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What crushed stone is needed for concrete. What fraction of crushed stone is needed for concrete. Dependence of the brand of concrete mortar on the brand of crushed stone

The concrete mixture includes several components that allow it to give everything required characteristics. 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 or even more. This value will depend on the type of material and its characteristics. Therefore, let’s consider what crushed stone can be used to prepare a concrete mixture.

Types of crushed stone for preparing concrete

Types of crushed stone: a) cube-shaped; 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 waste. 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 you can reduce creep and shrinkage of soil, increase the strength and durability of concrete.

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

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

Needled and flat crushed stone reduces the strength of the mixture, while increasing cement consumption. Its use reduces the frost resistance of concrete, so when carrying out construction work, crushed stone of this form is avoided. Let's take a closer look at 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 filler 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 granite crushed stone 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 the 5-20 mm fraction, which is used for the construction of reinforced concrete products, bridges and road surfaces. When used, the reliability of the composition and its high performance parameters are guaranteed.

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

Granite material is quite durable in its performance characteristics. Its grade is in the range of 1200-1400, and frost resistance is up to 400 cycles.

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Gravel and limestone material

Crushed gravel is obtained by sifting quarry rock or by crushing natural rocks. In terms of performance characteristics, it is inferior to granite, but has more low price. This filler is widely used in the production of concrete products, in road construction and when constructing 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:

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

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Dependence of concrete grade on crushed stone grade

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

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

Table 1. Selecting a grade 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 is impossible to obtain M500 concrete from M400 cement. To get the right brand, you need to choose the right proportions of all components.

Before deciding what fraction of crushed stone is needed when performing concrete work for the production 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?

Crushed stone– a building material obtained by crushing dense rocks into fractions of 5÷70 mm or more. It has a non-rounded, 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 industrial 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.

Crushed stone fraction is a collection of particles of the same size or spacing.

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

Why is it necessary to add crushed stone to prepare 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 to minimize the processes of shrinkage and creep, increases the strength, density, water resistance 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 concrete being produced. One of the factors in obtaining high strength concrete is density - the denser the concrete, the stronger it is. To reduce the intergranular space (voids) in concrete, it is necessary to select such sizes of sand and crushed stone particles so that when compacting the concrete mixture, the space between large crushed stone particles is occupied by more fine particles coarse aggregate, and the voids between small particles of crushed stone were occupied by large particles of sand, etc. So for good concrete it is necessary to have coarse aggregate particles of several fractions. This approach to calculating the composition of concrete saves a significant amount of cement during construction. The saving of 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 crushed stone. As is known, particles large size have a smaller specific surface area than particles small size Therefore, if the crushed stone and sand are very fine, the amount of cement needed to “glue” it together will be greater, or the amount of cement will not be enough to envelop the surfaces (the strength will be significantly reduced). To verify the correctness of the above justification, it is necessary to calculate the composition of concrete (using the NIIZhB method) using first a large fraction, and then 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 fractions of crushed stone 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.
• Can also be produced according to individual order fractions 70 (80) ÷ 120 mm, 120 ÷ 150 mm.

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

However, to obtain high-quality concrete, not one, but two or three fractions of crushed stone are often used, or there are crushed stones of different sizes. At enterprises producing concrete and iron concrete structures according to GOST 8267-93, quality control laboratories (technical control department) determine the grain composition, the smallest d and the largest D 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, an additional condition is 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, that the total residue on the 0.5 sieve (D min + D max) be 30-80% by weight.

After sifting, a graph of grain sifting of crushed stone is built and if the curve of this crushed stone falls into the shaded area of ​​the graph ( rice. 1), which means that such crushed stone can be used in the manufacture of concrete and reinforced concrete. If the curve does not fall within the specified area, add another fraction and repeat the sifting.

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

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

table 2

Note. It is permissible to use a fraction of 3÷10 mm if sand is used as a fine aggregate with a particle size modulus of no more than 2.5.

In order not to plot 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, table 3.

Table 3

Largest aggregate size, mm	Content of fractions 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 option for selecting the required crushed stone fraction

Selection process correct faction crushed stone, which is briefly given above, concerns mainly enterprises that produce concrete and reinforced concrete products. But in most cases it is enough to know the following information given in table 4(based on SNiP 3.03.01-87 Load-bearing and enclosing structures).

