What kind of support should be under the floor slab. Monolithic ceiling with your own hands. How not to support prefabricated floor slabs

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During the construction of a building, such an important issue as the support of floor slabs must be taken into account.

Building codes have special instructions on this matter.

Important structural element

Floors - load-bearing elements buildings made of reinforced concrete structures. They receive and distribute the loads from their weight and the people and equipment in the building onto walls and supports. With their help, the internal space of the structure is divided into floors, and the attic and basement spaces are also separated.

Floors in a building must meet many requirements. They must be strong, rigid, have good soundproofing characteristics, not burn and not allow water to pass through.

The material used for the production of floor slabs is reinforced concrete. Basically, these are multi-hollow structures with voids of different shapes: polygonal, oval, round. Most often in construction, elements with round voids are used. They are highly durable, technologically advanced and completely ready for installation. Their load-bearing capacity is 800 kg/m². They are laid on load-bearing walls located at a distance of about 9 m from each other. They rest on two sides. They are distinguished by fire resistance, rigidity, long term operation. Brick, aerated concrete, foam blocks and reinforced concrete panels are used as materials for the walls on which such overlapping elements will be laid.

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Some calculations

To find the amount of support for the floor slab, the base on which it is planned to be laid is of great importance. It is imperative to take into account the length and weight of the structure, the thickness of the supporting wall, and the seismological stability of the building. In addition, the load and its nature must be taken into account, whether it will be temporary or permanent. Such calculations should be carried out by specialists. For an individual developer, when drawing up a project and installation, the main reference point is the manufacturer’s markings.

When using flat overlapping elements, the span can be calculated as follows: you need to sum up the thickness of this element and the distance between the two supports. As for the depth of support of the floor slab on the brick base, this value should be equal to the thickness of the structure itself, but not less than 70 mm. To calculate the minimum thickness of the outer wall, which will become the basis for the floor slabs, it is necessary to take into account the thermal insulation layer and facing material on the end parts of the latter. Thus, a structure with a thickness of 140 mm must be supported by a base whose thickness is at least 300 mm.

Installation of frequently ribbed structures that have liners requires a minimum deepening of the floor slabs onto the base - 150 mm. During installation, do not allow hollow liners to enter the wall. If the ribs are reinforced with two rods, then it is necessary to bend them through one on the support. If the rib has one rod, then the clamps will take shear stress.

Reinforced masonry structures are analogues of flat ones. Therefore, the minimum value of the support depth of these elements can be determined in the same way. They must be at least 90 mm thick and supported on both sides.

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Individual construction

In specialized literature on construction work the definition of the necessary standards regarding the depth of support of floor slabs is given. This figure is within the range of 90-120 mm. To more accurately determine this value, certain calculations must be made, which take into account the length and weight of the structure, the thickness of the supporting wall and the material from which it is made. The expected load must also be determined.

For example, the use of a slab 6 m long requires a depth of support on a brick base of at least 100 mm. When using structures made of reinforced concrete or steel, a depth of at least 70-75 mm is permissible, for walls made of foam blocks and aerated concrete - at least 120 mm.

In houses made of brick, concrete or concrete blocks, the floors are usually made of reinforced concrete. They provide exceptional strength and earthquake resistance to the structure, and are also very durable and do not burn, which is important. There are several ways to construct reinforced concrete floors. The most common and universal is laying factory-made floor slabs. Such slabs are ordered from concrete factories and then installed using a crane and a team of workers. In cases where using a crane on a construction site is difficult, or when the house has a non-standard layout and it is difficult to lay out finished slabs, a monolithic floor slab is installed. In fact, you can fill a monolithic slab not only when there is evidence for it, but also simply because you consider it more appropriate. In this article we will tell you how to lay floor slabs and how to pour a monolithic slab. Not all work can be done independently, but it is still worth familiarizing yourself with the technology, if only to control the process on the construction site.

DIY monolithic floor slab

Monolithic flooring has a number of advantages compared to flooring made from ready-made reinforced concrete slabs. Firstly, the structure is strong and monolithic without a single seam, which ensures an even load on the walls and foundation. Secondly, monolithic filling allows you to make the layout of the house more free, since it can rest on columns. Also, the layout can involve any number of corners and crannies for which it would be difficult to select floor slabs standard sizes. Thirdly, it is possible to safely equip a balcony without an additional support plate, since the structure is monolithic.

You can install a monolithic floor slab yourself; you don’t need crane or a large team of workers. The main thing is to follow the technology and not skimp on materials.

Like everything related to construction, monolithic flooring begins with a project. It is advisable to order a calculation monolithic slab ceilings in the design office and do not save on it. It usually involves calculating the cross-section of the slab for the bending moment at maximum load. As a result, you will receive the optimal dimensions for the floor slab specifically in your home, instructions on what reinforcement to use and what class of concrete. If you want to try to perform the calculations yourself, an example of calculating a monolithic floor slab can be found on the Internet. We will not focus on this. Let's consider the option when a regular country house with a span of no more than 7 m, so we will make a monolithic floor slab of the most popular recommended size: from 180 to 200 mm thick.

Materials for the manufacture of monolithic floor slabs:

• Formwork.
• Supports for supporting the formwork at the rate of 1 support per 1 m2.
• Steel reinforcement with a diameter of 10 mm or 12 mm.
• Concrete grade M 350 or separately cement, sand and crushed stone.
• Bending device for reinforcement.
• Plastic supports for fittings (clamps).

