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Is it possible to use crushed sandstone for concrete? What fraction of crushed stone is needed for concrete. Types of crushed stone for preparing concrete

Crushed stone is a bulk building material on an inorganic basis with different sizes fractions (grains). It is most often used as a non-binding filler in concrete, which allows future structures and their foundations to withstand operational stresses. Crushed stone is added to the solution both to reduce its cost and to give it strength and elasticity after hardening, reducing shrinkage and the risk of deformation.

The material is obtained by crushing rocks or industrial waste, as a result - the stones have various shapes and remain rough, which increases their adhesion to cement and sand. For fraction size crushed concrete influenced by the maximum size of individual elements, it is marked accordingly, which makes selection and purchase easier. Depending on this, the solution acquires certain properties, in particular - the addition of large rock increases the strength during setting, and small stones - density (promotes better filling of voids).

Varieties

This material is classified according to several criteria: the type of raw materials used, the size and shape of crushed particles, frost resistance and strength. Crushed stone is made from hard granite, metallurgical and construction slag, gravel and limestone rocks. The stronger the raw material, the stronger the load it can withstand future design. For example, the best option granite will be used for pouring the foundation, and gravel will be optimal in terms of price and quality. Depending on the shape of the crushed stone fractions and their percentage in the total mass, three types of mixtures are distinguished:

1. Cuboid (from 12 to 15%), the densest.
2. Regular (18–35%), with an average distance between particles.
3. Flaky (from 25%), with a high content of grains with a lamellar or needle shape (width and thickness less than 3 times the length).

But the main indicator that determines the properties of crushed stone for concrete remains the size of the fractions. The standard ones include: from 3 to 8 mm (eurocrushed stone), 5–20, 10–20, 20–40, 25–60, 20–70, 40–70. Crushed stone with fractions from 5 to 10, 10–15, 15–20, 70–120, 100–300 is considered non-standard. How smaller size, the more expensive the building material, this is due to the complexity technological process when crushing it and more thoroughly sifting. In some cases, it is possible to mix a coarse fraction of crushed stone and small stone chips, the latter being used as an accompanying filler.

The most popular sizes are 20–40 mm, this is due to the low cost of this brand, but for pouring a foundation or with high requirements for concrete strength, it is better to buy backfill with grains from 5 to 10 mm or 5–20.

Markings

Depending on the frost resistance of the material, they are distinguished: F15, F25, F50, F100, F150, F200, F300, F400. The grade is not related to the size of the crushed stone fraction and is determined by the number of cycles of thawing or defrosting or saturation and drying in a solution of sodium sulfate. For mixing concrete, the optimal frost resistance is 300 or more (this is sufficient for pouring foundations and frequently used external structures). This indicator must be written down in the crushed stone passport, as well as the size of the fractions.

The main marking implies the strength of the source material under compression, crushability when crushing rock and the degree of wear during grinding in a drum. The indicator indicates the resistance of concrete external influences like the weight of a vehicle or the load on a foundation. The high-strength grades of crushed stone are M1200–1400, the very weak are M200, the norm is the percentage of grains obtained from weak rocks (when compressed in a water-saturated state of up to 20 MPa). For comparison, in the M300 this figure exceeds 15%, in the M1400 - no more than 5.

The ratio of crushed stone and concrete grades

The strength of the solution is determined by its fillers, the characteristics of which are indicated in the attached instructions or on the packaging. In practice, this means that M400 cement will not produce M500 or higher concrete; the corresponding ratio of grades is presented in the table:

Determining the strength of concrete depending on the fraction of crushed stone is approximate, but deviations are allowed. The latter are regulated by changing the ratio of sand or cement, adding granite chips. But in general, to obtain the required grade, it is important to observe strict proportions of the solution. Before choosing what crushed stone is needed for concrete, it is recommended to determine intended purpose structures and calculate future loads. For a solution up to M250, gravel is sufficient; the use of non-standard sizes grains High-strength grades (M300 and more) require the addition of granite screenings, preferably with square fractions up to 20.

According to building codes, the content of crushed stone with increased coarseness in the mixture varies from 1 to 20%, and undersized crushed stone should be within 20% of the total mass. If crushed stone with a fraction of up to 20 mm is used for concrete, problems with the distribution of voids do not arise. When this indicator increases to 40 mm, another type of filler (with different, smaller grain sizes) should be added to the solution. Accordingly, choosing a filler with a grain size of up to 70 mm implies adding at least three different crushed stone fractions to the concrete. The size cannot be less than 2/3 of the distance between the longitudinal reinforcement, but also no more than 30% of the smallest element of the building structure.

