Articles

Why are we against aerated concrete?

What salesmen of aerated concrete are silent about?

Production of cellular concrete is currently experiencing a rebirth. Volumes of production are increasing, the market is growing. And it's all thanks to the new rules introduced by the thermal resistance of building structures prescribed in SNIP II- 3 -79 *, by which through the efforts of advertising campaigns was claimed by one of the main positive features of aerated concrete - good thermal resistance of the material. Managers of manufacturing companies, promoting the product, goods touted talent eastern market. But is it as good as it is told in advertisements? What is kept under wrap?

Cellular concrete - artificial stone with evenly distributed pores. Derived of cellular concrete are foam concrete, aerated concrete. The difference between these materials is determined by the technology of production of these materials.

Foam concrete - light cellular concrete, the result of curing of a solution consisting of cement, sand and water, as well as foam. The foam provides the required air content of the concrete, and its evenly spreading throughout the mass in the form of closed cells..

Aerated concrete - autoclaved cellular concrete, consists of quartz sand, cement, lime, aluminum powder and water. These components are mixed and fed into an autoclave, wherein under certain conditions, their foaming occurs (when corrosion of aluminum powder, with hydrogen evolution, which forms the pores) and the subsequent hardening.

Main components of these materials are practically identical. The only difference is the method of using the foaming blowing agent and the method of curing. The advantage of aerated concrete is that usage of autoclave process enables the production of material with a predetermined set of properties necessary and stable quality characteristics.

Further, in the text, I will use the term "aerated concrete", but the main conclusions are also applicable to foam concrete. foam concrete - a material that  meets the stated specifications, and in most case doesn’t require costly investments made in very questionable circumstances. The owners of foam concrete business often do not have their laboratories, certification material, with all the terms. Percentage of highly dubious "garage" foam production is very high, so before buying concrete blocks, you need to think well about who, where and how produced them.

Commercial production of autoclaved aerated concrete was started by firm "Siporex" (Sweden) in 1929.In Russia people started using cellular concrete steel in 50-60 years. In Moscow and the Baltic entire institutions existed, developing new technologies of its production. This article reviews the properties of autoclaved aerated concrete  in the form of blocks, since this material is the most popular and has demand on the market, primarily because it is stable with a set of factory-built permanent qualities. In addition, there are also blocks reinforced products, namely slabs, coating bridges, stairs, arched bridges.

So, what we were told by gas-silicate managery? Here is a cocktail of all the positive properties, usually dumped in the pile:

 - ecology (only natural, natural materials are used in production)

- fire protection (refers to incombustible matter)

 - high insulation qualities comply with all the thermal resistance standarts of single-layer design,

 -workability (material can easily be cutting, polishing up)

 - lightness

 - bearing capacity of high

- high water vapor permeability

- high resistance to frost (up to 200 cycles)

- no need for extra protection (plaster, painting)

 -has a wide range of densities with the given parameters,

-the lowest price

Solid benefits! But somehow we, fools, still haven’t built houses of this remarkable material, why? Why professional constructors are no so positive about gas-silicate? Why professional constructors somehow don’t see such good properties of aerated concrete as good insulation and bearing capacity

The answer is simple - the professionals are very familiar with the material, its properties to believe in all those ads and use gas-silicate based solely on science data and Building Code and Regulation. But private developers are far from such a fundamental attitude towards choosing building material, often swallow the hook of advertisements and very happy with their choice.

What kind of material is aerated concrete, really?

Based on the requirements of GOST 25485-89 (CELLULAR CONCRETE): Section 1.2.2: According to their intended use types of concrete are divided into:

• construction
• construction and thermal insulation
• thermal insulation.

According to its density aerated concrete is divided into:

• Thermal insulation - brand D300- D500
• Construction and thermal insulation - brand D500 - D900
• Construction - brand D1000 -B1200

GOST requirements suggest that concrete blocks whose density is 500 and below are solely thermal insulation blocks, while the 500 mark is on the border of the definitions and the main characteristics of the brand with such density are determined by the manufacturer and the test results. Currently, the most optimal and popular brands are blocks with density of 400-500 kg per cubic meter. Therefore, when buildng a house with a view of sustaining capability and good insulating properties, the brand D500 would be the best choice.

Let us consider the stated properties of aerated concrete more closely:

1. Sustaining capability.

The brand D500 is designed for constructing buildings no higher than 3 floors. Its sustaining capability is enough to withstand the load of the entire construction and floor panels. 