Table 4

Parameter	Parameter value
1. Number of fractions of coarse aggregate with grain size: up to 40 mm more than 40 mm	At least two At least three
2. Largest aggregate size for reinforced concrete products	No more than 2/3 of the smallest distance between reinforcement bars
3. Largest aggregate size for reinforced concrete slabs	No more than 1/2 the thickness of the slab
4. 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	No more than 0.33 internal diameter pipeline
6. When pumping with a concrete pump, the largest aggregate size, including grains largest size flakie and needle-shaped	No 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), when constructing massive hydraulic structures, it is allowed to use crushed stone and gravel of the following fractions:

• 120÷150 mm;
• more than 150 mm should be laid directly into the form (formwork) before laying the concrete mixture.

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

Table 5

Crushed stone fraction	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.	Laying the foundation, production industrial buildings and structures, concrete, concrete and reinforced concrete structures, construction of roads and railways
Sharing 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, more than 150 mm	Construction of massive foundations, industrial buildings and structures, used in landscape design: decor, finishing of pools, pond banks

A few more important notes you need to know in order to make the correct concrete mixture:

1. The quality (purity, absence of organic impurities) of the 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, clayey and organic impurities in coarse aggregate (crushed stone) should not exceed 1-3% by weight. If the crushed stone is still contaminated, you can wash it with water under pressure from a hose.
2. The strength of the aggregate also has a significant influence on the strength of concrete. Depending on the class of concrete, the grade of crushed stone from natural stone must be no lower (GOST 26633-91*, clause 1.6.7). - see table below, table 6.
3. It is not recommended to use recycled crushed stone to make the foundation.

Table 6

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

You were advised by an expert

Konev Alexander Anatolievich

Granite, limestone or gravel crushed stone is an indispensable component of concrete solutions that determines the brand building material and, accordingly, the strength and durability of a concrete structure or concrete product.

Crushed stone for preparing concrete mortar

As was said, the strength of crushed stone determines the strength of the final product. In other words, the strength of the foundation, floor slab, column or other structure cannot be higher than the strength of the filler.

In this regard, before considering the question of what fraction of crushed stone is needed for concrete, it is worth citing the dependence of the most popular brands of heavy concrete on the grade of crushed stone:

Crushed stone (brand)	Concrete (grade)
1200	M400-500
1000	M300
800	M200
600	M100

The numbers indicated in the grades of crushed stone and concrete represent the average compressive strength in kg/cm2. As follows from the data presented, for example, using M800 grade crushed stone as a filler, it is impossible to obtain concrete with a strength of 400-500 kg/cm2.

The principle of selecting crushed stone fraction sizes

To ensure a given grade (strength) of concrete, it is unacceptable to use crushed stone with the same fraction sizes. This will lead to the fact that the volume of structures will be filled unevenly, with a large number of voids. This reduces tensile, compressive and torsional strength.

Concerning optimal sizes granules, for pouring critical structures (foundations, columns, floor slabs), it is recommended to use a mixture of crushed stone standard sizes“grains” – 5-10 mm and 10-20 mm.

This costs a little more, but the end result is durable, read - durable design. For pouring non-critical (lightly loaded structures): blind area, garden paths etc., it is allowed to use crushed stone of any fraction, including inexpensive granite screenings, broken brick, furnace slag and other “stone” waste.

You should also pay attention to the characteristics of the configuration of individual crushed stone granules, the so-called flakiness. Flakiness characterizes the flatness of elements.

The use of crushed stone with a high degree of flakiness leads to an increase in cement consumption and a corresponding increase in construction costs. In addition, such concrete is difficult to compact. In this regard, crushed stone with a significant amount of “flaky” granules is not recommended for the construction of buried foundations. At the same time this excellent material for the construction of blind areas, paths and other concrete platforms.

• If you are limited in the choice of crushed stone fraction, focus on granite material with granule sizes from 5 to 20 mm. Concrete mortars based on it can be called “universal”, so they can be used for the construction of all types of concrete structures and products: bases, foundations, floor slabs, blind areas, columns, etc.
• If you decide to increase the density of concrete by introducing fine crushed stone into it, keep in mind that in this case the consumption of cement increases significantly. This will lead to a significant increase in construction costs. As you know, cement is the most expensive component of concrete.

