The strip foundation can easily be classified as the most popular - it is the one that is preferred in the vast majority of cases when building your own home outside the city. The diagram of such a foundation, the algorithm for its calculation, reinforcement drawings and pouring technology - all these issues have been worked out in practice to the smallest detail, and therefore many site owners very often prefer to undertake its construction

But correctly calculated calculations of the dimensions of the foundation strip, carefully dug trenches and firmly put together, reliable waterproofing and a perfectly aligned reinforcement cage according to the drawings will cost nothing if concrete that does not meet the conditions of future operation or is poorly prepared is used for pouring. Therefore, we offer the reader who is interested in the topic strip foundation, commit small excursion into the theory and practice of preparing the solution necessary for such purposes. This is useful to know both when planning to order ready-made concrete and for preparing the solution yourself at the construction site.

So, what brand of concrete is needed for a strip foundation, and how it can be made on your own.

Classification of concrete solutions

First of all, what exactly is concrete? This term is usually understood as a stone of artificial origin, which is obtained as a result of hardening of a mixture poured into a mold, consisting of correctly selected components, which include:

— binder (most often cement plays this role);

— aggregates (usually sand, gravel or crushed stone, but some concretes require the use of other materials).

- water, which acts both as a diluent and as a necessary component for the chemical reaction of the formation and maturation of concrete stone.

If you read the construction literature, you can see that the classification of concrete is very ramified and includes many varieties of them for various purposes. However, in this case we will not go deeper into it - we are only interested in those types that are suitable for building foundations. This means that from the whole variety we will highlight only heavy mortars based on Portland cement and dense aggregates.

If you look at it, an almost complete picture of the performance qualities of concrete is given by four main indicators of the solution: brand strength or class (can be indicated by the symbol M or B), frost resistance (F), water resistance (W) and mobility or workability (P). Some indicators are of decisive importance for the construction of the foundation, others, say, indirect, but you need to get to know them all.

Brands and strength classes of concrete

This is perhaps the main indicator that must be taken into account when choosing a specific solution.

The grade indicates what compressive load fully matured concrete can withstand, expressed in kgf/cm².

There are quite a few grades of concrete in terms of strength, from M50 to M800. However, in the field of private construction, and specifically when constructing foundations, developers, as a rule, will have to deal with grades from M100 to M300, less often - M400.

In reference literature and in construction technological instructions, very often, instead of the concepts of brand strength, they operate with an indicator of the class of concrete. These quantities are, to a certain extent, interconnected, and differ, rather, in their “coordinate system”.

The class is denoted by the letter B and a numerical indicator that corresponds to the load (expressed in megaPascals), which with a 95% probability will be withstood by the prototype - a cube with sides of 150 mm.

Thus, if the brand speaks about the average strength value, then the class speaks about the actual strength. Example - a solution of the same component composition, corresponding to a certain brand, but hardened and gaining strength under different conditions, can show a different strength class according to test results.

However, in conditions of independent private construction, knowledge of such subtleties is unlikely to be needed. It is important to understand the grades of concrete and how they roughly relate to classes. The following table will help with this.

Table 1. Correlation between grades and classes of concrete

Compressive strength of concrete, kg/cm2	Concrete strength class	The closest concrete grade in terms of strength
65.5	AT 5	M 75
98.2	B 7.5	M 100
131.0	B 10	M 150
163.7	B 12.5	M 150
196.5	B 15	M 200
261.9	B 20	M 250
294.4	B 22.5	M 300
327.4	B 25	M 350
392.9	B 30	M 400
458.4	B 35	M 450

Having such a “cheat sheet” at hand, it is easy to navigate both systems for assessing the strength of concrete.

Frost resistance of concrete

The frost resistance grade is designated by the letter symbol “F” and a number, which indicates the number of freezing and thawing cycles of material samples without any significant changes in its physical, mechanical and operational characteristics.

The method for determining frost resistance is established by GOST 10060.0-95. The same standard also provides for the classification of materials according to this indicator - markings vary from F25 to F1000.

When assessing frost resistance, unnecessary extremes are often taken:

• The number of freezing cycles does not at all mean the number of years, that is, alternations of winter and summer. During one cold season, there can be several such freeze-thaw cycles, and sometimes even several dozen - remember winters with an abundance of thaws.
• Frost resistance class, for example, F50 - this does not mean at all that after 50 cycles the concrete becomes completely unusable.

In principle, such an assessment of frost resistance applies to concrete that is saturated with water. For example, a dry wall structure made of, say, concrete with frost resistance grade F50 or F100, over the course of its many years of operation, can withstand thousands of freeze-thaw cycles, while remaining completely unharmed. That is, if the foundation is provided with the proper quality of waterproofing (and this is one of the key requirements for ensuring the durability of your own home), then the frost resistance grade will not be a critical indicator. Moreover, if the concrete preparation technology is followed, this indicator, even if desired, cannot be underestimated - make a smaller F100.

