We recently put down a slab for a small (3m x 3m) garden shed and it is around 80mm thick all around. It looks like we have mixed the incorrect ratio for the concrete, we used the numbers on the back of the cement bags but as we were using pre mixed sand + agg the ratios seem to be out.

We were doing 7 part sand + agg and 1 part cement. The slab has cured a bit but it doesn't seem very strong at all.

I am able to rub the slab with a stick and the concrete just wears away really easy. It has only been 24 hours since laying, should it still be this soft or will it not set properly at all?

What are my options if it isn't going to be strong enough? Re-lay (not ideal)? Or can I use a sealer of some description to protect the outer layers and stop it from wearing so easily?

Will it harden up over the next few days or will it have cured mostly by now?

Cheers,
Dean

Concrete takes 28 days to get to full strength but the strength after 7 days is around 90% of the final strength.

After a day its probably only around 20% strength so give it a few more days.

The ratio to make concrete is 1 x concrete sand, 3 x 14mm aggregate 1 x cement will achive around 20 MPa dependant on water content

So there is still hope for it? Is it normal for it to still be quite brittle/soft at this point? We had to use some premix bags too as we were a little short and the colour between the two is huge, the premix is a much darker grey where the stuff we mixed is much lighter. Some of the overflow of the premix is also really hard and the other is not so hard.

Hopefully it will harden up a bit in a couple of days, it is only for a shed so doesn't have to be perfect but I don't want it washing away either.

We got the mix from the back of the cement bags, I think it was 1 part builders cement, 3 parts sand and 5 parts agg for foundations and large masses. We were using blue circle builders cement (http://www.boral.com.au/productcatalogu ... oduct=2329).

The problem I think was that I didn't counter for the different sizes of sand/gravel, 2 3 buckets of sand and 5 buckets of gravel doesn't equal 8 buckets of the premix as the sand fills the voids that would usually be between the gravel if it was on its own, meaning that there would have been more sand.

I think we should have done about 5 to 1 ratio with the premix.



Lets see how it goes.

Thanks for all your help