25. Columnar foundations. Solid foundations.
Foundation - this is a supporting structure, a part of the building that takes all the loads from the structures above and transfers it to the base. Foundations are laid below the freezing depth of the soil in order to prevent their heaving. On heaving soils, when constructing light wooden buildings, shallow foundations are used.
Foundation designs come in various types : strip, columnar, slab (solid) and pile. The choice of foundation type depends on the structural system of buildings, the magnitude of the transferred loads, as well as the bearing capacity and deformability of the soil.
Columnar foundations in the form of prefabricated reinforced concrete pillars and pillows, they are used to transfer loads from the columns of frame buildings to the ground. Columnar foundations are erected mainly for houses without basements with light walls (wooden, panel, frame). They are also laid under brick walls when a deep foundation is required and a strip foundation is uneconomical. Columnar foundations are 1.5-2 times more economical than strip foundations in terms of material consumption and labor costs. It is preferable to construct columnar foundations on heaving soils , since at minimal cost they can be installed below the freezing depth.
Depending on the design of the building, the pillars for the foundation can be stone, brick, concrete, rubble concrete, reinforced concrete and other materials. Most often, when constructing columnar foundations, ready-made prefabricated concrete and reinforced concrete blocks are used. Columnar foundations must be installed under the corners of the house, at the intersections of walls, under frame posts, heavy and load-bearing partitions, beams and other places of concentrated load.
To reduce the pressure on weak soils, columnar foundations made of piece materials are widened in the lower part, making ledges at least two rows of masonry high.
To increase the stability of columnar foundations, to avoid their horizontal displacement and overturning, as well as to arrange the supporting part of the base, a grillage is made between the pillars. When installing columnar foundations For wooden buildings, the function of a grillage can be performed by a wooden frame made of logs or timber. In this case, the space between the planning ground mark (blind area) and the piping is filled with a fence.
The pillows of such foundations are made in the form of special glass-type blocks or various combinations of trapezoidal prefabricated pillows of strip foundations. For heavy loads, the column foundation can be supplemented with flat reinforced concrete slabs of the required dimensions. The external fencing of the underground space of buildings with columnar foundations is made of basement panels, which are supported on special consoles of columns of the outer rows or ledges of foundation pads.
Solid (slab) foundations They are used primarily in the construction of multi-story buildings on weak, unevenly compressed soils. Slab foundations are a type of shallow, or rather, non-buried foundations, the depth of which is 40-50 cm. Design slab foundation is associated with the consumption of concrete and reinforcement and may be advisable when constructing small and compact houses or other buildings when a high base is not required and the slab itself is used as a floor.
The foundation slab is designed flat or ribbed with the ribs located under load-bearing walls or columns. The ribbed structure reduces the consumption of steel and concrete, but is more labor intensive than a solid one. When making foundations from flat slabs, formwork and reinforcement work (rolling out ready-made reinforcement mesh) are extremely simplified, and concrete work is mechanized. Due to their lower labor intensity, foundations in the form of solid-section slabs are more common than ribbed ones. The thickness of the foundation slab is determined depending on the span (pitch) of the supporting structures and the type of slab itself and is 1/8-1/10 span for ribbed slabs, and 116-1/8 span for solid slabs.
A solid, non-buried slab as part of the spatial system “slab - superfoundation structure” ensures the perception of external force influences and possible deformations of the soil foundation and eliminates the need for various kinds of measures to prevent uneven deformations of the soil, which usually require significant resources in conditions of weak, sandy and heaving soils.
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, in fact, interests us, 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 level groundwater, 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 tapes. Another “centaur” suitable for such places is 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(at least twice as much as with 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 (one and a half floor options, attached garages, special solutions for flights of stairs and sites...), 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 main 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.
Slab foundation area cannot be smaller 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 specified 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 bases 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 pillow is artificial base, it is intended 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 pillow is covered with thick plastic film or other waterproofing materials, which during concreting will prevent leakage of laitance. 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
With all the modern variety of types of foundations and their advantages, many bathhouse builders still prefer monolithic ones. After all, what is whole is always stronger than prefabricated structures. And the construction process in this case is somewhat simpler. And the most popular foundation is a monolithic slab, which is so reliable that skyscrapers are even built on it.
