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How to pour a slab on a strip foundation

pour, slab, strip, foundation

First of all, to install the foundation with your own hands, you need to analyze the features of the soil on the site and calculate the carrying capacity of the slab. Ideally, it is better to entrust all calculations and design work to professionals. But if there is no such a possibility, you can use the following recommendations and guidelines:

  • To analyze the ground, it is necessary to take the ground from five points of the future slab. at its corners and in the center. Data will clarify the level of groundwater, the load-bearing capacity of the soil on the site and its composition. For information, you can use the data on the buildings already built in the region on similar foundations.
  • When calculating the carrying capacity of the slab according to the instructions, it is worth taking into account that it is recommended to use M-200 grade cement to fill the main slab, and M-50 grade cement can be used for filling the screed without reinforcement (the data is taken from SNiP 2.02.01).
  • If there is a high level of groundwater on the site it is worth to design a quality drainage system from the foundation.
  • Reinforcement should be purchased at the rate of 130 kg/m2 slab. In this case, three-quarters of the rods should have a section of 12-16 mm (for stringing mesh with a mesh size of 15-20 cm) and one-quarter of the bars with a section of 6-8 mm for the installation of longitudinal struts between the two horizontal meshes.

In addition, when performing calculations and soil analysis, you can use legislative documents:

  • GOST 25100 (to calculate the soil classification on the site);
  • SP 70.13330 (determining the bearing and settlement capacity of the house);
  • SP 28.13330 (determining the anti-core of the frame of the house);
  • VSN 29-85; 37-96 (rules of design and installation of a shallow-depth foundation MZI);
  • GOST R 52085; 52086 (proper step-by-step construction of the formwork).

Important: A monolithic slab distributes the weight of the house evenly over each cm2 of its area, thus effectively leveling the ground pressure on the foundation during the heaving seasons.

Preparation of the excavation

The depth to which you need to lay a slab foundation depends on the type of soil on your site.

  • If the soil consists of dense rocks, then on the marking should dig a foundation pit to a depth of 50 cm.
  • When the site is dominated by weak rocks of soil or what, they need to be removed. On such a site, the depth of the excavation must be a meter or more.
  • The bottom of the prepared pit is leveled horizontally.
  • For digging the excavation it is desirable to use special equipment, as digging it with your own hands is much longer and heavier. One should manually level walls, the bottom of the foundation and those defects that will remain after the excavator.
  • If the special equipment in some areas will dig deeper than expected, you should not pour the same soil into the pits, it is better to pour sand, as it does not give shrinkage.
  • Drainage system and water drainage must be planned and dug in advance taking into account possible height differences.

Combined foundation strip and slab Foundation strip-plate. Concrete Casting Slab or Tape Foundation with Floors on the Combined Tape and Plate Foundation

For normal functioning of the drainage you need to make a small drop in the direction of water drainage.

Advantages and features of a slab base

When deciding to mount a strip foundation with a monolithic slab, it is necessary to clearly understand what positive and negative sides of it the masters and owners of the future home will have to face. The main advantages of the monolithic foundation strip type are:

  • High strength of foundation. Reinforcement technology of concrete bed slabs increases the ability of foundation to withstand higher loads.
  • Possibility to erect the building both on simple soils and on the soil with unsuitable features for construction (e.g. clay, loam, high location of ground waters, etc.).).
  • The long service life of the foundation, counted in tens and hundreds of years.
  • Possibility to build buildings of any number of storeys and purposes on such foundation.
  • Reducing the cost of arranging the floor on the first floor, because the cast-in-place slab is already the floor.

But for all its positive aspects, the strip-type slab foundation also has some disadvantages. These are:

  • The need for more accurate and thorough complex calculations;
  • Excessive consumption of construction materials;
  • The need for additional workers or a large expenditure of time and effort for the construction of such a foundation-plate;
  • The laboriousness of the process in preparing the excavation for pouring a monolithic slab inside the strip foundation;
  • Relatively high cost of the work, as opposed to the device of a standard strip foundation.

Load calculation

To build a strong and secure foundation of strip type with a monolithic slab inside it, it is necessary to correctly calculate all the parameters of the future house and the basis for it

For the construction of a strong and reliable strip foundation with a monolithic slab inside it, it is necessary to correctly calculate all the parameters of the future house and the basis for it. It is unlikely that you can make such calculations on your own, without an engineering background. Therefore it is best to entrust this stage of the work to a professional architect.