Technology for pouring a monolithic floor slab includes the following steps:

1. Calculation of the floor slab if the span is more than 7 m, or the project involves supporting the slab on a column/columns.
2. Installation of deck type formwork.
3. Reinforcement of the slab with steel rods.
4. Pouring concrete.
5. Compacting concrete.

So, after the walls have been driven to the required height, and their level is almost perfectly leveled, you can begin to install a monolithic floor slab.

The construction of a monolithic floor slab assumes that concrete will be poured into horizontal formwork. Sometimes horizontal formwork is also called “deck”. There are several options for its arrangement. First - rental of ready-made removable formwork made of metal or plastic. Second - production of formwork on site using wooden planks or sheets of moisture-resistant plywood. Of course, the first option is simpler and preferable. Firstly, the formwork is collapsible. Secondly, it offers telescopic supports, which are needed to support the formwork at the same level.

If you prefer to make the formwork yourself, please note that the thickness plywood sheets should be 20 mm, and the thickness edged boards 25 - 35 mm. If you knock down panels from edged boards, then they need to be tightly adjusted to each other. If gaps are visible between the boards, then the surface of the formwork should be covered with waterproofing film.

Installation of formwork is carried out in this way:

• Vertical support posts are installed. These can be telescopic metal stands, the height of which can be adjusted. But you can also use wooden logs with a diameter of 8 - 15 cm. The step between the racks should be 1 m. The racks closest to the wall should be located at least 20 cm from the wall.
• Crossbars are placed on top of the racks (a longitudinal beam that will hold the formwork, an I-beam, a channel).
• Horizontal formwork is laid on the crossbars. If not ready-made formwork is used, but home-made one, then the longitudinal beams are laid on top cross beams, on which sheets of moisture-resistant plywood are placed on top. The dimensions of the horizontal formwork must be adjusted perfectly so that its edges rest against the wall without leaving gaps.
• The height of the pillar supports is adjusted so that the upper edge of the horizontal formwork coincides with the upper edge of the wall masonry.
• Vertical formwork elements are installed. Taking into account the fact that the dimensions of a monolithic floor slab must be such that its edges extend 150 mm onto the walls, it is necessary to construct a vertical fence exactly at this distance from inner edge walls.
• The last time the horizontal and even position of the formwork is checked using a level.

Sometimes, for the convenience of further work, the surface of the formwork is covered with a waterproofing film or, if it is made of metal, lubricated with machine oil. In this case, the formwork can be easily removed, and the surface concrete slab will be perfectly smooth. Usage telescopic stands preferable for formwork wooden supports, since they are reliable, each of them can withstand a weight of up to 2 tons, microcracks do not form on their surface, as can happen with wooden log or timber. Renting such racks will cost approximately 2.5 - 3 USD. per 1 m2 of area.

After arranging the formwork, a reinforcement frame made of two meshes is installed in it. For the manufacture of the reinforcement frame, steel reinforcement A-500C with a diameter of 10 - 12 mm is used. These rods are used to knit a mesh with a mesh size of 200 mm. To connect longitudinal and transverse rods, 1.2 - 1.5 mm knitting wire is used. Most often, the length of one reinforcing rod is not enough to cover the entire span, so the rods will have to be connected to each other lengthwise. To make the structure strong, the rods must be connected with an overlap of 40 cm.

The reinforcing mesh should extend onto the walls by at least 150 mm if the walls are made of brick, and by 250 mm if the walls are made of aerated concrete. The ends of the rods should not reach the vertical formwork along the perimeter by 25 mm.

Strengthening a monolithic floor slab is done using two reinforcing mesh. One of them - the bottom one - should be located at a height of 20 - 25 mm from the bottom edge of the slab. The second - the top - should be located 20 - 25 mm below the top edge of the slab.

In order for the lower mesh to be located at the required distance, special plastic clips. They are installed in increments of 1 - 1.2 m at the intersection of the rods.

The thickness of the monolithic floor slab is taken at the rate of 1:30, where 1 is the thickness of the slab, and 30 is the span length. For example, if the span is 6 m, then the slab thickness will be 200 mm. Considering that the grids should be located at a distance from the edges of the slab, the distance between the grids should be 120 - 125 mm (from a slab thickness of 200 mm we subtract two gaps of 20 mm and subtract 4 thicknesses of reinforcing rods).

To space the meshes at a certain distance from each other, they are made from 10 mm reinforcing rod using a special bending tool. special clamps - stands as in the photo. The upper and lower flanges of the clamp are 350 mm. The vertical size of the clamp is 120 mm. The installation step of the vertical clamps is 1 m, the rows should be staggered.

Next step - end clamp. It is installed in increments of 400 mm at the ends of the reinforcement cage. Serves to strengthen the support of the slab on the wall.

Another important element is connector of upper and lower mesh. You can see what it looks like in the photo. It is necessary so that the spaced grids perceive the load as one whole. The installation step of this connector is 400 mm, and in the area of ​​support on the wall, within 700 mm from it, in steps of 200 mm.

Pouring concrete

It is better to order concrete directly from the factory. This makes the task much easier. In addition, pouring the mortar from a mixer in an even layer will ensure exceptional strength of the slab. The same cannot be said about the slab, which was poured manually with breaks to prepare a new portion of the solution. So it is better to pour concrete immediately in a layer of 200 mm, without interruptions. Before pouring concrete into the formwork, it is necessary to install a frame or box for technological openings, for example, a chimney or ventilation duct. After pouring, it must be vibrated with a deep vibrator. Then leave to dry and gain strength for 28 days. During the first week, the surface must be moistened with water, only moistened, and not filled with water. After a month, the formwork can be removed. The monolithic floor slab is ready. For installation of floor slabs, the price includes the cost of reinforcement, concrete, rental of formwork and ordering a mixer machine, as well as a concrete pump. In fact, it comes out to about 50 - 55 USD. per m2 of flooring. You can see how the floor slab is poured with concrete in the video demonstrating the installation of floor slabs.

