Concrete mix: characteristics of the main components and key

Probably, everyone knows what a concrete mix consists of: we take cement, gravel, sand, mix everything with water until a uniform consistency and pour it into the mold. However, as in most cases, everything is just at first glance. In fact, in the process of preparing a solution, there are many nuances, because the ratio of the main components depends both on the substances used and on the purpose of the concrete.

Types of formulations

To destination

The proportions of the concrete mix are not constant: depending on what will be used as a binder and filler, as well as in what conditions the structure will be used, they may change. Below we consider the classification of concretes by a number of parameters.

Depending on the purpose, the compositions are divided into the following groups:

• Conventional Solutions. Used to form building blocks, as well as to fill monolithic structures. In most cases, do not contain additional components with the exception of modifiers that improve the natural properties of the material.
• Cellular materials. The concrete mixture for such a solution usually includes a pore-forming composition, which produces gas during the reaction with the components. This gas in the form of small bubbles spreads in the thickness of the poured concrete, providing an increase in its performance.

Note! Often in this group of materials include solutions with insulating fillers, such as polystyrene concrete, claydite, arbolit. Depending on the density, these composites can be used both for the erection of supporting structures with low thermal conductivity, and for insulation of already built walls.

• Hydrotechnical concretes. They are distinguished by minimal water permeability, they are not destroyed by contact with water for a long time. Usually used for facing dams, bridge trusses, etc., but can also be used in everyday life - for example, for arranging wells.
• Special formulations. Designed for use in extreme conditions. Includes components that can withstand operational loads for a long time. Fire-resistant (cement-concrete mixture contains alumina, slags, fireclay bricks), acid-resistant, anti-radiation materials, etc.
• Separately it is necessary to allocate auxiliary solutions, which include, for example, the starting mixture for the concrete pump. This composition is prepared in much the same way as building cement, but we add much more water (about 150 liters to 2 bags of cement). Liquid "concrete milk" wets pipelines and prevents the formation of traffic jams.

• Finally, a separate group consists of repair compounds.. These include fluid mixtures (used to restore horizontal surfaces) and low shrinkage thixotropic materials (used to eliminate defects on vertical walls). The main advantage of these products is effective adhesion with already polymerized concrete.

According to the main substance

Regardless of where the composition will be applied, it should be made on the basis of a binder - a substance that ensures the transformation of the mixture into a monolith.

By this parameter the following categories of materials are distinguished:

• Composition on cement binder. The simplest concrete mix according to GOST is made on the basis of Portland cement of various strengths. Pozzolan cement and slag Portland cement are also used for construction. This variety is the most common, and therefore below we will focus mainly on its characteristics.

Note! The same category includes repair compositions with non-shrinking cement, as well as refractory mixtures with a high content of alumina.

• Gypsum mortar is also included in the group of concretes. This material is used to finish the ceilings and erect partitions. The combination of gypsum with pozzolan cement provides a high level of water resistance, because such mixtures are used in the repair of bathrooms.
• Alkali solution. It is produced by alkali exposure to finely ground slags. It is used when filling the bases, production of building blocks, construction of monolithic designs. The reactions taking place in the solution make it possible to effectively bind even low-quality aggregates with a high content of clay and silt fractions.

• Polymer-cement complexes. Usually contain portland cement in a mixture with latex, polyurethane or epoxy resin. After polymerization, a film is formed on the surface of the material, closing the pores and preventing the penetration of moisture into the thickness of the material. Polymer cement is used in the finishing of floors, walls, garden paths, etc. The key drawback is the relatively high price.

Fillers

The parameters that characterize the concrete mix can also include the composition of the filler. Without this component at our disposal there will be only a cement-sand mortar, which is more expensive and inferior in strength to high-grade concrete.

We use as filler:

• Limestone. Cheap and easy to process, but has relatively low strength (up to 600). In principle, it can be used for the preparation of base compounds, however, low frost resistance limits the geography of use.

Note! Crushed limestone is usually taken as a filler for cement not stronger than M300.

• Gravel (strength 800-1000). Available, relatively cheap, has quite satisfactory performance. That gravel backfill is most often used in the preparation of concrete solution with his own hands.

• Granite. It is more expensive than gravel, but surpasses it in strength and heat resistance. Virtually not deformed even with sudden temperature changes, because the instruction recommends using it as a filler for structures with high strength requirements.

