Strength of concrete. Classes and grades of concrete by strength and how they are determined

The strength of concrete is the ability of a substance, after hardening, to withstand physical, chemical and mechanical loads and impacts. This is a key characteristic that plays a decisive role in determining methods of construction and further operation of concrete structures and structures.

Determining and establishing the strength properties of concrete is a very important aspect of construction. The developer is obliged to take this indicator into account before commissioning the property. The customer must also carefully consider the strength of the concrete in order to avoid life-threatening situations. First, let's look at the types of modern concrete.

There are several groups based on the weight of concrete:

• Super light;
• Lungs;
• Heavy;
• Very heavy.

The production and preparation of concrete mixture is an important process, on which the further characteristics of concrete products . In addition to the basic resources used to create the mixture, it is possible to use additives that can not only enhance the existing properties of the mixture, but also provide it with new ones. For more detailed information about Polytem ® additives, contact our specialists or go to the corresponding section of the site.

Concrete grades by strength

To determine the strength of concrete, a small cube is created from the prepared mixture, the sides of which are 15 cm. The sample is tested. Mechanical pressure is applied to it. The highest pressure value that the cube can withstand is indicated in the name of the concrete grade; the letter “M” is used to designate the grade. For example, concrete grade M100 can withstand pressure of 98 kgf/cm2 (kilogram-force). Today, there are seven most common brands used in a variety of fields.

Water in cement mixture

The strength of concrete directly depends on the amount of water. The more liquid, the more mobile the mixture, which becomes more plastic and is laid without the formation of voids, but at the same time the strength properties decrease.

Mobile mixtures include:

♦ sedentary - P1; ♦ universal – P2 – P3; ♦ movable, not requiring sealing - P4; ♦ casting - P5.

The most popular types are ready-mixed concrete P3 and P4; you can buy them from our company with delivery throughout St. Petersburg and the region.

Concrete strength class

The distribution of concrete into strength classes occurs in a similar way as with the distribution by grade. The letter “B” is used to determine the class. In this case, the unit of measurement changes and the physicochemical aspects of the fillers, sealers, binders, and pouring method used are taken into account. The unit of measurement is MPa (megapascal). Technical tests and trials are carried out in accordance with GOST 18105-2010 “Concrete. Rules for monitoring and assessing strength." For a more clear example, let's look at the ratio of concrete grade to class, main areas of application and permissible loads.

Strength of concrete

Concrete strength is a key indicator of the load-bearing capacity of concrete. It is calculated experimentally by determining the compression limit of the material - the maximum load limit as a result of which the sample begins to collapse.

The design resistance of concrete to axial compression means its resistance to loading influences. This indicator is associated with standard parameters and is used during design calculations.

Until 2003, designers relied on material grades, but then a new classification was introduced. The grade of concrete for compression is designated by the letter “M” and indicates the tensile strength expressed in kgf/cm2, and the class of concrete is designated by the letter “B” and is expressed in MPa.

The difference is not only in units of measurement. The main difference between the classifications is the guarantee of confirmation of the strength of the material. The grade indicates the average value, and the compressive class of concrete guarantees that in 95% of testing cases the specified strength is ensured, and the risk of deviation from the standard indicators is no more than 5%.

Current standards are reflected in SP 52−101−2003 “Concrete and reinforced concrete structures without prestressing reinforcement.” Modern classification helps to design concrete and reinforced concrete structures with optimal characteristics.

Using average strength indicators (grade of concrete) carries the risk that the actual characteristics of the material will be lower than the calculated ones. If the average indicators are used as the lowest, for reinsurance, it is necessary to increase the size of the concrete structure, which leads to its noticeable increase in cost.

Table of concrete grade and strength class

buildingbook.ru

In short, the following grades of concrete are recommended for the following building structures:

- footing or preparation of the base for a monolithic structure - B7.5;

- foundations - not lower than B15, but in some cases the water resistance grade should be no lower than W6 (concrete B22.5). Also, according to Appendix D to SP 28.13330.2012, which has not yet been adopted, the class of concrete for foundations must be no lower than B30. I recommend using concrete with a waterproof grade of at least W6, which will ensure the durability of the structure;

- walls, columns and other structures located on the street - frost resistance grade not lower than F150, and for an area with an estimated outside air temperature below -40C - F200.

- internal walls, load-bearing columns - according to calculations, but not lower than B15, for highly compressed ones not lower than B25.