Whatever concrete the foundation is poured from, its waterproofing is a must!

A big mistake is made by those owners who ignore this issue when building their house. Even the use of unreasonably expensive grades of concrete in the absence of reliable waterproofing does not guarantee the durability of the building’s foundation. But it’s not that difficult to do. More about this in a separate publication on our portal.

If we think sensibly, then the frost resistance class indicator in private construction is important only in two typical places where concrete products are used and there is no way to protect them from direct exposure to water.

• This is paving paths and pouring blind areas: here a frost resistance class of at least F300 is desirable
• The second case is cement roofing tiles, when produced by responsible manufacturers achieving frost resistance of F600÷F800.

Practice shows that for foundations (of course, with their waterproofing) the F150 indicator is quite sufficient, and for walls - even less, about F50÷F100

Cement prices

Concrete waterproof class

This indicator is indicated by the letter symbol W and a number (from W2 to W12). The numerical indicator indicates the maximum pressure, expressed in megaPascals, at which the prototype (cylinder 150 mm high) does not allow water to pass through.

From point of view practical application in the conditions of individual construction (in particular, when constructing a strip foundation), the significance of this indicator is small. By analogy with the frost resistance class, if you follow the requirements of the technology for preparing high-quality concrete mortar of the required grade (class) of strength, then the optimal water resistance parameters will be achieved on their own.

Table 2. Approximate correspondence of concrete indicators with standard optimal combinations of components and compliance with the technology of mixing solutions

Class (closest grade) of concrete by strength	Frost resistance	Waterproof
V-7.5 (M100)	F50	W2
V-12.5 (M150)	F50	W2
V-15 (M200)	F100	W4
V-20 (M250)	F100	W4
V-22.5 (M300)	F200	W6
V-25 (M350)	F200	W8
V-30 (M400)	F300	W10

Even if it seems to the owners that the soil at the construction site is oversaturated with moisture, it will still be much cheaper, faster and more reliable to carry out high-quality waterproofing than to order expensive concrete with a higher waterproof grade (W12 or even higher).

Indicators of workability of the solution

But this parameter is extremely important for ensuring the quality of foundation construction work. Moreover, in the conditions of a regular construction site, when mechanized means of laying concrete into formwork may not be available. In fact, this indicator hides how plastic the solution is, how it can be poured, distributed with complete filling of all cavities, and leveled to a given level.

There are several criteria for assessing workability, but in everyday construction practice the most commonly used indicator is the workability indicator, denoted by the letter symbol P and a digital value. Where does everything come from?

The mobility index of a solution shows how much it can spread under the influence of its own weight. This parameter is checked by a simple experiment - a special form in the form of a truncated cone is tightly filled with the prepared solution (base diameter - 200 mm, top hole diameter - 100 mm, height - 300 mm. Filling is carried out in three stages - equal in height layers, with mandatory multiple by bayoneting (piercing the solution to the surface of the base with a metal rod). Then the excess solution is removed from above, and the form is carefully removed with a progressive upward movement (this should take from 3 to 7 seconds). The form is installed next to the cone formed from the solution, which sags under own weight.

The measured amount of subsidence of the cone from the initial horizontal level (from the top of the mold) will lead to the desired indicator of the mobility of the solution.

Table 3. Indicators of concrete mobility and features of the use of solutions

Amount of cone subsidence	Mobility indicator	Note
0	Not regulated	Rigid concrete is a special area of ​​application.
- from 1 to 4 cm	P1	Slow-moving, semi-rigid concrete. It is held on a vertically located bayonet of a shovel. In private construction practice it is practically not used.
- from 5 to 9 cm	P2	The solution is of medium mobility - it slides smoothly from the bayonet of the shovel. It can be used when pouring a foundation, but with a fairly sparse arrangement of reinforcement bars and with mandatory deep vibration.
- from 10 to 15 cm	P3	Mobile solution - flows down from the vertically located bayonet of the shovel. It can probably be considered optimal for pouring a strip foundation with any frequency of arrangement of reinforcing rods. Requires the mandatory use of a vibrator.
- from 16 to 20 cm	P4	Concrete mortar of high mobility, practically fluid. Excellent for working with concrete pumping plants. A good option for strip foundation. The use of vibratory laying is desirable.
- over 20 cm	P5	Fully flowable solution. Mostly suitable for pouring monolithic walls and ceilings. For a strip foundation it will still be too mobile, and is usually not used for such purposes.

What kind of concrete should I order for a strip foundation?

So, the main criteria for evaluating concrete solutions used for individual construction were considered. It remains to be seen: what composition will be optimal for pouring a strip foundation.

