What crushed stone is needed for concrete? What fraction of crushed stone is needed for concrete Is crushed stone obligatory in concrete.

01.06.2018

Concrete is a modern building material that involves the use of a mixture of cement with water, sand and any other solid material. Most often, crushed stone is used from solid materials, due to its special properties and good opportunities for versatile use. But it can be different, and if you want to order crushed stone, you should decide what fraction of crushed stone is needed for concrete.

Why crushed stone is a good aggregate for concrete

Crushed stone is made from solid mountain deposits, the grain size of which is at the level of 0.05-0.7 cm according to European standards. This styling material is worth using because of the following advantages:

Crushed stone should be defined as a large aggregate, which makes it possible to nullify all the processes of unsteadiness and compaction of the structure. In this regard, its use contributes to an increase in the quality of the entire mixture.

The addition of crushed stone forms the skeleton of a concrete structure, in which the aggregate can be up to 90%.

Most of the financial resources are spent on cement. To save money, you need to strive to reduce costs while maintaining sufficient quality. The most important quality parameter and its indicator is strength, which depends on the total mass density. For this purpose, gravel of a special size is selected, which, when compacted, can be distributed into smaller ones. From this we can conclude that good concrete implies the presence of different fractions of the material.

Fractional division of crushed stone

To talk about which crushed stone for concrete is best suited, it is worth understanding what a fraction is. A fraction is defined as nothing more than the division of particles into groups of equal sizes.

After crushing the material, we obtain the following fractional indicators:

• 0.05-0.1; 0.05-0.2 cm;
• 0.1-0.15; 0.1-0.2 cm;
• 0.15-0.2 cm;
• 0.2-0.4 and 0.4-0.8 cm;

However, on an individual order, you can get crushed stone with a grain size of up to 1.5 cm.

A few words about the correct selection of fractions for the mixture

Most often, crushed stone is used for the mixture, the particles of which can be attributed to the first fraction. Although this option is not financially economical, it remains popular due to the high quality of the resulting mixture. When using larger fractions, concrete will not be uniformly filled with solid material, which will significantly reduce the strength of the structure.

An important aspect in determining the fractional component is the subsequent area of ​​​​use of the concrete mix. The following table will help you navigate:

As is clear from the table, the fraction is determined based on the scale of activity and the desired resistance of the hardened mixture.

The stability of the mixture is also affected by the hardness of the filling material, which must be selected as follows:

Do not take the given data as an unshakable truth. Deviations are more than possible and they are eliminated by changing the ratios of other components. For example, if high quality concrete is needed, and only low strength crushed stone is available, then more cement is added to the final mixture. This also works in the absence of aggregate of the required fractionation, but already in changing the amount of sand added.

From this we can conclude that the durability of concrete depends not so much on the selected materials, but on how the proportions for the mixture were selected.

You can buy crushed stone or concrete in Rostov-on-Don in our company "Concrete 61". You can be sure of the timely and high-quality provision of services.

Before deciding which crushed stone fraction is needed when performing concrete work for the manufacture of reinforced concrete products and structures, it is necessary to consider such concepts as crushed stone and its fraction.

What fraction of crushed stone should be used in the manufacture of reinforced concrete structures?

rubble- building material obtained by crushing dense rocks into fractions of 5 ÷ 70 mm or more. It has an unrolled, rough surface. After crushing the crushed stone, it is sieved into fractions.

Depending on the rock from which crushed stone is produced, there are the following types of crushed stone:

1. limestone;
2. gravel, or just gravel;
3. granite;
4. artificial (from production waste, for example, slag).

Granite aggregate is considered one of the best - it has high strength, but at the same time it is the most expensive in terms of cost.

Fraction of rubble is a collection of particles of the same size or interval.

So, then you need to figure out why crushed stone is added to the concrete mix? Briefly, we will consider the reasons why it is necessary to use crushed stone, and then we will consider which fractions must be added to the concrete mix in order to obtain a strong and durable structure.

Why is it necessary to add crushed stone for the preparation of concrete?

1. Crushed stone makes up the skeleton of concrete, which is approximately 80 ... 85% of the total volume. Crushed stone in concrete is called coarse aggregate.
2. The use of coarse aggregate in concrete allows minimizing the processes of shrinkage, creep, increases the strength, density, water tightness and crack resistance of the structure.
3. Since the most expensive component of concrete is cement, it means that it is necessary to strive to reduce its consumption, without deteriorating the properties of the produced concrete. One of the factors for obtaining high strength concrete is density - the denser the concrete, the stronger it is. In order to reduce the intergranular space (voids) in concrete, it is necessary to select such particle sizes of sand and crushed stone so that when compacting the concrete mixture, the space between large particles of crushed stone is occupied by smaller particles of coarse aggregate, and the voids between small particles of crushed stone are occupied by large particles of sand, etc. . This means that for good concrete it is necessary to have particles of coarse aggregate of several fractions. This approach to calculating the composition of concrete saves a significant amount of cement during construction. Saving cement is explained by the following: small particles of cement are like glue, which should cover and “glue” together all the surfaces of sand and gravel. As you know, large-sized particles have a smaller specific surface than small-sized particles, therefore, if crushed stone and sand are very fine, more cement is needed to “glue” it, or that amount of cement will not be enough to envelop the surfaces (strength will decrease significantly) . In order to verify the correctness of the above justification, it is necessary to calculate the composition of concrete (it can be done using the NIIZhB method) using first a large fraction, and then with several fractions, and then compare the cement consumption.

