The frost resistance of building materials shows how much a particular sample is able to maintain its properties after several successive cycles of freezing and thawing. In the case of concrete, the main reason for its destruction during these processes is solid water, 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, when tested for frost resistance of concrete, high stresses are created . The destruction begins with the protruding parts, and then continues in the upper layers and finally penetrates deep into.
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 stress.
The frost resistance of concrete is measured using methods that control the freezing and thawing procedures. The parameters of the studied parameter depend on the following factors: freezing temperature, duration of cycles, dimensions of the test sample, method of water saturation. For example, the process of concrete destruction occurs faster if freezing is carried out at the lowest possible temperatures in salt solutions.
The frost resistance of concrete is calculated until a certain number of repeated cycles reduces the sample mass by 5 percent and reduces its strength by 25 percent. It is the number of procedures that the building material has withstood that determines its brand. The degree of frost resistance is also assigned depending on the sphere 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 destruction process.
The frost resistance of concrete depends only on the number of macropores, since water in shallow pores does not freeze even at extremely low temperatures, therefore 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:
- By reducing large pores by increasing concrete density.
- The creation of 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 during normal water saturation. At the same time, unfrozen water displaced by ice will leak into the free space, and then the pressure will weaken.
The internal air volume in frost-resistant concrete should be between four and six percent. The amount of air depends not only on the consumption of cement and water, but also on coarse aggregate. The air volume in the inner pores of concrete increases when the flow of water and cement increases, and the size of the aggregate fractions, on the contrary, decreases.