Concrete

CONCRETE, an artificial construction material, consisting of a binding substance and particles of stone, brick, etc. As a binding substance, portland cement and others (see Cement) are usually used, which has the appearance of a powder and when mixed with water forms a liquid mass of greater or lesser density. As stone-like additives, sand, gravel, crushed stone, slag, pumice, diatomaceous earth are used. In the preparation of concrete, stone-like additives of various sizes are usually used, for example, sand and gravel or sand and crushed stone. The particles of the binding substance, in the form of a liquid cement mass, fill the voids between smaller grains (in sand); this results in the so-called solution, which in turn fills the voids between larger particles (in gravel or crushed stone). When the binding substance hardens, an artificial stone is formed, which bears the name concrete. The composition of concrete is usually denoted by three numbers indicating the volumetric ratio of its constituent parts, with the volume of cement taken as unity; the first number refers to cement, the second to sand, and the third to gravel or crushed stone. For example, concrete of composition 1:2:4 contains one volume part of cement, two parts of sand, and four parts of gravel or crushed stone. The smaller the number of volumes of sand per 1 volume of cement and the fewer voids the sand used contains, the richer the concrete will be; the fewer voids in the gravel between its particles or the less gravel in relation to the volume of the solution, the denser the concrete; and conversely, with larger amounts of voids and stone-like parts, the concrete becomes leaner and more porous. Thus, by selecting stone-like additives and their proper dosage, concrete with very diverse properties in terms of strength, durability, thermal conductivity, air and water impermeability, fire resistance, etc. can be obtained. In construction, concrete is widely used and is used for foundations, walls, columns, floors, ceilings, etc. In addition, in combination with steel rods located in its thickness, concrete is used on a very large scale for the construction of various reinforced concrete structures, which, by proper use of the mechanical properties of both materials, i.e., concrete and iron, acquire the ability to resist tensile, bending and shearing stresses, whereas concrete without steel rods is suitable, mainly, only for resisting compressive stresses. In practice, concrete of composition 1:2:4 is often used for important parts of structures and composition 1:4:6 for less important ones. The mixing of constituent parts in the preparation of concrete is done manually or with the help of special machines called concrete mixers. Depending on the amount of water added, stiff, plastic, or fluid concrete is obtained. The degree of mixing, the purity of sand from clay and earthy additives, the consistency of dosage, the purity and quality of stone-like additives are of great importance for the strength and durability of concrete. To give concrete parts of a structure a specific external shape, wooden forms (formwork) are used, which are filled with concrete mass and then removed after it hardens. The stiff and plastic concrete mass, when filling forms, is tamped to obtain a denser and stronger concrete; the liquid concrete mass (fluid concrete) does not require tamping. During the hardening of concrete, a chemical process of binding part of the water occurs, while the remaining part evaporates, and in its place voids are formed; as a result, fluid concrete is usually more porous than tamped concrete; with increasing porosity, the strength of concrete decreases. Low temperature slows down and, within certain limits, prevents 'setting' (hardening of concrete). To accelerate setting, it is useful to moisten (water) concrete. At the beginning of hardening, the strength of concrete increases very rapidly; then the process of strength gain slows down, but continues for several years. The most rapid increase in strength occurs during the first 28 days; the strength reached by this time increases by about 50% after six months, and after 2-3 years, by about 100%, amounting to approximately 60-250 kg per sq. cm. The bulk weight of hardened concrete varies within very wide limits, depending on the weight of stone-like additives. For example, 1 cubic meter of concrete with natural sand and gravel weighs 2.2-2.3 tons; when using brick crushed stone instead of gravel, the weight of 1 cubic meter decreases to about 2 tons, and the weight of concrete with slag or pumice, depending on its porosity, drops to 1.5 and even to 1 ton (lean slag and pumice concretes). The thermal conductivity of concrete, on the one hand, depends on the bulk weight of stone-like additives, and on the other hand, on the coefficient of thermal conductivity of such. The coefficient of thermal conductivity of concrete is approximately proportional to its bulk weight, since lighter stone-like particles are usually less conductive. The coefficient of thermal conductivity of concrete with sand and gravel is 1.1-1.2; lean porous slag and pumice concretes, as well as concrete with diatomaceous earth (so-called warm concretes) have a coefficient of thermal conductivity of 0.4-0.5. With increasing humidity of concrete, its coefficient of thermal conductivity increases. Thus, warm concretes represent a material suitable, from the point of view of thermal insulation, for the construction of exterior walls in residential buildings, which can be made monolithic, i.e., as if entirely cast in the form, or from separate, pre-made concrete stones. The more porous the concrete, the greater its air permeability; this property also makes it very suitable from a sanitary point of view for the construction of walls in residential buildings, but with increasing air permeability, its water permeability also increases. Therefore, when it is necessary to obtain air and water impermeable concrete enclosing surfaces, it is advisable to use as rich and dense concrete as possible, or to cover less dense concrete surfaces with water-impermeable layers (rich cement plaster, plaster with so-called emulsions, etc.). Concrete is subject to the general laws of expansion and compression with temperature fluctuations. The coefficient of expansion of concrete approximately coincides with that of iron and is on average taken as 0.000010. To avoid cracks, large concrete structures have to be cut by so-called expansion, or thermal joints, within which concrete can undergo temperature deformations without noticeable disruption of the connection between individual particles. The abrasion resistance of concrete is relatively large; as a result, dust formation, for example, on concrete floors is quite significant; to prevent abrasion and dust formation, the surface of concrete floors should be covered with a layer of rich cement mortar (1 part cement and 1-2 parts not too fine sand with sharp grains), compacted during its laying with iron floats. An even stronger layer is obtained by adding iron sawdust or fine iron shavings to the cement mortar; this results in so-called Kleinlogele steel concrete, which has 8 times less abrasion than an ordinary cement floor. Concrete does not resist strong mineral acids and acetic acid. Weak solutions of these acids, especially sulfuric acid, act on it very destructively when constantly renewed. Free CO₂ acts similarly. Alkalis, hydrates, oxides of potassium, sodium and ammonia do not act on it. Salts act differently. Magnesium chloride and sulfate salts act destructively. The action of sea water is very complex and in some cases destructive. Bog water with its impurities of sulfuric acid and free CO₂ acts destructively when constantly renewed. Mineral and ground waters, not containing free CO₂ and salts of H₂SO₄, but even if saturated with other salts, do not act on concrete. Thus, the main agents of destruction turn out to be H₂SO₄, its salts and free CO₂. All those impurities and processes that lead to their formation are dangerous. Concrete is insensitive to tar and mineral oils; fatty oils have an unfavorable effect, but with a sufficiently dense surface, their action is negligible. Concrete is resistant to oil. Municipal, fecal and domestic waters are generally harmless to concrete, but in case of anaerobic decay and formation of H₂S and free CO₂, their action on concrete is destructive. Waters of many industrial enterprises, containing the above-mentioned destructive compounds, act harmfully even in weak solutions when constantly renewed. Sulfur gases contained in combustion products and sewer gases cause destruction of concrete. High temperature acts harmfully on concrete, because when heated, the water of hydration is removed, and the binding force of cement is destroyed. The resistance to high temperature is the less, the richer, denser and younger the concrete. In addition, this resistance depends on the type of stone-like additives.

Concrete with volcanic origin crushed stone, with brick crushed stone, with slag is more fire-resistant, and the loss of strength begins when heated to approximately 300°; at a temperature of 1000° such concrete already has only about 50% of its original strength. The loss of strength of Concrete with limestone crushed stone and gravel begins at lower temperatures and reaches 50% already at a temperature of 500-600°.