# dry mix concrete - how many bags?



## RogerS (28 Nov 2006)

I need to concrete a small area and so will use bags of dry mix concrete. Does anyone know how I can convert from required concrete volume to dry weight in the bags?


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## StevieB (28 Nov 2006)

Guess  Actually that is all you usually can do since the bag mix is a powder and bears no relation to the volume once you have added water to make the mix. It should say on the bag what volume it will make up to and its never as much as you think in my experience. :roll: 

Steve.


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## RogerS (28 Nov 2006)

Mmmm..then I'd end up taking the excess bags to the tip  

I did find this formula on the web but couldn't make the leap to a bag of ready-dry-mix but I've just had a lightbulb moment and on the assumption that the mix in the bag is 1:2:4 then I should be able to work backwards 

Estimating Quantities of material needed.

1. Calculate the volume of concrete needed.
2. Estimate the total volume of dry material by multiplying the required volume of concrete by 1.65 to get the total volume of dry loose material needed (this includes 10% extra to compensate for losses).
3. Add the numbers in the volumetric proportion that you will use to get a relative total. This will allow you later to compute fractions of the total needed for each ingredient. (i.e. 1:2:4 = 7).
4. Determine the required volume of cement, sand and gravel by multiplying the total volume of dry material (Step 2) by each components fraction of the total mix volume (Step 3) i.e. the total amount cement needed = volume of dry materials * 1/7.
5. Calculate the number of bags of concrete by dividing the required volume of cement by the unit volume per bag of cement (0.0332 m3 per 50 kg bag of cement or 1 ft3 per 94 lb bag).


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## Argee (28 Nov 2006)

There is a simpler (but slightly less accurate) way, Roger.

Measure the dimensions of the bag. Divide the cubic result into that required in your job. Add 12.5% and hope for the best! I can tell you from experience that you won't be far out. 

Ray.


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## StevieB (28 Nov 2006)

Why can't you stop at step 2 Roger? If its a ready mix then you do not need to know the proportions, just multiply by 1.65 to get the required volume of dry mix. (You can measure a bag to get the volume of each bag) the proportins shouldn't come into it should they? Or am I being daft? :? 

Steve.


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## Adam (28 Nov 2006)

Mix up a single bag? Might cost you less than the cost of over/under ordering?

Adam


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## Jake (28 Nov 2006)

StevieB":2luwme5g said:


> Why can't you stop at step 2 Roger? If its a ready mix then you do not need to know the proportions, just multiply by 1.65 to get the required volume of dry mix. (You can measure a bag to get the volume of each bag) the proportins shouldn't come into it should they? Or am I being daft? :?
> 
> Steve.



If you buy the materials separately, they have a larger total volume than they do when mixed together - that's why the particle sizes are being mixed up in the first place, to fill the gaps where there would otherwise be air (or water, etc). 

In a ready mix bag, the gravel and sand and cement is already mixed so I think the change in volume when mixed with water will be much smaller than when combining separate volumes of sand gravel and cement with water.


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## StevieB (28 Nov 2006)

Jake - sure, I understand that, but in his first post Roger said that he was using bags of dry mix concrete - not bags of ballast and cement. If its ready mixed that is not an issue surely?

Steve.


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## Jake (28 Nov 2006)

I think that means that 1.65 is the wrong number to multiply by - it gives the you the total pre-mixed volume of the three separately.


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## StevieB (28 Nov 2006)

> I think that means that 1.65 is the wrong number to multiply by - it gives the you the total pre-mixed volume of the three separately.



Nope, its exactly right from the formula Roger provided. Although to be honest, the difference between component materials and a ready mix is going to be so small as to be negligeable unless the hole to be filled is an olympic swimming pool   

Steve.


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## Shultzy (28 Nov 2006)

The Wickes site on http://www.wickes.co.uk/content/ebiz/wi ... gil/13.pdf will show you the approx info

"For convenience and to ensure a perfect mix, a bagged Wickes concrete mix is available. This has the correct amount of cement and all-in ballast and all that is necessary is for clean water to be added. Between two and three litres of water will be needed for each bag of concrete mix.

As a rule of thumb, one bag of Wickes concrete mix will cover an area of approximately 0.0125 m3. It is difficult to give exact figures as the sub-base for concrete work, which is normally hardcore or broken bricks, varies so much in shape and has voids to be filled."

NUMBER OF BAGS OF WICKES CONCRETE MIX REQUIRED
Major bags Produces approximately
concrete mix this amount of concrete
15 0.19 cu.m
30 0.38 cu.m
45 0.56 cu.m
80 1.00 cu.m

PS :- Sorry I'm not sure how to tabulate the results


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## RogerS (28 Nov 2006)

Many thanks for all your help guys...The engineer in me wants to calculate (and probably will) all the various suggestions to see how close they all are and where it gets me in reality.

That Wickes data sheet is very helpful (for lots of other things as well). One minor niggle in that they don't say what the weight is of a major bag of concrete which would be very handy when comparing other suppliers bags (such as Travis Perkins).


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## Jake (28 Nov 2006)

StevieB":3vbkhg82 said:


> Nope, its exactly right from the formula Roger provided.



It doesn't matter, but step 4 wouldn't work if that was the case.



> Although to be honest, the difference between component materials and a ready mix is going to be so small as to be negligeable unless the hole to be filled is an olympic swimming pool
> 
> Steve.



20% or something would be my guess. Could be a bag or two depending on what he'd doing, which ooh, three quid or so?


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## RogerS (28 Nov 2006)

Jake":3rge6zze said:


> 20% or something would be my guess. Could be a bag or two depending on what he'd doing, which ooh, three quid or so?



  It's not the cost...it's the getting rid of the unwanted stuff afterwards !! My binmen are reluctant to take concrete


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## jasonB (28 Nov 2006)

From my surveying days:

1.6 tonnes of all in balast to 1cubic meter. 

Assuming a concrete mix of 1:6 by weight (1.6t div 6) = 0.27tonnes of cement which will fill voids and not add to the volume.

So 1cubic meter of drymix will weigh 1.87tonnes

We now come to the age old question - how big are the bags you intend to buy :?: 20kg, 25kg or 40kg if still allowed?

20kg bag = 0.011 cu mtr
25kg bag = 0.013 cu mtr approx wickes yield.

We wont go into bulking which you may get with wickes mix as their aggregate can be damp (separate pack of cement in bag) or if already mixed one would hope the aggregate was dry when they mixed it :wink: 

Jason


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