Concrete preparation for the foundation. Why is concrete preparation necessary for the future foundation? Why do you need a footing under the foundation slab

Laying the foundation is one of the most important components construction works. Not the least important in this process is the device of a special footing, which should be located under a solid foundation. The quality of its execution determines the stability and durability of the structure. Before carrying out work, it is necessary to determine what density of the concrete or crushed stone layer is needed for the construction of a particular building.

Definition

The footing is a layer of concrete mortar that must be prepared for an economical, convenient laying of the main foundation. This thin layer is poured to the main mass (for example, during the construction of the base using concrete slabs).

Purpose and thickness

A preparatory concrete or gravel base is made using different methods, means and building materials. But with any device, its purpose will remain unchanged - it consists in preparing and leveling the surface. Due to this, the main concrete pouring It is spent more economically due to uniform laying, which does not require additional costs of the mixture for unevenness and depressions in the ground. Concrete and crushed stone footing, which is required for construction work, is used in the following cases:

1. Surface preparation for ease of installation of structural parts. It is much easier to make markings on a leveled site, reinforcing cage etc. It is obvious that these works are more difficult to carry out on loose soil.
2. Waterproofing. Thin concrete bases are also necessary in order to retain moisture from the poured solution. Cracks appear on the frozen slab. Often they appear due to the fact that the liquid is unevenly distributed in the structure of the cement.
3. Protection of the foundation from groundwater. If the layer is laid under the slab base, The groundwater. In these cases, the footing absorbs some of the liquid, thereby preserving the load-bearing layer on which the building is based.

In addition, the presence of shock absorption allows you to increase the strength of the structure due to the fact that the base is better preserved. As for the thickness of the foundation monolith, it will depend directly on the dimensions and severity of the structure. On average, this indicator is in the range of 0.15-1 meter.

Types of training

Before starting the main work, it is necessary to prepare. The scope of work includes the following three stages:

• performing calculations;
• site leveling;
• surface preparation before laying the foundation.

The first preparatory stage involves careful calculations and work with documentation. Then the site needs to be cleared. From the territory you will need to remove debris, cut down and uproot trees, dig up shrubs and other plants. The next step involves creating a "pillow" on the surface, which will be located under the foundation of the future building. For this, the following building materials are used:

Preparation device (stages of work)

First you need to clear the surface of the soil and make a pit. Competent preparation of the deepening involves a series of works that are aimed at bringing the soil into the desired state. In other words, the soil must withstand the strong loads specified in the project. It is necessary that after the foundation is laid, the soil tightly wraps around the foundation from below. First, it is worth forming a pit, cleaning its bottom with a bulldozer, and then carefully compacting the base. In addition, during the compaction of the pit, the soil should be moistened or drained. Most trenches are dug by hand. In addition, builders visualize the surface of the base itself, set the corners of the walls using pegs. After the pit is dug, specialists proceed to the following stages of construction:

• marking the construction site for the foundation;
• site leveling;
• preparation the right amount gravel (layer - ten centimeters);
• compacting the pillow with the help of a vibration device;
• installation of the formwork (its height depends on the layer of the concrete mixture of the layer;
• pouring the solution to the top of the formwork;
• pillow reinforcement (section of rods - at least eight millimeters);
• ramming cement mixture vibrating plate;
• installation of a reinforcing cage that allows you to fasten the crushed stone pillow to the base (they should protrude above the poured concrete by about twenty to thirty centimeters).

Can be installed in concrete reinforced frame, but you can do without this material. In practice, the two options are almost indistinguishable. The only difference is that a pillow created without reinforcement is limited in size. will strengthen the bottom of the pillow, which perceives the pressure arising from the load of the structure and transmitted through the base.

The difference in the quality of the foundation of houses depends not only on the materials used for the construction, but also on how the site was prepared before construction began. When building a house, it is worth deciding whether the concrete preparation is suitable for the foundation, as the highest quality and most stable, or it is better to choose another one for this soil. Sand or crushed stone preparation is less reliable, more difficult to create, but cheaper in building materials and laying time.

What is training used for?

Most often, concrete preparation is used under the flood foundation or to increase the strength and stability of the base of the house.