In quarries for the extraction of granite or other crushed stone, according to GOST 8267-93, the following main crushed stone fractions are produced after crushing:

• 5 (3) ÷ 20 mm;
• 5 (3) ÷ 10 mm;
• 10 ÷ 15 mm;
• 10 ÷ 20 mm;
• 15 ÷ 20 mm;
• 20 ÷ 40 mm;
• 40 ÷ 80 (70) mm.
• Fractions 70 (80) ÷ 120 mm, 120 ÷ 150 mm can also be produced on an individual order.

What fractions are used to make concrete? How to choose the right crushed stone fractions?

However, to obtain high-quality concrete, not one, but two or three crushed stone fractions are often used, or there are crushed stones of different sizes. At enterprises for the manufacture of concrete and reinforced concrete structures, according to GOST 8267-93, in the laboratories of the Quality Control Department (technical control department), the grain composition is checked, the smallest d and largest D are the nominal sizes of crushed stone. These characteristics of crushed stone are determined by sifting crushed stone through standard sieves and weighing the residues on each sieve. Next, the total residues on each sieve are determined. The results obtained are compared with the values ​​in table 1 from GOST 8267-93.

Table 1

Note:

1. For crushed stone fractions 5(3) ÷ 10 mm and a mixture of fractions 5 (3) ÷ 20 mm, the condition is additionally applied under which the total residue on the lower sieve 2.5 (1.25) mm should be 95 ÷ 100%.
2. It is allowed, at the request of the consumer, so that the total residue on a 0.5 sieve (D min + D max) is 30-80% by weight.

After sieving, a graph of grain sieving of crushed stone is built, and if the curve of this crushed stone falls into the shaded area of ​​the graph ( rice. one), which means that such crushed stone can be used in the manufacture of concrete, reinforced concrete. If the curve does not fall within the specified area, add another fraction and repeat the sieving.

Rice. 1. Grain composition of gravel (crushed stone)

According to GOST 26633-91* “Concrete is heavy and fine-grained. Specifications ”the largest crushed stone size for a given reinforced concrete structure should be established in the standards, in standard projects, technical conditions or working drawings of reinforced concrete structures. Knowing what the largest grain size of crushed stone is provided for in our case, according to the table from the same GOST (clause 1.6.4.), We determine which fractions can be used, tab. 2.

table 2

Note. It is allowed to use a fraction of 3 ÷ 10 mm if sand is used as a fine aggregate with a particle size modulus of not more than 2.5.

In order not to build a graph of the grain composition of crushed stone, you can use the table from GOST 8267-93 (clause 1.6.5., Table 5), and check the content of individual fractions, tab. 3.

Table 3

The largest aggregate size, mm	Fraction content in coarse aggregate (crushed stone, gravel), %
	5(3)÷10 mm	10÷20 mm	20÷40 mm	40÷80 mm	80÷120 mm
10	100	–	–	–	–
20	25 – 40	60 – 75	–	–	–
40	15 – 25	20 – 35	40 – 65	–	–
80	10 – 20	15 – 25	20 – 35	35 – 55	–
120	5 – 10	10 – 20	15 – 25	20 – 30	30 – 40

A simplified version of the choice of the required crushed stone fraction

The process of choosing the correct fraction of crushed stone, which is briefly described above, mainly concerns enterprises that produce concrete and reinforced concrete products. But in most cases, it is enough to know the following information given in tab. 4(based on SNiP 3.03.01-87 Bearing and enclosing structures).

Table 4

Parameter	Parameter value
1. The number of fractions of coarse aggregate with grain size: up to 40 mm more than 40 mm	At least two At least three
2. The largest aggregate size for reinforced concrete products	No more than 2/3 of the smallest distance between rebars
3. The largest aggregate size for reinforced concrete slabs	Not more than 1/2 plate thickness
4. The largest aggregate size for reinforced concrete thin-walled products	No more than 1/3 - 1/2 of the thickness of the product
5. When pumping with a concrete pump, the largest aggregate size	Not more than 0.33 of the inner diameter of the pipeline
6. When pumping with a concrete pump, the largest aggregate size, including grains of the largest size of flaky and needle-shaped	Not more than 15% by weight
7. When concreting underground structures using injection and vibration injection methods (clause 2.7)	No more than 10÷20

According to SNiP 3.03.01-87 (clause 3.1), it is allowed to use crushed stone and gravel of the following fractions in the construction of massive hydraulic structures:

• 120÷150 mm;
• more than 150 mm, lay directly in the form (formwork) before laying the concrete mix.