Table 4. Areas of application of ready-mixed concrete of various grades

Class (grade) of concrete by strength	Concrete brand	Application of concrete
B7.5 (M100)	BSG V 7.5 P3 F50 W2	Preparation for the foundation
B10 (M150)	BSG V 10 P3 F50 W2	Preparation for the foundation, pouring a strip foundation for non-critical structures
B12.5 (M150)	BSG V 12.5 P3 F100 W2	Preparation for the foundation, pouring a strip foundation for non-critical structures (light outbuildings)
B15 (M200)	BSG V 15 P3 F100 W4	Monolithic strip and slab foundations, screeds, grillages.
B20 (M250)	BSG V 20 P3 F150 W4	Monolithic foundations - strips and slabs, crossbars, insulated Swedish slabs, underfloor heating screeds
B22.5 (M300)	BSG V 22.5 P3 F150 W6	Monolithic foundations of any type, monolithic reinforced concrete load-bearing structures, thin high-strength screeds for heated floors.
B25 (M350)	BSG V 25 P3 F200 W8	All of the above applications are in particularly difficult operating conditions
B30 (M400)	BSG V 30 P3 F200 W8	Monolithic foundations and load-bearing structures for heavy multi-storey buildings, including in difficult construction sites. More durable brands in individual construction are not used.

Let us pay special attention - for now we are talking about ready-mixed concrete, which is ordered from the relevant companies with delivery to the construction site. In this case, the mixture should be unloaded no later than 90 minutes after the initial mixing of the components with water - when transported in special concrete mixer trucks, and no more than 45 minutes - if the solution is transported in a dump truck, that is, without constant mixing. This means that it is necessary to assess in advance the fundamental possibility of such delivery and provide access roads to the place where the foundation is poured.

When ordering ready-mix concrete (RBG), it is necessary to take into account some more nuances. Unfortunately, the emergence of a considerable number of small companies practicing the production of mortars does not at all mean that they all work in compliance with standards or established technical conditions. Alas, the reality is such that not everyone can be trusted - you should familiarize yourself in advance with the reputation of nearby organizations of a similar profile. Very often you have to deal with the fact that, based on the results of concrete maturation, its final actual strength differs significantly from that declared when purchasing the solution, and it is very difficult to check this at the purchase stage. The desire of manufacturers to “save on cement” is quite understandable, but not at the expense of quality!

Which exit?

• The first is to trust only yourself, and with strict adherence to all necessary proportions (we will come to this issue a little later).
• Secondly, for insurance purposes, when ordering ready-mixed concrete, deliberately overestimate its strength class and all other relevant characteristics by one or two steps. By the way, they do the same thing if construction has to be carried out on a site with pronounced heaving soil.

Therefore, we can give another table based on the practice of construction in normal conditions of the “Russian outback”. The indicators given in it do not have any theoretical justification - these are only generalized average statistical data on the use of ready-mixed concrete specifically for pouring strip foundations.

Type of one-story house	Soils with good bearing capacity, low heaving	Heaving soils
Light wooden panel or frame houses	BSG V 15 P3 F100 W4 (M-200)	BSG V 20 P3 F150 W4 (M-250)
Houses made of timber, log cabins	BSG V 20 P3 F150 W4 (M-250)
Houses made of gas silicate blocks, expanded clay concrete, sawdust concrete (arbolite) and other similar materials	BSG V 22.5 P3 F150 W6 (M-300)	BSG V 25 P3 F200 W8 ​​(M-350)
Houses made of brick, stone or monolithic reinforced concrete	BSG V 25 P3 F200 W8 ​​(M-350)	BSG V 30 P3 F200 W8 ​​(M-400)

Note to table 5. In the case where it is planned to construct a building of more than one floor, it is recommended to order BSG of a higher grade. But in any case, no higher than B30 (M400).

We prepare concrete for a strip foundation ourselves

If there is no way to use the services of ready-mixed concrete manufacturers, or there is a complete lack of trust in the local “shabashniks,” you can entrust this very labor-intensive function to “your own shoulders.”

You must be prepared for the fact that the work ahead will be considerable and almost continuous, since it is recommended that the pouring of the strip foundation be carried out during one shift. Therefore, it is necessary to prepare in advance all the necessary components for preparing the desired brand of concrete, to provide for the procedure for their correct dosage (by volume or mass). It may be worth considering whether there will be enough manpower to carry out almost continuous loading and unloading of the concrete mixer.

Prices for concrete mixers

concrete mixer

So, to independently prepare a concrete solution for the foundation, you will need the previously mentioned components - cement, sand, crushed stone or gravel, water. The use of special additives that improve certain qualities of the finished solution cannot be ruled out. But additives are a separate matter, and they will be discussed in a separate publication on our portal. In the meantime, here are the basic components of concrete.

