The foundation is the most important part of any structure, it is responsible for the strength and reliability of the main structure. Therefore, determining the type of fundamental basis, calculating parameters and choosing building materials requires a responsible approach.
Of all types of foundations, developers very often prefer a foundation in the form of a monolithic concrete slab, despite its high cost.
Selection of materials for a monolithic concrete base
The strength and reliability of the entire structure depends on the quality of the materials that will be used to create a monolithic slab foundation. Therefore, you should approach this process very responsibly.
Concrete
Particular attention should be paid to the choice of concrete mortar, since to create a monolithic base it is recommended to use a special class of this building material. In particular, concrete must have the following characteristics:
- Brand - not lower than M300, which corresponds to strength class B22.5. Read an additional article about.
- The mobility of the mixture is P-3.
- Frost resistance – above F
- Water resistance – not less than W
Thermal insulation materials
Most often, a monolithic slab foundation is erected for buildings that are used all year round. Therefore, the choice of insulation for the foundation of a house should also be approached responsibly.
Waterproofing materials
In addition, roll waterproofing, for example, bitumen-polymer materials, can be used. They are distinguished by a higher quality composition, thanks to which the material can withstand high and low temperatures without compromising its quality characteristics.
Selection of fittings
Calculation of slab foundation parameters
A monolithic slab foundation is also called a floating foundation. This is explained by the properties of the slab to “float” during seasonal soil movements. However, to ensure such characteristics, it is necessary to accurately calculate the parameters of the slab foundation. Various factors must be taken into account.
When calculating the thickness of the concrete base, the following values are taken into account:
- The distance between the top and bottom rows of the reinforcement cage.
- The thickness of the concrete pour under and above the frame.
- Diameter of reinforcement bars.
In most cases, when adding these values, it turns out that the height of the slab is approximately 30 cm. The obtained result can be taken into account when constructing a monolithic slab base on hard and stable ground.
When performing calculations, you should take into account the material from which the main structure will be built and the number of floors. For example, 5-6 cm should be added to the obtained values if the walls of the house are brick. In addition, if there is a second floor in a brick house, the foundation slab increases by another 40 cm.
When calculating the depth of the pit, take the height of the slab as a basis and add to this the thickness of the drainage layer of 30 cm and a sand cushion 20 cm high. As a result, it turns out that 50-60 cm is added to the total height of the slab.
Based on the total height of the monolithic slab, the required amount of concrete, the total length of reinforcement and the load from the base to the ground can be calculated.
Technology for manufacturing a monolithic concrete slab for a foundation
Like any construction process, a monolithic slab foundation is built using a certain technology, which is divided into several stages.
Stage 1. Preparatory activities
The preparation process involves developing the site, tidying up the soil and collecting the necessary tools.
The work will be carried out using the following:
- Shovel and bayonet shovel.
- Building level.
- Marking cord or regular rope.
First, the working area is determined and the top fertile layer is removed from the designated area, using a bulldozer or shovel for this purpose.
Stage 2. Excavation work
Taking the parameters of the slab foundation as a basis, the dimensions of the pit are calculated. In this case, 1 meter is added on each side for more convenient work. It is important to understand that a large volume of soil must be removed for the foundation slab, so it is better to use construction equipment for this purpose.
The depth of the pit on average reaches 1.5 meters, therefore, the clay layer must be removed almost completely. The bottom of the pit is covered with sand or gravel, the surface is leveled, checking the horizontalness with a building level. At this stage, even slight slopes should be avoided, as this may cause the foundation slab to collapse.
Stage 3. Creation of formwork
To form a foundation slab, it is necessary to assemble a formwork structure; this will require strong boards more than 2.5 cm thick. The formwork is installed around the perimeter of the pit, with its outside put strong supports. After assembling the structure, you can test its strength; to do this, it is enough to apply several strong blows. If the formwork can withstand them, then there is no doubt about its strength. Otherwise, the design must be redone.
Stage 4. Insulation and waterproofing
When building a slab foundation, it is very important to remove moisture from its base; a drainage system is created for this purpose. The installation process is as follows:
- Trenches are dug across the pit to drain water.
- Geological textiles are placed in them, and the material should protrude slightly beyond the edges of the trenches.
- Then plastic perforated pipes are laid and wrapped with geotextile edges.
- Fine crushed stone is poured into the trenches over the pipes, leveling the surface to the same level.
Further actions involve waterproofing and thermal insulation of the base of the slab foundation:
- The bottom of the pit is covered with waterproofing material.
- Thermal insulation slabs are laid on top of it.
- Next comes another layer of waterproofing.
On this page you will learn what features a foundation made of a monolithic concrete slab has, and how it is calculated and designed.
The technology for constructing a slab foundation will also be discussed in detail and informative video materials will be provided to help better understand the construction process.
Fig 1.1: Monolithic concrete slab foundation
Advantages of a slab foundation
Among all existing types of foundations, a slab foundation is rightfully considered one of the most reliable. A monolithic foundation is universal; it can be built on almost any type of soil, in contrast to strip and pile foundations that are limited in use.
Slab foundations are excellent for building houses on the following types of soil:
- Peat;
- Sandy;
- Swampy;
- On the ground with high level location groundwater;
- On soils prone to seasonal heaving.
Heaving- this is the tendency of soil to change its volume during the cold season. This happens due to the freezing of groundwater (moisture increases in volume during crystallization), as a result of which structures located in the ground simply bulge outward.