But one issue needs to be taken into consideration.  In order for the floor panels not to cut through walls made of aerated concrete blocks, a special lacing of reinforced concrete should be made in the points of contact of floor panels with the walls and other elements under pressure.  In worst case scenarios it can be replaced by conventional brickwork or padding support made of reinforced concrete. At the same time, note that these loaded elements in the building become so-called cold bridges (discussed later). Buildings higher than three floors are hardly ever constructed with aerated concrete blocks, as the gas concrete used in such designs possesses higher density, which in turn dramatically decreases insulating properties of the material and increases construction costs. Another important point to consider is that aerated concrete is a rather brittle material. It has low cross-breaking strength, that is, it lacks elasticity. The slightest deformation of the foundation can lead to massive cracks throughout the structure. That is why a building made of cellular concrete requires monolithic strip foundation or a basement of normal-weight concrete, which entails considerable costs. It is simply unprofitable to build a strong and costly foundation for a small building. At the same time, one should never economize when laying the foundation for an aerated concrete cottage, as without a strong foundation there is no point in working with cellular concrete. That is why to work with aerated concrete blocks, a monolithic strip foundation is necessary, which is something very few construction companies can technologically afford, not to mention private real estate developers. More problems arise when one needs to fix massive objects on aerated concrete blocks. Regular fittings are not suitable for mounting on aerated concrete. One would need to purchase special, designed for brittle and porous materials fasteners, which are naturally more expensive. Generally, they are chemical capsules and special screw-in dowel pins of specific design. For instance, to secure insulation in a conventional brickwork or concrete base one would need five EJOT disk-shaped dowel pins, which cost 10 rubles each. At the same time, to conduct the same operation with aerated concrete blocks one would been special screw-in dowel pins costing 60 rubles each. In total, the cost for insulation mounting per 1 square meter of the wall is increased by 250 rubles, and if we assume that the facade of a common cottage has an area of about 500 square meters, construction cost may rise by about 125 thousand rubles!!! That is about half of the cost of all aerated concrete blocks needed for the cottage.

2. High thermal insulation properties.

As our manufacturers of aerated concrete assure us, according to modern standards of thermal resistance, aerated concrete blocks 380 millimeters thick are suitable for the midland (Moscow and Moscow region, to be precise, Rreq=3.15). That is quite a reasonable wall thickness. But they are being very cunning or so busy with sales that they simply forgot about the existence of methods for calculating thermal resistance, developed by the State Construction Committee of Russia. Here as well (Hebel) we are given thermal resistance figures for their material when dry (pay attention that they are not informing us of that) so we can multiply it by the coefficient of desired resistance of the construction and get "pretty" 380mm. This is the definition of consumer fraud!

So what wall thickness is actually needed?

Let us calculate the actual wall thickness for aerated concrete buildings on the basis of the current Building Regulations. We will be looking at two instances - minimum and maximum thickness.

We will not be taking into consideration different infractions that lead to lowered estimates, as everything must be conducted in accordance with a specific technique.

The calculation has its own rules and methods. Based on SNiP 23-01-99 "Building Climatology " and SNiP II- 3 -79* "Construction Thermal Engineering" we find out that the estimation for Moscow and Moscow (Rreq = 3,15) allows for "maximum allowable increment of calculated weight ratio of water up to 12% (condition B)", which in turn reduces the thermal conductivity of aerated concrete (we are calculating the data for brand D500 by linear interpolation between 400 and 600 brands) to 0.21. Some sources (picture) argue that the actual moisture content of aerated concrete in use is set within 4 - 5% (which corresponds to a thermal conductivity coefficient of 0.17 W /(m*deg C).

Now, using only data on humidity, we will calculate wall thickness: option 1 (minimum) - 535 mm option 2 (in accordance with building regulations) - 662 mm So where is the stated 380 mm thickness? But we will go further.

When calculating the required thickness of the walls it is also necessary to take into account heat loss in the process laying the blocks. In most cases, blocks are placed on conventional cement and sand mortar, which in turn reduces heat resistance of the structure by 25%. If the blocks are placed on the recommended special thin layer (3-5 mm) adhesive solution, the heat loss increases by about 10%. After accounting for masonry joints we have the following wall thickness: Option 1 -  588 mm, Option 2 - 827 mm. Now for the next step. We recall from paragraph 1 that in the cellular block masonry there are so called "cold bridges", i.e. joints, padding support, lacing. Different estimates provide that they decrease heat resistance of the masonry by 10 to 30%. As a result, we get the final wall thickness: In the minimal Option 1 it should be 647mm, and in the maximum Option 2 it equals 1072 mm (over one meter!!!)

