Monolithic reinforced concrete strip is most often used by individual developers for the construction of one-story and two-story buildings. In this regard, the question often arises - what brand of concrete for strip foundation will a private house be optimal? After all, the durability of the base depends on the main operational parameters of the solution used. In the future, competent selection of cement-crushed stone mixture will save the homeowner from carrying out unscheduled repair work, requiring large financial investments and strength.

Basic performance characteristics of concrete

Before ordering mixers with concrete mixture for pouring a monolithic foundation strip, you need to decide on its basic parameters:

• Brand;
• Compressive strength;
• Frost resistance;
• Moisture resistance.

It is their choice that determines how strong the base will be and how long the structure will last, regardless of operating conditions.

Before the adoption of SNiP 20301-84 in 1986, when designing not only strip foundations, but also other structures made of reinforced concrete, the grade of concrete was basic parameter. This term reflects the average compressive strength of an individual concrete sample. It is expressed by the value kgf/cm 2 - kilogram of force exerted per unit area. This is exactly what is followed today in private housing construction.

Currently, with the adoption of building codes and regulations, in all calculations of reinforced concrete structures, including strip foundations, it is customary to use not the grade of ready-mixed concrete, but its compressive and tensile strength class. This value reflects the guaranteed strength of the material when pressure or force is applied to it in MPa. And here it is not the average value that is taken, but the exact value with the minimum permissible error.

The grade and class of concrete are designated by the letters M and B, respectively, with numerical values. In the first case, the digital expression for indicates the average strength value in kgf/cm 2. In the second case, the value of the guaranteed strength in MPa with a small error (13.5%).

The main range includes values ​​of classes B7.5 - B40 and grades M100 - M500.

The lowest tensile strength for concrete is 100 kg/cm2. In private construction, it is not practical to use cement mixtures used for strip foundations with a value above 400 kg/cm2: higher grades are expensive, which makes their use in this area an unjustified waste.

Frost resistance of branded concrete

Frost resistance (F) is an indicator that reflects the number of freezing-thawing cycles that can withstand monolithic foundation tape type. Typically, concrete deteriorates mainly due to the expansion of water freezing in its pores. The more there are, the lower the F index. Such structures quickly wear out, losing their load-bearing capacity.

Characteristics of concrete for frost resistance
Brand	Characteristic
Less than F50 – low	Structures made of such concrete quickly crack under the influence of temperature changes. Compositions of this brand are used extremely rarely.
F50-F150 - moderate	The most common group. These brands are suitable for many years of use. This level of frost resistance is typical for mixtures with average compressive strength.
F150-F300 - increased	Structures built from concrete with increased frost resistance are capable of working for decades in harsh conditions characterized by significant temperature changes.
F300-F500 - high	This level of frost resistance is required in special cases. One example is the variable water level in the operation area of ​​the facility. This material is expensive and it is not practical to use it in private construction.
F-500 – extra high	Used in exceptional cases. Mixtures of the highest grades have such frost resistance. Structures made from them can last for centuries.

The numbers indicate the number of freeze-thaw cycles that monoliths can withstand.

Moisture resistance of concrete grades

Moisture resistance or hydrophobicity of concrete is its ability to resist the penetration of moisture under a certain pressure. This parameter is marked by a combination of the letter W and an even number - from 2 to 20. The numbers indicate the maximum pressure (MPa x 10-1) at which a test sample with a diameter of 150 mm and the same height will not allow moisture to pass through.

Correspondence of concrete grades with moisture resistance class
Waterproof class	Characteristic	Concrete grade
W2	The material is prone to absorption large quantity water. Not to be used without waterproofing.	M100-M200
W4	Absorbs a moderate amount of water; it is not recommended to use without waterproofing measures.	M250-M300
W6	Has reduced water permeability. Widely used in construction.	M350
W8	After absorbing moisture in an amount of 4.2% by weight of the material, the permeability of the brand begins to decrease.	M400
W10-W20	Resistant to moisture, used in construction hydraulic structures, water storage tanks, etc. Does not require waterproofing. High cost (4.5-5.3 thousand rubles/cubic m) is the main limiter for use in private construction	Over M400

The ideal option for pouring a strip foundation is M300 concrete. It has sufficient strength, frost resistance falls into the “increased” range and moderately absorbs moisture.