Main components of concrete mortar

Cement

Cement in concrete acts as a binding component. Most often, Portland cement (PC) is used for these purposes, obtained by finely grinding grains of special clay (clinker) and limestone sintered at a high temperature, with the dosed addition (from 1.3 to 3%) of gypsum.

Industrially produced cement is divided into grades - from 300 to 600. The grade itself indicates what the approximate compressive strength of a mortar prepared from cement and sand (in a ratio of 1:3) will be, with an optimal water-cement ratio and full maturation (28 days). To prepare concrete solutions for pouring strip foundations, PTs-400 cement is most often used, as it is the most affordable and quite universal for such purposes. PC-500 cement can also be used - it’s just a matter of changing the initial proportions, but PC-400 is simply easier to purchase due to its widespread availability, and the price-quality ratio speaks in its favor.

You can glean some more information from the cement markings.

• Thus, the symbol D indicates the presence of modifying additives: “TO” – no additives, another number – the percentage of additives by weight of cement;
• Symbol "B" - quick-hardening cement;
• — “PL” — plasticized cement;
• — “GF” is cement with hydrophobic properties.

You should always pay attention to the production date of the cement. Even under normal storage conditions, cement can lose up to 20% of its brand strength in the first three months; in six months such a loss will be 30%, and up to 40% within a year. The solution is either to use only fresh cement, in accordance with the proportions of the solution, or to make appropriate adjustments to the amount of cement added (proportional to the loss of strength) and increasing the time for preparing the solution (almost four times). It is much better, of course, to try to make do with fresh cement.

Sand

Sand is used as a mandatory fine aggregate in concrete. The optimal option is a quarry one, with a particle size modulus of 2 to 3.25 mm, but with a voids content of no more than 30%, that is, with the content of all fractions.

In private construction, when preparing concrete grades up to M300, it is allowed to use sand with a fine fraction (less than 1.6 mm) of no more than 20% of the total volume, and with an impurity content (clay) of up to 3% by weight. The content of dusty sand fractions (less than 0.6 mm) should also not exceed 3%.

For the correct water-cement ratio, it is better to use dry sand. The average density of sand is about 1500 kg/m³.

Prices for crushed stone

Gravel or crushed stone

Crushed stone or gravel is used as coarse concrete aggregate. The difference between these materials mainly lies in the fact that crushed stone is a derivative of forced crushing of rocks, and its edges usually have a sharp shape. Gravel is a product of natural destruction (weathering or water erosion) of rocks, and therefore its elements often have rounded outlines and smooth rounded edges.

Crushed stone and gravel can also vary in fraction size - usually this indicator can vary in the range from 5 to 70 mm (there are four fractions: 5÷10 mm, 10÷20 mm; 20÷40 mm and 40÷70 mm). According to existing standards, for high-quality preparation of concrete, its composition must include crushed stone or gravel of at least two different fractions - with a maximum of up to 40 mm, or three - with a maximum of up to 70 mm.

Wherein maximum size grains of crushed stone or gravel should not exceed two-thirds of the distance between the longitudinal reinforcement rods. In addition, the maximum size should not be more than 1/3 (or better yet, even 1/5) of any minimum size poured building structure. since we are talking about a strip foundation, then, for example, with a strip width of 200 mm, the size of the crushed stone or gravel fraction should not be higher than 40 mm.

In fact, when pouring strip foundations for small buildings, private developers often use large aggregate of one fraction - 20 mm. This, in principle, is quite enough, but working on small ones with larger crushed stone or gravel, and when pouring narrow strips with a fairly frequent arrangement of reinforcing rods, can be very difficult.

Crushed stone or gravel must also be clean - even small clay contamination can sharply, up to 30%, reduce the final actual strength of the finished concrete solution. So, if you have purchased material with obvious signs of heavy contamination with clay or soil, before starting work it is necessary to wash it in advance with directed jets of water from hoses.

Water

It would seem - but what’s so complicated here: pour water into the mixture you are mixing until you get the desired consistency...

It turns out that everything is not so simple.

• First of all, water is also subject to certain requirements regarding its purity and chemical composition. All of them are listed in detail in the specifications established by GOST 23732-79 - this is the absence of traces of petroleum products, acids, phenols, sugars, standard oxidability, maximum salt content, etc. True, in most cases, clean tap or well water is quite suitable for mixing.
• The second and, perhaps, most important thing is the optimal ratio of water and cement. Excess moisture is never good for the resulting “cement stone”. Just look at the following diagram for an example.

The curved lines in the diagram indicate changes in the brand strength of the resulting concrete from cement (in proportion with sand 1:3) of the corresponding brand. Let's take, for example, cement PC-400. With a water-cement ratio of 0.4, the strength at full maturation reaches over M400. But with an increase in the amount of water there is a sharp decrease, and with a ratio of 1.0, the strength of concrete is already estimated at only M100, that is, the loss in this indicator is fourfold!!!