Wall thickness YOU need lies between 64 cm and 1.07 m.

That is, of course, in accordance with current SNiP's and GOST's. If you are a private developer, you may make your walls thinner, but then you will have to additionally heat the atmosphere and make your own invaluable contribution to the greenhouse effect, that is your right! But in that case why do sellers of aerated concrete lie about the "warmness" of the material?

During the design, construction and state approval of buildings designers, customers and contractors cannot afford such wall thickness. As a result, in professional construction aerated concrete blocks are used solely for walling, and their wonderful properties of "thermal insulation" and "high sustaining capability" objectively and reasonably remain unused.

That is why the loud claims made by aerated concrete manufacturers about "high thermal insulation" properties are nothing but a MYTH.

3. High frost resistance and vapor permeability.

Frost resistance tests are being conducted in order to recommend use of unprotected aerated concrete on building facades. But let us look again at the properties, where stated frost resistance of the brand D500 equals 25 cycles (F25). We should not forget about humidity, which reduces thermal resistance. Aerated concrete is a strong moisture-absorbing agent, that is, it absorbs moisture from its surroundings very fast. What can be done if unprotected aerated concrete simply sucks in precipitation? Moreover, its moisture level by weight can reach 35%, which in turn will dramatically reduce its thermal resistance and the properties stated by manufacturer will simply disappear. The house will be cold. To assure that aerated concrete does not absorb moisture, vapor barrier needs to be created on the inside of the building. To achieve that, it is enough for the wall to be primed (deep penetration primer curbs vapor permeability of materials) and glazed, which is usually done. The only thing that should not be allowed is glazing without the use of primer and/or wallpaper - this traditional procedure leads to accumulation of moisture in aerated concrete blocks due to humidity inside the building and (due to linear deformation, expansion of residual lime) detaches finishing materials in a short time. It is necessary to at least make the surface of the facade water-repellent, and it must be done periodically - once every two to three years. Waterproofing prevents atmospheric moisture from quickly soaking into aerated concrete and, being a vapor-permeable, allows water vapor to withdraw from the wall to the atmosphere. Many people build walls out of aerated concrete and then lay brick over it. This should be done with caution. Brick has poor vapor permeability (vapor passes mostly through masonry joints). Therefore a ventilated gap should be left between the brick veneer and the concrete wall, protected from precipitation. But this gap creates a problem of anchoring. How can a layer of veneering bricks be "bound" to the supporting base, so a beautiful wall half-brick thin does not collapse? To ensure this, after every 4-5 rows of bricks special (!!!) plastic or stainless-steel anchors (regular reinforcement can corrode in about 6-8 years) should be placed and attached to the carrier aerated concrete wall. Low density of aerated concrete does not permit use of classic inexpensive hardware. If you do not leave a ventilation gap, there is a high risk of excessive moistening, with all its possible consequences. Maybe you should skip facade decoration. Frost resistance of many modern facade decoration materials must equal at least 50 cycles. Brand D500 does not reach this number, its frost resistance is only 25 cycles, but this registered fact does not prevent the majority "aerated concrete managers" from screaming about 200 cycles... They just keep silent about one thing, being that high frost resistance is achieved only in considerably dense aerated concretes, which are classified as structural and not thermal insulation.

Here is another interesting fact: "Handbook for CNiP" released by Science and Research Institute of Structural Physics of the            State Committee for Construction of the USSR and designed "For engineering and technical staff of research and design organizations". 1.1. ... when developing projects of walling constructions one should prefer options that while meeting regulatory requirements ensure reduction of energy and material resources 1.6. To prevent excessive moistening of external walling materials, it is recommended to add layers of high vapor resistance on the inside. 1.7. When constructing walls for rooms with high humidity levels, it is not recommended to use sand-lime brick, hollow stones, cellular concrete, wood, fiberboard, and other materials with low water- and bio-resistance. In addition, cellular concrete is defined as a material with low moisture and bio-resistance. How should we regard statements made by those defending aerated concrete that facade should not be protected,  if science says that even in areas like bathrooms and toilets (rooms with high humidity) laying aerated concrete blocks is NOT RECOMMENDED even inside?