For clarity, we present in tabular form the predominant use of crushed stone, depending on the fraction, tab. 5

Table 5

Fraction of rubble	Application area
5 (3) ÷ 20 mm; 5 (3) ÷ 10 mm; 10 ÷ 15 mm; 10 ÷ 20 mm; 15 ÷ 20 mm;	Production of concrete, concrete and reinforced concrete structures, bridge elements, floor slabs, etc.
20 ÷ 40 mm; 40 ÷ 80 (70) mm.	Foundation laying, production of industrial buildings and structures, concrete, concrete and reinforced concrete structures, construction of roads and railways
Joint use of several fractions with particles from 20 to 70 mm	Construction of massive industrial buildings and structures, bridges, tunnels, etc.
70 (80) ÷ 120 mm, 120 ÷ 150 mm, over 150 mm	Construction of massive foundations, industrial buildings and structures, used in landscape design: decoration, decoration of pools, banks of a reservoir

A few more important notes you need to know in order to make the right concrete mix:

1. The quality (purity, absence of organic impurities) of aggregate (crushed stone) has a significant impact on the strength of concrete. According to GOST 26633-91 * (clause 1.6.5), the content of dusty, clay and organic impurities in coarse aggregate (crushed stone) should not exceed 1-3% by weight. If, nevertheless, the gravel is contaminated, you can rinse it with water under pressure from a hose.
2. Also, the strength of the aggregate has a significant effect on the strength of concrete. Depending on the class of concrete, the brand of crushed stone from natural stone must be at least (GOST 26633-91 *, clause 1.6.7). - see table below tab. 6.
3. It is not recommended to use secondary crushed stone for the manufacture of the foundation.

Table 6

If you have any questions, write in the comments below.

You were advised by an expert

Konev Alexander Anatolievich

The concrete mixture includes several components that allow it to betray all the necessary characteristics. Without fail, it must contain 3 main components: cement, water and aggregate. Very often, crushed stone is used as a filler, because it has high performance characteristics.

To obtain high-strength concrete, high-strength types of crushed stone are used.

The strength of crushed stone can be 1000 MPa and even more. This value will depend on the type of material and its features. Therefore, consider what kind of crushed stone can be used to prepare a concrete mixture.

Varieties of crushed stone for the preparation of concrete

Types of crushed stone: a) cuboid; b) acute-angled; c) wedge-shaped; d) flaky.

Crushed stone for concrete can be artificial and natural. The first option is obtained from construction waste and household slag. It is a recycled material, so it has a low cost. In construction work, it is almost never used (they are used to strengthen weak soils in places where non-federal roads are being built). The second type is obtained from rocks by crushing them. With the help of crushed stone, it is possible to reduce the creep and shrinkage of the soil, increase the strength and durability of concrete.

The choice of crushed stone is carried out according to such a characteristic as the size of fractions (the size of individual particles that prevail in the composition of the material). The smaller the size of the fractions, the smaller will be its number.

When coarse-grained crushed stone is used to provide high strength when setting concrete, and fine-grained - to better fill voids and cavities.

Prickly and flat crushed stone reduces the strength of the mixture, while increasing the consumption of cement. Its use reduces the frost resistance of concrete, so they try not to use crushed stone of this form during construction work. Let us consider in more detail each type of crushed stone that is used in construction work.

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Granite crushed stone for concrete

This material is one of the most durable ore materials. It is made by crushing natural granite. Such crushed stone is the best aggregate for concrete, which should have a high grade:

• airfield and road surfaces;
• critical areas that are under heavy load (columns, walls, slabs);
• bridge deck and other types of bridge structures.

The quality of crushed granite can be determined by such characteristics as density, compressive strength and fraction, which should be in the range of 5-150 mm. The highest demand is for a fraction of 5-20 mm in size, which is used for the construction of reinforced concrete products, bridges and road surfaces. When it is used, the reliability of the composition and its high operational parameters are guaranteed.

The middle fraction has a size of about 40 mm, which is used in the construction of industrial facilities. The size of the coarse fraction required for the design of overall structures is 40-70 mm. Larger dimensions are used in the construction of rubble concrete foundations.

The material of granite in terms of its operational characteristics is quite durable. Its brand lies within 1200-1400, and frost resistance - up to 400 cycles.

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gravel and limestone material

Gravel crushed stone is mined by sifting quarry rock or by crushing natural rocks. In terms of performance, it is inferior to granite, but has a lower price. This filler is widely used in the production of reinforced concrete products, in road construction and in the design of foundations. Depending on the size of the fractions, it is divided into small (up to 10 mm), medium (10-20 mm) and large (up to 40 mm).

Crushed stone made from limestone is one of the cheapest types. According to the accuracy class, several groups of such material are distinguished:

• brand M600-M800 is the result of processing limestone or dolomite. It has high performance and has a large fraction size;
• brand M300-M600 is made of limestone;
• the M200 brand is almost never used for the manufacture of concrete, as it has a high price.

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Dependence of the brand of concrete on the brand of crushed stone

Any hard rock is suitable as the main filler: expanded clay, limestone, gravel, granite, asphalt or chipped brick. But there are some nuances here. Each filler has a certain strength, which leads to severe limitations on the possibility of application. For example, the manufacture of a critical structure is unthinkable using broken bricks.

