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Old 05-08-2014, 03:55 AM
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Default Concrete Calculations

Hi folks,

I'm trying to work out the weight/volume of ingredients I'd need for a given volume of concrete. I presume the volume of concrete you end up with is not the combined volume of the ingredients?

Edit:
I know the volume of the casts...

Do I just assume a certain density for the finished concrete and thus calculate the finished weight and then assume that the combined weight of the ingredients should equal this weight?

Last edited by di11on; 05-08-2014 at 04:12 AM.
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Old 05-08-2014, 05:46 AM
Tscarborough's Avatar
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Default Re: Concrete Calculations

The final volume is not the combined volumes of the ingredients, but the mass IS the combined mass.

Are you talking regular concrete or refractory castable?
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Old 05-08-2014, 05:54 AM
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Default Re: Concrete Calculations

Quote:
Originally Posted by Tscarborough View Post
The final volume is not the combined volumes of the ingredients, but the mass IS the combined mass.

Are you talking regular concrete or refractory castable?
I'm not sure what I'm talking about yet... I'm trying to get an idea of how much of what I'd need for the various options... since I'm going for a small oven, I don't think the cost difference will be much.

I would probably go with a refractory castable if I could find one here in France. Identification and availability of the materials may determine what I use in the end. I am currently looking at a mix of sand, clay, ciment fondu and crushed brick. I've read good things about this mix in France... assuming I can overcome the fast set time issue. Since it is a small build, I might get away with it if I have extra hands.

So I guess I just need an idea of the resultant concrete's density, then I can work out the final weight and thus the weight of the individual components I need.
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Old 05-08-2014, 03:05 PM
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Default Re: Concrete Calculations

Quote:
Originally Posted by di11on View Post
Hi folks,

I'm trying to work out the weight/volume of ingredients I'd need for a given volume of concrete. I presume the volume of concrete you end up with is not the combined volume of the ingredients?

Edit:
I know the volume of the casts...

Do I just assume a certain density for the finished concrete and thus calculate the finished weight and then assume that the combined weight of the ingredients should equal this weight?
Wet volume is approximately 80% of the dry material
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Old 05-08-2014, 04:44 PM
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Default Re: Concrete Calculations

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Originally Posted by Toomulla View Post
Wet volume is approximately 80% of the dry material
Thanks for that - using that formula says I need about 10% more than using my assumed density formula - I hadn't account for any error, so that gives a good number to work with.
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Old 05-08-2014, 04:58 PM
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Default Re: Concrete Calculations

The density of castable refractory also varies depending on brands and what type you get. Go for dense castable not insulating castable for the inner dome. The stuff I use is approx 2 Kg/litre or quoted as 2000 Kg/m3 Check the specs on the stuff you are getting.
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Old 05-08-2014, 05:34 PM
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Default Re: Concrete Calculations

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Originally Posted by david s View Post
The density of castable refractory also varies depending on brands and what type you get. Go for dense castable not insulating castable for the inner dome. The stuff I use is approx 2 Kg/litre or quoted as 2000 Kg/m3 Check the specs on the stuff you are getting.
Hmm... I was gonna go with this stuff... CIMENT FONDUģ but it says it has a bulk density of 1100kg/m3
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Old 05-15-2014, 03:55 AM
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Default Re: Concrete Calculations

Quote:
Originally Posted by Toomulla View Post
Wet volume is approximately 80% of the dry material
Just looking into this a bit more...

In most of the forums I've read that you multiply the desired volume by 1.54 (or 1.64 if allowing for 10% wastage). But I guess this is for regular portland based concrete. 80% as you quote would be equivalent to multiplying your desired volume by 1.25 which is quite a bit lower. Is the 80% figure specific to refractory cement?

My calculations show that I will need just less than or just more than one 50kg bag of grog (depending on which calculation you go with), so I'm wondering if I need to get a second 50kg bag or not (costing me an extra €75). What would the impact be of less grog and more sand? Less thermal mass? Weaker structure? At the same time, an advantage of getting a second bag of grog is that I would get a different granularity giving me a better distribution.

Thanks in advance
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Old 05-15-2014, 03:41 PM
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Default Re: Concrete Calculations

When I mixed concrete in a mixer on site rather then ready mixed the accepted ratio was 21= 18 which is about 85% or a 15% reduction this mix has aggregate so the mixing of a brew with more fine aggregate May result in less volume.

My experience is that more material is consumed by wastage and additional volume then measured , having poured many concrete slabs experience is that you always use more then the theoretical calculation so be conservative
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Old 05-15-2014, 04:53 PM
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Default Re: Concrete Calculations

Calculate the volume of your dome (using outside radius to calculate total volume, subtract inside volume using inside radius) get the volume required, but don't forget to halve this because it's a hemisphere, then add the 20% for reduction on mixing. The extra required where the oven door sticks out at the top will be about the same as what you lose in the actual door opening.if you also want to cast an entry you'll need more castable for this too.
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