When I was a boy we had a field trip to Red Reef Park where all of us little ones got to snorkel along a small reef in Boca Raton Florida. It was so neat to see the diversity of sea life that hovered around these miniature calcium condos that were created by marine invertebrates. Playing around the coral and swimming amongst the fishes left me with a sense of joy and an idea. I asked my builder of a Dad why humans didn’t construct their homes like these little sea creatures did.
I’m sure he found most of my inquiries quite humorous as he quickly responded. He said that humans do build with what he called 'Man made’ coral, which he considered concrete. He showed me a bare concrete block which in my mind held little resemblance to the beautiful curvy coral. This block was exactly that, a block, that was heavy and grey with little beauty. I was quite dissatisfied with man made coral as he put it. Good thing that my child like attention span had me quickly off to Legos or Transformers or something else to give him a respite.
Earlier this year I finally saw what I would consider to be ‘Man’ made coral, AKA Air-Crete. Air-crete is cement where the normal aggregate of sand and rock is replaced with tiny little bubbles. Air-crete works on the principle of Structured Cavity Effect, which I wrote about here. Structured Cavity Effect is how nature can economically create structures that are similar and geometrically self supporting.
Think of thousands of hexagons connected in a beehive or millions of little air pockets in a stand of coral. Self-similar shapes within a repeating pattern creates a domicile that is super stable and repeatable. The brilliance of air-crete is that it uses bubbles to create microcavuels within a medium, cement, that has literally reshaped our world the last 120 years.
For all of concrete's benefits there are some drawbacks that are exasperated in the tropics. For one, aggregate, the sand and stone that give concrete it’s body, volume, is extremely heavy to transport and often destroys our soft clay roads just getting to the site. Also, concrete is so hard that it acts like steel dowel within the soft tropical clay. Costa Rica is seismically active, just meaning the ground shakes, a lot. Well any good boat builder will frown at using a metal dowel to connect two pieces of wood. As their boat shakes the metal dowel will articulate between the wooden pieces widening the hole it’s been put through…..Bad Idea.
Where I live is like living on a boat so I build accordingly. Air-crete is light and more malleable than your standard concrete. I’m sure all the little contractors in the Coral Castles told their clients, ‘Hey, we need to be light, we are always in water and on a shifting sand bar.’ I can relate. The clay in Costa Rica is like plastic in that it changes it’s shape quickly. Engineering structure that can handle such a wet, changeable environment becomes much easier with such a light medium as air-crete.
One of the neatest things about this medium is how it’s made. By taking cement, which is a binder, and mixing it with champagne you get air-crete. The champagne bubblefies the cement into air-crete. Just kidding, no champagne bottles were hurt in the making of these structures. The real way is with a horizontal lying mixing bowl that is injected with air from a compressor at a very specific rate and pressure. The cement gets foamy with bubbles and than hardens with the little cavities taking the place of where the sand and rocks used to go.
Normally cement hardens from a alkaline chemical reaction that lasts anywhere from one day to a month depending on how thick it is. Apparently the middle of the Hoover Dam is still wet because of it’s thickness. Coral also hardens through an alkaline process, a calcification of sorts, with the addition of the aforementioned microcavuels(micro caves.) Most people don’t realize that cement is over 50% calcium by weight so the similarities between air-crete and coral are quite remarkable.
The perfect marriage of air-crete and building comes in the form of domes. This is biomimicry at it’s finest, when humans copy and repeat what nature does implicitly. Domes are essentially strong bubbles, hemispheres, that efficiently distribute mechanical load throughout their surface. By combining curved walls with a convex roof, domes minimize material usage because they have less surface area and don’t need super hard materials. Most of the time we don’t need things as hard as cement (remember the boat analogy) but we do want the longevity that standard concrete provides. Air-Crete is a wonderful building solution because it sets up like concrete without the weight.
Engineers within the sustainable building movement put curves into their structures usually because their building materials necessitate a superior geometry to create strength. Now with air-crete these curves can be expanded upon without any negative ramifications. Also, all the cozy little nooks that are heated from Rocket Mass Heaters will be much more efficient. That’s right, even Cozier Nooks!
This is what gets me excited about air-crete as a oven builder, insulation. This stuff at three inches of thickness is better than any insulation you can purchase. Woohoo, now my Rocket stoves will burn even cleaner and be able to distribute heat only to the thermal mass of the inbuilt furniture. Hot buns are what we want! Usually this mass is right under your bum so that’s a good thing.
On all fronts air-crete seems to be a win win because of it’s great performance and it’s low embodied energy. That’s a fancy way of saying a little can do a lot. I can for see this technology spreading to all areas of the world where low cost housing is a priority. My first Rocket Mass Heater with air-crete will be an excellent test of it’s unsulative qualities. With a goal of 180 F within 20 minutes of burning I will get to see what type of performance my little Coral Temeszcal can do.