This, however, is easy to explain. Water not involved in a chemical reaction with cement has only two ways:

- or it gradually evaporates, leaving behind large pores that sharply reduce the strength qualities of the material.

- or it remains unbound in the pores - and this is even worse, as when frost sets in, this moisture will begin its destructive effect.

So, when mixing concrete, it is necessary to strictly observe the optimal water-cement ratio. For reference, the average values ​​for different grades of concrete and the cement used for their production are given in the table:

The grade (class) of concrete planned for production	Portland cement PC 400	Portland cement PC 500
M100 (B7.5)	1.03	not applicable
M150 (B12.5)	0.85	not applicable
M200 (B15)	0.69	0.79
M250 (B20)	0.57	0.65
M300 (V22.5)	0.53	0.61

What to do if construction took place in the winter?

“Recipes” for concrete for pouring strip foundations and the procedure for mixing

Dosage of components for preparing concrete

So, it remains to consider what proportions must be observed to prepare this or that brand of concrete.

If desired, it would be possible to provide the entire methodology for calculating and theoretically justifying the ratio of the initial ingredients - by weight and by volume. But, probably, the reader is more interested in the practical side. Well, the preparation of this type of concrete suitable for pouring strip foundations is such a well-established issue in everyday practice that one can simply give tables of proportions.

Table 7. Dosage of components for the preparation of concrete based on PC-400 cement

M100 (B7.5)	1: 4.6: 7.0	10: 41: 61	7.8: 1
M150 (V10 ÷V12.5)	1: 3.5: 5.7	10: 32: 50	6.4: 1
M200 (B15)	1: 2.8: 4.8	10: 25: 42	5.4: 1
M250 (B20)	1: 2.1: 3.9	10: 19: 34	4.3: 1
M300 (B22.5)	1: 1.9: 3,7	10: 17: 32	4.1: 1
M400 (B30)	1: 1.2: 2.7	10: 11: 24	3.1: 1

Table 8 Dosage of components for the preparation of concrete based on PC-500 cement

Brand (class) of strength of manufactured concrete	Ratio by mass (cement: sand: crushed stone)	Volume ratio (cement: sand: crushed stone)	Volume ratio of ready-mixed concrete to cement
M100 (B7.5)	1: 5.8: 8.1	10: 53: 71	9.0: 1
M150 (V10 ÷V12.5)	1: 4.5: 6.6	10: 40: 58	7.3: 1
M200 (B15)	1: 23.5: 5.6	10: 32: 49	6.2: 1
M250 (B20)	1: 2.6: 4.5	10: 24: 39	5.0: 1
M300 (B22.5)	1: 2.4: 4.3	10: 22: 37	4.7: 1
M400 (B30)	1: 1.6: 3.2	10: 14: 28	3.6: 1

The amount of water can be easily determined for each specific case according to the recommendations in the water-cement ratio table located above.

Mixing ingredients in a concrete mixer

The main task is to create an enveloping layer of cement “dough” for all aggregate granules, regardless of their fraction. It is almost impossible to perform such an operation manually - the use of mechanical mixers makes it possible to increase the strength of the resulting solutions by up to 40% compared to mixing with a shovel in a container.

When preparing a solution in a concrete mixer, adhere to the following order of operations:

• At the very beginning of work, the container of the concrete mixer must be “lubricated,” so to speak. To do this, a thin solution of cement, sand and water is mixed for several minutes - it is then drained - you can find a use for it at an ongoing construction site.
• Then, the following are added sequentially to a large-volume working concrete mixer (0.5 m³ or more):

— 15÷20% of the planned amount of water;

— the required dosage amount of sand and cement;

— after mixing this load, coarse aggregate (crushed stone, gravel) and the remaining amount of water are added.

• If you are using a small-volume concrete mixer, then it is better to do this:

- the required proportion of dry sand is mixed with approximately half of the crushed stone;

- then cement is added, also on a dry basis;

- after thoroughly mixing the dry mixture, add water, and at the very end, in order to completely break up all possible lumps, add the remaining part of the crushed stone (gravel).

• You should never assume that the longer you stir, the better the solution. There must be moderation in everything, as it can turn out exactly the opposite: the longer the cycle, the more water will simply evaporate, and the plasticity of concrete will decrease.

The minimum mixing time is usually estimated at one to one and a half minutes. (It is interesting that German standards have their own opinion on this matter - the duration of mixing is generally estimated at 30 seconds).

Video: example of preparing M300 concrete in a concrete mixer

Application: Quick and accurate calculation of components for self-preparing concrete for pouring a strip foundation

Below is a calculator that will allow you to quickly calculate the required amount of ingredients to prepare the required amount of concrete.

