aircrete vs. ferrocement
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So I want to build a dome house (in a tropical climate), where sun, lots of rain, humidity, insects and earthquakes are to be considered.
I really like the aircrete idea, however some local builders pointed out to me that ferrocement
with a rebar structure would be more suitable. Offering both, better stability and flexibility of the structure, and it´s much better tested in terms of durability. Those domes can carry a lot of weight...Plus, there is always the risk that the aircrete blocks are not done the right way and you have no way to test their quality...
Any thoughts on that?
Thanks -
Great post. Great questions. I'm glad to hear that you are questioning standard building practices - thinking outside the box - thinking for yourself.
You want to build a dome. Domes appear to be good housing options for many reasons. Being essentially the strongest possible habitable type geometric structure, we have the option of using materials that don't need to be as strong as those used for making inherently weak geometric structures. Is a ferrocement dome of the same shape and wall thickness stronger than a aircrete dome? Absolutely. If the goal is to live in an above ground bomb shelter, then ferrocement is the better choice. And while my neighbor lives in his ferro-cement bomb shelter, I'll use the same budget and build 10 very safe, comfortable, aircrete domes of the same size. Ferrocement is definitely "much better tested" than aircrete in the sense that it is the popular building material. It is not better tested in the sense that it has not been tested as well - the consumer-industrial-capitalistic society tends to promote, research, and test those products that have the most profit making potential. Aircrete is to the building industrial complex, what herbs are to the medical industrial complex. Those who choose to build with aircrete are generally those who are not afraid to make choices for themselves - observe, orient, decide, act. Many others choose what is considered "well tested" - observe others, act.
Concrete can be poured the wrong way also - I've done it. While it's true that there are volumes of detailed testing methods prescribed by the American Society for Testing and Materials (ASTM) whose instructions are meticulously followed by well paid scientists in white lab coats inside of multi-million dollar ferro-cement laboratories, no sample of concrete from your project will get tested there. In reality, we test building materials by feeling it, looking at it, standing on it, jumping on it, hitting it with hammers and cutting it with saws.
Those are some of my thoughts on this topic.
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Zander, thanks for your reply. This is very interesting to learn and no, I definitely don't need a bomb shelter
But... can you walk on an aircrete dome? Or install a roof structure like on Steve Areen's dome? -
@claudettn We walk on our domes frequently. We've not attempted to add a roof structure like Steve's. Depending on the mass of the roof structure, the dome needs to be made with appropriately thick walls and shape (see catenary dome) to accommodate. Our typical domes use 3.5" thick blocks. I think for now our focus is on creating affordable, practical, reasonable living spaces. But there is lots of room out there for exploring even more artistic, creative endeavors. I would suggest building a small simple dome first, then another larger one, try connecting them, then play with more advanced concepts like roof structures and so on.
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@claudettn said
So I want to build a dome house (in a tropical climate), where sun, lots of rain, humidity, insects and earthquakes are to be considered.
Regarding sun: Aircrete is very insulative, ferrocement is not insulative. Most ferrocement domes are coated with a thick layer of spray foam insulation.
Regarding rain, humidity, and insects: Aircrete and ferrocement perform the same in this arena. Both are cementitious materials that are not affected by water, moisture, or insects and do not support the growth of molds.
Regarding earthquakes: My personal vote goes toward aircrete, though there may be room for some debate. Aircrete is well known for it's inherent flexibility. An aircrete dome wrapped with a polyesther fabric shell has amazing resilience to seismic movement. The dome complex I am most familiar with is near the epicenter of Hawaii's recent 6.9 earthquake, which was preceded and followed by literally hundreds of smaller quakes in the surrounding few weeks. During that period 4 and 5 degree quakes were considered normal daily occurrences, with 2 and 3 degree quakes being experienced up to several times per hour. As a result, some cracks (up to 1mm wide and up to 24 inches long) can be observed in the dome structure above the raw cutouts for the planned windows and doors. To be precise, I observed one such crack above each of these raw cutouts. The one window that was finished has no cracks whatsoever. Regardless of how earthquake proof a structure may be, there are no rules on how strong an earthquake may be. In a catastrophic event where a structure is overcome by an earthquake, I'd rather be in an aircrete structure because chunks of falling aircrete will hurt, like ouch!, but falling chunks of concrete sever, crush, and splatter.
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Here is a great forum post about how the same concrete is made differently by various concrete companies. This post questions the common notion that concrete is strictly regulated. It questions the validity/usefulness of concrete "PSI ratings". To me this post supports my earlier statement that "in reality, we test building materials [including concrete] by feeling it, looking at it, standing on it, jumping on it, hitting it with hammers and cutting it with saws [or jackhammers]".
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Is Aircrete strong enough, can you walk in it..
Yes..
If my entire 24' dome was the wet mixing weight it would be 55,800 pounds. Divided over the area of the
8" thick dome wall is only 7.5 pounds per square inch.. Now cure out the water weight and consider that my test samples cure out stronger than 160 psi at 30 days... Yeah it's strong enough to walk on and not going to fall over...Now considering the tensile strength of the fabric at the equator.. Apoc 482S is rated 35 psi.. The circumference is 904 inches. The total tensile strength is 31,667 pounds. Using g a fabric rated 90 PSI would yeild 81360 points of tensile strength. Now how much weight of the structure transfers through via compression and how much stress is actually put on the fabric... I have no idea...
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I built my home with earthbag walls and intersecting ferrocement catenary vaults for the roof. I was hoping that I would have enough thermal mass for the structure to perform like an earthship. The mass warms from an attached greenhouse as the only heat source even with winters that can reach 30 below F. Mine doesn’t but earthship regularly do-
I’m hoping to cover the 1,200 sq foot multiple-domed roof with aircrete. I didn’t want to use foam because it can burn and has been implicated in causing heart attacks among first responders and occupants, due to the cyanide gas released when foam insulation burns.