What is good about this type of foundation?
Monolithic foundations are always strong and can withstand heavy loads. They are not afraid of uneven soil movements, constant heavy rainfall, or severe freezing and thawing. The bathhouse will simply rise and fall along with the foundation, without destroying any supports. After all, it is known that concrete works only for compression - and not for expansion. That is why a foundation in the form of a monolithic slab is practically irreplaceable for heaving and sandy soils where the groundwater level is high.
Yes, for timber, frame and log bathhouses such a foundation in some cases is a luxury - if the soil is normal, then it is easier to make a shallow strip foundation. But the Russian bathhouse itself has long ceased to be just a hut - its own dimensional ones are becoming fashionable bath complexes with swimming pools and entire billiard rooms. And for a massive steam room, a slab monolithic foundation is what you need.
Types of monolithic foundation designs
There are several types of monolithic foundation. The most popular is the slab type, which is also divided into just a slab and a slab on a tape, similar to an inverted bowl, which is becoming more and more popular abroad day by day.
But in terms of building a bathhouse, this type of monolithic foundation has proven itself to be the best so far - a monolithic slab of a simple design. Its main advantage is that there is no need to install it below the freezing depth of the soil - and this is a significant reduction in costs Construction Materials and reliability during sudden changes in air temperature.
A slab monolithic foundation is essentially a solid reinforced concrete slab that is buried in the ground. Both the external and internal walls of the bathhouse are built directly on this slab. And thanks to the uniform distribution of the entire load over the slab area, the pressure on the ground is minimized - the same physical law applies here when a person in boots falls into the snow, but not on skis, because the pressure area is already larger. The design of the slab is so versatile that it is suitable even for open peat bogs and even swamps. And most importantly, any errors are practically excluded in the construction of such a foundation, and therefore it is ideally suited for private construction. Including for a bathhouse, because the volume of excavation work in this regard is minimal, and ground floor the steam room is not really needed.
Another type of monolithic foundation is a columnar monolithic foundation, which is built for light baths. In fact, this is a single structure made of a grillage and the pillars connected to it.
But a strip monolithic foundation with a basement is capable of withstanding quite large loads and feels good in the most unfavorable climatic conditions due to the fact that it copes well with subsidence, thawing and ground vibrations. Essentially, this is a reinforced concrete strip that runs along the entire perimeter of the building. It can be shallow or recessed. The first option is suitable for a bathhouse made of logs and timber, but the second is for two-story brick steam rooms, which have considerable weight.
Stages of construction of a reinforced concrete slab
The process of constructing a monolithic foundation is much simpler than constructing prefabricated ones. But there is an important point: all materials used must be the same High Quality, because more serious requirements are imposed on a monolithic foundation. But there is no need to use construction equipment!
Stage I. Site preparation
The first thing you need to do is clear the area well: remove the top layer of soil with vegetation, for which you can hire a bulldozer.
The thickness of such a foundation, or rather, a monolithic slab, can vary from 15 to 40 cm. This depends on the characteristics of the soil, the weight of the future bathhouse and what it will be filled with.
Stage II. Digging a pit
Typically, a pit for such a foundation is dug to a depth of 1.5 meters, clay is pulled out from there and replaced with gravel or sand. The surface should be leveled according to construction level– there can be no talk of any slopes, otherwise deformation and complete destruction of the future foundation cannot be avoided.
Stage III. Installation of formwork
Sometimes such foundations are built from ready-made monolithic reinforced concrete slabs, which can be seen during construction in a panel house. They already have a clearly calculated quality, but to install them you will have to call a crane and still make a concrete screed on top of everything. And such a structure will no longer be as rigid as an absolutely monolithic slab.
But for something built with your own hands, you initially need formwork. It will require boards with a thickness of at least 25 mm plus bevels. The formwork itself must be installed with supports - and it is advisable to initially check the rigidity of the entire structure. This can be done with a simple kick - if the formwork breaks, it is better at this stage, and not during concreting.
Stage IV. Insulation and waterproofing
Here it is worth mentioning the Swedish technology for constructing such a foundation - it involves the use of modern thermal and waterproofing materials. Such a base is called an insulated slab, which has amazing energy-saving properties with short construction times and low costs. Just right for a Russian bath!