Rice. 1.2: Cracks in the basement and walls of the house are a characteristic result of frost heaving of the soil on the foundation
Heaving is the main enemy of most foundations - in winter, when the soil freezes, heaving forces push them out of the soil, and in summer, when groundwater thaws, shrinkage of the foundation occurs. Such vibrations exert strong dynamic loads on the foundation, as a result of which the foundation collapses, the walls of the house crack, and the building gradually falls into disrepair.
Among the advantages of a slab foundation we can also highlight:
- High load-bearing capacity - according to this characteristic, slab foundations have no competitors; on such foundations both one-story and multi-story houses from heavy building materials - brick, concrete, gas silicate;
- The slab foundation makes it possible to equip a full-fledged basement floor;
- The technology for constructing a slab foundation is quite simple; it can be done with your own hands without any problems;
- A monolithic foundation slab has the maximum durability among all types of foundations; its service life exceeds 150 years;
- The arrangement of a slab base does not require a large amount of excavation work, since the depth of the slab is minimal.
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Important! This is why it is important to compare the characteristics of foundations before choosing a foundation: . |
Disadvantages of a slab foundation
The only significant drawback of monolithic slab foundations is the high cost of installation. Such foundations are durable and reliable, but they are expensive because their creation requires a large amount of building materials - concrete and reinforcement.
Rice. 1.3: The process of arranging a slab foundation
In general, if you decide to build “to last,” a slab foundation is the best choice.
Slab foundation calculation
A high-quality foundation is a necessary condition for the strength and durability of any building, therefore its calculation must be approached with maximum responsibility.
When calculating the thickness, the characteristics of the soil and the mass of the building being constructed are taken into account. According to building regulations and rules, the minimum thickness of the slab base for buildings made of lightweight materials, foam concrete or wood should be 20 centimeters, for buildings made of brick - 25 centimeters.
The main condition is that a distance of at least 7 centimeters be maintained between the contours of the reinforcement frame, and that the reinforcement frame itself be recessed into the concrete by at least 5 centimeters.
Expert advice! During this process, preliminary subsidence of the soil occurs; this is necessary so that subsidence does not occur during concrete hardening, since in this case cracks may appear along the slab.
Creating formwork for pouring concrete
Expert advice! The footing is necessary to provide a smooth, hydrophobic surface that will not absorb moisture and concrete slurry from the main concrete slab.
Rice. 2.0: Footing under the foundation slab
Creating a Reinforcement Frame
Next, a reinforcement frame is made consisting of an upper and lower contour. Hot-rolled A3 corrugated reinforcement is used as vertical frame rods. Its diameter depends on the total thickness of the foundation slab (ratio 1 to 20 - for a slab with a thickness of 200 cm, reinforcement with a diameter of 10 mm is used, for a thickness of 300 mm - 16 mm, etc.)
Fig 2.1: Reinforcement cage diagram for a monolithic concrete slab
The pitch of the rods in the reinforcement frame is 20-25 centimeters, the upper and lower contours are connected by jumpers made of A1 reinforcing rods. The minimum distance between the contours is 7 cm, the frame itself must be recessed into the concrete slab by 5 cm. To raise the frame above the lower edge of the slab, special “mushroom” type stands are used.
Rice. 2.2: Supports for reinforcement cage
Horizontal rods and vertical jumpers are connected using steel binding wire with a diameter of 2 mm.
Rice. 2.3: Reinforcement frame connections
Pouring a concrete slab
Pouring concrete is carried out simultaneously (using ready-made industrial concrete purchased in the required quantity). After pouring, the concrete is compacted with an in-depth vibrator; if it is not available, it is bayoneted with a reinforcing rod.
Expert advice! The concrete should not be allowed to dry out; to do this, every day it is necessary to lift the film and moisten the rag with water.
The slab reaches its design strength within 28-30 days, after which construction can continue.
Such a foundation, due to the presence of support piles, has maximum load-bearing capacity and resistance to the negative influences of the soil (including seismic loads). These characteristics allow the use of a pile-slab foundation for construction multi-storey buildings.
Rice. 2.6: Project of a pile-slab foundation for the construction of a multi-story building
DIY slab foundation - video
Arranging a slab foundation is a complex process that requires not only theoretical knowledge, but also experience. In order for you to get a more complete picture of the most important aspects of construction, watch the video materials provided, which explain in detail the most important aspects of the different stages of constructing a slab base.
Marking the site for the construction of a slab foundation
Digging a pit for a slab foundation
Creating a compacting pad
Making formwork
with their further immersion at the construction site. Prices for driving piles are presented on the page: prices for driving piles. To order work on driving reinforced concrete piles, leave a request:
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Among all types of foundations chosen by private developers for the construction of their country houses and outbuildings, the undisputed leader in frequency of use are foundations. However, quite often the specifics of the soil at the construction site, the climate in the region, the location and dynamics of changes in underground aquifers require an excessively deep laying of the base of the strip foundation, which makes it an unprofitable solution, especially if we are talking about the construction of a relatively small in size and total mass building. We have to look for other options that are more economically justified, but at the same time not inferior in terms of load-bearing capabilities.
One such solution could be monolithic slab, poured under the entire future building. The uniform distribution of the load falling on such a foundation over the entire considerable area makes it possible to use such a scheme on soils with low bearing capacity. And the comparative simplicity of constructing such a foundation makes it completely doable on one’s own. So, the topic of this publication is a DIY foundation slab step-by-step instruction, from calculations to practical implementation.
