Optimal grades of concrete for the construction of a foundation slab
Almost all low-rise and most high-rise buildings in Russia stand on strip or block foundations made of heavy concrete. This is not surprising, given that heavy concrete, due to its plasticity, high load-bearing capacity and affordable cost, is the preferred material for constructing foundations for low-rise and high-rise buildings.
The building structure - the foundation, is its basis, taking on the weight of: walls, roofing, facade cladding, interior decoration, floor slabs and other structures and elements of a private house, cottage, villa or small country house. Therefore, saving on materials for its construction is unacceptable!
The proportions of cement for the foundation, as well as the proportions of other components of concrete for pouring it, must correspond to the accepted brand of concrete. And the accepted grade of concrete must correspond to the load on the structure being built.
Classification of concrete solutions
The composition of concrete includes cement, water, filler and special additives . Crushed stone, sand, gravel and other bulk materials can be used as filler. The additive in the solution serves to reduce internal stress during hardening.
The strength of the finished monolith depends on the amount of water in the mixture. Concrete is classified according to characteristics, which in turn are determined by the ratio of components.
Criteria for assessing the quality of concrete:
M – grade from M100 to M500 . The indicator reflects the performance of the solution.
P – mobility of concrete (from P1 to P5) . For standard slab foundations, concrete with indicators P3 and P4 is chosen.
B – composition strength from B3.5 to B60 . The value of the indicator is chosen based on the maximum load on the foundation. Guaranteed strength means the load that a monolithic base can withstand in 95% of cases.
W – indicator of concrete permeability to water (from W2 to W12) . When choosing a solution class, they are guided by the level of groundwater below the supporting base area.
F – frost resistance from F25 to F1000 . Experts recommend using a class of concrete with a high degree of frost resistance to eliminate the possibility of cracking of the monolith as a result of sudden temperature changes.
Thus, the final quality of concrete depends on a number of different indicators, the optimal characteristics of which are selected based on the design conditions of construction.
Below are the grades of concrete whose characteristics are suitable for the construction of a slab foundation:
Degree of mobility
Frost resistance degree
Recommended mixture composition for foundation
For the construction of the foundation, a concrete mixture of the highest quality is used, since the durability of the garage structure depends on the foundation. When choosing components, you need to be guided by simple but important rules:
The base requires the best cement: without debris, lumps and unnecessary impurities. It is also not recommended to purchase material long before construction. If storage conditions are not observed, it quickly deteriorates. The best time to purchase is considered to be 1-2 weeks before the start of construction.
Sand is recommended to be used with a minimum moisture content. Before mixing concrete, the raw materials can be dried or the amount of water intended for the solution can be reduced. However, such calculations should be trusted to a professional.
Crushed stone or gravel for the foundation is not required to be large, no more than 2 centimeters in diameter.
Important! To create high-quality concrete, it is necessary to maintain proportions and take into account the influence of external data. It is recommended to entrust such a troublesome task to specialists.
Brands and strength classes suitable for slab bases
When choosing concrete, the first thing to pay attention to is the brand and strength of the building mixture . The indicators give an idea of the wear resistance of the finished foundation under the influence of operational loads.
For slab foundations, the optimal grades of concrete are M250–350. When construction is planned on clay soil or other type of soil with a high degree of heaving, it is advisable to use concrete M350–M450.
Experienced builders are guided by the principle that the heavier the structure, the higher the strength of the mortar for a monolithic slab under it should be.
The video will tell you about the brands and classes of concrete:
What to use when building a foundation
We have to remember that the soil composition may differ in certain characteristics. For dry soil, it is better to prepare a solution from cement grade 500 D0, and you will get a mixture of grade M 350.
You can use the help of experienced builders who will tell you which brand of cement is best to use in a particular case, in what proportions to mix, and what sand dispersion to choose.
As a rule, M 400 and PC 400 with a D20 indicator are used in private construction for the construction of a one-story house and utility room. For such foundations, the maximum load is not the main indicator, and cement of this brand is considered an ideal solution that saves money.
