- How to calculate the optimal formulations for concrete
- How to determine the required concrete composition
- Calculation of concrete composition empirically
- Filler for concrete mix
- Selection of the brand and the required amount of cement
- Coarse-porous concrete
- Lightweight concrete
- Compositions for especially lightweight concrete
In this article: the main components of the concrete mix; three types of concrete mass consistency; calculation of water-cement ratio; selection and calculation of filler by fractions; testing the concrete mass with a cone; selection and calculation of cement consumption; modern types of concrete; the main mistakes in the preparation of concrete mix.
How to calculate the optimal formulations for concrete
Despite the fact that concrete in its present form was only discovered 200 years ago, there are concrete formulations that are about 6,000 years old. Today, the recipe for Roman concrete is again known, which was used by builders in the Roman Empire for centuries – a lime solution played the role of a binder in it. By the way, silicate concretes, in which lime acts as a binder, are effective to this day..
In modern construction, concrete is used that is different in composition and on how correctly the calculation of the composition of concrete is made, its strength and durability depend.
How to determine the required concrete composition
The basic rules for the selection of the composition of concrete are given in GOST 27006-86. Any concrete consists of three main components: cement, filler of certain fractions and water. There are two prerequisites – the water must be clean and fresh, the filler (sand, gravel, etc.) must not contain contaminants (dirt particles seriously affect the strength properties of concrete).
Concrete can have a different consistency (density): a hard concrete solution (reminiscent of damp earth) will require compaction with effort; plastic (rather thick and at the same time mobile) requires less compaction; cast – practically does not require sealing, is mobile and fills the form by gravity.
First of all, you need to decide on the water / cement ratio and the main priority in this matter will be the required concrete strength. Water has two tasks in creating a concrete mixture: it enters into a chemical reaction with cement, leading to the setting and hardening of concrete; plays the role of a lubricant for concrete components (cement, sand and gravel). To accomplish the first task, it is enough to add 25 to 30% water to one part of the cement, but it would be difficult to put such a mixture of concrete into a mold – this composition will be dry and not amenable to ramming. For this reason, more water is added to concrete than is needed for its hardening – it is necessary to lower the strength of the future concrete to obtain a solution of greater plasticity. However, this causes another problem – a larger amount of water after its evaporation leaves air pores in the concrete, thereby affecting the strength of the concrete structure. Therefore, it is necessary to calculate the water content in the concrete mixture with the greatest accuracy, achieving its minimum content.
The next step is to determine the cement / filler ratio (fine and coarse). But first, it is required to calculate the ratio in the filler itself – the amount of its small and large components – the density and efficiency of the concrete mixture will depend on this. The calculation is made according to the ratio of the filler to a unit of weight or volume of cement, for example: a concrete mixture containing 20 kg of cement, 60 kg of sand and 100 kg of crushed stone will have such a composition by weight – 1: 3: 5. The water required for the preparation of the concrete mixture is indicated in fractions of the unit weight of the cement, i.e. if for the given example of a concrete composition 10 liters of water is needed, then its ratio to cement will be 0.5.
The exact determination of the ratio of water and cement for concrete is possible only empirically (more on this later). If the volume of concrete work is small, you can use this table:
Received concrete grade Cement grade 200 250 300 400 500 600 one hundred 0.68 0.75 0.80 – – – 150 0.50 0.57 0.66 0.7 0.72 0.75 200 0.35 0.43 0.53 0.58 0.64 0.66 250 0.25 0.36 0.42 0.49 0.56 0.60 300 – 0.28 0.35 0.42 0.49 0.54 400 – – – 0.33 0.38 0.46
Note: The water to cement ratio in the table is correct for gravel aggregate concrete. If crushed stone is used as a filler, then 0.03-0.04 units must be added to each of the given ratios of water to cement.
Calculation of concrete composition empirically
To test the characteristics of experimental concrete mixes, you will need a special sheet metal cone – its structure should not have seams, because it is especially important that its surface is perfectly smooth from the inside. The cone must have the following dimensions: height 300 mm, diameter of the lower base 200 mm, and the upper base 100 mm. On the sides on such a cone, two handles are fixed, two brackets (paws) are attached to the lower base for support with feet.