How to lay floor slabs correctly

The use of factory-made monolithic reinforced concrete floor slabs is considered more traditional. The most popular are PC slabs - slabs with round voids. The weight of such slabs starts from 1.5 tons, so laying floor slabs with your own hands is impossible. A crane is required. Despite the apparent simplicity of the task, there are a number of nuances and rules that must be followed when working with floor slabs.

Rules for laying floor slabs

The prefabricated floor slab is already reinforced at the factory and does not require additional reinforcement or formwork. They are simply laid in a span supported on the walls, following some rules:

• The span should not be more than 9 m. This is the length of the slabs that are the largest.
• Unloading and lifting of slabs is carried out using special equipment provided by the project. For this purpose, the slabs have mounting loops to which the mounting slings are hooked.
• Before laying floor slabs, the surface of the walls on which they will be laid must be leveled. Large height differences and distortions are not allowed.
• The slabs should rest on the walls by 90 - 150 mm.
• The slabs must not be laid dry; all cracks and technological seams must be sealed with mortar.
• The location of the slabs must be constantly monitored in relation to the walls and supporting surfaces.
• The slabs are laid only on load-bearing walls, all partitions are installed only after the installation of the floors.
• If you need to cut a hatch in the ceiling, then it must be cut at the junction of two slabs, and not in one slab.
• The plates should be located as close to each other as possible, but with a gap of 2 - 3 cm. This will ensure earthquake resistance.

If there are not enough floor slabs to cover the entire span, and there remains, for example, 500 mm, then there are different ways laying floor slabs in this case. The first is to lay the slabs end to end, leaving gaps along the edges of the room, then seal the gaps with concrete or cinder blocks. The second is laying slabs with uniform gaps, which are then sealed concrete mortar. To prevent the solution from falling down, formwork is installed under the gap (a board is tied up).

Floor slab laying technology

During the process of laying floor slabs, there must be clear coordination of actions between the crane operator and the team receiving the slab. To avoid injury on a construction site and to comply with all technological process and the rules described in SNiPs, the construction foreman must have routing installation of floor slabs. It indicates the sequence of work, the quantity and location of equipment, special equipment and tools.

It is necessary to start laying floor slabs from the flight of stairs. After laying the slabs, their position is checked. The slabs are laid well if:

• The difference between the lower surfaces of the plates does not exceed 2 mm.
• The height difference between the upper surfaces of the slabs does not exceed 4 mm.
• The height difference within the site should not exceed 10 mm.

As the installation diagram for floor slabs demonstrates, after laying the slabs, they must be connected to each other and to the walls using metal connecting parts. Work on connecting embedded parts and connecting parts is carried out by welding.

Don't forget to follow safety precautions. It is not allowed to carry out work using a crane in an open area with a wind of 15 m/s, as well as during ice, thunderstorms and fog. When moving the slab using a crane, the installation team should be away from the path along which the slab will move, on the opposite side of the feed. Despite the fact that using the services of a professional foreman and a team of installers significantly increases the cost of installing floor slabs, this is still not the case when you can save money. The foreman must provide the project.

Before ordering slabs from the factory, it is necessary to carry out preparatory work. It is better to coordinate the delivery time of the machine with slabs and the crane at the same time, so as not to overpay for downtime of special equipment. In this case, the installation of the slabs can be carried out without unloading, directly from the vehicle.

Preparatory work before laying floor slabs

First - flat supporting surface. The horizon should be almost ideal; a height difference of 4 - 5 cm is unacceptable. First of all, we check the surface of the walls, then, if necessary, level it with concrete mortar. Subsequent work can be carried out only after the concrete has acquired maximum strength.

Second - ensure the strength of the support area. If the walls are built from brick, concrete or concrete blocks, then no additional measures need to be taken. If the walls are built from foam blocks or gas blocks, then before laying the slabs it is necessary to fill in the reinforced belt. Correct styling floor slabs assumes that the supporting surface must be strong enough to support the weight of the slab and not deform along the abutment line. Neither aerated concrete nor foam concrete have the necessary strength. Therefore, formwork is installed along the entire perimeter of the building, a reinforcement frame made of 8 - 12 mm rod is installed in it, and then everything is filled with concrete with a layer of 15 - 20 mm. Further work can be continued only after the concrete has dried.

Third - install mounting towers. Telescopic supports, as described in the section on installing a monolithic floor slab, are installed in increments of 1.5 m. They are designed to take on the weight of the slab if it suddenly slips out of its place. After installation, these towers are removed.

Installation of hollow core slabs using a crane

After the freshly poured concrete has acquired sufficient strength and has dried, the installation of the floor slabs can begin. For this, a crane is used, the lifting capacity of which depends on the size and weight of the slab; cranes of 3 - 7 tons are most often useful.

Stages of work:

• Concrete mortar is applied to the supporting surface in a layer of 2 - 3 cm. The depth of application of the mortar is equal to the depth of support of the slab, i.e. 150 mm. If the slab rests on two opposite walls, then the solution is applied only to two walls. If the slab rests on three walls, then on the surface of three walls. The actual laying of the slabs can begin when the mortar reaches 50% of its strength.

