What is frost resistance and what are the methods for determining it? What are the frost resistance requirements for ceramic, wall and cladding materials?
Frost resistance is the ability 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. The frost resistance grade of a material is taken to be the greatest number of cycles of alternating freezing and thawing that material samples can withstand without reducing the compressive strength by more than 15%; After testing, the samples should not have visible damage - cracks, chipping (mass loss no 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 established 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 alternating freezing and thawing according to long-term meteorological observations.
Lightweight concrete, brick, ceramic stones for external walls usually have a frost resistance of 15, 25, 35. However, concrete used in the construction of bridges and roads should have a grade of 50, 100 and 200, and hydraulic concrete - up to 500. Impact of alternating freezing on concrete and thawing is similar to repeated exposure to repeated tensile loading, causing material fatigue. Testing the frost resistance of a material in the laboratory is carried out on samples of established shapes and sizes (concrete cubes, bricks, etc.) before testing, the samples are saturated with water. After this, they are frozen in a refrigerator from -15 to -20C so that the water freezes in the thin pores. Samples removed from the refrigeration chamber are thawed in water at a temperature of 15-20C, which ensures the water-saturated state of the samples. Basic - first (for all types of concrete, except concrete for road and airfield pavements) and second (for concrete for road and airfield pavements); accelerated during repeated freezing and thawing - the second and third; accelerated during single freezing - fourth (dilatometric) and fifth (structural-mechanical). To assess the frost resistance of a material, physical control methods and, above all, the pulsed ultrasonic method are used. With its help, you can trace the change in the strength or elastic modulus of concrete during cyclic freezing and determine the grade of concrete based on frost resistance in freezing and thawing cycles, the number of which corresponds to the permissible decrease in strength or elastic modulus.
Detailed studies on the influence of pore granulometry on the frost resistance of ceramic materials revealed the following points:
all pores in a ceramic material (from the point of view of frost resistance) can be divided into three categories: dangerous, safe and reserve;
dangerous pores fill with water when saturated in the cold. It is retained in them when the material is removed from water and freezes at a temperature of -15 to -20° C. The diameter of these pores is from 200 to 1 micron for plastic-pressed clay bricks, from 200 to 0.1 microns for semi-dry pressed clay bricks ;
When saturated in the cold, safe pores are not filled with water, or the water that fills them does not freeze at the specified temperatures. These are usually small pores. The water filling them becomes essentially wall-adsorbed moisture, having the properties of an almost solid body and the freezing point is significantly lower (--20 ° C);
When saturated in the cold, the reserve pores are completely filled with water, but when the sample is removed from the saturating vessel, water partially flows out due to low capillary forces. These are large pores with a diameter of more than 200 microns.
According to these studies, ceramic material will be frost-resistant if the volume of reserve pores in it is sufficient to compensate for the increase in the volume of freezing water in dangerous pores.
In terms of frost resistance, an ordinary clay brick saturated with water must withstand, without any external signs of destruction (delamination of edges, chipping of edges and corners, cracking), at least 15 repeated cycles of alternating freezing at a temperature of -75 ° C and below, followed by thawing in water at a temperature of 15 ±5°С.
Lightweight brick must withstand, without any temporary signs of destruction, at least 10 repeated cycles of alternating freezing at a temperature of -15 ° C and below, followed by thawing at a temperature of 15 ± 5 ° C.
The facing brick must withstand, without any signs of visible damage, at least 25 repeated cycles of alternating freezing followed by thawing in water.
Frost resistance is the property of a material in a water-saturated state to withstand repeated freezing and thawing without visible signs of destruction and without reducing strength and weight.
This property is most important for structures subject to variable moisture, which primarily include foundations and the roof of a construction site.
Frost resistance is determined on samples, the number of which must be at least six in the shape of a cube with a side length of 70,100,150 mm. The specified quantity is divided into 2 series of 3 samples each, one control series is not subjected to freezing and thawing, the second is. And after a certain number of cycles (until the material loses 25% of the initial strength or 5% of the mass), both series are tested for compression, and the absence of a decrease or increase within the limits of compressive strength characterizes the material as frost-resistant or not frost-resistant.
