Of all existing foundations, the slab has the greatest versatility. In industrial buildings it can be assembled from prefabricated factory-made elements, but in civil engineering, including low-rise buildings, this structure is always monolithic. Its demand is based on the large support area, which allows for maximum balancing of the forces acting on the soil; moreover, the slab is suitable for almost any type of soil and eliminates the need to form a subfloor. Let's consider what kind of concrete should be used for the foundation slab.
Concrete grade for the foundation slab
Monolithic foundation slabs are included in the class of non-buried foundations. Concrete is poured into a pit, along the perimeter of which formwork is placed, with a reinforcement cage installed, on a compacted sand and crushed stone cushion. The height of the slab is from 0.2 m. A base of this type can be used on various soils. This is an excellent option on unstable soils. In order for the constructed foundation to last as long as possible, to be strong and reliable, it is necessary to select the appropriate grade of concrete for the slab foundation. There are a number of factors to consider when making your choice. Material and labor costs will depend on its correctness.
Main stages of construction
In principle, the technological procedure for pouring a slab foundation is not particularly complicated. This work includes several basic steps:
- markings are made at the site chosen for the construction;
- excavation work is being carried out;
- a crushed stone-sand cushion is laid at the bottom of the dug pit;
- formwork is installed;
- the reinforcement frame is installed;
- The concrete mixture is poured.
At the final stage, the formwork is removed from the finished foundation, and then, after the concrete gains strength, the construction of the building itself begins.
Features of a slab foundation
The foundation slab is used mainly in areas with problematic, unstable soils, in places where groundwater is close to the surface. Also, such a base is used when there is a risk of landslides or on bulk soils.
Pouring the slab
The technology for creating a monolithic slab is implemented in several stages:
- conduct a geodetic survey of the construction site;
- dig a pit of the required depth;
- lay out the formwork;
- the bottom of the excavation is covered with a sand-crushed stone cushion, the thickness of which varies from 0.1 to 0.3 m;
- knit reinforcement in two rows;
- fill the form with concrete.
A slab-type foundation is also called “floating” because it can withstand quite significant ground movements without affecting the construction. Also, such a foundation allows the construction of multi-story, heavy buildings.
The large consumption of building materials, and therefore money, for the implementation of the project in practice is considered the main disadvantage of a monolithic foundation. Construction work is also accompanied by greater labor costs compared to the creation of other types of foundations.
Choosing the highest grade of concrete will provide a high-quality result in terms of reliability, strength and durability, but financial costs will increase significantly. In most cases, this will not be justified from the point of view of the existing loads, therefore, in each specific case, the choice of concrete grade should be approached taking into account the factors acting on the foundation being constructed.
Classification of concrete
The main components of concrete are cement, water, filler and various additives. The first serves as a binder, with the help of which other constituent elements are connected.
Scheme of concrete varieties
The following materials can be used as fillers:
Fillers are designed to reduce internal stress in concrete as it gains strength (hardening).
The final strength of the monolith depends on the presence of water in the required proportion in the solution. Excessive amount of liquid leads to mechanical weakening of the created structure.
Concrete is classified according to the criteria given in the table below.
It is recommended to use compounds with an increased degree of frost resistance to avoid the appearance of cracks after several years due to temperature changes. The range of parameter values is from F25 to F1000. Frost resistance is an important parameter on very wet soils, where there is constant contact of the supporting structure with the soil.
The value of the water resistance coefficient ranges from 2 to 20. When constructing the foundation, choose W2 - W12, focusing on the groundwater level.
The final quality of the pour is determined by many factors. One of the main ones is the correct ratio of incoming components. High quality material should be used.
Areas of use of different grades of material
Concrete grades
The grade of concrete determines the construction area in which it can be used. It shows how a foundation that has gained full strength will resist operating loads. The brand is the main characteristic when choosing a building material; its strength is shown in kgf/cm2. At the same time, water resistance and mobility are reduced.
Among the existing grades of concrete (from M50 to M1000), M100-M450 have gained practical distribution.
The table below shows the grades of material and their characteristics.
