Thread: Sla-On-Grade Curing/Loading

I've lost my ACI reference to the amount of time that should transpire between placement of the concrete in a slab-on-grade foundation and loading or framing the struture on it. Any hints?

Thanks,

Aaron

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Depends on the mix and the admixtures, if any.

In South Florida, if the right mix was used, they were allowed to stack block on the slab after just 12 hours. The easiest way to accomplish that, if you need 2500 psi concrete, would be to place 7000 psi mix concrete, that's probably 3500 psi in 12 hours.

That said, once, in Gainesville, Florida, they (local contractors all got together and coordinate this) constructed a stick built house in 11 hours, *INCLUDING* placing the steel and pouring the concrete - that concrete was curing almost faster than it could be placed, and you had almost zero time to screed it, float it, and trowel it before it was too stiff (hard) to do anything with ... had to, because the walls were being set on it before the concrete finishers were done. Remember, only 11 hours to completion of the home.

ECJ:

I understand that the mix can be adjusted for exceptional circumstances, but for a standard 3500 psi mix 24 hours is not long enough. Agreed?

Aaron

Jerry,
Do you remeber the address?

Originally Posted by Aaron Miller

I understand that the mix can be adjusted for exceptional circumstances, but for a standard 3500 psi mix 24 hours is not long enough. Agreed?

Aaron,

Define "standard" 3500 mix".

You can order a "standard 3500 mix" which is "high early strength", 12 hours might be long enough for that.

You would need to find the mix and if it was high early strength or not (that is a "standard mix"), and, if not, and if there was nothing else in there, 24 hours 'might not be' long enough - but I'm not a an ACI concrete guy, so I don't know. All I know is that wherever I've been, "the standard" has allowed for loading block after 12 hours, probably because they are all using 'high early strength' concrete.

Bruce,

No, don't remember the address, but, as I recall, it was in the SW section (which was not as big back then as it is now - that was 30 years ago).

That was at about the same time that a company in Texas (Dallas?) built a house in a mall/convention center as a promotion, raffled off the house, then took it down and re-constructed it on another lot. I believe it was 2 days to build, up for 5 days, 2 days to take it down, and another 2 days to put it back together. Not real sure of the time I have down for those, other than I remember for sure that it was only up for 5 days in the mall/convention center.

Aaron,

Not exactly the info requested but this does make mention about cure time.

http://www.icc-es.org/reports/pdf_files/UBC/1290.pdf

I don't think this would be much different for the hand-driven cut nails we see.

Barry:

I think that will do nicely. Though not exactly what I was after it lends itself to spin . . .

Thanks,

Aaron

That has nothing to do with loading block on a slab, it only has to do with installing anchors into the concrete (two completely different issues and concerns).

Besides, it says the same thing I've been saying ... it depends on the strength of the concrete, not the curing time!

4.5 Fasteners are installed in normal-weight concrete that has cured a minimum of seven days OR HAS REACHED a minimum ultimate compressive strength of 2,000 pounds per square inch (13.8 MPa).

Aaron, I know you are good at spinning things, but, first you need to know what the strength of that concrete is, and *I* would *NOT* use this for fastener pullout to relate anything to loading concrete block on a slab.

ECJ:

Don't worry about the concrete block. We only use that here for building BBQs.

The problem here lies in the fact that there is no paperwork relating to the exact mix used, ad-mixes, water added, etc. We can only go by a rebound hammer test (5500 psi) now that the slab has cured. That will not tell us with any degree of accuracy what the original mix was.

As with most residential (and some commercial) slabs-on-grade here there is little or no oversight. The correct preparation for and placement of the concrete is always left to the discretion of Juan y Jose. No engineers watch over their designs as they are implemented. Few municipal inspectors even walk the site prior to pour, much less do they stand there while it is being poured. No paperwork is required to be provided. It is a GD free-for-all.

The builder poured this slab and was planning to go full steam ahead with framing on it the next day. I doubt like hell that the compressive strength would have reached 2000 psi by then. I know you were touting high-strength mixtures, but these are expensive and never used on residential work here. They are mostly for TX DOT for when they need to load a road in less than three days.

Any suggestions now that I have provided more pieces to the puzzle?

Thanks,

Aaron

Originally Posted by Aaron Miller

the amount of time that should transpire between placement of the concrete in a slab-on-grade foundation and loading or framing the struture on it.

I was keying in on the loading part of your question, however, I think you are asking more of the framing part of it.

Originally Posted by Aaron Miller

Don't worry about the concrete block. We only use that here for building BBQs.

Okay, so it must be frame, either wood or metal framing.

The builder poured this slab and was planning to go full steam ahead with framing on it the next day. I doubt like hell that the compressive strength would have reached 2000 psi by then.

That part, for the framing, is where the ICC report Barry provided comes in - if they are using those Hilti fasteners.

Find out what fasteners are being used and then address through the manufacturer.

In the case of those Hilti fasteners, they require the concrete be cured to 2,000 psi minimum - so have the builder provide proof of that. Have him take a core sample and have it break tested, if it meets 2,000 psi, go for it, if not, wait until it does meet 2,000 psi at a break test.

Or.

Wait 7 days, then, regardless of what it has cured to (OMG ... but that what it says), then go for it.

In the case of those Hilti fasteners, they require the concrete be cured to 2,000 psi minimum - so have the builder provide proof of that. Have him take a core sample and have it break tested, if it meets 2,000 psi, go for it, if not, wait until it does meet 2,000 psi at a break test.

ECJ:

That's what I was saying. The slab was poured in August of last year. I stopped the builder from framing the next day. He could produce no satisfactory docs then, and will not be able to now. So, the Hilti ICC-ES doc will give me the leverage I need. He cannot show that the slab had reached the necessary hardness by the following day.

Spin-spin-spin-spin . . . . .


Aaron

Originally Posted by Aaron Miller

The slab was poured in August of last year. I stopped the builder from framing the next day.

Surely, though, it is cured by now?

He could produce no satisfactory docs then, and will not be able to now. So, the Hilti ICC-ES doc will give me the leverage I need. He cannot show that the slab had reached the necessary hardness by the following day.

Spin-spin-spin-spin . . . . .

Aaron,

No spin cycle required for that document, simply wash, dry, fold.

This would be the soil engineers decision at the time of construction.
Depends on soil conditions.

Rolland Pruner
Livermore, Calif