Wetting Concrete

[DingD]

Anyone here wet their concrete pours for a few days after to slow down the curing?

 

[vinn]

What size slab?
Indoors or outdoors?
Whats the slab for - how will it be used?

Heat and wind are enemies of even curing - spraying with cool water or wet blanketing are sometimes used to slow the dehydration/cure rate but it all depends on your variables.

 

[JohnD]

Wetting doesn't slow the curing. It enables it to continue and improves hardness and strength.

 

[DingD]

JohnD, yeh, that's what I mean't. Quite new to the concrete game so still learning the ropes.

Not doing the slab yet, but it'll be about 5.8m x 5.6m - slab will be for cars, will get a C25 mix, and will be indoors (once I've got the walls up).

I just did the trench today which was about 6.1m by 5.9m, 450mm wide and about 650 deep. I did wet it down, and will continue to do so for the next few days so that I can get maximum strength out of it.

 

[JohnD]

I try to keep mine wet for at least two weeks. Sheeting with weighted plastic will often do, but I spray it with water first. Look out for the sheeting lifting at edges.

After 28 days it will have reached a high proportion of its potential ultimate strength.

Once it has dried it stops gaining strength, and will not restart even if you try to wet it.

Concrete that stays wet permanently (tunnels, underground chambers, even fencepost fill in wet ground) can be astonishingly hard and strong, and difficult to break up.

If your trench is in damp ground, you may only need to cover the top.

The time/strength graph is logarithmic, so it takes longer and longer to get smaller and smaller improvements. 28 days is often considered a reasonable compromise.

 

[DingD]

Thanks JohnD, quite fascinating, that.
So how long do you wait before you start building up the walls for a newly poured trench? I plan to wait about a week, inclined to do 2 weeks but that's quite a bit of inactivity, and I'm eager to get it finished.

At least there is one use for British weather, eh?

 

[^woody^]

You don't need to wet concrete in the ground. It's damp enough down there.

You don't need to wet concrete above either unless you're in North Africa in June. Certainly not anyone in Scotland.

 

[DingD]

South England here, days are getting pretty warm down here Nothing like basking in the sun whilst wrangling with ready-mix concrete.

 

[JohnD]

I'm not a builder, but if you're a DIYer, you can often do the pour one weekend, and leave it covered and damp for two weeks, then start to build off it. You will be covering your new wall as well, so if you are only working weekends, you will get another week before you are ready to uncover and do some more. It's not difficult to keep it wet for 28 days if you can spray and recover it after each day's work if you are only working weekends.

Hot sunny days are very bad for concrete and for mortar. You may find you can break it with your fingers if it dries fast.

 

[^woody^]

And please confirm that you are not building a specialised cantilevered highway bridge that needs specific controlled strength, rather than an extension on the back of the house.

 

[r896neo]

The only bit you need to worry about is the surface crust. Concrete that dries to quick can leave a slightly weaker surface which is more likely to be damaged/ dusty.

This only applies to a slab that is having a float finished surface and if that's the case use a spray applied curing membrane.

Normal slabs that will be screeded are a bit like footings, not worth worrying about.

For footings it's irrelevant, they will be hard enough to build on the next day, scuff marks don't matter.

 

[tomfe]

If you are using a self compacting concrete and are after it being the finish you'll need to spray on a anti-evaporation layer. They normally come with the product. Hope form flow for example comes with it.