General information about the foundation - monolithic slab
Typical diagram of a monolithic slab foundation
A slab foundation does not require a deep location; rather, on the contrary, its load-bearing capacity and “floating” features will appear precisely when it is sufficiently close to the surface of the earth. In this case, even frost heaving of the soil will not have a destructive effect on the stability of the building - the slab itself, with its high-quality construction, together with the building erected on it, seems to “float” on the surface of the soil.
A schematic diagram of a monolithic foundation slab is shown in the illustration below:
1 – Compacted soil – the bottom of a pit dug for the foundation.
2 – A carefully compacted “cushion” of sand, sand-gravel mixture, or crushed stone, which promotes uniform distribution of loads, becomes a kind of damper that softens the effects of ground vibrations. Layer-by-layer filling and compaction of such a “pillow” is practiced, with one or another alternation of materials, or homogeneous, using ASG.
3 – A layer of geotextile (dornite), which will give the sand “cushion” a kind of “reinforcement”, will prevent it from silting or washing away on waterlogged soils. This illustration shows only one of the options for placing a geotextile layer, however, their number and position may vary, depending on specific conditions. Thus, such a layer is often placed between the surface of the compacted bottom of the pit and the first layer of sand “cushion” - to prevent the penetration of soil particles into it. A layer of geotextile also separates the sand and gravel layers of the backfill - again for reasons of reinforcement and avoiding interpenetration. In this case, the location of the gravel or crushed stone layer above the sand seems to be more optimal - because capillary “suction” of ground moisture from below is almost completely eliminated.
4 – A layer of so-called concrete preparation. This element of the overall slab foundation “pie” is often neglected for reasons of saving material and reducing the overall duration of work. Meanwhile, such concrete preparation plays a significant role - it allows you to achieve a “clear geometry” of the base for further pouring the foundation or laying insulation materials, and makes it possible to very efficiently install the hermetically sealed waterproofing required for the slab.
5 – The already mentioned layer of waterproofing, which is mandatory for such a foundation slab, protects the base of the building from moisture from below. The optimal solution is at least two layers of roll waterproofing materials on a polymer-bitumen basis.
6 – The monolithic slab itself with the calculated thickness.
7 – reinforcing belt of a concrete slab. Its classic design consists of two levels of reinforcement grids connected to each other to give the structure volume with special clamps. The arrangement of the reinforcement is planned in such a way that a layer of concrete of about 50 mm is created between the rods and the edges of the slab at the top, bottom and ends - in order to prevent the start of metal corrosion processes.
This is a general scheme, but there are several types of monolithic foundation slabs, used depending on certain specific construction features.
The easiest to implement and probably the most common option is a solid slab, the uniform thickness of which is maintained over its entire area.
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This is the scheme that is most often chosen when constructing houses and outbuildings on fairly stable soil. However, it has an obvious drawback - the thickness of the slab is usually small, and is partially located below the ground level, that is, the upper edge is located close to the ground, which is not very good for wall structures. Increasing the thickness of the slab because of this is not economically feasible, which means that you can consider another option - pouring a foundation with reinforcing ribs, which have some resemblance to a strip foundation. Moreover, these ribs can be located both above and below the slab.
Thus, a kind of foundation-grillage can be obtained if, simultaneously with the slab, stiffening ribs are poured, protruding above the surface of the slab, which is obtained as a “bowl”. Such grillages are placed along the lines of construction of the load-bearing walls of the house structure - after waterproofing their horizontal surfaces, this is where the masonry begins.
A similar scheme is still often practiced in cases where it is planned beneficial use semi-basement or ground floor - the slab simultaneously becomes the floor of these rooms. And from the grillages they begin laying the plinth.
If you don’t want to deepen the slab too much into the ground, and at the same time achieve its maximum bearing capacity without thickening, you can use a scheme in which the stiffeners are located facing down.
When preparing the surface, installing the formwork and reinforcing frame, deep “channels” are immediately provided, which, after pouring the slab, will turn into stiffening ribs facing the ground.
This also turns out to be a kind of “symbiosis” of slab and strip foundations. Stiffening ribs are planned under external walls and capital internal partitions. Well, if internal partitions are not provided, then the ribs should be located parallel to each other and the shorter side of the perimeter of the house, with a pitch not exceeding 3000 mm.
This scheme makes it possible to achieve serious savings in concrete, since if there are properly planned stiffening ribs, the thickness of the slab can be significantly reduced, by 100÷150 mm, without losing its load-bearing potential, and this, after all, is 1.0÷1.5 cubic meters of solution for every 10 square meters area.
In addition, wide possibilities for insulating the foundation slab open up - the same height difference on the main surface and on the stiffeners is often accomplished by laying durable thermal insulation material, for example, extruded. By the way, this approach is the key condition for the construction of one of the improved varieties of slab foundations - the so-called “insulated Swedish slab”.
Insulated Swedish stove (USP) - the basis for houses with minimal energy consumption
The trend of building houses with minimal, zero or even negative external energy consumption, widely used in modern world construction, leads to the emergence and development of innovative technologies, which include USP. The main nuances are discussed in detail in the corresponding publication on our portal.
