Concrete class of concrete based on tensile strength in bending

The areas of design and construction are strictly regulated by special provisions on concrete and reinforced concrete, GOST standards, in which the class and grade of concrete are important.
The ratio of the class and grade of concrete determines engineering calculations for the construction of foundations, walls, architectural structures and structures, where the parameters are based on laboratory strength testing data using a concrete cube of certain sizes. In the UK and some European countries, the compressive strength of concrete monoliths is measured using a cylinder. The class and grade characterize the strength of concrete, but when developing the composition of a concrete mixture, frost resistance (F), water resistance (W), and other indicators should be taken into account, taking into account the individual characteristics of construction projects.

The strength of concrete is not a constant characteristic; over a certain time (with correctly selected proportions) the mortar mixture hardens and gains its design strength. The minimum curing period for concrete is 28 days, but the process of developing strength after this period does not end: the quality of concrete increases over time, and the base hardens. The strength of concrete depends on the ratio of water and cement; the ideal composition is considered to be the proportions W/C = 0.3:0.5; if the ratio is lower, the concrete loses its plasticity; as the proportion of water increases, the strength decreases, but the mobility of the solution becomes higher.

Material quality

Adhesion refers to how well the cement stone adheres to the aggregate particles. In addition, the main qualities also include:

• frost resistance;
• waterproof;
• compressive and tensile strength.

When the material is at its design age, its strength characteristics can be judged by the latest parameters. Therefore, it is worth noting that during cooking it turns out to be heterogeneous.

Here is the correspondence of concrete grades and classes

Strength fluctuations are reduced with high-quality mixture preparation, as well as with a higher construction culture. Therefore, it is worth remembering that the manufactured material must not only have an average specified value, but also have an even distribution over the entire surface.

Class Definition

The above-described fluctuations can be taken into account in such an indicator as a class, which is understood as a percentage indicator of a property. For example, if it is indicated that a material has a strength class of 0.95, then in 95 cases and 100 it will have this indicator.

It is worth noting that according to GOST, the classification of concrete consists of 18 main classes of compressive strength indicators. In this case, at the beginning the name of the class is indicated as B1, followed by the numerical value of the tensile strength, displayed in MPa.

Product classification

For a more accurate perception, it is worth giving an example. So, let’s assume that we have concrete class B35. This means that in 95 cases out of 100 it provides a compressive strength of up to 35 MPa.

In addition, there are other strength classes:

• index B,, denotes axial tension;
• index Btb displays the tensile strength during bending.

Remember that the compressive strength can be 20 times higher than the tensile strength. Therefore, during construction, steel reinforcement is used, which increases the load-bearing capacity of the material, while the price increases.

Table of concrete grades and classes by compressive strength

Brand Definition

According to the CMEA standard 1406-78, the main indicator of the strength of products is their class. If this standard was not taken into account during the design of various products, their strength is described using a brand.

It is understood as any of its properties, expressed in a numerical characteristic, for the calculation of which the average results of the samples shown during testing are used. To designate the brand, the values ​​obtained during testing are used:

Tip: Be aware that the grade cannot show strength variations throughout the entire volume of the concrete product.

How to convert concrete grades to classes

Hydraulic concrete - what is it?

Building materials for structures that are constantly in contact with water must be highly durable and moisture resistant. Hydraulic concrete has suitable qualities and can withstand the destructive effects of water. Its parameters and composition are determined by the relationship between the level and pressure of water, the size of the building and temperature conditions.

The following types of hydraulic material are distinguished:

• surface – used for the part that is above the water level;
• underwater - for an area under water;
• concrete in an area where the water level is constantly changing.

Hydraulic concrete can also be massive or non-massive, for pressure and non-pressure structures. Separate requirements for each type are specified in GOST 26633–2012.

Specifications

The main properties of the material include:

• frost resistance;
• high level of water resistance;
• compressive, bending and tensile strength.

According to the degree of frost resistance, hydraulic concrete is divided into 5 grades: F50, F100, F150, F200, F300, where the digital value indicates the number of cycles of freezing and thawing of the material before it loses 25% of its strength. The test is carried out in special freezers. The value of this parameter must be taken into account when constructing buildings that will be exposed to low temperatures. In some cases, frost-resistant concrete of the F400 grade is produced for hydraulic structures; during its production, special impurities are added to the composition in certain proportions.