Frost resistance grades: the most commonly used designation is “F” with numbers from 50 to 1000 (example - F200), indicating the number of freezing-thawing cycles.
For example, concrete grades for frost resistance: F50, F75, F100, F150, F200, F300, F400, F500.
End of work -
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The ability of a water-saturated material to withstand repeated alternating freezing and thawing without signs of destruction or a significant decrease in density. Destruction occurs due to the fact that the water in the pores increases in volume by about 9% when frozen. The greatest expansion of water during the transition to ice is observed at a temperature of -4°C; a further decrease in temperature does not cause an increase in the volume of ice. When water freezes, the pore walls experience significant pressure and can collapse. When all pores are completely filled with water, destruction of the material can occur even with a single freezing. When the porous material is saturated with water, the macrocapillaries are mainly filled, the microcapillaries are partially filled with water and serve as reserve pores into which the water is squeezed out during the freezing process. Consequently, the frost resistance of building materials is determined by the size and nature of porosity and the conditions of their operation.
The lower the water absorption and the greater the tensile strength of the material, the higher it is. Dense materials are frost-resistant. Among porous materials, only those materials that mainly contain closed pores or water are frost-resistant. Occupies less than 90% of pores. A material is considered frost-resistant if, after establishing the number of freezing and thawing cycles in a water-saturated state, its strength has decreased by no more than 15-25%, and weight loss due to chipping does not exceed 5%. Frost resistance is characterized by the number of cycles of alternating freezing at -15, -17°C and thawing at a temperature of 20°C. The number of cycles (grade) that the material must withstand depends on the conditions of its future service in the structure and on climatic conditions. Based on the number of cycles of alternating freezing and thawing that can be withstood (degree of frost resistance), materials are divided into grades Mrz 10, 15, 25, 35, 50, 100, 150, 200 and more. In laboratory conditions, freezing is carried out in refrigeration chambers. One or two freezing cycles in the refrigeration chamber give an effect close to 3-5 years of atmospheric action.
THERMAL CONDUCTIVITY
The property of a material to transfer heat through thickness from one surface to another. Thermal conductivity is characterized by the amount of heat (J) passing through a material 1 m thick with an area of 1 m2 for 1 second when the temperature difference on opposite surfaces of the material is 1 ° C. The thermal conductivity of a material is directly dependent on its chemical composition, porosity, humidity and temperature at which heat transfer occurs. Fibrous materials have different thermal conductivities depending on the direction of heat relative to the fibers (in wood, for example, the thermal conductivity along the fibers is twice as high as across the fibers). Fine-porous materials and materials with closed pores have greater thermal conductivity than large-porous materials and materials with interconnected pores. This is due to the fact that in large and interconnected pores, heat transfer by convection is enhanced, which increases the total thermal conductivity.
With increasing humidity of the material, thermal conductivity increases, since water has a thermal conductivity 25 times greater than air. The thermal conductivity of the raw material increases even more with a decrease in its temperature, since the thermal conductivity of ice is several times greater than the thermal conductivity of water. The thermal conductivity of the material is of great importance when constructing building envelopes - walls, ceilings, floors, roofs. Light and porous materials have little thermal conductivity. The higher the volumetric weight of the material, the higher its thermal conductivity. For example, the thermal conductivity coefficient of heavy concrete with a volumetric weight of 2400 kg/m3 is 1.25 kcal/m-h-deg, and that of foam concrete with a volumetric weight of 300 kg/m3 is only 0.11 kcal/m-h-deg.
HEAT CAPACITY
The property of a material to accumulate heat when heated. When subsequently cooled, materials with high heat capacity release more heat. Therefore, when using materials with increased heat capacity for walls, floors, ceilings and other parts of the room, the temperature in the rooms can remain stable for a long time.
Heat capacity coefficient - the amount of heat required to heat 1 kg of material on a heating system. Building materials have a heat capacity coefficient less than that of water, which has the highest heat capacity (4.2 kJ/(kg°C)). As materials are moistened, their heat capacity increases, but at the same time, thermal conductivity also increases.