It makes sense to make one more remark. Slab foundations can be not only completely poured, monolithic, but also prefabricated, consisting of ready-made foundations laid close to each other. reinforced concrete structures. It would seem that this is much simpler, however, the absence of a rigid connection between adjacent slabs makes such a foundation unstable to possible ground vibrations. For this reason, such a scheme is not widely used, and is practically not used in private residential construction. The only exception may be small-sized outbuildings, the area of which is limited by the size of one standard slab, but this, as you understand, is extremely rare.
Application of slab foundation. Its main advantages and disadvantages
The use of a slab foundation will be fully justified on construction sites characterized by soils with reduced bearing capacity. It is usually resorted to where there is more simple circuits, such as shallow shallow or columnar laying - are simply impossible due to the peculiarities of “geology”: the tendency of soils to frost heaving, horizontal “movements”, close proximity of aquifers, etc.
In addition, such a foundation, with careful calculations and design, can become a very reliable basis for multi-storey construction. Uniform distribution of loads over a large base area gives very low pressure on the ground even when constructing massive buildings and engineering structures. True, this applies to a greater extent to construction work carried out on an industrial scale.
By the way, there is a lot of debate about the advantages and disadvantages of a slab foundation, both real and, frankly speaking, far-fetched. Let's try to list them and understand this issue a little.
What they say about merits ?
- There is a widespread belief that a monolithic slab foundation is an absolute “panacea” for all cases, that is, it can be built on any type of soil. Allegedly, such a slab of a house, even in a marshy area, will be a reliable basis for a heavy building, since due to its “buoyancy” it will vibrate along with the movements of the soil, without being subject to deformation.
It is certainly impossible to agree with this statement. Most likely, it would be more correct to say only that a slab foundation opens up expanded possibilities for construction in areas with complex soils, with insufficient load-bearing capacity for a strip foundation, and with average heaving rates.
But on obviously swampy, waterlogged soils, with the likelihood of subsidence, especially in regions with a harsh winter climate, probably only pile foundation, years, piles are driven (screwed) into dense, load-bearing rocks located significantly below the freezing level.
And a slab foundation, located almost on the surface, can indeed move within certain limits along with ground vibrations, that is, “float”. But the trouble is that in areas with pronounced soil instability, these vibrations can have a very high amplitude and are applied unevenly from below to the surface of the slab. Even if the soil is absolutely homogeneous over the entire area, this unevenness is explained by banal reasons - on the south side, freezing almost always occurs to a shallower depth, and thawing in the spring occurs much faster. This means that the slab willy-nilly will experience colossal internal bending stresses.
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As a rule, slab foundations have a very significant margin of safety, and perhaps the slab itself will withstand such loads without cracking, but small linear deformations are quite likely. They will certainly be transferred to the walls, and in addition, the tilt of the entire building from the vertical axis cannot be ruled out. For wooden buildings, it may not be so critical, due to a certain mobility of the structure. But the stresses on rigid stone (block) walls increase with height, that is, the lever of application of force. And it is possible that somewhere in the upper area of the wall a crack will suddenly appear and begin to expand.
So, speaking objectively, you shouldn’t overestimate the versatility of a slab foundation – that would be reckless. In any case, if there is no confidence in unconditional success, it would be more advisable to invite specialists to conduct a geological analysis of the site. In addition, it is always useful to familiarize yourself with the “history” of the use of slab foundations in the nearby area - what kind of houses were built on them and how long ago, what is the depth of the foundation and the thickness of the slab, are there any complaints about the operation, how the buildings survived seasonal ground fluctuations - these and other questions will help you make the right choice.
- Slab monolithic foundations allow the construction of large, even multi-level houses built from heavy materials.
This is true, and many multi-storey buildings in large cities are built on a similar basis. In terms of its ability to evenly distribute the load over a large area, such a foundation has no equal. Of course, all this is true with professionally carried out calculations, taking into account the characteristics of the development site, and high-quality execution.
So the common belief is that a slab foundation is only suitable for small compact houses, and that “his life is short” is limited to 35–50 years – this is nothing more than fiction. We repeat - everything depends on competent professional calculations and on the quality of execution in accordance with the project.
- The construction of a slab foundation minimizes the work of digging a pit - no deep penetration into the ground is required.
If we talk about a slab located on the ground surface or with a slight recess, then this is indeed the case - only the top fertile layer of soil is removed, and the depth of the pit is largely determined by the calculated height of the sand and gravel cushion. True, if this depth is also multiplied by the entire area (and the slab must be laid wider than the future building, plus insulated blind areas), then the volume of selected soil can still be considerable. So this advantage is not very obvious - with a shallow strip foundation, sometimes it is simpler in this regard.
Well, if you plan to use a deep monolithic slab, that is, to create a house with a full-fledged basement on its basis, then you will have to dig a corresponding pit, that is, it is very difficult to do without the use of special equipment.
- The use of a slab foundation automatically solves the problem of a reliable foundation for the floors of the first (or basement) floor.
It's really important advantage. And if, simultaneously with preparing the slab for pouring, you provide a high-quality thermal insulation belt, then the floors will also be pre-insulated. In "insulated" Swedish stove“In addition, water floor heating circuits are also installed immediately.
- Work on a slab foundation cannot in any way be classified as a task of an increased category of complexity.
An ambiguous statement, with which, nevertheless, one can agree to a certain extent. Indeed, the work itself on the stove does not involve operations that require the highest qualifications of workers. Digging a pit and compacting a sand and gravel cushion, knitting a reinforcement frame, installing formwork, pouring and distributing concrete, caring for a slab that is gaining strength and other stages - all this is either understandable from the beginning, or a novice craftsman can get the hang of it in a very short time.