Reasons to make a footing:

1. Additional waterproofing, which slows down the rise of moisture from the soil into the foundation.
2. flat construction site, removes any irregularities left when digging the foundation pit by equipment or builders, allows you to even out differences up to 10 cm.
3. Prevents "concrete milk" from soaking into the soil, due to the fact that concrete pouring is not carried out on open ground, and on a solid base, the base, when solidified, has the same density, which increases its performance.
4. Facilitates the installation of reinforcement, makes it possible to use clamps, protecting the foundation reinforcement from corrosion.
5. Eliminates bending of reinforcement due to being pressed into the ground under the weight of uncured concrete.

What are the types of training

When preparing a site for a monolithic or strip foundation, you can choose different materials depending on the desired characteristics, budget and difficulty in installing on your own. The main reasons to choose one or another option are the endurance of the soil, its moisture content, whether a basement or basement is planned. It is also important what type of support will be used, not buried, shallow or buried. From what materials, and in what way it will be built, poured or from ready-made blocks, tape or slab.

Unfortunately, SNiP 52-01-2003 and SP 50-101-2004 do not clear recommendations, in which case and what preparation to carry out, so the reason for choosing is the type of foundation and the materials used for it.

Sand preparation

The simplest and cheap way level the ground for construction, this is sand preparation for the base, for light wooden houses used quite often, but not suitable for houses with a design life of more than 10 years. It is unreliable, gradual mixing with the soil and germination by plant roots causes subsidence under the weight of the building, distortions and cracks in floors and walls are possible. Low moisture resistance requires additional waterproofing to prevent dampness in the premises.

Concrete preparation

"Skinny" concrete is used with a maximum of 6% cement content, the main filler of which is sand, sometimes crushed stone or gravel. It is poured in a thin layer over the entire area of ​​​​the future foundation, the thickness of the concrete preparation depends on the type of soil, the weight of the house and the level of occurrence ground water. Depending on these three factors thickness can be from 4 to 10 cm.

Preparation with membranes from the profile

A new way of preparing the foundation, easy to install and less expensive than concrete or crushed stone. Despite manufacturers' assurances of strength and similar durability, it is less in demand.

If possible, more often choose proven footings. Due to additional costs for thermal insulation, the cost may reach the price of standard preparation.

crushed stone preparation

Crushed stone preparation for the foundation is used if reinforcement or uneven weight distribution is not provided. Crushed stone is covered with a thickness of 15 to 20 cm, pouring it with bitumen until a film is formed or the layer is completely impregnated. For heavy houses, it is unreliable due to the heterogeneity of crushed stone; it can lead to distortion and cracks in the foundation and walls.

In the cold season, crushed stone preparation is almost never used due to the difficulty of creating desired temperature for bituminous filling.

Preliminary work before pouring the footing

The construction of the foundation and preparation for it begin with engineering and geological work to find out how deep the groundwater lies, and what pressure the earth can endure at the site of the future home. A construction plan is drawn up and the type of foundation is selected, monolithic or tape, poured or prefabricated.

Before creating a concrete cushion, it is necessary to mark the territory in order to dig a foundation pit. It should make it possible to make formwork for pouring, therefore, at least 30 cm is indented from the future support on each side.

The fertile soil layer is removed and transferred to another place where it can be used. In depth, the foundation pit should be at least 20 cm larger than the planned support height.

If thermal insulation is laid, then the depth of the pit is increased by the width of the additional layers.

Method for preparing lean concrete

Skinny concrete is called because of the very low content of cement, which, due to its brittleness, makes it useless for full-fledged construction, but ideal for concrete foundation preparations. Depending on the resistance, the choice is provided by 2 classes of concrete mixtures, these are B7.5 and B15, the latter is more difficult to use due to the high content of expanded clay.

To obtain a cubic meter of the finished solution from B7.5, you need:

• 160 kg of cement, these are 6.4 bags of cement weighing 25 kg each;
• 2.2 tons of sand;
• 75 liters of water.

All components must be mixed well and poured onto the prepared area.

In the manufacture of B15, a special technique is needed to obtain a uniform consistency of the finished mixture.

