Reinforced concrete solid foundation. Do-it-yourself foundation: step-by-step instructions for building a foundation yourself Types by design

They are, first of all, the type of house (brick one-story, brick more than 2 floors, foam concrete house, light frame or panel house, etc.) which determines the weight of the building, and accordingly its pressure on the ground. In addition, the determining parameter is also the type of soil, the depth of seasonal freezing and the groundwater level.

When the load on the foundation is high and the soil is weak, solid foundations are used. They are a continuous reinforced concrete slab under the entire building area.

There are two types of solid foundations: slabs and cross-strips. They are erected from monolithic reinforced concrete to give the foundation spatial rigidity.

Slab foundations, erected under the entire area of the building, are a solid or lattice slab made of monolithic reinforced concrete or prefabricated cross reinforced concrete beams with rigid sealing of butt joints. Most often they are used in weak, heterogeneous soils with a high groundwater level, as well as in cases where the load on the foundation is large and the foundation soil is not strong enough. Such a foundation can withstand all vertical and horizontal movements of the soil well; the slab moves with it, protecting the house from destruction. Thanks to this feature, it received another name - floating.

The use of foundations of this type is practiced mainly in low-rise construction, as well as in small and simple building shapes. They are usually built on problematic soils: wet or with a high groundwater level. The disadvantage of this type of foundation is its cost; they are quite expensive due to the high costs of excavation work, concrete and metal reinforcement. However, if the use of foundations of another type cannot provide the house structure with the necessary stability, it is advisable and, more than justified, to use solid foundations. This applies to small and compact houses or other buildings when a high base is not required, and the slab itself is used as a floor (for example, garages, bathhouses, etc.). For higher-class houses, foundations are often installed in the form of ribbed slabs or reinforced cross strips.

When constructing solid foundations, there are some points that need to be given special importance. First of all, this concerns the process of preparing the base for the foundation. The top fertile layer of soil has a very low bearing capacity and also tends to be compressed. Therefore, it must be removed to a depth of 0.5 meters. A mixture of crushed stone and sand in a ratio of 60/40, respectively, is used as a cushion. A well-laid and compacted cushion reduces the likelihood of swelling of the lower part of the foundation when the temperature drops. Secondly, it is necessary to evenly distribute the pressure on the ground and allow groundwater to pass unhindered under the building. The sand cushion does not accumulate moisture, and the deep-set building prevents the ground from freezing in winter, thereby providing greater stability to the house.

The thickness of reinforced concrete foundation slabs is calculated during design, usually 200-300 mm. Reinforcement is performed in the upper and lower parts of the slab from rods with a thickness of at least 12 mm in increments of 200*200 or 300*300 mm. The thickness of the sand cushion is selected depending on the bearing capacity of the soil, groundwater level and the weight of the house, ranging from 200 to 500 mm.

Foundations for low-rise construction are made from local building materials (natural stone, rubble concrete, red brick, etc.), and they also use monolithic concrete or prefabricated concrete and reinforced concrete blocks.

The plane of the lower part of the foundation is called sole(Fig. 3.1), its broadening is pillow, and the horizontal plane of the upper part of the foundation is with a sawn-off shotgun. In the absence of basements and large pits, shallow foundations are usually designed, the base of which is located at a depth of at least 0.5 m from the ground level. On soils that swell when frozen, the depth of the base of the foundation of the external walls is taken to be at least 0.2 m below the thickness of the freezing layer.

There is a certain relationship between the architectural and planning solution of a low-rise building, the design of the foundation and the condition of the soil. For example, if an architect envisions a basement, large pit, or basement in a house design, then the foundation must be of a strip structure in order to successfully serve as a basement wall. The condition of the soil can influence the choice of architectural solution for the underground part of the house. For example, if a house is placed on soils with a high level of groundwater, then the thickness of the walls of the strip foundation increases due to additional waterproofing elements, which leads to a slight reduction in the area of the underground premises. In addition, there may be a threat of the basement part along with the house or part of the house with the pit rising (“floating up”) under the influence of groundwater pressure. In this case, it is usually necessary to abandon the design of underground premises or to design an expensive foundation structure with anchors in the ground or a weighted floor of the underground premises.

The most important parameter on which the shape and volume of foundations depend is foundation depth.Foundation depth- Thisdistance from the ground surface to the base of the foundation.

The depth of foundations depends on many factors: the purpose of the building; its space-planning and design solutions; magnitude and nature of loads; quality of the base; surrounding buildings; relief; accepted foundation designs and methods of construction work. However, first of all, deepening will determine the quality of the foundation soil, groundwater level and soil freezing.

The minimum foundation depth for heated buildings is usually 0.7 m for external walls and 0.5 m for internal walls.