Another thing is that a number of operations require the use of special tools and equipment. Thus, for high-quality compaction you cannot do without a vibrating plate; for the quick and uniform production of reinforcing clamps, it will be necessary to build an appropriate device; waterproofing with rolled materials involves the use gas burner with a balloon. And considering that the volume of concrete to be poured can be considerable, and it is advisable to pour the slab in one day, it is hardly worth relying on - you will have to order it for delivery.
We can say that, provided that some operations involve outside forces and resources, the owner, who has enlisted the help of friends or relatives, can easily handle the bulk of the work. True, you need to be prepared for the fact that the work ahead will be quite long, physically difficult, and sometimes even tiresome and monotonous. But for a small team of several strong men, it is doable. Of course, with strict adherence to all technological recommendations.
It is interesting that in some publications devoted to slab foundations, this is presented not as an advantage, but as a disadvantage - they say that working on such a slab is an extremely difficult task. It is possible that it is simply a matter of different evaluation criteria - from what point of view this problem is viewed.
Now let's turn our attention to flaws slab foundation:
- It is quite obvious that this type of house foundation is suitable for construction on a relatively flat area. If there is a significant difference in height in the building area, then such a scheme either becomes extremely complicated, becomes impractical, or is considered completely impossible.
- The slab must rest completely, with its entire area, on the ground - this is precisely what lies in its increased load-bearing capacity, even on not completely stable soils. And this, in turn, means that there can be no talk of any basement or cellar under the slab itself.
The only exception may be the scheme already mentioned above, in which the slab itself becomes the floor of a full-fledged basement, semi-basement or basement. It, as a rule, has upward-directed stiffening ribs - grillages, or well-thought-out reinforcement backfills, from which further construction of the buried part of the walls is already underway, by analogy with a deep strip foundation. But this type of foundation is a very expensive “pleasure” that requires highly qualified calculations and practical execution.
- The construction of a slab foundation will require advance planning and installation of the necessary engineering communications, for example, water supply, and sometimes power cable.
It is unlikely that such requirements can be attributed to shortcomings - it is rather assessed only as a specific technological feature, and with well-planned work it will not particularly complicate the entire construction process.
- There is a lot of talk about the high cost of such a foundation, which can reach almost half of the entire construction estimate.
Such frightening indicators, apparently, will be valid only for the deep slab already mentioned above. If the foundation is practically not deepened, the picture is certainly not so “terrifying”.
Of course, even with a small thickness of the slab, but with a considerable total area, centimeters very quickly grow into cubic meters of concrete solution. Two-tier reinforcement will require a significant consumption of reinforcement, certainly more than when pouring strip base. However, we must not forget that, along with the foundation slab, the developer immediately receives a finished foundation - essentially, the subfloor of the first floor, with it already well-made, and sometimes with insulation. That is, these stages of work are already falling out of the overall estimate.
So the excessively high cost is not always an obvious drawback, and the simplicity of constructing the slab largely compensates for the increased consumption of building materials.
How to calculate a monolithic slab foundation
Any foundation requires calculations, and slab foundations are no exception in this regard. True, it should be especially noted that designing such structures is still the lot of professionals, especially if it is planned to build a full-fledged country mansion.
However, sometimes you can resort to calculations yourself, for example, when constructing non-residential structures - a garage, barn, bathhouse, and utility buildings. And one of the key calculation parameters is always the thickness of the monolithic slab. Too small a thickness may not cope with bending loads; excessive thickening is a waste of effort and money.
How is the optimal slab thickness calculated?
Calculations should ideally be preceded by an analysis of the soil on the building site, since it is necessary to have an idea in advance about the bearing capacity of the layer on which the foundation slab will rest. Usually, specialists with a drilling rig are invited to do this, who make several holes, for example, in the corners and in the center of the site.
This makes it possible to evaluate the composition and thickness of the layers, the presence of “overwater”, and the location of aquifers, on the basis of which further calculations can be made.
Any of the soils is characterized by its load resistance, that is, in fact, by its bearing capacity. This parameter can be expressed in kilopascals (kPa), but for calculations in the metric system it is more convenient to use the value kilogram-force per square centimeter (kgf/cm²).
| Soil type | Design soil resistance | |
|---|---|---|
| kPa | kgf/cm² | |
| Coarse soils, gravel, crushed stone | 500÷600 | 5.0÷6.0 |
| The sands are large and gravelly | 350÷450 | 3.5÷4.5 |
| Medium sand | 250÷350 | 2.5÷3.5 |
| Dense sands of fine or silty fraction | 200÷300 | 2.0÷3.0 |
| The same sands, but of medium density | 100÷200 | 1.0÷2.0 |
| Sandy loam, hard and plastic | 200÷300 | 2.0÷3.0 |
| Loams, hard and plastic | 100÷300 | 1.0÷3.0 |
| Clays with a hard structure | 300÷600 | 3.0÷6.0 |
| Plastic clays | 100÷300 | 1.0÷3.0 |
It is clear that the distributed pressure created by the mass of the planned house (taking into account also the external loads on it) and the mass of the slab itself should not go beyond the specified limits. However, such a calculation will still not be sufficiently objective.
When calculating the required slab thickness, it is better to use the values of the optimal specific pressure on a particular soil - these indicators are determined specifically for slab foundations. The calculated value of the load from the entire structure, including the weight of the slab, should be as close as possible to the optimal value, with a possible deviation not exceeding 20÷25%.
