Frost resistance of concrete: we estimate the behavior of

Many of those who build houses in the climatic zone with regular cooling, are interested in how to increase the frost resistance of concrete. This issue is very relevant, since a strong decrease in temperature, and even more so - its sharp drops lead to increased wear of structures and accelerate the process of their destruction.

Below we look at what happens to the concrete when it freezes, and how to prevent the negative consequences of this process.

Processes in the material

In order to understand what the resistance of the cement slurry to low temperatures depends on and how it can be improved, it is necessary to study the processes that take place in the material itself. And here it should be noted that with prolonged exposure to cold concrete rapidly loses its strength, especially in the surface part.

To date, there are two hypotheses explaining this phenomenon:

• According to one point of view, the cause of the destruction of the material from the inside are ice crystals. Moisture, which seeps into the pores of the material, under the influence of low temperatures freezes, increasing in volume by about 10-12%. Ice inclusions affect the pore walls, destroying them and reducing the density of the solution.
• According to other statements, the main harmful factor is not the ice itself, but the liquid that remains in the capillaries during freezing. Ice pushes against the remnants of water, which are practically not compressed, and they destroy channels with a diameter of 5 to 100 nanometers.

Note! Despite the fact that the second hypothesis enjoys great prestige among specialists, both of them do not contradict each other. In any case, the main reason is called an increase in the volume of the liquid during the transformation into ice.

• Important in this case is the fact that the expanding liquid and ice fill the pores of a fixed, and at the same time sufficiently small volume. It is for this reason that the frost resistance of aerated concrete will be higher than that of full-bodied cement compositions of a similar brand: the reserve volume of cavities allows you to compensate for the resulting load.

It should be noted that the destruction of structures due to the occurring internal stresses occurs unevenly:

• Initially, the shape of the protruding faces is broken, and the angles are chipped as well.
• Then there are microcracks on the flat areas of open surfaces, which soon merge into large damaged areas. This can lead to the peeling of concrete, and to the formation of large potholes.
• At the third stage, the liquid penetrates into the deep structures of the structure, and its accumulation in large cracks provokes strong destruction.

Separately, it is worth noting that the intensity of exposure is enhanced by the fact that different components of concrete have a different coefficient of temperature deformation. Differences in changes in the volume of the cement monolith, mineral aggregate and steel reinforcement lead to the fact that over time, areas with a lower density form at the places of their contact.

Material analysis

Cold Resistance Indicators

Cold resistance is usually understood as the ability of a material to withstand low temperatures without destruction and irreversible deformations. For numerical designation of this parameter, such a value is used as the frost resistance class of concrete (F) - the number of freezing / thawing cycles that concrete of this brand can withstand until its compressive strength decreases by 5%.

Thus, the frost resistance of concrete F200 means that before the beginning of a noticeable loss of strength, the material can freeze and thaw at least 200 times, which is quite a significant indicator. Such concretes can be successfully used in central Russia, which is characterized by frequent temperature changes in winter.

Note! The frost resistance of asphalt concrete and pavement on the cement binder is determined somewhat differently: the material should lose no more than 5% of the mass.

Since the ability to resist low temperatures largely depends on how strong the base itself is, there is a direct connection between the material class and such an indicator as the frost resistance concrete grade. The most common formulations and their characteristics are given in the table:

As you can see, the dependence is quite obvious. The higher the strength of the material (respectively, the more its price will be), the longer and more efficiently it will resist freezing.

 Characterization

Determination of the frost resistance of concrete according to GOST (GOST 10060.0) is carried out in this way:

• A sample of medium structure is taken from the batch of concrete (i.e., without adding or removing filler).
• Samples differ from this sample in the form - cubes with an edge of 100 or 200 mm.
• The sample is dried for 28 days for curing, after which it is saturated with water for 4 days.
• Then the concrete cubes are placed in a freezer, where they are subjected to alternate freezing (- 18⁰C) and thawing (+18⁰WITH).
• After the required number of cycles, a study of the mechanical properties of the material is performed using a press.
• Based on the change in the compressive strength index, depending on the duration of the temperature exposure, a conclusion is made about the degree of cold resistance of the material.

Note! Accelerated testing is also allowed with repeated or single freezing followed by a calculated determination of the parameters.

To facilitate the work, you can use a special device to determine the frost resistance of concrete. Such devices are completed with measuring chambers and reference samples, which allows to obtain information about the operational properties of the material with a minimum of labor costs.

Also, to determine the cold resistance can be applied ultrasonic method according to GOST 26134-84. It is less time consuming to implement, but involves the use of fairly complex equipment, because with your own hands you will not be able to cope here - you have to turn to specialists.

Increased resistance to low temperatures

If necessary, you can make frost-resistant concrete with your own hands.

For this purpose, apply the following techniques:

• First, it is necessary to thoroughly seal the solution when pouring. When compacted, the porosity of the material decreases, which means that the volume of fluid that falls into the concrete when it is saturated decreases.

Note! For this purpose, bayoning is not enough - it is desirable to use a high-capacity vibration compactor.

• Secondly, increasing the frost resistance of concrete is carried out by forming additional internal cavities. At the same time, a gas-forming or pore-forming component is added to the solution, which provides for the placement of microscopic bubbles in the material.

Tip! The optimum volume of air entrained in this case is from 4 to 6% of the total volume of concrete.

• Thirdly, you can use special additives that increase the resistance of the already polymerized concrete to low temperatures.. These additives include calcium salts, as well as urea (urea) - they reduce the ice content of the material by reducing the density of freezing water. The scaly ice formed during the freezing of the concentrated salt solution has a less destructive effect on the pore walls.
• Finally, in some cases it is sufficient to simply protect the surface from direct contact with moisture.. Here both polymeric impregnations-silings and front paints forming a dense film can be used.

Conclusion

The information given in the article about what happens in the solution when it freezes, how the frost resistance of concrete is determined according to GOST, and what can be done to increase it, is very important. Prolonged exposure to low temperature, as well as repeated freezing and thawing, can literally in a few years reduce the strength of a concrete structure from almost half.

If you want to know how to prevent it - carefully review the above recommendations, as well as watch the video in this article.