Aircrete

1 What is aircrete?
2 A brief history of aircrete
3 What is autoclaved aircrete?
4 Aircrete structures?
5 How is aircrete made?
- 5.1 Testing with aircrete
- 5.2 Aircrete recipe
6 Benefits and barriers of using aircrete
7 Related articles on Designing Buildings
8 External links

[edit] What is aircrete?

Aircrete is often used to describe any concrete product that contains significant pockets of air, usually it is prepared with water, cement, and a foaming agent, sometimes with sand but rarely with aggregates. The foaming agent in the case of aircrete, cellular concrete or foamed concrete as it is often called, the foaming agent is prepared into a foam which is then mixed into the slurry to introduce air bubbles into the finished product, making it lighter and having insulative properties. This is as opposed to other method which introduce air entraining foam agents, such as aluminium powder which are introduced earlier and create bubbles through a chemical reaction with the concrete.

This product might also be referred to as aerated concrete (AC), air entrained concrete (where bubbles are less than about 1mm in size), air entrapped concrete (where bubbles are larger than about 1mm in size), aerated cellular concrete (ACC), aerated lightweight concrete (ALC) or non-autoclaved aerated concrete, as well as aerated concrete blocks, insulation blocks, breeze blocks or lightweight blocks.

[edit] A brief history of aircrete

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.

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.

Aircrete, might generally be considered to be the same process but without the autoclaving, ie it is cured naturally, normally in a sealed plastic sheet. The term aircrete itself is said to have been coined by the organisation DomeGaia much later in 2014, and generally refers to air cured aerated, cellular, foamed, or lightweight concrete. The organisation was set up by the Hajjar Gibran a longtime engineer and design/build contractor, who with Steve Areen, constructed original domed structures in Thailand as housing units. The business is now run by his son and her partner and continues to build across the globe, whilst supporting enthusiasts with other bespoke products such as foam machines and designs.

[edit] What is autoclaved aircrete?

Aircrete might also be used, incorrectly, to describe Autoclaved Aerated Concrete (AAC), this product differs from aerated concrete, non-autoclaved concrete or aircrete in that part of the manufacturing process includes placing the blocks in an autoclave. Autoclaving is the process of heating something using steam and pressure, in the case of AAC Blocks they might be steamed at up to 200 degrees centigrade. Manufactured blocks may also have lime, pulverised fuel ash, or aluminum powder added to the mix.

These elements and processes tend to stabilise the blocks, creating smaller tighter air bubbles and increasing strength and durability against moisture damage in the future. 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 he UK from around the 1950's.

[edit] Aircrete structures?

Aircrete is quite often (but not always) associated more with a DIY philosophy in using the material, often involving on site pours to create cost effective curved structures. Where as (non-autoclaved and autoclaved) aerated concrete might be more associated with manufactured products such as blocks, panels and beams. One of the proponents of the term aircrete in its use to create insitu poured structures is the organisation DomeGaia, founded by Hajjar Gibran. DomeGaia use aircrete as a key product in the design and construction of domed affordable housing solutions and have produced guidance for those wishing to experiment with aircrete, as well as designing and selling foam machines to help individuals produce aircrete themselves.

[edit] How is aircrete made?

[edit] Testing with aircrete

To make an aircrete test on a small scale one can use two buckets, some water, cement (approx. 2 kgs), washing up liquid, a whisk (preferable electric), a little drop of glycerine, a ladle, a knife, some old drinks cartons and a sheet of waste plastic.