Also, there is no fire district where I live so I’ve built with being fireproof in mind.
My question regards how well I can make aircrete blocks adhere to the surface of the ferrocement, which is coated with elastomeric paint, and whether aircrete is really waterproof. I read on the site that it and ferrocement are waterproof, which of course is incorrect- Portland cement is hygroscopic unless a waterproofing additive is used, which I have with the ferrocement. Otherwise, even concrete acts as a sponge to hold water and corrode any metal reinforcement as well as require deeper foundations where it freezes- because the trapped water will freeze and crack the concrete or cause spalling on roofs. Before Portland cement there was lime based mortars and old world homes built with stones and lime didn’t need to be concerned about water freezing inside the structural components because lime releases water, as opposed to absorbs. At any rate, I understand that the aircrete can support a man’s weight, I’m just wondering if an ideal solution would be a thin ferrocement coat over the aircrete- I’m hoping to have a stairway to the roof as the views are great, so having a sturdy roof seems very practical.
Lastly, I’ve heard of people using air entraining agents in cement to be able to pour at colder temperatures. Is it advisable to mix aircrete when it is below freezing? Thanks for any input. -
Here's a conversation about making aircrete in cold temperatures.
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Aircrete is not waterproof - it does wick water to a certain degree. It has been demonstrated to float for years, which has created some confusion about being waterproof. It is mentioned on the site as being "waterproof, fireproof, termite proof, rot proof" which is to say that water doesn't hurt it, as compared to say drywall and particle board. The statement could be misleading for some, but I think it's not intended to be.
Perhaps a waterproofing agent could be added to aircrete. Might be worth an experiment.
I don't think that water absorbed into aircrete would cause freeze cracking. This is suggested by the fact that it floats for years - it continues to contain enough air to remain flexible against freezing. These videos furhter support the notion:
and -
Randy and Kathy Johnson have had great success with their pure aircrete patio, but overall I think it's best to top an aircrete walking surface with a 1/2 to 1 inch coat of concrete. I did one recently using coarse gravel in the top coat. If I were to do it again I would use fine gravel or sand only so that I could keep the thickness down to 1/2" or so.
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Another issue... it seems that thermal insulation is poor with brick concrete.
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there is always the risk that the aircrete blocks are not done the right way and you have no way to test their quality...
ASTM (pka American Society for Testing and Materials) has fully addressed aircrete, referring to it as Cellular Concrete.
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You are right in what you are saying. You should try to build according to your code if you are building in a code requirement area, which is virtually most of the USA now. Regardless what other people might try to imply, testing is neither difficult or expensive. You can test the strenght of your aircrete or regular concrete and is around maybe 100 dollars for sampling, which includes about 5 cylinder samples. When I did mine, they were around 75 per 5 cylinder sample, and that included the technician going to the site and gathering the sample before the pour, and doing a test ON the concrete that was going to be poured to predict if it would possibly pass the PSI of concrete ordered. You could collect your own samples, as the cylinders are readily available, and then bring them to any engineering testing firm and that would save you money. Aircrete, or what they want to call aircrete in this page is in reality cellular concrete. This have been discussed before in this forum. Real aircrete expansion is by a chemical ( using aluminum powders) mean and then you need an autoclave to hot cure it for hours, which give ultimate tensile strenght. Domegaia concrete is not commercial aircrete but cellular concrete. Its strenght in compression is good, but its tensile strenght not so much, so you would need something to give that strenght. It could be rebar , metal lath or even better basalt rebar, which is expensive but is mineral and have higher tensile strenght than metal. You could also consider a thin layer of magnesium concrete, which is much stronger than any other concrete and can be laid out thin, but it is expensive, and then over that layer put basalt rebar and then aircrete. Many ways to do it, but the cellular concrete by itself is not too strong by itself for a building that needs coding. Minimum compression strengh of a concrete building in most codes is 1500 psi for walls and 2500 for a concrete rebar ceiling. PSI in the hundreds for a building is not much different than mortered foam blocks. The ideal would be a middle point, using the magnesium concrete and basalt rebar to hold the tensile strengh, and the celular concrete to give insulation. There is also basalt fiber that is sold in mats or just the fibers. That could also be added to celular concrete for even more insulation. Even another solution would be to buy a concrete inflatable dome cheap in alibaba from a chinese suppliers. They use them for fairs and displays. Then use that filled with air as a holding structure, and using regular concrete by hand in a thin layer, or maybe shotcrete or gunite and rebar. This will also give you a stronger shell that can be worked over with celular concrete. Then at the end good coating of sealing paint to make it weatherproof. Whichever way you choose good luck.
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@HandyDan How many bags of cement did you use for the 24' dome?
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@Jaay 250 Bags due to the thickness of the builds.
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Hi Zander,
It's been a while since I worked with concrete in a modern fast track environment but I do remember we used to test our concrete with every delivery.
Most frequently using a slump gauge, see - https://www.sitebox.ltd.uk/concrete-testing-slump-cone-osc8100?paid=googlepaidproducts&gclid=EAIaIQobChMIj9yCwPqq7gIVRuR3Ch0cCAjmEAYYAiABEgKEM_D_BwE
That page also has moulds for casting test blocks. Which can then be tested to destruction in various ways. E.g. crushing to test compressibility, I've also seen people keep them in sealed containers of water for several months, and then cut them to see how far it has penetrated.
I dare say there are more tests you can do ...
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@HandyDan In regards to using air Crete for a floor, what reinforcing would you use and how thick would you make it. My current application would be a base for patio pavers but I’m also interested in using it for the floor elsewhere.
Thanks! -
Referenced by
Zander