When the material reaches the age of 180 days, the level of water resistance is determined. During testing, hydraulic concrete must not allow moisture to pass through. Brands W2, W4, W6, W8 have these properties. This means that it can withstand water pressure of 0.2, 0.4, 0.6, 0.8 MPa, respectively. By adding special plasticizer admixtures or changing the proportions of cement, the density and water resistance can be increased to W12.

1. Compressive strength.

Determined by axial compression of a cube measuring 15x15x15 cm. There are several classes, which are designated by the letter B and a number indicating the number of cycles of exposure. The most popular include B10–B40.

2. Flexural and axial tensile strength.

This indicator is important when cracks are not allowed to form in the structure or the work is determined by the strength of tensile concrete. Main classes: from Bt 0.4 to Bt 4 in increments of 0.2. Flexural strength is indicated by indicators from Btb 0.4 to Btb 8 with the same step. In some cases, additional parameters are taken into account: resistance to abrasion by sediments and water flows, deformability, slight shrinkage, etc.

Composition of hydroconcrete

The properties and characteristics of the structures being built are determined by the components included in the cement base:

1. plasticized – increases resistance to water and frost resistance;

2. Portland cement – ​​used for buildings with variable water levels;

3. hydrophobic – added to underwater parts of structures;

4. pozzolanic or slag – highly resistant to mineral-containing and fresh water;

5. sulfate-resistant – resistant to the aggressive influence of hard water.

The optimal fillers are quartz sands from natural deposits, washed to remove dust particles and clay, with a density of 2 t/m3. If the grain size exceeds 2 mm, the level of water resistance decreases. You can use crushed stone or gravel, but they must have high density, flakiness, water and frost resistance.

Areas of application

Concrete for hydraulic structures is laid in large volumes in a short time, this is due to the peculiarities of its use. To regulate the temperature stresses that arise during heat generation, crushed ice is added to the concrete composition, plasticizing and mineral additives are introduced, and the masonry itself is cooled with cold water pipes. Extensive work is carried out using heavy equipment. The choice of cement for mortar is influenced by construction conditions and the principle of functioning of buildings.

Hydraulic waterproof concrete is used in construction:

• bridges, metro tunnels;
• swimming pools, decorative mini-ponds and water amusement parks;
• sewer mines;
• embankment areas;
• hydraulic engineering and treatment facilities (berths, dams, breakwaters).

Individually, such material is used for arranging a basement or building a foundation in cases where groundwater flows through the area.

Cost of main types

The desired grade of hydraulic concrete is obtained by mixing certain proportions of cement, water, aggregate (crushed stone, sand) and additives. The higher the ratio of the first component in the mixture, the higher the grade. In Russia, the most common are M300 and M400.

It is better to purchase goods from trusted factories that have a good reputation. When purchasing material, you must clarify all the parameters and make a description of the future design. The cost is determined individually for each person depending on the requirements for the construction. The manufacturing company determines not only the basic characteristics, but also additional ones, so that the composition best suits the specific building. Items in price lists are not always suitable for the buyer.

The table shows the approximate cost in rubles per cubic meter of hydraulic concrete of popular brands that can be bought in Russia.

Compressive strength grade

1. This is one of the most commonly used characteristics of concrete structures.
2. The instructions require to determine it using cube-shaped samples with a length of one side of 150 mm.
3. The test is carried out during the conditional design age - in most cases this is 4 weeks.

Tip: if a series of three samples is taken, the tensile strength is calculated from the two largest of them. To express it, the following units are used: kgf/cm2.

1. Experts identify only 17 grades of heavy concrete depending on its compressive strength. To designate them, the index “M” is used, followed by a number. For example, grade M450 means that such concrete guarantees a minimum compressive strength of 450 kgf/cm2.
2. If we take into account the axial tensile strength, then there are much more grades - from Pt5 to Pt50 (adding 5 kgf/cm2 each time). For example, the grade of concrete Pt30 will mean that it is able to withstand axial tension of up to 30 kgf/cm2.
3. For concrete that will be used during the manufacture of bendable reinforced concrete structures, there is also a flexural tensile characteristic, which is displayed using the “Ptb” index.