To test the quality of the concrete mix, you will also need a flat platform; a sheet of plywood, plastic or steel is suitable for its creation. The test itself is carried out as follows: the site is wetted with water, a cone is installed on it, its base is pressed against the site with its feet, then it is filled with concrete mixture in three steps (three layers). Each layer of concrete (about 100 mm) must be compacted by bayonetting, using a 500 mm steel rod with a diameter of 150 mm – after laying out the next layer, it must be pierced at least 25 times.
Having filled the cone, you need to cut off the protruding mass of concrete at the level of the edges with a bayonet shovel, then grab the side handles and slowly raise the cone body strictly vertically. The concrete mass, no longer restrained by the walls of the cone, will gradually settle, taking a vague shape – you need to wait until the sediment completely stops. After that, put the metal shape of the cone next to the concrete mass extracted from it, install a flat rail on the upper base of the cone in a strictly horizontal position and measure the distance from it to the upper point of the settled concrete using a centimeter ruler.
The sediment of hard concrete will be from 0 to 20 mm, plastic – from 60 to 140 mm, cast – from 170 to 220 mm. An important point – there should be no release of water and the concrete solution should not delaminate.
Filler for concrete mix
It is important that the filler (gravel, sand and crushed stone) is of different fractions – such compositions for concrete form the strongest concrete stone, because there will be practically no air cavities in it, in addition, the creation of such concrete will require the least amount of cement and sand. According to building codes, the total volume of air voids with sand filler should not be more than 37% of the total volume of concrete, with gravel filler – no more than 45%, and with crushed stone – no more than 50%.
You can test the filler for the number of voids directly at the construction site – you will need a ten-liter bucket and water. You can test both the already prepared mixture of filler, and each of its components separately: you need to fill a clean bucket with them to the brim, then level the mixture around the edges of the bucket (without sealing!) And pour measured portions of water into it with a thin stream so that it fills bucket to the brim. The amount of water poured into a bucket with a filler will show the volume of voids – for example, if 5 liters are included, then the volume of voids is 50%.
There are two ways to select the fractional composition of the filler for the concrete mixture.
In the first method, the maximum filler fraction will be 40 mm, i.e. for sifting gravel (crushed stone), a sieve with a mesh of 40 mm is used. As sifting, remove to the side the remainder (it is called the top residue) that has not passed through the cells.
The sifted filler must be passed through a sieve with a mesh of a smaller diameter (20 mm) – we get the first fraction of the filler (not passed through the mesh of a sieve with a diameter of 21-40 mm). Then we sequentially sift the filler through sieves with a mesh of 10 and 5 mm, we get the second (grain 11-20 mm) and third fractions (grain 6-10 mm). After the final sifting, the bottom residue remains (grain from 5 mm and less) – we collect it separately.
We make up the total volume of the filler with coarse grains – we take 5% of the residues (upper and lower) and 30% of each of the three fractions. If the volume of the upper residue is insufficient, take 5% of the first fraction instead. It is possible to compose the filler in two fractions (the first – 50-65% and the third – 35-50%) or three (the first fraction – 40-45%, the second – 20-30% and the third – 25-30%).
Compositions for concrete with a filler of 20 mm fractions are formed as follows: for sifting, a sieve with a 20 mm mesh is taken, then sifting through a 10 mm sieve, we get the first fraction (grain 11-20 mm). The next stage is sifting through a 5 mm sieve to obtain the second fraction (grain 6-10 mm). Finally, we sift through a 3 mm sieve – the third fraction has a grain of 4-5 mm. If a finer sand filler is required, it is required to sequentially sift the sand through a sieve with a 2.5 mm cell, then through a 1.2 mm cell (first fraction), then through a 0.3 mm cell (second fraction).
The total volume of the filler is made up of the first fraction (20-50%) and the second (50-80%).
Having measured the required amount of filler for each fraction, it is required to combine them and thoroughly mix this composition to evenly distribute grains of different sizes throughout the entire volume of the filler.