The strength of a solid drive indicates its brand. A table with an approximate ratio of crushed stone is presented below.

Table 1. Choice of brand of crushed stone for concrete

Brand of crushed stone	Concrete grade
M1200	M400-M500
M1000	M300
M800	M200
M600	M100

This table may have some deviations in one direction or another, but they are acceptable and can be adjusted by the ratio of cement and sand.

In practice, concrete M250 and less is made using gravel, and M300 and above is made using granite.

Concrete cannot be stronger than its fillers, that is, it will not be possible to obtain M500 concrete from M400 cement. To get the right brand, you need to choose the right proportions of all components.

Most of us know that concrete is made from a mixture of cement, sand and water. Depending on the required strength of the future solution, the proportions of creating this mixture may be different, but the ingredients do not change. Sometimes, for a special type of concrete, other components are added to the cement mixture in small quantities, but in general, the principle of creating concrete does not change. Here, many people have a question: in what cases is crushed stone added to concrete and why is it needed?

Crushed stone is also a building material. It is obtained by crushing rock stones into small fractions ranging in size from 5 to 70 or more millimeters. As you know, crushed stone has an uneven surface.

There are several varieties of crushed stone, each of which is used in a certain type of construction work:

• granite;
• gravel;
• limestone;
• slag or artificial.

Granite crushed stone is considered the most optimal for use in construction. However, it should be borne in mind that it is also the most expensive in terms of price.

Crushed stone is added to concrete for several reasons, one of which is that crushed stone, as a cheaper material compared to cement, but with a high degree of density and hardness, perfectly replaces it when creating a concrete mixture. In addition, crushed stone has other advantages compared to cement. It reduces creep, and the percentage of shrinkage of the mixture. Crushed concrete is much less likely to crack than conventional cement mix, and its density and water resistance are much higher.

When adding crushed stone to concrete, some nuances should be taken into account. Despite the fact that crushed stone in construction terminology is a large filler and significantly increases the density of the finished concrete mix, it should be added carefully to the mixture of sand and cement. To prevent air pockets from appearing near individual fractions of crushed stone, it is best to mix large particles of crushed stone with small particles of crushed stone and sand. That is, if you want to make strong concrete, it is not enough to add only large granite crushed stone with increased hardness to it - it will be much more efficient to mix large, medium and small crushed stone with a mixture of sand and cement. This will allow you to produce really strong and reliable concrete, minimizing the amount of cement in the overall mixture.

Adding only small gravel is not a very effective undertaking. In order to get durable concrete, you will need not so little cement, so the effect will not be as significant as from the use of crushed stone of different sizes.

Most often, concrete using crushed stone is produced when creating hydraulic reinforced concrete structures, tunnels, bridge elements, stone supports, fences and various foundations. Gravel is widely used in decorative architecture and landscape design. Not a single construction of railways and roads can do without crushed stone of a large fraction.

If you are going to add crushed stone to concrete, you should take into account that the absence of dust and other impurities in crushed stone is of great importance for the strength of the concrete mixture. According to GOST, their content should not exceed 1-2% of the total mass of crushed stone. In order to achieve cleanliness of crushed stone, it can be washed with a pressure of water from a hose.

Despite the fact that in many cases crushed stone is added to concrete in order to reduce its cost by reducing the amount of cement, it is highly discouraged in construction work, especially in laying the foundation, to use secondary crushed stone. Despite the relative cheapness, its density is noticeably lower than that of new crushed stone.

Composition of concrete - how to choose the composition of concrete?

ready-mixed concrete tender

The Lenbeton company is the first tender site for the sale of concrete in St. Petersburg. Our Company was founded by a group of professionals with extensive experience in the construction industry. We believe that this format of work with a client is an optimal and honest scheme of partnership relations.

In the classic version, the composition of concrete includes such components as a binder, water and fillers. Today, the construction industry supplements it with various plasticizers, water repellents and other additives that allow construction work to be carried out in the off-season, as well as increase the technical characteristics of this material.

GOST and concrete

GOST strictly determines the proportions in the composition of concrete and, depending on this, divides this building material into types. The ratio of components depends on the brand of cement used, the moisture content of the sand, and the fractions of the filler. The most common brand of concrete is 200. This brand of concrete has the following composition: cement M400 - 1 part, water - 3 parts, filler - 5 parts. Since the main binders in concrete are water and cement, before buying concrete, it is necessary to deal with such a technical indicator as W / C (water-cement module or water-cement ratio)

The strength of concrete has an inverse relationship with W / C - the lower this indicator, the stronger the building material. For concrete, it is enough that the W / C is equal to 0.2, but such concrete will not be plastic enough, so when choosing concrete, stop at a water-cement ratio of 0.3-0.5.

GOST regulates concrete according to:

• purpose - for ASG, as well as special (decorative, hydraulic, road, heat-resistant, etc.);
• type of binder material - lean, fatty, marketable;
• filler type - here they are similar;
• structure - large-porous, cellular, dense and porous;
• hardening conditions - in natural or special conditions;
• volumetric mass - light, extra light, lightweight, heavy and extra heavy;

Why is crushed stone in concrete?

