It is difficult to name the sphere of construction, wherever rubble is used. This is an irreplaceable bulk material, which is distinguished by a huge variety of types, and in each case, you should use the material of one or another fraction, strength, flakiness, color and frost resistance. Let’s try to understand the volumetric classification and understand what types of crushed stone exist, what fractions and grades it can be, and how to choose crushed stone so that it best suits the scope of application.
# 1. How is crushed stone obtained and where is it used?
Crushed stone is a bulk material that can vary significantly in color, particle size, strength and other parameters. The main reason for such a wide discrepancy in characteristics lies in the origin.
By origin, crushed stone is divided into:
crushed stone from rocks, which is obtained during the extraction of stone. It can be gravel, granite, quartzite and limestone. This is the most preferred group of materials for use in construction work;
ore crushed stone is obtained when processing rocks of ferrous and non-ferrous metals;
secondary and slag crushed stone is obtained from construction waste and industrial processes. These can be pieces of brick, concrete, asphalt, etc.
The second two types of crushed stone, although less popular, still find application, since the scope of use of crushed stone is quite wide. It is used for the manufacture of concrete of various grades, for the construction of monolithic structures and foundations, for the production of block building materials, for drainage, for the creation of road surfaces of various types, incl. loose garden paths and highways. In addition, rubble is indispensable when arranging runways and railways, and the designers also fell in love with this material for the ability to decorate flower beds, reservoirs, embankments and indoor surfaces with it.
# 2. Crushed stone fractions
As a result of crushing the raw material, particles of different sizes are formed, or, in other words, different fractions of crushed stone. To separate one fraction from another, crushed stone is sieved through a special sieve called a screen. There are crushed stone particles from 3 mm to 300 mm and more.
There are even more classifications of crushed stone by fractions than the fractions themselves. The simplest is the division into five fractions:
- the first is fragments up to 5 mm, which cannot even be called rubble. Rather, it is a by-product of production; it can be used only for decorative finishing of paths;
- the second is crushed stone 5-20 mm in size. It is the most demanded material in the construction and production of reinforced concrete products; it is also used for road construction and foundations. Such crushed stone is irreplaceable in private construction when preparing concrete;
- the third – crushed stone 20-40 mm. It is used in drainage works, in the construction of tram and railway tracks, and can also be used in the arrangement of foundations for industrial buildings;
- the fourth – large crushed stone 40-70 mm in size can be used for drainage, added to concrete and concrete products;
- the fifth – the largest crushed stone up to 300 mm in size, the so-called bottle. Its use is very limited, but it is suitable for decorating fences, pools and ponds.
There is another, more detailed classification of crushed stone by size, which implies the separation of standard and non-standard fractions. The standard ones include such fractions as 3-8 mm, 5-10 mm, 10-20 mm, 20-40 mm, 25-60 mm, 20-70 mm, 40-70 mm. By agreement with the consumer, non-standard fractions of 10-15 mm, 15-20 mm, 80-120 mm and 120-150 mm can be produced.
No. 3. Flakiness of rubble, or shape geometry
Elements of different shapes are found in the mass of rubble. There are stones that are distinguished by the presence of flat, even edges, they have a plate or needle-like shape. Stones with conditionally equal faces are called cuboid. The higher the content of lamellar and needle-shaped elements in the crushed stone, the higher the flakiness indicator. The operational characteristics of crushed stone and the scope of its use depend on it.
By flakiness, crushed stone is divided into the following types:
- ordinary crushed stone with a flakiness of 25-35%;
- improved crushed stone, where the part of the needle-shaped grains is 15-25%;
- cube-shaped crushed stone with flakiness no more than 15%.
Crushed stone with a low flakiness index is used where it is important to compact the concrete mixture tightly. Needle grains create unnecessary voids in concrete, so more crushed stone has to be added, but even so, the compressive strength and finished material will be lower. Crushed stone with high flakiness is used in drainage works, used in road construction.
No. 4. Density and strength of crushed stone
The density of crushed stone ranges from 1.2 to 3 g / cm3 and strongly depends on the type of origin of the material. The higher the density index, the more versatile the material is. The density and strength of crushed stone are directly proportional indicators.
Strength is the ability of a material to withstand various types of mechanical stress. Research and determination of the strength grade are carried out by a series of tests, checking the crushed stone in the cylinder, shelf drum and compressing it. Such experiments allow simulating real conditions of future exploitation. According to the strength grade, crushed stone can be from M200 to M1600 (the numerical indicator means the maximum withstand load in kg / cm2) and is divided into the following classes:
- М200 – crushed stone of very weak strength, used for arranging drainage systems and creating unloaded dumping;
- М300-М600 – crushed stone of weak strength, used in the same areas as crushed stone М200;
- М600-М800 – crushed stone of medium strength, can be used to create unloaded structures, for example, curtain walls;
- М800-М1200 – durable crushed stone, the most versatile option, which is used to create foundations, bearing walls, supports, fences, etc.;
- М1200-М1400 – high-strength crushed stone, without which it is impossible to build the foundation of a multi-storey building, bridge supports and hydraulic structures;
- М400-М1600 – heavy-duty crushed stone, which is used quite rarely and only when creating especially critical structures.
In rubble, the amount of impurities of weak rocks is normalized, because the strength of the material directly depends on this. To determine this parameter, tests are carried out and a pressure of 20 MPa is created on the crushed stone. According to GOST 8267-93, some of the impurities of low-strength rocks are normalized as follows:
- for crushed stone М1600 – no more than 1%;
- for crushed stone М1000-М1400 – no more than 5%;
- for crushed stone М400-М800 – no more than 10%;
- for crushed stone М200-М300 – no more than 15%.