Scope of application of concrete of various grades

To understand what grade of concrete is needed for a strip foundation, let’s turn to the requirements of building codes and manufacturers’ recommendations:

• M100 is used in road construction (when installing curbs, for example). When constructing residential buildings, this brand is only suitable for pouring foundation pads;
• M150 is used for concreting various platforms, screeds, creating terraces, etc. Concrete grades M100 and M150 are generally not used for strip foundations. The exception is light structures - fences, light gazebos, etc.;
• Brand M200 is suitable for making stairs and pouring blind areas. This composition often used for the construction of supporting walls when it is necessary to terrace a site;
• M 300 can be used for pouring foundations of any type. Stairs, partitions, monolithic walls, ceilings - these are the main areas of use of this brand.

Grades of higher strength - M350, M400, etc. are used for production reinforced concrete products– hollow core or floor slabs of column-transom reinforced concrete frames of buildings, for the construction of bridges, bank vaults. In individual construction, these mixtures are practically not used.

The M200 concrete grade is also of little use for strip foundations. However, when constructing light one-story buildings country houses its use is quite acceptable - on sandstones with low levels groundwater.

Criteria for choosing the grade of concrete for pouring the foundation strip

To determine what brand of concrete is needed for a strip foundation, you need to analyze a number of the most important factors. These include the weight of the building, soil characteristics on construction site and the location of groundwater relative to the base of the house.

Building weight

The mass of everyone building materials involved in the construction of the house creates a load on foundation tape. It is this parameter that is decisive when choosing a concrete grade. For guidance, private builders can use the following rules:

• under a lightweight prefabricated panel house, you can fill the tape with M200 concrete mixture;
• log houses 2-3 floors high, buildings made of cellular concrete can be erected on concrete grades M200-M300;
• brick or monolithic structures require the use of solutions of a grade not lower than M300.

The above recommendations are approximate. In fact, the weight of the above-ground structure is not the only factor taken into account when selecting the grade of concrete. It is necessary to take into account wind and snow loads, the magnitude of which depends on the location of the building site.

Data on wind and snow loads can be found in SNiP or construction reference books. A surprise awaits an inexperienced builder: these data vary not only depending on the region, but also the design of the roof, the magnitude of its slope, etc.

Soil features

Without geological research It is impossible to make a final decision on the choice of concrete grade. The strength requirements for the strip foundation of a building depend on the type of soil:

• on sandstones and other stable soils (rocky, for example), M200 solutions work quite well (if the total load allows);
• on clays and loams susceptible to frost heaving, a grade an order of magnitude higher is required - M250-M300.

Since clay soils are very common, M300 grade mixtures are the most used in private construction.

Features of the location of groundwater

The location of groundwater makes significant adjustments to previous recommendations for choosing a concrete grade. If they lie below the depth of soil freezing, all of the above is quite true. If their level is high, the brand of working solution should be taken an order of magnitude higher.

What brand of concrete do experienced builders use for pouring strip foundations on water-saturated soils? In this matter, they are guided by the coefficient of water permeability of the material. At high level underground water layers best choice there will be a brand M350.

Proportions of different grades of concrete for pouring a strip foundation

In the formulations of concrete solutions, a portion of cement is taken as a unit, other components (sand and crushed stone) are taken in relation to it.

For ease of use, this table uses a volume of cement equal to 10 liters as a basis.

The correctly selected brand of concrete is a guarantee of the construction of a strip foundation of adequate strength. But it is equally important to choose the best option, otherwise you may incur unnecessary costs. The best thing is to order foundation calculations from experienced designers.

