Its raining on my parade!

Have at last poured my garage raft slab.
I've covered it for tonight as temps are forecast to be low.
It's raining now and is forecast for tomorrow, am I ok to uncover it in the morning and just let the rains come down?
I've seen a couple of your postings, Noseall, where you've said concrete loves rain but I think that was for footings, does the same apply to my slab?
I have used the magic screed and easy float on it before the rain started.

Dave

PS I admire you guys who do this for a living! 7cm into 6.5m x 6m shuttering - asked for 5m chute, came with 3m! Good job I had my two lads to help me!

concrete can set underwater - need i say any more.

temperatures below 4 degrees can be problematic. as long as the 'muck' has been down longer than 12 hours in temperate conditions, then you will be fine.

it is only when freshly mixed cement based products are immediately subject to freezing conditions, that problems occur.

Thanks Noseall

I've got bubblewrap, hessian and a tarp on it tonight but I'll uncover it in the morning for possibly another bath!

Dave

the cover has to do three things:

- prevent rain falling on it until it is set (24 hours s/be OK

- protect it from frost while it is setting

- prevent it from drying out while it cures (two weeks is not too long) polythene sheet is good for that

so no need to take the cover off until you want to build on it and it will remain wet from the mix.

covers tend to lift or blow up at the edges, so weigh them down,

best not to walk on it for a few days.

the longer you leave it covered without drying, the harder and stronger it will get (once it has dried, it does not harden any more even if you re-wet it)

Completely agree with JohnD. There's a lot of myth when it comes to concrete, not helped by the fact that most people who work with it are only interested in getting it down, covering it up and then moving on to the next job.

What you're interested in though is a nice job that will last for ages without cracking or spalling after a few years.

Sure concrete can cure under water and it goes pretty hard after 24 hours and you wouldn't see any rain damage at this stage, you also wouldn't necessarily see any frost damage at this stage but the damage is still there and the life of your finished job would be shortened, hence the need for care and protection.

To really understand concrete you need to know how it works and as with most things its not as simple as it first seems. When you add water to cement, the water reacts chemically and causes the cement particles to grow rather like lots of tiny crystals, these crystal 'fingers' then interlock with each other and with the sand and aggregate to make it go hard. You need water to make the crystals grow but you don't want too much because then you end up trapping large molecules of water in amongst the crystals and when it dries out you are left with gaps in the concrete which weakens it and also allows rain in that can freeze and then cause cracks. In a perfect world you want to mix in just enough water so that the chemical reaction starts but no more, then you end up with very few voids and a very strong concrete (will be very difficult to work with though). Interestingly these crystals never stop growing, although after about 3 weeks they have done 95% of their growth and by that time the concrete has obtained most of its strength. I could go on, but you'd all get bored.

Just thought you might like to know why you need to protect it from rain and frost!

Merry Xmas everyone and best of luck with the project isettaman!

Thanks Julesmountain

I'll just leave it covered for a bit.

Dave