Concrete is classified according to three main indicators: the type of binder, medium density and purpose. Thanks to the use of various recipes for the composition of the mixture, the introduction of certain additives for concrete, the use of special types of binders, etc. Manufacturers can achieve the required characteristics and parameters from the finished mixture.
Even at the stage of calculating the map for selecting the composition of the concrete mixture, production technologists can predict how it will behave ready solution in one situation or another. With quite high accuracy frost resistance, water resistance, density, brand and other important parameters of the future composition are calculated. In order for the forecasts to come true, technologists must take into account all possible nuances that are responsible for the quality of future concrete. Let's look at the main factors influencing classification concrete mixtures.
This factor is the most important, largely determining the properties of the concrete mixture. Based on the binder, mixtures are divided into silicate, gypsum, cement, polymer-cement, slag-alkaline and special. In addition there are combined types mixtures, mixed with a combination of some two or three binders. Such mixes are quite often found in the form of dry plaster mixes, which often combine cement, gypsum, lime, etc. in one composition. However, let's return to our sheep. So, the main types of concrete differ in binder.
The density value is primarily influenced by the characteristics of the coarse aggregate (diabase, granite, gravel, dolomite, limestone, expanded clay, etc.). Density is one of the most important factors responsible for the resistance of a concrete element to compression, its frost resistance, water resistance, etc. According to density parameters they are distinguished the following types concrete:
Depending on the operating conditions of future iron concrete structures Manufacturers of building materials produce concrete mixtures of various types. The main emphasis here is on the behavior of the future concrete products or monolithic design under specific conditions. In some places, high sulfate resistance is required, in others fire resistance, and in others resistance to vibration, shock loads, etc. So, depending on the operating conditions of the future reinforced concrete structure, the following types of mixtures are selected.
Separately, I would like to mention such an important parameter as the strength of concrete. This parameter largely depends on the amount of binder introduced into the composition when mixing the mixture. The more cement included in the mixture, the higher the grade and class of the future concrete. This is one of the most important parameters, taken into account in any types and types of mixtures, regardless of their classification.
Classification of concrete
Concrete - artificial stone obtained by molding and hardening a rationally selected mixture of binder, water and aggregates (sand and crushed stone or gravel). The mixture of these materials before hardening is called a concrete mixture.
Concrete is classified according to the following leading characteristics: by main purpose, type of binder and filler, and structure.
By purpose concretes are of the following types:
constructive - for concrete and reinforced concrete load-bearing structures of buildings And structures (foundations, columns, beams, slabs, floor panels, etc.);
special - heat-resistant, chemical-resistant, decorative, radiation-protective, heat-insulating, etc.,
concrete prestressing, concrete polymers, polymer concrete .
By type of binder concrete substances are: cement , made using hydraulic binders - Portland cement and its varieties; silicate - on lime binders in combination with silicate or aluminate com-ponets; plaster - using gypsum anhydrite binders and concretes on slag and special binding materials.
Concrete is produced using conventional dense fillers, on natural or artificial porous fillers; In addition, a variety is cellular concrete, which is a hardened mixture of binder, water and fine silica component. It is characterized by high porosity up to 80...90% with evenly distributed pores measuring 3 mm.
In this regard, concretes are also classified according to their structure: dense, porous, cellular Andlarge-porous.
By type of filler Concrete is distinguished: with dense aggregates, porous and special, meeting special requirements (protection from radiation, heat resistance, chemical resistance, etc.).
By strength indicators during compression, heavy concrete has grades from 100 to 800. The grade of concrete is one of the standardized values of a unified type of a given concrete quality indicator, taken according to its average value. For various types of concrete, requirements are established for indicators characterizing strength, average density, water resistance, resistance to various influences, elastic-plastic, thermophysical, protective, decorative and other properties of concrete.
Certain requirements are imposed on materials for preparing concrete (binders, additives, fillers), its composition and technological parameters for the manufacture of structures for their operation in specific conditions.