Selection of the brand and the required amount of cement
To obtain a given grade of concrete, it is necessary to use a grade of cement that will be 2-3 times higher than the required grade of concrete (for Portland cement – 2 times, for other types of cement – 3 times). For example, to get a concrete grade of 160 kgf / cm2 you will need cement, the brand of which is not lower than 400 kgf / cm2. It must be borne in mind that the volume of the finished mass of concrete is less than the volume of its dry components – from one m3 will come out 0.59-0.71 m3 ready-made concrete. For the calculation of the concrete composition, see the table:
Filler type Water-cement ratio Concrete composition by volume (cement: sand: gravel (crushed stone)) Ready concrete volume Material consumption for 1m3 cement, m3 sand, m3 coarse filler, m3 water, m3 Settlement when tested with a cone 30-70 mm gravel 0.50 1: 1.4: 3.1 0.68 320 0.37 0.88 160 rubble 1: 1.6: 3.1 0.59 360 0.46 0.89 180 gravel 0.55 1: 1.7: 3.4 0.68 290 0.42 0.83 160 rubble 1: 1.8: 3.3 0.60 328 0.49 0.90 180 gravel 0.60 1: 1.9: 3.6 0.69 266 0.42 0.80 160 rubble 1: 2.1: 3.5 0.61 300 0.52 0.87 180 Draft when tested with a cone 100-120 mm gravel 0.50 1: 1.3: 2.7 0.68 352 0.38 0.80 176 rubble 1: 1.4: 2.7 0.59 396 0.46 0.90 198 gravel 0.55 1: 1.4: 3.1 0.68 320 0.37 0.83 176 rubble 1: 1.7: 2.9 0.60 360 0.51 0.87 198 gravel 0.60 1: 1.6: 3.3 0.69 294 0.39 0.81 176 rubble 1: 1.9: 3.1 0.61 330 0.52 0.85 198 Draft when tested with a cone 150-180 mm gravel 0.50 1: 1.2: 2.6 0.67 370 0.37 0.81 185 rubble 1: 1.4: 2.5 0.59 414 0.48 0.86 207 gravel 0.55 1: 1.4: 2.1 0.67 338 0.39 0.82 185 rubble 1: 1.5: 2.8 0.60 376 0.47 0.88 207 gravel 0.60 1: 1.6: 3.2 0.67 310 0.44 0.82 185 rubble 1: 1.8: 2.9 0.61 345 0.52 0.84 207
The sequence of drawing up the concrete mixture is as follows: the measured portions of the coarse fractions of the filler are mixed with each other; a portion of sand fractions is measured separately, poured onto a clean wooden board (sheet of metal), forming a bed; a measured amount of cement is poured into a sand bed and thoroughly mixed with sand; a prepared mass of gravel (crushed stone) is introduced into the finished cement-sand mixture and thoroughly mixed until a homogeneous composition (in dry form).
Then a measured amount of water is introduced through a watering can, the mixture is repeatedly stirred until a homogeneous mass of concrete is formed. The ready-mixed concrete should be used within an hour from the moment water is introduced into it..
Carefulness when choosing a filler will allow you to get not only strong concrete, but the same grade of concrete when using different grades of cement (see table).
Concrete grade for 28 days, kgf / cm2 Received concrete hard, requiring a strong seal plastic, requiring vibration cast, not requiring styling Cone test settlement about 10 mm about 50 mm about 100 mm used cement grade 200 300 400 200 300 400 200 300 400 50 1: 3.4: 5 1: 3.8: 6.5 – 1: 3: 5 1: 3.7: 5.8 – 1: 2.8: 4.4 1: 3.5: 4.9 – 75 1: 2.3: 5 1: 2.8: 5.5 1: 3.5: 6 1: 2.3: 4 1: 2.7: 4.8 1: 2.7: 5.2 1: 2: 3.5 1: 2.5: 4 1: 3: 4.4 one hundred 1: 2.1: 4.3 1: 2.5: 5 1: 3: 5.5 1: 1.9: 3.6 1: 2.5: 4.3 1: 2.8: 4.9 1: 1.8: 3.1 1: 2.1: 3.6 1: 2.6: 4.2 150 – 1: 1.9: 4 1: 2.3: 4.5 – 1: 1.7: 3.3 1: 2.2: 4.2 – 1: 1.6: 3 1: 2: 3.5
Note: the composition of concrete is shown in the following proportion – cement: sand: gravel (crushed stone).
