Slab Floor Depth

Posted: Sat Apr 22, 2006 9:38 am

I met with a concrete contractor the other day to discuss the floor of the studio, the offices and a few other smaller pours around the property.

I'm sure that he's got both our best interests at heart... (Read, he doesn't want to price himself out of work.)

I had been designing the studio floor slabs as 8 inch rebar reinforced slab, with 10-12 inch footers. (12 inch on the front control room wall)

He countered with a flat 6 inch fiber reinforced concrete slab w/ .5 inch rebar. He's assured me that this will more than make the inspectors happy, and should be less susceptable to cracking as the weight of the footers is what adds stress to the center of the slab load... which has a tendancy to promote cracking.

Any opinions?

Also, would I benefit from closing in the rebar pattern from 10 inches to 6 inches?

To review... The load will be 2x6 walls with .75 mdf and two layers of 5/8 fire rated gypsum. Ceilings will be 2x6 w/2-3 layers of 5/8 fire rated gypsum.

Thanx,
Max

Posted: Sat Apr 22, 2006 11:23 am

Typically concrete floors are poured at 4". I believe that is the building code. So anything over 4" is more than enough. And should not crack.
If the floor guy is someone who you know or "feel" you can trust. Let him do as he thinks is best for you (both). Obviously just let him know what your main concerns are.

Posted: Sat Apr 22, 2006 11:44 am

MadMax wrote:

I met with a concrete contractor the other day to discuss the floor of the studio, the offices and a few other smaller pours around the property.

I'm sure that he's got both our best interests at heart... (Read, he doesn't want to price himself out of work.)

I had been designing the studio floor slabs as 8 inch rebar reinforced slab, with 10-12 inch footers. (12 inch on the front control room wall)

He countered with a flat 6 inch fiber reinforced concrete slab w/ .5 inch rebar. He's assured me that this will more than make the inspectors happy, and should be less susceptable to cracking as the weight of the footers is what adds stress to the center of the slab load... which has a tendancy to promote cracking.

Any opinions?

Also, would I benefit from closing in the rebar pattern from 10 inches to 6 inches?

To review... The load will be 2x6 walls with .75 mdf and two layers of 5/8 fire rated gypsum. Ceilings will be 2x6 w/2-3 layers of 5/8 fire rated gypsum.

Max,

He's right that an 8" slab is overkill in this case - and I won't use a footing under a slab - but I would not use a 6" thick slab to carry bearing walls either.

I would want a 6" thick slab - the #4 drb is ok - but make sure he uses chairs to keep them in the top 1/3 of the slab (the bars in a non structural slab are actually shrinkage control) 16" grid is fine = but where the walls will rest you want a 12" thick haunch - which is integral with the slab - and thus places no un-natural sources on the slab itself.

If you need a detail I can email you one.

Rod

Posted: Sat Apr 22, 2006 1:09 pm

Rod,

Please forgive my english... I'm an American Confused

I VERY incorrectly used footer... I really meant haunch. - DOH!

So you ARE advising a 12" haunch on the perimeter of each pad.

I don't see how I could get away without one... thus my questioning the contractor not insisting on their use. Even after informing him of the weights involved in the walls and ceiling, he didn't think that the uniform 6" pads would have a problem with the wall weight.

I showed him the drawings and he seemed to understand the plans, but I just get the feeling he didn't comprehend the total weights involved.

I'd love a detail - I'm using TurboCAD if that's any help. THANX!!!!!!!!

Max

Posted: Sat Apr 22, 2006 3:16 pm

Max,

detail is in your email inbox.........

If he gives you a hard time - tell him to just be quiet and do it........ he's absolutely wrong about this causing problems.

Rod

Posted: Sat Apr 22, 2006 3:43 pm

Man, this seems like a pretty elementary matter for a pro to be wrong on. Maybe you should find someone else?

Posted: Sat Apr 22, 2006 5:48 pm

David French wrote:

Man, this seems like a pretty elementary matter for a pro to be wrong on. Maybe you should find someone else?

David,

the fact that someone is a pro at pouring concrete does not suggest that he/she is a pro at structural design.

The same way that being a professional car driver doesn't mean you could design an engine - and most pro recording engineers can't build a recording console or design their own compressors, pre-amps - or effects boxes.

Sincerely,

Rod

Posted: Sat Apr 22, 2006 6:08 pm

Got it, thanks.

Posted: Sat Apr 22, 2006 9:38 pm

Rod,

Got the file... THANX!!

That's exactly what I figured - 12" depth...

Just out of curosity, what thickness of expansion joint would you recommend?

Max

Posted: Sun Apr 23, 2006 7:19 am

MadMax wrote:

Rod,

Got the file... THANX!!

That's exactly what I figured - 12" depth...

Just out of curosity, what thickness of expansion joint would you recommend?

Max

Max,

1" is fine.

Rod

Posted: Sun Apr 23, 2006 8:36 am

Rod,

Pandora's box... sorry.