Based on concrete strength indicators, their guaranteed values - classes - are established. According With ST SEV 1406-78 concretes intended for buildings and structures are divided into classes B, the main controlled characteristic of which is the compressive strength of cubes measuring 150XI50X150 mm and, accordingly, cylinders measuring 150X300 mm. To transfer from concrete class (MPa) with a standard coefficient of variation of 13.5%, use the formula
R avg.bet = B/0.778.
Durability concrete is assessed by the degree of frost resistance. According to this indicator, concrete is divided into grades from F15 to F1500. The quality of concrete is assessed by water resistance, which is determined by the maximum value of water pressure at which no leakage is observed through control samples manufactured and tested for water resistance in accordance with the requirements of current standards.
Materials for heavy concrete (START)!
Heavy concrete used for the manufacture of foundations, columns, beams, bridge spans and other load-bearing elements and structures of industrial and residential buildings and engineering structures must acquire a certain strength within a given curing period, and the concrete mixture must be easy to lay and economical. When used in structures not protected from the external environment, concrete must have increased density, frost resistance and corrosion resistance. Depending on the purpose and operating conditions of concrete in a structure, appropriate requirements are imposed on its constituent materials, which predetermine its composition and properties, influence the technology of production of products, their durability and efficiency. For the preparation of heavy concrete, Portland cement, plasticized Portland cement, Portland cement with hydraulic additives, Portland slag cement, rapid-hardening Portland cement (BTC), etc. are used. Cement is selected taking into account the requirements for concrete (strength, frost resistance, chemical resistance, water resistance, etc.), as well as technology for manufacturing products, their purpose and operating conditions.
The brand of cement is chosen V depending on the designed compressive strength of concrete:
To prepare the concrete mixture, drinking water is used, as well as any water that does not contain harmful impurities (acids, sulfates, fats, vegetable oils, sugar), which prevent the normal hardening of concrete. You cannot use swamp and waste water, as well as water contaminated with harmful impurities, having a pH value of less than 4 and containing sulfates calculated as SO 4 ions more than 2700 mg/l and all other salts more than 5000 mg/l. Sea and other water containing mineral salts can be used if the total amount of salts in it does not exceed 2%. The suitability of water for concrete is determined chemical analysis and comparative tests of the strength of concrete samples made with this water and with clean drinking water and tested at the age of 28 days during storage under normal conditions. Water is considered suitable if the samples prepared on it have a strength no less than that of samples on clean drinking water. Additives for concrete include inorganic and organic substances or mixtures thereof, through the introduction of which in controlled quantities the properties of concrete mixtures and concretes are specifically regulated or Concrete is given special properties. The classification of concrete additives is based on the effect of their action. Based on this criterion, concrete additives are divided into the following groups:
1. Regulatory rheological properties concrete mixtures. These include plasticizers, which increase the mobility of concrete mixtures; stabilizing, preventing delamination, and water-retaining, reducing water separation.
2. Regulating the setting of concrete mixtures and concrete hardening. These include additives that retard setting, accelerate setting and hardening, and anti-frost, i.e., ensuring hardening of concrete at subzero temperatures.
3. Additives, porosity regulating concrete mixture and concrete. These include air-entraining, gas-forming and foam-forming additives, as well as compacting (air-removing or clogging concrete pores).
4. Additives, giving concrete special properties: hydrophobizing, reducing wetting, increasing radiation protection, heat resistance; anti-corrosion, i.e. increasing resistance in aggressive environments; steel corrosion inhibitors that improve the protective properties of concrete to steel; additives that increase bactericidal and insecticidal properties.
5. Additives multifunctional action, simultaneously regulating various properties of concrete mixtures and concretes: plasticizing-air-entraining; plasticizing, increasing the strength of concrete, and gas-forming-plasticizing.
6. Mineral powders - cement substitutes. This group includes finely ground materials introduced into concrete V amount 5...20%. These are ash, ground slag, stone crushing waste, etc., which give concrete special properties (heat resistance, electrical conductivity, color, etc.).
Surfactants (surfactants) are the most widely used plasticizing additives.
TO cement hardening accelerators , increasing the strength increase of concrete, especially in early dates, include calcium chloride, sodium sulfate, nitrite-nitrate-calcium chloride, etc.
