The value of concrete in the construction industry is difficult to overestimate. The basis of the foundations among the indicators of concrete is its strength. Namely – compressive strength. As a consequence, concrete structures are calculated so that the concrete can take compressive loads. The strength of concrete is determined by the brand of concrete and the class of concrete. The table of the relationship of grades and classes of concrete allows you to find the right solution in the construction work.
The use of concrete in construction
The range of modern use of various concrete mixes in construction is constantly increasing. Concrete of high strength grades is considered promising, as well as special concrete with the following technical parameters: durability, low draft, frost resistance, heat resistance, low mobility, resistance to cracking.
The main use of concrete is prefabricated or monolithic concrete and reinforced concrete structures and structures. In each type of construction (foundations, columns, walls, etc.) it is necessary to use concrete of the appropriate class and brand. Their characteristics are provided by the project of the construction site.
Grades of concrete are basic indicators of its compressive strength. The higher their degree, the higher the requirements for concrete strength.
Table of correspondence of the class of concrete and the scope of its use:
|Scope of use||Massive foundations in dry soils||Massive foundations in wet soils||Massive foundations in water-saturated soils||Floor preparation|
|Scope of use||Exterior and basement stairs||Septic tanks, cesspools||Slabs, beams with rare reinforcement||Overlapping plates, beams with frequent reinforcement|
The strength of concrete depends on the components involved in the preparation of the mixture:
- cement. The amount of cement affects the strength of concrete up to a certain point. After that, the strength indices become insignificant, and other characteristics of concrete (creep, shrinkage), on the contrary, become worse. In this regard, the quantitative composition of cement in the cube of concrete should not exceed 600 kg. The higher the grade of cement contained in concrete, the stronger it is;
Helpful advice! When preparing the concrete mix yourself, you should use cement of a grade twice the grade of concrete.
- water cement module. Hardening of concrete with cement occurs with the participation of water from 15 to 25%. The workability of the mixture is possible, as a rule, at 40 – 70%. Excess water contributes to the formation of pores and, as a result, compressive strength decreases. Concrete with a low water-cement ratio gain strength much faster;
- placeholders Small fractions of aggregates, the presence in their composition of dust and clay, organic inclusions negatively affect the strength of concrete. The adhesion of coarse fractions of aggregates with cement has a positive effect on strength;
- mixing Compressive strength also depends on how carefully the components of the concrete mix are mixed and on what equipment they are mixed. Not the last role is played by compaction – by increasing the average density value by 1%, the strength index will increase to 5% (per 1 cubic meter of the mixture);
- age and temperature conditions of hardening. The increase in compressive strength over time is influenced by the mineral structure of the cement – different cements contribute to different increases in strength. The optimum temperature for curing the concrete mixture is 15–20 ° C, the degree of humidity is from 90 to 100%. To ensure sufficient moisture for curing, it is recommended to cover the concrete with a film. At temperatures below zero hardening almost does not occur. To achieve a decrease in freezing temperature for water, you can use special additives.
According to the use in the mixture of the binder component, concrete is subdivided into cement, lime, gypsum, asphalt, silicate, clay, etc.
The use of certain fillers divides the concrete into types:
- heavy (gravel, crushed stone from dense rocks, used in concrete and reinforced concrete structures);
- especially heavy (iron ore, barite, used for the military sphere, nuclear power plants, landfills);
- lungs (natural slag pumice, found application in coverings and fences);
- especially lightweight (foam concrete or aerated concrete).
The properties of concrete are divided into waterproof, frost-resistant and fire-resistant, the degree of density of the concrete mix divides them into hard and ductile.
In order to choose a concrete mix correctly, it is necessary to know the conformity of the grades and classes of concrete to the specific type of work performed. To use concrete more durable than the design requires, it will be irrational. Basically, concrete is used for construction, not exceeding the brand 500 in compressive strength.
Consider the table “Brand of concrete and class of concrete depending on the use of the finished mixture”:
|Brand and grade of concrete||Main application|
|M100, (B7.5)||Work of a preparatory nature before pouring basement slabs, curbs, thermal insulation of structures, as the underlying layer before reinforcement|
|M150, (B12,5)||Paths on summer cottages, small sites, pouring monolithic slabs of the foundation|
|M200, (B15)||Foundation under the foundation of a private house, blind area, strip foundation, cushion under the pavement, screed for the floor|
|M250, (B20)||Foundation of a private house, stairs, small floors, fences, fences, outbuildings|
|M300||Capital construction of a private house: walls, ceilings, foundations|
|M350||Foundations of high-rise buildings, beams, floors, columns|
|M400||Hydrotechnical structures, bridges, storage, military facilities|
From the table we find out what brand of concrete for the foundation can be used to build a house in the private sector.
If you plan a small outbuilding, you can use a mixture with a low rate (M200) grade concrete. For the foundation of a private house that has more than one floor, a higher brand is already used (M250, M300).
Helpful advice! When constructing the strip foundation, besides the brand of concrete, it is necessary to take into account the characteristics of the soil in which it will be located.
The brand of concrete is determined on the basis of the characteristics of the binder component, the water-cement ratio and the density of the filler. Concrete is classified into plain and light.
Table “The proportions of concrete per 1m?”. Quality concrete mixes.
The composition of the concrete solution. Strength Indicators. Conformity marks for concrete use. The calculation of the ingredients of the mixture. Preparation of the solution.
Correspondence table of grades and grades of concrete:Consider the relationship of brand and grade of concrete. Conformity table of grade and grade of concrete will help to transfer the mark to the grade and vice versa.
The compressive strength, measured in MPa, is assigned to the class of concrete. Thus, the definition of B20 shows: the letter B is the designation of the class, the figure 20 is the pressure of 20 MPa maintained by the tested cube.
Compliance with the strength of concrete in compression classes in MPa to concrete brands is appointed by the conditions of technical documentation.
Below are two tables “Class of concrete for compressive strength in MPa.”
Table 1 – from 4.5 (MPa) to 32.7 (MPa):
Table 2 – from 39.2 (MPa) to 78.6 (MPa):
Under the frost resistance of concrete, they understand the ability of a material to undergo repeated freezing and thawing, without breaking down and without losing its compressive strength.
The ability of water at low temperatures to expand, filling the voids of the material, leads to its destruction. The higher the porosity in concrete, the more water will fill the voids. It means that the frost resistance index directly depends on the density and structure of the material.
The frost resistance of concrete is an indicator that is especially taken into account in climatic zones, where concrete structures are repeatedly frozen and thawed. Then there is the risk of losing their bearing qualities and damage.
The reason for the destruction of concrete products – the brand does not meet the standards for frost resistance. If the grade of concrete for frost resistance is chosen correctly, the concrete structure will last for more than a century. For concrete with high frost resistance, the main indicators, in addition to the presence of cement, are the cement brand, water-cement factor, conditions for the mixture to harden and other criteria.
Helpful advice! Obtaining a higher grade of frost resistance concrete contributes to the addition to the mixture of antifreeze additives (PMD). Their function consists in reducing the required amount of water and compaction of concrete.
Frost resistance is designated by the letter F and a digital designation (from 50 to 100), where the numbers correspond to the number of freezing and thawing periods of concrete structures at which their properties do not deteriorate.
A high grade for strength corresponds to a higher grade of frost resistant concrete.
Table of the ratio of grade, grade and cold resistance of concrete:
Using tables of grades and grades of concrete, as well as their characteristics, it is possible to obtain high-quality, strong and durable concrete structures.