Is it code that the sand be there?

My current soil structure... AFAIK...

1. Gravel - 1"
2. Clay/earth

The plan so far then...

1. Starting at existing grade, dig 6 inches for the haunches, plus 2 inches for poly/gravel.
2. Place 2" of gravel in bottom of haunch.
3. Lay poly in haunches and over the rest of the pad(s)
4. Rebar in 16" grid on 4" saddles
4a. Rebar 2" from bottom of haunch (as per drawing)
5. Pour the pads to 6" above existing grade.

1" expansion joint along exterior perimeter.

As for the different pads. I can save a pure-T butt load o' money if I could do one pour. But I don't see how I could do it and not leave some sort of steel framing/forms in the floor.

Is there a way to do one pour? Or am I just plain stupid? (A loaded question for sure, huh?)

Thanx,

Max

Posted: Sun Apr 23, 2006 10:14 am

MadMax wrote:

Rod,

Pandora's box... sorry.

Is it code that the sand be there?

My current soil structure... AFAIK...

1. Gravel - 1"
2. Clay/earth

The plan so far then...

1. Starting at existing grade, dig 6 inches for the haunches, plus 2 inches for poly/gravel.
2. Place 2" of gravel in bottom of haunch.
3. Lay poly in haunches and over the rest of the pad(s)
4. Rebar in 16" grid on 4" saddles
4a. Rebar 2" from bottom of haunch (as per drawing)
5. Pour the pads to 6" above existing grade.

The sand isn't a code requirement - it's an isolation requirement.

The 2" gravel above clay is not going to do a good job of isolating the slabs - sand is a great damping material.

Quote:

1" expansion joint along exterior perimeter.

good

Quote:

As for the different pads. I can save a pure-T butt load o' money if I could do one pour. But I don't see how I could do it and not leave some sort of steel framing/forms in the floor.

No problem - you can layout the different slabs - and install the expansion joint using steel form pins - then just remove the form pins and fill the holes with slurry after pouring. We do that all the time.

Sincerely,

Rod

Posted: Sun Apr 23, 2006 10:50 am

Rod Gervais wrote:

The sand isn't a code requirement - it's an isolation requirement.

The 2" gravel above clay is not going to do a good job of isolating the slabs - sand is a great damping material.

I mis-quoted the depth of the existing gravel. It's not 1", it's more like 6-8" of crush-n-run... I just went out and tried to dig a test hole w/o much luck. Jebus is that stuff compacted and HARD!

Would 4" of sand in the haunch alone be sufficient?

Quote:

No problem - you can layout the different slabs - and install the expansion joint using steel form pins - then just remove the form pins and fill the holes with slurry after pouring. We do that all the time.

Got it... I thinks.

Since it's an existing closed structure, I can pour from the back of the structure out to the front existing opening, pulling the pins as we come out... backfilling with the slurry... DOH! Confused

Like I said, Pandora's box...

I'd like your opinion on the slab finish... I'll probably pick a dye as opposed to grey... but, how hard of a finish should I get the contractor to power trowel? I'm gonna suspect that the harder, the better, but it's not like I do this for a livin' or anything!

Rod, I can't thank you enough!!!

Max

Posted: Sun Apr 23, 2006 5:09 pm

MadMax wrote:

I mis-quoted the depth of the existing gravel. It's not 1", it's more like 6-8" of crush-n-run... I just went out and tried to dig a test hole w/o much luck. Jebus is that stuff compacted and HARD!

Would 4" of sand in the haunch alone be sufficient?

Well it might - but that's the best I can tell you - I have always used it under the entire slab- which then damps the entire slab.

You can try it if you wish......... but I can't give any promises.....

Quote:

Got it... I thinks.

Since it's an existing closed structure, I can pour from the back of the structure out to the front existing opening, pulling the pins as we come out... backfilling with the slurry... DOH! Confused

We don't usually pull the pins until the concrete has set up enough to begin finishing.

Quote:

I'd like your opinion on the slab finish... I'll probably pick a dye as opposed to grey... but, how hard of a finish should I get the contractor to power trowel? I'm gonna suspect that the harder, the better, but it's not like I do this for a livin' or anything!

Max,

colors are up to you - as far as finish goes - I would want as flat and smooth as is possible......... a very good steel trowel finish is what you should buy..........

Rod

Posted: Sun Apr 23, 2006 6:03 pm

[quote="Rod Gervais"]Well it might - but that's the best I can tell you - I have always used it under the entire slab- which then damps the entire slab.

You can try it if you wish......... but I can't give any promises.....[quote]

Understood... Looks like I get to remove some of that crush-n-run. Oh joy.

Quote:

colors are up to you - as far as finish goes - I would want as flat and smooth as is possible......... a very good steel trowel finish is what you should buy..........

Just wasn't sure if there was any reason to get the contractor to do otherwise.

Thanx again,

Max

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