The practice of operating low-rise residential buildings with shallow foundations has shown that soils that swell when frozen gradually push such foundations out of the ground. Over the course of several years, a house can rise above ground level by tens of centimeters, while different sections of the building usually rise by different amounts, which leads to skewing of windows, doors and even breaking of walls. This phenomenon occurs from the action of lateral friction forces of swelling soil on the surfaces of foundations, which exceed the resistance of the relatively small mass of the house. To neutralize the undesirable effect of swelling when the soil freezes, it is necessary to design houses without basements on shallow foundations with a base in the form of a sand cushion. When installing a sand cushion, the soil is removed to a depth below freezing of at least 0.2 m and the excavation is filled with coarse sand, poured with water and compacted layer by layer. Backfilling is carried out to a level of 0.5 m from the site planning level. Shallow foundations are installed on the artificial foundation obtained in this way. This technique allows you to achieve significant savings in materials and costs. For example, in the Kyiv region, the depth of soil freezing is 0.9 m, therefore, a shallow foundation will be 1.1 m high, and with a sand cushion - 0.5 m, i.e. with a sand cushion on soils that swell due to freezing, about 50% of the material for constructing the foundation is saved.

According to the method of construction, foundations can be industrial or non-industrial. In mass construction, industrial foundations are used, which are made from prefabricated large-sized concrete or reinforced concrete elements. These foundations allow work to be carried out without seasonal restrictions and reduce labor costs on the construction site. Non-industrial foundations can be made of monolithic concrete or reinforced concrete, as well as small-sized elements (brick, rubble stone, etc.). Foundations of this kind are used, as a rule, for non-standard buildings.

By the nature of their work, foundation structures can be rigid, working only in compression, and flexible, which are designed to absorb tensile forces. The first type includes all foundations, with the exception of reinforced concrete ones. The use of flexible reinforced concrete foundations that can withstand bending moments can dramatically reduce the cost of concrete, but sharply increases the consumption of metal.

According to the structural design, foundations are divided into strip, columnar, pile and solid.

Install under all load-bearing walls of the building strip foundations in the form of solid walls. They can serve not only as a load-bearing structure that transfers permanent and temporary loads from the building to the foundation, but also as an enclosing structure for basement premises.

Strip foundations they are installed under all main (load-bearing and self-supporting) walls, and in some cases under columns. They are strip-walls sunk into the ground with a rectangular or stepped cross-section.

Strip foundations have become widespread in residential construction for buildings up to 12 floors, built using a frameless design.

The shape in plan and section, as well as the dimensions of the strip foundation, are set so as to ensure the most even distribution of the load on the base. The size of the foundation base is determined by calculation depending on the mass of the above-ground part, the foundation material and the bearing capacity of the soil. The thickness of its wall is determined by calculating strength and depending on the technological features of the material, for example, a wall made of rubble concrete is made at least 0.35 m thick, depending on the size of the filling stones. It is necessary to ensure that the resultant of all loads from the building passes in the middle third of the width of the base of the foundation, i.e. e< 1/3 (рис.3.3). Этим самым исключается появление в фундаменте растягивающих усилий.

Depending on the magnitude and direction of the design loads, strip foundations can be symmetrical or asymmetrical (Fig. 7.3).

Fig.7.3. Strip foundations: a – plan and section of a strip foundation made of prefabricated concrete blocks for a building with a basement; b, c – options without a basement made of solid and hollow blocks; d, e, f – design of a rigid foundation with a minimal, normal and maximally widened base; g – asymmetrical foundation; and – transition from one foundation depth to another; k, l, m, - options for strip foundations made of monolithic concrete, rubble concrete and rubble; 1 – basement wall blocks; 2 - hollow wall blocks of basements; 3 - foundation pillows; 4 – walls; 5 – floors; 6 – basement floors; 7 – blind area; 8 – concrete foundation; 9 – rubble concrete foundation; 10 – rubble foundation; 11 – floor of the first floor.

For the manufacture of strip foundations, any building materials except wood are used. On rocky soils, monolithic concrete with the inclusion of rock fragments (rubble concrete) is more often used. This material better fills uneven surfaces of the rock base. Rubble stone foundation strips are characterized by lower cement consumption, but are more labor and material intensive. Due to the size of the stones, according to the standard, the minimum width of the strips is taken to be no less than 0.5 m. As a rule, the walls of strip foundations made of these materials for low-rise buildings do not have widening in the area of the soles. Strip foundations made of red brick are designed for dry, strong soils with a thickness of 0.25 - 0.51 m. It is better to make the brick foundation pad from monolithic reinforced concrete with a thickness of at least 0.1 m, which increases the durability of the structure.

In mass construction conditions, strip foundations are usually erected from prefabricated concrete or reinforced concrete elements. Prefabricated strip foundations are assembled from two types of blocks (Fig. 7.4) - foundation pillow blocks (FBP) and wall blocks (FSB). The latter are made solid from lightweight concrete (γ ≤ 1600 kg/m 3) or hollow from heavy concrete (γ > 1600 kg/m 3), which can be used for internal walls and for external ones in soils not saturated with water. Wall blocks are used in the following sizes: height 0.6 m, length up to 2.4 m and width 0.3, 0.4, 0.5 and 0.6 m.

