Shed base question

[TMC]

Hi All

Have dug out and shuttered for 2 100mm thick shed bases

area 10' x 10' and 10' x 6'

Paving expert site recommends a 6 to 1 mix for bases generally but this is not a big one

6 of ballast sand mix 1 of ordinary portland

I wonder if a weaker mix say 9 to 1 would be adequate as a 30% reduction in the cement cost would be handy

Due to access and having to do it on a sunday this will all be mixed on site

Regards

[chris]

Would not recommend weakening it as it will become v porous. Ship and happorth tar? chris

[Tim Watts]

With paving slabs on top or not?

If the sand mix is just as a bed for the slabs, 9:1 will be fine to stop it migrating.

If you are putting a shed on the sand, stick with the 6:1...

[Kevin Poole]

If the sub-base is well compacted hardcore or stone, and if the eventual appearance of an odd small crack isn't too critical. I'd drop the thickness to 3", but stick with 1:6. For simple wooden sheds used for ordinary domestic purposes, you'd get away with even less thickness than that.

It's easy to over-engineer these things, especially when it's someone else's money.

Don't mix it too wet (strength is a function of water/cement ratio), and compact it well to get rid of air bubbles, then cover it to prevent over-rapid drying out. A layer of polythene under the concrete keeps it wet while it's curing and dry once it's a shed floor, but it does need the sub-base to be blinded with sand, and care while laying the concrete, to avoid puncturing it.

[Phil L]

You do know that that's about 1.5m3 of concrete you have to mix? If it were mine, I'd have dug the top inch of grass and soil, then shuttered

3 inches deep, so that the surface was 2 inches above surrounding ground, I'd have saved half a cube of concrete and the end result wouldn't be as at risk of flooding etc. Saying that, the last one I built, a few months ago (for a large wooden wendy house, but the principal is still the same) I just scraped out three channels about 6 inches deep and mixed a fairly strong, dry concrete, this went about 3 inches deep and I laid three rows of bricks on the strips. I then laid paving slabs onto these minature walls, leaving inch gaps between the slabs, underneath being hollow. The end rusult was a free draining solid base, which cost about 20 quid and took 4 hours, given that we used 2nd hand slabs and old bricks.

the only visible parts of it are the dwarf wall and the outer edge of the slabs.

[TMC]

thanks for the reples

6:1 it will be and sticking with 100mm

Regards