As a rule, the calculations are carried out taking into account the following parameters:

  • Features of the soil (by taking it for analysis);
  • The size of the building according to the house plan;
  • Calculation of the possible roll of the building during its operation;
  • Calculations for the possible crushing of the foundation and its pushing through;
  • Calculation according to the scheme of deformation.
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This is the only way to calculate the size of the footing for the slab inside the strip foundation.

Construction features

The strip foundation is a reinforced concrete element poured all around the perimeter of the object. It is poured under each exterior wall, the cross section remains the same around the perimeter.

Foundation in the form of a strip can be monolithic or precast.

Monolithic strip is arranged by pouring mortar into the formwork. Prefabricated foundation is a construction of prefabricated block stones, manufactured in the factory. The main disadvantages of precast structures are transportation, loading and unloading operations, and installation of blocks.

According to the degree of load strip foundations are classified into buried and shallow, performed on the perimeter of the building.

Slab reinforced concrete under the strip foundation is considered the foundation of the monolithic type, is laid over the area of the entire object. The monolith is achieved by pouring concrete mortar into the formwork structure. The main feature of such a foundation. reducing the pressure of the building on the ground, ensuring the stability of the object.

To build such a structure, it is necessary to take into account the peculiarities of local deformations, the movement of the soil base at different temperatures.

The main requirements of this technology are compliance with the parameter of the thickness of the plate, the structure, the device waterproofing layer and insulation.

Scope of application and peculiarities of calculation

The main purpose. use as a base on weak soils with a limit value of bearing capacity up to 2.5 kg/cm2. This includes sands. fines and medium density, clays and loams plastic, thawed and moistened soils. And it is the moisture level of soil that largely determines its carrying capacity. if it gets wet, it literally “floats”, losing its strength properties. Therefore, the drainage, waterproofing and blind area are given an important role in the arrangement of this type of foundation.

One of the advantages often cited is the argument that the monolithic slab can be made with your own hands. But if we do not even consider the choice of type of concrete and parameters of the reinforcement cage, there is such an important step as the calculation of the dimensions of the slab. And if with the area there is some clarity (the size of the house plus on each side a small margin), the thickness of the slab is not easy to calculate.

The recommended range of thickness of the foundation slab for low-rise construction is within 10-40 cm.

But this range includes a “light structure” in the form of a garage or summer kitchen, and a two-story house with an attic. If you make the slab not enough “thick” or choose the wrong thickness of reinforcing bar (or mesh size), then it will not be able to withstand the cumulative load. Excessive reserve of strength leads to the appreciation of the already expensive foundation, and the excessive amount of concreting will make the entire structure heavier, and already the soil can not withstand the cumulative load.

There is another option with diametrically opposite sphere of application. the slab is built on rocky and coarse-clastic soils with very high bearing capacity. In this case, it rather levels the site for construction, rather than redistributing the load from the structure.

Plate foundation vs. strip foundation: which is better?

The foundation is the “head” of everything. This is what determines the longevity and strength of any construction. both a private house and any other option. If a mistake is made in the selection, design or creation of this part of the house, the correction of the resulting problems can cost a lot. That is why the choice and creation of the foundation should be treated with the utmost responsibility.

Most often in the construction of private houses, a strip foundation is used. It is considered to be the simplest and most reliable. However, now it is not uncommon to use the slab variant as well. Which foundation is better. a slab or strip foundation? Let us try to answer this question.

Materials for construction

To “a” to “z” to understand the arrangement of the slab foundation, you need to know what materials it is made of. According to the requirements of the guidelines for the design of slab foundations, in the construction of such a base use:

For concreting, a mixture of grade M300 is selected, in a mandatory manner it is taken into account resistance to moisture and low temperature conditions, special importance is given to the mobility coefficient.

If the level of groundwater is as close to the surface of the ground as possible, it is recommended to use a sulfate-resistant concrete mixture, which is very rare.

Reinforcement bars are chosen with a diameter of 1.2. 1.6 cm, the brand can be any, the connection to the grid is made by knitting wire. If you intend to use welding, then the brand of reinforcement should correspond to a500c or a240c.

For waterproofing use bitumen-polymer compositions, it is allowed to use roll polyethylene, bentonite mats. If it is arranged shallow or shallow foundation, synthetic materials are used as thermal insulation layer, one example of which is extruded polystyrene foam as the most durable and long-lasting insulator.

Particular attention is paid to insulation if the technology provides that the warm slab foundation is also the floor of the room, which is at the same level with the basement.

