concrete mixing using ballast

19 Jun 2007
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West Glamorgan
United Kingdom
I am getting confused reading around the net about mix quantity.
I am looking to mix some concrete to make a hard standing area for a Pizza oven & total weights would be approx 400kg. Area is 1m square. I had to dig down 30cm due to the amount of bedding under the paving slabs I removed.
I have a jumbo bag of ballast which is a lot more than I need, but worked out cheaper than buying 1/2 the amount bagged.

I am not sure what mix to use. I guess I want something C25 (ready mix not an option since the distance from the road is to far!)

Can anyone give me some advice? I am reading 1:3 for C25 to 1:8 from various sites!
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For general jobs, I've used 1:6, use a little water as you can.

Presume it's sat on a good bed of mot/hardcore.

You can put reinforcement mesh if needed.
sat on clay - so I should be ok not needing a sub-base. Is 6:1 going to be strong enough?
my confusing is around more ballast:cement is stronger or weaker?
The more cement, the stronger the mix, so 1:6 is stronger than 1:8.

I think you will still be advised to lay a depth of hardcore down, but I'd wait for others to comment, I'd always put hardcore down before I start.

Once laid, keep the top wet, with hose or watering can with rose.
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For such things I do 1:5 and never had a problem.
The 3:2:1 mix needs 2 part sand, 3 part aggregate, so as the ballast is sand and aggregate mixed, 1:5 is what i do.
I think you will still be advised to lay a depth of hardcore down, but I'd wait for others to comment, I'd always put hardcore down before I start.
Hardcore is just ballast without cement and graded fines so just use your concrete mix unless you want to thin the slab down and save on cement
As others say 1to 5 or 6 will be more than OK. Just to put things into perspective even if your oven was balanced on a support leg 4" square the crushing force on the slab would 0.04N/mm2 and c25 is rated at 25N/mm2. The main issue is the force that is transmitted down to the underlying ground which based on the above loading with a 300 thick slab would be 0.001 N/mm2. Very poor ground is OK at 1/2 tonne/ft2 = 0.5N/mm2. Personally I would have just bedded some paving slabs on some sand which would have been more than adequate
6/1 is strong enough. The older textbooks show 1/3/6 for foundations, which isn't 9/1 as some people think. The fines fill the voids in the gravel.
One of the arguments against using all in ballast was that you couldn't be sure if it contained the correct amount of fines.
I think I will go for 5:1
The flagstones outside my backdoor had already sunk in places, which is why I wanted to put in a solid base.

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