Due to its high cost, the M500 is not popular. The most widely used concrete grade among private developers is M350. It is the best option in terms of the price of the material and its quality.
Factors determining the choice of concrete for a monolithic slab
When building lightweight structures on a slab, such as a garage, gazebo, barn, small frame or wooden house, the foundation material can be selected with an approximate consideration of the existing loads. Concrete grades M200 and M250 are suitable. Very rarely a slab foundation is made for light structures. Often they make do with pile or columnar types of foundation.
Monolithic slab foundation on sandy soil
To choose which concrete is best suited for creating a reinforced monolithic slab in a particular case when constructing heavier buildings, it is necessary to analyze a number of factors:
- geological conditions at the construction site: soil type, its structure, groundwater level;
- climatic features of the region: temperature regime and degree of air humidity;
- the weight of the building being erected and associated loads from snow, furniture, residents, household appliances and equipment.
All the factors considered must be taken into account in order to correctly determine the appropriate brand of concrete. Independent construction of the foundation (the entire structure) requires certain knowledge and practical skills in performing the work.
If the necessary experience is not available, then it is better to use the services of specialists who will not only competently construct the object, but also select the material suitable for specific conditions.
Ready mix in bags
How to pour a slab under the foundation: digging a pit
After the markings are ready, they begin to dig a pit. When constructing slab foundations, this procedure is usually performed using special equipment. Digging a foundation pit for a house by hand, of course, will be quite problematic.
Marking for a floating foundation, as well as digging a hole under it, should take into account the fact that the crushed stone-sand cushion must subsequently protrude beyond the slab by 20 cm on all sides. That is, the area of the pit should be larger than the very foundation of the house.
Foundations that are too floating when pouring, as already mentioned, are usually not deepened. Under the house, owners of country plots most often pour a slab about 40 cm thick. At the same time, the thickness of the sand and crushed stone cushions in total usually does not exceed 30-40 cm. Thus, a pit with a depth of 70-80 cm is most often dug under the slab foundation.
Concrete grades for creating a monolithic slab
In hard, rocky and sandy types of soil, the entire load from the structure is distributed evenly. To build lightweight buildings on them, you can use concrete grade M150 or M200. Heavier buildings will require the use of M300 or M350.
Clay soil is characterized by a high degree of heaving; its volume increases greatly when it freezes unevenly in different directions. In this case, only the M350 - M450 varieties are suitable.
On any soil, the heavier the building being erected, the higher should be the grade of concrete used to build the foundation for it.
In most cases, to pour a monolithic slab foundation, you can use concrete with the following performance characteristics:
- from F200 degree of frost resistance;
- mobility P-3;
- waterproof coefficient value, starting from W8;
- brand M300;
- class B22.5.
How to make concrete mixture B20 (M250)
A mixture of the same class can be made from different cements. The grade of binder must be higher than the grade of concrete, and in this case cements M400 and M500 are used. In the first case, 1 part of the binder is filled with 2.1 parts of sand and 3.9 parts of crushed stone. When using M500 cement, the C:P:SH ratio will be 1:2.6:4.5. The water-cement ratio for concrete of this class is 0.61.
A large filler for heavy concrete is crushed stone from dense rocks and screenings from their crushing, as well as slag from thermal power plants and metallurgical waste with a grain density of 2000 kg/m³. For M250 concrete, crushed stone of fraction 10-20 is used, but its mixture with one of the “neighboring” fractions is also allowed: 5-10 or 20-40. The content of clay and dust particles in coarse aggregate should not exceed 1%.
The fine aggregate is sand of natural origin or extracted by screening crushed rocks. It is also allowed to use sand from ferrous and non-ferrous metallurgy slags and fly ash.
Water for mixing concrete should not contain chemical impurities and compounds that can affect the hardening and setting time, provoke corrosion of concrete, and worsen its performance characteristics. You cannot mix the mixture not only with industrial water, but also with waste or peat water.
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The total amount of additives should not exceed 5% by weight of cement. They are introduced into the composition with mixing water.
