in the polymer concrete, the concrete is called fibropolymer concrete. As disperse reinforcement used short thin filaments and fibers (fibers) of asbestos, metal, glass, rocks and polymers (synthetic fiber).
Polymer concrete has high mechanical strength (Rсж = 60 ... 120 MPa, Rр = 7 ... 20 MPa, Rizg = 16 ... 40 MPa). Strength characteristics are determined by the properties of the components and adhesive forces of adhesion of resins and fillers. Frost resistance of polymer concrete F200 ... F300; heat resistance - 100 ... 200 ° С. The main property of polymer concrete is high chemical resistance in both acidic and alkaline environments. Кроме того, полимербетоны обладают высокой плотностью, износостойкостью, водостойкостью, беспыльностью, гигиеничностью и отличной адгезией к другим материалам.
Наряду с этим полимербетоны характеризуются повышенной деформативностью (ползучестью) и невысокой термостойкостью. Отрицательным свойством полимербетонов является их старение, усиливающееся при действии попеременного нагревания и охлаждения. Кроме того, необходимо соблюдение специальных правил охраны труда при работе с полимерами и кислыми отвердителями, могущими вызвать ожоги. В частности, необходима хорошая вентиляция, обеспечение рабочих защитными очками, резиновыми рукавицами, спецодеждой.
Полимербетоны получают путем интенсивного перемешивания в бетоносмесителе подогретых заполнителей, которые должны быть чистыми и сухими, полимерной смолы и добавок. Расход связующего составляет 100…200 кг на 1 м3 полимербетона при соотношении к наполнителю 1:5-1:12 по массе. Недопустимо попадание воды в смесь и на свежеуложенный бетон.
Полученную массу помещают в форму и уплотняют штыкованием, вибрированием или прессованием. Твердеют полимербетоны при нормальной температуре не менее 3…7 суток, а при температуре 40-80 °С - 6…12 ч.
Стоимость полимербетонов в несколько раз выше цементных бетонов, поэтому их применяют там, где высокая стоимость полимербетонов будет оправдана: при строительстве цехов химической, пищевой, целлюлозно-бумажной промышленности, где требуется обеспечить коррозионной стойкость несущих и самонесущих конструкций, подвергающихся интенсивному износу и химическому воздействию (трубы для жидкостей с абразивными частицами, сборные емкости под высокоагрессивные жидкости, плиты для химически стойких полов, тюбинги, колонны, плиты перекрытий и т.п.). Срок службы таких конструкций из обычного бетона весьма невелик, а при использовании полимербетона может быть значительно увеличен.
Полимербетоны (полимеррастворы) хорошо склеиваются с цементным бетоном, поэтому его применяют для ремонта железобетонных конструкций. Полимербетоны применяют для защиты от радиации в ядерной технике и атомной промышленности. Защитные свойства полимерных соединений с повышенным содержанием водорода особенно эффективны против нейтронного потока. В этом отношении представляют значительный интерес баритобетон, обладающий высокими защитными свойствами против нейтронного и гамма-излучения.
10.3. Полимерцементные бетоны
Полимерцементные бетоны получают, добавляя полимер непосредственно в бетонную или растворную смесь. Полимерные добавки вступают в активное взаимодействие с цементом или продуктами реакции его с водой и участвуют в образовании структуры цементного камня и бетона, заметно улучшая его свойства - прочность на изгиб повышается в 1,5…3 раза, прочность при сжатии на 20…50 %. Полимер по мере удаления свободной воды из системы образует тонкую пленку, равномерно распределенную по поверхности зерен цемента и заполнителя. Последняя, обладая хорошими адгезионными свойствами, улучшает сцепление цементного камня с заполнителем и цементных зерен между собой; она повышает монолитность бетона и придает ему специфические свойства по сравнению с обычным бетоном.