The service life of any building is largely determined by the strength of the supporting structure. In private buildings, strip, slab, column and pile foundations. The first two types are especially in demand during the construction of a residential area. Therefore, the question of what grade of concrete is needed for a strip foundation always sounds relevant. After all, the value of the indicator directly affects the quality of the basic support of a private house.

Concrete grade

Concrete is classified according to classes and grades. Strength class is the main parameter influencing the choice in various fields applications. It is denoted by the Latin letter B and expressed in MPa. The indicated figure indicates the maximum permissible compression pressure. That is, class B30 concrete can withstand loads of up to 30 MPa. When constructing a strip foundation, the safety margin is always taken into account, because private and any other construction must have guaranteed reliability.

Important! Over time, the strength of concrete changes.

The design of a strip foundation is carried out taking into account the construction period and specific load at its various stages. On average, strength gain lasts 28 days after pouring concrete, after which the process slows down significantly.

The strength of concrete is also evidenced by its grade, which ranges from 50 to 1000 units. This figure indicates the withstand compression pressure, expressed in kg/cm2. That is, concrete grade M200 is designed for a load within 200 kg/cm 2.

GOST 366633-91 contains a table showing the correspondence between the class and grade of concrete. This information must be taken into account when designing a strip foundation:

When studying the topic of what kind of concrete is needed for a strip foundation, in addition to the grade, other indicators are also important:

• Private buildings in cold climates require paying attention to the frost resistance of concrete, that is, the ability to withstand cycles of freezing and thawing. The parameter is designated by the letter F, the number indicates the number of cycles after which the strip foundation will not lose its performance characteristics declared during the design. The climate of the middle zone allows the use of concrete with frost resistance grade F200.
• Groundwater located close to the surface also has a significant impact on the strength of concrete in a strip foundation. Therefore, it is important to consider its resistance to water pressure, which varies between 2-20 units. The indicator is marked with the letter W.

Factors determining the choice of brand

The grade of concrete for the foundation of a private house is selected taking into account several factors that influence the supporting structure. The basic principle is proportionality: the stronger the pressure exerted on the tape, the higher the grade of concrete used to prepare the solution.

So, when choosing a brand you should consider:

• The massiveness of the future private building. Cottages made of timber or gas silicate blocks have significantly less weight than brick buildings. Therefore, the grade of material for the foundation should not be the maximum value.
• Soil properties. This or that type of soil under a private house exerts different pressure on the strip base. If there is hard rocky soil or sandstone, it is enough to take M150 or M 250; loam will require greater strength, so a suitable grade reaches the M350 level.
• In addition to strip foundations, a combined version with piles is also used in private construction. It is especially relevant on soils prone to heaving. The design of the supporting base is taken into account when determining what kind of concrete is needed for it.
• UGV. The water permeability of the foundation is obvious. Information about seasonal fluctuations in groundwater will help you choose the right brand.

Advice! Waterproofing, which is used for the base and side surfaces of the strip foundation, is designed to protect the foundation of a private house from premature destruction. It comes in different forms - rolls, pasting or coating materials.

Composition of the solution

Other components of the solution also influence the question of what brand of concrete to use for a strip foundation. Cement used in frequent construction has several varieties depending on the presence of additives or without them:

• normalized;
• fast-hardening;
• with a high degree of frost resistance;
• having low moisture absorption;
• with protection from aggressive environments.

The strength of the strip foundation is ensured by the presence of reinforcement. In addition to common metal rods, there are also polymer rods that are not subject to corrosion processes. Reliability reinforcement cage depends on several factors:

• type of rods;
• brand;
• section;
• quality of material (no rust);
• installation method (welding or knitting);
• material for knitting (polymer or iron).

Comment! The reinforcement structure must be located in the thickness of the strip foundation, not in contact with the walls and bottom, but at the same time exceed the boundaries of the strip belt at the top.