The calculator allows you to calculate in two ways:

• The first is determining the amount of ingredients to prepare a specific volume of solution - this can be useful, say, for planning the loading of a concrete mixer of known one-time productivity.
• The second option allows you to determine the total amount of concrete to fill the entire strip foundation. In this case, we will rather talk about the volumes of ingredients that need to be purchased or prepared at the work site to prepare the solution yourself.

The calculator calculates the two most commonly used grades of mortar - M200 and M300, which are made from PC-400 cement.

The calculations include a “folk” unit of measurement - a bucket. This refers to a bucket with a capacity of 10 liters.

Building a house is a responsible, labor-intensive and costly process. Therefore, careful planning at all stages of building a house, including the process of selecting the necessary materials, is extremely important. When choosing the type of foundation, it is necessary to take into account the size and weight of the structure, as well as the type of soil.

The choice of type and quality of foundation construction will determine how strong, durable and reliable the structure will be. The strength of the foundation is ensured not only by the chosen type of foundation, but also by the quality of the materials used to fill it. Therefore, before starting construction work, it is necessary to find out what kind of concrete is needed for the foundation, its grades and classes.

Compound

Foundation concrete is made from many components, which can be divided into groups:

1. Mineral component. These mineral elements, in particular sand, gravel and stone skeleton are responsible for bearing capacity, and the resistance of the foundation to loads will depend on its quality. The share of this group in the solution ranges from 70 to 80%.
2. Active group. This group includes a cementitious gel-like adhesive that binds mineral elements together. This group affects the strength of the base and the degree of shrinkage.

If you correctly select the composition of concrete for the foundation, it will provide the necessary level of strength of the underground structure and will also withstand seasonal soil movements.

Stamps

There are various grades of concrete for foundations. Their choice depends on the type of foundation, the weight of the future structure and other indicators. Therefore, it is extremely important to know which grade of foundation concrete is suitable for different foundations and types of structures.

1. M 100. This composition can be used in initial stages construction. Sand concrete of this brand is used for making footings and constructing foundations for light garage structures, wooden dachas, fences, chicken coops, sheds and other dacha buildings.
2. M 150. This brand of sand concrete is recommended for preparatory work, as well as for the construction of a lightweight strip foundation. Can be used for one-story structures built from aerated concrete or cinder block.
3. M 200. A commonly used brand of concrete for the foundation of a private house. It can be used when high-strength concrete is needed, as well as for the production of reinforced concrete structures.
4. M 250. This composition has a high cement content, so it can be used to create fairly strong structures.
5. M 300. Allows you to build the foundation of a house up to five floors high. This brand has a high strength rating, so it can be used for monolithic floors.
6. M 350. It can be used in the construction of critical structures, for example, for the construction of curbs, roads and other structures that must withstand significant loads.
7. M 400. Used for the construction of buildings up to 20 floors high. Therefore, it has high strength and resistance to mechanical damage.
8. M 450. Used for the construction of high-strength load-bearing floor slabs. Recommended for various structures that must withstand very heavy loads.
9. M 500. This brand is rarely used due to its high cost. It is advisable to use it only for the construction of cash vaults or other special facilities. However, it is this material that is recommended to be used if extreme strength and reliability of the structure is required.

If already available finished project structures, it indicates what grade of concrete is needed for construction, so choose yourself optimal material won't have to. The house design indicates all the characteristics of the mixture, so you just need to purchase the material specified in the documents.

For the foundations of some lightweight buildings, the use of the cheapest concrete is allowed, and for the creation of structures that must have a good degree of reliability, high-grade material or even Portland cement may be required.

Class

Concrete marking includes not only the brand, but also the class of the material. The concrete class is designated by the letter B, and shows information about compression ratio, waterproofness and other characteristics. Concreting the foundation should be carried out only using material of the required class. When choosing it, you should consider the following characteristics:

1. Mobility. Denoted by the letter P. This characteristic of the material indicates its homogeneity and flow rate. In this case, even a slight increase in water in the mixture can reduce its strength, which will lead to significant problems with the foundation in the future.
2. Waterproof. Denoted by the letter W. The indicator ranges from 2 to 12. The value of this indicator should be taken into account if there is a groundwater.
3. Frost resistance. Denoted by the letter F. This indicator characterizes the number of frosts and defrosts that the material can withstand. Particular attention should be paid to this indicator in the northern regions.

Therefore, in addition to the brand of the base mixture, it is also necessary to take into account its class. Before choosing a material, you should take into account the load on the foundation, the climate of the region, as well as geodetic features and the degree of soil heaving.

Video

This video explains the difference between the class and grade of concrete.