Antifreeze additives- potash, sodium chloride, calcium chloride, etc. - lower the freezing point of water, which contributes to the hardening of concrete at subzero temperatures.
For setting retardation Sugar syrup and additives SDB, GKZh-10 and GKZh-94 are used.
Sand- a loose mixture of grains with a particle size of 0.16...5 mm, formed as a result of the natural destruction of massive rocks (natural sands). Natural sands according to their mineralogical composition are divided into quartz, feldspathic, limestone, and dolomite. Of the natural sands, quartz sands are most widely used for heavy concrete.
As fine aggregate, sands of increased coarseness are used, coarse, medium and fine - natural and enriched; sands from crushing screenings and enriched sands from crushing screenings.
The grain composition of sand is of particular importance for obtaining quality concrete. Sand for concrete should consist of grains of various sizes (0.16...5 mm) so that the volume of voids in it is minimal; The smaller the volume of voids in the sand, the less cement is required to obtain dense concrete. The grain composition of sand is determined by sifting dry sand through a standard set of sieves with hole sizes (from top to bottom) 10; 5; 2.5; 0.63; 0.315; 0.16 mm. A sand sample dried to a constant weight is sifted through sieves with round holes with a diameter of 10 and 5 mm. The residues on these sieves are weighed and calculated to the nearest 0.1%. CONTINUATION!
Materials for heavy concrete (END)!
From a sample of sand that has passed through the above sieves, weigh 1000 g (G) of sand and sift it successively through a set of sieves with holes of size 2.5; 1.25; 0.63; 0.315 and 0.16 mm. The residues on each sieve are weighed (G,) and calculated:
the partial residue on each sieve - as the ratio of the mass of the residue on a given sieve to the mass of the sieved sample (a;) - is calculated with an accuracy of 0.1%:
аi = (Gi/G) 100,
the total residue (L) on each sieve - as the sum of partial residues on all sieves with large openings plus the residue on a given sieve - is calculated with an accuracy of 0.1%:
Ai = a2.5 + a1.25 + ... + ai,
where a2.5, a1.25, ... are partial residues on sieves with large openings starting from a sieve with openings of 2.5 mm, %; a, - partial residue on a given sieve, %.
The sand fineness modulus Mk (without gravel fractions with grain sizes larger than 5 mm) is determined as the quotient of the sum of total residues on all sieves divided by 100, starting with a sieve with a hole size of 2.5 mm and ending with a sieve with a hole size of 0.16 mm ;
sand fineness modulus is calculated with an accuracy of 0.1%:
Mk = (A 2 ,5 +A 1, 25 + A ABOUT .63 + A0.315+Ao,16)/100.
According to the size of the fineness modulus, sand is divided into increased fineness M To - W...3.5, large with M To > 2.5, average M To = 2.5...2.0, small Mk = 2.0...1.5 and very small M To = 1,5...1,0;
the total residues on sieve No. 063 (% by weight) are respectively equal to: 65...75, 45...65, 30...45, 10...30 and less than 10.
The grain composition of the fine aggregate must correspond to that indicated on the graph (Fig. 6.1). In this case, only grains passing through a sieve with round holes with a diameter of 5 mm are taken into account.
AS A LARGE aggregate used for heavy concrete gravel and crushed stone from rocks or crushed stone from gravel with a grain size of 5...70 mm.
Gravel- grains of a rounded shape and a smooth surface with a size of 5...70 mm, formed as a result of the natural destruction of rocks. The quality of gravel is characterized by: grain composition and grain shape, strength, grain content of weak rocks, the presence of dust and clay impurities, petrographic characteristics, density, porosity, voids and water absorption. For concrete, the most suitable form of grains is low-rounded (crushed stone), ovoid (rounded) is worse, lamellar and needle-shaped, which reduce the strength of concrete, are even worse.
Often gravel occurs along with sand. When the sand content in gravel is 25...40%, the material is called a sand-gravel mixture. Gravel, like sand, may contain harmful impurities of dust, silt, clay, organic acids...