Perhaps I will not cover all the standards that may stipulate the requirements for choosing a brand of concrete, so I ask you to unsubscribe in the comments if there are any inaccuracies.

The main standardized and controlled indicators of concrete quality are:

— compressive strength class B;

— axial tensile strength class Bt;

— frost resistance grade F;

— waterproof grade W;

- medium density grade D.

Concrete compressive strength class B

Compressive strength class of concrete B corresponds to the cubic compressive strength of concrete in MPa with a probability of 0.95 (standard cubic strength) and is accepted in the range from B 0.5 to B 120.

This is the main parameter of concrete, which determines its compressive strength. For example, concrete class B15 means that after 28 days at a hardening temperature of 20°C, the concrete strength will be 15 MPa. However, a different figure is used in the calculations. The calculated compressive strength of concrete (Rb) can be found in table 5.2 SP 52-101-2003

Table 5.2 SP 52-101-2003

Why is strength measured after 28 days? Because concrete gains strength throughout its life, but after 28 days the increase in strength is no longer so great. One week after pouring, the strength of concrete can be 65% of the standard (depending on the hardening temperature), after 2 weeks it will be 80%, after 28 days the strength will reach 100%, after 100 days it will be 140% of the standard. When designing, there is a concept of strength after 28 days, and it is taken as 100%.

Also known is the classification by concrete grade M and numbers from 50 to 1000. The number indicates the compressive strength in kg/cm². The difference between concrete grade B and concrete grade M lies in the method for determining strength. For a concrete grade, this is the average value of compressive force during testing after 28 days of sample exposure, expressed in kg/cm². This strength is ensured in 50% of cases. Concrete class B guarantees concrete strength in 95% of cases. Those. The strength of concrete varies and depends on many factors; it is not always possible to achieve the required strength and there are deviations from the design strength. For example, the M100 concrete grade ensures concrete strength after 28 days of 100 kg/cm² in 50% of cases. But for design purposes this is somehow too small, so the concept of concrete class was introduced. B15 concrete guarantees a strength of 15 MPa after 28 days in 95% of cases.

In design documentation, concrete is designated only as class B, but in construction practice, the concrete grade is still used.

You can determine the class of concrete by grade and vice versa using the following table:

The class of concrete for axial tensile strength Bt corresponds to the value of concrete axial tensile strength in MPa with a probability of 0.95 (standard concrete strength) and is accepted in the range from Bt 0.4 to Bt 6.

It is allowed to take a different value for the strength of concrete in compression and axial tension in accordance with the requirements of regulatory documents for certain special types of structures (for example, for massive hydraulic structures).

The frost resistance grade of concrete F corresponds to the minimum number of cycles of alternating freezing and thawing that a sample can withstand during a standard test, and is accepted in the range from F 15 to F 1000.

The water resistance grade of concrete W corresponds to the maximum value of water pressure (MPa 10-1) withstood by the concrete sample during testing, and is accepted in the range from W 2 to W 20.

Average density grade D corresponds to the average volumetric mass of concrete in kg/m3 and is accepted in the range from D 200 to D 5000.

Concrete is also marked by mobility (P) or the cone settlement is indicated. The higher the P number, the more liquid the concrete and the easier it is to work with.

For prestressing concrete, a self-stressing grade is established.

Selection of concrete grade by strength

The minimum class of concrete for structures is assigned in accordance with SP 28.13330.2012 and SP 63.13330.2012.

For any reinforced concrete building structures, the concrete class must be at least B15 (clause 6.1.6 of SP 63.12220.2012).

For prestressed reinforced concrete structures, the class of concrete in terms of compressive strength should be taken depending on the type and class of prestressed reinforcement, but not lower than B20 (clause 6.1.6 SP 63.12220.2012).

A reinforced concrete grillage made from precast reinforced concrete must be made of concrete of at least class. B20 (clause 6.8 SP 50-102-2003)

The class of concrete for structures is assigned according to strength calculations for technical and economic reasons, for example, on the lower floors of a building, monolithic columns have greater strength because the load on them is higher; on the upper floors the concrete class is reduced, which makes it possible to use columns of the same section on all floors.

There are also recommendations from SP 28.13330.2012. According to Resolution 1521 of December 26, 2014, appendices A and D of SP 28.13330.2012 are not included in the mandatory list, i.e. are recommended, but I recommend that you pay attention to these applications because, perhaps, they will soon be mandatory for use. First of all, it is necessary to classify the structure according to the operating environment according to Table A.1 SP 28.13330.2012:

Table A.1 - Operating environments

Depending on the selected operating environment, we assign a concrete class for the structure according to table D.1 SP 28.13330.2012.

