The main types of crushed stone for concrete and its technical characteristics. What crushed stone is needed for concrete. Selecting a fraction What crushed stone is needed for concrete

When mixing the ingredients of concrete solutions - water, cement, sand, chemical additives - you need to decide what crushed stone is needed for concrete. He plays an important role in its composition.

Characteristics and significance of the material

Why is crushed stone needed in concrete? This component of the solution - bulk material in the form of irregularly shaped stones - is a large filler; it creates a frame that absorbs all the loads of the solution. Thanks to it, the finished solution shrinks less, becomes more elastic, durable, creep is much reduced, and the deformation of the product under load is reduced. The composition of concrete is cheaper with fillers, since cement is more expensive than crushed stone.

According to the standards, the content of high-fineness filler should be from 1 to 20% of the mass of the solution, and low-fineness filler - no more than 20%. The solution must contain crushed stone of at least two fractions with a maximum grain size of up to 40 mm or three - with 40–70 mm.

The maximum grain size should be less than 2/3 of the shortest distance between the reinforcement bars, and also no more than 1/3 of the smallest product size (for example, the width of the tape). This material has sharp edges and a rough surface, so it adheres well to the components of the solution.

Classification

It is customary to classify crushed stone for concrete by density, size, flakiness (flatness, needle-like particles in the body of the stone), compressive strength, and frost resistance.

Kinds

Crushed stone is often called granite material, and gravel is crushed stone from natural stone, But common name both are rubble. It is created from different breeds and comes in:

• granite;
• gravel;
• dolomite (limestone);
• slag;
• secondary.

The first two are used for medium and heavy concrete, the rest - for light, cellular solutions, foam, and aerated concrete. Granite material is more durable and is created by crushing blocks of natural granite. The stone is sorted by size through a special vibrating sieve.

Granite crushed stone is used for road and airfield pavements, critical areas under heavy loads, columns, walls, slabs, bridges. This is the most durable filler; it is found in high-grade durable concrete. Its grade is 1200-1400, and its frost resistance level is up to 400 cycles.

Is it better to use crushed stone or gravel? The second is formed by crushing ordinary natural stone rock and sifting it. It is less durable than crushed granite. If you need especially durable frost-resistant concrete, the latter is recommended. On the other hand, crushed gravel has advantages: low radioactive background, low price. It is most often used for private construction, foundations of mansions, and road construction.

Two types of gravel are produced:

• crushed;
• natural.

Limestone (dolomite) filler is obtained from sedimentary rocks - limestone. Its components are mainly carbonate, calcium and similar compounds. He becomes limp from the water. For serious structures undesirable, and some experts do not recommend this crushed stone for concrete at all.

Slag, recycled materials are used for very lightweight concrete, which serves as thermal insulation.

Form

There are four types of crushed stone according to their geometric appearance. The determining factor is the level of content of the forms of individual grains in percentage to volume. According to this criterion, there is the following crushed stone:

• cuboid, wedge-shaped – 12–15%;
• regular (acute-angled) – 18–25%;
• flakier – 25%. There are different degrees of flakiness (flatness) of the grains. In turn, the material comes with needle-like or lamellar grains.

Cuboidal or low-face concrete crushed stone is most often produced in quarries; it is the most durable. The mixture of its grains has the densest packing of all types of crushed stone. Concrete with it is much stronger than with needle or plate stones, since the latter increase the distance between particles in the solution - accordingly, they require more cement.

Factions

By fraction there is large, medium, small crushed stone. Small material is always more expensive and requires more effort when processing stone blocks. For construction concrete, a grain range of 5-70 mm is used. Faction is maximum size individual grain. The lower its number, the finer the crushed stone grains.

Categories

The following fractions of granite crushed stone can be distinguished.

One category is fine (5–10; 5–20; 10–20 mm), very fine (2–5) and gravel chips (1–3). The first is the most popular, used for reinforced concrete products, bridges, road surfaces, in private construction, and for foundations. The second is for artificial concrete stone, concrete decorative parts, thin paving slabs, and small structures. Crushed gravel is also used for paving slabs, self-leveling floors, products that do not require special strength and play a decorative role.

The second category - medium (20–40 mm) is also popular for private construction and industrial facilities. First of all, it is used for foundations, road structures, in the construction of houses, floors, load-bearing structures, in production concrete slabs, beams, for ordinary work in private construction.

The third category - large (40–70 mm) - is used for rubble concrete foundations, when preparing large quantities of mortar, for massive structures. It is usually ordered in large quantities and in advance, since it is not very popular and is not always in stock.

Gravel fractions:

• small (up to 10 mm);
• medium (10–20 mm);
• large (40 mm).

The areas of application are the same as for granite material, but take into account that gravel is much weaker (but much cheaper). It is used for low grades of concrete.

For structures that require frost resistance and special strength, granite is recommended, although crushed gravel is a standard component of concrete for the foundations of private houses.