Footing for a monolithic slab

10 cm of crushed stone or sand are poured onto the cleared space and tamped with a vibrating plate. This is done to remove excess moisture from the base, making it last longer.

A formwork is mounted on a crushed stone pillow, into which a concrete mass of B7.5 cement will be poured. The formwork is made of such a height that the poured concrete forms a flat top, on which the base of the house will be laid. It should be a minimum of 10 cm and a maximum of 30 cm in height.

Often, when installing the formwork, it is advised to make a concrete preparation for the foundation with a margin of 10-30 cm relative to the foundation. This is done in order to reduce the pressure on the ground and leave room for thermal insulation and Finishing work, which prevent the destruction of the foundation from the side of the walls.

Concrete is poured flush with the top of the formwork, leveled and sometimes tamped with a vibrating plate. For better adhesion to the poured foundation slab, rods 0.8 cm in diameter are installed in the footing, protruding 10 cm from the preparation, if the base is planned to be large, then the footing will have to be poured in parts in order to insert the binding reinforcement.

If a ready-made concrete slab is bought, and not poured, then the bars are not installed.

After that, a layer of thin concrete is left to dry for 7–21 days, depending on the weather. Sometimes, for heavy buildings, the substrate is reinforced with a mesh with reinforcement 0.8 cm in cross section. In addition, the size of such a substrate must be at least 15 cm.

If the connecting rods were not installed in the substrate, then it is advised to put layers of waterproofing, thermal insulation from XPS, and then waterproofing again on the finished flooring. After that, you can mount the formwork for pouring monolithic slab basics.

How concrete preparation is poured and insulated can be seen in the video:

Footing for a strip base

Concrete preparation for a strip foundation is different from preparation for a slab. They dig a trench that is 40 cm wider, on each side of the foundation tape. Level the bottom of the trench and lay the waterproofing. Mount the formwork with a height of 20–30 cm and pour the concrete mixture.

The width of the footing tape should be 15 cm more than the base on each side. If it is planned to use a flooded strip foundation, the footing is reinforced with rods for better adhesion.

When a recessed strip base is built from ready-made blocks, it is advisable to use preparation from crushed stone rammed with a vibrating plate impregnated with bitumen. Sometimes waterproofing is placed on top and a second layer of footing is poured for better resistance to damage.

With a shallow or shallow foundation on stable soils, it is possible to level with sand or compacted gravel without bitumen. Such a foundation, due to the lightness of the building, does not require serious strengthening.

Preparing for a column foundation

For a pile foundation, sand preparation is more often used because of the functions of the foundation itself. It is installed for light houses and stable soils. Most of the time it doesn't require additional gain. If the house under construction is heavy enough, then crushed stone filled with bitumen is used. A concrete substrate is used extremely rarely, due to the possibility of buying small concrete slabs, which are more reliable than poured ones.

Whatever the foundation and house, in most cases, concrete preparation will extend the service life and, having spent money during the construction phase, will prevent spending on more expensive repairs to the foundation.

In fact, it is very simple to figure out what a footing is. Here, even the name speaks for itself - this is a layer of thin concrete, which is prepared for a comfortable and economical pouring of the main volume of the mixture. In fact, this layer is placed under the main layer of concrete during the device slab foundations, for example.

But why exactly this needs to be done (and whether it is necessary at all) is worth understanding in detail. In addition, it will be useful to consider what the installation instructions for such a base are.

Let's start with the most important.

The purpose of the footing

It is worth noting that the preparatory layer can be made different ways and materials may vary. However, the meaning, purpose of this work remains the same ().

In addition, the presence of such a shock-absorbing layer improves the strength characteristics of the entire structure as a whole. At least due to the fact that the main layer dries out and gains strength better, better.

Please note that the footing device involves the use of inexpensive concrete mix. As a rule, "skinny" concrete B3.5-B7.5 is used. That is, it turns out that due to the preparation of such a layer, the total price of the project is reduced, since a significant volume of the total cubic capacity is filled with a composition with a lower cost.

So, for what such work is needed, it is probably clear. Now let's see how this is all done.

Methods for constructing a footing

Before we begin, it is worth noting that footing is, first of all, a very desirable measure, but not always mandatory ().