Examples of use

Fine-grained mixtures are needed in the manufacture of decorative products from concrete, artificial stone, paving slabs, and they are also added to the composition for asphalt or other road surfaces. The use of too fine filler for pouring foundations and structures load-bearing walls not economically feasible, plus the risk of shrinkage and deformation increases. The middle and most popular crushed stone fraction (20–40 and 10–20) is better suited for creating large and small concrete structures. This ideal option for pouring the foundation. For cooking quality solution A concrete mixer should be used, it is difficult to achieve uniformity when mixing crushed stone, sand and binder cement manually.

Material with a fraction from 40 to 70 is classified as coarse-grain, and is needed when constructing massive buildings and mixing concrete in large volumes. It is better not to add it to solutions when pouring the foundation without adding auxiliary fillers of other sizes. Such material is difficult to obtain, the required quantity is calculated in advance, and its delivery is ordered separately. Crushed stone with a coarse fraction of 100 mm or more is called rubble or building stone, it is used mainly for decorative purposes, and not for preparing concrete. The same applies to small granite chips, but the latter is indispensable when creating self-leveling floors and as an auxiliary additive.

Particular attention is paid to the choice of filler when making mortar for pouring the foundation. The base can withstand maximum loads, so this type of work requires concrete with a strength of at least M300. Accordingly, a granite filler of at least M1200 is needed, although the addition of gravel is allowed for country houses. Recommended fractions of crushed stone for the foundation: 5–10, 10–20, 20–40, it is advisable to buy grades with a square grain shape.

01.06.2018

Concrete is a modern building material that involves the use of a mixture of cement with water, sand and any other hard material. Most often, crushed stone is used from solid materials, due to its special properties and good opportunities for versatile application. 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 filler 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 installation material is worth using due to the following advantages:

Crushed stone should be defined as a coarse aggregate, which makes it possible to eliminate all processes of instability and compaction of the structure. In this regard, its use helps to increase the quality of the entire mixture.

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

The largest expenditure of financial resources is 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, crushed stone of a special size is selected, which, when compacted, can be distributed into smaller ones. From this we can conclude that good concrete assumes the presence of different fractions of the material.

Fractional division of crushed stone

To talk about what kind of crushed stone for concrete fits better All you need to do is understand what a faction is. A fraction is defined as nothing more than the division of particles into groups of the same size.

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, according to 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 classified as the first fraction. Although this option is not economical from a financial point of view, it remains popular due to the high quality of the resulting mixture. When using larger fractions, concrete will not be evenly filled with solid material, which will significantly reduce the strength of the structure.

An important aspect when determining the fractional component is the subsequent area of ​​use of the concrete mixture. 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 required durability of the hardened mixture.

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

The data presented should not be taken 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, but only low-strength crushed stone is available, then more cement is added to the final mixture. The same works in the absence of filler of the required fractionation, but only by changing the amount of sand added.

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

You can buy crushed stone or concrete in Rostov-on-Don from our company “Beton 61”. You can be confident in the timely and high-quality provision of services.

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 for 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 what is it for?

Crushed stone is also 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 types 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 is worth considering that it is also the most expensive in price.

Crushed stone is added to concrete for several reasons, one of which is that crushed stone, as a more cheap material Compared to cement, but with a high degree of density and hardness, it perfectly replaces it when creating a concrete mixture. In addition, crushed stone has other advantages over cement. It reduces creep and shrinkage of the mixture. Concrete using crushed stone is much less likely to crack than conventional concrete cement mixture, 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 the crushed stone construction terminology is a large filler and significantly increases the density of the finished concrete mixture; it should be added to the mixture of sand and cement carefully. 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 crushed stone and sand. That is, if you want to make strong concrete, it is not enough to add only large crushed granite stone, which has increased hardness, to it - it will be much more effective to mix large, medium and small crushed stone with a mixture of sand and cement. This will allow you to make truly strong and reliable concrete while minimizing the amount of cement in the overall mixture.

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

Most often, concrete using crushed stone is made 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 highways can be done without coarse crushed stone.

If you are going to add crushed stone to concrete, you should take into account that the absence of dust and other impurities in the 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 ensure the cleanliness of the crushed stone, it can be washed with water pressure 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 to use recycled crushed stone in construction work, especially in laying the foundation. Despite its relative cheapness, its density is noticeably lower than that of new 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 or natural. The first option is obtained from construction waste and household slag. It is a recycled material, so it has a low cost. It is almost never used in construction work (it is 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 It 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, it is necessary to select the correct proportions of all components.

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 make it possible to produce construction work in the off-season, and also increase technical specifications 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 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 unit, 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 during installation concrete structures working 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, as a result of the interaction, ammonia gas. 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 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

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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 the 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 construction concrete– cement concrete. From the name itself it is clear that cement is used as a binder in cement concrete. For brevity 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. Aggregates 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 a ready-made gravel or crushed stone 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 artificial stone 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 more. 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, one could also come across the term “plasticity”. 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 the concrete has. 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 in 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 cannot watch and wait to see if concrete begins 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 times 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, top 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. This liquid materials, which 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. finishing 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 or 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. What's the solution? 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. 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.