Основными составляющими для полимерцементных бетонов служат поливинилацетатная и акриловая эмульсия, латексы синтетических каучуков, а также кремнийорганические полимеры, фуриловый спирт с солянокислым анилином и др. Эти полимеры, вводимые в бетонную смесь или растворы, действуют как поверхностно-активные вещества. Некоторые из них увеличивают пластичность, уменьшают водосодержание бетонной и растворной смеси и снижают ее водопотребность.
Количество полимерной добавки от 1 до 30 % от массы цемента в зависимости от вида полимера и целей модификации бетона или раствора.
Concrete with additives of water-soluble resins have many positive properties, the most valuable of which is to increase the elasticity without reducing the strength and a significant increase in deformability under compression. However, they have an increased sensitivity to temperature fluctuations in structures.
Polymer cement concretes are recommended for use in concrete and reinforced concrete structures, to which they impose requirements of low permeability and high resistance to impact, dynamic effects, axial tensile strength, frost resistance and corrosion resistance. These are floorings of industrial buildings, runways of airfields, exterior decoration of brick and concrete surfaces, water tanks and oil products.
10.4. Concrete
Concrete is a concrete, impregnated after hardening with monomers or liquid oligomers, which, after appropriate processing, go into solid polymers that fill the pores of the concrete. As a result, concrete strength (Rсж = 80 ... 120 MPa), its frost resistance and corrosion resistance increase by more than 2 times. Concrete is practically waterproof. To obtain a concrete polymer, mainly styrene and methyl methacrylate are used, polymerizing in concrete, respectively, into polystyrene and polymethyl methacrylate.
Impregnation to a depth of 3 cm is carried out in special sealed chambers under pressure.
A significant disadvantage of concrete polymer is a significant complication of the concrete technology: hardened concrete product must be dried before impregnation, it is impregnated under vacuum. In addition, working with monomers requires careful adherence to safety regulations.
10.5. Wood based products
Plywood is a sheet material, glued together from three or more layers of peeled veneer, arranged so that the fibers of adjacent veneer sheets are mutually perpendicular. Plywood is also produced with the direction of the veneer fibers in the adjacent layers at an angle of 30 °, 45 ° or 60 °. According to the number of veneer layers, three-layer, five-layer and multi-layer plywood are distinguished from 1.5 to 18 mm thick and sheet sizes up to 24001525 mm.
Glue plywood is made of birch , beech, alder, maple, ash, oak, pine, spruce, cedar, larch. Packages of veneer, recruited according to a predetermined pattern from sheets, which are smeared with glue in a certain way, enter a hydraulic press, the plates of which are heated by steam. Curing of polymer glue takes place at a temperature of 120 ... 160 ° C and a specific pressing pressure of 1.4 ... 2 MPa for 20 ... 30 minutes.
Plywood, veneered with veneer. Plywood used for interior decoration, furniture production, etc., unlike general purpose plywood, is veneered over the outer layer or both layers with a veneer made of precious wood with a decorative texture (oak, ash, beech, mahogany, walnut, ilma, karagach, larch, yew).
Wood-laminated plastics are sheets or plates made of peeled birch veneer, impregnated and glued in the heat treatment process under high pressure with a rezol phenol-formaldehyde polymer. They differ from plywood with a higher density (1.25-1.33 g / cm3) and have high mechanical properties: tensile strength along the fibers of the "shirt" 140-260 MPa, bending 150-280 MPa, specific impact strength 3-8 MPa. These plastics are resistant to the action of oils, solvents, detergents.
Wood laminates are manufactured in the form of sheets from 1 to 12 mm thick and up to 1500–1500 mm in size, or plates from 15 to 60 mm thick and up to 1500–1500 mm in size.
Applied in building structures that require chemical resistance, non-magnetic, high abrasion resistance.
Chipboards ( Chipboards ) are obtained by mixing machine chips, crushed wood waste (shredded), sawdust, and sometimes specially made thin chips (all from soft hardwood), with urea-formaldehyde or phenol-formaldehyde resin. The polymer consumption is 8 ... 12% of the total mass. In order to impart bio-resistance and fire resistance to wood chipboards, hydrophobicity, antiseptics, flame retardants or hydrophobic substances are introduced into the binder or chips.