Technology of construction of insulated boards

Saving energy is becoming a really hot topic, so few people build foundations without insulation. Any slab foundation is a multi-layered structure, and in the case of thermal insulation the layers are even more. To achieve the right level of quality it is necessary to carefully perform each of the levels. Let’s dwell on each in detail.

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The structure of the foundation monolithic slab

Ben’s self build vlog 1. what is a strip foundation?

Base preparation

The size of the excavation for the monolithic slab should be larger than the building, at least by 1 meter. Fertile soil is completely removed in this area. Its thickness varies in different regions from 20-30 cm to 50 cm and more. In any case, everything must be removed.

Excavate the excavation with a reserve of 1 meter in all directions

Along the edge of the excavation, just below the general level of the bottom, drainage pipes are laid, diverting surface water into the drainage wells. This measure is necessary to prevent the walls and the foundation itself from getting soggy.

Full foundation scheme monolithic slab

Level the bottom, fill the holes, remove the humps, level everything carefully and seal it. On the levelled bottom roll out the geotextile. It must cover not only the bottom, but also the walls. Plot the panels with overlaps, glue the edges with reinforced adhesive tape. Geotextile prevents the roots of plants from sprouting, and also prevents sand from washing out, which serves as a damping cushion.

Leveling the bottom

Clean sand of medium granularity is poured on top of a laid geotextile. The layer of sand 20-30 cm. It is poured in thin layers, distributed evenly and compacted in layers. The layer of sand, which can be properly tamped with a hand vibratory plate 8-10 cm. This is how layers and lay the sand. It must also be paved at a level, an equal layer throughout the excavation.

The sand is poured, it must be poured and tamped

The thickness of the layer can be checked with the stretched cords. They are tied to the driven in stakes, specially made supports for benches, to the installed level formwork (see the photo below). All the cords must be in a horizontal plane. Knowing the original distance from the bottom of the pit to the stretched strings, you can determine the height of the filled layer.

Crushed stone is poured on the tamped sand. Pour the entire volume at once, evenly distributing it on the site. The levelled crushed stone is compacted to a high density.

Crushed stone is backfilled, embedding elements of sewage and water pipelines are installed

At this stage, sewer and water pipes are laid. Ditches of the required depth are dug in the already compacted crushed stone. They should be such that there is some space around the embedded elements. Pipes are laid in ditches, covered with sand, leveled, the sand is compacted with a shovel or board. Greater compaction can lead to cracks. Therefore pipes are laid after ramming.

Concrete preparation

Along the perimeter of the pit place the formwork. It is usually assembled from 40 mm thick board or 18-21 mm plywood. The height of the formwork for the monolithic slab is the total thickness of the remaining layers. By its edge, it is convenient to control the level of concrete during pouring, so the board must be trimmed. To save material, it is possible to expose the formwork to preparation only. Once the concrete has set, it is removed and set higher, and then used again to fill the main slab. But the time loss with this approach is considerable, so it’s not always done this way.

In any case, the formwork is supported from the outside by stops and struts. The structure needs to be rigid to withstand the weight of the concrete.

Pour a layer of concrete 100 mm on tamped gravel. It can be concrete of low grades B7,5 B10. Concrete preparation will be a solid base for laying waterproofing and insulation, also serves to distribute the load from the house more evenly.

The concrete preparation is poured

Waterproofing

Since the monolithic slab of the foundation is completely in the ground, it needs careful waterproofing. That is why usually two types of materials are used: capping and sheeting. The basis is first thoroughly dedusted, then impregnated with kerosene or solvent diluted primer (and the sides of the concrete preparation is also blotted). It’s sold very thick and doesn’t set well with concrete. As a result, roll-fed waterproofing is poorly adhered and the foundation will get wet. Diluted it becomes more fluid and penetrates deeper into the concrete. In this case it almost does not lose any properties.

When laying roll-fed waterproofing, it is released outside the foundation by 10-15 cm. The panels are rolled out with an overlap and the joining edges should be coated with bitumen mastic and pressed well. When spreading it, it must be ensured that there are no kinks or waves.

If the water table is high, you may need two layers of roll-up waterproofing. Then it is rolled out crosswise and also glued on the primer (bitumen waterproofing), but you can not dilute.

Waterproofing of the monolithic foundation slab with double coating and roll-fed

Of roll-fed waterproofing materials are best shown Hydroisol, Tekhnonikol Tekhnoelast EPP.4 on high density polystyrene. Technolnikol this grade has a high tensile strength of about 60 kg, which increases the chances that it will not be damaged during further work. Use roofing felt, no matter how much you want to save money, you should not. In the modern version is too thin and brittle, quickly loses its properties. You can not replace the waterproofing of the board, so you have to use a better material.