That is, simply put, if you have a non-scale project and draft base consists of an old screed and there is no loose soil or uneven terrain at all, then it is simply impractical to perform such work.

So objectively assess the situation - you should not spend money in vain where such extra work will not affect the quality in any way.

Let's return to the options for the device of such a layer. As good example Let's take preparation for pouring the slab foundation.

Installation of a simple footing

For work, we need this:

1. Rubble.
2. Ramming device. It is easiest to make it out of wood or metal.

1. liquid bitumen.
2. Good and durable shovels.

You can start.

1. The required amount of crushed granite is poured onto the working area. As a rule, the material is dumped from a dump truck and the result is a large pile of rubble.
2. With the help of shovels, the stones are evenly distributed over the entire surface.
3. A tamping device is taken and the crushed stone is compacted over the entire area.
4. A layer of bitumen is applied to the resulting plane.

Tip: you can save money and use not bitumen, but a more or less durable roofing material or even a film. In any case, this will not give particularly good waterproofing in the future, but at least these materials will allow concrete to harden qualitatively (without loss of moisture).

In general, the simplest version of the footing is implemented.

Now let's look at how to do it as correctly as possible.

The device of the capital footing

So, we have a working area with loose soil and the risk of groundwater.

1. We determine the highest point of groundwater - the footing will have to be done just up to this level (well, in any case, it is advisable to do just that).
2. With the help of shovels, we try to level, plan the soil.
3. We fill the bottom of the pit with sand and evenly distribute it over the surface, and then ram it.
4. We lay a film or roofing material on top of the sand.
5. From thin reinforcement we make a kind of “cage” with a cell size of about 60 by 60 cm.

1. If desired, we install guide lines - beacons. It is convenient to stretch the mixture along them to get the maximum flat surface eventually.
2. We stir cement, sand, crushed stone and water - a homogeneous thick mass should be obtained.
3. Pour the solution and use the rule to distribute it over the entire area.
4. After the mortar dries, it must be treated with bitumen and, in fact, the footing device is completed on this.

Tip: in almost every such design, holes for communications are then made. As a rule, for this, cutting of reinforced concrete with diamond wheels with a grinder or simply drilling with a standard hammer drill is used. But you can greatly facilitate and speed up the work if you "lay" such holes in advance - in still damp concrete.

But even if you simply simply forgot about such moments, and not only the lining layer, but also the main slab is already flooded, then you should not despair, because you can always use such a service as diamond drilling of holes in concrete with powerful industrial equipment.

Many novice builders ask themselves the question “concrete, what is it and why is it needed at all?”. It is not difficult to figure out what this design is, since it is clear from the name that this is a thin layer of material that covers the foundation pit before laying the foundation (often such a layer is also called a pillow). However, it is worth deciding why you need to do this, and whether it is necessary to perform concrete footing when building a house or a bath.

Why you need to mount the footing

The footing performs a number of useful functions, namely:

• Creates a layer of waterproofing. Due to this, in the process of pouring the foundation, liquid cement mortar will not leak. In addition, the moisture in the screed will be distributed evenly and the base will not crack when dried.
• Allows you to create for drafting concrete base flat surface. Due to this, the consumption cement-sand mortar decreases.
• Protects the foundation from groundwater.
• Redistributes the pressure exerted by the soil and ground parts of the building.
• Allows for better reinforcement.
• Eliminates shrinkage of the building.

In addition, such a shock-absorbing layer significantly improves the strength and durability of the entire structure. The foundation, laid on the footing, is easier to "survive" the winter.

From the foregoing, it becomes obvious why the footing is needed, so let's move on to the varieties of these slabs.

Types of simple footings

The slab for the foundation is of several types:

Shchebnevaya

Such "preparation" is considered more economical, since crushed stone is cheaper. cement composition. The crushed stone layer must be at least 20 cm high. In the process of laying the footing, careful tamping is a prerequisite (preferably using vibrotamping equipment).

If he talks about the shortcomings of "preparation", then the technology of laying crushed stone footing under the foundation is considered unreliable. The fact is that such a substrate is not rigid enough, therefore, further work on the installation of the foundation on such a basis will not be performed on the high level. However, if you do not plan to build multi-storey building, then such a footing will be quite enough for economic block or baths.

