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Re: Slab on grade

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I mentioned it in my first 2 posts.................

-----Original Message-----
From:   Jim Kestner [SMTP:jkestner(--nospam--at)]
Sent:   Thursday, October 08, 1998 9:32 AM
To:     seaint(--nospam--at)
Subject:        Re: Slab on grade

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
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
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

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
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

I hope this helps. Good Luck.


Jim Kestner
Somerville, Inc.
Green Bay, Wi

Brian K. Smith wrote:

> James F Fulton wrote:
> > For control joints, there is a rule of thumb that says space control
> > (contraction) joints in feet at between 2 to 3 times the slab thickness in
> > inches, for a 6" slab this amounts to 12' to 18' joint spacing.
> >
> The rule of thumb you are proposing is also printed in ACI and PCA
> publications; however, it is intended to control shrinkage cracks in
> non-reinforced slabs.  We use it frequently for area paving (parking
> lots) with close joint spacing and no reinforcement.
> To determine joint spacing for a reinforced slab, it is more appropriate
> to use the subgrade drag theory/equation.  I am surprised no one has
> mentioned this.
> Brian K. Smith, P.E.
> Bossier City, Louisiana