In addition to the required grade of concrete, a high-quality solution is ensured by the correct proportion of other components, which are selected depending on the characteristics of the soil and the planned load on the strip foundation. Possible components:

• sand;
• crushed stone or gravel;
• lime or clay;
• special additives that increase one or another quality of concrete mortar.

Increased soil moisture forces an increase in the proportion of cement. Information from reference books and manufacturers' recommendations will help you calculate the required proportion. The quality of concrete mortar is also influenced by the fractions of accompanying materials (crushed stone, sand or gravel). Cement that is not sold in a timely manner loses strength, as a result, the M500 grade gradually corresponds to the indicators characteristic of the M400 grade, and then is converted into the M300 grade. Therefore, the concrete for a strip foundation must be “fresh”.

When calculating the need for components for pouring a strip foundation, one feature should be taken into account: often the cost of the material is indicated for 1 ton, and a private developer requires the amount of volume in cubic meters. And here we must remember the regularity: there is more material with a fine fraction of 1 m 3 in mass than larger fractions.

Common grades of cement may have the following proportions:

Attention! A high-quality solution is obtained when the grade of Portland cement is twice as high as the grade of concrete for the strip foundation. That is, when forming a supporting structure in private construction, durable concrete grade M200 is sufficient; cement grade M400 is used for it.

More durable grades of concrete, starting from M300, require smaller ratios, so the proportion to the cement grade can be reduced to 1.5. For concrete grade M400, cement grade M600 will be sufficient.

Areas of use

The grade of concrete for the foundation is determined taking into account the planned load, implement right choice Manufacturers' recommendations will help:

• The M100 grade is optimal for installing curbs along the road; in private construction, concrete of similar strength can only be useful for preparing a cushion under a strip foundation.
• The strength of the M150 grade is not much higher than M100, therefore, in addition to the design of the pillow, in home construction it is used for concreting platforms and forming screeds. If you plan to build a lightweight one-story building, a strip foundation filled with grade 150 concrete will fully withstand the load. Although in this situation it is more practical to turn to a columnar support structure.
• The scope of use of the M200 brand is the formation of stairs, tape and slab foundations. The strength of concrete allows you to make blind areas and support structures from it when terracing the site.
• The more durable grade M300 is suitable for constructing foundations of all types, monolithic walls, ceilings and partitions, stairs and supports for fences.
• The large load-bearing load of the structure forces us to turn to the M 350 brand. The material can withstand even the pressure of an aircraft, so airfield slabs are made using M350.
• The scope of application of M400 concrete bypasses private construction. These are mainly bridges, bank vaults, supporting frames in seismic zones.

Comment! The shrinkage coefficient increases the quality of concrete: the lower the indicator, the higher the strength. The required parameters are achieved by introducing special additives.

Nuances of foundation arrangement

Concrete for the foundation plays a key role in the formation of a durable structure. M200 grade material will help achieve the goal, but there are a number of nuances that should be taken into account during private construction:

• The proximity of groundwater obliges us to turn to the M300 brand. This is explained by the ability of high-grade material not to lose strength during prolonged contact with moisture.
• Reinforcement of a strip foundation is an essential condition for creating a solid foundation. Corrugated rods with a cross-section exceeding 12 mm are optimally suited for the frame.
• The bundle of pieces of metal reinforcement is carried out staggered.
• The pouring of the strip foundation is organized in one step. This requirement is especially relevant when using low grade concrete.
• Slow down the evaporation of moisture in hot weather, thereby preserving strip base declared strength, covering newly poured concrete with burlap or sawdust will help. It is also recommended to regularly water the surface.

Private construction requires a balanced approach at every stage. An informed choice of concrete grade will help create a reliable strip foundation that will preserve the integrity of the entire structure for a long time.