Table E.1 - Requirements for concrete depending on the classes of operating environments

If you look at these requirements, then for the foundation you need to use concrete of at least B30 (XC2 medium). However, for now these are recommended requirements, which in the future will become mandatory (or not, who knows?)

Selection of concrete grade for water resistance

The grade of concrete for water resistance is selected according to tables B.1-B.8 SP 28.13330.2012, depending on the degree of aggressiveness of the environment. Data on soil aggressiveness are indicated in geotechnical surveys and the recommended grade for water resistance is usually written there.

For piles, it is necessary to use concrete of a waterproof grade not lower than W6 (clause 15.3.25 SP 50-102-2003). Concrete B22.5 has this grade, so you need to take this into account when selecting the class of concrete.

For above-ground structures exposed to atmospheric influences at a design negative outdoor temperature above minus 40 °C, as well as for external walls of heated buildings, the concrete grade for water resistance is not standardized (clause 6.1.9 SP 63.13330.2012).

Selection of concrete grade for frost resistance

The selection of concrete grades for frost resistance is carried out according to tables Zh.1, Zh.2 SP 28.13330.2012, depending on the design temperature of the outside air.

Table G.1 - Requirements for concrete structures operating in conditions of alternating temperatures

Table G.2 - Requirements for frost resistance of concrete wall structures

The calculated winter outside air temperature for the calculation of reinforced concrete structures is taken based on the average air temperature of the coldest five-day period with a probability of 0.98 depending on the construction area according to SP 131.13330.2012.

In soils with a positive temperature, below the freezing level by 0.5 m, frost resistance is not standardized (SP 8.16 SP 24.13330.2011)

For example, for Moscow, the temperature of the coldest five-day period with a probability of 0.98 is equal to minus 29 °C. Then the grade of concrete for frost resistance is equal to F150 (Characteristics of the mode - Possible episodic exposure to temperatures below 0 ° C a) in a water-saturated state, for example, structures located in the ground or under water).

Protective layer of concrete

To ensure that the reinforcement does not become exposed over time, there are requirements for a minimum thickness of the concrete layer to protect the reinforcement. According to the manual on the design of concrete and reinforced concrete structures made of heavy concrete without prestressing reinforcement SP 52-101-2003, the minimum thickness of the protective layer is determined according to table 5.1 of the Manual to SP 52-101-2003:

Table 5.1 Benefits to SP 52-101-2003

For prefabricated reinforced concrete elements, the thickness of the protective layer can be reduced by 5 mm from the data in Table 8.1 SP 52-101-2003 (clause 8.3.2).

For bored piles, the protective layer of concrete is at least 50 mm (clause 8.16 of SP 24.13330.2011), for bored piles of bridge foundations - 100 mm.

For bored piles used as protective fences, the protective layer of concrete is assumed to be 80-100 mm (clause 5.2.12 of the Methodological manual for constructing fences from bored piles).

Also, in all cases, the thickness of the protective layer cannot be less than the thickness of the reinforcement.

The protective layer of concrete is considered from the outer surface to the surface of the reinforcement (not to the axis of the reinforcement).

The protective layer of concrete is usually provided by using fixatives:

Design values ​​for concrete resistance

SP 63.13330.2012 Concrete and reinforced concrete structures. Basic provisions

The calculated values ​​of concrete resistance to axial compression Rb are determined according to formula 6.1 SP 63.13330.2012:

The calculated values ​​of concrete axial tensile resistance Rbt are determined using formula 6.2 SP 63.13330.2012:

The values ​​of the safety factor for concrete in compression γb are taken equal to:

for calculations based on the limit states of the first group:

1.3 - for heavy, fine-grained, prestressing and lightweight concrete;

1.5 - for cellular concrete;

for calculations based on limit states of the second group: 1.0.

The values ​​of the reliability coefficient for concrete in tension γbt are taken equal to:

for calculations based on the limit states of the first group when assigning a concrete class for compressive strength:

1.5 - for heavy, fine-grained, prestressing and lightweight concrete;

2.3 - for cellular concrete;

for calculations based on the limit states of the first group when assigning a concrete class for tensile strength:

1.3 - for heavy, fine-grained, prestressing and lightweight concrete;

for calculations based on limit states of the second group: 1.0.