According to the method of manufacture, flat pressing plates are distinguished, in which the wood particles are arranged parallel to the front surfaces of the plate, and extrusion (by squeezing out of the press die) molding with wood particles predominantly perpendicular to these surfaces. Molding occurs at elevated temperatures.
As a decorative finish that protects the plates from moisture and abrasion, use polymer film materials, paper, resin-impregnated. Often, the surface of the plates (pre-polished) is covered with waterproof phenolic or epoxy paints.
Depending on their structure, methods of molding, the density of chipboard of flat pressing can vary from 550 to 820 kg / m3, and their water resistance is directly related to the type of binder used and wood species of chip particles.
Flat pressing plates of medium and high density are used in furniture production, for the manufacture of building panels and structures, temporary structures, roofs, window sills and other structural bearing elements, as well as as a finishing and facing material. Slabs of low density are heat and sound insulating material.
Extrusion particle boards are lined with decorative paper, peeled or sliced veneer, which increases their strength by 15 ... 29 times. The density of extrusion plates ranges from 350 to 650 kg / m3 with a limit of strength during static bending from 5 to 10 MPa. Plates of this type are used for the manufacture of non-critical building parts and as a material for the manufacture of panel doors and partitions.
Fiber boards are manufactured by hot pressing at a temperature
pe 150… 165 ° C under pressure of 1..5 MPa pulp consisting of specially prepared wood fibers, water, fillers, polymer and special additives (antiseptics, fire retardants, water-repellent substances).
Wood fibers are obtained from waste woodworking industries and non-wood. Wood on chipping machines is processed into chips, which is boiled in 1-2% solution of caustic soda to neutralize resinous and sugary substances. Then the chips are crushed in defibrators and other machines to the state of fine fibers. After additional steam treatment (at a temperature of 150 ° C and a pressure of 0.6-1 MPa), the fibers are mixed with water and these additives. In the manufacture of superhard plates, phenol-formaldehyde polymer is introduced into the mixture. The prepared mass is transferred to the casting machine, which has an endless metal grid and a vacuum unit. Here, the mass is dehydrated, compacted, cut into plates, which are sent to a roller dryer if highly porous insulating plates are formed. To obtain solid plates, it is necessary to press the mass, which is carried out on hydraulic high-rise presses at a temperature of 150-165 ° C under a pressure of 1-5 MPa.
Solid plates are used for the device partitions, filing ceilings, flooring, for the manufacture of door panels and built-in furniture. Finishing plates are lined with synthetic film with a pad of textured paper in the color and texture of fine wood. They are also produced with a matte surface, painted with water-based polyvinyl acetate paints. Such plates serve as facing of walls and ceilings. The plates painted with enamels have a glossy surface and they are more water resistant. These plates are used for wall cladding in medical institutions, grocery stores, etc. Insulating fibreboards are widely used in the form of heat and sound insulation material. In construction practice, finishing (decorative) and heat and sound insulating fibreboards are the most common.
Wooden glued constructions are large-sized construction elements made by gluing together with waterproof high-strength polymeric adhesives (resorcin glues for gluing wood and epoxy glues for gluing wood with metals) of separate substandard scraps of high-quality spruce or pine wood followed by temperature treatment for curing.
Production of glued wooden structures began in the 50s. Combining wood with other materials, they make both bearing and enclosing glued structures of buildings and structures, different in shape and purpose. Bearings include beams, columns, arches, pillars, trusses of large sections, large lengths, measured in tens of meters (up to 60 or more), cylindrical vaults, shells and spherical domes. Of these elements are made of large spans. Three-layer panels, sheathed with plywood, chipboard or fibreboard, with a middle layer of foam, foamed directly into the cavity of the product, are also glued. Wall panels consisting of a glued framework, to which flat asbestos cement sheets are attached on one or both sides, are also widely used in construction practice.
The use of glued structures, successfully competing with reinforced concrete and steel, is one of the most cost-effective ways to use wood in modern construction.
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