• While the solution dries, the crane operator can hook the slings to the slab fasteners.
• When the crane operator is given a signal that the slab can be moved, the team of workers must move away from the place where the slab is moving. When the slab is very close, the workers hook it with hooks and turn it around, thereby dampening the oscillatory movements.

• The stove is directed to the right place, one person should stand on one wall, and the other on the opposite. The slab is laid so that its edges rest on the wall at least 120 mm, preferably 150 mm. After installation, the slab will squeeze out excess mortar and evenly distribute the load.

• If there is a need to move the slab, you can use a crowbar. Its position can only be aligned along the laying area; the slab cannot be moved across the walls, otherwise the walls may collapse. The slings are then removed and a signal is given to the crane operator to pick them up.
• The procedure is repeated for all slabs without exception. The rules for installing floor slabs suggest that the alignment of the slabs should be done along the bottom edge, since it is the bottom surface that will be the ceiling in the room. Therefore, the slab is laid with the wider side down and the narrower side up.

You may come across a recommendation that reinforcement must be placed in the area where the slab is supported. Proponents of this method say that it is more convenient and easier to move the stove. In fact, placing anything other than concrete mortar under the slab is prohibited by the technical map. Otherwise, the slab can easily move out of the support area, as it will slide along the reinforcement. In addition, the load will be distributed unevenly.

Laying floor slabs on a foundation is practically no different from laying interfloor ceilings. The technology is exactly the same. Only the surface of the foundation must be thoroughly waterproofed before laying the slabs. If the project provides for non-standard support of floor slabs, then special steel elements are used for this. Such work should not be carried out without a specialist.

Anchoring - tying the slabs together - can be done in two ways, depending on the project.

First - tying slabs with reinforcement. Reinforcing rods with a diameter of 12 mm are welded to the fastening embedded elements on the slab. For slabs from different manufacturers, the location of these elements may be different: in the longitudinal end of the slab or on its surface. The strongest connection is considered to be a diagonal connection, when the plates are connected to each other with an offset.

The slab must also be connected to the wall. Why is reinforcement built into the wall?

Second way - ring anchor. In fact, it looks like an armored belt. Formwork is installed around the perimeter of the slab, reinforcement is installed into it and concrete is poured. This method slightly increases the cost of laying floor slabs. But it's worth it - the slabs end up clamped on all sides.

After anchoring, you can begin to seal the cracks. The gaps between the floor slabs are called rustications. They are filled with concrete grade M150. If the gaps are large, then a board is tied from below, which serves as formwork. If the gaps are small, then the floor slab will be able to withstand the maximum load the very next day. Otherwise you need to wait a week.

All modern slabs with round voids are produced with the ends already filled. If you purchased slabs with open holes, then they need to be filled with something 25 - 30 cm deep. Otherwise the slab will freeze. You can fill the voids mineral wool, concrete plugs or simply fill with concrete mortar. A similar procedure must be performed not only on those ends that face the street, but also on those that rest on the internal walls.

The price for laying floor slabs depends on the amount of work, the area of ​​the house and the cost of materials. For example, the cost of PC floor slabs alone is approximately 27 - 30 USD. per m2. The rest is related materials, crane rental and workers, as well as the cost of delivering the slabs. Professional teams have very different prices for installation of floor slabs, from 10 to 25 USD. per m2, maybe more depending on the region. As a result, the cost will be the same as for pouring a monolithic floor slab.

Laying floor slabs: video example

On the wall is one of the indicators of the reliability, safety and long service life of the building. A lot depends on the proper installation of slabs, so all norms and rules are regulated government agencies. There is a special document - SNiP, which has compiled a set of these standards.

Purpose of floors

Floors are one of the main load-bearing structures of a building, so they receive a lot of attention during construction. The main function of reinforced concrete floors is to transfer and distribute the load to its own weight, and then to other elements of the building.

By location data building construction are divided into interfloor, above-basement and attic. The slabs are manufactured in a factory and come in several types:

• prefabricated monolithic;
• multi-hollow;
• made from heavy grades of concrete.

The main requirements that high-quality floors must have are strength, rigidity, fire resistance, sound and water resistance.

Most floor slabs are made with voids; this design is considered the most optimal in terms of weight and quality. Laying takes place on the load-bearing walls of the building, the pitch of which can be up to 9 m.

Parameters for support amount

The maximum and minimum support of the slab floor on the wall is determined by the following factors:

1. The purpose of the building is residential, industrial, administrative.
2. The material from which the load-bearing walls are made and their thickness.
3. The size of the overlapped span between the walls.
4. The size of the reinforced concrete floor slab and its weight.
5. Seismic indicators of the building location.

In accordance with SNiP data, the support of floor slabs on walls is from 9 to 12 cm, depending on the factors described above. The final size is determined by engineers when designing the building. It is important to correctly calculate the amount of overlap, otherwise the pressure of the ceilings can lead to gradual cracking and destruction of the building.

Supporting unit for a slab on a brick wall

When constructing brick buildings, the masonry is carried out close to the future ceiling, and it is important to leave small niches for installing ceilings. The support unit for the floor slab on the wall is created taking into account the following conditions:

• The ends of the slabs should not rest on the brickwork. For example, with an overlap of 12 cm, the width of the niche should be 13 cm;
• the composition of the mortar for laying and fixing the floors must be identical;
• voids formed in the channels should be filled with concrete liners. They are manufactured at the factory along with the plates.

