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Why is gravel added to the solution? What kind of crushed stone is needed for concrete. Crushed stone for preparing concrete mortar

When carrying out construction work, it is necessary to carry out a high-quality selection of building materials. It is important to understand their classification and production methods. For making concrete and iron concrete products the main component is crushed stone.

Varieties

Crushed stone is a non-metallic material that is obtained by crushing large pieces of hard rock. Extraction occurs in quarries and often through blasting.

Classification

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

Used as

• aggregate in concrete production;
• for the construction of automobile and railways;
• For landscape design;
• for decorative purposes.

It is important to distinguish this building material from gravel. Gravel is fragments of rock formed in natural conditions as a result of weathering. It has a rounded shape. In contrast, crushed stone is produced artificially, by crushing, and it has relatively sharp edges.

• Granite. The most expensive because it is the most resistant to exposure to conditions external environment and resistant to stress. It is used to produce particularly hard concrete.
• Gravel. They are mined in quarries or from the bottom of reservoirs ( Sand and gravel). This type has a lower price, but is inferior in quality to granite. Crushed stone obtained by crushing gravel from a quarry is preferable to that obtained from the bottom of a reservoir. This is due to the fact that its surface is rougher than that of the one mined from the bottom. This improves grip sand-cement mortar. In the construction of low-rise buildings and in dacha construction crushed gravel may be used.
• Limestone(dolomite). It is mined by crushing sedimentary rocks (limestones). It has relatively low strength. This type is used to create reinforced concrete products with light loads. The main advantage is low price.
• Secondary- obtained by crushing recycled concrete products.

Limestone crushed stone is susceptible negative impact water - dissolution (especially in an acidic environment).

The secondary type has a low price, but also the lowest quality. The use of this type of building materials in critical structures is not recommended.

An important characteristic of this building material Flakiness is an indicator of the shape of the grains (flat or needle-shaped).

The strength of a concrete product directly depends on the strength of the crushed stone used.

Classification by strength grades

• for concrete grade M400-500, crushed stone grade 1200 is used;
• for M300 - 1000;
• for M200 - 800;
• for M100 - 600.

The principle of selecting crushed stone fraction sizes

• 0-5 mm - a by-product of crushing rocks, used when filling paths;
• 5-10 mm - used for concrete products together with larger filler;
• 5-20 mm - the most popular, used in the creation of reinforced concrete products;
• 20-40 mm - middle fraction;
• 25-60 mm - used when creating bases railway tracks;
• 40-70 mm - for large concrete structures;
• -70-120 mm - rubble stone, used in the construction of foundations.

It is impossible to create strong concrete using poor crushed stone.

You cannot use crushed stone with the same fraction sizes. Because this can create voids in the concrete, which will reduce its quality and stability. When creating responsible reinforced concrete structures use a mixture of crushed stone of sizes 5-10 mm and 10-20 mm.

Reference! The 5-20 mm fraction is considered universal.

Pouring concrete using this fraction is more expensive, but its quality is higher. It will be more durable. This crushed stone is suitable for pouring: foundations, slabs, floors. For concrete garden paths any fractions apply. To fill floors and blind areas, crushed stone of a small fraction is used, since the thickness of the concrete layer is small.

This building material is an important component for the production of concrete. An incorrectly selected fraction of this building material can lead to the creation of a low-quality reinforced concrete product.

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 tasks solved during construction and operation ready-made structures made of concrete. 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 foundations multi-storey 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 fill the voids between the large ones, forming a dense mass. Such concrete has good strength and frost resistance, but increases cement consumption.

Suitable for foundations for small buildings crushed gravel middle fraction 20-40 mm. 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

Has the greatest strength granite crushed stone 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 manufacture 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.

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

Crushed stone of medium fraction 20 - 40 mm is used in the foundations of houses, floors, production of beams, floors, 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.

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

Why do you need crushed stone in concrete?

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

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

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

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

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

Main characteristics of crushed stone

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

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

Classification

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

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

Granite

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

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

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

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

Gravel

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

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

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

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

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

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

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

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

Limestone

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

There can be several grain sizes:

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

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

Slag

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

Depending on the size of crushed stone there are:

• large;
• average;
• small;
• elimination

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

Secondary

Recycled crushed stone is called secondary 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 road construction.

Criterias of choice

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

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

Fraction

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

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

The ratio of concrete grade and crushed stone grade

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

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

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. The size of crushed concrete fractions is 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. Eg, 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. High-strength crushed stone grades include M1200–1400, very weak grades include M200; the norm is considered to be 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 regulations, 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 using

Fine-grained mixtures are needed in the manufacture decorative items made of concrete, artificial stone, paving slabs, it is also added to the composition for asphalt or other road surface. 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 medium and most popular crushed stone fraction (20–40 and 10–20) is better suited for creating large and small concrete structures. This perfect 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.