What brand of concrete is needed for a strip foundation. Brand of concrete for strip foundation: choice depending on the weight of the building and soil characteristics What concrete is needed for the strip foundation of a house

Construction of a comfortable private house starts with creating the foundation. At the same time, you need a strong and reliable type of foundation, which will determine the fate of the building.

And it is precisely in order for private houses to stand firmly, for a long time and without problems, that technology has recently begun to be used strip foundation, which has a huge number of advantages over other similar technologies.

But the question remains open: what grade of concrete in terms of density or classes and grades of concrete for a strip-type foundation when building a private house are suitable? Fortunately, there is a detailed answer to this.

The classification of concrete and the characteristics of “M 150” are such that it is used for a lightweight strip foundation made of foam concrete, as well as for preparation at the stage of concrete work.

It can be used in cases where when it is necessary to build a one-story private house, usually made of cinder blocks, aerated concrete, or, in more rare cases, foam concrete. Sometimes the M 150 brand can be used to create garages and various agricultural buildings.

Classification of concrete and compliance with the characteristics “M 200” in terms of density are almost always used for the construction of single- and two-story house having a slight overlap appearance. The strength of this type of concrete “M 200” classifies it as structural, allowing it to be used, among other things, for the creation of reinforced concrete elements and products.

Concrete grades "M 250", "M 300" and "M 350 b25" having General characteristics in general, in practice they do not differ from each other. They are used to create the foundations of exclusively large private residential buildings.

Specifically, the “M 300” brand is used for pouring the foundation of two-story or multi-story cottages(up to 5 floors). When pouring monolithic floors, experts recommend using, first of all, the characteristics of the concrete “M 300” being described.

As already written earlier, this brand of expanded clay concrete “M 200” is a structural mixture and is used in the creation of reinforced concrete elements and products. However, for such a situation, concrete “M 150” of type “B12.5” is also suitable.

Definitely the “M 200”, but if it is not available, or it is impossible to use it for some reason, it is quite possible to get by with the “M 150” type.

The second fundamentally important parameter is the soil characteristics. So, if a 1-2 storey building is being erected on sand or rock, and a strip foundation is used, the most acceptable option is to use concrete grades “M 150” and “M 200”.

However, it is worth considering that This type of soil phenomenon is quite rare, and most often builders have to deal with clay, loam and other similar types of soil. In such cases, the most acceptable option would be to use so-called ready-mixed concrete for the foundation.

It should be remembered that clay in the ground rocks under the building being constructed is a rather serious construction problem. The problem is that it has a high antinode.

As soon as environment the temperature drops - the clay begins to gradually increase in volume. It is for this reason that shallow foundation buildings begin to deform and collapse in winter.

In this case, the deformation consists of a relative movement of the foundation structure, usually by several tens of millimeters. In addition, this kind of foundation deformation is characterized by unevenness, due to the fact that the degree of saturation with soil water always varies.

Therefore, when creating a foundation on soil with clay, additional consultations with specialists and an even more painstaking and selective selection of the appropriate grade of concrete are required.

Otherwise, the building may, in a fairly near future, not only “slide” or “sag,” but even completely collapse into the ground.

The last most important parameter necessary for the correct selection of the brand of expanded clay concrete or concrete is the type of foundation being created. For example, a strip foundation under a private house in many cases requires the use of lower grades of concrete (grade of concrete for strip foundations), whereas pile foundation- higher.

However, this is not an axiom, and in some cases exceptions and compromises are made. Let us remind you that the pile type of foundation used in the construction of high-rise buildings, where concrete is poured using a special mechanism - a drilling injection machine.

The brand of concrete for the foundation of a private house determines its strength characteristics, or more precisely, what maximum load it can withstand. If you use a solution of a lower grade than necessary, the base will deform and collapse under the weight of the rebuilt house. The class of the mixture and the quality of mixing affect the reliability and service life of the entire building.

The following factors need to be taken into account:

• load - the weight of the entire structure, the number of floors;
• soil type;
• climate;
• terms of Use;
• type of base structure (strip, pile, etc.).

Strength is one of the most important characteristics that you should pay attention to first. But no less important is the degree of its moisture resistance and frost resistance. All these properties vary depending on the proportions of the components and the addition of various additives that accelerate or retard hardening or setting.

The type of soil also has a significant influence on the choice. When building low-rise private houses on sandy and rocky soils, smaller classes of concrete can be used. For clayey soils, higher grades are needed (from M250), since during freezing such soil expands greatly and heaving occurs. Because of this, deformation and destruction of the foundation of a private house occurs. The foundation is erected below the freezing level of the soil.