(clause 6.1.11 SP 63.13330.2012)

If necessary, the calculated values ​​of the strength characteristics of concrete are multiplied by the following operating conditions coefficients γbt, taking into account the specifics of concrete operation in the structure (nature of load, environmental conditions, etc.):

a) γb1 - for concrete and reinforced concrete structures, introduced to the calculated resistance values ​​Rb and Rbt and taking into account the influence of the duration of the static load:

γb1 = 1.0 for short-term (short-term) load action;

γb1 = 0.9 with prolonged (long-term) load action. For cellular and porous concrete γb1 = 0.85;

b) γb2 - for concrete structures, introduced to the calculated resistance values ​​Rb and taking into account the nature of destruction of such structures, γb2 = 0.9;

c) γb3 - for concrete and reinforced concrete structures concreted in a vertical position with a concreting layer height of over 1.5 m, added to the design value of concrete resistance Rb, γb3 = 0.85;

d) γb4 - for cellular concrete, added to the calculated value of concrete resistance Rb:

γb4 = 1.00 - when the moisture content of cellular concrete is 10% or less;

γb4 = 0.85 - when the moisture content of cellular concrete is more than 25%;

by interpolation - when the moisture content of cellular concrete is more than 10% and less than 25%.

The influence of alternating freezing and thawing, as well as negative temperatures, is taken into account by the coefficient of concrete operating conditions γb5 £ 1.0. For above-ground structures exposed to atmospheric environmental influences at a design temperature of outside air during the cold period of minus 40 °C and above, the coefficient γb5 = 1.0 is taken. In other cases, the coefficient values ​​are taken depending on the purpose of the structure and environmental conditions in accordance with special instructions.

(clause 6.1.12 SP 63.13330.2012)

For pile foundations in accordance with SP 24.13330.2011 Pile foundations, clause 7.1.9

7.1.9 When calculating cast-in-place, drilled piles and barettas (except for pillar piles and bored piles) based on the strength of the material, the design resistance of concrete should be taken with a reduction factor for operating conditions γcb = 0.85, taking into account concreting in a narrow space of wells and casing pipes, and additional reduction factor γ'cb, taking into account the influence of the method of piling work:

a) in clayey soils, if it is possible to drill wells and concrete them dry without fastening the walls when the groundwater level during the construction period is below the heel of the piles, γ'cb = 1.0;

b) in soils in which wells are drilled and concreted dry using removable casing pipes or hollow augers, γ'cb = 0.9;

c) in soils in which wells are drilled and concreted in the presence of water using removable casing pipes or hollow augers, γ'cb = 0.8;

d) in soils in which well drilling and concreting is carried out under clay solution or under excess water pressure (without casing), γ'cb = 0.7.

Parameters for calculating reinforced concrete structures:

Parameters for the calculation of reinforced concrete structures are given in SP 63.13330.2012:

Table 6.7

Table 6.8

Table 6.11

You need to be more careful with this table - the data is given not in 10-3 MPa, but in MPa x 10-3, i.e. in GPa or 1000 MPa. For example, the modulus of elasticity for B25 concrete is 30 GPa = 30*1000 MPa. I don’t know why the compilers of this table were so clever, but beginners get caught in this.

Designation of concrete on drawings

In the specification, concrete is marked according to GOST 26633-2012. For example: Concrete B25 F200 W8 ​​means that the concrete is accepted for strength class B25, frost resistance class 200, and water resistance class W8.

On sections and sections, concrete is indicated by shading in accordance with GOST 2.306-68, but there is no shading of reinforced concrete. Nevertheless, in construction drawings, shading is used in accordance with GOST R 21.1207-97 (the standard has been canceled, but nevertheless these shadings are used).

Literature:

1. SP 52-101-2003 Concrete and reinforced concrete structures without prestressing reinforcement (pdf);
2. Manual for SP 52-101-2003 Manual for the design of concrete and reinforced concrete structures made of heavy concrete without prestressing reinforcement (pdf)
3. SP 63.13330.2012 (Updated edition of SNiP 52-01-2003) Concrete and reinforced concrete structures. Fundamentals (pdf);
4. SP 24.13330.2011 (Updated edition of SNiP 2.02.03-85) Pile foundations (pdf);
5. SP 28.13330.2012 (Updated edition of SNiP 2.03.11-85) Protection of building structures from corrosion (pdf);
6. SP 52-105-2009 Reinforced concrete structures in cold climates and on permafrost soils (pdf).