Minimum support of floor slabs on brick wall is not standardized if the reinforced concrete product is placed on the end walls with one side. The installation is carried out so that the masonry, which will be higher than the ceiling, does not fall on the formed extreme voids.

Floor installation

The installation of floors is carried out by a team of four people:

• crane operator who delivers the slab,
• a rigger who rigs slabs,
• two installers involved in coordinating the slab and placing it in a given location.

Supporting floor slabs on a brick wall is one of the most important procedures that requires strict compliance with standards.

Before installation work it is necessary to straighten the comb brickwork. If this is not done, the slab will be unstable. The gaps that appear between the slabs are sealed with cement mortar.

Features of installation of floors for buildings made of aerated concrete

The slab ceiling is supported on the wall by a circular reinforced belt, which is mounted along its perimeter. Such a monolithic concrete strip covering the entire building is required if the support value is less than 12 cm. The following parameters for the reinforced belt are recommended:

• thickness 12 cm;
• width 25 cm;
• the support depth is the same as for reinforced concrete floors.

Combined with durable reinforced concrete slabs the reinforced belt creates a rigid structure that provides sufficient resistance to the structure of emergency impacts, temperature changes and shrinkage deformations.

If the amount of support of the ceiling on the wall is more than 12 cm, then the building does not need an additional reinforced belt. In such cases, it is enough to construct a reinforced belt from a ring anchor along the outer perimeter of the slabs.

Calculation of the support parameter

Regulates the amount of support of floor slabs on the walls of SNiP (otherwise, a set of norms and rules), which distinguishes the following types of slab sizes:

• modular - the width of the span into which the structure is installed;
• constructive - the actual size of the ceiling slab from one end to the other.

For example, if the modular length of the floor is 6.0 m, then the actual length is 5.98 m. To obtain a room size of 5.7 m, a slab should be installed with a support of 12 cm. The optimal calculation of the support of the floor slab on the wall is also important for preserving warmth in the room. If the end is too close to outer surface walls, there will be penetration of cold air inside. This design gives a cold floor in winter time.

Floor covering

Installation of floor slabs for ground floor is the simplest. In order to achieve a flat surface for laying reinforced concrete structures, the upper edge of the foundation should be leveled. Then formwork boards are placed along the upper edge of the poured foundation. This design filled with concrete mortar. This creates a perfectly flat pad for installing the slabs.

Installed on a smooth surface, the slabs form a flat ceiling, in which you only need to seal the seams, after which it is ready for finishing.

Sealing seams between floors

After optimal size The support of the floor slabs on the walls has been determined, and the reinforced concrete structures themselves have been installed, you should start sealing the seams between them.

For this purpose it is used sand-cement mortar, if the gaps are small. If there are large gaps, use the following methods:

1. The formwork is constructed from wooden boards, into which the mortar is then poured.
2. Large gaps can be sealed with fragments of reinforcement, fragments of bricks and other materials. They are compacted into the cracks, which are then covered with concrete mortar.

It is important to seal the voids formed when installing slabs immediately. This makes it much easier Finishing work, which will be produced upon completion of construction.

From correct calculation The amount of support of the ceiling on the wall determines the future strength and durability of the structure. Therefore, this process is regulated by SNiP rules and is carried out by experienced designers.

Currently, in our country, the most popular are three methods of constructing floors in a house. This is the installation of floor slabs, the installation of a monolithic reinforced concrete floor and installation of ceilings on wooden (less often metal) beams. We will definitely talk about all these methods and more. And the first technology we will consider is the installation of ready-made floor slabs.

First, a little about the floor slabs themselves. Depending on their shape, all slabs can be divided into flat and ribbed. Flat ones, in turn, are divided into solid and hollow. We are now interested in empty ones, because... It is this type of slab that is used mainly in low-rise construction.

Hollow core slabs, in turn, are also classified according to various parameters, such as the shape and size of the voids, the thickness of the slabs, the manufacturing technology of the slabs, and the method of reinforcement.

I will not delve into the topic of classification. It is better to look for this information on the websites of manufacturers of reinforced concrete products (reinforced concrete products). We'd better talk directly about installation.

The very first point that you need to pay attention to even at the stage of designing your future home is the opportunity to purchase in your area exactly the slabs of the sizes that are included in the project. Each manufacturer has its own specific range of products and it is always limited. This is really important and I am surprised that very often developers forget about this recommendation and then they have to either cut one or more slabs or do it on the floor monolithic site. We'll talk more about this below.

Storing floor slabs at a construction site.

Of course, it’s great if you have the opportunity to lay floor slabs immediately upon delivery directly from the truck that brought them. But most often this does not happen. Or the driver insists that you unload the slabs as quickly as possible, because... he is in a hurry for the next order, or the slabs are not laid on the machine in the order you need, or you simply bought them in advance and are not going to lay them yet. In all these cases, the slabs will need to be stored on your site.

Try to choose for this flat surface. Never place slabs directly on the ground. Be sure to put something under the edges of the slab, for example, trimmings wooden beam. There should be only two pads, at a distance of approximately 25-40 cm from the edges. Pads cannot be placed under the middle of the slab.

The slabs can be stored in stacks up to 2.5 meters high. Make the shims for the first slab higher, so that if they are possibly pressed into the ground when laying subsequent slabs, the first slab should never touch the ground, otherwise it could easily break. It is enough to make all subsequent linings even from an inch (2.5 cm). They must be stacked strictly one above the other.

