How to build a concrete slab foundation in a proper way
What is a slab foundation? It is the kind of monolithic foundations in the form of concrete slab laid on a prepared surface of sand and rubble. Traditionally, it is performed mainly on weak soils: peat bogs, sand, saturated clays having a tendency to heave and shift. This foundation is usually not deepened to a depth of soil freezing, so during the seasonal shifting of the soil it is shifted together with the soil. For this feature it is also called “the floating slab foundation”. This structure does not experience point loads - all the changes are transmitted to the foundation wall evenly. In case you wonder how to build a slab foundation in a proper way, here is some information that may be useful to you.
The device of the slab foundation reliable and can be used to build houses in all kinds of soils and at any depth of the water table. This is a good option and if the construction is carried out on uneven, highly compressible, heaving soils or on sand cushions. Thanks to its sturdy construction – the monolithic plate that runs under the entire area of the building - the base plate is not afraid of any displacement of the soil.
Pros and cons of the concrete slab foundation
- it can be erected on the soft ground with a large depth of freezing;
- it does not require the deep excavation unit;
- the bearing capacity of this type of foundation is rather high;
- the ability to withstand the stresses and changes from ground deformations;
- the floating foundation is very durable - its service life is up to 150 years old;
- this foundation, thanks to its design, also serves as a rough floor of the basement.
- the slab foundation is more expensive due to the significant consumption of materials;
- the high time-consuming manual work and the need for construction machinery raise the price of the slab foundation.
The cost of the slab foundation
The cost of the slab foundation slab is quite high. The device requires serious excavation works, so the total cost of the monolithic slab foundation is high. In addition, you will have to deal with the high consumption of building materials, such as concrete and materials needed for reinforcement. It is advisable to use the slab foundation in the construction of small houses, when the plate itself serves as the basis of the floor.
For correct calculations you should take into account the following characteristics:
- determination of the thickness of your future concrete slab foundation;
- the depth of the structure;
- calculation of the base area - it must exceed the parameters of the entire building;
- the selection and quantity of the foundation depends on the number of floors of the building.
Technology of work
- The preparation of the site
This is the most time-consuming operation for the implementation of the slab foundation. You must completely remove the top layer of soil to a depth of the established calculation. The last layer is better to be removed and manually aligned to prevent irregularities and pits: any deepening may cause deformation due to uneven loading in future during the exploitation of the house. The pit should exceed the dimensions of the foundation for about 5 feet on all sides.
- The preparation of the pillow of sand and gravel
This is the second stage of the work. It is necessary to compensate for the deformation of the ground forces, as well as for the removal of ground water and capillary rise of exceptions to the base of the foundation. The thickness of the cushion depends on the characteristics of the soil:
- if the soil is sandy, it can be about 6 inches;
- if the soil is clay, it can be no less than 12 inches
The sand is poured into the prepared pit, then it is evenly distributed over the entire area of the basement and carefully compacted. For marshy or wet soils use the pillow made of rubble, it improves the waterproofing of the concrete.
- The device of the formwork
You should make the removable formwork made of planed boards with a minimum thickness of about 20 mm, fastened together at the corners by means of screws. On its external side the formwork must be strengthened with braces. Then you should equip the necessary tunneling for communications performing around the formwork. The pipes can also be put through penetration and brought out before the pouring of the foundation.
Why do we need waterproofing? Moisture is present in any soils, more or less. The lack of waterproofing will lead to very serious problems. Constantly being exposed to humid environment, the supporting structure will be a bit cracked. This will allow the water to get inside the “body” of the base structure. When the freezing appears, moisture will begin to expand and increase cracking. In the end it will gradually skew the structure. All this will lead constant slab foundation repairs and even to the collapse of the building. Waterproofing is traditionally performed using thick plastic film, geotextile or roofing material, putting it lapped at the bottom of the pit over the formwork. Waterproofing of the concrete monolithic foundation is more often performed with a roll of roofing material. It is not difficult. The rolls are placed directly on the base plate structure. The top layer of waterproofing is placed on a layer of insulation. The flooring will lay above. The insulated slab foundation will serve you for decades.
- Reinforcement of the foundation
Reinforcement of the foundation is a very important step, it will depend on the strength of not only the foundation, but also the building in general. For smaller buildings it can be carried out with the help of reinforcement wire mesh of about 4-6 inches. The places where the bearing walls will stand must be reinforced with metal bars. If the design of the building is more massive, you should use the reinforcement bars with a diameter of about 10-12 mm laid in a grid. The transverse rods are knitted together using the wire. Welding of reinforcement is not recommended, as the field of welding in the construction lead to the excessive stresses. Reinforcing mesh should be completely submerged in the concrete, so set it to the special guides. If the thickness of the foundation is significant you should better set several layers of reinforcement.
- The pouring with concrete
The pouring with concrete is produced in one stage, so the concrete will have to be mixed super quickly. It is better to have a team of 4-5 people. Concrete is poured into the formwork and then compacted by means of deep vibrator and a vibrating rail. After the piercing of the concrete and removing the air voids out of it, its surface should be well smoothed.
- Drying of the foundation
Drying of the foundation usually takes 4-5 weeks. During the process of drying you will need to ensure that the upper layer of the foundation is not dried out, for this it is convenient to use covering materials. After the drying of the concrete the foundation is insulated using polystyrene plates.
Reasons for the slab foundation repair
The main causes of the foundation damage usually are:
- the use of substandard building materials;
- incorrect construction work;
- subsidence of the foundation.
All of these mistakes may lead to serious consequences. So be sure to carry out repairs in time! The first sign of malfunctions is the presence of cracks on the surface of the structure. To check its growth paste a paper strip on the crack. After two weeks make a check-up: if the paper is not damaged you shouldn’t worry.
Slab foundation is often used in the construction of private houses. Its design, with all its complexity is quite simple, so you can perform a slab foundation with his own hands, bringing special equipment only for special responsibility and hard work.
We hope this article helped you to get the necessary information on slab foundations.
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