The most common aggregate in concrete is crushed stone. Depending on the size of the granite obtained as a result of crushing, it is graded from fine to coarse fraction. However, consumers are most often not aware that not only the particle size is regulated by SNiP. An important indicator is also the content of grains of acicular and lamellar forms per unit volume. It is the shape of the grain that determines the groups of crushed stone:

• cuboid - 12-15%;
• regular -18-25%;
• flaky - over 25%.

Here, the percentage determines the ratio of the mass of grains of a given surface to the mass per unit volume (density). Crushed stone must be added to concrete not only to save cement. This is mainly done for better adhesion of the mortar, since the rough surface of the crushed stone particles and their acute-angled shape contribute to the bonding of all components of the concrete.

Why is there reinforcement in concrete?

Even with a low load, concrete structures are destroyed. A steel rod in tension works 100-200 times better. Therefore, in order for the entire concrete structure to work as a whole, one or more reinforcing bars are introduced into the concrete. Further, under the action of vibrocompression, air pockets are almost completely removed from concrete, and at the same time, the adhesion force between steel bars and concrete increases.

As a result, the bending strength, compression and tension increase, as well as the temperature deformation of the concrete structure is extremely low. Depending on the diameter and profile of the section (with or without moon-shaped protrusions), the reinforcement is divided into classes from A-1 to At-7. And if class A-1 is used in non-stressed structures more often as a mounting element for welding grids, then At (smelted from thermally compacted steels) is used for mounting concrete structures operating in aggressive environments.

Whatever class of reinforcement or other embedded materials is used in concrete, this building material is economical, fire-resistant, technologically advanced, and also has significant indicators of biological and chemical resistance, frost-resistant.

Why is ammonia added to concrete?

If you need to buy concrete with delivery, then it is very important to study the certification documents for this material. Since unscrupulous manufacturers add various mixtures with a high content of calcium nitrate to the concrete to speed up the hardening process.

And although they contain a small amount of ammonium salts, which prevent the formation of lumps of calcium nitrate, ammonia gas is released as a result of the interaction. Moreover, the more ammonium salts added to the concrete, the more pronounced the smell of ammonia.

Living or working in such premises can result in irreversible health consequences, therefore, the selection of the composition of concrete consists not only in knowing the brand of this building material, but also in carefully studying the manufacturer's reputation, and seriously reading the certificate for the required products.

Reinforced concrete: a gift from a botanist to the construction industry

In 1867 French gardener Monnier discovered and patented reinforced concrete. In the manufacture of cement pots for plants, he accidentally added pieces of metal there, and was surprised at the solidity and durability of these products.

Today, reinforced concrete is the most important building material, which is a composite material that includes concrete and steel. The fact is that concrete itself works great in compression, and steel, as you know, in tension. Combining these materials into a single whole will achieve high levels of strength, endurance, seismic resistance, fatigue failure, and many others.

Order a call from the Lenbeton manager

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Crushed stone as a necessary element of concrete mortar

10/27/2014 Fillers for concrete are varied. Among them, crushed stone often leads. Let's consider this situation in more detail. The stone from which concrete is made has its own strength. This strength compared to the strength of the finished cement will be much higher. And it would seem that this is wonderful, because the work performed with such concrete would have lived for many centuries, like mountains, but there is a major nuance here. It would be very inconvenient, time-consuming and practically impossible to create buildings from such huge stone blocks, because mankind invented concrete, and actually for concrete there are already fillings. Crushed stone, acting as a filler, performs its functions at a height. It is also called coarse aggregate, fine aggregate is sand. The quality of the finished concrete will directly depend on the quality of the crushed stone produced. Because the placeholder, you should read information about it, about the size and its manufacturer. So why does concrete need crushed stone? By itself, concrete cannot be used without filler, since its volumetric shrinkage would not allow pouring the mortar and retaining its shape. Without filler, concrete could not be shaped, it would simply spread when hardened. To prevent this from happening when working with concrete, fillers such as crushed stone and sand are used. Crushed stone is classified as a large filler, sand as a fine one. The more work is required in the thickness of the poured concrete layer, the naturally thicker the filler is selected. The volume of the filler is also often called a fraction. Without a special filler, such as crushed stone, the strength of concrete could not be fixed. Crushed stone improves the overall strength of the solution, from which the required work is then produced. Crushed stone also has its own strength. It directly depends on the type of stone used in the manufacture of this product. If the question of working with high-quality concrete is now in your main place, do not buy crushed stone without thinking and reading about its characteristics. We have already managed to tell you why there is crushed stone in concrete, now we will tell you a little about its production. Almost any crushed stone, whether it be sandstone, granite rocks or limestone, can become a ready-made filler for concrete. So, depending on the type of stone used, crushed stone has a number of characteristics by which its strength, shape, grain size, and the amount of impurities contained can be determined. Crushed stone for concrete usually falls into three categories: 1. Granite crushed stone (this is crushed stone, in the production of which mainly granite was used) 2. Lime crushed stone (which is obtained as a result of crushing limestone) 3. Gravel crushed stone (a product obtained by crushing rocks or sieved in granite rock) The category of crushed stone itself can also differ according to radioactivity and grain sizes.