Fig.7.4. Prefabricated strip foundations: a – foundation design for weak soils; b – laying foundation blocks with dense soils and low loads; c, d - foundations of large-panel buildings; d – elements of prefabricated large-block concrete foundations; f, g – elements of large-panel foundations.

Installation of prefabricated concrete foundations is carried out using cement mortar and bandaging the seams. In case of weak soils, reinforced distribution belts are laid along the foundation pads and along the edge of the foundation (Fig. 7.4 a). For dense soils and light loads, foundation pads can be laid at intervals (Figure 7.4 b). The gaps should be filled with soil.

For low-rise buildings with low loads and strong foundations, when strip foundations are irrational, they are used columnar foundations. They are installed under all load-bearing and self-supporting walls, as well as under individual pillars and columns.

Columnar foundations are foundations consisting of pillars sunk into the ground and foundation beams resting on them, which take the load from the walls and transfer it to the pillars.

The pillars are installed at the intersections of the walls and in the spaces between them with a certain pitch, which is determined by calculation depending on the mass of the building and the bearing capacity of the soil. For low-rise buildings, the pitch of the foundation pillars is 2.5 - 3.0 m.

Structural options for foundation beams and their proportions depending on the pitch of the pillars are shown in Fig. 7.5. To eliminate the possibility of displacement of the foundation beam and the wall located on it due to soil heaving, a cushion of sand or slag 0.4 m thick is placed under the foundation beam.

Fig.7.5. Structural diagrams of foundation beams for columnar foundations: a – fragment of a general view of the foundation; 1 – wall; 2 – foundation beam; 3 – pillars; b – f – various types of foundation beams; 4 – prefabricated reinforced concrete; 5 – prefabricated reinforced concrete lintels (reinforced beams); 6 – monolithic reinforced concrete beam; 7 – ordinary reinforced brick beam; 8 – reinforced brick beam with steel frames in the vertical joints of the masonry.

Pillars with a square cross-section in diameter are made from prefabricated concrete blocks, monolithic concrete, red brick, and natural stone. The dimensions of the pillars are taken based on strength calculations (material and soil). For low-rise residential buildings, the size of the pillar cushion does not exceed 1 m, and the horizontal section of the pillar can be equal to the size of the base or be smaller. In the latter case, the height of the pillow is taken to be no more than 0.3 m.

In cases where it is necessary to transfer significant loads to soft soil, pile foundations .

Pile foundations are foundations consisting of reinforced concrete, concrete or metal pile rods immersed in the ground, caps - the upper widened end of the pile, and a grillage that combines the work of all piles

Pile foundations are used on weak compressible soils, with deep occurrence of strong continental rocks, heavy loads, etc. Recently, pile foundations have become widespread for conventional foundations, because... their use provides significant savings in excavation volumes and concrete costs.

According to the material, piles can be wooden, reinforced concrete, concrete, steel and combined. Depending on the method of immersion in the ground, driven, driven, shell piles, drilled and screw piles are distinguished (Fig. 7.6).

Driven piles immersed using pile drivers, vibrating hammers and vibrating pressing units. These piles are most widely used in mass construction. In cross-section, reinforced concrete piles can be square, rectangular or hollow round: ordinary piles with a diameter of up to 800 mm, and shell piles - over 800 mm. The lower ends of the piles can be pointed or flat, with or without widening, and hollow piles can be with a closed or open end and with a camouflage heel (Fig. 7.6 d).

Cast-in piles arranged by filling pre-drilled, punched or stamped wells with concrete or another mixture. The lower part of the wells can be widened using explosions (piles with a camouflage heel).

Bored piles They differ in that ready-made reinforced concrete piles are installed into the well and the gap between the pile and the walls of the well is filled with cement-sand mortar.

Depending on the nature of the work in the ground, two types of piles are distinguished: rack piles and hanging piles. Rack piles , cutting through the thickness of weak soil, their ends rest on strong soil (rock) and transfer the load from the building to it. They are used when the depth of solid soil does not exceed the possible length of the piles. Foundations on rack piles practically do not give rise to precipitation.

If solid soil is located at a considerable depth, use hanging piles , the bearing capacity of which is determined by the sum of the resistance of friction forces on the side surface and the soil under the tip of the pile. Pile foundations in plan may consist of:

single piles - for individual supports (Fig. 7.6 d);

strips of piles - under the walls of a building, with piles arranged in one, two or more rows;

bushes of piles - under heavily loaded supports;

continuous pile field - for heavy structures with loads evenly distributed throughout the entire building plan.

