# Help calculating Vol to mass for diff concrete types

Discussion in 'Building' started by champagnecharly, 9 May 2015.

1. ### champagnecharly

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There are lots of different types of concrete mixes.. what I would like is the density/ volume of sharp sand, aggregate and cement so given a specific volume and mix type (c1,c2 etc) I can estimate (accurately) the quantities required.
I've searched the net and quantities are always in KG but size of hole is obviously volumetric.
If I'm not mistaken densities are as follows:

Cement 1.4T/ m3
Sharp Sand 1.7 T /m3
Gravel 1.6T/m3
Lime 0.7t/m3 (for mortars)

data taken from http://www.diydata.com/materials/cement/cement.php

But differs to mix estimator by http://www.diynot.com/forums/viewtopic.php?t=33393

However what also confuses me is that the density of concrete is greater at >= 2.2t/m3

Obviously the chemical reaction has a significant part to play in this but how can the we work it out accurately. There must be a formula?

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3. ### RonnyRaygun

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Don't forget the water...

We generally take a mass of 2400kg/m³.

The difference in mass will be negligible as you have quoted dry weights where there is air in the voids. Well compacted concrete should have very few voids as any voids are filled by the cement/water paste.
Hence the mass will usually come out somewhere near 2400kg/m³.

Why is it important to be so accurate?

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4. ### champagnecharly

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Cause i'm tight fisted and lazy, like understanding the science behind and prefer rather sit and do the maths compared to carry extra bags that i have to spend more on...

5. ### RonnyRaygun

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It's not an exact science - prescribed mixes have a minimum cement content and the amount of water added should be sufficient to allow workability but not cause an excessive slump. 70mm is a standard slump, but if you are using pumped concrete more water will need to be added to make the concrete more workable, but then in order to maintain strength more cement needs to be added.

6. ### champagnecharly

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It's not the addition of water and relevant slump that is causing the issue it's the quantifying of dry parts.

7. ### RonnyRaygun

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You need a copy of BS5328 - Parts 1 and 2.

They will give you all the information you need to design a mix.

8. ### champagnecharly

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Thanks.. at £200/ edition for non members.. mmm

9. ### RonnyRaygun

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As I said above, the important thing is to get the cement content and water cement ratio correct - on top of that as a rule of thumb you will have twice as much large aggregate (gravel) compared to small aggregate (sand).

It's hard to help much more than that as I don't know what your concrete is for.

Unless you are mixing a tiny amount of concrete it will be more cost effective to order in bulk bags of all in balast or 3/4 to dust than worry about buying one or two too many bags at a fiver each.

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11. ### champagnecharly

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In this instance it's only for fence posts/steel box section. but I wanted something stronger than the norm as posts are going up around 2.5m of which 0.6 is retaining. Total volume aprox 0.4m3. I also prefer not to use postcrete.

12. ### RonnyRaygun

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In this instance the concrete strength is the least of your worries - more important to ensure that you have the concrete founded on decent enough soil and enough bearing area that the soil isn't going to fail in compression.
Any old concrete mix will be sufficient as a post foundation - concentrate on ensuring you have enough of it.

13. ### champagnecharly

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Thanks.
As i was writing the last post i got a gut feeling that this would be said... Would ST1 be enough?
Should be on dirty chalk & flint at this depth. Would 300mm diameter footings 850 deep (or Aprox 2/3 of height above ground where shorter) be ok? box section is 40mm 3mm wall at 1200 centres.

14. ### champagnecharly

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Actually visualizing 300x850 made me think there shoudlnt be too much of an issue.. well hope not cause i would like to have to get it back out...
I' think my auger is only 200mm would that be wide enough given the depth?

15. ### r896neo

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A 40mm box section isn't going to be much use as a retaining structure and by the time its up to 2.5m it will be very prone to bending unless its only holding up wire.

I don't think the concrete is your possible weak point.

16. ### champagnecharly

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Oh.. Actually my mistake it's 40 x 60.

17. ### RonnyRaygun

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Are you saying your box sections span between your fence posts, horizontally, with no gaps between them? So your fence posts are at 1200mm?
If this is the case I would think they would be OK, although galvanising them wouldn't be a bad idea.
I don't know what the answer would be if I did a calc, and presumably you will have wind load on the fence panels, but as it is only retaining 600mm I wouldn't be overly concerned, especially if it's only retaining soil in a garden?

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