If a concrete mixer is used to mix concrete, its volume is the guideline for which you need to calculate the number of parts. The mixing procedure is as follows:
- about 80% of the total amount of water is poured into the drum;
- a plasticizer is added to dissolve;
- the full volume of binder is poured in and mixed until laitance is obtained on the surface;
- sand is dosed;
- crushed stone is poured;
- the remaining 20% water is added.
The drum rotation time depends on the type of concrete mixer. For gravitational ones this is no more than 2.5 minutes, for forced rotation ones - 1.5-2 minutes. The time is counted from the moment the last ingredient is loaded. It is not worth increasing the rotation period of the drum: this will not improve the quality of concrete, and due to intense evaporation of water, it can negatively affect plasticity.
Calculation of the required volume of material for pouring the foundation
To determine the amount of concrete required for constructing a slab, you need to know its volume. It is found by multiplying the height by the width and length or by the area of the base.
The table below, according to GOST 7473-94, shows the correspondence of the class of concrete to its brand and the consumption of cement (M400) with a certain frost resistance, water resistance, mobility per 1 cubic meter of solution along with crushed stone, sand and water.
Knowing the volume of the slab, using the table you can determine the required amount of sand, crushed stone and cement to fill the foundation with the required strength. You can easily prepare the working mixture in the indicated proportions yourself.
The technology for constructing a slab foundation is presented in the video below. Recommendations for choosing crushed stone, sand and concrete grade for constructing the foundation are given in the video below. The correct choice of concrete grade used for the construction of a slab foundation will determine the service life, reliability and strength of the entire structure. The choice of material is determined by the geology of the site and the mass of the future building. Products with the required parameters can be purchased in the form of ready-made mixtures or prepared independently.
What is needed for construction
The choice of materials for pouring a slab under the foundation is a stage whose importance, of course, cannot be ignored in any case. Otherwise, the finished structure will be short-lived.
The owner of a suburban area who decides to build such a foundation for a house, some kind of outbuilding, or, for example, just a gazebo or porch, must first of all choose the right:
- brand of concrete;
- rods for tying reinforcement;
- crushed stone and sand for the substrate.
All these materials must meet certain requirements.
Choosing a brand of concrete for the foundation
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.
How to properly pour a foundation slab for a house: marking
In order for the finished foundation, and subsequently the building itself, to be smooth and neat, markings must be made at the selected location before starting construction work. For this procedure, among other things, you will need to prepare:
- pegs;
- a skein of non-elastic cord;
- hammer.
Marking for a slab foundation can be carried out, for example, using the two-arc method. In this case, a peg is driven into one of the proposed corners of the future building. Next, a straight line is drawn through the resulting “point”. Then the same distance is measured from the peg along the line in both directions and driven in here according to a temporary reference. At the next stage, take a not too long rope with nails tied at its end, fasten it one by one to the outermost landmarks, stretch it and draw arcs that will have to intersect opposite the central peg. Next, a line is drawn from the found point to the central peg, thus obtaining the required perpendicular.
At the next stage of marking, using a construction square, drive a peg into the remaining three corners of the future foundation and pull the cord. After this, the evenness of the corners is checked using the diagonal method. That is, they stretch the cords diagonally between the corners, tying the latter in the center. Then they check that the lengths of the quarters of the resulting “cross” match.
Compound
Foundation concrete is made from many components, which can be divided into groups:
- Mineral component. These are mineral elements, in particular sand, gravel and crushed stone. The stone skeleton is responsible for the load-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%.
- 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.
- M 100. This composition can be used in the initial stages of 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.
- 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.
- M 200. A frequently 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.
- M 250 . This composition has a high cement content, so it can be used to create fairly strong structures.
- 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.
- M 350. 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.
- M 400. Used for the construction of buildings up to 20 floors high. Therefore, it has high strength and resistance to mechanical damage.
- M 450. Used for the construction of high-strength load-bearing floor slabs. Recommended for various structures that must withstand very heavy loads.
- 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 you already have a ready-made construction project, it will indicate what grade of concrete is needed for construction, so you won’t have to choose the optimal material yourself . The house design indicates all the characteristics of the mixture, so you just need to purchase the material specified in the documents.
