Floor screed: how many bags of sand /cement?

I cannot believe the answer to this isn't somewhere in the archives, but I'll be damned if I can find it!

I'm about to lay a floor screed (approx 3m x 1.8m area) using a 3:1 mix of sharp sand:cement, using a mixer. I can easily come up with the volume of wet mixture I need; but where can I find out the number of bags of cement and sharp sand I need to pick up from Wickes?!!

Cheers David

Reply to
Lobster
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Can you calibrate the mix ...

Based on say a bucket of sand, mix up 3:1 then work out the % of the bucket the result fills. I think it will be less than a bucket ... say

85% full? You can get an approx sand/cement to volume multiplier then.

Approach may or may not be helpful...

Alex.

Reply to
AlexW

How about:

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Reply to
John Rumm

that it seems that if I make some assumptions, eg that my sharp sand will be equyivalent to 10mm aggregate for calculation purposes (is it?!), and if I change my mix from 1:3 to 1:4) and guess my concrete grade(?!) and slump(???!) I can at least get some figures out... all seems awfully overcomplicated though!

David

Reply to
Lobster

Sand has a bulk density of between 1.4 and 1.6 kg/l (T/m^3)As you are making a mortar, the higher value will apply. The cement fills in the interstices, so make no allowance for that or the water. Once you have calculated how many kilos of sand, decide on your mix. Calculating the cement should then be a doddle.

John Schmitt

Reply to
John Schmitt

Thanks John, that's the sort of answer I was hoping for! The figure I calculate is also close enough to the answer I came up with from t'other John's pointer to the pavingexpert site to give me confidence I'm ordering the right amount.

Out of interest, and for my possible future use, what's the rationale behind the following:

Thanks David

Reply to
Lobster

If the sand is dry it will be 1.6 if a bit damp the sand tends to behave like brown sugar and tends to "fluff up", hence 1.4. Wet (saturated)sand has an overall density of 2 because of the water in the voids, however the actual particle packing is almost exactly as for dry sand. I hope this is clear enough.

John Schmitt

Reply to
John Schmitt

I use the volume of the sand. I reason, rightly or wongly, that the sement sits in the gaps in the sand to "glue" it together.

Rick

Reply to
Rick

Most on-site mixing of concretes and mortars is done by volume. The readymix (supplied to site in mixer lorry) tend to use weighbins as they batch from their silos and a water meter to achieve better quality control. If a bad load goes into a big project, it can turn very expensive!

John Schmitt

Reply to
John Schmitt

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