Next, let’s talk about the compositions of some modern concretes..
This type of concrete consists exclusively of coarse aggregate – sand is completely absent in their composition. The structure of large-porous concrete contains a large number of voids between the grains of the filler, the binder is contained in it in a very small amount – all this leads to a reduction in the bulk density of such concretes, when compared with conventional ones. In addition, coarse concrete has low thermal conductivity..
Compositions for concrete of this type contain various fillers, both natural (crushed stone or gravel of heavy rocks, crushed stone pumice or tuff), and artificial (expanded clay and broken bricks, slag pumice, large fuel slag, etc.). The minimum fraction of fillers for coarse concrete is 5 mm, the maximum is 40 mm, its volumetric weight can be from 700 to 2000 kg / m3 (depends on the type of filler and cement consumption).
The main purpose of large-porous concrete is to create walls and partitions of buildings for various purposes.
When forming a concrete mixture, it is important to strictly monitor the dosage of water – any deviations in the water / cement ratio in coarse concrete seriously compromise its strength (to a greater extent than in other types of concrete). The following happens: more water causes the cement paste to flow from the filler surface, disrupting the homogeneity of the internal structure of the concrete; lack of water leads to uneven enveloping of the filler, sharply complicating the laying of the concrete mixture.
Mixing of large-porous concrete is carried out in free-fall concrete mixers or with forced mixing: when using a heavy filler – 2-3 minutes, with a light filler – 4-5 minutes. The readiness of the concrete mixture for use is indicated by a characteristic reflection on the filler grains covered with a uniform layer of cement paste.
One of the characteristic features of coarse concrete is the higher yield compared to conventional concrete. Replacing dense concrete with large-porous concrete, it is possible to achieve significant savings in the binder (cement): with the introduction of heavy fillers – by 25-30%, when using light fillers – up to 50%. At the same time, the strength properties of coarse concrete are fully consistent with dense concrete.
Due to its qualities – low thermal conductivity, low volumetric weight and economical consumption of cement – large-porous concrete is excellent for creating wall structures.
The advantage of this type of concretes lies in their low weight and excellent thermal insulation properties, which are inaccessible to conventional concrete. At the same time, lightweight concrete has low strength, but this does not have a particular effect on those building structures where they are used. The technology for the production of lightweight concrete does not differ from the scheme for creating conventional concrete solutions. Lightweight concrete includes pumice concrete, expanded clay concrete, slag concrete, etc..
Pumice is the only natural material used in lightweight concrete as a filler. Pumice concrete has a low bulk density (from 700 to 1100 kg / m3) and its thermal insulation properties are higher than that of other types of lightweight concrete.
Expanded clay acts as a filler in expanded clay concrete; this type of lightweight concrete is used to create large-sized panels. Its strength properties, mobility and behavior during paving are completely similar to the dependencies related to other types of concrete..
Clinker cement acts as a binding agent for slag concrete; slags from the metallurgical industry (blast furnaces – granular, dump and swollen) and fuel slags formed after the combustion of anthracite and coal are used as a filler. The slag used in cinder concrete as a filler must be free of garbage and earth inclusions, contain unburned coal particles in its structure (for anthracites – over 8-10%, for brown coals – over 20%).
It is possible to reduce the consumption of cement in the composition of slag concrete by introducing special additives that densify and dilute the cement. For example, such an additive can be lime, which allows not only to reduce the consumption of cement, but also to improve its quality. Ash, clay, stone flour, etc. are used as special additives. Due to the introduction of additives, the molding of the cinder-concrete mixture is improved, otherwise this would require the introduction of more cement.
Compositions for especially lightweight concrete
Particularly lightweight concretes have another name – aerated concretes, these include aerated concrete, large-porous concrete with a highly porous filler, foam silicate, foam concrete, etc. Aerated concretes are created by introducing foam-forming additives into their composition that create air pores. Thus, air filling the concrete cells becomes the main filler in especially lightweight concrete. Due to the high thermal insulation properties of air, cellular concrete has low thermal conductivity and volumetric weight, low water absorption and high frost resistance..
