Confused by Concrete Calculations

EasyGoingPat

EasyGoingPat

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Hi,

I am a competent DIY-er but am finding it surprisingly difficult to get concrete numbers when trying to estimate how much cement, sand and gravel to buy for my project.

The project is simple: I am extending a pre-existing concrete apron on which the car stands. I already have a good compacted sub-base. The rectangular slab of concrete I am now intending to put on top of this is 0.45 cubic metres at 100mm depth.

How do I translate this into bags of product I need to buy? This breaks down into a number of questions:

1. What ratio of cement/sand/gravel should I use to take the weight of a family car?

2. Should I even use sand and gravel or just buy ballast?

3. How do the answers above translate into bags (say from Jewson)? Confusingly, cement is sold by weight (25kg bags) but ballast is just called a 'Handy Bag', with no indication of roughly what weight or volume this represents.

4. Do I need steel reinforcement grid?

5. Do I need those fibres you can get to add to the mix to reduce the risk of cracking? (I've used concrete of this type before and have bee hugely impressed by its strength.)

I'd appreciate any advice anyone can offer for this project.

Kind wishes - Patrick
 
The confusion arises because you can calculate concrete by volume or by weight. Usually volume is easiest because that's how it will be mixed, but you need to order it by weight if buying the ingredients separately. The calculation isn't difficult but you need to get the concept.

You should definitely get a quote from a volumetric mix wagon - half a cube is a bit small but saves a lot of labour.


1. Ideally I'd go for a ST2 mix, 1:2:4 - this is a strong general purpose mix.

2. Ballast is the same material just with a predefined volumetric mix ratio. It should be good for ST2 on a 1:6 cement:ballast

3. Haven't tested it but this seems ok
https://constructioncalcs.source4me.co.uk/calculators/standard/concrete-mix.php or see below.

4. I would for a car slab.

5. I would not if you've got the steel mesh. It's an either or... That is, you don't need mesh and fibres. Mesh will always provide more strength and good crack resistance/roughness. Whereas fibres will provide excellent crack resistance/toughness but not increase strength. Because strength is better for any sub-base settlement, that's what I'd go for unless you've got the proper heavy equipment to compact the subbase.

Onto the quantities...

Concrete is about 2.4t/m^3.

Volume x 2.4 = 0.45 x 2.4 = 1.08t

There are 7 'parts' to the mix. 1 part cement to 6 parts ballast.
1/7 x 1.08t = 0.154t cement
6/7 x 1.08t = 0.926t ballast

On a 1:6 ratio that's 150kg cement and 926kg ofof ballast.

Edit Forgot to reduce concrete weight for the lack of water. Should use a weight of dry mix around 2.0 to 2.1t/m^3
 
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I did a similar sized slab last year.
1 bulk bag of ballast and 6 small bags. 6 bags cement.

Only get the bulk bag if you've got a wheelbarrow, but it does work out cheaper. (Maybe not if you need to buy a barrow, but job will be easier with a barrow, and you'll need a mixer)
 
EasyGoingPat said:
Hi,

I am a competent DIY-er but am finding it surprisingly difficult to get concrete numbers when trying to estimate how much cement, sand and gravel to buy for my project.

The project is simple: I am extending a pre-existing concrete apron on which the car stands. I already have a good compacted sub-base. The rectangular slab of concrete I am now intending to put on top of this is 0.45 cubic metres at 100mm depth.

How do I translate this into bags of product I need to buy? This breaks down into a number of questions:

1. What ratio of cement/sand/gravel should I use to take the weight of a family car?

2. Should I even use sand and gravel or just buy ballast?

3. How do the answers above translate into bags (say from Jewson)? Confusingly, cement is sold by weight (25kg bags) but ballast is just called a 'Handy Bag', with no indication of roughly what weight or volume this represents.

4. Do I need steel reinforcement grid?

5. Do I need those fibres you can get to add to the mix to reduce the risk of cracking? (I've used concrete of this type before and have bee hugely impressed by its strength.)

I'd appreciate any advice anyone can offer for this project.

Kind wishes - Patrick
Click to expand...
.45 cube is one bulk bag of ballast and 6-7 25kg bags of dust (cement) - plus water.

Ballast is (sharp) sand and gravel.
 
Imagine a bag of ballast is 300mm x 400mm in area at 100mm deep. So your area is x bags wide by y bags long. And one bag of cement for every four sand. You don't use fibres in concrete, and don't need reinforcement.