The heat capacity of a material is important in cases where it is necessary to take into account heat accumulation, for example, when calculating the thermal resistance of walls and ceilings of heated buildings in order to maintain the temperature in the room without sudden fluctuations when the thermal regime changes, when calculating the heating of material for winter work, when calculating the design of furnaces. In some cases, it is necessary to calculate the dimensions of the furnace using volumetric specific heat capacity - the amount of heat required to heat 1 m3 of material on the HS.
WATER ABSORPTION
The property of a material to absorb and retain water in direct contact with it. It is characterized by the amount of water absorbed by a dry material completely immersed in water, and is expressed as a percentage of the mass (water absorption by mass).
The amount of water absorbed by a sample divided by its volume is water absorption by volume. Water absorption by volume reflects the degree to which the pores of the material are filled with water. Since water does not penetrate into all closed pores and is not retained in open voids, volumetric water absorption is always less than true porosity. Volumetric water absorption is always less than 100%, and water absorption by mass can be more than 100%.
Water absorption of building materials varies mainly depending on the volume of pores, their type and size.
As a result of saturation with water, the properties of materials change significantly: density and water conductivity increase, and the volume of some materials (for example, wood, clay) increases. Due to the disruption of bonds between material particles and penetrating water particles, the strength of building materials decreases.
SOFTENING COEFFICIENT
The ratio of the compressive strength of a material saturated with water to the compressive strength of a material in a dry state. The softening coefficient characterizes the water resistance of the material. For easily soaked materials, such as clay, the softening coefficient is 0. For materials that fully retain their strength when exposed to water (metal, glass, etc.), the softening coefficient is 1. Materials with a softening coefficient of more than 0.8 are classified as waterproof. In places subject to systematic moisture, the use of building materials with a softening coefficient of less than 0.8 is not permitted.
MOISTURE RELEASE
A property that characterizes the drying rate of a material in the presence of environmental conditions (low humidity, heating, air movement). Moisture loss is characterized by the amount of water that a material loses per day at a relative air humidity of 60% and a temperature of 20°C. Under natural conditions, due to moisture transfer, some time after the completion of construction work, a balance is established between the moisture content of building structures and the environment. This state of equilibrium is called air-dry or air-wet equilibrium.
WATER PERMEABILITY
The ability of a material to pass water under pressure. The characteristic of water permeability is the amount of water passing through 1 m2 of the surface of the material within 1 second at a pressure of 1 MPa. Dense materials (steel, glass, most plastics) are waterproof. The method for determining water permeability depends on the type of building material. Water permeability is directly dependent on the density and structure of the material - the more pores in the material and the larger they are, the greater the water permeability. When choosing roofing and hydraulic engineering materials, it is most often not water permeability that is assessed, but water resistance, characterized by a period of time after which signs of water infiltration appear under a certain pressure or a limiting value of water pressure at which water does not pass through the sample.
AIR RESISTANCE
The ability of a material to withstand repeated systematic wetting and drying for a long time without significant deformation and loss of mechanical strength. A change in humidity causes many materials to change their volume - they swell when wet, shrink when dry, crack, etc. Different materials behave differently in relation to the action of variable humidity. Concrete, for example, with variable humidity is prone to destruction, since the cement stone shrinks when it dries, and the filler practically does not react - as a result, tensile stress arises, the cement stone is torn away from the filler. To increase the air resistance of building materials, hydrophobic additives are used.
HUMIDITY DEFORMATIONS
Changes in the size and volume of a material when its humidity changes. A decrease in the size and volume of a material during drying is called shrinkage or shrinkage, an increase is called swelling.
Shrinkage occurs and increases as a result of a decrease in the layers of water surrounding the particles of the material and the action of internal capillary forces tending to bring the particles of the material closer together. Swelling is due to the fact that polar water molecules, penetrating between particles or fibers, thicken their hydration shells. Materials with a highly porous and fibrous structure that can absorb a lot of water are characterized by high shrinkage (for example, cellular concrete 1-3 mm/m; heavy concrete 0.3-0.7 mm/m; granite 0.02-0.06 mm/m ; ceramic brick 0.03-0.1 mm/m.
