Concrete mix ratio confusion

[Vernonv]

Hi All,
I've been doing a bit of concreting lately I have notice something that is beginning to disturb me in regard to the ratio's I am using - compared to online calculators (like this http://www.source4me.co.uk/calculate_concrete_mix.htm).

I work on a 1:2:4 ratio, but as I an using "all in" (premixed sand and aggregate), I modified the ratio to 1:6. I measure all the ingredients with buckets - 1 x 5 litre bucket (ice cream container) of cement and 3 x 10 litre buckets of "all in", per mix.

Anyway I always seem to use about half of the cement the calculators are telling me I should be using.

What is going on?
Is my maths wrong?
Are the online calculators wrong (conspiracy by the cement producers)?
Am I doing something wrong?

[echnidna]

A lot of the sand fills the voids between the stones which is throwing your mix ratio out.
I'd use a 1:4 mix, you'll find that'll work out better.

[Vernonv]

Thanks for such a speedy reply Bob.
As I am measuring the ingredients by volume, wouldn't the final volume be the same as if I measured the sand and aggregate separately? - sorry, I'm just trying to understand the issue.

[Ivan in Oz]

I have a 20mm Blue Metal / Sand mix.
The ratio recommended was 5:1
I usually add a little more Cement, Just In case.

So perhaps ends up nearer 4:1

This is for small jobs with no real Structure issues

[echnidna]

If you throughly mix exactly 1 bucket of sand with exactly 1 bucket of stones you will have a total of less than 2 buckets of mix, because the sand fills up the spaces between the stones

The important ratio is the sand/cement ratio as this binds the stones together

[Vernonv]

Thanks guys.
I had some breakfast and my brain decided to kick into gear and i realised what you were getting at Bob ... those 4 buckets of straight gravel and 2 buckets of sand contain a significant amount of "air" (mainly in the 4 buckets of gravel), which my equivalent 6 buckets of "all in" don't (because as you said, the sand fills the voids).

Thanks again ... looks like 4:1 it is and next time I'll wait until after breakfast to start posting.

[MikeT]

Vernon - I'm curious how your previous pours are holding up given you haven't used quite enough cement. I've always been told to add a little more to make it stronger and more workable.

It does seem counter intuitive when you first see mix ratios. How is all that gravel going to be held together by such a little amount of sand and cement? One of those places that "common sense" fails. Incidentally, one figure I remember from study is that spherical balls of the same size will pack leaving 40% air in a given volume - this reduces with irregular angular shapes and reduces further with different sizes. I think this is why gravel (which varies acrous a range) with variable size river sand is good for concrete. The smaller and smaller size grains fill in all the smaller and smaller gaps - held together by the concrete where they all touch (like conglomerate) and there is no clear sheer pathway through the concrete like there would be with, say, single size large and small sizes.

[Barry_White]

I may be wrong but I always thought and used the ratio of 3:2:1 of gravel, sand and cement for concrete by volume be it buckets, cups or shovels using 10 to 20mm gravel.

[Vernonv]

MikeT - I've used that mix for post footings, small pads (less that 1 x 0.5) and a large water tank pad (3.75 x 3.75). I've not noticed anything adverse with that mix, however they are low/no traffic situations with static loads.

Barry - That mix is (from the research I've done) a higher strength mix used for high wear situations (eg truck driveways). I think the 1:2:4 mix is more a general purpose one.

[Ekim]

Vernon,

I'm not sure how it works in NSW but in WA the cement manufacturers show the ratios on their respective cement bags. Forget about on-line calculators and use the manufacturer's specifications.

HTH

Mike

[Terrian]

I may be wrong but I always thought and used the ratio of 3:2:1 of gravel, sand and cement for concrete by volume be it buckets, cups or shovels using 10 to 20mm gravel.

they are the correct measure, the sand fills the gape between the stone, the cement fills the gap between the sand