The strength properties of aerated concrete are greatly influenced by their volumetric weight, for example, having a volumetric weight of 800-1000 kg / m3, the strength of especially lightweight concrete will be 50-75 kgf / cm2, with a lower volumetric weight of 600 kg / m3 strength will be 25-30 kgf / cm2.
Unlike other types of concrete, aerated concrete can be easily processed with ordinary tools – a plane, an ax and a saw, allowing you to make various slabs, panels, shells for thermal insulation and protection of heating networks, etc..
Among cellular concrete, the latest innovation is aerated concrete. Compositions for aerated concrete contain sludge (grinding of a sand-lime mixture, lime in it – 1.5-2% of the mass of sand), cement and a gas-generating additive – aluminum powder.
The concrete mixture of aerated concrete is kneaded in a concrete mixer, into which slurry and cement are introduced alternately, then, after 3 minutes, a portion of aluminum powder. The mixture is stirred for 8 minutes, then poured into molds and kept in them from 8 to 10 hours. During the holding period, the mass of aerated concrete swells and forms a hump. After the expiration of the period, the hump is cut off, the molds with the casting of aerated concrete are placed in autoclaves for steam treatment at a temperature of about 100 ° C and a pressure of 10 atmospheres.
Aerated concrete has a bulk density in the range of 400-1000 kg / m3, you can get aerated concrete with a lower bulk density (less than 400 kg / m3), if nepheline (non-fired) cements are used as a binder.
Aerated concrete is used to create blocks and panels for residential and industrial construction projects.
Aerated concrete, one of the most popular types of aerated concrete, is created from a mixture of cement, sand, water and an air-entraining additive such as rosin soap. The mixture is whipped in a concrete mixer rotating at high speed – as a result, a foamy mass is formed, which is poured into molds for setting and hardening. There is another way to obtain foam concrete – the foam is produced separately, in a special apparatus for foaming, then it is added to the concrete solution in a conventional concrete mixer. The foam concrete obtained in this way is more uniform in density than that obtained in a high-speed mixer.
Foam concrete has a bulk density of 400-800 kg / m3. As with all types of aerated concrete, foam concrete shrinks significantly during hardening, therefore it needs either autoclave steaming or holding for several hours. In foam concrete that is not subjected to steaming in an autoclave, it is necessary to introduce a larger amount of cement (350-450 kg / m3), its shrinkage causes numerous cracks up to complete destruction in some cases. Autoclaved foam concrete contains a greater amount of sand, and steaming in an autoclave at high temperatures and pressures of 8-12 atmospheres allows it to completely avoid shrinkage and cracking. Crushed sand serves as a filler for foam concrete; instead, you can use tripoli (opal sedimentary rock), marshalite (ground pulverized quartz) or fly ash from power plants.
Foam silicate has the same production technology as foam concrete. Their difference is that in the production of foam silicate, ground lime (boiling water) acts as a binder.
To get one m3 steamed aerated concrete requires up to 280 kg of cement, and for one m3 foam silicate requires 150 kg of lime. The cellular structure of the foam silicate is obtained in the course of successive operations: dissolving the foaming agent in water; shaking the solution until foam forms; mixing the binder and filler with water; combining concrete solution with foam solution and mixing in a foam concrete mixer. The concrete mixer for mixing foam silicate consists of three drum sections: in the first drum the concrete solution is mixed; in the second – an aqueous solution of a foaming agent; when ready, the contents of the first two sections enter the third drum, where cellular foam silicate is formed. Next – pouring the ready-made mass of concrete into forms and steaming in autoclaves under a certain pressure and temperature.
The main mistakes when drawing up concrete:
- the introduction of excess water. Rigid concrete is much more difficult to lay than plastic or cast concrete, so some would-be builders prefer to add water and thereby facilitate their task. As a result, “excess” water, without reacting with the binder, retains its free state in the mass of concrete. It evaporates over time and leaves behind pores that reduce the strength properties of concrete;
- insufficient compaction of the laid concrete mass (laying is carried out without vibration). In this case, the concrete contains a large number of voids filled with air – they reduce the strength and grade of concrete.