Criterias of choice

When choosing the optimal grade of concrete for pouring the foundation, the following factors must be taken into account:

grounds

When deciding which brand of concrete to choose, you should consider whether the house will have a basement, as well as the degree of soil freezing. When choosing a foundation, the depth of groundwater is also of great importance. Therefore, one should take into account such a material indicator as water resistance. The M 350 brand of concrete for the foundation of a house has a much higher water resistance than M 200 sand concrete.
However, you can save on material if you use a mixture of grade M 200. However, it is additionally recommended to use additives that increase the water resistance.

Load accounting

To determine what brand of material is needed, you should calculate the load that the base will constantly experience. In this case, it is necessary to take into account the maximum indicators. The weight of walls, ceilings, roof with construction system, furniture and household appliances, as well as the weight of people are taken into account.

For light structures, you can use concrete made from material grade M 150 or M 200. For the foundation of a structure made of timber, logs, gas blocks or ceramic blocks, it is better to use concrete of at least grade M 250. The base of brick and other heavy structures is best made from material grade M 350.

Climate and geology

The choice of cement grade is also influenced by soil characteristics. One of the important factors is It is typical for clay and loamy soils. When clay soil freezes, it increases in size, causing the load on the foundation to be uneven. In this case, it is recommended to fill the foundation with material grade M 300 or higher.

If the soil has a low heaving rate, then you can use a foundation made of M 200 concrete. This applies to sandy - rocky soil. This type of soil has a homogeneous structure, so the load will be distributed evenly.

Conclusion

If you have some construction experience, independently calculate the approximate cost of construction, taking into account the selected foundation, walls, ceilings, blood and even interior and exterior decoration.

However, choosing the required brand of material on your own is quite difficult. It is best to focus on the data specified in the house design, or contact specialists.

In contact with

To form a solid foundation in construction, it is necessary to pay attention to a number of factors. Take into account the geological features of the area and the quality of the material that will be used to make the base of the structure. Properties of the finished structure such as strength, water resistance, frost resistance and other characteristics are determined by the grade of concrete for the foundation.

The main components of the building mixture are cement, water, aggregates and various additives (plasticizers). When asking what is needed for construction, they take into account the ratio of each substance in the solution, which determines the physical characteristics of the hardened mass. Modern concretes are produced under markings from M75 to M500.

The most popular cement for foundations is M400 grade cement. Its marking may also contain the following letter designations:

• “D” - there are additives that modify the structure of the material;
• “B” - quick-hardening cement;
• “PL” - plasticizing;
• "GF" - hydrophobized.

Sand, gravel and crushed stone are used as fillers. To prepare mixtures of grade M300 and below, it is better to use fine-grained inclusions. Gravel is used as a coarse-grained component, although for a belt-type base a private house such material is rarely used.

Like other substances, water is added in a certain proportion to the other elements. Excess moisture in the solution will contribute to the formation of large pores during evaporation or, remaining inside the block, will press from the inside when freezing. For this reason, thinner concrete does not mean better concrete.

What grade of concrete is needed for the foundation?

In the practice of private construction, a strip foundation is considered the most preferable. Such a foundation is sufficient to ensure the support of a relatively light structure in difficult soils, under conditions of dynamic loads. Concrete is used as a foundation mortar.
When choosing a brand of concrete for pouring a foundation, you can use the following recommendations:

• For houses with a lightweight type of construction (frame or panel) - M200-M250;
• M250 mixtures are suitable for wooden buildings; for heaving locations, it is preferable to select M300;
• Residential buildings based on gas silicate blocks and similar materials will require for operation a foundation made of grade concrete M300 and M350;
• Brick or stone cottages will serve for a long time on a M400 base.

Before deciding which grade of concrete to use, it is better to consult with engineers.

Selection by strength

When selecting a brand of concrete for the strip foundation of a private house, it is necessary to take into account that the strength properties increase along with the increase in the value in the marking of the mixture. So, if you plan to use the M300 brand, the strength of such a structure will be at least 320 kg per cm².

Hardness indicators, depending on the class of concrete, are calculated. The meaning of the parameters is how much load the foundation can experience without destruction. You can calculate the approximate weight of the building according to the project. It is also necessary to take into account the pressure from people, furniture, and things. For winter, it would be advisable to take into account the snow load.

Depending on the soil

Before developing the area, one cannot ignore the result geological surveys. Of course, you can use ready-made research on neighboring areas, but this is not recommended. Where no construction has taken place, the soil may not perform well. The more heaving the soil, the higher the grade of concrete used should be, not lower than M350. The same quality of foundation will be needed for clay soils. In rocky areas, the foundation of a private house will sit firmly and reliably, without increased quality requirements building materials.

For areas where the groundwater level exceeds standard levels, it is better to plan to purchase a mixture with improved water resistance.