Its strength and resistance to deformation depend on the water saturation of concrete. These parameters are also affected by the effects of air temperature and its changes. If there is excessive water content in concrete, it will crystallize at low temperatures. The ice has nowhere to go, resulting in excess internal pressure.
It leads to maximum 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 excess water content.
A decrease in the strength of concrete can also occur when water is unevenly distributed in the pores during production or when water vapor formed in it freezes. With increasing water saturation of concrete, the strength of cooled samples up to 400 and up to 600 first increases to a certain value, and then decreases significantly. The maximum strength of concrete 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 capillary system formed in the space of the cement stone. The structure of concrete can be improved by reducing the porosity of concrete and forming a closed pore system. Experiments have shown that microcracks that arose during preload, during the cycle of thawing and freezing, significantly accelerate the destruction of concrete.
High-strength concrete is produced using a certain technology and has a more even structure, due to which it has increased frost resistance. A decrease in the water permeability of such concrete is achieved by reducing porosity. Organic structure-forming additives in the form of resin are added to the concrete mixture, which are neutralized by air-entraining SNF. 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 thawing and freezing. The use of additives increases water permeability and frost resistance, but reduces the strength of concrete. Concretes with the addition of SNV and GKZh-94 are used in harsh climatic conditions. Such concrete has increased strength and frost resistance.
The method for determining the frost resistance of building materials relates to the field of testing building products, in particular bricks, silicate and ceramic stones. The method for determining the frost resistance of building materials includes saturation of samples in water or sodium chloride solution, surface cyclic freezing and thawing of samples and visual assessment of frost resistance, with freezing carried out for 5-10 minutes, and thawing for 3-5 minutes 0.1-0.2 parts of the test surface, the freezing and thawing regimes change at a speed of 30-40 degrees/min, and the samples are immersed in water and sodium chloride solution to 90-95% of their volume. The invention reduces test duration, reduces labor intensity, and increases the reliability of test results.
The invention relates to the field of testing building materials, in particular to determining their frost resistance. There is a known method for determining the frost resistance of building materials, including saturating samples in water or a sodium chloride solution, freezing the samples in air at a temperature of minus 20 o C for 2 - 4 hours and thawing the samples in an aqueous environment or sodium chloride solution at a temperature of 20 o C in for 1.5 - 2 hours, recording the number of freezing-thawing cycles until a 25% loss of strength of the samples or a 5% loss of mass is achieved or until external signs of destruction appear, by which the frost resistance of building materials is judged (1). The disadvantage of this method is the significant complexity and duration of the test and the need to use complex and bulky equipment. There is a known method for accelerated determination of the frost resistance of building materials by saturating samples with a steel rod embedded in them with water, freezing and thawing, and recording a sharp increase in the initial electrical potential of the steel rod, by which the frost resistance of the material is judged (2). There is a known method for determining the frost resistance of building material samples based on the ratio of structural and strength characteristics, characterized in that capillary and contraction porosity is taken as a structural characteristic, and the work of destruction of the samples is taken as a strength characteristic (3). The disadvantages of the known methods (2, 3) are the indirectness of the methods for determining frost resistance and, as a result, the low accuracy of the results. In addition, the disadvantages of methods (1, 2, 3) are that the determination of frost resistance under conditions of direct volumetric freezing does not correspond to the actual operating conditions of the building material, which is alternately exposed to negative and positive temperatures on only one side. Therefore, the test results of building materials lead to a wide spread in the frost resistance values of the material. There is a known method for determining the frost resistance of building materials by one-sided freezing in a freezer in a special container that provides heat removal from one side of the test samples, thawing in a bath of water, determining the structural and strength characteristics of the