This article discusses the issue of optimal sizes of crushed stone granules for structures filled with the most common, universal type of building material - heavy concrete. For concrete of other types of concrete (asphalt concrete, perlite concrete, silicate concrete, tuff concrete and other types), the recommended filler sizes will be different.

Concrete composition - how to choose the concrete composition?

ready-mixed concrete tender site

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

In the classic version, 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, and also improve the technical characteristics of this material.

GOST and concrete

GOST strictly defines 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, sand moisture content, and filler fractions. 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, you need to understand such a technical indicator as W/C (water-cement module or water-cement ratio)

The strength of concrete is reversed proportional dependence with W/C - the lower this indicator, the stronger the building material. For concrete, it is enough for the W/C to be equal to 0.2, but such concrete will not be sufficiently plastic, so when selecting concrete, stop at a water-cement ratio of 0.3-0.5.

GOST regulates concrete according to:

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

Why is there crushed stone in concrete?

The most common filler 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. 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 needle-shaped and lamellar grains per unit volume. It is the shape of the grain that determines the groups of crushed stone:

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

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

Why is there reinforcement in concrete?

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

As a result, the bending, compressive and tensile strength increases, and the temperature deformation of the concrete structure is also extremely low. Depending on the diameter and cross-sectional profile (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 meshes, then At (melted from thermally compacted steels) is used when installing 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, and frost resistance.

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. Because unscrupulous manufacturers add various mixtures with a high content of calcium nitrate to concrete to speed up the hardening process.

And although they contain a small content 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 concrete, the more pronounced the smell of ammonia.

Living or working in such premises can result in irreversible health consequences, so selecting the composition of concrete consists not only of knowing the brand of this building material, but also of carefully studying the reputation of the manufacturer, and seriously familiarizing yourself with the certificate for the required products.

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

In 1867, the French horticulturist Monier discovered and patented reinforced concrete. When making cement pots for plants, he accidentally added pieces of metal 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 perfectly in compression, and steel, as you know, works in tension. Combining these materials into a single whole will achieve high performance strength, endurance, seismic resistance, fatigue failure and many others.

Order a call from Lenbeton manager

tpbeton.ru

Crushed stone as a necessary element of concrete mortar

10/27/2014 There are various fillers for concrete. Among them, crushed stone is often the leader. 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 ready-made cement will be much higher. And it would seem that this is wonderful, because work done with such concrete would live for many centuries, like mountains, but there is a major nuance here. Creating buildings from such huge stone blocks would be very inconvenient, labor-intensive and practically impossible, which is why humanity invented concrete, and in fact, concrete was used for filling. Crushed stone, acting as a filler, performs its functions at a high level. 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. Therefore, the filler should read information about it, its size and its manufacturer. So why does concrete need crushed stone? Concrete itself cannot be used without filler, since its volumetric shrinkage would not allow pouring the solution and maintaining its shape. Without filler, concrete could not be shaped; it would simply flow as it 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 coarse filler, sand as a fine filler. The more work is required in the thickness of the layer of poured concrete, the naturally thicker the filler is selected. The volume of the filler is also often called the fraction. Without a special filler such as crushed stone, the strength of concrete could not be recorded. Crushed stone improves the overall strength of the mortar, 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 issue of working with high-quality concrete is now on your top priority, do not buy crushed stone without thinking about it and reading about its characteristics. We have already told you why there is crushed stone in concrete, now we will tell you a little about its production. Almost any crushed stone, be it 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 one can determine its strength, shape, grain size, and the amount of impurities contained. Crushed stone for concrete usually calves into three categories: 1. Granite crushed stone(this is crushed stone, in the production of which granite was mainly used) 2. Lime crushed stone (which is acquired as a result of crushing limestone) 3. Crushed gravel (a product obtained by crushing rocks or sifted through granite rock) The category of crushed stone itself can also differ according to radioactivity and size grains

Don’t be afraid to use such materials for the first time. This type of work is the most common, and simply by carefully making the solution and adhering to the required standards, the result will undoubtedly satisfy you.