You also need to pay attention to the waterproof brand. Marked with the letter W and a number after it, for example, W2. The higher the number indicated, the less moisture it absorbs. The degree of moisture permeability of the material depends on the proportion of components and brand of cement. For the construction of private houses in areas with high groundwater levels, W6 and higher are recommended.

The solution is also divided according to frost resistance classes. This characteristic indicates how much freezing and thawing the material can withstand. It is especially important to take into account the frost resistance class when building a private house in a harsh climate. Marked with the letter F and numbers - F35, F100 and so on. The higher the number, the more freezing and thawing the concrete will tolerate. If the base is well waterproofed, then you can choose F75. If the foundation is located in more severe conditions, for example, with high groundwater levels and severe frosts, it is recommended to use class F100.

What is a grade of concrete strength, and what are they?

The class determines the maximum load it can bear. This indicator is indicated by the letter B and a number after it, for example, B25 means that the material can withstand pressure up to 25 MPa. There is also a marking with the letter M, indicated in kg/cm2. M100 can withstand up to 100 kg/cm2. The range of brands for the construction of the foundation of a residential building and other buildings starts from M100 and reaches M500.

To convert a brand into a class, the following formula is used: B=M*0.787/10. The class varies depending on the composition. The more cement, the higher the strength.

For foundations, M100, M150, M200, M250, M300, M400 and M500 are used. M100 and M150 are selected for the construction of utility buildings, small garages, fences, private houses (no more than one floor) made of foam, aerated concrete or wood. Also as an initial layer for pouring a higher class solution. M200 is used in the construction of the foundations of small buildings made of gas concrete, foam concrete, cinder blocks with light floors. M250 and M300 are suitable for constructing the foundations of two-story houses and buildings with more floors, pouring monolithic structures. Concrete of this brand can withstand significant loads; most often it is chosen for the construction of private houses.

M350, M400 and M500 are suitable for multi-storey buildings. By building regulations It is not recommended to use a solution below M250.

The type of base affects the class; for example, a house with a basement will require stronger concrete than a conventional building. The grade for strip foundations may be low, but for pile foundations a higher grade is needed. If the groundwater level is high, M350 or more is selected for the pile foundation.

Only clean sand should be used. Either crushed stone or gravel with a fraction of 2-4 cm is selected as a filler. Broken bricks cannot be used, as this significantly reduces the strength characteristics. Cement is selected grade 400 or 500. The most common proportion of components is 1: 3: 4 (cement/sand/crushed stone). Everything is thoroughly mixed to a homogeneous consistency, there should be no lumps.

The denser the solution, the stronger the foundation will be. Because of the voids, the quality and strength of the entire structure decreases, so after pouring the mixture is treated with a vibration machine. It will evenly distribute the mixture over the entire surface, compact it and remove air.

The monolith should be poured in one go. If you do this in several stages, then the first batch will already have time to set. After pouring the second one, a “cold seam” will appear in the concrete. According to the principle of action, this defect is similar to cracks. As a result, the base will not be monolithic, but will consist of several parts. It may break due to the load.

During pouring, temperature conditions must be observed; the optimal temperature is considered to be +16°C. If construction is carried out during the winter season, then anti-frost components are added to the solution. If without them, then the foundation must be insulated and warmed, for example, with heat guns or electric current, thanks to this the water will not freeze.

IN summer time water evaporates too quickly from the top of the solution. As a result, the concrete dries unevenly and cracks appear. To avoid this, it is constantly watered with water and covered with opaque material. Or plasticizers are added at the stage of diluting the mixture.

During hardening, all conditions must be observed the necessary conditions– temperature, humidity and so on. Concrete gains its greatest strength after 28 days (at optimal temperature and humidity conditions).

For buildings that will be built from aerated concrete, monolithic foundations should be erected, since it has a weak bending property. Even with slight heaving of the soil, cracks appear in the blocks. If the foundation is monolithic, the load will be distributed evenly over the entire surface.

The cost depends entirely on the brand of concrete. The higher it is, the more cement was spent to make it. To determine the required volume of solution, it is better to contact specialists, since if there is a shortage of the mixture, the foundation will be poured more than once, which will lead to a deterioration in its quality, and if there is an excess, there will be an overpayment.

It is worth noting that no matter what type of foundation the owner of a country house chooses, he will always have the question of what type of concrete to choose for this very foundation and how the characteristics of concrete differ from each other.

Concrete composition

Any concrete includes 3 fundamental components - these are:

1. Cement- a component that binds other fillers.
2. Fillers— Crushed stone, gravel, sand, various bulk additives.
3. And directly water.

As a result of the synthesis of these groups of components, concrete is formed; it is worth noting that the quality of concrete directly depends on the percentage of our fillers in the cement. Fillers are used in order to reduce the stress of concrete as a result of hardening of the foundation, and in order to reduce the economic cost of the structure, since the cost of the building directly depends on the choice of concrete components.