M 400 and PC 400 D0 are almost similar in properties to D20, but do not contain impurities. And if you are building a foundation that must be used in a humid environment and in severe frosts, and do not know which cement is better - with or without additives, use “clean” material.
M 500 and PC 500 D20 are good cement for foundations for large industrial and administrative facilities, high-rise residential buildings. For construction in frosty and humid regions, it is better to use PC 500 D0.
Which one to choose for a monolithic slab?
For concreting a monolithic slab, use a solution of a class not lower than B22.5. In this case, the optimal water resistance coefficient should be W8 and higher. When implementing high-quality thermal and waterproofing of the base, this indicator in rare cases can be reduced to W6.
Under given conditions, choose concrete with frost resistance from F200 and a degree of mobility from P3 . If groundwater lies close to the surface, then sulfate-resistant mixtures are added to the solution, and waterproofing of the sole of the monolithic base must be considered.
To get a high-quality foundation, the builder must understand which concrete will be optimal under given conditions. In addition to a well-chosen ratio of components and adherence to manufacturing technology, tamping after pouring is of fundamental importance to remove air bubbles and maximize concrete compaction.
In this case, the process of pouring the mixture into the formwork itself must occur simultaneously, since during layer-by-layer concreting the surface has time to set and the structure of the finished monolith will be heterogeneous, and, therefore, less durable.
Recommendations for self-filling.
To make concrete, you should use a concrete mixer, a wooden box, an iron bath or wood flooring. It is important to ensure that no debris or other impurities get into the solution. First, pour in the dry ingredients, such as sand, crushed stone, cement, and then stir them well so that the composition is homogeneous. Then we begin to slowly add water, stirring until it is evenly moistened. Then you can start filling.
Proportions of cement and sand for the foundation of a house.
Keep in mind that cement cannot be stored for long; over time, its grade decreases as it begins to react with moist air. After the first month, it can lose about 10% of its strength, and after 3 - 2%; in six months, about 30% is lost.
For pouring a foundation yourself, the most popular unit of measurement is a bucket, so it is unlikely that you will accurately measure the correct proportions. It is not recommended to stir the mixture with a shovel, as this will not be effective and the concrete will turn out uneven. Thus, losses may occur, and the result will be the M100 brand, but this will be enough to build a small house or gazebo. M100 grade cement will withstand a load of 100 kg/cm2.
Reference! Even if you deviate quite a lot from the standards for pouring a foundation, working with your own hands, it will still be durable and will withstand quite heavy loads, so it will be suitable for building a house. But according to official standards, you should take cement grade 300 or 400 in order to get grade 200 or more.
Pour the foundation in warm weather, as freezing temperatures can cause some difficulties. For example, you will have to heat the water and the composition itself, since they may freeze before hardening begins. And when the hardening process begins, without heating the concrete will begin to freeze due to the water contained in it, and the resulting ice crystals will begin to destroy it from the inside.
Cement, sand, water proportions for the foundation.
If you correctly observe the ratio and dilute the composition diligently adhering to the standards, the solution will turn out to be homogeneous and as close as possible to the correct version, even at home, so do not worry about its strength.
Now you know what proportions are needed to prepare 1 cube of concrete. The process may seem complicated, but you just need to correctly calculate the ratio and try to follow the technology. Use the acquired knowledge to build a house or gazebo, because concrete is not the most difficult material to prepare, so just an attentive person can do everything on his own.
Factors determining choice
To determine which concrete is best suited for the design conditions, it is necessary to evaluate a number of factors:
Geological features of the site: type and physical and mechanical properties of the soil, level of occurrence of underground sources;
Climatic conditions: temperature in the region, average air humidity.
Design weight of the structure: the weight of the house itself, the load from snow, the weight of furniture, people, equipment, the weight of the roof, attic, etc.
Composition of foundation concrete.
The solution that is used to fill the base of the building has the following concrete composition.
Cement varieties: Portland cement, slag Portland cement mixture, pozzolanic Portland cement and their rapid hardening variants.