The strength of gravel is assessed by testing for crushability in a cylinder. The latter is determined by crushing a gravel sample in a cylinder with a static load. After this, the sample is sifted through a sieve with a hole size corresponding to the smallest grain size in the original gravel sample, and the amount of weight loss is determined. Depending on this value, gravel is divided into grades: Dr8 (with a loss in weight of up to 8%), Dr12 (over 8 to 12%), Dr16 (over 12 to 16%) and Dr24 (over 16 to 24%).
For the construction of industrial and civil buildings, the strength of gravel grains should be more than 1.5...2 times higher than the strength of concrete.
According to the degree of frost resistance, gravel is divided into grades F 15, 25, 50, 100, 150, 200 and 300. Frost resistance of gravel is determined by direct freezing or testing in a solution of sodium sulfate. Gravel is considered frost-resistant if, in a water-saturated state, it can withstand repeated (15 cycles or more) alternating freezing at a temperature of -17°C and thawing without destruction. In this case, the loss in mass after testing is more than 5%. For grades F 15 and 25, a weight loss of 10% is allowed
A good grain composition of gravel is considered to be one in which there are grains of different sizes, which creates the least voids. The grain composition of gravel is determined by sifting 10 kg of dry sample through a standard set of sieves with opening sizes of 70, 40, 20, 10 and 5 mm. The grain composition of each fraction or mixture of several fractions of gravel must be within the limits indicated in the graph in Fig. 6.3. The largest size of gravel grains, Dmax, is taken to be the size of the sieve openings, on which the total residue does not exceed 10% of the sample, and the smallest gravel size, Dmax, is the size of the opening of one of the upper sieves, through which no more than 5% of the sifted sample passes. Below are the values of total residues on control sieves when sifting gravel (cold) fractions from 5 (3) to 10 mm, over 10 to 20; over 20 to 40 and over 40 to 70 mm.
Crushed stone is produced by crushing massive rocks, gravel, boulders or artificial stones into pieces measuring 5... 120 mm. To prepare concrete, crushed stone is usually used, obtained by crushing dense rocks, gravel, blast furnace and open-hearth slag. Crushing is carried out in stone crushers. In this case, not only crushed stone grains are obtained, but also small fractions related in size to sand and dust. The crushed stone grains have an irregular shape. The best shape is considered to be one that approaches the cube and tetrahedron. Due to the rough surface, crushed stone grains adhere better to the cement stone in concrete than gravel, but the concrete mixture with crushed stone is less mobile.
In terms of crushability, frost resistance, grain composition, and wear, crushed stone has the same requirements as gravel.
Depending on the shape of the grains, GOST 8267-82 establishes three groups of crushed stone from natural stone: cuboid, improved and ordinary. The content of lamellar (flaky) and needle-shaped grains in them does not exceed 15, 25 and 35% by weight, respectively. Lamellar and needle-shaped grains include those in which their thickness or width is 3 times or more less than their length.
Properties of concrete mixture
Heavy concrete must acquire design strength by a certain date and have other qualities corresponding to the purpose of the structure being manufactured (water resistance, frost resistance, density, etc.). In addition, a certain degree of mobility of the concrete mixture is required, which would correspond to the accepted methods of laying it.
Concrete mixture is a complex multicomponent system consisting of new formations formed during the interaction of binder with water, unreacted clinker particles, filler, water, introduced special additives and entrained air. Due to the presence of interaction forces between dispersed particles of the solid phase and water, this system becomes connected and can be considered as a single physical body with certain rheological, physical and mechanical properties.
The determining influence on these properties will be exerted by the quantity and quality of cement paste, which, being a dispersed system, has a highly developed interface between the solid and liquid phases, which contributes to the development of molecular adhesion forces and increased cohesion of the system.
Workability is the ability to fill a mold with a given type of compaction. Har-sya mobility, rigidity and coherence.
Mobility of concrete mixture- its ability to spread under its own weight. To determine under. For visibility, a cone is used (Fig. 6.4), which is filled layer by layer in three steps with concrete mixture, compacted by bayonet. After compacting the latter, the mold is removed. The resulting cone of concrete mixture settles under the influence of its own mass. The amount of cone settlement (cm) serves as an estimate of the mobility of the concrete mixture. Based on this indicator, a distinction is made between flexible (plastic) mixtures with a cone settlement of 1...12 cm or more, and rigid mixtures, which practically do not give a cone settlement, but when exposed to vibration, the latter have different molding properties. To assess the hardness of these mixtures, they use their own methods.