Advice: parallels should not always be drawn between the brand of concrete and its class.

The concrete surface class according to SNiP has 4 parameters

Classes and brands

The fact is that a lot depends on how homogeneous the material is. The coefficient of variation is used to indicate this quantity.

The lower its numerical value, the more homogeneous the concrete is. When this indicator decreases, the class and grade of the material decrease accordingly. For example, M300, which has a coefficient of variation of 18%, will receive class B15, but if it decreases to a value of 5%, the class will increase to B20.

Advice: research results prove that during the production of a concrete mixture it is necessary to achieve its maximum homogeneity.

The numerical value of strength is influenced by many factors. The greatest is the quality of the initial components, as well as such an indicator as porosity.

Making the solution

It takes a significant amount of time for a material made with Portland cement to gain strength. In addition, for the process to proceed normally, certain conditions must be met.

Frost resistance

Using an indicator such as the frost resistance grade of concrete, you can determine how many freezing and thawing cycles a 28-day material can withstand, while losing no more than 15% of its compressive strength. The F index is used to denote this indicator, and there are 11 classes in total.

Advice: for concrete to have good frost-resistant properties, it must contain high-quality Portland cement, as well as its various modifications - sulfate-resistant, hydrophobic, etc.

However, there are certain restrictions on the percentage of tricalcium aluminate in Portland cement.

For example, for:

• F200 no more than 7% of such a substance is allowed;
• F300 – up to 5%, etc.

The presence of active mineral additives in cement is extremely undesirable, since their use increases the need for water. But reducing water demand is achieved through the use of surfactants.

Working with solution in cold weather

Advice: in hydraulic structures with frost resistance grade F 300, as well as filler with a diameter of no more than 20 mm, the volume of entrained air should be within 2-4%

Here are a few instructions to follow:

1. To obtain high-quality frost-resistant concrete, the most accurate ratio of all components must be observed.
2. They must be thoroughly mixed with your own hands, obtaining the most homogeneous mixture possible.
3. After this compact.
4. Provide the necessary good conditions during the hardening process.

Advice: make sure that thermal expansion of the concrete components does not occur, and that the water and air values ​​are within acceptable limits.

In situations where parts with a high degree of frost resistance (F200 and above) are being manufactured, it is worth remembering that the material must harden under positive ambient temperature conditions. In addition, its moisture should be maintained for about 10 days.

Water permeability

The grade for such an indicator as water resistance is determined by testing the material for limited permeability during unilateral water pressure. To designate it, use the index “W”, followed by a number.

Water permeability of the material

It denotes the maximum pressure (in kgf/cm2) that the test sample, the diameter and height of which is 150 mm, can withstand during certain tests. For example, brand W4 can withstand water pressure of 4 kgf/cm2. There are 10 brands in total - from W2 to W20 (adding 2 kgf/cm2).

There are methods by which you can increase the water resistance of the mixture during its preparation, laying and hardening of concrete, as well as methods that can increase this indicator of already hardened material.

Concrete strength control

In order for the concrete solution to exactly correspond to the specified parameters and withstand loads, its quality is monitored even at the preparation stage. Before preparing the mixture, be sure to study the recipe, the requirements for the components and their proportions.

Basic criteria for monitoring and testing concrete:

• Compliance of the cement used with the brands specified in the recipe - for example, M100 cement is definitely not suitable for preparing M300 concrete, even if its volume is large. The higher the number next to the letter M in the cement marking, the more durable the solution will be.
• The volume of liquid in the solution - the more water in the mixture, the more actively the moisture evaporates during the drying process and can provoke the appearance of voids when hardening occurs.
• The quality and fraction of fillers - rough particles of irregular shape provide the strongest adhesion of the ingredients in the concrete composition, which during the hardening process gives the required result in the form of high strength. Dirty aggregate can reduce the tensile and compressive strength properties of concrete.

• Thorough mixing of the components at all stages of preparing the solution - according to the technology, the solution is mixed in a working concrete mixer or in production for a long time.
• The qualifications of workers also play an important role, since even if a high-quality B20 mixture is used, for example, the strength can be reduced due to improper installation and lack of compaction (vibration increases the strength of concrete by 30%).
• Conditions for hardening and operation are best when the concrete hardens and becomes hard at an air temperature of +15-25 degrees and high humidity. In this case, we can talk about the exact conformity of the monolith to its brand - if B15 concrete was poured, then its technical characteristics will be demonstrated.