Fig.7.6. Pile foundations: a – plan and sections; b – types of piles depending on the design scheme – rack piles and hanging piles; c – elements of a pile foundation: 1 – grillage; 2 – criminal; 3 – pile; d – types of piles: 1 – four driven concrete and reinforced concrete piles – square, round, solid and hollow; 5.6 – printed regular and with a widened heel; 7, 8 – camouflage; 9 – with hinged opening stops; 10 – prismatic pile; 11 – pile-shell; 12 – pile in the leader well; 13 – wooden pile; 14 – screw pile; d – arrangement of piles: pile rows, pile bushes, pile field; e – option of a pile foundation without grillage; g, i – options for pile foundations without grillages and caps: 1 – cap; 2 – pile; 3 – base panel; 4 – floors; 5 – column; 6 - crossbar

For low-rise construction, short reinforced concrete driven piles are used, usually with a square section of 150 × 150 mm, 200 × 200 mm, or drilled piles with a diameter of 300, 400 mm or more. The depth of laying short piles is no more than 6 m.

The distance between the piles and their number are determined by calculation. Typically, the distance between hanging piles is taken to be (3 – 8)d, where d is the diameter of a round pile or the side of a square pile. The clear distance between shell piles must be at least 1 m.

Grillage beams have much in common with foundation beams. The same materials are used for their manufacture. There are two types of reinforced concrete grillage - monolithic and prefabricated. Its width is 250 × 250 or 300 × 300 mm, height – 400 – 500 mm.

Pile foundations are 32–34% more economical than strip foundations in terms of cost, 40% in terms of concrete costs and 80% in terms of the volume of excavation work. Such savings make it possible to reduce the cost of the building as a whole by 1–1.5%, labor costs by 2%, and concrete consumption by 3–5%. However, steel costs increase by 1 - 3 kg per m 2.

In cases where the load transferred to the foundation is significant and the foundation soil is weak, arrange solid foundations under the entire building area. They are usually built on heavy heaving and subsiding soils.

Solid foundations are foundations in the form of rigid solid beam or beamless concrete or reinforced concrete slabs, arranged under the entire area of the building.

Such foundations well level all vertical and horizontal movements of the soil.

The ribs of the beam slabs can face up or down. The intersections of the ribs are used to install columns in frame buildings. The space between the ribs in slabs with the ribs up is filled with sand or gravel, and a concrete screed is placed on top. Concrete slabs are not reinforced. Reinforced concrete reinforced according to calculation. If solid foundations are deeply buried and there is a need to ensure their greater rigidity, the foundation slabs can be designed with a box-shaped section and placed between the ribs and ceilings of the boxes of the basement rooms (Fig. 7.7).

Solid foundations are especially appropriate when it is necessary to protect the basement from the penetration of groundwater at a high level, if the basement floor is subjected to high hydrostatic pressure from below.

A solid foundation slab for low-rise buildings is designed only in cases of construction of buildings on soils with uneven settlement or swelling and with a high level of groundwater (in buildings with a basement). The slab is made of monolithic heavy reinforced concrete with a thickness of at least 100 mm. The thickness of the slab is determined by calculation depending on the mass of the building, the strength of the soil and the distance between the walls. For houses without a basement, the foundation slab is installed on a sand cushion, which reduces the uneven settlement of the soil. In buildings with a basement, the foundation slab simultaneously serves as the base of the floor.

Slab foundations are quite expensive due to the large volume of concrete and metal consumption for reinforcement.

The construction of a solid foundation is most often resorted to in cases where the area allocated for the construction of a house is located on soil with a high groundwater level. Sometimes a solid foundation is used on sand cushions and on swelling soils.

A solid foundation is a single reinforced concrete slab that goes deep into the ground. In this regard, this type of foundation is often called slab foundation. It is well suited for building houses made of bricks, concrete blocks or other heavy building materials. Often, the project also provides for a solid foundation in the case of the construction of industrial premises, which are subject to increased requirements in terms of load-bearing capacity. These include, for example, garages.

Due to the uniform distribution of the load applied to the foundation over its entire plane, the pressure on the ground is minimized. This allows the construction of country houses even on swelling, unstable soils.

The foundation of a solid structure is resistant to soil movement, which may occur due to precipitation or freezing. Its construction is possible on almost any soil, since a monolithic slab made of concrete or reinforced concrete actually moves along with the soil when it shifts, eliminating deformation of the structure built on it.

The main technological feature of a solid foundation is the fact that it, together with the formwork, forms a single integral structure. Taking into account the fact that a monolithic foundation is most often laid on problematic soils, special requirements are placed on it. That is why, when planning and constructing it, all technologies must be followed with special care.

A solid foundation slab can be simple or reinforced, ribbed or smooth, solid or lattice. The grade of concrete is selected depending on the characteristics of the project being implemented.

In terms of depth, a solid foundation can be deep or shallow. The first, in addition to better load-bearing properties, also allows you to organize a basement.

The foundation of a solid structure is laid on a compacted gravel-sand cushion, under which a drainage system is installed. To organize a deep foundation, it is necessary to first dig a pit. Before pouring concrete, reinforcement should be installed, a waterproofing layer should be laid, and, if necessary, a layer of insulation.

The article describes the features of solid slab foundations. The scope of their application, operational and design differences are discussed in great detail. Applied issues related to the technology of construction of foundation slabs are brought to the fore.