A professionally completed private house project, among many others, contains list of materials required for the construction of the foundation. For the construction of most known types of foundation, concrete is required, so the documentation must indicate its main characteristics: brand, class, plasticity, frost resistance, water resistance, as well as the optimal ratio of mortar components (sand, crushed stone, cement and water).

To determine what brand of concrete is needed for the foundation of a house, a developer conducting construction on his own must you need to navigate the characteristics of its brands and places of application.

The main requirements for the foundation are durability, strength and safety of the building. And the main functions of the foundation are the uniform distribution and transfer to the ground of loads created by the weight of the house.

Basic characteristics of concrete

In order to obtain the characteristics required from the foundation and not spend too much on building a super-powerful foundation where it is not necessary, we will analyze the meaning of concrete markings.

M - grade concrete A. Shows what average compression load 1 square meter can withstand. cm of completely hardened concrete. For example, the designation M100 indicates the ability to withstand 98 kgf/cm2, M250 - 245 kgf/cm2.

B – concrete class. This is a modern and more accurate characteristic of permissible loads. Note that our builders still use the brand, not the class, of concrete. For example: the M100 brand corresponds to B7.5, and M250 corresponds to B20 class.

The higher the indicator near the M and B indices, the stronger the hardened concrete.

F – frost resistance. It is characterized by numbers from 25 to 1000 and indicates the number of freezing/thawing cycles that a foundation in moist soil can withstand without destruction. In conditions low level groundwater and shallow freezing depth in low-rise construction F100 - F300 is enough. The highest indicator may indicate that there is no need for waterproofing work for finished foundation. However, this is a controversial issue in terms of cost and climatic features of the territory.

W – waterproof. The indicator next to the index - from 2 to 20 - indicates how much hydrophobic additives it contains and how the concrete allows water to pass through. The higher the indicator, the more stable the foundation will be in soil with high humidity, and the smaller the scale of waterproofing work can be.

P – mobility. This is an indicator of the fluidity of concrete or the ease of its installation. The coefficients near the index can be from 1 to 5, where 1 is thick concrete, and 5 is liquid. The grade of concrete for the foundation of a private house must have a mobility index of P2 or P3. More liquid concrete is prepared if the formwork under the foundation is reinforced with a dense row of rods, or if a special concrete pump is used for pouring. The mobility of concrete changes not by increasing the amount of added water, but by changing the proportions of the components of the solution.

To obtain concrete with specific characteristics (the desired grade), cement of a certain grade (higher than the grade of the future concrete solution) is used in a certain proportion with the other components (crushed stone, sand and water).

Choosing concrete correctly

Criteria by which concrete is selected for foundation work:

soil characteristics, groundwater level and soil freezing depth;

weight and number of storeys of the building;

selected foundation type.

Load

The weight of a frame building (even with external cladding) is several times less than brick or panel. Accordingly, heavy houses require a solid foundation and reinforced concrete of increased strength. For light buildings with a shallow foundation, you can choose brands from M200 to M350, and for heavy ones - M400 or M500.

The soil

For the construction of country houses, areas are often allocated that are unsuitable for agricultural work due to their characteristics. It can be rocks, sandstones, clay or sandy loam. The latter are characterized by high mobility and heaving. Water in heaving soils in winter it freezes, bursting the soil and pushing out the foundation. Regardless of the chosen brand of concrete, the foundation on such soils should be located below the freezing depth.

In the specific features of the site, it is important to take into account the level of groundwater. It affects the water resistance coefficient of concrete. Seasonal flooding of rivers or their drying out, leading to a rise or fall in groundwater levels, is also important. Special additives to concrete increase the water resistance of concrete, and additional waterproofing treatment of the finished base enhances this effect.

