Lightweight concrete: types, classification, properties, use

Saving energy is becoming a necessity, and one of the main items of expenditure in our climate is heating. In this regard, new materials are being developed that allow you to build warm houses and insulate existing ones. Lightweight concretes are becoming more and more popular. This is a whole group of materials with a fairly wide range of properties and characteristics. 

Types of lightweight concrete

The reduction in the mass of concrete occurs due to the formation of pores and the use of lightweight aggregates instead of traditional gravel, and sometimes sand. Sometimes pores are formed by using different processes. Depending on the production method, lightweight concretes are divided into three groups:

• Cellular or porous concrete. Obtained by a mixture of binder, water, sand (some brands do not have sand) and additives that form foam or promote gas formation. When using foam, foam concrete is obtained, when using gas-forming additives, aerated concrete is obtained. If in aerated concrete most of the binder is lime, gas silicate is obtained. The main difference between cellular materials is the absence of large aggregates.
  

• Ordinary lightweight concrete. Obtained from a mixture of binder, coarse and fine aggregate, water. They differ from ordinary concrete in the presence of light porous aggregate instead of crushed stone. Almost all the voids between the aggregate particles are filled, there are few air cavities in such a material – no more than 6%.
  

• Coarse porous lightweight concrete. Instead of sand and gravel, large porous aggregate is used, which is mixed with a binder diluted with water. There is no sand, because this material is also called sandless concrete. Fragments of the filler are glued together only in those places where they touch, leaving voids unfilled. Air voids can be up to 25%.
  

But in each of the groups there can be many varieties and compositions. Various aggregates and various binders are used. Traditionally, cements are used as a binder (on Portland cement, materials have better strength characteristics). The second most popular binder is lime, less often gypsum is used. Sometimes they can use a mixture of binders and use liquid glass.

Hardening technologies

There are three technologies for the manufacture of cellular concrete:

• Natural curing. The composition poured into the molds is left indefinitely in the formwork. After a certain time (depending on the composition and type), the formwork is removed. Using this technology, the material is the cheapest, but its characteristics are in the lowest part of the acceptable range, and sometimes even lower.
• Treatment in heat and moisture chambers at atmospheric pressure. The quality indicators are higher, but the costs and price are also higher.
• Autoclave curing. The material acquires excellent characteristics, but it also costs more due to expensive equipment and energy costs (to maintain temperature and pressure in the chamber).
  

Placeholders

By origin aggregates for light concretes can be divided into two groups: natural (natural) and artificial. Natural ones are obtained by grinding natural porous materials: shell rock, pumice, lava, turf, limestone, etc. The best of them are pumice and volcanic turf. They have a closed pore structure, which reduces the amount of moisture absorbed by the material.

Artificial aggregates for lightweight concrete are wastes of some technological processes (slags) or materials specially created from natural components (expanded clay, vermiculite, perlite, etc.) as well as some chemical aggregates (polystyrene).

Properties, characteristics, application

The main characteristics of lightweight concrete that you should pay attention to when choosing are density (bulk weight), strength, thermal conductivity and frost resistance.

The density of the material depends mainly on the characteristics of the filler, as well as the consumption of binder and water. It can vary over a wide range – from 500 to 1800, but most often it is in the range of 800-1500 kg / m³. An exception is porous or cellular concrete (foam and aerated concrete). Their density can be from 200 kg/m³.

The main operational characteristic is compressive strength. It is subdivided into classes, indicated in the specification by the Latin letter “B”, followed by numbers. These figures indicates the pressure that the material can withstand. For example, strength class B30 means that in most cases (according to GOST 95%) it can withstand a pressure of 30 MPa. But in the calculations they take a margin of safety of the order of 25%. And when calculating for class B30, a strength of 22,5-22,7 MPa is laid.

At the same time, such a characteristic as the compression limit is also used. It is denoted by the Latin letter “M”, and the numbers following it are taken equal to the volumetric mass of concrete in kg / m³.

The thermal conductivity of lightweight concrete has an inverse relationship with respect to density: the more air a material contains, the less heat it conducts. This parameter varies considerably from 0,07 to 0,7 W/(mx°C). The lightest materials with low density are used as thermal insulation. They sheathe the walls of buildings and outbuildings. Foam concrete insulation of balconies and loggias is very popular. But the greatest economic effect can be obtained when building from lightweight medium-density concrete. It has sufficient bearing capacity to build a two- or three-story house. In this case, additional insulation is not required.