This is a continuation of the series of articles about foundations, and we have already published a lot of interesting material. Therefore we recommend:

Strip foundation. Part 1: types, soils, design, cost
Strip foundation. Part 2: preparation, marking, excavation, formwork, reinforcement
Strip foundation. Part 3: concreting, final operations
Strip foundation. Part 4: assembly of concrete block structures

A slab foundation, also known as “solid”, also known as “floating”, or “Swedish, Scandinavian slab”, is a solid slab located under the entire area of the building, buried in the ground, or laid on it. There are several design options for slabs - box-shaped, flat, ribbed, prefabricated from road reinforced concrete products, monolithic, with extensions at the corners, with or without reinforcement, insulated and cold... They all have their own distinctive features and specific scope of application. For private suburban construction, in terms of economic and functional characteristics, flat monolithic reinforced concrete slabs with a thickness of 20 to 40 cm with insulation have proven themselves to be the best. We will talk about them further.

Why choose a slab foundation

In low-rise construction, which is what we are actually interested in, this type of foundation for many reasons will be preferable to its competitors (both strip and pile structures). This is explained by advantages of both a purely technical and construction-related nature.

Strengths of solid foundations

Universality in foundation geology. A floating structure can be correctly used on all types of soils, including weak-bearing, heaving, horizontally mobile, high groundwater levels, permafrost...

There are some restrictions on the terrain - it is difficult to build such a foundation on a slope; most likely, piles will be preferable. However, there are American-tested technologies for constructing slabs on hillocks, which in their design (in the lower part of the site) have elements of high monolithic strips. Another “centaur” suitable for such places is a pile foundation with a low grillage in the form of a monolithic slab.

Good load-bearing capacity. This quality is due to the specific mechanics of the “house/slab/soil” interaction. In the next chapter we will look at this point in detail. Briefly, the slab has a large support area, so the pressure on the foundation soil is very low (from 0.1 kgf/cm2). Consequently, a two-story stone house on a slab can be built with confidence. They say that the elevator shaft of the Ostankino Tower stands on a monolithic slab.

High spatial rigidity. It is due to the absence of seams and joints, the use of rigid reinforcement, the massiveness of the structure and high material consumption. A slab foundation is excellent for houses with “inelastic” walls, which are very afraid of even the smallest (1-3 mm) movements of the supporting structure - brick, aerated concrete, cinder block, shell rock and other mineral materials.

In the presence of excessively heaving soils and significant sensitivity of buildings to uneven deformations, it is recommended to build them on shallow and non-buried monolithic reinforced concrete slabs, under which cushions made of non-heaving materials are placed.

SP 50-101-2004 “Design and installation of foundations and foundations of buildings and structures.”

Good insulating properties. When properly executed, it does not allow water to pass through and prevents heat loss through the floor.

Simple construction technology, built quickly. Easy to mark, minimum excavation work, simplified formwork design, easy to reinforce and concrete. Can be manufactured by low-skilled builders.

Conditional disadvantages of a slab foundation

Technically, it is very difficult to combine a solid slab and a basement in a structure.

The slab can be poured only in favorable weather (it is slightly inferior to prefabricated and pile driven foundations).

High price. Increased material consumption (concrete, reinforcement), of course, leaves its mark. But if you look at the problem as a whole, the picture changes dramatically - we save a lot on other materials, construction stages, and production operations:

the slab becomes the subfloor of the first floor - no need to make an overlap;
You can lay a water heated floor in the mass of the slab, rather than pouring a separate screed for it;
for the manufacture and fastening of formwork panels, less boards or sheet materials are needed (at least twice as much as strip structures);
no need to pay for removal/planning of a large volume of selected soil;
the height of the external walls is reduced, since it is possible to obtain a lower base (and these are expensive facade finishing materials, labor costs...);
lifting equipment, concrete pumps, excavators, driving pile drivers, drilling machines are not needed, everything is limited to mixer vehicles;
you can build it yourself and not hire highly paid professional builders, there is less risk of suffering financially from the “human factor” (simpler technology).

It turns out that the main disadvantage of slab foundations is the low awareness of domestic developers about their advantages. But in the northern part of the USA and Scandinavian countries, monolithic slabs have become the No. 1 foundation.

The principle of operation of a slab foundation

Situation

The building density is growing, people increasingly have to build on “bad” soils (weak, constantly wet, heaving, frozen...).

Modern projects of country houses have become much more complex in terms of architectural and planning solutions: different parts of the building are built at different heights (options of one and a half floors, attached garages, special solutions for staircases and landings...), uneven distribution of load-bearing walls over the building area. Houses are now bigger, higher, heavier.

Problem

On top of the foundation and on the natural foundation there are uneven impacts from the house. From below, complex soils either tend to form local failures under the building, or forces of frost heaving push the building out, and then, when thawed, sag. There is a danger of deformation and destruction of supporting structures.