Do not be afraid for the first time to use such materials. This type of work is the most common, and just carefully making the solution adhering to the necessary standards, the result will undoubtedly satisfy you.

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Ode to concrete

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09.09.2012 21:03

WHAT YOU NEED TO KNOW ABOUT CONCRETE.

“Live a century - learn a century” - (proverb).

“I know that I know nothing” (ancient Greek thinker Socrates).

These epigraphs are intended for those builders and customers who have decided that they know everything about concrete, since they have been working at a construction site for several years. In addition, there is a stereotype in Russian society that a builder is the simplest profession, and concrete worker is the simplest of all construction specialties. Specialists of the company "Credo" will not enter into a dispute with those who think so. But they cannot indifferently observe how sometimes illiterate builders and non-builders handle concrete. And with their ignorance, they not only spoil high-quality and expensive material, which causes direct damage to the customer or themselves, but also discredits a conscientious concrete manufacturer, suggesting to the customer that the concrete was of poor quality.

For the convenience of the reader, the article will be structured in the form of questions and answers. At the same time, most of the questions were dictated by practice.

What is concrete?

It seemed like a simple question. But few can give the correct answer to it. Concrete is an artificial stone material. It uses the best properties of stone - its strength. But why can't you just use a stone? Because it is very laborious and expensive, and sometimes it is simply impossible to give the stone the desired shape or size. For example, there are stone (granite) curbs and there are concrete curbs. Everyone understands that a concrete curb is cheaper. It is easier to shape concrete than it is to process granite. Well, it’s hard to imagine an interfloor ceiling made of stone. Is it just vaulted ceilings made of stone on small spans. Or it is already quite difficult to find a stone 12 meters long or more. And we see reinforced concrete beams of this length on almost every bridge. In addition, both stone and concrete do not take tensile loads well. But if reinforcement is inserted into the concrete, then the reinforcement located inside the concrete will perceive the tensile forces on the concrete. Everyone understands that inserting rebar into the stone and gluing it to it is also very laborious and expensive.

What is in concrete?

Concrete consists of three main components - binder, water and aggregate. For brevity, we will simply refer to the binder as "astringent". We will talk about the most common building concrete - cement concrete. From the name itself it is clear that cement is used as a binder in cement concrete. For brevity, we will simply refer to cement concrete as "concrete". There are many types of cement. We will not consider its varieties. This is a topic for a separate study and is more interesting for concrete manufacturers and other specialists. The main types of aggregates are crushed stone, gravel and sand. Crushed stone differs from gravel in that it is a crushed material. In our area, most often it is a product of crushing the same gravel, but separated by fractions, that is, by size. Gravel concrete is slightly cheaper, as gravel is cheaper than crushed stone. Concretes up to certain grades are made from gravel. The main characteristics of crushed stone and gravel are size and strength. Sand is coarse-grained and fine-grained. Fillers must be selected in a strictly defined proportion. Simplistically, it can be imagined that the voids between the particles of crushed stone or gravel should be filled with sand, and the voids between the sand particles should be filled with cement. Builders are doing the right thing when they purchase ready-made gravel or crushed stone-sand mixture (GPS or SCHPS) for the preparation of concrete at the facility. In the manufacture of them in the factory, the ratio of crushed stone-sand or gravel-sand is optimal.

What qualities should concrete have?

The main physical characteristic of concrete is its strength. It is measured by special devices when the concrete reaches the age of 28 days. Strength is measured in units of pressure. The most understandable and familiar to most people is the unit of strength in kilograms per square centimeter (kg/cm2). For example, a strength of 100 kg/cm2 means that concrete collapses when subjected to a pressure of 100 kg/cm2. Previously, and often now, this strength meant the brand of concrete. For example, 100 kg / cm2 meant M100, etc. According to the new GOST, the concept of “concrete class” was introduced, which takes into account not only strength, but also some other characteristics. But in this article, for simplicity, we will equate the concept of “concrete grade” and “concrete class”. For example, the grade of concrete is M100, the class of concrete is B7.5. There are special tables for matching the grade and class of concrete. Many manufacturers, for the convenience of buyers, indicate in the price lists both the brand and the class of concrete. For example: concrete B 7.5 (M100). In addition to strength, concrete has other physical characteristics. For example, water resistance, frost resistance and others. The feature names speak for themselves. Frost resistance - means the number of alternating freezing and thawing that concrete can withstand without breaking down. Water resistance is the ability of concrete to prevent water from penetrating through it. Frost resistance and water resistance are closely related to each other.

What is cement and why is it needed in concrete?

The first mention of cement appeared relatively recently - in 1844. Although in one form or another (for example, volcanic ash), cement has been known since ancient times. Simplistically, the production of cement can be represented as follows. Crushed rock of a special composition (marl) is fired in a kiln. During the firing process, chemically combined water is removed from the marl. As a result, clinker is formed. It is ground in special ball mills to a powder state. This powder is the cement. When a strictly defined amount of water is added to cement, it turns back into stone.

