Thermal conductivity of concrete: features, coefficient and table

One of the most important characteristics of concrete, of course, is its thermal conductivity. This indicator can vary significantly for different types of material. The thermal conductivity of concrete, primarily, depends on the type of filler used in it. The lighter the material, the better it is from the cold.

What is thermal conductivity: definition

In the construction of buildings and structures, different materials can be used. Residential and industrial buildings in the Russian climate are usually insulated. That is, during their construction, special insulators are used, the main purpose of which is to maintain a comfortable indoor temperature. When calculating the required amount of mineral wool or expanded polystyrene, the thermal conductivity of the base material used for the erection of building envelopes is taken into account without fail.

Very often, buildings and structures in our country are built from different types of concrete. Also, brick and wood are used for this purpose . Actually, the thermal conductivity itself is the ability of a substance to transfer energy in its thickness due to the movement of molecules. A similar process can occur, both in the solid parts of the material, and in its pores. In the first case, it is called conduction, in the second - convection. The cooling of the material is much faster in its hard parts. Pore-filling air traps heat, of course, better.

What does the indicator depend on

Conclusions from the foregoing can be drawn as follows. The thermal conductivity of concrete, wood and brick, like any other material, depends on them :

• density;
• porosity;
• humidity.

With increasing density of concrete , the degree of its thermal conductivity also increases. The more pores are in the material, the better it is from the cold.

Types of concrete

In modern construction can be used in a variety of types of this material. However, all concrete available on the market can be classified into two large groups:

• heavy;
• light foamy or porous filler.

Thermal conductivity of heavy concrete: indicators

Such materials are also divided into two main groups. Concrete can be used in construction:

• heavy;
• especially heavy.

In the production of the second type of material, such fillers as metal scrap, hematite, magnetite, and barite are used. Particularly heavy concrete is usually used only in the construction of facilities whose main purpose is radiation protection. This group includes materials with a density of 2500 kg / m ³ .

Conventional heavy concretes are made using such types of filler as granite, diabase or limestone, made on the basis of mountain rubble. In the construction of buildings and structures, similar material is used with a density of 1600-2500 kg / m ³ .

What can be the thermal conductivity of concrete in this case ? The table below shows the characteristics of different types of heavy material.

Thermal conductivity of lightweight cellular concrete

Such material is also classified into two main varieties. Very often, concrete based on a porous filler is used in construction. Expanded clay, tuff, slag, pumice are used as the latter. In the second group of lightweight concrete, the filler is conventional. But in the process of kneading such material foams. As a result, after ripening, many pores remain in it.

The thermal conductivity of lightweight concrete is very low. But at the same time, and in terms of strength characteristics, such material is inferior to heavy . They use light concrete most often for the construction of various kinds of residential and utility buildings that are not exposed to severe loads.

Light concrete is classified not only by the method of manufacture, but also by purpose. In this regard, there are materials:

• heat-insulating (with a density of up to 800 kg / m3);
• structural and heat-insulating (up to 1400 kg / m3);
• constructional (up to 1800 kg / m3).

The thermal conductivity of cellular lightweight concrete of different types is presented in the table.

Thermal insulation materials

Such concrete blocks are usually used for cladding walls made of brick or poured from cement mortar. As can be seen from the table, the thermal conductivity of concrete in this group can vary in a fairly wide range.

Concretes of this variety are most often used as insulation materials. But sometimes various insignificant building envelopes are erected from them.

Structural and heat-insulating and structural materials

Of this group, foam concrete, slag and slag concrete, and slag concrete are most often used in construction. Some types of expanded clay concrete with a density of more than 0.29 W / (m ° C) can also be attributed to this variety.

Very often such concrete with low thermal conductivity is used directly as a building material. But sometimes it is used as an insulator that does not let the cold through.

How does heat conductivity depend on humidity?

Everyone knows that almost any dry material isolates from the cold much better than wet. This is due, first of all, to a very low degree of thermal conductivity of water. They protect concrete walls, floors and ceilings from low street temperatures , as we have found, mainly due to the presence of pores filled with air in the material. When wet, the latter is displaced by water. And, consequently, the coefficient of thermal conductivity of concrete is significantly increased . In the cold season, the water that gets into the pores of the material freezes. The result is that the heat-saving qualities of walls, floors and ceilings are reduced even more.

The degree of moisture permeability in different types of concrete may be uneven. According to this indicator, the material is classified into several brands.

Wood as an insulator

Both “cold” heavy and lightweight concrete, low thermal conductivity , of course, very popular and popular types of building materials . In any case, the foundations of most buildings and structures are built precisely from cement mortar mixed with crushed stone or rubble stone .

Concrete mix or blocks made from it are also used for erection of building envelopes. But quite often, other materials, such as wood, are used to assemble floors, ceilings and walls. A beam and a board are, of course, distinguished by a much lower strength than concrete. However, the degree of thermal conductivity of a tree, of course, is much lower. In concrete, this indicator, as we found out, is 0.12-1.74 W / (m ° C). In a tree, the thermal conductivity coefficient depends, inter alia, on this particular breed.

In other breeds, this indicator may be different. It is believed that the average thermal conductivity of wood across the fibers is 0.14 W / (m ° C) . Cedar isolates space best from the cold. Its thermal conductivity is only 0.095 W / (m C).

Brick as an insulator

Further, for comparison, we consider the characteristics with respect to thermal conductivity and this popular building material. In terms of strength, brick is not only inferior to concrete, but often surpasses it. The same goes for the density of this building stone. All brick used today in the construction of buildings and structures is classified into ceramic and silicate.

Both of these types of stone, in turn, can be:

• corpulent;
• with voids;
• slotted .

Of course, solid bricks retain heat worse than hollow and slotted.

The thermal conductivity of concrete and brick is, therefore, almost the same. Both silicate and ceramic stone isolate the premises from the cold rather weakly. Therefore, houses built from such material should be additionally insulated. Expanded polystyrene or mineral wool are most often used as insulators for cladding brick walls as well as those poured from ordinary heavy concrete. Porous blocks can also be used for this purpose.

How is the thermal conductivity calculated?

This indicator is determined for different materials, including concrete, according to special formulas. In total, two techniques can be used. The thermal conductivity of concrete is determined by the Kaufman formula. It looks like this:

• 0.0935x (m) 0.5x2.28m + 0.025, where m is the mass of the solution.

For wet (more than 3%) solutions, the Nekrasov formula is used: (0.196 + 0.22 m2) 0.5 - 0.14 .

To eramzitobeton with a density of 1000 kg / m3 has a mass of 1 kg. Accordingly, for example, according to Kaufman in this case, a coefficient of 0.238 is obtained. The thermal conductivity of concrete is determined at a mixture temperature of +25 C. For cold and heated materials, its performance may vary slightly.