They are identical in their natural composition, but each of them has a number of features. The most common is Portland cement, used for the construction of any buildings; the creation of a monolith is also possible.
What is the proportion of cement to sand for the foundation.
The Portland slag cement mixture has high moisture resistance, but low frost resistance and hardens more slowly.
The pozzolanic option is ideal for construction characterized by high levels of humidity - underwater and underground structures. When used in air, the shrinkage of such a base is too large, and there is a partial loss of strength.
The choice of cement grade depends on the design grade of the concrete mortar and has the following markings: 200, 300, 400, etc.
This indicator is based on the possible tensile strength achieved during compression in kg per square meter. cm cube of concrete with an edge of 20 cm after it hardens after 28 days.
The concrete grade should be 1.5-2 times less than the cement grade.
Sand material is a finely dispersed aggregate. It should be free of clay impurities, silt (their content should not be higher than 5%) and plants.
Sand and gravel mixture for foundation concrete.
Too much silt and clay will negatively affect the strength of the foundation. For a concrete mixture, aggregate with sand grains from 1.2 mm to 3.5 mm, not smaller, is used.
The purity of this filler is checked as follows: it is poured into a bottle, filled with water and shaken. The presence of clear or slightly cloudy water indicates its purity.
Very cloudy water or the presence of clay sediment when sand settles - preparing concrete using such sand is unacceptable.
For a more detailed look at the components of concrete, watch the video:
Crushed stone is crushed pure rocks without contamination with earth and clay, the fractions of which should be 5-70 mm.
Contamination affects the deterioration of the quality characteristics of the produced concrete for the base.
The gravel is selected clean without a large amount of clay impurities, the fraction of which is from 1 to 8 cm. This material serves as a substitute for crushed stone.
Pure tap water without impurities.
The cost of the solution will vary depending on the quality of the raw materials . Practicing builders do not recommend saving on this expense item, because the reliability and service life of the entire structure ultimately depends on the strength of the load-bearing structure.
The table below shows average market prices for concrete, which, under various given conditions, can be used to construct a slab foundation:
The proportion of sand and cement required for the foundation mortar.
Knowing what the result should be and how the concrete should be used, the ratio of all components of the concrete solution is calculated.
The most commonly used mixture is in proportions of 1:3, where 1 is cement powder and 3 is sand. When adding more sand, the solution becomes stronger, but loses its elasticity.
This will make it difficult to apply to the surface and increase the likelihood of it falling off even with little mechanical impact.
The quality of concrete is also affected by the brand of cement. Different construction jobs require different markings of cement. For example, when using composition M-100 and cement M 400, the proportion will be 1:4.
What brand is needed to build a private house?
When a solution is prepared in a factory, many associated factors are taken into account when applying markings.
Individual developers use generally accepted recommendations for the use of a particular brand of concrete. As a rule, a solution of M300 grade is prepared and brought to the desired specification using additives.
What does the choice depend on?
The slab foundation is suitable for the construction of residential two-story houses, one-story cottages and various lightweight buildings on soils with any degree of heaving and groundwater levels. To satisfy the design conditions, the builder must evaluate the input data and select the optimal parameters for the foundation.
In order to avoid mistakes that will affect the quality of the foundation, it is advisable to contact a specialized company.
But sometimes private developers do not have this opportunity, so you have to trust generally accepted standards for general conditions. The table shows the recommended grades of concrete for the construction of a monolithic slab for various purposes:
Type of construction
Lightweight structures: garage, gazebo, shed
One-story frame-panel wooden houses
Two-story buildings made of timber and foam concrete
Heavy houses with walls made of brick or reinforced concrete
Construction of heavy structures on unstable soils
Optimal proportions of concrete
During the manufacturing process of concrete, the composition, or rather the proportions of the main components, is determined by its original purpose. Some brands of concrete cannot be used in the construction of foundations due to their weak strength.
The general proportions for high-quality concrete look like this:
10 kg of cement;
30 kg of sand;
40-50 kg of gravel or crushed stone to choose from.
