Frost resistance of building materialsshows how much a sample is able to maintain its properties after several consecutive 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 considerable pressure on the walls of the microcracks and pores of the material.
In turn, the hardness of concrete does not givewater to expand freely, so when testing for frost resistance of concrete high stresses are created. Destruction begins with protruding parts, and then continues in the upper layers and finally penetrates deeper.
Factor, which accelerates the destruction of concrete, is also a different coefficient of thermal expansion of the elements of which the building material consists. This creates additional stress.
The frost resistance of concrete is measured bymethods that control the procedures for freezing and thawing. The parameters of the investigated parameter depend on the following factors: the freezing temperature, the duration of the cycles, the dimensions of the test sample, the method of water saturation. For example, the process of destruction of concrete occurs faster if freezing is performed at the lowest possible temperatures in salt solutions.
Frost resistance of concrete is calculated beforemoment until a certain number of repeated cycles does not reduce the mass of the sample by 5 percent and reduce its strength by 25 percent. It is the number of procedures that the building material has withstood, determines its brand. The degree of frost resistance is also determined depending on in which sphere 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 onthe number of macropores, since the water in fine pores does not freeze even at the lowest possible temperatures, so it does not create additional stress. Thus, the character, shape and volume of large pores have a great influence.
The frost resistance of concrete can be improved in the following ways:
- Reduction of large pores by increasing the density of concrete.
- Creation of additional air pores in concrete withby introducing certain additives. If the volume of such pores is a quarter of the volume of frozen water, 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 volume of air 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 a large aggregate. The volume of air in the internal pores of concrete increases when the flow of water and cement increases, and the size of the aggregate fractions, on the contrary, decreases.
