What is the frost resistance of building materials. Method for determining the frost resistance of building materials

Water resistance- the ability of the material to maintain its strength when saturated with water: It is estimated by the softening coefficient K SIZE, which is equal to the ratio of the ultimate strength of the material in compression in a state saturated with water R V MPa, to the ultimate strength of dry material R dry, MPa:

Quantitatively, water resistance is usually estimated by the mass of water (in%) absorbed by the sample (according to the so-called water absorption), or by relative. change to.-l. indicators (most often linear dimensions, electrical or mechanical lights) after a certain time in the water. As a rule, water resistance is characterized by a coefficient. softening Kp (the ratio of the tensile, compressive or bending strength of a material saturated with water to the corresponding indicator in its dry state). Materials are considered waterproof if Kp is greater than 0.8. These include, for example, many metals, sintered ceramics, glass.

Water permeability- the ability of the material to pass water under pressure. The characteristic of water permeability is the amount of water that has passed within 1 s through 1 m2 of the surface of the material at a given water pressure. To determine the water permeability, various devices are used to create the desired one-sided water pressure on the surface of the material. The method of determination depends on the purpose and type of material. Water permeability depends on the density and structure of the material. The more pores in the material and the larger these pores, the greater its water permeability.

Waterproof(English) water tightness) - a characteristic of a material, measured in SI in meters or pascals and showing at what values ​​​​of hydrostatic pressure this material loses its ability not to absorb or pass water through itself.

Determination of water resistance by "wet spot"; based on the measurement of the maximum pressure at which water does not seep through the sample;

Determination of water resistance by filtration coefficient; based on the determination of the filtration coefficient at constant pressure from the measured amount of filtrate and filtration time;

Accelerated method for determining the filtration coefficient (filter meter);

Accelerated method for determining the water resistance of concrete by its airflow.

1. Frost resistance of building materials. Definition methods. Designs with increased requirements for frost resistance.

Frost resistance- the property of a material saturated with water to withstand multiple alternate freezing and thawing without signs of destruction and a significant decrease in strength.

The destruction of the material occurs only after repeated alternate freezing and thawing.

Materials are tested for frost resistance by the method of alternate freezing and thawing of samples. The freezing temperature should be (-20 ± 2) °C. Defrosting should be carried out in water at a temperature of 15 - 20 °C. To determine frost resistance, ammonia refrigeration units are usually used.

Sample cubes or cylinders with dimensions of at least 5 cm (for homogeneous materials 3 and inhomogeneous 5 pieces) are marked and checked with a magnifying glass and a steel needle for cracks, damage, etc. on their surface. The samples are saturated with water to constant weight and weighed, then placed in a refrigerator and kept at (-20 2)°C for 4 hours. After this time, they are removed from the refrigerator and lowered to thaw in a bath of water at room temperature for 4 hours. After thawing, the samples are inspected for damage. If cracks or spalls appear, the test is terminated. If no defects are observed, the test is continued by placing the specimens back in the refrigerator for 4 hours.

Samples are subjected to sequential freezing, thawing and inspection as many times as provided for by the regulatory document for the material being tested.

After the end of the test, the samples are wiped with a damp cloth and weighed. Weight loss is calculated by the formula, %:

, (10)

where m is the mass of the sample dried before testing, g;

m 1 - the same, after the test, g.

The material is considered to have passed the test if, after the number of freezing and thawing cycles established by the normative document, it has no visible signs of destruction and loses no more than 5% of the mass. This method requires special equipment and a lot of time. If it is necessary to quickly evaluate the frost resistance of a material, an accelerated method is used using a solution of sodium sulfate.

accelerated method

The prepared samples are dried to constant weight, weighed, labeled and immersed in a saturated solution of sodium sulfate at room temperature for 20 hours. Then they are placed for 4 hours in an oven in which the temperature is maintained at 115 °C. After that, the samples are cooled to normal temperature, again immersed in a solution of sodium sulfate for 4 hours and again placed in an oven for 4 hours. Such alternating holding of samples in a solution of sodium sulfate and drying is repeated 3, 5, 10 and 15 times, which corresponds to 15, 25, 50 - 100 and 150 - 300 cycles of freezing and thawing. This method is based on the fact that a saturated solution of potassium sulphate, penetrating into the pores of the material during drying, becomes supersaturated and crystallizes, increasing in volume. In this case, stresses arise that are much higher than the stresses caused by freezing water. Therefore, 1 cycle of accelerated tests is equivalent to 5 - 20 cycles of conventional

OR ANOTHER VARIANT:

A material is considered frost-resistant if, after establishing the number of freezing and thawing cycles in a state saturated with water, its strength decreased by no more than 15-25%, and the weight loss as a result of chipping did not exceed 5%. Frost resistance is characterized by the number of cycles of alternate freezing at -15, -17°C and thawing at a temperature of 20°C. The number of cycles (brand) that the material must withstand depends on the conditions of its future service in the structure and on climatic conditions. According to the number of withstand cycles of alternate freezing and thawing (degree of frost resistance), materials are divided into grades Mrz 10, 15, 25, 35, 50, 100, 150, 200 and more. Under laboratory conditions, freezing is carried out in refrigerators. One or two cycles of freezing in a refrigerator give an effect close to 3-5 years of atmospheric action.