Hardness index concrete mixture is determined using a special device (Fig. 6. 5), which consists of a cylindrical vessel with an internal diameter of 240 mm and a height of 200 mm with a device attached to it for measuring the slump of the concrete mixture in the form of a guide stand, a rod and a metal rod and six holes. The device is installed on a vibration platform and tightly attached to it. Then a metal cone mold with a nozzle is placed in the vessel, which is fixed in the device using a special holder ring and filled with three layers of concrete mixture. Then the cone shape is removed by turning the tripod, a disc is placed on the surface of the concrete mixture and the vibrating platform is turned on. Vibration with an amplitude of 0.5 mm is continued until the release of cement paste from two holes of the disk begins. The vibration time (s) determines the hardness of the concrete mixture. The classification of concrete mixtures according to the degree of their rigidity (workability) is given in Table. 6.2.
Table 6.2.Classification of concrete mixtures
The mobility of a concrete mixture is influenced by a number of factors: type of cement, water and cement paste content, aggregate size, grain shape, sand content.
The introduction of a surfactant, such as SDB, into the concrete mixture increases the mobility of the concrete mixture and reduces its water demand. Superplasticizers (S-3, 10-03, 40-03, etc.) have a positive effect on the mobility of the mixture. Their efficiency is higher in mobile mixtures; they can reduce the water requirement of the mixture by 20...25%.
At the same time, it should be taken into account that the mobility of the mixture decreases over time due to the physicochemical interaction of cement with water.
Connectedness- characterizes the homogeneity of the concrete structure.
Concrete composition design
The design of the composition has the goal of establishing such a consumption of materials per 1 m 3 of concrete mixture, which most economically ensures the production of a workable concrete mixture and the specified strength of concrete, and in some cases the necessary frost resistance, water resistance and special properties of concrete.
The composition of the concrete mixture is expressed as a ratio by mass (less often by volume) between the amounts of cement, sand and crushed stone (or gravel), indicating the water-cement ratio. The amount of cement is taken as one. Therefore in general view The composition of the concrete mixture is expressed by the ratio cement: sand: crushed stone = 1: x:y at W/C = z (for example, 1:2.4:4.5 at W/C = 0.45).
There are two compositions of concrete: nominal(laboratory), accepted for materials in a dry state, and production(field) - for materials with natural humidity.
At the time of calculating the composition of the concrete mixture, it is necessary to determine the quality starting materials: cement, water, sand and crushed stone (gravel) - according to GOST requirements.
Depending on the conditions in which the concrete will be located in a building or structure, other requirements may also be imposed on it, for example, the degree of frost resistance, resistance to aggressive water, and water resistance. The high frost resistance and impermeability of densely laid concrete are regulated by W/C and binder consumption, hence the need to standardize W/C in hydraulic, road and other special concretes.
The calculation of the concrete composition is carried out in the following order: the cement-water ratio is determined, ensuring the production of concrete of a given strength and water consumption; calculate the required consumption of cement, and then crushed stone (or gravel) and sand; check the mobility (rigidity) of the concrete mixture if these indicators deviate from the design; make adjustments to the composition of the concrete mixture; prepare samples to determine strength and test them within specified periods; recalculate the nominal composition of the concrete mixture to the production composition.
Determination of cement-water ratio produced according to the following formulas:
for concrete with C/V = 2.5
Water flow determination. The optimal amount of water in the concrete mixture (water content, l/m3) should provide the necessary mobility (or rigidity) of the concrete mixture. The amount of water for hardening 1 m 3 of concrete mixture for all calculations in accordance with ONTP 07-85 is taken equal to 200 l, regardless of the type, hardness and mobility of the concrete mixtures.