To mount a simple base, follow these steps:

1. Prepare work surface and lay rubble on it.
2. Smooth it out with shovels.
3. Tamp and compact the pillow over the entire surface.
4. Apply a layer of bitumen to the resulting footing to obtain good waterproofing. If you want to save money, then roofing material or polyethylene can be used instead of bitumen, however, these materials have lower waterproofing performance.

Sandy

"Preparation" of sand allows you to best redistribute the load on the foundation. Such pillows are recommended to be laid in autumn and spring, when the soil undergoes changes. Due to the underlying layer of sand, the lower part of the foundation will be located above the groundwater level, so that the monolith will not suffer from harmful effects moisture. That is why most often sandy footings are mounted in areas with problematic soil.

To install such a layer, you must:

1. Remove a layer of soil.
2. Fall asleep instead river sand fraction of at least 1.5 cm.
3. Using the building level, level the pillow around the entire perimeter.
4. Tamp "preparation".

Healthy! To determine the desired thickness of the footing, as well as select desired type layers, it is necessary to take into account: the type of soil, the presence of surrounding buildings, seismicity and acting loads. Detailed requirements and calculations are set out in SNiP 2.02.01-83, as well as in SP 50-101-2004 and SP 63.13330.2012.

Sand and gravel pillows are not suitable for all buildings and are not very durable. If you want to make the most reliable foundation for a residential building, then, of course, you should give preference to a concrete slab.

concrete pad

The device of this type of footing requires large financial investments, however, it is such a foundation that is best suited for slab and strip foundation. The fact is that during the installation of such fundamental foundations, a heavy reinforcing frame of rigid steel rods is installed, which requires a stronger foundation.

Before starting the installation of the base, you need to consider a few tips:

• For the installation of footings, “thin concrete” of classes from B 3.5 to B 7.5 (M 50, 75, 100) is used. It makes no sense to use more durable cement, besides, it will cost many times more.
• To obtain a solid “preparation”, it is enough to lay a concrete pad 10 cm thick (provided that groundwater does not prevail on the site).
• Before laying the concrete mixture, it is necessary to pour a thin layer of sand or gravel onto the bottom of the pit or trench.
• If a reinforcing frame is not used when installing a concrete pad, then optimum thickness base will be 15-20 cm.

• Thanks to the reinforcement of the footing, the ground part of the structure will be located on secure foundation. For the armored belt, metal rods with a cross section of 8 mm are used. The rods are installed vertically and should protrude approximately 25-30 cm above the surface. In this case, the thickness of the "preparation" can be reduced by 6-10 cm.

Footing installation

Let's say you plan to build a house on a site with rather loose soil and an elevated level of groundwater. To do this, follow these steps:

1. Determine highest point groundwater - "preparation" is best done to this level. Some mount the "preparation" in such a way that it protrudes 10 cm beyond the underground structure.
2. Level and compact the soil.
3. Pour coarse sand to the bottom of the pit, spread it over the surface and carefully compact it. For a better effect, you can also lay a layer of rubble.
4. Lay roofing material or polyethylene on top of the sand.
5. Lay the reinforcement frame with cells 60 x 60 cm.
6. Install the guides, it will be more convenient to level the liquid mixture along them.

1. Mix cement, crushed stone, sand and water until a homogeneous thick mass is obtained.
2. Pour concrete mortar and align it to beacons using a rule.
3. After the base has hardened, treat the surface with bitumen.

Healthy! In order not to use expensive drilling equipment after the solidification of the monolith, it is recommended to consider openings for communications at the stage of laying raw concrete.

In custody

Depending on the type of construction, sand, gravel or concrete "preparation" can be laid. Such a pillow will give the entire structure additional strength and protect the foundation from groundwater.

Post Views: 11

The stability of foundations depends on the quality of foundation preparation. Its choice and arrangement is largely determined by the material of underground structures and the soil conditions of the building site. Concrete preparation for the foundation is considered the most reliable and expensive compared to sand and gravel pillows. Most often, it is performed under monolithic tapes and slabs, taking into account the requirements of the relevant SNiP and SP.

