If there is soil with poor bearing capacity on the territory of the site, then the construction of a slab foundation is economically justified. The base is also suitable for conditions where groundwater is high, the soil contains clay in large volumes, and the earth swells when it freezes. The main disadvantage of this design is its high cost. But the opponents of this technology do not take into account that the equipped slab also performs the function of overlapping (floor) on the ground. Ultimately, the technology is cheaper than the deep strip foundation.
Design Features
The design of the described foundation is carried out on the basis of a standard called the “Guideline”. It was published in 1977. By reading this document, you will be able to learn about the basic requirements that must be taken into account when creating a project. The described design is created for houses whose area is more than 100 m22. Quite often, plates are used in conjunction withindustrial and utility buildings.
Designing a slab foundation involves taking into account some factors, among which should be highlighted:
- size accuracy;
- deformation loads in a certain area;
- static and dynamic loads from construction;
- soil mechanics factors;
- consumption of building materials.
Characteristic features should be established by conducting geotechnical surveys. The design provides for the calculation of diaphragms, and the design must include the reinforcement of the base, the determination of roll, displacements and deformations. The calculation must take into account the operating conditions of the foundation group.
Calculation
The thickness of the slab foundation is limited to a small allowable value. This parameter should be equal to the limit from 150 to 300 mm. For outbuildings, for example, slabs of 100 mm are used, while large buildings are best installed on a slab up to 400 mm in thickness. However, this phenomenon can be called rare.
The calculation of the required thickness is based on the fact that it is first necessary to determine the pressure from the building, taking into account all possible loads. This will allow you to calculate the specific pressure on the ground. The dimensions of the slab should be larger than the dimensions of the building by about 100 mm on each side. The slab foundation for a brick building should be 5 cm thicker than the same foundation for a foam concrete building. If there is a second floor in a brick house, then the thicknessbase increases to 40 cm. The final value will depend on the configuration and weight of the house.
If we are talking about a two-story foam concrete building, then the mentioned thickness should be equal to 35 cm. The calculation of the slab foundation also provides for determining the thickness of the pillow. It is located throughout the area and consists of rubble, as well as sand. They are laid out on a previously leveled bottom of the pit. Crushed stone is usually laid in a thickness of 20 cm, then sand comes, its thickness can be 30 cm.
The most common cushion thickness is 0.5 m. The calculation of the slab foundation is the determination of the parameters of all layers. For light wooden buildings, a pillow is laid, the thickness of which is 15 cm, while for a garage this value will be 25 cm. 0.5 cm pillows should be laid if you plan to build a heavy brick building. Crushed stone will compensate for the heaving and low density of the soil, acting as excellent drainage. This is especially true if the area has clay soil where groundwater is high. Sand at the same time will guarantee uniform distribution of the load on the ground.
Calculation example
You can understand the principle of calculation on a specific example. These manipulations are performed to determine the volume of concrete used in pouring. To do this, the sole area must be multiplied by the thickness. If you plan to install a house with dimensions of 10 x 10 m on a slab foundation, and the thickness of the base is 0.25 m, then the volume of the slab will be 25cubic meters. This value can be obtained by multiplying the above figures. The same amount of concrete will be needed to pour the foundation.
You must also consider the installation of stiffeners, which are necessary to increase resistance to deformation. They are placed at a distance of 3 m. They will be along and across the plate, forming squares. In order to carry out the calculation of the slab foundation, it is necessary to determine the height and length of the stiffeners. The last indicator is selected taking into account the length of each side of the base. In this example, this value is 10 m. In total, 8 ribs will be needed, so their total length is 80 m.
Additional calculations
The cross section should be rectangular or trapezoid. The width of the ribs according to the standard is 0.8 of the height. For rectangular ribs, the volume is 16 cubic meters. You will get this value if you multiply the numbers 0, 25, 0, 8 and 80. If we are talking about trapezoidal ribs, then their lower base is 1.5 times the thickness of the foundation, and the upper one is 0.8.
As for the volume of all ribs, it is 12 m2: you will get this figure if you multiply 0, 15 and 80. From the above calculation of the slab foundation, it can be seen that for pouring the base, the thickness of which is 25 cm, will require 25 m2 concrete.
Markup
The slab is not poured over the fertile layer. But at the first stage it is necessary to carry out markup. For this, a cord and pegs are used, whichinstalled along the perimeter of the drainage system. The pit is dug in such a way that it is 0.5 m larger than the slab on each side. The base will protrude 10 cm beyond the dimensions of the building.
Earthworks
The amount of work will be small. As for the removal of the fertile layer, you will need to go deeper by 40 cm. Work can be done on your own, without involving construction equipment. At this stage, the monolithic structure should be protected from soil moisture. Drains are laid around the perimeter. They will consist of a geotextile that extends over the edges of the ditch. Next comes a layer of compacted rubble and a perforated pipe. The system is filled with a natural filter in the form of crushed stone, everything is covered with geotextiles.
