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• Concrete Ratio / Slab Calculations help????
• tom13
Member

Hi

Looking to lay a concrete slab in my garage this weekend. The Size is 2m x 3m x around 1.5 – 2 inches deep.

I am looking at getting a bulk bag of ballast (1 tonne) and then mixing cement in. I calculated the volume to be around 0.75 cubic meters. How much ballast and cement will I need for this? I am getting the tonne bulk bag as mentioned but how many bags of cement will I need. Im estimating around 9?

Should I be mixing the ratio 6:1 as well?

Any help appreciated cheers.

redmex
Member

3m x 2m x 0.05 comes to 0.3 of a m cube, 12 bags of cement /cube so go for 4 maybe 5 bags

natrix
Member

What do you want a 1.5 inch deep slab for????

slowoldman
Subscriber

What do you want a 1.5 inch deep slab for????

That’s what I’m thinking. It’s a bit thin.

Looks like redmex is assuming all cement.

What do you want a 1.5 inch deep slab for????

That’s not a slab, that’s a screed

bearnecessities
Subscriber

Maybe the wife was very slender.

redmex
Member

The question was how many bags of cement req’d, as as for the calculator it’s a wee bit out of date, maybe 25 years since cement was 50 kg

tom13
Member

Gave you all the wrong measurements by mistake. Its 5m x 3m x 1.5 – 2 inches thick. It is going to be a base for my garage which is up but the floor is a mixture of slab and compacted gravel/soil so I want a more consistent surface.

wrightyson
Member

“Tonne bags” are roughly 750-800 kg, for that you really want 6 bags ie 150kg of cement in my book.
2.2 tonne to the m3 roughly, that weight will include your cement but not water.
But, as noted above 50mm is no more than a screed, and you most certainly wouldn’t want to be using a 20mm ballast for that, 10mm would be much better.

natrix
Member

If you want a decent base, 2 inches is too thin. See Paving expert guidance as per link above.

40mpg
Subscriber

You want to sort the sub base out first. If it’s a mix of slab and compacted gravel/soil you will get differential movement and cracking. With a 50mm concrete screed on top it will look like crazy paving in no Time, you’re wasting your time and money I’m afraid.

I’d recommend removing whatever is there, lay minimum 150mm compacted type 1 or 2 evenly across the entire area, then a damp proof membrane, then your slab. Ideally minimum 100mm concrete with anti crack reinforcement mesh.

Wrightyson may suggest more, he’s a proper builder, I’m just a bean counter!

wrightyson
Member

You really can’t have a 50mm floor. As above it will crack and not stand any kind of load. You need to get your barrow and shovel out I’m afraid. 100mm of compacted type 1 will do, then some poly then 100mm of concrete. Again as 40mpg says a layer of mesh would also be beneficial, you could “get away” with one sheet (4.8×2.4) by using it nearest the door way where most of the traffic would be. You will honestly regret putting 40 or 50mm down. As for paving expert, you don’t need to trawl through that.

surfer
Member

I’d recommend removing whatever is there, lay minimum 150mm compacted type 1 or 2 evenly across the entire area, then a damp proof membrane, then your slab. Ideally minimum 100mm concrete with anti crack reinforcement mesh.

Agree, my summer house had this (bit thicker concrete slab)

Edit: make sure you put sand over the hardcore before the membrane to avoid puncturing it.

wrightyson
Member

: make sure you put sand over the hardcore before the membrane to avoid puncturing it.

Again good practice but not something that’s necessary with a 1200 poly and the intended use of the floor. In this case it’s more for concrete purposes than waterproofing.

make sure you put sand over the hardcore before the membrane to avoid puncturing it.

This is a blinding idea. 😉

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