Reduce capillary leakage through the board can also be using liquid impregnations such as Betonit. It greatly reduces the absorption of moisture. It can penetrate to a depth of 50-60 cm, so that the concrete preparation will soak through. Minus of this material is a high price, but the material has excellent properties.

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Insulation

To insulate a slab foundation use high-density extruded polystyrene foam. The thickness of the insulation layer of 10-15 cm, depending on the region (for the Midlands is enough 10 cm). Laying is carried out in at least two layers, overlapping the joints, which form the cold bridges. It requires more time, but the cost of heating will be less. If the boards will have an L-shaped lock, they can be laid in one layer.

Thermal insulation is laid

As the polystyrene foam is not friendly with oil products, it is spread a dense polyethylene film, and then lay the insulating material.

Reinforcement

For the reinforcing frame we use ribbed reinforcement of AIII class, 12-14 mm in diameter. It is laid along and across, with a spacing of 15-30 cm, it can have one or two layers. It all depends on the type of soil and the weight of the building. All reinforcement parameters are calculated separately.

From the edge of the board the reinforcement should be at least 5 cm. Therefore it is laid on special supports that provide the necessary gap.

The first row of reinforcement is tied together, some stands are exposed for tying the second chord

When reinforcing a cage is obtained, in each place of intersection the rods are tied together with a special soft steel wire. There are also connection techniques using plastic clamps or welding. Bind quickly with plastic clamps, but not everyone trusts them. Welding is not recommended, because the weld is the most vulnerable to rust, and too rigid connection is obtained. When using wires and clamps, the entire structure can play a little without destroying the bond, but in welding, such movements cause the joint to burst. As a result, the reliability of such reinforcement is low.

Casting the foundation slab with concrete

The thickness of the slab is calculated for each case and can be from 20 cm to 50 cm. When pouring concrete not less than grade B30 is used. The entire perimeter of the slab should be poured in one day, avoiding the appearance of vertical joints. That is why most often ready-mixed concrete is delivered for concreting of slab foundations: large volumes are required within a certain period of time.

Concrete is vibrated at the same time as it is distributed

Schedule the arrival of the machines so that you have time to distribute the first batch and compact it. For compaction, construction vibrators are used, which create high-frequency vibrations. As a result, all air is removed, the concrete mixes better, and becomes more fluid and pliable. The result of this process is not only a flat concrete surface, but also a higher moisture absorption class.

In extreme cases, the slab can be poured in horizontal layers. A vertical subdivision is not practical, since the joints are more likely to crack.

Concrete care

A sufficient level of moisture content of 90-100 percent and a temperature above 5°C are essential for the normal curing process of the concrete. Preferably, the slab should be poured in warm weather at a temperature of around 20°C. This temperature range is optimal for the curing process. The care of concrete monolithic slabs is to prevent mechanical damage and maintain moisture.

Immediately after pouring, cover the concrete with a sheet or tarpaulin. This prevents it from heating up in the sun, it is not affected by wind. The film is glued into large sheets. Stripes are laid with a 10-15 cm overlap, glued with scotch tape. It is desirable that there are as few unsealed joints as possible, that is, the shelter should consist of one or two pieces, if one is too inconvenient. In doing so, separate pieces of the film overlap each other by at least half a meter.

After pouring, the monolithic slab should be covered with a film

The size of the foil is such that the side surface of the formwork will also be covered, and it is possible to lay a load on the edges of the foil to prevent the wind from lifting it. Also, load planks-press the place of the preheating of the two strips, to reduce the sailing, they can be spread on the surface.

If the air temperature is above 5°C, about 8 hours after concreting, irrigate the concrete with water for the first time. The irrigation should be done in drips, not sprays. In order not to damage the surface by drops, you can lay a sacking or a layer of sawdust on it, and cover the top with foil. They water the covering material, and it keeps the concrete moist. In any case only water at temperatures above 5°C.

Watered surface

If there is a threat of frost, the slab and formwork should be additionally insulated. You can use any insulating materials, as prepared for the construction of the house, and sawdust, straw and other improvised means.

When to take down the formwork

For a monolithic slab, it is recommended that the formwork be removed after the concrete has reached 70% of its design strength. This term depends on the temperature at which the hardening takes place. This relationship is shown in the table.

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