(of building materials), the ability of building materials in a wet condition to withstand many cycles of freezing and thawing without disintegrating. The basic cause of the disintegration of materials acted upon by low temperatures is that the water filling the pores of the material expands when it freezes. Frost resistance depends primarily on the structure of the material: the larger the pores that water can penetrate, the lower frost resistance will be.
The concept of frost resistance and methods of testing for it were first proposed in 1886 by Professor N. A. Beleliubskii.
The degree of frost resistance is determined on the basis of laboratory tests of samples of the material. The frost resistance value is the number of cycles of freezing and thawing the material can undergo before losing 25 percent of its initial strength or 5 percent of its weight.
The frost resistance of building materials is improved by decreasing their water absorption—for example, by increasing the proportion of closed pores, increasing the density of the outer layers of the material, or waterproofing the material’s surface. To a large extent frost resistance determines the durability of the outside wall and roof elements of buildings and structures.
K. N. POPOV
in plants, the ability to survive short or long periods of frost; a type of winter hardiness. Wintering plants develop frost resistance each year as a result of their prolonged and complex preparation for winter. During the warm season, when the plants are growing, their frost resistance is negligible; during winter frosts it is maximal. During thaws, frost resistance falls sharply and then, if intensification of frosts proceeds slowly, it rises again. Sharp temperature fluctuations are dangerous, because the plants do not have time to go through repeated hardenings.
Frost resistance is governed by physical and chemical processes occuring in the cells, which hamper the freezing of intracellular water and increase cell resistance to the dehydration of the protoplasts and to mechanical deformations by extracellular ice. These processes are developed by hardening plants at low temperatures in several stages, beginning during dormancy. If the necessary processes do not occur in the plant cells at any given stage, the plants are insufficiently frost resistant and may perish.
Frost resistance is primarily hereditarily determined. Some species of plants die during mild frosts (for example, lemon trees die at temperatures from −5° to −12°C), while others are capable of surviving the most severe winters (for example, some apple trees tolerate frosts to — 40°C). Larches, birches, and other trees of Eastern Siberia can survive frosts of — 70°C.
Different varieties of the same plant species may differ in their frost resistance; for example, some varieties of winter wheat die at temperatures below — 15°C, and others die only below — 23°C. Thus, one of the most effective methods of increasing frost resistance is the development of frost-resistant varieties for certain regions. Soil and climatic conditions and agricultural methods that ensure plants optimal conditions of nourishment, water supply, and soil aeration also influence frost resistance.
Cultivated plants usually do not attain maximal frost resistance under natural conditions (field or orchard), because the conditions for winter preparation are often unfavorable. Winter wheat, for example, freezes at temperatures below — 15°C at the depth of the tillering node; after hardening under laboratory conditions it can tolerate frosts to −30°C. Apricot is damaged slightly at a temperature of −60°C after laboratory hardening of one-year seedlings, while the Antonovka variety of apple is still capable of blossoming after such a frost. After laboratory hardening, cuttings of the European black currant can root and develop even after exposure to temperatures as low as −253°C.
Evaluation of the frost resistance of plants is done in the field (according to the number of plants that wintered per unit area) or in the laboratory, where the temperature at which plants in refrigeration units begin to freeze can be determined and where frost resistance can be studied over a long period.
REFERENCETumanov, I. I. “O fiziologicheskom mekhanizme morozostoikosti rastenii.” Fiziologiia rastenii, 1967, vol. 14, issue 3.
G. A. SAMYGIN and I. I. TUMANOV