By type of foundation and structure

When purchasing concrete, a private developer usually focuses on the “M” grade indicator. The higher the value, the higher the strength of the finished structure will be. For example, if you plan to use the M350 grade of concrete for the strip foundation of a private house, its qualities will be quite enough for a brick house.

Typically, for residential private buildings, a lightweight type of base made of M200 or M250 materials is used. If you select a mixture for the foundation two-story house, it is better to start with M300 materials. When mixing the solution, there is one peculiarity: if you treat the concrete that has not yet hardened with vibrators, this will increase the strength of the material and improve its performance qualities.

Additional measures to strengthen the building include the formation of an underground floor. So, you can make not only the foundation, but also make a concrete base brick house or basement.

When discussing the topic of what brand of concrete mixture is needed for the foundation, it is best to proceed from the direct dependence of strength on the mass of the structure. For the heaviest houses, accordingly, M400 or higher is intended.

Other options

When choosing a brand of concrete mixture for the foundation of a house, they also take into account additional factors, which will affect performance characteristics. Today, manufacturers of mortars conduct tests for properties such as frost resistance and water resistance.

For definition, the “W” marking is provided, having a range from W2 to W20. The selection of a specific brand depends on the climatic conditions of the area.

Frost resistance, as another characteristic of concrete, determines the number of freezing and thawing cycles of the structure. The frost resistance grade is determined by the number after the letter “F”. For example, F150-F300 is preferable to use in the Far North. If the marking contains a frost resistance value above 500, it means that such concrete is intended for military and industrial facilities.

To build a reliable house, you need to build a solid foundation. Concrete, as the main component of any structure, provides the required resistance to mechanical stress, water resistance, and therefore increases the durability of the building and its foundation. Therefore, correct calculation and selection of brand allows you to build a high-quality foundation. Below are the main points of selecting a category for constructing the foundation of a residential building.

What does a concrete foundation consist of?

The composition of the ingredients in concrete for the base in a private house includes water, cement, sand, crushed stone or gravel. The fractional content of sand should consist of particles measuring 1.2-3.5 millimeters, but not smaller. The component must be free of silt and clay. However, impurity content up to 5% is allowed. Otherwise, the strength of concrete will decrease.

The size of gravel or crushed stone varies between 1 and 8 centimeters. The material must also be clean. Cement is selected from the following categories:

• Portland cement, suitable for the construction of any buildings on a monolithic foundation;
• slag Portland cement, characterized by increased moisture resistance, but instability at low temperatures and low speed;
• pozzolanic Portland cement, intended for the construction of foundations with close groundwater or in conditions of high humidity, for the construction of underwater and underground buildings, but in the air it loses its properties and causes severe shrinkage;
• quick-hardening derivatives of the categories of mixtures listed above, which shorten the maturation period of ready-made concrete.

The classic solution for the foundation of a private house consists of 80% fractional filler (sand, crushed stone or gravel) and 30% binder (cement with water).

Selecting a concrete grade based on strength characteristics

Concrete strength table.

In the presence of groundwater, solutions M350 and higher are taken for laying foundations from slabs and strips. It is recommended to use only chalked gravel as filler. For arrangement bored piles 200th and . It is suitable for small private houses and cottages. Their strength is sufficient if the mixing proportions are maintained exactly as required. This should result in a homogeneous solution that requires high-quality tamping by vibration. This will increase not only the strength of these brands, but also frost resistance and moisture resistance.

Other characteristics

To choose concrete, in addition to strength, soil quality, wall materials, other characteristics should be taken into account:

• Conditions of construction and operation.
• Frost resistance. The parameter is taken into account if the foundation is poured in an unheated house, for example, in private summer cottages for temporary residence. This value is also taken into account when constructing foundations and insulation around the perimeter of the house.
• . Compositions with this parameter, equal to W2, W4, are recommended for laying foundations on well-drained soils in the absence of groundwater. Class W6 is suitable for foundations on leaking soils. The remaining water resistance categories (W8, W10) are practically not used for private homes.
• Workability. This value determines the comfort of working with concrete. For pouring the foundation, a material with parameter P2 is recommended. If frequent reinforcement is carried out, the use of grade P3 is allowed. Category P1 concrete is also allowed for use, but this will require high-quality vibration of the masonry to settle it and achieve high homogeneity.

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 the strip foundation of a private house will 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 that requires 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 the 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 absorbing large amounts of 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). During construction residential buildings this brand is only suitable for pouring a foundation cushion;
• 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 is often used for the construction of supporting walls when it is necessary to terrace the 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 the production of reinforced concrete products - hollow cores or floors of column-transom reinforced concrete building frames, 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 groundwater levels.

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

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

Building weight

The weight of all the building materials involved in the construction of the house creates a load on the foundation strip. 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 mortars of at least M300 grade.

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 best option, otherwise you may incur unnecessary costs. The best thing is to order foundation calculations from experienced designers.