I *always* get this wrong. Ive not looked but I have a strong suspicion I've asked about this before.

I just considered a different way to work it out, and want to know why I'll fail this time...

Dumpy bags come in different fill levels etc.

I'm thinking that, rather than trying some sort of calculator, I'll assume the cement doesn't alter the volume, as it goes in the gaps, right?

Same, pretty much, with the water?

So all I need to do is physically measure the footprint of the dumpy bag of all in ballast, and it's fill level, and that's my answer?

I don't need to do anything else, right? Weight, mix, all that is irrelevant. I just need to know the physical volume of the ballast and that will give me the physical volume of the cement, right?

All I want to do is a thin slab. Somthing like 100mm, at about 7 foot by 4 foot. I don't even really mind if it fractures, as it's just for a lean to log store.

A bulk bag is approx 0.5m2 it that helps

Somthing like 100mm, at about 7 foot by 4 foot.

You can't beat a bit of British measurement.

You can't beat a bit of British measurement.

Exactly how I measure

A slab 7ft x 4ft x 0.3ft has a concrete volume with 10% waste allowance of 9.24ft3
The finished concrete density may be around 136lb/ft3
The total weight is in order of 1260lb [573Kg]
With a mix in ratios of 1:2:4 the quantities will be :-
Cement - 180lb [82Kg]
Sand - 360lb [164Kg]
Gravel - 720lb [327Kg]
Added water
Allow a water to cement ratio of 0.45
Water amount 81lb [8gallons] [37Kg]
The compressive strength of the mix should be around 3000lb/in2 [21N/mm2] after 28 days

The 'Paving Expert' site is to concrete what this place is to welding.

Their concrete calculators:

Brilliant website, I bought his book!

The 'Paving Expert' site is to concrete what this place is to welding.

Their concrete calculators:

Calculators

A listing of the various calculators made available throughout the pavingexpert web site

www.pavingexpert.com

so...they talk about everything except concrete work?

If we are talking about the builders bulk bags. they weigh app 750kg, that will make about .42 of a cubic metre of wet concrete by the time water and cement are added.

Bob

If we are talking about the builders bulk bags. they weigh app 750kg, that will make about .42 of a cubic metre of wet concrete by the time water and cement are added.

Bob

the problem with that is consistency though. when i was on hiabs for the big orange DIY shop, we weighed a few bags at random because we were finding the hiabs would do some bags further than others...we were getting readings from 690kg to 940kg for sand and 710kg 890 kg for ballast. the gravel and decorative stone bags seemed pretty consisent at 850kg give or take 10kg either side.

i dont buy from them even though the father in alw has discount there, i prefer a different merchant that are cheaper, quicker to deliver and the drivers are more customer focused from my experience

Have mixed lots of concrete with ballast from bulk bags. Generally works out around 2.3 to 2.4 bags per cubic metre.

I take the view that for an extra few quid its worth making a stronger mix.

You will soon get miffed if the slab starts breaking up over the winter. Not worth the hassle.

the problem with that is consistency though. when i was on hiabs for the big orange DIY shop, we weighed a few bags at random because we were finding the hiabs would do some bags further than others...we were getting readings from 690kg to 940kg for sand and 710kg 890 kg for ballast. the gravel and decorative stone bags seemed pretty consisent at 850kg give or take 10kg either side.

i dont buy from them even though the father in alw has discount there, i prefer a different merchant that are cheaper, quicker to deliver and the drivers are more customer focused from my experience

The merchant we use, uses bigger bags than the sheds for the same price.

The merchant we use, uses bigger bags than the sheds for the same price.

same with me, and they are far more consistent. i bought 2x bulk bags of 20mm cotswold stone a while back and when they were dilivered the hiab had a scale on it, 990kg for the first and 995kg the second...worked out 40 quid cheaper than buying 2x 850kg bags from the orange garden centre

Given up with dumpy bags, we go with dumpy boxes now...

Yes, I know we have the space but our local builders merchant stores stuff outside so they may be 850kg but that's sopping wet...

and if you dont have the space, a bit of self help...

significantly more than a dumpy, and about £20 a load.

Given up with dumpy bags, we go with dumpy boxes now...
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Yes, I know we have the space but our local builders merchant stores stuff outside so they may be 850kg but that's sopping wet...

Great for those with space, and easy access. Everything here is a nightmare, and therefore expensive

If it is for a lean to log store. Wouldn't it be easier to use council slabs. 600x900x50.
We always do bin stores in slabs. Significantly easier than pouring a slab. Usually works out alot cheaper aswell as it is a single day's work.

All I want to do is a thin slab. Somthing like 100mm, at about 7 foot by 4 foot. I don't even really mind if it fractures, as it's just for a lean to log store.

a 7x4 ft x 4" also mean you have to be exact on your height and no tamping or pounding gets you there unless you are using precise measurements and alot of taming

when buying bags make sure you have extra

its between 5-6 bags of cement per bag hardcore mix gravel and sand is different that depends on your required strength and your mixing preference