Another important characteristic is frost resistance. Denoted by the Latin letter F, followed by numbers, displaying the number of defrosting/freezing cycles that the material can endure without losing strength. In cases with lightweight concrete, its frost resistance directly depends on the amount of binder in the composition: the more it is, the more frost-resistant the concrete will be.

appointment

By appointment, lightweight concrete is divided into the following groups:

• Thermal insulation. They have a thermal conductivity of not more than 0,25 W / (mx ° C), a density of not more than 500 kg / m³.
• Structural and thermal insulation. Thermal conductivity not higher than 0,6 W / (mx ° C), density 500-1400 kg / m³, strength grade not lower than M35. In low-rise private construction, they are used for the construction of load-bearing walls, in high-rise buildings – for unloaded walls.
• Structural. Density from 1500 kg/m³ and higher, strength grade not less than M 50 and frost resistance not lower than F 15. They are used for the construction of load-bearing walls for buildings above 3 floors.
  

Advantages and disadvantages

If we talk about the use of lightweight concrete as a heater, then there are few minuses. The main one is high hygroscopicity, which, nevertheless, varies over a wide range and strongly depends on the filler and the type of material. The second not very pleasant moment is the need to select the appropriate finish. If we are talking about exterior decoration (from the side of the street), then when choosing materials or type of decoration, it is necessary to take into account the high vapor conductivity. In this regard, either special vapor-permeable plasters are used or sheathing is made with a ventilation gap.

But the advantages of lightweight concrete as a heater are more significant. It is easy to install, lightweight, easy to cut and saw, tolerates weather changes well, does not require the use of wind protection. To all this, add high thermal insulation properties and a low price.

If we talk about the use of lightweight concrete as a material for building houses, their advantages are as follows:

• High thermal insulation characteristics. This property makes it possible to refuse additional wall insulation and at the same time reduce the thickness of the walls.
• Small mass. Lightweight concrete walls weigh several times less than traditional “heavy” materials and are comparable in weight to the weight of wooden houses. A small mass leads to a “lightening” of the foundation and the possibility of using simpler structures. And this significantly reduces construction costs, as well as transportation costs (they consider, mainly, the delivery of building materials by tonnage).
• The low weight allows the production of large-sized building blocks and slabs, which, however, are stacked by hand. This leads to a reduction in construction time, as well as a decrease in the number of seams, which in this case are cold bridges.
  

• Plasticity of the material and ease of processing. Many lightweight concretes are easy to cut, saw, grind. This allows them to be used for the manufacture of various architectural and decorative elements, as well as to obtain parts of the required size right on the spot by sawing the existing blocks into smaller fragments.
• They tolerate changes in operating conditions well. Changes in humidity and temperature practically do not affect the material. They also hold constant loads well, and are not particularly sensitive to mechanical stress. Dents appear in the material, but it is difficult to break the integrity of the block.
• Production waste is often used as aggregates. This reduces the cost of the material while reducing the burden on the environment.
• Some types of lightweight concrete can be made independently (usually with aggregates from slag or expanded clay), reducing construction costs to a minimum.

As you can see, lightweight concrete has a lot of advantages as a building material. But not everything is so rosy. There are disadvantages that you should be aware of in order to make an informed decision:

• To increase the strength of the walls, frequent reinforcement is necessary. This is an additional cost for materials and time for laying reinforcement.
• Insufficient crack resistance. The heterogeneous structure of the material leads to the fact that in the presence of uneven loads (uneven shrinkage of the foundation, for example), cracks appear in the blocks. If they are thin, cobweb-like, they do not affect the strength of the structure, although they look intimidating.
• High moisture absorption. The thermal insulation characteristics of wet materials are significantly reduced. Therefore, during construction, it is important to make high-quality waterproofing. If it is planned to use in conditions of high humidity, it is recommended to use pumice, agloporite and expanded clay as fillers.
• The low density of materials leads to the fact that fasteners do not hold well in such walls. The material holds vertical loads well, but it is bad for pull-out. For lightweight and cellular concrete, special fasteners have been developed, but the best solution is to install mortgages in places where heavy objects are supposed to be fastened.
• Difficulty in choosing exterior finishes. As already mentioned, this is either a cladding with a ventilated facade, or special plasters.
• For interior decoration, a high-quality pre-primer of the walls may be required – for better adhesion to plaster or putty.
• Low degree of sound absorption. Due to the large number of voids and concrete “tracks” passing between them, sounds are transmitted very well. For normal sound insulation, the use of additional materials is required.