Concrete marking

1. M - concrete grade.
2. B - concrete class.
3. F - concrete resistance to frost.
4. W - characteristics of concrete water resistance.
5. P - mobility of concrete.

Concrete grade

Concrete grade- a fundamental characteristic that shows us how a frozen foundation will transfer compressive loads after its hardening process, that is, after 30 days.

Concrete M100 B7.5 — Concrete of very low quality is mainly used in preparatory work during construction for pouring concrete pads under the foundation.

Concrete M150 B12.5 — concrete not very good good quality Mainly used in the construction of concrete paths and rough screeds.

Concrete M200 B15 — This concrete is used for the construction of elements with low load, such as stairs or for pouring strip foundation slabs.

Concrete M300 B22.5 — The most popular brand of concrete in Russia, because the quality of this concrete can be rated at a strict 4! They are also used for the construction of good and high-quality country houses.

Concrete M350 B25 — high-quality concrete, which is used for the construction of concrete structures requiring extremely high strength.

Concrete M500 B40 — Quite rarely used in Russia, because concrete costs large investments; it is mainly used for the production of bank vaults or for the construction of special buildings. objects.

Concrete class

The concrete class is designated by the letter B and characterizes more detailed information about the degree of compression; we can see information about more accurate data above.

Mobility of concrete

Concrete mobility Letter P - an indicator that characterizes the liquid of a homogeneous concrete mixture and the flow coefficient. It is worth noting that even a slight excess of water in the composition of this concrete can result in a loss of strength, and as a result, major problems.

Waterproof

Water permeability - letter W. Characterizes the coefficient of water permeability of concrete by water, usually they range from 2 - the minimum value to 12 the maximum value. This coefficient should be taken into account if there is groundwater on your site and the foundation can “play” when flooded by ground underground sources. By the way, if you want to get rid of this problem, I recommend reading the article about and about, which will leave your foundation in perfect condition.

Frost resistance

Frost resistance letter F - an indicator that characterizes how much defrosting and freezing concrete is able to withstand.

Video - pouring a house with high-quality concrete

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 of low groundwater levels and shallow freezing depth in low-rise construction, F100 - F300 is sufficient. The highest indicator may indicate that there is no need for waterproofing work for the 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:

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

Just as the foundations for a wooden and brick house differ, so does the grade 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 M150. A brick structure with a monolithic ceiling requires a grade of at least M250. The grade of concrete for the foundation of a two-story brick house is concrete grade M300 and higher.

The basis of a reliable house is a strong foundation, and the strength of the foundation is largely determined by the sufficient safety margin of concrete, as well as its other characteristics: frost resistance, and in case of high groundwater levels, water permeability. In order for a house to stand for a long time and without problems, you need a correctly calculated grade of concrete for the foundation. What it is and how to define it will be discussed further.

Composition of concrete for the foundation

• Knitting. Most often it is cement (Portland cement). There is also non-cement concrete, but it is not used for foundations.
• Placeholders:
  • sand;
  • crushed stone or gravel.
• Water.

The grade of concrete is determined by the proportions of all these components, as well as the conditions of its hardening (setting). Optimal conditions for concrete to gain strength are created at a temperature of +20°C. Under such conditions, the process is very active in the first 7 days. During this time, concrete gains about 50% strength. With these parameters, it is already possible to continue construction further. The design strength, which is taken as 100% during design, under such conditions is achieved in 28-30 days. In reality, the process continues further, but at a very low speed. The strength gained after 30 days is not taken into account anywhere - it goes “in reserve”.

As the temperature decreases, the setting time increases significantly (at +15°C it takes about 14 days to achieve 50% strength). At a temperature of +5°C the process practically stops, and under such conditions winter concrete is needed - with appropriate additives and/or measures to increase the temperature (wrapped, heated in a mixer, using heating through the formwork or heating directly by attaching heating cables to the formwork from the inside ).

Cement

Portland cement is used to make concrete different types. The most common are:

Any of these types of binders can be used to prepare concrete. Only you will need to take into account the setting time of the solution - you need to lay it and vibrate it before hardening begins.

Fillers

The quality of concrete is also influenced by aggregates. It is necessary to adhere not only to the recommended proportions, but also to quality indicators - humidity and grain size.

Sand

Depending on the grain size, the following types of sand are distinguished:

For backfilling, they mainly use large and medium ones, less often small ones. The sand must be clean - not contain any foreign inclusions - roots, stones, plant debris, pieces of clay. Even the content of dust and silty substances is standardized - they should not exceed 5%. If you decide to “extract” the sand yourself, check the amount of pollutants.