Weight and number of floors

How are foundations for wood and foundations different? brick house, so the brand of concrete for the foundation one-story house differs from the brand for a foundation for two or more floors. Differences in the magnitude of permanent loads require different load-bearing strengths of concrete. For one-story wooden houses(without heavy external cladding) concrete grade is sufficient M150. Brick building with monolithic ceiling requires a grade of at least M250. Brand of concrete for the foundation two-story house made of brick - this is concrete grade M300 and higher.

As you know, the durability and strength of any structure depends on the quality of the foundation on which it is based. In private construction, as a rule, 3 types of structures are used: columnar or pile, slab and the most common strip, which we will talk about. The brand of concrete for the strip foundation of a private house is the most important indicator its quality.

What is a strip foundation?

The strip type of foundation structures is considered the most common.

It is a reinforced concrete strip buried in the ground and passing in a closed loop under all load-bearing walls and piers of the building.

• The instructions for arranging this type of structure are not very difficult.. A trench is dug, a reinforcing iron frame is laid in it, and the whole thing is filled with liquid concrete.
• But, despite the apparent simplicity, perhaps the most difficult point here is the correct determination of which brand of concrete to use for a strip foundation. This choice is influenced by a number of components.

What factors influence the choice of composition brand?

Unfortunately, there are no universal recipes; price is not always a fundamental factor. After all, even expensive formulations have their own recommendations and restrictions on use.

Dependence on the weight of the building

If you want to build a house with your own hands, but you don’t have finished project with the prescribed recommendations, then, first of all, you should pay attention to the total weight of the building. This is the first indicator from which to start when selecting a brand of composition.

• A prefabricated panel house with a height of 2 floors, in most of our country it will comfortably stand on reinforced concrete foundation, filled with M200 brand.
• For 2 - 3 storey log buildings, as well as for houses built from foam concrete, gas silicate blocks or other cellular concrete, grades M200 - M300 are recommended.
• For heavy permanent buildings built of brick, solid cast concrete structures or other heavy materials, it is recommended to use a grade of M300 and higher.

Soil characteristics

Much depends on the geology of the soil in a given area, its composition and how deep the groundwater lies.

• Sandstones and rocks are considered the most favorable. They can be poured with concrete M200 - M250.

• Clay soils and loams pose quite a big problem. The fact is that these soils are susceptible to heaving when they freeze. That is, they do not freeze evenly and are capable of squeezing out the shallowly buried strip foundation in some places, which leads to general deformation.
  In this case, you can do it in two ways.

• First, deepen the structure to a level exceeding average general soil freezing in a given region.
• Secondly, you can arrange a strip-column structure, good reinforced foundation. This is when at key points they are poured to a significant depth, which stabilizes the entire structure.
• If we talk about what brand of concrete is needed for a strip foundation laid on clay, then experts recommend taking a grade that is an order of magnitude higher, approximately M250 - M300.

Important: it should also be taken into account that starting from grade M200, if necessary, cutting of reinforced concrete with diamond wheels is already used, and for deepening, diamond drilling of holes in concrete is used.

Common grades of concrete

When producing different grades of concrete for foundations, M 400 cement is most often used, but in different proportions.

• M100 - used as an underlayment when building roads, laying deep foundations or pouring floor screeds.
• M200 – suitable for shallow foundations based on stable soils. It is used to fill basements and various open areas.
• M250 – used for casting fences, various kinds of stairs and strip-column structures.
• M300 is considered the most popular brand for pouring foundations. It can be successfully poured into the vast majority of known foundation structures and can be used in damp environments.
• M400 - used for pouring underwater structures, bridges and other permanent structures. Used for building houses in extreme conditions.
• M500 - used for pouring vertical structures, repairing roads and structures, or as an additive.

Important: the quality and strength of concrete is greatly influenced by the so-called shrinkage coefficient - the lower it is, the stronger the structure will be.
It can be reduced using special additives.
When using vibrating machines, harder and drier compounds can be poured.

What else is worth paying attention to

• There is also a marking characterizing the strength class of the hardened composition. It is designated by the letter “B”, characterizes the compressive strength margin and is measured in megapascals.