Strength of concrete: table

Concrete exhibits certain values ​​in terms of tensile strength, bending, and other loads. They do not always correspond to those specified in GOST and design documentation; there is often an error that can be disastrous for the monolith and the entire structure or have no effect.

Types of concrete strength (compressive, bending, tensile, etc.):

1. Design

   - the one that is indicated in the documents and assumes values ​​​​at full load on the concrete structure. It is considered in a hardened monolith, 28 days after pouring.

2. Normalized

   – value that is determined according to technical specifications or GOST (ideal).

3. Actual

   is the average value obtained from the tests performed.

4. Required

   – the minimum suitable indicator for operation, which is established in the laboratory of production and enterprises.

5. Vacation pay

   – when the product can already be shipped to the consumer.

6. Stripping

   – observed at the moment when the concrete product can be removed from the molds.

Types of strength related to the brand of concrete and its quality: compression and bending, axial tension, as well as transfer strength. Concrete resembles stone - the compressive strength of concrete is usually much higher than its tensile strength. Therefore, the main criterion for the strength of a monolith is its ability to withstand a certain load under compression. This is the most significant and important indicator.

So, for example, the indicators of concrete B25 (strength class) and grade M350: average compression resistance up to 350 kgf/m2 or up to 25 MPa. Actual values ​​are usually slightly lower, since strength is influenced by many factors. B30 concrete will have corresponding indicators, etc.

To determine these indicators, special sample cubes are created, allowed to harden, and then sent under a specially designed laboratory press. The pressure is gradually increased and recorded at the moment when the sample cracks or crumbles.

The determining condition for assigning a grade and class to concrete is the calculated compressive strength, which is determined after the monolith has completely set and hardened (the process takes 28 days).

It is after 28 days that the concrete reaches the calculated/design strength of the grade. Compressive strength is the most accurate indicator of the mechanical properties of a monolith and its resistance to loads. This is a kind of boundary of already hardened concrete to the mechanical force acting on it in kgf/m2. Concrete M800/M900 has the highest strength, M15 has the lowest.

Flexural strength increases with increasing grade index. Typically the bending/extension ratings are lower than the load capacity. Young concrete shows a value in the region of 1/20, old concrete – 1/8. This parameter is taken into account at the design stage of construction. Method of determination: a beam of 120x15x15 centimeters is poured from concrete, allowed to harden, then placed on supports (the distance between them is 1 meter), a load is placed in the center, increasing it gradually until the sample collapses.

Strength is calculated using the formula Rben = 0.1PL/bh2, here:

• L – distance between supports;
• P – load and sample mass;
• Н, b, h – width/height of the beam section.

Strength is calculated in Btb and is indicated by a number in the range of 0.4-8.

Axial tension is rarely taken into account in the design process. This parameter is important for determining the ability of a monolith not to develop cracks during noticeable changes in air humidity and temperature. Tensile is some component taken from the flexural strength. It is difficult to determine; beam samples are often stretched using special equipment. This is relevant for concrete that is used in areas that exclude the possibility of cracks.

Transfer strength is the standardized value of the strength of a concrete monolith of stressed elements when the tensile force of reinforcing elements is transferred to it. This indicator is provided for by regulatory documents and specifications for different types of products. Usually a minimum of 70% of the design grade is assigned, much depends on the properties of the reinforcement.

Strength of concrete on 7 and 28 days: GOST, table

There are different types of concrete. As a rule, all types by grade and class are divided into light, ordinary and heavy (often the last two groups are combined, since all ordinary concrete is considered heavy).

Main groups of concrete by strength:

1. Lungs

   – grades from M5 to M35 are suitable for pouring non-load-bearing structures, from M50 to M75 are used for preparatory work before pouring, M100 and M150 are relevant for lintels, structures, and low-rise construction.

2. Ordinary concrete

   – the most common and often used in repair and construction work: M200/M300 are used to make foundations, blind areas, floors, screeds, curbs, supports, stairs, etc. M250 B20 demonstrates a strength of 262 kgf/m2 and a pressure of 20 MPa. M350 and M400 are used for monolithic, load-bearing structures of multi-storey buildings, swimming pool bowls.

