Application & Construction

Performance of autoclaved aerated concrete

Modern construction material for practical applications

● Phd eng. Katarzyna Łaskawiec, Łukasiewicz Research Network, Institute of Ceramics and Building Materials, Department of Concrete Technology CEBET, Poland MSc eng. Lech Misiewicz, SOLBET Sp. z o.o., Poland

Today, Poland is the largest AAC producer in Europe and when erecting walls AAC is undoubtedly the building material number one [5]. The assortment, properties and quality of the manufactured products are at the highest level. For several years, housing construction in Poland has been dynamically developing and thus more and more building materials have been sold, including, of course, AAC. New plants manufacturing AAC are built and the existing ones are modernized. Additionally, research works are carried out.

Is the assumption that this trend will be continued and the producers of aerated autoclaved concrete can expect an infinite great future true? Although sooner or later there will be a breakdown of the market, each product has its own life cycle. In which phase is the AAC today?

Finding answers to these questions goes far beyond the scope of this paper. Our goal was to try to take the first step in this direction. The authors have collected information about the performance that is declared today as well as about that performance that was previously determined. Which of the properties and for whom are useful during building erection and building use? What information is missing? Due to the limited volume of the paper, in the article the authors concentrated solely on the analysis of wall elements.

AAC as building material

Thanks to its advantages, AAC has an established position on the market. For over a dozen years, it has been the most popular material for making masonry construction in Poland. It is a modern material that does not only facilitate construction but also ensures high energy efficiency of the buildings. Thanks to its properties, it should maintain this position and even increase its market share.

There are many features that make this building material user-friendly. The criteria for each target group are sometimes different but they are also common. For individual investors, the most important is the technical quality of the building, ecology - in the context of building material, energy efficiency - in the context of living in an energy-saving house and the costs associated with construction. Investors and developers value low prices, speed of delivery and fashion as a factor determining the attractiveness of the erected building. Architects pay attention to technical parameters, complementarity of the system and the possibilities of implementing their ideas and visions. In turn, for contractors, factors such as the ease of building, shortening the time of erection and complementarity (the system) are important. These are just some of the features that determine the attractiveness of a given technology and one could probably find and enumerate many others.

It is true that there is no all-purpose material and technology, however, the possibilities offered by AAC are extensive. Thanks to this modern building material energy-efficient and durable constructions can be built in an easy, quick and systemic way. Let’s pose a question: what are the criteria for the assessment of materials and construction products depending on the target group (investors, designers, contractors, users) and the country, region, tradition and experience? For some, the price is still the most important (products, transport, labour, etc.). For others what is important is the fulfilment of the requirements of energy saving and thermal insulation, ergonomics or environmental protection and the sustainable use of natural resources. Security and quality are almost always important. For specialists, the time of investment development, ease of use and systemic technical solutions are meaningful.

AAC products

AAC is a homogeneous material. This means that all physical parameters (e.g. thermal, acoustic, compressive strength) are the same in all directions. This is of great importance for the execution and maintaining wall parameters.

In addition to uniformity, an important feature of AAC is its porosity. The porosity makes it easily machined. Due to the homogeneity and porosity, it is possible to form any elements from AAC at the production and construction stages. The production of profiled blocks for tongues and grooves or locks, or with profiled mounting brackets is simple and the shapes of wall elements significantly facilitate the work on the construction site. Finished products are easy to cut and mill. Even complex shapes of wall elements can be easily obtained. In addition, AAC´s porosity influences many other features. The most important of them is a small volumetric weight and good thermal insulation.

Wall elements

Thanks to low volumetric weight of AAC, large wall elements can be produced. This, in turn, affects the ergonomics and construction economics. The elements can be easily cut, thus a wide range of products is not needed (e.g. corner elements, complementary wall elements, compensating elements, etc.). In Europe, small-sized elements (blocks, U-shapes, plates) are produced, placed on the market and applied on the basis of harmonized EN 771-4 [N1] standard.

Reinforced elements

Lintel beams are an important element of the walls made of AAC, especially with respect to thermal insulation of building in which the external partitions are a single-layer wall without insulation. In Europe, they are marketed on the basis of EN 845-2 [N2] standard. A separate group of products is reinforced panels. They are used for building walls, floors and roofs. Their properties are a combination of good fire protection, sound insulation and thermal insulation. In Europe, they are marketed based on EN 12602 [N3] standard.

Systematically construction

In every respect, the best way to build is to develop buildings systemically. This does not mean building solely with the use of AAC. However, it means that - in addition to the use of different types of products made of AAC - other materials and products are used that are adjusted in their properties to AAC. That is one reason why more and more often AAC systems are used. They contain a range of blocks, tiles, precast lintels and reinforced panels, fittings and specific mortars, as well as other construction chemicals (in the form of plasters, adhesives, etc.). They create transparent and handy systems that enable to apply any desired construction method and meet the applicable requirements without any problems. Systemic building also gives the certainty of correct construction details. Correct execution of details in terms of construction and building physics is the basic condition for flawless building (Fig. 1).