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beton-spb.ru

Ode to concrete

" Back

09.09.2012 21:03

WHAT YOU NEED TO KNOW ABOUT CONCRETE.

“Live forever and learn” - (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 on construction sites for many years. In addition, in Russian society there is a stereotype that a builder is the simplest profession, and a concrete worker is the simplest of all construction professions. The specialists of the Credo company will not enter into an argument with those who think so. But they cannot indifferently observe how ignorantly builders and non-builders sometimes handle concrete. And with their illiteracy they not only spoil high-quality and expensive material, causing direct damage to the customer or themselves, but also discredit a conscientious concrete manufacturer, convincing 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. Moreover, most of the questions were dictated by practice.

What is concrete?

It would seem a simple question. But few can give the correct answer to it. Concrete is an artificial stone material. It uses best properties stone - its strength. But why can't you just use stone? Because it is very labor-intensive 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 into the desired shape than to process granite. Well, it’s hard to imagine an interfloor ceiling made of stone. Perhaps only vaulted stone ceilings on small spans. Or it’s completely 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 withstand tensile loads well. But if reinforcement is inserted into concrete, then the tensile forces on the concrete will be absorbed by the reinforcement located inside the concrete. Everyone understands that inserting reinforcement inside a stone and gluing it to it is also very labor-intensive and expensive.

What is included in concrete?

Concrete consists of three main components - binder, water and aggregate. For brevity, we will simply call the astringent “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. To be short cement concrete we will simply call it “concrete”. There are many types of cement. We will not consider its varieties. This is a topic for separate study and is of more interest to concrete producers and other specialists. The main types of aggregates are crushed stone, gravel and sand. Crushed stone differs from gravel in that it is crushed material. In our area, most often it is a product of crushing the same gravel, but separated by fractions, i.e. by size. Gravel concrete is slightly cheaper because gravel is cheaper than crushed stone. Concrete up to certain grades is made from gravel. The main characteristics of crushed stone and gravel are size and strength. Sand can be coarse-grained or fine-grained. Fillers must be selected in strictly defined proportions. In a simplified way, we can imagine 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 do the right thing when they purchase ready-made gravel or crushed stone sand mixture (GPS or ShchPS) to prepare concrete on site. When producing them in a factory, the crushed stone-sand or gravel-sand ratio is optimal.

What qualities should concrete have?

The main physical characteristic of concrete is its strength. It is measured with special instruments 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 measurement of strength in kilograms per square centimeter(kg/cm2). For example, a strength of 100 kg/cm2 means that concrete collapses when exposed to a pressure of 100 kg/cm2. Previously, and often now, this strength meant the grade 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, concrete grade M100, concrete class B7.5. There are special tables for matching the grade and class of concrete. For the convenience of buyers, many manufacturers indicate both the brand and class of concrete in their price lists. 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 names of the characteristics speak for themselves. Frost resistance means the amount of alternating freezing and thawing that concrete can withstand without collapsing. 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. In a simplified way, cement production 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 cement. When a strictly defined amount of water is added to cement, it turns back into stone.

Why are crushed stone and sand needed in concrete?

Indeed, if you add water, the cement will turn into stone. Answer: do fake diamond only made from cement is both expensive and difficult. In addition, the cement itself shrinks greatly. Therefore, aggregates are added to concrete: crushed stone or gravel and sand.

What happens if you put an arbitrary amount of aggregates into concrete?

There will be concrete. But it is no longer the quality that the manufacturer wanted to achieve from it. 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 be obtained. If there is more sand 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. You can add more cement, i.e., in excess. But then the economy will suffer. This will be very expensive concrete. The proportions of the components in concrete are selected by specialists in the laboratory. These proportions are called “selections”.

How much water should be added to concrete?

The amount of water is also determined in the laboratory. For cement to turn into stone, only 13% of the weight of cement is water. But in fact, during the production of concrete they add large quantity. 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 be impossible to fit it into the structure. Such concrete is used mainly for 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? To reduce the amount of water in concrete and at the same time ensure such qualities as workability, chemical additives called “plasticizers” and “superplasticizers” are used.

How is workability measured?

The indicator of concrete that reflects its workability is called “workability”. Previously, the term “plasticity” could also be found. Mobility is measured with special instruments and designated as follows: P1, P2, etc.

