How to build a monolithic slab foundation
- Construction of a monolithic foundation
- Foundation calculation
- Selection of materials and tools
- Slab foundation construction technology
Construction of a monolithic foundation
One of the most popular types of foundation currently is slab foundation. Foundations of this type are erected for buildings of various heights, including multi-storey and high-rise. This type of foundation is also the most reliable. A slab foundation is a monolithic reinforced concrete structure made under the entire area of the building under construction.
Construction technology requires mandatory reinforcement with metal rods. Thanks to this, the foundation moves along with the soil masses: when it freezes, it rises, and when it thaws, it lowers. This prevents the building from collapsing. For this property, foundations of this type received their second name - “floating”.
Floating foundations are especially effective when there are large loads on the foundation, on weak soils, on unevenly compressed soils, in seismically active areas, and on deeply frozen soils. Slab foundations are a type of shallow foundations laid at a depth of 40-50 cm.
They differ from non-buried strip foundations in that the slab foundations are rigidly reinforced along the entire plane. The use of slab foundations allows you to reduce the volume of excavation work, reduce labor costs, reduce the volume of concrete and save building materials.
There are several types of slab bases: ribbed, solid and box-shaped. Solid ones are used if the future building will not have a base. In this case, the slab will serve as the floor. When constructing residential buildings, the slab design involves a ribbed surface or cross monolithic strips. The height of the ribs will be equal to the height of the base. The ribs are made of monolithic concrete or prefabricated reinforced concrete blocks.
For multi-story buildings, box foundations are used that can withstand even significant loads. The height of the ribs of such slabs is equal to the height of the walls of the underground part of the building. The ribs are connected to each other by floor slabs and form a closed box. Thanks to the large support area and solid structure, minimal pressure on the ground is achieved.
Slab foundations for seismically hazardous areas have one feature. Stiffening ribs are erected from monolithic concrete during concreting of the slab; the frame of the ribs is welded to the reinforcement frame of the concrete base. Slab foundations are also made for soils with deep freezing. In this case, this is the most reliable cheap foundation option.
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Foundation calculation
The construction of slab foundations begins with some preparatory work. These include the determination of the foundation pie and the method of reinforcing floating slabs. In addition, it is necessary to select waterproofing materials for the lower sections of wall structures and make calculations for reinforced belts.
In order to make the correct calculation of a monolithic foundation, the following must be taken into account. First you need to decide on the thickness of the slab, the total area of the slab and the depth to which it will be laid. The area of the foundation should be slightly larger than the area of the building. This will help improve the strength characteristics of the building and reduce the likelihood of the house subsiding.
The foundation area is calculated using the formula:
S > γ n F/γ c Ro,
Where,
γ=1.2 - reliability coefficient;
F – load on the foundation (total weight of the house and payload);
γc – coefficient of working conditions, depends on the type of soil (for plastic clay it is 1.0, for weakly plastic clay, low-moisture sands - 1.2, for coarse sands 1.2, for fine sands - 1.3);
R0 – conditional resistance of the foundation soil for shallow foundations (determined using special tables).
Don't forget about the load inside the building. It must be taken into account that the building will contain furniture, household appliances, and will also house people. Therefore, it is necessary to add another 150 kg per m2 to the obtained calculation result.
Slab foundations, by their properties, fall into the category of shallow and non-buried foundations. The thickness of the slab depends on the characteristics of the soil and the type of building.
After all the calculations have been completed, it is necessary to divide the resulting number by the area of the building under construction and relate it to the bearing capacity of the soil. Despite all the calculations, stiffening ribs must be provided under the base floors and walls.
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Selection of materials and tools
For construction you will need the following materials and tools:
- concrete mixture;
- concrete mixer;
- shovel;
- wooden formwork;
- sand;
- crushed stone;
- waterproofing materials;
- thermal insulation materials;
- reinforcing bars.
The concrete on which the monolithic slab will be based plays a very important role, which is not surprising, because all the pressure falls on the solid surface.
In addition, the following types of loads are exerted on the slab: pushing, bending, heaving and soil resistance. The foundation slab should not be too thick; usually 20 cm is enough. Such a foundation can support a standard two-story frame house. For light buildings, you can make a slab 15 cm thick. If the house has a large enough area, the thickness of the foundation increases to 25 cm.
If the house is used for living in winter, care must be taken in choosing materials for thermal insulation. Thermal insulation is installed above and below the slab. Extruded polystyrene foam is perfect for these purposes. When working with them, it is necessary to ensure that cold bridges do not form; this will lead to cooling of the slab in winter.
Waterproofing materials are selected depending on the soil moisture in the area. This indicator can be determined by drilling a small well. You can also study the existing well on the site. If groundwater lies below 1 m, the foundation is waterproofed using a simplified technology. In this case, special moisture-proof membranes are used.