Frost resistance of building materials shows how a particular sample is able to maintain its properties after several successive cycles of freezing and thawing. In the case of concrete, the main cause of its destruction during these processes is water in the solid state, which exerts significant pressure on the walls of microcracks and pores of the material.
In turn, the high hardness of concrete does not allow water to expand freely, therefore, high stresses are created during the frost resistance test of concrete. The destruction starts from the protruding parts, and then continues in the upper layers, and finally penetrates deep.
A factor that accelerates the destruction of concrete is also a different coefficient of thermal expansion of the elements that make up the building material. This creates additional tension.
The frost resistance of concrete is measured using methods that control freezing and thawing procedures. The indicators of the studied parameter depend on the following factors: freezing temperature, cycle duration, dimensions of the studied sample, method of water saturation. For example, the process of concrete destructionis faster if freezing is carried out at the lowest possible temperatures in s alt solutions.
The frost resistance of concrete is calculated until a certain number of repeated cycles reduces the mass of the sample by 5 percent and reduces its strength by 25 percent. It is the number of procedures that a building material has withstood that determines its brand. The degree of frost resistance is also assigned depending on the area in which this concrete will be used.
Frost-resistant concrete has a special structure. The nature of its porosity does not allow the volume of ice to create too much pressure and slows down the process of destruction.
The frost resistance of concrete depends only on the number of macropores, since water in small pores does not freeze even at the lowest possible temperatures, so it does not create additional stress. Thus, the nature, shape and volume of large pores have a great influence.
The frost resistance of concrete can be improved in the following ways:
- Reducing large pores by increasing the density of concrete.
- Creating additional air pores in concrete by introducing certain additives. If the volume of such pores is a quarter of the volume of frozen water, then it will not be filled in the process of ordinary water saturation. In this case, unfrozen water displaced by ice will seep into the free space, and then the pressure will weaken.
Internal air volume in frost-resistant concreteshould be between four and six percent. The amount of air depends not only on the consumption of cement and water, but also on the coarse aggregate. The volume of air in the internal pores of concrete increases when the consumption of water and cement increases, and the size of aggregate fractions, on the contrary, decreases.