Why is this being done? It is important not to go to two extremes. If the optimal load value is exceeded, there is a possibility that the slab will eventually begin to sink into the ground. However, no less dangerous is a significant decrease in pressure on the ground - a structure that is too light for specific conditions becomes too “floating”, that is, it can warp even with the slightest seasonal fluctuations in the ground.
Please note the following:
- The second table does not show all types of soils. The fact is that on soils with high load-bearing capacity, the construction of a slab foundation itself simply does not make much sense - you can get by with much cheaper options.
- In addition, two rows are highlighted in color in the table. In both of these cases, it is recommended to conduct an in-depth analysis of the technical and economic feasibility of constructing a slab foundation.
— In the case of sandy loams, it is possible that the construction of a conventional strip foundation may be much more profitable.
— Hard clays are singled out for the reason that the density of their structure can sometimes be deceptive. If there is a possibility of waterlogging of these layers, for example, by nearby aquifers with seasonal fluctuations in their filling, then a sharp loss of the bearing capacity of the soil cannot be ruled out. The slab, along with the building, will gradually begin to “sink.” It is worth considering the question of the possibly greater feasibility of using
So, in order to calculate the required slab thickness, you will have to determine what distributed load the building itself will exert on the base, then find the difference with the optimal pressure value, and cover the remaining “deficit” using the mass of the reinforced concrete slab. Knowing the specific density of reinforced concrete, it is easy to calculate the volume, and having the area of the slab as initial data, determine its optimal thickness. At the same time, do not forget to take into account that the slab must protrude beyond the perimeter of all walls outward by at least the amount of its calculated thickness or even more - this already depends on the features of the project.
Below the reader will be offered a calculator in which this calculation algorithm is implemented. Of course, this application cannot compete with the accuracy of calculations professional programs, but for “estimates” in the field of DIY construction it can provide a useful service.
The calculator assumes that the developer has design outlines for the future building, that is, it will not be difficult for him to decide on the initial data. You will need to know the material and area of the walls (minus window and door openings), the area and type of floors, the area of the roof and the steepness of its slopes (to take into account the snow load). The calculation program already includes average values of the specific gravity of building structure materials, and takes into account approximate operational loads (weight of finishing, furniture, large household units, dynamic loads from people living in the house, etc.).
How to correctly calculate the area of structures?
Since area values often appear in calculations, it is worth giving appropriate recommendations in this regard. They are presented in a special article on our portal dedicated to it, which, by the way, also contains convenient calculators.
It is best to prepare the data necessary for the calculation in advance, write it down in a separate table, and then proceed with the calculations.
Nowadays, when the field of building materials is rapidly developing, there are many types of foundations that differ in characteristics, type and purpose. But in Lately Slab foundations are becoming increasingly popular among professional developers. This type is especially convenient for owners of plots with land in poor condition. And also, this option is considered one of the available ones. Even an inexperienced person in repair matters can fill a foundation slab on their own without putting in much effort.
Areas Suitable for Slab Foundation Applications
The most important feature of a slab foundation is shallow base, which is made of a concrete pad. This property allows it to fulfill its purpose even on moving soils. Any other type of foundation will become covered with cracks when the soil moves, so in such areas it is necessary to use exclusively slab foundations.
This base is perfect for the construction of houses where harsh substances are used. It prevents deformation of the structure.
Buildings that have low floors compared to the ground must be erected using exclusively slab foundations. When using it, there is no need to construct a basement floor and grillage.
Slab foundation advantages and disadvantages
Before you start choosing a product, you need to weigh all its pros and cons. Let's start with positive qualities slab foundation:
- It is reliable and durable. And it's hard to argue with that. Interview specialists who understand building materials, and everyone will unanimously say that a tiled foundation is the most reliable type of foundation.
- It has a large area of support on the ground, and if it is laid correctly, the soil will not adversely affect the foundation in the cold season.
- The slab foundation is universal. The fact is that specifications which it possesses allow it to be laid on almost any soil. Therefore, this type is most often used in areas where no other foundation would cope with the task.
- The ability to install the foundation above the freezing depth of the ground. This plus was made possible thanks to the massive sand and gravel cushion, which prevents the impact of the soil in the cold winter.
- A high level of rigidity, which prevents the house from sagging, and if this happens, the room will not suffer significant damage.
- No need to install a subfloor.
- The ease of technology of the device, which allows you to do the work yourself and save on the services of a specialist.
- Suitable for any type of low-rise building. The only thing that needs to be taken into account is the thickness of the slab (the lighter the room, the thinner the slab).
- Can withstand ground traffic. It is possible to build a house on floating lands, since the slab foundation will not burst or tear, but will simply begin to move along with them. This property keeps the building intact.
It is worth considering that all the previously listed advantages will be valid only if the installation rules are strictly followed. If there are technology violations during the laying process, then the advantages may turn into disadvantages.
Like any foundation, there are some negative qualities:
- One of the main drawbacks that buyers note is the price. The high cost is justified by the large amount of concrete, reinforcement, sand, and crushed stone, unlike analogues. But on the other hand, the money spent quickly pays off. The slab foundation will not require frequent repairs or costly maintenance; moreover, it will function as the floors of the first floor, which will also reduce costs.