Important: the strength of concrete can change over time.
Active strength gain occurs within 28 days from the moment of pouring.
Further, these processes slow down significantly, but do not stop.

• The strength level and grade of concrete are related indicators.
  According to GOST, each brand of concrete corresponds to its own, here are some basic indicators:

• B7.5 corresponds to M100
• B15 corresponds to M200
• B20 corresponds to M250
• B22.5 corresponds to M300
• B25, B27.5 correspond to M350
• B30 corresponds to M400
• B35 corresponds to M450

• For our country, the frost resistance indicator is also relevant; it is marked with the letter “F” and a certain number after it. The number indicates how many times the structure can freeze. For a larger area of ​​the country, F200 is sufficient; the more thaws in winter, the higher this figure.
  
• It is also worth paying attention to the indicator of resistance to high humidity. It is indicated by the letter “W” and the number after it.

Conclusion

It is worth considering the fact that the higher the class, quality and of course the price of concrete, the faster it will set. Experts advise pouring high-quality compounds within 1 day. Accordingly, quickly pouring such a foundation with your own hands will be problematic, so here it is better to resort to the services construction companies. You can find more information in the video posted in this article.

To build a reliable foundation for a private house, you need to pay attention to Special attention for material for manufacturing. It’s easy to get lost in the types of concrete, but if you prepare carefully, determining what kind of concrete is needed to pour the foundation will not be a problem.

The grade of concrete provides information about its main characteristics when designing and purchasing, it is necessary to find out the following characteristics:

• class or grade (outdated designation) of concrete in terms of compressive strength;
• frost resistance grade;
• moisture resistance grade;
• mobility grade.

All these characteristics are indicated by the manufacturer of the concrete mixture. Each of these points must be taken seriously, since they together and individually affect the durability and reliability of the structure. Next in order.

Selection by material strength

For the non-professional, there is a lot of confusion due to the presence of two ways to classify concrete by strength. Because of this, it is difficult to determine what kind of solution is needed to build the foundation of a private house. Here the case is similar to steel reinforcement. Builders of the “old school” use one designation, and young specialists educational institutions They teach something else. This is where misunderstanding arises. To determine which mixture is best to use for the foundation, you need to understand the concepts of class and brand.

The grade of concrete is designated by the letter M. The number after the marking is the average compressive strength of the finished structure in kgf/cm². Strength tests are carried out on concrete cubes with an edge of 15 cm. Experiments can be carried out 28 days after manufacturing the sample, provided that its hardening occurred at an average daily temperature of 20ᵒC. The cubes are pressed under pressure and determine the average strength. It is necessary to know the brand of concrete in order to order it at the factory, since outdated designations are still often used there.

The concrete class is designated by the letter B. After the marking, the value of the guaranteed strength in MPa is indicated. The guarantee is 95%. This means that if you take 100 samples, 95 or more of them will withstand the load specified on the brand. It is the class of concrete that is given as a characteristic of compressive strength in new (updated) regulatory documents.

A comparison of various designations is given in GOST “Heavy and fine-grained concrete. Specifications» table 6, appendix 1.

Strength class	Nearest brand in terms of strength	Deviation of strength characteristics for a brand compared to a class
B 7.5	M 100	+1,8%
AT 10	M 150	+14,5%
At 12.5	M 150	-8,4%
At 15	M 200	+1,8%
IN 20	M 250	-4,5%
At 22.5	M 300	+1,8%
At 25	M 350	+6,9%
At 27.5	M 350	-2,8%

If calculations are carried out using the markings from the first column, and the order at the factory is carried out according to the second, then a deviation with a plus sign means that concrete of greater strength than necessary is ordered (will ensure greater reliability).

The table shows only those values ​​that are best used for pouring the foundation. The use of higher classes is acceptable, but economically unjustified. Classes below B 7.5 for load-bearing structures do not use. The foundation is an element that works primarily in compression. In this case, the strength of concrete under vertical load is high, so there is no reason to use particularly strong concrete; the tension is compensated by the reinforcement.