Why do we need crushed stone and sand in concrete?

Indeed, when water is added, cement will turn into stone anyway. Answer: making an artificial stone only from cement is both expensive and difficult. In addition, the cement itself shrinks a lot. Therefore, aggregates are added to concrete: crushed stone or gravel and sand.

What happens if you put an arbitrary number of aggregates in concrete?

There will be concrete. But not the quality that the manufacturer wanted to achieve from him. If you put an excess amount of crushed stone, then there will be voids in the concrete that are not filled with sand and cement. Accordingly, the required strength will not work. If the sand is more than normal, then the cement contained in the concrete will not be enough to “coat” each grain of sand, and the grains of sand will not stick to each other. Accordingly, strength will suffer again. It is possible with a margin, i.e. pour cement in excess. But then the economy will suffer. It will be very expensive concrete. The proportions of the components in concrete are selected by experts in the laboratory. These proportions are called "rebounds".

How much water should be added to concrete?

The amount of water is also determined in the laboratory. In order for cement to turn into stone, only 13% of water by weight of cement is enough. But in fact, in the production of concrete, a larger amount is added. The ratio of the amount of water to the amount of cement by weight is called the water-cement ratio (WC). In practice, it ranges from 0.3 to 0.4. If the VC is made smaller, it will be impossible to work with concrete manually. It will be very hard, thick, dry. It will not be possible to put it into the structure. Such concrete is mainly used in vibrocompression, for example, in the manufacture of paving slabs or curbs. But with an increase in the amount of water, the quality of concrete deteriorates: its strength, water resistance, frost resistance. What to do? Chemical additives called "plasticizers" and "superplasticizers" are used to reduce the amount of water in concrete while still maintaining qualities such as workability.

How is workability measured?

The indicator of concrete, reflecting its workability, is called "mobility". Previously, one could also come across the term "plasticity". Mobility is measured with special devices and is designated as follows: P1, P2, etc.

Is it possible to make selections for the composition of concrete, the same for the whole country?

No, because in each locality there are crushed stone, gravel, sand, water, and cement of different breeds and qualities. And all selections of concrete are made for each specific case. The quality of materials changes, it is necessary to change selections.

Why does concrete thaw?

The characteristic that reflects the resistance of concrete to alternate freezing and thawing is called "frost resistance". Frost resistance is measured by the number of cycles of alternate freezing and thawing, as a result of which the concrete begins to collapse. Frost resistance is designated as follows: F150, F200, etc. This means that concrete can withstand 150 cycles of alternate freezing and thawing, and then it can collapse. The more water in concrete, the lower its frost resistance. Therefore, vibropressed paving slabs have greater frost resistance. The worse the gravel, crushed stone or sand (dirty, fragile, non-frost-resistant), the lower the frost resistance of concrete. Many have watched concrete thaw out of dirty local river gravel.

Why can't water be added to the finished concrete that the supplier brought?

When ordering concrete, the buyer, in addition to the class of concrete, must indicate its mobility. The manufacturer, guided by economic considerations, produces concrete with the ordered characteristics with a minimum margin of safety. Therefore, when concrete arrives at the site, all the components in it are in the ratio and quantity that is necessary for this class of concrete, including water. By adding additional water, builders increase the VC and thereby reduce the ordered and paid characteristics. In other words, the buyer pays for a high class of concrete, and it gets into the structure with underestimated characteristics. Conclusion: it is impossible to add water at a construction site to the imported concrete. Sometimes, for various reasons, however, such a need arises. For example, the builders did not have time to prepare the formwork or for other reasons. The concrete has thickened. Then the buyer needs to contact the concrete supplier for advice. And the supplier's technologist (and a conscientious manufacturer should have such a specialist) will tell you what to do. You need to contact the technologist of the supplier from whom you bought the concrete. It is he who knows what components are used in the manufacture of this concrete and how to proceed in order to maintain the quality of concrete.

Does concrete need maintenance?

