Aerated concrete AC

[edit] What is Aerated concrete ?

Aerated concrete (AC) also called cellular concrete is often used to describe any concrete product that contains significant pockets of air in a controlled manner, usually it is prepared with water, cement, a foaming agent and perhaps a fine light aggregate such as sand, but rarely with aggregates. The foaming agent introduces air bubbles into the finished product, making it lighter and having insulative properties.

When the air content within concrete is about 1.5% - 3% and bubbles are relatively large in size, over around 1mm and randomly located throughout the mix it is referred to as entrapped air, mostly an unintended consequence of mixing concrete, with insufficient agitation. When the air is uniformly distributed in microscopic bubbles of less than 1mm it is referred to as air entrainment. When it makes up between 3%-8% by volume of the finished product it is often done so to improve the performance under freeze thaw conditions. Aerated concrete or cellular concrete however normally refers to concrete products that have a higher content around 25-25% of air bubbles, which may be higher is no structural performance is required.

[edit] A background to aerated concrete (AC)

As early as 1889 Czech Hoffman tested and patented a method of aerating concrete with the use of carbon dioxide, whilst in 1914 Aylsworth and Dyer used aluminium powder and calcium hydroxide to create and patent a porous cementitious mixture. This might be considered as a very early form of aerated concrete, though much of the credit is associated with Johan Axel Eriksson who developed both aerated and autoclaved aerated concrete as described below.

[edit] A background to autoclaved aerated concrete (AAC)

The Architect and inventor Johan Axel Eriksson is generally accepted to be credited with perfecting the technique of producing a limestone and ground slate, lime formula in the early 1920's with Professor Henrik Kreüger at the Royal Institute of Technology in Sweden. They found that the foamed product could withstand the moisture and pressure of autoclaving and that this sped up the curing process without causing shrinkage, improving performance. The technology was patented using ground slate or alum shale and started to be mass produced from the late 30's.

Sometime later in the 1970's it was discovered that natural uranium within the alum shale, caused some radioactive radon gas exposure, which led to a new recipe being developed. This formula contained quartz sand, calcined gypsum, lime(mineral), cement, water and aluminium powder, but no contaminated alum shale and is the base recipe of many aerated concrete blocks that are produced today.

The aluminium powder is a air entraining agent and reacts with the calcium hydroxide formed on hydration of cement to produce hydrogen gas bubbles. The agent is mixed with the a fine aggregate (usually sand or fly ash), cement, lime, gypsum, and water and reacts on hydration creating pockets. Autoclaved aerated concrete (AAC) is essentially the same but once poured and setting, is placed in an autoclave which applies steam and pressure, to speeds up the curing process and create a stronger product.

A further iteration or variation of AAC is RAAC which is Reinforced Autoclaved Aerated Concrete, this is essentially the same product but containing reinforcement bars, to allow AAC to be formed into panels for walls, roofs and floors. It was developed later than AAC, and started to be used widely in countries like the UK from around the 1950's. The lifespan of such products might be shorter than those without reinforcement because of the possibilities of corrosion.

[edit] Related articles on Designing Buildings

• Aircrete.
• Aircrete blocks.
• Alkali-activated binder.
• Alkali-aggregate reaction (AAR).
• Applications, performance characteristics and environmental benefits of alkali-activated binder concretes.
• Blockwork.
• Cellular concrete.
• Fly ash.
• Formwork.
• Precast concrete.
• Self-compacting concrete.
• Smart concrete.
• Types of concrete.
• Types of concrete specification.