It is important to know!
The slab foundation is also called a solid, floating or Swedish slab. Shallow solid foundations are very often used in low-rise housing construction. The laying of this foundation is carried out on literally any soil. Thanks to the slab, the cost of work is significantly reduced, and it is ideal for literally any structure. In Russia, reinforced concrete foundations of this type have only been used for a few decades, while in Europe they have been used for a long time.
Thanks to the complex production of the slab, it is possible to obtain an insulated, effective foundation with general engineering systems in a short time. The built-in stove has a large number of advantages, i.e. it has a fairly high load-bearing capacity and is simple in design. This foundation will not be subject to large deformations after changes in the condition of the soil under the building. Even if the soil is difficult, the slab will be the most rational, only here you should take into account which concrete for the foundation of the house will be the best.
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 the degree of compression, water resistance 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:
- Mobility. It is designated 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.
- Waterproof. It is designated by the letter W. The indicator ranges from 2 to 12. The value of this indicator should be taken into account if there is groundwater on the site.
- Frost resistance. It is designated 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.
The final stage of pouring
Immediately after pouring, experienced builders recommend covering the entire area with polyethylene over the monolithic foundation slab. Subsequently, the concrete structure should be watered with water from a hose twice a day for 2 weeks. Such measures will prevent the appearance of surface cracks on the finished slab. And this in turn will improve the performance of the foundation.
The formwork from the finished structure can be removed within 2-3 days after the pouring procedure. In the future, the foundation must stand for at least a month. It is allowed to begin building the walls of a house on it at least a month after pouring. During this time, the hardened concrete will gain sufficient strength to support the weight of the building frame and its roof.
Criterias of choice
When choosing the optimal grade of concrete for pouring the foundation, the following factors must be taken into account:
Base type
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 sand concrete M 200. However, you can save on the material if you use a mixture of the M 200 brand. 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 a strip foundation 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 grade M material 350.
Climate and geology
The choice of cement grade is also influenced by soil characteristics. One of the important factors is soil heaving. 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 and rocky soil. This type of soil has a homogeneous structure, so the load will be distributed evenly.
Principles for calculating foundation thickness
A monolithic slab is considered the most expensive type of foundation, although it is the least labor-intensive. A foundation slab for a house made of aerated concrete can be installed in the shortest possible time. All operations are mechanized: excavation work is carried out using specialized equipment - graders, bulldozers. Concrete is poured using an automatic mixer. However, the construction of the thinnest foundation will require about 25 cubic meters of concrete, and this is a huge mass - 50–55 tons.
It is advisable to install such a foundation in two cases: when you, together with a group of other summer residents, are developing a new site, and when the house is being built on specific soil: then there is simply no alternative to a monolithic slab.
Conditions for slab foundation
The slab foundation differs from all others in its amazing versatility. A monolithic concrete slab, despite its massiveness, remains stable on the most problematic soils.
Slabs, as foundation foundations, are indispensable in permafrost conditions, on swampy non-reclaimed lands, on peat bogs, and on mobile loess soils.
The monolithic concrete pad has a large area, so it does not sag on any soil. Moreover: if such a foundation is built on heaving soil, then it will absorb all the loads:
- when the soil freezes, when the ice crumbs increase in volume, the slab will rise along with the house;
- when the ground thaws, the slab will take its previous position - also together with the house.
Thanks to this property, a foundation made of monolithic slabs is called “floating”.
Construction of a monolithic foundation slab
The classic design of a monolithic foundation consists of the following elements:
- sand substrate 100–300 mm thick - a layer of sand that acts as a shock-absorbing lining;
- gravel or crushed stone layer 200 mm high - a mass that distributes the load;
- footing - a thin layer, no more than 20-50 mm, - a rough base for a concrete mass;
- insulation – 100 mm padding made of polystyrene plates; can also act as waterproofing;
- waterproofing layer - polymer or roofing material flooring;
- monolithic slab reinforced with a three-dimensional two-level steel mesh: its thickness can reach 600 mm.