samples, followed by calculation of frost resistance using formula (4). There is a known method for determining the frost resistance of building materials, which includes saturating the sample with water by cyclically introducing under pressure portions of water, calculated according to the empirical formula (5). The disadvantages of the known methods (4, 5) are the insufficiently high reliability of the test results due to the use of calculation formulas using empirical coefficients. The closest to the proposed method is a method for determining frost resistance, including one-sided freezing of brick or stone masonry at an air temperature of - 15 - 20 o C for 8 hours, thawing of the frozen side of the masonry by sprinkling at a water temperature of 15 - 20 o C for 8 hours, registration the number of freezing and thawing cycles until visible signs of destruction appear on the surface of the masonry (peeling, delamination, cracking, spalling), or by the loss of mass and strength, by which the frost resistance of building material samples is judged (6). The disadvantages of the known method are its high labor intensity, cost and long test duration, which does not allow for operational control of the products, and significant energy costs for creating freezing conditions. The technical result of the proposed invention is to reduce test duration, reduce labor intensity, and increase the reliability of test results. The technical result is achieved by the fact that in the known technical solution, including preliminary saturation of samples in water or sodium chloride solution, one-sided cyclic freezing and thawing of samples, and visual assessment of frost resistance, directional, spot freezing is carried out for 5 - 10 minutes and thawing for 3 - 5 min 10 - 20% of the open surface of the test samples, and the change of freezing and thawing modes is carried out at a speed of 30 - 40 o per minute, and the samples are immersed in water or sodium chloride solution to 90 - 95% of their volume. The method was carried out as follows. Samples intended for testing for frost resistance were pre-saturated in water or a solution of sodium chloride. Then three samples were installed in a T-shape in a container with the front surface facing up. After this, water or sodium chloride solution was poured into the container until the samples were immersed by 90 - 95% of their volume. Then, the joint of three samples was treated with a directed flow of cold air at a temperature of minus 15 - 20 o C, i.e. 10 - 20% of their surface for 5 - 10 minutes. Then, at a speed of 30 - 40 o C per minute, they switched to the heating mode and treated the same joint with a warm air stream at a temperature of 15 - 20 o C for 3 - 5 minutes and recorded the number of freezing and thawing cycles until visible signs of destruction (delamination, cracking, chipping, peeling), which were used to judge the frost resistance of building materials. The use in the proposed technical solution of spot, directional freezing for 5 - 10 minutes and thawing for 3 - 5 minutes of 10 -20% of the open surface of the tested samples allows us to create in a short time conditions for processes close to actual ones during operation. Due to a sharp (30 - 40 o C per minute) change in freezing and thawing regimes, a stressed state is created in the pores of the material, causing destructive processes, namely loosening of the structure, intensification of microcrack formation and, accordingly, an increase in permeability. Immersing samples in water or sodium chloride solution by 90 - 95% of the sample volume ensures conditions for constant migration of moisture to the open front surface of the test sample through capillaries and microcracks. All these techniques make it possible to quickly determine frost resistance, which is close to the actual one. Low energy costs, low labor intensity, accessibility and reliability of results allow for ongoing monitoring of manufactured products and timely detection of process violations. Sources of information 1. GOST 10090.1-95, GOST 10090.2-95 "Concrete. Methods for determining frost resistance. 2. A.S. USSR N 482676 M. class C 01 N 33/38, 1975 3. A.S. USSR N 435621 M. class C 01 N 25/02, 1975 4. A.S. USSR N 828849 M. class C 01 N 33/38, 1982 5. A.S. USSR N 1255921 M. class C 01 N 33/38, 1986 6. GOST 7025-91 Ceramic and silicate bricks and stones. Methods for determining water absorption, density and frost resistance control.
Formula of invention
A method for determining the frost resistance of building materials, including saturation of samples in water or a solution of sodium chloride, cyclic freezing and thawing of the open surface of the samples and visual assessment of frost resistance, characterized in that 10 - 20% of the surface of the test sample is frozen and thawed for 5 - 10 minutes, respectively. 3 - 5 minutes, and the change of freezing and thawing modes is carried out at a speed of 30 - 40 degrees. /min, while the samples are immersed in water or sodium chloride solution by 90 - 95% of their volume.