3. M450 and above

   – heavy concretes with high strength and density are used for special structures and various types of military facilities.

Table in MPa

The strength of concrete is the most important indicator, which directly affects all other technical characteristics of the material, scope of application, and ability to withstand expected loads. Therefore, in the process of choosing a brand and class, it is worth taking into account SNiP and GOST, and when checking the material for compliance, pay attention to the research results and relevant documents.

Classes and grades of concrete.

Depending on the purpose of reinforced concrete structures and operating conditions, concrete quality indicators are established, the main of which are:

• axial compressive strength class B; indicated in projects in all cases as the main characteristic;

for heavy concrete, the Standards establish the following series of classes: B7.5, B10, B12.5, B15, B20, B25, B30, B35, B40, B45, B50, B55, B60.

for fine-grained ones, depending on the group in the range from B7.5 to B60.

for lightweight concrete depending on the average density B3.5 - B40.

• axial tensile strength class Bt, assigned in cases where this characteristic is of primary importance and is controlled in production; Bt0.8; Bt1.2; Вt1.6; Вt2; Bt2.4; Вt2.8; Bt3.2;
• frost resistance grade F; prescribed for structures exposed to alternating freezing and thawing when wet; Characterizes the number of cycles of alternating freezing and thawing that concrete can withstand in a water-saturated state, provided that the reduction in strength is no more than 15%. For heavy and fine-grained concrete - F50, F75, F100, F150, F200, F300, F400, F500. For lightweight concrete - F25 - F500. For cellular - F15 - F100.
• waterproof grade W; prescribed for structures that are subject to limited permeability requirements (tanks, etc.); W2, W4, W6, W8, W10, W12. It characterizes the maximum water pressure (kg/cm2), at which it does not leak through the test sample within the requirements of the Standards.
• medium density grade D; are prescribed for structures that, in addition to strength requirements, are subject to thermal insulation requirements, and are monitored in production. Heavy concrete from D2200 to D2500; lightweight concrete from D800 to D2000; porous concrete from D800 to D1400.

The specified class and grade of concrete are obtained by appropriate selection of the composition of the concrete mixture, followed by testing of control samples.

The class of concrete in terms of axial compressive strength B (MPa) is the temporary compressive strength of concrete cubes with an edge size of 150 mm, tested in accordance with the standard at the age of 28 days when stored under standard conditions (at a temperature of 202С and a humidity of at least 60 % ) and accepted with a probability of 0.95.

Strength is the main property of concrete

The most important property of concrete is strength. Concrete resists compression best.
Therefore, structures are designed in such a way that concrete can withstand compressive loads. And only some designs take tensile or flexural strength into account. Compressive strength. The compressive strength of concrete is characterized by class or grade (which is determined at the age of 28 days). Depending on the time of loading of structures, the strength of concrete can be determined at another age, for example 3; 7; 60; 90; 180 days.

In order to save cement, the obtained tensile strength values ​​should not exceed the tensile strength corresponding to the class or grade by more than 15%.

The class represents the guaranteed strength of concrete in MPa with a probability of 0.95 and has the following values: Bb1; Bb1.5; Bb2; Bb2.5; Bb3.5; Bb5; Bb7.5; Bb10; Bb12.5; Bb15; Bb20; Bb25; Bb30; Bb35; Bb40; Bb50; Bb55; Bb60. The grade is the standardized value of the average strength of concrete in kgf/cm2 (MPah10).

Heavy concrete has the following compression grades: Mb50; Mb75; Mb100; Mb150; Mb200; Mb250; Mb300; Mb350; Mb400; Mb450; Мb500; Mb600; Mb700; Mb800.

There are dependencies between the class of concrete and its average strength with a coefficient of variation of concrete strength n = 0.135 and a safety factor t = 0.95:

Bb = Rbx0.778, or Rb = Bb/ 0.778.

2.2. Reinforcement for reinforced concrete structures Purpose and types of reinforcement.

As was shown in lecture No. 1, reinforcement in reinforced concrete structures is installed primarily to absorb tensile forces. The required amount of reinforcement is determined by calculating structural elements for loads and impacts.