Solution

Increase the supporting area of the foundation, reducing the load from the house on the natural foundation.
Maximize the spatial rigidity of the foundation and evenly redistribute the pressure from top to bottom.
Use a heat insulator to separate the heated rooms from the ground under the house - thus eliminating uneven freezing under the building (in winter, the ground under the slab does not thaw).

All these methods of dealing with “unevenness” are inherent in the principle of operation of an insulated monolithic slab. This is a kind of single platform under the house, which is not subject to local bending (if properly designed), and without deformation is able to actually move with the ground - “float”.

Features of designing a slab foundation

Slab design differs significantly from methods for developing other types of foundations. Here, engineers also take into account all the basic soil parameters and all loads (weight of structures, operating weight, snow pressure). SP 20.13330.2011 has not been canceled.

However, the slab foundation must be considered as a single, jointly working “slab-above-foundation part” structure. Therefore, in this case, special attention is paid to a detailed study of specific components of the building and the supporting structure as a whole; drawings of the house are created and calculated, indicating diagrams of load distribution and their directions.

The whole problem lies in the difficulty of competently modeling bending loads, possible rolls that the slab experiences, and, accordingly, calculating its thickness, configuration, and the need for reinforcement, including local reinforcement. The most efficient design of foundation slabs is carried out using special computer systems that produce very detailed working drawings. That is why we recommend ordering a foundation slab calculation from a specialized organization; the cost of such work will range from 5 to 10 thousand rubles.

The most widespread are slabs with a thickness of 20 to 40 cm, but one detail is very interesting: most calculations show that different slab thicknesses can be used for the same house if the percentage of reinforcement is correctly manipulated.

For example, a solid foundation for some abstract building. At 20 centimeters, it is necessary to carry out local “additional reinforcement” of especially loaded areas and not make mistakes in the calculations; at 25 centimeters, the frame can be knitted evenly, without much risk. But a 30-centimeter slab, when compared with a 25-cm structure, will not allow you to save on reinforcement, but it will use much more concrete.

Exceptionally competent calculation allows you to cast slabs even with a thickness of 15-18 cm.

Note that it is possible to significantly increase the resistance of the slab to punching, while reducing its overall thickness (read material consumption) by making local thickenings of the foundation in the area of the corners, the junction of load-bearing walls, along the entire perimeter, under the columns. Such reinforced slabs are often called “American”; in cross-section they look like a prism.

The slab foundation cannot be smaller in area than the house; all cantilever sections must be taken into account. For example, if the building will be faced with brick or other heavy materials, then the slab must be laid in large sizes to provide a supporting area for the cladding.

Slab foundation construction technology

Since slab foundations are often used in very difficult geological conditions, the most stringent requirements are imposed on the planning and construction of floating structures, which are stipulated by many regulatory documents, for example, SNiP 3.03.01-87 “Load-bearing and enclosing structures” or SP 50-101- 2004 “Design and installation of foundations and foundations of buildings and structures.” Naturally, only high-quality materials should be used for the construction of foundation slabs.

The construction of all solid foundations is carried out approximately according to the same scheme:

Design.
Marking (only the outlines of the building are taken into reality).
Removing turf, sampling soil (if cushion/drainage is necessary).
Laying buried communications (water, sewerage).
Installation of cushion and drainage.
Installation of hydro- and thermal insulation.
Assembling a “warm floor”.
Knitting and laying of reinforcement cage.
Assembling and unfastening formwork.
Concreting.
Stripping.

Let's look at these operations in more detail.

We have more or less figured out the design. If you are building something serious, it is better to order the development of a foundation project from engineers, and you will definitely save your nerves and money.

We have already discussed the issues of carrying out preparatory work and carrying out markings in nature in the article.

As for earthworks. If soil replacement (massive cushions) and insulation are not required, then it is enough to remove only the top fertile layer, otherwise, the soil of the natural foundation is removed in the required volume. Sometimes, before excavation, it makes sense to level the building area - to make bedding. Then the additional material is very carefully compacted with a vibrating plate.

The most important condition is that the bulk soil under the slab foundation should not be inferior to the mainland (natural) in any way.

The cushion is an artificial base, it is designed to replace “bad” soils. The material for the cushion is most often a mixture of sand and crushed stone, which have good drainage properties, have little compression, and do not heave. The sand and gravel cushion is laid in layers of 100 mm, and each layer is carefully compacted with a vibrating platform. If clean sand is used, it must be spilled with water.

It is necessary to periodically check the horizontalness of each layer of pillows.

In areas with unfavorable water balance, it is recommended to lay several drains under the slab (cushion) to drain water.

Most technological maps for the production of solid foundations suggest laying geotextiles under the cushion, which prevents sand and gravel from silting (read: losing properties that are important to us).

In order for the hydro- and thermal insulation to fit well and not be deformed by the mass of concrete, the upper part of the cushion must have the most even plane possible. Some manufacturers of floating foundations even prefer to make a preparation screed from sand concrete.

The cushion is covered with a thick polyethylene film or other waterproofing materials that will prevent laitance from leaking during concreting. The sheets are laid overlapping and glued/soldered.

