Autoclaved aerated concrete AAC, Aerated Cellular Concrete ACC,
Autoclaved Lightweight Concrete ALC. This precast, porous, foam concrete building material goes by
many different names. Its main ingredients are Cement which is a mixture of silicates and oxides
Lime which is calcium oxide and/ or calcium hydroxide
Gypsum anhydrite which is anhydrous calcium sulfate CaSO4
Finely ground sand or silicon dioxide SiO2 Or fly ash
Aluminium powder Water
Calcium oxide reacts with water in an exothermic reaction to form Calcium hydroxide and 65.2
kj/mol. Calcium hydroxide reacts with aluminum powder and water to generate hydrogen gas
and cause the mixture to expand like bread dough.
Finally, silicon reacts with calcium hydroxide to form tobermorite
or hydrated calcium silicate C5S6H5. AAC products are cured under heat and pressure in
an autoclave. Steam is fed into the autoclave at a high pressure of 800kPa and a temperature of 180C
or 350F. Steam curing allows the concrete block to retain its porous form and harden in just 8-12
hours, unlike normal air curing that can shrink materials and take longer. AAC is a very versatile
product that can be formed into solid blocks, cored blocks and U blocks. They can also be formed
into floor and roof panels, exterior wall panels, thinner curved or straight partition panels,
lintels and cladding panels. Installation is very similar to traditional masonry construction.
Cured blocks or panels of AAC are joined with thin bed mortar. Hebel,
one of the biggest manufacturers of AAC, sent me these samples for an unpaid, unbiased review.
AAC has millions of small air pores. In fact, it’s 80% air. It is 5 times lighter than traditional
concrete and has a very low density of 300 kg/m3 or 20 pounds/ft3. Its weight reduces
transportation and labor costs, and increases assembly time on site. AAC doesn’t compress,
decay or lose its shape over time. It has excellent dimensional stability
despite its porous nature. It is light enough to float in water.
Air is a poor conductor, so the closed cellular structure and high thermal mass of AAC makes it
an excellent insulator. An 8 inch thick AAC wall performs like a 2x6 wood stud frame wall with
R20 fiberglass batt insulation or an 8 inch CMU block wall with R8.6 rigid insulation.
AAC can provide up to 35% energy savings in both hot and cold climates. However,
it doesn’t provide all the insulation a building needs to comply with building codes,
so additional continuous insulation may need to be added to the exterior of the building.
Because it is so lightweight, its strength is 1/6 to 1/3 that of traditional concrete blocks and it
has a lower thermal mass. It is still strong enough to be used on floors, roofs and walls.
It has an allowable shear stress of 8-22 psi and compressive strength of 300-900 psi.
Conventional concrete has a shear stress of 40 psi and compressive strength of 1500 psi.
AAC is also quite brittle and requires special care during handling and
transporting to avoid breakage. AAC has an excellent fire rating,
as expected from a cementitious product.
It can withstand fire exposure up to 4 hours, without charring or losing its
structural integrity. They do not emit toxic fumes even under intense heat.
It makes me wonder why we are still constructing wooden stick built homes
in fire-prone areas like California. To measure the spread of heat, I bought this FLIR one thermal
imaging device on Amazon. I’ll link it in the description below. It plugs into your
iphone or android and allows you to measure the temperature of surrounding surfaces.
Even when the hottest part of the block was over 150 degrees Fahrenheit,
the area around it was cool to touch.
Since aerated concrete has a porous structure and high surface mass,
it is a great acoustic insulation and soundproofing material. It can dampen
mechanical energy vibration and reduce sound transmission from the exterior to interior
and room to room. It has an stc rating of 40 for 4 inch thickness and 45 for 8 inch thickness.
Hebel’s Sound Barriers can be used along roads, freeways and rails to reduce noise levels. They
can even be routed with custom designs. When aerated concrete comes in contact with
water, the tiny pores are immediately filled and the surface is discolored.
Since it is vapor open, it dries out pretty quickly and water doesn’t pass through the
brick reducing the possibility of condensation inside the wall. Unlike conventional concrete,
this can’t be used as a finish since it is porous. It must be covered with a finish;
thin brick, stucco, stone, metal, wood cement, or vinyl. If aerated concrete is used for basements,
the outer face should be coated with a waterproof material
since soil can quickly break it down. It’s inorganic and not a food source which makes
it termite, mildew and mold-resistant. Due to mass production in controlled factory
environments, all the blocks are of uniform quality and identical size. This speeds up
assembly on job sites and reduces waste. Routing electrical and plumbing lines is relatively
easy. It can also be easily cut, drilled or grooved with manual or power tools.
AAC buildings have been successful in seismically active zones like Japan. The Kobe earthquake
that struck in January 1995, measured 7.2 on the Richter scale. Out of 5578 AAC buildings in Kobe,
none were destroyed. They even survived the subsequent fires, while timber-framed
houses around them collapsed and burned. One of the drawbacks is the learning curve
associated with using AAC. Masons who are used to traditional concrete have pushed back on switching
to AAC. Also while you can screw and nail into aac blocks, the attachment is not as strong.
Screws can strip out, nails can twist., so it is recommended to use plastic anchors.
Hebel has designed special, large-headed, square shank cut nails with better holding power.
Aerated concrete is considered to be environmentally-friendly
even though the autoclaving process requires significant amounts of energy and water.
It is recyclable, non-toxic, long-lasting and has lower-embodied carbon levels compared to
traditional concrete. We have to weigh the pros and cons when using concrete.
Yes, cement can be destructive to the planet, but concrete structures are long lasting,
can lower energy consumption and can withstand natural disasters.
It is pretty popular in Europe and Asia, but it has struggled to gain popularity in North America
because it had trouble competing with cheap stick building which is about 2.5% cheaper. However,
the spike in lumber prices, floods, hurricanes, forest fires and termite damage is causing the
industry to finally look for better alternatives like AAC. Hebel opened up a factory in the US
in the 90s, but they struggled to gain a foothold in the market. They had to shut
down their manufacturing plant and move to Neuvo Leon in Mexico where labor and operation costs
are lower. Fortunately, they are hoping to open another factory in the States again soon, which
will reduce transportation costs of the material. I would love to see this material gain popularity
in North America not just as highway partitions, but as a building material for homes.
Let me know what you think about aerated concrete in the comments below and also let
me know if I should make a dedicated video on this awesome flir one thermal imaging camera.
Don’t forget to hit the like button and subscribe to my channel. I’ll also link my Patreon page in
the description, if you can support me I’d really appreciate it. Thanks for watching. See ya.