Fittings installed according to calculation are called working; those installed for structural and technological reasons are called installation fittings. Mounting reinforcement ensures the design position of the working reinforcement in the structure and a more uniform distribution of forces between the individual rods of the working reinforcement. In addition, mounting reinforcement can absorb forces that are usually not taken into account by calculations due to concrete shrinkage, temperature changes, etc.

Working and installation reinforcement is combined into reinforcement products - welded and knitted meshes and frames, which are placed in reinforced concrete structures in accordance with the nature of their work under load.

The fittings are classified according to 4 criteria:

1. depending on the manufacturing technology - rod and wire. By rod we mean reinforcement of any diameter within 6 40mm, regardless of how it is supplied by industry - in rods (D>12mm, length up to 13m) or in coils (weighing up to 1300kg).
2. depending on the method of subsequent hardening, hot-rolled reinforcement can be thermally strengthened, or hardened in a cold state - by drawing, drawing.
3. The shape of the surface can be periodic or smooth. Protrusions in the form of ribs on the surface of bar reinforcement of a periodic profile, reefs or dents on the surface of wire reinforcement significantly improve adhesion to concrete.
4. according to the method of application - prestressed and non-prestressed reinforcement.

Methods and testing of concrete for strength

To determine the grade and class of concrete, a variety of methods are used - all of them fall into the categories of destructive and non-destructive. The first group involves conducting tests in a laboratory through mechanical action on samples that were filled with a control mixture and completely aged within the specified time frame.

To conduct research, a special press is used, which compresses prototypes and demonstrates compressive strength. Destruction is the most reliable and accurate method of studying concrete for strength of types such as compression, bending, tension, etc.

Basic non-destructive research methods:

• Impact impact.
• Partial destruction.
• Ultrasound examination.

The impact impact can be different - the most primitive is considered to be the impact impulse, which records the dynamic impact in energy equivalent. Elastic rebound determines the hardness parameters of the monolith at the moment of rebound of the drum striker.

The method of plastic deformation is also used, which involves treating the area under study with special equipment that leaves imprints of a certain depth on the monolith (the degree of strength is determined from them).

Partial destruction can also be different - chipping, tearing and a combination of these methods. If the cleavage method is used for testing, the edge of the product is subjected to a special sliding action to break off the part and determine the strength. Tear-off involves the use of a special adhesive that is used to attach a metal disk to the surface and then tear it off. When combining these methods, the anchor device is attached to the monolith and then torn off.

Advantages of Concrete Classification

The designation of the degree of quality of concrete by classes and grades exists and functions closely with each other. Both classifications are based on the same parameter - the strength of concrete.

To mix different types of concrete, there is a specific calculation for all the components of the finished solution. Compliance with proportions cannot guarantee exact compliance with the declared stability characteristics. This characteristic also depends on the quality of the ingredients used: sand, filler, additives and water. An important point that must be taken into account are the conditions for pouring the cement mortar and the quality of its setting.

The composition of the same brand can vary significantly in strength, so the brand contains information about the average value. In order to more accurately determine this parameter, divisions into concrete classes were developed. This classification allows you to obtain the value of the guaranteed strength of the material.

During construction calculations, the class will provide more reliable information, therefore this parameter is indicated in regulatory documents. When making a purchase at a hardware store, concrete is classified by grade.

Correlation of classes with brands

Each class corresponds to a specific brand. The correspondence table allows you to easily translate one name into another.

Compliance with classes marked for frost resistance, moisture resistance

Determining frost resistance when choosing the type of concrete can play a fundamental role. Stability to sudden temperature changes is considered an important condition for product quality. This factor is especially important in northern climates.

The frost resistance range represents a scale from F50 to F1000. The number in the marking means the maximum number of freezing and thawing cycles that the material can allow without changing its structure and quality.

Moisture resistance is another important property characterizing the cement-sand composition. Markings are designated from W2 to W20. The number in the name of the species indicates the maximum permissible water pressure. This indicator is directly proportional to the cost of the material.

The summary table allows you to determine the compliance of the concrete class and grades for frost resistance and water resistance. The higher the strength class, the more resistant the composition is to cold and moisture.

Scope of application

For each type of construction work, its own class of concrete mortar is used. The higher the specified value of the material, the better its performance. Let's look at the most popular types.

