If on the territory of the site soil with unsatisfactory bearing capacity, the construction of a slab foundation is economically justified. The base is also suitable for conditions where groundwater is located high, the clay contains large amounts of clay, and the earth swells when it freezes. The main disadvantage of this design is its high cost. But opponents of this technology do not take into account that the equipped slab also performs the function of overlapping (floor) on the ground. Ultimately, technology is cheaper than a deep tape base device.
Design Features
The design of the described foundation is carried out on the basis of a standard called the “Guide”. It was published in 1977. After reading this document, you can learn about the basic requirements that must be considered when creating a project. The described construction is created for houses with an area of more than 100 m 2 . Quite often, slabs are used in conjunction with industrial buildings and facilities for economic purposes.
Designing a slab foundation provides for the consideration of several factors, among which are:
- dimensional accuracy;
- deformation loads in a particular area;
- static and dynamic loads from the building;
- soil mechanics factors;
- consumption of building materials.
Characteristic features should be established through geotechnical surveys. Design involves the calculation of diaphragms, and the project must include reinforcement of the base, the definition of roll, displacements and deformations. The calculation should take into account the operating conditions of the foundation group.
Calculation
The thickness of the slab foundation is limited by a small acceptable value. This parameter must be equal to the limit of 150 to 300 mm. For household buildings, for example, 100 mm slabs 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 first you need to determine the pressure from the building, taking into account all the 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 brick construction should be 5 cm thicker than the same base for a foam concrete building. If there is a second floor in a brick house, then the thickness of the base will increase 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 crushed stone, as well as sand. They are laid out on a pre-aligned bottom of the pit. Crushed stone is usually laid in a thickness of 20 cm, then there is sand, 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 equal to 25 cm. 0.5 cm pillows should be laid if you plan to build a heavy brick building. Crushed stone will compensate for heaving and low soil density, acting as an excellent drainage. This is especially true if there is clay soil in the territory where groundwater is high. At the same time, sand will guarantee uniform distribution of load on the ground.
Calculation Example
You can understand the principle of calculation using a specific example. These manipulations are performed to determine the volume of concrete used during pouring. To do this, the sole area must be multiplied by the thickness. If you plan to install a house with a size of 10 x 10 m on a slab foundation, and the thickness of the base will be 0.25 m, then the volume of the slab will be 25 cubic meters. This value can be obtained by multiplying the above figures. The same amount of concrete will be needed to fill the foundation.
You must also consider the installation of stiffeners , which are necessary to increase resistance to deformation. They are located at a distance of 3 m. They will be located 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 in the shape of a rectangle 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 trapezoid ribs, then the lower base is 1.5 times the thickness of the foundation, and the upper is 0.8.
As for the volume of all ribs, it is 12 m 2 : you will get this figure if you multiply 0.15 and 80. From the above calculation of the slab foundation it is clear that 25 m 2 concrete will be needed to fill the foundation, whose thickness is 25 cm .
Markup
Plate filling is not carried out on the fertile layer. But at the first stage, it is necessary to markup. For this, a cord and pegs are used, which are installed around 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 foundation will protrude 10 cm beyond the dimensions of the building.
Excavation
The amount of work will be small. As for the removal of the fertile layer, you will need to go deeper by 40 cm. You can do the work 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 geotextiles that go to the edges of the ditch. Next is a layer of compacted crushed stone and a perforated pipe. The system is covered with a natural filter in the form of rubble, everything is covered with geotextiles.
Drains do not fit under the foundation slab or concrete preparation. The height of the backfill is at the same level with the rubble cushion. After the slab is poured 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 deepen them below the freezing mark, since the thermal insulation of the sole will retain geothermal heat. Enough depth of 1.2 m.
Backing
The slab foundation is necessarily located in preparation. This allows you to reduce heaving forces. First, sand is poured and compacted with a layer of 10 cm. In parallel, it must be abundantly moistened. It is convenient to use a vibrating plate for tamping. Next is crushed stone, which also compacts well. Instead, you can use a mixture of ASG, which is laid to a depth of 40 cm and is well compacted. In this case, the monolithic base receives reliable support on the lower layer.
Concrete preparation and waterproofing works
A monolithic slab must be waterproofed from below. This eliminates the corrosion of concrete and reinforcement. Laying a layer of rolled materials is carried out on 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 bitumen base. If the base of the plate is laid on a flat surface, this will increase strength and stabilize the geometry.
The thickness of the screed will be 5 cm, it does not need to be reinforced. The minimum construction budget will be provided by roll materials. The strips should be laid with an overlap of 20 cm, the seams should be treated with cold or hot mastic on a bitumen basis. The edges of the carpet are released beyond the perimeter of the concrete preparation, so that after filling the slabs, run them up or sideways.
Thermal insulation and reinforcement
A monolithic plate must be insulated. An extruded polystyrene foam can act as a heat insulator. Its laying is carried out in 2 layers. If stiffeners are provided for in the project, then the first layer must be laid back to back, while the second layer creates gaps 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 form of a triangle or square. Next, you can begin to create a reinforcing mesh, it is based on bars of a periodic profile. Their diameter can be equal to the limit of 12 to 16 mm. Elements are interconnected by wire or welding.
On concrete pads, it will be necessary to lay the lower belt of rods with a cross section of 10 cm. The reinforcement for the slab foundation is recessed in concrete. On the lower grid are clamps that make up the upper belt. The top card is fixed on them. Separate rods inside the armored belt are undesirable. They are bent in curved sections, and in the places of the input nodes of communications they are connected with common grid maps. In order to save reinforcement, its cells are increased to 20 x 20 cm.
Additional reinforcement
When building a house on a slab foundation, you should definitely take care of strengthening it. To this end, reinforcement is carried out. For knitting, steel rods that have fins are used. Their diameter is selected taking into account the loads on the base of the house. It is important to pay attention also to the characteristics of the soil. The thickness of the plate itself must also be taken into account.
For a typical two-story building, a monolithic slab is poured with a thickness within 300 mm. The diameter of the reinforcement in this case varies from 12 to 14 mm. The device of the slab foundation in this case provides for laying a mesh, the cell of which sometimes increases to 25 cm. The first mesh is located on brick supports. Then an additional layer of bricks is laid on top, then comes the second layer of the reinforcing mesh. If you plan to use welding for the bundle of reinforcement, then the rods should be selected taking into account the marking, in it you should find the index "C".
Formwork installation
The construction of a slab foundation necessarily requires a formwork device. To do this, plywood sheets, OSB or particleboard are installed around the perimeter. The material should look like shields. Their inner surface is protected by roofing material or a film to prevent chipping of the material during formwork. Shields are installed around the perimeter. To protect the structure from freezing, a 10-cm expanded polystyrene plate is laid inside. Under the blind area, the same insulation should be laid in order 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 was completed, you can begin to pour 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. It is forbidden to distill concrete around the perimeter with a shovel; it is necessary to rearrange the mixer or use a concrete pump.
Vibration compaction is performed before the appearance of cement milk, the absence of bubbles and the concealment of rubble. The mixture is heated in the winter, for this, a cable is laid inside the formwork, you can use steam heating or cover the surface with film materials. It is forbidden to lean the vibrator nozzles against the armopoyas nets. On the seventh day, under normal conditions, stripping is performed. The concrete surface should be protected from precipitation by covering with burlap. If work is carried out in hot weather, then the base is wetted from a watering can.
Finally
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 allows to reduce heating costs in the winter. The 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 enough. For colder regions, the layer thickness increases by 2 times.