Concrete calculations- base for block built structure

[JohnHamer1977]

Hi all,

I’m building a out building/posh shed out of concrete blocks. It will be about 4m x 3m and about 2.4m high at the front.

I thought I’d get ready mix concrete delivered, but I’ve got no through way to barrow it into the back garden and ground pumping is too expensive so I’ll need to mix my own. I already have a good sub base (I put 10 tonnes of stone down).

Q1. Just to check... I couldn’t just build on 900 x 600 paving slabs could I?

If the above is out of the question, which I’m thinking it will be...

Q2. Can anyone help me work out how many bags of what I’d need to mix my own concrete. And point me in the right direction of where’s best to buy the bags.

thanks in advance - john

 

[^woody^]

No you can't build blocks off slabs

One tonne of ballast will make about 0.6m3 of concrete, so
work out the volume in metres cubed (length x width x depth in meters)
times it by 1.67 to get the weight in tonnes
times that by 100 to get the weight in kg
divide that by the weight of the bag of sand - 20, 25 or 40 kg

Cement will be 1 bag for every 4 bags of ballast

It seems like you are laying a slab and building on the edges? If so you should thicken the eadges to prevent the walls cracking the slab in shear. Principles here Raft Foundations for Home Extensions Explained | RPO (reinforcementproductsonline.co.uk)

Negotiate a price for the bags from a merchant. Don't pay the book price for such a large amount.

If you're on a budget, it may pay to have bulk ballast delivered, and then bag it up yourself to carry through the house.

 

[JohnHamer1977]

No you can't build blocks off slabs

One tonne of ballast will make about 0.6m3 of concrete, so
work out the volume in metres cubed (length x width x depth in meters)
times it by 1.67 to get the weight in tonnes
times that by 100 to get the weight in kg
divide that by the weight of the bag of sand - 20, 25 or 40 kg

Cement will be 1 bag for every 4 bags of ballast

It seems like you are laying a slab and building on the edges? If so you should thicken the eadges to prevent the walls cracking the slab in shear. Principles here Raft Foundations for Home Extensions Explained | RPO (reinforcementproductsonline.co.uk)

Negotiate a price for the bags from a merchant. Don't pay the book price for such a large amount.

If you're on a budget, it may pay to have bulk ballast delivered, and then bag it up yourself to carry through the house.

Thanks woody very helpful. I have took your advice and ordered bulk bags for sand and aggregate and just 25kg bags for the cement.

 

[^woody^]

Last time someone asked me how many bags of sand they needed, I said that I'd just done a similar job and had ordered 50 bags.

They phoned me up a few weeks later to ask if I was sure it was the same job, as they had ordered 50 bags but had 20 bags left over.

I said yes, and we had 20 bags left over too.