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Slab on Grade

Slab on Grade

Slab on Grade

(OP)
I was at an ACI Dinner/Meeting last night and the discussion was related to slabs on grade.  The speaker, Paul Okamoto, noted that sawcutting should occur within an hour of the slab surface starting to drop in temperature after the initial rise.  Is anyone familiar with this?  With a laser temperature measuring device it should be pretty easy to determine when to start.

Dik

RE: Slab on Grade

We specify that sawcutting must occur as soon as the slab will support the weight of the man and equipment without leaving imprints in the concrete.  Not sure how that corresponds to surface temp change.   

RE: Slab on Grade

dik - I've actually just worked with Paul on a recent project...small world.

 

RE: Slab on Grade

(OP)
He's pretty informed... His speech was about designing SOG, and he (intentionally) didn't mention the time for sawcutting, which to me is critical.  I was always of the opinion that shrinkage was the main reason for sawcutting at an early age, and not aware that temperature played a significant role.

He mentioned a free FEM program that he uses for slab design and I sent him eMail earlier today to get the name of it.

My wife's quite sick and I didn't have the time to stay and chat with him.  I thought he was very well informed, an excellent speaker and very practical with a lot of 'hands-on' experience.

..., and the meal was OK.

Dik

RE: Slab on Grade

Time after initial set can vary from 3-8 hours.  A good saw man will observe raveling and wait a bit.  A pair of good saw men can not keep up with slipform paver doing dual lane pass.  Retarder and accelerator can change the speed that the saw can get on the slab.

RE: Slab on Grade

Please let me know if sawcut was not done within 24 hours, and there are no cracks in slab on grade. If saw cut is done after 48 hours let say with a delay, what are the consequences. Is the sawcut effective or not.

RE: Slab on Grade

My understanding is the later its done the deeper the sawcut has to be and theres is some formula in the literature that relates to this all other things being equal.

One point for someone on site is that sawcuts if done to early will chip the concrete edge even though being capable of supporting man and machine.  

I believe its a bit of an art to determine when to sawcut because theres so many variables to consider on site.

Concrete will always crack in the wrong places anyway for whatever reason.

However just because its cracked doesn't mean it necessarily failed.

RE: Slab on Grade

The Soff-Cut saw is the best option for early slab sawing.  Can be used within 1 to 2 hours of placement without raveling the edges.  

RE: Slab on Grade

I agree with hokie66, I've seen the Soff-Cut saw used on a many jobs and it is really slick.  Here is their web site for further information: http://www.soffcut.com/  

RE: Slab on Grade

(OP)
ali07
12 to 24 hours is way too long; sawcutting should be done within 6 or 8 hours after finishing.  If not, then you may as well let it continue to crack and sawcut/rout the cracks afterwards.  Soffcut saws are great and allow you to sawcut within a couple of hours of finishing.

It is a bit of a myth that waiting for the next day and sawcutting deeper will improve the cracking.  Not the case.  IMHO, the three items for SOG construction are the strength, the slump and sawcutting at the correct time.  It is also a myth that using low strength concrete is best because of the minimal cement content.  Better to have high strength.  I usually spec a max 3" slump... contractors hate it, but it's good for the slab; superplasticisers work really well.

Dik

RE: Slab on Grade

Yes hokkie Ive heard of soft cut too, but unfortunately not many contractors have it from my part of the woods(I dont know of any at least 6 months ago!).  If any are around now they would probably be so busy that it wouldn't be possible to do the cut  the same day anyway.  Thats just the cold hard facts of concreting.
 

RE: Slab on Grade

civeng,

That's a shame, as the Soff-Cut saws are readily available in most countries.  Not cheap, but nothing is that is any good.  I have had contractors specifically request permission to use Soff-Cut to expedite their next operation, which should be curing, but unfortunately is not always the case.

I agree with dik about his 3 items, but would add curing and joint design, especially for industrial floors.

RE: Slab on Grade

I agree

Also of utmost importance is not to restrain the concrete pavement in any way, but again its great in theory but sometimes just not possible in practice to do. No matter how soon the concrete is cut or the strength or slump if its restrained in some way it will crack.

RE: Slab on Grade

I didn't mention it in my previous post, but we always specify the soff-cut saw method.  Have not experience ravelling with that method.

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