Posted in Reinforced concrete structures Tagged Concrete grade

How can you improve the class and grade of concrete?

In order to enhance the strength gain of concrete, the use of additional fillers, aggregates or binders is allowed. The most common example is the addition of polypropylene fibers to a concrete mixture, which increases the early and final strength of concrete.

We strongly recommend that you use a high-quality product, a concrete strength additive Polytem ® Force. This product is guaranteed to enhance the strength characteristics of your mixture, increase its hardness and service life.

Cement activity

The activity of cements refers to the tensile strength of the material made from them, which has settled for 28 days. The activity determines what kind of concrete will be produced. The indicators of this parameter determine what brand the cement belongs to. The activity of cement is influenced by the following factors:

Grinding and granule sizes. Concrete made from finely ground cement quickly gains strength. If the grind is medium, then strength is gained at the end of the hardening period. Coarsely ground Portland cement does not mix well with water; in such concrete, the cement forms lumps when hardening, which negatively affects the strength.

Chemical composition. If it contains quicklime, cement remains active for quite a long time.

Impurities. Even 2% magnesium oxide in Portland cement accelerates the development of strength. But with an increase in this substance, the activity of cement decreases.

Freshness. If cement was stored in a humid environment for about one month, the strength will decrease by 20%. When stored in such conditions for about three months, the reduction in strength characteristics reaches 60%. This happens because when moisture and carbon dioxide come into contact, new compounds appear in the cement that have a bad effect on the activity.

The activity can be reduced by adding quartz to the mixture, and increased by adding aluminates. When adding gypsum to the cement mixture, the period and setting time are adjusted, in this way you can influence the increase in the hardening rate. Gypsum is added to the dry mixture or ready-made cement mortar.

If we summarize the information, we get the following: to get durable concrete, you need to give preference to fresh and finely ground cement.

Scope of use depending on strength

Depending on whether it belongs to a particular brand or class, the mixture can be used for completely different purposes. Let's look at the most popular and in demand.

The most important characteristic of concrete is its compressive strength , determined by the grade of the concrete mixture. For each type of construction work, different types of concrete are used:

• M100 is a lightweight concrete used in the preparatory and initial stages of construction. With its help, preparation for pouring monolithic walls and reinforcement works is carried out. Curbs and curbs are installed from it;
• M150 – The range of applications coincides with the brand indicated above. Has higher strength;
• M200 is the most popular and most frequently used brand. Used for many purposes - from laying roads and sidewalks to constructing buildings with increased load;
• M250 – The area of ​​use coincides with the previous brand, has slightly higher strength indicators;
• M300 – production of load-bearing wall blocks, floor slabs, fences, etc. Used for monolithic filling;
• M350 – is highly durable, used for the construction of airfields and load-bearing elements;
• M400 – production of reinforced concrete products, construction of buildings and structures subject to higher loads. Hydraulic structures, factories, large buildings, etc.;
• M450 - construction of heavy and massive objects - dams, metro, etc.;
• M500 – construction of reinforced concrete structures.

As we can see, this material is produced and manufactured in various variations. Be sure to contact specialists for a free consultation. Well, we recommend that you read other articles in our information section and become familiar with the products with which you can improve the characteristics and properties of your mixture.

Methods for determining strength: concrete compression testing

There are two methods:

• destructive;
• non-destructive.

The first method measures the minimum force applied to break cubes and cylinders that are cut, sawed or drilled out of whole products. The rate of increase in load force is constant. After the test is completed, the final value of such efforts is calculated.

In the second method of finding the required indicator, a given place is affected mechanically (impact, tearing, chipping, indentation, tearing with chipping, elastic rebound). The point of application of the device should not be on the edge or opposite the fittings. Next, find the result based on the expressed gradation.

You should not count on complete truthfulness; there is an error of up to 10% for each type of check.

How samples are selected using the destructive method

1. Samples from concrete mixture.

Cubic and cylindrical samples are prepared for testing. A cube with a long side of 150 mm is considered the reference.

• All specimens are created in special molds, and the structure is lubricated with oil before use. Next, fill it with concrete mixture and compact it.
• Compact using a bayonet with a steel rod, a vibrating platform or an in-depth vibrator.
• After a day, all hardened samples are taken out and placed in a box with normal conditions (humidity - 95%, temperature - +20 ° C). Sometimes the workpieces are placed in an aqueous environment or in an autoclave.