Preparation for installation of floor slabs.

Preparation begins at the moment when the masons cast out the last rows of masonry. The slabs will lie flat and without differences if the top rows load-bearing walls will be level and will be in the same horizontal plane.

To achieve this, there must be horizontal level marks in all corners of the room being covered. They are installed during the construction of walls using a level, or laser level, or hydraulic level. And when the last row of masonry is done, the distance from the marks to the top of the walls is controlled with a tape measure. It should be the same in all corners. From my experience I can tell you for sure that some masons neglect this, especially when they do backfill masonry at the same time as the face masonry, which is done “under the rod”.

The top row of load-bearing walls must be bonded. That is, if you look from the inside of the covered room, then on the load-bearing walls (on which the floor slabs rest) in the very top row of the masonry, only the pokes should be visible.

If the slabs are placed on a load-bearing partition 1.5 bricks thick (that is, the slabs rest on it on both sides), then the top row of such a partition is laid out in one of two ways:

Before laying floor slabs on walls made of various blocks (foam concrete, gas silicate, slag, etc.), it is necessary to make a reinforced concrete belt (usually about 15-20 cm thick). Such a belt is made either by pouring concrete into the formwork, or using special U-shaped blocks along the entire perimeter of the house box, i.e. not only on load-bearing walls, but also on non-load-bearing ones.

When installing hollow core slabs, the holes in them must be sealed. It is much more convenient to do this in advance, while the slabs are still on the ground. In general, SNiP prescribes that voids must be sealed on the side of the slab that rests on outer wall(to reduce the likelihood of freezing of the slab), and from the side that rests on internal partition, only starting from the third floor from the top of the house and below (to increase strength). That is, if, for example, the house has a basement ceiling, a ceiling between the 1st and 2nd floors and attic floor above the 2nd floor, it is mandatory to fill the voids on the side of load-bearing partitions only in the basement floor.

I will say that we always seal holes when laying slabs. Moreover, in Lately More and more often, round-hollow slabs come from factories with holes already filled. It's comfortable. If the holes are not sealed, we insert a one-and-a-half brick (you can even use half) into them and fill the remaining cracks with mortar.

Also, before installing the slabs, it is necessary to prepare the site for the crane in advance. It’s good if in the place where the crane will stand the soil is, as they say, compacted. It's worse when the soil is bulky. If you have a basement, you should not place the faucet too close to the house, in order to avoid what is shown in the figure below:

In such cases, it is better to order a truck crane with a longer boom. Also, sometimes in the place where the crane will stand, you first have to put several road slabs (usually used ones are found somewhere). Often this has to be done in the fall in rainy and slushy weather, when the area is so “broken” that the crane simply gets stuck on it.

Laying floor slabs.

Three people are enough to install floor slabs. One hooks the slabs, two lay them down. If you want, you can do it together, although not always. It happens that when covering, for example, the second floor, the installers and the crane operator do not see each other. Then, at the top, in addition to the 2 people directly laying the slab, there must be another person who will give commands to the crane operator.

Laying begins from the wall on a layer of mortar no more than 2 cm. The mortar should be thick enough so that the slab does not squeeze it completely out of the seam. After the crane operator places the slab on the walls, he first leaves the slings taut. At the same time, using a crowbar, if necessary, it is not difficult to move the slab a little. If the upper surfaces of the load-bearing walls were made smooth, then the slabs will lie evenly, without differences, as they say, “from the first approach.”

Regarding the amount of support of the slabs on the walls, I will give an extract from the document “A manual for the design of residential buildings. Vol. 3 (to SNiP 2.08.01-85) 6. COLORS:

Paragraph 6.16.: The depth of support of prefabricated slabs on walls, depending on the nature of their support, is recommended to be no less than, mm: when supported along the contour, as well as two long and one short sides - 40; when supported on two sides and the span of slabs is 4.2 m or less, as well as on two short and one long sides - 50; when supported on two sides and the span of the slabs is more than 4.2 m - 70.

When assigning the depth of support for floor slabs, you should also take into account the requirements of SNiP 2.03.01-84 for anchoring reinforcement on supports.

In our practice, we try to make the support no less than 12 cm, fortunately now it is possible to purchase exactly the slabs that are needed. The step of their lengths is 10 cm.

I often hear debates about whether it is possible to support hollow-core floor slabs on three sides (two short and one long) and how far the long side of the slab can be placed on the wall. From what is written above, it follows that it is possible to support the slabs this way. But it is not so. If you read the specified SNiP, it says that slabs that rest on three sides have a different reinforcement pattern than those that rest on only two sides.

The vast majority of hollow core slabs that are currently produced by concrete factories are designed specifically to be supported on two short sides, so it is not recommended to place them on the wall with their long side. Under a certain load, this can lead to cracking of the slab. The reinforcement scheme and, therefore, the possibility of supporting the slab on a third side must be clarified with the manufacturer.

Another mistake associated with improper loading of the slab is covering two spans at once (see figure below):

Under certain unfavorable conditions, the slab may crack, and the location of the crack is completely unpredictable. If you still use such a scheme, use a grinder to make a cut (to the depth of the disk) on the upper surface of the slab strictly above the middle partition. Thus, if something happens, the crack will pass precisely along this cut, which, in principle, is no longer scary.

Of course, it’s good if we manage to cover ourselves exclusively with whole slabs. But circumstances are different and yet sometimes some slab (or even more than one) has to be cut lengthwise or crosswise. To do this you will need a grinder with diamond blade on concrete, a sledgehammer, a crowbar and not the weakest guy at a construction site.