A separate group consists of fillers that provide increased thermal insulation:

• Claydite mix can be used for monolithic casting, and in the form of building blocks. Expanded clay has a sufficient strength and moisture resistance, retains heat well.
• Polystyrene granules are introduced into the solution to improve its thermal performance. The more polystyrene, the less will be the density of the material, which is why the instruction recommends using some varieties of such concrete only as a lining.

• Sawdust and wood chips are used to produce the so-called wood concrete. At observance of manufacturing techniques the wood filler almost completely loses a susceptibility to moisture. In this case, the thermal conductivity of the masonry decreases many times.

The most important characteristics of the composition

Class и марка бетона

To assess the mechanical qualities of the composition is used such a parameter as strength. It is determined primarily by the quality of the cement binder, as well as the properties of the filler. Strength demonstrates how effectively a material can resist deformation.

This characteristic can be indicated by two values ​​- brand and class:

• The brand of concrete is the value that shows the compressive load (in kgf / cm²), которую может выдержать образец при условии полного набора прочности. К примеру, бетон марки M100 разрушается только при воздействии в 100 кгс/см².
• However, today more often use such a value as a class - an indicator of guaranteed (that is, according to GOST implemented in 95% of the tests) strength in MPa. The values ​​of the brand and class are related.

In the table below we give examples of the use of the most popular mixtures:

  Note! The use of concrete mixture of a particular strength is governed by regulations, such as GOST 7473-2010, SNiP 3.03.01-87, etc.

For the preparation of concrete of different grades, certain proportions of sand, rubble and water are usually taken.

For example, when using cement M400:

• For concrete grade M 100 -1 part of cement, 4.5 parts of sand, 7 parts of rubble.
• For concrete M300 - 1 part of cement, 2 parts of sand, 3.5 parts of crushed stone.

For cement M500, the ratio will be different:

• M100 concrete - 1 part of cement, 5.7 parts of sand, 8 parts of crushed stone.
• Concrete M300 - 1 part of cement, 2.4 parts of sand, 4.2 parts of crushed stone.

The volume of water is usually selected depending on the requirements for the fluidity of the solution, as we discuss below.

Mobility and sediment cone

When working with a mortar it is very important that it is sufficiently plastic: then we will be able to fill the formwork or the form with less labor. The mobility of the concrete mix is ​​determined by the value of the sediment cone. Under this indicator usually understand the degree of plasticity of the fresh solution.

Draft cone is one of the key characteristics of the composition, and is determined separately for each batch. Usually in documents it is denoted by the index P with a digital value from 1 to 5.

The greater the value, the easier the solution will spread:

• Concrete with mobility П2 –П3 is used for ordinary construction work. On the one hand, they keep their shape, and on the other, they are easily distributed over the formwork.
• Formulations P4 are used when filling the formwork of a complex configuration, as well as when using the automatic feed. In this case, the density of laying increases, which saves on vibration processing.
• The solution with mobility P5 can be used to repair small cracks on the surface of the concrete screed.

Moisture resistance

Another parameter that characterizes a concrete mix is ​​its moisture resistance. It is denoted by the index W with a digital value equal to the pressure of water in (kgf / cm²), which is able to withstand the standard sample.

Concrete from W8 to W12 is considered waterproof.

They can be used:

• For the formation of the walls of underground premises in areas with a high level of groundwater.
• For arrangement of wells, septic tanks, tanks, etc.
• For pouring pool bowls.
• For decoration of hydraulic structures.

To increase the level of water resistance in the manufacture of the solution in the home, you can:

• Use higher grade cement.
• Perform a vibratory compaction of the material after pouring it into a form or formwork.
• After polymerization, treat the concrete with special impregnations covering the pores.

It should also be noted that measures to increase moisture resistance lead to higher construction costs, because they need to be carried out only if necessary.

Low Temperature Resistance

Finally, an important factor for evaluating a concrete mix is ​​its frost resistance. It is determined by the number of cycles of defrosting and subsequent thawing, which takes place until concrete loses 5% strength.

Note! Frost resistance is usually denoted by the F index, but you can also find just an indication of the number of cycles.

For ordinary construction, concrete F100 - F250 is used. Materials with greater resistance to low temperatures are in demand either in the northern regions, or where the structure is subject to constant freezing and thawing.

Conclusion

Concrete mix is ​​a rather complex and multicomponent composition. Properly selecting the ingredients, and accurately calculating their ratio, you can get the material with the characteristics we need.

Of course, it will be difficult to master all the wisdom without proper experience, so we recommend that beginners explore the video in this article before starting work.