Is it possible to make selections of concrete composition that are uniform for the entire country?

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

Why does concrete defrost?

A 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 alternating 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 alternating freezing and thawing, and then can collapse. How more water in concrete, the less its frost resistance. Therefore, vibration-pressed paving slabs has great frost resistance. The worse the gravel, crushed stone or sand (dirty, fragile, not frost-resistant), the less frost resistance of concrete. Many have watched concrete from dirty local river gravel defrost.

Why can't you add water to the ready-mixed concrete that the supplier brought?

When ordering concrete, the buyer must indicate its mobility in addition to the class of concrete. The manufacturer, guided by economic considerations, produces concrete with the ordered characteristics with a minimum strength reserve. Therefore, when concrete arrives at the site, all components are in the ratio and quantity required for this class of concrete, including water. By adding additional water, builders increase the VT and thereby reduce the ordered and paid for characteristics. In other words, the buyer pays for a high-grade concrete, but it ends up in the structure with underrated characteristics. Conclusion: you cannot add water to delivered concrete at a construction site. Sometimes, for various reasons, such a need nevertheless 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 exact supplier from whom you purchased the concrete. He is the one who knows what components are used in the production of this concrete and what needs to be done to preserve the quality of the concrete.

Does concrete need to be maintained?

High-quality care of concrete is no less important than its high-quality production. Some customers and builders are mistaken in 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 preserving the water (or moisture) that is already present in the concrete. As already stated, in order for concrete to turn into stone, water is needed. If builders do not ensure that water is retained in the concrete laid into the structure, then it will not have the strength that was ordered. What needs to be done for this? The concrete needs to be covered. Especially in sunny or windy weather. The wind often causes more damage than the sun. When water evaporates from concrete, there will not be enough water in it for the concrete to gain strength. The concrete will “dry out” and will never gain the strength that was planned. With intense evaporation of water, concrete cracks, as it shrinks sharply. After concrete cracks, water evaporates even more intensely through the cracks. In the future, during operation, water may enter the cracks, and the concrete will defrost. Through cracks in the 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. Once it starts, 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 concrete the word “shake”. IN different time year, in different weather conditions, this time can range from several minutes to several hours. The experience, qualifications and dexterity of concrete workers are very important here. A mistake is made by those who replace covering the concrete with watering it. Firstly, cement is washed out from the concrete surface, and secondly, upper layer concrete becomes waterlogged (VC increases). The consequence is that the concrete will “crumble” and peel off. What should you cover with? Any vapor-proof material. For example, polyethylene film. But the covering process is very labor-intensive. It is necessary to cover the concrete so as not to disturb its surface, if possible. The film must be secured to prevent it from being blown away by the wind. It is necessary to constantly monitor the position of the film. This is especially labor-intensive large areas, for example, on floors, road surfaces etc. What is the way out? Very simple. Now many manufacturers of concrete additives produce concrete care products. These are liquid materials that are applied to the surface of the concrete as soon as it is shaken, using ordinary 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 and turns into a film. These materials are called “film-forming materials.” This film allows you to retain water in concrete both in the sun and in the wind. As you understand, 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 you count the cost polyethylene film, the complexity of its installation, preservation, cleaning, storage, take into account the disturbed concrete surface or the cost of water, work on its spraying, water damage, then it will become clear that the use of film-forming materials is beneficial. Subsequently, this film evaporates and any material can be applied to the concrete. Decoration Materials, including tiles, without additional preparation. Conscientious concrete manufacturers often sell these materials themselves. Most often, they do this not with the goal of making money, but with the goal of helping builders and thereby maintaining 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 base for concrete is crushed stone or sand. If this material is dry, it can absorb large amounts of water. For example, crushed stone from Gelendzhik quarries absorbs a very large amount of water. After laying the concrete, moisture from the 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 amazed builders, who have no choice but to blame the concrete manufacturer and cover up the cracks, which can change nothing. No amount of watering and covering will help, because shrinkage cracks form from the bottom of the concrete. The same thing happens when concrete comes into contact with dry wooden formwork. Which exit? The base for concrete must be moistened “to capacity,” 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 special materials, for example, emulsol, processing. This is done not only to prevent concrete from sticking to the formwork, but also to prevent moisture from being absorbed. If there is no emulsol or waste, then it is necessary to heavily moisten the formwork, again avoiding puddles on horizontal surfaces. The exception is formwork made of laminated plywood or metal. The water doesn't go anywhere in it.

