How long does cement take to dry?

[sophiebird69]

Hi,

I don't know if I am posting this on the right part, but wanted to have a rough estimate as to how long it would take a 11ft x 11ft square thats 3" thick to dry enough to put a swimming pool on it (pool holds 900 gallons of water)? also will a concrete square be suitable for this as im having a local man do this for me and i want to make sure that the job is being done correctly.


I don't want to put it up as soon as it's dried and make it crack!!

Thanks,

Sophie

 

[JohnD]

Concrete doesn't gain strength by drying, but by "curing" which it does for the period while it remains damp after laying (once it has dried out, it won't gain further strength even if you damp it again).

A good way to keep it damp while it cures is to put a sheet of polythene over it, well fastened-down at the edges. You can spray it with a hose if you want to.

It will gain strength fastest in the first 48 hours; if you can keep it damp for a week, or, better, two, it will have gained so much of its strength that you will not gain much by leaving it longer. If it is not strong enough after two weeks then it never will be. The rate of curing gets slower and slower and slower after the first couple of days.

Protect it from hot sun for the first day (in some countries they don't lay concrete until the afternoon to avoid the hot mid-day sun which causes cracking and dusty/flaky surfaces).


I don't know what sort of slab you need for a pool, though.

 

[sophiebird69]

Ok Thanks for tht,

I guess the man Im paying doesnt know what he is doing after all then, as he just said let it dry (infact wouldnt do it today in the damp) and then it will b ok. Also he is putting no slabs down, just doing the concrete to put pool ontop of.

Maybe I would be better with just sand then?

Oh I dont know, LOL, my £40 pool has cost me well over £100 in work already!!

Thanks anyway,

Sophie

 

[JohnD]

Have a look at the pool instructions, they should give a clue of what is needed.

(when i said "slab" I mean the concrete that your pour on the ground that hardens into a slab, I didn't mean paving slabs)

The thing you want to avoid most of all, is putting stresses on freshly-placed concrete while it is still weak, and cracking it, when it would later be strong enough to take the weight. Day-old concrete is very easy to break.

 

[Nige F]

you just can`t get the quality workers in the stix

 

[JohnD]

that's not true, outside London all the people are well suited to manual labour. They all wear wellingtons and drive to work on tractors.

 

[Jasonb]

Concrete will reach 2/3rds of its final strength in 7days and virtually all it s strength in 28days but the curve flattens out as it continues to cure after that.

Concrete cures by a reaction between the cement content and water, this causes heat to build up in the concrete and is called the "heat of hydration". As has been said keep the concrete damp for a week especially in this weather. That is why concrete test cubes are kept in a tank of water before crush testing, Ah brings back memories of matrerials testing at college some 20yrs ago.

3" thick sounds a bit thin, is there any mesh in the concrete? 900gal is just over 4 tonnes of water

Jason

 

[sophiebird69]

No there is just sand and cement mix ontop of my mud he has dug out and some paving slabs here and there cos he couldnt smash them to make hardcore!!

he isnt even mixing in a cement mixer , just tipping the sand and cement and mixing on the ground! HELP!! I just know this is going to be a disaster now - as he had alreadt said that we could put pool up as soon as it had dried!!

Thanks everyone for your replies!

Sophie

 

[OzStarFirebird]

he isnt even mixing in a cement mixer , just tipping the sand and cement and mixing on the ground! HELP!! I just know this is going to be a disaster now - as he had alreadt said that we could put pool up as soon as it had dried!!
Sophie

this is what happened to us. this is the exact method that was used by our "expert" in mixing our cement for our outdor cage.
We were told initially that the concrete base for our cage frame could be put up one day, and then the cage could be put up within three days. Then we were told that it required 2 weeks for the concrete to dry.
Now we are being told that WE did it all wrong, and that we would be lucky if it didnt turn to powder under us!

All of this was done at the instance of the same "expert" who insisted that we do it HIS way, or not at all!

So, if our base is 6 inches deep at it's deepest point, and about 4 inches wide how long DOES it take to be safe for our frame? It's also NOT been kept moist, just left to dry in the sun - again at the insistance of our expert!

 

[JohnD]

once it has dried out in the sun, it will never go any harder. the curing process only continues while it is damp.

find yourself another expert.

Looking on the bright side, if it is weak enough to dig out with a teaspoon, it will not be too hard to dig it out and start again.

You can use the weak crumbly stuff with hardcore under your concrete slab as part of the foundtion, if you like, as if it was sand and gravel, but it needs to be underground so it can't wash away.

It is possible to mix concrete by hand, but this is quite hard work as it has to be turned multiple times to get it throughly mixed, dry and wet, and you have to avoid mixing mud or dirt (which has no strength) into it, so it is best done on an existing slab or large piece of flat ply or plastic.

In the UK the ground is usually moist, so concete cast into a trench is quite slow to dry (except on the top) but your conditions may vary

 

[OzStarFirebird]

thanks for the response!
I have been worried this whole time that our "expert" didnt actually know what he was doing - especially since he hasnt ever built anything in the climate conditions that we live in. Its rather dry here. (we live near the equator after all LOL )

Im going to really be annoyed at having to re-do this. We have already paid out nearly a thousand dollars AU for this, and are no further along NOW then we were 6 months ago!