To check 200 cc. centimeters of sand are poured into a half-liter container (jar, bottle), filled with water. After a minute and a half, the water is drained, poured in again and the sand is shaken. The procedure is repeated until the water is clear. If there are 185-190 cubic meters of sand left. cm, it can be used - its dust content does not exceed 5%.

You also need to pay attention to the moisture content of the sand. All proportions are based on dry ingredients. Even dry and loose sand has a moisture content of at least 1%, regular sand - 5%, wet - 10%. This must be taken into account when dosing water.

Crushed stone and gravel

Crushed stone is obtained by crushing rocks. Depending on the size of the fragments, the following fractions are distinguished:

To prepare concrete, several fractions are used - this way the distribution of crushed stone by volume is more uniform, and the strength increases. The size of the largest fragments is standardized: it should not be more than 1/3 of the smallest size of the structure. In relation to foundations, the distance between the reinforcement bars is taken into account. SNiP also determines the amount of small crushed stone: it must be at least 1/3 of the total volume.

Gravel has approximately the same fractions and sizes, but when used, the water-cement ratio (water/cement or w/c) increases by 0.05 (you need to pour 5% more water).

Water

Drinkable water is used to prepare and water concrete. Including the one you can drink after boiling. sea ​​water It is permissible to use with Portland and alumina cement. Any other process water is not suitable.

Concrete grade and its choice for strength

Depending on the characteristics of concrete, it is divided into compression classes and corresponding grades. This correspondence is shown in the table.

The compressive strength of concrete is determined by testing. This parameter reflects the load that this concrete can withstand without signs of destruction during prolonged exposure. Based on this characteristic, concrete is selected depending on the load that the house will create (it depends primarily on the weight of the walls and ceilings, but a lot of factors are also taken into account, including snow loads).

When designing the foundation and determining its dimensions, the load from the building is calculated. This figure is one of the parameters for choosing a brand. Find in the middle column of the table a value close to the calculated load and determine the brand.

The grade of concrete for the foundation depending on the materials of the walls and soils

Projects and calculations are not always carried out. When building dachas or bathhouses, developers prefer not to spend money and develop the structures themselves. And although there are many grades of concrete, in private housing construction, mainly three are used for foundations:

You also need to make adjustments that take into account the type of soil under the foundation and the level of groundwater. If the soil is sandy or rocky, and the water is below the freezing depth, then all recommendations remain valid. If the groundwater level is high and the soil is heaving, the grade of concrete is taken one step higher: the conditions are more difficult and a larger safety margin is required.

Selection based on foundation type

Other characteristics

In addition to the main characteristic - strength, concrete has a number of other indicators that can affect the choice of brand under special construction or operating conditions.

As for workability, it affects how comfortable you will be working with the solution. For foundations, concrete with characteristic P2 is most often used (for frequent reinforcement, P3 is used). It is permissible to use P1 when it is necessary to process concrete with a vibrator - it settles the solution well, making it more homogeneous.

Remember that when building a two-story house, the grade of concrete increases by one step from those given in the table.

Proportions of components for concrete

In recipes, concrete compositions are given by weight or volume. Moreover, cement is always taken as 1, and all other components are taken in relation to it. Proportions for commonly used brands are shown in the table.

As you can see, the same concrete can be obtained using different brands of cement. Fall asleep as you have, only adhering to the recommended mass (or volume) portions. Mostly when pouring foundations, Portland cement M400 and M500 is used. When composing the mixture, remember that if gravel is used instead of crushed stone, 0.05 more water is added.

How to decipher the table instructions? For example, let’s take concrete grade M250, it will be made from Portland cement M400. We will focus on mass. Then select the corresponding line in the third column: 1/2,1/3,9. This means that for one kilogram of M400 cement, we take 2.1 kg of crushed stone and 3.9 kg of sand. The amount of water can be taken from the table below - it is 0.65 kg for crushed stone backfill.

If we compose the same concrete based on volume, for example a 10-liter bucket, then we take the proportions from the fourth column: 10/19/34. This means that for 1 bucket of cement, we put 1.9 buckets of sand, 3.4 buckets of crushed stone. The water/cement ratio remains the same: 0.65 buckets.

Proportions of concrete per cubic meter. The mass of the components is indicated in kilograms, parts are given in brackets.

Another table summarizes data on the consumption of M400 and M500 cement per cubic meter of the most popular grades of concrete during foundation construction.

The correct grade of concrete for the foundation is the key to its strength. But high-quality compounds are very expensive. If, according to all the recommendations given, you need to use M300 and higher, it is better to order one, which will confirm or refute your assumptions. This service costs about $100-$150, but can save you thousands if you want a lower grade.