Slab on Grade Reinforcement 2

[Samwise Gamgee]

I remember the area of steel required for SOG is As = FLw / (2 x fs). Can anyone provide the reference page for this in ACI 360R-06. Or was this something in the previous version of ACO 360R ?

 

[dik]

The minimum temperature steel should be 0.2% of the concrete area (0.18% in some jurisdictions). There are other criteria based on amount of cracking, shrinkage-drag, temperature, loading, etc.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[OldDawgNewTricks]

The subgrade drag equation you reference was in ACI 360-92, Equation 6.3. It was removed from the newer editions.

 

[dik]

Thanks, Dawg... didn't know that. OP- The rest of the stuff is still applicable.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[Samwise Gamgee]

ACI 360R-06 specifies that it should be 0.5% to avoid control joints. It also mentions 0.15% otherwise.

I don't see the 0.18% anywhere in that code.

 

[dik]

Not in the ACI, but some other code and I don't recall the one... I generally use 0.2% in compliance with CSA A23 and the ACI. I seem to recall that the ACI slab on grade publication notes that TS should be greater than 0.15% for some circumstances... just a distant recollection. From a more current ACI

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[Settingsun]

I have a job where someone is using an Australian manual that still has the subgrade friction equation. It gives 0.14% for 26m joint spacing. The issue I see with that is that the concrete itself is much stronger so shouldn't crack in the first place - sets off the BS detector for whether this method actually gives a meaningful result.

The slab is 280 thick so pushing 100 L/D (93 to be more precise), and also a bit outside the 1.5:1 aspect ratio (16m in the other direction).

Thoughts on what reinforcement is appropriate for this slab geometry, and what level of crack control will be achieved by the 0.16% specified by the designer?

 

[dik]

26m is a long way between joints (sure it's not 26cm just jokin'), unless you are doing a joint free floor. 0.14% is way too low... should be 0.5% or 0.6%.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[Greenalleycat]

@Steveh, the point of steel isn't to stop cracking occurring. PT is needed for that. It's just to control crack widths to acceptable limits. It's acknowledging that a 26m free space between joints will crack SOMEWHERE and just wanting to keep it reasonable when the crack occurs.

You can look up the CCANZ residential and commercial slab on grade guides for some more input on calculating crack widths (there are two parts to the document)

 

[dik]

Great outfit... CCANZ; I'd not heard of them. Some of their publications:

https://concretenz.org.nz/general/custom.asp?page=Publication_Legacy

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[Settingsun]

The kiwis seem to be in the camp of very little reinforcement. But I don't think this level of reinforcement will reliably cause distributed cracking. Quite possible to get one wide crack with the reinforcement just keeping the width to about 2/3 of what it would otherwise have been.

Dik, 0.6% would be my number for this.

Residential - these are 6mm@200mm and 7mm@200mm meshes if I've done the sums correctly.

Commercial - joint spacing presumed to be in metres.

 

[Greenalleycat]

Yep, you've got it, typical slab mesh here for a 100mm slab is SE62 (Square, Earthquake Rated, 6mm @ 200mm pitch both ways)
So, basically 0.15% reinforcement
From what I've seen, that seems to produce adequate results, but I'm sure there is the odd slab that heavily cracks in one location
I would approach things differently for a commercial floor than a residential slab though

 

[Settingsun]

This is for a heavy duty external slab that's gone to D&C. I'll just point to the concrete code that gives qualitative descriptions of the level of crack control. The contractor's reinforcement is at the 'minor' degree of crack control, while the reference design was 'strong degree'. I suspect the contractor will be asked for a discount.