Thread: Slab cure time

I had houses built in Sydney and it varied as to how long the builders left the slab to cure. Beechwood homes leave their slabs 4 weeks before commencing construction. On the other hand had one built where the builder started building after just one week.
Usually driveway construction guys advise keeping off the drive for a week before driving on it, so maybe a week is long enough. Depends how impatient you are

I left my garden shed slab to cure for a week, as recommended in something I downloaded from the Boral web site.

But, there's a new house being built up the road from me where they put up the frame the day after the slab was poured, which struck me as a bit dodgy.

A week is plenty of time to let it go off, I would be starting this weekend if it was me

28 days minimum under constant moisture control to which the best way is to cover with plastic sheeting and give it a daily dose of water by squirting the hose under the plastic. I know some will say "but its only a shed".

If you need to do work on it then you can always reveal the area you need to work at.

This comes from practical experience. In March 2006 I poured the slab for my house. Its size is 20m x 17m with 600mm deep edge thicking, it was poured monolithly. Imediatly after it was finished I covered it with plastic sheeting and left it on for at least 28 days in reality it was probably longer. To date there is absoluty no cracks in the slab.

Just after I done mine a slab was poured for a house being built across the road. The formwork was stripped at the end of the day and no water was applied to the slab. Acouple of days later the owner called me across to show me 4 large cracks in the slab. He was not a happy chappy.

Concrete will attain most of its strength in the first seven days. We poured a slab 11 days ago that will be holding a 13T boiler and 26m chimney stack, and that is going on tomorrow. We were chasing 32MPA,and used 40 to achieve this earlier.At 7 days the test cylinders were 29.9 and 31 MPA.

Tools

Originally Posted by rod1949

28 days minimum under constant moisture control to which the best way is to cover with plastic sheeting and give it a daily dose of water by squirting the hose under the plastic. I know some will say "but its only a shed".

If you need to do work on it then you can always reveal the area you need to work at.

I dont get the keeping it wet theory, the cement in the concrete goes off wet or dry.

Al

Originally Posted by ozwinner

I dont get the keeping it wet theory, the cement in the concrete goes off wet or dry.

Al

If the cement runs out of water before the reaction is complete, then the concrete is not full strength, so you need to keep it wet/stop it from drying out. Once the concrete has gone hard (after 1 day or so), it is even better to cure it under water! Not very practical but, so just stop it from drying out quickly. This time of year, that's not much of a problem.

Keeping it wet ensures that the cement has enough water to complete its chemical reaction, because some of the mix water evaporates especially at the surface. But, can't just increase the water content in the mix as that reduces the final strength and toughness. Weird. Minimum curing time can be 7 or 14 days.

As said, most of the strength develops at 7 days; the 28-day strength is used for specification in the design. Reason for both of these milestones is so that the testing lab doesn't have to work on Sunday; both the lab and the concrete supplier typically don't operate on Sundays, but same situation in countries with other reserved days. Concrete continues to gain strength forever, although the rate of gain diminishes rapidly after 28 days.

You can generally commence some other construction a few days after placement, provided you don't apply significant loads, as in the driveway mentioned by John (munruben), or install drilled-in anchors or tension cast-in anchor bolts. For the anchors, most designs are based on 28-day strength, and premature drilling and/or tensioning could promote blow-out thus voiding any warranty.

You can likely start building your frame components next weekend, but I'd suggest waiting another week or two to make the final attachment to the slab.

Joe

Thanks for the input everyone.

I will definitely be starting work on it this weekend, but I probably won't get around to "bolting" anything down until the weekend after. So that will give it just over 2 weeks of cure time before I start drilling holes in it.

This is all very exciting .

Originally Posted by Amb

If the cement runs out of water before the reaction is complete, .

Not true sorry to say, I have brickwork that I have done that has dried out in summer in about 3 or 4 hours with the heat and the wind, and the same wall goes as hard as the rest of the job.

Unless you are talking about minute differences that would be hard to pick up without the aide of scientific instruments.

Al

Originally Posted by ozwinner

Not true sorry to say, I have brickwork that I have done that has dried out in summer in about 3 or 4 hours with the heat and the wind, and the same wall goes as hard as the rest of the job.

Unless you are talking about minute differences that would be hard to pick up without the aide of scientific instruments.

Al

The thing with a slab Al is you have a huge surface area exposed to the air and if it isn't kept damp and covered until it cures you will end up with shrinkage cracks in the surface. The exposed surface area of the morter in a wall would be fairly insignificant under the circumstances.

When I was doing sheds our engineer alway specified 21 days for curing before constructing the shed because of the pressures on the hold down bolts.

I have seen bad cracks in a slab because the builder either didn't lap the reinforcing mesh or they failed to tie the mesh even if it was lapped.

Shrinkage cracks wont affect the performance of the slab but they look ugly especially if it is in an exposed area.

When I first started work with the old man, he always left everything between 1 and 3 months. I'm sure i've been told by someone that back in the good old days people tried to pour slabs in autum and build on them in spring?, not sure if this was because of concrete or because they wanted to do the final fix work in summer and out of the heat?

I remember they cured the 8 slab's(1200mm +thick, 100m3 heat proof moral clay mix) at work for our lead pans(420Tons of lead + settings about 500T I guess)for a month. I was one of the crane drivers carrying the 5 Ton hopper thingos.
They float on about 40~50 pylons.
We can fill or empty one of these pans in 1/2hr, strong concrete they take a beating!