Determination of cement consumption. With the C/V value determined from the formula and the accepted water demand of concrete mixture B, the approximate cement consumption is calculated, kg/m3 of concrete:
Cement consumption per 1 m 3 of concrete should be no less than the minimum. If the cement consumption per 1 m 3 of concrete is below the permissible level, then it is necessary to bring it up to ■ the norm or introduce a finely ground additive.
Determination of aggregate consumption(sand and crushed stone or gravel) per 1 m 3 of concrete. To determine the consumption of sand and crushed stone (gravel), two conditions are specified:
1) the sum of the absolute volumes of all components of concrete (l) is equal to 1 m 3 (1000 l) of compacted concrete mixture:
where C, V, P, Sh - the content of cement, water, sand and crushed stone (gravel)< кг/м 3 ; q u , q b , g n , Q m - плотности этих материалов,кг/м 3 ;
2) cement-sand mortar will fill the voids in the coarse aggregate with some spreading of the grains:
where Vost.sh(g) is the voidness of crushed stone or gravel in a standard loose state (substituted into the formula as a relative value); a is the coefficient of expansion of crushed stone grains (or excess solution); for rigid mixtures a= 1.05...1.20, for flexible mixtures a= 1.2...1.4 and more; q h . u (G > - bulk density of crushed stone (gravel), kg/l; Q m (g > - density of crushed stone (gravel), kg/l.
Coefficient a determines the ratio between sand and crushed stone in concrete.
After determining the consumption of crushed stone or gravel, calculate the consumption of sand (kg/m3) as the difference between the design volume of the concrete mixture and the sum of the absolute volumes of coarse aggregate, cement and water:
If gravel or crushed stone is made up of several fractions, then it is necessary to establish in advance the optimal ratio between them, using a graph of the best grain composition or selecting a mixture with a minimum amount of voids.
Checking the mobility of the concrete mixture. After a preliminary calculation of the concrete composition, a test batch is made and the cone settlement or rigidity is determined. If the concrete mixture turns out to be less mobile than required, then increase the amount of cement and water without changing the cement-water ratio. If the mobility is greater than required, then sand and coarse aggregate are added in small portions, keeping their ratios constant. In this way, the specified mobility of the concrete mixture is achieved.
Clarification of the calculated composition of concrete using trial batches. Experimental batches of concrete are produced at three values of the water-cement ratio, of which one is taken as calculated, and the other two are 10...20% more or less. The amount of cement, water, sand and crushed stone (gravel) for concrete with a water-cement ratio not equal to the design one is determined using the above method. From each prepared mixture, three cube samples measuring 20X20X20 cm are prepared, which are kept under normal conditions and tested at the age of 28 days to determine the class of concrete (or at other times). Based on the test results, a graph is drawn up of the dependence of concrete strength on the cement-water ratio, with the help of which a C/V is selected that ensures the production of concrete of a given strength.
During trial batches, the mobility or rigidity of the concrete mixture is also checked (it must satisfy the design one), its density is determined and, based on the test results of trial batches, appropriate adjustments are made to the calculated composition of concrete. When changing the content of sand and crushed stone (gravel), their moisture content is taken into account.
Properties of concrete
Strength of concrete. In the structures of buildings and structures, concrete can be exposed to various operating conditions, experiencing compression, tension, bending, and spalling. The compressive strength of concrete depends on the activity of the cement, the water-cement ratio, the quality of the aggregates, the degree of compaction of the concrete mixture and the hardening conditions. The main factors in this case are the activity of cement and the water-cement ratio.
To obtain a workable concrete mixture, the ratio of water to cement is usually taken as W/C = 0.4...,0.7, while the chemical interaction of cement with water requires no more than 20% of water by weight of cement. Excess water that has not entered chemical reaction with cement, evaporates from concrete, forming pores in it, which leads to a decrease in the density and, accordingly, the strength of concrete. Based on this, the strength of concrete can be increased by reducing the water-cement ratio and increasing compaction.
Along with the activity and quality of cement, the water-cement ratio and the quality of aggregates, the strength of concrete is significantly influenced by the degree of compaction of the concrete mixture, the duration and conditions of concrete hardening.
A separate view is worth mentioning heat-resistant concrete. It is used mainly in the metallurgical industry as a foundation for open-hearth furnaces.