A layer of insulation up to 100 mm thick is laid on the waterproofing. Previously, they used polystyrene foam, but now everyone has switched to extruded polystyrene foam. Some builders believe that insulation is not a necessary layer, but it reduces heat loss through the slab and does not allow the soil under the slab to thaw uncontrollably and unevenly even under heated rooms. If you want to use a warm floor, you will not heat the ground, but let all the heat into the house. In the technological maps of foreign companies, it is recommended to lay the insulation (and pillow) outside the slab.

Heated floor pipes are laid out directly onto EPS sheets using a special mesh; naturally, they are not insulated with any materials in order to better transfer heat. Some heating routes can also pass through this layer - they are carried out in sleeves and heat insulators. All ends are removed from the pit for communications, the system is ringed and crimped. Under pressure, air pumped into the pipes prevents them from deforming when pouring concrete.

Reinforcement is perhaps the most difficult operation in the construction of floating foundations. This is where the most mistakes are made, both technological and design.

Let's start with the main thing. According to SP 52-103-2007, the minimum percentage of reinforcement for a reinforced concrete slab is 0.3%. It is calculated as follows: take a cross section of the slab and calculate its area, calculate the total cut area of all reinforcing bars, and compare these indicators. If the metal content of concrete is insufficient, then increase the diameter of the reinforcement or the number of rods (reduce the pitch). For thick slabs, a third tier of metal is used, located in the thickness of the slab. Practice shows that most often it is enough to lay two layers of reinforcement with a diameter of 12-14 mm, and a pitch of 150-250 mm.

Do not forget that in loaded areas (columns, load-bearing wall inside a building...) additional reinforcement may be required by laying auxiliary longitudinal rods within the punching prisms.

Depending on the design of the building, it sometimes makes sense to install vertical reinforcement outlets under load-bearing walls and columns (SP 52-103-2007), which will provide additional rigidity to the “slab-above-foundation part” system.

The presence of a protective layer of concrete is a prerequisite for high-quality reinforcement. The reinforcement cage meshes are displayed on special polymer mushroom stands. The fungi of the lower tier are small, about 4-5 cm. The intermediate fungi (between two meshes) have a height depending on the thickness of the slab, so that about 5 cm of concrete (protective layer) remains above the upper reinforcement. The fungi are placed one above the other, their total number (step) should ensure sufficient resistance of the frame to the loads arising during concreting.

It is prohibited to use all kinds of linings made of wood, stone, and metal.

It is recommended (SP 63.13330.2012) to connect the ends of the frame, the upper and lower tier, with U-shaped elements made of reinforcement. The reinforcing bars should not come into contact with the formwork, since a protective layer of concrete with a thickness of at least 40 mm should be provided.

A frame of viscous reinforcing bars is made using wire. The use of electric arc welding is allowed, but then it is necessary to use class A500c fittings, or similar, with the index “C”.

Due to the large volume of reinforcement work, it may be advisable to use standardized factory-made welded mesh. The joints obtained after laying must be placed in a “checkerboard” order - the joints of the finished mesh of the lower tier of reinforcement must be overlapped by the entire mesh of the upper tier.

The floating foundation formwork is very easy to assemble; you just need to level each side of the perimeter. Please note that a lot of concrete is used, and the pressure on the shields will be quite serious - so lift them off the ground very well.

The formwork should be wrapped inside with polyethylene to prevent laitance from leaking through the cracks. As an option, you can lay EPS sheets near the formwork, then they will reliably “stick” to the concrete and provide vertical insulation of the slab.

Expanded polystyrene is also used to separate buildings adjacent to the house, which require their own foundation (garage, porch, terrace...).

A separate small formwork contour is made for the pit for communications.

You can read about formwork and reinforcement in the article “Strip foundation. Part 2: preparation, marking, excavation, formwork, reinforcement."

The nuances of making a monolith can be found in our publication.

Concreting must be done in one work shift. The most rational way would be to order the delivery of concrete with a mixer and pour the foundation directly from the tray. For concreting remote areas, you can use a homemade gutter.

Concrete must be compacted with an in-depth vibrator.

For the manufacture of slab foundations, concrete is used with characteristics that are regulated by SP 52-103-2007. Most construction companies producing floating foundations offer to order concrete with the following performance properties:

strength class from B22.5 (grade not lower than M300);
water resistance coefficient from W8;
frost resistance from F200;
mobility P-3;
possibly sulfate resistant if groundwater is high.

Taking into account domestic realities, it is better for a private developer to order concrete at least a grade higher than the standardized one - there will be a better chance of obtaining the design strength class.

Next, you should carry out manipulations to care for the concrete. When the slab reaches 50% strength, the formwork can be removed. We examined these works in detail in the article “Strip foundation. Part 3: concreting, final operations”, we will add that the next day after pouring the floating foundation, the upper plane of the slab should be rubbed down - this will be a good base before installing any floor coverings.