Purpose of each brand

Classes B 7.5, B 10, B 12.5 (M 100, M 150) classified as “lean” concrete. They are used for the manufacture of concrete screeds or preparations. B 12.5 can be used for light, small utility buildings or porch foundations; lower classes are not designed for significant loads. The main role played by such mixtures during construction is leveling and preparation.

Class B 15 (M 200) has great strength. Can be used for retaining walls, concrete preparations for foundations, foundations of small buildings and structures with low mass.

Class B 20 (M 250) one of the most common for pouring support structures individual house. Can be used for making foundation slabs for wooden or frame houses, as foundations for porches and entrance groups. The soil characteristics must be quite good.

Class B 25 (M 350) ranks first in popularity. Suitable for pouring any type of foundation for a two-story house made of massive materials (brick, concrete). The class is highly durable. It is also used for constructing floors or steps of porches. Suitable for foundations on weak heaving soils.

If you plan to build a particularly important facility with high loads, the answer to the question of what kind of concrete is needed for the foundation will be At 27.5 and higher. For private housing construction, the use of these brands is not recommended due to the high load-bearing capacity margin and a strong increase in financial costs for materials.

Choice by frost resistance

What brand of foundation is needed for frost resistance depends on the climatic conditions of the area and the area of ​​application. Frost resistance is the ability to withstand a certain number of cycles of alternating thawing and freezing. Marking is done with the letter F. The number after it means the minimum number of cycles. On average, the number of freeze-thaw cycles is equal to the number of years. That is, F50 concrete is designed for 50 seasonal changes from warm to cold (50 years). Determination of the frost resistance number is carried out according to GOST “Concrete. Methods for determining frost resistance".

The scope of concrete application, depending on the number of freeze-thaw cycles, can be summarized in a table.

Frost resistance	Area of ​​use
	Limited area of ​​application: for structures protected from contact with the external environment
F50-F150	The best option for private and mass housing construction. It is better to choose the average value F100
F150-F300	Used in harsh climatic conditions (for example, regions of the Far North)
F300-F500	Used for the construction of structures operated in damp conditions; unpopular for private housing construction
>F500	Not used for the construction of private houses, suitable for particularly critical structures with a long expected service life

Important! For any structures in contact with the external environment regulatory documents Frost resistance must be at least F50.

Choice by water resistance

This characteristic is determined according to GOST “Concrete. Methods for determining water resistance". According to GOST “Heavy and fine-grained concrete. Technical Specifications" there are 10 grades of concrete mixture for the foundation of a house from W2 to W20 in increments of 2 (W4, W6 and so on).

The letter in the marking indicates the waterproof classification. The number characterizes resistance to water pressure. For example, brand W4 can withstand water pressure up to 4 atmospheres, which is 0.4 MPa.

To answer what kind of solution is needed for the foundation of a house, it is necessary to consider the climatic features of the area and the water saturation of the soil. A universal option would be the composition designated W4.

Mixture mobility

Depends on the technology for performing foundation work. There are three options for the development of events:

• The concrete mixer's access to the foundation area is limited. If access is completely limited, the solution is supplied in buckets (for small objects), after pouring it into prepared containers. In this case, it is better to choose a solution with a mobility of at least P3.
• If the concrete solution is supplied to the formwork by a pump, then the mobility of the mixture is chosen to be at least P4.
• If access for mixers is free from all sides, P2 is sufficient.

Remember that you need to place concrete in the formwork no later than 2 hours from the moment the solution is prepared at the factory. And it’s better within 1.5 hours, so as not to ask questions on the forum later -.

Knowing the grade (class) and other characteristics of concrete (frost resistance, water resistance, mobility), you can safely contact the plant and order a concrete solution for monolithic work.