Another enemy of concrete is frost. In order for concrete to become stone, positive temperatures are required. In laboratory conditions, the temperature is maintained at around 20 degrees Celsius. It is under these 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, we must not forget about the need to preserve moisture in the concrete. But a rapid increase in temperature when heating concrete is also harmful. Internal (not visible to the eye) stresses and destruction occur in concrete. This is important to know not only for those who use concrete heating. When concrete hardens, a chemical reaction occurs that releases heat. For small structures this is only to the benefit of concrete. For very large massive structures(most often during industrial construction, for example, powerful foundations) the concrete heats up so much that it needs to be cooled, for example, by pouring water. Sometimes they lay inside the concrete special pipes, water is pumped through them and thereby cools it.

So, concrete must be preserved from low temperatures. This is achieved by covering the concrete with films, matting, snow, etc. or warm it up. Concrete must arrive at the site at a temperature of at least 5 degrees. Celsius. To protect concrete from freezing before it is covered or heated, special antifreeze additives are used during 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 work process. In the future, for concrete to harden, it needs a positive temperature, i.e. it is necessary either to cover and thus preserve the heat that concrete releases during hardening, or to heat it up.

In this article, we touched only on those rules, non-compliance with which by builders can damage the business reputation of the concrete manufacturer and damage customers. In fact, the science of concrete is a serious discipline that is constantly evolving and requires long-term study. Practicing builders need to have a much smaller amount of knowledge about concrete and the rules for its use than science has, but a much larger amount of information than is presented in this article. The goal of the authors of the article was to arouse interest among that part of the builders and customers who do not even have the information presented in this article, and to encourage them to self-study secrets of the concrete worker profession. For those who already know everything stated above, the authors can only point out two points: 1. repetition is the mother of learning; 2. nothing stands still, everything is developing, including construction science.

To make a concrete mixture, you need to use several basic components. This list includes sand, cement, water and aggregate. Crushed stone is most often used as the latter. This building material comes in several varieties, each of which has its own characteristics.

Why do you need crushed stone in concrete?

Crushed stone is bulk material, which is obtained by crushing rocks, including boulders and gravel. In other words, crushed stone is usually understood as irregularly shaped stones that can belong to different rocks.

When dealing with the question of why crushed stone is added to concrete, it is necessary to emphasize several features. This building material is not always used when preparing a concrete mixture, but its inclusion in the composition allows you to solve a number of problems:

1. Strengthening. Crushed stone particles have an irregular shape, which enhances the adhesion of materials and reduces creep.
2. Increased water resistance. Concrete mixtures with the addition of coarse filler is used if the finished coating will often come into contact with moisture.
3. High strength. If such a filler is present in the mixture, it will take on the role of a frame. Concrete on crushed stone can be subject to increased loads. For this reason, this technology is used in the construction of tunnels, bridges, hydraulic structures, and foundations for buildings that are subject to increased requirements.
4. Reducing construction costs. When making a solution, the filler partially replaces cement, which is characterized by a higher price.

If you decide to use a filler, you should consider the following nuances:

1. Presence of several fractions of crushed stone. If the mixture contains only large stones, then air pockets may appear during the laying process, and therefore the strength will decrease.
2. The maximum size of stones depends on the design features. The fraction must be less than ⅔ than the smallest distance between adjacent reinforcing bars. Another calculation option is based on width finished product— stones should not exceed ⅓ of this figure.