In this article:
Concrete - main material construction, prepared using a certain technology. Additional Ingredients in its composition help improve structural and technical specifications concrete.
This construction material It is customary to classify according to 6 main characteristics: purpose, type of binder, average density, strength, frost resistance and water resistance.
Release various types concrete mixtures depends on the conditions where future reinforced concrete structures. The conditions can be very specific: fire resistance, sulfate resistance, resistance to stress, shock, vibration.
According to their intended purpose, the following types of mixtures are distinguished:
The most important factor that determines the properties of a concrete mixture is the type of binder.
The main types of concrete in this category:
Based on gypsum it is obtained gypsum concrete, which is used in finishing elements, for the manufacture of suspended ceilings and internal partitions. Gypsum-cement-pozzolanic mixtures with high water resistance are widely used; they are used to create bathroom blocks and various designs low-rise buildings.
Based on cement components they produce cement concretes and mortars. The most common raw material component is Portland cement and its varieties. Concrete mixtures based on Portland slag cement and pozzolanic cement are also widely used. The use of this type is construction.
This category includes decorative concrete, which is produced on the basis of colored, white cements. The idea of creating a decorative building material came to us from Germany. Concerning color range concrete, it includes green, black, brown, blue, yellow, red and white shades. Considered especially expensive white concrete. There are also concrete mixtures based on non-shrinking, tensile and aluminous cement.
Recently they began to use in construction slag-alkaline concrete. It is produced from slags mixed with alkali solutions. This type of concrete is indispensable when creating massive objects.
This is explained by the fact that when creating large structures from the Portland cement mixture, separation is carried out large quantity heat, and the temperature of building elements can reach 80°. If such an object cools very quickly, cracks may appear. The use of slag-alkaline concrete avoids this problem.
With a mixed binder base one gets polymer cement concrete. In this case, the base contains latexes, water-soluble resins and cement. When this mixture cools, a film appears on its surface, swelling in the presence of a large amount of moisture.
There are two types - framed and filled.
Application.
The material is actively used for landscape design, outdoor and interior decoration walls, building facades and when installing floors. Polymer-cement concrete is convenient to use, it is easy to apply both mechanized and manual.
Special binders will be needed to obtain special concrete. To obtain acid-resistant and heat-resistant concrete, liquid glass, slag, glass-alkaline elements are used as binders.
A very rare type of concrete, practically unused in modern production - silicate concretes. Their production is based on the use of lime, where there is an autoclave hardening method.
Dense autoclaved silicate concrete is used to create load-bearing panels interior walls and large blocks, as well as floor panels. Especially durable building materials are used to create railway sleepers and slate, which do not contain asbestos. Also, silicate concrete can be used for constructing road bases and in tubes for mine construction. There are also combined types of concrete made from the combination of 2-3 binder components. Such compounds can often be found in the composition plaster mixtures, where lime, gypsum, cement and other elements are combined in a single composition.
The main factor influencing the water resistance, frost resistance, and resistance of a concrete structure to compression is density. The significance of density is determined by large aggregates: dolomite, expanded clay, diabase, gravel, granite, limestone. Following compliance with GOST, concrete grades are distinguished within the range M50-M800.
Based on density parameters, the following types of concrete can be distinguished:
Each brand of concrete determines its strength class. For the construction of the least critical structures, grades with lowest value— M50, M75, M100. For example, this is the least durable concrete suitable for the construction of a blind area. To screed floors or railway floors, you will need concrete of higher strength, for example grade M200.
M550 concrete is considered the most durable.
The different strengths of all types of concrete depend on the proportions of sand, cement and crushed stone in its composition. High strength is achieved by the impressive presence of cement.
There are also grades of concrete according to frost resistance, which in GOST are marked with the letter F. Frost resistance is characterized by the largest number freezing and thawing with a decrease in mass and strength by a certain amount. The densest concrete mixtures are always the most frost-resistant. In this category there are concrete grades from F25 to F1000.
The ability of concrete not to allow water to pass through under pressure is called waterproof.
Concrete grades according to this classification are W2, W4, W6, W8, W12. Several years ago, the Russian letter V was used to denote this parameter.
