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slabs on grade (sorry it is long)

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Not to beat a dead horse, but this whole slab on grade thing bothers me. Why
don't we know more about this stuff by 2003? It would seem we could have
some type of consensus after 150 years or so of slabs on grade, which are on
every house and driveway in the US. I believe Gail is helping to write a
book on the subject right now, so maybe she can chime in.

We have the complete set of 2002 "ACI Manual of Concrete Practice" which has
ACI 302 in it. Now it says place a layer of compactable fill OVER the vaper
RETARDER (not barrier..). It does not tell you what this type of material
may be , or the thickness. What Scott said about the layer sitting on site,
possibly getting rained on, then trapping moisture between the retarder and
the slab makes sense. But what the ACI says makes sense too, about the
ability of the water to bleed into a sub base rather then getting trapped by
the retarder. I guess it is an in the field timing issue. Ideally, place the
retarder, place the base over top, then pour the slab in a short amount of


How do you get a base over a retarder without using heavy equipment or other
equipment that would damage the retarder?

How do you compact it?

How much time can you allow or should you instruct to elapse between placing
the base and pouring the slab?

We have never specified this and have had no real problems that I know of,
should we bother changing?

Polypropolene Fibers- we have been specifying these in lieu of WWF for about
3 years with no problems that I know of. Not to say there is no cracking,
but no calls about them. We know they do not prevent cracks or keep them
from opening up, just limit them and prevent the small spider cracks... But
have you ever seen WWF done correctly on a slab on grade? I believe in WWF
if it is done properly. You can tell them 1.5" from the surface until you
are blue in the face, but will you get it? How do you keep the contractor
from using rolls of rusty WWF laid on the ground. Once they are rolled up,
you cannot get them flat, so they do the wave in the slab, half in the dirt.
Then the guy operating the shoot walks all over the WWF and meanwhile some
poor sap with a rake or his hands is trying to pull the WWF up....

Control Joints- I did not know the ACI reccs that close of spacing, 24-36 x
slab thickness, or 18ft. We will be making that adjustment. Is this what
everyone else has been doing? We do take special care to detail every
control joint in a building slab, sometimes even on a seperate plan. I also
read somewhere the joints should be cut within 4-6 hours or forget about it,
because the concrete is already setting and shrinking and the joints are

Laying tile over a crack: besides the great advice of a control joint in the
tile, I just saw on some home improvement show where they laid tile on a
cracked slab. They first put down a elastomeric or similar self leveling
substance that remains very flexible. They explained that it allows the tile
to "float" over the cracks so the cracks can open or close and the tile will
hopefully not do the same....

Any other info or opinions on SOG are welcome!!


Andrew Kester, EI
Longwood, FL

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