1. Samples from ready-made concrete products.

Instances for strength testing are obtained by cutting, sawing or drilling out from whole products. There should be no reinforcement at the extraction site at a point where extraction will not result in a reduction in load-bearing capacity. Samples are taken away from the joints and edges of the product. Samples are removed from the middle part of the sample as in the figure.

Preliminary preparation for testing

Before proceeding directly to testing, all samples are measured and inspected for cracks, chips, and potholes. If there are chips of more than 10 mm, potholes with a diameter of 10 mm or more and a depth of 5 mm, the samples are discarded.

Measurements are also taken to check for linear errors, discrepancies in the perpendicularity of nearby faces, and offsets from straightness and flatness. If such defects are found, the edges and planes are ground or leveled with a quick-hardening substance no more than 5 mm thick.

How concrete samples are tested

All prepared samples of one group are tested for strength for one hour. Power loading is carried out without interruption, with a constant rate of increasing load until failure. At the same time, the time from the start of loading to its end is at least 30 s.

During the inspection, special construction stands are used:

• samples are placed on the bottom plate of the press in the center;
• then combine the top plate and the specimen so that they are close to each other;
• then a force load is applied at a speed of 0.6±0.2 MPa/s.

How is the strength of concrete related to frost resistance and water resistance?

The strength of concrete depends on its density. At the same time, a high level of density is reflected in other properties of the material.

Despite its high density, concrete remains a porous material. It contains many pores and “capillaries” in which mold, fungi and microorganisms can develop. Such exposure has a negative impact on the material and can lead to its destruction.

If the concrete is regularly exposed to low temperatures. The moisture in its pores freezes and expands. With each cycle of freezing and thawing, cracks and damage become larger and more dangerous, ultimately leading to destruction.

This is why density is so important for concrete. The denser it is, the fewer pores it has. This applies not only to their number, but also to their size.

To improve the hydrophobic properties of concrete, special additives and mastics are used to impregnate the hardened stone.

Properties of concrete

Initially, it is a material with a rough and heterogeneous structure. However, manufacturers, at the request of customers, can set the necessary properties during its manufacturing process:

• strength;
• deformation;
• physical.

Strength properties imply standard or necessary design characteristics for:

• compression;
• stretching;
• adhesion to the reinforcement.

Deformability properties imply changes that occur during various external influences:

• compressibility or elongation under load;
• creep;
• shrinkage;
• swelling;
• temperature deformations.

The main physical properties of concrete include parameters for:

• waterproof;
• resistance to various temperatures, corrosion, acids and other aggressive environments;
• fire resistance;
• thermal conductivity;
• sound conductivity and others.

Concrete for reinforced concrete structures

In reinforced concrete structures (RCS) used in modern construction, concrete is divided into the following types:

• Heavy. This type of concrete with a dense structure is made using cement as a binder and coarse-grained dense aggregates. It hardens under any conditions and has an average density of 2200-2500 kg/cub.m;
• Fine grained. This type of heavy concrete with a dense structure is made using cement as a binder and fine aggregates. It hardens under any conditions and has an average density of more than 1800 kg/cub.m;
• Easy. This type of coarse concrete with a dense porous structure is made using cement as a binder and porous aggregates. It hardens under any conditions. When its basic physical properties coincide with heavy concrete, it is used together with it.
• Cellular. This type of concrete hardens when special treatment is applied.
• Special tensile concrete.

For what reason are light and lightweight types of concrete used when creating reinforced concrete structures? Among the main advantages of their use, modern builders cite the following capabilities:

• reduction by 25-40% of the mass of reinforced concrete;
• reduction in the cost of reinforced concrete structures;
• improving the soundproofing characteristics of reinforced concrete concrete;
• increasing the heat-shielding characteristics of reinforced concrete structures;
• increasing the seismological stability of reinforced concrete structures;
• increasing the fire resistance of reinforced concrete concrete.

Lightweight, cellular and porous types of concrete with an average density of less than 1400 kg/m3 are used to create reinforced concrete fences. Dense fine-grained ones can be used together with heavy types, as a filler for joints and seams of reinforced concrete concrete. Particularly heavy ones are used in the construction of special facilities, including military bunkers and nuclear power plants. The average density of concrete used at such facilities is more than 2500 kg/cub.m.