- It is not recommended to lay a slab foundation under a house that is located on a site with a large slope. In this case, the solution would be to level the ground level, or install a basement floor, but this is quite expensive.
- There are difficulties in constructing a basement. If you build a basement, you will have to pour a monolithic slab underneath it, which will cause significant losses to the owner.
- Difficulties arise when installing the foundation on winter days. To pour concrete in sub-zero temperatures, you will need to spend money on heating it and maintaining warmth around it.
- Difficulties in wiring communications. You need to think about the location of the water supply, electricity, etc. before pouring concrete because it will be impossible to do so afterwards.
As you can see, a slab foundation has a large number of advantages, and the disadvantages can be overcome, although this will cost additional costs.
Material calculation
Before starting work, it is necessary to calculate the slab foundation: dimensions of the formwork, quantity and diameter of reinforcement, volume of concrete. All these values can be found using a special calculator. To do this, you will need to enter the brand of concrete, the width and length of the slab (in meters) and its height (in cm). Next, the program itself will calculate the amount of materials required for arranging the foundation.
Types of slab base
Also, before starting work, it is worth deciding on the type of tiled base. There are several options. The main ones:
This type is used during the construction of buildings with a basement. A hole is dug, a concrete pad is placed, on which a base is subsequently installed. The entire load falls on the strip foundation.
It is the best option among all because it has high strength. For its construction it is required large quantity materials and effort, so its technical characteristics are superior to other types of foundations.
Preparation of materials and tools
Preparatory work also includes assembling a set of materials:
- concrete;
- steel rods;
- crushed stone;
- sand;
- material for waterproofing (geotextile is most often used).
You will also need some tools for a slab foundation:
Construction of a slab foundation
The instructions for constructing a slab foundation include several steps:
1. First of all, we carry out a geological study of the earth, identifying places with strong differences in elevation.
2. We mark the future foundation.
3. Remove the layer of soil and level the area where construction will be carried out.
The choice of pit depth directly depends on the type of structure (shallow or buried). If we consider a shallow foundation, then it will be enough to remove 50-70 cm of soil. The dimensions of the pit are selected taking into account that they should be 1-2 meters larger than the length/width of the foundation. After digging, be sure to compact the soil.
4. We design and install formwork (strong wooden boards are suitable for its construction).
5. We dig several trenches in the hole and lay waterproofing in them.
6. We lay plastic pipes on top of the waterproofing.
7. Fill the bottom with a mixture of sand and gravel, distribute it evenly and compact it.
8. We assemble a double reinforcement frame and tie it together using special wire.
Only rods with ribs are suitable for fittings. They will ensure high-quality adhesion between the frame and the concrete mixture, and also prevent stretching of the foundation slab. If you use smooth rods in your work, the base will crack during the first subsidence of the soil. The reinforcement is laid in increments of less than 30 cm. If you increase the distance, the strength of the base will significantly decrease.
9. We place plastic pipes in places where various communications will be located.
Before installing, they must be covered with sand and the backfill thoroughly compacted. The pipes must be laid before pouring begins, since making holes in the concrete is prohibited.
10. Fill the monolithic slab with concrete mortar.
The slab foundation is poured immediately. The concrete solution must be mixed once and in a large quantity, so that it is enough for the entire pour. This work is quite labor-intensive, so ideally it should be done by four people. We lay the concrete in even, horizontal layers. It must be poured in such a way that the slab does not deviate from the horizon. To prevent such slopes, we create stiffening ribs in the lower part. These are strips of concrete made in the shape of trapezoids. The last layer must be poured very quickly. Use a mixer or concrete pump for this.
11. Carefully smooth and level the surface.
12. Cover the resulting foundation with film and leave it for a couple of weeks. Do not forget to moisturize the structure during the first five days.
How to properly mix concrete for a foundation?
To reduce the cost of work, you can mix the concrete at the site of the foundation yourself. But in this case, you will need a special concrete mixer. It mixes all the ingredients perfectly without forming lumps. This unit operates at a power of 250 watts and above. At one time, you can get from 50 to 250 liters of solution. But the quantity greatly affects the operating time. If we take a small device as an example, it will take about 5 hours to create 1 cube of pouring.
An important point is the location of the concrete mixer; it should be close to the base. This will reduce the amount of force you apply.
Of course, there is an option to mix concrete for a slab foundation yourself using a shovel, but this will greatly affect the quality of the future foundation.
Of course, the best option there will be insulation of the foundation to achieve warm temperature indoors all year round. For this purpose, various insulation materials are used. The most popular among them is polystyrene foam. Just a 10 cm layer will be enough.
Also, there are different types insulation. We recommend using subfundamental. The insulating material is placed directly under the slab.
Use the tips below to make your work easier:
1. To create a high-quality foundation, you need to use only strong reinforcement and concrete mix.
2. The use of a shallow foundation will reduce the cost of building materials by approximately 35-45% of the cost of the underground premises.
3. It is very important to consider the thickness of the slab. For example, having chosen a base thickness of 20 cm, you will need to “reinforce” it in some places where the load is too heavy. At 25 cm, it becomes possible to knit the frame from the reinforcement uniformly, without resorting to additional reinforcements. A 30-centimeter thickness will help increase the strength and durability of the foundation, but at the same time it will force you to spend a lot on concrete.
4. When making a cushion of sand and gravel, the material must be laid in layers. One layer should not exceed 12 cm. After each laying, compact thoroughly. If your pillow consists exclusively of sand, be sure to moisten the layers.