Stage V. Reinforcement
The next step is to install the fittings. Sometimes a floor heating system is additionally attached to a special mesh.
It is best to take 16 mm reinforcement - in extreme cases, of course, you can use 14 mm. But calculating it is not so easy - it is better to do it in advance.
The reinforcement must be laid crosswise, in two rows. This will result in two grids - one from below, 5 cm from the surface of the sand cushion, and the second from above, 5 cm from the surface of the foundation slab. There should be exactly 20 cm between the bars in the mesh. You need to knit the reinforcement with regular steel wire.
Stage VI. Pouring the foundation
It needs to be poured in one step, and it itself must only be of a high strength class - from M300 by brand, with a water resistance coefficient greater than W8 and frost resistance from F200 and a mobility index of P3. There is an important point here - all materials used must be of the highest quality, because more serious requirements are imposed on a monolithic foundation. In total, at least 20 cubic meters of concrete will be needed.
As soon as the slab is dry, the concrete floors in the bathhouse will be completely ready for finishing. This is the biggest advantage of a monolithic foundation – minimum hassle, maximum result!
On soft soils, the characteristic feature of which is increased compression, the best option The foundation of the house is a solid foundation. Start construction work for the construction of a house is associated with determining the quality of the soil at the construction site, the depth of groundwater, the level of freezing and the material from which the construction will be carried out. In addition, it is necessary to determine the number of storeys of the building, because the load exerted directly on the base of the house depends on this. The construction of a solid foundation is necessary in cases where the load on weak ground quite big. Such a foundation is a monolithic concrete slab located under the entire area of the building.
Features of a monolithic foundation
The main feature of a solid monolithic foundation is that it can withstand a high degree of load, since the slab is made using a reinforced frame that occupies the entire area of the building. Such a base has a flat, smooth surface and therefore can serve as a basement floor.
To install a solid foundation, it is necessary to erect formwork and allows construction to be carried out on any soil.
Even moving soil is incapable of damaging the integrity of the structure, and the evenly distributed load makes it possible to erect buildings on such a foundation, both the lightest and the heaviest, consisting of two or more floors.
Installation of a solid foundation is justified when carrying out construction work on buildings:
- on soil with a high sand content;
- in wetlands;
- on subsidence and peat soils.
A solid foundation is also irreplaceable in areas where a characteristic feature is the presence of soil close to the surface.
The use of a solid foundation is necessary when constructing buildings in soils prone to significant swelling. A slab made of reinforced concrete is located over the entire area of the building being constructed and does not lose its strength and shape, moving, if necessary, along with the soil.
Work on the construction of a solid foundation
First of all, before starting work, you will need to perform a calculation to determine:
- slab thickness;
- slab laying depths;
- total base area.
In order to significantly increase the strength of the building, the area of its base is increased by one or even two meters in each direction. When performing calculations, it is necessary to take into account the bearing capacity of the soil and the increase in load due to interior walls, ceilings, installed furniture and equipment. To obtain more accurate results, add 150 kg/m2 to the weight of the building itself, then the resulting number must be divided by the area of the house. The brand of cement used to prepare concrete is also taken into account.
M500 grade cement makes it possible to obtain a composition that, when hardened, can withstand a load of 500 kg/m2; accordingly, the thickness of the base slab will be at least 50 centimeters.
Using reinforced concrete slabs, builders receive a reliable and durable foundation for light frame structures and heavy multi-story buildings.
Installation of a solid foundation
Pouring a monolithic slab Reinforced concrete solid foundations are erected in several stages:
- marking the site intended for construction;
- erection of formwork;
- installation of reinforcement frame;
- pouring concrete.
For construction small house In a regular shape, you can use ready-made reinforced concrete slabs, but if the project of the future building is drawn up taking into account the wishes of the owners and the house has non-standard shapes and sizes, then it is necessary to pour concrete in accordance with the available data.
Marking
Before you begin marking the site, you should carefully prepare the site, getting rid of debris and vegetation. Then you need to use a level to achieve a perfectly flat surface on which the markings will be carried out. Transferring the plan of the future house drawn up according to the project to the surface of the earth requires the use of special marks, pegs, and laces. The construction thread should not be made of nylon. A stretchable cord is not able to maintain its shape and size, which means that the markings made will be inaccurate. Watch the video on how to mark the foundation.
