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I'm building a rock climbing cave in my backyard. The average height is about 4 metres. It will be made out of wood (hardwood sleeper studs, 19mm ply for walls). Floor area roughly 5mx4m. I am not sure what the total weight is yet. I want to put down 4 concrete footings for the corners. I have a bag of cement, two bags of washed sand, and a whole bunch of (what I think is) gravel in a heap in my backyard. It is brown, appears inorganic and the largest pieces are about 2cm. I have not made concrete before but I read that you need 1 part cement to 2 part aggregate to 3 parts sand. The aggregate people use usually look bigger than this gravel I have, but some sources say it's OK, so I want to ask if it's OK to use this to make concrete for footings. It is also pretty sandy so I'm wondering if I could just use this and cement (which I'll need to buy more of). I will run some tests this week to see how the concrete turns out, but more want to know if this will have the strength to hold up as footings for years. I'm located in Melbourne, Australia if that helps. Would really appreciate your thoughts. Thanks.

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  • Just to be sure, you have a bag of cement not concrete mix? While it's certainly possible, here in the US, straight up cement is far more rare at the consumer level than dry concrete mix is. If it's really a bag of concrete mix, all you need to do is add water. Well, and probably a couple of extra bags...
    – FreeMan
    Sep 1 '21 at 14:42
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    Look a lot like ungraded limestone screenings; The small stuff separated from crushed limestone gravel . The gravel is concrete aggregate. You probably need some larger gravel also to mix with it to get best strength. Sep 1 '21 at 15:13
  • the bag says "general purpose cement - conforms to AS3972". Something like this bunnings.com.au/bastion-20kg-general-purpose-cement_p0760330
    – Aralox
    Sep 1 '21 at 22:34
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If you use this as-is, I would certainly cut back or even eliminate the sand as this looks to have a lot of sand-like material in it already. Alternatively you could "screen" this and take out the fine material in favor of the larger stuff.

For your application, however, it doesn't sound like strength is a huge consideration, so you can be a bit imprecise in your formulation since you're not trying to get the highest strength possible out of your mix.

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