Classifying characteristics

The physical and mechanical characteristics of concrete are directly influenced by:

• preparation method;
• type of binder;
• type of coarse aggregate;
• type of fine aggregate;
• water.

These characteristics are determined by the structure of the material, which creates certain conditions for its hardening.

Taking into account the requirements for basic physical properties, concrete is classified in the following areas:

I. Structure

Divided into:

• dense concrete - all the free space between the aggregate substances is occupied by a hardened binder;
• large-porous - the free space between the aggregate substances is not completely occupied by the hardened binder (usually there is little or no sand in it);
• porous - in the free space between the filler substances there is a hardened binder with special additives, as a result of which specific pores are formed;
• cellular - artificial closed pores are created in the free space between the filler substances.

The strength of concrete directly depends on the increase in the density of its structure. The manufacturer can select the density required by the customer using:

• choosing the optimal grain composition;
• mechanical additional compaction of the concrete mixture during production;
• using more cement;
• changes in the water/cement ratio in concrete.

A higher grade of cement requires less of it to achieve the required concrete strength.

II. Density

It is measured by the ratio of the mass of a material per unit volume. According to the degree of average density, it is divided into the following categories:

• especially heavy – more than 2500 kg/cub.m;
• heavy – 2200-2500 kg/cub.m;
• lightweight – 1800-2200 kg/cub.m;
• light – 500-1800 kg/cub.m.

III. Type of binder in concrete

Modern manufacturers use various substances as a binder, according to which it is divided into the following types:

• cement;
• polymer cement;
• silicate on lime;
• plaster;
• mixed;
• special using a variety of additives.

IV. Type of filler

The following are used as filler in production:

• dense natural material (gravel or crushed rock, quartz sand);
• porous natural material (perlite, pumice, shell rock);
• artificial material (expanded clay, slag);
• a special material that ensures concrete resistance to various thermal and chemical influences.

Crushed stone is a cheaper material that can more quickly provide a given strength.

Concrete is also divided into types based on the porous aggregate used in it:

• expanded clay concrete;
• slag concrete;
• perlite concrete;
• pumice concrete and others.

V. Grain composition

Divided into the following types:

• coarse-grained, in which large and fine aggregates are used;
• fine-grained, in which only fine aggregates are used.

VI. Curing conditions

Divided into the following categories:

• natural hardening;
• subjected to treatment under atmospheric pressure with heat and moisture;
• autoclaved under high pressure conditions.

Reinforcement

Concrete structures reinforced with metal reinforcement are the strongest and most durable. Sometimes volumetric reinforcement is used - they add different types of fiber, for example, polypropylene. As a result, the strength of the material increases significantly, and when it hardens, shrinkage decreases.

Many other factors also affect the strength of concrete. For example, density, which, in turn, affects the water resistance and frost resistance of the building material. And the strength is also affected by the continuous or intermittent laying order and the use of vibrators. If these points are taken into account, the building structure will retain its properties for a long time.

Destructive methods

For laboratory testing, samples are separated from hardened concrete using destructive methods. The procedure for conducting them is regulated by GOST 10180-2012. The following samples are selected:

• cubes cut from a monolith;
• cylinders (cores) cut out by drilling with a diamond bit.

The test procedure is quite simple. The samples are placed under a press and brought to destruction. The force required for this is recorded.

Destructive methods force some damage to the monolith and are quite labor-intensive, but they give the most accurate strength values. That is why these methods are mandatory in the manufacture of critical concrete structures. The number of samples taken depends on the volume of concrete work, the number of mortar batches and its possible variations.

External conditions

The curing procedure must take place while maintaining the required humidity and temperature. This ensures the hydration of all cement grains, the water does not evaporate rapidly, which results in concrete of the planned strength. The optimal temperature for proper hardening of the solution is from 15 to 20℃. The most suitable relative air humidity is 90-100%. If the mixture is laid under such conditions, then the strength parameters of the structure being built increase along with the hardening time.

Cement is classified as a water-hardening binder, so after pouring the solution, favorable conditions for hardening should be provided. Until the mixture reaches critical strength, it must be constantly maintained at high humidity so that the resulting structure is as strong as planned.

In large structures, the temperature of the concrete solution varies between the inside and outside. For example, during the construction of large underground tunnels, bridges, overpasses. In these cases, the temperature is equalized with liquid nitrogen, cooling the mixture so that the concrete solution hardens more or less evenly.