After the pit is ready, a sand and gravel cushion is placed at the bottom, which must be thoroughly compacted. Trenches are laid across the future foundation over its entire area, the bottom of which is lined with geotextiles, and then covered with gravel and crushed stone. This is necessary drainage.
Formwork and frame
The formwork for a solid foundation is placed, protruding beyond the pit by 20 cm along the entire perimeter. The bottom of the pit is covered with a layer of crushed stone, the thickness of which must be at least 20 centimeters, and a solution based on a cement-sand mixture is poured on top of it, performing the first screed and creating a flat surface. It is covered with rolled waterproofing materials and the construction of formwork begins. Along the entire perimeter of the pit, supports are dug in for boards or panels, from which the formwork will be erected. The work is carried out under level control. Watch the video on how to install formwork for a solid foundation.
A reinforced mesh is laid on the surface of the first screed, rods are installed vertically at a distance of 20 cm, to which the lower mesh and, later, another upper mesh are tied.
The structure is fastened using annealed wire. The use of welding will lead to the formation of bridges that promote the development of corrosion.
Pouring concrete
When starting the final stage of work, you must remember that to create a reinforced concrete slab, you can order a ready-made solution, or you can prepare it yourself. But the hardening time is only 3-5 hours, and therefore you may not have time to prepare the concrete yourself. Therefore, it is worth spending money and ordering a mixer with ready-made concrete. The supplied solution is distributed over the base area using a rule, and then compacted using a vibrator.
There should be no metal components visible above the surface of the finished slab, so using a level, even before pouring begins, the height corresponding to the thickness of the foundation is marked on the vertical rods.
Foundations of different types can be erected under suburban and multi-storey buildings. For example, in some cases, solid slab foundations are poured under houses. Such grounds, in turn, can also be classified into several types. Before starting to pour a solid foundation, of course, its design must be drawn up.
Necessity of application
Slab foundations are one of the most reliable types of house foundations. In this regard, they are superior to tape and columnar ones in any case. However, the area of this type of structure is very large. They are solid foundations - a single thick slab under the entire house.
The construction of such structures is, of course, very expensive. In addition, for example, during the construction of a low-rise country house foundations of this type, unlike others, cannot be filled with concrete using homemade means. In this case, the cement mortar has to be ordered ready-made. Poured liquid concrete into the formwork when constructing such a foundation from a tank using a hose. And this, of course, makes the construction of the foundation even more expensive.
Due to the high cost, foundations with a solid slab are built under houses quite rarely. Their construction is considered advisable mainly only when the building is built on non-static soils. In this case, a solid slab can maintain the integrity of other building structures during movements.
Also, foundations of this type can be erected under various types of small-area buildings. For example, sometimes garden gazebos are built on such a foundation. Most often, under such structures, of course, columnar foundations are built. Solid foundation, however, in this case it can also be a good solution.
The slab under the gazebo or small extension, of course, will be very small in size. If the concrete depth is shallow, it won’t take much. In addition, it will be possible to fill the slab under the gazebo without the use of special equipment and assistants - manually at a time.
Main types by filling method
When building houses, solid foundations can be erected:
not buried;
shallow;
strongly buried.
The first can only be used in areas where there is no frost heaving. They build exclusively lightweight houses of small area on shallow foundations. The thickness of such structures, depending on the type of soil, can vary between 30-50 cm. Sometimes heavy brick houses. But the use of non-buried foundation slabs under such structures is allowed only on rocky soils.
Shallow foundations are usually erected during the construction of small private houses. The pit under them is dug very shallow. In most cases, when pouring such a foundation on an area according to the markings, the top fertile layer of soil is simply removed. Deeply buried foundations are built only on heaving soils under heavy buildings.
Types by design
In this regard, solid foundations are distinguished:
monolithic;
lattice.
The first type of foundation is a regular concrete slab. Solid monolithic foundations are the simplest and most popular type of such structures. But on very unreliable soils, foundations with stiffeners can also be installed. The latter are poured directly under the slab.
Sometimes the ribs at the lattice bases can be directed upward. In this case, the walls of the building are erected on them using approximately the same technology as on strip foundations. When using this type of solid foundation in a building, among other things, it is possible to equip a basement. This is how deep slab foundations, for example, are often poured.