Frost resistance

Using this parameter, the grade of material is determined by the number of freezing/thawing cycles. Frost resistance can also be assessed by cycles of lowering it into sodium sulfate and drying. For this filler, there are frost resistance grades from F300 and higher.

Contrary to popular belief - the larger the concrete crushed stone, the stronger the solution, this is not always the case, for example, for structures that work in bending or are reinforced with a better fraction of a smaller fraction. To withstand vertical loads, coarse crushed stone is better.
The material must be clean without adhering dirt, clay, dust, soil, ores. It is sometimes washed at the construction site. The concrete must cover the grains completely on all sides.

Crushed stone is always taken two or three grades higher than the design grade of the mortar, since concrete gains strength and density over the course of six months, a year or more, but crushed stone does not have this property. Thus, these two indicators are balanced.

Grain size selection

Large grains make the solution stronger, fine grains make it denser. Large pebbles contribute to the formation of porosity, which is undesirable, while small pebbles fill voids better. Therefore, it is recommended to combine them. For example, a good fraction is 5–20, it contains large grains of 20 mm and smaller grains of 5 mm. It is this kind of concrete – ready-mixed – that fills well monolithic structures. This type of stone is recommended for reinforced foundations.

Concrete with large crushed stone is more difficult to compact, reinforce and pull apart on the formwork. For densely reinforced and complex structures It’s better to take a smaller stone and add more cement. To increase the strength of the mixture, if there is no coarse crushed stone, it is recommended to add more cement.

Application

For concrete floors and similar products use a solution with fine crushed stone, since the thickness of the floor is small. A flatter (flakier) stone increases the consumption of binder material and cement; such a solution is less compacted. It is better not to use such crushed stone for foundations; it is well suited for blind areas.

For foundations, it is better to use granite crushed stone of medium-sized fractions (5–20, 5–10) - the concrete will be dense and frost-resistant. Granite filler has high frost resistance, so it is the best for our climate. For low temperatures A value of at least F150 is recommended.

For concrete grades below M250, it is more appropriate, more economical and more profitable to use gravel. From M300 – granite crushed stone. Good choice there will be granite crushed stone 5–20. This size is more universal and can be used for both foundations and blind areas and floors.

When increasing the density, a finer fraction is used, but here you need to take into account that this will cost more, since more cement is needed and it itself is more expensive. To reduce the cost of the solution, you can mix granite and gravel concrete crushed stone. When using coarse crushed stone, you need more sand to fill the voids or combine it with finer sand.

Here is an approximate table of brand correspondences:

Concrete	Crushed stone
100	600
200	800
300	1000
400-500	1200

When starting construction work using concrete mixtures, it is necessary to clearly understand what fraction of crushed stone is needed for concrete. The mortar filler makes up the largest share of the volume, so crushed stone is no less important than cement for determining the quality of concrete. The durability and strength of concrete depends on crushed stone; it determines the plasticity of the solution, reduces shrinkage and cement consumption, reducing the cost of work.

Concrete crushed stone is selected based on the problems solved during the construction and operation of finished concrete structures. The pressure to which a concrete path is exposed differs many times from the load on the foundation of the building. Just as the conditions for the foundations of light buildings differ from the requirements for the foundations of multi-story buildings. And although many factors influence the choice of crushed stone for concrete, there are certain rules that must be followed.

To fill the foundation for the construction of a large structure, a mixture of small fractions of 5 - 20 or 5 - 10 mm granite crushed stone is used. Cuboid-shaped grains are well compacted among themselves. A large grain of 20 mm gives strength to the solution, and smaller grains of 5 mm or more fill the voids between the large ones, forming a dense mass. Such concrete has good strength and frost resistance, but increases cement consumption.

For foundations for small buildings, crushed gravel with a medium fraction of 20-40 mm is suitable. A solution with a medium grain size is less plastic and is harder to compact and reinforce, but has a margin of safety and a low price.

For concrete floors, paths, blind areas, and paving slabs, limestone and secondary types of crushed stone with fine grain grains and increased flakiness are used. Crushed stone with a lamellar grain shape increases cement consumption and is difficult to compact, but is the most economical.

The use of crushed stone from rocks is standardized by GOST 8267, from metallurgical slag by GOST 5578, and from thermal power plant slag by GOST 26644.

Since concrete gains strength over six months or more, the strength of crushed stone to guarantee reliability is taken three to five grades higher.

Concrete strength grade	Crushed stone strength grade

Granite crushed stone has the greatest strength; its strength corresponds to grades M1200 - M1400, and frost resistance to grade F400. It is used in critical structures requiring high-quality concrete, but is the most expensive.

Crushed gravel is slightly inferior in characteristics, but cheaper than granite, and is classified as M800 – M1000 and F200. It is used both independently and in a mixture with granite to reduce the cost of construction.

