Thread: Concrete Slab Curing Question

By no means as well informed or experienced in this area as many other members. However our experience with two extensions a good while back, on slabs, was that with the first small slab - 1 room - the builder waited about 3 weeks for the slab to go off a bit, then used concrete nails to secure the timber. He sais he preferred to let the slab dry out a bit - reduced the chance of moisture soaking into the timber.

Second slab, 3.5 by 10 metres - the builder waited a fortnight, for much the same reasons. Gave us time to stain and seal 4 million lining boards, fascia, etc.

Both extensions were done in February, which is about the hottest month in these parts.

The concrete nails worked a treat and certainly did not need pre-drilling - I suspect your builder might be having a lend of you as far as the extra $1000 goes.

Hope you can get a better answer to your question.
Jeff

Moe

My first advice is get a second opinion. Sounds like these blokes are yanking your chain mate.

I would let it cure for as long as you want. The longer the better and dont let them pour after 9.00 am. Too hot and the concrete goes off too quickly compromising the overall quality and finished slab.

It is your house and most importantly your money so make sure you get what you want and dont give in to these people because they are only interested in getting the job finished and moving on to the next house and next cheque of course!

Pete

28 days is a bit of overkill, I wouldn't get to worried about them starting after a few days. The slab should be pretty hard after about three days but will still be releasing moisture, why not ask them to wait a week before starting. They probably have themselves all organised to get moving and a month wait is probably in the view unreasonable, ask for a week and they will most likely humour you.

John

If you are saying that after 48 hrs they come and put nails in your slab ..well thats bizarre. Bolts maybe, but then they still have to drill...got me stumpted ? must be some fixing method I'm not familiar with.
Also think 4 weeks is a bit conservative, as the difference in strength in the concrete between 14 days and 28 days is minimal. However you are employing and expert and if you go against his advise and something does go wrong then its your fault so my advise is accept the 28 days.

Moe, have a look at this site.

http://www.concretenetwork.com/concr.../watercure.htm

They recommend 7 days of water curing before commencing construction to achieve greater strength. 3 days water cure if on a tight schedule.

My wife works for a builder who uses concrete slabs and this is their regular practice as well. Keeping the poured concrete wet adds greatly to its strength.

Good luck.

We use curing compound which is sprayed over the slab straight after finishing. This seals the top and stops moisture escape. We quite often then proceed with studwork the next day. The curing compound goes on like bondcrete, white then dries clear. Sika make a good one that does not interfere with any floor coverings [ie tiles].
The compound can also be aplied to any vertical surfaces when the boxing is stripped.
My reading of the BCA says that using concrete nails for wall hold down is no longer acceptable for exterior or braced walls.

Concrete achieves about 70-80% of it's ultimate strength in the first 7 days. Almost nobody understands that it needs lots of water to 'hydrate' or 'go off' properly. The best and most simple way to do this is to keep it wet 24/7 with sprinklers. If water supply is a problem, cover it as soon as it's hard enough to walk on with hessian, soak this and then cover with polythene/forticon/whatever. Keep it damp under there for 7 days. This will help reduce shrinkage cracking dramatically. OK to build after this.

D

Originally Posted by TassieKiwi

Concrete achieves about 70-80% of it's ultimate strength in the first 7 days. Almost nobody understands that it needs lots of water to 'hydrate' or 'go off' properly. The best and most simple way to do this is to keep it wet 24/7 with sprinklers. If water supply is a problem, cover it as soon as it's hard enough to walk on with hessian, soak this and then cover with polythene/forticon/whatever. Keep it damp under there for 7 days. This will help reduce shrinkage cracking dramatically. OK to build after this.

D

Strength gain depends upon the mix design. We work with 32MPa concrete and with high cement content and low flyash content get high strength gains in the order of 70% in 5 to 7 days. Most domestic mixes have high flyash content and would experience a much slower strength gain than that. Flyash is a lot cheaper than cement but given more time acts in the same way.

Keep the slab moist to assist hydration and strength gain.

From my experience a couple of weeks is the norm for getting onto a slab would be pretty safe.

Are we talking slab on ground?

I wouldnt think it would be a huge drama, we are not talking about a suspended slab and removing formwork etc. A spray on curing compound would be good, but no many builders use these, most just let it sit in the sun and dont cover it at all, with little or no surface cracking. It is not untill the roof goes on that the loading occurs. The edge beams taking the load under the ground and will cure anyway. In my opinion 3 days (tops) is more than adequate.

I second thebuildingsurv 3 to 4 days is fine especially for those chippies using their framing gun instead of concrete nails for fixing the bottom plates.
Generally the greater influence on the strength required for slab on ground construction is the amount of water added before not after.