Design
When developing drawings of a solid foundation, of course, first of all you should decide on its thickness. When constructing high-rise city buildings, such calculations are made exclusively by specialists using various kinds of formulas.
In individual construction, a design for a solid reinforced concrete foundation for a small house can be developed independently. In this case, most likely, you won’t even have to calculate anything. There are standard indicators for the thickness of such foundations for certain types of buildings, which can be used as a guide in the process of drawing up a project.
So, for example:
gazebos and light extensions are erected on solid foundations 100-150 mm thick;
under light frame private houses, as well as one-story log and cobblestone houses, foundations of this type are most often poured to a depth of 200-300 mm;
under concrete structures or with brick or two-story log buildings, solid foundations with a thickness of 250-350 mm are erected;
under two- or three-story houses made of brick or concrete, it is necessary to pour slab foundations at a depth of 300-400 mm.
Load collection
If you wish, of course, you can independently make a more accurate calculation of the solid foundation when building a country house. The collection of loads when pouring such a structure is determined taking into account:
not taken into account on sandy soils;
on clayey ones it is divided in half;
on quicksand it is taken into account completely.
constant pressure from the roof, ceilings, walls, etc.;
temporary loads - snow, furniture, people.
The permanent load is calculated depending on the materials used to assemble the building structures and their parameters. According to the standards, the mass of the walls should be taken minus the openings.
The weight of the slab itself when performing calculations of solid foundations:
The snow temporary load on the foundation is determined according to Table 10.1 SP. In this case, the parameter is taken for this specific area. Uniformly distributed loads for residential buildings are assumed to be 150 kg/m2. The weight of very heavy objects that are supposed to be placed in the house is taken into account separately.
Selection of materials
The collection of loads on such foundations is calculated, thus, in exactly the same way as on columnar and strip foundations. A solid foundation, like any other, is poured in most cases, of course, from a concrete mixture. Having determined the thickness of such a foundation, you can easily calculate the amount of material required for its construction.
Concrete for the construction of solid foundations is usually used grades B15-B25. You can, of course, pour slab foundations using a higher quality and durable mortar. However, this is usually considered impractical due to the increased cost of work. One of the undoubted advantages of slab foundations in any case is increased strength.
In addition to concrete, to build such a foundation you will also need materials such as sand, reinforcing bars and waterproofing. To assemble the formwork you will need to prepare boards. Use to create a pouring mold slab base According to the standards, houses require lumber with a thickness of at least 30 mm. Before pouring the solution, it is recommended to cover the formwork boards with plastic film.
Concrete and reinforcement
Calculate the amount of material required to fill such a base, in addition to the thickness of the slab, taking into account the fact that:
at the edges the foundation should extend beyond the building by at least 10 cm;
the reinforcement rods for the slab should be 6 cm shorter than it;
rods are installed when pouring in increments of 40 cm;
the sand cushion should also extend 10 cm beyond the building;
When pouring, the waterproofing material is laid with a small margin.
It is advisable to use roofing felt as a waterproofing agent for pouring such a foundation.
Work order
Slab foundations are poured in several steps. A pit of the designed depth is first dug on the site.
At the next stage, when arranging a solid slab foundation, a multi-tiered reinforcement frame connected with the use of wire is installed on the sand cushion. In order for the volumetric mesh to subsequently appear in the thickness of the concrete, special plastic stands or bars 5 cm thick are first placed at the bottom of the pit.
At the final stage, concrete from the tank is poured into the pit. During the process of laying the mixture, any defects that appear are manually eliminated. From time to time, the concrete layer in the pit is pierced with shovels to eliminate air bubbles. At the final stage, carefully level the surface of the slab.
To fill a lattice solid foundation, longitudinal trenches are dug in the pit before filling with crushed stone. The concrete poured into them subsequently forms ribs.
The final stage
After the foundation is poured, it is advisable to cover the slab with plastic film. Subsequently, the slab should be periodically moistened with water for 2 weeks. This will avoid the appearance of surface cracks. It is allowed to erect walls on such a foundation, as on any other, only after the concrete has fully matured. That is, approximately 28 days after pouring.