To recreate concrete mixtures, for example, grade M300 or M400, the total weight of all components must be twice the weight of the water used. Thus, in standard proportions, 80-90 kg of dry mixture is used, which will require 40-45 liters of clean water.
If, when making concrete with your own hands, the solution turns out to be too dense, you should add a little liquid to achieve the right look. It is extremely important that the solution has a high viscosity, otherwise the mixture can be considered spoiled.
The resulting mixture, in consistency, should be easy to mix with a shovel, but also require some effort. (see photo above)
It is worth noting that wet sand contains a certain amount of liquid. In order not to spoil the cement mortar, you should first dry the sand or subtract a few liters of water. Also, you should not leave sand in buckets for a long time so that it does not become saturated with moisture from the air.
Is it possible and worth mixing it yourself?
You can mix the solution with your own hands using a container and a shovel . This is a labor-intensive undertaking that will take a lot of time, but the developer will be able to save on renting a concrete mixer.
The main inconvenience is concentrated at the moment when the finished mixture must be quickly transferred to the place where the slab foundation is laid.
You can do the job efficiently with your own hands only if there is no need for a large amount of solution.
Since concreting must be done simultaneously in the shortest possible period of time, before the surface of the mixture has time to set, several more construction workers will have to be involved . Otherwise, it is impossible to achieve high density and strength of the monolith.
It is much easier and more convenient to rent special equipment for mixing components. This approach will ensure uniformity of the material, and will also save time and effort for the developer. Mixing sand, cement, crushed stone and other additives with water in a mixer will eliminate the possibility of lumps forming. In this case, it is convenient to install the unit in close proximity to the formwork.
The properties of concrete depend on the quality of the original components and the relationship between them . The main substance is cement, on which the strength of the finished monolith largely depends.
As a rule, Portland cement is used in private housing construction. There are other options, but they either do not match the characteristics or are too expensive.
Clean sand and crushed stone with a minimum amount of clay inclusions (up to 5% of the volume of the material) are used as filler. The optimal particle size for sand is 1.2–3.5 mm, crushed stone is 10–20 mm.
Water functions not only as a solvent, but also as a main reagent . Therefore, to prepare the mixture, it is necessary to use only purified water (possibly from a well) without impurities of petroleum products and oils.
If construction is carried out in winter, the water must be heated to 40C and vice versa, in hot weather it is cooled to a cool state.
The table shows the ratio of the main components for the preparation of concrete grades that are suitable for the construction of a slab foundation:
The amount of special additives is selected separately in each specific case, depending on the required characteristics of the mixture.
Mixing in a concrete mixer
Before using the mixer, you need to make sure that the equipment is on a level surface. First, the equipment is put into operation, and then the ingredients are added one by one .
The operator must ensure that the pear is tilted at an angle of 45 degrees during the kneading process.
Stages of preparing the solution:
Pour in water and add cement.
Pour out sand and add water if necessary.
Rubble is poured out.
The total time for preparing the mixture should not exceed 10 minutes, otherwise the solution will begin to dry out. The freshly prepared mixture is immediately used for concreting a monolithic foundation.
To begin with, let's define the concept of a strip base. This type of foundation is perhaps the most popular in the field of private construction. Such a base is supplied under all load-bearing walls of the building, including internal ones.
It is customary to distinguish 2 types of base:
Stages of installing a shallow monolithic tape
Recessed foundation for the construction of a cottage
Any tape is laid in the ground on a prepared substrate, which can be made of sand and crushed stone or filled with concrete. The strip foundation must be reinforced using steel or composite reinforcement. A monolithic strip foundation is poured into the formwork.
Pouring concrete mortar into prepared formwork
A very important point when forming a monolithic base is the use of the correct solution. It is the grade of concrete on the strip foundation that largely determines the strength and performance characteristics of the monolith.
We will tell you how to choose the right concrete below, and you can learn more about the instructions for pouring a shallow strip foundation by watching the video in this article.