Step 1; spoon approximately 100 grams of cement into a bucket, add about half a litre of water and stir to make a slurry.
Step 2; stir well and scrape the base of the bucket to make sure all cement and water are mixed well to a thick liquid.
Step 3; in a separate bucket pour about 15ml of washing-up liquid, the glycerine, and half a litre of water. Using the whisk create a foam, until it is quite stiff and keeps its shape, like whisked egg whites, denser foam makes better aircrete.
Step 4; gently fold the foam into the bucket with the slurry, to create thick foamy slurry mix.
Step 5; mix the cement slurry and the foam together thoroughly, making sure it is mixed to an even grey colour.
Step 6; at the bottom of your foam bucket you will see the last soapy liquid; use your whisk again to create the last bit of foam to add to your aircrete mix, so that all the liquid is foamed and added to the slurry.
Step 7; continue to mix, ensuring the slurry and foam are even, this should be about 3-5 litres of aircrete now.
Step 8; gather empty drinks cartons and cut one side of them off leaving an exposed brick sized mould.
Step 9; ladle the aircrete mix into the moulds until it is level with the cut edge of the carton
Step 10; once filled, place the cartons into a plastic bag or cover with plastic sheet. This is to let them cure in a moist environment and will take two to three days. Depending n the climate you may sprinkle some water on the blocks to help the dry slowly and cure. After a week or so the aircrete blocks should be cured and hardened.

On a slightly larger scale and to simplify numbers the recipe for a 2 litre mix

1 litre of water with 25ml soap (and a drop of glycerine)
1.2 litres of water with 2.25 kg of cement, mixed together as a slurry.

This 2-litre mix should yield about 8-9 litres of aircrete, which should fill a standard 12 litre bucket, this creates approximately one larger block size. Making the foam at this scale usually starts to require a foam machine, and the mixing is better with a paddle mixer.

[edit] Aircrete recipe

There are a many slight variations on aircrete recipes, but a well established one is published by Dome Gaia, which is in US units. Here the same recipe has been adapted by a factor of 1.172, so as to match with a standard 50 kg size bag of cement common to the UK.

50kg cement labelled ‘Portland Cement’, recommended because of quality.
26.6 litres of water mixed with the cement as a slurry
A Stable foam; produced from 22.2 litres of water with 554.5 ml of detergent soap, plus some glycerine if required or a specialised foaming agent. Different detergents tend to produce different foams which can significantly impact the results.
For best results from a dense foam, a foam making machine should be used and an electric paddle mixer.

This recipe should be prepared and mixed in an oversized (210 litre/ 55 gallon) barrel to produce around 191-200 litres of aircrete.

Whilst ingredients and procedures may vary, the general approach is the same, which is to produce a foam that has some self standing strength through fine dense bubble, which helps expand the volume of the mixture, meaning a greater volume of blocks can be made from less ingredients.

[edit] Benefits and barriers of using aircrete

Aircrete has a significant number of benefits over other construction solutions, some of which are listed here;

Efficiency- because aircrete minimises the amount of cement used and its volume is primarily made up of air it is a relatively efficient system to build with.
Ease of use- because it is based on a solid wall build-up with a single consistent material, once the making of that material is mastered it is relatively easy to use with simple details and construction approaches.
Insulative qualities - because of the entrapped / entrained air it has a certain thermal resistance that is better than the solid equivalent and because it is a single material, detail is simplified.
Noise reduction - because it is a massive solid product it has good level of sound dampening, making it suitable for housing.
Lightweight - the above make the product lightweight and easy to handle on site, where using blocks.
Resistance - it is resistant to moisture, temperature changes and and fire. It is also resistant to many pests where many other products are not for example where termites are an issue.
Environmental - Although aircrete does contain cement, which is a high emissions product, the amount required per m3 volume of material is significantly less than other similar solutions. t can also be made without the use of sand, which can have positive impacts in sand mining areas where standard blocks use sand and cement. What's more is alternative recipes can further replace cement with other pozzolans such as fly ash, which is a waste product.
Cost- because of many of the items above in particular its efficiency and that it is comprised of readily available materials it can be significantly cheaper than other alternatives, whilst remaining a flexible material able to produce orthogonal and shaped designs.

There are some barriers to using the material, which mainly lie in mastering the production of the material, its strength, when compared to other materials such as standard concrete, it is lighter than water and floats, which in some circumstance could be problematic.