To make work easier, it is better to place the slab on a lining. Moreover, this lining is placed exactly under the cut line. At a certain moment, the slab will simply break along this line from its own weight.

First of all, we make a cut on the upper surface of the slab with a grinder along the cut line. Then, striking with a sledgehammer from above, we cut a strip along the top of the slab. It is quite easy to penetrate concrete in void areas. Next, we use a crowbar to break through the lower part of the slab (also through the voids). When cutting a slab lengthwise (we always cut along the hole in the slab), it breaks quite quickly. When chopping across, if the slab does not break after breaking the lower part with a crowbar, a sledgehammer is used to strike the vertical partitions of the slab from the side until the victory is achieved.

During the cutting process, we cut the reinforcement that comes across. You can use a grinder, but it’s safer to weld or cutting torch, especially when the reinforcement in the slab is prestressed. The grinder disc can bite. To prevent this from happening, do not cut the reinforcement completely, leave a couple of millimeters and then tear it apart with a blow from the same sledgehammer.

Several times in our practice we had the opportunity to cut slabs lengthwise. But we have never used, so to speak, “stumps” less than 60 cm wide (less than 3 holes remain), and I do not advise you to. In general, when deciding to cut a slab, full responsibility for possible consequences You take full responsibility for this, because not a single manufacturer will officially tell you that cutting a slab is possible.

Let's now see what can be done if you still don't have enough slabs to completely cover the room:

Method 1- we place the first or last (maybe both) slabs without bringing the long side to the wall. We fill the remaining gap with bricks or blocks, hanging them no more than half from the wall (see figure):

Method 2— we make the so-called “monolithic section”. Plywood formwork is placed underneath the slabs, a reinforcement frame is made (see figure below) and the area between the slabs is filled with concrete.

Anchoring of floor slabs.

After all the slabs are laid, they are anchored. In general, if the construction of a house is carried out according to a project, then it must contain an anchoring scheme. When there is no project, we usually use the circuit shown in the figure:

The anchor is made by bending the end into a loop, which clings to the mounting loop of the slab. Before welding the anchors to each other and to the mounting loops, they must be tensioned as much as possible.

After anchoring, we immediately seal all mounting eyes in the slabs and rustications (seams between the slabs) with mortar. Try not to delay this so that construction debris does not get into the rustics, and water does not pour into the eyelets during rain and snow. If you suspect that water has gotten into the slabs (for example, you bought slabs with voids already filled in, and rainwater could have gotten in even while being stored at the factory), it is better to release it. To do this, after installation, simply drill one small hole in the slabs from below with a hammer drill, into the voids where the mounting lugs are located.

It is especially dangerous to find water in voids in winter, when the house is not yet heated (or not completed at all) and the slabs freeze below zero. Water saturates the bottom layer of concrete, and with repeated freezing-thawing cycles, the slab simply begins to collapse.

Another way to secure the slabs is to construct a so-called concrete ring anchor. This is a kind of the same monolithic reinforced belt, only it is not made under the slabs, but in the same plane with them, also around the entire perimeter of the house. More often this method is used on foam concrete and other blocks.

Let me make a reservation right away that we have never used it because it is much more labor intensive. I think that a ring anchor is justified in more earthquake-prone regions than our Nizhny Novgorod region.

At the end of the article, I suggest watching a short video that talks about choosing floor slabs:

The topic for this article is ripe - builders make a lot of mistakes.

What is a precast slab (hollow or ribbed)? It is primarily reinforced reinforced concrete structure, designed for a specific job. Any reinforced concrete can only work in such a way that the stresses in it can be picked up by working reinforcement.

In prefabricated slabs, working reinforcement is located only in the lower zone of the slab and only along the slab. What does it mean? This means that the slab without destruction can only bend in the longitudinal direction and only so that the bend of the slab is directed downward.

As can be seen from the figure, when the slab bends, its lower part is stretched, and the reinforcement picks up this tensile stress, because concrete is not capable of this. Concrete without reinforcement will only crack and collapse when bent. At the slightest bend, we need to install reinforcement that will take the tensile stresses of the bend upon itself.

Now let's get back to prefabricated slabs. We know that the working reinforcement of the slab is located only along the slab and only at its lower edge.

Let us consider below various situations for supporting floor slabs.

How to support prefabricated floor slabs

1) The classic way to support the slab: on both sides.

Here everything is maintained in the best traditions: the slab bends under the weight of the load, the working reinforcement picks up the bending stresses, and if the load does not exceed the load-bearing slab, no destruction occurs - everything works according to plan.

2) Supporting the slab on three sides (two short and one long).

This method of supporting is also called supporting the slab with a bolt on the wall. It can be used when slabs are not placed along the width of the span, and it is impractical to make a monolithic section. Compared to the previous option, this option for the operation of the stove is worse, but in principle, it is not prohibited. The main thing to remember: it is advisable not to insert the slab on the long side into the wall deeper than the height of the slab (with a slab height of 220 mm, do not rest the slab deeper than 220 mm), so as not to cause pinching. What pinching is, and how it is harmful to prefabricated slabs, will be discussed a little further in the article.

In this case, not the entire slab is bent, but only its free edge. But still, the longitudinal working reinforcement comes into play and picks up tensile stresses - just not in the entire slab, but in part of it.