The classification of concrete can be based not only on the type of main components of the solution and characteristics, but also take into account the scope of its use. For those who are not too well versed in chemistry building mixtures, this is the most convenient way decide on the material that is suitable for specific type works Only after this should you move on to searching for a brand with the required technical parameters.
The main division of concrete provides for only two types: general construction and special. A separate line includes lightweight porous materials, whose scope of application directly depends on density indicators. At the same time, the same grades of concrete can often be found in several groups at once according to their intended purpose, so this feature should always be taken into account and when choosing, focus not only on strength.
The largest group of building materials, which includes all types of mixtures and finished products, widely used in various fields of civil engineering, as well as for the production of reinforced concrete products. Foundations are cast from them, walls are erected, beams, ceilings and columns are formed. When choosing compositions for specific conditions, it is necessary to pay attention to brands that characterize concrete in terms of density, strength, as well as frost resistance and water resistance after hardening.
The strength of the monolith is indicated by the numbers after the letters “M” or “B”. In the first case, the data are given in units of kgf/cm2. And although such a classification is considered insufficiently accurate and outdated, it is still actively used. In the second option, the values are indicated in MPa, and these are no longer average figures with the error allowed by GOST, but guaranteed strength. From these simple records it is easy to determine the characteristics of mixtures without any tables or reference books.
Different types of concrete grades find their application in construction:
The use of other grades from M400 and above is already in the professional field, since their characteristics are more suitable for various special-purpose structures: from pool bowls and tunnels to bridges and dams.
In addition to strength, the classification of general construction concrete takes into account its other properties. For example, frost resistance not only determines the scope of use of the monolith, but also its durability:
The water resistance class is of particular importance when choosing building materials for installations, swimming pool bowls or fonts, as well as drinking and septic wells. It is designated by marks from W2 to W20 and indicates the pressure of the water column that concrete can withstand (units of measurement - atm·10 -1).
There is also a division of monoliths by density (letter D). The strength of concrete, and therefore the possibilities of its use, partly depends on it. Heavy varieties from D2000-D2500 kg/m3 are used for the construction of critical structures, lightweight ones - for general construction work. Light products up to D1200 kg/m3 go mainly as thermal insulation materials, because they have low bearing capacity, barely reaching the brand strength M50-M75.
Here, the varieties of concrete are as numerous as the scope of application of this building material. Let's look at the most common of them:
Made from finely ground components with increased content active silica or alumina. Works great under severe temperature changes and prolonged heating up to +700-1700 °C (depending on the own fire-resistant properties of the mineral fillers). Used in the construction of thermal power plants and metallurgical workshops, as well as industrial furnaces. It has good strength properties and is graded at least M250, but is susceptible to acid corrosion.
Frost-resistant (up to F300) type of concrete with minimal water permeability. Used in the production of sewerage and drainage systems, dams and some underground structures. Traditionally divided into additional subgroups: underwater and above-water, as well as variable-level concrete. They all work in different conditions environment, and therefore differ in composition and characteristics.
This group of concretes includes high-strength, weather-resistant mixtures. They are used as road surfaces, for the development of industrial sites with intensive use, as well as for the construction of runways (runways).
It also has low water absorption due to the addition of liquid glass. Withstands heating up to +1000 °C and is resistant to most aggressive media, except alkalis. Found widespread use in finishing objects chemical industry. However, as an independent building material it is almost never used due to its relatively low mechanical strength, not exceeding B12.5-15.
Has very high tensile and compression resistance. It is made on the basis of PC or ShPC with heavy fillers - usually metal-containing. The fine components here are barite ores, shot from cast iron or lead. All this can increase the density grade to D6000.
Lightweight concrete
There is another classification principle, which is more often applied to light and especially light varieties of cellular concrete. Here everything is tied to their density (or rather, porosity). It characterizes the thermal insulation properties artificial stone and allows you to automatically divide such concrete into groups according to purpose:
Various tables with technical characteristics concretes do not give a complete picture of the possibilities of their use in construction. Therefore, before choosing such materials, it is necessary to study the description of their operational properties and scope of application.
