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• To: seaint(--nospam--at)seaint.org
• Subject: Re: Slab on grade
• From: Jim Kestner <jkestner(--nospam--at)somervilleinc.com>
• Date: Thu, 08 Oct 1998 11:31:31 -0500

```First of all, I assume that you are in an area where clay swelling or
shrinkage is not a problem. This will cause very significant cracking of the
slab. If this is the case, you must provide additional reinforcement in the
slab to resist these forces. Also, I assume that you are using normal
concrete, not shrinkage compensating cement. My comments are as follows:

1. The old rule of thumb for control joint spacing (in feet) is 3 to 5 times
the slab thickness (in inches). Try to stay closer to the lower limit of 3.
This applies to slabs with no or minimal reinforcement like WWF or synthtic
fibers.

2. Obviously the more reinforcement that you put in a slab, the farther you
can go between control joints. With #4 @ 12" o.c., it seems reasonble to
use  approximately a 27 x 27 pattern. We recommend, however, that only 1/2
of the reinforcement be extended thru the joints, otherwise, you may get to
much shrinkage restraint accumulation along the total length of the pour.

3. There is a formula for calculating the area of steel required in
slabs-on-grade with a certain control joint spacing. It is As = f L w / 2 fs
where As = area of reinf. in sq. in. per foot, f = 1.5 subgrade drag
coefficient, L = joint spacing in feet, w = wt. of slab in psf, fs = 40,000
psi reinf. stress.

4. There are many other things that can cause cracking which you must also
control including curing, slump, surface evaporation, temperature, etc.

5. If the slab is going to be poured before the roof is on, I would
encourage you to use an evaporation retardant such as "Confilm" or
"Euco-Bar".

I hope this helps. Good Luck.

Regards,

Jim Kestner
Somerville, Inc.
Green Bay, Wi

Phil Hodge wrote:

> I am writing this not as an engineer, but as an owner/contractor.  My
> experience, both as SE and contractor, is exclusively with steel.  All I
> know about concrete is that it cracks.
>
> I am acting as engineer/contractor/owner/banker/inspector/anything-else-
> anyone-can-think-of on a 54'x105' hangar.  (I've always heard that
> anyone who acts as their own attorney is employing a fool, does the same
> apply to engineers?  It's fun wearing all the hats!)
>
> I plan the SOG to be 6" 3000 psi concrete with #4@12" both ways in the
> middle, and a 12"x12" thickened edge, tapered to normal slab thickness
> at 2:1, on 4" clean gravel.  The footings are not cast integrally with
> the slab.  Soil is well compacted red clay common to eastern Tennessee.
> Use will be for planes less than 3200#, plus dump trucks, backhoes, and
> any other toys my wife acquires.  The finisher is just that, a
> finisher.  He knows nothing about the stuff we're supposed to worry
> about - rebar placement, crack spacing, mix, etc.  Three questions:
>
> 1)  How often should I cut control joints.  I will form and pour the
> slab in two pours on seperate days, each 27'x105'.  I want the minimum
> number of joints, but I also don't want any random cracks.  Suggestions?
>
>
> 3)  Is there anything in the above that jumps out at those experienced
> with concrete and shouts "This guy is an idiot"?  Anything I should do
> different?  Again, suggestions?
>
> By the way, if there are any pilots out there comfortable on 1900' of
> grass, The 100 Aker Wood private field is at N85d46' W84o46'.  Drop in
> someday.
>
> TIA
>
> Phil Hodge
> phil(--nospam--at)joistdesign.com
>

```