Areas of application

In Poland AAC is used for the construction of any types of walls, practically in all types of both residential (Fig. 2) and non-residential buildings.

An important and often underestimated feature of AAC is its isotropy. In combination with the possibility of easy material treatment, walls with complex shapes can be made. Blocks can be re-laid and joined regardless of the direction of the embedded element. Arch-shaped walls or walls re-walled at an angle other than 90 degrees are not problematic to perform (Fig. 3 and 4). Walls in places of intricate shapes retain the same parameters as in any other place.

Performance of masonry elements

Most of more than 4 million m3 of AAC produced annually in Poland [5, 10] are products of the highest standard, not only in terms of properties but also quality. Tables 1 - 6 present the most important performance in relation to the densities produced [9-12, N1].

The insulating properties of AAC are the result of its porous microstructure generated in the first growing phase of the multi-component concrete mass and its stiffening as a result of lime hydration. In the production of AAC the maintenance of the structure (shape of pores close to spherical) formed during growth, and thus the appropriate selection of raw material composition and external factors accompanying the growth are extremely important. The presence of pores with spherical-like shape and obtaining a proper microstructural structure of AAC, above all a tobermorite with a fibrous structure, will have a positive effect on the formation of its strength. Such porosity will also have a beneficial effect on the AAC’s frost resistance as the pores are not completely saturated with water [13]. Frost resistance is a property that determines the durability of the material. The basic factor determining the frost resistance of AAC in operating conditions is its degree of moisture. Moisture should not exceed 30 % of weight after the autoclaving process. If permanent moisture in the wall construction is too high, damage may occur. The average moisture for partition walls made of AAC is from 2 % to 5 % (stabilized moisture). With such moisture, partition walls made of AAC are characterized by good thermal insulation properties. It is very important that the material from which the partition wall is made is in a sorptive moisture condition (air-dry condition). Possible capillary moisture may only be transient. The water vapour permeability - the coefficient of vapour diffusion resistance [13] - is responsible for the stabilization of the partition wall in terms of moisture. Values of water vapour diffusion resistance coefficient for wall elements can be declared by the manufacturer of products on the basis of tests (Table 4) or taken from Appendix A of the EN 1745 standard [N4].

The raw material composition of the material guarantees its resistance to fungi and moulds. Testing of AAC for susceptibility of occurrence of mould and bacteria performed on it in the simulation of unfavourable, humid tropical climate conditions - i.e. at +25 to +30°C and relative humidity of air from 95 to 98% showed that even under such conditions AAC is completely resistant to bacteria, moulds and fungi [14, 15]. The study of the behaviour of AAC executed in Poland in July 2007 right after the flood showed that the concrete flooded with water containing various organic substances and chemicals, and then free-drying (through airing) is not susceptible to the development of microorganisms (bacteria, fungi, mould). Only single colonies of fungi were found and solely on the surface of masonry elements [13].

The raw material composition as well as the correct process control affect the shrinkage value of AAC. The changes in volume conditioned by moisture changes depend on the microstructure of materials that are formed in the environment of saturated steam in the autoclaving process. It is advisable to obtain a well-crystallized and stable crystalline structure. AAC with amorphous structure easily absorbs and gives off large amounts of water. The result is a large shrinkage and, ultimately, low resistance to crack formation. When designing a structure, it is very important to consider the shrinkage of the AAC elements caused by their drying out. The shrinkage value of AAC - εcs, determined under laboratory conditions EN 680 [N5] in the moisture range from 30 %, i.e. close to the moisture of the masonry immediately after autoclaving, to 2-5 % of mass (and thus the moisture stabilized in the wall) is from 0.09 to 0.40 mm/m. The research [13] shows that the value of shrinkage in the range of moisture from 30 % to 6 % of mass is significantly differentiated depending on the origin of the product (Table 5).

Requirements for masonry elements

Requirements for performance are included in the harmonized standards which define the system of assessment and verification of performance for a given product as well as requirements related to this system. The harmonized standard for AAC masonry elements is EN 771-4 [N1].

In 2004 the harmonized standard EN 771-4 [N1] replaced the Polish standard PN-89/B-06258 [N6]. Table 7 provides a comparison of both standards.

In the former PN standard both the performance and classes, levels and ranges to be met were determined. Methods of testing were also given. Of course, the current harmonized standards for products primarily contain essential characteristics and associated performance that a manufacturer should declare in relation to the specific intended use. However, it is worth considering whether some of the previously determined properties would not be useful today, primarily during design - especially when meeting the basic requirements set out in the Regulation 305/2011 (CPR).