Why do you need a footing

First of all, the preparation of the site for the foundation is aimed at strengthening and leveling the foundation. But concrete layer it is also a barrier that protects the future underground monolith from the loss of cement laitance, which, when concreting the structure, can simply seep into the ground or the underlying layers of crushed stone and sand. The footing contributes to the retention of moisture in the mortar mass, which is required for the correct passage of the concrete curing process. If there is not enough water, the cement will not be able to fully reveal its properties, and this will lead to:

• to the appearance of cracks in the concrete structure;
• to a shortage of design strength;
• to further destruction of the foundation during operation.

The device at the base of the underground part of the house concrete pad in accordance with SNiP contributes to the maximum uniform distribution loads acting from the soil and the ground part of the structure. Preparing for a concrete foundation allows you to level the bottom of the excavation and stably position the reinforcing cage for a monolithic tape or slab in the formwork. In addition, the underlying layer virtually eliminates the occurrence of soil settlements due to the impact of large or point loads.

Another reason that determines why the footing is needed is that, according to a hard and even layer of preparation, the foundation device in winter period happens easier.

Regulatory documents - SNiP and Code of Rules (SP)

The device of any structures during the construction of civil and industrial facilities is subject to certain requirements specified in the relevant SNiP and other standards of state and industry significance. Concrete preparation for the foundation is carried out based on:

• SNiP 2.02.01-83;
• SP 63.13330.2012;
• SP 50-101-2004.

These documents define measures for the design and construction of foundations, taking into account:

• soil type;
• surrounding buildings;
• acting loads;
• seismicity;
• environmental requirements.

The thickness and width of the concrete preparation for the foundation is calculated, according to SNiP, according to bearing capacity and possible deformations. In the first case, the calculation is required if:

• significant compressive loads are assumed;
• the building is supposed to be placed near slopes, on a slope or embankment;
• under the sole of the foundation there are weak soils.

It should be noted that SNiP allows not to make calculations for the bearing capacity if the project provides for measures that exclude soil displacement for the previously named reasons.

All long-term and short-term forces transmitted from the structure, including the weight of the underground part of the structure, are taken as loads on the concrete preparation of the foundation. Possible combinations specified in SNiP.

Footing device

Regardless of the type of soil, at the first stage of work on the preparation of lean concrete for the foundation, the bottom of the excavation should be leveled. For your information, for a monolithic tape it will be enough to dig a trench, and for a slab a foundation pit will be required. When determining the depth of the excavation, the thickness of the footing and the thickness of the sand and gravel layer laid directly on the ground are taken into account.

Loose soil must be tamped, and sand and gravel should be added to the bottom of the excavation. The layers are also rammed. Crushed stone, in this case, will perform drainage function. Further, sheets of roofing material or films are laid on the surface as waterproofing.

Concrete preparation for a monolithic belt or foundation slab should protrude along the perimeter beyond the limits of the underground structure by 10-15 cm. With this in mind, a crushed stone-sand subbase is made and a formwork is performed for pouring the base. The height of the box is taken based on the thickness of the preparation for the foundation.

At the next stage, a lean concrete solution is mixed, which includes a small amount of cement. Gravel and sand are used as filler. After the concrete is poured into the formwork, it will need to be tamped to get rid of air bubbles and leveled. In the first few days, the surface of the concrete preparation is covered plastic wrap to prevent excessive drying of the surface.

It is important to know that the base for the foundation, which is not reinforced with reinforcement, has size limitations.

Reinforcement significantly strengthens the footing and increases the reliability of the underground part of the structure. The nets are knitted by means of wire from rods with a diameter of eight millimeters, and laid before pouring the solution. To ensure reliable conjugation of concrete preparation with the foundation, metal rods are vertically installed in the thickness of the base so that they protrude 20-30 cm above the concrete surface.

The optimal thickness of the footing without reinforcement is 15-20 cm. When constructing a reinforced concrete base for the foundation, the size of the preparation can be reduced to 6-10 cm. The SNiP indicates the maximum deviation of the horizontal surface - no more than 5 mm per meter when installing a monolithic tape and no more than 50 mm for a slab with a width of more than 25 meters.