Both types of crushed stone are used for the production of heavy concrete. Limestone, recycled and slag are used to produce lightweight concrete.

The shape of crushed stone grains affects the quality of concrete; the more it resembles a cube, the denser it will fit into a given volume and the greater its strength. The lamellar and needle-shaped grains form a loose solution, are less durable and are less compacted.

In concrete solutions, crushed stone with a grain size from 5 to 70 mm is used. The finer the crushed stone, the higher its cost, since it requires high production costs.

For pouring foundations under individual cottages more often they use small fractions of crushed stone 3 – 8 mm; 5 – 10 mm; 10 – 20 mm and 5 – 20 mm. They are used both alone and for economy, mixed with medium fractions. For pouring a reinforced belt, preference is also given to small fractions of 5 - 10 mm and 10 - 20 mm. Smallest size formwork rarely reaches 35 cm and the middle fractions fill worse reinforced belts and form many voids.

Crushed stone of medium fraction 20 - 40 mm is used in the foundations of houses, floors, the manufacture of beams, floors, and load-bearing structures. It is used both in private construction and for the construction of industrial facilities.

Large fractions of crushed stone 25-60 mm; 20-70 mm; 40–70 mm is used for large volumes concrete works during the construction of multi-storey buildings. And it is used in a mixture with crushed stone of medium and small fractions. For individual houses, this fraction is almost never used.

For the manufacture of concrete mixture several basic components must be used. This list includes sand, cement, water and aggregate. Crushed stone is most often used as the latter. This construction material is presented in several varieties, each of which has its own characteristics.

Why do you need crushed stone in concrete?

Crushed stone is a bulk material that 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 irregular shape, due to which the adhesion of materials is enhanced and creep is reduced.
2. Increased water resistance. Concrete mixtures with the addition of coarse filler are 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. Reduced costs for construction works. 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 should 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 iron 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, construction of 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 crushed stone for the reason that this material is obtained from construction waste, including old asphalt, bricks, 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. Universal option does not exist, therefore, 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 about selecting 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. Technical and performance characteristics finished products.

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

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

There are several 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 crushed stone, in construction terminology, is a large filler and significantly increases the density of the finished concrete mixture, it should be added to a mixture of sand and cement with caution. To prevent air pockets from appearing near individual fractions of crushed stone, it is best to mix large particles of crushed stone with small particles of crushed stone and sand. That is, if you want to make strong concrete, it is not enough to add only large 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 obtain durable concrete, you will need quite a bit of cement, so the effect will not be as significant as using crushed stone of different sizes.

Most often, concrete using crushed stone is made when creating hydraulic structures. 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 consider that great value The strength of the concrete mixture is determined by the absence of dust and other impurities in the crushed stone. 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.

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 the construction industry. We believe that this format of working with a client is an optimal and honest scheme for partnership relationships.

IN classic version Concrete contains components such as binders, 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 you buy concrete, you need to understand this 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, extra light, lightweight, heavy and extra 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 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-tensioned 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 allow achieving high levels of strength, endurance, seismic resistance, fatigue failure and many others.

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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 of 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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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 interfloor covering 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 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 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 special devices when the concrete reaches the age of 28 days. Strength is measured in units of pressure. The most understandable and familiar unit for 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: making artificial stone only 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, a larger amount is added. The ratio of the amount of water to the amount of cement by weight is called the water-cement ratio (WC). In practice, it ranges from 0.3 to 0.4. If the VC is made smaller, it will be impossible to work with concrete manually. It will be very hard, thick, dry. It will 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 each area there are different types and qualities of crushed stone, gravel, sand, water, and 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. The 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 by various reasons, nevertheless, such a need arises. For example, the builders did not have time to prepare the formwork or for other reasons. The concrete has thickened. Then the buyer needs to contact the concrete supplier for advice. And the supplier’s technologist (and a bona fide 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 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 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 surface of the concrete, and secondly, the top layer of concrete becomes waterlogged (the CV 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 solution? 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 a conventional construction or garden sprayer (sprayer). Most often it is a liquid that has the color and consistency of milk. After application to concrete, the liquid dries 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 calculate the cost of polyethylene film, the labor intensity of its installation, preservation, cleaning, storage, take into account the damaged 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 finishing materials, including tiles, can be applied to the concrete 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, then it is able to absorb a large number 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 with special materials, for example, emulsol, mining. 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, chemical reaction with heat release. For small structures this is only to the benefit of concrete. With very large, massive structures (most often in industrial construction, for example, powerful foundations), the concrete heats up so much that it needs to be cooled, for example, by pouring water. Sometimes special pipes are laid inside the concrete, water is pumped through them and thereby cools it.

So, concrete must be 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 cause damage business reputation concrete manufacturer and damage to 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 builders and customers who do not even have the information presented in this article, and to encourage them to independently study the secrets of the concrete 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 develops and building science including.