Information about material performance – available/needed

Designer

  • Moisture in a stabilized state (according to EN 772-10)
  • Absorption due to capillary action (EN 772-11)

In the declaration of performance (DoP) producers of masonry elements may or should (depending on the declared use of their products) declare the value of absorption. Water absorption should be determined and declared by the manufacturer for those products that, due to their use, are exposed to external environment in unprotected walls (non-plastered external walls, veneered walls) but also in other walls particularly exposed to strong moisture. Previously, other terms were used to determine the water content in masonry materials: absorbability and moisture. In the currently binding harmonized standards on the properties of masonry units (EN 771) [N1], the term absorbability has been replaced by absorption. Depending on the type of masonry element, the maximum moisture was determined: accumulative and emissive, sorptive, desorptive or stabilized moisture. The degree of moisture affects the thermal conductivity of AAC [14]. Therefore, designers should have access to information about the values of the Fm coefficient allowing the conversion of the thermal conductivity coefficient λ10 to the computational thermal conductivity coefficient λdesing.

Manufacturers should declare the value of their AAC’s shrinkage. In the EN 680 [N5] standard it is recommended that the shrinkage should be referential. Practice shows that these values are small in relation to the total shrinkage value and it might seem that they are safe as it comes to wall’s scratch resistance. However, it is not. Therefore, to provide the designer with more information about the properties of the material, both shrinkage values should be named, i.e. referential and total.

In case of masonry with mortar for thin joints, the actual properties of the mortar (from the constructor’s point of view) do not matter when calculating the compressive strength - the strength of the mortar is not given in the formula for the strength of the masonry (EN 1996-1-1 [N7]). But this is of great importance in the case of bending and shearing. Assuming these strengths based on the wording of EN 1996-1-1 [N7] gives poor results, which in practice very significantly limit the use of AAC for making walls loaded mainly horizontally. Therefore, AAC producers should offer sets, i.e. masonry elements with mortar and for such sets they should declare the appropriate performance.

Contractor

The manufacturer should give the value of maximum mass of the wall element so that the contractor has information whether health and safety regulations are met. The maximum and consistent mass of blocks (manual transport mass in continuous operation, carried by one employee) should not be higher than 30 kg. As a result, one AAC block replaces several ceramic, silicate or concrete elements. Handy wall elements are appreciated by contractors since they allow them carry out construction works in a quick manner. Erection of seven pieces of AAC blocks takes less time than building several dozen smaller wall elements of the same mass from a different material. The low weight of AAC also has a significant impact on the optimization of costs related to logistics. Transportation allows to use means of transport up to their maximum capacity. In practice, this means that one can load a lot more wall materials made of AAC on a truck than, for example, material made from silicates or ceramics. Thanks to this, transport is cheaper and the material is widely available. Wall elements nowadays are characterized by dimensional accuracy and this favours precise and quick building of walls. Precise wall elements can be laid on a thin-layered mortar. Contrary to appearances, building on a thin joint is not difficult. It is enough to follow a couple of rules of such method of bricklaying and with the use of basic tools and work proceeds quickly, efficiently and without errors.

Directions of further development

AAC’s technology undergoes continuous development. So far, this development has mainly consisted in increasing the type of basic raw materials. Scientific and research progress allowed for further improvement of the performance of AAC, primarily as a result of technical progress in production processes. Today the basic density of AAC is 500 or 600 kg/ m3. Thermal conduction coefficients for these densities range from 0.125 to 0.190 W/m∙K. Further development may refer primarily to the microstructure of AAC and will depend on changes in the pore structure [21]. It is influenced, among others, by the grain composition of Al powder and sand. In the case of sand grain composition, fractions less than 100 μm are of great importance. Chemical admixtures should also be used to a greater extent. Manufacturing costs are always of great importance to producers, which, in the case of AAC, are highly impacted by the cost of binder: cement and lime. For this reason, there are experiments to partially replace these binders with mineral additives with pozzolanic properties. The analysis of current research as well as the volume of AAC production in Poland show that the basic density will be 400/500 kg/m3, with thermal conductivity coefficient of 0.09-0.13 W/m∙K. And only a small share will probably go to the production of AAC with a density of 300 kg/m3, whose thermal conductivity will be less than 0.100 W/m∙K. These properties can be obtained primarily by changing the microstructure.

The mortar significantly influences the compressive strength of the masonry wall, both its type (thin-layered/ normal) and the method of laying (over the whole surface/streaked). However, the use of higher mortar classes has no greater impact on strength. The filling of vertical joints with mortar increases the shear strength of the masonry but reduces compressive strength [14]. Therefore, one should expect the declaration of AAC’s performance properties regarding strength and a clearly defined mortar.