Drains are not placed under the foundation slab or concrete preparation. The height of the backfill is on the same level as the crushed stone cushion. After pouring the slab into the building, it will be impossible to enter communications. In this regard, sewerage and cold water supply are laid at this stage. There is no need to bury them below the freezing mark, since the thermal insulation of the sole will retain geothermal heat. Enough depth of 1.2 m.
Laying the backing
The slab foundation must be located in preparation. This makes it possible to reduce the heaving forces. First, the sand is covered with a layer of 10 cm and compacted. In parallel, it must be abundantly moisturized. It is convenient to use a vibrating plate for ramming. Next comes the crushed stone, which is also well compacted. Instead, you can use a mixturePGS, which is laid to a depth of 40 cm and is well compacted. In this case, the monolithic base receives a reliable support on the lower layer.
Concrete preparation and waterproofing work
Monolithic slab must be waterproofed from below. This will prevent corrosion of concrete and reinforcement. The laying of a layer of rolled materials is carried out according to concrete preparation, since the material can be torn by a layer of crushed stone. This provides a flat surface on which it is easy to stick the bituminous base. If the base of the slab is laid on a level surface, this will increase strength and stabilize the geometry.
The thickness of the screed will be 5 cm, it is not necessary to reinforce it. The minimum construction budget will be provided with rolled materials. The strips should be laid with an overlap of 20 cm, the seams should be treated with cold or hot bituminous mastic. The edges of the carpet are released beyond the perimeter of the concrete preparation, so that after pouring the slab, run them up or to the side.
Heat insulation and reinforcement
Monolithic slab must be insulated. Extruded polystyrene foam can act as a heat insulator. Its laying is carried out in 2 layers. If stiffeners are provided in the project, then the first layer must be laid end-to-end, while in the second, gaps are created along the width of the rib. Reinforcement of the slab with an armored belt is carried out taking into account the regulatory documentation for concrete and reinforced concrete structures.
First, clamps are made from a smooth 6 mm bar, which is bent in the formtriangle or square. Then you can start creating a reinforcing mesh, it is based on bars of a periodic profile. Their diameter can be equal to the limit from 12 to 16 mm. The elements are connected by wire or welding.
It will be necessary to lay the lower belt of bars with a section of 10 cm on the concrete pads. The reinforcement for the slab foundation is recessed into the concrete. Clamps are placed on the lower grid, which will make up the upper belt. The top card is fixed on them. It is undesirable to use separate bars inside the armored belt. They are bent in curved sections, and in places of communication input nodes they are associated with common grid maps. In order to save reinforcement, its cells are increased to 20 x 20 cm.
Additional reinforcement
Building a house on a slab foundation, you should definitely take care of its strengthening. For this purpose, reinforcement is carried out. For knitting, steel rods are used, which have ribs. Their diameter is selected taking into account the loads on the base of the house. It is also important to pay attention to the characteristics of the soil. The thickness of the slab itself must also be taken into account.
For a typical two-story building, a monolithic slab is poured with a thickness of 300 mm. The diameter of the reinforcement varies from 12 to 14 mm. The device of the slab foundation in this case provides for the laying of a grid, the cell of which sometimes increases to 25 cm. The first grid is located on brick supports. Then an additional layer of bricks is laid on top, followed by a second layerreinforcing mesh. If you plan to use welding for a bundle of reinforcement, then the bars must be selected taking into account the marking, you must find the index "C" in it.
Formwork installation
The construction of a slab foundation necessarily provides for a formwork device. To do this, plywood sheets, OSB or chipboard are installed around the perimeter. The material should look like shields. Their inner surface is protected with roofing felt or film to prevent chipping of the material during stripping. Shields are installed around the perimeter. To protect the structure from freezing, 10 cm polystyrene foam boards are laid inside. The same insulation should be laid under the blind area to exclude lateral freezing. The material is located at the level of the sole, it is placed flush with the upper or lower layer of the heat insulator inside the formwork.
Pouring concrete
When the reinforcement of the slab foundation according to the above technology has been completed, you can start pouring concrete. It is better to fill the space in one go. Between serving portions of the mixture, the maximum interval is 2 hours in warm weather. Concrete must not be distilled around the perimeter with a shovel, it is necessary to rearrange the mixer or use a concrete pump.
Vibrocompaction is performed until the appearance of cement laitance, the absence of bubbles and the hiding of crushed stone. The mixture is heated in winter, for this a cable is laid inside the formwork, you can use steam heating or cover the surface with film materials. Stick to netsarmopoyas nozzle vibrators is prohibited. On the seventh day, under normal conditions, stripping is carried out. The concrete surface should be protected from precipitation by covering it with burlap. If work is carried out in hot weather, then the base is wetted from a watering can.
In conclusion
When erecting a monolithic foundation, the stage of thermal insulation should not be neglected. The insulation layer not only fulfills its intended purpose, but also reduces heating costs in winter. Insulation can be foam or extruded polystyrene foam. The material is fixed at the ends of the base by spraying. For a typical house, a 50 mm layer of thermal insulation will be sufficient. For colder regions, the layer thickness is doubled.