Quality care of concrete is no less important than its quality production. Some customers and builders are mistaken, believing that if the concrete is of high quality, then nothing can spoil it. The addition of water has already been written above. Now let's talk about the preservation of the water (or moisture) that is already present in the concrete. As already mentioned, in order for concrete to turn into stone, water is needed. If the builders do not ensure the preservation of water in the concrete laid in the structure, then it will not have the strength that is ordered. What needs to be done for this? Concrete needs to be covered. Especially in sunny or windy weather. The wind often does more damage than the sun. When water evaporates from concrete, it will not be enough for concrete to gain strength. The concrete will "dry out" and will never gain the strength that was planned. With intensive evaporation of water, concrete cracks, as it shrinks sharply. After cracking of concrete, water evaporates from it through cracks even more intensively. In the future, during operation, water may enter the cracks, and the concrete will defrost. Through cracks in concrete, water and air enter the reinforcement, and it rusts and collapses. You can't watch and wait to see if the concrete starts to crack. If it starts, then the process cannot be stopped. It is necessary to cover the concrete immediately after laying, as soon as the film of water disappears from its surface, we call this state of the concrete the word “shaken”. At different times of the year in different weather, this time can be from several minutes to several hours. The experience, qualifications and skill of concrete workers are very important here. The mistake is made by those who replace the covering of concrete by watering it with water. Firstly, the cement is washed out from the concrete surface, and secondly, the top layer of the concrete becomes waterlogged (the CC increases). Consequence - the concrete will "crumble", peel off. What should be covered? Any vapor barrier material. For example, polyethylene film. But the process of covering is very laborious. It is necessary to cover the concrete so that, if possible, its surface is not disturbed. The film must be fixed so that it is not blown away by the wind. The position of the film must be constantly monitored. This is especially laborious in large areas, for example, on ceilings, road surfaces, etc. What is the way out? Very simple. Now many manufacturers of additives for concrete produce products for the care of concrete. These are liquid materials that are applied to the surface of the concrete, as soon as it shakes, with a conventional construction or garden sprayer (sprayer). Most often it is a liquid that has the color and consistency of milk. After application to concrete, the liquid dries up and turns into a film. These materials are called "film-forming materials". It is this film that allows you to keep water in concrete both in the sun and in the wind. As you know, the wind does not blow it away. At first glance it seems that the use of this material is expensive. But this is at first glance. If we calculate the cost of a polyethylene film, the laboriousness of its laying, preservation, cleaning, storage, taking into account the disturbed surface of concrete or the cost of water, spraying, water damage, it becomes clear that the use of film-forming materials is beneficial. In the future, this film evaporates and any finishing materials, including tiles, can be applied to concrete without additional preparation. Conscientious concrete producers often sell these materials themselves. Most often, they do this not to make money, but to help builders and thereby maintain their business reputation, since the concrete will be better preserved and the customer will not have any complaints.

Often concrete loses moisture due to the fact that it is laid on an unprepared base or formwork. Sometimes the basis for concrete is crushed stone or sand. If this material is dry, then it is able to absorb a large amount of water. For example, rubble from Gelendzhik quarries absorbs a very large amount of water. After concrete is laid, moisture from concrete in the area of ​​its contact with the base is intensively absorbed into the base material. As a result, the concrete quickly dehydrates and cracks in front of the astonished builders, who have nothing left to do but blame the concrete manufacturer and cover up the cracks, which can no longer change anything. No watering and covering will help, because shrinkage cracks form from below the concrete. The same happens when concrete comes into contact with dry wooden formwork. Which exit? The base for concrete must be moistened "to failure", that is, until it stops absorbing water, while avoiding the formation of puddles on the base. Those builders who lightly sprinkle water on the base, for example, from a mixer, deceive themselves and the customer. This is not enough. The formwork must be lubricated with special materials, for example, emulsol, mining. This is done not only so that concrete does not stick to the formwork, but also so that moisture does not absorb. If there is no emulsol or mining, then it is necessary to strongly moisten the formwork, again avoiding puddles on horizontal surfaces. The exception is formwork made of laminated plywood or metal. Water doesn't go anywhere in it.

Another enemy of concrete is frost. In order for concrete to become stone, a positive temperature is necessary. Under laboratory conditions, the temperature is maintained in the region of 20 degrees Celsius. It is under such conditions that it is believed that concrete will gain its design strength after 28 days. The higher the temperature, the faster the concrete will gain strength. At the same time, one should not forget about the need to preserve moisture in the concrete. But the rapid rise in temperature when the concrete is heated is also harmful. There are internal (not visible to the eye) stresses and destruction in concrete. It is important to know this not only for those who use concrete heating. When concrete hardens, a chemical reaction occurs with the release of heat. With small structures, this only benefits concrete. With very large, massive structures (most often in industrial construction, for example, powerful foundations), the concrete heats up so much that it needs to be cooled, for example, by pouring water. Sometimes special pipes are laid inside the concrete, water is pumped through them and thereby cools it.

So, concrete must be kept from low temperatures. This is achieved by covering the concrete with films, matting, snow, etc. or warm it up. Concrete must arrive at the object with a temperature of at least 5 degrees. Celsius. To protect concrete from freezing before it is covered or heated, special antifreeze additives are used in its manufacture. They are designed for different temperatures: -5, -10, -15 degrees. etc. and significantly increase the cost of concrete. But these additives protect concrete from freezing only during the production process. In the future, in order for the concrete to harden, it needs a positive temperature, i.e. it is necessary either to cover and thus preserve the heat that the concrete emits during hardening, or to heat it up.

In this article, we touched only on those rules, non-observance of which by builders can damage the business reputation of a concrete manufacturer and damage customers. In fact, the science of concrete is a serious discipline that is constantly evolving and requires a long study. Practicing builders need to have much less knowledge about concrete and the rules for its use than science has, but much more information than is presented in this article. The purpose of the authors of the article was to arouse interest among that part of builders and customers who do not even know the information presented in this article, and to encourage them to independently study the secrets of the concrete worker profession. For those who already know everything that has been said, it remains for the authors to point out only two points: 1. repetition is the mother of learning; 2. Nothing stands still, everything develops, including building science.