4. Durability.

Manufacturers claim that aerated concrete is durable. But houses built with aerated concrete have appeared fairly recently, so it is not yet possible to argue that aerated concrete is durable. In contrast to the brickwork, which has been used for centuries, aerated concrete has been used in mass construction for only about 40 years, so all claims about its durability are purely theoretical.

5. Low Price.

We have already cited the example of increasing the total cost of construction, if there is a need for mechanical mounting of structures on aerated concrete masonry. Now, here is an example of construction of a gas-silicate house and the amount of money the customer would lose. We will do a technical and economic calculation, comparing aerated concrete masonry 860 mm wide with modern multilayer structures (facade insulation system based on foamed polystyrene) with the same coefficient of thermal insulation. Price of materials (with delivery to the site): *The price is approximate, all other design elements are not taken into account.

Aerated concrete blocks - 1600 rub/sq.m + 400 rub for laying

Cement-sand mortar - 2300 rub/sq.m

Sand-Lime brick - 7 rub/ pcs + 600 rub/sq.m for laying

Facade insulation system 100mm - 1300 rub/sq.m

Silicate-based primer - 75 rub/l

Silicate-based paint - 200 rub/l

1) 1 sq.m of silicate masonry wall, painted outside only with the primer and silicate-based paint, 860 mm thick costs - 2020 rubles

2) 1 sq.m of wall made of 250 mm silica brick masonry + 120mm of insulation system, a total thickness of 380 mm cost - 2100 rubles

As cost calculations show, declared cheapness of aerated concrete masonry when compared with (nominally) more expensive types of finishes is rather questionable. Let us continue to compare them with the help of a calculator. A two-story house with external dimensions of (excluding internal partitions) 10x14 m, when built with aerated concrete, will have internal area of 203 sq.m. A building of the same external dimensions,but built using the insulation system will have internal area of 244 sq.m. Keep in mind, that in real estate trade square meters are what matters. With a very modest price per square meter, on average about $700, if you use aerated concrete, you will lose $28,700 when selling such a cottage!

(*** Note! Pricing is done in late 2005)

So, a summary of what we are not told:

1. Capability of aerated concrete to absorb moisture fast, which dramatically reduces its thermal performance, leading to deformation, which spoils the finish. The only way to avoid this phenomenon is through a costly complex of reasonable engineering measures aimed to protect aerated concrete from excess moistening. Aerated concrete is not recommended for use in rooms with high humidity levels. Hence, its unprotected use on the facade is also strongly cautioned.

2. Claimed high frost resistance is nothing but a cheap commercial trick. Optimal density for use as a construction and insulation material is density of brand D500, whose frost resistance properties do not exceed 25 cycles, while facade decoration materials are required to withstand 50 cycles. Excessively high parameters indicated are characteristic of products with higher density, a fact which sellers of aerated concrete prefer to hold back.

3. Low mechanical durability, which limits the use of conventional fasteners, requiring the customer to buy expensive special fasteners specifically designed for cellular concrete.

4. The declared low cost of aerated concrete blocks turns out to be exaggerated after a comprehensive study, together with the guarantee of durability of the material.

5. If we were to follow the heat resistance regulations established by the State Construction Committee, the thickness of blocks (380mm), claimed by the manufacturers of aerated concrete, is not enough. If the rules are not observed, there will be an increased consumption of energy for heating and air conditioning. If one were to comply with all building regulations, the thickness of the wall should be at least 640 mm, depending on the particular design of a building . It should be noted that the thickness of blocks produced normally goes up to only up to 500 mm.

6. For aerated concrete masonry a monolithic strip foundation is essential in order to prevent shrinkage and the risk of massive cracks in walls.

7. If the SNiP and GOST regulations are to be followed completely when laying aerated concrete blocks, the property price is significantly reduced (by approximately 10-20% depending on configuration) due to reduction of the amount of useful square meters of the internal area of the building.

8. Residual lime in the concrete leads to rapid corrosion of metal elements (lacing, piping, lintels, frame).

Based on the foregoing it can be concluded that the talk of low cost, high thermal insulation capabilities of the walls made of aerated concrete blocks are greatly exaggerated, are of exceptionally intrusive advertising nature and are only good for convincing not only those poorly versed in construction.

Author: Gennady Emelyanov

Based on the materials at www.wdvs.ru