How not to support prefabricated floor slabs

As we remember, working reinforcement in the slab is only in the longitudinal direction. In the transverse direction there is only a small mesh that can absorb the load from the slab’s own weight during installation (when the loop is lifted by a crane using four loops). And if we support the slab on two long sides, under load it will begin to bend as in the figure, and there simply will not be enough reinforcement area in this direction - the slab will begin to crack. On initial stage the existing mesh can take the load, but (I repeat), the reinforcement area of ​​this mesh is designed only for the slab’s own weight.

2) Installation of additional support in the span of the slab.

We need to remember once and for all: prefabricated slabs work exclusively as single-span slabs. If a wall or column appears somewhere in the span, what happens is shown in the figure above. The slab between the supports bends downwards, and above the support there is a bend in the opposite direction - with a stretched zone at the top. But in the upper zone of the slab we do not have working reinforcement, and we have nothing to absorb tensile bending stresses. As a result, cracks appear in the upper zone of the slab, as shown in the figure. It may be just one crack, but it will be enough to lead to an emergency over time or immediately.

3) Supporting the prefabricated slab on two walls with the removal of part of the slab in the form of a balcony (console).

This situation is approximately the same as in the previous case. There is no upper reinforcement, there is nothing to absorb the tension. The greater the length of the console and the greater the load on it (especially at the edge), the faster the destruction will occur.

Overhanging the slab in a different direction will be just as dangerous as that shown in the figure.

4) Supporting the prefabricated slab on columns (point supports).

If you want to support the slab not on walls or beams, but directly on columns, remember: this cannot be done. The principle of operation of reinforcement in reinforced concrete is as follows: tensile reinforcement in a slab works only when its ends are placed on a support. If there is no support under the edge of the slab (and under the end of the reinforcing bar), such reinforcement becomes useless ballast.

In the picture we see the option of supporting the slab on 4 columns. Firstly, the slab bends not only in the longitudinal, but also in the transverse direction - and as we found out from point 1, in this case cracks can form. But this is not the worst thing - these cracks simply will not have time to form due to an emergency in the other direction. So, secondly, we only have two outer reinforcements on the support, the rest are “hovering in the air” and are not included in the work. This means that the area of ​​working reinforcement in the slab has decreased many times compared to the required one. Naturally, such a slab will tend to collapse.

The best way out of this situation would be to install beams in the desired place to support the slab - between closely spaced columns.

5) Pinching of the prefabricated floor slab.

What is pinching? In the case of supporting floor slabs, this means placing the slab on the wall by more than the height of the slab section and loading it on top with the wall. The fact is that clamped slabs work completely differently than hinged slabs. All prefabricated slabs are designed for hinged support (when the slab, sagging, seems to rotate on the support). The regulatory documents for prefabricated slabs clearly stipulate the depth of support, and it should not only not be less than specified - it should not be made too large.

Let's look at the figure to see what the pinching of the slab on the support leads to.

With hinged support, the slab simply turns a little on the support and stretches in the lower zone - that’s where the lower working reinforcement is activated.

When pinched, the slab is inserted too deeply to rotate; as a result, it bends in a tricky way when the lower zone of the slab is stretched in the center, and the upper zone is stretched at the supports. And in this upper zone we do not have enough reinforcement to absorb tensile forces. As a result, cracks form, which are especially dangerous because they are not visible (they are hidden under the floor), but over time they expand and lead to an emergency condition.

I hope this article has clearly demonstrated how prefabricated (hollow, ribbed and solid) slabs can be supported, and how not.

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Comments

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0 #91 Irina 12/14/2019 10:03

I quote Ksenia:

0 #92 Taras 02/06/2020 10:21

I quote Irina:

I quote Ksenia:

Hello! In our private home, a situation arose that made our blood run cold, and this is not an exaggeration. During the construction of the first floor, 27 years ago, two floor slabs were laid with gross violation; they lay on the wall only 5 cm on each side. The slabs are 4.20 long. Now these slabs, especially one, move the wall they rest on outward. In short, after some time everything will collapse. I don’t see any other way out except to build additional walls around the perimeter of the room inside the room. If I have time..... There is literally no money at all. If you remove the floor, make a foundation and lay out the walls in brick to create support for the slab, will this correct the situation? The house is full of children, there is nowhere to go, everything will have to be done without leaving home. Answer please.

Ksenia, write to me at yandex.ru

I'll advise you. For free

0 #94 Irina 02/06/2020 11:51

I quote Oleg:

Irina, since you are considering, among other things, the options for gross mistakes, then I think that the addition that you cannot place a reinforcing bar under the slab in the support zone (and even more so in the project!) will fit well into this article. Some builders, even with considerable experience, make this mistake and require young designers to provide appropriate support. I've seen this before. Builders do this supposedly to maintain the thickness of the mortar under the slab. If the project calls for a 10 mm mortar under the slab, then they, the pests, lay a rod with a diameter of 10 mm.
Why not? One of the main purposes of the mortar under the slab is to distribute the load evenly, and the reinforcing bar is a stress concentrator. Those. the load will be transmitted through such a metal wedge. The brick under this bar may crack/chip because... the solution will be poured either flush with the rod, or the excess solution will be squeezed out during leveling, and the slab will begin to rest on this metal wedge.

Ksenia, write to me at yandex.ru

I'll advise you. For free

Good afternoon, please describe how to solve the problem that Ksenia wrote about.
The question is important for many. Thank you in advance

In the event of an emergency, you first need a thorough examination to clarify all the factors. Without this, any instructions may lead to a worsening situation.