5. Before laying the insulation material, the pillow must be covered with a thick plastic film to prevent water leakage from the concrete solution. It is advisable to glue or solder polyethylene at the joints, or lay the sheets overlapping.
After reading this article, everyone can be convinced that constructing a slab foundation is a fairly simple process and can be done with your own hands. The main thing is to follow all the rules of pouring and construction technology. If this is done, then your foundation will serve well for many years.
Making a slab foundation with your own hands
Construction of houses Nowadays, when the field of building materials is rapidly developing, there are many types of foundations that differ in characteristics, type and
On wet heaving soils or soils with high groundwater rise ideal option for the foundation is a reinforced slab. This is especially true for buildings that are sensitive to deformation - buildings made of aerated concrete, brick, etc. If all technologies are strictly followed, you can make a monolithic slab foundation with your own hands without the involvement of specialists.
When can you use a slab foundation?
Unlike strip foundations, a 25-40 cm thick slab is laid not under load-bearing walls, but under the entire building. When the ground moves, such a “cushion” will rise slightly at the same time as the house, without causing any damage to it. Shallow slab foundations are most often used in northern Europe for light weight buildings.For the manufacture of such a base, frost-resistant concrete M300 with W8 water resistance. If the groundwater is too close, it is advisable to use special sulfate-resistant concrete.
Advice. If the structure is heavy, you can use it to protect the slab from cracking. structures equipped with ribs, directed towards the ground. Such ribs, like the base itself, are reinforced with metal rods.
Ribbed foundation slab
The most significant disadvantage of slab structures is the complexity of the construction of basements and ground floors. After all, during their construction, the cost of the insulation layer will be equal to the cost of the foundation itself.
Selection and installation of reinforcement
1. For foundations of this type, only ribbed reinforcement is used. The frame is made of rods 12-16 mm with cells measuring 20 cm. When constructing outbuildings (baths, small country houses, garages), rods 12 mm thick are used. Heavier and more massive one- and two-house houses are erected on a foundation reinforced with rods with a thickness of 16 mm or more.2. The reinforcement is laid in two layers. The upper and lower horizontal layers should consist of longitudinal and transverse rods. Vertical reinforcement connects both belts together.
Laying reinforcement
Advice. The strength of the reinforcement at the welding site is reduced, so it is recommended not to weld it, but to tie it. In addition, if the rods are fixed too tightly on moving soils, the rigid ligament may simply burst. Therefore, they must have a certain degree of freedom, which is provided by the ligament. However, in order to avoid displacement of the rods during pouring, they must adhere to the reinforcement quite tightly.
3. The reinforcement should be fastened knitting wire round section (at least 1-1.2 mm). It must be fired and bend easily.
Knitting reinforcement
4. To tie, fold the knitting wire in half, then wrap it around the reinforcement and twist its ends together with pliers, a special hook, a collet (a device with a sleeve for clamping) or a knitting gun. One knitting requires about 30 cm of wire.
Collet
Hook for tying reinforcement
5. To prevent the reinforcement from touching the formwork, it is better to strengthen it with plastic grannies.
Plastic sockets for fittings
The procedure for laying a slab foundation
1. The soil is removed over the entire area of the future foundation to the required depth. It must be carefully leveled, otherwise the soil pressure on the slab will be uneven. However, loosening or digging up the soil at the planting site is strictly prohibited.
Leveling the area for concreting
2. The prepared site should be 2-5 m wider than the dimensions of the building.
3. A monolithic reinforced concrete foundation slab is laid on a 10-30 cm “cushion” of sand and crushed stone, which serves to additionally cushion the structure and protect the base from moisture. The “pillow” is laid in layers, with each layer spilled with water and compacted.
Slab foundation diagram
4. After the cushion layer, communications (water, sewer and drainage pipes) are laid.
5. For arrangement drainage system Transverse trenches are dug under the future foundation, which are covered with geotextiles, and then perforated pipes with holes for collecting water are installed in them. The trench is filled with fine crushed stone and again covered with geotextiles.
Laying the drainage system
6. Place on top of the sand layer concrete screed 10 cm thick, and after it has completely dried, a layer waterproofing. It is better to use modern bitumen-polymer materials that are resistant to temperature changes: in this case, the foundation will last much longer.
Advice. To protect the concrete screed from moisture, it is advisable to additionally treat it with a diesel fuel primer with the addition of 40% bitumen.
7. An overlap of 30-50 cm is made along the edges of the slab (then the edges of the waterproofing material should be wrapped around the slab and soldered to its end). All its seams are soldered with a blowtorch.
Laying waterproofing
8. If the foundation slab itself will serve as a subfloor, it should be securely insulate using polystyrene foam or more durable extruded polystyrene foam. Since when this material gets wet, its thermal insulation properties decrease, it is advisable to lay a polyethylene film on top of it.
Location of the thermal insulation layer
9. Formwork is erected slightly higher than the height of the slab. It is knocked together from boards or made from shields. The outer side of the formwork is supported by boards (struts). It should be positioned strictly vertically (you can check this with a building level) and have right angles.
Formwork for slab foundation
Formwork diagram for a monolithic slab
10. After installing the reinforcement layer, the site is poured concrete mortar. It must have a sufficient degree of mobility so that the crushed stone does not get stuck between the reinforcement bars.
12. The poured solution is carefully leveled and left until completely dry.
Proportions for preparing a solution for a foundation made of M400 cement