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RE: slab on grade

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Nothing crazy about it.

Make sure the design method you use allows for sub-grade friction effects
for shortening, differential shrinkage and differential temperature between
the top and bottom surfaces. 

Use a higher strength concrete than normal to reduce shortening and
increase cracking strength.

Place the vapour barrier immediately below the slab on top of 2in of
levelled sand to reduce friction and make sure it is not punctured.

Definitely specify that 25% of prestress force (to each strand, not 100% to
25% of strands) be applied on the morning after the pour with the remainder
of the prestress force applied as soon as the on site measused concrete
strength reaches the recommended minimum for the anchorages.

Make sure you cure the slab well immediately it is possible, preferrably
with ponded water..

As you will probably be using unbonded strands, the addition of fibres
would not hurt as well.

At 12:19 4/01/99 -0600, you wrote:
>Crazy thought.  Post-tension it to limit the cracking.
> -----Original Message-----
>From: Jerry D. Coombs [SMTP:jdcoombs(--nospam--at)]
>Sent: Monday, January 04, 1999 2:02 PM
>To: 'seaint(--nospam--at)'
>Subject: RE: slab on grade
>Yeah, that's taken care of, I have soil info.  Out here in sunny NM, I
>very sandy, fine granular, low (-0-) PI high bearing soil with
>dry conditions.  I'll prep it to get good uniform bearing,    I just want
>it to hold together when it shrinks and have a reason for what I do
>than use a 5" slab with #5's @12"OCEW and call it good.  Seasonal (and
>daily) temperature swings are pretty drastic.  I know it'll crack, but
>where and how much.
> -----Original Message-----
>From: Cain, William [SMTP:bcain(--nospam--at)]
>Sent: Monday, January 04, 1999 11:47 AM
>To: 'seaint(--nospam--at)'
>Subject: RE: slab on grade
>Before you go any further, run, don't walk to a good geotechnical
>engineer to determine what your soil conditions are.  If you have an
>expansive soil, the slab design will need to be quite different than if
>it's not.  I imagine your fee is probably pretty low (compared to the
>risk of problems) and these types of projects can go south in a hurry.
>Bill Cain, SE
>Oakland, CA
> -----Original Message-----
> From: jdcoombs(--nospam--at) [SMTP:jdcoombs(--nospam--at)]
> Sent: Monday, January 04, 1999 10:35 AM
> To: 'seaint(--nospam--at)'
> Subject: slab on grade
> Happy New Year, y'all
> I need to put in a good slab for outdoor basketball courts.  The
> would like for them to be free of joints.  It will be an
>unloaded slab, no
> traffic.  The slabs will be 100' x 65'.  I can spec a proper
>subgrade spec.
>  How do I design for the slab thickness and reinforcing required
>to keep
> cracking controlled.  I have reviewed  ACI 302 and 360, and the
>PCA guide.
>  None really address the unloaded slab except fot the "subgrade
> equation", which is not really a qualitative approach.  I'm
>willing to add
> fibers to get a good crack control, but how does one balance
>between that
> and steel?  How thick a slab?  How much steel?  How much fibers?
> comments are appreciated.
> Jerry D. Coombs, PE
> Albuquerque

Regards  Gil Brock
Prestressed Concrete Design Consultants Pty. Ltd.
5 Cameron Street Beenleigh Qld 4207 Australia
Ph +61 7 3807 8022		Fax +61 7 3807 8422
email:	gil(--nospam--at)