What is calculated concrete resistance and how to calculate

As you know, concrete is a very heterogeneous material, as a result of its strength indicators may vary significantly even within several prototypes made from the same mixture. But, in this case, how to calculate the strength of a concrete structure, for example, for compression? To do this, use the calculated values, in this case it will be the calculated resistance of concrete to compression.

Next, we will look at what the calculated characteristics are and how to find them out, as well as look at some of the other parameters of this material.

How to get the calculated resistance

To ensure sufficient reliability of concrete structures, when performing calculations, use such strength values ​​of the concrete material, which in most cases are lower than the actual indicators in the structures. These values ​​are called calculated, respectively, they directly depend on the actual or otherwise - normative values.

Regulatory specifications

More recently (until 1984) the only characteristic of the strength of concrete was its brand (M). This parameter indicates the average temporal stability of the material in compression. But, with the advent of SNiP 2.03.01, compression strength classes were also introduced.

In essence, the class is the normative resistance to axial compression of reference cubes measuring 15x15x15 cm with a security of 0.95 or guaranteed confidence level of 95% and a risk of 5 percent. It must be said that in this case it is risky to take the average fortress, since there is a 50 percent probability that it will be lower than the average in a dangerous section of the structure.

At the same time, it is too costly to take the minimum indicator as a basis, since this will lead to a significant unnecessary increase in the cross section of the structure.

Thus, the main parameter of strength in our case is the class. But, in addition to axial compression, axial tension is also an important characteristic. Resistance to axial tension (if this parameter is not controlled) is determined depending on the class B:

Tip! The higher the class of material, the higher its price. Therefore, it is impractical to build structures with an unreasonable margin of safety.

Estimated characteristics

As mentioned above, to ensure the reliability of structures, perform the calculation with a certain margin of safety. To obtain this margin, the specific resistance of concrete is divided by a certain coefficient, and thus this indicator is reduced in calculations.

The calculated resistance of concrete to tension or compression can be calculated by the following formula - R = Rn / g, where g - is the coefficient of reliability for durability. Usually this value is 1.3. However, the less uniform the array, the greater this coefficient.

However, it is not necessary to perform the calculation, as the table of calculated concrete resistance to compression and tension allows obtaining the necessary values:

Tip! As a result of the high strength of concrete products, their machining causes certain difficulties. To simplify this procedure, use a power tool with diamond nozzles. In particular, builders often perform cutting of reinforced concrete with diamond circles, or diamond drilling of holes in concrete, as well as diamond grinding of concrete surfaces.

Other features

In addition to the above parameters, when performing some calculations, other characteristics of concrete are required.

Next we look at some of them:

• The specific electrical resistance of concrete (p) - is the resistance to the passage of electric current through a concrete cube measuring 1x1x1 cm. This parameter of the liquid phase is influenced by the alkali content in the cement and the ratio of the liquid. Depending on this, the value can vary from 4 to 20 ohms. The determination of this characteristic may be required when organizing the heating of the solution by electrodes with your own hands. The higher this value is, the correspondingly the mass heats up stronger.
• Permeability - this parameter indicates the highest water pressure that a material can withstand, i.e. under which water cannot seep through the concrete sample. In terms of water resistance, there are W2-W20 marks, while the brand numbers indicate pressure in kgf / cm2, at which the structure is able to withstand water.
• Airtightness - this characteristic depends on the density of the structure. The resistance of concrete to the penetration of air in accordance with GOST 12730.5-84 may be 3.1-130.2 s / cm3, depending on its brand for water permeability.
• Frost - the ability to tolerate multiple cycles of freezing and thawing without losing basic properties. There are grades ranging from F50 to F1000, where the numbers indicate the number of freeze / thaw cycles that the material can withstand. In practice, the average frost resistance in conventional construction is within F100-F200.
• Heat conductivity - is one of the most important parameters of enclosing structures, which depends on the density of the structure. The greater its porosity, the lower the thermal conductivity, since the air filling the pores is an excellent heat insulator. At a density of 1200 kg / m3 density, the thermal conductivity of the material is 0.52 W / (m - ° C). Therefore, light-weight gas or foam concrete blocks, which have a porous structure, are used as thermal insulation materials.

Conclusion

Design resistance is an extremely important parameter when designing responsible supporting structures. Instructions for calculating these values ​​are quite simple and come down to an underestimation of the normative characteristics by dividing them by the corresponding coefficients.

From the video in this article, you can get more information on this topic.