You need to tamp it well to give it it's strength, and normally lightly soft brush the surface to give slip resistance.
 
You get less in a bulk bag in winter than in summer

I believe bulk bags are done by weight, in winter the ballast is sopping wet


Cement adds nothing to the volume when calculating materials, so it’s the volume of the ballast

Note: ballast contains the correct ratio of lumps and fines so you don’t need anything else but cement added to make concrete
 
1 cement, 2 sand and 4 gravel is not equal to 1 cement and 6 ballast.

The gravel has air gaps between the stones that the sand mostly fits into. The equivalent would be 5 ballast.

I've no idea what grade I mix, but I use 10 shovels of ballast and two shovels of cement per load and it's definitely tough.

Ideally put your mixer on a stand next to the ballast, then you only shovel once into the mixer about a metre away. Then tip into barrow, wheel away and tip out. You can get a lot done if you take the time to set up well.

If delivered in sacks or a bulk bag, cut or tip out into a mound the day before, which saves loads of backache and messing about. If you need more than 2 or 3 bulk bags then it may cost less to get it delivered in a tipper and tipped out onto your drive.

There's no harm in using fibres and steel. The steel displaces a negligible volume of concrete, don't bother subtracting anything.
 
I'm pretty sure fibres and steel are used together in commercial projects. The steel is for strength, the fibres to reduce the chances of shrinkage cracks, though both do both jobs to a degree.
 
Ivor Windybottom said:
I'm pretty sure fibres and steel are used together in commercial projects. The steel is for strength, the fibres to reduce the chances of shrinkage cracks, though both do both jobs to a degree.
Click to expand...
Not in my experience and a steel mesh will comfortably get cracks widths and frequency below spec. I'm sure it's done on occasion but not on the advice of an engineer!

I was frequently asked if we could forgo the steel in favour of just using fibres but that is rarely an option. The steel fibres add a little more tensile strength but usually nothing in the range that means reinforcement can be omitted entirely. The best application of fibres that I've used is in composite metal decking because that only has top mesh for crack control (and reinforcement bars for fire resistance). In that instance it is better to get rid of the mesh... but then it is the composite deck which is performing the role of the main reinforcement.

The literature the concrete firms produce make it sound like a wonder product but fibres are quite limited in what they can offer. I also worry about end of life - you'll have countless ready made microplastics in the demo spoil.
 
EasyGoingPat said:
The rectangular slab of concrete I am now intending to put on top of this is 0.45 cubic metres at 100mm depth
Click to expand...
So you're adding another 4.5 square metres of apron? I ask because the existing would have to be a fair bit larger than this for the end result to realise a usable parking space

EasyGoingPat said:
How do I translate this into bags of product I need to buy?
Click to expand...
You don't. You call a local readymix co that has a mix-on-site, pay-for-what-you-use wagon. They come and pour your half cube, you give em a hundred quid and off they go. It beats paying 80 quid for a tonne of materials, 20 quid to hire a mixer and then messing about for several hours shovelling it all through the mixer
 
bsr said:
@George_engineer are you saying that even the structural fibres... aren't?

I thought poly microfibres were for crack control and the wiggly steel structural macrofibres replaced rebar?
Click to expand...
It really depends on the application. If the bending moments are significant then fibres can't adequately transfer the stresses. You are still limited by the (relatively low) tensile strength of the concrete because each fibre needs to strain on the surrounding concrete to transfer the force to the next fibre. So while fibre reinforced concrete does have a higher tensile strength than plain concrete, it is still a lot less than reinforced concrete. In reinforced concrete the steel bars have a much greater distance to transfer strains.

In ground supported slabs - when the bending moments are usually a lot lower - you can get away with only fibres. But I'd be wary for DIY because you are less likely to have a compacting roller plate and carry out plate loading tests which gives more certainty on the competence of the sub-base. So I see mesh as a more robust solution and what swings it for me is a mesh at 200mm centres provides perfectly good crack control so why not have strength + crack control.

Although to correct my post above ... I did ask a friend and they in their experience they have sometimes omitted the top of layer of mesh and used fibres. They needed to keep the bottom layer of mesh for strength. I've never done it because in my view, why add fibres to 100% of the concrete when you only need crack control in the top layer and you are handling and placing reinforcement anyway. But it does happen and that was OK'd by an engineer.
 
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