how many bags of 94lb cement needed approx. to build a 20ft. dia dome?

  • Hi Guys
    I attended a 1 day workshop back in March and left word with one of the trainees about the approximate number of 94 lbs bags of cement is needed for a 20 ft. diameter dome including the floor. We are prepping for the build and want to see how much shelter is needed to store the concrete along with other build materials.


  • Major Contributor

    @mundane9 how thick are you walls and floor going to be?

  • Little Dragon Tamer

    @mundane9 I wish I knew how to upload an excel spreadsheet because I have made one which calculates all the bags, water, soap per cubic inch volume of structure. It's based on the numbers gleaned from DomeGaia sight stats. I am building a 21' diameter Nubian arch dome, think large half of an egg.

    I don't know if this cut past image works so... If I add up my foundation, floor, dome shell, gutters I get 597 cu ft (rounded up) comes to 108+- bags of Portland.
    I will try to show my formulas below.
    [Cubic in ] [ Cubic ft ] [ Bags of ] [ Water gallons ] [Price ]
    ] [ ] [ Portland (94lbs) ] [ per Bag Portland ] [ ]
    [183504.8492 ] [ =B30.0005787037 ] [ =C3/D$2 ] [ =D3E$2 ] [=D3*$F$2 ]

    B3= the cubic inches
    D$2 is 5.5 which is the avg cubic feet for a bag of Portland. I got this off the internet so if anyone can correct me please do. This is a work in process. I still need to make a first batch.
    D3 is the value of =C3/D$2
    E$2 is 7 the gallons of water per bag of Portland
    F2 is $10.03 i.e. your local price of a bag of Portland.

    I know this will be confusing but I don't know how to load my spreadsheet. Hope it will at least give some insight. I will donate the worksheet to the group if someone will show me how.

    The Dome/Entrance arches plus floor comes to 93 bags of Portland. Note this is an exact calculation based on cubic inches. Count on some fudge factor due to wast mistakes etc. I figure a 3" thick floor over compacted clay and 5" thick blocks. I may use poured concrete for the foundation ring.
    Does anyone know what formula of aircrete would be good for a foundation on hard packed clay soil?

  • Thanks a lot Anthony and Kimo. I am still working the floor out as the land is sloped so one end will end up being thicker than the other. We also intend to back fill a lot of it with local a'a and blue rock to 'anchor' the dome somewhat. Because I live on the mountain, it would cost a lot to doze it level. As it stands, the lot was previously dozed but since the land sits on the side of a volcano - getting it water level flat will cost a lot of money and hours.

    Anthony, will I be able to consult you some more once I have more details on the floor and other related stuff?

    thank you so much,

  • @anthonydjones - HI Anthony - a few other questions:

    • what is the thickness of your floor for these estimates/
    • s your dome going to be 21 ft in height also? If not, what is the intended height?

    Thanks much again

  • HI Anthony - I just wanted to clarify that you included the 3 " aircrete floor in the cement calculations (or not).

    thx again

  • Little Dragon Tamer

    @mundane9 Sure, I will share any info I am able. Just note I am no professional but an avid follower and have studied everything I can on this system. I hope to start building soon and will document everything including the mistakes.
    On the floor being level, I have thought about laying "urbanite" in the floor area to take up volumn. That way I could just fill in with aircrete. You could mayby do that on the downhill side to help in the cost of materials. Every little bit counts. The "ubanite" would be free unless you had to pay for transporting it. I have not determined if the Aircrete would properly cement to older cured chunks of concrete. If anyone has tried this please let me know. I love the idea of using some one else's landfill trash or throwaways and building a house with it.

  • Little Dragon Tamer

    @mundane9 Yes. Also I would be covering it with tile/wood or some sealant to make it withstand foot traffic and such.

  • HI Anthony
    I am currently completing a floor using dried bags of concrete with regular cement mortar. So far - it has appeared to hold up. I was told to help adhering/bonding of wet cement to old/cured cement - to either wet the cured concrete surface with water or cement bonding latex. I have used that bonding latex on previous concrete projects and they have held up for 4 years.

  • Little Dragon Tamer

    @mundane9 Thanks I will take it into consideration.

    Anyone out there have info on foundations for there domes? I will be on solid red clay ground. Not fill but native down 20+feet.

  • @anthonydjones The size and depth on your foundation are going to be determined by where you live, and how large of a structure you are building. Generally, if you live in an area that is a cooler climate, that freezes regularly. Your foundation will need to be below the frost level in your area. 30-36 inches down from surface level will keep your foundation from shifting during the freeze and thaw cycles. A typical "footing" for a foundation is 12 inches by 12 inches. This is actually over kill for an air create dome, but will insure that it has a reliable foundation. Hope this helped.

  • Little Dragon Tamer

    @Junior Thank you for confirming my thoughts. That is just about what I have determined. If you take a 1' vertical slice out of the dome profile you get a total weight of 456 lbs. That based on the 19 lbs per cubic feet of the Aircrete mix from DomeGaia. I can determine this with my solid modeling software and setting the material properties. Any thoughts of that with regards to a foundation? Also assuming you have some expertise in that reply?

  • Little Dragon Tamer

    @kimo 5" thick blocks as stated in my comment above.

  • Little Dragon Tamer

    @mundane9 As to height. My design is not a pure spherical dome but more of a Nubian arch. I will have the central compass point for the horizontal dim radius of 10.5'. the vertical radius is offset 3' so it's radius is 13.5' from that 3' offset. Imagine a 13.5' sphere sliced down the middle and slide it 's center over 3'. that is what it will be. I hope this is not confusing but you have your standard pole/chain or what ever you are going to use as a device to set your block and angle. I will make mine adjustable and set each course of blocks with a second "chain" centered on that 3' offset to set my vertical then I just go around with my pole compass device. Really should be simple.

Log in to reply