Types and selection criteria of crushed stone
When building foundations in private and industrial housing construction, concrete is used, in which, in addition to mortar, crushed stone is added. Professional builders select the type of material, guided by GOST, and this article will help private developers working on their own to find out what rubble is needed for the foundation of the house.
In order for an individual developer to be able to independently decide which rubble is best used for the foundation, he will first have to study the basic points.
Material
Crushed stone is produced by grinding in special devices natural boulders and elements of old concrete structures. During the preparation of concrete bases, crushed stone of the following types is added to it:
Granite. This rock is one of the most durable materials. Granite is added when pouring concrete during the construction of structures with a high load on the base. It is not recommended to use granite stones in domestic construction under a small private house. The cost of such material is high, and the strength characteristics are excessive for modest buildings.
Gravel. Traditionally, the choice of individual developers, when planning the foundation, falls on gravel rubble. Gravel has acceptable strength, but it is not as expensive as granite.
Limestone. The foundation is erected not only for capital buildings, it is needed for other buildings. If you are planning to build a barn, a small bathhouse, a high fence or a garage, fill in the tape base with the addition of limestone stones.
Slag. This material is produced from burnt coal residues. It is suitable for casting foundation tapes for structures with small dimensions and weight.
Concrete. Also applies to low price materials. Suitable for casting slabs for small construction projects. To get fine fractions from ready-made concrete, old structural elements from concrete are destroyed in special devices.
Factionalism
During the processing of natural and artificial stones, large pieces are split into smaller ones, obtaining several types of gravel fractions for the foundation.
Finished fragments are divided into standard and not standard. The standard sizes fall: 40-80 mm, 20-40 mm, 15-20 mm, 5-10 mm, 3-8 mm. Large fragments ranging in size from 80 to 150 mm, which are produced by special order for certain types of concrete, are called non-standard.
Small particles remaining after processing large stones in a “screen” are called screening, its fractions do not exceed 40 mm. It is customary to fill in dirt roads in rural areas with such material.
Strength
The most important characteristic in construction is the strength of the material. To determine the degree of resistance of the material to loads, it is checked in laboratories. The selected samples are tested for compression, crushing and abrasion.
Having completed the verification, the sample is classified:
- special strength - M1200 - 1400;
- durable - M800 - 1200;
- medium strength - M600 - 800;
- low strength - M300 - 600;
- very low strength - M200.
What gravel is needed for the foundation? When casting load-bearing elements, it is recommended to use products marked with M800 and above. When a tall building is under construction - M1200, and for objects with a small load on the ground, the M800 brand will be enough. These grades include gravel and granite products.
Filler generated from waste shows a result of not more than M600. It is not used to form concrete foundation slabs. Limestone and crushed slag show even less strength.
Frost resistance
Frost resistance is indicated by the letter F and a digital code equal to the number of cycles of tests passed. This approach to determining frost resistance is used for all materials in construction. In a special chamber, the sample is frozen, then allowed to completely thaw. How many times the material withstands freezing and thawing, they put such a figure to the marking.
Frost resistance depends on the material:
- limestone withstands from 25 to 100 cycles (F25-100);
- concrete stone from 50 to 75 cycles (F50-75);
- slag stone from 75 to 200 cycles (F75-200);
- gravel stone from 200 to 300 cycles (F200-300);
- granite stone from 300 to 400 cycles (F300-400).
In the conditions of the European part of Russia, materials marked F250 are used for plates and blocks.
Flattery
Choosing the necessary material, one should understand not only what fraction of crushed stone is needed for the foundation, but also what shape the individual elements should be.
Builders distinguish several forms of stones. The best form for pouring the foundation are cuboid stones. The solution tightly fits cubic pebbles, practically avoiding the formation of voids in the monolith, which increases the strength of the structure.
Flatter and oblong stones are called flaky stones. A large number of flaky stones reduces the structural strength due to the uneven distribution of the solution and the formation of voids.
When purchasing material, you need to evaluate the proportion of cuboid and flaky stones. Three types of products are distinguished by the percentage of presence of flaky stones;
- With a share of up to 10%. An ideal stone for a monolith.
- With a share of 10 to 15%. It is also recommended for filling slabs and tapes, but exclusively for outbuildings, for example, for erecting a shed of lightweight gas silicate blocks.
- With a share of over 25%. Not suitable for the foundation.
Radioactivity
The radiation level indicator indicates the safety and purity of stones from an environmental point of view. Under natural conditions, the rocks used for crushing can be a source of weak radioactive background due to the content of uranium or cesium particles.
The content of radionuclides in crushed stone usually does not exceed acceptable values and is defined as weak, but some consumers are used to carefully studying all the properties. The certificates for construction products must contain information about the level of radiation in the material intended for the installation of private houses.
According to SNIP, crushed natural stones are divided into 3 classes according to the level of radionuclide content:
- For the first class, this indicator is valid up to 370 Bq / kg. It is applied to any tasks.
- In the second class, a value of up to 740 Bq / kg is allowed. It can be used in the laying of roads in cities and towns.
- The third class includes material with radiation over 740 Bq / kg. He's heading for the freeway.
Which rubble to choose
For a monolithic slab or tape, gravel or granite with an average fraction should be purchased, but such an expensive material is not required for a pillow, blind area and concrete slab.
For pillow
To form a pillow under a monolithic plate, crushed stone M600-M800 with a particle size of up to 40 mm.
Pillows for the monolith are covered with two layers (we gave the pillow device for the foundation here). The sand layer is laid first to a height of 300 mm, then the same layer of stones is poured on top. To obtain a reliable foundation for pouring concrete, tamping is carried out alternately: first the sand is compacted, and then crushed stone.
A pillow made of these materials is designed to compensate for powerful loads on the soil and reduce the risk of subsidence of the base under the mass of the object.
For footing
In order to protect concrete poured into the formwork from uneven drying and moisture loss, a safety base is arranged under it - a concrete block.
Concrete does not allow water to seep from the solution into the ground, creating conditions for an even set of concrete strength.
What fraction of crushed stone to choose for the foundation? Typically, the concrete is made with a thickness of not more than 100 mm, and the stones are selected with a fraction of not more than 20 mm. If liquid bitumen is used to fill the concrete pavement, the fraction can be increased up to 40 mm.
Strength characteristics for concrete footing are not important.
For the blind area
In order to prevent the foundation from being washed away by rainwater, a blind area is arranged around the perimeter of the building. When planning a blind area, it is important to provide for effective water drainage through a drainage system filled with crushed stone of the required type.
The drainage system is not subject to high loads, so it is filled with fine-grained concrete or limestone stones. Filling with a fraction of up to 20 mm will be easier to compact to the desired density.