When choosing a brand of material for frost resistance, the type of building structure, its operating conditions and the climate in the construction area are taken into account. Climatic conditions are characterized by the average monthly temperature of the coldest month and the number of cycles of alternating cooling and warming according to long-term meteorological observations. Frost-stability of lightweight concrete, brick, ceramic stones for the outer walls of buildings is usually in the range of 15-35, concrete for the construction of bridges and roads - 50-200, for hydraulic structures - up to 500 cycles. The durability of the building depends on frost resistance. materials in structures exposed to atm. factors and water.

Designs with increased demanding frost resistance: hydraulic structures (piles, bridges). Open swimming pool, open water supply, sewerage,

Materials intended for the construction of load-bearing structures must have some margin of durability. In general, durability is a property of the design, not the material. But for materials, there are also criteria for evaluating applicability for the construction of critical buildings with a long estimated service life.

To determine the durability of metal structures, the concept of corrosion resistance is used. For metals, corrosion protection methods are provided: coatings, alloying, protective layers of concrete around reinforcing bars. For polymers, resistance to depolymerization and embrittlement is sometimes standardized. However, polymers are almost never used as elements of load-bearing structures, so their durability has little effect on safe operation. For stone structures, the frost resistance grade of the material of the outer layer of masonry is used as a criterion for durability.

The main mechanism of stone aging is the exhaustion of the frost resistance resource by the outer layers of masonry exposed to rain and frost. The frost resistance of the material of the outer 12 cm of a single-layer masonry or the frost resistance of the outer layer of a layered wall, as well as the frost resistance of the material of the upper part of the stone foundations - for the entire thickness of the masonry are normalized (the requirements are set out in SP 15.13330.2012 "Stone and reinforced masonry structures").

If the stone structure is designed correctly - taking into account the inadmissibility of moisture accumulation in the thickness of the wall during the heating period - then the frost resistance of layers that are not directly exposed to precipitation becomes an unimportant factor.

Frost resistance is standardized through the frost resistance grade. For walls of residential and office buildings with an estimated service life of 100 years or more, the frost resistance of the stone must be at least grade F35. For buildings that are being built on the coast of the Arctic Ocean - not lower than F50. For thin stone facings, the requirements are stricter - F75.

What is a frost resistance grade? This is the number of laboratory cycles of freezing a water-saturated material to a temperature of -18 ° C, followed by thawing without drying, during which there is no decrease in the performance properties of the material. Criteria for checking the quality of cyclically frozen materials are different. For concrete, the loss of strength is checked (should be no more than 15%). The appearance of the brick is checked for preservation.

To assess the applicability of materials and the durability of their structures, it should be understood that the numerical value of the grade is in no way related to the expected number of years of trouble-free operation. It’s just that in the first half of the 20th century, when methods were being developed for assessing the applicability of stones for laying critical structures, it was determined empirically that stones that show 35 cycles in the laboratory, in natural conditions of the European part of Russia, provide more than a hundred years of unchanged properties of external walls.

For example, let's take buildings familiar to us from the mass development of Leningrad: brick 12-storey point houses with walls of 2 slotted bricks, built in the 1970s, built of brick with frost resistance for the most part F25–35; aerated concrete panel "ships" series 600.11 - from aerated concrete brand F25. Both those and others are operated for half a century without signs of destruction. Their resource is far from exhausted.

Conclusion: almost all stone materials on the market today have sufficient frost resistance to build houses that will last more than one generation of residents. It is important to ensure their proper operation: drainage from window sills and parapets, exterior finish that does not lock moisture in the thickness of the wall, normal humidity conditions in rooms enclosed by stone walls or vapor barrier on their inner surface.

February 8, 2011

Frost resistance is understood as the ability of a material saturated with water to withstand repeated freezing and thawing without signs of destruction, i.e. without cracking, chipping, delamination and without significant loss of strength and weight.

The water in the pores of the material, turning into ice, increases in volume by about 10%. In this case, large internal stresses arise in the material, which gradually destroy it. Therefore, it is necessary to make the outer surfaces of walls and roofs from frost-resistant materials.

Frost-resistant materials are dense or with low water absorption (up to 0.5%).

The frost resistance of materials depends not only on water absorption, but also on the softening coefficient. Materials with a softening coefficient below 0.7 are practically non-frost resistant.