B30

Concrete has a high density, so its use is advisable in those structures that carry a large load. The finished composition is used for the construction of bridges, underground and hydraulic structures, storage facilities in banks and other elements that have special requirements for strength and quality.

B25 and B27.5

Class B25 is a cement-sand composition with high physical and technical characteristics. It is widely used for constructing piles, monolithic walls and foundations, floors, various columns and beams. This type of concrete is used to fill the base for swimming pool bowls that bear heavy loads. For the same reason, reinforced concrete rings are made from class B27.5. These structures are often chosen for the construction of wells or sewers that are under high pressure.

B22.5

Class B22.5 concrete mortar is excellent for pouring monolithic walls and ceilings, staircase structures, installing fences, house paths and platforms. You should opt for this composition if you need to prepare and compact the soil for a strip foundation.

B12.5 and B15

Classes B12.5 and B15 are used for leveling surfaces and making concrete walls, floor coverings, foundations, screeds, concreting pillars, platforms and paths. This composition is most often used for the construction and improvement of private houses.

B7.5

Class B 7.5 mortar is otherwise called “lightweight concrete”. He received his recognition in the field of carrying out work to prepare for further finishing of premises, to arrange the soil for the foundation or to improve the area next to the house. The material is often used to lay a cement-sand cushion under the road surface or to imitate natural stone.

Classification by degree of stretching

There is an additional classification of the material by strength: by stretching in the direction of the axis and by the maximum limit of stretching when bending the material. This indicator is important during construction work in difficult conditions, in which external damage to the surface is unacceptable.

Generally, concrete products are not designed to be tensile. But, nevertheless, the differentiation of classes according to this parameter is of great importance. It is necessary to take into account the degree of stretching of the material at the design stage in order to correctly assess the load on the object.

This allows you to extend the life of the concrete structure and avoid significant violations of standards. Failure to comply with the parameters creates a high risk of chips and cracks.

Axial tension

The parameter of the tensile strength of the material in the axial projection is very important when installing objects and structures, the design of which categorically does not allow the appearance of cracks or other damage. These can be swimming pools, fountains and other structures exposed to water. For the construction of dams at hydroelectric power stations, this strength index is the most objective parameter.

Concrete compositions are designated by the Latin letters Bt. They are divided into classes based on tensile strength: Bt0.8; Bt1,2; Bt1.6; Вt2; Bt2.4; Bt2.8; Bt3,2. The higher the marking index, the higher the strength characteristic.

Flexural stretch

This classification of cement-sand mortars is used when choosing a material for laying concrete road surfaces and when constructing airport landing strips. Such construction work requires a high level of tensile strength.

The designation of classes is indicated using the abbreviation Bbt. The classification has 19 levels: Bbt0.4; Bbt0.8; Bbt1,2; Bbt1.6; Bbt2.0; Btb2.4; Вbt2.8; Bbt3.2; Bbt3.6; Bbt4.0; Bbt4,4; Bbt4.8; Bbt5.2; Bbt5.6; Bbt6.0; Bbt6.4; Bbt6.8; Bbt7.2; Вbt8.

Identification of classes of concrete mortar according to various characteristics (strength, tensile stability in axial projection and bending) allows us to evaluate the product from all sides. This makes it possible to select the necessary quality material that will meet all the requirements of the scope of its application.

Strength of concrete in bending and axial tension

Bending strength is of great importance for structures subject to bending forces (beams, purlins, floor panels).
This characteristic has been studied quite well for normal-hardening concrete. We tested the effect of autoclave treatment on the flexural strength on concrete with a composition of 1:2, 34:3.75 with W/C = 0.55 at a cement consumption of 320 kg/m3. The concrete was prepared to a plastic consistency with a workability of 20 seconds. Various cements were used. Their mineralogical composition is presented in Table. 1. Part of the clinker (25, 40, 50 and 60%) was replaced by quartz sand when grinding cement. The cement was ground to a specific surface area of ​​3000 cm2/g.

Concrete samples measuring 4x4x16 cm, made with these cements, were subjected to autoclave treatment under steam pressure of 9, 13, 17 and 21 atm for 8 hours and tested one day after steaming. The results of bending tests of samples are presented in Fig. 70. The mineralogical composition of clinker does not have a significant effect on the flexural strength of autoclaved concrete. Specimens made with alitic, low- and medium-aluminate cements acquired flexural strength that was only 11–13% greater than specimens made with medium-alite, high-aluminate cement.