In Northern Europe and the USA, floating foundations have been actively used for more than half a century; over time they have proven their reliability, functionality and economic attractiveness. In our country, the slabs also found their developer. From year to year, solid foundations are becoming more and more popular, since in many cases there is simply no alternative to them.

Turishchev Anton, rmnt.ru

Today, any more or less serious building, erected in accordance with current technological standards, requires a foundation. Depending on the characteristics of the soil, the number of storeys of the building and some external factors, the type of foundation used is selected.

A monolithic solid foundation is poured when a building is erected on loose soils with low bearing capacity and in places where groundwater is close to the surface. Examples of places where it is impossible to do without the use of such a foundation are old landfills, soils prone to swelling, and sandy areas. This type of foundation is classified as shallow, and its use allows the building to obtain an acceptable support area on a small plot of land. This type of foundation is quite universal - it is built both under heavy multi-storey buildings and under prefabricated panel structures of light weight. The main difference will be in the method of placing reinforcing rods and the arrangement of additional stiffeners.

Technology for constructing a solid (slab) foundation

A solid (slab) foundation is a solid slab of reinforced concrete placed over the area of the entire building. It has an increased load-bearing capacity and resists soil displacement well, actually moving with the soil. It also increases the house’s resistance to loads that are likely due to land subsidence or temperature fluctuations.

General characteristics

The slab foundation is focused on complex types of soil:

peated;
swampy;
water-saturated;
weak-supported;
heaving;
subsidence.

Stages of work

Work on the formation of a slab (solid) foundation allows both the pouring of concrete at the construction site and the use of standard reinforced concrete slabs, which are used for laying roads. The main condition is a thickness within 20-30 cm. Depending on this, construction includes several stages:

preparation;
site breakdown;
formwork formation;
reinforcement;
pouring concrete.

Preparation

It consists of developing documentation, calculating estimates, precise planning and clearing the territory. In order to ultimately receive a complete package of documents, it is best to contact specialists working in the relevant field. This will save time and money, as well as professionally approach the construction of the house, taking into account the type of recommended foundation. This is especially true when constructing rear structures on soils with surface moisture.

In addition, the basis for the foundation requires a balanced approach and clarification of all details. Ideal surface evenness is also important. To do this, the area is first cleared of shrubs and other vegetation, stumps and roots are removed, and large stones and boulders are collected. Next, level it with a shovel and level, removing protrusions and indentations.

Site breakdown

This stage consists of transferring the plan to the area. To do this, a geodetic breakdown is carried out and key marks of the future building are set. Next, the entire top layer of soil is removed. It has a low load-bearing capacity and a high tendency to compaction. That is why it is removed to a depth of up to half a meter. The work is carried out using an excavator.

To backfill the pit, a gravel-sand or crushed stone-sand mixture is used at a rate of 60:40, respectively. It is compacted tightly. This pillow:

makes it possible to reduce the force of frost heaving on the lower zone of the foundation;
allows ground moisture to pass freely under the house;
evenly distributes the pressure of the building on the ground.

In addition, the sandy base does not retain water, and the low-set structure does not allow the soil to freeze during cold periods, providing the structure with increased stability. Then trenches are laid across the foundation (for reservoir drainage) and lined with geotextiles. Crushed stone is poured on top of it.

Formation of formwork and reinforcement

At the corners of the resulting “structure”, rotary sealed wells are installed, since the slab foundations lie mainly on soils with a high moisture content and maximum proximity to groundwater. Then proceed to the basic formwork. According to calculations, it should extend beyond the perimeter of the foundation by 15 centimeters.

The bottom of the pit is covered with granite crushed stone of a fraction of 4-6 cm. The maximum layer thickness can reach 20 cm. A small 4-centimeter layer of concrete is formed on top of it, which is the first screed. But before this, the crushed stone is poured with a liquid mixture of sand and concrete so that the outer layer forms an even “crust”.

Next we move on to waterproofing. These can be special roll materials with adhesions or a regular bitumen primer that is used to cover cement. Any rolled waterproofing material is glued onto it. An adhesive built-up waterproofing layer is laid over the mastic in 2 layers. If desired, you can also make thermal insulation layers.

Then they begin to form the formwork for a monolithic reinforced concrete slab. To do this, racks are dug in around the entire perimeter of the structure and any plank materials are nailed to them. During this operation, a level is required.

The base of the foundation slab is a special metal frame with reinforcement over the entire area. For these purposes, two iron meshes are used - lower and upper. They are tied together with special hooks and annealed steel wire.

Then, additional ones are installed between the main mesh rods, at a distance of 20 cm from each other. Plastic compensators or clamps are also installed, ensuring the best location of the steel rods.

Pouring concrete

Pouring concrete is the final stage of work on a slab foundation. During its implementation, both ready-made dry mixtures and self-mixed solutions can be used - based on cement, sand and gravel (crushed stone). They fill the formwork strictly to the height of the sides. After drying, the slabs begin the next stage of construction.