AAC (autoclaved aerated concrete) also known as PAC (precast aerated concrete) has been used successfully in Europe for over 80 years and its popularity has grown tremendously across the world. This is because the first-class building material – now used in various applications such as internal and external walls, wall-bearing systems and roofing – addresses the key requirements made by today’s construction sector and offers a range of benefits in construction and in environmental protection.
AAC is produced from nearly 100 per cent natural raw material and has a porous composition. To obtain its porosity, around 0.5 kg of aluminium paste per cu m of AAC is used in the manufacturing process. This accounts for just 0.14 per cent of the totally solid raw material used in AAC.
Figure 1 shows the ratio of solid material content in one cubic metre of AAC. What is astonishing is that you get 5 cu m of solid AAC from just 1 cu m of raw material. Producing AAC with a density of 400 kg/cu m, thermal conductivity of 0.10 W/(mK) and compressive strength of 2.5 N/sq mm is simply state of the art.
AAC is usually used in a density range of 400 to 700 kg/cu m and has a compressive strength of 2.5 to 7.5 N/sq mm.
It must be noted that density, compressive strength and thermal conductivity are inter-dependent. Simply put, higher density leads to higher compressive strength, but reduced thermal insulation properties – the lower the density, the lower the compressive strength but the higher the thermal insulation values.
AAC used as blocks or panels offer the same physical properties, with panels of large dimensions requiring to be reinforced. Here, the reinforcement steel is fixed as a mesh inside the panel, which can be produced in lengths of up to 6 m and used for walls or in roof applications. A slab of 6 m span can be produced with a live load of 5.6 kN/sq m.
The good thermal insulation properties of AAC can be attributed to its porous structure. However, AAC is sometimes wrongly compared with sponge and thought to be highly water absorbent. However, on the contrary, AAC has low water absorption characteristics compared to other building materials due to its physical structure (Figure 2). A close look at the bubbles within its composition reveals capillaries that are not straight but sinuous, which works against the capillary action and gives the product its unique characteristic. This is also the reason why an AAC block will never sink if placed in water.
AAC is ranked as an incombustible building material, conforming to category A1 as specified by international standards. It has a fire rating ranging from 30 to 180 minutes, thus meeting the requirements of all fire resistance categories. Using AAC minimises the fire risk. In fact, when exposed to a fire, it has been proven that buildings using AAC have remained in a habitable condition when all others around it were destroyed.
AAC also has great sound-dampening features, which opens up avenues for utilising its structural potential in acoustic applications.
All these properties have been assimilated in AAC over a century of research and development in the quest for an ideal building material – that is, a material that is easy to handle, provides good thermal insulation, is fire resistant, saves natural resources thus being environment-friendly, is available everywhere and that can be produced at a reasonable cost.
The first tests to produce a building material of constant quality that could be mass-produced from sand and lime were carried out at the end of the 19th century. The first patents were issued in 1877 for ‘cooking’ of a sand-lime mortar, which was followed in 1880, by a patent when this blend was autoclaved for the first time thereby forming calcium silicate hydrate. In the next step, the material was aerated by using diluted hydrochloric acid with limestone powder. In 1914, a patent was issued for using aluminium powder to obtain the required porosity. All those inventions were the roots of the basic structure of AAC as we as know it today.
The next milestone in the product’s development was after the First World War, when a shortage of energy in Sweden urged the Swedish government to push for energy-efficient building materials that had high thermal resistance, low energy consumption in production, high durability, and was non-combustible and easy to use. Following this, in 1924, Axel Eriksson received a patent on AAC and in 1929, the first industrial production of AAC started. After the Second World War, a huge demand for building materials in Germany resulted in a shortage of raw materials. Germany then started using AAC that was developed in Sweden.
With major developments in production technology and the need for short erection time, large panels with reinforcement steel were developed for high strength and extended span. The accuracy achieved with block production has surpassed all expectations and the development of different profiles for panels and blocks has resulted in a wide range of applications with the advantages of short erection time and cost savings.