Main characteristics of crushed stone

Having figured out why crushed stone is in concrete, it is necessary to indicate several basic characteristics on which the technical indicators of ready-made mixtures depend:

1. Density. Affects the strength of the finished product. The optimal density can be called density, which varies between 1.4-3 g/cm3.
2. Flakiness. This term refers to the plane of the stones. Based on this indicator, several types of crushed stone are distinguished: cuboid, acute-angled, lamellar. Cuboid grains have maximum strength.
3. Compressive strength. This indicator is determined during the process of compression and crushing of rock.
4. Fraction. The characteristic is one of the main ones. It is usually understood as the size of stones (grains). There are 3 main categories: large, medium and small. Knowing the main characteristics and features, you can choose the optimal crushed stone fraction for, M100 and any other.
5. Radioactivity. Indicated on the packaging. It means what natural radioactive background the building material has. Class I can be used in any construction work, including the construction of buildings. II - for the construction of highways.
6. Frost resistance. Using this characteristic, they indicate at what temperature conditions the strength of the finished concrete is maintained. This indicator is written using a letter. A number is indicated next to it - this means how many freezing and thawing the crushed concrete can withstand. For the climate of Moscow and most other regions of Russia, it is better to choose material with an F300 index.

Classification

Bulk filler is usually divided into several categories, taking into account the rocks from which this material was produced:

• granite;
• gravel;
• limestone;
• slag;
• secondary.

Granite

This type of crushed stone is obtained by crushing nonmetallic rocks, most often it is a monolithic rock. The quality of granite building material is regulated by GOST 8267-93. This material is superior in strength to other types of filler.

Several categories can be distinguished depending on what fraction the crushed stone is:

• fine-grained (fraction 0-5 mm and 5-10 mm);
• medium-grained (5-20 mm and 20-40 mm);
• coarse-grained (the stones here reach 40-70 mm or 70-120 mm).

In construction work, the middle fraction (5-20 mm) is most often used. This option is considered optimal for laying highways, building foundations, and erecting reinforced concrete structures.

Gravel

Such bulk material is produced from rocks. Often, rock dispersion technology is used for mining after extraction from the quarry. Product quality must meet the requirements of GOST 8267-93.

The strength of these stones is lower than granite, but this variety has several advantages:

• low price (relative to other types of crushed stone);
• minimal radioactivity.

Depending on the extraction method, building materials are of the following types:

• gravel for concrete - these stones are of river and sea origin;
• crushed - it can be obtained by crushing rocks.

Fractions of crushed stone of this variety can be as follows:

• small - the size of the stone varies between 3-10 mm;
• medium - 2 options are presented here (5-20 mm and 5-40 mm);
• large - grains reach 20-40 mm.

Gravel material has found application in the construction of pedestrian paths, filling reinforced concrete products, building sites.

Limestone

Sedimentary rocks are used to produce crushed limestone. The stones in this case are calcium carbonate. The advantage is the minimum cost.

There can be several grain sizes:

• 5-20 mm (the smallest fraction);
• 20-40 mm (most popular);
• 40-70 mm.

Due to its low strength, limestone material has limited use. It is suitable for the construction of paths with light loads, work in the printing and glass industries, and the preparation of cement mortar.

Slag

Slag crushed stone is usually understood as stones of different sizes, which are obtained from slag used in production.

Depending on the size of crushed stone there are:

• large;
• average;
• small;
• elimination

You can use this filler for cooking. The most common application is thermal insulation.

Secondary

Recycled crushed stone is called secondary because this material is obtained from construction waste, including old asphalt, bricks, and concrete. These products must meet the criteria of GOST 25137-82. The main advantage is low cost.

Disadvantages include low strength and frost resistance. Despite this, recycled materials are often used to strengthen weak soils and build roads.

Criterias of choice

To achieve maximum strength and durability of concrete, crushed stone must be selected carefully. There is no universal option, so when selecting it is necessary to take into account why the concrete is being prepared.

First of all, you need to check compliance with GOST. To do this, the seller is asked to provide accompanying documents for the shipment of goods. Another requirement is the minimum content of dust, sand and other impurities. The maximum figure is considered to be 2% of the total mass.

Fraction

When choosing what kind of crushed stone is needed for concrete, you should be extremely careful in selecting the grain sizes. Large stones should be used in structures that must withstand vertical loads. Such crushed stone takes on the main load and acts as a frame.

Smaller fractions are chosen for reinforced products and those elements that must withstand a large deflection load.

The ratio of concrete grade and crushed stone grade

Before you purchase all the ingredients to create the mixture, you need to select the appropriate crushed stone for concrete. The technical and operational characteristics of the finished products will depend on this.

The table below shows the use cases recommended by the manufacturers.