To determine frost resistance, the material is frozen to a temperature- 15 ° C, and then immersed in water at room temperature to thaw. The number of cycles of alternate freezing and thawing of the material, provided that its strength as a result of this will decrease by no more than 30%, and characterizes the frost resistance of the material.

"Materials Science for Plasterers,
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A.V. Aleksandrovsky

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Frost resistance and its determining factors.

Frost resistance- this is the ability of a material in a water-saturated state to withstand repeated alternating freezing and thawing. The frost resistance of a material depends on its structure, the degree of filling of the pores with water, the shape and size of the pores, the presence of trapped air in the pores after water saturation, the ionic composition, temperature, etc. The frost resistance of the material is determined by the number of cycles of freezing (-18 (-\+2)) and thawing in water (+20 (-\+2)), after which the samples reduce strength by no more than 5% or weight by no more than 5% /

Frost resistance - the property of a material saturated with water to withstand alternate freezing and thawing. The frost resistance of the material is quantified by the frost resistance brand. For the brand of material in terms of frost resistance, the largest number of cycles of alternate freezing and thawing is taken, which the material samples can withstand without reducing the compressive strength by more than 15%; after testing, the samples should not have visible damage - cracks, chipping (loss of mass is not more than 5%). The durability of building materials in structures exposed to atmospheric factors and water depends on frost resistance.

The frost resistance grade is set by the project, taking into account the type of structure, its operating conditions and climate. Climatic conditions are characterized by the average monthly temperature of the coldest month and the number of cycles of alternate freezing and thawing according to long-term meteorological observations.

Lightweight concrete, bricks, ceramic stones for exterior walls usually have a frost resistance of 15, 25, 35. However, concrete used in the construction of bridges and roads must have a grade of 50, 100 and 200, and hydraulic concrete - up to 500.

Exposure of concrete to alternate freezing and thawing is similar to repeated exposure to repeated tensile loading, causing material fatigue.

The frost resistance test of the material in the laboratory is carried out on samples of the established shape and size (concrete cubes, bricks, etc.). before testing, the samples are saturated with water. After that, they are frozen in a refrigerator from -15 to -20C, so that the water freezes in fine pores. The samples removed from the refrigerator are thawed in water at a temperature of 15-20C, which ensures the water-saturated state of the samples.

To assess the frost resistance of the material, physical control methods are used, and above all, the pulsed ultrasonic method. It can be used to track the change in the strength or modulus of elasticity of concrete during cyclic freezing and determine the brand of concrete by frost resistance in cycles of freezing and thawing, the number of which corresponds to the allowable decrease in strength or modulus of elasticity.

The strength and resistance to deformation depends on the water saturation of concrete. Also, these parameters are affected by the effects of air temperature and its differences. If the concrete has an excessive water content, then at low temperatures it crystallizes. The ice has nowhere to go, resulting in excessive internal pressure.
It leads to the ultimate tensile stress in the pore walls. Such changes contribute to a decrease in the strength of concrete. After thawing of the formed ice in the pores, this will lead to a decrease in the strength of concrete only in cases of excessive water content.
A decrease in the strength of concrete can also occur when water is unevenly distributed in the pores during manufacture or when the water vapor formed in it freezes. With an increase in water saturation of concrete, the strength of cooled specimens up to 400 and up to 600 first increases to a certain value, and then decreases significantly. The maximum value of concrete strength is a function of the degree of temperature decrease and the amount of water contained in the pores. Note that after thawing, the strength of concrete decreases. It is also worth emphasizing that prolonged exposure to low temperatures (even with their fluctuations) leads to a gradual loss of concrete strength. It is known that if concrete has less moisture and greater strength before freezing, then with prolonged exposure to low temperatures in winter, the resistance of concrete is much higher. The possibility of water saturation of concrete depends on its structure, more precisely, on the system of capillaries formed in the space of the cement stone. It is possible to improve the structure of concrete by reducing the porosity of concrete and forming a closed system of pores. Experiments have shown that microcracks that have arisen under preload, during the thawing and freezing cycle, significantly accelerate the destruction of concrete.
High-strength concrete is made according to a certain technology, and has a more even structure, due to which it has increased frost resistance. Reducing the water permeability of such concrete is achieved by reducing the porosity. Organic structure-forming additives in the form of resin are added to the concrete mixture, which are neutralized by the air-entraining SNV. Thanks to the use of GKZH-94, air is drawn into the concrete mixture, and closed pores of very small diameter are formed.
The artificial formation of such pores significantly increases the strength of concrete during repeated defrosting and freezing. The use of additives increases water permeability and frost resistance, but reduces the strength of concrete. Concrete with the addition of START and GKZH-94 is used in harsh climatic conditions. Such concrete has increased strength and frost resistance.