Most of the shortcomings, rather, are features of operation, but they must be taken into account. Then there will be no unpleasant surprises, and all the features will be taken into account at the planning stage.

Where and how to use at a construction site, do-it-yourself examples

As could be understood from all that has been said, lightweight concrete can be used for any structure. Walls are built from them, used as insulation, floor slabs are poured, and a screed is made. But all these tasks require different characteristics. They are “recruited” by the selection of components.

How to choose a recipe

For example, floor screed requires strength, hydrophobicity and low thermal conductivity. Strength and a decrease in the amount of absorbed moisture gives the use of Portland cement as a binder. Since the best natural additives that provide low moisture absorption – pumice and volcanic turf – cannot be called generally available, expanded clay or polystyrene balls can be used to increase thermal conductivity. They also absorb little moisture.

Now about proportions. They are taken standard for a given brand. And depending on the type chosen (sandless or normal), the aggregate is replaced. For floor screed, ordinary lightweight concrete is most often used. In them, the gravel is replaced with the selected aggregate, which is added in the desired proportion. Only water is taken less, making the solution so dense or fluid that it can only be laid.

Even in production, the exact composition of lightweight concrete is determined experimentally each time. This is due to the fact that aggregates have very different characteristics both in terms of mass and density and other parameters. They make several small batches with different aggregate composition (large, small, their proportions, combine several different types of aggregate) and different amounts of water. After solidification, it is determined which one is best suited for a particular task. Using the same method, you can independently determine how much and what aggregate is better to pour, and then close large volumes.

An example of attic insulation with polystyrene concrete

See the video for an example of experimental selection for specific tasks. It was necessary to choose a composition for warming the attic floor. It was decided to use polystyrene concrete as warm and light. A sandless composition was chosen and only polystyrene balls were poured as a filler.

According to the chosen recipe, light concrete was mixed and the attic was insulated. The process can be seen below.

But this composition is only suitable for warming in places with a small load. If you need a screed with thermal insulation characteristics for the floor, take the traditional recipe with sand, and replace the filler with polystyrene balls. To increase the strength characteristics, reinforcing fibers, such as fiber fibers, can be added. To improve plasticity, you can add, as in the video fragment, a certain amount of dish detergent or liquid soap. In general, the optimal composition must be determined experimentally.

An example of pouring a polystyrene concrete screed can be seen in the following video. No news, except for a different composition: there is sand. The result is a more uniform structure with cavities filled with concrete mortar and small air bubbles.

What else you need to know is that it is better not to use crumbs for the production of polystyrene concrete. For normal characteristics, balls are needed, and not any, but those that will adhere well to the solution. They have a strong film on the surface and do not absorb cement laitance, due to which they have good thermal insulation properties. The crumb obtained by grinding defective plates has an uneven and torn structure. As a result, it is impregnated with cement milk. Naturally, such concrete will be warmer than ordinary, but not the same as with granular.

Expanded clay concrete in private housing construction

Another popular aggregate for the production of lightweight concrete at home is expanded clay. It is made of clay, to which substances are added that increase in volume when heated. This composition is loaded into the furnace, where swelling takes place and followed by firing. But, as studies have shown, many clays are fonat, as a result, expanded clay also has a radiation background, sometimes even unsafe for health. So you need to be ready for his choice – to have a dosimeter.

The order of selection of the composition here is similar to that described above. Only the ability to change the proportions of large and medium fractions is added. You can also add or not sand and get different results in structure and characteristics.

Expanded clay concrete is used for pouring into molds and obtaining building blocks, and it is also possible to erect walls with adjustable formwork. Unlike expanded clay concrete blocks, this technology can be used to build load-bearing walls.

And in this video – the experience of living in a house made of monolithic expanded clay concrete.

Houses made of sawdust concrete – wood concrete

Another natural aggregate that costs mere pennies and can be used for private housing construction is sawdust, or rather shavings with sawdust. A very small fraction is unsuitable for this material; waste from rounding of medium or large size is needed.

The composition in this case is sandless, but the proportions are preserved: 1-6 parts of aggregate are taken for 7 part of concrete. In this case, sawdust. To increase the hydrophobicity of the composition, water glass or calcium chloride is added.

The second option of kneading and proportions

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