Frost resistance of building materials, in particular natural stone, usually refers to its ability to withstand multiple cycles of freezing and thawing in a saturated water state without visible signs of damage and without significant reduction in strength, as regulated by current standards. Frost resistance is the most important operational property of natural stone, allowing for an assessment of its durability.
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🔍Examples of stone destruction with low frost resistance
Numerous observations and accumulated practical experience indicate a significant negative impact on stone materials from the combined action of water and cyclically recurring negative temperatures, causing the "loosening" of the rock structure and leading to the formation and development of closed micropores; these, connecting with each other, form a continuous porous system, facilitating further access of water into the stone. In these conditions, moisture transfer processes in the pore system of the stone occur in liquid or gaseous phase under the influence of capillary forces or hydrostatic pressure.
In the mechanism of formation and accumulation of damage during the freezing of water in the pores of the stone, the dominant role belongs to physical processes, as they determine the resistance of the rock to freezing and thawing. In other words, the freezing of the stone under the influence of negative temperatures is accompanied by a decrease in its mechanical strength due to structural damage and micro-cracking, while the degree of degradation is significantly determined by the level of water saturation of the rock and the cyclic repetition of the process.
Theoretically, when all the pores are filled with water and it transitions to ice with an increase in volume of 9%, one would expect massive destruction of the pore walls with a sharp decrease in the strength of the stone. However, this does not actually happen: studies have shown that, for example, in the climatic conditions of the central part of Russia during winter, no more than 10-15% of the pore water freezes - mainly in the form of external ice crusts and, to a lesser extent, in the capillaries with increased diameter (from 0.001 to 1 mm). At the same time, the unidirectional pressure of the growing ice crystals is negligible and is within the range of 0.04 - 0.06 MPa, while the hydrostatic pressure of the water, which arises from the change in volume of the "water-ice" system in a closed or semi-closed space, is associated with the emergence of hydraulic internal pressure, which can reach 200 MPa (at an air temperature of -22⁰C).
Another feature is that the thin film of water covering the surface of the pores, even after the water turns into ice, can cause the formation of a vapor phase in the pore system; in this case, hydraulic internal pressure develops in the smallest pores as the water cools; there is a diffusive movement of still unfrozen water from small pores to larger ones. This leads to more severe damage than freezing the stone in air. It is evident that the destructive stresses that arise in the stone during freezing will depend on the ratio between the rate of ice formation within it and the ease of dissipating the resulting local pressures using pore compensators (i.e., pores not filled with water). Studies conducted on limestones of various porosities suggest that the most frost-resistant types of stone should be characterized by isomeric pores of approximately equal sizes, freely interconnected with each other like a network of intersecting capillaries. For such pores, the degree of water absorption is not a criterion for frost resistance: frost-resistant rocks with high water absorption easily absorb water but also quickly release it (limestones "Myachkovsky," "Melekhovo-Fedotovsky," dolomite "Beryozovsky," Jurassic limestone, etc.). On the other hand, the least frost-resistant rocks will be those with a sharply reduced rate of water release compared to the rate of saturation. Frost resistance also depends on the strength of the cohesive bonds between the mineral grains of the rock, as well as on the ratio of narrow and wide open pores and many other factors.
Testing of natural stone for frost resistance
The main requirements for the frost resistance of natural stone are formulated in domestic standards (GOST 9479-2011, GOST 30629-2011) and foreign standards (EN 12371) and others. The rocks used for the production of blocks and architectural-construction products are divided into seven grades: F15, F25, F35, F50, F100, F150, F200. The frost resistance grade is indicated in the contract for the supply of rock blocks, the area of application of which is determined depending on the construction-climatic zone, the service life of the designed buildings and structures, operating conditions (humidity regime of premises and humidity zones of the construction area), as well as taking into account the current building regulations.
For frost resistance testing, rock samples are prepared in the form of cubes with an edge of 40-50 mm or cylinders with a diameter and height of 40-50 mm. The number of samples is determined based on 5 samples for each stage of testing (usually 15, 25, 50, 75, 100, etc. cycles). The duration of holding the samples in the chamber at a temperature of - (20±2)⁰C should be 4 hours, after which they are placed in a bath and held there until completely thawed (but not less than 2 hours). The freezing-thawing cycle is then repeated. After 15, 25, and every subsequent 25 cycles of alternating freezing and thawing, five water-saturated samples are tested in accordance with GOST 30629-2011 (in European standards, tests are also conducted for tensile strength in bending). The loss of strength of the samples as a result of cyclic freezing-thawing is calculated as a percentage by dividing the difference in compressive strength in dry and water-saturated states by the compressive strength in the dry state (each calculation is taken as the arithmetic mean of the test results for 5 samples). For rock samples with a pronounced layered texture, results obtained from testing along the layering and perpendicular to it are recorded separately. A rock meets the corresponding grade for frost resistance if the value of compressive strength loss (bending) after the established number of cycles of alternating freezing-thawing does not exceed 20%. After each cycle of testing, the samples are carefully inspected, recording any changes that have occurred: chipping of edges, corner spalling, piece loss, cracking, etc. Completely destroyed samples are excluded from the testing cycle: when calculating the average compressive strength after freezing, their strength is considered equal to zero. Partially destroyed samples are tested on general grounds.
In some cases, freezing samples in a freezer is replaced by impregnating them with various salts that expand upon crystallization (the so-called "accelerated tests"). Usually, salts of sulfuric acid are used for this purpose - sodium sulfate, magnesium sulfate, etc. It is believed that the action of crystallizing sulfate salts is similar to the action of freezing water in the pores. However, there are many opponents of this viewpoint who argue that there is no analogy here: the behavior of stone impregnated with a solution of sulfuric acid salts, and then crystallizing, is influenced not so much by the crystallization force acting during the growth of salt crystals, but rather by the polymorphic transformations of the substance during changes in the temperature of the humidification environment and the drying of the salt crystals. The difference in the behavior of the rock when freezing water and when exposed to a solution of sulfuric acid salts is that these salts do not act uniformly on the tested sample (as in freezing), but only affect its surface parts (where the crystallizing solution can penetrate). As a result, when testing low-strength rocks, there is not a general destruction of the samples, but rather the peeling of the outer parts.
It should be noted that the reduction in the strength of the stone due to climatic effects is only the first stage of the destruction of the cladding material, which will inevitably be followed by flaking, surface and volumetric macro-cracking, delamination, and subsequent destruction of the stone.
It is worth reminding that, for example, Jura Limestone is layered and there is many years of experience in using this stone. It is well known that some layers of Jura Marble do not meet the requirements for frost resistance and therefore are not recommended by conscientious suppliers as external cladding for exteriors. This can be seen with the naked eye in the open quarry of Jura Marble, when a wintered open pit with a cut through the layers shows visible damage in some layers.
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🔍Examples of destruction of Jura Limestone with low frost resistance and obvious structural defects (suture seams)
However, in a situation where the reduction in the strength of the stone has not yet reached critical values (25-30%), the development of destruction processes can be slowed down by implementing a set of measures to ensure the preservation of the stone cladding: systematic monitoring of the stone's condition using electronic non-destructive testing methods, periodic hydrophobization of the surface, sealing of joints, treatment with consolidants, fluorination, etc.