If the content of low-strength impurities is more than 20%, then the material is called gravel and is used only in irresponsible work, for example, when creating temporary structures, filling local roads, etc.
No. 5. Frost resistance of crushed stone
Frost resistance shows how many cycles of freezing and thawing the crushed stone can withstand without losing its basic strength properties. In laboratory conditions, this indicator is determined by drying and saturating the material in a solution of sodium sulfate. Strength is indicated in the marking with the letter F, followed by a number – an indicator of the number of freeze-thaw cycles. The frost resistance of crushed stone ranges from F15 to F400, and it is best to use crushed stone with frost resistance of at least F300 in building construction.
According to the level of frost resistance, the following types of crushed stone are distinguished:
unstable, F15-F Used as bedding, for drainage and for interior work in constantly heated rooms;
steady, F50-F150. Can be used for low-rise residential construction in the southern regions;
highly resistant, F200-F This is the most versatile type of crushed stone that can be used for the construction of critical structures, in high-rise construction, when creating various kinds of concrete products.
No. 6. Radioactivity level
Radioactivity is as important an indicator of the quality of crushed stone as strength or frost resistance. To a large extent, the natural radioactive background depends on what kind of rock the crushed stone is made of. Granite is characterized by increased radioactivity, while quartzite and limestone have a low natural background.
According to the level of radioactivity, crushed stone is divided into the following classes:
Class I is a material with a low radioactivity not exceeding 370 Bq / kg. A versatile material that is suitable for the construction of any kind of objects. This rubble is ideal for residential construction;
Class II – crushed stone with radioactivity up to 740 Bq / kg. It is only suitable for road construction.
The radioactivity of crushed stone must be documented, therefore, when choosing and buying a material, it does not interfere with making sure that the appropriate certificates and sanitary and epidemiological conclusions are available.
No. 7. Shade of rubble
The color of the crushed stone depends on the origin of the material. When it comes to the construction of foundations, drainage and the production of concrete products, the shade of crushed stone does not play any role. If the material is used for decorative work (for example, for interior decoration of walls, decoration of flower beds and ponds), then the color is of great importance, fortunately, there are a lot of different options. Crushed stone is distinguished by a huge variety of colors, from gray to reddish, brownish, pink and yellowish.
No. 8. The most popular types of rubble (by origin)
We have already mentioned that crushed stone can be of different origins, which determines its performance. Today, the most popular are the following types of crushed stone:
crushed granite is the most durable and versatile. It is obtained from rocks of magmatic origin, which may include crystals of feldspar, mica and quartz. Depending on the predominance of certain minerals, the color of such rubble can be gray, pink or even reddish. Granite crushed stone is very durable (М1400-М1600), frost-resistant (F300-F400), has a low flakiness, but may have an increased radioactive background, so when buying it, you should definitely look at the certificates. Granite crushed stone is the best in all respects, therefore the price for it is appropriate, but still it is most often used in construction. It is used in the production of reinforced concrete products, concrete, the creation of foundations, the construction of roads of all types, the construction of critical facilities, bridges, airfields, etc. Allocate fractions from 5-10 mm to 150-300 mm;
crushed gravel is obtained by explosive mining of stone rocks and screening of quarry rock. The shape of the particles can be streamlined and cornerstone, the color is predominantly gray, but there can be elements with a greenish, white and beige tint. In terms of the main indicator, gravel crushed stone is inferior to granite, but still its properties allow the material to be used in the construction of critical facilities. Maximum strength – M1200, it has good frost resistance and low radioactive background. Since there are many quarries for its extraction, and the process itself is not as laborious as in the case of granite, the price of crushed gravel is significantly lower. Fraction of 5-20 mm is used in the production of paving slabs, 5-40 mm – for the production of some concrete products, 20-40 mm – for the production of concrete and road filling;
quartzite crushed stone is obtained from rocks, the basis of which is the quartz mineral. In terms of strength, such crushed stone is practically not inferior to granite, has a slight radioactive background and boasts an excellent appearance, therefore it is often used when carrying out decorative work;
crushed limestone is obtained from sedimentary rocks. It has the lowest strength among all types of rock crushed stone. The color is white, can have shades of yellow, brown and gray. The main advantage is the low price. Due to its low performance, it is possible to use crushed limestone only in the construction of non-critical structures, for example, low-rise buildings and roads with a low load, as well as in the production of the simplest concrete products. It is also used for the manufacture of mineral fertilizers, Portland cement, etc .;
secondary crushed stone is obtained by processing construction waste, incl. bricks, concrete, asphalt, etc. The production process remains the same as when receiving primary crushed stone, with the exception of raw materials. The main advantage of such crushed stone is the price, but in terms of basic indicators it is inferior to the granite counterpart, however, there are areas where the most durable and frost-resistant crushed stone is not required. Maximum strength corresponds to approximately M800, frost resistance – F Secondary crushed stone is used in the construction of roads with light load, as a coarse aggregate for concrete, as well as for strengthening soft soils;
crushed slag is obtained by processing metallurgical slags. As a result, material comes out with particles of 5-120 mm in size. It is used in the production of cement concretes and for strengthening foundations in road construction.
As you can see, for any purpose of use, you can find crushed stone with suitable properties and the most favorable price. The main thing is to familiarize yourself with the documentation when buying.