Mixing the solution yourself
As already mentioned, to pour a monolithic foundation slab for small structures, the owners of suburban areas often prepare the concrete mixture themselves. When carrying out this procedure, it is important to follow a certain procedure.
Experienced builders recommend mixing the concrete mixture for the floating foundation of a building only in a concrete mixer. Unfortunately, it will not be possible to prepare a very high-quality solution in a trough using a shovel. Concrete mixed in a concrete mixer is as homogeneous as possible. And therefore, the supporting structure for the house itself will subsequently become very durable.
When using M400 cement to prepare a mortar, owners of suburban areas usually maintain a cement/sand/crushed stone ratio of 1/3/4. The sand is thoroughly sifted before being used to prepare concrete.
First, load the dry ingredients into the concrete mixer and swirl them thoroughly. Next, add water in 1-2 steps. The amount of the latter is calculated in such a way that the finished concrete does not flow off the shovel and does not crumble into pieces after being compressed in the hand.
What brand of concrete is needed for a slab foundation?
What kind of concrete is needed for a slab foundation is a question asked by most owners of suburban areas who are planning to build a house. Usually the foreman helps in selecting the brand of concrete when calculating the estimate, but how can you be sure of his competence and be confident in the quality of the constructed foundation?
In this article we will tell you how to choose the right brand of concrete for the construction of any type of foundation.
Determining the grade of concrete for the foundation based on the load.
First, you need to understand what material the building will be built from, since this parameter directly affects its mass and therefore the load of the foundation. During frame construction, the load is usually small and sometimes even M200 grade concrete is sufficient. If you are building a wooden house from logs or timber, then you need to choose a mixture not lower than M250. Walls made of foam concrete are similar in weight to wood, so you can choose M250. When using gas silicate blocks, stronger concrete is needed - M300. And if you decide to build a more solid structure from sand-lime brick or precast reinforced concrete, then the foundation must be strong and consist of concrete grade M350 or higher.
What is a pillow for?
During operation, slab foundations are known to be significantly exposed to the adverse effects of environmental factors. First of all, such structures are extremely negatively affected by soil moisture. Also, foundations, including slab foundations, are annually subjected to the destructive effects of spring heaving.
In order to minimize these negative factors, layers of sand and crushed stone should be placed under the foundation of the house. This step is mandatory when constructing a slab foundation for a house.
First, crushed stone is placed in the dug pit under the floating base. This material will subsequently serve as a kind of layer between the wet soil and the foundation slab. Crushed stone laid in the pit will prevent concrete deterioration due to capillary rise of moisture, that is, it will act as drainage. Sand is also poured on top of the layer of this material before pouring the floating foundation. Such a pillow will subsequently serve as an excellent shock absorber during spring heaving.
What kind of concrete is needed for a slab foundation depending on the soil?
The grade of concrete may vary depending on the condition of the soil. The most suitable soil for construction is sand or rocky soil - here the grade of the mixture can be left at the minimum calculated level relative to the wall material. When building on clay or loamy soils that are prone to movement, the grade of the concrete mixture should be increased by one level.
For example: you are building a country house made of foam concrete, calculations have shown that it is optimal to use M250 grade concrete, but the construction will take place on an area with clay soil - in this case, it is necessary to increase the strength of the slab foundation and choose the M300 class.
Which concrete to use for the foundation based on its type.
The type of foundation is one of the important factors for determining the grade of concrete. The main point when choosing is the presence of a basement that requires the proper level of waterproofing. Each brand of concrete has its own water resistance index, for example, for M250 it is two times less than for M350. And in order for the foundation to be reliable in every sense, it is necessary to take this factor into account.
Final provisions when determining the grade of concrete for the foundation.
Perhaps the most important factor, both when choosing a brand and during construction, is the right choice of contractor. An experienced master will professionally help you with your choice, explain why it was made in favor of a particular mixture, accept the concrete, control its quality and you will receive a reliable foundation without unnecessary overpayments for a high-class mixture. Our team specializes in the construction of foundations in St. Petersburg and the region. Strip, slab, foundation with a plinth - any type of foundation will be performed at the highest level.
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