Frost resistance of building materials, in particular natural stone, usually refers to its ability to withstand multiple cycles of freezing and thawing in a water-saturated state without visible signs of destruction 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, which cause the "loosening" of the rock structure and lead to the formation and development of closed microvoids; the latter, connecting with each other, form a continuous porous system, facilitating further access of water into the stone. In these conditions, the processes of moisture transfer in the pore system of the stone occur in either 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 stone, the dominant role belongs to physical processes, as it is they that determine the resistance of the rock to freezing and thawing. In other words, the freezing of stone under the influence of negative temperatures is accompanied by a decrease in its mechanical strength due to structural damage and micro-crack formation, 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 a volume increase of 9%, one would expect a massive destruction of the pore walls with a sharp decrease in the strength of the stone. However, this does not happen in reality: studies have shown that, for example, in the climatic conditions of the central strip of the Russian Federation 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, within 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 volume change 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; at the same time, in the smallest pores, hydraulic internal pressure develops as the water cools; there is a diffusive movement of unfrozen water from small pores to larger ones. This leads to more severe damage than freezing stone in the air. It is obvious 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 in a network-like pattern of intersecting capillaries. For such pores, the amount 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 established 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 codes.
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 kept 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-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 and perpendicular to the layering are recorded separately. A rock corresponds to the appropriate 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 to be zero. Samples that are partially destroyed are tested on common grounds.
In some cases, freezing samples in a freezing unit is replaced by impregnating them with various salts that expand upon crystallization (the so-called "accelerated tests"). Usually, sulfuric acid salts are used for this purpose - sodium sulfate, magnesium sulfate, etc. It is believed that the action of crystallizing sulfuric salts is analogous to the action of freezing water in the pores. However, there are many opponents of this viewpoint who argue that no analogy exists: the behavior of the stone impregnated with a sulfuric acid salt solution 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 temperature changes in the moisture environment and the drying of the salt crystals. The difference in the behavior of the rock when freezing water and when subjected to a sulfuric acid salt solution 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 off of the outer parts.
It should be noted that the reduction in the strength of the stone due to climatic influences is only the first stage of the destruction of the facing material, after which peeling, surface and volumetric macro-cracking, delamination, and subsequent destruction of the stone will inevitably follow.
It is worth reminding that, for example, Jurassic limestone is layered and there is many years of experience in using this stone. It is reliably known that some layers of Jurassic 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 experienced eye in the open workings of Jurassic marble, when a wintered open quarry with a cut through the layers shows visible damage in some layers.
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🔍Examples of the destruction of Jurassic 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 condition of the stone using electronic non-destructive testing methods, periodic hydrophobization of the surface, sealing of joints, treatment with consolidants, fluorination, etc.