Concrete Base Help needed.

[james26]

Hi,
im making a concrete base for a garage but i have a few questions i need answers to before i go ahead with it, so here i am!
Basically i have everything ready for the concrete, the frame is 18ft by 10ft5 and 6inches above the hardcore - i've got the hardcore in.

Firstly a mate told me today that you have to lay the concrete in layers overwise the concrete won't set at the bottom, i was under the impression that it was better (and stronger) if you lay it all in one go. Is there any truth in the layer idea or should i just lay it in one go?

Secondly i really need to lay the concrete in the next week but the weather forecast says it's going to be a scorching week (heat-wave) and i have read that it's best not to do it when its hot or the concrete will shrink and crack! Is there a safe way to lay it in really hot weather without the risk of it shrinking and cracking?

It's cost me alot of money to get all the cement and ballast and i can't afford to mess it up so your help would be greatly appreciated!

Many Thanks Guys

James

 

[noseall]

for a 6" or 150mm thick slab, you are best pouring it in one go.

maybe he was getting confused with much thicker concrete or the hardcore.

thick concrete as in dam production or huge poured concrete structures, needs to be poured in pre determined sections to avoid overheating and ultimately blowing itself apart.

when applied in deep situations, hard core needs to be compacted in layers of about every 6- 10 inches.

concrete loves water and, particularly in its early stages, (first 48 hours) it is best kept damp.

if you have to lay it in the sun make sure you put some damped hessian sacking over the top. keep the sacking damp with a misting from a hose.

 

[james26]

Thanks for your reply, you've answered all my questions perfectly!
I like your suggestion on how to keep it damp, but i don't have any hessian sacking. Any ideas on where to get it from and is it cheap?

Thanks again

 

[noseall]

hessian sacking is available at good builders merchants.

unfortunately it comes in (i think) either 25 or 50 metre rolls!

it is 1500mm wide. (1.5m).

it is more commonly used to drape over brickwork in frosty conditions.

you can use some old dust sheets to keep it damp.

 

[james26]

Ok thats great, thanks for the advice!

James

 

[DrewBD00]

thick concrete as in dam production or huge poured concrete structures, needs to be poured in pre determined sections to avoid overheating and ultimately blowing itself apart.

They reckon if they had built the Hoover Dam (built 20's or 30's) in one big pour - it would still be setting today!

As it was they did it multiple sections with cooling channels built in to help it set.

 

[JohnD]

If you have the chance to place it in the afternoon (after the sun has passed its peak) it will be less likely to overheat and crack on the first day. This isn't usually a problem in the UK, but in mediterranean countries this is the norm for summer work.

The wet sacking (or you can use a polythene cover) will keep it damp, but will mark the surface unless you give it time to stiffen first. Same with water spray, it will give you an unwanted exposed aggregate surface if you do it on the first day. Keep a special eye on the edges and keep them especially moist, for at least a week, preferably two.