The addition of sand in an amount of 25% increases the tensile strength of concrete in bending using various types of cement. With the addition of 40% sand, the strength of samples using alite cements is equivalent to the strength of concrete obtained using pure cements. With large additions of sand, the flexural strength decreases more intensely in concrete based on belite cements.

According to Reinsdorf, when ground sand is added to Portland cement, the ratio between the flexural strength and compressive strength of autoclaved concrete increases from approximately 1:7.5 to 1:10.2.

A significant factor influencing the tensile strength of autoclaved concrete during bending is the steam pressure during steaming. From Fig. 70 it can be seen that with an increase in steam pressure from 9 to 13 atm, the bending strength increases slightly, and with a further increase in steam pressure it decreases significantly.

Experiments on steaming concrete at 21 atm for various times (Fig. 71) showed that an intensive increase in strength is observed in the first hours of steaming. The maximum value of bending strength is achieved when steaming for 4-6 hours, but its absolute value is 10-20% lower than that obtained when steaming for 8 hours at 9 at. Increasing the steaming time at 21 atm beyond 6 hours reduces the bending strength of concrete. It should be noted that these results are valid only for these experimental conditions. With changes in cement grinding fineness and concrete composition, the optimal steaming time at 21 atm may change.

In Fig. 72 shows a curve of the dependence of the flexural strength on the compressive strength for concrete steamed at different steam pressures; For comparison, data are given for concrete that hardened for 28 days under normal conditions. At the same value of compressive strength, autoclave-cured concrete has lower flexural strength than normally cured concrete. Increasing the steam pressure during autoclaving beyond 13 atm further reduces the flexural strength and is therefore not recommended. Lower value of concrete flexural strength at the same

compressive strength indicates the increased fragility of autoclaved concrete, which increases as the steaming temperature increases.

At NIIZhB Cand. tech. Sciences V.S. Bulgakov and engineer. L.P. Girenko investigated the physical and mechanical properties of high-strength concrete of normal and autoclave hardening. Steaming of concrete samples, data on which are given in table. 28, was carried out 30 hours after manufacture according to the 3+8+3 h regime at 9 at. The samples were tested 14 days after steaming.

The tensile strength in bending was determined in accordance with GOST 10180-62 on beams measuring 15x15x55 cm. To measure deformations on the bottom and side faces of the sample, strain gauges were glued before testing (in the zone of maximum moments). The load was given by two loads in steps equal to 0.1 destructive. The tensile strength of concrete in bending was calculated using the formula In accordance with GOST 10180-62, the coefficient K in this formula for beams measuring 15x15x55 cm was taken equal to 1. The destruction of the beams occurred in the zone of maximum moments.

As compressive strength increases, flexural tensile strength also increases. At the same time, the ratio Rр*i/Rсж for autoclave-hardening concrete is only slightly lower than for normal-hardening concrete.

The axial tensile strength of concrete was determined by testing prismatic specimens with broadenings at the ends.

The working part of the sample was 10x10x40 cm in size. To prevent destruction of the samples in the heads, reinforcement cages were placed in them. The sample had a smooth transition from broadening to the working section. The heads had holes formed by tubes laid during the manufacture of the sample. During testing, the pin of the gripping device was inserted into these holes.

Before testing, strain gauges were glued to the side surfaces of the sample to determine tensile strains. The sample was strengthened in the press using grips. The load on the sample was also applied in steps equal to 0.1 destructive. The results of testing samples under axial tension are given in table. 29.

As can be seen from the table, the axial tensile strength increases slightly with increasing concrete grade. At the same time, the ratio of axial tensile strength to compressive strength of high-grade concrete is almost the same.

Taking into account the fact that during autoclave treatment the compressive strength of concrete is higher than that of normal-hardening concrete, the results of testing the flexural strength of concrete can be considered satisfactory. Limitation of the scope of application of autoclaved concrete and reinforced concrete products is possible by other indicators, and not by the limit of concrete resistance to bending or axial tension.

Prof. G.D. Tsiskreli, who studied the influence of the moisture conditions of concrete on its physical and mechanical properties, found that moisture increases over time the tensile strength of heavy concrete made from mobile mixtures.